JP2006231183A - Catalytic converter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a catalytic converter preventing or avoiding various problems caused by heat transfer from exhaust gas flowing a bypass passage penetrating through the center of the catalytic converter to a honeycomb body. <P>SOLUTION: The catalytic converter is provided with the catalyst-carrying honeycomb body 10 contained in a cylindrical casing 20, and the bypass passage B penetrating through the center of the honeycomb body 10. A double inner tube composed of a first inner tube 21 and a second inner tube 22 is provided in a periphery region of the bypass passage B and honeycomb body 10 surrounding it. Between the first and second inner tubes 21, 22, a hollow heat insulating layer 25 is secured preventing intrusion of exhaust gas from the upstream side, by fixing the upstream end of the first inner tube 21 to an upstream contact part 23a of a reduced part 23 of the second inner tube 22 at an upstream end of the double inner tube. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a catalytic converter that is a kind of an exhaust gas purification device of an engine or that is used by being disposed in a part of an exhaust path of an exhaust gas purification device.

2. Description of the Related Art Conventionally, an exhaust gas purification apparatus including a honeycomb body that supports an exhaust gas purification catalyst is known. For example, Patent Document 1 has a large number of mesh-like vent holes in the axial direction, which are manufactured by stacking and laminating flat plate strips and corrugated strip strips of thin metal plates and winding them together in a spiral shape. Disclosed is a honeycomb core for supporting a catalyst, which is formed by forming a cylindrical hollow portion at the center of winding thereof (that is, a honeycomb core having an annular cross section perpendicular to the axis). Usually, a metal cylinder (pipe) is disposed in a cylindrical hollow portion of the honeycomb core body, and the honeycomb core body is loaded in a cylindrical metal case to constitute a catalytic converter. When the catalytic converter is mounted on an actual vehicle, the pipe passing through the center of the honeycomb core body is used as a bypass passage for bypassing the honeycomb core body with exhaust gas from the engine.
No. 7-33875

  However, in the conventional catalytic converter described above, when the exhaust gas flows through the bypass passage constituted by a single pipe, the heat of the exhaust gas is transferred in large amounts to the pipe and the honeycomb core surrounding the pipe. Was causing serious problems. For example, heat transferred through the pipe wall causes non-uniform heat distribution (ie, excessive thermal gradient in the radial direction) between the inner and outer peripheral portions of the honeycomb core body, resulting in the honeycomb core. Circumferential tensile stress is generated in the body, and as a result, the metal strip in the wound and laminated state may be cut and chipped into a foil shape, or a part of the strip may be displaced and fall off. It was. Further, the honeycomb core body may be pressed and damaged between the pipe and the cylindrical metal case that are excessively expanded by the exhaust heat, or the pipe itself may be cracked.

  The present invention has been made in view of the above-described circumstances, and its object is to solve various problems caused by the heat of the exhaust gas flowing through the bypass passage passing through the center of the catalytic converter being transferred to the honeycomb body. It is an object of the present invention to provide a catalytic converter that can be prevented or avoided.

  In order to achieve the above-mentioned object, the invention according to claim 1 is a catalyst-supporting honeycomb body having a large number of vent passages extending in the axial direction, and a central passage that passes through the central part of the honeycomb body in the axial direction. The first inner pipe that defines the central passage and the first inner pipe are spaced from each other in a boundary region between the central passage and the honeycomb body that surrounds the central converter passage. A double inner pipe composed of a second inner pipe that surrounds and separates is provided, and an upstream end portion of the second inner pipe is formed with a reduced diameter portion over the entire circumference thereof, and the first inner pipe The upstream end of the first inner tube or the vicinity thereof and the reduced diameter portion of the second inner tube are fixed at the upstream end of the double inner tube between the second inner tube and the second inner tube. The main point is that a hollow insulation layer that prevents the ingress of exhaust gas from the upstream side is secured. That.

