Numerical analysis of the influence of the fuel injection timing and ignition position in a direct-injection natural gas engine

•Gas sphere injection model is implemented and applied in engine simulation.•Effects of fuel injection timing and ignition position on combustion is clarified.•Effect of fuel injection timing on combustion is weakened as ignition is delayed.•Altering mixture and velocity is the root cause of fuel injection timing’s effect.•Ignition location influences flame propagation space and mixture distribution. Large-eddy simulation of fuel injection and combustion in a direct-injection natural gas engine was conducted. The influence of the fuel injection timing and ignition position was numerically analyzed. The engine used in this study operates in lean burn mode with a fuel-air equivalence ratio of approximately 0.72. The combustion pressure and in-cylinder burned volume decrease as fuel is injected earlier using the same ignition timing, and the fuel consumption rate also decreases. As ignition is delayed, the influence of the fuel injection timing is weakened because of the over-mixed mixture during the late compression stoke. Fuel injection timing changes the global fuel-air equivalence ratio, which is not the primary cause of its effect on combustion. As fuel is injected later, the in-cylinder velocity magnitude increases and a relatively richer mixture is distributed around the ignition position, which contributes to better combustion. This is the main mechanism of how fuel injection timing influences combustion. The ignition position determines the background distribution of the velocity magnitude and mixture and confines the available space for flame development. Central ignition is the best choice for the engine used in this study.