Diesel Fuels - Goodheart
This sample chapter is for review purposes only. Copyright © The Goodheart-Willcox Co., Inc. All rights reserved.
Diesel Technology
Diesel Fuels
After studying this chapter, you will be able to:
K Explain diesel fuel grades.
K Discuss fuel properties and characteristics.
K Explain the reason for the use of fuel additives.
K Describe the proper procedures for handling and
storage of diesel fuel.
K Name the various alternative fuels and their
lubricating oil, and paraffin are obtained. Finally, only coke
and asphalt remain.
The type of hydrocarbons obtained will vary depending
on the original geographic location of the crude oil. This
heating of crude oil to obtain various hydrocarbons is
known as distillation. It is a highly complicated process and
precision control of pressure and temperature is required.
Figure 14-1. A fractionating column is used to separate crude oil into the various hydrocarbon products. As the vaporized products
rise in the tower, they settle onto trays at different levels and are then piped out to other equipment for further refining.
(Allis-Chalmers Engine Div.)
Diesel Fuel Grades
All engines have a few things in common. One of them
is that they all need fuel in order to operate. Most mobile and
stationary engines use fuels derived from crude oil. Diesel
engines use several variations of diesel fuel, depending on
their application. In this chapter, you will learn about the different types of diesel fuel used in diesel engines.
Hydrocarbon Fuels
The liquid fuel used to operate diesel engines is
obtained from crude oil. Crude oil consists of a mixture of
hydrocarbons (hydrogen and carbon) and compounds
such as benzene, petane, hexane, heptane, toluene,
propane, and butane. These compounds have different
relative volatility points because they will vaporize, or
flash, at different temperatures. Hydrocarbon fuels are separated in a fractionating column.
As crude oil is heated, these hydrocarbons are given
off as a vapor, Figure 14-1. After the natural gas is vaporized from the crude oil, the applied temperature is raised
and the hydrocarbon with the next highest vapor point is
obtained. The first hydrocarbon engine fuel obtained is
high octane aviation gasoline. If everything is working
properly and you continue to raise the temperature, other
hydrocarbon fuels such as commercial gasoline, kerosene,
diesel fuel, domestic heating fuel, industrial fuel oil,
While the American Society of Testing Materials
(ASTM) has divided diesel fuels into three classifications,
only two recommended grades are considered acceptable
for use in high-speed trucks and buses in North America.
These are the number 1D and number 2D classifications. Grade number 4D, the heaviest diesel fuel, is used in
large stationary constant-speed engines or in some marine
applications. Number 4 and bunker fuels are not used in
high-speed mobile diesel engines, which are continually
accelerating and changing speed. Grade number 3D was
discontinued a number of years ago and is obsolete.
The Canadian government 3-GP-6D diesel fuel has its
own fuel specifications recognizing five categories of diesel
fuels, with even more restrictive standards set by ASTM.
Each individual refiner and supplier attempts to produce diesel fuels that meet as closely as possible with
ASTM and American Petroleum Institute (API) standards,
Figure 14-2. Depending on the crude oil source, the diesel
fuel end product may be on either the high or low end of
the prescribed heat energy scale in Btus per gallon. This is
why individual diesel fuels grades may vary slightly from
one supplier to another.
Grade 1D is generally the most refined and volatile
diesel fuel available. It is a premium fuel used in high rpm
engines requiring frequent changes in load and speed. Grade
2D is more widely used in truck fleets due to its greater heat
value per gallon, particularly in warm to moderate climates.
Although Grade 1D fuel has better properties for cold303
Figure 14-2. General classification chart for diesel fuel. (Detroit Diesel)
weather operation, many fleets still use Grade 2D in the
winter. Other cold weather aids include a fuel heater/water
separator for easier starting, as well as fuel additive conditioners that can be added directly to the fuel tank.
Like gasoline, diesel fuels are blended on a seasonal
and geographical basis to satisfy anticipated temperature
conditions. It is usually best and cheapest to burn the heaviest fuel that will work under given circumstances. Heavier
grades of diesel can usually produce more energy than
light grades, just so long as the increased viscosity does not
make the fuel too thick to flow and inject properly.
It is important to remember that the wrong grade of diesel
fuel can affect the operation of the engine. If an engine is not
developing the proper horsepower, an improper grade of diesel
fuel could be the cause. An easy way to confirm this is to use a
diesel fuel quality tester such as shown in Figure 14-3.
Diesel Fuel Properties
In a diesel engine fuel system, the fuel itself performs
three functions. It supplies chemical energy to be transformed into mechanical energy, lubricates precision parts
in the fuel system components, and cools metal surfaces
operating in conditions of friction.
The properties or characteristics of diesel fuel must
meet these three if the engine is to perform with reliability.
This sample chapter is for review purposes only. Copyright © The Goodheart-Willcox Co., Inc. All rights reserved.
