Patent Publication Number: US-7222595-B2

Title: Internal combustion engine for vehicle

Description:
FIELD OF THE INVENTION 
   The present invention relates to an internal combustion engine for a vehicle, which comprises a valve system comprising mechanism for controlling the valve operation characteristics by an electric actuator. 
   BACKGROUND OF THE INVENTION 
   A variable valve system for an internal combustion engine capable of changing the opening and closing timings and the maximum lift amount of an engine valve, is disclosed in JP 2002-155716. The valve system comprises a varying mechanism for controlling the valve lift amount of an intake valve put into an opening operation by a swing cam swingably supported on a drive shaft, and a drive mechanism having an electric motor for rotationally driving a control shaft of a control mechanism for controlling the operating position of the varying mechanism. The electric motor of the valve system is disposed at a rear end portion of a cylinder head with a plate therebetween and substantially in parallel to the control shaft. The drive shaft of the electric motor is disposed substantially in parallel to the drive shaft which is rotatably supported on the cylinder head and which is rotationally driven by the crankshaft. 
   The electric motor according to the above-mentioned Japanese reference is disposed on the exterior of the cylinder head and exposed to the outside air. Accordingly, the motor, is cooled by a process in which the heat generated by the operation thereof is released into the outside air. This arrangement ensures that accurate operations of the electric motor are secured, and the durability of the electric motor is enhanced. Meanwhile, in an internal combustion engine mounted on a vehicle, when it is intended to promote the cooling of the electric motor by utilizing the running airflow for the purpose of enhancing the performance of cooling by heat radiation, it is necessary to ensure that the collision of the running airflow on the electric motor is not hampered by the cylinder head itself or members disposed in the vicinity of the cylinder head. The limitation restricts the layout of the electric motor and makes it difficult to achieve a compact layout of the electric motor in relation to the cylinder head. When the electric motor is disposed at a tip end portion, in the cylinder axis direction, of a head cover connected to the cylinder head, the valve system comprising the electric motor is enlarged in size in the cylinder axis direction and, hence, the internal combustion engine comprising the valve system is enlarged in size in the cylinder axis direction. 
   SUMMARY OF THE INVENTION 
   The present invention has been made in consideration of the above-mentioned circumstances. It is an object of the inventions to enlarge the degree of freedom in laying out an electric actuator of a valve characteristic varying mechanism and to layout the electric actuator at the cylinder head in a compact form while securing good performance of cooling the electric actuator. It is another object to enhance the performance of cooling a combustion chamber wall and to prevent a valve chamber from being heated to a high temperature. 
   The invention relates to an internal combustion engine for a vehicle, mounted on the vehicle, comprising a cylinder head connected to a cylinder and defining a combustion chamber and a valve chamber, and a valve system comprising a valve characteristic varying mechanism for controlling valve operation characteristics of an engine valve comprised of an intake valve or an exhaust valve, with an electric actuator of the valve characteristic varying mechanism being disposed in the exterior of the valve chamber. The cylinder head is provided, between the combustion chamber and the valve chamber, with a duct for leading a running airflow therethrough, and the electric actuator is disposed at a position which is adjacent to the valve chamber in the radial direction with respect to the cylinder axis of the cylinder and at which the running airflow having passed through the duct collides against the electric actuator. 
   According to this, the airflow is guided by the duct formed in the cylinder head and collides against the electric actuator as a cooling airflow, thereby cooling the electric actuator. Therefore, it is unnecessary to lay out the electric actuator at such a position that the running airflow collides directly on the electric actuator, while avoiding the cylinder head itself or members disposed in the vicinity of the cylinder head. In addition, the duct can be formed so as to match the position of the electric actuator, and the electric actuator disposed adjacent to the valve chamber in the radial direction with respect to the cylinder axis can be laid out close to the cylinder head in the radial direction. Further, since the duct is formed between the combustion chamber and the valve chamber, the combustion chamber walls are cooled by the running airflow distributed through the duct, and the heating of the valve chamber by the heat coming from the combustion chamber is restrained. 
   The invention also relates to an internal combustion engine for a vehicle wherein the electric actuator comprises an output shaft extending in parallel to the cylinder axis. According to this, the electric actuator can be laid out along the cylinder axis, so that the electric actuator as a whole can be laid out closer to the cylinder axis, as compared with the case where the output shaft extends in parallel to a plane orthogonal to the cylinder axis. 
   Since the electric actuator is cooled by the running airflow guided by the duct, good performance of cooling the electric actuator is secured, and it is unnecessary to lay out the electric actuator at such a position that the running airflow collides directly on the electric actuator. In addition, the duct can be formed so as to match the position of the electric actuator, so that the degree of freedom in laying out the electric actuator is enhanced. Moreover, since the electric actuator can be disposed close to the cylinder head in the radial direction with respect to the cylinder axis, the electric actuator can be laid out at the cylinder head in a compact form, and it is possible to prevent the valve system from being enlarged in size in the cylinder axis direction and, hence, to prevent the internal combustion engine from being enlarged in size in the cylinder axis direction. Furthermore, the performance of cooling the combustion chamber walls is enhanced, and the valve chamber is prevented from being heated to a high temperature. 
   In addition to the effects of the invention as set forth in the cited claim. The electric actuator as a whole can be disposed close to the cylinder axis, so that the electric actuator can be disposed at the cylinder head in a compacter form in the radial direction. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a general right side view of a motorcycle on which an internal combustion engine according to the present invention is mounted. 
       FIG. 2  is a sectional view, generally along arrow II—II of  FIG. 4 , of the internal combustion engine of  FIG. 1 , partly in section along a plane passing through the center axes of an intake valve and an exhaust valve and the center axis of a control shaft. 
       FIG. 3  is a sectional view, generally along arrow IIIa—IIIa of  FIG. 8 , of the internal combustion engine of  FIG. 1 , partly in section generally along arrow IIIb—IIIb. 
       FIG. 4  is a sectional view, generally along arrow IV—IV of  FIG. 2 , of a valve system in the internal combustion engine of  FIG. 1  with the head cover removed, partly with component members of the valve system in appropriate section. 
       FIG. 5  is a view of a camshaft holder mounted to a cylinder head in the internal combustion engine of  FIG. 1 , as viewed along the cylinder axis from the head cover side. 
       FIG. 6  shows the valve system for the internal combustion engine of  FIG. 1 , in which (A) is a view of an exhaust drive cam of a valve characteristic varying system as viewed in the camshaft direction, and (B) is a view of an exhaust link mechanism and an exhaust cam in the valve characteristic varying mechanism in an appropriately pivotally moved condition. 
       FIG. 7(A)  is a sectional view along arrow VIIA of  FIG. 6 ,  FIG. 7(B)  is a view along arrow VIIB of  FIG. 6 ,  FIG. 7(C)  is a sectional view along arrow VIIC of  FIG. 6 , and  FIG. 7(D)  is a view along arrow VIID of  FIG. 6 . 
       FIG. 8  is a view of the head cover in the internal combustion engine of  FIG. 1  as viewed along the cylinder axis from the front side, with a drive mechanism of the valve characteristic varying mechanism shown in partly broken state. 
       FIG. 9  is a sectional view along arrow IX—IX of  FIG. 10 . 
       FIG. 10  is a sectional view along arrow X—X of  FIGS. 4 and 9 . 
       FIG. 11  is an illustration of the valve operation characteristics of the intake valve and the exhaust valve effected by the valve system for the internal combustion engine of  FIG. 1 . 
