Engine power

Power is the rate of doing work. When applied to engines, power ratings may be calculated either on the basis of indicated power (i.p.), that is the power actually developed in the cylinder, or on the basis of brake power (b.p.), which is the output power measured at the crankshaft. The b.p. is always less than the i.p., due to frictional and pumping losses in the cylinders and the reciprocating mechanism of the engine.

Since the rate of doing work increases with piston speed, the engine’s power will tend to rise with crankshaft speed of rotation, and only after about two-thirds of the engine’s speed range will the rate of power rise drop off.

The slowing down and even decline in power at the upper speed range is mainly due to the very short time available for exhausting and for inducing fresh charge into the cylinders at very high speeds, with a resulting reduction in the cylinders’ mean effective pressures.

Different countries have adopted their own standardised test procedures for measuring engine performance, so slight differences in quoted output figures will exist. Quoted performance figures should therefore always state the standard used. The three most important standards are those of the American Society of Automotive Engineers (SAE), the German Deutsch Industrie Normale (DIN), and the Italian Commissione technica di Unificazione nell Automobile (CUNA).

The two methods of calculating power can be expressed as follows:

The imperial power is quoted in horsepower (hp) and is defined in terms of foot pounds per minute. In imperial units one horsepower is equivalent to 33 000 ft lb per minute or 550 ft lb per second. A metric horsepower is defined in terms of Newton-metres per second and is equal to 0.986 imperial horsepower. In Germany the abbreviation for horsepower is PS derived from the translation of the words ’Pferd-Sta¨rke’ meaning horse strength.

The international unit for power is the watt, W, or more usually the kilowatt, kW, where 1 kW = 1000 W.

Conversion from watt to horsepower and vice versa is:

1 kW = 1.35 hp and 1 hp = 0.746 kW