Shock-tube study of methane ignition under engine-relevant conditions: experiments and modeling

Abstract

A series of shock tube experiments was conducted to measure the ignition delay of homogeneous methane/air (CH4/air) mixtures at moderate temperatures (1000 to 1350 K) and elevated pressures (16–40 atm). The equivalence ratios of the test mixtures were varied from 0.7 to 1.3 with the focus on the slightly lean-to-stoichiometric region, which is most relevant to internal combustion (IC) engine conditions. Transitions from mild to strong ignition were observed at lower temperatures with increasing pressure. An analytical study of methane oxidation under the above conditions was conducted using a detailed chemical kinetic mechanism proposed by Petersen et al. The mechanism was modified and extended in this work, based on the experimental results and literature reviews. The current model improves the agreement between the calculated ignition delay time and experimental data. Sensitivity and reaction flow analyses show that the oxidation of methyl (to form methoxy radicals) is a main rate-limiting step in pre-ignition reactions for stoichiometric methane/air mixtures at 1250 K. At lower temperatures, the activation energy decreases due to a rapid rise in the rates of reactions CH3+O2⇔CH3O2 and CH3O2+CH3⇔2CH3O, which effectively promotes ignition. At still lower temperatures (below 1100 K), the depletion of methyl and hydroxyl radicals becomes increasingly rate-limiting and leads to a re-increase of the activation energy. Further study of chemical kinetics for methane oxidation at high pressures, particularly for fuel-lean mixtures, is suggested to improve the agreement between the model and experiments.