Patent Publication Number: US-2007116578-A1

Title: Control Device for a Vehicular Refrigeration, Vehicular Variable Displacement Compressor, and A Control Valve for the Vehicular Variable Displacement Compressor

Description:
FIELD OF THE INVENTION  
      The present invention relates to a control device for a vehicular refrigeration circuit, a variable displacement compressor, and a control valve for the variable displacement compressor.  
     BACKGROUND OF THE INVENTION  
      JP 03-43685 A discloses a conventional variable displacement compressor. The variable displacement compressor includes a crank chamber, an intake chamber, and a discharge chamber, in which discharge capacity can be changed by controlling pressure of the crank chamber.  
      In the variable displacement compressor, the intake chamber is connected to an evaporator through tubing, the evaporator is connected to an expansion valve through the tubing, the expansion valve is connected to a condenser through the tubing, and the condenser is connected to the discharge chamber of the variable displacement compressor. With this construction, the variable displacement compressor may be used together with an exterior refrigerant circulation circuit composed of the evaporator, the expansion valve, the condenser, and the tubing, and may constitute a refrigeration circuit for a vehicle or the like.  
      In the variable displacement compressor, the intake chamber and the crank chamber communicate with each other through a bleed passage, and the discharge chamber and the crank chamber communicate with each other through an air supply passage. Further, the variable displacement compressor includes a first control device and a second control device built therein. The first control device and the second control device are control devices used for the refrigeration circuit which is used together with the variable displacement compressor and the exterior refrigerant circulation circuit.  
      The first control device includes a primary valve mechanism capable of changing a primary opening which is the degree of opening of the bleed passage, and a bellows provided in the bleed passage. The bellows moves in an axial direction while causing a first load due to the pressure of the crank chamber. The primary opening of the bleed passage decreases when the pressure of the crank chamber is higher than a set pressure and increases when the pressure of the crank chamber is lower than the set pressure.  
      Further, the first control device includes an intermediate chamber communicating with the discharge chamber through a fixed restriction and an actuating rod extending from the discharge chamber through the intermediate chamber to a valve body of the primary valve mechanism. The actuating rod has a small diameter portion on the discharge chamber side and a large diameter portion on the primary valve mechanism side, the large diameter portion being larger in diameter than the small diameter portion. An end of the small diameter portion of the actuating rod is exposed to the discharge chamber and a middle portion thereof faces the intermediate chamber. The discharge chamber and the intermediate chamber communicate with each other through the fixed restriction formed of a gap between the actuating rod and an axial hole of a valve cylinder for accommodating the actuating rod. Further, provision of the axial hole enables the intermediate chamber to allow pressure to act on the large diameter portion of the actuating rod. Thus, the actuating rod moves in the axial direction while causing a second load opposing the first load with the pressure of the discharge chamber and the pressure of the intermediate chamber, and adjust the primary valve mechanism so that the primary opening decreases or increases.  
      The second control device includes an electromagnetic flow control valve capable of varying the pressure of the intermediate chamber. The electromagnetic flow control valve is provided in a communication passage for allowing the intermediate chamber and the intake chamber to communicate with each other. The electromagnetic flow control valve can change a secondary opening, that is, the degree of opening of the communicating passage, due to an energization control from the outside, thereby changing the pressure of the intermediate chamber.  
      In the refrigeration circuit structured as described above, since the pressure of the crank chamber is a pressure with a factor for controlling the discharge capacity, the bellows of the first control device appropriately moves due to the pressure of the crank chamber, and the primary valve mechanism changes the primary opening of the bleed passage. As a result, this determines the pressure of the crank chamber primarily.  
      Further, the second control device changes the pressure of the intermediate chamber through the energization control. The actuating rod of the second control device appropriately moves due to the pressure of the discharge chamber and the pressure of the intermediate chamber. Thus, the actuating rod adjusts the primary valve mechanism so that the primary opening of the bleed passage decreases or increases. As a result, the pressure of the crank chamber is determined secondarily.  
      In this manner, the discharge capacity of the variable displacement compressor is changed. This refrigeration circuit is aimed at obtaining precise air conditioning according to the driving state of the variable displacement compressor, the outside environment, and the like.  
      However, the above-mentioned refrigeration circuit allows the pressure of the discharge chamber to act on the actuating rod as it is. The pressure of the discharge chamber naturally fluctuates according to the driving state of the variable displacement compressor, the outside environment, or the like. Therefore, in this refrigeration circuit, the actuating rod can resist the first load of the bellows too much or not resist it enough, so the pressure of the crank chamber may not be maintained in an optimum state.  
