Abstract:
A piston-operated compressor, of swash plate type and using CO 2  as a refrigerant, having a casing member in which a cylinder bore is formed to have a cylindrical peripheral wall surface and a piston reciprocating for compression in the cylinder bore and being formed of an aluminum alloy. The outer peripheral surface of the piston is coated with a film of a fluororesin material, and a piston ring of an iron metal is fitted in the neighborhood of the top portion of the piston to permit the CO 2  refrigerant to be compressed under high pressure. A first oil groove is formed in peripheral direction in parallel to and below the vicinity of the groove at the top portion of the piston in which the piston ring is fitted, and a second oil groove is formed below the first oil groove extending along the axial direction in parallel with the central axis of the piston.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a swash plate type refrigerant compressor using CO 2  as a refrigerant. More particularly, the present invention relates to a swash plate type piston-operated refrigerant compressor incorporating therein pistons reciprocating to compress the refrigerant and having an improved sliding performance and an extended operating life. 
     2. Description of the Related Art 
     Generally, a single-headed piston operated swash plate type compressor used for a vehicle climate control system includes a swash plate or a cam plate mounted on the drive shaft in a crank chamber, so that the rotation of the swash plate cooperating with the drive shaft is converted into the linear motion of the pistons inserted in cylinder bores. With the reciprocation of the pistons, the refrigerant gas returning from an external refrigeration system is sucked into the cylinder bores from a suction chamber and, after being compressed, is discharged into a discharge chamber. Specifically, many single-headed swash plate type compressors are so configured that the refrigerant returned gas is introduced directly into the cylinder bores without passing through the crank chamber as described above. The lubrication of the sliding portions and elements arranged in the crank chamber, therefore, are primarily dependent on the lubricant supplied to the crank chamber together with the blow-by gas. 
     The amount of the blow-by gas depends on the size of the fitting gap between the cylinder bores and the pistons. For supplying enough lubricant to properly lubricate the sliding portions and elements in the crank chamber, the fitting gap is required to have an appreciable size. In such a case, the problem of reduced compression efficiency is posed. 
     The practical application of CO 2  as a replacement refrigerant has recently been favored for environmental protection. Nevertheless, with a compressor using CO 2  (carbon dioxide gas) as a refrigerant, it is difficult to satisfy the pressure requirements. In a compressor employing an ordinary simple seal method with the cylinder bores and the pistons snugly fitted with each other without using any special sealing means between them, the amount of blow-by gas extremely increases to deteriorate the compressing performance. In view of this, a piston ring, which has thus far attracted little attention for application to an air-conditioning compressor, has recently become important. 
     Even when the piston ring is used, however, the large difference of the pressure acting on the operating end and the rear end of each piston at the time of compression and the high density of the refrigerant gas increases the gas flow rate, in the same passage area, considerably over the conventional compressor using the fluorinated hydrocarbon gas. 
     When the pistons move from the bottom dead center toward the top dead center for compressing the refrigerant gas, the compression reaction force and the inertia force of the pistons act on the swash plate, and the force thus acting on the swash plate is exerted on the pistons as a reaction force. In view of the fact that the swash plate is inclined with respect to a plane perpendicular to the center axis of the drive shaft, part of the force acting on the pistons is exerted in such a direction as to press the pistons against the inner periphery of the cylinder bores. Namely, the respective pistons receive side forces from the inner peripheral surface of the corresponding cylinder bores. Especially in the case of the CO 2  refrigerant, the side force is so great that the pistons unavoidably come into direct contact with the cylinder bores even if piston rings are fitted on the pistons. 
     SUMMARY OF THE INVENTION 
     Accordingly, an object of the present invention is to provide a swash plate type piston-operated refrigerant compressor using the CO 2  refrigerant in which the blow-by gas amount is limited in cooperation with the piston ring mounted on the pistons while at the same time preventing direct contact between the cylinder bores and the pistons made of metals of the same type. 
