Axial multi-piston compressor having rotary valve for allowing residual part of compressed fluid to escape
This application is a continuation, of application Ser. No. 08/131,452, filed Oct. 4, 1993, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1) Field of the Invention 
     The present invention relates to an axial multi-piston compressor comprising a drive shaft, a cylinder block having cylinder bores formed therein and surrounding the drive shaft, and a plurality of pistons slidably received in the cylinder bores, respectively, wherein the pistons are successively reciprocated in the cylinder bores by a rotation of the drive shaft so that a suction stroke and a discharge stroke are alternately executed in each of the cylinder bores. 
     2) Description of the Related Art 
     Japanese Unexamined Patent Publication (Kokai) No.59(1984)-145378 discloses a swash plate type compressor as representative of an axial multi-piston compressor, which may be incorporated in an air-conditioning system used in a vehicle such as an automobile. This swash plate type compressor comprises: front and rear cylinder blocks axially combined to form a swash plate chamber therebetween, the combined cylinder blocks having a same number of cylinder bores radially formed therein and arranged with respect to the central axis thereof, the cylinder bores of the front cylinder block being aligned and registered with the cylinder bores of the rear cylinder block, respectively, with the swash plate chamber intervening therebetween; double-headed pistons slidably received in the pairs of aligned cylinder bores, respectively; front and rear housings fixed to front and rear end faces of the combined cylinder blocks through the intermediary of front and rear valve plate assemblies, respectively, the front and rear housings each forming a suction chamber and a discharge chamber together with the corresponding one of the front and rear valve plate assemblies; a rotatable drive shaft arranged so as to be axially extended through the front housing and the combined cylinder blocks; and a swash plate securely mounted on the drive shaft within the swash plate chamber and engaging with the double-headed pistons to cause these pistons to be reciprocated in the pairs of aligned cylinder bores, respectively, by the rotation of the swash plate. 
     The front and rear valve plate assemblies in particular have substantially the same construction, in that each comprises: a disc-like member having sets of a suction port and a discharge port each set being able to communicate with the corresponding one of the cylinder bores of the front or rear cylinder block; an inner valve sheet attached to the inner side surface of the disc-like member and having suction reed valve elements formed integrally therein, each of which is arranged so as to open and close the corresponding suction port of the disc-like member; and an outer valve sheet attached to the outer side surface of the disc-like member and having discharge reed valve elements formed integrally therein, each of which is arranged so as to open and close the corresponding discharge port of the disc-like member. Each of the front and rear valve plate assemblies is also provided with suction openings aligned with passages formed in the front or rear cylinder block, respectively, whereby the suction chambers formed by the front and rear housings are in communication with the swash plate chamber into which a fluid or refrigerant is introduced from an evaporator of an air-conditioning system, through a suitable inlet port formed in the combined cylinder blocks. 
     In the swash plate type compressor as mentioned above, the drive shaft is driven by the engine of a vehicle, such as an automobile, so that the swash plate is rotated within the swash plate chamber, and the rotational movement of the swash plate causes the double-headed pistons to be reciprocated in the pairs of aligned cylinder bores. When each piston is reciprocated in the aligned cylinder bores, a suction stroke is executed in one of the aligned cylinder bores and a compression stroke is executed in the other cylinder bore. During the suction stroke, the suction reed valve element is opened and the discharge reed valve element is closed, whereby the refrigerant is delivered from the suction chamber to the cylinder bore through the suction port. During the compression stroke, the suction reed valve element concerned is closed and the discharge reed valve element concerned is opened, whereby the delivered refrigerant is compressed and discharged from the cylinder bore into the discharge chamber, through the discharge reed valve element. 
     In this type compressor, the refrigerant includes a lubricating oil mist, and the movable parts of the compressor are lubricated with the oil mist during he operation. Also, the oil mist appears on the suction and discharge reed valve elements, and serves as a liquid-phase seal when each of the reed valve elements is closed. 
