Patent Publication Number: US-6334759-B1

Title: Control valve for variable displacement compressor

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a control valve for a variable displacement compressor, particular to a displacement control valve for a swash-plate-type variable displacement compressor which is applied to an on-vehicle air-conditioning unit or the like. The compressor has a devise for releasing a pressure in a crank chamber. 
     2. Related Art 
     Known displacement control valves for a swash-plate-type variable displacement compressor are disclosed in Japan patent Laid-open No. H. 3-53474, Japan Utility Model Laid-open No. H. 6-17010, and Japan patent Application Laid-open No. H. 8-177735. 
     The swash-plate-type variable displacement compressor having the control valve basically decreases in discharge displacement with increase of a crank chamber pressure of the compressor and increases in the displacement with the decrease of the crank chamber pressure. 
     The control valve opens and closes a communication passage communicating a suction port of the compressor with the crank chamber by using a valve element moving in response to a suction pressure of the compressor, thereby controlling the pressure of the crank chamber. 
     Furthermore, the valve element of the control valve is moved toward the open position by the discharge pressure of the compressor or by increasing a spring force of a correction spring resiliently biasing the valve element toward the open position with increase of the discharge pressure to vary the opening-closing switching point of the valve element in response to the discharge pressure. Thus, the control valve controls the discharge displacement in relation to an outer-air condition (or the discharge pressure). 
     Each known displacement control valve is constructed to achieve its object. However, a recent variable displacement compressor has a less leak flow rate of a fluid leaking into a crank chamber with regard to a discharge fluid in a piston/cylinder section. Thus, when the compressor provides a low discharge pressure, the crank chamber can not obtain an enough pressure for achieving a desired control performance as the displacement control valve, in which the control valve will not meet the compressor in their control performances. 
     Increase of the leak rate of a discharge pressure fluid leaking into the crank chamber would solve the problem in a low discharge pressure state of the compressor. However, this increases a pressure loss in a high discharge pressure state of the compressor, undesirably increasing a energy loss against an energy saving trend. 
     SUMMARY OF THE INVENTION 
     In view of the above-described disadvantage, an object of the invention is to provide an improved control valve for a swash-plate-type variable displacement compressor having a devise for releasing a pressure of a crank chamber. The improved control valve will not increase a pressure loss in a high discharge pressure state of the compressor and provides a solution of the problem in a low discharge pressure state of the compressor so as to have a control characteristic to meet with the performance of the compressor, enabling a displacement control in relation to an environmental condition. 
     For achieving the object, a first aspect of the invention is a control valve for variable displacement compressor which includes: 
     a valve housing having a communication passage communicating a suction port of the compressor with an crank chamber of the compressor, 
     a main valve provided in the valve housing for opening and closing the communication passage, 
     a spring member correction biasing the main valve toward its closed position, and 
     a pressure actuated unit for moving the main valve toward its open position by receiving a suction pressure of the compressor, 
     a flow adjustment valve disposed in the valve housing for adjusting an open degree of said leak passage and receiving the discharge pressure exerting a force on the leak flow adjustment valve toward its closed position, the leak flow adjustment valve increasing a leak flow rate of a fluid flowing from a discharge port of the compressor to a crank chamber through the leak flow adjustment valve when the compressor provides a lower discharge pressure, and 
     a correction spring disposed between the main valve and the leak flow adjustment valve, the correction spring resiliently biasing a valve element of the main valve toward the open position, the correction spring increasing in spring force with transfer of a valve element of the leak flow adjustment valve toward the closed position. 
     The flow adjustment valve may be a slide valve slidingly received in a valve receiving recess formed in the valve housing and defines a leak passage between an outer surface of the leak flow adjustment valve and an inner surface the valve receiving recess, the leak passage becoming shorter with the transfer of the valve element of the leak flow adjustment valve toward its open position, adjusting a leak flow rate of a fluid flowing from the compressor discharge port to the crank chamber. 
