A Numerical Investigation on NO₂ Formation in a Natural Gas–Diesel Dual Fuel Engine

This research numerically simulates the formation and destruction of nitrogen dioxide (NO₂) in a natural gas (NG)–diesel dual fuel engine using commercial CFD software converge coupled with a reduced primary reference fuel (PRF) mechanism consisting of 45 species and 142 reactions. The model was validated by comparing the simulated cylinder pressure, heat release rate (HRR), and nitrogen oxide (NO_x) emissions with experimental data. The validated model was used to simulate the formation and destruction of NO₂ in a NG–diesel dual fuel engine. The formation of NO₂ and its correlation with the local concentration of nitric oxide (NO), methane, and temperature were examined and discussed. It was revealed that NO₂ was mainly formed in the interface region between the hot NO-containing combustion products and the relatively cool unburnt methane–air mixture. The NO₂ formed at the early combustion stage is usually destructed to NO after the complete oxidation of methane and n-heptane, while NO₂ formed during the postcombustion process survives through the expansion process and exits the engine. The increased NO₂ emissions from NG–diesel dual fuel engines was formed during the post combustion process due to higher concentration of HO₂ produced during the oxidation process of the unburned methane at low temperature. A detailed analysis of the chemical reactions occurring in the NO₂ containing zone consisting of NO₂, NO, O₂, methane, etc., was conducted using a quasi-homogeneous constant volume (QHCV) model to identify the key reactions and species dominating NO₂ formation and destruction. The HO₂ produced during the postcombustion process of methane was identified as the primary species dominating the formation of NO₂ during the post combustion expansion process. The simulation revealed the key reaction path for the formation of HO₂ noted as CH₄ → CH₃ → CH₂O → HCO → HO₂, with conversion ratios of 98%, 74%, 90%, 98%, accordingly. The backward reaction of OH + NO₂ = NO + HO₂ consumed 34% of HO₂ for the production of NO₂.