     Another object of the invention is to provide a swash plate type refrigerant compressor in which superior lubrication of the piston sliding portion is secured and a sufficient amount of lubricant can be supplied to the sliding elements and portions including the swash plate, the shoes, the hinge mechanism and the bearings in the crank chamber. 
     In accordance with the present invention, there is provided a swash plate type refrigerant compressor which comprises: 
     at least a casing having at least a cylinder bore and a crank chamber; 
     a drive shaft supported rotatably on the casing; 
     a swash plate mounted around the drive shaft to be rotated simultaneously with the drive shaft in the crank chamber; and 
     at least a piston having a top portion inserted into the cylinder bore for compression operation; 
     wherein the piston operatively engaged with the swash plate acts in the cylinder bore to compress the CO 2  refrigerant in response to the rotation of the drive shaft; 
     wherein a peripheral wall extending around the cylinder bore and the piston is formed of an aluminum alloy as a base metal; and 
     wherein the piston has a central axis and an outer peripheral surface, formed around the central axis, coated with a film of fluororesin material, the piston being provided with a piston ring mounted at a position adjacent to the top portion of the piston. 
     In the described compressor, the blow-by gas amount is determined by the width of the closed gap of the piston ring and the fitting gap between the cylinder bores and the pistons. Since the fluororesin film is formed on the outer peripheral surface of the pistons, however, direct contact is surely avoided between the metals, of the same type, of the cylinder bores and the pistons. Thus, the fitting gap is minimized so that the blow-by gas amount, i.e. the leakage amount of the compressed refrigerant is reduced to prevent the reduced performance of the compressor. At the same time, the surface contact through the fluororesin film can sufficiently resist a large side force. 
     Preferably, the casing having the cylinder bores is formed of a hypereutectic aluminum-silicon alloy and the piston ring is made of an iron metal. 
     The use of a hyper eutectic aluminum-silicon alloy for the casing as described above makes it possible to sufficiently resist the sliding with the piston ring made of an iron metal. 
     Also, preferably, in a compressor having a first oil groove extending in the peripheral direction in parallel and below a piston ring groove in which the piston ring is mounted, and a second oil groove extending along an axial direction below the first oil groove, the lubricant passage area can be increased for a lower viscous resistance without increasing the gas flow rate. Therefore, the lubricant can be held in the fitting boundary with the cylinder bores. 
     Further, assume that the second oil groove is formed in such a position as to be partly exposed to the interior of the crank chamber at least when the pistons reach the bottom dead center. Even when the refrigerant compressor is of variable displacement type with an extremely small angle of inclination of the swash plate, the lubricant is positively supplied into the crank chamber from the second oil groove, and therefore superior lubrication is achieved. Furthermore, in the case where the second oil groove is formed on the outer peripheral surface of the pistons where the effect of the side force can be avoided as far as possible, the second oil groove is not strongly pressed against the cylinder bores. Therefore, the wear and damage to both the pistons and the cylinder bores can be prevented. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features and advantages will be made more apparent from the detailed description taken in conjunction with the accompanying drawings, wherein: 
     FIG. 1 is a longitudinal cross-sectional view of a swash plate type refrigerant compressor according to an embodiment of the present invention; 
     FIG. 2 is an enlarged sectional view of an essential portion of the compressor of FIG. 1, illustrating, with exaggeration, the piston tilted at the top dead center; 
     FIG. 3 is a perspective view of the piston according to an embodiment of the present invention; 
     FIG. 4A is a graphical view showing the relation between the rotational angle of the swash plate plotted along the abscissa and the magnitude of the side force acting on each piston plotted along the ordinate; and 
     FIG. 4B is a diagrammatic view to explain the phase around the piston provided with a second oil groove formed therein. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, a front housing  1  is coupled to the front end surface of a cylinder block  2 . A rear housing  3  is coupled to the rear end surface of the cylinder block  2  through a valve plate  4 . The front housing  1 , the cylinder block  2  and the rear housing  3  constitute members of a compressor casing. A suction chamber  3   a  and a discharge chamber  3   b  are formed between the rear housing  3  and the valve plate  4 . The refrigerant gas (CO 2 ) from an external refrigeration circuit (not shown) is introduced directly into the suction chamber  3   a  through an inlet port  3   c.    
