Influence of combustion chamber pit depth on the combustion process and emissions in diesel engines

Combustion chamber structures and shapes exert a considerable influence on in-cylinder airflows, air-fuel mixing, and the combustion process, especially in high pressure direct injection diesel engines. This study systematically compares stepped-lip and ω-type chambers, revealing a nonlinear relationship between pit depth and pollutant formation under high boost conditions. Using CONVERGE 3D simulations with a 1/8 cylinder model, we analyze how pit depth influences spray dynamics, air-fuel mixing, combustion phases, and emissions. Key findings: (1) For stepped-lip chambers, shallower pits (h = 13 mm) accelerate combustion, achieving 15% lower PM and 5% higher NOx versus baseline, offering optimal PM-NOx balance for Euro VII compliance. (2) ω-type chambers with 17 mm depth maximise efficiency, improving indicated thermal efficiency by 2.1% while maintaining emission control. Comparative analysis shows distinct combustion duration (3–5°CA differences) and combustion phasing shifts between configurations. The nonlinear pit depth-emission correlation provides critical guidance for combustion optimisation, demonstrating how structural adjustments can reconcile efficiency improvements (17% soot reduction in stepped-lip) with emission regulation requirements. These insights advance combustion chamber design strategies for next-generation engines under stringent environmental standards.