Abstract:
A valve train device for an engine is configured to pivot a rocker arm supported on a rocker arm support shaft to drive a valve which opens and closes a valve opening formed in a combustion chamber. The device comprises a valve drive device and a swing member pivotally supported on a swing member support shaft and driven to pivot about the swing member support shaft by the valve drive device. A control arm is disposed between a swing cam surface formed on the swing member and a rocker-side surface formed on the rocker arm. The control arm is configured for transferring motion of the swing cam surface to the rocker-side surface. A displacement mechanism is provided for displacing a contact point between the control arm and the swing cam surface and a contact point between the control arm and the rocker-side surface. The rocker-side surface has an arcuate shape which arcs about a center of pivoting motion of the swing member and wherein the rocker-side surface or an extension of the rocker-side surface about said center of pivoting motion of the swing member passes in substantially near a center of swing of the rocker arm.

Description:
PRIORITY INFORMATION  
       [0001]     This application is continuation of PCT Application No. PCT/JP2004/006428, filed on May 6, 2004, which claims priority under 35 U.S.C. § 119 to Japanese Patent Application No. 2003-304931, filed on Aug. 28, 2003 and Japanese Patent Application No. 2003-126257, filed on May 1, 2003, the entire contents of these applications are expressly incorporated by reference herein. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     This invention relates to a valve train device for an engine and, more particularly, to a valve train device which can continuously change valve opening duration and/or the amount of valve lift.  
         [0004]     2. Description of the Related Art  
         [0005]     It is known in the art how to provide engines with valve train devices that are capable of continuously changing intake valve opening duration and/or the amount of valve lift. An example of such a valve train device comprises a camshaft, which drives an intake valve to open and close through a rocker arm. This device is arranged in such a way that a swing member is pivoted by the camshaft. A control arm is interposed between a swing cam surface of the swing member and a rocker-side depressed surface of the rocker arm. The valve opening duration and the amount of valve lift is continuously varied by changing a position of the control arm that comes into contact with the swing cam surface and a position of the control arm that comes into contact with the rocker-side depressed surface (See e.g., JP-A-Sho 59-500002).  
       SUMMARY OF THE INVENTION  
       [0006]     Using the aforementioned constitution, in which the position of the control arm to come into contact with the rocker-side depressed surface is changed, in the conventional type of valve train device may result in a problem depending on where the rocker-side depressed surface is disposed. For example, there may be a low transfer efficiency of force, applied from the swing cam surface to the control arm, and transferred to the rocker arm and therefore to the valve.  
         [0007]     An object of an embodiment of the present invention is to address the situations with the prior art described above and provide a valve train device for an engine which can enhance transfer efficiency of the force, applied to the control arm, and transferred to the rocker arm and therefore to the valve.  
         [0008]     Therefore, one embodiment of the present invention comprises a train device for an engine that is configured to pivot a rocker arm supported on a rocker arm support shaft to drive a valve which opens and closes a valve opening formed in a combustion chamber. The device comprises a valve drive device and a swing member pivotally supported on a swing member support shaft and driven to pivot about the swing member support shaft by the valve drive device. A control arm is disposed between a swing cam surface formed on the swing member and a rocker-side surface formed on the rocker arm. The control arm is configured for transferring motion of the swing cam surface to the rocker-side surface. A displacement mechanism is provided for displacing a contact point between the control arm and the swing cam surface and a contact point between the control arm and the rocker-side surface. The rocker-side surface has an arcuate shape which arcs about a center of pivoting motion of the swing member and wherein the rocker-side surface or an extension of the rocker-side surface about said center of pivoting motion of the swing member passes in substantially near a center of swing of the rocker arm.  
         [0009]     For purposes of summarizing the invention, certain aspects, advantages and novel features of the invention have been described herein. It is to be understood that not necessarily all such advantages may be achieved in accordance with any particular embodiment of the invention. Thus, the invention may be embodied or carried out in a manner that achieves or optimizes one advantage or group of advantages as taught herein without necessarily achieving other advantages as may be taught or suggested herein. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]     A general architecture that implements various features of specific embodiments of the invention will now be described with reference to the drawings. The drawings and the associated descriptions are provided to illustrate embodiments of the invention and not to limit the scope of the invention.  
         [0011]      FIG. 1  is a sectional side view of a valve train device for an engine according to a first embodiment of the present invention.  
         [0012]      FIG. 2  is a perspective view of a control arm, rocker arm and rocker shaft of the first embodiment.  
         [0013]      FIG. 3  is a sectional side view for describing the functions of an embodiment of the invention.  
         [0014]      FIG. 4  is a schematic view showing an embodiment of a come-off prevention member of the first embodiment.  
         [0015]      FIG. 5  is a sectional side view for describing a second embodiment of the invention.  
