Patent Publication Number: US-10316701-B2

Title: Overhead valve actuation mechanism for engine

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2016-167158, filed on Aug. 29, 2016, the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a preferable overhead valve actuation mechanism used for an engine mounted on a vehicle, such as a motorcycle. 
     Description of the Related Art 
     Conventionally, for example as described in Patent Document 1, a valve gear includes a rocker shaft and a rocker arm. The rocker shaft has an axis line parallel to a camshaft inside a cylinder head. The rocker shaft is disposed at a position projecting from a coupling face as viewed from an axial direction of the camshaft and supported by the cylinder head. The rocker arm is turnably borne by the rocker shaft so as to swing following a rotation of an air exhaust side valve motion cam disposed in the camshaft.
     Patent Document 1: Japanese Laid-open Patent Publication No. 2009-243401   

     In a conventional valve gear, a camshaft is arranged in a side of an intake valve inside a cylinder head. Therefore, the cylinder head that holds a valve drive system in this air intake side becomes large in size to narrow a peripheral space of the cylinder head. Due to such positional relationship with the camshaft, it is difficult to ensure a close arrangement of a throttle body to which the cylinder head is coupled. Also, an intake port of the cylinder head needs to be formed long. Thus, a problem, such as a difficult layout of auxiliary machine components in the air intake side, is caused. 
     SUMMARY OF THE INVENTION 
     Considering the above-described problems, one of the objectives of the present invention is to provide an overhead valve actuation mechanism for an engine that simplifies a configuration especially in an air intake side to effectively contribute to ensuring a space. 
     An overhead valve actuation mechanism for an engine of the present invention includes a camshaft that is rotatably supported by the cylinder head and includes one or a plurality of valve cams. The camshaft operates opening and closing of an intake valve and an exhaust valve via the valve cam. The only one camshaft is disposed within the one cylinder head. The camshaft has an axis center biased to a side of the exhaust valve with respect to a cylinder axis line as viewed from an axial direction of the camshaft. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a left side view of a motorcycle according to an embodiment of the present invention; 
         FIG. 2  is a left side view of an engine according to the embodiment of the present invention; 
         FIG. 3  is a drawing around a cylinder head viewed from above where a cylinder head cover is removed in the embodiment of the present invention; 
         FIG. 4  is a cross-sectional view taken along a line I-I in  FIG. 3 ; 
         FIG. 5  is a cross-sectional view taken along a line II-II in  FIG. 3 ; 
         FIG. 6  is a perspective view illustrating an exemplary configuration around the cylinder head where an overhead valve actuation mechanism of the present invention is disposed; 
         FIG. 7  is a downward perspective view illustrating a main part according to the overhead valve actuation mechanism of the present invention; 
         FIG. 8  is a side view illustrating a main part according to the overhead valve actuation mechanism of the present invention; 
         FIG. 9A  is a side view illustrating an action according to the overhead valve actuation mechanism of the present invention; 
         FIG. 9B  is a side view illustrating an action of a conventional valve gear in comparison with the present invention; 
         FIG. 10  is an upward perspective view illustrating a main part according to the overhead valve actuation mechanism of the present invention; 
         FIG. 11  is a side view illustrating a main part according to the overhead valve actuation mechanism of the present invention; 
         FIG. 12  is a side view illustrating an operation of a rocker arm according to the present invention; 
         FIG. 13  is a cross-sectional view taken along a line III-III in  FIG. 11  illustrating around a supporting structure of the rocker arm according to the present invention; and 
         FIG. 14  is a perspective view illustrating an exemplary shifting adjustment method of the rocker arm according to the present invention. 
         FIG. 15  is a side view illustrating an exemplary arrangement of an engine peripheral configuration member in the motorcycle according to the overhead valve actuation mechanism of the present invention; and 
         FIG. 16  a side view illustrating another exemplary arrangement of an engine peripheral configuration member in the motorcycle according to the overhead valve actuation mechanism of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following describes preferable embodiments for an overhead valve actuation mechanism for an engine according to the present invention based on the drawings. 