       FIG. 12  shows the valve system for the internal combustion engine of  FIG. 1 , in which (A) is an illustration of an essential part of the valve characteristic varying mechanism when a maximum valve operation characteristic is obtained in regard of the intake valve, and (B) is an illustration of an essential part of the valve characteristic varying mechanism when a maximum valve operation characteristic is obtained in regard of the exhaust valve, corresponding to an essential part enlarged view of  FIG. 2 . 
       FIG. 13(A)  is a view corresponding to  FIG. 12(A)  when a minimum valve operation characteristic is obtained in regard of the intake valve, and  FIG. 13(B)  is a view corresponding to  FIG. 12(B)  when a minimum valve operation characteristic is obtained in regard of the exhaust valve. 
       FIG. 14(A)  is a view corresponding to  FIG. 12(A)  when a decompression operation characteristic is obtained in regard of the intake valve, and  FIG. 14(B)  is a view corresponding to  FIG. 12(B)  when a decompression operation characteristic is obtained in regard of the exhaust valve. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Now, an embodiment of the present invention will be described below, referring to  FIGS. 1 to 14 . 
   Referring to  FIG. 1 , an internal combustion engine E for a vehicle to which the present invention is applied is mounted on a motorcycle V representative of a vehicle. The motorcycle V comprises a vehicle body frame  1  having a front frame  1   a  and a rear frame  1   b , a steering handle  4  fixed to an upper end portion of a front fork  3  rotatably supported on a head pipe  2  connected to the front end of the front frame  1   a , a front wheel  7  rotatably supported on lower end portions of the front fork  3 , a power unit U supported on the vehicle body frame  1 , a rear wheel  8  rotatably supported on a rear end portion of a swing arm  5  swingably supported on the vehicle body frame  1 , a rear cushion  6  for connection between the rear frame  1   b  and a rear portion of the swing arm  5 , and a vehicle body cover  9  covering the vehicle body frame  1 . 
   The power unit U comprises a transverse layout type internal combustion engine E having a crankshaft  15  extending in the left-right direction of the motorcycle V, and a power transmission device having a transmission and transmitting the power of the internal combustion engine E to the rear wheel  8 . The internal combustion engine E comprises a crankcase  10  forming a crank chamber in which to contain the crankshaft  15  and serving also as a transmission case, a cylinder  11  connected to the crankcase  10  and extending forwards, a cylinder head  12  connected to a front end portion of the cylinder  11 , and a head cover  13  connected to a front end portion of the cylinder head  12 . The cylinder axis L 1  of the cylinder  11  extends forwards, and either slightly upwards relative to the horizontal direction (see  FIG. 1 ) or substantially in parallel to the horizontal direction. The rotation of the crankshaft  15  driven by a piston  14  (see  FIG. 2 ) to rotate is transmitted to the rear wheel  8  through speed change by the transmission, to drive the rear wheel  8 . 
   Referring to  FIG. 2  also, the internal combustion engine E is an SOHC type air-cooled single-cylinder four-stroke internal combustion engine, in which the cylinder  11  is provided with a cylinder bore  11   a  in which the piston  14  is reciprocatably fitted, the cylinder head  12  is provided with a combustion chamber  16  on the side of facing the cylinder bore  11   a  in the cylinder axis direction A 1 , and further with an intake port  17  having an intake opening  17   a  opening into the combustion chamber  16  and an exhaust port  18  having an exhaust opening  18   a  opening into the combustion chamber  16 . In addition, a spark plug  19  fronting on the combustion chamber  16  is inserted in a mount hole  12   c  formed in the cylinder head  12 , to be mounted to the cylinder head  12 . Here, the combustion chamber  16  constitutes a combustion space, together with the cylinder bore  11   a  between the piston  14  and the cylinder head  12 . 
   Further, the cylinder head  12  is provided with one intake valve  22  and one exhaust valve  23  serving as engine valves which are reciprocatably supported by valve guides  20   i ,  20   e  and are each normally biased in the valve closing direction by a valve spring  21 . The intake valve  22  and the exhaust valve  23  are put into opening and closing operations by a valve system  40  provided in the internal combustion engine E, to open and close the intake opening  17   a  and the exhaust opening  18   a  defined by valve seats  24 . The valve system  40 , exclusive of an electric motor  80  (see  FIG. 3 ) is disposed in a valve chamber  25  defined by the cylinder head  12  and the head cover  13 . 
   An intake system comprising an air cleaner  26  (see  FIG. 1 ) and a throttle body  27  (see  FIG. 1 ) is mounted to an upper surface  12   a , i.e., one side surface of the cylinder head  12  in which an inlet  17   b  of the intake port  17  is opened, for leading air taken in from the exterior to the intake port  17 . On the other hand, an exhaust system comprising an exhaust pipe  28  (see  FIG. 1 ) for leading an exhaust gas flowing out from the combustion chamber  16  via the exhaust port  18  to the exterior of the internal combustion engine E is mounted a lower surface  12   b , i.e., the other side surface of the cylinder head  12  in which an outlet  18   b  of the exhaust port  18  is opened. In addition, the intake system comprises a fuel injection valve which is a fuel supply device for supplying a liquid fuel into the intake air. 
   The air taken in through the air cleaner  26  and the throttle body  27  flows through the opened intake valve  22  to be taken into the combustion chamber  16  in the intake stroke in which the piston  14  is moved downwards, and the air thus taken in is compressed in the state of being mixed with the fuel in the compression stroke in which the piston  14  is moved upwards. The fuel-air mixture is combusted by ignition by the spark plug  19  at the final stage of the compression stroke, and the piston  14  driven by the pressure of the combustion gas, in the expansion stroke in which the piston  14  is moved downwards, drives the crankshaft  15  to rotate. In the exhaust stroke in which the piston  14  is moved upwards, the burned gas flows through the opened exhaust valve  23  to be discharged from the combustion chamber  16  into the exhaust port  18 , as an exhaust gas. 
   Referring to  FIGS. 2 to 5  and  FIG. 10 , the valve system  40  comprises an intake main rocker arm  41  as an intake cam follower abutting on a valve stem  22   a  of the intake valve  22  so as to put the intake valve  22  into opening and closing operations, an exhaust main rocker arm  42  as an exhaust cam follower abutting on a valve stem  23   a  of the exhaust valve  23  so as to put the exhaust valve  23  into opening and closing operations, and a valve characteristic varying mechanism M for controlling the valve operation characteristics including the opening and closing timings and the maximum lift amounts of the intake valve  22  and the exhaust valve  23 . 
   The intake main rocker arm  41  and the exhaust main rocker arm  42  are rockably supported on a pair of rocker shafts  43  fixed to a camshaft holder  29  at fulcrum points  41   a ,  42   a  at central portions thereof, respectively, abut on the valve stems  22   a ,  23   a  at adjustment screws  41   b ,  42   b  constituting action portions at one-side end portions thereof, and make contact with an intake cam  53  and an exhaust cam  54  at rollers  41   c ,  42   c  constituting contact portions at other-side end portions thereof, respectively. 
