Patent Publication Number: US-11649756-B2

Title: Saddle-riding type vehicle exhaust structure

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     Priority is claimed on Japanese Patent Application No. 2020-159521, filed on Sep. 24, 2020, the contents of which are incorporated herein by reference. 
     BACKGROUND 
     Field of the Invention 
     The present invention relates to a saddle-riding type vehicle exhaust structure. 
     Background 
     In the related art, a saddle-riding type vehicle exhaust structure is known which includes an exhaust pipe that is connected to an exhaust port connecting to a combustion chamber of an engine and a muffler that is connected to a downstream side of the exhaust pipe in an exhaust flow direction (for example, refer to Japanese Patent No. 6444352). 
     SUMMARY 
     However, there is room for improving a suitable arrangement of the exhaust pipe while improving the output of the engine. 
     An aspect of the present invention is intended to suitably arrange an exhaust pipe while improving the output of an engine. 
     A saddle-riding type vehicle exhaust structure according to a first aspect of the present invention includes: an exhaust pipe that is connected to an exhaust port connecting to a combustion chamber of an engine and has a circular cross-sectional shape which is orthogonal to an exhaust flow direction; and a muffler that is connected to a downstream side in the exhaust flow direction of the exhaust pipe, wherein the exhaust pipe includes a muffler connection part that is connected to the muffler, an exhaust pipe upstream part that is positioned on an upstream side in the exhaust flow direction of the muffler connection part, and an exhaust pipe downstream part that is positioned on a downstream side in the exhaust flow direction of the muffler connection part, a cross-sectional area that is orthogonal to the exhaust flow direction of the muffler connection part is larger than each of a minimum value of a cross-sectional area that is orthogonal to an exhaust flow direction of the exhaust pipe upstream part and a minimum value of a cross-sectional area that is orthogonal to an exhaust flow direction of the exhaust pipe downstream part, and a vehicle width direction size of the cross-sectional shape of the muffler connection part and a vertical direction size of the cross-sectional shape of the muffler connection part are different from each other. 
     A second aspect of the present invention is the saddle-riding type vehicle exhaust structure according to the first aspect described above, wherein the engine may include a crankcase and a cylinder that stands from the crankcase and that has the exhaust port, the exhaust pipe may be connected to the exhaust port, be curved, pass a side of the cylinder, then pass above the crankcase, and extend rearward and upward, and the vehicle width direction size of the cross-sectional shape of the muffler connection part may be smaller than the vertical direction size of the cross-sectional shape of the muffler connection part. 
     A third aspect of the present invention is the saddle-riding type vehicle exhaust structure according to the first or second aspect described above, wherein the vehicle width direction size of the cross-sectional shape of the muffler connection part may be smaller than a maximum value of the vehicle width direction size of the cross-sectional shape of the exhaust pipe upstream part. 
     A fourth aspect of the present invention is the saddle-riding type vehicle exhaust structure according to any one of the first to third embodiments described above, wherein the engine may be supported by a vehicle body frame, the vehicle body frame may include a main frame that extends rearward and downward from a head pipe and a pivot plate that extends downward from a rear end part of the main frame, and the muffler connection part may pass an inside in a vehicle width direction of the pivot plate and overlap the pivot plate when seen from a vehicle width direction. 
     A fifth aspect of the present invention is the saddle-riding type vehicle exhaust structure according to the fourth aspect described above, wherein a swing arm may be swingably supported by the pivot plate, the swing arm and the vehicle body frame may be connected by a rear cushion, and the muffler connection part may be arranged between the pivot plate and the rear cushion in the vehicle width direction. 
     A sixth aspect of the present invention is the saddle-riding type vehicle exhaust structure according to any one of the first to fifth aspects described above, wherein the engine may be supported by a vehicle body frame, a connection member that connects the vehicle body frame to the exhaust pipe may be provided, and the connection member may be welded to at least a surface of the muffler connection part having a larger one of the vehicle width direction size of the cross-sectional shape and the vertical direction size of the cross-sectional shape. 
     According to the saddle-riding type vehicle exhaust structure of the first aspect of the present invention, the structure includes: the exhaust pipe that is connected to the exhaust port connecting to the combustion chamber of the engine and has the circular cross-sectional shape which is orthogonal to the exhaust flow direction; and the muffler that is connected to the downstream side in the exhaust flow direction of the exhaust pipe, wherein the exhaust pipe includes the muffler connection part that is connected to the muffler, the exhaust pipe upstream part that is positioned on the upstream side in the exhaust flow direction of the muffler connection part, and the exhaust pipe downstream part that is positioned on the downstream side in the exhaust flow direction of the muffler connection part, the cross-sectional area that is orthogonal to the exhaust flow direction of the muffler connection part is larger than each of the minimum value of the cross-sectional area that is orthogonal to the exhaust flow direction of the exhaust pipe upstream part and the minimum value of the cross-sectional area that is orthogonal to the exhaust flow direction of the exhaust pipe downstream part, and the vehicle width direction size of the cross-sectional shape of the muffler connection part and the vertical direction size of the cross-sectional shape of the muffler connection part are different from each other. Thereby, the following advantage is achieved. 
     By the cross-sectional area that is orthogonal to the exhaust flow direction of the muffler connection part being larger than each of the minimum value of the cross-sectional area that is orthogonal to the exhaust flow direction of the exhaust pipe upstream part and the minimum value of the cross-sectional area that is orthogonal to the exhaust flow direction of the exhaust pipe downstream part, since it is possible to adjust the pulsation of the exhaust gas in the exhaust pipe and actively suction a combustion gas in the combustion chamber of the engine, it is possible to improve the output of the engine. Additionally, the vehicle width direction size of the cross-sectional shape of the muffler connection part and the vertical direction size of the cross-sectional shape of the muffler connection part are different from each other, and thereby, it is possible to use an arrangement that prevents an increase in size of the vehicle or an arrangement that prevents the effect of interference on another configuration component. Accordingly, it is possible to suitably arrange the exhaust pipe while improving the output of the engine. 
