Optimal design of catalytic converters for minimizing cold-start emissions

Abstract

A transient one-dimensional two-phase model is used to obtain an explicit light-off criterion and to estimate the transient time and cumulative cold-start emissions from a catalytic monolith for the case of nonuniform catalyst loading. The light-off criterion can predict the nature of ignition (front-end, middle or back-end). For a fixed amount of metal loading, redistribution of the catalyst appropriately (with more catalyst near the inlet) favors front-end ignition in the monolith and reduces the cumulative cold-start emissions substantially. It is also found that there exists an optimal washcoat thickness and/or metal loading, above which the performance of the converter does not improve significantly while below this optimum, the transient time and cumulative emissions can be reduced by increasing the washcoat thickness or metal loading. For the case of front-end ignition, it is also found that the total transient time is the sum of ignition and the front-propagation times. We present analytical expressions for both these times and show how they may be used to estimate cumulative cold-start emissions. Numerical simulations validate the analytical design criteria presented. Finally, some strategies for minimizing cumulative cold-start emissions are discussed.