The speed at which a piston engine operates will depend on its size. In general small units will operate at high speed and large units at low speed. However since in most situations a piston-engine-based power unit will have to be synchronised to an electricity grid operating at 50 or 60 Hz, the engine speed must be a function of one or other of these rates. Thus a 50 Hz high-speed engine will operate at 1000, 1500 or 3000 rpm while a 60 Hz machine will operate at 1200, 1800 or 3600 rpm.
Engines are usually divided into three categories, high-, medium- and slow-speed engines. High-speed engines are the smallest and operate up to 3600 rpm. The largest slow-speed engines may run as slow as 58 rpm. Typical speed and power ranges for each type of engine are shown in Table 6.1.
Engine performance varies with speed. High-speed engines provide the greatest power output as a function of cylinder size, and hence the greatest power density. However the larger, slower engines are more efficient and last longer. Thus the choice of engine will depend very much on the application for which it is intended. Large, slow- or medium-speed engines are generally more suited to base-load generation but it may be more cost effective to employ high-speed engines for back-up service where the engines will not be required to operate for many hours each year.
In addition to standby service or continuous output base-load operation, piston engine power plants are good at load following. Internal combustion engines operate well under part load conditions. For a gas-fired spark-ignition engine, output at 50% load is roughly 8-10% lower than at full load. The diesel engine performs even better, with output barely changing when load drops from 100% to 50%.
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