Patent Publication Number: US-6213728-B1

Title: Variable displacement compressor

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a variable displacement compressor for vehicle air-conditioning. 
     FIG. 8 shows a prior art variable displacement compressor. A drive shaft is rotatably supported in the housing  101 , which encloses a crank chamber  102 . A lip seal  104  is located between the housing  101  and the drive shaft  103  to prevent leakage of fluid from the housing  101 . 
     An electromagnetic friction clutch  105  is located between the drive shaft  103  and the engine Eg, which serves as a power source. The clutch  105  includes a rotor  106  that is coupled to the engine Eg, an armature  107  that is fixed to the drive shaft  103 , and an electromagnetic coil  108 . When the coil  108  is excited, the armature  107  is attracted to and contacts the rotor  106 . In this state, power of the engine Eg is transmitted to the drive shaft  103 . When the coil  108  is de-excited, the armature  107  is separated from the rotor  106 , which disconnects the power transmission from the engine Eg to the drive shaft  103 . 
     A lug plate  109  is fixed to the drive shaft  103  in the crank chamber  102 . A thrust bearing  122  is located between the lug plate  109  and the housing  101 . A swash plate  110  is coupled to the lug plate  109  via a hinge mechanism  111 . The swash plate  110  is supported by the drive shaft  103  such that the swash plate  110  slides axially and inclines with respect to the axis L of the drive shaft  103 . The hinge mechanism  111  causes the swash plate  110  to integrally rotate with the drive shaft  103 . When the swash plate  110  contacts the limit ring  112 , the swash plate  110  is positioned at the minimum inclination position. 
     The housing  101  includes cylinder bores  113 , a suction chamber  114 , and a discharge chamber  115 . A piston  116  is accommodated in each cylinder bore  113  and is coupled to the swash plate  110 . A valve plate  117  partitions the cylinder bores  113  from a suction chamber  114  and a discharge chamber  115 . 
     When the drive shaft  103  rotates, the swash plate  110  reciprocates each piston  116 . Accompanying this, refrigerant gas in the suction chamber  114  flows into each cylinder bore  113  through the corresponding suction port  117   a  and suction valve  117   b,  which are formed in the valve plate  117 . Refrigerant gas in each cylinder bore  113  is compressed to reach a predetermined pressure and is discharged to the discharge chamber  115  through the corresponding discharge port  117   c  and discharge valve  117   d,  which are formed in the valve plate  117 . 
     An axial spring  118  is located between the housing  101  and the drive shaft  103 . The axial spring  118  urges the drive shaft  103  frontward (leftward in FIG. 8) along the axis L and limits axial chattering of the drive shaft  103 . A thrust bearing  123  is located between the axial spring  118  and an end surface of the drive shaft  103 . The thrust bearing  123  prevents transmission of rotation from the drive shaft  103  to the axial spring  118 . 
     A bleed passage  119  connects the crank chamber  102  to the suction chamber  114 . A pressurizing passage  120  connects the discharge chamber  115  to the crank chamber  102 . A displacement control valve, which is an electromagnetic valve, adjusts the opening size of the pressurizing passage  120 . 
     The control valve  121  adjusts the flow rate of refrigerant gas from the discharge chamber  115  to the crank chamber  102  by varying the opening size of the pressurizing passage  120 . This varies the inclination of the swash pate  110 , the stroke of each piston  116 , and the displacement. 
     When the clutch  105  is disengaged, or when the engine Eg is stopped, the control valve  121  maximizes the opening size of the pressurizing passage  120 . This increases the pressure in the crank chamber  102  and minimizes the inclination of the swash plate  110 . As a result, the compressor stops when the inclination of the swash plate  110  is minimized, or when the displacement is minimized. Accordingly, since the displacement is minimized, the compressor is started with a minimal torque load. This reduces torque shock when the compressor is started. 
     When the cooling load on a refrigeration circuit that includes the compressor is great, for example, when the temperature in a vehicle passenger compartment is much higher than a target temperature set in advance, the control valve  121  closes the pressurizing passage  120  and maximizes the displacement of the compressor. 
