Patent Publication Number: US-11377990-B2

Title: Exhaust pipe

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims the benefit of Japanese Patent Application No. 2019-001929 filed on Jan. 9, 2019 with the Japan Patent Office, the entire disclosure of which is incorporated herein by reference. 
     BACKGROUND 
     The present disclosure relates to an exhaust pipe. 
     A known exhaust system for automobiles includes a sub-muffler provided between a catalyst, disposed upstream of a flow path of exhaust gases, and a main muffler, disposed downstream of the flow path of the exhaust gases. 
     For this sub-muffler, an exhaust pipe having a double pipe structure including an inner pipe and an outer pipe is used. Such an exhaust pipe exhibits a muffling effect due to a gap between the inner pipe and the outer pipe. In the exhaust pipe, hot exhaust gases flow inside the inner pipe, thereby causing a difference in thermal expansion between the inner pipe and the outer pipe. 
     To absorb the difference in thermal expansion between the inner pipe and the outer pipe, an exhaust pipe has been invented in which a ring-like holding member is disposed between the inner pipe and the outer pipe at one end of the double pipe (see, for example, Japanese Unexamined Patent Application Publication No. 2002-227642). 
     The holding member of the aforementioned exhaust pipe is slidably disposed relative to the inner pipe and the outer pipe, and thus is not fixed to neither of the inner pipe and the outer pipe. In the technique of the above-described publication, two projections are provided by pressing the inner pipe such that the holding member is interposed between the projections in order to inhibit the holding member from falling out from the ends of the double pipe. 
     SUMMARY 
     At an end of the above-described double pipe, exhaust gases that have flown past an end of the inner pipe spread into the outer pipe. Then, if the difference between the length of the inner pipe and that of the outer pipe in the radial direction is large, in other words, if the difference in level between the inner pipe and the outer pipe is large, a turbulent flow of the exhaust gases and air flow noises tend to be caused. 
     It is desirable that one aspect of the present disclosure provides an exhaust pipe with a double pipe structure that can reduce generation of the turbulent flow of exhaust gases. 
     One aspect of the present disclosure provides an exhaust pipe comprising a double pipe and a retention member. The double pipe comprises an inner pipe through which exhaust gases pass, and an outer pipe disposed so as to surround an outer circumferential surface of the inner pipe. The retention member is disposed in a gap provided between the outer circumferential surface of the inner pipe and an inner circumferential surface of the outer pipe. The retention member is disposed at at least one of a first end or a second end of the double pipe. 
     Moreover, at the at least one of the first end or the second end of the double pipe where the retention member is disposed, a radial clearance between the outer circumferential surface of the inner pipe and the inner circumferential surface of the outer pipe at an opening of the inner pipe is smaller than the radial clearance in an arrangement area where the retention member is disposed. 
     This structure can inhibit the retention member from falling off the end of the double pipe due to the clearance between the inner pipe and the outer pipe at the opening of the inner pipe being smaller than the radial clearance in the arrangement area. This structure can also reduce the difference in level in the radial direction between the inner pipe and the outer pipe at the end of the double pipe. Due to this structure, generation of the turbulent flow of the exhaust gases can be reduced while having the retention member in the double pipe. As a result, production of the air flow noises can be reduced. 
     In one aspect of the present disclosure, at the at least one of the first end or the second end of the double pipe where the retention member is disposed, an outer diameter of the inner pipe at the opening may be larger than an outer diameter of the inner pipe in the arrangement area. This structure can easily and reliably make the clearance at the opening of the inner pipe smaller than the radial clearance in the arrangement area. 
     In one aspect of the present disclosure, at the at least one of the first end or the second end of the double pipe where the retention member is disposed, an inner diameter of the outer pipe at a position where the outer pipe coexists with an opening of the inner pipe may be smaller than an inner diameter of the outer pipe in the arrangement area. This structure can also easily and reliably make the clearance at the opening of the inner pipe smaller than the radial clearance in the arrangement area. 
     In one aspect of the present disclosure, at the at least one of the first end or the second end of the double pipe where the retention member is disposed, the outer diameter of the inner pipe in the arrangement area may be larger than an outer diameter of the inner pipe in an area located inside relative to the arrangement area along an axis of the inner pipe. This structure can more reliably reduce generation of the turbulent flow of the exhaust gases. 
