An Application of Petrol Injection to Automobile Engines

Abstract

The paper recounts the results obtained in a series of experiments carried out with the object of examining the claims made for improved results, both in fuel consumption and maximum power, which could be obtained by substituting a petrol injection system for a carburettor.

The experiments, carried out on four-cylinder engines of approximately 1·5 litres capacity, led to the development of a satisfactory system capable of giving equal fuel distribution to all cylinders with a tolerance of ±2 per cent at all loads. They showed that, for engines of this order of size, manifold injection was superior to direct injection and established that petrol injection will not give any better combustion efficiency than carburation. Hence any saving in fuel consumption obtained can only be due to better fuel distribution between cylinders, better matching of fuel/air ratio to a desired value, or increase in compression ratio made possible by the elimination of the hot spot. Under certain conditions the resultant saving in fuel at part load on bench tests could amount to some 10–15 per cent.

Maximum power can be increased by some 15–20 per cent by elimination of the choke, and still more if the compression ratio is increased slightly to take advantage of the reduced temperature of the incoming charge. Road tests in general confirmed the results of bench tests, but brought out the effect of many other factors which affect fuel economy. In order to properly evaluate these a number of experiments were carried out which are of interest and value as affecting the problem of fuel economy in general.

Starting at low temperatures requires very considerable over-fuelling with a petrol injection system, just as in the case of a carburettor, but such a system may be made to give better control during the process of warming up. In the authors' view an engine fitted with petrol injection is pleasanter to drive as a result of its inherently better idling, more uniform and smoother torque and increased power at full throttle. The results obtained, however, while attractive, do not bear out some of the more exaggerated claims sometimes made by advocates of petrol injection systems. No attempt is made to urge the introduction of petrol injection and it is left to the industry to decide whether, in view of the results obtained, it is felt that a petrol injection system with its concomitant increase in first cost is worth adoption.