JP2010043554A - Variable valve gear for internal combustion engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To change operation characteristics of an engine valve with a simple and compact structure, in a variable valve gear for an internal combustion engine capable of changing operation characteristics of an engine valve according to change in the rotation speed of a camshaft. <P>SOLUTION: A high speed cam 60 including a base circle part having same radius as a base circle part of a low speed cam 59 and including a high position part of which projection quantity is larger than a high position part of the low speed cam 59 is supported on a camshaft 43A on which the low speed cam 59 is fixed in such a manner that the same can relatively rotate at a position adjacent to the low speed cam 59. A cam driven member 46 is supported by an engine body 11 in such a manner that the same can operate, following either of the low speed cam 59 or the high speed cam 60. A centrifugal clutch 61A connecting the high speed cam 60 to a camshaft 43A while adjusting phase to the low speed cam 59 when rotation speed of the camshaft 43A gets to prescribed rotation speed or higher, is provided between the camshaft 43A and the high speed cam 60. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an engine valve that is disposed in the engine body so as to be capable of opening and closing and is biased toward the valve closing side, a camshaft that is rotatably supported by the engine body, and a cam that rotates together with the camshaft. And a cam driven member that operates so as to transmit a valve opening force to the engine valve side, and is capable of changing an operating characteristic of the engine valve in accordance with a change in the rotational speed of the camshaft. The present invention relates to a variable valve device.

A cam cylinder in which two cam portions having different cam profiles are adjacent to each other in the axial direction of the camshaft is in contact with the cam driven member, and the other cam portion is in contact with the cam driven member. A variable valve operating apparatus is already known in Patent Document 1 that changes the operating characteristics of an engine valve by being driven in the axial direction by an actuator between the position to be moved.
Japanese Patent Laid-Open No. 61-212615

  However, in the structure disclosed in Patent Document 1, the actuator is located at a position separated from the camshaft, and the cam is driven in the axial direction by rotating the lever with the actuator, which makes the structure complicated. In addition to this, a space for arranging the actuator is also required, leading to an increase in the size of the valve operating device.

  The present invention has been made in view of such circumstances, and an object of the present invention is to provide a variable valve operating apparatus for an internal combustion engine that can change the operating characteristics of the engine valve with a simple and compact structure.

  In order to achieve the above object, the present invention provides an engine valve that is arranged to be openable and closable in the engine body and biased toward the valve closing side, and a camshaft that is rotatably supported by the engine body. A cam driven member that operates to transmit a valve opening force to the engine valve side by following a cam that rotates together with the cam shaft, and the operating characteristics of the engine valve according to a change in the rotational speed of the cam shaft. In the variable valve system for an internal combustion engine that can be changed, a low speed cam having an arc-shaped base circle centered on the axis of the camshaft and a high-order portion protruding outward from the base circle is fixed. A high speed cam having a base circle portion having the same radius as the base circle portion of the low speed cam and a high-order portion having a protruding amount larger than the high-order portion of the low-speed cam is provided on the camshaft provided. Next to cam for The cam driven member that is supported so as to be relatively rotatable at the position and can come into contact with both the low speed cam and the high speed cam is operated by being driven by either the low speed cam or the high speed cam. A centrifugal clutch that is supported by the engine body and connects the camshaft to the camshaft in phase with the cam for low speed when the camshaft rotation speed exceeds a predetermined rotation speed; It is characterized by being provided between the camshaft and the high-speed cam so as to be coaxial with the shaft.

  The intake valve 37 of the first embodiment and the intake valve 37 and the exhaust valve 38 of the second embodiment correspond to the engine valve of the present invention, and the intake side tappet 46 of the first embodiment and the intake side tappet of the second embodiment. 46 and the exhaust side tappet 47 correspond to the cam follower of the present invention, and the intake side low speed cam 59 of the first embodiment and the intake side low speed cam 59 and the exhaust side low speed cam 75 of the second embodiment of the present invention. The intake side high speed cam 60 of the first embodiment and the intake side high speed cam 60 and the exhaust side high speed cam 76 of the second embodiment correspond to the high speed cam of the present invention.