     When the compression stroke is finished in each of the cylinder bores, the corresponding discharge reed valve element is closed. At this point of time, a small part of the compressed refrigerant is inevitably left in a fine space defined between the piston head and the valve plate assembly and in the discharge port formed in the valve plate assembly, and the corresponding suction reed valve element is adhered to the valve seat thereof with the liquid-phase oil. Accordingly, just after the suction stroke is initiated, i.e., just after the corresponding head of the double-headed piston is moved from top dead center toward bottom dead center, the suction reed valve element cannot be immediately opened, i.e., the refrigerant cannot be immediately introduced from the suction chamber into the cylinder bore through the suction reed valve element, because the residual part of the compressed refrigerant has a higher pressure than that of suction chamber, and because and the adhesion force and resilient force of the suction reed valve must be overcome before the refrigerant can be introduced from the suction chamber to the cylinder bore through the suction port. Namely, at the beginning of the suction stroke, the residual part of the compressed refrigerant is merely expanded in the cylinder bore, and thus the introduction of the refrigerant from the suction chamber into the cylinder bore cannot take place until a differential between the pressures in the cylinder bore and the suction chamber exceeds a certain level. 
     Therefore, in the compressor as mentioned above, a practical suction volume of the refrigerant, which can be obtained during the suction stroke, is lower than a theoretical suction volume of the refrigerant due to the residual part of the compressed refrigerant, and thus it is impossible to sufficiently realize a theoretical performance from the compressor. 
     Japanese Unexamined Patent Publication (Kokai) No. 5(1993)-71467, corresponding to U.S. Pat. No. 5,232,349 issued on Aug. 3, 1993, discloses an axial multi-piston compressor constituted such that a theoretical suction volume of the refrigerant can be substantially obtained during the suction stroke. In this compressor, the suction reed valves are substituted for a single suction rotary valve slidably disposed in a central circular space formed in the cylinder block and joined to the drive shaft for rotation thereof. Namely, the valve plate assembly is provided with only the discharge reed valve elements and the discharge ports, and the suction reed valve elements and the suction ports are eliminated therefrom. The suction rotary valve is provided with an arcuate groove formed in a peripheral surface thereof, and the arcuate groove is in communication with the suction chamber. The suction rotary valve is further provided with a through passage extending diametrically therethrough. On the other hand, the cylinder block is provided with radial passages formed therein, and each of these radial passages is in communication with the corresponding cylinder bore at an end face thereof on which the discharge port is disposed. The inner ends of the radial passages are opened at an inner wall face of the central circular space of the cylinder block in which the suction rotary valve is slidably received. 
     In the compressor as disclosed in JUPP (Kokai) No. 5(1993)-71467 (U.S. Pat. No. 5,232,349), when the suction stroke is executed in each of the cylinder bores, the cylinder bore concerned is communicated with the suction chamber through the radial passage thereof and the arcuate groove of the suction rotary valve, so that the refrigerant is introduced thereinto. During the suction stroke, the communication is maintained between the cylinder bore and the suction chamber due to a given arcuate length of the arcuate groove. When the suction stroke is finished, i.e., when the piston reaches bottom dead center, the communication between the cylinder bore and the suction chamber is cut off. Then, the compression stroke is initiated, so that the piston stroke is moved from bottom dead center toward top dead center. When the compression stroke is finished, i.e., when the piston reaches top dead center, a part of the compressed refrigerant is inevitably left in a small volume of the cylinder bore defined by the piston head and the valve plate assembly, similar to the compressor as disclosed in JUPP (Kokai) No. 59(1984)-145378. However, just after the compression stroke is finished, i.e., just after the piston is moved from top dead center toward bottom dead center, the cylinder bore concerned is communicated with the diametrically opposed cylinder bore, in which the suction stroke is just finished, through the diametrical through passage formed in the rotary valve, and thus the residual part of the compressed refrigerant escapes from the cylinder bore concerned to the diametrically opposed cylinder bore not governed by the compression stroke. Accordingly, as soon as the cylinder bore concerned is made to communicate with the suction chamber through the radial passage thereof and the arcuate groove of the rotary valve, the refrigerant is introduced from the suction chamber the cylinder bore concerned, due to the escape of the residual part of the compressed refrigerant. As a result, a practical suction volume of the refrigerant, which can be obtained during the suction stroke, is substantially equal to a theoretical suction volume of the refrigerant, and thus it is possible to substantially realize a theoretical performance from the compressor. 
