Variable-valve-actuation apparatus for internal combustion engine

Abstract: In a VVA apparatus for an internal combustion engine wherein the position of contact of a cam face of a VO cam with respect to the top face of a valve lifter is changed in accordance with a change in a rocking fulcrum of a rocker arm to alter the valve lift, the maximum valve lift is obtained in a first rotated position of a control shaft where an axis of a control cam is adjacent to a driving shaft. The minimum valve lift is obtained in a second rotated position of the control shaft where the axis of the control cam is positioned near a first pivotal point of the rocker arm and a crank arm with respect to a first line connecting the axis of the control shaft and the axis of the control cam upon the maximum valve lift.

Description: BACKGROUND OF THE INVENTION The present invention relates to a variable-valve-actuation (VVA) apparatus for an internal combustion engine, which can change, particularly, the valve lift of an intake or exhaust valve in accordance with the engine operating conditions. One of the VVA apparatus is shown in JP-A 11-107725. This VVA apparatus, applied to intake valves, comprises a crank cam arranged on the outer periphery of a driving shaft rotated together with a crankshaft and having an axis eccentric to an axis of the driving shaft, and a valve operating (VO) cam to which torque of the crank cam is transmitted through a transmission mechanism to have a cam face coming in slide contact with the top face of a valve lifter arranged at the upper end of the intake valve for operation thereof. The transmission mechanism includes a rocker arm disposed above the VO cam and swingably supported to a control shaft, a crank arm having an annular base engaged with the outer peripheral surface of the crank cam and an extension rotatably connected to a first arm of the rocker arm through a pin, and a link rod having a first end rotatably connected to a second arm of the rocker arm through a pin and a second end rotatably connected to an end of the VO cam through a pin. Moreover, fixed on the outer peripheral surface of the control shaft is a control cam having an axis eccentric to an axis of the control shaft by a predetermined amount and rotatably fitted in a support hole formed substantially in the center of the rocker arm. The control cam changes a rocking fulcrum of the rocker arm in accordance with the rotated position to change the position of contact of the cam face of the valve operating cam with respect to the top face of the valve lifter, carrying out variable control of the valve lift of the intake valve. Specifically, when the engine operating conditions are in the high-rotation and high-load range, in order to urge an actuator to rotate the control cam in one direction through the control shaft for rotation of the control cam in the same direction, the rocking fulcrum of the rocker arm is moved to approach the driving shaft. Then, an end or a cam nose of the VO cam is pushed downward by the link rod, etc. to move the position of contact of the cam face of the VO cam with respect to the top face of the valve lifter to a lift section of the cam face. Thus, the intake valve is controlled to have the maximum valve-lift characteristic. On the other hand, when the engine operating conditions are in the low-rotation and low-load range, the actuator rotates the control shaft in another direction for rotation of the control cam in the same direction, moving the rocking fulcrum of the rocker arm to separate from the driving shaft. Then, the pivotal point of the rocker arm and the link rod is moved upward to draw up the cam nose of the VO cam, moving the position of contact of the cam face of the VO cam with respect to the top face of the valve lifter to separate from the lift section of the cam face. Thus, the intake valve is controlled to have the minimum valve-lift characteristic. Therefore, the VVA apparatus allows full achievement of the engine performance in accordance with the engine operating conditions, i.e., an improvement in fuel efficiency and in engine output. With the above VVA apparatus, however, though the valve-lift characteristic can be changed by changing the rocking fulcrum of the rocker arm in accordance with the rotated position of the control cam, a full consideration is not made with regard to the direction of rotation of the control cam, particularly, the direction of rotation from the maximum valve-lift control position to the minimum valve-lift control position, and the position of rotation for minimum valve-lift control. This may raise a problem that a full reduction is impossible in the minimum valve lift due to the direction of rotation of the control cam. Moreover, this may raise another problem that during minimum valve-lift control, the line connecting the axes of first and second end pins of the link rod and the line connecting the axis of the second end pin and the axis of the driving shaft form a straight line to produce locking of the link rod, disturbing smooth rotation of the link rod and the VO cam upon transition of operation of the intake valve from closing to opening. DE-19815112 A1 discloses a variable-valve actuation apparatus according to the preamble part of claim 1. It is, therefore, an object of the present invention to provide a VVA apparatus for an internal combustion engine, which contributes to an improvement in the engine performance and a smooth operation of the apparatus components. SUMMARY OF THE INVENTION The present invention provides a variable-valve-actuation apparatus for an internal combustion engine with a cylinder head, a crankshaft and a valve, comprising: a driving shaft rotated in synchronism with the crankshaft, said driving shaft including a crank cam on an outer periphery; a control shaft arranged substantially parallel to said driving shaft; a valve lifter movably arranged with respect to the cylinder head, said valve lifter including a top face; a valve operating (VO) cam swingably supported by said driving shaft, said VO cam opening and closing the valve through said valve lifter, said VO cam including a cam face; a crank arm including a base and an extension, said base being slidably engaged with an outer periphery of said crank cam; a rocker arm including first and second arms, said first arm being rotatably connected to said extension of said crank arm, which forms a first pivotal point; a link rod having a first end rotatably connected to an end of said VO cam, which forms a second pivotal point, and a second end rotatably connected to said second arm of said rocker arm, which forms a third pivotal point; and a control cam mounted to said control shaft on an outer periphery, said control cam having an axis eccentric to an axis of said control shaft, said control cam changing a rocking fulcrum of said rocker arm in accordance with a rotated position of said control shaft, whereby a position of contact of said cam face of said VO cam with respect to said top face of said valve lifter is changed in accordance with a change in said rocking fulcrum of said rocker arm to alter a lift of the valve, wherein a maximum lift of the valve is obtained in a first rotated position of said control shaft where said axis of said control cam is adjacent to said driving shaft, wherein a minimum lift of the valve is obtained in a second rotated position of said control shaft where said axis of said control cam is positioned near said first pivotal point of said rocker arm and said crank arm with respect to a first line connecting said axis of said control shaft and said axis of said control cam upon said maximum lift. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows a section taken along the line I-I in FIG. 2, showing a first embodiment of a VVA apparatus for an internal combustion engine according to the present invention; FIG. 2 is a side view, partly in section, showing the VVA apparatus; FIG. 3 is a plan view showing the VVA apparatus; FIG. 4 is a perspective view showing a crank cam; FIG. 5 is a graph illustrating a valve-lift characteristic of a VO cam; FIG. 6A-6B are views similar to PIG. 1, taken along the line VI-VI in FIG. 2, showing operation of the VVA apparatus when the engine is at low velocity and low load; FIG. 7A-7B are views similar to FIGS. 6A-6B, taken along the line VII-VII in FIG. 2, showing operation of the VVA apparatus when the engine is at high velocity and high load; FIG. 8 is a view similar to FIG. 5, illustrating the relation between valve timing and valve lift; FIG. 9 is a view similar to FIG. 8, illustrating the correlation between rotation phase and valve lift when rotating a control shaft in the normal direction and in the reverse direction; FIG. 10 is a view similar to FIGS. 7A-7B; FIG. 11 is a fragmentary plan view showing the VVA apparatus in FIG. 10; FIG. 12 is a view similar to FIG. 10, taken along the line XII-XII in FIG. 11; FIG. 13 is a view similar to FIG. 4, showing the VVA apparatus in FIG. 10; FIG. 14 is a view similar to FIG. 13; FIG. 15 is a view similar to FIG. 9, illustrating the relation between valve timing and valve lift; FIG. 16 is a view similar to FIG. 12, showing operation of the VVA apparatus when the engine is at low velocity and low load; FIGS. 17A-7B are views similar to FIG. 16, showing operation of the VVA apparatus when the engine is at high velocity and high load; FIG. 18 is a view similar to FIG. 15, illustrating the relation between valve timing and valve lift; FIG. 19 is a view similar to FIGS. 17A-17B, taken along the line XIX-XIX in FIG. 20; FIG. 20 is a view similar to FIG. 2, showing the VVA apparatus in FIG. 19; FIG. 21 is a view similar to FIG. 3, showing the VVA apparatus in FIG. 19; FIG. 22 is a view similar to FIG. 14; FIG. 23 is a view similar to FIG. 18, illustrating the relation between valve timing and valve lift; FIGS. 24A-24B are views similar to FIG. 19, taken along the line XXIV-XXIV in FIG. 20, showing operation of the VVA apparatus when the engine is at low velocity and low load; FIGS. 25A-25B are views similar to FIGS. 24A-24B, taken along the line XXV-XXV in FIG. 20, showing operation of the VVA apparatus when the engine is at high velocity and high load; FIG. 26 is a view similar to FIG. 23, showing the relation between valve timing and valve lift; FIG. 27 is a view similar to FIGS. 25A-25B; and FIG. 28 is a view similar to FIG. 27. DETAILED DESCRIPTION OF THE INVENTION Referring to the drawings, a description will be made with regard to a VVA apparatus for an internal combustion engine embodying the present invention. The VVA apparatus includes two intake valves per cylinder, and an alteration mechanism for altering the valve lift of the intake valves in accordance with the engine operating conditions. Referring to FIGS. 1-3, the VVA apparatus includes a pair of intake valves 12 slidably arranged with a cylinder head 11 through valve guides, not shown, a hollow driving shaft 13 rotatably supported by a bearing 14 arranged with the cylinder head 11 in the upper portion, a pair of drive or eccentric rotating cams 15 fixed to the driving shaft 13 through press fit, etc., a pair of VO cams 17 swingably supported on an outer peripheral surface 13a of the driving shaft 13 and coming in slide contact with valve lifters 16 disposed at the upper ends of the intake valves 12 to open them, a transmission mechanism 18 connected between the crank cams 15 and the VO cams 17 for transmitting torque of the crank cams 15 to the VO cams 17 as a rocker force, and an alteration mechanism 19 for altering the operating position of the transmission mechanism 18. The driving shaft 13 extends in the longitudinal direction of the engine, and has one end with a follower sprocket, a timing chain wound thereon, etc. not shown, through which torque is received from a crankshaft of the engine. The driving shaft 13 is rotated counterclockwise as viewed in FIG. 1. The bearing 14 includes a main bracket 14a arranged at the upper end of the cylinder head 11 for supporting the upper portion of the driving shaft 13, and an auxiliary bracket 14b arranged at the upper end of the main bracket 14a for rotatably supporting a control shaft 32 as will be described later. The brackets 14a, 14b are fastened together from above by a pair of bolts 14c. Referring to FIG. 4, the crank cams 15 are formed substantially like a ring, each including a small-diameter main body 15a and a flange 15b integrated with the outer end face thereof. A though hole 15c is formed axially to receive the driving shaft 13. An axis Y of the main body 15a is offset with respect to an axis X of the driving shaft 13 in the radial direction by a predetermined amount. Moreover, the crank cams 15 are press fitted to the driving shaft 13 through the through hole 15c on the outer sides where no interference occurs with the valve lifters 16. The main bodies 15a have outer peripheral surfaces 15d formed in the same profile. The valve lifters 16 are formed like a covered cylinder, each being slidably held in a hole of the cylinder head 11 and having a flat top face 16a with which the VO cam 17 comes in slide contact. Referring to FIGS. 1-3 and 6A-7B, the VO cam 17 is formed substantially like a raindrop, and has a support hole 20a at a substantially annular base end 20, through which the driving shaft 13 is arranged for rotatable support. The VO cam 17 also has a pin hole 21a on the side of a cam nose 21. The lower side of the VO cam 17 is formed with a cam face 22 including a base-circle face 22a on the side of the base end 20, a ramp face 22b circularly extending from the base-circle face 22a to the cam nose 21, and a lift face 22c extending from the ramp face 22b to a top face 22d with the maximum lift arranged at an end of the cam nose 21. The base-circle face 22a, the ramp face 22b, the lift face 22c, and the top face 22d come in contact with predetermined points of the top face 16a of the valve lifter 16 in accordance with the rocking position of the VO cam 17. Specifically, referring to FIG. 5, in view of the valve-lift characteristic, a predetermined angular range  1 of the base-circle face 22a corresponds to a base-circle section, and a predetermined angular range  2 of the ramp face 22b subsequent to the base-circle section  1 corresponds to a ramp section, and a predetermined angular range  3 of the ramp face 22b from the ramp section  2 to the top face 22d corresponds to a lift section. The transmission mechanism 18 includes a rocker arm 23 disposed above the driving shaft 13, a crank arm 24 for linking a first arm 23a of the rocker arm 23 with the crank cam 15, and a link rod 25 for linking a second arm 23b of the rocker arm 23 with the VO cam 17. Referring to FIG. 3, the VO cam 23 is formed substantially like a crank as viewed in a plan, and has in the center a cylindrical base 23c rotatably supported by a control cam 33 as will be described later. The first arm 23a protruding from an outer end of the cylindrical base 23c has a pin hole 23d for receiving a pin 26, whereas the second arm 23b protruding from an inner end of the cylindrical base 23c has a pin hole 23e for receiving a pin 27 for connecting the second arm 23b and a first end 25a of the link rod 25. The crank arm 24 includes a relatively-large-diameter annular base 24a and an extension 24b arranged in a predetermined position of the outer peripheral surface of the base 24a. The base 24a has in the center an engagement hole 24c rotatably engaged with the outer peripheral surface of the main body 15a of the crank cam 15. The extension 24b has a pin hole 24d for rotatably receiving the pin 26. As best seen in FIG. 1, the link rod 25 is formed substantially like a letter L having a concave on the side of the rocker arm 23, and has first and second ends 25a, 25b formed with pin holes 25c, 25d through which ends of the pins 27, 28 press fitted in the pin holes 23e, 21 a of the second arm 23b of the rocker arm 23 and the cam nose 21 of the VO cam 17 are rotatably arranged. An axis Z2 of the pin 28 forms a rocking fulcrum of the VO cam 17. Arranged at one ends of the pins 26, 27, 28 are snap rings 29, 30, 31 for restricting axial movement of the crank arm 24 and the link rod 25. The alteration mechanism 19 includes the control shaft 32 rotatably supported by the bearing 14 above the driving shaft 13 and the control cam 33 fixed on the outer periphery of the control shaft 32 for forming a rocking fulcrum of the rocker arm 23. The control shaft 32 is disposed parallel to the driving shaft 13 and in the longitudinal direction of the engine to be rotatable within a predetermined range of angle of rotation by an electromagnetic actuator, not shown, arranged at one end. The control cam 33 is formed like a cylinder, and has an axis P1 eccentric to an axis P2 of the control shaft 32 by an amount α corresponding to a thick portion 33a as shown in FIG. 1. Referring to FIGS. 