The mechanism of the increased ratio of nitrogen dioxide to nitrogen oxides in methanol/diesel dual fuel engines

In the heavy-duty transportation sector, using methanol to partially replace diesel in engines can help reduce carbon-based emissions and thus has the potential for widespread use. However, methanol/diesel dual fuel engines exhibit a high NO2/NOx ratio despite the small energy contribution from methanol, a phenomenon unique compared to diesel engines. To uncover the underlying mechanism causing the increased NO2/NOx ratio in methanol/diesel dual-fuel engines, which is limited in existing studies, this study first utilizes a three-dimensional (3D) computational fluid dynamics (CFD) model to reproduce the NO2 concentration surge phenomenon. Each cell in the combustion chamber is then assumed to be a zero-dimensional (0D) closed homogeneous batch reactor to conduct chemical kinetic analysis, focusing on the regions with NO2 formation. The 3D CFD simulation results show that, compared to pure diesel operation, methanol/diesel dual-fuel operation has two areas with significant NO2 formation during the late oxidation process: the near-squish zone and the periphery of the main diffusion flame in the bowl. The 0D kinetic analysis indicates that the elementary reaction NO+HO2↔NO2+OH is primarily responsible for converting NO to NO2. Additionally, in the region just outside the diffusion flame, where high temperatures are maintained due to heat transfer, methanol forms HO2, which converts NO to NO2. In the low-temperature regions of the near-squish zone, NO2 forms where CH3OH and NO meet due to flow motion during the piston downward movement, with HO2 generated from the low-temperature oxidation of CH3OH. Overall, these findings deepen the understanding of NO2 formation in methanol/diesel dual-fuel engines, laying a foundation for the development of effective NO2 emission control strategies and supporting the sustainable advancement of methanol/diesel dual-fuel technologies.