Numerical Investigation on NO to NO ₂ Conversion in a Low-Temperature Combustion CI Engine 2021-01-0506

Low temperature combustion (LTC) has been proved to overcome the trade-off between NOx and soot emissions in direct injection compression ignition engines. However, the lowered NOx emissions are accompanied by high hydrocarbon and CO emissions. Moreover, the NOx emissions under LTC has much higher NO₂ concentrations compared with traditional high temperature combustion conditions. Experimental investigations have been carried out to show the hydrocarbon impact on NOx emissions and NO-NO₂ conversion under various engine operation conditions, but the mechanism is less understood. The article includes numerical studies of the impact of hydrocarbons in the in-cylinder conversion of NO to NO₂ during low temperature conditions in a compression ignition engine. In the present work, a stochastic reactor model with detailed chemical kinetics is utilized to investigate the reaction pathways during the NOx reduction and NO₂ conversion processes. The test conditions are simulated for a compression ratio of 13.1:1 to match the existing experimental data. A mixture of propane at 1000 ppm is dosed with NO compositions varying between 40 ppm to 1000 ppm. The reaction pathways depict a dependence on temperature during the NO-NO₂ conversion. The results showed that NO-NO₂ conversion rates initially increase until NO concentration of 350 ppm and then start to rapidly decrease as the concentration is further increased. The investigation using chemical pathways is conducted to understand the conversion of NO to NO₂ under compression alone without ignition. The numerical study shows increasing concentration of NO to NO₂ conversion until 1000K after which the NO₂ concentration declines for initial NO dosing of 350 ppm. Furthermore, the study presents the other species responsible for the destruction of NO and has not been identified in earlier literature. This paper aims to understand the role of the dominant species and the process of conversion of NO to NO₂.