     According to the saddle-riding type vehicle exhaust structure of the second aspect of the present invention, the engine includes the crankcase and the cylinder that stands from the crankcase and that has the exhaust port, the exhaust pipe is connected to the exhaust port, is curved, passes a side of the cylinder, then passes above the crankcase, and extends rearward and upward, and the vehicle width direction size of the cross-sectional shape of the muffler connection part is smaller than the vertical direction size of the cross-sectional shape of the muffler connection part. Thereby, the following advantage is achieved. 
     Even in a case where the exhaust pipe passes above the crankcase and extends rearward and upward, since the muffler connection part does not occupy a space in the vehicle width direction, it is possible to prevent an increase in size in the vehicle width direction. Accordingly, it is possible to achieve both output improvement of the engine and prevention of an increase in size in the vehicle width direction. 
     According to the saddle-riding type vehicle exhaust structure of the third aspect of the present invention, the vehicle width direction size of the cross-sectional shape of the muffler connection part is smaller than the maximum value of the vehicle width direction size of the cross-sectional shape of the exhaust pipe upstream part, and thereby, the following advantage is achieved. 
     It is possible to further prevent an increase in size in the vehicle width direction. 
     According to the saddle-riding type vehicle exhaust structure of the fourth aspect of the present invention, the engine is supported by the vehicle body frame, the vehicle body frame includes the main frame that extends rearward and downward from the head pipe and the pivot plate that extends downward from the rear end part of the main frame, and the muffler connection part passes the inside in the vehicle width direction of the pivot plate and overlaps the pivot plate when seen from the vehicle width direction. Thereby, the following advantage is achieved. 
     Since a foot part of a rider is generally located on the side of the pivot plate, the muffler connection part passes the inside in the vehicle width direction of the pivot plate, and thereby, it is possible to reduce a thermal impact on the foot part of the rider. Additionally, the muffler connection part overlaps the pivot plate when seen from the vehicle width direction, and thereby, it is possible to further prevent an increase in size in the vehicle width direction. 
     According to the saddle-riding type vehicle exhaust structure of the fifth aspect of the present invention, the swing arm is swingably supported by the pivot plate, the swing arm and the vehicle body frame are connected by the rear cushion, and the muffler connection part is arranged between the pivot plate and the rear cushion in the vehicle width direction. Thereby, the following advantage is achieved. 
     It is possible to further prevent an increase in size in the vehicle width direction. 
     According to the saddle-riding type vehicle exhaust structure of the sixth aspect of the present invention, the engine is supported by the vehicle body frame, the connection member that connects the vehicle body frame to the exhaust pipe is provided, and the connection member is welded to at least a surface of the muffler connection part having a larger one of the vehicle width direction size of the cross-sectional shape and the vertical direction size of the cross-sectional shape. Thereby, the following advantage is achieved. 
     Since a surface having a larger one of the vehicle width direction size of the cross-sectional shape and the vertical direction size of the cross-sectional shape in the muffler connection part has a larger curvature radius than a surface having a smaller one of the vehicle width direction size of the cross-sectional shape and the vertical direction size of the cross-sectional shape in the muffler connection part, in comparison with a case where the connection member is welded to the surface of the muffler connection part having a smaller one of the vehicle width direction size of the cross-sectional shape and the vertical direction size of the cross-sectional shape, welding work is facilitated. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a right side view of a motorcycle according to an embodiment. 
         FIG.  2    is a right side view of an exhaust structure of the motorcycle according to the embodiment. 
         FIG.  3    is a front view that includes a cross section of  FIG.  1   . 
         FIG.  4    is a top view that includes a IV-IV cross section of  FIG.  1   . 
         FIG.  5    is a right side view of a first front pipe, a second front pipe, and a muffler according to the embodiment. 
         FIG.  6    is an enlarged view of a VI part of  FIG.  5    and is a right side view of the second front pipe, a third front pipe, and an inner pipe according to the embodiment. 
         FIG.  7    is a top view of the first front pipe, the second front pipe, and the muffler according to the embodiment. 
         FIG.  8    is a VIII-VIII cross-sectional view of  FIG.  7   . 
         FIG.  9    is a left side view that includes a IX-IX cross section of  FIG.  7   . 
         FIG.  10    is a view showing a simulation result of the exhaust structure of the embodiment together with a simulation result of an exhaust structure of a comparison example and is a view showing a relationship between an engine rotation speed and an output. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, an embodiment of the present invention will be described with reference to the drawings. In the following description, a motorcycle as an example of a saddle-riding type vehicle is described. In appropriate places in the drawing used in the following description, an arrow FR that indicates a vehicle frontward direction of the motorcycle of the present embodiment, an arrow LH that indicates a vehicle leftward direction, an arrow UP that indicates a vehicle upward direction, and a line CL that indicates a center in a right-to-left direction of a vehicle body are shown. 
     &lt;Entire Vehicle&gt; 
     As shown in  FIG.  1   , the motorcycle  1  (saddle-riding type vehicle) includes a front wheel  3  that is steered by a handle  2 , a rear wheel  4  that is driven by a power unit  10  including a power source, and a vehicle body frame  20  that supports the power unit  10 . Hereinafter, the motorcycle is simply referred to as a “vehicle”. 
     The vehicle body frame  20  includes: a head pipe  21  that steerably supports the handle  2 ; a pair of right and left main frames  22  that extend rearward and downward from the head pipe  21 ; a down frame  23  that extends rearward and downward from the head pipe  21  more steeply than the main frame  22 ; a pair of right and left lower frames  24  that extend rearward from a lower end part of the down frame  23 ; a pair of right and left pivot plates  25  that extend downward from a rear end part of the main frame  22  and are connected to a rear end part of the lower frame  24 ; a pair of right and left seat rails  26  that extend rearward from a rear end part of the main frame  22 ; and a pair of right and left rear frames  27  that extend rearward and upward from a middle part in a vertical direction of the pivot plate  25  and are connected to a rear end part of the seat rail  26 . 
     An axle  4   a  of the rear wheel  4  is supported by a rear end part of the swing arm  5  that extends in a front-to-rear direction. 
