Patent Publication Number: US-2009235885-A1

Title: Variable valve actuating apparatus

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to internal combustion engines, and particularly to variable valve actuating apparatuses or systems for varying at least a lift of an engine valve, such as an intake valve or exhaust valve, of an internal combustion engine. 
     Japanese Patent Application Publication No. 2002-38913 corresponding to U.S. Pat. No. 6,499,454 discloses a variable valve actuating system for varying at least a lift of an intake valve set of an internal combustion engine. This variable valve actuating system includes: a drive shaft rotated by a crankshaft; a drive cam fixedly mounted to an outer radial periphery of the drive shaft; a transmitting mechanism for converting rotary motion of the drive cam to swinging motion of a swing cam, the transmitting mechanism including a rocker arm, a link arm and a link rod; and the swing cam that slides on a top surface of a valve lifter for opening and closing an intake valve. The link arm links the drive cam with the rocker arm. The link rod links the rocker arm with the swing cam. The rocker arm is linked at one end portion with one end of the link arm and one end of the link rod, and has a relatively large hole at another end portion in which a control cam is rotatably supported. The control cam is fixed and eccentric with respect to a control shaft. The variable valve actuating system is configured to control the position of the control cam according to an engine operating state by rotating the control shaft by an actuator. Movement of the control cam causes a change in the range of motion of the swing cam, and thereby causes changes in the lift and operating angle of the intake valve. 
     SUMMARY OF THE INVENTION 
     For assembling the variable valve actuating system described above and disclosed in Japanese Patent Application Publication No. 2002-38913, it is necessary to attach the control cam to the rocker arm by inserting the control cam into the hole formed in the rocker arm in the longitudinal direction of the control shaft. This imposes some requirements on the entire assembling process, for example, for avoiding interference between components. 
     In view of the foregoing, it is desirable to provide a variable valve actuating apparatus or system which can be assembled more easily. 
     According to one aspect of the present invention, a variable valve actuating apparatus for an internal combustion engine, comprises: a drive cam adapted to be rotated by the internal combustion engine; a control shaft supported for rotation about a rotation axis; a control cam coupled to the control shaft, wherein the control cam is eccentric with respect to the rotation axis of the control shaft; a rocker arm linked with the drive cam, and arranged to swing about the control cam in response to rotary motion of the drive cam, the rocker arm including a recess slidably engaged with an outer radial periphery of the control cam; and a swing cam linked with the rocker arm, and arranged to swing in response to swinging motion of the rocker arm for opening and closing an engine valve of the internal combustion engine. The variable valve actuating apparatus may further comprise: a device for maintaining contact between the recess of the rocker arm and the outer radial periphery of the control cam; and a drive shaft adapted to be rotated by the internal combustion engine, wherein: the drive cam is fixedly mounted to an outer radial periphery of the drive shaft; change of a rotational position of the control shaft causes a movement of the control cam with respect to the drive shaft, and causes at least a change in a lift of the engine valve; and the rotational position of the control shaft is controlled according to an operating state of the internal combustion engine. The variable valve actuating apparatus may further comprise: a drive shaft adapted to be rotated by the internal combustion engine, wherein the drive cam is mounted for rotation therewith to an outer radial periphery of the drive shaft; and an actuator for controlling a rotational position of the control shaft according to an operating state of the internal combustion engine, wherein the recess of the rocker arm has an entrance directed opposite to a portion of the rocker arm which is linked with the swing cam. The variable valve actuating apparatus may further comprise: a drive shaft adapted to be rotated by the internal combustion engine, wherein the drive cam is mounted for rotation therewith to an outer radial periphery of the drive shaft; and an actuator for controlling a rotational position of the control shaft according to an operating state of the internal combustion engine, wherein when the engine valve is open, the recess of the rocker arm is pressed on the outer radial periphery of the control cam by an elastic force of a valve spring provided for the engine valve. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a front view of a variable valve actuating system according to a first embodiment of the present invention in a position for a minimum lift set point. 
         FIG. 2  is a front view of the variable valve actuating system of  FIG. 1  in the position for the minimum lift set point at a moment when an associated intake valve is opened. 
         FIG. 3  is a front view of the variable valve actuating system of  FIG. 1  in a position for a maximum lift set point at a moment when the intake valve is closed. 
         FIG. 4  is a front view of the variable valve actuating system of  FIG. 1  in the position for the maximum lift set point at a moment when the intake valve is opened. 
         FIG. 5  is a plan view of the variable valve actuating system of  FIG. 1 . 
         FIG. 6  is a side view of the variable valve actuating system of  FIG. 1 . 
         FIG. 7  is a graphic diagram showing lift curves of the intake valve which are achieved by the variable valve actuating system of  FIG. 1 . 
         FIG. 8  is a front view of a variable valve actuating system according to a second embodiment of the present invention. 
         FIG. 9  is a plan view of the variable valve actuating system of  FIG. 8 . 
         FIG. 10  is a side view of the variable valve actuating system of  FIG. 8 . 
         FIG. 11  is a graphic diagram showing lift curves of intake valves which are achieved by the variable valve actuating system of  FIG. 8 . 
         FIG. 12  is an enlarged view of a structure in which a rocker arm engages with a control cam in a variable valve actuating system according to a third embodiment of the present invention. 
         FIG. 13  is a plan view of the variable valve actuating system according to the third embodiment. 
         FIG. 14  is a side view of the variable valve actuating system according to the third embodiment. 
         FIG. 15  is a front view of a variable valve actuating system according to a fourth embodiment of the present invention. 
         FIG. 16  is a front view of the variable valve actuating system of  FIG. 15  in a position for a minimum lift set point at a moment when an associated intake valve is opened. 
         FIG. 17  is a front view of the variable valve actuating system of  FIG. 15  in a position for a maximum lift set point at a moment when the intake valve is closed. 
         FIG. 18  is a front view of the variable valve actuating system of  FIG. 15  in the position for the maximum lift set point at a moment when the intake valve is opened. 
         FIG. 19  is a plan view of the variable valve actuating system of  FIG. 15 . 
         FIG. 20  is a side view of the variable valve actuating system of  FIG. 15 . 
         FIG. 21  is a front view of a variable valve actuating system according to a fifth embodiment of the present invention. 
         FIG. 22  is a front view of the variable valve actuating system of  FIG. 21  in a position for a minimum lift set point at a moment when an associated intake valve is opened. 
         FIG. 23  is a front view of the variable valve actuating system of  FIG. 21  in a position for a maximum lift set point at a moment when the intake valve is closed. 
