Abstract:
An oil pump includes a relief valve. The relief valve adjusts a pump discharge pressure depending on a differential pressure between the pump discharge pressure and a back pressure chamber pressure. An electromagnetic switching valve includes a movable member moved electromagnetically. The movable member brings a ball valve element selectively into a state pressed onto a valve seat portion, to selectively close and open an oil passage. The electromagnetic switching valve selectively supplies the pump discharge pressure to a back pressure chamber of the relief valve and drains the back pressure chamber of the relief valve by selectively opening and closing the oil passage. A pilot valve is disposed downstream of the electromagnetic switching valve, and operates upon receipt of the pump discharge pressure or the back pressure chamber pressure, and controls an oil pressure supplied to the back pressure chamber of the relief valve.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to an oil pump for an internal combustion engine, and a relief pressure control apparatus for such an oil pump. For example, the oil pump is configured to supply oil to sliding parts and others of the internal combustion engine which is mounted on an automotive vehicle. 
         [0002]    International Application Publication WO 2012/100344 A1 discloses a relief pressure control apparatus for an oil pump for an internal combustion engine, wherein the relief pressure control apparatus is configured to control a pump discharge pressure to a suitable level by relief, when the pump discharge pressure becomes excessively high, for example, when the oil pump is operating at high speed. This relief pressure control apparatus includes a relief valve and a pilot valve. The relief valve is configured to control the pump discharge pressure outputted by the oil pump. The pilot valve is configured to control an internal pressure of a back pressure chamber of the relief valve via a directional switching valve, with receipt of the pump discharge pressure as a pilot signal. The relief valve includes: a housing including a valve accommodation chamber; a valve element slidably mounted in the valve accommodation chamber; and a relief spring configured to bias the valve element in one direction. The pilot valve includes: a supply port for supply the introduced pump discharge pressure to the directional switching valve; and a drain port for draining an oil pressure supplied from the directional switching valve to the supply port. The pilot valve is configured to selectively open and close the drain port depending on the oil pressure. The directional switching valve is configured to implement switching between fluid passages in an on-off manner by moving a valve element forward or rearward, to obtain two different levels of relief pressure. 
       SUMMARY OF THE INVENTION 
       [0003]    The relief pressure control apparatus disclosed in International Application Publication WO 2012/100344 may be confronted by the following problem. In the directional switching valve, both of the oil pressure introduced from the pilot valve and the oil pressure returned from the back pressure chamber are communicated in opposite directions. Accordingly, in a case that the valve element is of a ball-type, when the ball valve element is free from pressing action of a push rod, behavior of the ball valve element may become unstable so that the passage between the supply port and the drain port cannot be correctly opened and closed. 
         [0004]    In view of the foregoing, it is desirable to provide an oil pump with a relief pressure control apparatus, which is capable of precisely controlling a relief pressure with stabilized behavior of a valve element of an electromagnetic switching valve even if the valve element is of a ball-type. 
         [0005]    According to one aspect of the invention, an oil pump for an internal combustion engine, comprises: a relief valve including a back pressure chamber, and configured to adjust a pump discharge pressure depending on a differential pressure between the pump discharge pressure and a back pressure chamber pressure, wherein the pump discharge pressure is a discharge pressure of the oil pump, and wherein the back pressure chamber pressure is an oil pressure in the back pressure chamber; an electromagnetic switching valve including: an oil passage; a valve seat portion; a ball valve element; and a movable member configured to be moved electromagnetically, and bring the ball valve element selectively into a state pressed onto the valve seat portion and into a state non-pressed onto the valve seat portion, to selectively close and open the oil passage; wherein the electromagnetic switching valve is configured to selectively supply the pump discharge pressure to the back pressure chamber of the relief valve and drain the back pressure chamber of the relief valve by selectively opening and closing the oil passage; and a pilot valve disposed downstream of the electromagnetic switching valve, and configured to operate upon receipt of at least one of the pump discharge pressure and the back pressure chamber pressure, and control an oil pressure supplied to the back pressure chamber of the relief valve. The oil pump may be configured so that: the electromagnetic switching valve is configured to control the back pressure chamber pressure by selectively allowing and preventing fluid communication between a discharge passage and a back pressure passage by operating the ball valve element depending on an operating state of the internal combustion engine; the pump discharge pressure is applied to the discharge passage; and the back pressure passage includes an end open to the back pressure chamber of the relief valve. The oil pump may be configured so that the pilot valve includes: a supply port configured to receive supply of the pump discharge pressure from the discharge passage directly or through the back pressure passage; a discharge port configured to discharge the supplied pump discharge pressure; a cylinder, wherein the supply port includes an end open to the cylinder, and wherein the discharge port includes an end open to the cylinder; a pilot valve element slidably mounted in the cylinder, and including an end portion including a pressure-receiving surface configured to receive the supplied pump discharge pressure, and configured to vary an opening area of the discharge port by sliding in accordance with the supplied pump discharge pressure; and a biasing member configured to bias the pilot valve element in a direction to close the discharge port. The oil pump may be configured so that the pilot valve is configured to vary a cross-sectional area of fluid communication between the supply port and the discharge port by sliding of the pilot valve element. The oil pump may be configured so that the pilot valve is configured to vary a cross-sectional area of fluid communication between the back pressure passage and the discharge port by sliding of the pilot valve element. The oil pump may further comprise an oil filter disposed in a discharge passage and upstream of the relief valve and the electromagnetic switching valve. 