         [0016]      FIG. 6  is a schematic top plan view of the second embodiment. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0017]     An embodiment of the present invention will be described hereinafter with reference to the attached drawings.  
         [0018]      FIGS. 1-3  are describe a first embodiment of the invention.  FIG. 1  is a sectional side view of a valve train device according to this embodiment of the invention.  FIG. 2  is a perspective view of core parts of the valve train device.  FIG. 3  is a view for describing transfer efficiency of a force F in this embodiment of the invention.  
         [0019]     In  FIG. 1 , reference numeral  1  denotes an valve device for opening and closing an valve opening formed in a combustion chamber. An engine can be provided with two intake and exhaust valve devices. However, in  FIG. 1 , only a portion at an intake valve device is shown. A combustion recess  2   a  is provided on the mating face of a cylinder head  2  of the engine with the cylinder body. The combustion recess  2   a  forms a top ceiling of a combustion chamber. The combustion recess  2   a  includes left and right intake valve openings  2   b . Each intake valve opening  2   b  is merged with an intake port  2   c  and led to an external connection opening of an engine wall. Each intake valve opening  2   b  is opened and closed through a valve head  3   a  of an intake valve  3 . The intake valve  3  is constantly urged with a valve spring or biasing member (not shown) in closing direction.  
         [0020]     In the embodiments described below, reference will be made to the intake valve  3  and intake valve device  1 . However, it should be appreciated that certain features and aspects of these embodiments may also be applied to an exhaust device and exhaust valve. It should also be appreciated that various features, aspects and advantages of the present invent may be used with engines having more than one intake valve and/or exhaust valve, and any of a variety of configurations including a variety of numbers of cylinders and cylinder arrangements (V, W, opposing, etc.).  
         [0021]     A valve train device  7  is disposed above the intake valve  3 . The valve train device  7  is configured such that: (i) an intake camshaft  8  which serves as swing member drive device causes a swing member  9  to swing or pivot, (ii) the swing member  9  causes a rocker arm  11  to swing or pivot through a control arm  10 , and (iii) the swing of the rocker arm  11  causes the intake valve  3  to proceed and retract in the axial direction, and thus the intake valve opening  2   b  is opened and closed.  
         [0022]     Causing the control arm  10  to proceed and retract can continuously change a contact point between the control arm  10  and the swing member  9  and a contact point between the control arm  10  and the rocker arm  11 , thereby continuously changing the opening duration of the intake valve  3  and the amount of valve lift.  
         [0023]     The intake camshaft  8  may be arranged in parallel with a crankshaft (not shown). The intake camshaft may be supported to be rotatable and immobile in a direction perpendicular to the crankshaft and in the axial direction through a cam journal portion formed on the cylinder head  2  and a cam cap provided on an upper mating face of the journal portion. In the illustrated embodiment, the intake camshaft  8  is formed with a single cam nose  8   c  common to the left and right intake valves, including a base circle portion  8   a  having a specified diameter, and a lift portion  8   b  having a specified cam profile. Each cylinder is provided with a single cam nose.  
         [0024]     The swing member  9  has a pair of left and right swing arm portions  9   a ,  9   a , a swing cam surface  9   b , a roller shaft  9   c , and a swing roller  9   d . The pair of swing arm portions  9   a ,  9   a  is supported for free swinging movement by a swing shaft  12  arranged in parallel with the intake camshaft  8  so as to be immobilized in the direction perpendicular to the swing shaft and in the axial direction. The swing cam surface  9   b  is formed to connect the front (lower) ends of the swing arm portions  9   a . The roller shaft  9   c  is arranged in parallel with the swing shaft  12  and in the midsection between the left and right swing arm portions  9   a ,  9   a  to pass therethrough. The swing roller  9   d  is rotatably supported on the roller shaft  9   c . The swing roller  9   d  is constantly in rotational contact with the cam nose  8   c.    
         [0025]     Base (upper) portions of the swing arm portions  9   a  is fitted to and supported with the swing shaft  12  for free swinging movement. The swing shaft  12  is provided with a pair of left and right balance springs  13  (e.g., coil springs). Each balance spring  13  has a first end  13   a  retained between the swing shaft  12  of the swing arm portion  9   a  and the roller shaft  9   c . A other end  13   b  of each balance spring is retained by the cylinder head  2 . The balance spring  13  urges the swing member  9  such that the swing roller  9   d  of the swing member  9  comes into contact with the cam nose  8   c  of the intake camshaft  8 , thereby preventing the swing roller  9   d  from moving away from the cam nose  8   c  at the high engine speed. This avoids or reduces abnormal behavior of the swing member  9 .  
