Patent Publication Number: US-8985075-B2

Title: Valve timing control system of internal combustion engine

Description:
TECHNICAL FIELD 
     The present invention relates to a valve timing control system of an internal combustion engine for variably controlling valve timings (i.e., valve open timing and valve closure timing) of intake and exhaust valves. 
     BACKGROUND ART 
     A valve timing control system, which is configured to change an angular phase of a camshaft relative to a timing sprocket by virtue of hydraulic pressure, is generally known. In recent years, there have been proposed and developed various valve timing control systems in which an angular phase of a camshaft relative to a timing sprocket that is configured to rotate in synchronism with rotation of an engine crankshaft is changed by transmitting rotary motion (torque) of an electric motor through a speed reducer to the camshaft, so as to variably control intake-valve timing and exhaust-valve timing. 
     One such valve timing control system has been disclosed in Japanese Unexamined Patent Application Publication No. 2006-207398 (hereinafter is referred to as “JP2006-207398”), corresponding to U.S. Pat. No. 7,603,223, issued on Oct. 13, 2009. In the valve timing control system disclosed in JP2006-207398, two electric-motor-driven valve timing control devices are mounted respectively on the intake camshaft and the exhaust camshaft. 
     SUMMARY OF THE INVENTION 
     In the valve timing control system as disclosed in JP2006-207398, the intake valve timing control device tends to frequently operate over the entire engine operating range after the internal combustion engine has been started. In contrast, in the case of the exhaust valve timing control device, valve timing (an angular phase of the exhaust camshaft relative to the sprocket) is often held constant within an engine operating range except middle engine speeds. Therefore, the intake valve timing control device requires the improved operational responsiveness to a valve-timing change (an angular phase shift of the intake camshaft relative to the sprocket), whereas the exhaust valve timing control device requires the improved phase holding performance for a phase angle of the exhaust camshaft relative to the sprocket. 
     However, in the case of the valve timing control system disclosed in JP2006-207398, the speed reducers are the same in the intake valve timing control device and the exhaust valve timing control device. For the reasons discussed above, assuming that a higher priority is put on the operational responsiveness, the phase holding performance tends to deteriorate. Conversely, assuming that a higher priority is put on the phase holding performance, the operational responsiveness tends to deteriorate. There is a problem that two contradictory requirements (i.e., the improved operational responsiveness and the improved phase holding performance) cannot be balanced. 
     Accordingly, it is an object of the invention to provide a valve timing control system of an internal combustion engine, configured to reconcile and balance two contradictory requirements, that is, the improved operational responsiveness of an intake valve timing control device and the improved phase holding performance of an exhaust valve timing control device. 
     In order to accomplish the aforementioned and other objects of the present invention, a valve timing control system of an internal combustion engine, comprises an electric-motor-driven intake valve timing control device installed on an intake camshaft, the intake valve timing control device comprising a first electric motor provided to generate torque by energizing the first electric motor, and a first speed reducer configured to reduce a rotational speed of the first electric motor, and transmit the reduced rotational speed to the intake camshaft for changing intake valve timing, and an electric-motor-driven exhaust valve timing control device installed on an exhaust camshaft, the exhaust valve timing control device comprising a second electric motor provided to generate torque by energizing the second electric motor, and a second speed reducer configured to reduce a rotational speed of the second electric motor, and transmit the reduced rotational speed to the exhaust camshaft for changing exhaust valve timing, wherein the first speed reducer of the intake valve timing control device is configured to have a friction less than a friction of the second speed reducer of the exhaust valve timing control device. 
     According to another aspect of the invention, a valve timing control system of an internal combustion engine, comprises an electric-motor-driven intake valve timing control device installed on an intake camshaft, the intake valve timing control device comprising a first electric motor provided to generate torque by energizing the first electric motor, and a first speed reducer having a first toothed gear configured to reduce a rotational speed of the first electric motor, and transmit the reduced rotational speed to the intake camshaft for changing intake valve timing, and an electric-motor-driven exhaust valve timing control device installed on an exhaust camshaft, the exhaust valve timing control device comprising a second electric motor provided to generate torque by energizing the second electric motor, and a second speed reducer having a second toothed gear configured to reduce a rotational speed of the second electric motor, and transmit the reduced rotational speed to the exhaust camshaft for changing exhaust valve timing, wherein the first speed reducer of the intake valve timing control device is configured to transmit torque by repeated relocations of each of rolling elements rolling and relocating from one of two adjacent teeth of the first toothed gear to the other, and the second speed reducer of the exhaust valve timing control device is configured to transmit torque by meshed-engagement of the second toothed gear with another toothed gear. 
     According to a further aspect of the invention, a valve timing control system of an internal combustion engine, comprises an electric-motor-driven intake valve timing control device installed on an intake camshaft, the intake valve timing control device comprising a first electric motor provided to generate torque by energizing the first electric motor, and a first speed reducer configured to reduce a rotational speed of the first electric motor, and transmit the reduced rotational speed to the intake camshaft for changing intake valve timing, and an electric-motor-driven exhaust valve timing control device installed on an exhaust camshaft, the exhaust valve timing control device comprising a second electric motor provided to generate torque by energizing the second electric motor, and a second speed reducer configured to reduce a rotational speed of the second electric motor, and transmit the reduced rotational speed to the exhaust camshaft for changing exhaust valve timing, wherein a cogging torque of the first electric motor of the intake valve timing control device is set to be less than a cogging torque of the second electric motor of the exhaust valve timing control device. 
     According to a still further aspect of the invention, a valve timing control system of an internal combustion engine, comprises an electric-motor-driven intake valve timing control device installed on an intake camshaft, the intake valve timing control device comprising a first electric motor provided to generate torque by energizing the first electric motor, and a first speed reducer configured to reduce a rotational speed of the first electric motor, and transmit the reduced rotational speed to the intake camshaft for changing intake valve timing, and an electric-motor-driven exhaust valve timing control device installed on an exhaust camshaft, the exhaust valve timing control device comprising a second electric motor provided to generate torque by energizing the second electric motor, and a second speed reducer configured to reduce a rotational speed of the second electric motor, and transmit the reduced rotational speed to the exhaust camshaft for changing exhaust valve timing, wherein the first electric motor of the intake valve timing control device is constructed by a brushless motor, and the second electric motor of the exhaust valve timing control device is constructed by a brush-equipped direct-current motor. 
     The other objects and features of this invention will become understood from the following description with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a plan view illustrating the essential part of the first embodiment of a valve timing control system. 
         FIG. 2  is a view taken in the direction of the arrow A of  FIG. 1 . 
         FIG. 3  is a longitudinal cross-sectional view illustrating an intake valve timing control (VTC) device of the first embodiment. 
         FIG. 4  is a perspective disassembled view illustrating major component parts constructing the VTC device of the first embodiment. 
         FIG. 5  is a lateral cross section taken along the line B-B of  FIG. 3 . 
         FIG. 6  is a lateral cross section taken along the line C-C of  FIG. 3 . 
         FIG. 7  is a lateral cross section taken along the line D-D of  FIG. 3 . 
         FIG. 8  is a longitudinal cross-sectional view illustrating an exhaust VTC device of the first embodiment. 
         FIG. 9  is a lateral cross section taken along the line E-E of  FIG. 8 . 
         FIG. 10  is a lateral cross section taken along the line F-F of  FIG. 8 . 
         FIG. 11  is a characteristic diagram illustrating the difference between a friction of the intake VTC device and a friction of the exhaust VTC device in the first embodiment. 
         FIG. 12  is a plan view illustrating the essential part of the second embodiment of a valve timing control system. 
         FIG. 13  is a characteristic diagram, illustrating the difference between a cogging torque of an electric motor of the intake VTC device and a cogging torque of an electric motor of the exhaust VTC device in the third embodiment. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Embodiment 
     Referring now to the drawings, particularly to  FIGS. 1-2 , the valve timing control system of the first embodiment includes an intake camshaft  02  rotatably supported on a cylinder head  01  through camshaft-journal bearing members  06  fixedly connected onto the upper deck of cylinder head  01 , an exhaust camshaft  03  rotatably supported on the cylinder head  01  through the camshaft-journal bearing members  06  and arranged parallel to the intake camshaft  02 , an electric-motor-driven intake valve timing control device (hereinafter referred to as “intake VTC”)  04  installed on the front end of intake camshaft  02 , and an electric-motor-driven exhaust valve timing control device (hereinafter referred to as “exhaust VTC”)  05  installed on the front end of exhaust camshaft  03 . 