     Nevertheless, the compressor shown in U.S. Pat. No. 5,232,349 involves a problem to be solved. In particular, when the residual part of the compressed refrigerant escapes from the cylinder bore concerned to the diametrically opposed cylinder bore not governed by the compression stroke, the pressure of the escaped part of refrigerant is substantially lowered to a low pressure level in the suction chamber. Thus, the pressure of the escape part of refrigerant must be again raised during the compression stroke. Of course, this is a great loss in the efficiency of compression in the compressor. 
     SUMMARY OF THE INVENTION 
     Therefore, an object of the present invention is to provide an axial multi-piston compressor constituted such that a residual part of the compressed fluid escapes from the cylinder bore to bring a practical suction volume of the fluid as close to a theoretical suction volume as possible without a great loss in the efficiency of compression in the compressor. 
     In accordance with the present invention, there is provided an axial multi-piston compressor comprising: a drive shaft; a cylinder block having cylinder bores formed therein and surrounding the drive shaft; a plurality of pistons slidably received in the respective cylinder bores; a conversion means for converting a rotational movement of the drive shaft into a reciprocation of each piston in the corresponding cylinder bore such that a suction stroke and a discharge stroke are alternately executed therein, a fluid being introduced into the cylinder bore during the suction stroke, and during the compression stroke, the introduced fluid being compressed and discharged from the cylinder bore such that a residual part of the compressed fluid is inevitably left in the cylinder bore when the compression stroke is finished; and a valve means for allowing the residual fluid to escape from the cylinder bore into another cylinder bore governed by the compression stroke. 
     The valve means may comprise a rotary valve joined to the drive shaft to be rotated together therewith and having a groove formed in a peripheral surface thereof, and during the rotation of the rotary valve, a communication between the cylinder bores is established by the groove, whereby the residual part of the compressed fluid can escape from one of the cylinder bores into the other cylinder bore. The groove may be in the form of a closed loop. The rotary valve may include a passage means for introducing the fluid into each of the cylinder bores during the suction stroke. Preferably, the groove and the passage means are diametrically opposed to each other on the peripheral surface of the rotary valve. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The other objects and advantages of the present invention will be better understood from the following description, with reference to the accompanying drawings, in which: 
     FIG. 1 is a longitudinal sectional view showing a wobble plate type compressor according to the present invention; 
     FIG. 2 is a cross-sectional view taken along a line II--II of FIG. 1; 
     FIG. 3 is a development view showing an outer wall surface of a suction rotary valve and an inner wall surface of a central space formed in a cylinder block of the compressor and slidably receiving the suction rotary valve; 
     FIG. 4 is a development view similar to FIG. 3, in which the suction rotary valve is rotated from an angular position of FIG. 3; 
     FIG. 5 is a development view similar to FIG. 3, in which the suction rotary valve is further rotated from an angular position of FIG. 4; 
     FIG. 6 is a development view similar to FIG. 3, in which the suction rotary valve is rotated over an angle of 180 degrees measured from the angular position of FIG. 3; 
     FIG. 7 is a development view similar to FIG. 3, in which the suction rotary valve is rotated over an angle of 60 degrees measured from the angular position of FIG. 6; 
     FIG. 8 is a graph showing a variation of pressure in a compression chamber and a variation of volume thereof when rotating the suction rotary valve over an angle of 360 degrees. 
     FIG. 9 is a graph showing an operation cycle performed in each compression chamber of the compressor. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows a wobble-plate-type compressor as an axial multi-piston compressor in which the present invention is embodied, and which may be used in an air-conditioning system (not shown) for a vehicle such as an automobile. The compressor comprises a cylinder block 10, front and rear housings 12 and 14 securely and hermetically joined to the cylinder block 10 at front and rear end faces thereof through the intermediary of O-ring rings 16 and 18, respectively. The cylinder block 10 and the housings 12 and 14 are assembled as an integrated unit by six screws 19 (see FIG. 2). In this embodiment, as shown in FIG. 2, the cylinder block 10 has six cylinder bores 20A, 20B, 20C, 20D, 20E, and 20F formed radially and circumferentially therein and spaced from each other at regular intervals, and each of the cylinder bores slidably receives a piston 22. The front housing 12 has a crank chamber 24 defined therewithin, and the rear housing 14 has a central suction chamber 26 and an annular discharge chamber 28 defined therewithin and partitioned by an annular wall portion 14a integrally projected from an inner wall of the rear housing 14. In this embodiment, the suction chamber 26 and the discharge chamber 28 are in communication with an evaporator and a condenser of the air-conditioning system, respectively, so that a fluid or refrigerant is supplied from the evaporator to the suction chamber 26 and a compressed refrigerant is delivered from the discharge chamber 28 to the condenser. 
