EXHAUST PIPE

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit of Japanese Patent Application No. 2020-208805 filed on Dec. 16, 2020 with the Japan Patent Office, the entire disclosure of which is incorporated herein by reference.

BACKGROUND

The present disclosure relates to an exhaust pipe.

In exhaust systems of internal combustion engines, noise is reduced by mufflers provided in exhaust flow paths. For example, Japanese Unexamined Utility Model Application Publication No. S61-138813 discloses an exhaust system in which a branch pipe, which is divided from an exhaust pipe forming part of an exhaust flow path, functions as a side-branch muffler.

SUMMARY

In such an exhaust system, low-frequency resonance that is dependent on the entire length of the exhaust system may be generated as a type of noise. Unfortunately, in the exhaust system disclosed in S61-138813, the branch pipe, which functions as a side-branch muffler, is short, thereby hindering noise reduction in the low frequency region.

It is desirable that one aspect of the present disclosure provides a technique to facilitate noise reduction in the low frequency region.

One aspect of the present disclosure provides an exhaust pipe that comprises a tubular portion and a space forming portion. The tubular portion forms an exhaust flow path in an exhaust system of an internal combustion engine. The tubular portion has a tubular shape. The space forming portion is provided along at least one of an inner surface and an outer surface of the tubular portion and forms at least one branch space between the space forming portion and the tubular portion. The at least one branch space communicates with the exhaust flow path and has a function as a side-branch muffler. The at least one branch space includes a space with a length that is one third or more of an entire length of the exhaust system.

Such a configuration provides the branch space that functions as a side-branch muffler with a relatively long length, thereby reducing noise in the low frequency region.

In one aspect of the present disclosure, the tubular portion may be a longest tubular portion of a tubular body forming the exhaust flow path in the exhaust system.

In one aspect of the present disclosure, the at least one branch space may comprise a space with a length that is one third or more of the entire length of the exhaust system. Such a configuration provides an even longer branch space that functions as a side-branch muffler, thereby further facilitating noise reduction in the low frequency region.

In one aspect of the present disclosure, the at least one branch space may comprise two branch spaces with lengths different from each other. Such a configuration has a muffling effect on sound waves with a wider range of frequencies.

In one aspect of the present disclosure, the at least one branch space may comprise two branch spaces each having a length that is one fifth or more of the entire length of the exhaust system. Such a configuration has the muffling effect on sound waves with a wider range of frequencies in the low frequency region.

In one aspect of the present disclosure, the space forming portion may be provided along the inner surface of the tubular portion. Such a configuration allows the branch spaces to be formed with the outer shape of the tubular portion maintained.

In one aspect of the present disclosure, the space forming portion may comprise a tubular inner pipe member disposed inside the tubular portion and forming a double-wall pipe together with the tubular portion. The inner pipe member comprises an attached portion and a separated portion. The attached portion is attached to the inner surface of the tubular portion. The separated portion is spaced apart from the inner surface of the tubular portion. The at least one branch space may comprise a space formed between the separated portion and the tubular portion. With such a configuration, the branch space is formed by providing a double-wall pipe structure and, thus, can be relatively easily formed.

In one aspect of the present disclosure, an area defined by an outer edge of the separated portion in a cross-section perpendicular to an axial direction of the double-wall pipe may be encompassed by an area defined by an outer edge of the attached portion in the cross-section. Such a configuration allows the branch space to be formed with the outer shape of the tubular portion maintained.

In one aspect of the present disclosure, the tubular portion may comprise a curved portion. The curved portion forms a curve in the exhaust flow path.