     Each of camshaft-journal bearing members  06  is made from aluminum alloy. The front-end camshaft-journal bearing member  06  is formed integral with a chain cover  07  configured to partially cover both the intake VTC  04  and the exhaust VTC  05 . A cover member  3  is bolted to a part of the chain cover  07  on the side of intake VTC  04  for hermetically covering the front end of intake VTC  04 . 
     [Intake VTC] 
     As shown in  FIGS. 3-4 , the above-mentioned intake VTC  04  is comprised of a sprocket  1  (serving as a driving rotary member) that rotates in synchronism with rotation of an engine crankshaft, and a phase change mechanism (a phase converter)  2  (see  FIG. 3 ) installed between the sprocket  1  and the intake camshaft  02  for changing a relative angular phase between the sprocket  1  and the intake camshaft  02  depending on an engine operating condition. 
     Sprocket  1  is comprised of an annular sprocket body  1   a , a timing gear  1   b  formed integral with the outer periphery of sprocket body  1   a , and an internal-tooth structural member  19 . Sprocket body  1   a  is made from iron-based metal material, and formed with a stepped inner peripheral portion and formed integral with the timing gear  1   b . Timing gear  1   b  receives torque from the crankshaft through a timing chain (not shown) wound on both a sprocket on the crankshaft and the sprocket  1  on the intake camshaft. Internal-tooth structural member  19  is formed integral with the front end of sprocket body  1   a.    
     Also, sprocket  1  is rotatably supported by a large-diameter ball bearing  43  interleaved between the sprocket body  1   a  and a driven rotary member, simply, a driven member  9  (described later) fixedly connected to the front end of intake camshaft  02 , so as to permit rotary motion of intake camshaft  02  relative to sprocket  1 . 
     Large-diameter ball bearing  43  is comprised of an outer ring  43   a , an inner ring  43   b , and balls  43   c  confined between outer and inner rings  43   a - 43   b . The outer ring  43   a  is fixed to the inner periphery of sprocket body  1   a , whereas the inner ring  43   b  is fixed to the outer periphery of driven member  9  (described later). 
     Sprocket body  1   a  has an outer-ring retaining annular groove  60  formed and cut in its inner peripheral surface. Outer-ring retaining annular groove  60  is formed as a shouldered annular groove into which the outer ring  43   a  of large-diameter ball bearing  43  is axially press-fitted. The shouldered portion of outer-ring retaining annular groove  60  serves to position one axial end face (i.e., a forward end face, viewing  FIG. 3 ) of the outer ring  43   a  in place. 
     Internal-tooth structural member  19  is formed integral with the circumference of the front end of sprocket body  1   a , and formed into a cylindrical shape extended toward an electric motor  12  (described later) of phase converter  2 . Internal-tooth structural member  19  is formed on its inner periphery with a plurality of waveform internal teeth  19   a . The annular rear end face of an annular female screw-threaded member  6 , formed integral with a housing  5  (described later), and the annular front end face of internal-tooth structural member  19  are arranged to be axially opposed to each other. 
     An annular retainer plate  61  is located at the rear end of sprocket body  1   a , facing apart from the internal-tooth structural member  19 . Retainer plate  61  is made from a metal plate. As shown in  FIG. 3 , the outside diameter of retainer plate  61  is dimensioned to be approximately equal to that of the sprocket body  1   a . The inside diameter of retainer plate  61  is set or dimensioned to be less than the inside diameter of the outer ring  43   a  of ball bearing  43  and also dimensioned to be approximately equal to the outside diameter of the inner ring  43   b  of ball bearing  43 . 
     Hence, the inner peripheral portion  61   a  (see  FIG. 4 ) of retainer plate  61  is arranged to be axially opposed to the rearward end face  43   e  of the outer ring  43   a  of ball bearing  43  with a given clearance space in such a manner as to cover the rearward end face  43   e  of the outer ring  43   a . Also, the inner peripheral portion  61   a  of annular retainer plate  61  has a radially-inward protruding stopper  61   b  integrally formed at a given circumferential angular position of the inner peripheral portion  61   a.    
     As seen in  FIG. 6 , the radially-inward protruding stopper  61   b  is formed into a substantially sector. The innermost edge  61   c  of stopper  61   b  is configured to be substantially conformable to a shape of the circular-arc peripheral surface of a stopper groove  02   b  (described later) of the front end of camshaft  02 . The outer peripheral portion of retainer plate  61  is formed with circumferentially equidistant-spaced, six bolt insertion holes  61   d  (through holes) through which bolts  7  are inserted. 
     Furthermore, an annular spacer  62  is interleaved between the inside face (the left-hand side face) of retainer plate  61  and the rearward end face  43   e  of the outer ring  43   a  of ball bearing  43 . Spacer  62  is provided for applying a slight push from the inside face of retainer plate  61  to the rearward end face  43   e  of the outer ring  43   a , when the annular female screw-threaded member  6  (housing  5 ), the sprocket  1 , and the retainer plate  61  are integrally connected to each other by fastening them together with bolts  7 . 
     In a similar manner to the six bolt insertion holes  61   d  (through holes) formed in the retainer plate  61 , the outer peripheral portion of sprocket body  1   a  (internal-tooth structural member  19 ) is formed with circumferentially equidistant-spaced, six bolt insertion holes  1   c  (through holes). On the other hand, the annular female screw-threaded member  6  is formed with six female screw threads  6   a  configured to be conformable to respective circumferential positions of bolt insertion holes  1   c  (bolt insertion holes  61   d ). Hence, the annular female screw-threaded member  6  (the housing  5 ), the sprocket  1 , and the retainer plate  61  are integrally connected to each other by axially fastening them together with bolts  7 . 
     Outside diameters of the sprocket body  1   a , the internal-tooth structural member  19 , the retainer plate  61 , and the female screw-threaded member  6  are dimensioned to be almost the same. 
     As shown in  FIGS. 1 and 3 , chain cover  07  is laid out and bolted to an engine body in a manner so as to vertically extend for covering the timing chain (not shown) wound on the sprocket. Chain cover  07  has a substantially circular opening  07   a  configured to be conformable to the contour of intake VTC  04 . The opening  07   a  is formed in the annular wall of the front end of chain cover  07 . The annular wall has four boss sections  07   b  integrally formed on the inner periphery of the annular wall and circumferentially spaced from each other. Four female screw-threads  07   c  are machined in respective boss sections  07   b  such that female screw-threads  07   c  extend from the front end face of the annular wall into the respective boss sections. 
     As shown in  FIGS. 1 and 3 , cover member  3  is made from aluminum alloy and formed into a substantially cup shape. Cover member  3  is comprised of a cup-shaped cover main body  3   a  and an annular flange  3   b  formed integral with the circumference of the right-hand side opening end (viewing  FIG. 1 ) of cover main body  3   a . Cover main body  3   a  is configured to cover the front end of phase converter  2 . Cover main body  3   a  has a slightly axially-extending cylindrical wall portion  3   c  integrally formed at a given position deviated upward from the center of the frontal flat wall portion of cover main body  3   a . The cylindrical wall portion  3   c  has a retaining through-hole  3   d  formed therein. 
     Annular flange  3   b  is integrally formed with four tab-like portions  3   e , circumferentially spaced apart from each other at intervals of approximately 90 degrees. Four bolt insertion holes  3   f  (through holes) are bored in respective tab-like portions  3   e  of the annular flange  3   b . Cover member  3  is fixedly connected to the chain cover  07  by means of bolts  54 , which are inserted through the respective bolt insertion holes  3   f  and screwed into the female screw-threads  07   c  formed in the respective boss sections  07   b  of chain cover  07 . 
     Also, the inner periphery of the right-hand side opening end (viewing  FIG. 3 ) of cover main body  3   a  is formed as a shouldered oil-seal retaining annular groove  3   h . A large-diameter oil seal  50  is interleaved between the shouldered oil-seal retaining annular groove  3   h  of cover main body  3   a  and the outer peripheral surface of housing  5 . Large-diameter oil seal  50  is formed into a substantially C-shape in lateral cross section. Oil seal  50  is made from synthetic rubber (a base material), and also a core metal is buried in the base material. The cylindrical outer peripheral surface of oil seal  50  is fitted to the shouldered oil-seal retaining annular groove  3   h  of cover main body  3   a  in a fluid-tight fashion, whereas the inner periphery of oil seal  50  (that is, a spring-loaded single lip and a non-spring-loaded dust lip) is fitted onto the outer periphery of housing  5  in a fluid-tight fashion. 
     As shown in  FIGS. 3-4 , housing  5  is comprised of a housing main body  5   a  made from iron-based metal material and formed into a substantially cylindrical shape with a rear end face (a bottom face) by pressing, and a seal plate  11  made from synthetic resin (non-magnetic material) and provided for sealing the axially forward opening (the left-hand side opening end, viewing  FIG. 3 ) of housing main body  5   a.    
