Abstract:
A variable displacement type compressor in which a discharge-pressure region, a suction-pressure region and a pressure control chamber are defined, has a tiltable swash plate and a piston reciprocated by the swash plate in the pressure control chamber. The inclination angle of the swash plate and the piston stroke are changed by adjustment of pressure in the pressure control chamber thereby to control the displacement of the compressor. The compressor further comprises a supply passage for supplying refrigerant gas from the discharge-pressure region to the pressure control chamber, a release passage for releasing the refrigerant gas from the pressure control chamber to the suction-pressure region, a first control valve for adjusting a cross-sectional area of the supply passage from the discharge-pressure region to the pressure control chamber and a second control valve for adjusting cross-sectional area of the release passage. The second control valve includes a valve body for opening and closing the release passage whose cross-sectional area is set minimum when the valve body is located at the closed position and a valve spring for urging the valve body in a direction to open the release passage. When the second control valve is closed, pressure in the supply passage downstream the first control valve acts on the valve body in a direction to close the release passage and pressure in the suction-pressure region acts on the valve body in a direction to open the release passage.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a displacement control mechanism for a variable displacement type compressor which adjusts the pressure in a pressure control chamber by supplying refrigerant gas in a discharge-pressure region into the pressure control chamber and releasing the refrigerant gas in the pressure control chamber to a suction-pressure region, thereby controlling the displacement of the compressor. 
     In a variable displacement type compressor provided with a pressure control chamber having therein a swash plate whose inclination angle is variable, the inclination angle of the swash plate decreases with an increase of the pressure in the pressure control chamber. On the other hand, the inclination angle of the swash plate increases with a decrease of the pressure in the pressure control chamber. When the inclination angle of the swash plate decreases, the stroke of a piston decreases thereby to decrease the displacement of the compressor. When the inclination angle of the swash plate increases, the stroke of the piston increases thereby to increase the displacement of the compressor. 
     Since the refrigerant gas which is supplied to the pressure control chamber has been already compressed, the operating efficiency of the variable displacement type compressor deteriorates as the amount of refrigerant gas released from the pressure control chamber to a suction-pressure region of the compressor increases. Therefore, the cross-sectional area of a release passage through which the refrigerant gas is released from the pressure control chamber to the suction-pressure region should be small as much as possible in view of the operating efficiency. 
     If the compressor is left in a stopped state for a long time, the refrigerant gas is changed into a liquid state and the liquefied refrigerant is accumulated in the pressure control chamber. When the compressor is started in such a state, the liquefied refrigerant is not released rapidly to the suction-pressure region if the release passage has a fixed throttle with a small cross-sectional area. As a result, the liquefied refrigerant is vaporized in the pressure control chamber and the pressure in the pressure control chamber is increased excessively. Therefore, it takes a long time before the displacement of the compressor is increased to a desired level after the compressor is started 
     A variable displacement type compressor with a displacement control mechanism is disclosed in Japanese Patent Application Publication NO. 2002-21721 to solve the above problem. The displacement control mechanism in this Publication has a first control valve which adjusts the cross-sectional area of a refrigerant gas supply passage through which refrigerant gas is supplied from a discharge-pressure region of the compressor to the pressure control chamber and a second control valve which adjusts a cross-sectional area of a refrigerant gas release passage through which refrigerant gas is released from the pressure control chamber to a suction-pressure region of the compressor The first control valve is an electromagnetic control valve which is operable to adjust the opening degree of the valve by changing the electromagnetic force. When the first control valve is in deenergized state, the opening degree of the valve is maximum and the inclination angle of a swash plate is minimum. This state corresponds to the minimum displacement operation of the compressor in which the displacement thereof is fixed at minimum. When the first control valve is in energized state, the opening degree of the valve becomes smaller than the maximum and then the inclination angle of the swash plate becomes larger than the minimum. This state corresponds to an intermediate displacement operation in which the displacement is not fixed to the minimum. 