     The present invention is preferably applied for the engine mounted on a motorcycle or similar vehicle. In this embodiment, a motorcycle  100  illustrated in  FIG. 1  is an example.  FIG. 1  is a left side view of the motorcycle  100  according to the embodiment. First, an overall configuration of the motorcycle  100  will be described with reference to  FIG. 1 . Here, in the respective drawings including  FIG. 1  used in the following description, as necessary, the front side of a vehicle is indicated by an arrow Fr and the rear side of the vehicle is indicated by an arrow Rr. The lateral right side of the vehicle is indicated by an arrow R and the lateral left side of the vehicle is indicated by an arrow L. 
     The motorcycle  100  in  FIG. 1  may typically be, what is called for off-road. The motorcycle  100  includes a steering head pipe  101  disposed on an upper portion of a front side of the vehicle body, and a steering shaft (not illustrated) is turnably inserted into the steering head pipe  101 . Then, the steering shaft includes an upper end on which a handlebar  102  is firmly attached and a lower end on which a front fork  103  is mounted. The front fork  103  includes a lower end to which a front wheel  104  as a steering wheel is rotatably journaled. 
     From the steering head pipe  101 , a main frame  105  composed of a pair of right and left parts extends to a rear of the vehicle body inclining obliquely downward, and a down tube  106  extends downward approximately vertical. Then, the down tube  106  is branched to right and left near a lower portion as lower frames  106 A, and the pair of lower frames  106 A extend downward to be bent to the rear of the vehicle body at an approximately right angle. The pair of lower frames  106 A includes rear end portions coupled to each rear end portion of the main frame  105  via a pair of right and left body frames  107 . 
     In a space surrounded by the pairs of right and left main frames  105 , the down tube  106  and the lower frames  106 A, and the body frames  107 , a water-cooled engine  10  as a driving source is mounted. An air cleaner, a fuel tank, or similar component is disposed above the engine  10  as described later, and a seat  108  is disposed in a front-rear direction at the upper portion of the vehicle body. A radiator  109  is disposed ahead of the engine  10 . 
     On the pair of right and left body frames  107  disposed on the lower portion of the approximately center of the vehicle body in the front-rear direction, front end portions of swing arms  110  are supported swingable in a vertical direction by a pivot shaft  111 . The swing arm  110  includes a rear end portion to which a rear wheel  112  as a driving wheel is rotatably journaled. While detailed illustrations are omitted, the swing arm  110  is suspended on the vehicle body via a link mechanism  113  and a shock absorber (rear wheel suspension device) coupled to the link mechanism  113 . The engine  10  disposed ahead of the swing arm  110  includes an output end on which a drive sprocket is mounted, and the rear wheel  112  includes a wheel axis on which a sprocket is pivotably supported. A chain is wound around the drive sprocket and the sprocket to couple to one another. 
     The overhead valve actuation mechanism of the present invention is applied for the engine  10  mounted on such motorcycle  100 .  FIG. 2  is a left side view of the engine  10  as an application example of the embodiment. 
     The engine  10  may be, for example, a single cylinder SOHC gasoline engine. In this embodiment, what is called a four valve engine, which includes two intake valves  30  and two exhaust valves  31 , is an example (see  FIG. 6 ). In a basic configuration of the engine  10  in  FIG. 2 , a cylinder block  12 , a cylinder head  13 , and a cylinder head cover  14  are integrally joined together in order above a crankcase  11 . A cylinder axis line Z is oriented in an approximately vertical direction. In a crank chamber of the crankcase  11 , a crankshaft  15  (in  FIG. 2 , briefly illustrated with the axis line of the crankshaft  15 ) is rotatably journaled. On the other hand, while the illustrations are omitted, a piston is fitted movable in the cylinder axis line Z direction inside a cylinder bore  16  (see  FIG. 4 ) of the cylinder block  12 . A crank pin of the crankshaft  15  and a piston pin of the piston are mutually coupled via a connecting rod. The piston reciprocating along the cylinder axis line Z direction inside the cylinder bore  16  rotatably drives the crankshaft  15 . 