Chapter 14
Diesel Fuels
Diesel Technology
Flash Point
Figure 14-3. A typical diesel fuel quality tester. (Kent-Moore Tools)
Fuel processors, as well as engine manufacturers, run laboratory tests on all fuel used in diesel engines. These measured properties give a good indication of the way the fuel
will perform, however, there is no real substitute for an
actual engine test. The major diesel fuel properties
affecting engine performance are:
K Heat value.
K Specific gravity.
K Flash point.
K Volatility.
K Cetane number rating.
K Pour point.
K Cloud point.
K Viscosity.
K Carbon residue.
K Sulfur content.
K Fungus and bacterial contaminants.
K Oxidation and water.
Heat Value
The heat value of fuel is a general indication of how
heat energy is supplied to an engine and how well the
engine converts heat energy into work. The heat value can
be found by testing with a calorimeter. With this test, a premeasured amount of fuel is burned and the amount of heat
emitted is carefully measured in Btus per pound of fuel. A
British thermal unit (Btu) is the amount of heat required to
raise the temperature of one pound of water one degree
Fahrenheit. The metric equivalent of this unit is known as
a joule. To convert Btus into joules, multiply by 1054.8.
Specific Gravity
The specific gravity of fuel is a ratio of the fuel density to the density of water. It is measured using a hydrometer. Specific gravity affects the fuel’s spray penetration as
it is injected into the combustion chamber. Because water
is the standard, it has a specific gravity of one. Since oil
floats on water, a diesel fuel’s specific gravity is always less
than one. Diesel fuel’s specific gravity ranges from 0.8 to
0.94. Specific gravity is also a factor in measuring the heat
value of the fuel. In general, heavier fuels usually have a
greater heat value per gallon (Btus) than lighter fuels. Thus,
specific gravity is a good indicator of the amount of heat
(Btus) available in a given amount of fuel.
The American Petroleum Institute (API) employs
another scale to determine specific gravity. Water has a
specific gravity of 20 on the API gravity scale. Ten is the
lowest value on this scale, the reverse of the system just
described. Diesel fuels generally range from 20 to 45 on
the API gravity scale, with most ranging between 34 to 36
at 60°F (15.5°C).
A fuel’s flash point is the lowest temperature at which
it will give off flammable vapors in sufficient quantity to
flash or momentarily ignite when brought into contact with
an open flame. The flash point has no effect on engine performance or on its ignition qualities. It is specified simply
as an index of fire hazard⎯a fuel oil with an extremely low
flash point is dangerous to store and handle. Diesel fuel
flash points are not an indication of how they will ignite in
an engine cylinder, however. This depends on the ignition
quality of the fuel. For example gasoline, which has a very
low flash point, would be a very poor diesel fuel due to its
ignition quality.
Volatility is a fuel’s ability to change to a vapor. It is indicated by the air-vapor ratio that is formed at a specific temperature. Diesel fuel volatility is indicated by a 90%
distillation temperature (the temperature at which 90% of the
fuel is distilled off). As volatility decreases, carbon deposits
and engine wear increase. Depending on such factors as the
combustion chamber condition, more smoke will also affect
power output, performance, starting, and warm-up.
Cetane Number Rating
The ease of diesel fuel oil ignition and the manner in
which it burns determine the ignition quality of the fuel oil.
Diesel fuel oil is injected into the combustion chamber in
liquid form. The fuel must then be able to vaporize quickly
and ignite without a flame or spark. This ability to vaporize
and ignite easily is called ignition quality.
The ignition quality of a diesel fuel is determined by
its cetane number rating, or cetane value. The cetane
rating or value of a diesel fuel is based on the ability of the
fuel to ignite. The cetane rating of a fuel is determined by
comparing it with pure cetane, which is a test fuel, and is
identified by a cetane number, Figure 14-4. This cetane
number represents the percentage of pure cetane in a reference fuel which will exactly match the ignition quality of
the fuel being tested. The cetane rating scale ranges from 0
to 100, with 100 being the highest ignition quality.
In general, the higher a fuel’s cetane rating, the lower
the emissions. Currently, a 40 cetane or above rating is
standard for all on-highway diesel engines. (In some areas,
50 cetane and higher are current standards.) Newer diesel
engines may require higher cetane fuel. The diesel engine
service manual will specify what cetane number to use.
Improving Ignition Quality
Fuels with poor ignition qualities can be improved or
reformulated by blending them with fuels that have good
ignition properties. The cetane number of such blends are
an average of the cetane numbers of the individual fuels.
To meet the cetane number required by most on-highway
vehicles, cetane improvers are added to the blends. The
lower cetane compounds are less responsive to these
improvers than the higher cetane paraffin fuels.
