Patent Publication Number: US-10309267-B2

Title: Variable valve mechanism, engine, and automatic two-wheeled vehicle

Description:
INCORPORATION BY REFERENCE 
     This application is based upon and claims the benefit of priority from the corresponding Japanese Patent Application No. 2016-133290 filed on Jul. 5, 2016, the entire contents of which are incorporated herein by reference. 
     TECHNICAL FIELD 
     The present invention relates to a variable valve mechanism, an engine, and an automatic two-wheeled vehicle, and particularly, to a variable valve mechanism which can be applied to a single overhead camshaft (SOHC) type valve train, an engine, and an automatic two-wheeled vehicle. 
     BACKGROUND ART 
     Hitherto, as an engine of an automatic two-wheeled vehicle, there is known an engine including a variable valve mechanism changing operation characteristics (a valve opening/closing timing or a valve lift amount) of an intake valve and an exhaust valve in response to an engine rotation speed (for example, see JP 2012-225277 A). The variable valve mechanism described in JP 2012-225277 A is applied to a SOHC type valve train. Specifically, in JP 2012-225277 A, one cam shaft is provided with two kinds of cams (a low-speed cam and a high-speed cam) having different operation characteristics. Further, a rocker arm which drives an intake/exhaust valve is slidable in the axial direction of the cam shaft. When the rocker arm slides in response to an engine rotation speed, the low-speed cam and the high-speed cam can be switched. Thus, it is possible to obtain desired operation characteristics of the valve mechanism. 
     SUMMARY OF INVENTION 
     Technical Problem 
     However, JP 2012-225277 A has problems in that a configuration becomes complex due to diverse types of cams and an overall mechanism increases in size in the axial direction since a space for sliding the rocker arm needs to be ensured. 
     The invention has been made in view of the relevant points and an object of the invention is to provide a variable valve mechanism, an engine, and an automatic two-wheeled vehicle capable of realizing a simple and compact configuration. 
     Solution to Problem 
     An aspect of the present invention is summarized as a variable valve mechanism which changes an opening/closing timing of an intake valve or an exhaust valve in response to an engine rotation speed, the variable valve mechanism including: a cam sprocket which rotates in response to a rotation of a crank shaft; a first cam shaft which is integrated with any one of intake side and exhaust side cams; a second cam shaft which is integrated with the other cam; and a transfer member that transmits a rotation of the cam sprocket to the first and second cam shafts, wherein the first and second cam shafts are formed so that the other cam shaft is inserted through one cam shaft to be relatively rotatable; and the transfer member is provided to rotate along with any one of the first and second cam shafts and rotates relatively to the cam sprocket under a predetermined condition. 
     According to this configuration, since the transfer member rotates relatively to the cam sprocket under a predetermined condition, the first and second cam shafts rotate relatively through the transfer member. Accordingly, since a difference in rotation phase occurs between the intake side and exhaust side cams, the opening/closing timing of the intake valve or the exhaust valve can be changed without causing an increase in type of cam. In particular, since the other cam shaft is inserted through one cam shaft in the first and second cam shafts, the first and second cam shafts can be disposed to overlap each other in the axial direction. As a result, it is possible to prevent an increase in size in the axial direction of the overall variable valve mechanism. In this way, the variable valve mechanism can be realized with a simple and compact configuration. 
     In the above mentioned variable valve mechanism according to the present invention, the transfer member can rotate relatively to the cam sprocket when the engine rotation speed exceeds a predetermined rotation speed and rotates along with the cam sprocket when the engine rotation speed is a predetermined rotation speed or less. According to this configuration, since the transfer member is rotated relatively to or along with the cam sprocket in response to the engine rotation speed, the rotation phases of the first and second cam shafts can be changed. Thus, the valve timing can be appropriately adjusted in response to the engine state. 
     The above mentioned variable valve mechanism according to the present invention can further includes: an intermediate operation member that is able to switch a relative rotation or an integral rotation of the cam sprocket and the transfer member, wherein the intermediate operation member can engage with the cam sprocket and the transfer member and moves outward in the radial direction of the cam sprocket to rotate the transfer member relatively to the cam sprocket when the engine rotation speed exceeds a predetermined rotation speed. According to this configuration, since the intermediate operation member moves in response to the engine rotation speed, the transfer member can be rotated relatively to the cam sprocket. In this way, since the variable valve mechanism can be realized without using a separate actuator or the like, a configuration is simplified. 
     In the above mentioned variable valve mechanism according to the present invention, the intermediate operation member can include: a support portion which is supported by the cam sprocket to be rotatable; a weight portion which is formed to be separated from the support portion; and an engagement portion which engages with the transfer member; and the intermediate operation member can rotate about the support portion as a support point, when the weight portion moves outward in the radial direction in accordance with the rotation of the cam sprocket. According to this configuration, since the weight portion receives the centrifugal force accompanied by the rotation of the cam sprocket, the intermediate operation member rotates about the support portion as a support point. Accordingly, since the transfer member can be rotated relatively to the cam sprocket through the engagement portion, the valve timing can be adjusted with a simple and easy configuration. 
