Electrically heated honeycomb body with corrugated layers of different cell densities

Description

Technical area

[0001] The invention relates to a honeycomb body for an electrically heatable catalyst in an exhaust gas tract, having a plurality of flow channels through which flow can pass along a main flow direction, wherein the honeycomb body is formed from a plurality of metal foils stacked on top of one another to form a layer stack which is wound around at least one pivot point, wherein the layer stack has at least a first metal foil which has a first corrugation, and furthermore has at least one second metal foil which has a second corrugation, wherein the first metal foil and the second metal foil each form first and second corrugated layers, wherein the honeycomb body has a plurality of winding layers which, in the wound state, are spaced from one another in the radial direction of the honeycomb body by an air gap, wherein the first corrugation is not the same as the second corrugation. State of the art

[0002] Electric heaters, among other things, are used to heat the exhaust gases of an internal combustion engine. These help quickly raise the exhaust gas temperature to a specified minimum temperature, ensuring the fastest and most complete exhaust aftertreatment possible within an exhaust system. The minimum temperature results from the so-called light-off temperatures of the catalysts used in the exhaust system for exhaust aftertreatment. The catalytically active coating of the honeycomb structures used requires such a minimum temperature to allow the chemical conversion process to proceed as completely as possible.

[0003] A common form of electric heater is a metallic honeycomb structure connected to a voltage source. Applying current to the honeycomb structure generates heat by utilizing its ohmic resistance. This heat is transferred to the flowing exhaust gas and, in part, to the surrounding structures, thereby heating the system.

[0004] The metallic honeycomb bodies are formed from a plurality of different metallic foils. Smooth metal foils and metal foils that are fully or partially structured are used. By stacking these metallic foils on top of each other, a layer stack is created. This layer stack is subsequently wound around winding mandrels using a suitable tool, forming a mostly disc-shaped honeycomb body with a plurality of flow channels through which air can flow along a main flow direction.

[0005] The individual windings of the layer stack are spaced from each other in the finished heating disc by an air gap to prevent a short circuit between different points on the heating disc and to form a defined current path along the length of the layer stack.

[0006] US 2015 / 030509 A1 discloses an electrically heatable honeycomb assembly comprising an electrical connection of sheet metal layers at a connecting pin. A honeycomb body has a metal shell with an inner circumference, through which the connecting pin is guided in a bushing and electrically insulated. The sheet metal layers are alternating coarsely structured and finely structured or smooth sheets, which together form a stack with a top and bottom layer and channels for axial gas flow between the layers. The connecting pin extends radially approximately perpendicular to the layers and is metallically connected to at least two or all layers by at least one intermediate piece.

[0007] US 5,070,694 A discloses an improved electrically heated core for an electrically heated catalyst. The core is characterized by a plurality of corrugated thin metal strips attached to an electrically conductive central core or tube. Alternating with the corrugated thin metal strips are narrow strips of hard metal arranged in an axially offset pattern. These strips are spirally wound and fused to braze them together into a unitary monolith. The core is housed in a casing provided with insulated means for supplying and heating the monolith with electrical current.

[0008] EP 0 507 216 A1 discloses a core element for an electrically heated catalyst and an electrically heated catalyst incorporating the same. The core element is characterized by a corrugated thin metal foil strip, preferably in a herringbone pattern, and, in laminar relationship thereto, a flat thin metal foil strip made of either stainless steel or brazing metal, or a combination of both. The flat thin metal foil strip has a width less than the width of the corrugated thin metal foil strip. The strips are then spirally wound or accordion-folded to form a monolith and suitably connected to a voltage source to power the unit.

[0009] The flow cross-section of the exhaust gas path in the area of the heating disc of the electric heater is occupied by the metal foils and the flow channels formed between them, as well as the air gap between the windings. The metal foils and, due to their size, the flow channels also create a backpressure that counteracts the exhaust gas flow. In electric heaters with metallic honeycomb bodies, which are known from the prior art, the comparatively high pressure loss caused by the heating disc means that the exhaust gas preferentially flows through the air gap. This is disadvantageous because no or only minimal heating of the exhaust gas can take place there. The largest proportion of the heat is transferred to the passing exhaust gas in the flow channels, since the electric heater has the largest active surface area there. Description of the invention, task, solution, advantages

[0010] Therefore, it is the object of the present invention to provide a honeycomb body for an electric heater in an exhaust gas path of an internal combustion engine, which has an improved flow profile and in particular ensures the complete flow through the individual flow channels formed between the metal foils.

[0011] The problem with regard to the honeycomb body is solved by a honeycomb body having the features of claim 1.