     In one aspect of the present disclosure, the retention member may be disposed at a downstream end of the double pipe in a flow direction of the exhaust gases. Resonance pipes may be formed on an upstream side of the double pipe in the flow direction of the exhaust gases. A resonance chamber may be formed between the retention member and the resonance pipes. 
     In one aspect of the present disclosure, the retention member may be disposed at an upstream end of the double pipe in a flow direction of the exhaust gases. Resonance pipes may be formed on a downstream side of the double pipe in the flow direction of the exhaust gases. A resonance chamber may be formed between the retention member and the resonance pipes. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Some embodiments of the present disclosure will be described hereinafter by way of example with reference to the accompanying drawings, in which: 
         FIG. 1  is a schematic plane showing an exhaust system of an embodiment; 
         FIG. 2A  is a schematic side view showing the exhaust pipe in  FIG. 1  from a second end side; 
         FIG. 2B  is a schematic sectional view taken along a line IIB-IIB in  FIG. 2A ; 
         FIG. 3  is a schematic sectional view taken along a line in  FIG. 2B ; 
         FIG. 4A  is a partially enlarged schematic sectional view showing a vicinity of a first end of the exhaust pipe in  FIG. 2B ; 
         FIG. 4B  is a partially enlarged schematic sectional view showing a vicinity of a first end of an exhaust pipe according to an embodiment that is different from the exhaust pipe in  FIG. 4A ; 
         FIG. 5A  is a partially enlarged schematic sectional view showing a vicinity of a first end of an exhaust pipe according to an embodiment that is different from the exhaust pipes in  FIGS. 4A and 4B ; 
         FIG. 5B  is a partially enlarged schematic sectional view showing a vicinity of a first end of an exhaust pipe according to an embodiment that is different from the exhaust pipes in  FIGS. 4A, 4B, and 5A ; 
         FIG. 5C  is a partially enlarged schematic sectional view showing a vicinity of a first end of an exhaust pipe according to an embodiment that is different from the exhaust pipes in  FIGS. 4A, 4B, 5A, and 5B ; 
         FIG. 5D  is a partially enlarged schematic sectional view showing a vicinity of a first end of an exhaust pipe according to an embodiment that is different from the exhaust pipes in  FIGS. 4A, 4B, 5A, 5B, and 5C ; 
         FIG. 6A  is a partially enlarged schematic sectional view showing a vicinity of a first end of an exhaust pipe according to an embodiment that is different from the exhaust pipes in  FIGS. 4A, 4B, 5A, 5B, 5C, and 5D ; 
         FIG. 6B  is a partially enlarged schematic sectional view showing a vicinity of a first end of an exhaust pipe according to an embodiment that is different from the exhaust pipes in  FIGS. 4A, 4B, 5A, 5B, 5C, 5D and 6A ; 
         FIG. 6C  is a partially enlarged schematic sectional view showing a vicinity of a first end of an exhaust pipe according to an embodiment that is different from the exhaust pipes in  FIGS. 4A, 4B, 5A, 5B, 5C, 5D, 6A, and 6B ; 
         FIG. 6D  is a partially enlarged schematic sectional view showing a vicinity of a first end of an exhaust pipe according to an embodiment that is different from the exhaust pipes in  FIGS. 4A, 4B, 5A, 5B, 5C, 5D, 6A, 6B, and 6C ; 
         FIG. 6E  is a partially enlarged schematic sectional view showing a vicinity of a first end of an exhaust pipe according to an embodiment that is different from the exhaust pipes in  FIGS. 4A, 4B, 5A, 5B, 5C, 5D, 6A, 6B, 6C, and 6D ; 
         FIG. 7  is a schematic sectional view showing an exhaust pipe according to an embodiment that is different from the exhaust pipe shown in  FIGS. 2A and 2B ; 
         FIG. 8  is a schematic sectional view showing an exhaust pipe according to an embodiment that is different from the exhaust pipes shown in  FIGS. 2B, and 7 ; and 
         FIG. 9  is a schematic sectional view showing an exhaust pipe according to an embodiment that is different from the exhaust pipes shown in  FIGS. 2B, 7 , and  8 . 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     1. First Embodiment 
     [1-1. Structure] 
     An exhaust system  1  shown in  FIG. 1  forms an exhaust flow passage of an internal combustion engine. The exhaust system  1  comprises a catalyst converter  2 , an exhaust pipe  3  which is a sub-muffler, and a main muffler  4 . 