     A second aspect of the invention is a control valve for variable displacement compressor comprising: 
     a valve housing having a communication passage communicating a suction port of the compressor with a crank chamber of the compressor, 
     a main valve provided in the valve housing for opening and closing the communication passage, 
     a spring member resiliently biasing the main valve toward its closed position, and 
     a pressure actuated unit for moving the main valve toward its open position by receiving a suction pressure of the compressor, 
     an auxiliary biasing unit disposed in the valve housing and exerting a force on the main valve toward its valve open position by a differential pressure between a discharge pressure of the compressor and a pressure of the crank chamber, 
     a leak passage provided in said valve housing for communicating a discharge port of said compressor with said crank chamber, and 
     a leak flow adjustment valve disposed in the valve housing for adjusting an open degree of said leak passage and receiving a discharge pressure exerting a force on the leak flow adjustment valve toward the closed position, the leak flow adjustment valve increasing a leak flow rate of a fluid flowing from a discharge port of the compressor to a crank chamber through the leak flow adjustment valve when the compressor provides a lower discharge pressure. 
     The leak flow adjustment valve may be a variable flow orifice valve adjusting a leak flow rate of a fluid flowing from the discharge port of the compressor to the crank chamber, the flow rate being proportional to an open rate of the leak flow adjustment valve. Alternatively, the leak flow adjustment valve may be a check valve being open when the discharge pressure of the compressor is lower than a reference pressure. 
     Next, an operation of each invention aspect will be discussed. 
     In the control valve for variable displacement compressor of the first aspect of the invention, the main valve opens and closes the communication passage in response to the suction pressure of the compressor to control the crank chamber pressure. The valve element of the leak flow adjustment valve moves toward the valve closed potion with increase of a discharge pressure, which increases the spring force of the correction spring with increase of the discharge pressure. That is, the main valve varies in its open or close starting point in response to the discharge pressure, enabling a displacement control in relation to a load according to an environmental air condition (or in relation to the discharge pressure). 
     Furthermore, in a lower discharge pressure state of the compressor, the leak flow adjustment valve increases a leak rate of a fluid flowing from the discharge port to the crank chamber to increase the crank chamber pressure, thereby keeping a control characteristic of the displacement control valve when the compressor is providing a low discharge pressure. The control valve characteristic readily meets the performance of the compressor. 
     The leak flow rate of the fluid leaking from the compressor discharge port to the crank chamber is adjusted basically proportionally to the open degree of the leak adjustment valve, adjusting the crank chamber pressure in response to the discharge pressure. 
     In the control valve for the variable displacement compressor of the second aspect of the invention, the main valve opens and closes the communication passage in response to the suction pressure of the compressor to control the crank chamber pressure. The force exerted on the main valve by the auxiliary biasing unit increases with increase of the discharge pressure. That is, the main valve varies in its open or close starting point in response to the discharge pressure, enabling a displacement control in relation to the discharge pressure (a load according to an environmental air condition). Furthermore, in a lower discharge pressure state of the compressor, the leak flow adjustment valve increases the rate of a leak flowing from the discharge port to the crank chamber to increase the crank chamber pressure, thereby keeping a control characteristic of the displacement control valve when the compressor is providing a low discharge pressure. The control valve characteristic readily meets the performance of the compressor. 
     The leak rate of the fluid leaking from the compressor discharge port to the crank chamber is adjusted proportionally to the open degree of the orifice-type leak adjustment valve. This adjusts the crank chamber pressure according to the discharge pressure. 
     The check valve used for the leak adjustment valve opens to adjust the crank chamber pressure when the deferential pressure between the discharge pressure and the crank chamber pressure is smaller than the reference pressure. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a sectional view showing an embodiment of a variable displacement compressor having a control valve according to the invention; 
     FIG. 2 is a sectional view showing a first embodiment of a control valve according to the invention for a vehicle displacement compressor; 
     FIGS. 3A to  3 C each are parallel enlarged view illustrating an operation of a leak flow adjustment valve assembled in the control valve for the variable displacement compressor; 
     FIG. 4 is a graph showing a specific performance of the control valve of the variable displacement compressor with regard to a discharge pressure and an intake pressure according to the invention; 
     FIG. 5 is a sectional view showing a second embodiment of a control valve according to the invention for a variable displacement compressor; 
     FIGS. 6A to  6 C each are a partial enlarged view illustrating an operation of a leak flow adjustment valve assembled in the control valve for the variable displacement compressor in the second embodiment; 
     FIG. 7 is a graph showing a specific relation between a differential pressure and an open sectional area of the leak flow adjustment valve in the second embodiment; 
     FIG. 8 is a graph showing a discharge pressure relating to an intake pressure of the control valve of the variable displacement compressor according to the invention; 
     FIG. 9 is a sectional view showing a third embodiment of a control valve according to the invention for a variable displacement compressor; 
     FIG. 10 is a graph showing a specific relation between a differential pressure and an open sectional area of a leak flow adjustment valve in the third embodiment; and 
     FIG. 11 is a sectional view showing a fourth embodiment of a control valve according to the invention for a variable displacement compressor. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to the accompanied drawings, an embodiment of the present invention will be discussed hereinafter. 
