Abstract:
A control valve is provided to be mainly used in a clutch-less compressor of the type in which displacement of the compressor is varied depending on the inclination of a drive plate which varies depending on the crank pressure. The control valve includes biasing means that applies force to a valve body, a force transferring member that urges the valve body to forcibly open the valve, and a solenoid assembly to actuate the force transferring member. The valve remains closed when no electric current is supplied to the solenoid assembly, regardless of the crank pressure or the suction pressure. This facilitates minimum displacement operation of the compressor for a desired period of time and therefore makes the valve suitable for a clutch-less type compressor that is directly connected to an engine with a belt and/or a pulley.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to compressors and controls valves for compressors, and more particularly, to variable displacement compressors and control valves employed in such compressors. 
     A typical type of variable displacement compressor employs an inclinable drive plate housed in a crank chamber. The inclination of the drive plate is changed to vary the displacement of the compressor. A control valve adjusts the pressure in the crank chamber (crank pressure Pc) to alter the inclination of the drive plate. Japanese Unexamined Patent Publication No. 6-26454 describes a compressor that employs such a control valve. The compressor has a bleeding passage that connects a crank chamber to a suction chamber (which is connected to the outlet of an evaporator). The control valve is located in the bleeding passage and includes an electromagnetic coil, a bellows, a valve body attached to the bellows, a valve chamber accommodating the bellows and the valve body, and a valve port connecting the crank chamber and the suction chamber. The target of the pressure in the suction chamber (target suction pressure) is adjusted by changing the current flowing through the electromagnetic coil. The refrigerant gas in the suction chamber is drawn into the valve chamber. The pressure of the suction chamber (suction pressure Ps) communicated to the valve chamber moves the valve body and changes the opened area of the valve port. This adjusts the amount of refrigerant gas that is released into the suction chamber from the crank chamber and thus controls the crank pressure Pc. The force of the bellows acts on the valve body to close the valve port, while the crank pressure Pc acts on the valve body to open the valve port. 
     In automobile air-conditioning systems, clutchless variable displacement compressors are often employed since they are lighter than compressors having clutches. A clutchless compressor is directly connected to an external drive source, or engine, by a pulley and a transmission belt without using an electromagnetic clutch. Since engine power is constantly transmitted to the compressor, the displacement of the compressor must be minimized by moving the drive plate to a minimum inclination position when the passenger compartment does not require cooling or when the cooling load is extremely small. 
     The control valve described in the Japanese patent publication can be employed in a clutchless variable displacement compressor. However, it is rather difficult to maintain the drive plate at the minimum inclination position and operate the compressor in a minimum displacement state. This is because the control valve must be either completely closed or minimally opened to maximize the crank pressure Pc and hold the drive plate at the minimum inclination position. Since the crank pressure Pc acts to open the control valve, it becomes difficult to keep the control valve closed or minimally opened as the crank pressure Pc increases. As a result, the crank pressure Pc cannot be increased sufficiently to hold the drive plate at the minimum inclination position and maintain minimum displacement operation. If minimum displacement cannot be continued when cooling is not necessary, engine power is consumed by the compressor in an inefficient manner. This diminishes the merits of clutchless compressors. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an objective of the present invention to provide a control valve that regulates the release of gas from a crank chamber in a clutchless variable displacement compressor. It is a further objective of the present invention to provide a clutchless variable displacement compressor that can continue minimum inclination operation as long as necessary. 
     To achieve the above objectives, the present invention provides a control valve for use with a compressor. The compressor is generally of the type that has a drive plate that inclines with respect to the axis of a drive shaft. The drive plate connects a piston to the drive shaft to convert rotation of the drive shaft into linear reciprocation of the piston within a cylinder bore. The compressor has a crank chamber which accommodates the drive plate. The pressure of the crank pressure is a crank pressure. The compressor also has a suction chamber into which gas is introduced from an external refrigerant circuit. The pressure of the suction chamber is a suction pressure. The compressor also includes a bleeding passage that permits flow of gas from the crank chamber to the suction chamber. Displacement of the compressor is varied depending on the inclination of the drive plate, which varies depending on the crank pressure. 
