Patent Publication Number: US-8118003-B2

Title: Valve operating system

Description:
TECHNICAL FIELD 
     The present invention relates to a valve operating system of an engine which is configured to change lift characteristics of a valve for opening and closing a region between a combustion chamber and an intake port or a region between the combustion chamber and an exhaust port. 
     BACKGROUND ART 
     An engine includes a valve (intake valve, exhaust valve) for opening and closing a region between a combustion chamber and an intake port or a region between the combustion chamber and an exhaust port. The engine is configured to control lift characteristics of the valve such as opening and closing timings and opening and closing amounts (lift amount of valve) to change its characteristics. Also, a valve operating system is proposed to change lift characteristics of the valve depending on the engine (e.g., see Japanese Laid-Open Patent Application Publication No. 2005-180232 (especially see FIGS. 1 and 2), Japanese Laid-Open Patent Application Publication No. Hei. 06-74010 (especially see FIGS. 11 and 12)). 
     According to the disclosure of Japanese Laid-Open Patent Application Publication No. 2005-180232, a first arm member, a pivot roller portion and a locker arm are provided between a drive cam and a valve. During rotation of the drive cam, the pivot roller portion and the first arm member rotate respectively around separate axes, and the locker arm, which is pressed by the first arm member, is pivoted around a base end portion thereof such that a tip end portion thereof moves vertically, causing the valve to reciprocate. On the other hand, when a control shaft is angularly displaced around its center axis, the attitude of the pivot roller portion with respect to the first arm member is changed, changing the lift characteristics of the valve. 
     According to the structure disclosed in Japanese Laid-Open Patent Application Publication No. Hei 06-74010, a pivot arm and a pivot cam are provided between a drive cam and a valve. A roller is rotatably supported in close proximity to a tip end of the pivot arm and is configured to contact the drive cam. When the drive cam rotates, a driving power is transmitted to the pivot arm and the pivot cam via the roller, and the pivot cam presses the valve, causing the valve to reciprocate. 
     In the structure disclosed in Japanese Laid-Open Patent Application Publication No. 2005-180232, the pivot roller portion, the first am member and others are disposed between the drive cam and the valve in addition to the locker arm and they are all pivoted during the reciprocation of the valve as described above. Therefore, an inertia moment of movable members increases. If the inertia moment increases, then it may be difficult to attain a high engine speed, or a wear amount of slidable portions may increase. Also, in the structure disclosed in patent document 2, during the rotation of the drive cam, a drive member, a pivot camshaft, a support shaft, and others are pivoted in addition to a follower and the pivot cam, increasing the inertia moment of the movable members. 
     The structure of Japanese Laid-Open Patent Application Publication No. Hei. 06-74010 is directed to reducing a PV value (i.e., a multiplication value of a surface pressure (P) and a sliding speed (V) at contact portions) at the time of contact between the pivot arm and the drive cam by causing the pivot arm and the drive cam to contact with the roller interposed between them. However, since the inertia moment of the movable members increases because of a weight of the roller, the maximum value of the PV value which occurs while the drive cam is rotating once can not be sufficiently reduced. 
     Accordingly, an object of the present invention is to provide a valve operating system which is capable of reducing an inertia moment in movable members of the valve operating system. Another object of the present invention is to provide a valve operating system which is capable of simplifying a structure for changing a phase between a driven member and a pivot cam while reducing a PV value between the drive cam and the driven member. 
     SUMMARY OF THE INVENTION 
     In view of the aforesaid circumstances, the present invention has been made, and a valve operating system of an engine which is configured to change lift characteristics of a valve for opening and closing a port for air-intake or for air-exhaust, according to the present invention, comprising: a drive cam provided at a camshaft which is configured to rotate in association with a crankshaft; and a pivot cam mechanism which is provided between the drive cam and the valve; wherein the pivot cam mechanism includes: a pivot member which is angularly displaceably supported by a first support shaft and includes a pressing portion which is configured to press the valve by the angular displacement of the pivot member around the first support shaft, the pivot member causing the valve to reciprocate; and a driven member which is angularly displaceably supported by a second support shaft provided at the pivot member eccentrically from the first support shaft and has a sliding contact surface which is configured to slidably contact the drive cam, the driven member being configured to transmit displacement of the drive cam to the pivot member; and wherein the pivot cam mechanism causes the driven member to be angularly displaced around the second support shaft to change relative attitudes of the driven member and the pivot member and causes the pivot member and the driven member to be integrally pivoted around the first support shaft according to rotation of the drive cam. 
     In such a configuration, since the members which move during the rotation of the drive cam (i.e., reciprocation of the valve) are reduced in number, an increase in an inertia moment is suppressed. By changing the relative attitudes of the driven member and the pivot member according to the angular displacement of the driven member around the second support shaft, the way to transmit the driving power from the drive cam to the valve can be changed so that the lift characteristics of the valve can be changed. 
     The second support shaft may be eccentric to be closer to the camshaft than the first support shaft. In such a configuration, since the size of the driven member can be reduced, the increase in the inertia moment can be suppressed more effectively. 
     The pivot cam mechanism may further include a relative attitude changing unit for changing a relative attitude of the driven member with respect to the pivot member. The relative attitude changing unit may include an eccentric member which is provided eccentrically from a center axis of the first support shaft and is configured to change a phase thereof around a center axis of the first support shaft, and a lever portion which is provided at the driven member and is configured to contact the eccentric member to change a phase of the driven member around a center axis of the second support shaft according to change in the phase of the eccentric member. The relative attitude changing unit may be configured to change the relative attitude of the driven member with respect to the pivot member according to change in the phase of the eccentric member to change the lift characteristics of the valve which occur according to the rotation of the drive cam. In such a configuration, by rotating the first support shaft around its center axis, the phase of the eccentric member is easily changed, and the eccentric member presses the lever portion, thereby changing the relative attitude of the driven member with respect to the pivot member. As used herein, the term “phase” means an angular position of the eccentric member which occurs by the angular displacement of the eccentric member around the center axis of the first support shaft with respect to a predetermined reference position. 
     The pivot cam mechanism may include a shaft angle displacement means configured to be angularly displaced about the first support shaft around a center axis thereof and a biasing means configured to apply a force to the driven member in a direction to cause the sliding contact surface to contact the drive cam. In such a configuration, since the driven member and the drive cam are always kept in contact with each other, even in the case where the relative attitude of the drive member with respect to the pivot member is changed, it is possible to avoid a noise which would be generated in the configuration in which there is a clearance between the driven member and the drive cam. In addition, the relative attitudes of the drive cam and the driven member can be determined correctly as compared to the configuration in which there is a clearance. 
     The eccentric member may include a cylindrical roller and is supported by the first support shaft such that the eccentric member is rotatable around a center axis of the roller. In such a configuration, wear of the lever portion of the driven member and the roller which would occur due to sliding friction between them can be reduced. 
     The pivot member may include two ring-shaped portions which are arranged such that their center axes conform to each other and are rotatably externally fitted to the first support shaft. The eccentric member may be provided to protrude from a peripheral surface of the first support shaft and may be disposed between the two ring-shaped portions. In such a configuration, since the eccentric member can stop displacement of the pivot cam in the center axis direction of the first support shaft, there is no need to provide a stop member exclusively for inhibiting the displacement. 
     The first support shaft may be provided on a peripheral surface thereof with a recess between the two ring-shaped portions, the eccentric member being disposed in the recess. The lever portion of the driven member may be disposed between the two ring-shaped portions. In such a configuration, the displacement of the eccentric member in the center axis direction of the first support shaft is inhibited, and the eccentric member restricts the ring-shaped portions so as to inhibit the displacement of the pivot member. Further, the ring-shaped portions restrict the lever portion so as to inhibit the displacement of the driven member. 
     A coil spring may be wound around the first support shaft and may be configured to apply a force to the driven member in a direction to cause the sliding contact surface of the driven member to contact the drive cam. One end of the coil spring is wound around and supported by the second support shaft. In such a configuration, there is no need to provide a member exclusively for stopping the one end of the coil spring with respect to the driven member. 
     The valve operating system may further comprise a lower support portion configured to support the first support shaft from below; and an upper support portion which is coupled to the lower support portion from above and supports the camshaft from below such that the camshaft is rotatable. An opposite end of the coil spring may be retained in a recess which is formed so as to be sandwiched between the lower support portion and the upper support portion and so as to open outward. In such a configuration, there is no need to provide a member exclusively for stopping the opposite end of the coil spring. 
     One end and an opposite end of a coil spring may extend from a winding portion forming a coil main body such that the one end and the opposite end extend substantially parallel with each other and toward substantially the same direction. In such a configuration, it is possible to suppress an event that the winding portion of the coil spring contacts the first support shaft with a great force when the pivot cam mechanism is rotated according to the rotation of the drive cam. To be specific, when the pivot cam mechanism operates and the driven member and the pivot member are pivoted, one end portion of the coil spring generates a restoring force for restoring the pivot cam mechanism to its initial attitude, while at the same time, the drag against the restoring force is exerted on the opposite end of the coil spring. For example, when the one end and the opposite end of the coil spring extend in the opposite direction with respect to the winding portion, reactions acting on the winding portion by the forces exerted on the one end and to the opposite end are oriented in the same direction. Because of the reactions, the coil spring is pressed against the first support shaft strongly. On the other hand, when the one end and the opposite end extend from the winding portion substantially parallel with each other and substantially in the same direction as described above, the reactions acting on the winding portion due to the forces exerted on the one end and the opposite end are oriented in substantially opposite directions and cancelled. As a result, it is possible to suppress the event that the coil spring is pressed against the first support shaft strongly. 
     The engine may have a plurality of ports which are aligned. The pivot cam mechanism may be provided to correspond to each of the ports. The driven members included in at least two adjacent pivot cam mechanisms may be supported by one second support shaft. One end of each of the coil springs may be wound around and supported by both ends of the second support shaft. In such a configuration, since the coil springs provided at both ends of the second support shaft, so as to sandwich at least the two driven members, apply a force to the transmission cam, the coil springs can be reduced in number. By reducing the coil springs in number, the inertia moment of the pivot cam mechanism can be reduced. 
     In the valve operating system of the present invention, positions and shapes of the drive cam, the driven member, and the pivot member may be designed so that a valve maximum acceleration point at which an acceleration of the valve is a maximum is located in a front part of a valve acceleration period in which the acceleration of the valve has a positive value while the drive cam is rotating once. 
     In such a configuration, the valve maximum acceleration point is located where the valve starts positive acceleration and is displaced at a low speed (i.e., front part of the valve acceleration period), and the maximum value of the PV value can be reduced. To be specific, the surface pressure P is a value obtained by dividing the contact load between the drive cam and the driven member by the contact surface between them. The contact load is determined by the inertia force of the tappet and the followers (tappet, valve, etc), the inertia force of the pivot cam, a force of a valve spring, etc, and the PV value is at a maximum in the rear part of the valve acceleration period. Accordingly, by locating the valve maximum acceleration point in the front part of the valve acceleration period, the acceleration of the valve in the rear part of the valve acceleration period can be reduced, and the inertia force of the tappet and the followers at the point in time when the PV value is at its peak can be reduced. As a result, the maximum value of the PV value can be reduced. As used herein, the term “valve acceleration period in which the acceleration of the valve has a positive value” means a period during which the surface pressure P generated between the drive cam and the driven member is increasing. 