         FIG. 24  is a front view of the variable valve actuating system of  FIG. 21  in the position for the maximum lift set point at a moment when the intake valve is opened. 
         FIG. 25  is a plan view of the variable valve actuating system of  FIG. 21 . 
         FIG. 26  is a side view of the variable valve actuating system of  FIG. 21 . 
         FIG. 27  is a plan view of a variable valve actuating system according to a sixth embodiment of the present invention. 
         FIG. 28  is a front view of the variable valve actuating system of  FIG. 27 . 
         FIG. 29  is a front view of a variable valve actuating system according to a seventh embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIGS. 1 to 6  show a variable valve actuating apparatus or system according to a first embodiment of the present invention for an engine valve set of an internal combustion engine which is an intake valve set in this example. The variable valve actuating system according to the first embodiment generally includes an intake valve set which includes two intake valves  3 ,  3  per cylinder, a drive shaft  4 , a drive cam  5  per cylinder, a swing arm set which includes two swing arms  6 ,  6  per cylinder, a swing cam set which includes two swing cams  7 ,  7  per cylinder, a transmitting mechanism  8  per cylinder, and a control mechanism  9 . Each intake valve  3  is slidably mounted in a valve guide not shown in a cylinder head  1 , for opening and closing an intake port formed in cylinder head  1 . Drive shaft  4  is hollow, having a longitudinal axis extending in a longitudinal direction of the engine. Drive cam  5  is fixedly mounted to drive shaft  4 . Each swing arm  6  is arranged close to an upper end of intake valve  3 . Each swing cam  7  opens and closes intake valve  3  by moving the swing arm  6 . Transmitting mechanism  8 , which is of a multiple link type, links drive cam  5  with swing cams  7 ,  7 , and converts rotary motion of drive cam  5  into swinging motion of swing cams  7 ,  7 . Controlled according to an engine operating state, control mechanism  9  varies the lift of intake valves  3 ,  3  by moving a fulcrum of a rocker arm  15  of transmitting mechanism  8 . 
     Each intake valve  3  is provided with a valve spring  10 , and biased by valve spring  10  in a direction to close the intake port, as shown in  FIG. 6 . Each valve spring  10  is disposed between the bottom of a substantially cylindrical bore formed or provided in an upper end portion of cylinder head  1 , and a spring retainer provided in an upper end portion of a valve stem of intake valve  3 . 
     Drive shaft  4  is rotatably mounted in cylinder head  1 . Drive shaft  4  includes longitudinal ends rotatably supported on bearings provided in an upper portion of cylinder head  1 . Drive shaft  4  is adapted to be rotated by a crankshaft of the engine. Specifically, drive shaft  4  receives a torque from the crankshaft through a rotation transmitting mechanism which is, for example, a chain drive mechanism including a timing sprocket provided at one longitudinal end of drive shaft  4 , and a timing chain wounded around the timing sprocket. When driven by the crankshaft, the drive shaft  4  rotates in a clockwise direction as shown by an arrow in  FIG. 1 . 
     Drive cam  5  is arranged between swing cams  7 ,  7  in the longitudinal direction of drive shaft  4 , shaped like a circular disc, and formed with a hole extending in a longitudinal direction of drive cam  5  (in the longitudinal direction of drive shaft  4 ), as shown in  FIGS. 1 and 6 . Drive cam  5  is coupled to or fixedly mounted on an outer radial periphery of drive shaft  4  for rotation therewith, where drive shaft  4  extends through the hole of drive cam  5 . Drive cam  5  is thus adapted to be rotated by the crankshaft of the engine. The hole is positioned in drive cam  5  so that drive cam  5  has an eccentric circular cam profile. The central axis Y of drive cam  5  is offset in a predetermined radial direction from the central axis X of drive shaft  4  by a predetermined distance. 
     Each swing arm  6  extends in a lateral direction of drive shaft  4 , and has a first end portion  6   a  which includes a slightly recessed lower surface in contact with the stem end of intake valve  3 , and a second end portion  6   b  which includes a semispherically recessed lower surface in contact with and slidably supported by the tip of a hydraulic lash adjuster  11  retained in a retainer hole  2  formed in cylinder head  1 . Swing arm  6  is thus arranged to swing about the tip of hydraulic lash adjuster  11 . Swing arm  6  is provided with a needle roller  12  substantially at a center of swing arm  6 . Needle roller  12  is rotatably supported with respect to swing arm  6 , and is in rolling contact with swing cam  7 , serving to reduce the friction between swing cam  7  and swing arm  6 . 
     Hydraulic lash adjuster  11  has a general structure, including a body  13   a,  and a plunger  13   b.  Body  13   a  is cylindrically shaped with an open top and a closed bottom, and inserted and fixedly mounted in retainer hole  2 . Plunger  13   b  is mounted in body  13   a  for sliding through the top opening of body  13   a.  Plunger  13   b  has a semispherical upper end in sliding contact with the second end portion  6   b  of swing arm  6 . Hydraulic fluid is supplied from a reservoir through a check valve to a high pressure chamber defined between the inner bottom of body  13   a  and the outer bottom of plunger  13   b.  Hydraulic lash adjuster  11  serves to constantly maintain the clearance between the tip of plunger  13   b  and the second end portion  6   b  of swing arm  6  (and the clearance between swing cam  7  and needle roller  12 ) substantially equal to zero by suitable supply of hydraulic fluid. 
     Each swing cam  7  includes a fitting recess  7   a  which is generally U-shaped, and fit on the outer radial periphery of drive shaft  4 , so that swing cam  7  is swingably supported with respect to drive shaft  4 , i.e. swing cam  7  is supported for swinging about the central axis X of drive shaft  4  with the fitting recess  7   a  in sliding contact with the outer radial periphery of drive shaft  4 . Swing cam  7  has a cam surface  7   b  at the lower side which is adapted to be in contact with needle roller  12  of swing arm  6 . The cam surface  7   b  of swing cam  7  includes a base circle surface region closer to drive shaft  4 , a ramp surface region extending like a circular arc from the base circle surface region toward a cam nose  7   c,  and a lift surface region extending from the ramp surface region toward an apex of the cam nose which defines a possible maximum lift set point of intake valve  3 . The cam surface  7   b  abuts on the top surface of a corresponding portion of the outer radial periphery of needle roller  12  of swing arm  6 , and the contact point of the cam surface  7   b  shifts among the base circle surface region, ramp surface region and lift surface region in dependence on the swing position of swing cam  7 . Swing cam  7  is arranged so that when swing cam  7  swings in the same direction as drive shaft  4  (clockwise direction as viewed in  FIG. 1 ), the contact point of cam surface  7   b  shifts toward the lift surface region for increasing the opening of intake valve  3 . The cam nose  7   c  of swing cam  7  is formed with a pin hole  7   e  extending in the longitudinal direction of drive shaft  4 . A connecting pin  20  is inserted through pin hole  7   e  for connecting swing cam  7  and a link rod  17 . The structure that swing cam  7  is directly mounted to drive shaft  4  eliminates the necessity of an additional support shaft for supporting swing cam  7 . This is effective for cost reduction and downsizing of the variable valve actuating system. Swing cam  7  is linked with rocker arm  15 , and arranged to swing in response to swinging motion of rocker arm  15  for opening and closing the intake valve  3 . 