BRIEF DESCRIPTION OF THE DRAWINGS

Example embodiments of the present disclosure will be described hereinafter with reference to the accompanying drawings, in which:

FIG. 1 is a plan view showing an exhaust system of an internal combustion engine;

FIG. 2A is an enlarged view showing a connection between a second tubular member and a second catalytic converter;

FIG. 2B is an enlarged view showing a connection between a second tubular member and a muffler;

FIG. 3A is a cross-sectional view taken along a line IIIA-IIIA in FIG. 2A;

FIG. 3B is a cross-sectional view taken along a line IIIB-IIIB in FIG. 2A;

FIG. 3C is a diagram showing the cross-section of the attached portion in FIG. 3A and the cross-section of the separated portion in FIG. 3B in an overlapping manner;

FIG. 4 is a schematic view showing the exhaust system of the internal combustion engine of FIG. 1 in a simplified manner;

FIG. 5 is a graph showing a muffling effect of the exhaust system; and

FIG. 6 is a schematic diagram showing communication holes according to a modified example.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS
1. Configuration

Referring to FIG. 1, an exhaust system 1 is disposed under the floor of a vehicle. The exhaust system 1 forms an exhaust flow path that is a flow passage for exhaust gas discharged from an internal combustion engine 100 of the vehicle.

The exhaust system 1 comprises at least one exhaust part. As the at least one exhaust part, the exhaust system 1 comprises, in the order from the upstream side of the exhaust flow path (from the left side in FIG. 1), an exhaust manifold 2, a first catalytic converter 3A, a second catalytic converter 3B, and a muffler 4. The exhaust gas discharged from the internal combustion engine 100 flows through the exhaust manifold 2 and into the first catalytic converter 3A. Each of the first catalytic converter 3A and second catalytic converter 3B comprise a catalyst. The first and second catalytic converters 3A, 3B reform or collect environmental pollutants in the exhaust gas. The muffler 4 reduces noise in the exhaust system 1.

The exhaust system 1 also comprises tubular members. Each tubular member constitutes a tubular body forming the exhaust flow path. Each tubular member is connected to the at least one exhaust part at least at one end the tubular member. Each tubular member forms the exhaust flow path in the upstream or downstream of the at least one exhaust part of the exhaust system 1. As the tubular members, the exhaust system 1 comprises a first tubular member 5A, a second tubular member 5B, and a third tubular member 5C. The first, second, and third tubular members 5A, 5B, 5C are, for example, circular pipes each having a perfectly circular cross-section with an almost consistent outer diameter. The term “perfectly circular” as used herein does not mean to describe the shape of a perfect circle in a strict sense, but means to distinguish a circular shape from an oval shape. The same applies hereinafter.

The first tubular member 5A is connected to the first and second catalytic converters 3A, 3B and forms the exhaust flow path therebetween. The second tubular member 5B is connected to the second catalytic converter 3B and the muffler 4 and forms the exhaust flow path therebetween. The third tubular member 5C is connected to the muffler 4 and forms the exhaust flow path in the downstream of the muffler 4. The exhaust gas that flows through the muffler 4 is discharged from the third tubular member 5C to the outside of the exhaust system 1. In FIGS. 1 to 2B, the direction in which the exhaust gas flows through the exhaust flow path is from left to right, that is, the direction indicated by an arrow Y.

At least one of the tubular members comprises a curved portion. The curved portion forms a curve in the exhaust flow path in the exhaust system 1. The exhaust system 1 is disposed under the floor of a vehicle as mentioned above. Under the floor of a vehicle, there are many other components, such as a propeller shaft, cross members, side members, and a fuel tank of the vehicle, in addition to the exhaust system 1. Thus, in order to avoid interference with these components, the exhaust flow path of the exhaust system 1 comprises the curved portion and, thereby, the exhaust flow path is at least partly curved as in exhaust systems of common vehicles.

Specifically in the exhaust system 1, the first and second tubular members 5A, 5B are provided with the curved portion as shown in FIG. 1. Particularly, the second tubular member 5B is provided with curved portions 51A to 51D at the positions where a double-wall pipe, which will be described below, is formed.

