Development of a Large-Eddy Simulation Methodology for the Analysis of Cycle-to-Cycle Combustion Variability of a Lean Burn Engine

Abstract : Ultra-lean burn conditions (λ > 1.8) is seen as a way for improving efficiency and reducing emissions of spark-ignition engines. It raises fundamental issues in terms of combustion physics and its modeling, among which the significant reduction of the laminar flame speeds and increase of the laminar flame thickness, as well as an increased sensitivity to local fuel/air equivalence ratio variations are essential to be accounted for as compared to conventional stoichiometric mixture conditions. In particular, the effects of the modified laminar flame characteristics on flame stretch during the early flame development in a spark ignited gasoline engine can be expected to become of importance. In the present work, a Large-Eddy Simulation combustion approach is presented and applied to the study of the cycle-to-cycle combustion variations of a direct injection gasoline engine operating both in stoichiometric and ultra-lean burn conditions. The Coherent Flame Model approach is used and enriched via a correlation for the laminar flame velocity accounting for nonlinear stretch effects. The stretched flame calculations are validated against experimental results. Then, different engine operating points are computed in stoichiometric and ultra-lean burn conditions assessing the capacity of the approach to reproduce variations of combustion regimes. The results are analyzed in terms of cycle-to-cycle combustion variabilities and the influence of the spark-plug orientation is studied. Finally, a detailed analysis of the flame development is presented with a particular emphasis on the analysis of the initial flame kernel development accounting for stretch effects in lean conditions and the analysis of extreme cycles in lean burn. A strong reduction of the flame velocity by one third was observed for lean-burn conditions due to non-linear stretch effects occurring during the early stage of the flame development while almost no change was observed for stoichiometric conditions. Moreover, the proposed approach was capable of handling the various conditions featuring significantly different combustion regimes (one order of magnitude for the Karlovitz number) with only a minor change in the model parameterization.