     Transmitting mechanism  8  includes a rocker arm  15 , a link arm  16 , a link rod set which includes two link rods  17 ,  17  per cylinder, as shown in  FIG. 5 . Rocker arm  15  is arranged above drive shaft  4 , extending generally in the lateral direction of the engine. Rocker arm  15  is linked with drive cam  5 , and arranged to swing about control cam  26  in response to rotary motion of drive cam  5 , as detailed below. Link arm  16  links rocker arm  15  with drive cam  5 . Each link rod  17  links rocker arm  15  with the cam nose  7   c  of the respective swing cam  7 . 
     Rocker arm  15  includes a first longitudinal end portion defining a recess, and a second longitudinal end portion opposite to the first longitudinal end portion in a longitudinal direction of rocker arm  15 , wherein the second longitudinal end portion is linked with swing cam  7  for transmitting swinging motion of rocker arm  15  to swing cam  7 . Specifically, rocker arm  15  is generally A-shaped in a lateral direction of drive shaft  4  as shown in  FIG. 6 , and includes a first end portion  15   a,  and a longitudinal end portion set which is Y-shaped as viewed in  FIG. 5  and includes two second end portions  15   b,    15   b.  The first end portion  15   a  of rocker arm  15  includes a recess  21  which is slidably engaged with an outer radial periphery of a control cam  26  for allowing rotation of control cam  26 . The recess  21  of rocker arm  15  and the outer radial periphery of control cam  26  have shapes fit on each other. Each second  5  end portion  15   b  of rocker arm  15  includes a pin hole  15   c  extending in the longitudinal direction of drive shaft  4 . A connecting pin  22  extends through the pin holes  15   c,    15   c  of the second end portions  15   b,    15   b  which are coaxially positioned. 
     The recess  21  of rocker arm  15  has an inner cylindrical surface  21   b  which appears as a generally semicircular arc fit on the outer radial periphery of control cam  26  as viewed in the longitudinal direction of drive shaft  4  in  FIG. 1 . Recess  21  has an inner diameter that is slightly larger than the outer diameter of control cam  26 . Recess  21  includes an entrance  21   a  directed downward as viewed in  FIG. 1 , i.e. directed toward intake valve  3  or toward hydraulic lash adjuster  11 . The first end portion  15   a  of rocker arm  15  is thus slidably supported with respect to control cam  26  for rotating or swinging about control cam  26  so that the second end portions  15   b,    15   b  can move upward and downward. 
     The first end portion  15   a  of rocker arm  15  is biased by a compression coil spring  24  toward control cam  26  or toward drive shaft  4 . Compression coil spring  24  includes a first end fixed to a rocker cover  14 , and a second end in pressing contact with the outer periphery of first end portion  15   a.  Compression coil spring  24  serves as a device for mechanically pressing the recess  21  of rocker arm  15  on the outer radial periphery of control cam  26 , constantly maintaining contact between the inner cylindrical surface  21   b  of the recess  21  and the outer radial periphery of control cam  26 , and preventing the recess  21  of rocker arm  15  from escaping from control cam  26 . 
     Link arm  16  includes a circular portion  16   a,  and a projecting portion  16   b.  Circular portion  16   a  has a relatively large outer diameter, and has a fitting hole  16   c  at the center. The fitting hole  16   c  is slidably fit on the outer radial periphery of drive cam  5  for allowing relative rotation of drive cam  5 . Projecting portion  16   b  is projecting from circular portion  16   a  in a radial direction of circular portion  16   a,  and disposed between the second end portions  15   b,    15   b  of rocker arm  15  as viewed the lateral direction of drive shaft  4  in  FIG. 5 . Projecting portion  16   b  includes a pin hole  16   d  extending in the longitudinal direction of drive shaft  4  between both opposite surfaces, through which connecting pin  22  extends. Projecting portion  16   b  is thus rotatably supported with respect to second end portions  15   b,    15   b  through connecting pin  22 . 
     Each link rod  17  is composed of a single piece made by press forming and folding. Link rod  17  has a U-shaped cross section as viewed in  FIG. 5 , which is advantageous in compactness. Link rods  17 ,  17  are arranged outside of the second end portions  15   b,    15   b  of rocker arm  15  in the longitudinal direction of drive shaft  4  as viewed in  FIG. 5 . Link rod  17  has a first end portion  17   a  including a pin hole through which connecting pin  22  extends, and a second end portion  17   b  including a pin hole through which connecting pin  20  extends. Link rod  17  is rotatably connected at the first end portion  17   a  to the second end portion  15   b  of rocker arm  15  through connecting pin  22 , and rotatably connected at the second end portion  17   b  to the cam nose  7   c  of swing cam  7  through connecting pin  20 . 
     As described above, connecting pin  22  pivotally connects all of the projecting portion  16   b  of link arm  16 , the first end portions  17   a  of link rods  17 , and the second end portions  15   b,    15   b  of rocker arm  15 . Connecting pin  22  is provided with snap rings  22   a,    22   a  at both longitudinal ends for preventing the connected members from escaping from the connecting pin  22 . Also, each connecting pin  20  is swaged at both longitudinal ends so as to prevent the link rod  17  and swing cam  7  from escaping from connecting pin  20 . 
     Control mechanism  9  includes a control shaft  25 , control cam  26 , and an actuator  41 . Control shaft  25  is arranged above drive shaft  4 , extending in parallel to drive shaft  4 . Control shaft  25  is supported for rotation about a rotation axis which is a central axis P of control shaft  25 . Control cam  26  is coupled or fixedly mounted to control shaft  25  for serving as a fulcrum for rocking motion of rocker arm  15 . The rotational position of control shaft  25  is regulated or controlled by actuator  41 . 
