Kinetics of engine-generated nitrogen oxides and carbon monoxide

Abstract

For temperatures characteristic of combustion in the spark-ignition automotive engine chemical equilibrium calculation of combustion product composition yields significant quantities of carbon monoxide and nitric oxide as well as atomic hydrogen, atomic oxygen and the hydroxyl radical. For continuous, equilibrium of combustion products during the engine expansion process, atomic species and free radicals must recombine, nitric oxide must decompose and carbon monoxide should to a considerable extent be oxidized. However, measurements indicate that carbon monoxide and nitric oxide concentrations appearing in engine exhaust correspond more closely to combustion temperature equilibrium values than to exhaust temperature values. It is therefore apparent that certain of the chemical reactions pertinent to carbon monoxide oxidation and nitric oxide decomposition may be kinetically limited during the expansion process. A theoretical analysis of the chemical kinetics of the internal combustion engine expansion process has been undertaken as the objective of the present work. Fourteen coupled non-linear differential equations based on thirty-two elementary chemical reactions were integrated numerically through use of a Runge-Kutta procedure. The integration was performed in a step-wise manner beginning at the initial point of expansion. Initial conditions included a chemical equilibrium distribution of species corresponding to average conditions of real engine combustion products just prior to expansion. Results were obtained in the form of concentration-time histories throughout expansion for each of the thirteen chemical species considered. It was found that the atomic species and the hydroxyl radical fail to recombine at a rate sufficient to maintain a continuous chemical equilibrium. Consequently these species persist in excess quantities throughout the process. Further, it was found that the persistence of excess concentrations of these species severely inhibits the oxidation of carbon monoxide during expansion. It was found that of the five elementary reactions directly involving nitric oxide, none was sufficiently rapid to effect an appreciable decomposition of nitric oxide.