Patent Publication Number: US-2019195099-A1

Title: Exhaust device of engine

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to Japanese Patent Application No. 2017-251533 filed on Dec. 27, 2017, which is incorporated herein by reference in its entirety. 
     BACKGROUND 
     Technical Field 
     The present invention relates to an exhaust device of an engine provided in a vehicle or the like. 
     Related Art 
     Generally, a vehicle or the like provided with an engine includes an exhaust pipe through which an exhaust gas from the engine flows and a muffler provided on a downstream side of the exhaust pipe. The muffler has a function to reduce exhaust noise. That is, a communication path for causing a plurality of expansion chambers and expansion chambers to communicate with one another is formed in a casing of the muffler. The exhaust gas discharged from the engine and flowing into the muffler via the exhaust pipe is expanded and attenuated in stages in the process of passing through the expansion chambers and the communication path. As a result, the exhaust noise is reduced (see, for example, JP H1-290913 A below). 
     SUMMARY 
     Enhancement of the muffler exhaust noise reduction effect is demanded with recent strengthening of the noise regulation. The muffler exhaust noise reduction effect can be enhanced with a structure in which the number of expansion chambers of the muffler is increased and the expansion and attenuation of the exhaust gas is repeated many times. However, if the number of expansion chambers is increased, the muffler is increased in size. For example, to downsize the vehicle or the like, the increase in size of the muffler is not desirable. 
     Further, in general, an effect to suppress a decrease in engine output can be enhanced by increasing a volume of a first-stage expansion chamber connected to an exhaust pipe in a muffler. To secure a desired engine output, increasing the volume of the first-stage expansion chamber in the muffler is demanded. However, if the volume of the first-stage expansion chamber is increased, the muffler is increased in size. 
     The present invention has been made in view of such a problem, and an objective of the present invention is to provide an exhaust device of an engine, the exhaust device being capable of enhancing the exhaust noise reduction effect of a muffler and suppressing a decrease in engine output while suppressing an increase in size of the muffler. 
     To solve the above problem, the present invention provides an exhaust device of an engine, the exhaust device including: an exhaust pipe through which an exhaust gas from the engine flows; and a muffler provided on a downstream side of the exhaust pipe, the muffler including a casing, a first expansion chamber, a second expansion chamber, and a third expansion chamber separately formed in the casing, and a first communication pipe, a second communication pipe, and a third communication pipe provided in the casing, wherein the exhaust pipe is connected to the first expansion chamber, the first expansion chamber communicates with the third expansion chamber via the first communication pipe, the first expansion chamber communicates with the second expansion chamber via the second communication pipe, the second expansion chamber communicates with, via the third communication pipe, another chamber separately formed from any of the first expansion chamber, the second expansion chamber, and the third expansion chamber outside or inside the casing, and the third expansion chamber does not communicate with an outside of the third expansion chamber except that the third expansion chamber communicates with the first expansion chamber via the first communication pipe. 
     According to the present invention, the exhaust noise reduction effect of the muffler can be enhanced and the decrease in engine output can be suppressed while the increase in size of the muffler can be suppressed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is an explanatory diagram illustrating a motorcycle provided with an exhaust device of a first embodiment of the present invention; 
         FIG. 2  is an external view illustrating a muffler and the like in the exhaust device of the first embodiment of the present invention; 
         FIG. 3  is a sectional view illustrating the muffler as viewed from the direction of arrows in  FIG. 2 ; 
         FIG. 4  is a sectional view illustrating the muffler as viewed from the direction of arrows IV-IV in  FIG. 3 ; 
         FIG. 5  is a sectional view illustrating the muffler as viewed from the direction of arrows V-V in  FIG. 3 ; 
         FIG. 6  is an explanatory view schematically illustrating a structure of the muffler in the exhaust device of the first embodiment of the present invention; 
         FIG. 7  is an external view illustrating a muffler and the like in an exhaust device of a second embodiment of the present invention; 
         FIG. 8  is a sectional view illustrating the muffler as viewed from the directions of arrows VIII-VI It in  FIG. 7 ; 
         FIG. 9  is a sectional view illustrating the muffler as viewed from the direction of arrows IX-IX in  FIG. 8 ; 
         FIG. 10  is a sectional view illustrating the muffler as viewed from the direction of arrows X-X in  FIG. 8 ; and 
         FIG. 11  is a sectional view illustrating the muffler as viewed from the direction of arrows XI-XI in  FIG. 8 . 
     
    
    
     DETAILED DESCRIPTION 
     An exhaust device according to an embodiment of the present invention includes an exhaust pipe through which an exhaust gas from an engine flows, and a muffler provided on a downstream side of the exhaust pipe. The muffler includes a casing, a first expansion chamber, a second expansion chamber, and a third expansion chamber separately formed in the casing, and a first communication pipe, a second communication pipe, and a third communication pipe provided in the casing. 
