Patent Publication Number: US-11384672-B2

Title: Secondary air introduction device

Description:
CROSS REFERENCE TO THE RELATED APPLICATION 
     This application is based on and claims Convention priority to Japanese patent application No. 2020-171037, filed Oct. 9, 2020, the entire disclosure of which is herein incorporated by reference as a part of this application. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a secondary air introduction device configured to supply air into an exhaust passage of an engine to combust unburned gas contained in exhaust gas. 
     Description of Related Art 
     Some engines used as a drive source in a vehicle such as a motorcycle have been known in which a secondary air introduction device is disposed in an exhaust passage thereof (for example, JP Laid-open Patent Publication No. H10-030434). The secondary air introduction device includes a secondary air valve configured to open and close an air introduction passage for introducing air into the exhaust passage. In JP Laid-open Patent Publication No. H10-030434, the secondary air valve is attached to a vehicle body frame. 
     In the structure of the JP Laid-open Patent Publication No. H10-030434, the secondary air valve is attached to the vehicle body frame. Accordingly, the secondary air valve is likely to undergo a large displacement with respect to the engine. For this reason, it is necessary to provide a rubber mount to support the secondary air valve or to constitute a part of a pipe connecting the secondary air valve with the exhaust passage by a rubber tube in order to suppress vibration. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a secondary air introduction device capable of suppressing relative vibration between a secondary air valve and a cylinder head. 
     In order to achieve the above object, the present invention provides a secondary air introduction device configured to supply air into an exhaust passage of an engine to combust unburned gas contained in exhaust gas, the secondary air introduction device including: an air introduction passage configured to introduce air into the exhaust passage; and a secondary air valve configured to be attached to a cylinder head of the engine and to control opening and closing of the air introduction passage. The secondary air valve is configured to be attached to an outer lateral surface of the cylinder head on one side in an axial direction of a crankshaft, and the secondary air valve includes an air introduction inlet formed in a position opposite to the outer lateral surface of the cylinder head in the axial direction of the crankshaft. For example, the position, in which the air introduction inlet is formed in the secondary air valve, overlaps with the cylinder head when viewed in the axial direction of the crankshaft. 
     According to this constitution, the secondary air valve is attached to the cylinder head, and therefore, relative vibration between the secondary air valve and the cylinder head can be suppressed. This offers a greater freedom in a support structure for the secondary air valve and in material selection for a pipe connecting the secondary air valve with the exhaust passage. Further, the secondary air valve is formed with the air introduction inlet in the position opposite to the outer lateral surface of the cylinder head, and therefore, it is possible to prevent interference between intake and exhaust pipes and pipes connected to the secondary air valve and thus to easily arrange the secondary air valve on the cylinder head. 
     In the present invention, the secondary air valve may include an elastic valve body having a plate shape, and the secondary air valve may be arranged such that a thickness direction of the valve body corresponds to the axial direction of the crankshaft. According to this constitution, the secondary air valve protrudes less with respect to the cylinder head in the axial direction of the crankshaft, which makes it easy to dispose the secondary air valve on the outer lateral surface of the cylinder head. 
     In the present invention, the secondary air valve may be disposed on one side in the axial direction of the crankshaft with respect to a power transmission member configured to transmit power to a valve mechanism of the engine, and an exhaust port may be provided on the other side with respect to the power transmission member. According to this constitution, the secondary air valve is disposed away, in the axial direction of the crankshaft, from the exhaust port by a space occupied by the power transmission member. Thus, heat transfer from the exhaust port to the secondary air valve can be mitigated, and therefore, influence of exhaust heat can be suppressed. 
     In the present invention, an exhaust pipe of the engine may extend on the other side of the cylinder head in the axial direction of the crankshaft. According to this constitution, the secondary air valve is disposed in an area opposite to the exhaust pipe with the cylinder head interposed therebetween. Thus, the secondary air valve can be disposed at a position distant from the exhaust pipe. Therefore, heat transfer from the exhaust pipe to the secondary air valve can be mitigated, and therefore, influence of exhaust heat can be suppressed. 
     In the present invention, the secondary air valve may be configured to be attached to the cylinder head through a plug member configured to close a passage defined inside the cylinder head. This constitution eliminates the necessity of providing a dedicated structure for attaching the secondary air valve to the cylinder head. 
