JPH08284627A - Valve train for internal combustion engine - Google Patents

Abstract

(57) [Abstract] [Purpose] To improve the engine performance by variably controlling the valve lift characteristics in multiple stages while reducing the overall size and weight of the valve lifter. [Structure] Low, medium and high speed cams 5, 6 and 7 are provided on a cam shaft 1 supported on a cylinder head 2, and
The cam lift of the cam is transmitted to the intake valve 3 via one valve lifter 8. The valve lifter 8 has a pair of first and second sliders 14 and 15 which are relatively moved toward and away from the center in a sliding hole 13 formed between the upper wall portion 10 and the inner pad 12. There is. And the hydraulic circuit 23
With the coil spring 22, the first slider 14 is advanced and brought into contact with the medium speed cam 6 in the medium rotation range of the engine, and the second slider 15 is extended and brought into contact with the high speed cam 7 in the high rotation range. did.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a valve operating system for variably controlling valve lift and valve timing of an intake valve or an exhaust valve according to the engine operating condition.

[0002]

2. Description of the Related Art Conventionally, an internal combustion engine for an automobile has a lift of an intake valve or an exhaust valve depending on an operating state for the purpose of achieving both improvement of fuel consumption at low and medium speeds and improvement of output torque at high speeds. It is known that a valve operating device having different characteristics and capable of controlling the opening / closing timing and the opening amount of the intake / exhaust valve is provided (see Japanese Patent Laid-Open No. 63-117109, etc.).

This is because a low speed cam and a high speed cam having different profiles for opening two intake valves per cylinder against the spring force of a valve spring are integrally formed on the outer periphery of a cam shaft which is rotationally driven by a crankshaft. And a pair of direct-acting type first and second valve lifters for transmitting the lift of the low speed cam to the intake valve at the upper end of the valve stem of each intake valve. Also,
A third valve lifter, which slides according to the lift of the high speed cam, is provided between the valve lifters.
Furthermore, at each upper end of the first to third valve lifters,
A connecting means is provided which is formed of a guide hole formed in the cam shaft axial direction, a guide recess, and a pair of hydraulic pistons slidably provided in the guide hole.

When the engine is running at low speed, the supply of hydraulic pressure to the pressure receiving chamber at one end of each guide hole of the connecting means is cut off, and the connection of each valve lifter by each hydraulic piston is released. Therefore, each intake valve opens and closes according to the small valve lift characteristic of the low speed cam via the first and second valve lifters.

On the other hand, when shifting from the low speed range to the high speed range, the hydraulic pressure is supplied to the pressure receiving chambers so that the hydraulic pistons are inserted into the pair of guide recesses of the third valve lifter and the third valve lifter and the third valve lifter. 1. Connect the second valve lifter. Therefore, each intake valve opens and closes according to the large valve lift characteristic of the high speed cam transmitted to the first and second valve lifters via the third valve lifter.

As a result, the fuel consumption can be improved and the output torque can be improved according to the engine speed.

[0007]

However, in the above-mentioned conventional valve operating device, in addition to the first and second valve lifters on the low speed cam side, the third valve lifter on the high speed cam side is provided. As a result, the valve lifter is inevitably increased in size and weight. Therefore, the inertial force during vertical sliding becomes large, and smooth operation cannot be obtained during high-speed rotation. Therefore, in order to support such high speed rotation,
When the spring set load of the valve spring is increased, the sliding frictional resistance between the intake valve and the cam may increase via the valve lifter.

Further, the valve lift and the valve timing are merely variably controlled in two stages by a high speed cam and a low speed cam. Therefore, it is difficult to perform fine variable control according to changes in the engine speed, and as a result, excellent engine performance, that is, excellent performance such as output torque, fuel consumption, and exhaust emission cannot be obtained.

[0009]

SUMMARY OF THE INVENTION The present invention has been devised in view of the problems of the conventional valve operating system. The invention according to claim 1 is a camshaft rotatably driven by a crankshaft of an engine. And three or more cams provided on the outer periphery of the cam shaft and having different profiles for opening and closing the intake and exhaust valves, and slidably provided in guide holes formed in the cylinder head. A valve lifter for transmitting the lift of the valve to the intake and exhaust valves, and a transmission member that slides in the axial direction of the camshaft is provided in a sliding hole defined inside an upper end of the valve lifter. At the same time, an opening window is formed in the upper wall portion of the valve lifter where the low speed cam is in sliding contact with the upper surface so as to expose the outer peripheral portions of the other plurality of cams into the sliding holes, and the transmission member is placed in the engine operating state. Sliding accordingly So it is characterized in that a drive mechanism that relatively disjunction the upper surface of said transfer member with respect to the outer peripheral surface of the other of the plurality of cams.