      The refrigeration circuit also allows the pressure of the intermediate chamber to act on the actuating rod and allows the pressure of the intermediate chamber to be changed through energization control from the outside by the electromagnetic flow control valve. Thus, it is felt that this refrigeration circuit is aimed at solving malfunctions due to fluctuations in the pressure of the discharge chamber by the energization control.  
      However, in this refrigeration circuit, when an attempt is made to solve malfunctions due to fluctuations of the pressure of the discharge chamber, the energization control must be performed in response to unsteady discharge pressure, so control by using a controller becomes complicated. In particular, in this refrigeration circuit, the pressure of the intermediate chamber itself, which is changed by the energization control, is affected by the pressure of the discharge chamber to fluctuate, so control by using the controller becomes more complicated.  
      Thus, this refrigerator circuit involves considerable difficulties in truly performing precise air conditioning according to a driving state of the variable displacement compressor, the outside environment, and the like.  
     SUMMARY OF THE INVENTION  
      The present invention has been made in view of the above-mentioned conventional problems. An object to be solved by the present invention is therefore the relatively easy realization of precise air conditioning according to the driving state of the variable displacement compressor, the outside environment, and the like.  
      According to the present invention, a control device for a vehicular refrigeration circuit is used together with a variable displacement compressor, having a crank chamber, an intake chamber and a discharge chamber, in which discharge capacity can be changed by controlling a pressure of the crank chamber, and with an exterior refrigerant circulation circuit connected to the intake chamber and the discharge chamber of the variable displacement compressor.  
      The control device is characterized by including: a bleed passage for communicating the intake chamber and the crank chamber with each other; an air supply passage for communicating the discharge chamber and the crank chamber with each other; a primary valve mechanism capable of changing a primary opening which is the degree of opening of at least one of the bleed passage and the air supply passage; a first pressure sensing member which moves while causing a first load due to a state pressure which is a pressure having a factor for controlling the discharge capacity and operates the primary valve mechanism so that the primary opening increases or decreases; a constant pressure valve mechanism for maintaining a high pressure in a high pressure chamber communicated with the discharge chamber, at a constant correction pressure; a secondary valve mechanism capable of changing the correction pressure to a predetermined control pressure by external energization control; and a second pressure sensing member which moves while causing a second load opposing the first load with the control pressure to correct the primary opening.  
      The primary valve mechanism may also be capable of changing the primary opening which is the degree of opening of the bleed passage, of changing the primary opening which is the degree of opening of the air supply passage, or of changing the primary opening which is the degree of opening of both the bleed passage and the air supply passage.  
      The first pressure sensing member moves due to a state pressure. The state pressure is a pressure having a factor for controlling the discharge capacity. As the state pressure, it is possible to adopt the intake pressure, for example, inside the intake chamber or in a low pressure portion of the exterior refrigerant circulation circuit, the discharge pressure, for example, of the discharge chamber or in a high pressure portion the exterior refrigerant circulation circuit, the differential pressure between the discharge pressures, or the like. As the first pressure sensing member, it is possible to adopt a bellows, a diaphragm, a rod, or the like.  
      As the constant pressure valve mechanism, a pressure regulating valve of a known constant pressure valve (i.e., pressure reducing valve) or the like may be adopted. Note that, the high pressure chamber may be directly connected to the discharge chamber or may be indirectly connected to the discharge chamber so as to be directly connected to the high pressure portion of the exterior refrigerant circulation circuit or the like.  
      As the secondary valve mechanism, it is possible to adopt a piezoelectric element, an electromagnetic opening/closing valve, or the like.  
      The second pressure sensing member moves due to the control pressure. As the second pressure sensing member, it is also possible to adopt a bellows, a diaphragm, a rod, or the like. In a case where a bellows is adopted as the second pressure sensing member, the control pressure may be introduced into a chamber accommodating the bellows or may be introduced into the bellows.  
      The control device according to the present invention may comprise: a state pressure chamber communicating with the discharge chamber through a state pressure passage; and a differential pressure generating mechanism provided in the state pressure passage, for causing the state pressure to be turned into a first state pressure and a second state pressure having a difference therebetween, wherein the first pressure sensing member is provided in the state pressure chamber, and moves due to a differential pressure between the first state pressure and the second state pressure while causing the first load.  
