JPH10196331A - Variable valve system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To facilitate machining of a lifter guide hole and management of a tolerance of size and prevent the occurrence of deviated wear of a direct strike type valve lifter, in a variable valve system employing a solid cam and the direct strike type valve lifter. SOLUTION: A variable valve system comprises a cam shaft 1 having a solid cam 2; a displacement device 3 therefor; and a direct strike type valve lifter 10 to open and close a valve 4 reciprocated by the solid cam 2. The direct strike type valve lifter 10 comprises an inverted cap-form lifter body 14 consisting of a disc-form end wall part 11 and a cylindrical side wall part 13; and a follow-up contact part 21 making contact with the solid cam with it following the solid cam 2. Through rotation blocking structures 27 and 33 arranged at the inside of the side wall part 13, rotation of the follow-up contact part 21 togetherwith the lifter body 14 based on the solid cam 2 is blocked.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a variable valve mechanism for continuously or stepwise changing a valve timing and a lift from a low rotation to a high rotation of an internal combustion engine.

[0002]

2. Description of the Related Art Japanese Patent Publication No. 7-45803 discloses FIG.
As shown in FIG. 1, a camshaft 101 provided with a three-dimensional cam 100 whose cam profile changes in the axial direction, and a direct-hit valve lifter 103 that opens and closes one valve 102 by reciprocating by the three-dimensional cam 100. The disclosed variable valve mechanism is disclosed. The direct-acting valve lifter 103 is provided with a cup 10 that covers the upper end of each valve 102.
4, a saddle 105 placed on the upper surface of the cup 104 so as to be surrounded by a peripheral wall 109 erected on the upper surface of the cup 104, and a contact line that is swingably mounted on the saddle 105 and rotates with the three-dimensional cam 100. Three-dimensional cam 1 while following changes in angle
00 and a shoe 106 that comes in contact with the shoe.

Further, Japanese Patent Publication No. 6-84722 discloses that
A camshaft 101 also having a three-dimensional cam 100,
As shown by a two-dot chain line in FIG. 11, a hydraulic actuator 107 for displacing the camshaft 101 in the axial direction according to the rotation speed of the internal combustion engine, and a direct-acting valve lifter 103 for opening and closing each valve 102 are provided. The disclosed variable valve mechanism is disclosed.

According to these variable valve mechanisms, the valve timing and the lift amount are continuously changed from a low rotation to a high rotation of the internal combustion engine to perform precise control according to the operating condition of the internal combustion engine. There is an advantage that it can be performed.

[0005]

In the above-described variable valve mechanism, since it is necessary to prevent the shoe 106 from rotating with respect to the three-dimensional cam 100, the following rotation preventing structure is provided. Restriction walls 108 (for restricting movement of the shoe 106 in the longitudinal direction) provided at both ends in the longitudinal direction of the saddle 105 have extension portions 11 toward the outer peripheral side.
0 is protruding. On the other hand, a female screw hole (not shown) is formed by cutting out a part of a lifter guide hole (not shown) of the cylinder head, and a slit-shaped notch 112 is formed in the female screw hole in a longitudinal direction. Is screwed. The extension 110 is slidably inserted into the notch 112, thereby preventing the saddle 105 and the shoe 106 from rotating with respect to the three-dimensional cam 100.

As described above, the conventional rotation preventing structure has the following problem because a part of the lifter guide hole is cut off for screwing the screw 111. It is difficult to machine the lifter guide hole, and it is not possible to control the dimensional tolerance of the inner diameter. The area of the lifter guide hole with which the cup 104 contacts decreases, and the partial surface pressure increases, causing uneven wear of the cup 104. Since the thickness around the lifter guide hole 8 cannot be increased due to the restriction of the space of the cylinder head, the rigidity around the lifter guide hole 8 tends to decrease due to the cutout. Then, assembling distortion or thermal distortion occurs, and seizure or uneven wear of the side wall portion 13 occurs.

An object of the present invention is to change the valve timing and the lift amount continuously or stepwise from low rotation to high rotation of an internal combustion engine by using a three-dimensional cam so as to meet the operating conditions of the internal combustion engine. Precise control can be performed, so that various characteristics such as torque, output, fuel efficiency, cleanness of exhaust gas, etc. can be maximized over the entire rotation range, as well as machining of lifter guide holes and dimensional tolerance management. And to provide a variable valve mechanism capable of preventing the uneven wear of the direct hit type valve lifter.

