EP4259909A2

EP4259909A2 - Heating element

Info

Publication number: EP4259909A2
Application number: EP21835605.3A
Authority: EP
Inventors: Peter Hirth; Rolf BRÜCK
Original assignee: Vitesco Technologies GmbH
Current assignee: Vitesco Technologies GmbH
Priority date: 2020-12-11
Filing date: 2021-12-01
Publication date: 2023-10-18
Also published as: US20240102411A1; WO2022122516A2; CN116917601A; DE102020215753B4; WO2022122516A3; US12085003B2; DE102020215753A1

Abstract

The invention relates to a device for the post-treatment of waste gases, comprising a flow section through which exhaust gas can be passed and which has at least one honeycomb body (1) acting as a catalyst and at least one heating element (2), the heating element (2) being formed from a ceramic material through which a flow can be passed along a plurality of flow channels from an inflow side to an outflow side, the heating element being electrically conductive along the walls delimiting the flow channels and being connectable to a voltage source by means of an electrical contacting means (3), the heating element (2) running in meandering fashion over a cross-section of the flow section through which a flow can pass.

Description

description

Ceramic heating disk as heating element

technical field

The invention relates to a device for the aftertreatment of exhaust gases, with a flow path through which exhaust gas can flow, with at least one honeycomb body acting as a catalyst and at least one heating element, the heating element being made of a ceramic material that runs along a plurality of flow channels from an inflow side to an outflow side can be flowed through, the heating element being electrically conductive along the walls delimiting the flow channels.

State of the art

Heating elements are used to heat catalytic converters, for example in the exhaust lines of internal combustion engines, in order to reach the so-called light-off temperature of the catalytic converters at an early stage, from which point the chemical conversion of the exhaust gases functions particularly efficiently. For this purpose, the heating elements are formed, for example, by electrically conductive conductor structures, which are connected to a power source and thus generate heat using the ohmic resistance.

A large number of heating discs for use in exhaust systems of internal combustion engines are known in the prior art. Among other things, metallic honeycomb bodies are used, which are formed from a plurality of metallic foils which are stacked and wound up. As a result, honeycomb bodies are formed with a plurality of flow channels that can be flowed through, through which the exhaust gas can flow. The heating discs are connected to a voltage source via an electrical connection. As an alternative to this, heating disks made of ceramic materials are known which have a metallic conductor which is connected to a voltage source and can be heated using the ohmic resistance.

A particular disadvantage of the devices in the prior art is that complex insulation measures have to be taken in order to prevent the current from following an undesired path and causing electrical short circuits. Undesired hot spots can also occur along the heating element if the current flows along the shortest possible path, resulting in sections on the heating element through which the current flows significantly more and are therefore heated than other areas. Hot spots can be disadvantageous, particularly with regard to durability, and non-homogeneous heat distribution over the cross section of the flow-through catalytic converter is also disadvantageous with regard to the efficiency of the catalytic converter.

Presentation of the invention, task, solution, advantages

It is therefore the object of the present invention to create a device with a catalytic converter and with a heating element, the heating element being easy to produce, durable and easily adaptable to different requirements.

The object regarding the catalytic converter with an electric heating element is solved by a catalytic converter with the features of claim 1.

One exemplary embodiment of the invention relates to a device for the aftertreatment of exhaust gases, with a flow section through which exhaust gas can flow, with at least one honeycomb body acting as a catalyst and at least one heating element, the heating element being formed from a ceramic material that runs along a plurality of flow channels from an inflow side can be flowed through to an outflow side, the heating element being electrically conductive along the walls delimiting the flow channels and by means an electrical contact can be connected to a voltage source, the heating element running in a meandering manner over a cross-section of the flow path that can be flowed through.

The heating element can preferably be produced from a disk-shaped honeycomb body, which can be produced, for example, by extrusion. The disk-like honeycomb body can be machined, for example, using a machining process, and a desired shape can thus be produced. The heating element is preferably designed in a meandering manner and thus forms a heating section which extends over the cross section of the device for exhaust gas aftertreatment through which flow can take place. Alternatively, the heating element can also be produced by a suitable shaping process as a meandering shaped heating section.

