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The development of a stronger cordierite material with much less porosity than the current material allows the manufacture of substrates with the same strength as the current part but with proportionately thinner walls. In the process of analyzing the properties of this new substrate it was discovered that optimum performance can be realized by using specially chosen combinations of cell density and wall thickness. Using the properties of this new material and the equations which relate material and structural features to substrate performance, substrate designs have been identified which in one case minimize the back pressure while maintaining the catalyst performance at the present level, and in another case maximize the catalyst performance while maintaining the back pressure at the present level. Other optimum design points are also indicated.

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.

A concern has been expressed regarding the durability of the ceramic thin wall and ultra-thin wall substrates under severe thermal and mechanical conditions. Damage that might result from these conditions would most likely lead to a reduction in catalyst performance. One of the potential damage mechanisms for automotive catalysts is erosion resulting from the impingement of particles onto the front face of the catalyst system. A basic study of the particulate erosion phenomenon of cellular ceramic substrates was undertaken in order to determine, in a controlled setting, the substrate, particulate, and flow conditions that might bring this damage about. This report will discuss a room temperature study of the effects of particle size, particle density, gas flow rate, cellular part orientation, and cellular design parameters on the erosion of ceramic substrates.

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.

Previous papers have considered the role of the substrate in the catalyst system. It has been shown that the total catalyzed surface area of the substrate (defined as the substrate geometric surface area multiplied by the substrate volume) can act as a surrogate for the catalyst performance. The substrate affects the back pressure of the exhaust system and therefore, the available power. Relationships have been developed between the substrate physical characteristics, and both the pressure drop and total surface area of the substrate. The substrate pressure drop has also been related to power loss. What has been lacking is a means of quantitatively relating the substrate properties to the conversion efficiency. This paper proposes a simple relationship between the substrate total surface area and the emissions of the vehicle as measured on the FTP cycle.

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.

The light-off segment of the automotive Federal Test Procedure (FTP) cycle is receiving considerable attention because of the contribution this portion of the test makes to the overall emissions of the automobile, now that the emissions during the remainder of the cycle have been virtually eliminated. In order for the precious metal catalyst to react quickly to the temperature of the exhaust gas and reach conversion temperature as quickly as possible, the effects of all other heat sinks in the system need to be minimized. With the overall heat capacity of the system in mind, there have been several papers published in the past 10 years which analyze the heat capacity of parts of the catalyst system. Unfortunately, there are differences among the values for both specific and substrate heat capacity when all of these references are compared.

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.

In the preceding paper the specific heat capacity, substrate heat capacity, and energy requirements of two types of substrates were discussed in detail both from the standpoint of predictions from measured material property values as well as actual energy measurements on ceramic and metal products. This information is valuable for the catalyst designer because of the light-off impact of this energy requirement. Some material was also presented regarding the change in this energy requirement with washcoat loading. Other aspects of the substrate which could reasonably come into play to enhance the light-off characteristics of a catalyst are the rates of heat and mass transfer. The latter of these could reasonably be expected to drive the catalyst activity. In addition, the pressure drop which results from the substrate structure could influence and limit the choice of cell configurations and product shapes and sizes, thereby constraining the list of possible options.

Ceramic monolithic catalyst supports have been an integral part of automotive emissions control systems since the early 70's. This investigation examines the impact of physical (cell density, frontal area, volume) and material (porosity, thermal mass) design parameters on vehicle emissions and pressure drop. This study indicates that CO and HC emissions can be reduced by larger volume and/or higher cell density substrates. Materials changes have little or no impact on catalyst performance. Pressure drop is increased by using a longer substrate, and dramatically reduced with a larger frontal area part.