Patent Publication Number: US-8522736-B2

Title: Phase variable device for engine

Description:
FIELD OF THE INVENTION 
     The present invention relates to a phase varying device for an automobile engine having a mechanism for varying valve timing of the engine by advancing or retarding the phase angle of a camshaft relative to the crankshaft of the engine, the mechanism utilizing circular eccentric cams. 
     BACKGROUND ART 
     A similar device in the form of a valve timing control device has been disclosed in Patent Document 1 cited below. As disclosed in Patent Document 1 this device has a drive rotor  2  driven by a crankshaft (not shown) and a guide plate  27  (which corresponds to a first control rotor of the present invention) rotatable relative to the drive rotor  2 . The camshaft  1  of the device has a lever member  18  which is integral with the camshaft  1  and rotatably coupled at one end thereof to one end of a pair of link arms ( 16   a  and  16   b ) with a pin  25 . The other end of the link arm ( 16   a  and  16   b ) are rotatably connected to the front ends of operative members ( 14   a  and  14   b ) by means of pins  24 . The operative members are provided on the front ends thereof with protrusions  26  that engage with spiral guides  32  formed in the rear end of a guide plate  27 . The rear ends of the operative members ( 14   a  and  14   b ) are configured to engage guide grooves ( 11   a  and  11   b ) which extend substantially in the radial direction (the grooves hereinafter referred to as radial guide grooves.) 
     When the guide plate  27  is attracted by an electric magnet  29 , the guide plate  27  is retarded in rotation relative to the drive rotor  2 . Then, the protrusions  26  of the front ends of the operative members ( 14   a  and  14   b ) are displaced in the spiral guides  32 , while the rear ends are displaced along the radial guide grooves ( 11   a  and  11   b ) in the radially inward direction of the drive rotor  2 . In this case, the link arms ( 16   a  and  16   b ) are rotated about the pin  25  relative to the lever member  18  in the clockwise direction (as viewed from the guide plate  27 ). As a consequence, the phase angle of the camshaft  1  relative to the drive rotor  2  (that is, the relative phase angle between the crankshaft and the camshaft) is advanced in the direction R as shown in  FIG. 4  (the direction referred to as phase angle advancing direction), thereby varying the valve timing of the valves. 
     PRIOR ART DOCUMENT 
     
         
         PATENT DOCUMENT JPA H2001-041013 
       
    
     DISCLOSURE OF THE INVENTION 
     Problems to be Solved by the Invention 
     To allow valve timing to change over a wide range, it is preferable to make the variable range of the phase angle of the camshaft  1  relative to the drive rotor  2  as large as possible. In the device of Patent Document 1, the maximum range of the phase angle variation can be extended by making the length of the link arms ( 16   a  and  16   b ) longer and making the outer diameters of the drive rotor  2  and guide plate  27  larger. However, such modifications will disadvantageously make the phase varying device larger. On the other hand, the space available for the phase-varying apparatus is limited in the engine. 
     If in the device of Patent Document 1 accuracy of the connection of the link arms ( 16   a  and  16   b ) and pins ( 24  and  25 ) has a low accuracy, and accuracy of the engagement of the operative members ( 14   a  and  14   b ) with the spiral guides  32  has a low precision, it may happen that the link arms ( 16   a  and  16   b ) cannot smoothly rotate relative to the lever members  18 , which may prevent the operative members ( 14   a  and  14   b ) from undergoing smooth movement in the spiral guides  32 . Manufacturing these elements with a high degree of accuracy will entail disadvantageously high production cost. 
     In view of such prior art problem, the present invention is directed to an improved phase varying device for an automobile engine which has a larger variable range in phase angle than conventional devices, yet it is compact in size and can be easily manufactured. 
     Means for Solving the Problems 
     There is provided in accordance with the present invention a phase varying device for use with an automobile engine, having a drive rotor driven by the crankshaft of the engine; a first control rotor rotatable relative to the drive rotor under the action of a torque means; and a phase angle varying mechanism operably coupled to the first control rotor in rotational motion relative to the first control rotor, the drive rotor and first control rotor rotatably supported by a camshaft of the phase varying apparatus, the phase varying device adapted to vary the phase angle of the camshaft relative to the crankshaft by varying the phase angle of the camshaft relative to the drive rotor, the phase angle varying mechanism characterized by comprising: 
     a first circular eccentric cam integrated with the first control rotor and having a center eccentrically offset from the rotational axis of the camshaft; 
     a second circular eccentric cam integrated with the camshaft and having a center eccentrically offset from the rotational axis of the camshaft; 
     a cam guide member for rotatably coupling the first circular eccentric cams with the second circular eccentric cam and for converting the eccentric rotational motion of the first circular eccentric cam into the eccentric rotational motion of the second circular eccentric cam, thereby varying the phase angle of the camshaft relative to the drive rotor in accord with the eccentric rotational motion of the second circular eccentric cam relative to the first circular eccentric cam. 
     (Function) Under the action of the torque means, the first control rotor is rotated relative to the drive rotor in either the phase advancing direction (rotational direction of the drive rotor driven by the crankshaft) or the phase retarding direction (direction opposite to the phase advancing direction). The first circular eccentric cam rotates eccentrically about the rotational axis of the camshaft together with the first control rotor. The eccentric rotation of the first circular eccentric cam is converted into the eccentric rotation of the second circular eccentric cam by the cam guide member. Since the camshaft rotates together with the second circular eccentric cam relative to the drive rotor, its phase angle relative to the drive rotor (or crankshaft) is altered. 
