Patent Publication Number: US-7219636-B2

Title: Variable valve timing control system of internal combustion engine

Description:
TECHNICAL FIELD 
   The present invention relates to a variable valve timing control system employing a hydraulically-operated phase converter capable of varying a relative phase of a camshaft to a crankshaft of an internal combustion engine by supplying working fluid (hydraulic pressure) selectively to either one of a phase-advance hydraulic chamber and a phase-retard hydraulic chamber, for variably adjusting an open-and-closure timing of an engine valve depending on an engine operating condition. 
   BACKGROUND ART 
   In recent years, there have been proposed and developed various variable valve timing control systems each employing a phase converter, such as a hydraulically-operated vane-type timing variator, a hydraulically-operated helical-gear-type timing variator, and the like. A hydraulically-operated vane-type timing variator has been disclosed in Japanese Patent Provisional Publication No. 2001-271616 (hereinafter is referred to as “JP2001-271616”), corresponding to German patent application No. 101 01 938 and also corresponding to U.S. Pat. No. 6,345,595, issued on Feb. 12, 2002 and assigned to the assignee of the present invention. In the hydraulically-operated vane-type variable valve timing control system disclosed in JP2001-271616, a vane member is fixedly connected to a camshaft end and rotatably enclosed in a cylindrical housing of a timing pulley whose opening ends are enclosed with front and rear covers. A phase-advance hydraulic chamber and a phase-retard hydraulic chamber are defined between diametrically-opposing partition walls and two blades of the vane member. The hydraulically-operated phase converter operates to vary a relative angular phase between the camshaft and the timing pulley (engine crankshaft) by supplying hydraulic pressure discharged from a reversible pump selectively to either one of the phase-advance hydraulic chamber and the phase-retard hydraulic chamber by switching one of a normal-rotational direction and a reverse-rotational direction of the reversible pump to the other, for variably adjusting a valve open timing and/or a valve closure timing of the engine valve depending on an engine operating condition. 
   SUMMARY OF THE INVENTION 
   However, in the system disclosed in JP2001-271616, the phase-advance hydraulic chamber is connected directly to a first port of two ports of the reversible pump via a first fluid line. In a similar manner, the phase-retard hydraulic chamber is connected directly to the second port of the reversible pump via a second fluid line. As is generally known, a comparatively large magnitude of alternating torque is exerted on the camshaft owing to a spring force of a valve spring for each engine valve and a reaction force resulting from each valve lifting during operation of the engine. Due to the alternating torque, a pulse pressure is applied to the working fluid in each of the phase-advance and phase-retard hydraulic chambers. There is an increased tendency for the pulse pressure, arising from alternating torque exerted on the camshaft, to be transmitted from the phase-advance and phase-retard hydraulic chambers through the first and second fluid lines to the respective ports of the reversible pump. The pulsating pressure serves as an undesirable load (in other words, undesirable energy loss) carried on the motor shaft of the electric motor of the reversible pump. Such an undesirable load means the necessity of an increased torque capacity of the electric motor of the reversible pump, in other words, large-sizing of the system, or higher system costs. It would be desirable to provide a means by which the pulse pressure, arising from alternating torque exerted on the camshaft, may be avoided from acting as a load carried on the motor shaft of the electric motor of the reversible pump. 
   Accordingly, it is an object of the invention to provide a variable valve timing control system employing a hydraulically-operated phase converter, capable of preventing a pulse pressure, arising from alternating torque exerted on a camshaft, from being transmitted from either one of phase-advance and phase-retard hydraulic chambers to a first port of two ports of a reversible pump as a load carried on a motor shaft of the pump, and promoting the outflow of working fluid from the other hydraulic chamber to the second port by the pulse pressure so that the promoted outflow serves as an assistance force (an assistive drive source for the pump). 
   In order to accomplish the aforementioned and other objects of the present invention, a variable valve timing control system of an internal combustion engine comprises a rotary member adapted to be driven in synchronization with rotation of an engine crankshaft, and rotatably supported on a camshaft to permit relative rotation of the camshaft to the rotary member, a hydraulically-operated phase converter disposed between the rotary member and the camshaft, and having a phase-advance hydraulic chamber and a phase-retard hydraulic chamber for changing an angular phase of the camshaft relative to the rotary member, an electric pump that supplies working fluid to the phase-advance hydraulic chamber and the phase-retard hydraulic chamber through a phase-advance hydraulic line connected to the phase-advance hydraulic chamber and a phase-retard hydraulic line connected to the phase-retard hydraulic chamber, a directional control valve disposed between a first pair of fluid lines including a discharge line and an induction line of the pump and a second pair of fluid lines including the phase-advance hydraulic line and the phase-retard hydraulic line, for determining a path through which the working fluid is directed from the discharge line to a first one of the phase-advance hydraulic line and the phase-retard hydraulic line and simultaneously determining a path through which the working fluid is directed from the second hydraulic line to the induction line, a control unit configured to be electronically connected to at least the directional control valve, for controlling the directional control valve depending on an engine operating condition, and a check valve disposed in the discharge line for permitting flow in a direction that the working fluid flows from the pump to the directional control valve and preventing any flow in the opposite direction. 
   According to another aspect of the invention, a variable valve timing control system of an internal combustion engine comprises a rotary member adapted to be driven in synchronization with rotation of an engine crankshaft, and rotatably supported on a camshaft to permit relative rotation of the camshaft to the rotary member, a hydraulically-operated phase converter disposed between the rotary member and the camshaft, and having a phase-advance hydraulic chamber and a phase-retard hydraulic chamber for changing an angular phase of the camshaft relative to the rotary member, an electric pump that supplies working fluid to the phase-advance hydraulic chamber and the phase-retard hydraulic chamber through a phase-advance hydraulic line connected to the phase-advance hydraulic chamber and a phase-retard hydraulic line connected to the phase-retard hydraulic chamber, an electromagnetic solenoid-operated directional control valve disposed between a first pair of fluid lines including a discharge line and an induction line of the pump and a second pair of fluid lines including the phase-advance hydraulic line and the phase-retard hydraulic line, for determining a path through which the working fluid is directed from the discharge line to a first one of the phase-advance hydraulic line and the phase-retard hydraulic line and simultaneously determining a path through which the working fluid is directed from the second hydraulic line to the induction line, a control unit configured to be electronically connected to at least the solenoid-operated directional control valve, for controlling the solenoid-operated directional control valve depending on an engine operating condition, a bypass line intercommunicating the discharge line and the induction line, and a bypass check valve disposed in the bypass line for permitting flow in a direction that the working fluid flows from the induction line via the bypass line to the discharge line and preventing any flow in the opposite direction. 
   According to a further aspect of the invention, a variable valve timing control system of an internal combustion engine comprises a rotary member adapted to be driven in synchronization with rotation of an engine crankshaft, and rotatably supported on a camshaft to permit relative rotation of the camshaft to the rotary member, a hydraulically-operated phase converter disposed between the rotary member and the camshaft, and having a phase-advance hydraulic chamber and a phase-retard hydraulic chamber for changing an angular phase of the camshaft relative to the rotary member, an electric pump that supplies working fluid to the phase-advance hydraulic chamber and the phase-retard hydraulic chamber through a phase-advance hydraulic line connected to the phase-advance hydraulic chamber and a phase-retard hydraulic line connected to the phase-retard hydraulic chamber, an electromagnetic solenoid-operated directional control valve disposed between a first pair of fluid lines including a discharge line and an induction line of the pump and a second pair of fluid lines including the phase-advance hydraulic line and the phase-retard hydraulic line, for determining a path through which the working fluid is directed from the discharge line to a first one of the phase-advance hydraulic line and the phase-retard hydraulic line and simultaneously determining a path through which the working fluid is directed from the second hydraulic line to the induction line, a bypass line intercommunicating the discharge line and the induction line, a control unit configured to be electronically connected to at least the solenoid-operated directional control valve, for controlling the solenoid-operated directional control valve depending on an engine operating condition, the control unit comprising a pump-failure detection section that detects a failure in the pump, and the control unit executes a fail-safe operating mode when the failure in the pump is detected by the pump-failure detection section, for creating a phase-control assistance force needed to supply the working fluid through the bypass line selectively to either one of the phase-advance hydraulic chamber and the phase-retard hydraulic chamber by a pulse pressure arising from alternating torque exerted on the camshaft, by controlling the solenoid-operated directional control valve without using the pump. 
   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 system block diagram illustrating an embodiment of an automotive variable valve timing control system with a hydraulically-operated phase converter, cross-sectioned. 
       FIG. 2  is hydraulic circuit diagram illustrating a hydraulic circuit of the variable valve timing control system of the embodiment. 
       FIG. 3  is an explanatory view showing the variable valve timing control system controlled to a phase-advance position. 
       FIG. 4  is an explanatory view showing the variable valve timing control system controlled to a phase-retard position. 
       FIG. 5  is an explanatory view showing the variable valve timing control system held in an intermediate position during a phase-hold operating mode. 
       FIG. 6A  is a longitudinal cross-sectional view explaining the operation of an electromagnetic directional control valve incorporated in the variable valve timing control system of the embodiment, during the phase-advance operating mode. 
       FIG. 6B  is a longitudinal cross-sectional view explaining the operation of the electromagnetic directional control valve, during the phase-hold operating mode. 
       FIG. 6C  is a longitudinal cross-sectional view explaining the operation of the electromagnetic directional control valve, during the phase-retard operating mode. 
       FIG. 7  is a graph showing a waveform characteristic of alternating torque exerted on a camshaft of an internal combustion engine. 
