Abstract
The engine-out exhaust emissions of CO, HC, and NOx vary as a function of air-fuel ratio as well as several other parameters such as ignition timing and EGR. Here we shall consider only the relationship to air-to-fuel ratio (A/F). A general relationship between engine out CO, HC, and NOx emissions and A/F is shown in Figure 1. Engine operation at lean A/F (net oxidizing condition) results in lower HC and CO emissions and more O2. These conditions favor subsequent catalytic oxidation reactions. At lean A/F the exhaust contains insufficient reducing agents to react with all the O2and all the NO. By operating closer to the stoichiometric A/F more NO can react but the volume of CO which must be oxidized increases. The stoichiometric A/F occurs at about 14.6. Here, the concentrations of oxidizing gases and reducing gases are matched, and equilibration of the exhaust mixture would yield only CO2, H2O, and N2. The application of three-way catalytic converters has this objective. Three-way catalysts operate in a narrow A/F band between 14 and 15.
Published Version
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