Patent Publication Number: US-8528389-B2

Title: Rotation angle sensing assembly including attaching structure, variable valve mechanism for internal combustion engine using the attaching structure, and vehicle incorporating the same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present invention claims priority under 35 USC 119 based on Japanese patent application No. 2009-295156, filed on Dec. 25, 2009. The entire subject matter of this priority document, including specification claims and drawings thereof, is incorporated by reference herein. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a rotation angle sensing assembly including sensor attaching structure, to a variable valve mechanism for an internal combustion engine having such attachment structure, and to a vehicle incorporating the same. More particularly, the present invention relates to an attachment structure in which an outer race of a bearing is secured with a reduction gear and an inner race of the bearing is secured to a supporting wall provided on a cylinder head, and a rotation angle sensor is secured integrally to the reduction gear, and to a vehicle incorporating the same. 
     2. Description of the Background Art 
     There is a known rotation angle sensor attaching structure provided on a variable valve mechanism for an internal combustion engine, a rotation angle detection object shaft is supported at two places at the opposite ends thereof by bearings, and a rotation angle sensor for detecting the rotation angle of the shaft is connected to an end portion of the rotation angle detection object shaft. An example of such rotation angle sensor attaching structure is disclosed in the Japanese Patent Laid-Open No. Hei 5-202719. 
     In such a known rotation angle sensor attaching structure, as described above, where a configuration wherein the rotation angle detection object shaft rotates a plural number of times uses a rotation angle sensor having a detection range of one rotation) (360°), a reduction gear, for reducing the speed of rotation of the rotation angle detection object shaft, is required. As a configuration for providing such reduction gear, it seems appropriate to support the opposite ends of the reduction gear in its axial direction by means of bearings or the like. In this instance, however, in order to support the reduction gear, it is necessary to assure a space in the axial direction of the rotation angle detection object shaft. 
     The present invention has been made in view of such circumstances as described above. Accordingly, it is one of the objects of the present invention to make it possible to miniaturize a rotation angle sensor attaching structure. 
     SUMMARY OF THE INVENTION 
     In order to achieve the above objects, the present invention provides a rotation angle sensor attaching structure for detecting rotation of a rotation angle detection object shaft through a reduction gear, is characterized in that an outer race of a bearing is force-fitted in and secured to a central portion of the reduction gear and an inner race of the bearing is secured to a supporting wall by a bolt, and an attaching supporting portion of the rotation angle sensor which extends across a head portion of the bolt is secured integrally to a side face of the reduction gear. 
     With this structure, since the outer race of the bearing is force fitted in the reduction gear and the inner race is secured to the supporting wall by the bolt and besides the attaching supporting portion of the rotation angle sensor is integrally secured to the side face of the reduction gear, only one bearing is required for attaching the reduction gear. 
     Consequently, since a large space is not required to attach the reduction gear, the rotation angle sensor attaching structure can be miniaturized in the axial direction of the rotation angle detection object shaft. Further, since only one bearing is required, the number of parts can be reduced. Furthermore, since the attaching supporting portion of the rotation angle sensor is secured integrally to the side face of the reduction gear, the rotation angle sensor attaching mechanism can be miniaturized in the axial direction. 
     In the configuration described above, the rotation angle sensor may be a potentiometer. Accordingly, the rotation angle of the rotation angle detection object shaft can be detected using the potentiometer, which is less expensive than the known methods. 
     Further, the present invention provides a variable valve mechanism for an internal combustion engine, characterized in that the rotation angle sensor attaching structure described above is used, and that a rotary shaft provided in parallel to an actuator provided on a side face of a cylinder head is driven by the actuator to vary the phase and/or the lift amount of the valve and the rotary shaft is used as the rotation angle detection object shaft while a driving gear of the reduction gear is provided at an end portion of the rotary shaft and the rotation angle sensor is provided on a side face of the cylinder head which extends perpendicularly to the rotary shaft. 
     With this structure, since the actuator is provided on the side face of the cylinder head in parallel to the rotary shaft and the rotation angle sensor is provided on the cylinder head side face perpendicular to the rotary shaft, the actuator and the rotation angle sensor do not make an obstacle to the disposition of other parts and can be disposed compactly. 
     Further, in the configuration described above, the internal combustion engine may be a V-type internal combustion engine having cylinders disposed in a V shape, and the actuator may be disposed on the counter offset side of the front and rear cylinders. Accordingly, since the actuator is disposed on the counter offset side of the cylinders, the actuator can be disposed compactly and the horizontal width of the internal combustion engine can be suppressed small. 
     Furthermore, the rotation angle sensor may be disposed on the inner side of the V banks Accordingly, since the rotation angle sensor is disposed on the inner side of the V banks, the rotation angle sensor can be disposed compactly such that it may not make an obstacle to the disposition of other parts. 
     Further, the internal combustion engine may be an internal combustion engine of a motorcycle wherein the V banks are disposed in the forward and rearward direction of a vehicle body. Accordingly, since the rotation angle sensor is disposed on the inner side of the V banks disposed in the forward and rearward direction of the motorcycle, the rotation angle sensor can be prevented from being hit by a flying stone or the like. 
     EFFECTS OF THE INVENTION 
     With the rotation angle sensor attaching structure according to the present invention, since the outer race of the bearing is force fitted in the reduction gear and the inner race is secured to the supporting wall by the bolt and besides the attaching supporting portion of the rotation angle sensor is integrally secured to the side face of the reduction gear, only one bearing is required for attaching the reduction gear. 
     Accordingly, since a large space is not required to attach the reduction gear, the rotation angle sensor attaching structure can be miniaturized in the axial direction of the rotation angle detection object shaft. Further, since only one bearing is required, the number of parts can be reduced. Furthermore, since the attaching supporting portion of the rotation angle sensor is secured integrally to the side face of the reduction gear, the rotation angle sensor attaching mechanism can be miniaturized in the axial direction. 
     Further, the rotation angle of the rotation angle detection object shaft can be detected using the potentiometer which is less expensive. 
     Meanwhile, with the variable valve mechanism for an internal combustion engine which uses the rotation angle sensor attaching structure according to the present invention, since the actuator is provided on the side face of the cylinder head in parallel to the rotary shaft and the rotation angle sensor is provided on the cylinder head side face perpendicular to the rotary shaft, the actuator and the rotation angle sensor do not make an obstacle to the disposition of other parts and can be disposed compactly. 
     Further, since the actuator is disposed on the counter offset side of the cylinders, the actuator can be disposed compactly and the horizontal width of the internal combustion engine can be suppressed small. 
     Furthermore, since the rotation angle sensor is disposed on the inner side of the V banks, the rotation angle sensor can be disposed compactly such that it may not make an obstacle to the disposition of other parts. 
     Further, since the rotation angle sensor is disposed on the inner side of the V banks disposed in the forward and rearward direction of the motorcycle, the rotation angle sensor can be prevented from being hit by a flying stone or the like. 
     For a more complete understanding of the present invention, the reader is referred to the following detailed description section, which should be read in conjunction with the accompanying drawings. Throughout the following detailed description and in the drawings, like numbers refer to like parts. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a right side elevational view of a motorcycle to which a movable valve mechanism for an internal combustion engine according to an embodiment of the present invention is applied. 
         FIG. 2  is a cut-away side view of an internal structure of an engine as viewed from the right side. 
         FIG. 3  is a detail sectional view showing an internal structure of a front bank of  FIG. 2  in an enlarged scale. 
         FIG. 4  is a side elevational view partly in section showing a valve mechanism. 
         FIG. 5  is a vertical sectional view of the valve mechanism of the front bank as viewed from the rear side. 
         FIG. 6  is a vertical sectional view of a driving mechanism as viewed from the side face side. 
