Catalytic Converters
3.2 Effect of Strengthened Outer Layer
The substrate (C) had neither macroscopic displacement nor pushing out (2), but cracks were generated in the sheets after a heat cycle test performed by the authors. The cracks state is schematically drawn in Figure 6. Cracks penetrated the outermost flat sheet at the edge of the core / jacket joint and propagated in the z direction within the adjacent corrugated sheet and stopped on their way.
Figure 6: Schematic diagram of cracks in the substrates
Figure 7(a) shows equivalent plastic strains (ep) at the points slightly apart from the core / jacket joint in the outermost flat sheet for the substrate (A), (B), and (C), respectively. Figure 7(b) shows Ap at the edge of the core / jacket joint in the adjacent corrugated sheets. Those points correspond to areas cracks were generated. The Ap values became lower than those in Figure 7(b), as the location was apart from the cracked area within the same corrugated sheet.
Although the substrate (A) had the shortest life among those three substrates, the values of Ap
obtained from the simulation were the lowest. In the substrate (A), the honeycomb core and the outer jacket were joined via only the outermost flat sheet. Therefore, if cracks penetrate the sheet thickness, e.g. when external forces are applied to the substrate, the connection between the honeycomb core and the outer jacket is lost and macroscopic displacement of the honey- comb core occurs in spite of its low thermal stress and strain.
Outermost flat sheet Outer jacket
Core / jacket joint Outermost corrugated sheet
Crack
Figure 7: Equivalent plastic strain in the outer most flat and corrugated sheet as a function of time
On the other hand, the substrate (B) and (C) with the Strengthened Outer Layer seem to be disadvantageous for mechanical durability because the values of plastic strain are high and cyclic plastic deformation occurs. Initial cracks are indeed easily to be generated in those sub- strates. However, the Strengthened Outer Layer is able to hold the honeycomb core even if the crack penetrates the outermost flat sheet, unlike the substrate (A).
Figure 8 and 9 show normal stresses in the G and z direction in the outermost flat and corru- gated sheets, respectively, in the substrate C at the points same as those shown in Figure 7. In Figure 8, the stresses on the outer surface of the outermost flat sheet are shown, where the values on the inner surface were almost same. Figure 9(a) and 9(b) show the stresses on the outer surface and inner surface respectively. In contrast, the signs of positive and negative are completely opposite between the outer and inner surface especially in IG, which indicates cyclic bending occurred in the corrugated sheet.
Figure 8: IG and Iz in the outermost flat sheet as a function of time
Considering the direction of crack propagation, although stress state would be different after crack initiation, Iz is dominant in the outermost flat sheet. It can be estimated that crack gener- ated in the outermost flat sheet propagates in the r or G direction. In the outermost corrugated sheet, IG is dominant due to bending, which leads cracks in the z direction.
-400 -200 0 200 400
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IGor Iz / MPa
Iz IG
0 0.01 0.02 0.03 0.04 0.05 0.06
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Time t / sec Equ
ival ent Plas tic stra in Ap
B
C
0 0.01 0.02 0.03 0.04 0.05 0.06
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Time t / sec Equivalent Plastic strainAp
B A
C
(a) Outermost flat sheet (b) Outermost corrugated sheet
In the substrate (C), although micro cracks are inevitably generated in the sheets constituting the honeycomb core, the location of initial cracks and their propagation direction are con- trolled. Therefore, fatal destruction is avoided and high mechanical durability against heat cycles is achieved.
Figure 9: IG and Iz in the outermost corrugated sheet as a function of time. (a) inner surface, (b) outer surface
4 Conclusion
Thermal stresses and strains to be generated in metal substrates for catalytic converters were simulated by elastic-plastic analysis. The flat and corrugated sheets constituting a honeycomb structure were directly modeled by bilinear thick-shell elements. The model could show the pushing out of the honeycomb core where both gas inlet and outlet side are joined and cracks propagation state of the substrate with the Strengthened Outer Layer. Substrates having the Strengthened Outer Layer with asymmetric joint structure in which sheets are joined only in the gas inlet side have two effects for achieving high durability, which are permitting expan- sion and contraction of the sheets in the z direction at the gas outlet region and controlling the location of cracks initiation and the direction of cracks propagation.
5 Acknowledgement
The authors would like to thank Dr. Yasuo Takahashi, associate professor at Osaka University, for the useful comments on this study.
6 References
[1] Bode, H., Metal-Supported Automotive Catalytic Converters (Ed.: H. Bode), Werkstoff- Informationsgesellschaft mbH, 1997, 17–31
[2] T. Takada et al., SAE 910615, 1991 [3] K. P. Reddy et al., SAE 940782, 1994
0 500 1000 1500 2000 2500 Time t / sec
IG
Iz
(b) Outer surface
-400 -200 0 200 400
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IGorIz / MPa
IG
Iz
(a) Inner surface
Reijo Lylykangas, Heikki Tuomola Kemira Metalkat Oy
1 Abstract
To meet ever tightening exhaust emission limits, improved engines and exhaust gas after treatment systems must be developed. The principal trend is to install the catalytic converter as near as practically possible to the engine to hasten light-off performance of the catalyst. A second trend is to reduce the thermal mass of the converter by reducing wall thickness of the substrate, and thirdly, increasing the cell density to improve mass and heat transfer from the bulk gas to the catalyst surface.
