Abstract:
A compressor includes a housing; a suction region and a discharge region which are formed in the housing in a partitioned manner. The compressor also includes a shaft which is pivotally supported in the inside of the housing. The compressor further includes a compression mechanism which sucks a refrigerant from the suction region and discharges the refrigerant to the discharge region due to a rotational motion of the shaft. The compressor further includes a bypass passage which makes the suction region and the discharge region communicate with each other, a valve element 41 which is capable of closing the bypass passage 40 from a discharge region side, and a spring which biases the valve element toward the discharge region side.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a compressor of a refrigerating cycle used in a vehicle-use air conditioner or the like, and more particularly to a compressor having a function of suppressing the flowing out of lubrication oil in the compressor to the outside of the compressor due to the movement of a refrigerant in the refrigerating cycle caused by a change in temperature when the compressor is stopped. 
       BACKGROUND ART 
       [0002]    In a refrigerating cycle used in an automobile-use air conditioner, using a partition wall (fire wall) which partitions an engine room side and a cabin side from each other as a boundary, an evaporator is arranged in the cabin side, and a condenser, a compressor and the like are arranged in the engine room side. In such a refrigerating cycle, when a vehicle is warmed by sunbeams during daytime in a state where the operation of the compressor is stopped, as shown in  FIG. 6 , a temperature of an evaporator A arranged in the cabin side which is liable to be warmed by sunbeams is also elevated, while temperatures of a condenser B and a compressor C arranged in the inside of the engine room are not elevated to an extent that the temperature of the evaporator A is elevated. Further, the compressor C having a large thermal capacity is hardly warmed and hence, the compressor C is a place which exhibits the lowest temperature in the refrigerating cycle so that a refrigerant is condensed in the compressor. Accordingly, in the compressor, due to oil (lubrication oil) and liquefied refrigerant, a liquid whose amount exceeds an oil storage capacity stays. 
         [0003]    On the other hand, when evening arrives and the vehicle is no more heated by sunbeams, a temperature of the condenser having a small thermal capacity is gradually lowered, and the following relationship is established among the respective temperatures of the evaporator, the compressor, and the condenser. 
         [0000]      evaporator temperature&gt;compressor temperature&gt;condenser temperature 
         [0004]    Due to such temperature difference, a pressure in the condenser becomes the lowest in the refrigerating cycle thus giving rise a phenomenon where a refrigerant which remains in the compressor and into which oil is dissolved is pushed out to a condenser side due to a pressure from the evaporator. 
         [0005]    When such a phenomenon caused by a change in temperature happens repeatedly, oil held in the compressor is gradually conveyed to the outside of the compressor and, eventually, oil remaining in the compressor is depleted. Accordingly, when an air conditioner is operated again after a long period where the air conditioner is not used at all, there is fear that a seizure of the compressor arises due to lubrication failure. 
         [0006]    To overcome such a drawback, conventionally, as described in the following PTL 1, there has been proposed the constitution where a low-pressure pipe which is connected to a suction port of a compressor in a refrigerating cycle and a high-pressure pipe which is connected to a discharge port of the compressor are connected to each other by a bypass pipe passage, a check valve which allows the flow of a refrigerant to the bypass pipe passage when a refrigerant pressure on a low-pressure pipe side (evaporator side) becomes higher than a refrigerant pressure on a high-pressure pipe side (condenser side) is mounted in the bypass pipe passage. Due to such a constitution, in a state where an operation of the compressor is stopped, when the refrigerant pressure on the low-pressure pipe side becomes higher than the refrigerant pressure on the high-pressure pipe side, the check valve is opened due to the pressure difference so that the refrigerant on the low-pressure pipe side is made to flow to the high-pressure pipe side through the bypass pipe passage thus almost preventing the refrigerant from flowing into the inside of the compressor whereby the flowing out of oil from the compressor is prevented. 
