EP2636864A1

EP2636864A1 - Dual pipe exhaust manifold

Info

Publication number: EP2636864A1
Application number: EP13001125.7A
Authority: EP
Inventors: Kazuya Uchida; Tsukasa Minato
Original assignee: Calsonic Kansei Corp
Current assignee: Marelli Corp
Priority date: 2012-03-08
Filing date: 2013-03-06
Publication date: 2013-09-11
Also published as: JP2013213491A; US20130232960A1; CN103306795A

Abstract

A dual pipe exhaust manifold includes an inner pipe (2), an outer pipe (3) and a plurality of spacers (23a-23c). The inner pipe (2) includes a plurality of branch pipes (21 a-21 d) and a plurality of collector pipes (22a and 22b). The collector pipes (22a and 22b) are slidably connected to each other. One end of each of the branch pipe is fixed to a flange section (20) configured and arranged to be attached to a cylinder head (1a) of an engine (1), and the other end of each of the branch pipe is slidably connected to a corresponding one of the collector pipes (22a and 22b) so that each of the collector pipes (22a and 22b) is connected to a plural number of the branch pipes (21a-21d). The outer pipe (3) covers a circumference of the inner pipe (2). The spacers (23a-23c) are disposed between the outer pipe (3) and the inner pipe (2) to form a gap therebetween.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to Japanese Patent Application No. 2012-051412, filed on March 8, 2012 and Japanese Patent Application No. 2012-276065, filed on December 18, 2012 . The entire disclosures of Japanese Patent Application Nos. 2012-051412 and 2012-276065 are hereby incorporated herein by reference.

BACKGROUND

Field of the Invention

The present invention relates to a dual pipe exhaust manifold such that an outer side of an inner pipe, where exhaust gas which is discharged from an internal combustion engine flows, is covered by an outer pipe to have a gap.

Background Information

As a conventional dual pipe exhaust manifold, a dual pipe exhaust manifold which is described in Japanese Laid-Open Patent Application Publication No. 2010-255611 is known.
In the conventional dual pipe exhaust manifold disclosed in the above mentioned publication, opening sides of both a branch pipe and a collector pipe with an approximate Y shape, where an inner pipe as a thin wall is divided into above and below, are combined using polymerization, and a flange section is formed by welding at outer side ends of each of the branch pipe and the collector pipe. On the other hand, in the outer pipe, a box which is divided into above and below is formed by being polymerized and combined in a tunnel shape so as to form an outer covering for both the branch pipe and a collecting section of the inner pipe according to the shape of the branch pipe and the collecting section.
At an appropriate position of the branch pipe and the collector pipe of the inner pipe, the outer pipe forms an outer covering as a wire mesh with a ring shape, a gap to the extent of the thickness of the wire mesh is secured between the inner pipe and the outer pipe, and corresponds to distortion (stretching, shrinkage) in the inner pipe in an axial direction as a floating structure where the inner pipe and the outer pipe are able to move relative to each other in the axial direction.
In addition, at the appropriate position between the inner pipe and the outer pipe, an inner pipe holding member with a thick wall, which is joined to the inner pipe and the outer pipe by spot welding, is provided, and thermal expansion difference which is generated between the inner pipe and outer pipe is dissipated and alleviated.

