Page 23 - Piston Engine-Based Power Plants

16 Piston Engine-Based Power Plants

Dehydrogenation may also be carried out in order to create unsaturated
hydrocarbons. All these products have a higher octane rating than
naphtha and can be blended with it.

Another common process is cracking of the naphtha. This can be
carried out at high temperature and pressure without a catalyst or at
lower pressure and temperature with a catalyst. Cracking generally
breaks larger hydrocarbons into smaller ones but depending upon how
it is carried out it can also increase their complexity and may dehydro-
genate too.

The octane rating of a fuel is measured in a test engine. The hydro-
carbon iso-octane, which has a branched structure, is arbitrarily
assigned an octane rating of 100 (hence the name of the rating).
N-octane, a straight chain hydrocarbon has a rating of 0. The octane
rating of an unknown gasoline is then measured by comparing its per-
formance in the test engine to various mixtures of the two standards.
For example, if it performs like a mixture containing 10% n-octane
and 90% iso-octane then it has an octane rating of 90.

An engine is usually specified so that it uses the fuel with a mini-
mum octane rating consistent with smooth operation. The typical
octane rating for modern road cars is 95 although some high perfor-
mance vehicles use fuel with a rating of 98. There is no advantage in
running a car with fuel of higher octane rating than it requires.

Another important property of gasoline is its vapour pressure. This
should be high enough to allow the engine to start and operate at low
temperatures but not too high that it causes vapour locks in the fuel
feed system. Where seasonal temperature variations are wide, gasoline
with different properties may be supplied at different seasons. Gasoline
can deteriorate if stored for a long period as a result of oxidation. To
counter this, fuel stabilizers may be added.

In some regions gasoline or petrol is mixed with a small amount of
ethanol which is a cleaner, bio-derived fuel. Regulations mandate this
in Brazil where around 25% ethanol is blended with gasoline. Ethanol
is also widely used in the USA where a blend containing around 10%
is common. Some US states require fuel to contain ethanol. In Europe
the use of ethanol is most common in Sweden, Germany, France and
Spain.