Electrically heatable honeycomb body having a plurality of corrugated layers with different cell densities - Patent Application 20070122967

Abstract

The invention relates to a honeycomb body (1) for an electrically heatable catalyst in an exhaust gas section, the honeycomb body (1) having a plurality of flow channels through which a flow can pass along a main flow direction, the honeycomb body (1) being formed from a plurality of metal foils (5, 6, 7, 8, 9, 10) stacked one on top of the other to form a layer stack (2), the plurality of metal foils (5, 6, 7, 8, 9, 10) being wound around at least one rotation point, the layer stack (2) being made up of at least one first metal foil (7) having a first corrugation and also at least one second metal foil (8) having a second corrugation. (6, 9), the first metal foil (7) and the second metal foil (6, 9) respectively forming a first wavy layer and a second wavy layer, the honeycomb body (1) having a plurality of wound layers which are spaced apart from one another in the radial direction of the honeycomb body (1) by gaps (11) in the wound state, the first corrugation being different from the second corrugation, and the layer stack (2) having at least three wavy layers which are separated from one another by a third metal foil (5, 10), which is respectively smooth or microstructured.

Description

[Technical Field]

[0001] The present invention relates to a honeycomb body for an electrically heatable catalyst in an exhaust gas section, which has a plurality of flow channels through which gas can flow along the main flow direction, the honeycomb body being formed from a plurality of metal foils stacked one on top of the other to form a layer stack, the plurality of metal foils being wound around at least one rotation point, the layer stack having at least one first metal foil with a first corrugation and at least one second metal foil with a second corrugation, the first metal foil and the second metal foil forming a first corrugated layer and a second corrugated layer, respectively, the honeycomb body having a plurality of wound layers which, in the wound state, are spaced apart from one another in the radial direction of the honeycomb body by air gaps.

[0002] Prior art Electric heaters, among others, are used to heat the exhaust gases of internal combustion engines. The electric heaters serve to quickly heat the exhaust gases to a predetermined minimum temperature in order to ensure as rapid and complete an exhaust gas aftertreatment as possible in the exhaust gas section. The minimum temperature results from the so-called activation (light-off) temperature of the catalyst for exhaust gas aftertreatment used in the exhaust gas section. The catalytically active coating of the honeycomb body used requires such a minimum temperature in order to make the chemical conversion process as complete as possible.

[0003] A known form of electric heater is a metallic honeycomb body connected to a voltage source. By passing an electric current through the honeycomb body, heat is generated via an ohmic resistance. This heat is dissipated into the flowing exhaust gas and partly into the surrounding structure, thereby achieving heating.

[0004] Metal honeycomb bodies are made from several different metal foils, including smooth and fully or partially structured foils. These metal foils are stacked one on top of the other to produce a layer stack. This layer stack is then wound around a so-called winding axis using a suitable tool, resulting in a honeycomb body that is usually disk-shaped and has a number of flow channels through it that are capable of passing through along the main flow direction.

[0005] To prevent short circuits between different points on the heating disk and to create a predetermined current path along the length of the layer stack, the individual windings of the layer stack are separated from each other by air gaps in the finished heating disk.

[0006] The flow cross section of the exhaust gas section in the region of the heating disk of the electric heater is occupied by a number of metal foils, the channels formed between these foils, and the gaps between the windings. The metal foils, and therefore the channels, generate a counterpressure against the exhaust gas flow due to their size. In the case of electric heaters with metal honeycomb bodies known from the prior art, the relatively high pressure loss caused by the heating disk causes the exhaust gas to flow preferentially through the gaps. This is disadvantageous because no or only minimal heating of the exhaust gas can be achieved there. The majority of the heat is transferred to the flowing exhaust gas in the channels, because the electric heater has the largest active surface there.

[0007] Summary of the invention, problems, solutions, and advantages It is therefore an object of the present invention to provide a honeycomb body for an electric heater in the exhaust gas section of an internal combustion engine, which has an improved flow profile and, in particular, ensures complete flow of the individual channels formed between the metal foils.

[0008] The problem with honeycomb bodies is solved by a honeycomb body having the features of claim 1.

