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E - THEORY/OPERATION - TURBO
1995 Volvo 850
1995 ENGINE PERFORMANCE
Volvo - Theory & Operation
850 - Turbo
INTRODUCTION
This article covers basic description and operation of engine
performance-related systems and components. Read this article before
diagnosing vehicles or systems with which you are not completely
familiar.
AIR INDUCTION SYSTEM
TURBOCHARGERS
Turbo models use a water-cooled turbocharger, mounted
directly to exhaust manifold, with a wastegate assembly attached to
rear of turbine housing. Turbocharger consists of a turbine/compressor
assembly, oil supply system and wastegate. Other components include
impellers, impeller shaft, bearings and impeller housings.
The safety valve of system is a pressure-actuated wastegate
that prevents excessive intake boost pressure. The wastegate is
controlled by the turbo control valve. This 3-port solenoid valve
monitors boost pressure and prevents turbo lag. The control valve is
activated by turbo control unit, which receives signals from throttle
position sensor, fuel injection ECU and turbo pressure sensor.
If boost pressure exceeds safe limits, engine damage may
result. The wastegate opens when exhaust pressure exceeds a
predetermined limit and allows exhaust gases to by-pass compressor.
Turbocharger operation requires a large quantity of clean oil to
prevent bearing failure. Engine oil pressure provides constant
lubrication to system.
At idle and light throttle, turbo engine operates like a
standard engine. When more power is required, exhaust gases from
exhaust manifold enter turbocharger’s turbine housing and flow through
turbine blades. Exhaust flow and turbine speed increase as throttle
opens and RPM increases. Impeller turns with turbine and forces air
into compressor housing and intake manifold. As impeller and turbine
speed increases, boost pressure also increases.
COMPUTERIZED ENGINE CONTROLS
850 turbo models use a Motronic 4.3 injection and electronic
ignition system, equipped with self-diagnostic capabilities.
Systems use an Electronic Control Unit (ECU) that receives
input from engine monitoring sensors. These sensors include camshaft
sensor, coolant temperature sensor, oxygen sensor, knock sensor, mass
airflow meter/sensor, timing pick-up and throttle switch. ECU uses
these input signals to control air/fuel mixture for emission control,
fuel economy and good driveability. Ignition and fuel controls are
provided by a single ECU.
CONTROL UNITS
Electronic Control Unit (ECU) provide precise control of
fuel, ignition and turbo operation. System has self-diagnostic
capabilities.
850 turbo uses a single ECU for ignition and fuel controls,
located in right front of engine compartment.
NOTE:
Components are grouped into 2 categories. The first category
is INPUT DEVICES, which are components that control or
produce voltage signals monitored by control unit. The
second category is OUTPUT SIGNALS, which are components
controlled by control unit.
INPUT DEVICES
Available input signals include:
Acceleration Sensor
Acceleration sensor is mounted to firewall at right rear of
engine compartment. Sensor consists of a piezoelectric vibration pickup that detects vertical acceleration of vehicle, for example driving
on a bumpy road. ECU uses this sensor signal to determine if
irregularities in crankshaft rotation are due to cylinder misfiring or
due to driving on an uneven road surface.
A/C Switch
Signals fuel injection ECU of A/C operation. This allows fuel
injection ECU to control idle speed with idle valve.
Air Temperature Sensor
Information gathered from air temperature sensor is combined
with information from pressure sensor to calculate intake air mass.
Engine Coolant Temperature Sensor
Engine Coolant Temperature (ECT) sensor is a negative
temperature coefficient type, meaning its resistance lessens as
temperature increases. Four cylinder sensor unit has 2 resistors. One
resistor is connected to fuel injection ECU and the other resistor is
connected to the ignition ECU.
Knock Sensor
Knock sensor detects knocking and sends signal to Motronic or
ignition ECU. ECU is able to gradually retard ignition timing to each
individual cylinder. If knocking does not stop, a signal is sent to
Motronic ECU to enrich air/fuel mixture.
