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Technical Paper

Physical Durability of Thin Wall Ceramic Substrates

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.
Technical Paper

High Temperature Compressive Strength of Extruded Cordierite Ceramic Substrates

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].
Technical Paper

Systems Design for Ceramic LFA Substrates for Diesel/Natural Gas Flow- Through Catalysts

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.
Technical Paper

Impact of Washcoat Formulation on Properties and Performance of Cordierite Ceramic Converters

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.
Technical Paper

Dynamic Fatigue Data for Cordierite Ceramic Wall-Flow Diesel Filters

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.
Technical Paper

Thermal Shock Resistance of Oval Monolithic Heavy Duty Truck Converters

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.
Technical Paper

Durable Packaging Design for Cordierite Ceramic Catalysts for Motorcycle 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.
Technical Paper

New Developments in Packaging of Ceramic Honeycomb Catalysts

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.
Technical Paper

Measurement of Biaxial Compressive Strength of Cordierite Ceramic Honeycombs

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.
Technical Paper

Design Considerations for Diesel Flow-Through Converters

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.
Technical Paper

Cell Design for Ceramic Monoliths for Catalytic Converter Application

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.
Technical Paper

Design Considerations for Mounting Material for Ceramic Wail-Flow Diesel Filters

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.
Technical Paper

Thin Wall Ceramic Catalyst Supports

The stringent emissions regulations, notably for cold start, have led to design modifications in each of the converter components, notably the catalyst support. With the faster light–off requirement, the catalyst support must have a lower thermal mass so as to reach the 50% conversion temperature as quickly as possible. Simultaneously, for higher warmed–up efficiency, the catalyst support must offer higher geometric surface area. Similarly, for improved fuel economy and for preserving engine power, the catalyst support must exert lower back pressure. Indeed, these three performance requirements might be met by certain thin wall ceramic substrates, including 400/4.5 and 600/4.3, which have 22% lower thermal mass, 25% higher geometric surface area and 8% larger open frontal area than the standard 400/6.5 substrate. Testing by automakers and international laboratories on engine dynamometers has verified the above advantages of thin wall substrates.
Technical Paper

Performance Parameters for Advanced Ceramic Catalyst Supports

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.
Technical Paper

Advances in Durability and Performance of Ceramic Preconverter Systems

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.
Technical Paper

Design Considerations for Advanced Ceramic Catalyst Supports

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.
Technical Paper

High Temperature Creep Behavior of Ceramic and Metal Substrates

The high temperature creep data for radial specimens, cut from metal and ceramic substrates and subjected to compressive loads representative of mounting and thermal pressure are presented as function of load and temperature. These data show that the creep resistance of metallic specimens under sustained loading varies with temperature and is orders of magnitude lower than that of ceramic specimens. The observed creep deformation in metallic specimens reduces their open frontal area and hydraulic diameter with potentially adverse impact on pressure drop across the metallic substrate.