  According to the above configuration of the present invention, when the rotational speed of the camshaft is less than the predetermined rotational speed, the centrifugal clutch is in a power transmission cut-off state. In this state, the low-speed cam rotates together with the camshaft. As a result, the engine valve is idled around the axis of the camshaft, and the engine valve opens and closes with operating characteristics corresponding to the cam profile of the low-speed cam. On the other hand, when the rotation speed of the camshaft rises above a predetermined rotation speed, the centrifugal clutch connects the high speed cam to the camshaft in phase with the low speed cam, and the low speed cam and the high speed cam are in the same phase. The high-speed cam has a higher protrusion than the low-speed cam, so the cam follower does not come into contact with the high-speed cam. Therefore, the engine valve opens and closes with an operating characteristic corresponding to the cam profile of the high speed cam. In addition, the centrifugal clutch for switching the operating characteristics of the engine valve is provided between the camshaft and the high-speed cam so as to be coaxial with the camshaft, so that the operating characteristics of the engine valve can be changed with a simple and compact structure.

  DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described below based on one embodiment of the present invention shown in the accompanying drawings.

  1 to 5 show a first embodiment of the present invention. FIG. 1 is a longitudinal side view of an internal combustion engine, which is a cross-sectional view taken along line 1-1 of FIG. 2, and FIG. -2 is an enlarged sectional view, FIG. 3 is a transverse sectional view of the centrifugal clutch in a power transmission cut-off state, FIG. 4 is a sectional view corresponding to FIG. 3 of the centrifugal clutch in a power transmission state, and FIG. It is a figure which shows sequentially the operation state of the inhalation | air_intake side tappet according to this, (a) is a state of low rotation, (b) is a figure which shows the state of high rotation.

  First, in FIGS. 1 and 2, the internal combustion engine is configured as an air-cooled single cylinder so as to be used in, for example, a work machine, and the engine main body 11 includes a crankcase 12 and one of the crankcases 12. The cylinder block 13 and the cylinder head 14 are provided with a cylinder block 13 projecting obliquely upward from the side surface, a cylinder head 14 joined to the head of the cylinder block 13, and a head cover 16 coupled to the cylinder head 14. Are provided with a number of air cooling fins 13a, 14a,.

  The crankcase 12 is composed of a case main body 16 that is integrally formed with the cylinder block 13 and opened on one side, and a side cover 17 that is fastened to an open end of the case main body 16 and stores oil. A crank chamber 18 is formed in the crankcase 12. A crankshaft 19 integrally having a pair of counterweights 19a and 19b and a crankpin 19c connecting both counterweights 19a and 19b is rotatably supported on the crankcase 12. Thus, both end portions of the crankshaft 19 pass through the case main body 16 and the side cover 17 in the crankcase 12 so as to freely rotate and protrude outward, between the crankshaft 19 and the case main body 16. The ball bearing 20 and an annular seal member 21 disposed outside the ball bearing 20 are interposed, and between the crankshaft 19 and the side cover 17, a ball bearing 22 and the ball bearing 22 are interposed. And an annular seal member 23 disposed outside.

  The cylinder block 13 is formed with a cylinder bore 25 into which the piston 24 is slidably fitted, and a combustion chamber 26 that faces the top of the piston 24 is formed between the cylinder block 13 and the cylinder head 14. One end of a connecting rod 27 is connected to the piston 24 via a piston pin 28, and the other end of the connecting rod 27 is connected to a crank pin 19 c of the crankshaft 19. That is, the other end of the connecting rod 27 is provided with a semicircular recess 29 for fitting the substantially half circumference of the crank pin 19c, and a semicircular recess 30 for fitting the remaining substantially half circumference of the crank pin 19c. A crank cap 31 is fastened to the other end of the connecting rod 27 by a pair of bolts 32.

  In addition, the crank cap 31 is integrally provided with an oil dipper 33 that repels oil in the crank chamber 18 in accordance with the rotation of the crankshaft 19. The oil dipper 33 is accommodated in the crank chamber 18. Become.

  In addition, an intake port 35 and an exhaust port 36 that can communicate with the combustion chamber 26 are formed in the cylinder head 14, and an intake valve 37 as an engine valve that opens and closes between the intake port 35 and the combustion chamber 26, an exhaust port 36, and An exhaust valve 38 that opens and closes between the combustion chambers 26 is disposed so as to be able to open and close. The intake valve 37 is urged in a valve closing direction by a valve spring 39, and the exhaust valve 38 is urged in a valve closing direction by a valve spring (not shown). Be forced. Further, a spark plug 41 is attached to the cylinder head 14 so that the front end portion faces the combustion chamber 26.

