US20130019831A1

US20130019831A1 - Valve actuation apparatus of internal combustion engine

Info

Publication number: US20130019831A1
Application number: US13/470,383
Authority: US
Inventors: Tetsuo Yamazaki; Noriomi Hosaka
Original assignee: Hitachi Automotive Systems Ltd
Current assignee: Hitachi Astemo Ltd
Priority date: 2011-07-21
Filing date: 2012-05-14
Publication date: 2013-01-24
Also published as: JP2013024124A

Abstract

In a valve actuation apparatus of a multi-cylinder engine employing a drive shaft rotatably arranged to extend over a plurality of cylinders, a drive cam attached to the drive shaft, and a motion transmitter provided for each of the cylinders for converting a rotary motion of the drive cam into an oscillating motion, a rockable-cam structural member is provided for each of the cylinders in a state where a shaft is inserted through inner peripheries of the rockable-cam structural members, for operating engine valves by the transmitted oscillating motion. A plain bearing is provided in at least one axial position between the axially-spaced rockable-cam structural members. An inner periphery of the plain bearing is in contact with an outer periphery of the shaft, whereas an outer periphery of the plain bearing is in contact with an inner periphery of a bearing portion mounted on the engine.

Description

TECHNICAL FIELD

The present invention relates to a valve actuation apparatus of an internal combustion engine configured to variably control valve characteristics such as a valve lift and valve events of an engine valve (intake and/or exhaust valves) depending on an operating condition of the engine.

BACKGROUND ART

In recent years, there have been proposed and developed various variable valve actuation apparatus capable of variably controlling valve characteristics (e.g., a valve lift and valve events) depending on an engine operating condition. One such variable valve actuation apparatus has been disclosed in Japanese Patent Provisional Publication No. 2010-163980 (hereinafter is referred to as “JP2010-163980”).
The variable valve actuation apparatus disclosed in JP2010-163980 is provided with a drive shaft, a rockable-cam structural member, a motion transmission mechanism, and a control cam. The drive shaft is arranged in a longitudinal direction of a multi-cylinder internal combustion engine and has a plurality of drive cams mounted on the outer periphery. The rockable-cam structural member has a pair of rockable cams for operating two intake valves per cylinder against the spring forces of valve springs. The motion transmission mechanism is provided for converting rotary motion of the drive cam into oscillating motion and for transmitting the oscillating motion to the rockable-cam structural member. The control cam is formed on a control shaft arranged in the longitudinal direction of the engine for changing a fulcrum of the oscillating motion of the motion transmission mechanism.
Both axial ends of the drive shaft are rotatably supported by means of bearings installed on a cylinder head. A plurality of rockable-cam structural members, associated with a plurality of engine cylinders, are installed on the outer periphery of the drive shaft arranged in one cylinder row (or in one cylinder bank).
However, in the variable valve actuation apparatus disclosed in JP2010-163980, the previously-noted plurality of rockable-cam structural members are configured to be rotatably supported on the common drive shaft. Thus, when power (motion) is transmitted from the drive shaft to the rockable cams against the valve spring forces, there is a possibility of a radial deflection of the drive shaft, which is configured to longitudinally extend over the plurality of engine cylinders. Such a radial deflection causes a reduction of driving force of the drive shaft. This results in an undesirably great difference between a target valve lift amount (a desired valve lift) and an actual valve lift amount achieved by the rockable cam. Therefore, it would be desirable to minimize the difference between a target valve lift amount and an actual valve lift amount as much as possible.

SUMMARY OF THE INVENTION

It is, therefore, in view of the previously-described disadvantages of the prior art, an object of the invention to provide a valve actuation apparatus of an internal combustion engine configured to minimize the difference between a target valve lift amount and an actual valve lift amount, while suppressing a deflection of a drive shaft during operation of the engine.
In order to accomplish the aforementioned and other objects of the present invention, a valve actuation apparatus of an internal combustion engine having a plurality of engine cylinders comprises a drive shaft adapted to be driven by a crankshaft of the engine and rotatably arranged to extend over the plurality of engine cylinders in a direction of a cylinder row of the engine cylinders, a drive cam attached to the drive shaft and provided for each of the engine cylinders, a motion transmission mechanism provided for each of the engine cylinders for converting a rotary motion of the drive cam into an oscillating motion, a rockable-cam structural member provided for each of the engine cylinders in a state where a shaft is inserted through inner peripheries of the rockable-cam structural members, for operating at least one engine valve by an oscillating force transmitted from the motion transmission mechanism, and a cylindrical collar member provided in at least one axial position between the rockable-cam structural members spaced apart from each other in an axial direction of the shaft, an inner peripheral surface of the collar member being in contact with an outer peripheral surface of the shaft, and an outer peripheral surface of the collar member being in contact with an inner peripheral surface of a bearing portion mounted on the engine.
According to another aspect of the invention, a valve actuation apparatus of an internal combustion engine having a plurality of engine cylinders comprises a drive shaft adapted to be driven by a crankshaft of the engine and rotatably arranged to extend over the plurality of engine cylinders in a direction of a cylinder row of the engine cylinders, a drive cam attached to the drive shaft and provided for each of the engine cylinders, a motion transmission mechanism provided for each of the engine cylinders for converting a rotary motion of the drive cam into an oscillating motion, and a rockable-cam structural member provided for each of the engine cylinders in a state where a shaft is inserted through inner peripheries of the rockable-cam structural members, for operating at least one engine valve by an oscillating force transmitted from the motion transmission mechanism, wherein the shaft is rotatably supported at a plurality of points including at least one axial position between the rockable-cam structural members spaced apart from each other in an axial direction of the shaft.
According to a further aspect of the invention, a valve actuation apparatus of an internal combustion engine having a plurality of engine cylinders comprises a drive shaft adapted to be driven by a crankshaft of the engine and rotatably arranged to extend over the plurality of engine cylinders in a direction of a cylinder row of the engine cylinders, a drive cam attached to the drive shaft and provided for each of the engine cylinders, a motion transmission mechanism provided for each of the engine cylinders for converting a rotary motion of the drive cam into an oscillating motion, a rockable-cam structural member provided for each of the engine cylinders in a state where a shaft is inserted through inner peripheries of the rockable-cam structural members, for operating at least one engine valve by an oscillating force transmitted from the motion transmission mechanism, and the rockable-cam structural member comprising a pair of rockable cams configured to oscillate for operating a pair of engine valves by respective oscillating motions of the rockable cams, a first cylindrical portion formed integral with the rockable cams and arranged between the rockable cams so that an outer peripheral surface of the first cylindrical portion is rotatably supported by a first bearing portion mounted on the engine, and a second cylindrical portion configured to protrude axially outward from at least one of the rockable cams, wherein both of the shaft and the second cylindrical portion are rotatably supported by a second bearing portion mounted on the engine.
The other objects and features of this invention will become understood from the following description with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view illustrating an embodiment of a valve actuation apparatus, which is applied to the intake-valve side of a V-type internal combustion engine, partly cut away.