     A front end part of the swing arm  5  is supported by a lower part of the pivot plate  25  via a pivot shaft  25   a  to be swingable upward and downward. A link mechanism  19  having a link member  18  is provided between a lower part of the pivot plate  25  and a front end part of the swing arm  5 . A rear cushion  6  that extends in the vertical direction is provided between an upper part of the pivot plate  25  and the link member  18 . 
     The power unit  10  includes: an engine  11  which is an internal combustion engine that burns a combustible air-fuel mixture and obtains an output; an ACG (not shown) that functions as a generator; and a transmission (power transmission mechanism, not shown) that is connected to a crankshaft (not shown) and transmits power from the engine  11  to the rear wheel  4  which is a drive wheel. A fuel tank  7  that is supported by the right and left main frames  22  is provided above the engine  11 . A seat  8  that is supported by the right and left seat rails  26  is provided at the rear of the fuel tank  7 . 
     &lt;Engine&gt; 
     The engine  11  includes a crankcase  12  that accommodates a crankshaft (not shown) and a cylinder  13  that stands to be slightly tilted frontward from a front upper part of the crankcase  12  toward an upward direction. 
     As shown in  FIG.  2   , the cylinder  13  includes: a cylinder block  13   a  that is connected to the front upper part of the crankcase  12 ; a cylinder head  13   b  that is connected to an upper part of the cylinder block  13   a ; and a head cover  13   c  that is connected to an upper part of the cylinder head  13   b . An exhaust port  13   ex  that is connected to a combustion chamber  11   a  of the engine  11  is provided on a front wall of the cylinder head  13   b.    
     &lt;Exhaust Structure&gt; 
     An exhaust structure  29  includes: an exhaust pipe  30  that is connected to the exhaust port  13   ex ; and a muffler  50  that is connected to a downstream side in an exhaust flow direction of the exhaust pipe  30 . Here, the exhaust flow direction means a direction in which the exhaust gas from the exhaust port  13   ex  flows. Hereinafter, a cross-sectional shape that is orthogonal to the exhaust flow direction of the exhaust pipe  30  is also simply referred to as a “cross-sectional shape”. 
     &lt;Exhaust Pipe&gt; 
     The cross-sectional shape of the exhaust pipe  30  is a circular shape. Here, the circular shape includes a true circle shape, an oval shape, and an ellipse shape. The exhaust pipe  30  has a cylindrical shape that extends along the exhaust flow direction while changing the cross-sectional shape. The exhaust pipe  30  is connected to the exhaust port  13   ex , is curved, passes a right side of the cylinder  13 , then passes above the crankcase  12 , and extends rearward and upward. 
     The exhaust pipe  30  includes an exhaust pipe  31 , a first front pipe  32 , a second front pipe  33  (muffler connection part), a third front pipe  34  (refer to  FIG.  6   ), an inner pipe  40 , and a tail pipe  35  (refer to  FIG.  9   ). The first front pipe  32 , the second front pipe  33  (muffler connection part), the third front pipe  34  (refer to  FIG.  6   ), the inner pipe  40 , and the tail pipe  35  (refer to  FIG.  9   ) are provided in this order in the exhaust flow direction. 
     In a side view of  FIG.  2   , the exhaust pipe  31  includes: a first extension part  31   a  that is curved and extends frontward and downward from the exhaust port  13   ex  and then extends rearward and upward; and a second extension part  31   b  that is curved from a downstream end in the exhaust flow direction of the first extension part  31   a , passes a right side of the cylinder head  13   b  and above the crankcase  12 , and extends rearward and upward. In a top view of  FIG.  4   , the first extension part  31   a  extends frontward from the exhaust port  13   ex  and is then curved and extends rightward. 
     In the top view of  FIG.  4   , the second extension part  31   b  extends to be inclined inward in the vehicle width direction toward the rear from the downstream end in the exhaust flow direction of the first extension part  31   a.    
     In the side view of  FIG.  2   , the first front pipe  32  extends rearward toward the inside in the vehicle width direction of the right pivot plate  25  from the downstream end in the exhaust flow direction of the second extension part  31   b  of the exhaust pipe  31 . In a side view of  FIG.  5   , a rear end part of the first front pipe  32  defines a funnel shape that is enlarged rearward. In the top view of  FIG.  4   , the first front pipe  32  extends to be inclined inward in the vehicle width direction toward the rear from the downstream end in the exhaust flow direction of the second extension part  31   b . In a top view of  FIG.  7   , the rear end part of the first front pipe  32  defines a funnel shape that is enlarged frontward. In other words, the rear end part of the first front pipe  32  defines a funnel shape that narrows toward the downstream side in the exhaust flow direction. 
     In the side view of  FIG.  2   , the second front pipe  33  extends rearward and upward from the downstream end in the exhaust flow direction of the first front pipe  32 . The cross-sectional area orthogonal to the exhaust flow direction of the second front pipe  33  is a uniform size throughout to the downstream end from an upstream end in the exhaust flow direction of the second front pipe  33  (the entire extension direction). The cross-sectional shape of the second front pipe  33  has a uniform size throughout to the downstream end from the upstream end in the exhaust flow direction of the second front pipe  33 . In the top view of  FIG.  4   , the second front pipe  33  extends rearward from the downstream end in the exhaust flow direction of the first front pipe  32  and then extends to be inclined outward in the vehicle width direction toward the rear. 
     As shown in  FIG.  3   , the second front pipe  33  passes the inside in the vehicle width direction of the right pivot plate  25 . In the side view of  FIG.  2   , a portion of the second front pipe  33  that passes the inside in the vehicle width direction of the right pivot plate  25  overlaps the right pivot plate  25 . As shown in  FIG.  3   , the second front pipe  33  is arranged between the right pivot plate  25  and the rear cushion  6  in the vehicle width direction. 