       FIG. 8  is a flow chart showing an engine-stop-period phase control routine executed within a controller incorporated in the variable valve timing control system of the embodiment. 
       FIG. 9  is a flow chart showing a fail-safe routine executed within the controller in presence of a motor failure. 
       FIG. 10  is a cross-sectional view illustrating a modified automotive variable valve timing control system having a lubricating oil hole/passage. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring now to the drawings, particularly to  FIGS. 1 and 2 , the hydraulically-operated phase converter equipped variable valve timing control system of the embodiment is exemplified in an automotive vehicle with a vane-type timing variator. 
   As shown in  FIG. 1 , the variable valve timing control system of the embodiment is comprised of a disc-shaped sprocket  1 , a camshaft  2 , a phase converter  3 , and a hydraulic circuit  4 . Sprocket  1  serves as a rotary member, which is driven in synchronism with rotation of an engine crankshaft (not shown) via a timing chain. Camshaft  2  is provided to operate engine valves such that relative rotation between sprocket  1  and camshaft  2  is permitted. Phase converter  3  is disposed between sprocket  1  and camshaft  2  for converting or changing an angular phase of camshaft  2  relative to sprocket  1 . Hydraulic circuit  4  is connected to phase converter  3  to hydraulically operate phase converter  3 . 
   Sprocket  1  has an outer toothed portion  1   a  formed on its outer periphery and in meshed-engagement with the timing chain, and a central bore  1   b . Sprocket  1  is rotatably supported on the camshaft end by loosely fitting central bore  1   b  of sprocket  1  onto the outer peripheral surface of camshaft  2  in such a manner as to permit relative rotation of camshaft  2  to sprocket  1 . Phase converter  3  is located on the front end (the left sidewall of sprocket  1  in  FIG. 1 ). 
   Camshaft  2  is rotatably supported on a cylinder head (not shown) by means of cam bearings. Camshaft  2  has a series of cams formed integral with the camshaft, for opening and closing engine valves via valve lifters (not shown). 
   Phase converter  3  includes a substantially cylindrical phase-converter housing  5  fixedly connected to sprocket  1  and a vane member  7  fixedly connected to the camshaft end. In the shown embodiment, housing  5  is bolted to the front end of sprocket  1 , whereas vane member  7  is bolted to the front end of camshaft  2  with a vane mounting bolt  6  by tightening the bolt, so that vane member  7  is rotatably housed in the cylindrical housing  5 . In lieu thereof, in order to change a relative phase of camshaft  2  to sprocket  1 , vane member  7  is fixedly connected to the front end of sprocket  1  (the rotary member), whereas housing  5  is fixedly connected to the front end of camshaft  2 . As best seen in  FIG. 2 , housing  5  is integrally formed with four partition wall portions  8 ,  8 ,  8 , and  8  each protruding radially inwards from the inner periphery of the cylindrical housing and has a frusto-conical shape in lateral cross section. Four phase-retard hydraulic chambers  9 ,  9 ,  9 , and  9 , and four phase-advance hydraulic chambers  10 ,  10 ,  10 , and  10  are defined by the four partition walls  8  and vane member  7 . Vane member  7  and four partition wall portions  8  are cooperated with each other to partition the internal space of housing  5  into the first group of phase-advance hydraulic chambers  10  and the second group of phase-retard hydraulic chambers  9 . Housing  5  is comprised of a porous housing, which is made of a porous sintered metal member such as sintered alloy materials. As can be seen from the cross section of  FIG. 1 , the rear opening end of the cylindrical housing  5  is enclosed by the front end face of sprocket  1 , while the front opening end of housing  5  is hermetically covered by a disc-shaped front cover  11  by tightening four bolts. On the other hand, vane member  7  is comprised of a substantially annular ring-shaped vane rotor  12  and four radially-extending vanes or blades  13 ,  13 ,  13 , and  13 . Vane rotor  12  has an axially-extending central bore into which vane mounting bolt  6  is inserted for bolting vane member  7  to the front end of camshaft  2  by axially tightening the vane mounting bolt. Four blades  13  are formed integral with vane rotor  12 , so that four blades  13  are circumferentially spaced apart from each other, and that extend radially outwards from the outer periphery of vane rotor  12 . The two adjacent blades  13  and  13  are circumferentially spaced apart from each other by approximately 90 degrees. Each of four blades  13 ,  13 ,  13 , and  13  is disposed in an internal space defined between the two adjacent partition wall portions  8  and  8 . As best seen in  FIG. 2 , four seals  8   a ,  8   a ,  8   a , and  8   a  are fitted into respective seal grooves formed in apexes of four partition wall portions  8 ,  8 ,  8 , and  8 . Thus, vane rotor  12  is rotatably slidably supported by means of four seals  8   a . In a similar manner, four apex seals (not numbered) are fitted into respective seal grooves formed in apexes of four blades  13 ,  13 ,  13 , and  13 , so that each blade  13  is slidable along the inner peripheral wall surface of housing  5 . As can be seen from the cross section of  FIG. 2 , phase-retard hydraulic chamber  9  is defined between the first sidewall of each of blades  13  facing in the normal-rotational direction of blades  13  and the first sidewall of each of partition wall portions  8  opposing the first sidewall of blade  13 . Similarly, phase-advance hydraulic chamber  10  is defined between the second sidewall of each blade  13  facing in the reverse-rotational direction of blades  13  and the second sidewall of each partition wall portion  8  opposing the second sidewall of blade  13 . Four phase-retard hydraulic chambers  9 ,  9 ,  9 , and  9  are communicated with each other by means of a first group of communication holes or passages formed in vane rotor  12  and crossed to each other. Four phase-advance hydraulic chambers  10 ,  10 ,  10 , and  10  are communicated with each other by means of a second group of communication holes or passages formed in vane rotor  12  and crossed to each other. 
   As can be appreciated from the hydraulic circuit diagram of  FIG. 2 , hydraulic circuit  4  is formed as a closed-loop hydraulic circuit. Hydraulic circuit  4  functions to supply hydraulic pressure (working fluid) selectively to either one of phase-retard hydraulic chamber  9  and phase-advance hydraulic chamber  10 , and also functions to exhaust the hydraulic pressure (working fluid) from the other hydraulic chamber. Returning to  FIG. 1 , the closed-loop hydraulic circuit  4  is comprised of a phase-retard hydraulic line  14 , a phase-advance hydraulic line  15 , a discharge line  16 , a suction line (or an induction line)  17 , an electric motor-driven oil pump  18 , a hydraulic supply line  20 , a bypass line (or a communicating line)  21 , and an electromagnetic directional control valve  22 . Phase-retard hydraulic line  14  is provided to supply or exhaust hydraulic pressure to or from a specified one of four phase-retard hydraulic chambers  9 . Phase-advance hydraulic line  15  is provided to supply or exhaust hydraulic pressure to or from a specified one of four phase-advance hydraulic chambers  10 . As described later, in the system of the embodiment, pump  18  is constructed by a non-reversible pump having an outlet port  32   e  (see  FIG. 2 ) connected to the upstream end of discharge line  16  and an inlet port  32   f  (see  FIG. 2 ) connected to the downstream end of induction line  17 . As shown in  FIGS. 1 and 2 , supply line  20  is connected at its upstream end to a reservoir  19 , and connected at its downstream end to induction line  17 . Bypass line  21  is disposed between discharge line  16  and induction line  17  so as to directly intercommunicate them not through pump  18 . Directional control valve  22  is disposed between a first pair of fluid lines including the phase-advance and phase-retard hydraulic lines  15  and  14  and a second pair of fluid lines including the discharge and induction lines  16  and  17 , for determining the path through which a fluid traverses within a given circuit. As seen from the cross section of  FIG. 1 , the two hydraulic lines  14  and  15  are formed in a fluid-line structural block  30  fixedly mounted on the cylinder head, and arranged in parallel with each other. One end of phase-retard hydraulic line  14  is connected to phase-retard hydraulic chamber  9  via a first inclined line  11   a  formed in front cover  11 . In a similar manner, one end of phase-advance hydraulic line  15  is connected to phase-advance hydraulic chamber  10  via a second inclined line  11   b  formed in front cover  11  and arranged in parallel with the first inclined line  11   a . The other end of phase-retard hydraulic line  14  is connected to a first port  35   a  (described later in reference to  FIGS. 6A–6C ) of directional control valve  22 , whereas the other end of phase-advance hydraulic line  15  is connected to a second port  35   b  (described later in reference to  FIGS. 6A–6C ) of directional control valve  22 . As clearly shown in  FIG. 1 , each of discharge line  16  and induction line  17  is comprised of a comparatively long, vertically-extending oil line segment and a comparatively short, horizontally-extending oil line segment. The junction (or the substantially L-shaped fluid-line portion)  16   b  of the vertically-extending oil line segment and the horizontally-extending oil line segment both constructing discharge line  16  is located in the vicinity of pump  18 . The junction (or the substantially L-shaped fluid-line portion)  17   b  of the vertically-extending oil line segment and the horizontally-extending oil line segment both constructing induction line  17  is also located in the vicinity of pump  18 . A short vertically-extended bore  16   a  is formed in fluid-line structural block  30  in such a manner as to slightly extend downwards in a direction of acceleration of gravity from the junction  16   b  of the vertically-extending oil line segment and the horizontally-extending oil line segment both constructing discharge line  16 . In a similar manner, a short vertically-extended bore  17   a  is formed in fluid-line structural block  30  in such a manner as to slightly extend downwards in the direction of acceleration of gravity from the junction  17   b  of the vertically-extending oil line segment and the horizontally-extending oil line segment both constructing induction line  17 . Each of vertical bores  16   a  and  17   a  is closed at its lower end, and serves as a contaminant trap by which dust, dirt, or other contaminants, such as metallic debris, mixed in working fluid (oil), can be removed and captured or trapped by the aid of deadweights of the contaminants themselves. By virtue of the contaminant-capturing vertical bores  16   a  and  17   a , it is possible to prevent dust, dirt, or other contaminants such as metallic debris from entering directional control valve  4  (or phase converter  3 ). This contributes to a less possibility of the directional control valve sticking due to contaminants, and also enhances the reliability of operation of phase converter  3 . In the shown embodiment, the two contaminant-capturing vertical bores  16   a  and  17   a  are formed at the respective junctions  16   b  and  17   b . At least one contaminant-capturing vertical bore ( 16   a  or  17   a ) may be formed. 