6A and 7A, the range of angle of rotation of the control shaft 32 will be described. In terms of the axis P1 of the control cam 33, a first rotation-angle position So where the thick portion 33a of the control cam 33 approaches the axis X of the driving shaft 13 corresponds to the maximum valve-lift control position of the intake valve 12 due to connection of the transmission mechanism 18 and the VO cam 17. The axis P1 is rotatable clockwise as viewed in FIG. 1, i.e., from the first rotation-angle position So to a second rotation-angle position S 2 located at about 150° on the side of the pin 26 for connecting the rocker arm 18 and the crank arm 24. The second rotation-angle position S 2 corresponds to the minimum valve-lift control position of the intake valve 12. The control cam 33 is rotatable counterclockwise as viewed in FIG. 1 from the second rotation-angle position S 2 to the first rotation-angle position So by the control shaft 32, but in the same direction as the direction (arrow R) of opening of the intake valve 12 by the VO cam 17 from the first rotation-angle position So to the second rotation-angle position S 2 as shown in FIG. 6A. The actuator for rotating the control shaft 32 within the range between the first and second rotation-angle positions So, S 2 is driven in accordance with a control signal out of a controller, not shown, for determining the engine operating conditions. The controller determines the actual engine operating conditions in accordance with detection signals out of various sensors such as a crank angle sensor, an air flow meter and a coolant temperature sensor to output a control signal to the actuator. When the engine is at low velocity and at low load, the control shaft 32 is rotated clockwise by the actuator in accordance with a control signal out of the controller. This moves the thick portion 33a of the control cam 33 upward with respect to the driving shaft 13, so that the axis P1 of the control cam 33 is kept in the second rotation-angle position S 2 located in the top left direction of the axis P2 of the control shaft 32 as shown by the full lines in FIGS. 6A-6B. Thus, the pivotal point of the second arm 23b of the rocker arm 23 and the link rod 25 is moved upward with respect to the driving shaft 13, so that the VO cam 17, having the cam nose 21 forcibly drawn up through the link rod 25, is rotated counterclockwise in its entirety. With the VO cam 17, referring to FIG. 6A, the full line shows the maximally rocked position or the peak valve-lift position, whereas referring to FIG. 6B, the full line shows the excessively rotated or maximally jumping position or the non-valve-lift position. Referring to FIGS. 6A-6B, when rotation of the crank cam 15 pushes the first arm 23a of the rocker arm 23 upward through the crank arm 24, a valve lift L1, which is fully small as shown in FIG. 6B, is transmitted to the VO cam 17 and the valve lifter 16 through the link rod 25. Thus, in such low-velocity and low-load range, referring to FIG. 8, the intake valve 12 has smaller valve lift and delayed opening timing as shown by the broken line, resulting in small valve overlap with the exhaust valve. This allows improved fuel efficiency and stable engine rotation. On the other hand, when the engine is at high velocity and high load, the control shaft 32 is rotated counterclockwise by the actuator in accordance with a control signal out of the controller. Thus, referring to FIGS. 7A-7B, the control shaft 32 rotates the control cam 33 counterclockwise from the position as shown by the full line in FIGS. 6A-6B to the first rotation-angle position So, moving the axis P1 (thick portion 33a) downward. This moves the rocker arm 23 in the direction of the driving shaft 13 or downward in its entirety, which urges the second arm 23b to push the cam nose 21 of the VO cam 17 downward through the link rod 25, rotating clockwise the VO cam 17 in its entirety by a predetermined amount. Therefore, the position of contact of the cam face 22 of the VO cam 17 with respect to the top face 16a of the valve lifter 16 is moved to the right or on the side of the top face 22d as shown in FIGS. 7A-7B. Thus, the crank cam 15 is rotated as shown in FIG. 7A to push the first arm 23a of the rocker arm 23 upward through the crank arm 24, obtaining a large valve lift L2 with respect to the valve lifter 16 as shown in FIG. 7B. In such high-velocity and high-load range, the cam-lift characteristic is larger as compared with the low-velocity and low-load range, obtaining larger valve lift and advanced opening timing and delayed closing timing of the intake valve 12 as shown by the full line in FIG. 8. This results in an improvement in intake-gas filling efficiency, ensuring full engine output. When passing from the high-velocity and high-load range with maximum valve-lift control to the low-velocity and low-load range, the control cam 33 is rotated, as described above, from the first rotation-angle position So to the second rotation-angle position S 2 as shown in FIGS. 6A-6B. Control of the direction of rotation and the rotation-angle position of the control cam 33 allows a full reduction in the valve lift and a prevention of locking of the link rod 25. Referring to FIGS. 6A-6B, a consideration will be made with regard to the direction of rotation of the control cam 33. Control from the maximum valve-lift control position or the first rotation-angle position So to the minimum valve-lift position can be achieved by rotating the control cam 33 clockwise as shown by the full lines, which is the way of the first embodiment, or by rotating the control am 33 counterclockwise at the same angle of rotation as shown by the one-dot chain lines. When having counterclockwise rotation, the axis P1 of the control cam 33 is moved to S 2 ' as shown by the one-dot chain line in FIG. 6A, so that the pivotal point Mo of the rocker arm 23 and the crank arm 24 upon the maximum valve lift is moved to M' located in the top right direction of Mo, moving the rocking center of the rocker arm 23 upward. This moves a pivotal point Ko of the rocker arm 23 and the link rod 25 upon the maximum valve lift to K' located in the top right direction of Ko. Thus, the VO cam 17, having the cam nose 21 drawn up with movement of the link rod 25 to the upper position K', has the position of contact moved to the side going away from the top face 22d, obtaining the minimum valve lift. In that case, however, due to clockwise rotation of the rocker arm 23 in its entirety, the point K' is not fully high, so that the minimum valve lift cannot be made to fully approach zero. On the other hand, the control cam 33 is rotated clockwise, so that when the axis P1 of the control cam 33 is moved to the second rotation-angle position S 2 as shown in FIG. 6A, the pivotal point Mo is moved to M below and at the left of Mo, moving counterclockwise the rocker arm 23 in its entirety as shown by the full line in FIG. 6A. Thus, the pivotal point K is moved further in the top left direction of K', so that the VO cam 17, having the cam nose 21 relatively largely drawn up with movement of the link rod 25, has a portion close to the base 22a coming in contact with the top face 16a of the valve lifter 16. Therefore, the minimum valve lift can be made to fully approach zero. FIG. 9 shows the correlation between a rotation phase angle  and a valve lift L when rotating the control cam 33, i.e., the control shaft 32, clockwise or in the normal direction or counterclockwise or in the reverse direction. Suppose that the control shaft 32 is rotated from the maximum valve-lift control position So in the normal direction and in the reverse direction by the same amount d2. As described above, due to the positional relationship between the pivotal points K, K' of the rocker arm 23 and the link rod 25, in the minimum valve-lift control position  2 ' on the reverse-rotation side, the valve lift L1' cannot be made to fully approach zero. On the other hand, in the minimum valve-lift control position  2 on the normal-rotation side, the valve lift L1 can be made to fully approach zero. This allows an improvement in the valve-lift characteristic of the intake valve 12, resulting in improved engine performance. Moreover, when rotating the control shaft 32 in the normal direction, the pivotal point K of the rocker arm 23 and the link rod 25 is moved further to the left with respect to the pivotal point K' when rotating the control shaft 32 in the