     In one aspect of the present invention, a control valve includes a valve chamber that forms a part of the blending passage. A valve seat defines a crank chamber side region and a suction chamber side region in the valve chamber. A valve port is formed in the valve seat to connect the two regions. A valve body engages and disengages from the valve seat to close and open the valve port, respectively. The control valve also includes a force transferring member. One of the valve body and the force transferring member is located in the crank chamber side region while the other is located in the suction chamber side region. A first spring urges the valve body toward the valve seat. A solenoid assembly generates an electromagnetic biasing force that is dependent upon the level of an electric current supplied to the solenoid assembly. The solenoid assembly urges the valve body in a direction away from the valve seat in accordance with the biasing force. The valve body remains engaged with the valve seat to close the valve port, regardless of the crank pressure or the suction pressure, when no electric current is supplied to the solenoid assembly. 
     This aspect of the present invention facilitates minimum displacement operation of the compressor for a desired period of time and therefore makes the valve suitable for a clutch-less type compressor that is directly connected to an engine with a belt and/or a pulley. 
     Other aspects and advantages of the present 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 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 cross-sectional view showing a variable displacement compressor to which control valves according to the present invention are applied; 
     FIG. 2 is a cross-sectional view showing a control valve according to the first embodiment; 
     FIG. 3 is a cross-sectional view showing a control valve according to the second embodiment; 
     FIG. 4 is a cross-sectional view showing a control valve according to the third embodiment; and 
     FIG. 5 is a cross-sectional view showing a control valve according to the fourth embodiment. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Four control valves (four embodiments) for variable displacement compressors will now be described with reference to the drawings. Each control valve is employed in the compressor shown in FIG.  1 . In the drawings, like numerals are used for like elements throughout. 
     [First Embodiment] 
     As shown in FIG. 1, a variable displacement compressor includes a cylinder block  1  having a plurality of cylinder bores  1   a  (only one shown). A front housing  2  is fixed to the front end of the cylinder block  1 . The front housing  2  houses a crank chamber  3 . A rear housing  4  is fixed to the rear end of the cylinder block  1  with a valve plate  5  arranged in between. The cylinder block  1 , the front housing  2 , and the rear housing  4  define a compressor housing. A suction plate  6  having suction flaps  6   a  is arranged on the front side of the valve plate  5 , while a discharge plate  7  having discharge flaps  7   a  is arranged on the rear side of the valve plate  5 . The central portion of the rear housing  4  houses a discharge chamber  9 . A suction chamber  8  extends about the discharge chamber  9  in the peripheral portion of the rear housing  4 . A suction port  5   a  and a discharge port  5   b  extend through the valve plate  5  in correspondence with each cylinder bore  1   a.  Each suction port  5   a  connects the suction chamber  8  with the associated cylinder bore  1   a.  Each cylinder bore  1   a  is connected to the discharge chamber  9  by the associated discharge port  5   b.    
     A rotary shaft  12  is rotatably supported by a pair of bearings  13  in the cylinder block  1  and the front housing  2 . One end of the rotary shaft  12  is directly connected to an external drive source, or engine E, by a pulley  10  and a power transmission belt  11 , which are indicated by broken lines. A rotor  14  is fixed to the rotary shaft  12  in the crank chamber  3  to rotate integrally with the rotary shaft  12 . A thrust bearing  15  is arranged between the rotor  14  and the inner wall of the front housing  2 . A pair of arms  14   a  having elongated holes  14   b  extend from the rotor  14 . A pin  16  is inserted through the elongated holes  14   b  to pivotally connect the rotor  14  to a drive plate  17 , which permits the drive plate  17  to incline. 