     The internal combustion engine in which the exhaust system  1  is employed is not limited to a specific type. Examples of the internal combustion engine include those used for transportation vehicles, such as automobiles, railroad vehicles, ships, and construction machines, and those used for power generation facilities for driving purpose or power generation purpose. 
     The catalyst converter  2  is configured to reform or collect environmental contaminants in the exhaust gases. The catalyst converter  2  includes, for example, a catalyst. The main muffler  4  is configured to further reduce noises produced by the exhaust gases passing through the exhaust pipe  3 . 
     The catalyst converter  2  and the exhaust pipe  3  are connected by a first pipe  5 A. The exhaust pipe  3  and the main muffler  4  are connected by a second pipe  5 B. The exhaust gases that have passed through the main muffler  4  is discharged from a third pipe  5 C. 
     The exhaust pipe  3  serves as a muffler in the exhaust system  1 . 
     As shown in  FIGS. 2A and 2B , the exhaust pipe  3  comprises a double pipe  11  and a retention member  12 . 
     &lt;Double Pipe&gt; 
     The double pipe  11  comprises an inner pipe  7 , an outer pipe  8 , projections  9 , and a gap  10 . 
     &lt;&lt;Inner Pipe&gt;&gt; 
     The inner pipe  7  is a metal pipe in which the exhaust gases pass through. Specifically, the exhaust gases that have passed through the catalyst converter  2  are introduced into the inner pipe  7  from one of a first opening  71  and a second opening  72 , and discharged from the opening on the opposite side. 
     In the vicinity of the first opening  71  of the inner pipe  7 , the projections  9 , which will be described below, are formed. The inner diameter of the first opening  71  of the inner pipe  7  is larger than the diameter of the inner pipe  7  in an arrangement area where the retention member  12 , which will be described below, is disposed. 
     At the second opening  72  of the inner pipe  7 , a fixed portion  72 A is provided so as to be fixed to the inner circumferential surface of the outer pipe  8 . The fixed portion  72 A includes two concave portions  72 B,  72 C formed by a part of the wall of the inner pipe  7  being inwardly depressed. The concave portions  72 B,  72 C form openings that make the gap  10  and the second opening  72  of the inner pipe  7  communicated. 
     In other words, a part of the fixed portion  72 A in the circumferential direction is spaced apart from the inner circumferential surface of the outer pipe  8 . Moreover, the fixed portion  72 A closes the gap  10  in the axial direction of the inner pipe  7  by means of a part of the fixed portion  72 A excluding the concave portions  72 B,  72 C. 
     &lt;&lt;Outer Pipe&gt;&gt; 
     The outer pipe  8  is a metal pipe disposed to surround the outer circumferential surface of the inner pipe  7 . The inner diameter of the outer pipe  8  is larger than the outer diameter of the inner pipe  7 . 
     A first end  81  of the outer pipe  8  surrounds the first opening  71  of the inner pipe  7  and the projections  9 . The retention member  12 , which will be described below, is disposed inside of the first end  81 . The first end  81  extends to the outside of the inner pipe  7  in the axial direction of the inner pipe  7  away from the longitudinal center of the inner pipe  7 . The first end  81  forms a first end  11 A of the double pipe  11 . 
     A second end  82  of the outer pipe  8  surrounds the second opening  72  of the inner pipe  7 . The second end  82  is joined with the outer circumferential surface of the inner pipe  7  (specifically, with the concave portions  72 B,  72 C) by, for example, welding. The second end  82  extends to the outside of the inner pipe  7  in the axial direction of the inner pipe  7 . The second end  82  forms a second end  11 B of the double pipe  11 . 
