Patent Publication Number: US-8991300-B2

Title: Variable displacement swash plate type compressor

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a variable displacement swash plate type compressor capable of controlling displacement by controlling the inclination angle of the swash plate based on the pressure in a crank chamber. 
     A variable displacement swash plate type compressor includes a swash plate, which is accommodated in a crank chamber. The inclination angle of the swash plate is variable. High-pressure control gas is supplied to the crank chamber, and the pressure in the crank chamber is controlled by controlling the amount of the supplied control gas. Accordingly, the compressor displacement is controlled. Specifically, when the crank chamber pressure is raised, the inclination angle of swash plate is reduced, which reduces the stroke of the pistons in the cylinder bores. Accordingly, the displacement is reduced. In contrast, when the crank chamber pressure is lowered, the inclination angle of the swash plate is increased, which increases the stroke of the pistons in the cylinder bores. Accordingly, the displacement is increased. 
     However, high-pressure refrigerant gas, which has been compressed in compression chambers, may be introduced as blow-by gas into the crank chamber through between each piston and the corresponding cylinder bore (through side clearances). When such blow-by gas enters the crank chamber, the crank chamber pressure cannot be set to a control target value, and the inclination angle of the swash plate deviates from a desired angle. Desired displacement thus cannot be achieved. 
     In a case in which a variable displacement swash plate type compressor is installed in a refrigerant circuit (external refrigerant circuit) of a vehicle air conditioner, it is preferable that the amount of lubricant circulated in the refrigerant circuit be limited to improve the cooling efficiency. However, if the amount of lubricant circulated in the refrigerant circuit is reduced, lubrication between the pistons and the cylinder bores deteriorates, which will increase wear of the cylinder bores. As a result, the amount of blow-by gas entering the crank chamber is increased. 
     For example, Japanese Laid-Open Patent Publication No. 2003-206856 discloses a technology for reducing wear of cylinder bores. As shown in  FIG. 6 , a piston  90  disclosed in the document has a tapered surface  92  at the distal end of the outer circumferential surface of a columnar portion  91 . The piston  90  also has a chamfered portion  93 , which is continuous with the tapered surface  92 . The diameter of the outer circumferential surface of the columnar portion  91  decreases toward the distal end. When coating is applied to the outer circumferential surface of the piston  90 , the above described configuration prevents the coating material from remaining at the distal portion of the columnar portion  91 , so that no annular protrusion is formed at the distal portion. As a result, the cylinder bore is prevented from being scratched by such an annular protrusion. Wear of the cylinder bore is thus reduced. Further, the structure of the tapered surface  92 , the chamfered portion  93 , and the decreasing diameter toward the distal end of the piston  90  allows lubricant to enter between the piston  90  and the cylinder bore. 
     However, according to the document, the shape of the piston  90  changes from the distal end toward the proximal end, particularly sharply at a section from the chamfered portion  93  to the tapered surface  92 . As a result, the side clearance, which is formed between the piston  90  and the cylinder bore, sharply narrows. This makes it difficult for lubricant to enter between the piston  90  and the cylinder bore. Accordingly, the lubrication between the piston  90  and the cylinder bore deteriorates, and wear of the cylinder bore increases. As a result, the entering amount of blow-by gas will be increased. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a variable displacement swash plate type compressor that reduces wear of a cylinder bore and the amount of blow-by gas. 
     To achieve the foregoing objective and in accordance with one aspect of the present invention, a variable displacement swash plate type compressor is provided that includes a cylinder block, in which a plurality of cylinder bores are formed, a plurality of single-headed pistons, a drive shaft, a swash plate, a crank chamber, and a plurality of compression chambers. Each piston is accommodated in one of the cylinder bores and has a main body and a skirt. The skirt is formed at a position closer to a proximal end of the piston than the main body. The swash plate rotates integrally with the drive shaft and is engaged with the skirts. The crank chamber accommodates the swash plate. Each compression chamber is defined in one of the cylinder bores by the associated piston main body. The displacement of the compressor is controllable by controlling the inclination angle of the swash plate by changing the pressure in the crank chamber. Each piston main body has a distal portion located on an end corresponding to the compression chamber. A tapering portion and an arcuate portion are formed in the distal portion. The arcuate portion is continuous with an end of the tapering portion that is closer to the compression chamber. The tapering portion and the arcuate portion each have a diameter that increases toward the skirt. The tapering portion has a tapering angle that is in a range from 0.45 degrees to 1.5 degrees. The distance between the distal end of the piston main body and a starting point of the tapering portion on an end closer to the skirt is set in a range from 1.5 mm to 5.0 mm. 
     Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention, together with objects and advantages thereof, may best be understood by reference to the following description of the presently preferred embodiments together with the accompanying drawings in which: 
         FIG. 1  is a cross-sectional view illustrating a variable displacement swash plate type compressor according to one embodiment of the present invention; 
         FIG. 2  is a side view illustrating a piston of the variable displacement swash plate type compressor; 
         FIG. 3   a  is a graph showing the relationship between the length of a crown and the amount of blow-by gas in a low displacement state; 
         FIG. 3   b  is a graph showing the relationship between the length of the crown and the maximum contact surface pressure in the maximum displacement state; 
         FIG. 3   c  is a graph showing the relationship between the tapering angle and the maximum contact surface pressure; 
         FIG. 4  is a graph showing the relationship between the flowing amount of lubricant between the piston main body and the cylinder bore and the rotational angle of the drive shaft; 
         FIG. 5   a  is a graph showing the relationship between the position of an introduction groove and the flowing amount of blow-by gas; 
         FIG. 5   b  is a graph showing the relationship between the position of the introduction groove and the contact pressure applied to the cylinder bore; and 
         FIG. 6  is a partial cross-sectional view showing a piston of a background art. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     One embodiment of the present invention will now be described with reference to  FIGS. 1 to 5 . 
     As shown in  FIG. 1 , the housing of a variable displacement swash plate type compressor  10  (hereinafter, referred to as a compressor  10 ), which is mounted on a vehicle, includes a cylinder block  12 . A front housing member  11  is joined to an end of the cylinder block  12 , and a rear housing member  13  is joined to the other end with an intercalated member  14  in between. The front housing member  11  and the cylinder block  12  define a crank chamber  15 . The front housing member  11  and the cylinder block  12  rotationally support a drive shaft  16  via a radial bearing  30 . The drive shaft  16  extends through the crank chamber  15 . 
     A pulley  17  is rotationally supported by a distal outer wall of the front housing member  11  via an angular bearing  18 . The pulley  17  is coupled to the distal end of the drive shaft  16 . The pulley  17  is directly connected to a vehicle engine  20 , which serves as an external drive source, via a belt  19 . That is, no clutch mechanism such as en electromagnetic clutch is provided between the pulley  17  and the vehicle engine  20 . Thus, during operation of the vehicle engine  20 , the drive shaft  16  is rotated by drive force transmitted by the belt  19  and the pulley  17 , which function as a power transmission mechanism. In this manner, the drive shaft  16  receives rotational drive force from the vehicle engine  20  via the clutchless power transmission mechanism. 
     In the crank chamber  15 , the rotary support  22  is fixed to the drive shaft  16  to rotate integrally with the drive shaft  16 , and the rotary support  22  is supported by the front housing member  11  via a thrust bearing  44 . The drive shaft  16  supports a swash plate  23 , which is permitted to slide along the central axis N and be inclined relative to the drive shaft  16 . The rotary support  22  and the swash plate  23  are coupled to each other by a hinge mechanism  24 . The hinge mechanism  24  allows the swash plate  23  to rotate integrally with the drive shaft  16  about the central axis N of the drive shaft  16 . 
     A spring  26  is located between the rotary support  22  and the swash plate  23  to surround the drive shaft  16 . The spring  26  urges the swash plate  23  to tilt the swash plate  23  toward the cylinder block  12 . A stopper ring  28  is attached to the drive shaft  16  at a position between the swash plate  23  and the cylinder block  12 , and a spring  28   a  is fitted about the drive shaft  16  between the stopper ring  28  and the swash plate  23 . When compressed, the spring  28   a  urges the swash plate  23  to tilt toward the rotary support  22 . 
     When the swash plate  23  is inclined toward the rotary support  22  to a position where the swash plate  23  contacts the rotary support  22 , a further inclination of the swash plate  23  is restricted. In this restricted state, the inclination angle of the swash plate  23  is the maximum value. On the other hand, when the swash plate  23  is inclined toward the cylinder block  12  to contact and compress the spring  28   a , a further inclination of the swash plate  23  is restricted. In this restricted state, the inclination angle of the swash plate  23  is the minimum value, which is slightly greater than 0 degrees. 
     The cylinder block  12  has cylinder bores  12   a , which are arranged about the drive shaft  16 . Each cylinder bore  12   a  accommodates a single-headed piston  36 . The piston  36  is permitted to reciprocate and has a diameter of 28 to 40 mm. Each piston  36  is coupled to a peripheral portion of the swash plate  23  by a pair of shoes  23   a  and is reciprocated within the associated cylinder bore  12   a  through rotation of the swash plate  23 . The piston  36  defines a compression chamber  12   b  for compressing refrigerant gas in the cylinder bore  12   a.    