According to a second aspect of the present invention, the plurality of cams having different profiles are composed of a high speed cam, a medium speed cam and a low speed cam, and an opening formed in an upper wall portion of the valve lifter. The outer peripheral portions of the high-speed cam and the medium-speed cam are exposed from the window into the sliding hole, and the transmission member is formed by a pair of left and right sliders, and both sliders are relatively approached by the drive mechanism. Alternatively, it is characterized in that the upper surfaces of both sliders are brought into contact with and separated from the outer peripheral surfaces of the high-speed cam and the medium-speed cam, respectively.

The invention according to claim 3 is characterized in that the valve lifter is formed into a substantially perfect circular shape.

[0012]

When the engine is running at a low speed, the drive mechanism slides the sliders in a direction in which they are separated from each other, and the contact between the upper surfaces of the sliders and the outer peripheral surface of the high speed cam is released.
Therefore, the valve lifter slides according to the cam profile of the low speed cam that is in contact with the upper surface, whereby the intake and exhaust valves open and close according to the small valve lift characteristic.

At this point, the high-speed cam and the medium-speed cam are in a completely free state in which they abut the sliders by coming out of the opening window into the sliding holes, that is, without any other member. Sliding friction resistance does not occur because it is in idle rotation without sliding contact.

On the other hand, when the low-to-medium speed range is shifted to the medium-speed range, one slider is advanced and slid toward the center of the sliding hole by the drive mechanism, and at the time when the slider is maximally advanced, the upper surface is a medium speed cam. Abut the outer peripheral surface of the. Therefore, the valve lifter is switched from the low-speed cam to the medium-speed cam and slides according to the profile of the medium-speed cam, whereby the intake and exhaust valves are opened and closed according to the intermediate valve lift characteristic.

When the medium rotation range is shifted to the high rotation range, one slider is moved backward to the original position by the drive mechanism, and at the same time, the other slider is advanced toward the center of the sliding hole to maximize the movement. The upper surface comes into contact with the outer peripheral surface of the high-speed cam when it advances. Therefore, the valve lifter
The medium speed cam is switched to the high speed cam and slides according to the profile of the high speed cam, whereby the intake and exhaust valves are opened and closed according to the high valve lift characteristic.

Of course, when the medium speed cam or the high speed cam is operating, the upper surface of the valve lifter is not in contact with the outer peripheral surface of the low speed cam in the lift area.

[0017]

1 to 3 show an embodiment in which a DOHC (double overhead cam) type valve operating system according to the present invention is applied to the intake side of a multi-cylinder internal combustion engine, in which 1 is a cylinder head 2. A cam shaft which is rotatably driven by a crank shaft (not shown) at the upper end through a cam bracket, 3
Is one intake valve per cylinder that opens and closes the open end of the intake port in the cylinder head 2.

The camshaft 1 is a pair of low speed cams 5 and 5 arranged on both sides of the axial center of the valve stem 3a of the intake valve 3, and the center between the low speed cams 5 and 5. One middle speed cam 6 and one high speed cam 7 arranged in the
Are integrally provided. The low speed cams 5 and 5 are set to have the same profile so that the outer peripheral surfaces 5a and 5a have a small valve lift characteristic as shown by X in FIG. 5, and the uniform width dimension is relatively small. It is set. On the other hand, the middle speed cam 6 and the high speed cam 7 are
As shown by Y and Z in FIG. 5, the profile a is set to have a valve lift characteristic larger than that of the low speed cam 5.

The intake valve 3 is biased in the closing direction by the spring force of a valve spring 4 elastically contacted with a spring retainer 3c provided on the outer periphery of a cotter 3b fixed to the stem end, and at the same time, on the top of the stem end. End face and each cam 5,
Opening is performed against the spring force of the valve spring 4 by the lift of each of the high speed and low speed cams 5, 5, 6 and 7 via the direct acting type valve lifter 8 provided between the valve springs 5, 6 and 7. It is designed to be used.