     The drive plate  17  has a hub  17   a.  A sleeve  19 , which slides axially along the rotary shaft  12 , is connected to the inner wall of the hub  17   a  by two connecting pins  20  (only one shown in FIG.  1 ), which are arranged on opposite sides of the rotary shaft  12 . A wobble plate  18  is fitted He to the hub  17   a  and is supported so that it is rotatable relative to the drive plate  17 . A guide rod  21  extends through the crank chamber  3  to prohibit rotation of the wobble plate  18 , while guiding the inclination of the wobble plate  18 . A piston  22  is retained in each cylinder bore  1   a  and connected to the wobble plate  18  by a piston rod  23 . A coil spring  25  is arranged on the rotary shaft  12  between the sleeve  19  and a ring  24 , which is secured to the rotary shaft  12 . The spring  25  biases the drive plate  17  and the wobble plate  18  in a direction that increases their inclination. 
     When the power transmitted from the engine E rotates the rotary shaft  12 , the drive plate  17  rotates, while inclined at a certain angle, and produces undulated motion of the wobble plate  18 . This causes each piston rod  23  to reciprocate the associated piston  23  with a stroke corresponding to the inclination of the drive plate  17 . During the reciprocation of each piston  23 , refrigerant gas is drawn into the associated cylinder bore  1   a  from the suction chamber  8 , compressed, and then discharged into the discharge chamber  9  in a cyclic manner. 
     The drive plate  17  and the wobble plate  18  function as a drive mechanism or a swash plate. The parameters that determine the inclination of the drive plate  17  includes the moment of the centrifugal force produced during rotation of the drive plate  17 , the moment of the biasing force produced by the spring  25  and the moment of the refrigerant gas pressure. The product of inertia of the drive mechanism is determined and the spring  25  is selected such that the centrifugal force moment and the spring force moment constantly act to increase the inclination of the drive plate. The refrigerant gas pressure moment refers to the moment produced by the interrelation of the compression reaction acting on the pistons  22  of the cylinder bores  1   a  undergoing the compression stroke, the interior pressure of the cylinder bores  1   a  undergoing the suction stroke, and the interior pressure of the crank chamber  3  (crank pressure Pc) acting as a back pressure applied to the pistons  22 . When the crank pressure Pc is high such that the gas pressure moment (which acts to decrease the inclination of the drive mechanism) becomes greater than the moments acting to increase the inclination of the drive plate  17  (i.e., the centrifugal force moment and the spring force moment), the drive plate  17  moves to the minimum inclination position (e.g., the position where the angle between a plane perpendicular to the rotary shaft  12  and the drive plate  17  is 3° to 5°). The drive plate  17  can also be arranged at an arbitrary inclination angle between the minimum and maximum inclination angles by decreasing the crank pressure Pc and balancing the gas pressure moment with the centrifugal force and spring force moments. The crank pressure Pc is controlled to alter the inclination of the drive plate  17  in order to change the stroke of the pistons  22  and vary the displacement of the compressor. 
     As shown in FIGS. 1 and 2, the discharge chamber  9  and the suction chamber  8  are connected to each other through an external refrigerant circuit  30 . The external refrigerant circuit  30  and the compressor forms a cooling circuit of an automobile air-conditioning system. The external refrigerant circuit  30  includes a condenser  31 , an expansion valve  32 , and an evaporator  33 . A temperature detector  32   a  is located at the outlet of the evaporator  33 . The expansion valve  32  functions as a variable throttling element located between the condenser  31  and the evaporator  33 . In other words, the opening size of the expansion valve  32  is feedback controlled in accordance with the temperature detected by the temperature detector  32   a  and the vaporizing pressure (i.e., the pressure at the inlet or outlet of the evaporator  33 ). The expansion valve  32  functions to produce a difference between the pressure of the condenser  31  and that of the evaporator  33  and supplies the evaporator  33  with liquefied refrigerant, the amount of which corresponds to the thermal load. This adjusts the amount of refrigerant flowing through the external refrigerant circuit  30  such that the refrigerant is superheated to an appropriate level by the evaporator  33 . 
     As shown in FIG. 2, a further temperature sensor  34  is arranged in the vicinity of the evaporator  33 . The temperature sensor  34  detects the temperature of the evaporator  33  and sends evaporator temperature data to a computer  38 , which controls the air-conditioning system. In addition to the temperature sensor  34 , a passenger compartment temperature sensor  35  for detecting the temperature of the passenger compartment, a temperature adjustor  36  for setting the temperature of the passenger compartment, an air-conditioner switch  37  for actuating the air-conditioning system, and an electronic control unit (ECU) for electronically controlling the engine E are connected to the input side of the computer  38 . The output side of the computer  38  is connected to a drive circuit  39  which is used to energize a coil  67  of a control valve  40 A (described later). 
