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Fundamentals of
Motor Vehicle
Book 2
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Fundamentals of
Motor Vehicle
5th Edition
Book 2
V.A.W. Hillier, Peter Coombes & David Rogers
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Text © V. A. W. Hillier 1966, 1972, 1981, 1991, 2006, P. Coombes 2006,
D.R. Rogers 2006
The rights of V. A. W. Hillier, P. Coombes and D.R. Rogers to be identified as authors
of this work has been asserted by them in accordance with the Copyright, Designs
and Patents Act 1988.
All rights reserved. No part of this publication may be reproduced or transmitted in
any form or by any means, electronic or mechanical, including photocopy,
recording or any information storage and retrieval system, without permission in
writing from the publisher or under licence from the Copyright Licensing Agency
Limited, of 90 Tottenham Court Road, London W1T 4LP.
Any person who commits any unauthorised act in relation to this publication may
be liable to criminal prosecution and civil claims for damages.
First published in 1966 by:
Hutchinson Education
Second edition 1972
Third edition 1981 (ISBN 0 09 143161 1)
Reprinted in 1990 (ISBN 0 7487 0317 9) by Stanley Thornes (Publishers) Ltd
Fourth edition 1991
Fifth edition published in 2006 by:
Nelson Thornes Ltd
Delta Place
27 Bath Road
GL53 7TH
United Kingdom
06 07 08 09 10 / 10 9 8 7 6 5 4 3 2 1
A catalogue record for this book is available from the British Library
ISBN 0 7487 8099 8
Cover photograph: Aston Martin V12 Vanquish by David Kimber/Car and Bike
Photo Library
Page make-up by GreenGate Publishing Services, Tonbridge, Kent
Printed and bound in Slovenia by Korotan – Ljubljana Ltd
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List of abbreviations
Purpose of the transmission system
Transmission types
History of electronic control
Sensors and actuators used in
transmission systems
Clutch electronic control
Manual gearbox electronic control
Torque converter electronic control
Automatic gearbox transmission
Continuously variable transmission
Light hybrid powertrain technology
Electronic differential and four-wheel
drive control
Transmission diagnostics
Transmission summary
Introduction to electronic petrol
injection systems
Petrol injection system examples
(multi-point injection)
Single-point (throttle body)
petrol injection
Direct petrol injection
Emissions and emission control
(petrol engines)
Engine management (the conclusion)
Engine system self-diagnosis (on-board
diagnostics) and EOBD
Modern diesel fuel systems
The rotary diesel injection pump
Cold-start pre-heating systems
Electronic control of diesel injection
(common rail systems)
Emissions, reliability and durability
Electronic ignition systems
(early generations)
Computer controlled ignition systems
Distributorless and direct ignition
Spark plugs
Application of electronics and computers
‘Electronic systems’ or ‘computer
controlled systems’
Electronic control units (ECUs)
Sensors: a means of providing information
Examples of different types of sensor
Obtaining information from analogue
and digital sensor signals
Actuators: producing movement and
other functions
Examples of different types of actuators
ECU/actuator control signals
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four-wheel drive
automatic brake differential
anti-lock braking system
alternating current
analogue to digital
traction control
automatic transmission fluid
controller area network
capacitor discharge
compression ignition
carbon monoxide
carbon dioxide
central processing unit
cornering stability control
constantly variable transaxle (Ford)
continuously variable transmission
direct current
dynamic drift control
dynamisches repelprogramm – German for
dynamic control program
direct-shift gearbox
electronic brake force distribution
electronic control unit
electronic diesel control
electronic differential lock
European Economic Community (now EU)
exhaust gas recirculation
European on-board diagnostics
electronic stabilisation programme
European Union
European extra-urban driving cycle
evaporative emissions
grand touring
homogeneous charge compression ignition
heated exhaust gas oxygen (Ford)
high tension
internal combustion
integrated starter–generator
light emitting diode
limited operating strategy
limited slip differential
manifold absolute pressure
malfunction indicator lamp
mechatronics transmission module
nitric oxide
nitrogen dioxide
oxides of nitrogen
negative temperature coefficient
on-board diagnostics
overhead cam
powertrain control unit
parts per million
Porsche traction management
pulse width modulated
Society of Automotive Engineers (USA)
sports utility vehicle
revolutions per minute (abbreviated to
rev/min when used with a number)
traction control system
transmission control unit
top dead centre
variable bleed actuator
verteiler – German for distributor (VE is used
by Bosch for a type of diesel injection pump)
wide open throttle
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We should like to thank the following companies for
permission to make use of copyright and other material:
Audi AG
BMW (UK) Ltd
Robert Bosch Ltd
Haldex Traction AB
Haynes Publishing Group
Jaguar Cars Ltd
LuK GmbH & Co
Porsche Cars (GB) Ltd
Siemens VDO Automotive
Toyota (GB) Ltd
Volkswagen (UK) Ltd
Every effort has been made to trace the copyright
holders but if any have been inadvertently overlooked
the publishers will be pleased to make the necessary
arrangement at the first opportunity.
Although many of the drawings are based on
commercial components, they are mainly intended to
illustrate principles of motor vehicle technology. For this
reason, and because component design changes so
rapidly, no drawing is claimed to be up to date.
Students should refer to manufacturers’ publications for
the latest information.
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Chapter 1
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what is covered in this chapter . . .
Application of electronics and computers
‘Electronic systems’ or ‘computer controlled systems’
Electronic control units (ECUs)
Sensors: a means of providing information
Examples of different types of sensor
Obtaining information from analogue and digital sensor signals
Actuators: producing movement and other functions
Examples of different types of actuators
ECU/actuator control signals
1.1.1 The increased use of electronic
and computer controlled
Modern motor vehicles are fitted with a wide range of
electronic and computer controlled systems. This book
details most of these systems and explains their
operation, as well as giving guidance on maintenance,
fault finding and diagnosis.
However, it is important to remember that
electronic or computer control of a system is often
simply a means of improving the operation or efficiency
of an existing mechanical system. Therefore many
mechanical systems are also covered, especially where
their function and capability has been improved
through the application of electronics and computer
control. See Hillier’s Fundamentals of Motor Vehicle
Technology Book 1 for explanations of the basic
mechanical systems that still form a fundamental part
of motor vehicle technology.
There are of course many electronic systems that do
not influence or control mechanical systems; these pure
electric/electronic systems are also covered.
There are many reasons for the increased use of
electronic systems. Although vehicle systems differ
considerably in function and capability, they rely on the
same fundamental electrical and electronic principles
that must be fully understood before a vehicle technician
can work competently on a modern motor vehicle.
1.1.2 Why use electronics and
computer control?
Most people who witnessed the cultural and
technological changes that occurred during the last 30
years of the twentieth century would probably regard
the electronics revolution as having had the greatest
impact on their working lives, significantly affecting the
rest of their lives as well. Although we are primarily
concerned with the motor vehicle here, electronics have
had a substantial and fundamental impact on the way
we live and particularly on the way we work. Electronic
systems affect almost all aspects of our lives, with the
design and production of consumer products being
particularly affected. Domestic goods, entertainment
systems and children’s toys have all changed
dramatically because of electronics. While all of the
above examples are obvious and important, electronics
has also enabled computers to become everyday
commodities for professional and personal use.
Why have electronics had such an impact on our
lives and the things we buy and use? A simple answer
could be that they are now much more affordable, but
this alone would not be a complete answer. The
application of electronics to so many products has
enabled dramatic improvements in the capability and
function of almost all such products. A simple
example is the process of writing a letter, which
progressed from being hand written to being created
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Introduction to powertrain electronics
Fundamentals of Motor Vehicle Technology: Book 2
Figure 1.1 Components used in a typical modern electronic computer controlled vehicle system (engine management system)
on a mechanical typewriter. The mechanical
typewriter was improved by the use of electronics, but
the introduction of the computer allowed businesses
and then individuals to produce letters with much
greater stylistic freedom. The computer allows the
user to correct errors, check spelling, change the
layout and achieve a more professional letter than
was ever possible with any of the previous methods.
This book has been produced using computers, with
the author typing the original text and producing
some of the illustrations on computer. The original
documents were then passed electronically (by e-mail)
to the production company, which used computers to
create the final style and prepare the book ready for
printing (the printer also uses computers and
Apart from the quality improvements already
mentioned computers have brought greatly increased
speed; this book would have taken much longer to write
and produce without the benefit of electronics and
computers. This is true of virtually everything that
makes use of electronics. Speed and efficiency are
important, but improvements in almost every way can
be achieved using electronics and computers.
So if we go back and again ask the question ‘Why
use electronic control?’ we can perhaps now provide a
number of answers, including improvements in speed,
in capability or function and in quality. The fact that
electronics are now much more affordable and
electronic components considerably smaller than in
the past, facilitates wide use of electronics, resulting in
all of those benefits so far discussed and many more.
1.1.3 Why use electronics and
computer control on the motor
Since the late 1960s motor vehicles have been fitted
with an increasing range of electronics and computer
control. Cost and size reductions are obviously
important because of the production volumes of
vehicles, space considerations and the need to keep
down the price paid by consumers (the people and
companies that buy the vehicles).
Reducing emissions and improving safety
Electronics and electronic control (or computer
control) have become increasingly necessary in motor
vehicles. For example, without electronic control of
vehicle systems (primarily the engine management
and emission control systems), emissions from engines
could not have been reduced by so much. Legislation
has imposed tighter control on emissions; a balance
has been struck between what is wanted and what can
be achieved. The legislators seek continued reductions
in emissions and the vehicle manufacturers have been
able to achieve tremendous results, but without
electronics it would not have been possible to reduce
emissions to anywhere close to the current low levels.
Safety is another area where electronics have
enabled improvements. The design of a motor vehicle
is very dependent on computers that can analyse data
and then help to incorporate improved safety into the
basic vehicle structure. Safety systems such as anti-lock
brakes (ABS) and airbag systems could not function
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‘Electronic systems’ or ‘computer controlled systems’
Consumer demand
One other important issue is consumer demand or
expectation. Not very long ago, only the most expensive
vehicles had electronic or computer controlled luxuries.
However, it is now expected that cheaper high volume
vehicles will also have electronically controlled systems,
including the ABS and airbag systems. In fact ABS is now
standard on vehicles sold across Europe. Further
examples include: air conditioning with electronic
control (climate control), electric seat adjustment (often
using electronic control), sophisticated in-car
entertainment systems (CD and DVD systems, etc.), as
well as driver aids such as satellite navigation or
dynamic vehicle control systems. In fact, consumer
expectations for more and more electronically controlled
vehicle systems is only matched by the desire of vehicle
manufacturers to sell more and more of these systems to
the consumer. When new or improved systems and
features are developed, the vehicle manufacturing and
sales industries are only too willing to offer them to
consumers, who then develop an expectation.
Without electronics, almost all of these new safety
systems, the modern emission systems and other
systems would not be affordable, and would certainly
not be as functional or as efficient.
Key Points
anywhere like as efficiently or reliably without the use
of electronics.
Electronic controls are now used for almost all
vehicle systems
Emissions regulations are a key factor in the
increasing use of electronic and computer control
1.2.1 Different levels of
sophistication and
Electronic enhancement or computer control
Although different people will provide different
definitions of electronic systems and computer controlled
systems, it is possible for the purposes of this book to
clearly separate the two types of system, as follows.
Electronic systems
An electronic system uses electronics to improve the
safety, size, cost or efficiency of a system, but the
electronics do not necessarily control the system.
For example the evolution of motor vehicle lighting
systems shows how electronics can be used on a simple
system. Figure 1.2 shows a headlight circuit that is
switched on by the driver when the light switch is
turned to the appropriate position. When the switch is
in the correct position, it allows electric current to flow
from the battery directly to the light bulbs. The
disadvantage of this type of circuit is that all of the
current passes through the light switch and through all
of the wiring; the switch and wiring must therefore be
of high quality and able to carry the relatively high
current (which creates heat).
Figure 1.2 Simple headlight circuit
Figure 1.3 Simple headlight circuit with a relay
Figure 1.3 shows the light circuit fitted with a relay.
When the driver turns the light switch to the appropriate
position, it allows electric current to pass to the relay,
which is then ‘energised’. However, to energise the relay
requires only a very low current; therefore, the switch
and the wiring will be subjected to neither high current
nor heat, and can be produced more cheaply. When it is
energised, the relay contacts (or internal switch) are
forced to close (owing to the magnetic field created by
current flowing through the relay winding), which then
allows a larger electric current to pass from the battery
through to the light bulbs.
If the relay is located close to the light bulbs, the wire
carrying the high current is relatively short, and because
the longer length of wire between the switch and the
relay carries only a low current, it can cost less than the
wire required in Figure 1.2. As well as the reduced cost
of the wiring, the reduced current and heat passing
through the light switch and much of the wiring
provides a safety benefit, allowing a less expensive
switch to be used.
Figure 1.4 shows almost the same wiring circuit as
Figure 1.3 but the relay has been replaced by an
electronic module. The electronic module performs the
same task as the relay but does not contain any moving
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Introduction to powertrain electronics
Figure 1.4 Simple headlight circuit using an electronic module
parts: there are no contacts or internal switch. The
module can consist of very few simple electronic
components (transistors and resistors, etc.), which are
inexpensive and reliable.
Note, however, that the module does not control the
lighting circuit (as is also the case with the relay); it
simply completes the lighting circuit in response to
input from the driver (when the light switch is turned to
the appropriate position).
Computer controlled systems
A computer controlled system could generally be
defined as a system in which some of the actions or
functions are automated, as opposed to being
controlled by the driver or passenger. Using the simple
example of the light circuit again, computer control
could automatically switch on the lights when it
became dark, such as at night or when the vehicle
passes into a tunnel.
For control to be automated, the computer would
need information from a sensor. A light sensor can be
used to detect the amount of light and pass an electrical
signal (proportional to the amount of light) to the
computer. The computer would then respond to the
electrical signal; i.e. if the signal had a specific value or
went above or below a certain value, the computer
would then switch on the lights.
It is possible that a simple version of an automated
light system could use a sensor that is simply a switch,
which provides either an on or off signal to the
computer. When the light fades to a certain level, the
switch could close, thus completing the light circuit.
Figure 1.5 shows a headlight circuit where a light sensor
has been included between the light switch (operated
by the driver) and the electronic module. This is
effectively the same circuit as shown in Figure 1.4, with
the addition of a simple light sensor switch. In this
example, the sensor simply forms part of the circuit
between the main switch and the electronic module;
therefore if the light switch is in the on position, the
lights will be switched on when the natural light fades
below the specified level. This type of system would not
represent a fully computerised system.
However, Figure 1.6 shows a similar circuit where
the electronic module is replaced by a more
sophisticated computer module or electronic control
Fundamentals of Motor Vehicle Technology: Book 2
Figure 1.5 Headlight circuit with an electronic module and a light
sensor switch
Figure 1.6 Computer controlled headlight circuit with a light
unit (usually referred to as an ECU). In this example,
the light sensor is directly connected to the ECU and
provides a signal that varies with the amount of light,
i.e. the voltage generated by the sensor could increase
or decrease as the light reduces. The computer would
then effectively make the decision as to when the lights
were switched on.
It is then in fact possible to increase the functionality
of the computer by adding more sensors. For example, a
rain sensor could be fitted to the vehicle to provide
automatic operation of the windscreen wipers. The
signal from the rain sensor could then also be passed to
the light system ECU, thus allowing the ECU to switch
on the lights when the rain sensor detected rain.
Although the above example is relatively simple, it
shows that a modern computer controlled system uses a
computer or ECU to control actions and functions,
depending on the information received. Many computer
controlled systems make use of a large number of
sensors passing information to the ECU, which may in
turn be controlling more than one action or function.
The above examples of headlight circuits represent ECU
controlled functions, i.e. switching on a light bulb.
However, when an ECU controls an action, it usually
does so by controlling what is referred to as an actuator.
Electric motors and solenoids are typical actuators that
can be controlled by an ECU; a number of examples will
be covered and explained within this book.
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‘Electronic systems’ or ‘computer controlled systems’
Actuators that could be fitted to an engine
management system
Fuel injector solenoid (for fuel quantity control).
Idle speed stepper motor (for idle speed control).
Exhaust gas recirculation solenoid valve (part of an
emission control system).
Turbocharger wastegate solenoid valve (controlling
turbocharger boost pressure).
Ignition coil (in this instance, the ECU is in fact
controlling the ignition timing when it switches the
ignition coil on/off, although strictly speaking the
ignition coil is not an actuator).
Figure 1.7 ECU controlled circuit with a single sensor and single
Sensors that could be fitted to an engine
management system
Engine coolant temperature sensor.
Air temperature sensor (ambient).
Air temperature sensor (intake system).
MAP (manifold absolute pressure) sensor (an intake
manifold pressure/vacuum sensor for an indication
of engine load).
Crankshaft position sensor (identifies the crankshaft
position for ignition and fuel injection timing, and
also indicates engine speed).
Camshaft position sensor (providing additional
information for ignition and fuel injection timing).
Throttle position sensor (indicates the amount of
throttle opening and the rate at which the throttle is
opened or closed).
Boost pressure sensor (indicates the boost pressure
in the intake manifold that has been created by the
Lambda sensor 1 (indicates the oxygen content in
the exhaust gas passing into the catalytic converter,
which enables the ECU to correct the fuel mixture).
Lambda sensor 2 (indicates the oxygen content in
the exhaust gas leaving the catalytic converter,
which helps the ECU assess if the catalytic converter
is functioning efficiently).
The ECU controlled system shown in Figure 1.8 is in
fact typical of a modern engine management system,
although this example does not show all of the sensors
and actuators that could be fitted. The example does
however illustrate a number of sensors and actuators
that can be controlled on a typical vehicle system that is
fully computer controlled. The engine management
system is a good example of the absence of driver input
to the control of the system (apart from placing a foot
on the throttle to select the desired speed).
Key Points
An ECU controlled system
As shown above, an ECU receives information from
sensors, makes calculations and decisions, and then
operates an actuator (or provides signals for electronic
components such as digital displays).
The essential point to remember is that an ECU
cannot achieve its main objective, which is to operate
an actuator or electronic component, unless the
appropriate signals are received. This is true of all ECU
controlled vehicle systems, and almost all other
computers: some form of input signal is required before
a calculation and control process can take place. Even a
normal PC (personal computer) used to write a letter
requires inputs from the keyboard and mouse before the
words are displayed on the monitor or before the letter
can be printed or e-mailed.
Figure 1.7 shows the basic principles of almost all
ECU controlled systems, whereby a sensor produces
some form of electrical signal, which is passed to the
ECU. The ECU uses the information provided by the
signal to make the appropriate calculations, and then
passes an electric control signal to an actuator or digital
component such as the dashboard display.
Figure 1.8 shows a more complex arrangement for
an ECU controlled system. This example would be
typical of an early generation fuel injection system
where the ECU is controlling a number of actuators and
where a number of sensors are used to provide the
required information.
All complex systems can be considered as having
inputs, control and outputs
Sensors usually provide inputs, and actuators are
controlled by ECU outputs
Figure 1.8 ECU controlled circuit with multiple sensors and
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Introduction to powertrain electronics
Fundamentals of Motor Vehicle Technology: Book 2
See Hillier’s Fundamentals of Motor Vehicle Technology
Book 3 for more detailed information about the
electronic components used in an ECU.
1.3.1 Decision making process
The electronic control unit is often referred to by many
other names, such as electronic control module, black
box or simply the computer. However, the most
commonly used name is the electronic control unit,
which is generally abbreviated to ECU.
Although the ECU can provide a number of functions
and perform a number of tasks, it is primarily the ‘brain’
of the system because it effectively makes decisions. In
reality, however, an ECU makes decisions based on
information received (from sensors) and then performs
a predetermined task (which has been programmed into
the ECU). Whereas a human brain is capable of ‘free
thinking’, an ECU is very much restricted in its decision
making process because it can only make decisions that
it has been programmed to make.
To compare free thinking with programmed decision
making, imagine a car driver approaching a set of traffic
lights when the green ‘go’ light is replaced by the amber
‘caution or slow down’ light. The driver can make a
decision either to slow down, or to accelerate and get
across the lights before the red ‘stop’ light is
illuminated. This decision is based on an assessment of
the conditions; different drivers will make different
decisions, and in fact one driver could make different
decisions on different occasions even if the conditions
were identical. To make a similar decision as to whether
to slow down or accelerate, an ECU would also assess
conditions such as vehicle speed and distance to the
traffic lights, as well as road conditions (wet, icy, etc.).
The ECU would then make the decision based on the
programming. If the conditions (information) were the
same on every occasion, the ECU would always make
the same decision because the programming dictates
the decision (not free thinking).
In reality, ECUs and computers in general are
progressively becoming more sophisticated, and their
programming is becoming increasingly complex. ECUs
can adapt to changing conditions and can ‘learn’, which
allows alternative decisions to be made if the original
decision does not have the desired effect. A human can
make a decision based on knowledge or information; if
the first decision does not then produce the desired
result, an alternative decision can be made because the
human brain possesses the ability of free thinking.
Modern ECUs do have a similar capability but it is a
programmed one, designed by humans.
The decision making capability of an ECU is
therefore dependent on the volume and accuracy of
information it receives, and the level of sophistication of
the programming.
1.3.2 Control
Having been designed with the capacity to make a preprogrammed decision, an ECU can then be used to
control other components. A simple example is the use
of an ECU to switch on an electric heater when the
temperature gets cold. Information from a temperature
sensor would inform the ECU that the temperature was
falling; it could then switch on an electrical circuit for
the heater.
With a simple version of this system, the ECU could
be programmed to switch on the heater at a
predetermined low temperature, and switch off the
heater when the temperature has risen to a
predetermined high temperature. Such a system would
result in the temperature rising and falling in cycles as
the heater was turned on and off. Note that the
temperature sensor could be a simple switch that
opened or closed at a predetermined temperature,
providing an appropriate signal to the ECU.
A more sophisticated system could however be
designed to maintain the temperature at a more
constant level. If the ECU was designed so that it could
control the electric current passing to the heater, this
would enable the heater to provide low or high levels of
heat. The ECU program could include the assessment of
how quickly or slowly the temperature was falling or
rising, so that the ECU could switch on part or full
power to the heater. If the temperature was falling
rapidly, the ECU could switch on full power to the
heater. If the temperature was falling slowly, the ECU
would need only to switch on part power to the heater.
In this more sophisticated system, the temperature
sensor would have to indicate the full range of
temperature values to the ECU, i.e. the signal from the
sensor would have to change progressively with change
in temperature; the ECU could consequently assess the
rate at which temperature was changing.
With the appropriate information from one or more
sensors, the ECU can be programmed to provide the
appropriate control over a component (such as the
heater). The achievement of better or more
sophisticated control of a component inevitably requires
more sophisticated and complex programming of the
ECU. However, to achieve the required level of
sophisticated control usually requires a greater amount
of more accurate information, i.e. a greater number of
sensors, each of which should provide more accurate
For example, compare an older fuel injection system
with a modern engine management system. Because of
tighter emission regulations and continuous efforts to
improve economy and performance, the modern engine
management system ECU must carry out many more
tasks with greater levels of control than older systems.
Figure 1.9 identifies some of the components in an early
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Electronic control units (ECUs)
Figure 1.9 Earlier generation ECU controlled fuel injection system
type of computer controlled fuelling system, which has
relatively few sensors and relatively few actuators, so
that the ECU has only a small number of tasks or
control functions to perform.
Figure 1.10 lists the components from a modern
engine management system where the ECU has a much
larger range of tasks to perform. The number of sensors
and actuators is therefore much greater than on earlier
systems. On the modern system, the ECU controls a
much larger number of other components, and in fact
has some control over other systems such as the air
conditioning system (the engine management ECU can
influence the operation of the air conditioning
compressor, so that the compressor, which is driven by
the engine, is switched off when full engine power is
Figure 1.10 Modern engine management system. The system
has a large number of sensors and actuators and the ECU
therefore has a large number of tasks and control functions to
perform including influence of other systems
1.3.3 ECU components and
Hillier’s Fundamentals of Motor Vehicle Technology Book
3 provides a detailed explanation of the components
and operations of ECUs, but a brief explanation is
required at this stage to enable the reader to appreciate
the complexity of the ECU.
Figure 1.11 Modern ECU and components
Main casing
An ECU (Figure 1.11) is, amongst other things, a
computer. Readers who use PCs or laptops will know
that they produce a considerable amount of heat. In
many cases an electric fan is used to move cooling air
around the PC or laptop to remove some of the heat. The
more powerful the computer, the more heat it produces.
An ECU is a powerful computer, and therefore produces
heat that must be removed or dissipated. Although some
very early ECUs were located on the vehicle so that a
cooling fan could help remove some of the heat, ideally
they need to be located where they are unlikely to be
exposed to moisture, as well as being isolated from
vibrations and kept away from engine heat. In general,
therefore, although not always, ECUs are located within
the passenger compartment. The ECU main casing is
usually an alloy casting which, because it can be bolted
to the vehicle bodywork, should help to dissipate heat.
As previously mentioned, a computer is regarded as the
brain of a controlled system; the ECU contains one or
more microprocessors which are the main decision
making components. As with a normal PC or laptop, the
microprocessor receives information to enable it to
make calculations (effectively the decisions). The
microprocessor then provides an appropriate output
signal, which is used to control an actuator or influence
another system (usually by communicating with
another ECU). Figure 1.12 shows the essential functions
within the ECU and the essential tasks of the
If we refer back to the example of the ECU
controlling a heater (section 1.3.2), the decisions as to
when to switch on the heater, and whether part or full
power should be used for the heater, are calculated or
decided by the microprocessor.
Amplifier (output or driver stage)
Microprocessors operate using very weak signals, i.e.
low voltage and current, so would not be directly
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Introduction to powertrain electronics
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Figure 1.13 Power transistor functioning as a switch in a light
Figure 1.12 Signal processing in the ECU
connected to the heater (section 1.3.2), which uses
much higher voltages and currents. The same applies to
an ECU that is controlling a vehicle system; most
vehicle systems operate on 12 volts with relatively high
currents, which are much higher than the voltages used
within microprocessors. To overcome the problem, the
output or control signal from the microprocessor will
usually be passed to some form of amplifier. The
amplifier receives the control signal from the
microprocessor and then provides an amplified or
stronger signal to the actuator.
The final part of the amplifier system is often
referred to as the output, power or driver stage. The
driver stage amplifier often contains a power transistor,
which may be seen mounted on the outside of the ECU
casing to help with heat dissipation. A simple power
transistor can be regarded as a switch that will switch a
high power circuit on or off when an appropriate signal
is received from a low power circuit. Therefore, if the
transistor is connected into the 12 volt circuit for an
actuator, or, for this example, a light bulb, it will switch
the light bulb on or off when the appropriate low
voltage signal is received from the microprocessor. The
signal from the microprocessor could be a simple on or
off signal: the power transistor would then switch the
12 volt circuit on or off.
Figure 1.13 shows a simple circuit where a light bulb
is switched on or off using a power transistor. Note that
the transistor is switching the earth or return part of the
12 volt circuit. The transistor receives a signal from the
microprocessor and effectively emulates or copies the
signal onto the 12 volt circuit.
There are a number of ways in which a power
transistor can switch or affect a higher power circuit.
Although a simple on or off function is commonly used,
a transistor can emulate or copy a progressively
changing input signal. Therefore, if the signal passing
into the transistor progressively rises and falls in
strength, the transistor can progressively increase and
decrease the current flow passing through the high
power circuit.
High speed switching of circuits
The ECU on a modern vehicle system is often tasked
with switching a circuit on and off at very high speed
and frequency, such as when an ignition coil or fuel
injector is switched on and off (which could occur as
often as 100 times a second on an engine operating at
high revolutions per minute). Therefore the decision
making process in the microprocessor would produce
an output signal that switches on and off at this
frequency, and the power transistor would also switch
on and off the 12 volt or power circuit at the same
Computers, including ECUs, have a memory which is
stored in a memory microchip. There are different
types of memory, but all of them essentially store a
description of the tasks that the ECU must perform.
When the microprocessor is making calculations, it
will refer to the memory or ‘talk’ to the memory to
establish what task should be performed when certain
items of information are received. As an example, if
we again refer to the computer controlled heater
system covered in section 1.3.2, the information
received by the microprocessor could indicate a low
temperature; the microprocessor would then refer to
the memory to find out what task to perform. The
memory would indicate that the task is to switch on
the heater.
The memory contains all of the necessary operating
details applicable to the system being controlled by the
ECU. For example, if the ECU is controlling a fuel
injection system, all the information about the fuelling
requirements are contained within the memory.
Therefore, if the information passed to the
microprocessor includes engine speed, engine
temperature, throttle position, etc., the microprocessor
[647] Chapter 01
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Electronic control units (ECUs)
refers to the memory to find out how long an injector
should be switched on for (how long the injector should
remain open so that the correct quantity of fuel can be
delivered). These operating details are placed or
‘programmed’ into the memory either at the time of
ECU manufacture or at a later time using dedicated
equipment (in both cases, this is referred to as the
software program). In many cases, it is possible to
reprogram the memory using modified software, which
can be useful if it is found that the original program has
a minor fault, such as causing a hesitation when the
vehicle is under acceleration.
In the memory systems discussed so far, once the
memory chip has been programmed with the operating
details, this program remains permanently in the
memory chip. However, there are situations where the
memory details change. A simple example is when a
memory chip might receive information relating to the
number of miles or kilometres that the vehicle has
travelled; this information could be used to calculate
fuel consumption. However, when the driver resets the
memory, the information is then erased, i.e. it is not
permanent. The memory chips that store this type of
information can lose it when the power is switched off,
so it is often necessary to provide a back-up power
supply using a small battery (usually contained within
the ECU) to prevent loss of data. Note that some ECUs
have a permanent power supply from the vehicle
battery (even when the ignition is switched off). In
these cases, the memory will be retained as long as the
vehicle battery is not disconnected.
Analogue and digital signals
An analogue signal can be regarded as a signal or
indicator that continuously changes from one value to
another. A good example is a speedometer using a
needle to sweep around the gauge with changes in
speed: the visual display is an analogue type display,
which shows progressive change.
A signal that relies on a change in voltage can also
be analogue. An example is the change in voltage that
occurs when a simple lighting dimmer control is altered
from the ‘dark’ to the ‘bright’ position. If a voltmeter
were connected to the output terminal of the dimmer
control (which is usually a variable resistor), the voltage
would be seen to progressively increase and decrease
when the control was altered.
A voltage signal produced by many sensors can be
an analogue signal. An example is a throttle position
sensor, which uses a variable resistor in much the same
way as the light dimmer switch: when the throttle is
opened or closed, the voltage progressively increases or
decreases (Figure 1.14).
Although earlier electronic systems relied on
analogue signals and in fact the electronics were
analogue based, modern computers and electronic
systems are generally digital systems.
A digital signal provides a stepped or pulsed signal.
A digital display can be used on a speedometer to
Figure 1.14 Analogue voltage signal produced by a throttle
position sensor
display speed in steps. These steps could be in
increments of 5 km/h or 5 mile/h. In such a case the
driver would only see the display change when the
speed increased by 5 km/h or 5 mile/h. Digital
electronic signals are also structured in steps, which
generally consist of electrical pulses.
ECUs on modern vehicles operate using digital
electronics. However, in basic terms, the digital process
consists of on and off pulses. In effect there are only two
main conditions that the ECU works with: the on and
off parts of the digital signal.
Signals that are either passing into, passing out of or
passing within an ECU should ideally also be digital
signals. These on and off pulses can then be counted by
the ECU (counting either the on parts or the off parts of
the signal). Alternatively the on and off pulses can be
used as a reference by the ECU, which could result in the
ECU performing a predefined task. The ECU does in fact
examine the digital signal in a number of ways, which
allows the ECU to extract different information from the
signal such as speed or frequency (Figure 1.15a).
In reality, when a digital signal is being used as an
information signal passing into the ECU, it does not
necessarily have to be exactly on or off. An example
could be a light switch in a 12 volt circuit, which would
produce an on signal of 12 volts and an off signal of
zero volts. However, the ECU could be programmed to
accept any voltage above 9 volts as being on, and any
voltage below 3 volts as being off. Therefore, if the
signal voltage from a sensor progressively changes
between zero volts and 12 volts (an analogue signal),
the ECU could still respond to the same programmed
voltage thresholds of 9 volts as an upper limit and 3
volts as a lower limit (Figure 1.15b). We should not
therefore always refer to a digital signal as being fully
on or off, but regard it as having upper or lower
thresholds, which can be monitored by the ECU as
reference points.
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Introduction to powertrain electronics
Fundamentals of Motor Vehicle Technology: Book 2
Figure 1.15 Analogue and digital signals
a Digital signal, where the pulses could be used to provide
speed or frequency information
b Analogue signal where the ECU locks on to the 3 volt and
9 volt thresholds reference points
c Principle of analogue to digital converters
Analogue to digital converters
Because ECUs ideally require a digital signal, some form
of conversion is necessary to change the analogue
signal from a sensor into a digital signal.
An example could be a temperature sensor, which is
used as a means to switch on a cooling fan. The ECU
could switch on the cooling fan when the sensor signal
voltage reaches the 9 volt threshold, but the ECU would
not switch off the fan until the sensor voltage fell to the
3 volt threshold (Figure 1.15b). The ECU would
therefore ideally require a modified signal that only
identified or ‘locked on’ to the 9 volt and the 3 volt
thresholds. In effect, this modification process takes
place within the ECU: an analogue signal is passed to
the ECU, which contains a converter that converts the
analogue signal into a digital signal. Because many
sensors produce analogue signals that need to be
converted to digital signals to enable the
microprocessor to function, a device known as an
analogue to digital converter (A/D converter) is used.
Figure 1.15c shows the principle of an A/D
converter and an indication of a typical analogue signal
and a digital signal. Refer to Hillier’s Fundamentals of
Motor Vehicle Technology Book 3 for more information
on analogue and digital signals as well as on A/D
Note that an ECU can also contain converters that
change digital signals into analogue signals. This might
be necessary if the actuator operates using an analogue
signal. A simple example is a fuel gauge, which may
require an analogue signal to enable the gauge needle
to indicate the fuel level. Although the microprocessor
is accurately creating the applicable digital signal, it
would need to be converted to some form of analogue
signal to operate the gauge. In reality, more and more
actuators are using digital signals.
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Sensors: a means of providing information
computers: some form of input signal is required before
a calculation and control process can take place.
Note: Understanding of the ECU and an ECU controlled
system enables a technician to perform diagnostic
processes much more easily. If the function of each
sensor and each actuator is understood, a relatively
quick diagnosis can be carried out. Although specialised
test equipment can be used, knowledge of the system
operation greatly improves the ability to perform quick
and accurate diagnosis.
Hillier’s Fundamentals of Motor Vehicle Technology
Book 3 provides an in-depth examination of the
operation and construction of some sensors and
actuators. In other chapters details of specific sensors
and actuators are dealt with in relation to specific
systems. However, the following two sections provide a
general understanding of sensors and actuators
commonly used on vehicle systems.
Key Points
The complete ECU
A fully operational modern ECU will contain those
components detailed above. Although many other
electronic components are required to make an ECU
operate, those discussed so far are the main functional
In conclusion therefore, the ECU receives
information from sensors (the information might be
either digital or analogue). The digital information
passes directly to the microprocessor, but the analogue
information must be converted to a digital signal before
being passed to the microprocessor. The microprocessor
then assesses the information, refers to the programmed
memory to find out what tasks to perform, makes the
appropriate calculations and passes an appropriate
control signal to the relevant actuator (or provides
signals for an electronic component such as a digital
display). Where the actuator is operated using higher
voltages and currents (such as a fuel injector), the weak
digital signal from the microprocessor will need to be
amplified using a power transistor or final stage.
The essential point to remember is that an ECU
cannot achieve its main objective, which is to operate
an actuator or electronic component, unless the
appropriate signals are received. This is true of all ECU
controlled vehicle systems and almost all other
ECUs contain one or more microprocessors that
carry out calculations and follow lists of
ECUs contain A/D converters that act on sensor
inputs, and D/A converters, as well as driver
circuits to control outputs
1.4.1 Sensor applications
It has previously been explained that an ECU controlled
system requires information to enable the ECU to make
the appropriate calculations and decisions, which then
in turn enables it to control actuators or electronic
devices. The greater the amount of information that can
be supplied to the ECU, the greater the control
capability and number of different control functions.
An ECU controlling an earlier generation of
electronic fuel injection system may have required only
four or five sensors to provide the required information
to it. This is because the ECU would only have been
required to control the fuel injectors and therefore only
limited amounts of information were necessary.
However, later systems that also included control of the
ignition system, idle speed and emissions devices (thus
forming an engine management system) would have as
many as 20 sensors, or more in some cases. As well as
controlling more systems, modern ECUs require more
accurate information from the sensors in order to meet
stricter emissions legislation. Sensors have therefore
become more sophisticated as well as increasing in
Whatever a sensor might be required to measure, e.g.
temperature or movement, it must be able to provide a
signal to the ECU that can be interpreted by the ECU.
Although the different types of electrical signal are
covered later in this section, an example of change in the
electrical signal would occur when temperature changes
which, for most temperature sensors, results in an
increase or decrease in the signal voltage passed from
the sensor to the ECU.
Figure 1.16 indicates the more common examples of
parameters that sensors must detect or measure on
modern vehicle systems. Many other sensor
applications are not included in the chart, but it does
provide a good indication of the types of information
and the types of applications for many sensors.
From Figure 1.16, it is possible to appreciate that
sensors perform a wide variety of measurement tasks.
The parameters most commonly measured are:
temperature (of fluids or exhaust gas)
movement (angular and linear), including
rotational sensing such as crankshaft speed
position (angular and linear), primarily for partial
rotation of components or partial linear movement
but also including exact angular position of
rotational sensors, e.g. the angle of rotation of a
crankshaft at a given time
oxygen, using a specific type of sensor used to
measure the oxygen content in the exhaust gas.
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Introduction to powertrain electronics
Fundamentals of Motor Vehicle Technology: Book 2
Figure 1.16 Sensors and sensor applications
Measurement task
Common applications
Additional applications
Engine coolant temperature
Air flow (engine load sensing)
Air mass (engine load sensing)
Ambient air temperature
Fuel/ignition/engine management/emission control
Fuel/ignition/engine management/emission control
Fuel/ignition/engine management/emission control
Fuel/ignition/engine management/emission control
Cooling fan, driver information display
Intake air temperature
Engine oil temperature
Throttle position
Fuel/ignition/engine management/emission control
Fuel/ignition/engine management/emission control
Fuel/ignition/engine management/emission control
Engine speed
Fuel/ignition/engine management/emission control
Engine intake vacuum/pressure
(engine load sensing)
Crankshaft angle position sensor
Camshaft angle position sensor
Fuel pressure
Fuel tank pressure
Boost pressure
Fuel/ignition/engine management/emission control
Oxygen (oxygen content of
exhaust gas)
Exhaust gas temperature
Position sensor for exhaust
gas recirculation valve
Wheel speed (vehicle speed)
Brake pedal position (on or off)
Acceleration/deceleration sensing
(sideways movement as well as
forward and backward movement)
Steering angle
Fuel/ignition/engine management/emission control
Fuel/ignition/engine management/emission control
Fuel/ignition/engine management/emission control
Fuel/ignition/engine management/emission control
Driver information display/air
Automatic transmission/anti-wheel
spin/other vehicle stability control/air
Automatic transmission/anti-wheel
spin/other vehicle stability control
Automatic transmission
Note: Information from other engine
management sensors will also be used
for controlling turbo or superchargers
Fuel/ignition/engine management/emission control
Fuel/ignition/engine management/emission control
Fuel/ignition/engine management/emission control
Anti-lock brakes/vehicle stability control
Driver information (vehicle speed)/
automatic transmission/airbag
Anti-lock brakes/vehicle stability control
Anti-lock brakes/vehicle stability control
Airbag/other safety systems
Vehicle stability control
Power steering
Mechanical and electronic sensing devices
Although some sensors use a combination of
mechanical and electrical components, which respond
together to movement, position or pressure (and
occasionally temperature), wherever possible most
modern sensors only use electronic/electrical
components. A typical example is a pressure sensor,
which in the past used an aneroid capsule that
deformed when the pressure changed (Figure 1.17).
The deformation of the capsule caused a rod to move;
the rod could be connected to a variable resistor which
altered the voltage in the sensor’s electrical circuit.
Later types of pressure/vacuum sensor use an electronic
component with no moving parts. Exposure to pressure
or vacuum causes the resistance of the component to
change; this change in resistance then alters the voltage
in the signal circuit (see Figure 1.17).
Figure 1.17 Two types of pressure sensor
a Capsule type pressure sensor using mechanical components
b Electronic type pressure sensor
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Examples of different types of sensor
Note: The explanations contained within this section
cover a number of commonly used sensors with
examples of the types of signal they produce. Although
other types of sensor are used in automotive
applications, they will generally be adaptations of those
covered below. However, other sensors are covered in
applicable sections within this book. For those readers
wishing to have more detailed explanations of the
electrical and electronic background to these sensors,
see Hillier’s Fundamentals of Motor Vehicle Technology
Book 3, which provides advanced studies on electrical
and electronic theory.
1.5.1 Temperature sensors
Temperature sensors (Figure 1.18a) are used in a wide
variety of applications, especially in engine control
systems, i.e. ignition, fuel and engine management.
Additional applications include air conditioning systems,
automatic transmissions and any system where
temperature control or temperature measurement is
critical to the system operation.
Temperature sensors are manufactured using a
resistance as the main component. The value of this
resistance changes with temperature. This type of
resistor is called a thermistor: the term is an
amalgamation of therm (as in thermometer) and
resistor. Because the sensor resistance forms part of an
electrical ‘series resistance’ circuit (other resistances are
contained within the ECU), when the temperature and
therefore the resistance changes, the voltage and
current in the circuit also change. The ECU, which of
Figure 1.18 Temperature sensor
a Typical appearance. The example shown is a coolant
temperature sensor from an engine management system
b Wiring for a temperature sensor
course forms part of the circuit (Figure 1.18b) and
supplies the reference voltage, will now have a signal
voltage that changes with temperature.
As with almost all modern ECU controlled systems,
a reference or starting voltage is applied to the sensor
circuit. This reference voltage originates at the ECU,
which reduces the traditional 12 volt vehicle supply to a
stabilised or regulated voltage, typically around 5 volts.
Note however that, because this circuit is used only to
provide a low power signal (and not to operate an
actuator such as an electric motor), current flow in the
circuit is very low. The current flow passes from the
ECU, through the temperature sensor resistance and
then returns to the ECU. Because the circuit is a series
resistance circuit, when the sensor resistance changes
the current in the circuit also changes, thus providing
the required temperature related signal.
There are generally two main types of resistance
based temperature sensors:
With the first type, the resistance within the sensor
decreases when the temperature increases. This type
is referred to as having a ‘negative temperature
coefficient’ (NTC).
With the second type, the resistance increases when
the temperature increases. This type is referred to as
having a positive temperature coefficient (PTC).
Temperature sensor analogue signal
With very few exceptions, temperature sensors produce
an analogue signal. The exceptions are sensors using a
switch, or contacts which close or open at specified
temperatures. In these cases the signal will be either on
or off.
Figure 1.19 Analogue signal voltage for a typical temperature
sensor circuit. Note the progressive change in voltage as the
temperature rises and falls
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Introduction to powertrain electronics
The analogue signal voltage produced by sensors with a
thermistor progressively increases or decreases with
changes in temperature. Because it is common practice
to use NTC sensors, where the resistance reduces as the
temperature increases, the signal voltage will generally
also reduce as the temperature increases. The typical
signal voltage from a temperature sensor circuit ranges
from approximately 4.5 volts when the temperature is
low, down to approximately 0.5 volts when the
temperature is high. More specific values are quoted in
Chapter 3, which describes how these sensors are used
in a fuel injection system.
Figure 1.19 shows the typical analogue output
signal voltage from a temperature sensor circuit when
temperature changes occur. Note that because the
signal is analogue, the change in voltage is progressive.
1.5.2 Rotational speed sensors
Variable reluctance type
Rotational speed sensors are used to detect speed or
revolutions per minute (rev/min) of a component; two
common examples are an engine crankshaft and a road
wheel. In both cases, the rotational speed information is
required to enable the ECU to perform its calculations.
For an engine system, the crankshaft speed information
is used for the calculation of fuel and ignition
requirements, as well as for emission control. The wheel
speed information is used to enable calculations for
anti-lock braking, wheel spin control and other vehicle
stability systems. The wheel speed information can of
course also be used to calculate road speed or distance
travelled; this information is then displayed to the
driver or can be used to calculate fuel consumption and
other information.
In most cases, rotational speed sensors work on a
simple principle, similar to that of an electrical
Figure 1.20 Typical arrangement for simple rotational speed sensor.
a Crankshaft speed sensor with a number of reluctor teeth
(reference points)
b Wheel speed sensor
Fundamentals of Motor Vehicle Technology: Book 2
generator: when a magnetic field is moved through a
coil of wire it generates an electric current. The
rotational speed sensor uses an adaptation of this
principle, which relies on altering the strength of the
magnetic field (or magnetic flux). This is achieved by
passing a ferrous metal object (iron or steel) close to or
through the magnetic field. The strength of the
magnetic field or flux increases or decreases when the
metal object is moved close to or away from the
magnetic field; this change in magnetic flux causes a
small current to be generated or induced within the coil
of wire. These sensors are often referred to as inductive
or magnetic variable reluctance sensors.
Rotational speed sensors are often constructed with
a permanent magnet located inside or adjacent to a coil
of wire. When a metal component (reluctor) passes close
to the sensor, the magnetic field or flux is altered.
However, the reluctor often takes the form of a disc,
which has one or more ‘teeth’, each of which acts as a
reluctor. Therefore, as each tooth passes the sensor, it
causes an electric current to be produced within the coil
of wire.
As shown in Figure 1.20a, a crankshaft speed sensor
can be located adjacent to the front or back of the
crankshaft, and a disc with one or more teeth, mounted
on the crankshaft, can be used as the reluctor disc. For
wheel speed sensors, a similar arrangement is used, but
the reluctor disc is located on the rotating portion of the
wheel hub, and the sensor is mounted so that it is close
to the reluctor disc. Figure 1.20b shows a similar sensor
used to measure wheel speed rotation (ABS wheel
speed sensor); note that the reluctor has a large number
of reference points.
Rotational speed sensor analogue signal
When each reluctor tooth passes the sensor, the change
in the magnetic field or flux produces a small low
voltage electric current. As each tooth passes the sensor,
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Examples of different types of sensor
Figure 1.21 Analogue signal produced by a rotational speed
sensor. Note that the voltage progressively increases and
decreases as the reluctor tooth approaches and leaves the pole
of the sensor magnet
the voltage increases and decreases, resulting in a
continuously changing voltage as the crankshaft or
wheel rotates. In fact, the current flow oscillates one
way and then the other within the circuit, and the
voltage oscillates from positive to negative. The voltage
increase and decrease is shown in Figure 1.21; note that
the highest voltage is produced when the reluctor tooth
is approaching the pole of the sensor magnet, and the
lowest voltage is produced when the reluctor tooth is
leaving the magnet pole. If there is no movement of the
reluctor tooth, there will be no current or voltage
produced, irrespective of the position of the reluctor
tooth. The signal voltage progressively increases and
decreases with the rotation, so the signal is in analogue
form. The ECU, which has an inbuilt timer or clock, is
therefore able to count the number of pulses over a
given time, and thus calculate the speed of rotation.
It should be noted that there are variations in the
way in which some rotational position sensors operate.
Some sensors use an ‘exciter coil’ which has a small
voltage applied to it, allowing a stronger signal to be
produced. Other types use a Hall effect system to
produce a signal. Both of these types of sensor are
discussed in Hillier’s Fundamentals of Motor Vehicle
Technology Book 3.
Rotational angular position sensor
In some cases, it is beneficial to be able to calculate or
assess the position of a rotating component such as a
crankshaft. If there is a means by which the ECU can
determine the position of the crankshaft during its
rotation, it is possible to control accurately the timing
of ignition and fuelling. By adapting the previously
described rotational speed sensor system, it is in fact
relatively easy to provide an angular position reference.
If, for example, the crankshaft reluctor disc has only
one reluctor tooth, this tooth could be the reference to
crankshaft angle and could therefore indicate top
dead centre (TDC) for piston number 1. In fact, this
single tooth could also provide the speed reference as
well, although the signal will only be produced once
for every crankshaft rotation. It is, however, common
practice to provide a number of teeth around the
reluctor disc (60 teeth is not uncommon), and for
each tooth to represent a particular angle of
crankshaft rotation. If there were 60 reluctor teeth,
each tooth would represent 6º of crankshaft angle
rotation. However, to establish a master reference or
master position point, it is normal practice either to
miss out one tooth or make one tooth a substantially
different shape from the other teeth (Figure 1.22).
Whichever method was used, the signal from the
sensor would contain one voltage change that was
different from the rest of the signal, and therefore
provide a master reference point such as TDC for
number 1 piston.
With a possible 60 reference points (or more in some
cases), the ECU is now able to calculate crankshaft
speed and the rotational position of the crankshaft very
accurately. In fact, the ECU can assess any increase or
decrease in crankshaft speed as each tooth passes the
sensor. Assuming there were 60 teeth or reference
points on the crankshaft reluctor disc, this would enable
the ECU to assess the change in crankshaft speed at
every 6º of crankshaft rotation. Control of ignition
timing, fuelling and emissions would therefore be far
more accurate than if only one reluctor reference tooth
were used.
Figure 1.22 Variable reluctance crankshaft position/speed sensor
with master reference point
a Crankshaft reluctor disc with a master position reference point
(missing tooth)
b Note the different shape of the signal created by the missing
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Introduction to powertrain electronics
Fundamentals of Motor Vehicle Technology: Book 2
Figure 1.23 Hall effect pulse generator
a Hall effect pulse generator
b Digital signal produced by a Hall effect pulse generator
c Hall effect system located in an ignition distributor
Many engine management systems have a position
sensor, which indicates the rotational position of the
camshaft, in addition to a crankshaft speed/position
sensor. The camshaft sensor is included because a
crankshaft TDC position reference usually relates to
more than one cylinder, e.g. cylinders 1 and 4, or
cylinders 1 and 6, so the ECU is not able to calculate
which cylinder is on the compression stroke and which
cylinder is on the exhaust stroke, whereas a camshaft
only rotates once for every engine operating cycle, i.e. a
master reference for any of the cylinders will pass the
sensor only once for every engine cycle. Therefore a
camshaft position sensor can indicate to the ECU the
position of cylinder 1 only (or any other cylinder chosen
to be the master reference cylinder), so it is possible for
the ECU to control injectors individually, timing them
accurately to each cylinder. It is also necessary to have a
cylinder reference signal for the modern generation of
ignition systems that use individual ignition coils for
each cylinder (there is no distributor rotor arm to
distribute the high tension (HT) to each spark plug).
Rotational speed/angular position sensor (Hall
Although performing a similar task to the variable
reluctance type sensors described above, the Hall effect
sensor provides a digital signal as opposed to an
analogue signal.
Hall effect principle
Figure 1.23a shows a Hall integrated circuit (IC) or Hall
chip. When a small input electrical current is passed
across chip terminals A to B (input current), and the chip
is exposed to a magnetic field (magnetic flux), a small
current is then available across C to D (output current).
A permanent magnet is located close to the Hall chip,
but the magnetic flux can be prevented from reaching
the Hall chip if a metal object is placed between the
magnet and the chip. On the example shown in Figure
1.23a, the metal object that is used to block the
magnetic flux is in fact a rotor or trigger disc, which is
mounted on a rotating shaft. The rotor disc has a
number of vanes and cut outs which, when the rotor is
turning, alternately block and allow the magnetic flux to
reach the Hall chip. The result is that the flow of current
across the chip terminals C to D will be switched on and
off in pulses. This pulsed signal can provide a speed
reference signal to an ECU. Figure 1.23b shows a typical
digital signal produced by a Hall effect pulse generator.
Hall effect ignition trigger
On some earlier generations of electronic ignition
systems, but also on some engine management systems,
a Hall effect pulse generator was located in the ignition
distributor body (Figure 1.23c). The rotor disc had the
same number of cut outs and vanes as cylinders. The
rotor disc was mounted on the distributor shaft and
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Examples of different types of sensor
rotated at half engine speed, i.e. one complete rotation
of the rotor for every engine cycle, which is two
crankshaft rotations. If the rotor had four vanes and cut
outs (for a four-cylinder engine), it would provide four
pulses for every engine cycle. The pulsed digital signal
would be passed to an ignition amplifier or to an ECU,
which would then switch on and off the ignition coil
circuit, thus producing the high voltage for the spark
plugs (see Chapter 2).
1.5.3 Position sensors for detecting
small angles of movement
The rotational position sensors described above are
designed for use on fast rotating components such as
crankshafts. However, there are a number of
components that may only partially rotate, and not in
fact do so continuously. A very common example is a
throttle butterfly or throttle plate. The throttle butterfly
is located on a spindle and may rotate through less than
90 degrees, from idle through to the fully open position.
On engine management systems and on older fuel and
ignition systems, the ECU requires information relating
to the throttle position to make accurate calculations for
fuelling and ignition timing, as well as for some other
control functions.
Almost all modern throttle position sensors (see
Figure 1.24) use a potentiometer (variable resistance),
which is usually connected to the throttle butterfly
spindle, although some types are connected to the
throttle pedal or throttle linkage. The potentiometer
provides a signal voltage that increases and decreases
when the throttle is opened and closed, equipping the
ECU with information about the angular position of the
throttle butterfly. Additionally, the ECU can detect the
rate at which the voltage increases or decreases,
enabling the ECU to calculate how quickly the driver is
intending to accelerate or decelerate. Information about
rate of change of throttle position enables the ECU to
provide more accurate fuel and ignition timing control.
Throttle position sensor analogue signal
The throttle position sensor provides a progressively
increasing and decreasing voltage when the throttle is
opened and closed. As with many other sensors, the
throttle position sensor requires a reference voltage,
typically around 5 volts. The voltage is applied to the
potentiometer resistance track, and a wiper or moving
contact moves across the track when the throttle is
opened or closed. Because the resistance along the track
increases from a low value (possibly as low as zero
ohms) to a high value, the voltage at one end of the
track could be 5 volts whilst at the other end it could be
as low as zero volts. As the wiper moves along the track,
the voltage at the contact point (wiper onto the track)
will change as the wiper moves. The wiper moves with
the movement of the throttle; therefore different
throttle positions will result in different voltages at the
Figure 1.24 Throttle position sensor and potentiometer schematic
wiper contact point (see Figure 1.25). The wiper is then
connected back to the ECU, which uses the voltage
value as an indication of throttle position.
Although there are variations in the construction of
throttle position sensors and the signal voltages, it is
quite common to have a low voltage of around 0.5 volts
to indicate the throttle closed position and a higher
voltage around 4.5 volts to indicate that the throttle is
fully open.
Note that some throttle position sensors, especially
older designs, have contacts that open and close when
the throttle is opened and closed. In these sensors one
Figure 1.25 Analogue signal produced by a throttle position
sensor compared with angle of throttle opening
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Fundamentals of Motor Vehicle Technology: Book 2
set of contacts is arranged so that they close when the
throttle is fully closed. A second set is also used to
indicate when the throttle reaches a certain opening
point, e.g. 60% open, an indication that the driver is
accelerating or requires more power. Some throttle
sensors have a combination of contacts and a
potentiometer, although this type is now becoming less
There are other components fitted to ECU controlled
vehicle systems that also use position sensors similar to
the throttle position sensor, and these are dealt with in
the relevant chapters.
1.5.4 Pressure sensors
There are generally two main types of pressure sensor: a
mechanical type and an electronic type.
Mechanical type
One simple mechanical type makes use of either a
diaphragm or capsule, which is exposed to the pressure,
or depression (Figures 1.26a and 1.26b).
For example, a pressure sensor can be used to sense
engine intake depression (often referred to as engine
vacuum). Because engine intake depression varies with
engine load and throttle position (and other factors),
the sensor can pass a signal to the ECU that indicates
the engine load. As a result, the ECU can control fuel
quantity and ignition timing, although in fact
information is required from other sensors (including
engine speed and throttle position) to enable the ECU
to calculate the true engine load accurately.
If the diaphragm type sensor (Figure 1.26a) was
used to sense engine intake depression, the lower
chamber would be exposed to atmospheric pressure and
the upper chamber would be exposed to engine
depression (a lower pressure unless the engine has a
turbo or supercharger). When the upper chamber
pressure alters (with engine operating conditions) it
will cause the diaphragm to deflect or move within the
casing. The diaphragm can be connected to a lever,
which acts on a potentiometer, causing a voltage
change in the potentiometer circuit (using the same
principle as the throttle position sensor potentiometer
described in the previous section). The signal voltage
from the potentiometer is passed to the ECU, which is
then able to control functions such as fuelling or
ignition timing in response to the pressure changes.
Note that on the diaphragm type sensor with a
potentiometer, the signal is analogue and would
progressively change in the same manner as a throttle
position sensor, but in this case the changes occur with
changes in engine intake pressure.
The diaphragm type sensor is in most cases too
simple and inaccurate to be used for modern vehicle
systems such as an engine management system;
however, the principle of operation is used for some
applications. A more widely used type in the past was
the capsule type, whereby a capsule is sealed and
Figure 1.26 Pressure sensors and potentiometer circuits
a Diaphragm type
b Capsule type
therefore kept at a fixed pressure, and is subsequently
exposed to the vacuum or depression; when the
pressure outside the capsule is lower, the capsule
contracts, moving the rod and potentiometer slider.
There are other mechanical methods for converting
pressure change into an electrical signal, although
mechanical pressure sensors are rarely used on modern
Electronic type
Electronic pressure sensors are much more reliable and
accurate than mechanical sensors and have no moving
parts (Figure 1.27). A solid state component or silicon
chip is exposed to the pressure or depression, which
puts the chip under a strain; the strain alters with
pressure change. The change in strain causes a minor
change in length or shape of the crystal. The change in
shape or length alters the resistance of the chip;
therefore, if the chip forms part of an electrical circuit,
the result will be a change of voltage in that circuit.
Note that, on some electronic types, the component
under strain is effectively a thin diaphragm made of
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Examples of different types of sensor
Figure 1.27 Electronic type MAP sensor
Pressure sensors can be used to measure the
atmospheric pressure, fuel line and fuel tank pressure.
Pressure sensor analogue or digital signal
Electronic type sensors can produce an analogue or a
digital signal, depending on their design. The analogue
signals are generally simple voltage changes that
increase and decrease according to changes in pressure.
Typically, a voltage of around 0.5 volts would indicate a
strong engine intake depression (low pressure) such as
would occur at idle speed or low load conditions
(throttle closed or almost closed). When the throttle is
initially opened this allows the intake pressure to rise
(almost no depression), which results in an increase in
voltage to approximately 4.5 volts.
Note that not all analogue pressure sensors operate
in the same way; therefore voltage values may differ.
Some sensors may provide a high voltage when the
depression is strong, and a low voltage when there is
almost no depression.
Digital pressure sensors generally provide a digital
pulse, which has a frequency that changes with the
change in pressure. In effect, the signal provided to the
ECU is a simple one consisting of many on/off pulses.
The ECU effectively counts the pulses and compares
them against the in-built clock or timer within the ECU.
When the pressure changes, the frequency of pulses
provides the ECU with a reference to the pressure.
Refer to section 1.6 for examples of analogue and
digital signals.
MAP sensors
It is general practice to refer to the atmospheric
pressure as being zero; this is often the value shown
when a pressure gauge is not connected to a pressure
source, i.e. the pressure gauge is not being used. We
therefore refer to this as gauge pressure. However, the
atmospheric pressure is of course not zero, but is in fact
approximately 1 bar (approximately 14.5 lb/in2 or
101 kilopascals), even though a gauge may indicate this
as being zero. Therefore a gauge pressure of zero
indicates a pressure of around 1 bar. Note, however,
that some gauges are calibrated so that they indicate
the actual or ‘absolute’ pressure.
Absolute pressure is therefore the true pressure value
as opposed to the traditional gauge pressure. If a gauge
reading indicates 2 bar, this would in fact be 2 bar above
atmospheric pressure (which is already at 1 bar); the
absolute pressure is therefore 3 bar.
The same applies to a pressure that is lower than
atmospheric pressure. If the gauge pressure reading
were lower than zero, e.g. a negative value such as
‘minus 0.25 bar’, this would be equivalent to an
absolute pressure of 0.75 bar (1 bar minus 0.25 bar).
When a complete vacuum is formed (i.e. there is no
pressure at all) the absolute pressure is zero. For this
reason we should not refer to engine intake depression
as being a vacuum. Intake manifold depression is a low
pressure but it is not a true vacuum.
Sensing manifold absolute pressure
Pressure sensors that are used to sense engine intake
depression generally now measure absolute intake
pressure. These sensors are therefore referred to as
manifold absolute pressure sensors (MAP sensors). The
intake pressure is dependent on a number of factors
including: throttle opening angle, engine load, air
temperature and density, engine speed, etc. Engine
condition affects the intake pressure; therefore this
factor also affects the sensed pressure value. Therefore
the absolute pressure value provides a more accurate
indication of engine operating conditions.
MAP sensors are generally of the electronic type and
may still provide either an analogue or digital signal.
1.5.5 Airflow sensing
As an alternative to the MAP or pressure sensor method
of assessing engine load, many engine management
systems and older fuel and ignition systems used
airflow sensors. There are two types of commonly used
airflow sensors: mechanical or electrical/electronic.
Mechanical airflow sensors are usually referred to as
flap or vane type airflow meters. A hinged flap is
exposed to the airflow; because the flap is spring loaded
to the ‘closed’ or stationary position, increasing the
airflow will cause the flap to open to a greater angle
(Figure 1.28). The flap is connected to a sophisticated
potentiometer; as with a throttle position sensor
potentiometer, when the flap moves it results in a
change in voltage at the potentiometer wiper contact. A
more detailed explanation is provided in Chapter 2.
When an engine draws in increasing volumes of air
on the induction strokes, this causes an increase in the
air volume passing through the intake trunking, which
is where the airflow meter is located. Therefore changes
in throttle position and engine speed or load will affect
the airflow, thus enabling the airflow sensor to provide
a relevant voltage signal to the ECU. The ECU is then
able to calculate the engine load and provide the
required amount of fuel.
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Fundamentals of Motor Vehicle Technology: Book 2
Figure 1.28 Flap or vane type airflow sensor: cutaway view and picture/drawing
Vane type airflow sensor analogue signal
As described above, the airflow sensor contains a
potentiometer, which provides a signal voltage that
progressively rises and falls as the vane or flap is moved
by the increasing or decreasing airflow. The signal is
therefore an analogue signal and is similar in
appearance to the signal produced by a throttle position
sensor (Figure 1.25).
The principle of operation relies on the fact that, when
air passes across a pre-heated wire it will have a cooling
effect. As the temperature of the wire changes, so does
its resistance. On mass airflow sensors, the sensing wire
Measuring air volume not air mass
It is important to note that the flap type airflow sensor
measures air volume but not air mass. For a given
volume of air, the mass can increase or decrease along
with temperature and pressure changes. The greater the
mass of air, the greater the amount of fuel required to
maintain the correct air:fuel ratio. Through measuring
only the volume, the flap type sensor is slightly limited
in its capacity to provide totally accurate information to
the ECU. As an example, if for a given volume of
measured air the density were to reduce, this change
would not be registered by a flap type sensor and would
not therefore result in a reduction in fuel delivered to
the cylinders; in effect the mixture would be too rich.
The inaccuracies are quite small, but because emission
regulations demanded tighter controls, the flap type
sensor became less popular and was largely replaced by
the electrical/electronic types of airflow sensors
described below.
Electrical/electronic airflow sensors generally operate
on what is referred to as the hot wire principle. Hot
wire sensors are affected by air density and can
therefore provide an indication of airflow, which
accounts for the mass of air rather than just the volume.
These airflow sensors are often called mass airflow
sensors; an example is shown in Figure 1.29.
Figure 1.29 Hot wire airflow sensor
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Examples of different types of sensor
is heated by passing a current through the wire. When
changes in airflow cause a change in the temperature
and therefore changes in the resistance of the wire, the
voltage then changes in the electronic circuitry
contained within the sensor assembly. This circuitry
(explained in Chapter 3) compensates for the change in
sensing wire resistance and applies increased or
decreased current to the wire to maintain the desired
temperature. The change in required current flow is
converted to a voltage signal that can be monitored by
the ECU, i.e. the airflow mass or a change in the airflow
mass results in an appropriate voltage signal passing
from the sensor to the ECU.
On some types of hot wire system, the wire is
occasionally heated when the engine is switched off to a
much higher than normal temperature, which burns off
any contamination or deposits on the wire that could
otherwise affect measurement accuracy. A variation on
the hot wire system is a hot film sensor. The operation
is much the same as for the hot wire sensor but an
integrated film type heated sensing element is used
instead of the heated wire.
weight); this was generally referred to as the
stoichiometric air:fuel ratio, but is now referred to as
lambda 1.
Although the air:fuel ratio varies under different
operating conditions, e.g. cold running, light cruise or
load conditions, modern engines do operate close to the
ideal air:fuel ratio for much of the time. On a modern
engine, the engine management system uses the
information from various sensors to enable the ECU to
calculate the required amount of fuel, thus keeping the
air:fuel ratio as close as possible to the desired value.
The catalytic converter provides a further
combustion process (for those exhaust gases that have
not been completely burned within the engine’s
combustion process), this additional combustion
process also requires a correct air:fuel ratio. The
unburned or partially burned gases within the exhaust
contain unburned or partially burned petrol; therefore if
an amount of oxygen is added and the temperature
within the converter is high enough, those unburned
and partially burned gases will combine and ignite,
hopefully creating a complete combustion of those
gases (thus reducing the pollutants).
1.5.6 Oxygen (lambda) sensors
Monitoring the oxygen in the exhaust gas
In reality, the exhaust gas can contain enough oxygen to
enable the unburned and partially burned fuel to ignite.
However, to ensure that the correct amount of oxygen is
present in the exhaust gas, the air:fuel ratio supplied to
the engine must be precisely controlled, e.g. an excess
of petrol (rich mixture) would lead to reduced amounts
of oxygen being passed to the exhaust gas. The oxygen
sensor therefore senses the oxygen content of the
exhaust gas and passes a signal back to the ECU, which
if necessary can alter the fuelling to correct the air:fuel
ratio, thus resulting in the exhaust gas having the
correct oxygen content.
Although the previous explanation provides a brief
understanding of the purpose of the oxygen sensor, the
operation of the catalytic converter and the oxygen
sensor are in fact much more complex. These topics are
therefore explained in greater detail in Chapter 2,
dealing with petrol engine emissions control systems.
Reducing pollutants in the exhaust gas
Oxygen sensors (Figure 1.30) are used on modern motor
vehicles for a very specific task: measuring the oxygen
content of the exhaust gas. Whilst the oxygen sensor is
not critical to the direct efficiency of the engine, it is
critical to the efficiency of the exhaust emissions control
system (the control of which is generally integrated into
the engine management system). The catalytic converter
plays the major part in reducing the pollutants contained
within the exhaust emissions; the converter, in simple
terms, creates a combustion process. For a catalytic
converter to work efficiently, it must be fed with exhaust
gases that contain the required amount of oxygen. The
oxygen sensor is used to measure the oxygen content
and provide a signal to the ECU which will in turn
control fuelling to ensure that the exhaust gas has the
correct oxygen level.
Correct air/fuel mixture
As detailed in Hillier’s Fundamentals of Motor Vehicle
Technology Book 1, efficient combustion in an engine
relies on the air and petrol mixture (air:fuel ratio) being
correct. The theoretically correct mixture is
approximately 14.7 parts of air to 1 part of petrol (by
Figure 1.30 Typical appearance of an oxygen sensor
Oxygen measurement
(Refer to Chapter 3 for additional information.)
A typical oxygen sensor is illustrated in Figure 1.30.
The sensor uses a natural process that, when specific
quantities of oxygen are passed through a certain
material, a small voltage is produced. Zirconium oxide
is one commonly used material for an oxygen sensor
When the sensor is located in the exhaust pipe, one
side of the sensing element is exposed to the exhaust
gas whilst the other side is exposed to the atmosphere.
Around 20.8% of the atmosphere consists of oxygen,
whilst the exhaust gas typically has around 0.1% to
0.8% oxygen; therefore there is a substantial difference
in the oxygen levels on the two sides of the sensing
element, causing a small voltage to be produced. The
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exact voltage will depend on the amount of oxygen in
the exhaust gas. The voltage produced by the sensor is
then passed to the ECU, which can alter the fuelling as
necessary to ensure that the oxygen content is correct.
The process is almost continuous: the sensor monitors
the oxygen level and passes a signal to the ECU, which
corrects the fuelling; this fuel correction then changes
the oxygen level which is again monitored by the
oxygen sensor, and so the process continues in a loop.
This kind of process is often referred to as a closed loop
Note that for the sensors to operate efficiently, they
must be at a high temperature (typically above 350°C).
The exhaust gas will provide heat but some sensors
have electrical heating elements built in to the sensor
body to speed up and stabilise the heating process.
Because the oxygen sensor is effectively monitoring
what is now referred to as the lambda value, the oxygen
sensors are commonly referred to as lambda sensors.
However, different manufacturers (of vehicles and
sensors) do use different terminology. One example is
the widely used Ford term ‘heated exhaust gas oxygen’
(HEGO) sensor.
Pre-cat control
As detailed above, the combination of the lambda
sensor and the ECU effectively controls the fine tuning
of the air:fuel ratio to enable the catalytic converter to
operate efficiently. The lambda sensor is located
upstream (in front of) the catalytic converter and is
therefore able to measure the oxygen level in the
exhaust gas passing into the converter. The position of
the lambda sensor in front of the catalytic converter is
referred to as pre-cat control because the combinaton of
lambda sensor and ECU controls the oxygen content
before it reaches the catalytic converter. This
arrangement is shown in Figure 1.31.
Post-cat monitoring
European legislation (and legislation in other
continents) demands that an additional function is now
incorporated into emission control systems. This
function is part of a broad range of on-board
diagnostic (OBD) functions. One aspect of OBD is that
some form of monitoring should take place to ensure
that the catalytic converter is performing efficiently.
Fundamentals of Motor Vehicle Technology: Book 2
Figure 1.31 Arrangement of catalytic converter and oxygen
sensor with pre-cat exhaust gas monitoring
This can be achieved by placing a second oxygen sensor
after or downstream of the catalytic converter (postcat). This arrangement is shown in Figure 1.32.
If the catalytic converter is not working, the same
level of oxygen will exit the converter as entered it. The
second lambda sensor signal (post-cat) will therefore be
identical to the pre-cat lambda sensor signal. In such
cases the ECU will establish that the catalytic converter
is not working and will illuminate the dashboard
warning light. A fault related code or message would
also be accessible from the ECU using appropriate
diagnostic equipment.
Figure 1.32 Arrangement of catalytic converter with two lambda
sensors for pre-cat measurement and post-cat monitoring and
oxygen sensor
As discussed in section 1.3.3, a modern ECU uses digital
electronic processes. However, many sensors might
provide only an analogue signal, which must be
converted by the analogue to digital converter that is
contained within the ECU. Analogue signals produced
by sensors vary quite considerably, although essentially
they all provide a progressive change in voltage and can
therefore be treated in a similar way by the analogue to
digital converter (A/D converter).
Some examples of typical analogue signals
produced by some sensors are shown and discussed in
this section.
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Obtaining information from analogue and digital sensor signals
1.6.1 Temperature sensor signals
1.6.2 Throttle position sensor signals
Temperature sensors generally provide a signal voltage
that changes progressively with the change in
temperature (section 1.5.1). Therefore, when the
temperature increases, the voltage will either decrease
or increase (depending on whether the sensor is an NTC
or PTC type). The voltage levels on a temperature
sensor circuit generally range from a maximum of
approximately 5 volts to a minimum of zero volts
(although for normal operation a typical range is
approximately 4.5 to 0.5 volts).
In converting the analogue signal into a digital
signal, the ECU can use a number of voltage threshold
points as reference points, which in effect divide the
operating voltage range into steps (Figure 1.33). When
the temperature changes and the voltage consequently
decreases or increases, each step up or down could be
counted to give the ECU an indication of temperature. If
each step of 0.5 volts represented a 10° rise in
temperature, the ECU would be able to count the
number of steps up or down and relate this to a
temperature value, thus enabling changes in fuelling
and ignition timing, etc. In reality, if a greater number
of reference points or steps can be created between the
maximum and minimum voltages, the ECU is able to
assess smaller changes in temperature, thus providing
improved accuracy.
It is also of interest to note that if the typical sensor
signal voltage is between 0.5 volts and 4.5 volts (when
the engine and sensor are operating correctly), then any
voltage above or below those values could be regarded
as incorrect. An incorrect voltage is most likely to occur
as a result of a faulty component (sensor) or wiring
fault. The ECU could therefore be programmed to
illuminate a fault light on the dashboard and
furthermore to provide some form of coded message,
which could be read or interpreted by diagnostic
As highlighted earlier (section 1.5.3), a throttle position
sensor is used to indicate the angle of opening of the
throttle butterfly. Although some earlier throttle
position sensors relied on switches and contacts, almost
all modern types use a potentiometer (variable
resistor). The form of the output signal from a
potentiometer is very similar to that from a temperature
sensor, i.e. it progressively increases and decreases.
Therefore when the throttle is opened and closed, the
voltage increases and decreases.
Assuming that the progressive or analogue increase
and decrease in voltage is converted to a digital or
stepped signal (in the same way as a temperature
sensor analogue signal is converted into voltage steps),
the ECU can establish the angle of opening of the
throttle and the rate at which the throttle is opened and
closed (Figure 1.34). The ECU can count the up or
down steps in voltage to calculate the angle of opening,
but can also calculate the speed at which the steps
occur, thus providing an indication of how quickly the
throttle position is changing. The ECU can then provide
the appropriate adjustments to fuelling, ignition timing,
1.6.3 Airflow sensors and MAP
sensors (analogue)
Airflow sensors and MAP sensors can provide analogue
or digital signals depending on their design. The
analogue types produce a voltage that increases and
decreases when the airflow volume or mass changes
(airflow sensors) or when the manifold intake
vacuum/pressure changes (MAP sensors). As with
temperature and throttle position sensors, progressive
increases and decreases in voltage are converted into a
digital or stepped signal so that the ECU can monitor
the changes. The ECU can therefore adjust the fuelling,
ignition timing and other functions as necessary, when
airflow, air mass or intake manifold pressures change.
The analogue signals and the subsequent converted
digital signals are therefore similar to those created by
the throttle position sensor (Figure 1.34), although, for
the airflow sensor, it is the change in the airflow that
causes a change in the voltage.
1.6.4 Crankshaft/camshaft speed
and position sensors
Figure 1.33 Analogue temperature sensor signal with conversion
to a digital signal
As described in section 1.5.2, this type of sensor
generally uses the principle whereby moving or altering
a magnetic field or magnetic flux generates a small
voltage. Reluctor teeth are located on a rotating
component such as a crankshaft or camshaft, so when
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Figure 1.34 Analogue throttle position sensor signal with
conversion to a digital signal
the teeth approach or leave the magnetic field (created
by the magnet within the sensor), positive or negative
voltages are generated. The voltage changes form the
analogue signal that is then passed to the ECU. As with
other analogue signals, the A/D converter changes the
signal into a digital format that can then be used by the
ECU (Figure 1.35).
The ECU is able to count the number of pulses, and,
because it has a clock or timing device, is then able to
calculate the speed of rotation of the crankshaft or
whatever rotating component is used to generate the
signal. To achieve this speed calculation, there needs
only to be one tooth on the reluctor disc. However, if a
number of teeth are located around the reluctor disc,
including a master tooth (a missing or differently
shaped tooth), the ECU is then able to monitor each of
the individual pulses generated by each of the teeth.
The ECU is able to calculate how many degrees the
crankshaft has rotated from the master position. If, for
example, the master position is TDC for cylinders one
and four, the ECU can assess how many degrees of
rotation the shaft has rotated from TDC. This could
enable the ECU to implement other control functions
that are crankshaft position dependent, such as opening
a fuel injector.
It is also possible for the ECU to assess the speed of
the crankshaft as each tooth passes the sensor. When a
cylinder is on the power stroke, the crankshaft speed
will increase, but when the cylinder is on the
compression stroke, the speed will decrease.
Additionally, if a particular cylinder has a fault which
reduces its combustion efficiency, then the acceleration
of the crankshaft during the power stroke will be less
than for a good cylinder, leading the ECU to assume
that a fault exists which could prevent petrol from
burning (causing high emissions). The ECU can
therefore switch off the fuel injector for that cylinder.
Note that the ECU will also have information from
the oxygen sensor, which might indicate that the
Figure 1.35 Crankshaft speed/position sensor signal with
conversion to a digital signal
a Signal produced by a crankshaft speed sensor with a single
reluctor tooth
b Signal produced by a crankshaft speed sensor with many
reluctor teeth and one missing master reference tooth
oxygen content is too low, i.e. there is excessive unburnt
fuel. The ECU can use this information, along with the
crankshaft acceleration/deceleration information, to
decide whether the fuel injector for the defective
cylinder should be switched off.
1.6.5 Wheel speed sensors
Most wheel speed sensors are identical in operation to
the crankshaft speed/position sensors. The main
difference is that, although the rotating disc or reluctor
disc contains a number of reluctor teeth, there is no
master reference tooth. The ECU counts the pulses
generated by the teeth; by combining this information
with the in-built clock information, the speed and
acceleration or deceleration of the wheel can be
calculated. An ECU on an ABS system is therefore able
to establish whether a wheel is accelerating or
decelerating at a different rate from the other wheels,
which would indicate that a brake was locking one
wheel. Many other vehicle systems use the information
from the wheel speed sensors: these are discussed in the
relevant sections of the book.
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Obtaining information from analogue and digital sensor signals
Note that the analogue and converted digital signals
produced by a wheel speed sensor are virtually identical
to the crankshaft speed/position sensor (Figure 1.35b).
However, there is no master reference tooth; therefore
the signal from the wheel speed sensor is a continuous
series of pulses.
1.6.6 Engine knock pressure sensors
1.6.7 Oxygen (lambda) sensor signal
Owing to the complex nature of the oxygen (lambda)
sensor signal and the interpretation of the signal by the
ECU, a full explanation of the signal and how the ECU
responds to the signal is provided in Chapter 3 on petrol
engine emissions control systems.
Figure 1.36 Knock sensor
a Knock sensor located in the engine block
b Signal produced by knock sensor
Key Points
Although not previously covered in this chapter, the
engine knock sensors are electronic structure borne
vibration sensors. A solid state component or silicon
chip (usually referred to as a piezo chip or crystal) can
be used to sense pressure changes (section 1.5.4). If this
type of chip is built into a sensor that is attached to the
engine (cylinder head or cylinder block), it can be used
to detect high frequency vibrations in the engine
casings when ignition knock occurs (Figure 1.36).
Ignition knock is caused when isolated pockets of
spontaneous combustion occur within the combustion
chamber, as opposed to the progressive and controlled
combustion process that should occur. Because modern
engines operate very close to the limits at which
combustion knock can occur, any small variations in fuel
quality or hot spots within the combustion chamber can
very quickly cause knock to occur: in effect, the ignition
timing may be slightly advanced for the conditions at
that time. The knock sensor detects the knock and
passes a signal to the ECU, which in turn slightly retards
the ignition timing until the knock disappears.
Knock sensors are discussed in detail in Chapter 2,
but in simple terms, the sensor produces a small
electrical signal, which is dependent on the frequency of
the vibrations; this signal is then used by the ECU to
control the ignition timing. The signal provided by the
knock sensor is analogue but it is very irregular because
there is not a consistent rotation or movement of a
component to create the signal. Although the engine
does produce regular vibrations, the combustion process
also causes irregular vibrations to occur. The sensor
signal therefore contains voltage spikes caused by all
vibrations, which are filtered by the ECU so that it is able
to analyse correctly combustion knock should it occur.
Note that some knock sensors must be tightened to
the correct torque setting when fitted to the engine;
over- or under-tightening can affect the capacity of the
sensor to detect the appropriate vibration frequencies.
Sensors convert physical quantities into signals
Position sensing is often achieved using a simple
A knock sensor is an accelerometer
1.6.8 Hall effect pulse generator
As briefly described in section 1.5.2, Hall effect sensors
produce a digital signal that consists of on/off pulses.
Hall effect sensors can therefore be used to provide
speed or position related information to the ECU. Such
sensors are used on some ignition systems, where the
sensor is located in the distributor body, the sensor
having one cut out and plate for each cylinder
reference. Hall effect sensors are also used as camshaft
position sensors; in such cases, the rotor might contain
only one cut out or plate, which would result in one
master reference signal being passed to the ECU.
Because the sensor is mounted on the camshaft, the
ECU can determine the position (e.g. TDC) of one of
the cylinders on a multi-cylinder engine. This is not
possible with a crankshaft sensor, because a master
TDC reference on a crankshaft will usually represent
TDC on two cylinders, e.g. cylinders one and four or
one and six.
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Introduction to powertrain electronics
The signal produced by a Hall effect ignition trigger on
older systems needs only to provide a trigger signal for
spark timing. Therefore one pulse of the signal
corresponds to the ignition timing point for each
cylinder; there is no requirement for a master reference
(Figure 1.37a). On a four-cylinder engine, the ignition
coil would produce four high voltage outputs (to create
a spark at a spark plug), but the distributor rotor arm
would direct the spark to the appropriate cylinder.
On later ignition systems (usually integrated into an
engine management system), the distributor is no
longer used; there is often one individual coil for each
cylinder. The ECU therefore needs to be given
information regarding the position of one of the
cylinders, for example, which cylinder is on the
compression stroke. Once the ECU has established a
reference to one of the cylinders, it can provide the
ignition coil control for that cylinder; then the ECU can
control the rest of the coils in turn at the appropriate
intervals of crankshaft rotation. Remember that the
ECU will be receiving speed and angular position
information from a crankshaft sensor. However, to
provide the master reference for one of the cylinders, a
Hall effect pulse generator, attached to the camshaft, is
often used. The camshaft rotates once for every engine
cycle, so the sensor needs only to provide a single pulse
(Figure 1.37b), which indicates that the chosen cylinder
is on the compression stroke (or any other stroke or
position, so long as the ECU is programmed with this
Fundamentals of Motor Vehicle Technology: Book 2
Figure 1.37 Digital signals produced by a Hall effect pulse
a Signal produced by a Hall effect pulse generator with four
pulses per engine cycle (four-cylinder engine) which is used on a
simple ignition system as a trigger reference signal for the four
ignition sparks
b Signal produced by a Hall effect pulse generator with one
pulse per engine cycle. The signal is used as a master reference
for ignition or sequential injection timing
Note that injection system control can also rely on a
camshaft located Hall effect trigger. If the injectors are
operated in sequence, i.e. in the same sequence as the
cylinder firing order, the ECU will also require a master
reference signal.
1.7.1 Completing the computer
controlled task
If we re-examine the purpose of ECU controlled
systems, the objective is to control a function or task
using the speed and accuracy that a computer or ECU
provides. Therefore, when the ECU has received the
required information and made the appropriate
calculations, the ECU will provide a control signal to a
component, which will then perform a task. In general,
those components that receive a control signal and then
perform a function or task are referred to as actuators.
Mechanical and non-mechanical actuators
The term actuation is generally assumed to mean that
something is moved or actuated, and, in a high
percentage of cases with ECU controlled systems, this is
true. The ECU control signal that is passed to the
actuator causes some form of movement of a
component, such as opening an air valve or moving a
lever (Figure 1.38a). However, there are some cases
where mechanical movement does not occur, such as
Figure 1.38 ECUs and actuators
a ECU controlled circuit with a single sensor and single actuator
which performs a mechanical task
b ECU controlled circuit with a single sensor and single actuator
which performs an electrical task
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Actuators: producing movement and other functions
when a light bulb is switched on or off, or when an
ignition coil is switched on or off (Figure 1.38b).
Another example of non-mechanical actuation is where
the ECU provides a signal to a digital dashboard display
to enable the driver to view engine and vehicle speed as
well as other information. However, even when no
mechanical movement takes place, when an ECU
provides a control signal to a component, that
component will usually be referred to as the actuator.
Communication signals between different ECUs
Another example where an ECU provides a control
signal that does not result in mechanical movement is
the communication of one ECU with another, or with
another electronic device.
An example of ECUs communicating is when the
engine management system ECU provides output signals
to an automatic gearbox ECU (Figure 1.39); the engine
management ECU might provide a digital information
signal to the gearbox ECU that indicates engine load
information. The engine management ECU is able to
calculate engine load conditions because it receives
information from sensors such as the airflow sensor, the
throttle position sensor and temperature sensor.
Therefore the engine management ECU can provide a
single ‘engine load’ signal to the gearbox ECU that
provides sufficient information for the gearbox ECU to
make its own calculations (also using information from
other sensors on the gearbox system). In this example,
the engine management ECU is not directly providing an
actuator signal but it is providing a signal which assists
the gearbox ECU to make its own calculations, so that it
can provide a control signal to a gearbox actuator. In
reality, the engine management ECU is still providing
control signals to the engine management system
actuators, but the information signal that is being passed
to the gearbox ECU is an additional function that
reduces the need for the gearbox system to duplicate the
sensors used in the engine management system.