         [0006]    According to another aspect of the invention, a relief pressure control apparatus for an oil pump, comprises: a relief valve including: a valve accommodation chamber, wherein an introduction port includes an end open to the valve accommodation chamber, and wherein a drain port includes an end open to the valve accommodation chamber, and wherein the introduction port is configured to introduce a pump discharge pressure through a discharge passage, and wherein the pump discharge pressure is a discharge pressure of the oil pump, and wherein the drain port is configured to drain the introduced pump discharge pressure; a relief valve element slidably mounted in the valve accommodation chamber, and including a first end side configured to receive the pump discharge pressure through the introduction port, and configured to vary an opening area of the drain port by sliding in accordance with the received pump discharge pressure; a biasing member configured to bias the relief valve element in a direction to close the drain port; a back pressure chamber formed in the valve accommodation chamber at a second end side of the relief valve element; and a back pressure passage including an end open to the back pressure chamber; an electromagnetic switching valve hydraulically connected between the discharge passage and the back pressure passage, the electromagnetic switching valve including: an open-ended hole configured to receive supply of the pump discharge pressure from the discharge passage; a back pressure hole configured to supply the pump discharge pressure from the open-ended hole to the back pressure passage; a ball valve element configured to selectively open and close the open-ended hole; and a solenoid configured to operate the ball valve element; and a pilot valve hydraulically connected between the electromagnetic switching valve and the back pressure chamber of the relief valve, and configured to operate upon receipt of at least one of the pump discharge pressure and a back pressure chamber pressure, wherein the back pressure chamber pressure is an oil pressure in the back pressure chamber of the relief valve, and control an oil pressure supplied to the back pressure chamber of the relief valve. The relief pressure control apparatus may be configured so that the electromagnetic switching valve is configured to control the back pressure chamber pressure by selectively allowing and preventing fluid communication between the discharge passage and the back pressure hole by operating the ball valve element depending on an operating state of an internal combustion engine. The relief pressure control apparatus may be configured so that the pilot valve includes: a supply port configured to receive supply of the pump discharge pressure from the discharge passage directly or through the back pressure passage; a discharge port configured to discharge the supplied pump discharge pressure; a cylinder, wherein the supply port includes an end open to the cylinder, and wherein the discharge port includes an end open to the cylinder; a pilot valve element slidably mounted in the cylinder, and including an end portion including a pressure-receiving surface configured to receive the supplied pump discharge pressure, and configured to vary an opening area of the discharge port by sliding in accordance with the supplied pump discharge pressure; and a biasing member configured to bias the pilot valve element in a direction to close the discharge port. The relief pressure control apparatus may be configured so that the pilot valve includes: a supply port configured to receive supply of the pump discharge pressure from the discharge passage directly or through the back pressure passage; a branch port hydraulically connected to the back pressure passage; a discharge port configured to discharge an oil pressure supplied from the branch port; a cylinder, wherein the supply port includes an end open to the cylinder, and wherein the branch port includes an end open to the cylinder, and wherein the discharge port includes an end open to the cylinder; a pilot valve element slidably mounted in the cylinder, and including an end portion including a pressure-receiving surface configured to receive the supplied pump discharge pressure, and configured to vary a cross-sectional area of fluid communication between the branch port and the discharge port by sliding in accordance with the supplied pump discharge pressure; and a biasing member configured to bias the pilot valve element in a direction to close the discharge port. The relief pressure control apparatus may be configured so that: the relief valve element has a cylindrical shape including a bottom including a pressure-receiving surface at a longitudinal end of the relief valve element; and the pressure-receiving surface is applied with the introduced pump discharge pressure. The relief pressure control apparatus may be configured so that: the relief valve element has a cylindrical shape including a bottom; the valve accommodation chamber has a cylindrical shape; the open end of the introduction port is located at a longitudinal end portion of the valve accommodation chamber; and the open end of the drain port and the open end of the back pressure passage are located at a peripheral portion of the valve accommodation chamber, and arranged in a longitudinal direction of the valve accommodation chamber, and formed to extend in a radial direction of the valve accommodation chamber. The relief pressure control apparatus may be configured so that: the pilot valve includes: a supply port configured to receive supply of the pump discharge pressure from the discharge passage directly or through the back pressure passage; a branch port hydraulically connected to the back pressure passage; a discharge port configured to discharge an oil pressure supplied from the branch port; a cylinder, wherein the supply port includes an end open to the cylinder, and wherein the branch port includes an end open to the cylinder, and wherein the discharge port includes an end open to the cylinder; a pilot valve element slidably mounted in the cylinder, and including an end portion including a pressure-receiving surface configured to receive the supplied pump discharge pressure, and configured to vary a cross-sectional area of fluid communication between the branch port and the discharge port by sliding in accordance with the supplied pump discharge pressure; and a biasing member configured to bias the pilot valve element in a direction to close the discharge port; the pilot valve element has a cylindrical shape including a bottom; the cylinder has a cylindrical shape; the open end of the supply port is located at a longitudinal end portion of the cylinder; and the open end of the discharge port and the open end of the branch port are arranged in a longitudinal direction of the cylinder, and formed to extend in a radial direction of the cylinder. The relief pressure control apparatus may be configured so that: the electromagnetic switching valve includes an accommodation part therein; the accommodation part is disposed at a distal end portion of the electromagnetic switching valve, and has a cylindrical shape, and accommodates the ball valve element movably; the open-ended hole is formed in a distal end of the accommodation part in a longitudinal direction of the electromagnetic switching valve; the back pressure hole is disposed at a peripheral portion of the accommodation part, and extends in a radial direction of the accommodation part; the solenoid includes a push rod inserted in the accommodation part; and the push rod is configured to cause the ball valve element to move forward and rearward selectively to selectively close and open the open-ended hole. The relief pressure control apparatus may be configured so that the electromagnetic switching valve is configured to maintain the open-ended hole closed by the ball valve element at start of the oil pump. The relief pressure control apparatus may be configured so that: energization of the solenoid of the electromagnetic switching valve is controlled based on parameters of an internal combustion engine; and the parameters include an oil temperature, a water temperature, a rotational speed, and a load. The relief pressure control apparatus may be configured so that: the electromagnetic switching valve includes a drain hole; the drain hole is located at a portion of the accommodation part opposite to the open-ended hole; and the drain hole is configured to allow fluid communication between the back pressure passage and an outside through the back pressure hole, when the open-ended hole is closed by the ball valve element. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is a schematic diagram showing an oil pump system including an oil pump and a relief pressure control apparatus according to a first embodiment of the present invention. 