     In the present invention, a pipe constituting a downstream part of the air introduction passage and connecting the secondary air valve with the exhaust passage may be made of a metal material. According to this constitution, the pipe of the downstream part of the air introduction passage can have improved heat resistance. Since the secondary air valve and the exhaust passage (cylinder head) constitute a same vibration system as described above, it is possible prevent the pipe of the downstream part connecting these components from having greater vibration, or a larger amplitude of vibration. Thus, even where the pipe of the downstream part is made of a metal material having excellent heat resistance, it is possible to suppress stress occurring in the pipe support part. 
     In the present invention, a pipe constituting a downstream part of the air introduction passage and connecting the secondary air valve with the exhaust passage may extend in the axial direction of the crankshaft in an area lower than a top part of a cylinder head cover. This constitution makes it easy to ensure a clearance between the pipe and a component disposed above the cylinder head cover and thus to avoid interference between the air introduction passage and other components. 
     In the present invention, the secondary air introduction device may further include a bracket configured to attach the secondary air valve to the cylinder head, wherein the bracket may include a support part configured to support a pipe constituting an upstream part of the air introduction passage, the upstream part being located on an upper side with respect to the secondary air valve. According to this constitution, the bracket for attaching the secondary air valve also serves to support the pipe, and therefore, the number of components can be reduced. 
     Any combination of at least two constructions, disclosed in the appended claims and/or the specification and/or the accompanying drawings should be construed as included within the scope of the present invention. In particular, any combination of two or more of the appended claims should be equally construed as included within the scope of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will be more clearly understood from the following description of preferred embodiments thereof, when taken in conjunction with the accompanying drawings. However, the embodiments and the drawings are given only for the purpose of illustration and explanation, and are not to be taken as limiting the scope of the present invention in any way whatsoever, which scope is to be determined by the appended claims. In the accompanying drawings, like reference numerals are used to denote like parts throughout the several views: 
         FIG. 1  is a side view illustrating a front part of a motorcycle including an engine including a secondary air introduction device according to a first embodiment of the present invention; 
         FIG. 2  is a side view of the engine; 
         FIG. 3  is a plan view of the engine; 
         FIG. 4  is a side view of the engine, with the secondary air introduction device being removed; 
         FIG. 5  is a vertical section view of the engine along a plane parallel to a widthwise direction of the motorcycle; 
         FIG. 6  is an exploded view of a secondary air valve of the secondary air introduction device; and 
         FIG. 7  is an exploded view of a support structure for the secondary air introduction device. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Hereinafter, a preferred embodiment of the present invention will be described with reference to the drawings.  FIG. 1  is a side view illustrating a front part of a motorcycle including an engine including a secondary air introduction device according to a first embodiment of the present invention. In the present specification, the “left” and “right” sides correspond to the “left” and “right” sides as seen by a rider on a vehicle, respectively. The “front” and “rear” sides correspond to the “front” and “rear” sides with respect to a direction in which the vehicle moves, respectively. Further, the “upstream” and “downstream” sides correspond to the “upstream” and “downstream” sides with respect to a direction in which air flows, respectively. 
     A vehicle body frame FR of the motorcycle in the present embodiment includes a main frame  1  constituting a front half of the vehicle body frame FR and a rear frame  2  constituting a rear half of the vehicle body frame FR. The rear frame  2  is coupled to a rear part of the main frame  1 . 
     A front fork  6  is turnably supported by a head pipe  4  disposed at front end of the main frame  1  through a steering shaft (non-illustrated). A front wheel  8  is mounted to a lower end of the front fork  6 . A handlebar  10  is mounted to an upper end portion of the front fork  6 . 
     A swing arm bracket  12  is provided to a rear end portion of the main frame  1 . The swing arm bracket  12  supports a swing arm  14  in a vertically swingable manner. A rear wheel (not illustrated) is mounted to a rear end portion of the swing arm  14 . 
     An engine E, which is a drive source of the motorcycle, is mounted in a lower part of the main frame  1  and in front of the swing arm bracket  12 . The engine E is configured to drive the rear wheel through a power transmission member  18  such as a chain. A fuel tank  22  is disposed in an upper part of the main frame  1 , and a seat  24  for a rider is mounted on the rear frame  2 . 
