An experimental and theoretical study into nitric oxide production at elevated pressures and temperatures

An investigation has been carried out into the formation of nitric oxide in high pressure combustion environments. Experiments have been conducted using a constant volume combustion bomb, which enabled the effect of pressure to be decoupled from the effect of temperature. Experiments have also been performed using a single cylinder spark-ignition engine to provide data for comparison. A thermodynamic and multi-reaction chemical kinetics model has been developed to calculate burned gas temperatures and NO concentrations. The model has been used to assess the effects of thermochemical data, kinetic rate data, radical concentrations and various chemical reactions on predicted NO concentrations, and the results have been compared to measured NO data. A search of the literature revealed that existing models describing NO formation in engines often fail to predict measured concentrations of NO, especially under high load conditions. Although the effect of pressure on NO formation has previously been subject to theoretical study, the mechanisms of NO formation at high pressure remain subject to uncertainty and in some cases empirical modifications to theoretical data have been used to obtain agreement with experimental results. The combustion bomb experimental results showed clear evidence that an increase in pressure causes a decrease in NO concentrations under lean conditions. However, this effect was not observed in the engine owing to the high temperatures that were generated under high load conditions. It was found that the pressure effect was less significant under stoichiometric and rich conditions in the combustion bomb. The model showed that the commonly-used extended Zeldovich mechanism was unable to accurately predict NO concentrations in non-stoichiometric conditions in either the bomb or the engine. It was found that the time evolution of temperature had a significant effect on calculated NO emissions, with high temperatures at the end of combustion generating much higher NO concentrations than high temperatures earlier in the combustion process. An existing comprehensive model of NO kinetics, the super-extended Zeldovich mechanism, has been subject to a sensitivity study which found that only a small number of reactions play a significant role in NO formation and destruction under the conditions tested here. calculations showed that reactions involving N02 made a significant contribution to NO formation and destruction under lean conditions, and accurately predicted NO concentrations under lean conditions in the bomb. However, the same reactions were found to accelerate NO formation in the engine.