FR2473623A1 - CATALYTIC PURIFICATION ASSEMBLY FOR THE EXHAUST GASES OF INTERNAL COMBUSTION ENGINES OF AUTOMOBILES AND METHOD FOR MANUFACTURING THE SAME - Google Patents

Abstract

A catalytic exhaust emission control unit (10) has a double-shell housing (12), which encloses a monolithic ceramic catalyst support (24). A gap between the support (24) and the housing (12) located on the circumference is filled with an expandable material (30), which is compressed when the housing (12) is assembled, corresponding reaction forces being exerted on the support (24), which, although they increase when the unit (10) heats up, do not increase to such an extent that the support (24) is damaged. The gap is designed for minimum and maximum dimensions, which are correlated with the thickness of the mat composed of the material (30), so that the mat will be within a predetermined density range when assembled, in order to generate the said reaction forces.

Description

The present invention relates to catalytic purification assemblies, or converters, for exhaust gases from automobile internal combustion engines and, more particularly, assemblies in which the catalyst is supported by a monolithic body comprising gas passage channels at thin walls. Sets of this type are known in the prior art, but these raise certain problems.

Almost all such assemblies include a metal housing having an inlet and an outlet which can be connected to the exhaust pipe of an internal combustion engine, and one or more monolithic ceramic catalyst supports or of metal enclosed in the housing and comprising a plurality of thin-walled passage channels extending through the housing, in substantially direct communication with its inlet and outlet orifices. However, monolithic ceramic supports are very brittle and both ceramic supports and metal supports can be damaged by crushing. Since such supports are normally subjected to considerable stress during walking, due to vibrations and shocks not only must they not be in contact with the hard metal of the housing, but also be prevented from moving excessively both laterally and axially with respect to the gas passage channels, without being The problem is further complicated by the fact that the coefficients of thermal expansion of the supports and the housing are different, which results in differential expansions and contractions in the usual temperature range. encountered in use, i.e. between room temperature and temperatures up to about 9500C e t possibly around 12500C. It is the support which is subjected directly to such high temperatures, because of the heat produced in the catalytic purification process. Therefore, it is also most desirable to thermally insulate the support from the housing.

The problem is further complicated by the need to withstand thermal shocks, that is, the rapid increases and decreases in temperature encountered in operation, to have a certain durability, and to allow manufacturing as economical as possible.

The existence of the foregoing problems is reported in various publications, and various attempts have been made to solve them which have been found to be without notable success. Among the publications and patents relating to attempts to resolve these problems, the following documents may be cited
"Development Of Canistering System for Monolithic
Catalysts "- Abthoff, Oblander and Santiago- arti
cle SAE, Automobile Engineers Meeting, Dearborn,
Michigan, October 1976.

"Expandable Inorganics Mount / Protect Catalytic
Converter Element "- Hogan - Article in Design-News,
October 20, 1975.

U.S. Patents nO
3,938,232 Noda and others February 17, 1976
3,959,865 Close and others June 1, 1976
3,978,567 Vroman September 7, 1976
4,020,539 Vroman May 3, 1977
4,070,158 Siebels January 24, 1978
4,093,423 Neumann June 6, 1978
Published German patent application
25 15 732 Starker November 11, 1976
From the above it can be seen that various types of housing and means for supporting a monolithic ceramic catalyst support have been proposed. In one of these types, the support is supported in the housing by a metal screen which is interposed between the support and the housing.

Examples of such a construction are described in US Pat. Nos. 3,978,567 and No. 4,020,539 cited above. However, this type of assembly is not very satisfactory, because the screen, even if it is made of stainless steel, tends to soften and corrode or burn when the operating temperatures are high.

The problem is further aggravated because at high temperatures the housing expands more than the support, and the screen does not expand enough to maintain adequate support forces on the support and therefore loses its ability to properly maintain the support in Such a mounting failure results in destructive contact of the support with the housing or a relative movement between them which is detrimental.

In another type of assembly, the mineral material is in the form of foam and hardens in the space separating the support and the housing. An exemplary embodiment of this type is described in the patent of the United States of America No. 3,959,865 cited above. The problem is again to ensure adequate support of the support without exerting harmful compressive forces. Such support is most difficult to obtain with a certain consistency with materials which can form a foam.

 A comparable arrangement is described in U.S. Patent No. 3,938,232 cited above. In this case, a housing of the clam-shell type, comprising two halves divided longitudinally, is freely mounted around a support, a peripheral space separating them. A somewhat elastic refractory that can flow is introduced into the space, solidified and baked. The parts of the housing are then tightened so as to obtain a compression preload in the refractory supporting the support. The regular obtaining of a predetermined compression pre-stress in the refractory is, however, very difficult in this type of arrangement.