     The engine E in the present embodiment is a water-cooled single-cylinder engine. However, the engine is not limited to this type and may be an air-cooled engine or a multi-cylinder engine. The engine E includes: a crankcase  28  for rotatably supporting a crankshaft  26 ; a cylinder  30  protruding upward from the crankcase  28 ; a cylinder head  32  coupled to an upper part of the cylinder  30 ; and a cylinder head cover  34  attached to an upper part of the cylinder head  32 . The engine E is supported by the vehicle body frame FR and is disposed between the front wheel  8  and the rear wheel (not illustrated). In the present embodiment, the engine E is disposed between the head pipe  4  and the seat  24  in a front to rear (front-rear) direction or longitudinal direction of the motorcycle. 
     In the present embodiment, an axis  26   a  of the crankshaft  26  extends in a vehicle widthwise direction (a left to right direction) of the motorcycle. In the following description, a left side of the vehicle body is called as one side in an axial direction of the crankshaft  26 , and a right side of the vehicle body is called as the other side in the axial direction of the crankshaft  26 . A side stand (not illustrated) is provided on the left side of the vehicle (on the one side in the axial direction of the crankshaft  26 ). 
     The cylinder head  32  has a rear surface formed with an air intake port  36  and a front surface formed with an exhaust port  38 . A fuel supply device  40  is connected to the air intake port  36 , and an air cleaner  42  is connected to the fuel supply device  40 . The air cleaner  42  is configured to filter outside air to generate clean air. The fuel supply device  40  is configured to inject fuel into the clean air from the air cleaner  42  to generate air-fuel mixture and supply the mixture to the air intake port  36 . The fuel supply device  40  may be, for example, a throttle body or a carburetor with a fuel injection valve. 
     An exhaust pipe  44  is connected to the exhaust port  38 . The exhaust pipe  44  extends frontward from a front surface of the cylinder head  32 , then is curved rearward in a U-shaped manner, and then extends rearward on the right side of the cylinder head  32  to be connected to an exhaust muffler (not illustrated). A part of the exhaust pipe  44 , which extends on the right side of the cylinder head  32  (on the other side in the axial direction of the crankshaft  26 ), extends in an area above the axis  26   a  of the crankshaft  26  of the engine E and below an upper end of the cylinder head  32 . 
     As shown in  FIG. 4 , a combustion chamber  30   a  is defined inside the cylinder  30 , and an intake passage  32   a  and an exhaust passage  32   b  are defined inside the cylinder head  32 . The intake passage  32   a  connects the air intake port  36  with the combustion chamber  30   a , and the exhaust passage  32   b  connects the combustion chamber  30   a  with the exhaust port  38 . That is, the mixture is introduced to the combustion chamber  30   a  through the intake passage  32   a , and exhaust gas is discharged from the combustion chamber  30   a  through the exhaust passage  32   b.    
     An outlet of the intake passage  32   a  is opened and closed by an intake valve  46 , and an inlet of the exhaust passage  32   b  is opened and closed by an exhaust valve  48 . The cylinder head  32  is provided with a valve mechanism (not illustrated) configured to control the intake valve  46  and the exhaust valve  48 . The valve mechanism is a known component and may be constituted by, for example, a camshaft and a cam disposed therein. 
     The valve mechanism is configured to be driven by power of the crankshaft  26  ( FIG. 1 ) of the engine E. Specifically, the power of the crankshaft  26  ( FIG. 1 ) is transmitted to the valve mechanism by a power transmission member  50  as shown in  FIG. 5 . Examples of the power transmission member  50  may include a belt and a chain etc. The power transmission member  50  is disposed on the one side in the axial direction of the crankshaft  26  of the engine E (i.e., on the left side of the engine E in the present embodiment). More specifically, the power transmission member  50  is disposed in a housing space  52  provided on the left side with respect to the combustion chamber  30   a , the intake passage  32   a  ( FIG. 4 ) and the exhaust passage  32   b  of the engine E. That is, the cylinder head  32  has an axial distance L 1  from a cylinder axis CY to a left-side surface (i.e., a surface on the one side in the axial direction of the crankshaft  26 ) of the cylinder head, which is larger than an axial distance L 2  from the cylinder axis CY to a right-side surface (i.e., a surface on the other side in the axial direction of the crankshaft  26 ) of the cylinder head by a space occupied by the housing space  52  formed therein. 
