Fuels, fluids and lubricants 577 Crude oil and its products 578 Petrol 578 Gas fuels 580 Automotive diesel fuels 581 Alternative fuels 582 Lubricating oil 582 Grease 585 Transmission oils 586 Hydraulic brake fluids 587 Technical terms 588 Review questions 588
Trang 15 PART
32 Fuels, fluids and lubricants
33 Service and maintenance
34 Basic mechanics and diagnosis
Trang 3Fuels, fluids and lubricants
Chapter 32
Crude oil and its products
Petrol
Gas fuels
Automotive diesel fuels
Alternative fuels
Lubricating oil
Grease
Transmission oils
Hydraulic brake fluids
Technical terms
Review questions
Trang 4Automotive fuels include petrol, diesel fuel, LPG and
natural gas Petrol is used in the engines of most
passenger cars and light commercial vehicles, diesel
fuel is used in diesel engines, gas is used in petrol
engines and some diesel engines that have been
adapted to run on gas.
Along with many other products, these fuels are
produced from refined crude oil, with the exception of
natural gas, which is recovered directly from the
ground.
Crude oil and its products
The crude oil from which fuels are produced is known
as mineral oil because it comes from the ground Oil
was formed millions of years ago at the bottom of the
sea which then covered most of what is now dry land.
The oil is believed to have originated from the bodies
of marine life, both animals and plants, which were
coated with silt and eventually converted to oil.
Wells are drilled to tap the oil and bring it to the
surface The crude oil is then transported to a refinery
where it is converted into petrol, oil and many other
products The crude oil varies from yellow, through
green to black, and must be refined before it can be
used.
Products from crude oil
Crude oil undergoes a number of processes in the
refinery One of the first steps is called fractionation or
fractional distillation, in which the crude oil is broken
down into its various fractions The fractions are
determined by the temperature at which the various
particles of oil distil.
The crude oil is heated at the bottom of a tall
hollow tower and turns to a vapour As the vapour
rises, it cools and distils to a liquid At various levels
in the tower, fractions are collected The lighter ones
distil towards the top of the tower and the denser ones
distil closer to the bottom In this way, a variety of
petroleum products are produced (Figure 32.1).
Petroleum products
Briefly, the main petroleum products and their uses are
as follows:
1 Gas This is the lightest fraction It is used as a
domestic and industrial fuel and is also processed
into liquefied petroleum gas (LPG).
2 Solvents These are used in the paint and chemical
industries.
3 Motor spirit (petrol) A large proportion of the crude oil is processed as fuel for petrol engines.
4 Kerosene This is the basis of fuel for jet engines Kerosene is also used for heating and for cleaning.
5 Diesel fuels Lighter fuels are used in automotive diesel engines Heavier diesel fuels are used in some larger diesels.
6 Furnace oil Contains the densest fractions of the crude oil and is used for heating.
Of these products, petrol, gas and diesel fuel (distillate) are the ones used as automotive fuels.
Petrol
Petrol is a hydrocarbon It is made up of various compounds that contain hydrogen and carbon atoms When the air–fuel mixture burns in the combustion chamber of the engine, the hydrogen and carbon atoms unite with the oxygen atoms contained in the air This generates a large amount of heat.
■ Petrol is not a simple fuel It is a blend of various lighter fractions of the crude oil to which chemicals and dyes have been added.
Volatility of petrol
Volatility is the ease with which a liquid turns to a vapour The volatility of a simple compound like water can be found by increasing its temperature until it boils (vaporises).
figure 32.1 Products from distillation of crude oil
Trang 5chapter thirty-two fuels, fluids and lubricants 579
Petrol is not as simple as this It is blended from a
number of different hydrocarbon compounds, each
having a different volatility, or boiling point Petrol
needs to vaporise easily for easy starting, but not so
easily that it will affect the performance of the engine
or cause vapour lock.
