An RCM experimental and modeling study on CH₄ and CH₄/C₂H₆ oxidation at pressures up to 160 bar

The oxidation of CH₄ and CH₄/C₂H₆ mixtures were studied at pressures relevant to knocking in large bore natural gas engines. The experiments were carried out in a rapid compression machine (RCM) at end of compression (EOC) temperatures ranging between 885 and 940 K at compressed gas pressures of 105, 125, 150, and 160 bar at varying equivalence ratios (0.417, 0.526, and 1.0) and dilution percentages (0, 10, and 30% Exhaust Gas Recirculation – EGR) that were defined in a test matrix. This study describes the method and limitations of performing high-pressure experiments of this magnitude in an RCM, modeling, and validation of the kinetic mechanism against experimental data. While the recently published AramcoMech 2.0 could well predict the ignition delay times (IDTs) for CH₄ within the uncertainty ranges at comparatively higher pressures and lower temperatures (885–940 K), the predicted reactivity is, in general, lower than that of AramcoMech 1.3 as shown in our previous screening study. Based on the comparison between both mechanisms as well as sensitivity analysis on the predicted IDTs, the reaction rate constant for Ḣ-atom abstraction from CH₄ by HȮ₂ radical was optimized in order to achieve better agreement with the new data while maintaining the agreement to the previous data sets. The modified mechanism predicts well the IDTs and the trend of their variation caused by the change in pressure, equivalence ratio, dilution percentage, and mixture variation with C₂H₆.