   The valve characteristic varying mechanism M comprises an internal mechanism contained in the valve chamber  25 , and the electric motor  80  which is an external mechanism disposed in the exterior of the valve chamber  25  and is an electric actuator for driving the internal mechanism. The internal mechanism comprises: one camshaft  50  rotatably supported on the cylinder head  12  and driven to rotate in conjunction with the crankshaft  15 ; an intake drive cam  51  and an exhaust drive cam  52  which are drive cams provided on the camshaft  50  and rotated integrally with the camshaft  50 ; link mechanisms Mli, Mle as interlocking mechanisms pivotally supported on the camshaft  50  and swingable about the camshaft  50 ; the intake cam  53  and the exhaust cam  54  which are valve cams connected to the link mechanisms Mli, Mle and pivotally supported on the camshaft  50  so as to operate the intake main rocker arm  41  and the exhaust main rocker arm  42 , respectively; a drive mechanism M 2  (see  FIG. 3 ) comprising the electric motor  80  as a drive source for swinging the link mechanisms Mli, Mle about the camshaft  50 ; a control mechanism M 3  interposed between the drive mechanism M 2  and the link mechanisms Mli, Mle and controlling the swinging of the link mechanisms Mli, Mle about the camshaft  50  according to the drive force of the electric motor  80 ; and a pressing spring  55  as pressing energizing means for applying a torque about the camshaft  50  to the link mechanisms Mli, Mle for the purpose of pressing the link mechanisms Mli, Mle against the control mechanism M 3 . 
   Referring to  FIGS. 2 to 4 , the camshaft  50  is rotatably supported on the cylinder head  12  and a camshaft holder  29  connected to the cylinder head  12 , through a pair of bearings  56  disposed at both end portions thereof, and is driven to rotate in conjunction with the crankshaft  15  (see  FIG. 1 ) at a rotation speed of one half that of the crankshaft  15 , by the power of the crankshaft  15  transmitted through a valve power transmission mechanism. The valve power transmission mechanism comprises a cam sprocket  57  integrally connected to a portion near the tip end of a left end portion, or one-side end portion, of the camshaft  50 , a drive sprocket integrally connected to the crankshaft  15 , and a timing chain  58  wrapped around the cam sprocket  57  and the drive sprocket. The valve power transmission mechanism is contained in a power transmission chamber which is defined by the cylinder  11  and the cylinder head  12  and is located on the left side, or one lateral side, in relation to a first orthogonal plane H 1 , of the cylinder  11  and the cylinder head  12 . Of the power transmission chamber, a power transmission chamber  59  formed in the cylinder head  12  is adjacent to the valve chamber  25  in the radial direction with the cylinder axis L 1  as a center (hereinafter referred to as “the radial direction”) and in the direction A 2  of the rotational center line L 2  of the camshaft  50  (hereinafter referred to as “the camshaft direction A 2 ”). Here, the first orthogonal plane H 1  is a plane orthogonal to a reference plane H 0  which includes the cylinder axis L 1  and will be described later. 
   Incidentally, in the valve characteristic varying mechanism M, members relating to the intake valve  22  and members relating to the exhaust valve  23  include mutually corresponding members, and the intake drive cam  51 , the exhaust drive cam  52 , the link mechanisms Mli, Mle, the intake cam  53  and the exhaust cam  54  have the same basic structures; therefore, the following description will be centered on the members relating to the exhaust valve  23 , and the members relating to the intake valve  22 , related descriptions and the like will be parenthesized, if necessary. 
   Referring to  FIGS. 2 ,  3 ,  6 ,  7  and  12 , the exhaust drive cam  52  (intake drive cam  51 ) fixed by being press fitted to the camshaft  50  has a cam surface formed over the entire circumference of the outer circumferential surface thereof. The cam surface is composed of a base circle portion  52   a  ( 51   a ) for not swinging the exhaust cam  54  (intake cam  53 ) through the link mechanism Mle (Mli), and a cam crest portion  52   b  ( 51   b ) for swinging the exhaust cam  54  (intake cam  53 ) through the link mechanism Mle (Mli). The base circle portion  52   a  ( 51   a ) has an arcuate sectional shape with a fixed radius from the rotational center line L 2 , and the cam crest portion  52   b  ( 51   b ) has a sectional shape such that the radius from the rotational center line L 2  increases and then decreases in the rotational direction R 1  of the camshaft  50 . The base circle portion  52   a  ( 51   a ) sets the swing position of the exhaust cam  54  (intake cam  53 ) so that the exhaust main rocker arm  42  (intake main rocker arm  41 ) makes contact with a base portion  54   a  ( 53   a ) of the exhaust cam  54  (intake cam  53 ), whereas the cam crest portion  52   b  ( 51   b ) sets the swing position of the exhaust cam  54  (intake cam  53 ) so that the exhaust main rocker arm  42  (intake main rocker arm  41 ) makes contact with the base circle portion  54   a  ( 53   a ) and the cam crest portion  54   b  ( 53   b ) of the exhaust cam  54  (intake cam  53 ). 
   The link mechanisms Mli, Mle are constituted of the intake link mechanism Mli connected to the intake cam  53 , and the exhaust link mechanism Mle connected to the exhaust cam  54 . Referring to  FIG. 4  also, the exhaust link mechanism Mle (intake link mechanism Mli) comprises a holder  60   e  ( 60   i ) pivotally supported on the camshaft  50  and swingable about the camshaft  50 , an exhaust sub rocker arm  66   e  (intake sub rocker arm  66   i ) pivotally supported on the holder  60   e  ( 60   i ) and driven by the exhaust drive cam  52  (intake drive cam  51 ) to swing, a connection link  67   e  ( 67   i ) pivotally supported on the exhaust sub rocker arm  66   e  (intake sub rocker arm  66   i ) at one end portion thereof and pivotally supported on the exhaust cam  54  (intake cam  53 ) at the other end portion thereof, and a control spring  68  for pressing the exhaust sub rocker arm  66   e  (intake sub rocker arm  66   i ) against the exhaust drive cam  52  (intake drive cam  51 ). 
   The holder  60   e  ( 60   i ) supported on the camshaft  50  through a bearing  69  in which the camshaft  50  is inserted comprises a pair of first and second plates  61   e  ( 61   i ),  62   e  ( 62   i ) spaced from each other in the camshaft direction A 2 , and a connection member for connecting the first plate  61   e  ( 61   i ) and the second plate  62   e  ( 62   i ) to each other at a predetermined interval in the camshaft direction A 2  and for pivotally supporting the exhaust sub rocker arm  66   e  (intake sub rocker arm  66   i ). The connection member comprises a collar  63   e  ( 63   i ) determining the predetermined interval between both the plates  61   e  ( 61   i ),  62   e  ( 62   i ) and serving also as a support shaft for pivotally supporting the exhaust sub rocker arm  66   e  (intake sub rocker arm  66   i ), and a rivet  64  inserted in the collar  63   e  ( 63   i ) to integrally connect both the plates  61   e  ( 61   i ),  62   e  ( 62   i ) to each other. As shown in  FIGS. 4 and 6 , the plates  61   e  ( 61   i ),  62   e  ( 62   i ) are provided with mount holes  61   e   3  ( 61   i   3 ),  62   e   3  ( 62   i   3 ) in which to mount bearings  69  for swingably supporting the plates  61   e  ( 61   i ),  62   e  ( 62   i ) on the camshaft  50 . 
   Referring to  FIG. 3  also, an exhaust control link  71   e  (intake control link  71   i ) of the control mechanism  3  is pivotally mounted to the first plate  61   e  ( 61   i ), and the exhaust control link  71   e  (intake control link  71   i ) and the first plate  61   e  ( 61   i ) are so connected as to be capable of relative motions at their connection portions  71   e   2  ( 71   i   2 ),  61   e   1  ( 61   i   1 ). Specifically, a connection pin  61   e   1   a  ( 61   i   1   a ) fixed by being press fitted in a hole in the connection portion  61   e   1  ( 61   i   1 ) of the first plate  61   e  ( 61   i ) serving as a holder side connection portion is relatively rotatably inserted in a hole in the connection portion  71   e   2  ( 71   i   2 ) of the exhaust control link  71   e  (intake control link  71   i ) serving as a control mechanism side connection portion. 
