Experimental and kinetic study of autoignition in methane/ethane/air and methane/propane/air mixtures under engine-relevant conditions

The ignition delay of homogeneous methane/air mixtures enriched with small fractions of ethane/propane was measured using the reflected-shock technique at temperatures from 900 to 1400 K and pressures from 16 to 40 bar. The results show complex effects of ethane/propane on the ignition of methane, but a common trend observed with both hydrocarbons is an increased promotion effect for temperatures below 1100 K. A detailed kinetic mechanism was used to investigate the interaction between ethane/propane and the ignition chemistry of methane under the above conditions. It was found that at relatively low temperatures, the reactions between ethane/propane and methylperoxy (CH3O2) lead to an enhanced rate of formation of OH radicals in the initiation phase of the ignition. By systematically applying the quasi-steady-state assumptions to the intermediate species involved in the main reaction path identified, we have achieved an analytical description of the ignition process in the transitional temperature regime. The analytical solutions agree reasonably well with the detailed kinetic model and the experimental results for both ignition delay and concentrations of major intermediate species.