      In this case, the differential pressure between the discharge pressures is the state pressure. The larger the flow rate of the refrigerant flowing through the exterior refrigerant circulation circuit becomes, the larger the pressure loss per unit length of the exterior refrigerant circulation circuit. Therefore, when the first pressure sensing member moves due to the differential pressure between the first state pressure and the second state pressure, the discharge capacity is determined while taking into consideration the flow rate of the refrigerant flowing through the exterior refrigerant circulation circuit.  
      When a bellows is adopted as the first pressure sensing member, the primary valve mechanism can be operated at high precision by the state pressure. Further, the movement is limited to the axial direction, so the direction of the first load is also limited to the axial direction, thus the first load can oppose the second load in a favorable manner.  
      The control device according to the present invention may comprise a second pressure sensing chamber accommodating the second pressure sensing member and constituting a part of the air supply passage, wherein the second pressure sensing member is a bellows including therein a second control chamber to which the control pressure is introduced.  
      In the control device according to the present invention, the secondary valve mechanism may comprise: a high pressure passage for communicating the discharge chamber and the second control chamber with each other; a release passage for communicating the second control chamber and the air supply passage with each other; and an actuator operated by controlling energization with respect thereto from outside and capable of changing a secondary opening which is the degree of opening of the release passage.  
      In the control device according to the present invention, the secondary valve mechanism may comprise: a high pressure passage for communicating the discharge chamber and the second control chamber with each other; and an actuator operated by controlling energization with respect thereto from outside and capable of changing a secondary opening which is the degree of opening of the high pressure passage.  
      It is preferable that the actuator is formed of a piezoelectric element because, with such construction, it is possible to achieve downsizing of the actuator and to easily change the secondary opening due to energization control from the outside.  
      The control device according to the present invention preferably includes a controller for controlling a duty ratio of a voltage to be applied to the piezoelectric element. The reason for this is that the provision of the controller facilitates energization control with respect to the piezoelectric element according to the driving state of the variable displacement compressor, the exterior environment, and the like.  
      The variable displacement compressor according to the present invention comprises: a housing including a cylinder bore, a crank chamber, an intake chamber, and a discharge chamber formed therein; a piston reciprocatingly accommodated in the cylinder bore, for defining a compression chamber in the cylinder bore; a drive shaft driven by an exterior drive source and rotatably supported by the housing; a swash plate supported in the crank chamber so that the swash plate can rotate in synchronism with the drive shaft and can be inclined, for allowing the piston to be reciprocatingly driven; and a control mechanism for controlling a pressure in the crank chamber, capable of changing an inclination angle of the swash plate to change discharge capacity, the intake chamber and the discharge chamber being connected to an exterior refrigerant circulation circuit to constitute a vehicular refrigeration circuit, wherein the control mechanism is the control device according to the present invention.  
      By using the variable displacement compressor for the vehicular refrigeration circuit, it is relatively easy to realize precise air conditioning according to the driving state of the vehicle, the outside environment, and the like.  
      The control valve for the variable displacement compressor according to the present invention comprises: a valve housing fixed to the housing; the air supply passage and the high pressure chamber formed in the valve housing; and the primary valve mechanism, the first pressure sensing member, the constant pressure valve mechanism, the secondary valve mechanism, and the second pressure sensing member provided in the valve housing.  
      As the control valve is integral with the variable displacement compressor, by using the variable displacement compressor for the vehicular refrigeration circuit, it is possible to obtain the above-mentioned operational effects. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a schematic sectional view of a part of a variable displacement compressor or the like of a vehicular refrigeration circuit according to Embodiment 1 of the present invention;  
       FIG. 2  is a sectional view of a control valve of the vehicular refrigeration circuit according to Embodiment 1;  
       FIG. 3  is an enlarged sectional view of a main part of the control valve of the vehicular refrigeration circuit according to Embodiment 1;  
       FIG. 4  is a block diagram of a control system of the vehicular refrigeration circuit according to Embodiment 1;  
       FIG. 5  is a graph showing a relationship between time and a discharge pressure of the vehicular refrigeration circuit according to Embodiment 1;  
       FIG. 6  is a graph showing a control property of a controller of the vehicular refrigeration circuit according to Embodiment 1;  
       FIG. 7  is a schematic view of a main part of the control valve of the vehicular refrigeration circuit according to Embodiment 1;  
       FIG. 8  is an enlarged sectional view of a main part of a control valve of a vehicular refrigeration circuit according to Embodiment 2 of the present invention;  
       FIG. 9  is a schematic view of a main part of the control valve of the vehicular refrigeration circuit according to Embodiment 2;  
       FIG. 10  is a graph showing a control property of a controller of the vehicular refrigeration circuit according to Embodiment 2;  
       FIG. 11  is a schematic view of a main part of a control valve of a vehicular refrigeration circuit according to a modification of the present invention; and  
       FIG. 12  is a schematic view of a main part of a control valve of a vehicular refrigeration circuit according to another modification of the present invention. 
