Numerical analysis of piston bowl topologies in a hydrogen-powered commercial vehicle engine: Impact on thermal load and mixture formation using CFD

Abstract

The performance of hydrogen combustion engines can be significantly influenced by the geometry of the piston bowl. While geometric effects are well documented for conventional diesel and gasoline engines, systematic investigations distinguishing specific topological influences in hydrogen powered heavy duty engines remain limited. In this work, a systematic bowl radius sweep is performed to assess effects on thermal load and mixture formation, varying the bowl radius from 125 to 300 mm while maintaining a constant compression ratio. The engine, originally developed for commercial utility vehicles, has been specifically converted for hydrogen operation to accommodate the specific requirements of this fuel. Five distinct piston bowl geometries are modeled and evaluated through Computational Fluid Dynamics (CFD) simulations using the commercial software AVL FIRE™ M. A validated operating point based on a reference piston bowl geometry serves as the basis for comparison. A unique feature of this study is the explicit analysis of sub-stoichiometric zones; these persistent lean pockets are spatially mapped to identify geometry induced “dead zones” that critically impact ignition stability and local thermal wall loading. Special attention is placed on flow quality and mixture formation when using hydrogen as fuel. Potential adverse effects observed in conventional piston bowl configurations could be mitigated due to hydrogen’s exceptionally high diffusivity. This characteristic allows overcoming geometry induced drawbacks and emphasizes the positive aspects of particular bowl shapes. In particular, among the observed configurations, almost all result in an efficiency increase with a maximum of 1.2% in the observed operation point. A higher impact is seen in the emission control where a reduction of roughly 40% in NOx [ppm] was achieved, compared to reference geometry. The mentioned results indicate that the study provides a comprehensive understanding of how piston bowl topology influences thermal load, mixture formation, and flow quality in the context of hydrogen combustion engines.