   In the system of the embodiment shown in  FIGS. 1–2 , a one-way check valve  23  is disposed in discharge line  16 , for permitting only the working-fluid flow from discharge port  32   e  of pump  18  to a third port  35   c  (described later in reference to  FIGS. 6A–6C ) of directional control valve  22 . As clearly shown in  FIGS. 1–2 , a pressure switch  24  (a pressure sensor, a pressure detector, or pressure detection means) is connected to or provided in discharge line  16  and located near the discharge port of pump  18 , for detecting or sensing a change in hydraulic pressure in discharge line  16 . Pressure switch  24  closes electrical switching element when a predetermined pressure point is reached, so as to generate a pressure switch signal indicating that the hydraulic pressure in discharge line  16  is higher than the predetermined pressure point. A bypass check valve  25  is disposed in bypass line  21  in such a manner as to permit only the working-fluid flow from induction line  17  to discharge line  16 . In addition to check valves  23  and  25 , a reservoir check valve  26  is disposed in supply line  20  for permitting only the fluid flow from reservoir  19  to induction line  17 . An oil-purifying filter or strainer  27  is provided at the upper opening end of reservoir  19 , and located at a higher level than an oil level Lo of reservoir  19  in a direction of acceleration of gravity. Also, the level of installation of the reservoir  19  itself is set to be higher than the level of installation of the hydraulically-operated phase converter  3  in the direction of acceleration of gravity. 
   As best seen in  FIG. 2 , electric motor-driven pump  18  is comprised of a non-reversible electric motor  31  and a trochoid pump  32  driven by motor  31 . Motor  31  is designed to rotate in only one rotational direction. Motor  31  is controlled responsively to a control signal (or a control current) from a controller, particularly, an electronic control unit (ECU)  33 . Trochoid pump  32  is comprised of a pump housing  32   a , a pump shaft  32   b , and inner and outer rotors  32   c  and  32   d . Pump shaft  32   b  is fixedly connected to the motor shaft of motor  31  so that the pump shaft rotates in synchronism with the rotation of the motor shaft. Inner rotor  32   c  is fitted onto pump shaft  32   b  so that inner rotor  32   c  is driven by pump shaft  32   b . Inner rotor  32   c  has an outer toothed portion, whereas outer rotor  32   d  has an inner toothed portion in meshed-engagement with the outer toothed portion of inner rotor  32   c . Trochoid pump  32  has outlet and inlet ports  32   e  and  32   f  formed in pump housing  32   a . Pump outlet port  32   e  communicates with discharge line  16 , whereas pump inlet port  32   f  communicates with induction line  17 . Electric motor-driven pump  18  (trochoid pump  32 ) is properly driven and shifted from an inoperative state to an operative state, when an open-and-closure timing of the engine valve, for example, an intake valve open timing, often abbreviated to “IVO”, and an intake valve closure timing, often abbreviated to “IVC”, must be phase-changed depending on an engine operating condition. 
   As best seen in  FIGS. 6A–6C , electromagnetic directional control valve  22  is constructed by a single solenoid-actuated, four-way, three-position directional control valve. In more detail, directional control valve  22  is comprised of a valve housing  35 , a slidable valve spool  36 , a valve spring  37 , and an electromagnetic solenoid  38 . Valve housing  35  is closed at one end and substantially cylindrical in shape. Valve housing  35  is fitted into a valve mounting hole  34  formed in the cylinder head. Spool  36  has at least three lands  36   a ,  36   b , and  36   c  (described later) formed integral with the valve spool body. Each of the lands is loosely fitted to the cylindrical bore formed in valve housing  35 , so that spool  36  is axially slidable in valve housing  35 . When solenoid  38  is energized, solenoid  38  acts to attract or move the spool rightwards (viewing  FIGS. 6A–6C ) against the spring force of valve spring  37 . Valve housing  35  has five ports, namely the first port  35   a  connected to the other end of phase-retard hydraulic line  14 , the second port  35   b  connected to the other end of phase-advance hydraulic line  15 , the third port  35   c  connected to the downstream end of discharge line  16 , the fourth port  35   d  connected to the upstream end of induction line  17 , and the fifth port (or a drain port)  35   e  connected via a drain line  39  to the upstream end of induction line  17 . The first, second, third, fourth, and fifth ports  35   a ,  35   b ,  35   c ,  35   d , and  35   e  are formed in valve housing  5  as radial bores extending radially with respect to the axis of spool  36 . Drain port (the fifth port)  35   e  is located near the bottom of valve housing  35 . 
   Spool  36  is formed with the first, second, and third lands  36   a ,  36   b , and  36   c  axially spaced from each other, for properly opening and closing the ports  35   a – 35   e . Spool  36  is also formed with a communicating bore  40  comprised of a comparatively long, axially-extending central bore portion and a comparatively short, radial bore portion. The radial bore portion of communicating bore  40  communicates the fourth port  35   d  connected to induction line  17 , whereas the axial bore portion of communicating bore  40  communicates a spring chamber of valve spring  37 . The spring chamber of valve spring  37  is opened to the atmosphere. By virtue of communicating bore  40  intercommunicating the fourth port  35   d  connected to induction line  17  and the spring chamber opened to the atmosphere, it is possible to prevent a resistance to sliding movement of spool  36  from being generated or developed during operation of directional control valve  22 . 
   Electromagnetic solenoid  38  includes a solenoid housing  38   a , an electrically energized coil  38   b , and a plunger (or an armature)  38   c . As clearly shown in  FIGS. 6A–6C , solenoid housing  38   a  has a cylindrical bore closed at one end. Coil  38   b  is installed in the cylindrical bore of solenoid housing  38   a  and arranged annularly along the inner periphery of solenoid housing  38   a , so that plunger  38   c  is axially slidable in the coil. When coil  38   b  is energized, it creates an electromagnetic force that repels plunger  38   c , such that plunger  38   c  comes out of the solenoid housing. Coil  38   b  of electromagnetic solenoid  38  is electrically connected through a connector  38   d  to the output interface of ECU  33 , so that the axial position of spool  36  is controlled in response to a command signal (or a control pulse signal) generated from the output interface of ECU  33  to coil  38   b . Concretely, the axial position of spool  36  is controlled by way of pulse-width modulated (PWM) control for the exciting current (or the control pulse signal) applied to coil  38   b  of electromagnetic solenoid  38 . For instance, when the pulse-width modulated signal of a predetermined high duty ratio such as “100%” is applied to coil  38   b , as shown in  FIG. 6C , spool  36  slides axially rightwards against the spring force of valve spring  37  and is held at its maximum actuated position (or the rightmost position). With spool  36  held at the maximum actuated position, fluid communication between discharge line  16  and phase-retard hydraulic line  14  is established and simultaneously fluid communication between induction line  17  and phase-advance hydraulic line  15  is established. When the pulse-width modulated signal of a predetermined middle duty ratio such as an intermediate value, which is substantially midway between “100%” and “0%”, is applied to coil  38   b , the spring force of valve spring  37  and the repulsion force created by coil  38   b  are suitably balanced to each other and thus spool  36  is held at an intermediate axial position (see  FIG. 6B ). With spool  36  held at the intermediate axial position shown in  FIG. 6B , the first port  35   a  is closed or blocked by the first land  36   a , and simultaneously the second port  35   b  is closed or blocked by the second land  36   b , and thus directional control valve  22  is kept at the shutoff position and there is no fluid flow through directional control valve  22 . In contrast to the above, when the pulse-width modulated signal of a predetermined low duty ratio such as “0%” is applied to coil  38   b , in other words, there is no exciting current applied to coil  38   b , as shown in  FIG. 6A , spool  36  is held at its spring-offset position (or the leftmost position). With spool  36  held at the spring-offset position, fluid communication between discharge line  16  and phase-advance hydraulic line  15  is established and simultaneously fluid communication between induction line  17  (or drain line  39 ) and phase-retard hydraulic line  14  is established. 
   As can be seen from the cross section of  FIG. 1 , for the purpose of oil-leakage prevention, a plurality of oil seals are provided. Concretely, oil seals  41   a ,  41   b ,  41   b , and  41   b  are placed at the fitting portion between front cover  11  and fluid-line structural block  30 . A pair of oil seals  42   a  and  42   a  are placed at the fitting portions between the left-hand sidewall of phase-converter housing  5  and front cover  11  and between the right-hand sidewall of phase-converter housing  5  and the left-hand sidewall of sprocket  1 . An oil seal  42   b  is also placed at the fitting portion between the right-hand sidewall of vane rotor  12  and the left-hand sidewall of sprocket  1 . 
