Effects of reaction progress on the laminar flame speed of gasoline/air mixtures under engine-relevant conditions

Understanding flame propagation in a gasoline/air mixture with reaction progress is necessary to understand flame propagation in the cylinder and improve the combustion performance in spark-ignition engines. In this study, numerical simulations were performed to discuss the mechanisms of laminar flame speed evolution of flames propagating in gasoline/air mixtures with reaction progress. The simulation was conducted for stoichiometric and fuel-lean mixtures under high temperature and pressure conditions considering the in-cylinder condition of spark-ignition engines. Two reactors in Ansys Chemkin-Pro software were coupled. The first reactor simulates a homogeneous, adiabatic, isochoric reaction progress for a finite designated time. The pressure increase in the reaction progress period was taken into account as observed in practical engines. Then, the output of the first reactor was used as the inlet condition of the second reactor, which simulates a steady one-dimensional planar flame propagation. As a result, the laminar flame speed increased despite the pressure increase caused by the reaction progress in the first reactor. However, the rate of change in the laminar flame speed with respect to the reaction progress varied depending on the conditions, and a greater rate of increase was observed in fuel-lean conditions than in stoichiometric conditions. Also, the trend was related to low temperature chemistry. A comparative study was conducted to examine the thermal, pressure, and chemical effects of reaction progress on the laminar flame speed, and the temperature increase by reaction progress had the dominant increase effect, while pressure increase and reduced chemical enthalpy by chemical composition change had negative impacts on flame propagation. Furthermore, sensitivity analysis was conducted to investigate how the reaction in the flame changes as a result of reaction progress. The results indicate that the increase in pressure and compositional change influenced the flame chemistry and resulting changes in the laminar flame speed, and the equivalence ratio had a significant impact on the trend of sensitivity.