Impact of nitric oxide (NO) on n-heptane autoignition in a rapid compression machine

This paper investigates the impact of nitric oxide (NO) on n-heptane ignition in a rapid compression machine (RCM). The ignition delays of stoichiometric mixtures of n-heptane and varying amounts of NO (0-400 ppm) are measured in an argon diluent at 9 bar and over 650-1000 K. In the low temperature oxidation regime (≤ 720 K), ignition of n-heptane was promoted by NO up to 100 ppm, whilst higher NO levels do not further promote ignition. Stronger promoting effects are observed in the negative temperature coefficient (NTC) regime and at higher temperatures where the ignition delays are monotonically reduced with up to 400 ppm NO addition. Such impacts significantly suppress the NTC behaviour of pure n-heptane's oxidation. Kinetic modelling is then conducted using the mechanisms of Anderlohr et al. (2009) and Contino et al. (2013) to simulate the measured ignition delays, as well as the species profiles in a jet stirred reactor (JSR) reported in the literature. This modelling indicates that the reactions between NO and hydrocarbon species larger than C2 are important for reproducing both the RCM and JSR data. However, the rate constants for some of the reactions in the mechanism of Anderlohr et al. appear to be overestimated when compared to the literature, and also cause overprediction of the retarding effect on n-heptane autoignition at low temperatures. Use of these previously published rate constants in a revised mechanism is then found to significantly improve the agreement with the low temperature RCM data whilst retaining the good agreement with the higher temperature RCM data and the JSR data.