   In the system of the embodiment shown in  FIG. 1 , an oil pump  43  that supplies moving engine parts with lubricating oil, also serves as a supplementary working-fluid source for the hydraulically-operated phase converter  3 . In addition to electric motor-driven oil pump  18 , oil pump  43  constructs a part of hydraulic circuit  4 . Thus, by the use of oil pump  43  as well as electric motor-driven oil pump  18 , if needed, a comparatively small amount of lubricating oil (working fluid) discharged from oil pump  43  (the supplementary working-fluid source) can be supplied to each of phase-retard hydraulic chambers  9 ,  9 ,  9 , and  9  and phase-advance hydraulic chambers  10 ,  10 ,  10 , and  10 . In more detail, as clearly shown in  FIG. 1 , a supply line  44 , communicating the discharge port of oil pump  43 , is also formed in fluid-line structural block  30  and the engine cylinder head. The downstream end  44   d  of supply line  44  is communicated with a working-fluid chamber  45  defined between the front end face of vane rotor  12  and the inner periphery of each of blades  13 . In the shown embodiment, each of at least two blades ( 13 ,  13 ) of the four blades has an inclined oil passage  46  (see  FIG. 1 , and  FIGS. 3–5 ). As seen from the cross section of  FIG. 1 , the previously-noted working-fluid chamber  45  is communicated with each of four phase-retard hydraulic chambers  9  and four phase-advance hydraulic chambers  10  through inclined oil passages  46  and  46  and a side clearance space  47 . Side clearance space  47  is defined between the rear end face of each blade  13  and the front end face (the left-hand sidewall) of sprocket  1 . Side clearance space  47  serves as a fluid-flow constricting orifice (a fixed orifice). The supplementary working-fluid circuit, which is constructed by oil pump  43 , supply line  44 , working-fluid chamber  45 , inclined oil passages  46  and  46 , and side clearance space  47 , permits part of lubricating oil (working fluid) discharged from oil pump  43  to flow into each of hydraulic chambers  9  and  10  through supply line  44 , working-fluid chamber  45 , inclined oil passages  46  and  46 , and side clearance space  47 . This enables air mixed in working fluid in each of hydraulic chambers  9  and  10  to be forcibly exhausted through the porous housing  5  to the exterior space, and also enables compensation for the insufficiency of oil corresponding to the quantity of air exhausted. An oil-purifying filter  48  is also disposed in the upstream portion of supply line  44 . Additionally, a one-way check valve  49  is disposed in supply line  44  and located downstream of oil-purifying filter  48  in a manner so as to permit only the working-fluid flow to the downstream direction of supply line  44 . 
   Electronic control unit (ECU)  33  generally comprises a microcomputer. ECU  33  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 ECU  33  receives input information from various engine/vehicle switches and sensors, namely a crank angle sensor (or a crankshaft position sensor), a cam angle sensor (or a camshaft position sensor), an airflow meter, an engine temperature sensor (or an engine coolant temperature sensor), a throttle valve opening sensor (or a throttle position sensor), and an ignition switch. The crank angle sensor is provided for detecting revolutions of the engine crankshaft. Assuming that the number of engine cylinders is “n”, the crank angle sensor generates a reference pulse signal REF at a predetermined crank angle for every crank angle 720°/n, and simultaneously generates a unit pulse signal (1° or 2°). The processor of ECU  33  arithmetically calculates engine speed Ne based on the period of the reference pulse signal REF from the crank angle sensor. The cam angle sensor generates a cam-angle sensor signal indicative of an angular position θ CAM  of camshaft  2 . The airflow meter measures or detects a quantity Qa of fresh air entering the engine cylinders. Engine temperature sensor detects an engine temperature, such as an engine coolant temperature Tw. The throttle valve opening sensor is located near an electronically-controlled throttle to generate a throttle sensor signal indicative of a throttle opening TVO, which is generally defined as a ratio of an actual throttle angle to a throttle angle obtained at wide open throttle. The ignition switch generates an ignition switch signal indicative of whether the ignition switch is turned ON or turned OFF. Within ECU  33 , the central processing unit (CPU) allows the access by the I/O interface of input informational data signals from the previously-discussed engine/vehicle switches and sensors. The CPU of ECU  33  is responsible for carrying the valve timing control program (the motor/directional-control-valve control program for valve timing control or phase control based on an engine operating condition containing at least one of engine speed and engine load), the engine-stop-period phase control program (described later in reference to the flow chart shown in  FIG. 8 ), and the fail-safe program (described later in reference to the flow chart shown in  FIG. 9 ) stored in memories and is capable of performing necessary arithmetic and logic operations. Computational results (arithmetic calculation results), that is, calculated output signals are relayed through the output interface circuitry of ECU  33  to output stages, namely non-reversible electric motor  31 , and an electromagnetic solenoid (described later) of electromagnetic directional control valve  22 . The output interface circuitry of ECU  33  is also connected to a warning system having a warning buzzer and/or an instrument-cluster warning lamp  33   WL  (described later), which comes on in response to an alarm signal from ECU  33 . Basically, the processor of ECU  33  estimates or determines the current engine operating condition based on the latest up-to-date information from the engine/vehicle switches and sensors. Thereafter, the processor of ECU  33  executes motor current control for motor  31  of motor-driven oil pump  18  and simultaneously executes pulse signal control (PWM control) for electric current applied to coil  38   b  of electromagnetic directional control valve  22 . More concretely, the variable valve timing control system of the embodiment of  FIGS. 1–2  operates as follows. 
   In a low engine speed and low engine load range (or with the engine at an idle rpm) just after the engine has been started, a control current is applied to motor  31  so as to rotate motor  31  in such a manner as to achieve an engine valve timing suitable to low engine speed and low engine load operation, while coil  38   b  of directional control valve  22  is de-energized with the duty ratio of the PWM signal switched to “0%”. Under such a low speed and low load condition, as shown in  FIGS. 6A and 3 , spool  36  axially slides toward the leftmost spool position (the spring-offset position) by the spring force of valve spring  37 , and is held at the spring-offset position. Owing to the axial positions of lands  36   a–   36   c  with spool  36  held at the spring-offset position, fluid communication between discharge line  16  and phase-advance hydraulic line  15  is established, fluid communication between discharge line  16  and phase-retard hydraulic line  14  is blocked, and at the same time fluid communication between phase-retard hydraulic line  14  and induction line  17  (or drain line  39 ) is established. Thus, as appreciated from the arrow indicating the fluid-flow direction in  FIG. 3 , working fluid discharged from trochoid pump  32  flows into each of phase-advance hydraulic chambers  10  through discharge line  16 , check valve  23 , and phase-advance hydraulic line  15 , whereas working fluid in each of phase-retard hydraulic chambers  9  flows into induction line  17  through phase-retard hydraulic line  14  and then returns to the inlet port  32   f  (see  FIG. 2 ) of trochoid pump  32  (motor-driven oil pump  18 ). As a result of this, the hydraulic pressure in each of phase-retard hydraulic chambers  9  becomes relatively low, whereas the hydraulic pressure in each of phase-advance hydraulic chambers  10  becomes relatively high. As shown in  FIG. 3 , each of blades  13  of the hydraulically-operated vane-type phase converter  3  rotates in the same rotational direction (in the clockwise direction) as the valve operating mechanism including sprocket  1  and camshaft  2 , and therefore the angular phase of camshaft  2  relative to sprocket  1  can be converted and phase-advanced. Thus, an open-and-closure timing of the engine valve, for example, an intake valve open timing IVO and an intake valve closure timing IVC, can be phase-advanced. By virtue of the phase-advanced IVO and IVC suitable to the low speed and low load operation, it is possible to remarkably enhance the combustion efficiency, utilizing inertial intake-air mass, thereby enhancing the combustion stability and reducing the fuel consumption rate during the low speed and low load condition. Thereafter, when the maximum phase-advanced position of camshaft  2  has been reached by way of the previously-discussed phase-advance action, the cam angle sensor generates a cam-angle sensor signal indicative of the maximum phase-advanced position of camshaft  2  relative to sprocket  1 . Responsively to the cam-angle sensor signal indicative of the maximum phase-advanced position of camshaft  2 , ECU  33  operates to reduce the applied motor current value to “0” so as to stop non-reversible electric motor  31 . At the same time, ECU  33  controls the duty ratio of the PWM signal applied to coil  38   b  of electromagnetic directional control valve  22  to the previously-noted predetermined middle duty ratio, such that the spring force of valve spring  37  and the repulsion force created by coil  38   b  of the solenoid-actuated directional control valve are suitably balanced to each other and thus spool  36  is held at the intermediate axial position shown in  FIG. 6B . Holding spool  36  at the intermediate axial position of  FIG. 6B  by way of the PWM control for electric current applied to coil  38   b , means that the first port  35   a  is closed by the first land  36   a , and simultaneously the second port  35   b  is closed by the second land  36   b . As a result, vane member  7  can be maintained at its maximum phase-advanced angular position (see  FIG. 3 ). 