FIG. 2 is a lateral cross-sectional view taken along the line II-II of FIG. 1.

FIG. 3 is a longitudinal cross-sectional view illustrating the valve actuation apparatus of the embodiment.

FIG. 4 is an enlarged cross-sectional view illustrating the essential part of the valve actuation apparatus of the embodiment shown in FIG. 3.

FIGS. 5A-5B are explanatory views illustrating the operation of the valve actuation apparatus of the embodiment during minimum working-angle and minimum valve lift control.

FIGS. 6A-6B are explanatory views illustrating the operation of the valve actuation apparatus of the embodiment during maximum working-angle and maximum valve lift control.

FIG. 7 is a valve characteristic diagram (exactly, engine valve lift and event characteristic curves), obtained by the valve actuation apparatus of the embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, the valve actuation apparatus of the embodiment is exemplified in a variable valve event and lift control apparatus of the intake-valve side of a V-6 internal combustion engine whose cylinders are arranged in two banks of three cylinders. In particular, FIGS. 1-4 show the valve actuation apparatus of the embodiment, which is installed on three cylinders (i.e., No. 1 cylinder, No. 3 cylinder, and No. 5 cylinder) of one cylinder bank of the V-6 engine.
As clearly shown in FIGS. 1-3, the valve actuation apparatus of the embodiment is provided with two intake valves 3, 3 per cylinder for opening and closing a pair of intake ports 2, 2 formed in a cylinder head 1, a drive shaft 4, a rockable-cam structural member 7, a motion transmission mechanism (or a multi-nodular linkage motion converter) 8, a control mechanism 9, and a pair of hydraulic lash adjusters 10, 10. Drive shaft 4 is located above the three cylinders (#1, #3, #5 cylinders) of one cylinder bank and arranged in a longitudinal direction of the engine, and has three drive cams 5 (one drive cam per cylinder) mounted on the outer periphery of drive shaft 4. Rockable-cam structural member 7 is rotatably supported by a plurality of bearings and configured to operate (open and close) the intake valves 3, 3 via a pair of swing arms 6, 6. Motion transmission mechanism 8 is provided for converting rotary motion (a torque) of drive cam 5 into oscillating motion (an oscillating force) and for transmitting the oscillating motion to the rockable-cam structural member 7. Control mechanism 9 is provided for controlling a working angle and a valve lift amount (simply, a valve lift) of each of intake valves 3, 3 via the motion transmission mechanism 8. Hydraulic lash adjusters 10, 10, which are associated with respective intake valves 3, 3, are installed on the cylinder head 1 for automatically reducing the valve lash (or the valve clearance) between intake valve 3 and rockable-cam structural member 7 via respective swing arms 6, 6. Drive shaft 4 serves as a common shaft common to the three rockable-cam structural members 7, 7, 7.
For the sake of simplicity, in the following discussion, the component parts of the valve actuation apparatus for only one cylinder (e.g., #1 cylinder) are concretely explained.
Each of intake valves 3, 3 is slidably held on the cylinder head 1 via a valve guide 3 g. Each of intake valves 3, 3 is permanently biased in the valve-closing direction by means of a valve spring 12 compressed between a spring retainer 11 located near the valve-stem end 3 a and the upper flat face (serving as a spring seat) formed in the cylinder head 1.
As best seen in FIG. 3, drive shaft 4 has two shaft- end journal portions 4 a and 4 b formed at both axial ends. The left-hand journal portion 4 b (viewing FIG. 3) is rotatably supported on a front-end bearing portion 32, whereas the right-hand journal portion 4 a (viewing FIG. 3) is rotatably supported on a rear-end bearing portion 13. Drive shaft 4 has a driven connection with an engine crankshaft (not shown) so that torque is transmitted from the crankshaft via a timing pulley (not shown) installed on one axial end of drive shaft 4 and a timing belt (not shown) to the drive shaft. As seen in FIG. 3, drive cam 5 is fixedly connected to the outer periphery of drive shaft 4 by means of a connecting pin 44. As clearly shown in FIG. 2, the geometric center “Y” of drive cam 5 is radially displaced from the axis “X” of drive shaft 4. That is, drive cam 5 is an eccentric cam having a cylindrical cam profile.
Front-end bearing portion 32 is configured to rotatably support the front end of drive shaft 4 with a very small clearance defined between the inner peripheral surface of bearing portion 32 and the outer peripheral surface of front-end journal portion 4 a. These peripheral surfaces make a pair of bearing surfaces. Rear-end bearing portion 13 is configured to rotatably support the rear end of drive shaft 4 via a cylindrical bearing member 46 interleaved between the inner peripheral surface of rear-end bearing portion 13 and the outer peripheral surface of the rear end of drive shaft 4.
As best seen in FIG. 3, drive shaft 4 is formed into a substantially cylindrical-hollow shape. The axially-extending hollow portion of drive shaft 4 is formed as an axial oil-passage hole 33. Lubricating oil is supplied into the axial oil-passage hole 33 by way of an oil-passage hole 32 a formed in the front-end bearing portion 32. As a matter of course, lubricating oil, supplied from the oil-passage hole 32 a to the axial oil-passage hole 33, is used for lubrication for the front-end bearing portion 32 and the rear-end bearing portion 13. Lubricating oil, supplied into the axial oil-passage hole 33, is also used for lubrication for first bearing portions 41, 41, 41 (described later) by way of respective radial oil holes 36, 36, 36 and used for lubrication for second bearing portions 42, 42, 42 (described later) by way of respective radial oil holes 35, 35, 35.
As best seen in FIG. 2, the underside of one end 6 a of swing arm 6 is kept in abutted-engagement with the valve-stem end 3 a of intake valve 3, while the other end 6 b of swing arm 6 is kept in abutted-engagement with a plunger 25 (described later) of hydraulic lash adjuster 10. A roller 14 is rotatably supported on a roller shaft 14 a press-fitted into a central bore formed at the intermediate portion between both ends 6 a-6 b of swing arm 6.