         FIG. 7  is a horizontal sectional view of the engine as viewed from above. 
         FIG. 8  is a front elevational view of a sensor-supporting wall. 
         FIG. 9  is an enlarged view of peripheral members to a sensor in  FIG. 6 . 
         FIG. 10  is a sectional view of a collar. 
         FIG. 11A  is a plan view showing a sensor connecting member. 
         FIG. 11B  is a sectional view taken along line XI-XI of  FIG. 11A . 
     
    
    
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
     An embodiment of the present invention will now be described, with reference to the drawings. Throughout this description, relative terms like “upper”, “lower”, “above”, “below”, “front”, “back”, and the like are used in reference to a vantage point of an operator of the vehicle, seated on the driver&#39;s seat and facing forward. It should be understood that these terms are used for purposes of illustration, and are not intended to limit the invention. 
       FIG. 1  is a right side elevational view of a motorcycle  10  including a rotation angle sensor attaching structure according to an illustrative embodiment of the present invention. The motorcycle  10  includes a vehicle body frame  11 , a pair of left and right front forks  13  supported for turning motion on a head pipe  12  attached to a front end portion of the vehicle body frame  11 , a handle bar  15  for steering attached to a top bridge  14  which supports upper end portions of the front forks  13 , and a front wheel  16  supported for rotation on the front forks  13 . 
     The motorcycle  10  also includes an engine  17  as an internal combustion engine supported on the vehicle body frame  11 , exhaust mufflers  19 A and  19 B connected to the engine  17  through exhaust pipes  18 A and  18 B, a rear swing arm  21  supported for upward and downward rocking motion on a pivot  20  at a rear lower portion of the vehicle body frame  11 , and a rear wheel  22  supported for rotation at a rear end portion of the rear swing arm  21 . A rear shock absorber  23  is disposed between the rear swing arm  21  and the vehicle body frame  11 . 
     The vehicle body frame  11  includes a main frame  25  extending rearwardly downwards from the head pipe  12 , a pair of left and right pivot plates (also called center frames)  26  connected to a rear portion of the main frame  25 , and a down tube  27  extend downwardly from the head pipe  12  and then extending in a curved state until it is connected to the pivot plates  26 . A fuel tank  28  is supported in such a manner as to cross over the main frame  25 , and the main frame  25  extends rearwardly to a location above the rear wheel  22 , at which a rear fender  29  is supported. A seat  30  is supported between an upper portion of the rear fender  29  and the fuel tank  28 . As shown in  FIG. 1 , the motorcycle  10  also includes a radiator  31  supported on the down tube  27 , a front fender  32 , a side cover  33 , a headlamp  34 , a tail lamp  35 , and a rider&#39;s step  36 . 
     The engine  17  is supported in a space surrounded by the main frame  25 , pivot plates  26  and down tube  27 . The engine  17  is a V-type internal combustion engine. The engine  17  has is a front and rear V-type two-cylinder water-cooled four-cycle engine having cylinders banked on the front and the rear in a V-shape. The engine  17  is supported on the vehicle body frame  11  through a plurality of engine brackets  37  (only one is shown in  FIG. 1 ) such that a crankshaft  105  is directed in a horizontal vehicle width direction oriented transversely with respect to a longitudinal axis of the vehicle body. Power of the engine  17  is transmitted to the rear wheel  22  through a drive shaft (not shown) disposed on the left side of the rear wheel  22 . 
     The engine  17  is formed such that the included angle (also called bank angle) between a front bank  110 A (cylinder) and a rear bank  110 B (cylinder) which individually configure the cylinders is smaller than 90 degrees (for example, 52 degrees). Valve mechanisms of the banks  110 A and  110 B are configured as those of a 4-valve double overhead camshaft (DOHC) type. 
     An air cleaner housing  41  and a throttle body  42  which configure an engine intake system are disposed in a V bank space K ( FIG. 2 ) formed in the forward and rearward direction of the vehicle body between the front bank  110 A and the rear bank  110 B and having a V shape, as viewed in side elevation. The throttle body  42  supplies air cleaned by the air cleaner housing  41  to the front bank  110 A and the rear bank  110 B. Further, the exhaust pipes  18 A and  18 B which configure an engine exhaust system are connected to the banks  110 A and  110 B, and extend along the right side of the vehicle body such that the exhaust mufflers  19 A and  19 B are connected to rear ends of the exhaust pipes  18 A and  18 B, respectively. Consequently, exhaust gas is exhausted through the exhaust pipes  18 A and  18 B and the exhaust mufflers  19 A and  19 B. 
       FIG. 2  is a cut-away view of an internal structure of the engine  17  as viewed from the right side, and  FIG. 3  is a detail sectional view showing an internal structure of the front bank  110 A of  FIG. 2  in an enlarged scale. 
     Referring to  FIG. 2 , the front bank  110 A and the rear bank  110 B of the engine  17  each have a substantially similar structure. In  FIG. 2 , the front bank  110 A is shown in regard to peripheral members to a piston, and the rear bank  110 B is shown with regard to peripheral members to a cam chain. As shown in  FIG. 2 , the engine includes an intermediate shaft (rear side balancer)  121 , a main shaft  123 , and a countershaft  125 . These shafts  121 ,  123  and  125  and the crankshaft  105  are all disposed parallel to each other, while they are displaced in the vehicle body forward and rearward direction and upward and downward directions. In addition, a gear transmission mechanism for transmitting rotation of the crankshaft  105  to the intermediate shaft  121 , the main shaft  123  and the countershaft  125  in order is configured in a crankcase  110 C which supports the shafts  121 ,  123  and  125 . 
     As shown in  FIG. 2 , a front side cylinder block  131 A and a rear side cylinder block  131 B are disposed on an upper face of the crankcase  110 C of the engine  17  extending forwardly and rearwardly on the vehicle body, and oriented in such a manner as to define a predetermined included angle therebetween. A front side cylinder head  132 A and a rear side cylinder head  132 B are coupled to an upper face of the cylinder blocks  131 A and  131 B, respectively. Further, head covers  133 A and  133 B are mounted on an upper face of the cylinder heads  132 A and  132 B to configure the front bank  110 A and the rear bank  110 B, respectively, of the engine  17 . 
     A cylinder bore  135  is formed in each of the cylinder blocks  131 A and  131 B, and a piston  136  is inserted for sliding movement in each of the cylinder bores  135 . The pistons  136  are connected to the crankshaft  105  through connecting rods  137 . 
     A combustion recess  141  which configures the top face of a combustion chamber is disposed above the piston  136 , and is formed on a lower face of each of the cylinder heads  132 A and  132 B, and a spark plug  142  is disposed in each of the combustion recesses  141 , with an end thereof exposed to the combustion recess  141 . The spark plug  142  is provided substantially coaxially with a cylinder axial line C. 
     The engine  17  is a fuel-injection type engine, wherein fuel is injected directly into the combustion chamber from an associated injector  143  provided for each cylinder, with a respective tip end thereof extending into each of the combustion recesses  141 . The injectors  143  are inserted from V-bank inner side faces of the cylinder heads  132 A and  132 B, and disposed such that the tip ends thereof are exposed to the combustion recesses  141 . Each injector  143  is attached in a state wherein it is inclined with respect to the cylinder axial line C. 
     A fuel pump  144  is provided at an upper portion of the head cover  133 A. Fuel is supplied from the fuel pump  144  to the injectors  143  through a fuel pipe  144 A. 
     The cylinder heads  132 A and  132 B have intake ports  145  formed therein, which are communicated with the combustion recesses  141  by a pair of openings  145 A formed in the cylinder head, and exhaust ports  146  which are communicated with the combustion recesses  141  by a pair of openings  146 A formed in the cylinder head. The intake ports  145  are disposed between the cylinder axial line C and the injectors  143 . 
