Patent Publication Number: US-6662581-B2

Title: Variable displacement compressor and displacement control valve for variable displacement compressor

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a variable displacement compressor and a displacement control valve for the variable displacement compressor, and more particularly to a variable displacement compressor for compressing a refrigerant gas in a refrigeration cycle for an automotive air conditioner, and a displacement control valve for a variable displacement compressor, for use therein. 
     2. Description of the Related Art 
     A compressor used for compressing refrigerant in a refrigeration cycle for an automotive air conditioner is driven by an engine, and hence is not capable of controlling the rotational speed thereof. For this reason, a variable displacement compressor capable of changing the compression displacement for compressing refrigerant is employed so as to obtain adequate refrigerating displacement without being constrained by the rotational speed of the engine. 
     In the above-mentioned variable displacement compressor, compression pistons are connected to a wobble plate fitted on a shaft driven for rotation by the engine, and the angle of the wobble plate is changed to change the length of piston stroke for changing the delivery quantity of the compressor. 
     The angle of the wobble plate is continuously changed by introducing part of the compressed refrigerant into a gastight pressure-regulating chamber and changing the pressure of the introduced refrigerant, thereby changing a balance between pressures applied to the opposite sides of each piston. 
     A compression displacement control device disclosed e.g. in Japanese Laid-Open Patent Publication (Kokai) No. 2001-132650 has a solenoid control valve arranged between a discharge port and a pressure-regulating chamber of a compressor or between the discharge port and a suction port of the same. This solenoid control valve opens and closes the communication such that a differential pressure across the solenoid control valve is maintained at a predetermined value. The predetermined value of the differential pressure can be set from outside by a current value. As a result, when the engine rotational speed increases, the pressure introduced into the pressure-regulating chamber is increased to reduce the displacement for compression, and when the engine rotational speed decreases, the pressure introduced into the pressure-regulating chamber is reduced to increase the displacement for compression, whereby the pressure of refrigerant discharged from the compressor is maintained at a constant level. 
     Although refrigerant generally used in a refrigeration cycle of an automotive air conditioner is a chlorofluorocarbon alternative HFC-134a, there has recently been developed a refrigeration cycle which causes the refrigerant to perform refrigeration in a supercritical region where the temperature of the refrigerant is above its critical temperature, e.g. a refrigeration cycle using carbon dioxide as refrigerant 
     In the conventional solenoid control valve for the compression displacement control device, to minimize operating displacement of the variable displacement compressor, it is required to maximize the amount of refrigerant introduced into the pressure-regulating chamber, but if the size of the valve is small, the amount of refrigerant introduced is small, and hence transition to a minimum displacement operation takes time, which can degrade controllability of the compressor. 
     On the other hand, when the size of the valve is increased so as to increase the amount of refrigerant introduced, the pressure-receiving area of the valve is also increased, and hence a large solenoid force is required to control the valve. Particularly in the refrigeration cycle using carbon dioxide as the refrigerant, since the pressure of refrigerant is increased to the supercritical region, the discharge pressure of the refrigerant becomes very high, so that the solenoid force for controlling the valve is also increased. This requires a huge solenoid, which causes an increase in the size of the solenoid valve and a resultant increase in manufacturing costs. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in view of the above circumstances, and an object thereof is to provide a variable displacement compressor and a displacement control valve for the variable displacement compressor which are capable of performing transition between operating capacities in a reduced time period and operating without using a large solenoid force even when the size of the valve is increased so as to increase the amount of refrigerant. 
     In order to accomplish the above object, a variable displacement compressor including a wobble member arranged in a pressure-regulating chamber formed airtightly, such that an inclination angle of the wobble member can be changed with respect to a rotational shaft, and driven by rotation of the rotational shaft for wobbling motion, and pistons each connected to the wobble member for performing reciprocating motion in a direction parallel to the rotational shaft in accordance with the wobbling motion of the wobble member, to thereby draw refrigerant from a suction chamber into a cylinder, compress the refrigerant, and deliver the compressed refrigerant from the cylinder to a discharge chamber is provided. The variable displacement compressor is characterized in that a flow rate of the refrigerant flowing in a first refrigerant passage extending from the discharge chamber to the pressure-regulating chamber and a flow rate of the refrigerant flowing in a second refrigerant passage extending from the pressure-regulating chamber to the suction chamber are controlled in an interlocked fashion such that the first refrigerant passage and the second refrigerant passage are opened and closed, based on a change in a differential pressure between pressure in the suction chamber and pressure in the discharge chamber. 
     In addition, in order to accomplish the above object a displacement control valve for a variable displacement compressor, for controlling an amount of refrigerant introduced from a discharge chamber into a pressure-regulating chamber, such that a differential pressure between pressure in the suction chamber and pressure in the discharge chamber are maintained at a predetermined differential pressure, to thereby change an amount of the refrigerant discharged from the variable displacement compressor is provided. The displacement control valve for a variable displacement compressor is characterized by comprising the steps of: (a) first and second valve elements operated in an interlocked fashion for opening and closing a refrigerant passage extending between the discharge chamber and the pressure-regulating chamber and a refrigerant passage extending between the pressure-regulating chamber and the suction chamber, respectively; (b) a solenoid section for applying a solenoid force corresponding to the predetermined differential pressure to the first and second valve elements. 
     The above and other objects, features and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings which illustrate preferred embodiments of the present invention by way of example. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view schematically showing a variable displacement compressor to which is applied a displacement control valve according to the invention. 
     FIG. 2 is a central longitudinal cross-sectional view of the displacement control valve according to a first embodiment. 
     FIG. 3 is a cross-sectional view schematically showing a variable displacement compressor to which is applied another displacement control valve according to the invention. 
     FIG. 4 is a central longitudinal cross-sectional view of the displacement control valve according to a second embodiment. 
     FIG. 5 is a central longitudinal cross-sectional view of a displacement control valve according to a third embodiment. 
