Abstract:
A catalytic converter with electric heating includes a housing and at least one honeycomb body being disposed in the housing for conducting a flow of exhaust gas of an internal combustion engine through the honeycomb body in a given flow direction. The honeycomb body is electrically conductive at least in partial regions upon being heated by an electric current and it has a catalytically active coating at least in partial regions. The honeycomb body has at least two electrically heatable partial regions being disposed in succession as seen in the given flow direction. The electrically heatable partial regions have a different axial length and/or a different electrical resistance. The honeycomb body also has at least one other partial region being at least substantially blocked to the electric current and separating the electrically heatable partial regions from one another.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a Continuation of International Application Ser. No. PCT/EP94/00115, filed Jan. 18, 1994. 
    
    
     CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a Continuation of International Application Ser. No. PCT/EP94/00115, filed Jan. 18, 1994. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a catalytic converter with electric heating, including a housing containing at least one honeycomb body configuration through which an exhaust gas of an internal combustion engine can flow in a flow direction, the honeycomb body configuration being electrically conductive at least in partial regions, being heatable and having a catalytically active coating at least in partial regions. 
     Electrically heated catalytic converters are described, for instance, in U.S. Pat. No. 5,146,743. A more sophisticated construction, which is the point of departure for the present invention, is described in International Patent Application WO 92/02714, corresponding to U.S. Pat. No. 5,411,711. It is also known to heat the honeycomb bodies that are used in electrically heatable catalytic converters, nonhomogenously. To that end, International Patent Application WO 92/13635, corresponding partially to U.S. application Ser. No. 08/353,964, filed Dec. 12, 1994, proposes perforating metal foils used in the honeycomb body with slits or holes, in order to influence a current distribution. 
     The known structures for electrically heatable honeycomb bodies, while fundamentally suitable for most applications, nevertheless make it difficult, for various given peripheral conditions, to create standardized models that are mechanically stable and at the same time meet the electrical and thermodynamic demands made of them. 
     It has been found that electrically heatable catalytic converters have to be adapted to virtually each vehicle type, if optimal results are to be attained. In order to do so, first a maximum current intensity available for heating must be taken into account, and an electrical resistance of the honeycomb body, at a given supply voltage (usually 8 to 12 V) must orient itself thereto. Since an electrically heatable honeycomb body should be disposed as close as possible to and upstream of a following non-heatable catalytic converter, such as a precatalyst or a main catalyst, the diameter of the electrically heatable honeycomb body must be adapted to those given situations. Moreover, the available heating area in proportion to the heated mass is important, which makes it desirable to be able to vary that proportion over a wide range. It is precisely that which is quite difficult in the known models having a fixed resistance and a fixed diameter. In particular, certain peripheral conditions would require quite axially short heatable honeycomb bodies, which are not sufficiently stable mechanically and above all cannot withstand the vibrations that occur in motor vehicles. 
     SUMMARY OF THE INVENTION 
     It is accordingly an object of the invention to provide an electrically heated catalytic Converter, which overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type and which can be standardized with respect to its dimensions yet nevertheless enables adaptation to given peripheral conditions over wide ranges. A proportion between a heating surface area and a mass to be heated should be adjustable, and a proportion between heated surface areas and catalytically active surface areas should also be variable. 
     With the foregoing and other objects in view there is provided, in accordance with the invention, a catalytic converter with electric heating, comprising a housing; and at least one honeycomb body being disposed in the housing for conducting a flow of exhaust gas of an internal combustion engine through the at least one honeycomb body in a given flow direction; the at least one honeycomb body being electrically conductive at least in partial regions upon being heated by an electric current; the at least one honeycomb body having a catalytically active coating at least in partial regions; the at least one honeycomb body having at least two electrically heatable partial regions disposed in succession as seen in the given flow direction, the electrically heatable partial regions having a different axial length and/or a different electrical resistance; and the at least one honeycomb body having at least one other partial region being at least substantially blocked to the electric current, the at least one other partial region separating the electrically heatable partial regions from one another. 