As shown in FIG. 2, the valve lifter 8 is formed of a metal material in a substantially circular cylindrical shape with a lid, and a substantially circular guide hole 9 formed at the upper end of the cylinder head 2.
It is slidably installed inside. Specifically,
As shown in FIGS. 1 and 3, the valve lifter 8 includes a circular upper wall portion 10, a peripheral wall portion 11 formed vertically from an outer peripheral edge of the upper wall portion 10, and an inside of the peripheral wall portion 11. The fixed inner pad 12, the sliding hole 13 formed between the inner pad 12 and the upper wall portion 10, and the sliding hole 1
3 is composed mainly of a pair of left and right sliders 14 and 15 which are transmission members that slide in the camshaft axial direction. The first slider 14 on the left side in FIGS. The second slider 15 is a high speed cam 7.
Are appropriately abutted against each other.

The upper wall portion 10 is formed integrally with the peripheral wall portion 11, and the outer peripheral surfaces 5a and 5a of the low speed cams 5 and 5 are in sliding contact with both sides of the upper surface 10a in the camshaft axial direction. At the center thereof, a rectangular opening window 16 is formed so as to penetrate the outer peripheral portions of the medium speed cam 6 and the high speed cam 7 into the sliding hole 13 along the direction perpendicular to the camshaft axis.

As shown in FIG. 2, the peripheral wall portion 11 has inner surfaces 11a, 11a facing the camshaft 1 in the direction perpendicular to the axis.
Is formed in a flat shape, and communication holes 17a and 17b for communicating the sliding hole 13 and an oil passage 25, which will be described later, are formed in the center of the opposing wall in the axial direction of the camshaft 1 so as to penetrate through the diametrical direction. There is.

The inner pad 12 is formed in a circular shape having substantially the same shape as the upper wall portion 10, and cooperates with the peripheral wall portion 11 and the upper wall portion 10 to define a sliding hole 13, and an outer peripheral portion. Is fitted to the inner peripheral surface step portion 11b of the peripheral wall portion 11, and the outer peripheral edge thereof is liquid-tightly fixed to the inner peripheral surface of the peripheral wall portion 11 by welding or the like. The inner pad 12 has a protrusion-shaped stopper 12a formed at the center of the upper surface, and the lower surface contacts the upper end surface of the stem end of the intake valve 3 at a circular protrusion 12b formed at the center of the lower surface. Shim 19 with a U-shaped cross section
Has been fixed.

The sliders 14 and 15 have a block shape as shown in FIGS. 1 and 4, and the inner pad is relatively moved toward or away from the center of the sliding hole 13 in the sliding hole 13. It is able to move back and forth on the 12 upper surface. The width L1 of the sliding holes 13 of the sliders 14 and 15 in the axial direction of the camshaft is the width L2 of the gap C formed when the two 14 and 15 retreat from each other and are separated from each other at the maximum distance. Is set smaller than. In addition, the inner end surface abuts the stopper 12a of the inner pad 12 at the center of the lower end edge of the front end surfaces 14a and 15a facing each other,
Locking grooves 18a and 18b are formed to restrict further movement of the sliders 14 and 15 in the approaching direction.

The drive mechanism for moving the sliders 14 and 15 forward and backward relative to each other is, as shown in FIG. 1, a coil spring 22 for moving the sliders 14 and 15 in a direction in which they are separated from each other. The hydraulic circuit 23 is provided to move in the approaching direction, that is, to the center side.

As shown in FIGS. 1 and 2, the coil spring 22 has front end faces 14a, 15 of both sliders 14, 15.
Both ends are elastically held between the bottom surfaces of the concave grooves 14b and 15b formed in the center of a. As shown in FIG. 1, the hydraulic circuit 23 has oil passages 25, 26, one end of which communicates with an oil pan 24 through a cylinder block (not shown) and the inside of the cylinder head 2, and the oil passages 25, 26. An oil pump 27 provided at the upstream end, and oil passages 25, 26 and drain passages 28, 2 provided at the downstream side of the oil pump 27.
9, and first and second electromagnetic switching valves 30 and 31 for switching between 9 and 9. In addition, pressure control valves 35 and 36 are provided in the drain passages 28 and 29, respectively.

The respective oil passages 25, 26 have the other end portions 25a, 26a formed in the cylinder head 2 at the peripheral wall portion 11.
The pressure receiving chambers 32 and 33 formed between the back surfaces of the sliders 14 and 15 and the inner surface of the peripheral wall portion 11 are communicated with each other via the communication passages 17a and 17b.