     Control shaft  25  includes a pair of shaft parts on both sides of the center of the cylinder in the longitudinal direction of control shaft  25  as shown in  FIG. 5 . The ends of the shaft parts confronting each other are provided with flanges  25   a,    25   a  between which the first end portion  15   a  of rocker arm  15  is slidably supported. The flanges  25   a,    25   a  of control shaft  25  serves to prevent the first end portion  15   a  from inclining when rocker arm  15  is rocking. 
     Control cam  26  is formed in a cylindrical shape having a smaller outer diameter than control shaft  25 . Control cam  26  is supported by and arranged between the flanges  25   a,    25   a  of control shaft  25  as viewed in  FIG. 5 . Control cam  26  is eccentric with respect to the central axis P of control shaft  25 . Specifically, control cam  26  has a central axis Q which is offset from the central axis P of control shaft  25  by an eccentric distance a, where the eccentric distance a is substantially as large as the diameter of control shaft  25 , as shown in  FIG. 1 . Accordingly, control cam  26  has an outer portion  26   a  projecting beyond the outer radial periphery of control shaft  25 , where the outer portion  26   a  is substantially half of control cam  26 , as viewed in  FIG. 1 . Control shaft  25  and control cam  26  are thus formed as a unit shaped like a crank. 
     Actuator  41  in control mechanism  9  includes an electric motor, and a speed reducer, such as a ball screw mechanism. The electric motor is fixed to a rear end wall of cylinder head  1 . The speed reducer is arranged to transmit an output torque of the electric motor to control shaft  25 . The electric motor is a linear DC motor which is driven according to a control signal outputted from an electric controller not shown. The controller measures or calculates an engine operating state with reference to feedback signals from sensors, and controls the electric motor according to the engine operating state. The sensors include a crank angle sensor for measuring engine speed, an airflow meter for measuring intake air quantity, an engine coolant temperature sensor for measuring engine coolant temperature, and a potentiometer for measuring the rotational position of control shaft  25 . 
     The following describes operations of the variable valve actuating system according to the first embodiment. The rotational position of control shaft  25  is controlled according to the engine operating state, and change of the rotational position of control shaft  25  causes a movement of control cam  26  with respect to drive shaft  4 , and causes at least a change in the lift of intake valve  3 , as detailed below. 
     For example, when the engine is at idle or at low speed, the controller issues such a control signal to the electric motor of control mechanism  9  that the electric motor rotates and outputs a torque which is transmitted to control shaft  25  through the speed reducer, and accordingly, control shaft  25  rotates in a clockwise direction by a corresponding angle as viewed in  FIG. 1 . The rotation of control shaft  25  causes the central axis Q of control cam  26  to revolve about the central axis P of control shaft  25  to a position below and slightly on the left of the central axis P so that the outer portion  26   a  of control cam  26  moves away from drive shaft  4 . This moves the rocker arm  15  leftward so that an angle θ between rocker arm  15  and link arm  16  increases, and moves the connecting pin  22  generally in the counterclockwise direction about drive shaft  4  as viewed in  FIG. 1 . Accordingly, swing cam  7  is rotated through link rod  17  in the counterclockwise direction as viewed in  FIG. 1  so that the cam nose  7   c  of swing cam  7  is moved upward. When drive cam  5  rotates under the condition described above and shown in  FIG. 1 , the second end portions  15   b,    15   b  of rocker arm  15  are moved upward and downward through link arm  16 . When the second end portions  15   b,    15   b  are moved downward as shown in  FIG. 1 , swing cam  7  is moved downward through link rod  17 . Under this condition, the amount of depression of swing arm  6  caused by the cam surface  7   b  of swing cam  7  is relatively small. This setting achieves a minimum valve lift set point. 
     In this way, when the engine is at idle or at low speed, the lift of each intake valve  3  is set by the variable valve actuating system at the minimum lift set point L as shown in  FIG. 7 . This retards the opening timing of intake valve  3  (intake valve opening timing IVO), and produces no valve overlap in which both of intake valve  3  and an exhaust valve are opened. This provides the engine with a small pumping loss, an improved combustion process, an improved fuel efficiency, and stable rotation performance. 
     For example, when the engine shifts to a predetermined high speed region from a predetermined low speed region, the controller issues such a control signal that the electric motor and speed reducer of control mechanism  9  rotate in a reverse direction, and accordingly, control shaft  25  rotates control cam  26  in the counterclockwise direction as viewed in  FIG. 1 . The rotation of control shaft  25  causes the central axis Q of control cam  26  to revolve about the central axis P of control shaft  25  to a position below and on the right of the central axis P of control shaft  25  as shown in  FIGS. 3 and 4  so that the outer portion  26   a  of control cam  26  moves toward drive shaft  4 . This moves the rocker arm  15  rightward so that the angle θ between rocker arm  15  and link arm  16  decreases, and moves the connecting pin  22  generally in the clockwise direction about drive shaft  4  as viewed in  FIG. 3 . Accordingly, swing cam  7  is rotated through link rod  17  in the clockwise direction as viewed in  FIG. 3  so that cam nose  7   c  of swing cam  7  is moved downward. Accordingly, the contact point of cam surface  7   b  of swing cam  7  with respect to needle roller  12  of swing arm  6  moves toward the cam nose  7   c  (toward the lift region). When drive cam  5  rotates under the condition described above and shown in  FIG. 3 , the second end portions  15   b,    15   b  of rocker arm  15  are moved upward and downward through link arm  16 . When second end portions  15   b,    15   b  are moved downward as shown in  FIG. 4 , swing cam  7  is moved downward through link rod  17 . Under this condition, the amount of depression of swing arm  6  caused by the cam surface  7   b  of swing cam  7  is relatively large. This setting achieves a maximum valve lift set point. 
     In this way, when the engine is at high speed, the lift of intake valve  3  is set by the variable valve actuating system at the maximum lift set point L 1  as shown in  FIG. 7 . This advances the opening timing of intake valve  3  (intake valve opening timing IVO) so as to produce and increase a valve overlap in which both of intake valve  3  and the exhaust valve are opened, and retards the closing timing of intake valve  3  (intake valve closing timing IVC). This provides the engine with an improved intake air charging efficiency, and thereby, a high engine output. 
