An experimental and modeling study on the ozone-assisted low-temperature ignition of methylcyclohexane in an RCM

Abstract

Methylcyclohexane (MCH) is a significant component of practical fuels and has garnered significant research interest in recent years. However, due to the low reactivity of MCH, ignition data below the temperature of 680 K are still lacking. To extend the experimental condition of MCH ignition to a lower temperature range, ozone (O3) was used as an active additive to assist in the kinetic investigation of the low-temperature ignition chemistry of MCH. Ignition delay times (IDTs) and the first-stage IDTs for MCH containing varying concentrations of O3 (0, 100, and 500 ppm) were determined in a rapid compression machine (RCM) under the pressure of 20 bar, equivalence ratios of 1.0 and 1.5, and temperatures in the range of 616–864 K. Species profiles during the auto-ignition of MCH at 615 K and 795 K were recorded using a fast sampling system combined with the gas chromatography (GC) technique. A kinetic model for MCH ignition with O3 addition was developed with some key reaction rates updated based on our newly obtained data. The promotional effects of O3 on the ignition of MCH were observed through both the IDTs and species evolutions. The negative temperature coefficient (NTC) behavior of MCH ignition disappeared in the presence of 500 ppm O3. The production of intermediate species displayed varying tendencies, with the formation of acetaldehyde (CH3CHO) being favored over other small alkenes. Moreover, the production of ethane (C2H6) was detected under 615 K with 500 ppm O3 addition, which was not reported in previous works concerning MCH combustion. Model analysis revealed that the oxidation pathways for ROO radicals of MCH altered because the addition of O3 shifted the onset of the oxidation temperature to a lower region, making the generation of RO radicals significantly favored. Therefore, the experimental species profiles can provide additional kinetic information for the low-temperature oxidation chemistry of MCH, and the corresponding data effectively highlighted and constrained the reaction rates of ROO radicals yielding RO radicals. Further model simulation indicated that the intramolecular hydrogen immigration of ROO radicals and the following secondary O2 addition oxidation pathways of MCH can be nearly frozen if the onset oxidation temperature is shifted as low as 550 K with 1000 ppm O3.