Patent Publication Number: US-2022220876-A1

Title: Vehicle exhaust gas purification device, corresponding production method, exhaust line and vehicle

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is the US national phase of PCT/EP2020/063689, which was filed on May 15, 2020, which claims priority to FR 19 05202, filed May 17, 2019. 
    
    
     FIELD OF THE DISCLOSURE 
     The present disclosure relates to vehicle exhaust gas purification devices equipped with heating members. 
     BACKGROUND 
     A heating member can be made using metal sheets, stacked parallel to each other and rolled, as proposed by DE102007010758. 
     Such a heating member channels the exhaust gas in a laminar flow. This does not promote heat transfer from the heating member to the exhaust gas. 
     Furthermore, the heating member is not very rigid and must be attached to the purifier at multiple points. These attachment points are typically rods rigidly connected to the heating element and engaged in the channels of the purification member. This makes the use of a ceramic purifier problematic because of its fragility, and requires that it be made of metal instead. Such a purifier has a higher cost compared to a ceramic purifier. 
     In this context, the disclosure aims to provide a purification device that does not have the above shortcomings. 
     SUMMARY 
     To this end, the disclosure according to a first aspect relates to a vehicle exhaust gas purification device, the purification device comprising:
         a tubular shell having an inner surface and having a central axis;
           an electric heating member housed in the tubular shell, the electric heating member comprises a heating element made of an electrically conductive material which is permeable to exhaust gases; a power supply providing electrical power to the electric heating member; and   an attachment of the heating element is directly onto the inner surface.   
               

     The use of a heating element made of an electrically conductive material permeable to the exhaust gas facilitates heat transfer between the electric heating member and the exhaust gas. The power of the electric heating member can be reduced, which results in lower electrical consumption. 
     The electric heating member is attached directly to the inner surface of the tubular shell. It is not attached to the exhaust gas purification member. 
     This makes it possible to use a ceramic purifier, as it no longer has to bear the heating member attachments. 
     Direct attachment means that the attachment transmits at least 60% of the forces experienced by the heating element, typically at least 90% of the forces, directly to the inner surface. Thus, when the power supply comprises one or more conductor rods passing through the tubular shell to contact the heating element, only a small part of the forces experienced by the heating element are taken up by the conductor rods. The seal between the rod and the tubular shell is only moderately stressed. Its life span is increased. 
     The purification device may furthermore exhibit one or more of the following features, taken in isolation or in any combination that is technically possible:
         the heating element is made of a foam, the heating element having a central portion in which the foam has a first density and at least one reinforced edge area in which the foam has a second density greater than the first, the attachment securing the or each reinforced edge area to the inner surface;   the attachment comprises at least one support plate extending along a rim of the heating member and rigidly connected to the or a reinforced edge area of the heating member;   the attachment comprises a plurality of studs rigidly securing the or each support plate to the inner surface;   the attachment comprises a layer of electrically insulating material interposed between the heating element and the inner surface, the layer of electrically insulating material advantageously comprising a cylindrical portion radially interposed between a rim of the heating element and the inner surface;   the layer of electrically insulating material comprises at least one annular portion interposed axially between a major face of the heating element and the inner surface;   the attachment comprises a peripheral frame surrounding the heating element and attached to the tubular shell, with the layer of electrically insulating material directly interposed between the frame and the heating element;   the peripheral frame is divided into two half-frames arranged axially on either side of the heating element, each half-frame comprising a peripheral edge interposed between the rim of the heating element and the inner surface, the two peripheral edges being axially interlocked;   the attachment comprises at least one attachment member rigidly attached to the heating member at a distance from an outer edge of the heating element, the or each attachment member having axially protruding ends on either side of the heating element, the peripheral frame comprising arms rigidly attached to the ends of the or each attachment member;   the tubular shell has welding holes in the peripheral frame and welds rigidly connecting the peripheral frame to the tubular shell through the welding holes;   the peripheral frame is a part of the tubular shell;   the layer of electrically insulating material comprises two sub-layers stacked on each other;   the attachment comprises a support grid extending in a plane substantially perpendicular to the central axis and attachment members for attaching the grid to the inner surface, the grid having a large grid face to which the heating element is rigidly attached by using electrically insulated rods;   the heating element has a plurality of through slots, the purification device comprising at least one mask arranged opposite a large face of the heating element and having solid areas axially opposite the slots;   the heating element has first and second areas electrically connected to first and second terminals of the power supply, the heating element having through slots defining a single S-shaped path through the heating element for electrical current between the first and second areas;   the first and second areas are located on an outer edge of the heating element, and are symmetrical to each other with respect to a geometric centre of the heating element;   the slots are all parallel to one another;   the heating element has at least one through slot inclined at 5 to 30° relative to the axis C.       

