Page 24 - Piston Engine-Based Power Plants

Fuels and Energy Resources for Reciprocating Engines 17

The energy content of gasoline is around 47 MJ/kg, based on the
lower heating value. However the actual content will vary depending
upon supplier and the season.

DIESEL

The second important branch of piston engines is the diesel engine.
Diesel engines operate in a slightly different way to spark ignition
engines. In a diesel engine, air alone is admitted into the cylinder
before compression and it is compressed more highly than in a spark
ignition engine, elevating the temperature of the air more. Fuel is only
admitted at the end of the compression stroke when the air is at its
hottest, hot enough so that the charge of fuel ignites spontaneously
without the need for a spark. Since there is no fuel present when the
air in the cylinder is compressed, knocking is not a problem. However
the ignition properties of the fuel are still important. Diesel engines use
a heavier fuel that spark ignition engines, and this fuel is commonly
referred to as diesel.

There are a variety of diesel fuels in use today. The principal. his-
torical diesel was, and is made by fractional distillation of crude oil,
just like gasoline. However there are alternatives including synthetic
diesel and biodiesel. Synthetic diesel can be manufactured from carbon
based materials including coal, natural gas and biomass. Biodiesel is
obtained from vegetable oil (sometimes animal fat too) that is con-
verted into a liquid fuel suitable for use in diesel engines.

Petroleum diesel is the fraction that is condensed from the fractional
distillation of crude oil after gasoline. It is produced at a fractional dis-

tillation temperature between 200 C and 350 C and contains carbon
molecules containing, typically, between 8 and 21 carbon atoms. This
makes the fuel relatively heavier than gasoline and less volatile.
Ignition takes place in a diesel engine when droplets of fuel are
sprayed into the hot compressed air in the cylinder. How these droplets
burn determines the efficiency and cleanliness of the engine. Different
diesel fuels behave in different ways and this can be characterised, in
the case of diesel fuel, by a rating called the cetane number which indi-
cates its combustion quality. In this case the primary consideration is
the delay in ignition of the fuel after it has been injected into the