Using large-eddy simulation and multivariate analysis to understand the sources of combustion cyclic variability in a spark-ignition engine

The origins of cyclic combustion variability (CCV) in spark-ignition engines are investigated using large-eddy simulation (LES) of a stable (low CCV) and two unstable (high CCV) operating points of a specifically dedicated experimental test-rig set up around a four valve pentroof single cylindre spark-ignition engine fueled with a premixture of gaseous propane and air. The unstable points are obtained from the reference by reducing significantly the equivalence ratio and by an important dilution by nitrogen respectively. A LES methodology is proposed and shown to be able to reproduce the experimental findings concerning phase-averaged mean and statistical variations around it of a number of key engine combustion parameters. The CCV and factors causing it are first illustrated by comparing typical slow and fast burning cycles in combination with simple correlation plots of major engine parameters, this allows qualitatively showing how local and global sources concur to generate CCV. In a second step, single parameter and multivariate regressions build from the LES results allow quantifying the relative importance of different local and global CCV sources. Finally, the comparison of the obtained findings as to the relative importance of major parameters on CCV is compared with qualitative summary from an extensive experimental survey by Ozdor et al. The presented LES results overall confirm major findings from the survey, but also indicate that detailed causes of CCV depend on the type of engine and its operation.