     The computer  38  computes a current I for energizing the coil  67  based on external data, such as the evaporator temperature detected by the temperature sensor  34 , the passenger compartment temperature detected by the passenger compartment temperature sensor  35 , the desired passenger compartment temperature set by the temperature adjustor  36 , the ON/OFF state of the air-conditioner switch  37 , and information sent from the ECU that is related the engine E (i.e., whether the engine is running and the engine speed). The drive circuit  39  then receives a command from the computer  38  to supply the control valve  40 A with the current I to energize the coil  67  and adjust the opening size of the control valve  40 A. 
     The structure of the control valve  40 A, which adjusts the amount of refrigerant gas released from the crank chamber  3  to control the crank chamber Pc, will now be described with reference to FIG.  2 . In the compressor of FIG. 1, refrigerant gas enters the crank chamber  3  through the slight space between each piston  22  and the wall of the associated cylinder bore  1   a.  This gas is referred to as blowby gas. That is, blowby gas leaks into the crank chamber  3  through the space between the piston  22  undergoing the compression stroke and the wall of the associated cylinder bore  1   a.    
     The control valve  40 A includes a valve mechanism  42 , which is housed in a valve housing  41 , and a solenoid  60 , which is coupled to the housing  41 . A valve chamber  43  is defined in the valve housing  41 . 
     An annular valve seat  44  extends along the inner wall of the valve housing  41  at a mid-section of the valve chamber  43 . In the valve chamber  43 , an upper region (crank chamber side region)  43   a  is defined above the valve seat  44  and a lower region (suction chamber side region)  43   b  is defined below the valve seat  44 . A valve port  45  connecting the upper and lower regions extends through the center of the valve seat  44 . 
     An entrance port  48  extends through the wall of the at valve housing  41  at the upper region  43   a  of the valve chamber  43 . An exit port  49  extends through the wall of the valve housing  41  at the lower region  43   b  of the valve chamber  43 . A passage  50  extending through the compressor is connected with the entrance port  48 . The passage  50  connects the crank chamber  3  to the upper region  43   a.  A further passage  51  extending through the compressor is connected with the exit port  49 . The passage  51  connects the lower region  43   b  to the suction chamber  8 . Accordingly, a bleeding passage is defined between the crank chamber  3  and the suction chamber  8  by the passage  50 , the entrance port  48 , the valve chamber  43 , the exit port  49 , and the passage  51 . 
     A valve element  46  is retained in the upper region  43   a  of the valve chamber  43 . The valve element  46  is movable in the axial direction (vertical direction of the control valve  40 A in FIG. 2) such that it moves toward or away from the valve seat  44 . When the valve element  46  contacting the valve seat  44 , the valve element  46  closes the valve port  45  and disconnects the upper region  43   a  from the lower region  43   b.  The valve element  46  is cylindrical and has a step formed on its outer surface. A spring  47  is held between the step on the valve element  46  and a step formed on the inner wall of the valve housing  41 . The spring  47  constantly biases the valve element  46  toward the valve seat  44  (i.e., in a direction closing the valve port  45 ). 
     A bellows  52 , or pressure sensitive membrane device, is arranged in the upper region  43   a  of the valve chamber  43 . The effective area A of the bellows  52  is equal to the opening area B of the lower region  43   b  (A=B). The effective area A of the bellows  52  is the area that is effective in applying a net force to the bellows  52  as a result of the net pressure applied to the bellows  52 . An adjustor  53  is screwed into the top portion of the valve housing  41 . The upper end of the bellows  52  is fixed to the adjustor  53 . 