     Positions and shapes of the drive cam, the driven member, and the pivot member, may be designed so that an absolute value of an acceleration change rate of the valve per unit angular displacement of the drive cam is larger in the front part which is forward relative to the valve maximum acceleration point of the valve acceleration period than in a rear part which is rearward relative to the valve maximum acceleration point of the valve acceleration period. In such a configuration, since the acceleration of the valve can be reduced in the rear part (i.e., rear part which is rearward relative to the valve maximum acceleration point) of the valve acceleration period during which the valve is displaced at a relatively high speed, the inertia force of the tappet and the follower in this period can be reduced and the thus the maximum value of PV value can be reduced in this period. 
     In the valve operating system of the present invention, positions and shapes of the drive cam, the driven member, and the pivot member are designed so that a pivot member maximum acceleration point at which an acceleration of the pivot member is at a maximum is located in a front part of a pivot member acceleration period in which the acceleration of the pivot member has a positive value while the drive cam is rotating once. 
     In such a configuration, the pivot member maximum acceleration point is located where the pivot member starts positive acceleration and is displaced at a low speed (i.e., front part in the pivot member acceleration period). Therefore, the inertia force of the pivot cam which is one factor for determining the contact load between the drive cam and the driven member, can be reduced in the rear part of the period in which the peak of the PV value is located, and thus, the maximum value of the PV value can be reduced. As used herein, the term “pivot member acceleration period in which the acceleration of the pivot member has a positive value” is referred to as a period in which the surface pressure P generated between the drive cam and the driven member is increasing. 
     Positions and shapes of the drive cam, the driven member, and the pivot member may be designed so that the acceleration of the pivot member is substantially zero at a position of the drive cam where a PV value is at a maximum, the PV value being a multiplication value of a surface pressure and a sliding speed at contact portions of the drive cam and the driven member. In such a configuration, since the acceleration of the pivot member is substantially zero, the influence of the pivot cam inertia at the point in time when the PV value is at a maximum can be lessened, and the maximum value of the PV value can be reduced. As used herein, the phrase “the acceleration of the pivot member is substantially zero” means that the acceleration need not be zero in a strict sense so long as it is sufficiently smaller than the peak value of the acceleration of the pivot cam. For example, the acceleration may be 10% or less of the maximum value of the pivot cam and preferably 5% or less of the maximum value. 
     An angle formed between a line segment connecting a rotational center axis of the drive cam to a center of angular displacement of the pivot member and a line segment connecting the rotational center axis of the drive cam to a contact point between the drive cam and the driven member may be set to an acute angle. By causing the driven member and the drive cam to contact each other without the roller between them, the contact point can be made closer to the pivot center of the pivot member, and thus the inertia moment of the pivot cam mechanism can be reduced. Since the angle formed between the line segment connecting the rotational center axis of the drive cam to the center of angular displacement of the pivot member and the line segment connecting the rotational center axis of the drive cam to the contact point between the drive cam and the driven member is set to the acute angle, the size of the driven member can be reduced, and the inertia moment of the driven member can be reduced. As a result, the PV value can be reduced. 
     The set angle may be set in a range between 35 degrees and 45 degrees. As the set angle is made smaller, the distance from the center axis of the first support shaft to the contact point can be made smaller and the driven member can be made shorter. Therefore, the inertia moment of the driven member can be reduced. There is a likelihood that a cam top radius (i.e., distance from the center axis of the drive cam to the cam nose) of the drive cam can be reduced by setting the set angle smaller and thereby, the maximum value of the V value can be reduced. Supposing that the lift characteristics of the valve during one rotation of the drive cam are not changed, the contact portions of the drive cam and the driven member are closer to the center of the angular displacement of the pivot member due to reduction of the size of the driven member according to the reduced set angle, and therefore the PV value tends to be large, because the moment acting on the driven member needs to be invariable. Therefore, to reduce the PV value, the smaller set angle is not better but there is an optimal value of the set angle. The set angle is preferably an acute angle, which is more preferably, in a range between 35 degrees and 45 degrees. 
     The pivot members included in the pivot cam mechanisms for an intake port and for an exhaust port may have an identical shape and the driven members included in the pivot cam mechanisms for the intake port and for the exhaust port may have an identical shape. In such a configuration, since the pivot cam mechanisms provided in the respective ports can be formed to have an identical structure, a manufacturing cost can be reduced. 
     An engine of the present invention comprises the aforesaid valve operating system, a cylinder head and a cylinder head cover which are arranged in an axial direction of a cylinder, the cylinder head cover being removably attached to the cylinder head; wherein the cylinder head cover is moved in a direction perpendicular to the axial direction to removably attach the cylinder head cover to the cylinder head. 
     In such a configuration, by moving the cylinder head cover with respect to the cylinder head in one direction in the direction perpendicular to the axial direction, the cylinder head cover can be removed from the cylinder head. Therefore, when removing the cylinder head cover, it is not necessary to move the cylinder head cover upward a great amount with respect to the cylinder head. For this reason, even in a vehicle in which there is a small gap (e.g., gap between the cylinder head cover and the main frame) above the cylinder head cover, the cylinder head cover can be removed without unloading the engine from the vehicle. 
     To attach the cylinder head cover, the cylinder head cover is moved in the opposite direction in the direction perpendicular to the axial direction as in the case of removing the cylinder head cover. For this reason, even in a vehicle in which there is a small gap above the cylinder head cover, the cylinder head cover can be attached without unloading the engine from the vehicle. This can reduce the number of work steps during maintenance of the engine, and can improve operation efficiency. 
     In the above invention, the cylinder head cover may be dividable into one part and the other part in the direction perpendicular to the axial direction. In such a configuration, one of the parts into which the cylinder head cover is divided in the direction perpendicular to the axial direction can be moved in one direction in the direction perpendicular to the axial direction or the other can be moved in the opposite direction, even in the structure in which built-in components or members protrude into the cylinder head cover. As a result, the cylinder head cover can be moved in the direction perpendicular to the axial direction, without interference between the built-in components or members and the wall portion of the cylinder head cover which is formed to extend in the direction perpendicular to the axial direction. 
     The above and further objects and features of the invention will more fully be apparent from the following detailed description with accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a right side view of a motorcycle  1  equipped with an engine E including a valve operating system according to an embodiment of the present invention. 
         FIG. 2  is a right side view of the engine of  FIG. 1 , a part of which is illustrated in cross-section. 
         FIG. 3  is an enlarged cross-sectional view of an upper part of the engine of  FIG. 2 , showing valve operating systems as being enlarged. 
         FIG. 4  is an exploded perspective view of a pivot cam mechanism of  FIG. 3 . 
         FIG. 5  is a front view of a major part of an assembled pivot cam mechanism. 
         FIG. 6  is a perspective view of a major part of the pivot cam mechanism of  FIG. 5 . 
         FIG. 7  is a perspective view of a major part of the pivot cam mechanism of  FIG. 5  as viewed from another angle. 
         FIG. 8  is a plan view showing a state where a head cover is removed from the engine of  FIG. 3 . 
         FIG. 9  is a plan view showing a state where upper brackets and drive camshafts are further removed from the engine of  FIG. 8 . 
         FIG. 10  is a view showing an operation of the valve operating system in a state where the pivot cam mechanism is set in one mode. 
         FIG. 11  is a view showing an operation of the valve operating system in a state where the pivot cam mechanism is set in another mode. 
         FIG. 12  is a schematic side view of the valve operating system including the pivot cam mechanism, wherein  FIG. 12(   a ) is a view showing the positional relationship between a control shaft, a coupling pin, and a drive camshaft, and the relationship between forces acting on a driven member, and  FIG. 12(   b ) is a view showing a contact position of the driven member and the drive cam. 
         FIG. 13  is a graph showing an example of the relationship between a set angle of  FIG. 12  and a PV value. 
         FIG. 14  is a graph showing a change in an acceleration of a valve body in the valve operating system according to this embodiment, wherein  FIG. 15(   a ) shows a change in an acceleration of a valve body according to a comparative example, and  FIG. 15(   b ) shows a change in an acceleration of a valve body in the valve operating system according to this embodiment. 
         FIG. 15  is a graph showing a change in an angular acceleration of a pivot member according to this embodiment, in which  FIG. 15(   a ) shows a change in an angular acceleration of a pivot member according to a comparative example and  FIG. 15(   b ) shows a change in an angular acceleration of a pivot member of the valve operating system according to this embodiment. 
         FIG. 16  is a graph showing a change in the PV value at contact portions of the drive cam and the driven member with respect to the angular displacement of the drive cam in the valve operating system according to this embodiment, wherein a horizontal axis indicates the angular displacement of the drive cam, a vertical axis indicates the PV value, a thin line indicates those of a comparative example, and a bold line indicates those of the valve operating system of this embodiment. 
         FIG. 17  shows a relative speed of the contact portions of the drive cam and the driven member with respect to the angular displacement of the drive cam in the valve operating system according to this embodiment, wherein a horizontal axis indicates the angular displacement of the drive cam, and a vertical axis indicates a relative speed at the contact portions. 
         FIG. 18  is a bar graph showing a contact load at the contact portions of the drive cam and the driven member at a point in time when the PV value is at a maximum in the valve operating system according to this embodiment. 
         FIG. 19  is a perspective view showing another structure of the pivot cam mechanism which is applicable to the engine of  FIG. 1 . 
         FIG. 20  is a perspective view showing another structure of a coil spring applicable to a pivot cam mechanism according to an embodiment of the present invention. 
         FIG. 21  is a schematic view showing the coil spring of  FIG. 20 . 
         FIG. 22  is a view showing a pivot cam mechanism including a pivot member and a driven member having another structure, wherein  FIG. 22(   a ) shows the pivot cam mechanism set in one mode and  FIG. 22(   b ) shows the pivot cam mechanism set in another mode. 
         FIG. 23  is a view showing a pivot cam mechanism including a pivot member and a driven member having still another structure, wherein  FIG. 23(   a ) shows the pivot cam mechanism set in one mode and  FIG. 23(   b ) shows the pivot cam mechanism set in another mode. 
         FIG. 24  is a view showing a pivot cam mechanism including a pivot member and a driven member having still another structure, wherein  FIG. 24(   a ) shows the pivot cam mechanism set in one mode and  FIG. 24(   b ) shows the pivot cam mechanism set in another mode. 
         FIG. 25  is a plan view of a cylinder head and a cylinder head cover of the engine E of  FIG. 2 , as viewed in the direction of arrow A. 
         FIG. 26  is a plan view showing a state where a part of the cylinder head cover is moved in the state shown in  FIG. 25 . 
         FIG. 27  is an enlarged view showing a region in the vicinity of a dividing plane B-B of the cylinder head cover. 
         FIG. 28  is a cross-sectional view showing a region where a cam cover of the engine passes when the cam cover is moved in a rightward and leftward direction. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Hereinafter, a valve operating system of the present invention will be described with reference to the drawings. By way of example, a motorcycle in which an engine including the valve operating system is mounted will be described with reference to the drawings. 
     [Motorcyle] 
       FIG. 1  is a right side view of a motorcycle  1  equipped with an engine E including the valve operating system according to this embodiment of the present invention. As used herein, the direction used in Embodiments described below is such that a driving direction of the motorcycle  1  is forward and the other directions are referenced from a rider R mounting a motorcycle  1 . 
     As shown in  FIG. 1 , the motorcycle  1  includes a front wheel  2  and a rear wheel  3 . The front wheel  2  is rotatably mounted to a lower portion of a front fork  5  extending substantially vertically. The front fork  5  is mounted to a steering shaft (not shown) by an upper bracket (not shown) provided at an upper end portion thereof and an under bracket provided under the upper bracket. The steering shaft is rotatably mounted by a head pipe  6 . A bar-type steering handle  4  extending rightward and leftward is mounted to the upper bracket. By the rider R&#39;s operation for rotating the steering handle  4 , the front wheel  2  can be rotated in a desired direction around the steering shaft. 
