Patent Publication Number: US-9410514-B2

Title: Variable displacement oil pump

Description:
TECHNICAL FIELD 
     The present invention relates to a variable displacement oil pump for automotive internal combustion engines. 
     BACKGROUND ART 
     In recent years, as for a variable displacement oil pump, a two-stage discharge pressure characteristic is often required for supplying different apparatus and parts, whose required discharge pressures differ from each other, for example, moving engine parts and a variable valve actuation device configured to control engine-valve operating characteristics, with oil discharged from an oil pump. According to such a two-stage discharge pressure characteristic, the pump discharge pressure can be maintained at a first discharge pressure in a first pump speed range and also maintained at a second discharge pressure in a second pump speed range. One such variable displacement oil pump has been disclosed in Japanese Patent Provisional Publication No. 2008-52450 (hereinafter referred to as “JP2008-524500”), corresponding to International Publication No. WO 2006/066405 (A1). 
     To satisfy such a two-stage discharge pressure characteristic, the variable displacement oil pump, as disclosed in JP2008-524500, has a cam ring, which is moveable or pivotable against the spring force of a return spring. The variable displacement oil pump is configured to achieve the two-stage discharge pressure characteristic by supplying the discharge pressure (the pressurized working fluid) to a selected one of two pressure-receiving chambers defined on the outer peripheral surface of the cam ring and by changing an eccentricity of a geometric center of the cylinder bore of the cam ring with respect to the axis of rotation of a rotor (exactly, a vane rotor) 
     SUMMARY OF THE INVENTION 
     However, in order to suitably adjust or change a relative pressure difference between two different discharge pressures (low and high hydraulic pressure levels) of a two-stage discharge pressure characteristic depending on the sort of apparatus to which the variable displacement oil pump can be applied, the prior-art variable displacement oil pump requires a change of pressure-receiving areas of the cam ring, on which hydraulic pressure of working oil introduced into one of the two pressure-receiving chambers and hydraulic pressure of working oil introduced into the other of the two pressure-receiving chambers respectively act. In other words, depending on the sort of applied apparatus, the sizes of the first and second control oil chambers have to be changed. This means that the basic pump-body structure has to be redesigned and thus the pump body itself has to be newly manufactured. 
     Accordingly, it is an object of the invention to provide a variable displacement oil pump capable of easily but accurately adjusting or changing a relative pressure difference between two different discharge pressures (first and second discharge pressure levels) of a two-stage discharge pressure characteristic without changing a basic pump-body structure. 
     In order to accomplish the aforementioned and other objects of the present invention, a variable displacement oil pump comprises a pump structural unit adapted to be driven by an internal combustion engine for varying a volume of each of a plurality of working chambers and for discharging oil, drawn into an inlet portion, from a discharge portion, a variable-volume mechanism configured to vary a variation of the volume of each of the working chambers, which chambers open into the discharge portion, by a displacement of a moveable member included in the pump structural unit, a first biasing member for forcing the movable member in a direction that the variation of the volume of each of the working chambers increases, a control chamber configured to displace the moveable member in a direction that the variation of the volume of each of the working chambers decreases, by introducing the oil, discharged from the discharge portion, into the control chamber, a directional control valve configured to selectively switch between an oil-discharge from the control chamber and an oil-introduction from the discharge portion to the control chamber by moving a valve member in one direction by a biasing force of a second biasing member or by moving the valve member in the other direction against the biasing force of the second biasing member by a discharge pressure discharged from the discharge portion and applied at a port of the directional control valve, and a control mechanism configured to variably control timing at which switching between the oil-discharge from the control chamber and the oil-introduction to the control chamber occurs, with respect to the discharge pressure applied at the port of the directional control valve. 
     According to another aspect of the invention, a variable displacement oil pump comprises a pump structural unit adapted to be driven by an internal combustion engine for varying a volume of each of a plurality of working chambers and for discharging oil, drawn into an inlet portion, from a discharge portion, a variable-volume mechanism configured to vary a variation of the volume of each of the working chambers, which chambers open into the discharge portion, by a displacement of a moveable member included in the pump structural unit, a first biasing member for forcing the movable member in a biased direction that the variation of the volume of each of the working chambers increases, a control chamber configured to change a displaced position of the moveable member by introducing the oil, discharged from the discharge portion, into the control chamber, a directional control valve including a spool having a pressure-receiving section for receiving the discharge pressure and slidably installed in a close-fitting bore into which a communication passage opens and which communicates with the control chamber, and a second biasing member for forcing the spool in one sliding direction opposite to the other sliding direction of the spool corresponding to a direction of action of the discharge pressure acting on the pressure-receiving section of the spool, the directional control valve being configured to selectively switch between an oil-discharge from the control chamber and an oil-introduction from the discharge portion to the control chamber by a sliding movement of the spool resulting from a relative pressure force between a biasing force created by the discharge pressure and a biasing force of the second biasing member, and a control mechanism configured to control the sliding movement of the spool with a setting change in the biasing force of the second biasing member, occurring by displacing a movable support, which is provided for supporting one end of the second biasing member, depending on a pressure level of the discharge pressure. 
     According to a further aspect of the invention, a variable displacement oil pump comprises a pump structural unit adapted to be driven by an internal combustion engine for varying a volume of each of a plurality of working chambers and for discharging oil, drawn into an inlet portion, from a discharge portion, a variable-volume mechanism configured to vary a variation of the volume of each of the working chambers, which chambers open into the discharge portion, by a displacement of a moveable member included in the pump structural unit, a first biasing member for forcing the movable member in a biased direction that the variation of the volume of each of the working chambers increases, a control chamber configured to change a displaced position of the moveable member by introducing the oil, discharged from the discharge portion, into the control chamber, a directional control valve including a spool having a pressure-receiving section for receiving the discharge pressure, a sliding sleeve configured to slidably accommodate therein the spool and also configured to have a sliding-contact surface in sliding-contact with an outer periphery of the spool and at least one communication port formed in the sliding-contact surface of the sliding sleeve, and a second biasing member for forcing the spool in one sliding direction, the directional control valve being configured to selectively switch between an oil-discharge from the control chamber and an oil-introduction from the discharge portion to the control chamber by switching an oil-discharge from the communication port and an oil-introduction from the discharge portion to the communication port by moving the spool in the other sliding direction against the biasing force of the second biasing member by a discharge pressure discharged from the discharge portion and acting on the pressure-receiving section of the spool, and a control mechanism configured to enable the sliding sleeve to be displaced in the other sliding direction of the spool against the biasing force of the second biasing member as well as an inertia of the spool. 
     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 schematic diagram illustrating a variable displacement oil pump system of the first embodiment. 
         FIG. 2  is a longitudinal cross-sectional view illustrating component parts of the variable displacement oil pump of the first embodiment. 
         FIG. 3  is a front elevation view of a pump housing of the variable displacement oil pump of the first embodiment. 
         FIG. 4  is an explanatory view illustrating the operation of a pilot valve of the variable displacement oil pump system of the first embodiment during a steady-state engine operating mode. 
         FIG. 5  is an explanatory view illustrating the operation of the variable displacement oil pump system of the first embodiment during high-load operation. 
         FIG. 6  is a characteristic diagram illustrating the relationship between discharge pressure of the variable displacement oil pump and engine speed in the embodiments. 
         FIG. 7  is a schematic diagram illustrating a variable displacement oil pump system of the second embodiment. 
         FIG. 8  is an explanatory view illustrating the operation of a pilot valve of the variable displacement oil pump system of the second embodiment during a steady-state engine operating mode. 
         FIG. 9  is an explanatory view illustrating the operation of the variable displacement oil pump system of the second embodiment during high-load operation. 
         FIG. 10  is a schematic diagram illustrating a variable displacement oil pump system of the third embodiment. 
         FIG. 11  is an explanatory view illustrating the operation of a pilot valve of the variable displacement oil pump system of the third embodiment during a steady-state engine operating mode. 
         FIG. 12  is an explanatory view illustrating the operation of the pilot valve of the third embodiment during high-load operation. 
         FIG. 13A  is an explanatory view, in longitudinal cross section, illustrating the operation of a pilot valve of the variable displacement oil pump system of the fourth embodiment during the early stage of engine start-up,  FIG. 13B  is an explanatory view illustrating the operation of the pilot valve during a steady-state engine operating mode, and  FIG. 13C  is an explanatory view illustrating the operation of the pilot valve during high-load operation. 
         FIG. 14A  is an explanatory view, in longitudinal cross section, illustrating the operation of a pilot valve of the variable displacement oil pump system of the fifth embodiment during the early stage of engine start-up,  FIG. 14B  is an explanatory view illustrating the operation of the pilot valve during a steady-state engine operating mode, and  FIG. 14C  is an explanatory view illustrating the operation of the pilot valve during high-load operation. 
         FIG. 15A  is an explanatory view, in longitudinal cross section, illustrating the operation of a pilot valve of the variable displacement oil pump system of the sixth embodiment during the early stage of engine start-up,  FIG. 15B  is an explanatory view illustrating the operation of the pilot valve during a steady-state engine operating mode, and  FIG. 15C  is an explanatory view illustrating the operation of the pilot valve during high-load operation. 
         FIG. 16A  is an explanatory view, in longitudinal cross section, illustrating the operation of a pilot valve of the variable displacement oil pump system of the seventh embodiment during the early stage of engine start-up,  FIG. 16B  is an explanatory view illustrating the operation of the pilot valve during a steady-state engine operating mode, and  FIG. 16C  is an explanatory view illustrating the operation of the pilot valve during high-load operation. 
         FIG. 17A  is an explanatory view, in longitudinal cross section, illustrating the operation of a pilot valve of the variable displacement oil pump system of the eighth embodiment during the early stage of engine start-up,  FIG. 17B  is an explanatory view illustrating the operation of the pilot valve during a steady-state engine operating mode, and  FIG. 17C  is an explanatory view illustrating the operation of the pilot valve during high-load operation. 
         FIG. 18A  is an explanatory view, in longitudinal cross section, illustrating the operation of a pilot valve of the variable displacement oil pump system of the ninth embodiment during the early stage of engine start-up,  FIG. 18B  is an explanatory view illustrating the operation of the pilot valve during a steady-state engine operating mode, and  FIG. 18C  is an explanatory view illustrating the operation of the pilot valve during high-load operation. 
         FIG. 19A  is an explanatory view, partly in cross section, illustrating a flow-passage structure in which the width of one opening end of a flow passage, whose passage area can be changed depending on the axial position of a cylindrical land of the pilot-valve spool, is approximately equal to the axial length of the cylindrical land,  FIG. 19B  is an explanatory view, partly in cross section, illustrating another flow-passage structure in which the width of the opening end of the flow passage is less than the axial length of the cylindrical land, and  FIG. 19C  is an explanatory view, partly in cross section, illustrating a further flow-passage structure in which the width of the opening end of the flow passage is greater than the axial length of the cylindrical land. 
         FIG. 20A  is an explanatory view, partly in cross section, illustrating a flow-passage structure in which the width of one opening end of a flow passage, whose passage area can be changed depending on the axial position of a somewhat exaggerated, barrel-shaped land of the pilot-valve spool, is approximately equal to the axial length of the barrel-shaped land,  FIG. 20B  is an explanatory view, partly in cross section, illustrating another flow-passage structure in which the width of the opening end of the flow passage is less than the axial length of the barrel-shaped land, and  FIG. 20C  is an explanatory view, partly in cross section, illustrating a further flow-passage structure in which the width of the opening end of the flow passage is greater than the axial length of the barrel-shaped land. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings, particularly to  FIGS. 1-6 , the variable displacement oil pump of the embodiment is exemplified in an automotive internal combustion engine vane-type variable displacement oil pump for supplying working oil to a variable valve actuation device configured to valve timing of each individual engine valve of an automotive internal combustion engine and for supplying lubricating oil to moving engine parts, in particular, for providing lubrication between pistons and cylinder bores in the form of oil jets and for providing lubrication to crank journal bearings of an engine crankshaft. 
     [First Embodiment] 
     The pump body of the variable displacement oil pump of the embodiment is provided at the front end of a cylinder block (not shown) of the internal combustion engine. As shown in  FIGS. 1-2 , the pump body is mainly comprised of a pump housing  1 , whose one end (an opening end) is hermetically closed by a pump cover  2 , and which has a cylindrical bore closed at the other end, a drive shaft  3  adapted to be driven by an engine crankshaft (not shown) and configured to be rotatably fitted in a center bore (a bearing bore  1   c  described later) formed substantially in the center of pump housing  1 , a rotor  4  fixedly connected at its central portion to the drive shaft  3  and rotatably accommodated in the pump housing  1 , and a cam ring  5 , which ring is a moveable member pivotably installed on an outer periphery of rotor  4 . 
     Also provided are a pilot valve  7  installed in a control housing  6  made by aluminum alloy, and serving as a pilot-operated directional control valve for controlling pressure-supply/pressure-release of hydraulic pressure used to produce pivotal movement of cam ring  5 , and an electromagnetic solenoid operated directional control valve  8  provided at the front end of the cylinder block and serving as a control mechanism. 
     As best seen in  FIG. 2 , when installing the pump cover  2  and the pump housing  1  on the cylinder block, they are fastened together with four bolts  9 . Concretely, bolts  9  are inserted through respective bolt insertion holes (through holes)  1   a  formed in the pump cover  2  as well as the pump housing  1 , and then the male screw-thread parts of bolts  9  are screwed into respective female screw-threaded portions formed in the cylinder block. 
     Pump housing  1  is integrally formed by aluminum alloy. As clearly shown in  FIG. 3 , pump housing  1  has a recessed pump accommodation chamber  1   b  (serving as a working chamber) whose bottom end face precisely machined to ensure a greatly-precise flatness/surface roughness, thus permitting smooth sliding motion of one axial sidewall surface of cam ring  5 . 
     Also, pump housing  1  is formed with the bearing bore ic (the through hole) formed substantially in the center of the bottom face of pump accommodation chamber  1   b  for rotatably supporting one axial end of drive shaft  3  and a pivot-pin hole  1   d  bored in the pump housing at a predetermined position of the inner peripheral surface of pump accommodation chamber  1   b . A pivot pin  10 , serving as a pivot of cam ring  5 , is inserted into the pivot-pin hole  1   d . A circular-arc shaped sealing surface  1   e  is partly formed on the inner periphery of pump accommodation chamber  1   b  and arranged in the upper part of the inner periphery than a straight line “M” (hereinafter referred to as “cam-ring reference line”), extending from the axis of pivot pin  10  and passing through the center of pump housing  1  (that is, the axis of drive shaft  3 ), when viewed in an axial direction defined by the axis of drive shaft  3 . 
     A seal member  13 , fitted into a seal-retention groove  5   b  (described later) formed in the cam ring  5 , is permanently in sliding-contact with the previously-noted sealing surface  1   e , to provide a sealing action by which oil leakage from a control oil chamber  16  (described later) can be prevented. That is to say, a sealing mechanism (a sealing structure) is constructed by both the sealing surface  1   e  and the seal member  13 . 
     As seen from the front elevation view of  FIG. 3 , sealing surface  1   e  is formed into a circular-arc shape whose geometrical center is the pivot-pin hole  1   d  and whose distance from the center (the pivot-pin hole  1   d ) is equal to a specified radius “R” (i.e., a specified length). The specified radius “R” is set to a specified length dimension that permits permanent sliding-contact between the seal member  13  and the circular-arc shaped sealing surface  1   e  within a limited range of eccentric oscillating motion of cam ring  5 . 
     A substantially crescent-shaped recessed inlet port  11  (a suction port or an inlet portion) is formed in the bottom face of pump housing  1  and placed on the left-hand side of drive shaft  3  (bearing bore  1   c ). A substantially crescent-shaped discharge port  12  (an outlet port or a discharge portion) is formed in the bottom face of pump housing  1  and arranged at a given position diametrically opposed to the inlet port  11 , that is, on the right-hand side of drive shaft  3 . Concrete configurations of inlet port  11  and discharge port  12 , substantially diametrically opposed to each other, are described later. 
     Furthermore, a lubricating-oil groove  23  is formed in the inner peripheral surface of the bearing bore  1   c  of pump accommodation chamber  1   b , configured to rotatably support the drive shaft  3 , for supplying lubricating oil discharged from the discharge port  12 . Lubricating-oil groove  23  is formed to extend in the axial direction of drive shaft  3  over a given range from the circumferential edge of one opening end of bearing bore  1   c  to a substantially midpoint of the entire axial length of bearing bore  1   c . Lubricating oil, stored or kept in the lubricating-oil groove  23 , interpose a film of oil between the drive shaft  3  and the bearing bore  1   c , thus ensuring a lubrication performance for the rotating drive shaft  3  and also suppressing undesired wear/seizing, occurring due to sliding friction. 
     Returning to  FIG. 2 , pump cover  2  is made by aluminum alloy and formed as a substantially disc-shaped, front plate cover. Pump cover  2  has a bearing bore  2   a  (a through hole) formed substantially in the center of pump cover  2  for rotatably supporting the other axial end of drive shaft  3 . Also, pump cover  2  is integrally formed with a plurality of radially-outward extending boss-like portions in which the bolt insertion holes (through holes)  1   a  for respective bolts  9  are formed. In the shown embodiment, the inside wall surface of pump cover  2  is formed as a simple flat surface. In lieu thereof, in a similar manner to the bottom face of pump accommodation chamber  1   b  of pump housing  1 , the pump cover  2  may be configured to have diametrically-opposed inlet and discharge ports and oil-reservoir spaces, formed or defined in the inner peripheral wall surface of pump cover  2 . Also, pump cover  2  is positioned circumferentially with respect to the pump housing  1  by means of a positioning pin  14  fixedly connected to the pump housing  1 , such that coaxial alignment between the pump-housing side bearing bore  1   c  and the pump-cover side bearing bore  2   a , rotatably supporting both axial ends of drive shaft  3 , is ensured. As previously discussed, the pump cover  2  and the pump housing  1  are fastened together with four bolts  9 . By the way, the previously-discussed control housing  6  of pilot valve  7  is integrally connected to the outside face of pump housing  1 . The tip  3   a  (the outermost end) of drive shaft  3 , protruded from the pump cover  2 , is configured to be coupled with a motion-transmission mechanism, such as a gear mechanism, so as to rotate the rotor  4  in the direction of rotation (clockwise) indicated by the arrow in  FIG. 1  by input torque, transmitted from the engine crankshaft via the tip  3   a  of drive shaft  3 . As can be seen in  FIG. 3 , the left-hand half of pump accommodation chamber  1   b  of pump housing  1  serves as an inlet area (a suction area), whereas the right-hand half of pump accommodation chamber  1   b  of pump housing  1  serves as a discharge area (an outlet area). 
     As shown in  FIGS. 1-2 , rotor  4  has seven slits  4   a  formed to extend radially outward and has seven back-pressure chambers  24  formed at the respective basal portions of slits  4   a . Seven vanes  15  are fitted into respective slits  4   a  of rotor  4 , in a manner so as to be slidable (retractable and extendable) in the radial direction of rotor  4 . Each of back-pressure chambers  24  has a circular cross-section for introducing discharge pressure, introduced from the discharge port  12 , into the back-pressure chambers  24 . By virtue of pressure in each of back-pressure chambers  24  and a centrifugal force created by rotation of rotor  4 , each of vanes  15  can be pushed radially outward. 
     As best seen in  FIG. 2 , rotor  4  has a substantially I-shaped cross section. The I-shaped rotor  4  has a pair of vane-ring grooves  4   b  and  4   c  formed in respective sidewalls of the inner peripheral portion of rotor  4 . A pair of vane rings  18 ,  18  are installed in the respective vane-ring grooves  4   b  and  4   c . Vane rings  18 ,  18  are installed in the respective sidewalls of the inner peripheral portion of rotor  4 , so that sliding motions of vane rings  18 ,  18  relative to the respective sidewalls of the inner peripheral portion of rotor  4  are permitted. Each of the radially-inward ends (the basal portions) of vanes  15  is kept in sliding-contact with the outer peripheral surfaces of vane rings  18 ,  18 . During operation of the pump, each of the radially-outward ends (the tips) of vanes  15  is brought into sliding-contact with the inner peripheral surface  5   a  of cam ring  5 . One pump working chamber is defined between two adjacent vanes  15 . That is, seven variable-volume pump working chambers (simply, pump chambers)  19  are defined as seven internal spaces partitioned in a fluid-tight fashion and surrounded by vanes  15 , the inner peripheral surface  5   a  of cam ring  5 , the outer peripheral surface of rotor  4 , and two axially opposed sidewalls (i.e., the bottom face of the recessed pump accommodation chamber  1   b  of pump housing  1  and the inside face of pump cover  2 ). 