reverse direction. Thus, as shown by the full line in FIG. 6B, a line Q1 connecting the axes Z1, Z2 of the pins 27, 28 of the first and second ends 25a, 25b of the link rod 25 and a line Q2 connecting the axis Z2 of the pin 28 of the second end 25b and the axis X of the driving shaft 13 do not form a straight line, but an L-shaped line. Specifically, when rotating the control shaft 32 in the reverse direction to obtain the minimum valve lift, the pivotal point K' is not fully moved to the left as described above, the two lines Q1, Q2 form a substantially straight line, having possible locked state where the link rod 25 is fully extended. In the first embodiment, however, the two lines Q1, Q2 form an L-shaped line, which allows smooth rotation of the link rod 25 and the VO cam 17 upon transition of operation of the intake valve 12 from closing to opening, having no disturbance of smooth operation of the intake valve 12. Referring to FIG. 9, a complementary description will be made with regard to an angle  formed by the lines Q1, Q2. Suppose that the control shaft 32 is rotated by d2 from the maximum valve-lift position  1 . Upon normal rotation, the angle  2 is about 160° , which allows the L-shape with fully small minimum valve lift L1. Upon reverse rotation, the angle  2 ' is about 180° , having full extension and relatively large minimum lift L1'. Referring to FIGS. 10 and 13, the VVA apparatus includes a pair of intake valves 112 slidably arranged with a cylinder head 111 through valve guides, not shown, a hollow driving shaft 113 rotatably supported by a bearing 114 arranged with the cylinder head 111 in the upper portion, a crank cam 115 fixed to the driving shaft 113 through press fit, etc., a pair of VO cams 117 swingably supported on an outer peripheral surface 113a of the driving shaft 113 and coming in slide contact with valve lifters 116 disposed at the upper ends of the intake valves 112 to open them, a transmission mechanism 118 connected between the crank cam 115 and the VO cams 117 for transmitting torque of the crank cam 115 to the VO cams 117 as a rocker force, and an alteration mechanism 119 for altering the operating position of the transmission mechanism 118. The driving shaft 113 extends in the longitudinal direction of the engine, and has one end with a follower sprocket, a timing chain wound thereon, etc. not shown, through which torque is received from a crankshaft of the engine. The driving shaft 113 is rotated counterclockwise as viewed in FIG. 10. The bearing 114 includes a main bracket 114a arranged at the upper end of the cylinder head 111 for supporting the upper portion of the driving shaft 113, and an auxiliary bracket 114b arranged at the upper end of the main bracket 114a for rotatably supporting a control shaft 132 as will be described later. The brackets 114a, 114b are fastened together from above by a pair of bolts 114c. Referring to FIG. 14, the crank cam 115 includes a substantially annular main body 115a and a cylindrical portion 115b integrated with the outer end face thereof. A though hole 115c is formed axially to receive the driving shaft 113. An axis Y of the main body 115a is offset with respect to an axis X of the driving shaft 113 in the radial direction by a predetermined amount. Moreover, the crank cam 115 is press fitted to the driving shaft 113 through the through hole 115c on the outer side where no interference occurs with the valve lifters 116. The main body 115a has an outer peripheral surface 115d formed in the same profile. The valve lifters 116 are formed like a covered cylinder, each being slidably held in a hole of the cylinder head 111 and having a flat top face 116a with which the VO cam 117 comes in slide contact. Referring to FIGS. 10 and 13, the VO cam 117 is formed substantially like a raindrop, and has a support hole 120a at a substantially annular base end 120, through which the driving shaft 113 is arranged for rotatable support. The VO cam 117 also has a pin hole 121a on the side of a cam nose 121. The lower side of the VO cam 117 is formed with a cam face 122 including a base-circle face 122a on the side of the base end 120, a ramp face 122b circularly extending from the base-circle face 122a to the cam nose 121, and a lift face 122c extending from the ramp face 122b to a top face 122d with the maximum lift arranged at an end of the cam nose 121. The base-circle face 122a, the ramp face 122b, the lift face 122c, and the top face 122d come in contact with predetermined points of the top face 116a of the valve lifter 116 in accordance with the rocking position of the VO cam 117. Specifically, referring to FIG. 15, in view of the valve-lift characteristic, a predetermined angular range  1 of the base-circle face 122a corresponds to a base-circle section, and a predetermined angular range  2 of the ramp face 122b subsequent to the base-circle section  1 corresponds to a ramp section, and a predetermined angular range  3 of the ramp face 122b from the ramp section  2 to the top face 122d corresponds to a lift section. The transmission mechanism 118 includes a rocker arm 123 disposed above the driving shaft 113, a crank arm 124 for linking a first arm 123a of the rocker arm 123 with the crank cam 115, and a link rod 125 for linking a second arm 123b of the rocker arm 123 with the VO cam 117. Referring to FIGS. 10 and 13, the VO cam 123 has in the center a cylindrical base swingably supported by a control cam 133 as will be described later through a support hole 123c. The first arm 123a protruding from an outer end of the cylindrical base has a pin hole for receiving a pin 126, whereas the second arm 123b protruding from an inner end of the cylindrical base has a pin hole for receiving a pin 127 for connecting a first end 125a of the link rod 125. The crank arm 124 includes a relatively-large-diameter annular base 124a and an extension 124b arranged in a predetermined position of the outer peripheral surface of the base 124a. The base 124a has in the center an engagement hole 124c rotatably engaged with the outer peripheral surface of the main body 115a of the crank cam 115. The extension 124b has a pin hole 124d for rotatably receiving the pin 126. As best seen in FIG. 10, the link rod 125 is formed substantially like a letter L having a concave on the side of the rocker arm 123, and has first and second ends 125a, 125b formed with pin holes through which ends of the pins 127, 128 press fitted in the pin holes of the second arm 123b of the rocker arm 123 and the cam nose 121 of the VO cam 117 are rotatably arranged. An axis Z2 of the pin 128 forms a rocking fulcrum of the VO cam 117. Arranged at one ends of the pins 126, 127, 128 are snap rings 129, 130, 131 for restricting axial movement of the crank arm 124 and the link rod 125. The alteration mechanism 119 includes the control shaft 132 rotatably supported by the bearing 114 above the driving shaft 113 and the control cam 313 fixed on the outer periphery of the control shaft 132 for forming a rocking fulcrum of the rocker arm 123. The control shaft 132 is disposed parallel to the driving shaft 113 and in the longitudinal direction of the engine to be rotatable within a predetermined range of angle of rotation by an electromagnetic actuator or DC motor 134 arranged at one end. The control cam 133 is formed like a cylinder, and has an axis P1 eccentric to an axis P2 of the control shaft 132 by an amount α corresponding to a thick portion 133a as shown in FIG. 10. Referring to FIGS. 10-12, a first restriction mechanism 140 is arranged between the bearing 114 and the control shaft 132 to restrict excessive rotation of the control shaft 132 during minimum valve-lift control. The first restriction mechanism 140 includes a stopper pin 141 arranged with the control shaft 132 to protrude radially and a first stopper protrusion 142 arranged on one side face of the auxiliary bracket 114b of the bearing 114 to protrude axially with respect to the control shaft 132, with which the first stopper pin 141 comes in contact to restrict the maximally rotated position of the control shaft 132 during minimum valve-lift control. As shown in FIG. 12, the stopper pin 141 has a base end 141a press fitted in a fixing hole formed radially in the control shaft 132, the circumferential position of which is determined based on the relative angular position with respect to the first stopper protrusion 142. Specifically, as shown in FIG. 10, when the VO cam 117 