     As shown in  FIGS. 2 and 3 , each of the intake ports  145  includes a lower intake port  145 B provided integrally with the cylinder head  132 A or  132 B, and an upper intake port  145 C provided separately from the cylinder head  132 A or  132 B. The upper intake ports  145 C are attached to the lower intake ports  145 B at angles varied toward a direction in which they further approach the head covers  133 A and  133 B. 
     The intake ports  145  merge with each other at an intake chamber  43 , and the intake chamber  43  is connected to the throttle body  42 . For the throttle body  42 , a Throttle-By-Wire (TBW) system is adopted, which varies the sectional area of the throttle valve by electronic driving of an actuator. The exhaust port  146  of the cylinder head  132 A is operatively connected to an exhaust pipe  18 A, and the exhaust port  146  of the cylinder head  132 B is operatively connected to another exhaust pipe  18 B ( FIG. 1 ). 
     A pair of intake valves  147  (valves) are disposed in the cylinder heads  132 A and  132 B for operating, i.e., opening and closing the openings  145 A of the intake ports  145 . Similarly, a pair of exhaust valves  148  (valves) are disposed in the cylinder heads  132 A and  132 B for opening and closing the openings  146 A of the exhaust ports  146 . The intake valves  147  and the exhaust valves  148  are individually biased in a closing direction of the ports by respective valve springs  149 ,  149 . The valves  147  and  148  are driven by valve trains  50  (variable valve trains), which can vary valve operation characteristics such as the opening and closing timings, the lift amounts and so forth of the engine valves. The valve trains  50  include camshafts  151  and  152  on the intake side and the exhaust side which are supported for rotation on the cylinder heads  132 A and  132 B and rotate in an interlocking relationship with rotation of the crankshafts  105 . Here, the camshafts  151  and  152  rotate in counterclockwise direction ( FIGS. 2 and 4 ). 
     An intake cam  153  is formed integrally on each camshaft  151 . The intake cam  153  has a base portion  153 A which forms a circular cam face, and a cam lobe portion  153 B which forms a cam face which projects to the outer periphery side from the base portion  153 A. Further, an exhaust cam  154  is formed integrally on each camshaft  152 . The exhaust cam  154  has a base portion  154 A having a circular cam face, and a cam lobe portion  154 B projecting to the outer periphery side from the base portion  154 A and forming a mountain-shaped cam face. 
     As shown in  FIG. 2 , an intermediate shaft  158  is supported for rotation on one end side of each of the cylinder heads  132 A and  132 B in the widthwise direction, and intermediate sprockets  159  and  160  are secured to the intermediate shaft  158 . A driven sprocket  161  is secured to one end side of the camshaft  151 , and another driven sprocket  162  is connected to one end side of the camshaft  152 . Driving sprockets  163  are secured to the opposite end sides of the crankshaft  105 . A first cam chain  164  is wrapped between the sprockets  159  and  163 , and a second cam chain  165  is wrapped between the sprockets  160  to  162 . The sprockets  159  and  163  and the cam chains  164  and  165  are accommodated in a cam chain chamber  166  formed on one end side of each of the banks  110 A and  110 B. 
     The reduction gear ratio from the driving sprocket  163  to the driven sprockets  161  and  162  is set to 2, and if the crankshaft  105  rotates, then the driving sprocket  163  rotates integrally with the crankshaft  105  and the driven sprockets  161  and  162  rotate at a rotational speed equal to one half that of the crankshaft  105  through the cam chains  164  and  165 . Consequently, the valves  147  and  148  open and close the intake port  145  and the exhaust port  146  in accordance with the cam profile of the camshafts  151  and  152  which rotate integrally with the driven sprockets  161  and  162 . 
     A generator (not shown) is provided at a left end portion of the crankshaft  105 , and a driving gear wheel (hereinafter referred to as crank side driving gear wheel)  175  is secured to a right end portion of the crankshaft  105  on the inner side of the right side driving sprocket  163  (on the left side of the vehicle body). The crank side driving gear wheel  175  meshes with a driven gear wheel (hereinafter referred to as intermediate side driven gear wheel)  177  provided on the intermediate shaft  121  and transmits rotation of the crankshaft  105  at an equal speed to the intermediate shaft  121  to rotate the intermediate shaft  121  at the equal speed and in the opposite direction to that of the crankshaft  105 . 
     The intermediate shaft  121  is supported for rotation below the rear side of the crankshaft  105  and below the front side of the main shaft  123 . 
     At a right end portion of the intermediate shaft  121 , an oil pump driving sprocket  181 , the intermediate side driven gear wheel  177  and a driving gear wheel (hereinafter referred to as intermediate side driving gear wheel)  182  of a diameter smaller than that of the driven gear wheel  177  are attached in order. 
     The oil pump driving sprocket  181  transmits rotating force of the intermediate shaft  121  to a driven sprocket  186 , which is secured to a driving shaft  185  of an oil pump  184  disposed on the rear side of the intermediate shaft  121  below the main shaft  123 , through a transmission chain  187  to drive the oil pump  184 . 
     Further, the intermediate side driving gear wheel  182  meshes with a driven gear wheel (hereinafter referred to as main side driven gear wheel)  191  provided for relative rotation on the main shaft  123  to transmit rotation of the intermediate shaft  121  at a reduced speed to the main shaft  123  through a clutch mechanism (not shown). In particular, the reduction gear ratio from the crankshaft  105  to the main shaft  123 , that is, the primary reduction gear ratio of the engine  17 , is set by the reduction gear ratio between the intermediate side driving gear wheel  182  and the main side driven gear wheel  191 . 
     The main shaft  123  is supported for rotation upwardly of the rear side of the crankshaft  105 , and the countershaft  125  is supported for rotation substantially rearwardly of the main shaft  123 . A final drive gear group not shown is disposed over the main shaft  123  and the countershaft  125 , and a transmission is configured from the members mentioned. 
     The countershaft  125  is connected at a left end portion thereof to a drive shaft (not shown) extending in the forward and rearward direction of the vehicle body. Consequently, rotation of the countershaft  125  is transmitted to the drive shaft. 
       FIG. 4  is a side elevational view partly in section showing a representative one of the valve trains  50 , and  FIG. 5  is a vertical sectional view of the valve train  50  of the front bank  110 A, as viewed from the rear side thereof. 
     As shown in  FIG. 3 , the valve trains  50  are provided substantially symmetrically with respect to the cylinder axial line C, and independently of each other on the intake side and the exhaust side. Further, since the valve trains  50  of the front bank  110 A and the rear bank  110 B have a substantially same structure, in the description of the present embodiment, only the valve train  50  on the intake side of the front bank  110 A is described. 
     As shown in  FIGS. 4 and 5 , the valve train  50  includes the camshaft  151  (on the exhaust side, the camshaft  152 ), including the intake and exhaust cams  153 ,  154  which rotate integrally with the camshaft  151 , and rocker arms  51 ,  52  for respectively opening and closing the intake and exhaust valves  147 ,  148 . The valve train  50  also includes a valve cam  52  supported for relative rotation on the camshaft  151  for opening and closing the intake valve  147  through the rocker arm  51 , a holder member  53  rockable around the camshaft  151 , and a link mechanism  56  supported for rocking motion on the holder member  53  and provided to transmit valve driving force of the intake cam  153  to the valve cam  52  to rock the valve cam  52 . The valve train  50  further includes a driving mechanism  60  for rotating the holder member  53 . Further, the link mechanism  56  includes a sub rocker arm  54  connected to the holder member  53 , and a connect link  55  for connecting the sub rocker arm  54  and the valve cam  52  for rocking motion. 