Mass Airflow Meter
This meter measures intake air mass. Measure sensor is a
heated wire which is maintained at 250 F (120 C) warmer than intake
air. Motronic ECU is able to calculate mass of intake air by measuring
amount of current required to maintain wire temperature. When engine
is turned off, any contaminants on wire is burned off by heating wire
to greater than 1800 F (1000 C).
Mass Airflow Sensor
This sensor uses a hot film, rather than a heated wire to
measure intake air mass. Since working temperature is high at 338 F
(170 C), and flow and temperature-sensitive resistances are mounted on
side of hot film, a burn-off function is not required.
Heated Oxygen Sensor (HO2S)
Also known as a Lambda probe, this heated oxygen sensor
generates an electrical signal proportional to air/fuel mixture.
Motronic ECU uses this information to adjust amount of injected fuel.
The turbo uses 2 oxygen sensors.
Throttle Position (TP) Sensor
The TP sensor signals Motronic or ignition and fuel injection
ECUs when throttle is fully closed or fully open.
OUTPUT SIGNALS
ECU processes information from input sensors and sends
appropriate voltage control signals to control devices.
NOTE:
For theory and operation of each output component, refer to
system indicated after component.
CHECK ENGINE Light
See CHECK ENGINE LIGHT under SELF-DIAGNOSTIC SYSTEM.
EGR Solenoid Valve
See EXHAUST GAS RECIRCULATION (EGR) under EMISSION SYSTEMS.
Fuel Injectors
See FUEL CONTROL under FUEL SYSTEM.
Fuel Pump
See FUEL DELIVERY under FUEL SYSTEM.
Idle Valve
See IDLE SPEED under FUEL SYSTEM.
Ignition Control Unit
See IGNITION SYSTEM.
Power Transistor & Ignition Coil
See ELECTRONIC IGNITION under IGNITION SYSTEM.
FUEL SYSTEM
FUEL DELIVERY
Fuel Pump
The 850 is equipped with an in-tank fuel pump. Fuel pump is
equipped with check valves to hold fuel pressure in system when
ignition is off. Fuel from main pump is sent through an in-line fuel
filter. Fuel is then sent to fuel pressure regulator where pressure is
maintained at a constant pressure in relationship to manifold
pressure. Excess fuel returns to fuel tank via a return line.
Fuel Pressure Regulator
Pressure regulator is a sealed unit which is divided by a
diaphragm into 2 chambers (fuel and spring chambers). Fuel chamber
receives fuel through inlet side from injector fuel rail. Spring
chamber is connected to intake manifold vacuum. At idle, intake
manifold vacuum is high. Diaphragm is pulled back by intake manifold
vacuum. Any excessive fuel is returned to fuel tank. As throttle is
depressed, intake manifold vacuum decreases. Regulator spring
overcomes manifold vacuum, increasing fuel pressure.
FUEL CONTROL
ECU calculates base injection pulse width by processing
signals from various engine sensors. Information from crankshaft
position sensor (RPM) is used to trigger timing of fuel injection.
During normal driving conditions, injection duration is regulated in
reference to mass air meter/sensor, engine speed, oxygen content of
exhaust gases and coolant temperature. Under full throttle conditions,
a richer fuel mixture is provided for increased power and to reduce
combustion heat in engine and catalytic converter.
Fuel Injectors
Each injector incorporates a solenoid, plunger and needle
valve which opens and closes an orifice. Control unit supplies current
through auxiliary relay for a predetermined period, opening all
injectors simultaneously to inject atomized fuel. Injection takes
place twice per revolution while starter motor is running and once per
revolution under normal driving conditions. Fuel is injected into
intake manifold close to each intake valve.
IDLE SPEED
Engine idle speed is controlled by ECU depending upon engine
operating conditions. ECU senses engine operating conditions and
determines best idle speed. Idle speed is controlled by varying air
passage inside idle valve.
