Patent Publication Number: US-6217291-B1

Title: Control valve for variable displacement compressors and method for varying displacement

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to compressors for compressing and discharging gas, and more particularly, to a compressor that varies displacement in accordance with the difference between the pressure of a discharge chamber and the pressure of a crank chamber, a control valve for controlling the pressure difference, and a method for varying the displacement of the compressor. 
     FIG. 20 shows a prior art compressor  200 . An inclinable swash plate  201  is accommodated in a crank chamber  203 . The displacement of the compressor  200  varies in accordance with the inclination of the swash plate  201 . A control valve  202  controls the pressure of the crank chamber  203  to alter the inclination of the swash plate  201 . The inclination of the swash plate  201  changes the stroke of pistons  204 , which are retained in the compressor  200 . There are two types of control valves  202 , a self-controlled type and an externally controlled type. 
     A self-controlled type control valve detects the suction pressure of the compressor  200 . The control valve automatically controls its position in accordance with the difference between the detected suction pressure value and a threshold pressure value. The threshold value is determined by the characteristics of a pressure sensing member (bellows), which is retained in the control valve. Accordingly, in a self-controlled type control valve, the threshold value cannot be changed when the compressor is operating. 
     In a externally controlled control valve, the threshold value can be changed when the compressor is operating. Typically, the externally controlled valve has an electromagnetic actuator and a controller  207 . The electromagnetic actuator includes a solenoid  206  and other relevant parts (e.g., steel core). In the control valve, the solenoid  206  is arranged coaxially with a pressure sensing member. The controller  207  controls the electromagnetic actuator in accordance with data sent from various types of sensors (e.g., ambient temperature). The electromagnetic actuator is actuated to change the threshold value. The threshold value is changed to vary and optimize the displacement of the compressor under different conditions. 
     Since the prior art self-controlled control valve cannot change the threshold value, the displacement of a compressor using such a valve cannot be flexibly varied. Although the externally controlled control valve can change the threshold value in accordance with the conditions surrounding the compressor, the electromagnetic actuator, which includes the solenoid and other relevant parts, increases the size of the compressor and complicates the structure of the compressor. This increases the product costs of the compressor. Furthermore, an amplifier having a large electric capacity must be used to actuate the electromagnetic actuator, which is controlled by the controller. However, the employment of a compressor using a high-capacity amplifier in an automotive air conditioning system significantly increases the load applied to the vehicle. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an objective of the present invention to provide a control valve that easily varies the displacement of a compressor, a compressor using such a control valve, and a method for varying the displacement of a compressor. 
     To achieve the above objective, the present invention provides a control valve installed in a variable displacement compressor for compressing gas. The compressor includes a discharge pressure region, a suction pressure region, and a crank chamber. The pressure in the discharge pressure region is higher than that of the suction pressure region. The crank chamber accommodates a crank mechanism for compressing the gas. The control valve changes the displacement of the compressor by controlling a difference between the pressure in the crank chamber and the pressure in the discharge pressure region or the suction pressure region. The control valve has the following structure. A pressure sensitive chamber is connected to a control region, which is one of the discharge pressure region or the suction pressure region. A first passage connects the crank chamber to the control region. A valve chamber is located in the first passage. A valve body is accommodated in the valve chamber for selectively closing and opening the first passage. A displaceable pressure sensitive mechanism is connected to the valve body and accommodated in the pressure sensitive chamber. The displacement of the pressure sensitive mechanism causes the valve body to move between an open position and a closed position. The pressure sensitive mechanism produces a force for determining an initial threshold pressure value at which the valve body is switched between the open position and the closed position. A controller controls the pressure in the pressure sensitive chamber by supplying gas from the discharge pressure region to the pressure sensitive chamber or by discharging gas from the pressure sensitive chamber to the suction pressure region to change the threshold value from the initial value to a second threshold value. The pressure sensitive mechanism functions in accordance with the pressure of the pressure sensitive chamber. The valve body behaves in accordance with the threshold value selected by the controller. 
     The present invention further provides a variable displacement compressor for compressing gas. The compressor includes a discharge pressure region and a suction pressure region. The pressure in the discharge pressure region is higher than that of the suction pressure region. The compressor has the following structure. A crank chamber accommodates a crank mechanism for compressing the gas. A control valve changes the displacement of the compressor by controlling a difference between the pressure in the crank chamber and the pressure in a control region, which is one of the discharge pressure region or the suction pressure region. The control valve has the following structure. A pressure sensitive chamber is connected to the control region. A first passage connects the crank chamber to the control region. A valve chamber is located in the first passage. A valve body is accommodated in the valve chamber for selectively closing and opening the first passage. A displaceable pressure sensitive mechanism is connected to the valve body and accommodated in the pressure sensitive chamber. The displacement of the pressure sensitive mechanism causes the valve body to move between an open position and a closed position. The pressure sensitive mechanism produces a force for determining an initial threshold pressure value at which the valve body is switched between the open position and the closed position. The compressor further includes a controller that controls the pressure in the pressure sensitive chamber by supplying gas from the discharge pressure region to the pressure sensitive chamber or by discharging gas from the pressure sensitive chamber to the suction pressure region to change the threshold value from the initial value to a second value. The pressure sensitive mechanism functions in accordance with the pressure of the pressure sensitive chamber. The valve body behaves in accordance with the threshold value selected by the controller. 
     The present invention further provides a method for controlling a displacement of a variable displacement compressor installed in a vehicle. The compressor has a discharge pressure region, a suction pressure region, a crank chamber, which accommodates a crank mechanism for compressing gas, and a control valve. The pressure in the discharge pressure region is higher than that of the suction pressure region. The control valve has the following structure. A valve body selectively closes and opens a passage that connects the crank chamber to the discharge pressure region or the suction pressure region. A pressure sensitive chamber is connected to the discharge pressure region or the suction pressure region. The control valve changes the displacement of the compressor by regulating the difference between the pressure in the crank chamber and the pressure in the discharge pressure region or the suction pressure region. The method includes the steps of as follows: detecting a driving state of the vehicle; and supplying gas from the discharge pressure region to the pressure sensitive chamber to increase the pressure in the pressure sensitive chamber in response to the driving state. 