    
    
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS  
      Hereinafter, Embodiments 1 and 2 of the present invention will be described with reference to the drawings.  
     Embodiment 1  
      As shown in  FIG. 1 , a vehicular refrigeration circuit according to Embodiment 1 of the present invention includes a variable displacement compressor  1 , an evaporator  2 , an expansion valve  3 , a condenser  4 , and tubing  5  connecting them to each other.  
      The variable displacement compressor  1  includes a housing composed of a cylinder block  11 , a front housing  12 , a rear housing  13 , and a valve plate  14 . The cylinder block  11  is provided with a plurality of cylinder bores  11   a  aligned in a circumferential direction so as to be in parallel with one another. The plurality of cylinder bores  11   a  pass through the cylinder block  11  in an axial direction. Each of the cylinder bores  11   a  accommodates a piston  15  such that the piston  15  can reciprocate. The head of each of the pistons  15  defines a compression chamber in each of the cylinder bores  11   a.    
      One end side of the cylinder block  11  is connected to the front housing  12 . The cylinder block  11  and the front housing  12  are provided with axial holes passing through the cylinder block  11  and the front housing  12  and extending in the axial direction. Inner portions of the axial holes constitute a crank chamber  16 . The axial hole of the front housing  12  is provided with a sealing device S and a radial bearing  17 . The axial hole of the cylinder block  11  is provided with a radial bearing  18  and a thrust bearing  19 . A drive shaft  20  is supported by the seal device S, the radial bearing  17 , the radial bearing  18 , and the thrust bearing  19  so as to be rotatable. An end of the drive shaft  20  is positioned in a boss of the front housing  12 . The drive shaft  20  is driven by an engine EG for a vehicle through an electromagnetic clutch MG. The engine EG serves as an exterior drive source. In a case where a vehicle is not driven by the engine EG but is driven by a motor, the motor serves as the exterior drive source.  
      In the crank chamber  16 , a lag plate  21  is fixed to the drive shaft  20 , and a thrust bearing  22  is provided between the lag plate  21  and the front housing  12 . Further, on a rear side of the lag plate  21 , a swash plate SP, through which the drive shaft  20  is inserted, is supported so as to be rotatable in synchronism with the drive shaft  20  and to be capable of inclining relative to the drive shaft  20 . A bias spring  23  and a hinge mechanism  24  are provided between the lag plate  21  and the swash plate SP. On an outer peripheral side of the swash plate SP, there are provided pairs of shoes  25  for both front and rear sides. Both shoes  25  being respectively sandwiched by the pistons  15 .  
      The cylinder block  11  and the rear housing  13  are connected to each other through the intermediation of the valve plate  14  provided therebetween. On a front surface of the valve plate  14 , there is provided an intake valve plate  26 . On a rear surface of the valve plate  14 , there are provided a discharge valve plate  27  and a retainer  28 . The intake valve plate  26 , the valve plate  14 , the discharge valve plate  27 , and the retainer  28  are fastened together by a bolt  29  and a nut  30 . Further, a bias spring  31  is provided between the thrust bearing  19  and the intake valve plate  26 .  
      The rear housing  13  is provided with an intake chamber  32  and a discharge chamber  33  formed therein. The valve plate  14  is provided with an intake port passing therethrough for communicating with the intake chamber  32 . An intake reed portion of the intake valve plate  26  is positioned on the compression chamber side of the intake port. Further, the intake valve plate  26  and the valve plate  14  are provided with a discharge port passing therethrough for communicating with the compression chamber. A discharge reed portion of the discharge valve plate  27  is positioned on the discharge chamber  33  side of the discharge port.  
