Patent Publication Number: US-7913658-B2

Title: Valve actuating mechanism for an internal combustion engine, and cylinder head incorporating same

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit under 35 USC 119 of Japanese Application No. 2007-095090 filed Mar. 30, 2007, and the entire subject matter of this priority document, including specification, claims and drawings is incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a variable valve actuating mechanism for a four-stroke internal combustion engine, preferably applicable to a vehicle such as a motorcycle, in which the variable valve actuating mechanism includes a camshaft having a pair of cams for one engine valve, and can selectively use either one of the respective cams for performing an opening/closing operation of the engine valve, depending on engine operating conditions. 
     2. Description of the Background Art 
     Conventionally, it has been known to provide a valve actuating mechanism which includes a camshaft, a rocker arm shaft arranged in parallel with the camshaft, and a rocker arm supported on the rocker arm shaft such that the rocker arm is pivotally movable about an axis of the rocker arm shaft, and is also slidably axially movable of the rocker arm shaft. In response to the rotary motion of the camshaft, the rocker arm is brought into contact with either one of the respective cams, and is pivotally rocked to open or close the engine valve. At the same time, the rocker arm is suitably moved in the axial direction thus allowing the selective use of either one of the respective cams for performing opening/closing operation of the engine valve, as disclosed in Japanese published patent application JP-A-2001-20710. In this reference, although the rocker arm is moved in the axial direction using an engine oil pressure depending on an open/closed state of the engine valve, the camshaft pushes the engine valve downwardly by way of the rocker arm (valve open state) and hence, there arises a drawback that it is necessary to increase a force applied to move the rocker arm. Further, when an electric sensor, an electric control or the like is used for moving the rocker arm corresponding to an open/closed state of the engine valve, the arrangement of the valve actuating mechanism per se becomes complicated. 
     BRIEF SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide an engine valve actuating mechanism which can change a cam, for opening or closing an engine valve, by moving a rocker arm in the axial direction of a rocking shaft, wherein the valve actuating mechanism can move the rocker arm corresponding to an open/closed state of the engine valve with a relatively simple arrangement. 
     To overcome the above-mentioned drawbacks, an engine valve actuating mechanism according to a first illustrative embodiment hereof includes a camshaft that includes a pair of first and second cams for one engine valve, and a rocker arm which is supported on a rocker arm shaft arranged in parallel with the camshaft, such that the rocker arm is pivotally movable about an axis of the rocker arm shaft and is selectively axially movable of the rocker arm shaft, wherein the rocker arm is brought into contact with either one of the respective cams in response to the rotary driving of the camshaft, and is pivotally rocked to open or close the engine valve. The rocker arm is moved to either one of a first operation position, at which the rocker arm is brought into contact with the first cam in the axial direction, and a second operation position at which the rocker arm is brought into contact with the second cam in the axial direction, thus allowing the valve actuating mechanism to selectively use either one of the respective cams for performing actuation of the valve. 
     The improvement is characterized in that the valve actuating mechanism further includes a first rocker arm moving mechanism which moves the rocker arm from the first operation position side to the second operation position side, a second rocker arm moving mechanism which moves the rocker arm from the second operation position side to the first operation position side, and a trigger member which is supported on an engine structural body for restricting the movement of the rocker arm in the axial direction of the rocker arm in response to a rocking state of the rocker arm. The rocker arm and the trigger member respectively include a rocker-side key portion and a trigger-side key portion which are capable of being brought into contact with each other in the axial direction, and the respective key portions are configured to increase or decrease contact margins thereof in the axial direction in response to a rocking state of the rocker arm and eliminate the contact margin thereof when the engine valve is closed, and the rocker arm is moved to the corresponding operation position by either one of the respective rocker arm moving mechanisms. 
     The rocker-side key portion comprises a pair of first and second rocker-side key portions, and the trigger-side key portion comprises a pair of first and second trigger-side key portions corresponding to the respective rocker-side key portions, and one of the pairs of corresponding key portions of the respective rocker-side key portions and the respective trigger-side key portions eliminates the contact margin in the axial direction at the time of predetermined rocking of the rocker arm. The rocker arm is moved by a predetermined quantity to the corresponding operation position side due to either one of the respective rocker arm moving mechanism in response to the elimination of the contact margin. The respective key portions of one pair overlap by a predetermined quantity in the axial direction, wherein by continuously rocking the rocker arm in this state, the respective key portions of one pair operate the trigger portions so as to eliminate a contact margin of the respective key portions of another pair in the axial direction. Thus, the rocker arms are movable to the corresponding operation position. 
     The engine valve and the respective cams are provided in a pair for each one cylinder, and the rocker arm comprises a pair of first and second rocker arms which respectively correspond to the pair of engine valves and the respective cams and are pivotally movable relative to each other, and the first and second rocker-side key portions are respectively mounted on the first and second rocker arms. 
     Between the corresponding key portions of the respective rocker-side key portions and the respective trigger-side key portions, a predetermined gap is formed in the axial direction such that forces from the respective rocker arm moving mechanism are not communicated to the portions. 
     The trigger member is rockably supported on the engine structural body. Between the trigger member and the engine structural body is provided a resilient member which biases the trigger member toward a side at which the contact margin of the respective key portions in the axial direction is increased and, at the same time, a rocking restriction portion which restricts the rocking angle of the trigger member while also suppressing the deformation of the resilient member is provided. 
     By mechanically providing the restriction on the movement of the rocker arm and the removal of the restriction in response to the rocking state of the rocker arm, that is, in response to an open/closed state of the engine valve, it is possible to move the rocker arm. Particularly, the restriction on the movement of the rocker arm is removed when the engine valve is closed and hence, the force, which is imparted for moving the rocker arm, can be reduced. Further, an electric sensor, a control and the like for detecting the open/closed state of the engine valve become unnecessary and hence, it is possible to simplify the valve actuating mechanism per se. Here, the above-mentioned one engine valve may include a plurality of engine valves which corresponds to one cylinder and is operated simultaneously. 
     The pair of key portions mounted on the rocker arm and the pair of key portions mounted on the trigger member are respectively brought into contact with each other in the axial direction, and the restriction on the movement of the rocker arm brought about by the contact of the respective key portions is removed by the pair of respective key portions which overlap each other in the axial direction by a predetermined overlapping quantity and hence, no additional parts other than the rocker arm and the trigger member are necessary as parts for removing the restriction on the movement of the rocker arm attributed to the trigger member thus reducing the number of parts. 
     The opening/closing timings or the lift quantities of the respective engine valve can be set individually and, at the same time, the restrictions on the movement of the respective rocker arms can be removed individually in response to the rocking state of the respective rocker arms. 
     At the time of performing a usual operation of the engine to which forces are not imparted from the respective rocker arm moving mechanism, frictions generated between the respective key portions can be reduced and, at the same time, when the restriction on the movement of the rocker arms are removed, it is possible to ensure the moving quantity at the time of overlapping the respective key portions of one pair with a predetermined quantity in the axial direction. 
     In eliminating the contact margin of the respective key portions in the axial direction at the time of removing the restriction on the movement of the rocker arm, it is possible to reduce a load applied to the resilient member by reducing an excessive rocking angle of the trigger member thus contributing to the miniaturization, the reduction of weight and the reduction of cost of the resilient member. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a left side view, partially in cross-section, of a cylinder head of an engine according to an illustrative embodiment of the present invention. 
         FIG. 2  is a top plan detail view of an essential part of a variable valve adjustment mechanism of the engine at a time of low-speed operation thereof. 
         FIG. 3  is a top plan detail view of the essential part of the variable valve adjustment mechanism at a time of high-speed operation thereof. 
         FIG. 4(   a ) is a cross-sectional view taken along a line A-A in  FIG. 2 , and  FIG. 4(   b ) is a cross-sectional view taken along a line B-B in  FIG. 2 . 
         FIG. 5(   a ) is a cross-sectional view taken along a line C-C in  FIG. 3 , and  FIG. 5(   b ) is a cross-sectional view taken along a line D-D in  FIG. 3 . 
         FIG. 6(   a ) is a left side view of a trigger arm which is a component of the variable valve adjustment mechanism, and  FIG. 6(   b ) is a right side view of the trigger arm. 
         FIG. 7  is a left side view of left and right rocker arms of the variable valve adjustment mechanism, as viewed in an overlapped manner. 
         FIG. 8(   a ) is a left side view of a center collar of the variable valve adjustment mechanism, and  FIG. 8(   b ) is an exploded perspective view showing the center collar being assembled to the rocker arm shaft. 
         FIG. 9  is a top plan view corresponding to  FIG. 2  and showing the first manner of operation of the variable valve adjustment mechanism. 
         FIG. 10(   a ) is a cross-sectional view corresponding to  FIG. 4(   a ) and showing the first manner of operation of the variable valve adjustment mechanism, and  FIG. 10(   b ) is a cross-sectional view corresponding to  FIG. 4(   b ) and showing the first manner of operation of the variable valve adjustment mechanism. 
         FIG. 11(   a ) is a cross-sectional view corresponding to  FIG. 4(   a ) and showing the second manner of operation of the variable valve adjustment mechanism, and  FIG. 11(   b ) is a cross-sectional view corresponding to  FIG. 4(   b ) and showing the second manner of operation of the variable valve adjustment mechanism. 
