Mathematical modeling and numerical simulations of automotive catalytic converters

The major undesirable chemical species that constitute the main exhaust emissions produced by spark ignition engines are hydrocarbons (HC ), carbon monoxide (CO) and nitrogen oxides ( NOx). The catalytic converter is one of the important devices for the control of emission from spark-ignition engines. Several concurrent physical processes such as convective heat transfer, gas phase chemical reactions, surface reactions, flow oscillations, water vapor condensation and diffusion mechanisms add to the system's complexity. Under cold-start conditions, the catalytic converter does not become fully active during the first two minutes of the operation, allowing a significant fraction of the overall pollutants to be emitted. In the present study, a one-dimensional mathematical model that physically represents a single channel of the honeycomb structure has been developed. The effects of the geometrical parameters are investigated for the cold start regime. A multi-dimensional model has been developed to study the effect of the converter insulation. The results of the computations suggest new material-dependent designs to improve the conversion efficiency of the device and the cold-start performance. Moreover, from our model calculations, we have observed that the monolith's temperature and therefore the light-off time are greatly affected by the noble-metal distribution over the honeycomb walls of the monolith. We have demonstrated that the light-off time is significantly shortened, by approximately 35%, using a suitable step-function for the noble-metal distribution. Hence, the emissions of the exhaust gas are reduced without increasing the cost of noble-metal materials used in the converter. For a given converter geometry and an amount of noble metal, an optimum noble metal distribution is being investigated with the understanding that the optimum noble-metal distribution proposed has to be practical in the area of manufacturing. Since the main source of the exhaust emissions is during the warm-up period of converters from a cold-start, the reduction of emissions shown in our model calculations is quite substantial.