Large Eddy Simulation On The Knock Combustion Process And Its Influence Mechanism In A DISI Engine

The application of high compression ratio and boosted intake charge is a promising technique to improve the fuel economy of the direct-injection spark-ignition(DISI)engine.However,the increasing propensity of knock occurrence deteriorates the engine performance and may cause irreversible damage on the engine structure.In order to seek for effective means to suppress knock,a deep insight of knock combustion process and its influence mechanism have to be given.In the present work,the knock combustion process of a single-cylinder DISI engine is numerically studied by using the methodology of LES coupled with G-equation and a chemical kinetic model for primary reference fuel(PRF)oxidation.The combustion process of knock cycles with different intensities are firstly simulated and analyzed by adjusting the spark timing.It is found that as the spark timing is advanced,the spontaneous ignition occurs earlier,which leads to an intensified knock cycle or even a super-knock cycle.A conventional deflagration wave or subsonic auto-ignitive flame is always induced by the initial auto-ignition in the conventional knock cycle,while the super-knock is always related to a detonation wave.Subsequently,the effects of flame speed,turbulence intensity,thermal stratification and fuel stratification on auto-ignition propensity and knock intensity are investigated by changing the laminar flame speed coefficients in the G-equation model and remapping the initial in-cylinder field of velocity,temperature and concentration.The results reveal that Increasing flame speed as well as increasing turbulence intensity promote the knock intensity first and then reduce.Comparatively,intensive turbulence possesses a superior knock resistance than high flame speed.Both thermal stratification and fuel stratification essentially induce reactivity stratification.As the level of reactivity stratification increases,several combustion mode can be induced in the reaction front initiated by spontaneous ignition: thermal explosion,continuous auto-ignition,detonation and conventional deflagration.The corresponding knock intensity increases first and then decreases.However,there are some differences between thermal stratification and fuel stratification in the effects on initial auto-ignition occurrence.Thermal stratification has little effect on the spark-induced flame speed,thus can enhance the formation of initial AI kernel due to higher temperature in the local hot-spot.In contrast,fuel stratification reduces the flame speed,which leads to retarded auto-ignition in the end-gas.On the other hand,higher fuel concentration reduces the ignition delay in the hot-spot.The two factors determine the result that the occurrence of initial auto-ignition retards first and then advances as the fuel stratification level increases.The stand-alone LEM simulations are conducted to take further insight into the effects of turbulence intensity and turbulence length scale on the end-gas auto-ignition of different PRFs.It is found that increased turbulence intensity delays the formation of the initial AI and shrinks the kernel size at the onset of AI due to the enhanced heat and radicals dissipation.However,after the first AI kernel forms,the intensified turbulence accelerates the overall combustion and increases the fraction of spontaneous ignition.The analysis of cases with different turbulent integral length scales reveals that the dissipation effect of the turbulence with the similar length scale of the nascent AI kernel is more prominent while the small-scale turbulence is more likely to increase the fraction of deflagration.The dissipation and homogenization effects of turbulence on the high-octane fuel are more prominent than on the low-octane fuel.With an initial temperature falling in the NTC regime,the first AI kernel of the low-octane fuel appears in the boundary layer with a lower temperature instead of the core region.As the turbulence intensifies,the effect of NTC on the end-gas auto-ignition vanishes.