     The engine E is provided with a secondary air introduction device  54 . The secondary air introduction device  54  is configured to supply the air from the air cleaner  42  into the exhaust passage  32   b  and to combust unburned gas contained in the exhaust gas. As shown in  FIG. 1 , the secondary air introduction device  54  includes: an air introduction passage  56  for introducing the air into the exhaust passage  32   b ; and a secondary air valve  58  for controlling opening and closing of the air introduction passage  56 . 
     The secondary air valve  58  allows the air to flow only in a single direction. In the present embodiment, a reed valve is used as the secondary air valve  58 . However, the secondary air valve  58  is not limited to a reed valve. The secondary air valve  58  is attached to the outer lateral surface of the cylinder head  32  on the one side in the axial direction of the crankshaft  26  and is disposed on an outside of the cylinder head  32 . In the present embodiment, the entirety of the secondary air valve  58  overlaps with the cylinder head  32  when viewed in the axial direction of the crankshaft  26 . In the present embodiment, the secondary air valve  58  is disposed at a center part of the outer lateral surface of the cylinder head  32  in the front-rear direction. 
     Specifically, the secondary air valve  58  is disposed between an intake camshaft and an exhaust camshaft in the front-rear direction. In other words, the secondary air valve  58  is disposed in an area between the intake valve  46  and the exhaust valve  48 . 
     Specifically, as shown in  FIG. 5 , the secondary air valve  58  is attached to a lateral surface opposite to the exhaust port  38  ( FIG. 4 ) with the housing space  52  located therebetween. That is, the secondary air valve  58  is disposed on the one side in the axial direction of the crankshaft  26  (on the left side in the present embodiment) with respect to the power transmission member  50  of the engine E, and the exhaust port  38  is provided on the other side (on the right side in the present embodiment) with respect to the power transmission member  50 . The secondary air valve  58  is attached to a lateral surface, of the outer lateral surface of the cylinder head  32 , located opposite to the exhaust pipe  44 . That is, in the present embodiment, the secondary air valve  58  is attached to a left-side surface of the cylinder head  32 . The secondary air valve  58  will be described in terms of details of its features and a support structure therefor. 
     As shown in  FIG. 1 , the air introduction passage  56  includes: an upstream part  60  connecting the secondary air valve  58  with the air cleaner  42 ; and a downstream part  62  connecting the secondary air valve  58  with the exhaust passage  32   b  ( FIG. 5 ). The upstream part  60  of the air introduction passage  56  extends rearward above the cylinder head cover  34  along the main frame  1  and is connected to the air cleaner  44 . 
     The upstream part  60  of the air introduction passage  56  is constituted by, for example, a rubber tube. As shown in  FIG. 2 , the upstream part  60  of the air introduction passage  56  is provided with an adjustment valve  64 . The adjustment valve  64  is configured to adjust a timing for supplying the secondary air. For example, the adjustment valve may be an electromagnetic valve. The adjustment valve  64  is controlled by e.g. an electronic control unit (ECU). In the present embodiment, the adjustment valve  64  is disposed at a lower front position with respect to the secondary air valve  58  on the left side of the cylinder  30 . However, the position of the adjustment valve  64  is not limited to this position. The adjustment valve  64  may be omitted. 
     The downstream part  62  of the air introduction passage  56  is constituted by a pipe made of a metal material. In the present embodiment, the downstream part  62  of the air introduction passage  56  is constituted by a steel pipe. The downstream part  62  of the air introduction passage  56  extends upward from the secondary air valve  58  and then extends above the cylinder head cover  34  in the axial direction of the crankshaft  26  (i.e., in the vehicle widthwise direction) as shown in  FIG. 3 . The downstream part  62  of the air introduction passage  56  then extends obliquely rearward and downward over the cylinder head cover  34  and is connected to the exhaust passage  32   b  of the cylinder head  32 . 
     As shown in  FIG. 2 , the downstream part  62  of the air introduction passage  56  extends upward so as to be inclined obliquely forward. The downstream part  62  of the air introduction passage  56  then extends in an area lower than a top part  34   a  of the cylinder head cover  34  in the axial direction of the crankshaft  26  (i.e., in the vehicle widthwise direction). In other words, the downstream part  62  of the air introduction passage  56  extends upward and obliquely forward so as to avoid passing the top part  34   a  of the cylinder head cover  34 . As described above, the secondary air valve  58  is disposed obliquely, and therefore, the pipe extending upward and obliquely forward can be arranged so as to take a shortest path. 
