CALCULATION OF EQUILIBRIUM COMPOSITION OF AUTOMOTIVE EXHAUST GASES

Abstract

The influence of temperature (600 to 1500 K), pressure (1 to 60 atm), and air/fuel ratio (13.0 to 17.0) on the equilibrium composition of automotive exhaust gas is calculated by minimizing the total free energy with the assumption that the outlet components are known. Nitrogen oxides, principally as nitric oxide, appear to be formed in combustion chamber zones with high temperatures (1800 to 2000 K) and relatively high air/fuel ratios. Nitrogen oxide elimination, although easier in a reducing atmosphere, appears also possible in an oxidizing atmosphere. Ammonia formation, although negligible in the combustion chamber, appears significant at operating conditions of a highly efficient reducing catalyst. Its formation is favored at low temperatures and high pressures if the air/fuel ratio is less than stoichiometric or at high temperatures if the air/fuel ratio is greater than than stoichiometric. Carbon monoxide concentration decreases with increasing air/fuel ratios and increases with the temperature for a fixed air/fuel ratio. Its concentration is determined not only by the water-gas shift reaction but also by other reactions. The most oxidation-resistant hydrocarbons are methane, acetylene, ethylene, and benzene. Among the aldehydes, formaldehyde shows the highest concentrations for the oxidized hydrocarbon fraction.