A CFD-assisted investigation into the mechanism behind the surge in NO2 emissions from methanol/diesel dual fuel engines

In the pursuit of carbon neutrality, the adoption of methanol as a transportation fuel presents a viable strategy for decarbonization. Methanol/diesel dual fuel operation, notable for its reduction in the overall carbon content of the fuel mix, presents a compelling method to mitigate carbon-based emissions. Notably, this mode of combustion has been observed to exhibit an unusual phenomenon where the NO 2 /NOx ratio surpasses 50%, diverging from the typical ratio of below 20% observed in diesel engines. This study aims to elucidate the underlying mechanism responsible for the elevated NO 2 concentration in methanol/diesel dual fuel engines, a topic that remains inadequately understood in existing literature. Utilizing 3D CFD simulations for comparative analysis of diesel and methanol/diesel dual fuel operations, the research identifies consistent NO generation mechanisms across both engine operations, driven by thermal NO pathways. The observed increase in NO 2 concentrations in methanol/diesel dual fuel engines is linked to a higher conversion rate of NO to NO 2 within the combustion chamber. This phenomenon becomes apparent during the main combustion stage when a subset of premixed methanol, though not auto-igniting, engages in low-temperature oxidation, leading to a significant production of HO 2 radicals. These radicals facilitate the conversion of NO to NO 2 , particularly at the diffusion flame boundary during the main combustion stage. The pronounced rise in NO 2 emissions results from the continuous conversion of NO to NO 2 in the regions where NO and HO 2 radicals (produced from methanol oxidation) overlap during the late oxidation stage, persisting until the exhaust valve opens. This investigation offers deeper insights into the combustion behaviors of methanol/diesel dual fuel engines, emphasizing the role of low-temperature oxidation of premixed methanol in shaping unique NOx emission characteristics. • The CFD model successfully captures the NO 2 /NOx surge in methanol/diesel engines. • NO 2 formation increases at the diffusion flame boundary. • The late oxidation stage sees a surge in NO 2 concentration. • The HO 2 radical facilitates NO to NO 2 conversion in cooler areas. • The NO 2 /NOx ratio continues to increase until the exhaust valve opens.