     Suppose that when the compressor is operating at maximized displacement, it is stopped by disengagement of the clutch  105  or by shutting off the engine Eg. In this case, the control valve  121  quickly maximizes the opening size of the closed pressurizing passage  120  to minimize the displacement. Also, when the vehicle is suddenly accelerated while the compressor is operating at maximum displacement, the control valve  121  quickly maximizes the opening size of the pressurizing passage  120  to minimize the displacement and to reduce the load applied to the engine Eg. Accordingly, refrigerant gas in the discharge chamber  115  is quickly supplied to the crank chamber  102 . Though some refrigerant gas flows to the suction chamber  114  through the bleed passage  119 , the pressure in the crank chamber  102  quickly increases. 
     Therefore, the swash plate  110 , when at a minimum displacement position (as shown by the broken line in FIG. 8) is pressed against a limit ring  112 . Also, the swash plate  110  pulls the lug plate  109  in a rearward direction (rightward in FIG. 8) through the hinge mechanism  111 . As a result, the drive shaft  103  moves axially rearward against the force of the axial spring  118 . 
     When the drive shaft  103  moves rearward, the axial position of the drive shaft  103  with respect to a lip seal  104 , which is held in the housing  101 , changes. Generally, a predetermined contact area of the drive shaft  103  contacts the lip seal  104 . Foreign particles such as sludge exist on the peripheral surface of the drive shaft  103  that is outside the predetermined contact area. Therefore, when the axial position of the drive shaft  103  with respect to the lip seal  104  changes, the sludge will be located between the lip seal  104  and the drive shaft  102 . This lowers the sealing performance of the lip seal  104  and may cause leakage of refrigerant gas from the crank chamber  102 . 
     When the operation of the compressor is stopped by the disengagement of the clutch  105  and the drive shaft  103  moves rearward, the armature  107 , which is fixed to the drive shaft  103 , moves toward the rotor  106 . The clearance between the rotor  106  and the armature  107  when the clutch  105  is disengaged is set to a small value, for example, 0.5 mm. Accordingly, when the drive shaft  103  moves rearward, the clearance between the rotor  106  and the armature  107  is eliminated, which causes the armature  107  to contact the rotating rotor  106 . This may cause noise and vibration or may transmit power from the engine Eg to the drive shaft  103  regardless of the disengagement of the clutch  105 . 
     When the drive shaft  103  moves rearward, each piston  116 , which is coupled to the drive shaft through the lug plate  109  and the swash plate  110 , also moves rearward. This moves the top dead center position of each piston  116  toward the valve plate  117  which may permit the pistons  116  to collide with the valve plate  117 . Since the control valve  121  maximizes the opening size of the pressurizing passage  120  during sudden accelerations of the vehicle while the compressor is operating, the rearward movement of the drive shaft  103  accompanying the control may cause the pistons  116  to repeatedly collide with the valve plate  117 . This generates noise and vibration. 
     To prevent the rearward movement of the drive shaft  103 , the force of the axial spring  118  can be increased. However, increasing the force of the axial spring  118  lowers the durability of the thrust bearing  123 , which is located between the axial spring  118  and the drive shaft  103 , lowers the durability of the thrust bearing  122 , which is located between the housing  101  and the lug plate  109 , and increases the load placed on the engine by the compressor. 
     SUMMARY OF THE INVENTION 
     An objective of the present invention is to provide a variable displacement compressor that can prevents the pressure in a crank chamber from excessively increasing. 
     To achieve the above objective, the present invention provides a variable displacement compressor comprises a housing, a cylinder bore formed in the housing, a crank chamber, a suction chamber, a discharge chamber, A piston is accommodated in the cylinder bore. A drive shaft is rotatably supported in the housing. A drive plate is coupled to the piston for converting rotation of the drive shaft to reciprocation of the piston. The drive plate is tiltably supported on the drive shaft. The drive plate moves between a maximum inclination position and a minimum inclination position in accordance with the pressure in the crank chamber. The inclination of the drive plate determines the piston stroke and the displacement of the compressor. A pressure control mechanism controls the pressure in the crank chamber to change the inclination of the drive plate. A control passage connects the crank chamber to a selected chamber in the compressor. A pressure adjusting valve is located in the control passage. The pressure adjusting valve regulates gas flow in the control passage. A controller controls the pressure adjusting valve to limit the pressure in the crank chamber to prevent the pressure in the crank chamber from becoming undesirably high. 
