Study of Engine Knock in HCCI Combustion using Large Eddy Simulation and Complex Chemical Kinetics

This paper studied the knock combustion process in gasoline HCCI engines. The complex chemical kinetics was implemented into the three-dimensional CFD code with LES (Large eddy simulation) to study the origin of the knock phenomena in HCCI combustion process. The model was validated using the experimental data from the cylinder pressure measurement. 3D-CFD with LES method gives detailed turbulence, species, temperature and pressure distribution during the gasoline HCCI combustion process. The simulation results indicate that HCCI engine knock originates from the random multipoint auto-ignition in the combustion chamber due to the slight inhomogeneity. It is induced by the significantly different heat release rate of high temperature oxidation (HTO) and low temperature oxidation (LTO) and their interactions. Pressure wave occurrence can be explained by the fact of significant pressure gradients in HCCI combustion field, which caused by multipoint auto-ignition in constant-volume heat release. For instance, slightly higher temperature spots were formed before ignition due to wall heat transfer and turbulence conversion. The exceedingly exothermic HTO with rapid heat release (in microseconds) at these local zones leads to the high-temperature hot-spots. These hot-spots with adiabatic flame temperature strongly influence the LTO of surrounding mixture. The pressure waves propagate, reflect and overlap in the combustion chamber which leads to pressure oscillation. The higher amplitude of pressure wave was observed at the center of the cylinder due to pressure focusing.