     In a side view of  FIG.  6   , the third front pipe  34  extends rearward and upward from the downstream end in the exhaust flow direction of the second front pipe  33 . The cross-sectional area orthogonal to the exhaust flow direction of the third front pipe  34  is gradually decreased toward the downstream end from the upstream end in the exhaust flow direction of the third front pipe  34 . In the side view of  FIG.  6   , the third front pipe  34  defines a funnel shape that is enlarged toward the front lower direction. As shown in  FIG.  9   , for example, an outer circumference of a front end part of the third front pipe  34  is welded to an inner circumference of a front end part of a front cap  52  of the muffler  50  in a fitted state. 
     In the side view of  FIG.  6   , the inner pipe  40  extends rearward and upward from the downstream end in the exhaust flow direction of the third front pipe  34 . As shown in  FIG.  9   , the inner pipe  40  is arranged within the muffler  50 . The inner pipe  40  includes a first punching part  41 , a second punching part  42 , a third punching part  43 , a forward-direction cut-standing part  44 , and a reverse-direction cut-standing part  45 . 
     The first punching part  41  is provided on a front part of the inner pipe  40 . The first punching part  41  includes a plurality of first punching holes  41   a . The first punching hole  41   a  has a circular shape when seen from a radial direction of the inner pipe  40 . In an example of  FIG.  9   , the first punching part  41  has a configuration in which nine first punching holes  41   a  that are aligned in an axis direction of the inner pipe  40  are provided in a plurality of rows in the circumferential direction of the inner pipe  40 . A plurality of rows of the first punching holes  41   a  are alternately offset back and forth along the axis direction of the inner pipe  40  in the circumferential direction of the inner pipe  40 . 
     The second punching part  42  is provided on a part of the inner pipe  40  at a further rearward side than the first punching part  41 . The second punching part  42  includes a plurality of second punching holes  42   a . The second punching hole  42   a  has an elongated hole shape that extends in the circumferential direction when seen from the radial direction of the inner pipe  40 . The plurality of second punching holes  42   a  are arranged in a staggered configuration. In the example of  FIG.  9   , the second punching part  42  has a configuration in which twelve second punching holes  42   a  that are aligned in the axis direction of the inner pipe  40  and thirteen second punching holes  42   a  that are aligned in the axis direction of the inner pipe  40  are alternately provided in the circumferential direction of the inner pipe  40 . 
     The third punching part  43  is provided on a rear part of the inner pipe  40 . The third punching part  43  is provided on a part of the inner pipe  40  at a further rearward side than the second punching part  42 . The third punching part  43  includes a plurality of third punching holes  43   a . The third punching hole  43   a  has an elongated hole shape that extends in the circumferential direction when seen from the radial direction of the inner pipe  40 . The plurality of third punching holes  43   a  are arranged in a staggered configuration. In the example of  FIG.  9   , the third punching part  43  has a configuration in which three third punching holes  43   a  that are aligned in the axis direction of the inner pipe  40  are provided in a plurality of rows in the circumferential direction of the inner pipe  40 . A plurality of rows of the third punching holes  43   a  are alternately offset back and forth along the axis direction of the inner pipe  40  in the circumferential direction of the inner pipe  40 . 
     The forward-direction cut-standing part  44  is provided on a part of the inner pipe  40  at a further rearward side than the first punching part  41 . The forward-direction cut-standing part  44  is provided at a portion corresponding to the second punching part  42 . The forward-direction cut-standing part  44  has a plurality of forward direction standing pieces  44   a  that stand outward in the radial direction of the inner pipe  40  toward the rear side along the axis direction of the inner pipe  40 . The forward direction standing piece  44   a  has a triangular shape that protrudes rearward from a rear end of the second punching hole  42   a  along the axis direction of the inner pipe  40  when seen from the radial direction of the inner pipe  40 . 
     The reverse-direction cut-standing part  45  is provided on a rear part of the inner pipe  40 . The third punching part  43  is provided on a part of the inner pipe  40  at a further rearward side than the forward-direction cut-standing part  44 . The reverse-direction cut-standing part  45  is provided at a portion corresponding to the third punching part  43 . The reverse-direction cut-standing part  45  has a plurality of reverse direction standing pieces  45   a  that stand outward in the radial direction of the inner pipe  40  toward the front side along the axis direction of the inner pipe  40 . The reverse direction standing piece  45   a  has a triangular shape that protrudes frontward from a front end of the third punching hole  43   a  along the axis direction of the inner pipe  40  when seen from the radial direction of the inner pipe  40 . That is, the reverse direction standing piece  45   a  has a triangular shape facing a direction opposite to the forward direction standing piece  44   a  when seen from the radial direction of the inner pipe  40 . 
     In a side view of  FIG.  9   , the tail pipe  35  extends rearward and upward from a rear end of the inner pipe  40 , is then curved, and extends rearward and downward. For example, an outer circumference of a front end part of the tail pipe  35  is welded to an inner circumference of a rear end part of the inner pipe  40  in a fitted state. 
     &lt;Muffler&gt; 
     The muffler  50  includes a cylinder body  51 , a front cap  52 , a rear cap  53 , an inner cap  54 , and a tail cap  55 . 
     In the side view of  FIG.  9   , the cylinder body  51  defines a cylindrical shape that extends straight rearward and upward. An expansion room  56  is provided between the cylinder body  51  and the inner pipe  40 . 
     For example, sound-absorption heat-insulation materials  57  and  58  are provided in the expansion room  56 . In the example of  FIG.  9   , a plurality of sound-absorption heat-insulation materials  57  and  58  are provided in the expansion room  56 . For example, the plurality of sound-absorption heat-insulation materials  57  and  58  includes a first sound-absorption heat-insulation material  57  such as glass wool and a second sound-absorption heat-insulation material  58  such as metal wool for scattering prevention of the glass wool. The second sound-absorption heat-insulation material  58  is provided between the first sound-absorption heat-insulation material  57  and the inner pipe  40 . 
     In the side view of  FIG.  9   , the front cap  52  defines a funnel shape that is enlarged rearward. 
     For example, an outer circumference of a rear end part of the front cap  52  is welded to an inner circumference of a front end part of the cylinder body  51  in a fitted state. 
     In the side view of  FIG.  9   , the rear cap  53  defines a funnel shape that is enlarged frontward. For example, an outer circumference of a front end part of the rear cap  53  is welded to an inner circumference of a rear end part of the cylinder body  51  in a fitted state. A rear part of the rear cap  53  has a cylindrical standing part  53   a  having a cylindrical shape that stands frontward and upward. 