         [0026]     The swing cam surface  9   b  has a base circle portion  9   e  and a lift portion  9   f  formed together in a curved manner to have a connected surface and a generally plate-like shape. The swing member  9  is provided so that the base circle portion  9   e  is positioned nearer to a rocker shaft  14  and the lift portion  9   f  is positioned opposite the rocker shaft  14 . The base circle portion  9   e  has an arcuate shape of a radius R 1  centered on the axis of the swing shaft  12  as the center of swing (a). Thus, while the base circle portion  9   e  depresses the roller  10   c , the intake valve  3  is placed at a fully closed position and not lifted even if a swing angle of the swing member  9  increases.  
         [0027]     Meanwhile, the lift portion  9   f  lifts the intake valve  3  greatly as the lift portion  8   b  of the intake camshaft  8  at the portion close to the top depresses the swing roller  9   d , that is, as the swing angle of the swing member  9  increases. In this embodiment, the lift portion  9   f  includes a ramp zone which gives a constant speed, an acceleration zone which gives a varied speed, and a lift zone which gives generally a constant speed.  
         [0028]     The rocker shaft  14  includes a large-diameter portion  14   a  and an eccentric pin  14   b  having a smaller diameter than the diameter for the large-diameter portion. In the illustrated embodiment, the eccentric pin  14   b  is provided on a midsection of the large-diameter portion, while being offset from an axial center (b) of the rocker shaft  14  toward the outer side in the radial direction. The large-diameter portion  14   a  is rotatably supported with the cylinder head  2 . The eccentric pin  14   b  has an axial center (c) positioned such that part of the outer surface  14   b ′ protrudes outward in the radial direction from an outer surface  14   a ′ of the larger-diameter portion  14   a . To the rocker shaft  14  is connected a rocker shaft driving mechanism (not shown) for controlling an angular position of the rocker shaft  14  according to an engine load (throttle opening) and engine speed.  
         [0029]     The rocker arm  11  is formed with left and right rocker arm portions  11   a ,  11   a , a rocker coupling portion  11   b , and ring-shaped bearing portions  11   c ,  11   c . Lower-half portions on the distal end side of the left and right rocker arm portions  11   a ,  11   a  are coupled integrally with the locker coupling portion  11   b . The ring-shaped bearing portions  11   c ,  11   c  are formed integrally with the proximal ends of the left and right rocker arms  11   a ,  11   a . The bearing portions  11   c ,  11   c  are supported with the large-diameter portions  14   a ,  14   a  of the rocker shaft  14 . Part of the bearing portions  11   c  towards the rocker arm portions  11   a  is provided with a clearance recess  11   f  that conforms to the outwardly projecting shape of the eccentric pin  14   b.    
         [0030]     The control arm  10  has a schematic structure in which: a control-side depressing surface  10   b  is formed in an arcuate shape about the center of swing (a) on the lower face of the distal ends of the left and right bifurcated control arm portions  10   a, a ; the roller  10   c  in rotational contact with the swing cam surface  9   b  is pivoted between the distal ends of the control arm portions a, a; and the bifurcated, semi-circular bearing portion  10   d  is formed at the proximal ends of the control arm portions.  
         [0031]     On the topside of the rocker coupling portion  11   b  of the rocker arm  11 , left and right rocker-side depressed surfaces  11   d ,  11   d  are formed to come into sliding contact with the left and right control-side depressing surfaces  10   b ,  10   b . The rocker-side depressed surfaces  11   d ,  11   d  are formed in an arcuate shape of a radius R 2  about the center of swing (a) of the swing shaft  12 . As shown in  FIG. 4 , An extension line  11   d ′ of the rocker-side depressed surface  11   d  is so set as to pass in the vicinity of the center of sing (b) of the rocker arm  11 , and more preferably, to pass inside a rotation locus C of the axial center (c) of the eccentric pin  14   b.    
         [0032]     With reference to  FIG. 1 , the control arm  10  is placed such that it is interposed between the left and right rocker arm portions  11   a ,  11   a  of the rocker arm  11 . The semi-circular bearing portion  10   d  is rotatably supported with the eccentric pin  14   b  of the rocker shaft  14 . The come-off prevention spring  15  prevents the bearing portion and the eccentric pin from coming off.  
         [0033]     In one embodiment, the come-off prevention spring  15  is made of spring steel band member, and has a holding portion  15   a  curved into approximately a C-shape and a depressing portion  15   b  that extends from the front end of the holding portion  15   a  toward the distal end of the rocker arm  11 . The come-off prevention spring  15  is designed to retain a curved retaining portion  15   c , which is formed adjacent to the boarder between the holding portion  15   a  and the depressing portion  15   b , to a retained portion  10   e  of the control arm  10 . The come-off prevention spring  15  is also designed to retain an accurate retaining portion  15   d , which is formed opposite to the pressing portion  15   b , to the eccentric pin  14   b . Thereby, the come-off prevention spring  15  holds the bearing portion  10   d  and the eccentric pin  14   b  together for relative rotation while preventing them from separating from each other.  
