Investigation of knock combustion mechanism and injection angle optimization in a heavy-duty direct-injection hydrogen engine

Hydrogen engines are a key power source for achieving zero carbon emissions in commercial vehicles. However, the issue of knock combustion severely restricts their application. In this study, numerical simulation methods were adopted to investigate the mechanisms of knock combustion in a heavy-duty direct-injection spark-ignition hydrogen engine, as well as the optimization of injection angles under different equivalence ratios (φ). The results show that conventional knock is due to pressure oscillations formed by the superposition of circumferentially propagating pressure waves along the cylinder wall, which are initially generated by unstable combustion and propagate radially. In contrast, super knock is produced by high-pressure waves resulting from the auto-ignition of end-gas mixtures, which interact with the flame to form intense pressure oscillations. The auto-ignition of end-gas mixtures is determined by pressure waves intensity from the flame front and the mass fraction of H2 in the end-gas. The concentration distribution of the mixture governs the initiation location of conventional knock and the region of maximum pressure oscillations. Knock intensity does not vary monotonically. When injection angle is decreased to 47.5°, knock is mitigated: knock is eliminated when φ < 0.6, while super knock is suppressed at φ = 0.6.