   In addition, the second plate  62   e  ( 62   i ) is provided with a decompression cam  62   e   1  ( 62   i   1 ) (see  FIGS. 6 and 12 ) for facilitating the starting by lowering the compression pressure through slightly opening the intake valve  22  and the exhaust valve  23  in the compression stroke at the time of starting the internal combustion engine E. Further, the second plate  62   e  is provided with a detected portion  62   e   2  to be detected by a detecting portion  94   a  of the swing position detection means  94  (see  FIGS. 3 and 14 ). The detected portion  62   e   2  is composed of a teeth portion engaged in the swinging direction of the second plate  62   e  by being meshed with a teeth portion constituting the detecting portion  94   a . Incidentally, though not used in this embodiment, the second plate  61   i  is also provided with a portion  62   i   2  corresponding to the detected portion  62   e   2 . 
   The collar  63   e  ( 63   i ) is integrally provided with a first spring holding portion  76  for holding one end portion of a control spring  68  consisting of a compression coil spring having a straight hollow cylindrical shape in the natural state, and a movable side spring holding portion  78  for holding one end portion of the pressing spring  55  consisting of a compression coil spring having a straight hollow cylindrical shape in the natural state. Both the spring holding portions  76 ,  78  are disposed adjacently to a fulcrum portion  66   ea  ( 66   ia ) of the exhaust sub rocker arm  66   e  (intake sub rocker arm  66   i ) in the camshaft direction A 2  and are disposed at an interval along the circumferential direction of the collar  63   e  ( 63   i ) (see  FIG. 4 ). 
   In addition, the collar  63   e  ( 63   i ) is provided, at a position spaced from the swing center line L 3  of the exhaust sub rocker arm  66   e  (intake sub rocker arm  66   i ), with a projected portion  63   e   1  ( 63   i   1 ) to be fitted in a hole  62   e   4  ( 62   i   4 ) formed in the second plate  62   e  ( 62   i ). The projected portion  63   e   1  ( 63   i   1 ) and the hole  62   e   4  ( 62   i   4 ) constitute an engagement portion for inhibiting relative rotations, around the swing center line L 3 , of the second plate  62   e  ( 62   i ) and the collar  63   e  ( 63   i ). By the engagement portion, the pair of spring holding portions  76 ,  78  are provided, whereby the collar  63   e  ( 63   i ) on which torques in the same direction are exerted by the spring forces of the control spring  68  and the pressing spring  55  is inhibited from relative rotation relative to the first and second plates  61   e  ( 61   i ),  62   e  ( 62   i ), so that the application of torques about the camshaft  50  to the link mechanisms Mli, Mle by the pressing spring  55  and the pressing thereof against the exhaust drive cam  52  (intake drive cam  51 ) by the control spring  68  are performed assuredly. 
   Referring to  FIGS. 2 to 4 ,  6 ,  7  and  12 , in the camshaft direction A 2 , the exhaust sub rocker arm  66   e  (intake sub rocker arm  66   i ) disposed between the first and second plates  61   e  ( 61   i ),  62   e  ( 62   i ) together with the exhaust cam  54  (intake cam  53 ) and the exhaust drive cam  52  (intake drive cam  51 ) makes contact with the exhaust drive cam  52  (intake drive cam  51 ) at a roller  66   eb  ( 66   ib ) serving as a contact portion for contact with the exhaust drive cam  52  (intake drive cam  51 ), is swingably supported on the collar  63   e  ( 63   i ) at the fulcrum portion  66   ea  ( 66   ia ) at one end portion thereof, and is pivotally supported on a connection pin  72  fixed to one end portion of the connection link  67   e  ( 67   i ) at the other end portion thereof. Therefore, the exhaust sub rocker arm  66   e  (intake sub rocker arm  66   i ) is swung about the collar  63   e  ( 63   i ) due to the rotation of the exhaust drive cam  52  (intake drive cam  51 ) together with the camshaft  50 . 
   The exhaust cam  54  (intake cam  53 ) pivotally supported on a connection pin  73  fixed to the other end portion of the connection link  67   e  ( 67   i ) is composed of a swing cam supported on the camshaft  50  through the bearing  44  and thereby swingable about the camshaft  50 , and is provided with a cam surface at a part of the outer circumferential surface thereof. The cam surface is composed of the base circle portion  54   a  ( 53   a ) for maintaining the exhaust valve  23  (intake valve  22 ) in the closed state, and the cam crest portion  54   b  ( 53   b ) for pressing down and thereby opening the exhaust valve  23  (intake valve  22 ). The base circle portion  54   a  ( 53   a ) has an arcuate sectional shape with a fixed radius from the rotational center line L 2 , whereas the cam crest portion  54   b  ( 53   b ) has such a sectional shape that the radius from the rotational center line L 2  increases along the counter-rotational direction R 2  (rotational direction R 1 ) of the camshaft  50 . Therefore, the cam crest portion  54   b  ( 53   b ) of the exhaust cam  54  (intake cam  53 ) has such a shape that the lift amount of the exhaust valve  23  (intake valve  22 ) gradually increases along the counter-rotational direction R 2  (rotational direction R 1 ). 
   The exhaust cam  54  (intake cam  53 ), on one hand, is swung about the camshaft  50  together with the exhaust link mechanism Mle (intake link mechanism Mli) by the same swing amount, by the drive force of the drive mechanism M 2  transmitted through the control mechanism M 3 , and, on the other hand, is swung about the camshaft  50  by the exhaust sub rocker arm  66   e  (intake sub rocker arm  66   i ) swung by the exhaust drive cam  52  (intake drive cam  51 ). The exhaust cam  54  (intake cam  53 ) swung relative to the camshaft  50  swings the exhaust main rocker arm  42  (intake main rocker arm  41 ), thereby putting the exhaust valve  23  (intake valve  22 ) into opening and closing operations. Therefore, the exhaust cam  54  (intake cam  53 ) is swung by the drive force of the drive mechanism M 2  transmitted sequentially through the holder  60   e  ( 60   i ), the exhaust sub rocker arm  66   e  (intake sub rocker arm  66   i ) and the connection link  67   e  ( 67   i ), and is swung by the drive force of the exhaust drive cam  52  (intake drive cam  51 ) transmitted sequentially through the exhaust sub rocker arm  66   e  (intake sub rocker arm  66   i ) and the connection link  67   e  ( 67   i ). 
   The control spring  68  for generating a spring force for pressing the roller  66   eb  ( 66   ib ) of the exhaust sub rocker arm  66   e  (intake sub rocker arm  66   i ) against the exhaust drive cam  52  (intake drive cam  51 ) is disposed between the collar  63   e  ( 63   i ) and the exhaust cam  54 , and can be extended and contracted in the circumferential direction of the camshaft  50  according to the rocking of the exhaust sub rocker arm  66   e  (intake sub rocker arm  66   i ). One end portion of the control spring  68  is held by the first spring holding portion  76 , and the other end portion is held by a second spring holding portion  77  provided at a shelf-like projected portion which is integrally formed on the exhaust cam  54  (intake cam  53 ). 