         [0034]     The distal end of the depressing portion  15   b  of the come-off prevention spring  15  comes into contact with a depressing groove lie with a predetermined amount of spring force, the depressing grove being provided on the topside of the rocker coupling portion  11   b  of the rocker arm  11  and at the center in the axial direction. The depressing groove  11   e  is formed in an arcuate shape about the center of rotation (a) of the swing member  9 . In the manner as described, the control arm  10  is urged clockwise as shown in the drawing. The roller  10   c  comes into contact with the swing cam surface  9   b . A slight gap (d) is created between the rocker-side depressed surface lid and the control-side depressing surface  10   b.    
         [0035]     In the manner as described, a displacement mechanism is constituted such that rotating the rocker shaft  14  allows a contact point (e) between the roller  10   c  and the swing cam surface  9   b  as well as a contact point (f) between the control-side depressing surface  10   b  and the rocker-side depressed surface  11   d  to be displaced.  
         [0036]     In the displacement mechanism, displacement of the contact point relative to the rotation angle of the rocker shaft  14  in a high operation range in which the opening duration of the intake valve  3  is long and the amount of the valve lift is large (shown by solid lines in  FIG. 1 ) and in a low operation range in which the opening duration of the intake valve  3  is short and the amount of the valve lift is small (shown by chain double-dashed lines in  FIG. 1 ) is smaller than the displacement of the contact point in a medium operation range in which the opening duration of the intake valve  3  and the amount of the valve lift are medium. In other words, in the high operation range, the axial center of the eccentric pin  14   b  is positioned near the point identified by the reference number c 1  in  FIG. 1 , while near the point identified by reference number c 2  in the low operation range. When the eccentric pin  14   b  is adjacent to the points c 1  or c 2 , each displacement of the contact points e and f relative to the rotation angle of the rocker shaft  14  is smaller than that in another operation range. In contrast, in the medium operation range, the axial center of the eccentric pin  14   b  is positioned approximately between c 1  and c 2 . When the eccentric pin  14   b  is adjacent approximately between c 1  and c 2 , each displacement of the contact point e and f relative to the rotation angle of the rocker shaft  14  is larger than those in the other operation ranges.  
         [0037]     An axial end surface  10   f  of the bearing portion  10   d  is in sliding contact with an end surface  14   c  of the large-diameter portion  14   a  of the rocker shaft  14 , the end surface forming a step from the eccentric pin  14   b , thereby positioning the control arm  10  in the axial direction. In turn, an inner end surface  11   c ′ of the bearing portion  11   c  is in sliding contact with an opposite end surface to the end surface  10   f  of the bearing portion  10   d  of the control arm  10 , thereby positioning the rocker arm  11  in the axial direction.  
         [0038]     Description will be next made of the operations and effects of this embodiment.  
         [0039]     In the valve train device  7  of this embodiment, the rocker shaft driving mechanism controls a rotational angular position of the rocker shaft  14  in accordance with engine operation conditions determined based on the engine speed and load. For example, in a high-speed and high-load operation range, the angular position of the rocker shaft  14  is controlled to position the axial center of the eccentric pin  14  to point c 1  as shown by solid lines in  FIG. 1 . Thus, when the control arm  10  is positioned at the advanced end and the base circle portion  8   a  of the camshaft  8  comes into contact with the roller  9   d , the contact point e between the roller  10   c  of the control arm  10  and the swing cam surface  9   b  of the swing member  9  is positioned closest to the lift portion  9   f  . This results in maximizing both the opening duration of the intake valve  3  and the amount of valve lift.  
         [0040]     In turn, in a low-speed and low-load operation range, the angular position of the rocker shaft  14  is controlled to position the axial center of the eccentric pin  14  to point c 2  as shown by chain double-dashed lines in  FIG. 1 . Thus, the control arm  10  moves to the retracted end, and the contact point e between the roller  10   c  of the control arm  10  and the swing cam surface  9   b  of the swing member  9  is positioned farthest from the lift portion  9   f . This results in minimizing both the opening duration of the intake valve  3  and the amount of valve lift.  
         [0041]     In one embodiment, the rocker-side depressed surface  11   d  is formed such that the extension line  11   d ′ thereof passes in the vicinity of the center (b) of swing of the rocker arm  11 . In another embodiment, the structure describe herein allows the extension line  11   d ′ to pass inside the rotation locus C (see  FIG. 3 ) of the center point (c) of the eccentric pin  14 . In the illustrate embodiment, the control arm  10  is also interposed between the left and right rocker arm portions  11   a ,  11   a  of the rocker arm  11 , and the rocker-side depressed surface  11   d  is formed on the rocker coupling portion  11   b  for coupling the left and right rocker arm portions  11   a ,  11   a . This enhances positioning the extension line  11   d ′ of the rocker-side depressed surface  11   d  such that it passes in the vicinity of the center (b) of swing of the rocker arm  11 .  