     Housing main body  5   a  has a bottom  5   b  formed at its rear end. Housing main body  5   a  is formed in a substantially center of the bottom  5   b  with a large-diameter eccentric-shaft insertion hole into which an eccentric shaft  39  (described later) is inserted. An axially-leftward extending cylindrical portion  5   c  is formed integral with the annular edge of the eccentric-shaft insertion hole in a manner so as to somewhat extend in the axial direction of intake camshaft  02 . The previously-discussed annular female screw-threaded member  6  is formed integral with the outer periphery of the bottom  5   b  of housing  5 . 
     Intake camshaft  02  has two rotary drive cams (per cylinder) integrally formed on its outer periphery for operating the associated two intake valves (not shown) per one engine cylinder. Also, intake camshaft  02  has a flanged portion  02   a  integrally formed at its front end. As seen in  FIG. 3 , the outside diameter of flanged portion  02   a  is dimensioned to be slightly greater than that of a fixed-end portion  9   a  of driven member  9  (described later). Hence, after installation of all component parts, the circumference of the front end face of the flanged portion  02   a  of intake camshaft  02  is brought into abutted-engagement with the rearward end face of the inner ring  43   b  of large-diameter ball bearing  43 . Driven member  9  is fixedly connected to the front end of the flanged portion  02   a  by means of a cam bolt  10  under a condition where the front end face of the flanged portion  02   a  has been kept in abutted-engagement with the rear end face of the fixed-end portion  9   a  of driven member  9 . 
     As shown in  FIG. 6 , the outer periphery of the flanged portion  02   a  of intake camshaft  02  is partially machined or cut as the stopper groove  02   b  recessed along the circumferential direction. The radially-inward protruding stopper  61   b  of retainer plate  61  is circumferentially moveably installed in the stopper groove  02   b . Stopper groove  02   b  is formed into a circular-arc shape having a specified circumferential length to permit a circumferential movement of stopper  61   b  within a limited motion range determined based on the specified circumferential length. Hence, a maximum phase-advance position of intake camshaft  02  relative to sprocket  1  is restricted by abutment between the counterclockwise edge of stopper  61   b  and the clockwise edge  02   c  of stopper groove  02   b . On the other hand, a maximum phase-retard position of intake camshaft  02  relative to sprocket  1  is restricted by abutment between the clockwise edge of stopper  61   b  and the counterclockwise edge  02   d  of stopper groove  02   b.    
     As appreciated from the longitudinal cross section of  FIG. 3 , stopper  61   b  is kept in a spaced, contact-free relationship with the fixed-end portion  9   a  of driven member  9  in the axial direction, thus adequately suppressing undesirable interference between the stopper  61   b  and the fixed-end portion  9   a.    
     As appreciated from the longitudinal cross section of  FIG. 3 , cam bolt  10  is comprised of a head  10   a  and a shank  10   b  formed integral with each other, and an annular washer provided at the boundary of head  10   a  and shank  10   b . Shank  10   b  is formed on its outer periphery with a male-screw-threaded portion, which is screwed into a female-screw-threaded portion machined into the front end of intake camshaft  02  along the axis of intake camshaft  02 . 
     Driven member  9  is made from iron-based metal material. As seen from the longitudinal cross section of  FIG. 3 , the driven member  9  is comprised of the disk-shaped fixed-end portion  9   a , an axially-forward-extending cylindrical portion  9   b  formed integral with the front end face of disk-shaped fixed-end portion  9   a , and a substantially cylindrical cage  41 , which cage is formed integral with the outer periphery of disk-shaped fixed-end portion  9   a  and configured to serve as a roller holder for holding a plurality of rollers  48  (rolling elements). 
     The rear end face of disk-shaped fixed-end portion  9   a  is arranged to abut with the front end face of the flanged portion  02   a  of intake camshaft  02 , and fixedly connected to the flanged portion  02   a  by an axial force of cam bolt  10 . 
     As shown in  FIG. 3 , cylindrical portion  9   b  is formed with a central bore  9   d  into which the shank  10   b  of cam bolt  10  is inserted. A needle bearing  38  is mounted on the outer periphery of cylindrical portion  9   b.    
     As shown in  FIGS. 3-5 , cage  41  (the roller holder) is configured to further extend from the outer periphery of disk-shaped fixed-end portion  9   a , and bent into a substantially L shape in longitudinal cross section and formed into a substantially cylindrical shape extending in the same axial direction as the cylindrical portion  9   b  and having an annular bottom axially opposed to one sidewall of a ball-bearing outer ring  47   b  (described later). More concretely, the substantially cylindrical portion  41   a  of cage  41  is configured to extend toward the bottom  5   b  of housing  5  through an annular internal space  44  defined between the annular female screw-threaded member  6  and the axially-leftward extending cylindrical portion  5   c . Also, the substantially cylindrical portion  41   a  of cage  41  has a plurality of axially-protruding lugs. As a whole, the axially-protruding lugs are shaped into a substantially comb-tooth shape. That is, by virtue of the axially-protruding lugs, each having a substantially rectangular cross-section, a plurality of roller-holding holes  41   b  are configured to be equidistant-spaced from each other with a given circumferential interval in the circumferential direction of the outer periphery of disk-shaped fixed-end portion  9   a . Rollers  48  are rotatably held or installed in respective roller-holding holes  41   b . The substantially cylindrical portion  41   a  of cage  41  has one fewer roller-holding holes (in other words, one fewer rollers or one fewer axially-protruding lugs) than the number of internal teeth  19   a  of internal-tooth structural member  19 . 
     An inner-ring retaining annular groove  63  is machined and defined between the outer periphery of disk-shaped fixed-end portion  9   a  and the annular bottom of cage  41  formed integral with each other, for retaining the inner ring  43   b  of large-diameter ball bearing  43 . 
     Inner-ring retaining annular groove  63  is formed as a shouldered annular groove configured to be radially opposed to the outer-ring retaining annular groove  60  of sprocket body  1   a . Inner-ring retaining annular groove  63  is comprised of a cylindrical outer peripheral surface extending in the axial direction of intake camshaft  02  and a radially-extending shouldered annular surface configured to extend radially outward from the innermost end of the cylindrical outer peripheral surface. When assembling, the inner ring  43   b  of ball bearing  43  is axially press-fitted onto the cylindrical outer peripheral surface. At the same time, the forward end face of the press-fitted inner ring  43   b  is brought into abutted-engagement with the shouldered annular surface of inner-ring retaining annular groove  63 , to position one axial end face (the forward end face) of the inner ring  43   b  in place. 
     Phase converter  2  is mainly constructed by the electric motor  12  coaxially located at the front end of intake camshaft  02 , and a roller speed reducer  8  provided for reducing the rotational speed of the motor output shaft  13  of electric motor  12  and for transmitting the reduced motor speed (in other words, the increased motor torque) to the intake camshaft  02 . 
     As seen in  FIGS. 3-4 , electric motor  12  is a brush-equipped direct-current (DC) motor. Electric motor  12  is comprised of the housing  5  serving as a yoke and rotating together with the sprocket  1 , the motor output shaft  13  rotatably installed in the housing  5 , a pair of substantially semi-circular permanent magnets  14 - 15  fixedly connected onto the inner peripheral surface of housing  5 , and a stator  16  fixed to the seal plate  11 . 
     Motor output shaft  13  is formed into a shouldered cylindrical-hollow shape, and serves as an armature. Motor output shaft  13  is constructed by a large-diameter portion  13   a  of the intake-camshaft side and a small-diameter portion  13   b  of the brush-holder side through a shouldered portion  13   c  formed substantially at a midpoint of the axially-extending cylindrical-hollow motor output shaft. An iron-core rotor  17 , having a plurality of magnetic poles, is fixedly connected onto the outer periphery of large-diameter portion  13   a . Eccentric shaft  39  is axially press-fitted into the large-diameter portion  13   a , in a manner so as to be axially positioned in place by the inside annular face of shouldered portion  13   c.    
     An annular member  20  is press-fitted onto the outer periphery of small-diameter portion  13   b . A commutator  21  is axially press-fitted onto the outer peripheral surface of annular member  20 , in a manner so as to be axially positioned in place by the outside annular face of shouldered portion  13   c.    
     Furthermore, a plug  53  is fixed or press-fitted to the inner peripheral surface of small-diameter portion  13   b , for preventing or adequately suppressing undesirable leakage of lubricating oil, which oil is supplied into the cylindrical-hollow motor output shaft  13  and eccentric shaft  39  for lubrication of a ball bearing  37  (described later) as well as the previously-discussed needle bearing  38 , to the outside. 
