More about Piston Engines
The thermal efficiency of an engine is not a single value but depends on the design, the state of its ancillaries, and the duty it is performing. It is rare to find a need for constant load, we may be driving an air compressor, an electric generator, a vehicle, a drainage or irrigation pump; the quantities or pressures required may vary from minute to minute, or according to the seasons. The great merit of piston engines is their ability to vary speed and torque independently, without undue loss of efficiency. The engine must be chosen to satisfy the peak demand. The nominal power of an engine may be specified as that power which can be maintained safely for long periods; many engines are capable of 10% overloads for short periods without overheating or undue wear. They are often less efficient when on overload.
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