     The valve plate  4  includes suction ports  4   a,  a suction valve  4   b,  a discharge port  4   c  and a discharge valve  4   d.  A crank chamber  5  is formed between the front housing  1  and the cylinder block  2 . A drive shaft  6  is rotatably supported on the front housing  1  and the cylinder block  2  through a pair of bearings  7  and arranged through the crank chamber  5 . A support hole  2   b  is formed at the central portion of the cylinder block  2 . The rear end of the drive shaft  6  is inserted into the support hole  2   b,  and the rear end thereof is supported on the inner peripheral surface of the support hole  2   b  through the bearings  7 . 
     A lug plate  8  is fixed on the drive shaft  6 . A swash plate  9  is supported on the drive shaft  6  slidably and movably in the direction along the axis L thereof in the crank chamber  5 . The swash plate  9  is coupled to the lug plate  8  through a hinge mechanism  10 . The hinge mechanism  10  includes a support arm  19  formed on the lug plate  8  and a pair of guide pins  20  formed on the swash plate  9 . The guide pins  20  are slidably inserted into a pair of guide holes  19   a,  respectively, formed in the support arm  19 . The hinge mechanism  10  is adapted to rotate the swash plate  9  integrally with the drive shaft  6 . Further, the hinge mechanism  10  guides the swash plate  9  to move in the direction along the axis L and to be inclined. 
     A plurality of cylinder bores  2   a  are formed in the cylinder block  2  around the drive shaft  6  and extend in the direction along the axis L. A single-headed piston  11  is housed in the cylinder bores  2   a.  The tail of the piston  11  is formed with a groove  11   a.  The hemispherical portions of a pair of shoes  12  are fitted relatively movably within the opposed inner wall surfaces of the groove  11   a.  The swash plate  9  is held slidably between the flat portions of the shoes  12 . The rotational motion of the swash plate  9  is converted into the reciprocal linear motion of the piston  11  through the shoes  12 , so that the piston  11  longitudinally reciprocates in the cylinder bores  2   a.  In a suction stroke, when the piston  11  moves from its top dead center toward its bottom dead center, the refrigerant gas in the suction chamber  3   a  pushes a suction valve  4   b  from a suction port  4   a  to open the latter and flows into the cylinder bores  2   a.  In a compression stroke, when the piston  11  moves from the bottom dead center to the top dead center, on the other hand, the refrigerant gas in the cylinder bores  2   a  is compressed, pushes a discharge valve  4   d  from a discharge port  4   c  to open the port  4   c  and is discharged into a discharge chamber  3   b.    
     A thrust bearing  21  is arranged between the lug plate  8  and the inner surface of the front housing  1 . With the compression of the refrigerant gas, the compression reaction force is exerted on the piston  11 , This compression reaction force is received by the front housing  1  through the piston  11 , the swash plate  9 , the lug plate  8  and the thrust bearing  21 . 
     As shown in FIGS. 1 to  3 , the piston  11  is formed integrally with a stopper  22 . The stopper  22  has a peripheral surface of substantially the same diameter as the inner peripheral surface of the front housing  1 . The peripheral surface of the stopper  22  is in contact with the inner peripheral surface of the front housing  1  in order to prevent the rotation of the piston  11  about the center axis S. 
     As shown in FIG. 1, the compressor has a gas supply passage  13  fluidly connecting the discharge chamber  3   b  and the crank chamber  5 . Specifically, an end of the gas supply passage  13  is open to the crank chamber  5 , and the other end thereof is connected to an electromagnetic valve  14  mounted on the rear housing  3 . The gas supply passage  13  extends from the electromagnetic valve  14  to the discharge chamber  3   b.  In other words, the electromagnetic valve  14  is arranged midway in the gas supply passage  13 . 
     The electromagnetic valve or solenoid valve  14  has a solenoid  14   a.  Upon energization of the solenoid  14   a,  a valve body  14   b  closes a valve hole  14   c.  When the solenoid  14   a  is deenergized, on the other hand, the valve body  14   b  opens the valve hole  14   c.    