     In the rear portion of the crankcase  11 , a transmission case  17  is integrally joined together. While the illustrations are omitted, inside this transmission case  17 , a transmission configured as a multistage transmission system is disposed. The crankshaft  15  disposed inside the crankcase  11  and the transmission are mutually coupled. An output of the engine  10  is transmitted to the drive sprocket, which is a final output end, in a state where the output is changed to a desired transmission ratio via the transmission. 
     The engine  10  includes, while the illustrations are omitted, an air intake system, which supplies a clean air (intake air) supplied from the air cleaner, a fuel system, which supplies a fuel from a fuel supply device, and an exhaust system, which discharges an exhaust gas after burning inside the cylinder from the engine  10 . The engine  10  also includes a valve system, which drivingly controls an intake valve and an exhaust valve of the air intake system and the exhaust system, respectively, a cooling system, which cools down the engine  10 , and a lubricating system, which lubricates movable parts of the engine  10 . Furthermore, a control system (ECU; Engine Control Unit), which properly controls operations of each of these systems in accordance with a predetermined sequence, is attached. The control of the control system causes a plurality of function systems to collaborate with the above-described auxiliary machines and similar machine. In view of this, a smooth operation is executed as the whole engine  10 . 
       FIG. 3  is a drawing around the cylinder head  13  viewed from above where the cylinder head cover  14  is removed.  FIG. 4  is a cross-sectional view taken along a line I-I in  FIG. 3 .  FIG. 5  is a cross-sectional view taken along a line II-II in  FIG. 3 . In the air intake system of the engine  10 , an intake port  18  opens in a rear side of the cylinder head  13  in this example. A purified air is supplied from the air cleaner to the intake port  18  via a throttle valve. To this intake port  18 , a throttle body  19  (see, for example,  FIG. 2 ) is coupled. The throttle body  19  internally includes an intake passage that is opened and closed by the throttle valve, and an intake flow amount is controlled. In a downstream side of the throttle valve, an injector for fuel injection is fitted. The fuel in the fuel tank in the fuel system is supplied to the injector with a fuel pump. An air-fuel mixture made by mixing the air and the fuel passes the intake port  18  and then flows into a combustion chamber  20  (see  FIG. 4 ). A spark plug  21  is fitted to the combustion chamber  20 . The spark plug  21  ignites the air-fuel mixture, and the air-fuel mixture burns and explodes inside the combustion chamber  20 . 
     In the exhaust system, an exhaust port  22  opens in a front side of the cylinder head  13  in this example. This exhaust port  22  communicates with the combustion chamber  20 . The air-fuel mixture burnt and exploded inside the combustion chamber  20  is discharged as the exhaust gas from the engine  10  via the exhaust port  22 . 
     In the cylinder head  13 , as illustrated in  FIG. 3 , an overhead valve actuation mechanism  23  according to the present invention is disposed. The overhead valve actuation mechanism  23  configures the valve system of the engine  10 . In a basic configuration, the overhead valve actuation mechanism  23  includes a camshaft  24  that is rotatably supported by the cylinder head  13  and includes one or a plurality of valve cams. The camshaft  24  opens and closes valves in the air intake side and the air exhaust side via this valve cam. The overhead valve actuation mechanism  23  also includes rocker arms  25  and a rocker arm shaft  26 . The rocker arms  25  are swung by the valve cam of the camshaft  24  and act on the valves to perform opening and closing operations. The rocker arm shaft  26  is supported by the cylinder head  13  and swingably supports the rocker arms  25 . 
     The camshaft  24  is horizontally disposed in a right-left direction in a state of being biased to the discharge side of the cylinder head  13 , that is, in the front side of the cylinder head  13 , and is rotatably supported by bearings  27  at both ends on the right and left of the camshaft  24 . As a driving mechanism of the camshaft  24 , a cam sprocket  28  is installed on a left shaft end portion of the camshaft  24 . This cam sprocket  28  is coupled to the crankshaft  15  via a cam timing chain (not illustrated). In this case, the drive sprocket is installed on a left side shaft end portion of the crankshaft  15 . The cam timing chain is wound around and installed in a predetermined manner between this drive sprocket and the cam sprocket  28 . This cam timing chain runs by being driven by the crankshaft  15  as a driving source inside a cam timing chain chamber  29 . This rotatably drives the camshaft  24  synchronizing with the crankshaft  15 . 
