US3852041A

US3852041A - Low profile catalytic converter

Info

Publication number: US3852041A
Application number: US00262708A
Authority: US
Inventors: M Foster; A Moore; J Haggart
Original assignee: Motors Liquidation Co
Current assignee: Motors Liquidation Co
Priority date: 1971-12-23
Filing date: 1972-06-14
Publication date: 1974-12-03
Anticipated expiration: 1991-12-03

Abstract

A low profile catalytic converter is disclosed including substantially identical top and bottom housing plates and substantially identical top and bottom catalyst retaining plates sandwiched therebetween which define an inclined catalyst containing space of substantially uniform depth spaced inwardly at varying distances of the housing plates. The four plates have mating peripheral faces coterminous over substantially their entire peripheries adapted for edge-sealing and, further, define in combination peripherally spaced cylindrical contours adapted to receive exhaust inlet and outlet pipes wherein one of the contours is defined by a semi-cylindrical downwardly concave portion in the top housing plate, like-positioned downwardly concave portions in both retaining plates and an upwardly concave portion in the bottom housing plate, and the other contour is similarly defined by the housing plates but with upwardly concave portions in both retaining plates. In the preferred embodiment, the top and bottom housing plates are further characterized by a plurality of depressed portions therein bearing against the retaining plates and the structure includes a plurality of support studs extending between the housing plates at the depressed portions to provide a converter having high structural integrity and increased resistance to deformation.

Description

United States Patent [191 Moore et al.

[451 Dec. 3, 1974 LOW PROFILE CATALYTIC CONVERTER [75] Inventors: Albert J. Moore; James A. Haggart,

,lr., both of Davison; Michael R. Foster, Goodrich, all of Mich.

[73] Assignee: General Motors Corporation,

Detroit, Mich.

22 Filed: June 14, 1972 21 Appl. No.: 262,708

Related US. Application Data [63] Continuation-in-part of Ser. No. 2ll,306, Dec. 23, 1971, abandoned, which is a continuation-in-part of Ser. No. 177,947, Sept. 7, 1971, abandoned.

[52] US. Cl. 23/288 F, 60/299, 181/35 C [51] Int. Cl. F01n 3/14 [58] Field of Search 23/288 F; 60/299;

{56] References Cited UNITED STATES PATENTS 3,149,925 9/1964 Scheitlin 23/288 F 3,600,142 8/1971 Fessler 23/288 F 3,615,255 10/1971 Patterson et a1. 23/288 F 3,702,236 11/1972 Fessler 23/288 F Primary ExaminerJoseph Scovronek Assistant Examiner Michael S. Marcus Attorney, Agent, or Firm-George A. Grove 57 ABSTRACT A low profile catalytic converter is disclosed including substantially identical top and bottom housing plates and substantially identical top and bottom catalyst retaining plates sandwiched therebetween which define an inclined catalyst containing space of substantially uniform depth spaced inwardly at varying distances of the housing plates. The four plates have mating peripheral faces coterminous over substantially their entire peripheries adapted for edge-sealing and, further, define in combination peripherally spaced cylindrical contours adapted to receive exhaust inletand outlet pipes wherein one of the contours is defined by a semi-cylindrical downwardly concave portion in the top housing plate, like-positioned downwardly concave portions in both retaining plates and an upwardly concave portion in the bottom housing plate, and the other contour is similarly defined by the housing plates but with upwardly concave portions in both retaining plates. In the preferred embodiment, the top and bottom housing plates are further characterized by a plurality of depressed portions therein bearing against the retaining plates and the structure includes a plurality of support studs extending between the housing plates at the depressed portions to provide a converter having high structural integrity and increased resistance to deformation.

8 Claims, 11 Drawing Figures 9 PATENHL [353 31974 sum 10; 4

llllll u.. a o q k m 1 m .Am w ....v...... m \V Pmsmmc awn SHEET 20F &

LOW PROFILE CATALYTIC CONVERTER This application is a continuation-in-part of our application, Ser. No. 211,306, filed Dec. 23, 1971, and now abandoned, which in turn is a continuation-in-part 2.2.99;.anplegigmas tlq 17.79 7 @1291 p 1971, and now abandoned, and entitled Lowfirofile Catalytic Converter.

This invention relates to the treatment of automotive exhaust gases from an internal combustion engine and, more particularly, to a low profile catalytic converter for the conversion of undesirable constituents of exhaust gases of an internal combustion engine to an innocuous form.

The desirability of treating exhaust gases of an internal combustion engine to convert the undesirable constituents thereof,-particularly carbon monoxide, unburned hydrocarbons and oxides of nitrogen to an innocuous form is now well established. Of the various methods suggested for treating automotive exhaust gases, one of the more promising is the use of a catalytic converter. 1n automotive applications, a catalytic converter having a catalytic element positioned therein is placed in the engine exhaust stream whereby exhaust gases from the engine pass into the converter and through the catalytic element which contains a catalytic material effective to oxidize CO and unburned hydrocarbons to CO and water and/or reduce the oxides of nitrogen before being released to the atmosphere.

There are numerous requirements to be met in providing an effective, durable and practical catalytic converter. First, the converter must be designed for placement in the exhaust system of an automobile. This requires that the converter be thin enough to maintain the required clearance between the vehicle and the road but yet include a catalyst bed of sufficient volume for the effective treatment of the exhaust gases passing therethrough. Further, the converter must be light enough to be supportable in the exhaust system but so designed as to be self supporting and to resist deformation, such as bulging, resulting from the combined effect of the elevated operating temperatures of the converter, i.e., temperatures up to about 1600 F, and the exhaust gas pressure within the converter, which under some conditions can be as great as 25 inches of mercury. Secondly, in converters employing a catalyst material in the form of spheres or pellets, it is necessary that the catalyst bed be kept tightly packed during operation to prevent attrition of the catalyst arising from both road vibration and fluidization of the catalyst bed by the exhaust gases flowing therethrough. Thirdly, it is desirable that the flow of gases through the catalyst bed be substantially uniformly distributed throughout the bed for maximum catalyst life and optimum conversion. Fourthly, the converter must be designed to prevent differential temperatures throughout the converter which can lead to localized cracking and failure of the converter structure. Fifthly, in order for the converter to be economically p'ractical'it must be constructed from relatively low cost materials, such as steel, as opposed to the more expensive hightemperature resistant nickel or cobalt-base alloys; it must include only a minimum of parts; and it must be designed to afford ease of assembly.