     The second control valve has a spool (a valve body for adjusting the cross-sectional area of the release passage) defining a cylindrical space and a back pressure chamber in the spool chamber in which the spool is accommodated. The back pressure chamber communicates with a pressure region downstream of the first control valve and the cylindrical space communicates with the pressure control chamber through a release passage (bleed passage). The spool is urged toward the back pressure chamber by a spring. A bleed hole is formed in the spool so as to secure a minimum cross-sectional area of the release passage. When the variable displacement type compressor is started, the first control valve is closed and the spool of the second control valve is moved in direction which increases the cross-sectional area of the release passage. Thus, the liquefied refrigerant in the pressure control chamber is rapidly released to the suction-pressure region, thereby reducing the time before the displacement is increased to a desired level after the variable displacement type compressor is started 
     When the first control valve is in energized state and opened, the second control valve is closed (or its spool is seated against a valve seat) and the refrigerant gas is released from the pressure control chamber to the suction-pressure region only through the bleed hole. In this state, the compressor is operating under a displacement more than the minimum (i.e. intermediate displacement). 
     When the cross-sectional area of the bleed hole is adjusted to be small, the pressure in the cylindrical space when the second control valve is in the closed state becomes substantially the same as that in the pressure control chamber. Since the first control valve has a throttling function, the pressure in the back pressure chamber becomes a pressure corresponding to the pressure in the pressure control chamber that is slightly higher than that in the cylindrical space. 
     Since the refrigerant gas released from the pressure control chamber to the suction chamber needs to be stopped during compressor operation under the minimum displacement, the second control valve should be in the closed state (or the spool be seated against the valve seat). Furthermore, the pressure in the back pressure chamber is slightly higher than that in the cylindrical space. Accordingly, the spring force of the spool spring needs to be small so that the spool is seated against the valve seat by the differential pressure between the back pressure chamber and the cylindrical space during the compressor operation under the minimum displacement. 
     When the first control valve is changed from the opened state to the closed state, the spool is moved away the valve seat. If the spring force of the spool spring is too small, however, the spool movement may be hampered by any foreign matters present between the peripheral surface of the spool and its accommodation chamber This prevents the liquefied refrigerant in the pressure control chamber from being rapidly released when the compressor is started. 
     If the cross-sectional area of the bleed hole is made too large, an excessive amount of refrigerant gas is released from the pressure control chamber to the suction chamber, with the result that the operating efficiency is deteriorated. Therefore, the present invention is directed to providing a variable displacement type compressor with a displacement control mechanism according to which the time taken before the displacement of the compressor is increased to the desired level after a start-up of the compressor is reduced and also the operating efficiency of the compressor is improved. 
     SUMMARY OF THE INVENTION 
     A variable displacement type compressor in which a discharge-pressure region, a suction-pressure region and a pressure control chamber are defined, has a tiltable swash plate and a piston reciprocated by the swash plate in the pressure control chamber. The inclination angle of the swash plate and the piston stroke are changed by adjustment of pressure in the pressure control chamber thereby to control the displacement of the compressor The compressor further comprises a supply passage for supplying refrigerant gas from the discharge-pressure region to the pressure control chamber, a release passage for releasing the refrigerant gas from the pressure control chamber to the suction-pressure region, a first control valve for adjusting a cross-sectional area of the supply passage from the discharge-pressure region to the pressure control chamber and a second control valve for adjusting cross-sectional area of the release passage. The second control valve includes a valve body for opening and closing the release passage whose cross-sectional area is set minimum when the valve body is located at the closed position and a valve spring for urging the valve body in a direction to open the release passage. When the second control valve is closed, pressure in the supply passage downstream the first control valve acts on the valve body in a direction to close the release passage and pressure in the suction-pressure region acts on the valve body in a direction to open the release passage. 
