Patent Publication Number: US-9903367-B2

Title: Variable displacement oil pump

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a variable displacement oil pump used as an oil pressure source for supplying oil to sliding contact portions of an internal combustion engine, for example. 
     An example of the variable displacement pump is shown in JP 2014-105623A (corresponding to US2014/0219847A1). 
     To supply the oil discharged from the oil pump to various sections having different required oil pressure levels, such as sliding contact portions of an internal combustion engine and a variable valve actuating device for controlling an operating characteristic of an engine valve, there is a recent demand for a two-step or multistep characteristic having a lower pressure characteristic for a first rotational seed region, and a higher pressure characteristic for a second rotational speed region. 
     The variable displacement oil pump of the above-mentioned patent document is designed to satisfy such a demand, with first and second control oil chambers formed between a pump housing and a cam ring. By controlling the introduction of the discharge pressure into the first and second control oil chambers with a pilot valve in accordance with an urging force based on the internal pressure in the first control oil chamber, to urge the cam ring in a direction decreasing the eccentricity or eccentricity quantity of the cam ring (concentric direction), an urging force based on the internal pressure in the second control oil chamber, to urge the cam ring in a direction increasing the eccentricity of the cam ring (eccentric direction), and a spring force of a spring to urge the cam ring in the eccentric direction, this variable displacement oil pump controls the eccentricity of the cam ring in a manner of two steps in dependence on the engine speed, and thereby satisfies the different required discharge pressure levels. 
     SUMMARY OF THE INVENTION 
     In this variable displacement oil pump, no consideration is given to an urgent force based on the internal pressure of each pumping chamber (PR) although the operation oil pressure of the cam ring should be determined by the urgent forces based on the internal pressures of the first and second control oil chambers, the urgent force based on the resilient force of the spring, and the urgent force based on the internal pressures of the pumping chambers. 
     Therefore, specifically in a high speed region corresponding to the second speed region, there is a tendency of generation of air voids (aeration) during the suction, and the internal pressures of the pumping chambers in the discharge region for compressing and discharging the oil, might be deceased and cause the cam ring to move (swing) before attainment of a predetermined set pressure level. 
     The present invention has been devised in view of the above-mentioned technical problem in the variable displacement oil pump. It is an object of the present invention to provide a variable displacement fluid pump to maintain an adequate operation pressure of a cam ring despite occurrence of aeration, and to achieve a higher fluid pressure desirable for an internal combustion engine. 
     According to the present invention, there are provided a first control oil or fluid chamber to receive an operating fluid or oil discharged from a discharge portion and thereby to produce an urging force (T 1 ) to urge a movable member ( 15 ) in a direction to decrease a pumping volume variation quantity of the pumping chambers, a second control oil or fluid chamber ( 32 ) to receive the fluid discharged from the discharge portion and thereby to produce an urging force to urge the movable member in a direction to vary the pumping volume variation quantity, and a control mechanism or section operated before the pumping volume variation quantity becomes smallest, and arranged to discharge the fluid from the second control oil chamber or to supply the fluid to the second control oil chamber with increase in a discharge pressure of the fluid discharged from the discharge portion. In a higher pressure region higher than a predetermined or highest fluid pressure required by the internal combustion engine, an operation fluid pressure of the movable member is set higher than an operation fluid pressure of the control mechanism. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a hydraulic circuit diagram of a variable displacement pump according to an embodiment of the present invention. 
         FIG. 2  is an enlarged view of the variable displacement pump shown in  FIG. 1 . 
         FIG. 3  is a view showing a torque distribution of torques acting on a cam ring of the variable displacement pump shown in  FIG. 2 . 
         FIG. 4  is an enlarged view of a pilot valve shown in  FIG. 1 . 
         FIG. 5  is an enlarged view of a solenoid valve shown in  FIG. 1 . 
         FIG. 6  is a graphic view showing an oil pressure characteristic of the variable displacement pump according to this embodiment. 
         FIG. 7A  is a hydraulic circuit diagram showing the variable displacement pump according to this embodiment in a state in an interval “a” shown in  FIG. 6 .  FIG. 7B  is a hydraulic circuit diagram showing the variable displacement pump according to this embodiment in a state in an interval “b” shown in  FIG. 6 . 
         FIG. 8A  is a hydraulic circuit diagram showing the variable displacement pump according to this embodiment in a state in an interval “c” shown in  FIG. 6 .  FIG. 8B  is a hydraulic circuit diagram showing the variable displacement pump according to this embodiment in a state in an interval “d” shown in  FIG. 6 . 
         FIG. 9  is a view similar to  FIG. 6 , for illustrating the effect of the variable displacement pump according to this embodiment. 
         FIG. 10  is a view of a pressure-flow characteristic at the time of occurrence of aeration in the variable displacement pump according to the present invention. 
         FIG. 11  is a view of a pressure-flow characteristic at the time of occurrence of aeration in the variable displacement pump in another example according to the present invention. 
         FIG. 12  is a view similar to  FIG. 9 , for illustrating the effect of a variable displacement pump of a comparative example. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiment(s) of the present invention is explained hereinafter with reference to the drawings. In the illustrated embodiment, the variable displacement oil pump is adapted to supply a lubricating oil to various parts of an internal combustion engine for a motor vehicle, such as sliding contact portions, and a valve timing control apparatus used for control of opening/closing timings of engine valves. 
     An oil pump  10  shown in  FIG. 1  is provided at front end portion of a cylinder block (not shown) of an internal combustion engine, for example. As shown in  FIG. 1 , oil pump  10  includes a pump housing, a drive shaft  14 , a cam ring  15 , a pump element, a pilot valve  40 , and a solenoid valve  60 . 
     The pump housing includes a pump body  11  shaped like a cup having a cylindrical wall and a bottom or end wall dosing one end of the cylindrical wall, to define a pump receiving chamber  13  in the pump body  11 , and a pump cover (not shown) closing the open end of pump body  11 . The drive shaft  14  is supported rotatably by the pump housing, and arranged to extend through a center portion of the pump receiving chamber  13  and to be driven by a crankshaft (not shown) of the engine. 
     The cam ring  15  serves as a movable (or swingable) member received movably (or swingably) in the pump receiving chamber  13 , and constitutes a varying mechanism varying a volume variation or volume variation quantity of each of later-mentioned pumping chambers PR in cooperation with later-mentioned first and second control oil chambers  31  and  32 , and a coil spring  33 . 