The electromagnetic switching valves 30 and 31 are adapted to be switched relative to each other by an ON-OFF signal from a controller 34, and the controller 34 is operated by a crank angle sensor and an air flow meter. The current engine operating state is detected based on information signals such as the engine speed, the intake air amount, and the engine cooling water temperature from the water temperature sensor.

The operation of this embodiment will be described below.
First, in the low engine speed region, an OFF signal is output from the controller 34 to the electromagnetic switching valves 30 and 31, and the oil passage 2
5, 26 and the drain passages 28, 29 are communicated with each other, so that hydraulic pressure is not supplied to both pressure receiving chambers 32, 33, and the pressure is low. Therefore, as shown in FIG. 1, the sliders 14 and 15 are retracted by the spring force of the coil spring 22 in the direction in which they are separated from each other, that is, until both sides of the rear end face hit the rear end face of the sliding hole 13.

Therefore, the medium speed cam 6 and the high speed cam 7
Means that the outer peripheral surfaces 6a and 7a do not contact the upper surfaces 14c and 15c of the sliders 14 and 15 and
In the clearance C between the parts 15, the parts rotate in a non-contact state, that is, rotate idly.

As a result, in the low speed cams 5 and 5, the outer peripheral surfaces 5a and 5a have the upper surface 10a of the upper wall portion 10 of the valve lifter 8.
And slide the valve lifter 8 up and down according to the cam profile. Therefore, the intake valve 3 opens and closes according to the small valve lift characteristic shown by X in FIG.

As a result, the overlap with the exhaust valve becomes small, and the residual gas in the combustion chamber can be reduced, so that the combustion is improved and the fuel consumption is improved.

On the other hand, when the engine shifts from the low rotation speed region to the middle rotation speed region, the controller 34 outputs an ON signal only to the first electromagnetic switching valve 30 to cut off the communication between the first oil passage 25 and the drain passage 28. Therefore, the oil pressure pumped from the oil pump 27 is supplied into the first pressure receiving chamber 32 via the first oil passage 25 and the communication passage 17a. Therefore, the first
The pressure inside the pressure receiving chamber 32 becomes high, and the first slider 14 moves toward the center of the sliding hole 13 against the spring force of the coil spring 22.

Here, the first slider 14 has the front end surface 14a of the middle speed cam 6 when the middle speed cam 6 is lifted, that is, when the cam lift portion is located in the sliding hole 13.
Although it abuts on one side surface and does not move further in the advancing direction, it retreats from the sliding hole 13 in the cam base circle area, so that the inner end surface of the locking groove 18a is outside the stopper 12a as shown in FIG. Advance until you hit the edge. As a result, the outer peripheral surface 6 a of the middle speed cam 6 contacts the upper surface 14 c of the first slider 14.

Therefore, the valve lifter 8 slides up and down according to the profile of the middle speed cam 6 via the first slider 14 and the inner pad 12 as the middle speed cam 6 rotates (see FIG. 6). Therefore, the intake valve 3 is
It opens and closes according to the middle valve lift characteristic shown by Y in FIG. As a result, it is possible to secure sufficient torque while improving fuel efficiency.

At this time, the low-speed cams 5 and 5 only need to be slidably in contact with the upper surface 10a of the valve lifter 8 when the base circle outer peripheral surfaces 5b and 5b are in sliding contact only when the medium-speed cam 6 is in the base circle. Upper surface 10a during cam lift 6
Is separated from.

Further, when the engine speed range is changed from the middle speed range to the high speed range, the controller 34 causes the first electromagnetic switching valve 30 to move.
OFF signal is output to the second electromagnetic switching valve 31
An ON signal is output to. Therefore, the first oil passage 25 and the drain passage 28 are communicated with each other, the hydraulic pressure in the first pressure receiving chamber 32 is returned to the oil pan 24, and the communication between the second oil passage 26 and the drain passage 29 is blocked. Thus, the hydraulic pressure is supplied into the second pressure receiving chamber 33 via the second oil passage 26 and the communication passage 17b. Therefore, the first slider 14 is moved backward by the spring force of the coil spring 22 to release the contact with the middle speed cam 6, and at the same time, the second slider 15 is moved.
It moves forward toward the center of the sliding hole 13 against the spring force of the coil spring 22.