         FIG. 12(   a ) is a top plan view corresponding to  FIG. 2  and showing the second manner of operation of the variable valve adjustment mechanism, and  FIG. 12(   b ) is a top plan view corresponding to  FIG. 2  and showing the third manner of operation of the variable valve adjustment mechanism. 
         FIG. 13(   a ) is a cross-sectional view corresponding to  FIG. 4(   a ) and showing the fourth manner of operation of the variable valve adjustment mechanism, and  FIG. 13(   b ) is a cross-sectional view corresponding to  FIG. 4(   b ) and showing the fourth manner of operation of the variable valve adjustment mechanism. 
         FIG. 14(   a ) is a top plan view corresponding to  FIG. 2  and showing the fourth manner of operation of the variable valve adjustment mechanism, and  FIG. 14(   b ) is a top plan view corresponding to  FIG. 2  and showing the fifth manner of operation of the variable valve adjustment mechanism. 
         FIG. 15  is a top plan view corresponding to  FIG. 3  and showing the sixth manner of operation of the variable valve adjustment mechanism. 
         FIG. 16(   a ) is a cross-sectional view corresponding to  FIG. 5(   a ) and showing the sixth manner of operation of the variable valve adjustment mechanism, and  FIG. 16(   b ) is a cross-sectional view corresponding to  FIG. 5(   b ) and showing the sixth manner of operation of the variable valve adjustment mechanism. 
         FIG. 17(   a ) is a cross-sectional view corresponding to  FIG. 5(   a ) and showing the seventh manner of operation of the variable valve adjustment mechanism, and  FIG. 17(   b ) is a cross-sectional view corresponding to  FIG. 5(   b ) and showing the seventh manner of operation of the variable valve adjustment mechanism. 
         FIG. 18(   a ) is a top plan view corresponding to  FIG. 3  and showing the seventh manner of operation of the variable valve adjustment mechanism, and  FIG. 18(   b ) is a top plan view corresponding to  FIG. 3  and showing the eighth manner of operation of the variable valve adjustment mechanism. 
         FIG. 19(   a ) is a cross-sectional view corresponding to  FIG. 5(   a ) and showing the ninth manner of operation of the variable valve adjustment mechanism, and  FIG. 19(   b ) is a cross-sectional view corresponding to  FIG. 5(   b ) and showing the fifth manner of operation of the variable valve adjustment mechanism. 
         FIG. 20(   a ) is a top plan view corresponding to  FIG. 3  and showing the ninth manner of operation of the variable valve adjustment mechanism, and  FIG. 20(   b ) is a top plan view corresponding to  FIG. 3  and showing the tenth manner of operation of the variable valve adjustment mechanism. 
         FIG. 21  is a left side view corresponding to  FIG. 1  and showing a shaft drive mechanism of the variable valve adjustment mechanism. 
         FIG. 22  is a rear surface view of an essential part of the shaft drive mechanism; and 
         FIG. 23  is an exploded perspective view of the rocker arm shaft. 
     
    
    
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
     Hereinafter, an embodiment of the present invention is explained in conjunction with the drawings. Here, in the explanation made hereinafter, an arrow FR indicates the front direction, an arrow LH indicates the leftward direction, and an arrow UP indicates the upward direction in the drawings respectively. 
       FIG. 1  is a left side view of a cylinder head  2  of a 4-stroke DOHC parallel 4-cylinder engine  1 , which is used as a prime mover of a vehicle such as a motorcycle, for example. A head cover  3  is mounted on an upper portion of the cylinder head  2 . A valve chamber  4  is formed between the cylinder head  2  and the head cover  3 , and a valve actuation system  5  for driving intake and exhaust valves  6 ,  7  is housed inside of the valve chamber  4 . Here, the symbol C 1  in the drawing indicates a center axis (cylinder axis) of a cylinder bore of a cylinder body. 
     Intake and exhaust ports  8 ,  9  are formed in the cylinder head  2  for each cylinder, and combustion-chamber openings of the intake and exhaust ports  8 ,  9  are respectively opened and closed by the intake and exhaust valves  6 ,  7 . The respective intake and exhaust valves  6 ,  7  are configured such that rod-like stems  6   b ,  7   b  thereof extend from umbrella-shaped valve elements  6   a ,  7   a  which are fitted in the combustion chamber openings, with machined faces thereof oriented toward the valve chamber  4  side. The valve stems  6   b ,  7   b  are held in place in the cylinder head  2  by way of cylindrical valve guides  6   c ,  7   c , which allow the valves to move therein in a slidable reciprocating manner. 
     Retainers  6   d ,  7   d  are mounted on distal end portions of the stems  6   b ,  7   b  of the respective valves  6 ,  7 . The respective valves  6 ,  7  are biased upwardly due to a biasing force of valve springs  6   e ,  7   e  which are arranged between the retainers  6   d ,  7   d  and the cylinder head  2 , and the valve elements  6   a ,  7   a  close the combustion chamber-side openings. 
     On the other hand, the valve elements  6   a ,  7   a  of the respective valves  6 ,  7  are periodically moved away from the combustion chamber-side openings to open the combustion chamber-side openings by allowing the respective valves  6 ,  7  to perform a stroke downwardly against the biasing force of the valve springs  6   e ,  7   e  when the respective associated camshafts  11 ,  12  cause such downward movement of the valves. 
     The respective valves  6 ,  7  are arranged in the cylinder head  2  such that the stems  6   b ,  7   b  thereof are inclined in a V-shaped manner with respect to a cylinder axis C 1 , as viewed in a side view. An intake-side camshaft  11  and an exhaust-side camshaft  12  which extend along the lateral direction are respectively arranged above the respective stems  6   b ,  7   b . The respective camshafts  11 ,  12  are supported on the cylinder head  2  (including a shaft holder  2   a ) in a manner so as to be rotatable about the respective axes thereof. 
     When the engine  1  is driven, for example, the camshafts  11 ,  12  are rotated in an interlocking manner with a crankshaft, by way of a chain-type power transmission mechanism (neither the power transmission mechanism nor the crankshaft are shown in the drawing). Here, in the drawing, symbols C 2 , C 3  indicate center axes (cam axes) of the respective camshafts  11 ,  12 . In the depicted embodiment, the engine  1  is of a 4-valve type, and includes a pair of left and right intake and exhaust valves  6 ,  7  for every cylinder thereof. 
     The respective intake valves  6  are opened and closed by being pushed by a cam  11 A of the intake-side camshaft  11  by way of a respective rocker arm  13  provided for each cylinder. The rocker arm  13  is supported on a rocker arm shaft  14 , which is arranged in parallel to the intake-side camshaft  11  behind a distal end portion of the stem  6   b  of the intake valve  6  in a pivotally movable manner about an axis of the rocker arm shaft  14 . 
     On the other hand, the respective exhaust valves  7  are opened and closed by being pushed directly by a cam  12 A of the exhaust-side camshaft  12 , by way of a valve lifter  7   f , which is mounted on a distal end portion of the stem  7   b . Here, symbol C 4  in the drawing indicates a center axis (rocker axis) of the rocker arm shaft  14 . 
     An arm portion  13   b  of the rocker arm  13  extends from a cylindrical proximal portion  13   a , to contact a distal end portion of the stem  6   b  of the intake valve  6 . The cylindrical proximal portion  13   a  is affixed to the rocker arm shaft  14  of the rocker arm  13 . The cam  11 A of the intake-side camshaft  11  is in a slidable contact with a cam slide-contact portion  13   c  of the rocker arm  13 , and this slide-contact portion  13   c  is mounted on an upper portion of the distal end portion of the arm portion  13   b . At the same time, a valve pushing portion  13   d  of the rocker arm  13 , which can push the distal end portion of the stem  6   b  downwardly, is mounted on a lower portion of the distal end portion of the arm portion  13   b , opposite the cam slide-contact portion  13   c.    
     Further, when the intake-side camshaft  11  is rotatably driven during operation of the engine  1 , the cam  11 A of the camshaft  11  is brought into sliding contact with the cam slide-contact portion  13   c , thus suitably rocking the rocker arm  13 . Accordingly, the valve pushing portion  13   d  of the rocker arm  13  pushes the distal end portion of the stem  6   b  of the intake valve  6  downwardly, so that the intake valve  6  is suitably reciprocated along the stem  6   b  to open or close the combustion-chamber-side opening. Here, the rocker arm  13  may include a cam roller (not shown), which is brought into rolling contact with the cam  11 A of the intake-side camshaft  11 . 
     Further according to the depicted embodiment, the valve actuation system  5  includes a variable valve adjustment mechanism  5   a , which changes valve opening/closing timings and lift quantities of the respective intake valves  6 . For example, in a low-rotary speed range in which an engine rotary speed is less than 6000 rpm (revolution per minute), the variable valve adjustment mechanism  5   a  opens and closes the respective intake valves  6  using a low-speed cam lobe of the intake-side camshaft  11 . Alternatively, in a high-speed rotation range in which the engine rotary speed is 6000 rpm or more, the variable valve adjustment mechanism  5   a  opens and closes the respective intake valves  6  using a high-speed cam lobe of the intake-side camshaft  11 . 
     Hereinafter, the variable valve adjustment mechanism  5   a  corresponding to one cylinder is explained. In the explanation made hereinafter, however, assuming that other cylinders also have the substantially same variable valve adjustment mechanism as in the one described cylinder, repeated explanation is omitted. 