     The disclosure according to a second aspect relates to a method of manufacturing a purification device having the above features, the method comprising a step of attaching the peripheral frame to the tubular shell by welding through welding holes provided in the tubular shell on the peripheral frame. 
     The disclosure according to a third aspect relates to an exhaust line comprising a purification device having the above characteristics. 
     The disclosure according to a fourth aspect relates to a vehicle comprising an exhaust line having the above features. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further features and advantages will be apparent from the detailed description given below, by way of indication and not in any way limiting, with reference to the appended figures, among which: 
         FIG. 1  is a simplified schematic depiction of an exhaust line comprising a purification device according to the disclosure; 
         FIG. 2  is an exploded perspective view of a first embodiment of the purification device of  FIG. 1 ; 
         FIGS. 3 and 4  are perspective views of variant embodiments of the electrically insulating material layer of the device of  FIG. 1 ; 
         FIG. 5  is a cross-section view of a second embodiment of the purification device of the disclosure; 
         FIG. 6  is a similar view to  FIG. 5 , illustrating variants of the second embodiment; 
         FIG. 7  is a perspective view, illustrating a variant of the first embodiment; 
         FIG. 8  is a perspective view, illustrating the variant embodiment of  FIG. 7 , and further showing a refinement of this variant; 
         FIG. 9  is an exploded perspective view of a third embodiment of the purification device of the disclosure; 
         FIG. 10  is an enlarged, schematic view of a detail from  FIG. 9 , showing the attachment of the heating element to the support grid; 
         FIG. 11  is a perspective view, illustrating the variant embodiment of  FIG. 7 , and further showing a second refinement of this variant; and 
         FIG. 12  is a top view of a particularly advantageous embodiment of the heating element, which can be used in all embodiments of the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The purification device  1  shown schematically in  FIG. 1  is intended for purifying the exhaust gas of a vehicle, typically the exhaust gas of a car or truck. 
     It is inserted into the vehicle&#39;s exhaust line  3 . This comprises an exhaust manifold  5  which collects the exhaust gases leaving the combustion chambers of the vehicle&#39;s internal combustion engine  7 . 
     The purification device  1  is fluidly connected to the manifold  5  by an upstream conduit  9 , on which other equipment such as a turbocharger is typically interposed. 
     Downstream, the purification device  1  is fluidly connected by a downstream conduit  11  to a nozzle  13 . Other equipment, such as silencers or other purification equipment, are interposed between the purification device  1  and the nozzle  13 . The purified exhaust gas is released into the atmosphere through the nozzle  13 . 
     The purification device  1  comprises a tubular shell  15  having an inner surface  16  of the tubular shell  15  having a central axis C, an electric heating member  19  housed in the tubular shell  15 , and a power supply  21  electrically supplying the heating member  19 . 
     The tubular shell  15  has an inlet  23  and an outlet  25  for exhaust gases, connected to the upstream and downstream conduits  9  and  11  respectively. 
     The tubular shell  15  has any suitable shape. 
     The purification device  1  further comprises an exhaust gas purification member  17  housed in the tubular shell  15 . 
     The purification member  17  is, for example, an SCR catalyst, a three-way catalyst, an oxidation catalyst or a NOx trap. 
     As can be seen in  FIG. 1 , a retaining sheet  27  is interposed between the purification member  17  and the tubular shell  15 . 
     Typically, one or more purification members are placed in the tubular shell  15 . 
     The heating member  19  is advantageously placed opposite and close to the inlet side  29  of the purification member  17 . Alternatively, the heating member  19  is placed opposite and close to the outlet face  31  of the purifier  17 , i.e. downstream thereof. The inlet face  29  and the outlet face  31  are the faces through which the exhaust gas enters and exits the purification member  17 . 
     Alternatively, the heating member  19  is placed at a distance upstream of the purification member  17 . 
     As shown in  FIG. 2 , the heating member  19  comprises a heating element  33  made of an electrically conductive material permeable to the exhaust gas. 
     In one embodiment, the heating element  33  may be substantially flat and thin. Advantageously, the heating element  33  may be in the form of a plate. “Thin” means that the thickness is between 0.3 mm and 30 mm. 
     According to a first embodiment where the heating element  33  would be a grid, the thickness may range from 0.3 mm to 10 mm, preferably between 0.5 and 2 mm. According to a second embodiment where the heating element is a foam or honeycomb, the thickness may be from 5 mm to 30 mm, preferably between 10 mm and 20 mm. 