     A pair of right and left main frame members  7  forming a vehicle body frame extend rearward from the head pipe  6 . A pivot frame member (also referred to as a swing arm bracket)  8  extends downward from a rear portion of each of the main frame members  7 . A swing arm  10  extending in a forward and rearward direction is mounted at a front end portion thereof to a pivot  9  provided at the pivot frame member  8 . The rear wheel  3  is rotatably mounted to a rear end portion of the swing arm  10 . 
     A fuel tank  12  is disposed above the main frame members  7  and behind the steering handle  4 . A straddle-type seat  13  is disposed behind the fuel tank  12 . An engine E is mounted below and between the right and left main frame members  7 . A driving power of the engine E is transmitted to the rear wheel  3  via a chain (not shown). The rear wheel  3  rotates, enabling a propulsive force to be generated in the motorcycle  1 . A cowling  14  is provided integrally so as to cover a front part of the motorcycle  1 , to be precise, the head pipe  6 , front portions of the main frame members  7 , and side portions of the engine E. In the motorcycle  1  having such a construction, mounting the seat  13 , the rider R rides the motorcycle  1 . Gripping grips  4   a  provided at end portions of the steering handle  4 , and putting feet on steps  15  provided in the vicinity of the rear portion of the engine E, the rider R drives the motorcycle  1 . 
     [Engine] 
       FIG. 2  is a right side view of the engine of  FIG. 1 , a part of which is illustrated in cross-section. As shown in  FIG. 2 , the engine E includes as major components, a cylinder head  20 , a cylinder head cover  21 , a cylinder block  22 , and a crankcase  23 . The engine E is an inline four-cylinder double overhead camshaft (DOHC) engine in which cylinders are arranged in a vehicle width direction. 
     An intake port  20 A is provided on the rear portion of the cylinder head  20  to correspond to each cylinder and to open obliquely rearward and upward. An exhaust port  20 B is provided on the front portion of the cylinder head  20  to correspond to each cylinder and to open forward. In the engine E of this embodiment, two intake ports  20 A and two exhaust ports  20 B are provided for each cylinder. 
     A drive camshaft  24  for an air-intake system and a drive camshaft  25  for air-exhaust system are arranged in an upper portion of the cylinder head  20  such that their axes extend in the vehicle width direction. The drive camshafts  24  and  25  are rotatably retained by shaft support brackets  49  (see  FIG. 3 ) including lower brackets  81  and upper brackets  82  as described later. The cylinder head cover  21  is provided over the shaft support brackets  49  and is fastened to the cylinder head  20  by bolts. 
     Cylinder blocks  22  respectively accommodating pistons (not shown) are respectively coupled to the lower portion of the cylinder head  20 . The crankcase  23  accommodating a crankshaft  26  extending such that its axis extends in the vehicle width direction is coupled to the lower portions of the cylinder blocks  22 . In a right wall portion of the cylinder head  20 , the cylinder head cover  21 , the cylinder block  22 , and the crankcase  23 , a chain tunnel  27  is formed continuously, in which a driving power transmission mechanism  28  for transmitting a rotational driving power of the crankshaft  26  to the drive camshafts  24  and  25  is accommodated. An oil pan  29  for reserving oil for lubrication or hydraulically-powered devices is provided at the lower portion of the crankcase  23 . An oil filter  30  for filtering the oil suctioned up by the oil pan  29  is provided at the front portion of the crankcase  23 . 
     The driving power transmission mechanism  28  includes an intake cam sprocket  31 , an exhaust cam sprocket  32 , a crank sprocket  33 , and a timing chain  34 . To be specific, the right end portion of the drive camshaft  24  for an air-intake system protrudes into the chain tunnel  27 , and the intake cam sprocket  31  is provided at the end portion. In addition, the right end portion of the drive camshaft  25  for the air-exhaust system protrudes into the chain tunnel  27 , and the exhaust cam sprocket  32  is provided at the end portion. Furthermore, the right end portion of the crankshaft  26  protrudes into the chain tunnel  27 , and the crank sprocket  33  is provided at the end portion. 
     The timing chain  34  is installed around the intake cam sprocket  31 , the exhaust cam sprocket  32 , and the crank sprocket  33 . When the crank sprocket  33  rotates, the intake cam sprocket  31  and the exhaust cam sprocket  32  rotate in association with the rotation of the crank sprocket  33 . Therefore, through the rotation transmission mechanism  28  formed by the intake cam sprocket  31 , the exhaust cam sprocket  32 , the crank sprocket  33  and the timing chain  34 , the rotational driving power of the crankshaft  26  is transmitted to the drive camshafts  24  and  25 . 
     Inside the chain tunnel  27 , a movable chain guide  35  and a fixed chain guide  36  are provided. The fixed chain guide  36  extends vertically in front of the timing chain  34  and from a location in front of and in the vicinity of the crank sprocket  33  to a location below and in the vicinity of the exhaust cam sprocket  32 . The fixed chain guide  36  is configured to contact the timing chain  34  from front to support the timing chain  34  from front. 
     The movable chain guide  35  extends vertically behind the timing chain  34 . The movable chain guide  35  is mounted at a lower end portion thereof to the right wall portion of the crankcase  23  at a location above and in the vicinity of the crank sprocket  33 . An upper end portion of the movable chain guide  35  is located below and in the vicinity of the intake cam sprocket  31 . A hydraulically-powered tensioner  37  mounted to the rear wall portion of the cylinder head  20  causes the movable chain guide  35  to apply a force from behind to the timing chain  34  to make the timing chain  34  have a suitable tension. 
     An output gear  38  configured to output the rotation of the crankshaft  26  is mounted on the right portion of the crankshaft  26  such that the output gear  38  is rotatable integrally with the crankshaft  26 . A transmission chamber  39  is formed in the rear portion of the crankcase  23 , and accommodates therein an input shaft  40  and an output shaft (not shown) such that the input shaft  40  and the output shaft extend substantially in parallel with the crankshaft  26 . A plurality of gears  41  are mounted on the input shaft  40  and the output shaft, constituting a transmission  4 . 
     An input gear  43  is mounted on the right end portion of the input shaft  40  such that the input gear  43  is configured to mesh with the output gear  38  of the crankshaft  26  and is rotatable integrally with the input shaft  40 . Therefore, the driving power of the engine E is transmitted from the crankshaft  26  to the input shaft  40  via the output gear  38  and the input gear  43 , and its rotational speed is changed by the transmission  42 . The resulting driving power is output to the rear wheel  3  ( FIG. 1 ). 
     The engine E of this embodiment includes a trochoidal rotor type oil pump  44 . A driven gear  46  is mounted on an input shaft of the oil pump  44  and is configured to mesh with a drive gear  45  mounted on the input shaft  40  of the transmission  42 . According to the rotation of the crankshaft  26 , the oil pump  44  is driven. The engine E is provided with oil passages through which oil for lubrication or hydraulic powering flows to deliver oil  47  suctioned up by the oil pump  44  from the oil pan  29  to engine components. 
     The engine E includes a valve operating system  50 A configured to open and close the intake port  20 A and a valve operating system  50 B configured to open and close the exhaust port  20 B, in association with the rotation of the crankshaft  26 . The valve operating system  50 A is configured to control a flow rate and a timing of air-intake from the intake port  20 A to the combustion chamber  52 , while the valve operating system  50 B is configured to control a flow rate and a timing of air-exhaust from the combustion chamber  52  to the exhaust port  20 B. Hereinafter, the valve operating system  50 A or  50 B will be described in detail. 
     [Valve Operating System] 
       FIG. 3  is an enlarged cross-sectional view of an upper part of the engine E of  FIG. 2 , showing the valve operating systems  50 A and  50 B, and others, as being enlarged. As shown in  FIG. 3 , in the cylinder head  20 , there are provided an intake valve mechanism  51 A configured to open and close the combustion chamber  52  with respect to the intake port  20 A and an exhaust valve mechanism  51 B configured to open and close the combustion chamber  52  with respect to the exhaust port  20 B. In the engine E which is an inline four-cylinder, four combustion chambers  52  respectively corresponding to the cylinders are arranged in one line in the depth direction of the drawing sheet. The intake-side valve operating system  50 A causes the intake valve mechanism  51 A to perform an opening and closing operation (reciprocating operation), while the exhaust-side valve operating system  50 B causes the exhaust valve mechanism  51 B to perform an opening and closing operation (reciprocating operation). Since the intake valve mechanism  51 A and the exhaust valve mechanism  51 B have substantially the same structure and the valve operating system  50 A and the valve operating system  50 B have substantially the same structure in the air-intake system and in the air-exhaust system, the valve mechanism  51 A and the valve operating system  50 A in the air-intake system will be described hereinafter. 
     The intake valve mechanism  51 A has a known structure. The intake valve mechanism  51 A includes a valve body  53  including a valve plug  53   a  facing the combustion chamber  52  in the intake port  20 A and a stem  53   b  extending upward from the valve plug  53   a . A groove is formed at an upper end portion of the stem  53   b . A cotter  56  is inserted into the groove. A spring retainer  55  is mounted to the cotter  56 . A spring seat  54  is mounted to the cylinder head  20  below the spring retainer  55 . A valve spring  57  is mounted between the spring seat  54  and the spring retainer  55 . The valve spring  57  applies an upward force to the valve body  53  with the spring retainer  55  interposed therebetween, i.e., to close the intake port  20 A. 
     The valve operating system  50 A includes the drive camshaft  24  configured to operate in association with the rotation of the crankshaft  26  of the engine E, a drive cam  24   a  fixed to the drive camshaft  24 , and a pivot cam mechanism  48  which is configured to contact the drive cam  24   a  and to transmit the movement of the drive cam  24   a  to a tappet  58  of the intake valve mechanism  51 A. 
     The pivot cam mechanism  48  is configured to transmit the driving power exerted by the drive cam  24   a  to the intake valve mechanism  51 A. Thereby, the intake valve mechanism  51 A obtains the driving power for opening and closing the intake port  20 A according to the rotation of the crankshaft  26 . The entire outer shape of the pivot cam mechanism  48  is changed by a motor  87  in this embodiment, which is an example of a drive means forming a shaft angle displacement means for angularly displacing a control shaft  60  described later around its center axis  60   a . Thereby, the timing at which the driving power is transmitted from the drive cam  24   a  to the intake valve mechanism  51 A or a displacement amount of the intake valve mechanism  51 A is changed. Therefore, the opening and closing timings and opening and closing amounts of the intake port  20 A which are operative in association with the rotation of the crankshaft  26 , i.e., the lift characteristics of the intake valve mechanism  51 A can be changed as desired. 
     The drive cam  24   a  has a non-circular contour as viewed along the rotational center thereof. The drive cam  24   a  has a shape in which a distance between a location on the contour and the rotation center of the drive cam  24   a  changes along the counter. 
     [Pivot Cam Mechanism] 
       FIG. 4  is an exploded perspective view of the pivot cam mechanism  48  of  FIG. 3 .  FIG. 5  is a front view of a major part of an assembled pivot cam mechanism  48 .  FIG. 6  is a perspective view of a major part of the pivot cam mechanism  48  of  FIG. 5 .  FIG. 7  is a perspective view of a major part of the pivot cam mechanism  48  of  FIG. 5  as viewed from another angle. The valve operating system  50 A of this embodiment includes two pivot cam mechanisms  48  respectively corresponding to two intake valve mechanisms  51 A configured to open and close two intake ports  20 A provided for each cylinder. 