     To explain the variable valve adjustment mechanism  5   a  also in conjunction with  FIG. 2 , the cam  11 A of the intake-side camshaft  11  comprises the left and right first cams  15   a ,  16   a  for low-speed rotation range, and the left and right second cams  15   b ,  16   b  for high-speed rotation range, which correspond to the left and right intake valves  6 . That is, the intake-side camshaft  11  includes four cams in total consisting of the left and right first cams  15   a ,  16   a  and the left and right second cams  15   b ,  16   b , which respectively correspond to the left and right intake valves  6  for one cylinder. 
     Hereinafter, a pair of the first cams and a pair of second cams which respectively correspond to the left and right intake valves  6  are grouped to as left and right cam pairs  15 A,  16 A, where the first cam pair  15 A includes a first set of low and high-speed cams  15   a ,  15   b  and the second cam pair  16 A includes the second set of low and high-speed cams  16   a ,  16   b . The left and right cam pairs  15 A,  16 A are arranged at positions in substantially left-and-right symmetry with respect to the cylinder axis C 1 , which is sandwiched between the pairs. The left and right cam pairs  15 A,  16 A are spaced apart from each other, with a predetermined distance therebetween in the cam axis direction. Further, the left and right cam pairs  15 A,  16 A respectively arrange the first cams  15   a ,  16   a  close to each other in the cam axis direction, and also arrange the second cams  15   b ,  16   b  close to each other in the cam axis direction, such that the first cams  15   a ,  16   a  are arranged on a left side and the second cams  15   b ,  16   b  are arranged on a right side, respectively. 
     Further, the rocker arm  13  is supported on the rocker arm shaft  14  so as to be pivotally movable about an axis (about a rocker axis C 4 ) of the rocker arm shaft  14 , and the rocker arm is also movable sideways in the axial direction (in the direction along the rocker axis C 4 ) of the rocker arm shaft  14 . Further, the rocker arm  13  is divided into left and right rocker arms  17 ,  18  which are independently movable relative to each other (pivotally movable relative to each other about the axis and movable relative to each other in the axial direction). 
     The left and right rocker arms  17 ,  18  are respectively provided corresponding to the left and right intake valves  6 . In addition, the left and right rocker arms  17 ,  18  are separately and individually rocked by the left and right first (low-speed) cams  15   a ,  16   a  or by the second (high-speed) cams  15   b ,  16   b , thus opening and closing the left and right intake valves  6 . 
     Hereinafter, proximal portions of the left and right rocker arms  17 ,  18  near the shaft  14  are respectively indicated by numerals  17   a ,  18   a , while outwardly-extending arm portions of the left and right rocker arms  17 ,  18 , are respectively indicated by numerals  17   b ,  18   b , and these arm portions are spaced away from the shaft  14 . Cam slide-contact portions of the left and right rocker arms  17 ,  18  are respectively indicated by numerals  17   c ,  18   c , while valve-pushing portions of the left and right rocker arms  17 ,  18  are respectively indicated by numerals  17   d ,  18   d . Here, as seen in  FIG. 2 , the left and right arm portions  17   b ,  18   b , the cam slide-contact portions  17   c ,  18   c , and the valve pushing portions  17   d ,  18   d  are respectively offset in lateral outward directions of the cylinder, relative to the left and right proximal portions  17   a ,  18   a.    
     To explain the above-mentioned arrangement also in conjunction with  FIG. 4 , the first cams  15   a ,  16   a  and the second cams  15   b ,  16   b  include both evenly spaced zero-lift surfaces F 1  extending part way therearound, and hill-shaped lift surfaces (cam lobes) F 2 , which project radially outwardly from the level of the zero-lift surfaces F 1 , at predetermined rotary positions of the cams. The zero-lift surfaces F 1  use the cam axis C 2  as the center thereof, and have the same diameter throughout. When the zero-lift surfaces F 1  of the respective cams  15   a ,  16   a ,  15   b ,  16   b  face the cam slide-contact portions  17   c ,  18   c  of the left and right rocker arms  17 ,  18  in an opposed manner, the intake valves  6  remain completely closed (a lift quantity assumes 0). 
     In contrast, when the lift surfaces (cam lobes) F 2  are brought into sliding contact with the cam slide-contact portions  17   c ,  18   c  of the rocker arms  13 , the intake valves  6  are opened a predetermined amount (the intake valves  6  being moved a predetermined distance). 
     Projection quantities (lift quantities) of the lift surfaces F 2  of the first, low-speed cams  15   a ,  16   a  are set smaller than projection quantities (lift quantities) of the second, high-speed cams  15   b ,  16   b . Further, projection quantities and shapes of the lift surfaces F 2  of the second cams  15   b ,  16   b  of the left and right cam pairs  15 A,  16 A are set equal to each other. On the other hand, the projection quantity of the lift surface F 2  of the first cam  16   a  out of the right cam pair  16 A is set smaller than the projection quantity of the lift surface F 2  of the first cam  15   a  out of the left cam pair  15 A, for example. 
     Accordingly, an intake-air speed in the low-speed rotation range of the engine  1  is increased and, at the same time, the difference in intake-air quantity at the time of changing over the cams is increased. Accordingly, it is possible to emphasize the change of the intake-air characteristic. Here, the lift quantity of the first cam  16   a  of the right cam pair  16 A may be set to zero, if desired, or the projection quantities of the lift surfaces F 2  of the first cams  15   a ,  16   a  may be set equal to each other. 
     Under certain conditions to be described later herein, first and second rocker arm moving mechanisms  21 ,  22  (described later) slidably move the left and/or right rocker arms  17 ,  18  axially inwardly in a lateral direction of the cylinder. The rocker arms  17 ,  18  are integrally movably supported on the rocker arm shaft  14 , and are individually axially movable on the rocker arm shaft  14  such that the proximal portions  17   a ,  18   a  of the left and right rocker arms  17 ,  18  move in an axial direction of the rocker arm shaft  14 . The proximal portions  17   a ,  18   a  may be moved inwardly until they contact side edges of an enlarged center collar  37  formed integrally on the rocker arm shaft. 
     When the operation of the engine  1  is stopped, or when the engine  1  is operated in a low-speed rotation range, the left and right rocker arms  17 ,  18  assume leftward-movement limit positions in the axial direction. In such a state, the cam slide-contact portions  17   c ,  18   c  thereof are arranged at positions below the first cams  15   a ,  16   a  of the respective left and right cam pairs  15 A,  16 A where the slide-contact portions  17   c ,  18   c  can be brought into slide contact with outer peripheral surfaces (cam surfaces) of the first cams  15   a ,  16   a.    
     The valve pushing portions  17   d ,  18   d  of the left and right rocker arms  17 ,  18  are formed with lateral widths thereof set wider than lateral widths of the cam slide-contact portions  17   c ,  18   c . When the left and right rocker arms  17 ,  18  assume the above-mentioned leftward-movement limit positions, right end portions of the left and right rocker arms  17 ,  18  are arranged at positions where they can push the distal end portions of the stems  6   b  of the left and right intake valves  6 , respectively. Here, the positions of the left and right rocker arms  17 ,  18  in the axial orientation shown in  FIG. 2 , close to the left and right first (low-speed) cams  15   a ,  16   a , are referred to as first operation positions. 
     On the other hand, to explain the above-mentioned arrangement also in conjunction with  FIG. 3 , the left and right rocker arms  17 ,  18  move into the axial rightward-movement limit positions in the axial direction when the engine  1  is operated in a high-speed rotation range. In such a state, the cam slide-contact portions  17   c ,  18   c  of the left and right rocker arms  17 ,  18  are respectively arranged at positions below the second cams  15   b ,  16   b  of the left and right cam pairs  15 A,  16 A where the cam slide-contact portions  17   c ,  18   c  can be brought into slide contact with the outer peripheral surfaces (cam surfaces) of the second cams  15   b ,  16   b.    
     When the left and right rocker arms  17 ,  18  assume the above-mentioned rightward-movement limit positions, left end portions of the valve pushing portions  17   d ,  18   d  of the left and right rocker arms  17 ,  18  are arranged at positions where the left end portions can push the distal end portions of the stems  6   b  of the left and right intake valves  6 . Here, positions of the left and right rocker arms  17 ,  18  in the axial direction shown in  FIG. 3 , close to the left and right second (high-speed) cams  15   a ,  16   a , are referred to as second operation positions. 
     That is, in response to the engine rotary speed, the variable valve adjustment mechanism  5   a  operates the first and second rocker arm moving mechanisms  21 ,  22  to axially move the left and right rocker arms  17 ,  18  on the rocker arm shaft  14  to either the first operation position or the second operation position. Accordingly, the variable valve adjustment mechanism  5   a  allows for the selective use of either the first, low-speed cam set  15   a ,  16   a , or the second, high-speed cam set  15   b ,  16   b  in the operation of the left and right intake valves  6 . 
