• Air dilution reduces NO x emissions while keeping combustion stable at high λ • EGR dilution combined with moderate λ reduces performance and combustion stability • Multi-pulse injection increases NO x and lowers thermal efficiency slightly • Water injection reduces NO x but raises instability at high levels of λ This study experimentally investigates combustion and emission control strategies for a spark-ignition direct-injection hydrogen (DI-H 2 ) engine operating under high-load and high-speed conditions. The work systematically evaluates the influence of air dilution, exhaust gas recirculation (EGR), multi-pulse injection, and water injection on combustion phasing, knock tendency, and cycle-to-cycle variability. Results show that air dilution effectively reduces NO x emissions—particularly at high excess air ratios ( λ ≥ 2.2), while maintaining stable combustion and high efficiency. Advancing combustion phasing toward TDC improves indicated efficiency levels but increases knocking risk at lower dilution levels ( λ = 2.0 ). EGR dilution, at a fixed λ = 2.3 , further decreases NO x but leads to unstable operation and reduced gross indicated efficiency (GIE). Multi-pulse injection strategies, in which approximately 20% of the total injected fuel mass is shifted from the main pulse to an early pilot injection during the intake stroke, provide no efficiency gain and increase stability and NO x compared with single-pulse operation. In contrast, moderate water injection (water-to-hydrogen mass ratio of approximately 2) achieves over 50% NO x reduction with acceptable combustion stability, while excessive injection (water-to-hydrogen mass ratio of 3) degrades efficiency and stability. Overall, the results highlight air dilution as the most effective strategy for balancing efficiency, emissions, and stability, while water injection offers promising potential for additional NO x control and knock mitigation. These findings provide quantitative insights into optimizing combustion strategies for high-load DI-H 2 engines and support the development of efficient, low-emission hydrogen combustion systems.
Molina et al. (Sun,) studied this question.