     Iron-core rotor  17  is formed by a magnetic material having a plurality of magnetic poles. The outer periphery of iron-core rotor  17  is constructed as a bobbin having slots on which coil windings of an electromagnetic coil  18  is wound. 
     On the other hand, commutator  21  is formed as a substantially annular shape and made from a conductive material. Commutator  21  is divided into a plurality of segments whose number is equal to the number of magnetic poles of iron-core rotor  17 . Terminals of the coil winding (not shown) drawn out from electromagnetic coil  18  are electrically connected to each of segments of commutator  21 . That is, the terminals of the coil winding are sandwiched and electrically connected to the hemmed section formed on the periphery of commutator  21 . 
     As a whole, the substantially semi-circular permanent magnets  14 - 15  are formed into a cylindrical shape, and have a plurality of magnetic poles in the circumferential direction. The axial position of each of permanent magnets  14 - 15  is offset forward from the fixed position of iron-core rotor  17 . 
     As shown in  FIG. 7 , stator  16  is mainly comprised of a disk-shaped synthetic-resin plate  22 , a pair of synthetic-resin brush holders  23   a - 23   b , a pair of first brushes  25   a - 25   b , a radially-inside electricity-feeding slip ring  26   a , a radially-outside electricity-feeding slip ring  26   b , and pig-tale harnesses  27   a - 27   b . Disk-shaped synthetic-resin plate  22  is integrally connected to the inner periphery of seal plate  11 . Brush holders  23   a - 23   b  are attached onto the inside face of synthetic-resin plate  22 . The first brushes  25   a - 25   b  serve as current-supply switching brushes and supported by respective holders  23   a - 23   b  so as to be radially slidable. The radially-inward ends of first brushes  25   a - 25   b  are kept in sliding-contact (elastic-contact or electric-contact) with the outer peripheral surface of commutator  21  by respective spring forces of coil springs  24   a - 24   b . The radially-inside electricity-feeding slip ring  26   a  and the radially-outside electricity-feeding slip ring  26   b  are attached to the synthetic-resin plate  22 , such that the outside face (the left-hand side face, viewing  FIG. 3 ) of each of electricity-feeding slip rings  26   a - 26   b  is partially exposed and that the inside face (the right-hand side face, viewing  FIG. 3 ) of each of slip rings  26   a - 26   b  is buried in the front end face of synthetic-resin plate  22 . The first brush  25   a  and the electricity-feeding slip ring  26   b  are electrically connected to each other via the pig-tale harness  27   a , whereas the first brush  25   b  and the electricity-feeding slip ring  26   a  are electrically connected to each other via the pig-tale harness  27   b . The radially-inside annular slip ring  26   a  and the radially-outside annular slip ring  26   b  are laid out to be coaxial with each other with a given aperture. 
     The previously-discussed seal plate  11  is fitted into an annular groove cut in the inner periphery of the front end of the cylindrical housing main body  5   a  of housing  5 , and fixedly connected to the front end of housing main body  5   a  in place by caulking. Also, the subassembly ( 11 ,  22 ) of seal plate  11  and disk-shaped synthetic-resin plate  22  is formed in its center with a shaft insertion hole  11   a  into which one axial end (the left-hand axial end, viewing  FIG. 3 ) of motor output shaft  13  is partially inserted. 
     An integrally-molded synthetic-resin brush retainer  28  is fixedly connected to the cover main body  3   a . As shown in  FIGS. 3-4 , brush retainer  28  is formed into a substantially L shape in side view. Brush retainer  28  is comprised of a substantially cylindrical brush-retaining portion  28   a , a connector portion  28   b , a pair of laterally-extending tab-like brackets  28   c ,  28   c  (see  FIG. 4 ), and a pair of terminal strips  31 ,  31 . Brush-retaining portion  28   a  is inserted into the retaining through-hole  3   d . Connector portion  28   b  is formed integral with the upper end of brush-retaining portion  28   a . Tab-like brackets  28   c ,  28   c  are formed integral with both sides of brush-retaining portion  28   a . Most of terminal strips  31 ,  31  are buried in the synthetic-resin brush retainer  28 . 
     Terminal strips  31 ,  31  are arranged parallel with each other in the vertical direction and partly cranked. One end (the downward terminal  31   a ) of each of the crank-shaped terminal strips  31  is exposed to the bottom of brush-retaining portion  28   a . The other end (the upward terminal  31   b ) of each of terminal strips  31  is configured to protrude into a female fitting groove  28   d  of connector portion  28   b . The upward terminals  31   b ,  31   b  of the two parallel terminal strips  31 ,  31  are electrically connected to a control unit (not shown) via a male socket (not shown) fitted to the female fitting groove  28   d.    
     Brush-retaining portion  28   a  is configured to extend horizontally (axially). An upper hollow sleeve is press-fitted into an upper cylindrical-hollow through hole bored in the brush-retaining portion  28   a . In a similar manner, a lower hollow sleeve is press-fitted into a lower cylindrical-hollow through hole bored in the brush-retaining portion  28   a . A pair of second brushes  30   a ,  30   a  are supported by the respective hollow sleeves so as to be axially slidable. The tips of second brushes  30   a ,  30   a  are kept in sliding-contact (abutted-engagement or electric-contact) with respective slip rings  26   a  and  26   b.    
     Each of second brushes  30   a ,  30   a  is formed into a substantially rectangular parallelopiped shape. A second coil spring  32   a  is disposed between the downward terminal exposed to the bottom of the upper cylindrical-hollow through hole of brush-retaining portion  28   a  and the associated second brush  30   a  under preload. In a similar manner, a second coil spring  32   a  is disposed between the downward terminal exposed to the bottom of the lower cylindrical-hollow through hole of brush-retaining portion  28   a  and the associated second brush  30   a  under preload. Thus, the tips of second brushes  30   a ,  30   a  are permanently forced or biased toward respective slip rings  26   a  and  26   b  by the spring forces of second coil springs  32   a ,  32   a.    
     Additionally, a flexible pig-tale harness  33  is connected between the square base of second brush  30   a  and the downward terminal  31   a  exposed to the bottom of the upper cylindrical-hollow through hole of brush-retaining portion  28   a  by welding, to provide electric connection. In a similar manner, a flexible pig-tale harness  33  is electrically connected between the square base of second brush  30   a  and the downward terminal  31   a  exposed to the bottom of the lower cylindrical-hollow through hole of brush-retaining portion  28   a  by welding, to provide electric connection. The lengths of pig-tale harnesses  33 ,  33  are set to appropriate lengths sufficient to restrict maximum sliding movements (maximum axially-extended positions) of second brushes  30   a ,  30   a  relative to sleeves  29   a - 29   b  for preventing the second brushes  30   a ,  30   a  from falling out of the respective sleeves  29   a - 29   b  by the spring forces of coil springs  32   a ,  32   a.    
     An annular seal member  34  is interleaved between the outer periphery of the root (the basal end) of brush-retaining portion  28   a  and an annular groove formed in the opening end of the cylindrical wall portion  3   c  of cover main body  3   a.    
     As seen in  FIG. 4 , each of the diametrically-opposed tab-like brackets  28   c ,  28   c  is formed into a substantially triangular shape, and formed with a bolt insertion hole (a through hole)  28   e . Thus, brush retainer  28  is fixedly connected to the cover main body  3   a  by means of bolts (not shown), which are inserted through the respective bolt insertion holes of tab-like brackets  28   c ,  28   c  and screwed into respective female screw-threads (not shown) formed in the cover main body  3   a.    
     The previously-discussed motor output shaft  13  and eccentric shaft  39  are rotatably supported by means of the small-diameter ball bearing  37  and the needle bearing  38 . Small-diameter ball bearing  37  is installed on the outer peripheral surface of the root of the shank  10   b  near the head  10   a  of cam bolt  10 . On the other hand, needle bearing  38  is mounted on the outer peripheral surface of cylindrical portion  9   b  of driven member  9 , and arranged in close proximity to the right-hand side end (viewing  FIG. 3 ) of small-diameter ball bearing  37  such that these bearings  37 - 38  are juxtaposed to each other. 
     Needle bearing  38  is comprised of a cylindrical retainer  38   a  press-fitted into the inner peripheral surface of eccentric shaft  39  and a plurality of needle rollers  38   b  (rolling elements) rotatably retained inside of the retainer  38   a . Each of needle rollers  38   b  is in rolling-contact with the outer peripheral surface of cylindrical portion  9   b  of driven member  9 . 