     A valve plate assembly 30 is disposed between the rear end face of the cylinder block 10 and the rear housing 14, and defines compression chambers 32A, 32B, 32C, 32D, 32E, and 32F together with the heads of the pistons 22 slidably received in the cylinder bores 20A to 20F, as shown in FIG. 2. The valve plate assembly 30 includes a disc-like plate member 34, a reed valve sheet 36 applied to an outer side surface of the disc-like plate member 34, and a retainer plate member 38 applied to an outer side surface of the reed valve sheet 36. The disc-like member 34 may be made of a suitable metal material such as steel, and has six discharge ports 40 formed radially and circumferentially therein and spaced from each other at regular intervals, so that each of the discharge ports 40 is encompassed within an end opening area of the corresponding one of the cylinder bores 20A to 20F. Note, in FIG. 2, each of the discharge ports 40 is illustrated by a phantom line. The reed valve sheet 36 may be made of spring steel, phosphor bronze, or the like, and has six discharge reed valve elements 42 formed integrally therewith and arranged radially and circumferentially to be in register with the discharge ports 40, respectively, whereby each of the discharge reed valve elements 42 can be moved so as to open and close the corresponding discharge port 40, due to a resilient property thereof. The retainer plate member 38 may be made of a suitable metal material such as steel, and is preferably coated with a very thin rubber layer. The retainer plate member 38 has six retainer elements 44 formed integrally therewith and arranged radially and circumferentially to be in register with the discharge reed valve elements 42, respectively. Each of the retainer elements 44 provides a sloped bearing surface for the corresponding one of the discharge reed valve elements 42, so that each discharge reed valve element 42 is opened only by a given angle defined by the sloped bearing surface of the retainer element 44. 
     A drive shaft 46 extends within the front housing 12 so that a rotational axis thereof matches a longitudinal axis of the front housing 12, and one end of the drive shaft 46 is projected outside from an opening formed in a neck portion 12a of the front housing 10 and is operatively connected to a prime mover of the vehicle for rotation of the drive shaft 46. The drive shaft 46 is rotatably supported by a first radial bearing 48 provided in the opening of the neck portion 12a and by a second radial bearing 50 provided in a central passage formed in the cylinder block 10. A rotary seal unit 52 is provided in the opening of the neck portion 12a to seal the crank chamber 24 from the outside. 
     A drive plate member 54 is mounted on the drive shaft 46 so as to be rotated together therewith, and a thrust bearing 56 is disposed between the drive plate member 54 and an inner side wall portion of the front housing 12. Also, a sleeve member 58 is slidably mounted on the drive shaft 46, and has a pair of pin elements 60 projected diametrically therefrom. Note, in FIG. 1, only one pin element 60 is illustrated by a broken line. A cam plate member 62 is swingably supported by the pair of pin elements 60. As apparent from FIG. 1, the cam plate member 62 is in an annular form, and the drive shaft 46 extends through a central opening of the annular cam plate member 62. The drive plate member 54 is provided with an extension 54a having an elongated guide slot 54b formed therein, and the cam plate member 62 is provided with a bracket portion 62a projected integrally therefrom and having a guide pin element 62b received in the guide slot 54b, whereby the cam plate member 62 can be rotated together with the drive plate member 54, and is swingable about the pair of pin elements 60. A wobble plate member 64 is slidably mounted on an annular portion 66 projected integrally from the cam plate member 62, and a thrust bearing 68 is disposed between the cam plate member 62 and the wobble plate member 64. 
