Cycle to Cycle Combustion Variations in a Spark Ignition Engine Fuelled with Natural Gas

Cycle to cycle combustion variability is a prominent characteristic of spark ignition engine that makes the engine very difficult to control. In this study, the combustion pressure fluctuations in a spark ignition engine fueled with natural gas were investigated. The results showed that the shape of coefficient of variation in the cylinder pressure versus crank angle is independent of the experimental conditions. Three points of this curve are important. They correspond to the combustion beginning, combustion end and 50% mass fraction burned. The results also showed that the cycle to cycle variations indicated mean effective pressure decrease as the equivalence ratio increased in the lean mixture region, and increase as the equivalence ratio increased in the rich mixture region. The cylinder pressure cyclic dispersion showed that the combustion stability is optimum for spark timings and equivalence ratio corresponding to maximum torques. The effect of retarding the spark timing from maximum torque timing and rich mixture on cycle to cycle variations are respectively higher than the effect of advancing the spark timing from maximum torque timing and lean mixture.