Abstract:
A valve operating mechanism for an internal combustion engine for varying the operation of the valve dependent on engine operating conditions. A first rocker arm movable to follow a first cam and a second rocker arm movable to follow a second cam corresponding to a higher speed operating condition than that of the first cam are pivotally supported on a rocker arm shaft. A first connection switching device capable of switching the connection and disconnection between the third and first rocker arms and a second switching device capable of switching the connection and disconnection between at least one of the first and third rocker arms and the second rocker arm independently from the first connection switching device, are disposed at locations displaced circumferentially relative to the rocker arm shaft. Thus, it is possible to dispose a trigger mechanism and to dispose3 roller followers coaxially with the connection switching device, while avoiding the increase in width of the three rocker arms along their swinging axes.

Description:
This is a continuation application, of Ser. No. 08/121,992, filed Sep. 15, 1993, now U.S. Pat. No. 5,388,552 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a valve operating device for use in an internal combustion engine for varying operating characteristics of an engine valve depending on an operating condition of the engine. 
     2. Description of the Prior Art 
     One conventional valve operating device is known from Japanese Patent Application Laid-open No. 57805/88, for example. 
     In the valve operating device disclosed in the above Patent Application, three rocker arms are disposed in an adjacent arrangement, and the adjacent rocker arms in two sets are connected to and disconnected from each other. This allows a pair of engine valves connected to the outer rocker arms to be varied in their operating characteristics depending upon three operating conditions: a low-speed operating condition, a medium-speed operating condition and a high-speed operating condition of the engine. In the prior art device, however, a selective coupling means for connecting the adjacent rocker arms in one of the sets to each other and a selective coupling means for connecting the adjacent rocker arms in the other set to each other are disposed coaxially and hence, in order to insure an operational stroke of each selective coupling means, the width of the three adjacent rocker arms along their swinging axes must be set larger. Therefore, the valve-operating device has an increased size, and in-order to insure the rigidity at the time when the rocker arms are connected, it is necessary to increase the strength of the rocker arms. 
     There is also a conventionally known valve operating device including a trigger mechanism mounted to the selective coupling means for restraining the operational timing for the selective coupling means, as disclosed in Japanese Patent Application Laid-open No.31611/86. If the trigger mechanism is applied to a valve operating device of the conventional type described above, which includes a pair of selective coupling means disposed coaxially, however, the axial length of a connecting piston as a component of the selective coupling means should be set relatively long. For this reason, the width of the rocker arms along their swinging axes must be further increased. Further, there is a conventionally known valve operating device which includes rocker arms each movable to follow a cam through a roller follower in order to reduce the resistance of friction with the cam, and selective coupling means disposed coaxially with the roller follower, as disclosed in Japanese Patent Publication No.50286/90. In such a conventional valve operating device including a pair of the selective coupling means disposed coaxially, however, if the means are disposed coaxially with the roller followers, the width of the rocker arms must be further increased in order to insure a space occupied by the roller followers. 
     On the other hand, in the valve operating device disclosed in the above Japanese Patent Application Laid-open No. 57805/88, the operating characteristics of each of the engine valves is varied at two stages of a disconnection condition in which the engine valve is opened and closed in accordance to the operation of the rocker arm connected to the engine valve, and a connection condition in which the engine valve is opened and closed in accordance to the operation of the adjacent rocker arm. Therefore, in order to achieve the variation of the operating characteristics at three stages according to the operating conditions of the engine, a combination of these stages must be selected from a limited combination of connection and disconnection of two sets of rocker arms. In order to enable a more precise varying control of valve-operating characteristics according to the operating conditions of the engine, it is desirable to increase the freedom of selection of a combination of the valve-operating characteristics in each operating region. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is a first object of the present invention to provide a valve operating device for an internal combustion engine, wherein a trigger mechanism can be disposed, and a roller follower can be disposed coaxially with a selective coupling means, while avoiding an increase in width of three rocker arms along their swinging axes. 
     To achieve the above object, according to a first aspect and feature of the present invention, there is provided a valve operating device for use in an internal combustion engine for varying operating characteristics of an engine valve depending upon operating conditions of the engine, comprising a first rocker arm movable in response to a first cam, a second rocker arm movable in response to a second cam corresponding to a higher speed operating condition than that of the first cam, a third rocker arm operatively connected to an engine valve, the first, second and third rocker arms being pivotally supported on a rocker arm shaft, first selective coupling means capable of switching the connection and disconnection between the third and first rocker arms, and second selective coupling means capable of switching the connection and disconnection between the second rocker arm and at least one of the first and third rocker arms independently of the first selective coupling means, the first and second selective coupling means being disposed at locations displaced circumferentially relative to the rocker arm shaft. 
     With the above construction, it is possible to sufficiently insure an operational stroke of each selective coupling means, while the width of each rocker arm along its swinging axis remains set relatively small. Moreover, it is possible to dispose a trigger mechanism and to dispose the selective coupling means coaxially with roller followers, while such Width remains set relatively small, thereby providing a reduction in size of the valve operating device and sufficiently insuring a rigidity at the time when the rocker arms are connected. 
     In addition to the first feature of the present invention, if at least one of the first and second selective coupling means exhibits a spring force in an expanding direction thereof for expanding and contracting, and includes a connection piston operable in a direction parallel to an axis of the rocker arm shaft; and wherein the device further includes a trigger which is disposed for angular displacement relative to the rocker arms and is brought into detachable engagement with the connecting piston for restraining the operational timing, it is possible to provide a precise operational timing for the selective coupling means, thereby insuring a reliable operation thereof. 
     Further, in addition to the first feature of the invention, if the valve operating device further includes a third cam provided to correspond to the third rocker arm, and roller followers provided on the first, second and third rocker arms to come into contact with the first, second and third cams, respectively; and wherein either one of the first and second selective coupling means is disposed coaxially with the roller followers, it is possible to reduce the friction loss and to reduce the force for operating the valve, while avoiding the increase in size of the rocker arms. 
     It is a second object of the invention to provide a valve operating device for an internal combustion engine, wherein it is possible to increase the freedom of selection of the characteristics of operation of engine valves by the three rocker arms depending upon the operating conditions of the engine, and avoid the increase in width of the three rocker arms along their swinging axes. 
     To achieve the above second object, according to the present invention, there is provided a valve operating device for use in an internal combustion engine for varying operating characteristics of an engine valve depending upon operating conditions of the engine, comprising a first rocker arm movable in response to a first cam, a second rocker arm movable in response to a second cam corresponding to a higher speed operating condition than that of the first cam, a third rocker arm operatively connected to an engine valve, the first and second rocker arms being disposed on one side of the third rocker arm, first selective coupling means mounted astride the rocker arm adjacent the third rocker arm and between the third rocker arm and one of the first and second rocker arms which is remotest from the third rocker arm for switching the connection and disconnection between the third rocker arm and the one of the first and second rocker arm, and second selective coupling means mounted between at least two mutually adjacent ones of first through third rocker arms for switching the connection and disconnection between the adjacent rocker arms independently of the first selective coupling means. 