On many vehicles where the engine management
ECU passes information to the gearbox ECU, the reverse
also applies: the gearbox ECU passes information back
to the engine management ECU. For instance the
gearbox ECU might inform the engine management
ECU that a gear change is taking place, e.g. third to
fourth gear. The engine management ECU can then
momentarily reduce the engine power, which makes the
gear change smoother. The engine management ECU
can achieve this by slightly retarding the ignition timing
or slightly reducing the amount of fuel injected, and in
some cases (if the ECU also controls the throttle
Figure 1.39 Communication between engine management and
automatic gearbox ECUs
opening electronically) by slightly closing the throttle.
Each of these actions would result in a momentary
reduction in engine power.
1.7.2 Actuators and magnetism
There are essentially two types of mechanical movement
actuators: one type is the solenoid and the second is the
electric motor. There are a number of variations in
solenoids and electric motors, but, in general, solenoids
are used to achieve linear movement and motors are
used for rotary movement (although it is possible for
motors to be used to create linear movement, via a
mechanical mechanism, or it is possible for solenoids to
create rotary movement, via a linkage).
The operation of mechanical actuators (solenoid
and electric motor types) relies on magnetism. Hillier’s
Fundamentals of Motor Vehicle Technology Book 3
explains in detail the way in which magnetic fields are
created and used for electric motors, solenoids and
generators, etc. However, the essential fact is that,
when a current is passed through a coil of wire, a
magnetic field is created around that coil of wire. The
magnetic field can then be used to create movement.
Solenoid type actuators
In a simple solenoid (Figure 1.40a), a soft iron plunger
is located within the coil, but the plunger is free to move
with a linear motion. When an electric current is passed
through the coil of wire and the magnetic field is
created, this will cause the plunger to be attracted
towards or through the coil. When the current is
switched off, the spring will return the plunger back to
the start or rest position. Different designs and
Figure 1.40 Simple solenoids
a Simple solenoid
b Double acting solenoid
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Introduction to powertrain electronics
constructions of solenoids allow many different tasks to
be performed. For example, the double acting solenoid
(Figure 1.40b) uses two coils of wire. One coil creates a
magnetic field, which moves the plunger in one
direction, and the other coil creates a magnetic field,
which moves the plunger in the opposite direction.
It is also possible for the ECU to regulate or control
the average current flow and voltage passing through
the coil of wire by altering the duty cycle and frequency
of the control signal pulses (see section 1.8). With this
control process, it is possible to control or regulate the
strength of the magnetic field. If the plunger is moving
against a physical resistance such as a spring, it can be
moved further by increasing the strength of the
magnetic field. Reducing the magnetic field will result
in the plunger moving back slightly. Additionally, when
a double acting solenoid is used, the plunger movement
can be controlled in both directions; in fact one
magnetic field can be used to oppose the other. This
allows an ECU to move and position the plunger with
reasonable accuracy.
Solenoid plungers can be connected to a number of
different types of mechanisms or devices that will
perform different tasks or functions; various solenoid
actuators are covered in the relevant chapters within
this book.
Fundamentals of Motor Vehicle Technology: Book 2
Electric motor type actuators
A simple electric motor operates on similar principles to
the solenoid, but instead of the magnetic field causing a
plunger to move with a linear motion, the magnetic
field forces a shaft to rotate. Figure 1.41 shows a simple
electric motor, which in this example has a permanent
horseshoe shaped magnet with a north and south pole.
A single loop of wire, which would normally be
attached to a rotor shaft, is fed with an electric current,
thus creating an electromagnetic field around the loop
of wire. When the electromagnetic field is created,
north and south poles will exist around the loop of wire.
These north and south poles will either be attracted to
or repelled from the north and south poles of the
permanent magnet. Remember that like poles repel
each other and unlike poles attract each other.
When the current is initially passed through the wire
loop, e.g. from connection A to connection B on the
wire loop, if the electromagnet north pole is adjacent to
the permanent magnet north pole (and the two south
poles will also be adjacent to each other), this will force
the shaft to rotate (Figure 1.41a). When the shaft then
rotates through 180º, the north poles will be adjacent to
the south poles, and because unlike poles attract each
other, the motor will not rotate any further.
However, in the diagram it can be seen that the pair
of semi-circular segments (or commutator) is attached
to the ends of the wire loop and therefore rotates with
the loop. The electric current passes from the power
supply to contact brushes which rub against the
segments as the shaft rotates. Therefore, when the shaft
and the segments have rotated through 180º, the two
segments are now not in contact with the original
brushes, but they are in contact with the opposing
brushes. This means that the electric current will be
flowing from connection B to connection A (Figure
1.41b), which is in the opposite direction around the
wire loop. The result is that the north pole of the
electromagnet is now a south pole, and the south pole is
now a north pole, which will cause the shaft and wire
loop to rotate another 180º; the process is then
The simple electric motor in Figure 1.41 shows how
magnetism can provide continuous rotary movement;
the resulting rotary motion can operate various devices.
Simple examples include fuel or air pumps, and wiper
motors operate on the same principles.
Figure 1.41 Simple electric motor. Note that the primary and
secondary windings are wound around a soft iron core to
concentrate and intensify the magnetic field
a Current passes from A to B creating north and south poles on
the electromagnet. The like poles will cause the shaft and wire
loop to rotate
b When the rotor has turned through 180°, the commutator
arrangement causes the current to flow in the reverse direction
around the wire loop (from B to A), therefore changing the north
pole to a south pole and the south pole to a north pole. The like
poles will again repel and cause the shaft to rotate through
another 180°
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Actuators: producing movement and other functions
However, many of the electric motors used on ECU
controlled vehicle systems are often more complex and
sophisticated in the tasks they have to perform, and in
their design and construction. Many of the motors do
not in fact perform a complete rotation, or they may be
controlled so that they rotate in small angular steps.
These types of motors are controlled by using different
types of wire loops (usually coils of wire) and using
different designs of commutator. In addition, by
applying control signals from the ECU that have
changing duty cycles, pulse widths and frequencies, it is
possible to rotate motors partially so that they start and
stop in any desired position. The partial rotation can be
progressive from one position to another, or it can be
achieved in a series of steps.
The capacity to control the rotation of motors
accurately allows them to be used for a variety of tasks
such as opening and closing air valves in small
increments (used for idle speed control). Other
examples of ECU controlled motors are dealt with
individually in the following sections and in other
chapters of this book.
Magnetism and non-mechanical actuators
There is one main actuator used on motor vehicles that
uses the effects of a magnetic field but does not produce
mechanical movement – this is the ignition coil.
An explanation of how an ignition coil works is
provided in Chapter 2 of Hillier’s Fundamentals of Motor
Vehicle Technology Book 1. It is sufficient here to highlight
the basic principles of ignition coil operation, which rely
on the movement of a magnetic field or magnetic flux to
induce an electric current into a coil of wire.
When a current is passed thorough a coil of wire, it
creates a magnetic field; this is the same principle as
used in electric motors. Additionally, as is the case with
an electrical generator, when a magnetic field moves
through a coil of wire (or the coil is passed through a
magnetic field) it causes an electric current/voltage to
be generated within the coil of wire. The faster the
magnetic field moves relative to the wire, the greater
the voltage produced. An ignition coil relies on both
On most vehicles, the voltage in the vehicle
electrical system is only around 12 volts, which is not
sufficient to create a spark or electric arc at the spark
plug gap. The ignition coil must provide a way to
increase the voltage from 12 volts to many thousands of
volts. A principle that is used in electrical transformers
is also used for ignition coils: there are two coils of wire,
one of which has many more windings than the other.
In an ignition coil a secondary coil can typically have
100 times more windings than the primary coil (see
Figure 1.42).
The process
The process relies on current (using the vehicle’s 12 volt
supply) passing through the smaller coil or primary
winding to create a magnetic field. The build up of the
magnetic field is relatively slow, but once the magnetic
field has been established at full strength, it can be
maintained for a very brief period so long as the current
continues to flow. However, when the current is
switched off, the magnetic field collapses extremely
rapidly, in fact very much more quickly than the speed
at which it was created.
Whilst the magnetic field is collapsing, the lines of
magnetic force are collapsing across the same coil of
wire that created it (primary winding); this causes a
current/voltage to be produced within the primary
winding. Because the speed of collapse of the magnetic
field is very rapid, it causes a much higher voltage to be
produced within this coil of wire, sometimes as high as
200–300 volts. Therefore, the speed of collapse is used
to step up the voltage from 12 to typically 200 volts.
However, 200 volts are still not sufficient to provide the
spark at the spark plug under the conditions that exist
in the combustion chamber (high pressure and other
factors make it difficult for an arc to be created at the
plug gap).
To achieve the desired voltage necessary to create
the spark, a secondary winding is used, as mentioned
above. The secondary winding can be adjacent to the
primary winding, although one winding is often
wrapped around the other. When the magnetic field is
created, the secondary winding is also exposed to the
magnetic field. Therefore, when the magnetic field
collapses, as well as creating a voltage in the primary
winding, it also creates a voltage in the secondary
winding. Because the secondary winding may have 100
times the number of turns or windings, 100 times the
voltage can in theory be produced. If 200 volts could be
produced in the primary winding (owing to the rapid
speed of collapse of the magnetic field), then in the
secondary winding it should theoretically be possible to
produce 20 000 volts (100 times greater).
Figure 1.42 Simple construction of an ignition coil
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Introduction to powertrain electronics
For most petrol engines, the required voltage to produce
a spark at the spark plug (under operating conditions)
is around 7 000 to 10 000 volts; therefore a coil that is
able to produce 20 000 volts is more than capable of
producing a spark. There is therefore sufficient
additional voltage available to overcome many minor
faults such as a plug gap that is too large or
Fundamentals of Motor Vehicle Technology: Book 2
Key Points
[647] Chapter 01
Actuators convert electrical signals into actions
Common actuators, such as fuel injectors, are
solenoid operated
1.8.1 Solenoid type actuators
There are many different types of solenoid type
actuators used on motor vehicle systems, a number of
which are covered within this book. The following
examples deal with two types that are used for totally
different tasks.
The first example is a fuel injector, which provides
very rapid opening and closing of a small valve with the
result that fuel flow into the engine (into the intake
manifold or combustion chamber) can be accurately
controlled; the amount of movement required to open
and close the injector is very small.
The second example is the use of a solenoid as an air
valve. In this example, the valve forms part of a
pressure/vacuum circuit which is used to control a
turbocharger wastegate. The valve does not have to
operate at the same speed as the injector, but it will
require greater movement.
Fuel injector
Fuel injectors are high precision components used to
control the flow of fuel into the engine. The injectors
are usually located in the intake manifold and therefore
inject fuel in the region of the intake valves. On some
modern petrol engines, the injectors are located so that
Figure 1.43 Solenoid type petrol injector and basic wiring
fuel is injected directly into the cylinder. Modern diesel
engines that now use electronic control for the fuel
system also use electronically controlled solenoid
injectors that inject fuel directly into the combustion
Figure 1.43 shows a typical construction for a
solenoid type petrol injector. The injector has a 12 volt
supply from the vehicle’s electrical system, which is
usually a permanent supply (via a relay) whilst the
ignition is switched on (engine running). The earth
circuit for the injector passes through the ECU, which
acts as the control switch.
The injector must open and close very rapidly and at
high frequency. The opening and closing time can often
occur in around three thousandths of a second
(3 milliseconds or 3 ms), and injectors might open and
close more than 7 000 times a minute.
Solenoid air valve
The example shown in Figure 1.44 is a relatively simple
solenoid that is used to control the pressure acting on a
diaphragm. The pressure is produced by a turbocharger,
which causes the intake manifold to be subjected to
pressure (when the turbocharger is operating) as well
as the normal vacuum levels for low load engine
conditions (when the turbocharger is not operating).
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Examples of different types of actuators
Figure 1.44 Solenoid
operated air valve
When the pressure produced by the turbocharger
becomes too high for engine safety, the ECU will cause
the solenoid to operate and thus open the valve. This
will allow pressure from the intake manifold to act on
the diaphragm in the wastegate, which in turn will open
and allow pressure from the turbocharger to escape
(often into the exhaust system or other separate pipes
that lead to the atmosphere).
To switch the air valve, the solenoid receives a
permanent power supply whilst the engine is running,
and the ECU controls the earth circuit. The solenoid air
valve does not have to operate at the same speed and
frequency as the fuel injector, but the movement of the
valve is usually much greater.
in turn controls the volume of air passing into the
engine at idle speeds.
Continuous rotation fuel pump motor
The example shown in Figure 1.45 is a conventional
type electric motor, which is used to drive a fuel pump.
In this example, the motor and pump assembly are
mounted outside the fuel tank, although for many
applications, an adaptation of this type of pump is
located inside the fuel tank.
The pump will receive a power supply, which is
usually fed via a fuel pump relay (often forming part of
an engine management system relay). The pump will
usually have a permanent earth connection.
1.8.2 Examples of electric motor
type actuators
Electric motors used on modern vehicles systems can be
categorised into three main types: full and continuous
rotation; full rotation with controlled positioning; and
partial rotation with controlled positioning.
Continuous rotation motors are effectively
conventional electric motors; the example used in
this section is a motor that is used to drive a fuel
Full rotation motors with controlled positioning are
used to position a mechanism or device such as an
air valve or a throttle butterfly. In most cases, the
motor may rotate through more than one complete
turn, but it can be stopped at a desired position.
Some stepper motors used for idle speed control
operate on this principle.
Partial rotation motors use the same principles of
operation as a normal motor but the angle of
rotation is limited. The example used in this section
is a motor that is used to control an air valve, which
Figure 1.45 Electric motor driven fuel pump
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Introduction to powertrain electronics
Fundamentals of Motor Vehicle Technology: Book 2
Idle speed stepper motor with full rotation and
controlled positioning
In the example shown in Figure 1.46, the motor is able
to rotate fully (possibly for more than one complete
rotation) but it can be positioned electrically by
switching on and off the current passing into the motor.
The motor contains more than one set of magnets and
electromagnets. This construction enables the ECU to
switch on and off each electromagnet, which enables
the motor to rotate in small steps in either direction.
The different control signals to the stepper motor will
therefore have a series of on/off pulses which can be
positive or negative to achieve clockwise or anticlockwise rotation of the motor.
Idle speed partial rotation motor (rotary idle
In this design (Figure 1.47), the motor armature is
restricted by mechanical stops from rotating through
more than approximately 60°. Connected to the end of
the armature is an air flap or air valve assembly which,
when opened and closed, will regulate the air passing
into the engine, thus enabling idle speed to be
In the simplest type, a spring keeps the motor
armature rotated against one of the mechanical stops.
However, when an electric current is applied to the
motor (creating electromagnets), this will cause the
armature to rotate against the spring. The ECU controls
the average current flowing in the circuit by altering the
duty cycle of the control signal. The greater the average
current, the more the armature will rotate against the
spring force. By continuously altering the duty cycle it is
then possible to alter the angular position of the
Figure 1.46 Stepper motor
Figure 1.47 Rotary idle valve using partial rotation motor
1.9.1 ECU functioning as a switch in
a circuit
The control signal provided by the ECU to an actuator is
most commonly a digital signal, which effectively
switches the actuator on or off; this is achieved in most
cases by making the ECU a part of the actuator
electrical circuit. The ECU is therefore acting as a
sophisticated switch that makes or breaks (switches on
or off) the actuator circuit (Figures 1.48a and 1.48b).
As previously described (see the text about amplifiers in
section 1.3.3), the ECU usually contains a final stage
power transistor, which is effectively the actuator circuit
switch. The low voltage signal from the ECU’s
microprocessor simply controls the power transistor,
which then replicates or copies the control signal. But
because the power transistor is the switch within the
actuator circuit, when the microprocessor control signal
is on or off, it causes the power transistor to switch on
or off, thus making or breaking the actuator circuit.
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ECU/actuator control signals
Figure 1.48 Switching an actuator circuit
a Normal switch controlling an actuator circuit
b ECU acting as the switch and controlling the actuator circuit.
Note that the power transistor in the ECU directly switches the
circuit in response to the signal from the ECU microprocessor
Note that for most ECU controlled actuator circuits, the
ECU (power transistor) forms part of the earth or return
circuit (negative path). The positive path from the
power supply (the battery) can be directly connected to
the actuator or it may contain a switch such as an
ignition switch. Fuses and relays are also generally
connected into the positive side of the circuit. The ECU,
which provides the controlling function, is therefore
making and breaking (switching on and off) the earth
or negative side of the circuit.
Most control signals provided by the ECU are
therefore simple on/off pulses that cause the power
Figure 1.49 ECU control signal duration and frequency
a ECU control signal with equal on and off duration of 1⁄2 second
and a frequency of 1 hertz
b ECU control signal with on duration of 1⁄4 second and off
duration of 3⁄4 second but with a frequency that is still 1 hertz
c ECU control signal with equal on and off duration, but with a
frequency of 10 hertz
transistor to switch on and off the actuator circuit.
However, it is not just simple on or off control that is
provided by the ECU: most control signals will cause
the actuator to switch on or off for different lengths of
time and at different speeds or frequencies (measured
in hertz (Hz)).
When examining the control signal, the duration of
the on or off period can be referred to as pulse width. It
is however general practice that the pulse width refers
to the on time only.
Figure 1.49a shows an on/off control signal where
the on time or pulse width is 1⁄2 second, and the off time
is also 1⁄2 second. In this case the frequency is 1 hertz,
which means that the actuator is switched on and off
once every second.
Figure 1.49b shows a similar signal, but the on time
is 1⁄4 second, with the off time being 3⁄4 second. The
frequency is therefore still 1 hertz but the on and off
times are different.
Figure 1.49c shows a control signal with equal on
and off times but the frequency is 10 hertz.
The completion of the on and off process is one
complete cycle of operation or 1 cycle. Therefore, when
the signal completes one on and one off pulse, this is
also referred to as 1 cycle. If there are 10 cycles within
one second, this is a frequency of 10 cycles per second,
which is referred to as 10 hertz (10 Hz).
If the durations of the on and off times are the same,
this is referred to as a duty cycle of 50%, i.e. the on time
is 50% of one cycle. If however the on time is 1⁄4 of the
total cycle time then this is referred to as a duty cycle of
Figure 1.50a shows two control signals, each with a
50% duty cycle. Although the durations and frequencies
of the two signals are different, the duty cycles are 50%
in both cases. Figure 1.50b shows two control signals,
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Introduction to powertrain electronics
Fundamentals of Motor Vehicle Technology: Book 2
Figure 1.50 Duty cycles
a Control signals with 50% duty cycle
b Control signals with 25% duty cycle
each with a 25% duty cycle, and again, although the
durations and frequencies are different, the duty cycles
are the same.
Important note: In the control signal examples
illustrated, the off time is shown as the higher portion
of the pulse, i.e. as a voltage level. The on time is
therefore shown as zero volts. When a switch (in this
case the ECU) is connected into the earth or negative
path of an actuator circuit, the earth circuit will in fact
be at zero volts when the circuit is switched on and at
battery level voltage when the circuit is switched off. It
is important to note when using test equipment, such
measurements displayed may be the reverse of the
expected readings, e.g. the duty cycle could be shown
as 25% instead of 75%.
1.9.2 Using the signal to control the
By understanding that the duration (pulse width), duty
cycle and frequency of the control signal can be altered,
it is possible to understand how an actuator can be
controlled so that the task it performs can be varied. An
example is a fuel injector, which can be provided with a
control signal where the duty cycle or pulse width
varies. This means that the injector can be opened for
longer or shorter time periods, thus allowing different
quantities of fuel to be delivered to the engine.
The control signals affect how the actuator operates
in different ways because the actuator is altering the
current flow in the circuit. It was explained previously
that the ECU is effectively an on/off switch, but this is
only part of the whole story.
Altering the control signal duty cycle
Altering the duty cycle or pulse width has the effect of
altering the average current flow and applied voltage in
a circuit.
As an example, a simple 12 volt light circuit is
switched on and off by a simple switch (Figure 1.51a).
When the circuit is switched on, the voltage on the
power supply to the bulb will be 12 volts. Because the
light bulb has a 2 ohm resistance, the current will
therefore be 6 amps and the bulb will produce its
maximum light output. However, when the switch is
off, the voltage and current will both be zero and the
bulb will produce no light.
If the light switch could be switched on and off
very rapidly, for example at 100 times a second
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ECU/actuator control signals
(Figure 1.51b), and the duty cycle was 50%, i.e. the on
and off pulses were both 50% of 1 cycle (of equal
duration), the result would be that the light would be
on for only half of the time. This means that the
average voltage, the average current and the average
amount of light produced by the bulb would also be
50% of the maximum value had the bulb been
switched on all the time.
Figure 1.51 Altering the duty cycles to affect the average voltage
and current
a Simple 12 volt light circuit and switch
b Average voltage and current in a light circuit with a 50% duty
c Average voltage and current in a light circuit with a 25% duty
In this example where the bulb is rapidly switched on
and off, the average voltage on the power supply circuit
is 6 volts because this is 50% of the maximum supply
voltage (50% of 12 volts). The average current is
therefore 3 amps (50% of 6 amps). The total amount of
light produced by the light bulb should in theory be
50% of the light that would have been produced if the
bulb had been illuminated for all of the time.
If the duty cycle was changed so that the on time
was only 25% of the total cycle (Figure 1.51c), then
12 volts would be available to power the bulb for only
25% of the time, but zero volts would be supplied for
75% of the time. The average voltage would therefore
be 25% of 12 volts, i.e. 3 volts. The average current
would therefore also be 25% of the maximum 6 amps,
i.e. 1.5 amps. The average amount of light produced
would therefore in theory also be 25% of the maximum.
If this same process of altering the duty cycle is
applied to a control signal that is being used on an
actuator such as an electric motor, it is then possible to
alter the power produced by the motor. The same
applies to any actuator control signal, where altering
the duty cycle will influence the way in which the
actuator functions.
Altering the control signal timing and frequency
If the control signal consists of simple on and off pulses,
an actuator will also be switched on and off. It is
therefore possible to provide the on and off pulses at a
specified time. A common example is when a fuel
injector used on a modern fuel injection/engine
management system is required to open at a certain
time in the engine operating cycle. The injectors on
some modern systems will open just before, or at the
start of, the intake stroke (possibly just before or just as
the intake valve opens). A sensor (usually the camshaft
position sensor) is used by the ECU as a timing
reference to calculate when the intake stroke is about to
start, allowing it to provide the on pulse in the control
signal at the right time.
The frequency of the control signal also affects how
an actuator behaves. For instance, a simple solenoid
could be used to open and close a small valve (which
could be allowing fuel to pass through a pipe). If the
control signal had a 50% duty cycle, and provided on
and off pulses that occurred very slowly, e.g. every 10
seconds, the solenoid would open the valve for 10
seconds and close the valve for 10 seconds. Although
this would regulate the flow of fuel in the pipe, it is not
a very effective means of control. If, however, the
control signal pulses occurred 100 times every second
(100 hertz), this would mean that the solenoid would
be trying to open and close 100 times a second. The
solenoid would in fact adopt a half open position i.e. it
would never reach the fully open or fully closed
positions. Therefore, altering the duty cycle will affect
the average opening time of a solenoid controlled valve,
but it is more effective if the frequency is high (such
as 100 hertz) than it is if the frequency is low.
[647] Chapter 01
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Introduction to powertrain electronics
Fundamentals of Motor Vehicle Technology: Book 2
Web links
Teaching/learning resources
Engine systems information
www.kvaser.com (follow CAN Education links)
Online learning material relating to powertrain
Chapter 2
[647] Chapter 02
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what is covered in this chapter . . .
Emissions, reliability and durability
Electronic ignition systems (early generations)
Computer controlled ignition systems
Distributorless and direct ignition systems
Spark plugs
This section relates to systems covered in chapters 3, 4
and 5.
2.1.1 Emissions legislation
The introduction of electronic engine control systems
was a result of a number of factors, most of which still
apply. As noted in Chapter 1, affordable electonics
enabled vehicle manufacturers to make increasing use
of electronic components and electronically controlled
systems. However, in the 1960s through to the early
1990s, it was in many cases still less expensive to fit
more traditional fuel and ignition systems, such as
carburettors and contact breaker ignition. At some stage
therefore, vehicle manufacturers needed some
motivation to fit electronic systems that in the early
stages were still more expensive to produce and to
develop than the existing components at that time.
Probably the single biggest factor in the increasing
use of electronic systems was the introduction of
emissions legislation. It is generally accepted that the
USA was the leading country in introducing legislation
that forced a reduction in emissions levels produced by
engines and vehicles in general. Legislation forced
vehicle manufacturers to develop and fit electronic
systems to engines. This process really started towards
the end of the 1960s, when electronics had just reached
a level of capability and cost that enabled
manufacturers to start to build systems that used
electronic components.
What did make things slightly difficult was that
different states in the USA had different problems;
therefore they had different requirements and
different legislation. Smog is one particular problem
(Figure 2.1) that captured everyone’s attention. Smog is
a term that became commonly used with reference to
fog that was not naturally formed in the atmosphere: it
was created from smoke that had been produced by
factories, houses and of course cars. Burning fossil
based fuels such as coal, petrol, diesel and even wood
produces smoke, and many towns and cities around the
world suffered with smog. One of the most famous is
London, which had for many years had a serious smog
problem. In fact, ever since entertainment films have
been made, it has been common to depict London (even
in the 1800s) as having a serious smog or fog problem.
The smog in London had been present for too many
years for it to be blamed entirely on motor vehicles, but
when certain weather conditions existed (which could
have produced normal fog), smoke produced by coal or
wood fires in houses and smoke from the factory
chimneys added to the problem. Although motor
vehicles must have started to contribute to the problem,
the relatively low number of vehicles in use up until the
1960s was not sufficient to be the major cause.
Figure 2.1 Smog in a city
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Engine management – spark ignition
Fundamentals of Motor Vehicle Technology: Book 2
Nitric acid
The formation of photochemical smog
Figure 2.2 Heat and exhaust gases can cause smog and other
pollution, especially in certain types of geographical location
The smog problem that occurred in Los Angeles,
California, which became very serious in the 1950s and
1960s, occurred because Los Angeles lies in a valley
where there is often little or no wind and a lot of heat.
In isolation, this could be regarded as a natural
problem, but in reality smog developed because of
emissions from the burning of fossil fuels. Los Angeles
had a high concentration of motor vehicles that, even
in the 1960s, spent a lot of time in traffic jams in what
was a confined area. Therefore a combination of
location, weather conditions and burning fossil fuels
caused extremely serious problems.
Whilst smog can be quite thick and therefore makes
driving hazardous, the real problem is a health issue.
Serious illness, such as respiratory problems, can be
caused by smog and many deaths have also been
attributed to smog. The acids contained within vehicle
related smog cause damage to buildings as well as
people. Athens in Greece is a particular case where
acids are destroying many of the ancient buildings.
Motor vehicles were a contributing factor to smog
problems so they became a focus of attention for
legislators. Legislation dealing with factories and fires
in houses has also been brought in. In the UK, the Clean
Air Act restricted the use of many types of fossil fuels in
both homes and factories, and London is now
significantly free of serious smog.
Vehicle emissions are a broad subject and smog is
only one of a number of problems influenced or created
by such emissions. Therefore, within this chapter,
emission problems are referred to and explained where
applicable, but particular reference should be made to
the emissions section.
If we appreciate that vehicle emissions can cause or
contribute to serious health and environment problems,
then we must accept that any effort to reduce the
problem is justifiable, even at a cost. The Los Angeles
problem, amongst others, was without doubt a major
talking point that captured the attention of the public:
the consumers that buy motor vehicles. Therefore any
reasonable added cost for the vehicle became relatively
acceptable. The vehicle manufacturers were therefore
tasked by legislators to reduce the level of vehicle
emissions. It is perhaps coincidental or fortunate that at
a time when emissions problems were a major focus of
attention for legislators, electronics were becoming very
much more capable and affordable.
Note: In the USA, diesel engines are very rarely fitted to
passenger vehicles. The cost of petrol was and remains
low compared to most countries and the petrol engine
was more acceptable as a means of powering large
American cars. Emissions legislation and technical
developments in the USA were therefore focused on the
petrol engine rather than the diesel engine. Europe and
other regions were therefore able to take advantage of
legislation and technical changes made in the USA, and
it was not until more recent times in Europe that diesel
engines have become a target for substantial emissions
reduction (and therefore technical change).
Engine maintenance
Engine design in the 1960s had not really changed too
much for many years. Although improvements had
been made, such as overhead valves instead of side
valves, the main objective was to improve engine
performance, which at that time primarily meant more
power. In the USA, in particular, vehicles were generally
much larger than in the rest of the world, and because
petrol was very inexpensive in the USA, large fuel
thirsty engines fitted in large heavy vehicles were
accepted as normal.
Engines in the USA at that time were usually of V8
configuration with typical capacities of 4 litres to
7 litres. Emissions levels from these large engines were
very high, especially when the engine was at idle speed
(when measured as a percentage of the total exhaust
gas, some pollutants were at their highest levels at idle
speed). It was therefore obviously going to be very
difficult to change vehicle and engine design suddenly,
so the changes were generally planned over a relatively
long period of time.
However it was recognised that one major factor
that could help to reduce emissions was to ensure that
regular maintenance was performed correctly, but
ideally, the need for regular maintenance on the fuel
and ignition systems should be reduced or even
eliminated. For those readers who have never worked
on older vehicles with carburettors and contact breaker
(points) ignition systems, it may be difficult to
appreciate that it was necessary to clean and adjust the
carburettor regularly and to adjust or replace the
[647] Chapter 02
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Emissions, reliability and durability
contact breaker in ignition systems. Ignition or fuel
systems would not suddenly become inefficient at a
particular interval, such as 16 000 km or 10 000 miles:
the reality was that ignition and fuel systems
progressively deteriorated. Therefore, immediately
after ‘perfect’ maintenance had been carried out,
systems would progressively become less and less
efficient, until a time was reached when the
inefficiency was unacceptable. Hopefully, regular
maintenance was performed before systems became
too unacceptable.
See Hillier’s Fundamentals of Motor Vehicle
Technology Book 1 for an explanation of carburettors
and contact breaker ignition systems.
Loss of efficiency of fuel and ignition systems
Carburettors would progressively accumulate a build
up of deposits in the airways and petrol jets (small
holes through which fuel passed). This build up of
deposits would then alter the air/fuel mixture, which
could result in reduced combustion efficiency and high
emissions. It was also common for the carburettor to
require slight adjustment of the idle mixture setting, to
further compensate for build up of deposits, and even a
change in the weather. Another major factor that made
it necessary to adjust the idle mixture setting was the
deterioration in the way the engine was performing,
which was caused by wear of the contact breakers in
the ignition system.
Progressive wear on the contact breakers was an
acceptable part of the design, and it was possible to
adjust them to compensate for wear until such time as
the wear was too excessive. Contact breakers on very
old vehicles may have required replacement after as
little as 5 000 miles (8 000 km) but even the
improved versions still required regular adjustment
and were traditionally replaced at main service
intervals, which could be between 5 000 and
10 000 miles (8 000 and 16 000 km).
One particular problem related to wear on the
contact breakers is that it can reduce the quality or
strength of the spark, because a worn set of contact
breakers (or an adjustment that is out of specification)
will not allow sufficient time for the ignition coil to
build up a strong magnetic field. This results in weak
electrical output from the ignition coil and therefore a
weak spark. The result is that the combustion of the
air–fuel mixture will not be efficient; this results in
high emissions of pollutants.
A second problem with worn contact breakers is
that, as the contact breakers wear, the ignition timing
changes. See Hillier’s Fundamentals of Motor Vehicle
Technology Book 1 for a full explanation of this.
However, in simple terms, wear in the contact breaker
mechanism causes the contact breakers to open earlier
or later. The opening time of the contact breakers
causes the ignition coil to provide the electrical energy
that in turn causes a spark at the plug, so wear in the
contact breakers affects spark timing.
Incorrect ignition spark timing will reduce combustion
efficiency, with the result that emissions of pollutants
increases, power is reduced and more fuel is wasted.
When ignition timing is incorrect (usually retarded or
later than specified), the engine will not idle smoothly.
However, it is possible on older vehicles to adjust the
air/fuel mixture at idle speed, which helps to smooth
out the way the engine is operating. This is generally
achieved by making the mixture richer than normal, i.e.
an excess of fuel. This in turn can also create higher
A third problem with contact breakers is that, each
time the contacts open, a small arc can occur across the
contacts. Although this problem was very much reduced
by the introduction of a condenser or capacitor in the
contact breaker circuit, arcing progressively damaged
the two contact points on the contact breakers.
Other systems requiring maintenance
It was more than just simple carburettor and contact
breaker maintenance that resulted in increasing
emissions. Older engine designs included many
components and systems that required regular
maintenance, which involved cleaning, adjustment or
replacement. Those areas requiring regular maintenance
valve operating clearances
spark plugs, which required regular changing or
resetting of the plug gap
engine oil, which required regular changing
engine breather systems
contact breakers and ignition timing
fuel systems (carburettors).
Each of the above listed items would progressively
wear, or their operating performance would
progressively deteriorate. In turn, this would either
affect combustion efficiency (which would increase
exhaust emissions), or would result in excessive engine
oil fumes and emissions. Without regular and correct
maintenance, many of those items listed above would
end up operating ‘out of specification’, which means
they would be operating outside their intended design
limits. The net result was that the engine would be
operating very inefficiently and excessive emissions
would be produced.
2.1.2 Reliability and durability
Even if an engine was maintained at the recommended
regular intervals, wear and progressive deterioration in
operating performance still occurred between the
maintenance intervals, thus causing an increase in
emissions. Ideally therefore, those adjustable items, or
items that wear and deteriorate between maintenance
intervals should be redesigned to avoid any reduction in
operating performance between maintenance intervals
(or for longer if possible). In effect, designers were
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Engine management – spark ignition
Fundamentals of Motor Vehicle Technology: Book 2
trying to create much improved reliability and
durability of engine components and systems so that
emissions did not increase significantly between
maintenance intervals. In fact, one piece of legislation
called for ignition systems to be able to operate without
maintenance for 50 000 miles (80 000 km).
Ignition system developments
A number of design changes were made over a period of
time to various engine components and systems.
However, one particular engine system benefited from
the use of electronics: the ignition system.
Early designs of electronic ignition (fitted as original
equipment) were designed to eliminate the contact
breakers (see section 2.2). An electronic module
functioned as the on/off switch for the ignition coil
circuit, so the current passing through the coil circuit
was no longer being controlled by a mechanical switch;
arcing at the contacts was therefore eliminated.
Additionally, higher currents could be passed through
the electronic module, which enabled more magnetic
and electrical energy to be created within the ignition
With the contact breakers eliminated as a means of
switching the ignition coil circuit, the spark still had to
be provided at the correct time. The electronic module
therefore needed a reference or trigger signal so it
would be able to switch off the ignition coil circuit at
the appropriate time, thus creating the spark at the
spark plug. Most earlier ignition systems used an
‘inductive’ or magnetic sensor, which was located
within the distributor body (effectively in the same
place as the previously used contact breakers).
The inductive or magnetic sensor (often referred to
as a pulse generator) operated in the same way as
modern speed/position sensors or rotational speed
sensors (see section 1.5.2), but usually used one
reluctor tooth for each cylinder. When each reluctor
tooth passed the sensor magnet, it caused a small
electrical pulse to be induced into the coil of wire
(adjacent to or wound around the magnet). The
electrical pulses were used by the electronic module as
a reference point for each cylinder, thus allowing the
electronic module to switch the ignition coil circuit at
the appropriate time. The ignition system delivered
the high voltage to a rotor arm (as was the case with
contact breaker systems), and the rotor arm directed
the voltage to the appropriate spark plug.
Figure 2.3 shows a comparison between a typical
contact breaker ignition system and an early generation
electronic ignition system, whilst Figure 2.4 shows a
typical inductive ignition timing sensor and an early
type ignition module.
On the early generations of electronic ignition, the
automatic timing advance and retard mechanisms were
identical to those used on contact breaker systems. The
centrifugal ‘bob weight’ system was used to advance the
timing with increase in engine speed and the vacuum
operated system was used to retard or ‘back off’ the
Figure 2.3 Schematic layout of ignition systems
a A contact breaker ignition system
b An early generation electronic ignition system
timing when a high load was applied to the engine. The
advance and retard mechanisms were linked to the
inductive sensor base plate and to the rotor shaft (as on
contact breaker systems) enabling relative angular
movement between the inductive sensor magnet and
the reluctor teeth, which affects the triggering timing
(and therefore the ignition timing).
2.1.3 Progress in electronic system
Electronic ignition systems were certainly much more
reliable and efficient than contact breaker systems.
What we would now regard as simple electronics
allowed considerable improvements to be made in the
ignition system (as described above) but mechanical
devices were still relied on to alter the ignition timing
when engine speed and load changed. Fuel systems,
however, even into the 1980s (certainly in Europe)
continued to rely on the carburettor as the means by
which fuel and air were mixed in the correct
improvements to the capability and accuracy of
carburettors, by adding various mechanical devices
and some electronic control functions onto the basic
carburettor. The carburettor actually developed into a
complex and often unreliable device. There was
therefore a growing demand to find an alternative
method of delivering fuel to the engine.
[647] Chapter 02
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Emissions, reliability and durability
Figure 2.4 Inductive pulse generator and ignition module
a Two types of inductive pulse generator that were located in the distributor body
b Typical appearance of ignition amplifier/module
Emissions legislation in the USA, Europe and many
other countries was becoming increasingly tough for
vehicle manufacturers to comply with, but electronics
were developing at a very rapid pace, which enabled
many design changes to be made to ignition and fuel
systems. While the early use of electronics improved
reliability and durability, electronics latterly developed
to the level of being able to control the engine systems;
this was a fundamental turning point in vehicle
The remaining sections within this chapter detail
more modern ignition and petrol systems, which are
generally now integrated as part of an engine
management system. Ignition timing is now controlled
electronically instead of mechanically, and fuel systems
are also electronically controlled (via electronic fuel
injection). Although some fuel injection systems used
in the late 1970s (through to the early 1990s) were
mechanically based, even these systems were improved
by the use of electronic control. In the end, however,
mechanically based ignition or fuel systems were
effectively no longer able to provide the accuracy and
control (at a cost effective price) that are now
necessary to maintain the low emissions levels
demanded by legislation.
Other benefits
Although emissions reductions are often regarded as the
only motivation for using electronics on engine systems,
the truth is that there are many other benefits. In
general, when engine efficiency is improved, this
usually results in better fuel economy and higher engine
power, as well as improved engine smoothness and
reliability. Using electronically controlled systems
allows engine designers to change certain design
features so that there are fewer compromises.
For example, when ignition timing is electronically
controlled, compression ratios can be increased to a
point where they are almost at their extreme limits
(which improves combustion efficiency). High
compression can result in combustion knock or preignition in the engine (especially if the fuel quality is
poor); this problem can be accelerated if the ignition
timing is only very slightly incorrect. Because the old
mechanical timing controls were relatively inaccurate, it
was not possible to risk damage that could be caused by
high compression. Therefore ignition timing throughout
the speed and load ranges of an engine were generally
set on the ‘safe side’, slightly retarded from the ideal
However, with electronic control and monitoring,
the ignition timing can be more accurate, which reduces
the risk of detonation and knocking, and, if knock
sensors detect any combustion knock, then the timing
can be retarded slightly to reduce the problem.
The above is just one instance where engine design
can be improved through the application of electronics.
The net result is that engine efficiencies are improved
so that power and economy as well as emissions are all
substantially better than was the case with older non-
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Engine management – spark ignition
electronic systems. Although we do tend to focus on
ignition and petrol injection, other engine systems
have also been improved through electronic control;
examples include variable valve timing and even
engine cooling systems.
Whilst it is often the case that existing mechanical
systems are improved through electronic control,
sometimes totally new systems are introduced which
may not have been possible with mechanical control. A
good example is electronic diesel injection which is not
an evolution of mechanical diesel injection, but a new
way of delivering diesel fuel to the engine with
accuracies and control that were not previously
Growing similarities between diesel and petrol
Perhaps it is ironic to note that the modern electronic
‘common rail’ type diesel injection systems bear a close
relationship to petrol injection systems, with the use of
electronic injectors and many sensors that are almost
identical to those of a petrol system. Conversely, some
modern petrol engines make use of direct petrol
injection, whereby the injectors are positioned in the
cylinder rather than in the traditional location of the
2.2.1 Disadvantages and limitations
of mechanical ignition systems
Those readers who are not totally familiar with
mechanically based ignition systems (contact breaker
systems with mechanical advance and retard
mechanisms) should refer to Hillier’s Fundamentals of
Motor Vehicle Technology Book 1.
There are many disadvantages associated with
mechanical ignition systems; the main disadvantages
intake ports. Diesel engines have of course traditionally
always had direct injection into the cylinder (or more
accurately: into the combustion chamber).
Whilst the fundamental difference between petrol
and diesel engines remains the way in which ignition
occurs (spark for petrol and heat generated from high
compressions for diesel), the systems used for fuel
delivery are now very similar.
The continuous pace of development
Many electronically controlled engine systems are
covered within the rest of this chapter, but such is the
pace of development that innovations are introduced on
a regular basis. However, if the reader has an
understanding of the fundamental aspects of engine
operation along with an understanding of electronic
control, it is relatively easy to embrace any new
A key driver for emissions has been changes in
Regular maintenance and accurate settings of
ignition and fuel systems reduces emissions and
improves fuel consumption
This section deals initially with those ignition systems
that were not necessarily integrated into engine
management systems. However, the design of these
separate ignition systems incorporates many features
and components that were then used for ignition
control on engine management systems. By studying
many of these components and design features
independently, it becomes easier to understand the
complete operation of petrol engine management
systems, which are covered in section 3.2.
Fundamentals of Motor Vehicle Technology: Book 2
Key Points
the contact breaker mechanism wears (causing
incorrect ignition timing and low ignition coil
at higher engine speeds, there is insufficient time
(ignition dwell time) for the ignition coil to build up
a strong magnetic field thus reducing ignition coil
output; this is made worse by higher engine
operating speeds
there is arcing at the contact breaker contact faces
(causing reduced ignition coil output)
maximum current flow passing across the contact
breakers is limited because excessive current will
promote arcing and cause the contact breaker faces
to burn away
ignition timing control is inaccurate, which restricts
the potential for engines to operate close to their
limits of efficiency
at high engine speeds, contact breakers are not able
to open and close quickly and accurately: there is a
tendency for contact breaker bounce (points
bounce) to occur (this causes the contact breakers to
bounce open before they should do, thus causing
incorrect spark timing and reduced coil output).
Contact breaker systems are therefore not suited to
the high engine speeds that are now typical for the
modern engine
weaker fuel mixtures can be more easily ignited
with larger spark plug gaps but this requires more
energy to be produced by the ignition coil, and such
levels of energy are not available from contact
breaker systems under all operating conditions; the
following sections explain how greater energy levels
are produced.
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Electronic ignition systems (early generations)
All of the above limitations and problems reduce engine
efficiency, leading to high emissions, reduced engine
power and higher fuel consumption. It was necessary
therefore to replace the mechanical contact breaker with
an electronic switch for the ignition coil primary circuit.
Figure 2.5 is a schematic diagram of a contact
breaker ignition system. Note that, as with almost every
design of coil ignition system, the primary circuit for the
ignition coil is switched on the earth circuit, in this
example by the contact breakers.
2.2.2 Main requirements of
electronic ignition systems
The fundamental requirements of an ignition system
have not changed much since the day that ignition
systems were first used on a petrol engine. The primary
requirements are to provide a spark or arc at a spark
plug that is strong enough to ignite the air–fuel mixture;
this spark must occur at the correct time in the
operating cycle. There have been other requirements
introduced over the years such as suitable interference
suppression, higher engine speed operation, etc., but
the basic requirements are much as they always were.
What has changed however is the quality of spark
and the standards of performance (reliability and
timing accuracy). These changes continue through to
today’s ignition systems where they form part of an
engine management system. We can therefore look at
the overall requirements of an ignition system at this
stage, and as the reader progresses through this ignition
section (and the engine management section), it is then
possible to see how the later generations of ignition
system are able to provide improved quality and
standards of performance.
Figure 2.5 Contact breaker ignition
High voltage
A fundamental requirement of the ignition system is
that it should produce a sufficiently high voltage from
the ignition coil at all speeds to enable the air–fuel
mixture to be initially ignited under cylinder pressure
(compression pressure). The spark or arc produced at
the spark plug must produce sufficient heat to cause
ignition of the mixture. Many thousands of volts
(kilovolts, or kV) are used to create or initiate the spark.
Voltage requirement (firing voltage)
A typical voltage requirement for a modern engine is in
the region of 7 kV to 12 kV or slightly higher (assuming
all components are good), while on an older engine, the
requirements were slightly lower at around 6 kV to
10 kV. Note however that, irrespective of how much
voltage the coil can produce, the voltage delivered by
the coil is dependent on the conditions that exist in the
high voltage ignition circuit and in the combustion
chamber. Remember that electricity will take the easiest
path to earth, so if the circuit from the coil passed
directly to earth (in effect a short circuit), the energy or
voltage requirement would essentially be zero, because
this is the easiest route without resistance or barriers to
electrical current flow. If, however, gaps and resistances
exist in the circuit, a higher voltage will be required for
the flow of electricity to reach earth. The major factors
affecting ignition systems are listed below.
Plug gap – with a gap in the electrical HT circuit
(high tension or high voltage circuit), the energy
required for the electrical flow to jump the gap and
reach earth will be large. The larger the gap, the
higher the voltage requirement; plug gaps are
generally larger than in the past to assist in igniting
weaker fuel mixtures. It is also true that the plug
gap can become fouled or contaminated, which in
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Engine management – spark ignition
some cases will make it more difficult to create a
spark, this also increases the voltage requirement.
Resistance – if we add resistance to the plug leads
and possibly also to the spark plug (to provide
suppression), again this will mean that a higher
voltage is required. Additionally, as plug leads and
other HT components deteriorate, the resistance
generally increases, which increases the voltage
Cylinder pressures – with modern high
compression engines the high pressure at the plug
gap makes it more difficult for electricity to reach
earth and a higher voltage is again required. It is
also interesting to note that if the ignition timing
(spark timing) is slightly retarded so that the spark
occurs at TDC instead of slightly before, the cylinder
pressures at this time will be higher; this will also
create a higher voltage requirement. A worn engine
that has a low cylinder pressure will therefore
require a lower voltage.
Air:fuel ratio – although a minor factor, it is also
interesting to note that it is more difficult for a spark
to be generated across air than it is across vaporised
petrol. When a spark is created, it ionises the
mixture. Vaporised petrol ionises much more readily
than air, so it is more difficult to create a spark in a
weak mixture; again, a higher voltage is required.
Figure 2.6 Output voltage from an ignition coil through one ignition cycle
Fundamentals of Motor Vehicle Technology: Book 2
The important thing to remember is that electricity is
effectively ‘lazy’, but it will always try to reach earth,
i.e. complete the circuit. If the voltage requirement is
low, then the voltage delivered by the ignition coil will
be low. If however, there are resistances and gaps, etc.
in the circuit, these will result in a higher voltage being
delivered by the coil. Because electricity will always
attempt to reach earth, even if there are restrictions or
resistances, the voltage delivered by the coil will
increase in line with the requirements, until such time
as there is insufficient energy in the coil.
Figure 2.6 shows the output voltage produced by an
ignition coil during one ignition cycle for one cylinder.
Note that ‘firing voltage’ is at a high voltage value,
which is necessary to initiate the spark at the spark plug
gap (under operating conditions). The rest of the
voltage levels are detailed in the following paragraphs.
Maintaining the spark (spark duration)
Since the introduction of emissions regulations, because
engines generally operate on weaker or leaner mixtures
(a greater proportion of air), it is more difficult initially
to ignite the air/fuel mixture. Additionally, when a
weaker mixture is used, flame spread throughout the
mixture can be less effective at igniting all of the
mixture. To help overcome these problems, a slightly
higher initial spark voltage is required, but a spark of
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Electronic ignition systems (early generations)
longer duration is also required to maintain the
temperature at the spark plug, thus helping to ignite or
maintain the combustion of all of the mixture.
Modern ignition systems produce a spark that can
last typically around 2 ms (2 milliseconds or 2
thousandths of a second) or more, whilst older engines
(with relatively rich mixtures) had spark durations of
around 0.5 ms. The coil needs to produce sufficient
energy or voltage to create the spark initially, but there
must also be sufficient energy available from the coil to
maintain the spark for longer periods than was the case
with older systems that ignited richer mixtures.
The spark voltage and spark duration periods shown
in Figure 2.6 illustrate that once the spark has been
initiated, a much lower voltage is then required to
maintain the spark: once the ionisation process has
been started, maintaining that process does not require
such a high voltage.
When almost all of the coil energy has been used to
initiate and maintain the spark, the energy remaining in
the coil will be insufficient to keep the spark going;
therefore the spark will extinguish. The small amount of
remaining energy then tends to oscillate backwards and
forwards within the system until there is no usable
energy remaining.
Coil charge time (dwell period)
If the coil is visualised as a container that stores
electrical energy, it is clear that a fully charged coil will
be able to maintain a spark for longer than a partially
charged coil. Because the coil energy is now used to
initiate and then maintain the spark for longer periods,
modern ignition coils must be able to build up energy
(charge up) much faster than those on older systems.
Current flows through the coil primary winding are
increased by reducing the resistance of the coil primary
winding, which enables faster charging of the coil.
Additionally, the charge up period (previously referred
to as the ignition dwell period) is controlled on
electronic systems such that there is a longer time
available to build up the coil energy.
As well as ensuring the coil provides enough energy
to initiate and maintain the spark for longer, higher
engine speeds add another demand because they mean
less time for the coil to build up energy, so ignition
systems must be able to charge the coil (build up the coil
energy) in even less time.
In Figure 2.6, the indicated dwell period is the
period when current flows through the primary
winding, allowing energy to be built up within the coil.
A switch located in the primary circuit is used to switch
on and off the flow of current. On old systems this
switch was the contact breaker, and when the contact
breakers closed, this would complete the circuit and
allow current to flow. On modern systems, the contact
breakers are replaced by an electronic switch, which is
usually part of an amplifier or module (or in many
cases it is incorporated into the engine management
On older type contact breaker ignition systems it would
take as long as 10 ms for the current to reach its
maximum flow rate (typically 4 A maximum). As an
example, on an old type four-cylinder engine operating
at 6000 rev/min (Figure 2.7), the whole ignition cycle
for a cylinder (the time between the spark on one
cylinder to the spark on the next cylinder) would take
only 5 ms (5 thousandths of a second). However, it is
not possible to allocate the whole of the ignition cycle
to building up coil energy because some of that time
must be available for the coil to deliver the energy to
produce the spark. Typically, around half of the total
ignition cycle was used for building up coil energy; in
our example, this would mean 2.5 ms (2.5 thousandths
of a second). If it takes 10 ms to reach maximum
current flow in the coil primary circuit, then at high
engine speeds there would only be around one-quarter
of that time available, which would restrict coil energy.
On an engine operating on relatively rich mixtures,
the low coil energy was not so much of a problem but
on modern high speed engines with weaker mixtures,
the coil energy must be higher than on older ignition
systems. Modern coils operating with modern
electronically controlled ignition systems can build up
sufficient energy in around 3 or 4 ms or less. Although
the coils may not be fully charged at high engine
speeds, the charge level is sufficient. However, many
modern engines now use one coil for each cylinder,
which on a four-cylinder engine means that a coil has
four times as long to build up the energy as engines
using a single coil for all cylinders. This is especially
important on an engine with six, eight, ten or 12
cylinders, because the more cylinders there are, the less
time is available between ignition cycles. Remember, on
a four-cylinder engine, there are two sparks for every
crankshaft revolution. On a 12-cylinder engine there are
six sparks for every crankshaft revolution, which means
there is only one third of the time available compared to
a four-cylinder engine.
Dwell angles and dwell percentage
On old ignition systems that used contact breakers to
switch on and off the primary current the contact
breakers were set so that they were closed for typically
around 50% to 60% of the ignition cycle, i.e. the dwell
period. On an old type four-cylinder engine with contact
breakers and a distributor, the distributor shaft would
rotate through 90° for each ignition cycle (for one
cylinder). A dwell period of 50% would equate to 45° of
distributor rotation. It was therefore common in the past
to quote the charge-up time as being the ‘dwell angle’.
When contact breakers were fitted and adjusted, the
objective was to achieve the specified dwell angle; by
adjusting the position of the contact breaker within the
distributor, the correct dwell angle could be obtained.
Because the mechanical setting and operation of the
contact breakers dictated this percentage, it would not
change throughout the speed range of the engine. If the
percentage were any larger, at slow engine speeds, the
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Engine management – spark ignition
Fundamentals of Motor Vehicle Technology: Book 2
Figure 2.7 The charge up time (dwell period) on a four-cylinder engine
current flow in the primary circuit would be too long
and the circuit and coil would overheat. If the
percentage were any smaller, at high engine speeds there
would not be enough time to charge up the ignition coil.
The percentage used was therefore a compromise.
However, once the correct dwell angle was set, it
would not change with engine speed, so when the
engine speed increased, the whole ignition cycle had to
take place within less time. Therefore there was less
time to charge up the coil.
Modern electronically based ignition systems are
able to control the dwell period. In effect, the dwell
time is controlled so that it is ideally the same duration
at all engine speeds. Therefore, if the ignition coil takes
only 2 ms to build up the required energy, then the
dwell time would be 2 ms at all engine speeds. There
are situations where at high engine speeds there may
not be quite enough time available but a number of
other design changes have overcome this problem (as
discussed in the following sections).
Spark timing
The spark must occur at the correct time, at all engine
speeds and loads. When the mixture is initially ignited, it
must have sufficient time to combust and for the gases
to begin to expand before the piston has moved too far
down the cylinder on the power stroke. Ideally, the
mixture should initially be ignited just before the piston
reaches TDC on the compression stroke; this should
allow for the mixture to begin to combust and then for
the heat to cause the gases to start to expand just as the
piston reaches TDC. The subsequent expansion of the
gases then forces the piston back down the cylinder.
If the spark occurs too soon (with over-advanced
timing), the gases will start to expand before the piston
reaches TDC and the expansion will try to force the
piston down the cylinder before it has reached TDC,
which means that some of the energy created by the gas
expansion will be pushing against the rising piston; this
can result in combustion knock occurring because of the
premature ignition timing.
If the spark occurs too late (with over-retarded
timing), the expansion of gases will be late and the
piston may be already on its way down the cylinder (on
the power stroke). The effect of the gas expansion will
therefore be wasted.
Speed related timing advance
If we accept that it takes a certain period of time for the
fuel to ignite and then combust or burn sufficiently to
create the heat and expansion of the gases, then in
general this time period will theoretically not change
significantly when the engine speed changes (assuming
all other conditions remain the same). The burn time
does in fact change with engine speed, but for the
following example, it is assumed that the time remains
If we assume that the time allowed for the burning
process to take place and create the maximum pressure
in the cylinder is around 4 ms (4 thousandths of a
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Electronic ignition systems (early generations)
If in the following example (see Figure 2.8a) we assume
a burn time of 4 ms, then at 1000 rev/min the
crankshaft would rotate through 24° during the 4 ms
burn time. If we also then assume that maximum gas
pressure should occur at 14° of crankshaft rotation after
TDC, then the spark should occur at 10° before TDC.
If the engine speed is increased to 2000 rev/min, i.e.
doubled, 4 ms will still be needed for the burn time
(Figure 2.8b). Because the crankshaft is now rotating at
twice the previous speed, it will rotate through 48°
during the 4 ms burn time. Therefore, the spark must
occur 48° before the maximum pressure is required
(which remains at 14° after TDC). The spark must
therefore occur at 34° before TDC.
Further increases in speed would therefore also
require additional advances in the ignition timing, but
because the burn time does in fact not remain constant
(conditions within the cylinder change with speed), the
amount of additional advance required for increases in
engine speed gradually reduces. Figure 2.8c shows an
approximate advance curve related to engine speed.
Note that different engine designs and combustion
chamber designs will have different advance
characteristics, so the advance curve illustrated shows a
trend, rather than indicating exact values of spark
timing advance.
As previously mentioned, older type mechanical
advance mechanisms are not accurate enough to
provide the exact advance curve needed for modern
engines. Therefore electronic systems now control the
timing advance process as described later in section 2.3.
Figure 2.8 Ignition timing advance related to engine speed
a Burn time takes 4 ms. At 1000 rev/min, 4 ms gives 24° of
crankshaft rotation. If the maximum pressure must occur 14° after
TDC, then the spark must occur 10° before TDC.
b Burn time takes 4 ms. At 2000 rev/min, 4 ms gives 48° of
crankshaft rotation. If the maximum pressure must occur 14° after
TDC, then the spark must occur 34° before TDC.
c Typical ignition advance requirements
second), then the spark must occur 4 ms before the
maximum pressure is required. In reality, the maximum
pressure created by the gas expansion is usually
required just after the piston has passed TDC, i.e. it is
just beginning the downward stroke.
Load related timing advance/retard
When an engine is operated at light load, it is possible to
operate on weaker mixtures than when the engine is
operated under high load conditions. On modern
engines, the mixture is controlled by the engine
management system using the oxygen sensor
monitoring process (as discussed in sections 1.5.6 and
3.2), and there is not so much change between the
mixture settings for light and heavy load conditions
(compared with older engines). However, when weaker
mixtures are used for light load conditions this causes a
requirement for a different ignition or spark timing.
A weaker mixture takes longer to burn, so the spark
timing will require additional advance. Additionally,
under light load conditions, the throttle valve
(butterfly) is only partially open, thus restricting the
flow of intake air; this means that a lower volume of air
is drawn into the cylinder, and during the compression
stroke, the cylinder pressures and temperatures are
lower, which also has an effect on the burning process.
Under these conditions, it is therefore necessary to
advance the timing slightly to account for the slower
burn time.
When the engine is again placed under load and the
mixture is no longer weak (cylinder pressures will also
be higher), the amount of timing advance can be
reduced back to the load setting.
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Engine management – spark ignition
On older ignition systems with mechanical advance
mechanisms, the speed related timing advance
provided the main advance setting and when the
engine was under higher loads, a vacuum operated
system would retard the timing; this type of load
dependent system was referred to as a timing retard
system. In effect there was a speed related advance
system and a load related retard system.
Incorrect timing
If the ignition timing is incorrect, as previously
mentioned, it can cause two problems: first, overadvanced timing can cause combustion knock: the
premature expansion of the gases will be wasted
because the piston is still rising on the compression
stroke when the gases are expanding; second, overretarded timing will result in the maximum pressure
occurring when the piston has already travelled too far
down the cylinder: therefore the expansion of the gas
will again be wasted. In effect, both over-advanced and
over-retarded timing will cause a reduction in power.
The ignition timing can also affect the emissions;
over-advanced timing can cause incomplete combustion
of the mixture which will result in high levels of
unburned or partially burned fuel entering the exhaust
gas. In addition, higher temperatures will be created in
the cylinder (the pressure rise will be higher), which
can result in increases of oxides of nitrogen (see section
3.5). However, over-retarded timing can in some cases
help to reduce emissions, although this might also cause
poor combustion: some older emission control systems
used retarded timing under certain operating conditions
to ensure that combustion continued later in the engine
cycle, helping to burn some of the partially burned
exhaust gases.
Interference suppression
Ignition systems create considerable electrical noise;
this is the effect of providing a high voltage to a spark
plug, which causes radio frequency energy around the
wires carrying the high voltage. Suppression is achieved
by using resistances built into the high tension (HT)
cable and/or the spark plugs and rotor arm (where
fitted on older systems). Apart from the fact that the
interference causes electrical noise (crackles, etc.) on
the vehicle radio, television and other audiovisual
equipment can be affected (even if it is some
considerable distance from the vehicle). Legislation
limits the levels of interference.
2.2.3 Electronic switching of the coil
primary circuit
The coil primary circuit carries a relatively high
current for an extremely short period of time. On a
contact breaker system, the resistance of the coil
primary winding was typically around 3 ohms, which
would mean the current flow in the 12 volt primary
circuit would reach a maximum of 4 A (which was the
Fundamentals of Motor Vehicle Technology: Book 2
maximum that could be used on a contact breaker
system without causing excessive arcing and
damage). On older slow running engines, this current
level was sufficient to enable the coil to build up a
reasonably strong magnetic field and therefore
produce an acceptable output voltage. Note, however,
that fuel mixtures were relatively rich (containing an
excess of petrol) which enabled the mixture to be
ignited with a relatively weak spark. However, if a
stronger spark is required, this can be generated by
either increasing the current flow in the ignition coil
primary circuit (lowering the resistance of the coil
primary winding) or maintaining the current flow in
the primary circuit for longer periods (a longer dwell
time). Both options would result in accelerated
damage to the contact breakers. In addition, higher
engine speeds and multi-cylinder engines reduce the
available time for increasing the dwell period (the
coil charge time).
Using an electronic switch instead of a mechanical
switch provides a number of benefits. One is improved
reliability because there are no moving parts and no
arcing at the contacts, but an electronic switch also
enables higher current flows to exist for longer periods.
The simple type electronic switch, which is a power
transistor, simply switches on and off the coil primary
circuit at the appropriate time, but note that the
transistor will perform the switching task only when an
appropriate electrical trigger signal is provided.
Figure 2.9 shows a simplified example of how a
transistor functions as a switch by comparing a
transistor to a water valve. Note that the main water
flow from the collection point C cannot flow through
the valve and be emitted at E when the valve is closed.
However, if a small flow of water is allowed to pass into
the base of the valve B this will cause the valve to open
and allow the main water flow to pass from C to E.
For the transistor, the principle is much the same.
The main electrical circuit cannot flow through the
transistor from the collector to the emitter (C to E) until
a small voltage or current is applied at the base B. It is
therefore possible to provide a small electrical signal at
B (a low current or voltage) to control a much higher
current and voltage, which are passing through the
transistor from C to E.
See Hillier’s Fundamentals of Motor Vehicle
Technology Book 3 for a more detailed explanation of
how transistors operate.
2.2.4 Electronically assisted ignition
Some early generations of electronic ignition system
used a contact breaker to provide the small electric
signal to the transistor. Figure 2.10 shows a simplified
circuit where the transistor is the switch for the ignition
coil primary circuit. In this example, however, the
contact breaker is used only as a means of switching on
and off a low current signal to the transistor. Therefore
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Electronic ignition systems (early generations)
Figure 2.9 Principle of a transistor using a water valve as a comparison
a A water valve closed and open
b A transistor switching on and off
the contact breaker can be operated in exactly the same
way as on a traditional contact breaker ignition system,
but the contact breakers do not carry the high current. A
resistor is used in this example in the contact breaker
circuit to reduce the current flow passing to the B or
base terminal of the transistor.
Where a simple transistor is used to switch the coil
primary circuit, it is often referred to as the ‘ignition
module’ or ‘ignition amplifier’, although in truth the
transistor is not strictly speaking amplifying the trigger
signal but merely switching the coil primary circuit in
response to a trigger signal.
The contact breaker triggered system shown in
Figure 2.10 would eliminate the high currents passing
through the contact breaker, and higher currents could
then be allowed to flow in the ignition coil primary
circuit. However, the accuracy of the ignition timing is
still dependent on the contact breaker, which would still
suffer from mechanical wear. A non-mechanical trigger
mechanism is therefore required.
Figure 2.10 Using a contact breaker to control the switching of a transistor in an ignition circuit
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Engine management – spark ignition
Fundamentals of Motor Vehicle Technology: Book 2
2.2.5 Inductive ignition trigger
reluctor teeth passes the magnet and inductive coil
assembly, which will produce a small electrical pulsed
signal. This electrical signal is an analogue signal. The
pulsed signal is then passed to the ignition module
(power or switching transistor), which uses the pulses
as a trigger or reference point to switch the ignition coil
primary circuit.
The example in Figure 2.11 shows a magnet and
inductive coil assembly located to one side of the
reluctor. The iron reluctor is mounted on the distributor
shaft and therefore rotates with the shaft. The example
shown in Figure 2.12 operates in much the same way as
the example in Figure 2.11, but the construction is
slightly different. The magnet is a circular disc which is
located concentrically with the distributor shaft; the
inductive coil is also concentric with the magnet. The
stator, or pole for the magnet, consists of fingers (one
for each cylinder) which protrude upwards. A rotor or
reluctor, which also has one reluctor tooth for each
cylinder, is located on the distributor shaft; the reluctor
teeth are formed as fingers that protrude downwards
and pass adjacent to the stator fingers.
In both examples, when the reluctor teeth or fingers
pass the stator or stator fingers, this causes an electrical
signal to be produced as explained below.
Inductive pulse generator
Figures 2.11 and 2.12 show two different types of
inductive trigger or pulse generator. Both types were
located in the ignition distributor body and as such
physically replaced those components originally used
for contact breakers. On earlier generations of
electronic ignition, both types were also connected to
(inherited from the contact breaker systems).
Both examples operate on the same principle of
using a reluctor with the same number of reluctor teeth
(or triggering lugs) as the number of cylinders, i.e. four
reluctor teeth for four cylinders. A permanent magnet
and inductive coil (coil of wire) are located adjacent to
the reluctor. When the reluctor is rotating, each of the
Generating the pulse
When a reluctor tooth is aligned with the permanent
magnet or stator (as shown in Figure 2.11), it allows the
magnetic flux to flow from the stator, across the reluctor
and back again. When the distributor shaft is rotating,
the reluctor teeth will inevitably move away from the
stator, providing a gap between the reluctor teeth and
the stator. This gap results in a greater reluctance of the
magnetic field or magnetic flux, i.e. the flow will be less.
In effect, when the reluctor teeth approach the
stator, the flow of magnetic flux will increase. The flow
of magnetic flux will be at its maximum when the teeth
and stator are in alignment, and it will reduce when the
reluctor teeth move away from the stator. When the
flow of magnetic flux changes, i.e. increases or
There are a number of ways that an electrical signal can
be created that will act as a means of triggering the
power transistor in an ignition coil primary circuit. As
explained above, a contact breaker can be used to
switch on and off a trigger circuit, but more accurate
and reliable methods are generally non-mechanical.
The following section covers the most common types of
non-mechanical trigger mechanism that were used for
electronic ignition systems.
The vast majority of early generation electronic
ignition systems used either an inductive or Hall effect
trigger system, although a few systems did use an
optical trigger. Although we can refer to these systems
as being non-mechanical, mechanical components are
used, but the important fact is that the trigger signal
passed to the ignition module is created by a nonmechanical process, i.e. there is no mechanical switch.
A number of constructional and design variations
existed for all three trigger systems, but for each of the
examples mentioned in the following section, the
general operating processes form the basis for other
variants of each type.
Figure 2.11 Inductive pulse generator with the magnet and inductive coil located at the side of the reluctor
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Electronic ignition systems (early generations)
Figure 2.12 Inductive pulse generator with the magnet and inductive coil located concentrically around the reluctor
decreases (owing to rotation of the reluctor), this causes
a small electrical current to be produced in the
inductive coil. The voltage generated is at its greatest
when the change in flux flow is at its greatest; this
occurs just as the reluctor teeth are approaching or
leaving alignment with the stator.
Figure 2.13a shows the voltage output from the
inductive sensor, or variable reluctance sensor as it is
sometimes called. There are four electrical pulses (for a
four-cylinder engine) produced during one complete
rotation of the reluctor. A positive voltage is produced
as the reluctor teeth approach alignment with the
stator (magnet); as the reluctor teeth leave alignment
with the stator, a negative voltage is produced. The
output signal is therefore an analogue alternating
current (AC).
As described above, when the reluctor teeth
approach or leave the stator (Figure 2.13b) this causes a
large change in magnetic flux, which therefore produces
higher voltage (positive or negative). However, when
the reluctor teeth are close to alignment and in
alignment with the stator, the result is very little or no
change in the flux, which means that less voltage is
produced. When the reluctor is directly in alignment
with the stator, there is no change in magnetic flux:
therefore the voltage produced is zero.
Note: The gap between the reluctor teeth and the stator
is effectively set during manufacture. However, on some
types of construction it is possible to alter the gap. If the
gap is not correct, this will affect the magnetic flux and
the strength of the signal produced. Reference should
always be made to manufacturer’s specifications.
Reference point for ignition timing
It is normal practice to use the change or ‘switch over’
from positive to negative voltage, i.e. the zero voltage
point, as the reference point for ignition timing. The
ignition module will therefore use this ‘zero volt’ point
of the electrical signal as the reference to switch off the
ignition coil primary circuit, thus creating the high
voltage and a spark at the plug.
Figure 2.13 Rotation of the reluctor
a One rotation of the reluctor produces four pulses as an
analogue signal
b Voltage levels produced when the reluctor teeth are in
different positions relative to the magnet (stator)
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Engine management – spark ignition
The ignition coil will produce its high voltage output
when each reluctor tooth is aligned with the stator: on a
four-cylinder engine there will be four high voltage
outputs from the coil for every rotation of the
distributor shaft. The output from the coil must
therefore be distributed to the appropriate spark plugs
at the correct time (when each of the cylinders is close
to TDC on the compression stroke); this is achieved by
passing the high voltage from the ignition coil to the
centre of a rotor arm located within the distributor.
When the distributor shaft and rotor arm are rotating,
the rotor arm will direct the high voltage to the contact
segments in the distributor cap, which allows the
voltage to pass to each of the spark plug leads and spark
plugs in turn (Figure 2.14). It is therefore important to
note the exact location of each spark plug lead on the
distributor cap to ensure that the voltage is directed to
the correct spark plug at the correct time.
Wiring circuit for an inductive pulse generator
Inductive pulse generators generally have two wiring
connections to the ignition module. Effectively, these
two wires provide a positive and a negative path for the
electric current. However, remember that the current is
an alternating current which means that the flow
alternates within the wiring; each wire therefore
alternately carries positive and negative flows.
Figure 2.15 shows the wiring for a typical inductive
sensor and ignition module.
Fundamentals of Motor Vehicle Technology: Book 2
The wiring diagram (Figure 2.15) shows two wires
carrying the pulsed signal from the inductive pulse
generator to the ignition module. Because the module
forms part of the earth circuit for the ignition coil
primary circuit, the power supply from the ignition
switch passes to the coil positive terminal (usually
marked terminal 15) and then through the coil primary
winding to the ignition module. The module functions
as the switch for the primary circuit; therefore the
circuit must pass through the power or switching
transistor in the module before it is connected to earth.
If the ignition module contained only simple passive
electronics, no power supply would be required for the
module, but it contains active electronic components
that require an additional power supply and earth
On some applications, a third wire has been used
which is wrapped around the two signal wires. The
third wire is connected to earth or ground, acting as a
screen or shield against interference from other
electrical systems. Also note that when only two wires
are used, one of the wires may be wrapped around the
other, which provides a form of screening.
The inductive sensors can usually be classified as
self-generating, which means that no additional power
Figure 2.14 Rotor arm and distributor cap (allowing a high
voltage to be directed to the correct spark plug)
a Distributor cap
b Rotor
Figure 2.15 Wiring for a simple inductive trigger ignition system
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Electronic ignition systems (early generations)
supply is required to enable the pulsed signal to be
produced. Therefore the two wiring connections simply
provide a complete circuit for the inductive coil. There
are however some examples where an electric current is
used to enhance or create the magnetic field; in such
cases the same wiring that provides the current to the
sensor also carries the pulsed signal.
Advantages and disadvantages of inductive pulse
Inductive pulse generators are relatively inexpensive to
produce and generally reliable and accurate, even when
working in the harsh environment of a vehicle’s engine.
Inductive pulse generators produce an analogue signal,
which, when passed to an ECU that is operating with
digital electronics, will require an analogue to digital
converter (A/D converter). When the analogue signal is
supplied to the early types of ignition modules (a simple
power transistor switch), some simple circuitry within
the ignition module is used to reshape the pulse so that
the applicable reference points are recognised, i.e. the
zero volt timing reference point.
The inductive pulse sensor operates using the same
principles as a conventional electrical generator and, as
is the case with a conventional generator, the voltage
produced increases with the increase in rotational speed
of the rotor (or reluctor). Therefore, when the engine
speed increases, the voltage produced by the inductive
pulse generator also increases, ranging from around
0.5 volts at slow speeds to a possible 100 volts at high
speeds. If, therefore, there is any deterioration in the
strength of the magnetic flux, and the engine is turning
over very slowly during starting, it is possible that no
signal will be produced.
Also of note is that, on the example shown in Figure
2.11, where the magnet and inductive coil assembly are
located to one side, it is possible for an erratic or
unusable signal to be produced if wear exists in the
distributor shaft bearings. If a distributor shaft bearing
is worn, the shaft can wobble during rotation; this in
turn can result in the air gap between the reluctor teeth
and the stator changing. The usual problem
encountered is that the air gap for one reluctor tooth is
too small whilst the gap for the opposite tooth is too
large. It is not uncommon in these cases for the pulse
signal to be missing a trigger pulse for one and
sometimes two cylinders. This will mean that one or
two cylinders may not receive a spark at the spark plug.
A digital signal also has other advantages relating to the
very defined reference points that can be provided. In
the previous section (2.2.5) it was stated that the
analogue signal produced by the inductive sensor
would change with engine speed: the voltage produced
by the sensor increases with increases in engine speed.
In fact, the whole ‘shape’ of the signal changes.
However, with a digital signal such as the signal
produced by a Hall effect sensor, the reference points
are consistent, irrespective of engine speed.
Hall type systems are often referred to as Hall effect
switches or Hall effect pulse generators. Figure 2.16a
shows a typical construction of a Hall effect ignition
trigger located in an ignition distributor; Figure 2.16b
shows a separate view of the Hall trigger assembly.
2.2.6 Hall effect ignition trigger
As mentioned previously, when a sensor produces an
analogue signal (such as the inductive ignition trigger),
if the signal is then passed to an ECU that operates with
digital electronics, an analogue to digital converter is
required to enable the ECU to interpret the signal. It is
therefore an advantage if the sensor is able to provide a
digital signal.
Figure 2.16 Hall effect pulse generator
The rotor has four vanes which causes four pulses to be
produced (a digital signal) during one rotation of the rotor
a A Hall effect ignition trigger in an ignition distributor assembly
b A Hall effect pulse generator
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Engine management – spark ignition
Hall effect digital pulse
The construction and operation of the Hall effect sensor
is described in Chapter 1. The signal produced is a
square wave: Figure 2.17 shows a typical digital signal
produced by a Hall effect ignition trigger. Here, there
are four pulses produced during one rotation of the
rotor, which would indicate that the rotor has four
vanes and would be used on a four-cylinder engine.
With a digital signal, it is possible to make use of at
least two definitive reference points on the signal. One
reference point is when the voltage increases from zero
to 5 volts (or whatever voltage is used on the sensor).
The second option is when the voltage drops from
5 volts down to zero. Therefore, either the rise or the
fall in voltage can be used as the reference point for the
ignition module to switch off the ignition coil primary
circuit. When working on Hall effect systems, it is
therefore necessary to refer to the manufacturer’s
instructions to find out whether the ignition timing
point occurs when a vane is just leaving the air gap
(between the magnet and the Hall chip), or when the
vane is just entering the air gap, because this will
dictate whether the voltage is rising or falling.
A – switch on – no vane in gap
B – switch off – vane in gap
Fundamentals of Motor Vehicle Technology: Book 2
For most Hall effect systems, there will be the same
number of vanes as there are engine cylinders. This
means that on a four-cylinder engine with a single
ignition coil, there will be four vanes, and the ignition
coil will therefore produce four high voltage outputs for
every rotation of the Hall effect rotor/trigger disc. As
with the inductive ignition trigger system, it is therefore
necessary to pass the high voltage coil output to a rotor
arm in the distributor cap, which will then direct the
voltage to the four spark plugs in turn.
Wiring for Hall effect pulse generator
Figure 2.18 shows the wiring for a simple Hall effect
triggered ignition system. The Hall effect pulse
generator initially requires two wiring connections for
the input (a power supply and earth connection) to
enable it to function; Figure 2.16b shows the circuit
passing across the Hall chip terminals A and B.
However, the signal produced at the Hall chip should
also have two connections (positive and negative
paths), but the negative or earth path is shared with the
earth path of the input, which means that there is a
total of three connections between the Hall effect
sensor assembly and the ignition module.
Note that it is common practice to mark the
terminals on the Hall sensor connector plug with three
symbols: +, –, and 0. The + terminal is the power
supply (often stabilised at 5 or 8 volts), the – terminal
is the earth terminal and the 0 terminal is the output
terminal for the digital signal.
The ignition module will require a power supply and
earth connection, and the module again forms part of
the earth circuit for the ignition coil primary circuit.
2.2.7 Optical ignition trigger
Vane:gap ratio = 70:30
Figure 2.17 Digital signal produced by a Hall effect pulse
Hall effect
ignition module
Optical ignition triggers have been used on a number of
ignition systems and also for some other applications
where a digital signal is preferred to an analogue signal;
Coil low
with Hall effect
trigger unit
(4 cylinder)
Hall effect
power and signal
12 V
4 15
Figure 2.18 Wiring for a simple Hall effect triggered ignition
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Electronic ignition systems (early generations)
the optical system provides an alternative to the Hall
effect system.
Producing the optical trigger digital pulse
A light emitting diode (LED) produces a small light
when an electric current is passed through it;
conversely a phototransistor produces a small electric
current when it is exposed to light. Therefore, if an LED
is used to project light onto a phototransistor, the
phototransistor will produce an electric current.
As illustrated in Figure 2.19, an LED and
phototransistor can be located, as an assembly, inside
the distributor body. If a ‘chopper disc’ is located on the
distributor shaft so that the vanes or shutters of the
chopper disc pass between the LED and phototransistor,
the shutters will prevent the light from the LED from
reaching the phototransistor. If the chopper disc has
four shutters (as shown in the illustration), then when
the disc rotates (with the distributor shaft), each time a
shutter blocks the light from the LED then the
phototransistor will not produce an electric current.
However, when the gaps between the shutters are in
line with the LED and phototransistor, the light will
reach the phototransistor thus producing the electric
current. The four shutters and gaps would result in four
on/off pulses of current being produced by the
phototransistor in one rotation of the distributor shaft
and shutter disc; this version would therefore be used
on a four-cylinder engine.
The optical system produces a digital pulse (Figure
2.20), which can be at a lower voltage than that
provided by the Hall system (sometimes as low as
2.4 volts). However, the signal from the optical system
is sufficient for the ignition module to identify the rise
in voltage or the fall in voltage as the signal alternates
from on to off.
Figure 2.19 Optical ignition trigger assembly
1 pulse/cylinder
(4 cylinder)
+2 V
V signal
+0.2 V
360° crank angle
Figure 2.20 Digital signal produced by an optical ignition trigger
Wiring for optical ignition trigger
The LED requires a power supply and earth connections
(Figure 2.21). The power supply will usually be passed
from the ignition module to the LED. Note that the
voltage will be stabilised by components within the
module. The phototransistor will also require two
connections to the module (positive and negative
paths) to carry the signal produced by the
phototransistor. There will therefore be four
connections between the optical sensor and the ignition
module. The module will also require its own power
supply and earth connections, and, as with inductive or
Hall systems, the module will form part of the earth
circuit for the ignition coil.
Advantages and disadvantages of an optical
The optical ignition trigger system provides a true
digital signal, which is especially useful when the signal
is passed to an ECU that operates with digital
electronics. Optical systems are also generally reliable
and relatively inexpensive.
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Engine management – spark ignition
Fundamentals of Motor Vehicle Technology: Book 2
Coil LT
Optical ignition
tracer module
12 V
4 15
Supply wires
to LED
Signal wires from
Figure 2.21 Wiring for an optical ignition trigger
The main disadvantage with an optical system is the
importance of keeping the LED and phototransistor
clean. A build up of dirt or oil on the components would
reduce the efficiency or prevent the system from
2.2.8 Ignition modules/amplifiers:
early types
With earlier types of electronic ignition, the ignition
module simply functioned as the electronic switch for
the ignition coil primary circuit. Assuming that a trigger
mechanism such as an inductive trigger (as described in
section 2.2.5) was used as a means of providing the
timing reference signal, the module simply switched the
primary circuit on and off thus forming a nonmechanical circuit switch. There were several variations
of early systems but the basic objective was to eliminate
any mechanical switching, and this was achieved with
an inductive trigger (or Hall effect type) and a
transistor that acted as the primary circuit switch.
Ballast resistor and dwell period
Early generations of electronic ignition systems, often
had a ballast resistor in the primary circuit (as did many
contact breaker systems). The resistance of the ballast
resistor altered with temperature, and the temperature
change was dictated by the length of time that current
flowed in the primary circuit. As with a contact breaker
system, the dwell period was a fixed percentage of the
ignition cycle, e.g. 60%. Therefore, at slow engine
speeds when an ignition cycle took a relatively long
time, the current flowed through the primary circuit for
60% of this period, causing the resistor temperature
and its resistance to increase, thus reducing the current
flow. However, when the engine speed increased, the
time available for the current flow reduced thus
reducing the temperature of the resistor; the resistance
of the ballast resistor therefore also reduced thus
increasing the current flow in the primary circuit.
The action of the ballast resistor therefore allowed a
high current to flow in the primary circuit when there
was reduced time available at high engine speeds but,
when the engine speed was low, the current flow was
reduced to prevent overheating of the coil and wiring
(i.e. it is an output control ballast). The result was that
the ignition coil could build up to an acceptable energy
level at high as well as low speeds.
The ignition coils would have a primary winding
resistance of typically 1.5 ohms (or slightly less), which
would allow for a rapid build up of current flow. The
ballast resistor would also have a resistance of
approximately 1.5 ohms, so the total resistance in the
circuit was approximately 3 ohms, which, with the
12 volt supply, would result in a current of 4 A. When
the resistance of the ballast resistor increased at low
engine speeds, this would then reduce the current flow.
For many systems, the dwell percentage was often more
than 60%, which meant that there was a longer period
for charging the coil. So long as the ballast resistor
functioned correctly, then the system would not
overheat at low speeds.
Dwell periods on the early systems were generally a
fixed percentage (fixed dwell angle). In the same way
that a reference point on the trigger signal was used as
a reference to the timing point, it was also possible to
use another reference point on the signal as a reference
to starting the dwell period. Some systems might have
used an electronic device within the ignition module to
control the dwell period.
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Electronic ignition systems (early generations)
Ignition timing advance and retard
With early generations of electronic ignition, the ignition
timing advance and retard mechanisms remained much
the same as for a contact breaker system: mechanical
advance mechanism to alter the timing with engine
speed and a vacuum operated retard system to alter the
timing with changes in engine load. See section 2.2 in
this book and section 2.28.8 in Hillier’s Fundamentals of
Motor Vehicle Technology Book 1.
2.2.9 Ignition modules: later types
with dwell control and
constant energy function
Improving the coil output at all engine speeds
One of the main problems with early electronic modules
was that the dwell period was a compromise (as was
the case with contact breaker systems); this meant that
the dwell period was too long at slow engine speeds
and too short at high engine speeds (hence the use of a
ballast resistor as described in the previous section).
The next generation of ignition modules therefore
provided a facility to alter the percentage of dwell
depending on the engine speed. In effect, when the
engine was at low speeds and an ignition cycle lasted a
relatively long time, the dwell percentage was a small
percentage of the long ignition cycle time. When the
engine was at high speeds and the cycle time was much
shorter, the dwell percentage was increased.
Example of dwell control
As an example, to achieve a good quality spark, if we
assume that a current that flows for 2.5 ms is needed to
allow the coil to build up the required amount of energy
(magnetic field strength), then this in theory would be
the same irrespective of engine speed. At 1000 rev/min
on a four-cylinder engine, the ignition cycle for one
cylinder would last for 30 ms, so the required 2.5 ms
would represent one-twelfth of this period i.e. 8.33%.
If the engine speed is then increased to
2000 rev/min (twice the speed), then one ignition cycle
will now last for only 15 ms (half the time). However,
the coil would still require 2.5 ms of charge up time,
which represent one sixth of the total 30 ms i.e.
16.66%. In effect, the charge up time remains the same
but the percentage of the whole cycle changes in
proportion with the change in engine speed.
As a last part of the example, if the engine speed is
now increased to 6000 rev/min, the ignition cycle for
one cylinder will last for only 5 ms. The coil charge up
time will remain at 2.5 ms, which now represents 50%
of the ignition cycle.
It is therefore possible with this type of dwell time
control to operate an engine at high engine speeds and
still provide a long enough dwell period for the coil to
build up strong energy levels.
The actual control of the dwell period is not
necessarily as precise as in the explanation above, and
there are several variations in the exact dwell time
provided depending on the ignition system module
design. However, the objective is to ensure that the
dwell time is sufficient for all engine speeds, thus
allowing the ignition coil to provide a reasonably
consistent output at all speeds.
For engines with more than four cylinders, the dwell
time will have to be slightly less because of the shorter
time available for one ignition cycle for each cylinder.
However, on more modern ignition systems,
developments have included one coil to provide a spark
for two cylinders and more recently, systems now
provide one coil for each cylinder. In both cases, the
time available for each coil to charge up is considerably
increased. These systems are explained in greater detail
later in this section.
Controlling dwell for specific conditions
Although the system can control the dwell to suit
engine speed, there are some operating conditions
where it is an advantage to enable the coil to provide
greater energy levels than normal. The usual examples
are to provide a slight increase in dwell time at starting
and at low engine speeds. Starting inevitably requires a
strong spark, and at idle speed where emissions are
critical, a better quality spark helps to ensure improved
combustion. This is especially true with engines
operating on relatively weak mixtures, which then
benefit from long spark durations.