     A First Embodiment 
     FIG. 1 shows a variable displacement compressor having a displacement control valve embodying the present invention. FIG. 2 shows a first embodiment of the displacement control valve. 
     The variable displacement compressor  1  of a swash-plate type has a crank chamber  3  defined in a compressor body  2  and has a plurality of cylinder chambers  4  each communicating with the crank chamber  3  at a stroke end thereof. Each cylinder chamber  4  engages axially a slidable piston  5  that is coupled to an end of a piston rod  6  on the side facing the crank chamber  3 . 
     The compressor housing  2  supports rotatively a drive shaft  7  which is rotated through a drive belt (not shown) coupled to a pulley  8  by an engine (not shown). 
     The drive shaft  7  is joined to a swash plate (inclined plate)  9  within the crank chamber  3  through a conventional connection link (not shown) to be able to vary the mounting angle of the swash plate  9 . The swash plate  9  has a surface engaging with the piston rod  6  on the side defining the cylinder chamber  4  so as to exert a axial force of the piston rod  6 . 
     The swash plate  9  that is in an inclined state is rotated through the drive shaft  7 . Thereby, the piston  5  of each cylinder chamber  4  reciprocates with a stroke corresponding to an inclined angle of the swash plate  9 . The incline angle is automatically adjusted according to a difference between a pressure Pc in the crank chamber and a pressure in a suction pressure (a compressor suction pressure) Ps in each cylinder chamber  4 . 
     The incline angle of the swash plate  9  decreases with increase of the crank chamber pressure Pc, which decreases the stroke of the piston  5 . Thereby, the compressor  1  decreases in discharge capacity. On the contrary, the incline angle of the swash plate  9  increases with decrease of the crank chamber pressure Pc, which increases the stroke of the piston  5 . Thereby, the compressor  1  increases in discharge capacity until the crank chamber pressure Pc becomes substantially equal to the suction pressure Ps to bring the compressor  1  in a full load state. 
     Each cylinder  4  has a suction port  14  with a one-way intake valve  12  and has a discharge port  15  with a discharge valve  13 . The suction port  14  of each cylinder chamber  4  communicates with an intake connection port  17  through an intake passage  16 . The discharge port  15  communicates with a discharge connection port  19  through a discharge passage  18 . The intake connection port  17  and discharge connection port  19  communicate with a circulating line for a cooling cycle unit including an evaporator  20 , and expansion valve  21 , a condenser  22 , etc. 
     The compressor housing  2  has a valve recess  23  for receiving a control valve  30  according to the present invention to be secured therein. 
     The control valve  30  has a cylindrical valve housing  31  mounted in the recess  23 . 
     The valve housing  31  includes a main valve chamber  32 , an auxiliary valve chamber  33 , a suction pressure delivering port  34  opened toward the main valve chamber  33 , and a crank chamber pressure delivering port  35  opened toward the auxiliary valve chamber  33 , and a main valve port  36  disposed between the main valve chamber  32  and the auxiliary valve chamber  33 . 
     The main valve chamber  32  has a main valve element movable vertically. The main valve element  39  consists of a ball  38  and a ball retainer  37  holding the ball. The ball  38  of the valve element  39  opens and closes the main valve port  36  to allow or to shut down a flow communication between the suction pressure delivering port  34  and the crank chamber pressure delivering port  35 . 
     The housing  31  has a lower end which is positioned in the open side of the valve receiving recess  23  of the compressor body  2 . On the lower end of the housing  31 , there is mounted a diaphragm unit  41  which is a pressure actuated unit that is externally exposed from the valve receiving recess  23 . 
