Patent Publication Number: US-5836748-A

Title: Swash plate type variable displacement compressor utilizing a spool for controlling the inclination

Description:
This application is a 371 of PCT/JP94/01148 filed Jul. 13, 1994. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a swash plate type variable displacement compressor which is used in, for example, an air conditioning system for a vehicle. 
     2. Description of the Related Art 
     In general, an electromagnetic clutch is used to connect and disconnect the power transmission path from the engine to a compressor in a refrigerant circuit installed in a car. When an air conditioning system is switched on, the electromagnetic clutch is activated and engine power is transmitted to the compressor via a belt transmission mechanism and the electromagnetic clutch. 
     Frequent repetition of the connection and disconnection of the electromagnetic clutch to the compressor reduces the durability of the compressor, and also causes the entire refrigerant circuit to instantaneously vibrate when the compressor is activated. 
     In addition, whenever an electromagnetic clutch is used, the overall size and weight of a compressor is inevitably increased. This requires increased space to mount the compressor in the engine compartment and makes the mounting of the compressor difficult. Further, because the electromagnetic clutch when in action consumes considerable power, the battery in the vehicle must bear a great load. 
     As a solution to this shortcoming, a clutchless compressor has been proposed whose drive shaft is normally rotated with the engine. While the refrigerant gas is circulating between the compressor and the external refrigerant circuit, therefore, no significant problem arises. When there is an insufficient amount of the gas discharged to the external refrigerant circuit from the compressor, however, the circulation of the gas is stopped, which may cause insufficient lubrication of the sliding portions in the compressor. A clutchless swash plate type compressor designed to overcome this problem is disclosed in Japanese Unexamined Patent Publication No. Hei 3-37378. In this compressor, when the discharge of the gas to the external refrigerant circuit including an evaporator from the compressor is unnecessary, the valve which is connected to the suction chamber is closed to reduce the pressure in the suction chamber and the control valve for the passage between the discharge chamber and the crank chamber is opened. 
     When the gas discharge is unnecessary, the swash plate of the disclosed compressor is moved to have the minimum inclined angle to minimize the piston stroke. Further, the passage between the external refrigerant circuit and the compressor is blocked to suppress the load with respect to the rotation of the compressor. The slight reciprocation of each piston causes the gas to be discharged to the discharge chamber from the associated cylinder bore and to further flow into the crank chamber via the individual sliding portions from the discharge chamber. The gas in the crank chamber then flows to the suction chamber from which it is drawn into the associated cylinder bore. 
     According to the conventional compressor, as is apparent from the above, when the discharge of the gas to the external refrigerant circuit from the compressor is unnecessary, a small amount of refrigerant gas is circulated inside the compressor so that oil suspended in the refrigerant gas lubricates the sliding portions. As the gas in the compressor simply circulates, the amount of gas to be circulated is not enough to provide a sufficient amount of lubricating oil, causing insufficient or inadequate lubrication of the individual sliding portions, particularly, in the crank chamber. 
     Under low temperature conditions as in the winter or the like, when the compressor is stopped during large displacement operation, a large amount of gas remains, liquefied in the crank chamber. With this liquid refrigerant present, even when the compressor is activated, it is difficult to turn the liquid refrigerant to the gaseous state merely by the gas circulation in the compressor which is running now with a small displacement. When the liquid refrigerant is circulated in the compressor, therefore, the oil sticking on the individual sliding portions is washed out. This deteriorates the lubrication in the compressor and increases the power loss. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is a primary objective of the present invention to provide a clutchless swash plate variable displacement compressor, which will overcome the aforementioned conventional shortcomings and can improve the lubrication performance at the individual sliding portions in the compressor. 
     A housing having a crank chamber and a plurality of cylinder bores also has a discharge chamber and a suction chamber, which are communicatable with the cylinder bores. A drive shaft is supported in the housing, and pistons are accommodated in the respective cylinder bores. A swash plate is supported on the drive shaft in the crank chamber in such a manner that its inclined angle is changeable, so that the undulation of the swash plate causes the pistons to reciprocate. The compressor further comprises a mechanism for temporarily holding the swash plate to an inclined position from an upright position when the compressor is running when the swash plate would otherwise be caused to stay upright. 