   Thereafter, assuming that the engine operating condition has been changed from low speed and low load operation to high speed and high load operation, a control current is applied to motor  31  so as to change from the valve timing (IVO, IVC) suitable to low speed and low load operation to the valve timing (IVO, IVC) suitable to high speed and high load operation, while the PWM signal of a high duty ratio suitable to the high speed and high load operation is applied to coil  38   b  of directional control valve  22 . Under such a high speed and high load condition, as shown in  FIGS. 6C and 4 , spool  36  axially slides toward the rightmost spool position (the maximum actuated position) against the spring force of valve spring  37 , and is held at the maximum actuated position. Owing to the axial positions of lands  36   a–   36   c  with spool  36  held at the maximum actuated position, fluid communication between discharge line  16  and phase-retard hydraulic line  14  is established, fluid communication between discharge line  16  and phase-advance hydraulic line  15  is blocked, and at the same time fluid communication between phase-advance hydraulic line  15  and induction line  17  is established. Thus, as appreciated from the arrow indicating the fluid-flow direction in  FIG. 4 , working fluid in each of phase-advance hydraulic chambers  10  flows through phase-advance hydraulic line  15  and directly returns to induction line  17 , whereas working fluid discharged from trochoid pump  32  (motor-driven oil pump  18 ) flows into each of phase-retard hydraulic chambers  9  through discharge line  16 , check valve  23 , and phase-retard hydraulic line  14 . As a result of this, the hydraulic pressure in each of phase-retard hydraulic chambers  9  becomes relatively high, whereas the hydraulic pressure in each of phase-advance hydraulic chambers  10  becomes relatively low. As shown in  FIG. 4 , each of blades  13  of the hydraulically-operated vane-type phase converter  3  rotates in the opposite direction (in the anticlockwise direction) opposing to the rotational direction of the valve operating mechanism including sprocket  1  and camshaft  2 , and therefore the angular phase of camshaft  2  relative to sprocket  1  can be converted and phase-retarded. Thus, the IVO and IVC are phase-retarded. By virtue of the phase-retarded IVO and IVC suitable to the high speed and high load operation, it is possible to remarkably enhance the engine power output during the high speed and high load condition. Thereafter, when the maximum phase-retarded position of camshaft  2  has been reached by way of the previously-discussed phase-retard action, the cam angle sensor generates a cam-angle sensor signal indicative of the maximum phase-retarded position of camshaft  2  relative to sprocket  1 . Responsively to the cam-angle sensor signal indicative of the maximum phase-retarded position of camshaft  2 , ECU  33  operates to reduce the applied motor current value to “0” so as to stop the motor. At the same time, ECU  33  controls the duty ratio of the PWM signal applied to coil  38   b  of electromagnetic directional control valve  22  to the previously-noted predetermined middle duty ratio, such that the spring force of valve spring  37  and the repulsion force created by coil  38   b  of the solenoid-actuated directional control valve are suitably balanced to each other and thus spool  36  is held at the intermediate axial position shown in  FIG. 6B . Holding spool  36  at the intermediate axial position of  FIG. 6B  by way of the PWM control for electric current applied to coil  38   b , means that the first port  35   a  is closed by the first land  36   a , and simultaneously the second port  35   b  is closed by the second land  36   b . As a result, vane member  7  can be maintained at its maximum phase-retarded angular position (see  FIG. 4 ). 
   Thereafter, assuming that the engine operating condition has been changed from the high speed and high load operation to middle speed and middle load operation, by way of both of motor current control for electric motor  31  and PWM control for electric current applied to coil  38   b  of the solenoid-actuated directional control valve  22 , vane member  7  rotates clockwise from its maximum phase-retarded angular position toward its intermediate angular position shown in  FIG. 5 . Immediately when the intermediate angular position of vane member  7  has been reached (see  FIG. 5 ), ECU  33  controls the duty ratio of the PWM signal applied to coil  38   b  of directional control valve  22  to the previously-noted predetermined middle duty ratio, so that spool  36  is held at the intermediate axial position shown in  FIG. 6B . By holding spool  36  at the intermediate axial position of  FIG. 6B  by way of the PWM control for electric current applied to coil  38   b , vane member  7  can be maintained at the intermediate angular position (see  FIG. 5 ) located midway between the maximum phase-advanced angular position and the maximum phase-retarded angular position. As a result of this, it is possible to realize the optimal valve timing control suitable to the middle engine speed and middle engine load, thus balancing two contradictory requirements, namely reduced fuel consumption rate and enhanced engine power output during the middle speed and middle load condition. 
   Thereafter, suppose that the engine operating condition shifts from the middle speed and middle load operation (or the low speed and low load operation) to engine stop operation. During a time period of engine idling that the engine is shifting to the stopped state, as described later in reference to the flow chart of  FIG. 8 , ECU  33  executes the engine-stop-period phase control routine. Briefly speaking, during the time period of engine idling that the engine is shifting to the stopped state, the engine valve timing (IVO, IVC) is temporarily controlled to the phase-retard direction, that is, vane member  7  is temporarily controlled to its maximum phase-retarded angular position by way of both of motor current control for electric motor  31  and PWM control for electric current applied to coil  38   b  of the solenoid-actuated directional control valve. Exactly speaking, during a time period from a point of time when the ignition switch becomes turned OFF to a point of time when the engine stopped state has been completed, in other words, during the engine-stop-period phase control, electric motor-driven pump  18 , which is in operative (ON) during phase change, is further driven continuously for a brief moment, even after the ignition switch has been turned OFF, or electric motor-driven pump  18 , which is in inoperative (OFF) after completion of phase change, is driven momentarily for a brief moment, even after the ignition switch has been turned OFF. By momentarily driving electric motor-driven pump  18  for a brief moment after the turning-OFF action of the ignition switch, vane member  7  can be shifted or preset to such an engine-restart standby position (such an engine-restartable valve-timing position) as to be properly phase-advanced from the maximum phase-retarded angular position (the initial position or the reference phase-angle position) shown in  FIG. 4  to the intermediate angular position shown in  FIG. 5 . The engine-restart standby position substantially corresponds to a valve timing (a relative angular position of camshaft  2  to sprocket  1 ), which is preprogrammed to be suitable for an engine restarting period. Presetting vane member  7  to the engine-restart standby position enhances or improves the engine restartability. 
   As can be appreciated from the above, the hydraulically-operated phase converter equipped variable valve timing control system of the embodiment can provide the following operation and effects ( 1 )–( 12 ). 
   ( 1 ) As is generally known, for instance, in the low engine speed range, a comparatively large magnitude of alternating torque (see  FIG. 7 ) whose oscillation frequency is large, may be exerted on the camshaft owing to the spring force of the engine valve spring for each engine valve and the reaction force resulting from each engine valve opening and closing during operation of the engine. Owing to the alternating torque transmitted from camshaft  2  to vane member  7 , a pulse pressure is applied to the working fluid in each of phase-retard hydraulic chambers  9 ,  9 ,  9 , and  9 , and phase-advance hydraulic chambers  10 ,  10 ,  10 , and  10 . For the reasons discussed above, during the phase-retard operating mode, usually, there is an increased tendency for the pulse pressure to be transmitted from phase-retard hydraulic line  14  to discharge line  16 . In the system of the embodiment, on the one hand, check valve  23  is disposed in discharge line  16 . Thus, it is possible to effectively block or shut off the undesirable transmission of pulse pressure from phase-retard hydraulic line  14  to discharge line  16 , thereby effectively suppressing or preventing the load (i.e., undesirable energy loss) carried on the motor shaft of non-reversible electric motor  31  of pump  18  from undesirably increasing. In the system of the embodiment, on the other hand, there is no check valve disposed in induction line  17 . Thus, the pulse pressure can be permitted to be transmitted from phase-advance hydraulic chamber  10  through phase-advance hydraulic line  15  to induction line  17  during the phase-retard operating mode. The pulse pressure promotes the outflow of working fluid from phase-advance hydraulic chamber  10  through phase-advance hydraulic line  15  and induction line  17  to inlet port  32   f . The promoted outflow serves as an assistance force (an assistive drive source for electric motor-driven pump  18 ), thus effectively reducing electric load needed to drive the motor shaft of non-reversible electric motor  31  of pump  18 . In other words, during operation of the system of the embodiment, the system permits the pulse pressure to be transmitted from induction line  17  through inlet and outlet ports  32   f  and  32   e  of pump  18  to discharge line  16 , and whereby the pulse pressure acts as an assistance force (an assistive drive source for electric motor-driven pump  18 ). As a result, the system of the embodiment realizes small-sizing of electric motor-driven pump  18  and reduced manufacturing costs. 
   ( 2 ) When pump  18  is in inoperative during the phase-hold operating mode, each of the first and second ports  35   a  and  35   b  of directional control valve  22  are closed and thus there is no fluid flow through directional control valve  22  kept at the shut-off position. During the phase-hold operating mode, it is possible to certainly prevent the pulse pressure arising from the alternating torque from being transmitted from either one of phase-retard hydraulic line  14  and phase-advance hydraulic line  15  via directional control valve  22  to either one of discharge line  16  and induction line  17 . Thus, it is possible to avoid the electric load needed to drive the motor shaft of non-reversible electric motor  31  of pump  18  from being affected by the pulse pressure during the phase-hold operating mode. Thus, it is possible to reduce the amount of electric current supplied to motor  31  when re-driving the motor shaft of motor  31 . 
   ( 3 ) When the motor shaft of motor  31  of electric motor-driven pump  18  begins to rotate and thus the pumping action is insufficient or when pump  18  cannot be satisfactorily rotated owing to a pump failure (or a motor failure), bypass check valve  25  can be opened with the aid of the pulse pressure arising from the alternating torque and transmitted to induction line  17 . With bypass check valve  25  opened with the aid of the pulse pressure, the working-fluid flow from induction line  17  via bypass line  21  to discharge line  16  is permitted, thus enabling working-fluid supply from one of phase-retard hydraulic chamber  9  and phase-advance hydraulic chamber  10  via bypass line  21  to the other during low pump speed operation of pump  18  or in presence of a failure in pump  18 . Thus, it is possible to operate the hydraulically-operated phase converter  3  by suitably controlling electromagnetic directional control valve  22  depending on an engine operating condition, even during low pump speed operation of pump  18  or in presence of a failure in pump  18 . 