  A valve gear 42A that opens and closes the intake valve 37 and the exhaust valve 38 is configured to slide up and down according to the rotation of the camshaft 43A and the camshaft 43A that is rotatably supported by the crankcase 12. The intake side tappet 46 supported by the case main body 16 and the cylinder block 13 of the crankcase 13 and the case main body 16 and the cylinder block 13 are slid up and down according to the rotation of the camshaft 43A. An exhaust-side tappet 47, an intake-side pushrod 48 that extends vertically by connecting the lower end to the upper end of the intake-side tappet 46, and a lower-end portion that extends vertically from the upper end of the exhaust-side tappet 47 An exhaust-side push rod (not shown) that extends and an axis parallel to the crankshaft 19 are provided on the cylinder head 14. An intake-side rocker arm 51 that is swingably supported by the rocker shaft 50, and an exhaust-side rocker arm (not shown) that is swingably supported by the rocker shaft 52. One end of the intake-side rocker arm 51 is One end of the exhaust side rocker arm is in contact with the upper end of the intake side push rod 48, and the upper end of the exhaust side push rod is in contact with the other end of the intake side rocker arm 51 and the exhaust side rocker arm. The tappet screws 53 that are in contact with the head of the exhaust valve 38 are screwed together so that the advance / retreat position can be adjusted.

  Both end portions of the camshaft 43A are rotatably supported by the case body 16 and the side cover 17 in the crankcase 13, and the camshaft 43A and the crankshaft 19 are fixed to the crankshaft 19 between them. A timing transmission mechanism 57 is provided that includes a drive gear 55 and a driven gear 56 that is fixed to the camshaft 43A so as to mesh with the drive gear 55. The timing transmission mechanism 57 causes the crankshaft to rotate. 19 rotational power is transmitted to the camshaft 43A at a reduction ratio of 1/2.

  An exhaust side cam 58 is integrally provided on the camshaft 43A so that the lower end portion of the exhaust side tappet 47 is in sliding contact. An intake side low speed cam 59 is integrally provided on the cam shaft 43A, and an intake side high speed cam 60 is supported so as to be relatively rotatable at a position adjacent to the intake side low speed cam 59. The intake side tappet 46, which is a driven member, can contact both the intake side low speed cam 59 and the intake side high speed cam 60, and the intake side low speed cam 59 and the intake side high speed cam 60. The engine body 11 is supported by the case body 16 of the crankcase 13 and the cylinder block 13 so as to be able to operate.

  The intake-side low-speed cam 59 has an arc-shaped base circle portion 59a centering on the axis of the camshaft 43A and a high-order portion 59b that protrudes outward from the base circle portion 59a, and is integrated with the camshaft 43A. The intake-side high-speed cam 60 has a base circle portion 60a having the same radius as the base circle portion 59a of the low-speed cam 59 and has a larger protruding amount than the high-order portion 59b of the low-speed cam 59. And a high-order part 60b protruding from the base circle part 60a.

  Between the intake-side high-speed cam 60 and the camshaft 43A integrally provided with the intake-side low-speed cam 59, when the rotational speed of the camshaft 43A becomes a predetermined rotational speed or more, the intake air A centrifugal clutch 61A that connects the side high speed cam 60 to the camshaft 43A in phase with the intake side low speed cam 59 is provided coaxially with the camshaft 43A.

  In FIG. 3, the centrifugal clutch 61 </ b> A includes a disk-shaped drive plate 62, a hook-shaped clutch housing 63 that coaxially covers the drive plate 62, and the clutch according to the action of centrifugal force accompanying the rotation of the drive plate 62. Clutch weights 64 and 65 pivotally supported by the drive plate 62 so as to be able to frictionally engage at a plurality of, for example, two places at equal intervals in the circumferential direction of the inner circumference of the housing 63, and these clutches The clutch weights 64, 65 are disposed on the side where the frictional engagement between the ring-shaped side plate 66 (see FIG. 2) sandwiching the weights 64, 65 between the drive plate 62 and the inner periphery of the clutch housing 63 is released. The clutch weight 64 adjacent to the circumferential direction of the clutch housing 63 so as to exert a spring force for urging the clutch housing 63 Each comprise a clutch spring 67 provided between 65.