     With this structure, it is possible to lubricate components inside the compressor without intercepting the passage between the compressor and the external refrigerant circuit when the compressor is running with the zero displacement or a small displacement close to zero. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
    
    
     The features of the present invention that are believed to be novel are set forth with particularity in the appended claims. 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 vertical cross-sectional view of essential portions of a swash plate type variable displacement compressor according to one embodiment of the present invention; 
     FIG. 2 is a vertical cross-sectional view showing the overall compressor under zero-displacement conditions; 
     FIG. 3 is a horizontal cross-sectional view showing a hinge mechanism; 
     FIG. 4 is a cross-sectional view taken along the line IV--IV in FIG. 2; 
     FIG. 5 is a cross-sectional view showing an oil pump; 
     FIG. 6 is a vertical cross-sectional view showing an electromagnetic direction switching valve, an electromagnetic open/close valve and a pressure control valve; 
     FIG. 7 is a vertical cross-sectional view also showing the electromagnetic direction switching valve, the electromagnetic open/close valve and the pressure control valve; 
     FIG. 8 is a vertical cross-sectional view showing the overall compressor with a large displacement; and 
     FIG. 9 is a vertical cross-sectional view showing another compressor according to this invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Embodiment 
     A preferred embodiment of the present invention will now be described referring to the accompanying drawings. 
     As illustrated in FIG. 2, a front housing 2 is fixed to a cylinder block 1. A crank chamber 2a is formed in the front housing 2. A plurality of cylinder bores 1a are formed in the cylinder block 1. A rear housing 3 is secured to the cylinder block 1. A suction chamber 3a and a discharge chamber 3b are defined in the rear housing 3. The front housing 2, cylinder block 1 and rear housing 3 constitute the housing of a compressor. 
     A drive shaft 4 is rotatably supported in the cylinder block 1 and front housing 2 via radial bearings 5 and 6 in such a manner that the drive shaft 4 passes through the crank chamber 2a. A pulley 7 is secured to the free end portion of the drive shaft 4 outside the compressor. A belt 8 is wrapped around the pulley 7, so that the rotation of the engine is transmitted via the belt 8 and pulley 7 to the drive shaft 4. 
     A rotary plate 9 is secured to the drive shaft 4 inside the crank chamber 2a. A thrust bearing 10 is arranged on the inner wall of the front housing 2 to receive the thrust load applied to the rotary plate 9. A support arm 11, having a hole 11a, is integrally formed with the rotary plate 9. 
     A swash plate support 12 having a spherical surface 12a is reciprocatably supported on the drive shaft 4 along its axial direction. A swash plate 13 is supported on the swash plate support 12 to be tiltable along the spherical surface 12a. 
     A support pin 14 is fitted in the hole 11a of the support arm 11 so as to be rotatable about its own axis, with guide holes 14a formed in both end portions of the support pin 14, respectively. See FIG. 3. Two parallel projections 13a are integrally formed at the center portion of the swash plate 13 in such a position to sandwich the drive shaft 4. A pair of guide pins 15 and 16 are respectively fixed to the two parallel projections 13a, and have their free end portions slidably fitted in the guide holes 14a. In this embodiment, the support arm 11, the swash plate support 12, the guide pins 15 and 16 and the projections 13a allow the swash plate 13 and the rotary plate 9 to rotate together. Together, these components form a hinge mechanism that permits the variable inclination of the swash plate 13. 
     As shown in FIG. 2, a plurality of pistons 17 are reciprocatably placed in the respective cylinder bores 1a. A pair of shoes 18 are supported by each piston 17. The peripheral portion of the swash plate 13 is positioned between the individual shoes 18 of each pair. The undulating rotation of the inclined swash plate 13 causes the pistons 17 to reciprocate. A stopper 19 is secured to the outer surface of the drive shaft 4. When the swash plate support 12 abuts on the stopper 19, the swash plate 13 is held upright. A step 4a is formed on the outer surface of the drive shaft 4 to determine the maximum inclined angle at which the swash plate 13 can be positioned. 
     A valve plate 20 is secured between the cylinder block 1 and the rear housing 3. As best seen in FIG. 4, a suction hole 20a and a discharge hole 20b are formed in the valve plate 20 in association with each cylinder bore 1a. A discharge valve 21a and a suction valve 22a are formed in the valve plate 20, and are of the reed valve type. A retainer plate 23 (see FIG. 2) is placed over the valve plate 20. A retainer 23a is formed on the retainer plate 23 in association with each discharge valve 21a to prevent that discharge valve 21a from opening too much. As shown in FIG. 1, a center hole 1b is formed in the cylinder block 1. A cylindrical spool 24 is slidably supported on the outer surface of the drive shaft 4 in the center hole 1b. A lip seal 25 is provided on the inner surface of the center hole 1b between the radial bearing 6 (see FIG. 2) and the spool 24. A ring 21 is secured to the inner surface of the center hole 1b to hold the lip seal 25 in place. 