In another type of assembly, a veil of compressible, elastic ceramic fibers resistant to high temperatures is interposed, with a certain degree of compression between the support and the housing. In an arrangement described in US Patent No. 4,093,423 cited above, the web is wrapped around the support to give an overall cross-sectional area greater than that of the interior of the housing, and the The assembly is pushed longitudinally into the housing through a reduction funnel. But, not only is such a longitudinal assembly somewhat difficult, but also the maintenance of a predetermined degree of compression stress in the web after assembly The result is either inadequate support of the support or destructive compression stresses. Another arrangement using the elastic compressible web, resistant to high temperatures, made of ceramic fibers as support of the support, is described in the patent of the United States of America No. 4,093,423 and in patent of the United States of America No. 4,070,158 In this arrangement, the assembly is produced by winding the veil and the support t in a pre-laminated sheet shell having overlapping ends which are pulled together so as to exert the desired compression of the web. However, here again, it is difficult to regularly obtain a predetermined degree of compression stress in the web.

 It has also been suggested to use an intumescent material for supporting a support in a housing, the material being installed initially with a compression preload so as to properly support the support. Heating the material in service has the effect of causing it to swell to the point that there is more than compensation for the peripheral clearance between the housing and the support, and actually increases the compressive stress so as to properly support the support without cause harmful constraints. The two patents cited above, as well as the published German patent application, suggest such use of an intumescent material. However, they only suggest the use of a one-piece peripheral housing in which a pre-wound support is introduced longitudinally with a veil of tumescent material.

As in the case of United States patent No. 4,093,423, such a longitudinal assembly is not only difficult to obtain without damaging either the web or the support, but also does not allow the regular obtaining of a degree predetermined compression pre-stresses in the support material. Consequently, during subsequent heating, the compressive stresses acting on the support may be either inadequate or harmful.

 It is therefore an object of the present invention to provide an improved catalytic purification assembly for exhaust gases from automobile internal combustion engines which comprise a monolithic catalyst support enclosed in a metal housing.

Another object of the present invention is to provide an assembly of this type which is durable, adequately and non-detrimentally supports the support both before its installation and during its use in an automobile, ensures suitable thermal insulation between the support and its housing, which is simple to produce and inexpensive to manufacture and assemble.

The objects of the present invention are achieved by providing a housing of the clam-shell type, by wrapping a veil of tumescent material around a catalyst support, so that the overall transverse dimension of the wound support is greater than the transverse internal dimension of the housing, and by assembling the two halves of the housing around the rolled support, and by fixing the two halves together so as to compress the material.

The thickness of the web is related to the thickness of the peripheral space between the support and the housing so as to obtain an assembled web density which is within a range making it possible to create sufficient compressive forces to prevent movement detrimental of the support relative to the housing, forces which will increase during the expansion of the material by heating but not to the point of damaging the support.

The present invention will be clearly understood from the following description given in relation to the accompanying drawings in which
Figure 1 is a longitudinal sectional view of a catalytic purification assembly according to the present invention;
Figure 2 is an end view of the assembly shown in Figure 1;
Figure 3 is a view corresponding to Figure 1 of a variant of the present invention; and
Figure 4 is a sectional view of the assembly shown in Figure 3 taken along line 4-4.

 Referring now to Figures 1 and 2 of the drawings, there is shown a catalytic purification assembly 10 of the exhaust gases of internal combustion engines according to the present invention. The assembly 10 comprises a metal housing 12 in the form of a clam shell in two parts, having a large main cylindrical part 14, end parts 16 whose dimensions are reduced in an asymmetrical manner up to the circular orifices of inlet and outlet 18, which include flanges 20 allowing the assembly of the assembly 10 in the exhaust pipe (not shown) and an internal combustion engine. Although the main part 14 is presented as being cylindrical, it will be appreciated that it can have another shape in conventional section, and be for example, oval, elliptical, rectangular, square, etc. Furthermore, although the end parts 16 are shown as being asymmetrical, it will be understood that they can be symmetrical with the main part 14, that is to say be frustoconical.

As indicated above, the housing 12 comprises two parts which can have a similar force when it is cut by a plane of symmetry. The joining edges of the two parts have edges folded outwards 22. After assembly of the parts, as will be described later, the edges 22 are welded or else fixed together so as to avoid gas leaks and to constitute a housing. unitary 12.