     A gas withdrawal passage  6   a  is formed in the drive shaft  6 . The gas withdrawal passage  6   a  has an inlet open to the crank chamber  5 , forward of the drive shaft  6   a,  and an outlet open into the support hole  2   b,  rearward of the drive shaft  6   a.  A gas withdrawal hole  2   c  is connected to the interior of the support hole  2   b  and the suction chamber  3   a.  When the gas supply passage  13  is closed at the position of the valve hole  14   c  with the solenoid  14   a  energized, the high-pressure refrigerant gas in the discharge chamber  3   b  is not supplied to the crank chamber. Under this condition, the refrigerant gas in the crank chamber  5  only flows out into the suction chamber  3   a  through the gas supply passage  6   a  and the gas withdrawal hole  2   c,  so that the internal pressure of the crank chamber  5  approaches the low internal pressure of the suction chamber  3   a.  As a result, the difference is reduced between the internal pressure of the crank chamber  5  and the internal pressure of the cylinder bores  2   a,  and as shown in FIG. 1, the inclination angle of the swash plate  9  (the angle of inclination from a plane perpendicular to the axis of rotational of the drive shaft  6 ) becomes maximum, thereby maximizing the discharge capacity of the compressor. 
     AS long as the valve hole  14   c  is open with the solenoid  14   a  deenergized, the high-pressure refrigerant gas in the discharge chamber  3   b  is supplied through the gas supply passage  13  to the crank chamber  5  so that the internal pressure in the crank chamber  5  increases. As a result, the difference increases between the internal pressure of the crank chamber  5  and the internal pressure of the cylinder bores  2   a,  until finally the inclination angle of the swash plate  9  reaches a minimum thereby to minimize the discharge capacity of the compressor. 
     The swash plate  9  has a stop protrusion  9   a  formed on the front side thereof, which is brought into contact with the lug plate  8  and thus the swash plate is restricted to not exceed a predetermined maximum inclination angle. The swash plate  9  is also restricted to a minimum inclination angle by being brought into contact with a ring  15  mounted on the rear portion of the drive shaft  6 . 
     As described above, the intermediate portion of the gas supply passage  13  is closed and opened in response to the energization and deenergization of the solenoid  14   a  of the solenoid valve  14 . Thus, the internal pressure of the crank chamber  5  is regulated. With a change in the internal pressure of the crank chamber  5 , the difference also changes between the internal pressure of the crank chamber  5  exerted on the front surface (the left side in FIG. 1) of the piston  11  and the internal pressure of the cylinder bores  2   a  exerted on the rear surface (the right side subjected to compression in FIG. 1) of the piston  11 . Thus, the inclination angle of the swash plate  9  coupled to the piston  11  through the shoes  12  also undergoes a change. The change in the angle of inclination of the swash plate  9  causes a change in the stroke amount of the piston  11  to thereby regulate the discharge capacity of the compressor. The solenoid  14   a  of the electromagnetic valve  14  is energized or deenergized selectively in accordance with the information such as the cooling load under the control of a controller (not shown). In other words, the discharge capacity of the compressor is regulated in accordance with the cooling load. 
     As a feature of the present invention, the cylinder block  2  having the cylinder bores  2   a  and the piston  11  are fabricated of an aluminum alloy, or preferably a hyper eutectic aluminum-silicon alloy. In the neighborhood of the apex of the outer peripheral surface of the piston  11 , an annular groove  25   a  is formed, into which the piston ring  25  is fitted. A fluororesin (polytetrafluoroethylene) film is formed on the outer peripheral surface of the piston  11  for avoiding direct contact with a metal of the same type and minimizing the fitting gap K with the cylinder bores  2   a.    
     Further, each piston  11  is formed with a later-described oil groove for holding the lubricant against the corresponding cylinder bores  2   a  and assuring a positive oil supply into the crank chamber  5 . 