Out of the tubular members, the longest member is provided with a space forming portion, which will be described below. In the example shown in FIG. 1, the second tubular member 5B is the longest among the first, second, and third tubular members 5A, 5B, 5C. The second tubular member 5B is, thus, provided with the space forming portion. Hereinafter, the configuration where the second tubular member 5B is provided with the space forming portion is referred to also as an exhaust pipe 10 with muffler function. The exhaust pipe 10 with muffler function corresponds to one example of the exhaust pipe. Moreover, the tubular member provided with the space forming portion, namely the second tubular member 5B, corresponds to one example of the tubular portion.

The space forming portion is provided along at least one of the inner surface and the outer surface of the tubular portion, forming at least one branch space. The at least one branch space refers to a space that communicates with the exhaust flow path and functions as a side-branch muffler.

As an example of the space forming portion that is formed along the inner surface of the tubular portion, the second tubular member 5B is provided with a first inner pipe member 6A and a second inner pipe member 6B. Both the first and second inner pipe members 6A, 6B have a tubular shape. Specifically, the first and second inner pipe members 6A, 6B are circular pipes each having a perfectly circular cross-section.

Each of the first and second inner pipe members 6A, 6B is disposed inside the second tubular member 5B and forms the double-wall pipe together with the second tubular member 5B. In other words, in the portion of the second tubular member 5B where the first inner pipe member 6A or the second inner pipe member 6B is disposed, the double-wall pipe is formed in which the second tubular member 5B serves as the outer pipe and the first inner pipe member 6A or the second inner pipe member 6B serves as the inner pipe. The first and second inner pipe members 6A, 6B are not visible from the outside of the exhaust system 1, but shown with solid lines in the drawings to facilitate understanding.

As shown in FIGS. 2A, 3A, and 3B, the first inner pipe member 6A comprises an attached portion 61A and a separated portion 62A. FIG. 3A is a cross-sectional view showing a portion of the above-described double-wall pipe composed of the second tubular member 5B and the attached portion 61A in which the portion is cut along a plane perpendicular to the axial direction of the double-wall pipe. The cross-section in FIG. 3A (the cross-section taken along the line IIIA-IIIA in FIG. 2A) is hereinafter referred to as a first cross-section. FIG. 3B is a cross-sectional view showing a portion of the above-described double-wall pipe composed of the second tubular member 5B and the separated portion 62A in which the portion is cut along a plane perpendicular to the axial direction of the double-wall pipe. The cross-section in FIG. 3B (the cross-section taken along the line IIIB-IIIB in FIG. 2A) is hereinafter referred to as a second cross-section.

As shown in FIGS. 2A and 3A, the outer surface of the attached portion 61A is almost identical in shape to the inner surface of the second tubular member 5B. The outer surface of the attached portion 61A is in contact with the inner surface of the second tubular member 5B along the entire circumference in the direction around the axis of the second tubular member 5B. The outer surface of the attached portion 61A is attached to the inner surface of the second tubular member 5B by welding. Both the first inner pipe member 6A and the second tubular member 5B are, for example, circular pipes each having a perfectly circular cross-section. Thus, the outer diameter of the attached portion 61A is, for example, almost the same as the inner diameter of the second tubular member 5B.

As shown in FIG. 3A, the area defined by an outer edge S1 of the attached portion 61A in the first cross-section nearly corresponds to the area defined by an inner edge T of the second tubular member 5B in the first cross-section. The expression of two areas nearly corresponding to each other as used herein means that the two areas have almost the same size, that is, the shapes and surface areas of the two areas are almost the same.

As shown in FIGS. 2A and 3A, the outer surface of the separated portion 62A is situated inside of the inner surface of the second tubular member 5B in the exhaust flow path. The separated portion 62A and the second tubular member 5B are spaced apart from each other. Both the first inner pipe member 6A and the second tubular member 5B are, for example, circular pipes each having a perfectly circular cross-section. Thus, the outer diameter of the separated portion 62A is smaller than, for example, the inner diameter of the second tubular member 5B. The outer diameter of the separated portion 62A is, for example, almost consistent except the portion of the separated portion 62A situated on the upstream side of the exhaust flow path. The portion on the upstream side extends to the attached portion 61A as will be described below, and gradually expands in diameter toward the attached portion 61A.