  According to the first aspect of the present invention, the double inner pipe is composed of the first inner pipe that defines the central passage of the catalytic converter and the second inner pipe that surrounds the first inner pipe with a space therebetween. Thus, a hollow heat insulating layer (for example, an air layer) is secured in the boundary area between the central passage and the surrounding honeycomb body, that is, around the central passage. And this hollow heat insulation layer is exhausted from the upstream side by adhering the upstream end portion of the first inner tube or the vicinity thereof and the reduced diameter portion of the second inner tube at the upstream end portion of the double inner tube. Gas entry is disabled. For this reason, even when high-temperature exhaust gas flows through the central passage, heat transfer from the central passage to the honeycomb body is alleviated by the hollow heat insulating layer secured around the central passage. Therefore, an uneven heat distribution (that is, an excessive thermal gradient in the radial direction) hardly occurs between the inner peripheral portion and the outer peripheral portion of the honeycomb body, and the honeycomb body is temporarily caused by such a thermal distribution (thermal gradient). Even if tensile stress in the circumferential direction occurs, the tensile stress in the circumferential direction does not become excessive, so that the honeycomb body can be prevented from being damaged.

  In addition, the reduced diameter portion formed in the second inner pipe to fix the first inner pipe and the second inner pipe over the entire circumference at the upstream end of the double inner pipe enters the hollow heat insulating layer from the upstream side. It functions as an exhaust gas blocking means (blocking wall) that makes it impossible to enter the exhaust gas, and contributes to maintaining the heat insulating performance of the hollow heat insulating layer. In addition, since the reduced diameter portion of the second inner pipe and the first inner pipe are fixed to each other at the upstream end of the double inner pipe, it is within the range along the axial direction or the longitudinal direction of both inner pipes. The fixed location (fixed location) of both inner pipes is limited to only one location along the circumference corresponding to the reduced diameter portion (upstream end portion of the double inner tube). For this reason, even when a high-temperature exhaust gas flows through the first inner pipe and the first inner pipe expands greatly compared to the second inner pipe, the first inner pipe is restrained by the second inner pipe. Without thermal expansion in the axial direction. Therefore, the occurrence of thermal stress concentration based on the difference in thermal expansion between the first inner tube and the second inner tube can be avoided, and damage to the double inner tube due to thermal stress concentration can be prevented.

  According to a second aspect of the present invention, in the catalytic converter according to the first aspect, the reduced-diameter portion includes an upstream contact portion that can contact the entire upstream end of the first inner pipe or the entire periphery thereof. The gist of the present invention is that it is formed from a tapered portion that gradually expands from the upstream contact portion.

  According to the second aspect of the present invention, the upstream contact portion formed in the reduced diameter portion of the second inner pipe can be contacted over the entire circumference of the upstream end of the first inner pipe or the vicinity thereof. Therefore, by fixing the upstream contact portion and the upstream contact portion of the first inner pipe or the vicinity thereof in contact with each other over the entire circumference, the airtightness between the inner pipes is improved, The hollow heat insulating layer formed between the first inner pipe and the second inner pipe is more surely secured. Further, in fixing the first inner pipe and the second inner pipe at the upstream end of the double inner pipe, the first inner pipe and the upstream contact portion (second inner pipe) are brought into contact with each other and stacked. By setting them together, both inner pipes can be easily and reliably fixed.

  The invention according to claim 3 is the catalytic converter according to claim 1 or 2, wherein the downstream end of the second inner pipe is at or near the downstream end of the first inner pipe. The gist is that a downstream contact portion that can be contacted over the entire circumference of the downstream end portion or the vicinity thereof without being fixed is formed.

  According to the third aspect of the present invention, the downstream contact portion formed in the second inner pipe is located at the downstream end of the double inner pipe, and the downstream end of the first inner pipe or its end Since it can contact the whole circumference of the vicinity and close the downstream opening of the hollow heat insulating layer, it is possible to prevent the exhaust gas downstream of the catalytic converter from flowing back and entering the hollow heat insulating layer as much as possible. Further, the downstream contact portion of the second inner pipe is not fixed (adhered) to the first inner pipe, that is, the first inner pipe and the second inner pipe at the downstream end of the double inner pipe are: It is considered as a non-fixed state (non-fixed state). For this reason, the fixed location (fixed location) of both the inner pipes within the range along the axial direction or the longitudinal direction of both the inner tubes is one location along the circumference corresponding to the reduced diameter portion (of the double inner tubes). The structure that is limited to only the upstream end portion is maintained, and the effect (avoidance of thermal stress concentration) as mentioned in claim 1 is maintained.