     The pump element is received in, and surrounded by, the cam ring  15 . The pump element is arranged to be driven and rotated by drive shaft  14  in the clockwise direction in  FIG. 1  and thereby to perform a pumping action to increase/decrease the volumes of pumping chambers PR formed between the cam ring  15  and the pump element. The pilot valve  40  serves as a control mechanism provided on a downstream side of an oil main gallery MG of the internal combustion engine, and arranged to control the supply and drainage (or discharge) of the oil pressure (control pressure) to the first and second control oil chambers  31  and  32 . The solenoid valve  60  is provided in an oil passage (later-mentioned second introduction passage  72 ) branching off from the oil main gallery MG, and arranged to perform a selection or changeover control of the introduction of the control oil supplied to the pilot valve  40 . 
     The pump element or pump member (or rotary member) of this example includes a rotor  16 , a plurality of vanes  17  and a pair of ring members  18 . The rotor  16  is received rotatably in cam ring  15 , and mounted on drive shaft  14  so that the center portion of rotor  16  is fit over the outside surface of drive shaft  14 . Rotor  16  includes an outer circumferential portion formed with a plurality of slits  16   a  formed radially, and arranged to receive the vanes  17 , respectively. Each vane  17  can move radially in a corresponding one of the slits  16   a  (in the radial outward direction to project outward and in the radial inward direction to withdraw deeper). The ring members  18  are smaller in diameter than rotor  16 . The two ring members  18  are disposed on both sides of rotor  16  so that a radial inner portion of rotor  16  is sandwiched between the two ring members  18 . 
     The pump body  11  is a single integral member of aluminum alloy material. As shown in  FIG. 2 , a shaft hole  11   a  is opened substantially at a center position through the end wall or bottom of pump receiving chamber  13 , and arranged to support one end of the drive shaft  14  rotatably. In an area surrounding the central shaft hole  11   a , there are formed suction port  21   a  and discharge port  22   a  confronting each other diametrically across the central shaft hole  11   a . The suction port  21   a  serving as a suction portion is opened in a region (suction region) where the inside volume of each pumping chamber PR is increased in accordance with the pumping action of the pump element, and shaped to have an arc recess extending circumferentially like a circular arc. The discharge port  22   a  serving as a discharge portion is opened in a region (discharge region) where the inside volume of each pumping chamber PR is decreased, and shaped to have an arc recess extending circumferentially like a circular arc. 
     A support recess  11   b  is formed at a predetermined position in the inside circumferential wall of pump receiving chamber  13 . The support recess  11   b  is shaped to have a substantially semicircular cross sectional shape and to support a rod-shaped pivot pin  19  for supporting the cam ring  15  swingably. This inside circumferential wall of pump receiving chamber  13  is formed with a first seal slide surface  13   a  and a second seal slide surface  13   b . The first seal slide surface  13   a  is formed on an upper side of an imaginary straight line M (hereinafter referred to as a cam ring reference line) connecting the center of support recess  11   b  (or the axis of the pivot pin  19 ) and the center of central shaft hole  11   a  (or the axis of drive shaft  3 ), as viewed in  FIG. 2 . The first seal surface  13   a  is shaped to be always held in sliding contact with a later-mentioned first seal member  30   a . The second seal slide surface  13   b  is formed on a lower side of the cam ring reference line, in  FIG. 2  and shaped to be always held in sliding contact with a later-mentioned second seal member  30   b.    
     The suction port  21   a  is integrally formed with an introduction portion  23  extending radially so as to bulge, from a middle portion of the circumferentially extending suction port  21   a , toward a later-mentioned spring receiving chamber  28 . In the vicinity of the connecting portion between the suction port  21   a  and introduction portion  23 , there is formed a suction hole  21   b  extending through the end wall of pump body  11 , and opening to the outside. With this construction, the pump  10  functions to suck the oil stored in an oil pan T of the internal combustion engine, by the use of a negative pressure produced by the pumping action of the pump element, through the suction hole  21   b  and suction port  21   a , into the pumping chamber(s) PR in the suction region. The suction hole  21   a  is connected with the introduction portion  23 , and further connected with a lower pressure chamber  35  formed in an outer circumferential region of cam ring  15  in the suction region, and arranged to receive the oil of a lower pressure which is the intake pressure. 
     The discharge port  22   a  includes a leading end portion formed with a discharge hole  22   b  extending through the end wall of pump body  11  and opening to the outside. With this construction, the pump  10  functions to supply the oil pressurized by the pumping action and discharged to the discharge port  22   a , from the discharge hole  22   b  through the oil main gallery MG to the various sliding contact portions of the internal combustion engine and the valve timing control apparatus. 
     A suction port and a discharge port are formed in the inside surface of the pump cover (not shown), too, in the same manner as the suction port  21   a  and discharge port  22   a  formed in the inside surface of the end wall of pump body  11 , and arranged to confront axially the suction port  21   a  and discharge port  22   a  of pump body  11 . 
     The drive shaft  14  extends through the end wall of pump body  11 , to a shaft end portion connected the crankshaft (not shown). By receiving the rotational force transmitted from the crankshaft, the drive shaft  14  rotates the rotor  16  in the clockwise direction as viewed in  FIG. 2 . As shown in  FIG. 2 , a line N (hereinafter referred to as a cam ring eccentric direction line) is an imaginary straight line passing through the center of drive shaft  14  and intersecting the cam ring reference line M at right angles. This cam ring eccentric direction line N serves as a boundary between the suction region and the discharge region. 
     The rotor  16  includes the slits  16   a  extended radially outwards from a central portion of the rotor. Moreover, rotor  16  is formed with back pressure chambers  16   b  each formed at the radial inner end of one slit  16   a . In this example, each back pressure chamber  16   b  has an approximately circular cross section. The back pressure chambers  16   b  are arranged to receive the discharge oil pressure. The vanes  17  are pushed radially outwards by the centrifugal force due to the rotation of rotor  16  and the pressure in the back pressure chambers  16   b.    
     Each vane  17  includes a forward end sliding on the inside circumferential surface of cam ring  15  and an inner base end sliding on the outer circumferential surfaces of first and second ring members  18 . The ring members  18  are arranged to push each vane  17  radially outwards, away from the center of rotor  16 , so that the forward end of each vane  17  slides on the inside circumferential surface of cam ring  15  even when the centrifugal force is small and the pressure in the back pressure chambers  16   b  is low at low engine speeds. Thereby, the vanes  17  defines each of the pumping chambers PR liquid-tightly. 
     The cam ring  15  is an integral member shaped like a hollow cylinder, and made of sintered metallic material. Cam ring  15  includes a pivot portion  26  which extends axially, which is located at a predetermined position in the outer circumferential portion and which is formed in the shape of a substantially circular arc recess fit over the pivot pin  19  to define a fulcrum of eccentric swing motion. 
     Cam ring  15  further includes an arm portion  27  projecting radially from a portion diametrically opposite to the position of the pivot portion  26 , and having a portion abutting on the coil spring  33  which serves as an urging or biasing member and which is set to have a predetermined spring constant. This arm portion  27  is formed with a projection  27   a  formed on one side of arm portion  27  facing in the moving (rotational) direction, in the form of a substantially circular arc projection, and arranged to abut always on the forward end of coil spring  33 , and thereby to form a linkage between arm portion  27  and coil spring  33 . 
