The cylinder pressure varies considerably while the gas expands during the power stroke. Peak pressure will occur just after TDC, but this will rapidly drop as the piston moves towards BDC. When quoting cylinder pressure, it is therefore more helpful to refer to the average or mean effective pressure throughout the whole power stroke. The units used for mean effective pressure may be either kilonewtons per square metre (kN/m2) or bars (note: 1 bar ¼ 100 kN/m2).

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).