[0009] One embodiment of the present invention relates to a honeycomb body for an electrically heatable catalyst in an exhaust gas section, having a plurality of flow channels along the main flow direction, the honeycomb body being formed from a plurality of metal foils stacked one on top of the other to form a layer stack, the plurality of metal foils being wound around at least one rotation point, the layer stack comprising at least one first metal foil having a first corrugation and at least one second metal foil having a second corrugation, the first metal foil and the second metal foil forming a first corrugated layer and a second corrugated layer, respectively, the honeycomb body comprising a plurality of wound layers which, in the wound state, are spaced apart from one another in the radial direction of the honeycomb body by gaps, the first corrugation being different from the second corrugation, the layer stack comprising at least three corrugated layers separated from one another by a third metal foil, which may be smooth or microstructured.

[0010] The honeycomb body is manufactured by known methods, in which several metal foils are stacked one on top of the other, and the layer stack thus formed is wound around one or more winding axes. The layer stack then produces several winding layers, which are arranged radially next to each other and form the honeycomb body from the center of the winding toward the outer diameter of the honeycomb body. In the case of electrically heatable honeycomb bodies, such as those of the present invention, the individual winding layers are radially spaced apart by air gaps to prevent physical contact between the individual winding layers and thus, in particular, to prevent undesired current paths. The purpose is to allow current to flow through the honeycomb body from a predetermined current inlet to a predetermined current outlet.

[0011] Metal foils can be essentially divided into two categories: metal foils with at least partial corrugation and metal foils that are substantially smooth or have very little microstructure. At least partially corrugated metal foils have a regular, approximately sinusoidal wave shape. The corrugation is defined by a so-called pitch (p), which represents the distance between two minimum or maximum values ​​on the X-axis. Furthermore, the corrugation is defined by a wave width (w), which represents the distance between two minimum or two maximum values ​​on the Y-axis. The X-axis of the corrugation extends in the direction of the corrugation height, while the Y-axis extends in the direction of the corrugation width.

[0012] The corrugations create cells in the honeycomb through which fluid can flow along the honeycomb's axis. The cells are defined radially by metal foils. The density of cells per unit area is expressed as cells per square inch (cpsi). The higher this number, the more cells there are in one square inch. More cells per square inch means that each cell has a smaller cross-sectional area, which in turn means higher pressure loss.

[0013] The honeycomb body according to the present invention has at least two different corrugated layers formed by metal foils with different corrugations, which necessarily result in different cell densities per unit area and therefore different pressure losses for a medium, such as an exhaust gas flow, flowing through the cells.

[0014] Therefore, with the essential structural form of the invention, in which a plurality of radially spaced-apart winding layers are provided, at least three zones with different pressure losses are formed across the cross section of the honeycomb body. One is the gap between the winding layers, which creates the lowest pressure loss. There are also corrugated layers with a low cell density per unit area, and finally, regions of corrugated layers with a high cell density per unit area. The different pressure loss zones have a direct effect on the exhaust gas flow, which allows for improved flow through the electrically heated area of ​​the honeycomb body and therefore improved heating of the exhaust gas.

[0015] It is particularly advantageous if the first metal foil has a smaller corrugation than the second metal foil.

[0016] As already explained, different corrugations improve the flow through the honeycomb body and thus the heating of the exhaust gases. Smaller corrugations mean, inter alia, corrugations with smaller wave heights and / or widths, which results in corrugations with a higher cell density per unit area. To ensure optimal cell flow, the cell density of the corrugated layer should preferably vary within the range of 20 cpsi to 500 cpsi.

[0017] The corrugated layer formed from the first metal foil preferably has a cell density of 100 cpsi to 150 cpsi for the corrugated layer made from the first metal foil, or 25 cpsi to 85 cpsi for the corrugated layer made from the second metal foil. These cell densities are particularly advantageous for achieving a cell density advantageous for the honeycomb body for heating exhaust gases of an internal combustion engine, which allows particularly uniform and complete flow through the cells of the honeycomb body.

[0018] Particularly advantageously, the honeycomb body has a porosity of 94% to 97%, in which case the hydraulic diameter of the individual cells is preferably between 2 mm and 5 mm.

[0019] It is also advantageous if a corrugated layer formed from one first metal foil is used to accommodate the support structure. The corrugated layer made of the first metal foil has a higher cell density and therefore cells with a smaller hydraulic cross-sectional area. The corrugated layer is preferably formed to accommodate support pins, which support the honeycomb body while electrically insulating it from other catalysts.

[0020] A preferred embodiment is characterized in that corrugated layers formed from a second metal foil are used to increase the rigidity of the honeycomb body. A smaller cell density allows for the creation of corrugated layers with greater rigidity. These corrugated layers are particularly suitable for forming edge layers of a layer stack in order to create a dimensionally very stable honeycomb body.