Accordingly, it is the principal object of this invention to provide an improved catalytic converter suitable for treatment of automotive exhaust gases meeting the aforementioned requirements.

It is further an object of our invention to provide a low profile catalytic converter of high structural integrity and efficiency which is relatively simple in structure and is economical to manufacture.

Another object of the present invention is to provide an improved catalytic converter suitable for use on automobiles and characterized in its construction by the use of four sheet metal members so organized and arranged that edge seals only serve to define both the catalyst holding bed and the air flow passages.

Still another and more particular object of the present invention is to provide a catalytic converter construction having a pair of outer plates and a pair of inner plates sandwiched therebetween, the. inner plates having complementary dished portions defining a catalyst containing space, the unit defining cylindrical inlet and outlet passages in spaced relationship and each such passage being defined by cooperating upwardly concave and downwardly concave portions, with one plate thickness on one of such portions and three plate thicknesses on the other of such portions of each passage so that the inner plates serve cooperatively to define a gas flow path requiring the exhaust gas to pass through the catalyst bed.

1n the preferred embodiment of our invention, these and other objects are accomplished by providing a low profile catalytic converter having substantially identical top and bottom housing plates and substantially identical top and bottom catalyst retaining plates sandwiched therebetween which define an inclined catalyst containing space of substantially uniform depth adapted to retain catalyst material therein to form a catalyst bed spaced inwardly at varying distances of the housing plates. The four plates having mating peripheral faces coterminous over substantially their entire peripheries forming a four-layer sandwich suitable for edge sealing so as to define a sealed inlet plenum communicating with a cylindrical inlet at one end of the converter and a sealed outlet plenum communicating with a cylindrical outlet at the other end of the converter whereby exhaust gases entering the inlet flow into the inlet plenum enveloping the upper portion of the catalyst bed and then flow downwardly through the catalyst bed wherein the conversion reactions take place to be collected in the outlet plenum surrounding the lower portion of the catalyst bed and discharged from the converter through the outlet.

The cylindrical inlet and outlet are adapted to receive exhaust inlet and outlet pipes and are defined in combination by like-positioned, concave, substantially semi-cylindrical peripherally spaced portions of the four plates wherein one opening is defined by a downwardly concave portion in the top housing plate, downwardly concave portions in both retaining plates and an upwardly concave portion in the bottom housing plate, and the other opening is similarly defined by the housing plates but with upwardly concave portions in both retaining plates.

The top and bottom housing plates are further characterized by a plurality of depressed portions therein bearing against the top and bottom retaining plates and the structure includes a plurality of support studs extending through the retaining plates and between the housing plates at the depressed portions. The support studs have conformations cooperatively associated with openings in the retaining plates at the depressed portion of the housing plates and are anchored to the housing plates to maintain a predetermined spacing therebetween and to provide a converter having high structural integrity and increased resistance to deformation.

Other objects and advantages of our invention will become apparent from the following detailed description, reference being had to the accompanying drawings of which:

FIG. 1 is a side view with parts cut away of one form of the catalytic converter embodying our invention;

FIG. 2 is a top plan view of the converter of FIG. 1 with parts broken away to show the interior constructron;

FIG. 3 is a side elevational view of another form of the catalytic converter with attached exhaust inlet and outlet pipes;

FIG. 4 is a top plan view of the converter of FIG. 3 with parts broken away to show the interior constructron;

FIG. 5 is a cross-sectional view taken along line 55 of FIG. 4;

FIG. 6 is a cross-sectional view taken along line 66 of FIG. 3;

FIG. 7 is an enlarged fragmentary cross-sectional view taken along line 77 of FIG. 4;

FIG. 8 is an enlarged top plan view of a portion of FIG. 4 including a partial weld;

FIG. 9 is a cross-sectional view taken along line 9--9 of FIG. 8;

FIG. 10 is an enlarged fragmentary side elevational view taken along line 10-10 of FIG. 4 with a portion of the apparatus welded; and

FIG. 11 is an exploded perspective view of the catalytic converter of FIGS. 3 through 10.

Referring now to the drawings, the catalytic converter shown in FIGS. 1 and 2 comprises, generally, a housing 10 formed preferably of a high chromium stainless steel or coated or plated steel effective to resist the corrosive attack of high temperature salts and gases. The converter housing 10 is comprised of a top housing plate or shell 12 and a bottom housing plate or shell 14 which include generally vertically extending

side wall portions

16 and 18, respectively, with

peripheral flange portions

20 and 22, respectively, extending therefrom substantially about their entire peripheries in spaced relation to each other.

Mounted within the housing 10 is an inclined catalyst retaining element 24 comprised of a top plate 26 and a bottom plate 28. The top plate 26 includes a perforated or louvered top portion 30 spaced from the top housing plate or shell 12 having a plurality of regularly spaced perforations or louvers 32 of about U] 6 inch in size therethrough. Similarly, the bottom plate 28 includes a perforated or louvered bottom portion 34 spaced from the bottom housing plate or shell 14 having a plurality of regularly spaced perforations or louvers 36 extending therethrough. The

openings

32 and 36 may take several different forms. For example, they may be merely in the form of holes extending through the retaining plates or in the form of projections extending outwardly from the plates which are cupped in the direction of impinging gas flow, as shown in FIGS. 1 and 2, to direct the gases into and out of the element 24. In the preferred form, however, as shown in FIGS. 4 and 5 and in more detail in FIG. 7, the louvers 32' and 36' are die formed by cutting, splitting, and pushing areas of the metal out of the plane of the plate to form rows of regularly spaced, arcuate louvers having a height of about 0.065 inch, a width of about 0.15 inch and a length of about 0.5 inch with about 0.25 inch separation between the successive aligned louvers and 0.15 inch separation between rows.

Referring again to FIGS. 1 and 2 and to the catalyst retaining element 24, the

plates

26 and 28 also include gas impervious

side wall portions

38 and 40, respectively, spaced from the respective

side wall portions

16 and 18 of the housing 10 with mating

peripheral flange portions

42 and 44 extending therefrom. The

catalyst retaining plates

26 and 28 thus define an inclined catalyst retaining space 46 of substantially uniform depth spaced inwardly at varying distances of the

housing plates

12 and 14. The distance of the plate 26 from the housing plate 12 progressively decreases from left to right as seen in FIG. 1, so as to form a progressively decreased cross-section for incoming exhaust gas flow, thus tending to provide a fairly constant pressure gradient. Similarly, the space between plate 28 and plate 14 progressively increases from left to right so as to provide a progressively increased cross-section for outflowing exhaust gas flow, again providing a fairly constant pressure gradient.