     The inner cap  54  defines an annular shape having a flange on an outer circumference of the inner cap  54 . For example, the flange of the inner cap  54  is welded to an inner circumference of a front end part of the rear cap  53  in a fitted state. For example, a front part of the tail pipe  35  is welded to an inner circumference of the inner cap  54  in a fitted state. 
     In the side view of  FIG.  9   , the tail cap  55  defines a funnel shape that is enlarged rearward. For example, a rear part of the tail pipe  35  is welded to an inner circumference of a front end part of the tail cap  55  in a fitted state. For example, the cylindrical standing part  53   a  of the rear cap  53  is welded to an inner circumference of a rear end part of the tail cap  55  in a fitted state. 
     &lt;Action of Inner Pipe&gt; 
     As shown in  FIG.  9   , the inner pipe  40  includes the first punching part  41 , the second punching part  42 , and the third punching part  43  that are provided in this order from the third front pipe  34  to the tail pipe  35 , and the forward-direction cut-standing part  44  and the reverse-direction cut-standing part  45  that are provided to correspond to the second punching part  42  and the third punching part  43 , respectively. 
     For example, the exhaust gas via the third front pipe  34  is guided to the inside of the inner pipe  40  (refer to an arrow Ex 1  in  FIG.  9   ). Then, the exhaust gas is subject to the influence of friction (pipe wall friction) by a wall surface of the first punching part  41  (the plurality of first punching holes  41   a ). Accordingly, the flow rate of the exhaust gas is decreased due to the influence of the pipe wall friction. 
     Then, the exhaust gas passes through the second punching part  42  (the plurality of second punching holes  42   a ), flows along the forward-direction cut-standing part  44  (the plurality of forward direction standing pieces  44   a ), and is guided to the expansion room  56  (refer to an arrow Ex 2  in  FIG.  9   ). 
     Then, the exhaust gas flows along the reverse-direction cut-standing part  45  (the plurality of reverse direction standing pieces  45   a ), passes through the third punching part  43  (the plurality of third punching holes  43   a ), and is guided to the inside of the inner pipe  40  (refer to an arrow Ex 3  in  FIG.  9   ). 
     In this way, according to the configuration in which the inner pipe  40  includes the first punching part  41 , the second punching part  42 , and the third punching part  43  in this order from the third front pipe  34  to the tail pipe  35 , and the forward-direction cut-standing part  44  and the reverse-direction cut-standing part  45  corresponding to the second punching part  42  and the third punching part  43 , respectively, it is possible to smoothly guide the exhaust gas to the inside of the inner pipe  40  without disturbing the flow of the exhaust gas that flows to the inside of the inner pipe  40 . Therefore, it is possible to decrease a pressure loss while ensuring the same sound-absorbing performance as, for example, a configuration (a configuration in which an inner pipe has a forward-direction cut-standing part and a punching part in this order from the third front pipe  34  to the tail pipe  35 ) that does not have a reverse-direction cut-standing part. Accordingly, it is possible to further improve the output of the engine  11 . 
     &lt;Second Front Pipe&gt; 
     As shown in  FIG.  5   , the first front pipe  32  (exhaust pipe upstream part) is connected to the upstream end in the exhaust flow direction of the second front pipe  33 . For example, an outer circumference of a rear end part of the first front pipe  32  is welded to an inner circumference of a front end part of the second front pipe  33  in a fitted state. 
     As shown in  FIG.  9   , the third front pipe  34  (exhaust pipe downstream part) is connected to the downstream end in the exhaust flow direction of the second front pipe  33 . For example, an outer circumference of a rear end part of the second front pipe  33  is welded to an inner circumference of a front end part of the third front pipe  34  in a fitted state. A rear end part of the second front pipe  33  is connected to a front end part of the front cap  52  of the muffler  50  via the front end part of the third front pipe  34 . The second front pipe  33  also functions as a muffler connection part that is connected to the muffler  50 . 
     Hereinafter, a cross-sectional area orthogonal to the exhaust flow direction of the first front pipe  32  is defined as a “first flow path cross-sectional area A 1 ”, a cross-sectional area orthogonal to the exhaust flow direction of the second front pipe  33  is defined as a “second flow path cross-sectional area A 2 ”, and a cross-sectional area orthogonal to the exhaust flow direction of the third front pipe  34  is defined as a “third flow path cross-sectional area A 3 ”. 
     As shown in  FIG.  5   , the second flow path cross-sectional area A 2  is larger than each of a minimum value A1min of the first flow path cross-sectional area A 1  and a minimum value A3min of the third flow path cross-sectional area A 3  (A 2 &gt;A1min, A 2 &gt;A3min). Here, the minimum value A1min of the first flow path cross-sectional area A 1  means a first flow path cross-sectional area A 1  of a portion of the first front pipe  32  having the most reduced diameter. The minimum value A3min of the third flow path cross-sectional area A 3  means a third flow path cross-sectional area A 3  of a portion of the third front pipe  34  having the most reduced diameter. 
     As shown in  FIG.  8   , a vehicle width direction size W 2  of the cross-sectional shape of the second front pipe  33  and a vertical direction size H 2  of the cross-sectional shape of the second front pipe  33  are different from each other. In the present embodiment, the vehicle width direction size W 2  of the cross-sectional shape of the second front pipe  33  is smaller than the vertical direction size H 2  of the second front pipe  33  (W 2 &lt;H 2 ). For example, a ratio H 2 /W 2  of the vertical direction size H 2  of the second front pipe  33  to the vehicle width direction size W 2  of the cross-sectional shape of the second front pipe  33  can be preferably equal to or more than 1.1 and equal to or less than 5.0 and can be further preferably equal to or more than 1.5 and equal to or less than 2.5. 