Figure 14-4. Comparison of the relationship between octane
and cetane. Note that as the cetane number increases, the burn
rate increases.
ambient temperature, engine performance will be satisfactory. The most commonly available and recommended fuel
oil, grade 2D, has a cloud point or WAP of 10°F (-12°C),
while the thinner grade 1D has a cloud point of -20°F
(-28.8°C). In extremely cold areas, engine clogging can be
minimized by using only grade 1D in winter. In most
places, mixing a base of grade 2D with grade 1D, or other
special additives will work well in colder weather. As a
rule, grade 2D’s cloud point drops two degrees for every
10% of grade 1D that is added up to a ratio of 50:50.
One of the most effective chemical means for lowering the fuel’s cloud point is by blending #2 diesel fuel
with kerosene. Fuel suppliers will often do this themselves
and market it as “winter fuel.” Although this fuel will be
more viscous at lower temperatures, its performance will
still vary depending on where it was purchased. A 40/60
blend in one area may have a higher cloud point that a
40/60 blend in another. A fuel additive with a cloud point
depressant will have limited effect. At most, it will lower
the cloud point by 3-4°.
Pour Point
The improvers promote early, uniform ignition of fuel
and prevent high pressure increases in the combustion
cycle. Depending on the amount of cetane components in
the base fuel, typical alkyl nitrate additive treatment can
increase cetane by three to five numbers (1:1000 ratio). With
high natural cetane premium base fuels (containing a high
percentage of paraffins) and a 1:500 alkyl nitrate treatment
ration, cetane may increase as much as seven numbers.
Most improvers contain alkyl nitrates, which break
down readily to provide extra oxygen for combustion.
They also break down and oxidize fuel in storage.
However, they also generate organic particulates, water,
and sludge, all of which degrade fuel quality. Recently,
several new cetane improvers or reformulaters without
alkyl nitrates have been used and, properly blended, will
improve oxidation stability while providing a cetane
increase of two to five increments.
It is only recently that engineers have learned how
important the cetane level is in meeting the emissions
equation. The air-fuel mixing time in a direct-injected
diesel is about one-tenth the time of a carbureted gasoline
engine. Early ignition promotes smooth and complete
combustion, leading to reduced emissions. Excessive delay
can produce very high peak cylinder pressures which
make for rough and noisy engine operations. The higher
the cetane number, the shorter the ignition delay, which is
the time between the start of fuel injection into the cylinder
and the actual start of combustion.
Cloud Point
Diesel fuel contains paraffin, and in cold temperatures
wax crystals can start to form, accumulate, and clog engine
filters. The temperature at which this happens is referred to
as the cloud point or wax appearance point (WAP).
Generally, if the fuel’s cloud point is at least 10° below the
Pour point is another common way of measuring a
diesel fuel’s performance in cold weather. It is the minimum temperature at which fuel can flow and is expressed
as the temperature 5°F (8.8°C) above the level at which the
oil becomes solid or refuses to flow. The pour point averages about 10° lower than the cloud point. Fuel treated
with special additives called flow improvers or wax modifiers keep the wax crystals from forming clumps and
choking fuel lines. They will generally give satisfactory performance at 9° lower than untreated fuel. However, fuel
improvers do not have any impact on the fuel’s cloud
point, since they do not prevent the formation of wax or
paraffin crystals themselves.
Viscosity or stiffness is the property of a diesel fuel that
resists the force which causes the fluid to flow. It is related
closely to specific gravity and pour point. Two common
methods of measuring viscosity are the Saybolt test and the
kinematic centistokes test. Both tests involve heating the oil
to an exact temperature and measuring its flow rate
through a standard sized orifice.
The viscosity of diesel fuel is generally specified at
100°F (38°C). Viscosity of fuels for medium-speed and highspeed engines normally ranges from 2.4 to 4.1 centistokes
(cSt), or about 39 seconds Saybolt Universal (SSU). In general, any fuel with a viscosity lower that 2.4 cSt or 34 SSU,
when measured at 100°F (38°C), will be too thin and could
damage injectors as well as other parts of the fuel system.
Diesel fuel also has a low viscosity index, meaning that
it is thin when hot but gets thick when cold. A diesel fuel that
can go through an injection system easily in warm weather
may get too thick to flow properly in cold weather. As mentioned previously, diesel fuel is supplied in several grades.
Grade 1D is a winterized diesel fuel. It is thinner or less
This sample chapter is for review purposes only. Copyright © The Goodheart-Willcox Co., Inc. All rights reserved.
Chapter 14
Diesel Fuels
viscous than grade 2D, which is the fuel used at normal operating temperatures. Diesel fuel viscosity also affects the spray
pattern in the combustion chamber, Figure 14-5. Low viscosity creates a fine mist, while high viscosity results in
coarse or heavy atomization. In other words, viscosity affects:
K Lubrication capability at various temperatures.
K Atomization, or spray capability.
K Ignition and burning characteristics.
Therefore, viscosity is a very important consideration
when selecting a diesel fuel.
Carbon Residue
Ash or carbon residue is the deposit left in the combustion chamber due to incomplete combustion or the use
of fuels made from residual blends. It can be measured in
the laboratory by heating a measured fuel sample in a
closed container in the absence of air. Carbon residue is
the final product that remains in the container after
heating. This product is then expressed as a percentage by
weight of the original sample.