     As shown in  FIG. 3 , the downstream part  62  of the air introduction passage  56  according to the present embodiment includes: a tube body  62   a  constituted by a steel tube; and flanges  62   b ,  62   c  provided on opposite ends of the tube body  62   a . The tube body  62   a  and the flanges  62   b ,  62   c  are jointed by e.g. welding. 
     An upstream end of the downstream part  62  of the air introduction passage  56  is bolted to the secondary air valve  58  through the flange  62   b , and a downstream end of the downstream part  62  of the air introduction passage  56  is bolted to the cylinder head  32  through the flange  62   c . Further, a middle part of the tube body  62   a  of the downstream part  62  of the air introduction passage  56  is supported by the cylinder head cover  34 . Specifically, the middle part of the tube body  62   a  is provided with a support stay  66  fixed thereto by welding and is attached to the cylinder head cover  34  by a bolt  65  through the support stay  66 . 
     Thus, the downstream part  62  of the air introduction passage  56  is supported at the opposite ends and the middle part thereof, so that the downstream part  62  of the air introduction passage  56  is stably supported. However, the support structure for the downstream part  62  of the air introduction passage  56  is not limited to this configuration. 
     As shown in  FIG. 6 , the secondary air valve  58  includes a first casing  68 , a second casing  70 , and a valve member  69 , and the first and second casings  68 ,  70  are coupled to each other by a bolt  72  with the valve member  69  interposed therebetween. The first casing  68  has an air inlet part  68   a  formed at an upper part of the first casing  68 , and the second casing  70  has an air outlet part  70   a  formed at an upper part of the second casing  70 . The air inlet part  68   a  is formed with an air introduction inlet  68   aa  for supplying the air to the secondary air valve  58 . The air outlet part  70   a  is formed with an air introduction outlet  70   aa  for discharging the air from the secondary air valve  58 . In the present embodiment, the air inlet part  68   a  is constituted by a plug pin, and the air outlet part  70   a  is constituted by a flange. 
     The valve member  69  is a reed valve and includes a valve body  69   a  constituted by a plate member capable of opening only in a single direction. A valve chamber  71  is defined inside the first casing  68  and the second casing  70  and is partitioned by a valve body  69   a  to define an upstream valve chamber  71   a  and a downstream valve chamber  71   b . That is, the upstream valve chamber  71   a  is a space upstream of the valve body  69   a  in a direction of air flow, and the downstream valve chamber  71   b  is a space downstream of the valve body  69   a . In the present embodiment, the upstream valve chamber  71   a  is defined by the first casing  68 , and the downstream valve chamber  71   b  is defined by the second casing  70 . 
     The first casing  68  is formed with a bolt insertion hole  74  opened in the vehicle widthwise direction. In the present embodiment, there are three bolt insertion holes  74 . However, the number of the bolt insertion holes  74  may be larger than or smaller than three. 
     As shown in  FIG. 2 , the air inlet part  68   a  of the secondary air valve  58  is disposed so as to be opposite to the outer lateral surface of the cylinder head  32  in the axial direction of the crankshaft  26 . In other words, the air inlet part  68   a  of the secondary air valve  58  overlaps with the cylinder head  32  when seen in a side view. As shown in  FIG. 3 , the air introduction inlet  68   aa  and the air introduction outlet  70   aa  are arranged in a same orientation. In the present embodiment, the air introduction inlet  68   aa  and the air introduction outlet  70   aa  are directed substantially upward. More specifically, the air introduction inlet  68   aa  and the air introduction outlet  70   aa  are opened upward and obliquely forward (toward the side of the exhaust port  38 ). Such an inclined arrangement makes it possible to stably support the secondary air valve  58  at a middle position on the outer lateral surface of the cylinder head  32  in the front-rear direction while reducing the bending of the downstream part  62  constituted by a metal pipe. 