     FIG. 6 is a cross-sectional view schematically showing a variable displacement compressor to which is applied still another displacement control valve according to the invention. 
     FIG. 7 is a central longitudinal cross-sectional view of a displacement control valve according to a fourth embodiment. 
     FIG. 8 is a central longitudinal cross-sectional view of a displacement control valve according to a fifth embodiment. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. 
     FIG. 1 is a cross-sectional view schematically showing a variable displacement compressor to which is applied a displacement control valve according to the invention. 
     The variable displacement compressor includes a pressure-regulating chamber  1  formed airtightly and a rotational shaft  2  rotatably supported in the pressure-regulating chamber  1 . The rotational shaft  2  has one end extending outward from the pressure-regulating chamber  1  via a shaft sealing device, not shown, and having a pulley  3  fixed thereto which receives transmission of a driving force from an output shaft of an engine via a clutch and a belt. A wobble plate  4  is fitted on the rotational shaft  2  such that the inclination angle of the wobble plate  4  can be changed. A plurality of cylinders  5  (only one of which is shown in the figure) are arranged around the axis of the rotational shaft  2 . In each cylinder  5 , there is arranged a piston  6  for converting rotating motion of the wobble plate  4  to reciprocating motion. Each of the cylinders  5  is connected to a suction chamber  9  and a discharge chamber  10  via a suction relief valve  7  and a discharge relief valve  8 , respectively. The respective suction chambers  9  associated with the cylinders  5  communicate with each other to form one chamber which is connected to an evaporator of a refrigeration cycle. Similarly, the respective discharge chambers  10  associated with the cylinders  5  communicate with each other to form one chamber which is connected to a gas cooler or a condenser of the refrigeration cycle. 
     Further, in the variable displacement compressor, a displacement control valve  11  comprised of two valves is arranged at an intermediate portion of a refrigerant passage extending from the discharge chamber  10  to the pressure-regulating chamber  1  and in a refrigerant passage for communication between the pressure-regulating chamber  1  and the suction chamber  9 . There are formed orifices  12 ,  13  between the discharge chamber  10  and the pressure-regulating chamber  1  and between the pressure-regulating chamber  1  and the suction chamber  9 , respectively. It should be noted that although the orifices  12 ,  13  are formed in the body of the variable displacement compressor, they may be formed in the displacement control valve  11 . 
     In the variable displacement compressor constructed as above, as the rotational shaft  2  is rotated by the driving force of the engine, the wobble plat  4  fitted on the rotational shaft  2  rotates, which causes reciprocating motion of each piston  6  connected to the wobble plate  4 . As a result, refrigerant within the suction chamber  9  is drawn into a cylinder  5 , and compressed therein, and then the compressed refrigerant is delivered to the discharge chamber  10 . 
     At this time, during normal operation, responsive to a discharge pressure Pd of the refrigerant within the discharge chamber  10 , the displacement control valve  11  controls the amount of refrigerant introduced into the pressure-regulating chamber  1  (the pressure in the pressure-regulating chamber  1  at the time is represented by Pc 1 ) and the amount of refrigerant introduced from the pressure-regulating chamber  1  into the suction chamber  9  (the pressure in the pressure-regulating chamber  1  at the time is represented by Pc 2 ) in an interlocking fashion such that the differential pressure between the discharge pressure Pd and a suction pressure Ps is maintained at a predetermined differential pressure. As a result, the pressure Pc (=Pc 1 =Pc 2 ) in the pressure-regulating chamber  1  is held at a predetermined value, and the displacement of the cylinder  5  is controlled to a predetermined value. 
     When transition to the minimum displacement operation is performed, the displacement control valve  11  fully opens one valve thereof provided for introducing refrigerant from the discharge chamber  10  into the pressure-regulating chamber  1  and fully closes the other valve thereof provided for introducing refrigerant from the pressure-regulating chamber  1  into the suction chamber  9 , thereby shortening time for increasing the pressure Pc (=Pc 1 ) in the pressure-regulating chamber  1 . It should be noted that although the displacement control valve  11  fully closes the refrigerant passage extending from the pressure-regulating chamber  1  to the suction chamber  9  during the time period, there remains a flow of refrigerant at a minute flow rate via the orifice  13 . 
     For a maximum displacement operation of the compressor, the displacement control valve  11  fully closes the one valve thereof provided for introducing refrigerant from the discharge chamber  10  into the pressure-regulating chamber  1  and fully opens the other valve thereof provided for introducing refrigerant from the pressure-regulating chamber  1  into the suction chamber  9 , so as to maximize the amount of refrigerant introduced from the pressure-regulating chamber  1  into the suction chamber  9 , thereby shortening time for reducing the pressure Pc (=Pc 2 ) in the pressure-regulating chamber  1 . It should be noted that although the displacement control valve  11  fully closes the refrigerant passage extending from the discharge chamber  10  to the pressure-regulating chamber  1  during the time period, refrigerant is introduced into the pressure-regulating chamber  1  via the orifice  12 , whereby lubricating oil mixed into the refrigerant is supplied to the pressure-regulating chamber  1 . 
     Next, the displacement control valve  11  according to the invention will be described in detail. 
     FIG. 2 is a central longitudinal cross-sectional view of the displacement control valve according to a first embodiment. 
     The displacement control valve  11  is comprised of two valve elements  21 ,  22  integrally formed such that they are operated in an interlocked fashion. More specifically, a central shaft  25  axially movably held by a holder  24  fitted in a central opening portion of a body  23 , thin shafts  26 ,  27  formed to have a smaller thickness than the central shaft  25  and extending from the opposite ends of the same, and the valve element  21  positioned at a location downward of the thin shaft  26 , as viewed in the figure, are integrally formed with each other, and the other valve element  22  is arranged in abutment with the upper thin shaft  27 . The central shaft  25  held by the holder  24  has a pressure-receiving area smaller than respective effective pressure-receiving areas of the valve elements  21 ,  22  and forms a pressure-sensing portion. Further, the central shaft  25  is formed with a portion with a reduced diameter, on which a packing  30  formed e.g. of polytetrafluoroethylene is fitted. 