           [0008]      FIG. 2  is a schematic diagram showing the oil pump system of  FIG. 1  in a state where an electromagnetic switching valve is energized. 
           [0009]      FIG. 3  is a schematic diagram showing the oil pump system of  FIG. 1  in a state where the electromagnetic switching valve is de-energized. 
           [0010]      FIG. 4  is a schematic diagram showing the oil pump system of  FIG. 1  in a state where the electromagnetic switching valve is held de-energized. 
           [0011]      FIG. 5  is a characteristic diagram showing a relationship between a pump discharge pressure and a pump rotational speed (or engine rotational speed) in the oil system of  FIG. 1 . 
           [0012]      FIG. 6  is a schematic diagram showing an oil pump system including an oil pump and a relief pressure control apparatus according to a second embodiment of the present invention. 
           [0013]      FIG. 7  is a schematic diagram showing the oil pump system of  FIG. 6  in a state where an electromagnetic switching valve is energized. 
           [0014]      FIG. 8  is a schematic diagram showing the oil pump system of  FIG. 6  in a state where the electromagnetic switching valve is de-energized. 
           [0015]      FIG. 9  is a schematic diagram showing the oil pump system of  FIG. 6  in a state where the electromagnetic switching valve is held de-energized. 
           [0016]      FIG. 10  is a characteristic diagram showing a relationship between a pump discharge pressure and a pump rotational speed (or engine rotational speed) in the oil system of  FIG. 6 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0017]    In each present embodiment, an oil pump is embodied as one serving as a power source for a variable timing control (VTC) device for varying opening and closing timings of engine valves of an internal combustion engine of an automotive vehicle, or serving to supply lubricating oil to sliding parts of the internal combustion engine, specifically to supply lubricating oil to sliding part between each piston and the corresponding cylinder bore by oil jet, or serving to supply lubricating oil to bearings of a crankshaft of the internal combustion engine. 
       First Embodiment 
       [0018]    As shown in  FIGS. 1 to 4 , an oil pump system including an oil pump  1  of a trochoid type and a relief pressure control apparatus according to a first embodiment of the present invention includes a relief valve  3 , an electromagnetic switching valve  4 , and a pilot valve  5 . Relief valve  3  is disposed in a branch passage  2  branched from a discharge passage  1   a  of oil pump  1 , and is configured to adjust a pump discharge pressure of oil pump  1 . Electromagnetic switching valve  4  is configured to be controlled to selectively supply the pump discharge pressure to a back pressure chamber  10  of relief valve  3 , and prevent the pump discharge pressure from being supplied to back pressure chamber  10 . Pilot valve  5  is hydraulically connected between back pressure chamber  10  of relief valve  3  and electromagnetic switching valve  4 , and configured to receive supply of the pump discharge pressure via the electromagnetic switching valve  4 , and control an oil pressure supplied to back pressure chamber  10 . 
         [0019]    Oil pump  1  includes a housing, a pump shaft, an inner rotor, and an outer rotor. The housing is attached to a cylinder block  01  of an internal combustion engine. The pump shaft is rotatably inserted in the housing, and configured to receive input of a torque from a crankshaft of the internal combustion engine. The inner rotor is fixed to an outer periphery of the pump shaft by press-fitting or the like, and includes external teeth of a trochoid type. The outer rotor is disposed outside of the inner rotor, and meshes with the inner rotor by internal teeth of a trochoid type, wherein the number of the teeth of the outer rotor is greater by one than that of the inner rotor. 
         [0020]    Oil pump  1  is configured to allow the pump shaft to be rotated by rotational drive of the crankshaft, and thereby suck lubricating oil from an oil pan  6  through a suction passage  1   b  and an internal suction port, and discharge the lubricating oil through an internal discharge port to discharge passage  1   a . The lubricating oil is then supplied through a main oil gallery  27  to sliding parts of the internal combustion engine and the VTC device and others. An oil filter  28  is disposed at a portion of main oil gallery  27  dose to discharge passage  1   a.    
         [0021]    Relief valve  3  includes a valve accommodation chamber  8 , a relief valve element  9 , a back pressure chamber  10 , and a valve spring  11 . Valve accommodation chamber  8  is formed in a casing  7 , and has a cylindrical shape whose bottom opening is closed by a plug  8   a . Relief valve element  9  is slidably mounted in valve accommodation chamber  8 . Back pressure chamber  10  is defined in a lower part of the internal space of valve accommodation chamber  8 . Valve spring  11  is mounted in back pressure chamber  10 , and is configured to serve as a biasing member to bias the relief valve element  9  upward away from plug  8   a.    