     The spool 24 has a large-diameter portion 24a and a small-diameter portion 24b. When the spool 24 is in a retracted position as shown in solid lines in FIG. 1, the outer surface of the large-diameter portion 24a contacts the inner surface of the center hole 1b. A pressure chamber 26 is formed between the outer surface of the small-diameter portion 24b of the spool 24 and the inner surface of the center hole 1b. 
     When oil is supplied to the pressure chamber 26 with the large-diameter portion 24a in contact with the inner surface of the center hole 1b, the spool 24 is urged along to the drive shaft 4 and moved forward in the direction of the swash plate support 12. As a result, the swash plate support 12 moves in the same direction. This causes the swash plate 13 to move from the upright position to the inclined position shown in phantom lines in FIG. 1. 
     As shown in FIGS. 2 and 5, a trochoid pump 27 is located at the center of the rear housing 3. As shown in FIG. 5, this pump 27 has inner teeth 27a and outer teeth 27b. The inner teeth 27a are rotated by the drive shaft 4. As the inner teeth 27a rotate, the outer teeth 27b rotate in the same direction at a slower speed than the inner teeth 27a. 
     As shown in FIG. 5, a clearance 103 formed between the teeth 27a and 27b, shifts in the rotational direction of the teeth 27a and 27b. During this shift, the clearance undergoes a change in its volume due to the difference between the rotational speeds of the teeth 27a and 27b. Through the above-described action, the lubricating oil is led into the clearance 103 from an arcuate suction port 28a and is discharged through an arcuate discharge port 29a. 
     A first oil passage 30 is formed axially in the center of the drive shaft 4, as shown in FIG. 2. Branch oil passages are formed at a plurality of points of the first oil passage 30 allowing oil to be supplied to the bearings 5 and 6, the crank chamber 2a, the swash plate support 12, etc. A second oil passage 28 is connected between the bottom of the crank chamber 2a and the suction port 28a of the pump 27. A third oil passage 29 is connected between the discharge port 29a of the pump 27 and the first oil passage 30. As shown in FIGS. 1 and 2, a first valve assembly 31 is provided in the third oil passage 29. This first valve assembly 31 is connected via a fourth oil passage 32 to the pressure chamber 26. 
     As shown in FIG. 6, first and second valve chambers 34 and 35 are formed in a valve case 33. A columnar shaped first valve 36 and a spherical shaped second valve 37 are respectively disposed in the valve chambers 34 and 35. The second valve 37 is urged by a spring 38 in a direction to close the fourth oil passage 32. 
     An electromagnetic solenoid 39 is secured on the top of the case 33, and a fixed core 41 and a movable core 42 are retained in the solenoid 39. A hole 41a is formed in the fixed core 41, with a first rod 43 inserted in the hole 41a in a lengthwise direction. The first rod 43 has a first end portion fixed to the movable core 42 and a second end portion abutting on the second valve 37. The first valve 36 is secured to the second end portion of the first rod 43. 
     As shown in FIG. 6, with the solenoid 39 de-excited, the movable core 42 is separated from the fixed core 41 by the urging force of the spring 38. This allows the second valve 37 to be held at the position to close the fourth oil passage 32 and for the first valve 36 to be held at the position in order to open the third oil passage 29. The oil is supplied to the passage 30 of the drive shaft 4 from the passage 29 to lubricate the individual sliding portions in the compressor as shown in FIG. 1. 
     With the solenoid 39 excited, as shown in FIG. 7, the movable core 42 is attracted to the fixed core 41 against the urging force of the spring 38. This permits the first valve 36 to be shifted to the position to close the third oil passage 29. At the same time, the second valve 37 is shifted to the position to open the fourth oil passage 32. The oil provided from the pump 27 is not supplied to the first oil passage 30, but is supplied into the pressure chamber 26 via the fourth oil passage 32. The spool 24 therefore moves forward to shift the swash plate support 12 in the same direction. This causes the swash plate 13 to move to the inclined position from the upright position. 