    
    
     Other aspects and advantages of the present invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features of the present invention that are believed to be novel are set forth with particularity in the appended claims. The invention, together with objects and advantages thereof, may best be understood by reference to the following description of the presently preferred embodiments together with the accompanying drawings in which: 
     FIG. 1 is a cross-sectional view showing a compressor according to a first embodiment of the present invention; 
     FIG. 2 is a schematic enlarged cross-sectional view showing a control valve employed in the compressor of FIG. 1; 
     FIG. 3 is a graph showing the characteristics of the suction pressure threshold value in the compressor of FIG. 1; 
     FIG. 4 is a cross-sectional view combined with a block diagram showing a control valve employed in a compressor according to a second embodiment of the present invention; 
     FIG. 5 is a graph showing the characteristics of the suction pressure threshold value in the compressor of FIG. 4; 
     FIG. 6 is a schematic cross-sectional view showing a control valve employed in a compressor according to a third embodiment of the present invention; 
     FIG. 7 is a graph showing the characteristics of the suction pressure threshold value in the compressor of FIG. 6; 
     FIG. 8 is a schematic cross-sectional view showing a control valve employed in a compressor according to a fourth embodiment of the present invention; 
     FIG. 9 is a graph showing the characteristics of the suction pressure threshold value in the compressor of FIG. 8; 
     FIG. 10 is a schematic cross-sectional view showing a control valve employed in a compressor according to a fifth embodiment of the present invention; 
     FIG. 11 is a graph showing the characteristics of the suction pressure threshold value in the compressor of FIG. 10; 
     FIG. 12 is a schematic cross-sectional view showing a control valve employed in a compressor according to a sixth embodiment of the present invention; 
     FIG. 13 is a schematic cross-sectional view showing a control valve employed in a compressor according to a seventh embodiment of the present invention; 
     FIG. 14 is a schematic cross-sectional view showing a control valve employed in a compressor according to an eighth embodiment of the present invention; 
     FIG. 15 is a graph showing the characteristics of the suction pressure threshold value in the compressor of FIG. 14; 
     FIG. 16 is a schematic cross-sectional view showing a valve mechanism employed in a compressor according to an ninth embodiment of the present invention; 
     FIG. 17 is a schematic cross-sectional view showing a control valve and a valve mechanism employed in a compressor according to a tenth embodiment of the present invention; 
     FIG. 18 is a schematic cross-sectional view showing a control valve and a valve mechanism employed in a compressor according to an eleventh embodiment of the present invention; 
     FIG. 19 is a schematic cross-sectional view showing a control valve employed in a compressor according to a twelfth embodiment of the present invention; and 
     FIG. 20 is a cross-sectional view showing a prior art compressor. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention embodied in a variable displacement compressor  10  will now be described with reference to the drawings. To avoid a redundancy, like or same reference numerals are given to those components that are the same or similar in all embodiments. 
     First Embodiment 
     As shown in FIG. 1, a front housing  12  is fixed to the front end of a cylinder block  11 . A rear housing  14  is fixed to the rear end of the cylinder block  11  with a valve plate  13  arranged in between. A crank chamber  15  is arranged in the front housing  12  in front of the cylinder block  11 . 
     A rotatable drive shaft  16  extends through the crank chamber  15  between the front housing  12  and the cylinder block  11 . The front end of the drive shaft  16  projects out of the crank chamber  15 . A pulley  18  is secured to the projected end of the drive shaft  16 . The pulley  18  is supported by the front housing  12  by means of an angular bearing  17  and connected to an engine  20  by a belt  19 . In other words, the compressor  10  is a clutchless type variable displacement compressor. That is, a clutch is not used to connect the drive shaft  16  to an external drive source, or engine  20 . 
     A swash plate  23 , which serves as a cam plate, is supported such that it inclines and slides along the drive shaft  16  in the crank chamber  15 . A pair of guide pins  25  is fixed to the swash plate  23 . Round guides  25   a  are provided on the distal end of each guide pin  25 . A rotor  22  is fixed to the drive shaft  16  in the crank chamber  15  to rotate integrally with the drive shaft  16 . The rotor  22  has a support arm  24 , which extends toward the swash plate  23 . The support arm  24  has a pair of guide bores  24   a . Each guide bore  24   a  slidably accommodates one of the guide pins  25 . The engagement between the support arm  24  and the guide pins  25  rotates the swash plate  23  integrally with the drive shaft  16 , while permitting movement of the swash plate  23  along the surface of the drive shaft  16  and guiding inclination of the swash plate  23 . The inclination of the swash plate  23  decreases as it moves rearward toward the cylinder block  11 . The support arm  24  and the guide pins  25  define a hinge mechanism. The swash plate  23  has a counterweight  23   a  located on the opposite side of the drive shaft  16  from the hinge mechanism. 
     A first spring  26  is arranged between the rotor  22  and the swash plate  23 . The first spring  26  urges the swash plate  23  toward the rear (rightward in FIG.  1 ). A projection  22   a  is formed on the rear surface of the rotor  22 . When the swash plate  23  comes into contact with the projection, further inclination of the swash plate  23  is prohibited. In this state, the swash plate  23  is located at a maximum inclination position. 
     A central bore  27  extends through the cylinder block  11  along the axis of the drive shaft  16 . A cylindrical cup-like shutter  30  is accommodated in the central bore  27  and supported so that it slides along the axis of the drive shaft  16 . The shutter  30  has a peripheral surface with a stepped portion. The wall of the central bore  27  also has a stepped portion. A second spring  31  is arranged between the stepped portion of the shutter  30  and the stepped portion of the central bore  27  to urge the shutter  23  toward the swash plate  23 . 
     A radial bearing  32  is arranged between the rear end portion of the drive shaft  16  and the inner wall of the shutter  30 . A snap ring  33  prevents the radial bearing  32  from falling out of the shutter  30 . The radial bearing  32  moves together with the shutter  30  in the axial direction of the drive shaft  16 . Accordingly, the rear end portion of the drive shaft  16  is rotatably supported in the central bore  27  by the shutter  30  and the radial bearing  32 . The central bore  27  is connected with a suction passage  28 . A shutting surface  34  is defined on the rear end of the shutter  30 . When the shutter  30  moves rearward, the shutting surface  34  contacts the positioning surface  29 , which is defined on the valve plate  13 . In this state, the suction passage  28  is disconnected from the central bore  27 . 
     A thrust bearing  35  is arranged between the swash plate  23  and the shutter  30 . The thrust bearing  35  slides along the drive shaft  16  and is constantly clamped between the swash plate  23  and the shutter  30  by the forces of the first and second springs  26 ,  31 . 
     The inclination of the swash plate  23  decreases as it moves toward the rear. As the swash plate  23  moves rearward, the thrust bearing  35  moves the shutter  30  toward the positioning surface  29  against the force of the second spring  31 . When the shutting surface  34  contacts the positioning surface  29 , the swash plate  23  is located at a minimum inclination position, while the shutter  30  is located at a shutting position. In this state, the inclination of the swash plate  23 , with respect to a plane perpendicular to the axis of the drive shaft  16 , is slightly greater than zero degrees. 
     Cylinder bores  11   a  (only one shown) extend about the drive shaft  16  in the cylinder block  11 . A piston  36  is accommodated in each cylinder bore  11   a.  Each piston  36  is operably connected to the swash plate  23  by means of shoes  37 . The rotation of the drive shaft  16  is transmitted to the swash plate  23  by the rotor  22 . The shoes  37  convert the rotation of the swash plate  23  to reciprocal movement of each piston  36  in the associated cylinder bore  11   a.    
     An alteration in the inclination of the swash plate  23  changes the stroke of the pistons  36  and varies the displacement. The hinge mechanism (the support arm  24  and the guide pins  25 ) keeps the upper dead center position of each piston  36  at the same location regardless of the swash plate inclination. The distance between the head of each piston  36 , when located at the top dead center position, and the valve plate  13  is substantially null. 
     An annular suction chamber  38  is defined about the suction passage  28  in the central portion of the rear housing  14 . An annular discharge chamber  39  is defined about the suction chamber  38 . The suction chamber  38  is connected to the central bore  27  through a communication port  45  extending through the valve plate  13 . The suction chamber  38  and the suction passage  28  are disconnected from each other when the shutter  30  is located at the shutting position. 
     A suction port  40  and a discharge port  42  extend through the valve plate  13  in correspondence with each cylinder bore  11   a.  A suction flap  41  is provided on the valve plate  13  in correspondence with each suction port  40 . A discharge flap  43  is provided on the valve plate  13  in correspondence with each discharge port  42 . 
