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The deterioration of combustion stability as lean operating limits and misfire conditions are approached has been investigated experimentally. The study has been carried out on spark ignition engines with port fuel injection and four-valves-per-cylinder. Test conditions cover fully-warm and cold operation, and ranges of air/fuel ratio, exhaust gas recirculation rates and spark timing. An approximate method of calculating gas/fuel ratio is described. This is used to show that combustion stability, characterised by the coefficient of variation of i.m.e.p., is a function of calculated gas/fuel ratio and spark timing until near to the limit of stability. A rapid deterioration in stability and the onset of weak, partial burning occurs at a gas/fuel ratio between 24:1 and 26:1 under fully-warm operating conditions, and around one gas/fuel ratio lower under cold operating conditions.

An experimental investigation of combustion cycle-by-cycle stability under cold idling conditions has been carried out on a Dl diesel to examine the influence of pilot fuel injection strategy. The engine is a single cylinder variant of a multi-cylinder design meeting Euro 4 emissions requirements. The engine build had a swept volume of 500cc and a compression ratio of 18.4:1. Work output and heat release characteristics have been investigated at test temperatures of 10, 0, −10 and −20°C and speeds in the range from 600 to 1400rpm. At the lowest temperature, −20°C, stability is sensitive to the timing of main injection and is prone to deteriorate with increasing engine speed. The influence of the number of pilot injections and pilot fuel quantity on stability has been explored. Best stability was achieved by increasing the number of pilot injections as temperature is lowered, from one at 10°C to two at −10°C and between two and four at −20°C.