      The rear housing  13  is provided with a control valve  34  as a control device. The cylinder block  11 , the intake valve plate  26 , and the valve plate  14  are provided with a bleed passage  35  passing through those, for communicating the intake chamber  32  and the crank chamber  16  with each other through a fixed restriction  35   a . Further, the cylinder block  11 , the intake valve plate  26 , the valve plate  14 , and the rear housing  13  are provided with a pressure detecting passage  36   a  and air supply passages  36   b  and  36   c  passing through those, for communicating the discharge chamber  33  and the crank chamber  16  with each other through the control valve  34 . The pressure detecting passage  36   a  extends from the discharge chamber  33  to the control valve  34 . The air supply passage  36   b  extends from the discharge chamber  33  to the control valve  34  through a fixed restriction  33   a . The fixed restriction  33   a  serves as a differential pressure generating mechanism. The pressure detecting passage  36   a  and the air supply passage  36   b  also serve as state pressure passages. The air supply passage  36   c  extends from the control valve  34  to the crank chamber  16 .  
      The control valve  34  includes, as shown in  FIG. 2 , a valve housing composed of a first valve housing  37 , an adjusting screw  38 , and a second valve housing  39 . The adjusting screw  38  is threaded on an end of the first valve housing  37 , thereby forming a first pressure sensing chamber  40  serving as a state pressure chamber. The first pressure sensing chamber  40  accommodates therein a first bellows  41  serving as a first pressure sensing member. An end of the first bellows  41  is fixed to the adjusting screw  38 .  
      The adjusting screw  38  has a communication hole  38   a  passing therethrough, for communicating the pressure detecting passage  36   a  with a first control chamber  41   a  in the first bellows  41 . Further, the first valve housing  37  has a communication hole  37   a  passing therethrough, for communicating the air supply passage  36   b  with the first pressure sensing chamber  40 . The communication holes  38   a  and  37   a  also serve as the state pressure passages.  
      The other end side of the first valve housing  37  is connected to the second valve housing  39 . The first valve housing  37  and the second valve housing  39  form a second pressure sensing chamber  42  coaxial with the first pressure sensing chamber  40 . The first pressure sensing chamber  40  and the second pressure sensing chamber  42  communicate with each other through an axial hole  37   b . Further, the first valve housing  37  has a communication hole  37   c  passing therethrough, for communicating the air supply passage  36   c  with the second pressure sensing chamber  42 . Thus, the air supply passage  36   b  can communicate with the air supply passage  36   c  through the communication hole  37   a , the first pressure sensing chamber  40 , the axial hole  37   b , the second pressure sensing chamber  42 , and the communication hole  37   c.    
      The second pressure sensing chamber  42  accommodates therein a second bellows  43  serving as a second pressure sensing member. The other end of the second bellows  43  is fixed to a fixed member  44  provided in the second pressure sensing chamber  42 .  
      The other end of the first bellows  41  is fixed to a first rod  45  extending into the axial hole  37   b , while an end of the second bellows  43  is fixed to a second rod  46  extending toward the axial hole  37   b  and faces and abuts on the first rod  45 . The axial hole  37   b , the first rod  45 , and the second rod  46  constitute a primary valve mechanism having a structure in which the periphery of axial hole  37   b  serves as a valve seat, the second rod  46  serves as a valve body, and a primary opening which is the degree of opening of the axial hole  37   b  can be changed.  
      Further, the first valve housing  37  and the second valve housing  39  are provided with an accommodation chamber  48  formed therein. The accommodation chamber  48  connects to the first pressure sensing chamber  40  through a first high pressure passage  49 . The accommodation chamber  48  accommodates a constant pressure valve  100 . The constant pressure valve  100  includes a first cylindrical body  50 , a diaphragm  51 , and a second cylindrical body  52  which are coaxial with one another.  
      The first cylindrical body  50  includes, as shown in  FIG. 3 , a high pressure chamber  50   a  formed on one end side which is the first high pressure passage  49  side, a constant pressure chamber  50   b  formed on the other end side, and a valve seat  50   c  having an axial hole, which is held between the high pressure chamber  50   a  and the constant pressure chamber  50   b . Further, the first cylindrical body  50  and the first valve housing  37  are provided with a second high pressure passage  53  for communicating the constant pressure chamber  50   b  with the second pressure sensing chamber  42 . The high pressure chamber  50   a , the constant pressure chamber  50   b , the first high pressure passage  49 , and the second high pressure passage  53  are high pressure passages.  
      The first cylindrical body  50  and the second cylindrical body  52  sandwich the diaphragm  51  therebetween. A rod  54  extending into the axial hole of the valve seat  50   c  is fixed to the diaphragm  51 . An end of the rod  54  constitutes a valve body  54   a  positioned in the high pressure chamber  50   a  and having a larger diameter. Further, the second cylindrical body  52  accommodates therein a bias spring  55  for biasing the diaphragm  51  toward the constant pressure chamber  50   b.    