     The phase angle of the camshaft relative to the drive rotor is greatly changed in proportion to the distance traveled by the central axis of the second circular eccentric cam during the change. Thus, the variable range of the phase angle of the camshaft relative to the drive rotor (or crankshaft) may be further extended, without making the outer diameters of the first and second circular eccentric cams larger, by increasing the degree of eccentricity of the second circular eccentric cam (that is, making longer the distance between the rotational axis of the camshaft and the center axis of the circular eccentric cam). 
     It is noted that the phase angle of the camshaft relative to the drive rotor is smoothly altered by the eccentric rotations of the first and second circular eccentric cams via the cam guide member if the drive rotor is not very accurately mounted on the drive rotor. 
     As defined in claim  2 , the phase varying device of claim  1  may be configured in such a way that 
     the drive rotor is provided with radial guide grooves that extend in substantially radial directions perpendicular to the rotational axis of the camshaft; and 
     the cam guide member is provided with
         a pair of grip sections penetrating the radial guide grooves to grip the outer circumference of the first circular eccentric cam by the opposite sides, the grip sections movable in the radial guide grooves in response to the eccentric rotational motion of the first circular eccentric cam; and   an oblong circular hole extending in the direction perpendicular to the radial guide grooves and slidably accommodating therein the second circular eccentric cam so as to displace the second circular eccentric cam in the direction perpendicular to the radial guide grooves.       

     (Function) The cam guide member reciprocates in the direction perpendicular to the rotational axis of the camshaft due to the fact that the grip sections in engagement with the radial guide grooves of the drive rotor moves in the guide grooves in response to the eccentric rotation of the first circular eccentric cam inside the oblong hole. Since the oblong hole extends in the direction perpendicular to the radial guide grooves, the reciprocating cam guide member  33  causes the second circular eccentric cam, slidably held in the oblong hole, to rotate eccentrically. 
     The first circular eccentric cam, cam guide member, and second circular eccentric cam are arranged so that the paired first and second circular eccentric cams can slidably move on the inner walls of the grip sections and oblong hole. In this arrangement, the phase angle of the camshaft relative to the drive rotor can be smoothly changed since the paired cams can undergo smooth relative motions if the cams and the cam guide member are not formed with high precision. 
     The initial change in the phase angle of the camshaft relative to the drive rotor can be set to occur in either the phase advancing direction or phase retarding direction. This can be done by setting the initial angular positions of the axes of the first and second circular eccentric cams angularly offset in either the same direction with respect to the radial guide grooves of the drive rotor or the opposite directions across the guide grooves. In other words, if the center axes of the first and second circular eccentric cams are inclined in the same direction with respect to the radial guide grooves of the drive rotor, the second circular eccentric cam is eccentrically rotated in the same direction as the first circular eccentric cam, but rotated in the direction opposite to that of the first circular eccentric cam if they are inclined initially in the opposite directions with respect to the guide grooves. Thus, the direction of the initial change in phase angle from the initial angular position can be easily switched from the phase retarding direction to the phase advancing direction by simply changing the initial angular position of the center axis of the second circular eccentric cam. 
     As defined in claim  3 , the phase varying device of claim  1  or claim  2  may be configured in such a way that the torque means comprises: 
     a first control means for rotating the first control rotor in the phase retarding direction relative to the drive rotor (the direction being opposite to the rotational direction of the drive rotor rotated by the crankshaft); and 
     a reverse mechanism for rotating the first control rotor in the phase advancing direction relative to the drive rotor (the direction being the same as that of the drive rotor driven by the crankshaft). 
     (Function) The first control means alters the phase angle of the camshaft relative to the drive rotor (or crankshaft) either in the phase advancing direction or phase retarding direction, while the reverse mechanism alters the phase angle in the opposite direction. 
     As defined in claim  4 , the phase varying device of claim  3  may be further configured in such a way that 
     the reverse mechanism comprises:
         a second control rotor arranged rotatable relative to the camshaft;   a second brake means for putting a brake on the second control rotor so as to rotate the second control rotor in the phase retarding direction relative to the first control rotor; and   a ring mechanism for rotating the first control rotor in the phase advancing direction relative to the drive rotor when the second braking means is in operation, and wherein       

     the ring mechanism includes
         a first ring member in sliding contact with a first circular eccentric hole formed in the first control rotor;   a second ring member in sliding contact with a second circular eccentric hole formed in the second control rotor;   an intermediate rotor having a radial guide groove and being rotatable together with the camshaft; and   an eccentric coupling member passing through the radial guide groove of the intermediate rotor and movable in the radial guide groove, the eccentric coupling member having opposite ends rotatably coupled to the first and second ring members, respectively, to allow for relative eccentric rotations of the first and second ring members.       

     (Function) The second control rotor rotates the first control rotor in the phase advancing direction relative to the drive rotor via the ring mechanism in the manner as described below. As the second brake means puts a brake on the second control rotor, the second circular eccentric hole of the second control rotor eccentrically rotates about the center axis of the camshaft. In response to the eccentric rotational motion of the second eccentric hole, the second ring member rotates and reciprocates in the second circular eccentric hole, thereby displacing the eccentric coupling member in the radial guide groove of the intermediate rotor. The first ring member rotates and reciprocates in the first circular eccentric hole by the displacement of the eccentric coupling member. Through the rotation motion of the first ring member, the first control rotor is subjected to a torque which causes the first control rotor to be rotated in the phase advancing direction relative to the drive rotor. 