     In the present embodiment, the outer pipe  8  is a straight pipe having a constant diameter. In other words, the inner diameter of the first end  81  of the outer pipe  8  and the inner diameter of the second end  82  are the same. Moreover, the central axis of the outer pipe  8  corresponds to the central axis of the inner pipe  7 . Nevertheless, these central axes do not have to be coaxial. 
     &lt;&lt;Gap&gt;&gt; 
     The gap  10  is formed between the outer circumferential surface of the inner pipe  7  and the inner circumferential surface of the outer pipe  8 . The gap  10  is a space defined by the outer circumferential surface of the inner pipe  7 , the inner circumferential surface of the outer pipe  8 , the fixed portion  72 A, and the retention member  12 . 
     The gap  10  comprises a resonance chamber  10 A and two resonance pipes  10 B. The resonance chamber  10 A is formed between a part of the outer circumferential surface of the inner pipe  7  excluding the fixed portion  72 A (in other words, a part of the double pipe  11  excluding the second end  11 B) and the inner circumferential surface of the outer pipe  8 . The two resonance pipes  10 B are respectively formed between the concave portion  72 B of the inner pipe  7  and the inner circumferential surface of the outer pipe, and between the concave portion  72 C of the inner pipe  7  and the inner circumferential surface of the outer pipe. 
     The resonance pipes  10 B communicate with the exhaust flow passage in the inner pipe  7 , and the resonance chamber  10 A communicates with the exhaust flow passage via the resonance pipes  10 B, which makes the resonance pipes  10 B and the resonance chamber  10 A serve as a Helmholtz resonator. 
     &lt;&lt;Projections&gt;&gt; 
     The projections  9  are formed at a position on the inner pipe  7  located inside relative to the position of the retention member  12  in the axial direction toward the longitudinal center of the inner pipe  7 , and protrude radially outwardly from the outer circumferential surface of the inner pipe  7 . The projections  9  restrict inward movement of the retention member  12  in the axial direction of the inner pipe  7 . 
     As shown in  FIG. 3 , the exhaust pipe  3  comprises at least one projection  9 . In a case where there is more than one projection, the projections  9  are spaced apart from each other in the circumferential direction.  FIG. 3  shows an example in which six projections  9  are equidistantly disposed in the circumferential direction; nevertheless, the number of the projections  9  is not limited to six. Moreover, the intervals between two or more projections  9  do not have to be equal. Furthermore, the projections  9  may be wide in the axial direction of the double pipe  11 . In other words, the projections  9  may extend along the axis of the double pipe  11 . The projections  9  may have rounded shapes such as hemispheres, or may have angular shapes such as parallelepipeds. 
     &lt;Retention Member&gt; 
     As shown in  FIG. 2B , the retention member  12  is disposed in the gap  10  at the first end  11 A of the double pipe  11 . Specifically, the retention member  12  is inserted between the outer circumferential surface of the inner pipe  7  and the inner circumferential surface of the outer pipe  8 , but is not fixed to the inner pipe  7  and the outer pipe  8 . 
     The retention member  12  is disposed entirely along the outer circumferential surface of the inner pipe  7  and the inner circumferential surface of the outer pipe  8  in the circumferential direction. In other words, the retention member  12  is disposed so as to substantially block the space between the inner pipe  7  and the outer pipe  8  in the axial direction of the inner pipe  7 . To form a portion of the wall of the resonance chamber  10 A, it is desirable that the retention member  12  is formed in a ring-like shape that can reduce the gap between the inner pipe  7  and the outer pipe  8 . The retention member  12  may be provided with an opening/openings in a portion thereof in the circumferential direction to the extent that the function of the resonance chamber  10 A is not impaired. 
     The retention member  12  is only required to be able to define the gap  10 , that is, the resonance chamber  10 A, and to be slidable relative to at least one of the inner pipe  7  or the outer pipe  8 . Thus, the retention member  12  is not limited to a particular member. It is desirable that the retention member  12  is not breathable, but may be breathable to the extent that the function of the resonance chamber  10 A is not impaired. The retention member  12  is preferably a metal wire mesh, for example. Due to the retention member  12  being slidably disposed in the space between the inner pipe  7  and the outer pipe  8 , stress produced by the difference in thermal expansion between the inner pipe  7  and the outer pipe  8  is reduced. 
