Page 9 - Piston Engine-Based Power Plants

2 Piston Engine-Based Power Plants

The piston engines used for power generation are almost exclusively
derived from similar engines designed for motive applications. Smaller
units are normally based on car or truck engines while the larger
engines are based on locomotive or marine engines. Performance of
these engines vary. The small engines are usually cheap because they
are mass produced but they have relatively low efficiencies and short
lives. Larger engines tend to be more expensive but they will operate
for much longer. Large, megawatt-scale engines are among the most
efficient prime movers available, 1 with simple cycle efficiencies
approaching 50%.

The piston engine takes its name for the characteristic feature of the
engine design, a piston. This piston moves backwards and forwards (or
up and down) within a cylinder, that is sealed at one end, in response
to the expansion and contraction of a gas within the sealed chamber as
the gas is heated and cooled. The heating and cooling of the gas sealed
in the piston cylinder can be carried out by applying alternate heating
and cooling externally, in which case the engine is called an external
combustion engine. However in most engines of this type the heating
takes place via the combustion of a fuel in air inside the cylinder itself.
This type of engine is called an internal combustion engine.
There are two principle types of internal combustion reciprocating
engines, the spark ignition engine and the compression or diesel
engine. The latter was traditionally the most popular for power gen-
eration applications because of its higher efficiency. However it also
produces high levels of atmospheric pollution, particularly nitrogen
oxides. As a consequence spark ignition engines burning natural gas
have become increasingly popular units for power generation, at
least within industrialised nations. A third type of piston engine,
called the Stirling engine, is also being developed for some specia-
lised power generation applications. This engine is novel because it is
an external combustion engine.

The enduring popularity of the piston engine derives from it porta-
bility and flexibility. With a small reservoir of fuel the engine is self-
contained and can produce energy or work for extended periods. This

1 Slow-speed engines are the most efficient engines for converting fuel energy via heat into rotary
motion to generate electricity. Fuel cells, which turn chemical energy directly into electrical
energy, can be more efficient.