The catalyst retaining space 46 is filled with a desired catalyst material 48, as hereinafter fully explained, and the catalyst material 48 is tightly packed therein to provide what is termed a catalyst bed.

The catalyst can be used in any form, but is preferably in a pelleted, extruded or spherical form of about Va inch size with or without a support or carrier. Prefe rably, the catalyst is distributed on or impregnated in a support or carrier which may or may not contribute to the catalytic activity of the composite. Our invention is not limited to the use of any particular catalyst but includes the use of any suitable oxidation or reduction catalyst or catalyst capable of both oxidation and reduction which is effective at operating temperatures up to about l600 F over extended periods of time. It is desirable that the catalyst bed be relatively shallow to minimize back pressure in the exhaust system.'We have found that a bed of about 2 inch depth performs adequately. As may be seen most clearly in FIG. 7, the catalyst material 48 occupies the space beneath each arcuate louver 32 and is sufficiently large to be arrested by and not pass through the opening defined thereby.

As has heretofore been mentioned, the top and bottom housing plates or

shells

12 and 14, respectively, and the top and bottom

catalyst retaining plates

26 and 28, respectively, each have

peripheral flange portions

20, 22, 42, 44, respectively, extending outwardly thus providing the plates with mating peripheral faces 20', 22', 42 and 44 (FIG. 11) over substantially their entire peripheries as seen in plan view. As shown in FIGS. 1, 5 and 10, the mating

peripheral flange portions

42 and 44 of the catalyst element 24 are disposed between the

peripheral flange portions

20 and 22 of the housing 10 so as to form a four-layer sandwich suitable for edge-sealing by means of an external, substantially continuous peripheral weld 50, as further shown in FIG. 10, to provide an absolute internal and external seal. Accordingly, the catalyst element 24 is mounted and supported within the housing 10 by means of this construction with the

side wall portions

38 and 40 of the element 24 being spaced inwardly of the

side wall portions

16 and 18, respectively, of the housing 10 to provide continuous

peripheral spaces

52 and 54, respectively, therebetween.

In addition to the foregoing, the flange portions of the housing and the element 24 are formed into likepositioned, concave, substantially semi-cylindrical configurations at respective opposite ends of the converter to define an exhaust gas inlet 56 and an exhaust gas outlet 58 in combination with the housing 10. As shown in FIGS. 3 and 4, the inlet 56 and the outlet 58 are adapted to receive and be sealingly joined to the engine exhaust inlet pipe 60 and outlet pipe 62, respectively, for feeding exhaust gas into the converter and at the respective other end thereof for discharging the treated gases from the converter. In the preferred form, the inlet 56 is displaced slightly upwardly and the outlet tube 58 is displaced slightly downwardly in relation to the longitudinal axis of the converter to minimize gas flow restriction. It will be recognized that the displacement angle between the exhaust gas inlet 56 and the outlet 58 may be varied depending on the placement of the converter in the exhaust system.

Referring again to FIGS. 1 and 5, it maybe seen that at both the inlet 56 and the outlet 58 the top housing plate 12 is formed with a semi-cylindrical concave down portion 64, and similarly the bottom housing plate 14 is formed with a like-positioned semicylindrical concave up portion 66. However, at the inlet 56 the top retaining plate 26 and the bottom retaining plate 28 are both formed with like-positioned semi-cylindrical concave up portions68 and 70, respectively, adapted to mate with each other and with the concave up portion 66 of the bottom housing plate 14. At the exhaust outlet 58, the top retaining plate 26 and the bottom retaining plate 28 are both formed with like-positioned semi-cylindrical concave down

portions

72 and 74, respectively, adapted to mate with each other and with the concave down portion 64 of the top housing plate 12. Accordingly, the cylindrical contour of inlet 56 is defined by a three-layer or plate thickness concave up sandwich and an opposed single layer or plate thickness concave down plate, and the cylindrical contour of outlet 58 is defined by a three-layer concave down sandwich and an opposed single layer concave up plate.

There is thus provided within the housing 10 a sealed inlet plenum 76, FIGS. 1, 2, 4 and 5, communicating with the exhaust gas inlet 56 and including the peripheral space 52 surrounding the top catalyst retaining plate 26. Likewise there is provided within the housing 10 a sealed outlet plenum 78, FIGS. 1 and 5, communicating with the exhaust gas outlet 58 and including the peripheral space 54 surrounding the bottom retaining plate 28.

The low profile catalytic converter thus described has an inclined catalyst bed sloping upwardly from the converter inlet 56 to the converter outlet 58 wherein exhaust gases entering the converter through the inlet 56 must pass downwardly from the inlet plenum 76 through both

catalyst retaining plates

26 and 28 thereby coming into contact with the catalyst material 48. An inclined catalyst bed as above-mentioned provides a bed of relatively large volume with a relatively shallow depth. Accordingly, effective exhaust gas treatment can be realized with a converter having a relatively small height in relation to its length. As a result, the convertercan be easily mounted on the vehicle in the exhaust stream relatively close to the engine to achieve fast warm-up as opposed to converters which must be placed in the conventional muffler position. Further, the inclined bed and down flow of the exhaust gases provide for an even flow distribution of the exhaust gases through the converter while minimizing fluidization and movement of the catalyst particles. Although the converter has been described in terms of exhaust gases flowing downwardly through the catalyst bed sloping upwardly from the inlet to the outlet, it will be recognized that the gas flow could be reversed with the exhaust gases flowing upwardly through-the bed which now slopes downwardly from the inlet to the outlet.

