Engine-performance terminology

Engine-performance terminology

            To enable intelligent comparisons to be made between different engines’ ability to pull or operate at various speeds, we shall now consider engine design parameters and their relationship in influencing performance capability.

Piston displacement or swept volume

            When the piston moves from one end of the cylinder to the other, it will sweep or displace air equal to the cylinder volume between TDC and BDC. Thus the full stroke movement of the piston is known as either the swept volume of the piston displacement.

The swept or displaced volume may be calculated as follows:

 V = πd⌑2L / 4000

Where        V = piston displacement (cm3)

                    π = 3.142

                    d = cyclinder diameter (mm) 

                    L = cylinder stroke (mm)

Mean effective pressure:

            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 kilonewton per square metre (kN/m² ) or bars (note: 1 bar = 100 kN/m² ).

Engine torque:

            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 metres (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. 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.

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. 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