Effects of Exhaust Gas Recirculation Constituents on Methyl Decanoate Auto-Ignition: A Kinetic Study

Abstract

Biodiesel engines are found to have improved soot, hydrocarbon (HC), and carbon monoxide (CO) emissions, with modestly increased nitrogen oxides (NOx) emissions. Exhaust gas recirculation (EGR) could be used for the NOx emissions control, especially in the fuel-kinetics-dominated engine combustion concepts. A detailed chemical kinetic model of methyl decanoate (MD), a biodiesel surrogate fuel, was used here to simulate the two-stage auto-ignition process of biodiesel with EGR addition. The effects of EGR constituents, including carbon dioxide (CO2), water vapor (H2O), CO and H2, were identified in a constant-pressure ignition process and in a variable pressure, variable volume process. Firstly, numerical methods were used to isolate the dilution, thermal, and chemical effects of CO2 and H2O at a constant pressure. It was found that in the biodiesel auto-ignition processes, the dilution effects of CO2 and H2O always played the primary role. Their thermal and chemical effects mainly influenced the second-stage ignition, and the chemical effect of H2O was more significant than CO2. The triple effects of CO and H2 were also analyzed at the same temperature and pressure conditions. Additionally, the sensitivity analysis and reaction pathway analysis were conducted to elucidate the chemical effects of CO and H2 on the ignition processes at different temperatures. Finally, based on a variable pressure, variable volume model simulating the engine compression stroke, the effects of CO2, H2O, CO and H2 addition under the engine operational conditions were studied and compared to those under the constant pressure conditions.