Idle Valve
Idle valve uses a solenoid or motor to control by-pass air.
Signal from ECU determines idle speed by controlling amount of by-pass
air.
IGNITION SYSTEM
ELECTRONIC IGNITION
ECU computes correct timing of each ignition pulse in
response to signals from RPM sensor, Camshaft Position (CMP) sensor,
Mass Airflow (MAF) sensor, Engine Coolant Temperature (ECT) sensor,
Throttle Position (TP) sensor, Knock Sensors (KS) and Transmission
Control Module (TCM).
IGNITION TIMING CONTROL
Ignition Timing Advance Control
Ignition timing is totally controlled by an ECU. Ignition
timing is based on preprogrammed information and modified by inputs
from engine sensors.
Knock Sensor
Two knock sensors are used. Knock sensors are fitted to
cylinder block to sense detonation inside cylinders. When detonation
is detected, ECU retards ignition timing in each cylinder individually
until knocking stops. If knocking continues, ignition control unit
signals turbo control unit to reduce boost pressure in stages.
EMISSION SYSTEMS
FUEL EVAPORATIVE SYSTEM (EVAP)
Evaporative emissions system is designed to prevent fuel
vapor from entering atmosphere. Fuel system is completely sealed and
vented only through a carbon canister. System consists of
pressure/vacuum relief fuel filler cap, rollover valve, charcoal
canister, purge valve and various connecting hoses.
Fuel pressure/vacuum relief filler cap allows excessive tank
pressure to vent. It also allows air into fuel tank if vacuum should
become excessive due to a malfunction in fuel evaporation system. Fuel
tank vapor is vented by a line through rollover valve to charcoal
canister. Rollover valve is located in vent line close to fuel tank.
Valve is designed to prevent fuel spillage if vehicle rolls over.
Valve is open until vehicle is at a 45-degree angle or more from
horizontal position.
Canister Purge Valve
Charcoal canister is filled with activated charcoal. Fuel
vapor from tank is absorbed by charcoal when engine is not running.
When engine is running faster than idle, canister purge valve opens
and fuel vapor is drawn into engine and burned.
EXHAUST GAS RECIRCULATION (EGR)
EGR System
EGR system operates by returning some exhaust gases to engine
to be mixed with air/fuel mixture. This exhaust gas, which is
basically inert at this stage, lowers combustion temperature. Reducing
combustion temperature reduces amount of oxides of nitrogen (NOx)
released into atmosphere.
EGR Solenoid Valve
When engine coolant temperature is less than 115 F (45 C),
solenoid receives no current and EGR system is inactive. With engine
at operating temperature, solenoid receives current from relay and
opens vacuum line to EGR valve. EGR valve is opened completely by
negative pressure. Even the slightest throttle opening opens idle
switch. Time relay cuts current to solenoid, disconnecting EGR system
for about 5 seconds and avoiding HC and particle build-up during
acceleration from idle.
EGR Electronic Vacuum Regulator Valve (EVRV)
EGR valve is supplied with a vacuum control signal from
connection in lower section of the EVRV. Vacuum in intake manifold is
supplied to upper connection. EVRV stabilizes signal from intake
manifold and converts control module signal into a modified vacuum
signal for controlling EGR valve.
SELF-DIAGNOSTIC SYSTEM
Vehicle is equipped with self-diagnostic systems. A CHECK
ENGINE light glows to signal driver of a system malfunction. Fault
codes are retrieved through the diagnostic unit, located in right
front of engine compartment. The diagnostic unit is equipped with an
LED indicator, activation button and function select cable.
CHECK ENGINE LIGHT
NOTE:
CHECK ENGINE light is also known as Malfunction Indicator
Light (MIL).
CHECK ENGINE light is located on instrument panel. Light will
illuminate when ignition switch is turned to ON position (bulb check)
or when emission-related systems are malfunctioning during normal
engine operation.
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