     When the engine returns from the high speed region to the low speed region, the lift of the intake valve  3  is set to the minimum lift set point L. The central axis Q of control cam  26  as a pivot for swinging motion of the first end portion  15   a  of rocker arm  15  moves to the position shown in  FIG. 1  so as to move the link arm  16  in the counterclockwise direction about drive cam  5  with respect to the position for the maximum lift set point L 1 . This advances the lift peak phase so that the intake valve opening timing IVO advances little at about top dead center, and the intake valve closing timing IVC advances significantly, with respect to the condition of the maximum lift set point L 1 , as shown in  FIG. 7 . The intake valve opening timing IVO closer to top dead center for the condition of the minimum lift set point L is effective for allowing a suitable control of valve overlap. This makes it possible to reduce an increase of residual burned gas, and suppress adverse effects on fuel efficiency, at the minimum lift set point L. 
     In this way, the variable valve actuating system can control the lift and operating angle of the engine valve set continuously between the minimum lift set point L and the maximum lift set point L 1  by actuating the control shaft  25  according to engine operating state. 
     The provision of the recess  21  of the first end portion  15   a  of rocker arm  15  enables an operator to easily attach the rocker arm  15  to control cam  26  only by engaging the recess  21  of rocker arm  15  with control cam  26  in the radial direction of control cam  26 . This leads to a low manufacturing cost in view of efficiency of assembling operation. 
     The arrangement that the projecting portion  16   b  of link arm  16  and the first end portion  17   a  of link rod  17  are connected through the common connecting pin  22  at the second end portions  15   b,    15   b  of rocker arm  15 , is effective for allowing the reaction force of the valve spring  10  applied to link rod  17  and the driving torque applied from drive cam  5  to link arm  16  to cancel each other when intake valve  3  opens and closes under the condition of the maximum lift set point L 1 . Accordingly, the load acting on the central axis Q of control cam  26  from rocker arm  15  is relatively small, and the required driving torque of actuator  41  is relatively small accordingly. 
     The crank structure of control shaft  25  and control cam  26  allows a large eccentric distance a between the central axis P of control shaft  25  and the central axis Q of control cam  26 . This allows a large range of movement of the central axis Q as a fulcrum of rotation of rocker arm  15 , and a large amount of change of the lift, operating angle, and peak lift phase, of intake valve  3 . The crank structure of control shaft  25  and control cam  26  is also effective for reducing the outer diameter of control shaft  25 . This leads to downsizing of parts such as bearings and thereby downsizing of the entire variable valve actuating system. The simple structure of the first end portion  15   a  of rocker arm  15  also leads to downsizing of the entire variable valve actuating system. 
     The structure that the first end portion  15   a  of rocker arm  15  is sandwiched and slidably held between the flanges  25   a,    25   a  of control shaft  25 , is effective for preventing the first end portion  15   a  from inclining when rocker arm  15  is rocking, and thereby, is effective for preventing abnormal wear between control cam  26  and first end portion  15   a.    
     The structure of transmitting mechanism  8  where the elastic force of valve spring  10  and the driving force of drive cam  5  are well balanced is effective for preventing stress concentration and occurrence of deformation. 
     The provision of compression coil spring  24  is effective for keeping constant contact between the recess  21  of rocker arm  15  and the outer radial periphery of control cam  26 , and thereby efficiently transmitting the movement of control cam  26  to rocker arm  15 . 
     When the engine is stopping, an alternating torque occurs in the engine and acts on control cam  26  in the direction toward the minimum lift set point L. However, when actuator  41  applies no torque to control shaft  25 , the elastic force of compression coil spring  24  is effective for mechanically moving and holding the control cam  26  to a position for an intermediate lift set point between the minimum lift set point L and maximum lift set point L 1 , specifically, to a position slightly displaced from the position for the minimum lift set point L toward the position for the maximum lift set point L 1 . This enables the engine to restart well. 
     The structure that control cam  26  is located at the first end portion  15   a  of rocker arm  15  is effective for reducing the contact load between recess  21  and control cam  26  because the elastic force of valve spring  10  is absorbed by drive cam  5 . 
     The structure that the inner peripheral surface of recess  21  has substantially the same shape as the outer radial periphery of control cam  26 , is effective for providing a wide contact area between rocker arm  15  and control cam  26 , and thereby preventing stress concentration therebetween. 
       FIGS. 8 to 10  show a variable valve actuating system according to a second embodiment of the present invention. In the second embodiment, the structures of rocker arm  15  and swing cams  7 ,  7  are modified, and two drive cams  5 ,  5  and two link arms  16 ,  16  are provided for swing cams  7 ,  7 , with respect to the first embodiment. 
     Rocker arm  15  is shaped so that the entrance  21   a  of recess  21  is directed at an angle θ 1  of about 30 degrees downward with reference to a line connecting the central axis Q of control cam  26  and the central axis O of connecting pin  22 , and the second end portion  15   b  is single, not branched. The second end portion  15   b  has a tip having a small width formed with a pin hole  15   c  through which connecting pin  22  extends. The second end portion  15   b  is rotatably connected substantially to the center of connecting pin  22  in the longitudinal direction of connecting pin  22 . 
     Compression coil spring  24  for biasing the first end portion  15   a  of rocker arm  15  is located at the position shown in  FIG. 8  along the direction toward the central axis Q of control cam  26  so that the inner cylindrical surface  21   b  of the recess  21  is constantly pressed to the outer radial periphery of control cam  26 . 
     Each drive cam  5  is fixed to or integrated with drive shaft  4 , and is arranged at an interval from the other drive cam  5 , as shown in  FIG. 10 . Each link arm  16  includes a circular portion  16   a,  and a projecting portion  16   b,  as in the first embodiment. The circular portion  16   a  includes the fitting hole  16   c  in which drive cam  5  is rotatably supported. The projecting portion  16   b  is rotatably supported by connecting pin  22  extending through the pin hole  16   d.  Projecting portions  16   b,    16   b  are arranged on both sides of the second end portion  15   b  of rocker arm  15  in the longitudinal direction of drive shaft  4  as shown in  FIG. 9 . 
     The cam surfaces  7   b,    7   b  of swing cams  7 ,  7  have different profiles as shown in  FIG. 8 . The cam surface  7   b  of one swing cam  7  is concaved or offset with respect to the cam surface  7   b  of the other swing cam  7 . 
     The drive shaft  4  and control shaft  25  are rotatably supported on bearings which are arranged between transmitting mechanisms  8 ,  8  in the longitudinal direction. 