     As each piston  36  performs the suction stroke and moves from its top dead center position to its bottom dead center position in the associated cylinder bore  11   a,  the refrigerant gas in the suction chamber  38  enters the suction port  40 , opens the suction flap  41 , and enters the cylinder bore  11   a.  As each piston  36  performs the compression stroke and moves from the bottom dead center position to the top dead center position in the associated cylinder bore  11   a,  the refrigerant gas is compressed in the cylinder bore  11   a.  The compressed gas then enters the discharge port  42 , opens the discharge flap  43 , and flows out into the discharge chamber  39 . The compression reaction of the refrigerant gas produced during the compression stroke is received by the front housing  12  by way of the pistons  36 , the rotor  22 , and the thrust bearing  44 . 
     A relief passage  46  extends through the drive shaft  16  and connects the crank chamber  15  to the interior of the shutter  30 . A relief bore  47  extends through the cylindrical wall of the shutter  30  to function as a throttle valve. The relief bore  47  connects the central bore  27  to the interior of the shutter  30 . The refrigerant gas in the crank chamber  15  flows into the suction chamber  38  through the relief passage  46 , the relief bore  47 , and the central bore  27 . The relief passage  46 , the relief bore  47 , and the central bore  27  form a bleeding passage. 
     As shown in FIG. 1, pressurizing passages  48 ,  49 , which connect the discharge chamber  39  to the crank chamber  15  extend through the cylinder block  11  and the rear housing  14 . A control valve  60  is installed in the rear housing  14  between the pressurizing passages  48  and  49 . 
     A first intake passage  51 , which does not intersect with the pressurizing passages  48 ,  49 , extends through the rear housing  14  to connect the suction chamber  38  to the control valve  60 . An electromagnetic valve  73  connects the discharge chamber  39  to the control valve  60  through a second intake passage  52 . The electromagnetic valve  73  selectively connects and disconnects the discharge chamber  39  and the control valve  60 . 
     After the refrigerant gas is compressed to a discharge pressure in each cylinder bore  11   a  and sent into the discharge chamber  39 , the refrigerant gas is sent toward an external refrigerant circuit  54  through a gas outlet  53 . The external refrigerant circuit  54  includes a condenser  55 , an expansion valve  56 , and an evaporator  57 . The refrigerant gas circulates through the external refrigerant circuit  54  before re-entering the compressor  10  through the suction passage  28 . The external refrigerant circuit  54 , together with the compressor  10 , forms a refrigerant circuit in an automotive air conditioning system. 
     The structure of the control valve  60  will now be described in detail. As shown in FIG. 2, the control valve  60  has a valve housing  61 . The valve housing  61  accommodates a valve chamber  62  and a pressure chamber  63 . A guide bore  64  extends between the valve chamber  62  and the pressure chamber  63 . A rod  65  is slidably arranged in the guide bore  64 . 
     The pressure chamber  63  is located at the lower portion of the valve housing  61 , as viewed in FIG.  2 . The pressure chamber  63  is defined by the inner wall of the valve housing  61  and a lower cap  67 . A bellows  66  is accommodated in the pressure chamber  63 . The lower end of the bellows  66  is fixed to the lower cap  67 . The interior of the bellows  66  is under vacuum, or is de-pressurized to an extremely low pressure. A spring  68  is arranged in the bellows  66 . The spring  68  urges the top of the bellows  66  toward the rod  65 . This keeps the top surface of the bellows  66  in contact with the lower end of the rod  65 . 
     A fixed throttle  69  and a port  70  extend through the valve housing wall, which defines the pressure chamber  63 . The pressure chamber  63  is connected to the first intake passage  51  through the fixed throttle  69 . The refrigerant gas in the suction chamber  38  flows into the pressure chamber  63  through the fixed throttle  69  such that the pressure of the suction chamber (suction pressure Ps) is applied to the bellows  66 . The control valve  60  detects and controls the pressure of the suction chamber  38 , which is connected to the pressure chamber  63 . The discharge chamber  39 , or discharge pressure region, is connected to the second intake passage  52  through the port  70 . The second intake passage  52  includes a fixed throttle  71 , which is arranged in the wall of the rear housing  14 , a passage  72 , which connects the fixed throttle  71  to the port  70 , and the electromagnetic valve  73 . 
     The electromagnetic valve  73  is controlled by a controller  58 . The controller  58  stops applying voltage to the electromagnetic valve  73  to cause the valve  73  to open the second intake passage  52 . This permits the high-pressure refrigerant gas in the discharge chamber  39  to flow into the pressure chamber  63  through the second intake passage  52 . The controller  58  applies voltage to the electromagnetic valve  73  to close the second intake passage  52  with the valve  73 . This blocks the flow of high-pressure refrigerant gas from the discharge chamber  39  to the pressure chamber  63 . The electromagnetic valve  73  is normally opened. The controller  58  may be part of a control unit of the automotive air conditioning system. Alternatively, the controller  58  may be an electronic control unit (ECU) of the engine  20  that includes a program, executed in an interrupting manner, for controlling the electromagnetic valve  73 . The controller  58  controls the electromagnetic valve  73  based on data sent from various sensors and switches (not shown). 
     The valve chamber  62  is located at the upper portion of the valve housing  61 , as viewed in FIG.  2 . The top of the valve chamber  62  is sealed by an upper cap  77 . A spherical valve body  75  is arranged in the valve chamber  62 . A valve seat  74  is defined in the valve chamber  62 . The valve seat  74  and the valve body  75  divide the valve chamber  62  into an upper region and a lower region. The upper and lower regions are completely disconnected from each other when the valve body  75  contacts the valve seat  74 . 
     A spring  76  is arranged in the upper region. The spring  76  has an upper end engaging the upper cap  77  and a lower end engaging the valve body  75 . The spring  76  forces the valve body  75  toward the valve seat  74 . The upper end of the rod  65  is located in the lower region of the valve chamber  62 . 
     The valve housing  61  has a first port  78 , which leads into the upper region of the valve chamber  62 , and a second port  79 , which leads into the lower region of the valve chamber  62 . The upper region of the valve chamber  62  is connected to the discharge chamber  39  through the first port  78  and the pressurizing passage  48 . The lower region of the valve chamber  62  is connected to the crank chamber  15  through the second port  79  and the pressurizing passage  49 . 
     When the valve body  75  contacts the valve seat  74  and disconnects the pressurizing passages  48 ,  49  from each other, the flow of refrigerant gas through the pressurizing passages  48 ,  49  from the discharge chamber  39  to the crank chamber  15  is stopped. When the bellows expands against the force of the spring  76  and moves the valve body  75  with the rod  65 , the valve body  75  moves away from the valve seat  74 . In this state, the pressurizing passages  48 ,  49  are connected to one another, which permits the flow of refrigerant gas from the discharge chamber  39  to the crank chamber  15  through the pressurizing passages  48 ,  49 . 
     The operation of the control valve  60  will now be described. The pressure in the suction chamber  38  (suction pressure Ps) is applied to the pressure chamber  63  through the fixed throttle  69 . Thus, when the suction pressure Ps fluctuates, the pressure Pk of the pressure chamber  63  fluctuates. The length of the bellows  66  changes in accordance with the pressure Pk of the pressure chamber  63 . For example, the bellows  66  contracts if pressure Pk is higher than a predetermined threshold value and expands if pressure Pk is lower than the threshold value. The deformation of the bellows  66  is transmitted to the valve body  75  through the rod  65 . Therefore, the position, or opening size, of the control valve  60  is determined by the pressure Pk of the pressure chamber  63 . Changes in the position of the control valve  60  alter the inclination of the swash plate  23 . In that sense, the operating principle of the control valve  60  is the same as a typical prior art self-controlled control valve. 