     The valve operating system  50 A includes as major components a control shaft  60  which is an example of a first support shaft, two pivot members  61  which are angularly displaceably supported by the control shaft  60  and are configured to respectively press the tappets  58 , two driven members  63  which are angularly displaceably supported by a coupling pin  62  which is an example of a second support shaft supported by the pivot cams  61  and are configured to contact the drive cams  24   a , and two relative attitude changing mechanisms  64  configured to change relative attitude of the driven members  63  relative to the pivot members  61 . In this embodiment, the valve operating system  50 A includes two pivot cam mechanisms  48  each including one pivot member  61 , one driven member  63 , and one relative attitude changing mechanism  64 . 
     In the pivot cam mechanism  48 , the driven member  63  and the pivot member  61  are angularly displaced so as to be pivoted around a center axis  60   a  of the control shaft  60  to apply the driving power exerted by the drive cam  24   a  to the intake valve mechanism  51 A, opening and closing the intake port  20 A. The relative attitude changing mechanism  64  causes the driven member  63  to be angularly displaced around a center axis  62   d  of the coupling pin  62 , changing the relative attitude of the driven member  63  with respect to the pivot member  61 . By changing the relative attitude, the timing at which the driving power is transmitted from the drive cam  24   a  to the intake valve mechanism  51 A or the displacement amount of the intake valve mechanism  51 A are changed so that the lift characteristics of the intake valve mechanism  51 A are changed. 
     As shown in  FIG. 4 , the control shaft  60  has a substantially cylindrical shape. In this embodiment, a plurality of sub-shafts  67  are coupled to form the control shaft  60 . A fitting protrusion  67   a  protrudes from an end portion of one sub-shaft  67  of the sub-shafts  67  coupled to each other at a location deviated from the center axis of the sub-shaft  67 , while a fitting hole  67   b  conforming in shape to the fitting protrusion  67   a  is formed at an end portion of the other sub-shaft  67 . Each sub-shaft  67  has a circular insertion hole  67   c  formed to penetrate along the center axis in a location deviated from the center axis. 
     The sub-shafts  67  are coupled coaxially to form the control shaft  60  in such a manner that the end portions thereof are opposite to each other, the fitting protrusion  67   a  of one of the sub-shafts  67  is fitted into the fitting hole  67   b  of the other sub-shaft  67 , and a round-rod-like roller shaft  68  having a dimension which is a sum of the sub-shafts  67  is inserted into the insertion holes  67   c . Since the control shaft  60  is formed by the plural sub-shafts  67  in this manner, the insertion holes  67   c  provided in the respective sub-shafts  67  can be formed accurately. 
     Shaft cut portions  69  are respectively formed at plural specified locations in the longitudinal direction of the control shaft  60 . The shaft cut portions  69  form recesses which are recessed radially toward the center relative to the remaining portion. In this embodiment, the shaft cut portion  69  has a predetermined width B 1  and has a substantially semi-circular shape in cross-section which is perpendicular to the center axis  60   a . More specifically, the shaft cut portion  69  has a shape including a bottom wall surface  69   a  of a rectangular flat surface shape and side wall surfaces  69   b  of a substantially semi-circular shape which extend upward from both side portions of the bottom wall surface  69   a . The insertion hole  67   c  is formed to penetrate the side wall surfaces  69   b  of the cut portion  69  near a location distant from the bottom wall surface  69   a . In the control shaft  60 , peripheral portions  70  adjacent at right and left sides of the cut portion  69  have a larger outer dimension than the other portion. 
     A roller  71  which is supported by a roller shaft  68  is attached to the control shaft  60  as an eccentric member provided in a location eccentric from the center axis  60   a . In this embodiment, the roller  71  has a cylindrical shape, and is formed such that a dimension B 2  in the center axis direction thereof is substantially equal to the width B 1  of the shaft cut portion  69  (to be precise, the dimension B 2  in the center axis direction of the roller  71  is slightly smaller than the width B 1  of the shaft cut portion  69 ). An insertion hole  71   a  is formed in the center axis position of the roller  71  so as to have an inner diameter which is substantially equal to that of the insertion hole  67   c  of the control shaft  60 . The roller  71  is mounted in the control shaft  60  such that the roller shaft  68  is inserted into the insertion hole  71   a  of the roller  71  when the roller shaft  68  is inserted into the insertion hole  67   c  of the control shaft  60 . The roller  71  is supported by the roller shaft  68  such that the roller  71  is rotatable around the center axis of the roller shaft  68 . The roller  71  mounted in the manner described above is disposed with a slight gap with the right and left side surfaces  69   b  of shaft cut portion  69  and eccentrically from the center axis of the control shaft  60 . The center axis of the roller  71  is located inside the cross-section of the control shaft  60 . In this embodiment, the roller  71  partially protrudes outward from the outer peripheral surface of the control shaft  60 . 
     Two pivot cams  61  which are constituents of the valve operating system  50 A are externally fitted to the control shaft  60 . Each pivot cam  61  is supported by the control shaft  60  such that the pivot cam  61  is angularly displaceable around the center axis  60   a  of the control shaft  60 . The pivot cam  61  includes an outer fitting tubular portion  61   a  which is externally fitted to the control shaft  60  and is rotatably supported around the center axis  60   a  of the control shaft  60 , a bearing portion  61   b  protruding from the outer peripheral portion of the outer fitting tubular portion  61   a , and a tappet pressing portion  74  which extends outward from the outer fitting tubular portion  61   a  and is configured to press the tappet  58 . 
     The outer fitting tubular portion  61   a  forms a cylindrical shape and is provided with a circular through-hole  61   f  into which the control shaft  60  is inserted. A tubular cut portion  61   c  which is cut in a circumferential direction is formed in an intermediate axial portion of the outer fitting portion  61   a . As a result, ring-shaped portions  61   d  are provided at the outer fitting tubular portion  61   a  such that the ring-shaped portions  61   d  are spaced apart in the center axis direction with the tubular cut portion  61   c  interposed between them. In this embodiment, the width of the tubular cut portion  61   c  of the outer fitting tubular portion  61   a , i.e., a distance B 3  in the center axis direction between the ring-shaped portions  61   d  is substantially identical to the width B 1  of the shaft cut portion  69 . 
     The bearing portions  61   b  respectively protrude radially outward from the ring-shaped portions  61   d  and are respectively provided with through-holes  61   e  extending in the center axis direction, into which the coupling pin  62  is inserted. 
     The tappet pressing portion  74  extending from the outer fitting portion  61   a  includes a pressing wall portion  74   a  which has a predetermined thickness B 4  in the direction in which the tappet pressing portion  74  is applied with a force from the tappet  58  and is configured to contact the tappet  58 , and a rib  74   b  coupling the pressing wall portion  74   a  to the outer fitting tubular portion  61   a . The outer wall surface of the pressing wall portion  74   a  includes a base circular-arc surface  74   c  whose center coincides with the center axis of the ring-shaped portion  61   d , and a lift curved surface  74   d  which extends continuously from the base circular-arc surface  74   c  and changes a distance between the center axis of the ring-shaped portion  61   d  and the outer peripheral surface thereof changes, for example, increases in the direction closer to the tip end. The rib  74   b  extends from the pressing wall portion  74   a , is branched at an intermediate point in one direction and in an opposite direction in the center axis direction, and the branched portions are coupled to the ring-shaped portions  61   a  and the bearing portions  61   b.    
     The driven member  63  is supported by the pivot member  61  via the coupling pin  62  having a hollow pipe shape with a smaller diameter than the control shaft  60 . The driven member  63  includes an insertion portion  63   a  into which the coupling pin  62  is inserted, a lever portion  63   b  extending radially in one direction from the insertion portion  63   a , and a drive cam contact portion  75  which extends radially in an opposite direction from the insertion portion  63   a  and is configured to contact the drive cam  24   a . The insertion portion  63   a  has a width B 5  which is substantially equal to a distance B 3  between the right and left bearing portions  61   b  of the pivot cam  61  (to be precise, width B 5  which is slightly smaller than the distance B 3  of the bearing portion  61   b ), and has a through-hole  63   c  into which the coupling pin  62  is inserted. The outer peripheral surface of the drive cam contact portion  75  forms a circular-arc sliding contact surface  75   a  which has a center set in a position different from the center axis, for example, and changes a distance between the outer peripheral surface thereof and the center axis of the insertion portion  63   a  in a direction toward the tip end. The outer peripheral surface of the drive cam contact portion  75  is a surface subjected to a surface hardening treatment such as chromium plating. In this embodiment, the circular-arc sliding contact surface  75   a  is harder than the lift curved surface  74   d  of the pressing wall portion  74   a . The drive cam contact portion  75  of the driven member  63  which contacts the drive cam  24   a  has a predetermined thickness B 6  in a direction in which the drive cam contact portion  75  is applied with a force from the drive cam  24   a . The thickness B 6  is larger than the thickness B 4  of the pressing wall portion  74   a  of the pivot cam  61  which contacts the tappet  58  so that the drive cam contact portion  75  has high wear resistance to the contact with the drive cam  24   a  rotating at a high speed. In addition, a center axis dimension B 7  of the drive cam contact portion  75  is set larger than a center axis dimension B 8  of the pressing wall portion  74   a.    
     By inserting the coupling pin  62  into the through-holes  61   e  and  63   c  of the bearing portion  61   b  and the insertion portion  63   a  in a state where the insertion portion  63   a  of the driven member  63  is located between the right and left bearing portions  61   b  of the pivot member  61 , the through-holes  61   e  of the bearing portions  61   b  and the through-hole  63   c  of the insertion portion  63   a  are coaxial with each other. Thus, the driven member  63  is rotatably supported with respect to the coupling pin  62 . The coupling pin  62  is configured to support the two driven members  63  in the vicinity of the both end portions thereof. The coupling pin  62  has a structure in which a portion of the coupling pin  62  between the right and left driven members  63  (i.e., portion between right and left support portions  62   a ) has a smaller outer dimension than the right and left support portions  62   a  to which the bearing portions  61   b  and the insertion portion  63   a  are externally fitted and support the driven members  63 . Thus, a lightweight the coupling pin  62  is achieved. 
     Coil springs  77  which are an example of a biasing means are externally fitted to the control shaft  60 . One end of each coil spring  77  is supported at an end portion of the coupling pin  62 . In more detail, the coil spring  77  is formed by winding a metal-made round-rod member having a predetermined elasticity plural times. The inner diameter of a winding portion  77   a  forming a coil main body winding is slightly larger than the outer diameter of the control shaft  60 . One end  77   b  and an opposite end  77   b  of the coil spring  77  extend in opposite directions along a tangential direction of the outer peripheral surface of the winding portion  77   a . The one end  77   b  has a stop winding portion  77   d  which is wound in the direction opposite to the direction in which the winding portion  77   a  is wound and has a smaller diameter than the winding portion  77   a.    
     As an example of a stop portion, a stop groove portion  62   c  forming a recess extending in the circumferential direction and having a substantially semi-circular cross-section is provided at an end portion of the coupling pin  62  supporting the one end  77   b  of the coil spring  77 . The stop winding portion  77   d  is fitted into the stop groove portion  62   c , so that the one end  77   b  of the coil spring  77  is stopped by the coupling pin  62 . The opposite end  77   c  of the coil spring  77  is inserted into and retained in a recess  78   a  which is formed between the lower surface of the lower bracket  81  (see  FIG. 3 ) supporting the drive camshaft  24  from below and the upper surface of a mounting portion  78  which is provided at the upper portion of the cylinder head  20 , supports the control shaft  60  from below, and is attached with the lower bracket  81  from above (see  FIG. 21(   a )). That is, in this embodiment, by the mounting portion  78  which is an example of the lower support portion supporting the control shaft  60  from below and the lower bracket  81  supporting the drive cam  24  from below and attached to the mounting portion  78  from above, the opposite end  77   c  of the coil spring  77  is retained from above and from below. A recessed region is formed on the lower surface of the lower bracket  81  to open upward. The recess  78   a  is formed so as to open outward (toward the control shaft  60  in  FIG. 7)  and so as to be sandwiched between the mounting portion  78  and the lower bracket  81  which is attached to the mounting portion  78  from above. The opposite end  77   c  of the coil spring  77  is inserted into and retained in the recess  78   a  (se  FIG. 7 ). 