     Operation of the first and second rocker arm moving mechanisms  21 ,  22  will now be described. The first rocker arm moving mechanism  21  includes a first spring  23 , which is positioned on a left side of the proximal portion  17   a  of the left rocker arm  17 . The first spring  23  applies a biasing force to the proximal portion  17   a  directed towards the second (high-speed) operation position side. The first rocker arm moving mechanism  21  also includes a first spring-receiving collar  25 , which is positioned on a left side of the first spring  23  and is affixed to an outer periphery of the rocker arm shaft  14 , in a manner such that the first spring-receiving collar  25  is not axially movable relative to the rocker arm shaft  14 . 
     In a similar manner, the second rocker arm moving mechanism  22  includes a second spring  24 , which is positioned on a right side of the proximal portion  18   a  of the right rocker arm  18 , and which applies a biasing force to the proximal portion  18   a  directed towards the first (low speed) operation position side. The second rocker arm moving mechanism  22  also includes a second spring-receiving collar  26 , which is positioned on a right side of the second spring  24 , and which is affixed to the outer periphery of the rocker arm shaft  14 , in a manner such that the second spring-receiving collar  26  is not axially movable relative to the rocker arm shaft  14 . 
     Each of the respective springs  23 ,  24  is formed as a compression coil spring, which is arranged to wrap around the outer periphery of the rocker arm shaft  14  (to allow the rocker arm shaft  14  to penetrate the springs  23 ,  24 ). A right end portion of the first spring  23  is fitted in a left-side outer periphery of the proximal portion  17   a  of the left rocker arm  17 , and a left end portion of the first spring  23  is fitted in a right-side inner periphery of the first spring-receiving collar  25 . On the other hand, a left end portion of the second spring  24  is fitted in a right-side outer periphery of the proximal portion  18   a  of the right rocker arm  18 , and a right end portion of the second spring  24  is fitted in a left-side inner periphery of the second spring-receiving collar  26 . 
     Here, the rocker arm shaft  14  is supported on the cylinder head  2  such that the rocker arm shaft  14  is axially movable thereof and is also pivotally movable about an axis thereof. 
     When the operation of the engine  1  is stopped, or when the engine  1  is operated in a low-speed range, the rocker arm shaft  14  and the respective spring-receiving collars  25 ,  26  assume the leftward-movement limit position in the axial direction thereof (see  FIG. 2 ). Here, the left and right rocker arms  17 ,  18  assume the first operation position, and the respective springs  23 ,  24  are arranged between the proximal portions  17   a ,  18   a  of the left and right rocker arms  17 ,  18  and the spring-receiving collars  25 ,  26  in a compressed manner, such that a predetermined initial compression is applied to the springs  23 ,  24 . Here, initial loads which the respective springs  23 ,  24  possess are set equal to each other and hence, the left and right rocker arms  17 ,  18  can be held at the first operation position, as shown. 
     On the other hand, to explain the above-mentioned arrangement in conjunction with  FIG. 3 , when the engine  1  is operated in a high-speed range, the rocker arm shaft  14  and the respective spring-receiving collars  25 ,  26  assume the rightward-movement limit position in the axial direction thereof. Here, the left and right rocker arms  17 ,  18  assume the second operation position, and the respective springs  23 ,  24  are arranged between the proximal portions  17   a ,  18   a  of the left and right rocker arms  17 ,  18  and the respective spring-receiving collars  25 ,  26  in a contracted or compressed manner, such that an initial compression is applied to the springs  23 ,  24  in the substantially same manner as described above. Here, initial loads which the respective springs  23 ,  24  possess are set equal to each other and hence, the left and right rocker arms  17 ,  18  can be held at the second operation position. 
     Here, the movement quantities of the rocker arm shaft  14  and the respective spring-receiving collars  25 ,  26  in the axial direction are equal to the movement quantities of the left and right rocker arms  17 ,  18  in the axial direction (a movement quantity between the respective operation positions). 
     Further, by integrally moving the rocker arm shaft  14  and the respective spring-receiving collars  25 ,  26  in the axial direction with respect to the cylinder head  2 , a predetermined difference is generated in resilient force between the respective springs  23 ,  24 , such that the movements of the left and right rocker arms  17 ,  18  in the axial direction with respect to the cylinder head  2  are restricted by a rocker arm movement-restricting mechanism  31  (described later). 
     To be more specific, when the rocker arm shaft  14  and the respective spring-receiving collars  25 ,  26  are moved from the leftward-movement limit position to the rightward-movement limit position with respect to the cylinder head  2 , the first spring  23  is compressed by a quantity corresponding to the movement of the rocker arm shaft  14  and the respective spring-receiving collars  25 ,  26 , thus increasing the resilient force of the first spring  23 . At the same time, the second spring  24  is expanded by the quantity corresponding to the movement of the rocker arm shaft  14  and the respective spring-receiving collars  25 ,  26 , thus decreasing the resilient force of the second spring  24 . 
     On the other hand, when the rocker arm shaft  14  and the respective spring-receiving collars  25 ,  26  are moved from the rightward-movement limit position to the leftward-movement limit position with respect to the cylinder head  2 , the second spring  24  is compressed by a quantity corresponding to the movement of the rocker arm shaft  14  and the respective spring-receiving collars  25 ,  26  thus increasing the resilient force of the second spring  24  and, at the same time, the first spring  23  is expanded by a quantity corresponding to the movement of the rocker arm shaft  14  and the respective spring-receiving collars  25 ,  26  thus decreasing the resilient force of the first spring  23 . 
     In this manner, by making use of the difference in resilient force between the respective springs (hereinafter, referred to as the resilient force stored in either one of the respective springs  23 ,  24 ), the left and right rocker arms  17 ,  18  are moved from one operation position to the other operation position. Here, the initial compression quantities of the respective springs  23 ,  24  are used as the expanding quantities of the respective springs  23 ,  24 . 
     The rocker arm movement-restricting mechanism  31  is provided for restricting movement of the left and right rocker arms  17 ,  18  in the axial direction until a predetermined resilient force is stored in either one of the respective springs  23 ,  24 . The rocker arm movement-restricting mechanism  31  includes a trigger arm  33 , which is supported on the cylinder head  2  by way of a support shaft  32  arranged parallel to the rocker arm shaft  14 . The trigger arm  33  is pivotally movable about an axis of the support shaft  32 , but is not axially movable on the support shaft  32 . 
     By way of contrast, the center collar  37  is fixedly attached to the rocker arm shaft  14  between the proximal portions  17   a ,  18   a  of the left and right rocker arms  17 ,  18 , such that the center collar  37  is not relatively rotatable about the axis of the rocker arm shaft  14 , and is relatively axially movable of the rocker arm shaft  14 . 
     The trigger arm  33  is arranged behind the rocker arm shaft  14 , and the trigger arm  33  is arranged in left and right symmetry with respect to the cylinder axis C 1 , for example. The support shaft  32  of the trigger arm  33  is arranged at an oblique rear upper position with respect to the rocker arm shaft  14 , and an arm portion  33   b  having a U-shape in cross section which includes left and right wall portions  34 ,  35  and a rear wall portion  36  extends downwardly from a proximal portion  33   a  of the trigger arm  33  which receives the support shaft  32  through a bore formed therein. 
     To explain the above-mentioned arrangement also in conjunction with  FIG. 6 , left and right notched portions  34   a ,  35   a  which open toward a front side while having shapes different from each other as viewed in a side view are formed in the left and right wall portions  34 ,  35  of the arm portion  33   b  of the trigger arm  33 . To be more specific, the left notched portion  34   a  is formed in a semicircular shape, which spans between a lower side of the proximal portion  33   a  and a distal end side of the arm portion  33   b  as viewed in a side view. 
     On the other hand, the right notched portion  35   a  is configured such that a lower portion thereof is formed in a semicircular shape having a diameter smaller than the left notched portion  34   a , as viewed in a side view, and an upper portion thereof is formed in a mountain shape which projects rearwardly from the left notched portion  34   a  as viewed in a side view. The lower portion and the upper portion of the right notched portion  35   a  overlap each other by a predetermined quantity in the vertical direction. Hereinafter, the left and right wall portions  34 ,  35  of the trigger arm  33  are respectively referred to as left and right trigger-side key portions  34 ,  35 . 
     On a rear side of the proximal portion  33   a  of the trigger arm  33 , a substantially horizontal plate-shaped stopper portion  33   c  is formed, which extends rearwardly. To explain the above-mentioned arrangement in conjunction with  FIG. 1 , the stopper portion  33   c  receives a resilient force of a spring (compression coil spring)  33   d  which is arranged between the cylinder head  2  and the stopper  33   c  in a compressed manner from above and, at the same time, brings a lower surface thereof into contact with an upper surface of a stopper receiving portion  33   e  of the cylinder head  2  and as a result, the pivotal movement (rocking) of the trigger arm  33  in the clockwise direction (CW) as seen in  FIG. 1 ,  FIG. 4  and other drawings is restricted. 
     Here, the trigger arm  33  is biased in the clockwise direction in  FIG. 1 ,  FIG. 4  and other drawings due to the spring  33   d , and the trigger arm  33  is held such that the arm portion  33   b  is arranged close to the rocker arm shaft  14  from behind the rocker arm shaft  14 . This state of the trigger arm  33  is referred to as a pre-rocking state of the trigger arm  33 . 