     On the camshaft  24 , a single air intake side valve cam  32  and a pair of air exhaust side valve cams  33  that are disposed at both right and left sides of the air intake side valve cam  32  are disposed so as to integrally rotate. In this example (see, for example,  FIGS. 4 and 6 ), the intake valve  30  is applied with a rocker arm drive system. That is, the rocker arms  25  and the rocker arm shaft  26  that swingably supports the rocker arms  25  are provided. The rocker arm shaft  26  is, as illustrated in, for example,  FIGS. 3 and 4 , disposed in the rear side of the camshaft  24  at a height position approximately identical to that of the camshaft  24 . Fixed shaft portions  26   a  disposed at both the right and left ends of the rocker arm shaft  26  are supported by the cylinder head  13  via journal portions  34 . 
     The rocker arms  25  include a pivot portion  35 , a contact portion  36 , and pressing portions  37 . The pivot portion  35  is rotatably journaled by the rocker arm shaft  26  and serves as a swing center. The contact portion  36  projects from the pivot portion  35  to a side of the air intake side valve cam  32  to receive a pressure from the air intake side valve cam  32 . The pressing portions  37  project in an arm shape from the pivot portion  35  to the intake valves  30  to contact top portions of these intake valves  30  and press the intake valves  30  in swinging. 
     The contact portion  36  is constituted of a tappet roller  36 A rotatably installed at an end portion of the rocker arms  25  in the air exhaust side. 
     As illustrated in  FIG. 3 , the pressing portions  37  are oriented toward the two intake valves  30 . The pressing portions  37  extend branching in a forked shape from the pivot portion  35 . The pressing portions  37  have the respective distal ends abutting on upper end portions of the intake valve stems  38  of the intake valves  30 . 
     The intake valve  30  is configured to reciprocate in the axial direction together with the intake valve stem  38  (see, for example,  FIG. 6 ) by the intake valve stem  38  being guided by a valve guide. While detailed illustrations are omitted, the intake valve stem  38  is always biased upward by the elastic force of an intake valve spring  39  fitted between spring seats on a top and a bottom. The pressing portions  37  pressing down the intake valve stems  38  against the elastic force of the intake valve springs  39  biases the intake valve stems  38  downward, that is, causes the intake valves  30  to open. 
     The exhaust valve  31  is applied with a direct drive system, that is, the exhaust valve  31  is directly driven substantially by the air exhaust side valve cam  33 . 
     The exhaust valve  31  reciprocates in the axial direction together with an exhaust valve stem  40  by the exhaust valve stem  40  being guided by a valve guide. While detailed illustrations are omitted, the exhaust valve stem  40  is always biased upward by the elastic force of an exhaust valve spring  41  fitted between spring seats on a top and a bottom. The air exhaust side valve cams  33  pressing down the exhaust valve stems  40  against the elastic force of the exhaust valve springs  41  biases the exhaust valve stems  40  downward, that is, causes the exhaust valves  31  to open. 
     With reference to  FIG. 4 , between the air exhaust side valve cam  33  and an upper end portion of the exhaust valve stem  40 , a finger follower  42  that is swingably supported by the cylinder head  13  via a spindle  42   a  is disposed. The air exhaust side valve cams  33  pressing the finger followers  42  causes the exhaust valve stems  40  to be pressed down. 
     In this example, as described above, the diameter of the intake port  18  is formed to be larger than the diameter of the exhaust port  22 . Corresponding to this, the intake valves  30  are formed to be in diameters larger than those of the exhaust valves  31 . 
     In a basic operation of the overhead valve actuation mechanism  23 , the above-mentioned cam timing chain running inside the cam timing chain chamber  29  rotatably drives the camshaft  24  synchronizing with the rotation of the crankshaft  15  inside the cylinder head  13 . In view of this, the air intake side valve cam  32  and the air exhaust side valve cams  33  of the camshaft  24  drive the intake valves  30  and the exhaust valves  31 , respectively, for opening and closing at a predetermined timing. 