     In a typical self-controlled valve, the valve body moves away from the valve seat when the suction pressure Ps reaches a predetermined threshold value Pset. The threshold value Pset is determined solely by the force of the spring  68 . Thus, the threshold value Pset cannot be varied when the compressor  10  is operating. However, in the control valve  60  of the first embodiment, the high-pressure refrigerant gas in the discharge chamber  39  is selectively drawn into the pressure chamber  63 . This varies the threshold value Pset of the suction pressure when the compressor  10  is operating. 
     The threshold value Pset of the suction pressure is varied as described below. The pressure Pk of the pressure chamber  63  is equal to the suction pressure Ps when the electromagnetic valve  73  is closed. In this state, a first threshold value Pset 1  is determined by the force of the spring  68 . In the first embodiment, the first threshold value Pset 1  is the initial threshold value Pset. 
     The high-pressure refrigerant gas in the discharge chamber  39  flows into the pressure chamber  63  when the electromagnetic valve  73  is opened. Thus, the pressure Pk of the pressure chamber  63  may reach the first threshold value Pset 1  even if the pressure Ps of the suction chamber  38  is less than the first threshold value Pset 1 . In other words, when the electromagnetic valve  73  is opened, the suction pressure threshold value Pset decreases from the initial first threshold value Pset 1  to a second threshold value Pset 2 . That is, the threshold value Pset of the control valve  60  decreases when the discharge chamber  39  is connected to the pressure chamber  63 . 
     The graph shown in FIG. 3 indicates the relationship between the pressure Pd of the discharge chamber  39  and the threshold value Pset. The horizontal dashed line shows the relationship of the initial threshold value Pset 1  to the discharge pressure Pd. The solid line shows the relationship between the second threshold value Pset 2  and the discharge pressure. The sloping dashed line is plotted along the minimum values of the suction pressure Ps that prevents the formation of frost. When the electromagnetic valve  73  is opened, the force of the spring  69  is chosen such that the difference between the second threshold value Pset 2  and the frost limit value decreases as the discharge pressure Pd increases. When the electromagnetic valve  73  is closed, the force of the spring  69  is chosen such that the difference between the first threshold value Pset 1  and the frost limit value increases as the discharge pressure Pd increases. 
     The control valve  60  is operated as described below in a manner independent of the operation of the electromagnetic valve  73 . 
     The suction pressure Ps is high when there is a strong demand for cooling the passenger compartment. The bellows  66  contracts when the suction pressure Ps exceeds the threshold value Pset. Contraction of the bellows  66  causes the force of the spring  76  to move the valve body  75  downward until the valve body  75  contacts the valve seat  74 . Contact between the valve body  75  and the valve seat  74  disconnects the discharge chamber  39  from the crank chamber and stops the flow of high-pressure refrigerant gas from the discharge chamber  39  to the crank chamber  15 . In this state, the refrigerant gas in the crank chamber  15  gradually flows into the suction pressure region (the central bore  27 , the suction chamber  38 , and the suction passage  28 ) through the bleeding passage. This gradually decreases the pressure Pc of the crank chamber  15 . A decrease in the pressure Pc reduces the back pressure applied to the pistons  36 . When the back pressure applied to the pistons  36  decreases, the inclination of the swash plate  23  increases, which lengthens the stroke of the pistons  36 . This increases the displacement of the compressor  10 . 
     The suction pressure Ps is low when the demand for cooling the passenger compartment is small. The bellows  66  expands when the suction pressure Ps falls below the threshold value Pset. This moves the valve body  75  away from the valve seat  74  against the force of the spring  76  and connects the discharge chamber  39  to the crank chamber  15 . Thus, the high-pressure refrigerant gas in the discharge chamber  39  flows into the crank chamber  15 . In this state, the refrigerant gas in the crank chamber  15  gradually flows into the suction pressure region (the central bore  27 , the suction chamber  38 , and the suction passage  28 ) through the bleeding passage. However, the fixed throttle  47  restricts the flow rate of the refrigerant gas. Hence, the pressure Pc of the crank chamber  15  increases. An increase in the pressure Pc increases the back pressure applied to the pistons  36 . When the back pressure applied to the pistons  36  increases, the inclination of the swash plate  23  decreases, which shortens the stroke of the pistons  36 . This decreases the displacement of the compressor  10 . 
     When the swash plate  23  moves toward the minimum inclination position, the shutter  30  moves rearward until its shutting surface  34  comes into contact with the positioning surface  29 . As a result, the flow of refrigerant gas through the suction passage  28  from the external refrigerant circuit  54  to the suction chamber  38  is stopped. However, refrigerant gas is continuously discharged from the cylinder bores  11   a  and into the discharge chamber  39 . The refrigerant gas in the discharge chamber  39  flows through the pressurizing passages  48 ,  49 , the crank chamber  15 , the relief passage  46 , and the relief bore  47  and then enters the suction chamber  38 . The refrigerant gas in the suction chamber  38  is drawn into the cylinder bores  11   a  and is again discharged into the discharge chamber  39 . Accordingly, an internal refrigerant circuit is formed in the compressor even if the suction passage  28  is completely closed by the shutter  30 . The difference in pressure at different locations in the internal refrigerant circuit guarantees the circulation of the refrigerant gas. Atomized lubricant is suspended in the refrigerant gas. Therefore, the circulation of the refrigerant gas lubricates the interior of the compressor in a satisfactory manner. 
     The controller  58  selectively opens and closes the electromagnetic valve  73  to shift the threshold value Pset between Pset 1  and Pset 2 . Data related to the driving conditions of the vehicle are electrically input into the controller  58 . Such data includes the vehicle velocity, the accelerating rate, and the driving mode of the automatic transmission (AT). The controller  58  controls the electromagnetic valve  73  based on the input data. For example, if the vehicle is being driven at a substantially constant velocity while a normal mode of the AT is selected, the controller  58  does not feed current to the electromagnetic valve  73 , which keeps the electromagnetic valve opened. In this state, the suction pressure threshold value Pset is set at the relatively low second threshold value Pset 2 . Consequently, the displacement of the compressor  10  readily increases even if the demand for cooling is relatively low (i.e., the suction pressure Ps is relatively low). If the velocity of the vehicle is accelerating while an economy mode of the AT is selected, the controller  58  feeds current to the electromagnetic valve  73  to close the electromagnetic valve  73 . In this state, the suction pressure threshold value Pset is set at the relatively high first threshold value Pset 1 . Thus, a greater cooling demand (suction pressure Ps) is required to increase the displacement of the compressor. 
     The advantages of the first embodiment will now be described. When the engine load is relatively low, such as when the vehicle is running at a constant velocity, the controller  58  opens the electromagnetic valve  73  and sets the suction pressure threshold value Pset at the relatively low second threshold value Pset 2 . In this state, the displacement of the compressor increases easily. On the other hand, when the engine load is relatively high, such as during acceleration of the vehicle, the controller  58  closes the electromagnetic valve  73  and sets the suction pressure threshold value Pset at the relatively high first threshold value Pset 1 . In this state, a greater demand for cooling is required to increase the displacement of the compressor  10 . This reduces the time during which a large load is applied to the engine  20  by the compressor  10 . Accordingly, the displacement of the compressor is varied by changing the threshold value Pset of the electromagnetic valve  73  in accordance with the operating conditions of the vehicle and the engine  20 . 