  According to a fourth aspect of the present invention, in the catalytic converter according to any one of the first to third aspects, the catalytic converter further includes a cylindrical case that accommodates the honeycomb body. The inner peripheral surface of the honeycomb body is connected to the outer peripheral surface of the second inner pipe at one of the downstream end portion and the upstream end portion, and the outer peripheral surface of the honeycomb body is connected to the downstream end portion and the upstream end portion. The gist of the invention is that the honeycomb body is held between the cylindrical case and the double inner pipe by being connected to the inner peripheral surface of the cylindrical case at the other of the portions.

  According to the invention described in claim 4, high-temperature exhaust gas flows in the first inner pipe of the double inner pipe, and heat is transmitted to the honeycomb body surrounding it, so that the inner peripheral portion and the outer peripheral portion of the honeycomb body are separated. Even when a thermal gradient or a thermal difference occurs between the inner peripheral surface of the honeycomb body, the inner peripheral surface of the honeycomb body is only connected to the second inner pipe at the downstream end (or upstream end). And the second inner tube can be thermally expanded in the axial direction without being constrained to each other. Similarly, since the outer peripheral surface of the honeycomb body is only connected to the cylindrical case at the upstream end portion (or downstream end portion), the outer peripheral portion of the honeycomb body and the cylindrical case are not constrained to each other. Can expand in the direction. Therefore, there is a large thermal gradient or heat between the second inner tube, the inner peripheral portion of the honeycomb body, the outer peripheral portion of the honeycomb body, and the cylindrical case, which are concentrically arranged with respect to the central axis of the catalytic converter. Even if a difference occurs, there is no concentration of thermal stress in the inside of the honeycomb body or in the connection portion, and the honeycomb body is prevented from being damaged.

  Further, when the inner peripheral surface of the honeycomb body is connected to the outer peripheral surface of the second inner pipe at one of the downstream end and the upstream end, the outer peripheral surface of the honeycomb body is connected to the downstream end and The inner peripheral surface side connection position and the outer peripheral surface side connection position of the honeycomb body are shifted in the axial direction so that the other end of the upstream side ends is connected to the inner peripheral surface of the cylindrical case. Therefore, it is possible to stably hold the honeycomb body between the cylindrical case and the double inner tube while maintaining the effect of avoiding the thermal stress concentration.

(Supplementary note) In the catalytic converter according to any one of claims 1 to 4, "the honeycomb body is formed by stacking a thin metal plate-like strip and a corrugated strip and collecting them in a lump. It is preferably a catalyst-supporting honeycomb body having a large number of vent passages extending in the axial direction, which is produced by winding and laminating in the shape of a cylinder and forming a cylindrical hollow portion at the center of winding. In such a honeycomb body, a flat band material or the like forms a spiral laminated structure, and the independence of the band material constituting each layer at the inner periphery and the outer periphery of the honeycomb body is increased (that is, between the layers). Therefore, the operational effects based on the configuration as described in claim 4 are more effectively exhibited.

  According to the catalytic converter of the first to fourth aspects of the present invention, the double inner pipe including the first and second inner pipes is provided in the boundary region between the central passage and the honeycomb body surrounding the central passage. By securing a hollow heat insulating layer and relaxing heat transfer from the central passage to the honeycomb body, uneven heat distribution in the honeycomb body can be avoided and damage to the honeycomb body can be prevented. Further, as described above, the fixing structure (or non-fixing structure) between the first and second inner pipes in the double inner pipe and the honeycomb body holding structure between the cylindrical case and the double inner pipe are devised. This avoids concentration of thermal stress in each part of the catalytic converter and prevents the catalytic converter from being damaged.