Constant energy control and high energy coils
It is obviously an advantage to use an ignition coil that
has a rapid build or charge up time, and this can be
achieved by using a coil with a low primary winding
resistance (low inductance). Many modern ignition
coils have a primary winding resistance that is as low as
0.5 ohms, which is one-sixth of the resistance of older
contact breaker system coils. Therefore, the potential
current flow through the primary winding on a modern
coil could be as high as 24 A (12 volts through a
resistance of 0.5 ohms). In fact, such potentially high
current levels would be too high and could damage the
wiring and the coil winding. However, another major
benefit of the low resistance primary winding is that the
build up of current flow is much quicker than with
primary windings of higher resistance. With this fact in
mind, it is then possible to allow an initial rapid build
up of current flow but to then limit the current so that it
does not reach levels that are too high; the coil will be
able to produce high energy levels due to the rapid
build up time but without the problem of high currents
damaging the system components.
Later generations of electronic ignition modules and
modern ignition systems use a method of current control
or ‘current limiting’ in conjunction with low resistance
ignition coils. The process of current control is generally
carried out in one of two ways, both methods relying on
a ‘feedback’ or ‘closed loop’ system. In effect, the system
monitors the current flow in the primary circuit and
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Engine management – spark ignition
Fundamentals of Motor Vehicle Technology: Book 2
when it rises to a predetermined level, the current in the
primary circuit is then either restricted or the dwell time
is reduced; both methods will prevent overheating that
would otherwise be caused by an excessive current level
that lasts for long periods.
With dwell control systems, the same principle is used
as for current limiting systems but the voltage from the
sensing resistor is passed to the dwell control circuitry
within the module. Dwell control therefore depends on
the voltage at the sensing resistor.
Feedback system
The feedback systems are relatively simple in operation
and rely on a resistor that forms part of the primary
circuit, i.e. it is in series with the primary winding of the
ignition coil. The resistor (referred to as a sensing
resistor) is located within the ignition module, which is
of course functioning as a switch on the earth path for
the primary circuit. The voltage drop across the resistor
will change with changes in current flow and, by passing
the voltage signal from the resistor to other circuitry in
the module, it is then possible to either control the
current or control the dwell time accordingly.
With the current limiting systems, the ‘Darlington
pair’ within the module is used to switch the primary
current through to earth (Figure 2.22). When the
voltage signal from the resistor indicates that the
current has reached the predetermined level, the input
voltage to the Darlington pair is reduced, which in turn
reduces the primary circuit current flowing from C to E
(collector to emitter). When the current is reduced, the
voltage at the sensing resistor is also reduced and this
voltage is then again used as the reference to control the
input voltage to the Darlington pair. In effect, the
process is a continuous action of monitoring and
adjustment of voltages and current flow (i.e. it is a
closed loop).
A complete constant energy system
Figure 2.23 shows a simplified layout of a constant
energy ignition system. The process is as follows.
A trigger signal is passed from an inductive or Hall
effect trigger to the ignition module.
The signal will be processed by the pulse shaping
device; if the trigger signal is provided by an
inductive trigger, it will also be converted from
analogue to digital.
The processed signal is then passed through the
dwell control device and peak coil current cut off
The signal is then passed to the ‘driver’ which is
effectively a low current/voltage switching transistor
that is directly responding to the processed trigger
signal. The driver responds to the trigger signal and
in turn switches on and off the Darlington pair,
which contains the main power transistor.
The Darlington pair forms the final switching
element of the ignition module. The primary current
passes through the Darlington pair, so when the
Darlington pair switches on the primary circuit,
current will flow through the primary circuit thus
enabling the ignition coil to build up a magnetic
field. When the Darlington pair switches off the
primary current, the magnetic field in the coil will
collapse, thus providing a high voltage to the spark
Note that the voltage either side of the current
sensing resistor is passed to a comparator, which
then passes an appropriate signal either to the
driver (which controls the Darlington pair), or the
signal is passed to the dwell control device.
Different locations for ignition modules
There are several variations in the design of the
electronic ignition systems so far discussed. Whilst in
principle the systems will generally all function in the
same way, the different versions produced for different
vehicles have constructional variations that are either
design preferences or are dictated by installation
In general, there are three basic physical layouts:
Figure 2.22 Voltage feedback control
remote module – the ignition module is remotely
located away from the trigger mechanism
integrated with the distributor body – the module is
either located inside the distributor or mounted on
the outside of the distributor body
located on the ignition coil – the module is mounted
on the ignition coil casing.
Figure 2.24 shows some examples of different module
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Electronic ignition systems (early generations)
Peak coil current
Darlington pair
Inductive pulse
Figure 2.23 Layout of constant energy ignition system
Figure 2.24 Different physical layouts for ignition systems
a Ignition coil mounted retrofit system
b Remote mounting
c Integrated distributor body mounting
Ignition switch
HT cable
Existing wire
or block ballast
resistor (if fitted)
Four heat sink
mounting points
for self tapping
Heat sink
Page 60
Engine management – spark ignition
2.2.10 Capacitor discharge ignition
Capacitor discharge ignition systems have been used in
the past on some vehicles and were not uncommon on
high performance engines. The capacitor discharge
system is however not ideally suited to modern engines
and is therefore seldom used on today’s vehicles.
Capacitor discharge (CD) systems operate in a
slightly different way to more traditional types of
ignition. Although an ignition coil is still used, the coil
does not store energy as is the case with most systems:
the ignition coil on a CD system functions as a ‘pulse
transformer’. In effect, a short and relatively high
voltage pulse (typically of 400 volts) is passed through
the primary winding of the coil; this causes a very rapid
build up of a magnetic flux (magnetic field) in the
primary winding. Because the secondary winding is
exposed to the rapidly created magnetic field, a very
high voltage (typically around 40 kV) is then induced
into the secondary winding.
To create the short 400 volt pulse, a capacitor in the
module is charged (effectively during a dwell period).
However, when a trigger signal is provided, the
capacitor discharges its stored energy through the
primary winding.
A simple capacitor discharge circuit is shown in
Figure 2.25. Note that a pulse generator is still used and
the trigger signal from the pulse generator is passed to a
pulse shaper. The processed signal from the pulse
shaper is passed to the trigger stage, and the signal from
the trigger stage will cause the capacitor to discharge.
Figure 2.25 Capacitor discharge system
Fundamentals of Motor Vehicle Technology: Book 2
CD systems provide a short but very high voltage coil
output, which is typically only around 0.1 ms in
duration. This short duration spark is not effective in
maintaining the combustion process with weaker
air/fuel mixtures. However, the high intensity 40 kV
coil output is consistent across most engine speeds and
it is very effective at igniting relatively rich mixtures.
One advantage of a CD system with older high
performance and racing engines was that the high
voltage at the spark plug could burn off any
contaminants. On older racing engines, it was common
on cold engines for the mechanical clearances to be
large until the engine reached operating temperatures;
this allowed oil to pass the piston rings and valve guides
and therefore enter the combustion chambers. Oil
would therefore contaminate the spark plugs and cause
ignition and combustion problems. Additionally,
because racing engines operated with relatively rich
mixtures, carbon fouling of the spark plugs was a
common problem. The high voltage at the spark plug
produced by CD systems was very effective at keeping
the plugs free of contaminants.
Key Points
[647] Chapter 02
Electronic ignition systems use a pulse generator
to signal an amplifier that switches the coil on and
To maintain constant energy in a coil, the dwell is
varied. Dwell is the angle of distributor rotation
when the coil is switched on. It is also given as a
[647] Chapter 02
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Computer controlled ignition systems
2.3.1 A further improvement in
timing accuracy
Eliminating the problems of mechanical timing
Although earlier generations of electronic ignition
systems had improved reliability and spark quality, the
accuracy of ignition timing advance and retard
mechanisms was still dependent on mechanical and
vacuum operated systems that had not changed for
many years: the timing advance and retard mechanisms
were no different to those of contact breaker systems.
The problem again was tighter emissions regulations, a
requirement for improved fuel consumption and
continued demand for improved engine power.
As is often the case with emission requirements,
there are conflicts between achieving the desired
emissions and also achieving economy and power.
However, if the ignition timing can be more accurately
controlled and if the changes in timing (advance and
retard with speed and load) can be more rapidly
implemented, then combustion efficiency can be
improved under almost all conditions.
As mentioned in section 2.2.2, if the correct ignition
(spark) timing can be provided at exactly the right time,
Figure 2.26 Engine speed related timing advance curve and
mechanical advance mechanism
a Ignition advance curves
b Mechanical advance mechanism using unequal length springs
the gases will expand at exactly the right time and this
will allow maximum possible power to be achieved
(assuming all other conditions are good). However,
because of minor variations in fuel quality and in other
engine operating conditions, it was general practice to
set the timing slightly retarded so that combustion
knock and other overheating problems did not occur.
Importantly, the relative inaccuracy of mechanical
and mechanical/vacuum based timing mechanisms
prevented the timing being correct for all operating
conditions. Even when the components were new,
mechanical advance mechanisms and mechanical/
vacuum operated retard mechanisms could not provide
correct timing in all operating conditions.
Mechanical ‘engine speed’ related advance
Figure 2.26a shows two advance lines for engine
speed/timing advance; the curved line shows the
requirements for an engine, whilst the straight line
shows what was available from a mechanical advance
mechanism (bob weight and spring system). Note that
the kinks or angle changes in the straight line are
caused by using unequal strength springs on the bob
weight system (Figure 2.26b): the weaker spring acts
against the bob weights for the lower engine speeds and
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Engine management – spark ignition
then, as the bob weights are flung out under centrifugal
force, the stronger spring then also acts against the bob
The difference between the two advance lines
shown in Figure 2.26a illustrates the inability of a
mechanical advance system to provide the correct
timing at all engine speeds. It was therefore necessary
to try to achieve the best compromise for timing
advance, which meant that the timing was not correct
for much of the engine speed range.
As well as not being able to provide the correct
timing at all times when the system was new, when the
timing mechanisms began to wear matters became
worse and timing was often much too far away from the
desired value to enable the engine to operate efficiently.
Mechanical and mechanical/vacuum systems could not
respond quickly enough to the required changes in
timing and the accuracy deteriorated over time due to
wear. It was therefore necessary to eliminate
mechanical systems and to use electronic or computer
control for the ignition timing functions.
Load related vacuum retard
Figure 2.27 shows a simple vacuum operated timing
retard system. The illustration shows the mechanism
acting on a base plate onto which the contact breaker
assembly were mounted.
When the throttle was closed at idle speed or during
deceleration (upper left diagram), the vacuum was
blocked from reaching the diaphragm and therefore any
timing changes were dependent purely on the
mechanical bob weight system (engine speed related).
When the throttle was then partially opened (light
load conditions), high manifold depression acts on
the diaphragm, which in turn pulls the diaphragm
Figure 2.27 Vacuum advance/retard mechanism
Fundamentals of Motor Vehicle Technology: Book 2
against the spring. The movement of the diaphragm
pulls the linkage, which rotates the base plate in an
anticlockwise direction, against the direction of
rotation of the distributor shaft. This would have the
effect of advancing the trigger signal and timing.
When however the throttle was then opened
further (high load conditions), the intake depression
would initially reduce (higher pressure), the spring
would then force back the diaphragm and in turn, this
would allow the base plate to move back in a
clockwise direction (the same direction as the
distributor shaft rotation). The opening of the contact
breaker and timing would therefore retard.
Whilst the vacuum advance/retard system was
reasonably effective, it could not accurately control
timing for all the variations of load that occur and
again a compromise was inevitable. Adding the
compromise of a vacuum system to the compromise
of a mechanical advance system, resulted in an
inaccurate timing control system.
For electronic systems, near the trigger
mechanism located in the distributor (e.g. inductive,
Hall effect or optical). Refer to Figures 2.11, 2.12 and
2.19. The movement of the base plate and distributor
shaft altered the timing in the same way.
Changing operating conditions
Apart from the lack of accuracy of mechanical and
vacuum based timing systems, there are other reasons
why greater flexibility of timing control is needed.
Air/fuel mixtures need to be altered more rapidly, so the
ignition timing must be altered accordingly; in addition,
ignition timing requirements differ with temperature. In
effect, anything that affects the speed of combustion,
i.e. the flame speed through the combustion chamber
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Computer controlled ignition systems
and the burn time, will require a different timing
advance value.
Although many adaptations to basic mechanical and
vacuum systems were introduced, the accuracy and
speed of change of timing remained less than
satisfactory for modern requirements.
With computer control, and the use of a number of
sensors, it became possible to obtain much more
accurate control of ignition timing. Rapidly changing
operating conditions could therefore be sensed
(including rapid changes in engine speed), and the
computer or ECU could then alter the timing as
necessary. Systems were therefore introduced where the
switching action of the ignition module was controlled
by the ECU. A trigger signal is still produced by an
inductive or Hall effect trigger (or other type of pulse
generator), but the signal is passed to the ECU, which
modifies or ‘phases’ the signal to achieve timing control.
Figure 2.28 shows the basic layout of such a system.
Note that the ignition module can be separate from the
ECU or, in many cases, the module is integrated within
the ECU.
The simple example in Figure 2.28 shows an ECU
receiving a trigger signal provided by an inductive
sensor. A vacuum pipe connects the intake manifold
vacuum (depression) to a pressure sensitive component
in the ECU thus providing the ECU with engine speed
and load information. See section 1.5.4 for information
on electronic type pressure sensors.
Figure 2.28 Layout of a computer (ECU) controlled ignition system
2.3.2 Digital timing control
If we assume that an ECU receives a trigger signal from
some form of engine speed sensor, and load
information via the vacuum sensor, the ECU can then
calculate the required ignition timing. Within the ECU is
a ‘look up table’ or memory, which contains all the
relevant timing data applicable to the different load
conditions; by comparing the conditions with the data
in the look up table, the ECU can determine the timing
advance required.
The look up table effectively contains a threedimensional map of the timing requirements, a simple
example of which is shown in Figure 2.29. The spark
timing map provides the timing angle (crankshaft angle)
related to engine speed (revolutions/second) and engine
load (on a scale of 0 to 1). The example in Figure 2.29
shows a relatively small number of speed, load and
timing reference points, but many more reference points
can be included on a spark map.
Figure 2.30 shows the same spark timing map with
the engine speed at 32 rev/s (1920 rev/min) and the
engine load at 0.5 (half load). The 32 point mark on the
map is therefore followed across until it intersects with
the 0.5 load line; the intersect point identifies the
required spark advance, which in this case is 52°.
The timing advance characteristics are established
during many tests on development engines (before
they go into production). Once the timing
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Engine management – spark ignition
Fundamentals of Motor Vehicle Technology: Book 2
Figure 2.29 Simplified three-dimensional spark advance map
characteristics have been established, production ECUs
will then have a memory and look up tables containing
the applicable data.
Because accurate timing is so important to engine
efficiency, performance, emissions and economy,
improvements in all of these factors are achieved with
a greater number of timing reference points.
Figure 2.31 shows a more modern and complex spark
advance map (compared with the example in
Figure 2.29).
It can now be appreciated that considerable
accuracy of spark timing can be achieved with the
complex spark timing map. However, because the
trigger signal from the inductive or Hall effect triggers
(and other pulse generators) discussed so far provides
only one reference point per cylinder, this is a limiting
factor in the accuracy of ignition/spark timing.
Additionally, the trigger mechanisms so far discussed
are usually located within the distributor body, so the
rotation of the trigger mechanisms is driven by the
distributor shaft which in turn is driven by a timing
belt or chain. Any wear or maladjustment of the belt
or chain will cause inaccuracies in the timing
A more reliable and accurate means of providing a
trigger/reference signal would enable the spark timing
also to be more accurate. The following section (2.3.3)
covers some examples of more accurate timing
trigger/reference signal systems which usually have a
sensor and reluctor or trigger disc located on the
Figure 2.30 Simplified three-dimensional spark advance map showing the spark timing at 32 rev/s and half engine load
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Computer controlled ignition systems
Figure 2.31 Typical modern three-dimensional spark advance map
2.3.3 Crankshaft speed/position
Direct triggering reference from the crankshaft
Most crankshaft speed/position sensors are located
adjacent to the crankshaft and are usually inductive
sensors (Figures 2.32 and 2.33). In some cases Hall
effect sensors have been used. Locating a sensor
adjacent to a crankshaft allows a reluctor disc (trigger
disc with the reference points) to be mounted directly
on the crankshaft, with most versions being mounted
either at the front pulley or at the rear of the crankshaft
adjacent to the flywheel. Sometimes a reluctor disc is
mounted at a convenient point on the crankshaft
between the crankshaft webs, i.e. the disc is located in
the crankcase.
The obvious advantage of locating the
reluctor/trigger disc on the crankshaft is that there is no
drive linkage (belt, chain or other mechanism); this
means that the trigger or reference signal will be
Figure 2.32 Crankshaft speed/position sensor
accurately identifying the crankshaft speed or angular
position without any losses of accuracy that could be
caused by drive mechanisms.
Increased number of reference points
With a crankshaft mounted reluctor or trigger disc, it is
possible to use a larger diameter disc, which can more
easily contain a larger number of reference points
(reluctor teeth). Due to the fact that most of the
crankshaft speed position sensors are of the inductive
type, the reference points usually take the form of teeth
located around the disc.
In most cases the trigger disc is located directly at
the front or the rear of the crankshaft and can form part
Figure 2.33 Crankshaft speed/position sensor located adjacent to
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Engine management – spark ignition
of the front pulley or part of the flywheel assembly.
Because of these locations, especially if the disc has a
similar diameter to the flywheel, it is relatively easy to
locate a number of reference points around the disc.
Some earlier examples had only a small number of
reference points, e.g. two or four, but most modern
systems have as many as 60 reference points. One of the
reference points or teeth is usually either missing or is a
different shape to the rest of the teeth; this enables a
master reference signal to be produced.
Figure 2.33 shows a flywheel located inductive
crankshaft speed/position sensor. Note the missing
tooth, which is adjacent to the sensor in the illustration.
In fact, the example shown in Figure 2.33 has two
missing teeth; one missing tooth is a master reference
for cylinders 1 and 4, while the other missing tooth is a
master reference for cylinders 2 and 3. On this
particular system, with a single ignition coil, the high
voltage output from the coil is directed via a rotor arm
located at the end of the camshaft (OHC type).
Therefore the ECU does not need to receive a signal
relating to each cylinder, but it does receive signals
relating to the TDC position of each pair of cylinders (or
other predefined angular position of the crankshaft).
On this system therefore, the sensor would provide
the speed signal and the angular position of the
crankshaft as each tooth passes the sensor; the ECU can
count the number of signals as each tooth passes the
sensor thus enabling the ECU to identify the angle of
rotation from TDC (or from the master reference
position). It is also possible on this type of system for
the ECU to assess any changes in crankshaft speed as
each tooth passes the sensor.
Using Figure 2.28 as an example, the ECU receives a
load signal from the pressure sensor (pressure
transducer), which is connected by a vacuum pipe to
Fundamentals of Motor Vehicle Technology: Book 2
Optical pick-up
No. 1
Rotor plate
Figure 2.34 Optical speed/position sensor located in distributor
the intake manifold. This system also uses information
from a knock sensor (discussed in section 2.3.4). Also
note that a coolant temperature sensor provides
information to the ECU to enable changes in timing to
occur with changes in temperature (temperature
sensors are covered in section 1.5.1).
Note: Some speed/position sensor systems located
within a distributor also have many reference points,
one type being an optical system with 360 slots located
around a disc (Figure 2.34). However, locating a large
number of reference points around a relatively small
disc requires good manufacturing accuracy and the
problem still remains that any wear or maladjustment
of the drive linkage to the distributor will result in
incorrect timing references.
Crankshaft speed/position sensor: operation and
Section 2.2.5 provides an explanation of an inductive
pulse generator, and the inductive crankshaft
speed/position sensor operates in exactly the same way.
The crankshaft sensors, however, are usually
constructed so that the winding or coil is formed
around the magnet and this assembly is located in the
sensor body, which is then bolted or secured in some
way to the engine block or flywheel housing. The sensor
is located so that it will be affected by the movement of
the reluctor teeth (reference points) whilst the
crankshaft is rotating.
Figure 2.35 shows a typical analogue signal
produced by an inductive crankshaft speed/position
sensor. Note the different shaped pulse produced by the
missing tooth.
The analogue signal is passed to the ECU which
then converts it to a digital signal, thus enabling the
required speed and angular position information to be
Figure 2.35 Analogue signal produced by an inductive crankshaft
speed/position sensor
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Computer controlled ignition systems
2.3.4 Knock sensors
A knock sensor (Figure 2.36) is effectively a vibration
sensor that responds to those vibrations in the engine
that cause pressure waves to occur in the cylinder block
or cylinder head. By detecting the vibrations or pressure
waves, a knock sensor can detect the vibrations caused
by combustion knock.
The knock sensor is an electronic pressure sensor,
which with a pressure sensitive crystal that produces a
small electrical pulse when it is exposed to pressure
waves (such as the engine vibrations). Vibrations
caused by combustion knock will result in a slightly
different signal (frequency and voltage) being produced
by the sensor. When the ECU receives the signal from
the sensor, it is able to filter out the normal vibrations
and respond to the particular part of the signal that is
caused by combustion knock.
Figure 2.36 Knock sensor located in cylinder block
Although ECU controlled ignition timing should provide
ideal timing for all operating conditions, it is possible
that fuel quality could be poor (momentarily or
continuously). Other factors such as the temperature in
the combustion chamber can also cause short term
combustion knock. In most cases, slightly retarding the
ignition timing/spark advance will reduce and
eliminate combustion knock.
Therefore, when the ECU detects a combustion
knock signal, it will respond by retarding the spark
timing a predetermined number of degrees. If the
combustion knock is no longer detected, the ECU will
progressively advance the timing to its correct value (so
long as combustion knock does not reoccur).
An ECU can alter the timing for just the affected
cylinder. When the knock occurs (when combustion
occurs in the affected cylinder), the ECU will then
provide the correct timing for the remaining cylinders
(for example, the remaining three cylinders on a fourcylinder engine). When the affected cylinder is then due
to receive its next spark, the ECU can retard the timing
for just the affected cylinder.
Key Points
Other types of sensor signal
Almost all crankshaft speed/position sensors are of the
inductive type, but where a Hall effect or optical system
is used, these types will provide a digital signal in the
same way as the older ignition trigger systems
discussed previously in sections 2.2.6 and 2.2.7. The
only differences compared with the older ignition
trigger systems will be the number of signal pulses
produced which will depend on the number of reference
The main advantage of computer controlled
ignition is accurate timing – that stays accurate
over the life of the engine
The ideal timing setting is held in the ECU memory
in the form of a look up table
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Engine management – spark ignition
Fundamentals of Motor Vehicle Technology: Book 2
2.4.1 Limitations of distributor based
Restricted dwell time and wasted energy
All of the ignition systems covered so far have two
major disadvantages when it comes to providing high
energy from the ignition coil to the spark plugs; both
disadvantages arise because the ignition systems use a
single coil to provide a spark for all of the cylinders.
Time to build up coil energy
The first disadvantage when using a single coil for all
cylinders is that it limits the time available to build up
coil energy between each of the individual ignition
cycles: there is very little ‘dwell’ time available for the
current to flow through the primary winding of the coil
and build up a strong magnetic field (magnetic flux).
As previously explained, on a multi-cylinder engine
there is very little time between one cylinder firing and
the next; the faster the engine speed and the greater the
number of cylinders, the less time there is available for
the ignition coil to build up sufficient energy for the
next ignition cycle. On modern high speed engines
which operate with relatively weak mixtures, it is
essential that the energy available from the coil is
sufficient to produce a powerful and long duration
spark otherwise emissions and general performance
will not be acceptable.
Distributor and rotor arm wasting energy
When a single coil is used to produce a spark for a
number of cylinders, the energy from the coil (high
voltage) is passed via a high tension cable (HT lead) to
the distributor cap and rotor arm assembly (see section
2.28 in Hillier’s Fundamentals of Motor Vehicle
Technology Book 1). Note that the lead passing the
energy from the coil to the distributor cap is often
referred to as the ‘king lead’. The distributor cap
contains a number of contact points (referred to as
electrodes), which in turn are connected to each of the
spark plugs via additional HT leads. When the high
voltage from the coil passes along the king lead to the
centre electrode in the distributor cap, it is then passed
to the centre of the rotor arm; because the rotor arm
rotates with the distributor shaft, it is then able to pass
the energy to the individual HT leads and spark plugs.
Figure 2.37a shows a basic layout of an ignition
system with a single coil and Figure 2.37b shows a plan
view of the rotor arm and distributor cap.
One problem with the rotor arm system is that
voltage is lost or wasted when the current flow flows
through all of the HT leads, and especially when the
current flows across the rotor arm tip to each of the
electrodes. There is a necessary gap between the rotor
arm tip and the electrode, and this absorbs or uses
some of the energy produced by the coil.
However, although Figure 2.37b shows the rotor arm in
alignment with the electrode, in reality, the rotor arm
passes through quite a large angle during the period of
time that the spark exists (remember that the spark
may last 2 ms or more). When an engine is operating at
6000 rev/min, the rotor arm (which rotates at half
engine speed) will rotate 50 times in one second or one
rotation in 0.02 s (2 hundredths of a second). During
the spark duration of 2 ms (2 thousandths of a
second), the rotor arm will rotate through one-tenth of
a complete rotation i.e. 36° of rotation. There is
therefore quite a substantial gap between the rotor arm
tip and the electrodes when the energy from the coil is
passing to the spark plug. This gap inevitably uses
considerable amounts of valuable energy, which
reduces the energy available to maintain the spark.
Additionally, the rotor arm tip and electrodes will
progressively deteriorate due to the arcing that occurs
as the voltage or energy flows across the gaps.
It is also important to note that, when electricity
has to jump the gap at the rotor arm tip, this creates
electrical interference, which must be suppressed to
prevent interference with other electrical and
electronic devices.
Using multiple ignition coils (eliminating the rotor
The next progression in ignition system design was
therefore based on a desire to eliminate the distributor
cap and rotor arm assembly and use more than one
ignition coil. Note that there were some engines
produced (typically V8 and V12 engines) that did use
two coils, each of which provided sparks for half of the
cylinders. However, these systems still used one rotor
arm for each coil and group of cylinders: in effect there
were two ignition systems.
The ultimate ignition system would have one coil
for each cylinder, and this is the general rule for modern
engines, where an individual ignition coil is either
directly connected to the top of the spark plug or there
is an HT lead from each coil to the spark plug.
There is however another solution, which is still
used on some systems, and this design uses a single coil
to provide a spark at two spark plugs. Although these
systems are often referred to as ‘distributorless’ ignition
systems, the same terminology can be applied to
systems that use one coil for each cylinder. For the
purposes of differentiation, coils that provide sparks to
two cylinders at the same time are referred to within
this book as ‘wasted spark’ systems, the reason for
which will be made clear in section 2.4.2. Section 2.4.3
covers systems that use a ‘single coil per cylinder’.
For both the wasted spark and the single coil per
cylinder systems, the ignition systems do not require a
distributor and rotor arm assembly to distribute the
spark to the different cylinders: they are both therefore
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Distributorless and direct ignition systems
Figure 2.37 Ignition system with a single ignition coil
a System layout
b Distributor cap and rotor arm: plan view
‘distributorless’ ignition systems. The term ‘direct
ignition’ is also used but this would generally refer to
systems where the ignition coil is directly located onto
the spark plug, i.e. there is no HT lead between the coil
and spark plug.
next revolution of the crankshaft, the wasted spark will
be at cylinder 1 and cylinder 4 will receive the spark at
the correct time (i.e. at the top of its compression
stroke). Figure 2.38 shows the basic principle: a pair of
cylinders that rise and fall together (but on different
strokes) receive a spark at the same time.
2.4.2 Wasted spark ignition systems
Principle of operation of a wasted spark system
Providing a spark at two cylinders
For the majority of engines layouts where an even
number of cylinders is used, two of the pistons will rise
and fall within the cylinders at the same time. Using an
in line four-cylinder engine as an example: pistons 1
and 4 rise and fall together, but when cylinder 1
approaches TDC on the compression stroke and is
provided with the spark, piston number 4 is
approaching TDC on the exhaust stroke. However, at
the next full rotation of the crankshaft, the situation is
reversed. The same process is true for pistons 2 and 3.
Therefore, with an ignition coil that can provide a
spark at the spark plugs for cylinders 1 and 4 at the
same time, if cylinder 1 receives a spark at the correct
time (i.e. at the top of its compression stroke) cylinder 4
will receive the spark when it is on the exhaust stroke
and therefore the spark to cylinder 4 is wasted. On the
Ignition coil
Spark plug
Discharge circuit
Spark plug
Figure 2.38 Single ignition coil providing a spark to two
cylinders at the same time
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Engine management – spark ignition
Positive and negative sparks
The ignition coil on a wasted spark system operates in
the same way as the conventional coils so far
discussed. However, both ends of the secondary
winding in the coil are connected to an HT lead and a
spark plug (Figure 2.39), whereas on a conventional
coil only one end of the secondary is connected to an
HT lead and spark plug (the other end is connected
internally within the coil to the end of the primary
winding, which completes the earth path for the
secondary winding).
On the wasted spark ignition coil, when the
magnetic field collapses around the secondary
winding, one end of the winding will be positive whilst
the other end will be negative. The flow of current at
the positive end of the winding will pass from the coil
winding down to the spark plug and then across the
plug gap to earth (the engine acts as the earth point for
the spark plugs). Note that current flow is generally
regarded as passing from the higher voltage to the
lower voltage, and, if we assume that the output
voltage from the coil in this example is 10 000 volts,
current will flow from the 10 000 volts at the coil to
the zero volts at earth.
The flow of current at the negative end of the
winding will however be from earth (the earth
electrode at the spark plug) across the plug gap and
then through the HT lead to the coil. This direction of
flow is caused by the fact that the voltage at the
negative end of the coil winding is a minus value, e.g.
–10 000 volts, which is 10 000 volts lower than the
voltage at earth (which is zero volts); current is again
assumed to flow from the higher to the lower voltage.
Note: Electron flow is actually opposite to the
generally accepted convention: electrons flow from the
lower to the higher voltage rather than from the
Figure 2.39 Schematic view of a wasted spark ignition coil
Fundamentals of Motor Vehicle Technology: Book 2
assumed higher to the lower voltage. However, as with
most explanations, it is assumed that conventional
flow is from the higher voltage to the lower voltage.
Irrespective of which way the current is flowing across
the spark plug gap, a spark will be provided that is
sufficient to cause combustion in the combustion
chamber. However, because electrons flow naturally
from a hot surface, and the centre electrode is the
hotter of the two electrodes, it is easier to produce the
spark or arc if the current flows in a direction that
matches this natural electron flow (because the voltage
required is lower). Conventional coils are wired in such
a way that there is a ‘negative spark’ at the plug gap.
However, with the wasted spark coils, one spark will be
a negative spark and the other will be a positive spark.
The positive spark is still effective at creating
combustion but there is a tendency (because of
incorrect electron flow) for the electrodes to operate at
slightly lower temperatures, which can result in fouling
of the plug electrodes. It is therefore necessary either to
use different grades of spark plug for the positive and
negative sparked plugs (which is not desirable) or to
use a spark plug that is effective irrespective of the
polarity of the spark.
Increased dwell time
Although the term ‘dwell’ is perhaps not really
applicable to modern electronic ignition systems, it still
used to refer to the period of time available for
building up coil energy, i.e. the time for current to flow
through the primary winding. We can therefore
examine the ‘dwell time’ available for a single coil
system and for a wasted spark system for a fourcylinder engine to assess the increased amount of dwell
time available.
1 When a single coil is used to serve all four
cylinders, the coil is required to provide a spark
when each of the cylinders approach TDC. This
means that in two revolutions of the crankshaft, the
coil will be required to provide four sparks, which is
two sparks per revolution of the crankshaft. This
equates to one spark or one complete cycle of
operation for the coil for every half rotation of the
2 When a wasted spark system is used on a fourcylinder engine, there would be two wasted spark
coils: one coil for cylinders 1 and 4, and one coil for
cylinders 2 and 3. If we examine the operation of
the coil that serves cylinders 1 and 4, it will be
required to provide a spark each time that the two
pistons approach TDC, which equates to one
complete cycle of operation for each revolution of
the crankshaft.
With a wasted spark system on a four-cylinder engine,
the ignition coils now have twice as much time
available for the cycle of operation (between one spark
and the next), which means that it would be possible to
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Distributorless and direct ignition systems
provide twice as much dwell time (if necessary) in order
to build up the energy in the coil. Note that, whenever a
wasted spark system is used, there will be twice as
much time available compared with the same engine
using a single coil with the same number of cylinders.
Typical operation and layout of a wasted spark
On a wasted spark system, the coils are switched in
exactly the same way as on a single coil system: the ECU
calculates the ignition timing in the same way and the
ECU will cause the ignition module to switch each of the
wasted spark coils at the appropriate time. Because there
are effectively two coils on a four-cylinder engine, there
would need to be two switching modules, i.e. the ECU
will be controlling two modules, each of which will
switch the primary circuit for one of the coils. In reality,
the modules are usually integrated into the ECU and in
fact the ECU will contain two power stages (one for each
coil), with all other functions such as current and dwell
control being managed by the ECU.
The ECU will require information (from a crankshaft
speed/position sensor) to indicate when one pair of
cylinders is approaching TDC; in effect a master
reference. Once this master reference has been
established, the ECU can make one of the coils
provide the sparks to one pair of cylinders. If the
crankshaft sensor is providing many reference points
in addition to the master reference (because there is a
large number of teeth on the reluctor disc), the ECU
can then calculate the angle of rotation of the
crankshaft and make the other coil or coils provide
sparks to the other pair of cylinders. As with other
ECU controlled systems, a load sensor and possibly
other sensors, e.g. temperature sensors, can provide
information to enable the ECU to more accurately
calculate the correct ignition advance angle (spark
Figure 2.40a shows a basic layout of a wasted
spark system, with an example of a typical wasted
spark coil shown in Figure 2.40b (note that there
would be one of these coils for each pair of cylinders).
Figure 2.40 Wasted spark ignition
a Layout of system
b Example of a wasted spark ignition coil
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Engine management – spark ignition
Alternative construction and operation of wasted
spark system with diode control
A development of the wasted spark system used a ‘coil
pack’ with two primary windings but only one
secondary winding (Figure 2.41).
The ECU functioned as on the previous system by
switching the two primary windings alternately. The
polarity applied to the two primary windings is
different; therefore this will cause the magnetic field
created in the primary and secondary windings to
alternately reverse polarity. In turn this will cause the
output current from the secondary winding to also
alternately reverse polarity.
In effect, the HT voltage at point A on the secondary
winding (Figure 2.41) will alternately be positive then
negative, depending on which of the primary windings
is active on each of the ignition cycles; the same
situation will therefore occur at point B, so that when
the HT coil output at A is positive, it will be negative at
point B. This situation will reverse when the other
primary winding is active on the next cycle.
If we assume that, on one of the cycles the current is
flowing through the secondary winding from A to B
(from earth through A to B and then back to earth
again), then the diode at spark plugs 1 and 4 will
prevent current flow. However, the diodes at spark
plugs 2 and 3 will allow current to flow, thus allowing a
spark to occur at spark plugs 2 and 3.
On the next cycle, the other primary winding will be
active and this will cause the polarity at A to be
negative and B to be positive. Therefore the diodes on
spark plugs 1 and 4 will allow current to flow and
sparks to occur at those plugs. However the diodes on
spark plugs 2 and 3 will prevent current flow and
therefore there will be no spark at plugs 2 and 3.
This design of system allows for a more compact coil
assembly that will contain the two primary windings
and a single secondary winding, as well as the diodes.
The ECU still performs the same task as for the
previously described wasted spark system, in that it
controls the switching of the primary circuits at the
appropriate time (spark timing). The ECU also still
relies on the crankshaft speed/position sensor and a
Figure 2.41 Wasted spark ignition system with diode control
Fundamentals of Motor Vehicle Technology: Book 2
load sensor (usually a vacuum sensor) for the required
2.4.3 Single coil per cylinder and coil
on plug ignition systems
The logical development
A high percentage of modern petrol engines are fitted
with ignition systems that use one coil for each
individual cylinder. The basic principle of operation for
each coil remains the same as for previously described
coils. However, unlike wasted spark systems, each coil
can provide a negative spark.
One logical reason for using a single coil for each
cylinder is the fact that in recent years there have been
several engine designs with odd numbers of cylinders
(five cylinders and three cylinders). A wasted spark
system cannot effectively be used on engines with odd
numbers of cylinders, since such a system is suited to
engines where two pistons rise and fall at the same
time. Older distributor cap and rotor arm systems are
inefficient, so, on engines with odd numbers of
cylinders, it is necessary to use one coil for each
Another advantage of single coil per cylinder
systems is that the ignition coil can be mounted directly
onto the spark plug. This direct connection eliminates
the need for an HT lead and therefore reduces the
potential for lead and connection failure. Electrical
interference is also greatly reduced. However, some
single coil per cylinder systems do locate the coils
remotely from the spark plugs and therefore need an HT
lead. A basic layout for a single coil per cylinder system
is shown in Figure 2.42.
With single coil per cylinder systems, it is perhaps
obvious that the time available to build up coil energy is
greater than when a single coil is used to provide a
spark to all cylinders. As an example, on an eightcylinder engine with eight ignition coils, each coil will
have eight times longer to complete one ignition cycle
compared with the same engine using a single coil for
all cylinders. The available ‘dwell’ time is therefore also
up to eight times longer.
The operation of the system is much as for wasted
spark systems except that the ECU will now have one
power stage to switch each of the ignition coils. The
ECU will receive information from a crankshaft
speed/position sensor and other information from load
sensors, etc. In most cases, single coil per cylinder
systems form part of an engine management system
and the ECU is usually the same ECU that controls
fuelling and other functions.
Cylinder recognition
Single coil per cylinder systems require one additional
item of information from sensors compared with other
ignition systems and this is referred to as ‘cylinder
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Distributorless and direct ignition systems
1 Ignition lock
2 Ignition coil
3 Spark plug
5 Battery
Figure 2.42 Single coil per cylinder ignition system
Note that the module can be located within the ECU or separate
from the ECU. In some cases the module is located with the
ignition coil.
used, it requires only one reference point to provide the
required information to the ECU.
It is quite common for a trigger lug to be located on
the camshaft; this lug can look very similar to a cam
lobe but it functions as the reluctor tooth for the
inductive sensor; the sensor is therefore located on the
camshaft cover or housing in line with the trigger lug.
Each time that the trigger lug passes the sensor, a
single electrical pulse will be passed to the ECU, which
can then assess which cylinder is on which part of the
cycle. It does not matter which cylinder or which stroke
of a cylinder’s cycle the signal identifies, so long as the
necessary information is programmed into the ECU.
The ECU uses the cylinder identification signal to
establish when a particular cylinder is on the
compression stroke, and the spark will then be
provided to that cylinder at the appropriate time. The
ECU then uses the information from the reference
points on the crankshaft speed/position sensor to
calculate when the sparks should be provided to the
remaining cylinders.
Cylinder recognition sensors are used for most
modern petrol injection systems so, as is the case with
most sensors, the information is provided to the ECU
which is then able to control ignition and fuel systems.
Key Points
Although the ECU can receive a master reference signal
from the crankshaft speed/position sensor, the master
reference cannot identify an individual cylinder; on a
four-cylinder engine, a TDC reference for number 1
cylinder will also indicate TDC for number 4 cylinder.
Therefore a separate sensor is required to identify when
one particular cylinder is on a particular part of the
engine operating cycle, e.g. TDC on the compression
stroke for number 1 cylinder.
Because the camshaft rotates once for every
complete engine cycle and the crankshaft rotates twice,
the camshaft is the best place to locate a cylinder
identification sensor. The sensors are generally either
inductive or Hall effect and operate in exactly the same
way as previously described inductive and Hall type
pulse generators. However, whichever type of sensor is
Distributorless ignition uses the ‘lost or wasted
spark’ principle
Direct ignition systems use one coil for each
2.5.1 Function
The sparking plug provides the gap across which the
high tension current (coil energy) jumps, thus creating
an arc or spark that will then ignite the petrol–air
mixture. Since the Frenchman Etienne Lenoir invented
the anti-flashover ribbed insulator sparking plug in
1860, many detailed changes have been made, but the
basic construction has remained the same: a highly
insulated electrode is connected to the HT cable, and an
earth electrode joined to the plug body.
2.5.2 Spark plug requirements
The basic requirement is that a spark of sufficient
energy should be produced across the electrodes at all
times, irrespective of the pressure and temperature of
the gases in the combustion chamber. These two factors
of temperature and pressure create a very hostile
operating environment, as highlighted in the following
paragraphs, but in addition to these basic requirements,
the plug must be: resistant to corrosion, durable, gas
tight and inexpensive to produce.
Besides withstanding an operating pressure of about
70 bar during combustion, the plug must be also able to
produce the high energy spark when the gas pressure
within the cylinder is about 10 bar or more (at the top of
the compression stroke). The voltage required to do this
may be as high as 30 kV, so adequate insulation is
required to prevent leakage of electrical energy to earth.
The plug must be capable of withstanding temperatures
of between 350°C and 900°C for long periods of time.
The spark plug construction should ensure that
electrode temperature remains between these limits,
because if these limits are exceeded the plug will fail.
Above 900°C the high temperature of the electrodes
causes pre-ignition, whereas below 350°C carbon will
form on the insulator; this can cause fouling which
allows the electrical energy to find an easier route to
earth via the carbon rather than jumping the plug gap.
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Engine management – spark ignition
2.5.3 Construction
Figure 2.43 shows the construction of a typical spark
plug. The example shown consists of an alloy/steel
centre electrode and an aluminium oxide ceramic
insulator, which is supported in a steel shell. Gas leakage
past the insulator is prevented by ‘sillment compressed
powder seals’ and leakage between the cylinder head
and the plug shell is prevented either by a gasket (often
a copper or alloy gasket) or by using a tapered seating
(where the shell contacts the cylinder head).
An earth electrode (usually of rectangular cross
section) is welded to the shell. Whilst most plugs have
traditionally used a single earth electrode, there are
many designs where more than one earth electrode is
used. There is normally a hexagon formed on the shell to
enable a socket to be used for installing and removing
(tightening and loosening) the spark plug.
Ribs are formed on the outside of the insulator,
which increase the length of the flashover path, and also
improve the grip of the HT lead end covers (or coil end
covers) that are used to prevent moisture or dirt
gathering around the insulator.
Fundamentals of Motor Vehicle Technology: Book 2
and numbers stamped on the insulator give the
following information:
diameter and reach
seat sealing and radio interference features
centre electrode features, such as the incorporation
of a resistor or an auxiliary gap heat range
configuration of the firing end of the plug.
Figure 2.44 shows the position of the plug when it is
screwed into the cylinder head. The gasket (where
used) creates a difference between the seating height A
and the plug reach.
Heat range
Heat range indicates the temperature range in which a
plug operates without causing pre-ignition and without
causing plug fouling due to carbon or oil deposits on
the insulator.
Figure 2.45 shows the heat limits and the effect of
road speed on the temperature of a typical plug. In
addition to pre-ignition and carbon fouling, the graph
shows that an operating temperature in excess of about
Plug terms and identification
The length of the thread that screws into the cylinder
head is called the ‘reach’ and the diameter of the
threaded part indicates the ‘plug size’; common sizes
used are 10, 12, 14 and 18 mm.
Spark plug manufacturers use their own codes to
identify their products and the variations; the letters
Figure 2.44 Spark plug screwed into cylinder head
Figure 2.43 Spark plug construction
Figure 2.45 Spark plug operating temperature
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Spark plugs
750°C causes oxide fouling of the insulator and
excessive burning of the electrodes.
The operating temperature of a plug depends on the
four features shown in Figure 2.46, these are:
1 Insulator nose length. This is the distance from the
top of the electrode to the body. The length of this
heat flow path governs the temperature of the
insulator nose; therefore if the path is made short,
the plug will run relatively cool.
2 Projection of insulator. The amount that the
insulator protrudes into the combustion chamber
governs the amount of cooling obtained from the
incoming air/fuel charge.
3 Bore clearance. The clearance between the
insulator and the shell governs the amount of
deposit that can be accepted before the plug
electrodes are shorted out (the spark finds a shorter
route to earth).
4 Material. Rate of heat transfer depends on the
thermal conductivity of the materials used,
especially the material used for the insulator.
Figure 2.47 shows two plugs with different heat ranges.
The hot (or soft) plug has a long heat transfer path and
is recommended for cool running, low compression
engines, and other engines that are used continually at
low speed for short journeys. Unless this type of plug is
used on these engines, carbon will build up on the
insulator, thus causing misfiring to occur after a short
period of time.
Figure 2.47 Spark plug heat range
At the other end of the heat range is the cold (or hard)
plug. The cold plug has good thermal conductivity,
and is therefore used on engines where high
compressions and high combustion temperatures
would cause excessive temperatures at the spark plug.
This type of plug is therefore frequently used in high
performance engines or engines with high power
outputs for their size.
Spark plug manufacturers offer a wide range of
plugs, thus enabling engines to be fitted with an
appropriate type of plug to suit the operating
temperatures, etc.
2.5.4 Electrode features
Spark plugs have traditionally used nickel alloy for the
electrodes, which give good resistance to corrosive
attack by combustion products, and also good
resistance to the erosion that is caused by the high
voltage arc. Both electrodes must be robust to withstand
vibration from combustion effects and they must also be
correctly shaped to allow a spark to be produced with
minimum voltage (Figure 2.48a). Under normal
conditions, erosion eats away the electrodes, so after a
period of time the earth electrode becomes pointed in
shape (Figure 2.48b); in this state it requires a higher
voltage to produce a spark.
Figure 2.46 Spark plug features that affect temperature
Figure 2.48 Electrode wear
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Engine management – spark ignition
As previously stated, nickel alloy has traditionally been
used for the electrodes. Although increasing the
insulator nose length reduces the risk of carbon fouling
when the vehicle is operated on short journeys, the plug
can then overheat when high vehicle speeds are
maintained for long periods. This temperature problem
can be overcome by using more expensive materials and
alloys for part of the construction of the electrodes,
which can include materials such as platinum, iridium,
silver or gold–palladium alloy. However, a less
expensive material is copper, which can be used for the
core of the central electrode and which provides good
thermal conductivity (Figure 2.49).
Platinum and other materials can be used for the
electrodes to overcome the problems of erosion caused
by high voltage arcing. These expensive materials might
be used only as a coating over the base material, so care
should be taken not to scratch the coating.
Plug gaps
Spark plug gaps used to require regular adjustment, and
the electrodes would require regular cleaning and even
filing to ensure that their tips were the correct shape (an
incorrect shape would be caused by erosion). This was
very important on vehicles with older ignition systems
because the eroded electrode tips would often result in
larger gaps that required additional voltage for an arc to
form. However, older ignition systems (primarily
contact breaker systems) would produce lower voltages
due to wear and maladjustment of the contact breakers.
It was therefore common for poor starting and misfires
to occur if spark plug maintenance and ignition system
maintenance were not carried out.
Fundamentals of Motor Vehicle Technology: Book 2
On older engines, plug gaps were typically around
0.6 mm (0.24 in) but gaps on modern engines and
spark plugs are more likely to be 0.8 mm and larger.
These larger gaps are more suitable for engines
operating on mixtures that are weaker than in the past.
Of note is the fact that the larger gaps will inevitably
require greater voltages to initiate the arc (spark) and to
maintain the arc. It was therefore essential that the
modern generation of ignition systems was developed
to create and sustain the arc at the spark plug.
Electrode polarity
A lower voltage is needed to produce a spark at the plug
electrodes when the centre electrode is negative in
relation to the HT circuit polarity. A hot surface emits
electrons and, because the centre electrode is the hotter
of the two electrodes, there is a natural flow of electrons
from the centre electrode to the earth electrode.
Therefore, if the circuit is connected to give the same
direction of electron flow as the natural flow of
electrons, this will assist in producing a spark for a
lower voltage.
Because the direction of electron flow in the
secondary winding depends on the polarity of the
primary winding, it is important that the primary
winding is connected correctly. Most coils will have
markings to identify the positive and negative terminals
and these are usually marked as ‘15’ or ‘+ve’, and ‘1’ or
‘–ve’. A correctly connected primary winding will
therefore provide a ‘negative’ spark.
Key Points
[647] Chapter 02
Combustion takes place at different temperatures
in different engines. Spark plugs are designed to
operate at set temperatures so the correct heat
range plug must be used
Spark plug centre electrodes use materials such as
copper or even silver to aid temperature
Web links
Engine systems information
www.kvaser.com (follow CAN Education links)
Teaching/learning resources
Online learning material relating to powertrain systems:
Figure 2.49 Copper core central electrode
Chapter 3
[647] Chapter 03
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what is covered in this chapter . . .
Introduction to electronic petrol injection systems
Petrol injection system examples (multi-point injection)
Single-point (throttle body) petrol injection
Direct petrol injection
Emissions and emission control (petrol engines)
Engine management (the conclusion)
Engine system self-diagnosis (on-board diagnostics) and EOBD
The following sections deal with sensors, ECUs and
actuators that are covered in detail in other sections; see
the relevant sections in Chapters 1 and 2, as well as
other sections within Chapter 3.
Electronic injection systems have progressed
through many developments and variations, so it
would not be possible to cover all of these within this
book. However, section 3.1 provides a general
understanding of the systems and their components,
and sections 3.2–3.4 give examples of specific injection
systems along with the latest developments. Note that
later injection systems form part of engine
management systems, which are covered separately in
section 3.6.
3.1.1 Fuel system developments
From the carburettor to electronic injection
As with ignition systems (see sections 2.2–2.4), fuel
systems have evolved progressively since the motor
vehicle first appeared, but, with the introduction of
electronic control, fuel injection has become the
dominant method of fuel delivery for the petrol engine.
The carburettor (covered in Hillier’s Fundamentals
of Motor Vehicle Technology Book 1) was almost
universally used on petrol engines through until the
late 1970s, when fuel injection systems began to
appear on mass produced vehicles. Electronic injection
was, however, used in the late 1960s to overcome
emission control problems with some vehicles intended
for the American market. In the 1970s a Bosch
mechanical/hydraulic system (Bosch K-Jetronic)
gained favour with many European manufacturers: this
system tended to lead the way until the early 1980s,
when a new generation of electronic injection systems
progressively became more common. The Bosch KJetronic and its ECU controlled variant the KE-Jetronic
system are dealt with in Hillier’s Fundamentals of Motor
Vehicle Technology Book 1.
It is suggested that the first application of fuel
injection was on the engine that was used by the Wright
Brothers for the first manned flight of an aeroplane.
However, simple carburettors were very much the only
petrol delivery systems used on mass production vehicle
engines for many years. Although diesel engines used a
mechanical injection system until fairly recently, petrol
engines relied on carburettor systems because of cost,
simplicity and the fact that there was no need for high
pressure delivery of petrol.
A number of cars did use petrol injection, but these
were generally racing cars and not production vehicles.
One notable exception, however, was in the mid-1950s,
when Mercedes-Benz used an adaptation of a
mechanical diesel pump on its racing engines and also
on a limited production sports car (Mercedes 300SL); it
is interesting to note that these systems were
adaptations of diesel mechanical pump systems, which
injected fuel directly into the cylinder and not into the
intake system, which has been the general principle for
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Engine management – petrol
most electronic injection systems until recently.
Mercedes continued to develop its use of fuel injection
with petrol engines and, in the early 1960s, several of
the company’s production vehicles were available with
petrol injection.
So, although fuel injection was not widely used,
Mercedes and a number of vehicle and racing engine
manufacturers continued to develop the use of fuel
injection for petrol engines. Various mechanical and
electronic systems used from the early 1960s through to
the 1980s were developed to improve emissions for
vehicles sold in the United States, although there was
an increase in the use of injection systems for the
European market for higher performance vehicles and
for more expensive vehicles.
It was, however, the introduction of European
emissions regulations that effectively forced the use of
fuel injection systems on almost all petrol engines by
the early 1990s. By this time, electronic control was
becoming less expensive and it was therefore inevitable
that any mechanical systems would be replaced by
electronic injection systems.
Fuel injection and engine management
Almost all modern petrol engines for light cars are
equipped with engine management systems, which
control the fuel injection, the ignition and many other
functions. The engine management system is therefore
effectively a number of different systems controlled by a
single ECU (in most cases). The main systems (ignition
and fuelling) are covered separately within this book;
the integration of these systems is dealt with in the
engine management section (section 3.6).
3.1.2 Advantages of electronic petrol
Improved efficiency and control
Compared with the carburettor, there are numerous
benefits provided by a fuel injection system; most of
these will become obvious in the following sections.
However, almost by way of a conclusion, it is certain
that an electronic fuel injection system provides an
overall efficiency of fuel delivery and control of fuel
quantity that could not be achieved with a carburettor;
the result is improved combustion efficiency, improved
engine performance (power), improved economy and
reduced emissions.
Even when compared with later types of mechanical
injection systems, electronic control provides a superior
capacity to control fuel quantity and to embrace any
changes in fuelling needed to suit changing conditions.
However, a fully electronic fuel injection system also
provides the facility for integration and communication
with other vehicle systems, such as the ignition and
emission control systems.
Some specific advantages of electronic fuel injection
are covered in the following sub-sections.
Fundamentals of Motor Vehicle Technology: Book 2
Controlled pressure difference
A carburettor operates by using ‘pressure difference’. In
basic terms, fuel in the carburettor float chamber (the
fuel reservoir) is exposed to atmospheric pressure.
Then, when air flows through the carburettor body, this
creates a low pressure area around the venturi (located
in the carburettor body). Therefore, the fuel in the
chamber, which is at a higher pressure, flows to the
lower pressure area. The airflow through to the engine
then carries the fuel with it thus resulting in a mixing of
air and fuel in the combustion chambers. In effect, a
pressure difference is created by the air flow, so varying
quantities of petrol can be drawn into the engine,
depending on
the speed of airflow
the size of the holes or jets through which the petrol
the throttle opening (the angle of opening of the
throttle butterfly).
A fuel injector works on a similar principle of pressure
difference, but the fuel at the injector is at a higher
pressure than that of the atmosphere and therefore
much higher than in the intake manifold or in the
cylinder on the induction stroke. The fuel pressure is
created using a pump controlled by some form of
regulator, so it is always at a controlled pressure. There
is, therefore, no need to create a low pressure by using
a venturi, because the fuel pressure is always higher
than that of the intake system or cylinder at the time
when the fuel is delivered. Even on a turbocharged or
supercharged engine, where the intake system pressure
can be higher than atmospheric pressure, the fuel
pressure will always be higher by a ‘controlled’ pressure
difference. Fuel therefore flows into the intake system
or into the cylinder in a controlled way due to this
pressure difference.
Electronic petrol injection system pressures vary,
but typically they are in the region of 2.5 to 3 bar. This
pressure, forcing the petrol through the injector nozzle,
then assists in creating good atomisation of the petrol:
mixing of air and petrol is therefore much more
Figure 3.1 shows a simple carburettor and a fuel
injector, both of which rely on pressure difference as a
means of delivering fuel.
Intermittent injection to individual cylinders
With a carburettor, the flow of air creates the low
pressure that causes the petrol to flow from the
carburettor to the intake system. In theory, the petrol
mixes with the air: therefore, as the air enters the
cylinder, the petrol also enters the cylinder. However,
there is an inevitable delay from the time that the
inlet valve opens (the start of airflow of that cylinder)
to the time when the increasing airflow draws petrol
from the carburettor; for this and other reasons, it is
necessary to operate with an excess of petrol in the
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Introduction to electronic petrol injection systems
Figure 3.1 Pressure difference causing a flow of fuel
a in a carburettor
b in a fuel injection system
With the more commonly used mechanical injection
systems (Bosch K and KE-Jetronic) the injector is in the
intake port just ahead of the intake valve (Figure 3.2),
so petrol is delivered directly to each individual intake
port. However, the flow of petrol from the injectors on
the K and KE systems is continuous all the time that the
engine is running, so most of the petrol is being injected
whilst the inlet valve is closed. The petrol is therefore
‘waiting’ in the intake port until the inlet valve opens
and the air starts to flow into the cylinder. In reality, the
petrol flowing from the injector is atomised sufficiently
for it to mix with the air in the port, so this is a
considerable improvement over the carburettor.
One big advantage of electronic injection is that the
petrol injectors (located in the same place as in
mechanical injection systems) are opened and closed at
specific times, which in theory reduces the waiting time
before the petrol is drawn into the cylinder. Although
on earlier generations of electronic injection, there was
some ‘waiting’ time, most modern systems inject at
precisely the correct time so that petrol typically leaves
the injectors just before the intake valve opens. Note
that some modern systems inject petrol direct into the
cylinder during the intake stroke.
The injection on fully electronic systems is not
continuous, since the injectors open and close
intermittently at predetermined times, so it is
sometimes referred to as ‘intermittent injection’.
Figure 3.2 Injector located just ahead of the inlet valve
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Engine management – petrol
Precise fuel control
Electronic injection systems are controlled by a
computer (ECU), which not only switches the injectors
on and off at the appropriate time, but is also supplied
with a wide range of information (by various sensors)
that enable it to calculate the required fuel quantity for
all operating conditions. The ECU is therefore able to
deliver the correct fuel quantity at the correct time, and
change that quantity as the operating conditions
change. A schematic layout of a simple injection system
is shown in Figure 3.3.
The injection system ECU can also communicate
with other electronic systems, such as the ignition and
emissions systems. In fact, there is considerable
communication between the engine systems and chassis
systems such as ABS and transmission, as well as with
systems such as the air conditioning. This level of
communication enables the fuel injection ECU to assess
many aspects of a vehicle’s operation, thus helping to
improve the accuracy and efficiency of the engine. In
turn, the fuel injection ECU can pass information to
other vehicle systems, thus improving the efficiency of
those other systems.
Management of these systems can be integrated so
that they are all controlled by a single ECU: this process
is now almost universal for engines, where an engine
management system ECU controls virtually all engine
functions. A similar philosophy is used for chassis
systems, where the braking, and vehicle stability
systems are controlled by a single ECU. Since this ECU
communicates with the engine management ECU, the
next step is to use a single ECU for all vehicle
3.1.3 Main components and layout
of a multi-point, port type
electronic system
Note: This section deals with the main components
required for a simple multi-point injection system.
Additional components used for emission control and
Fundamentals of Motor Vehicle Technology: Book 2
for other functions are covered in subsequent sections
in this chapter. Single-point injection systems (often
referred to as ‘throttle body’ injection), where a single
injector is used to deliver fuel to all of the cylinders, are
covered in section 3.3. Direct injection, where the
injectors deliver fuel directly to the cylinders, is covered
in section 3.4.
Two sub-systems
An electronic petrol injection system effectively consists
of two sub-systems: an electrical/electronic system and
a fuel delivery system. This section deals with the subsystems for multi-point injection systems, where an
individual injector is used to deliver fuel to each
The main components of both sub-systems are listed
below and are also illustrated in Figure 3.4.
Electrical/electronic system (see section 3.1.4)
Injectors – electrically operated fuel valves that,
when open, allow petrol to flow into the engine.
ECU – the computer that calculates the required
amount of petrol and then opens the injectors for
the appropriate amount of time.
Sensors – provide the necessary information to the
ECU to enable it to calculate the fuel required for
different operating conditions.
Fuel system (see section 3.1.5)
Fuel pump – moves the fuel from the fuel tank to
the injectors; the pump provides an excess of fuel,
which results in pressure being developed in the fuel
Fuel filter – filters the fuel to remove dirt particles
that could damage the system components or block
the injectors.
Fuel pressure regulator and fuel rail – the regulator
controls the pressure of the fuel; the fuel rail acts as
the distribution pipe to pass fuel to the injectors.
Note that, in addition to the main sub-systems, an idle
speed control system forms part of many injection
systems. These systems are covered separately in
section 3.1.6.
Figure 3.3 Layout of a simple electronic fuel injection system
The schematic layout shows the ECU receiving information from
sensors and then controlling the fuel injectors.
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Introduction to electronic petrol injection systems
Figure 3.4 Simple electronic fuel injection system, showing the
fuel and electrical/electronic sub-systems
3.1.4 Electrical/electronic system
Injector solenoid valves
Petrol injectors used on electronic injection systems are
fuel valves that open and close to control fuel delivery.
The injectors are solenoids with a needle valve attached
to the solenoid armature, so that, when current flows in
the solenoid winding, the magnetic field moves the
armature, which in turn moves the needle valve off its
seating and allows the fuel to flow through the nozzle.
An example of an injector with a needle valve is shown
in Figure 3.5a, and a different type of injector, with a
disc rather than a needle is shown in Figure 3.5b. Note
that a fine mesh filter is used to filter out the very small
particles that can damage the injector nozzle seating.
The injector solenoid valve is connected to a fuel
supply rail (Figure 3.6), or in some cases is located
within the fuel rail. The fuel within the rail is regulated
at a predetermined pressure, which is altered to suit
operating conditions. However, the quantity of fuel
delivered is largely controlled by opening the injectors
for differing lengths of time.
Creating an atomised fuel spray
Fuel is fed to the injector under pressure (typically
around 3 bar); because the fuel is under pressure, when
it flows out through the injector nozzle a spray of finely
atomised fuel is formed that is able to mix easily with
the air. To further assist with creating a spray of
atomised fuel, the injector needle and needle seating
are designed so that the fuel is forced to exit the injector
in a particular spray pattern.
In general, the fuel exiting the injector nozzle is
directed so that it sprays against the back of the inlet
valve. There are now many different designs of nozzle
used to create a spray pattern for the fuel as it flows
through and exits the nozzle. Depending on the location
of the injector in the inlet port, spray pattern
requirements will differ: wide and narrow angle spray
patterns are used to suit the different engine
applications. Some injectors provide a dual spray
pattern, designed to suit engines with two inlet valves
per cylinder; each of the fuel sprays is directed to the
back of each of the inlet valves.
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Engine management – petrol
Discharge orifices
Fundamentals of Motor Vehicle Technology: Book 2
Rubber coil
Injector connector
From delivery
TE type nozzle
Solenoid coil
Figure 3.5 Fuel injectors
a with a needle valve
b with a disc valve
Figure 3.6 Injectors connected to the fuel rail
Many petrol injection systems now inject petrol directly
into the cylinder rather than into the intake port. These
systems are covered in section 3.4.
Speed of operation and opening time
With most systems, the injectors will open either once
or twice for each operating cycle of a cylinder (see
injector timing in the following paragraphs). Therefore,
if an engine is operating at 6000 rev/min, each cylinder
will complete 3000 cycles in one minute or 50 cycles in
one second. An injector might therefore open and close
as many as 50 times a second (once a cycle) or 100
times a second (twice a cycle).
An injector needs to open for sufficient time to allow
the required amount of fuel to enter the intake port
(or enter the cylinder with some types). Depending on
the amount of fuel required (for example, low load
and engine speed or high load), the injectors will
typically be open for durations of 1.5 ms to as much as
15 ms.
Electronic control unit (ECU)
As with an ECU controlled ignition system, the fuel
injection electronic control unit is the ‘brain’ of the
system (Figure 3.7). The ECU controls the fuel injectors
in response to the information received from the sensors.
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Introduction to electronic petrol injection systems
Figure 3.7 Typical appearance of a fuel injection ECU
An ECU contains a programmed memory, which, in an
injection ECU, contains data on how much fuel should
be injected under different operating conditions. When
information is received from the sensors, the ECU refers
to the programmed data and switches on the fuel
injectors so that they deliver the required amount of
petrol (Figure 3.8). See sections 1.2 and 1.3 for an
explanation of ECU operation and construction.
Fuel map and basic fuel program
Section 2.3.2 describes how a ‘map’ is used to provide
ignition timing values on modern ignition systems. The
same process is used for ECU controlled fuel injection
systems: a three-dimensional map provides the ECU
with the necessary references for the required quantity
of fuel.
Fuel supply
Intake air
Air flow
Fuel tank
Engine speed
Fuel pump
Idle to
full load
Throttle valve
Engine temperature
Intake air
Air temperature
Fuel injection
To engine
Signal processing
Figure 3.8 ECU processes in an injection system
Intake air quantity per unit of time and engine speed are the basic measured variables to which corrections are applied.
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Engine management – petrol
The map shown in Figure 3.9 gives references for
fuelling based on engine load and engine speed; the
fuelling references (vertical scale) are air:fuel ratios,
which are expressed as lambda (λ) values. Lambda
values are explained in section 3.5.1.
As noted above, the fuel map provides the ECU with
references for fuel quantities for different operating
conditions. However, there are some basic or
fundamental trends that dictate the overall mapping or
program strategy.
In theory, the amount of fuel injected is matched to
the mass of air injected so that the stoichiometric
air:fuel ratio is provided (the stoichiometric ratio is the
ideal ratio of air and fuel to provide complete
combustion). However, minor variations in air:fuel ratio
are necessary for different operating conditions, so the
ECU controls the injector opening time to suit the
various conditions as listed below.
Light load conditions – The injection duration is
long enough to provide the quantity of fuel needed
to give the theoretical stoichiometric ratio. Minor
increases or decreases in air mass (air drawn into
the engine) will result in minor changes in
injection duration.
Acceleration and high load – The ECU will
increase the injection duration so that the fuel
quantity increases to match the increase in air mass.
However, under heavy load and acceleration, a
slight excess of petrol is usually required (a rich
mixture), so the injection duration increases to
slightly more than would be required to achieve the
ideal stoichiometric ratio.
Cold running – When the engine is cold, the cold
surfaces of the intake port and combustion chamber
can cause slight condensation of the fuel and
prevent complete mixing of the fuel vapour and air.
Figure 3.9 Fuel mixture map
Fundamentals of Motor Vehicle Technology: Book 2
The injection duration is therefore increased slightly
to provide a rich mixture, thus ensuring that
sufficient fuel is available to mix with the air.
Idle – When the engine is idling, the air:fuel ratio on
modern engines is controlled at around
stoichiometric or lambda 1. It was, however, normal
on older engines for a slightly rich mixture to be
provided, which helped the engine to develop
sufficient power and to run smoothly.
Deceleration – During deceleration, no power is
required from the engine so fuel injection can be
completely cut off. Depending on engine speed and
whether the throttle is partially or completely closed
(indicated by the throttle position sensor), the fuel
injectors can be completely switched off, or the
injection duration reduced, so that very little fuel is
injected. Careful programming of the ECU map is
necessary because, when the throttle is reopened,
there can be a tendency for the engine to hesitate. A
progressive cut-off and reapplication of fuel
injection are necessary to ensure a smooth transition
from deceleration to acceleration.
ECU switching the injectors
The ECU contains the ‘power stages’ or power
transistors that are used to switch the injector electrical
circuits on and off. As with most computer controlled
systems, the ECU forms part of the earth circuit for the
injectors, so the ECU is switching on and off the earth
path. The injectors receive a battery voltage supply from
the battery via a relay.
Having made the necessary calculations, the low
current (and low voltage) microchips within the ECU
will provide an appropriate signal to the power stage,
which will cause the power stage to complete the earth
circuit to the injectors, thus switching them on and
allowing fuel to be delivered.
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Introduction to electronic petrol injection systems
Injector timing on later systems (sequential
With modern systems the injectors are usually opened
individually in sequence (to match the engine firing
order); this is known as sequential injection. The
injectors are typically opened just prior to the inlet
valve opening. All the required fuel is therefore
delivered in one ‘opening’ of the injector (Figure 3.11).
However, there are occasions where a very large
quantity of fuel is required, for example during full load
acceleration, where the injectors can be opened twice
for every operating cycle (half the fuel quantity is
delivered at each opening).
Although it is possible to use a signal from the
ignition system to trigger sequential injection, many
systems use separate sensors to identify one of the
cylinders, for example cylinder number 1; the ECU then
uses this signal as a master reference and operates the
injectors in sequence at the appropriate times. The
sensor is referred to as a ‘cylinder identification sensor’
or ‘phase sensor’: these are usually either inductive or
Hall effect sensors.
The camshaft rotates once for every engine cycle
(while the crankshaft rotates twice), so the cylinder
identification sensor is usually located adjacent to the
camshaft. Therefore, a single reluctor tooth or trigger
lug attached to the camshaft could then cause an
inductive sensor to provide a single reference signal
(see section 2.2.5). Alternatively a Hall effect rotor (see
section 2.2.6) attached to the camshaft could have a
single ‘cut out’, thus producing a single reference pulse.
A crankshaft speed/position sensor provides the
necessary crankshaft angle and speed information.
Injector timing on earlier systems
(simultaneous injection)
The ECU will switch on the injectors (by completing the
earth circuit) at a predefined time in the engine
operating cycle. On many earlier electronic injection
systems (typically through until the early 1990s), the
injectors were all opened at the same time (on fourcylinder engines), which is referred to as ‘simultaneous
injection’. With six-cylinder engines the injectors were
generally operated in two groups of three injectors; with
eight-cylinder engines the injectors were operated in
two groups of four; and with 12-cylinder engines there
were four groups of three injectors. All of the injectors in
a group would open and close at the same time.
It was also usual for all of the injectors to be opened
twice for every engine cycle, so half of the required
quantity of fuel was delivered each time the injectors
opened. On these older systems, the injector timing
was therefore not perfect because, while one cylinder
might have its injector opening when the inlet valve
was open, on the rest of the cylinders, the inlet valves
would be closed. As previously noted, the injected
petrol would therefore be ‘waiting’ for a short period
before it was drawn into the cylinder. Figure 3.10
shows a four-cylinder engine where the injectors are
opened simultaneously twice for every engine cycle.
On earlier systems, the injector opening would be
triggered by the ignition system, but the injection ECU
would switch on the injectors on alternate ignition
pulses: i.e., if the firing order was 1,3,4,2, then the
injectors would open when the ignition system was
firing numbers 1 and 4 cylinders or numbers 3 and 2
cylinders. Therefore, in a complete engine cycle (two
crankshaft revolutions), in which all cylinders would
have fired once, the injectors would have opened twice.
Figure 3.10 Injector timing for
simultaneous injection on a fourcylinder engine
Simultaneous injection
Injection timing
Fuel injection
Cylinder 1
Cylinder 2
Cylinder 3
Cylinder 4
180° 360° 540° 720°
Crankshaft angle
Independent injection
Injection timing
Fuel injection
Cylinder 1
Cylinder 2
Cylinder 3
Cylinder 4
180° 360° 540° 720°
Crankshaft angle
900° 1080°
Figure 3.11 Injector timing for
sequential injection on a fourcylinder engine
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Some systems used a trigger signal provided by a sensor
attached to a spark plug lead. The sensor generated a
small electrical pulse that was used by the ECU as the
master reference signal.
Injection duration
The ECU uses the information from the sensors to
calculate the operating conditions for the engine and
thus enable the correct volume of fuel to be injected. In
theory, the mass of air entering the engine is the main
item of information required by the ECU to enable the
correct quantity of fuel to be calculated. The air mass
can be measured with an air mass sensor but further
information is used to assist the ECU in the calculation
process. This additional information, supplied by other
sensors, is covered in the following sections.
When the ECU has calculated the quantity of fuel to
be delivered (effectively by noting the information from
the sensors and then referring to the programmed fuel
map within the memory), it will then switch on the
injectors for an appropriate length of time (the injection
duration). The duration will vary with the system’s
design and operating conditions, such as engine load,
engine speed, temperature, etc., but typical values are
between 1.5 ms and 15 ms.
Injectors are produced with differing nozzle sizes, so
different injectors will allow different quantities of fuel
to flow through the nozzle for a given opening
duration. The different nozzle sizes are produced to suit
larger and smaller engine cylinders, which will require
correspondingly larger or smaller quantities of fuel to
be delivered. A large nozzle injector used in a large
cylinder will have a similar opening duration to a small
nozzle injector used in a small cylinder.
Control signal
The ECU functions as the switch in the injector earth
circuit. The power stage within the ECU is the switching
component, and when the ECU calculates that the
injector should be switched on for a specific length of
time, the power stage will complete the injector earth
circuit for the appropriate time period.
As with any switch that is located in the earth
circuit, when the switch is ‘closed’ there is a complete
circuit, which means that the earth circuit voltage
should be 0 volts. When the switch on the earth circuit
is open, current does not pass to earth and there is an
‘open circuit voltage’ available at the negative terminal
of the injector and at the input terminal of the earth
circuit switch.
On an injector circuit, therefore, assuming that a
12 volt power supply is connected to the injector, the
input voltage to the injector positive terminal will be
12 volts. When the earth switch is closed (completing
the circuit through to earth), then the voltage at the
negative terminal of the injector and at the input
terminal of the power stage or switch will be 0 volts.
The switching action of the power stage therefore
results in the voltage on the earth path (negative
terminal of the solenoid) switching between 0 and
Fundamentals of Motor Vehicle Technology: Book 2
12 volts. In effect, this on/off switching action is the
control signal produced by the ECU: this control signal
is a simple on/off digital signal. The length of the on
pulse of the control signal will dictate the duration of
the opening of the injector.
Two examples of control signals are shown in Figure
3.12. These signals are typical of control signals when
observed with an oscilloscope.
The example in Figure 3.12a shows a signal
produced when the power stage switches on and off the
earth circuit. The spike at point C on the signal is
produced when the circuit is switched off, which causes
a ‘back EMF’ to be produced in the solenoid winding, i.e.
the rapidly collapsing magnetic field causes a voltage to
be induced in the winding. The duration of injection is
dictated by the pulse width B. Therefore points A, B and
C on the pulse can be described as follows:
A = injector earth circuit switched off. The open circuit
voltage can be measured at the injector negative
B = injector earth circuit switched on. The ECU
completes the earth circuit so the voltage in this
circuit is 0 volts. The width of this section of the
signal (pulse width) dictates the opening time or
duration of opening of the injector.
C = injector earth circuit switched off. At the end of
duration B, the ECU switches off the earth circuit
and back EMF is induced within the solenoid. The
EMF can reach figures in the region of 40 to
60 volts on some systems, as indicated at point C.
The example in Figure 3.12b is typical of systems
where current control is used by the ECU in a twostage control process. Stage 1 is where full current is
allowed to flow through the injector earth circuit (B2),
which allows rapid opening of the injector; the
duration of this full current period can be very short,
e.g. 0.5 ms. Stage 2 is where current limiting is
implemented by the ECU so that the current flow
through the earth circuit is limited (B3); however,
sufficient current will still flow to keep the injector
open for the required period. The injector duration is
varied by altering the width of the injector control
signal pulse in stage 2 (B3).
Injector earth circuit switched off. The open
circuit voltage can be measured at the injector
negative terminal.
B1 = Total duration of control signal (made up of B2 +
B2 = Injector circuit is switched on. At this point, the
ECU allows full current to flow through the earth
circuit, thus rapidly opening the injector. At the
end of period B2, the current is limited by the
ECU, which would in theory cause the injector to
B3 = When the current has been limited at the end of
B2, the ECU allows a reduced current to flow
which is sufficient to keep the injector open. The
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The main objective of any petrol delivery system is to
provide the correct mixture of air and petrol so that the
combustion process is efficient and produces maximum
power from the mixture. To achieve this, the fuel
injection system must provide the correct quantity of
petrol to suit the mass of air being drawn in by the
engine, i.e. the ratio of petrol to air must be correct.
width of the pulse at B3 alters, which controls the
duration of opening for the injector.
Injector earth circuit switched off. At the end of
duration B, the ECU switches off the earth circuit
and back EMF is produced within the solenoid.
The EMF can reach 40 to 60 volts on some
systems, as indicated at point C.
Injector (multi-point)
Injector (single-point)
Figure 3.12 Injector control signals
a A signal produced when the power stage switches on and off the earth circuit
b A signal typical of systems where current control is used by the ECU in a two-stage control process
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If we therefore assume that, at all times, the air:fuel
ratio is stoichiometric (as discussed in Book 1), then the
mixture delivered should be in the ratio of
approximately 14.7 parts of air to 1 part of petrol by
weight. This is the ratio that, in theory, will provide
complete combustion of the mixture.
However, there are numerous situations that
influence the air:fuel ratio: this means that the mixture
can vary above or below the stoichiometric value, i.e.
the mixture can have a slight excess of fuel or an excess
of air (rich or weak) to suit the conditions and
requirements. A simple example is with cold starting
and running, where a slightly rich mixture is required.
In other situations, a slightly weak mixture might be
provided for better fuel economy.
In reality, with modern engines, emissions
regulations force the use of a stoichiometric air:fuel
ratio for a high percentage of engine operating
conditions. However, the same rule applies with
modern and older engines: the injection system ECU
must initially calculate the fuel requirement based on
the mass of air flowing into the engine.
The main information required by the ECU is
therefore a measurement of the mass of air. Other
sensors are, however, required to enable the ECU to
‘fine tune’ the air:fuel mixture. As noted previously,
there are a number of factors that affect the air:fuel
ratio, such as:
emissions control
engine load
driver intentions
engine design
and other factors also slightly influence the mixture
finally delivered to the engine. Therefore many
additional sensors are used to give the ECU sufficient
information to enable it to finely adjust the fuel
quantity delivered by the injectors. The following list
highlights only those sensors that might be used on a
basic electronic injection system.
Note: Additional sensors are covered in the
examples of injections systems in section 3.2, whilst
other sensors used on modern systems are dealt with
in section 3.5 (emission control); also refer to
Chapter 1.
Airflow and mass airflow measurement
Airflow sensors (discussed later in this chapter) can be
used to measure and transmit a signal to the ECU
relating to either the volume or the mass of air that is
flowing through the intake system at a given time.
However, the ECU must calculate what mass of air is
flowing to each cylinder at any given time, so it also
requires engine speed information. Some examples of
airflow and mass airflow sensors are shown in
Figure 3.13.
Air mass and engine speed are the fundamental
items of information required by the ECU. Although
Fundamentals of Motor Vehicle Technology: Book 2
some systems use an air ‘mass’ sensor, others use
different sensors such as manifold absolute pressure
sensors (MAP sensors), which measure the manifold
depression; in this case, the ECU uses the MAP sensor
information along with engine speed, air temperature
and other information to establish the basic fuel
Coolant temperature (Figure 3.14a)
The engine coolant temperature information enables
the ECU to alter the fuel quantity (thus altering the
air:fuel ratio) so that, when the engine is cold, an excess
of fuel (a rich mixture) can be provided. The ECU can
slightly alter the fuel quantity and mixture over the
whole range of operating temperatures, thus allowing
‘fine tuning’ of the mixture.
Air temperature (Figure 3.14b)
The air temperature sensor information helps in
calculating the air mass, because air density changes
with temperature. Many air temperature sensors are
incorporated within the airflow sensors. Since a high
air temperature can cause ‘pinking’ or pre-ignition,
the ignition timing might be altered by the ignition
ECU to correct this, but it is also possible that the
injection ECU might slightly alter the fuel quantity if
Throttle position
On early basic electronic injection systems, a simple
‘throttle switch’ was used to indicate when the throttle
was in the closed position (idle). The switch also
indicated when the throttle was around 60% open,
which was a sign that the engine was under load. The
most widely used type of throttle switch on earlier
systems contained two sets of contacts, which closed
and opened at the relevant time as the throttle was
opened and closed. For both idle and load positions,
the ECU provided a slight enrichment of the mixture,
which helped stabilise the engine at idle and allowed
additional power to be developed under load. Note
that later engines, with improved emission control,
operate with weaker mixtures at idle and under load
conditions; the programming of the ECU and the
information required are altered to suit these
Later throttle position sensors use a potentiometer
or variable resistor instead of switches. With a
potentiometer based sensor, it is possible to send a
varying voltage analogue signal to the ECU; the signal
indicates all throttle opening angles and the ECU can
also calculate the rate at which the throttle is being
opened and closed. The ECU is therefore able to alter
the fuelling to suit the minor and major changes in
throttle position (load changes). The ECU can also
assess the driver’s intentions, such as the intention to
rapidly accelerate, by detecting the speed or rate at
which the throttle is being opened.
Figure 3.15 shows an example of a throttle switch
and a throttle position potentiometer.
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Figure 3.13 Examples of airflow and mass airflow sensors
a Flap type airflow meter
b Hot wire air mass meter
c MAP sensor
Silicon chip
Intake manifold pressure
Ignition trigger or crankshaft position trigger
As noted previously, early injection systems used
ignition pulses as a means of triggering the injectors.
These earlier injection systems were usually fitted to
engines that also had earlier designs of electronic
ignition with inductive or Hall effect triggers located in
a distributor, so it was common practice to provide a
signal from the ignition system to the injection ECU.
This signal was often taken directly from the coil
negative terminal, which would be the same terminal
used to provide a signal to a rev. counter. On some
ignition systems, the signal might have been a digital
signal provided by the ignition module.
Figure 3.14 Examples of temperature sensors
a Coolant temperature sensor
b Air temperature sensor
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Other sensors
The sensors described so far provide the essential
information to enable a simple injection system to
operate. In fact, many early systems used only these
The need for improved emission control and
efficiency led to other sensors being added, which are
covered in the following sections of this chapter.
3.1.5 Fuel system
Fuel pump
Providing sufficient fuel flow
The fuel pump moves the fuel from the fuel tank
through to the injectors. The pump must provide
sufficient fuel for the engine to operate at full load: it
delivers typically between 1 and 2 litres of fuel each
minute (depending on engine size).
The fuel pumps on most systems operate at full rate
all the time: there is no variation in the amount of fuel
delivered by the pump, irrespective of engine speed and
load. However, at low engine speeds and loads only a
small amount of fuel is used, so the excess fuel flows
back to the fuel tank.
Figure 3.15 Measurement of throttle position
a Throttle valve switch
b Throttle position potentiometer
c Output from potentiometer as throttle valve opens
With those later ignition and injection systems, a
crankshaft position sensor was often used. The sensor
transmitted the crankshaft speed and position data to
the ignition system ECU, and the ignition ECU then
transmitted a digital speed signal to the injection ECU.
Whichever triggering system is fitted, the ECU can
use the information as a triggering signal for opening
the injectors (the injector timing on simultaneous
injection systems). The ignition trigger signal also
provides the ECU with engine speed information, which
can be used to help calculate fuel requirements.
Figure 3.16 shows the process of an injection system
that is being triggered by an early ignition system.
Pressurising the system
Liquids cannot be compressed, but they can exist in an
enclosed system under pressure. With petrol injection
systems, a high volume of fuel delivered by the pump
flows to the injectors and, with only a small amount of
fuel able to escape through the injectors, this
continuous flow of fuel causes the system to build up
pressure. Although excess fuel is allowed to return to
the tank, a regulator valve is used (see below) to control
the pressure in the system. In theory therefore, the fuel
is always held in the system at a constant pressure (see
the following paragraphs dealing with pressure
regulators). This combination of a high volume of fuel
delivered by the pump and the pressure regulator
ensures that the fuel flowing through the injectors is at
a pressure that causes atomisation of the fuel when it
exits the injectors.
Construction and location
With most modern systems the fuel pump is located in
the fuel tank, although on many earlier systems the
pump was mounted externally (Figure 3.17). In some
cases, two pumps are used: one which initially moves
the fuel from the tank to the main pump (which might
be located too high in the vehicle for the fuel to flow
into it), the main pump then moves the fuel through the
fuel injection system.
The main pump is driven by an electric motor,
which turns a pumping element. A number of
different types of pumping element have been used:
the examples shown in Figure 3.18 cover both
positive displacement (a) and flow type pumping
elements (b).
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Ignition ECU
Crank sensor
Injection ECU receives speed /
trigger signals from ignition ECU
Injection ECU
Injection ECU is then able to provide
timed injection and calculate the
necessary fuel requirement
Fuel injectors
Figure 3.16 Ignition system used for triggering an earlier fuel injection system
One disadvantage of positive displacement types is that
pulses are produced by the pumping action, which can
cause noise and vibration in the fuel system. Flow type
pumps provide much quieter operation and are
therefore more widely used. However, with the latest
generation of direct injection systems, positive
displacement pumps are again becoming more widely
Fuel filter
The fuel filter is located in the fuel circuit after the fuel
pump. Although different manufacturers use slightly
different construction, the example shown in Figure
3.19 is typical of most filters.
The filter usually consists of a fine paper element
and a strainer, which can retain any larger particles.
Filters are often constructed in such a way that fuel
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Fundamentals of Motor Vehicle Technology: Book 2
should flow in one direction only; markings are usually
provided to indicate the fuel flow. Incorrect fitment can
result in collapse of the paper element.
Although fuel filters are designed to prevent any
impurities and dirt from reaching the injectors,
additional fine mesh filters are often also fitted in the
injector inlets.
1. Fuel tank
2. Electronic fuel pump
3. Fuel level sensor
4. Float
Relief valve
Figure 3.18 Fuel pump with positive displacement and flow type
pumping elements
a Roller cell pump
b Flow type pump
Check valve
Figure 3.17 Examples of fuel pumps
a Pump located in the fuel tank
b An external pump
Figure 3.19 Fuel filter
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Fuel pressure regulator and fuel rail
The fuel rail is simply a distribution pipe that allows
fuel to flow to all of the injectors. Fuel flows from the
tank through the pump and filter to the fuel rail,
where the injectors receive their supply of fuel. Until
relatively recently, on most systems the fuel rail had an
excess fuel pipe connection, which allowed unused or
excess fuel to flow back to the fuel tank. A pressure
regulator in the fuel rail maintains the pressure in the
fuel rail at the required value: excess fuel flows from
the regulator back to the fuel tank. An example of a
fuel system with an excess fuel return is shown in
Figure 3.20a.