       CITATION LIST 
     Patent Literature 
       [0007]    PTL 1: JP-A-7-218007 
       SUMMARY OF INVENTION 
     Technical Problem 
       [0008]    However, the check valve disclosed in the above-mentioned patent literature is configured such that a compression coil spring is arranged in a high-pressure pipe side of a valve element which constitutes a part of the check valve, and the valve element is biased in the closing direction by a biasing force of the compression coil spring. Due to such a constitution, unless a refrigerant pressure on a low-pressure pipe side (evaporator side) is increased by an amount corresponding to a spring force of the compression coil spring with respect to a refrigerant pressure on a high-pressure pipe side (condenser side), the bypass passage is not released. Accordingly, when a refrigerant pressure on the low-pressure pipe side (evaporator side) does not reach a pressure at which the valve element can be opened by pushing against the spring force of the compression coil spring although the refrigerant pressure on the low-pressure pipe side (evaporator side) is higher than the refrigerant pressure on the high-pressure pipe side (condenser side), a liquid refrigerant in the compressor is pushed out to the high-pressure pipe side (condenser side) together with oil due to the difference in pressure and hence, there still exists a possibility that oil in the compressor is depleted. 
         [0009]    The present invention has been made in view of such circumstances, and it is a primary object of the present invention to provide a compressor capable of effectively suppressing the flowing out of lubrication oil in a compressor to the outside of the compressor due to the movement of a refrigerant in a refrigerating cycle caused by a change in temperature in a state where an operation of the compressor is stopped. 
       Solution to Problem 
       [0010]    To achieve the above-mentioned object, a compressor according to the present invention includes: a housing; a suction region and a discharge region which are formed in the housing in a partitioned manner; a shaft which is pivotally supported in the inside of the housing; and a compression mechanism which sucks a refrigerant from the suction region and discharges the refrigerant to the discharge region due to a rotational motion of the shaft, wherein the compressor further includes: a bypass passage which makes the suction region and the discharge region communicate with each other; a valve element which is capable of closing the bypass passage from a discharge region side; and a spring which biases the valve element toward the discharge region side (toward the valve opening direction). 
         [0011]    Due to the above-mentioned constitution, in a state where the operation of the compressor is stopped, when a refrigerant pressure in the discharge region (high-pressure pipe side) which is defined in the housing and a refrigerant pressure in the suction region (low-pressure pipe side) which is defined in the housing are balanced with each other, the bypass passage is always opened. Accordingly, even when the refrigerant pressure on the suction region side (low-pressure pipe side) becomes slightly higher than the refrigerant pressure on the discharge region side (high-pressure pipe side) due to a change in temperature, it is possible to promptly release the refrigerant pressure on the suction region side (low-pressure pipe side) to the discharge region side (high-pressure pipe side) through the bypass passage and hence, there is no possibility that oil remaining in the compressor is taken out to the high-pressure pipe side together with a liquid refrigerant. 
         [0012]    It is preferable that, a valve port formed with a diameter smaller than an outer diameter of the valve element, and a valve accommodating hole formed on a discharge region side with respect to the valve port with a diameter larger than the outer diameter of the valve element are formed in the bypass passage directly or by way of a holder, and the valve element is accommodated in the valve accommodating hole in an axially movable manner. 
         [0013]    Due to such a constriction, when the operation of the compressor is started, the valve element accommodated in the valve accommodating hole moves in the valve accommodating hole promptly so that the bypass passage is closed by the valve element. This state is maintained until the compressor is stopped again so that the pressure difference between the discharge region and the suction region is decreased to an extent that the valve element is movable toward a discharge region side (toward the valve opening direction) due to a spring force of the spring. 
