Engine Torque | Engine Power
This is the turning-effort about the crankshaft's axis of rotation and is equal to the product of the force acting along the connecting-rod and the perpendicular distance between this force and the centre of rotation of the crankshaft. It is expressed in Newton meters (N m);
i.e. T = Fr
where T = engine torque (N m), F = force applied to crank (N) and r = effective crank-arm radius (m)
During the 180° crankshaft movement on the power stroke from TDC to BDC, the effective radius of the crank-arm will increase from zero at the top of its stroke to a maximum in the region of mid-stroke and then decrease to zero again at the end of its downward movement (Fig. 1.1-10). This implies that the torque on the power stroke is continually varying. Also, there will be no useful torque during the idling strokes. In fact some of the torque on the power stroke will be cancelled out in overcoming compression resistance and pumping losses, and the torque quoted by engine manufacturers is always the average value throughout the engine cycle.
The average torque developed will vary over the engine's speed range. It reaches a maximum at about mid-speed and decreases on either side (Fig. 1.1-11).
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 (Fig. 1.1-11).
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 standardized 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:
pLANn i:p: ¼ 60000
where i.p. ¼ indicated power (kW)
p ¼ effective pressure (kN/m2)
L ¼ length of stroke (m)
A ¼ cross-sectional area of piston (m2)
N ¼ crankshaft speed (rev/min)
and n ¼ number of cylinders
2pTN b:p: ¼ 60000
where b.p. ¼ brake power (kW)
p ¼ 3.142
T ¼engine torque (N m) and N ¼ crankshaft speed (rev/min)
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-meters 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-St¨arke' meaning horse strength.
The international unit for power is the watt, W, or more usually the kilowatt, kW, where 1 kW ¼ 1000 W.
Conversion from watt to horsepower and vice versa is: 1 kW ¼ 1.35 hp and 1 hp ¼ 0.746 kW