Relation between Fuel Injection and Heat Release in a Direct-Injection Engine and the Nature of the Combustion Processes

Abstract

Fuel injection and heat-release analysis for a direct-injection (D.I.) engine have been carried out over a wide range of load, speed, and injection timings using a digital computer. From these results emerge the broad principles governing the combustion in a D.I. engine. A model is proposed for relating quantitatively the rate of fuel injection to rate of heat release and hence cylinder-pressure diagram. It is found that the same law for correlation holds over wide range of load and speed and injection timing. The predicted cylinder-pressure diagram agrees reasonably well with the measured diagram over all test conditions, thus establishing the validity of at least the main features of the model. Whilst it is perhaps too optimistic as yet to predict i.m.e.p. (10 per cent error), the agreement in the rate of pressure rise (almost exact) and peak pressure (about 5 per cent error) is such that practical applications (for example, resulting noise) are already possible.

Arising from the heat-release analysis and high-speed photography a consistent picture of the combustion processes in a diesel engine is presented. It is shown that in general, and particularly in the case of D.I. engines, the highest rate of burning occurs immediately after ignition and the flame is essentially premixed in nature with low luminosity. This period of high rate of heat release is, however, very short and is followed by the main period which is essentially diffusion in character with high luminosity and comparatively lower rate. The effect of combustion-chamber type on these processes are discussed and comparison is made with heat release in a petrol engine.