     The vane-ring pair ( 18 ,  18 ) has a function that pushes or forces each of vanes  15  outwards in the radial direction of the rotor. Even during operation of the engine at low speeds, in which the centrifugal force, created by rotation of rotor  4 , and the pressure in each of back-pressure chambers  24  are both low, each of the radially-outward ends (the tips) of vanes  15  can be brought into sliding-contact with the inner peripheral surface  5   a  of cam ring  5  by means of the vane-ring pair ( 18 ,  18 ) and hence the pump chambers  19  can be partitioned in a fluid-tight fashion. 
     Cam ring  5  is made of easily-machined sintered alloy materials and integrally formed into a substantially cylindrical shape. As shown in  FIG. 1 , cam ring  5  has a pivot recessed portion  5   d  formed in its outer peripheral surface and arranged at the rightmost end of the previously-discussed cam-ring reference line “M”. Pivot pin  10 , which is fitted and positioned into the pivot recessed portion  5   b , serves as a fulcrum of oscillating motion of cam ring  5 . 
     Cam ring  5  has a substantially triangular integrally-formed protruding portion  5   e  configured in the upper left part of the outer periphery of cam ring  5  than the cam-ring reference line “M”. The previously-discussed seal-retention groove  5   b  is formed in the protruding portion  5   e  of cam ring  5  for retaining the seal member  13  therein. 
     As appreciated from the above, a pump structural unit is constructed by the drive shaft  3 , the rotor  4 , the cam ring  5 , the vanes  15 , and the vane rings  18 ,  18 . 
     The previously-discussed control oil chamber  16  is defined between the inner periphery of pump housing  1  and the upper part of the outer periphery of cam ring  5  (including the protruding portion  5   e ) than the cam-ring reference line “M”. 
     Control oil chamber  16  is configured such that, by way of hydraulic pressure introduced into the control oil chamber  6 , the cam ring  5  is displaced or forced against the bias of a first biasing member, simply a biasing member (a coil spring  28  described later) in a direction that an eccentricity of the geometric center of cam ring  5  to the axis of rotation of drive shaft  3  decreases. Control oil chamber  16  is also configured such that fluid-communication between the control oil chamber  16  and the discharge port  12  is established or blocked by means of the pilot valve  7 . Furthermore, control oil chamber  16  is sealed in a fluid-tight fashion such that oil leakage from the control oil chamber  16  can be prevented by the previously-discussed sealing mechanism, constructed by the sealing surface  1   e  of the inner periphery of pump housing  1  and the seal member  13  fitted into the seal groove  5   b  of cam ring  5  even during oscillating motion of cam ring  5 . 
     The outer peripheral surface of cam ring  5 , facing the control oil chamber  16 , functions as a pressure-receiving surface  20 . 
     The hydraulic pressure, introduced into the control oil chamber  16  and acting on the pressure-receiving surface  20 , serves as a force that the eccentricity of the geometric center of cam ring  5  to the axis of rotation of drive shaft  3  decreases by counterclockwise oscillating motion (viewing  FIG. 1 ) of the cam ring  5  about the pivot pin  10  serving as a fulcrum of oscillating motion of cam ring  5 . 
     Seal member  13  is made of a low-friction synthetic resin material and formed as an axially-elongated oil seal extending along the axial direction of cam ring  5 . Seal member  13  is retained and fitted into the seal-retention groove  5   b  formed in the outer peripheral surface of protruding portion  5   e  of cam ring  5 . A rubber elastic member or an elastomeric member (not numbered) is attached onto the innermost end face of the seal-retention groove  5   b . Thus, the seal member  13  of cam ring  5  is permanently forced toward the sealing surface  1   e  of pump housing  1  by the elastic force of the rubber elastic member. The sealing surface  1   e  of pump housing  1  and the seal member  13  of cam ring  5 , abutted each other, provide a good leakproof seal, thus suppressing an internal oil leakage from the control oil chamber  16  to the low-pressure side to a minimum. 
     As shown in  FIGS. 1-3 , inlet port  11  is configured to open into pump chambers  19  whose volumes increase during rotation of the rotor in an eccentric state of the geometric center of cam ring  5  to the axis of rotation of rotor  4 . Inlet port  11  is configured so that lubricating oil in an oil pan (not shown) is drawn through an inlet hole  11   a  into the inlet port  11  by a negative pressure produced by a pumping action of the pump structural unit. Inlet hole  11   a  is formed substantially at a midpoint of the crescent-shaped recessed inlet port  11 . Additionally, a working-fluid introduction portion  11   b  is formed substantially at a midpoint of the outer peripheral side of inlet port  11  in a manner so as to extend toward a spring chamber  27  (described later). Introduction portion  11   b  communicates with the inlet hole  11   a . The inlet hole  11   a , together with the introduction portion  11   b , communicates with a low-pressure chamber  22 . Inlet hole  11   a  is configured to supply working fluid (oil), which is drawn up from the oil pan through a suction passage (not shown) by a negative pressure produced by a pumping action of the pump structural unit, into the inlet port  11  so as to introduce the working fluid to pump chambers  19  whose volumes increase during rotation of the rotor. As discussed above, the inlet port  11 , the inlet hole  11   a , the introduction portion  11   b , the low-pressure chamber  22  constructs a low-pressure structural portion. 
     On the other hand, discharge port  12  is configured to open into pump chambers  19  whose volumes decrease during rotation of the rotor in an eccentric state of the geometric center of cam ring  5  to the axis of rotation of rotor  4 . A discharge hole  12   a  is formed in an upper portion of the crescent-shaped discharge port  12 . Discharge port  12  is configured so that oil is delivered from the inlet hole  12   a  through a discharge passage  12   b  and a main oil gallery  25  (described later) formed in a cylinder head into moving or sliding engine parts and a variable valve actuation device such as a variable valve timing control device. 
     Electromagnetic solenoid operated directional control valve  8  (detailed later) as well as pilot valve  7  (detailed later) is disposed in a branch passage  29 , branched from the main oil gallery  25 . 
     By the way, a first oil filter  51  is disposed in the main oil gallery  25  and placed in the vicinity of the discharge passage  12   b . A second oil filter  52  is disposed in the branch passage  29  near the branch point of the upstream side of main oil gallery  25  and branch passage  29 . Hence, oil, supplied to the directional control valve  8  as well as the pilot valve  7 , can be filtered doubly by means of these oil filters. 
     As a filtering element of each of oil filters  51 - 52 , a filter paper is used. To easily replace the filter clogged up, a replaceable cartridge-type oil filter or a replaceable filter-paper equipped oil filter is used. 
     As best seen in  FIG. 1 , cam ring  5  has an arm  26  integrally formed to extend radially outward from the outer peripheral surface of the cam-ring cylindrical main body and located on the opposite side to the pivot recessed portion  5   d . Arm  26  is comprised of a radially-outward protruding main arm body  26   a  having a substantially rectangular cross section and a substantially semi-spherical contacting surface protrusion  26   b  integrally formed on the lower face of main arm body  26   a . In more detail, a portion of the lower face of the arm main body  26   a  except the semi-spherical contacting surface protrusion  26   b  is formed as a flat surface. On the other hand, the outer peripheral surface of protrusion  26   b  is formed as a semi-spherical surface having a small radius of curvature. 
     Spring chamber  27  is arranged at the opposite position to the pivot-pin hole  1   d  of pump housing  1  and formed to face the underside of arm  26 . 
     Spring chamber  27  is formed into a substantially rectangular shape having longer opposite sides in the axial direction of pump housing  1 . Coil spring  28  (the biasing member) is installed in the spring chamber  27  for biasing such that the cam ring  5  is biased or forced through the arm  26  in the clockwise direction (viewing  FIG. 1 ), that is, in the direction that the eccentricity of the geometric center of cam ring  5  to the axis of rotation of rotor  4  increases. By the way, spring chamber  27  communicates with the low-pressure chamber  22  through the introduction portion  11   b  and the inlet port  11 . 
     When assembling, coil spring  28  is disposed between the semi-spherical protrusion  26   b  of arm  26  and the bottom face of spring chamber  27 , under preload. The top face of coil spring  28  is always kept in abutted-engagement with the semi-spherical protrusion  26   b  over the entire range of oscillating motion of cam ring  5  during operation of the pump. More concretely, the top face of coil spring  28  is kept in elastic-contact with the semi-spherical protrusion  26   b  of arm  26 , whereas the bottom face of coil spring  28  is kept in elastic-contact with the bottom face of spring chamber  27 . Thus, the arm  26  of cam ring  5  is permanently forced or biased by a given spring load W, produced by coil spring  28 , in the clockwise direction (viewing  FIG. 1 ) that the eccentricity of the geometric center of cam ring  5  to the axis of rotation of rotor  4  increases. 
     Under preload, in other words, under a spring-loaded state where the spring load W is applied to the arm  26 , coil spring  28  functions to permanently force or bias the arm  26  of cam ring  5  upward (viewing  FIG. 1 ) in a direction that the eccentricity of the geometric center of cam ring  5  to the axis of rotation of rotor  4  increases, that is, in a direction that the volume difference between a volume of the largest working chamber of pump chambers  19  and a volume of the smallest working chamber of pump chambers  19  increases, in other words, in a direction that the rate of change of the volume of each of pump chambers  19  increases. As can be seen from the characteristic diagram of  FIG. 6 , the given spring load W, produced by coil spring  28  with cam ring  5  kept at its initial setting position (i.e., the maximum-eccentricity angular position) shown in  FIG. 1 , is set to a spring force that cam ring  5  begins to move (oscillate) counterclockwise from the initial setting position when the discharge pressure from the pump (that is, the hydraulic pressure in the control oil chamber  16 ) reaches a hydraulic pressure P 1  required for the variable valve timing control (VTC) device. 
     A substantially semi-spherical motion-restriction protrusion if is integrally formed on the inner peripheral surface of pump housing  1  to be opposed to the spring chamber  27  in the axial direction of coil spring  28 . With cam ring  5  kept at its initial setting position (i.e., the maximum-eccentricity angular position or the spring-loaded original position) shown in  FIG. 1 , the semi-spherical motion-restriction protrusion if is brought into abutted-engagement with the upside of arm  26 , for restricting a maximum clockwise angular displacement of the arm  26  of cam ring  5 . 
     As seen from the right-hand cross-section of  FIG. 1 , pilot valve  7  is mainly comprised of a stepped cylindrical close-fitting bore  30 , a substantially cylindrical small-diameter valve spool (simply, a spool)  32  (a substantially cylindrical valve member), a substantially cylindrical large-diameter axially-movable spring-support slider  33 , and a valve spring  34  (serving as a second biasing member). Stepped cylindrical close-fitting bore  30  is formed in the control housing  6  to extend vertically. Stepped cylindrical close-fitting bore  30  is comprised of an upper small-diameter bore  30   a , a lower large-diameter bore  30   b , and a stepped or shouldered portion  30   c . The lowermost end of large-diameter bore  30   b  is hermetically closed by a lid member  31 . Spool  32  is vertically slidably installed in the small-diameter bore  30   a . Large-diameter spring-support slider  33  is vertically slidably installed in the large-diameter bore  30   b . Valve spring  34  is disposed between the spool  32  and the large-diameter spring-support slider  33  under preload such that the spool  32  and the large-diameter spring-support slider  33  are biased to be spaced from each other in the opposite axial directions. 
     The upper end of small-diameter bore  30   a  of stepped cylindrical close-fitting bore  30  communicates with the branch passage  29  through an oil introduction port  29   a  (a pilot pressure port) formed in the control housing  6 . One opening end  35   a  of a first communication passage  35  is configured to open into the upper portion of small-diameter bore  30   a  of stepped cylindrical close-fitting bore  30 . The other end of the first communication passage  35  communicates with the control oil chamber  16  through a communication bore  36  formed in the right-hand end wall of pump housing  1 . 
     The inside diameter of oil introduction port  29   a  is dimensioned to be less than that of small-diameter bore  30   a , in a manner so as to form a frusto-conical tapered valve-spool-land bearing or seating surface  29   b  between them. With a first land  32   a  (described later) of spool  32  seated on the tapered bearing surface  29   b , the oil introduction port  29   a  is closed. 
     One opening end of a drain passage  37 , which passage communicates with the oil pan, is configured to open into the lower portion of small-diameter bore  30   a.    
     Spool  32  is comprised of first and second lands  32   a - 32   b , and a small-diameter shaft  32   c  between them. The first land  32   a  constructs a valve element. The outside diameter of the second land  32   b  is dimensioned to be identical to that of the first land  32   a . Spool  32  has a cylindrical bore  32   d  closed at its upper end and extending along the axis of spool  32 . 
     The axial length of the first land  32   a  is dimensioned to be shorter than that of the second land  32   b . The opening end  35   a  of the first communication passage  35  is opened or closed depending on the axial position (axially sliding motion) of the first land  32   a  of spool  32 . The upper end  34   a  of valve spring  34  is kept in elastic-contact with the upper end face of cylindrical bore  32   d . By the way, the axial length of the first land  32   a  is dimensioned to be slightly greater than the inside diameter of the opening end  35   a  of the first communication passage  35 . 
     The second land  32   b  has an axially long outer peripheral surface that ensures a stable sliding motion of spool  32  in the small-diameter bore  30   a.    
     Small-diameter shaft  32   c  defines an annular groove  32   e  between first and second lands  32   a - 32   b . At the spool position shown in  FIG. 1 , the annular groove  32   e  is configured to face the opening end  35   a  of the first communication passage  35 . Also, small-diameter shaft  32   c  has a radial through hole  32   f  for communicating the annular groove  32   e  with the cylindrical bore  32   d  by way of the radial through hole  32   f.    
     On the other hand, large-diameter spring-support slider  33  has a spring-support bore  33   a  closed at its lower end and configured to retain the lower end of valve spring  34  such that the lower end  34   b  of valve spring  34  is kept in elastic-contact with the bottom end face of spring-support bore  33   a . A cylindrical small-diameter stopper protrusion  33   c  is integrally formed at the center of the underside  33   b  (serving as a large-diameter pressure-receiving surface) of large-diameter spring-support slider  33 . The stopper protrusion  33   c  is provided for restricting a maximum downward movement (i.e., lowermost axial position) of the large-diameter spring-support slider  33 . Also, the stopper protrusion  33   c  is configured to define a large-diameter pressure-receiving chamber  38  between the underside (large-diameter pressure-receiving surface  33   b ) of large-diameter spring-support slider  33  and the inside face of lid member  31 . The underside  33   b  receives hydraulic pressure introduced through the directional control valve  8  into the pressure-receiving chamber  38 , so as to cause an upward sliding motion of large-diameter spring-support slider  33 . 
     A second communication passage  39  is provided to communicate a supply-and-exhaust port  46  (described later) of directional control valve  8  with the pressure-receiving chamber  38  of pilot valve  7 . One opening end of the second communication passage  39  is configured to open into the lowermost end of large-diameter bore  30   b.    
     As seen in  FIG. 1 , electromagnetic solenoid operated directional control valve  8  is mainly comprised of a valve body  40 , a valve seat  42 , a metal ball valve  44 , and an electromagnetic solenoid  45 . Valve body  40  is press-fitted into a valve accommodation bore  1   g  formed in the cylinder block at a given position. Valve body  40  has an axially-extending inside stepped working bore  41 . Valve seat  42  is press-fitted into the upper end of valve body  40  (exactly, the upper large-diameter bore of working bore  41 ) and has a central solenoid control port  43 . Ball valve  44  is configured to seat on or lift from the valve seat  42 , for opening or closing the opening end of solenoid control port  43 . Solenoid  45  is integrally connected to the lower end of valve body  40 . 
     Valve body  40  has the supply-and-exhaust port  46  (a radial through hole) formed at the upper end and configured to communicate with the upper large-diameter bore of working bore  41 . Also, valve body  40  has a drain port  47  (a radial through hole) formed at the lower end and configured to communicate with the lower small-diameter bore of working bore  41 . Supply-and-exhaust port  46  always communicates with the pressure-receiving chamber  38  of pilot valve  7  through the second communication passage  39 . 
     Solenoid control port  43  communicates with the branch passage  29  through an oil passage  48  formed in the cylinder block. 
     Solenoid  45  includes a solenoid casing  45   a , an electromagnetic coil (not shown), a stationary iron core, and a movable iron core, all accommodated in the casing  45   a . A pushrod  49  is fixedly connected to the tip of the movable iron core and configured to axially slide in the small-diameter bore of working bore  41  for producing or removing a push on the ball valve  44 . 
     A cylindrical passage  50  is defined between the outer peripheral surface of pushrod  49  and the inner peripheral surface of the small-diameter bore of working bore  41 , for appropriately communicating the supply-and-exhaust port  46  with the drain port  47  by way of the cylindrical passage  50 . 
     When the electromagnetic coil of solenoid  45  is energized, the pushrod  49  extends such that the tip of pushrod  49  pushes the ball valve  44  upward. As a result, the ball valve  44  seats on the valve seat  42  so as to close the opening end of solenoid control port  43 . At the same time, the supply-and-exhaust port  46  is communicated with the drain port  47  by way of the cylindrical passage  50 . 
     Conversely when the electromagnetic coil of solenoid  45  is de-energized, as clearly shown in  FIG. 5 , the pushrod  49  retracts such that a push on the ball valve  44  is removed. As a result, fluid-communication between the solenoid control port  43  and the supply-and-exhaust port  46  is established. At the same time, fluid-communication between the cylindrical passage  50  and the drain port  47  is blocked. 
     Energization/de-energization (ON/OFF) of the electromagnetic coil of solenoid  45  is controlled responsively to a control command from an electronic control unit (not shown). 
     Although it is not clearly shown in the drawings, the electronic control unit (ECU) generally comprises a microcomputer. The control unit 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 an engine oil temperature sensor, an engine temperature sensor (e.g., an engine coolant temperature sensor), an engine speed sensor, an engine load sensor and the like. Within the control unit, the central processing unit (CPU) allows the access by the I/O interface of input informational data signals from the previously-discussed engine/vehicle sensors. The CPU of the control unit is configured to detect or determine an engine operating condition based on the input informational data and further configured to control, based on the determined engine operating condition (in particular, latest up-to-date information about engine speed), the operation of the electromagnetic coil of solenoid  45 . Concretely, when latest up-to-date information about engine speed is less than or equal to a predetermined reference engine speed “N” (see the characteristic diagram shown in  FIG. 6 ), the control unit generates a command (an ON signal) for energizing the electromagnetic coil of solenoid  45 . Conversely when latest up-to-date information about engine speed is greater than the predetermined reference engine speed “N”, the control unit generates a command (an OFF signal) for de-energizing the electromagnetic coil of solenoid  45 . However, in the case that latest up-to-date information about engine speed is less than or equal to the predetermined reference engine speed “N” but latest up-to-date information about engine load is greater than a predetermined reference engine load, that is, during high load operation, the control unit generates a command (an OFF signal) for de-energizing the electromagnetic coil of solenoid  45 . 
     [Operation of First Embodiment] 
     The operation of the variable displacement oil pump system of the first embodiment is hereunder described in detail. 
     When the engine is operating at low speeds, such as during an idling period following engine start-up, in other words, during the initial startup of the pump, cam ring  5  is spring-loaded or biased as shown in  FIG. 1  by the spring force of coil spring  28 , and thus arm  26  is brought into abutted-engagement with the motion-restriction protrusion if. Hence, cam ring  5  is kept at its maximum clockwise angular position (a cam-ring maximum-eccentricity angular position) at which the eccentricity of the geometric center of cam ring  5  to the axis of rotation of drive shaft  3  becomes a maximum value and thus the pump discharge flow rate also becomes a maximum value. 