  As shown in FIG. 2, a cylindrical cylindrical member 68 that coaxially surrounds the camshaft 43 </ b> A is fixed to the camshaft 43 </ b> A, and the drive plate 62 is fixed to the cylindrical member 68. One end of a cylindrical connecting cylinder 69 that coaxially surrounds the camshaft 43A is coaxially and integrally connected to the intake side high-speed cam 60, and the other end of the connecting cylinder 69 is connected to the clutch. The housing 63 is coaxially and integrally connected. That is, the drive plate 62 rotates with the camshaft, and the clutch housing 63 rotates with the intake side high speed cam 60.

  One end portions of the clutch weights 64 and 65 are pivotally supported on the drive plate 62 via support shafts 70 and 70, and the clutch weights 64 and 65 are provided on the inner periphery of the clutch housing 63. Friction materials 71 and 72 that can be frictionally engaged are attached. In addition, a locking recess 73 is provided at one location on the inner periphery of the clutch housing 63, and the friction material 71 of one of the clutch weights 64 and 65 can be engaged with the locking recess 73. An engaging protrusion 71a is provided.

  In such a centrifugal clutch 61A, when the rotational speed of the camshaft 43A is low, the centrifugal force acting on the clutch weights 64, 65 is smaller than the spring force of the clutch springs 67, and the rotational positions of the clutch weights 64, 65 As shown in FIG. 3, the friction members 71 and 72 are not positioned to be frictionally engaged with the inner periphery of the clutch housing 63. Thus, when the rotational speed of the camshaft 43A exceeds a predetermined rotational speed, the centrifugal force acting on the clutch weights 64, 65 becomes larger than the spring force of the clutch springs 67, and the clutch weights 64, 65 72 is rotated to a position for frictional engagement with the inner periphery of the clutch housing 63. At this time, an engagement projection 71 a is provided on the friction material 71 of the clutch weight 64, and when the engagement projection 71 a is in contact with the inner periphery of the clutch housing 63, the friction materials 71 and 72. Since the entire surface is not in contact with the inner periphery of the clutch housing 63, the drive plate 62 is rotated relative to the clutch housing 63, and the engagement protrusion 71a is, as shown in FIG. 63, the friction members 71 and 72 are brought into pressure contact with the inner periphery of the clutch housing 63 to be frictionally engaged, whereby the drive plate 62, that is, the camshaft 43A, and the clutch housing 63, that is, The intake-side high-speed cam 60 is connected to rotate integrally.

  In addition, in a state where the engaging protrusion 71a is engaged with the locking recess 73, the relative position between the camshaft 43A and the connecting cylinder 69, that is, the intake-side low-speed cam 59 provided integrally with the camshaft 43A and the above-mentioned The relative position of the intake side high speed cam 60 coupled to the connecting cylinder 69 is a relative position where the phases of the high position portion 59b of the intake side high speed cam 59 and the intake side high speed cam 60b are matched.

  Next, the operation of the first embodiment will be described. The intake shaft low speed cam 59 is fixed on the camshaft 43A, and the base circle portion having the same radius as the base circle portion 59a of the intake side low speed cam 59 is described. The intake-side high-speed cam 60 having a high-order portion 60b having a protrusion 60a and a protruding amount larger than the high-order portion 59b of the intake-side low-speed cam 59 is supported so as to be relatively rotatable at a position adjacent to the intake-side low-speed cam 59. Thus, the intake side tappet 46 can be in contact with both the intake side low speed cam 59 and the intake side high speed cam 60 and is driven by either the intake side low speed cam 59 or the intake side high speed cam 60. The camshaft 43 is supported by the case main body 16 and the cylinder block 13 of the crankcase 13 in the engine main body 11. A centrifugal clutch 61A that connects the intake-side high-speed cam 60 to the camshaft 43A in phase with the intake-side low-speed cam 59 when the rotational speed of the engine becomes equal to or greater than a predetermined rotational speed is coaxial with the camshaft 43A. The camshaft 43A and the intake side high-speed cam 60 are provided.

  Therefore, when the rotational speed of the camshaft 43A is less than the predetermined rotational speed, the centrifugal clutch 61A is in a power transmission cut-off state, and in this state, the intake-side low-speed cam 59 is combined with the camshaft 43A as shown in FIG. Although rotating, the intake-side high-speed cam 60 idles about the axis of the camshaft 43A, and the intake valve 37 opens and closes with an operating characteristic corresponding to the cam profile of the intake-side low-speed cam 59.

