Experimental Investigation on Cycle by Cycle Variations in a Natural Gas Fuelled Spark Ignition Engine

Experiments were conducted on a single cylinder, natural gas fuelled spark ignition engine. Air fuel ratio was varied from about stoichiometric to the lean limit at two different throttle positions with optimum spark timing. Subsequently the engine was tested at constant throttle and equivalence ratio with variable spark timing. COV (coefficient of variation) of IMEP (indicated mean effective pressure) and peak pressure increase with a reduction in equivalence ratio. When the engine starts to misfire there is a drastic increase in the COV of IMEP. Spark timing has a smaller effect on COV of IMEP than on COV of peak pressure. When the spark timing is advanced, COV of peak pressure attains a minimum value just before knock sets in. Prior cycle effects were seen when there was misfire. Spark timing had little influence on the frequency distribution of IMEPs of cycles, which was generally symmetrical about the mean. Individual cycle IMEPs are almost directly proportional to peak indicated torque at MBT (minimum advance for best torque) timing. Such a relationship does not exist between peak torque and peak pressure when there are misfiring cycles. However, the relationship between peak pressure or IMEP with individual cycle peak torque changes with spark timing, when throttle position and equivalence ratio are held constant.