     The honeycomb body configuration is subdivided by slits and/or electrically poorly conductive zones in such a way as to produce at least two electrically heatable partial regions. 
     In accordance with another feature of the invention, the electric current has a given direction, and the at least one honeycomb body has a multiplicity of slits formed therein extending approximately crosswise or obliquely to the direction of the electric current. 
     The invention proceeds from the fact that a catalytic conversion in a catalytic converter does not take place to a significant extend until beyond temperatures of approximately 300° to 450° C. If a large honeycomb body is therefore heated slowly, then initially a catalytic conversion does not occur anywhere. If instead one rapidly heats only a very axially short area with a small heated mass, then the catalytic conversion begins early there. The prerequisite therefor is that the surface of that area not be too large in proportion to the heating output and to the mass throughput of exhaust gas, which is still cold in the cold starting phase. In the rapidly heated area, an exothermic reaction ensues, which jointly activates the chemical energy contained in the exhaust gas during the cold starting phase in order to heat the catalytic converter. The electrical energy stored in the rapidly heated area, and the chemical energy converted there, heat the exhaust gas and directly following partial regions of the catalytic converter jointly, but from the end of the heated area on the temperature drops again, since the following part of the catalytic converter functions as a heat sink. At a short distance downstream of the heated area, the temperature therefore drops back below 300° C., so that catalytic conversion no longer occurs there. According to the invention, a further heatable partial region is provided there in order to raise the temperature again to a range which is suitable for catalytic conversion. However, while the first heatable partial region must heat the exhaust gas from 150° C. to 450° C., for instance, the second heatable partial region must raise the temperature only from 300° C. to 450° C., for instance. It is readily apparent that less electrical power is needed therefor than must be employed in the first heatable region. The second heatable partial region should therefore have higher resistance than the first heatable partial region, which can be achieved either by a different construction or by a shorter axial length. 
     In accordance with a further feature of the invention, the first electrically heated partial region has from twice to four times the axial length of the second electrically heated partial region. If the structure is otherwise the same in both partial regions, this accordingly raises the resistance in the second partial region by a factor of 2 to 4. 
     With regard to the mechanical strength of the honeycomb body configuration, it is advantageous if at least one non-directly heatable partial region is disposed between the electrically heatable partial regions. 
     In accordance with an added feature of the invention, in order to enable utilization of this surface area for the catalytic conversion, this partial region has a catalytically active coating. Although this area is not directly heatable, nevertheless it is jointly heated by the two directly electrically heatable partial regions, so that this directly heatable partial region still contributes to the catalytic conversion. 
     In accordance with an additional feature of the invention, in order to meet stringent environmental protection laws, a rapid onset of catalytic conversion is necessary, and therefore the first electrically heatable partial region is constructed, in terms of its axial length, its electrical resistance, its mass and its surface area, in such a way that at a given supply voltage, such as 8 to 12 V, it is heated in the exhaust gas stream of the engine during a cold-starting phase to approximately 450° C. within from 3 to 5 seconds. As discussed in further detail below in terms of exemplary embodiments, the structural form according to the invention still allows this condition to be met yet with a mechanically stable structure. 
     In accordance with yet another feature of the invention, the partial region, which is located downstream of the first electrically heatable partial region and is not directly heatable, should also have an axial length, mass and surface area such that the temperature drop during the cold-starting phase of the engine, over its axial length and taking into account exothermic reactions, is only approximately 50° to 150° C. for an inlet temperature of 450° C. In this kind of embodiment, practically the entire non-directly heatable partial region nevertheless contributes very early to the catalytic conversion and thus lowers pollutant emissions during the cold starting phase. 
     In accordance with yet a further feature of the invention, the second electrically heatable partial region is constructed with respect to its axial length, its electrical resistance, its mass and its surface area, in such a way that at a given supply voltage, during a cold-starting phase of the engine, at an inlet temperature of 400° C. and taking into account exothermic reactions in this partial region, it brings about a temperature increase of from 50° to 150° C. 