     At this time, the electromagnetic coil of directional control valve  8  becomes energized responsively to an ON signal from the control unit, and thus the pushrod  49  extends to push the ball valve  44  upward. As a result, the opening end of solenoid control port  43  is closed by the ball valve  44  and hence fluid-communication between the solenoid control port  43  and the supply-and-exhaust port  46  is blocked and fluid-communication between the supply-and-exhaust port  46  and the drain port  47  is established. Therefore, pressure-receiving chamber  38  of pilot valve  7  becomes communicated with the oil pan through the second communication passage  39 , the supply-and-exhaust port  46 , the cylindrical passage  50 , and the drain port  47  and thus there is no hydraulic pressure acting on the pressure-receiving surface  33   b  of large-diameter spring-support slider  33 . Large-diameter spring-support slider  33  is forced or biased downward by the spring force of valve spring  34 . At this time, a maximum downward displacement of large-diameter spring-support slider  33  is restricted by abutment of the stopper protrusion  33   c  with the inside face of lid member  31 . 
     On the other hand, spool  32  is forced or biased upward by the spring force of valve spring  34  and thus the circular top of the first land  32   a  is seated on the tapered bearing surface  29   b  and thus held at the uppermost axial position of spool  32 . Hence, fluid-communication between the oil introduction port  29   a  and the first communication passage  35  is blocked and fluid-communication between the first communication passage  35  and the drain passage  37  through the annular groove  32   e , the radial through hole  32   f , and the cylindrical bore  32   d  is established. 
     Therefore, control oil chamber  16  becomes communicated with the oil pan through the communication bore  36 , the first communication passage  35 , the annular groove  32   e , the radial through hole  32   f , the cylindrical bore  32   d , and the drain passage  37 , and thus there is no hydraulic pressure supplied or directed to the control oil chamber  16 . 
     Any counterclockwise displacement of cam ring  5  against the spring force of coil spring  28  does not occur, and hence cam ring  5  is held at its maximum-eccentricity angular position. Under these conditions, the pump discharge pressure as well as the pump discharge flow rate increases proportionally, as the engine speed increases (see the engine-speed versus hydraulic-pressure characteristic in a low-speed range “a” shown in  FIG. 6 ). By the way, the hydraulic pressure at this point of time becomes a hydraulic pressure level included within a required hydraulic pressure range for the VTC device. 
     When the risen hydraulic pressure is introduced or applied from the main oil gallery  25  through the branch passage  29  into the oil introduction port  29   a  of pilot valve  7 , spool  32  begins to move downward against the spring force of valve spring  34 . When the pump discharge pressure reaches the hydraulic pressure P 1 , spool  32  shifts to a slightly downward-displaced axial position. With the spool  32  slightly displaced downward from the uppermost spring-offset axial position, fluid-communication between the oil introduction port  29   a  and the opening end  35   a  of first communication passage  35  remains blocked by the first land  32   a . Thus, there is no hydraulic pressure supply to the control oil chamber  16 . 
     As previously described, the given spring load (the set spring force) of coil spring  28  (with cam ring  5  kept at its initial setting position) is set to a spring force that cam ring  5  begins to be displaced counterclockwise from the initial setting position by hydraulic pressure of the given hydraulic pressure level P 1  supplied to the control oil chamber  16  without any pressure reduction and then the geometric center of cam ring  5  and the axis of rotation of drive shaft  3  become concentric to each other, in other words, the eccentricity of the geometric center of cam ring  5  to the axis of rotation of drive shaft  3  becomes zero. Hence, when spool  32  is further displaced downward and thus the first land  32   a  reaches a further downward position shown in  FIG. 4 , the oil introduction port  29   a  becomes communicated with the first communication passage  35  by way of a small aperture (also serving as a flow-constriction orifice) defined with the first land  32   a  further downwardly displaced. Thus, the reduced hydraulic pressure is supplied through the first communication passage  35  to the control oil chamber  16 , and as a result the pump discharge flow rate can be adjusted by a counterclockwise displacement of cam ring  5  against the spring force of coil spring  28 . 
     When hydraulic pressure, supplied from the first communication passage  35  to the control oil chamber  16 , is excessively high, a counterclockwise displacement of cam ring  5  tends to become large, and thus the pump discharge flow rate decreases. As a result, a fall in hydraulic pressure, supplied to the main oil gallery  25 , occurs, and thus spool  32  can be displaced upward by the spring force of valve spring  34 . Hence, the flow passage area of the small aperture, defined by the first land  32   a  to communicate the oil introduction port  29   a  with the first communication passage  35 , becomes smaller and whereby the hydraulic pressure supplied to the control oil chamber  16  falls. 
     Conversely when hydraulic pressure, supplied from the first communication passage  35  to the control oil chamber  16 , is excessively low, a counterclockwise displacement of cam ring  5  tends to become small, and thus the eccentricity of the geometric center of cam ring  5  to the axis of rotation of drive shaft  3  becomes greater and the pump discharge flow rate excessively increases. As a result, a rise in hydraulic pressure, supplied to the main oil gallery  25 , occurs, and thus a downward movement of spool  32  against the spring force of valve spring  34  occurs. Hence, the flow passage area of the small aperture, defined by the first land  32   a  to communicate the oil introduction port  29   a  with the first communication passage  35 , becomes larger and whereby the hydraulic pressure supplied to the control oil chamber  16  rises. 
     In this manner, immediately when the pump discharge pressure reaches the given hydraulic pressure P 1 , fluid-communication between the oil introduction port  29   a  and the first communication passage  35  becomes established, and thereafter the hydraulic pressure in the control oil chamber  16  can be appropriately controlled or regulated by virtue of an appropriate change in the flow passage area of the small aperture, defined by the first land  32   a  to communicate the oil introduction port  29   a  with the first communication passage  35 , such that the pump discharge pressure can be held at the given hydraulic pressure P 1 . Additionally, the hydraulic pressure in the control oil chamber  16  can be appropriately controlled or regulated by a comparatively small axial movement of spool  32  (in particular, the first land  32   a ) without being almost affected by a spring constant of valve spring  34 . 
     That is to say, even when a slight fluctuation in hydraulic pressure (pump discharge pressure) occurs, it is possible to satisfactorily change the flow passage area of the small aperture defined by the first land  32   a . Thus, even when the engine speed increases, there is a less rise in hydraulic pressure. Hence, as can be seen from the engine-speed versus hydraulic-pressure characteristic indicated by the horizontal solid line “b” in  FIG. 6 , the hydraulic pressure (the pump discharge pressure) can be controlled or regulated to the given constant pressure level P 1 . 
     Furthermore, suppose that a change in the distribution of hydraulic pressure applied to the inner peripheral surface  5   a  of cam ring  5  occurs due to an engine speed change, a working-oil temperature change (an oil viscosity change), mixing of air into working oil (lubricating oil), and/or the occurrence of cavitation, and thus a fluctuation in hydraulic pressure, by which cam ring  5  can be displaced about the pivot pin, occurs. In such a case, after the given hydraulic pressure P 1  has been reached and thus fluid-communication between the oil introduction port  29   a  and the first communication passage  35  has been established, the hydraulic pressure in the control oil chamber  16  can be appropriately controlled or regulated by virtue of an appropriate change in the flow passage area of the small aperture defined by the first land  32   a , without being affected by such a change in the hydraulic-pressure distribution. 
     When the engine speed reaches the predetermined reference engine speed “N” shown in  FIG. 6 , the necessity of oil-jet injection for cooling reciprocating pistons occurs. Also, with wide open throttle (WOT) or during maximum engine torque output, the necessity of supply of hydraulic pressure of a high-pressure level P 2  to crank journal bearings of the engine crankshaft occurs. 
     By the way, when the engine is running at low speeds less than or equal to the predetermined reference engine speed “N” but the engine load is high, in other words, during high load operation, also, the necessity of oil-jet injection occurs. Therefore, even during the mid-speed but high-load operation as indicated by the broken line in  FIG. 6 , as well as in the high-speed range “c” as indicated by the solid line in  FIG. 6 , the hydraulic pressure (the pump discharge pressure) has to be risen to the given hydraulic pressure level P 2 . In this case, the control unit generates a command (an OFF signal) for de-energizing the electromagnetic coil of solenoid  45  of directional control valve  8 . 
     That is, as clearly shown in  FIG. 5 , the electromagnetic coil of solenoid  45  of directional control valve  8  is de-energized, a push on the ball valve  44  toward the solenoid control port  43  by extension of pushrod  49  is removed. Hence, the ball valve  44  moves in the opposite axial direction (i.e., downward) by hydraulic pressure in the solenoid control port  43  to block or prevent working-fluid flow through the cylindrical passage  50 . Thus, fluid-communication between the supply-and-exhaust port  46  and the drain port  47  through the cylindrical passage  50  is blocked and simultaneously fluid-communication between the solenoid control port  43  and the supply-and-exhaust port  46  is established. Therefore, hydraulic pressure in the main oil gallery  25  (the branch passage  29 ) is delivered into the pressure-receiving chamber  38 , since the supply-and-exhaust port  46  always communicates with the second communication passage  39  of pilot valve  7 . 
     The same hydraulic pressure in the branch passage  29  is delivered into both the pressure-receiving chamber  38  and the oil introduction port  29   a . However, the pressure-receiving area of the pressure-receiving surface  33   b  of large-diameter spring-support slider  33  is dimensioned to be greater than that of the top face (serving as a pressure-receiving section) of the first land  32   a , and hence three component parts, namely, spool  32 , valve spring  34 , and large-diameter spring-support slider  33  upwardly move together toward the oil introduction port  29   a . At this time, a maximum upward displacement of large-diameter spring-support slider  33  is restricted by abutment of the upper face of large-diameter spring-support slider  33  with the shouldered portion  30   c  formed between small-diameter bore  30   a  and large-diameter bore  30   b  (see  FIG. 5 ). 
     In accordance with the upward movement of spool  32 , as a matter of course, the first land  32   a  moves upward and reaches the uppermost axial position shown in  FIG. 1 . Thus, the first communication passage  35  becomes communicated with the drain passage  37  through the radial through hole  32   f  of small-diameter shaft  32   c . As a result of this, a fall in hydraulic pressure in the control oil chamber  16  occurs. Hence, by the spring force of coil spring  28 , cam ring  5  returns in the direction that the eccentricity of the geometric center of can ring  5  to the axis of rotation of rotor  4  increases. Therefore, the pump discharge flow rate increases and thus the hydraulic pressure discharged from the pump rises up to given hydraulic pressure level P 2  shown in  FIG. 6 . Owing to the increased pump discharge flow rate, hydraulic pressure in the main oil gallery  25  (the branch passage  29 ) also increases. Thus, spool  32  moves downward as shown in  FIG. 5  against the spring force of valve spring  34 . 
     In this manner, as soon as the pump discharge pressure reaches the given hydraulic pressure level P 2 , as discussed above, the first land  32   a  reaches the further downward position shown in  FIG. 4 . Hence, the oil introduction port  29   a  becomes communicated with the first communication passage  35  by way of a small aperture (also serving as a flow-constriction orifice) defined with the first land  32   a  of pilot valve  7  further downwardly displaced. As a result, the reduced hydraulic pressure is introduced through the first communication passage  35  to the control oil chamber  16 . 
     The hydraulic pressure in the control oil chamber  16  is controlled by the pilot valve  7  such that the pump discharge pressure can be held at the given constant pressure level P 2 . The control method and operation for holding the pump discharge pressure at the given constant pressure level P 2  are the same as those described previously for holding the pump discharge pressure at the given constant pressure level P 1 . 
     As discussed above, in the first embodiment, by energization/de-energization control (ON/OFF control) for the electromagnetic coil of solenoid  45  of directional control valve  8 , the discharge pressure from the pump to the main oil gallery  25  can be controlled or switched between two kinds of hydraulic pressure levels, namely, low hydraulic pressure level P 1  and high hydraulic pressure level P 2 . 
     Additionally, the controlled discharge pressure can be stably held at a given constant pressure level by virtue of an appropriate change in the flow passage area of the small aperture, defined by the first land  32   a  to communicate the oil introduction port  29   a  with the first communication passage  35 , regardless of engine operating conditions, such as a change in engine speed, a change in engine oil temperature and the like. 
     The relationship (containing a relative pressure difference) between the two different pump discharge pressures (that is, settings of two kinds of hydraulic pressure levels P 1  and P 2 ) can be determined by a quantity of expansion and contraction of valve spring  34  and a spring constant of valve spring  34 . The settings of two kinds of pump discharge pressures have to be varied depending on the type of internal combustion engine. In the shown embodiment, desired settings of two kinds of pump discharge pressures can be easily achieved by only a setting change (e.g., a spring-constant change) of valve spring  34 , without any structure change or any design change in other component parts (e.g., the cam ring and/or the pump housing). Therefore, it is unnecessary to redesign or newly manufacture a basic structure of the pump body from a beginning, thus greatly reducing manufacturing costs. Also, even when the desired settings of two kinds of pump discharge pressures cannot be supported by only a setting change (e.g., only a spring-constant change) of valve spring  34 , it is possible to satisfactorily support the desired settings by slightly modifying or changing the axial length of stopper protrusion  33   c  of large-diameter spring-support slider  33 , the formation position of shouldered portion  30   c  between small-diameter bore  30   a  and large-diameter bore  30   b , and/or the formation position of the stepped portion of tapered bearing surface  29   b  between oil introduction port  29   a  and small-diameter bore  30   a.    
     When hydraulic pressure in the pressure-receiving chamber  38  becomes high, large-diameter spring-support slider  33  is brought into abutted-engagement (into wall-contact) with the shouldered portion  30   c  to ensure a good seal between pressure-receiving chamber  38  and small-diameter bore  30   a . Therefore, it is unnecessary to strictly manage or control the accuracy or the quality concerning the clearance space between the inner peripheral wall surface of large-diameter bore  30   b  and the outer peripheral wall surface of large-diameter spring-support slider  33 . 
     Additionally, spool  32  and large-diameter spring-support slider  33  are two separate component parts. Thus, it is unnecessary to strictly manage or control the accuracy or the quality concerning the concentricity of small-diameter bore  30   a  and large-diameter bore  30   b . From the viewpoints discussed above, manufacturing or machining work becomes easy. 
     In the shown embodiment, the control unit is configured to perform ON/OFF control for the electromagnetic coil of solenoid  45  of directional control valve  8  based on the engine operating condition (in particular, latest up-to-date information about engine speed and/or engine load). Actually, the variable displacement oil pump system of the embodiment is configured to rise the pump discharge pressure up to the high-pressure level P 2  with the electromagnetic coil de-energized (kept in its OFF state), fully taking into account a fail-safe in the presence of a pump discharge pressure control system failure, for example undesirable breaking of the electromagnetic coil (see the engine-speed versus hydraulic-pressure characteristic indicated by the horizontal solid line “c” in  FIG. 6 ). 
     Furthermore, in the shown embodiment, first and second oil filters  51 - 52  are disposed near the branch point of the upstream side of main oil gallery  25  and branch passage  29 . Thus, it is possible to adequately prevent contaminants and/or metal debris from entering the pilot valve  7  and/or the directional control valve  8  by virtue of double filtering-out action by means of these oil filters. Hence, there is a less risk of undesirably poor operation (e.g., a sticking valve) of the pilot valve  7  and/or the directional control valve  8 , which may occur owing to contaminants and/or metal debris. 
     Assume that undesirable clogging of at least one of first and second oil filters  51 - 52  occurs. In such a case, due to the clogged oil filter, hydraulic pressure cannot be introduced to the control oil chamber  16 , and thus cam ring  5  can be maintained at its initial setting position (i.e., the maximum-eccentricity angular position) shown in  FIG. 1 . Although it is not clearly shown in the drawings, a relief valve (not shown) begins to operate, when the pump discharge pressure becomes excessively high owing to the maximum-eccentricity angular position. As a result, an excessive rise in pump discharge pressure can be suppressed. As discussed above, even in the presence of a pump discharge pressure control hydraulic circuit failure, concretely undesirable clogging of the hydraulic circuit, a high pump discharge pressure can be ensured. Hence, even during high-speed and high-load operation, it is possible to adequately suppress the engine from being damaged owing to an insufficient hydraulic pressure. 
     [Second Embodiment] 
     Referring now to  FIG. 7 , there is shown the variable displacement oil pump system of the second embodiment. The fundamental configuration of the second embodiment is similar to that of the first embodiment. Thus, the same reference signs used to designate components (elements) in the pump system shown in  FIGS. 1-5  will be applied to the corresponding reference signs used in the pump system shown in  FIGS. 7-9 , for the purpose of comparison of the two different pump systems. Detailed description of the same components (elements) will be omitted because the above description thereon seems to be self-explanatory. 
     Briefly speaking, in the second embodiment, a second control oil chamber  53  is further formed at the lower part of the outer periphery of cam ring  5  than the pivot pin  10 , serving as a fulcrum of oscillating motion of cam ring  5 . Additionally, the structure of spool  57  of pilot valve  7  of the second embodiment is changed from the structure of spool  32  of the first embodiment. 
     More concretely, the first control oil chamber  16  is defined between the inner periphery of pump housing  1  and the upper part of the outer periphery of cam ring  5  than the cam-ring reference line “M”, whereas the second control oil chamber  53  is defined between the inner periphery of pump housing  1  and the lower part of the outer periphery of cam ring  5  than the cam-ring reference line “M”. In  FIG. 7 , the lowermost end of the first control oil chamber  16  and the uppermost end of the second control oil chamber  53  are arranged near the pivot pin  10 , in a manner so as to sandwich the cam-ring reference line “M” between first and second control oil chambers  16  and  53 . 
     Regarding the first control oil chamber  16 , hydraulic pressure in the branch passage  29  is always directly introduced from an introduction passage  54 , branched from the branch passage  29 , through the first communication bore  36  to the first control oil chamber  16 . That is, the first control oil chamber  16  serves as an ordinarily-pressure-applied chamber. The hydraulic pressure, introduced to the first control oil chamber  16 , creates a force that rotates or biases the cam ring  5  against the spring force of coil spring  28  in the counterclockwise direction that the eccentricity of the geometric center of cam ring  5  to the axis of rotation of rotor  4  decreases. 
     Regarding the second control oil chamber  53 , hydraulic pressure in the branch passage  29  is introduced from the pilot valve  7  through a second communication bore  55 , formed parallel to the first communication bore  36 , to the second control oil passage  53 . The hydraulic pressure, introduced to the second control oil chamber  53 , creates a force that gives assistance to the spring force of coil spring  28  and rotates or biases the cam ring  5  in the clockwise direction that the eccentricity of the geometric center of cam ring  5  to the axis of rotation of rotor  4  increases. 
     Assume that the same hydraulic pressure is supplied to both the first control oil chamber  16  and the second control oil chamber  53 . In such a case, the force created by the same hydraulic pressure supplied to the first control oil chamber  16  and acting to rotate the cam ring  5  in the counterclockwise direction and the force created by the same hydraulic pressure supplied to the second control oil chamber  53  and acting to rotate the cam ring  5  in the clockwise direction tends to cancel out each other. Hence, in the case of the same hydraulic pressure supply to first and second control oil chambers  16  and  53 , there is a less hydraulic pressure that produces a counterclockwise displacement of cam ring  5  against the spring force of coil spring  28 . That is, first and second control oil chambers  16  and  53  (i.e., the ratio between the pressure-receiving area of a portion of the outer peripheral surface of cam ring  5 , associated with the first control oil chamber  16  and the pressure-receiving area of a portion of the outer peripheral surface of cam ring  5 , associated with the second control oil chamber  53 ) are designed such that cam ring  5  cannot be rotated or displaced against the spring force of coil spring  28  in the direction that the eccentricity of the geometric center of cam ring  5  to the axis of rotation of rotor  4  decreases, in the case of the same hydraulic pressure supply to first and second control oil chambers  16  and  53 . 