     As shown in FIG. 2, a first gas passage 44 is connected between the discharge chamber 3b and the crank chamber 2a. A second gas passage 45 is connected to the suction chamber 3a and the crank chamber 2a. As shown in FIG. 6, the first gas passage 44 and the second gas passage 45 are in partial communication with each other. A second valve assembly 46 is provided midway in the first gas passage 44. Both the first and second valve assemblies 31 and 46 utilize the electromagnetic solenoid 39. 
     As shown in FIG. 6, a valve chamber 48 is formed in a valve case 47 of the second valve assembly 46. A spherical valve 49 is disposed in the valve chamber 48. A spring 50 urges the valve 49 to close the passage 44. A second rod 51 is fixed to the movable core 42 and abuts on the valve 49. 
     With the solenoid 39 excited, as shown in FIG. 7, the second rod 51 moves, together with the movable core 42, away from the valve 49. Concurrently, spring 50 urges the valve 49 to close the passage 44. 
     When the solenoid 39 is de-excited during a compression cycle with the swash plate 13 inclined, the second rod 51, as shown in FIG. 6, is moved upward by the urging force of the spring 38 via the second valve 37, the first rod 43 and the movable core 42. This configuration is also shown in FIG. 8. Accordingly, the valve 49 is shifted to the position to open the passage 44. Consequently, the high-pressure gas is supplied via the passage 44 to the crank chamber 2a from the discharge chamber 3b, thus increasing the differential pressure Δp between the pressure in the crank chamber 2a and the pressure in the suction chamber 3a, which acts on each piston 17. The swash plate 13 is therefore forced to move to the upright position from the inclined position. 
     During the compressor&#39;s compression cycle, the displacement altering control is performed on the compressor in accordance with the cooling load. In accordance with the value of the suction pressure proportional to the cooling load, the degree of the opening of the passage 45, which is connected to the crank chamber 2a and suction chamber 3a, is adjusted. As a result, an adjustment is made to the differential pressure Δp acting on the piston 17. 
     As shown in FIG. 6, a third valve assembly 52 is located between the first and second valve assemblies 31 and 46. A retainer chamber 47a is formed in the case 47. A valve 53 is disposed in the retainer chamber 47a to open or close the passage 45. This valve 53 is urged by a spring 54 in a direction to close the passage 45. A chamber 55 is defined by the valve 53 and the case 47. The suction pressure in the suction chamber 3a is applied inside the chamber 55 via the passage 45. A chamber 75, formed in the case 47, communicates with the crank chamber 2a. 
     The degree of opening of the passage 45 is controlled based on the difference between the pressure in the chamber 55 and that in the chamber 75. This chamber 55 communicates with a chamber 47b, defined between the inner surface of the valve 53 and a bellows 56, via a passage 53a formed in the valve 53. The second rod 51 is slidably inserted through a hole 53b formed in the valve 53. 
     With a large cooling load and a high pressure in the suction chamber 3a, a large pressure is created in the chamber 55 causing the valve 53 to open. Therefore, a large amount of gas flow into the suction chamber 3a via the passage 45 from the crank chamber 2a. As a result, the differential pressure p acting on the piston 17 decreases, causing the inclined angle of the swash plate 13 to increase. This in turn results in an increase in the stroke of the piston 17 as well as in an increase in the compressor&#39;s displacement. 
     When the cooling load becomes smaller and the pressure in the suction chamber 3a falls, on the other hand, the pressure in the chamber 55 also drops. Accordingly, the valve 53 is shifted in a direction that restricts the passage 45. This increases the differential pressure p which in turn reduces the inclined angle of the swash plate 13 and the stroke of the piston 17. The compressor can thus operate with a small displacement. 
     A passage provided between the crank chamber 2a and the suction chamber 3a includes a restriction (not shown). Accordingly, blow-by gas entering the crank chamber 2a, passing between the inner surface of each cylinder bore 1a and the associated piston 17, circulates back to the suction chamber 3a. 
     As shown in FIG. 6, a controller 57 as control means, electrically connected to the coil 40, of the electromagnetic solenoid 39, comprises a central processing unit (CPU) 58 and a timer 59. The controller 57 receives various electrical signals from an ignition switch 62 for the engine, an air conditioning switch 63, a sensor 64 for detecting the temperature of the gas discharged from the compressor, a sensor 65 for detecting the temperature inside the vehicle, a suction pressure sensor (not shown), a discharge pressure sensor (not shown), and the like. The timer 59 executes a counting operation to set the operation start timing and the operation time for the electromagnetic solenoid 39. When the air conditioning switch 63 is switched on, the electromagnetic solenoid 39 is energized. 