     In accordance with yet an added feature of the invention, the second electrically heatable partial region is followed by a further partial region with a catalytically active coating that then likewise contributes early to the catalytic conversion. In principle, further electrically heatable and non-heatable partial regions can follow, with their effect each corresponding to the effects described above. 
     In accordance with yet an additional feature of the invention, it is especially advantageous if the entire honeycomb body configuration, or in other words both the heatable and the non-heatable partial regions, are coated with catalytically active material. 
     In principle it is possible for each partial region of the honeycomb body configuration to include a separate honeycomb body, with the honeycomb bodies being disposed one after the other. 
     However, in accordance with again another feature of the invention, the honeycomb body configuration includes a single honeycomb body, which is assembled from structured sheet-metal layers and is subdivided into at least three axially successive partial regions, namely a first electrically heatable partial region, a following partial region that is electrically non-continuously conductive at least in one direction as a result of a number of slits in the sheet-metal layers and therefore is not directly heatable, and a following second heatable partial region. This configuration is especially simple and economical to make and is especially stable mechanically. Unlike the disclosures of the prior art, in this case the slits are used in order to completely prevent a flow of current in a partial region. 
     In accordance with again a further feature of the invention, the slits are disposed crosswise or at an angle to the direction of the electrical potential, so that no electrical current flows in the sheet-metal regions between the slits. In most known structures for electrically heatable honeycomb bodies, especially that described in International Patent Application WO 92/02714, corresponding to U.S. Pat. No. 5,411,711, that means that the slits extend approximately in the flow direction or at an acute angle to it, thereby preventing an electrical flow of current crosswise to the flow direction. However, the axial stability of the partial region is preserved since the slits have virtually no effect on the stability. The heatable partial regions are joined together so axially stably that that body has a high axial mechanical strength even if the electrically heatable partial regions are axially very short. For the sake of axial stability, slits extending precisely parallel to the flow direction would be the most favorable, but slits extending slightly obliquely to the flow direction behave more favorably when steel layers or sheets are bent and especially when they are corrugated, so that the latter form is preferred. 
     In accordance with again an added feature of the invention, for many applications an axial length of all of the electrically heatable regions together of between 4 and 20 mm and preferably 6 to 16 mm is favorable. 
     In accordance with again an additional feature of the invention, the axial length of the first electrically heatable partial region is between 2 and 10 mm and preferably approximately 6 mm. 
     In accordance with still another feature of the invention, a range from 75 to 105 mm, and preferably approximately 90 mm, has proved to be favorable for the diameter of the honeycomb body configuration. 
     In accordance with still a further feature of the invention, the total axial length of the honeycomb body configuration should be between 12 and 40 mm and preferably approximately 25 mm, to attain a sufficient mechanical stability. 
     In accordance with still an added feature of the invention, the invention can be realized not only by honeycomb bodies assembled from individual metal layers but also by way of example by a single extruded honeycomb body, which is subdivided into axially successive partial regions having an electrical resistance that increases in the flow direction. It is known to make electrically conductive honeycomb bodies from metal powder or a mixture of ceramic powder and metal powder by extrusion, with the electrical resistance being adjustable by the mixture ratio between the metal powder and the ceramic powder. The resistance in individual regions of such a honeycomb body can also be varied by a subsequent treatment, such as oxidation or etching, so that partial regions of differing electrical resistance are easy to make. 
     Therefore an extruded honeycomb body can also have three axially successive partial regions, namely a first electrically conductive and heatable partial region, an electrically poorly conductive partial region following it that is therefore not directly heatable, and a second electrically conductive and heatable partial region following that. The effect of these partial regions is equivalent to the effects described above. 
     In accordance with still an additional feature of the invention, the extruded honeycomb body is formed of predominantly metal material in the highly conductive partial regions, and it is formed of predominantly ceramic material or metal material of high porosity in the poorly conductive regions. 