     When a decrease in the force that gives assistance to the spring force of coil spring  28  occurs owing to a fall in hydraulic pressure in the second control oil chamber  53 , as shown in  FIG. 9  cam ring  5  can be rotated or displaced against the spring force of coil spring  28  in the counterclockwise direction that the eccentricity of the geometric center of cam ring  5  to the axis of rotation of rotor  4  decreases. Additionally, for safety (for a fail-safe function), (i) the spring load W of coil spring  28  applied to the arm  26  and (ii) the ratio between the pressure-receiving area of a portion of the outer peripheral surface of cam ring  5 , associated with the first control oil chamber  16  and the pressure-receiving area of a portion of the outer peripheral surface of cam ring  5 , associated with the second control oil chamber  53  are set such that a rotary motion or a counterclockwise displacement of cam ring  5  against the spring force of col spring  28  can occur by the application of hydraulic pressure of approximately 1 MPa to both of the first control oil chamber  16  and the second control oil chamber  53 . 
     In order to form first and second control oil chambers  16  and  53 , in addition to the first sealing surface  1   e , a circular-arc shaped second sealing surface  1   i  is further configured or formed on the inner peripheral surface of an expanding portion  1   h  integrally formed to expand a part of the pump housing  1 . The second sealing surface  1   i  is configured to be almost point-symmetrical to the first sealing surface  1   e  with respect to the rotation axis of drive shaft  3 . In addition to the first protruding portion  5   e , cam ring has a second protruding portion  5   f  at a given angular position substantially corresponding to the expanding portion  1   h  of pump housing  1 . In a similar manner to the first seal-retention groove  5   b  formed in the first protruding portion  5   e  for retaining the first seal member  13 , a seal-retention groove is formed in the outer peripheral surface of the second protruding portion  5   f  for retaining a second seal member  56  so as to permit permanent sliding-contact between the second seal member  56  and the second sealing surface  1   i.    
     Other component parts are the same as the pump structural unit of the variable displacement oil pump system of the first embodiment, and also these operations are the same. 
     In a similar manner to the first embodiment, in the second embodiment, pilot valve  7  is formed with three cylindrical bores (that is, oil introduction port  29   a , small-diameter bore  30   a , and large-diameter bore  30   b ) having respective inside diameters differing from each other. A spool  57  of pilot valve  7 , involved in the variable displacement oil pump system of the second embodiment has axially-spaced three lands (that is, first, second, and third lands  57   a ,  57   b , and  57   c ), and a first small-diameter shaft  57   d  between first and second lands  57   a - 57   b , and a second small-diameter shaft  57   e  between second and third lands  57   b - 57   c . A first annular groove  57   h  between first and second lands  57   a - 57   b  is defined on the outer periphery of first small-diameter shaft  57   d , whereas a second annular groove  57   i  between second and third lands  57   b - 57   c  is defined on the outer periphery of second small-diameter shaft  57   e.    
     Spool  57  has a cylindrical bore  57   f  closed at its lower end and extending along the axis of spool  57 . Cylindrical bore  57   f  always communicates with the oil introduction port  29   a . The second small-diameter shaft  57   e  has a radial through hole  57   g  for communicating the second annular groove  57   i  with the cylindrical bore  57   f  by way of the radial through hole  57   g.    
     One opening end  58   a  of a third communication passage  58  is configured to open into the axially intermediate portion of small-diameter bore  30   a  of stepped cylindrical close-fitting bore  30 . The other end of the third communication passage  58  communicates with the second control oil chamber  53  through a second communication bore  55  formed in the right-hand end wall of pump housing  1 . One opening end  59   a  of a drain passage  59 , which passage communicates with the oil pan, is configured to open into the upper portion of small-diameter bore  30   a  than the opening end  58   a  of the third communication passage  58 . 
     The opening end  58   a  of the third communication passage  58  and the opening end  59   a  of the drain passage  59  are opened or closed relatively depending on the axial position of the sliding spool  57  (in particular, the axial position of the second land  57   b ), so as to establish or block fluid-communication between the oil introduction port  29   a  and the third communication passage  58  or fluid-communication between the third communication passage  58  and the drain passage  59 . 
     The other configuration of pilot valve  7  of the second embodiment is the same as the first embodiment. That is, valve spring  34  is disposed between the spool  57  and the large-diameter spring-support slider  33  under preload such that the spool  57  and the large-diameter spring-support slider  33  are biased to be spaced from each other in the opposite directions (see  FIG. 7 ). 
     As clearly shown in  FIG. 7 , spool  57  is upwardly forced or biased by the spring force of valve spring  34  and thus the annular top of the first land  57   a  of spool  57  is seated on the tapered bearing surface  29   b  formed between oil introduction port  29   a  and small-diameter bore  30   a . On the other hand, regarding large-diameter spring-support slider  33 , its stopper protrusion  33   c  is brought into abutted-engagement with the inside face of lid member  31  by the spring force of valve spring  34 , to define the large-diameter pressure-receiving chamber  38  between the underside (large-diameter pressure-receiving surface  33   b ) of large-diameter spring-support slider  33  and the inside face of lid member  31 , which lid member is provided for hermetically closing the lowermost end of large-diameter bore  30   b . At this time, valve spring  34  is disposed between the spool  57  and the large-diameter spring-support slider  33  under preload (i.e., under a specified set spring load). 
     The valve configuration of electromagnetic solenoid operated directional control valve  8  incorporated in the pump system of the second embodiment is identical to that of the first embodiment. The supply-and-exhaust port  46  of directional control valve  8  is configured to always communicate with the pressure-receiving chamber  38  of pilot valve  7  via the second communication passage  39 . 
     [Operation of Second Embodiment] 
     The operation of the variable displacement oil pump system of the second embodiment is hereunder described in detail in reference to the engine-speed versus hydraulic-pressure characteristic diagram of  FIG. 6 . 
     Referring now to  FIG. 7 , there is shown the initial working state of the pump system of the second embodiment during operation of the engine at low speeds, in other words, during the initial pump startup state where the pump discharge pressure is still low. At this time, the electromagnetic coil of directional control valve  8  becomes energized responsively to an ON signal from the control unit, and thus the pushrod  49  extends to push the ball valve  44  upward. As a result, the opening end of solenoid control port  43  is closed by the ball valve  44  and hence fluid-communication between the solenoid control port  43  and the supply-and-exhaust port  46  is blocked and fluid-communication between the supply-and-exhaust port  46  and the drain port  47  is established. Supply-and-exhaust port  46  is configured to always communicate with the second communication passage  39  of pilot valve  7 . Therefore, pressure-receiving chamber  38  of pilot valve  7  becomes communicated with the oil pan through the second communication passage  39 , the supply-and-exhaust port  46 , the cylindrical passage  50 , and the drain port  47 . There is no hydraulic pressure acting on the pressure-receiving surface  33   b  of large-diameter spring-support slider  33 . That is, the pressure-receiving chamber  38  becomes a low-pressure state. Stopper protrusion  33   c  of large-diameter spring-support slider  33  is kept in abutted-engagement with the inside face of lid member  31  by the spring force of valve spring  34 . 
     On the other hand, the annular top of the first land  57   a  of spool  57  is abutted or seated on the tapered bearing surface  29   b  by the spring force of valve spring  34 . With the spool  57  positioned at the uppermost axial position, the second annular groove  57   i  of second small-diameter shaft  57   e  becomes communicated with the third communication passage  58 , and thus fluid-communication between the third communication passage  58  and the oil introduction port  29   a  through the radial through hole  57   g  of second small-diameter shaft  57   e  is established. 
     The third communication passage  58  is configured to always communicate with the second communication bore  55 . Therefore, the second control oil chamber  53  is communicated with the oil introduction port  29   a , and thus kept in a state that hydraulic pressure in the main oil gallery  25  is delivered into the second control oil chamber  53 . 
     On the other hand, the first control oil chamber  16  is configured to always communicate with the main oil gallery through the first communication bore  36 , the introduction passage  54 , and the branch passage  29 . Thus, hydraulic pressure of the same pressure level is supplied from the main oil gallery  25  to both the first control oil chamber  16  and the second control oil chamber  53 . Any counterclockwise displacement of cam ring  5  against the spring force of coil spring  28  does not occur, and hence cam ring  5  is kept at its initial setting position (i.e., the maximum-eccentricity angular position) shown in  FIG. 7 . Under these conditions, the pump discharge pressure as well as the pump discharge flow rate increases proportionally, as the engine speed increases (see the engine-speed versus hydraulic-pressure characteristic in a low-speed range “a” shown in  FIG. 6 ). 
     When the risen hydraulic pressure is introduced from the main oil gallery  25  through the branch passage  29  into the oil introduction port  29   a  of pilot valve  7 , spool  57  begins to move downward against the spring force of valve spring  34 . When the pump discharge pressure reaches the hydraulic pressure P 1 , spool  57  shifts to a slightly downward-displaced axial position (see the axial position of spool  57  shown in  FIG. 8 ). With the spool  57  slightly displaced downward from the uppermost spring-offset axial position, fluid-communication between the third communication passage  58  and the oil introduction port  29   a  through the radial through hole  57   g  of second small-diameter shaft  57   e  becomes blocked by the inner peripheral wall surface of small-diameter bore  30   a . In contrast, fluid-communication between the third communication passage  58  and the drain passage  59  through the first annular groove  57   h  becomes established. As a result, hydraulic pressure in the second control oil chamber  53  is drained through the third communication passage  58  and the drain passage  59  into the oil pan and thus the second control oil chamber  53  becomes a low-pressure state. 
     The given spring load (the set spring force) of coil spring  28  (with cam ring  5  kept at its initial setting position) is set to a spring force that cam ring  5  is prevented from being displaced counterclockwise from the initial setting position with hydraulic pressure of the given hydraulic pressure level P 1  supplied to the second control oil chamber  53  without any pressure reduction. However, as the hydraulic pressure in the second control oil chamber  53  reduces, cam ring  5  begins to rotate counterclockwise against the spring force of coil spring  28  such that the pump discharge flow rate can be adjusted. 
     When hydraulic pressure in the second control oil chamber  53  is excessively low, a counterclockwise displacement of cam ring  5  tends to become large, and thus the pump discharge flow rate decreases. As a result, a fall in hydraulic pressure in the main oil gallery  25  (the branch passage  29 ) occurs, and thus spool  57  can be slightly displaced upward by the spring force of valve spring  34 . Hence, the flow passage area of the small aperture, defined by the second land  57   b  to communicate the first annular groove  57   h  with the opening end  58   a  of the third communication passage  58 , becomes smaller and whereby the amount of working fluid directed from the small aperture through the first annular groove  57   h  to the drain passage  59  decreases. As a result, hydraulic pressure in the second control oil chamber  53  rises. 
     Conversely when hydraulic pressure in the second control oil chamber  53  is excessively high, a counterclockwise displacement of cam ring  5  tends to become small, and thus the pump discharge flow rate increases. As a result, a rise in hydraulic pressure, supplied to the main oil gallery  25  (the branch passage  29 ), occurs, and thus a downward movement of spool  57  against the spring force of valve spring  34  occurs. Hence, the flow passage area of the small aperture, defined by the second land  57   b  to communicate the first annular groove  57   h  with the opening end  58   a  of the third communication passage  58 , becomes larger and whereby the amount of working fluid directed from the small aperture through the first annular groove  57   h  to the drain passage  59  increases. As a result, hydraulic pressure in the second control oil chamber  53  falls. 
     In this manner, immediately when the pump discharge pressure reaches the given hydraulic pressure P 1 , fluid-communication between the oil introduction port  29   a  and the second control oil chamber  53  through the third communication passage  58  becomes blocked and fluid-communication between the drain passage  59  and the third communication passage  58  becomes established, and thereafter the hydraulic pressure in the second control oil chamber  53  can be appropriately controlled or regulated by virtue of an appropriate change in the flow passage area of the small aperture, defined by the second land  57   b  to communicate the first annular groove  57   h  with the opening end  58   a  of the third communication passage  58 . 
     Additionally, the hydraulic pressure in the second control oil chamber  53  can be appropriately controlled or regulated by a comparatively small axial movement of spool  57  (in particular, the second land  57   b ) without being almost affected by a spring constant of valve spring  34 . 
     That is to say, even when a slight fluctuation in hydraulic pressure (pump discharge pressure) occurs, it is possible to satisfactorily change the flow passage area of the small aperture defined by the second land  57   b . Thus, even when the engine speed increases, there is a less rise in hydraulic pressure. Hence, in the second embodiment as well as the first embodiment, as can be seen from the engine-speed versus hydraulic-pressure characteristic indicated by the horizontal solid line “b” in  FIG. 6 , the hydraulic pressure (the pump discharge pressure) can be controlled or regulated to the given constant pressure level P 1 . 
     Also, in the same manner as the first embodiment, when the hydraulic pressure (the pump discharge pressure) has to be risen to the given hydraulic pressure level P 2 , the electromagnetic coil of solenoid  45  of directional control valve  8  becomes de-energized responsively to an OFF signal from the control unit, thereby permitting the pushrod  49  to retract so as to establish fluid-communication between the solenoid control port  43  and the supply-and-exhaust port  46  and simultaneously to block fluid-communication between the supply-and-exhaust port  46  and the drain port  47  through the cylindrical passage  50 . Hence, hydraulic pressure in the main oil gallery  25  (the branch passage  29 ) is delivered into the pressure-receiving chamber  38 . 
     Hydraulic pressure of the same pressure level is delivered from the main oil gallery  25  (the branch passage  29 ) into both the pressure-receiving chamber  38  and the oil introduction port  29   a . However, the pressure-receiving area of the pressure-receiving surface  33   b  of large-diameter spring-support slider  33  is dimensioned to be greater than that of the upper face (serving as a pressure-receiving section) of spool  57 , and hence three component parts, namely, spool  57 , valve spring  34 , and large-diameter spring-support slider  33  upwardly move together toward the oil introduction port  29   a . At this time, a maximum upward displacement of large-diameter spring-support slider  33  is restricted by abutment of the upper face of large-diameter spring-support slider  33  with the shouldered portion  30   c  between small-diameter bore  30   a  and large-diameter bore  30   b  (see  FIG. 9 ). 
     In accordance with the upward movement of spool  57 , as a matter of course, the second land  57   b  moves upward, and then the spool  57  reaches the uppermost axial position shown in  FIG. 7 . Thus, the third communication passage  58  becomes communicated with the oil introduction port  29   a  through the radial through hole  57   g  of the second small-diameter shaft  57   e . As a result of this, a rise in hydraulic pressure in the second control oil chamber  53  occurs. Hence, owing to the risen hydraulic pressure in the second control oil chamber  53  as well as the spring force of coil spring  28 , cam ring  5  returns in the direction that the eccentricity of the geometric center of cam ring  5  to the axis of rotation of rotor  4  increases. Therefore, the pump discharge flow rate increases and thus the hydraulic pressure discharged from the pump to the main oil gallery  25  also increases. Thus, spool  57  moves downward against the spring force of valve spring  34 . 
     In this manner, as soon as the pump discharge pressure reaches the given hydraulic pressure level P 2 , the second land  57   b  reaches the axial position, corresponding to the opening end  58   a  of the third communication passage  58 , as shown in  FIG. 9 . The first annular groove  57   h  becomes communicated with the third communication passage  58  by way of the small aperture, defined by the second land  57   b  to communicate the first annular groove  57   h  with the opening end  58   a  of the third communication passage  58 . Hence, fluid-communication between the drain passage  59  and the second control oil chamber  53  becomes established. As a result, hydraulic pressure in the second control oil chamber  53  falls. 
     The hydraulic pressure in the second control oil chamber  53  is controlled by the pilot valve  7  such that the pump discharge pressure can be held at the given constant pressure level P 2 . The control method and operation for holding the pump discharge pressure at the given constant pressure level P 2  are the same as those described previously for holding the pump discharge pressure at the given constant pressure level P 1 . 
     As discussed above, the engine-speed versus hydraulic-pressure characteristic and effects, achieved by the variable displacement oil pump system of the second embodiment, are the same as the first embodiment. Additionally, the second embodiment can provide the following further operation and effect. That is, even in the presence of a variable displacement oil pump system failure, more concretely, even in an abnormal situation where a mechanical problem, such as a locked pilot valve  7  and/or a locked directional control valve  8  (concretely, a sticking ball valve of the pilot valve  7  and/or a sticking spool of the directional control valve  8 ) occurs owing to contaminants, impurities and the like and thus hydraulic pressure supply from the main oil gallery  25  to both the first control oil chamber  16  and the second control oil chamber  53  is maintained, immediately when the supplied hydraulic pressure becomes a fail-safe pressure level (approximately 1 MPa), the pump system shifts to a fail-safe operating mode at which cam ring  5  begins to rotate in the counterclockwise direction that the eccentricity of the geometric center of cam ring  5  to the axis of rotation of rotor  4  decreases. 
     [Third Embodiment] 
     Referring now to  FIG. 10 , there is shown the variable displacement oil pump system of the third embodiment. The fundamental configuration of the third embodiment, such as the pump structural unit of the variable displacement oil pump, is similar to that of the second embodiment. However, the third embodiment somewhat differs from the second embodiment in that, in the third embodiment, the working-fluid flow passage for the first control oil chamber  16  is configured such that hydraulic pressure can be supplied or exhausted to or from the first control oil chamber  16  by way of the pilot valve  7 . Additionally, in the third embodiment, the given spring load W, produced by coil spring  28  with cam ring  5  kept at its initial setting position (i.e., the maximum-eccentricity angular position) shown in  FIG. 10 , is set to a spring force that the eccentricity of the geometric center of cam ring  5  to the axis of rotation of drive shaft  3  can be held at its maximum value when the engine is put in a stop state where the pump drive shaft  3  has stopped rotating. 
     In a similar manner to the first embodiment shown in  FIGS. 1-5 , in the pump system configuration of the third embodiment shown in  FIGS. 10-12 , one opening end  35   a  of the first communication passage  35  is formed near the oil introduction port  29   a  and configured to open into the upper portion of small-diameter bore  30   a  than the opening end  58   a  of the third communication passage  58 . The other end of the first communication passage  35  is configured to communicate with the first control oil chamber  16  through the first communication bore  36  formed in the right-hand end wall of pump housing  1 . 
     In a similar manner to the second embodiment shown in  FIGS. 7-9 , in the pump system configuration of the third embodiment shown in  FIGS. 10-12 , spool  57  of pilot valve  7  has axially-spaced three lands (that is, first, second, and third lands  57   a ,  57   b , and  57   c ), and the first small-diameter shaft  57   d  (i.e., the first annular groove  57   h ) between first and second lands  57   a - 57   b , and the second small-diameter shaft  57   e  (i.e., the second annular groove  57   i ) between second and third lands  57   b - 57   c.    
     The width (i.e., the axial length) of the first annular groove  57   h  is dimensioned to be approximately equal to the inside diameter (i.e., the opening width) of the opening end  35   a  of the first communication passage  35 . The width (i.e., the axial length) of the second annular groove  57   i  is dimensioned to be approximately equal to the inside diameter (i.e., the opening width) of the opening end  58   a  of the third communication passage  58 . The inside diameter (i.e., the opening width) of the opening end  59   a  of the drain passage  59  is dimensioned to be approximately equal to the width (i.e., the axial length) of the first annular groove  57   h . Also, the second small-diameter shaft  57   e  has the radial through hole  57   g  for communicating the second annular groove  57   i  with the cylindrical bore  57   f  by way of the radial through hole  57   g . Depending on the axial position of spool  57  (in particular, the second annular groove  57   i ), the radial through hole  57   g  can be appropriately communicated with the third communication passage  58 . 
     The valve configuration of electromagnetic solenoid operated directional control valve  8  incorporated in the pump system of the third embodiment is identical to that of the second embodiment. 
     Oil, discharged from the pump discharge passage  12   b , passes through the oil filter  51  or an oil cooler (not shown). The discharged oil flow enters the main oil gallery  25 . Then, the oil flow is directed or supplied through the main oil gallery  25  to moving or sliding engine parts and hydraulically-operated devices (i.e., a VTC device). 