As best seen in FIG. 1, rockable-cam structural member 7 is comprised of a cylindrical member 7 a whose inner peripheral surface is rotatably fitted onto the outer peripheral surface of drive shaft 4 with a radial clearance T, and a pair of rockable cams 7 b, 7 b formed integral with the cylindrical member 7 a. Rockable cams 7 b, 7 b are arranged at both ends of cylindrical member 7 a, such that each of rockable cams 7 b, 7 b is in alignment with the direction of motion of the associated intake valve 3. Radial clearance T is set to a clearance that the outer peripheral surface of drive shaft 4 can be kept out of contact with the inner peripheral surface of cylindrical member 7 a during operation of the engine.
The underside of each of rockable cams 7 b, 7 b is formed as a cam-contour surface 7 c including a base-circle surface, a ramp surface, a lift surface, and the like. The underside of rockable cam 7 b is contoured such that the base-circle surface, the ramp surface, and the lift surface are brought into rolling-contact with given positions of the upside of roller 14 of swing arm 6, depending on the oscillatory position of rockable cam 7 b.
Cylindrical member 7 a has a first journal portion 7 d (serving as a first cylindrical portion) arranged at its axial middle position, and a second journal portion 7 e (serving as a second cylindrical portion) arranged at one axial end (the left-hand axial end of cylindrical member 7 a, viewing FIG. 4) in a manner so as to protrude axially outward from one rockable cam (the left-hand rockable cam 7 b, viewing FIG. 4) of the rockable-cam pair. First and second journal portions 7 d-7 e are formed on the outer peripheral surface of cylindrical member 7 a. First and second journal portions 7 d-7 e are rotatably supported by means of three first bearing portions 41, 41, 41 and second bearing portions 42, 42, 42. Three first bearing portions 41 and three second bearing portions 42 are arranged between the rear-end bearing portion 13 and the front-end bearing portion 32.
Each of first bearing portions 41 is comprised of a half-round bearing-groove section ld (i.e., a lower bearing half) formed on the upper end of cylinder head 1 and a bearing bracket 41 a arranged above the half-round bearing-groove section 1 d and having a half-round bearing-groove section 41 c (i.e., an upper bearing half) opposing to the half-round bearing-groove section 1 d. First bearing portion 41 is configured to rotatably support the first journal portion 7 d of cylindrical member 7 a between the two opposing half-round bearing-groove sections, namely, the half-round bearing-groove section 1 d of cylinder head 1 and the half-round bearing-groove section 41 c of bearing bracket 41 a. Each of three bearing brackets 41 a is attached to the cylinder head 1 by two bolts (not shown). Rear-end bearing portion 13 is comprised of a half-round bearing-groove section 1 d (i.e., a lower bearing half) formed on the upper end of cylinder head 1, a main bracket 13 a arranged above the half-round bearing-groove section 1 d and having a half-round bearing-groove section 13 c (i.e., an upper bearing half) opposing to the half-round bearing-groove section 1 d, and a sub-bracket 13 b arranged above the main bracket 13 a. Sub-bracket 13 b and main bracket 13 a are mounted on the cylinder head 1 by fastening them together with bolts. In a similar manner to the rear-end bearing portion 13, each of three second bearing portions 42 is comprised of a half-round bearing-groove section 1 d (i.e., a lower bearing half) formed on the upper end of cylinder head 1, a main bracket 42 a arranged above the half-round bearing-groove section 1 d and having a half-round bearing-groove section 42 c (i.e., an upper bearing half) opposing to the half-round bearing-groove section 1 d, and a sub-bracket 42 b arranged above the main bracket 42 a. Sub-bracket 42 b and main bracket 42 a are mounted on the cylinder head 1 by fastening them together with bolts.
Rear-end bearing portion 13 is configured to rotatably support the journal portion 4 a of cylindrical bearing member 46 between the two opposing half-round bearing-groove sections, namely, the half-round bearing-groove section 1 d of cylinder head 1 and the half-round bearing-groove section 13 c of main bracket 13 a. On the other hand, an axial part of second bearing portion 42 (the axial right-hand side of second bearing portion 42, viewing FIGS. 3-4, more concretely, approximately one-third bearing section of second bearing portion 42 in the axial direction) is used to rotatably support the second journal portion 7 e of cylindrical member 7 a between the two opposing half-round bearing-groove sections, namely, the half-round bearing-groove section 1 d of cylinder head 1 and the half-round bearing-groove section 42 c of main bracket 42 a.
As clearly shown in FIG. 4, a plain bearing 43 is rotatably supported between the inner peripheral surface of the half-round bearing-groove section 42 c of main bracket 42 a and the outer peripheral surface of drive shaft 4. The remaining axial part of second bearing portion 42 (the axial left-hand side of second bearing portion 42, viewing FIGS. 3-4, more concretely, approximately two-third bearing section of second bearing portion 42 in the axial direction) is used to rotatably support the plain bearing 43 between the two opposing half-round bearing-groove sections, namely, the half-round bearing-groove section 1 d of cylinder head 1 and the half-round bearing-groove section 42 c of main bracket 42 a. That is, the plain bearing 43 together with the second journal portion 7 e of cylindrical member 7 a is rotatably supported between the two opposing half-round bearing-groove sections, namely, the half-round bearing-groove section 1 d of cylinder head 1 and the half-round bearing-groove section 42 c of main bracket 42 a. Plain bearing 43 is a cylindrical collar member (a ring-shaped member) formed of metal materials. In the shown embodiment, the collar-type plain bearing 43 has an annular groove and an oil hole communicating with the annular groove for lubricating-oil distribution around the entire outer peripheral surface of bearing 43 (see FIG. 3).
