ANALYSIS OF THE THERMAL EFFICIENCY AND CYCLIC VARIATIONS IN A SI ENGINE UNDER LEAN COMBUSTION CONDITIONS

There are significantly variations in cylinder pressure traces from cycle to cycle in spark ignition engines. The variations, named cyclic variations, exist even when engine is stable. Cyclic variability must be considered in the design and control of spark ignition engines and are magnified under certain engine operating conditions. In this work, effects of air excess coefficient on cyclic variability, engine performance characteristics and engine emission characteristics were investigated at lean operation conditions. The test engine is a spark ignition engine with electronically controlled fuel injectors. According to the test results, engine air excess coefficient can be increased to improve engine thermal efficiency and to decrease the harmful exhaust emissions under low load conditions without exceeding the limit of cyclic variability.