Effects of hydrogen injection timing and injection pressure on mixture formation and combustion characteristics of a hydrogen direct injection engine

In this study, the effects of injection pressure and injection timing on mixture formation and combustion characteristics are carefully investigated in a direct-injection (DI) hydrogen engine, using a three-dimensional (3D) numerical method. Firstly, the numerical models are comprehensively validated in terms of the hydrogen injection process and combustion process at different injection pressures. The results show that the homogeneity of the mixture distribution becomes worse as the start of injection (SOI) is delayed at a fixed injection pressure. And the rich mixture was distributed near the cylinder wall, which also leads to a decrease in the flame propagation velocity. At this work, as the injection pressure decreases at a fixed SOI of -220 CAD ATDC, the peak cylinder pressure increases. That is because for the low injection pressure, the injection pulse width rises significantly, the mixture stratification in the cylinder becomes more inhomogeneous, and the rich mixture is distributed near the spark plug, which leads to a significant increase in the flame propagation velocity. While the fast combustion leads to a higher combustion temperature and subsequently an increased NOx emission. At a fixed end of injection (EOI), the in-cylinder mixture became more inhomogeneous as the decrease of injection pressure. It is interesting that the injection pressure only influences the turbulence kinetic energy during the injection process. In addition, at the fixed SOI and EOI, the peak cylinder pressure has little influence on increasing the nozzle diameter. The mixture distributions of the two conditions are similar, in which the rich mixture region is close to the spark plug, thus the combustion rate is slightly increased. Overall, the present work confirms that the mixture formation and mixture distribution determine the combustion process for the hydrogen engine, and low injection pressure can also achieve better performance.