[0008]

In order to achieve the above object, a variable valve mechanism according to the present invention has a cam profile continuously changed in the axial direction from a cam profile for low rotation to a cam profile for high rotation. A camshaft having a three-dimensional cam, a displacement device for continuously or stepwise displacing the camshaft in the axial direction according to an operating condition such as the rotation speed of the internal combustion engine, and a reciprocating motion based on the cam profile of the three-dimensional cam And a direct-acting valve lifter that opens and closes the valve by opening and closing the valve. The direct-acting valve lifter is an inverted cup-shaped lifter having a disc-shaped end wall and a cylindrical side wall. A main body and a follow-up contact portion that contacts the three-dimensional cam while following a change in the contact line angle accompanying the rotation of the three-dimensional cam, and the follow-up contact portion rotates with the lifter body with respect to the three-dimensional cam. Rotation-inhibiting structure to prevent is characterized in that provided on the inside of the side wall portion.

Here, the phase, valve opening angle and lift amount of the valve timing in the cam profile for low rotation and the phase, valve opening angle and lift amount of the valve timing in the cam profile for high rotation are determined for each internal combustion engine. Can be set as appropriate according to the requirements in.
However, in many cases, the cam profile for low rotation has a small valve opening angle and lift amount, and the cam profile for high rotation has a large valve opening angle and lift amount.

When the camshaft is displaced in a stepwise manner by the displacement device, the camshaft may be changed in two steps. In such a case, it is preferable that the displacement in the two steps can be adjusted. More preferably, the camshaft is displaced in at least three stages. Most preferably, the camshaft is continuously displaced. The displacement device is not limited to a specific structure, but may be a device using hydraulic pressure, electromagnetic force, or the like.

As the rotation preventing structure, a guided portion provided on the inner wall surface of the side wall portion and a guide portion fixedly provided on the cylinder head side and entering the inside of the side wall portion allow reciprocating movement, and A structure that engages to prevent rotation can be exemplified.

The guide portion is provided at one portion of the lifter guide member, and at another portion of the lifter guide member, a fixed portion fixed to the cylinder head and fixed to the outer periphery of a valve guide for guiding the stem portion of the valve. Preferably, a spring seat portion for receiving one end of a valve spring for biasing the valve is provided.

It is further preferable that a valve oil seal member for oil-sealing the outer periphery of the stem is attached to the fixed portion.

In order to enable adjustment of the valve clearance, it is preferable to provide, for example, the following structures (A) and (B). (A) A structure in which a shim for adjusting valve clearance is mounted between a direct-acting valve lifter and an end of the valve. (B) The following contact portion is a replacement part for adjusting the valve clearance.

The variable valve mechanism of the present invention can be applied to either an intake valve or an exhaust valve, but is preferably applied to both.

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a variable valve mechanism in which the present invention is applied to both an intake valve and an exhaust valve will be described below with reference to the drawings. Therefore, in the embodiment, simply referring to a valve refers to both an intake valve and an exhaust valve.

First, FIGS. 1 to 7 show a variable valve mechanism according to a first embodiment. The camshaft 1 is formed with a three-dimensional cam 2 in which the cam profile is continuously changed in the axial direction from the right low-rotation cam profile to the left high-rotation cam profile in FIG. The three-dimensional cam 2 includes a base circular portion 2a and a nose portion 2b. The base circular portion 2a has the same radius in both the low-rotation cam profile and the high-rotation cam profile, and thus has a cylindrical surface without inclination. However, the nose part 2b
Are inclined like a conical surface because the valve opening operation angle and the lift amount are small in the low rotation cam profile and the valve opening operation angle and the lift amount are large in the high rotation cam profile.

At the end of the camshaft 1, there is provided a displacement device 3 for continuously displacing the camshaft 1 in the axial direction according to the operating conditions such as the rotation speed of the internal combustion engine. The displacement device 3 includes, for example, a guide portion of the camshaft 1 using a spline, and a driving portion of the camshaft 1 using a hydraulic pressure (both not shown), and a rotation sensor of an internal combustion engine, an accelerator opening sensor, and the like. Is controlled by a control device (not shown) such as a microcomputer that operates based on the above.

