Experimental and kinetic investigation of methyl formate oxidation using time-resolved CO measurements and ignition delay times

Methyl formate (MF, CH₃OCHO) is the simplest methyl ester and a promising carbon-neutral e-fuel produced from renewable methanol and CO₂. Reliable deployment of MF requires detailed kinetic mechanisms capable of accurately predicting its oxidation chemistry, for which time-resolved intermediate-species measurements provide stringent validation targets. In this study, the first time-resolved CO mole-fraction profiles for MF oxidation, together with ignition delay time (IDT) measurements, were obtained behind reflected shock waves. The experiments were conducted for MF/O₂ mixtures diluted in 99% Ar at equivalence ratios of φ = 0.5, 1.0, and 2.0 over a temperature range of 1231–1608 K and pressures of 1.77–2.03 atm. Relative to fuel-lean conditions, the measured IDTs increased modestly under stoichiometric conditions and approximately doubled under fuel-rich conditions. The measured CO time histories exhibited two distinct formation stages, consisting of a rapid initial rise followed by a slower second stage leading to a distinct peak. The time to reach the CO peak exhibited a similar dependence on equivalence ratio, whereas the peak CO concentration showed a much stronger dependence. Five literature mechanisms were evaluated. All overpredicted the IDTs and reproduced the overall two-stage CO profile and the rapid initial CO formation but systematically overpredicted the peak CO concentration while predicting longer CO peak times. Sensitivity and reaction-pathway analyses traced the mechanism discrepancies to the CH₃OH sub-mechanism, specifically the branching of CH₂OH between thermal decomposition and O₂-assisted oxidation. Guided by these analyses, the rate coefficients of three reactions in a selected mechanism (CH₃OH + H ⇄ CH₂OH + H₂, CH₂OH + M ⇄ CH₂O + H + M, and CH₂OH + O₂ ⇄ CH₂O + HO₂) were substituted with values already reported in the literature. The resulting case reproduced the measured IDTs and CO peak times more closely than any of the five mechanisms across all equivalence ratios, although a moderate overprediction of the peak CO concentrations remained. The present measurements provide new time-resolved CO measurements and IDT validation targets for the development and assessment of detailed kinetic mechanisms for MF oxidation.