     Other aspects and advantages of the 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 variable displacement compressor according to a first embodiment of the present invention; 
     FIG. 2 is a cross sectional view showing the displacement control valve of the compressor of FIG. 1; 
     FIG. 3 is a partial enlarged cross-sectional view showing the electromagnetic friction clutch of the compressor of FIG. 1; 
     FIG. 4 is a partial enlarged view showing the release valve of the compressor of FIG. 1; 
     FIG. 5 is a cross sectional view showing a variable displacement compressor according to a second embodiment; 
     FIG. 6 is a partial enlarged cross-sectional view showing a release valve in a third embodiment; 
     FIG. 7 is a partial enlarged cross-sectional view showing a release valve in a fourth embodiment; and 
     FIG. 8 is a cross sectional view of a prior art variable displacement compressor. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A single head type variable displacement compressor for vehicle air-conditioners according to a first embodiment of the present invention will now be described with reference to FIGS. 1-4. 
     As shown in FIG. 1, a front housing member  11  and a rear housing member  13  are coupled to a cylinder block  12 . A valve plate  14  is located between the cylinder block  12  and the rear housing member  13 . The front housing member  11 , the cylinder block  12 , and the rear housing member form a compressor housing. 
     As shown in FIGS. 1 and 2, the valve plate  14  includes a main plate  14   a,  a first sub-plate  14   b,  a second sub-plate  14   c,  and a retainer plate  14   d.  The main plate  14   a  is located between the first sub-plate  14   b  and the second sub-plate  14   c.  The retainer plate  14   d  is located between the second sub-plate  14   c  and the rear housing member  13 . 
     A crank chamber  15  is defined between the front housing member  11  and the cylinder block  12 . A drive shaft  16  passes through the crank chamber  15  and is rotatably supported by the front housing member  11  and the cylinder block  12 . 
     The drive shaft  16  is supported in the front housing member  11  through the radial bearing  17 . A central bore  12   a  is formed substantially in the center of the cylinder block  12 . The rear end of the drive shaft  16  is located in the central bore  12   a  and is supported in the cylinder block  12  through the radial bearing  18 . A spring seat  21 , which is a snap ring, is fixed to the inner surface of the central bore  12   a.  The thrust bearing  19  and the axial spring  20  are located in the central bore  12   a  between the rear end surface of the drive shaft  16  and the spring seat  21 . The axial spring  20 , which is a coil spring, urges the drive shaft frontward (leftward in FIG. 1) through the thrust bearing  19 . The axial spring  20  is an urging member. The thrust bearing  19  prevents transmission of rotation from the drive shaft  16  to the axial spring  20 . 
     The front end of the drive shaft  16  projects from the front housing member  11 . A lip seal  22 , which is a shaft sealing assembly, is located between the drive shaft  16  and the front housing member  11  to prevent leakage of refrigerant gas along the surface of the drive shaft  16 . The lip seal  22  includes a lip ring  22   a,  which is pressed against the surface of the drive shaft  16 . 
     An electromagnetic friction clutch  23  is located between an engine Eg, which serves as an external power source, and the drive shaft  16 . The clutch  23  selectively transmits power from the engine Eg to the drive shaft  16 . The clutch  23  includes a rotor  24 , a hub  27 , an armature  28 , and an electromagnetic coil  29 . The rotor  24  is rotatably supported by the front end of the front housing member  11  through an angular bearing  25 . A belt  26  is received by the rotor  24  to transmit power from the engine Eg to the rotor  24 . The hub  27 , which has elasticity, is fixed to the front end of the drive shaft  16  and supports the armature  28 . The armature  28  is arranged to face the rotor  24 . The electromagnetic coil  29  is supported by the front wall of the front hosing member  11  to face the armature  28  across the rotor  24 . 
     When the coil  29  is excited while the engine Eg is running, an attraction force based on electromagnetic force is generated between the armature  28  and the rotor  24 . 
     Accordingly, the armature  28  contacts the rotor  24 , which engages the clutch  23 . When the clutch  23  is engaged, power from the engine Eg is transmitted to the drive shaft  16  through the belt  26  and the clutch  23  (See FIG.  1 ). When the coil  29  is de-excited in this state, the armature  28  is separated from the rotor  24  by the elasticity of the hub  27 , which disengages the clutch  23 . When the clutch  23  is engaged, transmission of power from the engine Eg to the drive shaft  16  is disconnected (See FIG.  3 ). 