   The pressing spring  55  normally exerting on the exhaust link mechanism Mle (intake link mechanism Mli) a spring force for applying a torque directed in one sense of the swinging direction has its one end portion held by the movable side spring holding portion  78  of the holder  60   e  ( 60   i ), and has its other end portion held by a fixed side spring holding portion  79  provided in the camshaft holder  29  which is a fixed member fixed to the cylinder head  12 . 
   The spring force of the pressing spring  55  for pressing the exhaust link mechanism Mle (intake link mechanism Mli) toward the side of the cylinder  11  acts directly on the holder  60   e  ( 60   i ) to press the holder  60   e  ( 60   i ) in the direction toward the cylinder  11 , and the torque exerted on the holder  60   e  ( 60   i ) by the spring force is directed in the above-mentioned one sense. The one sense is set to be the same as the sense of the torque exerted on the exhaust cam  54  (intake cam  53 ) by the reaction force applied to the exhaust cam  54  (intake cam  53 ) from the exhaust valve  23  (intake valve  22 ) when the exhaust cam  54  (intake cam  53 ) opens the exhaust valve  23  (intake valve  22 ). Therefore, the sense in which the spring force of the pressing spring  55  normally presses the connection portion  61   e   1  ( 61   i   1 ) against the connection portion  71   e   2  ( 71   i   2 ) in the swinging direction is the same as the sense in which the above-mentioned reaction force presses the connection portion  61   e   1  ( 61   i   1 ) against the connection portion  71   e   2  ( 71   i   2 ) in the swinging direction, based on the torque applied from the exhaust cam  54  (intake cam  53 ) to the holder  60   e  ( 60   i ) through the connection link  67   e  ( 67   i ) and the exhaust sub rocker arm  66   e  (intake sub rocker arm  66   i ). 
   At the connection portions  71   e   2  ( 71   i   2 ),  61   e   1  ( 61   i   1 ) provided with slight gap due to the pivotal supporting, the connection portion  61   e   1  ( 61   i   1 ) on one side is normally pressed against the connection portion  71   e   2  ( 71   i   2 ) in the swinging direction by the pressing spring  55 ; therefore, when the first plate  61   e  ( 61   i ) is swung by the exhaust control link  71   e  (intake control link  71   i ), the influence of the gap (play) between the connection portion  71   e   2  ( 71   i   2 ) and the connection portion  61   e   1  ( 61   i   1 ) is eliminated, and the motion of the exhaust control link  71   e  (intake control link  71   i ) is accurately transmitted to the holder  60   e  ( 60   i ). 
   Here, referring to  FIGS. 2 ,  4 ,  6  and  12 , the spring holding portions  76 ,  77 ,  78 ,  79  will be further described. The spring holding portions  76 ,  77 ,  78 ,  79  have spring guides  76   a ,  77   a ,  78   a ,  79   a  which are each inserted into an end portion of the control spring  68  or an end portion of the pressing spring  55 . The spring guides  76   a ,  77   a ,  78   a ,  79   a  have the same basic structure in the point of having base portions  76   a   1 ,  77   a   1 ,  78   a   1 ,  79   a   1  and tapered portions  76   a   2 ,  77   a   2 ,  78   a   2 ,  79   a   2 , respectively. The base portions  76   a   1 ,  77   a   1 ,  78   a   1 ,  79   a   1  are each a portion over which the end portion of the control spring  68  or the pressing spring  55  is fitted in the state of being inhibited from moving in the radial direction, and the tapered portions  76   a   2 ,  77   a   2 ,  78   a   2 ,  79   a   2  are continuous with the base portions  76   a   1 ,  77   a   1 ,  78   a   1 ,  79   a   1  and are each tapered so as to obviate interference with the control spring  68  or the pressing spring  55  when the control spring  68  or the pressing spring  55  is curved and when the control spring  68  or the pressing spring  55  is in a substantially straight hollow cylindrical shape, due to the rocking of the exhaust sub rocker arm  66   e  (intake sub rocker arm  66   i ) or the swinging of the holder  60   e  ( 60   i ). 
   In this embodiment, the base portions  76   a   1 ,  77   a   1  of the spring guide  76   a ,  77   a  of the first and second spring holding portions  76 ,  77  are cylindrical, and have outside diameters roughly equal to or slightly greater than the inside diameter of the control spring  68 . The tapered portions  76   a   2 ,  77   a   2  are in a straight truncated conical shape with a bottom portion having an outside diameter equal to the base portions  76   a   1 ,  77   a   1 , and the outside diameter thereof decreases in the direction from the bas end portion  76   a   1 ,  77   a   1  toward the tip end. The degree of the taper of both the tapered portions  76   a   2 ,  77   a   2  is so set as to avoid interference with the control spring  68  when the control spring  68  is extended and simultaneously curved according to the rocking of the exhaust sub rocker arm  66   e  (intake sub rocker arm  66   i ) and when the control spring  66  is most contracted into a substantially straight hollow cylindrical shape. 
   The second spring holding portion  77  comprises the spring guide  77   a  having a mount portion  77   a   3 , in addition to the base portion  77   a   1  and the tapered portion  77   a   2  having the same functions as those in the first spring holding portion  76 . The spring guide  77   a  is fixed to the exhaust cam  54  (intake cam  53 ) by inserting the mount portion  77   a   3  into a hole in the projected portion mentioned above and then plastically deforming the mount portion  77   a   3  by caulking. In addition, the heights of the spring guides  76   a ,  77   a  from respective receiving surfaces of the first and second spring holding portions  76 ,  77  are nearly equal in this embodiment, but they may be set to be different, taking into account the strength of the control spring  68  or the like. 
   Besides, when the control spring  68  is curved due to the rocking of the exhaust sub rocker arm  66   e  (intake sub rocker arm  66   i ), the curvature of curving near the spring guide  77   a  of the second spring holding portion  77  which is the movable side spring holding portion movable relative to the first spring holding portion  76  is greater than the curvature of curving near the spring guide  76   a  of the first spring holding portion  76  which is the fixed side spring holding portion. Therefore, the degree of tapering of the tapered portion  77   a   2  is set to be greater than that of the tapered portion  76   a   2 , and, in this embodiment, the apex angle of the cone determining the conical surface of the tapered portion  77   a   2  is set to be smaller. 
   On the other hand, the base portions  78   a   1 ,  79   a   1  of the spring guide  78   a ,  79   a  of the movable side and fixed side spring holding portions  78 ,  79  are in a cylindrical shape with an outside diameter nearly equal to or slightly greater than the inside diameter of the pressing spring  55 . The tapered portions  78   a   2 ,  79   a   2  are each in a truncated conical shape with a bottom portion having an outside diameter equal to the base portion  78   a   1 ,  79   a   1 , and the outside diameter thereof decreases in the direction from the base portion  78   a   1 ,  79   a   1  toward the tip end. The degree of tapering of both the tapered portions  78   a   2 ,  79   a   2  is so set as to avoid interference with the pressing spring  55  when the pressing spring  55  is extended and simultaneously curved according to the swinging of the holder  60   e  ( 60   i ) and when the pressing spring  55  is most contracted into a substantially straight hollow cylindrical shape. 
   The fixed side spring holding portion  79  comprises, in an integral form, the spring guide  79   a  having a base portion  79   a   1  and the tapered portion  79   a   2  similar to those of the movable side spring holding portion  78 , a flange portion  79   b  having a receiving surface on which the pressing spring  55  abuts, and a mount portion  79   c . The fixed side spring holding portion  79  is fixed to the camshaft holder  29  by press fitting of its mount portion  79   c  into a hole  29   c  (see  FIG. 5  also) in the camshaft holder  29 . Besides, the heights of the spring guides  78   a ,  79   a  from respective receiving surfaces of the movable side and fixed side spring holding portions  78 ,  79  are nearly equal in this embodiments, but they may be set to be different, taking into account the strength of the pressing spring  55  or the like. 