Referring now particularly to FIG. 9, the top and

bottom housing plates

12 and 14 also include a plurality of depressed portions therein with openings 82 extending therethrough bearing against the top and bottom

catalyst retaining elements

26 and 28, respectively, which include like-positioned openings 83 therethrough. Support studs 84 extend through the element 24, the catalyst 48, the openings 83 in the element 24, and the openings 82 in the housing 10 at the depressed portions 80. The studs 84 include a shoulder portion 86 upon which seats the inner surface of the top and

bottom portions

30 and 34 of the

respective retaining plates

26 and 28. The housing is designed such that annular lip portions 88 extend outwardly from the housing about the studs 84. The lip portions 88 may be formed during forming of the

housing plates

12 and 14 to provide a relatively loose fit between the studs 84 and the lip portions 88 after which the lip portions 88 are welded-to the studs 84 and reinforcing washers 90 placed in the depressed portions 80 about the annular lip portions 88 by means of a continuous annular weld 94 (FIG. 8) which fuses the members to each other. The use of studs and reinforcing washers substantially strengthens the housing and the catalyst retaining element to provide a catalytic converter of high structural integrity resistant to deformation at elevated temperatures and pressures. As shown in FIG. 2, the top and

bottom portions

30 and 34 of element 24 may also include ribbing 96, if desired. However, we have found that high temperature induced sag of the element 24 and exhaust gas flow restriction can be reduced by eliminating the ribbing and increasing the number of louvers 32', as shown in FIG. 4.

The housing it) is also provided with a top cover 98 above the top housing plate 12 having a layer of insulation 100, for example a ceramic wool, therebetween for insulating the converter from heat loss and for protecting the underbody of the vehicle from the heat of the converter. Similarly, the housing 10 is also provided with a bottom cover 99 below the bottom housing plate 14 having another layer of insulation 101 therebetween to prevent heat loss through the bottom of the converter. The top cover 98 is joined to the housing 10 by means of a U-shaped joint 102 which is crimped about the edge-sealed

peripheral flange portions

20 and 22 of the housing 10 and a flange portion 97 of the bottom cover 99. The top cover 98 may also be provided with ribs 103 for strengthening the cover.

As mentioned above, it is among the principal objects of this invention to provide a practical and economical catalytic converter of high structural strength and increased resistance to deformation and cracking which includes only a minimum of parts'and which affords ease of assembly. In this regard, the forming and assembly operations will be described with reference being had to FIG. 11 and other appropriate Figures.

The converter housing 10 comprised of the top housing plate 12 and the substantially identical bottom housing plate 14 is formed by forming the plates including the depressions 80 and the holes 82 therethrough in a single die forming operation. Since the two plates are substantially identical, they may be formed in identical dies thereby achieving production economies and reduced parts inventory, although it will be recognized that the plates need not be identical to achieve the other aforementioned objects of our invention. Similarly, the

catalyst retaining plates

26 and 28 are formed in identical dies and it may be seen that the plates thus become interchangeable in assembly. Prior to assembly, if desired, one of the housing plates and its adjacent retaining plate and one of the insulation covers may be provided with

holes

104 and 105 and 107, respectively, in the side walls thereof, as shown in FIG. 11, through which a probe may be passed after assembly for monitoring the temperature of the catalyst bed or its activity. In the preferred embodiment, the plates differ only in this regard.

The four plates are then assembled together engine are the studs 84 passing therethrough and are placed in a fixture whereby the peripheral faces 20', 22, 42 the 44' are in mating engagement thus forming a four-layer sandwich. The outboard peripheral edge is then sealed by a single substantially continuous peripheral edge weld 50 (FIG. 10) to provide an absolute internal and external seal. Preferably, the welding sequence is automatic with the fixtured plates being turned on edge to expose the peripheral edge to a vertical mounted electron beam welder with the plates being rotated about the edge to form a substantially continuous weld.

The washers 90 are then positioned and fused to the studs 84 and lip portions 88 by a continuous weld 94 at the lip portions 88. Again an electron beam weld is preferred.

Next, the inlet and

outlet pipes

60 and 62 are inserted in the converter inlet 56 and outlet 58, respectively, and a continuous peripheral weld 106 is made to secure them therein, as shown in FIG. 3.

Finally, the catalyst material 48 is introduced into the catalyst containing space 46 through the fill hole 108, FIGS. and 11, which includes a threaded bushing 109 mounted and welded therein. Filling preferably includes vibration of the converter to achieve complete packing of the material therein. A removable plug 110 is then screwed into the bushing 109. The plug 110 is located in the converter bottom to allow for replacement of the catalyst when the converter is mounted on a vehicle.

lF desired, the layers of

insulation

100 and 101 can then be placed on and under the housing and the top cover 98 can be crimped about the weld peripheral edge and the flange portion 97 of the bottom cover 99.

It may thus be seen that the provision of substantially identical mating elements and the unique design and sealing method provides an integral sealed structure which may be easily, simply and inexpensively formed and assembled.

In operation, gases from an internal combustion engine are carried by the inlet pipe 60 into the inlet 56 and into the inlet plenum 76. The entering gases flow throughout the inlet plenum 76 enveloping the top plate 26 of the catalyst retaining element 24. The gases then flow downwardly through the louvers 32 in the perforated top portion 30 of the element 24 and pass through the catalyst material 48 wherein the oxidation and/or reduction reactions take place. The treated gases flow out through the louvers 36 in the perforated bottom portion 34 of element 24 and are collected in the outlet plenum 78 where they envelop the bottom plate 28 of the element 24. The treated gases then flow out the outlet 58 and are discharged through the outlet pipe 62 down the tailpipe to the atmosphere.

At the elevated operating temperature of the converter, i.e., temperatures up to about 1600 F, the material forming the converter inherently becomes deformable at a lower imposed stress. Thus, the housing and the catalyst retaining element have a tendency to bulge outwardly as a result of the gas pressure in the converter resulting from the restriction to gas flow presented by the catalyst bed. However, our improved converter design substantially eliminates this problem by the provision of a plurality of depressed portions in the housing 10 bearing against the element 24 with support studs extending therethrough between the

housing plates

12 and 14. The support studs act to maintain a predetermined spacing between the element plates and to maintain the housing plates against the retaining plates thereby rigidifying the structure and providing the element 24 with increased resistance to deformation. Further, however, the studs are secured to the housing by means of the reinforcing washers to provide pinning points, i.e., points in the housing plates that are relatively immovable in an outward direction in relation to each other thus yielding a pressure vessel effect in the housing plates between pinning points which substantially increases the stress needed to bulge the housing outward.

The catalytic reaction taking place in the catalyst bed is an exothermic reaction, and therefore, the temperature of the bed and of the treated exhaust gases rise from the heat of the reaction. As a result of the heated gases in the outlet plenum 78 enveloping the bottom plate 28 of the catalyst retaining element 24, the temperature of the bed is maintained and the temperature around the converter housing is equalized thereby preventing differential thermal stresses in the converter structure.