For cold climates, the fuel is blended with a greater
percentage of highly volatile fractions than the fuel for
hot climates Winter blends are often used, even in a
mild climate, to provide easy starting on cold mornings.
■ Volatile fractions are those that vaporise easily at
low temperatures.
Knocking
During normal combustion in the engine, the pressure
in the combustion chamber increases evenly as the
air–fuel mixture burns If the fuel burns too rapidly,
there will be a sudden increase in pressure and this will
cause a ‘knock’ When knocking, energy in the fuel is
wasted because it does not burn properly Also,
prolonged knocking will damage the engine.
Cause of knocking
During combustion, the spark at the spark plug starts
the burning process A wall of flame spreads rapidly in
all directions from the spark and travels through the
compressed air–fuel mixture until the charge is burnt.
This is called flame propagation.
1 Figure 32.2(a) shows the normal movement of the
flame wall through the combustion chamber.
During combustion, the pressure increases rapidly,
but steadily.
2 Figure 32.2(b) shows the effect if the flame travels
too fast There will be excessive pressure built up
towards the end of combustion This will cause the
last part of the charge to explode suddenly instead
of being burnt The explosion is referred to as
detonation and this causes knocking that can
actually be heard The sudden shock due to
detona-tion can damage engine parts if knocking is severe.
Knock sensors
Knock sensors are fitted to engines with electronic fuel
injection as part of the engine management system.
These detect knocking that is not evident to the driver.
When knocking occurs, the engine management
system makes adjustments to the ignition and fuel
systems to stop the knock This causes a loss of engine
power, but prevents harmful knocking.
Octane rating of petrol
Fuels are tested to determine how they will perform in
an engine and they are given a research octane number (RON) to denote their resistance to knocking A high-octane fuel resists knocking, while a low-high-octane fuel knocks easily.
The basic method of testing the octane rating of petrol is in laboratory tests and is known as the research method This uses a special single-cylinder test engine and a reference fuel.
The reference fuel is a mixture of iso-octane and heptane, and the proportion of iso-octane in the fuel is its octane rating For example, with 90% iso-octane and 10% heptane, the reference fuel would have an octane number of 90.
The test engine has an adjustable cylinder head that enables the compression ratio of the engine to be altered The fuel being tested is used to run the engine and the compression ratio is gradually increased until
figure 32.2 Combustion
(a) normal combustion with the mixture
burn-ing evenly (b) the last part of the charge tends to explode and
produce detonation
(a) Normal combustion
(b) Combustion and detonation
Trang 6knocking occurs Then, without changing the
compres-sion ratio, the engine is switched to the reference fuel.
The proportions of iso-octane and heptane in the
reference fuel are adjusted until knocking reaches the
same intensity as it did for the fuel being tested This
gives an octane number for the reference fuel, which
will be the same for the fuel being tested.
The two laboratory reference fuels, iso-octane and
heptane, are used only for testing purposes Iso-octane is
very resistant to knocking and so has been given an
octane number of 100 Heptane, the other reference fuel,
is a poor fuel which knocks very easily This has been
given an octane number of zero A mixture of the two
fuels can be used to produce a fuel of any octane number.
■ The RON (research octane number) is used for
petrol sold in service stations in Australia.
Grades of petrol
Petrol is available in a minimum of two grades – ULP
(unleaded petrol) and a premium ULP Many fuel
companies offer higher grades at the pump to suit
higher performance vehicles The grades are:
1 ULP – 91 RON
2 premium – 95–98 RON
3 premium extra – 100 RON.
Petrol that was marketed as super or leaded petrol
contained a small quantity of tetraethyl lead that was
used to give it a high octane number Lead produces
harmful emissions from the engine’s exhaust and so
emission-control regulations required all new petrol
engines produced after 1986 to operate on unleaded
petrol.
As well as reducing harmful emissions, unleaded
petrol enabled catalytic converters to be used in exhaust
systems and these are fitted to all new vehicles.
Catalytic converters remove other emissions, but cannot
be used with leaded fuel because the lead poisons the
catalyst in the converter and makes it ineffective.