     As shown in  FIG.  7   , the vehicle width direction size W 2  of the cross-sectional shape of the second front pipe  33  is less than a maximum value W1max of the vehicle width direction size W 1  of the cross-sectional shape of the first front pipe  32  (W 2 &lt;W1max). Here, the maximum value W1max of the vehicle width direction size W 1  of the cross-sectional shape of the first front pipe  32  means a vehicle width direction size of a portion of the first front pipe  32  having the most enlarged diameter in a top view. 
     As shown in  FIG.  4   , the vehicle width direction size W 2  of the cross-sectional shape of the second front pipe  33  is smaller than a diameter Dex of a rear end of the exhaust pipe  31  (W 2 &lt;Dex). Here, the diameter Dex of the rear end of the exhaust pipe  31  means an outer diameter of the downstream end in the exhaust flow direction of the second extension part  31   b.    
     &lt;Connection Member&gt; 
     As shown in  FIG.  2   , the exhaust pipe  30  is supported by the right rear frame  27  of the vehicle body frame  20  via a connection member  60 . For example, the connection member  60  is fixed to the right rear frame  27  via a fastening member such as a bolt. The connection member  60  is welded to at least a surface of the second front pipe  33  having a larger one of the vehicle width direction size W 2  of the cross-sectional shape and the vertical direction size H 2  of the cross-sectional shape (refer to  FIG.  8   ). 
     As shown in  FIG.  8   , the connection member  60  includes: a stay main body  61  having a penetration hole  61   a  through which a bolt is inserted; a first extension part  62  that extends toward an upper surface of the second front pipe  33  from an inner end in the vehicle width direction of a lower part of the stay main body  61 ; and a second extension part  63  that extends toward a right side surface of the second front pipe  33  from an outer portion in the vehicle width direction of a lower part of the stay main body  61 . 
     The penetration hole  61   a  penetrates through the stay main body  61  in the vehicle width direction. The first extension part  62  has a first curved surface  62   a  having an arc shape along a right upper end part of an upper surface of the second front pipe  33 . The second extension part  63  has a second curved surface  63   a  having an arc shape along an upper middle part of a right side surface of the second front pipe  33 . The curvature radius of the second curved surface  63   a  is larger than the curvature radius of the first curved surface  62   a.    
     The length of the second curved surface  63   a  along the outer circumference of the second front pipe  33  is larger than the length of the first curved surface  62   a  along the outer circumference of the second front pipe  33 . 
     The connection member  60  is welded to a side surface (a surface having a larger one of the vehicle width direction size W 2  of the cross-sectional shape and the vertical direction size H 2  of the cross-sectional shape) of the second front pipe  33  by each of the first extension part  62  and the second extension part  63 . Specifically, the connection member  60  is welded to the right upper end part of the upper surface of the second front pipe  33  by the first curved surface  62   a  and is welded to the upper middle part of the right side surface of the second front pipe  33  by the second curved surface  63   a.    
     &lt;Relationship Between Engine Rotation Speed and Output&gt; 
       FIG.  10    is a view showing a simulation result of the exhaust structure of the embodiment together with a simulation result of an exhaust structure of a comparison example and is a view showing a relationship between an engine rotation speed and an output. 
     In  FIG.  10   , the horizontal axis represents an engine rotation speed, and the vertical axis represents an output (output of the engine). In  FIG.  10   , reference numeral R 1  represents a graph showing output characteristics of a throttle opening degree of 37.5% of the embodiment, reference numeral R 2  represents a graph showing output characteristics of a throttle opening degree of 50% of the embodiment, reference numeral S 1  represents a graph showing output characteristics of a throttle opening degree of 37.5% of the comparison example, and reference numeral S 2  represents a graph showing output characteristics of a throttle opening degree of 50% of the comparison example. 
     The exhaust structure of the embodiment corresponds to the exhaust structure  29  described above. That is, as shown in  FIG.  2   , the exhaust structure of the embodiment includes: the exhaust pipe  30  that is connected to the exhaust port  13   ex  connecting to the combustion chamber  11   a  of the engine  11  and has a circular cross-sectional shape which is orthogonal to the exhaust flow direction; and the muffler  50  that is connected to the downstream side in the exhaust flow direction of the exhaust pipe  30 , wherein: the exhaust pipe  30  is connected to the exhaust port  13   ex , is curved, passes the right side of the cylinder  13 , then passes above the crankcase  12 , and extends rearward and upward; as shown in  FIG.  5   , the exhaust pipe  30  includes the second front pipe  33  that is connected to the muffler  50 , the first front pipe  32  that is connected to the upstream side in the exhaust flow direction of the second front pipe  33 , and the third front pipe  34  that is connected to the downstream side in the exhaust flow direction of the second front pipe  33 ; the cross-sectional area A 2  that is orthogonal to the exhaust flow direction of the second front pipe  33  is larger than the minimum value A1min of the cross-sectional area that is orthogonal to the exhaust flow direction of the first front pipe  32  and the minimum value A3min of the cross-sectional area that is orthogonal to the exhaust flow direction of the third front pipe  34  (A 2 &gt;A1min, and A 2 &gt;A3min); and as shown in  FIG.  8   , the vehicle width direction size W 2  of the cross-sectional shape of the second front pipe  33  is smaller than the vertical direction size H 2  of the cross-sectional shape of the second front pipe  33  (W 2 &lt;H 2 ). 
     The exhaust structure (not shown) of the comparison example is common to the exhaust structure  29  of the embodiment in that an exhaust pipe is connected to the exhaust port  13   ex , is curved, passes the right side of the cylinder  13 , then passes above the crankcase  12 , and extends rearward and upward. The exhaust structure of the comparison example differs from the exhaust structure  29  of the embodiment in that the cross-sectional area orthogonal to the flow direction of the exhaust pipe is uniform throughout the extension direction of the exhaust pipe and that the cross-sectional shape of the exhaust pipe is uniform throughout the extension direction of the exhaust pipe. 
     As shown in  FIG.  10   , it was confirmed that the exhaust structure (graphs R 1  and R 2 ) of the embodiment has greatly improved output characteristics of the throttle opening degree of 37.5% and the throttle opening degree of 50% near the engine rotation speed of 10000 [r/min] compared to the exhaust structure (graphs S 1  and S 2 ) of the comparison example. Accordingly, it was found that according to the exhaust structure of the embodiment, it is possible to improve the output of the engine. 
