Manufacturers of engines intended for stationary use, for example, to power a pump or a generator, usually can supply the purchaser of an engine with a performance curve for the engine, Figure 7.4.
The performance curve will show the rated power for an engine for different engine speeds ranging from the minimum speed recommended for a load to the maximum. However, the usable power is less than the rated power if one of several conditions exists, as explained below. The adjustment from rated power to usable power is called derating. Failure to derate an engine could shorten the life of the engine.
The factors used for derating depend on the engine type and the operating environment. Spark ignition and diesel engines are derated to allow for the effect of accessories, temperature, altitude, and type of service.
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