     A spring receiving chamber  28  for receiving and holding the coil spring  33  is formed in the pump body  11 , at a position confronting the support groove  11   b . The spring receiving chamber  28  extends, along the cam ring eccentric direction N shown in  FIG. 2 , at the position adjacent to the pump receiving chamber  13 . Coil spring  33  is disposed elastically between the end wall or bottom of spring receiving chamber  13  and the arm portion  27  (projection  27   a ), with a predetermined set load W 1 . The other end wall (upper wall) of spring receiving chamber  28  serves as a regulating portion  29  for regulating the swing range in the eccentric direction of cam ring  15 . The regulating portion  29  is arranged to abut on the other (upper) side of arm portion  27  and thereby prevent cam ring  15  from moving further in the eccentric direction. 
     The set load W 1  of coil spring  33  is so set that, in a high pressure region exceeding a maximum or highest engine requirement oil pressure required by the internal combustion engine (a later-mentioned third engine requirement oil pressure Pe 3 ), an operation oil pressure of cam ring  15  (a later-mentioned second operation oil pressure Pc 2 ) is higher than a changeover oil pressure of pilot valve  40  (a later-mentioned second changeover oil pressure Pv 2 ). With this setting, the second operation oil pressure Pc 2  of cam ring  15  does not become lower than the second changeover oil pressure Pv 2  of pilot valve  40  in any of situations such as dimension error of a spool valve element  43  of pilot valve  40  and nonuniformity of a set load W 2  of a valve spring  44  of pilot valve  40 . Therefore, this setting is a setting satisfying the later-mentioned third engine requirement oil pressure Pe 3  securely. 
     Thus, cam ring  15  is always urged by an urging force Ts (as shown in  FIG. 3 ) of coil spring  33  through the arm portion  27  in the direction increasing the eccentricity (the clockwise direction in  FIG. 1 , hereinafter referred to as the eccentric direction). Therefore, in an inoperative state, the cam ring  15  is held in the position at which the other (upper) side of arm portion  27  abuts against the regulating portion  29  and the eccentricity is greatest. 
     Cam ring  15  includes first and second seal forming portions  15   a  and  15   b  projected, respectively, to have seal surfaces curved in the form of a concentric circular arc with the first and second seal slide surfaces  13   a  and  13   b  formed in the inside circumferential surface of pump receiving chamber  13  of the pump housing ( 11 ). First and second seal members  30   a  and  30   b  are retained, respectively, in the seal surfaces of first and second seal forming portions  15   a  and  15   b . Each seal member  30   a  or  30   b  is a long member of a low friction material such as fluorine resin having a low friction characteristic, extending rectilinearly in the axial direction of cam ring  15 . Each of first and second seal members  30   a  and  30   b  is backed up by an elastic member of rubber material, and pressed on the confronting seal slide surface  13   a  or  13   b , as shown in  FIG. 2 , to form a liquid tight seal between the seal surface of the seal forming portion  15   a  or  15   b  and the seal slide surface  13   a  or  13   b.    
     The first and second control oil or fluid chambers  30   a  and  30   b  are defined around the cam ring  15 , by this seal structure. First control oil chamber  31  is defined between the pivot pin  19  and the first seal member  30   a  held by the first seal forming portion  15   a . Second control oil chamber  32  is defined between the pivot pin  19  and the second seal member  30   b  held by the second seal forming portion  15   b . The control pressure is introduced into first and second control oil chambers  31  and  32 , from a control pressure introduction passage  70  branching off from the oil main gallery MG, as an oil pressure in the engine. Specifically, the control pressure is the oil pressure in the engine resulting from a pressure decrease caused by passage of the pump discharge pressure through an oil filter (not shown). This control oil pressure is introduced into the first control oil chamber  31 , through a first introduction passage  71  which is a first branch passage branching off from the control oil pressure introduction passage  70 . The control oil pressure is introduced into the second control oil chamber  32 , through a second introduction passage  72  ( 72   a ,  72   b ) which is a second branch passage branching off from the control oil pressure introduction passage  70 . 
     First and second pressure receiving surfaces  15   c  and  15   d  are formed in the outer circumferential surface of cam ring  15  and arranged to face the first and second control oil chambers  31  and  32 , respectively. Therefore, cam ring  15  receives a moving force (swing force) by the application of the pressures in the first and second control oil chambers  31  and  32  on the first and second pressure receiving surfaces  15   c  and  15   d . The pressure receiving area of first pressure receiving surface  15   c  in first control oil chamber  31  is set smaller than the pressure receiving area of second pressure receiving surface  15   d  in second control oil chamber  32 . When the same oil pressure is applied to both of first and second pressure receiving surfaces  15   c  and  15 , the cam ring  15  is urged as a whole in the direction decreasing the eccentricity (the counterclockwise direction in  FIG. 1 , hereinafter referred to as the concentric direction). 
     Therefore, the cam ring  15  receives a torque (Tp) in the concentric direction, and a torque (Tm) in the eccentric direction. As shown in  FIG. 3 , the concentric direction torque (Tp) is made up of an urging force T 1  caused by the internal pressure in first control oil chamber  31  and an urging force TL caused by the internal pressures in pumping chambers PR in the downstream part of the discharge region. The eccentric direction torque (Tm) is made up of the urging force Ts caused by the set load of coil spring  33 , an urging force T 2  caused by the internal pressure in second control oil chamber  32 , and an urging force TU caused by the internal pressures in pumping chambers PR in the upstream part of the discharge region. Since the difference in the pressure receiving area of the pumping chambers PR between the upstream part and the downstream part of the discharge region is small, a resulting force of these urging forces TL and TU due to the internal pressures in pumping chambers PR becomes equal to zero or a very small torque in one direction (the concentric or eccentric direction). 
     When a resultant force Tt resulting from the urging forces T 1  and T 2  due to the internal pressures in first and second control oil chambers  31  and  32  is smaller as compared with the set load W 1  of coil spring  33 , the cam ring  15  is held in a most eccentric state. When the resultant force Tt resulting from the urging forces T 1  and T 2  due to the internal pressures in first and second control oil chambers  31  and  32  exceeds the set load W 1  of coil spring  33 , the cam ring  15  is rotated in the concentric direction in accordance with the resultant force Tt of urging forces T 1  and T 2  of the control pressures in first and second control oil chambers  31  and  32  (as shown in  FIG. 7B  and  FIG. 8B ). 