Enclosed in the main part 14 of the housing 12 is one or more conventional monolithic supports or bodies 24 of ceramic catalyst. Each support 24 is cylindrical and has a honeycomb structure, that is to say comprises a plurality of narrow channels and with thin walls 26 for the passage of gases which extend from one end to the other The walls of the channels 26 are coated or impregnated with a catalyst, as is conventional, so as to transform the undesirable harmful components of the exhaust gases of an internal combustion engine into harmless or less harmful substances. The supports 24 are, as might be expected, quite delicate as regards mechanical stresses, in particular compression stresses, although they resist high temperatures and the corrosive effects of exhaust gases.

As shown, the assembly 10 contains two supports 24 which are placed end to end in the housing 12, having an inner circumferential rib 28 between them. The transverse dimensions of the supports 24 are smaller than the corresponding internal dimensions of the main part 14 of the housing 12 so as to form a peripheral or circumferential gap between the supports and the housing. Placed in the gap is a somewhat elastic intu mescent material 30 which is compressed radially during the mounting of the housing parts, so as to pre-stress the material enough to hold the supports 24 end to end as well as to avoid movement radial or transverse detrimental to the housing 12. The intumescent material has a coefficient of thermal expansion greater than that of the housing. Consequently, when the assembly 10 is heated during operation, the expansion of the material 30 increases the compressive stresses so that there is an even greater resistance to the movement of the supports 24 relative to the housing 12 without increasing the stresses to the point that they would exert a decreasing effect or cause detrimental forces on the supports. Preferably, the intumescent material 30 is the so-called Interam material manufactured by the company called 3M Company.

Interam material is a matrix of ceramic fibers in which small pieces of intumescent material, i.e. mica, are mixed. The material is in the form of a sheet or a veil, the thickness of which increases for temperatures above about 400 "C. The Interam material has a high coefficient of expansion, resists heat up to temperatures of about 11000C , has good thermal insulation properties, and is elastic even at high temperatures. While the publication cited above entitled: "Expandable Inorganics Mount / Protect Catalytic Converter Element" suggests the use of Interam material to support the monolithic ceramic support of a catalytic purification assembly, it only suggests such use in an assembly where the support is wound with the material and where the wound support is introduced longitudinally into the open end of the housing. As indicated previously, such an assembly presents some problems.

It has been found after extensive testing that the density of the Interam material when assembled, before inflating as a result of heating, must be within a certain range of values for the desired results to be obtained. If this density is too low, the support cannot be prevented from moving relative to the housing 12, both before and during the high temperatures to which the Interam material is normally exposed during its use. If the density is too high, expansion of the Interam material by heating the assembly to operating temperatures will exert damaging compression stresses on the support. It has been found that the desired density range for monolithic ceramic substrates of 30 to 40 cells per square centimeter with a 0.3 mm thick veil was between approximately
3 0.64 g / cm3 and approximately 1.53 g / cm3.

The supports 24 of the type described above are manufactured by the manufacturers according to acceptable dimensional tolerances. For example, in a set of conventional dimensions, each support can have an outside diameter of the order of 100 mm, with a typical tolerance of +1.5 mm - 1 mm. Given the large variation in diameter, it is essential to design the assembly 10 so that there is a minimum peripheral gap between the support 24 and the housing 12 for a given thickness and density of the Interam veil. present in mind, the assembly 10 must also be designed so that there is a maximum peripheral gap between the support 24 and the housing 12, for these same thickness and density of the Interam web.

 For example, if the assembly 10 is designed to have a maximum clearance of 4 mm, an Interam veil 30 of 4.2 mm 2 thick and 0.256 g / cm in weight would have, when compressed in a such a gap of 4 mm during assembly, 3 a density of approximately 0.64 g / cm. A smaller gap would lead to a higher density after assembly for the same sail. Thus, the assembly must be designed so as to have a minimum gap which will not create a density, after assembly, greater than about 1.53 g / cm3. Research has also shown that there is a certain relationship between minimum and maximum interstices.

When we have arrived at a correct design of the gap, the thickness of the web is then calculated so that, during assembly, its density is within the range mentioned above. Conversely, if the veil is manufactured with 2 a given thickness and weight per cm, the assembly 10 is designed to have maximum and minimum interstices which will produce densities of the Interam product in the desired range after assembly. To assemble the parts, the webs 30 of correct thickness are wound around the supports 24, and the latter are placed in one of the parts of the housing.