     With the above construction, it is possible to vary the operating characteristics of an engine valve operatively connected to the third rocker arm at three stages; to increase the freedom of selection of the operating characteristics of the engine valve; to reduce the width in each rocker arm along its swinging axis by offsetting both the connection switching means around each rocker arm, and to increase the rigidity at the time when the rocker arms are connected. 
     In addition to the construction according to the second feature of the invention, if an engine valve other than the engine valve operatively connected to the third rocker arm is operatively connected to the first rocker arm, and wherein the second rocker arm is disposed between the first and third rocker arms, the driving load from the cams during operation of the engine at a high-speed can be applied to both the engine valves in a substantially equally distributed manner, thereby preventing an offset load from being generated. 
     The above and other objects, features and advantages of the invention will become apparent from the following description of preferred embodiments, taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a longitudinal sectional view, taken along a line 1--1 in FIG. 2, of a first embodiment of the present invention when being applied to an intake valve-side valve operating device in an SOHC internal combustion engine; 
     FIG. 2 is an enlarged plan view taken along an arrow 2 in FIG. 1; 
     FIG. 3 is an enlarged sectional view taken along a line 3--3 in FIG. 1; 
     FIG. 4 is an enlarged sectional view taken along a line 4--4 in FIG. 1; 
     FIG. 5 is a longitudinal sectional view similar to FIG. 1, but illustrating a modification to the first embodiment; 
     FIG. 6 is a longitudinal sectional view, taken along a line 6--6 in FIG. 7, of a second embodiment of the invention when being applied to an intake valve-side valve operating device in an SOHC internal combustion engine; 
     FIG. 7 is an enlarged plan view taken along a line 7--7 in FIG. 6; 
     FIG. 8 is an enlarged sectional view taken along a line 8--8 in FIG. 6; 
     FIG. 9 is an enlarged sectional view taken along a line 9--9 in FIG. 6; 
     FIG. 10 is a simplified plan view of a third embodiment of the invention; 
     FIG. 11 is a simplified plan view of a fourth embodiment of the invention; 
     FIG. 12 is a simplified plan view of a fifth embodiment of the invention; 
     FIG. 13 is a simplified plan view of a sixth embodiment of the invention; 
     FIG. 14 is a simplified plan view of a seventh embodiment of the invention 
     FIG. 15 is a simplified plan view of a eighth embodiment of the invention 
     FIG. 16 is a simplified plan view of a ninth embodiment of the invention; 
     FIG. 17 is a simplified plan view of a tenth embodiment of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of the present invention will hereinafter be described with reference to the drawings. 
     FIGS. 1 to 4 show a first embodiment of the invention. As shown in FIGS. 1 and 2, a cylinder head Hc1 of an SOHC internal combustion engine has a pair of intake ports 12 opening at an upper surface of a combustion chamber 11. A pair of engine valves or intake valves V 1 , V 2  for opening and closing the intake ports 12 individually is slidably fitted in a pair of respective guide sleeves 13 mounted in the cylinder head Hc1. The intake valves V 1 , V 2  have respective upper ends projecting from the respective guide sleeves 13. The intake valves V 1 , V 2  are normally urged upwardly, i.e., in a valve closing direction, by coil valve springs 15 disposed around the intake valves V 1 , V 2  between retainers 14 fixed to the projecting upper ends of the intake valves V 1 , V 2  and the cylinder head Hc1. 
     A cam shaft 16 1  that is shared by the intake valves V 1 , V 2  and exhaust valves (not shown) is operatively coupled to a crankshaft (not shown) for rotation at a speed reduction ratio of 1/2 with respect to the rotation of the crankshaft. First, second, and third rocker arms 17 1 , 18 1 , 19 1  are interposed between the cam shaft 16 1  and the intake valves V 1 , V 2  for converting rotary movement of the cam shaft 16 1  to opening and closing movement of the intake valves V 1 , V 2 . 
     As also shown in FIG. 3, the cam shaft 16 1  is rotatably supported by the cylinder head Hc1 and a plurality of holders 20 coupled to the cylinder head Hc1. The cam shaft 16 1  has a low-speed cam 21, a high-speed cam 22, and a raised portion 23 which are integrally formed with the cam shaft 16 1  and successively arranged in the order named. The low-speed cam 21 has a cam profile for opening and closing the intake valve V 1  in a low-speed operating range of the engine and opening and closing the intake valves V 1 , V 2  in a medium-speed operating range of the engine. The low-speed cam 21 comprises a base-circle portion 21a and a cam lobe 21b projecting radially outwardly from the base-circle portion 21a. The high-speed cam 22 has a cam profile for opening and closing the intake valves V 1 , V 2  in a high-speed operating range of the engine. The high-speed cam 22 comprises a base-circle portion 22a and a cam lobe 22b projecting more radially outwardly from the base-circle portion 22a than the cam lobe 21b of the low-speed cam 21 and having a larger angular extent than the cam lobe 21b. The raised portion 23 has a substantially circular profile around the axis of the cam shaft 16 1  for substantially disabling the intake valve V 2  in a low-speed operating range of the engine. The raised portion 23 includes a slightly radially projecting region in a position corresponding to the cam lobes 21b, 22b of the low- and high-speed cams 21, 22. 
     The first rocker arm 17 1  is operatively coupled to the intake valve V 1 , and the third rocker arm 19 1  is operatively coupled to the intake valve V 2 . The second rocker arm 18 1  is disposed between the first and third rocker arms 17 1 , 19 1  adjacent thereto so that the second rocker arm 18 1  can be freed from the intake valves V 1 , V 2 . The rocker arms 17 1  to 19 1  are rockably supported on a rocker arm shaft 24 1  positioned obliquely upwardly of the cam shaft 16 1  and having an axis parallel to the cam shaft 16 1 , the rocker arm shaft 24 1  being fixedly supported by the holders 20. 
     A cam slipper 25 held in sliding contact with the low-speed cam 21 is mounted on a lower portion of the lower end of the first rocker arm 17 1 . A cam slipper 26 held in sliding contact with the high-speed cam 22 is mounted on the lower surface of a lower portion of the second rocker arm 18 1 . A slipper 27 held in sliding contact with the raised portion 23 is mounted on a lower portion of the lower end of the third arm 19 1 . 