     The pilot valve  40  includes, as main components, a valve body  41 , a spool valve element  43  and a valve spring  44 , as shown in  FIG. 4 . The valve body  41  is shaped like a hollow cylinder extending (axially) from a first axial end portion formed with an introduction or intake port  50 , to a second axial end portion whose opening is closed by a plug  42 . Through the introduction port  50 , pilot valve  50  is connected with the first introduction passage  71 . The spool valve element  43  is slidably received in valve body  41 , and arranged to control the supply and drainage of the oil pressure for the first and second control oil chambers  31  and  32 . Spool valve element  43  includes first and second land portions  43   a  and  43   b  having larger diameter(s) and sliding on the inside circumferential surface of the valve body  41 . The valve spring  44  is elastically disposed, in valve body  41 , between the plug  42  and the spool valve element  43  with a predetermined set load W 2 , and arranged to urge the spool valve element  43  toward the first end formed with introduction port  50  always. 
     The valve body  41  includes a valve receiving portion  41   a  in the form of a cylindrical bore having an inside diameter approximately equal to an outside diameter of spool valve element  43  (the outside diameter of first and second land portions  43   a  and  43   b ), and extending axially between the first axial end portion and the second axial end portion of valve body  41 . Spool valve element  43  is slidably received in this valve receiving portion  41   a . The introduction port  50  is opened in the first axial end portion of valve body  41 , and adapted to be connected with first introduction passage  71  to introduce the control pressure from first introduction passage  71  into pilot valve  40 . The plug  42  is screwed in a female screw portion or internally threaded portion formed in the inside circumferential surface of the second axial end portion of valve body  41 . 
     The circumferential wall of valve body  41  defining the valve receiving portion  41   a  is formed with first and second connection ports  51  and  52 , a supply/discharge port  53  and a drain port  54 . The first connection port  51  is opened at a first axial position near the first end portion ( 50 ) and adapted to be connected with first control oil chamber  31 . The second connection port  52  is opened at a second axial position (or intermediate position) near the axial middle of the circumferential wall and adapted to be connected with second control oil chamber  32 . The supply/discharge port  53  is opened at a third axial position (near the second axial position) and adapted to be connected with the solenoid valve  60  through a downward passage  72   b  which is a downward segment of the second introduction passage  72  (as shown in  FIG. 1 ), for the supply/discharge of the control oil for the second control oil chamber  32 . The drain port  54  is opened at a fourth axial position near the second axial end portion ( 42 ) and arranged to drain the oil pressure conveyed through a later-mentioned inside passage  55  of spool valve element  43  from the first and second control oil chambers  31  and  32 . 
     The spool valve element  43  includes a smaller diameter shaft portion  43   c  connecting the first and second land portions  43   a  and  43   b  which are formed, respectively at both ends. In the valve receiving portion  41   a  of valve body  41 , the spool valve element  43  defines a pressure chamber  56 , a relay chamber  57  and a back pressure chamber  58 . The pressure chamber  56  is formed between the first land portion  43   a  and valve body  41 , and arranged to receive the control pressure through introduction port  50 . The relay chamber  57  is formed between first and second land portions  43   a  and  43   b , and arranged to serve as a portion for relay between the second connection port  52  and the supply/discharge port  53 . The back pressure chamber  58  is formed between the second land portion  43   b  and plug  42 , and arranged to drain the oil pressure conveyed through the inside passage  55 . 
     Spool valve element  43  further includes the inside passage  55  extending axially from the second end of spool valve element  43  (closer to plug  42 ), having a stepped shape decreasing the inside diameter stepwise toward the first end (closer to introduction port  50 ), and serving as a passage for discharging the oil pressure in first control oil chamber  31 . Specifically, inside passage  55  includes a small diameter section  55   a  near the first end and a large diameter section  55   b  extending from the second end of spool valve element  43  to the small diameter section  55   a  and receiving a first end portion of valve spring  44 . The small diameter section  55   a  is connected with the first connection port  51  through an annular groove  59   a  and a plurality of radial holes  59  extending to the annular groove  59   a  from the small diameter section  55   a  in the state in which spool valve element  43  is at an upper end position near the first end as shown in  FIG. 4  (or  FIG. 1 ). In the state in which spool valve element  43  is at a lower end position as shown in  FIG. 8B , the small diameter section  55   a  is disconnected from the first connection port  51 . The large diameter section  55   b  is connected with back pressure chamber  58  through the inside space of coil spring  44  received in large diameter section  55   b.    
     The thus-constructed pilot valve  40  assumes the following states in dependence on the control pressure introduced into the pressure chamber  56  through introduction port  50 . When the control pressure introduced into pressure chamber  56  through introduction port  50  is lower than or equal to a predetermined first changeover pressure Pv 1 , the spool valve element  43  is pushed by valve spring  44  toward the first end of valve receiving portion  41   a , and located at a first position (or first select or valve position) in a predetermined range on the first end&#39;s side of valve receiving portion  41   a  (cf.  FIG. 7A ). At the first position of spool valve element  43 , the first land portion  43   a  closes first connection port  51 , and disconnects first connection port  51  from introduction port  50 , and the relay chamber  57  connects second connection port  52  with the supply/discharge port  53 . 
     When the control pressure introduced into pressure chamber  56  becomes higher than the first changeover pressure Pv 1 , the spool valve element  43  moves, against the urging force of valve spring  44 , from the first position, in a direction toward the second end of valve receiving portion  41   a , to a second position (or second select or valve position) which is a middle or intermediate position in valve receiving portion  41   a  (cf.  FIG. 7A ,  FIG. 8A ). At the second position of spool valve element  43 , the first land portion  43   a  overlaps the first connection port  51  and thereby forms a throttle (V), so that first connection port  51  is connected with the introduction port  52  through the pressure chamber  56  by this throttle, and the relay chamber  57  holds the connection between second connection port  52  and supply/discharge port  53 . 
     When the control pressure introduced into pressure chamber  56  becomes higher than a second changeover pressure Pv 2 , the spool valve element  43  further moves, against the urging force of valve spring  44 , from the second position, in the direction toward the second end of valve receiving portion  41   a , to a third position in a predetermined range near the second end of valve receiving portion  41   a  (cf.  FIG. 8B ). At the third position, the first land portion  43   a  opens the first connection port  51  widely and connects first connection port  51  fully with introduction port  50  through pressure chamber  56 , and the second land portion  43   b  breaks the connection between second connection port  52  and supply/discharge port  53  through relay chamber  57 , and makes a connection between second connection port  52  and drain port  54  through inside passage  55 . 
