Tumble and swirl flow motion effects on hydrogen combustion and NOx formation in spark-ignition engines for passenger car engines

This study presents an experimental comparison of three in-cylinder flow strategies — tumble, swirl, and flat deck swirl (FD swirl) — in a hydrogen-fueled spark-ignition engine under lean operation. Two single-cylinder direct-injection research engines were used to represent tumble and swirl architectures, with a modified swirl configuration to obtain FD swirl. Three representative operating points at fixed lambda, comb phasing, and load degree were evaluated, after optimizing valve timing, injection phasing, and spark timing. The analysis focused on burn rate, combustion stability, knock tendency, thermal efficiency, and emissions (NO x and unburned H 2 ). Tumble delivered the fastest and most stable combustion, lowest knock intensity, and highest efficiency (up to 47%, compared to 38% for swirl). FD swirl showed intermediate behavior, which improved combustion relative to swirl. However, tumble produced higher NO x at high load and increased unburned H 2 . The results clarify how flow topology governs hydrogen combustion and provides empirical guidelines for the conversion from previous specialized designs to high-efficiency hydrogen engine development.