     As described above, the pivot cam mechanism  48  according to this embodiment is mainly comprised of relatively few constituents which are the control shaft  60 , the pivot member  61 , the driven member  63 , and the coil spring  77 . The pivot cam mechanism  48  is assembled in a procedure described below. First, the control shaft  60  is inserted into the ring-shaped portions  61   a  of each pivot member  61  and is disposed such that the tubular cut portion  61   c  of the pivot member  61  and the shaft cut portion  69  of the control shaft  60  conform to each other. In this state, the roller  71  is fitted to the shaft cut portion  69  of the control shaft  60  through the tubular cut portion  61   c  of the pivot member  61 , and the roller shaft  68  is inserted into the insertion hole  67   c  of the control shaft  60 . And, the roller shaft  68  is also inserted into the insertion hole  71   a  of the roller  71  to allow the roller  71  to be supported by the control shaft  60 . 
     At this time, the roller  71  protrudes from the outer peripheral surface of the control shaft  60  and is fixed. Therefore, the pivot member  61  with the roller  71  located between the right and left ring-shaped portions  61   a  is restricted in displacement in the rightward and leftward direction, but is angularly displaceable in a predetermined angle range around the center axis of the control shaft  60 . 
     Then, the driven member  63  is disposed between the right and left bearings  61   b  of the pivot member  61  such that the through-holes  61   e  and  63   c  conform to each other. The coupling pin  62  is inserted into the through-holes  61   e  and  63   c . Then, the coil springs  77  are externally fitted to the control shaft  60  from both sides of two sets of pivot members  61  and driven members  63 . The stop winding portion  77   d  at the one end  77   b  is wound around and stopped by the stop groove  62   c  at the end portion of the coupling pin  62 . The opposite end  77   c  is located in the recess  78   a  formed between the mounting portion  78  of the cylinder head  20  and the lower bracket  81  when the lower brackets  81  are attached to the cylinder head  20 . Thereby, the driven member  63  is subjected to a force applied from the coil spring  77  in the direction to cause the circular-arc sliding contact surface  75   a  to contact the drive cam  24   a . Thus, two pivot cam mechanisms  48  are assembled as shown in  FIGS. 6 and 7 . 
     The pivot cam mechanism  48  having the above described structure is like a locker arm which is provided between the drive cam  24   a  and the intake valve mechanism  51 A. To be specific, the locker arm which forms an elongated arm and is pivoted at an intermediate portion thereof is divided at a position closer to the drive cam  24   a  than the pivot position. A portion of the locker arm at the intake valve mechanism  51 A side including the pivot position is supposed to be the pivot member  61 , and a portion of the locker arm which is closer to the drive cam  24   a  is supposed to be the driven member  63 . The pivot member  61  and the driven member  63  are integrally angularly displaceable around the control shaft  60  during the rotation of the drive cam  24   a , while allowing the driven member  63  to change the relative attitude with respect to the pivot member  61 . 
     Since the stop winding portion  77   d  of the one end  77   b  of the coil spring  77  is wound around and stopped by the stop groove  62   c  formed in the coupling pin  62  as described above, a stop member for exclusive use need not be provided. In addition, since the stop groove portion  62   c  has a substantially semicircular cross-section, the contact surface pressure between the stop groove portion  62   c  and the stop winding portion  77   d  which is formed by the round rod member and has a substantially circular cross-section is reduced, lessening wear-out of these constituents. In addition, since the opposite end  77   c  of the coil spring is sandwiched between the mounting portion  78  and the lower bracket  81 , a member exclusively for retaining the opposite end  77   c  need not be provided. As a result, the components are reduced in number. 
     Since the coil spring  77  is mounted as described above, the driven member  63  is subjected to a force in one direction around the control shaft  60 , and the drive cam contact portion  75  contacts the drive cam  24   a . Also, the lever portion  63   b  contacts the roller  71 . As viewed along the center axis of the insertion portion  63   a  of the driven member  63 , the drive cam  24  and the roller  71  are located in one of two regions defined by a straight line L (see  FIG. 10 ) passing through the tip end of the drive cam contact portion  75  and the tip end of the lever portion  63   b . The circular-arc sliding contact surface  75   a  contacting the drive cam  24   a , and the surface of the lever portion  63   b  contacting the roller  71  are directed toward the one region with respect to the straight line L. 
     An output shaft of the motor  87  (see  FIG. 9 ) is coupled to the control shaft  60  of the pivot cam mechanism  48 . The motor  87  is driven so that the control shaft  60  is rotated a desired angle around the center axis  60   a  thereof so as to change the phase. As described later, when the control shaft  60  is rotated to change the phase of the roller  71  around the center axis  60   a , the lever portion  63   b  contacting the roller  71  moves, changing the relative attitude of the driven member  63  with respect to the pivot member  61 . According to the attitude change, the timing at which the driving power is transmitted from the drive cam  24   a  to the intake valve mechanism  51 A and the displacement amount of the intake valve mechanism  51 A are changed, changing the lift characteristics of the intake valve mechanism  51 A. In this way, the roller  71  and the lever portion  63   b  form a relative attitude changing mechanism  64  for changing the relative attitude of the driven member  63  with respect to the pivot member  61  to change the lift characteristics of the intake valve mechanism  51 A. 
     As shown in  FIG. 3 , the pivot member  61  and the driven member  63  included in the pivot cam mechanism  48  are configured to open toward the center in the forward and rearward direction of the engine E. To be specific, the tappet pressing portion  74  of the pivot member  61  extends from the control shaft  60  toward a center in the forward and rearward direction of the engine E. The drive cam contact portion  75  of the driven member  63  extends upward from the control shaft  60  toward the center in the forward and rearward direction of the engine E. Therefore, the pivot member  61  and the driven member  63  are configured to open at an acute angle from the control shaft  60  as a base end toward the center in the forward and rearward direction of the engine E. 
     In this embodiment, the drive cam  24   a  in the air-intake system of  FIG. 3  is configured to rotate counterclockwise, and the drive cam  24   a  at the exhaust side is configured to rotate counterclockwise as in the drive cam  24   a  at the intake side. 
     As shown in  FIG. 3 , a shaft support bracket  49  is provided on the upper surface of the cylinder head  20  to support the drive camshaft  24  such that the drive camshaft  24  is rotatable. The shaft support bracket  49  includes a lower bracket  81  protruding from the upper surface of the cylinder head  20  and an upper bracket  82  mounted to the lower bracket  81  from above by bolts  80 . The lower bracket  81  has a lower bearing recess  81   a  having a semicircular cross-section. The upper bracket  82  has an upper bearing recess  82   a  with a semi-circular cross-section, facing the lower bearing recess  81   a . The drive camshaft  24  is rotatably inserted and supported in a bearing space with a circular cross-section which is defined by the lower bearing recess  81   a  and the upper bearing recess  82   a.    
     A insertion hole  81   b  is formed on the lower bracket  81  to penetrate along the center axis direction of the drive camshaft  24 . An oil pipe  83  is inserted into the insertion hole  81   b . Therefore, there is no need to provide a member exclusively for supporting the oil pipe  83 . Thus, the number of components is reduced, and space saving is attained. Two oil pipes  83  are provided between the valve operating system  50 A included in the air-intake system and the valve operating system  50 B included in the air-exhaust system. A plurality of outlets  83   a  are formed on the peripheral wall of the oil pipe  83  such that they are spaced apart from each other in the longitudinal direction thereof. The outlets  83   a  are provided at locations respectively corresponding to the valve operating system  50 A. The oil flowing in the oil pipe  83  is ejected toward the valve operating system  50 A through the outlets  83   a.    
     The outlets  83   a  of the oil pipe  83  are located in close proximity to the tip end portion of the drive cam contact portion  75  of the driven member  63 . To be specific, the oil pipe  83  is disposed in a space formed between the pivot cam mechanism  48  in the air-intake system and the pivot cam mechanism  48  in the air-exhaust system. The outlets  83   a  of the oil pipe  83  are disposed to face contact surfaces of the driven member  63  and the drive cam  24   a  in at least one position in the movable range of the pivot cam mechanism  48 . 
       FIG. 8  is a plan view showing a state where the head cover  21  is removed from the engine E of  FIG. 3 .  FIG. 9  is a plan view showing a state where the upper brackets  82  and the drive camshafts  24  and  25  are further removed from the engine E of  FIG. 8 . As shown in  FIG. 8 , the valve operating system  50 A in the air-intake system is aligned in one line at one side relative to the combustion chambers  52  arranged in one line in the rightward and leftward direction, while the valve operating system  50 B in the air-exhaust system is aligned in one line at the other side. The drive camshafts  24  and  25  extend along the direction in which the valve operating systems  50 A and  50 B are aligned. As described above, the end portions of the drive camshafts  24  and  25  are respectively coupled to the cam sprockets  31  and  32  inside the chain tunnel  27 . 
     As shown in  FIG. 9 , the control shaft  60  extends along the direction in which the valve operating systems  50 A and  50 B are aligned. A gear chamber  85  is provided at an end portion of the engine E which is opposite to the chain tunnel  27 . A control gear  86  configured to mesh with the control shaft  60  is accommodated in the gear chamber  85 . The control gear  86  is driven by the motor  87  attached to the cylinder head  20 , and in association with this, the control shaft  60  rotates. The operation of the motor  87  is controlled by an ECU (electronic control unit) (not shown) which is built into the motorcycle  1 . 
     As shown in  FIGS. 8 and 9 , a pair of oil pipes  83  are disposed to extend along the direction (rightward and leftward direction) in which the valve operating systems  50 A and  51 A are aligned, between the pivot cam mechanisms  48  in the air-intake system and the pivot cam mechanisms  48  in the air-exhaust system. One end portion of the oil pipe  83  is coupled to a pipe connecting portion  88  provided at the upper surface of the cylinder head  20 . The pipe connecting portion  88  has an oil supply passage (not shown) in which the oil suctioned up by the oil pump  44  from the oil pan  29  flows. Through the oil supply passage, the oil is fed to the oil pipe  83 . 
     [Operation Principle] 
     Subsequently, the operation principle of the valve operating system  50 A according to this embodiment will be described.  FIG. 10  is a view showing an operation of the valve operating system  50 A in a state where the pivot cam mechanism  48  is set in one mode. In this mode, the pivot member  61  and the driven member  63  in the pivot cam mechanism  48  are open with a relatively large angle. As shown in  FIG. 10 , when the tip end portion (to be precise, tip end of a cam nose) of the drive cam  24   a  is located at an upper limit position, the base circular arc surface  74   c  of the tappet pressing portion  74  of the pivot member  61  is in contact with the tappet  58  (actually, there is a minute clearance between the base circular arc surface  74   c  and the tappet  58 ). Therefore, the lift amount of the tappet  58  (i.e., lift amount of the valve body  53 ) is substantially zero, and the valve body  53  closes the intake port  20 A. The drive cam contact portion  75  of the driven member  63  in this case is applied with a force by the coil spring  77  via the coupling pin  62  toward one direction (counterclockwise in  FIG. 10 ) around the center axis  60   a  of the control shaft  60  so that the drive cam contact portion  75  is pressed against the drive cam  24   a . In this case, the lever portion  63   b  of the driven member  63  is in contact with the roller  71 , and therefore, angular displacement of the insertion portion  63   a  in one direction around the center axis  60   a  is inhibited. 