     The solenoid valve  60  is received in a valve receiving hole (not shown) provided in the second introduction passage  72  at an intermediate position between both ends of second introduction passage  72 . As shown in  FIG. 5 , the solenoid valve  60  includes a valve body  61 , a seat member  62 , a ball valve element  63  and a solenoid  64 , as main components. The valve body  61  is a hollow cylindrical member having an inside axial passage  65  extending through valve body  61 . Valve body  61  includes a valve element receiving portion  66  formed by enlarging a part of inside axial passage  65  to have a larger diameter, in a first end portion of valve body  61  near a first end of valve body  61  (the left side end as viewed in  FIG. 5 , retaining the seat member  62 ). The seat member  62  is press fit and fixed in an outer end or first (or left side) end portion of the valve element receiving portion  66 . Seat member  62  includes a center opening defining an introduction port  67  which is an upstream opening connected with an upstream passage  72   a  of second introduction passage  72 . The upstream passage  72   a  is an upstream segment of second introduction passage  72 , as shown in  FIG. 1 . A valve seat  62   a  is formed in an inner open end of seat member  62 . The ball valve element  63  is disposed to be seated on and moved away from, the valve seat  62   a , and arranged to open or close the introduction port  67 . The solenoid  64  is provided in a second end portion of valve body  61  (a right end portion as viewed in  FIG. 5 ). 
     The valve element receiving portion  66  is formed in the first (left side) end portion of valve body  61  to receive the ball valve element  63 , and shaped to have a stepped enlarge shape having an inside diameter or dimension greater than the inside diameter or dimension of inside axial passage  65 . A step (annular step) formed between the valve element receiving portion  66  and the inside axial passage  65  is formed as a valve seat  66   a  which is similar to the valve seat  62   a  formed in seat member  62 , and which confronts axially the valve seat  62   a . The circumferential wall of valve body  61  is formed with a supply/discharge port  68  and a drain port  69 . The supply/discharge port  68  is opened near the forward or first end (left end in  FIG. 5 ) radially, into the valve element receiving portion  66 , and connected with the downstream passage  72   b  for supply and drainage of the oil pressure for pilot valve  40 . The drain port  69  is opened radially into the inside axial passage  65 , at a position closer to the position of solenoid  60 , and connected to the oil pan T. 
     The solenoid  64  includes a coil (not shown) in a casing  64   a . With an electromagnetic force produced by energization to the coil, the solenoid  64  moves an armature (not shown) disposed in the coil and a rod  64   b  fixed with the armature leftward as viewed in  FIG. 5 . Solenoid  64  receives the exciting current in accordance with engine operation condition(s) calculated or sensed from parameters such as an oil temperature, a water temperature and the rotational speed of the internal combustion engine, under the control of an ECU (not shown) mounted in the vehicle. 
     The thus-constructed solenoid valve  60  is operated in the following manner. When the solenoid  64  is energized, the solenoid moves the rod  64   b  outwards (leftwards) and presses the ball valve element  63  with the forward end of rod  64   b  against the valve seat  62   a  of seat member  62 . Therefore, the ball valve element  63  closes the introduction port  67  to break the connection between introduction port  67  and supply/discharge port  68 , and the inside axial passage  65  connects the supply/discharge port  68  with drain port  69 . When the solenoid  64  is not energized, the ball valve element  63  is moved backwards (rightward) by the control pressure introduced from the introduction port  67 , and pressed against the valve seat  66   a  of valve body  61 . Therefore, the introduction port  67  is connected with the supply/discharge port  68 , and the supply/discharge port  68  is disconnected from the drain port  69 . 
       FIG. 6 ˜ 11  are views for illustrating characteristic operations of the oil pump  10  according to this embodiment. 
     First,  FIG. 6  is used for explaining required oil pressures or requirement oil pressures of the internal combustion engine which are used as references for the discharge pressure control of the oil pump  10 . The example of  FIG. 6  employs three engine requirement pressures which are oil pressures required by the engine. In  FIG. 6 , a first engine requirement pressure Pe 1  is an oil pressure corresponding to an oil pressure required by a valve timing control device in the case of the internal combustion engine provided with the valve timing control device for improving the fuel consumption. A second engine requirement pressure Pe 2  is an oil pressure required by an oil jet for cooling the piston(s) in the case of the engine provided with the piston cooling oil jet device. The before-mentioned third engine requirement pressure Pe 3  is an oil pressure required for lubrication of bearing portions of the crankshaft at high engine speeds. A solid line connecting the points of Pe 1 , Pe 2  and Pe 3  in  FIG. 6  represents an ideal oil pressure (control pressure) varying with engine speed R of the internal combustion engine. A broken line in  FIG. 6  represents an actual oil pressure characteristic of the oil pump. 
     In  FIG. 6 , the first changeover oil pressure Pv 1  is an oil pressure at which the spool valve element  43  starts moving from the first position to the second position against the urging force caused by the set load W 1  of valve spring  44 . The second changeover oil pressure Pv 2  is an oil pressure at which the spool valve element  43  starts moving from the second position to the third position against the urging force of valve spring  44 . 
     In an interval or region “a” corresponding to an engine speed region from a start of the engine to a low engine speed in a low speed region as shown in  FIG. 6 , the control pressure P is lower than the first changeover pressure Pv 1 , and hence the spool valve element  43  of pilot valve  40  is located at the first position at which the first connection port  51  is disconnected from pressure chamber  56  by first land portion  43   a , and instead connected with the inside axial passage  55 , and the second connection port  52  is connected through relay chamber  57  with the supply/discharge port  53 . as shown in  FIG. 7A . Furthermore in this engine speed region, the solenoid  64  is supplied with the exciting current, and hence the solenoid valve  60  is put in the state the introduction port  67  is disconnected from the supply/discharge port  68 , and the supply/discharge port  68  is connected with the drain port  69 . Therefore, the oil in first control oil chamber  31  is discharged to oil pan T through the inside passage  55  and drain port  54 , and the oil in second control oil chamber  32  is also discharged to oil pan T through relay chamber  57 , supply/discharge port  53  and solenoid valve  60 , so that first and second control oil chamber  31  and  32  receive no oil pressure, and the internal pressures in first and second control oil chambers  31  and  32  are equal to the atmospheric pressure. As a result, the control pressure P is lower than the first operation pressure Pc 1 , the cam ring  15  is held in the greatest eccentricity state, and the control pressure P is increased substantially in proportion to the engine speed R. 
     When the engine speed R increases and the control pressure P reaches the first changeover pressure Pv 1  shown in  FIG. 6 , then, the solenoid  64  of solenoid valve  60  is held energized, and the spool valve element  42  in pilot valve  40  moves slightly toward plug  42  against the urging force of valve spring  44 , and by so doing moves from the first position to the second position as shown in  FIG. 7B . Therefore, the pilot valve  40  is put in the state in which the first connection port  51  is disconnected from the inside passage  55  by first land portion  43   a  and instead connected slightly with pressure chamber  56 , and the second connection port  52  is connected with oil pan T through relay chamber  57  as in the interval “a”. Therefore, the first control chamber  31  receives a control pressure Px slightly lowered from the first changeover pressure Pv 1  introduced through a throttle V formed by overlap of first connection port  51  and first land portion  43   a . The second control oil chamber  32  is held in no oil pressure state in which the oil is discharged from second control oil chamber  32  to oil pan T. Consequently, the urging force T 1  caused by the internal pressure in first control oil chamber  31  overcomes the urging force Ts of coil spring  33  because the first operation pressure Pc 1  is set lower than first changeover pressure Pv 1 , and the above-mentioned pressure Px is at a level capable of causing the operation, and the cam ring  15  moves slightly in the concentric direction. 