     The exhaust pipe is connected to the first expansion chamber. The first expansion chamber communicates with the third expansion chamber via the first communication pipe, and communicates with the second expansion chamber via the second communication pipe. The second expansion chamber communicates with an outside of the casing via the third communication pipe. Note that, in the casing, another chamber may be separately formed from any of the first expansion chamber, the second expansion chamber, and the third expansion chamber, and the second expansion chamber may communicate with the another chamber, instead of the outside of the casing, via the third communication pipe. The third expansion chamber does not communicate with an outside of the third expansion chamber except that the third expansion chamber communicates with the first expansion chamber via the first communication pipe. 
     The muffler of the exhaust device of the present embodiment has the structure in which the exhaust pipe is connected to the first expansion chamber, the first expansion chamber communicates with the third expansion chamber via the first communication pipe, and the third expansion chamber does not communicate with the outside of the third expansion chamber except that the third expansion chamber communicates with the first expansion chamber via the first communication pipe, whereby the first expansion chamber, the third expansion chamber, and the first communication pipe as a whole can be caused to function as a first-stage expansion chamber to which the exhaust pipe is connected. 
     As described above, a decrease in engine output can be enhanced by increasing the volume of the first-stage expansion chamber to which the exhaust pipe is connected in the muffler. In the muffler of the exhaust device of the present embodiment, a total of the volumes of the first expansion chamber, the third expansion chamber, and the first communication pipe is the volume of the first-stage expansion chamber. Therefore, the volume of the first-stage expansion chamber can be increased on the whole by increasing the volume of the first expansion chamber, increasing the volume of the third expansion chamber, or increasing the volume of the first communication pipe (increasing the length of the first communication pipe, for example). The first expansion chamber, the third expansion chamber, and the first communication pipe can be dispersed in the casing of the muffler in such a manner that the first expansion chamber is arranged in the rear in the casing, and the third expansion chamber is arranged in the front in the casing. Therefore, the volume of the first-stage expansion chamber in the muffler of the present embodiment can be increased while suppressing the increase in size of the muffler on the whole by selecting one or ones of the first expansion chamber, the third expansion chamber, and the first communication pipe, the volume or volumes of which can be increased, while suppressing the increase in size of the muffler, and increasing the volume or volumes, while taking the structural restrictions of the muffler into consideration, for example. Therefore, both the suppression of the increase in size of the muffler and the suppression of the decrease in engine output can be achieved. 
     Further, in the structure in which the first expansion chamber and the third expansion chamber are connected with the first communication pipe, the exhaust gas is attenuated when flowing in from the first expansion chamber or the third expansion chamber to the first communication pipe, and is expanded when flowing out from the first communication pipe to the third expansion chamber or the first expansion chamber. In this manner, the exhaust noise reduction effect can be enhanced with the structure that functions as the first-stage expansion chamber in the muffler of the present embodiment. Therefore, the exhaust noise reduction effect can be sufficiently enhanced by simply providing the second expansion chamber in addition to the first expansion chamber and the third expansion chamber that function as the first-stage expansion chamber, and a large number of expansion chambers is not necessary to obtain high exhaust noise reduction effect. Therefore, both the suppression of the increase in size of the muffler and the improvement of the exhaust noise reduction effect can be achieved. 
     First Embodiment 
       FIG. 1  illustrates a motorcycle  1  provided with an exhaust device  8  of a first embodiment of the present invention. Note that arrows respectively representing the front (F), rear (B), up (U), down (D), right (R), and left (L) are drawn as needed at the lower right in each of the drawings. When a direction is described in each embodiment, the direction follows these arrows. 
     In  FIG. 1 , a front wheel  4  is rotatably supported on a front portion of a vehicle body frame  2  of the motorcycle  1  via a front fork  3 . Further, a rear wheel  6  is rotatably supported on a rear portion of the vehicle body frame  2  via a swing arm  5 . Further, an engine  7  is supported on an intermediate portion in a front-rear direction of the vehicle body frame  2 . The exhaust device  8  of the present embodiment is provided in a portion from a lower side of the engine  7  to a right side of the rear wheel  6  in the motorcycle  1 . 
     The exhaust device  8  is a device that discharges an exhaust gas of the engine  7  to the atmosphere. The exhaust device  8  includes a front exhaust pipe  11 , a chamber  12 , a catalyst device  13 , a rear exhaust pipe  14 , and a muffler  15 . 