     The rocking restriction portion  33   e  of the cylinder head  2  is formed on a portion of an inner wall surface of the cylinder head, and is arranged behind the arm portion  33   b  of the trigger arm  33 . The rocking restriction portion  33   e  can be brought into contact with a rear surface of the trigger arm  33  when the trigger arm  33  is rotated in the counter-clockwise direction (CCW) in  FIG. 1 ,  FIG. 4  and other drawings. Due to such an arrangement, a rocking angle of the trigger arm  33  when the trigger arm  33  is rocked against the biasing force of the spring  33   d  can be restricted. Here, the rocking restriction portion may be situated behind a rear surface of the trigger arm  33 . 
     As shown in  FIG. 2 ,  FIG. 4  and  FIG. 7 , left and right rocker-side key portions  38 ,  39  are formed on rear sides of the proximal portions  17   a ,  18   a  of the left and right rocker arms  17 ,  18 . These left and right rocker-side key portions  38 ,  39  project rearwardly while having shapes different from each other as viewed in a side view. To be more specific, the left rocker-side key portion  38  is formed on a rear side of the right end portion of the left proximal portion  17   a  in a mountain shape as viewed in a side view. Further, the left rocker-side key portion  38  is formed in a wall-shape orthogonal to the lateral direction, and a lower portion of the left rocker-side key portion  38  is formed into an arcuate shape, which is brought into contact with a tangential line extending toward a lower end of the proximal portion  17   a  as viewed in a side view. 
     On the other hand, the right rocker-side key portion  39  is formed on a rear side of a left end portion of the right proximal portion  18   a  in a substantially trapezoidal shape as viewed in a side view. Further, the right rocker-side key portion  39  is formed in a wall-shape orthogonal to the lateral direction, and a rear portion of the right rocker-side key portion  39  is formed into an arcuate shape substantially coaxial with the rocker arm shaft  14  as viewed in a side view. 
     When the left and right rocker arms  17 ,  18  are arranged at the first operation position, the left-rocker-side key portion  38  is arranged adjacent to a left side of the left trigger-side key portion  34  of the trigger arm  33  (see  FIG. 2 ), while when the left and right rocker arms  17 ,  18  are arranged at the second operation position, the left rocker-side key portion  38  is arranged adjacent to a right side of the left trigger-side key portion  34  (see  FIG. 3 ). When the trigger arm  33  is in the pre-rocking state, the left trigger-side key portion  34  of the trigger arm  33  overlaps the left rocker-side key portion  38  by a predetermined quantity, as viewed in the axial direction. 
     On the other hand, when the left and right rocker arms  17 ,  18  are arranged at the first operation position, the right rocker-side key portion  39  is arranged adjacent to a left side of the right trigger-side key portion  35  of the trigger arm  33  (see  FIG. 2 ). In contrast, when the left and right rocker arms  17 ,  18  are arranged at the second operation position, the right rocker-side key portion  39  is arranged adjacent to a right side of the right trigger-side key portion  35  (see  FIG. 3 ). When the trigger arm  33  is in the pre-rocking state, the right trigger-side key portion  35  of the trigger arm  33  overlaps the right rocker-side key portion  39  by a predetermined quantity as viewed in the axial direction. 
     A predetermined clearance is defined in the axial direction between the left and right rocker-side key portions  38 ,  39  and the left and right trigger-side key portions  34 ,  35  which are respectively arranged adjacent to each other, such that the forces from the respective rocker arm moving mechanisms  21 ,  22  are not applied to the left and right rocker arms  17 ,  18  (a state that a predetermined initial compression is applied to the respective springs  23 ,  24 , in other words, a state that the forces which are applied to the left and right rocker arms  17 ,  18  from the respective springs  23 ,  24  are equal to each other) (see  FIG. 2 ,  FIG. 3 ). 
     As shown in  FIGS. 8A-8B , the center collar  37  is formed substantially in a ring shape, having a diameter substantially equal to diameters of the proximal portions  17   a ,  18   a  of the left and right rocker arms  17 ,  18 . The center collar  37  has a central bore  37   c  formed axially therethrough to slidably receive the rocker arm shaft  14  therein. The center collar  37  also includes a center cam portion  37   a , formed on a rear outer side of an upper portion of the center collar  37 . The center cam portion  37   a  extends rearwardly along a substantially horizontal tangential line. The center collar  37  also has a radial through hole  37   b  formed therein, which extends radially outwardly from the central bore  37   c  through the center collar  37  in opposite directions, as shown in  FIG. 8A . 
     In addition, a central slot  14   a  is formed in the rocker arm shaft  14  at a predetermined position, and this central slot  14   a  extends through the rocker arm shaft  14  in the radial direction while extending a predetermined length in the axial direction, as shown. 
     The center collar  37  is mounted on the rocker arm shaft  14  at the predetermined position, and these parts are assembled to each other by way of an engaging pin  37   d  which penetrates the through hole  37   b  and the central slot  14   a . Accordingly, the center collar  37  is supported on the rocker arm shaft  14  at the predetermined position such that the center collar  37  is not relatively rotatable about the axis of the rocker arm shaft  14 , but is relatively axially movable on the rocker arm shaft  14  by a quantity corresponding to the length of the central slot  14   a.    
     To explain the above-mentioned arrangement in conjunction with  FIG. 2  and  FIG. 4(   a ), when the left and right rocker arms  17 ,  18  are arranged at the first operation position, the center cam portion  37   a  is arranged inside of the notched portion  34   a  of the left trigger-side key portion  34  of the trigger arm  33 , and a distal end portion of the center cam portion  37   a  is arranged close to an upper inner peripheral surface of the left notched portion  34   a . On the other hand, to explain the above-mentioned arrangement in conjunction with  FIG. 3  and  FIG. 5 , when the left and right rocker arms  17 ,  18  are arranged at the second operation position, the center cam portion  37   a  is arranged in the inside of the notched portion  35   a  of the right trigger-side key portion  35  of the trigger arm  33 , and a distal end portion of the center cam portion  37   a  is arranged close to an upper inner peripheral surface of the right notched portion  35   a.    
     Here, the rocker arm shaft  14  is moved in the axial direction thereof with respect to the cylinder head  2  due to an operation of a shaft driving mechanism  41  described later, and at the same time, the rocker arm shaft  14  is also rotatable about the axis thereof. To be more specific, when the rocker arm shaft  14  is arranged at the leftward-movement limit position, the rocker arm shaft  14  is arranged at a counterclockwise rotation limit position about an axis thereof in  FIG. 4  and other drawings, and when the rocker arm shaft  14  is arranged at the rightward-movement limit position, the rocker arm shaft  14  is arranged at a clockwise rotation limit position about the axis thereof in  FIG. 4  and other drawings. 
     The center collar  37  is also rotated integrally (see  FIG. 10(   a )) along with the rotation of the rocker arm shaft  14 . At the same time, a position of the center collar  37  may be slidably adjusted in the axial direction with respect to the rocker arm shaft  14 , depending on the combination of the central slot  14   a  and the engaging pin  37   d.    
     Further, such that the left and right rocker arms  17 ,  18  are at the first operation position, to allow the first rocker arm moving mechanism  21  to store a predetermined force for moving the left and right rocker arms  17 ,  18  to the second operation position, first of all, as shown in  FIG. 9 , the shaft drive mechanism  41  is operated so as to move the rocker arm shaft  14  at the leftward-movement limit position in the rightward direction together with the respective spring-receiving collars  25 ,  26 . 
     Here, since a lower portion of the left rocker-side key portion  38  of the left rocker arm  17  and a lower portion of the left trigger-side key portion  34  of the trigger arm  33  overlap each other with a predetermined overlapping quantity as viewed in the above-mentioned axial direction, the lower portion of the left rocker-side key portion  38  and the lower portion of the left trigger-side key portion  34  are brought into contact with each other in the axial direction so that the rightward-movement of the left and right rocker arms  17 ,  18  at the portions relative to the trigger arm  33  (cylinder head  2 ) is restricted. 
     Here, although a rear portion of the right rocker-side key portion  39  of the right rocker arm  18  and a lower portion of the right trigger-side key portion  35  of the trigger arm  33  overlap each other with a predetermined overlapping quantity as viewed in the axial direction, a predetermined gap S is defined, in the axial direction, between the rear portion of the right rocker-side key portion  39  and the lower portion of the right trigger-side key portion  35 . 
     To explain the above-mentioned arrangement also in conjunction with  FIG. 10 , the rocker arm shaft  14  is rotated about an axis thereof in the clockwise direction, as shown in  FIG. 10  and other drawings, along with the movement thereof in the rightward direction. When the center collar  37  is rotated in the clockwise direction along with the rotation of the rocker arm shaft  14 , an outer peripheral surface formed on a distal end of the center cam portion  37   a  is brought into slidable contact with an upper inner peripheral surface of a notched portion  34   a  of the left trigger-side key portion  34  of the trigger arm  33  in the above-mentioned pre-rocking state and hence, the trigger arm  33  is rotated in the counter-clockwise direction shown in  FIG. 10  and other drawings, against a biasing force of the spring  33   d.    