     An annular discharge chamber  39  is defied between the rear housing member  13  and the intercalated member  14 . A suction chamber  38 , which is a zone of a lower pressure than the discharge chamber  39 , is defined at a position inward of the discharge chamber  39 . The intercalated member  14  has suction ports  40 , which communicate with the suction chambers  38 , suction valves  41 , which selectively open and close the suction ports  40 , discharge ports  42 , which communicate with the discharge chambers  39 , and discharge valves  43 , which selectively open and close the discharge ports  42 . 
     When each piston  36  moves from the top dead center to the bottom dead center, refrigerant gas in the corresponding suction chamber  38  is drawn into the cylinder bore  12   a  through the corresponding suction port  40  and the corresponding suction valve  41 . Refrigerant gas drawn into the cylinder bore  12   a  is compressed to a predetermined pressure as the piston  36  is moved from the bottom dead center position to the top dead center position. Then, the gas is discharged to the discharge chamber  39  through the corresponding discharge port  42  and the corresponding discharge valve  43 . 
     The rear housing member  13  has a discharge passage  50 , which communicates with the discharge chamber  39 , and a suction passage  32 , which communicates with the suction chamber  38 . The discharge passage  50  and the suction passage  32  are connected to each other via an external refrigerant circuit  75 . The external refrigerant circuit  75  includes a condenser  76 , which is connected to the discharge chamber  39  via the discharge passage  50 , an expansion valve  77 , which is connected to the condenser  76 , and an evaporator  78 , which is connected to the expansion valve  77 . The suction passage  32  is connected to the evaporator  78 . As described above, the compressor  10  is incorporated in a refrigeration cycle. 
     A bleed passage  34  for connecting the suction chamber  38  with the crank chamber  15  and a supply passage  48  for connecting the discharge chamber  39  with the crank chamber  15  are formed in the cylinder block  12  and the rear housing member  13 . A flow control valve  49  is located in the supply passage  48 . The flow control valve  49  is an electromagnetic valve, which selectively opens and closes the supply passage  48  in accordance with supply and stop of electricity to a solenoid. 
     The flow control valve  49  opens or closes the supply passage  48 , thereby changing the amount of high-pressure refrigerant gas supplied to the crank chamber  15  from the discharge chamber  39 . The pressure in the crank chamber  15  is changed in accordance with the relationship between the supplied amount of the refrigerant gas and the amount of refrigerant gas that is conducted to the suction chamber  38  via the bleed passage  34 . When the pressure in the crank chamber  15  is changed in this manner, the pressure difference between the crank chamber  15  and the cylinder bores  12   a  acts to alter the inclination angle of the swash plate  23 , so that the displacement is adjusted. 
     Specifically, when electricity supply to the flow control valve  49  is stopped, the flow control valve  49  fully opens the supply passage  48 , so that the discharge chamber  39  and the crank chamber  15  communicate with each other. Accordingly, high-pressure refrigerant gas in the discharge chamber  39  is supplied to the crank chamber  15  via the supply passage  48 , so that the pressure in the crank chamber  15  is released to the suction chamber  38  via the bleed passage  34 . This raises the pressure in the crank chamber  15  to minimize the inclination angle of the swash plate  23 . Accordingly, the displacement of the compressor  10  is minimized. 
     In contrast, when electricity is supplied to the flow control valve  49 , the opening degree of the supply passage  48  is made smaller than the fully open state in accordance with the supplied electricity. This reduces the amount of high-pressure refrigerant gas supplied from the discharge chamber  39  to the crank chamber  15  via the supply passage  48 . Also, the pressure in the crank chamber  15  is released to the suction chamber  38  via the bleed passage  34  and is thus lowered. Such pressure reduction increases the inclination angle of the swash plate  23  from the minimum inclination angle, so that the displacement of the compressor  10  is increased from the minimum displacement. 
     The piston  36  will now be described. 
     As shown in  FIG. 2 , the piston  36  has a skirt  36   a , which is engaged with the swash plate  23 , and a columnar piston main body  37 , which is formed integrally with the skirt  36   a . The skirt  36   a  is formed at a proximal end (left end as viewed in  FIG. 2 ) of the piston  36  with respect to the piston main body  37 . A proximal surface  37   a  is formed on an end of the piston main body  37  that corresponds to the skirt  36   a  (proximal end). A distal surface  37   b  is formed on an end of the piston main body  37  that is opposite to the skirt  36   a  (distal end). The proximal surface  37   a  and the distal surface  37   b  are flat. The distance between the proximal surface  37   a  and the distal surface  37   b , that is, the entire length of the piston main body  37 , is a piston length L. 