     The rocker arm  51  is formed wide, and the intake valves  147  in pair are opened and closed by the single rocker arm  51 . The rocker arm  51  is supported at an end portion thereof for rocking motion on a rocker arm pivot  51 A secured to the cylinder head  132 A. An adjusting portion  51 B of the screw type for abutting with an upper end portion of the intake valve  147  is provided at the other end portion of the rocker arm  51 , and a roller  51 C, for contacting the valve cam  52 , is supported for rotation at a middle portion of the rocker arm  51 . 
     As shown in  FIG. 5 , the camshaft  151  has a sprocket-securing portion  151 A formed thereon and having the driven sprocket  161  ( FIG. 2 ) secured to one end side thereof. Further, a positioning flange  151 B projecting to the outer periphery of the camshaft  151  and having a circular sectional shape, the intake cam  153 , a valve cam supporting portion  151 C for rotating the valve cam  52  for rocking motion, and a collar fitting portion  151 D, formed with a smaller diameter than the valve cam supporting portion  151 C, are provided on the camshaft  151 , arranged in the order listed, from the sprocket securing portion  151 A side. A camshaft collar  155 , which functions as a bearing for the camshaft  151 , is journaled outside of the collar fitting portion  151 D, and is held in place, and pressed against the valve cam  52  side, by a fixing bolt  156  fastened to the other end of the camshaft  151 . 
     The camshaft  151  is supported at the opposite ends thereof for rotation by camshaft supporting portions  201  and  202 . Particularly, the camshaft supporting portions  201  and  202  are configured such that caps  201 B and  202 B each having a supporting portion of a substantially semicircular shape are secured to head side supporting portions  201 A and  202 A of a semicircular sectional shape formed at an upper portion of the cylinder head  132 A, respectively. On the camshaft supporting portion  201  provided on the positioning flange  151 B side, a groove  201 C is formed in conformity with the shape of the positioning flange  151 B such that the position of the positioning flange  151 B is controlled by the groove  201 C thereby to position the camshaft  151  in the axial direction. 
     Further, holder supporting portions  201 D and  202 D for supporting the holder member  53  are provided on the face of the camshaft supporting portions  201  and  202  on the intake cam  153  side, respectively. 
     The valve cam  52  is supported for rotation on the valve cam supporting portion  151 C provided at an intermediate portion of the camshaft  151 . On the valve cam  52 , a base portion  52 A and a cam lobe portion  52 B are formed, as shown in  FIG. 4 . The base portion  52 A of the valve cam  52  is provided for selectively keeping the intake valve  147  in a closed state, and the cam lobe portion  52 B is provided for selectively pushing down the intake valve  147  to open the valve. A through-hole  52 C is formed in the cam lobe portion  52 B, as shown. 
     A valve cam return spring  57  ( FIG. 5 ), is disposed surrounding a portion of the camshaft collar  55 , as shown, and this spring is provided for biasing the valve cam  52  in a direction in which the cam lobe portion  52 B is spaced away from the roller  51 C of the rocker arm  51 , that is, in a direction in which the intake valve  147  is closed. One end  57 A of the valve cam return spring  57  is attached to the through-hole  52 C formed in the cam lobe portion  52 B of the valve cam  52 . 
     As seen in  FIG. 5 , the valve cam return spring  57  is a torsion coil spring, and is wrapped at a coiled portion  57 B thereof around the camshaft  151 . The valve cam return spring  57  is attached at the other end  57 C thereof to the grooved portion  69  formed at an end portion of the holder member  53 . The coiled portion  57 B is formed long in the axial direction beyond the grooved portion  69  and is wrapped at the other end  57 C thereof around the one end  57 A side in such a manner as to overlap with the coiled portion  57 B. Therefore, the valve cam return spring  57  can be disposed compactly in the axial direction while the number of turns thereof is assured. 
     The holder member  53  includes first and second plates  53 A and  53 B disposed in a predetermined spaced relationship from each other in the axial direction of the camshaft  151  sandwiching the intake cam  153  and the valve cam  52  therebetween, and a sub rocker arm holder  59  for connecting the first and second plates  53 A and  53 B to each other in the axial direction of the camshaft  151 . The first plate  53 A is disposed on one end side of the camshaft  151  to which the driven sprocket  161  is secured, and the second plate  53 B is disposed on the other end side of the camshaft  151 . 
     Further, the sub rocker arm holder  59  is configured to include shaft portions  59 A and  59 C extending in parallel to the camshaft  151  and a coupling portion  45  for coupling the shaft portion  59 A and the shaft portion  59 C integrally to each other. Further, a cylindrical accommodating portion  74  is formed at the coupling portion  45 , and a sub rocker arm return spring  58  (hereinafter referred to as return spring) for biasing the sub rocker arm  54  to the intake cam  153  side is accommodated in the cylindrical accommodating portion  74 . 
     At an end of the shaft portion  59 A on the first plate  53 A side, a sub rocker arm supporting portion  59 B (fulcrum) to which one end of the sub rocker arm  54  is connected is formed. The sub rocker arm supporting portion  59 B is a shaft formed with a smaller diameter than that of the shaft portion  59 A. 
     The first and second plates  53 A and  53 B and the sub rocker arm holder  59  are secured by a pair of bolts  53 D for fastening the first plate  53 A and the sub rocker arm holder  59  from the outer face side of the first plate  53 A and another pair of bolts  53 E for fastening the second plate  53 B and the sub rocker arm holder  59  from the outer face side of the second plate  53 B. Female thread portions  79  into which the bolts  53 D and  53 E are screwed are formed individually on the shaft portions  59 A and  59 C. 
     Further, a bolt hole  53 C is formed in the second plate  53 B. The bolt hole  53 C is operatively connected to the driving mechanism  60 . 
     The first and second plates  53 A and  53 B have respective shaft holes  157 A and  158 A formed therein through which the camshaft  151  extends. The circumferential edge portions of the shaft holes  157 A and  158 A form ring-shaped annular projections  157 B and  158 B which project toward the holder supporting portions  201 D and  202 D of the camshaft supporting portions  201  and  202 , respectively, as shown in  FIG. 5 . The holder member  53  is supported by the annular projections  157 B and  158 B fitted in the holder supporting portions  201 D and  202 D, respectively, and can be pivoted around the camshaft  151 . Further, the annular projections  157 B and  158 B are assembled coaxially to the camshaft  151 . 
     Further, a gap S is formed in the axial direction between an end of the cap  201 B and the bolt  53 D, and also between the cap  202 B and the bolt  53 E. The gaps S are set to a size with which, when the caps  201 B and  202 B are assembled from above to the head side supporting portions  201 A and  202 A, the caps  201 B and  202 B do not contact with the bolts  53 D and  53 E, respectively. Therefore, upon assembly operation, the bolts  53 D and  53 E do not make an obstacle and the assembly feasibility is good. 
     The sub rocker arm  54  is disposed together with the intake cam  153  and the valve cam  52  between the first and second plates  53 A and  53 B. The sub rocker arm  54  is supported at one end portion thereof on the sub rocker arm supporting portion  59 B of the sub rocker arm holder  59  such that it is rocked around the sub rocker arm supporting portion  59 B. A roller  54 A for contacting with the intake cam  153  to press the base portion  153 A and the cam lobe portion  153 B is supported for rotation at a central portion of the sub rocker arm  54 . The connect link  55  is connected at one end thereof to the other end portion of the sub rocker arm  54  through a pin  55 A which supports the connect link  55  for rocking motion, and the valve cam  52  is connected to the other end of the connect link  55  through a pin  55 B which supports the valve cam  52  for rocking motion. 