     Results of the Invention 
     According to the claimed inventions, a compact phase varying device may be obtained which has a wide range of variable phase angle for a camshaft relative to the crankshaft. 
     Although the phase varying mechanism for varying the phase angle between the camshaft and the drive rotor includes a multiplicity of circular eccentric cams, the mechanism has less elements and simpler structure than conventional devices. Thus, the accuracy of the mechanism can be easily achieved. Accordingly, the inventive phase varying device can operate more smoothly than those conventional devices utilizing link arm mechanisms and/or spiral guide grooves. Further, the inventive phase varying device can be easily manufactured at low cost. 
     It is noted that since the inventive mechanism is simpler in structure and has less elements, the mechanism operates smoothly if it did not have a higher accuracy than conventional mechanism that utilize link arms and/or spiral guide grooves. As a result, the phase varying device of the invention can be easily manufactured at low cost. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a phase varying device for automobile engine in accordance with a first embodiment of the invention. 
         FIG. 2  is an exploded perspective view of the device shown in  FIG. 1 . 
         FIG. 3  is an axial cross section of the device shown in  FIG. 1 . 
         FIG. 4  shows radial cross sections of the phase varying device of the first embodiment, set in the phase retarding mode. More particularly,  FIG. 1(   a ) shows a cross section, taken along Line A-A of  FIG. 3 , illustrating an arrangement of the first circular eccentric cam; and  FIG. 1(   b ) a cross section of a second circular eccentric cam, taken along Line B-B of  FIG. 3 , illustrating an arrangement of the second circular eccentric cam in the phase retarding mode. 
         FIG. 5  shows radial cross sections of the phase varying device in accordance with the first embodiment of the invention having an altered phase angle. In particular,  FIG. 5(   a ) shows the cross section taken along Line A-A of  FIG. 3 , and  FIG. 5(   b ) the cross section taken along Line B-B of  FIG. 3 . 
         FIG. 6  shows cross sections, taken along Line B-B of  FIG. 3 , of the second circular eccentric cam set in the phase advancing mode under a given initial condition ( FIG. 6(   a )) and the cross section having a certain change in phase angle ( FIG. 6(   b )). 
         FIG. 7  shows radial cross sections of a reverse mechanism under a given initial condition. More particularly,  FIG. 7(   a ) shows the cross section taken along Line C-C, 
         FIG. 7(   b ) taken along Line D-D,  FIG. 7(   c ) taken along Line E-E of  FIG. 3 . 
         FIG. 8  shows radial cross sections of the reverse mechanism having a certain change in phase angle. More particularly,  FIG. 8(   a ) shows the cross section taken along Line C-C,  FIG. 8(   b ) taken along Line D-D, and  FIG. 8(   c ) taken along Line E-E of FIG.  3 . 
         FIG. 9  is an axial cross section of a phase varying device for automobile engine in accordance with a second embodiment of the invention, having another reverse mechanism. 
         FIG. 10  is an axial cross section of a phase varying device for automobile engine in accordance with a third embodiment of the invention, having a further reverse mechanism. 
     
    
    
     NOTATIONS 
     
         
         
           
               30  phase varying device for automobile engine 
               31  drive rotor 
               33  cam guide member 
               34  first control rotor 
               34   f  first circular eccentric hole 
               35  first electromagnetic clutch (first brake means) 
               36 - 37  sprockets 
               40  drive cylinder 
               41  first circular eccentric cam 
               45  camshaft 
               46  second circular eccentric cam 
               47  radial guide grooves of drive rotor 
               48  grip sections of cam guide member 
               49  oblong hole of cam guide member 
               50  first ring member 
               51  intermediate rotor 
               52  movable coupling member 
               53  second ring member 
               54  second control rotor 
               54   c  second circular eccentric hole 
               56  second brake means 
               57  and  62  reverse mechanism 
               59  torsion coil spring (reverse mechanism) 
               60  control rotor 
               61  drive disc 
               65  phase angle varying mechanism 
               66  torque means 
               67  ring mechanism 
             L 0  rotational axis of camshaft 
             L 3  direction of radial guide groove of drive cylinder 
             L 4  direction of major axis of oblong hole 
           
         
       
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     The invention will now be described in detail by way of example (first through third embodiments) with reference to the accompanying drawings. 
     Each of the phase varying device in accordance with the first through third embodiments of the invention is mounted in an automobile engine. The device is adapted to not only transmit the rotational motion of the crankshaft of the engine to a camshaft so that the intake valves/exhaust valves of the engine are opened/closed in synchronism with the rotational motion of the crankshaft, but also vary the valve timing of the intake valves/exhaust valves in accord with the load and/or rpm of the engine. 