In the first inner pipe member 6A, the separated portion 62A is narrower than the attached portion 61A. The first inner pipe member 6A is, for example, a circular pipe having a perfectly circular cross-section. Thus, the inner diameter of the separated portion 62B is smaller than, for example, the inner diameter of the attached portion 61A.

FIG. 3C is a diagram showing the cross-section of the attached portion 61A in FIG. 3A and the cross-section of the separated portion 62A in FIG. 3B in an overlapping manner with the centers of these portions aligned. As shown in FIG. 3C, the area defined by an outer edge S2 of the separated portion 62A in the second cross-section is encompassed by the area defined by the outer edge S1 of the attached portion 61A in the first cross-section. A certain area is “encompassed” by another area as described herein means that the size of the certain area is smaller than that of the other area. In other words, it means that the certain area completely fits into the other area when these areas are overlapped.

For example as shown in FIGS. 1 and 2A, the attached portion 61A forms the upstream end of the double-wall pipe composed of the second tubular member 5B and the first inner pipe member 6A. The separated portion 62A forms the portion of the double-wall pipe situated downstream of the attached portion 61A. That is, a space 7A formed between the second tubular member 5B and the separated portion 62A is closed in the axial direction of the double-wall pipe at the upstream end of the double-wall pipe by the attached portion 61A. The space 7A formed between the second tubular member 5B and the separated portion 62A communicates with the exhaust flow path at the downstream end of the double-wall pipe. In other words, the space 7A formed between the second tubular member 5B and the separated portion 62A is included in the at least one branch space described above. Hereinafter, the space 7A formed between the second tubular member 5B and the separated portion 62A is referred to as a first branch space 7A.

As shown in FIG. 2B, the second inner pipe member 6B comprises an attached portion 61B and a separated portion 62B. The attached portion 61B and the separated portion 62B are configured similarly to the above-described attached portion 61A and separated portion 62A respectively. However, the portion of the separated portion 62B situated on the downstream side of the exhaust flow path, instead of the portion on the upstream side of the exhaust flow path, extends to the attached portion 61B, as will be described below. The outer diameter of the separated portion 62B is almost consistent except the portion on the downstream side. The portion on the downstream side gradually expands in diameter toward the attached portion 61B.

As shown in FIGS. 1 and 2B, the attached portion 61B forms the downstream end of the double-wall pipe composed of the second tubular member 5B and the second inner pipe member 6B. The separated portion 62B forms the portion of the double-wall pipe situated upstream of the attached portion 61B. That is, a space 7B formed between the second tubular member 5B and the separated portion 62B is closed in the axial direction of the double-wall pipe at the downstream end of the double-wall pipe by the attached portion 61B. The space 7B formed between the second tubular member 5B and the separated portion 62B communicates with the exhaust flow path at the upstream end of the double-wall pipe. In other words, the space 7B formed between the second tubular member 5B and the separated portion 62B is also included in the at least one branch space described above. Hereinafter, the space 7B formed between the second tubular member 5B and the separated portion 62B is referred to as a second branch space 7B.

FIG. 4 is a schematic view showing the exhaust system 1 of FIG. 1 in a simplified manner. For easier understanding of the length of the branch space, the exhaust system 1 in FIG. 4 is simplified in a manner such that both the first and second tubular members 5A, 5B have no curved portions and the exhaust parts are linearly arranged. The length of each branch space refers to the length from the portion where the branch space is closed to the portion where the branch space communicates with the exhaust flow path.