     The diaphragm unit  41  has a saucer-shaped upper cover  42  snap-fitted on the lower end of valve housing  31 , a saucer-shaped lower cover  44  joined to the upper cover  42  with a diagram  43  sandwiched therebetween, a cylindrical spring accommodating case  45  snap-fitted on the lower cover  44 , and an adjusting screw  46  screwed in the spring accommodating case  45 . 
     On one side of the diaphragm  43 , there is defined a diaphragm chamber  47  facing to the valve housing  31 . In the other side of the diaphragm  45 , there is defined a closed chamber  48  facing the spring case  45 . The diaphragm  43  is joined to the main valve  39  on the side defining the diaphragm chamber  47 . 
     At the side of the diaphragm  43  defining the closed chamber  48 , there are sequentially disposed an abutment plate  49 , a ball  50 , and a spring retaining member  51 . Between the spring retaining member  51  and the adjusting screw  46 , there is arranged a compression coil spring  52  resiliently biasing the main valve element  39  toward the valve closing position (upward) through the diaphragm  43  and the ball retainer  37 . 
     The diaphragm chamber  47  communicates with the suction pressure delivery port  34  of the main valve chamber  32  through a clearance (not shown) between the valve housing  31  and the ball retainer  37  to provide a suction pressure Ps to the suction pressure delivering port  34 . 
     In the auxiliary valve chamber  33 , there is disposed a leak flow adjustment valve  53  that is a slide valve slidably engaging with a valve receiving recess defined in the chamber  33 . An end cap  54  snap-fitted on an upper end portion of the valve housing  31  has a discharge pressure delivery port  55 . A discharge pressure Pd delivered to the discharge pressure delivery port  55  exerts a force on the leak flow adjustment valve  53  in the valve closing direction (downward). A crank chamber pressure Pc delivered to the crank chamber pressure delivering port  35  provides a force the leak flow adjustment valve  53  in the valve opening direction (upward). The end cap  54  has an air filter  56  attached thereto. 
     The auxiliary valve chamber includes a movable retainer  57  for supporting an auxiliary compression coil spring  58  sandwiched between the retainer  57  and the leak flow adjustment valve  53 . The auxiliary spring  58  resiliently biases the leak flow adjustment valve  53  toward the open position and resiliently biases the main valve  39  toward the open position through retainer  57 . 
     The leak flow adjustment valve  53  receives a force acting toward the valve closed position by a differential pressure ΔP (that is, Pd−Pc) between the discharge pressure Pd and the crank chamber pressure Pc. The leak flow adjustment valve  53  also receives another force acting toward the valve open position by the auxiliary spring  58 . The combination of the forces moves the valve element of the leak flow adjustment valve  53  between the fully opened position illustrated in FIG.  3 A and the closed position illustrated in FIG.  3 C. In the closed position, the valve element abuts against an auxiliary valve seat  59  disposed in the auxiliary valve chamber  33 . The leak flow control valve  53  has a leak flow passage  60  defined between an outer surface of the valve and an inner surface of the auxiliary valve chamber  33 . A travel of the valve toward the valve open position decreases the passage length L of the leak flow passage  60 , adjusting the rate of the leak flowing from the discharge pressure delivery port  55  to the crank chamber delivery port  35  substantially proportionally to the open degree of the leak flow valve. 
     If the crank chamber pressure Pc is assumed to be substantially constant, the flow adjustment valve  53  primarily responds to the discharge pressure Pd. Thus, the leak flow rate increases with decrease of the discharge pressure Pd. 
     Such configured control valve  30  is inserted and secured in a valve receiving recess  23  of the compressor body  2 . The suction pressure delivery port  34  communicates with the suction port  14  through an suction pressure passage  24 . The crank chamber pressure delivery port  35  communicates with the crank chamber  3  through the crank chamber pressure passage  25 . The discharge pressure port  55  communicates with the discharge port  15  through the discharge pressure passage  26 . Note that the suction pressure passage  24 , the discharge pressure passage  25 , and the discharge pressure passage  26  are passages defined in the compressor body  2 . 
     Next, operation of thus configured displacement control valve  30  will be discussed. 
