The impact of NOx addition on the ignition behaviour of n-pentane

Abstract

Modern engine concepts present several opportunities for nitrogen combustion chemistry, particularly the interaction of NO_x (NO + NO₂) with fuel fragments and products of partial combustion. Current mass-production internal combustion engines are routinely fitted with exhaust gas recirculation (EGR) systems which mix exhaust gases containing NO_x with a fresh charge of unburnt fuel and air. Further, interest in application of alkyl nitrates as reactivity enhancers in experimental engine concepts also leads to conditions in which the concentrations of NO_x and fuel or fuel fragments are high and the ensuing chemistry plays a major role in the mixture reactivity. In this work, ignition delay times for n-pentane doped with NO_x (NO + NO₂) were examined in a rapid compression machine. Blends of n-pentane and oxygen at stoichiometric ratios of 0.5, 1.0, and 2.0 were prepared in nitrogen or 1 : 1 nitrogen/argon bath gas blends at a dilution ratio of 7.52 : 1 diluent : oxygen and doped with either NO or NO₂ at concentrations of up to 1000 ppm. Ignition delay times were observed for post-compression pressures of 15 bar nominal and temperatures between 650 and 1000 K. A new chemical kinetic model is presented which is constructed upon recent, verified literature mechanisms for pentane combustion and for the combustion of small hydrocarbons and nitrogenated species. Additional recent developments in nitrogen combustion chemistry are applied to update the mechanism and new classes of reactions between fuel fragments and nitrogenated species are introduced and added systematically to the model. The reaction rates for the mechanism are taken from the literature or estimated by analogy and are then manually adjusted as informed by simulation results and sensitivity analysis. Further fine optimization of the model is accomplished utilizing an automated routine. Comparison is made with another pentane–NO_x model in the literature and associated data from jet-stirred reactor (JSR) experiments. The model presented in this work is found to have superior performance in predicting and modeling the ignition delay times and similar behaviour in reproducing JSR species profiles as compared with the baseline literature mechanism.