In FIG. 4, the sound pressures of resonance that can be generated in the exhaust system 1 are also shown. Such resonance is a type of noise in the exhaust system 1. The wavelength of resonance is dependent on a length La of the exhaust system 1. The term “length La of the exhaust system 1” as used herein means the entire length of the exhaust system 1. In other words, the term “length La of the exhaust system 1” as used herein means the entire length of the exhaust flow path in the exhaust system 1. In FIG. 4, the exhaust flow path in the exhaust system 1 is schematically shown in a linear manner. Thus, the length from an end Pa1 to an end Pa2 corresponds to the length La of the exhaust system 1. The end Pa1 is the end of the exhaust manifold 2 that is connected to the internal combustion engine 100. The end Pa2 is the end of the third tubular member 5C that is open to the outside of the exhaust system 1. In the actual exhaust system 1, the exhaust flow path is not completely linear as shown in FIG. 1. In such a case, the length along the shape of the exhaust flow path is the length of the exhaust system 1. Moreover, in the case where the exhaust flow path is, for example, curved inside the exhaust part(s), the length of the exhaust system 1 is larger than the length of the appearance of the exhaust system 1 due to the length(s) of the curve(s) and the like.

As the sound pressures of resonance that can be generated in the exhaust system 1, FIG. 4 shows specifically the sound pressures of a standing wave D1 in the first mode, a standing wave D2 in the second mode, and a standing wave D3 in the third mode. The standing wave D1 has an antinode at the end Pa1, a node at the end Pa2, and no further nodes therebetween. One quarter of a wavelength λd1 of the standing wave D1 is equal to the length La of the exhaust system 1 ((¼)λd1=La). The standing wave D2 has an antinode at the end Pa1, a node at the end Pa2, and one more node therebetween. One quarter of a wavelength λd2 of the standing wave D2 is equal to one third of the length La of the exhaust system 1 ((¼)λd2=(⅓)La). The standing wave D3 has an antinode at the end Pa1, a node at the end Pa2, and two more nodes therebetween. One quarter of a wavelength λd3 of the standing wave D3 is equal to one fifth of the length La of the exhaust system 1 ((¼)λd3=(⅕)La).

Side-branch mufflers having a side-branch space with a length Lbn that is one quarter of the wavelength λ of noise (Lbn=(¼)λ) have the muffling effect particularly on such noise. Noise in the exhaust system 1 at a lower frequency has a longer wavelength. In other words, noise in the low frequency region has a longer wavelength. From the perspective of reducing noise in the low frequency region, the exhaust system 1, thus, is provided with the branch space with a length that is one fifth or more of the length La of the exhaust system 1. From the perspective of further reducing noise in a lower frequency region, the exhaust system 1 may be provided with the branch space with a length that is one third or more of the length La of the exhaust system 1.

The first and second branch spaces 7A, 7B are designed, for example, to have lengths different from each other. Specifically, a length Lb1 of the first branch space 7A is designed to be one fifth of the length La of the exhaust system 1 (Lb1=(⅕)La). As described above, the first branch space 7A is closed on the upstream side of the exhaust flow path by the attached portion 61A and communicates with the exhaust flow path on the downstream side of the exhaust flow path. Thus, the length Lb1 of the first branch space 7A refers specifically to the length of the separated portion 62A. In other words, the length Lb1 of the first branch space 7A refers to the length from an end Pb1 of the separated portion 62A situated on the upstream side of the exhaust flow path to an end Pb2 of the separated portion 62A on the downstream side.

Moreover, a length Lb2 of the second branch space 7B is designed to be one third of the length La of the exhaust system 1 (Lb2=(⅓)La). As described above, the second branch space 7B is closed on the downstream side of the exhaust flow path by the attached portion 61B and communicates with the exhaust flow path on the upstream side of the exhaust flow path. Thus, the length Lb2 of the second branch space 7B refers specifically to the length of the separated portion 62B. In other words, the length Lb2 of the second branch space 7B refers to the length from an end Pb3 of the separated portion 62B situated on the upstream side of the exhaust flow path to an end Pb4 of the separated portion 62B on the downstream side.

