Piston engines or reciprocating engines (the two terms are often used inter- changeably) are by a wide margin the largest group of thermodynamic heat engines in use around the world. Their applications range from model airplanes to lawn mowers, and they include all the automotive power plants found in motorcycles, cars, trucks, and many other sorts of heavy machinery. They also power locomotives, ships, and many small aircraft, and they provide stationary electrical power and combined heat and power to numerous sites across the globe.
The number in use is enormous; the United States alone produces more than 35 million each year. Engines vary in size from less than 1 kW (model engines can be a few watts) 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. They are manufactured all over the globe and there is a large global base of expertise in their maintenance and repair.
In line with the wide range of engines available, the power generation appli- cations of piston engines are enormously varied. 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, such as in hospitals or to support highly sensitive computer installations like for air traffic control and the many server farms around the world. Commercial and industrial facilities use medium-size 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.
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 loco- motive 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%.
There are two main 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 oxide. As a consequence, spark-ignition engines burning natural gas have become the more popular units for power generation, at least within industrialized nations. A third type of piston engine, called the Stirling engine, is also being developed for some specialized power generation applications. This engine is novel because the heat energy used to drive it is applied outside the sealed piston chamber.