The amount of carbon residue considered acceptable in
diesel fuel oil varies depending on the combustion chamber
design, the injector adjustments, and the general condition of
the engine. This amount is generally more critical in small high
speed engines than in large slow speed industrial engines.
Soot ash, which originates from either additives or the
crude oil itself, also causes wear of fuel injection components, the pistons, and the piston rings. Standard requirements allow for a maximum of .001% soot ash content.
Due to air quality concerns, future legislation may further
limit the amount of soot ash a diesel engine can produce.
Sulfur Content
The wear of pistons, rings, and cylinders in a diesel
engine generally increases when there is an excessive
amount of sulfur in the fuel. Excess sulfur content also
causes varnish to form on the piston skirt and creates oil
sludge in the engine crankcase. Sulfur also combines with
water to form corrosives as a result of the combustion
process. These corrosives can etch finished surfaces, accelerate engine wear, attack softer metals (such as bearings),
and deteriorate engine oil. Similar corrosion damage from
sulfur is frequently found in the engine’s exhaust system.
Sulfur and aromatic content of diesel fuel are most responsible for harmful exhaust emissions. Sulfur dioxide emissions
are reduced when sulfur content is reduced. The reduction of
aromatics will reduce carbonaceous particle emissions.
Sulfur is removed from diesel fuel in a process called
hydrotreating. Hydrogen is used with a catalyst at temperatures between 500 and 800°F (260 and 430°C) to react
with the sulfur compounds present. The reaction forms
hydrogen sulfide, which is separated from the hydrocarbon
and sent to a sulfur plant to be converted to elemental
sulfur. The sulfur content can only be determined by chemical analysis of the fuel.
EPA regulations mandate the sulfur content in diesel
fuel. The 2004 standards called for a sulfur content of no
greater than 500 parts per million (.05% by weight) and a
maximum aromatic content of 35% in all on-highway
diesel fuels. EPA regulations for 2007 on-highway engines
require the use of ultra-low sulfur diesel fuel (ULSD). ULSD
has a dramatically lower sulfur content than previous onhighway grade diesel fuels. The sulfur content of ULSD
cannot exceed 15 parts per million (.0015% by weight).
Using ultra-low sulfur diesel dramatically reduces
diesel exhaust emissions. The lower sulfur content of ULSD
produces fewer sulfate emissions. It also enables use of
emissions reduction equipment, such as particulate traps
and catalytic converters, that lowers emissions of particles
and nitrogen oxides (NOx). When these systems are used
with ULSD, emissions of fine particulates can be reduced
by more than 90% and emissions of hydrocarbons can be
reduced to nearly undetectable levels.
Even without special emission reduction equipment,
using ULSD reduces sulfate pollutants. ULSD fuel provides
significant clean air benefits while ensuring the same energy
and performance standards as regular highway diesel. Ultralow sulfur, low aromatic, and high cetane number diesel
fuels enhance engine performance because improved engine
combustion results in easier starting, smoother running, less
noise, and less smoking during start-up.
Only off-highway, non-road diesel engines can currently use higher sulfur content fuels. These include farm
tractors, boats, locomotives, and stationary engines. The
2007 sulfur content standard for non-road, locomotive,
and marine fuel (NRLM) is 500 parts per million (ppm). In
2010, all on- and off-highway non-road (NR) diesel fuel
will need to meet a limit of 15 ppm sulfur, and in 2012,
locomotive and marine (LM) diesel fuel will also be regulated at 15 ppm.
Fungus and Bacterial Contaminants
Figure 14-5. Proper fuel viscosity results in a good spray pattern.
However, improper viscosity will give a poor spray pattern.
Fungus and bacteria are the most common contaminants found in diesel fuels. Fungus and bacteria live in the
Diesel Technology
water and feed on the hydrocarbons found in diesel fuel.
These contaminants are called hydrocarbon utilizing
microorganisms (HUM). HUM will spread through a fuel
system where moisture or trace amounts of water are present. The resulting bacterial problems can shorten engine
filter life. Draining the fuel system will reduce HUM activity,
but will not eliminate it. The only way to eliminate HUM
growth entirely is to treat the fuel system with a biocide.
parts; and others that are oxidation inhibitors. In addition,
remember that geographical locations, operating conditions, type of fuel storage, handling methods, and maintenance procedures are all factors that can determine
whether or not a fuel additive might help a diesel fuel. In
most cases, the fuel supplier or marketer will select and
add the additive they believe will best improve the quality
of their fuels. Fuels containing special additives may be
more expensive than those without additives.
Oxidation and Water
Precipitants and particulates are noncombustible
materials formed when either two incompatible fuels are
mixed or when a fuel oxidizes. Oxidation can occur when
an unstable fuel circulates through a diesel engine.