   ( 4 ) Furthermore, phase-converter housing  5  is comprised of a porous housing, which is made of a porous sintered metal member such as sintered alloy materials. Even when air has been mixed in working fluid in each of phase-retard hydraulic chamber  9  and phase-advance hydraulic chamber  10  owing to oil leakage from the interior of phase converter  3  or oil leakage from each of phase-retard hydraulic line  14  and phase-advance hydraulic line  15 , and discharge line  16  and induction line  17 , arranged between pump  18  and phase converter  3  in the engine stopped state, it is possible to exhaust the air through the porous housing  5  to the exterior space by operating pump  18  and by rising the hydraulic pressure in each of hydraulic chambers  9  and  10 . As a result, it is possible to prevent the control accuracy of variable valve timing control accomplished by means of phase converter  3  from being deteriorated owing to the air mixed in working fluid in each of hydraulic chambers  9  and  10 . From the property of the porous housing  5 , made of a porous sintered metal material, housing  5  permits only the air mixed in working fluid in each of hydraulic chambers  9  and  10  to be exhausted to the exterior space, but prevents undesirable leakage of working fluid having a comparatively high viscosity, thus avoiding a pressure drop in working fluid delivered from discharge line  16  to either one of hydraulic chambers  9  and  10 . 
   ( 5 ) Moreover, reservoir check valve  26  is disposed in supply line  20  (see  FIGS. 1–2 ) arranged between induction line  17  and reservoir  19 , to permit only the working-fluid flow from reservoir  19  via reservoir check valve  26  to induction line  17 , and to prevent any flow in the opposite direction. In the engine stopped state, it is possible to effectively charge and store or hold working fluid in induction line  17 , thus preventing air from being mixed in working fluid in induction line  17 . 
   ( 6 ) As can be seen from the system block diagram of  FIG. 1  and the hydraulic circuit of  FIG. 2 , the level of installation of reservoir  19  is set to be higher than the level of installation of the hydraulically-operated phase converter  3  in a direction of acceleration of gravity. By way of the setting of installation of reservoir  19  higher than the level of installation of the hydraulically-operated phase converter  3 , the working fluid in reservoir  19  can be sufficiently charged and stored in the fluid lines arranged between phase converter  3  and pump  18 , even in the engine stopped state. Thus, it is possible to prevent a vacuum from being created in the hydraulic pressure system laid out between phase converter  3  and pump  18 , thus preventing air from being undesirably mixed in working fluid in each of the hydraulic lines arranged between phase converter  3  and pump  18 . 
   ( 7 ) Additionally, oil-purifying filter  27  of reservoir  19  is installed at a higher level than the oil level Lo of working fluid stored in reservoir  19  in a direction of acceleration of gravity. The working fluid splashed during operation of the valve operating mechanism tends to be dripped onto the upper face of oil-purifying filter  27 . Thus, it is possible to effectively filter out or remove dust, dirt, or other contaminants mixed in the working fluid through oil-purifying filter  27 . The upper oil-purifying filter  27 , which is laid out at a higher level than the oil level Lo of reservoir  19 , never serves as a fluid-flow resistance, in other words, an undesirable load carried on the motor shaft of electric motor-driven pump  18  during working-fluid supply from reservoir  19  to pump  18 . This prevents or avoids the responsiveness of operation of electric motor-driven pump  18  from being deteriorated. 
   ( 8 ) To provide the fluid-tight sealing action for each of phase-retard hydraulic chambers  9 ,  9 ,  9 , and  9  and phase-advance hydraulic chambers  10 ,  10 ,  10 , and  10  of phase converter  3  and to prevent leakage of working fluid from at least hydraulic chambers  9  and  10 , oil seals  41   a ,  41   b ,  41   b , and  41   b  are placed at the fitting portion between front cover  11  and fluid-line structural block  30  formed with phase-retard hydraulic line  14 , phase-advance hydraulic line  15  and supply line  44 . Oil seals  42   a  and  42   a  are placed at the fitting portions between phase-converter housing  5  and front cover  11  and between phase-converter housing  5  and sprocket  1 . An oil seal  42   b  is also placed at the fitting portion between vane rotor  12  and sprocket  1 . Thus, it is possible to effectively prevent leakage of oil from at least phase-retard hydraulic chambers  9 ,  9 ,  9 , and  9  and phase-advance hydraulic chambers  10 ,  10 ,  10 , and  10 , in the engine stopped state, thereby preventing air from being mixed in working fluid in each of hydraulic chambers  9  and  10 . 
   ( 9 ) For the purpose of working fluid supply (or hydraulic pressure supply), electric motor-driven oil pump  18  is provided as a main working fluid source (or a main hydraulic pressure source). Also provided is oil pump  43  that supplies moving engine parts with lubricating oil and serves as a supplementary working-fluid source (or a supplementary pump operable independently of electric motor-driven pump  18 ) for the hydraulically-operated phase converter  3 . Phase converter  3  is formed with an air bleeder (air bleeding means) that acts to exhaust air undesirably mixed in working fluid in each of hydraulic chambers  9  and  10  to the exterior space. As discussed above, the porous phase-converter housing  5 , which is made of a porous sintered metal material, serves as the air bleeder. By the aid of the working fluid pressurized by and oil pump  43  and discharged and routed from oil pump  43  (the supplementary working-fluid source) through supply line  44 , working-fluid chamber  45 , inclined oil passages  46  and  46  and side clearance space  47  into each of phase-retard hydraulic chamber  9  and phase-advance hydraulic chamber  10 , it is possible to effectively forcibly exhaust the air mixed in working fluid in each of hydraulic chambers  9  and  10  through the porous housing  5  (serving as the air bleeder) to the exterior space. At the same time, by the aid of the pressurized working fluid discharged from oil pump  43  (the supplementary working-fluid source) to each of hydraulic chambers  9  and  10 , it is possible to suitably compensate for the insufficiency of oil (working fluid) corresponding to the quantity of air exhausted. As a result, it is possible to prevent the control accuracy of variable valve timing control (phase control) of phase converter  3  from being deteriorated owing to the air mixed in working fluid in each of hydraulic chambers  9  and  10 . Additionally, even when electric motor-driven pump  18  of two pumps  18  and  43  has been failed, it is possible to charge or feed working fluid from oil pump  43  to each of hydraulic chambers  9  and  10  of phase converter  3 . 
   ( 10 ) As previously described (see the effect ( 3 )), with bypass check valve  25  opened due to the pulse pressure, the working-fluid flow from induction line  17  through bypass line  21  to discharge line  16  is permitted, thus enabling working-fluid supply from one of hydraulic chambers  9  and  10  via bypass line  21  to the other during low pump speed operation of pump  18  or in presence of a failure of pump  18 . Moreover, in the system of the embodiment, even during low pump speed operation of pump  18  or in presence of a failure in pump  18 , it is possible to deliver or feed working fluid from oil pump  43  (the supplementary working-fluid source) through supply line  44 , working-fluid chamber  45 , inclined oil passages  46  and  46  and side clearance space  47  into each of hydraulic chambers  9  and  10 . Thus, it is possible to more certainly keep a sufficient working-fluid charged state wherein working fluid is satisfactorily charged and stored in each of hydraulic chambers  9  and  10 , even during low pump speed operation of pump  18  or in presence of a failure in pump  18 . 
   ( 11 ) Furthermore, the working fluid, discharged from oil pump  43  (the supplementary working-fluid source), and then routed through the working-fluid passage  44 – 46  into each of hydraulic chambers  9  and  10  can be greatly restricted or constricted by means of a fluid-flow constricting orifice (side clearance space  47 ) located downstream of the working-fluid passage  44 – 46  and intercommunicating both of phase-retard hydraulic chamber  9  and phase-advance hydraulic chamber  10 . By the provision of the fluid-flow constricting orifice (side clearance space  47 ), it is possible to prevent a pressure differential between phase-retard hydraulic chamber  9  and phase-advance hydraulic chamber  10  from being created during operation of oil pump  43 . That is to say, the fluid-flow constricting orifice (side clearance space  47 ) acts to avoid or prevent the hydraulically-operated phase converter  3  from being undesirably operated owing to the working-fluid supply from oil pump  43  to each of hydraulic chambers  9  and  10  of phase converter  3 . Additionally, the fluid-flow constricting orifice is comprised of side clearance space  47 , which is defined between the inner peripheral surface of phase-converter housing  5  and the end face of vane member  7  in sliding-contact with the inner peripheral surface of housing  5 . More concretely, the fluid-flow constricting orifice is comprised of side clearance space  47 , which is defined between the rear end face of each blade  13  of vane member  7  and the front end face (the left-hand sidewall) of sprocket  1 . In this manner, the fluid-flow constricting orifice (side clearance space  47 ) is simply formed or defined between the existing phase-converter housing  5  and vane member  7 . Thus, there is no necessity of an additional orifice. Side clearance space  47 , easily simply defined between vane member  7  and sprocket  1 , contributes to the simplified hydraulic circuit (or the simplified hydraulic system) for the hydraulically-operated phase converter  3 . 