      In the second pressure sensing chamber  42 , the fixed member  44  is sandwiched between the first valve housing  37  and the second valve housing  39 . The fixed member  44  separates the second pressure sensing chamber  42  into a first chamber  42   a  in which the second bellows  43  is located and a second chamber  42   b  as the rest of the fixed chamber  44 .  
      Formed in the fixed member  44  are a third high pressure passage  56  for communicating the second high pressure passage  53  with the second control chamber  43   a  in the second bellows  43 , a first release passage  57  extending in the axial direction, for communicating the second control chamber  43   a  with the second chamber  42   b , and a second release passage  58  for communicating the second chamber  42   b  with the first chamber  42   a  outside the second bellows  43 .  
      In the second chamber  42   b  of the second pressure sensing chamber  42 , there is provided an actuator  59  formed of a piezoelectric element. As shown in  FIG. 4 , the actuator  59  is connected to a controller  62  via a driver  61 , with a lead wire  60  fixed to the second valve housing  39 . The controller  62  is connected to a plurality of sensors  63  and  64 , such as a room temperature sensor, an outside air temperature sensor, and a sensor for the degree of opening of an accelerator of a vehicle, switches  65  and  66 , and the like.  
      In the variable displacement compressor  1 , as shown in  FIG. 1 , the intake chamber  32  is connected to the evaporator  2  through the tubing  5 , the evaporator  2  is connected to the expansion valve  3  through the tubing  5 , the expansion valve  3  is connected to the condenser  4  through the tubing  5 , and the condenser  4  is connected to the discharge chamber  33  of the variable displacement compressor  1 . With this construction, the variable displacement compressor  1  is used together with an exterior refrigerant circulation circuit composed of the evaporator  2 , the expansion valve  3 , the condenser  4 , and the tubing  5  to constitute the vehicular refrigeration circuit. In the vehicular refrigeration circuit, CO 2  is adopted as a refrigerant.  
      In the refrigeration circuit structured as described above, the drive shaft  20  of the variable displacement compressor  1  is rotated by the engine EG or the like. As a result, the swash plate SP rotates while being inclined at a certain angle with respect to the drive shaft  20 , and each of the pistons  15  reciprocates in each of the cylinder bores  11   a , so a low-pressure refrigerant is sequentially taken into the intake chamber  32  from the evaporator  2 . The refrigerant is compressed in the compression chamber, and is then discharged to the discharge chamber  33  to be discharged toward the condenser  4 . Air supplied to the evaporator  2  is provided for the air conditioning inside the vehicle.  
      During the above-mentioned process, the control valve  34  controls the pressure in the crank chamber  16  and changes the inclination angle of the swash plate SP to change the discharge capacity as follows.  
      First, the pressure of the discharge chamber  33  is a discharge pressure Pd which is high. However, the fixed restriction  33   a  exists between the discharge chamber  33  and the air supply passage  36   b , so, as shown in  FIG. 2 , pressure of the refrigerant in the pressure detecting passage  36   a  is high discharge pressure PdH, and pressure of the refrigerant in the air supply passage  36   b  is low discharge pressure PdL. The high discharge pressure PdH is a first state pressure and the low discharge pressure PdL is a second state pressure.  
      The high discharge pressure PdH in the pressure detecting passage  36   a  is introduced into the first control chamber  41   a  in the first bellows  41  through the communication hole  38   a . On the other hand, the low discharge pressure PdL in the air supply passage  36   b  is introduced into the first pressure sensing chamber  40  through the communication hole  37   a . Thus, the first bellows  41  moves while causing a first load F 1  due to a differential pressure ΔPd between the high discharge pressure PdH and the low discharge pressure PdL. Note that the high discharge pressure PdH and the low discharge pressure PdL fluctuate according to the driving state, the outside environment, or the like of the variable displacement compressor  1 , but the differential pressure ΔPd therebetween has little fluctuation range.  
      Accordingly, the second rod  46  changes the primary opening of the axial hole  37   b . As a result, in the refrigeration circuit, the pressure Pc of the crank chamber  16  is intended to be primarily determined while taking into consideration the flow rate of the refrigerant flowing through the exterior refrigerant circulation circuit.  