     As shown in FIG. 1, a lug plate  30  is fixed to the drive shaft  16  in the crank chamber  15 . A thrust bearing  67  is located between the lug plate  30  and the inner wall of the front housing member  11 . A swash plate  31 , which serves as a drive plate, is supported on the drive shaft  16  to slide axially and to incline with respect to the drive shaft  16 . A hinge mechanism  32  is located between the lug plate  30  and the swash plate  31 . The swash plate  31  is coupled to the lug plate  30  through the hinge mechanism  32 . The hinge mechanism  32  integrally rotates the swash plate  31  with the lug plate  30 . The hinge mechanism  32  also guides the swash plate  31  to slide along and incline with respect to the drive shaft  16 . As the swash plate  31  moves toward the cylinder block  12 , the inclination of the swash plate  31  decreases. As the swash plate  31  moves toward the lug plate  30 , the inclination of the swash plate  31  increases. 
     A limit ring  34  is attached to the drive shaft  16  between the swash plate  31  and the cylinder block  12 . As shown by the broken line in FIG. 1, the inclination of the swash plate  31  is minimized when the swash plate  31  abuts against the limit ring  34 . On the other hand, as shown by solid lines in FIG. 1, the inclination of the swash plate  31  is maximized when the swash plate  31  abuts against the lug plate  30 . 
     Cyclinder bores  33  are formed in the cylinder block  12 . The cylinder bores  33  are arranged at equal annular intervals about the axis L of the drive shaft  16 . A single head piston  35  is accommodated in each cylinder bore  33 . Each piston  35  is coupled to the swash plate  31  through a pair of shoes  36 . The swash plate  31  converts rotation of the drive shaft  16  into reciprocation of the pistons  35 . 
     A suction chamber  37 , which is a suction pressure zone, is defined in the substantial center of the rear housing member  13 . A discharge chamber  38 , which is a discharge pressure zone, is formed in the rear housing member  13  and surrounds the suction chamber  37 . The main plate  14   a  of the valve plate  14  includes suction ports  39  and discharge ports  40 , which correspond to each cylinder bore  33 . The first sub-plate  14   b  includes suction valves  41 , which correspond to suction ports  39 . The second sub-plate  14   c  includes discharge valves  42 , which correspond to the discharge ports  40 . The retainer plate  14   d  includes retainers  43 , which correspond to the discharge valves  42 . Each retainer  43  determines the maximum opening size of the corresponding discharge valve  42 . 
     When each piston  35  moves from the top dead center position to the bottom dead center position, refrigerant gas in the suction chamber  37  flows into the corresponding cylinder bore  33  through the corresponding suction port  39  and suction valve  41 . When each piston  35  moves from the bottom dead center position to the top dead center position, refrigerant gas in the corresponding cylinder bore  33  is compressed to a predetermined pressure and is discharged to the discharge chamber  38  through the corresponding discharge port  40  and discharge valve  42 . 
     A pressurizing passage  44  connects the discharge chamber  38  to the crank chamber  15 . A bleed passage  45 , which is a pressure release passage, connects the crank chamber  15  to the suction chamber  37 . The bleed passage  45  functions as a control passage that connects the crank chamber  15  to a selected chamber in the compressor, which is the suction chamber  37  in this embodiment. A displacement control valve  46  is located in the pressurizing passage  44 . The control valve  46  adjusts the flow rate of refrigerant gas from the discharge chamber  38  to the crank chamber  15  by varying the opening size of the pressurizing passage  44 . The bleed passage  45  and the control valve  46  form a pressure control mechanism. The pressure in the crank chamber  15  is varied in accordance with the relation between the flow rate of refrigerant from the discharge chamber  38  to the crank chamber  15  and that from the crank chamber  15  to the suction chamber  37  through the bleed passage  45 . Accordingly, the difference between the pressure in the crank chamber  15  and the pressure in the cylinder bores  33  is varied, which varies the inclination of the swash plate  31 . This varies the stroke of each piston  35  and the displacement. 
     The control valve  46  will now be described. 
     As shown in FIG. 2, the control valve  46  includes a valve housing  65  and a solenoid  66 , which are coupled together. A valve chamber  51  is defined between the valve housing  65  and the solenoid  66 . The valve chamber  51  accommodates a valve body  52 . A valve hole  53  opens in the valve chamber  51  and faces the valve body  52 . An opener spring  54  is accommodated in the valve chamber  51  and urges the valve body  52  to open the valve hole  53 . The valve chamber  51  and the valve hole  53  form part of the pressurizing passage  44 . 