  According to the invention described in claim 2, it is possible to secure the hollow heat insulating layer more reliably. According to the invention described in claim 2, the first inner pipe and the second inner pipe can be easily and reliably fixed at the upstream end portion of the double inner pipe. According to the third aspect of the invention, the downstream contact portion can prevent the exhaust gas downstream of the catalytic converter from flowing backward and entering the hollow heat insulating layer as much as possible. According to the invention described in claim 4, the honeycomb body can be stably held between the cylindrical case and the double inner tube.

  Hereinafter, an embodiment of the present invention will be described with reference to the drawings. As shown in FIGS. 1 and 2, the catalytic converter includes a catalyst-supporting honeycomb body 10, a cylindrical case 20 that accommodates the honeycomb body 10, and a double core that penetrates the center of the honeycomb body 10 in the axial direction. And inner pipes (21, 22).

  The catalyst-supporting honeycomb body 10 may be made of either metal or ceramics. However, the catalyst-supporting honeycomb body 10 is made of metal in that a cylindrical hollow portion for arranging the double inner tubes (21, 22) can be easily formed at the center thereof. It is preferable that the honeycomb body be made. The metal honeycomb body 10 is formed by stacking, for example, a flat strip 11 made of a heat-resistant thin steel plate and a corrugated strip 12 obtained by machining the heat-resistant thin steel plate into a corrugated shape. At the same time, the stacked strips 11 and 12 are wound into a collective spiral using a winding guide rod, and then the winding guide rod is extracted from the center. The honeycomb body 10 thus obtained has a substantially cylindrical shape having a large number of mesh-like vent holes 13 extending in the axial direction and a cylindrical hollow portion at the winding center. The honeycomb body 10 carries a predetermined catalyst material.

  The cylindrical case 20 is a cylindrical member made of, for example, stainless steel. The inner diameter of the cylindrical case 20 is slightly larger than the outer diameter of the honeycomb body 10, and the axial length (full length) of the cylindrical case 20 is set to be larger than the axial length of the honeycomb body 10. The honeycomb body 10 can be housed inside the center of the shaped case 20.

  The double inner pipe (21, 22) includes a first inner pipe 21 and a second inner pipe 22 that surrounds the first inner pipe 21 at a predetermined interval. The first inner pipe 21 is a straight cylindrical pipe-shaped stainless steel pipe. The second inner tube 22 has reduced diameter portions 23 and 24 in which both ends of a straight cylindrical pipe-shaped stainless steel tube are reduced in diameter. The total length of the first and second inner tubes 21 and 22 is substantially equal to the total length of the cylindrical case 20, and the radius of the second inner tube 22 is larger than the radius of the first inner tube 21.

  In this catalytic converter, the first inner pipe 21, the second inner pipe 22 and the cylindrical case 20 are arranged concentrically around a central axis L common to them (see FIG. 1). A housing space for the honeycomb body 10 is defined between the inner peripheral surface of the cylindrical case 20 and the outer peripheral surface of the second inner tube 22. Further, the first inner pipe 21 arranged at the most central portion defines and forms a bypass passage B as a central passage that passes through the central portion of the honeycomb body 10 in the axial direction. In the boundary area between the bypass passage B and the honeycomb body 10, double inner pipes (21, 22) are disposed, and between the first inner pipe 21 and the second inner pipe 22, An interval substantially equal to the radial difference between the tubes 21 and 22 is ensured over the entire circumference. That is, at the upstream end and the downstream end of the double inner pipe (21, 22), the first inner pipe 21 and the second inner pipe 22 are in contact and there is no gap, but the double inner pipe ( 21 and 22) except for both end portions, a space is formed between the first inner tube 21 and the second inner tube 22 to form an annular space (a hollow heat insulating layer 25 described later).

