Electric exhaust-gas heating device

Abstract

An electric exhaust-gas heating device which can be installed transversely to an exhaust gas flow in an exhaust gas line and through which the exhaust gas to be treated can flow axially is provided with a discoid, gas-permeable, electrically conductive, porous heating element (12) through which exhaust gas can flow axially and which forms a serpentine resistance heating conductor whose adjacent, parallel, elongate conductor segments (26, 28) are spaced apart and laterally delimited by gaps axially extending through the heating element (12). Electrode connections (18, 20) are also provided, which form between them a current path in which the conductor segments (26, 28) are located. The heating element (12) has a circumferential edge (14) having an increased density. Proceeding from each of the two electrode connections (18, 20), a first conductor portion (26) is defined in the current path since the circumferential edge portions (46, 48) into which the first conductor portion (26) transitions at its opposite ends are separated from one another. At least one of the two first conductor portions (26) has a larger cross-section transversely to its longitudinal extent and a lower electrical resistance than additional conductor portions (28) located between the first conductor portions (26).

Description

[0001] Electric exhaust gas heater

[0002] The invention relates to an electric exhaust gas heating device which can be mounted transversely to an exhaust gas flow in an exhaust gas line and through which the exhaust gas to be treated can flow axially, comprising a disc-shaped, gas-permeable, electrically conductive, porous heating element through which exhaust gas can flow axially and which forms a meandering resistance heating conductor, the adjacent, parallel, elongated conductor sections of which are spaced apart and laterally delimited by gaps extending axially through the heating element, a first electrode connection and a second electrode connection which form a current path between them in which the conductor sections lie, wherein the heating element has a peripheral edge with an increased density compared to a heating section framed by the peripheral edge, and wherein, starting from each of the two electrode connections, a first conductor section in the current path is defined in that the peripheral edge sections,into which the first conductor section merges at its opposite ends, are separated from each other.

[0003] Catalysts are typically installed in the exhaust systems of internal combustion engines to reduce emissions. To ensure optimal catalytic oxidation immediately after a cold start, it is common practice to provide heating devices that heat the catalyst to a reaction temperature.

[0004] For example, an electric heating device through which the exhaust gas flows can be used, which is arranged upstream of the catalyst and which heats the exhaust gas before it flows through the catalyst.

[0005] If the heating element of the heating device is to be heated by current flow due to the Joule effect and also be located in the exhaust stream, it must simultaneously exhibit high electrical resistance and low flow resistance. These requirements are met, for example, by heating elements made of an open-pore metal foam or a porous body composed of numerous, interconnected honeycombs that also conduct electricity.

[0006] It is crucial for these heating elements to be stable over the long term, i.e. they can be switched on at full heat output tens of thousands of times without the conductor sections breaking. However, the conductor sections are subject to high thermal and mechanical stress. For example, the downstream side of the conductor sections, i.e. the side not exposed to the flow, heats up to a maximum of 800°C during operation, whereas the upstream side, onto which the still cold exhaust gas flows, is considerably cooler. This means that the different temperatures result in different thermal expansions and high mechanical stresses in the conductor sections, particularly in the area of ​​the ends of the conductor sections where they merge into the peripheral edge. This peripheral edge serves to carry current from one conductor section to the adjacent conductor section and has a very low electrical resistance due to its increased density.This is also desired because no heating effect should occur in this area, as this area usually serves to mechanically fix the heating element in the exhaust pipe and little or no exhaust gas reaches it.

[0007] The object of the invention is to improve the known electric heating device so that it is more stable over time. Furthermore, the heating device should not become significantly more expensive as a result.