         [0042]     In a preferred embodiment, “such that the rocker-side depressed surface  11   d  or its extension line  11   d ′ passes in the vicinity of a center of swing (b) of the rocker arm  11 ” means that the rocker-side depressed surface  11   d  is approximated as close as possible to a straight line Lo that connects the center of swing (b) and a point (f) of application of force F transferred from the control arm  10  to the rocker arm  11 , thereby transferring the force F with high efficiency as the rotational force of the rocker arm  11 .  
         [0043]     The rocker-side depressed surface  11   d  is of the illustrated embodiment is therefore formed in such a manner that the extension line  11   d ′ thereof passes in the vicinity of the center (b) of swing of the rocker arm  11 . Thus, the force F transferred from the swing member  9  to the contact point (f) via the control arm  10  can be efficiently transferred to the rocker arm  11  and therefore to the valve  3 . In other words, in this embodiment, since the rocker-side depressed surface lid passes in the vicinity of the center (b) of swing of the rocker arm  11 , the rocker-side depressed surface  11   d  generally agrees with the straight line Lo. This increases a first component force F 1  of the force F. The first component force F 1  being perpendicular to the straight line Lo as a rotational force of the rocker arm  11  and the force F being transferred from the control arm  10  to the rocker arm  11 . Thus, the transfer efficiency of the force F from the control arm  10  to the rocker arm  11  enhances.  
         [0044]     The center (a) of swing of the swing member  9  is located at a point opposite to a valve shaft line L 1  with respect to a straight line L 2  parallel to the valve shaft line L 1  and passing the axial center (b) of the rocker shaft  14 , while being away from the straight line L 2  by a distance g. This provides an advantage to the extension line  11   d ′ of the rocker-side depressed surface  11   d  to pass in the vicinity of the center (b) of rotation of the rocker arm  11 . More specifically, as an angle formed between the direction of the force F applied to the rocker arm  11  and the straight line Lo that connects a point (f) of application of the force F and the center (b) of swing of the rocker arm  11  is closer to the right angle, the transfer efficiency of the force F increases. Since the center (a) of swing of the swing member  9  is located on the side opposite to the valve shaft line L 1 , the direction of the force F can be easily changed to be close to the direction perpendicular to the straight line Lo.  
         [0045]     The eccentric pin  14   b  provided on the midsection of the rocker shaft  14  is adapted to support the bearing portion  10   d  of the control arm portion a for free rotation, and the come-off prevention spring  15  holds the bearing portion  10   d  and the eccentric pin  14   b . This allows the opening duration of the valve  3  and the amount of valve lift to continuously change by using a very simple structure or solely rotating the rocker shaft  14 . This also facilitates work for coupling the control arm  10  and the eccentric pin  14   b.    
         [0046]     In the case of multi-cylinder engine, because uniform valve opening duration and amount of valve lift need be ensured for all cylinders, several control arms  10  within the dimensional tolerance range are prepared to be selected in combination with the rocker shaft  14  in order to uniform the valve opening duration and the amount of valve. Assemble and removal work when such a selective combination is required can be easily carried out.  
         [0047]     The depressing portion  15   b  in the illustrated embodiment is integrally formed with the come-off prevention spring  15 , the depressing portion  15   b  urging the control arm  10  by depressing the rocker arm  11 , such that the roller  10   c  comes into contact with the swing cam surface  9   b . Thus, the roller  10   c  of the control arm  10  can be constantly in contact with the swing cam surface  9   b  of the swing member  9  by a simple constitution. Also, a rolling contact of the roller  10   c  with respect to the motion of the swing cam surface  9   b  can be kept normal, thereby preventing the wearing of the swing cam surface  9   b  and the roller  10   c.    
         [0048]     Offset displacement of the eccentric pin  14   b  is preset so that the outer surface  14   b ′ of the eccentric pin  14   b  protrudes outward from the outer surface  14   a ′ of the rocker shaft  14  in the radial direction. This can increase the displacement of the control arm  11  without increasing the diameter of the rocker shaft  14 , thereby increasing the adjustment range for the valve opening duration and amount of valve lift.  
         [0049]     When the eccentric pin  14   b  protrudes outward, an inner peripheral surface of the bearing portion  11   c  supported with the rocker shaft  14  of the rocker arm  11  is formed with the clearance recess  11   f  which conforms with the amount of protrusion of the eccentric pin  14   b . Thus, while the clearance recess  11   f  of the rocker arm  11  fits the protrusion of the eccentric pin  14   b , the rocker arm  11  is displaced in the axial direction of the rocker shaft  14 , so that the rocker arm  11  can be assembled with the rocker shaft  14  without any problem.  