  As shown in FIG. 2, the reduced diameter portion 23 is formed over the entire circumference of the upstream end portion of the second inner pipe 22, and makes it impossible for the exhaust gas to enter the hollow heat insulating layer 25. It functions as (exhaust gas blocking means). The reduced diameter portion 23 includes a cylindrical upstream contact portion 23a that can contact the entire circumference of the upstream end portion of the first inner pipe 21, and a taper that gradually increases in diameter from the most downstream side of the upstream contact portion 23a. Part 23b. The inner diameter of the upstream contact portion 23 a is substantially equal to the outer diameter of the first inner tube 21. The tapered portion 23 b is inclined by connecting the upstream contact portion 23 a of the second inner tube 22 and the maximum outer peripheral portion of the second inner tube 22. At the upstream end of the double inner pipe (21, 22), the upstream end of the first inner pipe 21 and the upstream contact portion 23a of the second inner pipe 22 are in contact with each other, and both the inner pipes 21 are in contact with each other. , 22 are fixed by welding (the welded portion is not shown) over the entire circumference in a state where the upstream end portions are overlapped. Thus, the outer peripheral surface of the first inner pipe 21 and the second inner pipe are fixed by welding the upstream end portion of the first inner pipe 21 and the upstream contact portion 23a of the second inner pipe 22 over the entire circumference. A hollow heat insulating layer 25 that prevents the exhaust gas from entering from the upstream side of the catalytic converter is secured between the inner peripheral surface of 22 (excluding both ends of both inner pipes 21 and 22). In the present embodiment, the hollow heat insulating layer 25 exists as an air layer.

  As shown in FIG. 2, the reduced diameter portion 24 is formed over the entire circumference of the downstream end portion of the second inner tube 22. The reduced diameter portion 24 includes a cylindrical downstream contact portion 24a that can contact the entire circumference of the downstream end portion of the first inner pipe 21, and a taper that gradually increases in diameter from the most upstream side of the downstream contact portion 24a. Part 24b. The inner diameter of the downstream contact portion 24a is approximately equal to the outer diameter of the first inner tube, and is approximately equal to the inner diameter of the upstream contact portion 23a. The tapered portion 24 b is inclined by connecting the downstream contact portion 24 a of the second inner tube 22 and the maximum outer peripheral portion of the second inner tube 22. Also at the downstream end of the double inner pipe (21, 22), the downstream end of the first inner pipe 21 and the second end are the same as the upstream end of the double inner pipe (21, 22). The downstream contact portion 24 a of the inner tube 22 is in contact with each other, but the downstream contact portion 24 a is not fixed (fixed) to the downstream end portion of the first inner tube 21. That is, the upstream contact portion 23a is a fixed end (fixed end), while the downstream contact portion 24a is a free end, and the first end at the upstream end portion of the double inner pipe described above. Unlike the fixed state (fixed state) between the inner tube 21 and the second inner tube 22, the first inner tube 21 and the second inner tube 22 at the downstream end of the double inner tube are not fixed (unfixed). State). The downstream contact portion 24a contacts the entire circumference of the downstream end portion of the first inner pipe 22 to substantially close the annular opening on the downstream side of the hollow heat insulating layer 25, so that the exhaust gas downstream of the catalytic converter flows backward. Thus, it works to prevent entry into the hollow heat insulating layer 25 as much as possible.

  In the present embodiment, the first inner tube 21 and the second inner tube 22 (upstream contact portion 23a) are fixed to each other only at the upstream end, and the first inner tube 21 and the second inner tube 22 are downstream. There is no fixing at the end. For this reason, the fixed location of both the inner pipes 21 and 22 in the range along the axial direction or the longitudinal direction of both the inner pipes 21 and 22 is only one place along one circumference corresponding to the upstream contact portion 23a. Limited.

  Further, as shown in FIG. 2, the outer peripheral surface of the upstream end portion of the honeycomb body 10 is connected to the entire inner peripheral surface of the cylindrical case 20 via the annular brazing portion 26, and the honeycomb body The inner peripheral surface of the downstream end 10 is connected to the outer peripheral surface of the second inner tube 22 through the annular brazing portion 27. The upstream annular brazing portion 26 (the outer peripheral surface side connecting position of the honeycomb body) and the downstream annular brazing portion 27 (the inner peripheral surface side connecting position of the honeycomb body) are displaced in the axial direction. The honeycomb body 10 is stably held between the cylindrical case 20 and the double inner pipes (21, 22) in spite of being held only by the annular brazing portions 26, 27.