         [0014]    Assuming that, as shown in  FIG. 7 , a valve element A were positioned in an open space of the discharge region, when the operation of the compressor is started, a refrigerant gas which tends to flow into the suction region from the discharge region through a bypass passage B would pass through a gap between the valve element A and a peripheral edge of an opening end of the bypass passage B instead of passing along side of the valve element A, and directly flows into the suction region. Accordingly, the valve element A does not close the bypass passage B thus giving rise to a drawback that a refrigerant gas is blown off into the suction region from the discharge region. However, by adopting the above-mentioned constitution, a refrigerant which tends to flow to a suction region side from a discharge region side passes along a side surface of the valve element held in the valve accommodating hole and, thereafter, flows out to the suction region through the valve port. Accordingly, the flow of the refrigerant works to push the valve element toward a valve port side against a spring force, and this force allows the valve element to promptly close the valve port. Once the valve element closes the valve port, a discharge pressure and a suction pressure act on a front side and a rear side of the valve element respectively, and a closed state is maintained against a spring force of the spring due to such pressure difference. 
         [0015]    To acquire such an action more effectively, it is preferable that an area of a passage formed between an outer peripheral surface of the valve element and an inner peripheral surface of the valve accommodating hole is set smaller than an area of the valve port. Due to such a constitution, pressure drop in the bypass passage from the discharge region to the suction region is preferentially imparted to before and after a refrigerant passes along the valve element rather than to before and after a refrigerant passes through the valve port and hence, the pressure difference can be surely generated on an upstream side and a downstream side of the valve element whereby the valve element can be moved promptly. 
         [0016]    Further, in the compressor where the housing is constituted of shell members formed by dividing the housing in two in the axial direction of the shaft, the discharge region and the suction region are defined in the respective shell members, and a discharge port which is communicated with the discharge region and a suction port which is communicated with the suction region are formed in either one of the shell members, it is desirable that the bypass passage is formed in the above-mentioned one shell member in which the discharge port and the suction port are formed so as to make the above-mentioned refrigerant which moves due to a change in temperature flow toward a high-pressure pipe side without passing the inside of the compressor as much as possible whereby the flowing out of oil in the compressor is prevented. 
         [0017]    The above-mentioned constitution may be realized such that the valve element is formed of a steel ball, the spring accommodated in the valve port is formed of a compression coil spring, and a valve seat face on which the valve element is seated is formed on a portion shifting to the valve accommodating hole from the valve port. 
       Advantageous Effects of Invention 
       [0018]    As has been explained heretofore, according to the present invention, the suction region and the discharge region defined in the housing of the compressor are communicated with each other by forming the bypass passage, and the valve element which is capable of closing the bypass passage from a discharge region side and the spring which biases the valve element toward the discharge region side (toward the valve opening direction) are provided to the bypass passage. Due to such a constitution, even when a refrigerant pressure in the suction region, that is, on a low-pressure pipe side (evaporator side) is slightly increased also with respect to a refrigerant pressure in the discharge region, that is, on a high-pressure pipe side (condenser side), the refrigerant pressure on the suction region side (low-pressure pipe side) can be promptly released to the discharge region side (high-pressure pipe side) through the bypass passage whereby there is no fear that oil remaining in the compressor is taken out to the outside of the compressor together with a liquid refrigerant. 
         [0019]    The valve port formed with a diameter smaller than an outer diameter of the valve element, and a valve accommodating hole formed on a discharge region side with respect to the valve port with a diameter larger than the outer diameter of the valve element are formed in the bypass passage directly or by way of a holder, the spring is held on the valve port, and the valve element is accommodated in the valve accommodating hole in an axially movable manner. Due to such a constitution, the valve element can be moved against a spring force of the spring by a refrigerant which tends to flow in the bypass passage along a side of the valve element immediately after the operation of the compressor is started and hence, the valve port can be promptly closed and the closed state can be maintained in a stable manner. 
         [0020]    In this case, by setting the area of the passage formed between the outer peripheral surface of the valve element and the inner peripheral surface of the valve accommodating hole smaller than the area of the valve port, it is possible to make the valve element react more sensitively by a refrigerant which tends to flow the bypass passage immediately after the operation of the compressor is started and hence, the valve element can promptly and surely close the valve port. 