     The heating element  33  typically extends in a plane substantially perpendicular to the central axis C. 
     Typically, the heating element  33  heats by Joule effect. 
     It comprises a network of exhaust gas passages, generating a turbulent flow of exhaust gas through the heating element  33 . 
     The material constituting the heating element  33  is typically a metal, such as stainless steel, or a metal alloy, or a ceramic. For example, this material is an iron alloy, such as FeCrAl. Alternatively, the material is a nickel or copper alloy, such as NiCr. In another embodiment, the material is a ceramic made of silicon carbide SiC. 
     The heating element  33  is typically a foam, with open pores organised in a random or regular manner. 
     Alternatively, the heating element  33  is a wire mesh or grid, or a honeycomb material. 
     The pore density is typically between 5 ppi (pores per inch) and 40 ppi. The material typically has a developed surface of between 500 and 5000 m2/m3, preferably between 1000 and 3000 m2/m3, and even more preferably between 1500 and 2500 m2/m3. 
     Advantageously, the heating element  33  is coated with at least one coating with a catalytic function to contribute to the aftertreatment of the exhaust gas. This coating is intended for the oxidation and/or reduction of polluting compounds in the exhaust gas. It can be, for example, of the same type as those used in TWC (Three-Way Catalyst), DOC (Diesel Oxidation Catalysis), PNA (Passive NOx Absorber), LNT (Lean NOx Trap), SCR (Selective Catalytic Reduction) or for the hydrolysis of a reducing agent used for the reduction of nitrogen oxides. 
     Alternatively or additionally, this coating is intended to increase the surface roughness of the material, with a view to promoting turbulence and thus heat exchange. 
     Due to its porosity, the heating element  33  also acts as a particle filter. The heating element  33  is regenerated at each heating, with the trapped soot particles being removed. 
     Advantageously, the heating element  33  is integral. It is all one piece, made of the same material. 
     Typically, the heating element  33  is obtained by cutting a single piece of the electrically conductive material from a plate. 
     Alternatively, the heating element  33  is obtained by casting, extrusion, sintering, additive manufacturing (3D printing), etc. 
     The heating element  33  has a thickness of between 2 and 50 mm, preferably between 5 and 30 mm, and more preferably between 10 and 20 mm. 
     In other words, the heating element  33  is in the form of a slice of electrically conductive material, cut directly to the required shape. 
     According to the disclosure, the purification device  1  comprises an attachment  35  of the heating element  33  directly onto the inner surface  16  of the tubular shell  15 . 
     The attachment  35  transmits directly to the inner surface  16  of the tubular shell  15  at least 60% of the forces to which the heating element is subjected  33 , preferably at least 80% of the forces, and more preferably at least 90% of the forces. 
     In other words, the attachment  35  directly engages the inner surface  16  of the tubular shell  15  to lock the heating element  33  in position. 
     It transmits to the inner surface  16  of the tubular shell  15  the various forces to which the heating element is subjected: Forces resulting from the acceleration of the vehicle, reaction of the road, forces applied by the exhaust gas, etc. 
     According to a first embodiment, illustrated in  FIG. 2 , the attachment  35  comprises a layer  37  of electrically insulating material interposed between the heating element  33  and the inner surface  16  of the tubular shell  15 . This layer holds the heating element within the inner surface of the tubular shell. 
     The material is for example a fibrous refractory material (such as alumina, silica, etc.) or a stratified refractory material (such as mica type, etc.). 
     The layer  37  comprises a cylindrical portion  39  radially interposed between a rim  49  of the heating element  33  and the inner surface  16  of the tubular shell  15 . The cylindrical portion  39  is compressed between the rim  49  and the inner surface  16  of the tubular shell  15 . The radial pressure exerted by the cylindrical portion  39  of the electrically insulating layer onto the heating element  33  helps to lock it in position. 
     It preferably extends around the entire circumference of the heating element  33 , and axially along the entire length of the plate. 
     The layer of electrically insulating material  37  preferably comprises at least one annular portion  41  axially interposed between a large face of the heating element  33  and the inner surface  16  of the tubular shell  15 . 
     i. Typically, the layer of electrically insulating material  37  comprises two annular portions  41 , interposed axially between the two large faces  43 ,  45  of the heating element  33  and the inner surface  16  of the tubular shell  15 . 
     The or each annular portion  41  extends along the peripheral edge of the corresponding large face  43 ,  45 . It covers a small fraction of the radius of the heating element  33 , typically less than 20% of the radius, preferably less than 10% of the radius. 