     The control valve  60  of the first embodiment is obtained merely by adding the port  70 , through which high-pressure refrigerant gas is selectively drawn, to the prior art self-controlled valve. Since this eliminates the need for a large electromagnetic actuator, the control valve  60  of the first embodiment is compact and relatively inexpensive. Furthermore, since an electromagnetic actuator need not be connected to the compressor  10 , the installation of the control valve  60  is relatively simple. 
     Although the electromagnetic valve  73  requires the intake passage  52 , which includes the passage  72 , the cross-sectional area of the intake passage  52  is small. Thus, the electromagnetic valve  73  may be a small one that consumes little power. Furthermore, the fixed throttle  69  arranged in the first intake passage  51 , which connects the pressure chamber  63  and the suction chamber  38 , decreases the amount of refrigerant gas that flows out of the pressure chamber  63  when the electromagnetic valve  73  is opened. This is another factor that permits the employment of a more compact electromagnetic valve  73 . 
     The characteristics of the two threshold values Pset 1 , Pset 2 , that is, the inclination of the two curves Pset 1 , Pset 2  shown in the graph of FIG. 3, is correlated with the inner diameter D 1  of the fixed throttle  71  and the inner diameter D 2  of the fixed throttle  69 . Based on the experience of the inventors, it is believed that the inclination of the Pset 1  and Pset 2  curves increase as the inner diameter D 2  of the fixed throttle  69  increases, or as the leakage of refrigerant gas from the pressure chamber  63  increases. 
     In an air conditioning system employing a compressor, pressure loss normally occurs in accordance with the length of the piping between the outlet of the evaporator  57  and the inlet of the compressor  10 . Thus, an air conditioning system employing a compressor that incorporates a prior art self-controlled control valve must have the suction pressure threshold value Pset set differently for each type of vehicle in accordance with the length of the piping. More specifically, the force of the spring  68  must be changed for each type of vehicle. However, in the first embodiment, the threshold value Pset is shifted between at least the first and second threshold values Pset 1 , Pset 2  by adjusting the amount of the high-pressure refrigerant gas drawn into the pressure chamber  63 . This simplifies the structure of the air conditioning system in comparison to that of the prior art. 
     Second Embodiment 
     As shown in FIG. 4, the first port  78  extending from the upper region of the valve chamber  62  is connected to the crank chamber  15  through the pressurizing passage  49 . The second port  79  extending from the lower region of the valve chamber  62  is connected to the discharge chamber  39  through the pressurizing passage  48 . Thus, refrigerant gas pressurized to the discharge pressure Pd is constantly sent into the lower region of the valve chamber  62 . The refrigerant gas in the lower region of the valve chamber  62  has a tendency to flow toward the upper region of the valve chamber  62 . In other words, the flow direction of refrigerant gas in the valve chamber  62  is the same as the direction in which the valve body  75  moves away from the valve seat  74 . This direction, upward in FIG. 4, is the same as the urging direction of the spring  68 . Thus, the differential pressure produced between the discharge pressure Pd, which acts on the lower side of the valve body  75 , and the pressure Pc, which acts on the upper side of the valve body  75 , is added to the force of the spring  68 . As a result, in the control valve  60  of the second embodiment, the first threshold value Pset 1  curve, which represents the characteristics of the control valve  60  when the electromagnetic valve  73  is closed, is inclined upwardly to the right, as shown in FIG.  5 . 
     Since the first threshold value Pset 1  curve is inclined upwardly to the right, the difference between the first threshold value Pset 1  (initial value) and the second threshold value Pset 2  in the second embodiment, as shown in FIG. 5, is greater than the difference between the first threshold value Pset 1  and the second threshold value Pset 2  in the first embodiment, as shown in FIG.  3 . Therefore, in comparison to the control valve  60  of the first embodiment, the control valve  60  of the second embodiment varies the suction pressure threshold value Pset by a greater degree when the electromagnetic valve  73  is switched. Accordingly, the compressor  10  incorporating the control valve  60  of the second embodiment can be used with a larger number of vehicle types. 
     Third Embodiment 
     As shown in FIG. 6, three ports  81 ,  82 ,  83  extend through the wall of the pressure detecting chamber  63 . The first port  81  is connected to the discharge chamber  39  through a passage  84 . A fixed throttle  85  is arranged in the passage  84 . The second port  82  is connected to the suction chamber  38  through a passage  86 . A fixed throttle  87  is arranged in the passage  86 . The third port  83  is connected to the suction chamber  38  through a passage  88 . An electromagnetic valve  73  is arranged in the passage  88 . The controller  58  controls the electromagnetic valve  73  to selectively open and close the passage  88 . 
     When the passage  88  is closed by the electromagnetic valve  73 , refrigerant gas pressurized to pressure Pd flows into the pressure chamber  63  from the discharge chamber  39 . Some of the refrigerant gas flows into the suction chamber  38  through the passage  86 , throttled by the fixed throttle  87 . Thus, the pressure Pk of the pressure chamber  63  approaches the pressure of the discharge chamber  39 . On the other hand, opening the passage  88  with the electromagnetic valve  73  has the same effect as increasing the inner diameter of the fixed throttle  87 . Therefore, although relatively high pressure refrigerant gas flows into the pressure chamber  63  from the discharge chamber  39 , the refrigerant gas flows out of the pressure chamber  63  and into the suction chamber  38  through the passages  86 ,  88 . Consequently, the pressure Pk in the pressure chamber  63  approaches the pressure Ps of the suction chamber  38 . Closing the passage  88  with the electromagnetic valve  73  in FIG. 6 is substantially equivalent to opening the electromagnetic valve  73  in FIG.  2 . Opening the passage  88  with the electromagnetic valve  73  in FIG. 6 is substantially equivalent to closing the electromagnetic valve  73  in FIG.  2 . 
     The characteristics of the suction pressure threshold value Pset in the third embodiment are shown in the graph of FIG.  7 . When the electromagnetic valve  73  is closed, the threshold value Pset is set at the second threshold value Pset 2 . When the electromagnetic valve  73  is opened, the threshold value Pset is set at the first threshold value Pset 1 . The second threshold value Pset 2  is set such that it is as close as possible to the frost limit curve. 
     The refrigerant gas flowing through the electromagnetic valve  73  employed in the first embodiment is pressurized to a value substantially the same as the discharge pressure Pd, whereas the refrigerant flowing through the electromagnetic valve  73  employed in the third embodiment is only pressurized to a value substantially the same as the suction pressure Ps. Thus, the electromagnetic valve  73  of the third embodiment is more compact than the electromagnetic valve  73  of the first embodiment. 
     The compressor  10  incorporating the control valve  60  shown in FIG. 6 has the same advantages as the first embodiment. 
     Fourth Embodiment 
     As shown in FIG. 8, the first port  78  extending from the upper region of the valve chamber  62  is connected to the crank chamber  15  through the pressurizing passage  49 . The second port  79  extending from the lower region of the valve chamber  62  is connected to the discharge chamber  39  through the pressurizing passage  48 . Thus, relatively high pressure refrigerant gas from the discharge chamber  39  is constantly sent into the lower region of the valve chamber  62 . The refrigerant gas in the lower region of the valve chamber  62  has a tendency to flow toward the upper region of the valve chamber  62 . In other words, the flow direction of refrigerant gas in the valve chamber  62  is the same as the urging direction of the spring  68 . Thus, the differential pressure produced between the discharge pressure Pd, which acts on the lower side of the valve body  75 , and the crank chamber pressure Pc, which acts on the upper side of the valve body  75 , is added to the force of the spring  68 . 