[0021] It is also preferred if the layer stack forming the honeycomb body has at least three wavy layers, at least one central wavy layer being formed by one first metal foil, and at least both wavy layers forming the radial edges of the layer stack being each formed by one second metal foil.

[0022] The middle corrugated layer of the first metal foil has a relatively high cell density and is used to accommodate support structures, such as support pins. This corrugated layer preferably forms the center of the layer stack, while corrugated layers formed from the second metal foil are adjacent on both sides. These corrugated layers have a lower cell density, thus offering less resistance to the flow of exhaust gases and being more rigid, thereby improving the shape durability of the honeycomb body.

[0023] Between the wavy layer of the first metal foil and the wavy layer of the second metal foil, there is preferably arranged a smooth layer or a so-called microstructured layer, which separates the wavy layers from one another and in particular prevents adjacent wavy layers from sliding into each other.

[0024] Furthermore, it is advantageous if the layer stack forming the honeycomb body has exactly three corrugated layers, with the following successive layers arranged within the layer stack: 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. The central corrugated layer made of the first metal foil forms a receptacle for a support pin. The two laterally adjacent corrugated layers made of the second metal foil increase stability and simultaneously reduce pressure losses in the edge regions, thereby avoiding or significantly reducing air gaps, particularly between the wound layers.

[0025] Overall, a layer stack structure consisting of three corrugated layers is optimal for achieving the highest possible efficiency in heating exhaust gases, while keeping costs and manufacturing efforts as low as possible. Furthermore, for typical dimensions of honeycomb bodies for heating exhaust gases in the exhaust ducts of cars or trucks, choosing a three-layer structure is particularly advantageous. This allows particularly good adjustment of the ohmic resistance required to achieve sufficiently rapid and powerful heating of the exhaust gases using the supplied current.

[0026] Furthermore, it is advantageous if the second metal foils each form one wavy layer, and the wavy layer has a wave shape with a wave height and / or a wave width greater than at least one wavy layer formed by one first metal foil.

[0027] The variation in wave height can contribute to a reduction in cell density per unit area, which can also be achieved by varying the wave width. Preferably, the edge layers have a lower cell density per unit area compared to the central layers, which results in a desired improvement in the flow of cells in the honeycomb body and a desired reduction or prevention of exhaust gas flow through the formed voids.

[0028] It is also beneficial if the corrugated layers forming the radial edge regions of the layer stack create a lower pressure loss for the fluid flowing therethrough than corrugated layers located in the center of the layer stack.

[0029] Furthermore, it is advantageous if the cell density of the corrugated layer formed by the first metal foil is between 100 cpsi (cells per square inch) and 150 cpsi, and the cell density of the corrugated layer formed by the second metal foil is between 25 cpsi and 80 cpsi. The above ratio of cell densities forms a favorable optimum, especially for passenger car and truck applications, with respect to typical structure sizes, flow rates, temperatures, and available currents for heating.

[0030] Furthermore, it is advantageous if at least one first group of second metal foils is provided having a ratio (p / W) of wave height (p) to wave width (W) of less than 1.8, and / or if at least one second group of second metal foils is provided having a ratio (p / W) of wave height (p) to wave width (W) of greater than 1.8.

[0031] In this case, the ratio p / W is an important factor for the shape of each corrugated layer: the higher this ratio, the flatter the corrugations, which makes the layer more flexible, especially in the Y direction, i.e., across the corrugations, making it easier to roll the corrugated layer, but also making it more susceptible to undesired deformation and damage.

[0032] A very small ratio of p / W results in a high waviness, which reduces flexibility in the Y direction and achieves a greater stiffness of the corrugated layer.

[0033] Extensive research has revealed that there is a turning point for the stability of the corrugated layer in the region around the value of 1.8 for the p / W ratio. If the ratio is less than 1.8, the corrugated layer tends to be rigid, which is good from the standpoint of stability, because it is less susceptible to mechanical deformation that could damage or clog the cells. If the ratio is greater than 1.8, the corrugated layer tends to be soft, which is advantageous for absorbing radial forces and stresses and thus ensuring the durability of the corrugated layer or honeycomb body.

[0034] Advantageous developments of the invention are set forth in the dependent claims and in the following description of the drawings.

[0035] The present invention will now be described in detail based on examples with reference to the drawings. [Brief explanation of the drawings]

[0036] [Figure 1] 1 is a plan view of a honeycomb body according to the present invention, in which the channels formed by the honeycomb body extend along a surface normal to the plane of the drawing.

