Fast-NO emission analysis of different mixture formation strategies in a hydrogen single-cylinder heavy-duty engine

This study investigates nitrogen oxide emissions (NO\(_x\)) in a heavy-duty hydrogen engine by comparing Port Fuel Injection (PFI) with two Direct Injection (DI) configurations under various load conditions. A fast chemiluminescence detector (CLD) enables cycle-resolved nitrogen monoxide emission (NO) measurements, providing detailed insights into the emission characteristics of each injection strategy. The findings reveal that the PFI configuration consistently results in the lowest NO\(_x\) emissions due to superior air–fuel mixture homogenization. Additionally, it exhibits minimal cycle-to-cycle variations in both pressure traces and NO emissions. The indicated efficiency of the PFI setup is also higher compared to DI, likely due to the higher charge air pressures required to maintain a constant air–fuel ratio and reduced wall-heat losses. Conversely, the DI configurations, especially the 4-hole cap design, produce significantly higher NO\(_x\) emissions and show considerable variability between cycles. A strong exponential correlation between NO emissions and peak cylinder pressure (p\(_{max}\)), which directly influences in-cylinder temperature, is observed across all configurations. The DI setups exhibit faster combustion, driven by increased turbulent kinetic energy from the hydrogen jet, leading to higher in-cylinder pressures and temperatures. This rapid combustion process complicates emission control by increasing NO\(_x\) formation. Despite similar combustion behavior and efficiency between the 1-hole and 4-hole DI setups, the latter generates much higher NO\(_x\) emissions, highlighting the crucial role of mixture homogenization. Cycle-based analysis further indicates that DI configurations, particularly the 4-hole cap design, experience single-cycle NO emissions spikes, making consistent emission control more challenging. While PFI presents clear advantages in emission reduction and efficiency, DI setups provide comparable power output with lower charge air pressure requirements. However, challenges in mixture formation must be addressed to optimize DI strategies for hydrogen engines. Overall, the study underscores the significance of optimizing mixture formation to mitigate NO\(_x\) emissions in hydrogen engines.