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An advanced three-way converter system with significant improvements in light-off performance, conversion efficiency, thermal stability and physical durability at high operating temperature is described. The converter system is comprised of a light-weight ceramic substrate with high surface area triangular cell structure, a new catalyst formulation with enhanced thermal stability and good substrate compatibility, and a durable packaging design which together lead to consistent improvements in high temperature performance and durability. Experimental data including FTP performance, canning trials, and high temperature vibration and thermal shock tests for both the advanced and standard three-way converter systems are presented.

Ceramic preconverters have become a viable strategy to meet the California LEV and ULEV standards. To minimize cold start emissions the preconverter must light-off quickly and be catalytically efficient. In addition, it must also survive the more severe thermomechanical requirements posed by its close proximity to the engine. The viability of the ceramic preconverter system to meet both emissions and durability requirements has also been reported recently(1,2). This paper further investigates the impact preconverter design parameters such as cell density, composition, volume, and catalyst technology have on emissions and pressure drop. In addition, different preconverter/main converter configurations in conjunction with electrically heated catalyst systems are evaluated. The results demonstrate that ceramic preconverters substantially reduce cold start emissions. Their effectiveness depends on preconverter design and volume, catalyst technology, and the system configuration.

The shape and size of the unit cell of a ceramic monolith have a profound influence on its geometric and mechanical properties. These, in turn, affect the catalytic performance, converter durability and vehicle drive-ability. This paper presents the important relationships between cell geometry and monolith's open frontal area, geometric surface area, hydraulic diameter, bulk density, structural rigidity, strength and heat transfer characteristics of the monolith; both the square and triangular cells are considered. These relationships provide a rational basis for selecting the cell shape and size which will yield the best balance between the various performance requirements, i.e. light-off characteristics, conversion efficiency, back pressure and long-term dutability. It is shown that certain tradeoffs are necessary in selecting the final cell geometry which is best accomplished by prioritizing the various performance requirements.

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 with 95+% conversion efficiency requirements call for advanced ceramic catalyst supports with thinner walls, higher cell density and optimum cell shape. The extrusion technology for cellular ceramics has also made significant progress which permits the manufacture of advanced catalyst supports. Similarly, modifications in cordierite chemistry and the manufacturing process have led to improved microstructure from coatability and thermal shock points of view. The design of these supports, however, requires a systems approach to balance both the performance and durability requirements. Indeed as the wall gets thinner, the contribution of washcoat becomes more significant in terms of thermal mass, heat transfer, thermal expansion, hydraulic diameter and structural stiffness - all of which have an impact on performance and durability. For example, the thinner the wall is, the better the light-off performance will be.

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.

An important element of the diesel filter assembly is a resilient ceramic mat placed between the ceramic filter and the stainless steel can. It has four key functions: i) to provide adequate gripping pressure, ii) to permit free axial expansion of can, iii) to act as a seal for gases, and iv) to minimize temperature gradients in the filter, which require certain mat properties, namely low-to-medium compression modulus, low shear modulus, and low friction coefficient between mat and filter. This paper compares the properties and performance of two different mats, Interam® I and III, in “hot shake” and “exhaust gas simulator” tests. The results indicate that Interam® III is a superior material for diesel filter application and that a complete coverage by this mat will prolong the durability of the filter.

The stringent emissions standards in the late 1990's like NLEV, ULEV and SULEV have led to major modifications in the composition and design of ceramic substrates. These changes have been necessitated to reduce cold start emissions, meet OBD-II requirements, and to ensure 100,000 mile durability requirement in a cost-effective manner. This paper presents the key advances in ceramic substrates which include lower thermal expansion, lighter weight, higher surface area and improved manufacturing process all of which help meet performance requirements. In addition to above benefits, the compressive and tensile strengths of lightweight substrates, as well as their thermal shock resistance, are found to be adequate following the application of high surface area alumina washcoat. The strength properties are crucial for ensuring safe handling of the substrate during coating and canning and for its long term mechanical durability in service.

The dynamic fatigue data for two different cordierite ceramic wall-flow diesel filter compositions, EX-54 and EX-66, are obtained at 200° and 400°C using the 4-point bend test. These compositions offer larger mean pore size and experience lower pressure drop than the EX-47 composition, and hence are more desirable for certain applications. Their fatigue behavior in the operating temperature range is found to be equivalent or superior to that of EX-47 composition which helps promote filter durability. The fatigue data are used to arrive at a safe allowable stress, which would ensure the required 290K vehicle mile durability. The paper also discusses the impact of mean pore size on high temperature strength and fatigue properties and their effect on filter durability.