     The interior of the bellows  52  is in a vacuum, or is depressurized, and accommodates a spring  52   a.  The spring  52   a  biases the lower end of the bellows  52  downward. The refrigerant gas in the crank chamber  3  is drawn into the upper region  43   a  of the valve chamber  43  through the passage  50  and the entrance port  48 . Thus, the lower, movable end of the bellows  52  abuts against or moves away from the valve element  46  depending on the level of the crank pressure Pc. The location of the valve element  46  in the valve chamber  43  determines the opening size of the control valve  40 A (i.e., the opening size of the bleeding passage). 
     The solenoid  60 , which forms the lower part of the control valve  40 A, has a cup-like retainer  61 . A fixed steel core  62  is fitted into the upper portion of the retainer  61 . The fixed core  62  defines a solenoid chamber  63  in the retainer  61 . A movable steel core  64 , which serves as a plunger, moves axially in the solenoid chamber  63 . 
     A solenoid rod  65 , or force transferring member, extends through the center of the fixed core  62 . A bearing  68  is arranged between the fixed core  62  and the solenoid rod  65  so that the rod  65  is movable in the axial direction. A passage extends along the bearing  68  to equalize the pressures at the upper and lower sides of the bearing  68 . 
     The upper end of the solenoid rod  65  is located in the lower region  43   b  of the valve chamber  43 , to which the pressure of the suction chamber  8  (suction pressure Ps) is applied. The lower end of the solenoid rod  65  is located in the solenoid chamber  63  and fitted into a bore extending through the center of the movable core  64 . The movable core  64  and the solenoid rod  65  are fixed to each other. Thus, the movable core  64  and the solenoid rod  65  move integrally with each other in the axial direction. A spring  66  is arranged between the movable core  64  and the fixed core  62 . The spring  66  biases the movable core  64  and the solenoid rod  65  in the downward direction of FIG.  2 . 
     A coil  67  is wound about the fixed and movable cores  62 ,  64 . The computer  38  commands the drive circuit  39  so that current I flows through the coil  67 . This causes the coil  67  to produce an electromagnetic force corresponding to the current I. The electromagnetic force attracts the movable core  64  toward the fixed core  62  and moves the solenoid rod  65  away from the solenoid  60  in the axial direction. This, in turn, pushes the valve element  46  away from the solenoid  60 . The opening size of the control valve  40 A is determined by the distance between the valve element  46  and the valve seat  44 . 
     If the air-conditioner switch  37  is ON when the engine E is running, the computer  38  obtains the temperature of the evaporator detected by the temperature sensor  34  and the difference between the passenger compartment temperature detected by the passenger compartment temperature sensor  35  and the temperature set by the temperature adjustor  36 . The computer  38  then uses this data to compute the current I for energizing the coil  67  using a formula, which is predetermined by a control program. The drive circuit  39  is then commanded to energize the coil  67  in accordance with the computed current I. This produces an electromagnetic attraction, or upward biasing force F of the solenoid rod  65 . The biasing force F determines the opening size of the control valve  40 A and controls the crank pressure Pc and the suction pressure Ps. 
     The control valve  40 A serves to control the inclination of the drive plate by adjusting the crank pressure Pc. More specifically, if the coil  67  is energized to open the control valve  40 A, the gas in the crank chamber  3  is drawn into the suction chamber  8  through the bleeding passage. If the amount of blowby gas entering the crank chamber  3  becomes less than the amount of refrigerant gas flowing through the bleeding passage from the crank chamber  3  to the suction chamber  8 , the crank pressure Pc decreases. This increases the inclination of the drive plate  17 . If the amount of blowby gas entering the crank chamber  3  becomes greater than the amount of refrigerant gas flowing through the bleeding passage from the crank chamber  3  to the suction chamber  8 , the crank pressure Pc increases. This decreases the inclination of the drive plate  17 . If the amount of refrigerant gas entering the crank chamber  3  becomes equal to that leaving the crank chamber  3 , the crank pressure Pc becomes constant, which holds the drive plate  17  at its current inclination. 
     The control valve  40 A also serves to control the suction pressure Ps without influence from the crank pressure Pc. 