         [0022]    Valve accommodation chamber  8  is formed with an introduction port  12 , a relief port  13 , and a back pressure port  14 . Introduction port  12  is located at an upper end portion of valve accommodation chamber  8 , and is hydraulically connected to a downstream end of a feedback passage  2   a  branched from branch passage  2 . Relief port  13  is located at a peripheral portion of valve accommodation chamber  8  and closer to the upper end portion of valve accommodation chamber  8 , and is hydraulically connected to an upstream end of a relief passage  1   c  hydraulically connected to suction passage  1   b . Back pressure port  14  is located at a lower end portion of valve accommodation chamber  8 , and is hydraulically connected to back pressure chamber  10 . 
         [0023]    Relief valve element  9  has a cylindrical shape including a bottom. Specifically, relief valve element  9  includes an upper wall  9   a  having a disc-shape. The upper wall  9   a  includes an upper surface including a pressure-receiving surface  9   b  configured to receive the pump discharge pressure through the introduction port  12 . As a peripheral wall  9   c  of relief valve element  9  having a cylindrical shape slides on an inner peripheral surface of valve accommodation chamber  8 , the opening area of relief port  13  changes. 
         [0024]    Valve spring  11  is configured to constantly bias the relief valve element  9  upward by an elastic force, and thereby bias the relief valve element  9  in a direction to close the open end of relief port  13  by the outer peripheral surface of peripheral wall  9   c.    
         [0025]    Back pressure port  14  has an upstream end hydraulically connected to a downstream end of back pressure passage  15  hydraulically connected to electromagnetic switching valve  4 , and is constantly open to back pressure chamber  10  even when relief valve element  9  is moved downward maximally. 
         [0026]    Electromagnetic switching valve  4  includes a valve body  16 , a solenoid part  17 , and a ball valve element  19 . Valve body  16  has a cylindrical shape, and is inserted and fixed in a valve hole  02  formed in cylinder block  01 . Solenoid part  17  is disposed at a rear end of valve body  16 . Ball valve element  19  is movably mounted in a valve accommodation part  18  formed in valve body  16  and at a distal end of valve body  16 . 
         [0027]    Valve body  16  is hermetically sealed and held in valve hole  02  by a seal ring  20  that is fitted and fixed to an outer periphery of a rear end portion of valve body  16 . Valve body  16  is formed with a rod insertion hole  21  that extends in the longitudinal direction of valve body  16 , and has a distal end facing the valve accommodation part  18 . The distal end portion of valve body  16  is formed with an open-ended hole  22  which allows fluid communication between valve accommodation part  18  and branch port  2   b  of branch passage  2 , and extends through in the longitudinal direction of valve body  16 . The open-ended hole  22  has a stepped shape having a small diameter portion inside thereof, and the periphery of open-ended hole  22  serves as an annular seat portion configured to be in contact or out of contact with ball valve element  19 . 
         [0028]    The peripheral wall of the distal end portion of valve body  16  is formed with a plurality of back pressure holes  23  that extend radially of valve body  16 , and allow fluid communication between valve accommodation part  18  and back pressure passage  15  through an annular groove  23   a.    
         [0029]    Valve body  16  is formed with a plurality of drain holes  25 , wherein each drain hole  25  extends radially of valve body  16 , and is hydraulically connected to valve accommodation part  18  via a tubular passage  24  formed in part of rod insertion hole  21 . Each drain hole  25  is hydraulically connected to oil pan  6  via a groove  25   a.    
         [0030]    Valve accommodation part  18  has a cylindrical shape having a small diameter and extending in the longitudinal direction of valve body  16 , and is configured to allow ball valve element  19  to move in valve accommodation part  18  in the longitudinal direction of valve accommodation part  18 . 
         [0031]    Solenoid part  17  includes a cylindrical case  17   a  which accommodates an electromagnetic coil, a fixed iron core, a movable plunger, and other components. The movable plunger has a distal end fixed to a proximal end of push rod  26  that is a movable member configured to slide in rod insertion hole  21 . Push rod  26  has a distal end pressed onto ball valve element  19  in the longitudinal direction, or released from the pressing. 
         [0032]    When the electromagnetic coil of electromagnetic switching valve  4  is energized (or turned on) by a control unit not shown, push rod  26  is caused by the movable plunger to move forward to press the ball valve element  19 . This action causes ball valve element  19  to be in pressing contact with the annular seat portion, and thereby close the open-ended hole  22 , and simultaneously allows fluid communication between back pressure holes  23  and drain hole  25  through tubular passage  24 . 
         [0033]    On the other hand, when the energization to the electromagnetic coil of electromagnetic switching valve  4  is cut off, the movable plunger causes ball valve element  19  to move rearward by a return spring mounted in the movable plunger. This action causes ball valve element  19  to open the open-ended hole  22 , and simultaneously close the end of tubular passage  24  by the pump discharge pressure introduced through the open-ended hole  22 . Accordingly, fluid communication between open-ended hole  22  and back pressure holes  23  is allowed through the valve accommodation part  18 , whereas fluid communication between valve accommodation part  18  and drain hole  25  is prevented. 
         [0034]    The control unit is configured to selectively energize and de-energize the electromagnetic coil of electromagnetic switching valve  4  based on a sensed state of operation of the internal combustion engine which includes parameters including an oil temperature, a water temperature, an engine rotational speed, an engine load, etc. In the present example, the energization/de-energization of the electromagnetic coil is controlled mainly based on the engine rotational speed. 