         [0050]     In the low operation range in which the opening duration of the valve  3  is short and the amount of valve lift is small, the eccentric pin  14   b  is positioned at point c 2  so that the displacement of the contact point (e) relative to the rotation angle of the rocker shaft  14  is smaller than the displacement in the medium operation range in which the opening duration of the valve  3  and the amount of valve lift are medium. This, in the low engine speed range, can avoid abrupt variations in engine output due to slight variations in rotation angle of the rocker shaft  14 , and can provide smooth operations, thereby avoiding jerky feeling.  
         [0051]     In the high operation range in which the opening duration of the valve  3  is long and so forth, the eccentric pin  14   b  is positioned at (c 1 ), so that the displacement of the contact point (e) relative to the opening angle of the rocker shaft  14  is preset smaller than the displacement in the medium operation range in which the opening duration of the valve is medium and so forth. This, in the high engine speed range, can reduce a torque required for rotating rocker shaft  14 , and can provide smooth driving operations.  
         [0052]     The control arm  10  is brought into sliding contact with the step  14   c  from the eccentric pin  14   b  of the rocker shaft  14 , thereby positioning the control arm in the axial direction. The rocker arm  11  is brought into sliding contact with the axial end surface  10   f  of the control arm  10 , thereby positioning the rocker arm in the axial direction. Therefore, positioning of the control arm  10  and the rocker arm  11  in the axial direction can be achieved without any dedicate parts.  
         [0053]     In the description of the first embodiment, the come-off prevention member is a leaf spring. However, as shown in  FIG. 4 , the come-off prevention member of the invention may be a rod-shaped come-off prevention pin whose both ends are press-fitted through the outer ends of the bearing portion  10   d.    
         [0054]     In the description of the first embodiment, the control arm is included in the rocker arm. However, the control arm may be disposed externally to the rocker arm in the invention.  
         [0055]     For example,  FIGS. 5 and 6  are for describing a second embodiment in which a control arm is disposed externally to a rocker arm. In these figures, the same reference numerals as in FIGS.  1  to  4  designate the same or corresponding parts.  
         [0056]     A rocker arm  21  includes: a cylindrical bearing portion  21   a  supported with a large-diameter portion  24   a  of a rocker shaft  24 ; and left and right rocker arm portions  21   b ,  21   b  integrally extending forward from axially opposite ends of the bearing portion  21   a . Bottom surfaces of the distal ends of the rocker arm portions  21   b  come into contact with the top ends of left and right intake valves  3 ,  3 , respectively.  
         [0057]     Rocker-side depressed surfaces  21   d  are formed on the topside of the left and right rocker arm portions  21   b . The rocker-side depressed surfaces  21   d  are formed in an arcuate shape of a predetermined radius about an axial center of a swing shaft  12 . An extension line  21   d ′ of the rocker-side depressed surface  21   d  is so set as to pass in the vicinity of a center of swing (b) of the rocker arm  21 , and more preferably, to pass inside a rotation locus C of an axial center (c) of an eccentric pin  24   b.    
         [0058]     The control arm  20  includes a pair of left and right arm portions  20   a ,  20   a , a roller shaft  20   b  and proximal end portions  20   d  of the left and right arm portions  20   a ,  20   a . The roller shaft  20   b  rigidly connects the distal ends of the left and right arm portions  20   a ,  20   a  together. The proximal end portions  20   d , which are formed in a semi-circular, are coupled and supported with the eccentric pin  24   b  of the rocker shaft  24 , and retained together with the eccentric pin by the leaf spring, using the same constitution as in the first embodiment.  
         [0059]     The left and right arm portions  20   a ,  20   a  are positioned externally to their associated rocker arm portions  21   b ,  21   b  in the axial direction. Each arm portion and the associated rocker arm portion form a clearance between them to accommodate a roller  20   c . The rollers  20   c ,  20   c  are supported with the roller shaft  20   b  for free rotation. The rollers  20   c  are in rotational contact with a swing cam surface  9   b  of the swing arm  9 .  
         [0060]     The roller shaft  20   b  is in sliding contact with the left and right rocker-side depressed surfaces  21   d ,  21   d  of the rocker arm  21 . In other words, in this embodiment, the roller shaft  20   b  has a control-side depressing surface for depressing the rocker-side depressed surface  21   d.    
         [0061]     The second embodiment of the invention is constituted in a way such that: the arm portions  20   a  of the control arm  20  are placed externally to the rocker arm portions  21   b  of the rocker arm  21 , the roller  20   c  is placed between the arm portion and the rocker arm portion, and the roller shaft  20   b  depresses the rocker-side depressed surface  21   d . This enables the rocker-side depressed surface  21   d  to be formed such that an extension line  21   d ′ thereof passes in the vicinity of the center of swing (b) of the rocker arm  21 . This can enhance transfer efficiency of force from the control arm  20  to the rocker arm  21  as with the case in the first embodiment.  
