Spray-to-combustion interaction in hydrogen direct injection engines: Effects of injector structure and injection pressure

Hydrogen direct injection engines offer a promising pathway toward clean transportation by enabling flexible fuel–air control and supporting lean-burn operation. However, the coupling effects of injection pressure and injector design on spray morphology, in-cylinder mixture formation, and combustion behavior remain insufficiently understood. This study presents a systematic investigation into these interactions through a three-tiered methodology combining constant volume combustion chamber experiments, computational fluid dynamics simulations, and engine validation. Three types of injectors—including a needle-type single-hole, outward-opening, and outward-opening with a flow guide structure—were tested. The results demonstrate that spray structure and penetration were highly dependent on injector configuration. The needle-type single-hole injector exhibited strong axial penetration, while the outward-opening with a flow guide structure injector promoted broader lateral dispersion. Computational fluid dynamics simulations revealed that injection pressures of 40 and 50 bar optimized turbulent kinetic energy and mixture stratification near ignition timing. Engine tests confirmed that 50 bar achieved the highest in-cylinder pressure and combustion efficiency (Indicated mean effective pressure = 6.49 bar, Coefficient of variation = 0.65 %). All conditions maintained low nitrogen oxides emissions (<30 ppm). The experimental and numerical results show consistent trends, reinforcing that injection pressure must be carefully matched with injector geometry to ensure efficient and stable combustion. These findings provide valuable guidance for the calibration of hydrogen direct injection engines and highlight key considerations for developing low-emission hydrogen powertrains.