Relocation of the excess fuel return pipe
A disadvantage of returning excess fuel from the fuel
rail to the fuel tank is that the fuel carries heat from the
engine bay back to the fuel in the tank. The temperature
of the fuel in the tank therefore rises, which causes
evaporation, resulting in fumes that can escape to the
atmosphere. Emissions regulations require emissions
from the fuel tank to be controlled. A recent change in
fuel system design helps to reduce evaporative tank
emissions by eliminating the return pipe from the fuel
rail. The return pipe is relocated so that it collects the
excess fuel from a position much closer to the fuel tank:
it is connected just after the fuel filter. These systems
are often referred to as ‘returnless systems’, although in
reality a return pipe is still used. An example of a
returnless system is shown in Figure 3.20b.
Pressure regulator
One principle of operation of electronic injection
systems is that if the fuel pressure remains constant, i.e.
the fuel exiting from the injector is always at the same
pressure, then any variation in fuel quantity delivered to
the cylinders can be controlled by altering the duration
of the open time of the injectors. In effect, the injection
‘on time’ or injection duration is the only method
through which the fuel quantity is regulated, and the
duration is controlled by the ECU in response to
information from the sensors.
A fuel pressure regulator is therefore fitted to the
fuel delivery system to ensure that the fuel pressure
remains constant.
Fuel pressure and engine intake pressure
One factor that must be considered in maintaining a
constant fuel pressure is the change that occurs in the
engine intake system pressure.
While an engine is operating, the pressure in the
intake system (manifold and ports) varies with changes
in engine load, engine speed and throttle opening
angle. At one extreme, when the throttle is initially
opened, the intake pressure is almost the same as
atmospheric pressure (approximately 1 bar). At the
other extreme, for example when the engine is at high
speed and the throttle is suddenly closed, the intake
pressure will fall (often referred to as the intake
vacuum) due to the restriction of the closed throttle and
the strong suction created by the cylinders. The intake
pressure can reduce to a typical value of 0.5 bar, so the
pressure will have fallen by 0.5 bar.
If the fuel injection pressure remained constant at a
typical value of 3 bar, then the difference between the
injection pressure and the intake system pressure would
vary as the intake system pressure varied. The extremes
of effective or true injection pressure would therefore be
as follows:
1 intake system pressure = 1 bar
injection pressure
= 3 bar
pressure difference
= 2 bar.
The injection pressure will therefore be 2 bar.
2 intake system pressure
injection pressure
pressure difference
The true pressure of
2.5 bar.
Figure 3.20 Fuel delivery systems
a Fuel system with excess fuel return from fuel rail
b Returnless fuel system
= 0.5 bar
= 3 bar
= 2.5 bar.
injection will therefore be
Therefore, if the true injection pressure were allowed to
vary with the intake system pressure, the quantity of
fuel delivered would also vary: for the same duration of
injector opening time, if the true injection pressure
increased, the amount of fuel flowing through the
injector would also increase.
To overcome this problem, most pressure regulators
are fitted with a pressure/vacuum pipe connection to
the intake system. When the intake pressure reduces,
this lower pressure acts on the pressure regulator, which
in turn reduces the pressure in the fuel system. In effect,
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as the pressure in the intake system rises and falls, the
pressure in the fuel system also rises and falls by the
same amount.
The relationship between intake system and fuel
system pressure will therefore be as follows:
1 intake system pressure
injection pressure
pressure difference
The true pressure of
2 bar.
= 1 bar
= 3 bar
= 2 bar.
injection will therefore be
2 intake system pressure
injection pressure
pressure difference
The true pressure of
2 bar.
= 0.5 bar
= 2.5 bar
= 2 bar.
injection will therefore be
The following statement can therefore be made:
‘Fuel system pressure is maintained by the pressure
regulator at a constant pressure relative to intake
system pressure.’
Figure 3.21 shows a typical pressure regulator, where
intake system pressure (manifold pressure) is connected
to the regulator. The operation is as follows.
When fuel is delivered to the fuel rail and regulator,
it will flow to the underside of the regulator
diaphragm. A spring acts on the diaphragm and
valve assembly, which holds the valve closed.
However, the fuel entering the system will build up
pressure to a level where it will cause the diaphragm
to lift against the spring, and therefore the valve will
As soon as the valve opens, the excess fuel will
escape through the return port, which will cause the
pressure to reduce; the valve will therefore close. In
reality, the valve is constantly oscillating between
the open and closed positions, with the result that
Figure 3.21 Fuel pressure regulator
Fundamentals of Motor Vehicle Technology: Book 2
the pressure is maintained at a value that is
dependent on the strength of the spring.
When the manifold pressure reduces, this lower
pressure acts on top of the diaphragm and helps to
lift the diaphragm against the spring. Therefore the
fuel pressure will not need to be so high before it
lifts the diaphragm and opens the valve.
Therefore, when there is a low pressure in the intake
manifold, the fuel pressure beneath the diaphragm will
be lower when the valve opens. When the intake
manifold pressure is higher, then the fuel pressure
beneath the diaphragm will need to be higher to open
the valve.
3.1.6 Idle speed control
Stalling at idle speed
When an engine is operating at idle speed, many of the
engine’s processes are relatively inefficient. For
example, the air flows through the intake system at a
low speed, which does not help the air and fuel to mix.
For many engines therefore, especially older designs,
emission levels at idle speed were relatively high (as a
percentage of the total exhaust gas) and the power
developed by the engine was very low.
The regulations are very much focused on idle speed
emissions, so for many years it has been necessary to
operate engines on a weak mixture or at stoichiometric
air:fuel ratios. However, these air:fuel ratios at idle
speed do not enable the engine to produce good power
outputs leading to a tendency for the engine to stall
when any load is applied.
To overcome this problem, a means of controlling
the idle speed is used which relies on regulating the
airflow into the engine to ensure that the engine does
not stall or run too slowly at idle. One option is to rely
on the driver to control the idle speed with the throttle;
this is, of course a very imprecise and impractical
method. Therefore automated systems are used to
regulate the airflow. In effect these systems are
automatic throttle controls, and in some cases, they do
physically control the throttle butterfly. However, many
systems regulate the air flowing through a bypass port
by using an ECU controlled air valve.
Stepper motor idle valves
See section 1.8.2 for additional information on stepper
A stepper motor is effectively an electric motor that
can be stopped or positioned at selected angles of
rotation. Some stepper motors can be controlled so that
the motor or armature will rotate and stop in
increments of less than 1 degree of rotation.
Stepper motors are generally used in one of two
ways to control idle speed: either by acting on an air
bypass port or by acting on a linkage that connects to
the throttle butterfly (throttle valve or plate).
Figure 3.22a shows a stepper motor used to regulate the
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air through an air bypass port and Figure 3.22b shows a
stepper motor acting on a throttle butterfly linkage.
The control signals for stepper motors are generally
conventional digital control signals: they are on/off
pulses. A stepper motor has a number of windings, each
of which can be provided with an on/off control signal
from the ECU, which enables the armature to be
positioned accurately.
Figure 3.22 Electric motors used to control idle speed
a Construction of stepper motor idle control valve
b Stepper motor acting on throttle linkage
c Rotary idle valve
Rotary idle valve using partial rotation motors
Although this type of valve also uses a type of electric
motor, the rotation of the armature is restricted by
mechanical stops or limiters, so that it can only partially
rotate. Bosch produces the most widely used type of
rotary idle valve: see Figure 3.22c. The armature is
connected to a valve that controls the air flowing
through a bypass port (Figure 3.22c); the assembly is
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usually remotely located, so air pipes connect the valve
assembly to the intake system throttle body.
When the throttle is in the idle or closed position,
air is able to bypass the throttle butterfly by flowing
through the air pipes and the rotary idle valve; the idle
speed is therefore dictated by the valve position.
Earlier versions had two windings in the motor. By
transmitting a control signal through each winding, it
was possible to rotate the armature clockwise or anticlockwise, depending on which control signal provided
the higher current flow through the winding. Later
types use a spring to rotate the armature in one
direction, with a control signal flowing to a winding
that will then rotate the armature against the spring;
this type is shown in Figure 3.22c.
The ECU alters the duty cycle of the digital control
signal (see section 1.9), which alters the average
current in the circuit and the winding. This then causes
a stronger or weaker magnetic field to be produced,
which results in a stronger or weaker force to oppose
the armature spring.
Solenoid idle valves
Several system manufacturers have used solenoid
operated valves to regulate the air flowing through an
air bypass port. The principle of regulating air in a port
is the same as for stepper motor systems (see above),
but a linear solenoid is used to control the valve instead
of a rotary motor (Figure 3.23).
Although there are various designs of solenoid
systems, the basic principle relies on a solenoid
armature being connected to an air valve. When the
solenoid armature moves, it increases or decreases the
aperture in the air bypass port.
The solenoid is usually spring loaded in one
direction; current flowing through the winding causes
the armature to move against the spring. By increasing
or decreasing the current in the circuit and winding, it is
then possible to create a greater or weaker magnetic
field, which creates a stronger or weaker force on the
armature. Altering the current in the circuit therefore
moves the armature (against the force of the spring)
giving the required valve position to regulate the airflow
in the bypass port and the idle speed.
Although there are variations in the control signals
used for solenoid valves, a digital signal can be used
where the duty cycle is altered, which in turn alters the
average current flow (see section 1.9 for additional
information on control signals and solenoids).
Solenoid valve
Ignition switch
air pressure
air pressure
Air intake
To intake
air chamber
From air
cleaner side
Figure 3.23 Solenoid type idle speed control valves
From ECU
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This section explains the operation of different types of
electronic fuel injection systems. The examples are all
multi-point port injection systems: i.e. they have
individual injectors for each cylinder, with the injectors
located in the intake ports. Section 3.3 covers singlepoint systems.
3.2.1 Example 1: Simple multi-point
injection with airflow sensor
System components and layout
Figure 3.24 shows the main components used in the
Bosch LE system, which was widely used by many
vehicle manufacturers and was a system from which
many others evolved. In addition to the main injection
system components, an auxiliary air valve is used to
provide a fast idle speed when the engine is cold. The
air valve is not controlled by the ECU and its operation
is explained later in this section. The LE2 system
therefore has several sensors but only one set of
actuators: the injectors. The fuel delivery system on the
Bosch LE is identical to the example illustrated in Figure
3.20, so it is not described again in this section.
Airflow sensor with combined air temperature
The airflow sensor provides the ECU with an analogue
signal that indicates the volume of air being drawn into
the engine. Air density changes with temperature, so an
air temperature sensor is built into the airflow sensor
assembly. The ECU therefore receives airflow and air
temperature information.
Figure 3.24 Bosch LE injection system
Airflow sensor operation
The airflow sensor has a flap or vane that is forced to
move when the air flows through the sensor body
(Figure 3.25). The flap is connected to a hinge or shaft,
so the angle of the flap increases as the airflow
A potentiometer (variable resistor) is fitted to the
sensor assembly, and the potentiometer ‘wiper’ or
‘slider’ is connected to the shaft. Therefore, when the
flap moves, the potentiometer wiper moves around the
resistance track of the potentiometer. This changes the
voltage at the wiper, with the magnitude of this change
depending on the flap position. The voltage reading at
the wiper is sent to the ECU, which provides the ECU
with an indication of airflow.
One of the disadvantages of the flap system is the
change in angular movement that occurs with increased
airflow. When the airflow is low, the flap is almost at
right angles to the airflow and therefore the force acting
on the flap is relatively high; any small change in
airflow will cause a relatively large change in the flap
angle. However, when the airflow is already high, the
higher forces acting on the flap will have pushed it to a
position where it is almost in line with the airflow
(almost lying flat in the sensor body), so any further
small increases in airflow will not greatly affect the
angle of the flap: it will move only a little further.
When the flap is almost at right angles to the
airflow, the voltage change at the potentiometer will be
quite large for a small change in the airflow. When the
flap is almost in line with the airflow, however, small
changes in airflow will result only in very small changes
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Engine management – petrol
in voltage, which does not provide sufficient
information to the ECU. The ECU ideally requires large
changes in voltage to assess the airflow accurately.
The potentiometer used on the airflow meter is
potentiometers. A thick film resistance track is used,
made of several segments, each with a different
resistance. The resistance of the segments is designed to
compensate for the reducing angular movement of the
flap as the airflow increases: as the wiper moves across
the track, the output voltage is progressive and linear.
On this type of airflow sensor, 12 volts is applied to
the potentiometer, and, as the wiper moves across the
resistance track, the voltage at the wiper changes from
typically around 5 volts at low air volumes to around
9 volts at high air volumes.
Damping chamber
A second flap (attached to the first flap) is positioned in
a small chamber (referred to as a damping chamber).
Air is drawn or induced into the engine in pulses or
waves (each cylinder creates a single strong pulse
giving as many pulses in one engine cycle as there are
cylinders), so the first flap will also tend to pulsate
when the airflow passes through the sensor. The second
flap is also exposed to the pulsing action of the airflow,
but the airflow is directed against the second flap in
such a way that the pulsing on the second flap cancels
the pulsing of the first flap.
Figure 3.25 Cutaway views of vane type airflow sensor
a Air side
b Electrical connection side
Fundamentals of Motor Vehicle Technology: Book 2
If the compensating flap and damping chamber were
not used, the pulsations caused by the airflow would
also cause the signal from the sensor to pulsate.
Air temperature (see section 1.5.1)
An air temperature sensor is incorporated within the
airflow sensor. The temperature sensor is a thermistor,
which is a resistor that changes in resistance with
changes in temperature. Because the sensor forms part
of a series resistance circuit that has a reference voltage
applied to it, when the temperature changes, the
resistance and voltage in the circuit also change. The
change in voltage is used as a signal to the ECU.
The ECU uses the signal for the change in air
temperature in conjunction with the airflow signal
(because air mass for a given airflow changes with
Mixture adjustment
A bypass port is provided on older airflow sensors so
that mixture adjustments can be carried out at idle
speed. This facility is no longer required with modern
engine management systems, but on older engines it
was needed to ensure that the idle emissions were
within specified limits and to enable the engine to idle
Figure 3.25 shows the bypass port, which has an
adjusting screw at one end. If the adjusting screw is
screwed fully into the port it will block the bypass port,
which will force all of the air being drawn into the
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engine to flow through the main intake port and
therefore move the sensing flap. When the adjusting
screw is unscrewed, a small amount of air is able to flow
through the bypass port, so the flap will not be affected
by all of the airflow: the sensing flap will move back
slightly, which will reduce the signal voltage
transmitted to the ECU. Altering the adjusting screw
will therefore affect the amount of air flowing through
the bypass port and thus the amount of air affecting the
sensing flap. The signal voltage will alter, which causes
the ECU to adjust the quantity of fuel being injected.
Adjusting the bypass port screw will therefore affect the
fuel quantity and the mixture at idle speed.
Coolant temperature sensor
(See section 1.5.1.) The coolant temperature sensor is
positioned (usually in the cylinder head) so that it can
measure the temperature of the engine coolant. A signal
from the sensor is transmitted to the ECU so that the
fuel quantity can be altered for cold running (by
enriching the mixture) as well as for other minor
variations in fuelling that are required at different
coolant temperatures (to provide fine tuning of the
mixture). Figure 3.26 shows a typical coolant
temperature sensor.
The coolant temperature sensor operates in exactly
the same way as the air temperature sensor described
above. The sensor resistance changes with coolant
temperature, resulting in a voltage change in the circuit,
which the ECU can then use as an indication of coolant
The sensor has a negative temperature coefficient
(NTC), so its resistance reduces as the temperature
Figure 3.26 Coolant temperature sensor
increases. A typical resistance for the sensor is around
7000 ohms (7 kΩ) at 0°C, falling to around 250 ohms
at 100°C.
The voltage in the sensor circuit also reduces as the
temperature increases. A reference voltage is applied to
the sensor circuit, which on the Bosch LE2 system is
around 12 volts. However, when the sensor is
connected to the circuit, the resistances in the circuit
reduce the voltage to a value that depends on the
resistance of the sensor, which changes with
temperature. For normal operation, the voltage in the
circuit is around 9 volts for a very cold engine and
around 5 volts for a hot engine.
Throttle position switch
As previously described, the throttle switch consists of
two sets of contacts. One set closes when the throttle is
closed (the idle position). The second set of contacts
closes when the throttle is approximately 60% open
(this value will depend on the application).
With the Bosch LE2 system, 12 volts is applied to
the centre terminal of the switch. When a set of contacts
closes, the 12 volt signal is transmitted back to the ECU.
The ECU uses this signal as an indication of idle
position or load position. The air:fuel ratio is usually
enriched slightly to stabilise the idle speed and to
enable the engine to produce full power. Figure 3.27
shows a throttle switch and its construction.
Timing/trigger reference
The Bosch LE2 system relies on a signal from the
ignition system for information on engine speed (to
assist in fuelling calculations) and as a reference for
triggering the injectors. A signal is taken from the
ignition coil or direct from the ignition module. In
effect, every time the ignition module switches off the
ignition coil (spark timing) a signal is transmitted to the
Figure 3.27 Throttle position switch
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The injectors on an LE2 system operate simultaneously:
they all open and close together on a four-cylinder
engine. The ECU uses every alternate ignition pulse (on
a four-cylinder engine) as a reference to open the
injectors, so the injectors open twice for every engine
Injectors (actuator)
Simultaneous injection
The only true actuators on an LE2 system are the
injectors, which are exactly as described in section
3.1.4. On a four-cylinder engine, all four injectors are
opened and closed at the same time (simultaneous
injection). In fact, all four injectors are connected back
to the ECU at one terminal, so the power stage within
the ECU switches all four injectors together (more than
one power transistor might however be used). With
engines of more than four cylinders, the injectors might
be triggered and connected in groups of three or four.
LE2 injectors operate with a fuel pressure of around
2.5 bar above the intake manifold pressure (see section
Figure 3.28 shows the control signal provided by the
ECU. Note that the EMF created when the injectors are
switched off causes a voltage spike at around 60 volts.
Idle speed control/adjustment
The standard LE2 system did not provide an automated
idle speed control system. The idle speed was adjusted
manually using a bypass port adjuster on the throttle
body (Figure 3.29).
Manual idle speed adjustment
A bypass port was usually formed as part of the throttle
body (Figure 3.29). The bypass port allows intake
airflow to bypass the closed throttle butterfly (throttle
valve). The bypass port has an adjusting screw that can
be altered to allow more or less air to bypass the
throttle butterfly.
back ‘EMF’ spike
injector open
injector closed
Figure 3.28 Injector control signal
Manual idle air
bypass adjustment
air valve
Figure 3.29 Manual and auxiliary air valve bypass ports to
control idle speed
Therefore, if the adjusting screw is unscrewed, more air
is allowed to enter the engine, which will increase the
idle speed. Screwing in the adjuster will restrict the air,
thus reducing the idle speed. This enables the idle speed
to be set to the manufacturer’s specifications.
On the LE2 system, when either more or less air is
allowed to flow into the engine through the bypass port,
the air will still have to flow through the airflow sensor.
An increase in airflow will therefore cause the airflow
sensing flap to move, altering the sensor signal to the
ECU; the ECU will therefore increase the fuel quantity
to correspond with the increase in airflow, thus
maintaining the air:fuel ratio.
Auxiliary air valve (cold running)
When an engine is cold, all moving components have
higher levels of friction, and the cold oil can also cause
additional drag or resistance in the engine. To prevent
this additional friction and resistance from stalling the
engine when it is cold, the LE system provides additional
air to the engine, which results in a slight increase in
engine speed at idle (with the throttle closed).
An additional or auxiliary air valve is used which is
connected by air pipes to the throttle body. As with the
manual idle speed adjuster, the air valve is effectively a
bypass port, as shown in Figure 3.29. However, instead
of a manual adjusting screw, the auxiliary air valve has
a temperature sensitive plate valve which is open when
cold and closes when hot (Figure 3.30).
The valve assembly is exposed to two heat sources.
The first heat source is an electrical heating element
integrated into the air valve body. When the engine is
cold the valve plate is in the open position, so when the
engine is started, additional air flows through to the
engine, thus providing a fast idle speed. However, when
the engine is running, the full battery voltage is applied
to the heating element, which heats up a bimetallic strip
attached to the valve plate. As the bimetallic strip heats
up, it bends, which causes the valve plate to
progressively close the air bypass port.
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Petrol injection system examples (multi-point injection)
Bosch LE system, the injectors used the 12 volt supply
and the sensors were also provided with a 12 volt
reference voltage. Although all systems largely still use
the full battery voltage for actuators such as the
injectors, it is now normal practice to use a 5 volt
reference voltage for sensors.
A system relay provides the system components with
the 12 volt supply. The relay acts as a safety device and
will switch off the power supplied to the components
unless certain signals are received from the engine, etc.
The relay consists of contacts which, when closed,
connect the battery voltage direct to the system
components. Energising windings within the relay will
cause the contacts to close when a voltage is applied to
the windings.
Relay operation
The relay receives the full battery voltage supply direct
from the battery (possibly fused on some applications)
to terminal 30.
Figure 3.30 Auxiliary air valve located in the intake system
When the ignition is initially switched on, the
battery voltage will be applied to terminal 15 of the
relay (the voltage will be applied to the energising
winding). A timer circuit within the relay will cause
the relay to apply the battery voltage from relay
terminal 87b to the fuel pump for a few seconds
(allowing the pump to operate, thus ensuring that
the fuel system is under pressure). If the engine is
not cranked or started, the relay will switch off the
supply to the pump.
When the ignition switch is then placed in the
cranking position, the voltage will be applied from
the starter circuit to terminal 50 of the relay; this
will again cause the energising winding to close the
contacts and the battery voltage will now be applied
to all of the system components (the fuel pump,
injectors and sensors). The engine should now start.
When the engine starts, an ignition speed signal
(from the ignition coil or module) is transmitted to
the relay at terminal 1, which indicates that the
engine is running. Because the start signal from the
starter circuit will now switch off (the engine is no
longer cranking), the speed signal acts as a
replacement so that the relay will continue to
provide battery voltage to the injection system.
If for any reason the engine were to stop, the
ignition signal would disappear and the relay would
switch off the power supply to the injection system.
When the engine is switched off, in theory, the
bimetallic strip will cool down, allowing the valve plate
to reopen the port. However, the air valve is positioned
so that it is exposed to engine heat, so the valve body
stays hot until the engine cools down. So the port will
not open again until the engine is quite cool.
It takes typically around 3 minutes (depending on
the vehicle application) for the auxiliary air valve to
move from the fully open to the fully closed position. In
this time, the engine should have reached an operating
temperature that allows it to idle at normal speed.
Electrical systems and wiring (LE2)
Although the Bosch LE system is now an old one, its
basic elements are still relevant to today’s injection
systems. Therefore, in addition to the wiring circuit
shown in this section (Figure 3.31), the operation of the
circuit and some of the functions are also explained.
All injectors will receive battery voltage from relay
terminal 87 during starting and engine running. The
second terminal at each injector is then connected to
ECU terminal 12, which is the earth path for the
injectors. The circuit passes from terminal 12 through
the power stage of the ECU to earth. Therefore, when
the ECU switches on the injectors, the power stage will
complete the earth circuit for the injectors.
Power supply
Early electronic injection systems generally used battery
voltage for all aspects of system operation. On the
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Figure 3.31 Wiring diagram for Bosch LE2 injection system
Fuel pump
The fuel pump receives a power supply from relay
terminal 87b while the engine is starting and running.
Throttle switch
The throttle switch receives battery voltage at terminal
18 from relay terminal 87. When the idle contacts or the
load contacts in the switch are closed, the battery
voltage will then be applied from the switch terminals 2
or 3 to the ECU at terminals 2 or 3. The ECU will then
have an indication of idle or engine load.
Coolant temperature sensor
The coolant temperature sensor also has a battery
voltage supply from relay terminal 87. As previously
noted, the sensor is part of a series resistance circuit, so,
as the resistance of the sensor varies with temperature,
the signal from the sensor to ECU terminal 10 will
change, indicating the temperature to the ECU.
Airflow sensor
The airflow sensor is supplied with the battery voltage
at terminal 9 through relay terminal 87. This voltage is
applied across the air temperature sensor, which
operates in the same way as the coolant sensor
described above. The signal from the air temperature
sensor passes from terminal 8 to the ECU terminal. The
supply voltage is also applied across the potentiometer
within the airflow sensor; when the wiper on the
potentiometer moves (due to the airflow sensing flap
moving), the voltage on the wiper contact will change,
and this changing signal is transmitted from terminal 7
of the airflow sensor to terminal 7 of the ECU. Airflow
sensor terminal 5 is the earth connection for the
Auxiliary air valve
The valve is supplied with the battery voltage from relay
terminal 87 while the engine is starting and running;
this will cause the heating element in the valve
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Petrol injection system examples (multi-point injection)
assembly to become hot and remain hot while the
engine is running, which causes the air valve to close
(so there is no fast idle).
The ECU is also supplied with the battery voltage from
relay terminal 87. A speed signal is provided at terminal
1 (to enable the ECU to calculate the fuelling
requirements). The cranking or start signal is supplied
to terminal 4; the ECU can then provide additional
injection pulses or lengthen the duration of the
injection control signal (both of these will allow
additional fuel to be injected for starting).
Signals from the temperature and airflow sensors
are transmitted to terminals 7, 8 and 10, with the
throttle switch signals passing to terminals 2 and 3. The
injector control signal is provided at terminal 12, with
terminals 5 and 13 being earth connections for the ECU.
3.2.2 Example 2: Multi-point system
with added functionality
Note: This section should be studied in conjunction
with section 3.2.1. Note also that the fuel delivery
system on the Bosch M1.5 is identical to the example
covered in section 3.1.5 and illustrated in Figure 3.20. It
is therefore not described again in this section.
Figure 3.32 Bosch Motronic M1.5 system
System components and layout
The system featured in this second example is again
made by Bosch, but is a later system than the previously
covered LE2 system. The system is referred to as M1.5
(Figure 3.32), and features a number of improvements
and changes, as well as added functionality and
capability. In Bosch terminology, the ‘M’ tends to refer to
‘Motronic’, the Bosch term that is generally applied to
an engine management system. The M1.5 system
combines the ignition and fuel injection functions as
well as some other functions, which include control of
the idle speed via an ECU controlled air valve.
Although this section does not deal specifically with
engine management, the M1.5 system provides an
insight into later fuel system developments as well as
into early engine management systems. Not all of the
functions and components of the M1.5 system are dealt
with in this section: some are covered in greater detail
in the emissions section and in the engine management
Sensors and sensor reference voltage
Many of the sensors used on the M1.5 system are
developments of, or the same as, those used on the LE2
system (section 3.2.1). There are however some
additional sensors.
One major change to the system is that the reference
voltages used for the sensors are generally stabilised at
5 volts (as opposed to battery voltage). This is because
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Key Points
the M1.5 system uses digital electronics to a much
greater extent than previous systems and a 5 volt circuit
is more suitable for use with electronic components.
Standard multipoint gasoline injection systems use
solenoid type injectors mounted in the inlet port
spraying directly at the back of the inlet valve
The quantity of fuel delivered is a function of
injector opening duration as the pressure
differential across the injector is kept constant.
This is achieved via the fuel pressure regulator
which takes into account manifold pressure
Figure 3.33 Wiring diagram for Bosch M1.5 injection system
Fundamentals of Motor Vehicle Technology: Book 2
Airflow sensor with combined air temperature
The airflow sensor and the air temperature sensor
operate in much the same way as the sensor on the LE2
system (section 3.2.1). However, one major change is in
the idle mixture adjustment or CO (carbon monoxide)
adjustment. Although the task remains the same, the
adjuster on the M1.5 airflow sensor is a potentiometer
instead of an air bypass adjuster. Since the reference
voltage to the sensor is 5 volts, the output signal
voltages during normal operation will typically be
between 0.25 and 4.75 volts.
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The CO adjusting screw is connected to a small
potentiometer, so when the screw is adjusted it alters
the voltage at the potentiometer wipe connection. The
voltage across the potentiometer is applied to the ECU.
When the adjustment is made, the voltage changes and
the ECU alters the injected fuel quantity, which in turn
alters the mixture/CO setting.
In some vehicle applications, the system uses an
oxygen sensor (lambda sensor) to control the mixture,
so the CO adjuster is not used. Oxygen/lambda sensors
are covered in the section 3.5.
Coolant temperature sensor
The coolant temperature sensor is a negative
temperature coefficient (NTC) sensor, which operates in
exactly the same way as the version used on the LE2
system. Note that the reference voltage to the sensor is
5 volts.
Figure 3.34 shows the typical resistance values for
the sensor and typical voltages in the sensor circuit at
different coolant temperatures. Although the values
quoted are typical for many systems, some systems may
have sensor resistances and voltages that differ; always
refer to the appropriate specifications when testing.
Figure 3.34 Temperature against resistance and voltage for a
coolant temperature sensor
Temperature, °C
Resistance (ohms)
Signal voltage
Throttle position sensor
On the M1.5 system, the throttle position sensor (Figure
3.35) is a potentiometer instead of a switch. A 5 volt
reference is applied to the potentiometer. When the
throttle is opened and closed, the potentiometer wiper
(which is connected to the throttle butterfly shaft),
moves across the resistance track, thus providing a
change in voltage corresponding to the position of the
throttle. The output signal is transmitted to the ECU to
enable it to assess the angle of throttle opening.
When the throttle is closed (at idle) the voltage from
the sensor potentiometer should be at the specified
value, which is typically between 0.3 and 0.9 volts.
When the throttle is opened, the voltage rises and, at
full throttle, the voltage will be in the region of 4 to
4.5 volts.
Trigger/timing reference (engine speed sensor)
The M1.5 system is a combined injection and ignition
system, and a single sensor is used to provide the ECU
with crankshaft speed and angular position
information. The crankshaft speed/position sensor is an
inductive or variable reluctance sensor.
Figure 3.35 Throttle position potentiometer: Bosch Motronic
M1.5 system
See sections 1.5.2 and 2.2.3 for additional information
on variable reluctance sensors used to indicate
crankshaft speed and position.
On the M1.5 system there is a trigger or reluctor disc
on the crankshaft (different positions on the crankshaft
are used for different engine applications). The disc has
60 reference points or trigger teeth, although one tooth
is missing, which functions as the master reference.
Figure 3.36 shows the sensor and reluctor disc.
The sensor is constructed with a permanent
magnet and a winding. It is located next to the
reluctor disc and as each tooth passes the sensor, it
induces a small current into the winding. So, when the
crankshaft is rotating, the sensor will produce an
electrical pulse or signal as each tooth passes the
sensor. The missing tooth will create a slightly
Figure 3.36 Crankshaft speed/position sensor: Bosch Motronic
M1.5 system
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Fundamentals of Motor Vehicle Technology: Book 2
Crankshaft sensor engine idling
different pulse shape, which the ECU will use as the
master reference. Figure 3.37 shows part of the AC
analogue signal that would be seen when the sensor is
connected to an oscilloscope.
The ECU uses the master reference signal to
establish a master position for the crankshaft, e.g. TDC
for cylinders 1 and 4. This can be used as a trigger
reference for operating the injectors and as a master
reference for the ignition timing. The M1.5 system is a
simultaneous injection system, i.e. all injectors open
and close together. Also note that the ignition system
has a single coil for all cylinders and a rotor
arm/distributor cap (connected to the end of the
camshaft) to distribute the HT voltage to the
appropriate spark plugs.
The additional reference points on the reluctor disc
provide the ECU with angular rotation information for
the crankshaft: the ECU can determine crankshaft
speed as each tooth passes the sensor (each reference
tooth represents six degrees of crankshaft rotation).
Figure 3.37 Signal produced by crankshaft speed/position
sensor: Bosch Motronic M1.5 system
Injectors (actuator)
The injectors operate in exactly the same way as those
on the LE2 system (section 3.2.7). However, on the
M1.5 system, the injectors are connected in groups of
two on four-cylinder engines, but are still all opened
and closed at the same time.
Idle speed control valve (actuator)
(Also see section 3.1.6). The air valve used on the M1.5
system is referred to as a ‘rotary idle valve’. The valve is
operated by a type of electric motor that has its
rotation limited by mechanical stops; the motor is
therefore able to rotate only partially. Connected to the
motor is a flap or valve that is placed in a bypass port
through which air flows around (bypasses) the throttle
butterfly to the intake system.
Figure 3.38 ECU controlled air valve controlling the airflow
through a bypass port
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The electric motor is spring loaded in one direction of
rotation, and current flowing through the motor
windings will tend to rotate the motor in the opposite
direction. By varying the current, it is possible to
rotate the motor against the spring to achieve
different angles of rotation or positioning; this allows
varying volumes of air to flow through the port
(Figure 3.38).
Control signal
The ECU provides the earth path for the idle valve
circuit. However, the earth path passes through a
power stage in the ECU, which rapidly switches on and
off the earth circuit. The result is that a digital control
signal is produced with an on/off frequency of around
100 Hz (100 times a second). The ECU alters the duty
cycle (on/off ratio) of the control signal, which alters
the average current in the circuit; this in turn alters
the position of the motor and valve (see section 1.8 for
information about altering duty cycles to control
actuators). Figure 3.39 shows the typical control
Maintaining and increasing idle speed
The idle control system can control the idle speed in
two ways. First, when the engine is at normal
operating temperature, if certain loads are applied to
the engine, such as an electrical load (headlights,
heated rear window, etc.), the additional load would
normally cause the idle speed to reduce. The ECU,
which is receiving the speed signal from the crankshaft
sensor, will immediately detect a minor drop in engine
speed, and will change the control signal so that the
valve opens slightly, thus restoring the idle speed to the
specified value. This process is effectively continuous
and ensures that any minor change in engine idle
speed is corrected.
The second process for controlling the idle speed relies
on information from other sensors. For example, when
the engine is cold, the ECU assesses the engine
temperature from the coolant temperature sensor
information and opens the idle air valve slightly to
increase the engine speed and overcome the additional
friction and drag that exist in the engine at low
Other information can also be used by the ECU to
alter the idle speed: for example, when the air
conditioning system is switched on, the load of the air
conditioning compressor would slow the engine
down, but to drive the compressor also requires
considerable power that may not be available from
the engine at the normal idle speed. The ECU
therefore opens the air valve an increased amount
which increases the idle speed. Note that the air
conditioning system is connected to the ECU so, when
the ECU receives an appropriate signal from the air
conditioning system, the ECU can implement a faster
Ignition coil (actuator)
The ignition coil is not part of the fuel system, but the
same ECU controls the fuelling and ignition systems.
The ignition module effectively forms part of the ECU,
so the ECU can use the same information from the
various sensors to calculate the ignition timing, and
then switch the ignition module, which in turn switches
the ignition coil. See section 2.3 for information on
computer controlled ignition systems.
Electrical systems and wiring (M1.5)
Figure 3.33 shows the wiring of the M1.5 system.
Although some functions of the M1.5 system are
similar to the LE2 system, there are many significant
differences, as explained below.
Figure 3.39 Control signals for rotary idle valve
ch A: Frequency(Hz) 99.78
Idle speed control valve (rotary)
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Power supply and relay
The power supply to the M1.5 system is split into two
categories: the first category is the supply to the
actuators, which operate using full battery voltage; the
second category is the reference voltage for many of the
sensors, which is usually 5 volts.
The actuator supply is therefore via the system relay,
but the reference voltage is provided by the ECU, which
has a voltage stabiliser system to reduce the battery
voltage down to a stabilised 5 volts for the sensors.
Relay operation
The relay has two sets of contacts: one set switches the
power supply to the fuel pump and Lambda sensor
heater (where fitted); the second set switches the
power supply to the system actuators.
Compared with the LE2 system previously covered,
the relay on the M1.5 system operates slightly
differently. Each of the energising windings within the
relay is earthed via the ECU; therefore, the ECU controls
when the energising windings are able to close the
The relay receives full battery voltage direct from
the battery to relay terminals 30 and 86. When the
ignition is switched on, the ECU receives battery
voltage via the ignition switch (to ECU terminal 27),
which indicates that the driver intends to start the
engine. The ECU then completes the earth path for
the relay energising windings at ECU terminals 3
and 36 (connecting to relay terminals 85b and 85).
Both of the relay contacts will then close, providing
a power supply to the fuel pump (relay terminal
87b) and to the rest of the actuators (relay terminal
87). The fuel pump will run briefly to ensure that
there is fuel pressure.
If the engine is then not started, the ECU will switch
off the earth path to relay terminal 85b, thus
causing the pump contacts to open and switch off
the fuel pump.
When the engine is cranked over for starting, the
crankshaft position sensor will provide a signal to
the ECU, which will now have an indication that the
engine is being started; the ECU will then reconnect
the earth path for the energising winding (at relay
terminal 85b), thus causing the fuel pump contacts
to close again and provide power to the fuel pump.
The relay will continue to provide power supplies to
all components so long as the ECU is receiving the
ignition ‘on’ voltage and a signal from the crankshaft
position sensor.
If the engine were to stop, the signal from the
crankshaft position sensor would disappear and the
ECU would switch off the earth paths for the relay
energising windings. The relay contacts would then
open, causing all actuators to switch off.
All injectors will receive battery voltage from relay
terminal 87 during starting and engine running. Note
Fundamentals of Motor Vehicle Technology: Book 2
that the injectors are then connected to the ECU at
terminals 16 and 17; these are the earth paths for the
injectors. From terminals 17 and 18 the circuit passes
through the power stages of the ECU to earth.
Therefore, when the ECU switches on the injectors, the
power stages will complete the earth circuit for the
injectors. Although there are two groups of injectors, for
this application the injectors are still switched at the
same time.
Idle speed control valve
The idle control valve receives power from relay
terminal 87. The earth path for the valve is via ECU
terminal 4; this is the circuit within the ECU that
connects to the power stage and therefore provides the
control signal.
Ignition coil
The ignition coil receives a power supply from the
ignition switch, and the coil is switched to earth via
ECU terminal 1.
Fuel pump
The fuel pump receives power from relay terminal 87b
while the engine is starting and running.
Other actuators
There are some other actuators fitted, including a
Lambda sensor heater and an EVAP canister purge
valve. Although not all applications of M1.5 had these
components, they are emissions control components
fitted to many vehicles and are therefore covered in
section 3.5.
Coolant temperature sensor
The coolant temperature sensor is connected to the ECU
at terminals 45 and 26. Terminal 26 is an earth path
that is shared with other components. The reference
voltage (5 volts) is applied to the sensor from terminal
45. Because the sensor is part of a series resistance
circuit, the voltage at terminal 45 will then reduce,
depending on the temperature, and therefore also the
resistance value at the sensor.
Airflow sensor
The airflow sensor receives a 5 volt supply at terminal 3
from ECU terminal 12. The voltage is applied across the
airflow sensor potentiometer and when the wiper on
the potentiometer moves (as the airflow sensing flap
moves), the voltage on the wiper contact will change.
This changing voltage level is transmitted from terminal
2 of the airflow sensor to terminal 7 of the ECU.
The supply voltage is also applied to the CO
adjustment potentiometer (within the airflow sensor).
The wiper position on the CO potentiometer resistance
track depends on the adjuster screw position, and
therefore the voltage at the wiper also depends on the
adjuster screw position. However, the voltage at the
wiper is applied back to the ECU from airflow sensor
terminal 1 to ECU terminal 43. The voltage at these
terminals is used by the ECU to adjust the fuelling at
idle speed.
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The air temperature sensor (located within the airflow
sensor) operates in the same way as the coolant sensor
described above. The air temperature sensor has a
separate 5 volt supply (which is the reference voltage) at
sensor terminal 5. As with the coolant sensor, when the
air temperature sensor is connected in the circuit, the
resistance of the sensor alters the voltage in the circuit.
Therefore the voltage at airflow sensor terminal 5 or at
ECU terminal 44 depends on the air temperature.
All sensing elements within the airflow sensor
assembly connect through to earth via sensor terminal 4
to ECU terminal 26.
Throttle position sensor
The throttle position sensor or potentiometer receives a
5 volt supply to the potentiometer resistance at sensor
terminal 2 (supplied from ECU terminal 12). The earth
for the potentiometer resistance is via sensor terminal 1
to ECU terminal 26. The potentiometer wiper
connection passes to sensor terminal 3 and to the ECU
at terminal 53. Therefore, when the throttle is opened
and closed, the voltage at sensor terminal 3 and ECU
terminal 53 will increase and decrease, thus providing
an indication of throttle angle to the ECU.
Crankshaft position sensor
The crankshaft position sensor (or engine speed sensor)
is an inductive sensor that produces its own signal. The
two main connections from sensor terminals 1 and 2
connect to ECU terminals 48 and 49. These two
connections provide a complete circuit for the sensor
winding. The signal is transmitted via terminal 1 of the
sensor to ECU terminal 48; the other connection is
therefore the return or earth path.
Note that a third connection to sensor terminal 3
connects to ECU terminal 19 and to earth; this circuit
forms a screen or shield around the sensor wiring to
shield out other electrical interference.
The following list indicates the function of each
connection at the ECU terminals. Note that not all
terminals are used.
Terminal 1 Switched earth path for the ignition coil
Terminal 2 Earth connection
Terminal 3 Switched earth path for fuel pump relay
energising winding
Terminal 4 Switched earth path for the idle speed
control valve
Terminal 5 Switched earth path for the EVAP canister
purge valve (covered in section 3.5.1)
Terminal 6 Connection to automatic transmission
Terminal 7 Airflow sensor signal
Terminal 9 Signal from vehicle speed sensor
Terminal 10 Earth connection
Terminal 12 5 volt supply to airflow sensor and
throttle position sensor
Terminal 13 Connection to diagnostic plug (covered in
section 3.7.3)
Terminal 14 Earth connection
Terminal 16 Switched earth path for a group of
Terminal 17 Switched earth path for a group of
Terminal 19 Earth connection
Terminal 20 Earth connection (only connected if the
engine does not have a catalytic
converter, this connection effectively
‘programs’ the ECU to control fuelling
and ignition applicable to a vehicle
without a catalytic converter)
Terminal 21 Earth connection (only connected if the
vehicle has automatic transmission, this
effectively ‘programs’ the ECU to perform
certain functions differently)
Terminal 22 Connection to dashboard warning light
(illuminates the light if there is an engine
management system fault)
Terminal 24 Earth connection
Terminal 26 Earth circuit for various sensors
Terminal 27 Ignition on supply from ignition switch
Terminal 28 Signal from lambda/oxygen sensor
(covered in section 3.5.7)
Terminal 32 Signal to trip computer (to enable the
trip computer to calculate fuel
consumption, etc.)
Terminal 34 Connection to automatic transmission
Terminal 36 Switched earth path for relay energising
winding (to close main contacts)
Terminal 37 Battery voltage power supply from relay
main contacts
Terminal 40 Connection to air conditioning system
Terminal 41 Connection to air conditioning system
Terminal 43 Signal from CO adjuster (in airflow
Terminal 44 Air temperature sensor signal
Terminal 45 Coolant temperature sensor signal
Terminal 46 Connection to octane adjust plug
(connector plugs with different resistance
values are connected across this circuit;
this indicates to the ECU the octane grade
of fuel being used; the ignition timing
and fuelling may alter depending on
which octane plug is used)
Terminal 47 Earth connection (used on specific
applications if the vehicle has four-wheel
Terminal 48 Connection to crankshaft speed/position
Terminal 49 Connection to crankshaft speed/position
Terminal 51 Connection to automatic transmission
Terminal 53 Signal from throttle position sensor
Terminal 55 Connection to diagnostic plug (covered in
section 3.7.3).
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3.2.3 Example 3: Multi-point systems
with hotwire air mass sensors
This section should be studied in conjunction with
Example 2 (section 3.2.2).
System components
The major difference between the system detailed in
Example 2 and the examples in this section is the use of
a hotwire air mass sensor instead of a vane or flap
airflow sensor. The rest of the system is similar to, or the
same as, Example 2 and is therefore not repeated.
manufacturers and applications in each vehicle, so
additional wiring diagrams are not included in this
section, apart from the wiring for hotwire sensors.
However, the wiring of many systems will be similar to
that for the Bosch Motronic M1.5 system shown in
Figure 3.33, with the obvious exception that terminal
numbers at the ECU, sensors, actuators and relays, etc.
will be different. Most injection systems are designed to
perform the same basic tasks, so the components and
wiring requirements are generally similar.
Hotwire air mass sensor (see also section 1.5.5)
Hotwire air mass sensors perform a similar task to vane
or flap mechanical airflow sensors, except that air mass
sensors measure the mass of air as opposed to the
volume of air. Additionally, air mass sensors do not use
mechanical means of measurement, but rely totally on
electronic measurement of the air mass. The mass of air
measured changes with density and temperature (both
of which change with altitude). The mass of a given
volume of air therefore varies, and the volume of fuel
Figure 3.40 Example of hotwire mass airflow sensor
a Hotwire air mass meter
b Wiring for hotwire air mass sensor
Fundamentals of Motor Vehicle Technology: Book 2
provided should be dependent on the mass of air rather
than its volume; this means that mechanical airflow
sensors (which measure the volume of air) do not
provide sufficient information to the ECU. Any change
in altitude, or any other factor that affects the air
density, is not accounted for by mechanical airflow
Sensor operation
Hotwire air mass sensors use the cooling effect of the air
flowing through the intake system; the greater the mass
or density of that air, the greater the cooling effect.
Cooling changes the resistance of a heated wire
element; this resistance change is used to produce the
output signal from the sensor. A temperature sensing
element is also used.
Hotwire air mass sensors generally provide an
analogue signal, where the voltage rises and falls with
the change in air mass (increase and decrease of air
drawn into the engine). Some air mass sensors provide
a digital signal.
Some designs of hotwire air mass sensor use an
additional heating process to burn off contamination
that could build up on the sensing wire; this ‘burn off’
function is implemented after the engine is switched off
and lasts for a short period of around one second.
Figure 3.40 shows the typical appearance of a
hotwire air mass sensor and the sensing element, as
well as a wiring diagram for a sensor.
Typical wiring connections for this sensor would be:
a battery voltage supply connection via the injection
a battery voltage supply connection from the ECU
when burn off sequence is required
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Petrol injection system examples (multi-point injection)
an output voltage terminal giving mass air flow as
an analogue voltage
earth connection for burn off
sensor earth connection
spare terminal (used for CO adjustment when
Hot film air mass sensors
A development of the hotwire sensor is the ‘hot film’
sensor. Instead of using a heated wire, the hot film type
has a thin metallic film sensing element. The operation
and general appearance are similar to the hotwire
system, although the construction eliminates the need
for the ‘burn off’ process.
3.2.4 Example 4: Multi-point systems
with map sensors
This section should be studied in conjunction with
Example 2 (section 3.2.2).
System components and layout
The major difference between the system detailed in
Example 2 and the examples in this section is the use of
a MAP sensor instead of an airflow sensor. The rest of
the system is similar to, or the same as, Example 2 and
is therefore not repeated.
Map sensor
See also section 1.5.4.
Principle of operation
MAP sensors (Figure 3.41) are used as an alternative to
airflow sensors to enable the ECU to calculate the mass
of air entering the engine. The MAP sensor information
is used in conjunction with the engine speed information
to enable the ECU to make the appropriate calculations.
MAP sensor systems will therefore usually have a
crankshaft speed/position sensor with a large number of
Figure 3.41 Examples of MAP sensors
a Remotely mounted MAP sensor
b MAP sensor located on intake manifold
reference teeth on the reluctor disc, to provide very
accurate engine speed information to the ECU.
The MAP sensor measures the pressure (or
depression) in the intake manifold; the pressure
depends on engine load and throttle position, as well as
engine speed. However, intake pressure can be at a
certain value for many combinations of engine speed
and throttle position. For example, the pressure or
depression at idle speed with a closed throttle can be
similar to the depression when the engine is operating
at light load with a partially open throttle. Therefore the
engine speed and throttle position information are
provided separately to the ECU, so that different
operating conditions can be allowed for.
Modern MAP sensors use a pressure sensitive
component such as a piezo crystal that changes its
electrical resistance with presssure. The pressure
sensitive components form part of an electronic circuit
which then produces an electrical signal that will vary
with any variation in pressure.
The MAP sensors are connected to the intake
manifold via a pipe, although, for many applications,
the sensor is connected directly to the intake manifold
or plenum chamber. When the intake system pressure
changes, the signal produced by the MAP sensor also
changes. MAP sensors can provide digital or analogue
Analogue signal MAP sensor
Any analogue signal MAP sensor provides a voltage that
rises and falls with changes in intake pressure. Modern
MAP sensors are provided with a 5 volt supply or
reference voltage, and the output from the sensor
therefore generally varies between approximately 0.25
to 4.75 volts (depending on the intake pressure). Refer
to section 1.6.3.
An analogue MAP sensor will typically have a low
output at low pressure (i.e. high manifold vacuum), with
increasing voltage output as manifold pressure increases.
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Engine management – petrol
terminal 2 provides the analogue signal. Note that
different sensors will have different terminal numbers.
Digital signal MAP sensors
A digital MAP sensor operates in a similar way to an
analogue sensor, but it produces a digital signal instead
of an analogue signal.
The sensor produces a simple on/off signal.
However, when the intake pressure changes, the signal
frequency changes, i.e. the number of pulses produced
in a second will increase or decrease. As an example, at
idle speed where the manifold pressure is low (high
depression), the frequency could be around 90 Hz, but
when the throttle is opened and the intake pressure
rises, the frequency could rise to approximately 150 Hz.
The sensor power supply (typically 5 volts) is a
stabilised voltage supplied by the ECU and connected
via a terminal on the sensor. Additionally an earth (0
volt) connection terminal as well as a signal output
terminal will be available.
Key Points
In the example shown in Figure 3.42, the 5 volt supply
is provided by the ECU to terminal 1 of the sensor.
Terminal 3 is the earth connection (via the ECU) and
Fundamentals of Motor Vehicle Technology: Book 2
Figure 3.42 Analogue MAP sensor
Air mass or volume flow meters measure the air
consumption of the engine directly as they are
mounted in the inlet tract
Manifold pressure sensors are used in conjunction
with the throttle position signal and engine speed
to calculate engine air consumption indirectly
3.3.1 Simplified injection system
Compromise between a carburettor and multipoint injection
Single-point injection systems have a cost advantage
over multi-point injections systems, but they have many
of the features of multi-point injection systems that
allow them to provide much better fuel delivery and
mixture control than carburettors. However, there are
several limitations with single-point systems, including
certain limitations on emissions control and in the types
of engine that can efficiently operate with single-point
injection. These limitations are explained below.
A single-point injection system is in many ways
similar to a carburettor, because the fuel enters the
engine intake system from a single point in the throttle
body (Figure 3.43). However, whereas a carburettor
relies on the creation of a lower pressure area within the
venturi to draw in fuel from a reservoir, single-point
injection makes use of a single injector that injects fuel
(under pressure) directly above the throttle butterfly
(throttle valve or plate). Although the fuel pressure for
a single-point system is not as high as for multi-point
systems, the pressure is higher than that in the intake
system. Typical injection pressures for single-point
systems are around 1 bar or slightly less.
With single-point injection, all cylinders receive fuel
from the single injector. However, because the injector
is controlled by an ECU in the same way as on a multipoint injection system, it is possible to use sensors to
provide information to the ECU; this therefore provides
better control of fuel quantity than a carburettor, but
with reduced cost compared with a multi-point
injection system.
The disadvantages of a single-point system are in fact
not dissimilar to those of a carburettor; for example,
fuel/air separation when the air and fuel mixture flows
around corners in the intake system. Additionally, fuel
can still condense against the cold manifold walls
during cold running.
Single-point injection was quite widely used on
four-cylinder engines but these systems were not
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Single-point (throttle body) petrol injection
Figure 3.43 Single-point injection system
suitable on longer engines, such as straight six-cylinder
engines, because the different intake manifold lengths
result in uneven distribution of fuel. This is the same
problem that affected many carburettor engines where
the length of the intake pipe from the carburettor or
single injector to outer cylinders was much greater than
to the central cylinders; this resulted in the outer
cylinders running more weakly than the inner cylinders.
A rich mixture was therefore provided to ensure that all
cylinders developed reasonable power and could run
reasonably efficiently. However, the central cylinders
then operated with a slightly rich mixture, which causes
high emissions. Single-point injection is therefore
suitable for vehicles with smaller engines, although
some V8 engines were fitted with a single-point system;
this was possible because the location of the injector
within the centre of the V resulted in similar intake pipe
lengths to all cylinders.
One other major disadvantage relates to emissions
control and emissions control regulations. It is now
necessary on modern systems to stop delivery of fuel
to a cylinder if that cylinder is operating very
inefficiently. If the spark at the plug were very
inefficient or failed completely, unburned fuel would
flow through the cylinder and into the atmosphere as
pollution. Modern multi-point injection systems, can
detect which cylinder is operating inefficiently and
switch off the injector to that cylinder. This is not
possible on single-point systems where the injector
supplies fuel to all cylinders.
3.3.2 Operation of a single-point
injection system
Injector (actuator)
The injector (Figure 3.44) operates in much the same
way as an injector for a multi-point system. The
injector is a solenoid that, when energised, causes the
needle to lift off the seat (the typical needle lift is
approximately 0.06 mm). A control signal from the
ECU opens and closes the injector for a calculated
period of time (typically 1.25 ms to 8 ms, depending
on operating conditions). It is usual for the injector to
be opened at every ignition spark; i.e. on a fourcylinder engine, the injector would be opened each
time a spark occurred, which equates to four times for
every engine cycle.
Idle speed control (actuator)
As with multi-point injection systems, some form of
automated idle speed control is provided. A common
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method is to use a stepper motor (see section 3.1.6),
which acts on the throttle butterfly via some form of
linkage. The ECU controls the stepper motor to either
maintain or increase the idle speed for cold running or
when load is applied to the engine at idle.
Alternatively, the stepper motor can control a valve,
which alters the aperture in a bypass port. The port
allows air to bypass the throttle butterfly; therefore,
when the valve allows more air to flow through the
port, the idle speed increases. Bypass port systems are
covered in section 3.1.6.
The main information required for a single-point
injection system to calculate the required fuel quantity
is engine speed and throttle position (throttle opening
angle). These two signals provide sufficient information
for the ECU to calculate the required quantity of fuel to
suit the engine load. In effect, the ECU has an indication
of ‘air charge’ per cylinder from the engine speed and
throttle opening signals. Some systems have a MAP
sensor to provide additional information relating to
engine load.
Ignition trigger or speed signal
On earlier systems, a speed signal was received direct
from the ignition system (ignition coil or ignition
module). Later when injection and ignition were
combined, a signal was provided by a crankshaft
speed/position sensor.
Throttle position
A throttle position sensor (usually a potentiometer)
provides information relating to throttle angle opening
and the rate at which the throttle is being opened or
Figure 3.44 Injector for a single-point system
Fundamentals of Motor Vehicle Technology: Book 2
closed. The throttle position sensor operates in the
same way as those previously described for multi-point
injection systems (sections 3.1 and 3.2). It was,
however, common practice to use a set of contacts in
the throttle sensor to indicate the closed or idle throttle
Air temperature
An air temperature sensor is located in the throttle body
(Figure 3.43). Because air density changes with
temperature, the information from the sensor assists in
calculating the required fuel quantity to match the air
density. An air temperature sensor operates in an
identical way to air temperature sensors previously
covered under multi-point systems (sections 3.1 and
Coolant temperature
The operation and function of coolant temperature
sensors is the same as for multi-point injection systems
(sections 3.1 and 3.2). As with all fuelling systems,
enrichment (excess fuel) is needed during cold running,
and minor fuelling adjustments can be made for minor
changes in engine temperature: the coolant
temperature sensor provides the relevant information.
Other sensors
Figure 3.43 shows a lambda (oxygen) sensor and other
components that are applicable to emissions control.
These components are covered in section 3.5.
Fuel system
The fuel system of a single-point injection system is
similar to that of a multi-point system (Figure 3.43). A
fuel pump filter and regulator assembly are used, which
operate in much the same way as on a multi-point
system (see section 3.1.5). However there are two
major differences between single-point and multi-point
fuel systems. First, single-point systems operate at
lower fuel pressures, typically 1 bar.
The second difference is that, because the fuel is
injected ahead of (or upstream) of the throttle butterfly,
the fuel is injected into a pressure zone that does not
change significantly with throttle opening. In section
3.1.5 it was explained that, because a multi-point
injector injects fuel into the intake port, the injection
pressure is regulated so that it is always at a constant
pressure ‘above the pressure in the intake port’. The
pressure regulator is therefore connected to the intake
system pressure so that the regulator can ‘sense’ intake
system pressure.
On a single-point injector, the fuel is injected into a
relatively constant pressure zone above the throttle
butterfly (which is at atmospheric pressure) and
therefore the injection pressure does not need to be
altered when the intake pressure changes. The pressure
regulator therefore has no connection to the intake
The fuel supply system and pressure regulator are
shown in Figure 3.45.
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Direct petrol injection
Figure 3.45 Fuel system for a single-point system
Manifold heating
If atomised petrol condenses on cold surfaces, problems
can occur when the air:fuel mixture flows through the
intake manifold when the engine is cold. On some
applications, therefore, an electric heater is located at
the base of the intake manifold (Figure 3.46) to help
prevent the petrol from condensing.
The heater is switched on when the ignition is
initially switched on and during starting; the heater can
remain switched on for a number of minutes after
starting. During cold running, therefore, the air:fuel
mixture flowing from the throttle body is heated, which
helps to ensure that the fuel remains atomised.
Figure 3.46 Manifold heater on a single-point injection system
This section covers the basic principles of direct petrol
injection (also called gasoline direct injection or GDI).
Direct injection systems help to achieve overall
combustion efficiencies by operating in conjunction
with special combustion chamber designs and with
electronic throttle control. In addition, emissions
control for direct petrol injection systems is slightly
different from that for engines with multi-point port
injection. For these reasons, additional information
about direct petrol injection is provided in the
emissions section (section 3.5).
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3.4.1 Benefits of direct injection
Direct injection into the cylinder
Section 3.1.1 mentions older generations of petrol
injection systems which used injection pumps to
deliver petrol to injectors that were directly injecting
into the cylinders. This type of injection was used
with considerable success on aircraft engines during
the 1930s and 1940s, but the requirements for
injection on aircraft engines were slightly different
from those for the modern automobile. However,
while diesel engines have relied on direct injection
through almost the whole life of this type of engine,
petrol delivery systems for automobiles were able to
be much less sophisticated (i.e. to use carburettors)
until emissions regulations forced better control of
fuel delivery.
While cost was inevitably a major factor in using
relatively inexpensive petrol delivery systems, the
technologies and materials that were available at the
time also restricted the mass production of what we
would now consider to be the ideal fuel system. Modern
electronic control and materials have enabled designers
to develop fuel injection systems that can efficiently
deliver fuel direct to the cylinder, rather than to the
intake system.
Figure 3.47 compares multi-point port type
injection, single-point (throttle body) injection and
direct injection systems.
It is claimed that direct injection, when compared
with an equivalent engine with port injection, provides
a decrease in fuel consumption in the region of 15% to
20%, while engine power is slightly improved. The
details within this section provide an understanding of
how these benefits are achieved.
One other benefit is that direct injection systems
require very rapid vaporisation of the petrol to enable
it to mix quickly with the air. This rapid vaporisation is
achieved through the use of high fuel pressures and a
special injector nozzle design. Importantly, when a
liquid vaporises, it has the effect of drawing heat from
the surrounding air, i.e. it cools the surrounding air.
Therefore, when fuel is injected into the cylinder, the
vaporisation process reduces the temperature of the
air in the cylinder, reducing the potential for
combustion knock (which can occur if temperatures
are too high).
This reduced tendency for combustion knock
enables higher compression ratios of around 12:1 to be
used (which would otherwise raise cylinder
temperatures and cause combustion knock). Thus
combustion efficiency is improved, giving more power
as well as improved fuel consumption and emissions. In
addition, the cooling effect on the air in the cylinder
causes the air to become denser; the greater the air
density or mass within the cylinder, the greater the
power produced.
Fundamentals of Motor Vehicle Technology: Book 2
1 Fuel
2 Air
3 Throttle valve
4 Intake manifold
5 Injector
6 Engine
Figure 3.47 Comparison of different types of petrol injection
a Multi-point fuel injection (MPI)
b Throttle body fuel injection (TBI)
c Direct fuel injection (DI)
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Direct petrol injection
Key Points
Mixture formation
Until very recently, the vast majority of petrol engines
operated with the air and petrol mixed outside the
cylinder (e.g. port type injection); the air and petrol
mixture was then drawn into the cylinder during the
intake stroke. Although mixing can continue after the
air and petrol are inside the cylinder, the initial mixing
process starts in the intake manifold (carburettors and
single-point injection) or in the intake ports (port
injection). With direct injection, the air is still drawn
into the cylinder in the conventional manner, but the
petrol is injected directly into the cylinder, so mixing
occurs only within the cylinder.
Gasoline direct injection engines inject fuel directly
into the combustion chamber, in a similar way to a
diesel engine
These engines have two distinct operating modes
for combustion: one is similar to a standard
gasoline engine where increased power is needed;
the other is similar to diesel engine where
economy is most important (e.g. part-load)
One main advantage of mixing the air and petrol in the
cylinder is that different mixture formation processes
can be achieved using different injection timing.
Essentially, there are two types of mixture formation
used with direct injection systems: ‘homogenous’ and
Homogenous mixture formation
A homogenous mixture is one where the fuel mixes with
the air in such a way that the mix is uniform or
unvarying throughout the whole volume of air/petrol
mix (Figure 3.48a). This means that the whole volume
of mixture will have the same air:fuel ratio (no weak or
rich pockets of mixture). Therefore, when ignition
occurs, all of the mixture will ignite and burn (combust)
with equal efficiency and the flame created by initial
combustion will therefore spread through the whole
mixture (flame prorogation).
In general, a homogenous mixture will operate at or
around the stoichiometric air:fuel ratio of 14.7 parts of
air to 1 part of petrol (by weight). This is the theoretical
ideal ratio which will also provide low emissions of
most pollutants. It is possible to operate with weak
mixtures of up to 20:1 (or slightly higher) before
misfiring occurs. These weaker mixtures provide good
economy and low emissions of most pollutants. In
practice, maximum torque and power are usually
achieved with slightly richer air:fuel ratios of around
12:1 but with higher emissions of some pollutants.
Since the early 1990s in Europe (earlier in the USA),
emissions regulations have resulted in engines
operating with air:fuel ratios that are generally close to
the stoichiometric value for most operating conditions.
This allowed catalytic converters to convert most of the
pollutants into harmless gases (refer to emissions in
section 3.5). Operating at stoichiometric air:fuel ratios
Figure 3.48 Homogenous and stratified mixture formation
a Homogenous mixture formation
b Stratified mixture formation
throughout the mixture effectively means that the
mixture should be homogenous under all engine
operating conditions.
Stratified mixture formation
With stratified mixture formation, a small isolated
pocket or cloud of air:fuel mixture is created within the
cylinder; the remainder of the air is effectively pure
(Figure 3.48b). In reality, it is possible to have a pocket
of mixture with a stoichiometric air:fuel ratio (which
therefore burns normally), while the remaining air is
either completely free of any petrol or has a very small
amount of petrol mixed in, i.e. it is very weak.
The small pocket of mixture is directed by the
airflow within the combustion chamber so that it is
directly exposed to the spark plug. When the spark
occurs, therefore, it is only this pocket or cloud of
mixture that ignites and combusts. The combustion of
this isolated cloud of mixture is used to heat up all of
the remaining air, thus producing expansion of the gas
within the cylinder. If the remaining ‘fresh air’ does in
fact contain a small quantity of petrol (forming a very
weak mixture), it will combust slowly, which will in fact
assist in the expansion of the gases.
It should, however, be noted that a stratified
mixture formation will not produce as much energy or
force within the cylinder as a fully homogenous mix of
air and petrol, because only a small percentage of the
full charge of air in the cylinder is used to generate the
heat. With homogenous mixtures, the full charge of air
is mixed with petrol, and therefore all of the mix
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Engine management – petrol
combusts. It is also possible to alter the air:fuel ratio for
the small pocket of mixture so that this pocket also
operates on a weaker mixture, but the mixture must be
rich enough to achieve good combustion.
The important point to remember is that, although
the small localised pocket of mixture has an air:fuel
ratio that is rich enough to achieve combustion, the
overall mixture within the cylinder has an excess of air
because of the large volume of pure air in the rest of the
cylinder. It is in fact possible to achieve a total air:fuel
ratio of up to 40:1 (the total quantity of air compared
with the total quantity of petrol).
The obvious advantage of stratified mixture
formation is that the amount of fuel required is much
smaller than for homogenous mixtures and therefore
fuel consumption is much lower. However, a stratified
mixture formation cannot produce the same power as a
homogenous mixture, which means that stratified
mixes can be used for light engine load operation (idle
speed and light load cruising).
One disadvantage of stratified mixture formation is
that, at higher engine speeds, excessive turbulence is
created, which does not allow the formation of the
cloud or pocket of gas to localise around the spark plug
tip. This results in poor combustion. Additionally, if an
increase in power or torque is required, the air:fuel ratio
provided to the pocket of mixture must be richer. This
can lead to very small, but very rich, zones of mixture
(within the cloud) which can result in soot being
Stratified mixture formation is therefore ideal for
light load conditions and lower engine speeds, but,
when engine speeds increase above mid-range
(typically around 3000 rev/min) or increased engine
torque and power are required, the engine must operate
with a homogenous mixture.
Injection timing
Most direct injection petrol engines operate with
stratified and homogenous mixture formations
depending on operating conditions. This is achieved by
controlling the injection timing. Direct injection systems
generally have two distinct timing periods, which
provide different characteristics for mixing the air and
fuel. One timing period is during the induction (intake)
stroke; the other is at the end of the compression stroke.
Intake stroke injection timing
When petrol is injected during the intake stroke (while
the air is being drawn into the cylinder), the fuel will
mix with all of the air in the cylinder, resulting in
complete mixing or homogenous mixture formation.
Note that the intake ports can be designed to create
swirl or controlled turbulence of the air entering the
cylinder, which assists in mixing the petrol with the air.
The mixture is typically at or close to the stoichiometric
air:fuel ratio, thus enabling good power to be produced
with reasonably low emission of pollutants. The high
fuel injection pressures used and the design of the
injector nozzle create good atomisation of the petrol,
Fundamentals of Motor Vehicle Technology: Book 2
improving the mixing process, which continues during
the intake and compression strokes. Figure 3.48 shows
the injection of fuel during the intake stroke.
Compression stroke injection timing
A relatively small amount of petrol is injected at the end
of the compression stroke, just prior to ignition (Figure
3.48). The design of the combustion chamber includes
an area (usually in the top of the piston crown) which
promotes swirl or turbulence in a small, localised region.
This allows the injected fuel to mix with a small pocket
of air, forming a small pocket or cloud of mixed air and
petrol. The small pocket of mixture is then directed to
the spark plug tip, ensuring ignition of the mixture.
To create the small localised pocket of air:fuel
mixture requires special piston and combustion chamber
design. In addition, the location of the spark plug and
injector in the cylinder are critical. One specific design
features an additional flap in the intake tract (known as
a charge motion valve). This is used in conjunction with
a specially shaped piston crown and inlet manifold
design to provide the required gas behaviour in stratified
operation mode. This characteristic behaviour is known
as ‘tumble’. The flap valve is actuated electronically via a
stepper motor and is controlled by the ECU. The angle of
this valve reduces the cross sectional area of the inlet
manifold, thus increasing gas velocity and tumble
imparted to the incoming air charge during stratified
operation. During homogeneous operation this valve is
fully open and has no effect.
Using both timing periods
Direct injection systems in petrol engines generally use
both timing periods (intake stroke and compression
stroke timing) independently, depending on the
operating conditions. For light load driving and at idle,
compression stroke injection timing means that very
lean mixtures can be used (stratified mixture
formation), which provides low power but good
economy. When higher engine power is required or
when the engine is operating at higher speeds, the
injection timing changes to the intake stroke, providing
a full charge of mixed air and petrol to the cylinder
(homogenous mixture formation).
Because the injector timing is entirely controlled by
the ECU, it is possible to time the injection to any point
in the engine operating cycle. The exact time of
injection during the intake stroke period and the
compression stroke period can therefore be adjusted to
suit the exact operating conditions, such as speed,
temperature, etc.
There are also certain conditions under which
injection takes place on both the intake and the
compression strokes. A small quantity of fuel is delivered
on the intake stroke, which produces a homogenous but
weak mixture. Injection occurs again on the compression
stroke to produce a normal stratified charge (which will
have an air:fuel ratio that is close to stoichiometric).
With this dual injection process, the stratified charge
ignites and combusts normally which then creates
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Direct petrol injection
combustion in the rest of the air (which has a weak
homogenous mixture). This process produces more
power or torque than when the system is operating with
only a stratified mixture formation. This is used when
the system is changing from stratified to homogenous
operation to provide a smooth transition.
Throttle control and mixture control to regulate
power and torque
While direct injection has a number of advantages that
on their own help to improve engine efficiency and
reduce emissions, it is the fact that direct injection
allows ‘stratified mixture formation’ to be used that
provides the greatest benefit. With direct injection,
there are effectively two types of mixture formation and
combustion process that can be used at different times
(see injection timing above). However, getting the full
benefit of both these processes, especially the stratified
mixture formation, requires additional changes to
engine design and engine control.
Filling the cylinder with air
Ideally, a cylinder should be fully charged with air at the
end of the intake stroke (the largest possible volume of
air), causing higher pressures at the end of the
compression stroke. When combustion occurs, the heat
produced causes the air to expand, but when a higher
volume of air is compressed into the small combustion
chamber, the expansion will be greater.
Ideally therefore there should be no restrictions that
could prevent the cylinder from filling with air during
the intake stroke. Unfortunately, petrol engines have
traditionally had a throttle butterfly to regulate airflow
into the cylinder as a means of controlling engine
torque and power: when the engine is operating at light
loads the throttle is almost fully closed, restricting the
airflow into the cylinder. The cylinder is therefore only
partially filled with air, resulting in low efficiency (low
volumetric efficiency). Additionally, power is wasted by
the pumping action of the piston on the intake stroke,
which is trying to draw air through the restriction.
To avoid this, the throttle should remain as far open
as possible to enable improved volumetric efficiency
with subsequent improvements during the combustion
and expansion phases. This is in fact achievable with
direct injection by holding the throttle open during light
load conditions and then using an alternative means of
controlling torque and power. The throttle is electrically
operated using a stepper motor or similar device, which
is in turn controlled by the system ECU (see engine
management systems, section 3.1.6).
Controlling power by altering the air:fuel ratio
As explained earlier in this section, when an engine has
direct fuel injection, the stratified mixture formation
process is used during light load operation. It is possible
to alter the air:fuel ratio of the stratified charge (the
small pocket of mixture), which will alter the energy
produced during the combustion and expansion phases.
So, if a weak stratified charge mixture is used, less
energy will be produced compared with when the
mixture is at the ideal air:fuel ratio (or slightly richer).
Therefore if the throttle is held in the open position
(by controlling the stepper motor), a full charge of air
will fill the cylinder on each intake stroke, but the
energy produced on the power stroke will be regulated
by the air:fuel ratio in the small pocket of mixture.
Note that when the engine is required to produce
more power because the load is increasing (when
accelerating), it must operate with a homogenous
mixture formation. The stratified mixture formation
process operates with air:fuel ratios that are much too
weak to enable good torque and power to be produced.
Air:fuel ratios must be controlled within a fairly tight
tolerance for homogenous operation, so it is not
possible to use changes in air:fuel ratio to control
torque and power: the throttle must be used to control
the torque and power.
Different processes for different operating
Depending on the driving conditions, the engine
therefore operates with different processes as shown in
Figure 3.49.
3.4.2 Operation and components
Evolution from port injection systems
Direct injection systems have many similarities to the
port injection systems described earlier, so many of the
components (sensors and actuators) are identical or
Figure 3.49 Using the different operating processes for different operating conditions
Stratified or
Intake or compression
stroke injection
Power regulation:
throttle or air:fuel ratio
Light load and idle
(above mid-range engine speeds, the
system reverts to homogenous operation)
Load (torque and power)
Transition from stratified to homogenous
Cold running (warm-up phase)
air:fuel ratio
Stratified and homogenous
Intake and compression
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Engine management – petrol
very similar. In effect, direct injection is an evolution
from port injection. The main physical differences are
the fuel system pressures, fuel pumps and the location
of the injectors.
While the injectors are inevitably more robust to
cope with the harsh environment within the cylinder
(high pressure and temperatures), the principle of
operation is the same as for port type injectors.
The ECU is in principle the same as those used on
port type systems, but inevitably, different
programming is used to control the slightly different
operating processes. In this section, therefore, detailed
descriptions will be provided only for those
components, sub-systems and processes that are
significantly different from those of port injection
Injectors used in direct injection systems operate in
much the same way as for those in port injection
systems: the injector is constructed with a solenoid that
opens the injector valve by moving the needle off a seat,
thus allowing fuel to flow through the valve. The
opening time and opening duration of the solenoid are
controlled by the ECU so that the required quantity of
fuel is injected at exactly the correct time. Figure 3.50
shows an injector used on a direct injection system.
Vaporising the fuel
To provide a good mixture of air and fuel in both
stratified and homogenous mixture formations requires
finely vaporised petrol. The high pressures (typically up
to 5 bar) used by direct injection systems, in
1 Fuel inlet with fine
2 Electrical
3 Spring
4 Solenoid
5 Injector housing
6 Nozzle needle with
solenoid armature
7 Valve seat
8 Injector outlet
Figure 3.50 Direct injection high pressure injector
Fundamentals of Motor Vehicle Technology: Book 2
conjunction with the injector nozzle design, cause the
petrol to be delivered from the injector in very fine
droplets that can vaporise rapidly before they contact
the cylinder or piston surfaces, which could then cause
the fuel to return to a liquid state. The rapid
vaporisation provides much quicker mixing with the air.
However, an additional benefit is that, when
vaporisation occurs, it has a cooling effect on the air,
which lowers the potential for combustion knock.
High voltage for rapid opening
The injectors have a very limited time in which to
deliver the fuel to the cylinders. On port injection
systems, the whole of one engine cycle is available for
injecting fuel, i.e. two crankshaft revolutions (where
each cylinder can pass through the four strokes and fuel
can be injected at almost any time or all the time if
necessary). With direct injection, there is only limited
time on the induction stroke or on the compression
stroke to inject the fuel; the injectors must therefore
open as quickly as possible to maximise the time
available to be used for injecting fuel.
While the ECU produces the control signal in the
same way as on a port injection system, the control
signal is then transmitted to a driver module which is
usually separate from the ECU. The driver module
contains capacitors that are charged up while the
injector is switched off (closed). When the ‘on’ section
of the control signal is received by the driver module
(indicating the start of injection), the capacitor rapidly
discharges at between 50 and 100 volts (depending on
system design); this high voltage is discharged through
the injector circuit. This short high voltage discharge
from the capacitor causes a very rapid and strong buildup of the magnetic field in the injector solenoid
winding, which in turn causes the injector needle to
quickly lift off the nozzle seating. Once the injector is
open, current flow through the injector solenoid
winding is reduced and a ‘hold on’ voltage of around
7 volts is used to hold the injector open until it is time
to close the injector.
Injection pressure
In direct injection systems, fuel can be injected at the
end of the compression stroke, when cylinder pressures
can reach 20 bar. To obtain the required atomisation of
the fuel in this high pressure environment and to deliver
the required quantity of fuel quickly, it is necessary to
use a high fuel injection pressure. The pressure in the
fuel rail (to which all the injectors are connected), is
typically around 120 bar (see fuel delivery system later
in this section).
Throttle control
Section 3.4.1 explained that, when the engine is
operating with the stratified mixture formation process,
the throttle is held open and engine torque is controlled
using changes in the air:fuel ratio. The throttle must
therefore not be directly connected to the throttle pedal,
and is in fact controlled by the ECU, which sends
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Direct petrol injection
control signals to a motor (usually a stepper motor),
making the throttle open and close as required.
In effect, the driver selects a desired level of
performance or engine operation in the usual way by
moving the throttle pedal. The throttle pedal is
connected to a potentiometer and, as the throttle pedal
is moved, an analogue signal is sent to the ECU. The
ECU can control the opening of the throttle depending
on the driver input via the throttle pedal and on other
factors such as temperature, engine speed, etc.
However, when the system is operating using the
stratified mixture formation process, the throttle is held
open and engine power is controlled by changes in the
air:fuel ratio.
Figure 3.51 shows an electronically controlled
throttle assembly.
The sensors used for direct injection systems are
generally the same as those used for conventional
multi-point port injection systems, but an additional
sensor measures pressure in the fuel delivery rail. A
direct injection system forms part of a complete engine
management system, which also controls ignition,
emissions systems and other engine related systems, so
reference should be made to section 3.6, as well as
Chapter 1, which covers sensors used in modern fuel
injection systems.
Although the sensors used are generally the same as
those previously described for port injection systems,
there are slight differences relating to measurement of
the mass of air entering the cylinders.
Mass airflow measurement
It is important to note that, when the engine is
operating with an open throttle and engine power is
controlled by the air:fuel ratio, there is effectively little
restriction in airflow through the intake system, so
there is little reduction in pressure in the intake
manifold and ports. Remember that the low pressure or
depression in a throttled engine is caused by the
restriction of the throttle butterfly. Changes in manifold
depression are therefore not as significant as with a
throttled engine, but it is more difficult to calculate the
mass of airflow.
Figure 3.51 Electronically controlled throttle
Many direct injection systems use a hot film airflow
sensor (see sections 1.5.5 and 3.2.3), which can be used
in conjunction with an intake manifold pressure sensor
(section 1.5.4). An air temperature sensor is integrated
with the hot film sensor assembly. The combined
information from the sensors enables the ECU to make
appropriate adjustments for the mass of air entering the
Other systems use two pressure sensors: one
measures the atmospheric or ambient air pressure; the
second measures the intake manifold pressure. An air
temperature sensor is also used. The ECU uses the
information to make the appropriate air mass
The ECU uses its calculations about the mass of air
induced into the engine to provide the appropriate
signals to control the amount of fuel to be injected, and
also other functions such as ignition timing.
Fuel delivery system
The fuel delivery system has to provide fuel at much
higher pressures than on normal, port injection systems,
to enable fuel to be injected into the cylinder when
cylinder pressures are high (on the compression stroke).
Additionally, higher injection pressures help to create
better fuel atomisation and vaporisation.
Low pressure pumping system
A conventional low pressure electric pump (the same as
that on a port injection system) is used to move the
petrol from the tank to a high pressure pump. The low
pressure system operates at around 3 bar to 5 bar,
depending on the system design. A high pressure pump
is driven by the engine (usually from the camshaft) and
delivers fuel to the fuel rail at pressures up to 120 bar.
The low pressure system has a pressure regulator,
which is usually located in the fuel tank with the fuel
pump. When the fuel pressure exceeds the required
value, excess fuel is released from the regulator and is
allowed to flow back into the tank. A normal fuel filter
is also usually located in the tank. Figure 3.52 shows the
basic layout of a low pressure system which feeds fuel to
the high pressure pump.
Figure 3.52 Low and high pressure fuel pumping system for
direct petrol injection
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Engine management – petrol
High pressure pumping system
There are two main types of high pressure pumping
system, shown in Figures 3.53 and 3.54.
The example in Figure 3.53 is referred to as a
‘continuous delivery’ system. The engine driven pump
provides an excess of fuel to the fuel rail; i.e. it supplies
a greater quantity of fuel than is consumed by the
engine. The pump volume will increase with engine
speed, which will result in excessively high fuel
pressures. However, a pressure control valve (or
regulator valve) is fitted at the end of the fuel rail,
which allows excess fuel to flow back to the low
pressure system. The pressure control valve is
electrically operated (usually a solenoid type) and
controlled by the ECU, which provides a control signal
to the valve.
The ECU receives information from a pressure
sensor, which is also located on the fuel rail. In response
to the signal from the pressure sensor, the ECU controls
the regulator valve, adjusting the pressure to the
required value. The control valve also acts as a
mechanical safety valve in case fuel pressure exceeds
safe limits. The excess fuel flowing from the control
valve then flows back to the low pressure side of the
The example shown in Figure 3.54 is referred to as a
‘demand controlled’ system. The fuel pump contains a
fuel quantity control valve which regulates the flow of
fuel from the pumping element. In effect, the control
valve performs a similar function to the pressure control
valve described in the previous section, except that, in
this example, when the quantity control valve opens it
allows excess fuel to flow directly from the pump back
to the return line. In this system, it is therefore possible
Fundamentals of Motor Vehicle Technology: Book 2
to control the quantity of fuel delivered by the pump to
match engine requirements.
The ECU uses the pressure sensor signal to identify
fuel pressure and then control the ‘quantity control
valve’ to regulate pressure to the required value.
Three barrel pump
The three barrel pump shown in Figure 3.55 has three
pumping plungers which are forced to move along the
barrel through the rotation of a cam ring located on the
main pump shaft (an eccentric element). The pump is
usually mounted on the engine, with the pump shaft
driven by the engine camshaft. While the engine is
turning, the three plungers will move up the barrels due
to the action of the cam ring, and a return spring forces
the plungers to return down the barrels. This movement
of the plungers forces fuel (delivered by the low
pressure pump to the high pressure pump) to be
pumped out to the fuel rail at high volume and
pressure. A separate pressure control valve then
regulates fuel pressure.
The use of three plungers and the shape of the cam
ring ensure that the pumping action from the three
plungers overlaps; this reduces the pressure pulsations
and fluctuations produced by the individual plungers.
Single barrel pump
The single barrel pump is mounted so that a cam lobe
on the engine camshaft can act against a single plunger.
Fuel flows from the low pressure system into the high
pressure plunger, and, in the example shown in Figure
3.56, the pump uses the integral ECU controlled
quantity control valve to control the quantity of fuel
that is able to flow back to the return line. When the
valve is fully open, all of the fuel pumped by the
Figure 3.53 Continuous fuel
delivery system
1 High pressure pump
2 High pressure sensor
3 Fuel rail
4 Pressure control valve
5 High pressure fuel
6 Fuel tank with pump
module, including presupply pump
Figure 3.54 Demand controlled
fuel delivery system
1 High pressure pump
2 High pressure sensor
3 Fuel rail
4 Pressure limiter
5 High pressure fuel
6 Fuel tank with pump
module, including presupply pump
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Direct petrol injection
plunger flows back to the return line, but when the
valve is closed, all the fuel flows to the fuel rail. By
continuously moving the control valve between the
open and closed positions, the amount of fuel flowing
to the fuel rail can be controlled as required by the
A pulsation damper or pressure attenuator is used to
dampen the pressure pulses produced by the single
plunger. The damper has a diaphragm that is forced
against the fuel pressure by a spring. When pressure
pulses occur, the diaphragm moves against the spring,
creating a larger volume above the plunger which
slightly reduces the pressure. As the plunger rises and
falls, creating pressure pulses, the diaphragm also
moves to create larger and smaller volumes above the
plunger, thus damping the pressure pulses. In addition,
the fuel rail contains a large volume of fuel, which also
helps to reduce pressure fluctuations.
Figure 3.55 Three barrel high pressure pump
a Longitudinal section
b Cross-section
1 Fuel inlet (low pressure)
2 High pressure connection
to rail
3 Leakage return
4 Outlet valve
5 Inlet valve
6 Pump plunger
7 Piston seal
8 Pump barrel
9 Fuel quantity control valve
10 Fuel pressure attenuator
Figure 3.56 Single barrel high pressure pump
Eccentric element
Pump barrel
Pump plunger (hollow
piston, fuel inlet)
5 Sealing ball
6 Outlet valve
7 Inlet valve
8 High pressure
connection to rail
9 Fuel inlet (low pressure)
10 Cam ring
11 Axial seal (sleeve seal)
12 Static seal
13 Input shaft
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Engine management – petrol
Fundamentals of Motor Vehicle Technology: Book 2
Injection system pressure during starting
When the engine is being cranked for starting, there is
little or no residual pressure in the high pressure system.
Full pressure is not produced by the high pressure pump
until the engine is turning over at higher speeds. During
starting, the injectors are timed to inject during the
intake stroke when the cylinder pressure is low; it is
therefore possible to use low pressure fuel (that is
flowing out of the high pressure pump) for starting.
When the engine is turning at a sufficient speed, so the
pump is delivering high pressures, the pressure sensor
signal indicates the higher pressure value to the ECU,
which can then alter the injection timing to the
Key Points
compression stroke, provided, of course, that this is
appropriate for the operating conditions.
Injection pressure in a gasoline direct injection
engine is much higher than a port injected
gasoline engine. This pressure is monitored and
controlled by the ECU
The fuel supply system consists of a low pressure
pump to lift the fuel from the tank as well as a high
pressure pump to raise the pressure to injection
pressure levels. The high pressure pump is engine
driven; the low pressure pump is electrical
3.5.1 Lambda and the stoichiometric
air:fuel ratio
2 If there is too much air (a weak mixture) then the
excess air factor is greater than 1, which is expressed
as λ > 1 (lambda is greater than 1).
3 If there is too little air (a rich mixture) then the
excess air factor is less than 1, which is expressed as
λ < 1 (lambda is less than 1).
