Search Results

The substrate/washcoat systems which preserve both the mechanical and thermal attributes of cordierite substrates are most desirable for prolonged durability of automotive catalysts. This paper provides a micromechanics viewpoint of substrate/washcoat composite whose properties are predictable, measurable and relevant to catalyst durability. The micromechanics model helps quantify substrate/washcoat interaction which controls the long-term catalyst performance. Three different examples of substrate/washcoat systems are used here to illustrate the optimization process during the development of new substrates or washcoat technologies to meet the more stringent emission and durability requirements of advanced catalysts for the 1990s.

The isostatic strength of porous cordierite ceramic monoliths plays an important role during canning and subsequent operation of automotive catalysts. Its value depends on wall porosity, cell geometry, skin thickness and morphology, monolith size and contour, and substrate/washcoat interaction. If the stresses induced by canning loads and closure speeds exceed the isostatic strength, the monolith may exhibit either crushing or shear type failure. This paper presents the room temperature isostatic strength data for coated and uncoated ceramic monoliths of different contour, size, and cell geometry. The applied isostatic load on the monolith is translated into stresses in the porous cell wall using both an analytical model and finite element analysis. It is found that the failure criteria are governed by the fundamental tensile and compressive strengths of the cell wall.

The dual requirement of high conversion efficiency and 50K mile durability for cordierite ceramic converters is achievable through optimization of washcoat and catalyst formulation. This paper presents new data for high temperature physical properties, light-off performance, conversion efficiency and pressure drop through an oval cordierite ceramic converter with triangular cell structure and two different washcoat formulations; namely standard vs high-tech. Both of the washcoat systems have a beneficial effect on strength properties with nominal impact on thermal shock resistance. Both the standard and high-tech catalysts provide identical light-off performance for CO, HC and NOx conversion. The high-tech washcoat and catalyst system, in particular, provides consistently superior conversion efficiency for CO, HC and NOx. The pressure drop across the catalyst depends on hydraulic diameter and is only 8% higher for high-tech washcoat than for standard washcoat.

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.

The large frontal area cordierite ceramic flow-through converter for diesel emissions must meet the 290K vehicle mile durability requirement, almost a six fold increase over that of automotive converters. This paper compares the size, the geometry and the operating conditions of automotive vs. diesel converters and suggests ways to design the converter system to meet the challenging durability requirements without compromising its performance with respect to back pressure and conversion efficiency. It is shown that the mechanical durability of the system, which is critical for meeting the 290K vehicle mile durability, can best be met by ensuring good compatibility between the substrate and washcoat and by designing a rugged packaging system with positive mounting pressure under all driving conditions.

The thermal durability of a large frontal area cordierite ceramic wall-flow filter for light-duty diesel engine is examined under various regeneration conditions. The radial temperature distribution during burner regeneration, obtained by eight different thermocouples at six different axial sections of a 75″ diameter x 8″ long filter, is used together with physical properties of the filter to compute thermal stresses via finite element analysis. The stress-time history of the filter is then compared with the strength and fatigue characteristics of extruded cordierite ceramic monolith. The successful performance of the filter over as many as 1000 regenerations is attributed to three important design parameters, namely unique filter properties, controlled regeneration conditions, and optimum packaging design. The latter induces significant radial and axial compression in the filter thereby enhancing its strength and reducing the operating stresses.

This paper addresses the packaging design for monolithic cordierite ceramic converters to meet the new, stringent durability requirements of the 1990's, while minimizing warranty cost for the automaker. These objectives are best met by using a systems approach during the early phases of packaging design, i.e. by examining design interactions between the ceramic monolith, alumina coating, ceramic mat or wiremesh mounting material with seals, stainless steel can, heatshields, and associated peripheral components. Failure of any one of these components can prove detrimental to converter durability. In this paper we take advantage of overall understanding of the observed failure modes and individual component behavior, and present new data for optimizing the total converter durability through initial design. In particular, the impact of symmetric gas entry, monolith contour, clamshell anisotropy, mount density, stiffener ribs, and heatshield insulation on total durability is highlighted.

An approximate analytical solution for stress distribution in the rectangular lens of a glass headlamp due to static and impact loading is presented. Both low mass/high velocity and high mass/low velocity impact data and the resulting failure modes are discussed. Generally, glass headlamp lenses break either due to Hertz stress (front surface under high localized tension), or due to flexural stress (back surface under tension due to bending), or the combination of two. Failure due to flexural stress is illustrated by a star-crack, while that due to Hertz stress is illustrated by a Hertzian cone or “bullet hole” in the lens. The failure mode during low mass/high velocity impact is predominantly Hertzian while that during high mass/low velocity impact is flexural for lenses 0.120″ to 0.150′ thick. No significant differences are observed in the impact resistance of standard and light-weight lenses in this thickness range.

The high temperature dynamic fatigue data for the catalyst support composition, EX-20, 400/6.8, are presented. These data indicate that the fatigue effects are more severe when the substrate temperature in the peripheral region is near 200°C. The major impact of high temperature fatigue is the slow degradation of substrate’s initial strength while in service. Such a degradation must be taken into account in designing the total converter package to meet life requirements. For the EX-20, 400/6.8 substrate, approximately 50% of its initial strength is available to withstand the combined stresses from mechanical, thermal, and vibrational loads in service. At temperatures well above 200°C, the available design strength can be as high as 65% of substrate’s initial strength. The fatigue theory, the measurement technique, and the application of fatigue data to long term durability of cordierite substrates are discussed.

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.

The physical properties, including modulus of rupture, structural modulus and thermal expansion coefficient of segmented, large frontal area, ceramic wall-flow diesel filters are presented. The effect of cement composition, its coverage and segmentation pattern on these properties and on the failure modes during strength testing is discussed. Using these properties the mechanical and thermal integrities of LFA filter are computed and compared with those of monolithic filter,. The paper discusses both the high efficiency (EX-47, 100/17) and low efficiency (EX-66, 100/25) filter compositions.