     &lt;Action and Effect&gt; 
     As described above, the exhaust structure  29  of the motorcycle  1  of the embodiment described above includes: the exhaust pipe  30  that is connected to the exhaust port  13   ex  connecting to the combustion chamber  11   a  of the engine  11  and has a circular cross-sectional shape which is orthogonal to the exhaust flow direction; and the muffler  50  that is connected to the downstream side in the exhaust flow direction of the exhaust pipe  30 , wherein the exhaust pipe  30  includes the second front pipe  33  that is connected to the muffler  50 , the first front pipe  32  that is connected to the upstream side in the exhaust flow direction of the second front pipe  33 , and the third front pipe  34  that is connected to the downstream side in the exhaust flow direction of the second front pipe  33 , the cross-sectional area A 2  that is orthogonal to the exhaust flow direction of the second front pipe  33  is larger than each of the minimum value A1min of the cross-sectional area that is orthogonal to the exhaust flow direction of the first front pipe  32  and the minimum value A3min of the cross-sectional area that is orthogonal to the exhaust flow direction of the third front pipe  34 , and the vehicle width direction size W 2  of the cross-sectional shape of the second front pipe  33  and the vertical direction size H 2  of the cross-sectional shape of the second front pipe  33  are different from each other. 
     According to this configuration, by the cross-sectional area A 2  that is orthogonal to the exhaust flow direction of the second front pipe  33  being larger than each of the minimum value A1min of the cross-sectional area that is orthogonal to the exhaust flow direction of the first front pipe  32  and the minimum value A3min of the cross-sectional area that is orthogonal to the exhaust flow direction of the third front pipe  34 , since it is possible to adjust the pulsation of the exhaust gas in the exhaust pipe  30  and actively suction the combustion gas in the combustion chamber  11   a  of the engine  11 , it is possible to improve the output of the engine  11 . Additionally, the vehicle width direction size W 2  of the cross-sectional shape of the second front pipe  33  and the vertical direction size H 2  of the cross-sectional shape of the second front pipe  33  are different from each other, and thereby, it is possible to use an arrangement that prevents an increase in size of the vehicle or an arrangement that prevents the effect of interference on another configuration component. Accordingly, it is possible to suitably arrange the exhaust pipe  30  while improving the output of the engine  11 . 
     In the embodiment described above, the engine  11  includes the crankcase  12  and the cylinder  13  that stands from the crankcase  12  and that has the exhaust port  13   ex , the exhaust pipe  30  is connected to the exhaust port  13   ex , is curved, passes the right side of the cylinder  13 , then passes above the crankcase  12 , and extends rearward and upward, and the vehicle width direction size W 2  of the cross-sectional shape of the second front pipe  33  is smaller than the vertical direction size H 2  of the cross-sectional shape of the second front pipe  33 . Thereby, the following advantage is achieved. 
     Even in a case where the exhaust pipe  30  passes above the crankcase  12  and extends rearward and upward, since the second front pipe  33  does not occupy a space in the vehicle width direction, it is possible to prevent an increase in size in the vehicle width direction. Accordingly, it is possible to achieve the output improvement of the engine  11  and prevention of an increase in size in the vehicle width direction. 
     In the embodiment described above, the vehicle width direction size W 2  of the cross-sectional shape of the second front pipe  33  is smaller than the maximum value W1max of the vehicle width direction size W 1  of the cross-sectional shape of the first front pipe  32 , and thereby, the following advantage is achieved. 
     It is possible to further prevent an increase in size in the vehicle width direction. 
     In the embodiment described above, the engine  11  is supported by the vehicle body frame  20 , the vehicle body frame  20  includes the main frame  22  that extends rearward and downward from the head pipe  21  and the pivot plate  25  that extends downward from the rear end part of the main frame  22 , and the second front pipe  33  passes the inside in the vehicle width direction of the pivot plate  25  and overlaps the pivot plate  25  when seen from the vehicle width direction. Thereby, the following advantage is achieved. 
     Since a foot part of a rider is generally located on the side of the pivot plate  25 , the second front pipe  33  passes the inside in the vehicle width direction of the pivot plate  25 , and thereby, it is possible to reduce a thermal impact on the foot part of the rider. Additionally, the second front pipe  33  overlaps the pivot plate  25  when seen from the vehicle width direction, and thereby, it is possible to further prevent an increase in size in the vehicle width direction. 
     In the embodiment described above, the swing arm  5  is swingably supported by the pivot plate  25 , the swing arm  5  and the vehicle body frame  20  are connected by the rear cushion  6 , and the second front pipe  33  is arranged between the pivot plate  25  and the rear cushion  6  in the vehicle width direction. Thereby, the following advantage is achieved. 
     It is possible to further prevent an increase in size in the vehicle width direction. 
     In the embodiment described above, the connection member  60  that connects the vehicle body frame  20  to the exhaust pipe  30  is provided, and the connection member  60  is welded to at least a surface of the second front pipe  33  having a larger one of the vehicle width direction size W 2  of the cross-sectional shape and the vertical direction size H 2  of the cross-sectional shape. Thereby, the following advantage is achieved. 
     Since a surface having a larger one of the vehicle width direction size W 2  of the cross-sectional shape and the vertical direction size H 2  of the cross-sectional shape in the second front pipe  33  has a larger curvature radius than a surface having a smaller one of the vehicle width direction size W 2  of the cross-sectional shape and the vertical direction size H 2  of the cross-sectional shape in the second front pipe  33 , in comparison with a case where the connection member  60  is welded to the surface of the second front pipe  33  having a smaller one of the vehicle width direction size W 2  of the cross-sectional shape and the vertical direction size H 2  of the cross-sectional shape, welding work is facilitated. 
     MODIFIED EXAMPLE 
     The above embodiment is described using an example in which the exhaust pipe  30  is connected to the exhaust port  13   ex , is curved, passes a right side of the cylinder  13 , then passes above the crankcase  12 , and extends rearward and upward; however, the embodiment is not limited thereto. For example, the exhaust pipe  30  may be connected to the exhaust port  13   ex , be curved, pass below the crankcase  12 , and then extend rearward and upward. For example, the configuration of the exhaust pipe  30  can be changed in accordance with a requirement specification. 