■ Pre-1986 vehicles may need an upper cylinder
lubricant when re-fuelling to prevent engine valve
damage.
Petrol additives
Certain additives are used in petrol These can include:
1 Oxidation inhibitors – to help prevent the formation
of gum while the petrol is in storage.
2 Antirust agents – to protect the vehicle fuel system.
3 Detergents – to keep the fuel injectors clean.
4 Phosphorus compounds – to combat spark plug fouling.
5 Dye – for identification.
6 Octane improvers – to increase the octane rating.
In addition to chemicals that are added, the refining process is controlled to keep sulphur compounds and gum-forming substances to a minimum Excess sulphur compounds form sulphuric acids, which can damage metal parts and bearings Gum-forming substances could cause deposits in the fuel system and
on valves, pistons and rings.
Combustion of fuel
During combustion of the air–petrol mixture, water (H 2 O) and carbon dioxide (CO 2 ) are produced These are harmless and are carried from the exhaust However, all the fuel charge does not completely burn during combustion, and as a result, carbon monoxide (CO) is also formed This is a dangerous and poisonous gas.
Carbon monoxide has no colour, is tasteless and has practically no odour It takes only fifteen parts of carbon monoxide in 10 000 parts of air to make the air dangerous to breathe Higher concentrations can cause paralysis and death For this reason, an engine should never be operated in a closed space without some means of carrying the exhaust gas into the outside air Another effect of incomplete combustion is the presence of unburnt fuel hydrocarbons in the exhaust gas These, as well as carbon monoxide, cause pollution Emission control regulations are designed to limit the quantity of harmful emissions from the vehicle’s exhaust.
■ Enough carbon monoxide can be produced in a few minutes in a closed one-car garage to cause death Never run an engine with the garage doors closed.
Gas fuels
The two gas fuels that are used for motor vehicles are liquefied petroleum gas (LPG) and natural gas for vehicles (NGV) Both gases are hydrocarbons, as is petrol, but LPG and NGV have different chemical compositions.
LPG
LPG is obtained during the refining process of crude oil It consists mainly of propane and butane LPG is
Trang 7chapter thirty-two fuels, fluids and lubricants 581
normally a gas but it changes its state when it is
compressed and becomes liquid It is stored in liquid
form at a pressure of approximately 750 kPa.
When liquid LPG is allowed to expand, it increases
in volume and becomes a vapour The vapour will
occupy a space 270 times larger than it did as a liquid.
Pure LPG is colourless, odourless and tasteless, so
a chemical is added to give it a pungent odour This
enables the presence of LPG to be identified by smell
if there is a gas leak Low concentrations of LPG in the
air, even less than 0.5% (by volume) can be detected
by its odour.
■ LPG has a density close to twice the density of air,
so that leaking gas will tend to flow downward and
follow the ground It could concentrate in low
areas, such as work pits and drains.
Octane number of LPG
The octane requirements of LPG are defined by the
motor octane number (MON) method of rating fuels.
This gives octane numbers that are different to RON.
Automotive LPG has a MON of 92 and this can be
compared with ULP which has a MON of 82.
NGV
Natural gas for vehicles (NGV) is a natural gas that
consists mainly of methane Like oil, it comes from the
ground, where it was formed by decomposing
vegetable and other matter It undergoes a refining
process, but not to the extent of crude oil Some natural
gas is also obtained during the processing of crude oil.
NGV is compressed into a high-pressure cylinder,
but it will not change into a liquid unless it is also
reduced to an extremely low temperature This is not
practicable, so NGV is compressed to around 20 MPa
and stored as a gas in a heavy steel cylinder The
cylinder is drawn from a single billet of high alloy steel.
A cylinder of the type shown in Figure 32.3 has a
diameter of approximately 300 mm and is less than a
metre long It has a wall thickness of approximately
8 mm and weighs approximately 50 kg It is fitted with
a manual service valve at the top so that the cylinder can be closed off.