The heating element preferably has a structure similar to that of a ceramic honeycomb body of a catalytically active catalyst. A large number of fine channels runs through the honeycomb body from an inflow side to an outflow side, so that the honeycomb body and the heating element as a whole are gas-permeable along a defined main direction.

The electrical conductivity of the heating element is achieved by the channel walls delimiting the flow channels. For this purpose, the ceramic can be provided with an electrically conductive coating, for example. Alternatively, metallic particles can be admixed to the ceramic, so that the metallic-ceramic mixture as a whole is electrically conductive.

Due to its meandering structure, the heating element is particularly well suited to covering as large a part as possible of the cross-section through which flow can take place, in order to achieve heating of the flowing exhaust gas that is as homogeneous and strong as possible. In principle, the heating element can be constructed by alternating 180-degree deflections in a row, so that sections of the heating element running parallel to one another are formed. Alternatively, a spiral arrangement of the individual sections of the heating element Mentes are preferred, which are known for example from metallic honeycomb bodies for heating discs in the prior art.

The aim is to achieve the largest possible area of coverage of the cross-section through which flow can take place, without individual sections of the heating element coming into electrically conductive contact with one another.

The heating element can be arranged entirely in a single plane. As an alternative to this, the heating element can also be arranged, for example, in two or more planes that are spaced apart from one another. For this purpose, after a deflection, the heating element would run along the main flow direction of the honeycomb body and thus connect the two levels to one another. The heating element can thus also extend along the main flow direction of the honeycomb body.

It is particularly advantageous if the heating element has a number of deflections within one plane. Deflections through 180 degrees are particularly advantageous in order to achieve the best possible utilization of the available cross-sectional area of the honeycomb body and thus generate the highest possible heating output.

Furthermore, it is preferable that the heating element is formed of a ceramic honeycomb body. A heating element which is formed from a disc-shaped honeycomb body by a machining process is particularly advantageous.

It is also advantageous if the heating element has a cross-sectional thickening of the electrically conductive structure at the deflections compared to the remaining areas of the heating element. The electrically conductive structure is formed by the channel walls. In order to achieve a thickening of the cross section in the area of the deflection, more channel walls can be arranged in sections per unit area, for example, or the thickness of the channel walls can be increased in sections. Alternatively or additionally, the porosity of the channel walls can be different. High porosity allows more air per Unit volume bound in the material, resulting in less conductive material there overall. Decreased porosity thus results in more material per unit volume, which means there is relatively more material there.

A material thickening in the area of the deflections is advantageous in order to avoid the occurrence of so-called hotspots or hot spots. Due to the material thickening, there is locally more material through which the current can flow along the heating element. The heat generated at the heating element is thus distributed over more mass, which reduces the maximum local heating.

In a preferred embodiment, the cross-sectional thickening of the electrically conductive structure can also be formed to different extents over the cross-section of the heating element, so that, for example, areas that are close to the inner, smaller radius of curvature in the area of the deflection experience a smaller cross-sectional thickening, or even a cross-sectional reduction. while areas that are close to the outer larger radius of curvature in the area of the deflection experience a greater cross-sectional thickening. This can be advantageous in particular in order to influence the current flow along the heating element in a targeted manner and thus to avoid the occurrence of hot spots due to an increased current flow.

A preferred exemplary embodiment is characterized in that the heating element has a different thermal conductivity over the cross section of the heating element in the areas of the deflections than in the sections before and after the deflections.

The lower thermal conductivity counteracts the development of local hot spots. A reduced thermal conductivity can be produced, for example, by a special choice of material, for example by making the areas of the deflections from a different material than the rest of the heating element. The base material of the heating element, for example in the area of the deflections be mixed with another material or with another element.

The thermal conductivity at the inner edge of the deflection, adjacent to the smaller radii of curvature, is preferably different than at the outer edge of the deflection, adjacent to the larger radii of curvature. As a result, the thermal conductivity can be designed in such a way that the occurrence of hot spots, particularly in the area of the smaller radii of curvature, is reduced. The thermal conductivity can be influenced, among other things, by the choice of material, the porosity, the cross-sectional area of the heating element or the number and thickness of the channel walls.

