On the implications of nitromethane–NOx chemistry interactions for combustion processes

In this work, we report a detailed investigation of the CH₃NO₂ chemistry effect on fuel-NO interactions for the fuels methane and n-heptane using a recently developed and extensively validated H₂/O₂/CO/NO_x/NH₃/CH₃NO₂ baseline chemistry. In general, the model predictions show good agreement with temperature profiles of major and intermediate species in jet-stirred reactor experiments and they capture the subtle effect of NO addition. For both fuels, the CH₃NO₂ kinetics retard the system reactivity in the low temperature range by delaying the production of key radicals like OH and HO₂. This explains the retarding effect of NO for n-heptane low temperature ignition and the overprediction of reactivity enhancement by NO in earlier studies on methane combustion. For methane, the recently explored roaming mediated dissociation channel of CH₃NO₂ to CH₃O + NO is a major reaction pathway for CH₃NO₂ consumption. Our analysis suggests that at higher pressure, relevant to engine conditions, the two key intermediate species HONO and CH₃NO₂ feature strongly increased concentrations during n-heptane combustion and they may be detectable under such conditions in combustion experiments of this fuel-NO_x system. The results of this work call for detailed future investigations of the CH₃NO₂ chemistry effect in the context of exhaust gas recirculation, also with regard to the suppression of engine knock.