         [0035]    Pilot valve  5  has a similar configuration as relief valve  3 . Specifically, pilot valve  5  includes a cylinder  30 , a pilot valve element  31 , and a valve spring  32 . Cylinder  30  is formed in casing  7 , and has a cylindrical shape whose bottom opening is closed by a plug  30   a . Pilot valve element  31  is slidably mounted in cylinder  30 . Valve spring  32  is mounted in cylinder  30 , and is configured to serve as a biasing member to bias the pilot valve element  31  upward away from plug  30   a.    
         [0036]    Cylinder  30  is formed with a supply port  33 , a discharge port  34 , and a breather port  35 . Supply port  33  is located at a top end portion of cylinder  30 , and is hydraulically connected to a downstream end of a signal passage  29  branched from back pressure passage  15 . Discharge port  34  is located at a peripheral portion of an upper side portion of cylinder  30 , and is hydraulically connected to suction passage  1   b . Breather port  35  is located at a peripheral portion of a lower side portion of cylinder  30 , and is constantly hydraulically connected to the outside air, to ensure smooth sliding of pilot valve element  31 . Alternatively, breather port  35  may be formed in plug  30   a.    
         [0037]    Pilot valve element  31  has a cylindrical shape including a bottom. Specifically, pilot valve element  31  includes an upper wall  31   a  having a disc shape. The upper wall  31   a  includes an upper surface including a pressure-receiving surface  31   b  configured to receive supply of the pump discharge pressure through the signal passage  29 . As a peripheral wall  31   c  of pilot valve element  31  having a cylindrical shape slides on an inner peripheral surface of cylinder  30 , the opening area of discharge port  34  changes (namely, the cross-sectional area of fluid communication between supply port  33  and discharge port  34  changes). 
         [0038]    Valve spring  32  is configured to bias the pilot valve element  31  upward by an elastic force, and thereby bias the pilot valve element  31  in a direction to close the opening of discharge port  34  by the outer peripheral surface of peripheral wall  31   c . The elastic force of valve spring  32  is set such that when the pump discharge pressure equal to a second oil pressure value P2 as shown in  FIG. 5  is applied from supply port  33  to pilot valve element  31 , pilot valve element  31  is moved to a position to allow fluid communication between supply port  33  and discharge port  34 . 
         [0039]    &lt;Operation of First Embodiment&gt; The following describes operation of the oil pump with the relief pressure control apparatus described above. When the internal combustion engine is being started or operating at low speed, oil pump  1  is rotated by the internal combustion engine, and the electromagnetic coil of electromagnetic switching valve  4  is energized by the control unit. Accordingly, as shown in  FIG. 1 , push rod  26  is caused to press ball valve element  19  into pressing contact with the annular seat portion to close the open-ended hole  22 , and allow fluid communication between back pressure passage  15  and drain hole  25  through back pressure holes  23  and tubular passage  24 . By this action, the internal pressure of back pressure chamber  10  of relief valve  3  is lowered, so that relief valve element  9  is biased toward the maximum upward position in valve accommodation chamber  8  only by the elastic force of valve spring  11 , and the upper wall  9   a  is brought into pressing contact with a stepped annular surface of valve accommodation chamber  8 , and the relief port  13  is closed by the peripheral wall  9   c  of relief valve element  9 . 
         [0040]    When the engine rotational speed has risen with continuation of the energization of the electromagnetic coil so that the pump discharge pressure has reached a first oil pressure value P1, the pump discharge pressure applied to the pressure-receiving surface  9   b  of relief valve element  9  has become high and thereby cause relief valve element  9  to move downward by a certain amount against the elastic force of valve spring  11 . Under this condition, the outer peripheral surface of peripheral wall  9   c  of relief valve element  9  gradually increases the opening area of relief port  13  from zero toward the maximum position. By this action, an excess amount of working oil flowing from discharge passage  1   a  to branch passage  2  is returned to suction passage  1   b  through feedback passage  2   a , introduction port  12  and valve accommodation chamber  8  and relief port  13  of relief valve  3 , and relief passage  1   c . Accordingly, under that condition, as indicated by a broken line in  FIG. 5 , the pump discharge pressure is prevented from rising significantly above the first oil pressure value P1, so that the pump discharge pressure is suitably regulated to a substantially flat characteristic. The first oil pressure value P1 is set to satisfy a requested value used to drive the VTC device. 
         [0041]    When the internal combustion engine shifts thereafter into a state where high oil pressure is requested with increase of the engine speed and oil temperature, the control unit senses this condition, and then de-energizes the electromagnetic coil of electromagnetic switching valve  4 . By this action, as shown in  FIG. 3 , push rod  26  is caused to move rearward along with the movable plunger, to release the pressing contact between ball valve element  19  and the valve seat portion, so that the open-ended hole  22  is opened by ball valve element  19 , to allow fluid communication between branch passage  2  and back pressure passage  15 , and close the tubular passage  24 . Accordingly, the pump discharge pressure is supplied to back pressure chamber  10  of relief valve  3  through branch passage  2  and back pressure passage  15 , so that the internal pressure of back pressure chamber  10  becomes high. On the other hand, the pump discharge pressure is applied to pressure-receiving surface  9   b  of relief valve element  9  through feedback passage  2   a  and introduction port  12 . In this way, relief valve element  9  is applied with substantially identical oil pressures on upper and lower sides. As a result, relief valve element  9  is moved upward by the elastic force of valve spring  11 , so that the outer peripheral surface of peripheral wall  9   c  of relief valve element  9  closes the open end of relief port  13 , and the pressure-receiving surface  9   b  of upper wall  9   a  of relief valve element  9  closes the open end of introduction port  12 . 