     Tappet screws 28 are axially movable threaded in the respective other ends of the first and third rocker arms 17 1 , 19 1 , and held in abutment against the upper ends of the intake valves V 1 , V 2 . Therefore, the intake valves V 1 , V 2  are opened and closed in response to rocking movement of the first and third rocker arms 17 1 , 19 2 . 
     The second rocker arm 18 1  is resiliently urged in a direction to hold the cam slipper 26 in sliding contact with the high-speed cam 22 by a resiliently urging means (not shown). 
     The first selective coupling means 30 1  is disposed between the first and third rocker arms 17 1 , 19 1  for selectively connecting and disconnecting the first and third rocker arms 17 1 , 19 1 . A second selective coupling means 31 1  is disposed in the first through third rocker arms 17 1  to 19 1  at a position different from the first selective coupling means 30 1  around the axis of the rocker arm shaft 24 1  for selectively connecting and disconnecting the first through third rocker arms 17 1  to 19 1  independently of the first selective coupling means 30 1 . 
     The first rocker arm 17 1  has an integral joint arm 29 positioned more closely than the rocker arm shaft 24 1  to the intake valves V 1 , V 2  and extending across the second rocker arm 18 1  toward the third rocker arm 19 1 . The first selective coupling means 30 1  is arranged to selectively connect and disconnect the joint arm 29 and the third rocker arm 19 1 . The second selective coupling means 31 1  is provided in the first through third rocker arms 17 1  to 19 1  diametrically opposite to the first selective coupling means 30 1  across the rocker arm shaft 24 1 . 
     The first selective coupling means 30 1  comprises a coupling piston 32 capable of connecting the first and third rocker arms 17 1 , 19 1  to each other, a limiting member 33 for limiting movement of the coupling piston 32, and a return spring 34 for urging the coupling piston 32 and the limiting member 33 in a direction to disconnect the first and third rocker arms 17 1 , 19 1  from each other. 
     The joint arm 29 of the first rocker arm 17 1  has a bottomed guide hole 35 defined therein parallel to the rocker arm shaft 24 1  and opening toward the third rocker arm 19 1 . The coupling piston 32 is slidably fitted in the guide hole 35. A hydraulic pressure chamber 36 is defined between one end of the coupling piston 32 and the closed end of the guide hole 35. The first rocker arm 17 1  has a communication passage 37 communicating with the hydraulic pressure chamber 36. Within the rocker arm shaft 24 1  is provided a hydraulic pressure passage 38 connected to a hydraulic pressure supply source, not shown. The hydraulic pressure passage 38 is held in communication with the communication passage 37 and hence the hydraulic pressure chamber 36 irrespective of how the first rocker arm 17 1  may be angularly moved. 
     The coupling piston 32 is extensible and contractible while under a spring force from spring 43 tending to extend the coupling piston 32. The coupling piston 32 comprises a bottomed hollow cylindrical body 41 slidably fitted in the guide hole 35 and having a closed end facing the hydraulic pressure chamber 36, a short cylindrical body 42 slidably fitted in the guide hole 35, and a spring 43 compressed between the bottomed cylindrical body 41 and the short cylindrical body 42. The bottomed cylindrical body 41 has an annular recess defined in an outer surface of the open end thereof and defining a first annular engaging groove 44 between the bottomed cylindrical body 41 and one end of the short cylindrical body 42 when the open end of the bottomed cylindrical body 41 abuts against the end of the short cylindrical body 42. The bottomed cylindrical body 41 also has a second annular engaging groove 45 defined in an outer circumferential surface thereof. The set load of the spring 43 is smaller than that of the return spring 34. 
     The third rocker arm 19 1  has a bottomed guide hole 47 corresponding to the guide hole 35. The bottomed guide hole 47 opens toward the joint arm 29 and extends parallel to the rocker arm shaft 24 1 . The limiting member 33, which is in the form of a bottomed cylinder, is slidably fitted in the guide hole 47 and held in abutment against the other end of the short cylindrical body 42 of the coupling piston 32. The limiting member 33 has an open end facing toward the closed end of the guide hole 47 and a radially outwardly projecting flange 33a held in sliding contact with an inner surface of the guide hole 47. The return spring 34 is compressed between the closed end of the guide hole 47 and the closed end of the limiting member 33 for urging the coupling piston 32 and the limiting member 33 which are mutually abutted against each other toward the hydraulic pressure chamber 36. The closed end of the guide hole 47 has a communication hole 48 for bleeding air and oil. A retaining ring 49 is fitted in the inner surface of the guide hole 47 for engaging the flange 33a of the limiting member 33 to prevent the limiting member 33 from being dislodged out of the guide hole 47. 
     The first selective coupling means 30 1  is provided with a trigger mechanism 51 for controlling the timing for the first selective coupling means 30 1  to be operated. The trigger mechanism 51 has a trigger plate 52 swingable relatively to the rocker arms 17 1  to 19 1  about the axis of the rocker arm shaft 241 between a position in which the trigger plate 52 engages in the first engaging groove 44 or the second engaging groove 45 of the coupling piston 32 to limit movement of the coupling piston 32, and a position in which the trigger plate 52 disengages from the first engaging groove 44 or the second engaging groove 45 to allow movement of the coupling piston 32. 
     The first rocker arm 17 1  has a slit 53 defined in an upper portion thereof and a pair of guide plates 54 extending upwardly with one on each side of the slit 53. The slit 53 is positioned to align with the first engaging groove 44 when the bottomed cylindrical body 41 and the short cylindrical body 42 are displaced a maximum stroke toward the hydraulic pressure chamber 36, as shown in FIG. 3. When the bottomed cylindrical body 41 and the short cylindrical body 42 that abut against each other are moved a maximum stroke away from the hydraulic pressure chamber 36, the second engaging groove 45 is positioned in alignment with the slit 53. 
     A cylindrical collar 55 is fitted over the rocker arm shaft 241 between the first rocker arm 17 1  and one of the holders 20, and a cylindrical collar 56 is fitted over the rocker arm shaft 24 1  between the other holder 20 and the third rocker arm 19 1 . These collars 55, 56 prevent the rocker arms 17 1  to 19 1  from moving along the rocker arm shaft 24 1 . 
     The trigger plate 52 is supported on the collar 55 for angular movement relatively thereto. The trigger plate 52 has an integral engaging plate 52a whose opposite surfaces are held in sliding contact with the guide plates 54, respectively, and which extends from the slit 53 into releasable engagement in the first engaging groove 44 or the second engaging groove 45. 