Catalysts typically used in exhaust gas converters usually do not become effective until the temperature of the catalyst has risen to about 250 F. As a result, there is a time lapse between the emission of exhaust gas from the engine on start-up to when catalytic conversion of the gases begins through heating of the catalyst bed by the incoming exhaust gases. As previously stated, the catalytic reaction is exothermic and, therefore, the treated gases are heated to a temperature higher than the incoming gases in the converter structure. As may be seen, in our converter the bottom plate 28 of element 24 is surrounded by the treated exhaust gases in the peripheral spaces 54 whereby the bed temperature is maintained. Furthermore, the exiting gases contribute to heating of the incoming gases through the sealingly mated

flange portions

42 and 44 of the element 24 which form a common wall between space 54 and space 52 surrounding the top plate 26 of the element 24. In this manner, on start-up heat is contributed to the incoming gases by the treated gases thereby decreasing the time before the catalyst bed reaches startup temperature and thereby increasing the efficiency of the converter.

Although our invention has been described in terms of certain specific embodiments, it is to be understood that other forms may be adopted within the scope of the invention.

We claim:

ll. A catalytic converter for automobile use, comprising in combination: a top housing plate, a bottom housing plate, and a pair of catalyst retaining plates sandwiched therebetween, said four plates having mating peripheral faces coterminous over substantially their entire peripheries as seen in plan view so as to form a four-layer sandwich suitable for edge-sealing, said four plates further having like-positioned concave substantially semi-cylindrical peripherally spaced portions defining in combination cylindrical contours adapted to receive engine exhaust inlet and outlet pipes, respectively, said portions being both concave downwardly in the top plate, both concave upwardly in the bottom' plate, concave downwardly on both of the catalyst retaining plates at one cylindrical contour, and concave upwardly on both catalyst retaining plates at the other cylindrical contour, whereby exhaust gas passing from the exhaust inlet pipe to the exhaust outlet pipe must pass through both catalyst holding plates, the top and bottom plates being respectively dished upwardly and downwardly in relation to said peripheral faces so as to define a space for exhaust gas flow from the exhaust inlet pipe to the exhaust outlet pipe, and the catalyst retaining plates being respectively dished upwardly and downwardly inboard of said peripheral faces and, at varying distances in relation to said peripheral faces so as to define a catalyst containing space of substantially uniform depth disposed in a general slanting orientation between the exhaust inlet and the exhaust outlet, said catalyst retaining plates further having openings adapted to pass the exhaust gases while retaining catalyst material disposed in said space.

2. A catalytic converter for automobile use, comprising in combination: a top housing plate, a bottom housing plate, and a pair of catalyst retaining plates sandwiched therebetween, said four plates having mating peripheral faces coterminous over substantially their entire peripheries as seen in plan view so as to form a four-layer sandwich suitable for edge-sealing, said four plates further having like-positioned concave substantially semi-cylindrical peripherally spaced portions defining in combination cylindrical contours adapted to receive engine exhaust inlet and outlet pipes, respectively, said portions being both concave downwardly in the top plate, both concave upwardly in the bottom plate, concave downwardly on both of the catalyst retaining plates at one cylindrical contour, and concave upwardly on both catalyst retaining plates at the other cylindrical contour, whereby exhaust gas passing from the exhaust inlet pipe to the exhaust outlet pipe must pass through both catalyst holding plates, the top and bottom plates being respectively dished upwardly and downwardly in relation to said peripheral faces so as to define a space for exhaust gas flow from the exhaust inlet pipe to the exhaust outlet pipe, the catalyst retaining plates being respectively dished upwardly and downwardly inboard of said peripheral faces and at varying distances in relation to said peripheral faces so as to define a catalyst-containing space of substantially uniform depth disposed in a generally slanting orientation, between the exhaust inlet and the exhaust outlet, said catalyst retaining plates further having openings adapted to pass the exhaust gases while retaining catalyst material disposed in said space; and at least one support stud disposed in generally vertical orientation inboard said periphery and extending between the top and bottom plate and through the catalyst retaining plates, said support stud having shoulders cooperatively associated with the inner surface of the catalyst retaining plates to maintain a predetermined spacing therebetween and means effective to anchor the catalyst retaining plates against bowing away from said shoulders to prevent the four layer sandwich from bulging.

3. A catalytic converter for automobile use, comprising in combination: a top housing plate, a bottom housing plate, and a pair of catalyst retaining plates sandwiched therebetween, said four plates having mating peripheral faces coterminous over substantially their entire peripheries as seen in plan view so as to form a four-layer sandwich suitable for edge-sealing, said four plates further having like-positioned concave substan' tially semi-cylindrical peripherally spaced portions defining in combination cylindrical contours adapted to receive engine exhaust inlet and outlet pipes, respectively, said portions being both concave downwardly in the top plate, both concave upwardly in the bottom I plate, concave downwardly on both of the catalyst retaining plates at one cylindrical contour, and concave upwardly on both catalyst retaining plates at the other cylindrical contour, whereby exhaust gas passing from the exhaust inlet pipe to the exhaust outlet pipe must pass through both catalyst holding plates, the top and bottom plates being respectively dished upwardly and downwardly in relation to said peripheral faces so as to define a space for exhaust gas flow from the exhaust inlet pipe to the exhaust outlet pipe, the catalyst retaining plates being respectively dished upwardly and downwardly inboard of said peripheral faces and at varying distances in relation to said peripheral faces so as to define a catalyst containing space of substantially uniform depth disposed in a generally slanting orientation between the exhaust inlet and the exhaust outlet, said catalyst retaining plates further having openings adapted to pass exhaust gases while retaining catalyst material disposed in said space; and at least one support stud positioned between said catalyst retaining plates disposed in generally vertical orientation inboard said peripherally and extending through the top and bottom plates and through the catalyst retaining plates, the top and bottom plates including respectively upwardly and downwardly extending annular lip portions about said support stud, said support stud having conformations operatively associated with openings in the catalyst retaining plates so as to maintain a predetermined spacing therebetween and being anchored to reinforcing washersdisposed about said lip portions by continuous annular welds at said lip portions.

