Correlations between cycle-to-cycle variations and combustion parameters of a spark ignition engine

The coefficient of covariation (COV) of the in-cylinder pressure is determined on each crank angle of a number of cycles, in the case of a natural gas feed SI engine operating under lean conditions. The resulting curve of COV versus crank angle is explored. Its shape is independent of the experimental conditions and it is always the same for all the experimental points used. Three points of this curve show particular interest, as they correspond to the combustion beginning, combustion end and the point where the mass burned fraction is 50% (half combustion duration). The hypothesis concerning the combustion limits is proved by the comparison of the COV curves of fired and motored cycles and by the determination of the combustion beginning and end using different methods. The hypothesis of the half combustion duration point is verified from a burn rate analysis of the cycle. The integral of the COV curve in the combustion region is proposed as a criterion of the cyclic dispersion quantification. The integrals of the first and second combustion periods are explored as a function of the engine operating parameters.