     The sleeve member 58 is always resiliently pressed against the drive plate member 54 by a compressed coil spring 70 mounted on the drive shaft 46 and constrained between the sleeve member 58 and a ring element 72 securely fixed on the drive shaft 46, and thus the sleeve member 58 is resiliently biased against the drive plate member 54. 
     To reciprocate the pistons 22 in the cylinder bores 20A to 20F, respectively, the wobble plate member 64 is operatively connected to the pistons 22 through the intermediary of six connecting rod 74 having spherical shoe elements 74a and 74b formed at ends thereof, and the spherical shoe elements 74a and 74b of each connecting rod 74 are slidably received in spherical recesses formed in the wobble plate member 64 and the corresponding piston 22, respectively. With this arrangement, when the cam plate member 62 is rotated by the drive shaft 46, the wobble plate member 64 is swung about the pair of pin elements 60, so that each of the pistons 22 are reciprocated in the corresponding cylinder bore 20A, 20B, 20C, 20D, 20E, 20F. The crank chamber 24 can be in communication with the suction chamber 26 and/or the discharge chamber through a suitable control valve (not shown) so that a pressure within the crank chamber 24 is variable, whereby the stroke length of the pistons 22 is adjustable. 
     As shown in FIGS. 1 and 2, according to the present invention, a rotary valve 76 is slidably disposed in a circular space 78 formed by a part of the central passage of the cylinder block 10. The rotary valve 76 is coupled to the inner end of the drive shaft 46 so as to be rotated together therewith. To this end, as shown in FIG. 1, the rotary valve 76 is provided with a central hole 80 formed in one end face thereof and having a key slot 80a extending radially therefrom, and the drive shaft 46 is provided with a stub element 82 projected from the inner end face thereof and having a key 82a extending radially therefrom. Namely, the stub element 82 having the key 82a is inserted into the central hole 80 having the key slot 80a, so that the rotary valve 56 can be rotated together with the drive shaft 46. Note, in FIG. 1, a reference numeral 84 indicates a thrust bearing for the rotary valve 76, which is disposed in a central recess formed in the annular wall portion 14a of the rear housing 14. 
     The rotary valve 76 is also provided with a central hole 86 formed therein, and the central hole 86 is opened at the other end face of the rotary valve 76 so as to be in communication with the suction chamber 26 through a central passage of the thrust bearing 84. As best shown in FIG. 2, a sector-shaped groove 88 is formed in the rotary valve 76, and is in communication with the central hole 86. Thus, the sector-shaped groove 88 is in communication with the suction chamber 26 through the central hole 86. The rotary valve 76 is further provided with a closed loop groove 90 formed in a peripheral surface thereof. As is apparent from FIG. 3 in which an outer peripheral wall surface of the rotary valve 76 is shown as a development view, the closed loop groove 90 includes two parallel arcuate groove portions 90a and 90b coextended circumferentially along the outer peripheral surface of the rotary valve 76, and two side groove portions 90c and 90d connected between two sets of ends of the parallel arcuate groove portions 92a and 92b. On the other hand, as best shown in FIG. 2, the cylinder block 10 is provided with six radial passages 94A, 94B, 94C, 94D, 94E, and 94F formed therein and extended from the compression chambers 32A to 32F to the circular space 78 of the cylinder block 10, respectively. In FIG. 3, an inner peripheral wall surface of the circular space 78 is also shown in a development view to illustrate a relationship between the rotary valve 76 and the arrangement of the radial passages 94A, 94B, 94C, 94D, 94E, and 94F. 
     When the rotary valve 76 is rotated by the drive shaft 46 in a direction indicated by an arrow R (FIGS. 2 and 3), the radial passages 94A to 94F successively communicate with the suction chamber 26 through the central hole 86 and the sector-shaped groove 88. Also, during the rotation of the drive shaft 46, the pistons 22 are reciprocated in the cylinder bores 20A to 20F, so that a suction stroke and a compression stroke are alternately executed in each of the cylinder bores 20A to 20F. During the suction stroke, i.e., during movement of the piston 22 concerned from top dead center toward bottom dead center, the refrigerant is introduced from the suction chamber 26 into the corresponding compression chamber 32A, 32B, 32C, 32D, 32E, 32F through the central hole 86, the sector-shaped groove 88, and the corresponding radial passage 94A, 94B, 94C, 94D, 94E, 94F. During the compression stroke, i.e., during a movement of the piston 22 concerned from bottom dead center toward top dead center, the refrigerant is compressed in the corresponding compression chamber 32A, 32B, 32C, 32D, 32E, 32F, and is then discharged therefrom into the discharge chamber 28 through the corresponding reed valve 42. 