     A rear peripheral portion  37   c , which forms a right angle, is formed at the periphery of the proximal surface  37   a  of the piston main body  37 . A front peripheral portion  37   d , which has shape other than a right angle, is formed at the periphery of the distal surface  37   b  of the piston main body  37 . 
     A chamfered portion  37   h  is formed at the distal outer circumference of the piston main body  37 . The chamfered portion  37   h  forms a truncated cone the diameter of which decreases toward the distal end of the piston main body  37 . An arcuate portion  37   g , which is continuous with the chamfered portion  37   h , is formed on the outer circumferential surface of the piston main body  37 . The diameter of the arcuate portion  37   g  increases from the end closer to the distal end of the piston main body  37  (the distal surface  37   b ) toward the proximal end (the skirt  36   a ). Further, a tapering portion  37   f , which is continuous with the arcuate portion  37   g , is formed on the outer circumferential surface of the piston main body  37 . The diameter of the tapering portion  37   f  increases from the end closer to the distal end of the piston main body  37  (the distal surface  37   b ) toward the proximal end (the skirt  36   a ). That is, the chamfered portion  37   h , the arcuate portion  37   g , and the tapering portion  37   f  are continuously formed on the outer circumferential surface of the piston main body  37  from the distal end toward the proximal end. The chamfered portion  37   h , the arcuate portion  37   g , and the tapering portion  37   f  form a crown P. 
     The distance between a starting point T of the tapering portion  37   f  and the distal end of the piston main body  37  (the distal surface  37   b ), that is, the length E of the crown P, is set in a range from 1.5 mm to 5.0 mm. 
     When the displacement of the variable displacement swash plate type compressor  10  is low, the limit value of the blow-by gas amount in a range that does not affect the control of pressure in the crank chamber  15  (the limit value of the allowable blow-by gas amount) is represented by Bx. A limit value By is a blow-by gas amount that is less than the limit value Bx, and is more preferable. During a low displacement operation, the load acting on the piston main body  37  due to compression is small, and the side force (lateral force) is also small. Thus, the side force is received only by lubricant film between the piston main body  37  and the cylinder bores  12   a , and the piston main body  37  is scarcely tilted relative to the axis of the cylinder bore  12   a . Therefore, during a low displacement operation, unevenness of the side clearance between the piston main body  37  and the cylinder bores  12   a  is small, so that blow-by gas scarcely leaks. 
     The graph of  FIG. 3(   a ) shows the blow-by gas amount during a low displacement operation, in which blow-by gas is least likely to leak. The graph indicates that the longer the length E of the crown P, the greater the amount of blow-by gas becomes. Therefore, to prevent the blow-by gas amount from exceeding the limit value Bx, the length E of the crown P is preferably set less than or equal to 5.0 mm. To accurately control the displacement of the variable displacement swash plate type the compressor  10 , the limit value of blow-by gas amount is preferably set to the limit value By, which is lower than the limit value Bx. Accordingly, the length E of the crown P is preferably set to be less than or equal to 3.4 mm. In this manner, the upper limit value of the length E of the crown P is determined based on the limit values Bx, By of blow-by gas amount. 
     Regarding the contact surface pressure of the piston main body  37  acting on the cylinder bores  12   a , the maximum value in a range that does not affect the piston main body  37  and the cylinder bore  12   a  (the maximum value of allowable contact surface pressure) is represented by a maximum contact surface pressure Pa. A maximum contact surface pressure Pb is lower than the maximum contact surface pressure Pa. 
     The graph of  FIG. 3(   b ) shows the relationship between the contact surface pressure and the length E of the crown P during the maximum displacement operation. During the maximum displacement operation, the piston main body  37  receives a great load due to compression, and the side force is great. Accordingly, the piston main body  37  is easily tilted relative to the axis of the cylinder bore  12   a . The crown P functions most effectively in this situation. The surface pressure due to solid-to-solid contact between the piston main body  37  and the cylinder bores  12   a  is not generated when a lubricant film is formed between the piston main body  37  and the cylinder bores  12   a.    
     If the length E of the crown P is greater than or equal to 1.5 mm, a lubricant film is formed on the tapering portion  37   f , so that side force is received by the lubricant film. The contact surface pressure between the piston main body  37  and the cylinder bores  12   a  therefore does not exceed the maximum contact surface pressure Pa. Thus, to prevent the contact surface pressure from exceeding the maximum contact surface pressure Pa, the length E of the crown P is preferably set greater than or equal to 1.5 mm. Hence, to reduce the blow-by gas amount and prevent the contact surface pressure from exceeding the maximum contact surface pressure Pa, the length E of the crown P is preferably set in a range from 1.5 mm to 5.0 mm. 