SUMMARY

However, in the conventional dual pipe exhaust manifold, since there is a structure where the inner pipe has a thin wall and the branch pipe and collector pipe are joined by welding, there is a problem in that distortion occurs due to regulation from other parts as a result of thermal expansion in the entire inner pipe which reaches a high temperature due to high temperature exhaust gas which flows in an inner section and a reduction in longevity occurs at a portion where the distortion is concentrated.
The present invention focuses on the problems described above and the object is to provide a dual pipe exhaust manifold which is able to prevent a reduction in longevity of an inner pipe which is caused by concentration of distortion of the inner pipe due to thermal expansion.
In order to achieve this object, a dual pipe exhaust manifold according to a first aspect includes an inner pipe, an outer pipe and a plurality of spacers. The inner pipe includes a plurality of branch pipes and a plurality of collector pipes. The collector pipes are slidably connected to each other. One end of each of the branch pipe is fixed to a flange section configured and arranged to be attached to a cylinder head of an engine, and the other end of each of the branch pipe is slidably connected to a corresponding one of the collector pipes so that each of the collector pipes is connected to a plural number of the branch pipes. The outer pipe covers a circumference of the inner pipe. The spacers are disposed between the outer pipe and the inner pipe to form a gap therebetween.
A dual pipe exhaust manifold according to a second aspect is the dual pipe exhaust manifold according to the first aspect, wherein the collector pipes are fitted to each other so as to be slidable with respect to each other in a direction parallel to the flange section.
A dual pipe exhaust manifold according to a third aspect is the dual pipe exhaust manifold according to the first or second aspect, wherein each of the branch pipes is fitted in the corresponding one of the collector pipes from one of a direction parallel to the flange section and a direction perpendicular to the flange section.
A dual pipe exhaust manifold according to a fourth aspect is the dual pipe exhaust manifold according to any one of the first to third aspects, wherein each of the collector pipes is an integrally press-formed pipe in which opposing end portions are disposed on an opposite side from a surface to be hit by exhaust gas that has flowed into the collector pipe.
A dual pipe exhaust manifold according to a fifth aspect is the dual pipe exhaust manifold according to any one of the second to fourth aspects, wherein at least two of the branch pipes are respectively fitted in corresponding ones of the collecting sections in a direction parallel to the flange section and in directions opposed to each other.
A dual pipe exhaust manifold according a sixth aspect is the dual pipe exhaust manifold according to any one of the first to fifth aspects, wherein a tip end of each of the collector pipes and the branch pipes which is slidably inserted into a corresponding one of the collector pipes and the branch pipes is extended in a radial direction toward an inner circumferential surface of the corresponding one of the collector pipes and the branch pipes and folded back to reduce leakage of exhaust gas.
In the dual pipe exhaust manifold of the first aspect, since it is possible for there to be sliding between the branch pipe and the collecting section and between collecting sections relative to each other even when the inner pipe is thermally expanded due to high temperature exhaust gas, it is possible to absorb thermal expansion using the sliding and it is possible to prevent a reduction in longevity of the inner pipe which is caused by concentration of distortion of the inner pipe due to thermal expansion.
In the dual pipe exhaust manifold of the second aspect, it is possible to obtain a structure where it is possible to easily perform fitting and sliding in the collecting sections while the amount of protrusion in the perpendicular direction from the flange section of the inner tube and the outer tube is suppressed to be small.
In the dual pipe exhaust manifold of the third aspect, it is possible to absorb stretching in the direction which is parallel to the flange section and the direction which is perpendicular to the flange section with regard to the collector pipe based on thermal expansion of the inner pipe while the amount of protrusion in the perpendicular direction from the flange section of the inner tube and the outer tube is suppressed to be small.
In the dual pipe exhaust manifold of the fourth aspect, it is possible to manufacture the collector pipe at a low cost due to welding or the like being unnecessary, and even in this case, it is possible to prevent defects (reduction of the flow rate of exhaust gas and generation of noise, vibration, and the like) such as exhaust gas which has flowed into the collector pipe flowing as it is into the gap between the collector pipe and the outer pipe from the gap of the end portions which oppose each other.
In the dual pipe exhaust manifold of the fifth aspect, it is possible to suppress changes in the position of the collector pipe even when the branch pipes thermally expand and it is possible to suppress adverse effects of downstream parts of the exhaust system to be small.
In the dual pipe exhaust manifold of the sixth aspect, it is possible to reduce leaking of exhaust gas from portions of the collector pipes and branch pipes which are able to slide.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the attached drawings which form a part of this original disclosure:
Fig. 1 is a diagram illustrating a dual pipe exhaust manifold of a first embodiment of the present invention and is a perspective view where a portion of an outer tube of the dual pipe exhaust manifold has been cut off.
Fig. 2 is a diagram illustrating a portion of a collector pipe of a dual pipe exhaust manifold of the first embodiment.
Fig. 3 is a diagram where a portion of a collector pipe of the dual pipe exhaust manifold of the first embodiment is viewed from an engine side.
Fig. 4 is a cross section diagram illustrating a portion of the dual pipe exhaust manifold of a second embodiment of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS

Selected embodiments will now be explained with reference to the drawings. It will be apparent to those skilled in the art from this disclosure that the following descriptions of the embodiments are provided for illustration only and not for the purpose of limiting the invention as defined by the appended claims and their equivalents.
Below, aspects of embodiments of the present invention will be described in detail based on the embodiments which are illustrated in the drawings.

