Investigation of the mechanism behind the surge in nitrogen dioxide emissions in engines transitioning from pure diesel operation to methanol/diesel dual-fuel operation

Abstract

In diesel engines, nitrogen monoxide (NO) is the predominant component of nitrogen oxides (NOx) emissions. However, when transitioning to methanol/diesel dual-fuel operation, even with a small percentage of methanol replacing diesel energy (e.g. 10 %), the concentration of nitrogen dioxide (NO₂) increases significantly, becoming comparable to that of NO. This study employs multi-dimensional computational fluid dynamics (CFD) modeling to reproduce this NO₂/NOx surge ratio phenomenon and investigates the underlying mechanism driving the surge in NO₂ emissions, an area insufficiently explored in existing literature. By comparing CFD simulations with and without the NO+HO₂↔NO₂ + OH reaction in the chemical mechanism, the results reveal that the surge in NO₂ concentration disappears when this reaction is invalidated, while engine efficiency, combustion phasing, and overall NOx emissions remain largely unchanged. This indicates that the NO+HO₂↔NO₂ + OH reaction is the primary contributor to the sudden increase in the NO₂/NOx ratio. Further analysis during the main combustion stage shows that the diesel spray splits into two distinct regions after impinging on the bowl boundary, with one region deep within the bowl and the other near the squish region. During the late oxidation stage, the diffusion flame directed towards the deep bowl area remains a high-temperature zone with a high concentration of NO, whereas the flame near the squish region evolves into a low-temperature zone due to effective mixing with the low-temperature methanol/air mixture. In these low-temperature regions, almost all NO formed during the main combustion stage is converted to NO₂ during the late oxidation stage, leading to the observed NO₂/NOx ratio surge. Methanol oxidation contributes HO₂ radicals, which facilitate the NO-to-NO₂ conversion. Consequently, the low-temperature oxidation of methanol outside the high-temperature region does not lead to thermal ignition but is instead responsible for the rare occurrence of the NO₂ surge.