A Computational Study of In-Cylinder NO_x Reduction Strategies for a Compression-Ignition Engine Fueled With Diesel/Hydrogen Mixtures

Considerable efforts have been made to introduce alternative fuels for use in conventional diesel and gasoline engines. There is significant interest in adding hydrogen to a diesel engine to reduce emissions and improve efficiency. However, the main challenge associated with the use of hydrogen in diesel engines is high nitrogen oxide (NO_X) emissions. In the present study, a reduced chemical kinetics mechanism, consisting of 52 reactions and 29 chemical species for n-heptane fuel combustion, was incorporated with detailed chemical kinetics consisting of 29 reactions for hydrogen as well as additional nitrogen oxidation. This reaction mechanism was coupled with 3-D advanced CFD software to investigate the performance and emission characteristics of a diesel-hydrogen dual-fuel engine. Computational results showed good agreements with the experimental results for brake thermal efficiency, CO₂, CO, and NO_X emissions. The model was then employed to examine the effects of exhaust gas recirculation (EGR) and N₂ dilution on NO_X emissions. The computational results quantified the reduction in NO_X emissions with EGR and N₂ dilution, and a more remarkable reduction was found with 30% N₂ dilution. However, in terms of the N₂ dilution, a general decreasing trend was observed for both NO_X and CO₂ emissions, while CO emissions increased. In relation to the EGR, the NO_X emissions decreased while CO₂ and CO emissions significantly increased. Additionally, the results showed that the indicated mean effective pressure (IMEP) and indicated power decreased as the N₂ dilution increased. The same trend was observed for the EGR but the reduction was less compared to that of the N₂ dilution.