     Then, at a point of time when the rocker arm shaft  14  is moved to the above-mentioned rightward-movement limit position, and the rotation of the center collar  37  (brought about by the movement of the rocker arm shaft  14 ) and the rotation of the trigger arm  33  (brought about by the rotation of the center collar  37 ) are finished, the lower portion of the left rocker-side key portion  38  and the lower portion of the left trigger-side key portion  34  overlap each other while reducing an overlapping margin as viewed in the axial direction and, at the same time, the rear portion of the right rocker-side key portion  39  and the lower portion of the right trigger-side key portion  35  also assume an overlapping state while reducing an overlapping margin as viewed in the axial direction, in substantially the same manner. Here, a lower portion of the notched portion  35   a  of the right trigger-side key portion  35  assumes an arcuate shape substantially coaxial with the rocker arm shaft  14  as viewed in the axial direction. This state of the trigger arm  33  is referred to as a first rocking state of the trigger arm  33 . 
     At a point of time that the rocker arm shaft  14  and the respective spring-receiving collars  25 ,  26  are moved to the rightward-movement limit position from the leftward-movement limit position as described above, the first spring  23  which is positioned between the first spring-receiving collar  25  and a proximal portion  17   a  of the left rocker arm  17  whose movement is restricted is compressed by a predetermined quantity and hence, the first spring  23  assumes a state in which a resilient force sufficient for moving the left and right rocker arms  17 ,  18  to the second operation position from the first operation position is stored in the first spring  23 . 
     Then, assume that the left and right rocker arms  17 ,  18  are at the first operation position, the rocker arm shaft  14  is at the rightward movement limit position, and the trigger arm  33  is in the above-mentioned first rocking state. As shown in  FIG. 11 , the left and right first cams  15   a ,  16   a  rock the left and right rocker arms  17 ,  18  to a valve opening side from a valve closing side by rotatably driving the intake-side camshaft  11  (when the left and right cams  15   a ,  16   a  push the left and right rocker arms  17 ,  18  for lifting the left and right intake valves  6 ), for example, during a predetermined valve operation period which spans a point of time that the left and right intake valves  6  assume the maximum lift, an overlapping margin between the lower portion of the left rocker-side key portion  38  and the lower portion of the left trigger-side key portion  34  as viewed in the axial direction becomes 0 (the contact margin in the axial direction is eliminated) and hence, the restriction on the rightward-movement of the left and right rocker arms  17 ,  18  relative to the cylinder head  2  at the portions is removed. 
     Here, even when the left and right rocker arms  17 ,  18  are rocked when the trigger arm  33  assumes the above-mentioned pre-rocking state, the overlapping margin of the left rocker-side key portion  38  and the left trigger-side key portion  34  does not become 0. Accordingly, until the trigger arm  33  assumes the above-mentioned first rocking state (that is, until the first spring  23  acquires a predetermined force storing state), the restriction on the rightward movement of the left and right rocker arms  17 ,  18  are maintained. 
     On the other hand, the overlapping margin of the rear portion of the right rocker-side key portion  39  and the lower portion of the right trigger-side key portion  35  as viewed in the axial direction is, since the above-mentioned portions are formed coaxially with the rocker arm shaft  14 , hardly increased or decreased even when the left and right rocker arms  17 ,  18  rock. Accordingly, to explain the above-mentioned arrangement also in conjunction with  FIG. 12 , when the restriction on the rightward-movement of the left and right rocker arms  17 ,  18  between the left rocker-side key portion  38  and the left trigger-side key portion  34  is removed as described above, the left and right rocker arms  17 ,  18  (and the center collar  37 ) are moved in the rightward direction by a quantity corresponding to the above-mentioned gap S defined between the right rocker-side key portion  39  and the right trigger-side key portion  35 . 
     Here, by bringing the rear portion of the right rocker-side key portion  39  and the lower portion of the right trigger-side key portion  35  into contact with each other in the axial direction, the rightward movement of the left and right rocker arms  17 ,  18  relative to the cylinder head  2  is restricted. Here, the lower portion of the left rocker-side key portion  38  and the lower portion of the left trigger-side key portion  34  overlap each other by a quantity corresponding to the above-mentioned gap S in the axial direction. 
     Then, such that the left rocker-side key portion  38  and the left trigger-side key portion  34  overlap each other by a predetermined overlapping quantity in the axial direction as described above, when the left and right rocker arms  17 ,  18  are rocked or pivoted from a valve opening side to the valve closing side due to the continuous rotary driving of the intake-side camshaft  11 , as shown in  FIG. 13 , a lower outer peripheral surface of the left rocker-side key portion  38  is brought into slide contact with a lower inner peripheral surface of the notched portion  34   a  of the left trigger-side key portion  34  and hence, the trigger arm  33  is further rotated in the counter-clockwise direction shown in  FIG. 13  and other drawings from the above-mentioned first rocking state. 
     Further, to explain the above-mentioned arrangement also in conjunction with  FIG. 14 , at a point of time that the left and right rocker arms  17 ,  18  are rocked to a state in which a lift quantity of the intake valve  6  assumes 0 (valve full-closed state), an overlapping margin between a rear portion of the right rocker-side key portion  39  and a lower portion of the right trigger-side key portion  35  as viewed in the axial direction becomes 0 (a contact margin in the axial direction is eliminated) and hence, the restriction on the rightward-movement of the left and right rocker arms  17 ,  18  relative to the cylinder head  2  at the portions is removed. 
     Here, the restriction on the movement of the left and right rocker arms  17 ,  18  between the left rocker-side key portion  38  and the left trigger-side key portion  34  is also removed and hence, the left and right rocker arms  17 ,  18  (and the center collar  37 ) can be moved in the rightward direction whereby the left and right rocker arms  17 ,  18  are moved to the second operation position due to a resilient force stored in the first spring  23 . 
     When the movement of the left and right rocker arms  17 ,  18  to the second operation position is completed, the left and right rocker-side key portions  38 ,  39  and the left and right trigger-side key portions  34 ,  35  no longer overlap each other in the axial direction respectively and hence, the trigger arm  33  is rotated in the clockwise direction shown in  FIG. 13  and other drawings due to a biasing force of the spring  33   d  and returns to the above-mentioned pre-rocking state. 
     Next, such that the left and right rocker arms  17 ,  18  are at the second operation position, to allow the second rocker arm moving mechanism  22  to store a predetermined force for moving the left and right rocker arms  17 ,  18  to the first operation position, first of all, as shown in  FIG. 15 , the shaft drive mechanism  41  (in  FIG. 22 ) is operated so as to move the rocker arm shaft  14  at the rightward-movement limit position in the leftward direction together with the respective spring-receiving collars  25 ,  26 . 
     Here, since a lower portion of the left rocker-side key portion  38  of the left rocker arm  17  and a lower portion of the left trigger-side key portion  34  of the trigger arm  33  overlap each other with a predetermined overlapping quantity as viewed in the above-mentioned axial direction, the lower portion of the left rocker-side key portion  38  and the lower portion of the left trigger-side key portion  34  are brought into contact with each other in the axial direction so that the leftward-movement of the left and right rocker arms  17 ,  18  at the portions relative to the trigger arm  33  (cylinder head  2 ) is restricted. 
     Here, although a rear portion of right rocker-side key portion  39  of the right rocker arm  18  and a lower portion of the right trigger-side key portion  35  of the trigger arm  33  overlap each other with a predetermined overlapping quantity as viewed in the axial direction, the above-mentioned gap S is defined between the rear portion of the right rocker-side key portion  39  and the right trigger-side key portion  35 . 
     To explain the above-mentioned arrangement also in conjunction with  FIG. 16 , the rocker arms shaft  14  is rotated in the counter-clockwise direction shown in  FIG. 16  and other drawings about an axis thereof along with the movement thereof in the leftward direction. When the center collar  37  is rotated in the counter-clockwise direction shown in  FIG. 16  and other drawings along with the rotation of the rocker arm shaft  14 , an outer peripheral surface formed on a distal end of the center cam portion  37   a  is brought into slidable contact with an upper inner peripheral surface of a notched portion  35   a  of the right trigger-side key portion  35  of the trigger arm  33  in the above-mentioned pre-rocking state and hence, the trigger arm  33  is rotated in the counter-clockwise direction shown in  FIG. 16  and other drawings against a biasing force of the spring  33   d.    
     Then, at a point of time that the rocker arm shaft  14  is moved to the above-mentioned leftward-movement limit position, and the rotation of the center collar  37  brought about by the movement of the rocker arm shaft  14  and the rotation of the trigger arm  33  brought about by the rotation of the center collar  37  are finished, the lower portion of the left rocker-side key portion  38  and the lower portion of the left trigger-side key portion  34  overlap each other while reducing an overlapping margin as viewed in the axial direction and, at the same time, the rear portion of the right rocker-side key portion  39  and the rear portion of the right trigger-side key portion  35  also assume an overlapping state while reducing an overlapping margin as viewed in the axial direction in the same manner. Here, a lower portion of the notched portion  35   a  of the right trigger-side key portion  35  assumes an arcuate shape substantially coaxial with the rocker arm shaft  14  as viewed in the axial direction and hence, the trigger arm  33  assumes the above-mentioned first rocking state. 
     At a point of time that the rocker arm shaft  14  and the respective spring-receiving collars  25 ,  26  are moved to the leftward-movement limit position from the rightward-movement limit position as described above, the second spring  24  which is positioned between the second spring-receiving collar  26  and a proximal portion  18   a  of the right rocker arm  18  whose movement is restricted is compressed by a predetermined quantity and hence, the second spring  24  assumes a state in which a resilient force sufficient for moving the left and right rocker arms  17 ,  18  to the first operation position from the second operation position is stored in the second spring  24 . 