Automotive diesel fuels
Diesel fuels include some of the denser parts of refined crude oil Some of the less dense fractions in the fuel give ease of atomisation and vaporisation when sprayed into the combustion chamber Other fractions, being less volatile, give the fuel viscosity as well as providing power and economy.
The fuel must have a certain viscosity, so that a film will adhere to the injection pump and injector parts and act as a lubricant, but a low viscosity is necessary to allow it to be easily sprayed by the injectors.
Properties of diesel fuels
The properties of fuel for diesel engines are as follows:
1 Viscosity This must be low enough for the fuel to atomise readily when sprayed into the combustion chamber.
2 Heat value or energy This is higher for diesel-engine fuels than for petrol.
3 Flashpoint The flashpoint of a fuel is the temper-ature at which the fuel commences to give off a vapour that would ignite immediately if lit by a spark or flame For diesel fuel, this is about 55°C This is much higher than petrol, which vaporises at all normal atmospheric temperatures.
■ Because of its high flashpoint, diesel fuel is much safer to handle and store than petrol.
4 Self-ignition temperature This is the temperature at which the atomised fuel ignites and burns without the aid of a spark A low self-ignition temperature means that the engine will start easily and run with less diesel knock This temperature is around 250°C for diesel fuels.
5 Sulphur content All diesel fuels contain a certain amount of sulphur Too high a sulphur content results in excessive cylinder wear due to acid building up in the lubricating oil.
Diesel fuels have to be produced with a minimum amount of sulphur to reduce sulphur emissions from the exhaust system.
Cetane number
Diesel fuels are given a cetane number This is a measure of its ignition quality, or its readiness to burn
figure 32.3 A cylinder for NGV
975
60 litres (equal to 18.7 petrol litres) 54.5kg
Trang 8in the combustion chamber The minimum cetane
number for automotive diesel is 45.
Fuels with a low cetane number take longer to
ignite after being injected When ignition does take
place, there tends to be greater diesel knock, as the
accumulated fuel suddenly burns and builds up
pressure.
On the other hand, if the cetane number is
suffi-ciently high, the fuel ignites and commences burning
almost as soon as the injection spray commences.
There are also fewer exhaust emissions.
The cetane number is obtained by testing the fuel
in a test engine and comparing the results with a
reference fuel The reference fuel consists of the
chemicals cetane and a type of naphthalene Cetane
ignites easily and so is given the number 100, while
the naphthalene, which is slow burning, is given the
number zero.
The fuel being tested and the reference fuel are
used in the engine and compared The amount of
cetane in the reference fuel for the same engine
performance gives the cetane number for the fuel being
tested This is a similar procedure to testing the octane
number of petrol.
■ The aim is to have a fuel with power as well as
ignition qualities, so the highest possible cetane
number is not the only requirement for diesel fuel.
Cleanness of diesel fuel
Because the clearances in fuel injection parts are so
small, contamination from water, dust, rust or scale
must be prevented before the fuel is put in the tank of
the vehicle Contaminants can cause costly damage in
a very short time Care in handling and storing is
therefore most important.
The fuel tank of the vehicle should preferably be
kept full and not be allowed to stand partly empty, as
moisture will condense on the exposed metal surfaces
and allow drops of water to enter the fuel.
■ On the vehicle, good maintenance of the fuel system
is most important, with fuel filters and water
separators being regularly serviced.
Alternative fuels
There are a number of alternative fuels that have been
tried either alone or in conjunction with petrol and
some of these are being further developed Some
alternative fuels will be mentioned here because some
of these are likely to become more prominent as supplies of liquid fossil fuels are diminished.
Generally, the current automotive fuels have an advantage over the alternative fuels because of price, processing and also because of handling and storing.
1 Alcohol based fuels include methanol and ethanol Methanol can be obtained from natural gas, heavy crude oil and coal, but it is expensive to produce Ethanol can be produced from agricultural crops, particularly sugar cane.
