Influence of flow and ignition fluctuations on cycle-to-cycle variations in early flame kernel growth

Engine Cycle-to-Cycle Variations (CCVs) constitute a restriction for numerous concepts aimed at improving fuel consumption in spark-ignited engines. In consequence, engine CCV modeling has become an active research topic. Nevertheless, few fundamental studies have thus far examined the individual influences of the key parameters. In the present study, we performed 2D Direct Numerical Simulations (DNSs) of early flame kernel growth with semi-detailed chemistry solving to identify the controlling parameters of heat release fluctuations that are relevant to engine CCVs. Our objective was to assess how certain turbulence and flame parameters - namely turbulent flow motion pattern, turbulence intensity, integral length scale, mixture strength and initial kernel size - influence combustion variability. Two extreme mixture conditions were selected (stable operation at stoichiometry and unstable operation at the lean flammability limit), and computation conditions were derived from actual low load engine operating points. Results provide a classification of the key parameters, indicating initial kernel size and turbulent flow motion pattern as the main contributors. Additionally, DNSs highlighted the crucial role of mixture strength in the occurrence of local flame quenching and in the related amplification of heat release fluctuations.