FIRST EMBODIMENT

First, the overall configuration of a dual pipe exhaust manifold of a first embodiment will be described.
Fig. 1 illustrates a dual pipe exhaust manifold of the first embodiment. Here, in Fig. 1, an upper half portion of an outer pipe 3 is drawn in a state of being cut off in order for the structure of an inner pipe 2 which is an inner portion thereof to be easily seen. Here, the dual pipe exhaust manifold of the present embodiment is connected to a four-cylinder engine 1.
As shown in Fig. 1, the dual pipe exhaust manifold of the first embodiment is used in a vehicle, and is provided with the inner pipe 2 through which exhaust gas discharged from the engine 1 flows, and the outer pipe 3 which covers the inner pipe 2 from the outer circumference side.
The inner pipe 2 is manufactured with a thin wall as compared with the outer pipe 3 using metal which has high heat resistance and corrosion resistance, and is provided with a flange section 20, four branch pipes 21a to 21d, and two collector pipes 22a and 22b. The dual pipe exhaust manifold further includes three spacers 23a to 23c for securing a gap between the inner pipe 2 and the outer pipe 3.
Four exhaust gas inlet holes are provided in the flange section 20 along a longitudinal direction to match with four exhaust ports which are provided in a cylinder head 1a of the engine 1, and the cylinder head 1a is attached using a plurality of bolts (not shown in the drawings) which respectively penetrate a plurality of bolt holes 20a. The flange section 20 is fixed by being welded to an exhaust gas upstream side end section of the branch pipes 21a to 21d to match the exhaust gas inlet holes.
In the branch pipes 21a to 21d, the branch pipes 21a and 21d are formed at both end sides are J shaped and the two branch pipes 21b and 21c therebetween are formed in a linear shape. The branch pipes 21a and 21d at both end sides are arranged so that the exhaust gas downstream side end sections thereof oppose each other.
Each of the collector pipes 22a and 22b have an axial direction opening section which is provided on each of one end side and another end side thereof and a perpendicular direction opening section which is opened in the perpendicular direction to the collector pipes 22a and 22b toward the flange section 20 at a central position of both axial direction opening sections on an axis extending parallel along the longitudinal direction of the flange section 20.
The exhaust gas downstream side end section of the branch pipe 21a which is arranged in a reverse J shape as viewed from above is fitted in the axial direction opening section of one end side of the collector pipe 22a so as to be able to slide relatively in the axial direction, and the axial direction opening section of one end side of the collector pipe 22b is fitted in an axial direction opening section of another end side of the collector pipe 22a so as to be able to slide relatively in the axial direction. In addition, the exhaust gas downstream side end section of the branch pipe 21b is fitted in the perpendicular direction opening section of the collector pipe 22a so as to be able to slide relatively in the direction which is perpendicular to the flange section 20.
In the other collector pipe 22b, the exhaust gas downstream side end section of the branch pipe 21d which is arranged in a J shape as viewed from above is fitted in the axial direction opening section on the other end side so as to be able to slide relatively in the axial direction, and the exhaust gas downstream side end section of the branch pipe 21c is fitted in the perpendicular direction opening section so as to be able to slide relatively in the direction which is perpendicular to the flange section 20.
Here, a discharge pipe portion is integrally provided in a downward direction by a hole being provided in a lower surface at a position between the other end side of the axial direction opening section and the perpendicular direction opening section on the collector pipe 22b.
Here, the collector pipes 22a and 22b are formed by stamping and pressing a sheet of metal as shown in Figs. 2 and 3, and are formed into pipe shapes where both end portions face each other (to become opposing portions). The opposing portions are not fixed by welding or the like, and a perpendicular direction opening section is formed here. Accordingly, exhaust gas which flows inside of the branch pipes 21b and 21c flows in from the perpendicular direction opening section and hits a surface which is on the opposite side to the perpendicular direction opening section, and the flow direction thereof is changed to the axial direction.
Here, exhaust gas which flows in hits a concave portion 22c in Fig. 2. In addition, in Fig. 3, the branch pipe 21d is inserted in a left end side opening, the collector pipe 22a is inserted in a right end side opening, and the branch pipe 21c is inserted in a front side opening.
The outer pipe 3 is formed with a thick wall as compared with the inner pipe 2 using a metal material such as stainless steel with high rigidity, high heat resistance and corrosion resistance, and the branch pipes 21a to 21d and the collector pipes 22a and 22b are formed by combining and fixing an upper half portion and a lower half portion which respectively cover above and below in a state where a gap is formed between the pipes.
The spacer 23a is attached at a fitting portion of the one end side of the axial direction opening section of the collector pipe 22a and the exhaust gas downstream side end section of the branch pipe 21a, the spacer 23b is attached at a fitting portion of one end side of the axial direction opening section of the collector pipe 22b and the other end side of the axial direction opening section of the collector pipe 22a, and the spacer 23c is attached at a fitting portion of the other end side of the axial direction opening section of the collector pipe 22b and the exhaust gas downstream side end section of the branch pipe 21d, and a gap is formed between the inner pipe 2 and the outer pipe 3 by each of the spacers being supported by the inner surfaces of the outer pipe 3.