     In the example shown in  FIG. 2 , the layer of electrically insulating material  37  comprises two half-layers  47 , each having an L-shaped cross-section in a plane containing the central axis C. 
     Each half-layer  47  is made of one piece and comprises one of the two annular portions  41 , and half of the cylindrical portion  39 . 
     In the example shown in  FIG. 3 , the layer of electrically insulating material  37  comprises three elements independent of each other: The two annular portions  41 , and the cylindrical portion  39 . 
     In the example shown in  FIG. 4 , the layer of electrically insulating material  37  comprises two elements independent of each other: A first one-piece element comprising one of the two annular portions  41  and the cylindrical portion  39 , and a second element comprising the other of the two annular portions  41 . 
     The tubular shell  15  comprises a cylindrical portion  51 , opposite the rim  49 , against which the cylindrical portion  39  of the electrically insulating layer  37  bears. 
     The tubular shell  15  also comprises, for the or each annular portion  41 , a shoulder  53 , adjacent to the cylindrical portion  51 . 
     The or each shoulder  53  extends in a plane perpendicular to the central axis C. The or each annular portion  41  bears against the corresponding shoulder  53 . 
     To enable the mounting of the attachment  35 , the tubular shell  15  advantageously comprises first and second tubular sections  55 ,  57 . 
     The first and second tubular sections  55 ,  57  are independent parts, which are not integral with one another and which are attached to each other. 
     The first tubular section  55  defines one of the two shoulders  53  and a cylindrical section  59 . The second tubular section  57  defines the other shoulder  53  and a further cylindrical section  61 . The cylindrical section  59  has an outer cross-section corresponding to the inner cross-section of the other cylindrical section. It is tightly fitted into the other cylindrical section  61 . 
     The first and second tubular sections  55 ,  57  are rigidly connected to each other by any suitable connection: Peripheral welding, lugs, etc. 
     The first tubular section  55  has, for example, a cone  63  for connection to the upstream conduit  9 , delimiting the inlet  23 . 
     The second tubular section  57  has, for example, a tubular extension  65 , in which the purification member  17  is housed. 
     The cylindrical sections  59  and  61  together define the cylindrical portion  51  against which the cylindrical portion  39  of the electrically insulating layer bears. 
     The heating element  33  comprises two protrusions  67 ,  69 , electrically connected to the two terminals  71 ,  73  of the power supply  21 . 
     The protrusions  67 ,  69  are integral with the rest of the heating element  33 . 
     The protrusions  67 ,  69  project from the tubular shell  15  through holes  75 ,  77  in the tubular shell  15 . 
     A cover  79  made of an electrically conductive metal caps the projecting part of the protrusion  67 . It is attached to the protrusion  67  by any suitable connection allowing the passage of electric current, for example by brazing or welding. It carries a rod  81  for connection to a conductor electrically connected to the terminal  71  of the power supply  21 . 
     A boss  83  is rigidly attached to the outer surface of the tubular shell  15 . It surrounds the cover  79 . A layer  85  of electrical insulation is interposed between the cover  79  and the boss  83 . 
     Another cover  87  made of an electrically conductive metal caps the projecting part of the protrusion  69 . It is attached to the protrusion  69  by any suitable connection allowing the passage of electric current, for example by brazing or welding. It is also rigidly attached to the outer surface of the tubular shell  15 , by any suitable connection allowing the passage of electric current, for example by brazing or welding. 
     The tubular shell  15  in this case is the second terminal  73  of the power supply  21 . 
     A second embodiment of the disclosure will now be described, with reference to  FIGS. 5 and 6 . Only the points in which this second embodiment differs from the first will be detailed below. Elements that are identical or perform the same functions in both embodiments will be referred to by the same references. 
     In the second embodiment, the heating element  33  is a foam. 
     The heating element  33  has a central portion  89  in which the foam has a first density, and at least one reinforced edge area  91  in which the foam has a second density greater than the first. 
     The central part  89  has a maximum relative density of 20%, preferably between 5 and 10%, and the reinforced edge area has a minimum relative density of 40%, preferably more than 50%.
         The reinforced edge area  91  advantageously forms an equipotential connection between the legs of the heating element  33 , when the latter is of the spiral type having several legs, as shown in  FIG. 5 .       

     The attachment  35  secures the or each reinforced edge area  91  to the inner surface  16  of the tubular shell  15 . 
     Thus, forces are transmitted through the attachment  35  from the or each reinforced edge area  91 , which is more rigid than the central part  89  of the heating element  33 . 