The excess air factor
References have been made here and in Hillier’s
Fundamentals of Motor Vehicle Technology: Book 1 to the
stoichiometric air:fuel ratio, i.e. the ratio of air and fuel
that would in theory provide complete combustion. It is
usually quoted as 14.7 parts of air to 1 part of fuel by
weight (e.g. 14.7 grams of air for 1 gram of petrol). In
reality, it is oxygen contained within the air which is
required for the combustion process. To obtain the
appropriate quantity of oxygen, it is necessary to mix
the air and petrol in the stoichiometric ratio. It is
interesting to note that the quoted ratio is 14.7:1 by
weight, but if the volumes of the two elements are
compared, the volume of air is approximately 9500
times larger than the volume of petrol. Therefore every
litre of petrol burned in an engine requires 9500 litres of
air to be drawn in (assuming the stoichiometric ratio is
There has been a tendency in recent years to refer to
the correct ratio of petrol and air as the ‘air factor’ or
more precisely the ‘excess air factor’. The stoichiometric
air:fuel ratio should provide the correct amount of
oxygen, which can be regarded as 1. The Greek symbol
lambda (λ) is used to indicate the excess air factor as
shown below:
Emissions control systems often function efficiently
only when there is the appropriate amount of oxygen in
the exhaust gas (see section 3.5.6 and other parts of
section 3.5).
Comparison of lambda and air:fuel ratios
Figure 3.57 shows a comparison between the air:fuel
ratio scale and the lambda scale. Note that the
stoichiometric value of 14.7:1 relates to lambda = 1 (λ
=1). When the air/fuel mixture is weaker, the lambda
value increases, i.e. lambda is greater than 1 (λ > 1).
When the air/fuel mixture is rich, lambda is less than 1
(λ < 1).
Lambda window
Although the ideal air:fuel ratio is 14.7:1 (λ = 1), there
is a small tolerance or window for an air:fuel ratio that
results in low emissions and good combustion. The
amount of oxygen contained within the exhaust gas is
critical to the operation of catalytic converters (section
3.5.6) and some other emission reducing devices. To
ensure the correct amount of oxygen is contained
within the exhaust (for efficient catalytic converter
operation) it is necessary to operate the engine at the
stoichiometric air:fuel ratio (i.e. λ = 1). At this ratio,
1 If the mixture is correct, then the excess air factor is
correct, which is expressed as λ = 1 (lambda = 1).
Lambda window
Air:fuel ratio
Lambda scale
(lack of oxygen)
14.3:1 14.7:1 15.14:1
Figure 3.57 Lambda (excess air factor) compared with the air:fuel ratio scale
(excess oxygen)
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the CO, HC and O2 are in a balance so the catalytic
converter can function at its most efficient and reduce
pollutant levels. However, as mentioned above, there is
a window of tolerance for the air:fuel ratio, within
which the catalytic converter can still function
efficiently; this tolerance is referred to as the lambda
The lambda window range is generally quoted at a
lambda value of 0.97 to 1.03. Figure 3.57 provides an
indication of the air:fuel ratio corresponding to the
lambda window.
If the air:fuel ratio is controlled accurately so that
the excess air factor is always within the lambda
window, the catalytic converter will operate at
optimum efficiency at all times. However, section
3.5.4 explains that under certain engine operating
conditions, it is necessary to operate with air:fuel
ratios that are outside the lambda window.
3.5.2 Pollutants and the
Worldwide problem
Environmental considerations have forced many
countries to introduce regulations to limit the pollution
caused by motor vehicles. Emissions from motor
vehicles damage human health, plant life and the
environment. Problems are particularly severe in areas
where the geographic and climatic conditions create an
atmospheric envelope which traps the pollutants.
The first countries of the world to introduce
stringent emission controls were the USA, Australia,
Japan and Sweden. The EU has for many years
enforced strict emission regulations, especially since
the early 1990s. This has resulted in very significant
changes in automotive technology and design. Each
time new emission standards are introduced, the limits
are reduced. Manufacturers have to continually update
their vehicles to meet requirements adopted in the
country in which they are to be sold.
Although most industrial nations now recognise the
problems of pollution from vehicles, and therefore
impose legislation, not all countries or regions impose
the same standards. However, the main vehicle
producing nations have to manufacture vehicles that
comply with the tougher regulations imposed in the
countries where their vehicles are sold in volume, so
emissions control systems are still reasonably effective
even for those vehicles sold into countries where
legislation is weaker.
Pollutants and the petrol engine
With vehicles powered by petrol engines, there are
specific areas of the vehicle’s operation that cause
exhaust gas – can contain unburned fuel (HC),
partially burned fuel (CO), dangerous nitrogen
oxides (NOx) from combustion, and lead (Pb) from
petrol additives (leaded fuel is no longer widely
used on most modern vehicles)
crankcase – during engine operation, emissions are
passed into the crankcase, including some
combustion gases which pass the piston, vaporised
lubrication oil (HC) and corrosive acid compounds
fuel system including the fuel tank – the petrol that
is stored in the fuel tank gives off a vapour (HC).
Devices used to ‘clean up’ vehicle pollutants are costly,
and their use has often resulted in a lower power output
and higher fuel consumption. Consequently, the trend
has been for manufacturers to fit emission control
devices only if they are required to meet local country
regulations. However, modern technologies are
changing this situation, because most emission control
technologies are not as restrictive to engine power or as
‘fuel thirsty’ as older emission control systems. Emission
control is therefore becoming more consistent across
most regions of the world.
Conflicting requirements
Emission control and emissions reduction systems are
often complex in their operation, and frequently rely on
chemical and thermal reactions to achieve the desired
results. However, there have been, and still are, many
systems or designs which reduce one or more pollutants
but which increase the emission of other pollutants.
When this happens, it is necessary to treat the increased
pollutant, so, emission control systems must work
together to reduce the levels of all the various
It is also true that some emission reduction systems
reduce power or increase fuel consumption, but this is
becoming less of a problem owing to the technologies
available, and the philosophy of ‘reducing pollution at
source’; i.e. engines are now designed to be more
efficient or ‘pollution conscious’. This is a reversal of the
trend from the early days of emission control when
existing engine designs produced high emissions levels,
and treatment of the pollution occurred after the
pollutants had been created by the engine. However,
even the latest engines with exceptional combustion
efficiencies still produce pollutants that must be
reduced by subsequent means. This method of pollutant
reduction is often referred to as ‘after treatment’.
One conflicting area of emission control that is
increasingly becoming a focus of attention is the
improved efficiency of converting the pollutants into
the so-called harmless gases. Within the exhaust gas,
the three largest components are in fact not normally
regarded as pollutants:
nitrogen (N2 ) – approximately 71.5% of the exhaust
gas (but note that nitrogen forms almost 78% of the
atmosphere, and is not regarded as a pollutant)
water (H2O) – approximately 13.1% of the exhaust
gas (again, not regarded as a pollutant)
carbon dioxide (CO2 ) – a product of complete or
efficient combustion, representing approximately
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13.7% of the exhaust gas (not regarded as a harmful
pollutant but is now of concern, as explained
The primary concern with regard to the harmless gases
is carbon dioxide (CO2), which is not a toxic or directly
harmful pollutant but which does have an influence on
global warming. Unfortunately, the more complete the
combustion of fuel, the greater the level of CO2.
Note that CO2 naturally exists in the atmosphere;
animals breathe out CO2, and it is absorbed by plants.
The problem is that the level of CO2 in the atmosphere
has increased (due to the burning and combustion of
fossil fuels), which is upsetting the natural balance of
atmospheric gases. This is added to by the destruction
of plant life (especially the rainforests). The overall
increase in CO2 is a significant factor in the greenhouse
effect which leads to global warming.
The problem is that increased efficiency of
combustion leads to an increase of CO2. Furthermore,
when some of the true pollutants are treated, they are
also converted to CO2. In effect, during combustion, the
hydrocarbons in the petrol mix combine with oxygen in
the air to form CO2, and any unburned or partially
burned fuel emitted from the combustion chamber is
then treated in a catalytic converter and also turned
into CO2.
The only effective way of reducing production of
CO2 is to reduce the consumption of fuel. This can be
achieved by operating on leaner (weaker) air:fuel
ratios, such as are achieved with direct injection
Composition of the exhaust gas
Figure 3.58 shows the composition of exhaust gas when
the combustion process takes place with a
stoichiometric air:fuel ratio (λ = 1).
Emission regulations for petrol engines
Test programme
Before any new vehicle can be sold in the EU, and many
other countries, a number of examples of the vehicle
0.7% miscellaneous
(noble gases, oxygen, hydrogen
must be submitted to the regulating body of a member
state for tests to be carried out. This is to test whether
the vehicle type meets current legislated standards.
Each vehicle has to complete a test cycle which
reflects the vehicle being driven in an urban
environment. The process is similar in most countries
where emissions legislation is in force.
The vehicle is started from cold and subjected to
various speeds, including motorway type driving. To
monitor the emissions accurately the tests are carried
out with a vehicle dynamometer. The tests involve a
standardised driving procedure, including engine at
idle, as well as gear changing and braking, to simulate
driving conditions in a reasonably sized town.
Previous emission regulations limits were
determined on the cubic capacity of the engine.
However in 1992 the EU introduced EU Stage I
emission regulations with which all new vehicles had to
comply (Figure 3.59). In January 1996, the EU imposed
stricter emission limits with EU Stage II. In 2000,
vehicle manufacturers had to ensure that vehicles met
EU Stage III emission standards before they could be
sold. In 2005 all new vehicles had to comply with EU
Stage IV.
So far we have described engine combustion related
emissions. Vehicles are also subject to evaporative
emission testing, i.e. emissions that are emitted through
the vaporisation of fuel stored in the fuel tank and
contained in the fuel pipes. These tests are carried out
in a gas-tight chamber at various ambient temperatures,
with the engine stationary and running.
Engines submitted for test must also be designed to
run on unleaded petrol, to reduce lead based additives
in fuels. In addition the petrol tank filler pipe must be
designed to prevent the tank from being filled from a
petrol pump delivery nozzle with an external diameter
of 23.6 mm or greater. This regulation means that
nozzles of pumps supplying lead free petrol have to be
smaller than those used with pumps dispensing leaded
13.1% water (H2O)
13.7% carbon dioxide (CO2)
0.1% nitrous oxides (NOx)
0.2% hydrocarbons (HC)
0.005% particulates
0.7% carbon monoxide (CO)
71.5% nitrogen (N2)
Figure 3.58 Composition of exhaust gas
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Emissions and emission control (petrol engines)
Figure 3.59 EU Emissions regulations
Year of introduction
EU Stage I
EU Stage II
EU Stage III
EU Stage IV
July 1992
January 1996
January 2000
January 2005
CO g/km
HC g/km
NOx g/km
Different emissions reduction technologies
The method adopted by an engine designer to meet the
emission limits depends on the technology available to
a manufacturer. When it appeared likely that one option
to reduce emissions would involve the fitting of a
catalytic converter and its accompanying fuel mixture
control system, the likely cost for use on small and
medium sized cars would be relatively high compared
with the cost of the vehicle. This encouraged many
manufacturers to develop lean burn engines. Tests
show that this type of engine gives a good fuel economy
and a much lower emission level than conventional
engines. It was expected that the lean burn engine
would meet the expected future emission requirements,
but the introduction in 1989 of a more stringent
standard that was based on a new European extraurban driving cycle (EUDC) meant that to attain the
new limits, even the lean burn engine would need to be
fitted with an exhaust catalyst.
It is only recently that developments in direct petrol
injection systems met with the requirements to use less
fuel (for CO2 reductions), so lean burn processes are
again becoming viable. The use of stratified mixture
formation (see section 3.4) and different types of
catalytic converter design now enable very lean
mixtures to be used under light load and at lower
engine speeds.
NOx + HC g/km
3.5.3 The pollutants
Creation of pollutants
As noted earlier, when complete combustion occurs,
the fuel and oxygen combine to form carbon dioxide
(CO2). Put simply, one carbon atom combines with
two oxygen atoms. The thermal reaction within the
cylinder (the combustion process) is most efficient
when the air:fuel ratio is stoichiometric. In theory, if
this ratio is used all the time, all of the fuel and oxygen
combust and produce carbon dioxide. Therefore, in
theory there should be no unburned fuel or unburned
oxygen. Carbon dioxide is a product of complete
combustion. Water (H2O) and nitrogen (N2) are also
emitted. With the carbon dioxide they form just over
98% of the exhaust gas. However the remaining gases
include oxygen and hydrogen but also include
pollutants, which represent around 1% of the total
exhaust gas.
The 1% of exhaust gas that is regarded as pollution
can be broken down into three main pollutants (see
Figure 3.60): carbon monoxide (CO), hydrocarbons
(HC), and oxides of nitrogen (NOx). In addition, very
small percentages of ‘particulates’ exist which are
effectively soot, but for petrol engines the percentage is
exceptionally low and generally not of concern. Diesel
engines produce much more soot, which is regarded as
a diesel engine pollutant that must be treated.
Figure 3.60 Main pollutants from internal combustion engines
Carbon monoxide
Incomplete combustion or partially
burned fuel
Carbon (C)
Unburned fuel, vaporised fuel escaping
from fuel system
Partially burned fuel
Poisonous to human beings when inhaled, CO adheres
to haemoglobin in the blood and prevents oxygen being
carried to body cells
Irritates eyes and nose. Cancer risk. Odour
NOx (oxides of
nitrogen – NO
and NO2)
Very high combustion temperatures
cause nitrogen to combine with oxygen.
Highest when air:fuel ratio is just slightly
weaker than stoichiometric ratio
Added to petrol to raise octane rating.
Lead is no longer allowed as an additive
in fuel in most regions; it also damages
catalytic converters
Lead (Pb)
Smoke – restriction in visibility. Can carry carcinogens
(cancer causing agents). Odour
Toxic to humans. NO2 combines with water to form nitrous
acid, which causes lung disorders. It combines with
other exhaust products to give eye and nose irritants;
it also affects the nervous system. Component of smog
Toxic to humans, causing blood poisoning and nervous
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The exhaust gas also includes sulphur compounds in
very small quantities and these are not regulated by
legislation. However, the compounds (primarily
sulphur dioxide (SO2)) are produced because of
sulphates in the fuel. The amount of sulphate in the fuel
is subject to legislation which has progressively
restricted the level from 1000 ppm (parts per million)
down to 50 ppm.
Figure 3.60 shows the main pollutants that are
subject to emissions legislation and therefore subject to
reduction processes. Those pollutants that occur during
combustion are referred to as byproducts of
Treating or reducing the pollutants
There are effectively two main routes to reducing
1 The first route is to design the engine, fuel system
and ignition system so that lower levels of
Scale for
NOx (parts Scale for
per million) CO2 (%)
pollutants are produced during combustion. Much
progress has been made in these areas in recent
years and modern designs of combustion chambers,
fuel systems and ignition systems have all
contributed to substantial reductions in pollutants.
Importantly, providing the correct air:fuel ratio
has a major influence on the levels of pollutants.
Figure 3.61 shows how CO, HC and NOx emissions
are affected when the air:fuel ratio changes. Other
factors affect the levels of pollutants produced (such
as high temperatures) but the chart indicates the
changes in pollutants assuming other factors remain
the same. Note that the chart indicates the trend in
the gas values and not the actual values.
2 The second route to reducing pollutants is to change
the pollutants chemically once they have left the
combustion chamber (after treatment). There are a
number of ways in which this is achieved, described
in section 3.5.5 through to section 3.5.10.
Scale for
Scale for HC (parts
CO (%) per million)
Lambda window
Air:fuel ratio
Lambda scale
(lack of oxygen)
14.3:1 15.14:1
(excess oxygen)
2. When the excess air factor is
around lambda = 1 (within the
lambda window) the CO2 is at its
highest, indicating relatively
complete combustion.
3. It is only the NOx emissions
that are high when the excess air
factor is in the region of
lambda = 1 (theoretically correct
air:fuel ratio). Note that the NOx
peaks when the mixture is slightly
weak of 14.7:1 air:fuel ratio
(lambda = 1.05 approximately).
4. When the excess air factor is
less than lambda = 1, there is
virtually no O2. When the air:fuel
ratio is weaker (lambda greater
than 1), the O2 level rises
Lambda window
Scale for
CO (%)
1. It can be seen from the graphs
that when all the relevant gases
are taken into consideration, the
best compromise is when the
air:fuel ratio is at or around 14.7:1
(lambda =1).
Air:fuel ratio
Lambda scale
(lack of oxygen)
14.3:1 15.14:1
(excess oxygen)
Figure 3.61 The influence of air:fuel ratio (lambda/value) on pollutant levels
Note. The values shown for all of
the gases are typical for an
engine operating at medium load
conditions. The values will change
with load, engine design, fuel
and ignition system designs.
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Emissions and emission control (petrol engines)
Carbon monoxide (CO)
Carbon monoxide (CO) is formed when fuel is only
partially burned (incomplete combustion). A carbon
atom from hydrocarbons (fuel) combines with a single
oxygen atom (from the inducted air) in the cylinder to
form CO. Compared with CO2, CO lacks one oxygen
atom, due to a deficiency of oxygen in the combustion
mixture (or a pocket of mixture). Carbon monoxide is
therefore formed when a cylinder receives either a rich
or poor mixture of fuel and air, which leads to isolated
pockets of rich mixture. Carbon monoxide can also form
when the mixture is excessively weak, when fuel
droplets do not vaporise; however, formation of CO in a
weak mixture is not at the same level as in a rich
mixture. Carbon monoxide measurement is therefore a
good indicator of a rich mixture, but not of a weak
Methods of reducing CO include:
Control of mixture strength – control of the air:fuel
ratio by the fuel system, especially under slow
running and cold starting conditions. Engine
management control has improved this under all
Improved fuel distribution – multi-point fuel
injection has largely overcome this problem, with
direct injection providing further improvement.
Good distribution was difficult to achieve with a
carburettor or single-point injection system.
More precise engine tuning – engine management
systems ensure that the correct air/fuel mixture is
supplied to the engine during all operating
conditions; therefore no manual adjustment is
possible to alter the mixture strength.
Compact combustion chamber – modern engine
designs incorporate very compact combustion
chambers. Long narrow chambers associated with
an ‘over-square’ engine often gave a high CO
Improved mixing of air and fuel – intake port and
combustion chamber design (including piston
crown shape) can help to promote good mixing; this
is especially true for engines using direct injection.
Leaner air/fuel mixtures – the recent trend towards
lean mixture operation (stratified mixture
formation) has helped reduce CO levels.
Throttle positioner system – these open the throttle
slightly when the engine is at idle or when
Precise ignition timing – ensures that the spark
occurs at the correct time and remains constant
between servicing intervals. Computer controlled
ignition and engine management systems achieve
After treatment – catalytic converters and other
systems help to convert CO into CO2 (covered later
in this section).
Hydrocarbons (HC)
Hydrocarbons in the exhaust gas represent unburned
fuel from incomplete combustion. A rich mixture (lack
of oxygen or excess fuel), results in high levels of
hydrocarbons, because there is insufficient oxygen to
combine with the fuel during the combustion process.
Any reduction in combustion efficiency will result in
high levels of hydrocarbons, e.g. a cylinder misfire
caused through an ignition fault or reduced
compression (a mechanical fault). Excessively weak
mixtures can also result in high levels of hydrocarbons,
because excessively weak mixtures cannot support
complete combustion within the combustion chamber.
However, careful design of the engine and fuel systems
reduces this problem to a level where weak mixtures
produce very low levels of hydrocarbons.
Methods of reducing hydrocarbon emissions
Control of mixture strength – control of the air:fuel
ratio by the fuel system, especially under slow
running and cold starting conditions. Engine
management control has improved this under all
Improved distribution of the fuel – multi-point fuel
injection has largely overcome this problem, with
direct injection providing further improvement.
Good distribution was difficult to achieve with a
carburettor or single-point injection fuel system.
More precise engine tuning – engine management
systems ensure that the correct air/fuel mixture is
supplied to the engine during all operating
conditions; therefore no manual adjustment is
possible to alter the mixture strength.
Improved mixing of air and fuel – intake port and
combustion chamber design (including piston
crown shape) can help to promote good mixing; this
is especially true for engines using direct injection
Leaner air/fuel mixtures – the recent trend towards
lean mixture operation (stratified mixture
formation) helps reduce HC levels.
Mixture adjustment during deceleration – fuel
injection systems provide precise metering of the
fuel during deceleration (decel fuel cut off or
Precise ignition timing – ensures that the spark
occurs at the correct time and remains constant
between servicing intervals. The ignition timing is
retarded when the engine is slow running or
decelerating. Computer controlled ignition and
engine management systems achieve this.
After treatment – catalytic converters and other
systems help to convert HC into CO2 (this is covered
later in this section).
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Hydrocarbons are also formed when fuel vaporises and
escapes into the atmosphere from the fuel system, and
when unburned fuel passes the pistons into the
crankcase. These problems can be reduced in the
following ways:
Closed crankcase ventilation system – unburned
fuel passing the pistons and entering the crankcase
is prevented from escaping to the atmosphere by a
positive crankcase ventilation (PCV) system. The
unburned fuel is returned to the induction system.
Sealed fuel system – a fuel evaporative emission
control (EVAP) system seals the fuel tank, collects
the vaporised fuel, passes it through a charcoal filled
canister and delivers it to the induction manifold for
combustion in the engine cylinders (this is covered
later in this section).
Oxides of nitrogen (NOx)
Nitric oxide (NO) and nitrogen dioxide (NO2) are
grouped together under the term oxides of nitrogen
(NOx). The atmosphere consists of approximately
78% nitrogen and 21% oxygen. The air drawn into
the combustion chamber is heated during
combustion; under certain conditions the oxygen and
nitrogen can combine to form harmful NOx. The
formation of NOx occurs with combustion
temperatures above approximately 1300°C. However,
combustion temperatures can easily exceed 2500°C
during full load conditions when the production of
NOx reaches a critical limit. Formation of NOx is also
accelerated when the air:fuel ratio is slightly weaker
than stoichiometric.
Methods of reducing NOx include:
Combustion chamber shape – the shape of the
combustion chamber can be designed to increase
flame speed. Used in conjunction with lower
compression ratios, such chambers reduce NOx
formation, but normally only at the expense of fuel
economy and engine power.
Increase in air:fuel ratio – the highest flame speed
and NOx content occurs when the mixture is about
12% richer than the stoichiometric value. An engine
designed to operate on a weak mixture has reduced
emissions, but vehicle driveability suffers unless the
ignition timing and air/fuel mixture are set correctly.
However, direct injection systems with stratified
mixture formation help to overcome this problem.
Ignition timing – computer controlled ignition
systems can control the ignition timing to prevent
sudden advance in ignition timing (for a given brief
time period) when the throttle is snapped open.
Valve timing – by changing the inlet and exhaust
valve timing (i.e. the overlap period), the combustion
temperature can be lowered by inducing exhaust gas
into the intake port (the process of using exhaust
gases to reduce combustion temperatures is covered
later in this section). Variable valve timing can
optimise the overlap period during engine operation.
Fundamentals of Motor Vehicle Technology: Book 2
Intake air temperature – reducing the intake air
temperature can lower the combustion temperature
and therefore lower NOx production. If the engine is
fitted with a turbocharger, the fitting of an
intercooler can reduce NOx emissions significantly.
Decrease in flame speed – exhaust gas recirculation
(EGR) systems direct some exhaust gas back to the
induction manifold to slow down the combustion
when the engine is under certain load conditions.
The EGR system does however reduce the maximum
power of the engine.
After treatments – the fitting of a three-way catalyst
in the exhaust system reduces the level of NOx. More
recent designs of catalytic converter include NOx
storage catalysts, covered later in this section.
‘Catalyst’ is the name given to a material that produces
or hastens a chemical action without undergoing any
change itself.
Carbon dioxide (CO2)
In theory, if the correct amount of air combines with the
correct amount of fuel during perfect combustion, this
results in carbon dioxide (CO2), water (H2O) and
nitrogen (N2). Carbon dioxide is therefore a product of
complete combustion. Although it is not possible for
perfect combustion to occur in the ‘real world’, the more
efficient the combustion process, the higher the CO2
content in the exhaust. It is therefore necessary to
provide the correct air:fuel ratio to produce as perfect
combustion as possible. Any fault in the ignition system,
fuel system or combustion efficiency will lower the CO2
content in the exhaust gas.
Carbon dioxide is not directly harmful to humans
and is not regarded as a pollutant, but it is harmful in
the long term to the environment, and contributes to
global warming. Therefore, if the combustion process is
efficient and can be operated with weak mixtures, less
fuel should be used, reducing CO2 emissions.
Oxygen (O2)
Oxygen is an essential element to the combustion
process. During the combustion process the oxygen
should combine with hydrocarbons to form carbon
dioxide and water, leaving no oxygen or hydrocarbons
in the exhaust gas. Although not a harmful emission,
the exhaust gas contains a very small percentage of
oxygen (which is effectively unburned during the
combustion process). A rich mixture will result in no
oxygen in the exhaust gas whilst a weak mixture
(whether intended or caused through insufficient fuel
or an air leak in the inlet manifold) will result in high
oxygen content in the exhaust gas.
A reduction in combustion efficiency (e.g. a misfire)
will result in some of the fuel and oxygen not being
burned, which will increase the oxygen content in the
exhaust. Excess oxygen can combine with additional
gases to form other pollutants such as NO. If the air:fuel
ratio is chemically correct and the combustion process is
efficient, oxygen emissions should be almost zero.
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3.5.4 Influences of engine operation
on the pollutants
In theory, a petrol engine should operate at the
stoichiometric air:fuel ratio at all times, because this
ratio should result in complete combustion of the
oxygen and petrol: harmful emissions should be low
and power should be at its optimum level. However, an
engine is not a perfect machine. Other requirements,
such as reduced fuel consumption, dictate a need to use
air:fuel ratios that are either slightly richer or slightly
weaker than the stoichiometric value.
Although there are many factors dictating the exact
air:fuel ratio used in an engine, the following list
provides an understanding of the different
requirements. Note that inefficiencies in older engine
designs and older type fuel ignition systems
exaggerated many of the problems, such as poor mixing
of the air and fuel. Therefore some of the problems are
not as severe as in the past, although in most cases they
still occur to a limited extent.
Light load/part load
In general, under these conditions, an engine can
operate on mixtures where there is excess air, i.e. with
weak mixtures. With the modern generation of direct
fuel injection and stratified mixture formation, mixtures
can now be used with excess air ratios as weak as 40:1
during light load conditions. Note however that if an
engine is not operating with stratified mixture
formation and the emissions control includes a standard
three-way catalytic converter, it is necessary to operate
at around the stoichiometric ratio to enable the
converter to convert the pollutants into harmless gases.
Full load
To achieve maximum power, the stoichiometric ratio
should in theory be suitable; however, the air and fuel
do not completely or perfectly mix (especially on
carburettor systems) which results in some fuel not
mixing with the air; i.e. a weak mixture can exist in the
combustion chamber with any unmixed fuel unable to
burn efficiently. Therefore, a slightly richer mixture
must initially be provided so that enough fuel is able to
mix with the air and form a good combustible mixture.
Any excess fuel will cause some rich pockets of mixture
to exist. In addition, there are conditions where fuel will
condense on the intake manifold and cylinder walls,
which again will require additional fuel to ensure that
sufficient is available to form a good mixture.
For older engines with a less efficient design of
intake manifold and intake port, poor mixing of air and
fuel was a major problem. Excessively rich mixtures
were provided by the fuel system to ensure that
sufficient correctly mixed fuel was available during
combustion. Modern intake and fuel systems have
improved the mixing process. It is therefore possible to
reduce the amount of excess fuel for full load operation.
Cold starting/cold running
The problems associated with cold engine operation are
similar to the problems of operating at full load
conditions, i.e. poor mixing and fuel condensing on cold
surfaces. When the fuel is cold, it is more difficult to
vaporise and therefore does not mix as easily with the
cold air. In addition, the intake manifold and intake port
walls are cold, which causes atomised fuel to condense.
This problem is repeated when the fuel contacts the cold
cylinder walls. It is therefore necessary to provide an
excessively rich mixture to ensure that sufficient fuel can
mix with the air and achieve combustion.
Using port type fuel injection (as opposed to a
carburettor or single-point injection system) reduces the
problem of fuel condensing on the intake system walls.
A further improvement is gained by injecting directly
into the cylinder. However, cold cylinder walls still
cause condensation of fuel; therefore a rich mixture is
still required during cold starting.
When the engine has started, heat is passed to the
cylinder walls so it is possible to reduce the amount of
enrichment. On older engines, it was necessary to
maintain a rich mixture immediately after starting, but
the use of fuel injection into the ports or into the
cylinder reduces condensation problems so it is now
possible to operate using air:fuel ratios that are at or
close to stoichiometric immediately after cold starting.
It is also general practice to provide a slightly fast
idle speed after cold starting; this enables the engine to
overcome power losses caused by increased friction and
oil drag on cold engines. A fast idle also enables some
loads to be applied to the engine (e.g. electrical or autotransmission loads) which could otherwise cause the
engine to stall. During the warm-up period, the
increasing temperature of the engine allows
progressively weaker mixtures to be used and idle
speeds to be reduced.
Idle speed (normal operating temperatures)
Modern engines generally operate using air:fuel ratios
around the stoichiometric value or slightly weaker.
Most engines are relatively inefficient at idle speed,
especially engines with carburettors, where low air
speeds contribute to poor mixing of air and fuel. It is
therefore necessary to provide relatively rich mixtures
to ensure that there is sufficient fuel available to mix
with the air.
Modern engines are very efficient. Injection systems
and other design features improve the mixing of air and
fuel at idle speed. However, valve timing and other
design features are generally compromised so that an
engine is more efficient at normal operating speeds, i.e.
at mid-engine speeds where most driving takes place.
There is still a theoretical requirement for a slightly
richer mixture to be provided at idle speed. In reality,
the use of idle speed control systems and improved
efficiencies enable engines to operate at stoichiometric
(or close to stoichiometric), which is essential where
conventional three-way catalytic converters are used.
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When a driver changes the operating conditions of the
engine, e.g. from light load to full load, this is referred
to as a transition. One major problem with older
engines that used a carburettor was that air would
respond to a transition much more rapidly than petrol.
Because air is less dense than petrol, when the throttle
is suddenly opened, the volume of air flowing to the
engine increases rapidly but the petrol takes longer to
respond. It should also be remembered that the
increased flow of air created an increased depression in
the carburettor venturi which in turn caused additional
fuel to be drawn from the float chamber (fuel
reservoir). There was, however, a small but important
time delay in this process. Therefore, with most
carburettor systems, when the throttle is initially
opened, a momentarily weak mixture results.
Carburettors were fitted with different devices that
caused additional fuel to be delivered when the throttle
was opened, e.g. small pumps, directly operated by the
throttle linkage.
With injection systems, it is much easier to ensure
that the appropriate amount of fuel is injected at exactly
the same time the throttle is opened and the air volume
increases. It is possible, so sensitive is the control
system, to detect the rate at which the throttle is being
opened, so that the fuel can be increased in anticipation
of the air volume increasing.
During deceleration, when the load on the engine is
reduced by the driver closing the throttle, it is possible
to provide a weak mixture or to cut off fuel delivery
completely. Although this function was difficult to
achieve with carburettors, injection systems can control
the amount of fuel injected so that a progressive
reduction (or cessation) in fuel supply is achieved
which results in lower fuel consumption and reduced
3.5.5 Processes and devices for
emissions control and
Engine and engine control systems
As noted earlier, modern engine design and control
systems have been continuously improved with the
result that the whole fuel delivery, ignition and
combustion process is considerably more efficient than
in older engines. The overall effect is that engines now
produce far lower levels of pollutants. The use of
computer controlled fuel injection and ignition systems
has had a dramatic effect on overall efficiencies, but
further improvements have been achieved through
improved engine design and electronic control of
mechanical systems. Examples include changes to
combustion chamber and intake port design as well as
electronic control of valve timing (variable valve
timing). Higher compression ratios and four valve per
Fundamentals of Motor Vehicle Technology: Book 2
cylinder designs also help to reduce the levels of most
pollutants, although in some cases there is a risk of
increasing some pollutants, which then have to be
treated separately.
Most of the significant changes and improvements,
such as fuel injection and ignition systems, are dealt
with individually within this book, however, many
changes go almost unnoticed and simply form an
evolutionary part of general engine development. In
fact, many small changes or features might only be
applicable to one particular engine design and do not
necessarily justify individual explanation or coverage.
However, technicians will encounter many individual
design features when working on particular vehicles, so
reference should always be made to specific vehicle
information wherever possible.
Although engines now produce far less pollution
than in the past, even the low levels now produced are
regarded as excessive. Additional means are needed to
further reduce pollutant levels. Sections 3.5.6 to 3.5.10
cover the main pollutant reduction and control systems
which are referred to as ‘after treatment’ systems.
Lean burn technology
Lean burn technologies have been developed and
applied for many years, although there have been
certain limitations in the past, due to the control of NOx
Lean burn engines use very weak mixtures (excess
air) which is viable for light load engine operation. For
an engine to produce power or torque, a more enriched
mixture is required, closer to the stoichiometric air:fuel
ratio. Lean burn engines do, however, produce low
emissions of CO and HC, and use less fuel than engines
that have to operate around the stoichiometric ratio for
most of the time.
Lean burn engines generally use stratified mixture
formation (see section 3.4), whereby a small pocket
of rich mixture is created adjacent to the spark plug,
but the rest of the cylinder is filled with a weak
mixture or with air containing no petrol. The
stratified mixture principle results in the rich mixture
pocket igniting easily; the combustion of this pocket
then spreads through to the rest of the weak mixture
or simply heats the remaining air, thus causing gas
expansion. The overall mixture is weak which results
in low consumption of petrol and low CO and HC
The use of mixtures slightly weaker than
stoichiometric can cause high NOx emissions; however,
when the mixture is further weakened (by a large
excess of oxygen) such as on lean burn engines, NOx
levels reduce owing to lower combustion temperatures.
The problem is that the NOx does still exist in the
exhaust gas of a lean burn engine, so a catalytic
converter is needed for further reduction. However,
since there is an excess of oxygen in the exhaust gas
when the CO and HC are catalytically converted to CO2,
the oxygen molecules are taken from the excess oxygen
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Emissions and emission control (petrol engines)
and not from the NOx. This means that the NOx does not
lose its oxygen and is therefore not reduced to nitrogen.
More recent developments have enabled direct injection
systems to provide a high level of control over mixture
formation (see section 3.4), because direct injection can
switch from stratified mixture operation (an overall
weak mixture with a rich pocket) to homogeneous
mixture operation (a normal air:fuel ratio, close to
stoichiometric throughout). Additionally, recently
developed NOx accumulator catalytic converters
overcome the problem of untreated NOx escaping to the
atmosphere (see section 3.5.6).
After treatment systems
In many after treatment systems, a process of chemical
or thermal reaction is used to convert harmful gases
into less harmful or harmless gases. Whilst an in-depth
understanding of chemistry would be an advantage,
you need only to appreciate the basic process by which
chemical conversions take place.
For those systems that do not directly use chemical
or thermal reactions, individual explanations are
provided in the following sections.
Chemical change and thermal reactions
In very simple terms, a thermal reaction uses heat to
create a change in a substance, e.g. the change of water
into steam. However, thermal reactions can promote a
more fundamental change in substances and can cause
or assist different substances to combine (chemical
change); this phenomenon is used to reduce harmful
With exhaust gases, it is of interest to note that
much of the chemical change occurs by moving
oxygen atoms from one gas to another. Carbon
monoxide (CO) has one oxygen atom (hence
‘monoxide’); by adding another oxygen atom the
monoxide changes to dioxide. A similar process occurs
with hydrocarbons (HC). Two oxygen atoms can
combine with each carbon atom to form CO2.
Although oxygen atoms can be extracted from the
small amount of unburned oxygen in the exhaust gas,
there are also oxygen atoms contained within the
oxides of nitrogen (NOx). The chemical changes that
occur include removing oxygen from NOx; the oxygen
can then combine with CO and carbon from the HC to
form CO2. The advantage is that the NOx now loses
oxygen molecules to leave just nitrogen which is not a
pollutant. In reality, NOx in exhaust gas is generally
made up of nitric oxide (NO) and nitrogen dioxide
(NO2), of which the latter is the most harmful. When
oxygen atoms are removed some NO can remain,
which is relatively harmless unless it later meets with
more oxygen (usually when the gases leave the
exhaust pipe and enter the atmosphere).
The above brief explanation provides a very basic
understanding of the process of chemical change, which
in reality is much more complex. It is sufficient to
appreciate that such chemical changes occur, which
result in harmful gases being converted into relatively
harmless gases.
Oxidation process and catalysts
It has been mentioned previously that heat helps to
promote chemical change; this includes oxidation
processes (the reaction of oxygen with the gases). An
initial oxidation process occurs within the engine itself –
the combustion process in the cylinder. During this
process, oxidation of the fuel occurs and CO2 is formed.
In reality, perfect oxidation or combustion does not
occur, so partially or completely unburned fuel (CO and
HC) is produced.
By creating a secondary combustion or oxidation
process, it is possible to convert most of the remaining
CO and HC into CO2. Although various devices can
create a secondary oxidation, the most commonly used
type on modern vehicles is the three-way catalytic
converter; this is discussed in section 3.5.6. It should be
remembered that a catalyst is a substance that helps to
promote chemical change without actually changing
itself. So, making use of heat and a catalyst, a secondary
oxidation process converts the harmful gases into
harmless ones.
The important point to remember is that for an
oxidation process to be effective, spare oxygen must be
Other processes for treating pollutants
A number of other devices are used to convert or reduce
emissions of harmful gases.
Thermal afterburning – this process relies on
injection of air into the exhaust ports, which results
in continued combustion when the gases have left
the cylinder. It was used in the past to convert CO
and HC, and has been used in conjunction with
catalytic converters, the thermal afterburning being
effective during the warm-up phase, before the
catalytic converter has reached operating
Other thermal devices – some engines have in the
past been fitted with ‘hot spots’ in the exhaust
system. In effect a glow plug device is located in the
exhaust manifold or down pipe. The device can be
electrically heated but can also rely on exhaust heat.
When the device is at high temperature, it causes
secondary or continued combustion of the exhaust
gases, thus converting CO and HC to CO2. This type
of thermal device is no longer widely used.
Exhaust gas recirculation (EGR) – this is a process
whereby a controlled amount of the exhaust gas is
fed back into the intake system. The air drawn into
the cylinder therefore contains a percentage of
exhaust gas, which has the effect of reducing the
combustion temperature. Because the levels of NOx
increase with high combustion temperatures, the
use of EGR to lower the combustion temperatures
causes a reduction in the amount of NOx produced.
EGR is covered in detail in section 3.5.8.
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Evaporative emission control (EVAP control) –
evaporative emissions can be released into the
atmosphere from the fuel tank and fuel system.
Petrol vapour (HC) occurs naturally, so methods are
used to prevent its escape. The main system in use
relies on creating a sealed fuel system. Fuel vapours
are fed through to the engine intake system.
However, when the engine is not running, vapours
are collected in a charcoal canister, which releases
the vapour to the engine when it is running. EVAP
systems are covered in section 3.5.10.
Valve and ignition timing – although not strictly
regarded as after treatment, control of valve and
ignition timing can help to reduce pollutants. Valve
timing can be arranged so that the intake valve
opens before the exhaust valve closes (valve timing
overlap). In this way, during the induction stroke
some of the exhaust gas that has not been expelled
from the cylinder will mix with the fresh intake air.
As with controlled exhaust gas recirculation, the
exhaust gas helps to reduce the combustion
temperature and the formation of NOx. Variable
valve timing allows different overlap periods to be
used at different engine speeds. Although this
facility is aimed at producing improved torque and
power throughout the engine speed range, it also
assists in maintaining lower emissions.
Ignition timing can be altered to help improve
some emission levels. An appropriate example is
when ignition timing is retarded, so that the
combustion process is retarded, which means that
combustion continues when the exhaust valve
opens. The result is that excess oxygen in the exhaust
system can now combine with the CO and HC during
a combustion process that continues within the
exhaust ports. This process is very effective for cold
engines where a rich mixture is required (an excess
of CO and HC). The additional heat created by the
late combustion process also helps to heat up the
catalytic converter. The disadvantage is that the high
temperatures produced when the gases enter the
exhaust ports (where there is oxygen) increase the
formation of NOx.
3.5.6 Catalytic converters
Principle of operation
Catalytic converters are the most commonly used
devices for after treatment for CO, HC and NOx
reduction. As the name implies, catalytic converters
convert some of the exhaust gases, primarily the
pollutants, into harmless gases. Converters use heat to
change the gases, but a catalyst is used to accelerate the
During the 1970s, oxidation catalysts were fitted
(primarily for the US market) as a means of converting
CO and HC into CO2 (see the previous section). This
type of catalyst was also referred to as a ‘single-bed’
Fundamentals of Motor Vehicle Technology: Book 2
converter. The catalytic converter was located in the
exhaust system; the exhaust gases flowing through heat
the converter, enabling it to carry out the oxidation
process. However, the subsequent requirement to also
reduce levels of NOx resulted in an additional catalyst
being used, ahead (upstream) of the oxidation catalyst.
The two catalysts were often combined into a single
assembly, referred to as a ‘dual-bed’ catalyst because the
conversion processes still remained separate.
One problem with the oxidation catalytic converter
was that, if the system were fitted to an older engine
that operated with a relatively rich mixture, there was
insufficient oxygen in the exhaust gas to allow
oxidation to take place. This was also true if the NOx
reduction catalyst were fitted ahead of the oxidation
catalyst, because the NOx catalyst operated with a
relatively rich mixture. It was therefore necessary to use
an air injection system to ensure that the oxidation
catalyst had sufficient oxygen (Figure 3.62).
A significant development was the introduction of
the three-way catalytic converter which was able to
reduce the three main pollutants (CO, HC and NOx)
within one converter. The three-way converter became
the most widely used across the US, Europe and
Correct air:fuel ratios for different converter
As discussed in the previous section, there is a need for
free oxygen to enable CO and HC to be converted to
CO2. However, oxidation catalysts (and dual-bed
catalysts) were fitted to earlier vehicles with engines
that operated with relatively rich mixtures (a lack of
oxygen). These vehicles were often fitted with
carburettors as opposed to the more efficient fuel
injection systems, resulting in relatively poor mixture
formation. There was therefore insufficient spare
oxygen to allow the oxidation process to take place. To
provide the oxygen required by the catalytic converter
an air injection pump was usually fitted to pump air
into the exhaust system just ahead of the oxidation
However, NOx reduction operates more efficiently
with a lack of oxygen (a slightly rich mixture or λ < 1).
When the NOx flows through the NOx reduction catalyst,
the oxygen will readily separate from the NOx, owing to
the lack of free oxygen in the exhaust gas, thus leaving
just nitrogen (N2). However, if there is already an excess
of oxygen in the exhaust gas, there is a reduced
tendency for oxygen to separate from the NOx. The NOx
reduction process is therefore more suited to engines
operating on relatively rich mixtures in which oxygen is
lacking in the exhaust gas.
With the three-way catalytic converter, the process
of NOx reduction and CO/HC conversion takes place in
the combined assembly. Although some spare oxygen is
available in the exhaust gas, the oxygen required for CO
and HC conversion is also taken from NOx. The
important factor for the efficiency of a three-way
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Emissions and emission control (petrol engines)
Figure 3.62 Catalytic converter systems
a Single-bed oxidation catalyst
b Dual-bed catalyst
c Single-bed three-way catalyst
converter is the balance of the gases; i.e. the oxygen
content must not be excessive, otherwise NOx reduction
will not take place. At the other extreme, too little
oxygen in the exhaust gas will prevent the oxidation of
CO and HC (the CO and HC will not combust and
convert into CO2). Additionally, too much CO and HC in
the exhaust gas will mean that there will be insufficient
oxygen to facilitate full conversion to CO2. It is essential
that the balance of the gases is correct, which means
that the air:fuel ratio must also be correct. The correct
oxygen level in the exhaust gas is achieved by operating
the engine at the stoichiometric air:fuel ratio (λ = 1).
Figure 3.62 shows the three most commonly used
types of catalytic converter. A fourth type of converter is
now gaining popularity – an NOx accumulator
converter. This type is now being used on vehicles
operating with stratified mixture formation and direct
fuel injection. NOx accumulator converters are covered
later in this section.
Operating temperatures
To enable this chemical change in the gases, the
catalytic converter must operate at relatively high
temperatures. Because the conversion of CO and HC is
based on an oxidation process (in effect a second
combustion process), the temperature must be
sufficient to allow ignition of the gases. A typical ideal
temperature is between 400°C and 800°C for most types
of oxidation converter.
Heat is provided by the flow of exhaust gas through
the converter, but the oxidation process creates
additional heat. So long as the air:fuel ratio is correct,
the oxidation process maintains the heat to continue the
For most types of catalytic converter, excessive heat
will cause either loss of efficiency or permanent
damage. Consistent temperatures of 1000°C or more
will permanently damage the converter, and long
periods of operation at between 800 and 1000°C will
accelerate the ageing process of the converter. The
location of the converter is therefore critical to prevent
excessive heat being passed from the exhaust gases as
they exit the exhaust ports. Exhaust gases lose heat as
they flow through the exhaust system, so a location
further from the exhaust port is more suitable.
However, there is a need to heat up the converter
immediately after starting the engine. The closer the
converter is to the exhaust ports, the quicker this
happens. There has therefore been a tendency to use a
small ‘pre-cat’ located very close to the exhaust ports,
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Engine management – petrol
and a larger conventional converter in the exhaust
system under the vehicle. The pre-cat is designed for
higher temperature operation and stability, whilst the
larger converter is designed for lower temperature
operation. Note, however, that many vehicles are fitted
with a single high temperature converter located close
to the exhaust ports (usually just after the exhaust
manifold), as shown in Figure 3.63.
Construction of catalytic converters
Although there are a few variations in the construction
of catalytic converters, the majority of the three-way
types are constructed as detailed below and shown in
Figure 3.64. Note that the illustration also shows a
lambda sensor, which is discussed in section 3.5.7.
Maximum surface area
The objective is to expose the exhaust gases to the
catalyst material. To enable as much of the exhaust gas
as possible to be exposed to the catalyst, it is necessary
to use a series of small tubes that, together, have a large
surface area. If there is not enough surface area, the
conversion process will not treat sufficient amounts of
gas. It must be remembered that the exhaust gases flow
through the converter at high speed, so a large number
of small tubes ensures that the gas flow is not restricted,
but at the same time the large number of tubes allows
the maximum amount of surface area.
It is common practice to construct the converter
using a monolith or substrate that is a honeycomb made
of ceramic material (typically a magnesium aluminium
silicate). Note that some monoliths are produced using
a finely corrugated, thin metal foil.
The surfaces of the tubes formed by the honeycomb
are thinly coated with aluminium oxide (referred to as a
washcoat). The coating provides a rougher surface,
with a much larger surface area than a smooth surface
(imagine a smooth surface, such as a mirror, covered in
very small bumps or hills). The aluminium oxide can
increase the surface area by as much as 7000 times.
Fundamentals of Motor Vehicle Technology: Book 2
Coating of precious or noble metals
The active material, the catalyst material, is then added
to the washcoat. Active materials vary, but for the
oxidation catalysts (CO and HC conversion), platinum
and/or palladium are used. These two materials
accelerate the oxidation of CO and HC. In a three-way
converter, rhodium is also used to accelerate the
reduction of NOx.
These active materials are expensive and are
referred to as noble or precious metals. However, only
around 2 to 3 grams of these materials are used to coat
the surface areas of the converter, as the coating is
exceptionally thin.
The honeycomb or monolith is contained within a steel
casing. To protect the honeycomb from damage caused
by vibration, etc., it is mounted in matting that swells
when initially heated. The matting therefore forms a
protective layer around the honeycomb and also forms
a gas seal.
Potential faults and problems with catalytic
High temperatures and misfires
As was mentioned, catalytic converters operate ideally
between 400°C and 800°C, with temperatures much
higher than this causing accelerated ageing or
permanent damage.
One major factor that can increase the temperature
within the catalytic converter is an engine misfire.
When a misfire occurs, a quantity of petrol and air
remain unburned, and these flow out of the cylinder
and through the exhaust system into the catalytic
converter. The heat in the exhaust system and existing
combustion process within the converter will cause the
unburned air and fuel to ignite and combust and this
additional fuelling of the combustion process within the
converter creates excessive heat that can lead to
temperatures in excess of 1400°C. Such temperatures
1 Engine
2 Lambda oxygen sensor
upstream of the catalytic
converter (two-step sensor
or broadband sensor,
depending on system)
3 Three-way catalytic
4 Two-step lambda oxygen
sensor downstream of the
catalytic converter (only on
systems with lambda dualsensor control)
Figure 3.63 Location of catalytic converters
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Emissions and emission control (petrol engines)
Lambda oxygen sensor
Swell matting
Thermally insulated double shell
Washcoat (Al2O3 substrate
coating) with noble metal coating
5 Substrate (monolith)
6 Housing
Figure 3.64 Catalytic converter construction (three-way type)
will quickly melt the substrate or monolith in the
converter, causing permanent damage. Note, however,
that even brief misfires will accelerate the ageing
process of the converter.
Modern engine management systems can detect a
misfire using the various sensors. One indicator of a
misfire is a high oxygen level in the exhaust gas, which
is detected by the oxygen sensor (section 3.5.7). In
addition, the engine management ECU can check the
acceleration rate of the crankshaft using information
provided by the crankshaft speed/position sensor.
When the power stroke occurs within each cylinder, it
causes an acceleration of the crankshaft, but a misfire
by one cylinder will cause reduced acceleration
compared with the ‘good’ cylinders. The ECU can assess
which cylinder is misfiring and cut off the fuel supply
to the affected cylinder. Without delivery of fuel, there
will be no unburned fuel passing through to the
catalytic converter, although unburned oxygen will
Leaded fuel and other contaminants
Lead compounds restrict the ability of human cells to
absorb oxygen. Leaded petrol has been banned in the
EU since 2000. The amount of lead allowed in petrol
was previously subject to progressive reductions. Many
other countries have also banned leaded petrol.
With regard to leaded fuel and catalytic converters,
lead compounds clog the pores of the active materials,
reducing their efficiency. Excessive build-up of lead
stops the conversion processes from taking place.
Therefore, vehicles fitted with catalytic converters must
not be operated with leaded fuel.
It is also possible for the active materials in catalytic
converters to be affected by other contaminants, such as
deposits from the engine oil that pass by the piston
rings into the combustion chambers. A worn engine,
producing high emissions, can also contribute to the
rapid deterioration of its catalytic converter.
NOx accumulator converters
The increased use of lean burn technologies (primarily
stratified mixture formation with direct fuel injection),
results in an excess of oxygen in the exhaust gases when
very weak air/fuel mixtures are used. The excess
oxygen prevents effective reduction of NOx by the threeway catalytic converter (see lean burn technologies in
section 3.5.5). It is therefore necessary to find
alternative methods of reducing the amount of NOx.
NOx accumulator catalytic converters are now being
fitted to many vehicles with lean burn engines. The
accumulator type converter is not dissimilar to the
standard three-way converter in construction, but other
active materials are included, such as oxides of
potassium, calcium and barium. The converter can
operate in the same way as a three-way converter when
the air:fuel ratio is around the stoichiometric value.
However, when the engine is operating with very
weak mixtures (i.e. with excess oxygen in the exhaust
gas), the accumulator converter operates in a different
manner. The oxidation process causes the nitrogen and
NO to attract excess oxygen thus producing NO2.
However, the additional active materials cause a further
change which results in nitrates being formed. The
nitrates are then stored (accumulated).
Accumulation and conversion
As described above, the accumulator converter stores
NOx as nitrates, but at some stage the stored NOx must
be released; otherwise the converter will eventually
become over-saturated (its maximum storage capacity
will be reached). As the amount of NOx stored in the
accumulator converter increases, it impairs the device’s
ability to store more NOx so a means of assessing when
the stored NOx must be released is required. Two
methods can be used.
1 An NOx sensor downstream of the converter
indicates to the ECU when higher levels of NOx are
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Engine management – petrol
flowing out of the converter because the converter is
no longer storing the NOx in sufficient quantities as
it is almost full to capacity. At this stage the NOx is
released from the converter.
2 A
programming), which calculates the likely amount
of NOx that is stored (based on operating conditions
and temperature). At the calculated time the process
for NOx release is implemented.
When the accumulator converter is assessed to be full,
the engine is briefly operated using a rich mixture
(typically with an air:fuel ratio of 12:1 or richer),
resulting in an excess of CO, HC and H2 in the exhaust
gas. In simple terms, the oxygen in the NOx is lost,
leaving nitrogen. The oxygen combines with CO and HC
to give CO2 and H2O (water).
The end of the conversion process can be assessed
either using another ECU calculation (again using
conditions and time), or with an oxygen sensor located
after the converter: the end of the process will be when
the oxygen in the exhaust gas (after the converter) has
fallen to a predefined level.
An NOx accumulator converter can be used in
conjunction with a three-way converter. Figure 3.65
shows an arrangement where both converters are fitted
into an exhaust system. Note that the three-way
converter is fitted close to the exhaust manifold (in the
pre-cat location) with the NOx accumulator further
downstream. The NOx accumulator converter operates
at lower temperatures than the three-way converter
(ideally around 300–400°C, compared with 400–800°C
for the three-way type). This means that the two
converters must be separate.
3.5.7 Oxygen/lambda sensing
(controlling the air:fuel ratio)
Oxygen content: the critical factor
It has been highlighted a number of times within this
chapter that the oxygen content of the exhaust gas is
critical to the operation of the catalytic converter (as
Fundamentals of Motor Vehicle Technology: Book 2
well as to other emission control devices). Achieving
the correct oxygen content very much depends on the
air:fuel ratio, which should be stoichiometric (λ = 1).
Modern engine management systems are able to
provide very accurate control of the air:fuel ratio.
However, to attain the highest possible accuracy, it is
necessary to monitor the oxygen levels in the exhaust
An oxygen sensor (as shown in Figure 3.66) is used
to measure the oxygen content; this device then
transmits a signal to the ECU. If the oxygen content in
the exhaust gas is too high or low, the ECU is able to
change rapidly the fuelling as necessary, ensuring that
the oxygen level is restored to the correct value. When
the oxygen level is correct, λ = 1. The sensor is often
referred to as a ‘lambda sensor’.
When the oxygen content is at λ = 1 (or within the
lambda window described in 3.5.1), the catalytic
converter is able to convert CO and HC efficiently into
CO2 and also reduce NOx levels.
Figure 3.66 Oxygen/lambda sensor
Closed and open loop operation
Section 3.5.3 provides details of the air:fuel ratios used
for different engine operating and driving conditions.
Older engines operated using a wide range of air:fuel
ratios often far beyond the lambda window. For light
load conditions an air:fuel ratio of 17:1 or weaker was
not uncommon, and under full load 13:1 or richer
would have been used. However, such extreme ratios
are not suited to vehicles fitted with catalytic
converters, because they would dramatically reduce the
converter efficiency (which relies on the excess air
factor being λ = 1).
1 Engine with EGR system
2 Lambda oxygen sensor
upstream of the catalytic
3 Three-way catalytic converter
4 Temperature sensor
5 NOx accumulator type catalytic
converter (main cat)
6 Two-step lambda oxygen sensor,
optionally available with integral
NOx sensor
Figure 3.65 Location for an NOx accumulator converter and three-way converter
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Emissions and emission control (petrol engines)
For most engines since the early 1990s air:fuel ratios
are controlled as closely as possible within the lambda
window (ideally at λ = 1). It is not always possible to
avoid slight enrichment for full load conditions, and for
cold running it is also often necessary to briefly provide
some enrichment. However, light load conditions are
generally achieved using air:fuel ratios within the
lambda window. In general, every new or modified
version of an engine or an engine control system allows
increased operation within the lambda window,
enabling more efficient catalytic converter operation
and a greater reduction in pollutants.
Engines with stratified charge mixture formation
and direct fuel injection operate with exceptionally
weak mixtures under light load conditions; for further
information, see section 3.4 (direct fuel injection) and
also lean burn technology in section 3.5.5.
Closed loop
When an engine is operating under conditions where
the mixture is at or close to λ = 1, the oxygen or lambda
sensor monitors the oxygen content in the exhaust gas,
and the ECU responds to the sensor signal as necessary.
The process is referred to as ‘closed loop’ because there
is a continuous loop or repeat of the necessary actions,
i.e. monitoring and correction, as shown below and in
Figure 3.67.
Action 1
Action 2
Action 3
Oxygen/lambda sensor measures the
Signal sent by sensor to ECU.
ECU alters fuel quantity if necessary to
change the oxygen level.
Oxygen/lambda sensor measures the
appropriate signal to the ECU. If the oxygen content is
incorrect, the ECU makes a fine adjustment to the fuel
supply to enable the correct oxygen content to be
Open loop
When engine operating conditions or driving conditions
dictate that the air:fuel ratio should be outside the
lambda window (usually a rich mixture for full load or
cold running conditions), the lambda sensor signal is
effectively ignored. The fuel quantity will therefore
depend entirely on the ECU calculations from other
sensor information. The ECU will, however, revert back
to closed loop operation as soon as the operating and
driving conditions dictate.
Lambda/oxygen sensor operation (step type)
General principles
Although there are a few variants of lambda sensor, the
general principle of operation relies on comparing the
oxygen content in the exhaust gas to the oxygen content
in the air. In effect the oxygen content in the air acts as
a reference level against which the oxygen content in
the exhaust gas is compared.
In all types of lambda sensor in common use, an
electrical signal is produced by the sensor, depending
on the amount of oxygen within the exhaust gas. The
signal voltage changes with changes in oxygen level. A
signal is therefore transmitted to the fuel injection or
engine management ECU, which alters the fuel quantity
as necessary, until the oxygen content of the exhaust
gas is correct for efficient catalytic converter operation.
In a closed loop operation, the ECU calculates the
required fuel quantity based on information from the
various sensors (air mass, throttle position,
temperature, etc.) but the oxygen/lambda sensor
checks the actual oxygen content and provides the
Step type sensor operation
The more commonly used early generation of lambda
sensor is referred to as the step type or ‘narrow band’
sensor. The name originates from the characteristics of
the signal voltage produced by the sensor as explained
in the following paragraphs.
Figure 3.68 shows a schematic view of a simple step
type lambda sensor located in the exhaust pipe. In the
Figure 3.67 Closed loop operation
Figure 3.68 Schematic view of a step type lambda sensor
Action 1
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illustration, the sensor ceramic (item 1) is coated on
both sides with a layer of platinum that acts as
electrodes (items 2). The inner electrode is exposed to
the atmosphere, whilst the outer electrode is exposed to
the exhaust gas. The outer electrode has an additional
layer of porous ceramic (item 6) to protect it against the
exhaust gases that would eventually erode the platinum
At higher temperatures, the main ceramic (item 1)
allows oxygen ions through if there is a difference in the
oxygen content either side of the ceramic. When this
occurs it causes a small voltage to be produced across
the two platinum electrodes (shown as the small meter
connected across the electrodes in the illustration). The
voltage produced depends on the difference in oxygen
content on either side of the ceramic. Because one side
of the ceramic is exposed to the atmosphere (with an
oxygen content of around 20.8%) and the other to the
exhaust gas (with an oxygen content of around 0.2% to
0.3% in the region of λ = 1), there is a large difference
in oxygen content either side of the ceramic. The large
difference causes ions to be conducted through the
ceramic, which in turn results in a small voltage being
produced across the electrodes.
Figure 3.69 shows the voltage output from a step
type lambda sensor. An important point to note is that
when the air:fuel ratio is weaker than stoichiometric
(λ > 1), the voltage produced is typically lower than
100 mV (0.1 volts); in fact, the voltage may be as low as
50 mV and increase only very slightly as the mixture
becomes a little richer. When the air:fuel ratio is richer
than stoichiometric (λ < 1), the voltage produced is
around 950 mV (0.95 volts). The voltage only decreases
slightly as the mixture becomes weaker. However, when
the mixture changes to just slightly weaker or richer
than stoichiometric (just either side of λ = 1) the
voltage suddenly jumps from the weak mixture value of
approximately 100 mV to the rich mixture value of
approximately 950 mV.
In effect, the voltage produced by the sensor
changes little as a weak mixture becomes richer (i.e.
lambda is still greater than approximately 1.03).
However, when lambda reaches approximately 1.03,
the voltage suddenly jumps in a large step to a lambda
of approximately 0.97. If the mixture continues to
become richer (i.e. λ < 0.97), the voltage change is once
again very small.
The large step in voltage that occurs between about
λ = 0.97 and λ = 1.03 provides a very distinct signal
that enables the ECU to detect the change in oxygen
when the air:fuel ratio alters slightly away from λ = 1.
Conveniently, the lambda values of 0.97 and 1.03
effectively define the lambda window (this is covered in
section 3.5.1 and shown in Figure 3.57).
The signal voltage shown in Figure 3.69 is obtained
at a temperature of approximately 600°C. The voltage
produced also varies with temperature, so it is essential
that the ECU responds only to the sensor signal when
the sensor is at a defined operating temperature. The
Fundamentals of Motor Vehicle Technology: Book 2
Figure 3.69 Voltage output from a step type lambda sensor
sensor starts to function reliably when the temperature
temperature is around 600°C. Because the sensor
response to changes in oxygen content is very slow at
lower temperatures, the ECU is able to ignore these
slow responses and operate in ‘open loop’ mode. When
the sensor is at the required temperature and the
response time is quicker, the ECU then operates in
‘closed loop’ mode. Response times to changes in
oxygen level are around 50 ms for a fully hot sensor,
whilst for a cold sensor the response can take much
longer than 1 second, which is not suitable for
operating in closed loop because of the time delay.
The standard step sensor relies on heat from the
exhaust gas to reach operating temperature.
Immediately after starting and during warm-up from
cold the system will operate in open loop until the
sensor is at operating temperature. The sensor should
ideally be located close to the exhaust manifold to
obtain as much heat as possible after starting; however,
because of the potential for overheating and premature
deterioration, sensors are generally located some
distance from the manifold.
Heated step type lambda sensor
To overcome the problems of temperature variation,
which can change the accuracy and reliability of the
lambda sensor signal, heated sensors were developed.
There are two main types of heated sensor (step type),
as shown in Figure 3.70.
Figure 3.70a shows a direct development of the step
sensor described above. This type has an electrical
heater located in the sensing element of the sensor
assembly. When the engine is started and is running,
battery power is supplied via a relay to the heater. The
heater rapidly raises the temperature of the sensor so
that closed loop operation can start as little as 20 or
30 seconds after a cold start.
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Figure 3.70b shows a later type of heated step type
lambda sensor. Although this type operates in a similar
manner to the first type, the sensing element and heater
assembly are layered. The illustration shows this
layered construction.
Limitations of step type sensors
Step type sensors (heated and unheated) provide a
signal change that can only be effectively detected by
the ECU when the air:fuel ratio alters at the rich or
weak extremes of the lambda window. The sensor can
only clearly indicate when the lambda factor is at these
points, i.e. when there is a stepped change in the
voltage (as illustrated in Figure 3.69). These easily
identifiable stepped changes are easily detected by the
ECU, but it is not easy to measure the exact signal
corresponding to λ = l (which occurs at around
450 mV) because the voltage rise or fall is too rapid
when the value is close to λ = l.
Whilst the step type sensor signal is very effective
for vehicles fitted with three-way catalytic converters,
this narrow band of operation does not allow the
sensors to be effective when engines operate with a
wider range of air:fuel ratios. This is especially relevant
to the modern generation of direct injection engines
that operate with a stratified charge and air:fuel ratios
that may be as weak as 30:1 or 40:1. A sensor is
therefore required that can measure oxygen levels over
a broader range of air:fuel ratios; such sensors are
covered later in this section.
Figure 3.70 Heated lambda sensors
a An example of a heated lambda sensor
b Alternative type of sensor element with combined heater
Operating signals for step type lambda
It was highlighted earlier in this section that a
significant change in voltage is produced by the sensor
by changes around λ = 1. The approximate lambda
value range that causes the jump in voltage is λ = 0.97
to λ = 1.03 (see Figure 3.69).
If we assume that the engine and lambda sensor are
at full operating temperatures, the ECU will provide the
appropriate fuelling that should in theory result in an
air:fuel ratio of λ = 1. In reality, minor variations will
always occur; and the lambda sensor signal provides
the reference to the ECU regarding such deviations.
The measuring and correction process then passes
through the following phases (this is closed loop
operation, also referred to as ‘feedback control’).
1 If we assume that a very slightly rich mixture exists,
the lambda factor will be slightly low (λ < 1). The
sensor signal voltage will therefore rise to
approximately 950 mV, and the ECU will reduce the
fuel quantity slightly, which will cause a weakening
of the mixture and a reduction in the sensor voltage.
2 The sensor will now detect the weakening of the
mixture, which will produce a slightly increased
oxygen level. The lambda factor will increase
(λ > 1), which will cause the sensor voltage to fall
to approximately 100 mV. The ECU will detect the
reduced voltage and again alter the fuel quantity,
this time to provide a slight enrichment.
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3 The process will now start again, resulting in a
continuous increase and decrease (oscillation) in
the sensor voltage, as shown in Figure 3.71.
The frequency of the oscillations of the lambda sensor
signal depends on the speed of response for the sensor,
i.e. how quickly the sensor registers the change in
oxygen, which can be as rapidly as within 50 ms.
However, the fuelling change leads to an inevitable
delay in the alteration of the oxygen content, as this
change passes through the exhaust system to the sensor.
It is common to find that the frequency of change for
the sensor signal is around 1–3 Hz, depending on
engine speed, temperature and other factors.
The sensor signal in Figure 3.71 shows a very
consistent and regular oscillation which is seldom seen
in practice owing to minor variations that occur in the
oxygen content of the exhaust gas sample. The hash
that exists around the signal is typical of a lambda
sensor signal.
Figure 3.71 Continuous signal produced by a step type lambda
Broadband sensor
As mentioned earlier, the broadband sensor can be used
to measure the oxygen content over a very broad range
of air:fuel ratios, i.e. across a wide range of lambda
values from approximately 0.7 to 4. The broadband
sensor is therefore suitable for lean burn engines, such
as those using a stratified mixture formation (typically
on modern direct injection engines). In addition, the
broadband sensor can be used on diesel engines and
engines operating on gaseous fuels.
As well as offering a capability of measuring over a
wider lambda range, the broadband sensor provides a
progressive voltage change across the range of
operation, as opposed to the stepped voltage jump that
occurs with a narrow band step sensor.
Operating principles
Figure 3.72 shows the construction of the sensing
elements in a broadband sensor. The construction is
similar to the later type step sensors, with the different
materials arranged in layers.
Fundamentals of Motor Vehicle Technology: Book 2
Item 7 in the illustration is effectively a step type sensor
element or cell; however there is an additional cell,
referred to as the ‘oxygen pump cell’ (item 8). An
extremely small diffusion gap (item 6) separates the
pump cell from the step type sensing cell. There is
however a barrier (a porous diffusion barrier, item 11)
through which the oxygen from the exhaust gas must
flow before it reaches the diffusion gap. Once in the
diffusion gap the oxygen affects one electrode on the
step type sensing cell. The other electrode of the step
type cell is exposed to the reference air (oxygen in the
atmosphere), just as with a standard type step sensor.
The step type cell is measuring the oxygen content in
the diffusion gap, which would be lower than the
oxygen content in the air, thus causing a voltage to be
produced by the step type cell; this voltage is
transmitted to the control unit.
However, exhaust gas must flow through the pump
cell before it can reach the diffusion gap. The pump cell
also has the capacity to pump oxygen through the
diffusion barrier in either direction, so it can increase or
decrease the oxygen content in the diffusion gap. The
pumping action is created by providing a small
controlled voltage to the pump cell’s platinum
electrodes; the way the current flows dictates whether
oxygen is pumped in or out of the diffusion gap. The
objective is to use the pump cell to maintain the oxygen
content in the diffusion gap at a value of λ = 1. If the
diffusion gap contains too much or too little oxygen
(weaker or richer than λ = 1) because of high or low
oxygen levels in the exhaust gas, the pump cell then
decreases or increases the oxygen in the diffusion gap to
achieve λ = 1.
The process starts with the step type cell measuring
the oxygen content in the diffusion gap, creating a
voltage signal that is assessed by the electronic control
unit. Note that the initial oxygen level and oxygen level
for normal operation will depend on the oxygen content
in the exhaust gas, which will have flowed into the
diffusion gap. If the oxygen level in the diffusion gap is
too high or too low (higher or lower than λ = 1), the
electronic control applies the appropriate current at the
pump cell electrodes to pump oxygen in or out of the
diffusion gap as required to achieve λ = 1. The level of
current required is an indicator of the oxygen being
pumped in or out of the diffusion gap, and is therefore
also an indication of the oxygen content of the exhaust
If the oxygen content in the diffusion gap is at λ = 1,
there is no requirement for pumping oxygen, and the
current will be zero. However, if the oxygen content is
high (λ > 1) a negative current is used to pump oxygen
out of the diffusion gap; if the oxygen content is low
(λ < 1) a positive current is used to pump oxygen into
the diffusion gap. The electronic control unit produces a
sensor signal (voltage change) that is dependent on the
current level required to maintain the oxygen content in
the diffusion chamber at λ = 1.
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Figure 3.72 Broadband sensor (sensing element construction)
Sensor heating
Within the sensing unit assembly, a heater element is
constructed to heat the sensor rapidly and maintain a
relatively constant temperature that is not greatly
influenced by exhaust gas temperature. The function
and operation of the heater are the same as described
for the step type sensor. Broadband sensors generally
operate in the range 650–900°C.
Sensor output signal
Sensors produced by different manufacturers might
provide slightly different output signals to be monitored
by the engine management ECU. An example of a
broadband sensor signal is shown in Figure 3.73. Note
that the voltage change is progressive across a wide
lambda range; the illustration shows the voltage change
Figure 3.73 Output signal voltage for one type of broadband
within the range of λ = 0.7–1.5, although higher
lambda values are measured and provide slightly higher
voltage values.
Other types of lambda sensor
Some lambda sensors rely on a special semiconductor
device that responds to changes in oxygen content by
altering its resistance. When a voltage is applied across
this device, the resistance change affects the current in
the circuit (a series resistance circuit). This change is an
indicator of oxygen content. Thus it is possible to apply
a 5 volt reference voltage to the sensor, obtaining a
signal voltage that ranges from 1 to 4 volts, depending
on the oxygen content in the exhaust gas.
3.5.8 Exhaust gas recirculation
A method of reducing NOx
NOx formation
Within section 3.5, and in other sections of Chapter 3,
many references were made to oxides of nitrogen
(NOx). These are regarded as a pollutant and are formed
by the combination of oxygen molecules and nitrogen
molecules (both of which exist naturally in the air.
Whilst NOx formation increases when the air:fuel
ratio is very slightly weaker than the stoichiometric
value (i.e. λ = 1.05–1.1; see Figure 3.61), it is also true
that NOx formation increases significantly when
combustion temperatures rise. NOx formation occurs
because of the heat of the combustion process, which
enables a chemical change to take place. For NOx this
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can start when combustion temperatures are around
1300°C, but the formation of NOx increases at around
1800°C and accelerates if temperatures exceed around
When an engine is operating under load conditions,
especially when the air:fuel ratio is close to
λ = 1.05–1.1 (which can exist during part load
conditions), combustion temperatures will rise.
However, high load conditions, where a full charge of
air enters the cylinder (with the throttle fully open),
will inevitably cause higher combustion temperatures,
but under high load conditions the mixture is usually
slightly richer, which helps to reduce NOx formation.
When an engine is operating under part or medium
load conditions with an air:fuel ratio slightly weaker
than λ = 1, combustion temperatures are high and the
oxygen content of the exhaust gas enables high levels of
NOx to be formed. Although catalytic converters can be
used to reduce NOx levels after it has been formed
during combustion, it is often also necessary to use
other means to reduce the production of NOx during the
combustion process. In general, these devices or
processes are designed to reduce combustion
Using the exhaust gas
A very effective and well established method of
reducing combustion temperatures is to pass a
percentage of the exhaust gas back into the intake
system where the exhaust gas mixes with the new
charge of air entering the cylinder.
Exhaust gas is made up of a high percentage of inert
gases, such as water vapour (H2O) and carbon dioxide
(CO2). An inert gas does not combust, so when these
gases mix with the air flowing into the cylinder, they
cause a lowering of the combustion temperature and a
reduction in the formation of NOx.
Figure 3.74 Exhaust gas recirculation system (EGR)
Fundamentals of Motor Vehicle Technology: Book 2
A widely used method of enabling exhaust gas to mix
with the fresh intake air is to recirculate some of the
exhaust gas back into the intake manifold (Figure 3.74).
This process is referred to as exhaust gas recirculation
Note that a similar but less effective result can be
achieved when the valve timing is arranged so that
there is valve overlap, i.e. the intake valve opens before
the exhaust valve closes (at the end of the exhaust
stroke). The result is that some exhaust gas mixes with
the incoming fresh charge of air. The amount of valve
overlap on most engines was fixed, until recently.
However, variable valve timing mechanisms are now
used for many engines so that different overlap periods
can be used at different engine speeds. In reality,
variable valve timing systems are used to enable good
power or torque to be achieved over the whole engine
speed range, but the added benefit of valve overlap is to
facilitate the mixing of some exhaust gas with the fresh
charge of air, thus reducing combustion temperatures
and NOx.
Controlling the quantity of recirculated exhaust
For those engines operating with homogeneous mixture
formation (all of the air drawn into the engine is mixed
with fuel), it is possible to introduce around 10–15% of
the exhaust gas back into the fresh charge of air that is
drawn into the cylinder. This percentage does not
dramatically affect fuel consumption and power, but is
usually sufficient to reduce the NOx by a significant
amount. Slightly higher levels of exhaust gas
recirculation can be used on engines operating with a
stratified mixture formation (direct injection engines).
Importantly, the amount of exhaust passing through
to the intake system will depend on the exhaust gas
pressure and the intake manifold pressure (or
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Emissions and emission control (petrol engines)
depression). If the exhaust gas pressure is high, such as
when the engine is under high load (when the throttle
is fully open, high volumes of air are drawn into the
cylinder which results in high volumes of exhaust gas,
increasing the pressure in the exhaust system), a higher
flow of exhaust gas into the intake system will result. It
is therefore necessary to control the flow of exhaust gas
into the intake system to ensure that excessive
quantities of exhaust gas do not enter the intake system
and combustion chamber, which would cause poor
In general, the exhaust gas is recirculated during
part/medium load conditions but not when the engine
is at idle or at full throttle (when a richer mixture might
be used which would cause a reduction in NOx).
Figure 3.75 EGR valve with integrated solenoid
Figure 3.76 EGR system with vacuum
valve control
EGR valve
An EGR valve (Figure 3.75) is used to control the flow
of recirculated gas. Figure 3.74 shows an EGR valve
located in the pipe that feeds the exhaust gas to the
intake system. In this example, the valve is directly
opened and closed by a solenoid which is controlled by
the ECU. The ECU, which is receiving information from
various sensors, (e.g. engine speed, throttle position,
temperature, etc.) opens the valve the appropriate
amount to allow the required amount of exhaust gas to
be recirculated to suit engine operating conditions.
The EGR solenoid may be supplied with a digital
control signal that allows the gas valve to be accurately
positioned. The control signal duty cycle is altered to
provide an increased or decreased current/voltage to
the solenoid, which causes the valve to open or close to
a greater or lesser extent (see section 1.9).
Different types of EGR control system
The simplest type of system to control the flow of gases
in an EGR system is shown in Figure 3.74. This relies on
a single valve, directly controlled by the ECU. However,
other systems have been used, requiring additional
valves and sensors, that ensure that recirculated
exhaust gas is passed to the intake system.
Figure 3.76 shows a system where the main EGR
valve is opened and closed by ‘vacuum’ (low pressure or
depression) which is taken from the intake system. The
vacuum level applied to the EGR valve is in turn
regulated by an ECU controlled vacuum valve. On this
type of system, it is the vacuum control valve that
receives the digital control signal from the ECU.
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In some systems, the EGR valve is fitted with a sensor
that tells the ECU the amount the valve is open. Other
systems use a pressure sensor to detect the pressure in
the exhaust recirculation pipe. With these sensors, the
ECU is able to assess the amount of exhaust gas flowing
in the EGR system.
3.5.9 Secondary air injection
Previous parts of section 3.5 have highlighted the
problems of high emission levels during cold running.
Even modern engines require some enrichment during
the early phases of the warm-up period. Enrichment is
required to overcome condensing of the fuel on cylinder
walls and intake system walls, and to ensure that
sufficient fuel is able to vaporise in what is initially a
relatively cold environment.
However, a rich mixture will have insufficient
oxygen (λ of around 0.9 or less) which will result in
high levels of CO and HC. In many cases, these CO and
HC levels exceed what is permitted and must therefore
be reduced. Because the levels are so high, there is too
much CO and HC, and insufficient oxygen for the
catalytic converter to change the CO and HC into CO2.
Furthermore, in the early phases of engine start-up, the
catalytic converter is not at working temperature.
It is possible to inject air into the exhaust ports or
exhaust manifold, which enables oxygen to combine
with the CO and HC, owing to the temperature of the
exhaust gas. In effect, a combustion or oxidation
process occurs in the exhaust manifold. The secondary
air injection process is used only for short periods after
the engine is started from cold, but is sufficient to
reduce the CO and HC levels during this period. In
addition, the combustion of gases in the exhaust
manifold or ports adds additional heat to the exhaust
gas, which assists in quickly raising the temperature of
the catalytic converter, so it is able to function sooner.
Figure 3.77 Secondary air injection system
(with air pump)
Fundamentals of Motor Vehicle Technology: Book 2
Air pump injection
Figure 3.77 shows a layout for an air pump based air
injection system. The electrically driven air pump draws
air from the atmosphere (via the air filter) and pumps it
into the exhaust manifold through a control valve
which regulates the amount of air depending on
operating conditions. The control valve is connected to
the intake manifold (position A on the diagram) and
intake vacuum (depression) can therefore be passed to
the secondary air valve via a control valve. The vacuum
passing to the secondary air valve is regulated by a
control valve, which is controlled by the ECU.
When the engine is able to operate without mixture
enrichment and the catalytic converter is at working
temperature, the air pump is switched off.
Pulse air sysdems
Operating in much the same way as air pump injection
systems, the pulse air system relies on pressure pulses in
the exhaust pipe to draw in air, rather than using a
pump (Figure 3.78). The exhaust system is subject to
positive and negative pressure pulses when the engine
is running. Positive pulses occur when an exhaust valve
opens; negative pulses occur when the valve closes but
exhaust gases continue to move through the exhaust
A valve (one-way valve in the diagram) is used to let
the air into the exhaust manifold. It opens when the
pressure pulse in the exhaust system is low, and closes
when pulse pressure is high. The pulse pressure in each
exhaust port changes when the exhaust valve opens and
closes, so the valve opens and closes continuously and
rapidly during the warm period.
Vacuum from the intake manifold is fed to the pulse
air solenoid valve, which is controlled by the ECU.
When the ECU control signal opens the valve, vacuum is
passed to the ‘vacuum controlled valve’ which in turn
also opens, thus allowing air to flow to the one-way
valves and the exhaust manifold. The ECU can control
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Figure 3.78 Pulse air injection system
the pulse air solenoid valve so that the vacuum affecting
the control valve is regulated, thus enabling regulation
of the air flowing into the exhaust manifold.
3.5.10 Evaporative emission control
Figure 3.79 Evaporative emission control system (EVAP)
Key Points
Evaporative emission control systems (EVAP) are used
to prevent vapour from the fuel tank and fuel system
from escaping into the atmosphere. Figure 3.79 shows a
basic EVAP system.
A charcoal (carbon) canister is connected to the fuel
tank. The canister collects the fuel vapour which is
stored by the charcoal. When the engine is running,
vacuum (depression) in the intake system draws the
vapour from the canister and into the engine where it
mixes with the intake air. A ‘canister purge valve’ is
controlled by the ECU using a digital control signal
(section 1.9), to ensure that the flow of vapour to the
intake system is regulated. This allows a controlled
amount of vapour to enter the intake system for
different operating conditions. Typically, the vapour will
be drawn from the canister during light/medium load
engine conditions.
When the vapour mixes with the intake air, there
will be a slight enrichment of the air/fuel mixture. The
ECU will therefore adjust the injected fuel quantity to
compensate. However, because the process of drawing
vapour from the canister usually occurs only when the
lambda control system is operating in a ‘closed loop’,
the lambda sensor transmits an appropriate signal to
the ECU if the mixture is too rich, thus enabling the ECU
to make any corrections.
EGR reduces cylinder combustion temperature,
which in turn reduces NOx emission
The main way that emissions are reduced is by
maintaining accurate ignition and mixture strength
Evaporative emissions are reduced by storing them
in charcoal canisters and then burning them in the
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Fundamentals of Motor Vehicle Technology: Book 2
3.6.1 Combining the various systems
The requirement for integrated control
Each of the individual engine systems so far covered
within Chapter 3 (ignition, injection and emission)
have been subject to continuous development and
improvement. In isolation, there have been
improvements in engine power, fuel consumption and
lowering emission levels. However, the control of each
system needs to be integrated with the control of the
other systems to achieve the optimum results.
1 Activated charcoal canister
2 Hot film air mass sensor with integrated temperature
3 Throttle device (electronic throttle control)
4 Regeneration valve
5 Intake manifold pressure sensor
6 Fuel rail
7 Fuel injector
8 Actuators and sensors for variable valve timing
9 Ignition coil and spark plug
10 Camshaft phase sensor
11 Lambda sensor upstream of primary catalytic converter
12 Engine ECU
13 Exhaust gas recirculation valve
14 Speed sensor
Figure 3.80 Bosch ME Motronic engine management system
An example of integrated or combined control is the
need sometimes to alter ignition timing and fuel
quantity at exactly the same time to suit a change in
engine operating conditions. To achieve the best results,
communication between the ignition ECU and the fuel
system ECU is essential. Another example is the need to
alter timing and fuelling when certain emission control
functions are implemented: again, communication
between the different engine systems is essential to
achieve optimum system and engine efficiencies.
Knock sensor
Engine temperature sensor
Primary catalytic converter (three-way catalytic converter)
Lambda sensor downstream of primary catalytic converter
CAN interface
Fault indicator lamp
Diagnostics interface
Interface with immobiliser ECU
Accelerator pedal module with pedal travel sensor
Fuel tank
In-tank unit comprising electric fuel pump, fuel filter and
fuel pressure regulator
26 Main catalytic converter (three-way)
The on-board diagnostics system configuration illustrated by
the diagram reflects the requirements of EOBD
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Single ECU
During the late 1970s, some engine management
systems were produced with a single ECU. This type of
system became much more widely produced during the
1980s. Currently, nearly all light vehicle petrol engines
are fitted with a single ECU that controls most of the
functions. In effect, all of the engine related systems are
managed by a single ECU.
The next stage of integrated control systems would
be to control all vehicle functions from a single ECU: for
example, the engine and chassis systems (including
features such as anti-lock brakes), could all be
controlled from a single ECU. Although some attempts
have been made to produce a single ‘vehicle’ control
unit, the trend is to use high speed communication
networks that enable all of the vehicle system ECUs to
communicate and share information.
Figure 3.81 Bosch MED (direct fuel injection) Motronic engine
management system
3.6.2 Modern engine management
Apart from the diagnostic functions, almost all of the
individual systems that make up a modern engine
management system have been covered in the previous
sections of Chapter 3. The following two examples of
modern engine management systems illustrate the way
in which all of the previously discussed systems are
integrated using a single ECU.
Figure 3.80 shows a Bosch ME Motronic engine
management system which uses port type fuel injection
along with direct ignition and various emission control
Figure 3.81 shows a modern direct injection engine
management system (Bosch MED Motronic). This
system is similar to the example shown in Figure 3.80,
but note the differences in the fuel injection and
emission control systems.
Both systems feature European on-board diagnostics
(EOBD) connections which are covered in section 3.7.
Key Points
On early generations of engine management system,
ignition and fuel injection ECUs were separate, but
they communicated so that control functions were
harmonised. In addition, it was possible to share the
information from the various sensors. One
temperature sensor could be used to transmit a signal
to one of the ECUs, which in turn would transmit a
signal to another ECU. This approach reduced the
need to duplicate sensors and wiring, which inevitably
reduced cost.
Full engine management systems combine ignition
and fuel control as well as controlling many other
aspects such as EGR
Most modern engine management ECUs are
linked by CAN to other systems such as
transmission management
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3.7.1 Self-diagnosis of system faults
Reasons for self-diagnosis (on-board diagnostics)
Since the mid-1980s, an increasing number of European
market vehicles have been equipped with some form of
self-diagnosis. Self-diagnosis systems were in use
earlier, but mainly on American market products.
Self-diagnosis is the capacity of a vehicle system (or
more precisely the system ECU) to detect system
operating faults, and provide an indication that a
specific fault exists.
Self-diagnosis was initially fitted to fuel and ignition
systems, but it has become increasingly fitted to most
vehicle systems with electronic control. Self-diagnosis is
now found on vehicle systems such as ABS (and other
vehicle stability control systems), automatic gearboxes,
air conditioning, airbags and other safety systems. Selfdiagnosis is a function of the system computer or ECU,
which monitors and assesses all the input signals (from
sensors) as well as providing the output control signals.
The system ECU is able to distinguish correct from
incorrect signals (and operation) for much of the
system that is being controlled.
Self-diagnosis was introduced to overcome a
number of potential problems.
1 One aim of self-diagnosis is to overcome the
problems of working and diagnosing faults
associated with ‘new technology’. This is especially
important as each new vehicle system is introduced.