[0008] The problem is solved by an electric exhaust gas heating device of the type mentioned above, in which at least one of the two first conductor sections has a larger cross-section transverse to its longitudinal extent and a lower electrical resistance than further conductor sections located between the first conductor sections. It has been found that the first conductor sections are the most unstable. These first conductor sections border on so-called thermally dead conductor sections, through which at most minimal current flows because they are located at the very outside in the direction transverse to the longitudinal extent of the gap and do not have separate circumferential edge sections. Rather, the circumferential edge in these conductor sections runs along their entire extent, so that no or only a negligible current flows through these conductor sections. This means that they remain cold. However, these thermally dead conductor sections are fixed at the circumferential edge and are therefore very rigid.The immediately adjacent first conductor sections, which heat up and expand thermally, therefore meet the cold conductor sections at a transition point, so that the transition is subjected to heavy mechanical stress. Furthermore, the first conductor sections usually have the shortest axial length, so that when heated they are less able to compensate for the stresses caused by bending than the conductor sections located between the first conductor sections. By reducing the electrical resistance of the first or both of the first conductor sections, these do not heat up as much as the others and therefore expand less, which is accompanied by lower mechanical stress. The invention makes it possible to keep the overall resistance of the heating device the same as with previous heating devices, because the lower resistance of the first conductor sections can be compensated for by an increased resistance of the other conductor sections.

[0009] If the circumferential edge is closed except for the gap, elongated sections can arise in the areas where the electrode connections are provided, where the inner longitudinal edge is defined by a gap located entirely on the outside and the outer longitudinal edge by the circumferential edge. However, because the circumferential edge is denser and has no or no significant resistance, such an elongated section has no or no significant heating output. These elongated sections are therefore not the aforementioned first conductor sections, which are characterized by the fact that their ends merge into regions of the circumferential edge that are separated from one another, usually by a gap that completely severs the circumferential edge, so that the current does not flow parallel along the circumferential edge from one end to the opposite end, but only through the first conductor section.

[0010] Tests have shown that the exhaust gas heating devices according to the invention are significantly more stable than previous ones and can withstand many 1,000 switching operations more than previous exhaust gas heating devices.

[0011] The heating element of the exhaust gas heating devices according to the invention is preferably a metallic foam or a honeycomb body. The foam or honeycomb body is in each case a one-piece, electrically conductive body that also merges integrally into the peripheral edge. The metallic foam or honeycomb body preferably comprises FeCrAl or NiCrAl.

[0012] A simple and cost-effective option for realizing the peripheral edge is to assemble the heating element from two different parts, namely a part that forms the heating section, has a lower porosity and therefore a lower density, and a second part in the form of a ring with a higher density, which is connected to the heating section by sintering.

[0013] The typical design of the heating element provides for the gaps to alternately radiate from opposite halves of an outer peripheral surface of the peripheral edge. This means that, when viewed along the peripheral edge, one gap extends to the outer peripheral surface, whereas the neighboring gaps begin at a distance from the outer peripheral surface and thus do not extend to the outer peripheral surface. These gaps extend to the opposite half into the peripheral wall section there and run into it, but end at a distance from the outer peripheral surface, so that this region forms the electrical coupling and bridge between adjacent conductor sections, which also has a low and negligible electrical resistance.

[0014] The conductor sections preferably run linearly and parallel to each other. This allows the gaps to be easily produced by machining.

[0015] Another option for improving the stability of the heating element is for at least one of the two gaps that define a first conductor section to extend further into the circumferential edge with its end that terminates before the outer circumferential surface than gaps that only define the other conductor sections and also do not extend to the outer circumferential surface. The gaps are measured in their extension direction. In other words, the circumferential edge is cut deeper by at least one gap in a first or both first conductor sections than in the others, giving the conductor section more elasticity to bulge axially and deform during thermal deformation. This reduces the stress in the material. Tests have shown that this idea can be optimally combined with the aforementioned idea, according to which the first conductor section or both first conductor sections have a larger cross-section.However, this idea can also be used independently of the enlarged cross-section of the first conductor section(s) and represents a separate invention to increase the fatigue strength of the heating element.

[0016] In this context, the gap or gaps defining the first conductor sections on one side, which in an axial view (seen in the direction of flow) face away from a center of the heating element, can extend further into the circumferential edge with their end terminating in front of the outer circumferential surface than the other gaps. Since the gaps usually run alternately to the outer circumferential surface and end at a distance from it, it is normally sufficient for the outermost gap, i.e. the outermost gap perpendicular to the longitudinal extent of the conductor sections, which does not extend to the outer circumferential surface, to extend deeper into the circumferential edge than the other gaps.