   When the pressing spring  55  is curved due to the swinging of the holder  60   e  ( 60   i ) of the exhaust link mechanism Mle (intake link mechanism Mli), the curvature of curving near the spring guide  78   a  of the movable side spring holding portion  78  moved relative to the fixed side spring holding portion  79  is greater than the curvature of curving near the spring guide  79   a  of the fixed side spring holding portion  79 . Therefore, the degree of tapering of the tapered portion  78   a   2  is set to be greater than that of the tapered portion  79   a   2 , and, in this embodiment, the apex angle of the cone determining the conical surface of the tapered portion  78   a   2  is set to be smaller. 
   In the condition where the first and second spring holding portions  76 ,  77  are closest to each other, the control spring  68  assumes a substantially straight hollow cylindrical shape (see  FIGS. 12 and 13 ), and, in the condition where the movable side and fixed side spring holding portions  78 ,  79  are closest to each other, the pressing spring  55  assumes a substantially straight hollow cylindrical shape (see  FIG. 14 ). 
   Referring to  FIGS. 2 ,  3  and  12 , the control mechanism M 3  comprises a hollow cylindrical control shaft  70  as a control member driven by the drive mechanism M 2 , and control links  71   i ,  71   e  for transmitting the motion of the control shaft  70  to the link mechanisms Mli, Mle to thereby swing the link mechanisms Mli, Mle about the camshaft  50 . 
   The control shaft  70  is movable in parallel to the cylinder axis L 1 , i.e., movable in parallel to the reference plane H 0  which includes the rotational center line L 2  and is parallel to the cylinder axis L 1 . 
   The control links  71   i ,  71   e  are constituted of the intake control link  71   i  and the exhaust control link  71   e . The intake control link  71   i  is pivotally supported on the control shaft  70  at a connection portion  71   i   1  serving as a first intake connection portion, and is pivotally supported on the connection portion  61   i   1  of the first plate  61   i  of the intake link mechanism Mli at a connection portion  71   i   2  serving as a second intake connection portion. The exhaust control link  71   e  is pivotally supported on the control shaft  70  at a connection portion  71   e   1  serving as a first exhaust connection portion, and is pivotally supported on the connection portion  61   e   1  of the first plate  61   e  of the exhaust link mechanism Mle at a connection portion  71   e   2  serving as a second exhaust connection portion. The connection portion  71   i   1  of the intake control link  71   i  and the connection portion  70   a  of the control shaft  70  each have a hole into which one connection pin  71   e   3  fixed by being press fitted into a hole in the connection portion  71   e   1  of the exhaust control link  71   e  is relatively rotatably inserted, and are pivotally supported on the connection pin  71   e   3 , whereas the bifurcated connection portions  71   i   2 ,  71   e   2  (see  FIG. 7(D) ) have holes into which connection pins  61   i   1   a ,  61   e   1   a  of the connection portions  71   i   2 ,  71   e   2  are relatively rotatably inserted, and they are pivotally supported on the connection pins  61   i   1   a ,  61   e   1   a , respectively. At the connection portions  71   e   1  ( 71   i   1 ),  70   a  provided with slight gap due to the pivotal supporting, the connection portion  71   e   1  ( 71   i   1 ) is normally pressed against the connection portion  70   a  by the spring force of the pressing spring, so that the influence of the gap (play) between the connection portion  71   e   1  ( 71   i   1 ) and the connection portion  70   a  is eliminated, and the motion of the control shaft  70  is accurately transmitted to the exhaust control link  71   e  (intake control link  71   i ). 
   Referring to  FIGS. 3 and 8 , the drive mechanism M  2  for driving the control shaft  70  comprises an electric motor  80  capable of reverse rotation and mounted to the head cover  13 , and a transmission mechanism M 4  for transmitting the rotation of the electric motor  80  to the control shaft  70 . The control mechanism M 3  and the drive mechanism M 2  are disposed on the opposite side of the cylinder  11  and the combustion chamber  16 , with respect to a second orthogonal plane H 2  which includes the rotational center line L 2  and is orthogonal to the reference plane H 0 . 
   The electric motor  80  comprises a hollow cylindrical main body  80   a  in which a heating portion such as a coil portion is contained and which has a center axis parallel to the cylinder axis L 1 , and an output shaft  80   b  extending in parallel to the cylinder axis L 1 . The electric motor  80  is disposed on the outer side in the radial direction of the valve chamber  25 , in relation to the cylinder head  12  and the head cover  13 . The power transmission chamber  59  and an inlet portion  85   a  (described later) are disposed on the left side of the first orthogonal plane H 1 , and the main body  80   a , the spark plug  19  and an outlet portion  85   b  (described later) are disposed on the right side, i.e. the other side, of the first orthogonal plane H 1 . In the main body  80   a , a mounted portion  80   a   1  to be connected to a mount portion  13   a  formed in an eaves-like shape on the head cover  13  to project in the radial direction is provided with a through-hole  80   a   2 , and the output shaft  80   b  penetrates through the through-hole  80   a   2  to project to the exterior of the main body  80   a  and extends into the valve chamber  25 . The main body  80   a  is disposed at such a position that the whole part thereof is covered by the mount portion, as viewed in the cylinder axis direction A 1  from the side of the head cover  13 , or as viewed from the front side of the head cover  13  (see  FIG. 8 ). 
   Referring to  FIGS. 9 and 10  also, the main body  80   a  of the electric motor  80  overlapping with the cylinder head  12  and the head cover  13  in the cylinder axis direction A 1  and disposed on the outer side relative to the cylinder head  12  and the head cover  13  in the radial direction and in the exterior of the valve chamber  25  is disposed at a position which is adjacent to a circumferential wall  13   b  of the head cover  13  in the radial direction and at which the running airflow having passed through a duct  85  formed between the valve chamber  25  and the combustion chamber  16  in the cylinder head  12  collides on the main body  80   a  as a cooling airflow. The duct  85  has the inlet portion  85   a  (see  FIG. 4  also) having an inlet  85   a   1  opened toward the front side of the motorcycle V so as to take in the running airflow, the outlet portion  85   b  at which the spark plug  19  is disposed and which is opened at such a position that the running airflow (cooling airflow) coming from the inlet portion  85   a  collides on the main body  80   a , and a central portion  85   c  formed by duct walls including a combustion chamber wall  16   a  for communication between the inlet portion  85   a  and the outlet portion  85   b  and a valve chamber wall  25   a  opposed to the combustion chamber wall  16   a  in the cylinder axis direction A 1 . 
   The inlet portion  85   a  projects toward the outer side in the radial direction and the lower side relative to the head cover  13 , and the inlet  85   a   1  is opposed to the running airflow. Of the duct  85 , the portion opposed to the outlet portion  85   b  with the first orthogonal plane H 1  therebetween is closed by a chamber wall  59   a  of the power transmission chamber  59  which constitutes the duct wall of the central portion  85   c . Between the inlet portion  85   a  and the central portion  85   c , a restriction portion  85   d  smaller in passage area than those on the inlet portion  85   a  side and on the central portion  85   c  side is formed by a passage wall of a return oil passage  86  for a lubricating oil having lubricated the valve system  40  and by a boss provided with an insertion hole  87  for a head bolt. In addition, the restriction portion  85   d  is so shaped as to cause the running airflow coming from the inlet portion  85   a  to flow toward a portion, near the main body  80   a , of the outlet portion  85   b.    
