ENGINE EFFICIENCY.

Hawera Star, Volume XLVIII, 12 January 1929, Page 12

ENGINE EFFICIENCY.

INTERESTING FACTS. Roughly, only a quarter of the actual fuel value is used to drive the ear, no less than three-quarters, .approximately, being lost in various ways. The actual percentage of useful and wasted fuel varies with different designs, but ■ the 25 per cent, turned into work may Ibe taken as applying to the normal | touring ‘model car. | Before dealing with the why and wherefore of power losses, it is necessary to understand where the waste occurs. Only about 30 per cent, of the heat value of the fuel actually is used in the cylinders to force the pistons downwards. Of this 30 per cent, a further 5 per cent, is lost in bearing friction, in the inlet and exhaiist systems, and in transmission. Thus the result is- three-quarters waste, and onequarter of the fuel used. The thermal efficiency of an engine is the ratio of heat converted into work to the heat supplied. Assuming that a gallon of petrol has a heat value of 144,300 British Thermal Units, an engine which makes use of 43,290 British Thermal Units out of every gallon has a thermal efficiency of 30 per cent. Up to date the maximum thermal efficiency obtained is barely 40 per cent.; 30 per cent., however, is nearer the mark for ordinary production engines. Steam engines are even worse off in this respect, as their thermal efficiency may be set down at about 25 per cent. Where does the 70 per cent, of the heat value of the fuel go? The waterjackets and radiator absorb some 20 per'cent., and 41 per cent, passes-out i in the exhaust gases. At first- sight i it would appear that the cooliiig system I is an expensive and unnecessary part! of the engine if it causes such wastage, i but, as in the case of the majority of) engineering problems, it is a necessary i compromise. - t

In the absence of direct air or wfiter cooling, the iieat generated in the cylinders would grow until the oil temperature rose to such a pitch that its lubricating value was destroyed; this, of course, would mean seizure and distortion.

Mechanical efficiency is the ratio of energy given out to energy supplied. The average percentage here is much greater than in the case of thermal efficiency, being about 90 per cent, to 94 per cent. The 10 per cent, loss is caused by the friction of the pistons in the cylinders, the friction in the engine bearings, in the auxiliaries, in the gear box, and in the rear axle. Friction increases with the. engine speed ; therefore a study of mechanical efficiency is of great importance in connection with light car engines, which run at comparatively high rates of revolution. Foremost, by reason of its relative size, is piston friction. This loss represents a quantity some four times greater than that of the friction of the engine hearings; between 40 per cent, and 60 per cent, of the entire friction in the engine takes place in the cylinders. A large amount of the area of piston skirts is unnecessary' for taking wear, but heat conduction and the strength of the piston demand the presence of an adequate amount of metal.