2. Effects

According to the embodiment described in detail above, the following effects can be achieved.

(2a) In the exhaust system 1, the first and second branch spaces 7A, 7B are formed. The first branch space 7A has a length that is one fifth or more of the length La of the exhaust system 1. The second branch space 7B also has a length that is one fifth or more of the length La of the exhaust system 1. Such a configuration provides the branch spaces, which function as side-branch mufflers, with relatively long lengths, thereby reducing noise in the low frequency region. Such a configuration has the muffling effect particularly on the standing wave D3 in the third mode or sound waves with frequencies lower than that of the standing wave D3.

(2b) In the exhaust system 1, the second branch space 7B is formed. The second branch space 7B has a length that is one third or more of the length La of the exhaust system 1. Such a configuration provides the branch space, which functions as a side-branch muffler, with an even longer length, thereby further reducing noise in the low frequency region. Such a configuration has the muffling effect particularly on the standing wave D2 in the second mode or sound waves with frequencies lower than that of the standing wave D2.

(2c) In the exhaust system 1, the first and second branch spaces 7A, 7B are formed. The first and second branch spaces 7A, 7B have lengths different from each other. Such a configuration has the muffling effect on sound waves with a wider range of frequencies.

(2d) In the exhaust system 1, the first and second branch spaces 7A, 7B are formed. Each of the first and second branch spaces 7A, 7B has a length that is one fifth or more of the length La of the exhaust system 1. Such a configuration has the muffling effect on sound waves with a wider range of frequencies within a frequency region that includes the frequency of the standing wave D3 in the third mode and lower frequencies.

Referring now to FIG. 5 which is a graph showing the muffling effect in the exhaust system. In this graph, the horizontal axis represents the frequency of a sound wave while the vertical axis represents the amount of noise reduction. The solid line shows the muffling effect of the exhaust system 1. The broken line shows the muffling effect of an exhaust system with no branch space. The three dash-dotted lines show, in order from low to high, the frequency of the standing wave D1 in the first mode, the frequency of the standing wave D2 in the second mode, and the frequency of the standing wave D3 in the third mode that can be generated in the exhaust system 1. In the example shown in FIG. 5, the frequency of the standing wave D1 in the first mode is about 42 Hz, the frequency of the standing wave D2 in the second mode is about 125 Hz, and the frequency of the standing wave D3 in the third mode is about 208 Hz.

As shown in FIG. 5, noise in the low frequency region is reduced more in the exhaust system 1 than in the exhaust system with no branch space. The exhaust system 1 has a high muffling effect particularly on the standing wave D3 in the third mode and the standing wave D2 in the second mode. The exhaust system 1 has the muffling effect also on a sound wave with a frequency that is about three times higher than that of the standing wave D2 in the second mode.

(2e) The first and second inner pipe members 6A, 6B are disposed along the inner surface of the second tubular member 5B. Such a configuration allows the branch spaces to be formed with the outer shape of the second tubular member 5B maintained. It is, thus, possible to form the exhaust pipe 10 with muffler function that can be arranged in the same space as in the configuration where the second tubular member 5B merely functions as an exhaust flow path. The configuration where the second tubular member 5B merely functions as an exhaust flow path as mentioned herein refers to a configuration where the second tubular member 5B is not provided with the first and second inner pipe members 6A, 6B and the first and second branch spaces 7A, 7B are not formed.

(2f) Each of the first and second inner pipe members 6A, 6B forms the double-wall pipe together with the second tubular member 5B. The first branch space 7A is formed between the second tubular member 5B and the separated portion 62A of the first inner pipe member 6A. The second branch space 7B is formed between the second tubular member 5B and the separated portion 62B of the second inner pipe member 6B.

With such a configuration, the branch spaces are formed by providing a double-wall pipe structure and, thus, can be relatively easily formed.