   Therefore, the running airflow entering via the inlet  85   a   1  at the time of running flows through the inlet portion  85   a  into the central portion  85   c , cools the combustion chamber wall  16   a  and the valve chamber wall  25   a , then flows toward the outlet portion  85   b , cools the spark plug  19  at the outlet portion  85   b , and flows out via the outlet portion  85   b . Apart of the running airflow having flowed out of the outlet portion  85   b  collides on the main body  80   a , thereby cooling the main body  80   a.    
   Referring to  FIGS. 2 ,  3  and  8 , in the valve chamber  25 , the transmission mechanism M 4  disposed between the camshaft holder  29  and the head cover  13  in the cylinder axis direction A 1  is composed of a speed reduction gear  81  meshed with a drive gear  80   b   1  formed on the output shaft  80   b  penetrating through the head cover  13  and extending into the valve chamber  25 , and an output gear  82  which is meshed with the speed reduction gear  81  and is rotatably supported on the cylinder head  12  through the camshaft holder  29 . The speed reduction gear  81  is rotatably supported on a support shaft  84  supported by the head cover  13  and a cover  83  for covering an opening  13   c  formed in the head cover  13 , and has a large gear  81   a  meshed with the drive gear  80   b   1 , and a small gear  81   b  meshed with the output gear  82 . The output gear  82  has a hollow cylindrical boss portion  82   a  which is rotatably supported, through a bearing  89 , on a holding tube  88  connected to the camshaft holder  29  by bolts. 
   The output gear  82  and the control shaft  70  are drive connected to each other through a feed screw mechanism serving as a motion conversion mechanism by which the rotational motion of the output gear  82  is converted into a rectilinear reciprocating motion, parallel to the cylinder axis L 1 , of the control shaft  70 . The feed screw mechanism comprises a female screw portion  82   b  composed of a trapezoidal screw formed in the inner circumferential surface of the boss portion  82   a , and a male screw portion  70   b  composed of a trapezoidal screw formed in the outer circumferential surface of the control shaft  70  and meshed with the female screw portion  82   b . The control shaft  70  is slidably fitted over the outer circumference of a guide shaft  90  fixed to the boss portion  82   a , and can be advanced and retracted relative to the camshaft  50  in the cylinder axis direction A 1  through a through-hole  91  (see  FIG. 5  also) formed in the camshaft holder  29 , while being guided in the moving direction by the guide shaft  90 . 
   Referring to  FIG. 3 , the electric motor  80  is controlled by an electronic control unit (hereinafter referred to as ECU)  92 . For this purpose, detection signals are inputted to the ECU  92  from operating condition detection means  93 , which is composed of starting detection means for detecting the starting time of the internal combustion engine E, load detection means for detecting the engine load, engine speed detection means for detecting the engine speed, and the like and which detects the operating conditions of the internal combustion engine E, and from swing position detection means  94  (composed, for example, of a potentiometer) for detecting the swing position, or the swing angle relative to the camshaft  50 , of the holder  60   e  of the exhaust link mechanism Mle swung by the electric motor  80 , hence of the exhaust cam  54 . 
   Therefore, when the position of the control shaft  70  driven by the electric motor  80  is changed, the swing position which is the rotation position of the exhaust link mechanism Mle (intake link mechanism Mli) and the exhaust cam  54  (intake cam  53 ) relative to the camshaft  50  is changed according to the operating conditions, so that the valve operation characteristics of the exhaust valve  23  (intake valve  22 ) are controlled according to the operating conditions of the internal combustion engine E by the valve characteristic varying mechanism M controlled by the ECU  92 . 
   Details of the above will be described below. 
   As shown in  FIG. 11 , the intake valve and the exhaust valve are respectively put into opening and closing operations with arbitrary intermediate valve operation characteristics between maximum valve operation characteristics Kimax, Kemax and minimum valve operation characteristics Kimin, Kemin, with the maximum valve operation characteristics Kimax, Kemax and the minimum valve operation characteristics Kimin, Kemin as boundary values of basic operation characteristics of valve operation characteristics Ki, Ke controlled by the valve characteristic varying mechanism M for changing the opening and closing timings and the maximum lift amounts. Therefore, regarding the intake valve  22 , as the opening timing is continuously retarded on an angle basis, the closing timing is continuously advanced on an angle basis to continuously shorten the valve opening period, further, the rotational angle of the camshaft  50  (or the crank angle as a rotational position of the crankshaft  15 ) for obtaining the maximum lift amount is continuously retarded on an angle basis, and the maximum lift amount is continuously reduced. Simultaneously with the changes in the valve operation characteristics of the intake valve  22 , regarding the exhaust valve  23 , as the opening timing is continuously retarded on an angle basis, the closing timing is continuously advanced to continuously shorten the valve opening period, further, the rotational angle of the camshaft  50  for obtaining the maximum lift amount is continuously advanced on an angle basis, and the maximum lift amount is continuously reduced. 
   Referring to  FIG. 12  also, when the control shaft  70  driven by the drive mechanism M 2  and the intake control link  71   i  occupy first positions shown in  FIGS. 12(A) ,  12 (B), the maximum valve operation characteristic Kimax is obtained such that the opening timing of the intake valve  22  is at a most advanced angle position θ iomax, the closing timing is at a most retarded angle position θ icmax, and the valve opening period and the maximum lift amount are both maximized; simultaneously, the maximum valve operation characteristic Kemax is obtained such that the opening timing of the exhaust valve  23  is at a most advanced angle position θ eomax, the closing timing is at a most retarded angle position θ ecmax, and the valve opening period and the maximum lift amount are both maximized. 
   Incidentally, in  FIGS. 12 and 13 , the conditions of the exhaust link mechanism Mle (intake link mechanism Mli) and the exhaust main rocker arm  42  (intake main rocker arm  41 ) at the time when the exhaust valve  23  (intake valve  22 ) is closed are indicated by solid lines and broken lines, whereas the general conditions of the exhaust link mechanism Mle (intake link mechanism Mli) and the exhaust main rocker arm  42  (intake main rocker arm  41 ) at the time when the exhaust valve  23  (intake valve  22 ) is opened at the maximum lift amount are indicated by two-dotted chain lines. 
   During transition from the condition where the maximum valve operation characteristics Kimax, Kemax are obtained by the valve characteristic varying mechanism M to the condition where the minimum valve operation characteristics Kimin, Kemin are obtained, according to the operating conditions of the internal combustion engine E, the electric motor  80  drives the output gear  72  to rotate, and the control shaft  70  is advanced toward the camshaft  50  by the feed screw mechanism. In this instance, based on the drive amount of the electric motor  80 , the control shaft  70  swings the intake link mechanism Mli and the intake cam  53  in the rotational direction R 1  about the camshaft  50  through the intake control link  71   i , and, simultaneously, swings the exhaust link mechanism Mle and the exhaust cam  54  in the counter-rotational direction R 2  about the camshaft  50  through the exhaust control link  71   e.    