[0012] An embodiment of the invention relates to a honeycomb body for an electrically heatable catalyst in an exhaust gas system, having a plurality of flow channels through which flow can pass along a main flow direction, wherein the honeycomb body is formed from a plurality of metal foils stacked one on top of the other to form a layer stack, which is wound around at least one pivot point, wherein the layer stack has at least a first metal foil having a first corrugation, and furthermore at least a second metal foil having a second corrugation, wherein the first metal foil and the second metal foil each form first and second corrugated layers, wherein the honeycomb body has a plurality of winding layers which, in the wound state, are spaced from one another by an air gap in the radial direction of the honeycomb body, wherein the first corrugation is different from the second corrugation and the layer stack has at least three corrugated layers,wherein the corrugated layers are separated from each other by smooth or microstructured third metal foils.,

[0013] The honeycomb body is manufactured using a known method, whereby the metal foils are stacked on top of one another and the resulting layer stack is wound around one or more winding mandrels. The layer stack creates several winding layers that are arranged radially next to one another and form the honeycomb body from the winding center to the outer radius of the honeycomb body. In the case of an electrically heatable honeycomb body, as in the case according to the invention, the individual winding layers are spaced apart from one another in the radial direction by an air gap in order to prevent physical contact between the individual winding layers and thus, in particular, to prevent an unwanted current path. The aim is for a current to flow through the honeycomb body from a defined current introduction point to a defined current discharge point.

[0014] Metal foils can essentially be divided into two categories: those with at least partial corrugation, and those that are essentially smooth or have only very slight microstructuring. At least partially corrugated metal foils usually have an approximately sinusoidal corrugation. The corrugation is defined by the pitch (p), which describes the distance on the x-axis between two minima or maxima. The corrugation is also defined by the corrugation width (w), which describes the distance on the y-axis between two minima or two maxima. The x-axis of the corrugation runs in the direction of the height of the corrugation, while the y-axis runs in the direction of the width of the corrugation.

[0015] The corrugation creates cells in the honeycomb body, allowing air flow along its axial direction. The cells are radially delimited by the metal foils. The cell density per unit area is expressed in cells per square inch (cpsi). The higher this number, the more cells are arranged in a square inch. A high number of cells per square inch means a smaller cross-section of the individual cells and thus increased pressure loss.

[0016] The honeycomb body according to the invention has at least two different corrugated layers formed by the metal foils with different corrugations. The different corrugated layers inevitably have different cell densities per unit area. The different corrugated layers thus generate different pressure losses for a medium flowing through the cells, such as, in particular, an exhaust gas stream.

[0017] With regard to the design essential to the invention, which provides radially spaced winding layers, at least three zones of varying pressure loss are created across the cross-section of the honeycomb body. Firstly, the air gaps between the winding layers generate the lowest pressure loss. Further, the corrugated layers have a low density of cells per unit area, and finally, the area of corrugated layers with a high density per unit area. The zones of varying pressure loss directly influence the exhaust gas flow, allowing for improved flow through the electrically heated areas of the honeycomb body and thus improved heating of the exhaust gas.

[0018] It is particularly advantageous if the first metal foil has less corrugation than the second metal foils.

[0019] As already described, the different corrugations lead to improved airflow through the honeycomb body and thus to improved heating of the exhaust gas. A lower corrugation specifically refers to a corrugation with a lower corrugation height and / or a smaller corrugation width, thereby achieving a corrugation layer with a higher cell density per unit area. The cell density of the corrugation layers should preferably be in a range of 20 cpsi to 500 cpsi to ensure optimal airflow through the cells.

[0020] The corrugated layers formed from the first metal foil preferably have a cell density of 100 cpsi to 150 cpsi in the case of the corrugated layer made from the first metal foil, or a cell density of 25 cpsi to 85 cpsi in the case of the corrugated layer made from the second metal foil. These cell densities are particularly advantageous for achieving advantageous cell densities for honeycomb bodies used to heat exhaust gases from an internal combustion engine, allowing particularly uniform and complete flow through the cells of the honeycomb body.

[0021] The honeycomb body is particularly advantageous for a porosity of 94% to 97%. The hydraulic diameter of the individual cells is preferably between 2 mm and 5 mm.

[0022] It is also advantageous if the corrugated layer, formed from a first metal foil, serves to accommodate support structures. The corrugated layer made from a first metal foil has a higher cell density and thus cells with a smaller hydraulic cross-section. The corrugated layers are preferably designed to accommodate the support pins, which support the honeycomb body in an electrically insulated manner from another catalyst.

[0023] A preferred embodiment is characterized in that the corrugated layer, formed from a second metal foil, serves to increase the rigidity of the honeycomb body. The lower cell density allows for the production of corrugated layers with greater rigidity. These corrugated layers are particularly suitable for producing a highly dimensionally stable honeycomb body, especially when they form the edge layers of the layer stack.

