Effect of Injection Strategy on the Combustion and Knock in a Downsized Gasoline Engine with Large Eddy Simulation

Strategies to suppress knock have been extensively investigated to pursue thermal efficiency limits in downsized engines with a direct-injection spark ignition. Comprehensive considerations were given in this work, including the effects of second injection timing and injector location on knock combustion in a downsized gasoline engine by large eddy simulation. The turbulent flame propagation is determined by an improved G-equation turbulent combustion model, and the detailed chemistry mechanism of a primary reference fuel is employed to observe the detailed reaction process in the end-gas auto-ignition process. The conclusions were obtained by comparing the data to the baseline single-injection case with moderate knock intensity. Results reveal that for both arrangements of injectors, turbulence intensity is improved as the injecting timing is retarded, increasing the flame propagation speed. It can be found that knock intensity is greatly affected by the location of the injector since the distribution of the end gas is determined by the direction of the fuel injection. In present study, when injector is mounted on the opposite side as the spark plug, double injection strategies induce knock to higher intensity, and the auto-ignition even causes super knock when second injection timing is -100 CAD. Conversely, knock is suppressed by double injection strategies under the condition with opposite injector, because limited space and insufficient consumable gas in the end region prevent severe pressure oscillations. This work will give a new insight into suppressing knock mechanism and improving the power performance by different injection strategies.