     The front exhaust pipe  11  is a metal-made pipe connecting an exhaust port of the engine  7  and the chamber  12 , and sends the exhaust gas discharged through the exhaust port to the chamber  12 . The chamber  12  is provided on a lower side of the engine  7 . An expansion chamber is formed in the chamber  12 , and the exhaust gas sent from the front exhaust pipe  11  flows through this expansion chamber. The catalyst device  13  is a device that reduces harmful substances contained in the exhaust gas, and is provided on an upstream side or a downstream side of the expansion chamber in the chamber  12 , for example. The rear exhaust pipe  14  is a metal-made pipe connecting the chamber  12  and the muffler  15 , and sends the exhaust gas flowing out of the chamber  12  to the muffler  15 . The muffler  15  is provided on a right side of the rear wheel  6 . As will be described below, the exhaust gas sent from the rear exhaust pipe  14  flows through a plurality of expansion chambers provided in the muffler  15  and is then discharged to the atmosphere. In the exhaust device  8  of the present embodiment, exhaust noise is reduced by the chamber  12  and the muffler  15 . 
       FIG. 2  illustrates a downstream side of the rear exhaust pipe  14  and the muffler  15  in the exhaust device  8 .  FIG. 3  illustrates a cross section of the muffler  15  as viewed from the direction of arrows in  FIG. 2 .  FIG. 4  illustrates a longitudinal section of the muffler  15  as viewed from the direction of arrows IV-IV in  FIG. 3 , and  FIG. 5  illustrates a longitudinal section of the muffler  15  as viewed from the direction of arrows V-V in  FIG. 3 . 
     As illustrated in  FIGS. 2 to 5 , the muffler  15  includes a casing  21 , a first expansion chamber R 1 , a second expansion chamber R 2 , and a third expansion chamber R 3  separately formed in the casing  21 , and a first communication pipe  31 , a second communication pipe  32 , and a third communication pipe  33  provided in the casing  21 . 
     Specifically, as illustrated in  FIG. 1 , the casing  21  is formed in a cylindrical shape long in an axial direction and is arranged such that an axis X the casing  21  faces from a front lower side to a rear upper side in a right-side region of the rear wheel  6  of the motorcycle  1 . Further, as illustrated in  FIG. 2 , the casing  21  includes a casing body  22  formed in a cylindrical shape. As illustrated in  FIG. 3 , the casing body  22  has a double cylinder structure including an outer cylinder  23  and an inner cylinder  24 . Further, as illustrated in  FIG. 4 , a front lid portion  25  is provided on a front end portion of the casing body  22 , and an opening in a front end of the casing body  22  is closed by the front lid portion  25 . Further, a hole portion  25 A for allowing the downstream side of the rear exhaust pipe  14  to be inserted into the casing  21  is formed in the front lid portion  25 . Further, a rear lid portion  26  is provided on a rear end portion of the casing body  22 , and an opening in a rear end of the casing body  22  is closed by the rear lid portion  26 . Further, a hole portion  26 A for allowing the third communication pipe  33  described below to pass through is formed in the rear lid portion  26 . Further, a cover  27  is attached to the rear lid portion  26 , and a hole portion  27 A for allowing the third communication pipe  33  to pass through is also formed in the cover  27 . These portions of the casing  21  are formed of a metal material such as stainless steel, iron, or titanium, and are joined by means such as welding or screwing. 
     Further, as illustrated in  FIG. 4 , the first expansion chamber R 1 , the second expansion chamber R 2 , and the third expansion chamber R 3  are formed in the casing  21 . That is, a rear partition plate  28  and a front partition plate  29  are provided in the casing  21 , and a columnar space in the casing  21  is divided into three spaces by the rear partition plate  28  and the front partition plate  29 . Then, of the three spaces, the rearmost space serves as the first expansion chamber R 1 , the intermediate space in the front-rear direction serves as the second expansion chamber R 2 , and the frontmost space serves as the third expansion chamber R 3 . In other words, in the casing  21 , the first expansion chamber R 1  is formed closest to an outlet side, the third expansion chamber R 3  is formed closest to an inlet side, and the second expansion chamber R 2  is formed between the first expansion chamber R 1  and the third expansion chamber R 3 . 
     The rear partition plate  28  and the front partition plate  29  are formed of, for example, a metal material in a disk shape or a cylindrical shape short in the axial direction with a lid. In the present embodiment, the rear partition plate  28  is arranged at a rear position with respect to an intermediate portion in the front-rear direction in the casing body  22 . Specifically, the rear partition plate  28  is arranged at a front position from a rear end of the casing body  22  by about a quarter of the entire length of the casing body  22 . Further, the front partition plate  29  is arranged near the intermediate portion in the front-rear direction in the casing body  22 , specifically, at a slightly front position with respect to the intermediate portion in the front-rear direction. Further, the rear partition plate  28  and the front partition plate  29  are fixed to an inner peripheral surface of the inner cylinder  24  by means of welding or screwing. Further, an insertion hole  28 A for allowing the rear exhaust pipe  14  to pass through, an insertion hole  28 B for allowing the first communication pipe  31  to pass through, an insertion hole  28 C for allowing the second communication pipe  32  to pass through, and an insertion hole  28 D for allowing the third communication pipe  33  to pass through are formed in the rear partition plate  28 . Further, an insertion hole  29 A for allowing the rear exhaust pipe  14  to pass through, and an insertion hole  29 B for allowing the first communication pipe  31  to pass through are formed in the front partition plate  29 . 