jumps maximally with the intake valve 112 being subjected to minimum valve-lift control by rotation control of the control shaft 132 as will be described later, an angle formed by a line Q1 connecting the axes Z1, Z2 of the pins 127, 128 and a line Q2 connecting the axis X of the driving shaft 113 and the axis Z2 of the pin 128 is equal to an angle  4 that allows a full prevention of locking between the VO cam 117 and the link rod 125. The angle  4 is determined to be about 165° . Referring to FIG. 10, a second restriction mechanism 143 is arranged on the outer surface of the cylindrical base of the rocker arm 123 on the side of the VO cam 117. The second restriction mechanism 143 includes a second stopper protrusion 144 (see FIG. 16) arranged on the outer surface of the cylindrical base of the rocker arm 123. The second stopper protrusion 144 comes in contact with the top face of the VO cam 117 on the side of the cam nose 121 to restrict further rocking motion of the rocker arm 123. The second stopper protrusion 144 is formed like a sphere, and has an amount of protrusion determined such that when coming in contact with the VO cam 117 as shown by the two-dot chain line in FIG. 10, an angle formed by the lines Q1, Q2 is equal to an angle  5 that is slightly larger than the angle  4 , but allows a prevention of above locking. A third restriction mechanism 145 is arranged opposite to the first restriction mechanism 140 to restrict the maximally rotated position of the control shaft 132 in the reverse direction or during maximum valve-lift control. The third restriction mechanism 145 includes a third stopper protrusion 146 for restricting the rotated position of the stopper pin 141. The third stopper protrusion 146 is arranged on one side face of the auxiliary bracket 114b of the bearing 114 on the side opposite to the first stopper protrusion 142 with respect to the control shaft 132 to protrude axially with respect thereto. The third stopper protrusion 146 is positioned to define an angle that allows a prevention of possible locking between the VO cam 117 and the link rod 125 in the counterclockwise maximally rotated position of the control shaft 132 as viewed in FIGS. 10 and 12 during maximum valve-lift control by excessive rotation of the control shaft 132. Referring to FIG. 13, the actuator 134 for rotating the control shaft 132 within the range between the first and second rotation-angle positions is driven in accordance with a control signal out of a controller 135 for determining the engine operating conditions. The controller 135 determines the actual engine operating conditions in accordance with detection signals out of various sensors such as a crank angle sensor, an air flow meter, a coolant temperature sensor, and a potentiometer to output a control signal to the actuator 134. When the engine is at low velocity and at low load, the control shaft 132 is rotated clockwise as shown in FIG. 10 by the actuator 134 in accordance with a control signal out of the controller 135 until the stopper pin 141 comes in contact with the first stopper protrusion 142. This moves the thick portion 133a of the control cam 133 upward with respect to the driving shaft 113, so that the axis P1 of the control cam 133 is kept in the second rotation-angle position located in the top left direction of the axis P2 of the control shaft 132 as shown by the full lines in FIGS. 6A-6B. Thus, the pivotal point of the second arm 123b of the rocker arm 123 and the link rod 125 is moved upward with respect to the driving shaft 113, so that the VO cam 117, having the cam nose 121 forcibly drawn up through the link rod 125, is rotated counterclockwise in its entirety. Referring to FIGS. 10 and 16, when rotation of the crank cam 115 pushes the first arm 123a of the rocker arm 123 upward through the crank arm 124, a valve lift L1, which is fully small as shown in FIGS. 10 and 16, is transmitted to the VO cam 117 and the valve lifter 116 through the link rod 125. Thus, in such low-velocity and low-load range, referring to FIG. 18, the intake valve 112 has smaller valve lift and delayed opening timing as shown by the broken line, resulting in small valve overlap with the exhaust valve. This allows improved fuel efficiency and stable engine rotation. Further, during minimum valve-lift control, the control shaft 132 is held in the rotated position where excessive rotation is restricted by the stopper pin 141 coming in contact with the first stopper protrusion 142 as described above. Thus, the angle formed by the lines Q1, Q2 when the VO cam 117 jumps maximally is restricted to the angle  4 . This allows a sure prevention of locking between the VO cam 117 and the link rod 125 when the link rod 125 urges to rotate the VO cam 117 downward by eccentric torque of the crank cam 115. Thus, smooth operation of the VO cam 117 and the link rod 125 is obtained, resulting in smooth opening of the intake valve 112 during minimum valve-lift control. Still further, the first restriction mechanism 140, having the stopper pin 141 coming in contact with the first stopper protrusion 142 as described above, serves to merely restrict further rotation of the control shaft 132, and not to directly restrict the rocking position of the VO cam 117 that rocks fiercely during engine operation. Thus, there is no occurrence of hammering due to interference of the VO cam 117 with a member for restricting the rocking position thereof, allowing the maintenance of silence. Furthermore, when, after a long period of time of operation of the apparatus, the angle  4 is increased due to abnormal wear of the stopper pin 141 and the first stopper protrusion 142, which causes a change in the position of contact between the two, or due to abnormal wear of slide portions even though no change occurs in the above position of contact, the second restriction mechanism 143 functions so that the VO cam 117 has the top face on the side of the cam nose 121 coming in contact with the second stopper protrusion 144 as shown by the two-dot chain line in FIG. 10, obtaining a restriction of further rocking motion thereof. Thus, the angle formed by the lines Q1, Q2 can be restricted to the angle  5 that causes no locking between the VO cam 117 and the link rod 125, resulting in smooth operation of the intake valve 112 during a long period of time. Particularly, the second restriction mechanism 143 includes the second stopper protrusion 144 that can directly restrict excessive rocking motion of the VO cam 117, allowing a stable and sure prevention of locking between the VO cam 117 and the link rod 125. On the other hand, when the engine is at high velocity and high load, the control shaft 132 is rotated counterclockwise by the actuator 134 in accordance with a control signal out of the controller 135 until the stopper pin 141 comes in contact with the third stopper protrusion 146. Thus, referring to FIGS. 17A-17B, the control shaft 132 rotates the control cam 133 counterclockwise from the position as shown in FIG. 16 to the first rotation-angle position, moving the axis P1 (thick portion 33a) downward. This moves the rocker arm 123 in the direction of the driving shaft 113 or downward in its entirety, which urges the second arm 123b to push the cam nose 121 of the VO cam 117 downward through the link rod 125, rotating clockwise the VO cam 117 in its entirety by a predetermined amount. Therefore, the position of contact of the cam face 122 of the VO cam 117 with respect to the top face 116a of the valve lifter 116 is moved to the right or on the side of the top face 122d as shown in FIGS. 17A-17B. Thus, the crank cam 115 is rotated to push the first arm 123a of the rocker arm 123 upward through the crank arm 124, obtaining a large valve lift L2 with respect to the valve lifter 116 as shown in FIG. 17A. In such high-velocity and high-load range, the cam-lift characteristic is larger as compared with the low-velocity and low-load range, obtaining larger valve lift and advanced opening timing and delayed closing timing of the intake valve 112 as shown by the full line in FIG. 18. This results in an improvement in intake-gas filling efficiency, ensuring full engine output. During maximum valve-lift control also, the stopper pin 141 comes in contact with the third stopper protrusion 146 to allow a reduction in the angle formed by the lines Q1, Q2 when the VO cam 117 is largely rotated as shown in FIG. 17B, resulting in a sure prevention of locking