[0017] To ensure maximum elasticity of the conductor sections while maintaining sufficient stability of the peripheral edge, the gap(s) whose end terminates before the outer peripheral surface extends further into the peripheral edge than the others can extend at least 50% further into the peripheral edge than the others. However, these gaps should preferably extend to a maximum of the center of the peripheral edge to ensure sufficient stability in this area of ​​the peripheral edge. It is important, however, that no hot spots are created at the peripheral edge due to gaps that are too deep and thus have insufficient conductive cross-sectional area.

[0018] The heating element is in particular a cylindrical disc, preferably of uniform thickness, which is adapted to the cross-section of the exhaust pipe.

[0019] In order to achieve sufficient thermal relief of the first conductor section with an enlarged cross-section, this section should have a cross-section that is at least 5% larger than the other conductor sections. The first conductor section(s) with a larger cross-section therefore have, as desired, a lower electrical resistance. However, this also reduces the heating output in these conductor sections. To compensate for this, a variant of the invention provides that the at least one first conductor section has a larger cross-section at its ends (longitudinal ends) than in its middle. This means that in the region of the transition from the conductor section to the peripheral edge, which is mechanically particularly sensitive, the heating element is heated less due to the lower electrical resistance.However, in the middle section in the longitudinal direction of the conductor section, the resistance increases, and here an acceptable heating power can be achieved to partially compensate for unevenness in the heating power over the area of ​​the heating section.

[0020] This idea can be implemented by having at least one of the gaps that border the first conductor section with a larger cross-section have an arc shape in the axial view, so that this first conductor section widens in the axial view towards an opposite end and has a larger cross-section there than in its middle. The cross-section of the conductor section therefore increases continuously towards the middle and not abruptly in order to reduce stress peaks in the conductor section. It is possible for only one of the gaps to have this arc shape so that the remaining gaps can be produced very easily, for example by machining. An arc-shaped gap can be created, for example, by waterjet cutting or laser cutting.

[0021] Electrodes are attached to the electrode terminals, for example, by soldering or welding. In the simplest case, the electrode terminals are merely points on the outer circumferential surface.

[0022] To position the heating element, the peripheral edges can each be axially covered by an elastic ring, so that the heating element is clamped axially in the exhaust line between the elastic rings and thus positioned axially and radially. Further features and advantages of the invention will become apparent from the following descriptions and the following drawings, to which reference is made. In the drawings:

[0023] Figure 1 is a plan view of a first embodiment of the electric exhaust gas heating device according to the invention;

[0024] Figure 2 is a plan view of a second embodiment of the electric exhaust gas heating device according to the invention;

[0025] Figure 3 is a partial view of two electric exhaust gas heating devices, on the left according to the prior art and on the right according to the present invention with a widened first conductor section;

[0026] Figure 4 is a partial view of two further electric exhaust gas heating devices, on the left according to the prior art and on the right according to the present invention with a deeply cut gap at the first conductor section;

[0027] Figure 5 is a partial view of two further electrical exhaust gas heating devices, on the left according to the prior art and on the right according to the present invention, with both a widened first conductor section and a deeper cut gap on the first conductor section; and

[0028] Figure 6 is a partial view of yet another electric exhaust gas heating device with a first conductor section widened towards the ends.

[0029] Figure 1 shows an electric exhaust gas heating device 10, which is mounted in an exhaust line of an internal combustion engine upstream of a catalytic converter, transversely to the exhaust flow, and through which the exhaust gas to be heated flows axially. This electric exhaust gas heating device 10 is intended to heat the exhaust gas and the downstream catalytic converter when it is cold in order to improve exhaust emissions. The exhaust gas heating device 10 is therefore an electric resistance heater and comprises a disc-shaped, here circular-cylindrical, heating element 12 made of a metal foam or a honeycomb body.

[0030] The heating element 12 is formed in one piece and has a peripheral edge 14, which has a density that is a multiple of the density of a heating section 16 surrounded by the peripheral edge 14. In the case of a metallic foam, for example, the peripheral edge 14 has a density of 3000 kg / m 3 plus or minus 25% and the heating section 16 has a density of 750 kg / m 3plus or minus 25%.