     Then, the decrease of eccentricity of cam ring  15  due to movement of cam ring  15  in the concentric direction causes the control pressure P to decrease and become lower than the first changeover pressure Pv 1 . Consequently, the spool valve element  43  in pilot valve  40  is pushed back by the urging force of valve spring  44  from the second position to the first position. Therefore, as mentioned before, the oil in first control oil chamber  31  is discharged, the urging force T 1  due to the internal pressure of first control oil chamber  31  becomes smaller than the urging force Ts of coil spring  33 , and cam ring  1  is brought again to the state of the greatest eccentricity as shown in  FIG. 7A . 
     Thus, the connection state of first connection port  51  leading to first control oil chamber  31  is changed over repeatedly by the spool valve element  43  between the connection of first connection port  51  with the introduction port  50  though pressure chamber  56  and the connection of first connection port  51  with drain port  54  through inside passage  55 . Therefore, pilot valve  40  adjusts the control pressure P so as to hold the control pressure P at the level of first changeover pressure Pv 1 , and hence the characteristic of control pressure P of oil pump  10  becomes substantially flat (as shown in the interval “b” in  FIG. 6 ). 
     When the engine speed R further increases in the state in which spool valve element  43  of pilot valve  40  is in the second position, as shown in  FIG. 8A , first the solenoid  64  is deenergized, so that the introduction port  67  is connected with the supply/discharge port  68 , and the supply/discharge portion  68  is disconnected from drain port  69 . Since the control pressure P is still lower than second changeover pressure Pv 2 , and hence the spool valve element  43  is held at the first position, the pilot valve  40  is put in the state in which the first connection port  51  is connected with introduction port  50  through pressure chamber  56  and the second connection port  52  is connected with supply/discharge port  53  through relay chamber  57 . Therefore, the control pressure Px reduced by the throttle V formed by first land portion  43   a  is supplied to first control oil chamber  31 , and the control pressure P is supplied through second introduction passage  8   b  to second control oil chamber  32 . Therefore, the urging force Tm in the eccentric direction resulting from the urging force Ts of coil spring  33  and the urging force T 2  of the internal pressure in second control oil chamber  32  becomes greater than the urging force T 1  of the internal pressure in first control oil chamber  31  in the concentric direction. Consequently the cam ring  15  is pushed back in the eccentric direction and the control pressure increases again with a greater rate (the interval “c” in  FIG. 6 ). 
     When the control pressure P increases with this increasing characteristic and reaches the second changeover pressure Pv 2  (shown in  FIG. 6 ), then, as shown in  FIG. 8B , the solenoid  64  remains deenergized, and the spool valve element  43  in pilot valve moves toward plug  42  by the control pressure P introduced into pressure chamber  56  through introduction port  50 , against the urging force of valve spring  44 , and thereby moves from the second position to the third position. Therefore, the first connection port  51  is connected through a sufficiently wide opening, with introduction port  50  via pressure chamber  56 , and the second connection port  52  is disconnected from relay chamber  57  by second land portion  43   b  and instead connected through inside passage  55  with drain port  54 . As a result, the oil pressure is supplied sufficiently to first control oil chamber  31  and the oil is drained from second control oil chamber  32  through inside passage  55  and drain port  54  to oil pan T, so that the hydraulic pressure is applied only in first control oil chamber  31 . Therefore, the urging force T 1  by the internal pressure in first control oil chamber  31  in the concentric direction exceeds the urging force Ts of coil spring  33  in the eccentric direction, and cam ring  15  moves in the concentric direction. 
     With this movement of cam ring  15  in the concentric direction, the control pressure P is decreased by the decrease of the eccentricity of cam ring  15 , and the control pressure P becomes lower than second changeover pressure Pv 2 . As a result, spool valve element  43  is pushed back by the urging force of valve spring  44  from the third position to the second position. Therefore, as mentioned before, the control pressure P is supplied again into second control oil chamber  32 . Therefore, the urging force Tm in the eccentric direction resulting from urging force Ts of coil spring  33  and urging force T 2  of the internal pressure in second control oil chamber  32  becomes greater than the urging force T 1  of the internal pressure in first control oil chamber  31  in the concentric direction. Consequently the cam ring  15  is pushed back in the eccentric direction ( FIG. 8A ) and the control pressure P increases with a greater rate. 
     Thus, the connection state of second connection port  52  leading to second control oil chamber  32  is changed over repeatedly by the spool valve element  43  between the connection of second connection port  52  with the supply/discharge port  53  (introduction port  67 ) through relay chamber  57  and the connection of second connection port  52  with drain port  54  through inside Passage  55 . Therefore, pilot valve  40  adjusts the control pressure P so as to hold the control pressure P at the level of second changeover pressure Pv 2 , and hence the characteristic of control pressure P of oil pump  10  becomes substantially flat (as shown in the interval “d” in  FIG. 6 ). 
     In the earlier technology, in the swing motion control of the cam ring, no consideration is given to a decrease of the internal pressures in the pumping chambers PR due to aeration or involvement of air voids in the oil sucked into the pumping chambers PR. Therefore the air voids mixed in the oil during the suction causes a decrease of the modulus of volume elasticity of the oil and causes the oil to have compressibility. Consequently, in the compression process in the discharge region following the expansion process in the suction region, merely the air voids are compressed in the pumping chambers PR and the internal pressures in the pumping chambers are not increased directly. Accordingly, the urging force TL based on the internal pressures of the pumping chambers PR in the downstream part of the discharge region becomes greater than the urging force TU based on the internal pressures of the pumping chambers PR in the upstream art of the discharge region. 
     This relative increase of the urging force TL acting in the concentric direction, due to the internal pressures of the pumping chambers PR on the downstream side in the discharge region makes the torque Tp in the concentric direction greater than the torque Tm in the eccentric direction. Therefore, the second operation oil pressure Pc 2  is decreased to a value Pc 2 ′, as shown by a one-dot chain line in  FIG. 12 , in comparison with the condition free of the aeration (as shown by a broken line in  FIG. 12 ). Therefore, in the high speed region, the pump might be unable to satisfy the third engine requirement oil pressure Pe 3  or the maximum engine requirement oil pressure. 