     The holder 20 which confronts the first rocker arm 17 1  has a stopper pin 57 extending toward the first rocker arm 17 1 . A stopper 52b engageable upwardly with the stopper pin 57 projects from the trigger plate 52. A torsion spring 58 disposed around the collar 55 has one end engaging the stopper pin 57 and the other end engaging the trigger plate 52 downwardly. The trigger plate 52 is normally urged by the torsion spring 58 in a direction to cause the stopper 52b to abut against the stopper pin 57. The stopper pin 57 is positioned such that with the stopper 52b abutting against the stopper pin 57, when the first rocker arm 17 1  is at rest, the engaging plate 52a can extend from the slit 53 into engagement in the engaging grooves 54, 55, and when the first rocker arm 17 1  is angularly moved in a valve opening direction, the engaging plate 52 is released from the slit 53. 
     As shown in FIG. 4, the second selective coupling means 31 1  comprises a coupling piston 61 capable of connecting the first and second rocker arms 17 1 , 18 1  to each other, a coupling pin 62 capable of connecting the second and third rocker arms 18 1 , 19 1  to each other, a limiting member 63 for limiting movement of the coupling piston 61 and the coupling pin 62, and a return spring 64 for urging the coupling piston 61, the coupling pin 62, and the limiting member 63 in a direction to disconnect the first, second, and third rocker arms 17 1 , 18 1 , 19 1  form each other. 
     The first rocker arm 17 1  has a bottomed guide hole 65 defined therein parallel to the rocker arm shaft 24 1  and opening toward the second rocker arm 18 1 . The coupling piston 61 is slidably fitted in the guide hole 65. A hydraulic pressure chamber 66 is defined between one end of the coupling piston 61 and the closed end of the guide hole 65. The first rocker arm 17 1  has a communication passage 67 communicating with the hydraulic pressure chamber 66. The rocker arm shaft 24 1  has a hydraulic pressure passage 68 connected to the hydraulic pressure supply source(not shown) and isolated from the hydraulic passage 38 of the first selective coupling means 30 1 . The hydraulic pressure passage 68 is always communicated with the communication passage 67 and hence the hydraulic pressure chamber 66 irrespective of how the first rocker arm 17 1  is angularly moved. 
     The second rocker arm 18 1  has a guide hole 70 corresponding to the guide hole 65 and extending between its opposite sides parallel to the rocker arm shaft 24 1 . The coupling pin 62 having one end abutting against the other end of the coupling piston 61 is slidably fitted in the guide hole 70. 
     The third rocker arm 19 1  has a bottomed guide hole 71 corresponding to the guide hole 70. The bottomed guide hole 71 opens toward the second rocker arm 18 1  and extends parallel to the rocker arm shaft 24 1 . The limiting member 63, which is in the form of a bottomed cylinder, is slidably fitted in the guide hole 71 and held in abutment against the other end of the coupling pin 62. The limiting member 63 has an open end facing toward the closed end of the guide hole 71 and a radially outwardly projecting flange 63a held in sliding contact with an inner surface of the guide hole 71. The return spring 64 is compressed between the closed end of the guide hole 71 and the closed end of the limiting member 63 for urging the mutually abutted coupling piston 61, the coupling pin 62, and the limiting member 63 toward the hydraulic pressure chamber 66. The closed end of the guide hole 71 has a communication hole 72 for bleeding air and oil. A retaining ring 73 is fitted in the inner surface of the guide hole 71 for engaging the flange 63a of the limiting member 63 to prevent the latter from being dislodged out of the guide hole 71. 
     Operation of the first embodiment will be described below. In the low-speed operating range of the engine, no hydraulic pressure is developed in the hydraulic chambers 36, 66 of the first and second selective coupling means 30 1 , 31 1 . In the first selective coupling means 30 1 , the coupling piston 32 is moved a maximum stroke toward the hydraulic pressure chamber 36 under the force of the return spring 34, disconnecting the first and third rocker arms 17 1 , 19 1  from each other. In the second selective coupling means 31 1 , the coupling piston 61 and the coupling pin 62 are moved a maximum stroke toward the hydraulic pressure chamber 66 under the force of the return spring 64, disconnecting the first, second and third rocker arms 17 1 , 18 1 , 19 1  from each other. At this time, in the first selective coupling means 30 1 , the trigger plate 52 engages in the first engaging groove 44 with the first rocker arm 17 1  at rest. And the mutually abutting surfaces of the coupling piston 32 and the limiting member 33 are positioned between the joint arm 29 integral with the first rocker arm 17 1  and the third rocker arm 19 1 . In the second selective coupling means 31 1 , the mutually abutting surfaces of the coupling piston 61 and the coupling pin 62 are positioned between the first and second rocker arms 17 1 , 18 1 , and the mutually abutting surfaces of the coupling pin 62 and the limiting member 63 are positioned between the second and third rocker arms 18 1 , 19 1 . Consequently, the first, second, and third rocker arms 17 1 , 18 1 , 19 1  are angularly displaceable with respect to each other. 
     While the first and second coupling means 30 1 , 31 1  are thus in a position to disconnect the rocker arms, rotation of the cam shaft 16 1  causes the first rocker arm 17 1  to swing based on sliding contact with the low-speed cam 21, and the intake valve V 1  is opened and closed with timing and lift according to the cam profile of the low-speed cam 21. The third rocker arm 19 1  held in sliding contact with the raised portion 23 substantially stops its swinging movement, thereby substantially keeping the other intake valve V 2  closed. However, since the raised portion 23 has a slightly projecting region at a position corresponding to the cam lobes 21b, 22b of the low- and high-speed cams 21, 22, the intake valve V 2  is not completely closed, but is slightly opened when the intake valve V 1  is opened. Therefore, the intake valve V 2  is prevented from sticking to its valve seat. The second rocker arm 18 1  swings based on sliding contact with the high-speed cam 22. However, the swinging movement of the second rocker arm 18 1  does not affect the first and third rocker arms 17 1 , 19 1 . 
     In the medium-speed operating range of the engine, the second selective coupling means 31 1  is in a position to disconnect connect the rocker arms as no hydraulic pressure is built up in the hydraulic pressure chamber 66, and the hydraulic pressure chamber 36 of the first selective coupling means 30 1  is supplied with a high hydraulic pressure. The coupling piston 32 of the first selective coupling means 30 1  tends to move in a direction to increase the displacement of the hydraulic pressure chamber 36 against the bias of the return spring 34. When the first rocker arm 17 1  is at rest, however, the coupling piston 32 is prevented from moving as the trigger plate 52 engages in the first engaging groove 44. When the first rocker arm 17 1  starts moving in a valve opening direction, the engaging plate 52b of the trigger plate 52 disengages from the first engaging groove 44, allowing the coupling piston 32 to move. The coupling piston 32 slightly moves toward the third rocker arm 19 1 , shifting the first engaging groove 44 out of alignment with the slit 53. Therefore, the trigger plate 52 does not engage in the first engaging groove 44 after the first rocker arm 17 1  starts moving in the valve opening direction. 