The spacers 23a to 23c are, for example, configured as a wire mesh ring or the like.
In the dual pipe exhaust manifold of the first embodiment which is configured as described above, exhaust gas is discharged from the four exhaust ports of the cylinder head 1a to the corresponding branch pipes 21a to 21d by the engine 1 being operated.
The exhaust gas flows into each of the corresponding branch pipes 21a to 21d, enters from the branch pipe 21a into the one end side of the axial direction opening section of the collector pipe 22a along the axial direction, flows from the branch pipe 21b into the perpendicular direction opening section of the collector pipe 22a in the direction which is perpendicular to the axial direction, and changes direction to the axial direction by hitting the perpendicular direction opening section, that is, a surface which is on the opposite side of the opposing portion. The exhaust gas moves toward the axial direction opening section on the one end side of the collector pipe 22b.
After this, the gas is discharged from a discharge pipe portion in a downward direction.
In a similar manner, the exhaust gas flows in from the branch pipe 21c into the perpendicular direction opening section of the collector pipe 22b in a direction which is perpendicular to the axial direction and is discharged from the discharge pipe portion in a downward direction after changing direction to the axial direction toward the axial direction opening section on the other end side of the collector pipe 22b by hitting the perpendicular direction opening section, that is, the surface which is on the opposite side of the opposing portion. In addition, the exhaust gas is discharged in a downward direction from the discharge pipe portion after flowing from the branch pipe 21d toward the collector pipe 22a side from the axial direction opening section on the other end side of the collector pipe 22b in the axial direction.
The inner pipe 2 which is directly exposed to high temperature exhaust gas changes shape due to thermal expansion, and in particular, changes shape so as to become larger in the longitudinal direction thereof. However, expansion in the direction which is perpendicular to the flange section 20 of the branch pipes 21a to 21d is where the collector pipes 22a and 22b stretch and move in both directions of the branch pipes 21a and 21d. At this time, the fitting portions of the perpendicular direction opening sections of the collector pipes 22a and 22b and the exhaust gas downstream side end sections of the branch pipes 21b and 21c absorb by sliding relatively when difference is generated in the stretching of the branch pipes 21b and 21c and the collector pipes 22a and 22b.
In addition, expansion in the direction which is parallel with regard to the flange section 20 of the branch pipes 21a to 21d is absorbed by relative sliding of the fitting portion of the exhaust gas downstream side end section of the branch pipe 21a and one end side of the axial direction opening section of the collector pipe 22a, the fitting portion of the other end side of the axial direction opening section of the collector pipe 22a and one end side of the axial direction opening section of the collector pipe 22b, and the fitting portion of the other end side of the axial direction opening section of the collector pipe 22b and the exhaust downstream side end section of the branch pipe 21d.
Here, since the outer pipe 3 covers the inner pipe 2 with a gap so as not to reduce the temperature of the exhaust gas which flows in the inner pipe 2, it is easy for the exhaust gas which is maintained at a high temperature to be purified using a catalyst which is positioned downstream of the manifold.
In addition, the outer pipe 3 prevents leakage of the exhaust gas, which has leaked from the fitting portions of the collector pipes 22a and 22b and the branch pipes 21a to 21d with the collector pipes 22a and 22b which configure the inner pipe 2, to the outside of the outer pipe 3.
As is understood from the above, it is possible for the dual pipe exhaust manifold of first embodiment to obtain the effects described below.
Since the fitting portions of the collector pipes 22a and 22b and the branch pipes 21a to 21d with the collector pipes 22a and 22b which configure the inner pipe 2 are able to slide in a direction which is parallel to the flange section 20 and a direction which is perpendicular to the flange section 20, it is possible to absorb the stretching even in a case where the inner pipe 2 thermally expands due to the temperature of the exhaust gas, and as a result, it is possible to achieve an improvement in high temperature resistance and an increase in the longevity of the inner pipe 2.
In addition, since the collector pipes 22a and 22b are fitted so as to be able to slide in a direction which is parallel to the flange section 20, it is possible to suppress the amount of protrusion from the cylinder head 1a of the exhaust manifold to be small.
In addition, since the exhaust gas downstream side end section of the branch pipes 21a and 21d is fitted in the collecting sections 22a and 22b from a direction which is parallel to the flange section 20 from a direction where they relatively oppose each other, it is possible to suppress movement in the parallel direction where collecting sections 22a and 22b from becoming larger during thermal expansion of the branch pipes 21a to 21d, and as a result, it is possible to reduce adverse effects on other sections.
Since the collector pipes 22a and 22b are integrally press-formed pipes which have end portions which oppose each other at the opposite side to the surface which is hit by exhaust gas which has flowed in, it is possible not only to reduce production costs by welding being unnecessary but also to avoid the exhaust gas which has flowed in from the branch pipes 21b and 21c directly hitting the opposing portions which are not fixed and converting the direction to the axial direction.