     The heating element  33  has, for example, two reinforced edge areas  91  ( FIG. 5 ). Each reinforced edge area  91  is continuous and extends over a 25% to 50% fraction of the plate&#39;s periphery. The two reinforced edge areas  91  are symmetrical to each other with respect to the geometric centre of the heating element  33 .
         The attachment  35  in this case comprises at least one support plate  93 ,  95  extending along the rim  49  of the heating element  33  and rigidly attached to the or a reinforced edge area  91  of the heating element  33 .       

     The attachment  35  further comprises a plurality of studs  97  rigidly securing the or each support plate  93 ,  95  to the inner surface  16  of the tubular shell  15 .
         In the example shown in  FIG. 5 , the attachment  35  comprises two support plates  93 ,  95  extending along the rim  49  of the heating element  33  and each rigidly attached to one of the reinforced edge areas  91 .       

     The support plate  93  is made of an electrically conductive material. It is electrically connected to the first terminal  71  of the power supply  21 . 
     Viewed in cross-section perpendicular to the central axis C, the support plate  93  is interposed between the heating element  33  and the tubular shell  15 . It conforms to the shape of the outer edge of the heating element  33 . Thus, when the heating element  33  is circular, the support plate  93  is circular in shape. 
     The studs  97  are electrically insulating. They have a sandwich structure and each comprise, for example, two metal layers  98  rigidly attached to the inner surface  16  of the tubular shell  15  and to the support plate  33  respectively, separated from each other by an electrically insulating layer  99 . 
     A connector  101  passes through the tubular shell  15  via an opening in the tubular shell  15 . A ring, not shown, electrically isolates the connector  101  from the tubular shell  15 . The connector  101  provides the electrical connection from the support plate  93  to the first terminal  71  of the power supply  21 . 
     The support plate  95  is also made of an electrically conductive material. 
     It is electrically connected to the tubular shell  15 , which is the earth for the power supply  21 . 
     Viewed in cross-section perpendicular to the central axis C, the support plate  95  is interposed between the heating element  33  and the tubular shell  15 . It conforms to the shape of the outer edge of the heating element  33 . Thus, when the heating element  33  is circular, the support plate  93  is circular in shape. 
     The attachment studs  95  of the support plate  95  are electrically conductive. They are rigidly attached on one side to the inner surface  16  of the tubular shell  15  and on the other to the support plate  95 . 
     Each support plate  93 ,  95  extends substantially along the entire corresponding reinforced edge area  91 . It is attached by its entire surface to the reinforced edge area  91 . 
     In a variant embodiment shown in  FIG. 6 , a connector  103  passes through the tubular shell  15  via an opening in that tubular shell  15 . A ring not shown electrically isolates the connector  103  from the tubular shell  15 . The connector  103  provides the electrical connection from the support plate  95  to the second terminal  73  of the power supply  21 . The support plate  95  is not electrically connected to the tubular shell  15 . 
     The studs  97  securing the support plate  95  to the inner surface  16  of the tubular shell  15  are electrically insulating. They have the sandwich structure described above. 
     According to another variant embodiment shown in  FIG. 6 , the heating element  33  comprises a plurality of reinforced edge areas  91 , each extending over a small fraction of the plate&#39;s periphery, for example between 2% and 15%. The heating element  33  has, for example, at least four reinforced edge areas  91 . The reinforced edge areas  91  are spaced apart from each other and are, for example, evenly distributed around the heating element  33 . 
     In this case, each support plate  93 ,  95  is rigidly attached to at least two reinforced edge areas  91 , as shown in  FIG. 6 . 
     In yet another variant, not shown, the heating element  33  has a single reinforced edge area extending around substantially 100% of the periphery of the heating element. 
     In yet another variant embodiment, the support plates  93 ,  95  are used to attach a heating element which is not foam, but is of any other type: grid, honeycomb, etc. 
     A variant of the first embodiment of the disclosure will now be described, with reference to  FIGS. 7 and 8 . Only the points in which this variant differs from the one in  FIGS. 2 to 4  will be detailed below. Elements that are identical or perform the same functions in both variants will be referred to by the same references. 
     The attachment  35  comprises a peripheral frame  105  surrounding the heating element  33  and attached to the inner surface  16  of the tubular shell  15 . The layer of electrically insulating material  37  is directly interposed between the frame  105  and the heating element  33 . 
     The frame  105  completely surrounds the heating element  33 . It is radially interposed between the heating element  33  and the tubular shell  15 . Its shape corresponds to the shape of the outer edge of the heating element  33 . 
     Viewed in cross-section in a plane containing the central axis C, the frame  105  is U-shaped and open towards the heating element  33 . It thus defines a circular groove, in which the layer of electrically insulating material  37  is received. 