         [0062]     According to the embodiments described herein, as shown in  FIG. 3 , the control arm  10  is designed to transfer the motion of the swing cam surface  9   b  of the swing member  9  to the rocker-side depressed surface lid of the rocker arm  11 . In this case, the rocker-side depressed surface  11   d  is formed in an arcuate shape about the center of swing (a) of the swing member  9 , such that the rocker-side depressed surface  11   d  or its extension line  11   d ′ passes in the vicinity of the center of swing (b) of the rocker arm  11 . Thus, the force F applied from the swing member  9  to the control arm  10  can be efficiently transferred to the rocker arm  11  and therefore to the valve  3 .  
         [0063]     To be more specific, the force F transmitted from the control arm  10  to the rocker arm  11  is divided into a first component force (rotational force of the rocker arm) F 1  perpendicular to the direction of a straight line Lo that connects a point (f) of application of the force F and the center of swing (b) of the rocker arm, and into a second component force F 2  in the direction of the straight line Lo. In the embodiments described herein, since the rocker-side depressed surface  11   d  or its extension line  11   d ′ passes in the vicinity of the center of swing (b) of the rocker arm  11 , the rocker-side depressed surface  11   d  generally agrees with the straight line Lo. This decreases the second component force F 2  while increasing the first component force F 1 , which results in enhanced transfer efficiency of the force F from the control arm  10  to the rocker arm  11 .  
         [0064]     According to the illustrated embodiment of  FIGS. 1-3 , the control arm  10  is interposed between the left and right rocker arm portions  11   a ,  11   a  of the rocker arm  11 , and the rocker-side depressed surface lid is formed on the rocker coupling portion  11   b  for coupling the left and right rocker arm portions  11   a ,  11   a . This facilitates placing the rocker-side depressed surface  11   d  or its extension line  11   d ′ such that it passes in the vicinity of the center of swing (b) of the rocker arm  11 , thereby achieving enhanced transmission efficiency of the force from the control arm  10  to the rocker arm  11 .  
         [0065]     According to the embodiments of  FIGS. 5-6 , the control arm  20  is provided with the roller  20   c  which comes into contact with the swing cam surface  9   b  such that the roller is located externally to the rocker arm portion  21   b  of the rocker arm  21 , and the roller shaft  20   b  for supporting the roller  20   c  is designed to depress the rocker-side depressed surface  21   d  of the rocker arm portion  21   b . This facilitates the rocker-side depressed surface  21   d  or its extension line  21   d ′ being formed to pass in the vicinity of the center of swing (b) of the rocker arm  21 , thereby achieving enhanced transfer efficiency of the force from the control arm  20  to the rocker arm  21 .  
         [0066]     According to the embodiment of  FIGS. 1-3 , the proximal end of the control arm portion  10   a  is rotatably coupled with the eccentric pin  14   b  provided on the midsection of the rocker shaft  14 , and rotating the rocker shaft  14  allows displacing the contact point between the roller  10   c  and the swing cam surface  9   b  and the contact point between the control-side depressing surface  10   b  and the rocker-side depressed surface  11   d . This allows the opening duration of the valve  3  and the amount of the valve lift to continuously change by using a very simple structure that can be actuated by solely rotating the rocker shaft  14 .  
         [0067]     To the illustrated embodiments, the rocker-side depressed surface  11   d  or its extension line  11   d ′ passes inside the rotation locus C of the axial center (c) of the eccentric pin  14   b , which is generated by rotating the rocker shaft  14 . Thus, enhanced transmission efficiency of the force from the control arm  10  to the rocker arm  11  can be more certainly achieved.  
         [0068]     According to the embodiment of  FIGS. 1-3 , offset displacement of the eccentric pin  14   b  is preset so that the outer surface  14   b ′ of the eccentric pin  14   b  protrudes outward from the outer surface  14   a ′ of the rocker shaft  14  in the radial direction. This can increase the displacement of the control arm  11  without increasing the diameter of the rocker shaft  14 , thereby increasing the adjustment range for the valve opening duration and amount of the valve lift.  
         [0069]     For the eccentric pin  14   b  protruding outward, an inner peripheral surface of the bearing portion  11   c  of the rocker arm  11 , which is supported on the rocker shaft  14 , is formed with the clearance recess  11   f  which conforms with the amount of protrusion of the eccentric pin  14   b . Thus, while the clearance recess  11   f  fits the protrusion of the eccentric pin  14   b , the rocker arm  11  is displaced in the axial direction of the rocker shaft  14 , so that the rocker arm  11  can be assembled to the rocker shaft  14  without any problem.  