     The characteristics of the intake pressure threshold value Pset in the control valve  60  of the fourth embodiment are shown in FIG.  9 . The first threshold value Pset 1  curve, which is selected when the electromagnetic valve  73  is opened, is inclined more upwardly to the right in comparison to the first threshold value Pset 1  curve of the third embodiment shown in FIG.  7 . Accordingly, the difference between the first threshold value Pset 1  and the second threshold value Pset 2  in the fourth embodiment, as shown in FIG. 9, is greater than that of the third embodiment, as shown in FIG.  7 . Therefore, in comparison to the control valve  60  of the third embodiment, the control valve  60  of the fourth embodiment varies the suction pressure threshold value Pset by a greater degree when the electromagnetic valve  73  is switched. Accordingly, the compressor  10  incorporating the control valve  60  of the fourth embodiment can be applied to a larger number of vehicle types. 
     Fifth Embodiment 
     As shown in FIG. 10, the control valve  60  of the fifth embodiment is similar to that of the third embodiment (FIG.  6 ). A boss  61   a  extends from the valve housing  61  of the control valve  60 . The boss  61   a  houses a differential pressure valve mechanism  90 . The differential valve mechanism  90  includes a valve chamber  91 , a spherical valve body  92  accommodated in the valve chamber  91 , and a spring  93 . The valve chamber  91  has an opening that is sealed by a cap  94 . One end of the spring  93  is fixed to the cap  94 , while the other end is fixed to the valve body  92 . The spring  93  urges the valve body  92  toward the valve seat  91   a . When the valve body  92  comes into contact with the valve seat  91   a , the second port  82  is completely closed in the side of the valve chamber  91 . A bore  95  extends through the center of the cap  94 . The valve sensing chamber  63  is connected to the suction chamber  38  through the differential pressure valve mechanism  90 . 
     The valve chamber  91  is always connected with the suction chamber  38 . Thus, the pressure of the valve chamber  91  is equal to the suction pressure Ps. The pressure Pk in the pressure chamber  63  acts on the side of the valve body  82  that is closer to the second port  82 . Relatively high pressure refrigerant gas is continuously sent into the pressure chamber  63  from the discharge chamber  39  through the fixed throttle  85 . Accordingly, the pressure Pk of the pressure chamber  63 , which is applied to the valve body  92 , acts in a direction causing the valve body  92  to open the second port  82 . The position of the valve body  92  in the valve chamber  91  is determined by the force of the spring  93  and the difference between the suction pressure Ps and the pressure Pk of the pressure chamber  63 . For example, if the pressure Pk of the pressure chamber  63  is higher than a predetermined value, the valve body  92  moves away from the valve seat  91   a  and opens the second port  82 . This gradually decreases the value of the chamber pressure Pk. If the pressure Pk of the pressure chamber  63  falls below the predetermined value, the valve body  92  contacts the valve seat  91   a  and closes the second port  82 . This gradually increases the value of the pressure Pk. In this manner, the differential valve mechanism  90  automatically changes the size of its opening such that the difference between the suction pressure Ps and the pressure Pk of the pressure chamber  63  (Pk−Ps) is maintained at a substantially constant value. 
     Like the third embodiment, the electromagnetic valve  73  is normally closed in the control valve  60  of the fifth embodiment. In this state, relatively high pressure refrigerant gas flows into the pressure chamber  63  from the discharge chamber  39 . The pressure Pk of the pressure chamber  63  is determined by the differential pressure valve  90 . Closing the electromagnetic valve  73  in the fifth embodiment of FIG. 10 is like closing the electromagnetic valve  73  in the third embodiment illustrated in FIG.  6 . When the electromagnetic valve  73  is opened, the pressure Pk of the pressure chamber  63  approaches the pressure Ps of the suction chamber  38 , since the pressure chamber  63  and the suction chamber  38  are connected to each other through the passage  88 . 
     The characteristics of the suction pressure threshold value Pset in the fifth embodiment are shown in the graph of FIG.  11 . When the electromagnetic valve  73  is closed, the threshold value Pset of the suction pressure Ps is set at the second threshold value Pset 2 . When the electromagnetic valve  73  is opened, the threshold value Pset is changed from the second threshold value Pset 2  to the first threshold value Pset 1 . The second threshold value Pset 2  is set such that it is as close as possible to the frost limit curve. 
     As shown in the graph of FIG. 11, the first threshold value Pset 1  curve is substantially parallel to the second threshold value Pset 2  curve. This differs from the first to fourth embodiments (FIGS. 3,  5 ,  7 , and  9 ). In the first to fourth embodiments, the difference between the first threshold value Pset 1  curve and the second threshold value Pset 2  decreases as the discharge pressure Pd decreases. Accordingly, the control valve  60  of the fifth embodiment is advantageous if the suction pressure threshold value Pset must be varied by switching the valve  73  when the discharge pressure Pd is relatively low. 
     In the fifth embodiment, the differential pressure valve mechanism  90  maintains the same difference between the pressure Pk of the pressure chamber  63  and the suction pressure Ps. Thus, the difference between the first threshold value Pset 1  and the second threshold value Pset 2  is kept substantially constant regardless of the compressor displacement. As a result, the compressor displacement is variably controlled in accordance with the conditions of the vehicle and the engine  20  by shifting the suction pressure threshold value Pset even if the displacement is small. This decreases the load applied to the engine  20  and prevents the engine  20  from stalling, for example, when the engine  20  is idling (a state in which the engine speed is low and it is preferable that the compressor displacement is small) or when the vehicle is stopped suddenly. 
     Sixth Embodiment 
     As shown in FIG. 12, in the control valve  60  of the sixth embodiment, the number of ports extending through the valve housing  61  is less than that of the fifth embodiment. A single port  83  extends from the pressure chamber  63 . The pressure chamber  63  is connected to the suction chamber  38  solely by passage  96 . The valve body  75  is fixed to the upper end of the rod  65 . A narrow passage  97  extends through the valve body  75  and the rod  65 . The passage  97  connects the upper region of the valve chamber  62  to the pressure chamber  63 . Thus, refrigerant gas is continuously sent into the pressure chamber  63  from the discharge chamber  39  through the passage  97 . The passage  97  functions as a fixed throttle for restricting the flow of the refrigerant gas from the discharge chamber  39  to the pressure chamber 
     The electromagnetic valve  73  is arranged in the passage  96  at the rear portion of the rear housing  14 . The electromagnetic valve  73  includes a valve body  73   a , a spring  73   b , and a coil  73   c . A valve seat  96   a  is formed in the passage  96  to receive the valve body  73   a . The valve body  73   a  closes the passage  96  when in contact with the valve seat  96   a . The spring  73   b  urges the valve body  73   a  toward the valve seat  96   a . Excitation of the coil  73   c  moves the valve body  73   a  away from the valve seat  96   a  against the force of the spring  73   b . The controller  58  controls the electromagnetic valve  73  to selectively open and close the passage  96  and control the flow of refrigerant gas between the pressure chamber  63  and the suction chamber  38 . 
     The valve body  73   a  moves in accordance with the equilibrium between the force produced by the suction pressure Ps and the spring  73   b  and the force produced by the pressure Pk of the pressure chamber  63 , even if the coil  73   c  is not excited. The size of the passage  96  opened by the valve body  73   a  is varied in accordance with the movement of the valve body  73   a . Thus, the electromagnetic valve  73  functions as a variable throttle and maintains the difference between the suction pressure Ps and the pressure Pk at a substantially constant value. Accordingly, when current is fed to the coil  73   c , the electromagnetic valve  73  completely opens the passage  96 . When the coil  73   c  is de-excited, the electromagnetic valve  73  adjusts the opening size of the passage  96  based on the pressure Pk of the pressure chamber  63  and the suction pressure Ps. 