     The suction pressure Ps of the compressor  1  is delivered from the inlet port  14  to the suction pressure delivery port  34 , the main valve chamber  32 , and the diaphragm chamber  47  through the suction pressure delivery passage  24 . Thereby, the diaphragm unit  41  moves the main valve  39  toward the valve open position. 
     Since the spring force of the auxiliary spring  58  is substantially constant, the combination of the valve opening force exerted on the diaphragm  43  by the suction pressure Ps and the valve closing spring force due to the compression coil spring  52  opens or closes the main valve  39 . 
     Therefore, when the suction pressure Ps becomes lower than a set pressure (a reference set pressure Pss) determined by the compression coil spring  52 , the spring force of the compression coil spring  52  moves upward the main valve  39  toward the valve closed position to close the main valve port  36 . 
     The closing of the main valve port  36  interrupts communication of the suction port  34  with the crank chamber  3 , thereby increasing the crank chamber pressure Pc so that the compressor  1  becomes in an unload operation state. 
     Meanwhile, when the suction pressure Ps becomes higher than the reference set pressure Pss, the diaphragm  43  moves downward (in the drawing) against the spring force of the compression coil spring  52 . Thereby, the main valve  39  moves toward the valve open position to open the main valve port  36 . 
     The opened main valve port  36  delivers the suction pressure Ps to the crank chamber  3 , so that the crank chamber pressure Pc becomes equal to the suction pressure Ps and the compressor  1  operates in a full load condition. 
     As described above, when the auxiliary spring  58  is constant in spring force with no correction of the spring force in response to a high discharge pressure, the compressor  1  becomes in a displacement control condition in which the suction pressure Ps becomes constant to be equal to the reference pressure Pss as shown in a chain line in FIG.  4 . 
     Next, an actual operation of the displacement control  30  having the leak flow adjustment valve  53  and the auxiliary valve  58  will be discussed. 
     (a) When the discharge pressure Pd is in a lower range (Pd≦Pd1): 
     The valve closing force due to the differential pressure Δp (that is, Pd−Pc) is lower than an initial force of the auxiliary spring  58  so that the spring force of the auxiliary spring  58  brings the leak flow adjustment valve  53  into its full open condition as illustrated in FIG.  3 A. 
     Thereby, the auxiliary spring  58  keeps a predetermined initial force acting on the main valve  39  toward the valve open position so that a comparatively high constant pressure Ps is required to open the main valve  39  (in a region illustrated in FIG.  4 A). 
     Meanwhile, the leak flow adjustment valve  53  is in its full open state with the passage length L of the leak flow passage  60  being minimum. In this state, the maximum rate leak flows from the discharge pressure delivery port  55  to the crank chamber pressure delivery port  35 . 
     Thereby, when the compressor provides a low discharge pressure, the crank chamber pressure Pc increases. Thus, the displacement control valve keeps the control characteristic for adjusting the crank chamber pressure Pc when the discharge pressure Pd is in the lower range. The control characteristic meets the specific performance of the compressor. 
     (b) When the pressure Pd is between Pd1 and Pd2 (Pd1&lt;Pd&lt;Pd2): 
     A force closing the valve due to the deferential pressure ΔP (that is, Pd−Pc) becomes larger than the initial set force of the auxiliary spring  58 . Thereby, the leak flow adjustment valve  53  moves toward the closed position against the spring force of the auxiliary spring  58  as illustrated in FIG.  3 B. 
     This increases the spring force of the auxiliary spring  58  which moves the main valve  39  toward the open position. With the increase of the discharge pressure Pd, the main valve  39  decreases in its open-close switching pressure (in a range illustrated in FIG.  4 B). The control characteristic of the displacement control compressor meets the discharge pressure Pd that is responsive to a load condition of a system including the compressor. 
     Meanwhile, the leak flow adjustment valve  53  moves from the full open position toward the closed position. The increase of the travel distance of the control valve  53  increases the passage length L of the leak passage  60 , decreasing the rate of the leak flowing from the discharge pressure delivery port  55  to the crank chamber pressure delivery port  35 . 