     The exhaust pipe  3  may be connected to the first pipe  5 A at the first end  11 A of the double pipe  11 , that is, the first end  81  of the outer pipe  8 , or may be connected to the first pipe  5 A at the second end  11 B of the double pipe  11 , that is, the second end  82  of the outer pipe  8 . In other words, the retention member  12  may be disposed at a downstream end of the double pipe  11  in the flow direction of the exhaust gases, or may be disposed at an upstream end of the double pipe  11  in the flow direction of the exhaust gases. Accordingly, a portion of the first end  81  that extends to the outside of the inner pipe  7  in the axial direction of the inner pipe  7  is connected to the first pipe  5 A or the second pipe  5 B (see  FIG. 4A ). 
     &lt;Clearance Between Inner Pipe and Outer Pipe&gt; 
     As shown in  FIG. 4A , at the first end  11 A where the retention member  12  of the double pipe  11  is disposed, a radial first clearance D 1  is provided along the first opening  71  of the inner pipe  7  between the outer circumferential surface of the inner pipe  7  and the inner circumferential surface of the outer pipe  8 . The first clearance D 1  is smaller than a radial second clearance D 2  in an arrangement area  11 C where the retention member  12  is disposed. 
     In other words, the first clearance D 1  is smaller than the thickness of the retention member  12  in the radial direction of the double pipe  11 . This inhibits the retention member  12  from falling off the first end  11 A of the double pipe  11 . 
     In the present embodiment, at the first end  11 A, the outer diameter of the inner pipe  7  at the first opening  71  is larger than the outer diameter of the inner pipe  7  in the arrangement area  11 C where the retention member  12  is disposed. In the arrangement area  11 C, the outer diameter of the inner pipe  7  and the inner diameter of the outer pipe  8  are constant. 
     In the present embodiment, the diameter of the inner pipe  7  is increased on the outer side of the arrangement area  11 C of the first end  11 A, thereby making the first clearance D 1  smaller than the second clearance D 2 . The inner pipe  7  further comprises a straight portion  7 A and an enlarged diameter portion  7 B. The straight portion  7 A extends parallel to the outer pipe  8  after the increase in diameter of inner pipe  7  and reaches the first opening  71 . The enlarged diameter portion  7 B is formed between the arrangement area  11 C and the straight portion  7 A. It is desirable that the enlarged diameter portion  7 B is shaped such that the diameter thereof is gradually increased toward the straight portion  7 A, but may be shaped so as to be bent in a step-by-step manner and connected to the straight portion  7 A. 
     To increase the diameter of the inner pipe  7 , an outer die and a conical center die can be used, for example. The outer die includes several separate pieces formed by circumferentially dividing a cylindrical body, which has a constant outer diameter and an inner diameter reduced along the axial direction. First, the outer die is inserted into the inner pipe  7  in the axial direction, and the center die is inserted into a hollow portion of the inserted outer die in the axial direction from the small-diameter side. The inner pipe  7  is thereby expanded radially outward so as to form the straight portion  7 A and the enlarged diameter portion  7 B. 
     If at least one projection is provided on the outer circumferential surfaces of the divided pieces, at least one projection  9  can be concurrently formed on the inner pipe  7  when the inner pipe  7  is expanded. 
     [1-2. Operation] 
     In a case where the first end  11 A of the double pipe  11  is located on the upstream side of the exhaust pipe  3 , the exhaust gases flowing from the first pipe  5 A to the double pipe  11  enter the inner pipe  7  and the gap between the inner pipe  7  and the outer pipe  8 . By making the first clearance D 1  between the inner pipe  7  and the outer pipe  8  small, the exhaust gases tend to flow into the inner pipe  7  rather than into the gap between the inner pipe  7  and the outer pipe  8 . 
     The exhaust gases flowing in the inner pipe  7  pass through the straight portion  7 A and flow to the enlarged diameter portion  7 B. The exhaust gases passing through the enlarged diameter portion  7 B is facilitated to flow along the shape of the enlarged diameter portion  7 B toward radially inside of the inner pipe  7 , in other words, toward the axial center of the inner pipe  7 . Accordingly, the exhaust gases pass through the inside of the inner pipe  7 , and flow from the second opening  72  of the inner pipe  7  to the second pipe  5 B through the second end  82  of the outer pipe  8 . 