  On the other hand, when the rotational speed of the camshaft 43A rises above a predetermined rotational speed, the centrifugal clutch 61A connects the intake side high speed cam 60 to the camshaft 43A in phase with the intake side low speed cam 59, and FIG. As shown in (b), the intake-side low-speed cam 59 and the intake-side high-speed cam 60 rotate in the same phase, and the high position portion 60b of the intake-side high-speed cam 60 Since the protruding amount is larger than that of the high position portion 59b, the intake side tappet 46 does not come into contact with the high position portion 59b of the intake side low speed cam 59 but operates by being driven by the high position portion 60b of the intake side high speed cam 60. Thus, the intake valve 37 opens and closes with an operating characteristic corresponding to the cam profile of the intake-side high-speed cam 60.

  Moreover, the centrifugal clutch 61A for switching the operation characteristics of the intake valve 37 is provided between the cam shaft 43A and the intake side high-speed cam 60 so as to be coaxial with the cam shaft 43A, so that the operation of the intake valve 37 can be performed with a simple and compact structure. Characteristics can be changed.

  In the first embodiment, the case where only the operating characteristic of the intake valve 37 of the intake valve 37 and the exhaust valve 38 changes according to the change in the rotation speed of the camshaft 43 </ b> A has been described, but the intake valve 37 and the exhaust valve 38. A second embodiment of the present invention in which both of the operating characteristics are changed will be described with reference to FIG. 6. The parts corresponding to the first embodiment are indicated by the same reference numerals. Detailed description will be omitted.

  A valve operating device 42B that opens and closes an intake valve 37 and an exhaust valve 38, which are engine valves, is rotatably supported by the crankcase 12 and a camshaft 43B provided with a timing transmission mechanism 57 between the crankshaft 19 and the camshaft 43B. The intake side tappet 46 supported by the case body 16 and the cylinder block 13 of the crankcase 13 so as to slide up and down according to the rotation of the camshaft 43B, and up and down according to the rotation of the camshaft 43B. And an exhaust side tappet 47 supported by the case body 16 and the cylinder block 13.

  The camshaft 43B is integrally provided with an intake side low speed cam 59 and an exhaust side low speed cam 75, and an intake side high speed cam 60 and an exhaust side high speed cam 76 are provided with the intake side low speed cam 59 and It is supported so as to be relatively rotatable at a position adjacent to the exhaust-side low-speed cam 75. Moreover, the intake-side high-speed cam 60 is disposed on the exhaust-side low-speed cam 75 side and adjacent to the intake-side low-speed cam 59, and the exhaust-side high-speed cam 76 is on the intake-side low-speed cam 59 side. 75 is disposed at a position adjacent to 75.

  The intake side tappet 46, which is a cam driven member, allows the intake side low speed cam 59 and the intake side high speed cam 60 to be in contact with each other, and the intake side low speed cam 59 and the intake side high speed cam 60. The exhaust side tappet 47, which is supported by the case body 16 and the cylinder block 13 of the crankcase 13 in the engine body 11 and is a cam passive member, can be operated by being driven by any one of the engine body 11. It is possible to make contact with both the cam 75 and the exhaust-side high-speed cam 76 and to operate by following either the exhaust-side low-speed cam 75 or the exhaust-side high-speed cam 76. The main body 11 is supported by the case main body 16 and the cylinder block 13 of the crankcase 13.

  Between the intake-side high-speed cam 60 and the exhaust-side high-speed cam 76 and the intake-side low-speed cam 59 and the exhaust-side low-speed cam 75 provided integrally with the camshaft 43B. When the rotational speed of 43B becomes equal to or higher than a predetermined rotational speed, the intake side high speed cam 60 and the exhaust side high speed cam 76 are in phase with the intake side low speed cam 59 and the exhaust side low speed cam 75. A centrifugal clutch 61B connected to the camshaft 43B is provided coaxially with the camshaft 43B.

  According to the second embodiment, when the rotational speed of the camshaft 43B is less than the predetermined rotational speed, the centrifugal clutch 61B is in a power transmission cut-off state, and the intake side low speed cam 59 and the exhaust side low speed cam 75 are connected to the camshaft. 43B, the intake side high speed cam 60 and the exhaust side high speed cam 76 idle around the axis of the cam shaft 43B, and the intake valve 37 operates according to the cam profile of the intake side low speed cam 59. The exhaust valve 38 opens and closes with an operating characteristic corresponding to the cam profile of the exhaust-side low-speed cam 75.