Precipitants and particulates will normally settle out in low
or slow flow areas of the fuel system or become part of the
tank’s bottom sludge. Both can get mixed into the fuel
system during fuel delivery and can plug fuel filters.
Water causes the greatest concern because it is the
most common form of diesel fuel contaminant. Water is
found in the fuel system in two forms, free and dissolved.
Most diesel fuels contain some dissolved moisture. Diesel
fuel has a saturation level of water at any given temperature. As the temperature goes down, the fuel will hold less
dissolved moisture. There is currently no common method
of removing dissolved water from diesel fuel.
Free or nondissolved water in the fuel tank usually
comes from bulk storage tanks, from condensation, or from
dissolved water. Free water mixes with the fuel as storage
tank bottoms become agitated while dispersing or
receiving fuel. When water in vapor form is present in air,
it is called humidity. As air replaces fuel in storage tanks or
vehicle tanks when the tanks disperse or receive fuel, moisture in the form of humidity finds its way into the fuel
system. From there, moisture may condense as hot fuel
returning from the injectors flows back into the cooler fuel
tank. Free water in liquid form is heavier than diesel fuel
and settles on the low flow or slow flow areas of the fuel
system. The reverse is true when water freezes. Ice is
lighter than diesel fuel and floats in the system to create
plugs in fuel separators, filters, even going as far as the fuel
pump injectors.
Fuel Additives
Generally, no fuel additives are necessary when a
good quality, clean, and properly selected fuel is used.
Certain fuel characteristics, however, can be improved by
treatment with a fuel additive or conditioner. As already
mentioned, additives are used to improve fuel flow properties under cold or winter-like conditions, increase the
cetane number of the fuel, and to introduce a biocide to
the fuel to prevent fungi and bacteria from growing in the
diesel fuel.
There are a number of commercially available fuel
additives that will reduce smoke and corrosion of vital
Fuel Handling and Storage
The importance of clean fuel in the operation of a
diesel engine cannot be overemphasized. Diesel fuel is
generally delivered clean and free of impurities. However,
every time it is transferred or handled, the risk of contamination increases. While fuel storage and its handling are
not normally within the engine technician’s job classification, the results of improper storage and handling certainly
are. A problem often found by the engine technician when
troubleshooting a diesel engine is dirty fuel.
One of the major problems either in storage or in the
engine’s fuel system is leakage. Each connection or fitting
in either system is a potential source of leakage. There are
two phases to the leakage problem:
K The visible leakage of fuel from a line during
operation and shutdown.
K The opening in a suction line or fitting which
pulls foreign material into the system during
operation and may or may not present a visible
leak during shut-down.
Visible leakage should be stopped as soon as it is discovered. Loss of flow volume generally means a loss of
pressure. The leak which allows fuel contamination by
pulling in air, water, or dirt is more difficult to locate and
can be more damaging. Foreign materials, such as air, sediment (dirt), and water present problems and can enter the
fuel system in various ways.
Air drawn into the fuel system may make pick-up of
fuel much more difficult or prevent it entirely. In addition,
it may show up as:
K Low power.
K Gear pump wear.
K Rough operation.
K Soft or non-responsive throttle.
Air enters the fuel in any one or more of the following
K Suction leaks (lines, fittings, filters, or tank).
K Fuel turbulence, especially when coupled with
poor tank venting.
K Fuel pick-up point near return.
K Combustion gases entering injector.
This sample chapter is for review purposes only. Copyright © The Goodheart-Willcox Co., Inc. All rights reserved.
Chapter 14
Diesel Fuels
Dirt is as damaging to moving parts in the fuel system
as it is in any other part of the engine. Dirt can:
K Clog filters.
K Abrade metal surfaces.
K Increase combustion chamber deposits.
Dirt enters the fuel when:
K A suction leak is in an exposed area.
K Dirt collection in the tank allows it to be picked
up with the fuel.
K Careless filling and handling permits mud, dust,
or grime to enter tank or lines.
The technician may unintentionally add dirt to the
fuel system by:
K Using containers and tools exposed to dirt.
K Using lines and fittings that have accumulated
dirt or dust without cleaning them out.
K Allowing dirt to enter a line while it is removed
for repair. This is especially critical between filter
and pump.
Water in the fuel may show up as low power, also, but
it is even more damaging as already stated when it simply
contributes to corrosion on fuel system components. Water
enters the fuel system through:
K A suction leak in an exposed location.
K As condensation due to warm fuel.
K Careless storage and handling.
Careless storage and handling of diesel fuels can be a
major concern in the engine’s operation, Figure 14-6. It is
important to periodically perform inspection, cleaning and
maintenance checks on all fuel handling and filtering
equipment. Many state and local regulations include
record keeping and periodic inspections.
Diesel Technology
Storage Tanks
Diesel fuel tanks may be underground storage tanks
(UST), or above-ground storage tanks (AGST) systems.
Fuel stored in AGSTs will deteriorate more quickly than the
fuel stored in USTs due to sunlight exposure and the greater
temperatures found above ground.