     More specifically, as shown in FIG. 3, a first oil groove  16  is formed extending along the peripheral direction in parallel to and in the area below the annular groove  25   a  formed in the outer peripheral surface of the piston  11 . According to this embodiment, the first oil groove  16  is formed in annular fashion around the whole periphery of the piston  11 . The first oil groove  16  is not exposed into the crank chamber  5  from inside the cylinder bores  2   a  when the piston  11  moves to the bottom dead center thereof. 
     The piston  11  is further formed with a second oil groove  17 . Specifically, the second oil groove  17  is formed extending from the area further below the first oil groove  16  along the center axis S of the piston  11 . The second oil groove  17  is provided and configured as described hereinbelow. 
     As shown in FIG. 4B, suppose a straight line M is drawn extending through the center axis L of the drive shaft  6  and the center axis S of the piston  11  when the piston  11  is viewed from the side thereof where the rotational direction R of the drive shaft  6  indicated by the arrow is clockwise (when the piston  11  is viewed from the tail thereof in FIG.  4 B). Of the intersections P 1 , P 2 . between the straight line M and the peripheral surface of the piston  11 , the intersection P 1  far from the center axis L of the drive shaft  6  is assumed to be the 12 o&#39;clock position. In this case, the second oil groove  17  is formed in the range E of the 9 o&#39;clock position to the 10:30 position on the peripheral surface of the piston  11 . Further, the second oil groove  17  is formed at such a position and with such a length as not to be exposed to the interior of the crank chamber  5  when the piston  11  moves to the vicinity of the top dead center. 
     In the compressor described above, when the piston  11  moves from top dead center to bottom dead center in suction stroke, the refrigerant gas in the suction chamber  3   a  is sucked into the cylinder bores  2   a.  In the process, part of the lubricant contained in the refrigerant gas attaches to the inner peripheral surface of the cylinder bores  2   a.  In the compression stroke when the piston  11  moves from the bottom dead center to the top dead center, on the other hand, the refrigerant gas in the cylinder bores  2   a  is compressed and discharged into the discharge chamber  3   b.  At the same time, part of the refrigerant gas that has passed through the closed gap of the piston ring  25  leaks into the crank chamber  5  as a blow-by gas through the limited fitting gap K between the outer peripheral surface of the piston  11  and the inner peripheral surface of the cylinder bores  2   a.    
     The lubricant that has entered the fitting gap K together with the blow-by gas, on the other hand, is trapped and stored in the first oil groove  16  with the movement of the piston  11 . When the piston  11  is in a compression stroke, the internal pressure of the oil groove  16  increases due to the blow-by gas in the fitting gap K. The second oil groove  17 , however, is exposed at least partially in the crank chamber  5  in other than the case where the piston  11  moves to the vicinity of the top dead center. The internal pressure of the second oil groove  17 , therefore, is equal to or only slightly higher than the internal pressure of the crank chamber  5 . Thus, the differential pressure between the oil grooves  16 ,  17  in spaced opposed relation to each other through the fitting gap K causes the lubricant in the first oil groove  16  to flow into the second oil groove  17 . In the process, unlike the refrigerant gas constituting a compressive fluid, the viscous resistance of the oil component high in viscosity is affected by the length. In view of this, the length is reduced by forming the second oil groove  17 , while at the same time enlarging the area of the lubricant passage in the long seal portion thereby to attenuate the viscous resistance. In this way, a smooth sliding motion is secured in the fitting boundary with the cylinder bores  2   a.  Also, the lubricant in the second oil groove  17  is supplied, through the groove portion exposed in the crank chamber  5 , to the sliding portions in the crank chamber  5 , i.e. the relative sliding portions of the swash plate  9 , the shoes  2  and the piston  11 , thereby to lubricate those portions sufficiently. 
     The reaction force (hereinafter referred to as the side force) is exerted on the piston  11 , while in reciprocal motion, from the inner peripheral surface of the cylinder bores  2   a  due to the compression reaction force and its own inertia. As a result, the second oil groove  17  is preferably formed at a position on the peripheral surface of the piston  11  as free of the effect of the side force as possible. 
