Experimental and modeling investigations to improve the performance of the near-zero NOx emissions direct-injection hydrogen engine by injection optimization

High NOx emissions present a significant constraint on the power and efficiency enhancement of direct-injection hydrogen engines. To address this issue, a 2.0 L turbocharged direct-injection hydrogen engine is optimized to achieve near-zero NOx emissions and high engine performance through injection optimization. Both experimental and simulation results indicate that early injection of supersonic hydrogen increases the turbulence energy, while late injection leads to the clustering of rich mixing gases, resulting in elevated combustion temperature and increased NOx emissions. The NOx emissions can keep near-zero while the brake thermal efficiency improves from 40.4% to 41.7% with appropriate retarded injection timing. Therefore, an optimal injection timing exists, and the experiments revealed that adjusting the end timing of injection to 80°CA BTDC resulted in the approximately 1.5% increase in the brake thermal efficiency compared to other injection strategies with near-zero NOx emissions.