     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 longitudinal cross-sectional view of a clutchless variable displacement type compressor according to a first preferred embodiment of the present invention; 
         FIG. 2  is an enlarged fragmentary longitudinal cross-sectional view of the variable displacement type compressor of  FIG. 1 ; 
         FIG. 3  is a longitudinal cross-sectional view similar to that of  FIG. 2 , but showing a different state of the variable displacement type compressor; 
         FIG. 4  is an enlarged fragmentary longitudinal cross-sectional view of a clutchless variable displacement type compressor according to an alternative embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The first preferred embodiment of a clutchless variable displacement type compressor according to the present invention will now be described with reference to  FIGS. 1 through 3 . The compressor is generally designated by numeral  10 . The left side and the right side of the compressor  10  as viewed in  FIG. 1  correspond to the front side and the rear side thereof. As shown in  FIG. 1 , the compressor  10  includes a cylinder block  11  and a front housing  12  connected to the front end of the cylinder block  11 . A rear housing  13  is connected to the rear end of the cylinder block  11  through a valve plate  14 , valve forming plates  15 ,  16  and a retainer forming plate  17 . The cylinder block  11 , the front housing  12  and the rear housing  13  cooperate to form the entire housing of the variable displacement type compressor  10 . 
     The front housing  12  and the cylinder block  11  define therebetween a pressure control chamber  121 . A rotary shaft  18  is rotatably supported by the front housing  12  and the cylinder block  11  through radial bearings  19 ,  20 . Part of the rotary shaft  18  extending out of the pressure control chamber  121  is connected to an external drive source E (not shown), e.g. a vehicle engine, and receives a rotational drive force therefrom. 
     A lug plate  21  is secured to the rotary shaft  18 . A swash plate  22  is supported by the rotary shaft  18  in facing relation to the lug plate  21  so as to be slidable in and inclinable relative to the axial direction of the rotary shaft  18 . 
     The lug plate  21  has formed therethrough a pair of guide holes  211 . A pair of guide pins  23  are provided on the swash plate  22  and slidably fitted in the paired guide holes  211 , respectively. The guide holes  211  and the guide pins  23  cooperate to allow the swash plate  22  to incline relative to the axis of the rotary shaft  18  and rotate with the rotary shaft  18 . The inclination of the swash plate  22  is guided by the guide pins  23  slidably fitted in the guide holes  211  and the rotary shaft  18  slidably supporting the swash plate  23 . 
     As the center of the swash plate  22  moves toward the lug plate  21 , the inclination angle of the swash plate  22  increases. The maximum inclination angle of the swash plate  22  is restricted by the contact between the swash plate  22  and the lug plate  21 . The swash plate  22  shown by solid line in  FIG. 1  is positioned at the minimum inclination angle. The swash plate  22  shown by chain double-dashed line in  FIG. 1  is positioned at the maximum inclination angle. The minimum inclination angle of the swash plate  22  is set slightly larger than 0°. 
     The cylinder block  11  has formed therethrough a plurality of cylinder bores  111  and a piston  24  is slidably received in each cylinder bore  111 . Rotation of the swash plate  22  is converted to reciprocation of each piston  24  in its cylinder bore  111  through a pair of shoes  25 . 
     The rear housing  13  has formed therein a suction chamber  131  as a suction-pressure region and a discharge chamber  132  as a discharge-pressure region. The valve plate  14 , the valve forming plate  16  and the retainer forming plate  17  have formed therethrough a suction port  26 . Similarly, the valve plate  14  and the valve forming plate  15  have formed therethrough a discharge port  27 . The valve forming plate  15  has formed therein a suction valve  151  and the valve forming plate  16  has formed therein a discharge valve  161 , respectively. The cylinder bore  111 , the valve forming plate  15  and the piston  24  cooperate to define a compression chamber  112  in the cylinder block  11 . 
     Refrigerant gas in the suction chamber  131  is drawn into the compression chamber  112  through the suction port  26  while pushing open the suction valve  151  as the piston  24  moves toward the bottom dead center or leftward in  FIG. 1 . The refrigerant gas flowed into the compression chamber  112  is compressed and then discharged into the discharge chamber  132  through the discharge port  27  while pushing open the discharge valve  161  as the piston  24  moves toward the top dead center or rightward in  FIG. 1 . The discharge valve  161  is brought into contact with a retainer  171  of the retainer forming plate  17 , thus the opening degree of the discharge valve  161  being restricted. 
     When the pressure in the pressure control chamber  121  is decreased, the inclination angle of the swash plate  22  is increased and the displacement of the variable displacement type compressor is Increased, accordingly. On the other hand, the inclination angle of the swash plate  22  is decreased with an increase of the pressure in the pressure control chamber  121  and the displacement of the variable displacement type compressor is decreased, accordingly. The suction chamber  131  is connected with the discharge chamber  132  through an external refrigerant circuit  28 . The external refrigerant circuit  28  includes a condenser  29  for removing heat from the compressed refrigerant gas, an expansion valve  30  and an evaporator  31  for transferring ambient heat to the refrigerant. The expansion valve  30  is a temperature-sensitive valve operable to control the flow rate of refrigerant in accordance with the temperature of the refrigerant at the outlet of the evaporator  31 . A stop device is provided between the discharge chamber  132  and the external refrigerant circuit  28 . When the stop device is opened, the refrigerant gas in the discharge chamber  132  flows out to the external refrigerant circuit  28  and returns to the suction chamber  131 . 
     As shown in  FIG. 2 , an electromagnetic first control valve  33 , a second control valve  34  and a check valve  35  are disposed in the rear housing  13 . The first control valve  33  has a solenoid  39  having a fixed core  40  which is energized by an electric current supplied to a coil  41  of the solenoid  39  thereby to attract a movable core  42  toward the fixed core  40 . The electromagnetic force of the solenoid  39  urges a valve body  37  in the direction to close a valve hole  38  against the spring force of a spring  43 . Supply of electric current to the solenoid  39  is controlled by a controller C (duty-ratio controlling being performed in this preferred embodiment). 
     The first control valve  33  includes a pressure sensing device  36  having therein a bellows  361 , a pressure sensing chamber  362  and a spring  363 . The pressure in the suction chamber  131  (or suction pressure) is applied to the bellows  361  through a suction pressure passage  44  and the pressure sensing chamber  362 . The valve body  37  is connected to the bellows  361 . The pressure in the bellows  361  and the spring force of the spring  363  urge the valve body  37  in the direction which causes the valve hole  38  to be opened A valve chamber  50  is formed in the first control valve  33  in communication with the valve hole  38  and also with the discharge chamber  132  through a first supply passage  51 . 
     The second control valve  34  includes a valve housing  45  having therein a valve body  46  and a valve spring  47  urging the valve body  46 . The valve housing  45  includes a disc-shaped end wall  48  and a peripheral wall  49  integrally formed with the end wall  48 . The end of the peripheral wall  49  located remote from the end wall  48  is connected to the retainer forming plate  17 . 
     The valve body  46  includes a disc-shaped base portion  461 , a cylindrical sliding portion  462  integrally formed with the base portion  461  at the peripheral portion thereof and a pillar-shaped contact portion  463  integrally formed with the base portion  461  and extending from the center of the base portion  461  towards the retainer forming plate  17 . The valve body  46  is fitted in the valve housing  45  so that the sliding portion  462  is in sliding contact with the inner peripheral wall  49  of the valve housing  45 . The interior of the valve housing  45  is divided by the valve body  46  into a back pressure chamber  451  and a second control valve chamber  452 . The contact portion  463  of the valve body  46  is contactable at the distal end surface thereof with the retainer forming plate  17 . The end surface of the sliding portion  462  adjacent to the base portion  461  thereof is contactable with the end wall  48  of the valve housing  45 . The valve spring  47  is interposed between the retainer forming plate  17  and the base portion  461 . The valve spring urges the valve body  46  towards the back pressure chamber  451 . 
     The back pressure chamber  451  communicates with the valve hole  38  of the first control valve  33  through a second supply passage  52 . The peripheral wall  49  of the valve housing  45  has formed therethrough a communication hole  492  which is opened and closed by the sliding portion  462  of the valve body  46 . 
     The second control valve chamber  452  communicates with the pressure control chamber  121  through a second throttle passage  53  formed through the retainer forming plate  17 , the valve plate  14  and the valve forming plate  15 ,  16  and through a second bleed passage  54  formed through the cylinder block  11 . The second control valve chamber  452  communicates also with the suction chamber  131  through a bleed hole  491  formed through the peripheral wall  49  of the valve housing  45 . When the contact portion  463  of the valve body  46  is in contact with the retainer forming plate  17  as a valve seat defining the second control valve chamber  452 , the second throttle passage  53  is closed thereby to block the fluid communication between the pressure control chamber  121  and the second control valve chamber  452 . 
     The second bleed passage  54 , the second throttle passage  53 , the second control valve chamber  452  and the bleed hole  491  cooperate to form a second release passage  55  between the pressure control chamber  121  and the suction chamber  131 . 
     As shown in  FIG. 1 , the pressure control chamber  121  communicates with the suction chamber  131  through a first bleed passage  56  formed through the cylinder block  11  and a first throttle passage  57  formed through the retainer forming plate  17 , the valve plate  14  and valve forming plates  15 ,  16 . The first bleed passage  56  and the first throttle passage  57  serve as the first release passage  58  providing constant refrigerant gas communication between the pressure control chamber  121  and the suction chamber  131 . The second release passage  55  and the first release passage  58  are arranged in parallel relation to each other. 
     As shown in  FIG. 2 , the check valve  35  includes a check valve housing  59  having therein a check valve body  60  and a check valve spring  61  urging the check valve body  60  in the direction to close a check valve hole  591  formed in the housing  59 . The check valve hole  591  communicates with the communication hole  492  of the second control valve  34  through a third supply passage  62 . When the second throttle passage  53  is closed by the valve body  46  of the second control valve  34 , the communication hole  492  is opened by the sliding portion  462  of the valve body  46 , thus allowing the communication between the back pressure chamber  451  and the check valve hole  591 . A check valve chamber  592  is formed in the check valve  35  which communicates with the pressure control chamber  121  through a fourth supply passage  63  formed through the retainer forming plate  17 , the valve plate  14 , valve forming plates  15 , 16  and the cylinder block  11 . 
     The first supply passage  51 , the second supply passage  52  and the fourth supply passage  63  form a part of a supply passage  64  for supplying refrigerant gas from the discharge chamber  132  to the pressure control chamber  121 . The controller C operable to control the operation of the solenoid  39  of the first control valve  33  (by duty ratio) supplies electric current to the solenoid  39  when the air conditioning switch  65  is turned on and stops supplying the electric current when the air conditioning switch  65  is turned off. The controller C is electrically connected to a room temperature setting device  66  and a room temperature detector  67 . With the air conditioning switch  65  turned on the controller C controls the electric current supplied to the solenoid  39  based on the temperature difference between a target temperature set by the room temperature setting device  66  and the actual temperature detected by the room temperature detector  67 . 
     The opening and closing of the valve hole  38  of the first control valve  33 , i.e. the degree of valve opening in the first control valve  33 , depends on the balance among various forces such as the electromagnetic force generated by the solenoid  39 , the spring force of the spring  43  and the urging force of the pressure sensing device  36 . The degree of valve opening in the first control valve  33  can be continuously adjusted by changing the electromagnetic force. Specifically, as the electromagnetic force increases, the degree of valve opening in the first control valve  33  decreases. Furthermore, as the suction pressure in the suction chamber  131  increases, the degree of valve opening in the first control valve  33  decreases. Thus the first control valve  33  is operable to adjust the cross-sectional area of the supply passage from the discharge-pressure region to the pressure control chamber  121 . On the other hand, as the suction pressure in the suction chamber  131  decreases, the degree of valve opening in the first control valve  33  increases. The first control valve  33  controls suction pressure to a set pressure in accordance with the electromagnetic force. 
       FIG. 2  shows the state of the compressor in which with the air conditioning switch  65  turned off, supplying of electric current to the solenoid  39  is stopped (duty ratio=0), so that the degree of valve opening in the first control chamber  33  is the maximum. In this state, the inclination angle of the swash plate  22  is the minimum that is slightly larger than 0° and, therefore, refrigerant gas is being discharged from the cylinder bore  111  to the discharge chamber  132 . It is so arranged that the stop device  32  is closed thereby to stop the circulation of refrigerant in the external refrigerant circuit  28  when the swash plate  22  is at the minimum inclination angle. Part of the refrigerant gas discharged from the cylinder bore  111  to the discharge chamber  132  flows into the back pressure chamber  451  in the second control valve  34  through the valve hole  38  in the first control valve  33 . The valve body  46  of the second control valve  34  is moved by the pressure in the back pressure chamber  451  so as to close the second throttle passage  53 . 
     Refrigerant gas in the back pressure chamber  451  flows into the check valve chamber  592  through the communication hole  492 , the third supply passage  62  and the check valve hole  591  of the check valve  35  while pushing open the check valve body  60 . Thus the refrigerant gas flows into the pressure control chamber  121  through the fourth supply passage  63 . In other words, part of the refrigerant gas in the discharge chamber  132  flows into the pressure control chamber  121  through the supply passage  64 . Refrigerant gas in the pressure control chamber  121  flows out thereof through the first release passage  58  and is drawn into the suction chamber  131  and then into the cylinder bore  111  to be compressed. Refrigerant gas compressed is discharged into the discharge chamber  132 . 
     The inclination angle of the swash plate  22  is minimum in the state of  FIG. 2  and the variable displacement type compressor  10  operates under the minimum displacement. In this state, since the stop device  32  is closed, no circulation of refrigerant gas occurs in the external refrigerant circuit  28 . 
       FIG. 3  shows the state in which with the air conditioning switch  65  turned on, supplying of electric current to the solenoid  39  is maximum (duty ratio=1) thereby to close the valve opening in the first control valve  33 . Unless the variable displacement type compressor  10  operates under the minimum displacement (unless the inclination angle of the swash plate  22  is minimum), the stop device  32  is opened and the refrigerant circulates in the external refrigerant circuit  28 . 
     When the valve opening of the first control valve  33  is zero (When the valve hole  38  is closed), no refrigerant gas in the discharge chamber  132  flows into the back pressure chamber  451  of the second control valve  34  through the supply passage  64 . Accordingly, the valve body  46  of the second control valve  34  is positioned so as to open the second throttle passage  53  and also to close the communication hole  492  by the resultant force of the pressure (or suction pressure) in the second control valve chamber  452  in communication with the suction chamber  131  and the spring force of the valve spring  47 . The check valve body  60  is positioned so as to close the check valve hole  591  by the spring force of the check valve spring  61 . 
     In the state of  FIG. 3 , the supply passage  64  is closed and no refrigerant gas in the discharge chamber  132  flows into the pressure control chamber  121  through the supply passage  64 . Also, since the second release passage  55  is opened, the refrigerant gas in the pressure control chamber  121  flows out to the suction chamber  131  through both the first release passage  58  and the second release passage  55 . In this state, the inclination angle of the swash plate  22  is maximum and, therefore, the variable displacement type compressor  10  is operated under the maximum displacement. 
     When the air conditioning switch is turned on and the electric current supplied to the solenoid  39  of the first control valve  33  is neither 0 nor maximum (duty ratio being more than 0 but less than 1), refrigerant gas flows from the discharge chamber  132  to the back pressure chamber  451  of the second control valve  34 . Accordingly, the valve body  46  of the second control valve  34  is positioned so as to close the second throttle passage  53  thereby to close the second release passage  55 . Namely, refrigerant gas in the pressure control chamber  121  flows to the suction chamber  131  through the first release passage  58 , and the refrigerant gas flowed from the discharge chamber  132  to the back pressure chamber  451  flows into the pressure control chamber  121  through the check valve  35 . In this state, the inclination angle of the swash plate  22  becomes more than the minimum so that the suction pressure becomes the pressure set in accordance with the duty ratio, so that the variable displacement type compressor  10  is operated under the intermediate displacement. 
     When the first control valve  33  changes from the closed state shown in  FIG. 3  to the opened state, the pressure in the discharge chamber  132  propagates to the back pressure chamber  451  thereby to change the valve body  46  of the second control valve  34  from the opened state shown in  FIG. 3  to the closed state shown in  FIG. 2 . In this case, after the valve body  46  closes the second throttle passage  53 , the check valve  35  opens. Thus, the relation between the timing of closing the second control valve  34  and the timing of opening the check valve  35  is set so that the check valve  35  is opened after the valve body  46  of the second control valve  34  is closed in response to the pressure change taking place in the back pressure chamber  451  when the first control valve  33  changes from the closed state to the opened state. 
     When the first control valve  33  changes from the opened state to the closed state shown in  FIG. 3 , the pressure in the back pressure chamber  451  decreases and the valve body  46  of the second control valve  34  is moved from the closed position shown in  FIG. 2  to the opened position accordingly. 
     The following effects are obtained in the first preferred embodiment.
     (1) When the valve body  46  of the second control valve  34  is in the closed position thereby to close the second release passage  55 , the valve body  46  is urged by the resultant force of the pressure in the second control valve chamber  46  and the spring force of the valve spring  47  toward the position where the second release passage  55  is opened by the valve body  46 . On the other hand, the valve body  46  is urged by the pressure in the back pressure chamber  451  (part of the supply passage  64 ) located downstream of the first control valve  33  toward the opposite position where the second release passage  55  is closed by the valve body  46 . When the valve body  46  closes the second release passage  55 , the pressure in the back pressure chamber  451  is substantially the same as the pressure in the pressure control chamber  121  because the pressure in the pressure control chamber  121  propagates through the fourth supply passage  63  into the back pressure chamber  451  located downstream of the first control valve  33  with a throttle function. On the other hand, since the second control valve chamber  452  communicates with the suction chamber  131  through the bleed hole  491 , the pressure in the second control valve chamber  452  is substantially the same as the suction pressure. That is, in the compressor operation under an intermediate displacement, the differential pressure between the second control valve chamber  452  and the back pressure chamber  451  across the valve body  46  is substantially the same as the differential pressure between the suction pressure and the pressure in the pressure control chamber  121 .   

     As compared with the case of the Japanese Patent Application Publication NO. 2002-21721, the differential pressure between the second control valve chamber  452  (suction pressure) and the back pressure chamber  451  (control pressure) is higher than that in the case of the above prior art [the differential pressure between the pressure in the back pressure chamber (corresponding to the control pressure) and the pressure in the cylindrical space (control pressure)]. The structure according to which the differential pressure between the second control valve chamber  452  and the back pressure chamber  451  can be increased over the prior art enables the spring force of the valve spring  47  to increase. Such increased spring force of the valve spring  47  permits the valve body  46  to move from the closed position to the opened position more reliably even if any foreign matters enter into a clearance between the peripheral wall  49  of the valve housing  45  and the sliding portion  462 . This contributes to rapid release of refrigerant gas in the pressure control chamber  121  into the suction chamber  131  at a start-up of the compressor.
     (2) Since the second release passage  55  is closed during the compressor operation under an intermediate displacement, the cross-sectional area of the second throttle passage  53  forming a part of the second release passage  55  can be made relatively larger in light of the operating efficiency. This also contributes to rapid release of refrigerant gas from the pressure control chamber  121  into the suction chamber  131  at a start-up of the compressor.   

     Since the first release passage  58  is always opened (is kept opened), refrigerant gas in the pressure control chamber  121  flows out to the suction chamber  131  through the first release passage  58  during the operation under an intermediate displacement. The cross-sectional area of the first throttle passage  57  forming a part of the first release passage  58  can be made as small as possible thereby to decrease the amount of refrigerant gas flowing from the pressure control chamber  121  to the suction chamber  131  within the range where smooth compressor operation under an intermediate displacement is achievable without affecting its operation efficiency. In other words, the amount of the refrigerant gas compressed in the discharge chamber  132  and returning to the suction chamber  131  through the pressure control chamber  121  can be reduced for improvement of the operating efficiency.
     (3) When the first control valve  33  changes from the opened state to the closed state during the intermediate displacement operation under a high discharge pressure, the pressure in the pressure control chamber  121  may not decrease as desired due to the leakage of refrigerant gas from the cylinder bore  111  to the pressure control chamber  121 . If the pressure which fails to decrease in the pressure control chamber  121  is propagated into the back pressure chamber  451  through the supply passage  64 , the resultant force of the suction pressure in the second control valve chamber  452  and the spring force of the valve spring  47  may not exceed the pressure in the back pressure chamber  451  with the result that the valve body  46  of the second control valve  34  may fail to move from the closed position to the opened position.   

     The check valve  35  is provided to prevent the pressure failing to be decreased in the pressure control chamber  121  from being propagated into the back pressure chamber  451 . Therefore, when the first control valve  33  changes from the opened state to the closed state, the valve body  46  of the second control chamber  34  moves from the closed position to the opened position more reliably.
     (4) If the check valve  35  opens before the valve body  46  closes the second throttle passage  53 , the pressure in the pressure control chamber  121  is propagated into the back pressure chamber  451  before the valve body  46  closes the second throttle passage  53 , so that the pressure in the back pressure chamber  451  becomes substantially the same as the pressure in the pressure control chamber  121 . As a result, the valve body  46  may be stopped on its way between the opened position and the closed position before reaching the closed position.   

     The check valve  35  is opened after the valve body  46  of the second control valve  34  has been moved to the closed position. Therefore, the pressure in the pressure control chamber  121  will not propagate into the back pressure chamber  451  and the pressure in back pressure chamber  451  remains the pressure of the discharge-pressure region of the compressor before the valve body  46  closes the second throttle passage  53 . Thus, the valve body  46  is moved by the pressure of the discharge-pressure region in the back pressure chamber  451  to the position to close the second throttle passage  53 . 
     The present invention may be embodied in various ways as exemplified below. As shown in  FIG. 4 , the third supply passage  62  of the check valve  35  may be connected to the second supply passage  52  between the first control valve  33  and the second control valve  34 . According to this embodiment, the same advantageous effects as those in the first preferred embodiment are obtained. 
     The check valve  35  in the first preferred embodiment may be dispensed with. In this case, the same advantageous effects as (1) and (2) in the first preferred embodiment (the advantageous effects (1) and (2) of the first preferred embodiment) are obtained. A control valve having a pressure sensing device and operable to adjust the opening degree of its valve body in accordance with the differential pressure between two different points in the discharge-pressure region of the compressor may be used as the first control valve  33 . In other words, any control valve that is operable to increase the opening degree of its valve body with an increase of the refrigerant flow rate in the discharge-pressure region and to decrease the opening degree with a decrease of the refrigerant flow rate in the discharge-pressure region may be used as the first control valve  33 . 
     The first control valve  33 , the second control valve  34  and the check valve  35  may be arranged outside the housing of the variable displacement type compressor and these three valves may be arranged in communication with the suction chamber and the discharge chamber in the variable displacement type compressor through any suitable conduits. 
     The present invention may be applied to a variable displacement type compressor receiving power from an external drive source through a clutch. With the clutch engaged in such variable displacement type compressor, the refrigerant circulates In the external refrigerant circuit even during operation under the minimum displacement. With the clutch disengaged, the circulation of refrigerant in the external refrigerant circuit is stopped.