     More specifically, as shown in FIG. 2, when the piston  11  is in the vicinity of top dead center, the compression reaction force exerted on the piston  11  reaches a maximum. This compression reaction force and the force of inertia of the piston  11  act on the swash plate  9 . Therefore, the piston  11  is subjected to a large reaction force Fs corresponding to the resultant force of the compression reaction force and the force of inertia from the swash plate  9  tilted with respect to the plane perpendicular to the center axis L of the drive shaft  6 . This reaction force Fs can be decomposed into a component force F 1  along the direction of movement of the piston  11  and a component force f 2  along the center axis L of the drive shaft  6 . The component force f 2  causes the tail of the piston  11  to tilt toward the component force f 2 . For this reason, the peripheral surface of the tail of the piston  11  is pressed against the inner peripheral surface in the vicinity of the opening of the cylinder bores  2   a  with a force corresponding to the component force f 2 . In other words, the peripheral surface of the tail of the piston  11  is subjected to a large reaction force (side force) Fa corresponding to the component force f 2  from the inner peripheral surface in the vicinity of the opening of the cylinder bores  2   a.    
     The position at which the side force Fa acts on the piston  11  changes with the reciprocal motion of the piston  11 . During the period from the time point when the piston  11  is located at the top dead center to the time point when the swash plate rotates by 90° in the direction of arrow R, for example, the compressed refrigerant gas staying in the cylinder bores  2   a  is expanded again with the movement of the piston  11  from top dead center to bottom dead center. After the end of the reexpansion, the refrigerant gas starts to be sucked into the cylinder bores  2   a.  In the process, the compression reaction force is not exerted on the swash plate  9 , and the force F 0  acting on the swash plate  9  is substantially equal to the force of inertia of the piston  11 . Thus, the piston  11  is subjected to the reaction force Fs mainly based on the force of inertia from the swash plate  9 . This reaction force Fs can be decomposed into a component force f 1  along the direction of movement of the piston  11  and a component force f 2  substantially along the rotational direction R of the swash plate  9 , in accordance with the inclination angle of the swash plate  9 . The component force f 2  causes the tail of the piston  11  to tilt in the direction of the component force f 2 . As a result, the piston  11  is subjected to the side force Fa corresponding to the component force f 2  from the inner peripheral surface in the vicinity of the opening of the cylinder bores  2   a.  Actually, however, under this condition, the force F 0 acting on the swash plate  9  becomes substantially zero. Therefore, the side force Fa is not substantially exerted on the piston  11 . 
     When the swash plate  9  rotates by 90° in the direction of the arrow R and the piston  11  comes to the bottom dead center thereof, the direction of the component force f 2  exerted on the piston  11  is reversed from the case of FIG. 2 (where the piston  11  is located at top dead center). Thus, the piston  11  is subjected to the side force Fa in the reverse direction to the case of FIG. 2 from the inner surface in the vicinity of the opening of the cylinder bores  2   a.  In the process, the magnitude of the side force Fa is smaller than in the case of FIG.  2 . 
     FIG. 4A is a graph showing the relation between the rotational angle of the swash plate  9  (the coverage of the piston  11 ) and the magnitude of the side force Fa acting on the piston  11 . In this graph, the rotational angle of the swash pate  9  when the piston  11  is at top dead center is assumed to be 0°. 
     As shown in FIG. 4A, during the period from the time point when the piston  11  is located at top dead center to the time point when the swash plate  9  rotates by 90°, the side force Fa may assume a negative value. This indicates that the direction of each force described above becomes reversed. 