The other part is then subjected to the first part by compression of the webs 30 to a value included in the desired density range. The flanges 22 are then welded or fixed to each other according to another technique so as to form a gas-tight seam and create a unitary housing 12. The webs 30 will then be subjected to a compressive stress sufficient to maintain suitably supports 24 in the housing and prevent their movement as lateral as longitudinal or axial. During its use in an automobile, the assembly 10 is heated to a temperature which will have the effect of expanding the webs 30 enough to increase the forces holding the supports 24 in place but not to the point of producing detrimental forces on these supports.

 The sails 30 of intumescent material will not only maintain the supports 24 adequately, but also will have a cushioning effect or will absorb all the mechanical vibrations or all the shocks transmitted via the sails to the supports as a result of the inherent elasticity of the sails.

 In connection with Figures 3 and 4, there is shown an assembly 40 according to the present invention, which comprises a type of housing 42 in the shape of a clam somewhat modified. The housing 42 is similar to that of FIGS. 1 and 2 in that it comprises a main cylindrical part 44 containing only a simple monolithic support 46 of ceramic catalyst of the conical end parts 48, provided with connection flanges 50. The extreme parts 48 are, however, symmetrical with respect to the main part 44. The housing 42 is again made up of two parts divided longitudinally, but instead of each part having edges facing outwards, only one part has edges 52 along its longitudinal edges which overlap longitudinal side portions of the other portion.

 The assembly 40 is suitably proportioned, in the same way as for the assembly 10, and mounted in the same manner as this assembly 10, except however that the flanges 52 which cover the lateral parts of the other part of the housing have the advantage of minimizing possible damage to the web 54 by extrusion between the meeting edges of the parts of the housing during assembly.

 The appreciation of some of the measurement values indicated above must take into account the fact that they come from the conversion of Anglo-Saxon units into metric units.

 The present invention is not limited to the exemplary embodiments which have just been described, it is on the contrary liable to variants and modifications which will appear to those skilled in the art.

Claims (8)

1 - Catalytic purification assembly for exhaust gases for internal combustion engines of automobiles, characterized in that it comprises  - A monolithic catalyst support comprising generally parallel gas passage channels, a peripheral surface surrounding the channels, and a catalyst agent on the walls of the channels;  a metallic housing surrounding, and of the same general outline, the peripheral surface and distant from it of a generally uniform value, the housing comprising two half-parts joined substantially in a plane generally parallel to the channels; and  - an intumescent material filling the space between the housing and the peripheral surface, this material being compressed between the housing and the peripheral surface at a density within a range which will result in compression forces making it possible to prevent harmful relative movement of the support relative to the housing and, when the assembly is heated to normal operating temperatures, the expansion of the material will have the effect of increasing these forces but by an amount insufficient to damage the support.  2 - An assembly according to claim 1, characterized in that the support is ceramic.  3 - assembly according to claim 2, characterized in that the intumescent material comprises a veil of ceramic fibers, and in that the width of the gap between the support and the housing is related to the thickness of the veil so as to obtain the density range.  4 - assembly according to claim 2, characterized in that the intumescent material is a matrix of ceramic fibers in which are mixed small pieces of mica.  5 - assembly according to claim 4, characterized in that the density range is between about 3 3 0.64 g / cm3 and about 1.53 g / cm3.  6 - Method for manufacturing an assembly for catalytic purification of exhaust gases for internal combustion engines, characterized in that it comprises the following steps - the supply of a monolithic catalyst support comprising a plurality of generally parallel gas passage channels which extend from one end to the other of the assembly, a peripheral surface surrounding the channels, and a catalytic agent on the walls of the canals;  the provision of a metal housing surrounding, and of the same general outline, the peripheral surface of the support from which it is distant along a certain gap, the housing comprising two semi-parts that can be joined in a plane generally parallel to the passage channels of gas;; - the supply of a veil of intumescent material carrying ceramic fibers of thickness greater than that of the gap between the housing and the support;  - winding the veil around the support;  - The mounting of the two parts of the housing around the rolled veil and the securing of the parts so as to create compression forces in the veil sufficient to prevent by friction a significant movement of the support relative to the housing;  - the junction of the two parts of the housing while they are thus subject; and  - the correlation of the thickness of the web with the dimensions of the gap so as to obtain a density of the compressed web located within a range which will result in compressive forces making it possible to prevent a harmful movement of the support relative to the housing and, when the assembly is heated to normal operating temperature with consequently an expansion of the intumescent material, the increase in forces will not be sufficient to damage the support,  7 - Method according to claim 6, characterized in that the veil is a matrix of ceramic fibers in which are mixed small pieces of mica. 8 - Method according to claim 6, characterized in that the support is a ceramic.