     The downstream side of the rear exhaust pipe  14  is connected to the first expansion chamber R 1 . That is, the rear exhaust pipe  14  extends rearward from the chamber  12 , and a rear end side (downstream side) of the rear exhaust pipe  14  is inserted into the casing  21  through the hole portion  25 A formed in the front lid portion  25  of the muffler  15 , as illustrated in  FIG. 4 . Further, the rear end side of the rear exhaust pipe  14  linearly extends rearward parallel to the axis X of the casing  21  in the casing  21 , and enters the first expansion chamber R 1  from the hole portion  25 A across the third expansion chamber R 3  and the second expansion chamber R 2 . Then, an opening portion in a rear end of the rear exhaust pipe  14  opens to the first expansion chamber R 1 . Further, the rear end side of the rear exhaust pipe  14  is inserted into the insertion hole  29 A in the front partition plate  29  and the insertion hole  28 A in the rear partition plate  28  and is supported by the front partition plate  29  and the rear partition plate  28  in the casing  21 . Further, as illustrated in  FIG. 3 , the rear exhaust pipe  14  is arranged at a position close to the inner peripheral surface of the inner cylinder  24  with respect to a center of the casing  21 , where the axis X of the casing  21  passes through, specifically, at a lower side of the center of the casing  21 , in the casing  21 . Further, the rear end side of the rear exhaust pipe  14  is formed in a linearly extending cylindrical shape, and the shape of an opening portion in the rear end is substantially a perfect circle. 
     As illustrated in  FIG. 5 , the first communication pipe  31  connects the first expansion chamber R 1  and the third expansion chamber R 3  in the casing  21 , and the first expansion chamber R 1  communicates with the third expansion chamber R 3  via the first communication pipe  31 . The first communication pipe  31  is a metal-made pipe formed in a tubular shape having a substantially perfect circular shape in cross section. The first communication pipe  31  linearly extends parallel to the axis X of the casing  21  between the first expansion chamber R 1  and the third expansion chamber R 3  across the second expansion chamber R 2 . Further, an inner diameter of the first communication pipe  31  is constant from a front end to a rear end of the first communication pipe  31 . Further, a rear end side of the first communication pipe  31  enters the first expansion chamber R 1 , and an opening portion in a rear end of the first communication pipe  31  open to the first expansion chamber R 1 . Further, in the present embodiment, the rear end side of the first communication pipe  31  enters up to a vicinity of an intermediate portion in the front-rear direction in the first expansion chamber R 1 . Meanwhile, a front end side of the first communication pipe  31  enters the third expansion chamber R 3 , and an opening portion in a front end of the first communication pipe  31  opens to the third expansion chamber R 3 . Further, in the present embodiment, a front end side of the first communication pipe  31  enters up to a vicinity of an intermediate portion in the front-rear direction in the third expansion chamber R 3 . Further, the first communication pipe  31  is inserted into the insertion hole  29 B in the front partition plate  29  and the insertion hole  28 B in the rear partition plate  28  and is supported by the front partition plate  29  and the rear partition plate  28  in the casing  21 . Further, in the present embodiment, the length of the first communication pipe  31  is set to a value larger than half of the entire length of the casing body  22 , for example. 
     Further, as illustrated in  FIG. 3 , the first communication pipe  31  is arranged on the center side of the casing  21  with respect to the rear exhaust pipe  14  in the casing  21 . Further, a sectional area (opening area) of a flow path of the first communication pipe  31  is set to be equal to or larger than a sectional area (an opening area of the rear end) of a flow path on the rear end side of the rear exhaust pipe  14 . Further, the inner diameter of the first communication pipe  31  is set to be equal to or larger than an inner diameter of the rear exhaust pipe  14 . Note that the sectional area of the flow path of the first communication pipe  31  is significantly smaller than respective inner diameters of the first expansion chamber R 1  and the third expansion chamber R 3 . 
     As illustrated in  FIG. 4 , the second communication pipe  32  connects the first expansion chamber R 1  and the second expansion chamber R 2  in the casing  21 , and the first expansion chamber R 1  communicates with the second expansion chamber R 2  via the second communication pipe  32 . The second communication pipe  32  is a metal-made pipe formed in a tubular shape having a substantially perfect circular shape in cross section. The second communication pipe  32  linearly extends parallel to the axis X of the casing  21  between the first expansion chamber R 1  and the second expansion chamber R 2 . An inner diameter of the second communication pipe  32  is constant from a front end to a rear end of the second communication pipe  32 . Further, a rear end side of the second communication pipe  32  enters the first expansion chamber R 1 , and an opening portion in a rear end of the second communication pipe  32  opens to the first expansion chamber R 1 . Further, in the present embodiment, the rear end of the second communication pipe  32  is located on a rear side with respect to the rear end of the first communication pipe  31  in the first expansion chamber R 1 . Meanwhile, a front end side of the second communication pipe  32  enters the second expansion chamber R 2 , and an opening portion in a front end of the second communication pipe  32  opens to the second expansion chamber R 2 . Further, in the present embodiment, the front end side of the second communication pipe  32  enters up to a vicinity of an intermediate portion in the front-rear direction in the second expansion chamber R 2 . Further, the second communication pipe  32  is inserted into the insertion hole  28 C in the rear partition plate  28  and is supported by the rear partition plate  28  in the casing  21 . Further, the second communication pipe  32  is shorter than the first communication pipe  31 . 