         [0070]     According to the embodiments described above, the displacement of the contact point relative to the rotation angle of the rocker shaft  14  in a low operation range, in which the opening duration of the valve  3  is short and the amount of the valve lift is small, is preset smaller than the displacement of the contact point in a medium operation range in which the opening duration of the valve  3  and the amount of the valve lift are medium. This, in the low engine speed range, can avoid abrupt variations in engine output due to slight variations in rotation angle of the rocker shaft  14 , and can provide smooth operations, thereby avoiding jerky feeling.  
         [0071]     The displacement of the contact point in a high operation range, in which the opening duration of the valve  3  is long and so forth, is preset smaller than the displacement of the contact point in a medium operation range. This, in the high engine speed range, can reduce a torque required for rotating rocker shaft  14 , and can provide smooth driving operations.  
         [0072]     According to embodiment shown in  FIG. 4 , the semi-circular-shaped bearing portion  10   d  is formed at and integrally with the proximal end of the control arm portion a, and rotatably supported with the eccentric pin  14   b , and the come-off prevention member is provided for preventing the bearing portion  10   d  and the eccentric pin  14   b  from separating from each other. This facilitates work for coupling the control arm  10  and the eccentric pin  14   b.    
         [0073]     To be more specific, in the case of multi-cylinder engine, adjustments for uniform valve opening duration and amount of the valve lift are needed for all cylinders. Therefore, several control arms  10  within the dimensional tolerance range are prepared for selecting a combination to uniform the valve opening duration and the amount of the valve lift. Assembly and removal of the control arm to be carried out for selecting the combination are required to be easy. The illustrated embodiments can meet such a requirement.  
         [0074]     According to the embodiment of  FIGS. 1-3 , the come-off prevention member is a leaf spring  15  for holding the bearing portion  10   d  of the control arm portion  10   a  and the eccentric pin  14   b . This further facilitates the assembly/removal of the control arm  10  to/from the rocker shaft  14 .  
         [0075]     Also, the leaf spring  15  has the depressing portion  15   b  integrally formed therewith and urging the control arm  10  by depressing the rocker arm  11  such that the roller  10   c  comes into contact with the swing cam surface  9   b . Thus, the roller  10   c  of the control arm  10  can be constantly in contact with the swing cam surface  9   b  of the swing member  9  with a simple constitution. Therefore, a rolling contact of the roller  10   c  with respect to the motion of the swing cam surface  9   b  can be kept normal, thereby preventing the wearing of the swing cam surface  9   b  and the roller  10   c.    
         [0076]     According to the embodiments described above, the control arm  10  is brought into sliding contact with the step  14   c  from the eccentric pin  14   b  of the rocker shaft  14 , thereby being positioned in the axial direction. Also, the rocker arm  11  is brought into sliding contact with the axial end surface  10   f  of the control arm  10 , thereby being positioned in the axial direction. Therefore, positioning of the control arm  10  and the rocker arm  11  in the axial direction can be achieved without any dedicate parts.  
         [0077]     According to the embodiments described above, the center of swing (a) of the swing member  9  is located at a point opposite to the valve shaft line L 1  with respect to the straight line L 2  parallel to the valve shaft line L 1  and passing the axial center (b) of the rocker shaft  14 . This gives advantage to the rocker-side depressed surface lid or its extension line  11   d ′ to pass in the vicinity of the center of rotation (b) of the rocker arm  11 . More specifically, as an angle formed between the direction of the force F applied to the rocker arm  11  and the straight line Lo that connects the point (f) of application of the force F and the center of swing (b) of the rocker arm  11  is closer to the right angle, the transfer efficiency of the force increases. Since the center of swing (a) of the swing member  9  is located on the side opposite to the valve shaft line L 1 , the direction of the force F can be easily set perpendicular to the direction of the straight line Lo.  
         [0078]     Although this invention has been disclosed in the context of certain preferred embodiments and examples, it will be understood by those skilled in the art that the present invention extends beyond the specifically disclosed embodiments to other alternative embodiments and/or uses of the invention and obvious modifications and equivalents thereof. In addition, while a number of variations of the invention have been shown and described in detail, other modifications, which are within the scope of this invention, will be readily apparent to those of skill in the art based upon this disclosure. It is also contemplated that various combinations or subcombinations of the specific features and aspects of the embodiments may be made and still fall within the scope of the invention. Accordingly, it should be understood that various features and aspects of the disclosed embodiments can be combine with or substituted for one another in order to form varying modes of the disclosed invention. Thus, it is intended that the scope of the present invention herein disclosed should not be limited by the particular disclosed embodiments described above, but should be determined only by a fair reading of the claims that follow.