Effect of exhaust gas recirculation on the cycle-to-cycle variations in a natural gas spark ignition engine

This study investigates the effect of exhaust gas recirculation (EGR) on the cycle-to-cycle variations (CCV) in combustion in a natural gas spark ignition engine. The engine is operated at 2000 rpm and a stoichiometric fuel-air mixture is used. The EGR level is changed from 0% to 5%, 10%, 15%, and 20%. For each EGR level, a continuous wavelet transform is used to analyze the time series of the indicated mean effective pressure (IMEP) over 200 cycles. The dominant oscillatory modes of the CCV are identified and the engine cycles over which these modes may persist are delineated. The results reveal that the CCV of the IMEP occur on multiple timescales and exhibit complex dynamics. With no EGR, mainly high frequency intermittent fluctuations are observed. As the EGR level is increased, more persistent low frequency variations tend to develop. In addition, the spectral power increased with an increase in the EGR level. At the EGR level of 20%, the spectral power is found to increase significantly indicating that EGR has a pronounced effect on increasing the CCV. Knowledge of the dominant modes of variability may be useful to develop effective control strategies for reducing the CCV and improving engine performance in the presence of EGR.