4. A catalytic converter for automobile use, comprising in combination: a top housing plate, a bottom housing plate, and a pair of catalyst retaining plates sandwiched therebetween, said four plates having mating peripheral faces coterminous over substantially their entire peripheries as seen in plan view so as to form a four-layer sandwich suitable for edge'sealing, said four plates further having like-positioned concave substantially semi-cylindrical peripherally spaced portions defining in combination cylindrical contours adapted to receive engine exhaust inlet and outlet pipes, respectively, said inlet and outlet pipes being displaced above and below the longitudinal axis of said converter, respectively, said portions being both concave downwardly in the top plate, both concave upwardly in the bottom plate, concave downwardly on both of the catalyst retaining plates at one cylindrical contour, and concave upwardly on both catalyst retaining plates at the other cylindrical contour, whereby exhaust gas passing from the exhaust inlet pipe to the exhaust outlet pipe must pass through both catalyst holding plates, the top and bottom plates being respectively dished upwardly and downwardly in relation to said peripheral faces so as to define a space for exhaust gas flow from the exhaust inlet pipe to the exhaust outlet pipe, the catalyst retaining plates being respectively dished upwardly and downwardly inboard of said peripheral faces and at varying distances in relation to said peripheral faces so as to define a catalyst containing space of substantially uniform depth disposed in generally slanting orientation between the exhaust inlet and the exhaust outlet, said top and bottom catalyst retaining plates further having arcuate openings extending vertically toward the top and bottom plates, respectively, adapted to pass the exhaust gases while retaining catalyst material disposed in said space, a bottom cover in spaced relation to said bottom plate adapted to house a layer of insulation therebetween and including an outwardly extending flange portion, a top cover in spaced relation to said top plate adapted to house a layer of insulation therebetween and including a U- shaped flange portion disposed about the peripheral faces of said top and bottom plates and said flange portion of said bottom cover, and at least one support stud disposed in generally vertical orientation inboard said periphery and extending between the top and bottom plates and through the catalyst retaining plates, said support stud having conformations cooperatively associated with openings in the catalyst retaining plates so as to maintain a predetermined spacing therebetween and being anchored to the top and bottom plates, respectively.

5. A catalytic converter for automotive use comprising a four plate sandwich construction that includes a top plate, a bottom plate and two intermediate plates, said top plate being of concave downward configuration and having a peripheral skirt portion and a pair of peripherally spaced downwardly concave semicylindrical inlet and outlet portions, said bottom plate being of concave upward configuration having a pe ripheral upstanding skirt portion and a pair of peripherally spaced upwardly concave semi-cylindrical inlet and outlet portions, the bottom plate'being complementary to the top plate so as to form an enclosure having mating peripheral edges and full cylindrical inlet and outlet portions, said two intermediate plates also have peripheral portions complementary to and sandwiched between the peripheral portions of the top and bottom plates, one intermediate plate being upwardly concave and the other intermediate plate downwardly concave so as to define a catalyst retaining space therebetween said intermediate plates being perforated at the catalyst retaining space to allow and require gases entering the inlet portion to pass through the catalyst retaining space and out the outlet portion, both of said intermediate plates having a downwardly concave semi-cylindrical portion at the one end and an upwardly concave semi-cylindrical portion at the other end in nesting relation with each other and the top plate at said one end and with each other and the bottom plate at said other end.

6. A catalytic converter for automotive use comprising a four plate sandwich construction that includes a top plate, a bottom plate, and upper and lower catalyst retaining plates, the plates being complementary to each other so as to define a four layer periphery suitable for edge affixation to define a unitary structure, the top plate and the bottom plate inboard the periphery defining an exhaust gas flow passage so that exhaust gas travels into the space between said plates at one peripheral point and out of such space at another peripheral point, the upper and lower catalyst retaining plates being juxtaposed against each other and the top plate at one point in said flow passage so as to permit flow only underneath three layers formed by the top plate and the catalyst retaining plates and being juxtaposed against each other and the lower plate at another point in said flow passage so as to permit flow only over three layers formed by the bottom plate and the catalyst retaining plates, whereby the two catalyst retaining plates define a barrier to direct flow through the converter, said catalyst retaining plates having vertically spaced portions between said one point in the flow passage and the other point in the flow passage defining a catalyst retaining space and being perforated to permit and require exhaust gas flowing through the converter to pass through the catalyst.

7. A catalytic converter for automotive use comprising a four plate sandwich construction that includes a top plate, a bottom plate, and upper and lower catalyst retaining plates, the plates being complementary to 'each other so as to define a four layer periphery suitable for edge affixation to define a unitary structure, the top plate and the bottom plate inboard the periphery defining an exhaust gas flow passage so that exhaust gas travels into the space between said plates at one peripheral point and out of such space at another peripheral point, the upper and lower catalyst retaining plates being juxtaposed against each other and the top plate at one point in said flow passage so as to permit flow only underneath three layers formed by the top plate and the catalyst retaining plates and being juxtaposed against each other and the lower plate at another point in said flow passage so as to permit flow only over three layers formed by the bottom plate and the catalyst retaining plates, whereby the two catalyst retaining plates define a barrier to flow through the converter, said catalyst retaining plates having vertically spaced portions between said one point in said flow passage and said other point in saidflow passage defining a catalyst retaining space said catalyst retaining plates being perforated to permit and require exhaust gas flowing through the converter to pass through the catalyst, the vertical spacing between the top plate and the upper catalyst retaining plate progressively decreasing in the direction from said one point towards the other point over a substantial distance and the vertical spacing between the bottom plate and the lower catalyst retaining plate progressively increases from said one point towards the other point over said substantial distance so as to define a substantially uniform cross section for gas flow at all points over said substantial distance.

8. A catalytic converter for automotive use comprising a four plate sandwich construction that includes a top plate, a bottom plate, and upper andlower catalyst retaining plates, the plates being complementary to each other so as to define a four layer periphery suitable for edge affixation to define a unitary structure, the top plate and the bottom plate inboard the periphery defining an exhaust gas flow passage so that exhaust gas travels into the space between said platesat one peripheral point and out of such space at another peripheral point, the upper and lower catalyst retaining plates being juxtaposed against each other and the top plate at one point in said flow passage so as to permit flow only underneath three layers formed by the top plate and the catalyst retaining plates and being juxtaposed against each other and the lower plate at another point in said flow passage so as to permit flow only over three layers formed by the bottom plate and the catalyst retaining plates, whereby the two catalyst retaining plates define a barrier to flow through the converter, said catalyst retaining plates having vertically spaced portions between said one point in said flow passage and said other point in said flow passage defining a catalyst retaining space, said catalyst retaining plates being perforated to permit and require exhaust gas flowing through the converter to pass through the catalyst, the vertical spacing between the top plate and the upper catalyst retaining plate progressively decreasing in the direction from said one point in said flow passage towards said other point in said flow passage over a substantial distance and the vertical spacing between the bottom plate and the lower catalyst retaining plate progressively increases from said one point towards the other point over said substantial distance so as to define a substantially uniform cross section for gas flow at all points over said distance, and said top plate and the upper catalyst retaining plates and the bottom plate and the lower catalyst retaining plates being substantially juxtaposed in vertical orientation immediately in board their peripheries so as to define two layer vertical peripheral walls.