         [0021]    Further, in the compressor where the housing is constituted of the shell members formed by dividing the housing in two in the axial direction of the shaft, the discharge region and the suction region are defined in the respective shell members, and the discharge port which is communicated with the discharge region and the suction port which is communicated with the suction region are formed in either one of the shell members, by forming the bypass passage in one shell member in which the discharge port and the suction port are formed, even when the bypass passage is formed in the compressor, a refrigerant passes only in the vicinity of the port without passing the inside of the compressor as much as possible and hence, it is possible to surely prevent oil from being taken out from the compressor. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0022]      FIG. 1(   a ) is a cross-sectional view showing an example of a compressor according to the present invention in which a bypass passage is formed, and  FIG. 1(   b ) is a view of a rear-side shell member as viewed from an inner side in the axial direction. 
           [0023]      FIG. 2(   a ) and  FIG. 2(   b ) are conceptual views for explaining the flow of a refrigerant in the compressor according to the present invention, wherein  FIG. 2(   a ) is a side cross-sectional view of the compressor, and  FIG. 2(   b ) is a cross-sectional view taken along a line B-B in  FIG. 2(   a ). 
           [0024]      FIG. 3  is a cross-sectional view of the rear-side shell member, and is also a view taken along a line A-A in  FIG. 1(   b ) (a view showing the compressor in cross section such that the bypass passage can be observed). 
           [0025]      FIG. 4(   a ) and  FIG. 4(   b ) are enlarged cross-sectional views showing a portion of the bypass passage where a valve element and a spring are provided, wherein  FIG. 4(   a ) is the view showing a state where the valve element is away from a valve seat face, and  FIG. 4(   b ) is a view showing a state where the valve element is seated on the valve seat face. 
           [0026]      FIG. 5  is an enlarged cross-sectional view showing a portion of the bypass passage where the valve element and the spring are provided, and is also a view for explaining the flow of a refrigerant which flows through the bypass passage and the behavior of the valve element. 
           [0027]      FIG. 6  is a view showing a graph which indicates changes in temperature in an evaporator, a condenser, and the compressor in a refrigerating cycle mounted on a vehicle, and a schematic view for explaining the flow of a refrigerant and the flow of oil in a state where the compressor is stopped. 
           [0028]      FIG. 7  is a view for explaining the flow of the refrigerant and the behavior of the valve element when the valve element is positioned in an open space of a discharge region. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0029]    Hereinafter, an embodiment of the present invention is explained by reference to attached drawings. 
         [0030]    In  FIG. 1  and  FIG. 2 , a compressor  1  is a reciprocating-type compressor employed in a refrigerating cycle where a refrigerant is used as a working fluid. The compressor  1  includes: a front-side cylinder block  2 ; a rear-side cylinder block  4  which is assembled to the front-side cylinder block  2 ; a front-side shell member  6  which is assembled to a front side (a left side in the drawing) of the front-side cylinder block  2  with a valve plate  5  interposed therebetween; and a rear-side shell member  8  which is assembled to a rear side (a right side in the drawing) of the rear-side cylinder block  4  with a valve plate  7  interposed therebetween. The front-side shell member  6  and the rear-side shell member  8  respectively have opening end sides thereof extended so as to cover the cylinder blocks  2 ,  4  which are arranged close to the front-side shell member  6  and the rear-side shell member  8  respectively and are engaged with each other in the axial direction by fitting engagement. The front-side shell member  6 , the valve plate  5 , the front-side cylinder block  2 , the rear-side cylinder block  4 , the valve plate  7  and the rear-side shell member  8  are fastened to each other in the axial direction using fastening bolts  9 . The front-side shell member  6  and the rear-side shell member  8  constitute a housing of the compressor which is divided in two in the axial direction. 
