Study on the effect of engine operation parameters on cyclic combustion variations and correlation coefficient between the pressure-related parameters of a CNG engine

► We test a CNG engine using two methods on setting ignition timing. ► Trends of COVIMEP versus operation parameters under the two kinds of ignition timing are different. ► The correlation coefficients are affected by operation conditions. ► R(pmax, θpmax) and R(pmax, (dp/dφ)max) indicate stable, strong linear correlations. ► R(IMEP, pmax) and R(IMEP, (dp/dφ)max) indicate obvious linear correlations in some cases.