e UNETED STATES PATENT @FFIEE 5/69) cmwmeme @F eeemmmm Patent N0. o 41 Dated Decembez' 3 a 19 74 Albert J., Moore James A, Haggert, Jr., and

Inventor(s) Mi'cHEei Ra reefer It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

gm In the specification, column 2 line 34 "having" should g read have celumn 7, lines 24 and 25, "engine are" should read with ----w In the claims, column 10, line 49, "peripherally" should read periphery Signed and sealed this 18th day of February 1975.

(SEAL) Attest 2 C, MARSHALL DANN RUTH C. MASON Commissioner of Patents Attesting Officer and Trademarks

Claims

1. A CATALYTIC CONVERTER FOR AUTOMOBILE USE, COMPRISING IN COMBINATION: A TOP HOUSING PLATE, A BOTTOM HOUSING PLATE, AND A PAIR OF CATALYST RETAINING PLATES SANDWICHED THEREBETWEEN, SAID FOUR PLATES HAVING MATING PERIPHERAL FACES COTERMINOUS OVER SUBSTANTIALLY THEIR ENTIRE PERIPHERIES AS SEEN IN PLAN VIEW SO AS TO FORM A FOUR-LAYER SANDWICH SUITABLE FOR EDGE-SEALING, SAID FOUR PLATES FURTHER HAVING LIKE-POSITIONED CONCAVE SUBSTANTIALLY SEMI-CYLINDRICAL PERPHERALLY SPACED PORTIONS DIFINING IN COMBINATION CYLINDRICAL CONTOURS ADAPTED TO RECEIVE ENGINE EXHAUST INLET AND OUTLET PIPES, RESPECTIVELY, SAID PORTIONS BEING BOTH CONCAVE DOWNWARDLY IN THE TOP PLATE, BOTH CONCAVE UPWARDLY IN THE BOTTOM PLATE, CONCAVE DOWNWARDLY ON BOTH OF THE CATALYST RETAINING PLATES AT ONE CYLINDRICAL CONTOUR, AND CONCAVE UPWARDLY ON BOTH CATALYST RETAINING PLATES AT THE OTHER CYLINDRICAL CONTORU, WHEREBY EXHAUST GAS PASSING FROM THE EXHAUST INLET PIPE TO THE EXHAUST OUTLET PIPE

2. A catalytic converter for automobile use, comprising in combination: a top housing plate, a bottom housing plate, and a pair of catalyst retaining plates sanDwiched therebetween, said four plates having mating peripheral faces coterminous over substantially their entire peripheries as seen in plan view so as to form a four-layer sandwich suitable for edge-sealing, said four plates further having like-positioned concave substantially semi-cylindrical peripherally spaced portions defining in combination cylindrical contours adapted to receive engine exhaust inlet and outlet pipes, respectively, said portions being both concave downwardly in the top plate, both concave upwardly in the bottom plate, concave downwardly on both of the catalyst retaining plates at one cylindrical contour, and concave upwardly on both catalyst retaining plates at the other cylindrical contour, whereby exhaust gas passing from the exhaust inlet pipe to the exhaust outlet pipe must pass through both catalyst holding plates, the top and bottom plates being respectively dished upwardly and downwardly in relation to said peripheral faces so as to define a space for exhaust gas flow from the exhaust inlet pipe to the exhaust outlet pipe, the catalyst retaining plates being respectively dished upwardly and downwardly inboard of said peripheral faces and at varying distances in relation to said peripheral faces so as to define a catalyst containing space of substantially uniform depth disposed in a generally slanting orientation between the exhaust inlet and the exhaust outlet, said catalyst retaining plates further having openings adapted to pass the exhaust gases while retaining catalyst material disposed in said space; and at least one support stud disposed in generally vertical orientation inboard said periphery and extending between the top and bottom plate and through the catalyst retaining plates, said support stud having shoulders cooperatively associated with the inner surface of the catalyst retaining plates to maintain a predetermined spacing therebetween and means effective to anchor the catalyst retaining plates against bowing away from said shoulders to prevent the four layer sandwich from bulging.

3. A catalytic converter for automobile use, comprising in combination: a top housing plate, a bottom housing plate, and a pair of catalyst retaining plates sandwiched therebetween, said four plates having mating peripheral faces coterminous over substantially their entire peripheries as seen in plan view so as to form a four-layer sandwich suitable for edge-sealing, said four plates further having like-positioned concave substantially semi-cylindrical peripherally spaced portions defining in combination cylindrical contours adapted to receive engine exhaust inlet and outlet pipes, respectively, said portions being both concave downwardly in the top plate, both concave upwardly in the bottom plate, concave downwardly on both of the catalyst retaining plates at one cylindrical contour, and concave upwardly on both catalyst retaining plates at the other cylindrical contour, whereby exhaust gas passing from the exhaust inlet pipe to the exhaust outlet pipe must pass through both catalyst holding plates, the top and bottom plates being respectively dished upwardly and downwardly in relation to said peripheral faces so as to define a space for exhaust gas flow from the exhaust inlet pipe to the exhaust outlet pipe, the catalyst retaining plates being respectively dished upwardly and downwardly inboard of said peripheral faces and at varying distances in relation to said peripheral faces so as to define a catalyst containing space of substantially uniform depth disposed in a generally slanting orientation between the exhaust inlet and the exhaust outlet, said catalyst retaining plates further having openings adapted to pass exhaust gases while retaining catalyst material disposed in said space; and at least one support stud positioned between said catalyst retaining plates disposed in generally vertical orientation inboard said peripherally and extending through the top and bottom plates and through the catalyst retaining plates, the top and bottom plates includiNg respectively upwardly and downwardly extending annular lip portions about said support stud, said support stud having conformations operatively associated with openings in the catalyst retaining plates so as to maintain a predetermined spacing therebetween and being anchored to reinforcing washers disposed about said lip portions by continuous annular welds at said lip portions.