     In accordance with a concomitant feature of the invention, the honeycomb body can also include five partial regions, of which either three or two are directly heatable electrically. 
     Other features which are considered as characteristic for the invention are set forth in the appended claims. 
     Although the invention is illustrated and described herein as embodied in an electrically heated catalytic converter, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. 
     The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a fragmentary, diagrammatic, longitudinal-sectional view of a portion of an exhaust gas system in a motor vehicle, having a honeycomb body configuration according to the invention and a precatalyst disposed directly downstream thereof; 
     FIG. 2 is a diagram illustrating a temperature course during heating in a cold starting phase; 
     FIG. 3 is a fragmentary, longitudinal-sectional view of a structure of a catalyst configuration according to the invention; 
     FIG. 4 is a fragmentary, elevational view of a piece of a sheet-metal strip or layer which is suited to a honeycomb body configuration according to the invention; and 
     FIG. 5 is a longitudinal-sectional view of a structure of an extruded honeycomb body with three partial regions. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the figures of the drawing in detail and first, particularly, to FIG. 1 thereof, there is seen a diagrammatic, longitudinal section of a portion of an exhaust gas system of a motor vehicle, specifically a honeycomb body configuration 10, that is electrically heatable in partial regions and is located directly upstream of a precatalyst 6. Exhaust gas enters the honeycomb body configuration 10, which is accommodated in a housing 2, through an inflow diffusor 1, in a flow direction S. The precatalyst 6 is disposed in a housing 3 following the housing 2, and only a very small gap 5 with a width A is present between the honeycomb body configuration 10 and the precatalyst 6. The precatalyst 6 is followed by a diffusor 4, downstream of which a non-illustrated main catalyst is disposed. The honeycomb body configuration 10 has a diameter d, and the precatalyst 6 has a diameter D. An axial length LW of the honeycomb body configuration 10 and axial lengths LH1, LH2 and LH3 of heatable 10 partial regions are shown once again as being spread apart in the axial direction, in FIG. 1. 
     FIG. 3 shows the honeycomb body configuration 10 in a diagrammatic, longitudinal section. The honeycomb body configuration 10 has a first electrically heatable partial region 11, a following non-heatable other partial region 12, a following second partial region 13 that is again electrically heatable, a following other partial region 14 that is non-heatable, and a third electrically heatable partial region 15. However, simpler configurations according to the invention may include fewer partial regions that are disposed correspondingly. The honeycomb body configuration can have either a non-heatable partial region or a heatable partial region as its frontmost disk. Preferably, however, it will have a heatable partial region as its frontmost disk. 
     FIG. 2, which is related in spatial terms to FIG. 3, shows a temperature course in the honeycomb body configuration during a cold starting phase, in which the electrically heatable partial regions are heated. Cold exhaust gas flows to the partial region 11 at a temperature of 150° C., for instance, and is initially heated to 300° C. At that temperature, an exothermic catalytic conversion begins, so that further heating takes place through both electrical and chemical energy, and a following temperature rise to approximately 500° C. is somewhat steeper. In the following, non-heatable partial region 12, the temperature then drops from 500° C. to 300° C., but is still above the temperature which is necessary for the catalytic conversion, so that a catalytic conversion takes place within the entire partial region 12. The electrically heatable partial region 13 raises the temperature back to 500° C., while in the following, non-heatable partial region 14, the temperature drops again to approximately 300° C. The electrically heatable partial region 15 raises the temperature to 500° C. again, and the temperature then drops in the gap 5 and in the following precatalyst 6 to 300° C. again. A catalytic conversion thus takes place over an entire length LK, or in other words from approximately the middle of the first electrically heatable partial region 11 to the inside of the precatalyst 6. An entire electrically heatable length LH is composed of the three lengths LH1, LH2 and LH3 of the heatable partial regions 11, 13, 15. As can be seen, within certain limits the length LW of the honeycomb body 10 is independent of the total length LH of the heatable regions. Moreover, the length LK over which a catalytic conversion takes place is greater than the total length LH of the heatable regions. 