         [0042]    When the engine speed further rises from that condition, the high pump discharge pressure is supplied to the back pressure chamber  10  with the relief valve element  9  maintained at the maximum top position shown in  FIG. 3 , so that the pump discharge pressure exceeds the first oil pressure value P1 and reaches the second oil pressure value P2 as shown in  FIG. 5 . 
         [0043]    When the engine speed further rises and the pump discharge pressure exceeds the second oil pressure value P2, the pump discharge pressure is applied to pressure-receiving surface  31   b  of pilot valve element  31  through signal passage  29  and supply port  33  of pilot valve  5 , to cause pilot valve element  31  to move downward against the elastic force of valve spring  32 . This action allows fluid communication between supply port  33  and discharge port  34 , and thereby causes the working oil to be drained from back pressure passage  15  to the outside, and thereby adjusts the internal pressure of back pressure chamber  10  of relief valve  3 . When discharge port  34  is closed by pilot valve element  31  by the elastic force of valve spring  32 , the oil pressures in the upstream side and downstream side of electromagnetic switching valve  4  become equal to each other. However, when discharge port  34  is opened by pilot valve element  31  by the pump discharge pressure at the second oil pressure value P2, the oil pressure is drained from discharge port  34  so that the oil pressure in back pressure passage  15  is reduced by a pressure loss due to a flow resistance in electromagnetic switching valve  4 , for example, at open-ended hole  22 . The amount of oil drained from discharge port  34  is small so that the gradient of rise of the pump discharge pressure with respect to rise of the rotational speed is slightly reduced or substantially equal to the gradient to the second oil pressure value P2. 
         [0044]    In this way, pilot valve  5  adjusts the oil pressure in a portion of the back pressure passage  15  downstream of electromagnetic switching valve  4  (identical to the back pressure of relief valve  3 ) to the second oil pressure value P2. Accordingly, this oil pressure is applied in addition to the elastic force of valve spring  11  of relief valve  3 , so that when the pump discharge pressure at open-ended hole  22  reaches a third oil pressure value P3 (=P1+P2), the oil pump system is in the state shown in  FIG. 4 . As a result, the pump discharge pressure has a characteristic that the pump discharge pressure slightly ascends from the point of P3 in  FIG. 5 . 
         [0045]    The third oil pressure value P3 is set to obtain an oil pressure for lubrication of the crankshaft when the engine is operating at high speed, or an oil pressure for injecting an oil jet to the engine piston. 
         [0046]    In the present embodiment, the first oil pressure value P1 that is a valve open pressure of relief valve  3  and the second oil pressure value P2 that is a valve open pressure of pilot valve  5  can be changed only by changing the properties of valve spring  11  and valve spring  32 . Accordingly, even if the first oil pressure value P1 and second oil pressure value P2 are to be set depending on specifications of the engine, the components of the oil pump system except valve spring  11  and valve spring  32  can be used for different engines. This serves to reduce the manufacturing cost. 
         [0047]    When the energization of electromagnetic switching valve  4  is switched from on-state to off-state while the engine speed is increasing, the oil pump system shifts from the characteristic indicated by the broken line to the characteristic indicated by the solid line as indicated by a short dashed long dashed line in  FIG. 5 . 
         [0048]    In this way, according to the present embodiment, it is possible to precisely control the relief pressure of oil pump  1  by exclusive open/close control of open-ended hole  22  and the open end of tubular passage  24  by ball valve element  19  by outputting the on-signal and off-signal from the control unit to electromagnetic switching valve  4  depending on the engine speed. 
         [0049]    Especially, the flow of lubricating oil (pump discharge pressure) through the ball valve element  19  is unidirectional from branch passage  2  to back pressure passage  15  without inverse flow from back pressure passage  15  to branch passage  2 . Namely, no bidirectional flow is caused in electromagnetic switching valve  4 . This serves to stabilize constantly behavior of ball valve element  19  without causing the ball valve element  19  unstable and choking and others. This serves to further precisely control the relief pressure of oil pump  1 , and enhance the pumping efficiency. 
         [0050]    Although pilot valve  5  has a substantially identical configuration as relief valve  3  in the present embodiment, pilot valve  5  may be implemented by an electromagnetic valve or the like, if pilot valve  5  is configured to relieve the oil pressure in back pressure passage  15  depending on the oil pressure in back pressure passage  15  (the back pressure of relief valve  3 ). 
       Second Embodiment 
       [0051]      FIGS. 6 to 9  show a second embodiment which differs from the first embodiment mainly in that the arrangement of oil filter  28  is modified, and a relief valve element  40  replaces the relief valve element  9  in relief valve  3 , and a pilot valve element  50  replaces the pilot valve element  31  in pilot valve  5 , and the flow paths to relief valve  3  and pilot valve  5  are modified. 
         [0052]    In the second embodiment, oil filter  28  is located at an upstream portion of discharge passage  1   a , and main oil gallery  27  and an upstream end of branch passage  2  is connected to a downstream portion of discharge passage  1   a.    