Below the camshaft 1, a direct-acting valve lifter 10 that opens and closes the valve 4 by reciprocating up and down based on the cam profile of the three-dimensional cam 2 is provided. The stem 4 a of the valve 4 is guided by being inserted into a valve guide 25 fixed to the cylinder head 7.

The direct hit type valve lifter 10 includes an inverted cup-shaped lifter body 14 having a disk-shaped end wall 11 and a cylindrical side wall 13 extending downward from the end wall 11. The side wall 13 is vertically slidably guided by a lifter guide hole 8 formed in the cylinder head 7. A relief recess 15 for releasing the rotation trajectory of the three-dimensional cam 2 is formed around the lifter guide hole 8 of the cylinder head 7. A central portion of the lower surface of the end wall portion 11 is a pressing portion 12 for pressing the end portion of the valve 4.
A shim 9 for adjusting the valve clearance is interposed between the valve and the end of the valve 4. The valve 4 is attached to a valve retainer 5 attached near the upper end of the valve 4.
The upper end of the valve spring 6 for urging to the zero side is in contact. A spring recess 26 for holding the lower end of the valve spring 6 is formed in the cylinder head 7 directly below the lifter guide hole 8.

If only the guide by the lifter guide hole 8 is used, the following contact portion 21 described later rotates together with the lifter main body 14 with respect to the three-dimensional cam 2. 13 are provided inside.

At the lower end of the inner wall surface of the side wall portion 13, approximately 1
A guided portion 27 having a predetermined width is projected from two places separated by 80 degrees, and a guided portion 28 having a predetermined width is provided at two places approximately 180 degrees apart without the guided portion 27. The guided portion 27 and the guided portion 28 are formed by cutting the entire periphery of the lower end of the opening of the side wall portion 13 inward, and then cutting off two portions of the narrowed portion. On the other hand, the cylinder head 7 just below the lifter guide hole 8 has
For example, a lifter guide member 30 made of stainless spring steel is provided. The lifter guide member 30 is
An annular plate-shaped spring seat portion 31 provided on the lower surface portion is fitted into the spring concave portion 26, and a stepped cylindrical fixing portion 32 erected from the inner peripheral portion of the spring seat portion 31 is used as a valve guide. The cylinder 25 is positioned and fixed to the cylinder head 7 by being press-fitted into the outer periphery of the cylinder 25 in a non-rotatable manner (may be fitted in a different shape). The spring seat portion 31 functions as a seat that receives the lower end of the valve spring 6. The spring seat portion 31 is erected from two places approximately 180 degrees apart from each other on the outer peripheral portion, so that a predetermined width fixed to the cylinder head 7 side (a width smaller by a minute clearance than the width of the to-be-entered portion 28). The guide portion 33) enters the inside of the side wall portion 13 from the entered portion 28. Then, the side end surfaces of the respective guide portions 33 are slidably engaged with the side end surfaces of the respective guided portions 27 to allow reciprocation of the direct hit valve lifter 10 and to allow the follow-up contact portion 21 and the lifter body 14 The rotation is blocked.

As described above, since the rotation preventing structure is provided inside the side wall portion 13, there is no need to cut out a part of the lifter guide hole for screwing the screw 111 unlike the conventional example of FIG. . Therefore, the following effects can be obtained. Processing of the lifter guide hole 8 is easy, and dimensional tolerance control of the inner diameter can be easily performed. The area of the lifter guide hole 8 with which the side wall 13 contacts does not decrease, and uneven wear of the side wall 13 does not occur. Since the rigidity around the lifter guide hole 8 does not decrease and assembly distortion and thermal distortion hardly occur, seizure and uneven wear of the side wall portion 13 do not occur.

On the outer periphery of the fixed portion 32, a valve oil seal member 34 is externally fitted at the lower large-diameter portion 35 and is attached and integrated by welding or the like, and the upper small-diameter portion 36 is attached to the stem of the valve 4 by a stem. The outer periphery of the stem portion 4a is oil-sealed by contacting the outer periphery of the portion 4a and fastening the small-diameter portion 36 from the outer periphery by the fastening ring 37.
Therefore, the engine oil adhering to the upper portion of the stem portion 4a is blocked by the valve oil seal member 34 and does not enter the inside of the valve guide 25, so that the combustion chamber (not shown).
There is no danger of entering and wasting.