     In the overhead valve actuation mechanism  23  of the present invention, as illustrated in, for example,  FIG. 7 , the rocker arms  25  include a slipper  43  that projects from the pivot portion  35  in outer side in the axial direction to the camshaft  24 . On the other hand, the camshaft  24  includes a stopper cam  44  at a position opposing the slipper  43 . 
     In this case, as illustrated in  FIG. 8 , the slipper  43  is disposed in an opposite side of the contact portion  36  of the rocker arms  25  by interposing a virtual line that connects a rotation axis L 1  of the rocker arms  25  and a camshaft central axis Lc as viewed from the rotation axis direction of the rocker arm. In view of this, when the rocker arms  25  swing by the contact portion  36  being pressed by the air intake side valve cam  32 , the slipper  43  comes close to the stopper cam  44 . 
     The stopper cam  44  includes a stopper portion  45  as a swing restricting portion. The stopper portion  45  is formed on an outer peripheral surface of the stopper cam  44 . The stopper portion  45  is a portion with which the slipper  43  comes in contact when the rocker arms  25  swing and reaches a predetermined position, for example, when the rocker arms  25  swing over a maximum swing position that corresponds to a top portion  32   a  of the air intake side valve cam  32 . 
     More specifically, the stopper portion  45  of the stopper cam  44  is formed to be a continuous surface such that an interval between the slipper  43  and the stopper portion  45  is kept within a predetermined range when the rocker arms  25  swing in a range of a cam lobe  32 A (indicated by a one dot chain line in  FIG. 8 ) including the top portion  32   a  of the air intake side valve cam  32 . 
     The stopper portion  45  includes a first portion  45 A and a second portion  45 B. The first portion  45 A corresponds to the cam lobe  32 A of the air intake side valve cam  32 . The second portion  45 B corresponds to the range of the air intake side valve cam  32  other than the range of the cam lobe  32 A. The second portion  45 B is formed to be approximately circumferential. The first portion  45 A is formed to be inside the circumference of the second portion  45 B and have a minimum radius at a position that corresponds to the top portion  32   a  of the air intake side valve cam  32 . 
     In the case of above, the stopper portion  45  is disposed around a whole circumference of the stopper cam  44 , and the interval between the slipper  43  and the stopper portion  45  is formed so as to be kept within the predetermined range at every rotation position of the rocker arm shaft  26  including when the rocker arms  25  do not swing. 
     As illustrated in  FIG. 8 , the slipper  43  is disposed in the opposite side of the contact portion  36 , which receives the pressure from the air intake side valve cam  32  of the rocker arms  25  by interposing the virtual line that connects the rotation axis L 2  of the rocker arms  25  and the camshaft central axis Lc as viewed from the rotation axis direction of the rocker arm. Thus separately disposing the slipper  43  and the contact portion  36 , which are formed to project to the camshaft avoids interference with one another and improves a freedom of designing the interval between the slipper  43  and the stopper portion  45 . 
     In this example, the slipper  43  is disposed on the rocker arms  25  in the air intake side. Typically, the intake valve  30  is large in size compared with the exhaust valve  31 , and a weight of valve movable parts in the air intake side is larger than those in the air exhaust side. Accordingly, the air intake side is easily susceptible to the inertia force. However, disposing the above-described slipper  43  on this rocker arm in the air intake side ensures reducing this influence of the inertia force. 
     In the overhead valve actuation mechanism  23  of the present invention, especially, only one camshaft  24  is disposed within one cylinder head  13 , and an axial center of this camshaft  24  is biased to the exhaust valve  31  side with respect to the cylinder axis line Z as viewed from the axial direction of the camshaft  24  as illustrated in  FIG. 4 . 
     Here, as illustrated in  FIG. 4 , an angle between the intake valve  30  and the exhaust valve  31  is a valve included angle θ. The valve included angle θ is a sum of an inclination angle α of an axis line Li of the intake valve  30  (the intake valve stem  38 ) with respect to the cylinder axis line Z and an inclination angle β of an axis line Le of the exhaust valve  31  (the exhaust valve stem  40 ) with respect to the cylinder axis line Z. Here, in this example, it is formed to be α&lt;β, that is, the inclination angle of the axis line Li of the intake valve  30  is formed to be small and come close to the cylinder axis line Z compared with the axis line Le of the exhaust valve  31 . 
