THERMAL EFFICIENCY

Press, Volume CI, Issue 29921, 7 September 1962, Page 9

THERMAL EFFICIENCY

VACH gallon of petrol, contains a set amount of potential energy, in the form of heat, when burnt in an engine. The thermal efficiency of an engine is the ratio between the energy actually obtained from burning the fuel in it and the total amount of energy available from the fuel. It is the relationship of work done compared with the total amount of heat contained in or available from •he fuel. The thermal efficiency of ar internal combustion petrol engine is comparatively low. It has been calculated that of the total amount of energy available in petrol only about 15 per cent, is actually used to propel the car. About 70 per cent, of the

available energy is lost in the cooling water and oil and in the exhaust gases. About 15 per cent, of the remainder is lost in friction, both in the engine itself and in the transmission.

It is the heat losses that account for the greatest reduction in the thermal efficiency of an engine. The estimated 15 per cent, of heat that remains after losses from friction and cooling causes the gases developed in combustion to expand, pushing on the pistons in the engine and producing power. The thermal efficiency of an engine is usually about 22 per cent. The maximum so far developed is estimated at Little more than 26 per cent. At Le Mans, the famous 24hour race, there is a special

award for the car which shows the greatest ■thermal efficiency during the race, and every year there is strong competition among the smaller cars for this award. The smaller cars, of course, concentrate on producing the greatest power and hence the greatest thermal efficiency, from the smallest engine and lowest fuel consumption. At present the thermal efficiency of internal combustion petrol engines is limited by practical mechanical and other considerations. However, many engineers believe thermal efficiency will be increased considerably in the future and until the limit is definitely reached it cannot be said that the petrol engine, as found in cars today, has reached the limit, of its development.