Effects of intake dilution on the combustion and emission characteristics of a hydrogen internal combustion engine and multi-objective dilution strategy optimization

Hydrogen internal combustion engines (H2ICEs) are promising zero-carbon power sources for transportation because of their wide flammability limits, high flame speed, and zero CO2 emissions. However, high reactivity can produce excessive nitrogen oxides (NOx), creating a trade-off between performance and emissions. This study experimentally investigated the combined effects of excess air ratio (λ) and exhaust gas recirculation (EGR) on a six-cylinder multi-point port-fuel-injected H2ICE at 1200, 1500, and 1800 r/min over a wide range of λ and EGR rates. Combustion characteristics, performance, and emissions were evaluated, including ignition delay, rapid combustion duration, brake torque, brake thermal efficiency (BTE), coefficient of cyclic variation of indicated mean effective pressure (COV of IMEP), NOx, and unburned H2. Results showed that EGR mainly prolonged the post-combustion phase, and λ = 2.2 is a critical stability threshold (COV≤2%). In low-speed rich conditions (λ < 2.0), high EGR effectively suppressed knock and optimized ignition timing. At mid speed, medium EGR (10–15%) balanced combustion phasing and stability, while at high speed with λ > 2.4, EGR below 10% avoided excessive post-combustion heat release. BTE peaks at λ = 2.2–2.4, reaching 37.98%, 33.90%, and 29.02% at 1200, 1500, and 1800 r/min, respectively. NOx was reduced by over 90% in rich conditions with high EGR, whereas unburned H2 rose sharply when λ exceeded 2.4, particularly at high EGR rates. A game-theory-based multi-objective optimization framework was further used to obtain speed-dependent optimal λ-EGR combinations under stability constraints. The results indicated a relatively high dilution level at 1200 r/min (EGR = 25%, λ = 2.12), with both optimal λ and EGR decreasing as speed increased (1500 r/min: EGR = 13%, λ = 2.09; 1800 r/min: EGR = 12%, λ = 2.01). This study proposes speed-specific λ–EGR strategies that jointly optimize efficiency, power, combustion stability, and emissions, offering engineering guidance and theoretical insights for developing high-performance, low-emission H2ICEs.