As described above, the valve oil seal member 3
The lifter guide member 30 in which the lifter 4 is integrated has a positioning and fixing function to the cylinder head 7 via the valve guide 25, a rotation preventing function of the follow-up contact portion 21 and the lifter body 14, a seat function of the valve spring 6, and a stem. Part 4
Since the oil seal function of a is exhibited by one part, the number of parts can be reduced, the structure can be simplified, and the weight can be reduced.
Enables higher rotation speed than before.

A follow-up contact mechanism 17 constructed as follows is provided on the end wall portion 11. At the center of the upper surface of the end wall portion 11, a raised portion 18 which is long in a direction perpendicular to the axis of the three-dimensional cam 2 is provided.
Are formed integrally, and a semi-cylindrical inner surface seat 19 extending in the same direction is recessed in the raised portion 18. Both ends of the semi-cylindrical inner seat 19 are open so as to penetrate, and there is no obstacle such as the regulating wall 108 of the conventional example in FIG. 11, so that the precision machining of the semi-cylindrical inner seat 19 can be performed easily and accurately. Can be. An engagement recess 20 is provided at a substantially central portion in the longitudinal direction of the semi-cylindrical inner surface seat 19. Since the inner bottom surface of the engaging recess 20 is flat, the working of the engaging recess 20 can be easily performed, and the polishing of both inner surfaces of the engaging recess 20 can be easily and accurately performed. The semi-cylindrical inner seat 19 swings (rolls) on the semi-cylindrical inner seat 19.
A semi-cylindrical follow-up contact portion 21 including a semi-cylindrical surface 22 that makes possible contact and a flat contact surface 23 that makes contact with the three-dimensional cam 2 is swingably fitted. At a substantially central portion in the longitudinal direction of the semi-cylindrical surface 22, a fan-shaped engaging projection 24 is integrally provided, and the engaging projection 24 is engaged with the engaging concave portion 20 and is swingably sandwiched. . This engagement structure constitutes a follow-up contact portion regulating structure that regulates the longitudinal movement of the follow-up contact portion 21 in a state where both longitudinal end surfaces of the follow-up contact portion 21 appear.

The follow-up contact portion 21 comes into contact with the three-dimensional cam 2 on the contact surface 23 while following a change in the contact line angle accompanying the rotation of the three-dimensional cam 2 by swinging at a small angle. . At this time, the three-dimensional cam 2 slidably contacts the contact surface 23 of the following contact portion 21 in the longitudinal direction.
As described above, since the movement of the following contact portion 21 in the longitudinal direction is regulated, the following contact portion 21 does not come off the semi-cylindrical inner surface seat 19. In addition, since the longitudinal movement of the follow-up contact portion 21 is regulated in a state where both longitudinal end surfaces of the follow-up contact portion 21 appear, the regulation wall is provided at both longitudinal end surfaces as in the conventional example of FIG. There is no need to provide 108. For this reason, the cam contact surface length of the following contact portion 21 (the length in the longitudinal direction of the contact surface 23) can be maximized to a length substantially equal to the outer diameter of the direct-acting valve lifter 10. By increasing the height of the nose portion 2b, the lift amount of the valve 4 can be maximized.

The variable valve mechanism according to the present embodiment operates as follows. First, when the internal combustion engine is running at a low speed, the camshaft 1 is displaced leftward by the displacement device 3 as shown in FIG. 4, and the right low-speed cam profile of the three-dimensional cam 2 corresponds to the following contact portion 21. I do.

As shown in FIG. 4A, when the base circle portion 2a comes into contact with the contact surface 23 of the follow-up contact portion 21, the contact line angle is parallel to the axis of the three-dimensional cam 2. The contact surface 23 does not tilt with respect to the end wall portion 11 and contacts the base circle portion 2a. However, as shown in FIG. 4B, when the nose portion 2 b comes into contact with the contact surface 23 of the follow-up contact portion 21, the contact line angle is inclined, for example, by several degrees to several tens degrees with respect to the axis of the three-dimensional cam 2. Therefore, the follow-up contact portion 21 swings by the same angle, and the contact surface 23 makes good contact with the nose portion 2b.