     For example, when the piston 22 received in the cylinder bore 20A reaches top dead center, the rotary valve 76 is at an angular position, as shown in FIG. 3, with respect to the six radial passages 94A, 94B, 94C, 94D, 94E, and 94F. At this point of time, in the cylinder bore 20A or compression chamber 32A, the compression stroke is just finished so that a part of the compressed refrigerant is inevitably left in a small volume of the compression chamber 32A defined by the piston head (22) and the valve plate assembly 30. On the other hand, in the diametrically opposed cylinder bore 20D or compression chamber 32D, the piston 22 reaches bottom dead center, and thus the suction stroke is just finished. Also, each of the cylinder bores 20B and 20C or compression chambers 32B and 32C is subjected to the compression stroke, and each of the cylinder bores 20E and.20F or compression chambers 32E and 32F is subjected to the suction stroke. Further, in the situation shown in FIG. 3, the side groove portion 90c of the closed loop groove 90 bounds on the opening of the radial passage 94A, and the side groove portion 90d of the closed loop groove 90 partially lies over the opening of the radial passage 94C so that the compression chamber 32C communicates with the closed loop groove 90. 
     As soon as the rotary valve 76 is rotated from the angular position shown in FIG. 3 to an angular position as shown in FIG. 4, the side groove portion 90c of the closed loop groove 90 comes over the opening of the radial passages 94A so that the closed loop groove 90 communicates with the compression chamber 94A. On the other hand, the communication is still maintained between the closed loop groove 90 and the compression chamber 32c. Accordingly, the compression chambers 94A and 94C communicate with each other through the closed loop groove 90, so that the residual part of the compressed refrigerant escapes from the compression chamber 94A into the compression chamber 94C. In the situation shown in FIG. 4, since the compression chamber 94C is still subjected to the compression stroke, the pressure of the escaped part of the refrigerant cannot be considerably lowered, so that the escaped part of the refrigerant can be efficiently re-compressed in the compression chamber 94C. 
     When the rotary valve 76 is further rotated from the angular position shown in FIG. 4 to an angular position as shown in FIG. 5, the communication is still maintained between the radial passage 94A and the closed loop groove 90, but the communication is cut off between the radial passage 94C and the closed loop groove 90, so that the compression chamber 94A is not in communication with the compression chamber 94C. This premature cutting off of the communication between the radial passage 94C and the closed loop groove 90 is significant to prevent a part of the refrigerant from being returned from the compression chamber 94C to the compression chamber 94A. In particular, in the compression chamber 94C governed by the compression stroke, the pressure thereof is rapidly increased, whereas, in the compression chamber 94A governed by the suction stroke, the pressure thereof is rapidly lowered. Accordingly, the communication must be cut off between the compression chambers 94A and 94C at a suitable timing, before the return of the refrigerant from the compression chamber 94C to the compression chamber 94A can be prevented. 
     Just after the side groove portion 90c of the closed loop groove 90 passes through the opening of the radial passage 94A, the sector-shaped groove 88 communicates with the radial passage 94A, and thus the refrigerant can be immediately introduced from the suction chamber 26 into the compression chamber 32A due to the escape of the residual refrigerant therefrom. 
     When the rotary valve 76 is rotated over an angle of 180 degrees measured from the angular position of FIG. 3, the rotary valve 76 is at an angular position as shown in FIG. 6, and this situation is equivalent to that of FIG. 3. Namely, in the cylinder bore 20D or compression chamber 32D in which the piston 22 reaches top dead center, the compression stroke is just finished, and in the cylinder bore 20A or compression chamber 32A in which the piston 22 reaches bottom dead center, the suction stroke is just finished. 