As shown in FIGS. 3-4, plain bearings 43, 43, 43 are provided to rotatably support the drive shaft 4 via the associated second bearing portions 42, 42, 42. As best seen in FIG. 4, the inner peripheral surface of plain bearing 43 is movably loosely fitted onto the outer peripheral surface of drive shaft 4 with a very small clearance T1, so as to permit a rotary motion as well as an axial sliding motion of the plain bearing 43 relative to the drive shaft 4. The axial length of plain bearing 43 is dimensioned to be almost the same axial length of second bearing portion 42. The axial end face (the right-hand end face, viewing FIG. 4) of an axial part 43 a of each of three plain bearings 43, rotatably supported or held by the associated second bearing portion 42, is in abutted-engagement with the end face (the left-hand end face, viewing FIG. 3) of the associated second journal portion 7 e. The axial end face (the left-hand end face, viewing FIG. 3) of the remaining axial part 43 b of the leftmost (foremost) plain bearing 43, protruding from the associated second bearing portion 42 and facing apart from the associated second journal portion 7 e, is in abutted-engagement with the stepped sidewall face of the front end of drive shaft 4. The axial end face (the left-hand end face, viewing FIG. 3) of the remaining axial part 43 b of each of the right-hand side two plain bearings 43, both protruding from the associated second bearing portions 42 and facing apart from the associated second journal portions 7 e, is in abutted-engagement with the sidewall face of the associated drive cam 5, facing rearwards. Accordingly, rockable-cam structural member 7 is sandwiched between the plain bearing 43 and the drive cam 5 in the axial direction, thereby restricting an axial movement of rockable-cam structural member 7.
The contact-surface area of the outer peripheral surface of plain bearing 43 in contact with the inner peripheral surfaces of the two opposing half-round bearing-groove sections, namely, the half-round bearing-groove section 42 c of main bracket 42 a of second bearing portion 42 and the half-round bearing-groove section 1 d of cylinder head 1) is dimensioned to be greater than the contact-surface area of the outer peripheral surface of the second journal portion 7 e of cylindrical member 7 a in contact with the two opposing half-round bearing- groove sections 42 c and 1 d. By virtue of the contact-surface area of plain bearing 43 in contact with the two opposing half-round bearing- groove sections 42 c and 1 d, dimensioned to be greater than that of the second journal portion 7 e in contact with the half-round bearing- groove sections 42 c and 1 d, it is possible to enhance a mechanical strength of the supporting structure for the drive shaft 4.
In the shown embodiment, the outside diameter of plain bearing 43 and the outside diameter of second journal portion 7 e are set to be identical to each other, thereby effectively suppressing undesirable rattling motion, which may occur between the plain bearing 43 and the second journal portion 7 e, both located within the two opposing half-round bearing- groove sections 42 c and 1 d.
By virtue of a very small clearance T2 between the outer peripheral surface of plain bearing 43 and the inner peripheral surfaces of the two opposing half-round bearing- groove sections 42 c and 1 d, together with the very small clearance T1 between the inner peripheral surface of plain bearing 43 and the outer peripheral surface of drive shaft 4, drive shaft 4 is rotatably supported.
Regarding the very small clearance T1 between the inner peripheral surface of plain bearing 43 and the outer peripheral surface of drive shaft 4 and the very small clearance T2 between the outer peripheral surface of plain bearing 43 and the inner peripheral surfaces of the two opposing half-round bearing- groove sections 42 c and 1 d, a radial dimension of the summed clearance T3 (=T1+T2) of the very small clearances T1 and T2 is dimensioned to be less than the radial clearance T between the inner peripheral surface of cylindrical member 7 a of rockable-cam structural member 7 and the outer peripheral surface of drive shaft 4, that is, T1+T2=T3<T.
An oblique oil-passage hole 37 is formed in the foremost second bearing portion 42, associated with the leftmost (foremost) plain bearing 43, in a manner so as to communicate with the axial oil-passage hole 33 formed in the drive shaft 4. Lubricating oil can be supplied toward a control shaft 21 (described later) by way of the oblique oil-passage hole 37.
As best seen in FIGS. 2-3, motion transmission mechanism 8 is comprised of a rocker arm 15 laid out above the drive shaft 4, a link arm 16 mechanically linking one end 15 a of rocker arm 15 to the drive cam 5, and a link rod 17 mechanically linking the other end 15 b of rocker arm 15 to one (the right-hand side rockable cam 7 b, viewing FIGS. 3-4) of the two adjacent rockable cams 7 b, 7 b.
The substantially central, cylindrical basal portion of rocker arm 15 is supported on a control cam 22 (described later), such that a cylindrical support bore formed in the basal portion of rocker arm 15 is rotatably or pivotably fitted onto the cylindrical outer peripheral surface of control cam 22. The one end 15 a of rocker arm 15 is rotatably linked to the link arm 16 via a pin 18. The other end 15 b of rocker arm 15 is rotatably linked to the upper end of link rod 17 via a connecting pin 19.
The substantially annular basal portion of link arm 16 has a central fitting bore 16 a into which the cam body of drive cam 5 is rotatably fitted. The protruding end of link arm 16 is linked to the one end 15 a of rocker arm 15 via the pin 18.
The lower end of link rod 17 is rotatably linked to the cam-nose portion of the right-hand side rockable cam 7 b (viewing FIGS. 3-4) of the two adjacent rockable cams 7 b, 7 b via a connecting pin 20.
By the way, a valve-lift adjust mechanism 23 is provided between the other end 15 b of rocker arm 15 and the upper end of link rod 17 for fine-adjustment of a lift amount of each of intake valves 3, 3 when assembling respective component parts of the valve actuation apparatus.
Control mechanism 9 is comprised of the control shaft 21 arranged above the drive shaft 4 in a manner so as to extend parallel to the drive shaft 4 in the longitudinal direction of the engine, and the control cam 22 fixedly connected onto or integrally formed with the outer periphery of control shaft 21. Control cam 22 is rotatably fitted into the support bore of the basal portion of rocker arm 15 and thus serves as a fulcrum of oscillating motion of rocker arm 15.