     The CPU 58 has a memory 66 which stores various kinds of data. 
     The operation of the thus constituted variable displacement compressor will be now described. 
     When the engine starts while the air conditioning switch 63 is switched off, the drive shaft 4 of the compressor rotates. The ON signal from the engine ignition switch 62 is transferred to the controller 57, causing the electromagnetic solenoid 39 to be excited for a predetermined time, e.g., 10 minutes. This time is set by the timer 59, under the control of the CPU 58. 
     Consequently, as shown in FIG. 7, the movable core 42 is attracted to the fixed core 41, so that the first valve 36 is positioned to close the third oil passage 29, and the second valve 37 is positioned to open the fourth oil passage 32. Therefore, the pump 27 supplies the oil to the pressure chamber 26 via the fourth oil passage 32. 
     As a result, the spool 24 moves forward, shifting the swash plate 13 forward to the inclined position indicated by the chain line in FIG. 1 from the upright position indicated by the solid line in this diagram. The spool 24 moves forward until it hits against the stopper 19. At this time, the large-diameter portion 24a comes off the center hole 1b, permitting the oil in the pressure chamber 26 to flow into the crank chamber 2a. As indicated by the chain line in FIG. 1, the inclined angle of the swash plate 13 is not large. 
     As the drive shaft 4 rotates, the piston 17 reciprocates in the associated cylinder bore 1a, causing the gas to be supplied into the cylinder bore 1a from the external refrigerant circuit via the suction chamber 3a. The supplied gas is compressed in the cylinder bore 1a and is then discharged to the external refrigerant circuit via the discharge chamber 3b. Because the inclined angle of the swash plate 13 is not large at this time, the compressor will run with a small displacement. 
     The excited electromagnetic solenoid 39 moves the second rod 51 together with the movable core 42 downward in FIGS. 6 and 7. At the same time, the spring 50 urges the valve 49 of the second valve assembly 46 to a position to close the passage 44. Consequently, the gas supply to the crank chamber 2a from the discharge chamber 3b is stopped. 
     After the elapse of a predetermined time (e.g., about 10 minutes), the timer 59 de-excites the electromagnetic solenoid 39 upon time-up. Consequently, the first valve 36 of the first valve assembly 31 is urged in the direction that opens the third oil passage 29, while the second valve 37 is urged in the direction that closes the fourth oil passage 32 as shown in FIG. 6. This inhibits the oil supply to the pressure chamber 26, thereby freeing the spool 24. 
     The second rod 51 causes the valve 49 of the second valve assembly 46 to move in the direction to open the passage 44 against the urging force of the spring 50. This allows high-pressure gas to be supplied into the crank chamber 2a from the discharge chamber 3b, thus increasing the differential pressure Δp that acts on the piston 17. As a result, the swash plate 13 is forced to return to the upright position, causing the compressor to be switched to the zero-displacement operation mode. 
     Under the zero-displacement operation, the pressure in the discharge chamber 3b drops, reducing the differential pressure Δp. Since the center of gravity of the swash plate 13 lies opposite the hinge mechanism with respect to the drive shaft 4, the swash plate 13 is held at the upright position due to the centrifugal force acting on the gravitational center of the swash plate 13. 
     As described above, when the compressor is activated, the compressor temporarily runs with a small displacement, e.g., 10%. Therefore, the oil laden gases in the condenser and evaporator are supplied to the suction chamber 3a. The oil laden gas is also blown by to the crank chamber 2a from the compression chamber in the associated cylinder bore 1a, passing through the clearance between the outer surface of the associated piston 17 and the inner surface of the associated cylinder bore 1a. 
     The liquid refrigerant in the crank chamber 2a, together with that gas, flows via the passage 45 into the suction chamber 3a from which it is supplied into the cylinder bore 1a. The liquid refrigerant is then discharged from the cylinder bore 1a. Accordingly, the lubricating oil-containing liquid refrigerant in the crank chamber 2a is gradually gone. In the above-described manner, the gas circulates between the compressor and the external refrigerant circuit, so that the oil and gas in the external refrigerant circuit return to the compressor. 