     Thus, with increase of the discharge pressure Pd, the leak flow rate decreases, so that the crank chamber pressure Pc is adjusted in an appropriate value by applying the discharge pressure Pd. Therefore, the characteristic of the control valve meets the performance of the compressor. 
     (c) When the discharge pressure is higher than Pd2 (Pd≧Pd2): 
     A force acting toward the value closed position by the deferential pressure ΔP (that is, Pd−Pc) becomes larger than the initial spring force of the auxiliary spring  58 . The leak flow adjustment valve  53  engages with the auxiliary valve seat  59  against the spring force of the auxiliary spring  58  to be in the closed state as illustrated in FIG.  3 C. 
     Thereby, the auxiliary spring  58  exerts a maximum force (a constant load) on the main valve  39  toward the valve open position, and a comparatively low constant pressure Ps is required to open the main valve  39  (in a region illustrated in FIG.  4 C). 
     In addition, the leak flow adjustment valve  53  has been in the closed position so that a minimum leak flows from the discharge pressure delivery port  55  of the leak passage  60  to the crank chamber delivery port  35 . This decreases a pressure loss in a high pressure discharge state of the compressor. 
     A Second Embodiment 
     FIG. 5 shows a second embodiment of a displacement control valve according to the invention. In FIG. 5, the same components as those shown in FIG. 2 are denoted by the same reference numerals and are not discussed again. 
     The auxiliary valve chamber  33  receives slidably auxiliary biasing unit  61 . The auxiliary biasing unit  61  receives a downward force by the discharge pressure Pd delivered to the discharge pressure delivery port  55  and an upward force due to the crank chamber pressure Pc delivered to the crank chamber delivery port  35 . Thus, the auxiliary biasing unit  61  exerts a force due to the differential pressure ΔP (that is, Pd−Pc) between the discharge pressure Pd and the crank chamber pressure Pc on the main valve  39  toward the valve open position. 
     The auxiliary biasing unit  61  is resiliently biased toward the main valve  39  by a compression coil spring (a biasing spring)  63  disposed between the biasing unit  61  and a retainer ring  62  secured to the valve housing  31 . Thus, the auxiliary biasing unit  61  exerts a force on the main valve  39  toward the valve open position according to the spring force of the compression coil spring  63  and the differential pressure ΔP between Pd and Pc. 
     Note that since the crank chamber pressure Pc is substantially constant, the auxiliary biasing unit  61  responds primarily to the discharge pressure Pd. Hence, the discharge pressure Pd provides a force on the main valve  39  toward the valve open position. 
     The auxiliary biasing unit  61  is assembled in the leak flow adjustment valve  64 . The leak flow adjustment valve  64  has a conical head  64   a  and is a variable orifice valve increasing an effective open sectional area of the leak flow control port  65  in response to the valve open degree (valve lift degree). The discharge pressure Pd delivered to the discharge pressure  55  exerts a downward force on the leak flow adjustment valve  64  toward the closed position. The crank chamber pressure Pc delivered to the crank chamber pressure delivery port  35  exerts an upward force on the leak flow adjustment valve  64  toward the valve open position through a passage  61   a  defined in the auxiliary biasing unit  61  and through the leak flow control port  65 . 
     The leak flow adjustment valve  64  is resiliently biased by a valve opening spring  66  toward the valve open position. The leak flow adjustment valve  64  receives the spring force of the valve opening spring  66  and a valve closing force due to the deferential pressure ΔP (that is, Pd−Pc) of the discharge pressure Pd and the crank chamber pressure Pc. The combination of the forces moves the leak flow adjustment valve  64  between the full open position in which the valve engages with the retainer ring  62  as illustrated in FIG.  6 A and the closed position in which the valve engages with an auxiliary valve seat  67  formed in the auxiliary biasing unit  61  as illustrated in FIG.  6 C. This increases the effective open sectional area of the leak flow control port  65  with decrease of the differential pressure ΔP in a way illustrated in FIG.  7 . 
     Note that since the crank chamber pressure Pc is substantially constant, the leak flow adjustment valve  64  primarily responds to the discharge pressure Pd to increase the effective open sectional area of the leak flow control port  65  with decrease of the discharge pressure Pd. 
     Next, operation of such configured displacement control valve  30  will be discussed. 