     When the drive cam  24   a  rotates (rotates counterclockwise in  FIG. 10 ), and the cam nose moves down, the drive cam contact portion  75  of the driven member  63  is pressed down by the drive cam  24   a . At this time, since the lever portion  63   b  is in contact with the roller  71 , and therefore the angular displacement of the driven member  63  in one direction (counterclockwise direction in  FIG. 10 ) around the coupling pin  62  is inhibited, the driven member  63  causes the coupling pin  62  to be angularly displaced around the control shaft  60 . The driven member  63  and the pivot member  61  which are coupled to each other via the coupling pin  62  are integrally angularly displaced and pivoted around the control shaft  60 . In this construction, the lift amount of the tappet  58  is zero while the base circular arc surface  74   c  of the pivot cam  61  is sliding on the upper surface of the tappet  58 . When the pivot cam  61  further rotates and the lift curved surface  74   d  slides on the upper surface of the tappet  58 , the tappet  58  is pressed down according to the rotation of the pivot member  61 , and at the same time, the valve body  53  is displaced downward, increasing the lift amount. As a result, the intake port  20 A is opened. 
     As described above, there is a minute clearance between the base circular arc surface  74   c  of the pivot member  61  and the upper surface of the tappet  58 . Therefore, they do not slide and the base circular arc surface  74   c  moves with respect to the upper surface of the tappet  58  in the state where the pivot member  61  and the tappet  58  face each other with the clearance during a period in which the base circular arc surface  74   c  is opposite to the upper surface of the tappet  58 , to be precise. 
     The outlets  83   a  of the oil pipe  83  are oriented to face sliding portions of the driven member  63  and the drive cam  24   a  (without being disturbed by the drive cam contact portion  75  of the driven member  63 ) in at least a position of a movable range of the pivot cam mechanism  48  operable as described above. In this structure, during the operation of the valve operating system  50 A, oil  47  ejected through the outlets  83   a  of the oil pipe  83  is directly applied to the sliding surfaces of the driven member  63  and the drive cam  24   a . Thus, the oil  74  is sufficiently fed to the sliding surfaces and an oil film is formed stably on the sliding surfaces. As a result, durability of the valve operating system  50 A against wear-out and the like can be improved. 
       FIG. 11  is a view showing an operation of the valve operating system  50 A in a state where the pivot cam mechanism  48  is set in another mode. As shown in  FIG. 11 , when the control shaft  60  rotates counterclockwise in  FIG. 11 , the roller  71  moves according to the rotation. Thereby, the contact position of the lever portion  63   b  of the driven member  63  with respect to the roller  71  changes, changing the relative attitude of the driven member  63  with respect to the pivot member  61 . In the mode shown in  FIG. 11 , the pivot member  61  and the driven member  63  in the pivot mechanism  48  are open with a smaller angle than the pivot member  61  and driven member  63  in the mode shown in  FIG. 10 . 
     Thereby, the operation timing and lift amount of the valve body  53  which is pressed down by the pivot member  61  via the tappet  58  are changed. To be specific, as shown in  FIG. 11 , the lift amount is smaller and the open time of the intake port  20 A which is opened by the valve body  53  is shorter. Even when the relative attitude of the driven member  63  with respect to the pivot member  61  is changed as shown in  FIG. 11 , the outlets  83   a  of the oil pipe  83  are disposed to face sliding portions of the driven member  63  and the drive cam  24   a  in at least one position in the movable range of the pivot cam mechanism  48  (without being disturbed by the drive cam contact portion  75  of the driven member  63 ). Therefore, even in this mode, the oil film can be formed stably. 
     As should be understood from the structure and operation of the pivot mechanism  48  described above, the members of the pivot mechanism  48  of this embodiment which move during the rotation of the drive cam  24   a  are advantageously fewer. In addition, the coupling pin  62  has a hollow pipe shape and is lightweight. Thereby, an increase in an inertia moment during the operation can be suppressed. Furthermore, by changing the relative attitude of the driven member  63  with respect to the pivot member  61  according to the angular displacement of the control shaft  60 , the lift characteristics of the intake valve mechanism  51 A can be changed. 
     In the pivot cam mechanism  48  according to this embodiment, the control shaft  60  and the roller  71  are separate members. By suitably selecting the roller  71  from among the rollers having various shapes and dimensions and supporting it by the roller shaft  68 , various lift characteristics are easily attainable. 
     Whereas in this embodiment, the valve operating systems  50 A and  50 B have substantially the same structure as described above, for example, the outer shapes of the drive cams  24   a  (contours as viewed along the center axis direction of the drive camshaft  24 ) may be different between the air-intake system and the air-exhaust system. This can make the flow rates and timings for air-intake and air-exhaust different from each other while using the pivot members  61  and the driven members  63  which are identical in shape in the air-intake system and in the air-exhaust system. With regard to the relative attitude changing mechanism  64  including the lever portion  63   b , the roller  71 , and others, the members, which are identical in shape, may be used in the air-intake system and in the air-exhaust system, or otherwise, the outer shape of one or both of the pivot member  61  and the driven member  63  may be made different between the air-intake system and the air-exhaust system. 
     [Mechanical Structure of Pivot Cam Mechanism] 
       FIG. 12  is a schematic side view of the valve operating system  50 A including the above described pivot cam mechanism  48 , in which  FIG. 12(   a ) is a view showing the positional relationship between the control shaft  60 , the coupling pin  62 , and the drive camshaft  24 , and the relationship between the forces acting on the driven member  63 , and  FIG. 12(   b ) is a view showing the contact position of the driven member  63  and the drive cam  24   a . As described above, the outer peripheral surface of the tappet pressing portion  74  of the pivot member  61  supported by the control shaft  60  set in a predetermined phase is in contact with the tappet  58 . The lever portion  63   b  of the driven member  63  supported by the pivot member  61  via the coupling pin  62  is in contact with the roller  71  and the circular-arc sliding contact surface  75   a  of the drive cam contact portion  75  is in contact with the drive cam  24   a.    
     In the positional relationship between the control shaft  60 , the coupling pin  62 , and the drive camshaft  24  in  FIG. 12(   a ), the coupling pin  62 , which is an example of the second support shaft, is located closer to the drive camshaft  24  than the control shaft  60 , which is an example of the first support shaft. Since the coupling pin  62  supporting the driven member  63  is separate from the control shaft  60 , the size of the insertion portion  63   a  (see  FIG. 4)  can be reduced by reducing the diameter of the coupling pin  62 . This contributes to reduction of the size of the driven member  63 . By reducing the size of the driven member  63  and the coupling pin  62 , the weight of the portion distant from the control shaft  60  is reduced, enabling reduction in the inertia moment around the control shaft  60 . 
     In the relationship of the forces acting on the driven member  63  in  FIG. 12(   a ), the contact point between the circular-arc sliding contact surface  75   a  of the driven member  63  and the drive cam  24   a  form a force point P 1 , the center axis position of the coupling pin  62  by which the driven member  63  is rotatably supported form a force application point P 2 , and the contact point between the lever portion  63   b  of the driven member  63  and the roller  71  form a fulcrum point P 3 . In the valve operating system  50 A of this embodiment, the force application point P 2  is located between two straight lines L 1  and L 3  respectively passing through the force point P 1  and the fulcrum point P 3  so as to cross at a right angle a line segment connecting the force point P 1  to the fulcrum point P 3 . Thus, the force application point P 2  is set closer to the fulcrum point P 3  than the force application point P 1  to enable the driving power to be efficiently transmitted from the drive cam  24   a  to the pivot member  61 . 
     Since the force application point P 2  is set closer to the fulcrum point P 3  than the force application point P 1 , the driving power can be efficiently transmitted from the drive cam  24   a  to the pivot member  61 , while reducing the size of the driven member  63  as compared to the configuration in which the force application point P 2  is located outside the range between the straight lines L 1  and L 3 . In addition, by reducing the size of the driven member  63 , the inertia moment of the driven member  63  can be reduced, and a PV value can be reduced. 
     Subsequently, the contact position of the driven member  63  and the drive cam  24   a  will be described with reference to  FIG. 12(   b ). When the line segment connecting the center axis of the drive cam  24   a  to the center axis of the pivot member  61  is expressed as a first line segment L 4  and the line segment connecting the contact point (force point P 1 ) between the drive cam  24   a  and the driven member  63  to the center axis of the drive cam  24   a  is expressed as a second line segment L 5 , a set angle A 1  formed between the line segments L 4  and L 5  is set to an acute angle (i.e., 90 degrees&gt;A 1 &gt;0) which is more preferably in a range between 35 degrees and 45 degrees. In more detail, the set angle A 1  is set to an angle formed between the line segments L 4  and L 5  when the pivot member  61  rotates to a maximum degree (in other words, when the contact point between the driven member  63  and the drive cam  24   a  is closest to the center axis of the drive cam  24   a ) in the state where the control shaft  60  is set in a maximum rotation amount in an angular displacement direction (counterclockwise in  FIG. 12(   b )) for increasing the maximum lift amount of the valve body  53  when the drive cam  24   a  rotates and the angle formed between the pivot member  61  and the driven member  63  is set to a maximum value. This makes it possible to reduce the PV value at the contact portions of the drive cam  24   a  and the driven member  63 , i.e., a multiplication value (P×V) of the surface pressure (P) and the sliding speed (V) at the contact portions. As a result, wear resistance at the contact portions is improved. 
       FIG. 13  is a graph showing an example of the relationship between the set angle A 1  and the PV value. Herein, the PV value indicates a maximum value occurring when the set angle A 1  is set to a certain value. As shown in  FIG. 13 , the maximum value of the PV value has a minimum value when the set angle A 1  is near 40 degrees and increases as the set angle A 1  increases from near 40 degrees and decreases from near 40 degrees. The PV value is less than a predetermined value in a range between 35 degrees and 45 degrees. 
     Now, the relationship between the set angle A 1  and the PV value will be considered. As the set angle A 1  is reduced, the distance from the center axis of the control shaft  60  to the contact point P 1  can be reduced and the driven member  63  can be made short, so that the inertia moment can be reduced. There is a likelihood that a cam top radius (distance from the center axis of the drive cam  24   a  to the cam nose) of the drive cam  24   a  can be reduced by reducing the set angle A 1  in the structure according to this embodiment, thereby reducing the maximum value of the V value. Supposing that the lift characteristics of the valve body  53  during one rotation of the drive cam  24   a  are fixed, the contact portions of the drive cam and the driven member are closer to the center of the angular displacement of the pivot member due to reduction of the dimension of the driven member  63  according to the reduced set angle A 1 , and therefore the PV value tends to be large, because the moment acting on the driven member  63  needs to be invariable in principle. 
     As should be understood from the above, to reduce the PV value, it cannot be said that as small a set angle as possible is preferred, rather there is an optimal value of the set angle A 1 . In light of this, the inventors discovered that the set angle A 1  is preferably an acute angle, which is more preferably, in a range between 35 degrees and 45 degrees. Alternatively, a simulation program may be used to obtain the set angle A 1  with which the maximum PV value becomes the smallest, and the respective members and constituents may be designed so that the set angle A 1  becomes close to the obtained set angle A 1 . 
     The parameters which may affect the PV value include the shape of the circular-arc sliding contact surface  75   a  of the drive cam contact portion  75  of the driven member  63 , the shape of the outer peripheral surface of the drive cam  24   a , the dynamic friction coefficient of the contact portions, etc, as well as the above described set angle A 1 . Nonetheless, the degree (sensitivity) in a change of the PV value occurring when the set angle A 1  is changed is relatively large. Therefore, the PV value is easily reduced by controlling the set angle A 1  rather than controlling these parameters. Having described above the valve operating system  50 A associated with the intake port  20 A, the same advantages are achieved with the same configuration, in the valve operating system  50 B associated with the exhaust port  20 B. 