     As clearly shown in  FIG. 10 , solenoid control port  43  of directional control valve  8  and oil introduction port  29   a  of pilot valve  7  are both connected to the main oil gallery  25  (the branch passage  29 ). In lieu thereof, these ports  43  and  29   a  may be connected to the discharge port  12  or the discharge passage  12   b.    
     Supply-and-exhaust port  46  of directional control valve  8  is connected to the second communication passage  39  of pilot valve  7 . 
     The first annular groove  57   h  of spool  57  of pilot valve  7  is configured to open into the drain passage  59 . The second annular groove  57   i  is configured to communicate with the cylindrical bore  57   f  through the radial through hole  57   g , and further communicate with the oil introduction port  29   a . In the same manner as the previously-described first and second embodiments, also in the third embodiment, first and second drain passages  37  and  59  of pilot valve  7  and drain port  47  of directional control valve  8  are all configured to communicate with the oil pan. 
     [Operation of Third Embodiment] 
     The operation of the variable displacement oil pump system of the third embodiment is hereunder described in detail in reference to the engine-speed versus hydraulic-pressure characteristic diagram of  FIG. 6 . 
     Referring now to  FIG. 10 , there is shown the initial working state of the pump system of the third embodiment during operation of the engine at low speeds, in other words, during the initial pump startup state where the pump discharge pressure is still low. 
     At this time, the electromagnetic coil of directional control valve  8  becomes energized responsively to an ON signal from the control unit, and thus the pushrod  49  extends to push the ball valve  44  upward. As a result, the opening end of solenoid control port  43  is closed by the ball valve  44  and hence fluid-communication between the solenoid control port  43  and the supply-and-exhaust port  46  is blocked and fluid-communication between the supply-and-exhaust port  46  and the drain port  47  is established. Supply-and-exhaust port  46  is configured to always communicate with the second communication passage  39  of pilot valve  7 . Therefore, pressure-receiving chamber  38  of pilot valve  7  becomes communicated with the oil pan through the second communication passage  39 , the supply-and-exhaust port  46 , the cylindrical passage  50 , and the drain port  47 . There is no hydraulic pressure acting on the pressure-receiving surface  33   b  of large-diameter spring-support slider  33 . That is, the pressure-receiving chamber  38  becomes a low-pressure state. 
     Stopper protrusion  33   c  of large-diameter spring-support slider  33  is kept in abutted-engagement with the inside face of lid member  31  by the spring force of valve spring  34 . 
     On the other hand, the annular top of the first land  57   a  of spool  57  of pilot valve  7  is abutted or seated on the tapered bearing surface  29   b  of small-diameter bore  30   a  by the spring force of valve spring  34 . With the spool  57  positioned at the uppermost axial position, fluid-communication between the first communication passage  35  and the first drain passage  59  becomes established, since the first annular groove  57   h  of the first small-diameter shaft  57   d  becomes communicated with both the first communication passage  35  and the first drain passage  59 . 
     The third communication passage  58  becomes communicated with the oil introduction port  29   a  through the radial through hole  57   g  of second small-diameter shaft  57   e . The first communication passage  35  is configured to always communicate with the first communication bore  36  of pump housing  1 . Fluid-communication between the first control oil chamber  16  and the drain passage  59  becomes established and thus there is no hydraulic pressure supply to the first control oil chamber  16 . The third communication passage  58  is configured to always communicate with the second communication bore  55 . Fluid-communication between the second control oil chamber  53  and the oil introduction port  29   a  through the second annular groove  57   i  and the radial through hole  57   g  becomes established, and thus hydraulic pressure in the main oil gallery  25  is supplied to the second control oil chamber  53 . 
     As discussed above, hydraulic pressure is supplied from the main oil gallery  25  through the branch passage  29  to only the second control oil chamber  53 . Hence, cam ring  5  cannot rotate counterclockwise against the spring force of coil spring  28  and thus cam ring  5  remains kept at its initial setting position (i.e., the maximum-eccentricity angular position) shown in  FIG. 10 . Under these conditions, the pump discharge pressure as well as the pump discharge flow rate increases proportionally, as the engine speed increases (see the engine-speed versus hydraulic-pressure characteristic in a low-speed range “a” shown in  FIG. 6 ). 
     When the risen hydraulic pressure is introduced from the main oil gallery  25  through the branch passage  29  into the oil introduction port  29   a  of pilot valve  7 , spool  57  begins to move downward against the spring force of valve spring  34 . 
     When the pump discharge pressure reaches the hydraulic pressure P 1 , spool  57  shifts to a slightly downward-displaced axial position (see the axial position of spool  57  shown in  FIG. 11 ). 
     The inside diameter (i.e., the opening width) of the opening end  35   a  of the first communication passage  35  and the width (i.e., the axial length) of the first land  57   a  are dimensioned to be approximately equal to each other. At the unique axial position of spool  57  shown in  FIG. 11 , a unique flow path configuration for the first communication passage  35  can be selectively switched between (i) a flow path from the first communication passage  35  via the first annular groove  57   h  to the drain passage  59  and (ii) a flow path from the oil introduction port  29   a  to the first communication passage  35 . Additionally, a unique flow path configuration for the third communication passage  58  can be selectively switched between (i) a flow path from the third communication passage  58  via the first annular groove  57   h  to the drain passage  59  and (ii) a flow path from the oil introduction port  29   a  via the second annular groove  57   i  and the radial through hole  57   g  to the third communication passage  58 . Switching of the flow path configuration for the first communication passage  35  between the two different flow paths and switching of the flow path configuration for the third communication passage  58  between the two different flow paths can be carried out substantially at the same time. 
     As previously discussed, the first control oil chamber  16  always communicates with the first communication passage  35  through the first communication bore  36 , whereas the second oil chamber  53  always communicates with the third communication passage  58  through the second communication bore  55 . Hence, at the unique axial position of spool  57  shown in  FIG. 11 , switching of the flow path configuration for the first control oil chamber  16  (i.e., the first communication passage  35 ) between (i) the flow path from the first control oil chamber  16  via the first annular groove  57   h  to the drain passage  59  and (ii) the flow path from the oil introduction port  29   a  to the first control oil chamber  16 , and switching of the flow path configuration for the second control oil chamber  53  (i.e., the third communication passage  58 ) between (i) the flow path from the oil introduction port  29   a  via the second annular groove  57   i  and the radial through hole  57   g  to the second control oil chamber  53  and (ii) the flow path from the second control oil chamber  53  via the first annular groove  57   h  to the drain passage  59  can be carried out in concurrence with each other. By virtue of the two different flow-path switching actions, which can be carried out in concurrence with each other, a counterclockwise displacement of cam ring  5  against the spring force of coil spring  28  occurs, thereby adjusting the pump discharge flow rate. 
     When hydraulic pressure in the first control oil chamber  16  is excessively high or hydraulic pressure in the second control oil chamber  53  is excessively low, a counterclockwise displacement of cam ring  5  tends to become large, and thus the pump discharge flow rate decreases. As a result, a fall in hydraulic pressure in the main oil gallery  25  (the branch passage  29 ) occurs, and thus spool  57  can be slightly displaced upward by the spring force of valve spring  34 . Owing to the slight upward movement of the first land  57   a , the flow passage area of the small aperture, defined by the first land  57   a  to communicate the oil introduction port  29   a  with the opening end  35   a  of the first communication passage  35 , becomes smaller. As a result, a fall in hydraulic pressure in the first control oil chamber  16  occurs. At the same time, owing to the slight upward movement of the second land  57   b , the flow passage area of the small aperture, defined by the second land  57   b  to communicate the first annular groove  57   h  with the opening end  58   a  of the third communication passage  58 , becomes smaller and thus the amount of working fluid directed from the small aperture through the first annular groove  57   h  to the drain passage  59  decreases. As a result, a rise in hydraulic pressure in the second control oil chamber  53  occurs. 
     Conversely when hydraulic pressure in the first control oil chamber  16  is excessively low or hydraulic pressure in the second control oil chamber  53  is excessively high, a counterclockwise displacement of cam ring  5  tends to become small, and thus the pump discharge flow rate increases. As a result, a rise in hydraulic pressure in the main oil gallery  25  (the branch passage  29 ) occurs, and thus spool  57  can be slightly displaced downward against the spring force of valve spring  34 . Owing to the slight downward movement of the first land  57   a , the flow passage area of the small aperture, defined by the first land  57   a  to communicate the oil introduction port  29   a  with the opening end  35   a  of the first communication passage  35 , becomes larger. As a result, a rise in hydraulic pressure in the first control oil chamber  16  occurs. At the same time, owing to the slight downward movement of the second land  57   b , the flow passage area of the small aperture, defined by the second land  57   b  to communicate the first annular groove  57   h  with the opening end  58   a  of the third communication passage  58 , becomes larger and thus the amount of working fluid directed from the small aperture through the first annular groove  57   h  to the drain passage  59  increases. As a result, a fall in hydraulic pressure in the second control oil chamber  53  occurs. 
     In this manner, immediately when the pump discharge pressure reaches the given hydraulic pressure P 1 , fluid-communication between the oil introduction port  29   a  and the first communication passage  35  (the first control oil chamber  53 ) becomes established and simultaneously fluid-communication between the drain passage  59  and the third communication passage  58  (the second control oil chamber  53 ) becomes established. Thereafter, the hydraulic pressure in the first control oil chamber  16  can be appropriately controlled or regulated by virtue of an appropriate change in the flow passage area of the small aperture, defined by the first land  57   a  to communicate the oil introduction port  29   a  with the first communication passage  35 , and simultaneously the hydraulic pressure in the second control oil chamber  53  can be appropriately controlled or regulated by virtue of an appropriate change in the flow passage area of the small aperture, defined by the second land  57   b  to communicate the drain passage  59  with the third communication passage  58 . 
     Also, in the third embodiment, hydraulic-pressure control for the first control oil chamber  16  and hydraulic-pressure control for the second control oil chamber  53  can be carried out simultaneously by means of first and second lands  57   a - 57   b . Hence, as compared to the first embodiment ( FIGS. 1-5 ) and the second embodiment ( FIGS. 7-9 ), in the third embodiment ( FIGS. 10-12 ), hydraulic pressures in first and second control oil chambers  16  and  53  can be more precisely appropriately controlled or regulated by a further smaller axial movement of spool  57  (in particular, the first and second lands  57   a - 57   b ) without being entirely affected by a spring constant of valve spring  34 . 
     Also, in the same manner as the first and second embodiments, in the presence of a requirement of hydraulic-pressure rise to the given hydraulic pressure level P 2 , the electromagnetic coil of solenoid  45  of directional control valve  8  becomes de-energized responsively to an OFF signal from the control unit. The control method and operation for holding the pump discharge pressure at the given constant pressure level P 2  are the same as described previously for the first and second embodiments. 
       FIG. 12  shows the specific state of pilot valve  7  where the pump discharge pressure is controlled or regulated to the given constant pressure level P 2 . In this case, as can be seen from the cross section of  FIG. 12 , hydraulic pressure in the main oil gallery  25  (the branch passage  29 ) is delivered via the second communication passage  39  into the pressure-receiving chamber  38 . By means of large-diameter spring-support slider  33 , spool  57 , valve spring  34 , and large-diameter spring-support slider  33  upwardly move together toward the oil introduction port  29   a . Hence, large-diameter spring-support slider  33  is brought into abutted-engagement (into wall-contact) with the shouldered portion  30   c  between small-diameter bore  30   a  and large-diameter bore  30   b , such that a maximum upward displacement of large-diameter spring-support slider  33  is restricted by abutted-engagement with the shouldered portion  30   c . In accordance with the upward movement of spool  57 , as a matter of course, first and second lands  57   a - 57   b  move upward, and then the spool  57  reaches the uppermost axial position shown in  FIG. 10 . Thus, the third communication passage  58  becomes communicated with the oil introduction port  29   a  through the radial through hole  57   g  and simultaneously the first communication passage  35  becomes communicated with the drain passage  59  through the first annular groove  57   h . As a result of this, a rise in hydraulic pressure in the second control oil chamber  53  and a fall in hydraulic pressure in the first control oil chamber  16  simultaneously occur. Hence, owing to the risen hydraulic pressure in the second control oil chamber  53  as well as the fallen hydraulic pressure in the first control oil chamber  16 , cam ring  5  returns in the direction that the eccentricity of the geometric center of cam ring  5  to the axis of rotation of rotor  4  increases. Therefore, the pump discharge flow rate increases and thus the hydraulic pressure discharged from the pump to the main oil gallery  25  also increases. Thus, spool  57  moves downward against the spring force of valve spring  34 . In this manner, as soon as the pump discharge pressure reaches the given hydraulic pressure level P 2 , the first land  57   a  reaches the axial position, corresponding to the opening end  35   a  of the first communication passage  35 , and simultaneously the second land  57   b  reaches the axial position, corresponding to the opening end  58   a  of the third communication passage  58 , as shown in  FIG. 12 . As a result, a fall in hydraulic pressure in the second control oil chamber  53  and a rise in hydraulic pressure in the first control oil chamber  16  simultaneously occur. As discussed above, the pump discharge pressure can be kept at the given constant pressure level P 2 . 
     [Fourth Embodiment] 
     Referring now to  FIGS. 13A-13C , there is shown a modified pilot valve structure (a sleeve-equipped pilot valve structure, described later) incorporated in the variable displacement oil pump system of the fourth embodiment, and modified from the pilot valve  7  (the large-diameter spring-support slider-equipped pilot valve) of the first embodiment shown in  FIGS. 1-5 . The pump-body structure of the variable displacement oil pump of the fourth embodiment is identical to that of the first embodiment. Also, the structure of electromagnetic solenoid operated directional control valve  8  of the fourth embodiment is identical to that of the first to third embodiments. 
     In the first embodiment, pilot valve  7  is configured to change a switching pressure when switching the flow path configuration for the first communication passage  35  (i.e., the control oil chamber  16 ) between (i) the pressure-release flow path (i.e., the oil-discharge flow path) connected to the drain passage  37  and (ii) the pressure-supply flow path (i.e., the oil-introduction flow path) connected to the oil introduction port  29   a , by shifting the axial position of large-diameter spring-support slider  33  and by changing the entire axial length of valve spring  34  (in other words, the spring load of valve spring  34 ). 
     In the second embodiment, pilot valve  7  is configured to change a switching pressure when switching the flow path configuration for the third communication passage  58  (i.e., the second control oil chamber  53 ) between (i) the pressure-supply flow path connected to the oil introduction port  29   a  and (ii) the pressure-release flow path connected to the drain passage  59 , by shifting the axial position of large-diameter spring-support slider  33  and by changing the entire axial length of valve spring  34  (in other words, the spring load of valve spring  34 ). 
     In the third embodiment, pilot valve  7  is configured to change a switching pressure when switching the flow path configuration for the first communication passage  35  (i.e., the first control oil chamber  16 ) between (i) the pressure-release flow path connected to the drain passage  59  and (ii) the pressure-supply flow path connected to the oil introduction port  29   a , and simultaneously switching the flow path configuration for the third communication passage  58  (i.e., the second control oil chamber  53 ) between (i) the pressure-supply flow path connected to the oil introduction port  29   a  and (ii) the pressure-release flow path connected to the drain passage  59 , by shifting the axial position of large-diameter spring-support slider  33  and by changing the entire axial length of valve spring  34  (in other words, the spring load of valve spring  34 ). 
     In contrast to the above, in the fourth embodiment, the modified pilot valve  7  is configured to change a switching pressure by changing or shifting a port position of the pilot valve. 
     As clearly seen in  FIGS. 13A-13C , in the fourth embodiment, a sleeve  60 , which has a plurality of communication ports  61  (described later), is disposed between the stepped cylindrical close-fitting bore  30  of the modified pilot valve  7  (simply, pilot valve  7 ) and the spool  32 . An axial displacement of sleeve  60  is produced by energization/de-energization control (ON/OFF control) for the electromagnetic coil of solenoid  45  of directional control valve  8 , thereby enabling the entire axial length (i.e., the spring load) of valve spring  34  to be changed. This ensures switching of the pump discharge pressure between two-stage pressure levels P 1  and P 2 . 
     More concretely, sleeve  60  is comprised of a cylindrical small-diameter portion  60   a , and a radially-extending flanged large-diameter portion  60   b  formed integral with the lowermost end of small-diameter portion  60   a . The outer periphery of small-diameter portion  60   a  is machined to axially slide in the small-diameter bore  30   a  with a very small radial clearance between the inner peripheral surface of small-diameter bore  30   a  and the outer peripheral surface of small-diameter portion  60   a . In a similar manner, the outer periphery of flanged large-diameter portion  60   b  is machined to axially slide in the large-diameter bore  30   b  with a very small radial clearance between the inner peripheral surface of large-diameter bore  30   b  and the outer peripheral surface of flanged large-diameter portion  60   b . Additionally, two lands  32   a - 32   b  of spool  32  are machined to axially slide in the close-fitting cylindrical bore of small-diameter portion  60   a  with a very small radial clearance. 
     Small-diameter portion  60   a  of sleeve  60  has a plurality of communication ports  61  (circumferentially equidistant-spaced radial through holes) at a given axial position substantially corresponding to the first communication passage  35 . The opening width (i.e., the axial length) of the opening end  35   a  of the first communication passage  35  is dimensioned such that the first communication passage  35  always communicates with the communication ports  61  over the entire range of axial displacement of ports  61 . 
       FIG. 13A  shows an initial state of pilot valve  7  of the pump system of the fourth embodiment under a particular state where there is no pump discharge pressure application through the oil introduction port  29   a  to the top face of spool  32 , for example, with the engine put in a stop state, or during the early stage of engine start-up (i.e., during the initial startup of the pump). An annular spring seat  62  is integrally formed on the inner periphery of the lower portion of sleeve  60 . Under preload (i.e., under a specified set spring load), a sleeve spring  63  (serving as a third biasing member) is disposed between the spring seat  62  and the inside face of lid member  31 , which lid member is configured to hermetically close the lowermost end of large-diameter bore  30   b . A given spring load (a set spring force) of sleeve spring  63  (with sleeve  60  kept at its initial setting position or a spring-loaded original position) is set to a spring force that a downward displacement of sleeve  60  does not occur by the application of hydraulic pressure through the oil introduction port  29   a . Hence, in the initial state of pilot valve  7 , sleeve spring  63  forces the sleeve  60  into abutted-engagement with a shouldered bearing surface  30   d  formed the uppermost end of small-diameter bore  30   a.    
     Lid member  31  has a center drain port  31   a  (an axial through hole) bored in the axial direction of spool  32 . The structure of spool  32  of pilot valve  7  of the fourth embodiment, is the same as the first embodiment. That is, spool  32  has first and second lands  32   a - 32   b  and small-diameter shaft  32   c  between them. Spool  32  has the cylindrical bore  32   d  closed at its upper end and extending along the axis of spool  32 . Small-diameter shaft  32   c  defines the annular groove  32   e  between first and second lands  32   a - 32   b . Also, small-diameter shaft  32   c  has the radial through hole  32   f  for communicating the annular groove  32   e  with the cylindrical bore  32   d  by way of the radial through hole  32   f.    
     Valve spring  34  is disposed between the upper closed end face of cylindrical bore  32   d  of spool  32  and the inside face of lid member  31 , for biasing or forcing the spool  32  in the direction for closing of the oil introduction port  29   a.    
     An annular pressure-receiving chamber  64  is defined between the shouldered portion  30   c  of stepped cylindrical close-fitting bore  30  and the stepped portion of the small-diameter portion  60   a  and the flanged large-diameter portion  60   b  of sleeve  60 . One opening end of the second communication passage  39  is configured to open into the annular pressure-receiving chamber  64 . Supply-and-exhaust port  46  of directional control valve  8  communicates with the annular pressure-receiving chamber  64  of pilot valve  7  through the second communication passage  39 . 
     Communication ports  61  (circumferentially equidistant-spaced radial through holes) of sleeve  60  are configured to always communicate with the large-diameter first communication passage  35 . 
     In the initial state of pilot valve  7 , as shown in  FIG. 13A , the first communication passage  35  is communicated with the internal space of sleeve  60  (spool  32 ) through the communication ports  61 , the annular groove  32   e , and the radial through hole  32   f , and also communicated with the drain port  31   a  of lid member  31 . 