     When the air conditioning switch 63 is switched on, the controller 57 keeps the electromagnetic solenoid 39 excited, permitting the continual supply of the lubricating oil to the pressure chamber 26 from the pump 27. The spool 24 is therefore held at the forward position. At this time, the swash plate 13 turns while its inclined angle is being adjusted in accordance with the cooling load. This reciprocates the piston 17 to execute the gas compression stroke. 
     During this operation, the degree of opening of the passage 45 is adjusted by the third valve assembly 52 in accordance with a change in the suction pressure that is proportional to the cooling load. Consequently, the differential pressure Δp acting on the piston 17 is adjusted. In accordance with the cooling load, therefore, the inclined angle of the swash plate 13 is altered to adjust the discharge displacement. 
     When the temperature inside the vehicle is low and the cooling load is small, the pressure in the suction chamber 3a is low so that the degree of opening of the passage 45 is reduced by the valve 53 of the third valve assembly 52. As a result, the differential pressure Δp acting on the piston 17 is kept large so that the swash plate 13 is held at the minimum inclined angle for the 10% displacement operation. 
     When the cooling load is large, on the other hand, the pressure in the suction chamber 3a is high so that the degree of opening of the passage 45 is increased by the valve 53 of the third valve assembly 52, thus reducing the differential pressureΔp. Consequently, the swash plate 13 is moved away from the spool 24 to the maximum inclination side. When the cooling load is large, generally speaking, the air conditioning switch 63 is switched on, so that the compressor runs with a large displacement upon the activation of the engine. 
     When the zero-displacement operation continues for a predetermined time, i.e., when the de-excitation state of the electromagnetic solenoid 39 continues for a given time (e.g., 10 to 30 minutes) after the air conditioning switch 63 is switched off, the swash plate 13 is temporarily inclined by the timer 59 to accomplish the operation under compression. In other words, the electromagnetic solenoid 39 is excited for a predetermined time, e.g., 10 minutes, set by the timer 59, under the control of the CPU 58. 
     Consequently, as mentioned earlier, the movable core 42 is attracted to the fixed core 41, so that the first valve 36 is positioned to close the third oil passage 29, and the second valve 37 is positioned to open the fourth oil passage 32. Therefore, the pump 27 supplies the oil to the pressure chamber 26 via the fourth oil passage 32. 
     As a result, the spool 24 moves forward, shifting the swash plate 13 to the inclined position indicated by the chain line in FIG. 1 from the upright position indicated by the solid line in this diagram. As the drive shaft 4 rotates, therefore, the gas circulates between the compressor and the external refrigerant circuit, ensuring rich lubrication inside the compressor. 
     When the predetermined time elapses, the timer 59 de-excites the electromagnetic solenoid 39 upon time-up. Consequently, the second rod 51 causes the valve 49 of the second valve assembly 46 to move in the direction to open the passage 44 against the urging force of the spring 50. High-pressure gas is therefore supplied into the crank chamber 2a from the discharge chamber 3b, thus increasing the differential pressure Δp that acts on the piston 17. As a result, the swash plate 13 is forced to return to the upright position, causing the compressor to be switched to the zero-displacement operation mode. 
     In short, when the compressor according to this embodiment is called upon to run in the zero-displacement mode when the air conditioning switch 63 is set off, the compressor is shifted to the small-displacement state every given period of time, allowing the gas to circulate between the compressor and the external refrigerant circuit. The sliding portions inside the compressor are therefore well lubricated. Because this periodic operation every given period of time is carried out with a small displacement, the load on the engine is small. 
     This invention is not limited to the above-described embodiment, but may be embodied in the following forms without departing from the spirit and scope of the invention. 
     (1) A spring 133 may be disposed in the pressure chamber 26 to urge the spool 24 forward while the compressor is not running, as shown in FIG. 9. 
     With this structure, when the compressor is running, the inclined angle of the swash plate 13 is controlled as per the above-described embodiment. When the compressor is not running, the swash plate 13 is held at the minimum inclination position by the spring 133. When the engine is activated, therefore, the compressor spontaneously starts running with a small displacement, thus further improving the lubrication performance at the individual sliding portions in the crank chamber 2a. 
     (2) In the above-described embodiment, the valve 53 to open or close the second passage 45 is opened or closed in accordance with the suction pressure. Instead of the use of the suction pressure, an electromagnetic valve may be used so that the valve 53 is opened or closed by an externally supplied signal.