     Further, as illustrated in  FIG. 3 , the second communication pipe  32  is arranged on an upper side of the center of the casing  21  in the casing  21 . Further, the second communication pipe  32  is roughly arranged on an opposite side of the rear exhaust pipe  14  across the first communication pipe  31 . Further, a sectional area (opening area) of a flow path of the second communication pipe  32  is set to be less than the sectional area (opening area) of the flow path on the rear end side of the rear exhaust pipe  14 , and is set to be less than the sectional area (opening area) of the flow path of the first communication pipe  31 . Further, the inner diameter of the second communication pipe  32  is set to be less than an inner diameter of the rear exhaust pipe  14 , and is set to be less than the inner diameter of the first communication pipe  31 . 
     As illustrated in  FIG. 5 , the third communication pipe  33  has a front end side connected to the second expansion chamber R 2  in the casing  21  and has a rear end side extend outward from the casing  21 . As a result, the second expansion chamber R 2  communicates with the outside of the casing  21  via the third communication pipe  33 . The third communication pipe  33  is a metal-made pipe formed in a tubular shape having a substantially perfect circular shape in cross section. The third communication pipe  33  extends outward from the second expansion chamber R 2  to the outside of the casing  21  across the first expansion chamber R 1 . A front portion of the third communication pipe  33  linearly extends parallel to the axis X of the casing  21 , and a rear portion of the third communication pipe  33  is gently curved downward. Further, an inner diameter of the third communication pipe  33  is constant from the front end to the rear end of the third communication pipe  33 . Further, the front end side of the third communication pipe  33  enters the second expansion chamber R 2 , and an opening portion in a front end of the third communication pipe  33  opens to the second expansion chamber R 2 . Further, in the present embodiment, the front end of the third communication pipe  33  is located on a front side with respect to the front end of the second communication pipe  32  in the second expansion chamber R 2 . Meanwhile, the rear end side of the third communication pipe  33  passes through the hole portion  26 A formed in the rear lid portion  26  and the hole portion  27 A formed in the cover  27  and extends up to a position facing the outside of the casing  21 , and an opening portion in the rear end of the third communication pipe  33  opens to the outside of the casing  21 . Further, the third communication pipe  33  is inserted into the insertion hole  28 D in the rear partition plate  28  and the hole portion  26 A in the rear lid portion  26  and is supported by the rear partition plate  28  and the rear lid portion  26 . 
     Further, as illustrated in  FIG. 3 , the third communication pipe  33  is arranged on an upper side of the center of the casing  21  in the casing  21 . Further, the third communication pipe  33  is roughly arranged on an opposite side of the rear exhaust pipe  14  across the first communication pipe  31 . Further, a sectional area (opening area) of a flow path of the third communication pipe  33  is set to be equal to or smaller than the sectional area (opening area) of the flow path of the second communication pipe  32 . Further, the inner diameter of the third communication pipe  33  is set to be equal to or smaller than the inner diameter of the second communication pipe  32 . 
       FIG. 6  schematically illustrates a structure of the muffler  15  in the exhaust device  8  of the present embodiment described in detail. The muffler  15  has the following structural characteristics. That is, as illustrated in  FIG. 6 , in the first expansion chamber R 1 , the rear exhaust pipe  14  serves as an inflow path of the exhaust gas and the second communication pipe  32  serves as an outflow path of the exhaust gas. As described above, the first expansion chamber R 1  has the inflow path of the exhaust gas and the outflow path of the exhaust gas, which are different from each other. Further, in the second expansion chamber R 2 , the second communication pipe  32  serves as an inflow path of the exhaust gas and the third communication pipe  33  serves as an outflow path of the exhaust gas. As described above, the second expansion chamber R 2  has the inflow path of the exhaust gas and the outflow path of the exhaust gas, which are different from each other. In contrast, the pipe connected to the third expansion chamber R 3  is the first communication pipe  31  only. That is, the third expansion chamber R 3  does not communicate with the outside of the third expansion chamber R 3  except that the third expansion chamber R 3  communicates with the first expansion chamber R 1  via the first communication pipe  31 . Therefore, in the third expansion chamber R 3 , the first communication pipe  31  serves as an inflow path of the exhaust gas and also serves as an outflow path of the exhaust gas. Further, the sectional area of the flow path of the first communication pipe  31  connecting the first expansion chamber R 1  and the third expansion chamber R 3  is equal to or larger than the opening area in the rear end of the rear exhaust pipe  14  and is larger than the sectional area of the flow path of the second communication pipe  32 . With such a structure, a combination of the first expansion chamber R 1 , the third expansion chamber R 3 , and the first communication pipe  31  in the muffler  15  as a whole similarly functions to the first-stage expansion chamber to which the exhaust pipe is connected in the typical muffler. Rc in  FIG. 6  functionally illustrates the first-stage expansion chamber formed by the first expansion chamber R 1 , the third expansion chamber R 3 , and the first communication pipe  31 . 