     Likewise, when the limit value of the blow-by gas amount is set to By, the upper limit value of the length E of the crown P is set to be less than or equal to 3.4 mm. In  FIG. 3(   b ), when the maximum contact surface pressure is Pb, the lower limit value of the length E of the crown P is set to 2.8 mm. Accordingly, the length E of the crown P is more preferably set in a range from 2.8 mm to 3.4 mm. 
     A piston  36  in which a maximum contact surface pressure Pa is obtained when the length E of the crown P is 1.5 mm is denoted by sample A, and a piston  36  in which a more preferable maximum contact surface pressure Pb is obtained when the length E of the crown P is 2.8 mm is denoted by sample C. Further, a piston  36  in which a maximum contact surface pressure smaller than the maximum contact surface pressure Pb is obtained when the length E of the crown P is 3.4 mm is denoted by sample D, and a piston  36  in which a maximum contact surface pressure less than the maximum contact surface pressure of sample D is obtained when the length E of the crown P is 5.0 mm is denoted by sample B. In the piston main body  37 , a line that extends parallel with the central axis PL and is located on the circumferential surface of the piston main body  37  is defined as a tangent F. The angle between the tangent F and the tapering portion  37   f , or a tapering angle, is denoted by θ 1 . 
     In the case of sample A, the contact surface pressure between the piston main body  37  and the cylinder bore  12   a  does not exceed the maximum contact surface pressure Pa when the tapering angle θ 1  is in the range from 0.45 degrees to 1.5 degrees, as shown in  FIG. 3(   c ). Also, in the case of sample B, the contact surface pressure between the piston main body  37  and the cylinder bore  12   a  does not exceed the maximum contact surface pressure Pa when the tapering angle θ 1  is in the range from 0.45 degrees to 1.5 degrees. If the tapering angle θ 1  is less than 0.45 degrees, minute projections and recesses on the piston main body  37  and the cylinder bore  12   a  form a restriction between the cylinder bore  12   a  and a part closer to the distal surface  37   b  than the starting point T. Accordingly, lubricant does not reach a part closer to the proximal surface  37   a  than the restriction, so that no lubricant film is formed there. This reduces the length of the lubricant film formed on the tapering portion  37   f  along the central axis PL, and the pressure of the lubricant film is not raised. That is, solid-to-solid contact occurs between the piston main body  37  and the cylinder bore  12   a , which increases the contact surface pressure. 
     On the other hand, if the tapering angle θ 1  is greater than 1.5 degrees, although lubricant is allowed to enter the tapering portion  37   f , the clearance of the piston main body  37  in the circumferential direction is widened. Thus, lubricant flows in the circumferential direction, and lubricant film is hard to form. As a result, solid-to-solid contact occurs between the piston main body  37  and the cylinder bore  12   a , which increases the contact surface pressure. 
     Therefore, when the length E of the crown P is set as described above, the angle of the tapering portion  37   f  is preferably set in a range from 0.45 degrees to 1.5 degrees. 
     Further, when the length E of the crown P is set as described above, the angle of the tapering portion  37   f  is more preferably set in a range from 0.5 degrees to 1.3 degrees. 
     The arcuate portion  37   g  is formed to be gently arcuate, and the chamfered portion  37   h  has a shape that changes more gently than the arcuate portion  37   g . In the piston main body  37 , a line that extends parallel with the central axis PL and is located on the circumferential surface of the piston main body  37  is defined as a tangent F. The angle between the tangent F and the chamfered portion  37   h , or an inclination angle, is denoted by θ 2 . The inclination angle θ 2  is preferably set approximately to 30 degrees. Therefore, the piston main body  37  has a barrel-like shape with its diameter gradually decreasing toward the distal surface  37   b.    
     As shown in  FIG. 2 , an introduction groove  37   k  is formed on the outer circumferential surface of the piston main body  37 , at a position closer to the proximal surface  37   a  than the tapering portion  37   f . The introduction groove  37   k  extends along the entire circumference of the piston main body  37 . The position of the introduction groove  37   k  is preferably determined such that the distance X between the proximal surface  37   a  and introduction groove  37   k  and the piston length L satisfies the expression 0.6&lt;X/L&lt;0.8. 