     The first embodiment will be described in detail below with reference to  FIGS. 1  though  8 . A phase varying device  30  of the first embodiment for automobile engine consists of a drive rotor  31 , center shaft  32 , phase angle varying mechanism  65 , and torque means  66 , all arranged coaxial with a rotational axis L 0 . The phase angle varying mechanism  65  consists of a first circular eccentric cam  41 , cam guide member  33 , and second circular eccentric cam  46 . The torque means  66  consists of a first electromagnetic clutch  35  and a reverse mechanism  57 . In what follows the term “front end” of the device refers to the end of the device where a second electromagnetic clutch  56  is provided as shown in  FIG. 2 , and “rear end” refers to the other end where sprockets  36  is provided. The clockwise direction of rotation of the drive rotor  31  as viewed from the front end will be referred to as direction D 1  or phase advancing direction, while the counterclockwise direction will be referred to as direction D 2  or phase retarding direction. 
     It is now supposed that under the initial condition the center shaft  32 , cam guide member  33 , and first control rotor  34  are in rotation together with the drive rotor  31  driven by the crankshaft (not shown) in direction D 1  about the rotational axis L 0 . 
     The drive rotor  31  consists of two sprockets ( 36 ,  37 ) and a drive cylinder  40 . Formed at the centers of the sprockets  36  and  37  are circular holes  36   a  and  37   a , respectively. Provided inside and near the rear open end of the circular hole  37   a  is an inner flange  37   b . Reference numeral  37   c  indicates a circular hole formed on the inside of the inner flange  37   b , in which a multiplicity of disc springs  42  are coaxially stacked in the axial direction L 0 . Each of the disc spring  42  has a circular hole  42   a . Fitted from front into the circular hole  37   a  is a holder  43  having at the center thereof a circular hole  43   a.    
     On the other hand, the drive cylinder  40  is an integral body that includes a circular cylindrical section  40   a  and a bottom section  40   b . Formed in the bottom section  40   b  are a circular hole  40   c  and a pair of guide grooves  47  extending in substantially radial directions (the grooves referred to as radial guide grooves). The circular hole  40   c  is located at the center of the bottom section  40   b , and a middle cylinder  32   b  of the center shaft  32  is passed through the hole as described in detail below. The paired radial guide grooves  47  are symmetrically arranged across the circular hole  40   c . In what follows a phantom extension line passing through the rotational axis L 0  of the drive cylinder  40  and extending along the radial grooves  47  will be referred to as extension line L 3  ( FIG. 4 ). 
     The sprocket  36  is integrated with the sprocket  37  by means of coupling pins  38  inserted in a multiplicity of pin holes  36   b . The sprocket  37  is then integrated with the drive cylinder  40  by means of coupling pins  39  inserted in a multiplicity of pin holes  37   d  formed in the sprocket  37  and pin holes  40   d  formed in the drive cylinder  40 . 
     Thus, the center shaft  32  comprises a sequence of a small cylinder  32   a  followed by the middle cylinder  32   b , second circular eccentric cam  46 , and a large cylinder  32   c  arranged along the rotational axis L 0 . The outer diameter of the large cylinder  32   c  is substantially the same as the inner diameters of the circular holes  36   a ,  42   a , and  43   a . The second circular eccentric cam  46  has a center axis center axis L 2  offset from the rotational axis L 0  of the center shaft  32  by a distance d 2 , and eccentrically rotates about the L 0  together with the center shaft  32 . 
     By inserting the drive rotor  31  in the circular hole  36   a ,  42   a , and circular hole  43   a  of the center shaft  32 , the drive rotor  31  is rotatably supported by the center shaft  32 . The center shaft  32  is provided at the center thereof with a bolt insertion hole  32   d  and at the rear end with a coupling hole  32   e . There is provided a camshaft  45  having a cylindrical section  45   a  at the leading end thereof and a flange section  45   b  contiguous with the cylindrical section  45   a . By inserting the cylindrical section  45   a  of the camshaft  45  into the coupling hole  32   e  with the drive rotor  31  supported by the large cylinder  32   c , the center shaft  32  is coupled to the camshaft  45 , and is securely fixed to the camshaft  45  by tightening bolts  44  inserted in the threaded sections (not shown) of the camshaft  45  through the bolt insertion hole  32   d  from front. The drive rotor  31  is arranged between the second circular eccentric cam  46  and flange section  45   b  of the camshaft  45 , and is rotatable about the center axis L 0  relative to the camshaft  45 . 
     On the other hand, the cam guide member  33  has a pair of grip sections  48  and an oblong hole  49 . The paired grip sections  48  are formed across the rotational axis L 0  to project forward from the front end of the outer circumference of the cam guide member  33 . Further, the grip sections  48  have substantially the same width as those of the radial grooves  47  of the drive cylinder  40 , and are spaced apart from each other by the same distance as that of the radial grooves  47 . The oblong hole  49  is oblong in the direction L 4  perpendicular to the line that connects the grip sections  48  ( FIG. 4(   b )). The oblong hole  49  receives the second circular eccentric cam  46  such that the upper and lower end of the outer circumferential surface of the  46  is in sliding contact with the inner circumferential surface of the oblong hole  49 . 
     The cam guide member  33  is arranged between the sprocket  37  and the drive cylinder  40  and is supported by the center shaft  32  via the second circular eccentric cam  46  inserted in the oblong hole  49 . The grip sections  48  are engaged with the radial grooves  47 , with the leading ends of the grip sections  48  projecting forward from the radial grooves  47 . As the second circular eccentric cam  46  undergoes an eccentric rotation, the grip sections  48  are moved in the radial grooves  47  and in radial directions of the drive cylinder  40 . 