     The camshaft  24  is disposed with its whole body positioning in the exhaust valve  31  side with respect to the cylinder axis line Z. 
     As illustrated in  FIG. 4 , the camshaft  24  is disposed with a part of the camshaft  24  overlapping with respect to the axis line Le of the exhaust valve  31  as viewed from the axial direction of the camshaft  24 . 
     The overhead valve actuation mechanism  23  of the present invention includes the rocker arms  25  that act on the intake valves  30  to open and close via the air intake side valve cam  32  from the camshaft  24  and the rocker arm shaft  26  that rotatably supports these rocker arms  25 . 
     In this case, the rocker arms  25  include, by interposing the cylinder axis line Z in between, the pressing portions  37 , which are operating portions relative to the intake valves  30 , in the air intake side and the contact portion  36  with the air intake side valve cam  32  in the air exhaust side. 
     The rocker arms  25  have the axis center of the rocker arm shaft  26  biased to the exhaust valve  31  side with respect to the cylinder axis line Z as viewed from the axial direction of the rocker arms  25 . 
     Furthermore, the overhead valve actuation mechanism  23  of the present invention can include a supporting structure of the rocker arms  25  in which the rotation axis of the rocker arms  25  parallelly and slightly separates with respect to the center axis line of the fixed shaft portions  26   a  on the cylinder head side of the rocker arm shaft  26 . 
       FIG. 10  is a perspective view illustrating the rocker arm shaft  26  that rotatably supports the rocker arm  25  and the rocker arm  25 .  FIG. 11  is a side view viewed from the axial direction of the rocker arm shaft  26 . Here, as illustrated in  FIG. 11 , the rocker arm shaft  26  is formed such that the rotation axis L 2  of the rocker arms  25  slightly separates upward with respect to the center axis line L 1  of the fixed shaft portions  26   a  on the cylinder head side. 
       FIG. 12  is a side view illustrating an operation of the pivot portion  35 , the contact portion  36 , the pressing portion  37 , and the slipper  43  of the rocker arms  25  in association with the rotation of the fixed shaft portion  26   a  on the cylinder head side of the rocker arm shaft  26 . 
       FIG. 13  is a cross-sectional view taken along a line III-III in  FIG. 11  that illustrates around the supporting structure of the rocker arms  25 . Both ends of the rocker arm shaft  26  are supported by the journal portions  34  and fastened and secured with caps  46  covering the journal portions  34  from above. 
     The pivot portion  35  of the rocker arms  25  is rotatably journaled by a pair of supporting portions  47  disposed in the rocker arm shaft  26 . 
     The supporting portions  47  are formed such that the rotation axis L 2  of the rocker arms  25  parallelly and slightly separates (shifts) upward with respect to the center axis line L 1  of the fixed shaft portions  26   a  of the rocker arm shaft  26 . In this case, the rotation axis L 2  of the rocker arms  25  is eccentric with respect to the center axis line L 1  of the fixed shaft portions  26   a  of the rocker arm shaft  26  as viewed from the axial direction of the rocker arm shaft  26 . Accordingly, the rocker arm shaft  26  has, what is called an “eccentric supporting structure” in the supporting portion of the rocker arms  25 . 
     A shifting direction of the rotation axis L 2  with respect to the center axis line L 1  is adjustable as necessary by changing a fixed state (position and posture) of the rocker arm shaft  26  to the cylinder head  13 . For example, as illustrated in  FIGS. 10 and 11 , the fixed shaft portion  26   a  of the rocker arm shaft  26  is configured to rotate on the journal portion  34  of the cylinder head  13  and a groove  26   b  is disposed at one shaft end portion of the rocker arm shaft  26 . Inserting a tool in the groove  26   b  and rotating about the center axis line L 1  rotatably moves the rotation axis L 2  of the rocker arms  25  about the center axis line L 1 . In association with this, the pivot portion  35 , the contact portion  36 , the pressing portions  37 , and the slipper  43  of the rocker arms  25  also rotatably move about the center axis line L 1  (see  FIG. 12 ). Such structure that performs adjustment by rotating the rocker arm shaft  26  ensures easy adjustment even in a state where the rocker arm shaft  26  is supported by the cylinder head  13 . 