  Examples of the brazing material used in the brazing portions 26 and 27 include silver brazing, copper brazing, and brass brazing. An example of a brazing method that can be used is brazing in a vacuum furnace.

  According to this embodiment, the following operations and effects can be obtained.

  In the double inner pipe composed of the first inner pipe 21 and the second inner pipe 22, the boundary between the bypass passage B and the honeycomb body 10 surrounding the bypass passage B is connected to the upstream end of the first inner pipe 21 and the second inner pipe. By securing the upstream contact portion 23a of the inner pipe 22 over the entire circumference, the hollow heat insulating layer 25 can be secured more reliably. Therefore, even when high-temperature exhaust gas flows through the bypass passage B, heat transfer from the bypass passage B to the honeycomb body 10 is relaxed by the hollow heat insulating layer 25 secured around the bypass passage B, and the inner peripheral portion of the honeycomb body 10 and Uneven heat distribution (that is, excessive thermal gradient in the radial direction) hardly occurs on the outer periphery. Therefore, even if a tensile stress in the circumferential direction is generated in the honeycomb body 10 due to the heat distribution (thermal gradient), the circumferential tensile stress is not excessively increased, and the metal constituting the honeycomb body 10 is made. It is possible to prevent the strips 11 and 12 from being cut and chipped off in the form of foil (breakage of the honeycomb body).

  The fixed locations of the inner pipes in the range along the axial direction or the longitudinal direction of the first and second inner pipes 21 and 22 are the upstream end of the first inner pipe 21 and the reduced diameter part of the second inner pipe 22. 23 is limited to one place along one circumference corresponding to the upstream side contact portion 23a of the 23, so that the high temperature exhaust gas flows in the first inner pipe 21 and the first inner pipe 21 is the second inner pipe. Even in the case where the first inner tube 21 expands greatly compared to the second inner tube 22, the first inner tube 21 can be thermally expanded in the axial direction without being constrained by the second inner tube 22. It is possible to avoid thermal stress concentration based on the difference in thermal expansion between the two. If there are two or more places where both inner pipes are fixed within the range along the axial direction or longitudinal direction of both inner pipes 21 and 22 (for example, not only the upstream end of the double inner pipe but also the double inner pipe In the case where the entire periphery is also fixed at the downstream end portion of the pipe), the axial thermal expansion of the first inner pipe 21 is hindered by the two fixing portions, resulting in stress concentration. In this respect, according to the present embodiment, since the axially fixed portion of both the inner pipes 21 and 22 is only one place, the above-described thermal stress concentration does not occur, and the double inner tube (21 22) can be prevented.

  Furthermore, according to the present embodiment, high-temperature exhaust gas flows in the first inner pipe 21, heat is transferred to the honeycomb body 10 surrounding it, and a thermal gradient is generated between the inner peripheral portion and the outer peripheral portion of the honeycomb body 10. Even if a thermal difference occurs, the inner peripheral surface of the honeycomb body 10 is only connected to the second inner tube 22 via the downstream annular brazing portion 27, so that the inner peripheral portion of the honeycomb body 10 is (In particular, the metal strips 11 and 12 constituting the group 13 of vent holes arranged in the innermost circumference) and the second inner tube 22 can be thermally expanded in the axial direction without being constrained to each other. Similarly, since the outer peripheral surface of the honeycomb body 10 is only connected to the cylindrical case 20 through the upstream-side annular brazing portion 26, the outer peripheral portion of the honeycomb body 10 (especially the vent holes arranged on the outermost periphery). The metal strips 11 and 12) constituting the 13th group and the cylindrical case 20 can be thermally expanded in the axial direction without being constrained to each other. Therefore, between the four of the second inner tube 22, the inner peripheral portion of the honeycomb body 10, the outer peripheral portion of the honeycomb body 10, and the cylindrical case 20, which are concentrically arranged with respect to the central axis L of the catalytic converter. Even if a large thermal gradient or thermal difference occurs, thermal stress does not concentrate inside the honeycomb body 10 or at the connection site, and damage to the honeycomb body 10 can be prevented.

