A critical test for models of the nitric oxide formation process in spark-ignition engines

Abstract

An entirely satisfactory model for the formation of nitric oxide (NO) in spark-ignition engineshas not yet been developed. To further elucidate the NO-formation process, various quantities of pure NO were added to the intake air of a single-cylinder engine at five different air-fuel ratios. The experimental results contradict the predictions of the equilibrium-at-peak-cycle-temperature model and also the predictions of the formation-rate-controlling model. Models in which formation and decomposition processes are both important may explain the results of these experiments. The formation and decomposition processes seem to be of equal kinetic importance at theair-fuel ratio corresponding approximately to peak exhaust NO concentration (93% of stoichiometric fuel). The formation process becomes more important at leaner mixtures, and the decomposition process becomes more important at richer mixtures. In addition, a change in chemical mechanism is suggested at the stoichiometric air-fuel ratio.