     Moreover, though the internal pressure in each pumping chamber PR tends to be increased by a backward flow of the oil pressure from the discharge port  22   a , the pumping chambers PR rotate with their internal pressures remaining low and the lower pressure region expands when the rotational speed is higher in the high engine speed region. As a result, with increase of the engine speed, the concentric direction urging force TL caused by the internal pressures in the pumping chambers PR in the downstream part of the discharge region becomes higher as compared to the eccentric direction urging force TU, and the second operation oil pressure Pc 2  is further decreased. 
     By contrast, in the oil pump  10  according to this embodiment, in consideration of the decrease of the pressure in each pumping chamber PR due to the aeration, the second operation pressure Pc 2  is higher than the second changeover pressure Pv 2  where the second operation pressure Pc 2  is an operation pressure of cam ring  15  in the high pressure region exceeding the third or maximum engine requirement pressure Pe 3 , and the second changeover pressure Pv 2  is an operation pressure of pilot valve  40 . Therefore, the oil pump  10  can attain the third or maximum engine requirement pressure Pe 3  even when the internal pressures in the pumping chambers PR become lower due to the aeration as shown by the one-dot chain line in  FIG. 9 , and hence the discharge pressure (control pressure) is decreased by the decrease of the eccentricity of cam ring  15  due to the decrease of the internal pressures in the pumping chambers PR, as well as when there is no aeration as shown by the broken line in  FIG. 9 . 
     In this way, with the setting of the second operation pressure Pc 2  in the high pressure region exceeding the highest or third engine requirement pressure Pe 3   t , higher than the second changeover pressure Pv 2  of pilot valve  40 , the oil pump  10  according to this embodiment can satisfy the highest or third engine requirement pressure Pe 3  even if the discharge pressure (control pressure) is decreased by aeration, and secure the proper performance of the internal combustion engine. 
     Moreover, the operation pressures Pc 2  and Pv 2  of cam ring  15  and pilot valve  40  can be set by two urging or biasing members in the form of coil spring  33  and valve spring  44 . Therefore, the setting of the relationship between the operation pressures Pc 2  and Pv 2  is easy and advantageous for securing the satisfactory productivity of oil pumps and reducing the production cost. 
     Furthermore, the oil pump  10  of the illustrated embodiment has a two-step characteristic holding the first operation pressure Pc 1  in a predetermined low or lower engine speed region, and holding the second operation pressure Pc 2  higher than the first operation pressure in a predetermined high or higher engine speed region, as to the operation of cam ring  15 , and the oil pump  10  is arranged to satisfy the maximum engine requirement pressure Pe 3  in the high engine speed region. Accordingly, the oil pump  10  can prevent a decrease in the discharge pressure (control pressure) especially in the high rotational speed region in which the operation pressure of cam ring  15  tends to become lower. 
     In this embodiment, the adjustment of the second operation pressure Pc 2  is achieved by adjusting the set loads W 1  and W 2  of coil spring  33  and valve spring  44 . However, the adjustment of the second operation pressure Pc 2  can be achieved by various other means. For example, the adjustment of the second operation pressure Pc 2  can be achieved by adjusting a pressure receiving area difference between the pressure receiving area of first pressure receiving surface  15   c  of first control oil chamber  31  and the pressure receiving area of pressure receiving surface  15   d  of the second control chamber  32 . These pressure receiving areas can be set flexibly in accordance with various parameters such as specification data items of the pump and the vehicle to employ the pump. When the relationship of the operation oil pressure Pc 2  with respect to the operation pressure Pv 2  is adjusted by the pressure receiving area difference between the pressure receiving surfaces  15   c  and  15   d , the desired setting of operation pressure Pc 2  of cam ring  15  can be achieved without the need for changing the set loads W 1  and W 2  of springs  33  and  44 . 
       FIGS. 10 and 11  are views showing pressure-flow characteristics at the time of occurrence of aeration in examples of the variable displacement pump according to the present invention. In each of these views, a solid line represents a characteristic in the state free from aeration, a broken line represents a characteristic in the state suffering aeration, a one-dot chain line is an engine resistance line representing a resistance in the engine. In the example of  FIG. 10 , the second operation pressure Pc 2 ′ at the time of occurrence of aeration is invariably higher than the third engine requirement oil pressure Pe 3 . In the example of  FIG. 11 , the second operation pressure Pc 2 ′ at the time of occurrence of aeration may become lower than or equal to the third engine requirement oil pressure Pe 3 , but the third engine requirement oil pressure Pe 3  can be satisfied because the discharge flow rate is sufficient to afford to satisfy the requirement. The example of  FIG. 11  is included in the purview of the present invention as well as the example of  FIG. 10 . 
     Besides the oil pump  10  in the illustrated example, the present invention is applicable to various other oil pumps having different cam ring control structures. For example, the present invention is applicable to an oil pump having first and second springs  33  and  34  serving as a pair of coil springs for controlling the swing motion of a cam ring, as shown in FIG. 4 of JP2013-130090A (corresponding to US2013/164162A). This figure and related explanation of this patent document are herein incorporated by reference. In the oil pump having the first and second springs  33  and  34 , by adjusting the urging forces of the first and second springs  33  and  34  and the valve spring  44  of the pilot valve and/or adjusting the areas of the pressure receiving surfaces  15   j  and  15   k , it is possible to set the second operation pressure (Pc 2 ) in the higher pressure region higher than the third engine requirement pressure Pe 3 , higher than the second changeover pressure Pv 2  of a changeover control valve  40  in consideration of decrease of the oil pressure in the pumping chambers due to aeration, and thereby to achieve the effects and operations of the present invention as mentioned before. 
     The present invention is not limited to the illustrated examples. Various modifications and variations are possible within the purview of the present invention. For example, the engine requirement oil pressures Pe 1 ˜Pe 3 , the first and second changeover oil pressures Pv 1  and Pv 2 , and the structures and the arrangement of the oil passages of pilot valve  40  and solenoid valve  60  can be modified or varied flexibly in accordance with specification date items or parameters of the internal combustion engine of the vehicle in which the oil pump is installed, and the valve timing control apparatus or other apparatus. 
     In the illustrated example, the variable displacement pump is arranged to vary the discharge quantity by swing motion of the cam ring  15 . However, the varying means or mechanism to vary the discharge quantity is not limited to the means based on the swing motion. For example, the varying means may be configured to increase and decrease the discharge quantity or a pumping volume variation quantity of the pumping chambers PR (or a displacement or amount of fluid pumped per revolution), with rectilinear movement of the movable member or cam ring  15  in the radial direction. The motion of the movable member or the cam ring  15  is not limited to the swing motion. 