   ( 12 ) Also provided is oil-purifying filter  48  disposed in the upstream portion of supply line  44  of oil pump  43  (the supplementary working-fluid source). By the provision of oil-purifying filter  48  disposed in supply line  44 , it is possible to effectively filter out or remove dust, dirt, or other contaminants contained in working fluid discharged from oil pump  43  through oil-purifying filter  48 . Oil-purifying filter  48  disposed in supply line  44  serves as a fluid-flow resistance, thus producing a slight energy loss (i.e., a slight pressure drop). However, there is no problem, since oil pump  43  itself functions as the supplementary working-fluid source that supplies a slight amount of working fluid to the hydraulically-operated phase converter  3 , if needed. 
   Referring now to  FIG. 8 , there is shown the engine-stop-period phase control routine executed within the processor of ECU  33  incorporated in the variable valve timing control system of the embodiment. The engine-stop-period phase control routine shown in  FIG. 8  is executed as time-triggered interrupt routines to be triggered every predetermined time intervals such as 10 milliseconds. 
   At step S 1 , a check is made to determine whether switching from the turned-ON state of the ignition switch to the turned-OFF state occurs during idling of the engine. When the answer to step S 1  is in the affirmative (YES), that is, when the ignition switch becomes turned OFF, the routine proceeds from step S 1  to step S 2 . When the answer to step S 1  is in the negative (NO), that is, when the ignition switch remains turned ON, the routine returns to the main program to execute the usual variable valve timing control based on the current engine operating condition. 
   At step S 2 , the latest up-to-date information concerning engine speed Ne, determined based on the sensor signal from the crank angle sensor, is read. 
   At step S 3 , a check is made to determine whether the current engine speed Ne is less than or equal to a predetermined engine-speed lower limit N THL , such as 50 rpm. When the answer to step S 3  is negative (NO), that is, in case of Ne&gt;N THL , the routine proceeds from step S 3  to step S 4 . 
   At step S 4 , under the condition defined by the inequality of Ne&gt;N THL , the hydraulically-operated vane-type phase converter  3  (exactly, each of blades  13  of vane member  7  of phase converter  3 ) is controlled from the initial position (or the reference phase-angle position) obtained at the beginning of the engine starting period to the previously-discussed engine-restart standby position, properly phase-advanced from the maximum phase-retarded angular position shown in  FIG. 4  to the intermediate angular position shown in  FIG. 5 . The initial position of phase converter  3  corresponds to the maximum phase-retard position of camshaft  2  (in other words, the maximum phase-retarded angular position of vane member  7 ), since vane member  7  tends to rotate toward the maximum phase-retarded angular position due to its inertia at the beginning of the engine starting period. After step S 4 , step S 5  occurs. 
   At step S 5 , a check is made to determine, based on the latest up-to-date information concerning cam angle θ CAM  determined based on the cam angle sensor signal, whether the engine-restart standby position of phase converter  3  (i.e., the intermediate angular position of vane member  7  shown in  FIG. 5 ) has been reached. When the answer to step S 5  is negative (NO), that is, when the engine-restart standby position of phase converter  3  has not yet been reached, the routine returns from step S 5  again to step S 4 , so as to succeedingly control the hydraulically-operated phase converter  3  to the phase-advance side (exactly, the engine-restart standby position). Conversely when the answer to step S 5  is affirmative (YES), that is, when the engine-restart standby position of phase converter  3  has been reached, the routine proceeds from step S 5  to step S 6 . 
   At step S 6 , in order to achieve the phase-hold operating mode and to retain the engine-restart standby position of phase converter  3  unchanged, electromagnetic directional control valve  22  is controlled to its shut-off position (i.e., the intermediate axial position of spool  36  shown in  FIG. 6B ) by way of the PWM control. After step S 6 , the routine flows from step S 6  to step S 2  to repeatedly execute the engine-stop-period phase control routine. 
   Returning to step S 3 , conversely when the answer to step S 3  is affirmative (YES), that is, in case of Ne≦N THL , the routine proceeds from step S 3  to step S 7 . 
   At step S 7 , an engine-stop timer is set to a predetermined delay period during which the shut-off position of electromagnetic directional control valve  22  is retained unchanged, for measuring an elapsed time from the point of time when switching to the ignition-switch turned-OFF state has occurred. 
   At step S 8 , the duty ratio of the PWM signal applied to coil  38   b  of electromagnetic directional control valve  22  is fixed to the predetermined middle duty ratio so as to hold electromagnetic directional control valve  22  at the shut-off position. 
   At step S 9 , a check is made to determine whether the predetermined delay period of the engine-stop timer initialized at step S 7  has expired. When the answer to step S 9  is negative (NO), that is, when the predetermined delay period of the engine-stop timer has not yet expired, the routine returns from step S 9  to step S 8  in order to succeedingly hold electromagnetic directional control valve  22  at the shut-off position. Conversely when the answer to step S 9  is affirmative (YES), that is, when the delay period of the engine-stop timer has expired, the routine advances from step S 9  to step S 10 . 
   At step S 10 , the engine-stop timer is reset. After step S 10 , steps S 11  and S 12  occur. 
   At step S 11 , the electric current applied to non-reversible electric motor  31  of motor-driven oil pump  18  is controlled to “0” to stop electric motor  31  of pump  18 . 
   At step S 12 , the duty ratio of the PWM signal applied to coil  38   b  of electromagnetic directional control valve  22  is controlled to the predetermined low duty ratio such as “0%”, so as to de-energize electromagnetic directional control valve  22 . 
   As set out above, in accordance with the engine-stop-period phase control shown in  FIG. 8 , the angular phase of camshaft  2  relative to sprocket  1  can be properly converted and phase-controlled to a predetermined phase-advanced position corresponding to the previously-noted engine-restart standby position (see  FIG. 5 ) of vane member  7 , properly phase-advanced from the maximum phase-retarded angular position (see  FIG. 4 ). Presetting vane member  7  to the engine-restart standby position enhances or improves the engine restartability. Therefore, the engine-restart standby position of vane member  7  means a better restartability angular-phase position. 
   Referring now to  FIG. 9 , there is shown the fail-safe routine executed within the processor of ECU  33  in presence of a failure in non-reversible electric motor  31  or a failure in electric motor-driven pump  18 . The fail-safe routine shown in  FIG. 9  is also executed as time-triggered interrupt routines to be triggered every predetermined time intervals such as 10 milliseconds. 
   At step S 21 , just after switching to the ignition-switch turned-ON state, a system-failure detection timer is set to a predetermined delay period representing the time allowed for pressure switch  24  to be switched ON if there is no system failure, more concretely, if electric motor-driven pump  18  and/or non-reversible electric motor  31  is unfailed and operating normally. 
   At step S 22 , the solenoid-actuated directional control valve  22  is shifted to its operative state. Actually, coil  38   b  of directional control valve  22  is energized and de-energized by a duty cycle pulsewidth modulated (PWM) signal at a controlled duty ratio, so that the axial position of spool  36  of the solenoid-actuated directional control valve  22  is controlled and axially slid for a phase change (a phase advance or a phase retard), which is determined based on the current engine operating condition. For instance, when the phase advance is required, the duty ratio of the PWM signal applied to coil  38   b  is set to the predetermined low duty ratio such as “0%”, such that spool  36  is controlled to the spring-offset position in which fluid communication between discharge line  16  and phase-advance hydraulic line  15  is established and simultaneously fluid communication between induction line  17  (or drain line  39 ) and phase-retard hydraulic line  14  is established, in order to attain the phase-advance operating mode. Conversely when the phase retard is required, the duty ratio is set to the predetermined high duty ratio such as “100%”, such that spool  36  is controlled to the maximum actuated position in which fluid communication between discharge line  16  and phase-retard hydraulic line  14  is established and simultaneously fluid communication between induction line  17  and phase-advance hydraulic line  15  is established, in order to attain the phase-retard operating mode. 
   At step S 23 , electric motor  31  is energized. 
   At step S 24 , a switch signal from pressure switch  24  is read. 
   At step S 25 , a check is made to determine whether the switch signal from pressure switch  24  is high, in other words, pressure switch  24  is switched ON. When the answer to step S 25  is negative (NO), that is, when pressure switch  24  is switched OFF, the routine proceeds from step S 25  to step S 26 . Conversely when the answer to step S 25  is affirmative (YES), that is, pressure switch  24  is switched ON, the routine proceeds from step S 25  to step S 33 . The processor of ECU  33  determines, based on the state of pressure switch  24  switched OFF, that the hydraulic pressure in discharge line  16  is not satisfactorily risen. On the contrary, the processor of ECU  33  determines, based on the state of pressure switch  24  switched ON, that the hydraulic pressure in discharge line  16  is satisfactorily risen. 
   At step S 26 , a check is made to determine whether the predetermined delay period of the system-failure detection timer initialized at step S 21  has expired. When the answer to step S 26  is negative (NO), that is, when the predetermined delay period of the system-failure detection timer has not yet expired, the routine returns from step S 26  to step S 24  in order to repeatedly execute steps S 24 –S 25 . Conversely when the answer to step S 26  is affirmative (YES), that is, when the delay period of the system-failure detection timer has expired, the routine advances from step S 26  to step S 27 . When the flow from step S 25  via step S 26  to step S 27  occurs, the processor of ECU  33  determines that there is a less amount of working fluid discharged from pump  18  in spite of electric motor  31  already energized. This is because the hydraulic pressure in discharge line  17  does not yet reach the predetermined pressure point after the predetermined elapsed time has expired with motor  31  energized. That is, steps S 25 –S 26  and the system-failure detection timer and pressure switch  24  serve as an abnormal-condition detection means or a system-failure detection means that detects an abnormal-condition of motor  31  of electric pump  18  (or a motor/pump failure). In particular, steps S 25 –S 26  serves as a pump-failure detection section of the processor of ECU  33  that detects a pump failure or determines that pump  18  is failed when the hydraulic pressure detected by pressure switch  24  remains at a pressure level less than the predetermined pressure point after electric motor  31  of pump  18  has been energized and thereafter the predetermined delay period (a set time of the system-failure detection timer) has expired. 