     The main valve  39  receives a correction force through the auxiliary biasing unit  61 . The correction force is due to the deferential pressure ΔP (that is, Pd−Pc) multiplied by the pressure receiving area Ah of the auxiliary biasing unit  61 . Since the crank chamber pressure Pc is substantially constant, the valve opening force of Ah·Pd relating to the discharge pressure Pd is added to the main valve  39 . 
     Meanwhile, the compression coil spring  52  has been selected to have a spring force corresponding to a reference set pressure Pss when the reference discharge pressure is Pds. Due to the valve opening force of Ah·Pd exerted on the main valve  39 , the suction pressure Ps required for opening the main valve increases with decrease of the discharge pressure Pd (when the discharge pressure Pd is lower than the reference discharge pressure Pds), while the suction pressure Ps required for opening the main valve decreases with increase of the discharge pressure Pd (when the discharge pressure Pd is higher than the reference discharge pressure Pds). 
     These are summarized by the following formula. 
     
       
         ps=Pss−Ah(Pd−Pds)/Ad  
       
     
     This, as illustrated in a solid line in FIG. 8, achieves the specific control that the suction pressure Ps decreases substantially proportionally to the increase of the discharge pressure Pd. Thereby, the discharge pressure Pd related to a load of a system including the displacement control compressor meets the control characteristic of the compressor. 
     Next, operation of the leak flow adjustment valve  64  will be discussed. 
     (a) When the discharge pressure Pd is in a lower state (Pd≦Pd1): 
     The valve opening force due to the deferential pressure ΔP (that is, Pd−Pc) is not larger than the spring force of the valve opening spring  66 , so that the spring force of the valve opening spring  66  moves the leak flow adjustment valve  64  to the full open position as illustrated in FIG.  6 A. 
     Thus, the leak flow control port  65  has a maximum effective flow sectional area to provide a maximum rate of the leak flow flowing from the discharge pressure delivery port  55  to the crank chamber pressure displacement port  35 . 
     As a result, the crank chamber pressure Pc increases in the low state of the discharge pressure Pd. This achieves a specific control of the displacement control valve adjusting the crank chamber pressure during the low state of the discharge pressure Pd, which is appropriate for the specific performance of the compressor. 
     (b) When the discharge pressure is between Pd1 and Pd2: 
     The valve closing force due to the deferential pressure ΔP (that is, Pd−Pc) is larger than the spring force of the valve opening spring  66 , so that the leak flow adjustment valve  64  moves to the closed position as illustrated in FIG. 6B against the valve opening spring  66 . 
     Thus, the leak flow control port  65  has a decreased effective flow sectional area in response to the movement of the leak flow adjustment valve  64  to provide a decreased rate of the leak flow flowing from the discharge pressure delivery port  55  to the crank chamber pressure delivery port  35 . 
     As a result, the leak flow decreases in response to the increase of the discharge pressure, so that the crank chamber pressure Pc is kept in an appropriate state in response to the discharge pressure Pd. In this pressure range of the discharge pressure Pd, the specific control of the control valve is appropriate for the specific performance of the compressor. 
     (c) When the discharge pressure Pd is higher than Pd2 (Pd≧Pd2): 
     The valve closing force due to the deferential pressure ΔP (that is, Pd−Pc) is larger than the spring force of the valve opening spring  66 , so that the leak flow adjustment valve  64  engages with the auxiliary valve seat  67  to close it as illustrated in FIG. 6C against the spring force of the valve opening spring  66 . 
     Thus, the leak flow control port  65  provides a minimum rate of a leak flow flowing from the discharge pressure delivery port  55  to the crank chamber pressure delivery port  35 . This decreases a pressure loss during the high pressure discharge state of the compressor. 
     A Third Embodiment 
     FIG. 9 shows a third embodiment of a displacement control valve according to the invention. The same components as those shown in FIG. 5 are denoted by the same reference numerals and are not discussed again. 
     The third embodiment includes a check valve having an on-off operation characteristic illustrated in FIG. 10 in place of the variable orifice valve as the leak flow adjustment vale  64  assembled in the auxiliary biasing unit  61 . 