     The frame  105  thus has a cylindrical bottom  107  applied against the inner surface  16  of the tubular shell  15 , and two annular wings  109  integral with the bottom  107 . The wings  109  extend in respective planes perpendicular to the central axis C. They extend on either side of the large faces  43 ,  45  of the heating element  33 , opposite the outer edge of the heating element. 
     The cylindrical portion  39  of the electrically insulating layer  37  bears against the bottom  107 . 
     The annular portions  41  of the electrically insulating layer  37  each bear against one of the wings  109 . 
     For ease of assembly, the peripheral frame  105  is advantageously divided into two half-frames  111  arranged axially on either side of the heating element  33 . 
     Each half-frame  111  comprises a peripheral edge  113  interposed between the rim  49  of the heating element  33  and the inner surface  16  of the tubular shell  15 . 
     The two peripheral edges  113  are axially interlocked. They are rigidly attached to each other by any suitable connection: peripheral welding, lugs, etc. 
     The two peripheral edges  113  together define the bottom  107  of the frame. 
     Each half-frame  111  comprises, in addition to the peripheral edge  113 , one of the two wings  109 . 
     The two half-frames  111  are independent parts, which are not integral with one another and which are attached to each other. 
     Advantageously, the attachment  35  comprises at least one fastening member  115  rigidly attached to the heating element  33  at a distance from an outer edge  117  of the heating element  33  ( FIG. 8 ). 
     Only one attachment member  115  has been shown in  FIG. 8 . Alternatively, the attachment  35  comprises a plurality of attachment members  115 , distributed over the entire surface of the heating element  33 .
         The or each attachment member  115  has ends  119  protruding axially on either side of the heating element  33 .       

     The peripheral frame  105  comprises arms  121  rigidly attached to the ends  119  of the or each attachment member  115 . 
     This stiffens the heating element  33 . 
     The arms  121  typically extend from the wings  109 , and are integral with the wings  109 . 
     In the example shown, the attachment  35  comprises a single attachment member  115 , attached to the geometric centre of the heating element  33 . The peripheral frame has four arms  121  opposite each large face  43 ,  45 , forming  900  angles between them. 
     One of the half-frames  111  is as described above. The other half-frame  111  has an upstanding peripheral edge  122 , extending axially from the wing  109  away from the heating element  33 . 
     The attachment members  115  are of any suitable type. These are, for example, elongated attachment members, such as screws or tie rods. 
     According to one variant of the first embodiment, the layer  37  of electrically insulating material comprises two sub-layers stacked on one another. 
     The first underlayer, in contact with the heating element  33 , is for example a ceramic layer overmolded on the heating element  33 . 
     The second underlayer is interposed between the first underlayer and the frame  105  or between the first underlayer and the inner surface  16  of the tubular shell  15 . 
     The second underlayer is for example a preformed fibre web, of the type of fibre web  27  holding the purification member  17  in position in the tubular shell  15 . Such a web is known as “canning”. 
     Alternatively, the second underlayer is made of a braided electrical insulation material. 
     In another embodiment, the second underlayer is a fibre rope or sleeve. 
     In another embodiment, the second underlayer is made of mica and is preformed. 
     Advantageously, a layer of electrical insulation is arranged in the through slots  135  of the heating element, so as to guide the electrical current. 
     Such a design allows for electrical insulation and highly effective attachment. It increases the force applied to the heating element, thus absorbing vibrations and expansion of the heating element. It increases the compactness of the heating system. 
     In a further embodiment of the first embodiment, the tubular shell  15  has weld holes in the peripheral frame  105  and welds joining the peripheral frame  105  to the tubular shell through the weld holes. 
     The peripheral frame  105  advantageously has tabs P provided for this purpose, visible in  FIG. 8 . The tabs P are pressed against the inner surface  16  in front of the welding holes. They project axially from the upstanding peripheral edge  122 . 
     These holes allow the peripheral frame  105  to be welded to the tubular shell from the outside of the shell. Thus, there is no molten metal splash inside the tubular shell. In addition, the heat exchange surface between the heating element and the gas is increased. 
     The disclosure also relates to a method of attaching the peripheral frame  105  to the tubular shell  15  by welding through welding holes in the tubular shell  15  at the peripheral frame  105 . 
     In another embodiment of the first embodiment, the peripheral frame  105  is a part of the tubular shell  15 . 
     In this case, the layer  37  of electrically insulating material is preformed, and pre-installed in the peripheral frame  105 . 
     The peripheral frame  105  in this case is typically a ring, radially U-shaped in cross-section. The layer  37  is pre-installed at the bottom of the U. 
