Effects of Partial Oxidation in an Unburned Mixture on a Flame Stretch under EGR Conditions

The purpose of the present study is to find a way to extend a combustion stability limit for diluted combustion in a spark-ignition (SI) gasoline engine which has a high compression ratio. This paper focuses on partial oxidation in an unburned mixture which is observed in the high compression engine and clarifies the effect of partial oxidation in an unburned mixture on the behavior of a flame stretch and the extinction limit. The behavior of the flame stretch was simulated using the detailed chemical kinetics simulation with the opposed-flow flame reactor model. In the simulation, the reactants which have various reaction progress variables were examined to simulate the flame stretch and extinction under the partial oxidation conditions. The mixtures were also diluted by complete combustion products which represent exhaust gas recirculation (EGR). The simulation result shows that low-temperature oxidation (LTO) in reactants extends the extinction limit and mitigates a decrease in flame temperature of the stretched flame. The detailed analysis was also conducted from the viewpoint of molecular diffusion and chemical reactions of the partially oxidized reactants. The result reveals that the extension of the extinction limit for the partially oxidized mixture results from the acceleration of the chemical reactions rather than the diffusion effect. The LTO reactions in a pre-flame zone decompose fuel into smaller molecules before entering a reaction zone, and consequently subsequent reactions accelerate in the reaction zone. The result of sensitivity analysis is also discussed.