     According to the muffler  15  in the present embodiment having such structural characteristics, the exhaust noise reduction effect can be enhanced while the increase in size of the muffler  15  is suppressed. 
     That is, as illustrated in  FIG. 6 , the exhaust gas flowing out of the chamber  12  passes through the rear exhaust pipe  14  and flows into the first-stage expansion chamber Rc in the muffler  15 . Then, expansion of the exhaust gas and interference of a pressure wave in the exhaust gas occur in the first-stage expansion chamber Rc. Further, when the pressure in the second expansion chamber R 2  is lower than the pressure in the first-stage expansion chamber Rc, the exhaust gas flows from the first-stage expansion chamber Rc through the second communication pipe  32  into the second expansion chamber R 2 . Then, the expansion of the exhaust gas and the interference of the pressure wave in the exhaust gas occur in the second expansion chamber R 2 . Further, the exhaust gas in the second expansion chamber R 2  flows out to the outside of the casing  21  through the third communication pipe  33 . The exhaust noise is reduced by the expansion and interference of the exhaust gas occurring in the first-stage expansion chamber Rc and the second expansion chamber R 2 , and attenuation of the pressure wave of the exhaust gas occurring when the exhaust gas passes through the second communication pipe  32  or the third communication pipe  33 , and the like. 
     Furthermore, in the muffler  15  of the present embodiment, the first-stage expansion chamber Rc is functionally formed by the first expansion chamber R 1 , the third expansion chamber R 3 , and the first communication pipe  31  as a whole. In this first-stage expansion chamber Rc, the exhaust gas flowing through the rear exhaust pipe  14  first flows into the first expansion chamber R 1 , most of the exhaust gas flows through the first communication pipe  31  having the largest sectional area of the flow path, of the communication pipes opening to the first expansion chamber R 1 , into the third expansion chamber R 3 . Thereafter, the exhaust gas is moved between the third expansion chamber R 3  and the first expansion chamber R 1  via the first communication pipe  31  due to a difference in pressure between the first expansion chamber R 1  and the third expansion chamber R 3 . Such a flow of the exhaust gas in the first-stage expansion chamber Rc causes the expansion, interference, or attenuation of the exhaust gas. Thereby, the exhaust noise is reduced. 
     According to the muffler  15  in the present embodiment, the exhaust noise reduction effect can be enhanced by the first expansion chamber R 1  and the third expansion chamber R 3  that form the first-stage expansion chamber Rc, and the second expansion chamber R 2 . That is, a large number of the expansion chambers are not required to enhance the exhaust noise reduction effect. Therefore, the exhaust noise reduction effect can be enhanced while the increase in size of the muffler  15  is suppressed. 
     Further, according to the muffler  15  having the above-described structural characteristics, the decrease in engine output can be suppressed and desired engine output can be secured while the increase in size of the muffler  15  is suppressed. 
     That is, in general, the effect to suppress the decrease in engine output is enhanced when the volume of the first-stage expansion chamber to which the exhaust pipe is connected is increased in the muffler. In the muffler  15  of the present embodiment, the first-stage expansion chamber Rc is functionally formed by the first expansion chamber R 1 , the third expansion chamber R 3 , and the first communication pipe  31  as a whole. Therefore, the volume of the first-stage expansion chamber Rc is a value of a total of the respective volumes of the first expansion chamber R 1 , the third expansion chamber R 3 , and the first communication pipe  31 . Therefore, the volume of the first-stage expansion chamber Rc can be increased by increasing the volume of the first expansion chamber R 1 , increasing the volume of the third expansion chamber R 3 , or increasing the volume of the first communication pipe  31  (increasing the length of the first communication pipe  31 , for example). Since the first expansion chamber R 1 , the third expansion chamber R 3 , and the first communication pipe  31  are dispersed in the casing  21  of the muffler  15 , the volume of the first-stage expansion chamber Rc can be increased while the increase in size of the muffler  15  is suppressed, by selecting one or ones of the first expansion chamber R 1 , the third expansion chamber R 3 , and the first communication pipe  31 , the volume or volumes of which can be increased, while suppressing the increase in size of the muffler  15 , and increasing the volume or volumes, while taking the structural restrictions of the muffler  15  (for example, arrangement of another expansion chamber) or the structural restrictions of the motorcycle  1  (for example, arrangement of other parts in a rear portion of the motorcycle  1 ) into consideration, for example. Therefore, the decrease in engine output can be suppressed and desired engine output can be secured while the increase in size of the muffler  15  is suppressed. 
