Piston engines or reciprocating engines (the two terms are often used interchangeably to describe these engines) are used throughout the world in applications ranging from lawn mowers to cars, trucks, locomotives, ships, and for power and combined heat and power generation. The number in use is enormous; the US alone produces 35 million each year. Engines vary in size from less than 1 kW to 65,000 kW. They can burn a wide range of fuels including natural gas, biogas, LPG, gasoline, diesel, biodiesel, heavy fuel oil and even coal.
The power generation applications of piston engines are enormously varied too. Small units can be used for standby power or for combined heat, and power in homes and offices. Larger standby units are often used in situations where a continuous supply of power is critical; in hospitals or to support highly sensitive computer installations such as air traffic control. Many commercial and industrial facilities use medium-sized piston-engine-based combined heat and power units for base-load power generation. Large engines, meanwhile can be used for base-load, grid-connected power generation while smaller units form one of the main sources of base-load power to isolated communities with no access to an electricity grid.
Piston engines used for power generation are almost exclusively derived from 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. Smaller 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 probably the most efficient prime movers available,1 with simple cycle efficiencies approaching 50%.
There are two principle types of reciprocating engines, the spark-ignition engine and the compression or diesel engine. The latter was traditionally the most popular for power generation 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 gas have become the more 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 specialised power generation applications. This engine is novel because the heat energy used to drive it is applied outside the sealed piston chamber.
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