      In this case, when the effective sectional area of the first bellows  41  is A, the load of thrust resulting from flow rate differential pressure is derived from the equation: A·ΔPd. When the effective sectional area of the first rod  45  and the second rod  46  is B, the load caused by high-pressure correction is derived from the equation: B·(PdL−Pc). Thus, the first load F 1  is derived from the equation: A·ΔPd+B·(PdL−Pc).  
      As shown by the broken line of  FIG. 5 , the low discharge pressure PdL fluctuates according to the driving state of the variable displacement compressor  1 , the outside environment, or the like. Therefore, in the refrigeration circuit, the low discharge pressure PdL is not introduced into the second control chamber  43   a  as it is. As shown in  FIG. 3 , the low discharge pressure PdL in the first pressure sensing chamber  40  is introduced into the high pressure chamber  50   a  of the constant pressure valve  100  through the first high pressure passage  49 . The low discharge pressure PdL in the high pressure chamber  50   a  is introduced into the constant pressure chamber  50   b  through the axial hole of the valve seat  50   c . Note that the constant pressure in the high pressure chamber may have a certain degree of error.  
      For example, when the low discharge pressure PdL in the constant pressure chamber  50   b  is higher than a desired correction pressure PdL 0 , the constant pressure chamber  50   b  presses the diaphragm  51  against the bias force of the bias spring  55 , so the valve body  54   a  causes the degree of opening of the axial hole of the valve seat  50   c  to decrease. On the other hand, when the low discharge pressure PdL in the constant pressure chamber  50   b  is lower than the desired correction pressure PdL 0 , the constant pressure chamber  50   b  pulls the diaphragm  51  while yielding to the bias force of the bias spring  55 , so the valve body  54   a  causes the degree of opening of the axial hole of the valve seat  50   c  to increase. In this manner, as shown in  FIG. 5 , the pressure in the constant pressure chamber  50   b  is maintained at the correction pressure PdL 0 .  
      The correction pressure PdL 0  in the constant pressure chamber  50   b  is introduced into the second control chamber  43   a  of the second bellows  43  through the second high pressure passage  53  and the third high pressure passage  56 .  
      On the other hand, from the sensors  63  and  64 , the switches  65  and  66 , and the like, information on the driving state of the variable displacement compressor  1 , the outside environment, and the like is transmitted to the controller  62 . The controller  62  controls energization with respect to the actuator  59  through the driver  61  according to a duty ratio shown in  FIG. 6 .  
      In this case, when the actuator  59  causes the secondary opening of the first release passage  57  to increase due to the energization control by the controller  62 , the control pressure PdLx in the second control chamber  43   a  in the second bellows  43  passes through the first release passage  57  and the second release passage  58  to reach the first chamber  42   a  of the second pressure sensing chamber  42 , and passes through the communication passage  37   c  and the air supply passage  36   c  in the stated order to reach the crank chamber  16 . Therefore, the control pressure PdLx in the second control chamber  43   a  becomes lower. Thus, the second load F 2  of the second bellows  43  decreases, so the pressure Pc of the crank chamber  16  is greatly affected by the movement of the first bellows  41  and the first rod  45 .  
      In contrast, when the actuator  59  causes the secondary opening of the first release passage  57  to decrease due to the energization control by the controller  62 , the control pressure PdLx in the second control chamber  43   a  in the second bellows  43  becomes higher. Thus, the second load F 2  of the second bellows  43  increases, so the pressure Pc of the crank chamber  16  becomes less prone to be affected by the movement of the first bellows  41  and the first rod  45 .  
      Thus, as shown in  FIG. 7 , the second bellows  43  opposes the first load F 1  of the first bellows  41  with the control pressure PdLx thereof to an appropriate degree to suitably correct the second rod  46  such that the primary opening decreases or increases.  
      In this case, when the effective sectional area of the second bellows  43  is C, the second load F 2  serving as a thrust is derived by the equation: C·PdLx. As a result, the following formula is established.  
      (Formula 1)
 
 A·ΔPd+B ·( PdL−Pc )= C·PdLx 
 
      By using this formula, the pressure Pc of the crank chamber  16  is secondarily determined. In the variable displacement compressor  1 , the high pressure refrigerant in the discharge chamber  33  is supplied to the crank chamber  16  such that the resultant condition is satisfied, while an excess of the refrigerant in the crank chamber  16  is delivered to the intake chamber  32  through the bleed passage  35 . As a result, the pressure Pc of the crank chamber  16  is easily turned into an optimum state. According to the pressure in the crank chamber  16 , the inclination angle of the swash plate SP changes, thereby appropriately changing the discharge capacity.  