     In the sixth embodiment (FIG.  12 ), the first threshold value Pset 1  curve is substantially parallel to the second threshold value Pset 2  curve in the same manner as the fifth embodiment (FIG.  11 ). The difference between the first threshold value Pset 1 , which is affected by the force of the spring  68 , and the frost limit is greater than the difference between the second threshold value Pset 2 , which is affected by the amount of refrigerant gas drawn into the pressure chamber  63  from the discharge chamber  39 , and the frost limit. The second threshold value Pset 2  curve approaches the frost limit curve as the discharge pressure Pd increases. 
     In the sixth embodiment, the electromagnetic valve  73  shifts the threshold value between two values. Furthermore, the passage  97 , which extends through the valve body  75  and the rod  65 , decreases the number of passages in the compressor  10 . This decreases the number of machining processes required during the production of the compressor  10  and reduces the number of seals required to seal spaces between the control valve  60  and such passages. Additionally, since the number of passages are decreased, the rear housing  14  has a smaller size. Thus, the compressor  10  is more compact. 
     Seventh Embodiment 
     As shown in FIG. 13, in the same manner as the sixth embodiment, the electromagnetic valve  73  is installed in the rear portion of the rear housing  14 . A port  107  extends from the pressure chamber  63 . The port  107  is connected to a passage  84 , which leads into the discharge chamber  39 . A fixed throttle  108  is defined in the port  107 . The pressurizing passage  49 , which extends from the crank chamber  15 , is connected to the valve chamber  62  through a port  109 . A passage  98 , which extends from the suction chamber  38 , is connected to the valve chamber  62  through a port  110 . The pressure Ps of the suction chamber  38  is always applied to the valve chamber  62 . The valve chamber  62  is connected to a valve sensing chamber  63  by way of a port  112 , a passage  99 , the electromagnetic valve  73 , and the port  83 . 
     The valve chamber  62  houses the valve body  75 . The valve body  75  is formed integrally with the rod  65 . The spring  68  arranged in the bellows  66  urges the valve body  75  toward the port  109 . The valve body  75  and the rod  65  are moved by the deformation of the bellows  66 . For example, if the pressure Pk of the pressure chamber  63  is high, the bellows  66  contracts and causes the valve body  75  to open the port  109 . If the pressure Pk of the pressure chamber  63  is low, the bellows  66  expands and closes the port  109  with the valve body  75 . Accordingly, the suction chamber  38  and the crank chamber  15  are connected and disconnected from each other in accordance with the pressure Pk of the pressure chamber  63 . 
     The electromagnetic valve  73  is arranged in the passage  99 . When the valve body  73   a  contacts a valve seat  99   a , which is formed in the passage  99 , the valve body  73   a  closes the passage  99 . Like the sixth embodiment, the electromagnetic valve  73  opens the passage  99  when the coil  73  is excited. When the coil  73   c  is de-excited, the electromagnetic valve  73  functions as a variable throttle and maintains the difference between the suction pressure Ps and the pressure Pk of the pressure chamber  63  at a substantially constant value. 
     The operation of the control valve  60  will now be described. High-pressure refrigerant gas is gradually drawn into the pressure chamber  63  from the discharge chamber  39  through the port  107 . Thus, the pressure Pk of the pressure chamber  63  approaches the discharge pressure Pd. 
     Excitation of the coil  73   a  causes the valve body  73   a  to open the passage  99  and release the high-pressure refrigerant gas from the pressure chamber  63 . As a result, the pressure Pk of the pressure chamber  63  decreases to a value slightly higher than the suction pressure Ps. In th is state, the difference between the suction pressure Ps and the pressure Pc of the crank chamber  15  scarcely affects the behavior of the spring  68 . Thus, the first threshold value Pset 1  curve decreases more gradually than that of the sixth embodiment. 
     If the coil  73   c  is de-excited and the suction pressure Ps of the suction chamber  38  is high, the electromagnetic valve  73  remains closed until the difference between the pressure Pk of the pressure chamber  63  and the suction pressure Ps of the suction chamber  38  reaches a predetermined value . Therefore, the pressure Pk of the pressure chamber  63  increases. When the pressure Pk of the pressure chamber  63  exceeds a predetermined value P 0 , the bellows  66  contracts against the force of the spring  68 . This causes the valve body  75  to open the port  109  and release the refrigerant gas in the crank chamber  15  into the suction chamber  38  through the valve chamber  62 . 
     If the coil  73   c  is de-excited and the suction pressure Ps of the suction chamber  38  is relatively low, the electromagnetic valve  73  is opened such that the difference between the pressure Pk of the pressure chamber  63  and the suction pressure Ps of the suction chamber  38  becomes equal to a predetermined value. When the pressure Pk of the pressure chamber  63  falls below the predetermined pressure P 0 , the force of the spring  68  expands the bellows  66 . This closes the port  109  with the valve body  75  and stops the flow of refrigerant gas in the valve chamber  62  from the crank chamber  15  to the suction chamber  38 . 
     As described above, the valve body  73   a  throttles the passage  99  and restricts the amount of refrigerant gas released into the suction chamber  38  from the pressure chamber  63  when the coil  73   c  is de-excited. Therefore, the pressure Pk of the pressure chamber  63  is higher than the suction pressure Ps by a predetermined value. Accordingly, in the same manner as the sixth embodiment, the second threshold values Pset 2  are lower than the first threshold values Pset 1  by a predetermined amount. 
     The electromagnetic valve  73  automatically adjusts the pressure Pk of the pressure chamber  63  such that the difference between the pressure Pk and the suction pressure Ps remains constant. Further, the controller  58  selectively connects and disconnects the crank chamber  15  and the suction chamber  38  with the electromagnetic valve  73 . In other words, the controller  58  shifts the threshold value between the first threshold value Pset 1  and the second threshold value Pset 2 . 
     In the seventh embodiment, the valve chamber  62  is located between the passages  99 ,  98  that connect the pressure chamber  63  to the suction chamber  62 . This decreases the number of passages extending between the control valve  60  and the discharge chamber  39 . Like the sixth embodiment, this decreases the number of machining processes required during the production of the compressor  10  and reduces the number of seals required to seal spaces between the control valve  60  and such passages. Additionally, since the number of passages are decreased, the rear housing  14  has a smaller size. Thus, the compressor  10  is more compact. 
     Eighth Embodiment 
     As shown in FIG. 14, in the control valve  60  of the eighth embodiment, a switching valve  130  is arranged in the pressurizing passages  48 ,  49 . The switching valve  130  is controlled by a controller  58  to switch the connections between the discharge chamber  39 , the crank chamber  15 , and the valve chamber  62 . 
     FIG. 14 shows the switching valve  130  in a normal position, or first position. In the first position, the discharge chamber  39  is connected to the first port  78 , while the second port  79  is connected to the crank chamber  15 . When the switching valve  130  is moved to a second position, the discharge chamber  39  is connected to the second port  79 , while the first port  78  is connected to the crank chamber  15 . Regardless of whether the switching valve  130  is in the first position or the second position, the high-pressure refrigerant gas in the discharge chamber  39  is sent to the crank chamber  15  through the valve chamber  62 . However, the flow direction of gas in the valve chamber  62  is reversed by the switching valve  130 . That is, if the switching valve  130  is in the first position, the refrigerant gas flows downward in the valve chamber  62 , as viewed in FIG.  14 . If the switching valve  130  is in the second position, the refrigerant gas flows upward in the valve chamber  62 . 