     A pressure sensitive chamber  55  is formed in the valve housing  65 . The pressure sensitive chamber  55  is connected to the suction chamber  37  through a pressure detection passage  47 . A bellows  56 , which is a pressure sensitive member, is accommodated in the pressure sensitive chamber  55 . A spring  57  is located in the bellows  56 . The spring  57  determines the initial length of the bellows  56 . The bellows  56  is coupled to and operates the valve body  52  through a pressure sensitive rod  58 , which is integrally formed with the valve body  52 . 
     A plunger chamber  59  is defined in the solenoid  66 . A fixed iron core  60  is fitted in the upper opening of the plunger chamber  59 . A movable iron core  61  is accommodated in the plunger chamber  59 . A follower spring  62  is located in the plunger chamber  59  and urges the movable core  61  toward the fixed core  60 . A solenoid rod  63  is integrally formed at the lower end of the valve body  52 . The distal end of the solenoid rod  63  continuously abuts against the movable core  61  by the forces of the opener spring  54  and the follower spring  62 . In other words, the valve body  52  moves integrally with the movable core  61  through the solenoid rod  63 . The fixed core  60  and the movable core  61  are surrounded by a cylindrical electromagnetic coil  64 . 
     As shown in FIG. 1, the suction chamber  37  is connected to the discharge chamber  38  through an external refrigerant circuit  71 . The external refrigerant circuit  71  includes a condenser  72 , an expansion valve  73 , an evaporator  74 . The external refrigerant circuit  71  and the variable displacement compressor constitute a refrigeration circuit. 
     A controller C is connected to an air-conditioner switch  80 , which is a main switch of the vehicle air-conditioner, a temperature adjuster  82  for setting a target temperature in a passenger compartment, and a gas pedal sensor  83 . The controller C is, for example, a computer, which is located on current supply lines between a power source S (a vehicle battery) and the clutch  23  and between the power source S and the control valve  46 . The controller C supplies electric current from the power source S to the electromagnetic coils  29 ,  64 . The controller C controls current supply to each coil  29 ,  64  based on information including the ON/Off state of the air-conditioner switch  80 , a temperature detected by the temperature sensor  81 , a target temperature set by the temperature adjuster  82 , and the gas pedal depression degree detected by the gas pedal sensor  83 . 
     When the engine Eg is stopped (when the ignition switch is positioned at the accessory off position), most of the current supply to the electric equipment of the vehicle is stopped. Accordingly, the supply of current from the power source S to each coil  29 ,  64  is stopped. That is, when the operation of the engine Eg is stopped, the current supply lines between the power source S and each coil  29 ,  64  are disconnected upstream of the controller C. 
     Operation of the control valve  46  will now be described. 
     The controller C supplies a predetermined electric current to the coil  29  of the clutch  23  when the air-conditioner switch  80  is turned on during the operation of the engine Eg, and the temperature detected by the temperature sensor  81  is higher than the target temperature set by the temperature adjuster  82 . This engages the clutch  23  and starts the compressor. 
     The bellows  56  of the control valve  46  is displaced in accordance with the pressure in the suction chamber  37 , which is connected to the pressure sensitive chamber  55 . The displacement of the bellows  56  is transmitted to the valve body  52  through the pressure sensitive rod  58 . On the other hand, the controller C determines the electric current value supplied to the coil  64  of the control valve  46  based on the temperature detected by the temperature sensor  81  and the target temperature set by the temperature adjuster  82 . When an electric current is supplied to the coil  64 , an electromagnetic attraction force in accordance with the value of the current is generated between the fixed core  60  and the movable core  61 . The attraction force is transmitted to the valve body  52  through the solenoid rod  63 . Accordingly, the valve body  52  is urged to reduce the opening size of the valve hole  53  against the force of the opener spring  54 . 
     In this way, the opening size of the valve hole  53  by the valve body  52  is determined by the equilibrium of the force applied from the bellows  56  to the valve body  52 , the attraction force between the fixed core  60  and the movable core  61 , and the force of each spring  54 ,  62 . 
     As the cooling load on the refrigeration circuit increases, for example, as the temperature detected by the temperature sensor  81  becomes higher than the target temperature set by the temperature adjuster  82 , the controller C instructs the control valve  46  to increase the current supply to the coil  64 . This increases the attraction force between the fixed core  60  and the movable core  61  and increases the force that urges the valve body  52  toward the closed position of the valve hole  53 . In this case, the bellows  56  operates the valve body  53  targeting a relatively low suction pressure. In other words, as the current supply increases, the control valve  46  adjusts the displacement of the compressor to maintain a relatively low suction pressure (corresponding to a target suction pressure). 