[0031] As can be seen in Figure 1, electrode terminals 18, 20 for electrodes 22, 24 are provided on the peripheral edge 14, in this embodiment diametrically opposed to each other. These electrodes can be coupled to an electrical energy source. A meandering current path through the heating element 12 is formed between the electrode terminals 18, 20.

[0032] This current path is composed of parallel, elongated conductor sections 26, 28, which are laterally spaced from one another by gaps 30, 32 extending axially (i.e., in the direction of flow) through the heating element 12. Thus, the heating element 12 is a resistance heating conductor.

[0033] The gaps 30, 32 run essentially parallel to each other and run circumferentially alternately from opposite halves 34, 36 (indicated by the arrows) of an outer circumferential surface 38 of the circumferential edge 14 to the other half 36, 34. On this other half, the gaps 30, 32 penetrate into the circumferential edge 14, but end there without reaching the outer circumferential surface 38 on this half.

[0034] As a result, conductor sections 26, 28 lying side by side on the circumferential edge 14 are electrically and mechanically coupled to one another by a U-shaped bridge, whereby this bridge has no significant electrical resistance due to the significantly higher density of the circumferential edge 14 and therefore, when current flows along the current path, heating only occurs in the area of ​​the conductor sections 26, 28.

[0035] Starting from the electrode terminals 18, 20, there is a so-called thermally dead conductor section 40 that experiences no or no significant heating when current flows. This is because these conductor sections 40 are continuously bordered by the peripheral edge 14, so that current, starting from the respective electrode terminal 18, 20, flows through the peripheral edge 14 to the adjacent conductor section 26 due to the low electrical resistance of the peripheral edge 14, as symbolized by arrows 42. The material region of the conductor sections 40 with lower density thus leads to no or no significant heating.

[0036] The conductor sections 26 are each the first conductor sections in the current path starting from the electrode terminals 18, 20, in which the peripheral edge sections 46, 48, which are provided at opposite ends of the conductor section 26 and into which the conductor section merge, are also separated from each other.

[0037] In the case of the conductor sections 40, there is only one peripheral edge section, which, however, electrically connects and thus short-circuits the ends of the conductor section 40 in the longitudinal direction (ie in the direction of the gaps 30, 32).

[0038] The remaining conductor sections 28 then connect to these two first conductor sections 26. In other words, the first conductor sections 26 are always those at the ends of the current path, which are noticeably heated when current flows.

[0039] These first conductor sections 26 are also shorter in the longitudinal direction than the conductor sections 28.

[0040] If heating occurs due to current flow, the downstream end face of the heating element 12 is heated to a higher temperature than the upstream end face because it is exposed to the cold exhaust gas and cooled. Thus, the conductor sections 26, 28 have different temperatures at their end faces, so that the thermal expansion on the downstream end face causes mechanical stress and bending of the conductor sections 26, 28 in the axial direction between their ends. However, since the conductor sections 26 are shorter in the longitudinal direction, these mechanical stresses have a greater effect than on the adjacent conductor sections 28, which have greater flexibility due to their increased length.

[0041] In order to reduce the mechanical stresses at the ends of the conductor sections 26, ie at the transition points to the peripheral edge 14, one or more of the following measures are provided.

[0042] Figure 3 shows, on the left, a section of a heating element 12 in the region of an electrode terminal 18 and the first conductor section 26. The heating element 12 is manufactured as in the prior art, specifically with conductor sections 26, 28 of identical and constant cross-section across the heating element 12. This means that the gaps 30, 32 run parallel to one another at the same distance. Since the heating element 12 is plate-shaped, at least in the region of the heating section 16, and of uniform thickness except for the gaps 30, 32, the widths of the conductor sections 26, 28, as well as their cross-sections, are identical.

[0043] In the right-hand variant in Figure 3, however, the respective first conductor section 26 is wider than the conductor sections 28 and thus has a greater width B compared to the width b of the conductor sections 28, with an identical thickness compared to the conductor sections 28. The electrical resistance of the first conductor sections 26 is therefore smaller than that of the conductor sections 28, which is associated with a somewhat lower heating output but also lower mechanical stresses.