     Further, the sub rocker arm  54  is biased by the return spring  58 , and the roller  54 A of the sub rocker arm  54  is normally kept pressed against the intake cam  153 . 
     The sub rocker arm  54  has a holder connecting portion  54 B connected to the sub rocker arm supporting portion  59 B and extends perpendicularly to the camshaft  151 , an eccentric portion  54 C extending from the holder connecting portion  54 B and curved downwardly so as to extend along the outer diameter of the camshaft  151 , and a link portion  54 D connected to the valve cam  52  through the connect link  55 . 
     The eccentric portion  54 C is formed eccentrically in the axial direction of the camshaft  151  so as to bypass the intake cam  153  from the first plate  53 A side to the second plate  53 B, and a stepped portion  76  in the form of a plate is formed on a side face of the eccentric portion  54 C such that it is swollen in the axial direction of the camshaft  151 . The stepped portion  76  is provided so as to be curved along a lower edge portion of the sub rocker arm  54 . The return spring  58  is received at a lower end thereof by the stepped portion  76  through a spring washer  77  ( FIG. 4 ). The return spring  58  is received at an upper end thereof by a circlip  78  which engages with the accommodating portion  74 . 
     The link portion  54 D is provided continuously to an end of the eccentric portion  54 C and is connected to the valve cam  52  through the connect link  55 . Since the eccentric portion  54 C is eccentric in this manner, the sub rocker arm  54  connects the intake cam  153  and the valve cam  52 , which are provided at different positions in the axial direction on the camshaft  151 , to each other. 
     Now, operation of the valve train  50  is described. 
     Referring to  FIG. 4 , if the camshaft  151  in the valve train  50  configured in such a manner as described above is rotated in the counterclockwise direction ( FIG. 4 ), then the sub rocker arm  54  is pushed up by the cam lobe portion  153 B of the intake cam  153 , which rotates integrally with the camshaft  151 , through the roller  54 A and rocked around the shaft portion  59 A. Together with this, the valve cam  52  is rotated in the clockwise direction in  FIG. 4  around the camshaft  151  through the connect link  55 . Then, by the rotation of the valve cam  52 , the cam lobe portion  52 B pushes the rocker arm  51  through the roller  51 C to push down the intake valve  147  through the rocker arm  51  to open the intake valve  147 . 
     Further, in a state wherein the base portion  153 A of the intake cam  153  contacts with the roller  54 A by further rotation of the camshaft  151 , the sub rocker arm  54  is pushed down by the return spring  58  and the valve cam  52  is rotated in the counterclockwise direction in  FIG. 4  by the valve cam return spring  57  until the base portion  52 A is brought into contact with the roller  51 C. Consequently, the intake valve  147  is pushed up into a closed state by the valve spring  149  ( FIG. 2 ). 
     As shown in  FIG. 4 , a driving mechanism-connecting member  63  is connected to the holder member  53  in the valve train  50 . Further, the driving mechanism-connecting member  63  is connected to the driving mechanism  60  ( FIG. 6 ), and the holder member  53  is rocked in the direction indicated by an arrow mark A and the direction indicated by another arrow mark B by driving of the driving mechanism  60 . 
     If the holder member  53  is rocked in the direction indicated by the arrow mark A, then the position of the sub rocker arm supporting portion  59 B varies together with the holder member  53 , and the link mechanism  56  is rocked in the clockwise direction around the axis of the camshaft  151  while the roller  54 A is rocked in the clockwise direction and the valve cam  52  is rocked in the clockwise direction. 
     On the other hand, if the holder member  53  is rocked in the direction indicated by the arrow mark B, then the link mechanism  56  is rocked in the counterclockwise direction around the axis of the camshaft  151  together with the holder member  53 , and the roller  54 A is pivoted in the counterclockwise direction while the valve cam  52  is rocked in the counterclockwise direction. 
     In this manner, the valve train  50  is configured such that valve operation characteristics of the intake valve  147  and the exhaust valve  148 , that is, the opening and closing timings, the opening and closing periods and the lift amount of the intake valve  147  and the exhaust valve  148 , can be controlled by varying the position of the roller  54 A and the initial position of the rocking movement of the valve cam  52 . 
     Here, the initial position of the rocking motion of the valve cam  52  indicates a rocking position of the valve cam  52  in a state wherein the roller  54 A contacts with the base portion  153 A of the intake cam  153  and the sub rocker arm  54  is not pushed up by the cam lobe portion  153 B. On the other hand, the opening and closing timings of the intake valve  147  and the exhaust valve  148  indicate the timings of opening and closing movements of the intake valve  147  and the exhaust valve  148  with respect to rotation of the camshafts  151  and  152 , that is, the phases of the opening and closing movements of the intake valve  147  and the exhaust valve  148 . 
     For example, if the holder member  53  on the intake side is rocked further in the direction of the arrow mark A (in the clockwise direction in  FIG. 4 ), then the roller  54 A and the valve cam  52  are rotated in the clockwise direction, and the cam lobe portion  52 B comes close to the roller  51 C. If the camshaft  151  is rotated in this state, then the starting timing of the pushing up of the roller  54 A by the cam lobe portion  153 B becomes earlier and the period within which the cam lobe portion  52 B pushes down the roller  51 C and the pushing down amount increase. Consequently, the opening timing of the intake valve  147  is hastened and the opening period and the lift amount of the intake valve  147  increase. 
       FIG. 6  is a vertical sectional view of the driving mechanism  60  as viewed from the side.  FIG. 7  is a horizontal sectional view of the engine  17  as viewed from above. It may be noted that, in  FIG. 7 , the banks  110 A and  110 B are shown in a view as viewed along the cylinder axial line C (FIG.  2 ) from above the engine  17 . Further, as shown in  FIG. 7 , the rear bank  110 B is disposed at a rear portion of the crankcase  110 C with the front bank  110 A rotated by 180° around the cylinder bore  135 , and here, detailed description of the rear bank  110 B is omitted. 
     As shown in  FIG. 6 , the driving mechanism  60  is connected to the holder member  53  through the driving mechanism-connecting member  63 . The driving mechanism  60  has a bar-shaped ball screw  61  (rotational angle detection object shaft, rotary shaft) disposed over the camshaft  151  and the camshaft  152 , two sliders  62  provided on the intake side and the exhaust of the ball screw  61  and threadably movable in the axial direction, an electric actuator  70  ( FIG. 7 ) for rotating the ball screw  61 , and the driving mechanism-connecting member  63 . The driving mechanism-connecting member  63  is provided between each of the sliders  62  and each of the holder members  53 . 
     A gear  64  is securely mounted at a front end portion of the ball screw  61  on the camshaft  152  side, and the electric actuator  70  is connected to the gear  64  through a gear train provided so as to extend inwardly and outwardly across an upper portion side wall of the cylinder head  132 A. The electric actuator  70  is controlled by an electronic control unit (ECU) of the vehicle. As the ECU drives the electric actuator  70 , the holder member  53  is rocked through the ball screw  61  and the driving mechanism-connecting member  63 , and the operation characteristics of the intake valve  147  and the exhaust valve  148  are controlled in response to an operation state of the engine  17 . 
     The electric actuator  70  includes an electric motor  71 , a driving shaft  72  of the electric motor  71 , and an intermediate shaft  73  to which driving force of the electric motor  71  is transmitted from the driving shaft  72 . The electric motor  71  is secured to an outer side face  139  in the vehicle widthwise direction of an upper portion of the cylinder head  132 A such that the driving shaft  72  thereof extends substantially in parallel to the ball screw  61 . The electric actuator  70  is disposed at a front portion of the outer side face  139 , and the air cleaner housing  41  extends in the vehicle forward and rearward direction in such a manner as to continue to a rear portion of the electric actuator  70 . 