     Since the valve operating system  50 A or  50 B described above is mainly comprised of relatively few constituents, which are the control shaft  60 , the pivot member  61 , the driven member  63 , and the coil spring  77 , assembly precision can be improved and a manufacturing cost can be reduced. In addition, since the coupling pin  62  supporting the driven member  63  is attached to the pivot member  61 , the relative positions of the control shaft  60  supporting the pivot member  61  and the coupling pin  62  can be determined accurately. 
     Since the roller  71  constituting the relative attitude changing mechanism  64  is mounted in the vicinity of the center axis of the control shaft  60  rather than distant from the control shaft  60 , the inertia moment around the control shaft  60  is reduced in the pivot cam mechanism  48 . Furthermore, since the circular-arc sliding contact surface  75   a  of the drive cam contact portion  75  of the driven member  63  is a surface subjected to a hardening process and the oil  47  is directly fed to the sliding portions of the drive cam  24   a  and the circular arc sliding contact surface  75   a , the oil film is stably formed between the drive cam  24   a  and the driven member  63 . 
     [Acceleration Curve of Valve Body and Pivot Member] 
       FIG. 14  is a graph showing a change in an acceleration of valve body  53  in the valve operating system  50 A or  50 B as described above, in which  FIG. 14(   a ) shows a change in an acceleration according to comparative example, and  FIG. 14(   b ) shows a change in an acceleration in the valve operating system  50 A or  50 B according to this embodiment. In  FIGS. 14(   a ) and  14 ( b ), a horizontal axis indicates a displacement angle of the drive cam  24   a  and a valve acceleration period in which the valve body  53  has a positive acceleration while the drive cam  24   a  is rotating once, and a vertical axis indicates an acceleration of the valve body  53 .  FIG. 15  is a graph showing a change in an angular acceleration of the pivot member  61 , wherein  FIG. 15(   a ) shows a change in an angular acceleration according to a comparative example and  FIG. 15(   b ) shows a change in an angular acceleration in the valve operating system  50 A or  50 B according to this embodiment. In  FIGS. 15(   a ) and  15 ( b ), a horizontal axis indicates a displacement angle of the drive cam  24   a  and a pivot member acceleration period in which the pivot member  61  has a positive acceleration while the drive cam  24   a  is rotating once, and a vertical axis indicates an angular acceleration of the pivot member  61 .  FIG. 16  is a graph showing a change in the PV value at contact portions of the drive cam  24   a  and the driven member  63  with respect to the angular displacement of the drive cam  24   a  in the valve operating system  50 A or  50 B according to this embodiment, wherein a horizontal axis indicates the angular displacement of the drive cam, a vertical axis indicates the PV value, a thin line indicates those according to a comparative example, and a bold line indicates those of the valve operating system  50 A or  50 B of this embodiment. 
     When the drive cam  24   a  rotates, the contact portions of the drive cam  24   a  and the driven member  63  slide and the contact portions of the pivot member  61  and the valve body  53  (to be precise, tappet  58 ) slide. As shown in  FIG. 14(   a ), in the valve operating system according to the comparative example, in the valve acceleration period in which the acceleration of the valve has a positive value while the drive cam is rotating once, a valve maximum acceleration point X 2 A at which the acceleration of the valve body  53  is a maximum value Y 2 A is located in a rear part which is rearward relative to an intermediate point X 1 A. In the valve acceleration period, a change rate of the acceleration of the valve body  53  is larger in a rear part which is rearward relative to the valve maximum acceleration point X 2 A than in a front part which is forward relative to the valve maximum acceleration point X 2 A. As shown in  FIG. 15(   a ), in the pivot member acceleration period (substantially conforming to the valve acceleration period) in which the angular acceleration of the pivot member has a positive value while the drive cam is rotating once, a pivot member maximum acceleration point X 4 A at which the angular acceleration of the pivot member  61  is a maximum value Y 4 A is located in a rear part which is rearward relative to an intermediate point X 3 A. In the pivot member acceleration period, a change rate of the angular acceleration of the pivot member  61  is larger in a rear part which is rearward relative to the pivot member maximum acceleration point X 4 A than in a front part which is forward relative to the pivot member maximum acceleration point X 4 A. As shown by the comparative example (thin line) of  FIG. 16 , there is a tendency that the PV value at the contact portions is at a maximum in the rear part of each of the valve acceleration period and the pivot member acceleration period. As used herein, the phrase “the acceleration is positive” in the “valve acceleration period in which the acceleration of the valve has a positive value” and “pivot member acceleration period in which the acceleration of the pivot member has a positive value” means that the acceleration occurring when the surface pressure P generated between the drive cam  24   a  and the pivot member  63  increases. 
     In view of the circumstances, in the valve operating systems  50 A or  50 B according to this embodiment, the positions and shapes of the drive cam  24   a , the driven member  63 , the pivot member  61 , and the roller  71  are designed so that the acceleration of the valve body  53  in the rear part of the valve acceleration period is smaller, to be precise, the valve maximum acceleration point is located in the front part of the valve acceleration period, rather than the rear part of the valve acceleration period in which the PV value tends to be maximum. In addition, the positions and shapes of the drive cam  24   a , the driven member  63 , the pivot member  61 , and the roller  71  are designed so that the angular acceleration of the pivot member  61  in the rear part of the pivot member acceleration period is smaller, to be precise, the valve maximum acceleration point is located in the front part of the pivot member acceleration period, rather than the rear part of the pivot member acceleration period in which the PV value tends to be maximum. 
     Further, the positions and shapes of the drive cam  24   a , the driven member  63 , the pivot member  61 , and the roller  71  are designed so that the absolute value of an acceleration change rate of the valve body  53  per unit angular displacement of the drive cam  24   a  is larger in the front part which is forward relative to the valve maximum acceleration point of the valve acceleration period than in the rear part which is rearward relative to the valve maximum acceleration point of the valve acceleration period. Moreover, the positions and shapes of the drive cam  24   a , the driven member  63 , the pivot member  61 , and the roller  71  are designed so that the angular acceleration of the pivot member  61  is substantially zero at the position of the drive cam  24   a  where the PV value is at a maximum. 
     The details will be described with reference to  FIGS. 14 and 15 . As shown in  FIG. 14(   b ), in the valve operating system  50 A or  50 B according to this embodiment, a valve maximum acceleration point X 2 B at which the acceleration of the valve body  53  is a maximum value Y 2 B is located in the front part which is forward relative to the intermediate point X 1 B of the valve acceleration period. Also, in the valve acceleration period, the change rate of the acceleration of the valve body  53  is smaller in the rear part which is rearward relative to the valve maximum acceleration point X 2 B than in the front part. As shown in  FIG. 15(   b ), in the valve operating system  50 A or  50 B according to this embodiment, the pivot member maximum acceleration point X 4 B at which the acceleration of the pivot member  61  is a maximum value Y 4 B is located in the front part which is forward relative to the intermediate point X 3 B in the pivot member acceleration period. Also, the angular acceleration of the pivot member  61  is substantially zero at a position where the PV value is at a maximum in the rear part which is rearward relative to the pivot member maximum acceleration point X 4 B in the pivot member acceleration period. 
     Thereby, regarding the acceleration of the valve body  53  which occurs when the PV value is at a maximum, the acceleration Y 3 B of the valve operating system  50 A or  50 B according to this embodiment of  FIG. 14(   b ) is smaller than the acceleration Y 3 A of the comparative example of  FIG. 14(   a ). In addition, regarding the angular acceleration of the pivot member  61  which occurs when the PV value is at a maximum, the angular acceleration Y 5 B of the valve operating system  50 A or  50 B according to this embodiment of  FIG. 15(   b ) is smaller than the angular acceleration Y 5 A of the comparative example of  FIG. 15(   a ). 
       FIG. 17  shows the surface pressure P and the relative speed V of the contact portions of the drive cam  24   a  and the driven member  63  with respect to the angular displacement of the drive cam  24   a  in the valve operating system  50 A or  50 B according to this embodiment, wherein a horizontal axis indicates the angular displacement of the drive cam  24   a , and a vertical axis indicates the speed V and the surface pressure P of the contact portions. The surface pressure P is indicated by a bold line and the speed V is indicated by a thin line.  FIG. 18  is a bar graph showing a contact load at the contact portions of the drive cam  24   a  and the driven member  63  at a point in time when the PV value is at a maximum in the valve operating system  50 A or  50 B according to this embodiment. 
     As indicated by the bold line of  FIG. 17 , by setting the acceleration curves as shown by  FIGS. 14(   b ) and  15 ( b ), a peak of the surface pressure P at the contact portions of the drive cam  24   a  and the driven member  63  is located in the front part of the valve acceleration period, and as shown in  FIG. 18 , the contact load at the point where the PV value is at a maximum in the rear part of the valve acceleration period is reduced in the valve operating system  50 A or  50 B according to this embodiment as compared to the comparative example. As a result, even though the speed V increases toward the rear part of the valve acceleration period as shown by the thin line of  FIG. 17 , the maximum value of the PV value in the valve operating system  50 A or  50 B, which is located in the rear part of the valve acceleration period, is reduced as compared to that of the comparative example (broken line), as shown by a bold line in  FIG. 16 . 
     In the case of using the structure of the valve operating system  50 A including the drive cam  24   a , the driven member  63 , the pivot member  61 , the roller  71 , and others as described with reference to  FIGS. 4 to 6  and  FIG. 12 , a person skilled in the art can suitably design the positions and shapes of the drive cam  24   a , the driven member  63 , the pivot member  61 , and the roller  71  so that the valve maximum acceleration point is located in the front part of the valve acceleration period, the valve maximum acceleration point is located in the front part of the pivot member acceleration period, the acceleration of the valve body  53  is set smaller in the rear part which is rearward relative to the valve maximum acceleration point of the valve acceleration period than in the front part, and the acceleration of the pivot member  61  is set smaller in the rear part which is rearward relative to the pivot member maximum acceleration point in the pivot member acceleration period than in the front part. For example, in the state where the center axis of the drive cam  24   a  and the control shaft  50  are fixed in predetermined positions, the above described lift characteristics and pivot characteristics are attained by suitably designing the shapes of the drive cam  24   a , the driven member  63  and the pivot member  61 . Alternatively, using a simulation program which is commercially available or separately created, a condition of the members for attaining desired lift characteristics and pivot characteristics can be easily determined without manufacturing a trial model. Therefore, the design of the positions and shapes will not be described in detail. 
     In the manner described above, in the valve operating system  50 A or  50 B according to this embodiment, the PV value of the contact portions of the drive cam  24   a  and the driven member  63  and the PV value of the contact portions of the pivot member  61  and the valve body  53  (to be precise, tappet  58 ) are reduced. 
     [Another Structure of Pivot Cam Mechanism] 
     Having described the structure of the valve operating system  50 A, in which two sets of drive cams  24   a , driven members  63  and pivot members  61  are provided to correspond to the two intake ports  20 A, a different structure may be alternatively used. 
       FIG. 19  is a perspective view showing another structure of the valve operating system which is applicable to the engine E. As shown in  FIG. 19 , a valve operating system  90  includes one set of pivot cam mechanism  48  identical to that of Embodiment 1 shown in  FIG. 6  and another one set of pivot cam mechanism  90   a  which is different in structure from the pivot cam mechanism  48 . To be specific, the pivot cam mechanism  48  including the drive cam  24   a , the driven member  63 , the pivot member  61 , the roller  71  (not shown in  FIG. 19 ) and others is provided to correspond to one intake port  20 A (see  FIG. 3 ), whereas the pivot cam mechanism  90   a  consisting of the pivot member  61  without the drive cam  24   a , the driven member  63  and the roller  71 , is provided to correspond to the other intake port  20 A. 