     When the axes of the guide holes 35, 47 are aligned with each other, i.e., when the rocker arms 17 1  to 19 1  return to an at rest position, the coupling piston 32 fits into the guide hole 47, connecting the first and third rocker arms 17 1 , 19 1  to each other. In this condition, the second engaging groove 45 is aligned with the slit 53. When the first rocker arm 17 1  is held at rest, the trigger plate 52 engages in the second engaging groove 45. 
     With the first and third rocker arms 17 1 , 19 1  connected to each other, the third rocker arm 19 1  swings with the first rocker arm 17 1  that is held in sliding contact with the low-speed cam 21. Consequently, the intake valves V 1 , V 2  are opened and closed with timing and lift according to the cam profile of the low-speed cam 21. 
     In the high-speed operating range of the engine, a high hydraulic pressure is introduced into both the hydraulic pressure chambers 36, 66 of the first and second selective coupling means 30 1 , 31 1 . The first selective coupling means 30 1  continues to keep the rocker arms connected. The hydraulic pressure developed in the hydraulic pressure chamber 66 of the second selective coupling means 31 1  pushes the coupling piston 61 against a force of the return spring 64. When the axes of the guide holes 65, 70, 71 are aligned with each other, the coupling piston 61 fits into the guide hole 70 and the coupling pin 62 fits into the guide hole 71, thereby connecting the first, second and third rocker arms 17 1  to 19 1  to each other. 
     When the first to third rocker arms 17 1  to 19 1  are connected to each other, the first and third rocker arms that are operatively coupled to the intake valves V 1 , V 2  swing with the second rocker arm 18 1  that is angularly moved by the high-speed cam 22. The intake valves are therefore opened and closed with timing and lift according to the cam profile of the high-speed cam 22. 
     After the rocker arms have been connected by the second selective coupling means 31 1  in the high-speed operating range of the engine, the hydraulic pressure may be released from the hydraulic pressure chamber 36 of the first selective coupling means 30 1  to cause the first selective coupling means 30 1  to disconnect the rocker arms. 
     When the engine switches from the high-speed operating range to the low-speed operating range, or when the hydraulic pressure is released from the hydraulic pressure chamber 36 of the first selective coupling means 30 1  after the rocker arms have been connected by the second selective coupling means 31 1  in the high-speed operating range of the engine, the coupling piston 32 is pushed toward the hydraulic pressure chamber 36 under the resiliency of the return spring 34. When the first rocker arms 17 1  is at rest, the trigger plate 52 engages in the second engaging groove 45, and hence the coupling piston 32 is prevented from moving. When the first rocker arm 17 1  starts to move in the valve opening direction, the trigger plate 52 disengages from the second engaging groove 45, and the bottomed cylindrical body 41 first moves toward the hydraulic pressure chamber 36 under the bias of the return spring 43. At this time, the short cylindrical body 42 does not return to the guide hole 35 due to frictional forces produced in the guide holes 35, 47 upon rocking movement of the first rocker arm 17 1 . When the first rocker arm 17 1  is then returned to an at rest position, the short cylindrical body 42 returns to the guide hole 35, disconnecting the first and third rocker arms 17 1 , 19 1 . In the second selective coupling means 31 1 , when the axes of the guide hole 65, the guide hole 70, and the guide hole 71 are aligned with each other, the coupling piston 61 returns to the guide hole 65, and the coupling pin 62 returns to the guide hole 70, disconnecting the rocker arms. 
     Consequently, in the low-speed operating range of the engine, the intake valve V 1  is opened and closed with timing and lift according to the cam profile of the low-speed cam 21, and the other intake valve V 2  is substantially disabled for reducing fuel consumption. In the medium-speed operating range of the engine, the intake valves V 1 , V 2  are opened and closed with timing and lift according to the cam profile of the low-speed cam 21 for producing an output torque matching the medium-speed operating range. In the high-speed operating range of the engine, the intake valves V 1 , V 2  are opened and closed with timing and lift according to the cam profile of the high-speed cam 22 for increasing the engine output power. Accordingly, the valve operating device can provide valve operating characteristics depending respectively on the low-, medium- and high-speed operating ranges of the engine. 
     The intake valve V 2  operatively coupled to the third rocker arm 19 1  can be operatively coupled to the first rocker arm 17 1  across the second rocker arm 18 1 . The intake valve V 2  is substantially disabled in the low-speed operating range of the engine, opened and closed by the low-speed cam 21 in the medium-speed operating range of the engine, and opened and closed by the high-speed cam 22 in the high-speed operating range of the engine. The intake valve V 2  is therefore capable of varying its operating characteristics in three steps. This, together with the fact that the intake valve V 1  operatively coupled to the first rocker arm 17 1  can vary its operating characteristics in two steps, i.e., is opened and closed by the low-speed cam 21 and opened and closed by the high-speed cam 22, permit the operating characteristics of the intake valves V 1 , V 2  which are variable depending on the operating conditions of the engine to be selected in combinations more freely than heretofore. 
     In this valve operating device, inasmuch as the second rocker arm 18 1  slidingly contacting the high-speed cam 22 is disposed between the first and third rocker arms 17 1 , 19 1  that are operatively coupled individually to the intake valves V 1 , V 2 , the drive load from the high-speed cam 22 can substantially uniformly be distributed to the intake valves V 1 , V 2 , which are thus prevented from suffering irregular loads. 
     The coupling piston 32 of the first selective coupling means 30 1  is relatively long in its axial direction because it is composed of the bottomed cylindrical body 41 facing the hydraulic pressure chamber 36, the short cylindrical body 42, and the spring 43 compressed between the bottomed cylindrical body 41 and the short cylindrical body 42 for extension and traction under the spring force tending to extend the coupling piston 32 in order to disengageably engage the trigger plate 52 of the trigger mechanism 51. The first and second selective coupling means 30 1 , 31 1  are displaced relative to each other around the axis of the rocker arm shaft 24 1 , and the first selective coupling means 30 1  is disposed between the third rocker arm 19 1  and the joint arm 29 extending from the first rocker arm 17 1  toward the third rocker arm 19 1 . Therefore, the required length of the coupling piston 32 can be accommodated in the first rocker arm 17 1  without increasing the width of the latter along the axis of the rocker arm shaft 24 1 , and the first selective coupling means 30 1  can be actuated with proper timing by the trigger mechanism 51. A sufficient operating stroke is available for the first and second selective coupling means 30 1 , 31 1  without increasing the width of the rocker arms 17 1 , 18 1 , 19 1 . 