     In the above mentioned variable valve mechanism according to the present invention, the intermediate operation member can be formed to be slidable along a guide groove formed in the cam sprocket and the transfer member; and when the intermediate operation member moves outward in the radial direction in accordance with the rotation of the cam sprocket, the transfer member can rotate relatively to the cam sprocket. According to this configuration, the intermediate operation member moves outward in the radial direction along the guide groove due to the centrifugal force accompanied by the rotation of the cam sprocket. Accordingly, since the transfer member can be rotated relatively to the cam sprocket, the valve timing can be adjusted with a simple and easy configuration. 
     Further, the engine according to the invention may include the variable valve mechanism. 
     Further, the automatic two-wheeled vehicle according to the invention may include the engine. 
     According to the invention, since the intake side and exhaust side cam shafts are coaxially disposed to overlap each other, the variable valve mechanism can have a simple and compact configuration. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side view illustrating a schematic configuration of an automatic two-wheeled vehicle including an engine that adopts a variable valve mechanism according to an embodiment; 
         FIG. 2  is a perspective view of a valve train according to the embodiment; 
         FIG. 3  is a perspective view illustrating the variable valve mechanism according to the embodiment; 
         FIG. 4  is an exploded perspective view of the variable valve mechanism illustrated in  FIG. 3 ; 
         FIG. 5  is an exploded perspective view of a cam shaft assembly (cam shaft) according to the embodiment; 
         FIG. 6  is a cross-sectional view of the variable valve mechanism illustrated in  FIG. 3 ; 
         FIGS. 7A and 7B  are diagrams illustrating an operation of the variable valve mechanism according to the embodiment; 
         FIG. 8  is a perspective view illustrating a variable valve mechanism according to a modified example; 
         FIG. 9  is a cross-sectional view of the variable valve mechanism illustrated in  FIG. 8 ; and 
         FIGS. 10A and 10B  are diagrams illustrating a part of components of the variable valve mechanism according to the modified example. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, an embodiment of the invention will be described in detail with reference to the accompanying drawings. Further, in the following description, an example in which a variable valve mechanism according to the invention is applied to an engine of an automatic two-wheeled vehicle will be described, but the application target can be changed without limitation. For example, the variable valve mechanism according to the invention may be also applied to engines of other automatic two-wheeled vehicles, buggy type automatic three-wheeled vehicles, or automatic four-wheeled vehicles. Regarding the direction, the front side of the vehicle will be denoted by an arrow FR, and the rear side of the vehicle will be denoted by an arrow RE. Further, in the drawings below, a part of the configuration will be omitted for convenience of the description. 
     Referring to  FIG. 1 , a schematic configuration of an automatic two-wheeled vehicle that employs an engine according to the embodiment will be described.  FIG. 1  is a side view illustrating a schematic configuration of an automatic two-wheeled vehicle including an engine that employs the variable valve mechanism according to the embodiment. 
     As illustrated in  FIG. 1 , an automatic two-wheeled vehicle  1  has a configuration in which an engine  2  is suspended on a vehicle body frame  10  formed of aluminum alloy or a steel product equipped with a power unit, an electricity system, and the like. The engine  2  is, for example, a single cylinder four-cycle engine. The engine  2  has a configuration in which a cylinder assembly  20  (hereinafter, simply referred to as a cylinder  20 ) obtained by a combination of a cylinder block or a cylinder head is attached to an upper portion of a crank casing  21 . 
     Components such as a piston (not illustrated) or a valve train  5  (see  FIG. 2 ) are received inside the cylinder  20 . Although it will be described later in detail, the valve train  5  according to the embodiment is configured as a single overhead camshaft (SOHC) type valve train. Further, various shafts that transmit a rotation of a crank shaft (not illustrated) are received inside the crank casing  21  in addition to the crank shaft. 
     An exhaust pipe  11  is connected to a front exhaust port of the engine. The exhaust pipe  11  extends downward from the exhaust port, is bent under the crank casing  21 , and extends toward the rear side of the vehicle body. A muffler  12  is attached to a rear end of the exhaust pipe  11 . An exhaust gas which is produced after combustion is discharged to the outside through the exhaust pipe  11  and the muffler  12 . 
     A fuel tank  13  is disposed at an upper portion of the vehicle body frame  10 . A driver seat  14  and a passenger seat  15  are disposed at the rear side of the fuel tank  13  along with a rear cowl  16 . A pair of left and right front forks  30  are supported by a front head portion of the vehicle body frame  10  to be steerable along with a handlebar  31 . A head lamp  32  is provided at the front side of the handlebar  31 . A front wheel  33  is supported by a lower portion of the front fork  30  to be steerable and an upper portion of the front wheel  33  is covered by a front fender  34 . 