     A valve seat  28  for the valve element  21  is formed by the lower end, as viewed in the figure, of the body  23  holding the holder  24 . The valve seat  28  has a valve hole whose inner diameter is slightly larger than that of a portion of the holder  24  holding the central shaft  25 . 
     A valve seat  29  for the valve element  22  is formed by the upper end, as viewed in the figure, of the holder  24 . The valve seat  29  has a valve hole whose inner diameter is slightly larger than that of the portion of the holder  24  holding the central shaft  25 . The valve element  22  is urged in a valve-closing direction by a spring  32  arranged between a spring-receiving member  31  fitted in the upper opening end, as viewed in the figure, of the body  23  and the valve element  22  itself. 
     The body  23  is fitted in an upper opening of a body  33 . The body  33  has a central opening portion in which are fixedly fitted respective upper ends of a fixed core  34  and a sleeve  35  of a solenoid section. The fixed core  34  has a central opening portion forming a guide for axially slidably holding a shaft  36  of the solenoid section. The lower end of the shaft  36  is axially slidably held by a guide  38  arranged in a stopper  37  closing the lower end of the sleeve  35 , and a movable core  39  of the solenoid section is fitted on the lower portion of the shaft  36 . The movable core  39  has an upper end thereof held in abutment with a stopper ring  40  fitted on the shaft  36 , and is urged upward, as viewed in the figure, by a spring  41  arranged between the guide  38  and the movable core  39  itself. Further, the sleeve  35  is surrounded by a solenoid coil  42 . 
     The body  23  has a hole communicating with a central space through which the thin shaft  26  extends, and the hole forms a port  43  for receiving the discharge pressure Pd from the discharge chamber  10 . A strainer  47  is mounted on the outer edge of the port  43 . Further, the body  23  has a hole communicating with a central space through which the thin shaft  27  extends, and the hole forms a port  44  for receiving the suction pressure Ps from the suction chamber  9 . The body  33  has a hole communicating with a space in which the valve element  21  is arranged, and the hole forms a port  45  for introducing the pressure Pc 1  into the pressure-regulating chamber  1 . The spring-receiving member  31  has a hole communicating with a space in which the valve element  22  is arranged, and the hole forms a port  46  for introducing the pressure Pc 2  from the pressure-regulating chamber  1 . A strainer  47   a  is mounted on a distal end of the body  23 . 
     The body  23  has O rings  48 ,  49  fitted thereon at respective locations upward and downward of the port  44 , while the body  33  has O rings  50 ,  51  fitted thereon at respective locations upward and downward of the port  45 . 
     Now, the relationship of pressures in the displacement control valve  11  will be described. First, the discharge pressure Pd received from the discharge chamber  10  via the port  43  acts on the central shaft  25  and the valve element  21  in the opposite directions of the axis. When the effective pressure-receiving area of the valve element  21  is represented by A, and that of the central shaft  25  by B, a force of Pd·A acts downward, as viewed in the figure, on the valve element  21 , while a force of Pd·B acts upward, as viewed in the figure, on the central shaft  25 . Between the effective pressure-receiving area A of the valve element  21  and the effective pressure-receiving area B of the central shaft  25 , A&gt;B holds, and hence, after all, a force of Pd (A−B) acts on the valve element  21  and the central shaft  25  in the downward direction, as viewed in the figure, for opening the valve. The difference (A−B) corresponds to the effective pressure-receiving area of the conventional valve element, and conventionally, the flow rate of refrigerant is limited by the effective pressure-receiving area. According to the present invention, however, although the valve element  21  has the large effective pressure-receiving area A which can allow an increased amount of refrigerant to flow, the force acting on the valve element  21  in the valve-opening direction is limited to the small force Pd (A−B). Further, since the pressures Pc 1 , Pc 2  (Pc 1 =Pc 2 ) in the pressure-regulating chamber  1  are axially applied to the valve elements  21 ,  22  from the respective opposite sides via the respective ports  45 ,  46 , the influence of the pressure Pc upon the valve element  21  is canceled. Thus, the central shaft  25  having a different pressure-receiving area from that of the valve element  21  is integrally formed with the valve element  21 , and this makes it possible to form a valve having a small pressure-receiving area of (A−B), irrespective of the valve size. 
     Similarly, a force of Ps (A−B) acts on the valve element  22  and the central shaft  25  in the valve-opening direction, and the pressures Pc 1 , Pc 2  (Pc 1 =Pc 2 ) in the pressure-regulating chamber  1  are axially applied to the valve elements  21 ,  22  integral with each other from the respective opposite sides, which cancels the influence of the pressure Pc upon the valve element  22 . It should be noted that the ratio between the effective pressure-receiving area of the valve element  22  and that of the central shaft  25  is configured to be equal to the ratio between the effective pressure-receiving area of the valve element  21  and that of the central shaft  25 . Therefore, the valve elements  21 ,  22  form a differential pressure valve which operates in response to a differential pressure between the discharge pressure Pd and the suction pressure Ps. 
     Further, the pressure Pc 1  received via the port  45  is supplied to a gap between the sleeve  35  and the movable core  39  as well as to a gap between the movable core  39  and the stopper  37  via a clearance between the fixed core  34  and the shaft  36 . In short, the inside of the solenoid section is filled with the pressure Pc 1 . 
     In the displacement control valve  11  having two valve structures interlocked as described above, when no control current is supplied to the solenoid coil  42  of the solenoid section, as shown in FIG. 2, the valve element  21  between the discharge pressure Pd and the pressure Pc 1  from the pressure-regulating chamber  1  is fully open, whereas the valve element  22  between the pressure Pc 2  and the suction pressure Ps is fully closed. Further, the movable core  39  of the solenoid section is held away from the fixed core  34  due to a balance between spring load of the spring  32  and that of the spring  41 . Therefore, the value of the pressure Pc 1  in the pressure-regulating chamber  1  is held close to that of the discharge pressure Pd, and hence the difference between pressures applied to the opposite faces of the piston  6  is minimized, whereby the wobble plate  4  is inclined at an inclination angle which minimizes the length of stroke of the piston  6 , thus controlling the variable displacement compressor to the minimum displacement operation. 