         [0031]    In the inside of the front-side cylinder block  2  and the rear-side cylinder block  4 , a swash plate accommodating chamber  10  is formed in a defined manner by assembling the front-side cylinder block  2  and the rear-side cylinder block  4  to each other. In the swash plate accommodating chamber  10 , a shaft  12  is rotatably supported by shaft support holes  11  formed in the front-side cylinder block  2  and the rear-side cylinder block  4  respectively by way of radial bearings  19 . One end of the shaft  12  projects from the front-side shell member  6 , and a relay member  14  is fixed to the portion of the shaft  12  projecting from the front-side shell member  6  in a state where the relay member  14  is mounted in the axial direction using a bolt  13 . A boss portion  6   a  which extends toward a front side so as to cover the shaft is integrally formed with the front-side shell member  6 , and a pulley  16  which is connected to a drive source not shown in the drawing by way of a belt is rotatably fitted on the boss portion  6   a  by way of a bearing  15 . The pulley  16  transmits rotational power to the shaft  12  by way of the relay member  14 . 
         [0032]    A plurality of cylinder bores  17  which are arranged parallel to the shaft support hole  11  and are arranged equidistantly on the circumference about the shaft  12  are formed in the respective cylinder blocks  2 ,  4 . In the inside of each cylinder bore  17 , a double-headed piston  18  having head portions on both ends thereof is inserted and is slidable in a reciprocating manner, and compression chambers  25  are defined between the double-headed piston  18  and the valve plates  5 ,  7  respectively. 
         [0033]    A swash plate  20  which is accommodated in the swash plate accommodating chamber  10  and is rotated integrally with the shaft  12  is fixed to the shaft  12 . The swash plate  20  is rotatably supported on the front-side cylinder block  2  and the rear-side cylinder block  4  by way of thrust bearings  21 , and a peripheral edge portion of the swash plate  20  is sandwiched between a pair of semispherical shoes  22  which is engaged with an engaging recessed portion  23  formed on a center portion of the double-headed piston  18  such that a front side and a rear side of the peripheral portion is sandwiched by the semispherical shoes  22 . Accordingly, when the shaft  12  is rotated so that the swash plate  20  is rotated in an oscillating manner, such an oscillating rotation motion is converted into a reciprocating motion of the double-headed piston  18  by way of the shoes  22  so that a capacity of the compression chamber  25  is changed. 
         [0034]    In each of the respective valve plates  5 ,  7 , a suction hole  26  which is opened or closed by a suction valve not shown in the drawing which is mounted on a cylinder-block-side end face of the valve plate  5 ,  7 , and a discharge hole  27  which is opened or closed by a discharge valve not shown in the drawing which is mounted on a shell-member-side end face of the valve plate  5 ,  7  are formed corresponding to each cylinder bore  17 . A suction chamber  28  for accommodating a refrigerant to be supplied to the compression chamber  25  and a discharge chamber  29  for accommodating a refrigerant to be discharged from the compression chamber  25  are defined in the front-side shell member  6  and the rear-side shell member  8  respectively. 
         [0035]    The suction chambers  28  formed in the respective shell members  6 ,  8  are connected with the swash plate accommodating chamber  10  through low-pressure passages  30 , and the swash plate accommodating chamber  10  is communicated with a suction port  32  which is formed in the rear-side shell member  8  and is connected to an external cycle through a suction passage  31  connected to the swash plate accommodating chamber  10 . 
         [0036]    The discharge chambers  29  formed in the respective shell members  6 ,  8  are communicated with a discharge port  34  formed in the rear-side shell member  8  through a discharge passage  33 , and the discharge chambers  29  are connected to the external cycle through the discharge port  34 . 
         [0037]    Accordingly, during a suction stroke where a volume of the compression chamber  25  is increased along with the reciprocating movement of the double-headed piston  18 , a refrigerant which is introduced into the suction chamber  28  from the suction port  32  through the suction passage  31 , the swash plate accommodating chamber  10 , and the low-pressure passage  30  is sucked into the compression chamber  25  through the suction hole  26 . On the other hand, during a compression stroke where the volume of the compression chamber  25  is decreased, a refrigerant which is compressed in the compression chamber  25  is discharged into the discharge chamber  29  through the discharge hole  27 , is introduced to the discharge port  34  formed in the rear-side shell member  8  from the discharge chamber  29  through the discharge passage  33 , and is pumped out to the external cycle from the discharge port  34 . 
         [0038]    As shown also in  FIG. 3 , in the rear-side shell member  8 , a bypass passage  40  is formed through which an area in the vicinity of the suction port  32  of the suction passage  31  which forms a suction region and an area in the vicinity of the discharge port  34  of the discharge passage  33  which forms a discharge region are communicated with each other. The bypass passage  40  is constituted such that a passage forming hole  40   a  is formed in the rear-side shell member  8  from an inner face of the discharge passage  33 , a passage forming hole  40   b  is formed in the rear-side shell member  8  from an inner surface of the suction passage  31 , and distal end portions of the respective passage forming holes  40   a,    40   b  are communicated with each other. A valve element  41  and a spring  42  are accommodated in the passage forming hole  40   a  which is formed in the rear-side shell member  8  from the discharge passage  33 . 
         [0039]    To be more specific, as shown in  FIG. 4 , a valve port  43  which is formed with a diameter smaller than an outer diameter of the valve element  41  and a valve accommodating hole  44  which is formed on a discharge region side (a discharge passage  33  side) with respect to the valve port  43  with a diameter larger than the outer diameter of the valve element  41  are directly formed in the passage forming hole  40   a  of the bypass passage  40  formed from the discharge passage  33  along the axial direction of the passage forming hole  40   a  of the bypass passage  40 , and a valve seat face  45  on which the valve element  41  is seated is formed on a portion shifting to the valve port  43  from the valve accommodating hole  44  in a tapered shape such that a diameter of the valve seat face  45  is gradually decreased. 
         [0040]    In this example, the valve element  41  is formed of a steel ball, and the spring  42  is formed of a compression coil spring. The spring  42  is accommodated in and held by the valve port  43 , the valve element  41  is accommodated in and held by the valve  2   0  accommodating hole  44 , and the valve element  41  is always biased toward a discharge region side (a discharge passage  33  side) by the spring  42 . A stopper member mounting hole  46  which opens at an inner peripheral face of the valve accommodating hole  44  and extends in the direction different from the axial direction of the valve accommodating hole  44  is formed in the vicinity of an opening end of the valve accommodating hole  44  which opens at the discharge passage  33 . A stopper member  47  which is press-fitted in the stopper member mounting hole  46  is fixed in a projecting manner such that the stopper member  47  traverses the valve accommodating hole  44 . Accordingly, the valve element  41  is held in the inside of the valve accommodating hole  44  in a state where the valve element  41  is movable along the axial direction while a moving range of the valve element  41  is restricted by the stopper member  47 . 
         [0041]    An area of a passage between an outer peripheral surface of the valve element  41  and an inner peripheral surface of the valve accommodating hole  44  (a value obtained by subtracting a cross-sectional area of the valve element  41  from a cross-sectional area of the valve accommodating hole  44  on a plane perpendicular to an axis of the valve accommodating hole  44 ) is set smaller than a cross-sectional area of the valve port  43 . 
         [0042]    In the above-mentioned constitution, in a state where the operation of the compressor  1  is stopped, when a refrigerant pressure in the compressor on a high-pressure pipe side and a refrigerant pressure in the compressor on a low-pressure pipe side are approximately balanced with each other, there exists substantially no pressure difference between front and rear sides of the valve element  41  in the bypass passage  40  which is formed between the discharge region and the suction region, and the valve element  41  is in a state where the valve element  41  is away from the valve seat face  45  by being pushed toward a discharge region side by a spring force of the spring  42  (a state shown in  FIG. 4(   a )). 
         [0043]    In this state, when a temperature of a refrigerating cycle apparatus is increased due to the influence of sunbeams and, thereafter, the vehicle is no more heated by sunbeams so that the relationship of evaporator temperature&gt;compressor temperature&gt;condenser temperature is established among a temperature of the evaporator, a temperature of the compressor and a temperature of the condenser, a refrigerant tends to flow toward a high-pressure pipe side from a low-pressure pipe side through the compressor  1  due to a pressure from the evaporator. However, the valve element  41  of the bypass passage  40  is in a state where the valve element  41  is away from the valve seat face  45  and hence, an area of the suction region in the vicinity of the suction port  32  and an area of the discharge region in the vicinity of the discharge port  34  are communicated with each other through the bypass passage  40 . Accordingly, a refrigerant on the low-pressure pipe side can be made to flow promptly toward the high-pressure pipe side through the bypass passage  40  without passing through the inside of the compressor. Accordingly, there is no possibility that a refrigerant which flows from the low-pressure pipe side takes out oil in the compressor to the outside of the compressor thus preventing oil in the inside of the compressor from being depleted. 
         [0044]    On the other hand, immediately after the operation of the compressor is started, a refrigerant which is compressed by the piston and is discharged into the discharge region is discharged to the outside of the compressor and some of the refrigerant tends to flow in the  2   0  suction region through the bypass passage. In this case, as shown in  FIG. 5 , the refrigerant which tends to flow in a suction region side from a discharge region side passes along the side of the valve element  41  which is accommodated in the valve accommodating hole  44  and, thereafter, tends to flow out to the suction region through the valve port  43 . However, the area of the passage between the valve element  41  and the valve accommodating hole  44  is set smaller than the cross-sectional area of the valve port  43  and hence, a pressure of the refrigerant is largely dropped at the place whereby the pressure difference is surely generated between an upstream side and a downstream side of the valve element  41 . Accordingly, the refrigerant pushes the valve element  41  toward a valve port  43  side against a spring force of the spring  42  so that the valve element  41  is promptly seated on the valve seat face  45  due to such a force and closes the valve port  43 . Once the valve element  41  closes the valve port  43 , a discharge pressure and a suction pressure act on front and rear sides of the valve element  41  respectively so that a closed state is maintained in a stable manner against a spring force of the spring  42  due to such pressure difference. 
         [0045]    This state is maintained until the compressor  1  is stopped again so that the pressure difference between the discharge region side and the suction region side is sufficiently decreased and the valve element  41  starts to move toward the discharge region side (toward the valve opening direction) due to the spring force of the spring  42 . 
         [0046]    Particularly, in the above-mentioned constitution, the area of the passage between the outer peripheral surface of the valve element  41  and the inner peripheral surface of the valve accommodating hole  44  is set smaller than the area of the valve port  43  and hence, immediately after the operation of the compressor  1  is started, the pressure difference is surely generated between the front and rear sides of the valve element  41  due to the refrigerant which tends to flow in the bypass passage  40  along the side of the valve element  41  and hence, the valve element can be moved promptly thus surely closing the valve port  43 . 
         [0047]    In the above-mentioned constitution, the constitution where the valve port  43 , the valve accommodating hole  44 , and the valve seat face  45  are integrally formed on a middle portion of the bypass passage  40  by working an inner wall of the passage is taken as an example. However, it may be possible to adopt the structure where a cartridge is formed by accommodating the spring  42  and the valve element  41  in a holder in which the valve port  43 , the valve accommodating hole  44  and the valve seat face  45  are formed, and the cartridge is mounted in a middle portion of the bypass passage  40 . 
         [0048]    Further, in the above-mentioned example, as the compressor, the reciprocating-type compressor which makes use of a double-headed piston is exemplified. However, the compressor is not limited to such a reciprocating-type compressor, and other piston-type compressors or compressors of other types may adopt the substantially same constitution. 
       Reference Signs List 
       [0000]    
       
           1 : compressor 
           6 : front-side shell member 
           8 : rear-side shell member 
           12 : shaft 
           31 : suction passage 
           32 : suction port 
           33 : discharge passage 
           34 : discharge port 
           40 : bypass passage 
           41 : valve element 
           42 : spring 
           43 : valve port 
           44 : valve accommodating hole 
           45 : valve seat face