As best seen in FIG. 3, control shaft 21 is rotatably supported by means of four third bearing portions 45, 45, 45, 45. The rightmost (rearmost) third bearing portion 45 is constructed by an upper half-round bearing section formed at the lower end of sub-bracket 13 b and a lower half-round bearing section formed at the upper end of main bracket 13 a of rear-end bearing portion 13. Each of the remaining three third bearing portions 45 is constructed by an upper half-round bearing section formed at the lower end of sub-bracket 42 b and a lower half-round bearing section formed at the upper end of main bracket 42 a of second bearing portion 42. The angular position of control shaft 21 is controlled by means of an actuator (not shown). The axially-extending hollow portion of control shaft 21 is formed as an axial oil-passage hole 38. Lubricating oil is supplied into the axial oil-passage hole 38 of control shaft 21 by way of the oblique oil-passage hole 37 formed in the foremost second bearing portion 42. Axial oil-passage hole 38 is configured to feed lubricating oil to the two opposing half-round bearing sections of each of third bearing portions 45 by way of radial oil holes (through holes) 39 formed in the control shaft 21 in a manner so as to penetrate the outer periphery of control shaft 21 and the inner periphery of the hollow portion of control shaft 21. Axial oil-passage hole 38 is also configured to feed lubricating oil to the clearance space defined between the inner peripheral surface of the support bore of the basal portion of rocker arm 15 and the outer peripheral surface of control cam 22 by way of radial oil holes (through holes) 40 formed in the control cam 22 in a manner so as to penetrate the outer periphery of control cam 22 and the inner periphery of the hollow portion of control shaft 21. On the other hand, control cam 22 is an cylindrical eccentric cam whose geometric center is radially displaced from the axis of control shaft 21 by a predetermined eccentricity.
The control-shaft actuator (not shown) is comprised of a housing (not shown) fixedly connected to cylinder head 1, an electric motor fixed to one end of the housing, and a ball-screw motion-transmitting mechanism (simply, a ball-screw mechanism) installed in the housing so as to transmit a motor torque created by the electric motor to the control shaft 21. The electric motor is constructed by a proportional control type direct-current (DC) motor. Rotary motion of the electric motor (in the normal-rotational direction or in the reverse-rotational direction) is controlled in response to a control command signal from an electronic control unit (not shown) whose signal value is determined based on an engine operating condition.
As shown in FIG. 2, each of hydraulic lash adjusters 10, 10, associated with respective intake valves 3, 3, is comprised of (i) a substantially cylindrical lash-adjuster body 24 closed at its bottom end and held and fitted into a cylindrical retaining hole 1 a formed in cylinder head 1, (ii) the plunger 25 slidably accommodated or fitted in the body 24, and (iii) a one-way check valve (not shown). For instance, the hydraulic lash adjuster of the embodiment uses a ball check valve having a check ball and a return spring. Plunger 25 is also formed into a cylindrical-hollow shape but has a partition wall (not shown) at its bottom to define a hydraulic oil reservoir chamber in the cylindrical-hollow plunger 25. The partition wall of plunger 25 has a small center communication hole (not shown), such as a small through hole. On the other hand, the body 24 has a high-pressure chamber (not shown) defined at the lower end of body 24. The high-pressure chamber of body 24 is communicated with the hydraulic oil reservoir chamber of plunger 25 through the communication hole of the partition wall of plunger 25. The check valve is disposed in the high-pressure chamber so that the check valve permits free-flow of working fluid (hydraulic oil) in the reservoir chamber of plunger 25 via the communication hole of the partition wall of plunger 25 to the high-pressure chamber of body 24, but prevents any working-fluid flow in the opposite direction. By the way, cylinder head 1 has an oblique through hole 1 b formed therein for exhausting working fluid accumulated in the cylindrical retaining hole 1 a of cylinder head 1.
Although it is not clearly shown in FIG. 2, body 24 of lash adjuster 10 has (i) a first annular recessed groove 24 g formed in the outer peripheral surface of the intermediate portion between both ends of body 24 and (ii) a first oil-passage hole (a through hole) formed in the peripheral wall of the first annular recessed groove 24 g. One opening end of the first oil-passage hole of body 24 communicates via the first annular recessed groove 24 g with the downstream end of an oil passage 30 formed in the cylinder head 1, whereas the other opening end of the first oil-passage hole of body 24 communicates with the inside of body 24.
In a similar manner to the lash-adjuster body 24, although it is not clearly shown in FIG. 2, plunger 25 of lash adjuster 10 has (i) a second annular recessed groove formed in the outer peripheral surface of plunger 25 and configured to be substantially conformable to the position of formation of the first annular recessed groove 24 g of body 24 and (ii) a second oil-passage hole (a through hole) formed in the peripheral wall of the second annular recessed groove. One opening end of the second oil-passage hole of plunger 25 communicates with the second annular recessed groove, whereas the other opening end of the second oil-passage hole of plunger 25 communicates with the reservoir chamber of plunger 25.
As clearly seen in FIG. 2, the upstream end of oil passage 30 of cylinder head 1 is configured to communicate with a main oil gallery 31 formed in the cylinder head 1 for lubricating-oil supply to moving engine parts. Pressurized working fluid (pressurized lubricating oil) is force-fed from an oil pump (not shown) to the main oil gallery 31 during operation.
When hydraulic pressure in the high-pressure chamber of body 24 becomes lowered owing to extension of plunger 25, working fluid, supplied from the oil passage 30 into the cylindrical retaining hole 1 a of cylinder head 1, is flown into the reservoir chamber of plunger 25 through the first annular recessed groove 24 g and the first oil-passage hole, both formed in the body 24 and the second annular recessed groove and the second oil-passage hole, both formed in the plunger 25. Furthermore, the working fluid, supplied into the reservoir chamber of plunger 25, forces the check ball against the spring force of the return spring in a direction for opening of the check valve, and then the working fluid is further flown into the high-pressure chamber of body 24. In this manner, plunger 25 of lash adjuster 10 acts to always push up the other end 6 b of swing arm 6 so as to automatically adjust each of the clearance between rockable cam 7 b and roller 14 of swing arm 6 and the clearance between the one end 6 a of swing arm 6 and the valve-stem end 3 a of intake valve 3 to zero lash.

