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This paper reviews the development and successful application of ceramic catalytic converters for controlling automotive exhaust emissions. It presents the scientific rationale for designing the high surface area substrate to meet both performance and durability requirements. This is followed by a step-by-step design process for each of the converter components. The initial design stage focuses on understanding automaker's requirements and optimizing component design commensurate with them. The intermediate stage involves laboratory testing of converter components in simulated environment and ensuring component compatibility from durability point of view. The final design stage addresses the critical tests on converter assembly to ensure performance and field durability. In addition, it examines the necessary trade-offs and associated design modifications and evaluates their impact on warranty cost for system failure.

Stringent emissions standards for HC, CO and NOx have necessitated the development of thin wall ceramic substrates which offer higher surface area, larger open frontal area and lower thermal mass. Such substrates offer the additional benefit of being compact which make them ideal for manifold mounting in the engine compartment. These attributes of ceramic substrates, following washcoat and catalyst application, translate directly into quick light-off, high conversion efficiency and low back pressure. To preserve these advantages at high operating temperature and still meet 100,000 mile vehicle durability, the thermomechanical interaction between the substrate and thin wall washcoat system must be managed carefully via formulation, % loading and the calcination process. This paper presents the physical properties data for thin wall ceramic substrates before and after the washcoat application.

The motorcycle emissions regulations for both two-stroke and four-stroke engines, which are receiving worldwide attention, will go into effect in the very near future. To meet these regulations, the motorcycles will require a catalyst in conjunction with the muffler due to space limitations. The combination of high engine speeds, high vibrational acceleration, high HC and CO emissions, high oxidation exotherms, and stringent durability requirements, points to cordierite ceramic substrate as an ideal catalyst support. However, as an integral unit within the muffler, its packaging design must be capable of withstanding isothermal operating conditions which may exceed the upper intumescent temperature limit of the ceramic mat. This paper describes a durable packaging design for the ceramic catalyst which employs a hybrid ceramic mat, special end rings and gaskets, and high strength stainless steel can.

The 290,000 vehicle-mile durability requirement for diesel/natural gas oxidation catalysts calls for robust packaging systems which ensure a positive mounting pressure on the ceramic flow-through converter under all operating conditions. New data for substrate/washcoat interaction, intumescent mat performance in dry and wet states, and high temperature strength and oxidation resistance of stainless steels, and canning techniques insensitive to tolerance stack-up are reviewed which help optimize packaging durability. Factors contributing to robustness of converter components are identified and methods to quantify their impact on design optimization are described. CERAMIC FLOW-THROUGH catalysts for diesel exhaust aftertreatment have met with much success since their introduction in 1993.

Thermal stresses constitute a major portion of the total stress which the ceramic wall flow filter experiences in service. The primary source of these stresses is the temperature gradients, both in radial and axial directions, which attain their maximum values during regeneration. The level of particulate loading, the flow rate, the filter size and the mounting design govern the severity of temperature gradients which, together with physical properties and aspect ratio of the filter, dictate the magnitude and distribution of thermal stresses. The filter, the mounting, and the regeneration conditions should be so designed as to minimize these stresses to insure reliable and fracture free performance of the filter throughout the lifetime of the vehicle. In this paper we present a thermal stress model, based on finite element method, which computes stresses in the axisymmetric filter subjected to linear or step temperature gradients in radial and axial directions.

Ceramic honeycomb structures have been used successfully as catalyst supports in gasoline-powered vehicles for the past ten years. They are currently the leading candidate for trapping and oxidizing the carbonaceous particulate emissions in diesel-powered vehicles. In both of these applications the long term durability of the ceramic substrate is of prime importance. This, in turn, depends on the physical properties of cellular structure, cyclic nature of service loads and design of the mounting assembly. This paper examines the nature and dependence of both the mechanical and thermal stresses in the substrate on its geometry, properties, mounting parameters, and the operating conditions. It also compares the observed failure modes with those predicted by the theory. The paper concludes with a set of recommendations for optimal systems design and acceptable operating conditions which will promote the long term durability of the ceramic substrate.