     FIG. 4, which is also related in spatial terms to FIG. 3, illustrates a basic structure of a sheet-metal strip or layer 17 with slits 16 of a kind that are suitable for making a honeycomb body configuration 10 corresponding to the structure of International Patent Application WO 92/02714, corresponding to U.S. Pat. No. 5,411,711, which is hereby entirely incorporated by reference. If a voltage is applied to non-illustrated ends of the sheet-metal strip or layer 17, then the current flows practically only within the regions 11, 13 and 15, but not in the regions 12 and 14, since there the slits prevent any significant flow of current. The foil 17 can be either a smooth foil or a corrugated foil. The precise form and configuration of the slits 16 is not of decisive importance, as long as in their entirety they prevent a flow of current which is crosswise to the flow direction in the regions 12 and 14. 
     FIG. 5 diagrammatically shows a structure of an extruded honeycomb body configuration 20 with a first electrically heatable partial region 21, a non-electrically-heatable partial region 22, and a second electrically heatable partial region 23. The partial region 21 has a high proportion of metal, while the region 22 has a higher proportion of ceramic. In principle, in extruded honeycomb bodies, an electrical resistance that increases continuously in the flow direction can be attained by suitably varying the mixture of metal and ceramic components in the extrusion, and thus again, in an especially favorable way, it attains the object of the invention. 
     The following tables show how honeycomb body configurations according to the invention can be standardized and how broad the allowable.ranges .for the various parameters are. The tables relate to structural forms in accordance with International Patent Application WO 92/02714, corresponding to U.S. Pat. No. 5,411,711, and the adjacent columns disposed alongside one another relate to models for electrical outputs of 750 W, 1000 W, 1500 W, 2000 W and 3000 W. In the rows disposed one below another, three different diameters d of the honeycomb body configuration are shown, namely 76 mm, 86 mm and 96 mm, pertaining to corresponding diameters D of precatalysts of 80 mm, 90 mm and 100 mm, respectively. 
     
                                           TABLE 1__________________________________________________________________________   Unit  750   1000  1500   2000  3000No.   Meas. W     W     W      W     W__________________________________________________________________________1  mm    D = 80          D = 80                D = 80 D = 80                             D = 80   mm    d = 76          d = 76                d = 76 d = 76                             d = 76   mm    LW = 17.5          LW = 17,5                LW = 17,5                       LW = 17.5                             LW = 26   mm    LH = 6,5          LH = 8,5                LH = 13                       LH = 17.5                             LH = 26   g     m = 10          m = 13                m = 19 m = 26                             m = 39   m.sup.2    FH = 0,04          FH = 0,05                FH = 0,08                       FH = 0,11                             FH = 0,17   layers    7     7     7      7     7   corrugated    3     3     3      3     3   cm.sup.3    V = 32          V = 42                V = 65 V = 88                             V = 1302  mm    D = 90          D = 90                D = 90 D = 90                             D = 90   mm    d = 86          d = 86                d = 86 d = 86                             d = 86   mm    LW = 17,5          LW = 17.5                LW = 17,5                       LW = 17.5                             LW = 28   mm    LH = 6          LH = 8.5                LH = 14                       LH = 17.5                             LH = 28   g     m = 13          m = 18                m = 30 m = 38                             m = 60   m.sup.2    FH = 0.06          FH = 0,08                FH = 0.13                       FH = 0,16                             FH = 0,26   layers    9     9     9      9     9   corrugated    4     4     4      4     4   cm.sup.3    V = 38          V = 54                V = 89 V = 111                             V = 1783  mm    D = 100          D = 100                D = 100                       D = 100                             D = 100   mm    d = 96          d = 96                d = 96 d = 96                             d = 96   mm    LW = 17.5          LW = 17.5                LW = 17.5                       LW = 20                             LW = 30   mm    LH = 7          LH = 10                LH = 14                       LH = 20                             LH = 30   g     m = 16,7          m = 24                m = 33,5                       m = 48                             m = 72   m.sup.2    FH = 0.07          FH = 0.1                FH = 0.141                       FH = 0,21                             FH = 0,31   layers    9     9     9      9     9   corrugated    4     4     4      4     4   cm.sup.3    V = 55          V = 78.5                V = 110                       V = 157                             V = 235,5__________________________________________________________________________ 
    
     
