On multi-stage autoignition of n-propylcyclohexane at low temperatures

Abstract

n-Propylcyclohexane (nPCH) has been widely used as a surrogate component representing alkylated cycloalkanes in conventional transportation fuels. Since the autoignition characteristics of nPCH at low temperatures have not been extensively studied in the literature, a series of rapid compression machine (RCM) experiments were conducted using nPCH/air mixtures over a wide range of engine-relevant conditions. Specifically, ignition delay times were measured over the compressed temperature range of TC = 615‒750 K, compressed pressures of pc = 10‒30 bar, and equivalence ratios of φ = 0.25‒2.0. For the first time, a three-stage ignition phenomenon of nPCH was observed at ultra-fuel-lean conditions (φ = 0.25). Using the chemical kinetic model of nPCH developed in our recent work (Combustion and Flame 238 (2022) 111944), experimental results were simulated to better understand the changes in temperature and heat release rate at each stage of autoignition. In addition, the simulated concentrations of key species (ȮH, HȮ2, CO, and CO2) and values of the heat production rate per reaction were analyzed to identify the controlling chemistry at each stage. The current model can well capture the three-stage, two-stage, and one-stage ignition for different mixtures.Based on experimental pressure traces, heat release rate (HRR) profiles of representative one-, two-, and three-stage ignition cases were derived, showing that the trends of the deduced HRR characteristics agree well with those predicted by RCM simulations using the current model. Moreover, the main heat production chemistry of different stages in the multi-stage ignition is summarized in this work. For the three-stage ignition, the low-temperature chain-branching reactions of nPCH oxidation dominate at the first stage, while the intermediate-to-high temperature chain termination reaction of Ḣ + O2 (+M) 〈=〉 HȮ2 (+M), the chain branching reaction of H2O2 (+M) 〈=〉 ȮH + ȮH (+M), and the reaction sequence of ĊH2CHO → CH2O → HĊO → CO play the major role at the second and third stages. When increasing the equivalence ratio from 0.25 to 0.5, a two-stage ignition is observed and its heat production chemistry is different from those of the three-stage ignition. The reactions related to the Ḣ-atom abstraction reaction from small species by O2 and ȮH radical and the formation reaction of C2 species (CHX1*O2J =〉 Ċ2H3 + CH2CO + C2H4) are dominant at the first stage, while the high-temperature reactions involving the competitive reactions of Ḣ + O2 (+M) 〈=〉 HȮ2 (+M) and Ḣ + O2 〈=〉 Ö + ȮH and the exothermic reaction of CO + ȮH 〈=〉 CO2 + Ḣ play an essential role at the second stage. When pressure increases from 10 bar to 20 bar, only one-stage ignition behavior is observed. The heat production analysis also shows that the high-temperature chemistry is dominated by the Ḣ-atom associated reactions, namely Ḣ + O2 (+M) 〈=〉 HȮ2 (+M) and Ḣ + O2 〈=〉 Ö + ȮH, as well as the CO conversion reactions such as CO + ȮH 〈=〉 CO2 + Ḣ.