between the VO cam 117 and the link rod 125. Therefore, this cooperates with operation of the first restriction mechanism 140 to always ensure smooth opening of the intake valve 112 during minimum and maximum valve-lift controls, resulting in a prevention of lowered engine performance. Further, under the normal service conditions, the VO cam 117 may not collide with the second stopper protrusion 144, having no occurrence of hammering, resulting in a maintenance of silence. If above abnormal wear is produced, hammering will occur, but locking can surely be prevented between the VO cam 117 and the link rod 125. It is to be noted that such hammering is useful to give warning to a driver. Still further, in the second embodiment, the crank cam 115 and the VO cam 117 are mechanically linked with each other by the crank arm 124 and the link rod 125 through the rocker arm 123. Thus, excessive rocking motion or jumping of the VO cam 117 during engine high rotation can be restricted by the link rod 125, etc. This always ensures excellent link between the VO cam 117 and the crank cam 115, allowing a stable and sure prevention of above locking. Referring to FIGS. 19-21, the VVA apparatus includes a pair of intake valves 212 slidably arranged with a cylinder head 211 through valve guides, not shown, a hollow driving shaft 213 rotatably supported by a bearing 214 arranged with the cylinder head 211 in the upper portion, a pair of drive or eccentric rotating cams 215 fixed to the driving shaft 213 through press fit, etc., a pair of VO cams 217 swingably supported on an outer peripheral surface 213a of the driving shaft 213 and coming in slide contact with valve lifters 216 disposed at the upper ends of the intake valves 212 to open them, a transmission mechanism 218 connected between the crank cams 215 and the VO cams 217 for transmitting torque of the crank cams 215 to the VO cams 217 as a rocker force, and an alteration mechanism 219 for altering the operating position of the transmission mechanism 218. The driving shaft 213 extends in the longitudinal direction of the engine, and has one end with a follower sprocket, a timing chain wound thereon, etc. not shown, through which torque is received from a crankshaft of the engine. The driving shaft 213 is rotated counterclockwise as viewed in FIG. 19. The driving shaft 213 has an oil passage 213b formed axially to communicate with an oil main gallery, not shown, and hydraulic holes 213c formed radially, each having one end communicating with the hydraulic passage 213b and another end communicating with a clearance between an outer peripheral surface 213a of the driving shaft 213 and an inner peripheral surface of a support hole 220a of the VO cam 217 as will be described later. The bearing 214 includes a main bracket 214a arranged at the upper end of the cylinder head 211 for supporting the upper portion of the driving shaft 213, and an auxiliary bracket 214b arranged at the upper end of the main bracket 214a for rotatably supporting a control or support shaft 232 as will be described later. The brackets 214a, 214b are fastened together from above by a pair of bolts 214c. Referring to FIG. 22, the crank cams 215 are formed substantially like a ring, each including a small-diameter main body 215a and a flange 215b integrated with the outer end face thereof. A though hole 215c is formed axially to receive the driving shaft 213. An axis Y of the main body 215a is offset with respect to an axis X of the driving shaft 213 in the radial direction by a predetermined amount. Moreover, the crank cams 215 are press fitted to the driving shaft 213 through the through hole 215c on the outer sides where no interference occurs with the valve lifters 216. The main bodies 215a have outer peripheral surfaces 215d formed in the same profile. The valve lifters 216 are formed like a covered cylinder, each being slidably held in a hole of the cylinder head 211 and having a circular top face 216a formed in the cross direction of the engine, with which the VO cam 217 comes in slide contact. Referring to FIGS. 19-21 and 24A-25B, the VO cam 217 is formed substantially like a letter U, and has the support hole 220a at a substantially annular base end 220, through which the driving shaft 213 is arranged for rotatable support. The VO cam 217 also has a pin hole 221a on the side of a cam nose 221. The lower side of the VO cam 217 is formed with a cam face 222 including a base-circle face 222a on the side of the base end 220, a ramp face 222b circularly extending from the base-circle face 222a to the cam nose 221, and a lift face 222c extending from the ramp face 222b to a top face 222d with the maximum lift arranged at an end of the cam nose 221. The base-circle face 222a, the ramp face 222b, the lift face 222c, and the top face 222d come in contact with predetermined points of the top face 216a of the valve lifter 216 in accordance with the rocking position of the VO cam 217. Specifically, referring to FIG. 23, in view of the valve-lift characteristic, a predetermined angular range  1 of the base-circle face 222a corresponds to a base-circle section, and a predetermined angular range  2 of the ramp face 222b subsequent to the base-circle section  1 corresponds to a ramp section, and a predetermined angular range  3 of the ramp face 222b from the ramp section  2 to the top face 222d corresponds to a lift section. The transmission mechanism 218 includes a rocker arm 223 disposed above the driving shaft 213, a crank arm 224 for linking a first arm 223a of the rocker arm 223 with the crank cam 215, and a link rod 225 for linking a second arm 223b of the rocker arm 223 with the VO cam 217. Referring to FIG. 21, the VO cam 223 is formed substantially like a crank as viewed in a plan, and has in the center a cylindrical base 223c rotatably supported by a control cam 233 as will be described later. The first arm 223a protruding from an outer end of the cylindrical base 223c has a pin hole 223d for receiving a pin 226, whereas the second arm 223b protruding from an inner end of the cylindrical base 223c has a pin hole 223e for receiving a pin 227 for connecting the second arm 223b and a first end 225a of the link rod 225. The crank arm 224 includes a relatively-large-diameter annular base 224a and an extension 224b arranged in a predetermined position of the outer peripheral surface of the base 224a. The base 224a has in the center an engagement hole 224c rotatably engaged with the outer peripheral surface of the main body 215a of the crank cam 215. The extension 224b has a pin hole 224d for rotatably receiving the pin 226. As best seen in FIG. 19, the link rod 225 is formed like a straight line with a predetermined length, and has first and second ends 225a, 225b formed with pin holes 225c, 225d through which ends of the pins 227, 228 press fitted in the pin holes 223e, 221a of the second arm 223b of the rocker arm 223 and the cam nose 221 of the VO cam 217 are rotatably arranged. An axis Z2 of the pin 228 forms a rocking fulcrum of the VO cam 217. Referring to FIGS. 25A-25B, due to circular formation of the top face 216a of the valve lifter 216, the position of a normal corresponding to the direction of a reaction force of a valve spring that acts on the point of contact between the VO cam 217 and the valve lifter 216 during rotation of the crank cam 215 is changed between a first position near the driving shaft 213 and a second position opposite thereto with respect to the axis Z2 of the pin 228 in the maximum valve-lift range of the intake valve 212. Specifically, in the base-circle range of the VO cam 217, as shown in FIG. 25A, a normal hi of a reaction force F 1 of the valve spring extends vertically or in the axial direction of a valve stem of the intake valve 212, and is positioned near the driving shaft 213 with respect to the axis Z2. In the lift range of the VO cam 217, as shown in FIG. 25B, a normal h 2 of a reaction force F 2 of the valve spring is positioned opposite to the driving shaft 213 with respect to the axis Z2 and at a distance I therefrom, since the VO cam 217 comes in contact with the outer peripheral edge of the valve lifter 216. On the other hand, in the small rotation-angle range of the intake valve 212, as shown in FIGS. 24A-24B, since the VO cam 217 always comes in contact with substantially the center of the top face 216a of the valve lifter 216, the normals hi, h 2 are positioned near the driving shaft 213 with respect