     The auxiliary biasing unit  61  of the third embodiment provides the same modification on the main valve  39  in the open-close operation as the second embodiment. As is similar to the control characteristic shown by a solid line in FIG. 8, the auxiliary biasing unit  61  achieves the specific control that the suction pressure Ps decreases substantially proportionally to the increase of the discharge pressure Pd. The discharge pressure Pd related to a load of a system including the displacement control compressor meets the control character of the compressor. 
     When the discharge pressure Pd is in a lower state, the valve opening force due to the deferential pressure ΔP (that is, Pd−Pc) is not larger than the spring force of the valve opening spring  66 , so that the spring force of the valve opening spring  66  moves the leak flow adjustment valve  64  to the full open position. 
     Thus, the leak flow control port  65  opens to provide a maximum rate of the leak flow flowing from the discharge pressure delivery port  55  to the crank chamber pressure delivery port  35 . 
     As a result, the crank chamber pressure Pc increases in the low state of the discharge pressure Pd. This achieves a specific control of the displacement control valve adjusting the crank chamber pressure during the low state of the discharge pressure Pd, which is appropriate for the specific performance of the compressor. 
     In a higher state of the discharge pressure Pd, when the valve closing force due to the deferential pressure ΔP (that is, Pd−Pc) becomes larger than the spring force of the valve opening spring  66 , the leak flow adjustment valve  64  engages with the auxiliary valve seat  67  to close it against the spring force of the valve opening spring  66 . 
     Thus, the leak flow control port  65  provides a minimum rate of a leak flow flowing from the discharge pressure delivery port  55  to the crank chamber pressure delivery port  35 . This decreases a pressure loss during the high pressure discharge state of the compressor. 
     In any of the aforementioned embodiments, the pressure actuated unit has been the diaphragm unit  41 . However, the pressure actuated unit may be a bellows of a closed structure or the like. A fourth embodiment of a control valve having such a closed bellow to modify the first embodiment of the control valve for the variable-displacement-type compressor. 
     A Fourth Embodiment 
     FIG. 11 shows a fourth embodiment of a displacement control valve according to the invention. The same components as those shown in FIG. 2 are denoted by the same reference numerals and are not discussed again. 
     In the fourth embodiment, the pressure actuated unit includes a closed bellows  70 . The bellows  70  is mounted in a bellows accommodation case  68  secured to the valve housing  31 . The bellows  70  has a bellow body  72  unitarily provided with an end plate  71  and has another end plate  73  closing the other end thereof. The bellow body  71  is under a negative inside pressure. Within the bellow body  72 , there is disposed an abutting plate  75  adjacent to the end plate  71 . Between the abutting plate  75  and the end plate  73 , there is mounted a compression spring  74  resiliently biasing the abutting plate  75  and the end plate  73  to part them, that is, in the expansion direction (or the valve closing direction) of the bellows  70 . The abutting plate  75  has a stopper surface  75   a  abutting against an opposing surface  73   a  of the end plate  73  to limit the maximum contraction movement of the bellows  70 . 
     The bellow accommodation case  68  has an adjustment screw  69  screwed therein. The adjustment screw  69  supports one end of the bellows  70  through a ball joint structure consisting of a ball  76  positioned around an axial center line of the adjustment screw  69  and a spherical concave  73   b  defined in the end plate  73  around the center thereof (around the center of the bellows). That is, the bellows  70  and the bellows accommodation case  68  are coupled to each other through the adjustment screw  69  and the ball joint structure. 
     The end plate  71  of the bellows  70  is abutting against a ball retainer  37  of the main valve  39  so that the expansion and contraction motions of the bellows  70  are directly applied to the main valve  39 . 
     The bellows accommodation case  68  communicates with the suction pressure delivery port  34 . The bellows  70  receives the suction pressure Ps delivered to the suction pressure delivery port  34  in the value opening direction. The bellows  70  expands or contracts according to the deferential pressure between the suction pressure Ps and the bellows inner pressure. 
     In addition, between the ball retainer  37  of the main valve  39  and the valve housing  31 , there is mounted a weak compression spring  77  resiliently biasing the main valve  39  toward the open position. 
     In the fourth embodiment, the leak flow adjustment valve  53 , the movable spring retainer  57 , the auxiliary spring  58 , etc. each have a configuration similar to that described in the first embodiment.