2 Synthetic fuels are derived from coal and oil shale.
In this process, the oil that is locked in the shale has to be removed by special processes This is difficult and much more expensive than refining crude oil.
3 Engines will operate on hydrogen, but it requires special storage conditions at high pressures and low temperatures This presents difficulties in using it
as a fuel for motor vehicles.
4 Electric vehicles and hybrid vehicles are already in use These require large heavy batteries that occupy
a lot of space.
Hybrid vehicles have small internal combustion engines as well as an electric motor The electric motor provides the power for normal light-load conditions and the engine provides assistance when starting off and when operating under load The electric motor draws from the battery, but some recharging takes place during operation of the vehicle The distance that can be travelled on the electric motor is limited and external recharging is necessary, for example overnight.
5 Biodiesel is a term used to denote diesel fuel that has been produced from agricultural crops.
Lubricating oil
A satisfactory lubricating oil has to:
1 be the correct viscosity
2 resist oxidation
3 avoid carbon formation
4 prevent corrosion
5 resist extreme pressures
6 avoid foaming.
To give the oil all these properties, a number of additives are introduced into the oil during the manu-facturing process.
Trang 9chapter thirty-two fuels, fluids and lubricants 583
Most oils are mineral oils and these are produced
by refining Other oils are called synthetic oils because
they are obtained by chemical conversion of raw
materials rather than by the usual refining methods.
Viscosity of oil
Viscosity refers to the resistance of oil to flow A thick
or heavy oil has a high viscosity A thin or light oil has
a low viscosity To understand viscosity simply, oil
can be described by two characteristics: body and
fluidity.
1 Body refers to the resistance of the oil to
pene-tration during heavy loads For example, during the
power stroke, the load on the connecting-rod
bearing increases, and the body in the oil prevents it
from being squeezed out of the bearing.
This property also cushions shocks and helps to maintain a seal between the pistons and the cylinder
walls.
2 Fluidity is the ease with which the oil flows through
oil lines and spreads over bearing surfaces.
■ To some extent, fluidity is the opposite of body, and
the viscosity of engine oils is a balance between
these two properties.
Viscosity is affected by the temperature of the oil.
It is reduced when hot and increased when cold An
engine oil needs fluidity when cold while retaining its
body at high engine temperatures.
The viscosity of the oil is determined by a
visco-meter This is a device which determines the time that
a specified amount of oil takes to flow through a hole
of a particular diameter Temperature is taken into
account during the test.
Oil additives
Oils do not come direct from the refining process, but
are improved with chemical additives during
manu-facture These are:
1 viscosity-index improvers
2 pour-point depressants
3 oxidation inhibitors
4 corrosion and rust inhibitors
5 foam inhibitors
6 detergents
7 dispersants
8 extreme-pressure additives.
Viscosity-index improvers
The viscosity of oil changes with temperature To provide an accurate measure of this, a viscosity-index scale is used An oil with a high viscosity index will have less viscosity change with temperature than an oil with a low viscosity index.
Special additives are used to improve the viscosity index of oils Oils with viscosity-index improvers have relatively little change in viscosity between low and high temperatures.
Pour-point depressants
At low temperatures, oil could become so thick that it would not pour at all Certain additives can be put into oil to depress, or lower, the temperature at which the oil becomes too thick to flow.
These additives keep the oil fluid at low temperatures so that there is adequate engine lubrication during cold-weather starting and initial operation.
Oxidation inhibitors
When oil is heated and agitated, oxygen in the air tends to combine with oil and oxidise it This is the treatment that engine oil receives in use – it is heated and agitated as it passes through the engine, so some oil oxidation is bound to occur.
Oil that is oxidised breaks down to form various harmful substances, some of which can coat engine parts with an extremely sticky, tar-like material This can clog oil channels and tend to restrict the action of piston rings and valves Oil oxidation can also coat engine parts with a varnish-like substance.