to the axis Z2. Arranged at one ends of the pins 226, 227, 228 are snap rings 229, 230, 231 for restricting axial movement of the crank arm 224 and the link rod 225. The alteration mechanism 219 includes the control shaft 232 rotatably supported by the bearing 214 above the driving shaft 213 and the control cam 233 fixed on the outer periphery of the control shaft 232 for forming a rocking fulcrum of the rocker arm 223. The control cam 233 is formed like a cylinder, and has an axis P1 eccentric to an axis P2 of the control shaft 232 by an amount α corresponding to a thick portion 233a as shown in FIG. 19. The control shaft 232 is disposed parallel to the driving shaft 213 and in the longitudinal direction of the engine to be rotatable within a predetermined range of angle of rotation by an electromagnetic actuator, not shown, arranged at one end. The actuator is driven in accordance with a control signal out of a controller, not shown, for determining the engine operating conditions. The controller determines the actual engine operating conditions in accordance with detection signals out of various sensors such as a crank angle sensor, an air flow meter and a coolant temperature sensor to output a control signal to the actuator. When the engine is at low velocity and at low load, the control shaft 232 is rotated clockwise by the actuator in accordance with a control signal out of the controller. This moves the thick portion 233a of the control cam 233 upward with respect to the driving shaft 213, so that the axis P1 of the control cam 233 is kept in a second rotation-angle position located in the top left direction of the axis P2 of the control shaft 232 as shown in FIGS. 24A-24B. Thus, the rocker arm 223 is moved upward with respect to the driving shaft 213, so that the VO cam 217, having the cam nose 221 forcibly drawn up through the link rod 225, is rotated counterclockwise in its entirety. Referring to FIGS. 24A-24B, when rotation of the crank cam 215 pushes the first arm 223a of the rocker arm 223 upward through the crank arm 224, a valve lift L1, which is fully small as shown in FIG. 24B, is transmitted to the VO cam 217 and the valve lifter 216 through the link rod 225. Thus, in such low-velocity and low-load range, referring to FIG. 26, the intake valve 212 has smaller valve lift and delayed opening timing as shown by the broken line, resulting in small valve overlap with the exhaust valve. This allows improved fuel efficiency and stable engine rotation. In the small-valve-lift range, the normals h 1 , h 2 of the reaction forces F 1 , F 2 of the valve spring are positioned near the driving shaft 213 with respect to the axis Z2, so that forces f 1 , f 2 acting, from the inner peripheral surface of the support hole 220a of the VO cam 217, on the outer peripheral surface 213a of the driving shaft 213 are applied to the whole area of a lower end 220b of the inner peripheral surface and a lower end 213d of the outer peripheral surface 213a. However, at that time, due to small valve lift of the VO cam 217, a reaction farce of the valve spring is small, having less occurrence of wear between the lower ends 220b, 213d. Moreover, at that time, the normals h 1 , h 2 are moved only within the rocking-fulcrum-side range of the VO cam 217, resulting in achievement of smaller valve lift. On the other hand, when the engine is at high velocity and high load, the control shaft 232 is rotated counterclockwise by the actuator in accordance with a control signal out of the controller. Thus, referring to FIGS. 25A-25B, the control shaft 232 rotates the control cam 233 counterclockwise from the position as shown in FIGS. 24A-24B, moving the axis P1 (thick portion 33a) downward. This moves the rocker arm 223 in the direction of the driving shaft 213 or downward in its entirety, which urges the second arm 223b to push the cam nose 221 of the VO cam 217 downward through the link rod 225, rotating clockwise the VO cam 217 in its entirety by a predetermined amount. Therefore, the position of contact of the cam face 222 of the VO cam 217 with respect to the top face 216a of the valve lifter 216 is moved to the right or on the side of the top face 222d as shown in FIGS. 25A-25B. Thus, the crank cam 215 is rotated as shown in FIG. 25A to push the first arm 223a of the rocker arm 223 upward through the crank arm 224, obtaining a large valve lift L2 with respect to the valve lifter 216 as shown in FIG. 25B. In such high-velocity and high-load range, the cam-lift characteristic is larger as compared with the low-velocity and low-load range, obtaining larger valve lift and advanced opening timing and delayed closing timing of the intake valve 212 as shown by the full line in FIG. 26. This results in an improvement in intake-gas filling efficiency, ensuring full engine output. Moreover, referring to FIG. 25B, in the large-valve-lift range or the range of large reaction force of the valve spring, the position of contact of the cam face 222 with respect to the top face 216a of the valve lifter 216 is located near the edge of the top face 216a in the vicinity of the maximum lift. Thus, the normal h 1 of the reaction force F 2 of the valve spring is positioned outside with respect to the axis Z2, i.e. opposite to the driving shaft 213 with respect thereto. Therefore, the VO cam 217 is subjected to a counterclockwise moment M about the axis Z2 in its entirety, and is pushed downward by the load f 2 . Thus, the load acting direction is reversed such that an upper end 220c of the inner peripheral surface of the support hole 220a comes in slide contact with an upper end 213e of the outer peripheral surface 213a of the driving shaft 213. This prevents the lower end 220b of the inner peripheral surface of the support hole 220a from coming in slide contact with the lower end 213d of the outer peripheral surface 213a of the driving shaft 213, having no occurrence of local heat generation and wear of the lower ends 220b, 213d. Further, lubricating oil is supplied between the inner peripheral surface of the support hole 220a and the outer peripheral surface 213a of the driving shaft 213, having improved lubrication performance of the two surfaces. Still further, upon above load reversion, there produce not only a so-called restricted-film effect of lubricating oil, which contributes to an improvement in the load performance of lubricating oil, but a forced supply thereof to a contact portion between the two surfaces, which allows a further prevention of occurrence of wear therebetween. Furthermore, the VO cam 217 has a rocking range restricted by the transmission mechanism 218, particularly, by the link rod 225, allowing a restriction of excessive rocking motion even at high rocking speed during high rotation of the engine, etc. FIG. 27 shows the link rod 225 being formed like a circular arc having a concave on the side of the driving shaft 213, and the top face 216a of the valve lifter 216 is formed flat. Circular formation of the link rod 225 allows the normal h 2 of the reaction force F 2 of the valve spring to be positioned outside with respect to the axis Z2 in the maximum valve-lift range of the intake valve 212. Therefore, this produces an effect of preventing an occurrence of local wear between the outer peripheral surface 213a of the driving shaft 213 and the inner peripheral surface of the support hole 220a of the VO cam 217. FIG. 28 shows the VO cam 217 being supported by a support shaft 300 that is a member different from the driving shaft 213. Moreover, the support shaft 300 has an oil passage 301 formed axially, and an oil hole 302 formed radially and having one end communicating with the support hole 220a of the VO cam 217. The force f 2 out of the VO cam 217 does not act on the driving shaft 213, preventing local wear of the driving shaft 213, resulting in improved durability thereof. As for the support shaft 300, the wear resistance is improved on the same principle as described. In the case of engines with lower maximum valve-lift requirements, referring to FIG. 9, the maximum valve-lift position So may slightly be displaced on the normal-rotation side  1 ' with respect to  1. Further, the restriction mechanism may be constructed to restrict excessive upward motion of the second arm of the rocker arm. Still further, the present invention is applicable to VVA apparatus with no alteration mechanism. Furthermore, the present invention is applicable to the exhaust valve.