As described above, the follow-up contact portion 21 is
Swings by a small angle for each rotation of the camshaft, contacts the three-dimensional cam 2 while following the change in the contact line angle, and is pressed by the nose portion 2b. Therefore, the direct hit type valve lifter 10 reciprocates up and down based on the cam profile for low rotation, and as shown by the curve L in FIG. 6, the valve 4 on the exhaust side and the intake side is opened with a small valve opening operation angle and a lift amount. Open and close to increase low-speed torque and improve fuel economy.

When the internal combustion engine is running at high speed, the camshaft 1 is displaced rightward by the displacement device 3 as shown in FIG. Corresponding to 21.

Then, the follow-up contact portion 21 is arranged as shown in FIG.
As shown in (b), the swinging motion is performed once for each rotation of the three-dimensional cam 2, and the three-dimensional cam 2 contacts the three-dimensional cam 2 while following the change in the contact line angle, and is pressed by the nose portion 2b. Therefore, the direct-acting valve lifter 10 reciprocates up and down based on the cam profile for high rotation, and as shown by the curve H in FIG. 6, the exhaust-side and intake-side valves 4 are opened at a large valve opening operation angle and a large lift amount. Open and close, increase the intake volume, increase the high-speed output.

The camshaft 1 is continuously displaced by the displacement device 3 in accordance with the operating conditions such as the number of revolutions and the degree of opening of the accelerator, even during the above-mentioned low-speed rotation to high-speed rotation. Among them, the cam profile of the intermediate portion corresponds to the following contact portion 21. Accordingly, the direct-acting valve lifter 10 reciprocates up and down based on the cam profile, and opens and closes the valve 4 at an intermediate valve opening angle and lift amount as shown by a curve M in FIG. Generate the corresponding torque and output.

As described above, according to the variable valve mechanism of the present embodiment, the valve timing and the lift amount are continuously changed from the low rotation to the high rotation of the internal combustion engine, and the operating condition of the internal combustion engine is changed. , And various characteristics such as torque, output, fuel efficiency, and cleanliness of exhaust gas can be maximized over the entire rotation range. In addition, the change can be performed smoothly and quietly by the displacement of the camshaft 1.

FIG. 7 shows the torque characteristic of the internal combustion engine obtained by the present embodiment by a solid line, but is shown by a dashed-dotted line, which is a conventional variable valve mechanism which is not a conventional variable type as shown by a broken line. In contrast to such a variable valve mechanism of the type that changes in two stages at low and high rotations, the torque is increased over the entire rotation range, and no valley is generated. In addition, depending on the setting, the fuel efficiency is up to 15 to
It is thought that it is improved by about 20%.

FIG. 8 shows a variable valve mechanism according to the second embodiment. In the present embodiment, when the position of the valve oil seal member 34 is high due to the structure of the internal combustion engine, the valve oil seal member 34 is formed separately from the lifter guide member 30 and is fixed to the upper end of the valve guide 25. Only the point is different from the first embodiment, and the other is the same as the first embodiment.

FIGS. 9 and 10 show a variable valve mechanism according to a third embodiment. In this embodiment, instead of the guided portion 27, four ridges are provided which extend vertically from the lower end of the inner wall surface of the side wall portion 13 to an intermediate height (preferably 10 mm or more) or from the lower end to the upper end. Guided part 29
Is different from the first embodiment only in the point that is provided, and the other is the same as the first embodiment. The side end surfaces of the respective guide portions 33 are slidably engaged with the side end surfaces of the two guided portions 29, so that the direct hit valve lifter 1
Zero reciprocation is allowed and the rotation is prevented.

Note that the present invention is not limited to the configuration of the above-described embodiment, and may be embodied by, for example, changing as follows without departing from the spirit of the invention. (1) The camshaft 1 is displaced stepwise. (2) The configuration and control method of the displacement device 3 are appropriately changed. (3) In each of the above embodiments, the shim 9 for adjusting the valve clearance is interposed between the pressing portion 12 and the end of the valve 4, but the shim 9 is omitted. In this case, it is preferable to prepare a follow-up contact portion 21 having a thickness slightly different from that of the contact portion 21 and appropriately select and replace the follow-up contact portion 21 to adjust the valve clearance.