     As the opening size of the valve hole  53  is reduced by the valve body  52 , the flow rate of refrigerant gas from the discharge chamber  38  to the crank chamber  15  through the pressurizing passage  44  is reduced. On the other hand, refrigerant gas in the crank chamber  15  continuously flows to the suction chamber  37  through the bleed passage  45 . This gradually decreases the pressure in the crank chamber  15 . Accordingly, the difference between the pressure in the crank chamber  15  and the pressure in the cylinder bores  33  is decreased, which increases the inclination of the swash plate  31  and the displacement of the compressor. 
     As the cooling load on the refrigeration circuit decreases, for example, as the difference between the temperature detected by the temperature sensor  81  and the target temperature set by the temperature adjuster  82  decreases, the controller C reduces the current supply to the coil  64 . This weakens the attraction force between the fixed core  60  and the movable core  61  and reduces the force that urges the valve body  52  toward the closed position of the valve hole  53 . In this case, the bellows  56  operates the valve body  52  targeting a relatively high suction pressure. In other words, as the current supply decreases, the control valve  46  adjusts the displacement of the compressor to maintain a relatively high suction pressure (corresponding to a target suction pressure). 
     As the opening size of the valve hole  53  increases, the flow rate of refrigerant gas from the discharge chamber  38  to the crank chamber  15  is increased, which gradually increases the pressure in the crank chamber  15 . This increases the difference between the pressure in the crank chamber  15  and the pressure in the cylinder bores  12   a  and reduces the inclination of the swash plate  31  and the displacement of the compressor. 
     A structural characteristic of the present embodiment will now be described. 
     As shown in FIG. 1, a pressure release passage  90  is independent from the bleed passage  45  and connects the crank chamber  15  to the suction chamber  37 . The release passage  90  functions as a control passage, which connects the crank chamber  15  to a selected chamber, which is the suction chamber  37  in this embodiment. As shown in FIGS. 1 and 4, a release valve  95 , which is an electromagnetic valve in this embodiment, is located in the release passage  90 . The release valve  95  includes a solenoid  95   a,  which is controlled by the controller C, and a valve body  95   b,  which varies the opening size of the release passage  90 . When the solenoid  95   a  is excited, the valve body  95   b  closes the release passage  90  (See FIG.  1 ). When the solenoid  95   a  is de-excited, the valve body  95   b  opens the release passage  90  (See FIG.  4 ). 
     When the air-conditioner switch  80  is turned off during the operation of the compressor, the controller C stops the current supply to the coil  29  and disengages the clutch  23  and simultaneously stops the current supply to the coil  64  of the control valve  46 . Further, the controller C stops the current supply to the solenoid  95   a  of the release valve  95 . 
     When the gas pedal depression degree, which is detected by the gas pedal sensor  83 , is greater than a predetermined value during the operation of the compressor, the controller C judges that the vehicle is being quickly accelerated and stops the current supply to the coil  64  of the control valve  46  and to the solenoid  95   a  of the release valve  95  for a predetermined period. 
     When the engine Eg is stopped during the operation of the compressor, the current supply lines between the power source S and each coil  29 ,  64  and between the power source S and the solenoid  95   a  are disconnected upstream of the controller C. Accordingly, the current supply to the coil  29  is stopped and the clutch  23  is disengaged, which stops the current supply to the coil  64  and the solenoid  95   a.    
     When the clutch  23  is disengaged or the engine Eg is stopped, the current supply to the coil  64  of the control valve  46  is stopped. Then, the attraction force between the fixed core  60  and the movable core  61  disappears, and the control valve  46  fully opens the pressurizing passage  44 . This increases the pressure in the crank chamber  15  and minimizes the inclination of the swash plate  31 . As a result, the compressor is stopped when the inclination of the swash plate  31  is minimized, or when the displacement is minimized. Accordingly, since the compressor is started from the minimum displacement state, which produces a minimum torque load, the torque shock of starting the compressor is limited. 
