Effects of residual burnt gas heterogeneity on early flame propagation and on cyclic variability in spark-ignited engines

This paper investigates the effects of Residual Burnt Gas (RBG) heterogeneity on premixed early flame propagation. The aim is to quantify the contribution of RBG spatial fluctuations on Cycle-to-Cycle Variations (CCVs) observed in Spark-Ignited (SI) engines under stoichiometric operation. We performed Direct Numerical Simulations (DNSs) of turbulent flame propagation with a semi-detailed kinetic mechanism for iso-octane (29 species and 48 reactions). To obtain relevant results for SI engines, aerothermochemical conditions (pressure, temperature, turbulence characteristics) representative of low-load engine operation were extracted from experimental data and engine Large Eddy Simulation computations. The main findings are as follows: (1) RBG heterogeneity combines two opposing effects. Hot temperature pockets tend to locally increase the laminar flame speed, while diluted mixtures slow down flame propagation. (2) RBG heterogeneities locally enhance flame propagation because of the dominant effect of temperature over dilution. (3) RBG heterogeneities are able to create additional flame wrinkling even without the introduction of sensible enthalpy fluctuations. (4) DNS results contain valuable information to further our understanding of previously misinterpreted experimental observations in engines [Sztenderowicz and Heywood, SAE paper 902142, 1990]. The influence of RBG heterogeneities on the flame development process is minor compared to their contribution on heat release fluctuations and therefore on engine CCVs as a result of the non-linear chemical response to local mixture conditions.