     In the illustrated example, the variable displacement oil pump is a variable displacement vane pump employing the cam ring  15  as the movable member to vary the displacement. However, the present invention is not limited to the vane pump. It is possible to employ various other types of the variable displacement oil pump. For example, the variable displacement oil pump according to the present invention may be a trochoid pump. In this case, an outer rotor forming an external gear corresponds to the movable member instead of the cam ring  15 . The outer rotor is disposed in a manner enabling eccentric motion, and there are provided, around the outer rotor, the control oil chamber(s) and spring(s) to vary the position of the movable member. 
     In one of possible interpretations, a variable displacement oil pump according to the present invention comprises a basic structure comprising: a pump or pumping element to vary inside volumes of pumping chambers to suck the oil through a suction portion or suction port and to discharge the oil through a discharge portion or discharge port; a varying mechanism or varying section or means to increase and decrease a pumping volume variation quantity (or displacement or amount of fluid pumped per revolution), with movement of a movable member (such as a cam ring of a vane pump or an outer rotor of a trochoid pump); an urging mechanism or urging section or means to urge the movable member in an increasing direction to increase the pumping volume variation quantity (such as an eccentric direction increasing the eccentricity of the cam ring); a housing member or housing section or means ( 11 ,  30   a ,  30   b ) to define a first control oil chamber to receive the oil discharged from the discharge portion and thereby to produce an urging force to urge the movable member in a decreasing direction to decrease the pumping volume variation quantity (such as a concentric direction decreasing the eccentricity of the cam ring), and a second control oil chamber to receive the oil discharged from the discharge portion and thereby to produce an urging force to urge the movable member in a direction to vary the pumping volume variation quantity; and a pressure control section or mechanism or means to control at least one of pressures in the first and second control oil chambers. The variable displacement oil pump according to the present invention may have any one or more of following features. 
     First feature; an operation oil pressure of the movable member is set higher than an operation oil pressure of the pressure control section at least in a predetermined operating region. Second feature; an operation oil pressure of a cam ring included in the varying mechanism is set to satisfy a maximum or highest engine requirement oil pressure required by the internal combustion engine in consideration of resistance in the internal combustion engine. Third feature; the pressure control section is configured to control the pressures in the first and second control oil chambers to hold the discharge pressure of the oil pump at a predetermined higher pressure level (Pc 2 , for example) in a predetermined first engine operating region (such as a predetermined higher engine speed region). Fourth feature; the pressure control section is configured to control the pressures in the first and second control oil chambers to hold the discharge pressure of the oil pump at a predetermined lower pressure level in a predetermined second engine operating region (such as a predetermined lower engine speed region). 
     Fifth feature; the pressure control section is arranged to receive, as a control pressure (P), the discharge pressure of the oil pump through an introduction section or an introduction passage, and to assume an operative state (or fourth) state (such as the state shown in  FIG. 8B ) to supply the control pressure to the first control oil chamber (or to supply the control pressure only to the first control oil chamber and drain the second control oil chamber) (to urge the movable member in the decreasing direction) when the control pressure (P) becomes equal to or higher than a predetermined changeover pressure (such as Pv 2 ) (or the operation oil pressure of the pressure control section) and an inoperative state (or third) state (such as the state shown in  FIG. 8A ) to supply the control pressure to the first control oil chamber (through a limited opening or throttled opening V in the illustrated example) and to supply the control pressure to the second control oil chamber (to urge the movable member in the increasing direction) when the control pressure (P) is lower than the predetermined changeover pressure (such as Pv 2 ). Sixth feature; the movable member (such as cam ring  15 ) is arranged to move from an inoperative position (such as the position of cam ring  15  shown in  FIG. 8A  or  FIG. 7A ) to an operative position (such as the position of cam ring  15  shown in  FIG. 8B  or  FIG. 7B ) when the control pressure (P) supplied into the first control oil chamber becomes higher than a predetermined operation pressure (such as Pc 2 )(or the operation oil pressure of the movable member) which is set higher than the predetermined changeover pressure (such as Pv 2 ). Seventh feature; the movable member is arranged to be held in an inoperative position (such as the position of cam ring  15  shown in  FIG. 8A  or  FIG. 7A ) (without moving to an operative position (such as the position of cam ring  15  shown in  FIG. 8B )) when the control pressure is higher than or equal to the changeover pressure (Pv 2 ) but lower than the predetermined operation pressure (Pc 2 ). 
     Eighth feature; the pressure control section is arranged to alternate between the inoperative state and the operative state to hold the discharge pressure at the predetermined operation pressure (such as Pc 2 ) by moving the movable member between the inoperative position and the operative position, to hold the discharge pressure at a predetermined pressure level. Ninth feature; the pressure control section is arranged to assume a second state or operative state (such as the state shown in  FIG. 7B ) to supply the control pressure to the first control oil chamber when the control pressure (P) becomes equal to or higher than a predetermined changeover pressure (such as Pv 1 ) and a first state or inoperative state (such as the state shown in  FIG. 7A ) to supply the control pressure neither to the first control oil chamber nor to the second control oil chamber when the control pressure (P) is lower than the predetermined changeover pressure (such as Pv 1  lower than Pv 2 ). Tenth feature; the movable member (such as cam ring  15 ) is arranged to move from the inoperative position (such as the position shown in  FIG. 7A ) to the operative position (such as the position shown in  FIG. 7B ) when the control pressure (P) supplied into the first control oil chamber becomes higher than a predetermined operation pressure (Pc 1  lower than Pc 2 ) (which is set lower than the predetermined changeover pressure (such as Pv 1  in the example shown in  FIG. 6 ). Eleventh feature; the pressure control section is arranged to alternate between the first state and the second state to hold the discharge pressure at the predetermined changeover pressure (such as Pv 1 ) by moving the movable member between the inoperative position and the operative position. Twelfth feature; the pressure control section includes a first section (which may include a control or pilot valve ( 40 )) to control the supply and drainage of the oil to and from the first and second control chambers, respectively, and a second section (such as a solenoid valve and/or an electronic control unit) configured to control the oil pressures in the first and second control oil chambers in a higher pressure mode ( FIGS. 8A and 8B , intervals “c” and “d” in  FIG. 6 ) or a lower pressure mode ( FIGS. 7A and 7B , intervals “a” and “b”). Thirteenth feature; the pressure control section may include a pilot valve and a solenoid valve. The pressure control section may further include a control unit to set the pressure control section in a first mode (solenoid on) and a second mode (solenoid off) in accordance with an operating condition of the engine. 
     This application is based on a prior Japanese Patent Application No. 2014-255685 filed on Dec. 18, 2014. The entire contents of this Japanese Patent Application are hereby incorporated by reference. 
     Although the invention has been described above by reference to certain embodiments of the invention, the invention is not limited to the embodiments described above. Modifications and variations of the embodiments described above will occur to those skilled in the art in light of the above teachings. The scope of the invention is defined with reference to the following claims.