     The inner ring of small-diameter ball bearing  37  is retained between the annular front end face of cylindrical portion  9   b  of driven member  9  and the annular washer  10   c  of cam bolt  10 . On the other hand, the outer ring of small-diameter ball bearing  37  is press-fitted to the stepped portion defined between the small-inside-diameter section and the large-inside-diameter section of eccentric shaft  39 , in a manner so as to be axially positioned in place by abutment with the inside annular face of the stepped portion of eccentric shaft  39 . 
     A small-diameter oil seal (a seal member)  46  is interleaved between the outer peripheral surface of large-diameter portion  13   a  of motor output shaft  13  (eccentric shaft  39 ) and the inner peripheral surface of axially-leftward extending cylindrical portion  5   c  of housing  5 , for preventing leakage of lubricating oil from the inside of speed reducer  8  toward the inside of electric motor  12 . 
     The control unit (not shown) includes an input/output interface (I/O), memories (RAM, ROM), and a microprocessor or a central processing unit (CPU). The input/output interface (I/O) of the control unit receives input information from various engine/vehicle sensors, namely, a crank angle sensor, a cam shaft angle sensor, an airflow meter, an engine temperature sensor (an engine coolant temperature sensor), an accelerator opening sensor, and the like. Within the control unit, the CPU allows the access by the I/O interface of input informational data signals from the engine/vehicle sensors. The CPU is responsible for carrying the engine control program (i.e., the ignition-timing/throttle/fuel-injection/valve-timing control program) stored in memories, and is capable of performing necessary arithmetic and logic operations, depending on the current engine/vehicle operating condition, determined based on latest up-to-date informational data signals from the engine/vehicle sensors. Computational results (arithmetic calculation results), that is, calculated output signals are relayed through the output interface circuitry of the control unit to output stages (actuators), for electronic spark control, control of an electronically-controlled throttle valve, control of the fuel-injection system, and control of the VTC system. Concretely, the control unit is configured to detect an actual relative phase of intake camshaft  02  to sprocket  1  responsively to input informational signals from the crank angle sensor and the cam angle sensor and also configured to determine a desired relative phase of intake camshaft  02  to sprocket  1  depending on the current engine/vehicle operating condition. The control unit is further configured to perform rotational speed control of motor output shaft  13  by controlling electric-current supply to the electromagnetic coil  18  of electric motor  12 . The rotational speed of motor output shaft  13  is reduced by means of the speed reducer  8 . In this manner, the actual relative phase of intake camshaft  02  to sprocket  1  can be controlled and brought closer to the desired value. 
     As seen from the cross sections of  FIGS. 3 and 5 , and the perspective disassembled view of  FIG. 4 , speed reducer  8  is mainly comprised of the eccentric shaft  39  (constructing a part of the eccentric rotation member) that performs eccentric rotary motion, a middle-diameter ball bearing  47  (constructing the remainder of the eccentric rotation member) installed on the outer periphery of eccentric shaft  39 , a plurality of rollers (serving as rolling elements)  48  rotatably installed on the outer periphery of middle-diameter ball bearing  47  and circumferentially arranged substantially at regular intervals, the cage  41  configured to partition, retain and guide these rollers  48 , kept in rolling-contact with an outer ring  47   b  (described later) of middle-diameter ball bearing  47 , in the circumferential direction by respective roller-holding holes  41   b  (in other words, respective axially-protruding lugs), while permitting a slight radial displacement (a slight oscillating motion) of each of rollers  48 , and the driven member  9  formed integral with the cage  41 , and the internal-tooth structural member  19  with the waveform internal toothed portion  19   a.    
     Eccentric shaft  39  is formed into a shouldered cylindrical-hollow shape. Eccentric shaft  39  is constructed by a small-diameter portion  39   a  (at the front end) and a large-diameter portion  39   b  (at the rear end). The small-diameter portion  39   a  of eccentric shaft  39  is press-fitted into the inner peripheral surface of large-diameter portion  13   a  of motor output shaft  13 . The large-diameter portion  39   b  of eccentric shaft  39  is a substantially cylindrical cam. The geometric center “Y” of the cam contour surface of the outer periphery of large-diameter portion  39   b  of eccentric shaft  39  is slightly displaced from the axis “X” (i.e., the rotation center “X” shown in  FIGS. 3 and 5 ) of motor output shaft  13  in the radial direction. 
     As viewed from the longitudinal cross section of  FIG. 3 , middle-diameter ball bearing  47  is comprised of an inner ring  47   a , the outer ring  47   b , and balls  47   c  rotatably disposed and confined between them. The inner ring  47   a  of ball bearing  47  is press-fitted onto the outer peripheral surface (i.e., the eccentric-cam contour surface) of large-diameter portion  39   b  of eccentric shaft  39  in a manner so as to be axially positioned in place. In contrast to the inner ring  47   a , the outer ring  47   b  is not securely fixed in the axial direction. That is, the outer ring  47   b  is free and therefore is able to move contact-free. Concretely, the left-hand sidewall (viewing  FIG. 3 ) of the outer ring  47   b , facing the electric-motor side, is kept out of contact with the housing  5  of electric motor  12 , while the right-hand sidewall of the outer ring  47   b , axially opposed to the annular bottom of cage  41 , is kept out of contact with the inside wall surface of the annular bottom of cage  41 . More concretely, a very small axial clearance “Caxial” is defined between the right-hand sidewall of the outer ring  47   b  and the inside wall surface of the annular bottom of cage  41 , axially opposed to each other. Rollers  48 , interleaved between the outer periphery of outer ring  47   b  of middle-diameter ball bearing  47  and the waveform internal toothed portion  19   a  of internal-tooth structural member  19 , are held in rolling-contact with the outer peripheral surface of outer ring  47   b . A crescent-shaped annular clearance “Cannular” is defined between the outer peripheral surface of outer ring  47   b  and the substantially comb-tooth shaped protruding portion (the substantially cylindrical portion  41   a ) of cage  41 . Owing to eccentric rotary motion of eccentric shaft  39 , middle-diameter ball bearing  47  is radially moved or displaced by virtue of the crescent-shaped annular clearance “Cannular”. That is, the crescent-shaped annular clearance “Cannular” permits a slight radial displacement (a slight oscillating motion) of middle-diameter ball bearing  47 . 
     Each of rollers  48  is made from iron-based metal material, and formed as a cylindrical solid roller. Owing to the eccentric displacement (oscillating motion) of middle-diameter ball bearing  47 , the radially-inward contact surface of each of rollers  48 , included within a given area, is brought into abutment (rolling-contact) with the outer peripheral surface of the outer ring  47   b  of middle-diameter ball bearing  47 . On the other hand, the radially-outward contact surfaces of some of rollers, associated with the given area, are fitted into some troughs of internal teeth  19   a  of internal-tooth structural member  19  (serving as a toothed wheel or a toothed gear). That is, in the eccentric position of the eccentric rotation member (namely, the middle-diameter ball bearing  47  and eccentric shaft  39 ) shown in  FIG. 5 , roller  48 , located at the 12 o&#39;clock position, is brought into completely fitted-engagement (full tooth engagement) with the inner face of the trough between the uppermost two adjacent internal teeth  19   a ,  19   a . In contrast, roller  48 , located at the 6 o&#39;clock position, is brought out of engagement. That is, owing to the eccentric displacement (oscillating motion) of the eccentric rotation member (i.e., the middle-diameter ball bearing  47  and eccentric shaft  39 ), rollers  48  can radially oscillate, while being circumferentially guided by respective axially-protruding lugs (respective roller-holding holes  41   b ) of cage  41 . 
     To ensure smooth operation of the electric-motor-driven phase-converter equipped VTC apparatus, lubricating oil is supplied into the internal space of speed reducer  8  by lubricating-oil supply means. As shown in  FIG. 3 , the lubricating-oil supply means is comprised of an annular oil supply passage (not numbered), which is annularly grooved in the outer periphery of the journal of intake camshaft  02  rotatably supported by camshaft-journal bearing members  06  mounted on the cylinder head  01  and to which lubricating oil is supplied from a main oil gallery (not shown), an axial oil supply hole  51 , a small-diameter axial oil hole  52 , and large-diameter oil drain holes (not shown). Axial oil supply hole  51  is formed in the front end of intake camshaft  02  to communicate the annular oil supply passage via an oil groove, cut in the front end face of intake camshaft  02  and configured to communicate the downstream end of axial oil supply hole  51 . Small-diameter axial oil hole  52  is formed as a through hole in the driven member  9 , such that one end of small-diameter axial oil hole  52  is opened into the axial oil supply hole  51  through the oil groove cut in the camshaft end face and the other end of small-diameter axial oil hole  52  is opened into the internal space defined near both the needle bearing  38  and the middle-diameter ball bearing  47 . Large-diameter oil drain holes (not shown) are formed in the driven member  9  as oil outlets. 