     Alternatively, as illustrated in  FIG. 14 , a structure may form an adjustment hole  48  at the shaft end portion of the rocker arm shaft  26  in the radial direction and include an adjuster  49  that is insertable into this adjustment hole  48 . Appropriately turning the adjuster  49  inserted into the adjustment hole  48  as an arrow B illustrated in  FIG. 14  can adjust the shifting direction of the rotation axis L 2  with respect to the center axis line L 1 . 
     Next, main operational effects of the overhead valve actuation mechanism  23  of the present invention will be described. In this example, as described above, the camshaft  24  is arranged biased to the exhaust valve  31  side as viewed from the axial direction of the camshaft  24 . 
     Thus disposing the camshaft  24  close to the air exhaust side can simplify the configuration of an upper peripheral region of the intake valves  30  in the cylinder head  13  and constitute the intake port  18  to be short and compact. Then, it is easy to dispose the throttle body  19  and similar component close to the cylinder head  13 . 
     Such structure is easy to dispose the intake passage close to the cylinder head and effective in the aspect of ensuring a freedom of layout in what is called, a downdraft type air intake structure, compared with a conventional structure in which the camshaft is disposed in the air intake side. The downdraft type air intake structure is, for example, an air cleaner  114  is disposed above the cylinder head  13  and an air is fed and supplied from the air cleaner  114  to the throttle body  19  in a lower side. The air cleaner  114  and the throttle body  19  are coupled via a coupling pipe  115 . A fuel tank  116  is mountable at an appropriate position in the rear of the engine  10  as illustrated in  FIG. 15 . 
     Alternatively, as illustrated in  FIG. 16 , it is possible to dispose the fuel tank  116  above the cylinder head  13  and mount the air cleaner  114  at an appropriate position in the rear of the engine  10 . The air cleaner  114  and the throttle body  19  are coupled via the coupling pipe  115 . 
     The camshaft  24  is disposed with its whole body positioning in the exhaust valve  31  side with respect to the cylinder axis line Z. Furthermore, the camshaft  24  is disposed with a part of the camshaft  24  overlapping with respect to the axis line of the exhaust valve  31  as viewed from the axial direction of the camshaft  24 . 
     Disposing the camshaft  24  close to the air exhaust side can simplify the configuration of an upper peripheral region of the intake valves  30  in the cylinder head  13  and constitute the intake port  18  to be short and compact. 
     Furthermore, in the case of above, the rocker arms  25  include, by interposing the cylinder axis line Z in between, the pressing portions  37 , which are operating portions relative to the intake valves  30 , in the air intake side and the contact portion  36  with the air intake side valve cam  32  in the air exhaust side. The rocker arms  25  have the axis center of the rocker arm shaft  26  biased to the exhaust valve  31  side with respect to the cylinder axis line Z. 
     Thus, collectively disposing main constitution parts of the overhead valve actuation mechanism  23  in the air exhaust side enables to effectively ensure a space in the air intake side to effectively utilize the empty space. 
     On the other hand, in this example, as described above, the slipper  43  is disposed on the rocker arms  25 , and the stopper cam  44  including the stopper portion  45  as the swing restricting portion of the rocker arms  25  is disposed in the position opposing the slipper  43  in the camshaft  24  side. 
     This structure regulates further swinging of the rocker arms  25  by the slipper  43  of the rocker arms  25  and the stopper portion  45  coming in contact when what is called a valve jump is generated, to prevent the valve jump from becoming excessively large. The valve jump is a phenomenon in which the rocker arms  25  separate from the cam lobe of the air intake side valve cam  32  due to the inertia force of movable parts (such as the rocker arms  25 ) of a valve mechanism, that is, the overhead valve actuation mechanism  23  becoming larger than the reactive force received from the intake valve springs  39  during a high rotation of the engine  10 . 
     This structure regulates a movement so as not to let the rocker arms  25  excessively jump (a slight jump is allowed) by the slipper  43  coming in contact with the first portion  45 A of the stopper portion  45  when the rocker arms  25  and the air intake side valve cam  32  separate by a certain clearance or more, thereby keeping the jump within a slight range. 