     When a maximum control current is supplied to the solenoid coil  42  of the solenoid section, the movable core  39  is attracted toward the fixed core  34  and moved upward, as viewed in the figure, whereby the valve element  21  between the discharge pressure Pd and the pressure Pc 1  from the pressure-regulating chamber  1  is fully closed, and the valve element  22  between the pressure Pc 2  and the suction pressure Ps is fully opened. As a result, in addition to refrigerant being introduced from the pressure-regulating chamber  1  into the suction chamber  9  via the orifice  13 , refrigerant flows from the port  46  communicated with the pressure-regulating chamber  1 , and passes between the valve element  22  and the valve seat  29  therefor, followed by being introduced into the suction chamber  9  via the port  44 . Since the amount of refrigerant introduced from the pressure-regulating chamber  1  into the suction chamber  9  is increased, it is possible to increase a speed at which the operating displacement is maximized. 
     During execution of normal control in which a predetermined control current is supplied to the solenoid coil  42  of the solenoid section, the movable core  39  is attracted toward the fixed core  34  and moved upward, as viewed in the figure, according to the magnitude of the control current. As a result, the valve element  22  is opened from its closed state only when the differential pressure between the discharge pressure Pd and the suction pressure Ps exceeds a predetermined reference value. In short, during execution of the normal control, the displacement control valve  11  operates as a differential pressure valve. 
     FIG. 3 is a cross-sectional view schematically showing a variable displacement compressor to which is applied another displacement control valve according to the present invention. In FIG. 3, component parts and elements similar to those appearing in FIG. 1 are designated by identical reference numerals, and detailed description thereof is omitted. 
     In the variable displacement compressor, a displacement control valve  60  comprised of two valves is arranged at an intermediate portion of a refrigerant passage extending from a discharge chamber  10  to a pressure-regulating chamber  1  and in a refrigerant passage for communication between the pressure-regulating chamber  1  and a suction chamber  9 . Further, there are formed orifices  12 ,  13  between the discharge chambers  10  and the pressure-regulating chamber  1  and between the pressure-regulating chamber  1  and the suction chamber  9 , respectively. 
     In the variable displacement compressor constructed as above, as a rotational shaft  2  is rotated by the driving force of an engine, a wobble plate fitted on the rotational shaft  2  rotates, which causes reciprocating motion of each piston  6  connected to the wobble plate  4 . This causes refrigerant within the suction chamber  9  to be drawn into a cylinder  5  and compressed therein, and then the compressed refrigerant is delivered to the discharge chamber  10 . 
     At this time, during normal operation, responsive to a discharge pressure Pd of the refrigerant within the discharge chamber  10 , the displacement control valve  60  controls the amount of refrigerant introduced into the pressure-regulating chamber  1  (the pressure in the pressure-regulating chamber  1  at the time is represented by Pc 1 ) and the amount of refrigerant introduced from the pressure-regulating chamber  1  into the suction chamber  9  (the pressure in the pressure-regulating chamber  1  at the time is represented by Pc 2 ) in an interlocking fashion such that the differential pressure between the discharge pressure Pd and a suction pressure Ps is maintained at a predetermined differential pressure. As a result, the pressure Pc (=Pc 1 =Pc 2 ) in the pressure-regulating chamber  1  is held at a predetermined value, and the displacement of the cylinder  5  is controlled to a predetermined value. 
     When transition to the minimum displacement operation is performed, the displacement control valve  60  fully opens one valve thereof provided for introducing refrigerant from the discharge chamber  10  into the pressure-regulating chamber  1  and fully closes the other valve thereof provided for introducing refrigerant from the pressure-regulating chamber  1  into the suction chamber  9 , thereby shortening time for increasing the pressure Pc (=Pc 1 ) in the pressure-regulating chamber  1 . 
     For a maximum displacement operation of the compressor, the displacement control valve  60  fully closes the one valve thereof provided for introducing refrigerant from the discharge chamber  10  into the pressure-regulating chamber  1  and fully opens the other valve thereof provided for introducing refrigerant from the pressure-regulating chamber  1  into the suction chamber  9 , so as to maximize the amount of refrigerant introduced from the pressure-regulating chamber  1  into the suction chamber  9 , thereby shortening time for reducing the pressure Pc (=Pc 2 ) in the pressure-regulating chamber  1 . 
     Next, the displacement control valve  60  for executing the above control will be described in detail. 
     FIG. 4 is a central longitudinal cross-sectional view of the displacement control valve according to a second embodiment. 
     In the displacement control valve  60 , the two valve elements  61 ,  62  are opposed to each other via a transmission shaft  63  on an identical axis such that they can move along the axis. The valve element  61  arranged at an upper location, as viewed in the figure, is integrally formed with a piston  64  forming a pressure-sensing portion and a shaft  65  connecting between the valve element  61  and the piston  64 . Further, the one-piece member formed by the valve element  61 , the shaft  65  and the piston  64  is formed therethrough with a communication hole  66  extending along the axis thereof. Similarly, the valve element  62  arranged at a lower location, as viewed in the figure, is integrally formed with a piston  67  forming a pressure-sensing portion and a shaft  68  connecting between the valve element  62  and the piston  67 . Further, the one-piece member formed by the valve element  62 , the shaft  68  and the piston  67  is formed therethrough with a communication hole  69  extending along the axis thereof. Each of the valve elements  61 ,  62  has an end face thereof in abutment with the transmission shaft  63 , and the end face is formed with a step for allowing communication between the communication hole  66  ( 69 ) and a space where the valve element  61  ( 62 ) is located, even in the abutted state. 
