Page 22 - Piston Engine-Based Power Plants

Fuels and Energy Resources for Reciprocating Engines 15

Knocking can occurs when the gas in the cylinder is compressed by
the piston during part of the engine cycle. When a gas is compressed it
heats up. How much it heats up how hot it gets will depend upon
the compression ratio; the more highly the gas is compressed the hotter
it will get. In a spark ignition engine this gas is a mixture of air and
fuel and if the temperature becomes too high, the fuel may ignite spon-
taneously. Thus the composition of the fuel must be tailored to the
engine design and compression ratio.
The octane rating of gasoline depends on its composition. Simple,
straight chain saturated hydrocarbons (alkanes) have the lowest resis-
tance to knocking and ignite the most readily. More complex branched
hydrocarbons and aromatic hydrocarbons (these contain complex car-
bon ring structures) have higher resistance and therefore a higher
octane rating. Some of the processes used during the refining of oil can
convert the simpler hydrocarbons into more complex molecules.
Gasoline is usually produced by blending refinery products from differ-
ent treatment processes together to give a fuel with the desired
properties.

The initial process carried out during the refining of oil is fractional
distillation. This involves heating the crude oil slowly and allowing
components to evaporate. The most volatile components tend to evap-
orate first and these are collected, followed by later fractions that have
relatively higher boiling points. Straight run gasoline or naphtha, is the
first product of direct distillation of crude oil. During the fractional
distillation it has a starting boiling point of around 35 C and a finish-

ing boiling point of 200 C. The naphtha collected between these boil-
ing points has a relatively low octane rating. Its octane rating can be
increased by using additives. The main historical additive used to
increase octane rating is tetraethyllead. The use of this started during
the 1920s but it was phased out towards the end of the century after
the environmental and health damage caused by the lead was identi-
fied. Modern gasolines instead achieve a suitable octane rating by
blending hydrocarbons with different properties.
In order to obtain these other hydrocarbons, naphtha is reformed.
Reforming is usually performed using a catalyst, hence its common name
of catalytic reforming. The process converts the straight chain hydrocar-
bons in the naphtha into branched chain and cyclic hydrocarbons.