Here, the second slider 15 has the sliding hole 1 at the time of cam lift of the high speed cam 7, that is, the cam lift portion.
3, the front end surface 15a contacts one side surface of the high speed cam 7 and does not move further in the advancing direction.
In the cam base circle area, since it retreats from the sliding hole 13, it advances until the inner end surface of the locking groove 18b hits the outer peripheral edge of the stopper 12a as shown in FIG. As a result, the outer peripheral surface 7a of the high-speed cam 7 has the second slider 1
5 abuts on the upper surface 15c. Therefore, the valve lifter 8 moves the second slider 15 as the high speed cam 7 rotates.
And, it moves up and down according to the profile of the high-speed cam 7 via the inner pad 12 (see FIG. 8). For this reason,
The intake valve 3 opens and closes according to the large valve lift characteristic shown by Z in FIG. As a result, intake charging efficiency is improved and high torque is obtained.

Further, at this time, the low speed cams 5 and 5 can be moved by simply sliding the base circle outer peripheral surfaces 5b and 5b onto the upper surface 10 of the valve lifter 8 only when the high speed cam 7 is in the base circle. It is separated from the upper surface 10a during the cam lift. If the engine speed shifts to the low rotation range from this point, an OFF signal is output to both electromagnetic switching valves 30 and 31, and the sliders 14 and 15 move backward.

As described above, in this embodiment, the valve lifter 8 for raising and lowering the pair of low speed cams 5 and 5 and medium and high speed cams 6 and 7 can be made common and can be unified. The overall size and weight can be reduced. For this reason, the inertial force at the time of vertical sliding can be made as small as possible, and it becomes possible to sufficiently cope with high rotation.

Moreover, in the low engine speed region, the outer peripheral surfaces 6a and 7a of the medium and high speed cams 6 and 7 are not in contact with the sliders 14 and 15, so-called idle rotation, so that the sliding friction resistance is greatly reduced. Can be realized.

Further, since the valve lift and the valve timing can be variably controlled in three stages of low, medium and high depending on the engine speed, the engine performance such as output torque, fuel consumption and exhaust emission can be sufficiently brought out. It will be possible.

Furthermore, since the guide hole 9 and the valve lifter 8 are formed into a substantially circular shape, the working work is facilitated, the manufacturing work efficiency is improved, and the cost is reduced.

By the way, depending on the specifications of the internal combustion engine, 1
Two intake valves are provided per cylinder, and one intake valve is deactivated in order to improve fuel efficiency at low revolutions, and both intake valves are activated to give priority to output torque during medium and high revolutions. is there. Therefore, such 2
It is also conceivable to apply the variable mechanism shown in FIG. 10 in combination with the valve type. That is, it is possible to apply the variable mechanism of the above embodiment to one intake valve 3 and the variable mechanism shown in the figure to the other intake valve 37.

The variable mechanism shown in FIG. 10 does not have a low speed cam on the camshaft 1, but has only a medium speed cam 6 and a high speed cam 7, and has the same structure as the above embodiment. There is. Therefore, the same reference numerals are given and a detailed description is omitted.

Therefore, in the low engine speed region, the intake valve 3 on one side is opened and closed by the low speed cams 5 and 5 as described above, but the intake valve 37 on the other side is opened and closed as shown in FIG. Since the middle and high speed cams 6 and 7 only idle in the sliding hole 13 through the opening window 16, they do not operate at all and enter a rest state. On the other hand, when the engine shifts from the low rotation speed region to the middle rotation speed region and from the middle rotation speed region to the high rotation speed region, this intake valve 30 operates in the same manner as the intake valve 3 on the one side by the same action as in the above embodiment. It opens and closes with medium and high valve lift characteristics (see Fig. 11).

Therefore, by using this variable mechanism, the fuel consumption can be greatly improved in the low rotation speed range, and the engine performance in the middle and high rotation speed range can be improved.

This variable mechanism is used in combination with the variable mechanism of the above-described embodiment and has no variable mechanism in the intake valve on one side, or in one of a plurality of cylinders in the low engine speed region. It is also possible to apply it to the one that deactivates the cylinder.