     Further, in the casing  21  of the muffler  15  of the present embodiment, the second expansion chamber R 2  is arranged between the first expansion chamber R 1  and the third expansion chamber R 3 . With such an arrangement, the first expansion chamber R 1  and the third expansion chamber R 3  can be separated from each other, and the length of the first communication pipe  31  can be increased. The volume of the first-stage expansion chamber Rc can be increased by increasing the length of the first communication pipe  31 . Therefore, the effect to suppress a decrease in engine output can be enhanced. 
     Further, the long first communication pipe  31  can be provided without extending the casing  21  by arranging the second expansion chamber R 2  between the first expansion chamber R 1  and the third expansion chamber R 3 , and providing the first communication pipe  31  across the second expansion chamber R 2 . 
     Further, in the casing  21  of the muffler  15  of the present embodiment, the first expansion chamber R 1  is arranged on the rear side of the third expansion chamber R 3 . With such an arrangement, the first expansion chamber R 1  can be separated from the engine  7  while the entire casing  21  as a whole is brought close to the engine  7 . As a result, the engine  7  and the muffler  15  can be arranged close to each other, and at the same time, the exhaust pipe connecting the engine  7  and the first expansion chamber R 1 , more specifically, the rear exhaust pipe  14  connecting the chamber  12  and the first expansion chamber R 1  can be made long. 
     Further, in the muffler  15  of the present embodiment, the first expansion chamber R 1  is arranged in a rearmost part of the casing  21 , and the third expansion chamber R 3  is arranged in a frontmost part of the casing  21 . By arranging the first expansion chamber R 1  in the rearmost part in the casing  21 , the engine  7  and the muffler  15  can be arranged close to each other, and the effect to increase the length of the rear exhaust pipe  14  can be further enhanced. Further, the length of the first communication pipe  31  connecting the first expansion chamber R 1  and the third expansion chamber R 3  can be further increased, and the effect to increase the volume of the first-stage expansion chamber Rc and suppress the decrease in the engine output can be further enhanced, by respectively arranging the first expansion chamber R 1  and the third expansion chamber R 3  in the rearmost part and the frontmost part in the casing  21 . 
     Further, the opening area of the rear end opening to the first expansion chamber R 1  in the first communication pipe  31  of the muffler  15  of the present embodiment, is equal to or larger than the opening area of the rear end opening to the first expansion chamber R 1  in the rear exhaust pipe  14 . Thereby, the resistance of the exhaust gas flowing from the first expansion chamber R 1  to the third expansion chamber R 3  via the first communication pipe  31  can be made small. Therefore, a rise of pressure (back pressure) on the exhaust side of the engine  7  can be suppressed, and the decrease in engine output can be suppressed. 
     Further, in the casing  21  of the muffler  15  of the present embodiment, the rear exhaust pipe  14  is arranged at the position close to the inner peripheral surface of the inner cylinder  24  with respect to the center of the casing  21 , the first communication pipe  31  is arranged on the center side of the casing  21  with respect to the rear exhaust pipe  14 , and the second communication pipe  32  and the third communication pipe  33  are roughly arranged on the opposite side of the rear exhaust pipe  14  across the first communication pipe  31 . With the arrangement, wasteful detouring of the exhaust gas in the casing  21  can be suppressed. Therefore, an effect to suppress the rise of the back pressure of the engine  7  can be enhanced. 
     Second Embodiment 
       FIG. 7  illustrates a downstream side of a rear exhaust pipe  54  and a muffler  55  in an exhaust device of a second embodiment of the present invention.  FIG. 8  illustrates a cross section of the muffler  55  as viewed from the directions of arrows VIII-VIII in  FIG. 7 .  FIG. 9  illustrates a longitudinal section of the muffler  55  as viewed from the direction of arrows IX-IX in  FIG. 8 ,  FIG. 10  illustrates a longitudinal section of the muffler  55  as viewed from the direction of arrows X-X in  FIG. 8 , and  FIG. 11  illustrates a longitudinal section of the muffler  55  as viewed from the direction of arrows XI-XI in  FIG. 8 . Note that, in the exhaust device of the second embodiment of the present invention, the same constituent element as the exhaust device  8  of the first embodiment of the present invention is denoted by the same reference numeral and description of the constituent element is omitted. 
