Supercritical autoignition of methane in an ultra-high-pressure rapid compression machine at up to 160 bar: real-fluid effects and two-stage ignition behavior

Although methane autoignition under ultra-high-pressure and supercritical conditions is highly relevant to advanced high-pressure combustion systems, such conditions have rarely been studied in RCMs. In this work, supercritical methane ignition is investigated in a newly established ultra-high-pressure rapid compression machine (UHP-RCM) under compressed pressures of 120–160 bar and compressed temperatures of 887–1037 K, with particular focus on real-fluid effects and the chemical origin of the observed two-stage ignition behavior in methane, a fuel usually regarded as exhibiting single-stage ignition without pronounced NTC behavior. The experiments show that increasing compressed pressure and methane/oxygen loading both promote methane ignition reactivity under supercritical conditions. A distinct two-stage ignition behavior is observed for the more diluted mixture, whereas the less diluted mixture mainly exhibits single-stage ignition due to its stronger overall reactivity. Simulations are performed to quantify the influence of real-fluid effects. The results show that real-fluid treatment can increase the compressed temperature derived by up to approximately 11 K and shorten the simulated ignition delay time by > 15%, with the largest difference approaching 50%. These effects are comparable to or larger than the experimental uncertainties, indicating a non-negligible impact of real-fluid behavior under the present conditions. Kinetic analysis is further conducted to elucidate the two-stage ignition chemistry, where the first-stage ignition is found to originate from H 2 O 2 decomposition, whereas the subsequent reduction in reactivity with increasing temperature is attributed to the increasing importance of methyl-radical termination pathways that convert ĊH 3 back to CH 4 or promote its recombination to form C 2 H 6 . Final ignition occurs when the temperature becomes sufficiently high for the Ḣ + O 2 = ȮH + Ӧ reaction to accelerate rapidly and trigger the main ignition event. The present work provides new data and mechanistic understanding of methane autoignition under ultra-high-pressure, supercritical conditions, and highlights the importance of incorporating both real-fluid effects and pressure-dependent chemistry in the modeling of such systems under high-pressure conditions. Novelty and significance statement. The present work investigates methane ignition under a scarcely explored supercritical RCM regime at pressures up to 160 bar, for which available methane RCM data remain limited. The novelty of this work lies in the combination of new high-pressure methane ignition measurements, explicit consideration of real-fluid effects, and a detailed stage-resolved kinetic interpretation of the observed two-stage ignition behavior in methane, a phenomenon more commonly discussed for fuels with stronger low-temperature chemistry such as propane and dimethyl ether. This is significant because it provides new mechanistic insight into methane autoignition under supercritical conditions and improves the physical basis for combustion modeling at ultra-high pressures.