     When the gas pedal depression degree detected by the gas pedal sensor  83  is greater than a predetermined value, the current supply to the coil  64  is stopped. This causes the control valve  46  to fully open the pressurizing passage  44 . As a result, the inclination of the swash plate  31  is minimized and the compressor is operated at the minimum displacement with relatively low torque load. Therefore, the load on the engine Eg is reduced and the vehicle is smoothly accelerated. 
     When the current supply to the coil  64  is stopped while the compressor is operated at maximum displacement, the control valve  46  quickly maximizes the opening size of the closed pressurizing passage  44 . This permits relatively high-pressure refrigerant gas in the discharge chamber  38  to flow quickly to the crank chamber  15 . Since the amount of refrigerant gas that flows from the crank chamber  15  to the suction chamber  37  through the bleed passage  45  and the through hole  91   a  of the release valve  91  is limited, the pressure in the crank chamber  15  is quickly increased. 
     However, when the pressure in the crank chamber  15  increases to an excessive degree by the discontinuation of the current supply to the coil  64 , the current supply to the solenoid  95   a  is simultaneously stopped, which causes the release valve  95  to open the release passage  90  as shown in FIG.  4 . Therefore, a relatively large amount of gas flows from the crank chamber  15  to the suction chamber  37  through the release passage  90 . As a result, an excessive increase of the pressure in the crank chamber  15  is limited, which prevents the swash plate from being pressed against the limit ring  34  by an excessive force when at the minimum inclination position. Also, the swash plate  31  does not strongly pull the lug plate  30  rearward (rightward in FIG. 1) through the hinge mechanism  32 . As a result, the drive shaft does not move axially against the force of the axial spring  20 . 
     When the vehicle is quickly accelerated while the compressor is operating at maximum displacement, the load on the engine Eg can be reduced by disengaging the clutch  23 . However, shock is produced when engaging or disengaging the clutch  23 , which lowers the vehicle performance. However in this embodiment, the clutch  23  is not disengaged when the vehicle is quickly accelerated, which improves the vehicle performance. 
     The present embodiment has the following advantages. 
     Excessive increases of the pressure in the crank chamber  15  are prevented by opening the electromagnetic release valve  95  in the release passage  90 . As a result, the drive shaft  16  is prevented from moving axially against the force of the axial spring  20 . 
     The drive shaft  16  does not move with respect to the lip seal  22 . That is, the position of the drive shaft  16  with respect to the lip ring  22   a  of the lip seal  22  does not change. Therefore, sludge does not get in the space between the lip ring  22   a  and the drive shaft  16 . This extends the life of the lip seal  22  and prevents leakage of gas from the crank chamber  15 . 
     The armature  28  of the clutch  23  moves with respect to the rotor  24  in the direction of axis L and contacts or separates from the rotor  24 . In the present embodiment, since the axially rearward movement of the drive shaft  16  is prevented, a desirable clearance is ensured between the rotor  24  and the armature  28  when the clutch  23  is disengaged. Accordingly, power transmission between the rotor  24  and the armature  28  is disrupted without fail while the electromagnetic coil  29  of the clutch  23  is de-excited. This prevents noise, vibration, and heat that are caused by contact between the rotor  24  and the armature  28 . 
     Each piston  35  is connected to the drive shaft  16  through the lug plate  30 , the hinge mechanism  32 , the swash plate  31  and the shoes  36 . The axially rearward movement of the drive shaft  16  is prevented, which prevents the pistons  35  from moving toward the valve plate  14 . As a result, the pistons  35  are prevented from colliding with the valve plate  14  at the top dead center position. Therefore, noise and vibration caused by the collision between the piston  35  and the valve plate  14  are suppressed. 
     The opening size of the pressurizing passage  44  is varied by controller C based on the information including the passenger compartment temperature, the target temperature, and the gas pedal depression degree. Compared to a compressor having a control valve that operates in accordance with only suction pressure, a sudden change of displacement from the maximum to the minimum can occur in the compressor including the control valve  46 , that is, the pressure in the crank chamber  15  can be quickly increased. Therefore, the release valve  95  of the compressor of FIG. 1 effectively prevents sudden increases of the pressure in the crank chamber  15 . 
     Compared to a pressure difference valve that opens or closes the release passage  90  according to a difference of pressure between the crank chamber  15  and the suction chamber  37 , the release valve  95 , which is an electromagnetic valve operated by external instructions, responsively opens the release passage  90  without fail. Accordingly, the release valve  95  limits the pressure in the crank chamber  15 . 