     FIG. 5 shows a modification of the first embodiment. The first selective coupling means 30 1  that is operable independently of the second selective coupling means 31 1  is disposed between the first and third rocker arms 17 1 , 19 1  in a position such that the central angle α formed between the first selective coupling means 30 1  and the point where the load is applied from the low-speed cam 21 to the first rocker arm 17 1  is approximately 90°. 
     When the first selective coupling means 30 1  is actuated to connect the rocker arms, since the third rocker arm 19 1  is angularly moved by the first rocker arm 17 1 , the first rocker arm 17 1  undergoes a rotational moment acting about a straight line that interconnects the junction between the first and third rocker arms 17 1 , 19 1  perpendicular to the axis of the rocker arm shaft 24 1 . However, inasmuch as the first selective coupling means 30 1  is disposed in a position such that the central angle α formed between the first selective coupling means 30 1  and the point where the load is applied from the low-speed cam 21 to the first rocker arm 17 1  is approximately 90°, the above rotational moment is produced in a plane substantially parallel to the slidingly contacting surfaces of the cam slipper 25 and the low-speed cam 21. Therefore, irregular loads are prevented from acting on the slidingly contacting surfaces of the cam slipper 25 and the low-speed cam 21. 
     FIGS. 6 to 9 show a second embodiment of the invention. As shown in FIGS. 6 and 7, first, second, and third rocker arms 17 2 , 18 2 , 19 2  are interposed between a pair of intake valves V 1 , V 2  openably and closably supported by a cylinder head Hc2 of a DOHC internal combustion engine and a cam shaft 16 2  for converting rotary movement of the cam shaft 16 2  to opening and closing movement of the intake valves V 1 , V 2 . 
     As also shown in FIG. 8, the cam shaft 16 1  has a medium-speed cam 75, a high-speed cam 22, and a low-speed cam 21 which are integrally formed with the cam shaft 16 2  and successively arranged in the order named. The medium-speed cam 75 has a cam profile for-opening and closing the intake valve V 1  in a low-speed operating range of the engine and opening and closing the intake valves V 1 , V 2  in a medium-speed operating range of the engine. The medium-speed cam 75 comprises a base-circle portion 75a and a cam lobe 75b projecting radially outwardly from the base-circle portion 75a. The cam lobe 75b projects more radially outwardly than the cam lobe 21b of the low-speed cam 21 and less radially outwardly than the cam lobe 22b of the high-speed cam 22 and has a larger angular extent than the cam lobe 21b and a smaller angular extent than the cam lobe 22b. 
     The first rocker arm 17 2  has one end operatively coupled to the intake valve V 1 , and the third rocker arm 19 2  has one end operatively coupled to the intake valve V 2 . The second rocker arm 18 2  is disposed between the first and third rocker arms 17 2 , 19 2  adjacent thereto. The rocker arms 17 2  to 19 2  are rockably supported on a rocker arm shaft 24 2  positioned obliquely downwardly of the cam shaft 16 2  and having an axis parallel to the cam shaft 16 2 . 
     A first selective coupling means 30 2  is disposed between the first and third rocker arms 17 2 , 19 2  for selectively connecting and disconnecting the first and third rocker arms 17 2 , 19 2 . A second selective coupling means 31 2  is disposed in the first to third rocker arms 17 2  to 19 2  at a position different from the first selective coupling means 30 2  around the axis of the rocker arm shaft 24 2  for selectively connecting and disconnecting the first to third rocker arms 17 2  to 19 2  independently of the first selective coupling means 30 2 . 
     As also shown in FIG. 9, the first and third rocker arms 17 2 , 19 2  have respective integral joint arms 76, 77 positioned remotely from the intake valves V 1 , V 2  across the rocker arm shaft 24 2  and extending across the second rocker arm 18 2  in confronting relationship to each other. The first selective coupling means 30 2  is disposed between the joint arms 76, 77. The second selective coupling means 31 2  is located in the first to third rocker arms 17 2  to 19 2  between the position in which the first and third rocker arms 17 2 , 19 2  are operatively coupled to the intake valves V 1 , V 2  and the rocker arm shaft 24 2 . 
     The first selective coupling means 30 2  comprises a coupling piston 82 capable of connecting the joint arms 76, 77, a limiting member 83 for limiting movement of the coupling piston 82, and a return spring 84 for urging the coupling piston 82 and the limiting member 83 in a direction to disconnect the joint arms 76, 77 from each other. 
     The joint arm 76 of the first rocker arm 17 2  and the joint arm 77 of the third rocker arm 19 2  have respective guide hole 85, 89 defined therein parallel to the rocker arm shaft 24 2 . 
     The coupling piston 82 is slidably fitted in the guide hole 85, with a hydraulic pressure chamber 86 being defined between the coupling piston 82 and the closed end of the guide hole 85. The first rocker arm 17 2  has a communication passage 87 communicating with the hydraulic pressure chamber 86. The rocker arm shaft 24 2  has a hydraulic pressure passage 88 communicated to a hydraulic pressure supply source (not shown). The hydraulic pressure passage 88 is always communicated with the communication passage 87 and hence the hydraulic pressure chamber 86 irrespective of how the first rocker arm 17 2  is angularly moved. 
     The limiting member 83, which is in the form of a bottomed hollow cylinder, is slidably fitted in the guide hole 89 and prevented from being dislodged out of the guide hole 89 by a retaining ring 90 fitted in an inner surface of the guide hole 89. The return spring 84 is disposed under compression between the closed end of the guide hole 89 and the limiting member 83. The closed end of the guide hole 89 has a communication hole 91 for bleeding air and oil. 
     The first to third rocker arms 17 2  to 19 2  move in response to the respective cams 75, 22, 21 through respective roller followers 92, 93, 94. These roller followers are mounted on the respective rocker arms 17 2  to 19 2  between the rocker arm shaft 24 2  and the intake valves V 1 , V 2 . The second rocker arm 18 2  is normally urged to hold the roller follower 92 in contact with the high-speed cam 22 by a resiliently urging means (not shown). 
     The roller follower 92 comprises an inner race 95 having an axis parallel to the rocker arm shaft 24 2  and fixedly fitted over the first rocker arm 17 2 , an outer race 96 held in contact with the medium-speed cam 75, and a plurality of rollers 97 interposed between the inner and outer races 95, 96. The roller follower 93 comprises an inner race 98 having an axis parallel to the rocker arm shaft 24 2  and fixedly fitted over the second rocker arm 18 2 , an outer race 99 held in contact with the high-speed cam 22, and a plurality of rollers 100 interposed between the inner and outer races 98, 99. The roller follower 94 comprises an inner race 101 having an axis parallel to the rocker arm shaft 24 2  and fixedly fitted over the third rocker arm 19 2 , an outer race 102 held in contact with the low-speed cam 21, and a plurality of rollers 103 interposed between the inner and outer races 101, 102. The inner races 95, 98, 101 are fixedly fitted over the respective rocker arms 17 2  to 19 2  such that they are aligned with each other when the rocker arms 17 2  to 19 2  are at rest. 
