Cylinder Pressure Variations in Petrol Engines

Abstract

The paper deals with a detailed research programme which was carried out in an endeavour to find out the cause of the cylinder pressure fluctuations from cycle to cycle which occur in a petrol engine. Every working variable was altered in turn, and its effect on combustion observed on an electronic engine indicator. The development of the cylinder pressure over a large number of cycles was recorded at each setting with the aid of an oscillograph camera. For more detailed observations, the photographed traces were replotted on a volume scale and individual P-V diagrams were studied and in some cases drawn as log P-log V graphs. The experiment showed that the fluctuations in peak pressures and i.m.e.p. were caused by differences in the heat-release rate from cycle to cycle. All hydrocarbon fuels exhibit similar variations, but town gas with its more rapid burning produces steadier pressure development. The air/fuel ratio of the mixture is the main factor which influences the magnitude of the maximum pressure variations from cycle to cycle. The steadier mixture ratio is in the order of 13 and on either side of this the fluctuations increase rapidly, especially on the weak end. Using several simultaneous ignition sources, it is possible to decrease the individual flame front path and therefore increase the resulting combustion speed, which reduces the fluctuations.