     A valve seat  70  for the valve element  61  is formed by the lower end, as viewed in the figure, of a body  71  axially slidably holding the piston  64 . The valve seat  70  has an inner diameter which is slightly larger than the inner diameter of a cylinder holding the piston  64 . The valve element  61  is urged in the valve-opening direction by a spring  72 . 
     The body  71  is fitted in an upper opening of a body  73 . The body  73  is formed with a hole extending downward from the upper opening, the hole having four stepwise sequentially reduced-diameter portions. A first reduced-diameter portion has a holder  74  fitted therein for axially movably holding the transmission shaft  63 , and an edge of opening formed in a step to a next reduced-diameter portion forms a valve seat  75  for the valve element  62 . A next reduced-diameter portion forms a cylinder for axially slidably holding the piston  67 , and a next reduced-diameter portion forms a guide for axially slidably holding a shaft  76  of a solenoid section. Further, the lower portion of the body  73  forms a fixed core  78  of the solenoid section. 
     The body  73  is screwed in an upper opening of a body  79 . The upper end of a sleeve  80  is fixed to a lower opening of the body  79 . The sleeve  80  has a lower end thereof closed by a stopper  81 . Within the sleeve  80 , the lower end of the shaft  76  is axially slidably held by a guide  82  provided in the stopper  81 . A movable core  83  is fitted on the lower portion of the shaft  76 . The movable core  83  has an upper end thereof held in abutment with a stopper ring  84  fitted on the shaft  76 , and is urged upward, as viewed in the figure, by a spring  85  arranged between the guide  82  and the movable core  83  itself. Further, the outer periphery of the sleeve  80  is surrounded by a solenoid coil  86 . 
     The body  71  has a hole communicating with a central space through which the shaft  65  extends, and the hole forms a port  87  for receiving the discharge pressure Pd from the discharge chamber  10 . A strainer  88  is mounted on the port  87 . The body  73  has a hole communicating with a space in which the valve element  61  is located, and the hole forms a port  89  for introducing the pressure Pc 1  into the pressure-regulating chamber  1 . The body  73  also has a hole communicating with a space in which the valve element  62  is located, and the hole forms a port  90  for introducing the pressure Pc 2  from the pressure-regulating chamber  1 . Further, the body  73  is formed with a hole for communication with a central space through which the shaft  68  extends, and the body  79  is formed with a hole such that this hole communicates with the hole of the body  73 , whereby the two holes form a port  91  communicating with the suction chamber  9  under the suction pressure Ps. 
     The body  73  has O rings  92 ,  93  fitted thereon at respective locations upward and downward of the port  89 , while the body  79  has O rings  94 ,  95  fitted thereon at respective locations upward and downward of the port  91 . Further, portions of the body  73  and the body  79  in contact with each other, closer to the solenoid section with respect to the port  91 , are sealed by an O ring  96 . 
     Now, the relationship of pressures in the displacement control valve  60  will be described. First, the discharge pressure Pd received from the discharge chamber  10  via the port  87  is applied to the piston  64  and the valve element  61  in the opposite directions of the axis. When the effective pressure-receiving area of the valve element  61  is represented by A, and that of the piston  64  by B, a force of Pd·A acts downward, as viewed in the figure, on the valve element  61 , while a force of Pd·B acts upward, as viewed in the figure, on the piston  64 . Between the effective pressure-receiving area A of the valve element  61  and the effective pressure-receiving area B of the piston  64 , A&gt;B holds, and hence, after all, a force of Pd (A−B) acts on the valve element  61  and the piston  64  in the downward direction, as viewed in the figure, for opening the valve. The difference (A−B) corresponds to the effective pressure-receiving area of the conventional valve element, and conventionally, the flow rate of refrigerant is limited by the effective pressure-receiving area. According to the present invention, however, although the valve element  61  has the large effective pressure-receiving area A which can allow an increased amount of refrigerant to flow, the force acting on the valve element  61  in the valve-opening direction is limited to the small force Pd (A−B). Furthermore, the pressure Pc 1  received via the port  89  is also applied to a back pressure chamber-side face of the piston  64  via the central communication hole  66 , so that the influence of the pressure Pc 1  upon the valve element  61  is canceled. Thus, the piston  64  having a different pressure-receiving area from that of the valve element  61  is integrally formed with the valve element  61 , which makes it possible to form a valve having a small pressure-receiving area, irrespective of the valve size. 
     Similarly, a force of Ps (A−B) acts on the valve element  62  and the piston  67  in the valve-opening direction, and the pressure Pc 2  received via the port  90  is also applied to a back pressure chamber-side face of the piston  67  via the central communication hole  69 , so that the influence of the pressure Pc 2  upon the valve element  62  is canceled. It should be noted that the ratio between the effective pressure-receiving area of the valve element  62  and that of the piston  67  is configured to be equal to the ratio between the effective pressure-receiving area of the valve element  61  and that of the piston  64 . Therefore, the valve elements  61 ,  62  in opposed arrangement form a differential pressure valve which operates in response to a differential pressure between the discharge pressure Pd and the suction pressure Ps. 
     Further, the pressure Pc 2  received via the port  90  is supplied via the communication hole  69  to a space forming the back-pressure chamber of the piston  67 , a clearance between the fixed core  78  and the shaft  76 , a space between the fixed core  78  and the movable core  83 , a clearance between the sleeve  80  and the movable core  83 , and a clearance between the movable core  83  and the stopper  81 , and hence the internal part of the displacement control valve  60  closer to the solenoid section with respect to the O ring  96  is filled with the pressure Pc 2  (=Pc). 