4. A catalytic converter for automobile use, comprising in combination: a top housing plate, a bottom housing plate, and a pair of catalyst retaining plates sandwiched therebetween, said four plates having mating peripheral faces coterminous over substantially their entire peripheries as seen in plan view so as to form a four-layer sandwich suitable for edge-sealing, said four plates further having like-positioned concave substantially semi-cylindrical peripherally spaced portions defining in combination cylindrical contours adapted to receive engine exhaust inlet and outlet pipes, respectively, said inlet and outlet pipes being displaced above and below the longitudinal axis of said converter, respectively, said portions being both concave downwardly in the top plate, both concave upwardly in the bottom plate, concave downwardly on both of the catalyst retaining plates at one cylindrical contour, and concave upwardly on both catalyst retaining plates at the other cylindrical contour, whereby exhaust gas passing from the exhaust inlet pipe to the exhaust outlet pipe must pass through both catalyst holding plates, the top and bottom plates being respectively dished upwardly and downwardly in relation to said peripheral faces so as to define a space for exhaust gas flow from the exhaust inlet pipe to the exhaust outlet pipe, the catalyst retaining plates being respectively dished upwardly and downwardly inboard of said peripheral faces and at varying distances in relation to said peripheral faces so as to define a catalyst containing space of substantially uniform depth disposed in generally slanting orientation between the exhaust inlet and the exhaust outlet, said top and bottom catalyst retaining plates further having arcuate openings extending vertically toward the top and bottom plates, respectively, adapted to pass the exhaust gases while retaining catalyst material disposed in said space, a bottom cover in spaced relation to said bottom plate adapted to house a layer of insulation therebetween and including an outwardly extending flange portion, a top cover in spaced relation to said top plate adapted to house a layer of insulation therebetween and including a U-shaped flange portion disposed about the peripheral faces of said top and bottom plates and said flange portion of said bottom cover, and at least one support stud disposed in generally vertical orientation inboard said periphery and extending between the top and bottom plates and through the catalyst retaining plates, said support stud having conformations cooperatively associated with openings in the catalyst retaining plates so as to maintain a predetermined spacing therebetween and being anchored to the top and bottom plates, respectively.

5. A catalytic converter for automotive use comprising a four plate sandwich construction that includes a top plate, a bottom plate and two intermediate plates, said top plate being of concave downward configuration and having a peripheral skirt portion and a pair of peripherally spaced downwardly concave semi-cylindrical inlet and outlet portions, said bottom plate being of concave upward configuration having a peripheral upstanding skirt portion and a pair of peripherally spaced upwardly concave semi-cylindrical inlet and outlet portions, the bottom plate being complementary to the top plate so as to form an enclosure having mating peripheral edges and full cylindrical inlet and outlet portions, said two intermediate plates also have peripheral portions complementary to and sandwiched between the peripheral portions of the top and bottom plates, one intermediate plate being upwardly concave aNd the other intermediate plate downwardly concave so as to define a catalyst retaining space therebetween said intermediate plates being perforated at the catalyst retaining space to allow and require gases entering the inlet portion to pass through the catalyst retaining space and out the outlet portion, both of said intermediate plates having a downwardly concave semi-cylindrical portion at the one end and an upwardly concave semi-cylindrical portion at the other end in nesting relation with each other and the top plate at said one end and with each other and the bottom plate at said other end.

7. A catalytic converter for automotive use comprising a four plate sandwich construction that includes a top plate, a bottom plate, and upper and lower catalyst retaining plates, the plates being complementary to each other so as to define a four layer periphery suitable for edge affixation to define a unitary structure, the top plate and the bottom plate inboard the periphery defining an exhaust gas flow passage so that exhaust gas travels into the space between said plates at one peripheral point and out of such space at another peripheral point, the upper and lower catalyst retaining plates being juxtaposed against each other and the top plate at one point in said flow passage so as to permit flow only underneath three layers formed by the top plate and the catalyst retaining plates and being juxtaposed against each other and the lower plate at another point in said flow passage so as to permit flow only over three layers formed by the bottom plate and the catalyst retaining plates, whereby the two catalyst retaining plates define a barrier to flow through the converter, said catalyst retaining plates having vertically spaced portions between said one point in said flow passage and said other point in said flow passage defining a catalyst retaining space said catalyst retaining plates being perforated to permit and require exhaust gas flowing through the converter to pass through the catalyst, the vertical spacing between the top plate and the upper catalyst retaining plate progressively decreasing in the direction from said one point towards the other point over a substantial distance and the vertical spacing between the bottom plate and the lower catalyst retaining plate progressively increases from said one point towards the other point over said substantial distance so as to define a substantially uniform cross section for gas flow at all points over said substantial distance.

8. A catalytic converter for automotive use comprising a four plate sandwich construction that includes a top plate, a Bottom plate, and upper and lower catalyst retaining plates, the plates being complementary to each other so as to define a four layer periphery suitable for edge affixation to define a unitary structure, the top plate and the bottom plate inboard the periphery defining an exhaust gas flow passage so that exhaust gas travels into the space between said plates at one peripheral point and out of such space at another peripheral point, the upper and lower catalyst retaining plates being juxtaposed against each other and the top plate at one point in said flow passage so as to permit flow only underneath three layers formed by the top plate and the catalyst retaining plates and being juxtaposed against each other and the lower plate at another point in said flow passage so as to permit flow only over three layers formed by the bottom plate and the catalyst retaining plates, whereby the two catalyst retaining plates define a barrier to flow through the converter, said catalyst retaining plates having vertically spaced portions between said one point in said flow passage and said other point in said flow passage defining a catalyst retaining space, said catalyst retaining plates being perforated to permit and require exhaust gas flowing through the converter to pass through the catalyst, the vertical spacing between the top plate and the upper catalyst retaining plate progressively decreasing in the direction from said one point in said flow passage towards said other point in said flow passage over a substantial distance and the vertical spacing between the bottom plate and the lower catalyst retaining plate progressively increases from said one point towards the other point over said substantial distance so as to define a substantially uniform cross section for gas flow at all points over said distance, and said top plate and the upper catalyst retaining plates and the bottom plate and the lower catalyst retaining plates being substantially juxtaposed in vertical orientation immediately inboard their peripheries so as to define two layer vertical peripheral walls.