Investigation of combustion chamber geometry on mixture formation and combustion characteristics in a heavy-duty direct-injection hydrogen engine

Hydrogen is a promising alternative for decarbonizing the heavy-duty transport sector. However, inhomogeneous mixture distribution hinders its practical application. Optimizing combustion chamber geometry is an effective strategy to enhance mixture distribution and engine performance. Based on a 14.8 L turbocharged heavy-duty direct-injection hydrogen engine, this study investigates the effects of four combustion chamber geometries (original, cross-shaped, petal-shaped, and wave-shaped) on mixture formation, combustion characteristics, and emission characteristics under lean-burn conditions using three-dimensional numerical simulations. The results indicate that the combustion chamber geometry plays a decisive role in mixture stratification and turbulent kinetic energy development by guiding the hydrogen jet and organizing the in-cylinder macroscopic flow field. Specifically, the wave-shaped chamber sustains a strong tumble motion during the compression stroke, creating a high-turbulence zone near the spark plug and an ideal “rich-core and lean-periphery” stratified mixture at the ignition timing. At λ = 2.0, the wave-shaped chamber achieves a gross indicated thermal efficiency of 45.9%, representing an increase of 3.2 percentage points compared to the original chamber. As λ increases, the wave-shaped chamber exhibits excellent adaptability. At λ = 2.5, it achieves a gross indicated thermal efficiency of 43.6% and NOx emissions of 695 ppm, representing the optimal trade-off between performance and emissions. Additionally, the cross-shaped chamber approaches near-zero emissions at λ = 2.7. These findings provide critical theoretical support for optimal design of combustion chambers in heavy-duty hydrogen engines.