     In the displacement control valve  60  having the two valve structures interlocked as described above, when no control current is supplied to the solenoid coil  86  of the solenoid section, as shown in FIG. 4, the valve element  61  between the discharge pressure Pd and the pressure Pc 1  from the pressure-regulating chamber  1  is fully open, whereas the valve element  62  between the pressure Pc 2  and the suction pressure Ps is fully closed. Further, the movable core  83  of the solenoid section is held away from the fixed core  78  due to a balance between spring load of the spring  72  and that of the spring  85 . Therefore, the value of the pressure Pc 1  in the pressure-regulating chamber  1  is held close to the value of the discharge pressure Pd, and hence the difference between pressures applied to the respective opposite faces of the piston  6  is minimized, whereby the wobble plate  4  is inclined at an inclination angle which minimizes the length of stroke of the piston  6 , thus controlling the variable displacement compressor to the minimum displacement operation. 
     When a maximum control current is supplied to the solenoid coil  86  of the solenoid section, the movable core  83  is attracted toward the fixed core  78  and moved upward, as viewed in the figure, whereby the valve element  61  between the discharge pressure Pd and the pressure Pc 1  from the pressure-regulating chamber  1  is fully closed, and the valve element  62  between the pressure Pc 2  and the suction pressure Ps is fully opened. As a result, in addition to refrigerant being introduced from the pressure-regulating chamber  1  into the suction chamber  9  via the orifice  13 , refrigerant flows from the port  90  communicated with the pressure-regulating chamber  1 , and passes between the valve element  62  and the valve seat  75  therefor, followed by being introduced into the suction chamber  9  via the port  91 . Since the amount of refrigerant introduced from the pressure-regulating chamber  1  into the suction chamber  9  is increased, it is possible to increase a speed at which the operating displacement is maximized. 
     During execution of normal control in which a predetermined control current is supplied to the solenoid coil  86  of the solenoid section, the movable core  83  is attracted toward the fixed core  78  and moved upward, as viewed in the figure, according to the magnitude of the control current. As a result, the valve element  62  is opened from its closed state only when the differential pressure between the discharge pressure Pd and the suction pressure Ps exceeds a predetermined reference value. In short, during execution of the normal control, the displacement control valve  60  operates as a differential pressure valve. 
     FIG. 5 is a central longitudinal cross-sectional view of a displacement control valve according to a third embodiment. In FIG. 5, component parts and elements similar to those appearing in FIG. 4 are designated by identical reference numerals, and detailed description thereof is omitted. 
     The displacement control valve  60   a  according to the third embodiment has a different structure for canceling the influences of the pressures Pc 1 , Pc 2  upon respective valve elements  61 ,  62 , from that of the FIG. 4 displacement control valve  60  shown in FIG.  4 . More specifically, a one-piece member formed by the valve element  61 , a piston  64 , and a shaft  65 , and a one-piece member formed by the valve element  62 , a piston  67 , and a shaft  68  are each formed as a solid member having no communication hole axially extending therethrough. On the other hand, a body  71  is formed with a communication hole  97  for introducing the pressure Pc 1  into a back pressure chamber of the piston  64 . Further, a body  73  is formed with a communication hole  77  opening into a space forming a back pressure chamber of the piston  67  and a clearance between respective portions of a fixed core  78  and a sleeve  80  closer the solenoid section with respect to an O ring  96 . The displacement control valve  60   a  constructed as above operates similarly to the displacement control valve  60  of the second embodiment. 
     FIG. 6 is a cross-sectional view schematically showing a variable displacement compressor to which is applied still another displacement control valve according to the present invention. In FIG. 6, component parts and elements similar to those appearing in FIGS. 1 and 3 are designated by identical reference numerals, and detailed description thereof is omitted. 
     In the variable displacement compressor, a displacement control valve  100  comprised of two valves is arranged at an intermediate portion of a refrigerant passage extending from a discharge chamber  10  to a pressure-regulating chamber  1  and in a refrigerant passage for communication between the pressure-regulating chamber  1  and a suction chamber  9 . The two refrigerant passages share the portion between the displacement control valve  100  and the pressure-regulating chamber  1 . 
     In the variable displacement compressor constructed as above, as the rotational shaft  2  is rotated by the driving force of the engine, the wobble plate fitted on the rotational shaft  2  rotates, which causes reciprocating motion of each piston  6  connected to the wobble plate  4 . As a result, refrigerant within the suction chamber  9  is drawn into a cylinder  5  and compressed therein, and then the compressed refrigerant is delivered to the discharge chamber  10 . 
     At this time, during normal operation, responsive to discharge pressure Pd of the refrigerant within the discharge chamber  10 , the displacement control valve  100  controls the amount of refrigerant introduced into the pressure-regulating chamber  1  and the amount of refrigerant which is part of refrigerant to be introduced into the pressure-regulating chamber  1  but supplied into the suction chamber  9  in a bypassing member, such that the differential pressure between the discharge pressure Pd and a suction pressure Ps is maintained at a predetermined differential pressure. As a result, the pressure Pc in the pressure-regulating chamber  1  is held at a predetermined value, and the displacement of the cylinder  5  is controlled to a predetermined value. Thereafter, the pressure Pc in the pressure-regulating chamber  1  is returned to the suction chamber  9  via an orifice  13 . 
     When transition to the minimum displacement operation is performed, the displacement control valve  100  fully opens one valve thereof provided for introducing refrigerant from the discharge chamber  10  into the pressure-regulating chamber  1  and fully closes the other valve thereof provided for introducing refrigerant from the pressure-regulating chamber  1  into the suction chamber  9 , thereby shortening time for increasing the pressure Pc in the pressure-regulating chamber  1 . 
     When transition to the maximum displacement operation is performed, the displacement control valve  100  fully closes the one valve thereof provided for introducing refrigerant from the discharge chamber  10  into the pressure-regulating chamber  1  and fully opens the other valve thereof provided for introducing refrigerant from the pressure-regulating chamber  1  into the suction chamber  9 , so as to maximize the amount of refrigerant introduced from the pressure-regulating chamber  1  into the suction chamber  9 , thereby shortening time for reducing the pressure Pc in the pressure-regulating chamber  1 . 