     The structure of recess  21  that entrance  21   a  is directed downward by the angle of about 30 degrees with respect to the reference line is effective for providing a tight contact between the rocker arm  15  and control cam  26 , and allowing the rocker arm  15  to follow the eccentric movement of control cam  26  well. Even when the difference between the profiles of the cam surfaces  7   b,    7   b  of swing cams  7 ,  7  causes the rocker arm  15  via connecting pin  22  to incline, the structure of recess  21  is also effective for resisting the moment of inclination of rocker arm  15  and connecting pin  22 . This enables the variable valve actuating system to precisely control the lift and operating angle of intake valve  3 . 
     The structure that two drive cams  5 ,  5  are arranged close to each other in the longitudinal direction of drive shaft  4  per cylinder, is effective for preventing the connecting pin  22  and rocker arm  15  from inclining, even when drive cams  5 ,  5  are shaped to have different cam profiles so as to differentiate the lifts of intake valves  3 ,  3  from each other. 
     The structure that the cam surfaces  7   b,    7   b  of swing cams  7 ,  7  have different profiles, is effective for slightly differentiating the lifts of intake valves  3 ,  3  from each other under the condition of the minimum lift set point L as shown in  FIG. 11 , promoting in cylinder intake swirl, improving the combustion process, stabilizing the engine rotation, and enhancing the fuel efficiency. 
       FIGS. 12 to 14  show a variable valve actuating system according to a third embodiment of the present invention. This variable valve actuating system is presented by modifying the first embodiment so that two control cams  26 ,  26  are provided per cylinder, and accordingly, rocker arm  15  has two branched first end portions  15   a,    15   a.  Control shaft  25  and drive shaft  4  are rotatably supported by bearings not shown. 
     Each first end portion  15   a  of rocker arm  15  has a C-shaped recess  21 , where the entrance  21   a  of recess  21  has a narrower entrance width Z than the outer diameter of control cam  26 , as shown in  FIG. 12 . On the other hand, each control cam  26  has flat surfaces  26   b,    26   b  at the outer radial periphery which are parallel to each other, as shown in  FIG. 12 . The distance between flat surfaces  26   b,    26   b  is smaller than the outer diameter of control cam  26 , and slightly smaller than the entrance width Z of recess  21 . The flat surfaces  26   b,    26   b  of one control cam  26  is located in the same circumferential position as those of the other control cam  26 . In this way, the outer radial periphery of control cam  26  partly has a thickness in a radial direction of control cam  26  which is narrower than the entrance  21   a  of recess  21  of rocker arm  15 , and control cam  26  is engaged with rocker arm  15  in such a rotational position that control cam  26  is prevented from escaping through the entrance  21   a  from the recess  21  of rocker arm  15 . 
     Attaching the rocker arm  15  to control cams  26 ,  26  is implemented by allowing the edges of entrance  21   a  of each recess  21  to be in contact with the edges of the flat surfaces  26   b,    26   b  of control cam  26 , then sliding the rocker arm  15  in the radial direction of control cams  26 ,  26  so as to fit the inner cylindrical surface  21   b  of each recess  21  to a portion of the outer radial periphery of control cam  26  between the flat surfaces  26   b,    26   b,  finally rotating the rocker arm  15  about the control cams  26 ,  26  to a predetermined rotational position. Each recess  21  is thus engaged with the circular outer radial periphery of control cam  26 , and prevented from escaping from control cam  26 . In this way, attaching the rocker arm  15  to control cam  26  is simply implemented so as to achieve stable and reliable engagement between rocker arm  15  and control cam  26 . According to the structure described above, compression coil spring  24  for maintaining the rocker arm  15  in contact with control cam  26  is omitted, in contrast to the first embodiment. This leads to reduction in the number of parts, and thereby leads to reduction in manufacturing cost, and improvement in assembling operation. 
     The structure that rocker arm  15  is rotatably supported by control cams  26 ,  26  which are arranged at an interval, is effective for preventing the rocker arm  15  from inclining, even when the cam surfaces  7   b,    7   b  of swing cams  7 ,  7  have different profiles. 
       FIGS. 15 to 20  show a variable valve actuating system according to a fourth embodiment of the present invention. In this embodiment, two transmitting mechanisms  8 ,  8  are provided for intake valves  3 ,  3  independently of each other. Specifically, as shown in  FIGS. 19 and 20 , two drive cams  5 ,  5  are provided per intake valve  3 , and the total four drive cams  5  are attached to drive shaft  4  at predetermined intervals. Accordingly, link arm  16  is provided per drive cam  5 . 
     Control shaft  25  are provided with two control cams  26 ,  26  for two intake valves  3 ,  3 . Control shaft  25  has flat surfaces  25   b,    25   b  at the outer radial periphery each of which is located outside of control cam  26  in the longitudinal direction. Each control cam  26  is linked with the respective one of two rocker arms  15 ,  15 . As in the third embodiment, each first end portion  15   a  has a C-shaped recess  21 , where the entrance  21   a  of recess  21  has a narrower entrance width Z than the outer diameter of control cam  26 . On the other hand, each control cam  26  has flat surfaces  26   b,    26   b  at the outer radial periphery which are parallel to each other. The distance between flat surfaces  26   b,    26   b  is smaller than the outer diameter of control cam  26 , and slightly smaller than the entrance width Z of the recess  21  of rocker arm  15 . The entrance  21   a  of each recess  21  is directed substantially in the longitudinal direction of rocker arm  15 , or in the longitudinal direction of the first end portion  15   a  of rocker arm  15  toward the longitudinal end of rocker arm  15 , opposite to connecting pin  22  which is linked with swing cam  7  and is related to actuation of intake valve  3 . Control shaft  25  and drive shaft  4  are supported by a bearing  33 . 
     Each rocker arm  15  has a pin hole  15   c  at the second end portion  15   b  through which connecting pin  22  passes. Rocker arm  15  is rotatably linked with the first end portion  17   a  of link rod  17  through the connecting pin  22 . Each rocker arm  15  also has a pin hole  15   d  substantially at the center of rocker arm  15  in the longitudinal direction through which a respective connecting pin  28  passes. Rocker arm  15  is rotatably linked with the projecting portions  16   b,    16   b  of link arms  16 ,  16  through the connecting pin  28 . One pair of link arms  16 ,  16  are arranged on both sides of one combination of rocker arm  15  and link rod  17 , and the outer pair of link arms  16 ,  16  are arranged on both sides of the other combination of rocker arm  15  and link rod  17 , in the longitudinal direction of drive shaft  4 , as shown in  FIGS. 19 and 20 . 