     A driving gear wheel portion  72 A is formed on the driving shaft  72 , and a first intermediate gear  73 A which meshes with the driving gear wheel portion  72 A and a second intermediate gear  73 B which meshes with the gear  64  provided on the ball screw  61  are secured to the intermediate shaft  73 . 
     As shown in  FIG. 7 , the engine  17  is configured such that the front bank  110 A and the rear bank  110 B are offset to the cam chain chambers  166  side with respect to the cylinder bores  135 . In particular, the front bank  110 A and the rear bank  110 B are configured such that, where the cylinder bores  135  are regarded as the center, side walls thereof on the cam chain chambers  166  side are swollen in the vehicle widthwise direction from the outer side faces  139  of the cam chain chambers  166  on which the electric actuators  70  are disposed. 
     According to the present illustrative embodiment, since the electric actuators  70  are disposed on the counter offset sides Q of the front bank  110 A and the rear bank  110 B, the size in the vehicle widthwise direction of the front bank  110 A and the rear bank  110 B including the electric actuators  70  can be prevented from increasing one-sidedly to one side in the vehicle widthwise direction, and the electric actuators  70  can be disposed compactly. 
     Further, since the electric actuators  70  are provided on the outer side of the cylinder heads  132 A and  132 B and a space can be assured in the cylinder heads  132 A and  132 B, the ball screw  61 , slider  62  and so forth can be disposed efficiently in the cylinder heads  132 A and  132 B. 
     Each ball screw  61  extends perpendicularly to the camshafts  151  and  152  and is disposed on the other end side of the camshafts  151  and  152 , that is, on the side opposite to the side on which the driven sprockets  161  and  162  are secured. Since the ball screw  61  does not extend in the upward and downward direction of the engine  17  but is disposed so as to lie over the camshaft  151  and the camshaft  152 , the height of the engine  17  can be suppressed low. 
     The ball screw  61  is supported at the opposite ends thereof for rotation by ball screw supporting portions  203 . As shown in  FIG. 5 , the ball screw supporting portions  203  are each configured by securing a cap  203 B having a supporting portion of a semi-circular sectional shape to a camshaft side supporting portion  203 A formed at an upper portion of the camshaft supporting portion  202 . 
     As shown in  FIG. 6 , helical screw threads  61 A and  61 B and helical shaft thread grooves  61 C and  61 D are formed on the intake side and the exhaust side on an outer circumferential face of the ball screw  61 . The screw threads  61 A and  61 B and the shaft thread grooves  61 C and  61 D are set such that the screw winding directions on the intake side and the exhaust side are opposite to each other. 
     The slider  62  is formed as a block and has a through-hole  62 A through which the ball screw  61  extends. A helical nut screw thread  62 B corresponding to the screw thread  61 A or  61 B and a helical nut thread groove  62 C corresponding to the shaft thread groove  61 C or  61 D are formed on an inner circumferential face of the through-hole  62 A. A plurality of balls  65  are disposed for rolling movement between the nut thread groove  62 C and the shaft thread groove  61 C or  61 D. When the ball screw  61  is rotated, the slider  62  moves in the axial direction on the ball screw  61  through the balls  65 . 
     The driving mechanism-connecting member  63  has an arm member  86  connected to the slider  62 , and a connecting member  87  for connecting the arm member  86  to the second plate  53 B of the holder member  53 . The connecting member  87  is configured from a bolt and a nut for connecting the second plate  53 B and the arm member  86  to each other. The arm member  86  is formed in a substantially L shape as viewed in side elevation, and is secured at one end  86 A thereof to the second plate  53 B through the connecting member  87  and at the other end  86 B for rocking motion to the slider  62 . More particularly, the other end  86 B of the arm member  86  extends to the opposite side faces of the slider  62  and is connected in such a manner as to sandwich the slider  62  from the opposite side faces. 
     If the arm member  86  is moved in the axial direction of the ball screw  61  integrally with the slider  62 , then it is rocked around the other end  86 B thereof while it pulls the holder member  53  to rock the holder member  53  connected to the one end  86 A thereof. 
     Meanwhile, the slider  62  and the arm member  86  have same parts which are disposed symmetrically to each other with respect to a middle portion in the axial direction of the ball screw  61  on the camshaft  151  and the camshaft  152 . If the ball screw  61  is rotated, then the sliders  62  move in the opposite directions to each other to rock the holder members  53  on the intake side and the exhaust side. 
     As shown in  FIG. 6 , a rotation angle sensor  80  (hereinafter referred to as a sensor  80 ) is provided on a wall portion of the cylinder head  132 A in the proximity of an end of the ball screw  61  on the camshaft  151  side, for detecting the angle of rotation which is the amount of rotation of the ball screw  61 . The ball screw  61  is a rotation angle detection object shaft whose angle of rotation is detected by the sensor  80 . The ECU described hereinabove calculates the rocking amount of the holder members  53 , based on the angle of rotation of the ball screw  61  detected by the sensor  80 , and utilizes the calculated value for control of the valve operation characteristics. 
     The sensor  80  is supported by a sensor-supporting wall  88  provided on the cylinder head  132 A. 
       FIG. 8  is a front elevational view of the sensor-supporting wall  88 . 
     As shown in  FIGS. 6 ,  7  and  8 , the sensor-supporting wall  88  is a plate formed so as to have a substantially L shape as viewed in side elevation, and has a wall portion  89  extending in the upward and downward direction of the cylinder head  132 A, and a base portion  90  provided at a lower end of the wall portion  89  and secured to an upper face  132 A 1  of the side wall of the cylinder head  132 A. 
     A thick portion  89 A is provided at an upper portion of the wall portion  89  such that it projects to the V bank space K side, and a sensor supporting hole  89 B for supporting the sensor  80  is formed in the thick portion  89 A. The sensor supporting hole  89 B is a circular opening penetrating through the thick portion  89 A. Meanwhile, a female thread portion  89 C is formed in the thick portion  89 A below the sensor supporting hole  89 B. An outer edge portion  89 D of the wall portion  89  is configured from left and right side edge portions formed so as to taper upwardly and an upper edge portion in the form of a curved face continuing to upper portions of the opposite side edge portions. The outer edge portion  89 D serves as a catch pan for a gasket  67  for sealing between the head cover  133 A and an upper face  132 A 1  of the cylinder head  132 A. 
     A pair of positioning pins  90 A for being fitted with hole portions (not shown) formed on the upper face  132 A 1  side are provided on the base portion  90 , and the sensor-supporting wall  88  is secured integrally to an upper portion of the cylinder head  132 A by a pair of bolts  66  extending through the positioning pins  90 A and fastened to the upper face  132 A 1 . 
     The sensor-supporting wall  88  is provided continuously to inner side walls  140  (cylinder head side walls) of the cylinder heads  132 A and  132 B which define the V bank space K, and forms a part of the side walls of upper portions of the cylinder heads  132 A and  132 B. The inner side wall  140  is a side wall of the cylinder head  132 A extending perpendicularly to the ball screw  61 . Further, the sensor-supporting wall  88  is disposed at an end of the inner side wall  140  on the electric actuator  70  side and is positioned on an extension line in the axial direction of the ball screw  61 . 
     As shown in  FIG. 6 , the sensor  80  has a cylindrical body portion  80 A, an inputting portion  80 B provided on one end side of the body portion  80 A for receiving rotation of an inspection object inputted thereto, and a stay portion  80 C projecting from the body portion  80 A on the other end side. The inputting portion  80 B is connected to the detection object and rotated integrally with the detection object, and the angle of rotation of the detection object is obtained by detecting the angle of rotation thereupon. 