During the manufacturing process, chemicals known as oxidation inhibitors are included so that engine oils have a resistance to oxidation.
Corrosion and rust inhibitors
At high temperatures, acids can form in the oil and corrode engine parts, especially bearings Corrosion inhibitors are added to the oil to prevent this.
Rust inhibitors function in two ways: they displace water from metal surfaces so that oil coats them, and they have an alkaline reaction to neutralise combustion acids.
Foam inhibitors
The churning action in the engine crankcase tends to cause the engine oil to foam If this occurs, the oil will not provide normal lubrication of bearings and other
Trang 10moving parts To prevent foaming, antifoaming
additives are mixed with the oil.
Detergents
The combustion process leaves deposits of carbon on
pistons, rings, valves and other internal parts of the
engine Also, some oil oxidation may take place,
causing other deposits To prevent the formation of
these deposits, engine oils contain a detergent additive.
The detergent loosens the deposits of carbon, gum
and dirt and the oil then carries the particles of material
away The larger particles drop to the bottom of the
crankcase, but smaller ones tend to remain suspended
in the oil These impurities, or contaminants, are
drained out when the oil is changed.
Dispersants
To prevent the particles from clotting, and to keep
them in a finely divided state, a dispersant is added to
the oil Without the dispersant, the particles would tend
to collect, and form larger particles, which might block
the oil filter and reduce its effectiveness.
Particles could also build up in oil passages and
restrict oil flow The dispersant prevents this and
increases the amount of contaminants that the oil can
carry.
Dispersants and detergents work together If the
dispersant can keep the contaminants suspended in
the oil as small particles, they will not deposit on
engine parts and there is less need for detergent action.
Extreme-pressure additives
An automotive engine subjects the lubricating oil to
very high pressures, not only in the bearings, but also
in the valve train To prevent the oil from being forced
from between the surfaces, extreme-pressure additives
are put into the oil They form very strong films of oil
that resist pressure.
Classifications of oil
There are two different classifications of oil One is the
SAE rating of its viscosity, the other is a service
classification, which is related to its performance.
SAE ratings of oil
The SAE rating of oil was developed by the Society of
Automotive Engineers in USA It is based on the
viscosity of the oil SAE ratings are usually referred to
as grades.
The normal grades are from SAE 20 to SAE 50.
Winter grades, suitable for use at lower temperatures, have a suffix ‘W’ These grades are SAE 10W and SAE 20W.
■ The SAE grade does not relate to the quality of the oil, only to its viscosity.
Multigrade oils
Many oils are rated as multigrade oils, and these have a viscosity range equivalent to several single-grade oils For example, an SAE 20W–50 oil is a multigrade oil that covers a range of grades from SAE 20 to SAE 50.
A 20W–50 oil has the characteristics of an SAE 20 oil to keep it thin when the engine is cold and it has the characteristics of an SAE 50 oil so that it will not become too thin when the engine is hot.
Service classifications of oil
As distinct from a viscosity rating, lubricating oils are also given a service classification This is based on the type of service for which the oil is suited.
API classifications, developed by the American Petroleum Institute, are used These have ‘S’ classifications for petrol engines and ‘C’ classifications for diesel engines.
Oil classifications for petrol engines
Classifications for petrol engines commenced with SA, then went to SB and so on to SJ and beyond Each classification provides for more severe service and engine protection than the previous one For example, oils classified for SJ service provide greater protection against oxidation, high-temperature deposits, sludge and corrosion than oils with earlier classifications These oils also provide for extended periods between oil changes.
Classifications for diesel oils
For diesel engines, classifications commenced at CA and have reached CG and beyond Each classification
is designed for more severe service than the previous one For example, oil classified CF is designed for diesel engines that operate under severe service conditions such as: continuous low temperatures and light loads, or continuous high-temperatures and heavy-load conditions.
Combined classifications
Some lubricating oils fulfil the requirements of both petrol and diesel classifications These are given a dual classification, such as SJ/CF.