[0024] It is also preferable if the layer stack forming the honeycomb body has at least three corrugated layers, wherein at least one middle corrugated layer is formed by a first metal foil and at least the two corrugated layers forming the radial edges of the layer stack are each formed by a second metal foil.

[0025] The middle corrugated layer, made of a first metal foil, has a comparatively higher cell density and serves to accommodate the support structures, such as the support pins. This corrugated layer preferably forms the central corrugated layer of the layer stack, while corrugated layers made of second metal foils adjoin it on both sides. These have a lower cell density, thus offering lower resistance to the exhaust gas flowing through them, and are also stiffer, which can achieve improved dimensional stability of the honeycomb body.

[0026] Between the corrugated layers of the first metal foil and the corrugated layers of the second metal foil, smooth layers or so-called microstructured layers are preferably arranged, which separate the corrugated layers from one another and in particular prevent the adjacent corrugated layers from slipping into one another.

[0027] Furthermore, it is advantageous if the layer stack forming the honeycomb body has exactly three corrugated layers, with a third metal foil, a second metal foil, a third metal foil, a first metal foil, a third metal foil, a second metal foil, and a third metal foil being arranged successively within the layer stack. The central corrugated layer made of the first metal foil forms the receptacle for the support pins. The two laterally adjoining corrugated layers made of the second metal foils increase stability and simultaneously reduce the pressure loss occurring at the edge region, thereby preventing or significantly reducing airflow through the air gap formed between the winding layers.

[0028] Overall, the structure of the layer stack consisting of three corrugated layers is optimal, both to achieve the highest possible efficiency in heating the exhaust gas and to keep costs and manufacturing effort as low as possible. Furthermore, given the typical dimensions of the honeycomb body for heating exhaust gases in the exhaust system of a car or truck, a three-layer structure is particularly advantageous. This allows for the necessary ohmic resistance to be adjusted particularly well to achieve sufficiently rapid and powerful heating of the exhaust gases with the available electrical currents.

[0029] Furthermore, it is advantageous if the second metal foils each form a corrugated layer which has a corrugation with a greater corrugation height and / or greater corrugation width than the at least one corrugated layer formed by a first metal foil.

[0030] Varying the corrugation height can contribute to a reduction in cell density per unit area. This can also be achieved by varying the corrugation width. Preferably, the edge layers have a lower cell density per unit area than the central layers, which specifically improves the flow through the cells of the honeycomb body and reduces or prevents the exhaust gas from flowing past the formed air gaps.

[0031] It is also expedient if the corrugated layers forming the radial edge region of the layer stack generate a lower pressure loss for a fluid flowing through them than the corrugated layers arranged in the center of the layer stack.

[0032] Furthermore, it is advantageous if the cell density of the corrugated layers formed by the first metal foils is between 100 cpsi (cells per square inch) and 150 cpsi, and the cell density of the corrugated layers formed by the second metal foils is between 25 cpsi and 80 cpsi. This ratio of cell densities represents a preferable optimum, particularly for car and truck applications, for the typical sizes, flow velocities, temperatures, and electrical currents available for heating.

[0033] Furthermore, it is expedient if at least a first group of the second metal foils is provided which has a ratio (p / W) of corrugation height (p) to corrugation width (W) of less than 1.8 and / or at least a second group of the second metal foils is provided which has a ratio (p / W) of corrugation height (p) to corrugation width (W) of greater than 1.8.

[0034] The p / W ratio is a key factor in determining the shape of each corrugated layer. The higher the ratio, the flatter the wave, making the layer more flexible, especially in the Y direction, i.e., the direction perpendicular to the corrugation. This makes the corrugated layer easier to wind, but also more susceptible to unwanted deformation and damage.

[0035] A very low p / W ratio results in high corrugation, which reduces flexibility in the Y direction and achieves higher stiffness of the corrugation layer.

[0036] Extensive research has shown that the p / W ratio around 1.8 represents the inflection point for corrugated sheet stability. For ratios below 1.8, the corrugated sheet tends to be stiff, which is beneficial in terms of stability because it is less susceptible to mechanical deformation, which could damage or close cells. For ratios above 1.8, the corrugated sheet tends to be soft, which is beneficial for absorbing radial forces and stresses, thus ensuring the durability of the corrugated sheet or honeycomb structure.

[0037] Advantageous further developments of the present invention are described in the subclaims and in the following description of the figures. Short description of the drawings

[0038] The invention is explained in detail below using an exemplary embodiment with reference to the drawing. The drawing shows: Fig. 1 a view of a honeycomb body according to the invention, wherein the flow channels formed by the honeycomb body run along a surface normal to the plane of the drawing. Preferred embodiment of the invention

[0039] The Figure 1 shows a plan view of a honeycomb body 1, which is formed from a layer stack 2 by winding. The layer stack 2 is supported by support pins 4 relative to a catalyst 3 arranged behind it.