     Here, for the comparison,  FIG. 9B  illustrates the case where a conventional valve mechanism is appied (explanation is made using reference numerals identical to this example). Since the conventional valve mechanism does not originally include the slipper  43  and the stopper portion  45 , when the above-described valve jump is generated, the rocker arms  25  excessively swing by the inertia force and makes a significant jump as an arrow A indicates. Therefore, there is a possibility that the inertia force causes unstable operation of the intake valves  30 . In order to avoid this, it has been conventionally typical to design the maximum number of revolution of the engine  10  to be reduced to the extent where the valve jump is unlikely to be generated. 
     In contrast to this, in the present invention, the slipper  43  of the rocker arms  25  and the stopper cam  44  in which the stopper portion  45  is formed are disposed to restrain the rocker arms  25  from making an excessive jump in its course (a slight jump is allowed). This can substantially make the inertia mass of the rocker arms  25  that is subsequently applied to the intake valves  30  zero (0). In view of this, the inertia force from the rocker arms  25  that affects the intake valves  30  in opening and closing is reduced, thereby further stable valve actuation is ensured even when the engine  10  is in a high revolution. 
     Therefore, the range of revolution number of the engine  10  in which the valve gear can properly operate expands to a side of the high-revolution, thereby ensuring the engine with a further high revolution. 
     Even in the case where the number of revolutions of the engine  10  rises and a jump amount increases in the conventional structure, the jump amount can be reduced to a certain amount or less, thereby reducing an impact that the intake valves  30  receive from the valve seats of the cylinder head  13  to small when closing. Similarly, the rocker arms  25  that are retreated by the intake valves  30  gently abuts on the air intake side valve cam  32  when retreating, thereby also reducing an impact to the air intake side valve cam  32  to small. 
     As described above, the overhead valve actuation mechanism  23  in this example has a structure in which the rocker arms  25  operate the intake valves  30  and the slipper  43  is disposed on the rocker arms  25 . Typically, the intake valve  30  is large in size compared with the exhaust valve  31 , and the weight of the valve movable parts in the air intake side is larger than those in the air exhaust side. Accordingly, the air intake side is easily susceptible to the inertia force compared with the air exhaust side, thereby reducing the operational limit number of revolutions of the valve mechanism. In contrast to this, application of the overhead valve actuation mechanism  23  of this example reduces the influence of the inertia force to air intake side valve drive parts. Compared with the case where the overhead valve actuation mechanism  23  is applied to the air exhaust side, the proper operational number of revolutions of the valve mechanism can be effectively increased. 
     Especially, even in a structure in which the camshaft  24  positions in the exhaust valve  31  side with respect to the cylinder axis line Z and the rocker arms  25  are formed long so as to cross over the cylinder axis line Z as this example shows, by reducing the influence of the inertia force with the overhead valve actuation mechanism  23  in this example ensures achieving both a compact configuration of the upper peripheral region of the intake valves  30  and an effect of increasing the proper operational number of revolutions of the valve mechanism. 
     The stopper cam  44  on which the stopper portion  45  is formed is disposed on the camshaft  24  that positions in the exhaust valve  31  side with respect to the cylinder axis line Z. The slipper  43  that opposes to the stopper portion  45  also positions in the exhaust valve  31  side. Thus, the mechanism that reduces swinging of the rocker arms  25  being aggregated in the exhaust valve  31  side effectively ensures the space in the air intake side to effectively utilize the empty space. 
     While the present invention has been described using various embodiments above, the present invention is not limited only to these embodiments. Changes and similar modification are possible within the scope of the present invention. 
     As described above, while the embodiment of the present invention has described the example of the four valve engine, the present invention is applicable to the case where a two valve engine that includes one intake valve and one exhaust valve in the air intake side and the air exhaust side, respectively. 
     According to the present invention, disposing the camshaft close to the air exhaust side can simplify a configuration of an upper peripheral region in the air intake side of the cylinder head and constitute the intake port to be short and compact. Then, it is easy to dispose the throttle body and similar component close to the cylinder head.