   At step S 27 , the system-failure detection timer is reset. After step S 27 , a series of steps S 28 –S 32  occur. 
   At step S 28 , the duty ratio of the PWM signal applied to coil  38   b  of electromagnetic directional control valve  22  is controlled to the predetermined low duty ratio such as “0%”, so as to de-energize electromagnetic directional control valve  22 . 
   At step S 29 , the electric current applied to non-reversible electric motor  31  of motor-driven oil pump  18  is controlled to “ 0 ” to stop electric motor  31  of pump  18 . 
   At step S 30 , an electric motor-driven pump failure indicative flag (simply, a pump failure flag) is set to “ 1 ”. 
   At step S 31 , an electromagnetic-directional-control-valve (OCV) control map change from a normal-condition OCV control map (suitable to the absence of the pump failure) to an abnormal-condition OCV control map (suitable to the presence of the pump failure) occurs. Therefore, after switching to the abnormal-condition OCV control map, it is possible to keep bypass line  21  opened, and thus to return working fluid, which is flown from either one of phase-retard hydraulic chamber  9  and phase-advance hydraulic chamber  10  into induction line  17 , utilizing the pulse pressure arising from alternating torque exerted on camshaft  2  and applied to the working fluid in each of hydraulic chambers  9  and  10 , via bypass line  21  to discharge line  16 , by continuously controlling electromagnetic directional control valve  22  based on the current engine operating condition in accordance with the abnormal-condition OCV control map. By the use of abnormal-condition OCV control map, it is possible to supply working fluid (hydraulic pressure) from induction line  17  through bypass line  21  and discharge line  16  selectively to hydraulic chambers  9  and  10 , either one of which requires a hydraulic pressure rise, utilizing the pulse pressure, thus creating a phase-control assistance force by the pulse pressure without using pump  18 , even in presence of the pump failure. 
   At step S 32 , ECU  33  outputs an alarm signal to the warning system (warning means) having the warning buzzer and/or instrument-cluster warning lamp  33   WL , so that the warning buzzer and/or instrument-cluster warning lamp  33   WL  comes on in response to the alarm signal from ECU  33 , and thus a visual and/or audible warning concerning the pump failure is signaled to the driver. The warning system energized (warning lamp  33   WL  lighting) allows the vehicle to dock for quick repairs. 
   Returning to step S 25 , when pressure switch  24  is switched ON, ECU  33  determines that electric motor-driven pump  18  is operating normally, and thus the routine flows from step S 25  to step S 33 . After step S 33 , a series of steps S 34 –S 38  occur. 
   At step S 33 , the system-failure detection timer is reset. 
   At step S 34 , electromagnetic directional control valve  22  is controlled based on the current engine operating condition (the latest up-to-date information about engine speed and/or engine load) in accordance with the normal-condition OCV control map. 
   At step S 35 , a deviation (or an error signal) of an actual angular phase of vane member  7  of phase converter  3  from a desired angular phase determined based on the current engine operating condition is calculated or computed. 
   At step S 36 , a check is made to determine whether the deviation (the error signal value) between the actual angular phase and the desired angular phase is within a predetermined dead zone. When the answer to step S 36  is affirmative (YES), that is, the deviation is within the dead zone, the routine proceeds from step S 36  to step S 37 . Conversely when the answer to step S 36  is negative (NO), that is, the deviation is out of the dead zone, the routine returns from step S 36  to step S 35 , so as to repeatedly execute steps S 35 –S 36 . 
   At step S 37 , a check is made to determine whether a drive signal of non-reversible electric motor  31  of electric motor-driven pump  18  is generated from the output interface of ECU  33 , in other words, motor  31  is energized (ON). When the answer to step S 37  is affirmative (YES), the routine proceeds from step S 37  to step S 38 . Conversely when the answer to step S 37  is negative (NO), the routine returns from step S 37  to step S 35 , so as to repeatedly execute steps S 35 –S 37 . 
   At step S 38 , the system-failure detection timer is set again. After step S 38 , the routine returns to step S 24 , so as to repeatedly execute the fail-safe routine. 
   As discussed above in reference to the flow chart of  FIG. 9 , even when electric motor-driven pump  18  has been failed, it is possible to selectively supply working fluid (hydraulic pressure) to hydraulic chambers  9  and  10 , either one of which requires a hydraulic pressure rise, by continuously controlling only the electromagnetic directional control valve  22  by means of ECU  33 . This enables continuous executions of phase control for angular phase of camshaft  2  relative to sprocket  1 , even in presence of the pump failure. 
   The processor of ECU  33  incorporated in the system of the embodiment is also programmed to execute engine-stall-period phase control similar to the engine-stop-period phase control routine shown in  FIG. 8 , so as to enhance or improve the restartability of the engine, even when a sudden engine stall takes place without turning the ignition switch OFF. Actually, the processor of ECU  33  phase-controls vane member  7  of phase converter  3  to the previously-noted engine-restart standby position by way of both of motor current control for electric motor  31  and PWM control for electric current applied to coil  38   b  of the solenoid-actuated directional control valve  22 , executed simultaneously with a point of time when the engine is restarted after an engine stall has occurred. 
   As a modification modified from the variable valve timing control system of the embodiment, an air bleeder (or air bleeding means) may be provided in a hydraulic pressure system laid out between the hydraulically-operated phase converter  3  and electric motor-driven pump  18  (a main working-fluid (hydraulic pressure) source) in order to exhaust or extract air mixed in working fluid in the hydraulic system to the exterior space. By means of the air bleeder, it is possible to effectively exhaust or extract undesirable air, which has been mixed in working fluid in each of hydraulic chambers  9  and  10  of phase converter  3  or in the hydraulic pressure system laid out between phase converter  3  and pump  18  due to leakage of working fluid in the engine stopped state, through the air bleeder to the exterior space. As a result of this, it is possible to prevent the control accuracy of variable valve timing control of the hydraulically-operated phase converter  3  from being deteriorated owing to the air mixed. In the system of the shown embodiment, phase-converter housing  5 , which is comprised of a porous housing formed of a porous sintered metal material, serves as the air bleeder (air bleeding means). In lieu thereof, the air bleeder may be provided in the hydraulic pressure system laid out between the hydraulically-operated phase converter  3  and electric motor-driven pump  18 , except the phase-converter housing. In such a case, a certain portion of the hydraulic pressure system may be formed of a porous sintered structural part. By the use of the porous sintered structural part, it is possible to effectively exhaust or extract only the air mixed in working fluid in the hydraulic pressure system for phase converter  3 , while preventing leakage of working fluid (oil) having a comparatively high viscosity, as much as possible. This avoids a pressure fall in working fluid delivered from pump  18  through discharge line  16  to either one of hydraulic chambers  9  and  10 . 
   Referring now to  FIG. 10 , there is shown another modification modified from the variable valve timing control system of the embodiment. In the modification shown in  FIG. 10 , as an oil-lubricating circuit, an axial oil hole  44   a , first and second radial oil holes  44   b  and  44   c , and an annular groove  44   g  are provided. More concretely, axial oil hole  44   a  is formed and bored in vane mounting bolt  6  in the axial direction of bolt  6  from the bolt head. The downstream end  44   d  of supply line  44  is communicated with axial oil hole  44   a  as well as working-fluid chamber  45 . The first radial oil hole  44   b  is also formed and bored in vane mounting bolt  6  in the radial direction of bolt  6  in such a manner as to crossing axial oil hole  44   a . The second radial oil hole  44   c  is formed and bored in the front end of camshaft  2  in the radial direction of camshaft  2 . Annular groove  44   g  is formed on the inner periphery of the female screw-threaded portion of the front end of camshaft  2  into which vane mounting bolt  6  is screwed. The first and second radial oil holes  44   b  and  44   c  are communicated with each other through annular groove  44   g . Therefore, during operation of phase converter  3 , the lubricating circuit  44   a ,  44   b ,  44   c  and  44   g  permits working fluid (lubricating oil) to be supplied through the lubricating circuit ( 44   a ,  44   b ,  44   c ,  44   g ) to friction bearing surfaces  50  between the outer peripheral surface of the end of camshaft  2  and the inner peripheral surface of the central bore  1   b  of sprocket  1 , which is rotatably supported on the camshaft end, so as to permit a constant flow of lubricating oil across the friction bearing surfaces  50 . 
   In the shown embodiment, the hydraulically-operated phase converter  3  is constructed by hydraulically-operated vane-type timing variator. The fundamental concept of the invention can be applied to a variable valve timing control system employing a hydraulically-operated helical-gear-type timing variator. Furthermore, in the shown embodiment, the variable valve timing control system is exemplified to control a phase (intake valve open timing IVO and/or intake valve closure timing IVC) of the intake valve. In lieu thereof, the variable valve timing control system of the invention may be applied to each exhaust valve of an exhaust system so as to control a phase (exhaust valve open timing EVO and/or exhaust valve closure timing EVC) of the exhaust valve. 
   The entire contents of Japanese Patent Application No. 2004-149890 (filed May 20, 2004) 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.