(2g) The area defined by the outer edge S2 of the separated portion 62A in the second cross-section is encompassed by the area defined by the outer edge S1 of the attached portion 61A in the first cross-section. Such a configuration allows the branch space to be formed with the outer shape of the second tubular member 5B maintained. Thus, the external dimension of the exhaust pipe 10 with muffler function is reduced as compared with a configuration where a branch space is formed by expanding the tubular portion in the circumferential direction of the double-wall pipe.

(2h) The second tubular member 5B is provided with curved portions 51A to 51D at the positions where the double-wall pipe is formed, that is, where the first inner pipe member 6A or the second inner pipe member 6B is disposed. Such a configuration allows the branch spaces to be formed irrespective of with or without the curved portions in the tubular portion, thereby providing longer branch spaces.

3. Other Embodiments

An embodiment of the present disclosure has been described hereinabove. The present disclosure, however, should not be limited to the above-described embodiment and may be carried out in variously modified manners.

(3a) In the above-described embodiment, two branch spaces, namely the first and second branch space 7A, 7B, are formed in the second tubular member 5B. However, there may be one branch space, or three or more branch spaces formed in the tubular portion.

(3b) In the above-described embodiment, the first and second branch spaces 7A, 7B are formed in the second tubular member 5B. The first and second branch spaces 7A, 7B have lengths different from each other. In the case where two or more branch spaces are formed in the tubular portion, each branch space may have the same length as the other branch space(s) or may have a length different from the other branch space(s). That is, each of the branch spaces may reduce noise at the same frequency as the frequency reduced in other branch spaces, or may reduce noise at a frequency different from the frequencies reduced in other branch spaces.

(3c) In the above-described embodiment, the first branch space 7A reduces noise at the frequency of the standing wave D3 in the third mode. The second branch space 7B reduces noise at the frequency of the standing wave D2 in the second mode. In this manner, the aforementioned at least one branch space may comprise a branch space that reduces noise at the frequency of at least one standing wave out of the standing waves in the first to third modes.

(3d) In the above-described embodiment, the first and second branch spaces 7A, 7B have lengths that are one fifth or more of the length La of the exhaust system 1. However, the lengths of the branch spaces formed in the tubular portion are not limited to one fifth or more of the length of the tubular portion. For example, the lengths of the branch spaces formed in the tubular portion may be, for example, one third or more, one half or more, two thirds or more, three quarters or more, or four fifths or more of the length of the tubular portion. Moreover, the lengths of the branch spaces formed in the tubular portion may be, for example, the same as the length of the tubular portion.

(3e) In the above-described embodiment, each space formed between the second tubular member 5B and the first inner pipe member 6A and between the second tubular member 5B and the second inner pipe member 6B is closed at one end of the double-wall pipe and communicates with the exhaust flow path at the other end, thereby forming the first and second branch spaces 7A, 7B. However, the branch spaces may be formed in a different way.

For example, the branch space may be formed as in an exhaust pipe 11 with muffler function shown in FIG. 6. In the exhaust pipe 11 with muffler function, the space formed between the second tubular member 5B and an inner pipe member 6C is closed at both ends of the double-wall pipe and communicates with the exhaust flow path through communication holes 8 formed in the middle portion of the double-wall pipe. Thus, two branch spaces 7C, 7D are formed.

In FIG. 6, a group of perfectly circular holes is shown as the communication holes 8. However, the number of holes constituting the communication holes is not particularly limited. The shape of the holes is likewise not particularly limited, and may be, for example, an oval shape, a polygonal shape, a polygonal shape with round corners, or a star shape. The inner pipe member 6C is not visible from the outside of the exhaust pipe 11 with muffler function, but shown with solid lines in FIG. 6 to facilitate understanding.

(3f) In the above-described embodiment, the first inner pipe member 6A or the second inner pipe member 6B forms the double-wall pipe together with the second tubular member 5B. However, the tubular portion and the space forming portion may form a multiple-wall pipe with, for example, triple walls or more.