     During operation, lubricating oil is constantly fed from the discharge port of an oil pump (not shown) into the oil supply hole  51  via the main oil gallery formed in the cylinder head. Hence, by the previously-discussed lubricating-oil supply means, lubricating oil can be fed via the oil supply hole  51  to the internal space  44  and stays in the internal space  44 . Then, the lubricating oil is supplied from the internal space  44  to moving parts, namely, middle-diameter ball bearing  47  and rollers  48  for lubrication, and further flows into the eccentric shaft  39  and the internal space of motor output shaft  13 , for lubrication of moving parts, such as needle bearing  38  and small-diameter ball bearing  37 . By the way, undesirable leakage of lubricating oil, staying in the internal space  44 , to the inside of the electric-motor housing  5  can be prevented or adequately suppressed by means of the small-diameter oil seal  46 . 
     The fundamental operation of intake VTC  04  incorporated in the VTC system of the embodiment is hereunder described in detail. 
     When the engine crankshaft rotates, sprocket  1  rotates in synchronism with rotation of the crankshaft through the timing chain (not shown). On one hand, torque flows from the sprocket  1  through the internal-tooth structural member  19  via the annular female screw-threaded member  6  to the housing  5  of electric motor  12 , and thus permanent magnets  14 - 15  and stator  16 , all attached to the inner periphery of housing  5 , rotate together with the housing  5 . On the other hand, torque flows from the sprocket  1  through the internal-tooth structural member  19  via the rollers  48 , cage  41 , and driven member  9  to the intake camshaft  02 . Thus, intake camshaft  02  is rotated to operate (open/close) the intake valves against the spring forces of the valve springs by the intake-valve cams. 
     During a given engine operating condition after the engine start-up, an electric current is applied from the control unit through the terminal strips  31 ,  31 , pig-tale harnesses  33 ,  33 , second brushes  30   a ,  30   a , and slip rings  26   a - 26   b  to the electromagnetic coil  18  of electric motor  12 . Hence, motor output shaft  13  is driven. Then, the output rotation from the motor output shaft  13  is reduced by means of the speed reducer  8 , and thus the reduced motor speed (in other words, the multiplied motor torque) is transmitted to the intake camshaft  02 . 
     That is, when eccentric shaft  39  rotates eccentrically during rotation of motor output shaft  13 , each of rollers  48  moves (rolls) and relocates from one of two adjacent internal teeth  19   a ,  19   a  to the other with one-tooth displacement per one complete revolution of motor output shaft  13 , while being held in rolling-contact with the outer ring  47   b  of middle-diameter ball bearing  47  and simultaneously radially guided by the associated axially-protruding lug (the associated roller-holding hole  41   b ) of cage  41 . By way of the repeated relocations of each of rollers  48  every revolutions of motor output shaft  13 , rollers  48  move in the circumferential direction with respect to the waveform internal toothed portion  19   a  of internal-tooth structural member  19 , while being held in rolling-contact with the outer ring  47   b  of middle-diameter ball bearing  47 . In this manner, torque is transmitted through the driven member  9  to the intake camshaft  02 , while the rotational speed of motor output shaft  13  is reduced. The reduction ratio of this type of speed reducer  8  can be determined by the number of rollers  48 , in other words, the number of roller-holding holes  41   b  (i.e., the number of axially-protruding lugs of cage  41 ). The fewer the number of rollers  48 , the lower the reduction ratio. That is, the reduction ratio can be arbitrarily set depending on the number of rollers  48 . 
     As discussed above, by execution of rotational speed control of motor output shaft  13 , intake camshaft  02  is rotated in a normal-rotational direction or in a reverse-rotational direction with respect to the sprocket  1 , and thus an angular phase of intake camshaft  02  relative to sprocket  1  is changed, and as a result intake valve open timing (IVO) and intake valve closure timing (IVC) can be phase-advanced or phase-retarded. 
     As discussed above, the speed reducer  8 , incorporated in the intake VTC  04 , is configured such that the rotational speed of motor output shaft  13  of electric motor  12  can be reduced by virtue of the repeated relocations of each of rollers  48  every revolutions of motor output shaft  13 , rollers  48  moving in the circumferential direction with respect to the waveform internal toothed portion  19   a  of internal-tooth structural member  19 , while being held in rolling-contact with the outer ring  47   b  of middle-diameter ball bearing  47 . Hence, as seen from the characteristic diagram of  FIG. 11 , a friction F 1  of intake VTC  04  during operation (in other words, during speed-reduction of the roller speed reducer  8 ) becomes adequately reduced. Thus, it is possible to enhance or improve the phase-conversion responsiveness for the angular phase shift of intake camshaft  02  relative to sprocket  1  in the phase-advance direction or in the phase-retard direction. 
     As clearly shown in  FIG. 6 , the clockwise rotary motion (normal-rotational motion) of intake camshaft  02  relative to sprocket  1  is restricted by abutment between the counterclockwise edge of stopper  61   b  and the clockwise edge  2   c  of stopper groove  2   b . On the other hand, the counterclockwise rotary motion (reverse-rotational motion) of intake camshaft  02  relative to sprocket  1  is restricted by abutment between the clockwise edge of stopper  61   b  and the counterclockwise edge  2   d  of stopper groove  2   b.    
     [Exhaust VTC] 
     As shown in FIGS.  1  and  8 - 10 , the above-mentioned exhaust VTC  05  is comprised of a driving rotary member  70  that rotates in synchronism with rotation of the engine crankshaft, and a phase change mechanism (a phase converter)  71  installed between the driving rotary member  70  and the exhaust camshaft  03  for changing a relative angular phase between the driving rotary member  70  and the exhaust camshaft  03  depending on an engine operating condition. 
     Driving rotary member  70  is comprised of a sprocket  75  and a gear member  80  formed into a substantially cylindrical shape with an annular bottom. The sprocket  75  and the gear member  80  are integrally connected to each other by axially fastening them together with bolts. 
     Phase converter  71  is mainly constructed by the electric motor  72 , an electric-motor energization control circuit  73 , and a planetary-gear speed reducer  74  provided for reducing the rotational speed of a motor output shaft  72   a  of electric motor  72  and for transmitting the reduced motor speed to the exhaust camshaft  03 . Electric motor  72  and electric-motor energization control circuit  73  serve as a phase-control torque generating system. 
     For instance, electric motor  72  is a brushless motor. The control torque to be applied to the motor output shaft  72   a  is generated by energizing a coil of electric motor  72 . Energization control circuit  73  is constructed by a microcomputer, a motor driver, and the like, and located outside of the electric motor  72 . Energization control circuit  73  is electrically connected to the electric motor  72 , for controlling energization of electric motor  72  depending on the engine operating condition. In accordance with the controlled energization mode, electric motor  72  is driven so as to hold, increase, or decrease the control torque applied to the motor output shaft  72   a.    
     Planetary-gear speed reducer  74  is comprised of a driven rotary member  76 , a substantially cylindrical-hollow planet carrier  77 , elastic members (resilient members)  78 ,  78 , and a planet rotor  79 . 
     The peripheral wall section of gear member  80  is formed with a driving internal toothed portion  81  whose addendum circle is located radially inside of a root circle. Sprocket  75  has a plurality of radially-outward protruding teeth  75   a . The timing chain (not shown) is wound on both the teeth  75   a  of sprocket  75  and a plurality of teeth of the sprocket on the crankshaft, such that torque from the crankshaft is transmitted to the sprocket  75 . Therefore, when the output torque from the crankshaft is inputted through the timing chain to the sprocket  75 , sprocket  75  rotates in synchronism with rotation of the crankshaft, while holding the angular phase of the sprocket  75  relative to the crankshaft. At this time, the direction of rotation of sprocket  75  becomes a counterclockwise direction in  FIGS. 9-10 . 
     As shown in  FIGS. 9-10 , the driven rotary member  76  of planetary-gear speed reducer  74  is formed into a substantially cylindrical shape with an annular bottom. As clearly shown in  FIG. 8 , driven rotary member  76  is fitted to the inner periphery of sprocket  75 . The peripheral wall section of driven rotary member  76  is formed with a driven internal toothed portion  82  whose addendum circle is located radially inside of a root circle. As seen from the longitudinal cross section of  FIG. 8 , the driven internal toothed portion  82  is configured to be displaced axially rightward from the driving internal toothed portion  81 , and the geometric center of driven internal toothed portion  82  and the geometric center of driving internal toothed portion  81  are arranged coaxially with each other. The diameter (exactly, the pitch-circle diameter) of driven internal toothed portion  82  is dimensioned to be less than that of driving internal toothed portion  81 , and the module of driven internal toothed portion  82  and the module of driving internal toothed portion  81  are the same. Thus, the number of teeth of driven internal toothed portion  82  is fewer than that of driving internal toothed portion  81 . 