(Modification) The embodiment of the present invention may be modified as follows.
As a mounting method for holding the honeycomb body 10 between the cylindrical case 20 and the double inner pipe (21, 22), a brazing mode as shown in FIG. 3 is used instead of the brazing mode shown in FIG. May be adopted. That is, the inner peripheral surface of the upstream end portion of the honeycomb body 10 is connected to the outer peripheral surface of the second inner tube 22 via the annular brazing portion 28, and the outer peripheral surface of the downstream end portion of the honeycomb body 10 is connected. The entire circumference of the cylindrical case 20 may be connected via the annular brazing portion 29. Also in this case, the upstream annular brazing portion 28 (the inner peripheral surface side connection position of the honeycomb body) and the downstream annular brazing portion 29 (the outer peripheral surface side connection position of the honeycomb body) are displaced in the axial direction. Therefore, the honeycomb body 10 is stably held between the cylindrical case 20 and the double inner pipes (21, 22) despite being held only by the two annular brazing portions 28, 29. .

  In the above embodiment, the upstream contact portion 23a of the second inner pipe 22 is fixed to the upstream end of the first inner pipe 21, but is fixed to the vicinity of the upstream end of the first inner pipe 21. It is good. Moreover, in the said embodiment, although the downstream contact part 24a of the 2nd inner pipe 22 was made to contact over the perimeter of the downstream end part of the 1st inner pipe 21, the downstream end part of the 1st inner pipe 21 is made. You may make it contact over the perimeter of the vicinity.

It is a front view which shows the outline of a catalytic converter. It is sectional drawing which expands and shows the longitudinal cross section in the AA of FIG. It is sectional drawing equivalent to FIG. 2 in the example of a change.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Catalyst holding honeycomb body 13 Honeycomb body mesh-like ventilation path 20 Cylindrical case 21 1st inner pipe (1st inner pipe)
22 Second inner pipe (second inner pipe)
23 Reduced diameter portion 23a Upstream contact portion 23b Tapered portion 24 Reduced diameter portion 24a Downstream contact portion 24b Tapered portion 25 Hollow heat insulating layer 26, 27, 28, 29 Annular brazing portion B Bypass passage as a central passage L Central axis

Claims (4)

A catalytic converter comprising a catalyst supporting honeycomb body having a large number of vent passages extending in the axial direction, and a central passage that passes through the central part of the honeycomb body in the axial direction,
In the boundary region between the central passage and the honeycomb body surrounding the central passage, the first inner pipe that defines the central passage and the second inner pipe that surrounds the first inner pipe with a space therebetween. A double inner pipe is formed, and an upstream end portion of the second inner pipe is formed with a reduced-diameter portion over the entire circumference thereof, between the first inner pipe and the second inner pipe. In the upstream end portion of the double inner tube, the upstream end portion of the first inner tube or its vicinity and the reduced diameter portion of the second inner tube are fixed to each other so that the exhaust gas enters from the upstream side. A catalytic converter characterized in that a hollow heat-insulating layer is secured.   The diameter-reduced portion is formed of an upstream contact portion that can contact the entire circumference of the upstream end of the first inner pipe or the vicinity thereof, and a tapered portion that gradually increases in diameter from the upstream contact portion. The catalytic converter according to claim 1.   The downstream end of the second inner pipe is not fixed to the downstream end of the first inner pipe or the vicinity thereof, but the entire circumference of the downstream end of the first inner pipe or the vicinity thereof. The catalytic converter according to claim 1, wherein a downstream contact portion capable of contacting is formed.   The catalytic converter further includes a cylindrical case that accommodates the honeycomb body, and the outer peripheral surface of the second inner pipe is disposed at one of the downstream end portion and the upstream end portion of the inner peripheral surface of the honeycomb body. And connecting the outer peripheral surface of the honeycomb body to the inner peripheral surface of the cylindrical case at the other of the downstream end portion and the upstream end portion thereof, the honeycomb body becomes a cylindrical case. The catalytic converter according to any one of claims 1 to 3, wherein the catalytic converter is held between the inner pipe and the double inner pipe.

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