At the same time as above, demands for improved mechanical durability of the converter are required. These improvements have not only to withstand higher temperatures but also higher accelerations caused by engine vibration and exhaust gas pulsation.
A new type of metallic substrate is introduced to meet more stringent requirements. The es- sential feature of the substrate is that flow channel is not straight and is specifically designed to mix gas flow instead of the traditional laminar flow pattern. This factor improves heat and mass transfer.
The new substrate has excellent mechanical durability and superior performance durability compared to that of conventional metallic and ceramic substrates.
Theoretical calculations related to mechanical durability together with practical emission and mechanical durability tests are presented.
2 Introduction
The new stringent emission standards have hastened technical development of catalytic con- verters. Both wash coat formulations and substrates are in the fast development process. Cell densities of ceramic converters are increasing from 400 cpsi to 900 cpsi and wall thickness is reducing from 0,153 mm (6 mils) to 0,052 mm (2 mils). Metallic converters are increasing cell densities up to 2000 cpsi and reducing foil thickness to 0,02 mm. Targets have been to increase geometric surface area and to minimise thermal mass of a converter.
To speed up light off time converters are installed as near the engine as possible. In this lo- cation converters have to face very demanding conditions. Temperature is higher and accelera- tion created by engine vibration and exhaust gas pulsation greater than in the under floor posi- tion. In a close-coupled position temperature can arise over 1100 °C and acceleration can reach 100g.
The weakest links in the ceramic converter systems are the assembling mat and thermal shock. The ceramic blanket does not stand higher temperature than 800 °C [1]. Thermal shock can brake the substrate in case temperature gradient in axial or in radial direction is too high.
Material Aspects in Automotive Catalytic Converters, Hans Bode Copyright © 2002 Wiley-VCH Verlag GmbH &Co. K aA ISBN: 3-527-30491-6G
In brazed metallic converters the most critical failure risks are:
· The joint between the shell and the substrate is the most critical area. The matrix is heating and cooling faster than the much thicker shell. This causes different thermal expansion rate and high tensile stress to the brazed joints.
· The melting point of the brazing material is about 1160 °C. It is near the operating tem- perature.
· Failures of metallic converters start often so that the straight foils start to break. Uneven temperature and flow distribution and high vibration create high tensile stress inside the matrix. A corrugated foil can bend easier than a straight foil can stretch. This can cause the straight foils to break. [2,3].
Light off time of a converter has the most essential influence to the CVS- emission results.
Thermal mass of a converter and heat transfer from the exhaust bulk gas to the catalyst surface has vital influence to light off time.
After light off period mass transfer dominates reaction rate.[4,5]
The flow channels in the catalytic converters are small and straight. In these channels a laminar gas flow develops in the first few millimeters of the substrate. When the flow is lami- nar mass and heat transfer is not effective. By using discontinuous flow channels can be im- proved mass and heat transfer in a converter.
The main target of this project was to develop mechanically sufficient substrate for the most demanding close-coupled car applications. Simultaneously we were trying to reduce thermal mass of the substrate and improve mass and heat transfer between bulk gas and the catalyst surface.
3 A New Metallic Substrate
Metallic substrates have some essential advantages compared to ceramic substrates in design- ing the structure of the converter. The channel shape can be discontinuous or channels can be in connection with each other. This gives opportunity to improve mass and heat transfer be- tween bulk gas and the catalyst surface. Also it is possible to mix exhaust gas inside the con- verter and improve gas flow distribution when gas is passing through the substrate. If a con- verter locates immediately after a manifold the gas flow is not mixed properly when it comes in the converter. Fast reactions in the converter can cause local overheating.
Kemira Metalkat has developed a new kind of metallic substrate called EcoXcell. The structure is based on the classical principle of a static mixer. Corrugated foils are stacked one above the other so that culmination lines are crossing each other. Figure 1 shows the structure.
Foils are fixed together in the cross over points by using the new welding process. The ma- trix made this way is fixed to the shell by using laser welding through the grooves made in the shell and the matrix. The welding are made relative near to each other to avoid failures caused by different expansion of the shell and the matrix during heating and cooling phases. The latest laser welding technology is used to achieve maximal mechanical durability.
The shape of the substrate, shell density and all dimensions can be easily varied. In this con- verter there is no straight foil. Gross angle of the culmination lines can be also changed.
A converter with two substrates can be installed so that stacked foil layers have 90° angle between each other. Then converter is mixing gas flow efficiently way and expanding out gas flow in all directions, shown in the figure 2.
Figure 1: New welded structure
Figure 2: New welded two substrate structure
4 Theoretical Calculation
Laboratory test offers durability evaluation for catalytic converter substrate, which could pro- vide pass/failure information under certain vibration environment. Comparing to laboratory test, FEM is another tool, which could be employed for improving durability performance while reducing the product development cost and time. FEM not only evaluates the durability performance, but also explores many possible design options and promises to yield in optimum design with significant cost and time saving. The FEM analysis of a catalytic converter in- cludes calculating on anisotropic material model, thermal stress, and natural frequencies. In
this paper an anisotropic non-traditional superelement model is present and all calculations are developed based on MSC.visualNastran for Windows.
5 Load Types
Typical load types on the catalytic converter are:
· high temperature
· exhaust gas pressure fluctuations
· mechanical vibrations from engine and road excitation
· high temperature gradient between center of matrix to mantle
· thermal shocks due to exhaust gas temperature fluctuations and high gas flow
· external temperature shocks due to splash water