     At the second end  11 B located on the downstream side of the double pipe  11 , the resonance pipes  10 B respectively having openings on the downstream side are formed, and the resonance chamber  10 A coupled with the openings of the resonance pipes  10 B on the upstream side is further formed. Thus, noises are muffled due to the Helmholtz resonance. 
     At the first end  11 A of the double pipe  11 , the exhaust gases cannot easily enter the gap between the inner pipe  7  and the outer pipe  8 , as described above, because the first clearance D 1  between the inner pipe  7  and the outer pipe  8  is smaller than the second clearance D 2 . The exhaust gases, therefore, are less likely to contact the retention member  12 . 
     On the other hand, in a case where the first end  11 A of the double pipe  11  is located on the downstream side of the exhaust pipe  3 , the exhaust gases flowing from the first pipe  5 A to the double pipe  11  enter the inside of the inner pipe  7  and the gap between the inner pipe  7  and the outer pipe  8 , that is, the resonance pipes  10 B. Due to the resonance pipes  10 B being formed in portions of the inner pipe  7  and the outer pipe  8  in the circumferential direction, the cross-sections of the resonance pipes  10 B are narrower than other areas of the double pipe  11 . The exhaust gases, thus, tend to flow into the inner pipe  7  rather than into the resonance pipes  10 B. 
     The exhaust gases flowing in the inner pipe  7  spread radially outward in the enlarged diameter portion  7 B of the inner pipe  7  along the shape of the enlarged diameter portion  7 B, and flow toward the downstream side of the straight portion  7 A. On the downstream side of the straight portion  7 A and at the first end  81  of the outer pipe  8 , the exhaust gases spread radially outward and flow toward the second pipe  5 B. On the other hand, the exhaust gases flowing through the resonance pipes  10 B enter the resonance chamber  10 A. 
     The resonance pipes  10 B are formed on the upstream side of the double pipe  11 , and the resonance chamber  10 A coupled with the openings on the downstream side of the resonance pipes  10 B is formed. Thus, noises are muffled due to the Helmholtz resonance. 
     [1-3. Effect] 
     The following effects are achieved by the embodiment described in detail hereinabove. 
     ( 1   a ) Due to the first clearance D 1  between the inner pipe  7  and the outer pipe  8  at the first opening  71  of the inner pipe  7  being smaller than the second clearance D 2  in the arrangement area  11 C where the retention member  12  is disposed, the retention member  12  can be inhibited from falling off the first end  11 A of the double pipe  11 . This structure can also reduce the difference in level in the radial direction between the inner pipe  7  and the outer pipe  8  at the first end  11 A of the double pipe  11 . Accordingly, while having the retention member  12  in the double pipe  11 , generation of the turbulent flow of the exhaust gases can be reduced, which in turn reduces the production of the air flow noises. 
     ( 1   b ) In a case where the first end  11 A of the double pipe  11  is located on the downstream side of the exhaust pipe  3 , the exhaust gases that have flown through the inner pipe  7  spread in accordance with the size of the inner diameter at the end of the inner pipe  7  and also spread inside of the first end  81  of the outer pipe  8 . Thus, the exhaust gases are unlikely to accumulate around the first opening  71 , which in turn limits an increase in pressure loss of the exhaust gases. 
     ( 1   c ) Due to the first clearance D 1  between the inner pipe  7  and the outer pipe  8  at the first opening  71  of the inner pipe  7  being smaller than the second clearance D 2  in the arrangement area  11 C where the retention member  12  is arranged, the exhaust gases are less likely to strike the retention member  12 . As a result, deterioration of the retention member  12  can be inhibited. 
     ( 1   d ) The resonance pipes  10 B are formed in the double pipe  11 , and the resonance chamber  10 A is formed between the retention member  12  and the resonance pipes  10 B. Thus, generation of the turbulent flow of the exhaust gases can be reduced in the double pipe  11  that comprises the Helmholtz resonator. Moreover, the enlarged diameter portion  7 B located at the end of the inner pipe  7  serves also as a stopper that limits the outward movement of the retention member  12 , which forms a portion of the wall of the resonance chamber  10 A, in the axial direction of the inner pipe  7 . 