SECOND EMBODIMENT

Next, a dual pipe exhaust manifold of a second embodiment of the present invention will be described.
The configuration of the sliding portions of the branch pipes 21a to 21d and the collector pipes 22a and 22b is different to the configuration of first embodiment in the dual pipe exhaust manifold of the second embodiment.
That is, as shown in Fig. 4, the exhaust gas downstream side end section of the branch pipe 21a is stretched towards an inner circumferential surface of the exhaust gas upstream side end section of the collector pipe 22a by being extended to a radial direction outer side, and the tip end section is folded back so as to be curled. A curled portion 21a1 is set so as to be in line contact at an inner circumferential surface of the collector pipe 22a.
In addition, although not shown in the drawings, the sliding portion of the downstream side end section of the branch pipe 21b and the upstream side end of the collector pipe 22a, the sliding portion of the upstream side ends of the branch pipe 21c and the collector pipe 22b, and the sliding portion of the upstream side ends of the branch pipe 21d and the collector pipe 22b have the same configuration as described above.
Other configurations are the same as in first embodiment.
Accordingly, it is easy for the exhaust gas to be ejected by increasing the gap in the sliding portions due to assembly rattling of the branch pipes 21a to 21d and the collector pipes 22a and 22b, thermal expansion of the pipe due to high temperature exhaust gas, vibration from the engine, and the like. However, leakage of the exhaust gas is reduced by narrowing the gap by the curled portion of the end section of the branch pipes 21a to 21d being in line contact with the inner circumferential surface of the collector pipes 22a and 22b in the dual pipe exhaust manifold of second embodiment which is configured as described above.
In this case, the curled portion 21a1 has a spring function and the gap is maintained to be narrow due to thermal expansion. In addition, the curled portion 21a1 also is suitable for sliding.
As a result, temperature of the outer pipe is increased by storing exhaust gas which has leaked from the gap in a space between the inner pipe 2 and the outer pipe 3 in the dual pipe exhaust manifold of second embodiment, and as a result, it is possible to prevent defects such as damage to the outer pipe 3 by heat distortion being generated in the outer pipe 3 due to repetition of rising and falling of the temperature of the outer pipe 3.
Above, the present invention has been described based on each of the embodiments described above but the present invention is not limited to the embodiments and the present invention includes cases where there are design changes and the like in a scope which does not depart from the gist of the present invention.
For example, it is possible to appropriately change the shapes or materials of the inner pipe 2 and the outer pipe 3.
In addition, the curled portions are provided in the sliding portions at the branch pipes 21a to 21d side in second embodiment, but the curled portions may also be provided at the sliding portion side of the collector pipes 22a and 22b.
In this case, it is not absolutely necessary to provide the curled portion in all of the sliding portions of the branch pipes 21a to 21d and the collector pipes 22a and 22b, and may be provided only in sliding portions of the branch pipes 21a and 21d and the collector pipes 22a and 22b where leakage is severe.
In addition, the dual pipe exhaust manifold of the present invention is not limited to a four-cylinder engine, and may be applied to, for example, a six-cylinder engine or the like.
In addition, the discharge pipe portion may be provided at a different position.