     Next, the displacement control valve  100  for executing the above control will be described in detail. 
     FIG. 7 is a central longitudinal cross-sectional view of the displacement control valve according to a fourth embodiment. 
     In the displacement control valve  100 , the two valve elements  101 ,  102  are arranged opposed to each other on an identical axis such that they can move along the axis. The valve element  101  arranged at an upper location, as viewed in the figure, is integrally formed with a piston  103  forming a pressure-sensing portion and a shaft  104  connecting between the valve element  101  and the piston  103 , and the one-piece member formed by the valve element  101 , the shaft  104 , and the piston  103  is formed with a communication hole  105  axially extending therethrough. Similarly, the valve element  102  arranged at a lower location, as viewed in the figure, is integrally formed with a piston  106  forming a pressure-sensing portion and a shaft  107  connecting between the valve element  102  and the piston  106 , and the one-piece member formed by the valve element  102 , the shaft  107 , and the piston  106  is formed with a communication hole  108  axially extending therethrough. The valve elements  101 ,  102  have respective end faces thereof in abutment with each other, and the end faces are each formed with a step for allowing communication between the communication hole  105  ( 108 ) and a space where the valve elements  101  ( 102 ) is located, even when the valve elements  101 ,  102  are in abutment with each other. 
     A valve seat  109  for the valve element  101  is formed by the lower end, as viewed in the figure, of a body  110  axially slidably holding the piston  103 . The valve seat  109  has an inner diameter which is slightly larger than the inner diameter of a cylinder holding the piston  103 . The valve element  101  is urged in the valve-opening direction by a spring  112  arranged between an E-shaped stopper ring  111  fitted on the valve element  101  and the body  110 . 
     The body  110  is fitted in an upper opening of a body  113 . The body  113  is formed with a hole extending therethrough downward from the upper opening and having three stepwise sequentially reduced-diameter portions. An edge of opening formed in a step to a first reduced-diameter portion forms a valve seat  114  for the valve element  102 . A next reduced-diameter portion forms a cylinder for axially slidably holding the piston  106 , and a next reduced-diameter portion forms a guide for axially slidably holding a shaft  115  of a solenoid section. Further, the body  113  has a communication hole  116  formed therein which extends parallel with the axis thereof from the upper opening, and a lower end of the communication hole  116  has a communication hole laterally formed thereacross, for communication with an opening forming the guide of the shaft  115  and an outer periphery of the body  113 . Further, the lower portion of the body  113  forms a fixed core  117  of the solenoid section. 
     The body  113  is screwed in the upper opening of a body  118 . The upper end of a sleeve  119  is fixed to a lower opening of the body  118 . The sleeve  119  has a lower end thereof closed by a stopper  120 . Within the sleeve  119 , the lower end of the shaft  115  is axially slidably held by a guide  121 . A movable core  122  is fitted on the lower portion of the shaft  115 . The movable core  122  has an upper end thereof held in abutment with a stopper ring  123  fitted on the shaft  115 , and is urged upward, as viewed in the figure, by a spring  124  arranged between the guide  121  and the movable core  122  itself. Further, the outer periphery of the sleeve  119  is surrounded by a solenoid coil  125 . 
     The body  110  has a hole communicating with a central space through which the shaft  104  extends, and the hole forms a port  126  for receiving the discharge pressure Pd from the discharge chamber  10 . A strainer  127  is mounted on the port  126 . The body  113  has a hole communicating with a central space formed in the upper opening portion thereof, and the hole forms a port  128  for introducing the pressure PC into the pressure-regulating chamber  1 . Further, the body  113  has a hole communicating with a central space through which the shaft  107  extends and the body  118  is formed with a hole such that this hole communicates with the hole of the body  113 , whereby the two holes form a port  129  communicating with the suction chamber  9  under the suction pressure Ps. 
     The body  113  has an O ring  130  fitted thereon at a location between the port  126  and the port  128 , while the body  118  has O rings  131 ,  132  fitted thereon at respective locations upward and downward of the port  129 . Further, portions of the body  113  and the body  118  in contact with each other, closer to the solenoid section with respect to the port  129 , are sealed by an O ring  133 . 
     Now, the relationship of pressures in the displacement control valve  100  will be described. First, the discharge pressure Pd received from the discharge chamber  10  via the port  126  is applied to the piston  103  and the valve element  101  in the opposite directions of the axis. When the effective pressure-receiving area of the valve element  101  is represented by A, and that of the piston  103  by B, a force of Pd·A acts downward, as viewed in the figure, on the valve element  101 , while a force of Pd·B acts upward, as viewed in the figure, on the piston  103 . Between the effective pressure-receiving area A of the valve element  101  and the effective pressure-receiving area B of the piston  103 , A&gt;B holds, and hence, after all, a force of Pd (A−B) acts on the valve element  101  and the piston  103  in the downward direction, as viewed in the figure, for opening the valve. The difference (A−B) corresponds to the effective pressure-receiving area of the conventional valve element, and conventionally, the flow rate of refrigerant is limited by the effective pressure-receiving area. According to the present invention, however, although the valve element  101  has the large effective pressure-receiving area A which can allow an increased amount of refrigerant to flow, the force acting on the valve element  101  in the valve-opening direction is limited to the small force Pd (A−B). Moreover, the pressure Pc received via the port  128  is also applied to a back pressure chamber-side face of the piston  103  via the central communication hole  105 , so that the influence of the pressure Pc upon the valve element  101  is canceled. Thus, the piston  103  having a different pressure-receiving area from that of the valve element  101  is integrally formed with the valve element  101 , which makes it possible to form a valve having a small pressure-receiving area, irrespective of the valve size. 