     A swing arm (not illustrated) is connected to a rear portion of the vehicle body frame  10  to be swingable up and down. A rear wheel  40  is supported by the rear portion of the swing arm to be rotatable. A driven sprocket (not illustrated) is provided at the left side of the rear wheel  40  and power of the engine  2  is transmitted to the rear wheel  40  by a drive chain (not illustrated). An upper portion of the rear wheel  40  is covered by a rear fender  41  provided at a rear portion of the rear cowl  16 . 
     Next, the valve train according to the embodiment will be described with reference to  FIG. 2 .  FIG. 2  is a diagram illustrating a state where a cylinder head cover is separated from the engine and is a perspective view of the valve train according to the embodiment. 
     As illustrated in  FIG. 2 , the valve train  5  which controls the opening/closing of an intake valve  50  and an exhaust valve  51  is provided at an upper portion of the cylinder  20 . As described above, the valve train  5  is an SOHC type valve train and has a configuration in which a cam shaft assembly  6  (hereinafter, simply referred to as a cam shaft  6 ) is disposed above the intake valve  50  and the exhaust valve  51 . 
     Two intake valves  50  are disposed at the cam shaft  6  on the rear side of the vehicle to be arranged in the left and right direction (the vehicle width direction). Further, two exhaust valves  51  are disposed at the cam shaft  6  on the front side of the vehicle to be arranged in the left and right direction. Each of the intake valves  50  and the exhaust valves  51  is provided with a valve spring  52 . The intake valves  50  and the exhaust valves  51  are constantly urged upward (in a closing direction) by the valve springs  52 . 
     The cam shaft  6  extends in the left and right direction. An intake cam  62  and an exhaust cam  63  are provided at the cam shaft  6  to be arranged in the left and right direction. Specifically, as illustrated in  FIGS. 2 and 3 , the left side in the axial direction is the intake cam  62  and the right side in the axial direction is the exhaust cam  63 . Further, a right end of the cam shaft  6  is provided with a cam sprocket  53 . A cam chain (not illustrated) which transmits the rotation of the crank shaft is wound around the cam sprocket  53 . 
     The cam shaft  6  is obtained by coaxially assembling an intake cam shaft  60  (a first cam shaft) and an exhaust cam shaft  61  (a second cam shaft) (see  FIG. 4 ). Although it will be described later in detail, the cam shaft  6  and the peripheral components constitute a variable valve mechanism  7  which changes the opening/closing timings of the intake valve  50  and the exhaust valve  51 . 
     An intake rocker arm  54  which opens and closes the intake valve  50  and an exhaust rocker arm  55  which opens and closes the exhaust valve  51  are provided above the cam shaft  6  (the intake cam  62  and the exhaust cam  63 ). The intake rocker arm  54  is supported to be swingable by an intake rocker shaft (not illustrated) extending in the left and right direction. Specifically, the intake rocker arm  54  includes a support portion  54   a  which serves as a swing support point, a contact portion  54   b  which contacts the intake cam  62 , and a pressing portion  54   c  which presses the intake valve  50 . 
     The support portion  54   a  has a cylindrical shape through which the intake rocker shaft is insertable. The contact portion  54   b  extends forward and downward from the support portion  54   a  and a roller  54   d  is attached to a front end thereof. An outer surface of the roller  54   d  is in contact with an outer surface of the intake cam  62 . The pressing portion  54   c  is bifurcated backward and downward from the support portion  54   a  and front ends thereof are in contact with an upper end of the intake valve  50 . 
     The exhaust rocker arm  55  is also supported to be swingable by an exhaust rocker shaft (not illustrated) extending in the left and right direction. Specifically, the exhaust rocker arm  55  includes a support portion  55   a  which serves as a swing support point, a contact portion  55   b  which contacts the exhaust cam  63 , and a pressing portion  55   c  which presses the exhaust valve  51 . 
     The support portion  55   a  has a cylindrical shape through which the exhaust rocker shaft is insertable. The contact portion  55   b  extends backward and downward from the support portion  55   a  and a roller  55   d  is attached to a front end thereof. An outer surface of the roller  55   d  is in contact with an outer surface of the exhaust cam  63 . The pressing portion  55   c  is bifurcated forward and downward from the support portion  55   a  and front ends thereof are in contact with an upper end of the exhaust valve  51 . 