     Then, assume that the left and right rocker arms  17 ,  18  are at the second operation position, the rocker arm shaft  14  is at the leftward movement limit position, and the trigger arm  33  is in the above-mentioned first rocking state. As shown in  FIG. 17 , the left and right second cams  15   b ,  16   b  rock the left and right rocker arms  17 ,  18  to a valve opening side from a valve closing side by rotatably driving the intake-side camshaft  11  downward, for example, during a predetermined valve operation period which spans a point of time that the left and right intake valves  6  assume the maximum lift, an overlapping margin between the lower portion of the left rocker-side key portion  38  and the lower portion of the left trigger-side key portion  34  as viewed in the axial direction becomes 0 and hence, the restriction on the leftward-movement of the left and right rocker arms  17 ,  18  relative to the cylinder head  2  at the portions is removed. 
     Here, even when the left and right rocker arms  17 ,  18  are rocked when the trigger arm  33  assumes the above-mentioned pre-rocking state, the overlapping margin of the left rocker-side key portion  38  and the left trigger-side key portion  34  does not become 0. Accordingly, until the trigger arm  33  assumes the above-mentioned first rocking state (that is, until the second spring  24  acquires a predetermined force storing state), the restriction on the leftward movement of the left and right rocker arms  17 ,  18  are maintained. 
     On the other hand, the overlapping margin of the rear portion of the right rocker-side key portion  39  and the lower portion of the right trigger-side key portion  35  as viewed in the axial direction is hardly increased or decreased even when the left and right rocker arms  17 ,  18  rock. Accordingly, to explain the above-mentioned arrangement also in conjunction with  FIG. 18 , when the restriction on the leftward-movement of the left and right rocker arms  17 ,  18  between the left rocker-side key portion  38  and the left trigger-side key portion  34  is removed as described above, the left and right rocker arms  17 ,  18  are moved in the leftward direction by a quantity corresponding to the above-mentioned gap S. 
     Here, by bringing the rear portion of the right rocker-side key portion  39  and the lower portion of the right trigger-side key portion  35  into contact with each other in the axial direction, the leftward movement of the left and right rocker arms  17 ,  18  relative to the cylinder head  2  is restricted. Here, the lower portion of the left rocker-side key portion  38  and the lower portion of the left trigger-side key portion  34  overlap each other by a quantity corresponding to the above-mentioned gap S in the axial direction. 
     Then, such that the left rocker-side key portion  38  and the left trigger-side key portion  34  overlap each other by a predetermined overlapping quantity in the axial direction as described above, when the left and right rocker arms  17 ,  18  are rocked from a valve opening side to the valve closing side due to the continuous rotary driving of the intake-side camshaft  11 , as shown in  FIG. 19 , an outer peripheral surface of a lower portion of the left rocker-side key portion  38  is brought into slide contact with a lower inner peripheral surface of the notched portion  34   a  of the left trigger-side key portion  34  and hence, the trigger arm  33  is further rotated in the counter-clockwise direction shown in  FIG. 19  and other drawings from the above-mentioned first rocking state. 
     Further, to explain the above-mentioned arrangement in conjunction with  FIG. 20 , at a point of time that the left and right rocker arms  17 ,  18  are rocked to a state in which a lift quantity of the intake valve  6  assumes 0, a rear portion of the right rocker-side key portion  39  and a lower portion of the right trigger-side key portion  35  overlap each other as viewed in the axial direction with an overlapping margin of 0 and hence, the restriction on the leftward-movement of the left and right rocker arms  17 ,  18  at the portions relative to the cylinder head  2  is removed. 
     Here, the restriction on the movement of the left and right rocker arms  17 ,  18  between the left rocker-side key portion  38  and the left trigger-side key portion  34  is also removed and hence, the left and right rocker arms  17 ,  18  (and the center collar  37 ) can be moved in the leftward direction whereby the left and right rocker arms  17 ,  18  are moved to the first operation position due to a resilient force stored in the second spring  24 . 
     When the movement of the left and right rocker arms  17 ,  18  to the first operation position is completed, the left and right rocker-side key portions  38 ,  39  and the left and right trigger-side key portions  34 ,  35  no longer overlap each other in the axial direction respectively and hence, the trigger arm  33  is rotated in the clockwise direction shown in  FIG. 19  and other drawings due to a biasing force of the spring  33   d  (see  FIG. 1 ) and returns to the above-mentioned pre-rocking state. 
     In this manner, by suitably changing (varying) the actuation (opening/closing) timing or the valve lift quantity of the intake valve  6  depending on whether the rotary speed of the engine  1  (rotary speed of the crankshaft) is in a stop or low-speed rotary range or in a high-speed rotary range, it is possible to reduce a valve overlapping quantity and to suppress a lift quantity in the low-speed rotary range of the engine  1 , while it is possible to increase the valve overlapping quantity and the lift quantity in the high-rotary range of the engine  1 . Here, it is needless to say that a variable valve adjustment mechanism similar to the above-mentioned variable valve adjustment mechanism may be also applied to an exhaust side of the engine. In this case, it is possible to realize efficient intake and exhaust operations at respective rotary ranges of the engine  1 . 
     As shown in  FIG. 21  and  FIG. 22 , the shaft drive mechanism  41  includes an electrically-operated motor  42  which constitutes a drive source, a speed-reduction gear shaft  43  which is arranged parallel to a drive shaft  42   a  of the electrically-operated motor  42 , and a connecting rod  44  which connects an eccentric shaft  43   a  of the speed-reduction gear shaft  43  and one end side of the rocker arm shaft  14 . 
     The electrically-operated motor  42  is mounted on a left (or right)-side surface of the cylinder head  2 , and is arranged to be orthogonal to a cylinder axis C 1  when a drive shaft axis C 5  is viewed in a side view. A drive gear  42   b  is formed on an outer periphery of the drive shaft  42   a  of the electrically operated motor  42 , and the drive gear  42   b  is meshed with a large-diameter gear  43   b  mounted on one end side of the speed reduction gear shaft  43 . A rotary drive force of the electrically-operated motor  42  is transmitted to the speed reduction gear shaft  43  with the speed reduction by way of the respective gears  42   b ,  43   b , and the eccentric shaft  43   a  of the speed reduction gear shaft  43  is displaced laterally so as to allow the rocker arm shaft  14  to perform a stroke in the lateral direction (in the axial direction). Accordingly, a resilient force is stored in either one of the first rocker arm moving mechanism  21  and the second rocker arm moving mechanism  22 . In  FIG. 22 , symbol C 6  indicates a rotary center axis of the speed reduction gear shaft  43 , symbol C 7  indicates a center axis of the eccentric shaft  43   a  when the rocker arm shaft  14  is moved in the rightward direction, and symbol C 7 ′ indicates a center axis of the eccentric shaft  43   a  when the rocker arm shaft  14  is moved in the leftward direction. 
     To explain the above-mentioned arrangement also in conjunction with  FIG. 23 , on one end portion of the rocker arm shaft  14 , an end rod  45  coaxial with the rocker arm shaft  14  is mounted by way of an end collar  46 . The end rod  45  has one end portion thereof rotatably connected to a distal end portion of the connecting rod  44  by way of a connecting pin  45   a  parallel to the eccentric shaft  43   a , and has another end portion thereof held by the end collar  46  such that another end portion is not axially movable but is rotatable about an axis thereof. 
     The end collar  46  rotatably holds the end rod  45  about an axis thereof using a plurality of engaging pins  46   a . On the other hand, the end collar  46  by way of a connecting pin  46   b , which penetrates the rocker arm shaft  14 , and the end collar  46  fixedly hold one end portion of the rocker arm shaft  14  in the radial direction. Here, in the drawing, symbol  45   b  indicates an engaging groove formed in an outer periphery of the end rod  45 , which is engaged with engaging pins  46   a  formed on an inner periphery of the end collar  46  in a projecting manner. Further, the end collar  46  allows, in the same manner as the above-mentioned first spring-receiving collar  25 , a left end portion of the first spring  23  to be fitted in a right-side inner periphery thereof. That is, the end collar  46  also functions as the first spring-receiving collar  25  of the left outer cylinder of the engine  1 . 
     The rocker arm shaft  14  is formed of a single body, which extends astride the respective cylinders of the engine  1 . For example, on another end portion of the rocker arm shaft  14 , a rotary collar  47 , which has formed therein, a helical engaging groove  47   a  in an outer periphery thereof, is fixedly mounted by way of a connecting pin  47   b  which penetrates the rocker arm shaft  14  and the rotary collar  47  in the radial direction. 
     The rotary collar  47  is inserted into and supported by a support hole not shown in the drawing, which is formed in the cylinder head  2  such that the rotary collar  47  is rotatable about an axis thereof and is axially movable. An engaging pin  47   c  which projects toward an inner periphery of the above-mentioned support hole are suitably engaged with the engaging groove  47   a  formed in the rotary collar  47 . Due to such an arrangement, when the rocker arm shaft  14  performs a stroke, in response to such a stroke, the end collar  46 , the rocker arm shaft  14 , the rotary collar  47 , the first spring-receiving collar  25 , and the second spring-receiving collar  26  are suitably rotated. The rotary collar  47  allows, in the same manner as the second spring-receiving collar  26 , a right end portion of the second spring  24  to be fitted in the left-side inner periphery thereof. That is, the rotary collar  47  also functions as the second spring-receiving collar  24  in the right outer cylinder of the engine  1 . 