     The variable valve actuating system described above operates as follows. For example, when the engine is at low speed and low load, control shaft  25  is controlled to rotate to the position shown in  FIGS. 15 and 16  as in the first embodiment, so that the lift of intake valve  3  is set at the minimum lift set point L. When the engine shifts to a predetermined high speed region, control shaft  25  is controlled to rotate in the counterclockwise direction as shown in  FIGS. 17 and 18 , so that the lift of intake valve  3  is set at the maximum lift set point L 1 . 
     Attaching the rocker arm  15  to control cam  26  is implemented as in the third embodiment by allowing the edges of entrance  21   a  of recess  21  to be in contact with the edges of the flat surfaces  26   b,    26   b  of control cam  26 , then sliding the rocker arm  15  in the radial direction of control cam  26  so as to fit the inner cylindrical surface  21   b  of recess  21  to a portion of the outer radial periphery of control cam  26  between the flat surfaces  26   b,    26   b,  finally rotating the rocker arm  15  about the control cam  26  to a predetermined rotational position. Each recess  21  is thus engaged with the circular outer radial periphery of control cam  26 , and prevented from escaping from control cam  26 . In this way, attaching the rocker arm  15  to control cam  26  is simply implemented so as to achieve stable and reliable engagement between rocker arm  15  and control cam  26 . According to the structure described above, compression coil spring  24  for maintaining the rocker arm  15  in contact with control cam  26  is omitted, in contrast to the first embodiment. 
     The structure that the entrance  21   a  of recess  21  is directed opposite to connecting pin  22  is effective for maintaining tight contact between rocker arm  15  and control cam  26  without compression coil spring  24 , because the force from valve spring  10  is transmitted to act the rocker arm  15  in the direction from the inner cylindrical surface  21   b  of recess  21  toward the outer radial periphery of control cam  26 . 
     The structure that each intake valve  3  is provided with the respective drive cam  5  and rocker arm  15  is effective for allowing to adjust the lift of intake valve  3  independently of the other intake valve  3  by adjusting the profiles of drive cam  5  and control cam  26  independently of the other drive cam  5  and control cam  26 . 
     The structure that the projecting portion  16   b  of link arm  16  is connected to substantially the center of rocker arm  15  is effecting for expanding the range of movement of link rod  17  and swing cam  7 . Simultaneously, the load acting on the recess  21  of rocker arm  15  increases. However, the increased load can be resisted because the recess  21  is rightly and stably engaged with control cam  26 . 
       FIGS. 21 to 26  show a variable valve actuating system according to a fifth embodiment of the present invention. In this embodiment, drive cam  5  is oval-shaped as viewed in the longitudinal direction of drive shaft  4  in  FIG. 21 . Rocker arm  15  is arranged to extend generally in the horizontal direction above the drive cam  5 , and includes a roller support shaft  30  on which a roller  29  is rotatably supported. Roller  29  is in rolling contact with the outer radial periphery of drive cam  5 . 
     Specifically, drive cam  5  is prepared separately from or independently of drive shaft  4 , including a sleeve  5   c  as shown in  FIG. 25 . Drive cam  5  is fixed to the outer radial periphery of drive shaft  4  by a fixing pin  35  which passes through the sleeve  5   c  and drive shaft  4 . Each cylinder is provided with one drive cam  5 . Drive cam  5  has such an asymmetrical cam profile that at least at the maximum lift set point L 1 , intake valve  3  accelerates more rapidly when ascending to a peak of lift than when descending from the peak. Drive cam  5  includes an ascending flank  5   a  and a descending flank  5   b  which have different profiles as shown in  FIG. 21 . The ascending flank  5   a  is substantially straight, while the descending flank  5   b  is curved outwardly. Accordingly, the acceleration of intake valve  3  is larger when ascending (opening) than when descending (closing). 
     Rocker arm  15  is integrally formed, and shaped like a crank as viewed in  FIG. 25 . The first end portion  15   a  of rocker arm  15  is formed with a recess  21  which is directed upward as viewed in  FIG. 21  and slidably engaged with control cam  26 . The second end portion  15   b  of rocker arm  15  is rotatably supported with respect to the first end portion  17   a  of link rod  17  through the connecting pin  22  which passes through the pin hole  15   c.  Recess  21  is U-shaped as in the first embodiment, and the entrance  21   a  is directed upward in the direction of gravity or opposite to a direction in which a force of gravity acts. 
     Roller  29  is rotatably supported on roller support shaft  30  in a space  31  which is defined in the center of rocker arm  15  as shown in  FIG. 25 . 
     Swing cams  7 ,  7  are arranged at a predetermined interval, and connected to each other by a cylindrical member  32 , as shown in  FIG. 25 . Each swing cam  7  is triangularly shaped, including a cam surface  7   b  at the bottom edge. Cylindrical member  32  is rotatably supported on the outer radial periphery of drive shaft  4 . 
     Cylindrical member  32  includes a journal  32   a  at the center in the longitudinal direction as shown in  FIG. 25 , and rotatably supported on bearing  33  which is provided at an upper end portion of cylinder head  1 . Drive shaft  4  is rotatably supported inside of and by cylindrical member  32 . 
     Link rod  17  is liked with swing cam  7  by connecting pin  20  which passes through the hole of second the end portion  17   b  of link rod  17  and a projecting portion  7   d  of swing cam  7 , where the projecting portion  7   d  is formed opposite to the cam nose  7   c.    
     Compression coil spring  24  is arranged between the roller  29  and the tip of the second end portion  15   b  of rocker arm  15 . Compression coil spring  24  has a relatively large spring constant, and pushes the second end portion  15   b  of rocker arm  15  downward. Accordingly, roller  29  is pressed on the outer radial periphery of drive cam  5  in a radial direction of drive cam  5 . Simultaneously, the first end portion  15   a  of rocker arm  15  is pushed upward, and the inner cylindrical surface  21   b  of recess  21  is pressed on the bottom surface of control cam  26 , while roller  29  serves as a fulcrum. 
     For example, when the engine is operating at low speed and low load, control shaft  25  is controlled to rotate so that the central axis Q of control cam  26  is substantially in a position just above the central axis P of control shaft  25 , as shown in  FIGS. 21 and 22 . Accordingly, the second end portion  15   b  of rocker arm  15  is moved to a relatively low position so that swing cam  7  is rotated in the counterclockwise direction via link rod  17 , and a portion of cam surface  7   b  closer to the base circle is in contact with needle roller  12  of swing arm  6 . Under the condition described above, the range of movement of the contact point of swing cam  7  is close to the base circle as shown in  FIGS. 21 and 22 , so that the lift of intake valve  3  is set at the minimum lift set point L. 