     The air introduction inlet  68   aa  and the air introduction outlet  70   aa  are aligned in the axial direction of the crankshaft  26 . In the present embodiment, the air introduction outlet  70   aa  is located closer to the cylinder head  32  than the air introduction inlet  68   aa . This makes it possible to shorten the downstream part  62  constituted by a metal pipe and to dispose the upper pipe  60  constituted by a rubber hose away from the cylinder head  32 . Further, as shown in  FIG. 6 , the valve body  69   a  of the secondary air valve  58  is disposed such that a thickness direction of the valve body  69   a  coincides with the axial direction AX of the crankshaft  26 . 
     The support structure for the secondary air valve  58  will be described. As shown in  FIG. 2 , the secondary air valve  58  is attached to the outer lateral surface of the cylinder head  32  through a bracket  76 . Specifically, the bracket  76  is removably attached to the cylinder head  32  by bolts  94 , and then the secondary air valve  58  is removably attached to the bracket  76  by bolts  102 . 
     As shown in  FIG. 7 , the bracket  76  according to the present embodiment is formed by bending a metal plate member. The bracket  76  may be produced by any other method. The bracket  76  according to the present embodiment is shaped in an elongated form in the front-rear direction and is formed with bolt insertion holes  80  opened in the vehicle widthwise direction at opposite end portions of the bracket in the front-rear direction. 
     Further, the bracket  76  is formed with a threaded hole  82  opened in the vehicle widthwise direction at a position corresponding to that of the bolt insertion hole  74  of the secondary air valve  58 . In the present embodiment, there are threaded holes  82  each constituted by a cylindrical boss  84  joined to the bracket  76 . The constitution of the threaded hole  82  is not limited to the cylindrical boss  84 . 
     The bracket  76  has a front end portion formed with a support part  86 . The support part  86  supports the upstream part  60  of the air introduction passage  56  ( FIG. 2 ). The support part  86  according to the present embodiment includes: a protruding piece  86   a  which protrudes frontward from the bracket  76 ; and a through-hole  86   b  defined in the protruding piece  86   a . The constitution of the support part  86  is not limited to this structure. The support part  86  may be omitted. 
     The cylinder head  32  is formed with threaded holes  88  in the outer lateral surface of the cylinder head  32 . There are two such threaded holes  88  aligned in the front-rear direction. In the present embodiment, the threaded holes  88  are constituted by plug holes for processing an oil passage in the cylinder head  34 , and the plug holes are formed with female threads. The oil passage connects the intake valve  46  and the exhaust valve  48  with the housing space  52  and collects oil after valve lubrication in the housing space  52  ( FIG. 5 ). Thus, use of the plug holes formed during the manufacturing process of the engine E makes it possible to reduce the number of components and the number of operation steps. However, the threaded holes  88  are not limited to the plug holes. 
     Plug members  90  are fitted to the threaded holes  88 . The plug members  90  close a passage (the oil passage in the present embodiment) defined inside the cylinder head  32 . The plug members  90  are removably attached to the threaded holes  88  through O rings  92 . Each of the plug members  90  includes: a male thread part  90   a  configured to be fastened to the threaded hole  88 ; and a tubular boss part  90   c  having a female thread  90   b  formed thereinside. That is, the plug members  90  are fitted to the threaded holes  88  (plug holes) to close the oil passage and, at the same time, constitute mount parts for the secondary air valve  58 . 
     The bracket  76  is removably attached to the female threads  90   b  of the plug members  90  by the bolts  94 . Specifically, flanged collars  96  are inserted into the bolt insertion holes  80  of the bracket  76  from an inner side of the bracket in the vehicle widthwise direction, and the collars  96  are then inserted into tubular dampers  98  constituted by elastic members fitted into the bolt insertion holes  80 . Then, the bolts  94  are inserted into hollow holes of the collars  96  from an outer side of the bracket in the vehicle widthwise direction and is fastened to the female threads  90   b  of the plug members  90 . It should be noted that in the present embodiment, there are washers  100  interposed between heads  94   a  of the bolts  94  and the dampers  98 . 
     Further, the secondary air valve  58  is removably attached to the bracket  76  by the bolts  102 . Specifically, the bolts  102  are inserted into the bolt insertion holes  74  of the secondary air valve  58  from an outer side of the secondary air valve  58  in the vehicle widthwise direction and are fastened to the threaded holes  82  of the bracket  76 . In this way, the secondary air valve  58  is attached to the cylinder head  32  through the bracket  76  and the plug members  90 . 