GENERAL INTERPRETATION OF TERMS

In understanding the scope of the present invention, the term "comprising" and its derivatives, as used herein, are intended to be open ended terms that specify the presence of the stated features, elements, components, groups, integers, and/or steps, but do not exclude the presence of other unstated features, elements, components, groups, integers and/or steps. The foregoing also applies to words having similar meanings such as the terms, "including", "having" and their derivatives. Also, the terms "part," "section," "portion," "member" or "element" when used in the singular can have the dual meaning of a single part or a plurality of parts. The terms of degree such as "substantially", "about" and "approximately" as used herein mean a reasonable amount of deviation of the modified term such that the end result is not significantly changed.
While only selected embodiments have been chosen to illustrate the present invention, it will be apparent to those skilled in the art from this disclosure that various changes and modifications can be made herein without departing from the scope of the invention as defined in the appended claims. For example, the size, shape, location or orientation of the various components can be changed as needed and/or desired. Components that are shown directly connected or contacting each other can have intermediate structures disposed between them. The functions of one element can be performed by two, and vice versa. The structures and functions of one embodiment can be adopted in another embodiment. It is not necessary for all advantages to be present in a particular embodiment at the same time. Every feature which is unique from the prior art, alone or in combination with other features, also should be considered a separate description of further inventions by the applicant, including the structural and/or functional concepts embodied by such feature(s). Thus, the foregoing descriptions of the embodiments according to the present invention are provided for illustration only, and not for the purpose of limiting the invention as defined by the appended claims and their equivalents.

Claims

A dual pipe exhaust manifold comprising:
an inner pipe (2) including a plurality of branch pipes (21a-21d) and a plurality of collector pipes (22a and 22b), the collector pipes (22a and 22b) being slidably connected to each other, one end of each of the branch pipes (21a-21d) being fixed to a flange section (20) configured and arranged to be attached to a cylinder head (1a) of an engine (1), and the other end of each of the branch pipes (21a-21d) being slidably connected to a corresponding one of the collector pipes (22a and 22b) so that each of the collector pipes (22a and 22b) is connected to a plural number of the branch pipes (21a-21d);

an outer pipe (3) covering a circumference of the inner pipe (2); and

a plurality of spacers (23a-23c) disposed between the outer pipe (3) and the inner pipe (2) to form a gap therebetween.
The dual pipe exhaust manifold according to claim 1, wherein
the collector pipes (22a and 22b) are fitted to each other so as to be slidable with respect to each other in a direction parallel to the flange section (20).
The dual pipe exhaust manifold according to claim 1 or 2, wherein
each of the branch pipes (21a-21d) is fitted in the corresponding one of the collector pipes (22a and 22b) from one of a direction parallel to the flange section (20) and a direction perpendicular to the flange section (20).
The dual pipe exhaust manifold according to any one of claims 1 to 3, wherein
each of the collector pipes (22a and 22b) is an integrally press-formed pipe in which opposing end portions are disposed on an opposite side from a surface to be hit by exhaust gas that has flowed into the collector pipe.
The dual pipe exhaust manifold according to any one of claims 2 to 4, wherein
at least two of the branch pipes (21a-21d) are respectively fitted in corresponding ones of the collecting pipes (22a and 22b) in a direction parallel to the flange section (20) and in directions opposed to each other.
The dual pipe exhaust manifold according to any one of claims 1 to 5, wherein
a tip end of each of the collector pipes (22a and 22b) and the branch pipes (21a-21d) which is slidably inserted into a corresponding one of the collector pipes (22a and 22b) and the branch pipes (21a-21d) is extended in a radial direction toward an inner circumferential surface of the corresponding one of the collector pipes (22a and 22b) and the branch pipes (21a-21d) and folded back to reduce leakage of exhaust gas.