[Basic Operation of Valve Actuation Apparatus of the Embodiment]

For instance, in a low engine-speed range, such as when the engine is idling, the electric motor of the control-shaft actuator is driven in the normal-rotational direction by a control current (a control command signal) generated from the control unit, and thus torque is transmitted via the ball-screw mechanism to the control shaft 21. Owing to the rotary motion of control shaft 21, control cam 22 rotates together, and hence the geometric center of control cam 21 revolves around the axis of control shaft 21 along a circle with a center corresponding to the axis of control shaft 21 and a predetermined radius corresponding to the predetermined eccentricity of the geometric center of control cam 22 from the axis of control shaft 21. As a result of this, that is, owing to a given angular displacement of the geometric center of control cam 22 about the axis of control shaft 21 in the normal-rotational direction, as shown in FIGS. 5A-5B, the thick-walled portion of control cam 22 is displaced in an upper right direction in a manner so as to be spaced apart from the drive shaft 4. Hence, the pivot (i.e., connecting pin 19) of the other end 15 b of rocker arm 15 and the link rod 17 is displaced upward with respect to the drive shaft 4. Accordingly, the cam-nose portion of each of rockable cams 7 b, 7 b of the rockable-cam structural member 7 is forcibly pulled up via the link rod 17.
Under these conditions, when the one end 15 a of rocker arm 15 is pushed up via the link arm 16 owing to rotary motion of drive cam 5, an upward displacement (in other words, a lift amount) of the one end 15 a of rocker arm 15, is transmitted through the link rod 17 and the rockable- cam pair 7 b, 7 b to the swing- arm pair 6, 6. Thus, intake valves 3, 3, associated with respective swing arms 6, 6, are opened against the spring forces of valve springs 12, 12. By virtue of an attitude change of the motion transmission mechanism (the multi-nodular linkage motion converter) 8, a valve lift of each of intake valves 3, 3 becomes an adequately small lift amount (see the valve characteristic L of FIG. 7). This valve characteristic L corresponds to a minimum working angle and minimum valve lift control mode.
Thereafter, when the engine operating condition has changed to a high engine-speed range, the electric motor of the control-shaft actuator is driven in the reverse-rotational direction by a control current (a control command signal) generated from the control unit, and hence the ball-screw mechanism also rotates in the reverse-rotational direction. As a result of this, that is, owing to a given angular displacement of the geometric center of control cam 22 about the axis of control shaft 21 in the reverse-rotational direction, as shown in FIGS. 6A-6B, the thick-walled portion of control cam 22 (or the geometric center of control cam 22) moves toward the drive shaft 4. Hence, rocker arm 15, as a whole, moves toward the drive shaft 4. Accordingly, the cam-nose portion of each of rockable cams 7 b, 7 b of the rockable-cam structural member 7 is forcibly pushed down via the other end 15 b of rocker arm 15 and the link rod 17. As can be appreciated from comparison between the attitude of rockable cam 7 b shown in FIG. 5A and the attitude of rockable cam 7 b shown in FIG. 6A, and comparison between the attitude of rockable cam 7 b shown in FIG. 5B and the attitude of rockable cam 7 b shown in FIG. 6B, the angular position of the rockable- cam pair 7 b, 7 b of FIGS. 6A-6B is displaced anticlockwise by a given angular displacement from the angular position of the rockable-cam pair of FIGS. 5A-5B. Therefore, as can be appreciated from comparison between the rolling-contact position of cam-contour surface 7 c of rockable cam 7 b in rolling-contact with the outer peripheral surface of roller 14 of swing arm 6 at a peak lift shown in FIG. 5B and the rolling-contact position of the same cam-contour surface 7 c in rolling-contact with the outer peripheral surface of roller 14 of swing arm 6 at a peak lift shown in FIG. 6B, in the case of the attitude of the multi-nodular linkage motion transmission mechanism 8 shown in FIGS. 6A-6B, the rolling-contact position (the abutted-engagement position) of cam-contour surface 7 c of rockable cam 7 b in rolling-contact with the outer peripheral surface of swing-arm roller 14 shifts toward the cam-nose portion.
Under these conditions, when the one end 15 a of rocker arm 15 is pushed up via the link arm 16 owing to rotary motion of drive cam 5 during the open period of intake valve 3, a comparatively greater upward displacement (in other words, a greater lift amount) of the one end 15 a of rocker arm 15, is transmitted through the link rod 17 and the rockable- cam pair 7 b, 7 b to the swing- arm pair 6, 6. Thus, intake valves 3, 3, associated with respective swing arms 6, 6, are more greatly opened against the spring forces of valve springs 12, 12 (see the valve characteristic L1 of FIG. 7). This valve characteristic L1 corresponds to a maximum working angle and maximum valve lift control mode. The working angle and valve lift amount can be varied continuously from the minimum working angle and minimum valve lift characteristic L to the maximum working angle and maximum valve lift characteristic L1 depending on the control current generated from the control unit to the electric motor of the control-shaft actuator.
For instance, during an open period of each of intake valves 3, 3 of #1 cylinder in a high engine-speed range as shown in FIGS. 3 and 6B, the one end 15 a of rocker arm 15 is pushed up by application of a push (a force) from the drive cam 5 via the link arm 16 to the one end 15 a of rocker arm 15, and simultaneously the other end 15 b of rocker arm 15 is pushed down, and thus the cam-nose portion of each of rockable cams 7 b, 7 b is also pushed down. Roller 14 of each of swing arms 6, 6 is also pushed down by the lift surface of cam-contour surface 7 c of each of rockable cams 7 b, 7 b, thereby opening each of intake valves 3, 3. Under these conditions, a large reaction force FC1 of the valve spring 12 associated with one intake valve 3 of the two adjacent intake valves and a large reaction force FC2 of the valve spring 12 associated with the other intake valve 3 are transmitted or applied through the swing arm pair 6, 6, the rockable-cam structural member 7, the link rod 17, the rocker arm 15, and the link arm 16 to the drive cam 5 in the form of a pushing-down force (a large load) F1.
For the reasons discussed above, drive shaft 4 tends to slightly deflect downward (that is, toward the side of intake valves 3, 3), while reducing (i) the radial clearance T between the outer peripheral surface of drive shaft 4 and the inner peripheral surface of cylindrical member 7 a of rockable-cam structural member 7, (ii) a clearance between the outer peripheral surface of cylindrical member 7 a (in particular, the first journal portion 7 d) and the inner peripheral surface of the first bearing portion 41, and (iii) a clearance between the outer peripheral surface of cylindrical member 7 a (in particular, the second journal portion 7 e) and the inner peripheral surface of the second bearing portion 42, and (iv) the very small clearance T1 between the inner peripheral surface of plain bearing 43 and the outer peripheral surface of drive shaft 4.
At this time, that is, during the open period of each of intake valves 3, 3 of #1 cylinder in a high engine-speed range as shown in FIGS. 3 and 6B, each of intake valves 3, 3 of #3 cylinder, which cylinder is located adjacent to the #1 cylinder, is controlled to a valve-closed state. However, owing to a slight downward deflection of drive shaft 4 as described previously, there is a risk for each of intake valves 3, 3 of the adjacent #3 cylinder to be downwardly opened slightly via the rockable- cam pair 7 b, 7 b and the swing- arm pair 6, 6.
In the case of the variable valve actuation apparatus disclosed in JP2010-163980, somewhat similar to the present invention in construction but not having plain bearings 43, a comparatively great clearance T exists between the drive shaft 4 and the cylindrical member 7 a of rockable-cam structural member 7. Additionally, each of rockable-cam structural members 7 is supported at two points, that is to say, by means of the associated first and second bearing portions 41 and 42 via first and second journal portions 7 d and 7 e both constructing the cylindrical member 7 a. Thus, by the construction of the variable valve actuation apparatus disclosed in JP2010-163980, it is possible to suppress a radial displacement of each of rockable cams 7 b, 7 b. During a period of rolling-contact between the base-circle surface of cam-contour surface 7 c of each of rockable cams 7 b, 7 b and the outer peripheral surface of swing-arm roller 14, in other words, during a closed period of each of intake valves 3, 3 of #3 cylinder, it is possible to suppress each of intake valves 3, 3 of #3 cylinder from being undesirably slightly opened. However, owing to a slight downward deflection of drive shaft 4, drive cam 5 itself is also displaced downward, and thus an upward displacement of the one end 15 a of rocker arm 15 via the link arm 16 tends to reduce, and hence an actual valve lift amount also tends to become less than a target valve lift amount.
