used in a modern car, whether petrol, diesel or even LPG (
), has to meet high standards of purity if the car's
Modern fuels have to be sufficiently volatile to ignite quickly, even under
adverse conditions, and they have to have the correct blend of
burn evenly enough to develop useful power in an
internal combustion engine
The fuel must also have the right
value to prevent pinking (detonating
too early) which can cause engine damage.
engines are tailored to run on a particular grade of
fuel and tuned to run quite close to the limits which that fuel can support, so
consistent quality is important in modern fuels.
Both petrol and diesel are derived from crude oil, which is a complex
mixture of many hundreds of different hydrocarbons as well as other products
which need to be removed during refining. Crude oils vary from source to
source; they generally contain light volatile liquids, petrol among others, as
well as much heavier, almost solid constituents, such as bitumen.
Separating out and purifying petrol and diesel from crude oil requires
complex processes, which are carried out in an oil refinery.
The oil is refined into its constituents by a process called fractional
. This separates out the different constituents of crude oil by
making use of the fact that they boil off and
The first process is carried out in a fractionating column, a tall
cylindrical tower up to 250ft (75m) high, inside which are between 30 and 40
trays called fractionating trays, stacked one above the other. The bottom of
the column is kept very hot but the temperature drops the further you move up
the column, so that each tray is slightly cooler than the one below it.
The crude oil is preheated to between 315°C and 370°C so that all except the
heaviest constituents vaporize. It is then fed in towards the bottom of the
fractionating column as a mixture of gas and liquid. The oil vapour rises up
the column through devices, such as bubble caps, in the fractionating trays
which cause it to be thoroughly mixed with the liquid already there. The
heavier, still-liquid oil passes down to the bottom of the column.
As the vapour rises it cools in line with the falling temperature of the
trays. Whenever the vapour passes up to and bubbles through a tray containing
liquid whose temperature corresponds to the boiling point of one of the
constituents of the vapour, that constituent
out on the tray. The
other, higher-boiling-point vapours carry on up the column.
In this way, each constituent in the vapour meets a tray on which it
condenses out. The result is a series of separated constituents, called
fractions, which can be drawn off from the column through pipes.
There are six main fractions. The lightest, which is still a gas when it
reaches the top of the column, is called refinery gas and is used as a fuel by
the refinery itself.
The rest is further processed in an additional plant. The lightest of the
resulting liquid fractions are very volatile and are used for blending into
Then come naphtha (used for further processing into petrochemicals or
alternatively for blending into petrol), kerosene (which is basically
paraffin), diesel oils and light and heavy oils used for industrial
lubricating, and then the heaviest fraction, bitumen, which is left as a
The basic fractional distillation process divides up crude oil into its pure
chemical hydrocarbons. But some of these hydrocarbons are more valuable than
others. In particular, the demand for petrol is much greater than for bitumen,
or indeed for diesel. So some of the heavier fractions are converted at the oil
refinery into petrol. This is done by a process called cracking.
Thermal cracking uses heat in the process, while catalytic cracking uses a
In thermal cracking the hydrocarbons are heated to between 450°C and 540°C
. The result is a low-grade fuel which is then refined again at
higher temperatures and pressures to produce petrol of good enough quality to
use in car engines.
Catalytic cracking is more useful than thermal cracking since it gives a
higher yield of useful products. By adding a catalyst (usually an
aluminium-silica powder) to the oil during the pre-heating phase, the heavy
fractions can be broken into a mixture of lighter ones, which are then fed into
a fractionating column to be separated out.
These light fractions then undergo further processes called conversion
processes to produce the correct mix of hydrocarbons.
These conversions are followed by treatment stages where the appropriate
are introduced to make the blended petrol suitable for winter or
To be useful for running internal combustion engines, petrol must have
certain properties. It must burn smoothly in the engine over a wide range of
speeds and power outputs without
. This manifests itself by
'knocking' and, if allowed to persist, can result in serious engine damage.
The petrol must have some readily volatile constituents which allow the
engine to start easily in cold weather. But the petrol must not be so volatile
that it vaporizes too easily and causes a lock in the fuel system, or even
(see sideline, right).
The performance of petrol is gauged primarily by its octane number. To work
this out, the petrol is compared with two standard fuels of known performance
called n-heptane and iso-octane, both of which are hydrocarbons. N-heptane is a
poor fuel for internal combustion engines and causes heavy knocking; it has an
octane number of 0. Isooctane is the opposite, a very high quality fuel, and
has an octane number of 100.
If petrol has an octane number of 90 this means that it gives a performance
equivalent to a mixture of 90 parts iso-octane to 10 parts nheptane. Most car
engines require petrol with an octane rating of between 90 and 100.
As a further anti-knock measure, it is still normal practice to add a small
quantity of tetraethyl or tetra-methyl lead to the petrol. However, this is
steadily being curtailed because of the poisonous nature of lead.
The maximum amount of lead allowed in petrol was lowered from 0.4 to 0.15
grammes per litre in 1986, and unleaded petrol is starting to appear on the
European market. This is petrol to which no lead
have been added.
Diesel fuel is more viscous and heavier than petrol, is less volatile and
comes off the fractionating column at a lower level.
Diesel fuel is not graded by octane rating like petrol; instead it is given
a cetane rating. This is derived by comparing diesel with two other
hydrocarbons, cetane and alpha-methylnaphthalene.
High-quality diesel fuel used in road-going vehicles has a cetane rating of
about 50, while slower engines such as those used in large ships may run on
fuel with a lower cetane rating. The higher the cetane rating, the easier the
starting, the smoother the combustion and the lower the level of 'diesel
Some low-grade diesel (called gas oil) used for stationary or off-road work
is dyed for identification and is thus known as red diesel. Only white diesel,
which has had Road Tax Duty paid on it, can legally be used on the road.
Diesel fuel, like petrol, normally has important additives. Anti-freezing
and anti-waxing agents have to be added to diesel fuel used in cold weather to
stop it clogging in
From the oil refinery, the petrol and diesel are transported to their garage
and service station outlets by road or rail in specially designed tankers.
The fuels are usually stored in under
tanks below the forecourt at the
point of sale - the filling station. Petrol and diesel fuels are stored in
separate tanks, as are different grades of petrol, until they are lifted to
above ground level and metered for sale by the
Petrol pumps can normally call up fuel from any of the underground tanks and
blend it to supply the users with whichever grade is needed. However, leaded
and unleaded petrol must not be mixed so they need separate tanks and pumps
which must be clearly marked to show what they contain.
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The fuel supply system is generally reliable and any problems that do occur are usually traceable to the pump, the fuel filter or the tank. Occasionally, however, the fault lies in the fuel line between the tank and the engine.