Abstract:
A catalytic converter mat having a high density in areas which coincide with the ends of a substrate to be wrapped thereby and low density between the ends is disclosed which reduces cost of the mat component of a catalytic converter while retaining the benefits of proper mat pressure at the inlet and outlet ends of the substrate to retain and support the substrate without mat erosion and insulation of the substrate from the canister.

Description:
TECHNICAL FIELD 
     The present invention relates to producing catalytic converters. More particularly, the invention relates to producing catalytic converters while reducing the cost of the intumescent mat employed. 
     BACKGROUND OF THE INVENTION 
     Catalytic converters regularly employ a housing or canister, a catalytic substrate material and an intumescent mat placed between the canister and the substrate to hold the substrate in place and insulate the canister from the heat of the substrate during operation. Many different methods of wrapping the mat around the substrate exist and many different thicknesses and densities of these mats are designed into different catalytic converters. 
     Typical prior art methods for wrapping the substrate material around the catalytic substrate use a single thickness of mat material. This is true whether a single layer of mat is employed or multiple layers of mat are used. The method works extremely well and is reliable but is expensive to manufacture from the standpoint of material. Because lower housing temperatures are increasingly desirable, thicker intumescent mats are being employed on a regular basis. Additionally, more dense mat is being used to prevent erosion of the edges of the mat while the catalytic converter is in service. With thickness and density comes higher expense. Avoiding expense increase is therefore a desirable interest. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to reduce the cost of mat materials needed to manufacture a catalytic converter. 
     It is a further object of the invention to produce a catalytic converter having a lower temperature outer housing. It is yet a further object of the invention to accomplish the above objects while retaining mat mounting densities at the inlet and outlet ends of the substrate. 
     Advantageously, the method for producing catalytic converters and the catalytic converter produced thereby achieve the above objects by providing a single mat and partially cutting and folding the mat upon itself in specific locations to create a structure having variable thickness and therefore discrete densities after being packed into the housing or canister. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will now be described, by way of example only, with reference to the accompanying drawings in which: 
     FIG. 1 is a plan view of a mat illustrating lines upon which a fold is taught; 
     FIG. 2 is an end view of the mat of FIG.  1  and illustrates the partial cuts of the material at the illustrated fold line; 
     FIG. 3 is a plan view of the mat of FIG. 1 in the folded condition; 
     FIG. 4 is an end view of the mat of FIG. 3; 
     FIG. 5 is a plan view of a mat with an alternate fold pattern of the invention comprising a second embodiment; 
     FIG. 6 is an end view of the mat of FIG. 5; 
     FIG. 7 is a plan view of the folded mat of the second embodiment; 
     FIG. 8 is a end view of the folded mat of the second embodiment; 
     FIG. 9 is a cross-sectional view of a catalytic converter of the invention employing the mat of the FIG. 5 (second) embodiment; and 
     FIG. 10 is a graph representing thermal conductivity of the mat for density versus temperature. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention achieves all of the foregoing objects by employing a single weight mat and creating specific areas of mat which when the substrate and mat are mounted in the converter canister, become more dense than areas other than the specific areas. By providing the specific areas where density is higher at the inlet and outlet ends of a catalytic substrate, the substrate can be properly mounted and the area between the inlet and outlet ends can be of less dense material. Thus, the substrate is properly mounted in the canister at the inlet and outlet ends thereof with mount densities created by layered material and which are at densities generally accepted as required for good substrate retention and minimized erosion. The middle area of the mat relative to the substrate provides excellent insulation properties due to the low density, and therefore low thermal conductivity of the material. Because less material is employed in the central area due to the configuration of the invention all design parameters of the catalytic converter are met while reducing cost. 
     Referring to FIG. 1 a first embodiment of the mat of the invention is illustrated in plan view. Each mat part  10  is cut from large roll of mat with a certain basis weight in grams per square meter. After this mat is cut to the desired shape and installed in a converter assembly, it has a density in grams per cubic centimeter, determined by the basis weight of the mat used, and the annular space in which it is installed. The method of folding the mat is dictated by the ratio of mat density needed at the inlet and outlet ends of the catalyst coated substrate, to the density needed in the central area of the catalyst coated substrate. The embodiment illustrated in FIGS. 1-4 is for a mat that requires a density in the finished product of for example 1.0 grams per cubic centimeter at the inlet and outlet ends of the converter and a density of 0.5 grams per cubic centimeter in the central area of the catalytic converter. One way to achieve this result is to provide mat  10  of FIG. 1 which includes body  12 , tongues  14  and wings  16 . 