Tanks are made of steel, fiberglass, or steel clad with
fiberglass. Galvanized copper tanks are not used to store
diesel fuel. There is a chemical reaction between the fuel
and copper that produces a residue which will quickly clog
fuel filters. The term “tank systems” includes the piping
connected to the tank(s).
In some large industrial operations, diesel fuel is fed from
the main supply tank located (above or in the ground) outside
of the building to a one day tank in the engine room,
Figure 14-7. This tank has a sufficient capacity to provide fuel
for one day of operation. The main storage normally stores
only a one week supply of fuel. Fuel stored for longer periods
of time can form gum which, along with corrosive compounds, causes deterioration. Many refineries are also using
additives that help stabilize the fuel in order to reduce gum formation. Moisture condensation inside a fuel storage tank
increases as the tank size increases. Condensation in a large
tank can be reduced by using a series of smaller tanks and connecting them into the line as needed. The size of tanks is generally controlled by the Bureau of Fire Underwriters and local
Alternative Fuels
The Clean Air Act of 1990 requires that all centrally
fueled on-highway transportation operations of 10 or more
vehicles in certain high-emission areas are to phase in
alternative fuel vehicles beginning in 1998. The act does
not mandate any specific alternative fuels such as gasoline
Figure 14-7. Schematic of a stationary fuel system with day tank and storage tank. (Waukesha Engine Div.)
or diesel. The alternative fuels now being considered for
use with compressed (diesel) type engines are compressed
natural gas (CNG), liquefied petroleum gas (LPG),
liquefied natural gas (LNG), and alcohol based products
such as methanol and ethanol.
The Clean Air Act regulations are closely tied to clean
fuel vehicle programs in each state’s air quality implementation plan. Since the supply of certain fuels varies across
the country, not all states favor the same fuel. Southern
states are a big supporter of natural gas, while some
western states favor liquid petroleum gas, and a number of
midwestern states actively tout ethanol.
Some engine companies are devoting research and
testing to natural gas. Many of their initial products are
being offered for urban buses, which are on a stricter emissions schedule than medium and heavy duty trucks. Engine
makers face the challenge of designing new engines with
efficiency as close as possible to that of a diesel without
making major component changes, Figure 14-8.
On-board fuel tanks for the storage of CNG while the
vehicle is on the road is a problem that needs to be
addressed. For example, experimental taxi cab operations,
using two or three tanks placed in the trunk, had a range of
only 120-160 miles (192-256 km).
Liquefied petroleum gas (LPG) is mainly propane and
butane, along with small amounts of other gases. It has
combustion qualities equal to or better than high grade
diesel. LPG is a vapor or gas at normal room temperature
and atmospheric pressure. This presents some design problems. In fact, since LPG is not a liquid, the entire fuel
system must be redesigned to handle it, Figure 14-9.
Compressed and Liquefied Natural Gas
Figure 14-6. Care must be taken whenever filling a storage tank. A⎯Below ground tanks can present as much danger as aboveground tanks. B⎯Always take care when filling any vehicle’s tank. (Heavy Duty Trucking and Andrew Ryder)
Natural gas has been used for 60 years to power industrial
internal combustion engines. Natural gas is one of the lightest
fractions (parts) of crude oil. Chemically, it is very similar to
gasoline. Unlike gasoline, natural gas contains several light
gases, heavy gases, and other impurities. For natural gas to be
used as an alternative fuel for on-highway vehicles, it must be
refined into either compressed natural gas or liquefied.
Compressed natural gas (CNG) is composed primarily
of methane and hydrocarbons that have a high carbon-to
hydrogen ratio. Hydrogen is an ideal fuel that burns well
while producing little pollution. It is the main reason CNG
is a very desirable fuel. It produces good power, economy,
and low exhaust pollution levels.
Figure 14-8. Natural gas engines are used in place of basic
diesel engines in some mobile and stationary applications.
(Diesel & Gas Turbine Publications)
This sample chapter is for review purposes only. Copyright © The Goodheart-Willcox Co., Inc. All rights reserved.
Chapter 14
Diesel Fuels
Diesel Technology
Figure 14-9. Simple schematic for a liquefied petroleum gas (LPG) fuel system.
Figure 14-11. Schematic showing typical CNG tank arrangement under a bus. (Diesel & Gas Turbine Publications)
LPG has operating characteristics almost identical to
those of gasoline and diesel. However, because it is
already a gas, the problem of breaking up liquid fuel is
eliminated. Since the fuel enters the intake manifold and
combustion chamber as a vapor, combustion is much more
efficient. For this reason, LPG is generally an excellent
alternative fuel. Liquefied natural gas (LNG) has the same
characteristics as LPG.
A heavy-duty truck would need too many fuel storage
tanks to make it a feasible concept at this time.