     When the rotary valve 76 is rotated over an angle of 60 degrees measured from the angular position of FIG. 6, the rotary valve 76 is at an angular position as shown in FIG. 7, and this situation is also equivalent to that of FIG. 3. When the rotary valve 76 is further rotated from the angular position shown in FIG. 7, the compression chamber 94A is supplied with an additional part of refrigerant that escaped from the compression chamber 94E, as is apparent from the descriptions referred to FIGS. 4. 
     FIG. 8 is a graph showing a variation in pressure in the compression chamber 32A, represented by a curve P, and a variation in volume of the compression chamber 32A, represented by a curve V, when rotating the rotary valve 76 over an angle of 360 degrees. In this graph, it is assumed that a rotational angle of the rotary valve 76 is zero when the piston 22 is at top dead center in the cylinder bore 20A (FIG. 3). 
     As soon as the rotation of the rotary valve 76 is initiated, the side groove portion 90c of the closed loop groove 90 comes over the opening of the radial passage 94A so that communication is established between the compression chamber 32A and the closed loop groove 90. In the graph of FIG. 8, reference PT 1  indicates a period of time over which the communication is maintained between the compression chamber 32A and the closed loop groove 90, and reference PTc indicates a period of time over which the radial passage 94C communicates with the side groove portion 90d of the closed loop groove 90. In an overlapped and hatched area of the periods PT 1  and PT c , the compression chambers 32A and 32C communicate with each other (FIG. 4), and thus the residual part of the compressed refrigerant is fed from the compression chamber 32A to the compression chamber 32C, so that the pressure P is rapidly lowered. 
     Just after the side groove portion 90c of the closed loop groove 90 passes through the opening of the radial passage 94A, the compression chamber 32A communicates with the suction chamber 26 through the central hole 86, the sector-shaped groove 88 and the radial passage 94A. In the graph of FIG. 8, reference PT 2  indicates a period of time over which the communication is maintained between the compression chamber 32A and the suction chamber 26, and the suction stroke is executed over the period of time PT 2 . During the suction stroke, the pressure P is kept constant, and the volume V of the compression chamber 94A reaches a maximum peak at the end of the suction stroke. After suction stroke is finished, i.e., after the compression stroke is initiated, the pressure is gradually increased. 
     In the graph of FIG. 8, reference PT indicates a period of time over which the communication is maintained between the compression chamber 32A and the closed loop groove 90, and reference PT indicates a period of time over which the radial passage 94E communicates with the side groove portion 90c of the closed loop groove 90. In an overlapped and hatched area of the periods PT 3  and PT E , the compression chambers 32A and 32E communicate with each other, and thus the residual part of the compressed refrigerant is fed from the compression chamber 32E to the compression chamber 32A, so that the pressure P is abruptly increased. 
     Thereafter, the pressure P is rapidly increased in response to a decrease of the volume V of the compression chamber 32A, shown in the graph of FIG. 8. When the pressure P reaches the maximum value, the discharge reed valve is opened so that the compressed refrigerant is discharged from the compression chamber 32A into the discharge chamber 28, and thus the maximum value of the pressure P is kept at constant. 
     Note, although only the cylinder bore 20A or compression chamber 32A has been referred to in the above-description, the same is true for other compression chambers 32B, 32C, 32D, 32E, 32F. 
     FIG. 9 shows an operation cycle performed in the compression chambers 32A, 32B, 32C, 32D, 32E, 32F. In this cycle, references A and B indicate top dead center and bottom dead center. The suction stroke is executed in a section indicated by A→B, and the compression stroke is executed in a section indicated by B→A. In the compressor disclosed in U.S. Pat. No. 5,232,349, the compression stroke is executed along a broken line shown in FIG. 9. The efficiency of the compressor according to the present invention is improved by a differential indicated by a hatched area in FIG. 9. 
     In the embodiment described, although the present invention is applied to a wobble plate type compressor as an axial multi-piston compressor, the present invention may be embodied in another type axial multi-piston compressor. 
     Finally, it will be understood by those skilled in the art that the foregoing description is of a preferred embodiment of the disclosed compressor, and that various changes and modifications may be made to the present invention without departing from the spirit and scope thereof.