[0040] The layer stack 2 is wound around two winding mandrels, giving the layer stack 2 an S-shaped configuration. The individual winding layers are radially spaced from each other by the air gap 11.

[0041] The layer stack 2 has a central corrugated layer formed from a first metal foil 7. This corrugated layer is enclosed by third metal foils 5, 10, which have a microstructure. Alternatively, a completely smooth third metal foil could also be provided.

[0042] The third metal foils 5, 10 are followed by two corrugated layers of second metal foils 6, 9. The second metal foils 6, 9 have a slightly larger corrugation height (pitch) and a significantly larger corrugation width, whereby the corrugated layers formed by the second metal foils 6, 9 have a lower cell density than the corrugated layer formed by the first metal foil 5, 10.

[0043] The second metal foils 6, 9 are followed by third metal foils 5, 10, which also have a microstructure and, in alternative embodiments, could also be formed by completely smooth metal foils.

[0044] The air gap 11 serves to electrically separate the individual winding layers from each other and thus to prevent short circuits and unwanted current conduction paths.

[0045] The embodiment of the Figure 1 In particular, it is not restrictive and serves to clarify the inventive concept. List of reference symbols

[0046] 1. Honeycomb body 2. Layer stack 3. Honeycomb body 4. Support pin 5. Third metal foil 6. Second metal foil 7. First metal foil 8. Third metal foil 9. Second metal foil 10. Third metal foil 11. Air gap

Claims

1. Honeycomb body (1) for an electrically heatable catalytic converter in an exhaust gas tract, having a plurality of flow channels through which flow can occur along a main through flow direction, wherein the honeycomb body (1) is formed from a plurality of metal foils (5, 6, 7, 8, 9, 10) stacked on one another to form a layer stack (2), which is wound around at least one pivot point, wherein the layer stack (2) has at least one first metal foil (7) which has a first corrugation, and furthermore has at least one second metal foil (6, 9) which has a second corrugation, wherein the first metal foil (7) and the second metal foil (6, 9) respectively form first and second corrugated layers, wherein the honeycomb body (1) has a plurality of winding layers, which are spaced apart from one another in the radial direction of the honeycomb body (1) in the wound state by an air gap (11), wherein the first corrugation is different from the second corrugation, characterized in that the layer stack (2) has at least three corrugated layers, wherein the corrugated layers are each separated from one another by smooth or microstructured third metal foils (5, 10).

2. Honeycomb body (1) according to Claim 1, characterized in that the first metal foil (7) has a lesser corrugation than the second metal foils (6, 9).

3. Honeycomb body (1) according to one of the preceding claims, characterized in that the corrugated layer which is formed from a first metal foil (7) is used to accommodate support structures (4).

4. Honeycomb body (1) according to one of the preceding claims, characterized in that the corrugated layer which is formed from a second metal foil (6, 9) is used to increase the rigidity of the honeycomb body (1).

5. Honeycomb body (1) according to one of the preceding claims, characterized in that the layer stack (2) forming the honeycomb body (1) has at least three corrugated layers, wherein at least one middle corrugated layer is formed by a first metal foil (7) and at least the two corrugated layers forming the radial edges of the layer stack (2) are each formed by a second metal foil (6, 9).

6. Honeycomb body (1) according to one of the preceding claims, characterized in that the layer stack (2) forming the honeycomb body (1) has precisely three corrugated layers, wherein in succession a third metal foil (5, 8), a second metal foil (6, 9), a third metal foil (5, 8), a first metal foil (7), a third metal foil (5, 8), a second metal foil (6, 9), and a third metal foil (5, 8) are arranged inside the layer stack (2).

7. Honeycomb body (1) according to one of the preceding claims, characterized in that the second metal foils (6, 9) each form a corrugated layer, which has a corrugation having greater wave height and / or greater wave width than the at least one corrugated layer formed by a first metal foil (7).

8. Honeycomb body (1) according to one of the preceding claims, characterized in that the corrugated layers forming the radial edge area of the layer stack (2) generate a lower pressure loss for a fluid flowing through than the corrugated layers arranged in the centre of the layer stack (2).

9. Honeycomb body (1) according to one of the preceding claims, characterized in that the cell density of the corrugated layers formed by the first metal foils (7) is between 100 cpsi (cells per square inch) and 150 cpsi and the cell density of the corrugated layers formed by the second metal foils (6, 9) is between 25 cpsi and 80 cpsi.

10. Honeycomb body (1) according to one of the preceding claims, characterized in that at least one first group of the second metal foils (6, 9) is provided which has a ratio (p / w) of wave height (p) to wave width (w) of less than 1.8 and / or at least one second group of the second metal foils (6, 9) is provided which has a ratio (p / w) of wave height (p) to wave width (w) of greater than 1.8.