A Computational Study of Hydrogen Mixing and Combustion in a High-Speed Direct-Injection Spark-Ignition Engine

Hydrogen has emerged as a promising alternative fuel due to its high reactivity, fast flame speed, and potential to enable clean and sustainable energy systems. Direct injection is the preferred fueling strategy for hydrogen engines, as it enhances power density while addressing safety concerns. However, the low density of hydrogen necessitates a large molar quantity of fuel, leading to strong fuel-air stratification and posing challenges for mixing in a confined chamber with complex turbulent flow and jet/wall interactions. This challenge is exacerbated in the present study, where the evaluated racing engine operates at a high engine load (indicated mean effective pressure of about 20 bar) and an extremely high speed (7500 revolutions per minute). Furthermore, to restrict abnormal combustion behaviors and inhibit oxides of nitrogen emissions, a lean-burn combustion strategy with an overall equivalence ratio of 0.4 was applied, where diffusive-thermal (DT) instability effects would matter. To elaborate on the complicated fundamentals dynamics from injection to combustion for this high-speed direct-injection spark-ignition hydrogen engine, this study intends to establish a well-validated computational framework based on measured engine combustion data. Two flamelet-based combustion models, including the G-equation and Extended Coherent Flamelet Model (ECFM), were applied and analyzed. Specifically, for both models, the DT instability effects were considered for the generation of tabulated flame speeds and flame thicknesses. Additionally, the impact of mesh resolution, turbulent Schmidt number, and chemical kinetics mechanisms on the hydrogen combustion characteristics were evaluated. Combustion regimes were also characterized to improve the understanding of hydrogen combustion under extreme operating conditions.