NOx reduction in hydrogen-fueled direct-injected spark ignition (DISI) engine using post-injection strategy: Experimental and chemical kinetics approaches

The increasing demand for sustainable transportation has emphasized the need for advanced technologies to mitigate greenhouse gas (GHG) and nitrogen oxide (NOx) emissions. Hydrogen internal combustion engines (HICEs) are promising candidates for zero-emission vehicles thanks to no CO₂ emissions. However, managing NOx emissions remains a critical challenge, particularly under near-stoichiometric conditions. This study investigates a post-H₂ injection strategy during the expansion stroke of HICEs to reduce NOx emissions inspired by selective non-catalytic reduction (SNCR). Post-H₂ injection was performed at 40–160° aTDC in 20° CA intervals, with an injection pressure of 30 bar. The post-injection rate was varied from 10 % to 40 % of the total hydrogen flow rate at 2000 rpm and from 20 % to 40 % at 4000 rpm. Experimental results showed that NO reduction rates exceeded 80 % with a 20 % post-injection rate, reaching over 90 % at 30 % injection rates. The 0-D simulations revealed Zeldovich reactions (N + NO ⇌ N₂ + O and H + NO ⇌ N + OH) play a significant role in NO reduction under high initial NO concentrations, while its effectiveness increases at lower temperatures. For moderate NO concentrations, HNO formation (N + NO + M ⇌ HNO + M) becomes the dominant pathway to reducing NO, whereas the significance of Zeldovich reactions diminishes. In addition, It was also confirmed that the NO reduction caused by additional hydrogen post-injection did not lead to the pathway that increases NO₂ and N₂O generation. These findings suggest that post-H₂ injections could significantly reduce the size of after-treatment systems or be selectively applied during transient operating conditions to achieve substantial reductions in NOx emissions.