         [0053]    Relief valve element  40  is mounted in valve accommodation chamber  8  of relief valve  3  and configured to slide upward and downward in the longitudinal direction of valve accommodation chamber  8 . Relief valve element  40  is of a spool-valve type and includes a pressure-receiving part  40   a , a valve shaft  40   b , a valving part  40   c , and an annular passage  40   d . Pressure-receiving part  40   a  has a disc-shape facing the introduction port  12 . Valving part  40   c  has a cylindrical shape having a bottom, and is integrally coupled to pressure-receiving part  40   a  via valve shaft  40   b  at the center. Annular passage  40   d  is formed in the outer periphery of valve shaft  40   b . Relief valve element  40  is biased constantly toward introduction port  12  by the elastic force of valve spring  11 . 
         [0054]    The peripheral portion of the upper side portion of valve accommodation chamber  8  is formed with a downstream portion  41   b  of a return passage  41  having an upstream portion  41   a  hydraulically connected to discharge passage  1   a , wherein upstream portion  41   a  is located upstream of oil filter  28 . Relief port  13  has an opening on the lower side of downstream portion  41   b  of return passage  41  in the longitudinal direction of valve accommodation chamber  8 , and the opening of relief port  13  is hydraulically connected to relief passage  1   c.    
         [0055]    The downstream portion  41   b  of return passage  41  constantly faces annular passage  40   d  over a predetermined range of sliding of relief valve element  40 . On the other hand, the opening area of the open end of relief port  13  is changed by the outer peripheral surface of valving part  40   c  depending on the position of relief valve element  40  in the sliding range. Namely, when relief valve element  40  is held at the maximum upper position shown in  FIGS. 6 and 8  by the elastic force of valve spring  11 , the open end of relief port  13  is completely closed by the outer peripheral surface of valving part  40   c . On the other hand, when relief valve element  40  is positioned slightly below the maximum upper position as shown in  FIGS. 7 and 9 , relief port  13  is opened by valving part  40   c  so that the opening area of relief port  13  is controlled to a small value, and relief port  13  and return passage  41  are hydraulically connected to each other via annular passage  40   d.    
         [0056]    Pilot valve element  50  is of a spool-valve type similar to relief valve element  40 , and includes a pressure-receiving part  50   a , a valve shaft  50   b , a valving part  50   c , and an annular passage  50   d . Pressure-receiving part  50   a  has a disc-shape facing the supply port  33 . Valving part  50   c  has a cylindrical shape having a bottom, and is integrally coupled to pressure-receiving part  50   a  via valve shaft  50   b  at the center. Annular passage  50   d  is formed in the outer periphery of valve shaft  50   b . Pilot valve element  50  is biased constantly toward supply port  33  by the elastic force of valve spring  32 . 
         [0057]    The peripheral portion of the upper side portion of cylinder  30  is formed with a downstream portion  51   a  of a branch port  51  branched from back pressure passage  15 . The open end of discharge port  34  is located on the lower side of downstream end  51   a  in the longitudinal direction of cylinder  30 . 
         [0058]    Signal passage  29  has an end connected to feedback passage  2   a  and another end connected to supply port  33 . 
         [0059]    As shown in  FIGS. 6 to 8 , when the pump discharge pressure supplied via branch passage  2 , feedback passage  2   a  and signal passage  29  is lower than or equal to a predetermined value, pilot valve element  50  is held at the maximum upper position by the elastic force of valve spring  32  so that the downstream end  51   a  of branch port  51  is closed. On the other hand, when the supplied pump discharge pressure is higher than the predetermined value, pilot valve element  50  is moved downward to open the downstream end  51   a  of branch port  51 , and thereby allow fluid communication between branch port  51  and discharge port  34  through annular passage  50   d  as shown in  FIG. 9 . Namely, the cross-sectional area of fluid communication between branch port  51  and discharge port  34  is changed depending on the oil pressure in supply port  33 . 
         [0060]    Breather port  35  has an end open to the lower end portion of cylinder  30 . 
         [0061]    &lt;Operation of Second Embodiment&gt; The following describes operation of the oil pump with the relief pressure control apparatus according to the second embodiment. When the internal combustion engine is being started or operating at low speed, oil pump  1  is rotated by the internal combustion engine, and the electromagnetic coil of electromagnetic switching valve  4  is energized by the control unit. Accordingly, as shown in  FIG. 6 , push rod  26  is caused to press ball valve element  19  into pressing contact with the annular seat portion to close the open-ended hole  22 , and allow fluid communication between back pressure passage  15  and drain hole  25  through back pressure holes  23  and tubular passage  24 . By this action, the internal pressure of back pressure chamber  10  of relief valve  3  is lowered, so that relief valve element  40  is biased toward the maximum upward position in valve accommodation chamber  8  only by the elastic force of valve spring  11 , and the upper wall  40   a  is brought into pressing contact with a stepped annular surface of valve accommodation chamber  8 , and the relief port  13  is closed by the peripheral wall  40   c  of relief valve element  40 . 
         [0062]    When the engine rotational speed has risen with continuation of the energization of the electromagnetic coil so that the pump discharge pressure has reached the first oil pressure value P1, the pump discharge pressure applied to the pressure-receiving part  40   a  via discharge passage  1   a , oil filter  28 , branch passage  2 , and feedback passage  2   a  has become high and thereby cause relief valve element  40  to move downward by a certain amount against the elastic force of valve spring  11 . Under this condition, the outer peripheral surface of peripheral wall  40   c  of relief valve element  40  gradually increases the opening area of relief port  13  from zero toward the maximum position. By this action, an excess amount of working oil flowing from discharge passage is through return passage  41  to annular passage  40   d  is returned to suction passage  1   b  through relief port  13  and relief passage  1   c . Accordingly, under this condition, as indicated by a broken line in  FIG. 10 , the pump discharge pressure is prevented from rising significantly above the first oil pressure value P1, so that the pump discharge pressure is suitably regulated to a substantially flat characteristic. 
