Catalytic converter design for minimisation of cold-start emissions

The axial catalyst distribution in a monolithic converter that minimises co ld-start pollutant emissions is investigated numerically, under the constra int of fixed total catalyst surface area. Various warm-up mechanisms can be present during the transient period. The catalyst distribution affects gre atly which mechanisms prevail. For the optimal distribution, a large amount of catalyst is required in the upstream section of the monolith. This ensu res that the hot spot is kept at the monolith inlet throughout the warm-up period, and hence heat transfer by convection dominates. A result of practi cal significance is that the evolution and the steady-state value of the te mperature of the exhaust gas stream at the monolith inlet do not affect sig nificantly the form of the optimal distribution. Even though the local cata lyst surface area of the optimal distribution in the downstream section of the converter is reduced as compared to the uniform distribution, steady-st ate performance is not adversely affected. A converter with two-zones, each having a different bur uniform catalyst loading is also examined. It is sh own that such a design can closely approximate the optimal distribution. Fi nally, performance degradation due to sintering is shown to be more severe for the case of the uniform catalyst distribution.