[0037] Preferred Embodiments of the Invention 1 shows a plan view of a honeycomb body 1 formed by winding a layer stack 2. The layer stack 2 is supported by support pins 4 against a catalyst 3 arranged behind it. The layer stack 2 is wound around two winding axes, so that the layer stack 2 has an S-shaped extension. The individual wound layers are radially spaced from one another by air gaps 11.

[0038] The layer stack 2 has a central corrugated layer formed from a first metal foil 7. This corrugated layer is surrounded by a microstructured third metal foil 5, 10. Alternatively, it would be possible to provide a completely smooth third metal foil.

[0039] Adjacent to the third metal foil 5, 10 are two corrugated layers made of second metal foils 6, 9. The second metal foils 6, 9 have a slightly larger wave height (pitch) and a significantly larger wave width, which results in the corrugated layers formed by the second metal foils 6, 9 having a lower cell density than the corrugated layers formed by the first metal foils 5, 10.

[0040] Adjacent to the second metal foils 6, 9 are again third metal foils 5, 10, respectively, which may also have a microstructure or, in an alternative embodiment, may be formed by a completely smooth metal foil.

[0041] The air gaps 11 are used to electrically isolate the individual winding layers from each other, thus preventing short circuits and undesired current conduction paths.

[0042] The embodiment of FIG. 1 has no particular limiting nature and is used to illustrate the idea of ​​the present invention. [Explanation of symbols]

[0043] 1 Honeycomb body Two-layer stack 3 Honeycomb body 4 support pins 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 void

Claims

1. A honeycomb body (1) for an electrically heatable catalyst in an exhaust gas section, the honeycomb body having a plurality of flow channels that are passable along a main flow direction, The honeycomb body (1) is formed from a plurality of metal foils (5, 6, 7, 8, 9, 10) stacked one on top of the other to form a layer stack (2), and the plurality of metal foils (5, 6, 7, 8, 9, 10) are wound around at least one rotation point; The layer stack (2) comprises at least one first metal foil (7) having a first corrugation and at least one second metal foil (6, 9) having a second corrugation, the first metal foil (7) and the second metal foil (6, 9) form a first wavy layer and a second wavy layer, respectively; The honeycomb body (1) has a plurality of wound layers, and the plurality of wound layers are spaced apart from one another in the radial direction of the honeycomb body (1) by gaps (11) when wound. In the honeycomb body (1), the first corrugation is different from the second corrugation, and the layer stack (2) has at least three corrugated layers; The corrugated layers are separated from each other by a third metal foil (5, 10), which may be smooth or microstructured, The layer stack (2) forming the honeycomb body (1) has three corrugated layers, A honeycomb body (1), characterized in that 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 adjacent to one another within the layer stack (2).

2. 2. The honeycomb body (1) according to claim 1, wherein the first metal foil (7) has a smaller corrugation than the second metal foil (6, 9).

3. 3. The honeycomb body (1) according to claim 1 or 2, wherein the corrugated layer formed from one first metal foil (7) is used to accommodate a retaining structure (4).

4. 3. The honeycomb body (1) according to claim 1 or 2, wherein the corrugated layer formed from one second metal foil (6, 9) is used to increase the rigidity of the honeycomb body (1).

5. The second metal foils (6, 9) each form one corrugated layer; The corrugated layer formed by the second metal foil (6, 9) has corrugations having a larger wave height and / or wave width than the corrugated layer formed by the first metal foil (7). A honeycomb body (1) according to claim 1 or 2.

6. 3. The honeycomb body (1) according to claim 1 or 2, wherein the corrugated layers forming the radial edge regions of the layer stack (2) generate a smaller pressure loss for a fluid flowing therethrough than the corrugated layers arranged in the center of the layer stack (2).

7. the cell density of the corrugated layer formed by the first metal foil (7) is between 100 cpsi (cells per square inch) and 150 cpsi; the cell density of the corrugated layer formed by the second metal foil (6, 9) is between 25 cpsi and 80 cpsi; A honeycomb body (1) according to claim 1 or 2.

8. at least one first group of said second metal foils (6, 9) is provided, the ratio of wave height (p) to wave width (w) (p / w) being less than 1.8; and / or at least one second group of said second metal foils (6, 9) is provided with a ratio (p / w) of wave height (p) to wave width (w) greater than 1.8; A honeycomb body (1) according to claim 1 or 2.

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