Effects of different excess air coefficients and spark timing on the performance and emissions of a heavy-duty hydrogen SI engine under lean conditions

• A critical threshold is identified at λ = 2.2, beyond which intake pressure supersedes the excess air coefficient as the dominant control factor for combustion. • When hydrogen is burned in a lean mixture, delaying spark timing can reduce NOx emissions by up to 50 %. • The combined effect of advanced spark timing and a high excess air coefficient (λ ≥ 2.3) advances the combustion phase. When coupled with the piston's upward compression effect, this effectively offsets the cylinder pressure loss caused by ultra-lean combustion, while enhancing power output. • The λ–SPK synergistic strategy achieves an indicated thermal efficiency of over 46 %, while achieving near-zero nitrogen oxide emissions. This study utilized CONVERGE software to establish a three-dimensional combustion chamber simulation model for a 12.5 L port fuel injection (PFI) heavy-duty hydrogen-fueled internal combustion engine and validated the reliability of the model through experimental data. Based on this simulation model, the effects of excess air coefficient (λ = 2.0–2.3) and spark timing (SPK) on combustion, performance, and emission characteristics were systematically investigated under a 1200 r/min operating condition. The results indicate that, under fixed SPK timing conditions, as the excess air coefficient increases from 2.0 to 2.3, combustion intensity is significantly weakened, with peak cylinder pressure and heat release rate (HRR) decreasing by 12.3 % and 39.3 %, respectively, while NOx emissions decrease by 88 %. The influence of intake pressure on the combustion process increases with increasing λ. Advancing the SPK timing significantly improves combustion efficiency. When λ is 2.0 and 2.3, the corresponding cylinder pressure increases by 23.7 % and 10.6 %, respectively, and the HRR increases by 27.4 % and 10.4 %, respectively. However, this results in increased NOx emissions. Delaying the SPK timing causes NOx emissions to drop sharply, with a maximum reduction of 50 %. The combined effect of advanced SPK timing and a high excess air coefficient (λ ≥ 2.3) leads to a shift in the combustion phase. When this is coupled with the piston’s upward compression effect, it effectively increases cylinder pressure and enhances power output. This study provides theoretical support for understanding the performance of large-displacement hydrogen-fueled internal combustion engines under lean-burn conditions.