Conversions on city buses, however, are presently being
done in several big cities, since fuel tank storage is not as
great a problem, Figure 14-10. In most cases there is adequate clearance underneath the bus, Figure 14-11, to allow
for several tanks. In addition, most city buses do not accumulate as many miles in a day as would a long-haul
tractor/trailer. Buses can also be fueled up locally at their
gaining support in the transportation industry. At present, the
most popular is a biodiesel blend of number 1 diesel fuel and
soybean oils. There is also especially strong interest in
biodiesel from the agricultural community because used
cooking oils in addition to soybean and processed animal
fats can be used in the making of biodiesel fuels.
Industry interest is high in biodiesel since it is not a
stand-alone fuel like the other alternative fuels, but rather a
blend with existing diesel fuels. This means little or no alterations to the fuel injection system are necessary, nor is any
special training of service personnel needed. The two most
popular blends today are 20-30% soybean oil to
80-70% diesel oil. In physical properties, biodiesel has
almost the same btu/gallon ratio as number 1 diesel. Further,
it has slightly better lubricating qualities than number 1
diesel, can handle winterizers and has a gel point slightly
depot by a fast-charge facility, which takes about the same
length of time as it would to fuel up a diesel-powered bus.
Alcohol-Based Fuels
Alcohol-based fuels are being studied as an alternative
fuel for the transportation industry. The two types used to
power internal combustion engines are ethyl alcohol (ethanol)
and methyl alcohol (methanol). Alcohols are especially desirable as an automotive fuel because they can be manufactured
from sources other than crude oil. Alcohol intended as an
automotive fuel must be almost pure. Quite often, several
refining steps are needed to approach this purity.
Denatured ethanol or “grain alcohol” is colorless,
harsh tasting, and highly flammable. It can be made from
numerous farm crops such as wheat, corn, sugar cane,
potatoes, fruit, oats, soy beans, or any material rich in carbohydrates. Crop wastes are also a source. One of the
major drawbacks of using ethanol, clean burning as it is, is
that it does emit carbon dioxide (CO2).
Methanol or wood alcohol can be made from wood
chips, coal, oil shale, tar sands, cornstalks, garbage, or
even manure. Like ethanol, methyl alcohol is a colorless,
odorous, flammable liquid. Methanol is currently being
used in several urban diesel powered bus fleets with some
success. It is one of the major contenders as a feasible alternative fuel for heavy-duty high-speed diesel engines.
higher than number 2. It has very low toxicity if ingested and
is actually biodegradable by some standards.
The main drawback to biodiesel is its relative high
cost in comparison to regular diesel fuel. Biodiesel is now
being tested in some city buses and is seeing increased
popularity for use in marine diesel engines. In the near
future, biodiesel fuels may play a major role in the future
in reducing engine exhaust emissions, Figure 14-12.
More money and time must be spent on alternative
fuel engine development, on-board fuel storage, and
fueling facilities before they can become a reality. At the
present, diesel fueled engines are still cheaper and more
efficient than any of the proposed alternatives. With the
federal mandate of low-sulfur diesel fuel, as well as reformulation procedures, diesel engines will be performing
well into the 21st century.
Biodiesel Fuels
Figure 14-10. Buses in some major cities are now able to
operate on compressed natural gas (CNG). (Pierce Transit, Inc.)
Biodiesel is a comparative latecomer to the alternative
fuels scene, but as on-highway, off-highway, stationary and
even marine engine manufacturers continue to investigate
any and all alternative sources of fuel, biodiesel is quickly
Figure 14-12. In the future, biodiesel fuels can play a role in reducing exhaust emissions. (National Soy Diesel Development Board)
This sample chapter is for review purposes only. Copyright © The Goodheart-Willcox Co., Inc. All rights reserved.
Chapter 14
Diesel Fuels
Only two recommended grades of diesel fuel are considered acceptable for use in high-speed trucks and buses
in North America. These are the number 1D and number
2D classifications. Grade 1D is generally the most refined
and volatile diesel fuel available. It is a premium quality
fuel used in high rpm engines requiring frequent changes
in load and speed. Grade 2D is more widely used in truck
fleets due to its greater heat value per gallon, particularly
in warm to moderate climates.
Diesel fuels are blended on a seasonal and geographical basis to satisfy anticipated temperature conditions. In a diesel engine fuel system, the fuel-supplied
chemical energy is transformed into mechanical energy.
Diesel fuel also cools and lubricates precision parts in the
fuel system.
The heat value of fuel is a general indication of how
heat energy is supplied to an engine, and thus, how well
the engine converts heat energy into work. Specific gravity
is a ratio of the density of fuel to the density of water.
The cetane rating or value of a diesel fuel is based on
the ability of the fuel to ignite. There is currently no
common method of removing dissolved water from diesel
fuel. Free or nondissolved water in the fuel tank usually
comes from bulk storage tanks, from condensation, or from
dissolved water. Free water gets into the fuel as storage
tank bottoms become agitated while dispersing or
receiving fuel.
Fuel additives are commonly used to improve diesel
fuel flow properties under cold or winter-like conditions.