     As shown in  FIG. 8 , the annular bottom wall of driven rotary member  76  is formed with a coupling section  76   a  which is coaxially arranged and fixedly connected to the front end of exhaust camshaft  03 . Hence, driven rotary member  76  is able to rotate in synchronism with rotation of exhaust camshaft  03 , while holding the angular phase of the driven rotary member  76  relative to the exhaust camshaft  03  constant. Additionally, driven rotary member  76  is configured such that relative rotation of driven rotary member  76  with respect to sprocket  75  is permitted. 
     By the way, in  FIGS. 9-10 , the direction “Padvance” indicates a relative-rotation direction in which driven rotary member  76  is phase-advanced with respect to the sprocket  75 , whereas the direction “Pretard” indicates a relative-rotation direction in which driven rotary member  76  is phase-retarded with respect to the sprocket  75 . 
     As shown in  FIGS. 8-10 , the inner peripheral portion of the cylindrical-hollow planet carrier  77  is configured to define an input portion  83  to which the control torque is inputted from the motor output shaft  72   a  included in the phase-control torque generating system. 
     Input portion  83  is arranged coaxially with respect to the geometric center of driving internal toothed portion  81 , the geometric center of driven internal toothed portion  82 , and the axis of motor output shaft  72   a . Input portion  83  has at least two grooves  84  (i.e., radially-inward cutouts or openings). Planet carrier  77  has a joint  85  which is fitted to the grooves  84 . By virtue of the joint  85  fitted to the grooves  84 , planet carrier  77  is mechanically connected to the motor output shaft  72   a . Hence, planet carrier  77  is able to rotate together with the motor output shaft  72   a . Additionally, planet carrier  77  is configured such that relative rotation of planet carrier  77  with respect to each of driving rotary member  70  (i.e., gear member  80  and sprocket  75 ) and driven rotary member  76  of planetary-gear speed reducer  74  is permitted. 
     Part (the right-hand half, viewing  FIG. 8 ) of the outer peripheral portion of the cylindrical-hollow planet carrier  77  is configured as an eccentric portion  86  whose geometric center (an eccentricity axis “E”) deviates from the geometric center of driving internal toothed portion  81  (that is, the geometric center of driven internal toothed portion  82 ). Eccentric portion  86  has a pair of recesses  87  (i.e., two radially-outward cutouts or openings). As best seen in  FIG. 9 , the previously-discussed resilient members  78 ,  78  are accommodated in respective recesses  87 ,  87 . 
     Planet rotor  79  is constructed by combining a planetary bearing  88  and a planetary gear  89 . Planetary bearing  88  is a radial bearing comprised of an outer ring  88   a , an inner ring  88   b , and ball rolling elements  88   c  confined between outer and inner rings  88   a - 88   b.    
     In the shown embodiment, the outer ring  88   a  is concentrically press-fitted onto the inner periphery of the center bore  89   a  of planetary gear  89 . On the other hand, the inner ring  88   b  is concentrically fitted onto the outer periphery of eccentric portion  86  of planet carrier  77 . With this arrangement, planetary bearing  88  is supported by the planet carrier  77  from the inner peripheral side of planetary bearing  88 . Additionally, planetary bearing  88  is configured to exert restoring forces, which are applied from respective resilient members  78 ,  78 , on the center bore  89   a  of planetary gear  89 . 
     Planetary gear  89  is formed into a stepped cylindrical shape, and arranged concentrically with the eccentric portion  86 . Thus, planetary gear  89  is arranged eccentrically to both the geometric center of driving internal toothed portion  81  and the geometric center of driven internal toothed portion  82 . Planetary gear  89  has a large-diameter section and a small-diameter section, which are integrally formed with each other and configured to respectively define a driving external toothed portion  90  and a driven external toothed portion  91 , each having an addendum circle located radially outside of a root circle. The numbers of the external teeth of the driving external toothed portion  90  and the driven external toothed portion  91  are respectively set to be smaller than the numbers of the internal teeth of the driving internal toothed portion  81  and the driven internal toothed portion  82  by the same tooth number. Actually, in the shown embodiment, the driving external toothed portion  90  has one fewer external teeth than the number of internal teeth of driving internal toothed portion  81  (see  FIG. 10 ). In a similar manner, the driven external toothed portion  91  has one fewer external teeth than the number of internal teeth of driven internal toothed portion  82  (see  FIG. 9 ). The module of driven external toothed portion  91  and the module of driving external toothed portion  90  are the same. Thus, the number of teeth of driven external toothed portion  91  is fewer than that of driving external toothed portion  90 . 
     As appreciated from the cross section of  FIG. 10 , driving external toothed portion  90  is meshed with the driving internal toothed portion  81  on the inner periphery of driving internal toothed portion  81 . As seen from the longitudinal cross section of  FIG. 8 , the driven external toothed portion  91  is configured to be displaced axially rightward from the driving external toothed portion  90 , and the geometric center of driven external toothed portion  91  and the geometric center of driving external toothed portion  90  are arranged coaxially with each other. As appreciated from the cross section of  FIG. 9 , driven external toothed portion  91  is meshed with the driven internal toothed portion  82  on the inner periphery of driven internal toothed portion  82 . The geometric center of driven external toothed portion  91  and the geometric center of driving external toothed portion  90  correspond to the eccentricity axis “E” of eccentric portion  86  of planet carrier  77 . Hence, planetary gear  89  can perform a planetary motion so as to revolve in the direction of rotation of eccentric portion  86 , while revolving on the eccentricity axis “E” of driving external toothed portion  90  (i.e., the eccentricity axis “E” of driven external toothed portion  91 ). As appreciated from the cross sections of  FIGS. 9-10 , planetary-gear speed reducer  74  is a cycloid planetary-gear speed reducer. 
     Phase converter  71 , configured as previously discussed, changes an angular phase of exhaust camshaft  03  relative to sprocket  75  depending on the control torque inputted from the motor output shaft  72   a  to the input portion  83  of planet carrier  77 , thereby achieving exhaust-valve timing (exhaust-valve open timing EVO and exhaust-valve closure timing EVC) suited to the engine operating condition. 
     Concretely, when planet carrier  77  does not rotate relatively to the sprocket  75  due to the control torque held constant, the driving external toothed portion  90  and the driven external toothed portion  91  of planetary gear  89  rotate together with respective rotary members  70  and  76 , while holding the meshed positions thereof with the internal toothed portions  81  and  82 , respectively. Thus, the relative angular phase between the sprocket  75  and the exhaust camshaft  03  does not change and as a result the exhaust valve timing is held constant. 
     When planet carrier  77  rotates relatively to the sprocket  75  in the direction “Padvance” responsively to an increase in the control torque in the direction “Padvance”, the driving external toothed portion  90  and the driven external toothed portion  91  of planetary gear  89  unitarily perform the planetary motion, while changing the meshed positions with the internal toothed portions  81  and  82 , respectively. Thus, driven rotary member  76  rotates relatively to the sprocket  75  in the direction “Padvance”. Accordingly, the angular phase of exhaust camshaft  03  relative to sprocket  75  changes toward the phase-advance side and as a result the exhaust valve timing is controlled to the phase-advance side. 
     Conversely when planet carrier  77  rotates relatively to the sprocket  75  in the direction “Pretard” responsively to an increase in the control torque in the direction “Pretard”, the driving external toothed portion  90  and the driven external toothed portion  91  of planetary gear  89  unitarily perform the planetary motion, while changing the meshed positions with the internal toothed portions  81  and  82 , respectively. Thus, driven rotary member  76  rotates relatively to the sprocket  75  in the direction “Pretard”. Accordingly, the angular phase of exhaust camshaft  03  relative to sprocket  75  changes toward the phase-retard side and as a result the exhaust valve timing is controlled to the phase-retard side. 
     As discussed above, exhaust VTC  05  is configured such that driven rotary member  76  (i.e., exhaust camshaft  03 ) rotates relatively to sprocket  75  by virtue of the planetary motion of planetary gear  89  with changes in the meshed positions of the external toothed portions  90  and  91  with the respective internal toothed portions  81  and  82 , occurring due to an increase in the control torque of motor output shaft  72   a  in the direction “Padvance” or in the direction “Pretard”. That is to say, exhaust VTC  05  is configured such that relative rotation of driven rotary member  76  (i.e., exhaust camshaft  03 ) to sprocket  75  occurs by virtue of both the meshed-engagement of driving external toothed portion  90  with driving internal toothed portion  81  and the meshed-engagement of driven external toothed portion  91  with driven internal toothed portion  82 . Hence, as seen from the characteristic diagram of  FIG. 11 , a friction F 2  of the exhaust VTC  05  during operation (in other words, during speed-reduction of the planetary-gear speed reducer  74 ) becomes comparatively greater. Thus, on one hand, the phase-conversion responsiveness for the angular phase shift of exhaust camshaft  03  relative to sprocket  75  in the phase-advance direction or in the phase-retard direction tends to deteriorate. On the other hand, the phase holding performance for the phase angle of exhaust camshaft  03  relative to sprocket  75  can be improved by the comparatively greater friction F 2  of the exhaust VTC  05 . 
