Page 25 - Piston Engine-Based Power Plants

18 Piston Engine-Based Power Plants

combustion chamber (usually but not always the cylinder). A higher
cetane number equates to a more rapid ignition, a shorter ignition time
and more complete combustion of the fuel. Shorter ignition time and
complete combustion lead to more efficient and cleaner engines. The
cetane number is usually derived from tests involving the fuel density
and boiling or evaporation points.

Regular diesel fuel has a typical cetane rating of 48 and premium
diesel 55. Biodiesel, depending upon the blend has a rating of 50 to 55
and the typical rating of synthetic diesel is 55. As with the octane rat-
ing, there is no advantage to running an engine using a fuel of higher
cetane rating than it requires. Most modern diesel engines for road use
operate with fuel of cetane rating between 45 and 55.

In addition to the cetane rating, diesel fuels are divided into three
grades. Diesel No. 1 is a relatively volatile fuel containing molecules
with between 8 and 19 carbon atoms. This is typically used in smaller,
high speed diesel engines that operate at varying speeds and loads such
as those for buses. Diesel No. 2 has a lower volatility, with molecules
containing 9 to 21 carbon atoms. It is also used in high speed diesel
engines but is usually restricted to those with a relatively constant
speed and load. Diesel No. 4 is the heaviest diesel, often made by
blending heavy distillate diesel and residual fuel oils. Typically it con-
tains hydrocarbons with more than 25 carbon atoms. This type of fuel
is used in medium and slow speed diesel engines that operate at con-
stant load such as stationary power generation as well as ships and
locomotives.

LIQUEFIED PETROLEUM GAS

Liquefied petroleum gas (LPG) is a third fuel that can be exploited to
fire piston engines. It is used in spark ignition engines. The gas, nor-
mally supplied in liquid form under pressure, is a mixture of two
hydrocarbons, propane (C 3 H 8 ) and butane (C 4 H 10 ). It may also con-
tain iso-butane which has the same chemical composition as butane
but a different structure. LPG is attractive because it has relatively
lower carbon dioxide emissions than either gasoline or diesel. Its
octane rating is between 90 and 110 depending on the proportion of
the different hydrocarbons.