     2. Second Embodiment 
     [2-1. Structure] 
     An exhaust pipe according to a second embodiment has the same structure as that of the exhaust pipe  3  according to the first embodiment, except for the structure of the first end  11 A. 
     Similarly to the first embodiment, at the first end  11 A in the second embodiment, the first clearance D 1  between the inner pipe  7  and the outer pipe  8  at the first opening  71  of the inner pipe  7  is smaller than the second clearance D 2  in the arrangement area  11 C where the retention member  12  is disposed. 
     Moreover, at the first end  11 A, a third clearance D 3  is larger than the second clearance D 2  in the arrangement area  11 C as shown in  FIG. 4B . The third clearance D 3  is located between the inner pipe  7  and the outer pipe  8  in an inside area  11 D located inside relative to the arrangement area  11 C where the retention member  12  is disposed. 
     Specifically, the outer diameter of the inner pipe  7  in the arrangement area  11 C is larger than the outer diameter of the inner pipe  7  in the inside area  11 D. In other words, the diameter of the inner pipe  7  increases from the inside area  11 D toward the arrangement area  11 C and further increases from the arrangement area  11 C toward the first opening  71 . 
     The inner diameter of the outer pipe  8  in the arrangement area  11 C is smaller than the inner diameter of the outer pipe  8  in the inside area  11 D. In other words, the diameter of the outer pipe  8  decreases from the inside area  11 D toward the arrangement area  11 C. 
     [2-2. Effect] 
     The following effects are achieved by the embodiment described in detail hereinabove. 
     ( 2   a ) The multi-step increase in diameter of the inner pipe  7  enables reduction of the turbulent flow generated at each enlarged diameter portion, which in turn enables more reliable reduction of the turbulent flow of the exhaust gases generated in the entire double pipe  11 . 
     ( 2   b ) The increase in diameter of the inner pipe  7  in the arrangement area  11 C can inhibit the inner pipe  7  from being crushed (in other words, flattened) when the projections  9  are formed. Moreover, due to the reduction in diameter of the outer pipe  8  in the arrangement area  11 C, the gap between the retention member  12  and the inner pipe  7  and the outer pipe  8  can be narrowed. 
     3. Other Embodiments 
     The embodiments of the present disclosure have been described hereinabove. Nevertheless, it goes without saying that the present disclosure is not limited to the aforementioned embodiments and may be embodied in various forms. 
     ( 3   a ) As shown in  FIG. 5A , the exhaust pipe  3  according to the aforementioned embodiments may be configured such that the end of the inner pipe  7  including the first opening  71  is formed into a flared shape. In other words, the inner pipe  7  may be configured without any straight portion that extends in parallel to the outer pipe  8  after an increase of the diameter, and may be formed in a shape in which the cross-sectional area of the inner pipe  7  increases toward the edge of the end portion. The flared shape can be formed by pressing the end of the inner pipe  7  in the axial direction. 
     ( 3   b ) As shown in  FIG. 5B , the exhaust pipe  3  according to the aforementioned embodiments may be configured such that the inner diameter of the outer pipe  8  in an area of the double pipe  11  in which the outer pipe  8  coexists with the first opening  71  of the inner pipe  7  is smaller than the inner diameter of the outer pipe  8  in the arrangement area  11 C. 
     In other words, the diameter of the outer pipe  8  may be reduced on the outside relative to the arrangement area  11 C in the first end  11 A. Alternatively, as shown in  FIG. 5C , a combination of the increase of the diameter of the inner pipe  7  and the reduction of the diameter of the outer pipe  8  may be employed. In the examples shown in  FIGS. 5B and 5C , the amount of increase in diameter of the inner pipe  7 , in other words, change in cross-sectional area of the inner pipe  7  is none or small. This can further reduce the turbulent flow. 