     As previously discussed, communication ports  61  are configured to be circumferentially equidistant-spaced from each other so as to always communicate with the first communication passage  35  regardless of the sense of sleeve  60  in the direction of rotation, in other words, even in the presence of a rotational displacement of sleeve  60  about the axis of spool  32 . 
     A switching action of the flow path configuration for the first communication passage  35  (i.e., the control oil chamber  16 ), carried out within the pump system of the fourth embodiment, between (i) a pressure-release flow path connected to the oil pan and (ii) a pressure-supply flow path connected to the oil introduction port  29   a  is the same as the first embodiment. The fundamental operation of the variable displacement oil pump of the fourth embodiment employing the sleeve-equipped pilot valve is similar to that of the variable displacement oil pump of the first embodiment employing the large-diameter spring-support slider-equipped pilot valve. Thus, in the same manner as the first embodiment, the variable displacement oil pump system of the fourth embodiment employing the sleeve-equipped pilot valve can provide the two-stage pump discharge pressure characteristic shown in  FIG. 6 . 
       FIG. 13B  shows the working state of pilot valve  7  of the pump system of the fourth embodiment during a steady-state engine operating mode, at which the pump discharge pressure rises up to the given hydraulic pressure level P 1 . As clearly shown in  FIG. 13B , spool  32  is downwardly displaced toward the lid member  31  against the spring force of valve spring  34  with hydraulic pressure, applied through the oil introduction port  29   a  to the top face of spool  32 . The width (i.e., the axial length) of the first land  32   a  is dimensioned to be approximately equal to the opening width of each of communication ports  61 . Therefore, when the first land  32   a  downwardly moves to the axial position of the communication ports  61 , fluid-communication between the communication ports  61  and the drain port  31   a  through the annular groove  32   e  and the radial through hole  32   f  becomes blocked and fluid-communication between the communication ports  61  and the oil introduction port  29   a  becomes established. That is, switching of the flow path configuration for the first communication passage  35  (i.e., the control oil chamber  16 ) from (i) the pressure-release flow path connected to the drain port  31   a  to (ii) the pressure-supply flow path connected to the oil introduction port  29   a  occurs. Hence, hydraulic pressure is introduced through the oil introduction port  29   a  and the first communication passage  35  to the control oil chamber  16  and as a result the pump discharge flow rate can be adjusted by a counterclockwise displacement of cam ring  5  against the spring force of coil spring  28 . 
     When hydraulic pressure in the control oil chamber  16 , is excessively high, a counterclockwise displacement of cam ring  5  tends to become large, and thus switching of the flow path configuration for the first communication passage  35  from the pressure-supply flow path connected to the oil introduction port  29   a  to the pressure-release flow path connected to the drain port  31   a  occurs. 
     Conversely when hydraulic pressure in the control oil chamber  16 , is excessively low, a counterclockwise displacement of cam ring  5  tends to become small, and thus switching of the flow path configuration for the first communication passage  35  from the pressure-release flow path connected to the drain port  31   a  to the pressure-supply flow path connected to the oil introduction port  29   a  occurs. 
     In this manner, immediately when the pump discharge pressure reaches the given hydraulic pressure P 1 , fluid-communication between the oil introduction port  29   a  and the first communication passage  35  becomes established, and thereafter the hydraulic pressure in the control oil chamber  16  can be appropriately controlled or regulated by appropriate switching between (i) the pressure-release flow path connected to the drain port  31   a  and (ii) the pressure-supply flow path connected to the oil introduction port  29   a  by virtue of slight upward and downward axial displacements of the first land  32   a , such that the pump discharge pressure can be held at the given hydraulic pressure P 1 . 
     At this time, the electromagnetic coil of directional control valve  8  becomes energized responsively to an ON signal from the control unit. Thus, the annular pressure-receiving chamber  64  of pilot valve  7  becomes communicated with the oil pan through the second communication passage  39 , the supply-and-exhaust port  46 , the cylindrical passage  50 , and the drain port  47 . There is no hydraulic pressure acting on the annular upper sidewall surface (serving as a pressure-receiving surface) of flanged large-diameter portion  60   b  of sleeve  60 . That is, the annular pressure-receiving chamber  64  becomes a low-pressure state. Hence, sleeve  60  (communication ports  61 ) can be kept at the spring-loaded original position shown in  FIG. 13B  by the spring force of sleeve spring  63 . At this time, a switching pressure when switching the flow path configuration for the first communication passage  35  (i.e., the control oil chamber  16 ) between (i) the pressure-release flow path connected to the drain port  31   a  and (ii) the pressure-supply flow path connected to the oil introduction port  29   a , becomes the given hydraulic pressure level P 1  shown in  FIG. 6 . 
     Also, when the hydraulic pressure (the pump discharge pressure) has to be risen to the given hydraulic pressure level P 2 , the electromagnetic coil of solenoid  45  of directional control valve  8  becomes de-energized responsively to an OFF signal from the control unit, thereby permitting the pushrod  49  to retract so as to establish fluid-communication between the solenoid control port  43  and the supply-and-exhaust port  46  and simultaneously to block fluid-communication between the supply-and-exhaust port  46  and the drain port  47  through the cylindrical passage  50 . Thus, hydraulic pressure is supplied through the branch passage  29  into the annular pressure-receiving chamber  64 . Hence, sleeve  60  begins to move downward from the spring-loaded original position of  FIG. 13B  against the spring force of sleeve spring  63  (exactly, against the biasing force of valve spring  34  and an inertia of the spool  32  as well as the biasing force of sleeve spring  63 ) and thus the annular lower sidewall surface of flanged large-diameter portion  60   b  of sleeve  60  is brought into abutted-engagement with the inside face of lid member  31  by the supplied hydraulic pressure, while compressing the sleeve spring  63  (see  FIG. 13C ). The annular lower sidewall surface of flanged large-diameter portion  60   b  and the inside face of lid member  31 , abutted with each other, provide a good leakproof seal, and hence hydraulic pressure in the annular pressure-receiving chamber  64  becomes high. 
     By the way, to more certainly enhance a leakproof seal performance, it is preferable to machine or produce the radial clearance space between the inner peripheral surface of small-diameter bore  30   a  and the outer peripheral surface of small-diameter portion  60   a  as small as possible. Machining the radial clearance space between small-diameter bore  30   a  and small-diameter portion  60   a  as small as possible, permits the radial clearance space between large-diameter bore  30   b  and flanged large-diameter portion  60   b  to be machined somewhat looser. By virtue of such a somewhat looser radial clearance, it is unnecessary to strictly manage or control the accuracy or the quality concerning the concentricity of small-diameter portion  60   a  and flanged large-diameter portion  60   b.    
     As can be seen from the cross section of  FIG. 13C , in accordance with the downward movement of sleeve  60 , as a matter of course, the communication ports  61  move downward. Thus, the first land  32   a  of spool  32 , together with the communication ports  61 , moves downward, while compressing the valve spring  34  by the circular top of the first land  32   a . Hence, the spring load of valve spring  34  becomes a higher spring load. At this time, a switching pressure when switching the flow path configuration for the first communication passage  35  (i.e., the control oil chamber  16 ) between (i) the pressure-release flow path connected to the drain port  31   a  and (ii) the pressure-supply flow path connected to the oil introduction port  29   a , becomes the given hydraulic pressure level P 2  shown in  FIG. 6 . 
     The other operation and effects of the pump system of the fourth embodiment are the same as the first embodiment. However, in the fourth embodiment shown in  FIGS. 13A-13C , it is possible to enhance a wear-and-abrasion resistance to the sliding-contact portion of sleeve  60  with the spool  32  by producing the sleeve  60  by iron-based materials. Additionally, the sliding-contact surface area of the outer periphery of sleeve  60  with the stepped cylindrical close-fitting bore  30  of control housing  6  made by aluminum alloy is greater than the sliding-contact surface area of the inner periphery (the sliding-contact surface) of sleeve  60  with the spool  32 , thus enhancing a wear-and-abrasion resistance. 
     [Fifth Embodiment] 
     Referring now to  FIGS. 14A-14C , there is shown another modified sleeve-equipped pilot valve structure incorporated in the variable displacement oil pump system of the fifth embodiment. The fifth embodiment is a modification that the sleeve-equipped pilot valve structure, somewhat similar to the fourth embodiment, is applied to the pump system of the second embodiment. The pump-body structure of the variable displacement oil pump of the fifth embodiment is identical to that of the first embodiment. Also, the structure of electromagnetic solenoid operated directional control valve  8  of the fifth embodiment is identical to that of the first to third embodiments. 
     As clearly seen in  FIGS. 14A-14C , in the fifth embodiment, the sleeve  60  is vertically slidably disposed between the stepped cylindrical close-fitting bore  30  of pilot valve  7  and the spool  57 . An axial displacement of sleeve  60  is produced by energization/de-energization control (ON/OFF control) for the electromagnetic coil of solenoid  45  of directional control valve  8 , thereby enabling the entire axial length (i.e., the spring load) of valve spring  34  to be changed. This ensures switching of the pump discharge pressure between two-stage pressure levels P 1  and P 2 . 
     Small-diameter portion  60   a  of sleeve  60  has a plurality of communication ports  61  (circumferentially equidistant-spaced radial through holes) at a given axial position substantially corresponding to the third communication passage  58 . Also, the small-diameter portion  60   a  of sleeve  60  has a plurality of drain ports  65  (circumferentially equidistant-spaced radial through holes) at an upper axial position than the communication ports  61  and substantially corresponding to the drain passage  59 . That is, the drain port  31   a  of lid member  31  of the pilot valve structure of the fourth embodiment ( FIGS. 13A-13C ) is replaced with the drain ports  65  of the pilot valve structure of the fifth embodiment. 
     Spool valve  57  is upwardly forced or biased by the spring force of valve spring  34  in the direction for closing the oil introduction port  29   a . On the other hand, sleeve  60  is forced or biased by the spring force of sleeve spring  63  in the direction of abutted-engagement with the shouldered bearing surface  30   d  formed the uppermost end of small-diameter bore  30   a.    
     In the initial state of pilot valve  7 , as shown in  FIG. 14A , the communication ports  61  are communicated with the oil introduction port  29   a  through the radial through hole  57   g  of second small-diameter shaft  57   e , and also communicated with the third communication passage  58 . Thus, hydraulic pressure in the main oil gallery  25  is delivered into the second control oil chamber  53 . 
       FIG. 14B  shows the working state of pilot valve  7  of the pump system of the fifth embodiment during a steady-state engine operating mode, at which the pump discharge pressure rises up to the given hydraulic pressure level P 1 . As clearly shown in  FIG. 14B , spool  57  is downwardly displaced toward the lid member  31  against the spring force of valve spring  34  with hydraulic pressure, applied through the oil introduction port  29   a  to spool  57 . The width (i.e., the axial length) of the second land  57   b  is dimensioned to be approximately equal to the opening width of each of communication ports  61 . Therefore, when the second land  57   b  downwardly moves to the axial position of the communication ports  61 , fluid-communication between the communication ports  61  and the oil introduction port  29   a  through the second annular groove  57   i  and the radial through hole  57   g  becomes blocked and fluid-communication between the communication ports  61  and the drain passage  59  through the first annular groove  57   h  and the drain ports  65  becomes established. That is, switching of the flow path configuration for the third communication passage  58  (i.e., the second control oil chamber  53 ) from (i) a pressure-supply flow path connected to the oil introduction port  29   a  to (ii) a pressure-release flow path connected to the drain ports  65  occurs. Hence, a fall in hydraulic pressure in the second control oil chamber  53  occurs and as a result the pump discharge flow rate can be adjusted by a counterclockwise displacement of cam ring  5  against the spring force of coil spring  28 . 
     When hydraulic pressure in the second control oil chamber  53  is excessively low, a counterclockwise displacement of cam ring  5  tends to become large, and thus switching of the flow path configuration for the third communication passage  58  from the pressure-release flow path connected to the drain ports  65  to the pressure-supply flow path connected to the oil introduction port  29   a  occurs so as to rise hydraulic pressure in the second control oil chamber  53 . 
     Conversely when hydraulic pressure in the second control oil chamber  53  is excessively high, a counterclockwise displacement of cam ring  5  tends to become small, and thus switching of the flow path configuration for the third communication passage  58  from the pressure-supply flow path connected to the oil introduction port  29   a  to the pressure-release flow path connected to the drain ports  65  occurs so as to fall hydraulic pressure in the second control oil chamber  53 . 
     In this manner, immediately when the pump discharge pressure reaches the given hydraulic pressure P 1 , fluid-communication between the drain passage  59  and the third communication passage  58  becomes established, and thereafter the hydraulic pressure in the second control oil chamber  53  can be appropriately controlled or regulated by appropriate switching between (i) the pressure-supply flow path connected to the oil introduction port  29   a  and (ii) the pressure-release flow path connected to the drain passage  59  by virtue of slight upward and downward axial displacements of the second land  57   b , such that the pump discharge pressure can be held at the given hydraulic pressure P 1 . 
     At this time, the electromagnetic coil of directional control valve  8  becomes energized responsively to an ON signal from the control unit. Thus, the annular pressure-receiving chamber  64  of pilot valve  7  becomes communicated with the oil pan through the second communication passage  39 , the supply-and-exhaust port  46 , the cylindrical passage  50 , and the drain port  47 . There is no hydraulic pressure acting on the annular upper sidewall surface of flanged large-diameter portion  60   b  of sleeve  60 . That is, the annular pressure-receiving chamber  64  becomes a low-pressure state. Hence, sleeve  60  (communication ports  61  and drain ports  65 ) can be kept at the spring-loaded original position shown in  FIG. 14B  by the spring force of sleeve spring  63 . At this time, a switching pressure when switching the flow path configuration for the third communication passage  58  (i.e., the second control oil chamber  53 ) between (i) the pressure-supply flow path connected to the oil introduction port  29   a  and (ii) the pressure-release flow path connected to the drain passage  59 , becomes the given hydraulic pressure level P 1  shown in  FIG. 6 . 
     Also, when the hydraulic pressure (the pump discharge pressure) has to be risen to the given hydraulic pressure level P 2 , the electromagnetic coil of solenoid  45  of directional control valve  8  becomes de-energized responsively to an OFF signal from the control unit. Thus, hydraulic pressure is supplied through the branch passage  29  into the annular pressure-receiving chamber  64 . Hence, sleeve  60  begins to move downward from the spring-loaded original position of  FIG. 14B  against the spring force of sleeve spring  63  and thus the annular lower sidewall surface of flanged large-diameter portion  60   b  of sleeve  60  is brought into abutted-engagement with the inside face of lid member  31  by the supplied hydraulic pressure, while compressing the sleeve spring  63  (see  FIG. 14C ). 
     As can be seen from the cross section of  FIG. 14C , in accordance with the downward movement of sleeve  60 , as a matter of course, the communication ports  61  move downward. Thus, the second land  57   b  of spool  57 , together with the communication ports  61 , moves downward, while compressing the valve spring  34  by the underside of the third land  57   c . Hence, the spring load of valve spring  34  becomes a higher spring load. At this time, a switching pressure when switching the flow path configuration for the third communication passage  58  (i.e., the second control oil chamber  53 ) between (i) the pressure-supply flow path connected to the oil introduction port  29   a  and (ii) the pressure-release flow path connected to the drain passage  59 , becomes the given hydraulic pressure level P 2  shown in  FIG. 6 . 
     By the way, the opening width of the opening end  58   a  of the third communication passage  58  is set or dimensioned such that the third communication passage  58  always communicates with the communication ports  61  over the entire range of axial displacement of ports  61 . Additionally, the opening width of the drain passage  59  is set or dimensioned such that the drain passage  59  always communicates with the drain ports  65  over the entire range of axial displacement of ports  65 . 
     The other operation and effects of the pump system of the fifth embodiment are the same as the second embodiment. However, in the fifth embodiment shown in  FIGS. 14A-14C , it is possible to enhance a wear-and-abrasion resistance to the sliding-contact portion of sleeve  60  with the spool  57  by producing the sleeve  60  by iron-based materials. Additionally, the sliding-contact surface area of the outer periphery of sleeve  60  with the stepped cylindrical close-fitting bore  30  of control housing  6  made by aluminum alloy is greater than the sliding-contact surface area of the inner periphery of sleeve  60  with the spool  57 , thus enhancing a wear-and-abrasion resistance. As previously described, the oil pump of the fourth embodiment employing the sleeve-equipped pilot valve is superior to the oil pump of the first embodiment employing the large-diameter spring-support slider-equipped pilot valve, in enhanced wear-and-abrasion resistance. In a similar manner, the oil pump of the fifth embodiment employing the sleeve-equipped pilot valve is superior to the oil pump of the second embodiment employing the large-diameter spring-support slider-equipped pilot valve, in enhanced wear-and-abrasion resistance. 
     [Sixth Embodiment] 
     Referring now to  FIGS. 15A-15C , there is shown a further modified sleeve-equipped pilot valve structure incorporated in the variable displacement oil pump system of the sixth embodiment. The sixth embodiment is a modification that the sleeve-equipped pilot valve structure, somewhat similar to the fourth and fifth embodiments, is applied to the pump system of the third embodiment. The pump-body structure of the variable displacement oil pump of the sixth embodiment is identical to that of the first embodiment. Also, the structure of electromagnetic solenoid operated directional control valve  8  of the sixth embodiment is identical to that of the first to third embodiments. 
     As clearly seen in  FIGS. 15A-15C , in the sixth embodiment, the sleeve  60  is vertically slidably disposed between the stepped cylindrical close-fitting bore  30  of pilot valve  7  and the spool  57 . The small-diameter portion  60   a  of sleeve  60  has a plurality of first communication ports  61  (circumferentially equidistant-spaced radial through holes) at a given axial position substantially corresponding to the first communication passage  35 . The small-diameter portion  60   a  of sleeve  60  has a plurality of drain ports  66  (circumferentially equidistant-spaced radial through holes) at a lower axial position than the first communication ports  61  and substantially corresponding to the drain passage  59 . Also, the small-diameter portion  60   a  of sleeve  60  has a plurality of second communication ports  67  (circumferentially equidistant-spaced radial through holes) at a lower axial position than the drain ports  66  and substantially corresponding to the third communication passage  58 . 
     An axial displacement of sleeve  60  is produced by energization/de-energization control (ON/OFF control) for the electromagnetic coil of solenoid  45  of directional control valve  8 , thereby enabling the entire axial length (i.e., the spring load) of valve spring  34  to be changed. This ensures switching of the pump discharge pressure between two-stage pressure levels P 1  and P 2 . 
     In the initial state of pilot valve  7 , as shown in  FIG. 15A , spool  57  is forced into abutted-engagement with the shouldered bearing surface  30   d  by the spring force of valve spring  34 . At the same time, sleeve  60  is forced into abutted-engagement with the shouldered bearing surface  30   d  by the spring force of sleeve spring  63 . 
     In the initial state of pilot valve  7 , as shown in  FIG. 15A , the first communication passage  35  is communicated with the drain ports  66  through the first communication ports  61  and the first annular groove  57   h , whereas the third communication passage  58  is communicated with the oil introduction port  29   a  through the second communication ports  67  and the radial through hole  57   g . Thus, hydraulic pressure in the main oil gallery  25  is delivered into the second control oil chamber  53 . 