     The valve operating system  90  having such a structure is capable of operating as in the above described valve operating system  50 A, and of achieving advantages as described above. In the valve operating system  90  shown in  FIG. 19 , the constituents having the same structures as those of the valve operating system  50 A are identified by the same reference numbers and will not be further described. 
     The coil spring may have a structure different from that of the above described coil spring  77 .  FIG. 20  is a side view showing another structure of the coil spring which is applicable to the valve operating system  50 A or  90 . As shown in  FIG. 20 , a coil spring  91  is formed by winding in close contact a round-rod member which is made of metal and has a predetermined elasticity as in the coil spring  77 , and one end  91   b  and an opposite end  91   c  of the round-rod member extends from a winding portion  91   a  forming a wound coil main body. In the coil spring  77 , the one end  77   b  and the opposite end  77   c  extend in opposite directions such that they are located on the tangential line contacting the outer peripheral surface of the winding portion  77   a , whereas the one end  91   b  and the opposite end  91   c  of the coil spring  91  of  FIG. 20  extend from two points  91   g  and  91   h  which are located at the opposite sides with respect to a center axis  91   f  of a coil main body  91   a  on the outer periphery of the winding portion  91   a . To be more specific, the one end  91   b  extends linearly in this embodiment from the point  91   g  along a tangential line  91   d  of the winding portion  91   a  at the point  91   g  as viewed from the direction along the center axis  91   f . Also, the opposite end  91   c  extends from the point  91   h  in substantially the same direction as the one end  91   b  along a tangential line  91   e  of the winding portion  91   a  at the point  91   h , and then in the opposite direction in which the winding portion  91   a  is wound. 
     The coil spring  91  having such a structure is capable of reducing the contact pressure generated by the contact of the winding portion  91   a  of the coil spring  91  with the control shaft  60  when the pivot cam mechanism  48  or  90  operates according to the rotation of the drive cam  24   a . That is, when the pivot cam mechanism  48  or  90  operates, the coil spring  90  generates a restoring force to restore the pivot cam mechanism  48  or  90   a , while at the same time, drags F 1  and F 2  against the restoring force are exerted on the one end  91   b  and the opposite end  91   c , respectively. As shown in  FIG. 21(   a ), in the coil spring  77  of  FIG. 6  whose one end  77   b  and opposite end  77   c  extend in the opposite direction with respect to the coil main body, the drags F 1  and F 2  are oriented in substantially the same direction with respect to the coil spring, causing the coil spring to contact the control shaft  60  with a resultant force of F 3 . On the other hand, in the coil spring  91  whose one end  91   b  and opposite end  91   c  extend in substantially the same direction with respect to the coil main body  91   a , the drag F 1  exerted on the one end  91   b  of the coil spring  91  and the drag F 2  exerted on the opposite end  91   c  of the coil spring  91  are oriented in substantially opposite directions and are cancelled, thereby reducing the force generated by contact of the coil spring  91  with the control shaft  60  to less than the resultant force F 3 , as shown in  FIG. 16(   b ). 
     Having described that the phase changing mechanism  64  includes the roller  71  and the lever portion  63   b  in the pivot cam mechanism  48  or  90   a , this is exemplary. For example, a follower  63  and the coupling pin  62  may be fixedly coupled and the coupling pin  62  may be configured to be rotatable. Alternatively, the phase of the driven member  63  around the coupling pin  62  may be changed via the gear. 
     The pivot cam mechanisms  48  and  90   a  may be provided in number to correspond to the intake valve mechanism  51 A and the exhaust valve mechanism  51 B provided in one cylinder. The present invention is applicable to an engine including one intake valve mechanism  51 A and one exhaust valve mechanism  51 B in one cylinder, or an engine including three or more intake valve mechanisms  51 A and three or more exhaust valve mechanisms  51 B in one cylinder. 
     The structure of the pivot member  61  and the structure of the driven member  63  are not limited to the above described structures.  FIGS. 22 to 24  are views showing a pivot cam mechanism including a pivot member and a driven member having another structure.  FIG. 22(   a ),  FIG. 23(   a ), and  FIG. 24(   a ) show the pivot cam mechanism set in one mode, and  FIG. 22(   b ),  FIG. 23(   b ), and  FIG. 24(   b ) show the pivot cam mechanism set in another mode. In  FIGS. 22 to 24 , the same components and members as those of the pivot cam mechanism  48  as described above are identified by the same reference numerals and will not be further described. 
     A pivot cam mechanism  100  shown in  FIG. 22  includes a pivot member  101  and a driven member  101  which are different in structure from the pivot member  61  and the driven member  63  of the pivot cam mechanism  48 . To be specific, the pivot member  101  is formed such that a phase difference B 10  around the control shaft  60  between a bearing portion  103  supporting the driven member  102  via the coupling pin  62  and a tappet pressing portion  104  is smaller than a phase difference around the control shaft  60  between the bearing portion  61   b  and the tappet pressing portion  74  of the pivot member  48 . The driven member  102  is formed such that a maximum width B 11  of the drive contact portion  105  is larger than a width of the drive cam contact portion  75  as viewed from the direction along the center axis  60   a . The meaning of the “phase difference B 10  around the control shaft  60  between the bearing portion  103  and the tappet pressing portion  104 ” is the same as the meaning of the acute angle formed between a line segment connecting a center axis  103   a  of the bearing portion  103  to the center axis  60   a  of the control shaft  60  and a line segment connecting a tip end  104   a  of the tappet pressing portion  104  to the center axis  60   a.    
     As shown in  FIG. 23 , a pivot cam mechanism  110  includes the pivot member  101  having the same structure as that shown in  FIG. 22 , and a driven member  111  having a structure different from those of the driven members  63  and  101  described above. A drive cam contact portion  112  of the driven member  111  has a predetermined thickness B 12  in the direction in which the drive cam contact portion  112  is pressed by the drive cam  24   a  and extends from the insertion portion  63   a  into which the coupling pin  62  is inserted. The drive cam contact portion  112  consists of a sliding contact wall portion  105   b  forming a circular-arc sliding contact surface  105   a  in the drive cam contact portion  105  of the driven member  102  of  FIG. 22 . That is, a support wall portion  105   c  (see  FIG. 17 ) connecting the sliding contact wall portion  105   b  to the insertion portion  63  is omitted. The driven member  111  having such a structure attains lightweight and can suppress an increase in an inertia moment during the rotation of the drive cam  24   a  as compared to the driven member  102  shown in  FIG. 22 . 
     As shown in  FIG. 24 , a pivot cam mechanism  120  includes a pivot member  121  and a driven member  122 . The pivot member  121  includes a tappet pressing portion  123  extending radially outward from an outer fitting tubular portion  61   a  and a bearing portion  124  extending from the tappet pressing portion  123  toward the drive cam  24   a  to support the coupling pin  62  at a tip end portion thereof. The driven member  122  has a circular-arc shape which is curved such that a longitudinal intermediate portion is closer to the drive cam  24   a . A base end portion  122   a  of the driven member  122  is pivoted to the coupling pin  62 , and a tip end portion  122   b  is in contact with the roller  71 . A circular-arc sliding contact surface  122   c  which is configured to slidably contact the drive cam  24   a  is formed in the outer peripheral surface of the driven member  122 , i.e., the outer surface of the circular arc. 
     The pivot cam mechanisms  100 ,  110 , and  120  shown in  FIGS. 22 to 24  are capable of reducing the inertia moment during the rotation of the drive cam  24   a  by reducing the components and members in number, as in the pivot cam mechanisms  48  and  90   a.    
     Hereinafter, the cylinder head cover  21  and the cylinder head  20  will be described in detail with reference to  FIGS. 25 and 26  as well as the other figures. The cylinder head cover  21  is a casing having a bottomed tubular shape with a rectangular cross section and being open in one direction. The cylinder head cover  21  is dividable in the rightward and leftward direction. In this embodiment, the cylinder head cover  21  is divided into a cam cover  21 A and a chain cover  21 B at a dividing plane B-B shown in  FIG. 25  (see  FIG. 27 ). The cam cover  21 A (cam mechanism cover portion) is disposed at the left side of the cylinder head  20  and is configured to cover the drive camshaft  24 , the pivot cam mechanism  48  and others. The chain cover  21 B (transmission mechanism cover portion) is disposed at the right side of the cylinder head  20  and is configured to cover the rotation transmission mechanism  28 . 
       FIG. 27  is an enlarged view showing a region surrounding the dividing plane B-B of the cylinder head cover  21 . The dividing plane B-B will be described in detail. As shown in  FIG. 9 , the dividing plane B-B is a plane passing through the chain tunnel  27 . In this embodiment, the dividing plane B-B is located at substantially the center in the vehicle width direction of the chain tunnel  27  and is substantially perpendicular to the rightward and leftward direction. By locating the dividing plane B-B in this position, the cylinder head cover  21  can move without contacting the components and members in the interior of the chain tunnel  27 , if the intake cam sprocket  31  and the exhaust cam sprocket  32  are formed to have a larger width, and the portion of the chain tunnel  27  is formed to have a larger width in the forward and leftward direction than the remaining portion. 
     The cam cover  21 A has a structure in which front and rear inner walls thereof extend substantially vertically and extend in the rightward and leftward direction. For this reason, when the cam cover  21 A is moved to the right or to the left with respect to the cylinder head  20 , the inner walls of the cam cover  21 A will not contact the built-in components such as the valve operating system. Since the inner walls of the cam cover  21 A extend in the rightward and leftward direction and extend vertically, the portion of the inner walls passes through the same region (see region  200  in  FIG. 28 ) when the cam cover  21 A is moved to the right or to the left with respect to the cylinder head  20 . This can lessen a region where the cam cover  21 A passes. 
     The cam cover  21 A of the cylinder head cover  21  having such a structure is fastened to the cylinder head  20  by bolts  99  at six positions which are at right and left sides and at front and rear sides at the center. The chain cover  21 B is fastened to the cylinder head  20   a  at a right end portion thereof by bolts  99   a . The cam cover  21 A and the chain cover  21 B are fastened such that their end portions which are opposite to each other with respect to the dividing plane B-B are fastened to each other by bolts  99   b  with a seal member interposed therebetween. 
     Since the covers  21 A and  21 B are respectively mounted to the cylinder head  20  in the manner described above, one of the covers  21 A and  21 B can be removed and the other can be kept fastened. There is no need to remove both of the covers  21 A and  21 B during maintenance. In a mounting operation, one of the covers  21 A and  21 B can be mounted based on the other which is kept fastened as a reference. Since there is no need to position the covers  21 A and  21 B together, the mounting operation is facilitated. 
     Having described the motorcycle  1  as an example in the above described embodiments, the present invention may be applied to valve operating systems used in engines mounted in other vehicles, such as four-wheeled vehicles, small watercraft, or off-road vehicles. In particular, the present invention is suitably applicable to straddle-type vehicles which tend to be smaller than seat-type vehicles. The structure of the valve operating system of the present invention is not limited to the above embodiments. For example, the valve operating system may be used in objects other than vehicles, and change, addition, or deletion of the structure of the valve operating system may be carried out without departing from a scope of the present invention. 
     As this invention may be embodied in several forms without departing from the spirit of essential characteristics thereof, the present embodiments are therefore illustrative and not restrictive, since the scope of the invention is defined by the appended claims rather than by the description preceding them, and all changes that fall within metes and bounds of the claims, or equivalence of such metes and bounds thereof are therefore intended to be embraced by the claims.