     The first control rotor  34  has a circular form having an outer diameter substantially the same as the inner diameter of the inner circumference  40   e  of the cylinder  40  so that the first control rotor  34  can be fitted in the circular cylindrical section  40   a  of the cylinder  40 . The first control rotor  34  is rotatable about the rotational axis L 0  relative to the drive cylinder  40  with its outer circumferential surface  34   a  supported in the inner circumferential surface  40   e  of the cylinder  40   e . The first control rotor  34  is provided with a circular hole  34   b  for passing therethrough the middle cylinder  32   b  of the center shaft  32  and the first circular eccentric cam  41 . 
     The first circular eccentric cam  41  is formed on the rear face of the first control rotor  34  to project rearward therefrom. The first circular eccentric cam  41  has a enter axis L 1  (eccentric center) which is offset from the center axis L 0  of the first control rotor  34  by a distance d 1 , whereby the first circular eccentric cam  41  eccentrically rotates about the rotational axis L 0  together with the first control rotor  34 . The first circular eccentric cam  41  is gripped by the grip sections  48  projecting from the radial guide grooves  47 , in slidable contact with the radially inner surface of the grip sections  48 . 
     Under the initial condition (prior to any phase change), the eccentric center of the first circular eccentric cam  41  (or the center axis L 1  of the cam) is located at an inclined position angularly offset from the upward extension line L 3  in the counterclockwise direction D 2 , as shown in  FIG. 4 . The grip sections  48  of the cam guide member  33  are arranged in the respective radial grooves  47  such that, in the example shown herein, the upper one is in slidable contact with a stopper  47   a  formed at the upper end of the upper radial groove  47 . 
     On the other hand, the eccentric center (center axis L 2 ) of the second circular eccentric cam  46  is initially located at a position which is either angularly offset in the counterclockwise direction D 2  with respect to the upward extension line L 3 , just like the center axis L 1  of the first circular eccentric cam  41  ( FIG. 4(   b ), or alternatively in the clockwise direction D 1  unlike the center axis L 1  ( FIG. 6(   a )). 
     If the center axis L 2  of the second circular eccentric cam  46  is angularly offset in the direction D 2  with respect to the upward extension line L 3  like the center axis L 1  as shown in  FIG. 4(   b ), the phase angle of the camshaft  45  is altered from its initial phase angle in the phase retarding direction D 2  relative to the drive rotor  31 . But if the center axis L 2  is offset in the opposite direction (that is, phase advancing direction D 1 ) with respect to the upward extension line L 3  as shown in  FIG. 6(   a ), the phase angle of the camshaft  45  is altered from its initial phase angle in the phase advancing direction D 1 . (In what follows an initial arrangement of the center axis L 2  inclined in the same direction as the center axis L 1  will be referred to as phase retarding mode, while the arrangement of the center axis L 2  inclined in the direction opposite to the center axis L 1  referred to as phase advancing mode.) Thus, in the embodiments shown herein, the phase retarding mode and phase advancing mode can be switched over by simply changing the initial angular positions of the first circular eccentric cam  41  and second circular eccentric cam  46 , or changing the inclination of the center axis L 2  relative to the extension line L 3 . 
     The first electromagnetic clutch  35  and reverse mechanism  57  are arranged ahead of the first control rotor  34 . The first electromagnetic clutch  35  is securely fixed to the engine casing (not shown), facing the front face (or contact face  34   c ) of the first control rotor  34 . When a coil  35   a  is energized, the first electromagnetic clutch  35  will attract and bring the contact face  34   c  of the first control rotor  34  in rotation with the drive rotor  31  into sliding contact with the friction member  35   b.    
     While the contact face  34   c  is in sliding contact with the friction member  35   b , the first control rotor  34  is retarded with respect to the drive rotor  31 , that is, the first control rotor  34  is rotated in the phase advancing direction D 2  relative to the drive rotor  31  ( FIGS. 2 and 4 ). On the other hand, when the reverse mechanism  57  is enabled in the manner described below, the first control rotor  34  is rotated in the phase advancing direction D 1 , which is the opposite rotational direction as compared with that brought by the first electromagnetic clutch  35 . 
     The reverse mechanism  57  comprises a second control rotor  54 , a ring mechanism  67 , and the second electromagnetic clutch  56 . The ring mechanism  67  comprises, in addition to the second control rotor  54 , a first ring member  50  disposed in the stepped circular hole  34   d  formed in the front end of the first control rotor  34 , an intermediate rotor  51 , a movable member  52 , and a second ring member  53  disposed in the circular stepped hole  54   c  formed in the rear end of the second control rotor  54 . 
     The first control rotor  34  has in the front end thereof the stepped circular hole  34   d . Formed in the bottom section  34   e  of the stepped circular hole  34   d  is a stepped first circular eccentric hole  34   f . The first circular eccentric hole  34   f  has a center O 1  offset from the rotational axis L 0  of the center shaft  32  by a distance d 3 . The first ring member  50  has an outer diameter which is substantially equal to the inner diameter of the first circular eccentric hole  34   f , and is slidably engaged with the first inner circumference of the first circular eccentric hole  34   f . The first ring member  50  is formed with a first engagement hole  50   a  that is open in the forward direction. 
