Search Results

The long term durability of a heavy duty gasoline truck converter is addressed by examining thermal stresses due to radial temperature gradients under three different driving schedules. The pertinent physical properties of a catalyzed cordierite ceramic converter, with triangular cell structure, are first measured as function of temperature. These are followed by thermal mapping of mid-bed temperatures with the aid of thermocouples under various driving cycles on the truck dynamometer. Both the physical properties and the temperature distribution are then used as input parameters in the finite element thermal stress model to compute stresses in the oval converter.

The monolithic, large frontal area (LFA), extruded ceramic substrates for diesel flow-through catalysts offer unique advantages of design versatility, longterm durability, ease of packaging and low Cost [1, 2]*. This paper examines the effect of cell density and cell size on catalyst light-off performance, back pressure, mechanical and thermal durability, and the steady-state catalytic activity. The factors which affect these performance characteristics are discussed. Certain trade-offs in performance parameters, which are necessary for optimum systems design, are also discussed. Following a brief discussion of design methodology, substrate selection, substrate/washcoat interaction and packaging specifications, the durability data for ceramic flow-through catalysts are summarized. A total of over 18 million vehicle miles have been successfully demonstrated by ceramic LFA catalysts using the systems design approach.

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.

The typical ceramic catalyst support for automotive application has a total volume of 1640 cm3. Approximately 10% of this volume is subjected to tensile thermal stresses due to a radial temperature gradient in service [1]*. These stresses are kept below 50% of the substrate strength to minimize fatigue degradation and to ensure long-term durability [2]. However, the tensile strength measurements are carried out in 4-point bending using 2.5 cm wide x 1.2 cm thick x 10 cm long modulus of rupture bars in which the specimen volume subjected to tensile stress is merely 3.2 cm3 or 0.2% of the total substrate volume [3]. Thus, a large specimen population is often necessary (50 specimens or more) to obtain the strength distribution representative of full substrate. This is particularly true for large frontal area substrates for diesel catalyst supports with an order of magnitude larger stressed volume. In this paper, the modulus of rupture data are obtained as function of specimen size.

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.

Significant advances in composition and the manufacturing process have led to thin wall cordierite ceramic substrates with low thermal mass, high surface area, and large open frontal area-properties that are critical for fast light-off, high conversion efficiency and low back pressure. Indeed, such substrates are ideal catalyst supports for meeting the ever-stringent emissions regulations, ala SULEV and ULEV, as demonstrated by recent performance data1. This paper focuses on the physical durability of 400/4 and 600/4 cordierite ceramic substrates. In particular, it presents strength, fatigue, and modulus data which influence the mechanical durability. In addition, it presents thermal expansion data which impact the thermal durability. Both of these durabilities are examined as a function of operating temperature.

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.

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.

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 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 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.

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 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.

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

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 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.

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 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.

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.

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.