Repair technicians need to be able to identify faults
relatively quickly and easily.
2 Fast and accurate diagnosis should help to reduce
warranty costs, while retaining customer loyalty and
3 The self-diagnosis system can be used during vehicle
assembly to ensure that each vehicle leaves the
assembly plant without detectable faults.
4 Importantly, modern engine management systems
and emission control systems are fitted to vehicles to
ensure that emission levels remain low and within
legislated limits. Many faults that would result in
unacceptable emissions can remain undetected by
the driver. With a self-diagnostic system, it is
possible to provide a warning to the driver or the
workshop technician, along with an indication of
what the fault is.
5 Modern vehicle systems rely on complex software
programs within the ECU as well as electrical and
electronic systems. In a high percentage of cases,
technicians are not familiar with these ‘new’
technologies. Self-diagnostic systems are designed
to support technicians on unfamiliar aspects of a
new motor vehicle.
Self-diagnostic facilities should help to reduce the
length of time that faults remain undetected.
Self-diagnosis systems form only a small part of the
whole facility. The next section highlights other
functions that can be implemented by the system
computer to further assist in reducing emissions when
faults occur, and to assist a technician to test or check
vehicle system operation. The self-diagnosis facility is
often referred to as OBD (on-board diagnostics).
Vehicle manufacturers appoint authorised dealers or
repair workshops to sell and maintain vehicles. These
usually have very sophisticated, dedicated equipment
containing code reading or scan tools as part of the
overall equipment package. However, independent
repair workshops usually purchase general purpose
code reading equipment, designed to function on many
makes and models of vehicle. The general purpose
equipment is usually designed so that cartridges or pods
can be inserted into the code reader; the different
cartridges contain different software programs that
enable the code reader to communicate and operate
with different vehicles.
Fault codes, blink codes and fault related
Once the ECU has recognised that a fault exists on a
vehicle system, it is able to implement other functions.
The following list covers the common functions
performed by the ECU when a fault is recognised.
1 The ECU can illuminate a dashboard warning light
to indicate to the driver that a fault exists.
2 A system failure, such as a sensor fault, could cause
the engine to run poorly and generate high
emissions. Under these conditions, the ECU might
be able to control the engine management system so
that it operates in a ‘limp home’ or ‘fail safe’ mode.
When the ECU implements limp home operation, it
normally substitutes a preset value for the failed
sensor, which should ensure reasonable engine
3 The ECU can provide some form of coded output
(fault code) which is accessed or retrieved by a ‘fault
code reader’ or through other means such as a ‘blink
code’ transmitted via the dashboard warning light or
via an LED based test tool. A fault code usually
consists of a number (or series of numbers),
although some systems provide a series of letters or
even a short message. As explained later in this
section, some standardisation has been adopted
across all vehicles for fault code systems, whereby
specific faults are allocated a dedicated code
number. This standardisation generally only applies
to emissions related faults, but inevitably embraces
the engine management system.
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Methods of retrieving fault codes
As mentioned above, fault codes can be retrieved from
the vehicle ECU using a fault code reader which, when
connected to the ECU, displays a code number related
to the fault.
Many systems, however (especially older systems
and non-engine management systems), provide what is
often referred to as a ‘blink code’. A blink code can be
output by the ECU to the dashboard warning light, and
the number of flashes on the warning light corresponds
to the relevant code number. Some systems use an LED
light (often located on the ECU) or codes might be
accessed by connecting a separate LED tester (or a volt
or dwell meter). When a fault code number has been
retrieved, the technician refers to the appropriate
information source to look up the code number and
establish the nature of the fault. Most code readers
indicate the messages within the code reader software.
To access a blink code, it is normally necessary to
perform a set procedure which effectively ‘instructs’ the
ECU to output the code. The procedure may involve
linking two terminals on a connector plug or other
similar procedures which will then cause the ECU to
output the code.
When a code reader is connected to the appropriate
connector plug, it communicates with the vehicle ECU,
and by following the instructions supplied with the
code reader (or the instructions on the code reader
screen display), the technician can make the code
reader transmit an instruction to the ECU which causes
the ECU to output the codes. The instruction from the
code reader may take the form of a password; when the
instruction or password is sent to the ECU, the ECU
provides the coded output, thus allowing the code
reader to display the code number.
Although some inexpensive code readers effectively
act as a means of accessing the simple blink code (i.e.
they simply provide a code number), most elaborate
readers also display a message which details the nature
of the fault associated with the displayed code.
When a blink code is read via a warning light or
LED, it is normal practice to count the number of times
the light flashes. When more than one code exists,
different systems will use slightly different methods of
separating the different codes. There are different
methods of displaying a code number. For instance,
code 12 could simply be displayed by 12 flashes, but
alternatively could be displayed by providing one flash,
a pause, then two flashes.
The example in Figure 3.82 shows how two codes
might be displayed (i.e. code 12 and then code 23). A
brief pause is provided between the ‘tens and units’ as is
the case between the 1 and the 2 of code 12, and
between the 2 and the 1 of code 23. However, a longer
pause (in this case 2 seconds) is provided between the
two different codes, as is the case between codes for 12
and 23.
Some inexpensive code readers simply provide a
flashing LED; the flashes are counted in the same way
as a normal blink code. Most code readers provide a
display screen that indicates the code as a number. A
code reader might provide a relevant message in
addition to the code number.
Two-digit code
One-digit code
0.5 2 s
4.5 s
4.5 s
T or TE1 terminal ON
T or TE1 terminal ON
(Trouble codes 12 and 23 shown)
(Trouble codes 2 and 3 shown)
0.5 2 s
4.0 or 4.5 s
0.5 2 s
1.5 s
0.5 2 s
2.5 s
1.5 s
T or TE1 terminal ON
Figure 3.82 Example of a blink code (code 12 followed by code 21)
4.0 or 4.5 s
0.5 2 s
2.5 s
T or TE1 terminal ON
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Fundamentals of Motor Vehicle Technology: Book 2
3.7.2 How the ECU recognises and
overcomes system faults
Expected operating values
Virtually all the circuits connected to the system
computer (ECU) carry a voltage or signal that should lie
within certain operating values. These values are
programmed into the ECU, which can compare actual
signal values with expected or programmed values.
For example, the power supply for the ECU might be
expected to fall between lower and upper limits of
9 volts and 15 volts. If the voltage is above or below the
expected limits, this would be recorded as a fault by the
ECU – the starting point for self-diagnosis.
Many sensors on computer controlled vehicle
systems operate by providing a voltage or signal voltage
that should normally lie within certain easily defined
limits. The ECU can thus recognise that a fault exists,
assuming that the fault causes the signal voltage to fall
outside the expected limits or tolerance.
Example of fault detection using a simple
temperature sensor circuit
One example of a sensor circuit that should usually
provide a voltage within defined limits is the engine
coolant temperature sensor. It functions by changing its
resistance as the coolant temperature changes. The
sensor forms part of a series resistance circuit, which
means that when the resistance of the sensor changes, it
affects the voltage in the circuit (Figure 3.83).
The ECU provides a reference voltage to the
temperature sensor circuit which, on modern systems, is
normally 5 volts. When the circuit is complete (sensor
plug connected) the voltage on the section of the circuit
between the ECU and the sensor is reduced (by the
action of the resistances). The exact voltage depends on
the value of the resistances and, although the ECU
resistance remains the same, the sensor resistance
changes, thus affecting the voltage in the circuit.
On nearly all temperature sensor circuits, a typical
operating value is around 3.0–3.5 volts when the
coolant temperature is low (a cold engine). The voltage
then falls to around 0.3–0.7 volts when the engine is
hot. It is possible for the voltage to reach higher or
lower values, but this would mean an extremely cold or
hot engine.
Assuming that the sensor and associated wiring are
in good condition, and that extreme cold and hot
temperatures are reached, it is possible (although very
unlikely) for the voltage in the circuit to reach as high as
4.5 volts or as low as 0.2 volts. These values can be
used by the ECU as maximum and minimum values,
and the only likely situation that would cause the
voltage to lie outside this range would be a fault in the
wiring, sensor or ECU itself. In reality, a tolerance must
be allowed slightly outside of the expected maximum
and minimum values: in our example we will use
0.1 volts as the minimum and 4.8 volts as the
maximum. Figure 3.84 shows typical values for normal
operation and for detecting a fault.
There are two main faults that the ECU will easily
identify: a break in the circuit and a short in the circuit.
Figures 3.85 and 3.86 show a circuit break (open
circuit) and a short circuit. In both cases the fault is
shown as a wiring problem. However, any part of the
circuit, including the sensor resistance itself, could
suffer a short circuit or a circuit break, which would
provide the same results.
Figure 3.84 Example of temperature sensor circuit operating
voltages and voltages that would be regarded as a fault condition
Sensor signal voltage
Extremely hot
(engine running at very high
temperature but this condition
must be allowed for)
Extremely cold
(engine running at very low
temperature but this condition
must be allowed for)
Fault condition
(ECU detects voltage values
that are outside normal
expected values)
minimum value may
be as low as 0.2 volts
maximum value may
be as high as 4.5 volts
less than 0.1 volt
greater than 4.8 volts
Voltage drops
to zero indicating
short to ECU
Voltage signal
water temperature
Short circuit
Water temperature sensor
Figure 3.83 Schematic layout of temperature sensor circuit
Figure 3.85 Temperature sensor circuit with a short to earth
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Short circuit
When there is a short circuit (Figure 3.85) that results in
the signal wire shorting through to earth, the 5 volt
reference supplied by the ECU is connected directly to
earth. The current has already flowed through the
resistance in the ECU, so, when the circuit is connected
to earth, the voltage falls to zero.
The circuit is now the same as a light circuit in
which a voltage is applied to a bulb and then from the
bulb to earth. The voltage on the earth side of the bulb
is zero. The resistance in the ECU acts in the same way
as the light bulb.
The ECU monitors the voltage in the signal circuit,
which would normally sit within acceptable operating
limits of 0.2–4.5 volts. Because the voltage detected is
now zero, the ECU recognises this as a fault. The ECU is
able to allocate a fault code to this particular fault.
Note that the ECU might provide a fault code for a
fault on the sensor circuit. However, many systems will
provide a code that states: ‘the voltage level in the
temperature sensor circuit is too LOW’.
The ECU cannot determine the exact nature of the
fault; it can only establish that a fault exists causing a
low voltage. Therefore the technician must still carry
out a detailed investigation of the circuit.
Open or broken circuit
A broken or open circuit (Figure 3.86) prevents the flow
of current through the circuit. Without a current flow, a
resistance does not have any effect on the voltage level
in the circuit. Therefore, the 5 volts, applied as a
reference voltage by the ECU, remains at 5 volts.
The ECU is again monitoring the voltage at point A
(this is the voltage in the signal circuit which would
normally be within the acceptable operating limits of
0.2–4.5 volts). Because 5 volts is now detected at point
A, the ECU recognises this as a fault, and allocates an
appropriate fault code.
Note that the ECU might provide a fault code which
indicates that a fault exists on the sensor circuit.
However, many systems will provide a code that states:
‘the voltage level in the temperature sensor circuit is too
Voltage rises to
equal supply voltage
indicating open
circuit to ECU
Break in wiring giving
an open circuit
Figure 3.86 Temperature sensor circuit with a break in the wiring
(open circuit)
The ECU cannot determine the exact nature of the fault:
it can only establish that a fault exists, causing a high
voltage. Therefore the technician must still carry out a
detailed investigation of the circuit.
Self-diagnosis on other types of sensor or circuit
The system ECU is able to monitor any of the circuits
connected to it. The ECU is effectively pre-programmed
with the acceptable values for the various circuits, and
is therefore able to identify a fault when values lie
outside acceptable limits.
Many sensors on a system provide a digital signal,
i.e. a signal that consists of on/off pulses, such as the
signal from a Hall effect trigger used for vehicle speed
sensors, ignition triggers, etc. The ECU can monitor the
pulses and detect the correct operating voltage for the
signal, or whether the pulse is acceptable or unavailable.
Other sensors provide analogue signals, which again
might consist of a series of pulses. A crankshaft
speed/position sensor usually provides a series of pulses
which the ECU is able to detect as being at a certain
voltage. Again, the ECU can detect whether the signal is
acceptable or unavailable.
The ECU also provides operating signals to actuators
such as injectors or idle speed control valves. If the
actuator and associated circuits are good, an electric
current will flow through the ECU in order to control
the actuator. The ECU is therefore able to recognise
many of the faults owing to the fact that, if there is a
fault, the current flow might be incorrect, or there
might be no current flow at all if the circuit is broken.
In all cases where a fault is identified by the ECU,
the technician should attempt to gain as much
information as possible about the operation of the
system. Knowledge of the way in which the ECU detects
particular faults and provides substitute values can
assist in accurate diagnosis.
Limp home/emergency operation
In a large number of cases when a fault is detected by
the ECU in a sensor circuit the ECU might be able to
substitute a value for the failed circuit. If the
temperature sensor signal voltage is outside the
expected values, the ECU will detect the fault. The ECU
has programmed substitution values for certain faults,
which can be used to ensure that the engine can still
run. With a temperature sensor fault, the ECU could
substitute a temperature value, such as a warm running
value, that enables the car to be driven to a repair
workshop. However, the engine would be difficult to
start from cold because the substituted value is for a
warm engine.
Modern systems can implement a more complex
substitution process. Again using the temperature
sensor fault as the example, the ECU timer facility
enables it to calculate the length of time the engine has
been running, and then internally substitute a
temperature value depending on that time period (e.g.
if the engine has been running for an hour, the
substitution value would be equivalent to normal
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engine temperature). When the engine is switched off
for a long period, the ECU would calculate that the
engine was cold, and provide a cold running
temperature value. At various time intervals, the ECU
can substitute progressively warmer temperature values
until the engine is assumed to be at normal operating
It is possible for the ECU to use substitution values
for many of the sensors. It is often the case that the
substitution process allows the engine to operate
without any indications that there is a fault. The driver
would often be unaware that a fault exists, except that
the warning light on the dashboard illuminates.
There are however a number of faults that will not
allow a substitute value to be used. An obvious example
is a power supply or earth circuit fault. Another sensor
that cannot be easily substituted is the main crankshaft
position/speed sensor. Without the signal from this
sensor, the ECU has no reference to engine speed or
exact crankshaft position, but if additional position
sensors are fitted to the engine (such as a camshaft or
cylinder identification sensor) these can allow the ECU
to provide a limited means of operating the engine
There are a number of terms used to describe the
process whereby an ECU substitutes operating values;
examples include:
limp home
limited operating strategy (LOS)
emergency operating strategy.
Faulty signal values that are within expected
Basic self-diagnosis and fault code systems (including
many of the systems produced before the end of the
1990s) have certain limitations, because they only
define a fault when the signal is outside expected or
acceptable limits. This method of diagnosis can lead to
situations where a fault does exist, but the ECU cannot
recognise it.
Again, using the temperature sensor circuit as an
example, if the sensor itself failed in such a way that the
sensor resistance remained at a value corresponding to
a normal engine temperature, the voltage in the sensor
circuit would also be at an acceptable value. The ECU
would not then recognise this failure.
If, for example, the sensor failed, so that the
resistance of the sensor (and therefore the voltage in the
circuit) corresponded to 40°C (a warm engine) the ECU
would only detect this value, and provide a slightly rich
fuel mixture which is applicable to the warm engine
temperature. That the voltage in the circuit is ‘stuck’ at
this value will not be registered as a fault by these older
However, as described in the previous paragraphs
dealing with limp home/emergency operation, most
modern systems now have a facility to recognise that
the value is unchanging: the ECU can then provide a
substitute value.
Fundamentals of Motor Vehicle Technology: Book 2
Many systems up until the late 1990s (and possibly
later) did not have the facility to recognise faults for
which the value remained within acceptable limits.
Therefore technicians should always be prepared for
self-diagnosis and fault code systems that do not
indicate a fault, even if a fault exists in the system.
Overcoming the limitations of less sophisticated
self-diagnosis systems
The previous paragraphs highlighted the fact that less
sophisticated systems have certain limitations that must
be accounted for when carrying out diagnosis.
However, most of these limitations have now been
overcome by the system manufacturers, by increasing
the capability of the self-diagnostic section of the
More capable systems operate on what is sometimes
referred to as ‘improbable or implausible’. In effect, the
ECU is programmed with a degree of ‘intelligence’,
which enables it to judge whether a signal or value on a
circuit is probable or plausible.
For example, a temperature sensor value not
altering from a cold value when the engine has been
running for a considerable amount of time would be
regarded as improbable or implausible. The ECU can
use more than one item of information from the other
sensors or from programmed logic to make a judgment.
An example of programmed logic would be: ‘a cold
temperature value cannot be correct when the engine
has been running for a long time’.
A signal would not be plausible if:
1 the throttle position sensor indicates that the
throttle has just opened fully
2 the engine speed has increased
3 the airflow sensor indicates that there has been no
change in the air being drawn into the engine.
The above situation would indicate a likely airflow
sensor fault, which could in fact be overridden by the
ECU; in this case, it could ignore the airflow sensor fault
and rely on throttle position and engine speed as a
means of calculating the fuelling and ignition
requirements. Therefore, the ECU on a modern system
has far better ability to diagnose a fault and, where
necessary, override the faulty inputs.
Adaptive strategy
Adaptive strategy provides the ECU with the ability to
relearn the values provided by the sensor circuits.
A simple example is the signal voltage provided by a
throttle position sensor when the throttle is in the idle
or closed position. The ECU might initially be
programmed to expect a certain voltage, e.g. 0.5 volts
(with a small tolerance or range), when the throttle is
closed or in the idle position.
However, wear in the linkage and other changes
that can occur over a period of time, can result in a
change in the voltage when the throttle is closed. If,
over a period of time or at a set time during the system
operation, the ECU is able to detect that this voltage is
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now different, it can replace the original expected value
with the new value.
Although not directly related to fault code reading,
it is worth noting that an adaptive strategy can have a
major effect on the way the system identifies or deals
with a fault. Additionally, it will have a direct effect on
the success of rectification work. The particular
example of the throttle position sensor voltage (and
many other examples of the ECU being able to relearn
values) may not have a direct effect on the selfdiagnostic process but if a sensor or a circuit does fail on
the vehicle and a new sensor or component is fitted, the
ECU must go through a period of ‘relearning’ the new
values in order to be able to provide the correct control
over the engine.
It is worth noting that, after rectification work is
carried out, or even if a battery has been disconnected,
the ECU might need to relearn the values of the sensors,
etc. For example, some engine management systems
can take more than 16 km (10 miles) to relearn sensor
values. It is also a common requirement that an engine
must go through a full cycle of operation, such as cold
start, load and cruise conditions, etc., before it can fully
relearn the system signals. The engine may therefore
not perform properly until the ECU has relearnt and
adapted to the new values.
When a vehicle battery is disconnected the ECU
might suffer memory loss, so it is advisable to test drive
the vehicle after the battery has been reconnected to
ensure that the engine is performing normally.
3.7.3 Live data/serial data and other
additional functions of selfdiagnostic systems
Live data/serial data
In addition to providing fault codes, modern systems
can give other information to the technician about the
operation of the vehicle system. Engine management
systems, for example, are able to provide live data via
the fault code reading equipment.
The ECU is already monitoring the circuits of the
engine management system in order to control the
system. The ECU relies on signals from the sensors, etc.
to control fuelling, ignition timing, emissions and other
functions. It is relatively easy for the ECU to output or
transmit those same signal values to a code reader or
similar item of test equipment.
If a capable code reader is connected to the engine
management system diagnostic plug, the code reader is
able to display a considerable range of measurements
relating to the operation of the engine management
system. Different systems provide different items of
data, but the following list indicates just a few of the
sensor measurements that can be accessed via the live
data systems, with the use of a suitable code reader or
similar equipment:
battery voltage
engine speed
airflow sensor signal voltage
coolant temperature sensor signal voltage
throttle position sensor voltage
lambda (oxygen) sensor signal voltage
air temperature sensor signal voltage
MAP (vacuum) sensor signal.
Additionally, the ECU can also provide control signal
values covering:
ignition timing
injection control
EVAP system solenoid control
idle speed valve control duty cycle (on/off ratio).
In fact, a wide range of control signal values and sensor
signal values can be accessed on modern systems.
Additionally, operational information can be accessed,
such as whether the system is operating in closed or
open loop for emission control (see section 3.5.7).
With the live data information, it is possible for the
technician to check on ECU operation and ensure that
the system is operating as intended.
Note that some of the values may be converted by
the code reader into a more acceptable form. The
coolant temperature sensor voltage could be displayed
in the normal way, as a voltage, but can be converted to
give the coolant temperature in degrees, e.g.
0.9 volts = 85°C.
Live data allows the technician to view a
considerable range of readings that would otherwise
need to be accessed by more traditional measurement
tools, such as multimeters or oscilloscopes.
In addition to the examples of live data shown
above, the ECU can also output information about
computer or ECU calculations. This can include the
calculated load and other values which are usually
displayed in a format that does not assist technicians
unless they have access to dedicated data.
The live data output and other information
delivered by the ECU is often referred to as ‘serial data’
or ‘data stream information’. These terms simply refer to
the way in which a code reader or other test equipment
communicates with the ECU. As with personal
computers, data communication (such as between the
PC and a printer) is achieved by transferring
information in series along a wire, i.e. with one piece of
information following another.
Using live data for diagnosis
There is no doubt that live data is an aid to diagnosis. In
previous sections we have noted that fault codes are
primarily of use if the fault causes the signal values to
lie outside acceptable limits. However, if a fault results
in signal voltages or values that are within acceptable
limits, it may not be recognised by the ECU. Therefore,
if an engine management system has an undetected or
unrecognised fault, but the engine is obviously not
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running properly, it is possible to examine the live data
to see if the signal values are correct for the engine
operating conditions.
Again referring to the coolant temperature sensor as
an example, the sensor signal voltage might indicate
‘0.9 volts = 85°C’, but this could not be correct if the
engine had not been run for some hours. In other
words, the engine is cold but the temperature sensor is
indicating that the coolant is almost up to the hot
operating temperature. The values would not be
recognised as a fault on many systems, but the live data
display allows the technician to compare values against
what should be expected for the conditions.
As another example, if the ECU detects a signal that
is not plausible or probable, such as the airflow sensor
signal discussed previously (see section 3.7.2), the ECU
might not provide a fault code that indicates the exact
fault. However, reference to the live data should enable
an experienced technician to identify that the airflow
sensor signal is incorrect under certain operating
Snapshot or playback mode
Various terms are used to describe the snapshot or
playback facility. This facility enables the fault code
reader to display fault codes, or in some cases live data
that were recorded earlier.
The exact capability varies depending on the
system; however, it is possible for the code reader (in
conjunction with the ECU) to capture codes or data at a
time when a fault occurred. This facility is extremely
useful when trying to trace intermittent faults that do
not necessarily show up during workshop tests. The test
equipment can be connected to the vehicle, which is
driven until the intermittent fault occurs.
It is often the case that the code reader/ECU
combination can record a range of measurements or live
data at the time a fault occurs. The measurements and
data are monitored while the vehicle is being driven or
the engine is running. It may be necessary for someone
to press a start button on the code reader when the fault
occurs (such as a noticeable misfire). The
measurements received from just prior to the fault until
just after it are stored.
Some vehicle systems might have already registered
a fault code relating to an intermittent fault. The code
reader/ECU combination can be programmed to
capture and store the data next time the particular
intermittent fault occurs. In some cases, the ECU will
automatically store applicable data when a fault occurs,
and it is then simply a matter of connecting the test
equipment at some later stage to obtain the data that
was stored at the time the fault occurred.
The final result is that the data can be analysed after
an intermittent fault has been noticed, thus enabling
the technician to perform an accurate diagnosis.
Not all vehicle systems and not all code readers
allow the snapshot or playback facility to function.
Fundamentals of Motor Vehicle Technology: Book 2
Service adjust mode
Some vehicle systems, notably engine management
systems, have a service adjust mode which can be used
for some basic service set procedures. On some older
systems, idle speed and other operating values could be
set or adjusted, but with most modern systems this
facility is not required.
There are occasions, however, when some basic
settings can be altered, such as in a country with low
quality fuel. It is possible on some vehicles to effectively
reprogram the ECU so that it alters the timing and
fuelling characteristics to suit different fuel qualities. In
reality, the ECU has a number of programmed
characteristics already located in its memory: it is
simply a matter of selecting the appropriate program by
using the code reader or other suitable equipment.
Actuator simulation tests
It is becoming increasingly common for systems to
incorporate an actuator test facility. As well as
monitoring the electrical circuits for an actuator such as
an injector, modern ECUs are able to provide a control
signal to the actuator (in effect a simulation of the
actuator signal). This control signal has a known value,
so that the operation of the actuator can be checked. It
is usual to implement this facility using the code reader:
the technician selects the appropriate function, which
causes the actuator signal to be transmitted from the
ECU to the actuator, which then operates.
With modern systems, the ECU can provide a
control signal to a number of components or actuators
in the system. In engine management systems, for
example, the ECU can control the injectors, the idle
valve, the EVAP canister purge valve and possibly other
items (such as the exhaust gas recirculation system
valve). During an actuator test, the ECU is instructed to
operate these components independently and normally,
when the engine is not running.
In this way it is possible for the technician to hear or
observe operation of the various actuators, which helps
to establish whether these components are functioning
correctly, and that the associated wiring is good.
When an actuator test is carried out on the injectors,
it is likely that fuel will be injected into the intake
manifold or into some cylinders. It is advisable that the
fuel pump is disconnected to prevent excessive fuel
entering the engine. As a minimum precaution, the
spark plugs should be removed and the engine cranked
over to evacuate excess fuel.
‘Set procedure’ testing
Several engine management systems rely on the
technician following a set procedure or test routine that
requires a number of actions to be performed by the
technician. These set procedures are usually dictated to
the technician via the code reader. In effect, instructions
are provided for the actions that the technician must
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When the code reader is connected to a diagnostic plug,
various messages displayed on the code reader provide
the technician with relevant instruction to perform a
certain task, before going on to the next phase of
testing. On some systems, the whole self-diagnostic
routine is based around the need for the technician to
perform certain procedures all the way through the test
Typical procedures that need to be carried out
include altering the engine speed, turning the steering
(to check power steering pressure switch operation)
and depressing the brake pedal (to check brake switch
If the procedures are not carried out as instructed,
the test results will be incorrect, and the ECU might
diagnose a fault that does not in fact exist. In other
cases, the ECU will not proceed to the next set of tests
until the correct responses or readings have occurred or
been recorded.
Clearing codes
It is important that, when a fault has been rectified, the
original fault codes are removed from the ECU memory.
The process for clearing codes varies with each make
and model of vehicle, and will also depend on whether
blink codes are being accessed or a code reader is being
Reference should always be made to the specific
information on each vehicle to ensure that the correct
process is used to clear old fault codes.
Common problems with the use of code reading
Although self-diagnostic systems, code reading and
retrieval of live data are generally reliable facilities, a
number of problems can arise which are often blamed
on the code reader, but may in fact be due to common
problems such as operator error or system faults that
prevent good test results being obtained.
The following lists those common faults that often
result from what are termed ‘communication errors’
between the code reader and the system ECU.
1 All instructions provided by the equipment
manufacturer might not have been followed exactly.
When certain procedures are not performed in the
correct order or as instructed, the ECU might not
allow further tests to continue or might provide
incorrect results.
2 Instructions provided on the code reader display
might not be followed accurately. If any procedure is
carried out incorrectly or not at the correct time, the
ECU might not allow tests to continue or incorrect
results might be obtained.
Minor changes are often made to systems
because of modifications that occur throughout the
production life of a vehicle. This can result in small
changes to the procedures that have to be carried
out by the technician when accessing fault codes,
etc. Although the technician might be conversant
with a particular system, a variant of that system
might require slightly different procedures or even
use different fault codes.
3 All connections between the code reader and the
diagnostic plug must be checked. Some systems and
code readers require a separate power supply or
earth connection to be made (usually an adapter
lead or harness is provided to connect the
equipment to the battery). Check whether the
system being tested requires the use of a separate
4 The code reader or the application cartridge/card
must be correct for the system being tested. It is
unfortunately not uncommon for a code reader (or
the application cartridge) to be designed to operate
with a particular range of systems, and for the
vehicle manufacturer to then modify the system in
production, which can prevent communication with
the original code reader or cartridge.
5 All wiring between the code reading equipment and
the diagnostic plug, and between the plug and the
ECU must be checked. It is also necessary to check
for poor earth connections to the ECU and check the
power supply to the ECU and to the code reader.
A reduced power supply (flat battery) can cause
communication to cease if the voltage level falls below
a certain limit.
Different terminology for code reading
A scanner or scan tool is another term (often used to
describe American equipment) applied to code reading
equipment. In general, the term code reader is applied
to something that simply extracts codes from the
vehicle system. A scanner is an item of equipment
which effectively scans the ECU memory for
However, the terms code reader and scanner are
loosely used by the industry as product names, as well
as being general terms to identify equipment that
communicates with a vehicle system ECU.
3.7.4 Use of test equipment to
access fault codes
The test equipment (code reader or scan tool) provides
specific instructions about equipment operation and the
connection processes. It is useful to understand some
typical processes for equipment use and for accessing
blink codes, fault codes and live data.
Blink codes via the dashboard warning lights
A blink code is the simplest form of fault code reading,
especially when displayed via the dashboard warning
light. Dashboard warning lights are now fitted to
virtually all passenger vehicles, with a typical
appearance as shown in Figure 3.87.
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Fundamentals of Motor Vehicle Technology: Book 2
Figure 3.87 Dashboard warning light
It is necessary to refer to the specific vehicle
information to find out how its blink codes can be
retrieved, but the following example shows how blink
codes are retrievable on one particular range of
Example of blink code retrieval
In this example, it is necessary to instruct the ECU to
provide a blink code output to the dashboard warning
light. Here the instruction to the ECU is provided by
linking two terminals on the diagnostic harness plug
(Figure 3.88). When the two terminals are linked and
the ignition is switched on, the ECU causes the warning
light to flash an appropriate number of times,
depending on whether there is a fault code stored in the
ECU memory.
As discussed in section 3.7.1, the number of flashes
of the warning light represents the fault code: counting
the flashes enables the technician to read the code.
There are many other methods used by vehicle
manufacturers for instructing the ECU to output fault
codes via the warning light, so refer to specific
instructions for each vehicle.
Accessing blink codes using an LED probe
On some vehicles (mainly older models), it is possible
to access the blink code using an LED probe (Figure
3.89). The process usually involves connecting the
probe tip to a terminal or connection of the ECU, or to a
terminal of a wiring harness plug, while the other
connection for the probe (the crocodile clip in Figure
3.89) remains connected to earth.
As with blink code retrieval using the warning light,
it is usually necessary to instruct the ECU to output the
codes. Refer to specific vehicle information for the
correct procedure and to establish where the LED probe
tip should be connected.
One example of a procedure for instructing the ECU
to output a blink code using an LED probe is where two
terminals on a connector or harness plug are linked
using a switch. This process was used in some older
generation Audi models.
Retrieving blink codes via a vehicle system LED
Some vehicles use an LED located on the ECU (or in
other locations on the vehicle) to enable a blink code
to be read by the technician. As with the warning light
blink codes, a specific procedure needs to be
performed to instruct the ECU to output the code to
the LED light.
Figure 3.89 An LED probe for accessing the blink code
Link tool
Figure 3.88 Example of
linking terminals on the
diagnostic plug to instruct
the ECU to provide blink
code output
a Link tool inserted into
connector to initiate
diagnostic sequence
b Plan view of connector
Check connector
Diagnostic plug
Diagnostic connectors
to be linked
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A switch is connected across the two specified
terminals, and then the LED probe connected across
two other terminals of the harness plug. The specific
instructions will indicate when the switch should be
closed or open. This action makes or breaks the circuit
across the two terminals. If the switch is opened and
closed at the appropriate times, the ECU will output the
fault codes as a blink code on the LED probe.
Obtaining codes and fault related information
using a code reader/scan tool
Using a code reader/scan tool is the most informative
method of retrieving fault related information from the
A code reader/scan tool must be connected to the
ECU to obtain the information; connection is made to a
diagnostic plug (Figure 3.90), which in most modern
vehicles (with engine management systems) is located
inside the passenger compartment. However, some
other vehicle systems, such as ABS, have a diagnostic
plug located in the engine compartment or elsewhere.
Note that European regulations now specify that the
engine management diagnostic plug is to be located
within a defined region of the passenger compartment;
this is explained more fully in section 3.7.5.
Appropriate vehicle application software
Many code readers can use different cartridges, pods or
software to enable them to function on different makes
and models of vehicle and on different makes of vehicle
system. Although some code readers are designed to
operate on only one (or a few) makes and models, most
have the facility to use different application software,
Figure 3.90 Connection of a
code reader to the vehicle
diagnostic plug
which can be purchased from the code reader
manufacturer. Depending on the equipment design, the
software is contained within replaceable pods or
cartridges. In some cases new software can be
downloaded via the Internet.
Most code readers are capable of operating with
many different makes and models of vehicle. Software
upgrades enable newer models to be covered without
necessarily requiring a complete new code reader.
Starting the communication
For almost all vehicle systems, the ECU requires some
form of password before it will output the fault codes or
live data to the code reader. The password is usually an
electronic code that is transmitted from the code reader
to the ECU. In most cases, the appropriate software
cartridge is installed in the code reader, or the
appropriate vehicle is selected from the menu on the
code reader, causing the correct password to be sent to
the ECU. When the ECU has received the password, the
ECU and the code reader can then communicate.
Retrieving fault codes and data
Once communication has been established between the
code reader and ECU, the capacity to retrieve codes and
information will depend on the equipment being used
and the system being tested. In general, however,
instructions are provided on the code reader display, or
are included in an operator manual.
Following the correct procedures will then allow
fault codes, messages or live data to be retrieved, as
well as enabling other functions to be performed, such
as actuator tests and intermittent fault detection.
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Different vehicle systems will provide different levels of
information. Some may provide only simple fault codes,
whilst modern engine management systems provide a
considerable depth of live data, etc. Additionally, lower
cost code readers might not be able to display all of the
information available from the ECU, and may not be
able to initiate many of the other ECU test functions,
such as actuator testing.
3.7.5 EOBD (European on-board
Emissions related legislation affecting on-board
The term on-board diagnostics refers in general to the
facilities already described relating to self-diagnosis.
However, originally in America and more recently in
Europe, legislation has imposed certain standards for
on-board diagnostic systems, with particular regard to
emission control systems. In reality, because emission
systems are now under the control of the engine
management system, the operation of the engine
management system is influenced by the legislation.
The American market has had on-board diagnostic
legislation for some years (OBD 1 and OBD 2). In
Europe, legislation was introduced in January 2000
that applied to new models (subsequently embracing
diesel engines as well as petrol engines). In Europe, the
on-board diagnostic legislation is referred to as EOBD
(European on-board diagnostics) and is an adaptation
of the American OBD 2, which was originally
introduced in 1996. EOBD is part of a range of
regulations introduced by the EC aimed at controlling
or reducing the emissions from motor vehicles. There
are many aspects to EOBD regulations, but the relevant
issues are discussed within this section.
Whilst the regulations are primarily aimed at
emissions control systems/engine management
systems, other vehicle systems also affect emission
levels, such as the air conditioning. The air conditioning
system may be operating at idle speed, thus causing the
idle speed to reduce as well as demanding engine
power. An engine management system must ensure that
the idle speed is maintained, and also ensure the
air:fuel ratio is controlled, so power can be developed
without increasing emissions significantly. It is common
to find fault codes on modern systems that embrace air
conditioning functions, automatic transmission
functions, and also codes for other vehicle systems that
communicate or co-operate with the engine
management system.
Long term monitoring of emissions from the
One objective of the regulations is to ensure that
emissions levels from motor vehicles are maintained at
acceptable levels for the operating life of the vehicle,
and also that, when the emissions levels are
Fundamentals of Motor Vehicle Technology: Book 2
unacceptable because of a fault, vehicle systems are
able to detect the incorrect emissions and identify the
fault. In effect, the regulations impose a requirement for
monitoring emissions and emissions control systems
(including the engine management system). This
requirement effectively defines the technical functions
of the engine management system and some of the ECU
diagnosis software. EOBD regulations resulted in the
introduction of post-cat monitoring, i.e. an oxygen or
lambda sensor located after the catalytic converter
(section 3.5.7). This additional lambda sensor allowed
the ECU to monitor the efficiency of the catalytic
converter process. A second lambda sensor signal
indicates the oxygen content of the exhaust gas in the
catalytic converter.
The self-monitoring and self-diagnostic capability of
the modern engine management ECU is highly
complex, and the software program embraces many
variables that can occur during engine management
system operation. As a simple example, if the signal
voltage from a temperature sensor exceeds the
maximum programmed limit (e.g. 4.8 volts, as used in
earlier examples in section 3.7.2), this would be
regarded as a fault on older systems. However, EOBD
systems must have a built-in tolerance that allows the
signal voltage to exceed the maximum limit for a very
brief period of time.
ECU programming must therefore permit an
occasionally faulty signal, so long as it occurs only for
an exceptionally short period and on a limited number
of occasions; under these specified conditions, the ECU
would not regard the fault as permanent, so no fault
code would be stored and the warning light would not
be illuminated.
Standardisation of fault codes, communication
and diagnostic plugs
Standardised codes
Another significant aspect of EOBD is the
standardisation of many engine system related fault
codes. Prior to EOBD, vehicle manufacturers used their
own coding system to indicate faults – there was no
consistency across makes or models. Whilst vehicle
manufacturers are still allowed to use their own fault
code system and codes (which can be used in their
authorised repair workshops in conjunction with the
dedicated vehicle test equipment), there is a
standardised list of codes and messages that apply to
emissions related system faults. The standardised
system of codes must be accessible to general purpose
code reading equipment, so that equipment
manufacturers can produce code reading equipment
that provides the same fault codes and messages
irrespective of the vehicle being tested.
The standardised codes, which number in excess of
500, are referred to as P Zero or P0 codes. Vehicle
manufacturers’ own coding systems are referred to as
P1 codes. The P0 codes must still be accessible even if a
vehicle manufacturer uses its own P1 coding system.
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The letter P denotes the power train (effectively the
engine and emission systems). Other code letters refer
to different aspects of the vehicle, as shown below:
power train (standardised EOBD codes)
chassis system codes
body system codes
network system codes (this relates to the
communication networks where different
computers or ECUs communicate with each
A zero (0) following a letter (as in P0) indicates a
standardised code such as EOBD, or a code established
by the Society of Automotive Engineers (SAE) or other
organisations. A number 1 or 2 following a letter (as in
P1 or P2) indicates a manufacturer’s own code. Note
that EOBD does not at this time embrace
standardisation or access to live data, but only fault
codes and fault messages.
Figure 3.91 shows the structure of the standardised
P0 fault codes and messages.
Fault (00–99)
1. Fuel and air metering
2. Fuel and air metering (injector circuit)
3. Ignition systems or misfire
4. Auxiliary emission controls
5. Vehicle speed control and idle control system
6. Computer output circuit
7. Transmission
8. Transmission
0. SAE
1. MFG
B. Body
C. Chassis
P. Powertrain
N. Network
Key Points
Figure 3.91 Examples of EOBD standard P0 fault codes
OBD2 and EOBD are similar – but not identical –
take care when interpreting results!
Modern OBD systems use a standard diagnostic
plug/socket located in easy reach of the driver’s
Standardised communication
EOBD rules identify a common ‘language’ that should
be used in the computer system, so that it
communicates with a standard type or general purpose
code reader. The standardised language is in fact
referring to the computer protocol used when the code
reader and ECU communicate. Included in the
standardisation are the passwords to enable the code
readers to gain access to the information and data
within the ECU.
Standardised diagnostic connector plug
EOBD specifies a standard diagnostic plug which has 16
terminals or connector pins (Figure 3.92). The function
of the pins is also standardised, i.e. some of the 16 pins
are designated as part of the EOBD system and are
allocated specific functions such as battery voltage,
power supply and earth connection for the code reader
communication terminals (the terminals through which
the codes and data are transmitted from the ECU to the
code reader). This allows a single code reader to
connect to any modern vehicle with a single connector
harness and plug. The result is that vehicle testing
stations, police forces and workshops can have a
standard code reader to retrieve EOBD information
from any vehicle.
The location of the diagnostic plug has also been
defined within EOBD regulations. In general, it is sited
close to the driver’s seat, approximately between the
centre line of the vehicle and the steering column. The
connector plug is found on many vehicles just under the
dashboard, adjacent to the driver’s leg, although in
some vehicles it is on the centre console.
Malfunction indicator lamp
The malfunction indicator lamp (MIL) is another term
for the engine warning light. The MIL is intended for
use when emissions/engine management system faults
occur, or emissions are outside predefined limits. The
MIL must be positioned on the dashboard and must not
be red when illuminated. When the MIL is illuminated,
it indicates that a system fault exists, and that the
engine management system might be operating using
the ‘limp home’ function. There are two main operating
strategies for the MIL.
1 If the MIL illuminates permanently (no flashing)
when the engine is running, it indicates that the
ECU has detected a fault that could allow excessive
emissions to be produced. It also indicates that a
fault code or message is stored in the ECU memory.
The fault code can be accessed using a code reader
or scan tool.
2 If the MIL flashes continuously when the engine is
running, it indicates the ECU has detected an engine
misfire. In such cases, it is possible for excess oxygen
and fuel to ignite in the catalytic converter, causing
permanent damage, or at least to accelerate its
ageing. If the MIL is flashing, the driver should
ideally reduce engine speed and load, and take the
vehicle to a repair workshop as soon as possible.
Figure 3.92 16-pin diagnostic connector plug
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Misfire detection
Another feature of EOBD systems is the facility to detect
misfires. There is a strong possibility that the catalytic
converter might be damaged because of a misfire, and
high emissions of HC are produced when misfires occur,
so the engine management system must be able to
detect misfires and, in some cases, cut off fuel to the
affected cylinder.
Misfires can be detected using several methods as
listed below.
1 Monitoring
acceleration – when the combustion process occurs
in a cylinder, it produces power that forces the
crankshaft to accelerate. If the combustion process
in a cylinder is not efficient, or produces less power
than the other cylinders, the acceleration at the
crankshaft will not be as great as for the other
cylinders. This difference enables the ECU to
establish which cylinder is misfiring or running less
2 Spark detection is a method whereby the ECU
monitors the voltage in the ignition coil. When a
misfire occurs, the changes in secondary circuit
firing and spark voltage can also affect the voltage
in the primary circuit. The process is not dissimilar
to examining the ignition circuit voltages using an
oscilloscope or test meter. By comparing voltages
across the different cylinders, and referencing
expected or previous voltages for that cylinder, the
ECU can identify which cylinder is misfiring.
3 A misfire causes an increase in the unburned
oxygen content of the exhaust gas. The
lambda/oxygen sensor will detect the high levels of
oxygen, and the ECU will register this as a fault.
This process alone does not identify which cylinder
is misfiring, but by analysing the misfire detection
Fundamentals of Motor Vehicle Technology: Book 2
information provided by other methods such as in
(1) or (2), the ECU can assess which cylinder is at
Cylinder isolation
If exhaust oxygen levels are excessively high, which will
usually be accompanied by high levels of unburned fuel
(HC), it is possible for the ECU on many modern
systems to cut off fuel delivery to the affected cylinder.
As well as reducing the high HC emissions, cutting off
the fuel supply to the affected cylinder reduces the risk
of excessive combustion of fuel and oxygen in the
exhaust system and catalytic converter, combustion
which could create high temperatures and damage the
catalytic converter.
Web links
Engine systems information
www.kvaser.com (follow CAN Education links)
Teaching/learning resources
Online learning material relating to powertrain
Chapter 4
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what is covered in this chapter . . .
Development of modern diesel fuel systems
Development and electronic control of the rotary type diesel injection pump
Cold-start pre-heating systems
Electronic control of diesel injection (common rail systems)
4.1.1 Emissions, economy and
engine performance
As with the petrol engine, legislation has forced
designers of diesel engines to reduce emissions of
pollutants, whilst at the same time consumer demand
for improved engine performance and economy has
potentially conflicted with emission requirements.
However, again as with petrol engines, improved
engine design and the development of the fuel systems
has resulted in improvements in almost all areas.
In comparison with most petrol engines, the diesel
engine operates with very weak air:fuel ratios. Apart
from some modern direct petrol injection engines
operating at light load, a petrol engine generally
operates with a lambda value (excess air factor) of close
to 1. Diesel engines, however, always operate with a
lambda value greater than 1 and as high as lambda 1.4.
Because of this high excess of air, carbon monoxide
(CO) emissions are exceptionally low, with values as
low as 0.01% at idle and with a typical maximum of
around 0.2% (200 parts per million) at full load.
Hydrocarbon (HC) emissions are also low at less than
50 parts per million (ppm) at full load, but this can rise
to approximately 500 ppm at idle speed, which is in fact
higher than emissions from petrol engines.
Emissions of oxides of nitrogen (NOx) are high with
diesel engines and, as with petrol engines, levels can
reach 2500 ppm.
The diesel engine does not generally use any form
of air volume or air mass control, i.e. there is normally
no throttle control to regulate the volume/mass of air
entering the cylinder. The volume of air drawn into the
cylinders is entirely dependent on engine speed, load
and design; therefore power or torque control is totally
dependent on fuel quantity (air:fuel ratio). Engine
torque and power are therefore controlled by the
quantity of fuel injected into the cylinder. Exceptionally
weak mixtures will result in low power and torque
levels, suitable for light load operation. However, when
it is necessary for the engine to produce higher levels
of torque and power, the quantity of fuel must be
increased (a richer air:fuel ratio).
Since engine torque and power are dependent on
the air:fuel ratio, when power or torque are required a
rich mixture (reduced oxygen content) is provided
which results in an increase in CO and also in NOx
emissions (due to combustion temperature increase).
Additionally, the levels of soot emitted increase with
richer mixtures (soot can be regarded as fuel droplets
that have not vaporised, compared with HC emissions,
which are vaporised fuel).
In recent years, legislation has imposed
progressively tougher emissions limits, with the Euro 4
legislation for 2005 dictating further reductions in
emissions. Various emissions reduction techniques are
therefore used on modern diesel engines with particular
focus on electronic control of diesel injection systems.
In fact, the introduction of ‘common rail’ diesel injection
systems is an interesting development, with their
similarity to direct injection petrol systems that have
also been relatively recently introduced.
The subject of diesel emissions is frequently
debated by the politicians and scientists, with the
result that there is a continuously changing view as to
whether diesel or petrol engines produce the more
harmful emissions.
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Engine management – diesel injection
Key Points
Economy and engine performance
Traditionally, because diesel engines operate with
relatively weak air:fuel ratios (compared with petrol
engines), fuel consumption is generally much lower
than an equivalent petrol engine. However, diesel
engines have generally produced less power than an
equivalent size of petrol engine, although torque
levels are usually higher. The diesel engine is,
however, ideally suited to turbocharging, which can
enable the diesel to produce high torque and high
power outputs.
Many modern passenger car diesel engines with
turbocharging can in fact match the performance of
their petrol equivalents (even in the sportier models).
Equivalent performance with reduced fuel consumption
is a major consumer benefit.
Another significant factor for the consumer is the
cost of diesel fuel. In general, in Europe, diesel fuel is
less costly than petrol (with the notable exception of the
UK). For this reason, many European countries have
high percentages of diesel cars, with in excess of 50%
being normal in some countries. In the UK, there has
been a trend towards a higher percentage of vehicle
sales being diesel (beginning around 2000), with up to
30% of the total cars sold now being diesel engine
vehicles (especially for smaller cars).
One other factor that has noticeably changed with
the diesel engine is noise. Modern injection systems
and engine designs have resulted in a considerable
reduction in the traditional diesel knock, an important
factor for the consumer. The intrusion of diesel engine
noise into passenger compartments is much lower and
there is also a significant reduction in noise outside
the vehicle.
In comparison with petrol/gasoline engines,
diesels operate with very weak mixtures. This
results in very low carbon monoxide emissions
Fundamentals of Motor Vehicle Technology: Book 2
Inevitably, the improvements have been very much
forced by environmental considerations (emissions
legislation), but with the addition of consumer benefits,
the diesel engine has become a much more accepted
engine type, especially in countries where the petrol
engine was traditionally the preferred option for
passenger vehicles.
4.1.3 Decline of in-line pump and
rotary pump injection systems
Electronic control for in-line diesel injection pumps is
generally used only on large engines in heavy
commercial vehicles. Passenger cars and light
commercial vehicles increasingly used rotary type
pumps through the 1980s and 1990s. With legislation
forcing further emissions reductions, the trend has
more recently been towards electronic control for unit
injectors and more frequently, common rail injection
systems which are similar in layout and operation to a
modern petrol injection system.
Figure 4.1 shows an in-line type pump (for a sixcylinder engine), which has electronic control to take
care of functions that were previously mechanically or
hydraulically/pneumatically controlled. Injection
timing (start of delivery) and fuel quantity are
controlled by the ECU which passes a control signal to
solenoids or other actuators that then alter the
position of the mechanical devices within the pump.
In effect, the pump operates in much the same way as
older designs but electronic systems enhance its
capability and accuracy. The in-line pump is not
covered in this book because of its lack of use in
passenger and light vehicles. For an understanding of
the operation of in-line pumps, see Chapter 2 in
Hillier’s Fundamentals of Motor Vehicle Technology
Book 1.
Diesel engine performance was generally inferior
to that of petrol/gasoline engines but the addition
of a turbocharger makes the performance similar.
However, diesel economy is still better
4.1.2 Diesel engine developments
As with the petrol engine, fundamental diesel engine
design has improved. General improvements embrace
the combustion chamber and the use of four valves per
cylinder, as well as improved intake port designs.
However, it is fuel systems that have provided the
greatest change to the diesel engine. With the
progressive introduction of electronic control, which
was initially used to enhance the operation of
traditional diesel injection pumps (in-line as well as
rotary), through to fully electronic common rail
systems, electronic control has made some very
significant improvements to the diesel engine.
Figure 4.1 Bosch in-line diesel injection pump
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The rotary diesel injection pump
4.2.1 Rotary pumps ‘without’
electronic control
This section covers just one type of rotary pump. See
Chapter 2 in Hillier’s Fundamentals of Motor Vehicle
Technology Book 1 for additional information on the
operation of diesel injection pumps.
Figure 4.2 shows a VE pump fitted in a self-bleeding
fuel system layout similar to that used on light vehicles.
As with other rotary pumps, this type has one pumping
element and a number of high pressure outlets, one for
each engine cylinder.
In addition to the basic features associated with modern
distributor rotary pumps, various add-on modules can
be fitted to the VE pump; these include:
a solenoid operated fuel cut-off to give the driver a
key start/stop operation
an automatic cold starting module to advance the
a fast idle facility to give even running during warmup
torque control for matching the fuel output with the
fuel requirement.
The section through the pump (Figure 4.3) shows the
layout of the basic sub-systems; these include:
Figure 4.2 Bosch distributor pump fuel system
Figure 4.3 Bosch VE distributor pump
a low pressure fuel supply
a high pressure fuel supply and distributor
a fuel shut-off solenoid
a distributor plunger drive
an automatic injection advance unit
a pressure valve
a mechanical governor.
Low pressure fuel supply
Driven at half crankshaft speed by a drive shaft, a
transfer pump with four vanes delivers fuel to the
pumping chamber at a pressure set by the regulating
This fuel pressure, which rises with engine speed, is
used to operate the automatic advance unit. It also
gives an overflow through the pump body, which aids
cooling and provides the self-bleeding feature. After
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Engine management – diesel injection
passing through a small restriction at the top of the
pump the surplus fuel is returned to the fuel tank.
High pressure fuel supply
Figure 4.4 is a simplified view of the pumping chamber
with part of the distributor head cut away to show the
pump plunger. Besides rotating in the head to give a
valve action, the plunger is reciprocated through a
constant stroke to produce the high pressure. The axial
movement is provided by a cam plate moving over a
roller ring. The quantity of high pressure fuel delivered
to the injector via the outlet bore is controlled by the
position of the control spool. The control spool varies
the effective pumping stroke: the stroke increases as the
spool is moved towards the distributor head and
therefore increases the quantity of fuel delivered.
Figure 4.4 Principle of the VE pumping unit
Fundamentals of Motor Vehicle Technology: Book 2
In the position shown in Figure 4.4a the rotation of the
plunger has caused one of the metering slits to open the
inlet passage. At this point all outlet ports are closed.
Prior to this, the plunger had moved down the chamber
to create a condition for the fuel to enter and fill the
high pressure chamber.
Slight rotation of the plunger closes the inlet port
and causes the single distributor slit in the plunger to
open one of the outlet ports. Whilst in this position the
plunger is moved up the chamber to pressurise the fuel
and deliver it through the outlet bore to the injector.
The position of the plunger at the end of the
injection period is shown in Figure 4.4b. At this point,
the control spool has already allowed a considerable
movement of the plunger before the cut-off bore in the
plunger has been uncovered. The exposure of this port
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The rotary diesel injection pump
instantly reduces the pressure and terminates the
injection. Further pumping movement of the plunger
causes the fuel in the pumping chamber to be returned
to the pump cavity. With the spool set in this maximum
fuel position, which corresponds to the fuel requirement
for starting, a movement of the control spool to an
extreme position away from the distributor head
reduces the output to a minimum; this is the spool
setting for slow running.
Fuel shut-off
The ‘no fuel’ or ‘stop’ position is provided by a solenoid
operated valve. The solenoid cuts off the fuel supply to
the inlet passage when the ‘ignition’ key is switched off.
Distributor plunger drive
The plunger must be rotated and reciprocated. Figure
4.5 illustrates how this is done.
port stops; when the pump element ceases to supply
fuel to the outlet port, the delivery valve closes which
immediately causes the pressure to drop in the high
pressure line which, in turn, causes immediate closure
of the injector. However, pressure remains sealed in the
high pressure line.
Automatic injection advance unit
The roller ring assembly is not fixed rigidly to the casing;
instead it can be partially rotated through an angle of up
to 12º to allow the automatic advance mechanism
shown in Figure 4.6 to vary the injection timing.
When the pump is rotated, fuel under pressure from
the transfer pump is delivered to the timing advance
chamber via the pump cavity. A rise in the pump speed
causes the transfer pump pressure and flow to increase.
The increase in pressure moves the timing advance
Figure 4.5 Plunger drive
The distributor pump driveshaft is rotated at half
crankshaft speed (for a four-stroke engine), and is
transmitted via a yoke and cam plate to provide rotary
motion to the pump plunger.
Reciprocating motion is provided by the rotation of
a cam plate as it moves over four roller followers fixed
to a roller ring. In a pump suitable for a four-cylinder
engine, four lobes are formed on the cam plate and
contact between the plate and rollers is maintained by
two strong plunger return springs. A yoke positioned
between the driveshaft and the cam plate allows the
plate to move axially whilst still maintaining a drive.
Pressure valve
A delivery valve is fitted in the distributor head at the
connection point to the high pressure fuel lines (see
4.3). The valve is used to seal the pressure in the high
pressure line when the fuel delivery to the fuel outlet
piston against its spring, which in turn, causes the
actuating pin to rotate the roller ring in a direction
opposite to the direction of rotation of the driveshaft.
The rotation of the roller ring advances the injection
The VE pump is fitted with either a two-speed or an all
speed governor. The layouts of these types of governor
are similar, but differ in the arrangement of the control
Figure 4.7 shows the main construction of a twospeed governor, which controls the engine during the
idling and maximum speed operation. At other times
the driver has near direct control of the quantity of fuel
delivered and hence the power output of the engine.
The centrifugal governor, which consists of a series
of flyweights, is driven from the driveshaft through
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Engine management – diesel injection
Figure 4.6 Principle of the automatic advance
Figure 4.7 Governor – mechanical type
Fundamentals of Motor Vehicle Technology: Book 2
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The rotary diesel injection pump
gears with a ratio that steps up the speed. The high
speed flyweight rotation given by this ratio ensures
good sensitivity of the governor, especially during the
idling phase.
An increase in engine speed, and the associated
centrifugal action on the flyweights, produces an
outward force that pushes a sliding sleeve against a
control lever system. This lever, which is connected at
its lower end to the control spool on the pumping
plunger, can move only when the sliding sleeve is able
to overcome the reaction of the spring that is in use at
that time.
With the accelerator pedal half depressed and the
governor stationary, the starting spring pushes the
sliding sleeve towards the flyweights and moves the
control spool to the maximum fuel position.
When the engine starts, the release of the accelerator,
combined with the outward movement of the
flyweights, causes the lever to move the control spool to
the minimum fuel position. When the engine is
operating in this phase, smooth idling is obtained
through the interaction of the flyweights and idling
With the accelerator pedal lever against the
adjustable idling stop, any small speed increase causes
the flyweights to exert a larger force on the sliding
sleeve. This slightly compresses the idling spring and, as
a result, the spool control lever moves the spool and
reduces the fuel delivery.
Any slight drop in engine speed produces the
opposite action, so smooth idling under governor
control is obtained.
Mid-range operation
Once the idling range has been exceeded, the larger
governor force puts the idling and starting springs out
of action. At this stage the intermediate spring comes
into use to extend the idle control range and so smooth
the transition from idle to mid-range operation. The
intermediate spring is stronger and provides a flexible
link between the driver’s pedal and the control spool
lever, so that, when the accelerator pedal is depressed, a
slight delay in engine response is introduced.
Beyond this phase any movement of the accelerator
produces a direct action on the control spool.
Maximum speed
During mid-range operation, the pre-load of the main
governor spring causes the spring assembly to act as a
solid block. However, when the engine reaches its
predetermined maximum speed, the force given by the
flyweights equals the spring pre-load. Any further speed
increase allows the flyweights to move the spool control
lever. This reduces the quantity of fuel being delivered
and so keeps the engine speed within safe limits.
4.2.2 Injectors
The purpose of the injector is to break up the fuel to the
required degree (i.e. to atomise it) and deliver it to the
combustion region in the chamber. This atomisation
and penetration is achieved by using a high pressure to
force the fuel through a small orifice.
Many vehicles use a type of injector that
incorporates a valve. The closed system is responsive to
pump pressure; raising the pressure above a
predetermined point allows the valve to open, and stay
open until the pressure has dropped to a lower value.
The ‘snap’ opening and closing of the valve gives
advantages, which make this system popular.
The complete injector, shown in Fig. 4.8a, consists
of a nozzle and holder, which is clamped to form a gastight seal in the cylinder head. A spring, compressed by
an adjusting screw to give the correct breaking
(opening) pressure, thrusts the needle on to its conical
seat. Fuel flows from the inlet nipple through a drilling
to an annular groove about the seat of the needle. A
thrust, caused by fuel acting on the conical face X, will
overcome the spring and lift the needle when the
pressure exceeds the breaking pressure. The opening of
the valve permits discharge of fuel until the pressure
drops to the lower limit. Any fuel which flows between
the needle and body acts as a lubricant for the needle
before being carried away by a leak-off pipe.
4.2.3 Injector nozzle types
There are three main types of nozzle:
single hole
Single hole nozzle
See Figure 4.8b. A single orifice, which may be as small
as 0.2 mm (0.008 in), is drifted in the nozzle to give a
single jet form of spray. When this nozzle is used with
indirect injection systems, a comparatively low injection
pressure of 80–100 bar is used.
Multi-hole nozzle
See Figure 4.8c. Two or more small orifices, drilled at
various angles to suit the combustion chamber, produce
a highly atomised spray form. Many engines with direct
injection systems use a four-hole nozzle with a high
operating pressure of 150–250 bar. A long stem version
of this type makes it easier to fit the injector in the head.
Pintle nozzle
See Figure 4.8d. Swirl chambers can accept a soft form
of spray, which is the form given by a pintle nozzle
when it is set to operate at a low injection pressure of
110–135 bar.
A small cone extension on the end of the needle
produces a conical spray pattern and increases the
velocity of the fuel as it leaves the injector. This type
tends to be self-cleaning.
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Engine management – diesel injection
Fundamentals of Motor Vehicle Technology: Book 2
where the electronic control influenced the operation of
a rotary pump. One example of this type of electronic
control is therefore detailed below and illustrated in
Figure 4.9. The general term used to describe these
systems is electronic diesel control (EDC).
An electronic diesel control system can give the
following advantages:
Figure 4.8 Injectors
The elimination of heater plugs on some small indirect
injection engines has been made possible by the
invention of a special pintle nozzle known as the
‘pintaux’ type, as shown in Figure 4.8e. Starting
conditions produce a small needle lift, and so fuel
passes through the small auxiliary hole and is directed
to the hottest part of the chamber. Under normal
running pressures, the full lift of the needle discharges
the fuel through the main orifice.
4.2.4 Rotary pumps with electronic
With an ever increasing demand on the compression
ignition engine to develop more power with lower
emissions, together with an increase in fuel economy,
electronic control of the diesel fuel system has now
become the standard for passenger vehicles with diesel
Although the very latest generations of electronic
diesel systems are in fact very similar to petrol injection
systems, i.e. the injectors are directly controlled by the
system ECU (section 4.3), technicians may still
encounter early generations of electronic diesel systems
lower emissions
lower soot emissions
increased engine output.
The non EDC Bosch VE pump accurately controls the
quantity of fuel delivered by the injectors with the use
of the control spool as well as a governor and an
automatic advance unit. However, external influences,
such as engine temperature and air density, will affect
the engine performance and also the emissions. Precise
control of the fuel system can be achieved with the use
of electronic diesel control.
The EDC electronic control unit (ECU) controls the
fuel system by using two actuators, a solenoid operated
control spool and a solenoid operated timing advance
unit, which are located in the distributor pump (Figure
4.9). The pump uses many of the components that are
fitted to the VE-type distributor pump, including the
fuel shut-off valve and the fuel delivery plunger.
The ECU monitors the engine operating conditions
from information supplied by sensors and provides the
correct control signals to the actuators, giving precise
control of the fuel delivered to the injectors. The EDC
system uses sensors very similar in operation to those
used with petrol fuel injection systems (see Figure 4.10).
An accelerator cable between the throttle pedal and
the distributor pump is no longer required to control the
fuel volume. The position of the throttle pedal is
monitored by the EDC ECU with the use of a throttle
position sensor fitted to the throttle pedal linkage. The
ECU controls the volume of fuel delivered to the
injectors by using a solenoid operated control spool.
The engine speed is monitored by a sensor fitted to
the engine crankshaft; the sensor is usually of the
inductive type. An additional sensor is fitted to the
distributor pump, which monitors the speed and
position of the fuel control spool in relation to
crankshaft position. The ECU uses the information from
these sensors, together with additional sensor
information to determine the volume of fuel and fuel
injection timing.
A manifold absolute pressure (MAP) sensor enables
the ECU to monitor the volume of air entering the
engine. The ECU calculates the air density from the
MAP sensor signal in conjunction with the intake air
temperature sensor signal. The MAP sensor signal is
also used to monitor and control the turbo boost
pressure. The ECU controls the turbo boost pressure
with a waste gate actuator solenoid.
Two temperature sensors are used: an engine coolant
temperature sensor to monitor engine temperature and
an intake air temperature sensor. The ECU uses
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The rotary diesel injection pump
Figure 4.9 An electronically controlled diesel rotary pump (EDC)
Injector needle motion
Temperature sensors coolant/air
Control spool position sensor
Air mass sensor
Engine speed
Manifold absolute pressure sensor
Control spool solenoid
Timing advance unit solenoid
EGR solenoid
Glow plug control module
Vehicle speed
Accelerator pedal position sensor
Figure 4.10 Inputs and outputs for an EDC system
temperature information for fuel volume control. This
information is also used to control the length of time
that the glow plugs operate during starting.
An injector motion sensor is fitted to one of the
injectors (Figure 4.11), usually to number 1 cylinder. At
the start of fuel injection, when the fuel pressure
increases and lifts the injector valve from the seat, the
sensor produces a signal. The start of injection
influences engine starting, combustion noise, fuel
consumption and emissions. The ECU monitors the
sensor signal and determines, in conjunction with the
engine speed sensor information, the fuel injection
timing control.
To enable the modern diesel engine to meet
emission regulations, many engines are fitted with an
exhaust gas recirculation (EGR) system. During certain
engine operating conditions, the exhaust gases are
mixed with the fresh air in the induction system, which
lowers the combustion temperature, thus reducing the
harmful emissions produced by the engine. The volume
of EGR is measured with a mass air flow sensor, either a
hot wire or a hot film type. The ECU controls the EGR
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Engine management – diesel injection
Figure 4.11 An injector with a motion sensor
valve actuator accordingly to ensure the correct volume
of exhaust gases are recirculated to provide the correct
emission levels.
The position of the control spool in relation to the
distributor plunger determines the volume of fuel
delivered to each injector, in the same manner as
previously described with the Bosch VE pump. The
volume of fuel delivered dictates the engine speed and
engine power. A mechanical governor is no longer fitted
to the distributor pump; the position of the control
spool is electronically controlled by the EDC ECU with a
solenoid. Depending on the position of the spool, the
volume of fuel is either increased or decreased. The
position of the spool can be altered to provide
maximum fuel for full load through to zero fuel to
prevent fuel from being supplied to the injectors. The
exact position of the control spool is monitored by the
ECU with a position sensor.
As with the VE pump, the fuel pressure inside the
pump is relative to engine speed. The timing advance
unit functions in a similar manner to that of the VE
Fundamentals of Motor Vehicle Technology: Book 2
pump, except that the fuel pressure applied to the
advance unit is controlled by the EDC ECU with the use
of the timing advance unit solenoid. The fuel injection
timing can either be advanced or retarded by altering
the control signal to the solenoid.
The EDC ECU controls the engine idle speed by
controlling the volume of fuel delivered. To ensure that
the engine idle is as smooth as possible, the ECU will
slightly vary the volume of fuel to each cylinder by the
corresponding amount.
The EDC ECU also incorporates a diagnostic
function similar in operation to that of a petrol engine
management system. If a fault occurs with the system,
the ECU will if possible operate with a limited operating
strategy (LOS). If a sensor circuit fails, the ECU will
substitute the value of the sensor circuit, to provide
limited emergency operation of the system. If the ECU
detects a system fault, it illuminates a warning lamp in
the instrument panel to alert the driver that a fault has
occurred; the fault will also be stored in the memory of
the ECU in the form of a code. To diagnose the system
fault, the fault information can be retrieved from the
EDC ECU memory with the appropriate diagnostic test
Many modern vehicles are prevented from being
driven by the fitting of an engine immobiliser system.
Early immobiliser systems prevented diesel engines
from being started by isolating the power supply to the
distributor pump stop solenoid, preventing fuel from
entering the plunger. Modern electronically controlled
diesel fuel systems are immobilised within the ECU. If
the ECU receives an incorrect immobiliser code from the
driver, it prevents fuel from being supplied to the
injectors by isolating the control signals to the
distributor pump solenoids.
Key Points
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Electronic diesel control (EDC) systems can vary
timing and fuel quantity by acting on the
automatic advance unit and the control spool
Inputs to an EDC system are similar to those used
for petrol/gasoline engine management
4.3.1 Cold starting
Direct injection engines
All compression ignition engines require some special
provision for cold starting, although modern direct
injection engines (injecting directly into the main
combustion chamber, as opposed to into a pre-chamber)
may require cold-start assistance only at low ambient
temperatures. The heat generated during compression,
even under cranking conditions, is usually sufficient to
cause ignition of the vaporised fuel.
For most cold-start conditions, the direct injection
engines used in most modern passenger cars are able to
start relatively easily: the injection of a larger quantity
of fuel (a rich mixture), and the greater amount of
easily ignitable fractions contained in the injected
charge, are generally sufficient to start a cold direct
injection engine. However, with many modern direct
injection engines, assistance during cold starting is
needed to reduce harmful emissions when the engine is
initially started.
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Cold-start pre-heating systems
Figure 4.12 Indirect diesel injection into a pre-chamber
Indirect injection engines
With indirect injection (Figure 4.12), the pre-chamber is
not exposed to the same amount of heat as the main
combustion chamber (heat is lost to the cylinder walls
and combustion chamber walls). These greater heat
losses therefore dictate that indirect injection engines
have extra provision to ensure ignition of the fuel
during cold starting. A significant design difference is
that higher compression ratios are used for indirect
injection engines: ratios of about 16:1 are used with
direct injection engines, while indirect injection engines
use higher ratios of the order of 22:1, and in some cases
a ratio as high as 30:1 is used. These higher
compression ratios increase the heat produced during
compression, which aids cold starting. A high
compression ratio is also used in indirect injection
engines to raise their thermal efficiency, and hence
economy, unlike direct injection engines; this tends to
counteract the greater heat loss caused by the larger
surface areas of an indirect injection combustion
4.3.2 Cold-start assistance
Manifold heaters
Seldom used on modern passenger car engines,
manifold heaters are electrical units fitted to pre-heat
the air as it passes through the inlet manifold to the
operate for is usually dependent on engine
temperature: the colder the engine, the longer the glow
plugs function.
The glow plugs are usually controlled automatically
by a timer relay or an electronic control unit (ECU).
When the ignition is switched on, the controller usually
illuminates a glow plug warning light in the instrument
panel to warn the driver when the glow plugs are
operating. Modern direct injection engines have a glow
plug fitted to each cylinder. The glow plugs used on
many modern diesel engines can remain switched on for
a few minutes after the engine has started, ‘post-glow’,
normally with a reduced electrical current to prevent the
glow plugs from overheating and burning out. This
provides additional heat in the combustion chamber to
improve the combustion process and therefore lower
emissions when the engine is started from cold.
Modern diesel injection systems are electronically
controlled, so it is convenient and more effective for the
diesel fuel system ECU also to control the glow plug
operation. The ECU is already receiving information
from the temperature sensors and other sensors so it
can control the glow plug operating time, the current
flow (reduced current and heating after starting), and
the warning light (the indication to the driver when the
engine can be started).
Modern glow plugs have heating elements that are
effectively resistances with a positive temperature
coefficient or PTC; the PTC increases the resistance of
the heating element as the element temperature rises,
thus progressively reducing the current flow. In effect,
the current flow is self-regulated to prevent overheating
of the heating element, while still allowing an initial
high current to heat up the element for the cold start.
Figure 4.13 shows a glow plug in a combustion
chamber of a direct injection engine. Figure 4.14 shows
the construction of a modern glow plug and Figure 4.15
shows the arrangement for controlling the operation of
Pintaux injector
A Pintaux injector is a pintle injector which has an
auxiliary hole to direct fuel down the throat of the prechamber during the cranking period (see Figure 4.8e).
This type of injector is suitable for indirect injection
Heater plug
Still the most widely used form of cold-start assistance
on diesel engines, the ‘glow plug’ or ‘heater plug’ is
fitted in the combustion chamber and is effectively an
electric heater that can be used during cold starting and
in the early phases of cold running. When the air is
cold, the air in the combustion chamber is heated by an
electrical heating element for a few seconds prior to
starting a cold engine. The time that the glow plugs
Figure 4.13 Glow plug location on a direct injection engine
combustion chamber
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Fundamentals of Motor Vehicle Technology: Book 2
the glow plug. Note that in the example (Figure 4.15),
the ECU (which is receiving information from the
temperature sensors) is effectively controlling the ‘glow
plug control unit’; the control unit regulates the high
current passing to the glow plugs.
Electrical connector terminal
Insulating washer
Glow plug shell
Glow tube
Control filament
Filling powder
Helical heating wire
Heater element gasket
Double gasket
Round nut
Figure 4.14 Glow plug construction
Sheathed-element glow plug
Glow control unit
Glow-plug and starter switch
To battery
Indicator lamp
Control line to the engine ECU
Diagnosis line
Figure 4.15 Glow plug control
Key Points
Most diesel systems use a glow plug in the
combustion chamber to help increase the
temperature of the injected fuel
Glow plugs are controlled by a timer relay or an
ECU so that the optimum heat time is used
depending on the engine temperature
Note: Modern electronic diesel systems operate in a
very similar way to electronic petrol injection systems,
especially direct petrol injection. See sections 3.1 to 3.4
for information on sensors, actuators and control
signals for electronic injection systems. Chapters 1 and
2 include additional information about sensor and
actuator operation.
4.4.1 Advantages and disadvantages
of direct injection
Direct injection systems have been used on larger diesel
engines for many years, especially for heavy
commercial applications. Since the 1980s, light
passenger cars have also increasingly been fitted with
smaller direct injection engines. Direct injection is a
more efficient method of fuel delivery: it develops more
power and has a lower fuel consumption than indirect
injection (where the fuel is delivered into a precombustion chamber). Direct injection does, however,
have one major disadvantage: the combustion noise is
higher than that of indirect injection, which is
undesirable in passenger motor vehicles.
The combustion noise is generally referred to as
combustion knock and is caused by ignition of the fuel
after injection has initially started. The short delay
between ‘start of injection’ and the ignition of the fuel
means that there is a relatively large quantity of fuel
that initially ignites; this causes a rapid combustion and
pressure rise in the early phases of the combustion
process, which causes an audible knock. On most fuel
injection systems used up until the late 1990s, which
were relying on mechanical pumps to generate the
pressure for injection, it was relatively difficult to
precisely control the fuel quantity delivered during the
early injection phase. If the initial quantity of fuel
injected is too large, ignition is rapid and initial gas
expansion is rapid, thus causing the combustion knock.
Therefore an objective with the modern direct
injection diesel fuel system is to control the initial
injection quantity in what is termed the ‘pilot injection
phase’, which is when a small quantity of fuel is
injected, ahead of the main injection period. This small
quantity of fuel causes a small rise in pressure during
the pilot phase, thus reducing the rapid speed of
combustion, which in turn reduces combustion knock.
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Electronic control of diesel injection (common rail systems)
4.4.2 Common rail
Common rail diesel fuel systems have been widely used
in commercial vehicle and large diesel engine
applications for many years. Common rail refers to a
system whereby the injectors receive fuel from a
common supply rail, which is fed fuel under pressure by
an engine driven pump. On early common rail systems
the pump provided fuel at a relatively low pressure; the
fuel was then passed to the injectors through a pipe (or
even through holes or tubes built into the cylinder
head). The injectors contained a pumping element
(usually driven from the engine camshaft via a rocker
system), which produced the high pressure necessary
for injecting the diesel fuel. Some common rail systems
delivered fuel to a separate pumping element which
then passed the fuel at high pressure to the injector.
One advantage of using a common delivery rail is
that the high pressure pumping element is located in or
close to the injector, so the high pressure created by the
pumping element does not have to pass through a long
delivery pipe, which is the case for traditional diesel
fuel pump systems, where the pump is a considerable
distance from the injectors. With a long delivery pipe
carrying high pressure, when the pump delivers the fuel
it causes a pressure wave to travel along the delivery
pipe (which is full of fuel); the time delay in the high
pressure wave reaching the injector causes timing
inaccuracies of injector opening and closing. With the
short or non-existent high pressure delivery pipe on
common rail systems, this delayed pressure wave
problem is eliminated or reduced: see the following
paragraphs dealing with unit pumps and unit injectors.
Unit pumps and unit injectors
Figure 4.16 shows a relatively recent type of ‘unit
injector’, where the fuel injector contains a pumping
element that is driven by a cam and rocker system. The
cam lobe can be part of the normal camshaft used to
operate the inlet and exhaust valves. The injector is fed
with fuel at a relatively low pressure from a common
supply rail (feeding all injectors). The high pressure is
then created by the pumping element in the injector
itself. Unit injectors can deliver fuel at typical pressures
of 2000 bar.
With this type of unit injector, a solenoid attached to
the injector controls a valve arrangement that opens or
closes an outlet or spill port. When the outlet port is
open, the pumping element will still function and build
up pressure, but the fuel will pass straight out of the
outlet port back to the low pressure fuel system. When
the outlet port is closed by the solenoid, this will cause
pressure to build up above the injector nozzle (due to
the action of the pumping element). The pressure buildup will then cause the injector nozzle to open and
deliver fuel. At the appropriate time, the solenoid will
open the outlet port again, which will cause an
immediate drop in pressure above the nozzle. This will
then allow the spring in the injector nozzle to return to
1 Actuating cam
2 Pump plunger
3 High pressure solenoid valve
4 Injection nozzle
Figure 4.16 Unit injector with combined pumping element for a
common rail system
the closed position (a closing spring is located in the
nozzle, as in older injectors – see Figure 4.8).
The solenoid is controlled by an ECU, which
functions in much the same way as a petrol injection
system ECU: i.e. the ECU receives information from
sensors and is therefore able to control the opening and
closing of the injector, thus controlling fuel quantity
(the injection duration).
Figure 4.17 shows a similar arrangement to that of
the unit injector but the pumping element is separate
1 Injection nozzle
2 Nozzle holder
3 High pressure line
4 High pressure solenoid valve
5 Pump plunger
6 Actuating cam
Figure 4.17 Unit pump system for a common rail system
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Engine management – diesel injection
from the injector; a short delivery pipe is therefore
required to deliver the high pressure fuel from the
pumping element to the injector. This type of system is
generally referred to as a ‘unit pump’ system and is
suited to engines where the camshaft location may not
allow a combined unit injector and pumping element
to be used.
For both the unit injector and the unit pump systems,
the common rail supply (from the low pressure pump)
can be via a low pressure delivery pipe or through a hole
or port system built into the cylinder head.
4.4.3 Electronically controlled
common rail systems using a
single high pressure pump
The logical evolution of the common rail system is to use
a single high pressure pump feeding the common supply
rail. The injectors will therefore not contain a pumping
element (or require separate individual pumping
elements), but can still be controlled using a solenoid
which regulates the outlet port of the injector. Figure
4.18 shows the general layout of this type of system.
Note that this type of system is virtually identical in
principle to a ‘direct injection’ petrol injection system
(section 3.4). Although fuel at high pressure will pass
from the common fuel rail to the injectors, the opening
and closing of the injectors is not dependent on pressure
waves passing through the pipe: it is totally dependent
on the solenoid action, which causes the injector outlet
port to open or close (as on the unit injector).
With this type of common rail system, as well as
controlling the injector timing and injector duration
(fuel quantity control), the pressure at which the fuel is
Fundamentals of Motor Vehicle Technology: Book 2
injected into the combustion chamber can also be
altered to suit the engine operating conditions and
cylinder pressure. The fuel delivered to the common
fuel supply rail (by the engine driven high pressure
pump) can be monitored by a pressure sensor and
controlled using a pressure regulator. The pressure of
the fuel delivered to the injectors can therefore be
controlled so that it is always at the desired value.
Typical injection pressures are around 1600 bar.
With electronic control, the injection timing can be
accurately controlled to allow the fuel to ignite and
burn correctly within the combustion chamber.
Conventional diesel fuel systems inject the total volume
of fuel required by the cylinder during one injector
opening. For this type of common rail system with
higher fuel pressure, the volume of fuel can be injected
into the combustion chamber in stages: a pilot
injection, main injection and sometimes a post
injection. The pilot injection period can be controlled,
so that only very small quantities of fuel are injected,
thus reducing combustion knock. Post injection can be
used to aid the control of emissions: a small quantity of
fuel is injected at the end of the power stroke or even on
the start of the exhaust stroke, the fuel vaporises and
passes through to an NOx catalyst which then reduces
NOx emissons.
Using the information from the various system
sensors, the ECU determines the volume of fuel and the
point in time at which the fuel is to be injected to
provide the required power from the engine. Figure 4.19
shows a typical single high pressure pump common rail
system (sometimes referred to as a common rail
accumulator system), whilst Figure 4.20 shows a
complete system with sensors and actuators; note that
the illustration shows a turbocharged engine, where the
boost pressures are also controlled by the ECU.
4.4.4 Fuel pressure system
Low pressure system
With a low pressure system, the fuel pressure is
produced by a low pressure pump that supplies fuel
from the fuel tank via a fuel filter to a high pressure
pump. The ECU controls the operation of the low
pressure pump. The pump location and design are very
similar to those of a petrol fuel injection system. Figure
4.21a shows the layout of the fuel system. Figure 4.21b
shows a typical low pressure fuel pump. Figures 4.21c
and 4.21d show roller cell and gear type low pressure
pump elements. Note that a conventional fuel filter is
fitted in the low pressure system (Figure 4.22).
1 High pressure pump
2 Rail (high pressure
fuel acccumulator)
3 High pressure solenoid valve
4 Injector
5 Injection nozzle
Figure 4.18 Electronically controlled common rail system using a
single high pressure pump
High pressure system
The high pressure pump, normally driven at half
crankshaft speed by the camshaft, generates the high
fuel pressure which is stored in the common fuel rail,
hence the name ‘common rail’ (Figure 4.23). The ECU
varies the pressure produced by the high pressure pump
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Electronic control of diesel injection (common rail systems)
Figure 4.19 Common rail accumulator injection system
up to a maximum pressure of typically 1300 to 1600 bar
with electrically operated solenoid valves within the
pump assembly. The ECU varies the fuel pressure
according to engine operating conditions; the fuel
pressure is not relative to engine speed (apart from
during engine cranking). The diesel fuel lubricates the
internal cams and plungers of the high pressure pump.
The high pressure pump (Figure 4.24) is driven by
the engine and is usually located in the same position as
a traditional diesel pump. The fuel pressure is
monitored by the ECU with a fuel pressure sensor
situated in the common rail (Figure 4.23), so the fuel is
always delivered at the correct pressure to suit the
engine operating conditions. The fuel passes from the
fuel rail to the injectors through metal fuel pipes. These
pipes are approximately the same in length and
manufactured without excessively sharp bends which
might restrict fuel flow. Note that if any of the fuel pipes
are disconnected during service or repair, they should
be renewed. The pipes are made from steel which
deforms thus providing some flexibility for fitting and
to allow for vibration as well as small movements that
will occur due to engine expansion caused by heat. The
high pressure pump connections, when tightened,
ensure a fuel tight seal.
The common rail acts as an accumulator or reservoir
of fuel, damping pressure fluctuations in the high
pressure system due to the pumping action and
injection. The fuel rail is also fitted with a fuel pressure
regulator (Figure 4.25), so if the fuel pressure becomes
abnormally high, the excess pressure will pass through
the pressure limiting valve and return to the fuel tank. A
mechanical fuel pressure limiting valve was used on
early common rail fuel systems. With later systems, the
ECU controls the fuel rail pressure with an electrically
operated solenoid valve. Note that the ECU monitors
the fuel pressure in the fuel rail and controls the
pressure with a solenoid valve on the side of the high
pressure pump.
4.4.5 Fuel injection system
(See Chapters 1 and 2 for information on electronic
system sensors and actuators, and on electronic
actuator control signals.)
The ECU controls the injectors by making use of a
similar principle to that of petrol fuel injection. The
common rail fuel system uses many of the sensors that
provide information for electronically controlled
distributor pump diesel fuel systems and for petrol
injection systems. These include:
an engine speed sensor fitted to the crankshaft
a camshaft position sensor
an accelerator pedal position sensor
a MAP sensor
an engine coolant and intake air temperature sensor
an air mass sensor.
These sensors are used to monitor engine operating
conditions. The accelerator sensor provides the ECU
with driver requirements: whether the driver wishes to
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Engine management – diesel injection
Fundamentals of Motor Vehicle Technology: Book 2
Figure 4.20 A complete common rail injection system (single high pressure pump system)
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Electronic control of diesel injection (common rail systems)
Key to Figure 4.20
Engine, engine management, and high pressure fuel injection components
17 High pressure pump
18 Metering unit
25 Engine ECU
26 Fuel rail
27 Rail pressure sensor
28 Pressure control valve (DRV 2)
29 Injector
30 Glow plug
31 Diesel engine (01)
M Torque
Sensors and setpoint generators
Pedal-travel sensor
Clutch switch
Brake contacts (2)
Operator unit for vehicle speed controller (cruise control)
Glow-plug and starter switch (‘ignition switch’)
Road speed sensor
Crankshaft speed sensor (inductive)
Camshaft speed sensor (inductive or Hall sensor)
Engine temperature sensor (in coolant circuit)
Intake air temperature sensor
Boost pressure sensor
Hot-film air mass meter (intake air)
B Interfaces
13 Instrument cluster with displays for fuel consumption,
engine speed, etc.
14 Air-conditioner compressor with operator unit
15 Diagnosis interface
16 Glow control unit
CAN Controller Area Network
(on-board serial data bus)
C Fuel-supply system (low-pressure stage)
19 Fuel filter with overflow valve
20 Fuel tank with pre-filter and Electric Fuel Pump,
EFP (presupply pump)
21 Fuel-level sensor
Additive system
Additive metering unit
Additive control unit
Additive tank
Air supply
Exhaust gas recirculation cooler
Boost pressure actuator
Turbocharger (in this case with variable turbine
geometry (VTG))
35 Control flap
36 Exhaust gas recirculation actuator
37 Vacuum pump
Exhaust-gas treatment
Broadband lambda oxygen sensor, type LSU
Exhaust gas temperature sensor
Oxidation type catalytic converter
Particulate filter
Differential pressure sensor
NOx accumulator type catalytic converter
Broadband lambda oxygen sensor, optional NOx sensor
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Engine management – diesel injection
Fundamentals of Motor Vehicle Technology: Book 2
Key to Figure 4.21a
Figure 4.21 Low pressure fuel system and low pressure pumps
Fuel tank
Presupply pump
Fuel filter
Low-pressure fuel
High-pressure pump
High-pressure fuel
Fuel rail
Fuel return line
glow plug
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Electronic control of diesel injection (common rail systems)
Figure 4.22 Fuel filter
accelerate, decelerate or allow the engine to idle whilst
stationary. The ECU uses the sensor information to
calculate the desired fuel pressure, injection volume
and duration to produce the required engine power
and torque.
