Exhaust Emission Characteristics

Abstract

The engine-out exhaust emissions of CO, HC, and NO_x 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 NO_x 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 O₂ and 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.