     In the valve train  5  with such a configuration, when the cam shaft  6  rotates along with the rotation of the crank shaft, the contact portion  54   b  (the contact portion  55   b ) slides along a cam surface (the outer surface) of the intake cam  62  (the exhaust cam  63 ). Especially, the contact portion  54   b  (the contact portion  55   b ) is pressed upward by a protruding portion of the intake cam  62  (the exhaust cam  63 ). For this reason, the intake rocker arm  54  (the exhaust rocker arm  55 ) rotates about the support portion  54   a  (the support portion  55   a ) as a support point and the pressing portion  54   c  (the pressing portion  55   c ) moves downward. 
     At this time, the pressing portion  54   c  (the pressing portion  55   c ) presses the intake valve  50  (the exhaust valve  51 ) downward (in an opening direction) against an urging force of the valve spring  52 . As a result, the intake valve  50  (the exhaust valve  51 ) is opened. When the contact portion  54   b  (the contact portion  55   b ) gets over the protruding portion of the intake cam  62  (the exhaust cam  63 ), the intake valve  50  (the exhaust valve  51 ) is urged upward by the urging force of the valve spring  52 . As a result, the intake valve  50  (the exhaust valve  51 ) is closed. In this way, the opening and closing of the intake valve  50  and the exhaust valve  51  is controlled. 
     Incidentally, as a valve train, there is known a valve train including a variable valve mechanism that changes operation characteristics (including a valve timing or a valve lift amount) of the intake valve and the exhaust valve in response to the engine rotation speed. For example, in the variable valve mechanism applied to the SOHC type valve train, one cam shaft is provided with two kinds of cams (a low-speed cam and a high-speed cam) having different operation characteristics. Further, the rocker arm for driving the intake/exhaust valve is slidable in the axial direction of the cam shaft. 
     In this case, since the number of types of cams increases in order to change the operation characteristics, a complex configuration is obtained. Alternatively, since a structure for sliding the rocker arm or a slide space is required, a problem arises in that the overall mechanism increases in size in the axial direction of the cam shaft in addition to the complex configuration. 
     Here, in the embodiment, the intake cam shaft  60  is formed to be inserted through the exhaust cam shaft  61  (see  FIG. 5 ) and the intake cam shaft  60  and the exhaust cam shaft  61  are disposed to overlap each other in the axial direction. Accordingly, an increase in length of the cam shaft  6  in the axial direction is prevented. Further, since the intake cam shaft  60  and the exhaust cam shaft  61  are rotated relatively to each other in response to the engine rotation speed, the rotation phase of the intake cam shaft  60  can be displaced. Accordingly, it is possible to realize the variable valve mechanism  7  capable of changing the operation characteristics (including a rotation phase) of the intake cam  62  (see  FIG. 5 ) by a simple and compact configuration without a particular cam having different operation characteristics. 
     Next, the variable valve mechanism according to the embodiment will be described with reference to  FIGS. 3 to 6 .  FIG. 3  is a perspective view illustrating a part of the variable valve mechanism according to the embodiment.  FIG. 4  is an exploded perspective view of the variable valve mechanism illustrated in  FIG. 3 .  FIG. 5  is an exploded perspective view of the cam shaft assembly (the cam shaft) according to the embodiment.  FIG. 6  is a cross-sectional view of the variable valve mechanism illustrated in  FIG. 3 . 
     As described above, the valve train  5  according to the embodiment (see  FIG. 2 ) includes the variable valve mechanism  7  which changes the opening and closing timing of the intake valve  50  or the exhaust valve  51  (see  FIG. 2 ) in response to an engine rotation speed. Specifically, as illustrated in  FIG. 3 , the variable valve mechanism  7  is a so-called governor type variable valve timing mechanism which advances the valve timing of the intake valve  50  by using the centrifugal force generated by the rotation of the cam shaft  6  (the cam sprocket  53 ). 
     As illustrated in  FIGS. 3 and 4 , the variable valve mechanism  7  has a configuration in which a governor flange  70  or a pair of governor arms  71  are attached to a right surface of the cam sprocket  53  provided at the right end of the cam shaft  6  through bolts  72  and  73 . Although it will be described later in detail, the governor arm  71  can be rotated by the centrifugal force generated by the rotation of the cam shaft  6 . 
     A circular hole  53   a  is formed at the center of the cam sprocket  53 . Further, a side surface of the cam sprocket  53  is provided with two penetration holes  53   b  which serve as rotation support points of the governor arms  71 . Two penetration holes  53   b  are formed at the opposite positions with the circular hole  53   a  interposed therebetween. The cam sprocket  53  is attached to the exhaust cam shaft  61  to be rotatable together through a sprocket flange  66  to be described later. 
     The governor flange  70  includes a circular portion  70   a  which engages with the intake cam shaft  60  to be described later and a flange portion  70   b  which is widened outward in the radial direction from the outer periphery of the circular portion  70   a.  A circular hole  70   c  is formed at the center of the circular portion  70   a.  When the bolt  72  passes through the circular hole  70   c  and the bolt  72  is threaded into the intake cam shaft  60 , the governor flange  70  is fixed to the intake cam shaft  60 . 