It is also preferable if the heating element has a different heat capacity over the cross section of the heating element in the areas of the deflections than in the sections before and after the deflections. The local generation of a higher heat capacity also counteracts the development of local hot spots, which means that a more homogeneous heat distribution is achieved overall. Here, too, the heat capacity generated by the selection of the parameters in the area of the smaller radii of curvature is preferably different from the heat capacity in the area of the larger radii of curvature.

It is also advantageous if the heating element has a different electrical resistance over the cross section of the heating element in the areas of the deflections. The specific electrical resistance can also be influenced by the material properties already mentioned. Advantageously, the specific electrical resistance is also adjusted over the cross section of the heating element, so that the occurrence of hot spots is reduced or completely avoided. The heating element preferably has a higher electrical resistance in the area of the smaller or narrower radii of curvature than in the area of the larger or wider radii of curvature. Advantageously, this leads to a shift of the main route of flow of the flow from the inside of the deflection to the outside of the deflection. It is also advantageous if the proportion of electrically conductive material per unit area is different in some areas. In this way, particularly well conductive areas and less well conductive areas can be produced within the heating element. In this way, the flow of current along the heating element can be advantageously influenced.

Advantageous developments of the present invention are described in the dependent claims and in the following description of the figures.

Brief description of the drawings

The invention is explained in detail below using exemplary embodiments with reference to the drawings. In the drawings show:

1 is a top view of a heating element which is arranged on one of the end faces of a heating disk formed by a honeycomb body,

2 shows a top view of a deflection area of the heating element, the cell density in the area of the deflection being higher than in the rest of the heating element,

3 shows a plan view of a deflection area of the heating element, the walls delimiting the flow channels being thicker in the area of the deflection than in the rest of the heating element, and FIG

4 shows a plan view of a deflection area of the heating element, the walls delimiting the flow channels being formed in sections from a material with different material properties. Preferred embodiment of the invention

FIG. 1 shows a flow cross section 1 of the device for exhaust gas aftertreatment in a schematically indicated manner. Within this flow cross section 1, a heating element 2 is arranged, which has a plurality of flow channels Baren flow through which can be flowed along a main flow direction, which is parallel to a surface normal on the plane of the drawing. The individual flow channels are delimited by walls in a direction transverse to the main flow direction. Since the heating element 2 is made of a ceramic material, it is either provided with an electrically conductive surface coating or has a certain proportion of electrically conductive material. The ceramic can be mixed with metallic particles, for example, in order to generate sufficient electrical conductivity.

The heating element 2 is heated by energizing the heating element 2. For this purpose, the heating element 2 can be connected at the end via electrical contacts 3 to a voltage source. Using the ohmic resistance, the heating element 2 is thus heated if current flows through the heating element 2 .

The meandering heating element 2 in the exemplary embodiment in FIG. 1 can be produced, for example, by a machining process from a honeycomb body designed as a disc.

In the exemplary embodiment in FIG. 1, the heating element 2 is arranged in a meandering manner over the cross section 1 of the catalytic converter that is upstream or downstream in the direction of flow (not shown in FIG. 1). Arrangements deviating from this can also be provided in order to optimally utilize the cross-sectional area of the catalytic converter and to ensure the best possible heat transfer and the most homogeneous heat generation possible. Deviating from the exemplary embodiment in FIG. 1, the heating element can also run in a direction which follows a surface normal on the plane of the drawing. This is particularly important in the case of a heating element arranged in several levels one behind the other.

The heating element 2 has a plurality of deflection areas 4 in which it changes its direction. These deflection areas 4 are preferably designed in such a way that the generation of local hot spots is avoided or at least significantly reduced. For this purpose, the deflection area 4 can be thickened, for example, have a reduced porosity compared to the rest of the heating element 2, have a lower thermal conductivity or have an increased thermal capacity. The material used or the wall thickness can also vary over the cross section of the deflection area 4 .