     As has been explained previously, the valve actuating mechanism  5  of the engine  1  according to the above-mentioned embodiment includes the intake-side camshaft  11  which includes the pair of first cams  15   a ,  16   a  and the second cams  15   b ,  16   b  for one intake valve  6 , and the left and right rocker arms  17 ,  18  which are supported on the rocker arm shaft  14  which is arranged in parallel with the intake-side camshaft  11  such that the left and right rocker arms  17 ,  18  are pivotally movable about the axis of the rocker arm shaft  14  and are axially movable of the rocker arm shaft  14 , wherein the left and right rocker arms  17 ,  18  can be brought into contact with either one of the respective cams  15   a ,  16   a ,  15   b ,  16   b  in response to the rotary driving of the intake-side camshaft  11  and are pivoted to open or close the intake valve  6 , and the left and right rocker arms  17 ,  18  are moved to either one of the first operation position at which the left and right rocker arms  17 ,  18  can be brought into contact with the first cams  15   a ,  16   a  in the axial direction and the second operation position at which the left and right rocker arms  17 ,  18  can be brought into contact with the second cams  15   b ,  16   b  in the axial direction thus allowing the valve actuating mechanism  5  to selectively use either one of the respective cams  15   a ,  16   a ,  15   b ,  16   b  for performing opening/closing operation of the intake valve  6 . The valve actuating mechanism  5  having such an arrangement further includes the first rocker arm moving mechanism  21  which moves the left and right rocker arms  17 ,  18  from the first operation position side to the second operation position side, the second rocker arm moving mechanism  22  which moves the left and right rocker arms  17 ,  18  from the second operation position side to the first operation position side, and the trigger arm  33  which is supported on the cylinder head  2  for restricting the movement of the left and right rocker arms  17 ,  18  in the axial direction of the rocker arms  17 ,  18  in response to the rocking state of the left and right rocker arms  17 ,  18 , and the left and right rocker arms  17 ,  18  and the trigger arm  33  respectively include the left and right rocker-side key portions  38 ,  39  and the left and right trigger-side key portions  34 ,  35  which are capable of being brought into contact with each other in the axial direction, and the respective key portions  38 ,  39 ,  34 ,  35  are configured to increase or decrease the contact margins thereof in the axial direction in response to the rocking state of the left and right rocker arms  17 ,  18  and eliminate the contact margins thereof when the intake valve  6  is closed, and the left and right rocker arms  17 ,  18  are moved to the corresponding operation position by either one of the respective rocker arm moving mechanisms  21 ,  22 . 
     Due to such an arrangement, by mechanically changing over the restriction on the movement of the left and right rocker arms  17 ,  18  and the removal of the restriction in response to the rocking state of the left and right rocker arms  17 ,  18 , that is, in response to the open/close state of the intake valve  6 , it is possible to move the left and right rocker arms  17 ,  18 . Particularly, the restriction on the movement of the left and right rocker arms  17 ,  18  is removed when the intake valve  6  is closed and hence, it is possible to reduce the forces which are imparted for moving the left and right rocker arms  17 ,  18 . Further, an electric sensor for detecting the open/closed state of the intake valve  6 , a control and the like become unnecessary and hence, it is possible to simplify the valve actuating mechanism per se. 
     Further, the above-mentioned valve actuating mechanism  5  is characterized in that one of pairs of corresponding key portions (respective key portions  38 ,  34 ) of the respective left and right rocker-side key portions  38 ,  39  and the respective left and right trigger-side key portions  34 ,  35  eliminates the contact margin in the axial direction at the time of predetermined rocking of the left and right rocker arms  17 ,  18 , and the left and right rocker arms  17 ,  18  are moved by a predetermined quantity to the corresponding operation position side due to either one of the respective rocker arm moving mechanisms  21 ,  22  in response to the elimination and, at the same time, the respective key portions  38 ,  34  of one pair overlap by a predetermined quantity in the axial direction, in such a state, by continuously rocking the left and right rocker arms  17 ,  18 , the respective key portions  34 ,  38  of one pair operate the trigger arm  33  so as to eliminate the contact margin of the respective key portions  35 ,  39  of another pair in the axial direction, whereby the left and right rocker arms  17 ,  18  are movable to the corresponding operation position. 
     Due to such an arrangement, the pair of key portions  38 ,  39  which is mounted on the left and right rocker arms  17 ,  18  and the pair of key portions  34 ,  35  which is mounted on the trigger arm  33  are respectively brought into contact with each other in the axial direction, and the restriction on the movement of the left and right rocker arms  17 ,  18  brought about by the contact of the respective key portions  34 ,  35 ,  38 ,  39  is removed by the respective key portions  34 ,  38  of the pair which overlap each other by a predetermined quantity in the axial direction and hence, no additional parts other than the left and right rocker arms  17 ,  18  and the trigger arm  33  are necessary as parts for removing the restriction on the movement of the left and right rocker arms  17 ,  18  attributed to the trigger arm  33  whereby the number of parts can be reduced. 
     Further, the valve actuating mechanism  5  is characterized in that the intake valve  6  and the respective cams  15   a ,  16   a ,  15   b ,  16   b  are provided in a pair for each cylinder, and the left and right rocker arms  17 ,  18  respectively correspond to the pair of intake valves  6 . The respective cams  15   a ,  16   a ,  15   b ,  16   b  and are provided such that the left and right rocker arms  17 ,  18  are pivotally movable relative to each other, and the left and right rocker-side key portions  38 ,  39  are mounted on the left and right rocker arms  17 ,  18 , respectively. 
     Due to such an arrangement, the opening/closing timings or the lift quantities of the respective intake valves  6  can be set individually and, at the same time, the restrictions on the movement of the respective left and right rocker arms  17 ,  18  can be removed individually in response to the rocking state of the respective rocker arms  17 ,  18 . 
     Further, the valve actuating mechanism  5  is characterized in that between the corresponding key portions of the respective rocker-side key portions  38 ,  39  and the respective trigger-side key portions  34 ,  35 , a predetermined gap is formed in the axial direction such that forces it away from the respective rocker arm moving mechanisms  21 ,  22  are not imparted. 
     Due to such an arrangement, at the time of performing a usual operation of the engine  1  to which the forces are not imparted from the respective rocker arm moving mechanisms  21 ,  22  (when the changeover of the valve driving cam is not performed), frictions generated between the respective key portions  34 ,  35 ,  38 ,  39  can be reduced and, at the same time, when the restriction on the movement of the left and right rocker arms  17 ,  18  are removed, it is possible to ensure the amount of travel at the time of overlapping the respective key portions  34 ,  38  of one pair with a predetermined quantity in the axial direction. 
     Further, the above-mentioned valve actuating mechanism  5  is also characterized in that the trigger arm  33  is rockably supported on the cylinder head  2 , and between the trigger arm  33  and the cylinder head  2 , the spring  33   d  which biases the trigger arm  33  toward a side at which the contact margin of the respective key portions  34 ,  35 ,  38 ,  39  in the axial direction is increased and, at the same time, a rocking restricting portion  33   f  which restricts a rocking angle of the trigger arm  33  for suppressing the deformation of the spring  33   d  is arranged. 
     Due to such an arrangement, in eliminating the contact margin of the respective key portions  34 ,  35 ,  38 ,  39  in the axial direction at the time of removing the restriction on the movement of the left and right rocker arms  17 ,  18 , it is possible to reduce a load applied to the spring  33   d  by reducing an excessive rocking angle of the trigger arm  33  thus contributing to the miniaturization, the reduction of weight and the reduction of cost of the spring  33   d.    
     Here, the present invention is not limited to the above-mentioned embodiment. For example, the present invention may adopt the arrangement which restricts the operation of the rocker arm by the respective rocker arm moving mechanisms  21 ,  22  until the respective springs  23 ,  24  acquire a predetermined force storing state without using the trigger arm  33  for restricting the movement of the rocker arm. Further, the present invention may adopt the arrangement, which stores the force in the respective springs  23 ,  24  by suitably moving only the respective spring-receiving collars  25 ,  26  without moving the rocker arm shaft  14  in the axial direction. Further, the respective springs  23 ,  24  may be formed of a tensile or torsional coil spring or a leaf spring, and may be formed of a resilient material other than metal. Still further, the present invention may adopt the arrangement which, without moving the rocker arm in two stages, moves the rocker arm between the respective operation positions at a stroke when recessed portions and projecting portions of the respective key portions are aligned or engaged with each other. 
     Further, the engine to which the present invention is applied is not limited to a 4-valve-type engine, and may be a 2-valve-type or a 3-valve-type engine, and may adopt a single rocker arm which cannot perform relative rocking at intake and exhaust sides of one cylinder. Further, the engine to which the present invention is applied is not limited to a DOHC engine but may be an OHC or OHV engine. Further, the engine to which the present invention is applied may be applicable to a parallel plural cylinder engine other than the 4-cylinder engines, a single-cylinder engine, or various types of reciprocating engines such as a V-type plural-cylinder engine. 
     The embodiments of the present invention have been described as above. The present invention is not limited to the above embodiments, but various design changes can be made without departing from the present invention as set forth in the claims.