     An upstream end of the downstream part  62  of the air introduction passage  56  is coupled to the air outlet part  70   a  (flange) of the secondary air valve  58  through the flange  62   b  by means of bolts  104 . There is a gasket  105  interposed between the flange  62   b  and the air outlet part  70   a  of the secondary air valve  58 . As described above, the secondary air valve  58  is attached to the cylinder head  32  through the dampers  98  constituted by the elastic members. Thus, even where the downstream part  62  of the air introduction passage  56  is constituted by a metal pipe as in the present embodiment, inclination of the pipe can be accommodated. 
     A downstream end of the upstream part  60  of the air introduction passage  56  as shown in  FIG. 1  is coupled to the air inlet part  68   a  of the secondary air valve  58 . Specifically, a rubber tube constituting the upstream part  60  is fitted to the air inlet part  68   a  (plug pin) of the secondary air valve  58  and is fastened by a cramp member  106  ( FIG. 2 ) from its outer periphery so as to be retained. 
     The upstream part  60  of the air introduction passage  56  passes through the adjustment valve  64 , extends upward on an outer side of the cylinder head  32  and the cylinder head cover  34  and then extends rearward to be connected to the air cleaner  42 . The upwardly extending part of the upstream part  60  on the outer lateral side of the cylinder head  32  is supported by the support part  86  of the bracket  76 . Specifically, a grip member  108  is attached to the through-hole  86   b  of the support part  86  of the bracket  76 , and the grip member  108  supports the rubber tube constituting the upstream part  60 . 
     The effects of the secondary air introduction device according to the present embodiment will be described. When the engine E as shown in  FIG. 1  starts operating, the mixture M of fuel and air is supplied to the engine E from the air cleaner  42  through the fuel supply device  40 . The mixture M is combusted in the combustion chamber  30   a  shown in  FIG. 4 , and exhaust gas G generated thereby is discharged from the exhaust port  38  to the outside of the engine through the exhaust passage  32   b . The exhaust gas G discharged from the inside of the combustion chamber  30   a  into the exhaust passage  32   b  contains unburned gas. 
     At this point, the inside of the engine has negative pressure, and the valve body  69   a  ( FIG. 6 ) of the secondary air valve  58  is open. Thus, when the adjustment valve  64  is opened in response to a command from the electronic control unit (ECU), air A (secondary air) from the air cleaner  42  is introduced into the exhaust passage  32   b  inside the cylinder head  32  shown in  FIG. 4  through the air introduction passage  56 . Thanks to the secondary air A, the unburned gas contained in the exhaust gas G is combusted, so that the contents of hydrocarbon (HC) and carbon monoxide (CO) in the exhaust gas are reduced. 
     According to the above constitution, the secondary air valve  58  shown in  FIG. 1  is attached to the cylinder head  32 , and therefore, relative vibration between the secondary air valve  58  and the cylinder head  32  can be suppressed. This offers a greater freedom in the support structure for the secondary air valve  58  and in material selection for the pipe (the upstream part  60  of the air introduction passage  56 ) connecting the secondary air valve  58  with the exhaust passage  32   b  ( FIG. 4 ). Further, the air inlet part  68   a  of the secondary air valve  58  is formed in the position opposite to or away from the outer lateral surface of the cylinder head  32 , and therefore, it is possible to prevent interference between pipes connected to the intake and exhaust ports  36 ,  38  (i.e., an intake pipe (the fuel supply device  40 ) and the exhaust pipe  44 ) and the pipes connected to the secondary air valve  58  (i.e., the upstream part  60  and the downstream part  62 ) and thus to easily arrange the secondary air valve  58  on the cylinder head  32 . 
     Since the secondary air valve  58  is provided on the outer lateral surface of the cylinder head  32 , a clearance between the engine E and a member disposed above the engine can be more easily ensured, as compared with a case where the secondary air valve is provided on an upper part of the cylinder head cover  34 . The engine E of the present embodiment is a single-cylinder engine which has, for example, an ignition plug (not illustrated) on a right-side surface of the cylinder head  32  (a lateral surface opposite to the left-side surface where the secondary air valve  58  is attached). Thus, the secondary air valve  58  does not disturb incoming wind to be introduced into the area in the vicinity of the ignition plug. 