     The inputting portion  80 B of the sensor  80  is provided on the axis of the body portion  80 A and rotates around the axis. Here, the sensor  80  is a potentiometer. More particularly, the sensor  80  is a potentiometer having a detectable angular range not of a plurality of rotations but of an angle within one rotation) (360°), and can be acquired at a low cost because it is comparatively simple in structure. 
     The sensor  80  is supported in the sensor supporting hole  89 B of the sensor-supporting wall  88  and disposed in a state wherein the inputting portion  80 B is directed to the inside of the cylinder head  132 A. An O-ring  80 D made of rubber is interposed between the body portion  80 A and the sensor supporting hole  89 B. Further, the sensor  80  is secured to the sensor-supporting wall  88  by a sensor fixing bolt  80 E which extends through the stay portion  80 C and is fastened to the female thread portion  89 C. 
     The sensor  80  is supported on the sensor-supporting wall  88  and disposed with a rear portion thereof exposed to the inside of the V bank space K. As shown in  FIG. 7 , a connecting portion  42 A between the intake chamber  43  and the throttle body  42  is provided in the V bank space K, and the sensor  80  is disposed in a space between the connecting portion  42 A and the inner side wall  140  of the front bank  110 A. Further, the sensor  80  of the rear bank  110 B is disposed in a space between a fuel pipe  180 , which connects the fuel pipe  144 A and the injectors  143 , and the inner side wall  140  of the rear bank  110 B. 
     Since the sensors  80  are disposed between the inner side wall  140  and the connecting portion  42 A and between the inner side wall  140  and the fuel pipe  180  in the V bank space K in this manner, the sensors  80  can be disposed compactly such that they may not make an obstacle to disposition of other parts such as the intake chamber  43 , throttle body  42  and fuel pipe  180 . Further, since the sensors  80  are covered from forwardly and rearwardly with the engine  17 , they can be prevented from being hit by a flying stone and so forth. 
       FIG. 9  is an enlarged view of peripheral elements of the sensor  80  shown in  FIG. 6 . 
     A rotation transmitting portion  91  is provided between the ball screw  61  and the sensor  80 , and rotation of the ball screw  61  is transmitted to the sensor  80  by the rotation transmitting portion  91 . 
     The rotation transmitting portion  91  has an output power shaft  92  formed at one end of the ball screw  61  on the opposite side to the gear  64 , a gear supporting shaft  93  disposed in parallel to the output power shaft  92  below the output power shaft  92 , a reduction gear  94  supported on the gear supporting shaft  93  and held in meshing engagement with the output power shaft  92 , and a sensor attaching supporting portion  95  (also referred to as a sensor-connecting element) secured integrally to the reduction gear  94  and attached to the sensor  80 . 
     The output power shaft  92  has a driving gear  92 A which meshes with the reduction gear  94 , and this driving gear  92 A is formed by shaping an outer circumferential face of an end portion of the ball screw  61  so as to have a shape of a gear. The number of teeth of the reduction gear  94  is greater than that of the driving gear  92 A so that the speed of rotation of the ball screw  61  is reduced by the reduction gear  94 . 
     More particularly, the reduction gear ratio between the driving gear  92 A and the reduction gear  94  is set corresponding to the maximum angle of rotation of the ball screw  61 , that is, to the range of rocking motion of the holder member  53  and is set such that, also where the ball screw  61  rotates by a plural number of times and the holder member  53  is rocked fully over the range of rocking motion thereof, the angle of rotation of the reduction gear  94  is smaller than one rotation. 
     The gear supporting shaft  93  has a supporting bolt  96  screwed in the ball screw supporting portion  203 , and a collar  97  fitted with the shaft portion  96 A of the supporting bolt  96  and secured to the ball screw supporting portion  203 . The supporting bolt  96  is a hexagon bolt having a shaft portion  96 A and a hexagonal head portion  96 B. The supporting bolt  96  is secured to a supporting wall  210  which is a wall portion of the ball screw supporting portion  203  on the side opposing to the sensor-supporting wall  88  and is provided in parallel to the ball screw  61  immediately below the ball screw  61 . 
     A female thread portion  210 A into which the supporting bolt  96  is screwed and a positioning hole  210 B formed with a greater diameter than that of the female thread portion  210 A and with high accuracy on the front face side of the supporting wall  210  are formed on the supporting wall  210 . The supporting wall  210  is positioned on the inner side of the cylinder head  132 A with respect to an end of the output power shaft  92  and is provided substantially in parallel to the sensor-supporting wall  88 , and a space M in which the rotation transmitting portion  91  can be disposed is formed between the supporting wall  210  and the sensor-supporting wall  88 . 
       FIG. 10  is a sectional view of the collar  97 . 
     The collar  97  has a cylindrical collar shaft portion  97 A, a positioning portion  97 B formed with a smaller diameter than that of the collar shaft portion  97 A and with high accuracy on one end side of the collar shaft portion  97 A, a flange portion  97 C formed with a greater diameter than that of the collar shaft portion  97 A on the other end side of the collar shaft portion  97 A, and an inner diameter portion  97 D which fits with the shaft portion  96 A of the supporting bolt  96 . 
     The collar  97  is positioned to the supporting wall  210  by engaging the positioning portion  97 B thereof with the positioning hole  210 B, and is secured to the supporting wall  210  by the head portion  96 B which contacts with an end face of the flange portion  97 C by fastening force of the supporting bolt  96 . The head portion  96 B of the supporting bolt  96  is disposed in an opposing relationship to the inputting portion  80 B of the sensor  80 , and the axis of the supporting bolt  96  and the axis of the sensor  80  substantially coincide with each other. 
     The reduction gear  94  is supported on the collar  97  through a bearing  98  provided at a central portion thereof. 
     The bearing  98  is a ball bearing and is configured including an outer race  98 A on the outer diameter side, an inner race  98 B on the inner diameter side, and a plurality of balls  98 C provided between the outer race  98 A and the inner race  98 B. 
     The reduction gear  94  is formed in a shape of a disk, and a force fitting hole  94 A open in a circular shape is formed at a central portion of the reduction gear  94 . The bearing  98  is provided integrally with the reduction gear  94  by the outer race  98 A being force fitted in a force fitting hole  94 A. 
     The bearing  98  is attached to the collar  97  with the inner race  98 B thereof fitted with an outer circumferential face of the collar shaft portion  97 A and is secured by being pressed against the supporting wall  210  by the flange portion  97 C acted upon by the fastening force of the supporting bolt  96 . 
     In this manner, the reduction gear  94  is supported in a cantilever fashion by the single supporting bolt  96  fastened to the supporting wall  210  below the ball screw  61 . Therefore, a large space in the axial direction of the supporting bolt  96  is not required to provide the reduction gear  94 , and the reduction gear  94  can be disposed compactly. 
     Further, since the collar  97  is secured by the fastening force of the supporting bolt  96  in the state wherein it is positioned and supported by the positioning portion  97 B and the positioning hole  210 B which are formed with high accuracy, even if it is supported in a cantilever fashion, the collar  97  and the bearing  98  can be secured with certainty and the reduction gear  94  can be supported with certainty. 
       FIGS. 11A and 11B  are views showing the sensor-connecting element  95 .  FIG. 11A  is a plan view showing the sensor-connecting element  95 .  FIG. 11B  is a sectional view taken along line XI-XI of  FIG. 11A . It may be noted that, in  FIG. 11A , the reduction gear  94  is indicated by an alternate long and two short dashes line together with the sensor-connecting element  95 . 
     As shown in  FIGS. 9 ,  11 A and  11 B, the sensor-connecting element  95  is formed such that a central portion of a plate member substantially in the form of a disk is swollen in a convex manner, and has a base portion  99  formed in an annular shape for contacting with the outer side face  94 B of the reduction gear  94 , and a swollen portion  100  swollen in a convex shape at a central portion thereof. An accommodating space F is formed on the inner side of the swollen portion  100 . 
