Numerical study on early flame kernel development in spark ignition engines

The development and control of spark ignition engines with increased efficiency and reduced engine-out emissions requires simulation tools capable of predicting cycle-to-cycle variations (CCV). While the Large-Eddy Simulation (LES) technique has this potential, available LES methods typically fail to reproduce cyclic variability observed in experiments. Although evidence for the correlation between early flame kernel development and CCV was provided long ago, there is still a lack of fundamental understanding of the initial combustion phase, and accurate LES models do not exist. In this work, these shortcomings have been addressed in two different ways. First, multi-cycle LES with low numerical discretization error have been realized by integrating state-of-the-art methods and models into an existing Cartesian LES code designed to minimize artificial dissipation. Validation results for several sub-models are provided and accurate predictions of engine-relevant quantities of interest such as tumble ratio, inflow pressure loss, and global burning rate are shown. The application of the enhanced LES framework to compute CCV in a production-type engine and to simulate early flame kernel development on a locally refined mesh are demonstrated. Second, early flame kernel/turbulence interactions and differential diffusion effects during flame kernel development have been analyzed based on an engine-relevant direct numerical simulation (DNS) database, which was specifically designed for this purpose. Run-to-run variations in the global heat release rate of flame kernels are shown to be caused by flame kernel area dynamics. A novel analysis reveals that the related occurrence of excessive curvature variance is due to flame kernel distortion by large-scale turbulent flow motion with characteristic length scales greater than the flame kernel size. Differential diffusion is shown to substantially reduce the global burning rate of flame kernels in mixtures of common transportation fuels and air, thus increasing CCV tendency. The analysis shows that the large positive global mean curvature intrinsic to the flame kernel configuration may detrimentally affect the local mixture state inside the reaction zone, which can be compensated by spark energy supply during the most critical initial stage. Further, it is shown that differential diffusion effects under engine-typical Karlovitz numbers are not weakened by small-scale turbulent mixing, which is undesirable for the engine application, but can be favorable in terms of modeling