[0044] A further measure for reducing the voltages in the conductor sections 26 is shown in Figure 4, right. Figure 4, left, again shows a section of a heating element 12 according to the prior art, as in Figure 3, left.

[0045] Tests have shown that, in particular, the outer gaps 30 extending from the electrode terminals 18, 20 in the direction of the current path, which delimit the respective first conductor section 26 and end in the peripheral edge 14, i.e., at this half 34 or 36, do not extend to the outer peripheral surface 38, must extend deeper into the peripheral edge 14 to create more flexibility in these areas. These gaps 30 define the respective first conductor sections 26 on one side, which, viewed in the axial direction, face away from a center of the heating element 12. Surprisingly, this deeper cut in the gap 30 does not create a weak point, but rather leads to greater fatigue strength at this otherwise most critical point.

[0046] In Figure 4, on the right, it can be seen that the gap 30 extends significantly deeper into the peripheral edge 14 at the top (with depth T) than the adjacent gap 32 extends into the peripheral edge 14 at the bottom (with depth t). The respective depths T, t must, of course, be measured from the same side of the gap in order to be able to compare them, because the peripheral edge 14 is circular.

[0047] Preferably, these gaps 30 extend at their end, at which they remain spaced from the outer peripheral surface 38, at least 50% further into the peripheral edge 14 than the remaining gaps 32, which also do not reach the outer peripheral surface 38.

[0048] However, in order to ensure sufficient mechanical stability, these gaps 30 should only cut through the peripheral edge 14 up to its center at most.

[0049] In the embodiment according to Figure 5, the left-hand illustration again shows a section of the heating element 12 according to the prior art, whereas the right-hand illustration combines the measures according to Figures 3 and 4, each on the right. This means that the gaps 30 extend deeper into the peripheral edge 14 than the other gaps 32, and the first conductor sections 26 have a larger cross-section, here a greater width, than the remaining conductor sections 28.

[0050] In general, it is preferred if the first conductor sections 26 have a cross-section that is at least 5% larger than the remaining conductor sections 28.

[0051] In the embodiment according to Figure 6, the gap 30 is not straight in axial view, but arcuate, whereas the gap 32, which is closer to the center (in axial view) of the heating element 12 and delimits the conductor section 26 on the other side, runs linearly here. The arcuate shape is selected such that the ends (in the longitudinal direction) of the conductor section 26 have a larger cross-section than the center of the conductor section 26. The width and thus the cross-section of the conductor section 26 thus varies continuously, with the electrical resistance in the region of the ends, where the mechanical weak point is located, being lower than in the center. This measure ensures that the first conductor section 26 has better heating performance in the region of its center. For example, the width of the conductor section 26 in its center can be the same as the width b of the other conductor sections 28, whereas the ends have a larger width B.Of course, the gap 32 could also be curved.

[0052] Overall, the remaining conductor sections 28 do not need to have an identical cross-section. Rather, it is also possible to provide some of these conductor sections with a different cross-section than neighboring conductor sections. For example, the conductor sections could become increasingly narrower toward the center of the exhaust gas heating device, as they are also longer.

[0053] The embodiment according to Figure 2 corresponds to that according to Figure 1, although here the electrode terminals 18, 20 are not diametrically opposed, but rather on the same half 34. Generally, it should be emphasized that the electrode terminals can be provided at a variety of locations, not only at the locations shown in the drawings. Preferably, the electrode terminals are located at the level of the thermally dead conductor sections 40 or in the region of the first conductor sections 26.

[0054] Here too, however, both measures are provided to increase the long-term stability of the heating element 12, ie the first, extra-wide conductor sections 26 and the gaps 30 projecting deeper into the peripheral edge 14.

[0055] In general, it should be emphasized that, alternatively or in addition to the gaps 30, the immediately adjacent gap 32, which also delimits the conductor section 26, can be cut deeper into the peripheral edge 14 to increase the elasticity of the conductor section 26. This would then be the case at points 50 in Figure 2.