     The second selective coupling means 31 2  comprises a coupling piston 105 capable of connecting the first and second rocker arms 17 2 , 18 2 , a coupling pin 106 capable of connecting the second and third rocker arms 18 2 , 19 2 , a limiting member 107 for limiting movement of the coupling piston 105 and the coupling pin 106, and a return spring 108 for urging the coupling piston 105, the coupling pin 106, and the limiting member 107 to disconnect the rocker arms. 
     The coupling piston 105 is slidably fitted in the inner race 95 of the roller follower 92, with a hydraulic pressure chamber 109 defined between one end of the coupling piston 105 and the first rocker arm 17 2 . The first rocker arm 17 2  has a communication passage 110 communicating with the hydraulic pressure chamber 109. The rocker arm shaft 24 2  has a hydraulic pressure passage 112 communicated to a hydraulic pressure supply source(not shown) and isolated from the hydraulic passage 87 of the first selective coupling means 30 2 . The hydraulic pressure passage 112 is always communicated with the communication passage 110 and hence the hydraulic pressure chamber 109 irrespective of how the first rocker arm 17 2  may be angularly moved. 
     The coupling pin 106 whose one end abuts against the other end of the coupling piston 105 is slidably fitted in the inner race 98 of the roller follower 93. 
     The limiting member 107, which is in the form of a bottomed hollow cylinder, abuts against the other end of the coupling pin 106 and is slidably fitted in the inner race 101 of the roller follower 94. The return spring 108 is compressed pressed between the third rocker arm 19 2  and the limiting member 107. The third rocker arm 19 2  has a communication hole 113 coaxial with the inner race 101. 
     According to this second embodiment, in the low-speed operating range of the engine, the first and second selective coupling means 30 2 , 31 2  are actuated to disconnect the joint arms and the rocker arms. The intake valve V 1  is opened and closed with timing and lift according to the cam profile of the medium-speed cam 75, and the other intake valve V 2  is opened and closed With timing and lift according to the cam profile of the low-speed cam 21. In the medium-speed operating range of the engine, the first selective coupling means 30 2  connects the joint arms, and the second selective coupling means 31 2  still disconnects the rocker arms. The intake valves V 1 , V 2  are opened and closed with timing and lift according to the cam profile of the medium-speed cam 75. In the high-speed operating range of the engine, at least the second selective coupling means 31 2  connects the rocker arms, and the intake valves V 1 , V 2  are opened and closed with timing and lift according to the cam profile of the high-speed cam 22. 
     Therefore, depending on the operating conditions, i.e., the low-, medium-, and high-speed operating ranges, of the engine, the operating characteristics of the intake valves V 1 , V 2  may be varied for reducing fuel consumption in the low-speed operating range and increasing the engine output power in all of the operating ranges of the engine. Furthermore, since the intake valve V 2  operatively coupled to the third rocker arm 19 2  can be operatively coupled to the first rocker arm 17 2  across the second rocker arm 18 2 , the intake valve V 2  is opened and closed by the low-speed cam 21 in the low-speed operating range of the engine, opened and closed by the medium-speed cam 75 in the medium-speed operating range of the engine, and opened and closed by the high-speed cam 22 in the high-speed operating range of the engine. The intake valve V 2  is therefore capable of varying its operating characteristics in three steps. This, together with the fact that the intake valve V 1  operatively coupled to the first rocker arm 17 2  can vary its operating characteristics in two steps, i.e., is opened and closed by the medium-speed cam 75 and opened and closed by the high-speed cam 22, permit the operating characteristics of the intake valves V 1 , V 2  which are variable depending on the operating conditions of the engine to be selected in combinations more freely than heretofore. 
     As the first and second selective coupling means 30 2 , 31 2  are displaced from each other in the circumferential direction relative to the rocker arm shaft 24 2 , the axial length of the coupling piston 105, the coupling pin 106, and the limiting member 107 Of the second selective coupling means 31 2  may be set longer without increasing the width along the axis about which the rocker arms 17 2  to 19 2  swing than would be if a pair of selective coupling means for selectively connecting and disconnecting adjacent rocker arms were coaxially arranged. Therefore, without increasing the width of the rocker arms 17 2  to 19 2 , the roller followers 92 to 94 may be positioned coaxially with the second selective coupling means 31 2  for reducing the frictional resistance between the cams 75, 22, 21 and the rocker arms 17 2  to 19 2  for achieving a reduction in the power required to operate the valves. 
     FIG. 10 shows a third embodiment of the present invention. An SOHC internal combustion engine has a first rocker arm 17 1  operatively coupled to-an intake valve V 1  and held in sliding contact with a low-speed cam, a third rocker arm 19 1  operatively coupled to an intake valve V 2  and held in sliding contact with a raised portion for substantially disabling the intake valve V 2  and a second rocker arm 18 1  disposed between the first rocker arm 17 1  and the third rocker arm 19 1  and held in sliding contact with a high-speed cam. A first selective coupling means 30 1  is disposed between the first and third rocker arms 17 1 , 19 1 , and a second selective coupling means 31 3  which is of basically the same structure as the first selective coupling means 30 2  in the second embodiment is disposed between second and third rocker arms 18 1 , 19 1 . 
     In the low-speed operating range of the engine, the first and second selective coupling means 30 1 , 31 3  disconnect the rocker arms to cause the intake valve V 1  to be actuated by the low-speed cam, and disable the other intake valve V 2 . In the medium-speed operating range of the engine, only the first selective coupling means 30 1  connects the rocker arms to cause the intake valves V 1 , V 2  to be actuated by the low-speed cam. In the high-speed operating range of the engine, the first and second selective coupling means 30 1 , 31 3  connect all three of the rocker arms to cause the intake valves V 1 , V 2  to be actuated by the high-speed cam. 
     FIG. 11 shows a fourth embodiment of the invention. An SOHC internal combustion engine has a first rocker arm 17 1  operatively coupled to an intake valve V 1  and held in sliding contact with a low-speed cam, a third rocker arm 19 1  operatively coupled to an intake valve V 2  and held in sliding contact with a raised portion for substantially disabling the intake valve V 2 , and a second rocker arm 18 2  disposed between the first rocker arm 17 1  and the third rocker arm 19 1  and held in sliding contact with a high-speed cam. A first selective coupling means 30 1  is disposed between the first and third rocker arms 17 1 , 19 1 , and a second selective coupling means 31 3  is disposed between the first and second rocker arms 17 1 , 18 2 . 