     The intermediate rotor  51  is provided at the center thereof with a square hole  51   a , and outside the square hole  51   a  with a guide groove  51   b  extending in a substantially radial direction (the groove referred to as radial guide groove). A phantom line that extends through the rotational axis L 0  of the intermediate rotor  51  and along the radial grooves  51   b  will be referred to as extension line L 5 . The intermediate rotor  51  is unrotatably fixed to the center shaft  32  by fitting the flat engaging faces  32   f  and  32   g  of the center shaft  32  in the square hole  51   a.    
     The second control rotor  54  has a central circular hole  54   a  and a second circular eccentric bore  54   c  formed in the rear end of the control rotor  54 . The second control rotor  54  is rotatably mounted on the center shaft  32  via the small cylinder  32   a  inserted in the circular hole  54   a . The second circular eccentric bore  54   c  is eccentrically offset from the rotational axis L 0  by a distance d 4 , like the center O 2  of the second circular eccentric bore  54   c . The second ring member  53  has an outer diameter which is substantially the same as the inner diameter of the second circular eccentric bore  54   c , and slidably engaged in the second circular eccentric bore  54   c . The second ring member  53  is provided in the rear end thereof with a second engagement bore  53   a . The first and second ring members  50  and  53 , respectively, are arranged such that their centers O 1  and O 2  are located on the opposite sides of the extension line L 5 . 
     The movable member  52  consists of a central thin cylindrical shaft  52   a  coaxially inserted in a thick hollow cylindrical shaft  52   b . The opposite ends of the thin cylindrical shaft  52   a  are slidably engaged with the first and second engagement holes  50   a  and  53   a  of the first and second ring members  50  and  53 , respectively, to couple them together. The thick hollow cylindrical shaft  52   b  is movable in the radial grooves  51   b.    
     Arranged at the leading end of the small cylinder  32   a  of the  32  projecting from the circular hole  54   a  is a holder  55 . As shown in  FIG. 2 , except for the center shaft  32 , all the elements between the holder  55  and the sprocket  36  inclusive are fixedly mounted on the camshaft  45  by means of a bolt  44 , which is passed through the central holes of the respective elements from front and then tightened ( FIG. 3 ). 
     The second electromagnetic clutch  56  is mounted on the engine casing (not shown), facing the front end of the second control rotor  54 . When the coil  56   a  of the second electromagnetic clutch  56  is energized, the clutch  56  attracts the contact face  54   b  of the second control rotor  54  and brings it into contact with the friction member  56   b , thereby putting a brake on the second control rotor  54 . 
     Incidentally, the movable member  52  may be equipped with bearings, or may be replaced by balls. In that case, since the movable member  52  can roll in the radial grooves  51   b  in the radial direction with less friction, energy consumption by the electromagnetic clutches  35  and  56  will be reduced. The intermediate rotor  51  is preferably formed of a non-magnetic material. If the intermediate rotor  51  is made of a non-magnetic material, the magnetic force for attracting one of the control rotors  34  and  54  is not transmitted to the other one, so that it is possible to avoid a problem that both of the first and second control rotors  34  and  54 , respectively, will be simultaneously attracted by one electromagnetic clutch. 
     Next, referring to  FIGS. 2 through 7 , operations of the phase varying device  30  in accordance with the first embodiment will now be described. Under the braking action of the first electromagnetic clutch  35 , the first control rotor  34  is retarded in rotation, that is, rotated in the counter clockwise direction D 2 , relative to the drive rotor  31 , center shaft  32  and cam guide member  33 . 
     The first circular eccentric cam  41  shown in  FIG. 4(   a ) undergoes an eccentric rotation together with the first control rotor  34  in the counterclockwise direction D 2  about the rotational axis L 0 . The grip sections  48  of the cam guide member  33  are displaced in the downward direction D 3  in the radial grooves  47  by the first circular eccentric cam  41  which is in sliding contact with the radially inner surfaces of the grip sections. The cam guide member  33  is moved in the downward direction D 3  together with the grip sections  48 . Operations of the elements of the device up to this point are the same both in the phase retarding mode and phase advancing mode. 
     As the cam guide member  33  is moved downward in the phase retarding mode, the second circular eccentric cam  46  is acted upon by a force exerted by the wall of the oblong hole  49  which lowers simultaneously with the cam guide member  33 , and is eccentrically rotated in the counterclockwise direction D 2  as shown in  FIG. 4(   b ). Since the center shaft  32  (or camshaft  45 ) is integral with the second circular eccentric cam  46 , it is rotated in the direction D 2  relative to the drive rotor  31 . As a result, the phase angle of the camshaft  45  relative to the drive rotor  31  (or the crankshaft) is altered from the initial angular position to a new phase retarded position offset therefrom in the counterclockwise direction D 2 . 
     On the other hand, in the phase advancing mode, if the cam guide member  33  is moved downward as shown in  FIG. 6(   a ), the second circular eccentric cam  46 , initially set to a phase advancing position, is rotated eccentrically in the clockwise direction D 1 , contrary to the phase retarding mode. As a consequence, the center shaft  32  (or camshaft  45 ) is rotated in the direction D 1  relative to the drive rotor  31 . Accordingly, the phase angle of the camshaft  45  relative to the drive rotor  31  (or crankshaft) is changed from the initial angular position to a new position offset therefrom in the clockwise phase advancing direction D 1 . 
     On the other hand, to reduce the change in phase angle (that is, to bring back the angular position to a position closer to the initial angular position), the reverse mechanism  57  is enabled to rotate the first control rotor  34  in the phase advancing direction D 1  relative to the drive rotor  31 . 