     When the current supply to the coil  64  of the control valve  46  is stopped, the current supply to the solenoid  95   a  is simultaneously stopped and the valve body  95  opens the release passage  90 . In other words, the pressure in the crank chamber  15  when the pressurizing passage is fully opened is limited by opening the release passage  90 . This is an advantage of the electromagnetic release valve  95 , which cannot be achieved by the pressure difference valve. 
     The control valve  46  varies the displacement of the compressor by changing the flow rate of refrigerant gas from the discharge chamber  38  to the crank chamber  15  by changing the opening size of the pressurizing passage  44 . The compressor of FIG. 1 can more quickly increase the pressure in the crank chamber  15  than a compressor that only adjusts the flow of refrigerant from the crank chamber  15  to the suction chamber  37  to vary the displacement. Accordingly, when the compressor is stopped, the displacement is quickly minimized. When the compressor is restarted right after the previous stop, the compressor is started at the minimum displacement without fail. The release valve  95  is especially effective for the compressor of FIG. 1, which tends to excessively increase the pressure in the crank chamber  15 . 
     For example, the structure of the control valve  46  may be changed such that the attraction force between the fixed core  60  and the movable core  61  operates the valve body  52  to increase the opening size of the valve hole  53 . In this case, the current supply from the power source S to the coil  64  must be maximized to minimize the displacement especially when the engine Eg is stopped. In other words, it is necessary to maintain the current supply line between the power source S and the coil  64 . This requires a drastic change from the existing electrical system. 
     In contrast, the control valve  46  of the present embodiment only stops the current supply from the power source S to the coil  64  to minimize the displacement when the engine Eg is stopped. Accordingly, it does not matter that the current supply line between the power source S and the coil  64  is disconnected when the engine Eg is stopped. Therefore, the displacement is minimized without changing the structure of existing vehicle electric systems. 
     The illustrated embodiments can be varied as follows. 
     As shown in FIG. 5, the valve body  95   b  may not completely close the release passage  90  when the solenoid  95   a  is excited. This permits restricted gas flow through the space between the release passage  90  and the valve body  95   b  when the difference between the pressure in the crank chamber  15  and the pressure in the suction chamber  37  is smaller than predetermined value. Therefore, the release passage  90  releases gas from the crank chamber  15  with restriction and prevents an excessive increase of the pressure in the crank chamber  15 . Accordingly, the bleed passage  45  is not required. 
     As shown in FIG. 6, a through hole  95   c  that is smaller than the cross-sectional area of the release passage  90  may be formed in the valve body  95   b  of the release valve  95 . When the difference between the pressure in the crank chamber and the pressure in the suction chamber  37  is smaller than predetermined value, or when the solenoid  95   a  is excited, the through hole  95   c  releases gas from the crank chamber  15  in a restricted manner. Therefore, the release passage  90  releases gas from the crank chamber  15  and prevents an excessive increase of the pressure in the crank chamber  15 . Therefore, the bleed passage  45  is not required. 
     As shown in FIG. 7, instead of the release passage  90 , a pressure limiting passage  100 , which limits the pressure in the crank chamber  15 , may be provided between the discharge chamber  38  and the crank chamber  15 . The release valve  95  is located in the pressure limiting passage  100 . The pressure limiting passage  100  is independent from the pressurizing passage  44 . When the pressure in the crank chamber  15  increases excessively, the release valve  95  decreases the opening size of or completely closes the pressure limiting passage  100 , which limits the supply of refrigerant gas to the crank chamber  15 . 
     As shown in FIG. 1, the release valve  95  may open the release passage  90  only when the current supply to the coil  64  is stopped while the compressor is operated at the maximum displacement. In other words, when the current supply to the coil  64  is stopped while the compressor is operating at the maximum displacement, the release valve  95  is not opened. 
     In any of the embodiments shown in FIGS. 1-4, when the gas pedal depression increases, the controller C judges that the vehicle is being quickly accelerated. Instead, the controller C may judge that the vehicle is being quickly accelerated when the engine speed of the engine Eg is greater than a predetermined value. 
     The present invention may be applied to a compressor that varies the displacement by adjusting the flow of refrigerant gas from the crank chamber  15  to the suction chamber  37  by the control valve  46 . In this case, the control valve  46  is located in a passage that connects the crank chamber  15  to the suction passage  37 . 
     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. 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.