       FIG. 15B  shows the working state of pilot valve  7  of the pump system of the sixth embodiment during a steady-state engine operating mode, at which the pump discharge pressure rises up to the given hydraulic pressure level P 1 . As clearly shown in  FIG. 15B , spool  57  is downwardly displaced toward the lid member  31  against the spring force of valve spring  34  with hydraulic pressure, applied through the oil introduction port  29   a  to spool  57 . The width (i.e., the axial length) of the first land  57   a  dimensioned to be approximately equal to the opening width of each of first communication ports  61 . Also, the width (i.e., the axial length) of the second land  57   b  dimensioned to be approximately equal to the opening width of each of second communication ports  67 . Therefore, when the first land  57   a  downwardly moves to the axial position of the first communication ports  61  and simultaneously the second land  57   b  downwardly moves to the axial position of the second communication ports  67 , a unique flow path configuration for the first communication ports  61  (i.e., the first communication passage  35 , in other words, the first control oil chamber  16 ) is switched from (i) a pressure-release flow path connected to the drain ports  66  to (ii) a pressure-supply flow path connected to the oil introduction port  29   a  and simultaneously a unique flow path configuration for the second communication ports  67  (i.e., the third communication passage  58 , in other words, the second control oil chamber  53 ) is switched from (i) a pressure-supply flow path connected to the oil introduction port  29   a  to (ii) a pressure-release flow path connected to the drain ports  66 . Hence, a rise in hydraulic pressure in the first control oil chamber  16  and a fall in hydraulic pressure in the second control oil chamber  53  occur simultaneously and as a result the pump discharge flow rate can be adjusted by a counterclockwise displacement of cam ring  5  against the spring force of coil spring  28 . 
     When hydraulic pressure in the first control oil chamber  16  is excessively high or hydraulic pressure in the second control oil chamber  53  is excessively low, a counterclockwise displacement of cam ring  5  tends to become large, and thus switching of the flow path configuration for the first control oil chamber  16  (i.e., the first communication passage  35 ) from the pressure-supply flow path connected to the oil introduction port  29   a  to the pressure-release flow path connected to the drain ports  66  and switching of the flow path configuration for the second control oil chamber  53  (i.e., the third communication passage  58 ) from the pressure-release flow path connected to the drain ports  66  to the pressure-supply flow path connected to the oil introduction port  29   a  occur simultaneously so as to fall the hydraulic pressure in the first control oil chamber  16  and simultaneously rise the hydraulic pressure in the second control oil chamber  53 . 
     Conversely when hydraulic pressure in the first control oil chamber  16  is excessively low or hydraulic pressure in the second control oil chamber  53  is excessively high, a counterclockwise displacement of cam ring  5  tends to become small, and thus switching of the flow path configuration for the first control oil chamber  16  (i.e., the first communication passage  35 ) from the pressure-release flow path connected to the drain ports  66  to the pressure-supply flow path connected to the oil introduction port  29   a  and switching of the flow path configuration for the second control oil chamber  53  (i.e., the third communication passage  58 ) from the pressure-supply flow path connected to the oil introduction port  29   a  to the pressure-release flow path connected to the drain ports  66  occur simultaneously so as to rise the hydraulic pressure in the first control oil chamber  16  and simultaneously fall the hydraulic pressure in the second control oil chamber  53 . 
     In this manner, immediately when the pump discharge pressure reaches the given hydraulic pressure P 1 , fluid-communication between the oil introduction port  29   a  and the first communication passage  35  and fluid-communication between the drain passage  59  and the third communication passage  58  become established, and thereafter the hydraulic pressure in the first control oil chamber  16  can be appropriately controlled or regulated by appropriate switching between (i) the pressure-release flow path connected to the drain passage  59  and (ii) the pressure-supply flow path connected to the oil introduction port  29   a  by virtue of slight upward and downward axial displacements of the first land  57   a , and simultaneously the hydraulic pressure in the second control oil chamber  53  can be appropriately controlled or regulated by appropriate switching between (i) the pressure-supply flow path connected to the oil introduction port  29   a  and (ii) the pressure-release flow path connected to the drain passage  59  by virtue of slight upward and downward axial displacements of the second land  57   b , such that the pump discharge pressure can be held at the given hydraulic pressure P 1 . 
     At this time, the electromagnetic coil of directional control valve  8  becomes energized responsively to an ON signal from the control unit. Thus, there is no hydraulic pressure acting on the annular upper sidewall surface of flanged large-diameter portion  60   b  of sleeve  60 . That is, the annular pressure-receiving chamber  64  becomes a low-pressure state. Hence, sleeve  60  (first communication ports  61 , drain ports  66  and second communication ports  67 ) can be kept at the spring-loaded original position shown in  FIG. 15B  by the spring force of sleeve spring  63 . At this time, a switching pressure when switching the flow path configuration for the first communication passage  35  (i.e., the first control oil chamber  16 ) between (i) the pressure-release flow path connected to the drain passage  59  and (ii) the pressure-supply flow path connected to the oil introduction port  29   a , and simultaneously switching the flow path configuration for the third communication passage  58  (i.e., the second control oil chamber  53 ) between (i) the pressure-supply flow path connected to the oil introduction port  29   a  and (ii) the pressure-release flow path connected to the drain passage  59 , becomes the given hydraulic pressure level P 1  shown in  FIG. 6 . 
     Also, when the hydraulic pressure (the pump discharge pressure) has to be risen to the given hydraulic pressure level P 2 , the electromagnetic coil of solenoid  45  of directional control valve  8  becomes de-energized responsively to an OFF signal from the control unit. Thus, hydraulic pressure is supplied through the branch passage  29  into the annular pressure-receiving chamber  64 . Hence, sleeve  60  begins to move downward from the spring-loaded original position of  FIG. 15B  against the spring force of sleeve spring  63  and thus the annular lower sidewall surface of flanged large-diameter portion  60   b  of sleeve  60  is brought into abutted-engagement with the inside face of lid member  31  by the supplied hydraulic pressure, while compressing the sleeve spring  63  (see  FIG. 15C ). 
     As can be seen from the cross section of  FIG. 15C , in accordance with the downward movement of sleeve  60 , as a matter of course, first and second communication ports  61  and  67  move downward. Thus, the first land  57   a , together with the first communication ports  61  and the second land  57   b , together with the second communication ports  67 , move downward, while compressing the valve spring  34  by the underside of the third land  57   c . Hence, the spring load of valve spring  34  becomes a higher spring load. At this time, a switching pressure when switching the flow path configuration for the first communication passage  35  (i.e., the first control oil chamber  16 ) between (i) the pressure-release flow path connected to the drain ports  66  and (ii) the pressure-supply flow path connected to the oil introduction port  29   a  and simultaneously switching the flow path configuration for the third communication passage  58  (i.e., the second control oil chamber  53 ) between (i) the pressure-supply flow path connected to the oil introduction port  29   a  and (ii) the pressure-release flow path connected to the drain ports  66 , becomes the given hydraulic pressure level P 2  shown in  FIG. 6 . 
     By the way, the opening width of the opening end of the first communication passage  35  is set or dimensioned such that the first communication passage  35  always communicates with the first communication ports  61  over the entire range of axial displacement of ports  61 . Additionally, the opening width of the drain passage  59  is set or dimensioned such that the drain passage  59  always communicates with the drain ports  66  over the entire range of axial displacement of ports  66 . 
     [Seventh Embodiment] 
     Referring now to  FIGS. 16A-16C , there is shown another modified sleeve-equipped pilot valve structure incorporated in the variable displacement oil pump system of the seventh embodiment. The basic pilot valve structure of the seventh embodiment is similar to the fourth embodiment. The seventh embodiment somewhat differs from the fourth embodiment in that the structure (the cross section) of the sleeve  60 , disposed between the stepped cylindrical close-fitting bore  30  and the spool  32 , is changed. More concretely, in the seventh embodiment, sleeve  60  has an upper wall portion  60   c  formed integral with the uppermost end of small-diameter portion  60   a . Upper wall portion  60   c  of sleeve  60  has a large-diameter communication bore  60   d  (an axial through hole) formed substantially at the center of the upper wall portion  60   c . Large-diameter communication bore  60   c  is configured to always communicate with the oil introduction port  29   a . The underside of upper wall portion  60   c  serves as a second bearing surface  60   e  that restricts a maximum upward movement of spool  32 . The structure of spool  32  of pilot valve  7  of the seventh embodiment is identical to that of the first embodiment. 
     Small-diameter portion  60   a  of sleeve  60  has a plurality of communication ports  61  (circumferentially equidistant-spaced radial through holes) at a given axial position substantially corresponding to the first communication passage  35 . 
     Lid member  31  has a stepped upwardly-protruding portion  31   b  integrally formed at the center of the upside of lid member  31 . The inner peripheral surface of the lower end of sleeve  60  is slidably guided by the cylindrical outer peripheral surface of protruding portion  31   b . Also, lid member  31  has a center drain port  31   a  (an axial through hole) bored in the axial direction of spool  32 . Under preload (i.e., under a specified set spring load), valve spring  34  is disposed between the upper closed end face of cylindrical bore  32   d  of spool  32  and the top face of protruding portion  31   b  of lid member  31 , for biasing or forcing the first land  32   a  of spool  32  into abutted-engagement with the second bearing surface  60   e  of sleeve  60  by the spring force of valve spring  34 . Also, sleeve  60  is biased or forced into abutted-engagement with the shouldered bearing surface  30   d  of stepped cylindrical close-fitting bore  30  by the force that upwardly pushes the first land  32   a  in the direction for closing of the oil introduction port  29   a.    
     A substantially annular pressure-receiving chamber  64  is defined between the underside of flanged large-diameter portion  60   b  of sleeve  60  and the stepped portion of the large-diameter disk-shaped lid portion and the stepped portion of lid member  31 . One opening end of the second communication passage  39  is configured to open into the substantially annular pressure-receiving chamber  64 . 
     A back-pressure chamber  68  is defined between the stepped portion between the shouldered portion  30   c  of small-diameter bore  30   a  and large-diameter bore  30   b  and the stepped portion of the small-diameter portion  60   a  and the flanged large-diameter portion  60   b  of sleeve  60 . Back-pressure chamber  68  is configured to communicate with the drain port  31   a  of lid member  31  through a back-pressure drain hole  69  formed in the lower portion of small-diameter portion  60   a  of sleeve  60 . 
     In the initial state of pilot valve  7 , as shown in  FIG. 16A , control oil chamber  16  is communicated with the drain port  31   a  through the first communication passage  35 , the communication ports  61 , the annular groove  32   e , the radial through hole  32   f  of small-diameter shaft  32   c  and the cylindrical bore  32   d  defined in the sleeve  60 . Communication ports  61  are configured to always communicate with the first communication passage  35 , regardless of the sense of sleeve  60  in the direction of rotation, in other words, even in the presence of a rotational displacement of sleeve  60  about the axis of spool  32 . 
     The pump-body structure of the variable displacement oil pump of the seventh embodiment (see  FIGS. 16A-16C ) is identical to that of the first embodiment (see  FIGS. 1-5 ). Also, the connecting path configuration between the pump body and the electromagnetic solenoid operated directional control valve  8  in the pump system of the seventh embodiment is identical to that of the first embodiment, thus providing the two-stage pump discharge pressure characteristic shown in  FIG. 6 . However, the electromagnetic solenoid operated directional control valve  8  incorporated in the pump system of the seventh embodiment shown in  FIGS. 16A-16C , is changed to a different directional-control-valve specification that supplies hydraulic pressure from the branch passage  29  through the directional control valve via the second communication passage  39  to the annular pressure-receiving chamber  64  when energized (ON), and also blocks the pressure-supply flow path from the branch passage  29  via the second communication passage  39  to the annular pressure-receiving chamber  64  and simultaneously switches the flow path configuration for the second communication passage  39  to a pressure-release flow path from the second communication passage  39  through the directional control valve to the drain port  47  when de-energized (OFF). 
       FIG. 16B  shows the working state of pilot valve  7  of the pump system of the seventh embodiment during a steady-state engine operating mode, at which the pump discharge pressure rises up to the given hydraulic pressure level P 1 . As clearly shown in  FIG. 16B , spool  32  is downwardly displaced toward the lid member  31  against the spring force of valve spring  34  with hydraulic pressure, applied through the oil introduction port  29   a  and the large-diameter communication bore  60   d  of sleeve  60  to the top face of spool  32 . Therefore, the first land  32   a  of spool  32  downwardly moves away from the second bearing surface  60   e  of sleeve  60 , and hence, there is no valve-spring force acting on the upper wall portion  60   c  (or the second bearing surface  60   e ) of sleeve  60 . At this time, the electromagnetic coil of the directional control valve  8 , whose specification is changed, becomes energized responsively to an ON signal from the control unit. Thus, during the steady-state engine operating mode of  FIG. 16B , hydraulic pressure is supplied through the branch passage  29  into the annular pressure-receiving chamber  64 . By the way, the pressure-receiving area of flanged large-diameter portion  60   b  of sleeve  60  that receives hydraulic pressure introduced into through the directional control valve  8  into the pressure-receiving chamber  64  is configured or dimensioned to be greater than the pressure-receiving area (substantially corresponding to the lateral cross section of small-diameter portion  60   a ) of the annular pressure-receiving section of upper wall portion  60   c  of sleeve  60 . Thus, when the same hydraulic pressure in the main oil gallery  25  (the branch passage  29 ) has been supplied to both the upside of sleeve  60  and the underside of sleeve  60 , sleeve  60  is upwardly displaced and thus kept at its hydraulically-actuated original position shown in  FIG. 16B  under pressure by virtue of the pressure-receiving area difference between the upside of sleeve  60  and the underside of sleeve  60 . Hence, the upper wall portion  60   c  of sleeve  60  is kept in abutted-engagement with the shouldered bearing surface  30   d  of stepped cylindrical close-fitting bore  30 . By the way, the width (i.e., the axial length) of the first land  32   a  is dimensioned to be approximately equal to the opening width of each of communication ports  61 . Therefore, when the first land  32   a  downwardly moves to the axial position of the communication ports  61 , fluid-communication between the communication ports  61  and the drain port  31   a  through the annular groove  32   e  and the radial through hole  32   f  becomes blocked and fluid-communication between the communication ports  61  and the oil introduction port  29   a  becomes established. That is, the flow path configuration for the first communication passage  35  (i.e., the control oil chamber  16 ) is switched from (i) the pressure-release flow path connected to the drain port  31   a  to (ii) the pressure-supply flow path connected to the oil introduction port  29   a . Hence, hydraulic pressure is introduced through the oil introduction port  29   a  and the first communication passage  35  to the control oil chamber  16  and as a result the pump discharge flow rate can be adjusted by a counterclockwise displacement of cam ring  5  against the spring force of coil spring  28 . In this manner, immediately when the pump discharge pressure reaches the given hydraulic pressure P 1 , fluid-communication between the oil introduction port  29   a  and the first communication passage  35  becomes established, and thereafter the hydraulic pressure in the control oil chamber  16  can be appropriately controlled or regulated by appropriate switching between (i) the pressure-release flow path connected to the drain port  31   a  and (ii) the pressure-supply flow path connected to the oil introduction port  29   a  by virtue of slight upward and downward axial displacements of the first land  32   a , such that the pump discharge pressure can be held at the given hydraulic pressure P 1 . 
     Also, when the hydraulic pressure (the pump discharge pressure) has to be risen to the given hydraulic pressure level P 2 , the electromagnetic coil of directional control valve  8  becomes de-energized responsively to an OFF signal from the control unit, thereby permitting hydraulic pressure to be directed from the pressure-receiving chamber  64  to the oil pan. Hence, sleeve  60  begins to move downward from the hydraulically-actuated original position of  FIG. 16B  with hydraulic pressure, applied through the oil introduction port  29   a  to the annular pressure-receiving section of upper wall portion  60   c  of sleeve  60 , due to working fluid drained from the pressure-receiving chamber  64 , in other words, a pressure drop in the pressure-receiving chamber  64  (see  FIG. 16C ), and thus the annular lower sidewall surface of flanged large-diameter portion  60   b  of sleeve  60  is brought into abutted-engagement with a stepped stopper face  31   c  of the stepped portion between the protruding portion  31   b  and the large-diameter disk-shaped lid portion of lid member  31 . At this time, a maximum downward displacement of sleeve  60  is restricted by abutment of the annular lower sidewall surface of flanged large-diameter portion  60   b  with the stopper face  31   c . As a result, as appreciated from the cross section of  FIG. 16C , the volume of pressure-receiving chamber  64  becomes a minimum volumetric capacity. 
     As can be seen from the cross section of  FIG. 16C , in accordance with the downward movement of sleeve  60 , as a matter of course, the communication ports  61  move downward. Thus, the first land  32   a  of spool  32 , together with the communication ports  61 , moves downward, while compressing the valve spring  34  by the circular top of the first land  32   a . Hence, the spring load of valve spring  34  becomes a higher spring load. At this time, a switching pressure when switching the flow path configuration for the first communication passage  35  (i.e., the control oil chamber  16 ) between (i) the pressure-release flow path connected to the drain port  31   a  and (ii) the pressure-supply flow path connected to the oil introduction port  29   a , becomes the given hydraulic pressure level P 2  shown in  FIG. 6 . By the way, the opening width of the opening end of the first communication passage  35  is set or dimensioned such that the first communication passage  35  always communicates with the communication ports  61  over the entire range of axial displacement of ports  61 . 
     The other operation and effects of the pump system of the seventh embodiment are the same as the fourth embodiment. However, in the seventh embodiment, in contrast to the above, when there is no hydraulic pressure supply to the pressure-receiving chamber  64  with the electromagnetic coil of directional control valve  8  de-energized (OFF), the pump discharge pressure becomes set or held at the given hydraulic pressure level P 2  (the high pressure level), thus ensuring a fail-safe effect in the presence of undesirable clogging of the flow paths of the hydraulic circuit of the pump system. 
     [Eighth Embodiment] 
     Referring now to  FIGS. 17A-17C , there is shown a further modified sleeve-equipped pilot valve structure incorporated in the variable displacement oil pump system of the eighth embodiment. The basic pilot valve structure of the eighth embodiment is similar to the fifth embodiment. 
     The eighth embodiment somewhat differs from the fifth embodiment in that, in a similar manner to the seventh embodiment, the structure (the cross section) of the sleeve  60  of pilot valve  7  of the eighth embodiment, disposed between the stepped cylindrical close-fitting bore  30  and the spool  57 , is changed. 
     More concretely, in the eighth embodiment, sleeve  60  has an upper wall portion  60   c  formed integral with the uppermost end of small-diameter portion  60   a . Upper wall portion  60   c  of sleeve  60  has a large-diameter communication bore  60   d  (an axial through hole) formed substantially at the center of the upper wall portion  60   c . Large-diameter communication bore  60   c  is configured to always communicate with the oil introduction port  29   a . The underside of upper wall portion  60   c  serves as a second bearing surface  60   e  that restricts a maximum upward movement of spool  57 . The structure of spool  57  of pilot valve  7  of the eighth embodiment is identical to that of the fifth embodiment. 
     Small-diameter portion  60   a  of sleeve  60  has a plurality of communication ports  61  (circumferentially equidistant-spaced radial through holes) at a given axial position substantially corresponding to the third communication passage  58 . Also, the small-diameter portion  60   a  of sleeve  60  has a plurality of drain ports  65  (circumferentially equidistant-spaced radial through holes) at an upper axial position than the communication ports  61  and substantially corresponding to the drain passage  59 . 
     In the initial state of pilot valve  7 , as shown in  FIG. 17A , spool  57  is forced into abutted-engagement with the second bearing surface  60   e  of sleeve  60  by the spring force of valve spring  34 . At the same time, sleeve  60  is forced into abutted-engagement with the shouldered bearing surface  30   d  by the spring force of valve spring  34 . 
     In the initial state of pilot valve  7 , as shown in  FIG. 17A , the communication ports  61  are communicated with the oil introduction port  29   a  through the radial through hole  57   g , and thus the third communication passage  58  (i.e., the second control oil chamber  53 ) is communicated with the oil introduction port  29   a . Thus, hydraulic pressure in the main oil gallery  25  is delivered into the second control oil chamber  53 . The pump-body structure of the variable displacement oil pump of the eighth embodiment (see  FIGS. 17A-17C ) is identical to that of the fifth embodiment (see  FIGS. 14A-14C ). Also, the connecting path configuration between the pump body and the electromagnetic solenoid operated directional control valve  8  in the pump system of the eighth embodiment is identical to that of the fifth embodiment, thus providing the two-stage pump discharge pressure characteristic shown in  FIG. 6 . However, the electromagnetic solenoid operated directional control valve  8  incorporated in the pump system of the eighth embodiment shown in  FIGS. 17A-17C , is changed to a different directional-control-valve specification that supplies hydraulic pressure from the branch passage  29  through the directional control valve via the second communication passage  39  to the annular pressure-receiving chamber  64  when energized (ON), and also blocks the pressure-supply flow path from the branch passage  29  via the second communication passage  39  to the annular pressure-receiving chamber  64  and simultaneously switches the flow path configuration for the second communication passage  39  to a pressure-release flow path from the second communication passage  39  through the directional control valve to the drain port  47  when de-energized (OFF). 