     Similarly, a force of Ps (A−B) acts on the valve element  102  and the piston  106  in the valve-opening direction, and the pressure Pc received via the port  128  is also applied to a back pressure chamber-side face of the piston  106  via the central communication hole  108 , so that the influence of the pressure Pc upon the valve element  102  is canceled. It should be noted that the ratio between the effective pressure-receiving area of the valve element  102  and that of the piston  106  is configured to be equal to the ratio between the effective pressure-receiving area of the valve element  101  and that of the piston  103 . Therefore, the valve elements  101 ,  102  in opposed arrangement form a differential pressure valve which operates in response to a differential pressure between the discharge pressure Pd and the suction pressure Ps. 
     Further, the pressure Pc received via the port  128  is also supplied via the communication hole  116  formed through the body  113  to a gap between the sleeve  119  and the fixed core  117  and the movable core  122 , a space between the fixed core  117  and the movable core  122 , and a gap between the movable core  122  and the stopper  120 , and hence the inside of the solenoid section is filled with the pressure Pc. 
     In the displacement control valve  100  having the two valve structures interlocked as described above, when no control current is supplied to the solenoid coil  125  of the solenoid section, as shown in FIG. 7, the valve element  101  between the discharge pressure Pd and the pressure Pc from the pressure-regulating chamber  1  is fully open, whereas the valve element  102  between the pressure Pc and the suction pressure Ps is fully closed. Further, the movable core  122  of the solenoid section is held away from the fixed core  117  due to a balance between spring load of the spring  112  and that the spring  124 . Therefore, the value of the pressure Pc in the pressure-regulating chamber  1  is held close to the value of the discharge pressure Pd, and hence the difference between pressures applied to the respective opposite faces of the piston  6  is minimized, whereby the wobble plate  4  is inclined at an inclination angle which minimizes the length of stroke of the piston  6 , thus controlling the variable displacement compressor to the minimum displacement operation. 
     When a maximum control current is supplied to the solenoid coil  125  of the solenoid section, the movable core  122  is attracted toward the fixed core  117  and moved upward, as viewed in the figure, whereby the valve element  101  between the discharge pressure Pd and the pressure Pc from the pressure-regulating chamber  1  is fully closed, and the valve element  102  between the pressure Pc and the suction pressure Ps is fully opened. As a result, in addition to refrigerant being introduced from the pressure-regulating chamber  1  into the suction chamber  9  via the orifice  13 , refrigerant flows from the port  128  communicated with the pressure-regulating chamber  1 , and passes between the valve element  102  and the valve seat  114  therefor, followed by being introduced into the suction chamber  9  via the port  129 . Since the amount of refrigerant introduced from the pressure-regulating chamber  1  into the suction chamber  9  is increased, it is possible to increase a speed at which the operating displacement is maximized. 
     During execution of normal control in which a predetermined control current is supplied to the solenoid coil  125  of the solenoid section, the movable core  122  is attracted toward the fixed core  117  and moved upward, as viewed in the figure, according to the magnitude of the control current. As a result, the valve element  102  is opened from its closed state only when the differential pressure between the discharge pressure Pd and the suction pressure Ps exceeds a predetermined reference value. In short, during execution of the normal control, the displacement control valve  100  operates as a differential pressure valve. 
     FIG. 8 is a central longitudinal cross-sectional view of a displacement control valve according to a fifth embodiment. In FIG. 8, component parts and elements similar to those appearing in FIG. 7 are designated by identical reference numerals, and detailed description thereof is omitted. 
     The displacement control valve  100   a  according to the fifth embodiment has a different structure for canceling the influence of the pressure Pc upon valve elements  101 ,  102 , from that of the FIG. 7 displacement control valve  100 . shown in FIG.  7 . More specifically, a one-piece member formed by the valve element  101 , a piston  103  and a shaft  104 , and a one-piece member formed by the valve element  102 , a piston  103  and a shaft  107  are each formed as a solid member having no communication hole axially extending therethrough. On the other hand, a body  110  is formed with a communication hole  134  for introducing the pressure Pc into a back pressure chamber of the piston  103 . Further, a body  113  is formed with a communication hole  116  opening into a space forming a back pressure chamber of the piston  106  and a gap between respective portions of a fixed core  117  and a sleeve  119  closer to the solenoid section with respect to an O ring  133 . The displacement control valve  100   a  constructed as above operates similarly to the displacement control valve  100  of the fourth embodiment. 
     As described heretofore, the displacement control valve according to the present invention is comprised of first and second valve elements which are operated in an interlocked fashion for opening and closing passages communicating, respectively, between a discharge chamber and a pressure-regulating chamber and between the pressure-regulating chamber and a suction chamber, and a solenoid section which applies a solenoid force corresponding to a predetermined differential pressure to the first and second valve elements. This enables control to the minimum operating displacement in which introduction of refrigerant from the pressure-regulating chamber to the suction chamber is inhibited and refrigerant is introduced at a maximum flow rate from the discharge chamber to the pressure-regulating chamber as well as control to the -maximum operating displacement in which introduction of refrigerant from the discharge chamber to the pressure-regulating chamber is inhibited and refrigerant is introduced at a maximum flow rate from the pressure-regulating chamber to the suction chamber, thereby making it possible to sharply shorten time for transition between operating capacities. 
     Further, in the present invention, the first and second valve elements are integrally formed with a central shaft forming a pressure-sensing portion having a smaller pressure-receiving area than those of the first and second valve elements. This makes it possible to make the pressure-receiving areas of the respective first and second valve elements substantially equal to a difference in pressure-receiving area between the first or second valve element and the central shaft, and hence even if the size of the valve is increased so as to increase the amount of refrigerant permitted to flow during transition between operating capacities, the respective substantial pressure-receiving areas of the first and second valve elements can be reduced, irrespective of the valve size, by reducing the difference in pressure-receiving area between each of the first and second valve elements and the central shaft. Therefore, it is not required to increase the solenoid force for controlling the first and second valve elements, which makes it possible to reduce the size of a solenoid section. 
     The foregoing is considered as illustrative only of the principles of the present invention. Further, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and applications shown and described, and accordingly, all suitable modifications and equivalents may be regarded as falling within the scope of the invention in the appended claims and their equivalents.