Computational study of the influence of in-cylinder flow on spark ignition–controlled auto-ignition hybrid combustion in a gasoline engine

In this research, computational fluid dynamics simulations were performed to investigate the effect of in-cylinder flow motion on the in-cylinder conditions and spark ignition–controlled auto-ignition hybrid combustion. It is proved in this study that asymmetric intake valve events could be used to generate the swirl-dominated flow motion. However, the macroscopic flows, such as the swirl and tumble, show very weak correlations with zone-to-zone conditions and hybrid combustion process. The detailed investigation on the in-cylinder zone-to-zone conditions indicates that the in-cylinder turbulent kinetic energy level and the mean flow velocity (Vm) around the spark plug would directly affect the early flame propagation process, which in turn affect the subsequent auto-ignition process through changing the heat transfer between central burned gas and end-gas. In addition, the increased temperature inhomogeneity of the spherical zones caused by the in-cylinder flow motion would prolong the auto-ignition combustion. The structures of the flame front and auto-ignition sites also demonstrate the significant impact of in-cylinder motion on the combustion process. It is found that the combustion mode transition is very sensitive to the in-cylinder turbulent kinetic energy, Vm, and temperature and its inhomogeneity, indicating that these flow and thermal conditions could be used to optimize the hybrid combustion mode operation. It also proves the fluctuations of the in-cylinder flow, and thermal conditions could be the reasons leading to significant cycle-to-cycle variations in spark ignition–controlled auto-ignition hybrid combustion.