                                           TABLE 2__________________________________________________________________________   Unit  750   1000   1500   2000   3000No.   Meas. W     W      W      W      W__________________________________________________________________________1  mm    D = 80          D = 80 D = 80 D = 80 D = 80   mm    d = 76          d = 76 d = 76 d = 76 d = 76   mm    LW = 17.5          LW = 17.5                 LW = 17.5                        LW = 17.5                               LW = 26   mm    LH = 6,5          LH = 8,5                 LH = 13                        LH = 17.5                               LH = 26   g     m = 9 m = 12 m = 18 m = 24 m = 36   m.sup.2    FH = 0.04          FH = 0,053                 FH = 0.08                        FH = 0,106                               FH = 0,159   layers    7     7      7      7      7   corrugated    3     3      3      3      3   cm.sup.3    V = 30.2          V = 44 V = 60.4                        V = 88 V = 120.82  mm    D = 90          D = 90 D = 90 D = 90 D = 90   mm    d = 86          d = 86 d = 86 d = 86 d = 86   mm    LW = 17,5          LW = 17.5                 LW = 17.5                        LW = 17,5                               LW = 28   mm    LH = 6          LH = 8,5                 LH = 14                        LH = 17.5                               LH = 28   g     m = 13,5          m = 18 m = 27 m = 36 m = 54   m.sup.2    FH = 0.05          FH = 0.065                 FH = 0,1                        FH = 0.13                               FH = 0.195   layers    9     9      9      9      9   corrugated    4     4      4      4      4   cm.sup.3    V = 41.3          V = 55 V = 82.5                        V = 111                               V = 1653  mm    D = 100          D = 100                 D = 100                        D = 100                               D = 100   mm    d = 96          d = 96 d = 96 d = 96 d = 96   mm    LW = 17.5          LW = 17.5                 LW = 17,5                        LW = 20                               LW = 30   mm    LH = 7          LH = 10                 LH = 14                        LH = 20                               LH = 30   g     m = 16          m = 21.5                 m = 32.5                        m = 43 m = 65   m.sup.2    FH = 0.07          FH = 0.094                 FH = 0.141                        FH = 0.187                               FH = 0.282   layers    9     9      9      9      9   corrugated    4     4      4      4      4   cm.sup.3    V = 59          V = 78.5                 V = 118                        V = 157                               V = 235.5__________________________________________________________________________ 
    
     The abbreviations in the tables have the following meanings: 
     LW=length of the honeycomb body configuration; 
     LH=total axial length of the heated regions; 
     m=mass; 
     FH=heatable surface area; and 
     V=volume. 
     The tables differ in the mass and the heatable surface area. The tables also show how many intertwined layers are used to form such a honeycomb body, and how many of these layers are corrugated. High-temperature-corrosion proof metal layers of iron-chromium-aluminum alloys are typically used, with a thickness of from 0.04 to 0.1 mm, as the sheet-metal layers. As can be seen from the tables, the honeycomb bodies according to the invention are especially suitable for the lower electrical power ranges of 750 W, 1000 W or 1500 W. In the case of the higher power ranges, the standardization can be extended by heating the entire honeycomb body electrically (or in other words by no longer subdividing it into partial regions) and optionally increasing its axial length. However, it is precisely for the lower power ranges, with standard diameters and standard lengths of the honeycomb body configuration, the possibility arises of achieving favorable properties in terms of the warmup time in the cold starting phase.