Additives can also help eliminate fuel contaminants.
The importance of clean fuel in the operation of a
diesel engine cannot be overemphasized. Foreign materials such as air, sediment (dirt), and water presents various
problems and can enter the fuel system in various ways
such as suction leaks, careless filling and handling, and
dirty fittings, and lines. It is important to periodically perform inspection, cleaning, and maintenance checks on all
fuel handling and filtering equipment.
The method of storing the fuel depends on the size of
storage tanks or drums, the frequency and method of fuel
delivery, climatic conditions, and local regulations. Fuel
stored above ground will deteriorate more quickly than
fuel stored underground due to the greater temperature
ranges found above ground. Underground storage tanks as
well as above ground storage tank systems must meet rigid
safety and environmental regulations.
Storage tanks should be made of either steel, fiberglass, or steel clad with fiberglass. Galvanized and copper
tanks should not be used to store diesel fuel. The most
promising alternative fuels are compressed natural gas,
liquefied petroleum gas, liquefied natural gas, methanol
and ethanol.
Important Terms
Relative volatility
Heat valve
British thermal unit (BTU)
Foreign materials
Specific gravity
Flash point
Underground storage tank
Above-ground storage
tank (AGST)
Alternative fuel vehicles
Ignition quality
Cetane number rating
Ignition delay
Pour point
Compressed natural gas
Liquefied petroleum gas
Carbon residue
Soot ash
Liquefied natural gas (LNG)
Ethyl alcohol
Methyl alcohol
Cloud point
Review Questions—Chapter 14
Do not write in this text. Place your answers on a separate sheet of paper.
1. Which of the following grades of diesel fuel are recommended for use in high-speed trucks and buses?
(A) number 1D.
(B) number 2D.
(C) number 4D.
(D) Both A & B.
2. Like gasoline, diesel fuels are blended on a ______
and ______ basis to satisfy anticipated temperature
3. Heavier grades of diesel fuel usually produce ______
energy than lighter grades.
4. The value(s) used to measure the amount of heat
emitted is called:
(A) British thermal unit (Btu).
(B) joule.
(C) flash point.
(D) Both A & B.
Diesel Technology
5. Volatility of diesel fuel is indicated when ______ or
the fuel is distilled off.
(A) 75%
(B) 90%
(C) 50%
(D) 100%
6. Sulfur is removed from diesel fuel by:
(A) distillation.
(B) hydrotreating.
(C) filtering.
(D) None of the above.
7. Which of the following types of tanks should not be
used to store diesel fuel?
(A) Steel.
(B) Fiberglass.
(C) Copper.
(D) Steel clad with fiberglass.
8. Fuel stored above ground will deteriorate ______
than fuel stored underground due to the greater temperature ranges found above ground.
9. Name three ways air commonly enters diesel fuel.
10. True or False? Flow improvers help redissolve paraffin
crystals that have formed in the diesel fuel.
ASE-Type Questions
1. Technician A says that grade 1D diesel fuel is generally the most refined and volatile diesel fuel available.
Technician B says that grade 2D is more widely used
in truck fleets due to its greater heat value per gallon,
particularly in warm to moderate climates. Who is
(A) A only.
(B) B only.
(C) Both A & B.
(D) Neither A nor B.
2. The _____ of fuel is a general indication of how heat
energy is supplied to an engine and how well the
engine converts heat energy into work.
(A) heat value
(B) viscosity
(C) flash point
(D) cetane number rating
3. All of the following are contaminants of diesel fuel
(A) water.
(B) dirt.
(C) cetane.
(D) air.
4. All of the following are diesel fuel properties EXCEPT:
(A) heat value.
(B) pour point.
(C) specific gravity.
(D) octane rating.
5. The temperature at which wax crystals begin to form
in diesel fuel is known as the ______.
(A) cloud point
(B) pour point
(C) viscosity point
(D) specific gravity level
6. The viscosity of diesel fuel affects all of the following
(A) the lubrication capability of the fuel at different
(B) the cloud point.
(C) the burn capability.
(D) the atomization capability.
7. The
_____ of diesel fuel is based on the ability of the
to ignite.
specific gravity
cetane number
8. Technician A says that fungus and bacterial growth in
diesel fuel can be eliminated by draining settled water
out of the fuel tanks at regular intervals. Technician B
says that the only way to eliminate bacterial growth
entirely is to treat the fuel system with a biocide. Who
is right?
(A) A only.
(B) B only.
(C) Both A & B.
(D) Neither A nor B.
9. Diesel fuel tanks can be made from ______.
(A) galvanized copper
(B) aluminum
(C) steel clad with neoprene
(D) fiberglass
10. Technician A says that as the temperature of diesel
fuel goes down it will hold less dissolved water.
Technician B says that there is no common method
now available for removing dissolved water from
diesel fuel. Who is right?
(A) A only.
(B) B only.
(C) Both A & B.
(D) Neither A nor B.
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