Q Is there any truth in the claim that
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refrigeration could be used to increase the power output of a diesel engine? Of allied interest, why is the inlet manifold of a typical petrol engine so arranged that it is heated by the exhaust manifold? Do different rules apply to the two types of unit?
AAn intercooler is fitted to a number of
turbocharged diesel engines between the turbocharger and the inlet manifold to cool down the air after it has been heated by compression in the turbocharger. The density of the air is reduced by a rise in temperature and as the output of an engine is proportional to the weight of air that is aspirated, cooling the air increases the output given that sufficient fuel is injected into the cylinders. Refrigeration may be regarded as an extreme form of intercooling and has been applied experimentally with notable success, as much as 30/40 per cent more power having been obtained with a very small increase in peak cylinder pressures and with a worthwhile reduction in specific fuel consumption. To date, however, the weight, cost and complexity of established types of refrigerating system debar their use in motor vehicles.
A petrol engine also benefits from cooling the air so long as mixture distribution is not affected. It is customary to heat the manifold to assist vaporization of the fuel droplets and thus aid distribution, good distribution being particularly difficult to obtain when the engine is ticking over. At higher speeds it is less of a problem and if the vehicle is mainly employed on longer runs at the normal working temperature, locating the intake of the air cleaner in a cooler area may well improve performance and fuel consumption. Some operators of naturally-aspirated diesel engines have fitted an electric fan under the bonnet to blow cool air across the intake and claim that it gives a substantial increase in performance.