     The control valve  60  functions in the same manner as the control valve  60  shown in FIG. 2 when the switching valve  130  is in the first position. When the switching valve  130  is maintained in the first position, the electromagnetic valve  73  is controlled to shift the suction pressure threshold value Pset between the first threshold value Pset 1  and the second threshold value Pset 2 , as shown in the graph of FIG.  15 . The control valve  60  functions in the same manner as the control valve  60  shown in FIG. 4 when the switching valve  130  is in the second position. When the switching valve  130  is maintained in the second position, the electromagnetic valve  73  is controlled to shift the suction pressure threshold value Pset between a third threshold value Pset 3  (corresponding to the first threshold value Pset 1  in the embodiment illustrated in FIG. 4) and the second threshold value Pset 2 , as shown in the graph of FIG.  15 . Furthermore, when the electromagnetic valve  73  is closed, the switching valve  130  is controlled to shift the suction pressure threshold value Pset between the first threshold value Pset 1  and the third threshold value Pset 3 . Accordingly, the controller  58  controls the electromagnetic valve  73  and the switching valve  130  such that the threshold value Pset is shifted between three values, as shown in FIG.  15 . 
     Ninth Embodiment 
     In a ninth embodiment according to the present invention, the electromagnetic valve  73  employed in the embodiments of FIGS. 2 and 4 may be replaced by a valve mechanism  120  shown in FIG.  16 . The valve mechanism  120  has a first chamber  121  and a second chamber  122 . The first chamber  121  is connected to the discharge chamber  39  by way of a fixed throttle  71 . The first chamber  121  is connected to the second chamber  122  through a communication bore  123 . A spherical valve body  125  is accommodated in the first chamber  121 . A spool  124  is slidably accommodated in the second chamber  122 . The spool  124  divides the second chamber  122  into a right region (rightward of the spool  124 ) and a left region (leftward of the spool  124 ). The right region is always connected with the pressure chamber  63  through the port  70 . The left region is connected to an intake passage  126 , which leads to the engine. A spring  127  is arranged in the left region to urge the spool  124  to the right, as viewed in FIG. 16. A connecting rod is fixed to the right end of the spool  124 . The spherical valve body  125  is connected to the spool  124  by the connecting rod. The valve body  125  opens the communication bore  123  when the spool  124  moves toward the right and closes the communication bore  123  when the spool  124  moves toward the left. 
     When the vehicle is being driven at a constant speed and the engine speed is substantially constant, the valve mechanism  120  of FIG. 16 de-pressurizes the left region of the second chamber  122  due to the vacuum pressure produced by the flow of intake air in the intake passage  126 . However, the force of the vacuum pressure is weaker than the force of the spring  127 . Thus, the valve body  125  does not close the communication bore  123 . When the engine speed increases (e.g., during acceleration of the vehicle) and causes the vacuum pressure to apply a force on the spool  124  that is stronger than the force of the spring  127 , the spool  124  moves toward the left and closes the communication bore  123  with the valve body  125 . In this state, the flow of refrigerant gas through the passage  72  is stopped. Accordingly, the valve mechanism  120  may be used in lieu of the electromagnetic valve  73  employed in the embodiments of FIGS. 2 and 4 to shift from the second threshold value Pset 2  to the first threshold value Pset 1  during acceleration of the vehicle. 
     Tenth Embodiment 
     In a tenth embodiment, as shown in FIG. 17, the sixth embodiment may be modified such that the lower region of the valve chamber  62  is connected to the discharge chamber  39  and the upper region of the valve chamber  62  is connected to the crank chamber  15 . In this structure, the force produced by the difference between the discharge pressure Pd and the pressure Pc is applied to the valve body  75  in addition to the force of the spring  68 . Further, a clearance  128  extends between the wall of the guide bore  64  and the rod  65  to connect the lower region of the valve chamber  62  with the pressure chamber  63 . Thus, the high-pressure refrigerant gas that enters the valve chamber  62  from the discharge chamber  39  further flows into the pressure chamber  63 . In this structure, a simple machining process is carried out to connect the valve chamber  62  and the pressure chamber  63  to each other. 
     Eleventh Embodiment 
     In an eleventh embodiment, as shown in FIG. 18, the seventh embodiment may be modified such that the suction chamber  38  is connected to the top end of the valve chamber  62  and such that the crank chamber  15  is connected to the side of the valve chamber  38 . Like the seventh embodiment, the refrigerant gas in the crank chamber  15  is released toward the suction chamber  38  based on the pressure Pk of the pressure chamber  63 . 
     Twelfth Embodiment 
     In a twelfth embodiment, as shown in FIG. 19, the seventh embodiment may be modified such that the valve chamber  62  is arranged in the passage  49 , which connects the suction chamber  38  and the crank chamber  15 . In this embodiment, the amount of high-pressure refrigerant gas sent into the pressure chamber  63  from the discharge chamber  39  is varied to change the suction pressure threshold value Pset. 
     It should be apparent to those skilled in the art that the present invention may be embodied in many other specific forms without departing from the spirit or scope of the invention. For example, the present invention may be embodied as described below. 
     In the sixth embodiment, the passage  97  extending through the valve body  75  and the rod  65  may be replaced by a communication passage extending through the valve housing  61  to connect the upper region of the valve chamber  62  to the pressure chamber  63 . In this structure, the high-pressure refrigerant gas in the discharge chamber  39  flows into the pressure chamber  63  through the communication passage. Thus, this structure has the same advantages as the sixth embodiment. 
     The electromagnetic valve  73  employed in the first to eighth embodiments may be replaced by an electromagnetic valve that can be controlled to maintain a partially opened state. In such structure, the suction pressure threshold value Pset is selected from three values. Furthermore, the power of the engine  20  is distributed appropriately between the power train and the compressor  10 . Thus, the driving performance of the vehicle and the cooling performance are both maintained at a high level. 
     The electromagnetic valve  73  employed in the first to eighth embodiments is shifted between two positions. However, an electromagnetic valve that continuously varies its opening size in accordance with a supply current may be employed instead of the electromagnetic valve  73 . In this case, the controller  58  may vary the level of the current. In this structure, the suction pressure threshold value Pset is varied continuously. Thus, the operation of the compressor  10  may be more finely controlled. 
     In the first to eighth embodiments, the control valve  60  need not be incorporated in the compressor  10 . 
     In the first to eighth embodiments, the pressure chamber  63  may be connected with the central bore  27  or the suction passage  28 . 
     In the first to eighth embodiments, the valve chamber  62  may be connected to the central bore  27  or the suction passage  28 . 
     The present invention may also be applied to a wobble plate type compressor. Furthermore, the compressor may be connected to the engine by an electromagnetic clutch. 
     In the first to eighth embodiments, the control valve  60  is actuated in accordance with the suction pressure Ps communicated to the pressure chamber  63 . However, a control valve that is actuated in accordance with the crank pressure Pc communicated to the pressure chamber  63  may be employed instead. In this case, the suction pressure Ps is varied in accordance with changes in the threshold value. 
     Therefore, the present examples and embodiments are to be considered as illustrative and not restrictive, and the invention is not to be limited to the details given herein, but may be modified within the scope and equivalence of the appended claims.