  On the other hand, when the rotational speed of the camshaft 43B rises above a predetermined rotational speed, the centrifugal clutch 61B connects the intake side high speed cam 60 to the camshaft 43B in phase with the intake side low speed cam 59, and the exhaust side The high-speed cam 76 is connected to the camshaft 43B in phase with the exhaust-side low-speed cam 75, the intake valve 37 opens and closes with operating characteristics corresponding to the cam profile of the intake-side high-speed cam 60, and the exhaust valve 38 The opening / closing operation is performed with an operation characteristic corresponding to the cam profile of the exhaust-side high-speed cam 76.

  Moreover, the centrifugal clutch 61B for switching the operation characteristics of the intake valve 37 and the exhaust valve 38 is provided between the camshaft 43B, the intake-side high-speed cam 60, and the exhaust-side high-speed cam 76 so as to be coaxial with the camshaft 43B. Therefore, the operation characteristics of the intake valve 37 and the exhaust valve 38 can be changed with a simple and compact structure.

  Although the embodiments of the present invention have been described above, the present invention is not limited to the above-described embodiments, and various design changes can be made without departing from the present invention described in the claims. It is.

FIG. 3 is a longitudinal side view of the internal combustion engine of the first embodiment, and is a cross-sectional view taken along line 1-1 of FIG. FIG. 2 is an enlarged sectional view taken along line 2-2 of FIG. It is a cross-sectional view of the centrifugal clutch in a power transmission cut-off state. It is sectional drawing corresponding to FIG. 3 of the centrifugal clutch in a power transmission state. It is a figure which shows sequentially the operation state of the suction side tappet according to rotation of a cam shaft, (a) is a state of low rotation, (b) is a figure which shows the state of high rotation. It is sectional drawing of the internal combustion engine corresponding to FIG. 2 of 2nd Example.

Explanation of symbols

DESCRIPTION OF SYMBOLS 11 ... Engine main body 37 ... Intake valve 38 which is an engine valve ... Exhaust valves 43A, 43B which are engine valves ... Cam shaft 46 ... Intake side tappet 47 which is a cam driven member Exhaust side tappet 59 which is a cam follower member ... Intake side low speed cam 59a, 60a ... Base circular part 59b, 60b ... High position part 60 ... Intake side high speed cam 61A, 61B ... Centrifugal Clutch 75... Exhaust side low speed cam 76... Exhaust side high speed cam

Claims (1)

  An engine valve (37, 38) that is disposed on the engine body (11) so as to be openable / closable and is biased toward the valve closing side, and a camshaft (43A) rotatably supported by the engine body (11) , 43B) and a cam (59, 60; 75, 76) that rotates with the cam shaft (43A, 43B), and operates to transmit a valve opening force to the engine valve (37, 38) side. In a variable valve operating apparatus for an internal combustion engine, comprising a cam driven member (46, 47) and capable of changing an operating characteristic of the engine valve (37, 38) in accordance with a change in the rotational speed of the cam shaft (43A, 43B). A low-speed cam having an arc-shaped base circle part (59a) centering on the axis of the camshaft (43A, 43B) and a high-order part (59b) projecting outward from the base circle part (59a) (59,75) is fixed The cam shaft (43A, 43B) has a base circle portion (60a) having the same radius as the base circle portion (59a) of the low speed cam (59, 75) and the low speed cam (59, 75). A high speed cam (60, 76) having a high position portion (60b) having a larger protruding amount than the high position portion (59b) is supported at a position adjacent to the low speed cam (59, 75) so as to be relatively rotatable. The cam follower members (46, 47) capable of contacting both the low speed cam (59, 75) and the high speed cam (60, 76) are the low speed cam (59, 75) and the high speed cam (60). , 76) and is supported by the engine body (11) so that it can be operated and the cam for high speed when the rotational speed of the camshaft (43A, 43B) exceeds a predetermined rotational speed. (6 , 76) is connected to the camshaft (43A, 43B) in phase with the low speed cam (59, 75), and the centrifugal clutch (61A, 61B) is coaxial with the camshaft (43A, 43B). A variable valve operating apparatus for an internal combustion engine, which is provided between the cam shaft (43A, 43B) and the high speed cam (60, 76).

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