Claims: A variable-valve-actuation apparatus for an internal combustion engine with a cylinder head (11), a crankshaft and a valve (12), comprising: a driving shaft (13) rotated in synchronism with the crankshaft, said driving shaft (13) including a crank cam (15) on an outer periphery; a control shaft (32) arranged substantially parallel to said driving shaft (13); a valve lifter (16) movably arranged with respect to the cylinder head, said valve lifter (16) including a top face (16a); and a valve operating cam (17) swingably supported by said driving shaft (13), said valve operating cam opening and closing the valve (12) through said valve lifter (16), said valve operating cam (17) including a cam face (22); characterized in that the variable-valve-actuation apparatus further comprises: a crank arm (24) including a base (24a) and an extension (24b), said base (24a) being slidably engaged with an outer periphery of said crank cam (15); a rocker arm (23) including first and second arms (23a,23b), said first arm (23a) being rotatably connected to said extension (24b) of said crank arm (24), which forms a first pivotal point; a link rod (25) having a first end rotatably connected to an end of said valve operating cam (17), which forms a second pivotal point, and a second end rotatably connected to said second arm (23b) of said rocker arm (23), which forms a third pivotal point; and a control cam (33) mounted to said control shaft (32) on an outer periphery, said control cam (33) having an axis (P1) eccentric to an axis (P2) of said control shaft (32), said control cam (33) changing a rocking fulcrum of said rocker arm (23) in accordance with a rotated position of said control shaft (32), whereby a position of contact of said cam face (22) of said valve operating cam (17) with respect to said top face (16a) of said valve lifter (16) is changed in accordance with a change in said rocking fulcrum of said rocker arm (23) to alter a lift of the valve (12), wherein a maximum lift of the valve (12) is obtained in a first rotated position of said control shaft (32) where said axis (P1) of said control cam (33) is adjacent to said driving shaft (13), wherein a minimum lift of the valve (12) is obtained in a second rotated position of said control shaft (32) where said axis (P1) of said control cam (33) is positioned near said first pivotal point of said rocker arm (23) and said crank arm (24) with respect to a first line connecting said axis (P2) of said control shaft (32) and said axis (P1) of said control cam (33) upon said maximum lift. A variable-valve-actuation apparatus as claimed in claim 1, wherein said minimum lift is obtained by rotating said axis (P1) of said control cam (33) from a first position upon said maximum lift to a second position on the side of said pivotal point of said rocker arm (23) and said crank arm by a predetermined angle of rotation. A variable-valve-actuation apparatus as claimed in claim 1, further comprising a first restriction mechanism (140) restricting an angle formed by a second line connecting an axis of said driving shaft (13) during lift control of the valve (12) and said second pivotal point of said link rod (25) and said valve operating cam and a third line connecting said second pivotal point and said third pivotal point of said rocker arm (23) and said link rod (25) to less than a first predetermined angle. A variable-valve-actuation apparatus as claimed in claim 3, wherein said first restriction mechanism (140) comprises a stopper (141) defining a maximally rotated position of said control shaft in one direction. A variable-valve-actuation apparatus as claimed in claim 3, further comprising a second restriction mechanism (143) arranged between said valve operating cam (17) and said rocker arm (23), said second restriction mechanism (143) restricting said angle to a second predetermined angle greater than said first predetermined angle of said first restriction mechanism. A variable-valve-actuation apparatus as claimed in claim 1, wherein the direction of a reaction force of a valve spring acting on a point of contact between said valve lifter (16) and said valve operating cam (17) during one rotation of said crank cam (15) is changed between a third position near said driving shaft (13) with respect to said second pivotal point and a fourth position opposite to said driving shaft (13) with respect to said second pivotal point. A variable-valve-actuation apparatus as claimed in claim 1, further comprising an alteration mechanism (19) altering said rocking fulcrum of said rocker arm (23) in accordance with operating conditions of the engine. A variable-valve-actuation apparatus as claimed in claim 7, wherein said driving shaft (13) has an oil passage formed axially, and an oil hole formed radially for hydraulic communication between said oil passage and an inner peripheral surface of a support hole of said valve operating cam (17). A variable-valve-actuation appaatus as claimed in claim 8, wherein said top face of said valve lifter (16) is formed like a circular arc. A variable-valve-actuation apparatus as claimed in claim 1, wherein said link rod (25) is formed like a letter L having a concave on the side of said rocker arm (23).

Inventors: HIBI TSUTOMU, MIYAZATO YOSHIAKI, NAKAMURA MAKOTO, NOHARA TSUNEYASU, TAKEMURA SHINICHI, YAMADA YOSHIHIKO, HIBI, TSUTOMU, MIYAZATO, YOSHIAKI, NAKAMURA, MAKOTO, NOHARA, TSUNEYASU, TAKEMURA, SHINICHI, YAMADA, YOSHIHIKO, Hibi, Tsutomu, c/o Unisia Jecs Corp., Miyazato, Yoshiaki, c/o Unisia Jecs Corp., Nakamura, Makoto, c/o Unisia Jecs Corp., Yamada, Yoshihiko, c/o Unisia Jecs Corp.
Applicants: ATSUGI UNISIA CORP, NISSAN MOTOR, NISSAN MOTOR CO., LTD., UNISIA JECS CORPORATION
Application year: 2000
Main classification: F01L13
IPC codes: F01L13