(3g) In the above-described embodiment, the first inner pipe member 6A or the second inner pipe member 6B forms the double-wall pipe together with the second tubular member 5B. However, the space forming portion does not have to form a multiple-wall pipe, such as a double-wall pipe, together with the tubular portion.

In other words, each of the first inner pipe member 6A and the second inner pipe member 6B is arranged inside the second tubular member 5B, covering the inner surface of the second tubular member 5B along the entire circumference in the direction around the axis of the second tubular member 5B. However, the space forming portion may be provided inside the tubular portion, covering the inner surface of the tubular portion along part of the circumference in the direction around the axis of the tubular portion.

(3h) In the above-described embodiment, each of the first and second inner pipe members 6A, 6B is provided along the inner surface of the second tubular member 5B. However, the space forming portion may be provided along the outer surface of the tubular portion. In this case, the space forming portion may be provided outside the tubular portion, covering the outer surface of the tubular portion along the entire circumference in the direction around the axis of the tubular portion. Alternatively, the space forming portion may be provided outside the tubular portion, covering the outer surface of the tubular portion along part of the circumference in the direction around the axis of the tubular portion.

(3i) In the above-described embodiment, out of the first, second, and third tubular members 5A, 5B, 5C, the longest member, namely the second tubular member 5B, is provided with the space forming portion. When the tubular member to be provided with the space forming portion is determined based on the length in this manner, the length of each tubular member is acquired as follows. Specifically, if the tubular member forms the exhaust flow path between the internal combustion engine and the exhaust part disposed at the uppermost stream position, the length of the tubular member is from the end of the tubular member connected to the internal combustion engine to the end connected to the exhaust part. If the tubular member forms the exhaust flow path between two exhaust parts, the length of the tubular member is from the end of the tubular member connected to one of the exhaust parts to the end connected to the other exhaust part. If the tubular member forms the exhaust flow path on the downstream side of the exhaust part disposed at the lowermost stream position, the length of the tubular member is from the end of the tubular member connected to the exhaust part to the end open to the outside of the exhaust system.

(3j) In the above-described embodiment, out of the first, second, and third tubular members 5A, 5B, 5C, the longest member, namely the second tubular member 5B, is provided with the space forming portion. However, the tubular member provided with the space forming portion is not limited to the longest tubular member out of multiple tubular members. That is, the tubular portion does not have to be the longest portion of the tubular body forming the exhaust flow path in the exhaust system.

(3k) In the above-described embodiment, the second tubular member 5B consists of a single component. However, the tubular portion may be composed of two or more components. For example, the tubular portion may be a member comprising two or more tubular components coupled to each other.

(3l) In the above-described embodiment, the first, second, and third tubular members 5A, 5B, 5C and the first and second inner pipe members 6A, 6B have perfectly circular cross-sections. However, the cross-sectional shapes of the tubular members and an inner pipe member are not particularly limited. The cross-sectional shapes of these components may be, for example, oval shapes, polygonal shapes, or polygonal shapes with round corners. Moreover, these cross-sectional shapes and the cross-sectional areas thereof may be variable.

(3m) In the above-described embodiment, the exhaust system 1 comprises the muffler 4; however, the muffler 4 may be optional.

(3n) In the above-described embodiment, the exhaust system 1 is used in the internal combustion engine 100. However, the exhaust system 1 may be used in internal combustion engines of, for example, water vessels, aircrafts, and general-purpose machines.

(3o) Functions of one component in the aforementioned embodiments may be achieved by two or more components, and a function of one component may be achieved by two or more components. Moreover, functions of two or more components may be achieved by one component, and a function achieved by two or more components may be achieved by one component. Some of the components of the above embodiments may be omitted. At least part of the configurations of the aforementioned embodiments may be added to or replaced with other configurations of the aforementioned embodiments.

EXHAUST PIPE

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