     Specifically, the second electromagnetic clutch  56  shown in  FIG. 2  is enabled to put a brake on the second control rotor  54  shown in  FIG. 7(   a ), thereby causing the second control rotor  54  to be rotated in the phase retarding direction D 2  relative to the intermediate rotor  51  and first control rotor  34 . The second ring member  53  then slidably rotates within the second circular eccentric bore  54   c  in the direction D 1 , thereby displacing the movable member  52  downward in the radial grooves  51   b  (in the direction D 3  shown in  FIG. 7(   b )). As the movable member  52  is displaced in the direction D 3 , the first ring member  50  shown in  FIG. 7(   c ) is slidably rotated in the direction D 2  within the first circular eccentric hole  34   f , generating a torque that causes the first control rotor  34  to be rotated in the direction D 1 . As a consequence, the first control rotor  34  is rotated in the phase advancing direction D 1  relative to the intermediate rotor  51  and second control rotor  54 , contrary to the case where the first electromagnetic clutch  35  is enabled. 
     As the first control rotor  34  rotates in the phase advancing direction D 1  relative to the drive rotor  31 , the first circular eccentric cam  41  is eccentrically rotated in the clockwise direction D 1  about the rotational axis L 0  as shown in  FIG. 5(   a ), thereby moving the grip sections  48  and cam guide member  33  in the upward direction D 4  in the radial grooves  47 . In the phase retarding mode, the second circular eccentric cam  46  (center shaft  32 ) shown in  FIG. 5  is rotated in the phase advancing direction D 1  relative to the drive rotor  31  as the cam guide member  33  moves upward. As a consequence, the crankshaft is returned relative to the drive rotor  31  towards its initial angular position, reducing its phase angle relative to the drive rotor  31 . On the other hand, in the phase retarding mode, the second circular eccentric cam  46  (or center shaft  32 ) is rotated in the phase retarding direction D 2  relative to the drive rotor  31  as the cam guide member  33  moves upward, as shown in  FIG. 6(   b ). As a consequence, the phase angle of the crankshaft relative to the drive rotor  31  is reduced, that is, the crankshaft is returned towards its initial angular position. 
     Referring to  FIG. 9 , there is shown a phase varying device in accordance with a second embodiment of the invention. The phase varying device of the second embodiment has the same structure as the first embodiment except that the reverse mechanism  57  of the first is replaced with a torsion coil spring  59  in the second embodiment. 
     Thus, the reverse mechanism is simple in structure. The torsion coil spring  59  has one end  59   a  securely fixed to the drive cylinder  40  and the other end  59   b  fixed to the first control rotor  34 . The first control rotor  34  constantly urges the first control rotor  34  in the direction D 1  opposite to the rotational direction (phase retarding direction D 2  in  FIG. 2 ) of the braking torque exerted by the first electromagnetic clutch  34 . 
     The first control rotor  34 , which rotates together with the drive cylinder  40 , is rotated in the phase retarding direction D 2  relative to the drive cylinder  40  if it is subjected to a braking torque exerted by the first electromagnetic clutch  35  that exceeds the urging toque exerted by the torsion coil spring  59 , thereby changing the phase angle of the center shaft  32  (camshaft  45 ) in a predetermined direction (either in the phase advancing direction D 1  or phase retarding direction D 2 ) relative to the drive rotor  31 . The rotational motion of the first control rotor  34  relative to the drive cylinder  40  is stopped at a position (referred to as balancing position of the first control rotor  34 ) where the urging torque of the torsion coil spring  59  acting on the first control rotor  34  balances out the braking torque of the first electromagnetic clutch  35 . Since the phase angle of the camshaft  45  relative to the drive rotor  31  is determined by the balancing position of the first control rotor  34 , the phase angle can be adjusted by controlling the amount of electricity supplied to the first electromagnetic clutch  35 . 
     On the other hand, if the first electromagnetic clutch  35  is disabled, the first control rotor  34  is rotated in the phase advancing direction D 1  relative to the drive cylinder  40  until it returns to its initial angular position by the urging torque of the torsion coil spring  59 . 
     Incidentally, the camshaft  45 , which is in rotation together with the crankshaft (not shown), is periodically subjected to reactive forces of the valve springs (not shown). Such reactive forces generate torques (hereinafter referred to as external disturbing torques) that cause the camshaft  45  to be rotated in either the phase advancing direction D 1  or the phase retarding direction D 2  relative to the drive rotor  31 . Any of these external disturbing torques can arise an unexpected change in relative phase angle between the drive rotor  31  and camshaft  45 . 
     It should be appreciated that the phase varying devices in accordance with the first and second embodiments have a self locking mechanism for preventing such unexpected phase change caused by an external disturbing torque in that the camshaft  45  is rendered inoperative or locked relative to the drive rotor  31  when subjected to an external disturbing torque. 
     The self locking mechanism will now be described in detail below. An external disturbing torque exerted by the valve springs on the camshaft  45  is transmitted to the second circular eccentric cam  46  as an eccentric torque acting on the cam  46 . As the second circular eccentric cam  46  in the oblong hole  49  is subjected to such torque, the cam guide member  33  is acted upon by a force in the direction along the extension line L 3 , since the grip sections  48  are guided by the radial guide grooves  47  of the drive cylinder  40 . The first circular eccentric cam  41  integral with the first control rotor  34  is acted upon by a force exerted by the grip sections  48  in the direction of the extension line L 3  passing through the rotational axis L 0  at a right angle. 