     The introduction groove  37   k  is provided to supply lubricant between the piston main body  37  and the cylinder bore  12   a  to the entire circumference of the piston main body  37  and to urge the piston main body  37  away from the cylinder bores  12   a . If the depth of the introduction groove  37   k  is less than 0.1 mm, the amount of lubricant retained in the introduction groove  37   k  is reduced so that it will be difficult for the introduction groove  37   k  to spread lubricant to the entire circumference of the piston main body  37 . Thus, the depth of the introduction groove  37   k  is preferably greater than or equal to 0.1 mm. Setting the depth of the introduction groove  37   k  to a value greater than or equal to 0.1 mm allows the introduction groove  37   k  to spread lubricant over the entire circumference of the piston main body  37 , so that unevenness of lubricant film is prevented. As a result, the lubricant film limits tilting of the piston main body  37  to eliminates the unevenness of the side clearance. This reduces the increase in the flowing amount of the blow-by gas due to the side clearance. 
     If the opening width of the introduction groove  37   k  along the axis of the piston main body  37  is less than 0.5 mm, the amount of lubricant in the introduction groove  37   k  is reduced and the above described urging effect is lowered. In contrast, if the opening width of the introduction groove  37   k  is greater than or equal to 1.5 mm, the sealing performance of the lubricant film formed by the lubricant in the introduction groove  37   k  is lowered. Therefore, the opening width of the introduction groove  37   k  along the axis of the piston main body  37  is preferably set greater than or equal to 0.5 mm and less than 1.5 mm. 
     Operation of the compressor  10  will now be described. 
     When the drive shaft  16  rotates as the engine  20  operates, each piston  36  moves from the top dead center position to the bottom dead center position. Accordingly, refrigerant gas in the suction chamber  38  is drawn into the cylinder bore  12   a  via the suction port  40  and the suction valve  41 . At this time, the rear peripheral portion  37   c  of the piston main body  37  slides along the cylinder bore  12   a . Since the rear peripheral portion  37   c  forms a right angle, a small clearance is maintained between the cylinder bores  12   a  and the piston main body  37 . This reduces the likelihood of lubricant leaking to the crank chamber in a great amount. 
     In the graph of  FIG. 4 , the solid line indicates the flowing amount of lubricant in a case in which the pistons  36  of the present embodiment are employed. The line formed by a long dash alternating with one short dash indicates the flowing amount of lubricant in a case in which the rear peripheral portion  37   c  of the piston main body  37  and the front peripheral portion  37   d  of the compression chamber are both chamfered (a piston of Comparison Example 1). As shown in  FIG. 4 , compared to the case of the piston of Comparison Example 1, the flowing amount of lubricant between the cylinder bore  12   a  and the piston main body  37  is small at any rotational angle in the case of the piston  36  of the present embodiment. This indicates that, at the rear peripheral portion  37   c  of the piston main body  37 , likelihood of lubricant leaking in a great amount is reduced. As a result, it is possible to retain lubricant between the piston main body  37  and the cylinder bores  12   a.    
     Refrigerant gas drawn into the cylinder bore  12   a  is compressed to a predetermined pressure as the piston  36  is moved from the bottom dead center position to the top dead center position. Then, the gas is discharged to the discharge chamber  39  via the corresponding discharge port  42  and the corresponding discharge valve  43 . During the period from suction to discharge of refrigerant gas, the piston main body  37  receives side force, which acts to tilt the piston main body  37 . However, since the length E and the tapering angle θ 1  of the crown P are set to appropriate values, a lubricant film is formed between the piston main body  37  and the cylinder bores  12   a . The lubricant film receives the side force to limit the tilting of the piston main body  37 . 
     During the compression stroke, high-pressure refrigerant gas, which has been compressed at the top dead center position, flows as blow-by gas toward the crank chamber  15  through between the piston  36  and the cylinder bores  12   a  (through the side clearance). 
     As described above, the piston main body  37  has the tapering portion  37   f  and the arcuate portion  37   g , and the length E and the tapering angle θ 1  of the crown P are set to appropriate values. The piston  36  is tilted in relation to the central axis PL when receiving compression reaction force. However, during the compression stroke, lubricant is drawn into between the cylinder bore  12   a  and the piston main body  37  by the wedge effect. As a result, a lubricant film is formed between the cylinder bores  12   a  and the piston main body  37 , and the pressure of the lubricant film is increased by the wedge effect. Although a small amount of lubricant is allowed to leak due to the surface roughness of the piston main body  37  and the cylinder bores  12   a , the repulsive force of the lubricant film urges the piston main body  37  away from the cylinder bore  12   a . Thus, the contact surface pressure due to solid-to-solid contact between the cylinder bore  12   a  and the piston main body  37  is lowered, and wear of the cylinder bore  12   a  is reduced. 
     Since the crown P of the piston main body  37  has the chamfered portion  37   h , the arcuate portion  37   g , and the tapering portion  37   f  arranged from the distal end toward the proximal end, the shape of the crown P is gradually changed. Thus, the side clearance between the distal end of the piston main body  37  and the cylinder bore  12   a  gradually decreases toward the proximal end, so that lubricant is reliably drawn into the side clearance when the piston  36  reciprocates. Therefore, lubricant film is formed and maintained between the piston main body  37  and the cylinder bores  12   a.    
     Since the introduction groove  37   k  is provided to supply lubricant between the piston main body  37  and the cylinder bore  12   a  to the entire circumference of the piston main body  37 , unevenness of the lubricant film in the circumference direction is reduced. This allows the lubricant film to reliably exert urging force. As a result, tilting of the piston main body  37  caused by the thickness of the lubricant film (the pressure of the lubricant film) is reduced, which reduces the likelihood of the piston main body  37  unevenly contacting the cylinder bores  12   a . In consequence, the unevenness of the side clearance along the entire circumference of the piston main body  37  is limited. This reduces an increase in the flowing amount of blow-by gas due to the side clearance. 
     The position of the introduction groove  37   k  is determined such that the distance X and the piston length L satisfy the expression 0.6&lt;X/L&lt;0.8. By determining the position of the introduction groove  37   k  in this manner, the flowing amount of the blow-by gas is lower than that in a case in which no introduction groove  37   k  is formed as shown in  FIG. 5(   a ) (reference line J). Further, as shown in  FIG. 5(   b ), by determining the position of the introduction groove  37   k  in this manner, the contact surface pressure between the piston main body  37  and the cylinder bore  12   a  is also lower than that in a case in which no introduction groove  37   k  is formed (reference line J). 
     The above described embodiment has the following advantages. 
     (1) Based on analysis of the amount of blow-by gas and the maximum contact surface pressure, the tapering angle θ 1  of the tapering portion  37   f  in the piston main body  37  is set in a range from 0.45 degrees to 1.5 degrees, and the length E of the crown P is set in a range from 1.5 mm to 5.0 mm. This reduces wear of the cylinder bores  12   a  and the amount of blow-by gas. 
     (2) The tapering portion  37   f  is formed in the crown P of the piston main body  37  to have a tapering angle θ 1  in a range from 0.45 degrees to 1.5 degrees. This allows a lubricant film to be reliably formed between the cylinder bores  12   a  and the piston main body  37 , thereby reducing solid-to-solid contact between the piston main body  37  and the cylinder bores  12   a . Therefore, the contact surface pressure is prevented from reaching the maximum contact surface pressure Pa, and wear of the cylinder bore  12   a  is reduced. 
     (3) The length E of the crown P is set in a range from 1.5 mm to 5.0 mm. When the piston main body  37  is not significantly affected by side force, for example, during a low displacement operation, setting the length E of the crown P to be less than or equal to 5.0 mm ensures a length along the central axis PL of lubricant film formed between the cylinder bore  12   a  and the piston main body  37 . This limits tilting of the piston main body  37  caused by side force and the amount of blow-by gas that flows through between the cylinder bores  12   a  and the piston main body  37 . On the other hand, when the piston main body  37  is greatly affected by side force, for example, during a large displacement operation, setting the length E of the crown P to be greater than or equal to the lower limit value of 1.5 mm reliably forms lubricant film, which receives the side force. As a result, the contact surface pressure is prevented from reaching the maximum contact surface pressure Pa while reducing the amount of blow-by gas, and wear of the cylinder bore  12   a  is reduced. 
     (4) The tapering portion  37   f  formed in the piston main body  37  exerts the wedge effect. Due to the wedge effect, lubricant is drawn into between the cylinder bore  12   a  and the piston main body  37 , and the pressure of the lubricant film is increased. The repulsive force of the lubricant film urges the piston main body  37  away from the cylinder bore  12   a . Thus, the contact surface pressure due to solid-to-solid contact between the cylinder bore  12   a  and the piston main body  37  is lowered, and wear of the cylinder bore  12   a  is reduced. 
     (5) The position of the introduction groove  37   k  is set such that the distance X and the piston length L satisfy the expression 0.6&lt;X/L&lt;0.8. If the introduction groove  37   k  is excessively close to the distal end of the piston main body  37 , lubricant cannot be readily supplied to the entire piston main body  37 . The configuration prevents such a drawback. That is, by arranging the introduction groove  37   k  at an appropriate position, lubricant film can be formed substantially over the entire space between the piston main body  37  and the cylinder block  12 , so that the contact pressure between the cylinder bore  12   a  and the piston main body  37  is reduced. 