In contrast to the above, according to the valve actuation apparatus of the embodiment, drive shaft 4 is rotatably supported by three plain bearings 43, 43, 43, which bearings are axially spaced apart from each other and located adjacent to respective cylindrical members 7 a of rockable-cam structural member 7, in addition to the front-end bearing portion 32 and the rear-end bearing portion 13. That is, drive shaft 4 is rotatably mounted on the cylinder head 1 with the predetermined very small clearance T1 between the inner peripheral surface of each of plain bearings 43, 43, 43 and the outer peripheral surface of drive shaft 4. Additionally, drive shaft 4 is rotatably supported with the predetermined very small clearance T2 between the outer peripheral surface of plain bearing 43 and the inner peripheral surfaces of the two opposing half-round bearing- groove sections 42 c and 1 d.
Hence, according to the valve actuation apparatus of the embodiment, it is possible to restrict a downward deflection of drive shaft 4 within a radial dimension of the summed clearance T3 (=T1+T2) of the very small clearances T1 and T2. Therefore, it is possible to more effectively suppress the actual valve lift amount from becoming less than the target valve lift amount. In other words, it is possible to reduce a deviation of the actual valve lift amount from the target valve lift amount, as much as possible. As previously described, a radial dimension of the summed clearance T3 (=T1+T2) of the very small clearances T1 and T2 is dimensioned to be less than the radial clearance T between the inner peripheral surface of cylindrical member 7 a of rockable-cam structural member 7 and the outer peripheral surface of drive shaft 4. For the reasons discussed above, a downward deflection of drive shaft 4 within an area of the second bearing portion 42 is limited to a value less than that of drive shaft 4 within an area of the cylindrical member 7 a of rockable-cam structural member 7.
Returning to FIG. 3, the more concrete operation and effects obtained by the valve actuation apparatus of the embodiment, that is, the previously-discussed drive-shaft radial deflection limiting action, achieved by the clearance T3 (=T1+T2) appropriately tuned relatively to the radial clearance T, is hereunder explained in detail.
For instance, during the maximum working-angle and maximum valve lift control mode (suited for a high engine-speed range) shown in FIGS. 6A-6B, each of intake valves 3, 3 of the reference cylinder (that is, #3 cylinder) becomes controlled to a valve-closed state, while each of intake valves 3, 3 of the adjacent cylinder, for example, #1 cylinder becomes controlled to a valve-open state (see the valve-open state of the intake-valve pair (3, 3) of #1 cylinder at a peak lift shown in FIG. 6B). Under these conditions, a large load F1, arising from the previously-described reaction forces FC2-FC2, acts on the drive cam 5 of the adjacent engine-cylinder side (i.e., #1 cylinder). Assume that a maximum radial displacement (a maximum downward displacement) ΔS of the drive cam 5, arising from a radial deflection of drive shaft 4, takes place when the large load F1 is acting on the drive cam 5.
According to the valve actuation apparatus of the embodiment, a radial dimension of the summed clearance T3 (=T1+T2) of the very small clearance T1 between the inner peripheral surface of plain bearing 43 and the outer peripheral surface of drive shaft 4 and the very small clearance T2 between the outer peripheral surface of plain bearing 43 and the inner peripheral surfaces of the two opposing half-round bearing- groove sections 42 c and 1 a becomes less than the maximum radial displacement ΔS of the drive cam 5. By virtue of the clearance T3 (=T1+T2), a radial deflection of drive shaft 4 never reaches the maximum radial displacement ΔS of the drive cam. As discussed above, by the drive-shaft radial deflection limiting action, in other words, by the drive-cam radial displacement suppressing action, achieved by the clearance T3 (=T1+T2) appropriately tuned relatively to the radial clearance T, it is possible to adequately suppress or adequately reduce a deviation of the actual valve lift amount from the target valve lift amount.
As a result of this, the valve actuation apparatus of the embodiment ensures normal and stable operation of the engine. The previously-discussed more concrete operation and effects obtained by the valve actuation apparatus of the embodiment is exemplified in a maximum working-angle and maximum valve lift control mode (i.e., during a high valve-lift control mode suited for a high engine-speed range) shown in FIGS. 6A-6B. As can be appreciated, the valve actuation apparatus of the embodiment can provide the same operation and effects, that is, the drive-shaft radial deflection limiting action, in other words, the drive-cam radial displacement suppressing action, even during a low valve-lift control mode (see FIGS. 5A-5B) suited for idling operation of the engine.
Additionally, in the shown embodiment, lubricating oil, supplied into the axial oil-passage hole 33 of drive shaft 4 by way of the oil-passage hole 32 a of the front-end bearing portion 32, can be distributed around the entire inner peripheral surface of front-end bearing portion 32 as well as around the entire inner peripheral surface of rear-end bearing portion 13. Also, lubricating oil, supplied into the axial oil-passage hole 33 of drive shaft 4, can be distributed into a clearance space defined between the outer peripheral surface of the cylindrical member 7 a (in particular, the second journal portion 7 e) of rockable-cam structural member 7 (and the outer peripheral surface of plain bearing 43) and the inner peripheral surfaces of the two opposing half-round bearing- groove sections 42 c and 1 d of second bearing portion 42 and cylinder head 1, and a clearance space between the outer peripheral surface of the cylindrical member 7 a (in particular, the first journal portion 7 d) of rockable-cam structural member 7 and the inner peripheral surfaces of the two opposing half-round bearing-groove sections 41 c and ld of first bearing portion 41 and cylinder head 1, by way of radial oil holes 35, 35, 35 and radial oil holes 36, 36, 36. Furthermore, lubricating oil, supplied via the oblique oil-passage hole 37 of the foremost second bearing portion 42 into the axial oil-passage hole 38 of control shaft 21, can be distributed into a clearance space between the two opposing half-round bearing sections of each of third bearing portions 45 by way of radial oil holes 39 of control shaft 21. Also, lubricating oil, supplied into the axial oil-passage hole 38 of control shaft 21, can be distributed into a clearance space between the inner peripheral surface of the support bore of the basal portion of rocker arm 15 and the outer peripheral surface of control cam 22 by way of radial oil holes 40 of control cam 22. Thus, moving (rotating) parts of the valve actuation apparatus can be effectively lubricated.
It will be appreciated that the invention is not limited to the particular embodiments shown and described herein, but that various changes and modifications may be made. The valve actuation apparatus of the shown embodiment is applied to an intake-valve side of an internal combustion engine. In lieu thereof, the fundamental concept of the valve actuation apparatus of the embodiment may be applied to an exhaust-valve side.
Furthermore, the valve actuation apparatus of the embodiment is exemplified in a V-6 internal combustion engine. In lieu thereof, the fundamental concept of the valve actuation apparatus of the embodiment may be applied to a V-8 internal combustion engine, an in-line four-cylinder internal combustion engine, and the like.
Moreover, in the shown embodiment, drive shaft 4, which has a plurality of drive cams 5 provided for each individual engine cylinder, also serves as a shaft inserted through inner peripheries of rockable-cam structural members 7 provided for each individual engine cylinder. In lieu thereof, the shaft for the rockable-cam structural members 7 and the drive shaft 4 for the drive cams 5 may be configured separately from each other. In such a case, to more effectively suppress each of intake valves 3, 3 from being undesirably slightly opened even during a closed period of each of intake valves 3, 3, plain bearing 43 has to be installed so that the inner peripheral surface of plain bearing 43 is in contact with the outer peripheral surface of the shaft for the rockable-cam structural members 7, and the outer peripheral surface of plain bearing 43 is in contact with the inner peripheral surfaces of two opposing half-round bearing-groove sections, namely, the half-round bearing-groove section 1 d of cylinder head 1 and the half-round bearing-groove section 42 c of main bracket 42 a of second bearing portion 42.
By the way, the valve actuation apparatus of the embodiment uses a roller-type swing arm 6, serving as a roller cam follower. In lieu thereof, a typical bucket-type valve lifter, serving as a flat-face follower, may be used.
The entire contents of Japanese Patent Application No. 2011-159458 (filed Jul. 21, 2011) are incorporated herein by reference.
While the foregoing is a description of the preferred embodiments carried out the invention, it will be understood that the invention is not limited to the particular embodiments shown and described herein, but that various changes and modifications may be made without departing from the scope or spirit of this invention as defined by the following claims.