     An engagement pin  70   d  is attached to the circular portion  70   a  to be located at a position separated from the center in the radial direction. The engagement pin  70   d  protrudes toward the cam shaft  6 . When the engagement pin  70   d  engages with the engagement groove  60   b  of the intake cam shaft  60 , the governor flange  70  and the intake cam shaft  60  rotate together. The flange portion  70   b  is provided with two engagement pins  70   e  which protrude outward (rightward) in the axial direction. Each engagement pin  70   e  engages with an engagement hole  71   d  of the governor arm  71 . The governor flange  70  with this configuration serves as a transfer member that transmits the rotation of the cam sprocket  53  to the intake cam shaft  60 . 
     The governor arm  71  is formed in a substantially crescent shape to follow the circumferential direction of the cam sprocket  53 . Specifically, the governor arm  71  includes a support portion  71   a  which is supported by the cam sprocket  53  to be rotatable, a weight portion  71   b  which is formed to be separated from the support portion  71   a,  and an engagement portion  71   c  which engages with the governor flange  70  (the engagement pin  70   e ). 
     The support portion  71   a  has a cylindrical shape through which the bolt  73  is insertable. The governor arm  71  extends from the support portion  71   a  toward the front side in the rotation direction and a front end thereof is slightly bent inward in the radial direction. The bent front end portion is formed as the weight portion  71   b.  Further, the engagement portion  71   c  slightly extends from the support portion  71   a  toward the rear side in the rotation direction and a rear end thereof is slightly located at the inside in the radial direction in relation to the support portion  71   a.  The rear end portion of the engagement portion  71   c  is provided with the engagement hole  71   d  which can engage with the engagement pin  70   e.  The engagement hole  71   d  is formed in a substantially S-shape to be long in the radial direction. 
     The pair of governor arms  71  are attached to the cam sprocket  53  to be rotatable when the bolt  73  is inserted through the penetration hole  53   b  of the cam sprocket  53  and the support portion  71   a  and the bolt  73  is threaded into the sprocket flange  66  while the engagement pin  70   e  of the governor flange  70  engages with the engagement hole  71   d.  Although it will be described later in detail, the governor arm  71  serves as an intermediate operation member capable of switching the relative rotation or the integral rotation between the cam sprocket  53  and the governor flange  70 . 
     Further, the pair of governor arms  71  are provided with a pair of governor springs  74  which urge the weight portions  71   b  inward in the radial direction. The governor spring  74  is formed as, for example, a compression coil spring. One end of the governor spring  74  engages with a base end (a front bent portion of the governor arm  71 ) of the weight portion  71   b  of any one governor arm  71 . Further, the other end of the governor spring  74  engages with a rear end portion (between the support portion  71   a  and the engagement portion  71   c ) of the facing other governor arm  71 . 
     Next, a detailed configuration of the cam shaft  6  will be described. As illustrated in  FIG. 5 , the cam shaft  6  has a configuration in which a sprocket flange  66  is attached to a right end of the exhaust cam shaft  61  through the cylindrical exhaust cam shaft  61  and the bearing  65  of the intake cam shaft  60 . 
     The intake cam shaft  60  is formed in a hollow shape and extends in the left and right direction. The intake cam  62  is integrally formed at the left end of the intake cam shaft  60 . A screw hole  60   a  for a bolt  72  (see  FIG. 4 ) is provided at the right end of the intake cam shaft  60 . Further, an engagement groove  60   b  which engages with an engagement pin  70   d  of the governor flange  70  is formed at the outer peripheral side of the right end of the intake cam shaft  60 . 
     Further, a portion which is located on the right side of the intake cam  62  in the intake cam shaft  60  and is received inside the exhaust cam shaft  61  is formed so that a base end and a right end are larger (thicker) than an intermediate portion  60   e  in the radial direction. The thick portion of the intake cam shaft  60  serves as a support portion  60   c  which supports the exhaust cam shaft  61 . Specifically, an outer diameter of the support portion  60   c  is substantially equal to an inner diameter of the exhaust cam shaft  61 . Further, an outer surface of the support portion  60   c  is provided with an annular groove  60   d.  The annular groove  60   d  and the intermediate portion  60   e  serve as an oil supply path which supplies oil to a sliding surface between the intake cam shaft  60  and the exhaust cam shaft  61 . 
     The exhaust cam shaft  61  is formed such that the exhaust cam  63  is integrally formed at a left end, that is, an end opposite to the sprocket flange  66  and has a cylindrical shape through which the intake cam shaft  60  is insertable. Specifically, an inner diameter of the exhaust cam shaft  61  is set to be slightly larger than an outer diameter of the intake cam shaft  60 . A length of the exhaust cam shaft  61  is substantially the same as a length of the intake cam shaft  60  on the right side of the intake cam  62 . Further, the exhaust cam shaft  61  and the intake cam shaft  60  are formed to be rotatable relatively to each other. 
     The sprocket flange  66  which is provided at the right end of the exhaust cam shaft  61  is provided with two screw holes  66   a  which correspond to the penetration holes  53   b  of the cam sprocket  53 . The sprocket flange  66  is attached to the exhaust cam shaft  61  to be rotatable together and the cam sprocket  53  is fixed thereto. 
     Next, an operation of the variable valve mechanism according to the embodiment will be described with reference to  FIGS. 7A and 7B .  FIGS. 7A and 7B  are diagrams illustrating an operation of the variable valve mechanism according to the embodiment.  FIG. 7A  illustrates a state where the governor arm is closed and  FIG. 7B  illustrates a state where the governor arm is opened. Additionally, the governor spring or a part of configurations are not illustrated in  FIGS. 7A and 7B  for convenience of the description. 
     In the variable valve mechanism  7 , as illustrated in  FIGS. 7A and 7B , the governor arm  71  is urged inward in the radial direction of the cam sprocket  53  by the governor spring  74  (not illustrated). For example, when an engine rotation speed is a predetermined rotation speed or less, a centrifugal force generated in the weight portion  71   b  is smaller than an urging force of the governor spring  74  as illustrated in  FIG. 7A . For this reason, the governor arm  71  does not rotate about the support portion  71   a  as a support point. 
     Further, the weight portion  71   b  is located at a closed position which does not protrude outward in the radial direction from the outer edge of the cam sprocket  53 . At this time, the engagement pin  70   e  of the governor flange  70  contacts the inner end of the engagement hole  71   d  in the radial direction. In this case, the governor flange  70  and the cam sprocket  53  rotate together without any relative rotation. Accordingly, the intake cam shaft  60  and the exhaust cam shaft  61  (see  FIG. 5 ) which engage with the governor flange  70  also rotate together with the cam sprocket  53 . As a result, in the valve train  5  (see  FIG. 2 ), the opening and closing of the intake valve  50  and the exhaust valve  51  is controlled at a normal valve timing. 
     Meanwhile, when the engine rotation speed exceeds a predetermined rotation speed, a centrifugal force generated in the weight portion  71   b  becomes larger than an urging force of the governor spring  74 . For this reason, as illustrated in  FIG. 7B , the governor arm  71  rotates about the support portion  71   a  as a support point and the weight portion  71   b  moves outward in the radial direction. Accordingly, the weight portion  71   b  is located at an opened position which protrudes outward in the radial direction from the outer edge of the cam sprocket  53 . 
     Further, the engagement portion  71   c  moves inward in the radial direction by the rotation of the governor arm  71 . Accordingly, the governor flange  70  rotates in a direction opposite to the cam sprocket  53  while the engagement pin  70   e  contacts the outer end of the engagement hole  71   d  in the radial direction. As a result, the opening/closing timing of the intake valve  50  is adjusted. In this way, when the governor arm  71  is rotated so that the governor flange  70  and the cam sprocket  53  rotate relatively in response to the engine rotation speed in the variable valve mechanism  7 , the opening/closing timing of the intake valve  50  can be changed. 
     In this way, in the variable valve mechanism  7  according to the embodiment, the intake cam shaft  60  and the exhaust cam shaft  61  rotate relatively to each other through the governor flange  70  when the governor flange  70  rotates relatively to the cam sprocket  53  under a predetermined condition. Accordingly, since a difference in rotation phase occurs in the intake cam  62 , it is possible to change the opening/closing timing of the intake valve  50  without increasing the number of types of cams. Particularly, when the intake cam shaft  60  is inserted through the exhaust cam shaft  61 , the intake cam shaft  60  and the exhaust cam shaft  61  can be disposed to overlap each other in the axial direction. As a result, it is possible to prevent an increase in size of the overall variable valve mechanism  7  in the axial direction. In this way, the variable valve mechanism  7  can be realized with a simple and compact configuration. 
     Further, the governor flange  70  rotates relatively to the cam sprocket  53  when the engine rotation speed exceeds a predetermined rotation speed and rotates along with the cam sprocket  53  when the engine rotation speed is a predetermined rotation speed or less. In this way, when the governor flange  70  is rotated relatively to or along with the cam sprocket  53  in response to the engine rotation speed, the rotation phase of the intake cam shaft  60  can be changed. Thus, the valve timing can be appropriately adjusted in response to the engine state. 
     Further, when the governor arm  71  moves in response to the engine rotation speed in the variable valve mechanism  7 , the governor flange  70  can be rotated relatively to the cam sprocket  53 . Thus, since the variable valve mechanism  7  can be realized without a particular actuator or the like, a configuration is simplified. Further, since the weight portion  71   b  receives a centrifugal force accompanied by the rotation of the cam sprocket  53 , the governor arm  71  rotates about the support portion  71   a  as a support point. Accordingly, since the governor flange  70  can be rotated relatively to the cam sprocket  53  through the engagement portion  71   c,  the valve timing can be adjusted with a simple and easy configuration. 
     Next, a variable valve mechanism according to a modified example will be described with reference to  FIGS. 8 to 10 .  FIG. 8  is a perspective view of the variable valve mechanism according to the modified example.  FIG. 9  is a cross-sectional view of the variable valve mechanism illustrated in  FIG. 8 .  FIGS. 10A and 10B  are diagrams illustrating a part of the components of the variable valve mechanism according to the modified example.  FIG. 10A  is a diagram illustrating the cam sprocket when viewed from the right side and  FIG. 10B  is a diagram illustrating the circular plate when viewed from the left side. Further, in the modified example, since the configuration of the cam shaft assembly  6  (the cam shaft  6 ) is substantially the same as that of the embodiment, the same reference numeral will be given to the same component and a description thereof will be omitted. 
     As illustrated in  FIGS. 8 to 10 , the variable valve mechanism  9  according to the modified example has a configuration in which a circular plate  91  and a spline flange  92  are attached to a right surface of a cam sprocket  90  provided at a right end of the cam shaft  6 . Although it will be described later in detail, the circular plate  91  and the spline flange  92  are rotatable by the centrifugal force generated by the rotation of the cam shaft  6 . 
     The cam sprocket  90  is attached to the exhaust cam shaft  61  through the sprocket flange  66  to be rotatable together. The center of the cam sprocket  90  is provided with a circular hole  90   a.  Further, a plurality of spherical grooves  90   b  (fifteen spherical grooves in  FIGS. 10A and 10B ) are formed in a right surface of the cam sprocket  90  at the same interval in the circumferential direction. Specifically, the spherical groove  90   b  has an oval shape which is long in the radial direction in the side view. Further, the longitudinal direction of the spherical groove  90   b  is slightly inclined backward in the rotation direction relatively to the radial direction of the cam sprocket  90 . Although it will be described later in detail, a left half portion of a ball  93  is received in each spherical groove  90   b.  Further, the number of the spherical grooves  90   b  is not limited to the above-described example and can be modified appropriately. 
     The circular plate  91  has substantially the same diameter as that of the cam sprocket  90  and the center thereof is provided with a circular hole  91   a.  The circular plate  91  is attached to face the right surface of the cam sprocket  90 . A plurality of spherical grooves  91   b  (which are fifteen spherical grooves similarly to the spherical groove  90 b) are formed in the side surface (the left surface) of the circular plate  91  facing the cam sprocket  90  at the same interval in the circumferential direction to correspond to the spherical grooves  90   b  of the cam sprocket  90 . Specifically, the spherical groove  91   b  is formed in an oval shape to be long in the radial direction in the side view. Further, the longitudinal direction of the spherical groove  91   b  matches the radial direction of the cam sprocket  90 . Although it will be described later in detail, a right half portion of the ball  93  is received in each spherical groove  91   b.    
     A cylindrical spline flange  92  is attached to the circular hole  91   a  of the circular plate  91 . A spline (not illustrated) is formed in the inner surface of the circular hole  91   a  and the outer surface of the spline flange  92 . When the circular plate  91  and the spline flange  92  are spline-fitted to each other, the circular plate  91  and the spline flange  92  rotate together. When the bolt  94  is inserted through the center of the spline flange  92  and the bolt  94  is threaded into the screw hole  60   a  (see  FIG. 5 ) of the intake cam shaft  60 , the spline flange  92  is fixed to the intake cam shaft  60 . 
     Additionally, the spline flange  92  is provided with an engagement pin (not illustrated). When the engagement pin engages with the engagement groove  60   b  (see  FIG. 5 ) of the intake cam shaft  60 , the circular plate  91  and the spline flange  92  rotate along with the intake cam shaft  60 . The circular plate  91  and the spline flange  92  with this configuration serve as a transfer member that transmits the rotation of the cam sprocket  90  to the intake cam shaft  60 . 
     The balls  93  are received in the spherical grooves  90   b  and  91   b  between the cam sprocket  90  and the circular plate  91 . The ball  93  has a size in which the ball is slidable along the spherical grooves  90   b  and  91   b.  That is, the spherical grooves  90   b  and  91   b  constitute a guide groove  95  which guides the movement of the ball  93 . Further, although it will be described later in detail, the ball  93  serves as an intermediate operation member capable of switching the relative rotation or the integral rotation between the cam sprocket  90  and the circular plate  91  (the spline flange  92 ) of the governor arm. 
     A spring washer  96 , a ring spacer  97 , and a C-ring  98  are sequentially attached to the right surface of the circular plate  91 . Specifically, the spring washer  96  and the ring spacer  97  pass through the spline flange  92 . Further, the C-ring  98  engages with the outer surface of the spline flange  92  to regulate the rightward movement of the ring spacer  97  and the spring washer  96  in the axial direction. The spring washer  96  has an urging force in the axial direction and urges the circular plate  91  leftward toward the cam sprocket  90 . Accordingly, the balls  93  are sandwiched by the cam sprocket  90  and the circular plate  91 . 
     In the variable valve mechanism  9  with this configuration, when the engine is not started or the engine rotation speed is a predetermined rotation speed or less as illustrated in  FIG. 9 , the ball  93  is located at the inside of the guide groove  95  in the radial direction. At this time, the cam sprocket  90  and the circular plate  91  (the spline flange  92 ) can rotate together without any relative rotation. Thus, in the valve train  5  (see  FIG. 2 ), the opening and closing of the intake valve  50  and the exhaust valve  51  is controlled at a normal valve timing. 
     Meanwhile, when the engine rotation speed exceeds a predetermined rotation speed, the ball  93  moves outward in the radial direction along the guide groove  95  due to the centrifugal force of the ball  93 . At this time, since the spherical groove  90   b  of the cam sprocket  90  is inclined backward in the rotation direction with respect to the radial direction, the circular plate  91  rotates backward in accordance with the movement of the ball  93 . That is, the circular plate  91  (the spline flange  92 ) rotates relatively to the cam sprocket  90 . Accordingly, the intake cam shaft  60  rotates relatively to the cam sprocket  90  so that the opening/closing timing of the intake valve  50  is adjusted. In this way, in the variable valve mechanism  9  according to the modified example, the ball  93  (the intermediate operation member) moves outward in the radial direction along the guide groove  95  by the centrifugal force accompanied by the rotation of the cam sprocket  90 . Accordingly, since the circular plate  91  (the transfer member) can be rotated relatively to the cam sprocket  90 , the valve timing can be adjusted with a simple and easy configuration. With this configuration, even in the modified example, the valve timing can be adjusted by using the centrifugal force. 
     Additionally, the invention is not limited to the above-described embodiment and can be modified into various forms. In the above-described embodiment, the sizes or the shapes illustrated in the accompanying drawings are not limited thereto and can be appropriately changed without departing from the effect of the invention. In addition, an appropriately modification can be made without departing from the object of the invention. 
     For example, in the above-described embodiment, the single cylinder engine  2  has been exemplified, but the invention is not limited to this configuration. For example, the valve train  5  (the variable valve mechanisms  7  and  9 ) according to the embodiment may be also applied to a multi-cylinder engine. 
     Further, in the above-described embodiment, the single cylinder engine has a so-called four valve type valve train in which each of the intake valve  50  and the exhaust valve  51  is provided at two positions so that four valves are provided in total, but the invention is not limited to this configuration. The number of the intake valves  50  and the exhaust valves  51  can be appropriately changed. 
     Further, in the above-described embodiment, a configuration has been described in which one of engagement portions is formed as an engagement pin and the other thereof is formed as an engagement hole or a groove, but the invention is not limited to this configuration. For example, one of the engagement portions may be formed as an engagement hole or a groove and the other thereof may be formed as a protrusion such as an engagement pin. 
     Further, in the above-described embodiment, the variable valve mechanism  7  is used to adjust the opening/closing timing of the intake valve  50 , but the invention is not limited to this configuration. The variable valve mechanism  7  may be used to adjust the opening/closing timing of the exhaust valve  51 . 
     Further, in the above-described embodiment, a predetermined centrifugal force (an engine rotation speed) during the operation of the variable valve mechanism  7  (during the rotation of the governor arm  71 ) can be appropriately changed in response to a desired valve timing. 
     INDUSTRIAL APPLICABILITY 
     As described above, the invention has an effect that a simple and compact configuration can be realized and is particularly useful for a variable valve mechanism applicable to a single overhead camshaft (SOHC) type valve train, an engine, and an automatic two-wheeled vehicle. 
     REFERENCE SIGNS LIST 
       1  automatic two-wheeled vehicle 
       2  engine 
       5  valve train 
       50  intake valve 
       51  exhaust valve 
       53 ,  90  cam sprocket 
       6  cam shaft 
       60  intake cam shaft (first cam shaft) 
       61  exhaust cam shaft (second cam shaft) 
       62  intake cam (intake side cam) 
       63  exhaust cam (exhaust side cam) 
       7 ,  9  variable valve mechanism 
       70  governor flange (transfer member) 
       71  governor arm (intermediate operation member) 
       71   a  support portion 
       71   b  weight portion 
       71   c  engagement portion 
       91  circular plate (transfer member) 
       92  spline flange (transfer member) 
       93  ball (intermediate operation member) 
       95  guide groove