     (6) Further, by setting the position of the introduction groove  37   k  in a manner described above, the introduction groove  37   k  is prevented from being excessively close to the proximal end of the piston main body  37 . In other words, the introduction groove  37   k  is prevented from being excessively far from the compression chamber  12   b . Therefore, flow of blow-by gas is restricted at the distal surface  37   b  of the piston main body  37 , so that the amount of blow-by gas flowing to the crank chamber  15  is effectively reduced. 
     (7) The rear peripheral portion  37   c  of the piston main body  37  forms a right angle. Thus, the side clearance between the rear peripheral portion  37   c  of the piston main body  37  and the cylinder bores  12   a  is maintained at a constant value without being widened. This reduces the likelihood of lubricant leaking in a great amount at the rear peripheral portion  37   c  of the piston main body  37 . As a result, lubricant is retained between the piston main body  37  and the cylinder bores  12   a  to ensure the thickness of lubricant film, which reduces the likelihood of solid-to-solid contact between the piston main body  37  and the cylinder bores  12   a.    
     (8) The crown P is not simply formed on the piston main body  37 . Instead, the parameters such as the position and angle of the tapering portion  37   f  and the position of the introduction groove  37   k  are considered and determined comprehensively to reduce wear of the cylinder bore  12   a  and the amount of blow-by gas flowing to the crank chamber  15 . 
     (9) The piston  36  has at the distal end of the piston main body  37  the chamfered portion  37   h , the arcuate portion  37   g , which is continuous with the chamfered portion  37   h , and the tapering portion  37   f , which is continuous with the arcuate portion  37   g . Thus, the shape of the piston main body  37  gradually changes from the distal end toward the proximal end. Therefore, the side clearance between the distal end of the piston main body  37  and the cylinder bore  12   a  gradually decreases toward the proximal end, so that lubricant is reliably drawn into the side clearance when the piston  36  reciprocates. As a result, the lubricant film between the piston main body  37  and the cylinder bores  12   a  is maintained, and the amount of the blow-by gas leaking to the crank chamber  15  is reduced by the sealing performance of the lubricant film. 
     (10) The tapering angle of the tapering portion  37   f  is set to be small in a range from 0.45 degrees to 1.5 degrees, and the arcuate portion  37   g  and the chamfered portion  37   h  form a predetermined space between the cylinder bores  12   a  and the piston main body  37 . This allows adequate amount of lubricant to be reliably supplied to the tapering portion  37   f . Also, when the piston  36  is installed in the cylinder bore  12   a , the chamfered portion  37   h  prevents a corner of the tapering portion  37   f  from forming a dent in the cylinder bores  12   a  by scratching. If blow-by gas passes through such a dent in the cylinder bores  12   a , the amount of blow-by gas will be increased. The above described embodiment prevents such a possible drawback, thereby allowing the amount of blow-by gas to be reliably controlled. 
     (11) The tapering angle θ 1  of the tapering portion  37   f  is more preferably set in a range from 0.5 degrees to 1.3 degrees, and the length E of the crown P is more preferably set in a range from 2.8 mm to 3.4 mm. These settings reduce the amount of blow-by gas to a level lower than the maximum value that is allowable during a low displacement operation, and reduce the maximum contact surface pressure to a level lower than the maximum value in a range that does not affect the piston main body  37  and the cylinder bore  12   a.    
     The above described embodiment may be modified as follows. 
     In the above described embodiment, the crown P is formed by the chamfered portion  37   h , the arcuate portion  37   g , and the tapering portion  37   f . However, the chamfered portion  37   h  may be omitted, for example, so that the crown P is formed only by the arcuate portion  37   g  and the tapering portion  37   f.    
     In the above described embodiment, the chamfered portion  37   h  forms a truncated cone the diameter of which decreases toward the distal end of the piston main body  37 . However, the chamfered portion  37   h  may be formed such that the radius of curvature is gradually increased toward the distal end of the piston main body  37 . 
     In the above described embodiment, the rear peripheral portion  37   c  of the piston main body  37  forms a right angle. However, the rear peripheral portion  37   c  may be arcuate or tapered. 
     In the above described embodiment, the compressor  10  receives rotational drive force from the vehicle engine  20  via the clutchless power transmission. However, the compressor  10  may receive rotational drive force from the vehicle engine via a clutch-type power transmission mechanism. 
     Therefore, the present examples and embodiments are to be considered as illustrative and not restrictive and the invention is not to be limited to the details given herein, but may be modified within the scope and equivalence of the appended claims.