Experimental investigation of NOx emissions and SCR optimization in hydrogen internal combustion engines under full-range operating conditions

Hydrogen internal combustion engines (HICEs) are emerging as a key technology in achieving carbon neutrality due to their zero carbon emissions and superior combustion properties. This study presents a systematic investigation of NOx formation mechanisms and emission characteristics in a lean-burn spark-ignition HICE across a wide range of operating conditions. Experimental results reveal that increasing the relative air-fuel ratio (λ > 2.4) effectively reduces NOx emissions to below 0.2 g/kW∙h, with combustion stability influenced primarily by cycle-to-cycle variation at low loads and intake limitations at high loads. Additionally, the NOx composition is strongly dependent on both load and equivalence ratio, with a significant increase in N₂O fraction under low-load, ultra-lean conditions. To further mitigate NOx emissions at high loads, a Cu-CHA-based selective catalytic reduction (SCR) system was evaluated under varying temperatures, space velocities (SV), and NH₃/NOx ratios. Results indicate that ammonia storage capacity is more sensitive to SCR center temperature than SV, and achieving near-complete NOx conversion (≈100 %) requires precise control of ammonia dosing, especially under high SV conditions to avoid excessive NH₃ slip. These findings offer foundational insights into optimizing combustion strategies and aftertreatment control for hydrogen-powered engines in passenger vehicle applications.