     In the first embodiment as explained previously in reference to  FIGS. 1-11 , regarding the intake VTC  04 , the speed reducer  8  is configured such that the rotational speed of electric motor  12  can be reduced by virtue of the repeated relocations of each of rollers  48  every revolutions of motor output shaft  13 , rollers  48  moving in the circumferential direction with respect to the waveform internal toothed portion  19   a , while being held in rolling-contact with the ball-bearing outer ring  47   b . Hence, as seen from the characteristic diagram of  FIG. 11 , the friction F 1  of intake VTC  04  during speed-reduction of the roller speed reducer  8  becomes adequately reduced, thereby improving the phase-conversion responsiveness for the angular phase shift of intake camshaft  02  relative to sprocket  1  in the phase-advance direction or in the phase-retard direction. 
     In contrast, regarding the exhaust VTC  05 , as seen from the characteristic diagram of  FIG. 11 , the friction F 2  of exhaust VTC  05 , caused by both the meshed-engagement of driving external toothed portion  90  with driving internal toothed portion  81  and the meshed-engagement of driven external toothed portion  91  with driven internal toothed portion  82 , becomes greater than the friction F 1  of intake VTC  04 , thereby improving the phase holding performance for stably holding the phase angle of exhaust camshaft  03  relative to sprocket  75 . 
     Therefore, according to the valve timing control system of the first embodiment, it is possible to reconcile and balance two contradictory requirements, namely, the improved operational responsiveness of intake VTC  04  for the angular phase shift of intake camshaft  02  relative to sprocket  1  in the phase-advance direction or in the phase-retard direction by virtue of the roller speed reducer  8 , and the improved phase holding performance of exhaust VTC  05  for stably holding the phase angle of exhaust camshaft  03  relative to sprocket  75  by virtue of the planetary-gear speed reducer  74 . 
     Second Embodiment 
     Referring now to  FIG. 12 , there is shown the essential part of the valve timing control system of the second embodiment. The VTC system of the second embodiment differs from the first embodiment, in that in the second embodiment the roller speed reducer  8  is applied to the intake VTC  04  and a roller speed reducer  8 ′ similar to the roller speed reducer  8  incorporated in the intake VTC  04  is applied to the exhaust VTC  05 . Furthermore, in contrast to the first embodiment, in the second embodiment an electric motor  100  of intake VTC  04  is constructed by a brushless motor, whereas an electric motor  101  of exhaust VTC  05  is constructed by a brush-equipped motor. 
     Regarding intake VTC  04 , a housing  100   a  of electric motor  100  is fixedly connected to a sprocket  102 , to which torque is transmitted from the crankshaft, by means of bolts, such that the housing  100   a  always rotates in synchronism with rotation of the sprocket  102 . 
     Regarding exhaust VTC  05 , electric motor  101  is not directly connected to a sprocket  103 . That is, motor  101  is configured so as not to be affected by rotation of sprocket  103 . 
     As appreciated from the above, in the intake VTC  04 , housing  100   a  always rotates together with the sprocket  102  during operation of the engine, such that a dynamic friction arises. Hence, when intake camshaft  02  is rotated relatively to the sprocket  102  via the roller speed reducer  8  by rotating the electric motor  100  depending on a change in the engine operating condition, a starting speed of relative rotation of intake camshaft  02  to sprocket  102  tends to become faster, because of the dynamic friction. As a result of this, it is possible to improve the phase-conversion responsiveness for the angular phase shift of intake camshaft  02  to sprocket  102 . 
     Additionally, in the second embodiment, electric motor  100  of intake VTC  04  is constructed by a brushless motor, which has a less sliding friction resistance in comparison with a brush-equipped motor. Therefore, by the synergistic effect of the dynamic friction and the less sliding friction, it is possible to greatly improve the operational responsiveness of intake VTC  04  for the angular phase shift of intake camshaft  02  relative to sprocket  102 . 
     In contrast to the above, in the exhaust VTC  05 , even when sprocket  103  is rotating in synchronism with rotation of the crankshaft during operation of the engine, the motor output shaft of electric motor  101  is kept in a non-rotational state, that is, remains stationary, until such time that a control signal has been outputted from the control unit to the electric motor  101 . Hence, when electric motor  101  begins to operate depending on a change in the engine operating condition, on one hand, the phase-conversion responsiveness for the angular phase shift of exhaust camshaft  03  to sprocket  103  tends to deteriorate, because of a static friction resistance/drag of electric motor  101 . On the other hand, by virtue of the static friction of electric motor  101 , it is possible to improve the phase holding performance for the relative angular phase of exhaust camshaft  03 , thereby enabling the phase angle of exhaust camshaft  03  relative to sprocket  103  to be stably held at a desired relative-rotation position. 
     Additionally, in the second embodiment, electric motor  101  of exhaust VTC  05  is constructed by a brush-equipped motor, and thus a sliding friction resistance acts between two surfaces of a brush and a slip ring in sliding-contact with each other. Therefore, by the synergistic effect of the static friction and the comparatively greater sliding friction, it is possible to greatly improve the phase holding performance. 
     Third Embodiment 
     Referring now to  FIG. 13 , there is shown the characteristic diagram illustrating the cogging-torque difference between a permanent-magnet electric motor applied to intake VTC  04  and a permanent-magnet electric motor applied to exhaust VTC  05  in the valve timing control system of the third embodiment. In the third embodiment, the same type of speed reducer (e.g., a roller speed reducer) as the second embodiment is used for each of intake VTC  04  and exhaust VTC  05 , but a cogging-torque characteristic of the direct-current (DC) motor incorporated in the intake VTC  04  and a cogging-torque characteristic of the direct-current (DC) motor incorporated in the exhaust VTC  05  are set to be different from each other, so as to reconcile and balance two contradictory requirements, namely, the improved operational responsiveness of intake VTC  04  for the angular phase shift of the intake camshaft relative to the intake-side sprocket, and the improved phase holding performance of exhaust VTC  05  for stably holding the phase angle of the exhaust camshaft relative to the exhaust-side sprocket. 
     Concretely, the number of magnetic poles of the electric motor of intake VTC  04  is set to be greater than that of the electric motor of exhaust VTC  05 . Hence, as appreciated from the phase versus cogging-torque characteristic diagram of  FIG. 13 , the cogging torque T 1  of the electric motor of intake VTC  04  can be set to be less than the cogging torque T 2  of the electric motor of exhaust VTC  05 . As a result of this, a starting speed of rotation of the electric motor of intake VTC  04  having the relatively less cogging torque T 1  tends to become faster, thus improving the phase-conversion responsiveness for the angular phase shift of the intake camshaft to the intake-side sprocket. In contrast, regarding the exhaust VTC  05 , on one hand, the phase-conversion responsiveness for the angular phase shift of the exhaust camshaft to the exhaust-side sprocket tends to deteriorate, because of the relatively greater cogging torque T 2 . On the other hand, the phase holding performance for the phase angle of the exhaust camshaft relative to the exhaust-side sprocket can be improved by the relatively greater cogging torque T 2 . 
     As will be appreciated from the above, the invention is not limited to the particular embodiments shown and described herein, but various changes and modifications may be made. For instance, the configuration of each of electric motors to be applied to intake VTC  04  and exhaust VTC  05  and the configuration of each of speed reducers to be applied to intake VTC  04  and exhaust VTC  05  may be further modified in order to reconcile and balance two contradictory requirements, namely, the improved operational responsiveness of intake VTC  04  for the angular phase shift of the intake camshaft relative to the intake-side sprocket, and the improved phase holding performance of exhaust VTC  05  for stably holding the phase angle of the exhaust camshaft relative to the exhaust-side sprocket. 
     The entire contents of Japanese Patent Application No. 2013-021947 (filed Feb. 7, 2013) are incorporated herein by reference. 
     While the foregoing is a description of the preferred embodiments carried out the invention, it will be understood that the invention is not limited to the particular embodiments shown and described herein, but that various changes and modifications may be made without departing from the scope or spirit of this invention as defined by the following claims.