     In the low-speed operating range of the engine, the first and second selective coupling means 30 1 , 31 3  disconnect the rocker arms to cause the intake valve. V 1  to be actuated by the low-speed cam, and disable the other intake valve V 2 . In the medium-speed operating range of the engine, only the first selective coupling means 30 1  connects the rocker arms to cause the intake valves V 1 , V 2  to be actuated by the low-speed cam. In the high-speed operating range of the engine, the first and second selective coupling means 30 1 , 31 3  connect all the rocker arms to cause the intake valves V 1 , V 2  to be actuated by the high-speed cam. 
     FIG. 12 shows a fifth embodiment of the invention. A DOHC internal combustion engine has a first rocker arm 17 2   operatively coupled to an intake valve V 1  and held in sliding contact with a medium-speed cam, a third rocker arm 19 2  operatively coupled to an intake valve V 2  and held in sliding contact with a low-speed cam, and a second rocker arm 18 2  disposed between the first rocker arm 17 2  and the third rocker arm 19 2  and held in sliding contact with a high-speed cam. A first selective coupling means 30 2  is disposed between the first and third rocker arms 17 2 , 19 2 , and a second selective coupling means 31 3 , is disposed between the first and second rocker arms 17 2 , 18 2 . 
     According to the fifth embodiment, in the low-speed operating range of the engine, the first and second selective coupling means 30 2 , 31 3  disconnect the rocker arms to cause the intake valve V 1  to be actuated by the medium-speed cam, and also to cause the intake valve V 2  to be actuated by the low-speed cam. In the medium-speed operating range of the engine, only the first selective coupling means 30 2  connects the rocker arms to cause the intake valves V 1 , V 2  to be actuated by the medium-speed cam. In the high-speed operating range of the engine, the first and second selective coupling means 30 2 , 31 3  connect the rocker arms to cause the intake valves V 1 , V 2  to be actuated by the high-speed cam. 
     FIG. 13 shows a sixth embodiment of the invention. A DOHC internal combustion engine has a first rocker arm 17 2  operatively coupled to an intake valve V 1  and held in sliding contact with a medium-speed cam, third rocker arm 19 2  operatively coupled to an intake valve V 2  and held in sliding contact with a low-speed cam, a first selective coupling means 30 2  disposed between the first rocker arm 17 2  and the third rocker arm 19 2 , a second rocker arm 18 2  disposed between the first rocker arm 17 2  and the third rocker arm 19 2  and held in sliding contact with a high-speed cam, and a second selective coupling means 31 3  disposed between the second and third rocker arms 18 2 , 19 2 . 
     FIG. 14 shows a seventh embodiment. In this embodiment, a DOHC internal combustion engine has a single intake valve V operatively coupled to a third rocker arm 19 2  in a position corresponding to a second rocker arm 18 2  held in sliding contact with a high-speed cam, a first selective coupling means 30 2  disposed between a first rocker arm 17 2  and the third rocker arm 19 2 , and a second selective coupling means 31 3  disposed between the first and second rocker arms 17 2 , 18 2 . 
     According to the seventh embodiment, in the low-speed operating range of the engine, the first and second selective coupling means 30 2 , 31 3  disconnect the rocker arms to cause the intake valve V to be actuated by the low-speed cam engaged by third rocker arm 19 2 . In the medium-speed operating range of the engine, only the first selective coupling means 30 2  connects the rocker arms to cause the intake valve V to be actuated by the medium-speed cam engaged by first rocker arm 17 2 . In the high-speed operating range of the engine, the first and second selective coupling means 30 2 , 31 3  connect all the rocker arms to cause the intake valve V to be actuated by the high-speed cam engaged by the second rocker arm 18 2 . The third rocker arm 19 2  operatively coupled to the intake valve V is positioned adjacent to the second rocker arm 18 2  that is angularly moved by the high-speed cam. Therefore, in the high-speed operating range of the engine, the point where the load is applied from the high-speed cam to the second rocker arm 18 2  and the point where the load is applied therefrom to the valve V may be located closely to each other, as shown in FIG. 14, for minimizing the generation of irregular loads. 
     FIG. 15 shows an eighth embodiment of the invention. A first selective coupling means 30 2  is disposed between a first rocker arm 17 2  held in sliding contact with a medium-speed cam and a third rocker arm 19 2  operatively coupled to an intake valve V and held in sliding contact with a low-speed cam, and a second selective coupling means 31 3  is disposed between a second rocker arm 18 2  held in sliding contact with a high-speed cam and the third rocker arm 19 2  and is disposed between the first and third rocker arms 17 2 , 19 2 . 
     FIG. 16 shows a ninth embodiment. In this embodiment, a first rocker arm 17 3  slidably contacted by the medium-speed cam and operatively coupled to the intake valve V, as well as a second rocker arm 18 3  slidably contacted by the high-speed cam are disposed at opposite sides of a third rocker arm 19 3  which is operatively coupled to the intake valve V and is slidably contacted by the low-speed cam. The first selective coupling means 30 2  which is capable of connecting and disconnecting the first and third rocker arms 17 3 , 19 3  and the second selective coupling means 31 2  which is capable of connecting and disconnecting the first, second and third rocker arms 17 3  to 19 3  are disposed at locations displaced circumferentially relative to the rocker arm shaft 24 2 . 
     According to the ninth embodiment, in the low-speed operating range of the engine, the first and second selective coupling means 30 2 , 31 2  disconnect the rocker arms to cause the intake valve V to be opened and closed by the low-speed cam. In the medium medium-speed operating range of the engine, the first selective coupling means 30 2  connects the rocker arms to cause the intake valve V to be opened and closed by the medium-speed cam. In the high-speed operating range of the engine, the second selective coupling means 31 2  connects the rocker arms to cause the intake valve to be opened and closed by the high-speed cam. 
     FIG. 17 shows a tenth embodiment. As in the preceding embodiment, the first selective coupling means 30 2  which connects and disconnects a third rocker arm 19 4  slidably contacted with the low-speed cam and a first rocker arm 17 4  slidably contacted with the medium-speed cam, and the second selective coupling means 31 3  which connects and disconnects the third rocker arm 19 4  slidably contacted with the high-speed cam and the third rocker arm 19 4  may be disposed at locations displaced circumferentially relative to the rocker arm shaft 24 2 . 
     Although various embodiments of the invention have been described in detail, the invention should not be limited to the above embodiments, but various design modifications may be made without departing the invention as defined by the scope of claims. For example, the present invention is also applicable to a valve operating device for operating exhaust valves.