Further, in the above-mentioned embodiment, the case where it is applied to the intake valve side has been shown, but it is possible to apply it to the exhaust valve side and also to both the intake and exhaust valves. is there. The valve lifter 8 in each embodiment is
The maximum upward movement position is determined by the length of the valve spring 4, the upper surface 10a is set to be substantially the same as the height of the upper surface of the cylinder head 2, and the middle speed cam 6 that constantly faces the sliding hole 13 is formed. High speed cam 7 has an opening window 16
The free rotation is controlled by contacting the edge of the hole.

Further, in the above-described embodiment, if the variable mechanism of the low / high two-stage switching type as in the conventional example is provided on the exhaust valve side, the engine can be controlled with higher accuracy and the engine performance can be further improved. Can be improved.

[0051]

As is apparent from the above description, according to the present invention, a plurality of cams are not individually handled by respective valve lifters, but a single common valve lifter is used. Therefore, it is possible to reduce the size and weight of the entire valve lifter.

Therefore, the inertial force when the valve lifter slides up and down becomes small, and it is possible to sufficiently cope with high speed rotation, so that the spring force of the valve spring can also be made small. Therefore, the sliding frictional resistance between the intake / exhaust valve and each cam is reduced.

Further, the valve lift characteristic of the intake / exhaust valve can be variably controlled in a plurality of stages by using a plurality of cams having three or more different profiles, so that optimum engine performance according to the engine speed can be obtained. Will be possible.

[Brief description of drawings]

FIG. 1 is a sectional view taken along line AA of FIG. 2 showing an embodiment of the present invention.

FIG. 2 is a sectional view taken along line BB of FIG.

3 is a cross-sectional view taken along the line CC of FIG.

FIG. 4 is a perspective view of a slider used in this embodiment.

FIG. 5 is a valve lift characteristic diagram of the present embodiment.

FIG. 6 is a cross-sectional view taken along the line AA of FIG. 2 showing the operation of this embodiment.

FIG. 7 is a valve lift characteristic diagram of the present embodiment.

FIG. 8 is a sectional view taken along the line AA of FIG. 2 showing the operation of this embodiment.

FIG. 9 is a valve lift characteristic diagram of the present embodiment.

FIG. 10 is a cross-sectional view showing another variable mechanism applied in combination with the embodiment of the present invention.

FIG. 11 is a valve lift characteristic diagram of the other variable mechanism.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Cam shaft 2 ... Cylinder head 3 ... Intake valve 5 ... Low speed cam 6 ... Medium speed cam 7 ... High speed cam 8 ... Valve lifter 9 ... Guide hole 10 ... Upper wall 11 ... Peripheral wall 12 ... Inner pad 13 ... sliding holes 14, 15 ... sliders 14c, 15c ... upper surfaces 14d, 15d ... rear end surfaces 14e, 15e ... front end surface 16 ... opening window 22 ... coil spring (driving mechanism) 23 ... hydraulic circuit (driving mechanism)

Claims (3)

[Claims] 1. A cam shaft rotatably driven by a crank shaft of an engine, and a cam shaft provided on an outer periphery of the cam shaft,
A valve lifter which is slidably provided in three or more cams having different profiles for opening and closing the intake and exhaust valves and a guide hole formed in the cylinder head to transmit the lift of each cam to the intake and exhaust valves. And a transmission member that slides in the camshaft axial direction is provided in a sliding hole defined inside the upper end of the valve lifter, and a low speed cam slides on the upper surface of the valve lifter. An opening window is formed in the upper wall portion of the plurality of cams so as to expose the outer peripheral portions of the other plurality of cams into the sliding holes, and the transmission member is slid in accordance with the operating state of the engine. A valve mechanism for an internal combustion engine, which is provided with a drive mechanism that relatively separates and contacts the outer peripheral surfaces of a plurality of other cams. 2. A plurality of cams having different profiles are composed of a high-speed cam, a medium-speed cam and a low-speed cam, and the high-speed cam is formed from an opening window formed in an upper wall portion of the valve lifter. The outer peripheral portions of the medium-speed cam and the medium-speed cam are exposed in the sliding hole, the transmission member is formed by a pair of left and right sliders, and both sliders are relatively approached or separated by the drive mechanism. 2. The valve operating system for an internal combustion engine according to claim 1, wherein the upper surfaces of the sliders are brought into contact with and separated from the outer peripheral surfaces of the high speed cam and the medium speed cam, respectively. 3. The valve operating system for an internal combustion engine according to claim 2, wherein the valve lifter is formed in a substantially circular shape.