[0056] The peripheral edge 14 also represents the location used to attach the heating element 12 in an exhaust line. For this purpose, elastic rings are located on the two opposite ends of the peripheral edge 14, which are compressed by stops in the exhaust line, clamping the peripheral edge 14 between them and positioning the heating element 12 radially and axially in the exhaust line.

Claims

Claims 1. An electrical exhaust gas heating device which can be mounted transversely to an exhaust gas flow in an exhaust gas line and through which the exhaust gas to be treated can flow axially, comprising a disc-shaped, gas-permeable, electrically conductive, porous heating element (12) through which exhaust gas can flow axially and which forms a meandering resistance heating conductor, the adjacent, parallel, elongated conductor sections (26, 28) of which are spaced apart and laterally delimited by gaps extending axially through the heating element (12), a first electrode connection (18) and a second electrode connection (20) which form a current path between them in which the conductor sections (26, 28) lie, wherein the heating element (12) has a peripheral edge (14) with an increased density compared to a heating section (16) framed by the peripheral edge (14), and wherein, starting from each of the two electrode connections (18,20) a first conductor section (26) is defined in the current path in that the peripheral edge sections (46, 48) into which the first conductor section (26) merges at its opposite ends are separated from one another, characterized in that at least one of the two first conductor sections (26) has a larger cross-section transverse to its longitudinal extent and a lower electrical resistance than further conductor sections (28) located between the first conductor sections (26).

2. Electric exhaust gas heating device according to claim 1, characterized in that the heating element (12) is a metallic foam or a honeycomb body.

3. Electric exhaust gas heating device according to claim 2, characterized in that the peripheral edge (14) is a region of greater density than the heating section (16).

4. Electric exhaust gas heating device according to one of the preceding claims, characterized in that the gaps (30, 32) start alternately from opposite halves (34, 36) of an outer peripheral surface (38) of the peripheral edge (14) and run to the opposite half (36, 34) into the peripheral edge section (48) there and end at a distance from the outer peripheral surface (38).

5. Electric exhaust gas heating device according to claim 4, characterized in that at least one of the two gaps (30) which delimit a first conductor section (26) projects further into the peripheral edge (14) with its end terminating in front of the outer peripheral surface (38) than gaps (32) which delimit only the remaining conductor sections (28) and end in front of the outer peripheral surface (38).

6. Electric exhaust gas heating device according to one of the preceding claims, characterized in that those gaps (30) which define the first conductor sections (26) on a side which, in an axial view, are facing away from a center of the heating element (12), project with their end terminating in front of the outer circumferential surface (38) further into the circumferential edge (14) than the remaining gaps (32).

7. Electric exhaust gas heating device according to claim 5 or 6, characterized in that the gap or gaps (30) which project further into the peripheral edge (14) with their end terminating in front of the outer peripheral surface (38) project at least 50% further into the peripheral edge (14), preferably wherein these gaps (30) extend at most to the middle of the peripheral edge (14).

8. Electric exhaust gas heating device according to one of the preceding claims, characterized in that the heating element (12) has a uniform thickness in the axial direction in the heating section and is plate-shaped in this region.

9. Electric exhaust gas heating device according to one of the preceding claims, characterized in that the heating element (12) is designed as a cylindrical disc, preferably with a uniform thickness.

10. Electric heating device according to one of the preceding claims, characterized in that the at least one first conductor section (26) with a larger cross section has a cross section that is at least 5% larger than the other conductor sections (28).

11. Electric exhaust gas heating device according to one of the preceding claims, characterized in that the at least one first conductor section (26) has a larger cross-section at its ends than in its middle.

12. Electric exhaust gas heating device according to claim 11, characterized in that at least one of the gaps (30) which delimits the first conductor section (26) with a larger cross-section has an arcuate shape in axial view, so that this first conductor section (26) widens in axial view towards its opposite ends and has a larger cross-section there than in its middle.

13. Electric exhaust gas heating device according to one of the preceding claims, characterized in that electrodes (22, 24) are attached to the electrode terminals (18, 20).

14. Electric exhaust gas heating device according to one of the preceding claims, characterized in that the peripheral edge (14) is covered on both axial sides by an elastic ring and that the heating element (12) is clamped axially in the exhaust gas line between the elastic rings and is thereby positioned.

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