Effects of Engine Geometries on the Combustion Characteristics of a Heavy-Duty Hydrogen Spark-Ignition Engine

For heavy-duty applications, hydrogen (H2) internal combustion engines offer a practical solution for future transportation. However, the influence of cylinder head flow characteristics and piston geometry on lean H2 combustion remains insufficiently understood. This study presents a comprehensive computational investigation of three engine configurations characterized by distinct in-cylinder flow dynamics: mild swirl and tumble (Engine a), strong tumble (Engine b), and strong swirl (Engine c). High-fidelity three-dimensional computational fluid dynamics simulations were performed for both port-fuel injection (PFI) and direct injection (DI) strategies. The impact of piston geometry was evaluated by comparing the baseline piston with a flat piston, while the spark timing was optimized to achieve favorable combustion phasing. Combustion and NOx formation were modeled using a G-equation-based combustion framework incorporating diffusive-thermal instability effects and a validated in-house H2 chemical mechanism. Turbulence-flame interactions were further characterized using Borghi-Peters diagrams. Under PFI operation, the strong-tumble configuration (Engine b) generated the highest turbulent kinetic energy (TKE), resulting in faster flame propagation, more advanced combustion phasing, and improved thermal efficiency. The flat piston further enhanced efficiency by reducing mixture confinement within piston-induced recirculation zones. Under DI operation, H2 injection significantly increased turbulence intensity, and a flat piston promoted higher TKE near spark timing in Engines b and c by reducing mixture-wall interaction, leading to faster combustion compared with the baseline piston. In contrast, the original piston produced higher TKE within the piston bowl in Engine a due to stronger recirculation. Additionally, the strong-tumble configuration achieved the most homogeneous mixture distribution under DI conditions. These results demonstrate that in-cylinder flow structure, piston geometry, and DI injection strongly affect turbulence generation, mixture formation, and combustion performance. The strong-tumble configuration shows the greatest potential for achieving high thermal efficiency with controlled emissions in lean H2 spark ignition engines.