       FIG. 17B  shows the working state of pilot valve  7  of the pump system of the eighth embodiment during a steady-state engine operating mode, at which the pump discharge pressure rises up to the given hydraulic pressure level P 1 . As clearly shown in  FIG. 17B , spool  57  shifts to a slightly downward-displaced axial position (see the axial position of spool  57  shown in  FIG. 17B ). The width (i.e., the axial length) of the first land  57   a  and the inside diameter (i.e., the opening width) of each of the communication ports  61  are dimensioned to be approximately equal to each other. At the unique axial position of spool  57  shown in  FIG. 17B , a unique flow path configuration for the communication ports  61  (i.e., the third communication passage  58 ) is switched from (i) a pressure-supply flow path connected to the oil introduction port  29   a  to (ii) a pressure-release flow path connected to the drain passage  59 . Hence, a fall in hydraulic pressure in the second control oil chamber  53  occurs and as a result the pump discharge flow rate can be adjusted by a counterclockwise displacement of cam ring  5  against the spring force of coil spring  28 . In this manner, immediately when the pump discharge pressure reaches the given hydraulic pressure P 1 , fluid-communication between the drain passage  59  and the third communication passage  58  becomes established, and thereafter the hydraulic pressure in the second control oil chamber  53  can be appropriately controlled or regulated by appropriate switching between (i) the pressure-supply flow path connected to the oil introduction port  29   a  and (ii) the pressure-release flow path connected to the drain passage  59  by virtue of slight upward and downward axial displacements of the second land  57   b , such that the pump discharge pressure can be held at the given hydraulic pressure P 1 . 
     Also, when the hydraulic pressure (the pump discharge pressure) has to be risen to the given hydraulic pressure level P 2 , the electromagnetic coil of directional control valve  8  becomes de-energized responsively to an OFF signal from the control unit, thereby permitting hydraulic pressure to be directed from the pressure-receiving chamber  64  to the oil pan. Hence, sleeve  60  begins to move downward from the hydraulically-actuated original position of  FIG. 17B  with hydraulic pressure, applied through the oil introduction port  29   a  to the annular pressure-receiving section of upper wall portion  60   c  of sleeve  60 , due to working fluid drained from the pressure-receiving chamber  64 , in other words, a pressure drop in the pressure-receiving chamber  64  (see  FIG. 17C ), and thus the annular lower sidewall surface of flanged large diameter portion  60   b  of sleeve  60  is brought into abutted-engagement with the stepped stopper face  31   c  of the stepped portion between the protruding portion  31   b  and the large-diameter disk-shaped lid portion of lid member  31 . At this time, a maximum downward displacement of sleeve  60  is restricted by abutment of the annular lower sidewall surface of flanged large-diameter portion  60   b  with the stopper face  31   c . As a result, as appreciated from the cross section of  FIG. 17C , the volume of pressure-receiving chamber  64  becomes a minimum volumetric capacity. 
     As can be seen from the cross section of  FIG. 17C , in accordance with the downward movement of sleeve  60 , as a matter of course, the communication ports  61  move downward. Thus, the first land  57   a  of spool  57 , together with the communication ports  61 , moves downward, while compressing the valve spring  34  by the underside of the third land  57   c . Hence, the spring load of valve spring  34  becomes a higher spring load. At this time, a switching pressure when switching the flow path configuration for the third communication passage  58  (i.e., the second control oil chamber  53 ) between (i) the pressure-release flow path connected to the drain passage  59  and (ii) the pressure-supply flow path connected to the oil introduction port  29   a , becomes the given hydraulic pressure level P 2  shown in  FIG. 6 . By the way, the opening width of the opening end of the third communication passage  58  is set or dimensioned such that the third communication passage  58  always communicates with the communication ports  61  over the entire range of axial displacement of ports  61 . Additionally, the opening width of the drain passage  59  is set or dimensioned such that the drain passage  59  always communicates with the drain ports  65  over the entire range of axial displacement of ports  65 . 
     [Ninth Embodiment] 
     Referring now to  FIGS. 18A-18C , there is shown a still further modified sleeve-equipped pilot valve structure incorporated in the variable displacement oil pump system of the ninth embodiment. The ninth embodiment is a modification that the sleeve-equipped pilot valve structure, somewhat similar to the seventh and eighth embodiments, is applied to the pump system of the sixth embodiment. That is, in the ninth embodiment, the outer periphery of small-diameter portion  60   a  is machined to axially slide in the small-diameter bore  30   a  with a very small radial clearance. In a similar manner, the outer periphery of flanged large-diameter portion  60   b  is machined to axially slide in the large-diameter bore  30   b  with a very small radial clearance. Additionally, three lands  57   a - 57   c  of spool  57  are machined to axially slide in the close-fitting cylindrical bore of small-diameter portion  60   a  with a very small radial clearance. Upper wall portion  60   c  of sleeve  60  has a large-diameter communication bore  60   d  (an axial through hole) formed substantially at the center of the upper wall portion  60   c . The underside of upper wall portion  60   c  serves as a second bearing surface  60   e  that restricts a maximum upward movement of spool  57 . 
     The small-diameter portion  60   a  of sleeve  60  has first communication ports  61  (radial through holes) configured to communicate with the first communication passage  35 , drain ports  66  (radial through holes) configured to communicate with the drain passage  59 , and second communication ports  67  (radial through holes) configured to communicate with the third communication passage  58 . 
     In the initial state of pilot valve  7 , as shown in  FIG. 18A , spool  57  is forced into abutted-engagement with the second bearing surface  60   e  of sleeve  60  by the spring force of valve spring  34 . At the same time, sleeve  60  is forced into abutted-engagement with the shouldered bearing surface  30   d  by the spring force of valve spring  34 . 
     In the initial state of pilot valve  7 , as shown in  FIG. 18A , the first communication passage  35  is communicated with the drain ports  66  through the first communication ports  61  and the first annular groove  57   h , whereas the third communication passage  58  is communicated with the oil introduction port  29   a  through the second communication ports  67  and the radial through hole  57   g . Thus, hydraulic pressure in the main oil gallery  25  is delivered into the second control oil chamber  53 . The pump-body structure of the variable displacement oil pump of the ninth embodiment (see  FIGS. 18A-18C ) is identical to that of the third embodiment ( FIGS. 10-12 ) and the sixth embodiment (see  FIGS. 15A-15C ). Also, the connecting path configuration between the pump body and the electromagnetic solenoid operated directional control valve  8  in the pump system of the ninth embodiment is identical to that of the third embodiment and the sixth embodiment, thus providing the two-stage pump discharge pressure characteristic shown in  FIG. 6 . However, the electromagnetic solenoid operated directional control valve  8  incorporated in the pump system of the ninth embodiment shown in  FIGS. 18A-18C , is changed to a different directional-control-valve specification that supplies hydraulic pressure from the branch passage  29  through the directional control valve via the second communication passage  39  to the annular pressure-receiving chamber  64  when energized (ON), and also blocks the pressure-supply flow path from the branch passage  29  via the second communication passage  39  to the annular pressure-receiving chamber  64  and simultaneously switches the flow path configuration for the second communication passage  39  to a pressure-release flow path from the second communication passage  39  through the directional control valve to the drain port  47  when de-energized (OFF). 
       FIG. 18B  shows the working state of pilot valve  7  of the pump system of the ninth embodiment during a steady-state engine operating mode, at which the pump discharge pressure rises up to the given hydraulic pressure level P 1 . As clearly shown in  FIG. 18B , spool  57  shifts to a slightly downward-displaced axial position (see the axial position of spool  57  shown in  FIG. 18B ). The width (i.e., the axial length) of the first land  57   a  and the inside diameter (i.e., the opening width) of each of the first communication ports  61  are dimensioned to be approximately equal to each other. Additionally, the width (i.e., the axial length) of the second land  57   b  and the inside diameter (i.e., the opening width) of each of the second communication ports  67  are dimensioned to be approximately equal to each other. Therefore, when the first and second lands  57   a - 57   b  downwardly move to respective axial positions of the first and second communication ports  61  and  67 , at the unique axial position of spool  57  shown in  FIG. 18B , a unique flow path configuration for the first communication ports  61  (i.e., the first communication passage  35 ) is switched from (i) a pressure-release flow path connected to the drain ports  66  to (ii) a pressure-supply flow path connected to the oil introduction port  29   a , and simultaneously a unique flow path configuration for the second communication ports  67  (i.e., the third communication passage  58 ) is switched from (i) a pressure-supply flow path connected to the oil introduction port  29   a  to (ii) a pressure-release flow path connected to the drain ports  66 . Hence, a rise in hydraulic pressure in the first control oil chamber  16  and a fall in hydraulic pressure in the second control oil chamber  53  occur simultaneously and as a result the pump discharge flow rate can be adjusted by a counterclockwise displacement of cam ring  5  against the spring force of coil spring  28 . In this manner, immediately when the pump discharge pressure reaches the given hydraulic pressure P 1 , fluid-communication between the oil introduction port  29   a  and the first communication passage  35  and fluid-communication between the drain passage  59  and the third communication passage  58  become established, and thereafter the hydraulic pressure in the first control oil chamber  16  can be appropriately controlled or regulated by appropriate switching between (i) the pressure-release flow path connected to the drain passage  59  and (ii) the pressure-supply flow path connected to the oil introduction port  29   a  by virtue of slight upward and downward axial displacements of the first land  57   a , and simultaneously the hydraulic pressure in the second control oil chamber  53  can be appropriately controlled or regulated by appropriate switching between (i) the pressure-supply flow path connected to the oil introduction port  29   a  and (ii) the pressure-release flow path connected to the drain passage  59  by virtue of slight upward and downward axial displacements of the second land  57   b , such that the pump discharge pressure can be held at the given hydraulic pressure P 1 . 
     Also, when the hydraulic pressure (the pump discharge pressure) has to be risen to the given hydraulic pressure level P 2 , the electromagnetic coil of directional control valve  8  becomes de-energized responsively to an OFF signal from the control unit, thereby permitting hydraulic pressure to be directed from the pressure-receiving chamber  64  to the oil pan. Hence, sleeve  60  begins to move downward from the hydraulically-actuated original position of  FIG. 18B  with hydraulic pressure, applied through the oil introduction port  29   a  to the annular pressure-receiving section of upper wall portion  60   c  of sleeve  60 , due to working fluid drained from the pressure-receiving chamber  64 , in other words, a pressure drop in the pressure-receiving chamber  64  (see  FIG. 18C ), and thus the annular lower sidewall surface of flanged large-diameter portion  60   b  of sleeve  60  is brought into abutted-engagement with the stepped stopper face  31   c  of the stepped portion between the protruding portion  31   b  and the large-diameter disk-shaped lid portion of lid member  31 . At this time, a maximum downward displacement of sleeve  60  is restricted by abutment of the annular lower sidewall surface of flanged large-diameter portion  60   b  with the stopper face  31   c . As a result, as appreciated from the cross section of  FIG. 18C , the volume of pressure-receiving chamber  64  becomes a minimum volumetric capacity. 
     As can be seen from the cross section of  FIG. 18C , in accordance with the downward movement of sleeve  60 , as a matter of course, first and second communication ports  61  and  67  move downward. Thus, the first land  57   a , together with the first communication ports  61  and the second land  57   b , together with the second communication ports  67 , move downward, while compressing the valve spring  34  by the underside of the third land  57   c . Hence, the spring load of valve spring  34  becomes a higher spring load. At this time, a switching pressure when switching the flow path configuration for the first communication passage  35  (i.e., the first control oil chamber  16 ) between (i) the pressure-release flow path connected to the drain ports  66  and (ii) the pressure-supply flow path connected to the oil introduction port  29   a  and simultaneously switching the flow path configuration for the third communication passage  58  (i.e., the second control oil chamber  53 ) between (i) the pressure-supply flow path connected to the oil introduction port  29   a  and (ii) the pressure-release flow path connected to the drain ports  66 , becomes the given hydraulic pressure level P 2  shown in  FIG. 6 . 
     By the way, the opening width of the opening end of the first communication passage  35  is set or dimensioned such that the first communication passage  35  always communicates with the first communication ports  61  over the entire range of axial displacement of ports  61 . Additionally, the opening width of the drain passage  59  is set or dimensioned such that the drain passage  59  always communicates with the drain ports  66  over the entire range of axial displacement of ports  66 . 
     As will be appreciated from the above, according to the inventive concept, the electromagnetic solenoid-operated directional control valve  8  is configured to cooperate with either the slider  33  or the sleeve  60 , for automatically changing the spring-load setting (the preload setting) of spool-valve spring  34  and for variably controlling timing at which switching between the oil-discharge flow path from the control chamber ( 16 ;  16 ,  53 ) and the oil-introduction flow path to the control chamber occurs, with respect to the discharge pressure applied at the oil introduction port  29   a  of the pilot valve  7  (see the variable displacement oil pump employing the large-diameter spring-support slider-equipped pilot valve structure in the first to third embodiments and the variable displacement oil pump employing the ported-sleeve-equipped pilot valve structure in the fourth to ninth embodiments). 
     Also, as appreciated from the above, in the case of the variable displacement oil pump employing the large-diameter spring-support slider-equipped pilot valve structure in the first to third embodiments, the axial position of the movable spring-support slider  33  can be appropriately changed or switched between a given first axial position (a spring-loaded original position) and a given second axial position (a maximum displaced position) by using the electromagnetic solenoid-operated directional control valve  8 , depending on a pressure level of the discharge pressure. During a low discharge pressure operating mode of the pump at the pressure level P 1 , with the directional control valve  8  energized (ON), the slider  33  is kept at its spring-loaded original position, thereby ensuring a comparatively low load resistance to sliding movement of the spool, sliding against the spring bias of valve spring  34  by the discharge pressure applied at the oil introduction port  29   a . This means the ease of sliding of the spool against the spring bias of valve spring  34  with the discharge pressure applied at the oil introduction port  29   a , but such a low load resistance matches the pressure level P 1 , in other words, a low-pressure setting of valve spring  34 . Conversely during a high discharge pressure operating mode of the pump at the pressure level P 2 , with the directional control valve  8  de-energized (OFF), the slider  33  is kept at its maximum axially-displaced position, thereby ensuring a comparatively high load resistance to sliding movement of the spool, sliding against the spring bias of valve spring  34  by the discharge pressure applied at the oil introduction port  29   a . This means the difficulty of sliding of the spool with the discharge pressure applied at the oil introduction port  29   a , but such a high load resistance matches the pressure level P 2 , in other words, a high-pressure setting of valve spring  34 . In the shown embodiments, timing of switching between the low-pressure and high-pressure settings is variably controlled electrically by ON/OFF control for the electromagnetic solenoid-operated directional control valve  8 . In other words, the electromagnetic solenoid-operated directional control valve  8  is configured to control a load resistance to the sliding movement of the spool with a change in the spring bias of valve spring  34 , occurring by displacing the slider  33 , which is provided for supporting the lower end of valve spring  34 , depending on a pressure level of the discharge pressure. 
     Furthermore, in the shown embodiments, the communication passage (e.g., the first communication passage  35 ) of the spool is configured to be temporarily closed when switching a flow path configuration for the communication passage between an oil-introduction flow path from the discharge portion (e.g., the discharge port  12 ) via the communication passage to the control chamber (e.g., the first control oil chamber  16 ) and an oil-discharge flow path from the control chamber via the communication passage to a low-pressure portion (e.g., the drain passage  37 ), thus ensuring high-precision switching between the oil-introduction flow path and the oil-discharge flow path. 
     Moreover, in the fifth (see  FIGS. 14A-14C ), sixth (see  FIGS. 15A-15C ), eighth (see  FIGS. 17A-17C ), and ninth (see  FIGS. 18A-18C ) embodiments, the discharge pressure is applied at one part of an internal space defined in the sliding sleeve  60 , facing the pressure-receiving section of the spool, whereas atmospheric pressure is applied at the other part of the internal space, facing apart from the pressure-receiving section of the spool. This ensures smooth sliding motion of the spool during operation of the pump. 
     Referring now to  FIGS. 19A-19C , there are shown various flow-passage structures differing from each other in width dimensions concerning the opening width of the opening end of the first communication passage  35  with respect to the width (the axial length) of the first land  32   a  (the cylindrical land), for example, in the first embodiment.  FIG. 19A  shows the first flow-passage structure in which the opening width of the opening end  35   a  of the first communication passage  35  and the width (the axial length) of the first land  32   a  are dimensioned or configured to be approximately equal to each other.  FIG. 19B  shows the second flow-passage structure in which the opening width of the opening end  35   a  of the first communication passage  35  is dimensioned to be slightly less than the width (the axial length) of the first land  32   a .  FIG. 19C  shows the third flow-passage structure in which the opening width of the opening end  35   a  of the first communication passage  35  is dimensioned to be slightly greater than the width (the axial length) of the first land  32   a . In this manner, by relatively changing the opening width of the opening end  35   a  of the first communication passage  35  with respect to the width (the axial length) of the first land  32   a , it is possible to arbitrarily control or adjust a hydraulic-pressure supply (i.e., an amount of working oil) to the control oil chamber  16 , for the same stroke (i.e., the same axial displacement) of spool  32 . 
     Referring now to  FIGS. 20A-20C , there are shown various flow-passage structures differing from each other in width dimensions concerning the opening width of the opening end  35   a  of the first communication passage  35  with respect to the width (the axial length) of the first land  32   a  (the barrel-shaped land). As can be appreciated, the shape of the first land  32   a  of  FIGS. 20A-20C  differs from that of  FIGS. 19A-19C . More concretely, the first land  32   a  of  FIGS. 19A-19C  is a cylindrical land. On the other hand, the first land  32   a  of  FIGS. 20A-20C  is a barrel-shaped land. As seen in  FIGS. 20A-20C , both axial ends of the first land  32   a  are chamfered (see two chamfered portions  32   g - 32   h  in  FIGS. 20A-20C ). In  FIGS. 20A-20C , these chamfered portions  32   g - 32   h  are exaggerated. By the use of the barrel-shaped land (i.e., due to the barrel-like cross section), even when the width (the axial length) of the first land  32   a  is dimensioned to be slightly greater than the opening width of the opening end  35   a  of the first communication passage  35 , a slight clearance exists between the outer peripheral curved surface of the first land  32   a  and the inner peripheral wall surface of small-diameter bore  30   a , and hence three flow-path directions (namely, the flow passage through the oil introduction port  29   a , the flow passage through the annular groove  32   e , and the flow passage through the opening end  35   a  of the first communication passage  35 ) cannot be completely shut off. By selecting either the cylindrical land or the barrel-shaped land, it is possible to appropriately change the relationship between a change in the flow passage area of the small aperture, defined by the first land  32   a  to communicate the oil introduction port  29   a  with the first communication passage  35 , and a spool stroke (a spool axial displacement). Thus, it is preferable to use or select a suitable one from different land shapes (i.e., a cylindrical land and a barrel-shaped land), depending on a specification of the pump body and/or a working pressure of the pump. 
     As can be appreciated from the above, by appropriately selecting either a cylindrical land or a barrel-shaped land and/or by relatively changing the opening width of the opening end of the communication passage with respect to the width (the axial length) of the associated land, it is possible to appropriately change a rate of change in the flow passage area of the small aperture, defined by the land, with respect to spool stroke (spool axial displacement). The concept of the modified flow-passage structure with respect to the valve-spool land and the concept of the modified valve-spool land cross-section, explained in reference to  FIGS. 19A-19C  and  FIGS. 20A-20C  and exemplified in the first embodiment, can be applied to all of the shown embodiments. 
     The entire contents of Japanese Patent Application No. 2012-196712 (filed Sep. 7, 2012) 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.