     In an exhaust device  8  of the present embodiment, as illustrated in  FIG. 9 , a catalytic device  53  is provided in a portion located in a casing  21  of the muffler  55  on a downstream side of the rear exhaust pipe  54 . Further, as illustrated in  FIG. 10 , two first communication pipes  61 A and  61 B are provided in the casing  21 . These first communication pipes  61 A and  61 B are arranged in parallel between a first expansion chamber R 1  and a third expansion chamber R 3 , and the first expansion chamber R 1  and the third expansion chamber R 3  are connected by first communication pipes  61 A and  61 B. Further, as illustrated in  FIG. 8 , in the casing  21 , the rear exhaust pipe  54  is arranged on a lower left side of a center of the casing  21 , through which an axis X of the casing  21  passes, the one first communication pipe  61 A is arranged on a upper right side of the center of the casing  21 , and the other first communication pipe  61 B is arranged on a upper left side of the center of the casing  21 . Further, a second communication pipe  32  is arranged on a lower right side of the center of the casing  21 , and a third communication pipe  33  is arranged on an upper side of the center of the casing  21 . Further, a total of a sectional area (opening area) of a flow path of the one first communication pipe  61 A and a sectional area (opening area) of a flow path of the other first communication pipe  61 B is equal to or larger than the opening area in the rear end of the rear exhaust pipe  14 . Further; the sectional area (opening area) of the flow path of at least one of the first communication pipes  61 A and  61 B is larger than a sectional area (opening area) of a flow path of the second communication pipe  32 . 
     According to the muffler  55  in the present embodiment having such a configuration, functions and effects similar to those of the muffler  15  in the first embodiment of the present invention can be obtained. Further, according to the muffler  55  in the present embodiment, the configuration is provided with the two first communication pipes  61 A and  61 B, whereby the muffler  55  can be downsized. That is, in the case of providing a single thick first communication pipe having a desired flow path sectional area in the casing  21 , a large space in one place needs to be secured in the casing  21 . In contrast, in the case of providing the two first communication pipes  61 A and  61 B having the desired flow path sectional area as the total flow path sectional area in the casing  21 , relatively small two spaces are provided in the casing  21  in a dispersed manner. Therefore, the cross-sectional area of the casing  21  can be made small, and the muffler  55  can be downsized. 
     Note that, as described in each of the above embodiments, in the muffler  15  ( 55 ), the muffler  15  can be downsized by arranging the second expansion chamber R 2  between the first expansion chamber R 1  and the third expansion chamber R 3 . However, the arrangement (the direction of the arrangement, the arrangement order, or the like) of the first expansion chamber R 1 , the second expansion chamber R 2 , and the third expansion chamber R 3  is not limited to the expansion. Further, the ratio of the respective volumes of the first expansion chamber R 1 , the second expansion chamber R 2 , and the third expansion chamber R 3  is not limited to that illustrated in  FIG. 4  and the like. Further, the respective positions of the front partition plate  29  and the rear partition plate  28  in the front-rear direction in the casing  21  are not limited to those illustrated in  FIG. 4  and the like. Further, another chamber independent of any of the first expansion chamber R 1 , the second expansion chamber R 2 , and the third expansion chamber R 3  may be added in the casing  21 , and the downstream side of the third communication pipe  33  may be connected to the another chamber. Further, the third expansion chamber R 3  may be formed in a casing different from a casing in which the first expansion chamber R 1  and the second expansion chamber R 2  are formed, and these two casings may be connected by the first communication pipe  31 . 
     Further, the number of the first communication pipes  31  may be three or more. Further, the number of the second communication pipes  32  or the third communication pipes  33  may be two or more. Further, the shape of the cross section of each communication pipe is not limited to the perfect circle, and may be another shape such as an elliptical shape. Further, the shape of the cross section of the casing  21  is not limited to the perfect circle, and may be another shape such as an elliptical shape, a quadrangular shape, or a triangular shape. Further, the position at which the muffler  15  is provided in the motorcycle  1  is not limited to the position illustrated in  FIG. 1 . 
     Further, in the second embodiment of the present invention, the sectional area (opening area) of the flow path of at least one of the two first communication pipes  61 A and  61 B may be made equal to or larger than the opening area in the rear end of the rear exhaust pipe  14 . Thereby, the exhaust gas can be easily moved between the first expansion chamber R 1  and the third expansion chamber R 3 . 
     Further, the present invention can also be applied to an exhaust device without a chamber or an exhaust device without a catalyst device. Further, application of the exhaust device of the present invention is not limited to motorcycles, and the exhaust device of the present invention can also be applied to other types of saddled vehicles such as a three-wheeled motor vehicle, other types of vehicles such as a four-wheeled motor vehicle, and types other than vehicles such as an outboard motor. 
     Further, the present invention can be appropriately changed within a scope not contrary to the gist or idea of the invention that can be read from the claims and the entire specification, and the exhaust device of the engine accompanied by such change is also included in the technical idea of the present invention.