[0039]

Since the variable valve mechanism of the present invention is constructed as described above, the valve timing and the lift amount are continuously or stepwise controlled by the three-dimensional cam from the time of low rotation to the time of high rotation of the internal combustion engine. To precisely control the operating conditions of the internal combustion engine by changing the characteristics of the internal combustion engine, thereby maximizing the characteristics such as torque, output, fuel consumption, and cleanliness of exhaust gas over the entire rotation range. In addition to this, there is an excellent effect that processing of the lifter guide hole and dimensional tolerance management can be facilitated, and uneven wear of the direct-acting valve lifter can be prevented.

[Brief description of the drawings]

FIG. 1 is a perspective view showing a variable valve mechanism according to a first embodiment of the invention.

FIG. 2 is an exploded perspective view of main members of the variable valve mechanism.

FIG. 3 is a sectional view of the variable valve mechanism.

FIG. 4 is a sectional view showing the variable valve mechanism when the internal combustion engine is running at a low speed.

FIG. 5 is a cross-sectional view showing the variable valve mechanism when the internal combustion engine is rotating at a high speed.

FIG. 6 is a graph showing valve timing and lift amount by the variable valve mechanism.

FIG. 7 is a graph showing torque characteristics of an internal combustion engine obtained by the variable valve mechanism in comparison with a conventional example.

FIG. 8 is a cross-sectional view illustrating a variable valve mechanism according to a second embodiment.

FIG. 9 is a cross-sectional view illustrating a variable valve mechanism according to a third embodiment.

FIG. 10 is a sectional view taken along line XX of FIG. 9;

FIG. 11 is a perspective view showing a conventional variable valve mechanism.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Camshaft 2 Solid cam 3 Displacement device 4 Valve 4a Stem part 6 Valve spring 7 Cylinder head 8 Lifter guide hole 9 Shim 10 Direct-acting valve lifter 11 End wall part 12 Pressing part 13 Side wall part 14 Lifter main body 17 Follow-up contact mechanism 18 Uplift Part 19 Semi-cylindrical inner surface seat 20 Engagement concave part 21 Following contact part 22 Semi-cylindrical surface 23 Contact surface 24 Engagement convex part 25 Valve guide 26 Spring concave part 27 Guided part 28 Entry part 29 Guided part 30 Lifter guide member 31 Spring seat part 32 Fixed part 33 Guide part 34 Valve oil seal member 37 Tightening ring

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁶ Identification code FI F02F 1/24 F02F 1/24 F

Claims (6)

[Claims] 1. A camshaft having a three-dimensional cam in which a cam profile is continuously changed in an axial direction from a low-rotation cam profile to a high-rotation cam profile, and according to an operating condition such as a rotation speed of an internal combustion engine. A variable valve mechanism comprising: a displacement device that continuously or stepwise displaces the camshaft in the axial direction; and a direct-acting valve lifter that opens and closes a valve by reciprocating based on a cam profile of the three-dimensional cam. The direct-acting valve lifter follows an inverted cup-shaped lifter body composed of a disc-shaped end wall and a cylindrical side wall, and follows a change in a contact line angle accompanying rotation of the three-dimensional cam. A follow-up contact portion that contacts the three-dimensional cam while the rotation-preventing structure that prevents the follow-up contact portion from rotating with the lifter body with respect to the three-dimensional cam is provided on the side wall portion. Variable valve mechanism, characterized in that provided on the inner side. 2. The reciprocation of the rotation preventing structure includes a guided portion provided on an inner wall surface of the side wall portion, and a guide portion fixedly provided on a cylinder head side and entering the inside of the side wall portion. The variable valve mechanism according to claim 1, wherein the variable valve mechanism is configured to engage so as to allow movement and prevent the rotation. 3. The lifter guide member is provided at one portion of the lifter guide member, and at another portion of the lifter guide member, an outer periphery of a valve guide fixed to the cylinder head for guiding a stem portion of the valve. And a spring seat portion for receiving one end of a valve spring for biasing the valve.
The variable valve mechanism as described in the above. 4. The variable valve mechanism according to claim 3, wherein a valve oil seal member for oil-sealing an outer periphery of the stem portion is attached to the fixed portion. 5. The variable valve mechanism according to claim 1, wherein a shim for adjusting valve clearance is mounted between the direct-acting valve lifter and an end of the valve. 6. The variable valve mechanism according to claim 1, wherein the follow-up contact portion is a replacement part for adjusting a valve clearance.