   When the control shaft  70  and the exhaust control link  71   e  occupy second positions shown in  FIGS. 13(A) ,  13 (B), the minimum valve operation characteristic Kimax is obtained such that the opening timing of the intake valve  22  is at a most retarded angle position θ iomin, the closing timing is at a most advanced angle position θ icmin, and both the valve opening period and the maximum lift amount are minimized; simultaneously, the minimum valve operation characteristic Kemin is obtained such that the opening timing of the exhaust valve  23  is at a most retarded angle position θ eomin, the closing timing is at a most advanced angle position θ ecmin, and both the valve opening period and the maximum lift amount are minimized. 
   During transition of the control shaft  70  from the second position to the first position, the electric motor  80  drives the output gear  82  to rotate in the reverse direction, and the control shaft  70  is retracted away from the camshaft  50  by the feed screw mechanism. In this instance, the control shaft  70  swing the intake link mechanism Mli and the intake cam  53  in the counter-rotational direction R 2  about the camshaft  50  through the intake control link  71   i , and, simultaneously, swing the exhaust link mechanism Mle and the exhaust cam  54  in the rotational direction R 1  about the camshaft  50  through the exhaust control link  71   e.    
   In addition, when the control shaft  70  occupies a position between the first position and the second position, regarding the exhaust valve  23  (intake valve  22 ), innumerable intermediate valve characteristics are obtained such that the opening timing, the closing timing, the valve opening period and the maximum lift amount are set at values respectively between the opening timing, the closing timing, the valve opening period and the maximum lift amount at the maximum valve operation characteristic Kemax (Kimaxa) and those at the minimum valve operation characteristic Kemin (Kimin). 
   The intake valve and the exhaust valve are put into opening and closing operations with auxiliary operation characteristics, in addition to the above-mentioned basic operation characteristics, by the valve characteristic varying mechanism M. Specifically, the fact that decompression operation characteristics as the auxiliary operation characteristics can be obtained will be described referring to  FIGS. 14(A) ,  14 (B). During the compression stroke upon the starting of the internal combustion engine E, the electric motor  80  drives the output gear  82  to rotate in the reverse direction, and the control shaft  70  occupies a decompression position where it is retracted beyond the first position so as to be located away from the camshaft  50 . In this case, the exhaust link mechanism Mle (intake link mechanism Mli) and the exhaust cam  54  (intake cam  53 ) are swung in the rotational direction R 1  (counter-rotational direction R 2 ), the decompression cam  62   e   1  ( 62   i   1 ) of the second plate  62   e  ( 62   i ) makes contact with a decompression portion  42   d  ( 41   d ) provided in the vicinity of the roller  42   c  ( 41   c ) of the exhaust main rocker arm  42  (intake main rocker arm  41 ), the roller  42   c  ( 41   c ) parts from the exhaust cam  54  (intake cam  53 ), and the exhaust valve  23  (intake valve  22 ) is opened at a small decompression opening. 
   Now, the functions and effects of the embodiment configured as above will be described below. 
   The cylinder head  12  for forming the combustion chamber  16  and the valve chamber  25  is provided with the duct  85 , for guiding the running airflow, between the valve chamber  25  and the combustion chamber  16 , and the electric motor  80  is disposed at a position which is outside the valve chamber  25  and at which the running airflow having flowed through the duct  85  collides on the electric motor  80 . This configuration ensures that the running airflow is guided by the duct  85  to collide on the electric motor  80  as a cooling airflow, thereby cooling the electric actuator, so that good performance of cooling the electric motor  80  is secured. In addition, it is unnecessary to lay out the electric motor  80  at such a position that the running airflow collides directly on the electric motor  80 , while avoiding the cylinder head  12  and members disposed in the vicinity of the cylinder head  12 . The duct  85  can be formed to match the position of the electric motor  80 , so that the degree of freedom in laying out the electric motor  80  is enhanced. In addition, since the electric motor  80  disposed adjacent to the valve chamber  25  in the radial direction with respect to the cylinder axis L 1  can be laid out close to the cylinder head  12  and the head cover  13  in the radial direction, the electric motor  80  can be laid out at the cylinder head  12  and the head cover  13  in a compact form in the radial direction. Besides, it is possible to prevent the valve system  40  comprising the valve characteristic varying mechanism M having the electric motor  80  from being enlarged in size in the cylinder axis direction A 1  and, hence, to prevent the internal combustion engine E from being enlarged in size. Further, since the duct is formed between the combustion chamber  16  and the valve chamber  25 , the combustion chamber wall  16   a  is cooled by the running airflow passing through the duct  85 , and the heating of the valve chamber  25  by the heat transferred from the combustion chamber  16  is restrained, so that the performance of cooling the combustion chamber wall  16   a  is enhanced, and the valve chamber  25  is prevented from being heated to a high temperature. 
   Since the electric motor  80  comprises the output shaft  80   b  extending in parallel to the cylinder axis L 1 , the electric motor  80  can be laid out along the cylinder axis L 1 . Further, the electric motor  80  as a whole can be disposed closer to the cylinder axis L 1 , as compared with the case where the output shaft  80   b  extends in parallel to an orthogonal plane which is orthogonal to the cylinder axis L 1 . As a result, the electric motor  80  can be laid out at the cylinder head  12  in a compacter form in the radial direction. 
   In the cylinder head  12 , the power transmission chamber  59  and the inlet portion  85   a  are disposed on the left side of the first orthogonal plane H 1 , and the main body  80   a  of the electric motor  80 , the spark plug  19  and the outlet portion  85   b  are disposed on the right side of the first orthogonal plane H 1 , whereby the main body  80   a  and the power transmission chamber  59  occupying a comparatively large volume are disposed distributedly on both sides of the first orthogonal plane H 1 . In this point, also, the electric motor  80  is disposed at the cylinder head  12  and the head cover  13  in a compact form in the radial direction. 
   The electric motor  80  is mounted to the mount portion  13   a  formed on the head cover  13 , and the main body  80   a  of the electric motor  80  is disposed at such a position that the whole part thereof is covered by the mount portion  13   a , as viewed from the front side of the head cover  13 , whereby the electric motor  80  is shielded by the mount portion  13   a . Therefore, foreign matter such as a small stone kicked up by the front wheel  7  or the like during the running of the motorcycle V is prevented from colliding against the main body  80   a.    
   Of the duct  85 , the portion opposed to the outlet portion  85   b  with the first orthogonal plane H 1  therebetween is closed by the chamber wall  59   a  of the power transmission chamber  59  constituting the duct wall of the central portion  85   c , whereby it is ensured that the running airflow entering into the central portion  85   c  mostly flows toward the outlet portion  85   b , so that the spark plug  19  and the main body  80   a  are efficiently cooled by a large quantity of the running airflow. Between the inlet portion  85   a  and the central portion  85   c , the restriction portion  85   d  is formed in such a shape as to cause the running airflow coming from the inlet portion  85   a  to flow toward the portion, near the main body  80   a , of the outlet portion  85   b , whereby it is made easier for the running airflow to collide on the main body  80   a . In this point, also, the performance of cooling the main body  80   a  is enhanced. 
   Now, an embodiment obtained by partly changing the constitution of the above-described embodiment will be described below, in special regard of the modifications. 
   The internal combustion engine E may be a multi-cylinder internal combustion engine. Further, the internal combustion engine E may be an internal combustion engine in which one cylinder is provided with a plurality of intake valves and one or a plurality of exhaust valves, or may be an internal combustion engine in which one cylinder is provided with a plurality of exhaust valves and one or a plurality of intake valves. 
   The electric motor  80  may be mounted to the cylinder head  12 . The swing position detection means  94  may detect the swing position of the holder  60   i  of the intake link mechanism Mli.