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The current automotive catalytic converter is highly dependable and provides excellent emissions reduction while at the same time it offers little resistance to the flow of gasses through the exhaust system. As automobile performance requirements increase, and as the allowable tailpipe emissions are tightened, there is a need on the one hand to reduce the back pressure even further, and on the other, to increase the already excellent catalytic performance. This paper will analyze the substrate factors which influence the pressure drop and conversion efficiency of the catalyst system. The converter frontal area has the most significant influence on both pressure drop and conversion efficiency, followed in order by part length, cell density, and wall thickness.

The diesel oxidation catalyst is required to remove hydrocarbons and carbon monoxide from the diesel engine exhaust stream while minimizing the impact of all other features such as cost, space, pressure drop, weight, fuel consumption, etc. The challenge of designing a catalytic converter for a particular application then becomes to: first, understand the emissions and other performance targets and requirements for the engine; second, understand the influence each of the converter parameters has on the overall system performance and; third, optimize the system using these relationships. This paper will explore some of the considerations with respect to the second of the above challenges.

A significant advantage of monolithic cellular catalytic converters is the high substrate specific surface area offered for catalyst distribution. It has been shown elsewhere that the substrate total surface area can act as a surrogate for other catalyst parameters in estimating overall catalytic performance. Lacking in the literature, however, are indications of how this surface area influence changes with aging time and temperature. Also, there has been a tacit assumption that all surface areas are equivalent and that the underlying material and cell structure play no significant role. For these reasons, aging studies were carried out on two substrate configurations (extruded square cell ceramic and wrapped foil metal) to establish the surface area influences over time at temperatures of interest to the automotive companies. It is anticipated that the results of this study will be used to more effectively design catalysts to meet increasingly demanding durability requirements.

Several features of the automotive catalyst support contribute to the performance of a catalytic converter system. Certainly the very high surface area and straight and uniform channels allow for an active catalytic surface while still providing a comparatively low back pressure. Other properties of the substrate such as mass and specific heat capacity prove deleterious to the rapid attainment of high conversion efficiency. The size and shape of the channel also can have positive or negative effects, depending on the relative values of these factors, which contribute to both the back pressure and the heat/mass transfer. In turn, the mass transfer is directly related to the catalyst performance. This paper examines the inter-relationships among these substrate parameters and performance properties using both calculations from measured substrate properties and measured substrate performance properties for comparison.