      During the above-mentioned processes, in the refrigeration circuit, the constant pressure valve  100 , which is different from the actuator  59  whose energization is controlled from the outside, maintains the high pressure in the high pressure chamber  50   a  to be at the constant correction pressure PdL 0 . Therefore, it suffices that the controller  62  controls energization with respect to the actuator  59  on a condition that the constant correction pressure PdL 0  is maintained, thereby facilitating the control.  
      Consequently, according to the refrigeration circuit of Embodiment 1, it is possible to relatively easy realization of precise air conditioning according to the driving state of the variable displacement compressor  1 , the outside environment, and the like. This enhances utility of the refrigeration circuit and the variable displacement compressor  1 .  
      Further, the refrigeration circuit adopts the first bellows  41  and the second bellows  43 , so it is possible to operate the second rod  46  with high precision. Further, the movement is limited to the axial direction and the first load F 1  and the second load F 2  are also limited to the axial direction, so the first load F 1  and the second load F 2  can oppose each other in a favorable manner.  
      Further, the refrigeration circuit adopts the actuator  59  formed of a piezoelectric element, so it is possible to achieve both downsizing of the variable displacement compressor  1  and easy control of energization.  
     Embodiment 2  
      A refrigeration circuit according to Embodiment 2 is different from the refrigeration circuit according to Embodiment 1 in that it includes a fixed member  70  as shown in  FIGS. 8 and 9 . The fixed member  70  includes a third high pressure passage  71  for communicating the second high pressure passage  53  with the second chamber  42   b , a fourth high pressure passage  72  extending in the axial direction, for communicating the second chamber  42   b  with the second control chamber  43   a , and a release passage  73  for communicating the second control chamber  43   a  with the first chamber  42   a  outside the second bellows  43  formed in the fixed member  70 . The release passage  73  also serves as the fixed restriction. The controller  62  controls energization with respect to the actuator  59  through the driver  61  according to the duty ratio shown in  FIG. 10 . Other constructions are the same as those of Embodiment 1. The same structures are denoted by the same reference numerals, and detailed descriptions of those will be omitted.  
      In this case, the correction pressure PdL 0  in the constant pressure chamber  50   b  is introduced into the second chamber  42   b  of the second pressure sensing chamber  42  through the second high pressure passage  53  and the third high pressure passage  71 . Thus, when the actuator  59  causes the secondary opening of the fourth high pressure passage  72  to increase due to the energization control by the controller  62 , the control pressure PdLx in the second chamber  42   b  is introduced into the second control chamber  43   a  in the second bellows  43 , and the control pressure PdLx in the second control chamber  43   a  increases. Accordingly, the second load F 2  of the second bellows  43  increases, so the pressure Pc of the crank chamber  16  is less prone to be affected by the movement of the first bellows  41  and the first rod  45 .  
      On the other hand, when the actuator  69  causes the secondary opening of the fourth high pressure passage  72  to decrease due to the energization control by the controller  62 , the control pressure PdLx in the second control chamber  43   a  becomes lower. Accordingly, the second load F 2  of the second bellows  43  decreases, so the pressure Pc of the crank chamber  16  is greatly affected by the movement of the first bellows  41  and the first rod  45 .  
      Thus, also in the refrigeration circuit according to Embodiment 2, the same operational effects as those of Embodiment 1 can be obtained.  
      Hereinbefore, Embodiments 1 and 2 of the present invention are described. However, the present invention is not limited to Embodiments 1 and 2. It is needless to say that the present invention may be appropriately modified for application without departing from the scope of the present invention.  
      For example, as shown in  FIG. 11 , the second bellows  43 , which is set such that an interior thereof is maintained at a predetermined pressure, may be accommodated in the second pressure sensing chamber  42 , the control pressure PdLx may be introduced to an exterior of the second bellows  43 , and the actuator  59  may be provided to a portion where the second pressure sensing chamber  42  is communicated with the crank chamber  16 .  
      Further, as shown in  FIG. 12 , the second bellows  43 , which is set such that the interior thereof is maintained at a predetermined pressure, may be accommodated in the second pressure sensing chamber  42 , the control pressure PdLx may be introduced to the exterior of the second bellows  43  through the actuator  59 , and a fixed restriction  73  may be provided to a portion where the second pressure sensing chamber  42  is communicated with the crank chamber  16 .  
      The present invention may be applied to a vehicular refrigeration circuit, a vehicular variable displacement compressor, and the like.