     The bottom of the U defines part of the outer surface of the tubular shell. The legs of the U project inwards from the tubular shell  15 . 
     According to this variant, the tubular shell  15  advantageously comprises several independent tubular sections, the peripheral frame  105  being interposed between two sections. The tubular sections are fitted and fastened axially on either side of the peripheral frame  105 . For example, one of the sections corresponds to the inlet cone of the tubular shell  15 , and the other corresponds to the cylindrical ferrule in which the purification member  17  is housed. 
     The peripheral frame  105 , for ease of assembly around the heating element  33 , is advantageously divided into a number of angular sectors, for example two semicircles, which are attached to each other. 
     Advantageously, the peripheral frame  105  has at least one attachment hole for an electrical supply electrode, electrically connected to the heating element  33 . 
     This variant has the advantage of being very compact in thickness and diameter. The weight of the purification device is reduced. It facilitates the installation of the heating element and the integration of the purification device into the exhaust line. 
     A third embodiment of the disclosure will now be described, with reference to  FIGS. 9 and 10 . Only the points in which this third embodiment differs from the first will be detailed below. Elements that are identical or perform the same functions in both embodiments will be referred to by the same references. 
     The attachment  35  comprises a support grid  123  extending in a plane substantially perpendicular to the central axis C and attachment members  125  for attaching the grid  123  to the inner surface  16  of the tubular shell  15 . 
     The grid  123  has a large grid face  126  to which the heating element  33  is rigidly attached. 
     The support grid  123  extends substantially across the entire cross-section of the tubular shell  15 . It has a high exhaust gas permeability. It is made of metal or ceramic. 
     The heating element  33  is not pressed against the support grid  123 . A gap remains between the heating element and the support grid  123 . 
     The attachment members  125  are, for example, tabs integral with the support grid and bent against the inner surface  16  of the tubular shell  15 . They are typically welded to the inner surface  16  of the tubular shell  15 . 
     The heating element  33  is rigidly attached to the support grid  123  for example by rods  127 . Each rod  127  at one end is rigidly attached to the heating element  33 . At its opposite end, each rod  127  is engaged in an attachment hole  129 , corresponding to a mesh of the grid ( FIG. 10 ). 
     If the support grid  123  is electrically insulating, the rods  127  may be made of an electrically insulating or conductive material, but if the grid is electrically conductive, the rods must be electrically insulating 
     The insulator is typically a ceramic (glass, magnesium oxide, alumina). 
     The support grid  123  comprises for example a plurality of transverse bars  131 , and a plurality of longitudinal bars  133  perpendicular to the transverse bars  131 . 
     The transverse bars  131  are arranged in pairs, with the two transverse bars  131  in a pair having a small longitudinal distance between them. The pairs of transverse bars  131  have a relatively larger longitudinal spacing between them. 
     Similarly, the longitudinal bars  133  are arranged in pairs, with the two longitudinal bars  133  in a pair having a small transverse spacing between them. The pairs of longitudinal bars  133  have a relatively larger transverse spacing between them. 
     The attachment holes  129  are defined by the crossings between the pairs of transverse bars  131  and the pairs of longitudinal bars  133 , as shown in  FIG. 10 . 
     The rods  127  are distributed over the entire surface of the heating element  33 . 
     The rods  127  embedded in the support grid  123  have for example a shape with a groove arranged to be wedged between the bars  131 ,  133 . Alternatively, the rods  127  are welded. 
     One advantage of the low-mass bar attachment, apart from the low back-pressure, is the flexibility to accommodate thermal expansion of the heating element. The bars, their shapes, and the spacing between the bars are chosen to allow the expansion of the support and the heating element without adding stress. 
     According to a variant applicable to all embodiments of the disclosure, the heating element  33  has a plurality of through-slots  135  (visible, for example, in  FIGS. 5, 6, 7 ), the purification device  1  comprising a mask  137  arranged opposite one of the large faces  43 ,  45  of the heating element  33  and comprising solid areas  139  axially opposite the slots  135  ( FIG. 11 ). 
     A through-slot  135  is understood here to mean a slot passing through the heating element  33  over its entire axial thickness, from the large face  43  to the large face  45 . 
     Each through-slot  135  is open on the large side  43  and on the large side  45 . It therefore provides a preferred escape route for the exhaust gas through the heating element  33 . It is advantageous to limit the leakage rate through the through-slots  135  in order to improve heating quality. This is done with the help of mask  137 . 
