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

Natural gas combustion experiments were conducted in a lean burn CNG engine to uncover the regularities of cyclic combustion variations (CCVs) in different operating conditions. The effects of the engine operation parameters on the correlations between pressure related characteristic parameters of CCVs were also investigated. In the study, the level of CCVs was indicated by coefficient of variation of the indicated mean effective pressure (COVIMEP) and the investigation was conducted by two different ignition timing setting methods, which were fixed ignition timing method and the minimum COVIMEP (MCV) ignition timing method. The results show that the minimum COVIMEP appears at the optimal ignition timing while other operation condition parameters were fixed. And the trends of COVIMEP versus equivalent ratio, throttle opening position and engine speed separately vary significantly by setting ignition timing at different fixed value, but the trends were not disturbed as MCV ignition timing method is used. Furthermore, by computing the correlation coefficient, two pairs of parameters, which were the maximum cylinder pressure (pmax) versus its appearance time (θpmax) and pmax versus the maximum rate of pressure rise ((dp/dφ)max), showed stable and strong linear correlations. However, other two pairs of parameters, which were IMEP versus pmax and IMEP versus (dp/dφ)max, were influenced obviously by the equivalence ratio, engine speed and throttle opening.