     The graph of FIG. 4A indicates that when the rotational angle of the swash plate  9  is 0°, i.e. when the piston  11  is at top dead center, the side force Fa acting on the piston  11  becomes a maximum. The position on the peripheral surface of the piston  11  where the maximum side force Fa is exerted is the 6 o&#39;clock position as shown in FIG.  4 B. When a large side force Fa is exerted at the 6 o&#39;clock position on the peripheral surface of the piston  11 , the range E 1  of 3 o&#39;clock to 9 o&#39;clock positions with the 6 o&#39;clock position at the center thereof is where the piston  11  is pressed, strongly against the inner peripheral surface of the cylinder bore  2   a.  In the case where a second oil groove  17  is formed in the range E 1 , therefore, the opening edge of the second oil groove  17  is strongly pressed against the inner peripheral surface of the cylinder bores  2   a,  thereby sometimes wearing or damaging the piston  11  or the cylinder bores  2   a.  Preferably, therefore, the second oil groove  17  is formed in the range other than the range E 1  of 3 o&#39;clock to 9 o&#39;clock positions, i.e. in the range E 2  of 9 o&#39;clock to 3 o&#39;clock positions on the peripheral surface of the piston  11 . 
     To avoid the effect of the side force Fa, the second oil groove  17  is preferably formed in the part of the range E 2  of 9 o&#39;clock to 3 o&#39;clock where the side force Fa exerted on the peripheral surface of the piston  11  is minimum. The graph of FIG. 4A indicates that the side force Fa acting on the piston  11  is smaller when the piston  11  is in suction stroke (when the rotational angle of the swash plate  9  is 0° to 180°) than when the piston  11  in compression stroke (when the rotational angle of the swash plate  9  is 180° to 360°). 
     At the end of the reexpansion of the residual refrigerant gas in the cylinder bores  2   a  in a suction stroke, no compression reaction force is exerted on the swash plate  9  but most of the force exerted on the swash plate  9  is the force of inertia of the piston  11 . Particularly, when the rotational angle of the swash plate  9  is 90° as shown in FIG. 4A, substantially no side force Fa acts on the peripheral surface of the piston  11  at the 9 o&#39;clock position on the peripheral surface of the piston  11 . The side force Fa acting on the piston  11 , therefore, is smaller in suction stroke than in compression stroke when the compression reaction force occurs. In other words, in the range E 2  of 9 o&#39;clock to 3 o&#39;clock on the peripheral surface of the piston  11 , the side force Fa exerted in the range of 9 o&#39;clock to 12 o&#39;clock is smaller than that exerted in the range of 12 o&#39;clock to 3 o&#39;clock. 
     In addition, as shown in FIG. 4A, when the piston  11  is located at the bottom dead center, a comparatively large side force Fa acts at the 12 o&#39;clock position on the peripheral surface of the piston  11 . The piston  11 , when moved to the neighborhood of bottom dead center, may become unstable as the length supported by the cylinder bores  2   a  becomes shorter. Therefore, the second oil groove  17  is preferably not formed in the neighborhood of the 12 o&#39;clock position on the peripheral surface of the piston  11 . 
     Taking the foregoing facts into consideration, according to this embodiment, as shown in FIG. 4B, the second oil groove  17  is formed in the range E of 9 o&#39;clock to 10:30 on the peripheral surface of the piston  11 . 
     It will be understood from the foregoing description that, in the swash plate type compressor according to the present invention, the peripheral wall of the cylinder bores and the piston are fabricated of an aluminum alloy, direct contact between metals of the same type is avoided by the fluororesin film formed on the outer peripheral surface of the piston, and the fitting gap with the cylinder bores is minimized. As a result, coupled with the use of a piston ring, the amount of the blow-by gas can be limited to minimum. Thus, the CO 2  gas can be employed as a refrigerant gas without reducing the compression performance. 
     Also, in the swash plate type compressor according to this invention, when the first and second oil grooves are formed in the outer peripheral surface of the piston, the viscous resistance of the oil component can be reduced to secure a smooth sliding motion of the piston without increasing the gas flow rate through the fitting gap with the cylinder bores. Further, a sufficient amount of oil can be supplied to the sliding portions in the crank chamber through these oil grooves. 
     Furthermore, in the case where the second oil groove is formed in a phase minimizing the effect of the side force on the outer peripheral surface of the piston, the second oil groove can be sufficiently protected from wear and damage and the side force can be positively supported by the fluororesin film.