High temperature modulus of rupture (MOR) data, published previously, show that the ceramic catalyst supports get stronger with temperature due to the absence of water vapor and closure of microcracks which would otherwise act as stress concentrators [1, 2 and 3]*. The increased MOR value is partially responsible for the excellent durability of ceramic catalyst supports at high temperature. In this paper, we will present the compressive strength data of ceramic substrates at high temperature, namely the crush strength along B-axis and biaxial compressive strength of the whole substrate. Since the honeycomb strength is directly related to that of the individual cell wall, the compressive strength should also increase with temperature similar to the modulus of rupture. Accordingly, the ceramic substrates are capable of supporting higher mounting pressures exerted by the intumescent mat at high temperature [4].

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.

Under certain operating conditions when the combined stresses in a ceramic wall-flow diesel filter from mechanical, thermal, and vibrational loads exceed its threshold strength, the fatigue effects become important. This paper reviews the theory of static and dynamic fatigue, and presents fatigue data for Coming's high efficiency filter composition (EX-47, 100/17) in the temperature range 25° - 400°C which is representative of the stressed peripheral region during regeneration. The measurement and analysis of fatigue data, together with the implication on long-term durability of cordierite ceramic filters, is discussed.

The high temperature strength and deformation behavior of ceramic and two different metal substrates were measured in the 25°-1200°C temperature range in uniaxial and biaxial bending using rectangular bars and circular discs, respectively, prepared from the substrates. The data show that both of the metal substrates exhibit permanent deformation and lose their load carrying capability by an order of magnitude above 800°C. The ceramic substrate, on the other hand, preserves its strength and behaves elastically over the entire temperature range exhibiting neither permanent deformation nor cell distortion. These data suggest that the upper use temperature for metal substrates could be significantly lower than that for ceramic substrates to meet 50-100K vehicle mile durability

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.

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 stringent durability requirements approaching 100,000 vehicle miles for automotive substrates and 290,000 vehicle miles for large frontal area diesel substrates for 1994+ model year vehicles call for advanced packaging designs with thick ceramic mats and high mount densities. The latter result in high mounting pressure on the substrate and enhance its mechanical integrity against engine vibrations, road shocks and back pressure forces. A novel measurement technique which applies a uniform biaxial compressive load on the lateral surface of ceramic substrates, thereby simulating canning loads, is described. The biaxial compressive strength data obtained in this manner help determine the maximum mounting pressure and mat density for a durable packaging design. The biaxial compressive strength data for both round and non round substrates with small and large frontal area are presented.

The emissions regulations for the decade of 1990s are not only more stringent but are also required of vehicles other than passenger cars, for example both diesel and gasoline trucks as well as motorcycles. These latter applications involve different operating conditions in terms of space velocities, temperature profiles, and vibrational loads than those typical of passenger cars [1]*. In addition, the performance and durability requirements for these applications call for lower back pressure and longer service life. Furthermore, the space availability and the operating temperature range differ vastly so as to require special packaging designs to meet the durability requirements. This paper provides new data for ceramic insulating mats, both intumescent and non-intumescent [2,3], and ceramic substrates with thin and thick walls and square and triangular cell geometries [4], which are under development for non-passenger car applications indicated above.

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 stringent emissions legislation has necessitated advances in the catalytic converter system comprising the substrate, washcoat technology, catalyst formulation and packaging design. These advances are focused on reducing light-off emissions at lower temperature or shorter time, increasing FTP efficiency, reducing back pressure and meeting the mechanical and thermal durability requirements over 100,000 vehicle miles. This paper reviews the role of cordierite ceramic substrate and how its design can help meet the stringent emissions legislation. In particular, it compares the effect of cell geometry and size on performance parameters like geometric surface area, open frontal area, hydraulic diameter, thermal mass, heat transfer factor, mechanical integrity factor and thermal integrity factor - all of which have a bearing on emissions, back pressure and durability. The properties of advanced cell configurations like hexagon are compared with those of standard square cell.

As emissions regulations become more stringent, catalyst supports with higher cell density, smaller wall thickness, higher surface area and lower thermal mass become more desirable for faster light off and higher conversion efficiency. Simultaneously, however, washcoat formulation and loadings have to be adjusted to yield higher and more stable B.E.T. area at operating temperatures representative of close-coupled application. The thermal mass contribution of advanced washcoat system to catalyst supports with 600/4 and 900/2 cell structures may approach or even exceed that of uncoated substrates. Under such high washcoat loadings, the composite properties of advanced catalysts may be affected adversely in terms of their physical durability, notably in close-coupled application. This paper focuses on potential solutions to light-off performance and FTP efficiency, via optimization of substrate/washcoat interaction, geometric design and the mounting system.