Analyses about parameters that affect cyclic variation in a spark ignition engine

The combustion stability of a small spark-ignition engine has been investigated by means of both experimental tests and numerical analyses. The cyclic variation in combustion has been measured at part load considering several combustion chamber geometries. For each considered operating point, the in cylinder phenomena characterizing the engine have been reproduced by CFD analysis performed by means of a RANS approach. The properties of the mean flow calculated in the spark region at the spark time seem to affect the extent of the measured cyclic dispersion. In particular, for the investigated engine, the measured coefficient of variation in the indicated mean effective pressure seems correlated with both the calculated mean laminar flame speed and the calculated mean turbulence intensity.