     The above embodiment is described using an example in which the exhaust pipe  30  includes the second front pipe  33  that is connected to the muffler  50 , the first front pipe  32  that is connected to the upstream side in the exhaust flow direction of the second front pipe  33 , and the third front pipe  34  that is connected to the downstream side in the exhaust flow direction of the second front pipe  33 ; however, the embodiment is not limited thereto. For example, the exhaust pipe may include a muffler connection part that is connected to the muffler  50 , an exhaust pipe upstream part that is positioned on an upstream side in the exhaust flow direction of the muffler connection part, and an exhaust pipe downstream part that is positioned on a downstream side in the exhaust flow direction of the muffler connection part. That is, the exhaust pipe may not be a member in which the first front pipe  32 , the second front pipe  33 , and the third front pipe  34  are formed of a separate member and connected together and may be a member (an integrated object) in which the exhaust pipe upstream part, the muffler connection part, and the exhaust pipe downstream part are integrally formed of the same member. For example, the configuration of the exhaust pipe upstream part, the muffler connection part, and the exhaust pipe downstream part can be changed in accordance with a requirement specification. 
     The above embodiment is described using an example in which the vehicle width direction size W 2  of the cross-sectional shape of the second front pipe  33  is smaller than the vertical direction size H 2  of the cross-sectional shape of the second front pipe  33 ; however, the embodiment is not limited thereto. For example, the vehicle width direction size W 2  of the cross-sectional shape of the second front pipe  33  may be larger than the vertical direction size H 2  of the cross-sectional shape of the second front pipe  33 . For example, the vehicle width direction size W 2  of the cross-sectional shape of the second front pipe  33  and the vertical direction size H 2  of the cross-sectional shape of the second front pipe  33  may be different from each other. 
     The above embodiment is described using an example in which the vehicle width direction size W 2  of the cross-sectional shape of the second front pipe  33  is less than the maximum value W1max of the vehicle width direction size W 1  of the cross-sectional shape of the first front pipe  32 ; however, the embodiment is not limited thereto. For example, the vehicle width direction size W 2  of the cross-sectional shape of the second front pipe  33  may be a size equal to or more than the maximum value W1max of the vehicle width direction size W 1  of the cross-sectional shape of the first front pipe  32 . For example, the size relationship between the vehicle width direction size W 2  of the cross-sectional shape of the second front pipe  33  and the vehicle width direction size W 1  of the cross-sectional shape of the first front pipe  32  can be changed in accordance with a requirement specification. 
     The above embodiment is described using an example in which the second front pipe  33  passes the inside in the vehicle width direction of the pivot plate  25  and overlaps the pivot plate  25  when seen from the vehicle width direction; however, the embodiment is not limited thereto. For example, the second front pipe  33  may pass the outside in the vehicle width direction of the pivot plate  25 . For example, the second front pipe  33  may be provided at a position that does not overlap the pivot plate  25  when seen from the vehicle width direction. 
     The above embodiment is described using an example in which the second front pipe  33  is arranged between the pivot plate  25  and the rear cushion  6  in the vehicle width direction; however, the embodiment is not limited thereto. For example, the second front pipe  33  may be arranged in a region other than the space between the pivot plate  25  and the rear cushion  6  in the vehicle width direction. For example, the arrangement configuration of the second front pipe  33  can be changed in accordance with a requirement specification. 
     The above embodiment is described using an example in which the connection member  60  that connects the vehicle body frame  20  to the exhaust pipe  30  is provided, and the connection member  60  is welded to the right side surface (a surface having a larger one of the vehicle width direction size W 2  of the cross-sectional shape and the vertical direction size H 2  of the cross-sectional shape) of the second front pipe  33 ; however, the embodiment is not limited thereto. For example, the connection member  60  may be welded to the left side surface (an inside surface in the vehicle width direction) of the second front pipe  33 . For example, the connection member  60  may be welded to an upper surface or a lower surface (a surface having a smaller one of the vehicle width direction size W 2  of the cross-sectional shape and the vertical direction size H 2  of the cross-sectional shape) of the second front pipe  33 . For example, the connection member  60  may be joined to the second front pipe  33  by means other than welding. For example, the joint configuration of the connection member  60  with the second front pipe  33  can be changed in accordance with a requirement specification. 
     The above embodiment is described using an example in which the engine  11  is a single cylinder engine; however, the embodiment is not limited thereto. For example, the engine  11  may be a multi-cylinder engine. For example, the configuration of the engine  11  can be changed in accordance with a requirement specification. 
     The above embodiment is described using a motorcycle in which an engine is mounted on the vehicle body side as an example of a saddle-riding type vehicle; however, the embodiment is not limited thereto. For example, the saddle-riding type vehicle may be a unit-swing-type motorcycle. For example, the configuration of the saddle-riding type vehicle can be changed in accordance with a requirement specification. 
     The above embodiment is described using a configuration in which a transmission transmits the drive force of the engine  11  to the rear wheel  4 ; however, the embodiment is not limited thereto. For example, a configuration may be used in which the transmission transmits the drive force of the engine  11  to the front wheel  3 . For example, the configuration in which the drive force of the engine  11  is transmitted to the drive wheel can be changed in accordance with a requirement specification. 
     The present invention is not limited to the embodiment described above. For example, the saddle-riding type vehicle includes all types of vehicles on which a driver rides by straddling a vehicle body and includes not only a motorcycle (including a motorized bicycle and a scooter-type vehicle) but also a vehicle having three wheels (including a vehicle having two front wheels and one rear wheel in addition to a vehicle having one front wheel and two rear wheels). Further, the present invention is applicable to not only a motorcycle but also a vehicle having four wheels such as an automobile. 
     The configurations in the embodiment described above are examples of the present invention, and various changes such as replacing the constituent elements of the embodiment with known constituent elements can be made without departing from the scope of the present invention.