The fuel within the common rail is at a constant
pressure during injection and therefore the volume of
fuel injected is also constant during the injector opening
period. Therefore a precise volume of fuel can be
delivered during the injector opening period.
The EDC ECU determines the injector opening time
period (injector duration) from sensor information and
provides a control signal to the injector accordingly. The
high fuel pressure exerts a great force at the injector
needle valve, and therefore a very high voltage and
current are required to initially open the injector. The
injector driver control module provides the necessary
high voltage control signal to the injector. The module
might be located within the ECU, or in some cases fitted
as a separate unit. The ECU uses the engine speed
sensor to provide the timing control for each injector.
Additional information is required to synchronise each
injector with the cylinder cycle. A cylinder recognition
sensor monitors the camshaft position, which provides
the ECU with the information necessary to control the
phasing of the injectors. The injectors are situated in the
cylinder head and spray fuel into the swirling air within
the combustion chamber, which is normally integrated
into the crown of the piston.
If the current is switched off to the solenoid circuit
the injector is not energised and the injector needle
valve is closed, which prevents the pressurised fuel
leaving the injector nozzle (Figure 4.26). The high
pressure fuel is applied to the needle valve at the lower
section of the injector and also a control chamber which
is located on top of the injector needle valve within the
top section of the injector. The pressure of the solenoid
spring and the needle valve spring is higher than the
fuel pressure applied and therefore the needle valve
remains closed.
The ECU determines the injection period during
which the injector opens and injects a volume of fuel
into the combustion chamber. The ECU provides the
injector with a control signal that energises the injector
solenoid (Figure 4.26). The solenoid valve lifts,
allowing the fuel pressure to escape from the control
chamber into the chamber above. The fuel passing to
the chamber returns to the tank via the fuel return
system. The initial current required to lift the solenoid is
high, because of the pressure of the spring. Once the
solenoid is open, a smaller current is required to
maintain the solenoid position: the ECU applies a
holding current.
An orifice restriction prevents the high pressure fuel
from rapidly re-entering the control chamber; the
control chamber pressure is lower than the fuel pressure
1 Fuel rail
2 Pressure
control valve
3 Return line
from fuel rail to
fuel tank
4 Inlet from high
pressure pump
5 Rail pressure
6 Fuel line to
Figure 4.23 High pressure fuel system
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Engine management – diesel injection
Pump housing
Engine cylinder head
Inlet connection
High pressure inlet
Fundamentals of Motor Vehicle Technology: Book 2
Return connection
Pressure control valve
Barrel bolt
Shaft seal
Eccentric shaft
1 Drive shaft
2 Eccenter
3 Pump element with
pump plunger
4 Inlet valve
5 Outlet valve
6 Fuel inlet
Figure 4.24 High pressure pump
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Electronic control of diesel injection (common rail systems)
Figure 4.25 Fuel pressure regulator
Figure 4.26 Common rail injector
Electrical connections
Valve spring
Valve housing
Solenoid coil
Valve ball
Support ring
High pressure fuel supply
Valve body
Drain to low pressure circuit
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Engine management – diesel injection
Fundamentals of Motor Vehicle Technology: Book 2
applied to the needle valve. The difference in fuel
pressure between the control chamber and at the needle
valve causes the valve to lift from the seat and fuel is
expelled through the injector nozzle into the
combustion chamber. The pressure of the injected fuel is
equal to the pressure in the fuel rail. The high fuel
pressure, together with the design of the injector
nozzle, allows excellent atomisation of the fuel injected,
which promotes good mixing of the air and fuel within
the combustion chamber. Thoroughly mixing the air
and fuel reduces hydrocarbon and soot emissions.
To end the injection of fuel, the ECU switches off the
current flow through the injector solenoid circuit,
allowing the solenoid plunger and valve to return to its
seat. The closing of the solenoid valve allows the
control chamber to refill with high pressure fuel from
the fuel rail. The high fuel pressure in the control
chamber, together with the force of the needle valve
spring, exerts a greater force than that of the high fuel
pressure at the base of the needle valve, so the needle
valve returns to its seat and injection ceases.
Pilot injection
Earlier designs of diesel fuel systems (in-line and rotary
pump systems) generally inject the total volume of fuel
during one injector opening period for one cylinder
cycle. There is a time period between the start of
injection and the start of ignition of the fuel. When the
fuel ignites, the cylinder pressure rapidly increases,
which pushes the piston down the cylinder. The sharp
rise in cylinder pressure is heard and referred to as
diesel or combustion knock.
The common rail fuel system normally injects the
total volume of fuel (for the combustion process) in two
injection stages, often referred to as pilot injection and
main injection.
A small volume of fuel is injected before the piston
reaches TDC. This small volume of fuel, typically
between 1 and 4 mm3 is used to condition the cylinder
before the main volume of fuel is injected. The pilot
injection raises the cylinder pressure slightly due to the
combustion of the fuel: therefore the temperature
within the cylinder also rises. If the pilot injection
occurs too early in the compression stroke, the fuel will
adhere to the cold cylinder walls and the crown of the
piston, increasing the hydrocarbons and soot in the
exhaust gases.
Figure 4.27 shows the difference in the combustion
chamber pressure rise when pilot injection is used
compared with when there is no pilot injection. Note
the steeper rise in pressure that occurs just after TDC
when there in no pilot injection; it is this steep pressure
rise that is creating the diesel or combustion knock.
Main injection
The time delay between the points at which the fuel is
injected and ignited is reduced because the pilot
injection provides a slightly higher cylinder temperature
and pressure. The rate at which the combustion
Figure 4.27 Combustion pressures
a without pilot injection
b with pilot injection
pressure increases is less severe, resulting in a reduction
in combustion noise, lower fuel consumption and lower
emission levels.
The length of time that the injector is opened
(injector duration), together with the pressure at which
the fuel is injected, dictates the volume of fuel delivered
to the cylinder. It should be noted that although
changing the duration of injector opening time would
affect the volume of fuel delivered, an increase in the
fuel pressure is generally used as the primary means to
increase the volume of fuel delivered to a cylinder. At
high engine speeds, insufficient time exists between the
stages of injection and it is not possible to provide pilot
injection. The ECU combines both pilot and main stages
of injection and uses a single injector opening period to
inject the volume of fuel required.
The ECU monitors any imbalance between the
torque generated between cylinders. After each
injection period, the power stroke occurs, which
accelerates the speed of the crankshaft. The ECU
monitors the acceleration speed of the crankshaft
through the engine speed sensor signal. If all cylinders
are producing an equal amount of power, the
acceleration of the crankshaft between each cylinder
power stroke should also be equal. Engine wear will
affect the power produced by each cylinder, and the
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Electronic control of diesel injection (common rail systems)
Turbocharger boost pressure control
If a turbocharger is fitted, the ECU controls the
turbocharger boost pressure. The ECU monitors the
inlet manifold pressure with a pressure sensor. If the
pressure is too high (over-boost) the ECU regulates the
pressure by using a waste gate in the exhaust manifold.
Some later vehicles are fitted with a variable geometry
turbocharger, where the ECU alters the geometry inside
the exhaust turbine to vary the boost.
Exhaust gas recirculation
The ECU controls exhaust gas recirculation (EGR). This
returns some of the exhaust gases into the induction
system to reduce the harmful emissions emitted from the
exhaust, i.e. oxides of nitrogen or NOx (see section 3.5).
The ECU monitors the air mass sensor signal situated in
the air induction system, and the sensor provides an
indication of the volume of exhaust gas recirculated.
Unlike earlier generations of diesel engine, many
induction systems are fitted with a throttle plate in the
induction system. When the throttle plate (butterfly) is
used in a petrol engine, it alters the air volume entering
the engine and therefore alters engine power. However,
a throttle plate in a diesel engine is used to alter the rate
of EGR. At low engine speeds, the angle of the throttle
plate is adjusted to provide a depression in the
manifold, which induces the rate of EGR. At high
engine speeds and loads the throttle plate is fully open
to prevent restriction to the flow of air into the engine.
The throttle plate is either operated by a stepper motor
or by the modulation of a vacuum switching valve.
Key Points
ECU can alter the fuel volume and injection timing to
cylinders to equalise the power each cylinder produces
at low engine speeds. Unequal power between cylinders
is very apparent at idle, but the ECU stabilises the
engine speed, ensuring a smooth engine idle.
Sensors used in common rail injection systems are
very similar to those used for petrol/gasoline
A high pressure engine-driven pump supplies fuel
to electronically controlled injectors
Common rail systems can operate at pressures up
to 1600 bar
Web links
Engine systems information
www.kvaser.com (follow CAN Education links)
Teaching/learning resources
Online learning material relating to powertrain
Chapter 5
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what is covered in this chapter . . .
Purpose of the transmission system
Transmission types
History of electronic control
Electronic transmission control as a black box
Sensors and actuators used in transmission systems
Clutch electronic control
Manual gearbox electronic control
Torque converter electronic control
Automatic gearbox transmission management (epicyclic, fixed gear and CVT)
Light hybrid powertrain technology (starter–generator)
Electronic differential and four-wheel drive control
Transmission diagnostics
Future developments
Any vehicle equipped with a combustion engine as its
prime mover requires a transmission system to transmit
torque at an appropriate speed to the driving wheels.
Fundamentally, the transmission system is needed
because internal combustion engines have quite a
limited speed range at which useable torque is
produced (this varies, but generally lies in the range
1500–5000 rev/min). Operation within this speed
range is also important for the engine to achieve
maximum efficiency. This is clearly beneficial for fuel
economy and to minimise exhaust emissions. There are
several other reasons why a transmission system must
be incorporated in the powertrain:
to provide variable torque at varying speeds
selectable by the driver for the appropriate
condition, such as low speeds, overcoming
gradients, comfortable cruising speeds
to provide a ‘neutral’ state, i.e. a situation where the
engine can run without being connected to the
driving wheels, for example, when a vehicle is
so that the vehicle can be driven backwards (in
reverse), for manoeuvring, parking, etc.
to provide torque multiplication: the internal
combustion engine cannot produce any torque at
zero speed. Therefore a transmission (including a
clutch of some sort) is needed to overcome vehicle
inertia (resistance to change of speed) at standstill.
This provides a smooth application of tractive force
in a manner that can be controlled by the driver, and
consequent movement of the vehicle from zero
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Transmission types
The various types of vehicle powertrain transmissions
can be classified according to their operating
Multi-stage transmissions have a number of fixed
gear ratios which can be selected manually by the
driver, or automatically by a mechanical or
electrical control system according to the vehicle
operating status.
Continuously variable transmissions (CVTs) are
infinitely variable between certain boundary limits
achieved through hydraulic or mechanical means.
They can also be classified by their construction.
In-line transmissions have an input shaft on one
end and an output shaft at the other. They are
predominantly used in front engine, rear wheel
drive applications.
Dual shaft transmissions have their input and
output shafts misaligned or eccentric, typically for
front wheel drive applications.
Multi-stage transmissions rely on fixed, geometrically
locked elements (i.e. gears), whereas CVTs use friction
locking principles to achieve the necessary ratios. This
friction locking function needs an additional energy
input (for example, from the oil pump used to
generate hydraulic pressure for gearbox operation in
an automatic gearbox or CVT), which reduces the
overall efficiency of the gearbox itself. This
inefficiency is offset by the fact that, because of the
infinitely variable transmission ratios, the engine can
operate closer to its maximum efficiency, which
increases the efficiency of the powertrain as a
complete unit.
Another factor that differentiates transmissions
systems is their level of automation. In Europe,
traditional manual transmissions are dominant,
whereas in America or Asia, automatic shifting
hydraulic, or electrohydraulic, transmissions have the
largest share of the market. These traditional
transmissions are now being displaced by modern
developments in mechatronics and software such that
they have the efficiency of traditional manual
transmissions with all the benefits of an automatic
shifting transmission. In addition, they can operate in
manual or semi-automatic mode according to driver
preference and it is possible to integrate the shifting of
transmission ratios with other vehicle control or safety
systems for maximum driver benefit.
Table 5.1 summarises the common transmission
types and their characteristic features.
Key Points
All vehicles with an internal combustion engine
require a clutch and gearbox for reasons of
performance and efficiency
The main types of gearbox are manual or
automatic and both types can be electronically
Table 5.1 Transmission types
Transmission type
ratio via
hydraulic shift
Toroidal variator drive
Automatic shifting
manual transmission
Dual clutch
Fixed gears
Planetary gears and
torque converter
Belt/chain type drive
Friction wheel variator
Very High
Fixed gears, electromechanical actuation
Fixed gears, electrohydraulic actuation
comfort factorb
Efficiency when compared to an automatic, multistage transmission at given operating point with a petrol/gasoline engine
Measure of the quality of the change of transmission ratio: 1 = completely smooth, 0.1 = rough transition
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Fundamentals of Motor Vehicle Technology: Book 2
5.3.1 First developments in electrical
control for transmissions
Traditionally, transmissions and powertrains in
vehicles were purely mechanical systems. One of the
first developments to incorporate an electrical control
element was the overdrive system. This system was
fitted to many sporting or GT cars in the 1950s and
1960s and was also available as an option on many
other car models during that period.
The overdrive was a self-contained unit, fitted to the
existing gearbox casing, providing an additional gear
ratio which could be engaged in third or fourth gear.
The extra gear ratio was less than one, giving the
output shaft a higher speed than the input shaft.
(Usually, transmissions fitted at that time had a top gear
ratio of 1:1, known as ‘straight through drive’). Thus,
with overdrive selected, for a given road speed, engine
speed was reduced and this provided more relaxed
cruising and better economy. An inhibitor switch on the
gearbox prevented engagement of overdrive in first and
second gears.
The heart of the overdrive system was a single
epicyclic gear set (as used in many automatic
gearboxes) with engagement effected via a
hydraulically actuated cone clutch (Figure 5.1). This
provided the most redeeming feature of the system:
even though the system provided an extra gear, it was
not necessary to declutch (i.e. disconnect the engine
torque from the gearbox input shaft) in order to engage
or disengage the extra gear. This provided improved
drivability and a sporty overtone to the vehicle. The
driver could just ‘flick’ a switch to engage or disengage
the extra gear.
The engagement of the gear was implemented
hydraulically via an oil pump to generate pressure,
which caused an actuator to engage or disengage the
clutch. The control of the hydraulic circuit was
implemented electrically via a simple solenoid valve and
Figure 5.1 An early overdrive assembly
a Direct drive
b Overdrive
this is where the electrical control begins. The system
allowed the driver to engage overdrive via a switch on
the gear knob. This was a simple circuit, with an
interlocking switch, to ensure that overdrive could be
selected only in third or fourth (top) gear.
This is a very simple control circuit (Figure 5.2) to
ensure that the overdrive operates only when the
correct conditions exist. This was only the start of the
integration of electrical control into vehicle
transmissions: from this point, the growth in
sophistication developed rapidly.
5.3.2 Integration of electronics for
transmission control
In the 1980s the next major step forward for
transmission system control was the integration of
electronic control for automatic gearboxes. This was a
logical progression which allowed greater degrees of
freedom and flexibility, such as adapting shift control to
driving style, simplified hydraulics in the gearbox and
reduced costs.
A further advantage of this system, exploited during
the late 1980s and 1990s, was the integration of
transmission control with engine control. Control units
could share information from common sensors,
reducing costs. Strategies to improve gear shifts by
using engine control parameters (such as ignition
timing) could also be used to improve drivability and
These systems have been developed in combination,
even further in more recent years. Tighter integration
management, brought about by increasingly stringent
emission regulations, means that complete control of
the powertrain is essential for modern vehicles. No
longer can the engine and transmission be considered
as separate units. Current developments in vehicle
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Manual switch
2 4
1 3
dynamic control systems require open, fast
communication between control elements (steering,
braking, transmission, engine, etc.) and tightly
integrated systems to achieve high levels of vehicle
safety and performance.
5.3.3 Future developments
Future developments will involve improvements in
control performance. One of the latest engine
technologies under development (homogeneous charge
compression ignition, or HCCI) requires dynamic real
time control to achieve the required performance and to
operate within, for example, emissions limits. This level
of control performance, in combination with fast
communication between electronic control units, allows
further refinement of transmission and powertrain
Transmissions for hybrid engine technologies require
special consideration. Electric motors can produce
maximum torque at zero speed and hence do not need a
gearbox as such. When these motors are used in
conjunction with a combustion engine, a sophisticated
powertrain control system must be used. This uses the
appropriate prime mover according to the driving
conditions and operates the whole system as efficiently
as possible (the control system needs to provide energy
recovery, battery management, switch over from
electric to IC engine power and vice versa, etc.)
There is no doubt that this technology will evolve
and change shape as powertrains are developed to
produce more efficient, cleaner vehicles with
increasingly higher performance and drivability.
Key Points
Figure 5.2 Basic overdrive circuit diagram
Engine and transmission electronic systems must
communicate to improve efficiency
Powertrains are constantly evolving
5.4.1 Integrated electronic control
The transmission system can be improved and made
more sophisticated by integrating the transmission
control unit (TCU) with the engine control unit (ECU).
This is a purely electronic control/software
development. The transmission, engine and equipment
remain unchanged.
Linking together the operation of these two units
management. This is now common practice with
modern vehicles and it is a logical step forward to
improve the vehicle powertrain system as a whole unit
rather than considering engine and transmission as
separate systems. Some vehicle manufacturers are now
opting to produce a single electronic control unit for
engine and transmission called a PCU, or powertrain
control unit. This represents the ultimate step in
harmonisation and physical integration of the control
Typical technology adopted by most manufacturers
for this type of integration and communication is via
the controller area network (CAN) bus. Most electronic
control units have inbuilt CAN capability and the level
of system integration and interaction is chosen by the
manufacturer but not limited. Integration of the vehicle
control systems can provide a number of advantages.
Maximum efficiency and performance can be fully
realised through integrated control of the engine,
transmission and powertrain components. For
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example, the gearbox shift function can be
incorporated with the engine torque control and this
can be adjusted during the shift to optimise
Sensor information can be shared and used for
transmission operation and optimisation as well as
engine functions (for example, engine speed,
throttle position and engine temperature). Overall
sensor count is minimised (reducing manufacturing
costs) and wiring system complexity is reduced.
Vehicle control systems can be harmonised to work
together such that their function and operation can
be complementary. This is particularly important
with safety systems. Operation of transmission
components such as four-wheel drive, active
differential locks and torque distribution can be fully
integrated with safety systems such as stability
control, traction control, anti-lock braking and
brake force distribution, and engine control
functions such as throttle position and torque.
For more detailed information about the technology for
interfacing ECUs, see Hillier’s Fundamentals of Motor
Vehicle Technology Book 3.
5.4.2 Electronic transmission control
as a black box
Electronic control of the transmission is an inevitable
step forward for motor manufacturers, who have
already implemented full electronic control and
management of the engine fuel, ignition and control
systems. This step is necessary to optimise the overall
efficiency of the vehicle power unit. No longer can
engine and vehicle be considered separately, so full
integration and overall electronic control of the
complete powertrain is a reality today. This enables
manufacturers to achieve the levels of performance,
drivability and economy that the market demands.
It is important to remember though that the
powertrain or transmission control is a simple system! It
is similar to any other system on the vehicle. As such,
understanding of its operation can be broken down into
manageable elements. This is particularly important
with fault diagnosis.
The powertrain or transmission control ECU can be
considered as the central component in this system. It is
supplied information about the powertrain status from
a number of strategically placed sensors. This
information would typically include pressure,
temperatures, rotational speeds (wheels or gearbox
shafts), linear speeds (vehicle speed) and driver
requirements (throttle position or gear lever position).
The ECU processor runs a software program in real time
which responds to these inputs and calculates
corresponding actions to be taken. These actions are
implemented by actuators connected to and driven by
the ECU. For example, these could be solenoid valves to
supply pressurised oil to brake bands or clutches in the
gearbox, or signals to other engine systems to
implement some required action (for example, to retard
the ignition during a gear shift).
Sophistication in transmission and powertrain
technology is shifting from mechanical to electronic
mechanically based control systems (such as a
hydraulic valve block for an automatic transmission)
are being replaced with software and electronics with
much higher degrees of freedom and flexibility. The
consequence of this is that the remaining mechanical
Output speed sensor
Crank sensor
Control relay
Throttle position
Oil temperature
Inhibitor switch
Solenoid actuator 1
Solenoid actuator 3
Vehicle speed sensor
Air conditioning
Idle switch
Solenoid actuator 4
Up-shift switch
Down-shift switch
Figure 5.3 Powertrain control unit showing inputs and outputs
Lock up clutch
Gear position
Stop lamp switch
Selector switch
Solenoid actuator 2
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clutch operation and gear selection can be implemented
using electroactuators. A system like this uses all
electronic control and therefore offers many possibilities
for the use of advanced control methodology and
communication with other vehicle systems.
The first developments in electronic control of
transmissions consisted of replacing hydraulic control
with electronic control, but still using planetary gears,
brake bands and clutches; so the mechanical automatic
gearbox was still of conventional construction. After
this step, integration with engine control and
management systems became usual to increase
efficiency and drivability. Current technology is such
that less direct control is given to the driver in many
vehicle systems. Brakes and dynamic stability are
electronically controlled and monitored. Throttle and
traction control are fully electronic. In line with this
trend, gear selection and clutch control of a manual
gearbox can be implemented and greatly improved
with the addition of electronic control and monitoring
(for example, a Tiptronic or a direct shift gearbox
(DSG)). We are now reaching the point where the
difference between an automatic gearbox and a
manual gearbox is just in software function rather than
hardware or construction.
parts become simpler, since there is less intelligence
needed within them. This means that the skills of the
motor vehicle technician or engineer must also shift in
the same direction. These days it is not possible to avoid
the prospect of facing electronic or electrical faults on
vehicle powertrain or transmission systems. This does
not need to be a problem! A logical thought process in
conjunction with simple functional decomposition or
breakdown of the system will overcome any fault that
could occur.
Figure 5.4 shows a simple functional breakdown of
a powertrain control system.
5.4.3 General comments
Key Points
The above developments in electronic control systems
have reduced the mechanical complexity of the gearbox
design itself. The distinction between automatic and
manual gearbox construction is becoming less clear. For
example, in the 1970s, the construction of an automatic
gearbox was completely different from that of a
manual, constant mesh gearbox. The automatic
gearbox was expensive to manufacture. In place of the
manual gearbox’s clutch there was a torque converter.
In place of the gears and synchromesh there were
planetary gears and in place of a gear lever there was a
simple selector lever connected to an extremely
sophisticated and complex all hydraulic system of
valves to control gear shifting and selection.
Developments in electronics, sensors and actuators
have simplified the mechanical construction of
gearboxes to the point where a modern, automatic
gearbox is very similar to a manual gearbox, except that
All complex systems can be broken down and
represented in a diagram showing inputs and
Some systems use a single ECU for engine and
transmission control. Others allow the ECUs to
communicate using a CAN system
Electronic control unit
position switch
Air flow sensor
Vehicle speed
Selector lever
PRND 3 2 1
Program switch
Input-signal conditioning
Engine speed
Components used
for transmission
Input signals for
Input signals for
Input signals for
both systems
Shift valves pickup
Figure 5.4 Electronic control of transmission shown as a block diagram
Reverse gear protection
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Fundamentals of Motor Vehicle Technology: Book 2
Value to be
for example)
Sensor converts
the value to a
voltage (or other
electrical signal)
Signal is
converted from
analogue to digital
ECU acts on
the information
received and
sends output
signals to the
Figure 5.5 Representation of sensor operation
The sensors and actuators provide the essential data
and control elements of the powertrain control system
(Figure 5.5).
5.5.1 Sensors
Most of the sensors used in powertrain or transmission
systems are similar in technology to the equipment used
for engine management or engine control systems, so
they are subjected to a similar harsh operating
environment. They have to work reliably in extremes of
temperatures and pressures and perform their required
task for as much of the life of the vehicle as possible.
Normally the sensor has a simple transfer function
relating or converting the measured value to an output
signal. In most cases, for an automotive system, the
output signal would be a voltage. This voltage would be
connected to the ECU input. An analogue to digital
converter inside the ECU digitises this signal ready for
processing by the central processing unit (CPU). The
process for driving the actuator is similar but reversed.
The CPU outputs the demand value digitally to a digital
to analogue converter, and then the analogue signal
output will operate the actuator or actuator controller
to move the actuator to the required position (note that
the actuator controller could be inside the ECU; for
example, it could be a stepper motor driver circuit).
An important point to consider is the development
of smart sensor technology. Sensors fitting this
description are being developed and implemented more
frequently in modern vehicles. A smart sensor could be
described as one that has ‘local intelligence’. This means
that, rather than just being essentially a converter,
converting some physical quantity into an electrical
quantity, this sensor can have additional functions, such
communication with the driver, self monitoring, a
plausibility check, etc. The development of this
technology has only been possible because of the
dramatic miniaturisation of CPU technology. The
advantages are that:
there is distributed intelligence in the powertrain
control system; a greater overall intelligence in the
system; and less load on the ECU CPU
signal transmission can be digital, with a vastly
improved signal quality and reliability, and is less
susceptible to interference
the bus system can be standardised; sensors can be
daisy chained on the bus; and there is a significant
reduction in cabling/wiring.
Powertrain sensors are similar in their technology to
sensors used for engine control (for example, speed of
rotation sensors). Below, there is a brief overview of the
specific technology.
Speed (rotation) sensor
Speed sensors are used in engine control. For example,
all ECUs need a crankshaft position sensor and most
need a camshaft position sensor as well. Speed sensing
of gearbox shafts is important for auto shift
functionality and to provide closed loop feedback for
shift quality control. The two main technologies used
are inductive analogue sensors and digital Hall effect
sensors. Both of these, when used as rotation sensors,
measure relative velocity and form part of an
incremental rotation sensing system. They are used in
conjunction with an encoder or toothed wheel. A
typical inductive sensor’s cross section is shown in
Figure 5.6.
Figure 5.6 Inductive type speed sensor
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Sensors and actuators used in transmission systems
The inductive sensor is simple and reliable; it works on
the principle that a moving magnetic flux will induce a
voltage into an adjacent conductor. The construction
shown includes a permanent magnet and soft iron core,
around which an induction coil is wound. The sensor is
mounted in close proximity to an encoder wheel or a
gear wheel with a tooth profile. When this gear wheel is
rotated, the movement (passing) of the teeth in close
proximity to the sensor disturbs the magnetic flux and
hence induces a small voltage in the coil. The voltage
approximates a sine wave; its amplitude increases as
the speed increases (up to a saturation point). The
frequency is a function of the number of teeth on the
wheel (which is fixed) and the speed of the wheel. Thus
the frequency of the signal is proportional to the
instantaneous speed of the encoder or gear wheel. The
air gap and tooth profile dictate the overall output
signal profile and a reference mark can be provided
with a missing tooth or teeth. Through special pulse
configuration and teeth profiles, the shaft position and
direction of rotation, as well as speed, can be measured.
One limitation of this sensor is that it is passive in
nature: it cannot detect zero movement or position.
The Hall effect sensor (Figure 5.7) is similar in
outward appearance and use to the inductive sensor.
The main difference is the fundamental operating
principle. The Hall effect is widely used in industrial
sensing technology as well as automotive sensing
applications and can be described simply as a voltage
produced by the interaction between the current flowing
and the magnetic field around a current carrying
conductor. This voltage is a function of the magnitude
(size and direction) of the current and magnetic flux
strength, but also of the material of the current carrying
conductor. In all automotive sensor applications the
sensing element (the current carrying conductor) is a
semiconductor and hence optimised to provide a high
quality output signal (normally a voltage).
Hall effect sensors are also used for current sensing
applications, where they can sense the current flowing
in a cable via the magnetic field around it. The main
advantages of a Hall effect sensor when used as an
incremental rotation sensor are that:
the output signal it produces has a fixed amplitude;
only the frequency varies
the signal is normally a square wave, which is easily
processed with simple electronic circuitry in the
the sensor is active and can be used for position
sensing: it can sense zero position, because the
encoder wheel does not have to be moving to
generate a signal.
One example of the use of these sensors in transmissions
is to provide information about torque converter turbine
speed. This would be used in the control strategy for
shifting, for torque converter lock up control and also for
determining the correct line pressure during shifts.
Additionally these sensors would be used in CVT
applications for determining actual gear ratios.
Gearbox RPM sensors comply with two main
the signal frequency is proportional to the speed
the frequency and pulse width modulated (PWM)
signals give information about speed (including
zero speed, i.e. standstill) and direction of rotation.
Temperature sensors
Powertrain or transmission control systems need to
monitor the temperature of the transmission. A
sophisticated system will be calibrated to adjust system
pressures and responses as the temperature changes to
achieve the optimum drivability under all engine
conditions. Such a system will also act to prevent failure
of the transmission from excessive temperatures in
extreme working conditions.
Supply voltage
Measuring resistor
Figure 5.7 Hall effect speed sensor
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The sensing technology is similar to that used in engine
temperature sensors (Figure 5.8). The sensors are
continuous function devices that change resistance with
temperature (they are resistive elements with a negative
temperature coefficient or thermistors). A temperature
sensor would typically be mounted near the torque
converter, or the oil sump of the transmission, or both in
automatic transmission applications. For automated
manual transmission (such as Tiptronic or DSG) there is
a mechatronic assembly mounted on the gearbox. This
unit houses an electrohydraulic control assembly as well
as the transmission ECU itself. This is a harsh
environment, so temperature sensors are mounted not
only to sense oil temperature but also to sense ECU
control unit temperature. The latter is integrated within
the assembly.
Transmission temperature sensing will become more
commonplace as engine and transmission controllers
become more tightly integrated (with full powertrain
control), and also, as emission legislation and onboard
diagnostic legislation become more comprehensive.
Fundamentals of Motor Vehicle Technology: Book 2
driving mode could set the transmission in third gear
and disable torque converter lock up. For driving in
slippery conditions this mode reduces torque at the
wheels to prevent wheel spin. Another typical selection
is ‘sport’ mode, which changes the shift point selection
and kick down trigger point to optimise acceleration
and give a sporty feel to gear shifting.
Driver’s lever position sensor
The lever position sensor is the main user interface to
the driver (Figure 5.9). Depending on the manufacturer
and the vehicle, it will have a number of functions:
mode selection for the desired gear (park, reverse,
neutral, etc.)
reversing light operation
selection of overdrive
shift up and down for sequential gearboxes.
Additional switches (Figure 5.10) can be incorporated
for the driver to select different shift modes to suit
driving styles or conditions. These will change the shift
strategy, line pressure and torque converter control
according to the selected mode. For example, winter
Figure 5.9 Transmission gear selection lever
Figure 5.10 Selection switch
Figure 5.8 Temperature sensor
Pressure sensor
It is important to control pressure in the transmission
transmissions, to ensure good performance of the
control and actuator system. The valves and actuators
all need the hydraulic pressure to be correct to maintain
optimum system performance under all conditions. As
temperature changes, the viscosity of the lubricant can
change, which in turn can affect the system pressure.
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Sensors and actuators used in transmission systems
The technology used is similar to that used for engine
control (Figure 5.11): a pressure sensing element (a
thick film or semiconductor) is mounted in a housing
and exposed to the system pressure via a channel in a
fitting or mounting screw. With this sensor technology
additional circuitry can be included for linearisation
and temperature compensation of the raw signal. This
signal can be amplified to provide an appropriate
output voltage signal level.
2 cm
changes in permeability by measuring changes in its
own magnetic field.
One design of magnetoelastic sensor is constructed
as a thin ring of steel tightly coupled to a stainless steel
shaft. This assembly acts as a permanent magnet whose
magnetic field is proportional to the torque applied to
the shaft. A magnetometer converts the generated
magnetic field into an electrical output signal that is
proportional to the torque being applied. In another
proposed design (Figure 5.12), a portion of the shaft is
magnetised and the non-contact sensor measures
changes in the magnetic field caused by the torsional
forces in the shaft.
Magnetised shaft
Figure 5.11 Pressure sensor principle
Drivetrain torque sensor
A recent development is a sensor to measure drivetrain
torque in production vehicles. This is particularly
useful, since it could provide true closed loop feedback
and control of drivetrain torque distribution under
operational conditions. Additionally, real time limit
monitoring of transmission torque throughput could
help prevent damage to any transmission or powertrain
components. Such a sensor is also useful for monitoring
the efficiency of the powertrain. The sensor technology
used is non-contact magnetoresistive. Sensor units have
been developed that can be integrated into existing
installations with minimal design changes.
Torque sensing can use one of two principles:
measurement of stress in the shaft material (this is a
function of the shaft torque); or measurement of
angular displacement due to torsion between two
points on the shaft. Torque sensors for powertrain
applications use the former method and measure this
stress via a magnetoelastic principle.
The ability of the shaft material to concentrate
magnetic flux (i.e. its magnetic permeability) varies
with torque: a magnetoelastic torque sensor detects
Flux gates measure the axial magnetic field
generated by the shaft under torsional load
based on the magnetoelastic principle
Figure 5.12 Magnetoelastic torque sensor measurement
Position sensors
For certain applications a number of sensors could be
mounted in strategic positions to confirm that an action
has taken place, or to provide an input signal. For
example, sensors could detect gear lever movement for
sequential shift application, or travel sensors could
detect movement of shift forks in an electronically
automated manual gearbox. For these applications
small Hall sensors are commonly used for their
robustness and reliable operation (Figure 5.13).
Indirect sensor signals for
transmission/powertrain control
A number of sensor inputs to the transmission or
powertrain control are necessary to fully integrate and
harmonise the operation of the engine and
transmission. Even though these signals may not be
obviously linked to operation of a gearbox, the current
trend to close integration of engine and transmission
to provide an integrated powertrain unit means that
this type of integration becomes essential to support
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Fundamentals of Motor Vehicle Technology: Book 2
overall targets of low fuel consumption, good
drivability and optimum performance. Typical inputs
appropriate for an electronically controlled automatic
gearbox are as follows.
Key Points
Engine coolant temperature – This is sensed via a
negative temperature coefficient (NTC) sensor for
transmission system certain functions, such as the
torque converter lock-up clutch, are disabled until
the engine reaches operating temperature.
Engine speed (rev/min) via crank position
sensor – A fundamental input to the transmission
controller, as well as to the engine control system,
this signal is the transmission input speed. The
signal is a conditioned version of the ECU
crankshaft position sensor signal. It is needed to
optimise the transmission strategy for the various
engine operating states.
Brake on/off – This simple digital state signal is
derived from the brake pedal in a similar way to the
brake light switch. It is normally provided via a
separate switch mounted on the brake pedal or via a
double pole brake light switch, for safety reasons.
This signal is used to disengage the torque converter
clutch under braking conditions.
Air conditioning status – This signal is used by the
powertrain control to compensate for the additional
load placed on the engine by the air conditioning
system. It is derived from the compressor clutch
signal. The transmission control line pressure is
trimmed slightly according to the additional load.
Mass air flow/manifold pressure – This signal is
used as a fundamental indication of engine load. It
is measured by an air mass flow sensor. Certain
manufacturers prefer to derive engine load from
manifold pressure and throttle position. In either
case, this load signal is a parameter used in the
control of line pressure and lock up clutch function.
Throttle position – Another fundamental input to
the engine and transmission control, this signal is
important not only for absolute throttle position but
also for rate of change during fast changing driver
demand (e.g. tip in or WOT). The signal will be
used to control shift scheduling, and to control the
line pressure and torque converter.
Vehicle speed – The vehicle speed is one of the
main powertrain outputs (in addition to torque) and
hence is a critical parameter to monitor and feed
back. The existing vehicle speed sensor is used for
this purpose to determine shift scheduling and line
pressure control.
Most sensors convert a physical variable to an
electrical signal
The main types of sensor used in transmission are:
speed, temperature, pressure and position
Torque sensors are under development for use
with transmission control systems
Indirect sensors supply information from other
vehicle systems
5.5.2 Actuators
sensors for
Sensor control
Hall sensors
Figure 5.13 Gear lever sensor control unit
Actuators are devices that convert the low level electrical
signal from the ECU into actual, physical movement.
This could be continuous, cyclic movement or
movement to a set position in accordance with a
demand value. With the current trend towards indirect
operation of powertrain components (drive by wire),
actuators form an important element of the powertrain
control system. Clearly, an actuator needs to activate a
physical output to the required position, in the required
time. The sensor must be able to repeat this as and when
desired, reliably, for many operating cycles throughout
the life of the vehicle. The actuator must also be capable
of operating at the extremes of temperature that could
be encountered around the globe.
The current trend is towards totally electrical
actuation as opposed to electrical servo operation. For
example, rather than having an electrical solenoid
open a hydraulic valve to provide oil pressure to a
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Sensors and actuators used in transmission systems
hydraulic actuator for physical movement, the actuator
is totally electrical/electronic, with an electrical
mechanism providing the physical movement (i.e.
direct rather than indirect electrical actuation). The
reason for this is that, as electronics and electrical
technology has developed, it has become possible to
produce reliable highly efficient electroactuators
cheaply, with the required performance. These do not
need large amounts of electrical power, yet can still
provide the required force.
These actuators are compact and self-contained
(they need only electrical signal and power cables: no
additional power source (such as oil pressure) is needed
for the necessary force to be generated). Hence
integrating these units into existing powertrain
installations or new installations where space is at a
premium (always the case for modern vehicles) is less of
an issue for manufacturers. Another important benefit,
becoming increasingly significant, is that these actuators
can have ‘local intelligence’. The benefits of this are
clear: the actuator can provide reliable feedback to the
control system about its performance. An actuator can
provide not just positional feedback (which is important
for closed loop control) but also status monitoring, such
as temperature or a signal plausibility check. This allows
complex safety and redundancy to be integrated in the
overall control system. For example, a clutch actuator
could monitor the system for wear of the clutch and
inform the driver when clutch replacement will soon be
needed. Such actuator intelligence can be used by the
control system to provide real time monitoring of the
powertrain components, checking for faults and
efficiency of operation. This technology will become
essential as requirements for on board diagnostics and
system monitoring become more sophisticated and as
legislation becomes tighter.
The following section discusses the different types
and basic operating principles of commonly found
powertrain actuators.
Electrohydraulic actuators
Simple on/off actuator
Simple solenoid actuators (Figure 5.14) are commonly
used within the transmission system to direct flow of oil
into and out of components as required, typically to
implement gear shifting and for torque converter
control. Their basic construction consists of a spool
valve to control fluid flow connected to a simple
solenoid arrangement. Current supply to the solenoid
changes the valve state. They can be constructed for use
as on/off or change over valve (normally open or
closed). They are usually switched at battery voltage
because they demand a high current at initial actuation
(pull in of the solenoid). The benefits of these actuators
include their simple, robust construction.
Variable position bleed actuator
The variable position bleed actuator is used in
applications where a variable movement and position
Figure 5.14 Solenoid actuator
are required for fluid control (see Figure 5.15). For
transmission applications, such an actuator would be
used in a gearbox as a bleed valve, for example in
pressure control applications where the output pressure
from the valve would be a function of the supply current.
An important design criterion is to reduce ‘stiction’ or
static friction in the valve to allow precise positioning
with minimal error. This can be achieved through careful
port design. An electronic control circuit with pulse
width modulation would typically be used to operate
this valve by supplying the appropriate varying current
according to demand. An important feature of the circuit
is that it should be able to provide an appropriate
current to activate the valve quickly and then supply
sufficient current to hold the valve position without
overheating (different currents for pull in and holding
are provided by an intelligent driver circuit). This is
similar to a fuel injector driver circuit in the engine ECU.
It is very common to find all the required
electrohydraulic actuators mounted in a complete
assembly on or in the gearbox casing. This has a
number of advantages:
interconnecting hydraulic paths between the valves
are short, minimising pressure drop
one casting contains all the valves, reducing
manufacturing costs
the system is easier to integrate into the gearbox
assembly or design
its single interface point is easier to integrate into
the system electronics.
Figure 5.15 Schematic of a variable position bleed actuator
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Another recent development is to combine within the
electrohydraulic assembly, the electrical and electronic
parts, including the ECU! This creates a complete, selfcontained mechatronic unit for control and
management of the transmission. Very close interfacing
of hydraulics and electronics gives greater reliability
and reduces the installation space needed. In addition,
the complete transmission assembly forms a compact,
modular unit that can be easily integrated into current
or new vehicle designs. A typical example is shown in
Figure 5.16.
Figure 5.16 Mechatronic unit
Clutch actuator
There are several designs for electronic control and
electrical actuation of the clutch. The type chosen by
each manufacturer depends greatly on whether an
existing gearbox design is being adapted, or whether
the gearbox design is new and will integrate electrical
clutch operation from inception.
Figure 5.17 shows a current design used in a
production vehicle. This basically consists of a package
containing a brushless DC motor with feedback of
position. The rotary motion is converted to linear
Figure 5.17 Worm gear drive actuator
Fundamentals of Motor Vehicle Technology: Book 2
motion via a worm driven crank assembly. This in turn
operates a piston inside the unit which provides
hydraulic pressure to a clutch slave cylinder assembly. In
this case, therefore, the existing clutch actuator (a
electroactuator replaces the master cylinder assembly.
Clearly this system also incorporates an ECU and a
clutch pedal position sensor. The main advantage of
such a system is that it can be (and has been) introduced
in a production vehicle. Minimal changes or adaptations
are needed in the vehicle design to incorporate this
The disadvantage is that retaining an existing
hydraulic system means that this system still needs to be
maintained. In addition, the dynamic response of such a
system is reduced because there is more inertia in the
system due to the increased component count and the
interfaces between the actuator and the clutch itself.
An improvement is to dispense with the existing
actuator system and mount the electroactuator as close
as possible to the clutch itself. This is feasible for a
completely new design of gearbox or powertrain. Design
of the actuation mechanism can also improved with
greater operating force and higher efficiency. One design
applies the force directly at the release bearing via a
concentric release mechanism (Figure 5.18). It uses an
axial screw ball bearing track (a helix) with a rotating
collar such that, as the collar turns, an axial force is
applied to the clutch release bearing via the ball
bearings. The collar is rotated by a brushless DC motor
with a position feedback signal. This motor drives a lead
screw assembly to convert rotary to linear motion. This
linear force is applied to the collar via a Bowden cable
(shown in Figure 5.19).
An alternative mechanism is to apply the force to
the release bearing via a lever mechanism (Figure
5.20). In this case the actuator can also be mounted
close to the clutch on the bell housing. The actuator
uses a brushless DC motor with positional feedback.
This provides a high level of reliability and longevity.
The rotary motion is converted to linear movement via
a lead screw or spring band system incorporated in the
Figure 5.18 Direct acting clutch actuator
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Sensors and actuators used in transmission systems
Lead screw drive
Bowden cable
Brushless electric
Mechanical concentric
release system
Figure 5.19 Mechanical concentric clutch release system for
clutch actuator
Clutch bell housing
Figure 5.20 Dual clutch actuation system
flipper controls as in racing cars). The driver tells the
ECU that a shift is required; the ECU then calculates the
best time to do this and therefore has the ultimate
control over the process, taking into account other
factors, such as vehicle stability, braking, steering and
other dynamic factors.
There are a number of methods of gear shift
actuation depending on the gearbox type. The main
differentiating factors are whether the transmission is
retrofitted with actuators or is a new design where this
technology will be used. Another factor is whether the
transmission is an auto shift gearbox (a manual
transmission with electroactuation of the shifting
process) or is a parallel shift gearbox (such as the
Volkswagen direct shift gearbox (DSG), where the next
gear is engaged while the vehicle is in the existing gear;
this requires a twin clutch arrangement).
The system shown in Figure 5.21 is a retrofit unit
used with an existing gearbox design for a small front
wheel drive vehicle. In this particular application gear
selection is via a push pull and twist rod. This rod enters
the gearbox and has a selector ‘finger’ attached which
pushes forward and backward the appropriate selector
rail for the required gear (Figure 5.22).
The actuator unit consists of two brushless DC
motors, one to provide forward and backwards
movement (for gear selection) via a quadrant gear. The
other provides movement side to side (across the
gearbox ‘gate’) via a rack mechanism. The existing gear
interlock mechanism is employed to prevent selection
of two gears at the same time. It is important to note
that this actuator assembly was designed for the first
generation of automatic shift gearboxes, where
modularity (i.e. the ability to reuse existing components
in gearbox designs) was the most important factor.
actuator. This is transmitted by a simple lever
arrangement as a force at the release bearing. This
system is capable of transmitting high forces (for
clutches transmitting up to 900 Nm) and can also be
used in dual clutch applications.
Gear shift actuator
Clearly, with a normal automatic transmission, gears are
changed via a number of actuators and a combination of
brake bands and clutches, which operate hydraulically.
For electronic control applications, they are activated by
gearbox system oil pressure which is switched via
solenoid type valves as described above. Therefore the
actuators are hydraulic but controlled electrically. In this
section we will focus specifically on electroactuators:
actuators where the force is generated by some electrical
means and controlled electronically.
Actuators of this type are used in the latest
generation of highly efficient automatic shifting
transmissions. These transmissions are generally, in
construction, similar to a manual gearbox, except that
the driver has only indirect control of the gear shift
process. Most vehicles using this system receive driver
input from switches on the steering wheel (typically
Figure 5.21 Retrofit gear shift actuator
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Current designs of actuation mechanism are in line with
current trends in transmission and powertrain design:
manual constant mesh gearboxes are used, but with
electrical actuation of clutch and gear selection. Hence
the housings for these actuators will be designed in
from concept. The choice of whether the gearbox
behaves as a manual or automatic transmission (or
both) will simply be controlled via software and
perhaps the driver interface (such as whether or not
there is a clutch pedal; whether the gear lever is one
with a manual style gate, or is an auto-style selector
lever, a sequential selector lever or a steering wheel
paddle change).
Figure 5.23 shows a shift drum with an integral
motor which could be installed directly into a gearbox
This selection mechanism is similar to those used in
most motor cycles, in that the drum has grooves
forming tracks into which the gear selector forks
engage. The tracks are designed such that, as the drum
rotates, the appropriate selectors move the
synchromesh hubs in and out of engagement to select
the appropriate gear. A brushless DC motor is used,
integrated within the drum, complete with a planetary
gearbox to provide the required operating speed and
force. The advantage of this design is its compactness
and ease of installation into the gearbox housing. If a
single motor is used to drive the assembly the system
can be relatively low in cost but there are some points to
the gearbox design has to accommodate the
arrangement from concept; it cannot be retrofitted
the shift drum arrangement means that gear shifts
have to be sequential; arbitrary shifts cannot be
made (for example, from fourth to second gear)
Figure 5.22 Manual gear shift actuator for front-wheel drive
Fundamentals of Motor Vehicle Technology: Book 2
Integrated brushless motor
Figure 5.23 Double shift drum with internal drive
the latter can be avoided by using a double shift
drum arrangement: Figure 5.24 shows an example
with external drive motors.
Reduction and torque multiplication are provided by a
spur gear. For these systems both motors must be
capable of sufficient force to provide gear selection
reliably, compared with the previous retrofit design,
where only one motor needs to be capable of generating
this force (the other just moves across the gearbox
‘gate’). This factor increases the overall cost of the
assembly. Another important point is that interlocking
of the gear selection must be prevented: in theory, both
actuators could engage a gear at the same time.
Interlocking can be implemented with a mechanical
arrangement as well as through software.
A double shift actuator arrangement is currently
more appropriate for parallel shift gearboxes or DSG
systems where maximum advantage can be gained from
an optimised shift process. By using two actuators, with
active interlocks, reduced gear change times can
significantly improve vehicle performance and
acceleration. This provides a clear sales argument to the
customer for the additional cost of this technology.
Figure 5.24 Double shift drum with external drive
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commonplace on modern vehicles and will certainly be
more common in the future. Current actuator drive
units, brushless DC motors, provide reliable technology
that can last the life of the vehicle, and they are suitable
for use in safety critical applications (as above). The
compact design of the motors means that the actuators
require no greater space than the manual mechanisms
they replace. Additional technology can be integrated in
the actuator design, such as:
Key Points
Clutch electronic control
Actuators (most of them) convert an electrical
signal into a physical action
A common transmission actuator is a solenoid
operated valve that controls fluid pressure
The current trend is towards full electrical
operation, so that the actuator, for example, acts
directly on the component and no fluid pressure is
Smart actuators are being developed that contain
local intelligence
incremental travel measurement
electronic commutation
position sensing.
DC motors are popular as they can even be used
to retrofit for ‘manual’ gear shifts
This means that additional sensor mounting to support
operation of these actuators is not specifically required
because it is built in.
5.6.1 Introduction
The clutch is a simple device which can be employed
in any rotating drive system where it is necessary to
disengage or re-engage the transmission of torque. A
friction clutch is used for vehicle transmission systems.
The clutch system consists of a driven and a
driving member (Figure 5.25). In a transmission, the
driving member is usually also the engine flywheel.
The driven member is a friction disc which connects
via a spline to the gearbox input shaft. The friction
disc is clamped by a spring disc in the rest position,
and hence drive and torque can be transmitted from
driving to driven member. The spring clamping can be
relaxed as required by the operator (in a vehicle
transmission, via the clutch pedal); this allows the
friction disc to spin freely and hence no torque is
transmitted. If the spring pressure is gradually
released, the clamping force can be gradually applied
to the friction disc and hence torque can be
progressively transferred between the driving and
driven members as required. This allows the smooth
take up of drive and transfer of torque between the
engine and gearbox. The clutch in a vehicle
transmission allows the effective disconnection of the
engine and gearbox as required by the driver. This is
needed for three main reasons:
as mentioned, it allows the smooth application of
torque from the engine to the gearbox, which is
necessary for the vehicle to start from stationary,
because a combustion engine has to be running at
a minimum speed to produce power and torque
to allow disconnection of the engine, so that no
torque is transmitted through the gearbox; this
allows the driver to shift from one gear to another
in a fixed ratio manual gearbox
to provide a temporary neutral condition for stop
and start driving, for example, in traffic.
5.6.2 Electronic clutch management
For many years, mechanical friction clutches, operated
directly via a foot pedal (mechanically or hydraulically)
have remained completely unchanged. As the clutch
forms such a critical part of the transmission/powertrain
system, simple mechanical operation directly by the
driver leaves much room for improvement. Electronic
control of the clutch, in combination with full
integration of clutch operation with the overall
powertrain control system provides several possible
improved drivability, with more comfortable and
smoother clutch and gear shift action, and with antistall protection in automatic mode
effortless operation, particularly when fitted to
vehicles with high engine torque
improved pedal feel for the driver, with reduced leg
pressure required
improved reliability, which is achieved through
reduced driveline wear and tear, and the total
absence of mechanical linkages for clutch actuation
an automatic shifting mode when used with a
suitable gearbox
improved crash protection for occupants because
the pedal box is less intrusive
data acquisition of clutch use for maintenance
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Figure 5.25 Standard vehicle manual clutch
Such a system has been developed and can be fitted to
existing production models as well as to new designs of
powertrain systems for road vehicles. Such a system
replaces the existing hydraulic or mechanical actuation
assembly with an electromechanical actuator (see Figure
5.26). A pedal position sensor is attached to the clutch
pedal for driver input and the whole system is controlled
by an ECU integrated in the powertrain control system,
which takes additional information on vehicle behaviour
from the engine and powertrain ECUs and sensors. The
ECU controls the clutch actuator directly and thus the
mechanical link between the clutch pedal and clutch is
completely eliminated.
The basic functions of this unit are exactly the same
as those of a manual transmission: to accelerate the
vehicle from rest and stop it. Movement from rest
requires precise control of clutch engagement and this
can vary according to the operating conditions (a hill
start will require different control from a start on the
level). Changing gear is a highly dynamic process and
requires precise control for the gears to change
smoothly and seamlessly. Stopping (declutching) is a
simple process when carried out manually, but adding
electronic control opens up possibilities during extreme
conditions for clutch operation to be more controlled
and fully integrated with the other vehicle safety
systems (such as stability control, ABS and traction
control) to improve vehicle safety.
Additional functions can be added to the system
according to the application: for example, one could
drivetrain condition monitoring
creep control for accurate slow speed movement
active vibration damping for powertrains without a
dual mass flywheel.
The system can implement clutch operation in less than
one-tenth of a second to optimise safety and driver
comfort. Additionally, it operates seamlessly to correct
driver errors. The pedal stroke and resistance are fully
adjustable to suit the driver and the vehicle type. The
system’s compact design can save weight and offers
greater flexibility of installation in a vehicle.
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Clutch electronic control
Powertrain efficiency
Enhanced driver comfort using a mechatronic clutch
Actuator optimises
clutch operation
CBW transmission system
Clutch-by-wire (CBW)
supersedes the mechanical link
between the pedal and the clutch
control unit
5.6.3 Twin clutch arrangements
Figure 5.26 Electronic clutch control system
effectively spilt into two halves. During operation, i.e.
upshift or downshift, an appropriate gear can be preselected while the torque is still transmitted through an
existing gear. This dramatically reduces the gear shift
time, torque transmission to the wheels is practically
uninterrupted and acceleration times can be
significantly improved. This technology will be discussed
in more detail in the appropriate section of this book.
In this application, clutch engagement and
disengagement are handled electronically. Normally,
wet clutches are used, actuated by hydraulics. The
electronic controls are interfaced via VBA valves, driven
with a PWM signal which is generated from the ECU
according to demand.
DSG technology will be discussed and explained in
more detail in a later section of this chapter.
Key Points
The latest developments in gearbox automation (DSG or
parallel-shift gearboxes) require a twin-clutch
arrangement (Figure 5.27), because the gearbox is
The system reduces foot travel
and the force required to operate
the pedal. The slightest mistake in
controlling the clutch (for example,
sudden pedal release) is
The engine does not stall if the driver fails to disengage
the clutch, for example in emergency stop situations
The driver can select the clutch operation
mode – with or without the pedal
• Offers automatic and semi-automatic clutch modes
• Small system dimensions enhance driver leg protection in
front-end impacts
• Technology is designed for integration with other systems
(ABS, ESP, etc.)
• Enhances vehicle’s reliability by reducing transmission wear
(by automatically correcting operating mistakes) and
eliminating mechanical linkages
• Eliminates noise and vibration feedback into the cabin
• Enhances driving comfort. CBW makes clutch use easy,
provides smoother gear changes, and eliminates the risk of
stalling in automatic mode
Force feel system
Parallel shift boxes pre-select the next gear while
an existing gear is still in use
Electronic clutch control improves efficiency and
Clutch control can be fully electrical or an actuator
can be used to operate the slave cylinder, for
Figure 5.27 Twin multiplate clutch in section
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5.7.1 Introduction
Manual gearboxes (Figure 5.28) have not been
dramatically affected by developments in electrical and
electronic systems until recently. Early sliding mesh
gearboxes with straight cut gears were noisy and harsh,
and were difficult to use. Later, constant mesh
gearboxes with helical gears were quieter, but still
difficult to use (double de-clutching was required).
Modern synchromesh gearboxes are quiet and easy to
use, but gears cannot be engaged unless they are
synchronised. These developments all happened in the
early years of the mass produced car industry: since the
introduction of the synchromesh gearbox on mass
produced vehicles in the 1960s little has changed in
manual gearboxes, apart from their increasing number
of gears (initially from four to five, and now six gears
are commonplace).
Engine electronic systems have developed rapidly
over the past 20 years. Now these developments are
starting to impact on the transmission, even on a
completely mechanical device, the manual gearbox.
Drive-by-wire systems are becoming more common
Figure 5.28 Constant mesh gearbox
within the industry and in production vehicles. The next
step with manual transmission (already available at the
top end of the market) will be the introduction of
electronic shift control and actuation (even if manually
controlled by the driver) in conjunction with electronic
clutch actuation and monitoring (where the driver
controls the clutch position with virtual clutch pedal).
This technology is particularly interesting in
applications where improved fuel economy is the main
target: for maximum economy, driver behaviour
becomes more critical. Therefore, by automating the
gear shift process, the powertrain system can be
optimised to give the best possible fuel consumption:
the system can eliminate some driver errors and driving
styles which can compromise performance and fuel
economy. Where economy is the ultimate target, it is
much better to use automated manual transmissions
than other types of transmission, such as CVT or
hydraulic automatic transmission. This is because
manual transmissions do not suffer from the internal
losses (due to the hydraulic power needed for shift
actuation) associated with automatic gearboxes and
hence offer greater efficiency.
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Manual gearbox electronic control
Contrary to the above reasons of economy, automated
manual shift gearboxes are often used in high
performance vehicles because the manual gearbox is
more efficient than other types! Hence, for ultimate
vehicle performance, it is the preferred choice. With
electronic control of shifting and clutch actuation, shift
times can be optimised and improved considerably
compared with manual control by the driver. Advanced
strategies for acceleration from rest can be implemented
to improve vehicle performance. In addition, as noted
previously, the transmission control can be integrated
with other vehicle control systems for stability and
traction to provide the ultimate in high performance
driving. For example, in a critical situation such as
downshifting on a slippery surface, the clutch can be
released instantaneously if there is excessive engine drag
torque, so that the car will not lock the driving wheels
and slide.
This technology has been developed and marketed
successfully by manufacturers of high-performance
vehicles, such as BMW, Ferrari, and Aston Martin, and it
is widely used in motor sport, where the goal is ultimate
vehicle speed and performance.
Table 5.2 shows gear shift times for different
vehicles with automated manual gearboxes.
Table 5.2 Gear shift times
BMW SMG 1 (M3 E36)
BMW SMG 2 (M3 E46)
Ferrari F1 (575M)
Ferrari F1 (360 F1)
Ferrari F1 (Maserati 4200GT)
Bugatti Veyron
Aston Martin Vanquish
Alfa Romeo Selespeed
Minimum shift time (ms)
An important additional feature of electronic control is
its ‘self-learning’ capability. The transmission control
software can be designed to be adaptive to a person’s
driving style, and a driver can select a driving mode,
such as sport mode. BMW calls this feature ‘Drivelogic’,
when it is offered with their sequential manual gearbox
(SMG) on their high performance M series models.
This function allows the driver to choose the
transmission shift characteristics from 11 different
driving programs. These range from a balanced
dynamic program (program S1) to a very sporty
program (S5). Finally, the driver can also choose a
program (S6) where the system’s dynamic stability
control (DSC), which comes standard, is deactivated.
Here, the transmission will shift and respond with a
dynamic performance similar to that of a racing car,
thus giving the driver the ‘ultimate driving machine’
The highlights of the BMW SMG system are as
fast and precise gear shifting within 80 milliseconds
sequential shift actuation via a selector lever or
steering wheel buttons
shift warning lights on the dashboard
self-learning adaptation of the gearbox over time
‘Drivelogic’ personal settings – 11 driving programs
ranging from balanced dynamic to strictly sport
a sequential shift mode
a fully automatic shift mode
special functions – slip recognition, a climb assistant
and an acceleration assistant.
5.7.2 Case study: VW electronic
manual gearbox
Volkswagen has produced the world’s first production
3 litre car! That is, the company has developed a car
that uses only 3 litres of fuel every 100 km. The factors
affecting fuel economy are many and varied: namely
aerodynamics, rolling resistance, powertrain design and
vehicle mass. These all have to be optimised to achieve
the required efficiency from the vehicle to lower the fuel
consumption consistently to this level.
The vehicle used is a VW Lupo, adapted to give
greater powertrain efficiency. One of the most important
adaptations was to implement electronic switching in
the transmission (Figure 5.29). This was done to reduce
the possibility of increased fuel consumption caused by
drivers’ gear shifting habits. This system also ensures
that the vehicle is in the correct gear, to give the best fuel
consumption, at all times relative to the driving
conditions. The system covers three main component
Mechanical – The transmission is the manual
system used in current VW production small cars.
The gearbox was made lighter via additional
drillings in internal components and by reducing the
oil capacity. Additional mechanical components in
the system include the selector mechanism shaft
and levers
Hydraulics – The manual shift mechanism was
completely replaced by an electro-hydraulic unit.
The clutch is operated through an actuator
mechanism. Hydraulic power for the shift and
clutch is provided by an electric hydraulic pump and
pressure accumulator
Electronics – The driver uses a throttle pedal sensor
and selector lever, with the gear shift and clutch
operated via electro-hydraulic valves. Information
about shift position and selector lever position is fed
back through potentiometers and micro switches. At
the heart of the system is the transmission ECU.
The system can operate in manual, sequential mode or
automatic mode. Pushing the selector lever sends a shift
demand signal to the ECU via micro switches, and the
lever assembly contains a potentiometer to detect the
absolute position of the lever for selection of neutral,
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Figure 5.29 VW electronic manual gearbox overview
reverse or automatic shift mode. When a shift is made,
the ECU calculates if this is appropriate from a number
of input signals. These are:
engine speed and torque
throttle position
brake pedal and pressure
vehicle speed.
The signals are provided by sensors connected directly
to the transmission ECU or via the CAN bus interface to
the engine ECU. The process of changing gear is exactly
the same as a driver would implement manually. First
the clutch is opened via the hydraulic clutch actuator
(Figure 5.30); pressure is supplied and controlled at the
actuator via a solenoid valve operated by the ECU. The
clutch position is fed back to the ECU through a
movement sensor mounted on the actuator. The clutch
limit positions are monitored by the ECU at regular
intervals to compensate for clutch wear.
Once the clutch is open, gears are shifted by the
hydraulic pistons in the electro-hydraulic shift actuator
(Figure 5.31), and controlled via solenoid valves. There
are two pistons for gear selection and two for gate
selection (each piston pair provides forward and
backward force), each piston has its own controlling
solenoid valve to apply or release hydraulic pressure
smoothly and progressively. This is essential for smooth
synchronisation during gear shifting. Potentiometers
are fitted for gate and gear selector movement and
these send the measured position back to the ECU.
Once the next gear is fully engaged the clutch
actuator is released and the clutch closes to reinstate
torque into the gearbox. During transient operation
the clutch is kept approximately 20% open to ensure
good response during gear changing and to reduce
transition times.
Figure 5.30 Clutch slave cylinder
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Manual gearbox electronic control
Figure 5.31 Selector assembly
An additional feature of the system is the stop/start
function. This eliminates fuel wastage when the vehicle
is idling. When the vehicle is stationary, the engine
stops if the brake pedal is pressed for more than 3
seconds. When the pedal is released, the engine is
restarted automatically and first gear is engaged so
that, when the driver presses the accelerator pedal, the
vehicle accelerates immediately. This feature is
managed by the transmission ECU and is an integrated
part of the overall fuel consumption reduction strategy
for this vehicle.
5.7.3 Case study: Volkswagen direct
shift gearbox (DSG)
The ultimate gearbox would be one that combines the
best attributes of both types with the latest in control
and system integration technology. VW has proposed
this in the form of their direct shift gearbox (DSG); this
technology is also known as a parallel shift gearbox
(PSG). This attempts to combine the transmission
concepts of automatic and manual systems into a
completely new generation of gearbox.
The main system highlights are:
There are a number of advantages and disadvantages of
automatic and manual transmissions.
Manual gearbox – It is the most efficient type of
gearbox with minimal power losses; the driver has
full control over shifting and hence is provided with
sportier driving.
Automatic gearbox – This has the greater level of
smoothness and comfort, with no interruption in
torque transmission during driving.
a six speed synchromesh gearbox (plus reverse)
selectable, pre-programmed driving modes (sports,
a sequential shift via a lever or steering wheel
a completely integrated mechatronic control unit
which houses ECU electronics and electro-hydraulic
controls, mounted on the gearbox, providing a
system with minimal external interface connections
a hill-holder function and creep regulation, with
enhancements for low-speed driving and
system fault handling, with full electronic diagnostic
capability, and a limp home mode.
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Installation position in gearbox
Twin clutch
Output shafts
Input gear of differential
Parking lock gear
Figure 5.32 Gearbox shaft layout
Mechanical construction
The torque is transmitted into the two transmission
units via an integrated twin clutch assembly with
hydraulic actuation for both clutches. A dual-mass
flywheel is used to insulate the transmission from
engine torsional vibrations (see the diagram in the
Clutch section). The two input shafts are combined
concentrically, each one fitted with a pulse wheel so the
ECU can detect rotating speed (Figure 5.32). The two
output shafts hold the gear synchromesh units and both
transmit torque to the differential gear. The differential
also includes a gear wheel for a locking pawl to provide
a ‘park’ position (with the wheels locked by the
Gear selection is via selector forks (Figure 5.33), in
a similar way to a normal, manual gearbox, except
that, in this case, the selector forks are hydraulically
actuated through oil pressure. A small piston mounted
at each end of each selector fork is supplied with
pressurised oil according to shift requirements from
the control system. A small permanent magnet fitted
to each selector fork allows the ECU to detect the
precise fork position and hence gear engagement via a
sensor in the gearbox. Once the selection is made the
pressure is released and the selector fork is held in
position by a locking mechanism.
Figure 5.33 Selector mechanism
crown wheel
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Manual gearbox electronic control
An important part of the gearbox is the oil lubrication
system. This provides not just lubrication and cooling
but also hydraulic power for the actuators to shift gears
and operate the clutches. The oil pump is driven
directly from the engine input through a shaft. The
lubrication system also incorporates its own filter and
heat exchanger, since it is so critical to correct system
operation and performance.
Electronics and control system
At the heart of the system is the mechatronics
transmission module (MTM) (see Figure 5.34). The
robust construction of modern electronic technology
makes it commonplace with electronic transmissions to
integrate the electronics, electrics, hydraulics and
mechanics into a single module, mounted at the
transmission itself. This has the advantage that it
provides the highest level of component integration with
the minimum number of external interfaces and
connections to the vehicle, which greatly improves
In the DSG system the MTM is at the centre of the
system and all sensors and actuators are connected to it,
since all actions are initiated and monitored by it. This
unit also houses the ECU itself.
The sensors in the system measure the following:
clutch oil temperature – at this position in the
gearbox the lubricant is under the greatest thermal
stress, and by monitoring the temperature at this
point the control unit can regulate the flow of oil
gearbox input speed – this is basically the same as
the engine speed
input shaft speed – a speed sensor on each input
shaft monitors the speed input to each half of the
gearbox; they are mounted on the opposite side of
the clutch to the above sensors, which allows the
system to monitor the clutch status and slip ratio
output shaft speed – two sensors are mounted on a
single pulse wheel but with phase shift between
them; they monitor output shaft speed and direction
of vehicle travel (by offsetting the two signals)
clutch pressure – sensors are used in the regulation
of clutch operation
gearbox oil temperature – sensors protect the
gearbox from overload and are used to initiate a
warm-up function
control unit temperature – sensors protect the
system electronics
selector fork travel – sensors monitor the actual
position of the selector fork for gear selection
selector lever – sensors provide information about
absolute position (park, reverse, neutral, etc.) and
sequential shift (up, down).
The electro-hydraulic part of the control system consists
of a number of actuators:
a pressure control valve – this modulating valve
regulates the main system pressure according to
engine torque
clutch pressure control valves – modulating valves
control the clutch operation
an oil pressure control valve – a modulating valve
controls cooling oil flow
gear actuator valves – on/off solenoid valves engage
and disengage gears via selector forks
a multiplexer control valve – an on/off valve
controls the position of the so-called multiplexer;
this unit is used together with the gear actuator
valves for gear selection and reduces the number of
gear actuator valves required
safety valves – modulating valves isolate hydraulic
pressure in sections of the gearbox in the event of a
safety related fault; they also allow rapid opening of
each respective clutch, if necessary, when an
overpressure occurs.
Additional interfaces are provided via the CAN to:
Figure 5.34 Mechatronics module
the anti-lock braking system (ABS), the electronic
differential lock (EDL) and the traction/stability
control (ESP) system
the diesel or gasoline engine management system
the selector lever control unit
the steering column electronic control unit.
Basic principle of operation
The DSG consists in essence of two manual,
synchromesh gearboxes in one unit. Each one has its
own clutch and torque input from the engine. These
clutches are wet, multi-plate clutches that are actuated
hydraulically under the control of the transmission ECU.
Look at Figure 5.35: the first, third, fifth and reverse
gears are within transmission unit 1 and the second,
fourth and sixth gears are within transmission unit 2.
The fundamental principle behind splitting the
transmission in this way is that one transmission can be
engaged (i.e. in gear, transmitting torque) while the
other transmission can be in the next gear (i.e. in gear
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Transmission unit 2
Multi-plate clutch K2
Engine torque
Multi-plate clutch K1
Figure 5.35 DSG basic principle
Transmission unit 1
manufacturers use twin plate dry clutches with fully
electrical actuation of the clutches and gear shifting.
This improves efficiency as there are no power
requirements if the gearbox is in a quiescent condition
(i.e. a gear is fully engaged, with the gearbox
transmitting torque).
Key Points
but not transmitting torque), in preparation for the next
gear change. This gear change can be made very
rapidly, just by switching torque transmission from one
clutch to the other. It can be implemented in a
controlled manner, such that there is minimal loss of
torque at the road wheels.
With its double-clutch design and sophisticated
electronic control, the system is as comfortable to use as
an automatic transmission. In addition, its capacity to
implement lightning quick gear shifts means no loss of
torque transmission at the wheels, so performance
driving is particularly rewarding.
One important point to note though is that, because
hydraulic power is required to operate the gearbox,
there are some small parasitic losses, which reduce the
overall efficiency slightly. Parallel shift gearboxes that
are currently under development by other
Manual gearboxes under full electronic control are
effectively fully automatic – but are more efficient
than epicyclic or CVT gearboxes
A clutch actuator is needed to control manual
boxes automatically
Most manual systems can operate in manual,
sequential mode or automatic mode
Electronic control reduces shift times considerably
5.8.1 Introduction
A torque converter (Figure 5.36) is standard in
automatic transmissions of all types and basically
replaces the clutch. It converts a high speed/low torque
input from the engine into high torque/low speed
output to drive the transmission and therefore allows
the smooth take-up of the vehicle from rest. Also,
because of the ‘slip’ effect, it infinitely multiplies the
number of gear ratios available by effectively
interpolating between each fixed gear ratio.
A torque converter is basically an opposed pumpturbine unit, enclosed in a casing partially filled with
hydraulic oil.
The pump side of a torque converter is directly
connected to the prime mover (the combustion engine)
and the rotation torque circulates the oil inside the
casing. This imparts energy into the fluid in the form of
Figure 5.36 Torque converter
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Torque converter electronic control
kinetic energy. The turbine is placed directly in the path
of the moving fluid (i.e. directly opposite the pump), so
the dynamic energy in the fluid is recovered by the
turbine and converted back into torque. This simple
system is known as a fluid flywheel. The torque
converter (Figure 5.36) has an impeller mounted
between the pump and turbine to improve efficiency.
The impeller (or stator) is mounted to the casing via a
one-way clutch which allows it to rotate in one
direction only. The stator redirects the fluid path under
certain conditions to ensure that the fluid strikes the
turbine at the correct angle and speed under all
conditions, thus greatly improving the overall torque
output of the unit.
5.8.2 Torque converter lock-up clutch
Most modern automatic transmission systems
incorporating a torque converter will use a lock-up
clutch across the converter to improve fuel economy.
The basic problem with a torque converter is that it
relies on a speed difference between the input and
output shafts to be able to operate and transfer or
multiply torque. If the speeds are the same,
hydrodynamic oil circulation will not take place and
torque will not be transmitted. Loss of speed in the
torque converter is known as slip, and causes some
power to be lost as heat.
For overall powertrain efficiency, losses must be
minimised so a lock-up clutch is fitted to the converter
between the turbine and impeller (on the casing)
(Figure 5.37). When required by the control system (for
example, when a vehicle is cruising in top gear) the
lock-up clutch can be engaged and the converter
bypassed. This clearly prevents slip and the parasitic
losses that are unavoidable in the converter itself during
normal operation. Integrating operation of this clutch is
particularly easy when the control system itself is
The clutch can be engaged or disengaged by the
transmission ECU with an electro-hydraulic control
system. This could be a simple on/off arrangement such
that the clutch is engaged only in top gear, or it could be
engaged in top and second-to-top gears. This would be
a relatively simple way of increasing the overall
transmission efficiency in cruise conditions only.
5.8.3 Optimisation of lock-up
clutch – slip control
A further improvement in efficiency can be gained by
activating the torque converter lock-up as often as
possible or practical. This would increase the efficiency
of the transmission even further but would also create a
problem. The torque converter also has a damping
function: it isolates the transmission and powertrain
components from engine induced vibrations and hence
improves the drivability and smoothness of the vehicle.
When the torque converter lock-up clutch is engaged,
this damping is lost, which could severely affect the
driving comfort at low speeds. Therefore, with a simple
lock-up clutch on/off arrangement, a compromise
between efficiency and economy has to be found.
A recent development, made possible by advances
in control electronics, is a provision for the lock-up
clutch to operate progressively. This is implemented via
PWM VBA valves to control hydraulic pressure to the
lock-up clutch, allowing progressive, controlled
input shaft
Lock-up clutch
Figure 5.37 Torque converter with lock-up clutch
Non lock-up position
(piston released)
Lock-up position
(piston engaged)
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engagement and disengagement of the clutch. With
sophisticated control algorithms and methodologies, a
clutch slip control system can be created to optimise
converter efficiency, while still providing the superior
drivability of an automatic gearbox.
A typical system uses the converter and clutch
together and monitors the power distribution via the
slip speed. This is established from the speed difference
between the engine and torque converter turbine.
Hence the power transmission distribution can be
controlled and optimised, so that the system can
provide a distribution ratio that gives instantaneously
the best balance between power loss and
noise/vibration. In particular, this feature can improve
transmission efficiency at low vehicle speeds, with a
consequent reduction in fuel consumption.
Fundamentals of Motor Vehicle Technology: Book 2
The torque converter remains an important part of
current transmission system developments and its
efficiency can be improved further with the integration
of electronic control. Using the latest computer
modelling techniques at the design stage can increase
the basic unit efficiency and specific power capacity
considerably by optimising internal fluid flows and flow
paths. Further enhancements and developments made
possible through the flexibility of electronic control are:
idle disconnection of the torque converter – an
additional clutch controlled via the transmission
ECU, mounted between the engine and torque
converter, can be designed into the pump housing
(it is similar to a lock-up clutch); this allows
Key Points
5.8.4 Torque converter developments
disconnection of the converter and gearbox
internals to reduce drag and friction losses when the
engine is idling
a reverse torque converter – modifying the basic
construction of the torque converter to allow the
stator to transmit force (rather than being mounted
on a one-way clutch fixed to the casing) means that,
with the turbine locked, the reaction force against
the stator could be used to provide a reverse
rotation, which would be useful for CVT
applications, where a planetary gear set is still
needed within the gearbox to provide this function
at significant extra cost
starter–alternator – a multi-function unit that
combines traditional engine starting and electrical
power requirements, which, with electronic control,
provides additional features, such as traffic
start/stop functions, electrical assistance during
acceleration, hybrid drive options and power
regeneration, improving the overall efficiency and
adaptability of the powertrain.
Almost all torque converters now contain a lockup clutch. Using electronics, the slip can be
controlled to improve efficiency and drivability
Under electronic control a torque converter can be
made to produce a reverse rotation and be
disconnected completely at idle
Integrating the torque converter with a
starter–alternator provides features such as traffic
stop/start and acceleration assistance
5.9.1 Introduction
Automatic transmissions have been available in vehicles
since the early days of the development and mass
production of cars. In the UK, the automatic gearbox
has always been seen as a luxury option, while, in the
US, the automatic gearbox is standard on most
passenger cars.
The basic automatic transmission (see Figure 5.38)
consists of a sophisticated hydraulic–mechanical system
incorporating a torque converter (a hydrodynamic
device) in place of the clutch. A system of planetary
(epicyclic) gears provides the various forward and
reverse ratios. The whole system is controlled via a
sophisticated mechanism of valves supplying
pressurised oil to brake bands and clutches for
engagement of the appropriate gears. Such systems
worked well and were fully adopted by motor
manufacturers before the revolution in microelectronics
which brought in sophisticated engine management
systems. The additional degrees of freedom provided by
electronic control have enabled the shift processes to be
optimised and have resulted in improvements in the
operation and efficiency of automatic gearboxes.
Where electronic control is implemented the basic
mechanical arrangement of the gearbox remains the
same as in hydraulically controlled units (Figure 5.39):
drive into the gearbox from the engine via a torque
converter, planetary gear sets to provide the fixed
ratios, with gear changes implemented via brake bands
and one-way clutches. With electronic systems, an
electronic control unit (ECU) with a central processing
unit (CPU) has a number of inputs from the vehicle,
engine and driver. The required gear is calculated and
then implemented via the electrical actuation of
hydraulic valves to provide pressurised oil to the
appropriate brake bands or clutches.
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Automatic gearbox transmission management
Figure 5.38 Exploded view of ZF 6 speed automatic gearbox fitted to some Jaguar models
Figure 5.39 Gear train layout in the Borg Warner 55 gearbox
(a previous generation)
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Fundamentals of Motor Vehicle Technology: Book 2
The main advantages of using electronic control are:
multiple shift patterns can be selected by the driver
(such as sport, and winter patterns, etc.)
gear shifting can be made smoother
systems can be easily adapted to different vehicles,
reducing integration costs for manufacturers
the hydraulic control system is simplified, with no
need for one-way clutches.
The main technical features of an electronically
controlled transmission (Figure 5.40) are as follows.
With electro-hydraulic selection and implementation
of gearshifts, shift points are determined by the
control system according to driver requirements and
vehicle conditions. Gears are changed via a number
of electro-hydraulic solenoid valves that feed
hydraulic pressure to the brake bands and clutches.
Electronic control algorithms can optimise the
engagement and disengagement of these internal
components to allow for inertia, drag, etc. This
makes for fast, smooth gear changes. Most recent
systems dispense with internal overrunning clutches,
since shifts can be performed smoothly and safely
purely with electronic control. This reduces the
weight and size of the transmission.
sophisticated torque converter lock-up strategy to
optimise transmission efficiency yet still provide the
high degree of shift quality needed.
The number of gears has increased to extend the
range of gear ratios. With electronic control, shifts
are seamless and of high quality, so the number of
ratios can be extended to five or more (mechanically
controlled transmissions have three or four gears).
The system pressure is controlled and adapted
according to the transmission status via an electrohydraulic valve. This improves shift quality and
transmission efficiency over the life of the vehicle
conditions). It ensures that wear of the friction
components (clutches and brake bands) does not
impair the performance of the transmission.
Trondo inc.
Figure 5.40 Electronically controlled automatic transmission
Key Points
In some electronic control systems, the basic
mechanical arrangement of the gearbox remains
the same as with hydraulically controlled units
Electronic control of an automatic box allows
different modes to be selected by the driver
Electronics simplify the hydraulic control systems
Converter lock-up strategy can be improved
5.9.2 Automatic transmission
Basic requirements for electronic transmission
The control system must be capable of providing the
following features:
gear selection – shifting to or selection of the
correct gear ratio for the current driving conditions,
taking into account the system information from the
sensors at all times
shift quality – adapting system pressure control
dynamically to provide seamless shifts, and
implementing torque converter lock-up for
maximum efficiency
driver input – allowing additional input from the
driver, such as kick-down or sequential shifts
fault handling – detecting system faults and errors,
ensuring all shift operations are plausible,
preventing shifts operations that could cause
dangerous driving conditions, and providing a limp
home capability
an adaptive response – the ability to recognise and
adapt to individual driver styles and driving
The basic control functions of an automatic
transmission electronic control are as follows.
Shift point control
Generally, the actual shift point is determined from a
number of shift maps stored in the ECU which can be
pre-selected by the driver with a manual switch.
Typically, these maps would allow for driving modes
such as ‘sport’, ‘economy’ or ‘winter’. The shift points are
a function of accelerator position and driving speed and
take into account boundary limitations such as engine
speed limits (maximum and minimum). They
incorporate an element of hysteresis to prevent
unnecessary shifting, which could reduce driver comfort.
The shifting operation is time-critical: the finite time
taken to release and apply the friction components is an
important factor and, in the most sophisticated
applications, is taken into account in the software
calibration. The latest generation of transmissions with
no overrun clutches require overlap control of the
hydraulic clutch operation to allow smooth transition
from one gear to the next. This is particularly
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Automatic gearbox transmission management
demanding on the control system and requires high
CPU capability and real-time performance.
A further, more recent development is adaptive shift
point and transmission control. This means that the
ECU itself must be capable of adapting the shift points
according to driver and vehicle conditions. From
evaluation of the driver inputs (kick-down, use of the
brake, accelerator and selector lever) the basic shift
points can be adapted to suit the current driver style.
The driving conditions can be established via interfaces
to other control systems (such as the ABS and engine
management ECU). The following conditions can easily
be recognised and control adapted accordingly:
gradients are detected by comparing current and
requested acceleration with engine torque; the
system adapts by moving gear shifts to give higher
engine speeds
cornering is detected through differences in wheel
speeds, so requested shifts can be delayed or
prevented to optimise vehicle stability
in winter driving in ice or snow, wheel slip is
detected by measuring wheel speeds, so lower gears
are avoided to reduce tractive force on surfaces with
low friction coefficients
for traction control the shift strategy is adapted to
provide maximum traction.
Torque converter lock-up control
As mentioned previously, the torque converter usually
has a lock-up clutch to bypass it under certain
conditions. This improves the efficiency of the
transmission but must be implemented carefully so that
shift quality and drivability are not adversely affected:
the torque converter acts as a damping element for
torsional vibrations at lower engine speeds. The
converter lock-up clutch has three states: open, closed
and controlled. These states are defined and determined
in a similar manner to the gear shift point control and
are a function of engine speed and throttle demand. An
optimised characteristic curve for the converter lock-up
process is available for each gear and is stored in the
ECU calibration. The settings take into account the need
to optimise fuel consumption and tractive force.
Engine torque control during shifting
The evolution of automatic transmission with electronic
control has resulted in certain developments such as
torque converter lock-up and an increased number of
gears. These make it possible to design a sophisticated
and efficient powertrain system. However, these
developments place additional demands on the control
system to produce an efficient and smooth shifting
process, which can only be realised via a harmonised
engine and transmission control system. The shift
process can be optimised through engine intervention
during shifting using torque control. Of course, this
requires an interface between the engine and
transmission controls and, as discussed previously,
current technology supports this easily (via CAN).
The main aims of engine intervention control are:
to improve shift smoothness and drivability
to reduce wear by shortening slip times and forces
to transmit higher power
to improve synchronisation during shifts.
Torque can be controlled in one of two ways. Most
commonly it has been controlled by retarding the
ignition angle from the set position. This can be done
easily and has a fast response but clearly can only be
applied to gasoline engine vehicles. With more recent
technology, where there is a torque based functional
structure for engine control, with a CAN interface,
torque can be controlled through a torque interface,
which would be used with a number of other vehicle
control systems, such as ABS and TCS.
Pressure control
System pressure control is an important factor in shift
comfort, second only to torque control. It is responsible
for controlling the forces in the friction elements in the
gearbox during shifts and is a key factor in maintaining
consistent performance in shift quality throughout the
life of the vehicle. As with shift point control, adaptive
algorithms can be used to allow for life cycle variations
in the transmission (the wear of friction components)
and engine (changing tolerances), as well as changes in
response caused by variations in the temperature of the
automatic transmission fluid (ATF). The system
compares actual shift times with stored reference values
and uses this as the basis for making incremental
adjustments to the system pressure up to a maximum
deviation of ≠ 10%. This limit is imposed for reasons of
operational reliability. The adjustment values are stored
in the ECU memory so that they can be reinstated each
time the system powers on when the vehicle starts up.
Safety functions
Several safety related features must be included in the
transmission control system; these generally prevent
critical driving conditions caused by driver error or
failed components. Uncontrolled shifting is particularly
undesirable, especially downshifting, which could cause
serious problems for the driver, or destruction of the
Monitoring of the electronics system and
components themselves is particularly important. The
CPU in the ECU is monitored via internal and external
circuits; the software code execution is monitored for
plausibility during run time; and sensors and actuators
are continuously checked for correct and plausible
If a sensor or actuator fails, in most cases substitute
values can be used and the system switched to ‘limp
home’ mode. For example, the transmission output
speed signal can be substituted by a wheel speed signal,
temperature sensor values can be replaced with fixed
values. This would be sufficient to allow the vehicle to
be driven home or for repair. These safety
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enhancements and error handling methods give an
increased degree of confidence and acceptance in the
marketplace for full electronic control systems.
Diagnostic functions
The diagnostic capability and functionality of current
systems now forms approximately 30% of the system
software resources. The main reason for this is the
requirement for vehicles to comply with on board
diagnostics (OBD) in order to meet current and future
legislation. The diagnostic function handles the storage
of fault information and the communication of this data
to the service tester. The fault memory inside the ECU is
sub-divided into:
primary fault memory – non-volatile memory that
contains fault code and type, plus a warm-up
counter and system flags (an OBD requirement)
secondary fault memory – one memory slot per fault
code, containing filters, time stamps and flags
back-up memory – optional memory with deleted
fault codes from the primary fault memory
snapshot memory – for use with diagnostic
The most important monitoring functions for the
transmission are:
solenoid valve monitoring
pressure regulator monitoring
run-time monitoring of program code.
Key Points
In the past, access to fault information has been
manufacturer specific but, with the introduction of
OBD, a standardised protocol for communication to a
tester, with standard fault codes, is now in general use.