     As a consequence, when an external disturbing torque acts on the camshaft  45 , the first control rotor  34  is acted upon by a force in the direction perpendicular to the rotational axis L 0 , so that the outer circumferential surface  34   a  of the first control rotor  34  comes into frictional contact with the inner circumferential surface  40   e  of the drive cylinder  40 , thereby generating a frictional force that renders the first control rotor  34  unrotatable, or self-locked, relative to the drive cylinder  40 . 
     If the first control cylinder  34  and drive cylinder  40  are unrotatably locked to each other, the first circular eccentric cam  41 , cam guide member  33 , and second circular eccentric cam  46  become altogether unrotatable or locked, thereby preventing a further change in the phase angle between the camshaft  45  and the drive rotor  31 . 
     For this reason, it is preferable to provide a certain clearance between the outer circumferential surfaces of the middle cylinder  32   b  of the center shaft  32  and the inner circumferential surfaces of the circular holes  34   b  and  40   c  of the first control rotor  34  and drive cylinder  40 , respectively. Otherwise, in the event where such self-locking should takes place, the inner circumferential surface of the circular hole  34   b  of the first control cylinder  34  comes into contact with the outer circumferential surface of the middle cylinder  32   b  and is subjected to a rotational force (torque) that acts on the center shaft  32  before the outer circumferential surface  34   a  comes into touch with the inner circumferential surface  40   e  of the cylinder. Such torque will weaken the local frictional force generated by the outer circumferential surface  34   a  of the first control rotor  34 . To avoid this, a certain clearance is favored between the outer circumferential surface of the middle cylinder  32   b  and the respective inner circumferential surfaces of the circular holes  34   b  and  40   c.    
     Referring to  FIG. 10 , there is shown a phase varying device in accordance with a third embodiment of the invention. This phase varying device is the same in structure as the second embodiment, except that the first control rotor  34  and the drive rotor  40  of the second embodiment shown in  FIG. 9  are replaced by a control rotor  60  having a different configuration than that of the first control rotor  34  and a drive disc  61 , and that the torsion coil spring  59  is removed in the third embodiment. The phase varying device of the third embodiment has a reverse mechanism  62  which consist of the control rotor  60  and the drive disc  61 . The control rotor  60  is rotatably mounted on the middle cylinder  32   b  of the center shaft  32  inserted in the circular hole  60   b . The drive disc  61  is obtained from the drive cylinder  40  by removing the cylinder  40   b . The reverse mechanism  62  is adapted to rotate the control rotor  60  in the direction D 2  relative to the drive rotor  31  as shown in  FIG. 2  by utilizing an external disturbing torque exerted on the camshaft  45 . In operation, the reverse mechanism functions as follows. 
     The drive disc  61  has the same shape as the drive cylinder  40  shown in  FIG. 9  with the cylinder  40   b  removed therefrom. The drive disc  61  does not have such an inner circumferential surface as the inner circumferential surface  40   e  for supporting the outer circumferential surface  34   a  of the control rotor  34  of the second embodiment. As a consequence, the control rotor  60  is rotatably supported by the middle cylinder  32   b  of the center shaft  32  inserted in the central circular hole  60   b.    
     It is noted that a self-locking mechanism is not provided between the first control rotor  34  and drive disc  61 . Now that the drive disc  61  does not have an inner circumferential surface like the inner circumferential surface  40   e  of the first embodiment on which the control rotor  60  can abut, no self-lock function takes place on the outer circumferential surface  60   a  of the control rotor  60  if an external disturbing torque is applied to the camshaft  45 . As a consequence, the control rotor  60  is subjected to torques that arise from external disturbing torques and act on the camshaft  45 . These torques (referred to as relative rotational torques) tend to rotate the control rotor  60  relative to the drive disc  61 . 
     Since the relative rotation torques externally transmitted from valve springs (not shown) appear to pulsate on the camshaft  45  in synchronism with the engine rotation, they acts on the control rotor  60  both in the phase advancing direction and phase retarding direction alternately. However, the relative rotational torques are larger when they appear in the direction D 1  than in the direction D 2 . As a consequence, upon receipt of an external disturbance from the camshaft  45 , the control rotor  60  is rotated in the phase advancing direction D 1  relative to the crice disc  61 . 
     As a result, the control rotor  60  is rotated in the phase retarding direction D 2  relative to the drive disc  61  if it is acted upon by a braking force of the first electromagnetic clutch  35  in excess of the external torque acting in the direction D 1 . If the first electromagnetic clutch  35  is disabled, the control rotor  60  undergoes a relative rotation in the phase advancing direction D 1  by the external disturbing torques. The relative rotation of the control rotor  60  relative to the driver disc  61  will be stopped at a point where the braking torque of the first electromagnetic clutch  35  balances out the external disturbing torque. The camshaft  45  is rotated relative to the drive rotor  31  by the first electromagnetic clutch  35  in either the phase advancing direction D 1  or phase retarding direction D 2  to change the phase angle of the camshaft  45 , and rotated by the external disturbing torque in the direction opposite to that caused by the first electromagnetic clutch  35 . Thus, the phase angle of the camshaft  45  is fixed by balancing out the braking torque of the electromagnetic clutch with the external disturbing torque.