     On the other hand, when the engine shifts to a predetermined high speed and high load region, control shaft  25  is controlled to rotate in the clockwise direction so that the central axis Q of control cam  26  is moved to a position below and on the right of the central axis P of control shaft  25 , as shown in  FIGS. 23 and 24 . Accordingly, the second end portion  15   b  of rocker arm  15  is moved to a relatively high position so that swing cam  7  is rotated in the clockwise direction via link rod  17 , and a portion of cam surface  7   b  closer to the lift region is in contact with needle roller  12  of swing arm  6 . Under the condition described above, the range of movement of the contact point of swing cam  7  is close to the cam nose  7   c  as shown in  FIGS. 23 and 24 , so that the lift of intake valve  3  is set at the maximum lift set point L 1 . 
     When the central axis Q of control cam  26  is just above the central axis P of control shaft  25  as shown in  FIGS. 21 and 22 , rocker arm  15  is moved to the left as compared to the position shown in  FIGS. 23 and 24 . This movement is opposite to the direction of rotation of drive shaft  4 . Accordingly, the peak lift phase of intake valve  3  advances as shown in  FIG. 7 , as compared to the position shown in  FIGS. 23 and 24 . This changes the intake valve opening timing IVO little, and changes the intake valve closing timing IVC significantly. This is effective as in the first embodiment. The construction that the acceleration of intake valve  3  is larger when ascending (opening) than when descending (closing), is effective for preventing the drive cam  5  from bounding out of contact with roller  29 , and thereby, preventing the intake valve  3  from disordered movement. 
     The structure that the recess  21  of rocker arm  15  is directed upward in the direction of gravity, is effective for allowing the recess  21  to collect lubricating fluid which is scattered inside the housing of the variable valve actuating system, and thereby improving the condition of lubrication between rocker arm  15  and control cam  26 . The recess  21  of rocker arm  15  according to this embodiment is also effective in view of assembling and downsizing of the variable valve actuating system as in the first embodiment. When intake valve  3  is open, the recess  21  of rocker arm  15  is also pressed on the outer radial periphery of control cam  26  by the elastic force of 10. 
       FIGS. 27 and 28  show a variable valve actuating system according to a sixth embodiment of the present invention. In this embodiment, the structure of swing cam  7  is modified with respect to that in the fifth embodiment. Specifically, an upper portion of the structure of cylindrical member  32  and swing cams  7 ,  7  is cut off so as to form a U-shaped recess  34  whose inner diameter is slightly larger than the outer diameter of drive shaft  4  so that the recess  34  engages with the outer radial periphery of drive shaft  4 . The U-shape of cylindrical member  32  is possible, because the projecting portion  7   d  is located opposite (about 180 degrees) to the cam nose  7   c  with respect to cylindrical member  32 , so that cylindrical member  32  is resistant to inclination with respect to drive shaft  4 , although projecting portion  7   d  is provided offset to one swing cam  7 . Cylindrical member  32  is provided with ribs  34   b,    34   b  which are projecting radially from cylindrical member  32 , in order to compensate for a fall in bending rigidity of cylindrical member  32  due to the U-shape. 
     The construction described above is effective for allowing to attach the swing cams  7 ,  7  to drive shaft  4  by engaging the drive shaft  4  with recess  34  through entrance  34   a  in the radial direction of drive shaft  4 , not in the longitudinal direction of drive shaft  4 , and thus making it easy to attach the swing cams  7 ,  7  to drive shaft  4 , although drive shaft  4  is provided with flanges at the longitudinal ends for positioning so that it may be difficult to attach the swing cams  7 ,  7  to drive shaft  4  in the longitudinal direction. The integrated structure of drive cam  5  and drive shaft  4  is effective for reducing the sizes of the structure in the radial direction and longitudinal direction, and thereby reducing the size of the entire variable valve actuating system. The variable valve actuating system according to the sixth embodiment produces other advantageous effects as in the fifth embodiment. 
       FIG. 29  shows a variable valve actuating system according to a seventh embodiment of the present invention. In this embodiment, the shape of drive cam  5  is modified into a circular disc as in the first embodiment, as compared to the fifth and sixth embodiments. 
     Specifically, drive cam  5  in the form of a circular disc is attached to drive shaft  4  in a manner that the central axis Y of drive cam  5  is arranged eccentric by a predetermined distance from the central axis X of drive shaft  4 , and the outer radial periphery of drive cam  5  is slidably fit on the circular portion  16   a  of link arm  16 . 
     Link arm  16  is rotatably supported with respect to rocker arm  15  by connecting pin  28  which passes through the pin hole  15   c.  Pin hole  15   c  is formed in a substantially central portion of rocker arm  15  in the longitudinal direction of rocker arm  15 . 
     The first end portion  15   a  of rocker arm  15  is pressed by compression coil spring  24  upward and leftward to control cam  26  in the radial direction of control cam  26 , so that the inner cylindrical surface  21   b  of recess  21  is constantly maintained in contact with the outer radial periphery of control cam  26 . Rocker arm  15  is linked at the second end portion  15   b  with the projecting portion  7   d  of swing cam  7  through connecting pins  20  and  22  and link rod  17 , as in the fifth embodiment. 
     The variable valve actuating system according to the sixth embodiment operates as in the fifth embodiment, but differs from the fifth embodiment in that the spring constant of compression coil spring  24  may be small, because movement of rocker arm  15  is supported by drive cam  5  via link arm  16 . The spring constant of compression coil spring  24  may be small, if the first end portion  15   a  of rocker arm  15  as a fulcrum of movement of rocker arm  15  can be softly supported by compression coil spring  24 . 
     The structure that compression coil spring  24  presses the first end portion  15   a  of rocker arm  15  from below, is effective for eliminating the necessity of arrangement of compression coil spring  24  above rocker arm  15 , and thereby reducing the height of the entire variable valve actuating system. 
     The variable valve actuating system according to the present embodiments may be applied to exhaust valves. The locations of control shaft  25  and control cam  26  may be adjusted according to specifications and sizes of the variable valve actuating system. 
     The entire contents of Japanese Patent Application 2008-074824 filed Mar. 24, 2008 are incorporated herein by reference. 
     Although the invention has been described above by reference to certain embodiments of the invention, the invention is not limited to the embodiments described above. Modifications and variations of the embodiments described above will occur to those skilled in the art in light of the above teachings. The scope of the invention is defined with reference to the following claims.