     One of ordinary skill in the art will recognize that tongues  14  are provided to span the larger circumferential distance traversed by an outer layer of material in a two or more layered mat when being wrapped around a catalytic substrate. In order to render folding of the wings easier and to ensure that the mat when folded is substantially flat, wings  16  are partially severed from body  12  as illustrated in FIG.  2  and identified by slits  18 . The depth of each slit  18  is preferably about one-half the thickness of the mat  10 . Without slits  18 , it would be very difficult to fold the mat at the proper location. 
     In the invention, folding of mat  10  is easy and non-binding and easily creates a mat of the structure illustrated in FIGS. 3 and 4. Each wing  16  is folded away from slit  18  thus opening the mat material along each slit  18 . 
     The folding motion is continued in this direction until surface  20   a  of body  12  and each surface  20   b  are in contact with one another. It should be understood that surface  20  has been artificially divided into surfaces  20   a  and  20   b  to illustrate the pattern of folding and in fact constitutes a single surface of mat  10 . When surfaces  20   b  are in contact with surface  20   a  (reference to FIG. 4 being made) the inlet and outlet edges  22  and  24  of mat  10  (these are interchangeable) are twice the thickness of the central area indicated as  26 . Twice the density is, thus, achieved upon installing the mat  10  into a converter shell with a substrate. The density of edges  22  and  24  is calculated in advance to provide proper support for a catalytic substrate. The density of central section  26  is not as important for mounting purposes and so is calculated to provide sufficient insulating properties. While more insulation generally provides more insulative properties, less insulation can often match the insulative properties of more insulation in a fixed space. This is because higher density insulation is of higher thermal conductivity and less dense material is of reduced thermal conductivity. A graph reflecting thermal conductivity of relative densities is provided in FIG.  10 . 
     Since the material is of lower density at the central section of the substrate, a cost savings is realized while retaining proper mounting pressures at the inlet and outlet ends of the mat by providing local higher density material as described. 
     In another embodiment of the invention, referring to FIGS. 5-9, another method of folding is illustrated which produces a mat with density in the finished catalytic converter of a central section two thirds as great as at the inlet-outlet ends of the substrate around which the mat is wrapped. One preferred method of folding a mat to produce the desired result is illustrated beginning with FIG.  5 . 
     Initially, the mat material  10  is cut into a shape which facilitates the folding operation and enables proper wrapping of the mat around the substrate of a catalytic converter. The shape, it will be appreciated, should take into account the longer circumferential path of the layers of material that will be positioned more radially outwardly relative to the catalytic substrate. Flap of tongue  32  shall be the longest piece of mat in the layup. In this embodiment, two surfaces must be identified and artificially broken up as they were in the previous embodiment for purposes of clarity. In this embodiment, surface  34  is identified as  34 A and  34 B and a fold line  36  is shown. Along fold line  36  is slit  38  which extends inwardly into the thickness of mat  10  from a surface on the reverse side of mat  10  from surface  34 . For continuity from the discussion of the previous embodiment, the relevant surface is a surface shown generally at  20 A. It should be noted that because flap  32  is added in this embodiment surface  20 A is larger in this embodiment than in the previous embodiment. The additional section of surface  20 A occasioned by flap  32  is identified as surface  20 C. Slit  38  begins inwardly into mat  10  from surface  20  at the parting line between surface  20 A and  20 C. Slit  38  is preferably about one-half the thickness of mat  10 . Slits  18  are also provided, as in the previous embodiment, at the line where wings  16  join body  12  and are to the same depth as previously disclosed. Slits  18  extend from surface  34 B into the mat  10 . 
     The first fold is carried out along fold line  36  and opens slit  38 . From the view of FIG. 5, the folding is of the right half of the drawing onto the left half of the drawing using fold line  36  as the reference line. In this embodiment, preferably flap  32  is longer than body  12  and will extend beyond edge  40  of body  12  when surface  34 B is in overlying contact with surface  34 A. The extended portion is visible in FIG. 7 as the structure extending beyond edge  40 . Wings  16  are folded as they were described in the previous embodiment yielding an end view of the mat as shown in FIG.  8 . The folded and wrapped mat  10  is schematically illustrated in a finished product catalytic converter in FIG. 9 wherein catalyst substrate  42  is mounted within housing  44  by mat  10  in folded form. For clarity, the above discussed numerals are employed in FIG. 9 to show the different portions of mat  10 . As one of skill in the art should now appreciate, the inlet  48  and outlet  50  ends of the substrate  42  in FIG. 9 are mounted with more density of mat  10  whereas the central area has only two layers of mat and therefore is less dense. 
     It will be understood that a person skilled in the art may make modifications to the preferred embodiment shown herein within the scope and intent of the claims. While the present invention has been described as carried out in a specific embodiment thereof, it is not intended to be limited thereby but is intended to cover the invention broadly within the scope and spirit of the claims.