     The mask  137  has a solid area  139  opposite each through-slot  135 . The solid area  139  has substantially the same shape as the corresponding through-slot  125 . It therefore has an elongated shape, substantially the same length and substantially the same width as the corresponding through-slot  125 . It has the same design as the through-slot  125 . 
     The mask  137 , between the solid areas  139 , is hollowed out, so as not to create excessive back-pressure. Advantageously, spacers  141  connect the solid areas  139  to each other, to stiffen the mask. 
     In total, at least 80% of the surface of the mask  137  is preferably hollow. 
     In the example shown in  FIG. 11 , the mask  137  is integral with the peripheral frame  105 . Alternatively, the mask  137  is a member mechanically independent of the heating element  33  and/or the attachment  35 . The mask  137  is, for example, a plate rigidly attached to the tubular shell  15 . 
     The mask  137  is placed upstream or downstream of the heating element  33 . 
     In an embodiment applicable to all embodiments of the disclosure, the heating element  33  has first and second areas  143 ,  145  electrically connected to first and second terminals  71 ,  73  of the power supply  21 , the heating element  33  having through-slots  147  defining a single S-shaped path through the heating element  33  for electrical current between the first and second areas  143 ,  145  ( FIG. 12 ). 
     The first and second areas  143 ,  145  are located on the outer edge  117  of the heating element  33 , and are symmetrical to each other about a geometric centre of the heating element  33 . 
     The geometric centre is the barycentre of all points on the plate. When the heating element is circular, it is the centre of the circle. 
     The first and second areas  143 ,  145  divide the outer edge  117  into two opposing parts  149  and  151 . 
     A through-slot  147  is understood here to mean a slot passing through the heating element  33  over its entire axial thickness, from the large face  43  to the large face  45 . 
     Each through-slot  147  is open on the large side  43  and on the large side  45 . 
     The slots  147  are all parallel to each other. They all run in a transverse direction. 
     The slots  147  follow each other longitudinally, i.e. they are all longitudinally offset from each other. 
     Each slot  147  extends transversely from either part  149  or part  151  of the outer edge  117 . It is open at the said part. It extends over more than 50% of the transverse width of the heating element taken at said slot, preferably over 75% of the width. 
     The slots  147  extend alternately from part  149  and from part  151  of the outer edge  117 . In other words, two longitudinally successive slots  147  will extend one from part  149  and the other from part  151  of the outer edge  117 . 
     Typically, the first and second areas  143 ,  145  are substantially aligned longitudinally. Alternatively, the line through the first and second areas  143 ,  145  makes a small angle with the longitudinal direction, typically less than 45°. 
     The electric current thus follows a path comprising a plurality of transverse branches  153 , connected to each other by U-shaped areas  155  alternately oriented in opposite directions. 
     The number of slots  147  depends on the size of the heating member  33 . It is typically between 4 and 20. 
     Each slot has a small width, for example between 1 and 3. 
     The first and second areas  143 ,  145  are typically reinforced edge areas of the type described above. 
     A heating element offering the electric current such an S-path has the advantage that its resonance frequencies are relatively high, and are significantly higher than the excitation frequencies generated by the vehicle&#39;s combustion engine. The heating element is therefore not very sensitive to vibrations generated by the engine, and its robustness and durability are correspondingly improved. 
     The disclosure has been described above for a circular heating element. The heating element can have any other suitable shape: Oval, racetrack, elliptical, rectangular, TV screen (i.e. rectangle with rounded corners and/or edges), etc. 
     The disclosure has been described above for a heating member mounted in the same tubular shell as an exhaust gas purification member. Alternatively, the heating member and the exhaust gas purification member are mounted in different tubular shells, fluidly connected by a conduit. 
     According to an advantageous variant, applicable to all embodiments, the heating element  33  has at least one through-slot inclined at 5 to 30° with respect to the axis C. Typically, the heating element  33  has several through slots inclined by 5 to 30° relative to the axis C. These slots are open on the large face  43  and on the large face  45 . 
     Each slot is elongated, along a straight or sinuous centre line. The straight sections of the slot, taken perpendicular to the centre line, are bounded by two opposite edges, substantially parallel to each other. These two edges each form an angle of between 5 and 30° with the axis C. 
     This has the advantage of diverting the exhaust gas flowing through the heating member and increasing the contact area between the gas and the heating member. This improves the heat transfer between the gas and the heating element. 
     In all embodiments of the disclosure and in all variant embodiments envisaged, a layer of electrical insulation is advantageously arranged in any through-slots in the heating element so as to guide the electrical current. 
     Although various embodiments have been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this disclosure. For that reason, the following claims should be studied to determine the true scope and content of this disclosure.