     Three attaching portions  99 A are formed at different places of the base portion  99  by being partly projected in a diametrical direction, and each of the attaching portions  99 A has a hole  99 B formed therein so as to extend therethrough. The attaching portions  99 A are disposed in a spaced relationship by a substantially equal distance from each other such that they divide an outer circumference of the base portion  99  equally into three portions. 
     The swollen portion  100  has a top portion  100 A forming a disk-shaped end face of the swollen portion  100  and opposing to the head portion  96 B, and a cylindrical outer wall portion  100 B which interconnects an outer edge of the top portion  100 A and the base portion  99 . A plurality of communicating holes  100 C are formed in the outer wall portion  100 B such that they extend through the outer wall portion  100 B and are communicated with the accommodating space F. 
     A sensor connecting stem  101  arranged perpendicular to the plane of the top portion  100 A is provided uprightly at the top portion  100 A of the sensor-connecting element  95 . A key portion  101 A is formed on the sensor connecting stem  101  by cutting away part of a circular cross section of the sensor connecting stem  101  in the axial direction. 
     As shown in  FIG. 11A , threaded hole portions  94 C are formed at positions of the reduction gear  94  which correspond to the holes  99 B of the sensor-connecting element  95 . The sensor-connecting element  95  is secured integrally to the reduction gear  94  by fixing bolts  102  fitted in the holes  99 B from the swollen portion  100  side and fastened to the threaded hole portions  94 C. 
     The sensor-connecting element  95  is secured to the outer side face  94 B of the reduction gear  94  across the head portion  96 B of the supporting bolt  96  in such a manner as to cover the head portion  96 B, and the head portion  96 B of the supporting bolt  96  and the flange portion  97 C of the collar  97  are accommodated in the accommodating space F between the swollen portion  100  and the outer side face  94 B. 
     The sensor-connecting element  95  is secured such that the axis of the sensor connecting stem  101  is in register with the axis of the supporting bolt  96 , that is, with the center of rotation of the reduction gear  94 . The sensor connecting stem  101  rotates at an equal speed to that of the reduction gear  94  integrally with the reduction gear  94 . 
     The sensor connecting stem  101  of the sensor-connecting element  95  is inserted in the inputting portion  80 B of the sensor  80  supported on the sensor-supporting wall  88  and is connected to the inputting portion  80 B, and the inputting portion  80 B rotates integrally with the sensor connecting stem  101 . The inputting portion  80 B is formed in a shape corresponding to the key portion  101 A, and the sensor connecting stem  101  is connected with certainty to the inputting portion  80 B by the key portion  101 A. 
     Here, operation of the sensor  80  and the reduction gear  94  is described. 
     If the electric actuator  70  is driven in accordance with an instruction of the ECU to rotate the ball screw  61 , then the output power shaft  92  rotates integrally with the ball screw  61 , and the rotation of the output power shaft  92  is transmitted to the reduction gear  94 . Here, the speed of the rotation of the output power shaft  92  is reduced by the reduction gear  94 . Then, by the rotation of the reduction gear  94 , the sensor-connecting element  95  is rotated integrally with the reduction gear  94 , and the inputting portion  80 B of the sensor  80  is rotated at an equal speed to that of the reduction gear  94  by the sensor connecting stem  101 . Then, the ECU calculates the speed of rotation of the ball screw  61  based on the speed of rotation of the reduction gear  94  detected by the sensor  80 . 
     In the present illustrative embodiment, the reduction gear  94  is supported in a cantilever fashion on one end side in the axial direction of the supporting bolt  96  and the sensor connecting stem  101  of the sensor-connecting element  95  provided on the other end side of the supporting bolt  96  is connected to the inputting portion  80 B of the sensor  80 . Consequently, a large space is not required for supporting of the reduction gear  94  and transmission of rotation of the reduction gear  94  to the sensor  80 . Therefore, the attaching structure for the sensor  80  can be miniaturized in the axial direction of the ball screw  61  and the gear supporting shaft  93 . 
     The attachment of the sensor  80  can be carried out by inserting the sensor  80  into the sensor supporting hole  89 B of the sensor-supporting wall  88  to connect the inputting portion  80 B to the sensor-connecting element  95  and then fastening the sensor fixing bolt  80 E to the female thread portion  89 C of the sensor-supporting wall  88 . Therefore, maintenance of the sensor  80  can be carried out from the outer side of the cylinder head  132 A, and the maintenance operability is good. 
     Further, the O-ring  80 D is interposed between the body portion  80 A and the sensor supporting hole  89 B so that displacement from alignment between the sensor connecting stem  101  and the inputting portion  80 B by assembly error or the like can be absorbed by deformation of the O ring  80 D. Consequently, strain can be prevented from appearing between the sensor  80  and the sensor-connecting element  95  and the friction can be reduced. 
     As described above, according to the illustrative of the present invention, the outer race  98 A of the bearing  98  is forced fitted in the reduction gear  94  and the inner race  98 B of the bearing  98  is secured to the supporting wall  210  by means of the supporting bolt  96 . Besides, the sensor-connecting element  95  is secured integrally with the outer side face  94 B of the reduction gear  94  and the sensor-connecting element  95  is connected to the sensor  80 . Consequently, only one bearing  98  is required for attachment of the reduction gear  94 . Therefore, since a large space for attachment of the reduction gear  94  is not required, the attaching structure for the sensor  80  can be miniaturized in the axial direction of the ball screw  61 . 
     Further, since only one bearing  98  is required to support the reduction gear  94 , the number of parts can be reduced. Furthermore, since the sensor-connecting element  95  integrally secured to the reduction gear  94  is connected to the sensor  80 , the attaching structure for the sensor  80  can be miniaturized in the axial direction. 
     Further, the angle of rotation of the ball screw  61  can be detected using the sensor  80  which is a less expensive potentiometer. 
     Further, since the electric actuator  70  is provided in parallel to the ball screw  61  on the outer side face  139  of the cylinder head  132 A and the sensor  80  is provided on the inner side wall  140  extending perpendicularly to the ball screw  61 , the electric actuator  70  and the sensor  80  do not make an obstacle to the disposition of other parts and can be disposed compactly. 
     Furthermore, since the electric actuator  70  is disposed on the counter offset side Q of the front bank  110 A and the rear bank  110 B, it can be disposed compactly, and the size in the vehicle widthwise direction of the engine  17  can be suppressed small. 
     Also, since the sensor  80  is disposed on the inner side of the V bank space K, it can be disposed compactly such that it does not make an obstacle to the disposition of other parts such as the intake chamber  43 , throttle body  42  and fuel pipe  180 . 
     Further, since the sensor  80  is disposed on the inner side of the V bank space K disposed in the forward and rearward direction of the motorcycle  10  and is surrounded by the engine  17 , it can be prevented from being hit by a flying stone or the like. 
     It may be noted that the embodiment described above indicates a mode to which the present invention is applied. However, the present invention is not limited to the embodiment described above. 
     While, in the embodiment described above, the valve train  50  of the engine  17  varies the opening and closing phases and the lift amounts of the intake valve  147  and the exhaust valve  148 , the present invention is not limited to this. The valve mechanism may be otherwise configured, such that only the opening and closing phases are varied, or only the lift amounts are varied. Also other detailed configurations can naturally be modified as needed. 
     In other words, although the present invention has been described herein with respect to a number of specific illustrative embodiments, the foregoing description is intended to illustrate, rather than to limit the invention. Those skilled in the art will realize that many modifications of the illustrative embodiment could be made which would be operable. All such modifications, which are within the scope of the claims, are intended to be within the scope and spirit of the present invention.