Claims

1. A valve actuation apparatus of an internal combustion engine having a plurality of engine cylinders comprising:

a drive shaft adapted to be driven by a crankshaft of the engine and rotatably arranged to extend over the plurality of engine cylinders in a direction of a cylinder row of the engine cylinders;

a drive cam attached to the drive shaft and provided for each of the engine cylinders;

a motion transmission mechanism provided for each of the engine cylinders for converting a rotary motion of the drive cam into an oscillating motion;

a rockable-cam structural member provided for each of the engine cylinders in a state where a shaft is inserted through inner peripheries of the rockable-cam structural members, for operating at least one engine valve by an oscillating force transmitted from the motion transmission mechanism; and

a cylindrical collar member provided in at least one axial position between the rockable-cam structural members spaced apart from each other in an axial direction of the shaft, an inner peripheral surface of the collar member being in contact with an outer peripheral surface of the shaft, and an outer peripheral surface of the collar member being in contact with an inner peripheral surface of a bearing portion mounted on the engine.

2. The valve actuation apparatus as claimed in claim 1, wherein:

the shaft, which is inserted through the inner peripheries of the rockable-cam structural members, is the drive shaft.

3. The valve actuation apparatus as claimed in claim 2, wherein:

the drive shaft has a pair of shaft-end journal portions formed at both axial ends, the shaft-end journal portions being rotatably supported by a pair of shaft-end bearing portions mounted on the engine; and

the bearing portion, which is mounted on the engine, is arranged between the shaft-end bearing portions for rotatably supporting the collar member.

4. The valve actuation apparatus as claimed in claim 3, wherein:

the collar member is provided for each of the engine cylinders; and

the bearing portion, which is mounted on the engine for rotatably supporting the collar member, is also installed for each of the engine cylinders for rotatably supporting the collar members by the respective bearing portions.

5. The valve actuation apparatus as claimed in claim 3, wherein:

the collar member is arranged between the drive cam and the rockable-cam structural member, so that one axial end of the collar member is in abutted-engagement with the drive cam and the other axial end of the collar member is in abutted-engagement with the rockable-cam structural member.

6. The valve actuation apparatus as claimed in claim 4, wherein:

an axial movement of the rockable-cam structural member is restricted by both the collar member and the drive cam.

7. The valve actuation apparatus as claimed in claim 4, wherein:

the rockable-cam structural member comprises:

a pair of rockable cams configured to oscillate by the oscillating force transmitted from the motion transmission mechanism for operating a pair of engine valves by respective oscillating motions of the rockable cams, resulting from the transmitted oscillating force;

a first cylindrical portion formed integral with the rockable cams and arranged between the rockable cams so that an outer peripheral surface of the first cylindrical portion is rotatably supported by a bearing portion mounted on the engine; and

a second cylindrical portion configured to protrude axially outward from at least one of the rockable cams,

wherein an outer peripheral surface of the second cylindrical portion, together with the outer peripheral surface of the collar member, is in contact with the inner peripheral surface of the bearing portion, which is mounted on the engine for rotatably supporting the collar member.

8. The valve actuation apparatus as claimed in claim 7, wherein:

a contact-surface area of the outer peripheral surface of the collar member in contact with the inner peripheral surface of the bearing portion, which is mounted on the engine for rotatably supporting the collar member, is dimensioned to be greater than a contact-surface area of the outer peripheral surface of the second cylindrical portion in contact with the inner peripheral surface of the bearing portion, which is mounted on the engine for rotatably supporting the collar member.

9. The valve actuation apparatus as claimed in claim 8, wherein:

an outside diameter of the collar member and an outside diameter of the second cylindrical portion are set to be identical to each other.

10. The valve actuation apparatus as claimed in claim 7, wherein:

the outer peripheral surface of the second cylindrical portion of the rockable-cam structural member is rotatably supported by the bearing portion, which is mounted on the engine for rotatably supporting the collar member.

11. The valve actuation apparatus as claimed in claim 2, wherein:

the collar member is loosely fitted onto the drive shaft to permit a rotary motion as well as an axial sliding motion of the collar member relative to the drive shaft.

12. The valve actuation apparatus as claimed in claim 11, wherein:

a clearance between the inner peripheral surface of the collar member and the outer peripheral surface of the drive shaft is dimensioned to be less than a clearance between an inner peripheral surface of the rockable-cam structural member and the outer peripheral surface of the drive shaft.

13. The valve actuation apparatus as claimed in claim 12, wherein:

the clearance between the inner peripheral surface of the rockable-cam structural member and the outer peripheral surface of the drive shaft is set to a clearance that there is no contact between the inner peripheral surface of the rockable-cam structural member and the outer peripheral surface of the drive shaft during operation of the engine.

14. The valve actuation apparatus as claimed in claim 1, wherein:

the motion transmission mechanism comprises:

a control shaft arranged to extend over the plurality of engine cylinders;

a control cam provided for each of the engine cylinders and attached to the control shaft, a geometric center of the control cam being radially displaced from an axis of the control shaft;

a rocker arm pivotably supported on the control cam of the control shaft; and

a link arm mechanically linking the drive cam and the rocker arm for converting the rotary motion of the drive cam into the oscillating motion and for transmitting the oscillating motion to the rocker arm,

wherein the control shaft is rotatably supported by a further bearing portion configured to extend from the bearing portion, whose inner peripheral surface is in contact with the outer peripheral surface of the collar member, toward the control shaft.

15. A valve actuation apparatus of an internal combustion engine having a plurality of engine cylinders comprising:

a motion transmission mechanism provided for each of the engine cylinders for converting a rotary motion of the drive cam into an oscillating motion; and

a rockable-cam structural member provided for each of the engine cylinders in a state where a shaft is inserted through inner peripheries of the rockable-cam structural members, for operating at least one engine valve by an oscillating force transmitted from the motion transmission mechanism,

wherein the shaft is rotatably supported at a plurality of points including at least one axial position between the rockable-cam structural members spaced apart from each other in an axial direction of the shaft.

16. The valve actuation apparatus as claimed in claim 15, wherein:

17. A valve actuation apparatus of an internal combustion engine having a plurality of engine cylinders comprising:

the rockable-cam structural member comprising:

a pair of rockable cams configured to oscillate for operating a pair of engine valves by respective oscillating motions of the rockable cams;

a first cylindrical portion formed integral with the rockable cams and arranged between the rockable cams so that an outer peripheral surface of the first cylindrical portion is rotatably supported by a first bearing portion mounted on the engine; and

wherein both of the shaft and the second cylindrical portion are rotatably supported by a second bearing portion mounted on the engine.

18. The valve actuation apparatus as claimed in claim 17, wherein: