Patent Publication Number: US-9903352-B2

Title: Swash plate type variable displacement compressor

Description:
FIELD OF THE INVENTION 
     The present invention relates to a swash plate type variable displacement compressor. 
     BACKGROUND OF THE INVENTION 
     Patent Document 1 discloses a conventional swash plate type variable displacement compressor (hereinafter referred to as a compressor). This compressor includes a front housing member, a cylinder block, and a rear housing member, which form a housing. The front housing member and the rear housing member each include a suction chamber and a discharge chamber. The rear housing member also includes a control pressure chamber. 
     The cylinder block includes a swash plate chamber, a plurality of cylinder bores, and a main shaft through hole. Each cylinder bore includes a first cylinder bore formed in the rear part of the cylinder block and a second cylinder bore formed in the front part of the cylinder block. The main shaft through hole is formed in the rear part of the cylinder block and communicates with the swash plate chamber and the control pressure chamber. 
     The drive shaft is inserted in the housing and is rotationally supported in the cylinder block. The swash plate chamber accommodates a swash plate, which is rotatable through rotation of the drive shaft. A link mechanism, which allows change of the inclination angle of the swash plate, is arranged between the drive shaft and the swash plate. The inclination angle is defined as the angle of the swash plate with respect to a direction perpendicular to the rotation axis of the drive shaft. 
     Each cylinder bore reciprocally accommodates a piston. More specifically, each piston includes a first piston head that reciprocates in the first cylinder bore and a second piston head that reciprocates in the second cylinder bore. Thus, the first cylinder bore and the first piston head form a first compression chamber, and the second cylinder bore and the second piston head form a second compression chamber. A conversion mechanism reciprocates each of the pistons in the associated one of the cylinder bores by the stroke corresponding to the inclination angle through rotation of the swash plate. An actuator is capable of changing the inclination angle and controlled by a control mechanism. 
     The actuator is arranged in the swash plate chamber closer to the first cylinder bores relative to the swash plate. The actuator includes a non-rotational movable body, a movable body, a thrust bearing, and the control pressure chamber. The non-rotational movable body is arranged in the main shaft through hole not to rotate integrally with the drive shaft and covers the rear end of the drive shaft. The inner circumferential surface of the non-rotational movable body rotationally and slidably supports the rear end of the drive shaft. The outer circumferential surface of the non-rotational movable body slides in the main shaft through hole along the rotation axis so that the non-rotational movable body moves in the main shaft through hole in the front-rear direction. However, the non-rotational movable body does not slide about the rotation axis of the non-rotational movable body. The movable body is coupled to the swash plate and is movable along the rotation axis. The thrust bearing is located between the non-rotational movable body and the movable body. 
     Since the non-rotational movable body is arranged in the main shaft through hole, the main shaft through hole is partitioned into a rear end portion that communicates with the control pressure chamber and a front end portion that does not communicate with the control pressure chamber. The rear end portion of the main shaft through hole communicates with the control pressure chamber and functions as part of the control pressure chamber. The rear end portion has a pressing spring, which urges the non-rotational movable body forward. 
     The control mechanism includes a control passage and a control valve provided in the control passage. The control passage connects the control pressure chamber to the discharge chamber. The control valve adjusts the opening degree of the control passage to change the pressure in the control pressure chamber so that the non-rotational movable body and the movable body are movable along the rotation axis. 
     The link mechanism has a movable body and a lug arm fixed to the drive shaft. A rear end portion of the lug arm has an elongated hole, which extends in a direction perpendicular to the rotation axis of the drive shaft from the radially outer side toward the rotation axis. A pin is received in the elongated hole and supports the swash plate at a position forward to the swash plate such that the swash plate is allowed to pivot about a first pivot axis. A front end portion of the movable body also has an elongated hole, which extends in the direction perpendicular to the rotation axis of the drive shaft from the radially outer side toward the rotation axis. A pin is passed through the elongated hole and supports the swash plate at the rear end of the swash plate such that the swash plate is allowed to pivot about a second pivot axis, which is parallel to the first pivot axis. 
     The control valve of this compressor is capable of controlling the pressure in the control pressure chamber by the pressure of discharge refrigerant in the discharge chamber through adjustment of the opening degree of the control passage. Thus, the actuator of this compressor changes the inclination angle of the swash plate to allow change in the displacement per rotation of the drive shaft. 
     PRIOR ART DOCUMENTS 
     Patent Documents 
     Patent Document 1: Japanese Laid-Open Patent Publication No. 5-172052 
     SUMMARY OF THE INVENTION 
     In the above-mentioned conventional compressor, when the inclination angle of the swash plate is changed, the discharge refrigerant directly flows into the control pressure chamber through the control mechanism. Thus, the actuator of this compressor is susceptible to pulsation of the discharge refrigerant. This makes the inclination angle unstable and makes the compressor hard to operate at a suitable displacement in accordance with the operating condition of, for example, a vehicle to which the compressor is mounted. 
     Accordingly, it is an objective of the present invention to provide a swash plate type variable displacement compressor that is capable of operating at a suitable displacement. 
     To achieve the foregoing objective and in accordance with one aspect of the present invention, a swash plate type variable displacement compressor is provided that includes a housing in which a suction chamber, a discharge chamber, a swash plate chamber, and a cylinder bore are formed, a drive shaft that is rotationally supported by the housing, a swash plate that is rotational in the swash plate chamber by rotation of the drive shaft, a link mechanism, a piston reciprocally received in the cylinder bore, a conversion mechanism, an actuator, and a control mechanism that controls the actuator. The link mechanism is arranged between the drive shaft and the swash plate and allows change of an inclination angle of the swash plate with respect to a direction perpendicular to a rotation axis of the drive shaft. The conversion mechanism causes the piston to reciprocate in the cylinder bore by a stroke corresponding to the inclination angle of the swash plate through rotation of the swash plate. The actuator changes the inclination angle of the swash plate. The control mechanism controls the actuator. The housing has a pressure regulation chamber. The actuator includes a fixed body that is located in the swash plate chamber and fixed to the drive shaft, a movable body that is provided on the drive shaft and is capable of changing the inclination angle of the swash plate by moving along the rotation axis of the drive shaft, and a control pressure chamber defined by the fixed body and the movable body. The control pressure chamber changes the volume of the control pressure chamber by the pressure of refrigerant in the discharge chamber to move the movable body. The control mechanism includes a control passage that connects together the discharge chamber, the pressure regulation chamber, and the control pressure chamber, and a control valve that adjusts an opening degree of the control passage to change the pressure in the control pressure chamber to allow the movable body to move. The refrigerant in the discharge chamber flows into the control pressure chamber via the pressure regulation chamber. The pressure regulation chamber functions as a muffler that reduces pulsation of the refrigerant. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of a-compressor according to a first embodiment at the maximum displacement; 
         FIG. 2  is a schematic diagram showing a control mechanism of the compressor according to the first embodiment; 
         FIG. 3  is a cross-sectional view of the compressor according to the first embodiment at the minimum displacement; 
         FIG. 4  is a cross-sectional view of a compressor according to a second embodiment at the maximum displacement; 
         FIG. 5  is a schematic diagram showing a control mechanism of the compressor according to the second embodiment; and 
         FIG. 6  is a cross-sectional view of the compressor according to the second embodiment at the minimum displacement. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First and second embodiments of the present invention will now be described with reference to the drawings. A compressor according to the first embodiment is a double-headed swash plate type variable displacement compressor. A compressor according to the second embodiment is a single-headed swash plate type variable displacement compressor. These compressors are installed in vehicles and each is included in the refrigeration circuit in the air conditioner for a vehicle. 
     First Embodiment 
     As shown in  FIG. 1 , the compressor according to the first embodiment includes a housing  1 , a drive shaft  3 , a swash plate  5 , a link mechanism  7 , pistons  9 , pairs of shoes  11   a ,  11   b , an actuator  13 , and a control mechanism  15 , which is illustrated in  FIG. 2 . 
     As shown in  FIG. 1 , the housing  1  has a front housing member  17  at a front position in the compressor, a rear housing member  19  at a rear position in the compressor, first and second cylinder blocks  21 ,  23 , which are arranged between the front housing member  17  and the rear housing member  19 , and first and second valve forming plates  39 ,  41 . 
     The front housing member  17  has a boss  17   a , which projects forward. The boss  17   a  accommodates a shaft sealing device  25 . A first suction chamber  27   a  and a first discharge chamber  29   a  are formed in the front housing member  17 . The first suction chamber  27   a  is located radially inward in the front housing member  17 . The first discharge chamber  29   a  is formed into an annular shape and is located radially outward of the first suction chamber  27   a  in the front housing member  17 . 
     The front housing member  17  further includes a first front communication passage  18   a . The front end of the first front communication passage  18   a  communicates with the first discharge chamber  29   a , and the rear end of the first front communication passage  18   a  is open in the rear end of the front housing member  17 . 
     The control mechanism  15  is received in the rear housing member  19 . A second suction chamber  27   b , a second discharge chamber  29   b , and a pressure regulation chamber  31  are formed in the rear housing member  19 . The pressure regulation chamber  31  is formed in the middle of the rear housing member  19 . The second suction chamber  27   b  is formed into an annular shape and is located radially outward of the pressure regulation chamber  31  in the rear housing member  19 . The second discharge chamber  29   b  is also formed into an annular shape and is located radially outward of the second suction chamber  27   a  in the rear housing member  19 . That is, the pressure regulation chamber  31  is formed radially inward of the second discharge chamber  29   b  and the second suction chamber  27   b  in the rear housing member  19 . The rear housing member  19  corresponds to a cover according to the present invention. 
     Since the pressure regulation chamber  31  is formed in the rear housing member  19 , the pressure regulation chamber  31  is located at the rear end of the drive shaft  3 . 
     The rear housing member  19  further includes a first rear communication passage  20   a . The rear end of the first rear communication passage  20   a  communicates with the second discharge chamber  29   b , and the front end of the first rear communication passage  20   a  is open in the front end of the rear housing member  19 . 
     A swash plate chamber  33  is defined between the first cylinder block  21  and the second cylinder block  23 . The swash plate chamber  33  is arranged substantially in the middle of the housing  1  in the front-rear direction. 
     The first cylinder block  21  includes first cylinder bores  21   a  arranged at equal angular intervals in the circumferential direction and parallel to a rotation axis O of the drive shaft  3 . The first cylinder block  21  has a first shaft hole  21   b , through which the drive shaft  3  is passed. The first shaft hole  21   b  accommodates a first slide bearing  22   a . Instead of the first slide bearing  22   a , a roller bearing may be provided. 
     The first cylinder block  21  further includes a first recess  21   c  that communicates with the first shaft hole  21   b  and is coaxial with the first shaft hole  21   b . The first recess  21   c  communicates with the swash plate chamber  33  and forms part of the swash plate chamber  33 . The diameter of the first recess  21   c  is reduced in a stepwise manner toward the front end. A first thrust bearing  35   a  is arranged at the front end in the first recess  21   c . The first cylinder block  21  also includes a first connection passage  37   a , through which the swash plate chamber  33  and the first suction chamber  27   a  communicate with each other. The first cylinder block  21  also includes first retainer grooves  21   e  that limit the maximum opening degree of first suction reed valves  391   a , which will be discussed below. 
     The first cylinder block  21  further includes a second front communication passage  18   b . The front end of the second front communication passage  18   b  is open in the front end of the first cylinder block  21 , and the rear end of the second front communication passage  18   b  is open in the rear end of the first cylinder block  21 . 
     As in the first cylinder block  21 , a plurality of second cylinder bores  23   a  are formed in the second cylinder block  23 . Each of the second cylinder bores  23   a  form a pair with the corresponding one of the first cylinder bores  21   a  in the front-rear direction. The first cylinder bores  21   a  and the second cylinder bores  23   a  have the same diameter. 
     A second shaft hole  23   b , through which the drive shaft  3  is inserted, is formed in the second cylinder block  23 . The second shaft hole  23   b  communicates with the pressure regulation chamber  31 . The second shaft hole  23   b  accommodates a second slide bearing  22   b . Instead of the second slide bearing  22   b , a roller bearing may be provided. The first shaft hole  21   b  and the second shaft hole  23   b  correspond to a shaft hole according to the present invention. 
     In this compressor, the pressure regulation chamber  31  has a diameter greater than those of the first and second shaft holes  21   b ,  23   b . Thus, when the second cylinder block  23  and the rear housing member  19  are joined via the second valve forming plate  41 , the pressure regulation chamber  31  is placed over the second shaft hole  23   b.    
     The second cylinder block  23  further includes a second recess  23   c  that communicates with the second shaft hole  23   b  and is coaxial with the second shaft hole  23   b . The second recess  23   c  also communicates with the swash plate chamber  33  and forms part of the swash plate chamber  33 . The diameter of the second recess  23   c  is reduced in a stepwise manner toward the rear end. A second thrust bearing  35   b  is arranged at the rear end in the second recess  23   c . The second cylinder block  23  also has a second connection passage  37   b , through which the swash plate chamber  33  and the second suction chamber  27   b  communicate with each other. The second cylinder block  23  also includes second retainer grooves  23   e  that limit the maximum opening degree of second suction reed valves  411   a , which will be discussed below. 
     The second cylinder block  23  includes a discharge port  230 , a merged discharge chamber  231 , a third front communication passage  18   c , a second rear communication passage  20   b , and a suction port  330 . The discharge port  230  and the merged discharge chamber  231  communicate with each other. The discharge port  230  and the merged discharge chamber  231  are formed at a position closer to the front end of the second cylinder block  23  and are located at substantially the middle of the housing  1  in the front-rear direction. The merged discharge chamber  231  is coupled to a non-illustrated condenser, which forms a conduit, via the discharge port  230 . 
     The front end of the third front communication passage  18   c  is open in the front end of the second cylinder block  23 , and the rear end of the third front communication passage  18   c  communicates with the merged discharge chamber  231 . The first cylinder block  21  is joined to the second cylinder block  23  so that the third front communication passage  18   c  communicates with the rear end of the second front communication passage  18   b.    
     The front end of the second rear communication passage  20   b  communicates with the merged discharge chamber  231 , and the rear end of the second rear communication passage  20   b  is open in the rear end of the second cylinder block  23 . 
     The suction port  330  is formed at a position closer to the front end of the second cylinder block  23  and is located at substantially the middle of the housing  1  in the front-rear direction. The swash plate chamber  33  is coupled to a non-illustrated evaporator, which forms a conduit, via the suction port  330 . 
     The first valve forming plate  39  is located between the front housing member  17  and the first cylinder block  21 . The second valve forming plate  41  is located between the rear housing member  19  and the second cylinder block  23 . 
     The first valve forming plate  39  includes a first valve plate  390 , a first suction valve plate  391 , a first discharge valve plate  392 , and a first retainer plate  393 . The first valve plate  390 , the first discharge valve plate  392 , and the first retainer plate  393  include first suction holes  390   a , the number of which is the same as that of the first cylinder bores  21   a . The first valve plate  390  and the first suction valve plate  391  also include first discharge holes  390   b , the number of which is the same as that of the first cylinder bores  21   a . Furthermore, the first valve plate  390 , the first suction valve plate  391 , the first discharge valve plate  392 , and the first retainer plate  393  include a first suction communication hole  390   c . The first valve plate  390  and the first suction valve plate  391  also include a first discharge communication hole  390   d.    
     The first cylinder bores  21   a  communicate with the first suction chamber  27   a  through the corresponding first suction holes  390   a . The first cylinder bores  21   a  also communicate with the first discharge chamber  29   a  through the corresponding first discharge holes  390   b . The first suction chamber  27   a  and the first connection passage  37   a  communicate with each other through the first suction communication hole  390   c . The first front communication passage  18   a  and the second front communication passage  18   b  communicate with each other through the first discharge communication hole  390   d.    
     The first suction valve plate  391  is located on the rear surface of the first valve plate  390 . The first suction valve plate  391  includes the first suction reed valves  391   a , which are capable of opening and closing the corresponding first suction holes  390   a  by elastic deformation. The first discharge valve plate  392  is located on the front surface of the first valve plate  390 . The first discharge valve plate  392  includes first discharge reed valves  392   a , which are capable of opening and closing the corresponding first discharge holes  390   b  by elastic deformation. The first retainer plate  393  is located on the front surface of the first discharge valve plate  392 . The first retainer plate  393  limits the maximum opening degree of the first discharge reed valves  392   a.    
     The second valve forming plate  41  includes a second valve plate  410 , a second suction valve plate  411 , a second discharge valve plate  412 , and a second retainer plate  413 . The second valve plate  410 , the second discharge valve plate  412 , and the second retainer plate  413  include second suction holes  410   a , the number of which is the same as that of the second cylinder bores  23   a . The second valve plate  410  and the second suction valve plate  411  include second discharge holes  410   b , the number of which is the same as that of the second cylinder bores  23   a . Furthermore, a second suction communication hole  410   c  is formed through the second valve plate  410 , the second suction valve plate  411 , the second discharge valve plate  412 , and the second retainer plate  413 . A second discharge communication hole  410   d  is formed through the second valve plate  410  and the second suction valve plate  411 . 
     The second cylinder bores  23   a  communicate with the second suction chamber  27   b  through the corresponding second suction holes  410   a . The second cylinder bores  23   a  communicate with the second discharge chamber  29   b  through the corresponding second discharge holes  410   b . The second suction chamber  27   b  and the second connection passage  37   b  communicate with each other through the second suction communication hole  410   c . The first rear communication passage  20   a  and the second rear communication passage  20   b  communicate with each other through the second discharge communication hole  410   d.    
     The second suction valve plate  411  is located on the front surface of the second valve plate  410 . The second suction valve plate  411  includes the second suction reed valves  411   a , which are capable of opening and closing the corresponding second suction holes  410   a  by elastic deformation. The second discharge valve plate  412  is located on the rear surface of the second valve plate  410 . The second discharge valve plate  412  includes second discharge reed valves  412   a , which are capable of opening and closing the corresponding second discharge holes  410   b  by elastic deformation. The second retainer plate  413  is located on the rear surface of the second discharge valve plate  412 . The second retainer plate  413  limits the maximum opening degree of the second discharge reed valves  412   a.    
     In this compressor, the first front communication passage  18   a , the first discharge communication hole  390   d , the second front communication passage  18   b , and the third front communication passage  18   c  form a first communication passage  18 . The first rear communication passage  20   a , the second discharge communication hole  410   d , and the second rear communication passage  20   b  form a second communication passage  20 . 
     In this compressor, the first and second connection passages  37   a ,  37   b  and the first and second suction communication holes  390   c ,  410   c  connect the first and second suction chambers  27   a ,  27   b  to the swash plate chamber  33 . This substantially equalizes the pressure in the first and second suction chambers  27   a ,  27   b  and the pressure in the swash plate chamber  33 . Low-pressure suction refrigerant sent from the evaporator flows into the swash plate chamber  33  via the suction port  330 . As a result, the pressure in the swash plate chamber  33  and the pressure in the first and second suction chambers  27   a ,  27   b  are lower than the pressure in the first and second discharge chambers  29   a ,  29   b.    
     The drive shaft  3  includes a drive shaft main body  30 , a first support member  43   a , and a second support member  43   b . The drive shaft main body  30  extends rearward from the front of the housing  1 , is inserted in the boss  17   a  toward the rear end, and is inserted in the first and second slide bearings  22   a ,  22   b . Thus, the drive shaft main body  30 , or the drive shaft  3 , is rotationally supported by the housing  1  about the rotation axis O. The front end of the drive shaft main body  30  is located inside the boss  17   a  and the rear end of the drive shaft main body  30  is located inside the pressure regulation chamber  31 . 
     The swash plate  5 , the link mechanism  7 , and the actuator  13  are provided on the drive shaft main body  30 . The swash plate  5 , the link mechanism  7 , and the actuator  13  are arranged in the swash plate chamber  33 . 
     The first support member  43   a  is press-fitted to the front end of the drive shaft main body  30 . When the drive shaft  3  is rotated about the rotation axis O, the first support member  43   a  slides in the first slide bearing  22   a . The first support member  43   a  has a flange  430  that contacts the first thrust bearing  35   a  and an attachment portion (not shown) through which a second pin  47   b  is passed as will be described below. Furthermore, the front end of a first restoration spring  44   a  is secured to the first support member  43   a . The first restoration spring  44   a  extends along the rotation axis O from the first support member  43   a  toward the swash plate chamber  33 . 
     The second support member  43   b  is press-fitted to the rear end of the drive shaft main body  30 . When the drive shaft  3  is rotated about the rotation axis O, the second support member  43   b  slides in the second slide bearing  22   b . The second support member  43   b  also has a flange  431  that contacts the second thrust bearing  35   b . The flange  431  is arranged between the second thrust bearing  35   b  and the actuator  13 . 
     The swash plate  5  is shaped as a flat annular plate and has a front surface  5   a  and a rear surface  5   b . The front surface  5   a  faces forward of the compressor in the swash plate chamber  33 . The rear surface  5   b  faces rearward of the compressor in the swash plate chamber  33 . 
     The swash plate  5  is fixed to a ring plate  45 . The ring plate  45  is shaped as a flat annular plate. The ring plate  45  includes a through hole  45   a  at the central portion. The drive shaft main body  30  is inserted in the through hole  45   a  in the swash plate chamber  33  so that the swash plate  5  is mounted on the drive shaft  3 . 
     The link mechanism  7  has a lug arm  49 . The lug arm  49  is arranged forward of the swash plate  5  in the swash plate chamber  33  and located between the swash plate  5  and the first support member  43   a . The lug arm  49  substantially has an L shape extending from the front end to the rear end. As illustrated in  FIG. 3 , the lug arm  49  comes into contact with the flange  430  of the first support member  43   a  when the inclination angle of the swash plate  5  with respect to the rotation axis O is minimized. This compressor thus allows the lug arm  49  to maintain the swash plate  5  at the minimum inclination angle. A weight portion  49   a  is formed at the rear end of the lug arm  49 . The weight portion  49   a  extends in the circumferential direction of the actuator  13  over approximately half the circumference. The shape of the weight portion  49   a  may be changed as necessary. 
     As shown in  FIG. 1 , the rear portion of the lug arm  49  is coupled to a portion on a first side of the ring plate  45  via a first pin  47   a . This configuration supports the front portion of the lug arm  49  to be capable of pivoting about the axis of the first pin  47   a , which is a first pivot axis M 1 , relative to the first side portion of the ring plate  45 , or in other words, relative to the swash plate  5 . The first pivot axis M 1  extends perpendicular to the rotation axis O of the drive shaft  3 . 
     The front portion of the lug arm  49  is coupled to the first support member  43   a  with the second pin  47   b . This configuration supports the rear portion of the lug arm  49  to be capable of pivoting about the axis of the second pin  47   b , which is a second pivot axis M 2 , relative to the first support member  43   a , or in other words, relative to the drive shaft  3 . The second pivot axis M 2  extends parallel to the first pivot axis M 1 . The lug arm  49  and the first and second pins  47   a ,  47   b  correspond to the link mechanism  7  according to the present invention. 
     The weight portion  49   a  extends in the rear end of the lug arm  49 , that is, opposite to the second pivot axis M 2  with respect to the first pivot axis M 1 . Thus, the lug arm  49  is supported by the ring plate  45  with the first pin  47   a  so that the weight portion  49   a  passes through a groove portion  45   b  of the ring plate  45  and is located on the rear surface of the ring plate  45 , that is, rearward of the rear surface  5   b  of the swash plate  5 . As a result, the centrifugal force produced by rotation of the swash plate  5  about the rotation axis O is applied to the weight portion  49   a  at the rear surface  5   b  of the swash plate  5 . 
     In this compressor, the swash plate  5  is allowed to rotate together with the drive shaft  3  by connection between the swash plate  5  and the drive shaft  3  through the link mechanism  7 . The inclination angle of the swash plate  5  is changed through pivoting of the opposite ends of the lug arm  49  about the first pivot axis M 1  and the second pivot axis M 2 . 
     The pistons  9  each include a first piston head  9   a  at the front end and a second piston head  9   b  at the rear end. The first piston heads  9   a  are respectively accommodated in the first cylinder bores  21   a  to be capable of reciprocating in the first cylinder bores  21   a . The first piston heads  9   a  and the first valve forming plate  39  define first compression chambers  21   d  respectively in the first cylinder bores  21   a . The second piston heads  9   b  are respectively accommodated in the second cylinder bores  23   a  to be capable of reciprocating in the second cylinder bores  23   a . The second piston heads  9   b  and the second valve forming plate  41  define second compression chambers  23   d  respectively in the second cylinder bores  23   a . Since the first cylinder bores  21   a  and the second cylinder bores  23   a  have the same diameter as described above, the first piston heads  9   a  and the second piston heads  9   b  have the same diameter. 
     Each of the pistons  9  has an engaging portion  9   c  at the middle. Each of the engaging portions  9   c  accommodates the pair of hemispherical shoes  11   a ,  11   b . The shoes  11   a ,  11   b  convert rotation of the swash plate  5  into reciprocation of the pistons  9 . The shoes  11   a ,  11   b  correspond to a conversion mechanism according to the present invention. The first and second piston heads  9   a ,  9   b  thus reciprocate in the corresponding first and second cylinder bores  21   a ,  23   a  by the stroke corresponding to the inclination angle of the swash plate  5 . 
     The compressor shifts the top dead center positions of the first piston heads  9   a  and the second piston heads  9   b  by varying the stroke of the pistons  9  in accordance with change in the inclination angle of the swash plate  5 . More specifically, as shown in  FIG. 1 , when the inclination angle of the swash plate  5  and the stroke of the pistons  9  are maximized, the top dead center position of each first piston head  9   a  is the closest to the first valve forming plate  39 , and the top dead center position of each second piston head  9   b  is the closest to the second valve forming plate  41 . As shown in  FIG. 3 , as the inclination angle of the swash plate  5  is decreased and the stroke of the pistons  9  is decreased, the top dead center position of each second piston head  9   b  is gradually separated away from the second valve forming plate  41 . However, the top dead center position of each first piston head  9   a  scarcely changes from the case in which the stroke of the pistons  9  is maximized and is maintained in the vicinity of the first valve forming plate  39 . That is, the compressor shifts the top dead center position of each second piston head  9   b  by a greater amount than the top dead center position of each first piston head  9   a  as the inclination angle of the swash plate  5  is decreased. 
     As shown in  FIG. 1 , the actuator  13  is arranged in the swash plate chamber  33 . The actuator  13  is located rearward of the swash plate  5  to be able to enter the second recess  23   c . The actuator  13  includes a movable body  13   a , a fixed body  13   b , and a control pressure chamber  13   c . The control pressure chamber  13   c  is defined between the movable body  13   a  and the fixed body  13   b.    
     The movable body  13   a  includes a main body portion  130  and a circumferential wall  131 . The main body portion  130  is located at the rear part of the movable body  13   a  and extends radially in a direction to separate from the rotation axis O. The circumferential wall  131  is continuous with the periphery of the main body portion  130  and extends rearward from the front. A coupling portion  132  is formed on the front end of the circumferential wall  131 . The main body portion  130 , the circumferential wall  131 , and the coupling portion  132  form the movable body  13   a  into a cylindrical cup shape. 
     The fixed body  13   b  has a disk-like shape the diameter of which is substantially equal to the inner diameter of the movable body  13   a . A second restoration spring  44   b  is provided between the fixed body  13   b  and the ring plate  45 . More specifically, the rear end of the second restoration spring  44   b  is secured to the fixed body  13   b , and the front end of the second restoration spring  44   b  is secured to a portion on a second side of the ring plate  45 . 
     The drive shaft main body  30  is inserted in the movable body  13   a  and the fixed body  13   b . At this time, the movable body  13   a  is accommodated in the second recess  23   c  and faces the link mechanism  7  with the swash plate  5  located in between. The fixed body  13   b  is arranged in the movable body  13   a  rearward of the swash plate  5  and is surrounded by the circumferential wall  131 . This defines the control pressure chamber  13   c  between the movable body  13   a  and the fixed body  13   b . The control pressure chamber  13   c  is partitioned from the swash plate chamber  33  by the main body portion  130  of the movable body  13   a , the circumferential wall  131 , and the fixed body  13   b.    
     In addition to the main body portion  130  and the circumferential wall  131  of the movable body  13   a  and the fixed body  13   b , the drive shaft  3 , the rear housing member  19 , and the second cylinder block  23  partition the pressure regulation chamber  31  from the control pressure chamber  13   c.    
     In this compressor, since the drive shaft main body  30  is inserted in the movable body  13   a , the movable body  13   a  is rotational with the drive shaft  3  and is permitted to move along the rotation axis O of the drive shaft  3  in the swash plate chamber  33 . The fixed body  13   b , however, is secured to the drive shaft main body  30  with the drive shaft main body  30  inserted in the fixed body  13   b . This permits the fixed body  13   b  to only rotate with the drive shaft  3  and prevents the fixed body  13   b  to move like the movable body  13   a . Thus, the movable body  13   a  moves relative to the fixed body  13   b  when moving along the rotation axis O. 
     The second side portion of the ring plate  45  is coupled to the coupling portion  132  of the movable body  13   a  with a third pin  47   c . Thus, the second side portion of the ring plate  45 , that is, the swash plate  5  is pivotally supported by the movable body  13   a  about the axis of the third pin  47   c , which is an operation axis M 3 . The operation axis M 3  extends parallel to the first and second pivot axes M 1 , M 2 . The movable body  13   a  is thus held in a state connected to the swash plate  5 . When the inclination angle of the swash plate  5  is maximized, the movable body  13   a  contacts the flange  431  of the second support member  43   b.    
     The drive shaft main body  30  has an axial passage  3   a , which extends forward from the rear end along the rotation axis O, and a radial passage  3   b , which extends radially from the front end of the axial passage  3   a  and has an opening in the outer peripheral surface of the drive shaft main body  30 . The rear end of the axial passage  3   a  has an opening in the pressure regulation chamber  31 . The radial passage  3   b  has an opening in the control pressure chamber  13   c . Thus, the control pressure chamber  13   c  communicates with the pressure regulation chamber  31  via the radial passage  3   b  and the axial passage  3   a.    
     A threaded portion  3   d  is formed at the distal end of the drive shaft main body  30 . The drive shaft  3  is connected to a non-illustrated pulley or a non-illustrated electromagnetic clutch through the threaded portion  3   d.    
     As shown in  FIG. 2 , the control mechanism  15  includes a low-pressure passage  15   a , a high-pressure passage  15   b , a control valve  15   c , an orifice  15   d , the axial passage  3   a , and the radial passage  3   b . The axial passage  3   a  and the radial passage  3   b  correspond to a variable pressure passage according to the present invention. Furthermore, the low-pressure passage  15   a , the high-pressure passage  15   b , the axial passage  3   a , and the radial passage  3   b  form a control passage according to the present invention. 
     The low-pressure passage  15   a  is connected to the pressure regulation chamber  31  and the second suction chamber  27   b . The low-pressure passage  15   a , the axial passage  3   a , and the radial passage  3   b  connect the control pressure chamber  13   c , the pressure regulation chamber  31 , and the second suction chamber  27   b  with one another. The high-pressure passage  15   b  is connected to the pressure regulation chamber  31  and the second discharge chamber  29   b . The discharge refrigerant in the second discharge chamber  29   b  flows through the high-pressure passage  15   b . The high-pressure passage  15   b , the axial passage  3   a , and the radial passage  3   b  connect the control pressure chamber  13   c , the pressure regulation chamber  31 , and the second discharge chamber  29   b . The high-pressure passage  15   b  also has the orifice  15   d.    
     Since the second suction chamber  27   b  and the second discharge chamber  29   b , the pressure regulation chamber  31 , and the control pressure chamber  13   c  are connected as described above, the pressure regulation chamber  31  is located between the control pressure chamber  13   c  and both the second suction chamber  27   b  and the second discharge chamber  29   b . Furthermore, the pressure regulation chamber  31  is a space that has a cross-sectional area that is greater than the cross-sectional area of any of the low-pressure passage  15   a , the high-pressure passage  15   b , the axial passage  3   a , and the radial passage  3   b.    
     The control valve  15   c  is arranged in the low-pressure passage  15   a . The control valve  15   c  is capable of adjusting the opening degree of the low-pressure passage  15   a  in accordance with the pressure in the second suction chamber  27   b.    
     In the compressor shown in  FIG. 1 , a pipe coupled to the evaporator is coupled to the suction port  330 , and a pipe coupled to the condenser is coupled to the discharge port  230 . The condenser is coupled to the evaporator via a pipe and an expansion valve. The compressor, the evaporator, the expansion valve, and the condenser are included in the refrigeration circuit in the air conditioner for a vehicle. The illustration of the evaporator, the expansion valve, the condenser, and the pipes is omitted. 
     In the compressor having the above-described configuration, the drive shaft  3  rotates to rotate the swash plate  5 , thus reciprocating the pistons  9  in the corresponding first and second cylinder bores  21   a ,  23   a . This varies the volume of each first compression chamber  21   d  and the volume of each second compression chamber  23   d  in correspondence with the piston stroke. The compressor thus repeatedly performs a suction stroke for drawing in the suction refrigerant into the first and second compression chambers  21   d ,  23   d , a compression stroke for compressing the suction refrigerant in the first and second compression chambers  21   d ,  23   d , and a discharge stroke for discharging the compressed suction refrigerant from the first and second compression chambers  21   d ,  23   d  as the discharge refrigerant. 
     During the suction stroke, the suction refrigerant that has been drawn from the evaporator into the swash plate chamber  33  through the suction port  330  flows through the first connection passage  37   a  to the first suction chamber  27   a . The suction refrigerant that has reached the first suction chamber  27   a  is drawn into the first compression chambers  21   d  as the first suction reed valves  391   a  open the first suction holes  390   a  by the pressure difference between the first compression chambers  21   d  and the first suction chamber  27   a . Similarly, the suction refrigerant that has been drawn into the swash plate chamber  33  from the evaporator through the suction port  330  flows through the second connection passage  37   b  to the second suction chamber  27   b . The suction refrigerant that has reached the second suction chamber  27   b  is drawn into the second compression chambers  23   d  as the second suction reed valves  411   a  open the second suction holes  410   a  by the pressure difference between the second compression chambers  23   d  and the second suction chamber  27   b.    
     Furthermore, during the discharge stroke, the suction refrigerant that has been compressed in the first compression chambers  21   d  is discharged into the first discharge chamber  29   a  as the discharge refrigerant and flows through the first communication passage  18  to the merged discharge chamber  231 . Similarly, the suction refrigerant that has been compressed in the second compression chambers  23   d  is discharged to the second discharge chamber  29   b  as the discharge refrigerant and flows through the second communication passage  20  to the merged discharge chamber  231 . The discharge refrigerant that has reached the merged discharge chamber  231  is discharged to the condenser through the discharge port  230 . 
     During the suction stroke or the like, a rotor that is formed by the swash plate  5 , the ring plate  45 , the lug arm  49 , and the first pin  47   a  receive the piston compression force acting to decrease the inclination angle of the swash plate  5 . Through such change of the inclination angle of the swash plate  5 , displacement control is carried out by selectively increasing and decreasing the stroke of each piston  9 . 
     More specifically, when the control valve  15   c  of the control mechanism  15  shown in  FIG. 2  increases the opening degree of the low-pressure passage  15   a , the pressure in the pressure regulation chamber  31  and thus the pressure in the control pressure chamber  13   c  become substantially equal to the pressure in the second suction chamber  27   b . The piston compression force acting on the swash plate  5  thus moves the movable body  13   a  of the actuator  13  forward of the swash plate chamber  33  as shown in  FIG. 3 . Thus, in this compressor, the movable body  13   a  approaches the lug arm  49  and reduces the volume of the control pressure chamber  13   c.    
     Consequently, the second side portion of the ring plate  45 , that is, the second side portion of the swash plate  5  pivots clockwise about the operation axis M 3  against the urging force of the second restoration spring  44   b . Also, the rear end of the lug arm  49  pivots counterclockwise about the first pivot axis M 1  and the front end of the lug arm  49  pivots counterclockwise about the second pivot axis M 2 . The lug arm  49  thus approaches the flange  430  of the first support member  43   a . In this manner, the swash plate  5  pivots with the operation axis M 3  serving as a point of application and with the first pivot axis M 1  serving as a fulcrum. This reduces the inclination angle of the swash plate  5  relative to the rotation axis O of the drive shaft  3  and reduces the stroke of the pistons  9 . Thus, the displacement of the compressor per rotation of the drive shaft  3  is reduced. The inclination angle of the swash plate  5  shown in  FIG. 3  corresponds to the minimum inclination angle in the compressor. 
     The swash plate  5  of this compressor receives the centrifugal force acting on the weight portion  49   a . Thus, the swash plate  5  easily moves in such a direction as to decrease the inclination angle. Since the movable body  13   a  moves forward of the swash plate chamber  33 , the front end of the movable body  13   a  is located inward of the weight portion  49   a . As a result, when the inclination angle of the swash plate  5  is decreased, the weight portion  49   a  overlaps with approximately a half the front end of the movable body  13   a.    
     When the inclination angle of the swash plate  5  is reduced, the ring plate  45  contacts the rear end of the first restoration spring  44   a . This elastically deforms the first restoration spring  44   a , and the rear end of the first restoration spring  44   a  approaches the first support member  43   a.    
     When the inclination angle of the swash plate  5  is reduced, and the stroke of the pistons  9  is reduced, the top dead center position of each second piston head  9   b  is separated away from the second valve forming plate  41 . Thus, when the inclination angle of the swash plate  5  approaches zero degrees, compression work is not performed in the second compression chambers  23   d  while compression is slightly performed in the first compression chambers  21   d.    
     When the control valve  15   c  shown in  FIG. 2  reduces the opening degree of the low-pressure passage  15   a , the pressure in the pressure regulation chamber  31  is increased, and the pressure in the control pressure chamber  13   c  is increased. Thus, the movable body  13   a  of the actuator  13  moves rearward of the swash plate chamber  33  against the piston compression force acting on the swash plate  5  as shown in  FIG. 1 . Thus, in this compressor, the movable body  13   a  is separated away from the lug arm  49 , and the volume of the control pressure chamber  13   c  is increased. 
     Consequently, the movable body  13   a  pulls the lower part of the swash plate  5  rearward of the swash plate chamber  33  via the coupling portion  132  at the operation axis M 3 . This pivots the second side portion of the swash plate  5  counterclockwise about the operation axis M 3 . Furthermore, the rear end of the lug arm  49  pivots clockwise about the first pivot axis M 1 , and the front end of the lug arm  49  pivots clockwise about the second pivot axis M 2 . The lug arm  49  is thus separated from the flange  430  of the first support member  43   a . This pivots the swash plate  5  in the opposite direction to the direction in the case where the inclination angle decreases, with the operation axis M 3  and the first pivot axis M 1  serving as the point of application and the fulcrum, respectively. The inclination angle of the swash plate  5  with respect to the rotation axis O of the drive shaft  3  is thus increased. This increases the stroke of the pistons  9 , thus raising the displacement of the compressor per rotation of the drive shaft  3 . The inclination angle of the swash plate  5  shown in  FIG. 1  corresponds to the maximum inclination angle in the compressor. 
     As described above, in this compressor, when the pressure in the control pressure chamber  13   c  is increased, and the movable body  13   a  is separated away from the fixed body  13   b , the volume of the control pressure chamber  13   c  is increased. When the pressure in the control pressure chamber  13   c  is reduced, and the movable body  13   a  approaches the fixed body  13   b , the volume of the control pressure chamber  13   c  is reduced as shown in  FIG. 3 . That is, the displacement of the compressor per rotation of the drive shaft  3  is increased as the volume of the control pressure chamber  13   c  is increased. In contrast, the displacement per rotation of the drive shaft  3  is reduced as the volume of the control pressure chamber  13   c  is reduced. 
     In this compressor, the pressure regulation chamber  31  formed in the rear housing member  19  functions as a muffler that reduces the pulsation of the discharge refrigerant and the suction refrigerant. In this compressor, the volume of the pressure regulation chamber  31  is greater than the volume of the control pressure chamber  13   c  when the displacement is minimized and until the displacement is increased to a certain amount from the minimum. 
     In this compressor, the pressure regulation chamber  31  is arranged between the control pressure chamber  13   c  and both the second suction chamber  27   b  and the second discharge chamber  29   b . Thus, in this compressor, when the discharge refrigerant in the second discharge chamber  29   b  flows into the control pressure chamber  13   c  via the pressure regulation chamber  31 , the pulsation of the discharge refrigerant is reduced in the pressure regulation chamber  31  before flowing into the control pressure chamber  13   c.    
     In this compressor, the pressure regulation chamber  31  also reduces the pulsation of the suction refrigerant in the second suction chamber  27   b . Since the actuator  13  is unlikely to be influenced by the pulsation of the discharge refrigerant and the suction refrigerant when changing the inclination angle of the swash plate  5 , the compressor is allowed to stabilize the inclination angle of the swash plate  5 . 
     Since the pressure regulation chamber  31  has a diameter greater than those of the first and second shaft holes  21   b ,  23   b  and a passage cross-sectional area greater than that of any of the low-pressure passage  15   a , the high-pressure passage  15   b , the axial passage  3   a , and the radial passage  3   b , the volume of the pressure regulation chamber  31  is sufficient. Thus, the pressure regulation chamber  31  favorably functions as a muffler and is allowed to sufficiently reduce the pulsation of the discharge refrigerant and the suction refrigerant. 
     In particular, in this compressor, as the inclination angle of the swash plate  5  approaches zero degrees, the volume of the control pressure chamber  13   c  is reduced. Furthermore, when the inclination angle approaches zero degrees, no compression work is performed in the second compression chambers  23   d . Thus, when the inclination angle approaches zero degrees, the actuator  13  is apt to be significantly affected by the pulsation of the discharge refrigerant and the suction refrigerant. In this respect, since the pressure regulation chamber  31  reduces the pulsation of, for example, the discharge refrigerant as described above, the inclination angle of the swash plate  5  is stable even when the volume of the control pressure chamber  13   c  is small, or the displacement is small. 
     Thus, the compressor of the first embodiment is capable of operating at a suitable displacement. 
     Second Embodiment 
     As shown in  FIG. 4 , a compressor according to a second embodiment includes a housing  201 , a drive shaft  203 , a swash plate  205 , a link mechanism  207 , pistons  209 , pairs of shoes  211   a ,  211   b , an actuator  213 , and a control mechanism  16 , which is illustrated in  FIG. 5 . 
     As shown in  FIG. 4 , the housing  201  has a front housing member  217  at a front position in the compressor, a rear housing member  219  at a rear position in the compressor, and a cylinder block  221  and a valve forming plate  223 , which are arranged between the front housing member  217  and the rear housing member  219 . 
     The front housing member  217  includes a front wall  217   a , which extends in the vertical direction of the compressor on the front side, and a circumferential wall  217   b , which is integrally formed with the front wall  217   a  and extends rearward from the front of the compressor. The front housing member  217  is formed into a substantially cylindrical cup shape with the front wall  217   a  and the circumferential wall  217   b . Furthermore, the front wall  217   a  and the circumferential wall  217   b  define a swash plate chamber  225  in the front housing member  217 . 
     The front wall  217   a  has a boss  217   c , which projects forward. The boss  217   c  accommodates a shaft sealing device  227 . The boss  217   c  has a first shaft hole  217   d , which extends in the front-rear direction of the compressor. The first shaft hole  217   d  accommodates a first slide bearing  229   a.    
     The circumferential wall  217   b  has a suction port  250  that communicates with the swash plate chamber  225 . The swash plate chamber  225  is connected to a non-illustrated evaporator through the suction port  250 . 
     A part of the control mechanism  16  is received in the rear housing member  219 . The rear housing member  219  includes a first pressure regulation chamber  32   a , a suction chamber  34 , and a discharge chamber  36 . The first pressure regulation chamber  32   a  is located in the central part of the rear housing member  219 . The discharge chamber  36  is located radially outward of the rear housing member  219  in an annular form. Also, the suction chamber  34  is formed into an annular shape between the first pressure regulation chamber  32   a  and the discharge chamber  36  in the rear housing member  219 . The discharge chamber  36  is connected to a non-illustrated discharge port. The rear housing member  219  also corresponds to a cover according to the present invention. 
     The cylinder block  221  includes cylinder bores  221   a , the number of which is the same as that of the pistons  209 . The cylinder bores  221   a  are arranged at equal angular intervals in the circumferential direction. The front ends of the cylinder bores  221   a  communicate with the swash plate chamber  225 . The cylinder block  221  also includes retainer grooves  221   b  that limit the maximum opening degree of suction reed valves  61   a , which will be discussed below. 
     The cylinder block  221  further includes a second shaft hole  221   c , which communicates with the swash plate chamber  225  and extends in the front-rear direction of the compressor. The second shaft hole  221   c  accommodates a second slide bearing  229   b . The first shaft hole  217   d  and the second shaft hole  221   c  also correspond to a shaft hole according to the present invention. 
     The first pressure regulation chamber  32   a  of this compressor has a diameter greater than those of the first and second shaft holes  217   d ,  221   c . Thus, when the cylinder block  221  and the rear housing member  219  are joined via the valve forming plate  223 , the first pressure regulation chamber  32   a  is placed over the second shaft hole  221   c  also. 
     The cylinder block  221  further has a spring chamber  221   d . The spring chamber  221   d  is located between the swash plate chamber  225  and the second shaft hole  221   c . The spring chamber  221   d  accommodates a restoration spring  237 . The restoration spring  237  urges the swash plate  205  forward of the swash plate chamber  225  when the inclination angle is minimized. The cylinder block  221  also includes a suction passage  239  that communicates with the swash plate chamber  225 . 
     In this compressor, the swash plate chamber  225  communicates with the suction chamber  34  through the suction passage  239 . Thus, the pressure in the suction chamber  34  is substantially equal to the pressure in the swash plate chamber  225 . Since low-pressure suction refrigerant that has passed through the evaporator flows into the swash plate chamber  225  via the suction port  250 , the pressures in the swash plate chamber  225  and the suction chamber  34  are lower than the pressure in the discharge chamber  36 . 
     The valve forming plate  223  is located between the rear housing member  219  and the cylinder block  221 . The valve forming plate  223  includes a valve plate  60 , a suction valve plate  61 , a discharge valve plate  63 , and a retainer plate  65 . 
     The valve plate  60 , the discharge valve plate  63 , and the retainer plate  65  include suction holes  60   a , the number of which is equal to that of the cylinder bores  221   a . Furthermore, the valve plate  60  and the suction valve plate  61  include discharge holes  60   b , the number of which is equal to that of the cylinder bores  221   a . The cylinder bores  221   a  communicate with the suction chamber  34  through the suction holes  60   a  and communicate with the discharge chamber  36  through the discharge holes  60   b . Furthermore, the valve plate  60 , the suction valve plate  61 , the discharge valve plate  63 , and the retainer plate  65  include a first communication hole  60   c  and a second communication hole  60   d . The first communication hole  60   c  connects the suction chamber  34  to the suction passage  239 . 
     The suction valve plate  61  is provided on the front surface of the valve plate  60 . The suction valve plate  61  includes suction reed valves  61   a  that are capable of opening and closing the suction holes  60   a  by elastic deformation. The discharge valve plate  63  is located on the rear surface of the valve plate  60 . The discharge valve plate  63  includes discharge reed valves  63   a  that are capable of opening and closing the discharge holes  60   b  by elastic deformation. The retainer plate  65  is provided on the rear surface of the discharge valve plate  63 . The retainer plate  65  limits the maximum opening degree of the discharge reed valves  63   a.    
     The drive shaft  203  is inserted in the boss  217   c  toward the rear of the housing  201 . The front portion of the drive shaft  203  extends through the shaft sealing device  227  in the boss  217   c  and is supported by the first slide bearing  229   a  in the first shaft hole  217   d . The rear portion of the drive shaft  203  is supported by the second slide bearing  229   b  in the second shaft hole  221   c . In this manner, the drive shaft  203  is supported to be rotational about the rotation axis O relative to the housing  201 . The second shaft hole  221   c  and the rear end of the drive shaft  203  define a second pressure regulation chamber  32   b . The second pressure regulation chamber  32   b  communicates with the first pressure regulation chamber  32   a  through the second communication hole  60   d . The first and second pressure regulation chambers  32   a ,  32   b  form a pressure regulation chamber  32 . 
     Sealing rings  249   a ,  249   b  are provided on the rear end of the drive shaft  3 . The pressure regulation chamber  32  is sealed by the sealing rings  249   a ,  249   b  so that the swash plate chamber  225  does not communicate with the pressure regulation chamber  32 . 
     The link mechanism  207 , the swash plate  205 , and the actuator  213  are mounted on the drive shaft  203 . The link mechanism  207  includes a lug plate  251 , a pair of lug arms  253  formed on the lug plate  251 , and a pair of swash plate arms  205   e  formed on the swash plate  205 . In the drawing, only one of the lug arms  253  and one of the swash plate arms  205   e  are shown. The same applies to  FIG. 6 . 
     As shown in  FIG. 4 , the lug plate  251  has a substantially annular shape. The lug plate  251  is press-fitted to the drive shaft  203  and rotates integrally with the drive shaft  203 . The lug plate  251  is located at the front section in the swash plate chamber  225  and is located forward of the swash plate  205 . A thrust bearing  255  is located between the lug plate  251  and the front wall  217   a.    
     The lug plate  251  has a cylinder chamber  251   a  that extends in the front-rear direction of the lug plate  251 . The cylinder chamber  251   a  extends from the rear end surface of the lug plate  251  to a position in the lug plate  251  that corresponds to the interior of the thrust bearing  255 . 
     The lug arms  253  extend rearward from the lug plate  251 . The lug plate  251  includes a sliding surface  251   b  at a position between the lug arms  253 . 
     The swash plate  205  is shaped as a flat annular plate and has a front surface  205   a  and a rear surface  205   b . The front surface  205   a  has a weight portion  205   c , which projects forward of the swash plate  205 . When the inclination angle of the swash plate  205  is maximized, the weight portion  205   c  contacts the lug plate  251 . Furthermore, a through hole  205   d  is formed at the center of the swash plate  205 . The drive shaft  203  is inserted in the through hole  205   d.    
     The swash plate arms  205   e  are formed on the front surface  205   a . The swash plate arms  205   e  extend forward from the front surface  205   a . The swash plate  205  also has a substantially semicircular projection  205   g , which projects from the front surface  205   a  and is integrally formed with the front surface  205   a . The projection  205   g  is located between the swash plate arms  5   e.    
     In this compressor, the swash plate arms  205   e  are inserted between the lug arms  253  so that the lug plate  251  and the swash plate  205  are coupled with each other. Thus, the swash plate  205  is rotational in the swash plate chamber  225  together with the lug plate  251 . Coupling the lug plate  251  with the swash plate  205  in this manner causes the distal ends of the swash plate arms  205   e  to contact the sliding surface  251   b . The swash plate arms  205   e  slide along the sliding surface  251   b  so that the swash plate  205  is allowed to change the inclination angle relative to the direction perpendicular to the rotation axis O from the maximum inclination angle shown in the drawing to the minimum inclination angle shown in  FIG. 6  while substantially maintaining the top dead center position T. 
     As shown in  FIG. 4 , the actuator  213  includes the lug plate  251 , a movable body  213   a , and a control pressure chamber  213   b . The lug plate  251  forms the link mechanism  207  as described above and also functions as a fixed body according to the present invention. 
     The movable body  213   a  is fitted to the drive shaft  203  and is movable along the rotation axis O while sliding on the drive shaft  203 . The movable body  213   a  has a cylindrical shape that is coaxial with the drive shaft  203  and has a diameter smaller than that of the thrust bearing  255 . The movable body  213   a  is formed such that the diameter increases from the rear end toward the front end. 
     An operation portion  234  is formed integrally with the rear end of the movable body  213   a . The operation portion  234  extends vertically from the rotation axis O toward the top dead center position T of the swash plate  205  and is in point contact with the projection  205   g . This allows the movable body  213   a  to rotate integrally with the lug plate  251  and the swash plate  205 . 
     The movable body  213   a  can be fitted to the lug plate  251  by inserting the front end of the movable body  213   a  in the cylinder chamber  251   a . In a state in which the front end of the movable body  213   a  is inserted to the innermost position in the cylinder chamber  251   a , the front end of the movable body  213   a  is located at a position that corresponds to the interior of the thrust bearing  255  in the cylinder chamber  251   a.    
     The control pressure chamber  213   b  is defined by the front end of the movable body  213   a , the cylinder chamber  251   a , and the drive shaft  203 . The control pressure chamber  213   b  is partitioned from the swash plate chamber  225  and the pressure regulation chamber  32  by the movable body  213 , the lug plate  251 , and the drive shaft  203 . 
     The drive shaft  203  has an axial passage  203   a  and a radial passage  203   b . The axial passage  203   a  extends from the rear end of the drive shaft  203  toward the front end along the rotation axis O. The radial passage  203   b  extends in a radial direction from the front end of the axial passage  203   a  and opens in the outer circumferential surface of the drive shaft  203 . The rear end of the axial passage  203   a  is open in the pressure regulation chamber  32 . The radial passage  203   b  is open in the control pressure chamber  213   b . The axial passage  203   a  and the radial passage  203   b  connect the pressure regulation chamber  32  to the control pressure chamber  213   b.    
     The drive shaft  203  is connected to a non-illustrated pulley or an electromagnetic clutch by a thread portion  203   e  formed at the distal end like the compressor according to the first embodiment. 
     The pistons  209  are respectively accommodated in the corresponding cylinder bores  221   a  and are capable of reciprocating in the corresponding cylinder bores  221   a . Each piston  209  and the valve forming plate  223  define a compression chamber  257  in the corresponding cylinder bore  221   a.    
     The pistons  209  respectively have engaging portions  209   a . Each engaging portion  209   a  accommodates the hemispherical shoes  211   a ,  211   b . The shoes  211   a ,  211   b  convert rotation of the swash plate  205  into reciprocation of the pistons  209 . The shoes  211   a ,  211   b  also correspond to a conversion mechanism according to the present invention. The pistons  209  thus reciprocate in the corresponding cylinder bores  221   a  by the stroke corresponding to the inclination angle of the swash plate  205 . 
     As shown in  FIG. 5 , the control mechanism  16  includes a low-pressure passage  16   a , a high-pressure passage  16   b , a control valve  16   c , an orifice  16   d , the axial passage  203   a , and the radial passage  203   b . The axial passage  203   a  and the radial passage  203   b  correspond to a variable pressure passage according to the present invention. Furthermore, the low-pressure passage  16   a , the high-pressure passage  16   b , the axial passage  203   a , and the radial passage  203   b  form a control passage according to the present invention. 
     The low-pressure passage  16   a  is connected to the pressure regulation chamber  32  and the suction chamber  34 . The low-pressure passage  16   a , the axial passage  203   a , and the radial passage  203   b  connect the control pressure chamber  213   b , the pressure regulation chamber  32 , and the suction chamber  34  to one another. The high-pressure passage  16   b  is connected to the pressure regulation chamber  32  and the discharge chamber  36 . The discharge refrigerant in the discharge chamber  36  flows through the high-pressure passage  16   b . The high-pressure passage  16   b , the axial passage  203   a , and the radial passage  203   b  connect the control pressure chamber  213   b , the pressure regulation chamber  32 , and the discharge chamber  36 . The high-pressure passage  16   b  also has the orifice  16   d.    
     In this manner, the suction chamber  34  and the discharge chamber  36 , the pressure regulation chamber  32 , and the control pressure chamber  213   b  are connected so that the pressure regulation chamber  32  is located between the control pressure chamber  213   b  and both the suction chamber  34  and the discharge chamber  36 . Furthermore, the pressure regulation chamber  32  is a space with a cross-sectional area that is greater than the passage cross-sectional area of any of the low-pressure passage  16   a , the high-pressure passage  16   b , the axial passage  203   a , and the radial passage  203   b.    
     The control valve  16   c  is arranged in the low-pressure passage  16   a . The control valve  16   c  is capable of adjusting the opening degree of the low-pressure passage  16   a  in accordance with the pressure in the suction chamber  34 . 
     In this compressor, a pipe coupled to the evaporator is coupled to the suction port  250  shown in  FIG. 4 , and a pipe coupled to the condenser is coupled to the discharge port. Like the compressor of the first embodiment, the compressor of the present embodiment is included in the refrigeration circuit of the air conditioner for a vehicle together with the evaporator, the expansion valve, and the condenser. 
     In the compressor having the above-described configuration, the drive shaft  203  rotates to rotate the swash plate  205 , thus reciprocating each piston  209  in the corresponding cylinder bore  221   a . This varies the volume of each compression chamber  257  in accordance with the piston stroke. Thus, the suction refrigerant that has been drawn from the evaporator into the swash plate chamber  225  through the suction port  250  flows through the suction passage  239  and the suction chamber  34  and is compressed in the compression chambers  257 . The suction refrigerant that is compressed in the compression chambers  257  is discharged to the discharge chamber  36  as discharge refrigerant and is discharged to the condenser through the discharge port. 
     Like the compressor of the first embodiment, the compressor of the present embodiment is capable of performing displacement control by changing the inclination angle of the swash plate  205  to selectively increase and decrease the stroke of the pistons  209 . 
     More specifically, when the control valve  16   c  of the control mechanism  16  shown in  FIG. 5  increases the opening degree of the low-pressure passage  16   a , the pressure in the pressure regulation chamber  32  and thus the pressure in the control pressure chamber  213   b  become substantially equal to the pressure in the suction chamber  34 . The piston compression force that acts on the swash plate  205  causes the movable body  213   a  of the actuator  213  to slide in the cylinder chamber  251   a  along the rotation axis O from the swash plate  205  toward the lug plate  251  as shown in  FIG. 4 . This reduces the volume of the control pressure chamber  213   b . The front end of the movable body  213   a  thus enters the cylinder chamber  251   a.    
     Simultaneously, the swash plate arms  205   e  slide along the sliding surface  251   b  to separate away from the rotation axis O. Thus, the bottom dead center portion of the swash plate  205  pivots clockwise while substantially maintaining the top dead center position T. The inclination angle of the swash plate  205  relative to the rotation axis O of the drive shaft  203  is thus increased. This increases the stroke of the pistons  209  and thus increases the displacement of the compressor per rotation of the drive shaft  203 . The inclination angle of the swash plate  205  shown in  FIG. 4  corresponds to the maximum inclination angle in the compressor. 
     When the control valve  16   c  shown in  FIG. 5  reduces the opening degree of the low-pressure passage  16   a , the pressure in the pressure regulation chamber  32  is increased, and the pressure in the control pressure chamber  213   b  is increased. As shown in  FIG. 6 , since the movable body  213   a  slides in the cylinder chamber  251   a  along the rotation axis O toward the swash plate  205  while separating away from the lug plate  251 , the volume of the control pressure chamber  213   b  of the actuator  213  is increased. 
     This causes the operation portion  234  to push the projection  205   g  toward the rear of the swash plate chamber  225 . The swash plate arms  205   e  thus slide along the sliding surface  251   b  to approach the rotation axis O. This causes the bottom dead center portion of the swash plate  205  to pivot counterclockwise while substantially maintaining the top dead center position T. The inclination angle of the swash plate  5  relative to the rotation axis O of the drive shaft  203  is thus decreased. This reduces the stroke of the pistons  209  and the displacement of the compressor per rotation of the drive shaft  203 . The inclination angle of the swash plate  205  shown in  FIG. 6  corresponds to the minimum inclination angle in the compressor. 
     Like the compressor of the first embodiment, the pressure regulation chamber  32  of the compressor of the present embodiment functions as a muffler that reduces the pulsation of the discharge refrigerant and the suction refrigerant. In this compressor, the volume of the pressure regulation chamber  32  is greater than the volume of the control pressure chamber  213   b  when the displacement is maximized and until the displacement is reduced to a certain amount from the maximum. 
     In the compressor of the present embodiment, the pressure regulation chamber  32  is located between the control pressure chamber  213   b  and both the suction chamber  34  and the discharge chamber  36 . Thus, when the discharge refrigerant in the discharge chamber  36  flows into the control pressure chamber  213   b  via the pressure regulation chamber  32 , the pulsation is reduced in the pressure regulation chamber  32  before the discharge refrigerant flows into the control pressure chamber  213   b . The pressure regulation chamber  32  also reduces the pulsation of the suction refrigerant in the suction chamber  34 . Since the actuator  213  is unlikely to be influenced by the pulsation of the discharge refrigerant and the suction refrigerant when changing the inclination angle of the swash plate  205 , the compressor is allowed to stabilize the inclination angle of the swash plate  205 . 
     The first pressure regulation chamber  32   a  and the second pressure regulation chamber  32   b  form the pressure regulation chamber  32 , and the first pressure regulation chamber  32   a  has a diameter greater than those of the first and second shaft holes  217   d ,  221   c . Furthermore, the pressure regulation chamber  32  is a space with a cross-sectional area that is greater than the passage cross-sectional area of any of the low-pressure passage  16   a , the high-pressure passage  16   b , the axial passage  203   a , and the radial passage  203   b . Due to these reasons, the pressure regulation chamber  32  also has a sufficient volume. Thus, the compressor is also capable of sufficiently reducing the pulsation of the discharge refrigerant and the suction refrigerant with the pressure regulation chamber  32 . 
     In particular, as the inclination angle of the swash plate  205  is increased, the volume of the control pressure chamber  213   b  is reduced. When the inclination angle of the swash plate  205  is maximized, that is, when the displacement is maximized, the volume of the control pressure chamber  213   b  is minimized. Thus, unlike the compressor of the first embodiment, the actuator  213  is apt to be significantly affected by the pulsation of the discharge refrigerant and the suction refrigerant when the displacement of the compressor of the present embodiment is changed to be reduced from the maximum state. However, since the pressure regulation chamber  32  also reduces the pulsation of the discharge refrigerant as described above, even when starting to change the displacement from the maximum displacement state, the inclination angle of the swash plate  205  is stable. The other operations of the compressor are the same as the corresponding operations of the compressor of the first embodiment. 
     Although only the first and second embodiments of the present invention have been described so far, the present invention is not limited to the first and second embodiments, but may be modified as necessary without departing from the scope of the invention. 
     For example, regarding the control mechanism  15  of the compressor according to the first embodiment, the control valve  15   c  may be provided in the high-pressure passage  15   b , and the orifice  15   d  may be provided in the low-pressure passage  15   a . In this case, the control valve  15   c  is capable of adjusting the opening degree of the high-pressure passage  15   b . This allows the high-pressure in the second discharge chamber  29   b  to promptly increase the pressure in the control pressure chamber  13   c  and to promptly reduce the displacement. The same applies to the control mechanism  16  of the compressor according to the second embodiment. 
     Also, in the compressor of the second embodiment, the swash plate arms  205   e  and the lug arms  253  may be pivotally coupled with, for example, a coupling pin to couple the lug plate  251  to the swash plate  205 . 
     Furthermore, in the compressor of the first embodiment, the pressure regulation chamber  31  is formed only in the rear housing member  19 . However, the pressure regulation chamber  31  may be formed in the rear housing member  19  and the second cylinder block  23 , or may be formed in only the second cylinder block  23 . 
     Additionally, in the compressor of the second embodiment, the pressure regulation chamber  32  may be formed with only the first pressure regulation chamber  32   a  in the rear housing member  219 , or may be formed with only the second pressure regulation chamber  32   b  in the cylinder block  221 .