Numerical Simulation of Autoignition of Gasoline-Ethanol/Air Mixtures under Different Conditions of Pressure, Temperature, Dilution, and Equivalence Ratio.

A numerical simulation of autoignition of gasoline-ethanol/air mixtures has been performed using the closed homogeneous reactor model in CHEMKIN® to compute the dependence of autoignition time with ethanol concentration, pressure, temperature, dilution, and equivalence ratio. A semi-detailed validated chemical kinetic model with 142 species and 672 reactions for a gasoline surrogate fuel with ethanol has been used. The pure components in the surrogate fuel consisted of n-heptane, isooctane and toluene. The ethanol volume fraction is varied between 0 to 85%, initial pressure is varied between 20 to 60 bar, initial temperature is varied between 800 to 1200K, and the dilution is varied between 0 to 32% at equivalence ratios of 0.5, 1.0 and 1.5 to represent the in-cylinder conditions of a spark-ignition engine. The ignition time is taken to be the point where the rate of change of temperature with respect to time is the largest (temperature inflection point criteria). The results are validated against experimental data for pressures up to 60 bar. Since no experimental data on autoignition of multicomponent fuels at higher pressures is available to date, the results are extrapolated to 120 bar. The autoignition time is found to increase with ethanol concentration at lower temperatures but is found to decrease marginally at higher temperatures. The autoignition time is also found to decrease with pressure and equivalence ratio but increase with dilution. A correlation is proposed to calculate autoignition time of gasoline/air mixtures at varying ethanol concentrations, pressures, temperatures, dilution, and equivalence ratio that can be used in developing parametric burn rate and knock combustion models for engine simulation.