     Since the thickness direction of the valve body  69   a  of the secondary air valve  58  as shown in  FIG. 6  coincides with the axial direction AX of the crankshaft  26 , the secondary air valve  58  as shown in  FIG. 3  protrudes less with respect to cylinder head  32  in the axial direction of the crankshaft  26 , which makes it easy to dispose the secondary air valve  58  on the outer lateral surface of the cylinder head  32 . 
     As shown in  FIG. 5 , the secondary air valve  58  is attached to the lateral surface opposite to the exhaust port  38  with the power transmission member  50  located therebetween. That is, the secondary air valve  58  is disposed on the left side of the power transmission member  50 , and the exhaust port  38  is provided on the right side. Thus, the secondary air valve  58  is disposed away from the exhaust port  38  by the housing space  52  of the power transmission member  50 . This makes it possible to suppress influence of heat of the exhaust gas from the exhaust port  38  on the secondary air valve  58 . 
     The secondary air valve  58  is attached to the lateral surface opposite to the exhaust pipe  44  on the outer lateral surface of the cylinder head  32 . That is, the secondary air valve  58  is disposed in an area on the left side of the cylinder head  32 , and the exhaust pipe  44  is disposed in an area on the right side of the cylinder head  32 . This makes it possible to suppress influence of heat from the exhaust pipe  44  on the secondary air valve  58 . 
     As shown in  FIG. 7 , the secondary air valve  58  is attached to the cylinder head  32  through the plug members  90 . This eliminates the necessity of providing a dedicated structure for attaching the secondary air valve  58  to the cylinder head  32 , and therefore, the number of processing steps for the cylinder head  32  can be reduced. 
     The downstream part  62  connecting the secondary air valve  58  with the exhaust passage  32   b  ( FIG. 4 ) in the air introduction passage  56  as shown in  FIG. 2  is constituted by a metal pipe. Due to heat radiation from the engine E, the downstream part  62  of the air introduction passage  56  tends to have high temperature, so that the downstream part  62  is required to have improved heat resistance. By constituting the downstream part  62  by a metal pipe according to the above constitution, the downstream part  62  of the air introduction passage  56  can have improved heat resistance. In the present invention, since the secondary air valve  58  and the exhaust passage  32   b  (cylinder head  32 ) constitute a same vibration system, it is possible to prevent the pipe of the downstream part  62  connecting these components  58 ,  32  from having greater vibration, or a larger amplitude of vibration. Thus, even where the downstream part  62  is made of a metal material having excellent heat resistance, it is possible to suppress stress occurring in the pipe support part. 
     The downstream part  62  of the air introduction passage  56  extends in the vehicle widthwise direction in an area lower than the top part  34   a  of the cylinder head cover  34 . This constitution makes it easy to ensure a clearance between the downstream part  62  and a component disposed above the cylinder head cover  34  and to avoid interference between the air introduction passage  56  and other components. 
     The secondary air valve  58  is attached to the cylinder head  32  through the bracket  76 , and the bracket  76  is provided with the support part  86  for supporting the upstream part  60  of the air introduction passage  56 . Thus, the bracket  76  for attaching the secondary air valve  58  also serves to support the pipe, and therefore, the number of components can be reduced. 
     The present invention is not limited to the above embodiments, and various additions, modifications, or deletions may be made without departing from the scope of the present invention. For example, although the above embodiments are described with reference to an example in which the secondary air introduction device according to the present invention is applied to a motorcycle, the secondary air introduction device according to the present invention may also be applied to engines of vehicles other than motorcycles. Further, the secondary air introduction device according to the present invention may also be applied to engines of vehicles other than wheeled vehicles as well as to stationary engines on the ground. Accordingly, such variants are included within the scope of the present invention. 
     REFERENCE NUMERALS 
     
         
         
           
               26  . . . crankshaft 
               32  . . . cylinder head 
               32   b  . . . exhaust passage 
               34  . . . cylinder head cover 
               38  . . . exhaust port 
               44  . . . exhaust pipe 
               50  . . . power transmission member 
               54  . . . secondary air introduction device 
               56  . . . air introduction passage 
               58  . . . secondary air valve 
               60  . . . upstream part 
               62  . . . downstream part 
               68   a  . . . air inlet part 
               69   a  . . . valve body 
               76  . . . bracket 
               86  . . . support part 
               90  . . . plug member 
             E . . . engine