     ( 3   c ) As shown in  FIG. 5D , the exhaust pipe  3  according to the aforementioned embodiments may be configured such that the diameter of the outer pipe  8  is increased at the first end  11 A to the extent where the first clearance D 1  is kept smaller than the second clearance D 2 . The increase in diameter of the outer pipe  8  increases the diameter of a joint portion of a pipe to be connected to the first end  11 A by, for example, welding, which in turn enhances the joint strength between the pipes. 
     ( 3   d ) As shown in  FIGS. 6A-6E , the exhaust pipe  3  according to the aforementioned embodiments may be configured such that the projections  9  protrude radially inwardly from the inner circumferential surface of the outer pipe  8 . Moreover, the projections  9  may be provided to both of the inner pipe  7  and the outer pipe  8 . 
     ( 3   e ) In the exhaust pipe  3  according to the aforementioned embodiments, the resonance pipes  10 B do not have to be formed. For example, in an exhaust pipe  103  shown in  FIG. 7 , the outer diameter of an inner pipe  107  at a second end  111 B of a double pipe  111  is uniform. In other words, the inner pipe  107  does not have, at a second opening  172 , the fixed portion  72 A (specifically, the concave portions  72 B,  72 C) that is fixed to the inner circumferential surface of the outer pipe  8 . In the double pipe  111 , the resonance chamber  10 A is directly led to the exhaust flow passage in the inner pipe  107 . 
     For another example, as shown in an exhaust pipe  203  in  FIG. 8 , the entire circumference of a second end  282  of an outer pipe  208  at a second end  211 B of a double pipe  211  may be welded to an inner pipe  207 . The inner pipe  207  does not have the concave portions  72 B,  72 C at a second opening  272 . In the double pipe  111  in  FIG. 7  and the double pipe  211  in  FIG. 8 , the resonance chamber  10 A serves as a side branch. 
     ( 3   f ) In the exhaust pipe  3  according to the aforementioned embodiments, the inner pipe  7  may be provided with one or more communication hole(s). For example, an exhaust pipe  303  shown in  FIG. 9  comprises an inner pipe  307  of a double pipe  311  provided with communication holes  73 A,  73 B for communication between the gap  10  and the inside of the inner pipe  307 . 
     The communication holes  73 A,  73 B are provided at positions spaced apart from each other in the axial direction (in other words, in the longitudinal direction) of the inner pipe  307 . Moreover, the communication holes  73 A,  73 B are located between the first end  81  and the second end  82  of the outer pipe  8  in the axial direction of the inner pipe  307 . 
     The communication holes  73 A,  73 B are only required to have enough surface areas for the side branch type muffler to serve its function, but the shapes thereof are not limited to perfect circles. The shapes of the communication holes  73 A,  73 B may be, for example, ellipses, polygons, rounded polygons, and stars. Moreover, the communication holes  73 A,  73 B may each comprise separate small holes (in other words, a collection of small holes). 
     In a case where air column resonance occurs in a second exhaust flow passage (in other words, an exhaust flow passage in the entire exhaust system  1  in  FIG. 1 ) formed by the components of the exhaust flow passage including the exhaust pipe  303 , the communication holes  73 A,  73 B are provided at positions corresponding to the positions of antinodes of the standing wave produce in the second exhaust flow passage. 
     Moreover, in the exhaust pipe  103  in  FIG. 7  or the exhaust pipe  203  in  FIG. 8 , the communication holes  73 A,  73 B may be provided to the inner pipe  107  or the inner pipe  207 . 
     ( 3   g ) The retention member  12  may be provided to at both of the first end  11 A and the second end  11 B of the double pipe  11 . In this case, it is desirable that the first clearance D 1  is smaller than the second clearance D 2  at both of the first end  11 A and the second end  11 B. 
     ( 3   h ) In the exhaust pipe  3  according to the aforementioned embodiments, the double pipe  11  does not have to have the projections  9 . 
     ( 3   i ) Functions of one component in the aforementioned embodiments may be distributed to two or more components. Functions of two or more components may be integrated and achieved by one component. A part of the structures of the aforementioned embodiments may be omitted. At least a part of the structures of the aforementioned embodiments may be added to or replaced with other structures of another one of the aforementioned embodiments. It should be noted that any and all modes that are encompassed in the technical ideas identified by the languages in the claims are embodiments of the present disclosure.