Transmission management is the equivalent of
engine management – but for the transmission
Transmission and engine ECUs are linked to allow
control of engine torque during shifting
ECUs communicate using the CAN protocol
Modern systems can recognise and adapt to
individual driver styles
Figure 5.41 Tiptronic gearbox
Fundamentals of Motor Vehicle Technology: Book 2
5.9.3 Case study: Tiptronic gearbox
Tiptronic is the name used by Porsche to describe their
sequential shift automatic gearbox with intelligent shift
strategy adaptation. This technology was a joint
development between Porsche, ZF and Bosch. The name
‘Tiptronic’ is also used by other manufacturers to
describe this technology. The system consists of a
standard automatic transmission (i.e. torque converter,
epicyclic gear sets) with full electronic control. The
driver has the choice of driving in automatic mode, or of
using the manual lever (Figure 5.41) for sequential
shifting. Later versions (Tiptronic-s) also incorporate
steering wheel buttons for up and down shifts.
When the system is driven in automatic mode an
intelligent driving program runs. This takes into
account basic information about vehicle and engine
speed, vehicle acceleration and throttle position, and
adapts this according to the dynamics of the vehicle
(road resistance) and the ambient conditions (altitude).
This information is made available to the Tiptronic
control unit, which then decides which shift map to use
and what adjustments are necessary to it. This process
is carried out continuously and ‘steplessly’ and is
invisible to the driver.
Additional functions are:
a warm-up map – an optimised transmission
operation for fast warm up of the engine and
catalyst; upshift points are delayed, and the
converter clutch remains open
an active shift to sports map – rapid movement of
the accelerator initiates the sports map; the system
shifts back to the economy map automatically
kick-down – this does not initiate a change of shift
downshift during braking – this allows engine
braking assistance
overheat protection – this is automatically initiated
by restricting engine torque via the engine ECU
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Automatic gearbox transmission management
Throttle valve
Engine speed
Vehicle speed
Shift-curve adaption (transmission and converter lockup clutch)
• Data summing
• Filtering
• Averaging
• Weighting
Shift curves
CLC curves
Short-term adjustment
Special function
Inhibition of
trailing-throttle upshifts
ahead of curves
Manual tip (nudge) shifting
Active shifting
jump to shift
curve SK5
Gear retention
in curves
Upshift during
braking on low μ
+ Upshifting
– Downshifting
Computer support to
prevent excess rpm
Figure 5.42 Tiptronic shift strategy
gear hold in manoeuvres – lateral acceleration is
detected and the current gear is maintained for
maximum stability
torque reduction during shift – for smooth gearshifts
torque converter lock-up – from second gear up,
depending on engine load and shift characteristic
Figure 5.42 illustrates the shift curve adaptation process
for the Tiptronic system.
5.9.4 Case study: Honda’s
four-speed all clutch-to-clutch
The Honda clutch-to-clutch automatic transmission
system (Figure 5.43) is of particular interest as it is a
departure from the usual design of an automatic
gearbox. A traditional automatic gearbox has epicyclic
gear sets, with brake bands and clutches to provide the
appropriate forward and reverse gears. These are all
activated by electro-hydraulics and controlled by an
ECU. For a traditional front-engine–rear-drive vehicle
this can be easily accommodated in the powertrain
layout. For transverse front-engine–front-drive vehicles,
this arrangement becomes difficult to fit within the
engine compartment. Honda’s solution is closer to a
manual gearbox in form and is specifically designed for
use in front-wheel drive Honda cars.
Of particular interest is the gear shifting and
selection mechanism. The system does not use epicyclic
planetary gears, but is closer in design to a standard
constant mesh gearbox. The system provides four
forward gears plus reverse. The schematic is shown in
Figure 5.44.
The shaft layout consists of an input or mainshaft,
which drives a secondary shaft via an intermediate gear.
Between these two shafts is the countershaft. The
output from this shaft drives the road wheels via the
differential. The countershaft has all the fixed, forward
gears mounted on it, as well as a servo-operated dog
clutch selector for selecting forward or reverse.
The fixed gears on the countershaft mate with
corresponding freewheeling gears (of different ratios)
mounted on the main and secondary shafts. The
freewheeling gearwheels are engaged or disengaged
with the shaft via hydraulic, multi-plate clutches, one
for each gear. When a gear is required, the appropriate
clutch is engaged and torque is transmitted via that
gear. Changing gear is simply a matter of disengaging
one clutch and engaging another.
A simple analogy is to compare this unit with a
normal manual gearbox. The countershaft of this
gearbox can be compared to the layshaft of a simple inline manual transmission and the main/secondary shaft
can be compared to the primary/mainshaft. Instead of
the required gear being selected and engaged manually
via a synchromesh dog clutch from the gear lever, the
gears are changed by using a small clutch inside the
gearbox for each gear, activated hydraulically and
controlled electronically with an ECU.
Torque input to the gearbox from the engine is
through a traditional torque converter with a lock-up
clutch for maximum efficiency. The gear selector
clutches are engaged via hydraulic oil pressure and
controlled with solenoid operated shift valves. These
shift valves in turn are activated by the automatic
transmission ECU, which changes to the appropriate
gear for driving conditions. It is also possible for the
driver to select the required gear manually (a semi-
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Fundamentals of Motor Vehicle Technology: Book 2
automatic). Table 5.3 shows the operating conditions
of the clutches for each of the available gears.
The latest development of this gearbox has reduced
its overall length by 22 mm. This does not sound a lot,
but in a modern engine bay, where space is already at a
premium, this could be very advantageous. In addition,
the internal component count is reduced by dispensing
with the first gear one-way clutch. This means that the
gearbox needs only four clutches and a
servomechanism to provide all the required gears. In
the development of this latest version specific problems
had to be overcome.
In previous versions of the gearbox a one-way
clutch was fitted to the first gear clutch to improve shift
quality by transferring some torque during upshift.
Without this element a drop in torque during the shift
could be perceived by the driver, which adversely
affects drivability. This happens because hydraulic
clutches have a small, finite, delayed response caused
by the necessary refilling of the piston cavity with
hydraulic oil before clutch pressure is generated. The
piston cavity is emptied of oil through a check valve
after each operation to prevent displacement of the
piston due to centrifugal effects. To overcome this, a
centrifugal cancellation mechanism is fitted to the first
and second gear clutch hydraulics (Figure 5.45), which
allows precise operation and timing of the first and
second gear clutch operation for smooth upshifting.
This precision timing is achieved with highperformance linear solenoids, which can give the
required degree of control. In addition, the mainshaft
speed and acceleration are monitored by the automatic
transmission ECU, so that shifting can be monitored
and optimised in real time operation by the ECU.
Main shaft
Counter shaft
Secondary shaft
Table 5.3 Operating conditions
Elements Engaged
Actual car
1st 2nd 3rd 4th
Idle gear
Torque output to tyre
Figure 5.44 Gear train schematic
Figure 5.43 Four-speed all clutch-to-clutch system
3rd and 4th gear
with clutch packs
Main shaft
Final drive
1st and 2nd gear
with clutch packs
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Catastrophic failure of the gearbox at speed and a
consequential downshift to first gear due to control
system failure would cause over-revving of the engine
and possible road-wheel lock. This was previously
prevented by using a one-way clutch to stop drive
through the first gear being engaged under these
conditions. Since the current design dispenses with the
one-way clutch, a safety mechanism must be used to
prevent engagement of first gear. This is done with
direct hydraulic control using linear solenoids and new
shift logic with three solenoids, plus a fail-safe valve.
Thus gears are shifted through a combination of eight
solenoid signals (Figure 5.46) and this provides the
fail-safe mechanism.
The overall dimensions and form of this gearbox
are ideal for front-wheel drive vehicles, providing
safety and improved drivability for the user.
Clutch piston
Key Points
Tiptronic is the name used by Porsche (and
others now) to describe a sequential shift
automatic gearbox with intelligent shift strategy
Features such as ‘gear hold in manoeuvres’ can be
provided. With this feature, lateral acceleration is
detected and the current gear is maintained for
maximum stability
Piston cavity
For transverse front-engine–front-wheel drive, a
solution in use by Honda is closer to a manual
gearbox in form
Compensation cavity
Figure 5.45 Clutch piston mechanism
Pressure 3rd
switch clutch clutch
accumulator accumulator
2nd Pressure
valve E
CPC valve B
Shift solenoid B
Shift valve B
Shift solenoid C
LG solenoid
Shift valve C
Shift solenoid A
Shift valve A
Shift valve D
CPC valve A
Linear solenoid B
Linear solenoid A
Modulator valve
Regulator valve
Figure 5.46 Hydraulic control system
ATF pump
Servo valve
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Fundamentals of Motor Vehicle Technology: Book 2
5.10.1 Introduction
A problem with any combustion engine is that
maximum power, torque and fuel economy all occur at
different engine speeds within the range of normal
operation. In a conventional, selectable, fixed ratio
gearbox, in any particular gear, the engine speed and
vehicle speed have a fixed relationship, so maximum
torque, power or economy can be achieved only at one
road speed per target in each gear. Driving a car along a
road is transient in nature, so achievement of any of
these targets for any reasonable period of time, will be
nearly impossible.
Figure 5.47 shows that only a limited fit to the ideal
curve can be achieved with a standard transmission. A
continuously variable transmission (or CVT), as its
name implies, can provide an infinite number of ratios
between some absolute limits. Thus this transmission
system can give a powertrain performance curve that is
capable of matching the ideal tractive effort curve for a
particular vehicle.
CVT transmissions can be operated mechanically,
hydraulically or electrically and various designs have
been developed, proposed and utilised, although they
have never made the market breakthrough that was
anticipated. One popular design, adopted by DAF and
Volvo for small cars, was the Variomatic system shown
simplified in Figure 5.48.
The Volvo system satisfied the basic requirements
for a CVT for light vehicle applications. However, the
technology was considered ‘quirky’ within the market
place and the system layout was not easily adaptable
for general application in the front-engine–rear-drive
cars that were popular at the time that the system was
Figure 5.47 Tractive effort curves
a Ideal tractive effort curve
b Curve for conventional stepped transmission
developed. The system needed a specific powertrain
layout (a front engine, with rear drive via a transaxle
(Figure 5.49)). Composite rubber drive belts were used,
which needed adjustment and replacement. These belts
limited the maximum torque transfer possible, so the
system was suitable only for small cars.
The next generation of CVT developed for compact
cars used a steel belt instead of a rubber one. The
packaging is clearly different as the application is for
front-wheel drive vehicles (Figure 5.50) but the
principle of operation is exactly the same as that of the
Variomatic system. One important difference is that this
steel belt is actually used in compression to push, rather
than pull, the drive force. A major advantage of this
system is that parasitic losses inside the gearbox are
significantly reduced compared with the losses in a
traditional automatic gearbox.
The system incorporates the differential assembly
and forms a compact single unit. The steel belt limits
the torque that can be transferred, so the system can be
used only with smaller engines of up to 1.6 litres
Figure 5.48 CVT system
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Continuously variable transmission (CVT)
Figure 5.49 Variomatic transmission
Figure 5.50 Ford CTX transmission
Basic outline of CVT transmission operation
The CVT systems described above both work on the
same basic principle. They have similarly constructed
driving and driven members that consist of opposed
cones both mounted on the same shaft. These
effectively form pulleys with v-shaped grooves, with a
belt running between them to transmit torque. The
cones that form the pulley on the primary (driving) side
can be moved closer together or further apart: it is
through this mechanism that the overall gear ratio can
be varied infinitely. In operation, with a fixed width
belt, if the pulley sides (cones) are moved apart, the
effective working diameter of the belt is reduced. If they
are moved together, the diameter is increased (Figure
Gear ratios are shifted via an actuator: by moving
the relative positions of the cones on the primary pulley,
the actuator changes the effective working diameter of
this pulley and consequently the gear ratio. The
secondary (driven) pulley has the cones spring-loaded
against each other, which maintains the correct belt
tension as the ratio varies with the fixed length belt.
The CVT has great potential to reduce fuel
consumption and emissions because the engine can be
operated continuously at its optimum operating point.
The greatest problem is the power loss caused by the
internal energy requirements of the transmission. With
electronic systems, oil flow and pressure can be
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a ‘Low’ ratio
b ‘Overdrive’ ratio
1 Input (primary) pulley
2 Push-belt or chain
3 Output (secondary) pulley
a1, b1 ‘Low’ ratio
a2, b2 ‘Overdrive’ ratio
Figure 5.51 Simple CVT
Basic functions of the transmission
As well as the variable-ratio pulley assembly, additional
components are needed in the transmission system for
it to be suitable for road vehicles. These are:
mechanisms to provide a neutral gear
(disengagement of the drive to the road wheels) and
a reverse gear
an arrangement to allow the progressive take-up of
drive from standstill, such as an electric or multiplate clutch or torque converter, which should also
allow the vehicle to ‘creep’ at low speeds for
an appropriate fixed gear assembly to drive the final
drive/differential at the appropriate speed and
divide torque equally between the driving wheels
a suitable control system to select the correct ratio
according to driver’s requirements and driving
conditions. This could be a hydraulic system, but the
preferred solution for a modern vehicle would be
electronic or electro-hydraulic.
Electronic control functions
The control system for a CVT could also incorporate the
following specific requirements.
Contact pressure control – this provides adjustment
of the belt clamping force in relation to load forces.
Key Points
controlled accurately to suit the working conditions of
the gearbox, so overall efficiency can be significantly
improved. A most important factor is the availability of
reliable sensors and actuators to support reliable
implementation of the control strategy.
It prevents excessive forces being used, which waste
power and reduce efficiency, but maintains
sufficient pressure to prevent belt slip.
A driving program – this enables preselected driving
modes to be implemented as well as adaptive
functions, and the selection of fully automatic or
semi-automatic (sequential) modes.
Torque converter lock-up – this improves
transmission efficiency by bypassing torque
converter slip.
Pump control – control of the pump flow rate
improves transmission efficiency and prevents
excessive flow at high speed.
Limp home mode – in the event of failure, limp
home and fail-safe features must be built into the
control system structure, in addition to diagnostic
monitoring capability.
A CVT system has only two gears – forward and
CVT transmission can be operated mechanically,
hydraulically or electrically
Gear ratio shifting is implemented through an
actuator which alters the relative positions of the
In most CVT boxes the drive belt is pushed – not
CTX (the Ford transmission) stands for constantly
variable transaxle
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Continuously variable transmission (CVT)
5.10.2 Case study: Audi Multitronic
Audi has taken the principle of the basic CVT system
and extended its capability to make it suitable for
applications in larger vehicles where the torque transfer
requirements are much higher. By adapting and
improving some of the basic mechanical parts,
particularly those that limit the capabilities of the
traditional CVT, and with the addition of sophisticated
electronic control, Multitronic transmission (Figure
5.52) gives improved fuel consumption compared with
a traditional automatic transmission. The acceleration
performance of the vehicle is also marginally improved
when compared with a normal five speed manual
Overall the system provides very wide ranging gear
ratios of up to 6.05 to 1, both higher and lower ratios
than any other automatic transmission. With this
system Audi has produced a highly efficient CVT with
broader application possibilities and some important
technical developments and improvements.
System highlights
Link plate chain drive – The component that limits
torque transfer in a CVT is the connecting element
between the driving and driven parts of the variator.
Early designs used rubber belts (DAF 66, Volvo
340). Later designs, as mentioned above, use a steel
thrust belt designed by Van Doorne. Both are limited
in terms of the maximum torque that they can
transfer. For the use in the Audi application the belt
must transfer nearly 300 Nm of torque! The solution
was the development of a link plate chain drive,
designed jointly with LuK (Figure 5.53). This chain
is constructed of 1025 links with 75 pins, all made
Figure 5.53 Link plate chain
Starting clutch
Link-plate chain
Oil pump
Hydraulic control
Electronic control
Figure 5.52 Multitronic system overview
of high-strength steel. Torque is transferred through
the contact between the pulley flanks and the ends
of the pins.
Multi-plate clutch – A wide range of gear ratios is
available, so the torque converter is replaced by a
much more efficient, hydraulically operated, wet
multiplate clutch. This avoids the traditional losses
associated with a torque converter but also, with
electronic control, allows implementation of a
number of starting strategies according to driver
preference. These strategies are established via
monitoring of throttle demand and rate of change.
An additional feature is a ‘creep’ function that is
automatically initiated by the electronic control
system for low speed manoeuvres.
Dual piston variator with torque sensor – This
technology ensures that the variator grips the chain
with sufficient pressure, depending on torque
transmitted, to prevent slip but with no more
pressure than is necessary. This is measured by the
transmission ECU monitoring the clamping forces
(via the torque sensor) and variator speeds (via
speed sensors); the hydraulic pressures are then
adjusted accordingly. The advantage is that this
keeps the internal gearbox power requirements to
the absolute minimum, thus increasing efficiency, as
well as reducing heat build-up. The dual piston
arrangement for the variator (one piston for
clamping the chain and a smaller piston for ratio
change) ensures that the required dynamic response
can be obtained with a small, more efficient
hydraulic pump (Figure 5.54).
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selected by the driver manually with the shift lever
or steering wheel mounted buttons. This gives the
driver more control over shifting and a more sporty
driving experience (Figure 5.55).
5.10.3 Case study: Nissan electronic
Figure 5.54 Variator with torque sensor
DRP (dynamisches regelprogramm – dynamic
control program) – The transmission control
module (mounted on the gearbox itself) has a
highly dynamic program for calculating the target
transmission input speed. Based on driver input and
operating conditions, the system can determine the
appropriate driving style and shift pattern selection,
for example performance or economy. The driving
feel is designed to be similar to the feel of a manual
shift mode. The system also eliminates the so-called
‘rubber band’ effect typical of traditional CVTs: it
ensures that the engine speed increases
proportionally with road speed in a similar way to a
manual transmission when accelerating. Probably
the most intriguing feature is the manual, sequential
shift mode. This allows six predefined ratios (even
though there are an infinite number available) to be
Driver input
Evaluation of signal from the
accelerator pedal module.
Acceleration rate and position
of accelerator pedal
Nissan has recently been at the forefront of the
development, improvement and integration of CVT
technology in its vehicle range. As noted earlier, this
technology is only suitable for smaller, lower-power
vehicles. Nissan has added significant technical features
to the basic belt drive CVT to extend its application to
more powerful vehicles up to the 2.0 litre engine class.
In addition, the inclusion of an electronic control system
has increased the efficiency, improved performance and
extended the driving appeal of a CVT equipped vehicle.
Technical highlights
The main focus of the development was to:
improve power and economy
improve acceleration in terms of performance and
provide a manual, sequential shift mode.
To achieve these targets the technical developments
were as follows.
1 Torque converter with lock-up clutch – This
component was added to the transmission system to
provide improved acceleration from rest; an
additional benefit was smoother transmission
through the damping properties of the torque
Vehicle operating state
Constant speed
Vertical section of route
Evaluation of road speed and
road speed changes
Evaluation of road speed and
road speed changes
Calculation of target transmission input speed
Influencing factors
(e.g. engine warmup)
Transmission control
Actual transmission input speed (and hence engine speed)
Figure 5.55 Dynamic control program
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Continuously variable transmission (CVT)
The system shows measurable benefits in terms of
economy and performance. The fuel economy is
improved by 20% compared with the standard four
speed automatic. This is possible because of the
extended range through which the lock-up clutch can
operate (reducing slip losses) and because the system
hydraulic pressure adapts to engine load (via the
electronic control system) to reduce internal energy
The torque multiplication capability of the torque
converter improves acceleration performance by 30%,
and drivability is increased through the smooth, stepless
torque delivery of the CVT system. The manual shift
mode is an enhancement which will be appreciated by
drivers who want a greater level of interaction for a
more rewarding driving experience.
Key Points
converter. An electronically controlled lock-up
clutch ensures that the efficiency of the transmission
remains high. This clutch also features a highly
durable facing material that allows lock-up at lower
speeds than is normal (down to 20 km/h), which
gives a further improvement in economy.
2 Expanded belt width – The belt width is increased
by 25%, and, by optimisation of the pulley ranges,
the wider belt can easily accommodate the torque
generated by a 2.0 litre engine.
3 Electronic control system – The system consists of
sophisticated electro-hydraulic elements with a fully
electronic control system (ECU). This system
monitors and controls all aspects of transmission
operation to optimise performance and economy.
The system overview is shown in Figure 5.56.
4 Manual mode – To enable greater driver
interaction, a multi-speed manual mode is included.
This has sequential shifting to allow the driver to
select and hold gears during sports driving.
The Audi Multitronic system provides a very wide
range of gear ratios (from 6.05:1 to 1:1). The
system does not use a torque converter
The Nissan system monitors and controls all
aspects of transmission operation to improve
performance and economy
Using a wider belt can increase the torque capacity
and allow CVT systems to be used with larger
oil pump
Input signal system
pulley speed
Shift control
pulley speed
control unit
Line pressure
regulator valve
regulator valve
Torque converter
regulator valve
control valve
Control valve assembly
Lubrication and
cooling systems
Figure 5.56 System overview
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5.11.1 Introduction
A hybrid powertrain is one with more than a single
prime mover and can incorporate a number of
technologies for power conversion and energy
accumulation. The prime objective of a hybrid drive is
to harness the advantages of each particular drive
technology under its optimum operating conditions.
The consequential increase in efficiency of the
powertrain as a whole offsets the increased initial cost
and reduces the harmful exhaust emissions from the
The internal combustion engine and electric motor
can both be considered as torque sources or prime
movers; their respective attributes are show in Table
Table 5.4 Internal combustion engine and electric motor
Internal combustion
Electric motor
Slow: > 300 ms
at low speed
Maximum torque
not available
at lower speeds
Fast: < 5 ms
Maximum torque
available at lower
speeds and from zero
The hybrid drive solution offers great potential for
improving fuel consumption and reducing emissions
during low- to medium-speed operation of the vehicle.
This is because the internal combustion engine has
greatly reduced efficiency under part-load conditions. It
is likely that hybrid powertrain designs will become
commonplace in future as the technology develops and
improves. Figure 5.57 shows the classification of hybrid
A full hybrid drive powertrain with integrated
control needs to be a fundamental part of a vehicle’s
initial design. It is is impossible to introduce a hybrid
drive retrospectively into an existing vehicle without
considerable reworking of the powertrain (to
accommodate both the internal combustion engine and
the electric drive) as well as the vehicle chassis (to
accommodate the energy storage medium – the
5.11.2 Light hybrids
A compromise can be found in the form of the
integrated starter–generator (ISG), which is a natural
progression in the development of automotive electrical
systems because of the continuously growing demand
for electrical power in the modern vehicle. It is expected
that electrical power of up to 10 kW could be needed in
future and the standard 14 V electrical system will need
Hybrid electric vehicle system functionality classification
Full hybrid
Pure electrical driving
Mild hybrid
Micro hybrid
Cold start
Direct start
Coasting +
belt tensioner
Battery and
Electric boost
and launch
Figure 5.57 Hybrid electric vehicle classification
• Combustion engine
• E-machine(s)
• Combination
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Light hybrid powertrain technology (starter–generator)
to be supplemented or replaced by a 42 V system
(Figure 5.58). The separate starter and generator units
used in modern vehicles can be easily integrated into a
single unit to start the IC engine, provide on-board
electrical power and recharge the battery. This unit,
depending on the installation, could also provide a
number of additional features such as:
a short-term power boost function to supplement
the internal combustion engine during acceleration,
particularly in the lower speed ranges
a retarder to supplement the brakes, which could
also regenerate energy during deceleration: the
electric motor/generator is highly efficient at doing
this and can achieve efficiencies of up to 80% under
these conditions
an engine stop/start function for use in stationary
traffic to reduce harmful emissions; engine starting
times of less than 0.5 seconds can be achieved.
A considerable advantage of this technology is that it
can be integrated into an existing design with minimal
effort. Basically the system consists of:
a three-phase AC motor integrated with the internal
combustion engine design
an AC/DC converter which rectifies the AC
electricity generated by the three-phase motor
a DC/DC converter that provides the required
voltage levels
a control electronics system for the ISG powertrain
an energy management system controlling the ISG
and the vehicle power requirements.
There are two principal designs for an ISG currently
proposed or under development by manufacturers (see
Figure 5.59), which are discussed in more detail below.
Micro hybrid
Belt driven SG
Combustion engine
Electric machine (SSG)
Air cooled ECU
Smart switching unit
14 V battery
DLC (supercapacitors)
Combustion engine
Water cooled ECU
DLC (supercapacitors)
Electric machine
Figure 5.59 Micro and mild hybrid systems
Hybrid drive
42 V
Low level
14 V
DC link capacitor
CAN bus
Control ECU
Figure 5.58 Integrated starter–generator control system
Mild hybrid
Intergrated SG
Micro hybrid functionalities
+ Boost
+ Regeneration
High level
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5.11.3 Belt driven starter–generator
The belt driven starter–generator is also known as a
micro-hybrid system (Figure 5.60). It consists of a belt
driven electric machine mounted at the front of the
engine, with a control unit and belt tensioner. It provides
start/stop functionality as well as starting and charging
functions. The system works at 14 V and as such does
not require major modifications to the vehicle electrical
system: the standard starter battery technology is used.
This means that the system can be retrofitted to existing
engine designs, since it can be fitted in place of a
conventional alternator using the same fixings, freeing
up the space where the starter would normally appear
on the engine, which also gives a small weight saving.
In starter mode, the machine can start the engine
silently (with its belt drive) and up to three times
quicker than a traditional starter with a ring gear. In
generator mode, the machine is over 80% efficient, up
to 15% more so than a standard alternator because of
the advanced power management system.
In operation, when the vehicle is at a standstill in
traffic, the engine cuts out, so all sources of noise and
pollution are eliminated and fuel consumption is zero.
When required, the engine restarts automatically and
immediately in less than half a second; this is possible
because of the high torque/inertia ratio of the electric
machine. In modern traffic conditions, in suburban
areas, vehicles are standing for up to 35% of journey
time: overall fuel consumption in these conditions can
be improved by up to 10%.
Figure 5.61 Flywheel starter–generator
that, with these constraints, this technology must be
incorporated into the powertrain design. It would be
more difficult to incorporate this system than some
others and retrofit would not be feasible.
The system has a high performance, providing up to
200 Nm of torque for short durations. The unit is
capable of generating up to 8 kW of electrical power
continuously for the electrical system. A flywheel
starter–generator is usually fully integrated with a
vehicle’s power management system. Managed by the
powertrain control system as an available torque
source, the system is integrated via a standard interface,
such as a controller area network (CAN). A torque
management system will then distribute the torque
from the starter–generator and the internal combustion
engine according to driver requirements and driving
In addition to providing engine starting and
stop/start functions (as does the belt drive
starter–generator), the flywheel starter–generator can
be configured to improve efficiency, because it is far
more integrated with the powertrain control system.
Additional features that can be added to do this include
the following.
Figure 5.60 Belt driven starter–generator
5.11.4 Flywheel starter–generator
Also known as a mild hybrid system, the flywheel
starter–generator is mounted in place of the vehicle
flywheel and acts directly at the crankshaft between the
engine and transmission (Figure 5.61). Typically this
unit is a highly efficient synchronous or asynchronous
AC machine running at up to 60 V. Normally a system of
this type is integrated within a vehicle electrical power
system incorporating a higher voltage line, at 42 V, for
more efficient transmission and conversion. It is clear
A torque booster can provide additional torque to
complement an internal combustion engine under
certain conditions. The torque booster also gives the
vehicle designer the option of using a smaller
internal combustion engine. An important factor
here though is the battery state: the charge
condition of the battery must be monitored carefully
to avoid any significant deterioration in vehicle
Regenerative retarder – The ISG can be used to
recover energy during braking, and can be
incorporated into an active brake management
system for maximum benefit. Again though, the
battery’s state of charge is a critical factor: there
must be enough capacity to store all the energy
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The ISG system can be part of an intelligent powertrain
management system, controlling and managing all
available torque sources, and deploying the most
efficient one depending on operating conditions. As
well as managing traction demands, the system also
manages the vehicle’s power usage. It controls the
entire drive train by enabling communication between
all electronic units in the vehicle. It constantly monitors
and exchanges data between the internal combustion
engine, the powertrain, and the electrical control
systems, such that the auxiliary drive system always has
sufficient electrical power for the engine to be able to
self-start when required.
Key Points
Electronic differential and four-wheel drive control
A hybrid powertrain is one with more than one
prime mover (an engine and a motor)
ISG stands for integrated starter–generator
A mild hybrid can provide a short-term power
boost, a retarder function to supplement brakes
and an engine stop/start function to reduce
harmful emissions in static traffic
A micro-hybrid system has a belt driven electric
machine mounted at the front of the engine (a
combined starter and alternator)
5.12.1 Introduction
The rapid development of electronic systems controlling
the modern motor vehicle powertrain has progressively
extended from the engine, to the transmission and more
recently to the final drive system. Electronic control is
integrated into modern vehicles to ensure that torque
delivered by the powerpack (engine and transmission)
is efficiently and effectively transferred as tractive force.
Vehicle electronic control systems interact to maximise
the potential of the vehicle propulsion system to
improve safety and performance.
Vehicle wheels are often in contact with surfaces
where the adhesion varies significantly, so optimisation
of the tractive force is essential. Nearly all vehicles have
a standard mechanical differential, which, under
normal driving conditions, provides uniform torque
distribution between the driving wheels, giving
predictable vehicle behaviour. However, the force is
always distributed evenly, irrespective of speed; so, if
one wheel lacks grip and slips, no force is transmitted,
so the other wheel also transmits no force and the
vehicle is immobilised. Stability is another important
issue with modern vehicles. Powerful engines can
produce more force to accelerate the vehicle than the
system can transmit. Depending on the road surface
and driver behaviour, the wheels can spin, leading to
loss of traction and erratic performance.
Several electronic control solutions are available to
deal with these problems, and to enhance vehicle
performance as a whole.
Automatic brake differential (ABD)/traction
control (ASR)
An automatic brake differential is not a differential
lock in the traditional sense: it is not like a mechanical
lock, which positively locks and prevents differential
action completely. ABD uses the anti-lock brake system
infrastructure (wheel speed sensors, hydraulics, etc.)
and detects any difference between the rotational
speeds of the driving wheels. When it detects a speed
difference, the system can activate the brake on the
offending wheel to counteract the slip. This is done in
a progressive manner, allowing controlled transfer of
torque via the open differential to the driving wheel
with the greatest adhesion. There are some limitations
with this system: it cannot operate effectively at zero
speed and therefore is of no use for true off-road
applications; and because of the temperature limits of
braking systems, it is not available at higher vehicle
speeds (greater than 25 mile/h).
The system normally operates in conjunction with
the engine control electronics to provide active traction
control: as well as decelerating the slipping wheel with
the brake, the ECU can actively reduce engine power to
further control the torque distribution. The engine
torque is usually controlled via the electronic throttle
(e-gas) actuator and the ignition timing (spark retard).
The control units communicate using the high speed
CAN bus line.
Electronic control of differential
The above system is not specifically suitable for off-road
vehicles or sports utility vehicles (SUVs). So, for such
use, it needs to be supplemented by another system,
capable of sustained prevention of the differential
action on the driving axles for maximum traction at low
speed. Mechanical solutions here include differential
locks and limited slip differentials (LSDs). However,
these solutions require manual intervention by the
driver (the former) or can only respond in a simplistic
way (the latter). An electronic control system provides
the following benefits:
a smart system that can respond proactively, not
only to speed differences, but also to other
parameters monitored by the vehicle electronic
control systems (wheel speeds, throttle demands,
engine load, etc.)
a differential action that can easily be integrated
with other vehicle systems, such as ABS, traction
control and stability control.
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Fundamentals of Motor Vehicle Technology: Book 2
The system can be configured in a number of ways but
Figure 5.62 shows a solution for a live rear axle. It
consists of a standard differential, but with the
addition of multi-plate clutches between the planet
wheels and differential cage. These clutches are
engaged electromagnetically by a current in the coil.
This current determines and is proportional to the
torque transfer. When activated, the current coil
creates a magnetic field which pulls the cone into
frictional engagement with the differential cage. The
frictional torque created by the cone causes the balls
to ride up a ramp machined into the side gear. The
lateral movement of the side gear applies a force onto
the centre block and that load is transmitted from the
centre block to the opposite side gear, compressing the
clutch pack and locking the differential.
These systems could either be directly integrated in a
powertrain control unit (PCU), or they could
communicate with each other on a high-speed bus
system (e.g. CAN).
Key Points
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Differential control systems can operate in
conjunction with the engine control electronics to
provide active traction control
The differential action can be integrated with ABS,
traction control and stability control
Multi-plate clutches between the planet wheels
and differential cage are usually controlled
electromagnetically via current in a coil
5.12.2 Case study: Porsche traction
management (PTM)
Multiplate clutch
The Porsche Cayenne is equipped with one of the most
sophisticated four-wheel drive systems. The system
mechanics are similar to any off-road or SUV with fulltime 4WD, and the system intelligence lies in the
electronic controls. The fundamental principle is
extremely simple: to actively distribute torque to those
wheels that can utilise it most efficiently.
Current coil
Locking balls
Dynamometer test showing the relationship between bias
torque and coil current. The tight wheel was held to zero
RPM, the differential was driven at 50 RPM, and the loose
wheel was allowed to freely rotate at 100 RPM. The coil
current was slowly increased until the tight wheel torque
reached 18,000lb in.
This design is particularly intelligent as it does not
require any additional hydraulic or pneumatic power
and hence is highly efficient. It can easily be
integrated into an existing installation simply by
adding the hardware and driver stage to an existing
powertrain ECU. It does not need to have its own
specific control unit.
Figure 5.63 shows excitation current plotted against
locking torque.
Integration of safety systems
A clear benefit of these solutions is that it is possible to
connect the differential control system to other vehicle
dynamics controllers to provide a harmonised efficient
system with maximum tractive force control, providing
greater safety. The systems that could be linked include:
anti-lock braking (ABS)
traction control (ASR)
brake force distribution (EBD)
cornering stability control (CSC)
dynamic drift control (DDC).
Bias torque, lb in
Figure 5.62 Electronically controlled differential
Current, amps
Figure 5.63 Control current: excitation current v locking torque
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Electronic differential and four-wheel drive control
The system consists of:
a transfer case incorporating epicyclic differential
and range selection, with an electrically operated
progressive differential lock via a multiplate clutch
(Figure 5.64)
a front axle with electrical (ABD) lock capability
a rear axle with electrical, progressive differential
lock via a clutch
harmonised electronic control and monitoring of
brakes, traction and stability.
Drive to
rear axle
Drive to
front axle
Figure 5.64 Four-wheel drive traction management unit
The system provides permanent torque distribution
between the front and rear axles which, under normal
conditions, is a 38/62% front/rear split. This rear wheel
bias provides stable grip on almost any surface and
gives the driver the level of road feel and chassis
feedback that would be expected from a highperformance vehicle.
The transfer boxes provide high- and low-gear ratios
for on/off road use. The set of ratios is selected with a
toggle switch near the gear lever. The functions are
progressive and sequentially selected and shift between,
for example, on- and off-road conditions.
The first actuation of the switch selects ‘low’ range
through the transfer box. This is engaged via a
selector fork and electric motor actuator. In
addition, with the vehicle sensors, the active
electronic system continuously measures traction at
the wheels, as well as vehicle speed, lateral
acceleration, steering angle and operation of the
accelerator pedal. From this, the system
automatically calculates the optimal degree of
locking for the differentials at the drive axles. In this
way, more power is applied at the front or rear
wheels, depending on the driving situation.
The second actuation of the switch fully locks the
centre differential using a clutch and an electric
motorised actuator.
The third actuation of the switch fully locks the rear
differential, again using a clutch and actuator for
maximum off-road traction.
The front final drive does not have a mechanical
differential lock. Potential slip of one front wheel is
prevented by the automatic brake differential (ABD, see
above). The negative effects of a mechanical differential
lock, such as increased weight and limitations on
steering and handling can thus be avoided. If both
wheels on one axle are in danger of slipping, the control
system intervenes through the electronic engine
management and reduces power in order to maintain
grip. By combining these functions, the PTM ensures
optimum traction on almost every surface.
All the PTM functions are fully automatic, resulting
in inherently more dynamic handling characteristics, as
well as greater active safety.
5.12.3 Case study: Haldex coupling
Four-wheel drive transmissions (4WD) have been in
production models of performance cars for some 25
years. During this time the mechanics of these
transmissions have evolved to improve further the
traction efficiency of the system as a whole. The heart
of a 4WD system is the centre differential which divides
the torque between the front and rear axles. Over the
years this technology has evolved from simple locking
differentials, to limited slip differentials, to viscous
differentials, and, the latest iteration, the Torsen
differential. This development process has improved the
performance of 4WD vehicles, but all of these systems
suffer from the same shortfall: wheel slip can be
recognised only by a difference in shaft speeds between
the front and rear axles. The problem is that although
these systems can sense slip, they are not able to detect
the cause of it.
Developments in electronics have moved this
technology forward significantly with the introduction
of the Haldex coupling, which is now used by a number
of large motor manufacturers. Owing to the
mechatronic nature of this system, the coupling is fully
controllable and can take into account not only slip but
also vehicle dynamic state.
Overview and operation
The Haldex coupling is mounted in the rear differential
assembly and is driven by a propshaft connected to the
front transaxle (Figure 5.65). Engine torque is
transmitted via this propshaft from the front transaxle,
directly from the front differential drive. There is
effectively no centre differential. Inside the unit the
input shaft is connected to the output shaft and rear axle
via the Haldex multi-plate coupling; torque to the rear
axle is provided and controlled through this coupling.
The basic operation of the mechanics and hydraulics
of the system are as follows (Figure 5.66). When a
speed difference occurs between the input and output
shafts of the coupling, the swash plate drives the small
oil pump plungers and this generates oil pressure. This
pressure is used in the clutch pistons to compress the
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clutch plate pack and hence engage the clutch. Once the
clutch is locked then 100% of the torque is transmitted
through the coupling. The clutch pressure is maintained
by a regulating valve. The system includes an electric oil
pump which runs at engine speeds above 400 rev/min.
This provides a background system pressure of 4 bar
which pre-loads the clutch pack to remove any play and
ensures a minimum system response time.
Transverse FWD layout
Electronic control system
The ECU for the 4WD system/Haldex coupling is
mounted directly at the rear axle casing. This is in line
with current trends for powertrain control systems:
there is a fully integrated mechatronic control module
including electronics and hydraulics, with minimal
external connections and interfaces, to improve
reliability. Figure 5.67 shows the system overview.
Propshaft to rear axle
The sensors and interfaces used in the system are:
Rear axle including
Haldex coupling
a CAN interface to the engine ECU, providing
information about engine speed and load through
crankshaft position and throttle position sensors
Wet multi-plate clutch
Clutch piston
piston pump
throttle valve
Figure 5.65 Four-wheel drive control layout
Figure 5.66 Hydraulically controlled clutch assembly
Engine torque
Engine speed
Accelerator position
Other onboard
Haldex LSC
Oil temperature sensor
Hand brake switch
Oil pressure sensor
Brake light switch
Steerwheel angle
Figure 5.67 Electronics control system links
4x wheel speeds
Brake light switch
ABS active
ESP active
Yaw rate
Lateral acceleration
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Transmission diagnostics
a CAN interface to the ABS ECU, providing
information about the vehicle dynamic state,
including wheel speed, vehicle acceleration, brake
status (on or off) and handbrake status (on or off)
a temperature sensor for the Haldex coupling,
enabling the coupling to react to changes in oil
temperature, and to detect high temperature or
overload conditions so the system can protect itself
in extreme conditions.
The actuators are:
a positioning motor for the regulator valve
(controlled directly by the ECU) which controls the
clutch pressure and hence the operation and control
of the system
an oil pump, which provides background system
pressure and lubrication while the unit is in standby
The system’s main features and advantages are:
active torque distribution via an electronically
controlled multi-plate clutch, with instant activation
and high torque transfer for maximum traction
when required; an alternative separate off-road
mode (switch) can lock the coupling
Key Points
an extremely fast responding, highly dynamic
system but with the feel of a normal two-wheel
drive car for predictable handling
no strain on system components or the vehicle
during low-speed manoeuvring
compatibility with different tyre sizes, for example,
when a space saving spare wheel has to be fitted; an
algorithm in the ECU detects differences in the
diameters of the tyres and adjusts the characteristics
no restriction on towing or testing (a chassis
dynamometer or brake test) of the vehicle, because
the system is inactive when the engine is not
full compatibility with other vehicle dynamic
control systems such as ABS, EDL, TCS EBD and
ESP; the coupling communicates on-line with safety
systems in the vehicle.
The fundamental purpose of four-wheel drive
traction management is to distribute actively
torque to the wheels that can utilise it most
Four-wheel drive control systems are fully
integrated with other dynamic control systems for
maximum driver safety and benefit
5.13.1 Introduction
Although they are sophisticated pieces of modern
electromechanical technology, like anything else,
electronically controlled transmissions can and do go
wrong. Generally, considering the environment in
which they operate, these units are extremely reliable.
In most cases, failure of a sensor or actuator initiates
the limp home failure mode which means that the
vehicle can still be driven (although with reduced
performance) for repair or to get the occupants home.
An important point though is that the working
components are expensive and can be difficult to
replace if they are deeply embedded inside the
transmission unit. So correct diagnosis of the cause of
faults is important to prevent wasted time and the
unnecessary replacement of parts.
5.13.2 Typical diagnostic procedure
A logical approach is essential! For diagnosis, analysis of
a system should be broken down into its elemental,
functional parts. A typical test procedure would involve
some or all of the following.
Simple, basic manual and visual checks – The
most basic checks should be done first, for example,
the lubricating fluid level and condition. The
transmission should be at the correct temperature
when the fluid level is checked (see manufacturers’
data). Then a visual check should be made for
obvious problems, such as fluid leaks or damaged
electrical connections.
A road test and report – A complete understanding
of the problem is essential in order to be able to
diagnose a fault efficiently. This can be achieved
through a test drive. Findings should be recorded
efficiently as a report during or after the test drive.
Malfunctions and correctly functioning components
should be noted. Particular attention should be
given to shift quality, both downshifts and upshifts.
A stall test should be performed. It is necessary to
test the correct operation of different shift programs
and converter lock-up clutch operation (by driving
System mechanical checks – Basic checks should
be performed on the mechanical and hydraulic
components. The whole system can operate
correctly only if the system pressure is correct. If
[647] Chapter 05
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Fundamentals of Motor Vehicle Technology: Book 2
A5S 440Z
GS 8.55
GS 8.60.2
Range display
Program display
Diagnosis – programming
Shift lock
MV 3
MV 2
Manual gate
Prog switch
Kick down
MV 1
8.55 (CAN 60)
Oil temp
Input speed
Output speed
Figure 5.68 Electronic system overview
Valve body
[647] Chapter 05
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Transmission summary
Check resistances of sensors and actuators
Depending upon the nature of the problem, it may be
necessary to check various components for correct
resistance readings with a multimeter. Manufacturer
information is essential for this to compare the
measured value with the correct value. Changes in
ambient temperature can affect resistance readings in
electromagnetic coils, so a realistic tolerance must be
allowed for this. To prevent damage to sensitive
electronic components and false readings, general
multimeter etiquette must be followed, with
components completely disconnected from the circuit
when they are checked.
Check signals (waveforms) and voltages at
sensors and actuators
However, just because a sensor or actuator has the
correct resistance doesn’t mean that it is working
properly when installed. Intermittent faults are
particularly difficult to find as they usually occur only
when the vehicle is being operated. Therefore, if
possible, measure the signal at the sensor or actuator
under running conditions. In most cases, an
oscilloscope will give much more information than a
multimeter. The voltage signals at some sensors and
most actuators are quite dynamic in nature and a
multimeter is not able to process the signal
appropriately. There are many portable, cheap
oscilloscopes on the market ideal for this purpose and
robust enough for workshop use.
Check system electronics via diagnostic
Nearly all vehicle electronic systems have a diagnostic
interface. In newer vehicles (2001 on), this interface is
a standardised on-board diagnostic (OBD) connector
and there are a number of generic scan tools available
that can access fault codes from the transmission ECU
(as well as from the engine ECU). The fault codes have
a standardised protocol, which aids diagnostics quite
considerably. Older vehicles tend to have diagnostic
interfaces specific to their manufacturer. Generally,
specific equipment protocols exist, which must be used
to access the information. On some vehicles
(particularly Japanese ones) there is a blink code
indication, which, although limited in the information it
provides, can be a useful aid.
Diagnostics summary
Irrespective of its type (CVT, DSG, etc.) an electronic
transmission is a complex unit and a systematic
approach is needed when dealing with any technical
problems. An electronic transmission is, though, simply
a system (Figure 5.68). It has inputs which are
processed, giving outputs with the desired reactions or
responses. From first principles, therefore, the inputs to
the system must be confirmed as correct. If this is so,
then the outputs can be checked for correct action. If
actions are incorrect, then the actuators should be
checked or tested. If they are OK, then the connection
between the actuators and ECU should be checked, etc.,
etc. This approach will solve most problems that might
Another point to consider when diagnosing faults is
that a good understanding of the system and its inputs
and outputs is especially important, with so many
variables changing at the same time in such a complex
way. Make your life easy! Get as much information as
possible about the system. Try to get a good
understanding of the system in overview and how all
the components fit together and work together in
normal operation. This information can be obtained
from the manufacturer through training documentation
or from workshop manuals – as well as from good
textbooks of course!
Key Points
possible a pressure gauge should be used to take
readings under operating conditions, so that these
can be compared with manufacturer specifications.
If there is a throttle cable, adjustment of this should
be checked. Incorrect adjustment can cause
problems with shifting (shift points).
A logical approach is essential for diagnostics
Consider systems as black boxes and check all
inputs and outputs
Understand the system operation as well as the
diagnostic procedures
Check sensor signals and supply signals using an
oscilloscope as well as a multimeter
5.14.1 Outline of electronic control
It is clear that the development of electronic control for
transmission systems will continue to move at a swift
pace. Technical developments in mechatronics and
microelectronics now allow great freedom and
flexibility in terms of packaging of control units and
interfacing between vehicle control systems. These
developments are making electronic control of the
transmission, even automatic shifting, more attractive
in markets where an automatic gearbox would once
have been shunned, for example, in high performance
Highly integrated vehicle systems can provide
sophisticated control algorithms to improve driver
[647] Chapter 05
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comfort and safety. Together with adaptive strategies
that take into account additional operating parameters
and boundary conditions, systems can optimise vehicle
In summary, the main types of transmission
currently available are:
manual constant mesh transmissions
automatic transmissions, with a planetary gearbox
and torque converter
continuously variable transmissions, with a belt
driven variator.
Electronically controlled transmissions, in conjunction
with advanced powertrain control systems, can provide
improved vehicle performance and efficiency. The
following powertrain systems have all seen the benefits
of the implementation of electronic controls:
manual transmission – electronic clutch and shifting
for improved economy and/or vehicle performance
automatic transmission – sophisticated shift
strategies and control systems to improve efficiency
and driver interaction
continuously variable transmission – control of
shifting to improve driver feel, and more control
over hydraulics for greater efficiency
four-wheel drive – improved traction and extended
functionalities through integration with other
control systems
axle/traction control – better tractive force
management on surfaces with limited or no grip.
Current developments show that integrated powertrain
control will be the basis of future developments. To
improve performance, as well as to meet legislative
requirements on harmful vehicle emissions, it is no
longer appropriate to treat the powertrain system as
separate, mechanically connected units (the combustion
engine, gearbox, axle, etc.). The powertrain system,
consisting of these elements, must be considered as a
single unit and calibrated and optimised during
development for significant improvements to be made.
5.14.2 Future developments
Vehicle manufacturers are under increasing pressure to
reduce vehicle emissions. Advanced powertrains will
play a major part in achieving future goals of emission
reduction and improved efficiency to conserve natural
resources. A critical factor to be overcome is the reduced
efficiency of the combustion engine at low speeds, and
its polluting effect at idle where no power is required at
all by the vehicle for motion.
This is an area of great interest, with large potential
for improvement, and the current trend is towards
hybrid powertrains. The hybrid vehicle combines the
positive attributes of an electric drive (high efficiency,
quiet, full torque at low/zero speed) with those of a
Fundamentals of Motor Vehicle Technology: Book 2
combustion engine (high specific power to size ratio,
long driving range between refuelling). This technology
needs a sophisticated control system for the prime
movers to work together in a harmonious and efficient
way. This can easily be achieved with an electronic
control system but there are a number of factors that
impede the acceptance of this technology in the market
battery technology – current units are expensive and
heavy, and this technical hurdle must be overcome
to move the technology forward significantly
the existing electrical system voltage – the standard
voltage for vehicle systems is 12 volts, which is far
too low to transmit power efficiently at the level
required for tractive force; higher voltage systems
will be developed to cope with increasing power
demands, which will be essential for the adoption of
full hybrid systems
motor size – although highly efficient, modern
electric motors are still quite large relative to their
power output; as technology develops, motors of
the appropriate power rating will become smaller
and will be easier to integrate into a combined
vehicle propulsion system.
Manufacturers are currently working hard to overcome
these issues, as well as introducing light hybrid systems
that can be integrated within current vehicle ranges as
an intermediate step. This allows them to introduce the
technology into the market place and gauge customer
reaction to it.
Key Points
Transmission management is used primarily to
improve economy and vehicle performance, as
well as driver interaction
Hybrid solutions are likely to be the future …
Web links
Transmission systems information
Teaching/learning resources
Online learning material relating to transmission
[647] Index.qxp
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absolute pressure sensors 19
AC motors, ISG systems 227
pedal position sensors 177
sensing 12
stoichiometric air:fuel ratios 84
accelerometers, knock pressure
sensors 25
accumulator converters 135,
actuators 26–33
assemblies 197–8
common rail diesel systems 175
communication between ECUs
control signals 32–5
electrohydraulic assemblies
197–8, 209
Haldex coupling 233
injector 100, 106
non-mechanical 26–7
outputs 4–5
resistances, checking 235
simulation tests 156
single-point injection 113–14
status monitoring 197
transmission systems 196–201
wiring, M1.5 system 108
adaptive strategies 154–5
adaptive transmission control
214, 215
advance mechanisms 40, 47–8,
after treatment systems 128–30,
air bypass ports 94–5, 96
air conditioning control 3, 12
status signals 196
air:fuel ratios 21–2, 39
and dwell period 45–6
and pollutant levels 128,
and power control 119
and spark ignition 44
control 138–43
converter processes 134–5
diesel systems 163
fuel mixture maps 84
see also lambda values; mixtures
air injection 114, 146–7
air mass control
diesel systems 163, 177, 185
sensors 12, 20, 177, 185
hot wire/film 110–11
variables 83, 86
air temperature sensors 5, 12, 88,
97–9, 104–5, 114, 155
air valves 100–1, 102
airbag systems 2–3, 12
airflow, cylinder volumetric
efficiency 119
airflow sensors 12, 19–21, 88,
airflow/temperature sensors
97–9, 104–5
analogue 23
direct injection systems 121
live data systems 155
M1.5 system 108
power supply 101
alternating current (AC)
analogues 51
ECUs 7–8
ignition 56–9
analogue sensor signals 9–10, 19,
MAP sensors 23, 111–12
rotational speed 14–15
speed/position 66
temperature 13–14, 23
throttle position 23
analogue to digital converters 10,
23–4, 192
aneroid capsules, pressure sensors
angular displacement torsion
sensing 195
angular movement/position
sensors 11–12
anti-lock braking systems (ABS)
electronics 2–3
interface, VW DSG 209
sensors and 12
arcing, contact faces 42
atmospheric pressure sensors 19
Audi Multitronic transmission
automatic brake differential (ABD)
229, 231
automatic gearboxes
electronic control 188
transmission management
automatic injection advance units
167, 168
automatic shifting hydraulic
transmissions 187
automatic transmission 200, 207,
sensors and 12
auxiliary air valves 100–1, 102
ballast resistor ignition modules
basic fuel program, injection
systems 83–4
hybrid vehicles 236
live data systems 155
memory backup 9
belt/chain drive transmissions
belt driven starter–generators 228
blink codes 150–1, 157–9
Drivelogic feature 205
SMG system 205
bob weight and spring systems
boost pressure control,
turbochargers 185
boost pressure sensors 5, 12
Borg Warner 55 gearbox 213
in-line diesel injection pump
K & KE Jetronic injection
systems 77, 79
LE/LE2 systems 97–103
M1.5 system 103–9
Motronic engine management
systems 148, 149
VE distributor pump 165
[647] Index.qxp
Page 238
on/off signals 196
pedal position sensors 12
braking downshift, Tiptronic
gearbox 216
broadband lambda sensors 142–3
burn off function, hotwire air mass
sensors 110
burn times, ignition timing 46–7
butterflies, and cylinder
volumetric efficiency 119
bypass port, mixture adjustment
98–9, 100
cam and rocker system pump
drives 175
camshaft position sensors 5, 12,
16, 23–4, 25, 177, 181
camshaft speed/position sensors
capacitor discharge ignition (CD)
capsule type pressure sensors 18
carbon dioxide emission 125–6,
carbon monoxide emission 127,
diesel systems 163
carburettors 77, 78
alternatives to 40–2
and fuel efficiency 38–9
vs single-point injection 112–13
casing mounted ignition modules
catalysts 133, 136
catalytic converters 134–8
and mixture formation 117
operating temperatures 135–6
oxygen monitoring 5, 21–2
pre-cat control 22
post-cat monitoring 22
sensors 135–6
CD systems 60
centrifugal cancellation, hydraulic
clutches 218–19
centrifugal governors, diesel
pumps 167–9
chain drive, Multitronic
transmission 223
charge time see dwell period
chemical treatment, pollutants
ECUs 7–8
fault recognition 153
monitoring 153
power transistors 8
switching 8, 32–4
Fundamentals of Motor Vehicle Technology: Book 2
closed crankcase ventilation
systems 130
closed loop systems 57–8
operation 138–9
clutches 201
actuators 197, 198–9
electronic control 201–3
Haldex coupling 232
piston mechanisms 219
pressure control VW DSG MTM
slave cylinders 206
clutch-by-wire (CBW)
transmission systems 201–3
clutch-to-clutch automatic
transmission systems 217–19
code readers 150, 157, 159–60
coil on plug ignition systems
electronic switching 48
energy build up time 68
output improvement 57
wasted spark systems 71, 72–3
cold running
auxiliary air valve 100–1
emission levels 131, 146
stoichiometric air:fuel ratios 84
cold starting
diesel 165, 172–4
emission levels 131, 146
efficiency 41, 116, 130
NOx formation 144
pilot injection 184
combustion chambers
injection timing 118
diesel systems 164
pollutant reduction 129, 130
combustion knock 41, 116, 174
diesel knock 164, 174
sensors 25, 67
common rail diesel fuel systems
communication, standardisation
commutators 28
complex systems 5
components, ECU control of 6–7
compression ratios 41, 116
compression stroke injection
timing 118–19
compressors, air conditioning 7
computer controlled systems 4
computers see electronic control
units (ECUs)
concentric clutch release systems
constant energy ignition systems
contact breakers
coil primary circuit 48–50
dwell angles 45–6
elimination 40
ignition systems 38–9
wear 42
contact brushes 28
contact-based temperature sensors
contaminants, catalytic converters
continuous fuel delivery systems
continuous rotation motors 31
continuously variable
transmissions (CVTs) 187,
control signals 32–4
altering 34–5
M1.5 system 107
sequential systems 86
control spools, EDC systems
control unit temperature sensors,
controlled pressure difference 78
controller area network (CAN)
systems 189–90
coolant temperature sensors 5,
12, 13, 88, 170–1, 177, 196
LE2 systems 99, 101
live data systems 155
M1.5 system 105, 108
single-point injection 114
cooling systems 42
cornering detection 215
crank position sensors 196
crankcase emissions, pollutant
position sensors 5, 12, 109
position triggers 89–90
rotational sensors 14, 15–16,
speed/acceleration monitoring
speed/position sensors 11–12,
23–4, 65–6, 106
speed sensors 177, 181
creep regulation, VW DSG 207
current control systems
feedback 57–8
contact breaker systems 42
injection systems 77, 78, 79,
[647] Index.qxp
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isolation 162
position reference points 15–16
pressure requirements 44
recognition sensors 72–3, 85,
valves, diesel systems 164
volumetric efficiency 119
damping chambers 98
dashboard warning lights 157–8,
DC/DC converters, ISG systems
deceleration, stoichiometric
air:fuel ratios 84
delivery valves, diesel pumps 167
capabilities, VW DSG 207
connector plugs, standardisation
EDC systems 172
electronic transmission control
on-board 150–1
transmissions 233–5
diaphragm type pressure sensors
diesel engines
emissions legislation 38
injection systems 42, 77,
knock 164, 174
differential control, electronic
differentials, 4WD systems 231
digital pulses, Hall effect 54
digital signals 9–10, 19, 22, 25–6
MAP sensors 112
optical ignition triggers 54–6
digital timing control 63–4
digital to analogue converters 10,
diode control, wasted spark
systems 72
direct acting clutch actuators 198
direct fuel injection (DI) 115,
diesel 174–5
engine management system 149
pre-heating systems 172
direct ignition systems 68–73
direct shift gearboxes (DSG) 191,
199–201, 207–10
disc valves 81, 82
distance travelled information 14
distributor-based systems 68–9
distributor body located sensors
distributor caps 52
distributor plunger drives 167
distributorless ignition systems
distributors, Hall effect pulse
generators 25–6
double shift actuators 200–1
Tiptronic gearbox 216
uncontrolled 215
drive-by-wire systems 204
Drivelogic feature (BMW) 205
driver information 12, 190–1
driver input 5, 214
driver stage amplifiers, ECUs 7–8
driver’s lever position sensors 194
drivetrain torque sensors 195
driving members 201
driving modes, selectable, VW
DSG 207
dual clutch systems 187, 203
actuation 198–9
dual-bed catalysts 134
duty cycles 33–5
dwell angles 45–6, 56
dwell control, ignition modules
dwell period 45–6, 68, 70–1
dynamic control program (DRP),
Multitronic transmission 224
dynamic stability control (DSC)
dynamic vehicle control systems 3
EGR see exhaust gas recirculation
electric motor type actuators 4,
28–9, 31–2
electric vehicles, hybrid 226–9,
electrical actuation vs servo
operation 196–7
electrical control, transmissions
electrical/electronic injection
systems 77, 80, 81–90
electrical systems
LE2 system 101–3
M1.5 system 107–9
electro-hydraulic shift mechanisms
electroactuators 198–201
electrodes, spark plugs 74, 75–6
electrohydraulic actuator
assemblies 197–8
electrohydraulic transmissions
electromechanical actuators,
clutch systems 202–3
electronic airflow sensors 20–1
electronic and mechanical sensing
devices 12
electronic clutch management
electronic control units (ECUs)
4–5, 6–11
accumulator converters 138
actuator control signals 32–4
common rail systems 174–85
communication between 27
diesel systems 177–85
glow plugs 173–4
Haldex coupling 232–3
injection systems 80, 82–9,
integrated 148–50, 188–91,
lookup tables 63–4, 67
M1.5 system 109
monitoring, EDC systems 170–2
power supply, LE systems 103
powertrain control systems
sensor signals 192
shift control 205–6
system fault recognition 152–5
transmissions 188–91, 197–8,
214, 235–6
VW DSG 209
electronic diesel control (EDC)
systems 170–2
electronic differential control
electronic differential lock (EDL)
interface, VW DSG 209
electronic ignition systems 40–8
electronic manual gearboxes
electronic petrol injection systems
electronic shift control, manual
gearboxes 204–5
electronic switching, coil primary
circuit 48
electronic system overview 234
electronically assisted ignition
emergency operation 153–4
emission control 2, 6–7, 37–9,
125–8, 132–4, 236
after treatment 128–30
cold running 146
CVT systems 221
diesel systems 163
EDC systems 170
engine operation 131–2
(EVAP) systems 130
[647] Index.qxp
Page 240
injection systems 78
integrated 148–50
legislation 37–8, 160
petrol engines 124–47
sensors 12, 90
system actuators 5
contact breaker systems 42
production, stratified mixture
formation 117–18
regeneration, ISG systems 227
waste, distributor based systems
engine breather systems
maintenance 39
engine intake pressure 12, 18,
engine knock pressure sensors 25
engine maintenance, and
emissions 38–9
engine management systems 6,
11, 12
and fuel injection 78
load sensing 5, 12
interface, VW DSG 209
integrated 148–50, 188–91,
engine oil
changing 39
temperature sensors 12
engine output, EDC systems 170
engine speed
contact breaker systems 42
EDC systems 170
live data systems 155
sensors 5, 12, 105–6, 177, 181,
timing advance 61–2
trigger references 99–100
engine stop/start function, ISG
systems 227
engine torque control 215
engine tuning, pollutant reduction
environmental pollutants 125–7
see also emission control
epicyclic gears, automatic
transmission 212
EU emissions legislation 126–7,
160, 163
European on-board diagnostics
(EOBD) 160–2
evaporative emission (EVAP)
control 134, 147
live data systems 155
M1.5 system 108
excess air factor, lambda scale
Fundamentals of Motor Vehicle Technology: Book 2
excess fuel return pipes 93
exciter coils, rotational speed
sensors 15
exhaust gas
composition 125–6
oxygen sensors 5, 11–12, 21–2
pressure, and EGR 144–6
temperature sensors 11–12
exhaust gas recirculation (EGR)
systems 130, 133, 143–6,
common rail systems 185
control systems 145–6
sensors 12
valves 5, 145
external drives, double shift drums
fault codes 150–1
accessing 157–60
memory, OBD system 216
standardisation 160–1
fault handling, transmissions 214
fault recognition 152–5
feedback current control systems
firing end configuration, spark
plugs 74
firing voltage requirements 43–4
fixed dwell angle systems 56
fixed gear transmissions 187
flame speed 130
flap type airflow meters 19–20
flow type pumping 90–1
fluid temperature sensors 11–12
flyweights, centrifugal governors
as driving members 201
sensor locatedion 65–6
starter–generators 228
Ford CTX transmission 221
four-speed all clutch-to-clutch
systems 217–19
four-wheel drive control 229–33
Haldex coupling 232
friction clutches 201
friction wheel variators 187
front-wheel drive vehicles
CVT systems 220
gearboxes 217
fuel consumption/economy
and CO2 emission 130
CVT systems 221
diesel systems 164
direct injection systems 116–19
electronic manual gearbox 205,
powertrain automation 204–5
stratified mixture formation
fuel delivery systems 77–8, 90–4
common rail diesel systems
control, precise 80
cut-offs, solenoid operated 165,
diesel 163–4
filters 80, 91–2
flow 90–1
gauges 10
pollutant reduction 125, 128,
quantity control actuators 5
sealed 130
sensors 12, 18, 19
see also fuel injection systems
fuel emissions see emission
fuel evaporative emission control
(EVAP) systems 130
fuel injection systems 6–7, 30, 42,
77–8, 80, 83–4, 97–112
comparison 116–19
control 5, 177–85
driver control modules 181
duration control 86
EDC systems 181
live data systems 155
timing, direct injection
systems 118–19
diesel 42, 163–85
direct fuel injection (DI) 115,
116–24, 149
diesel 174–5, 177–85
GDI systems 115–24
pre-heating systems 172
electrical/electronic 77, 80,
engine intake pressure 93–4
fuel/ignition control
combination 148–9
fuel maps 83–4
fuel sprays 81–2
indirect, pre-heating 173
intermittent 78
multi-point 80, 97–112, 116
nozzle injectors 169–70
pilot/main 184–5
sensors 5, 11, 80, 87–90,
110–12, 121, 177–85
simplified 112–13
single-point 114–15
solenoid actuators 5
valves 80
vaporisation 116, 120
[647] Index.qxp
Page 241
volume control, EDC systems
see also injection pumps;
fuel mixtures see air:fuel ratio;
fuel pressure
regulators 80, 93–4, 183
sensors 12, 19
systems, diesel 176–7
fuel pumps 90–1
continuous rotation motors 31
diesel rotary pumps 165–7
power supply, LE systems 101
wiring, M1.5 system 108
fuel rails 80, 93–4
common rail diesel systems
injector solenoid valves 81
rail sensors 121
fuelling references 84
full rotation motors 31, 32
gas temperature sensors 11–12
gasoline direct injection (GDI)
systems 115–24
gear actuator valves, VW DSG
MTM 209
gear ratios
automatic transmission 214
Multitronic transmission 223
gear selection
electronic transmission control
Honda system 217–18
Porsche PTM system 231
VW DSG 208–9
gear shift
actuators 199–201, 221
high-performance vehicles 205
gear trains
Borg Warner 55 gearbox 213
Honda system 217–18
automatic, electronic control
electronic manual 204–10
information sharing 190–1
lightening 205
mode selection 205–6
RPM sensors 193
sensors 194, 196
sequential manual (SMG) 205
Tiptronic 216–17
transmission management
VW DSG 208
glow plugs 133, 170, 173–4
governors, rotary diesel injection
pumps 167–9
gradients detection, adaptive
control 215
Haldex coupling 231–3
Hall effect 16
digital pulses 54
ignition triggers 16–17, 53–4
phase sensors 85
pulse generators 25–6, 53–4
rotational speed sensors 15,
16–17, 192, 193
speed/position sensors 65, 67
switches 53–4
Tiptronic gearbox operation 196
headlight circuits 3–4
heat ranges, spark plugs 75
heated exhaust gas oxygen
(HEGO) sensors 22
heated step type lambda sensors
heater plugs 170, 173–4
heating, broadband sensors 143
high energy coils, ignition
modules 57–8
high pressure pumps 181, 182
diesel 176, 177
direct injection systems 122–3
high speed switching, circuits 8
high tension cables, ignition 68
high voltage rapid opening
injectors 120
hill-holder function, VW DSG 207
homogeneous charge compression
ignition (HCCI) 189
homogenous mixture formation
Honda clutch-to-clutch automatic
transmission system 217–19
hot film sensors 111, 121
hot wire sensors 20–1, 110–11
hybrid electric vehicle 236
classification 226
powertrain technology 226–9
transmissions 189
hydraulic clutches 218–19
Haldex coupling 232
hydraulic pump and pressure
accumulators 205
hydraulic shift transmissions 187
hydraulic system checks 233–4
hydraulics, Haldex coupling
hydrocarbon emissions 127
diesel systems 163
reduction methods 129–30
idle speed
and pollutants 131–2
control 5, 94–6, 113
EDC systems 172
LE2 system 100–1
live data systems 155
M1.5 system 106–7, 108
crankshaft sensor 106
disconnection, torque converters
partial rotation 32
stepper 5, 32
rotary diesel injection pumps
stoichiometric air:fuel ratios 84
idle valves 94–5
ignition advance curves 61
ignition coils 29
as actuators 5, 27
M1.5 system 107, 108
monitoring 162
output voltage 44
ignition systems 40
computer controlled 61–7
constant energy 58
electronically assisted 48–50
fuel control combination 148–9
and fuel efficiency 38–9
modules/amplifiers 56–9
pollutant reduction 128
sensors 5, 12
ignition timing 45–8
actuators 5
advance/retard 57
as after treatment 134
and cylinder pressure
requirements 44
dwell time 42
pressure sensors 18
and engine intake depression
Hall effect pulse generators
live data systems 155
pollutant reduction 129, 130
ignition triggers 89–90
Hall effect 16–17
inductive 50–3
single-point injection 114
immobiliser systems, EDC systems
improbable/implausible values
in-car entertainment systems 3
in-line injection pumps, diesel
in-line transmissions 187
inductive sensors
[647] Index.qxp
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analogue rotation 192–3
crankshaft speed/position 66
ignition systems 40
phase sensors 85
pulse generators 50–3
self-generating 52–3
injection pumps 116
high pressure 176
rotary diesel 167, 169
injection systems see fuel injection
injectors 80–2
LE2 system 100
M1.5 system 106
single-point injection 113
common rail systems 175–6,
direct injection systems 120
ECU switching 84
LE systems power supply 101
motion sensors, EDC systems
171, 172
nozzles 81–2
operation speed 82
common rail diesel systems
rotary diesel injection pumps
solenoid valves 81–2
timing 85–7
input shaft speed sensors, VW
input signals, ECUs 4–5
shift control 206
insulators, spark plugs 75
intake air temperature
NOx reduction 130
sensors 12, 170–1, 177
intake manifold pressure sensors
map sensors 111
and EGR 144–6
intake ports
diesel systems 164
injection systems 77–80
pollutant reduction 129
intake system, auxiliary air valve
integrated control
emission systems 148–50
ignition modules 58–9
powertrain systems 189–90,
starter–generator (ISG) systems
Fundamentals of Motor Vehicle Technology: Book 2
intelligent powertrain
management 229
Haldex coupling 232–3
VW DSG 209
interference suppression 48
intermittent injection 79
internal drives, double shift drums
Jaguar 6 speed automatic gearbox
knock see combustion knock
lambda sensors 5, 138–43
live data systems 155
M1.5 system 108
single-point injection 114
step type 141–2
see also oxygen sensors
lambda values
diesel systems 163
excess air factor 124–5
fuel mixture maps 84
lambda 1 air:fuel ratio 21
lambda window 124–5
and pollutant levels 128
see also air:fuel ratios
LE2 systems (Bosch) 97–103
lead emissions, origin/effects 127
leaded fuel, and catalytic
converters 137
lean burn technology
broadband sensors 142
emissions control 132–3
pollutant reduction 129, 130
LED optical ignition triggers 55
LED probes, blink codes 158–9
light hybrid powertrain
technology 226–9
lighting systems 3–4, 9, 26–7
limited slip differentials (LSDs)
limp home mode 153–4, 215, 207
linear movement sensors 11–12
linear solenoids 96
linear speed information sharing
link plate chain drives 223
live data systems 155–7
live rear axle systems 230
and pollutants 131
sensing 5, 12
stoichiometric air:fuel ratios 84
timing advance/retard 47
vacuum retard 62
locations, ignition modules 58
lock-up clutches 211–12, 215
lookup tables, ECU 63–4, 67
Los Angeles smog 38
low pressure pumping systems
lubricant viscosity, and system
pressure 194
M1.5 system 103–9
magnetic fields
actuators and 27–30
inductive pulse generators 50–1
magnetic sensors
ignition systems 40
rotational speed sensors 14–15
magnetoelastic torque sensors
main injection 184–5
malfunction indicator lamps (MIL)
manifold absolute pressure (MAP)
sensors 5, 19, 88, 170
analogue 23, 111–12
diesel fuel injection systems
live data systems 155
heating 173
single-point injection systems
pressure 93–4
direct injection systems 121
signals 196
manual checks, transmission
diagnostics 233
manual idle speed adjustment
manual transmissions 187, 207
clutches 201–2
gear shift actuators 200
gearbox electronic control
selection 200
MAP sensors see manifold absolute
pressure sensors
mass airflow measurement 88
direct injection systems 121
sensors 20–1
signals 196
master cylinder assemblies 198
master references
single coil systems 72–3
wasted spark systems 71
mechanical advance mechanisms
mechanical concentric clutch
[647] Index.qxp
Page 243
release systems 198–9
mechanical friction clutches 201
mechanical ignition systems 42–3
mechanical injection systems 77
mechanical system checks 233–4
mechanical timing systems 61–3
mechanical type sensors 12
airflow sensors 19–20
pressure 18
mechatronic control units,
integrated, VW DSG 207
mechatronic transmission
management units 197–8, 209
memory, ECUs 8–9, 83, 216
Mercedes-Benz, fuel injection
systems 77, 78
micro/mild hybrid systems 227
and catalytic converters 136–7
monitoring 162
adjustment, LE2 system 98–9
pollutant reduction 129, 130
power/torque regulation 119
enrichment, cold running 146
formation, direct injection
systems 117–18
lean burn technology 132
rich, and cold-start 172
see also air:fuel ratios
mode selection, gearboxes 205–6
modulating valves, VW DSG MTM
monoliths, catalytic converters
Motronic engine management
system (Bosch) 103–9, 148
movement sensors 11–12
multi-plate clutches 203, 223
multi-point fuel injection (MPI)
97–112, 116
components 80
hotwire air mass sensors
MAP sensors 111–12
nozzle injectors 169–70
multi-stage transmissions 187
multimeters, duty cycle readings
multiple ignition coil systems
control valves, VW DSG MTM
transmissions 189–91
Multitronic transmission (Audi)
narrow band sensors 139–42
needle valves 81, 82
negative sparks 70
negative temperature coefficient
(NTC) sensors 13–14, 196
Nissan CVT system 224–5
nitrogen, in exhaust gas 125
nitrogen oxides (NOx) 127
accumulator converters 135,
emissions, diesel systems 163
formation 143–4
reduction methods 130
noise, direct injection systems
non-mechanical actuators 26–7
OBD system, fault memory 216
off-road vehicles, differential
control 229–33
oil pressure control valves, VW
on/off actuators 197
on-board diagnostics (OBD) 22,
open circuits, fault recognition
open loop operation 138–9
opening time, injectors 82
operating conditions 62–3, 131
optical ignition triggers 54–6
optical speed/position sensors 66
oscillation frequency, step type
lambda sensors 142
oscilloscope readings 34, 86, 87
outlet port valves, common rail
diesel systems 175
output shaft speed sensors, VW
output signals
amplifiers, ECUs 7–8
broadband sensors 143
D/A converters 10
ECU systems 4–5
sensors 192
overdrive systems 188, 189
oxidation 133
catalysts 134
oxides of nitrogen see nitrogen
oxygen reduction, exhausts 130
oxygen sensors 11–12, 21–2,
analogue signals 25
single-point injection 114
see also lambda sensors; lambda
parallel shift gearboxes 210
partial rotation motors 31, 95–6
passwords, code readers 159
phase sensors 85
photochemical smog formation
phototransistors 55
pilot injection 184
phase control 174, 176
Pintaux injectors 173
pintle nozzle injectors 169–70,
planetary gears 187, 212
playback mode, data systems 156
plug gaps 43–4, 76
points ignition systems, and fuel
efficiency 38–9
polarity, spark plug electrodes 76
pollutants see emission control
Tiptronic gearbox operation
191, 196, 216–17
traction management (PTM)
port type injection systems 80,
117, 119–20
position reference points 15–16
position sensors 11–12, 17–18,
23–4, 195
positive crankcase ventilation
(PCV) systems 130
positive displacement pumping
positive temperature coefficient
(PTC) sensors 13–14
airflow sensors 19, 97–8, 104–5
position sensing 25
throttle position 17–18, 88–9
power amplifiers, ECUs 7–8
power supply
boost function, ISG systems
LE systems 101
loss, CVT systems 221–2
M1.5 system 108
regulation, mixture/throttle
control 119
power transistors 8, 48–50
control units 190–1
light hybrid 226–9
transmission types 187
pre-heating systems, diesel 172–4
pre-ignition detection 41
pre-programmed driving modes,
selectable, VW DSG 207
pressure accumulators 205
[647] Index.qxp
Page 244
pressure control
carburettors 78
gearboxes 215
fuel 90, 93–4
pressure sensors 12, 18–19, 67
analogue 25
direct injection systems 121
information sharing 190–1
pressure/vacuum sensors 11–12
transmission system 194–5
pressure systems, diesel 176–7
pressure valves, diesel pumps 167
programming, ECUs 6
pulley assemblies, CVT systems
pulse air systems 146–7
pulse generators
Hall effect 16–17, 53–4
ignition systems 40
inductive 50–3
signals 9–10, 33
pulse width modulated (PWM)
signals 33, 193
pump based air injection systems
pump-turbine unit, torque
converters 210–11
common rail diesel systems
direct injection systems 121–3
elements 90–1
high pressure, diesel 176, 177,
181, 182
single high pressure 178–80
purge valve wiring, M1.5 system
racing engines
capacitor discharge ignition 60
fuel injection systems 77, 78
radio interference, spark plugs 74
reference points
crankshaft sensors 65–6
ECU monitoring 9–10
ignition timing 51–2
rotational angular position
sensors 15
reference signal systems 65–7
sequential injection systems
reference voltages
LE systems 101
ECU systems 13
references, master
single coil systems 72–3
wasted spark systems 71
Fundamentals of Motor Vehicle Technology: Book 2
regenerative retarders, ISG
systems 228
relay operation
headlight circuits 3
LE systems 102
M1.5 system 108
reliability/durability issues 39–40
crankshaft sensors 65–6
inductive pulse generators 50–1
rotational speed sensors 14–15
remote ignition modules 58–9
requirements 44
throttle position sensors 17
ballast 56
spark plugs 74
temperature sensors 13
variable 9
retarder function, ISG systems
retrofit gear shift actuators 199
returnless systems 93
reverse torque converters 212
road speed information 14
road tests, transmission
diagnostics 233
road wheel rotational speed
sensors 14
roller cell pumps 92
roller ring assemblies 167
rotary diesel injection pumps
rotary idle valves 32, 95–6
M1.5 system 106–7
rotary injection pumps, diesel
rotational sensors 11–12
angular position 15–18
rotational movement 11–12
speed sensors 14–18, 190–1,
rotor arms 52
elimination 68–9
energy waste 68
rotor discs, Hall effect sensors
safety systems
electronic transmission control
integration 190, 230
satellite navigation systems 3
scan tools 150, 157, 159–60
seat sealing, spark plugs 74
secondary air injection 146–7
assembly, electronic manual
gearbox 207
control unit interface, VW DSG
sensors, VW DSG MTM 209
self-bleeding fuel systems 165
self-diagnosis systems 150–1, 154
self-generating inductive sensors
self-learning capability 205
semiconductor pressure sensing
elements 195
sensors 4, 11–13
applications 11–12
ECU input 4–5, 6
failure substitute values 215–6
inductive, self-generating 52–3
information sharing 190
MAP 111–12
M1.5 system 103–4
resistance checking 235
signals 22–6
analogue to digital converters
types 13–22
sequential injection 85
sequential manual gearbox (SMG)
serial data systems 155–7
series resistance circuits 13
service adjust mode, data systems
servos 196–7
shaft layout, VW DSG 208
shaft torque sensing 195
sheathed-element glow plugs 174
shift control
automatic transmission 214–15
electro-hydraulic 205
mode selection switches 194
quality 214
sensors 206
uncontrolled 215
shift curve adaptation, Tiptronic
system 217
short circuits, fault recognition
shut-off valves, solenoid 167
sideways movement sensing 12
signals 5, 9–10, 22–5, 192
analogue to digital converters
broadband sensors 143
checking 154, 197, 235
ECUs communication 27
stepped 23, 141–2
[647] Index.qxp
Page 245
see also sensors; specific
silver spark plug electrodes 75–6
simple multi-point injection with
airflow sensor 97–103
simplified injection systems
simulation tests, actuator 156
simultaneous injection 85
LE2 system 100
M1.5 system 106
single barrel pumps 122–3
single-bed converters 134
single coil systems 68, 72–3
single high pressure pump system
components 178–80
single hole nozzle injectors
single-point petrol injection
sliding mesh gearboxes 204
slip control, lock-up clutches
slip differentials, limited (LSDs)
smart sensor technology 192
smog 37–8
snapshot mode, data systems 156
solenoid actuators 4, 5, 27–8,
30–1, 196–7
solenoid injectors, single-point
injection 113
solenoid operated fuel cut-offs
solenoid valves
air 30–1
idle valves 95–6
injector 81–2
pulse air 146–7
solid state components, pressure
sensors 18–19
soot emissions, diesel systems
advance maps 63–4
detection 162
duration 44–5
plugs 39, 73–6
positive and negative 70
quality 43
timing 46–8
speed, injector operation 82
speed/position sensors 192–3
analogue 23–5
crankshaft 65–6
single-point injection 114
speed-related timing advance
spin control, sensors and 12
sports map shift, Tiptronic gearbox
sports utility vehicles (SUVs)
spring pressure, clutch operation
stability control, sensors and 12
stalling, at idle speed 94
standardisation, on-board
diagnostics 160–1
starter integration
starter–alternator torque
converters 212
starter–generator systems (ISG)
direct injection systems 124
electronic manual gearbox 207
rotary diesel injection pumps
voltages see reference voltages
status monitoring, actuators 197
steering angle sensing 12
steering column control unit
interface, VW DSG 209
step type sensors 139–42
stepped signals 9–10, 23
stepped transmission, tractive
effort curves 220
stepper motors 29, 32
as actuators 5
idle valves 94–5
stoichiometric air:fuel ratios 21,
84, 117–18, 124–5
stratified mixture formation
lean burn technology 132–3
pollutant reduction 129
substitute values, sensors 154,
substrates, catalytic converters
supercharger control sensors 12
switch-based temperature sensors
and relays 3
and sensors 4
circuits, high speed 8
coil primary circuit 48
digitally controlled 9
ECU functioning as 32–4
Hall effect 53–4
power transistors 8, 48–50
throttle 88–9
wasted spark systems 71
synchromesh gearboxes 204
six speed, VW DSG 207
synchromesh transmissions 187
system components, M1.5 system
103, 105–9
system fault handling, VW DSG
system fault recognition 152–5
system faults, self-diagnosis
system layout, M1.5 system 103,
system LED, blink codes 158
system pressure, automatic
transmission 214
temperature control, ECUs 6
temperature sensors 11–12, 13–14
analogue signals 23
coolant 99
EDC systems 170–1
fault recognition in 152–3
information sharing 190–1
and catalytic converters 135–7
extremes, actuator operation
spark plugs and 73
terminology, code reading
equipment 157
actuator simulation 156
emission regulations 126–7
fault codes access 157–60
routines 156–7
thermal afterburning 133
thermistors 13
thick film pressure sensing
elements 195
three barrel pumps 122
three-phase AC motors, ISG
systems 227
three-way catalytic converters
thresholds, digital signals 9–10
throttle body fuel injection (TBI)
112–15, 116
throttle butterflies 17
throttle control
direct injection systems 120–1
power/torque regulation 119
throttle linkages, stepper motor
action 94–5
throttle position sensors 5, 12, 17,
information sharing 190–1
live data systems 155
M1.5 system 105, 109
pedal sensors 205
signals 23, 196
single-point injection 114
variable resistors 9
[647] Index.qxp
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throttle positioner systems 129
throttle switches 88–9
LE systems 99, 102
timing advance
advance/retard 47
characteristics 63–4
EDC systems 172
rotary diesel injection pumps
timing control 61–4
ECU lookup tables 63–4, 67
ignition systems 40
injection systems 79
timing trigger systems see triggers
Tiptronic gearbox operation 191,
196, 216–17
top dead centre (TDC) 15, 16, 25
toroidal variator drive (Torotrack)
transmissions 187
torque 186
and emissions, diesel systems
boosters, ISG systems 228
diesel rotary pumps 165
mixture/throttle control 119
converters 187, 212
electronic control 210–12
lock-up control 196, 211,
215, 224
turbine speed information
distribution, Porsche PTM
system 231
drivetrain 195
Multitronic transmission 223
transfer, CVT systems 223
torque/inertia ratio, electric
motors 228
torsion angular displacement
sensing 195
traction control
adaptive control 215
(ESP) system interface, VW DSG
efficiency, 4WD systems 231
tractive effort curves 220
transient operation, clutches 206
transistors, switching 48–50
transmission systems 186–236
diagnostics 233–5
electronic switching 205
integration 188–91, 205
sensors 192–6, 195–6
splitting principle 209–10
Fundamentals of Motor Vehicle Technology: Book 2
temperature monitoring 193–4
transverse drive vehicles,
gearboxes 217
circuits 50–3
discs 16–17, 65–6
Hall effect 16–17, 53–4
injection systems 89–90
optical 54–6
timing 65–7
LE2 system 99–100
M1.5 system 105–6
turbine/pump unit, torque
converters 210–11
boost pressure control 5, 185
sensors 12
twin clutch systems 187, 203
actuation 198–9
two-speed governors, rotary diesel
injection pumps 167–9
two-stage current control process
unit injectors, common rail diesel
systems 175–6
USA, emissions legislation 37, 38,
vacuum advance/retard
mechanisms 62
vacuum sensors 11–12
valve timing
electronic 42
NOx reduction 130, 134
variable control 149
valves 39
actuator, VW DSG MTM 5, 209
rotary idle 32
airflow sensors 19–20, 97–8
Hall effect pulse generators 54
variable position bleed actuators
variable reluctance sensors
14–15, 51
variable resistors 9
variable-ratio pulley assemblies
variables, injection ECUs 83
Variomatic CVT system 220, 221
VE distributor pumps 165–72
vehicle application software 159
vehicle speed monitoring 196
information sharing 190–1
speed sensors 12
vehicle stability control sensors 12
venturi 78
visual checks, transmission
diagnostics 233
direct shift gearbox (DSG) 199,
electronic manual gearbox
analogue pressure sensors 19
checking, sensors/actuators
ECUs 7–8
feedback control 57–8
ignition systems 29–30, 43–4
oscillation, rotational speed
sensors 14–15
rapid opening injectors 120
threshold points, temperature
sensors 23
throttle position sensors 17
volumetric efficiency, cylinders
Volvo Variomatic CVT system 220
warm-up maps, Tiptronic gearbox
washcoats, aluminium oxide 136
wasted spark ignition systems 68,
water in exhaust gas 125
waveform checking 235
weak mixtures, and CO2 emission
wet clutches
twin clutch arrangement 203
VW DSG 209–10
wheel slip detection 215
wheel speed sensors 12, 24–5
information sharing 190–1
wheel spin control 12
windings, secondary 29
windscreen wipers 4
wipers, potentiometer 17, 97–8,
wiring circuits
Hall effect pulse generators 54
inductive pulse generators 52–3
LE2 system 101–3
M1.5 system 107–9
optical ignition triggers 55–6
worm gear drive actuators 198
Wright Brothers 77
zero voltage points 51–2
zirconium oxide oxygen sensors
[647] Index.qxp
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[647] Index.qxp
Page 248
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