FIG. 2 shows a detailed view of a deflection area 4, with a greater cell density being provided in the curved area 6 of the deflection 4 in comparison to the remaining structure 5 of the heating element 2. The area 6 has more flow channels per unit area than the rest of the heating element 2. As a result, compared to the remaining structure 5 of the heating element 2, the properties as an electrical conductor in the deflection area 4 are changed.

Figure 3 shows an alternative embodiment of the deflection area 7, the existing walls in the curved area 8 of the deflection 7, which delimit the flow channels, have a greater wall thickness than in the remaining structure 5 of the heating element 2. The increased wall thickness also increases the electrical Conductivity affected.

FIG. 4 shows a further alternative embodiment of a deflection area 9, the walls delimiting the flow channels being formed from different materials in the curved area 10 of the deflection 9. FIG. The walls arranged in the area 11 of the smaller inner radius are formed from a first material, while the walls arranged in the area 12 of the larger outer radius are formed from a second material. The materials can differ from one another in particular in terms of the specific electrical resistance, the metal content, the porosity, the surface coating or a combination of the aforementioned properties.

All of the designs shown in the exemplary embodiments in FIGS. 1 to 4 can be combined with one another as desired. In particular, it can be provided that individual variations are limited to limited areas within the deflection. For example, the inner area at the smaller bending radii is structured differently than the outer area at the larger bending radii.

The aim is in particular to produce a suitable influencing of the current flow within the heating element 2 in order to avoid the occurrence of local hot spots. For this purpose, the specific resistance can be adjusted in particular areas in order to locally limit or promote the flow of current. In principle, it is advantageous if there is an increased current flow in the outer areas of the deflection, in order to prevent hot spots from developing on the inner radius. Since the current follows the principle of least resistance, the targeted influencing of the resistance ensures that the current is guided appropriately.

Increasing the number of walls, thickening the walls, as well as reducing porosity all result in an overall increase in the cross-sectional area through which current can flow, which changes the electrical resistance of the respective area, which in turn affects the conduction of electricity, in particular the current distribution is changed over the cross section of the heating element.

In addition to influencing the resistance to influencing the power line, the changes described above can also directly influence the thermal conductivity of the heating conductor, which means that heat can be better dissipated and can be distributed, which can also reduce the occurrence of hot spots.

The exemplary embodiments of FIGS. 1 to 4 in particular do not have any restrictive character and serve to clarify the idea of the invention.

Claims

patent claims

1. A device for the aftertreatment of exhaust gases, with a flow section through which exhaust gas can flow, with at least one honeycomb body acting as a catalyst and at least one heating element (2), the heating element (2) being formed from a ceramic material that runs along a plurality of flow channels can be flowed through from an inflow side to an outflow side, the heating element (2) being electrically conductive along the walls delimiting the flow channels and being connectable to a voltage source by means of an electrical contact (3), the heating element (2) being connected in a meandering manner via a through- flowable cross section (1) of the flow path runs.

2. Device according to claim 1, d a d u r c h g e k e n n z e i c h n e t that the heating element (2) has a plurality of deflections (4, 7, 9) within one plane.

3. Device according to one of the preceding claims, d a d u r c h g e k e n n z e i c h n e t that the heating element (2) is formed from a ceramic honeycomb body.

4. Device according to one of the preceding claims, d a d u r c h g e k e n n z e i c h n e t that the heating element (2) at the deflections (4, 7, 9) compared to the remaining areas of the heating element (2) has a cross-sectional thickening of the electrically conductive structure.

5. Device according to one of the preceding claims, d a d u r c h g e k e n n z e i c h n e t that the heating element (2) in the areas of the deflections (4, 7, 9) has a different thermal conductivity across the cross section of the heating element (2) than in the sections before and after the Deflections (4, 7, 9).

6. Device according to one of the preceding claims, characterized in that the heating element (2) in the areas of the deflections (4, 7, 9) has a different heat capacity across the cross section of the heating element (2) than in the sections before and after the deflections (4, 7, 9).

7. Device according to one of the preceding claims, d a d u r c h g e k e n n z e i c h n e t that the heating element (2) in the areas of the deflections (4, 7, 9) has a different electrical resistance over the cross section of the heating element (2).