         [0063]    When the internal combustion engine shifts thereafter into a state where high oil pressure is requested with increase of the engine speed (or pump rotational speed) and oil temperature, the control unit senses this condition, and then de-energizes the electromagnetic coil of electromagnetic switching valve  4 . By this action, as shown in  FIG. 8 , push rod  26  is caused to move rearward along with the movable plunger, to release the pressing contact between ball valve element  19  and the valve seat portion, so that the open-ended hole  22  is opened by ball valve element  19 , to allow fluid communication between branch passage  2  and back pressure passage  15 , and close the tubular passage  24 . Accordingly, the pump discharge pressure is supplied to back pressure chamber  10  of relief valve  3  through branch passage  2  and back pressure passage  15 , so that the internal pressure of back pressure chamber  10  becomes high. On the other hand, the pump discharge pressure is applied to pressure-receiving part  40   a  of relief valve element  40  through feedback passage  2   a  and introduction port  12 . In this way, relief valve element  40  is applied with substantially identical oil pressures on upper and lower sides. As a result, relief valve element  40  is moved upward by the elastic force of valve spring  11 , so that the outer peripheral surface of peripheral wall  40   c  of relief valve element  40  closes the open end of relief port  13  and the pressure-receiving part  40   a  of relief valve element  40  closes the open end of introduction port  12 . 
         [0064]    When the engine speed further rises from that condition, the high pump discharge pressure is supplied to the back pressure chamber  10  with the relief valve element  40  maintained at the maximum top position shown in  FIG. 8 , so that the pump discharge pressure exceeds the first oil pressure value P1 and reaches the second oil pressure value P2 as shown in  FIG. 10 . 
         [0065]    When the engine speed further rises and the pump discharge pressure exceeds the second oil pressure value P2, the pump discharge pressure is applied to pilot valve element  50  through signal passage  29 , to cause pilot valve element  50  to move downward against the elastic force of valve spring  32 . This action allows valving part  50   c  to open the open end of discharge port  34 , and thereby allows fluid communication between the downstream end  51   a  of branch port  51  and discharge port  34  via branch port  51   d . Accordingly, the pump discharge pressure in back pressure passage  15  is drained to the outside, to adjust the oil pressure in back pressure passage  15 . 
         [0066]    When discharge port  34  is closed by pilot valve element  50  by the elastic force of valve spring  32 , the oil pressures in the upstream side and downstream side of electromagnetic switching valve  4  become equal to each other. However, when discharge port  34  is opened by pilot valve element  50  by the pump discharge pressure at the second oil pressure value P2, the oil pressure is drained from discharge port  34  so that the oil pressure in back pressure passage  15  is reduced by a pressure loss due to a flow resistance in electromagnetic switching valve  4 , for example, at open-ended hole  22 . As a result, relief valve  3  moves downward against the elastic force of valve spring  11  to reach the condition shown in  FIG. 9 . 
         [0067]    The second oil pressure value P2 is set to obtain an oil pressure for lubrication of the crankshaft when the engine is operating at high speed, or an oil pressure for injecting an oil jet to the engine piston. 
         [0068]    In this way, pilot valve  5  adjusts the oil pressure in a portion of the back pressure passage  15  downstream of electromagnetic switching valve  4  (identical to the back pressure of relief valve  3 ) so that the oil pressure in main oil gallery  27  becomes equal to the second oil pressure value P2. Accordingly, the oil pressure is set to a substantially flat characteristic from the point of P2 as indicated by a solid line in  FIG. 10 . Namely, back pressure chamber  10  receives supply of the pump discharge pressure that is obtained after passing through the oil filter  28  and the throttled open-ended hole  22 , and then being drained through branch port  51  and discharge port  34 , so that the pump discharge pressure is prevented from rising above the second oil pressure value P2, and thereby becomes flat. 
         [0069]    When the energization of electromagnetic switching valve  4  is switched from on-state to off-state while the engine speed is increasing, the oil pump system shifts from the characteristic indicated by the broken line to the characteristic indicated by the solid line as indicated by a short dashed long dashed line in  FIG. 10 . 
         [0070]    The other configuration and operation of the second embodiment is similar to that of the first embodiment. The second embodiment also serves to stabilize behavior of ball valve element  50  and thereby achieve a precise relief pressure control, and enhance the pumping efficiency. 
         [0071]    Also in the second embodiment, pilot valve  5  may be implemented by an electromagnetic valve or the like, if pilot valve  5  is configured to adjust or relieve the oil pressure in back pressure passage  15 . 
         [0072]    The first and second embodiments may be modified variously. For example, relief valve  3  and pilot valve  5  may be implemented by another combination than the shown combinations of the first and second embodiments. 
         [0073]    Oil pump  1  may be of another type than the trochoid type, such as a vane type or an external gear type. 
         [0074]    The relief pressure control apparatus may be formed integrally with the housing of oil pump  1  or provided separately from the housing of oil pump  1 . 
         [0075]    The entire contents of Japanese Patent Application 2013-217150 filed Oct. 18, 2013 are incorporated herein by reference. 
         [0076]    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.