     The downward biasing force of the bellows  52  (including the spring  52   a ) is represented by f 0 , the downward biasing force of the spring  47  is represented by f 1 , the downward biasing force of the spring  66  is represented by f 2 , and the electromagnetic attraction of the movable core  64  generated when the coil  67  is energized (i.e., the upward biasing force of the solenoid rod  65 ) is represented by F. As described above, the effective area of the bellows  52  is represented by A and the opening area of the lower region  43   b  of the valve chamber  43  is represented by B. 
     The biasing force applied to the valve element  46  by the solenoid  60  in the valve opening (upward) direction is represented by (F−f 2 ). The biasing force applied to the valve element  46  by the valve mechanism  42  in the valve closing (downward) direction is represented by (f 0 −Pc×A+f 1 ). The biasing force applied to the valve element  46  by the difference between the pressures of the upper and lower regions  43   a,    43   b  of the valve chamber  43  is represented by (Pc−Ps)B. The relationship between the three biasing forces is indicated by equation (1). Equation (2) is derived from equation (1). 
     
       
           F−f   2   =f   0   −Pc×A+f   1 +( Pc−Ps ) B   (1) 
       
     
     
       
           PsB=f   0   +f   1   +f   2   −F+Pc ( B−A )  (2) 
       
     
     The effective area A is equal to the opening area B. Thus, the suction pressure Ps can be represented as indicated by equation (3), which is derived from equation (2). 
     
       
           Ps =( f   0   +f   1   +f   2   −F )/ B   (3) 
       
     
     In equation (3), the biasing forces f 0 , f 1 , and f 2  are predetermined constants and the biasing force F is a function of the current I for energizing the coil  67 . Thus, the suction pressure Ps varies in accordance with the current I of the coil  67  and is not affected by the crank pressure Pc. The biasing force f 0  of the bellows  52  can be changed by adjusting the position of the adjustor  53 . 
     The computer  38  computes the current I for energizing the coil  67  based on the input data to control the opening size of the control valve  40 A. This adjusts the inclination of the drive plate and varies the displacement of the compressor. Furthermore, the pressure of the suction chamber  8  (suction pressure Ps), which is substantially the same as the outlet pressure Ps′ of the evaporator  33 , is adjusted and maintained at a value close to the target suction pressure Pset. Thus, the control valve  40 A and the computer  38  vary the displacement of the compressor such that the outlet pressure Ps′ of the evaporator  33 , which reflects the cooling load, is stabilized at a value close to the target suction pressure Pset. The solenoid  60  of the control valve  40 A and the computer  38  function to control the opening of the control valve  40 A such that the suction pressure Ps becomes substantially the same as the target suction pressure Pset. Furthermore, the solenoid  60  and the computer  38  change the target suction pressure Pset by controlling the current I for energizing the coil  67 . 
     If the air-conditioner switch  37  is OFF when the engine E is running or if the cooling load is small when the switch  37  is ON, the computer  38  controls the drive circuit  39  to stop energizing the coil  67 . This eliminates the electromagnetic attraction between the cores  62 ,  64  and nullifies the upward biasing force F of the solenoid rod (F=0). As a result, the downward biasing force f 2  of the spring  66  in the solenoid  60  moves the movable core  64  and the solenoid rod  65  downward and separates the upper end of the solenoid rod  65  from the valve element  46 . In this state, the biasing force f 1  of the spring  47  and the biasing force (Pc−Ps) B of the differential pressure between the upper and lower regions  43   a,    43   b  of the valve chamber  43  cause the valve element  46  to contact the valve seat  44 . 
     If the crank pressure Pc is greater than the biasing force f 0  of the bellows  52  (f 0 ≦Pc×A) when cooling is not required (the Coil  67  being de-energized), the movable lower end of the bellows  52  separates from the valve element  46  and thus does not bias the valve element  46 . On the other hand, if the biasing force f 0  of the bellows  52  is greater than the crank pressure Pc (f 0 &gt;Pc×A) when cooling is not required, the lower end of the bellows  52  biases the valve element  46  in the direction that closes the control valve  40 A. In each case, the crank pressure Pc does not act to bias the valve element  46  in the direction opening the control valve  40 A and the valve element  46  is kept in contact with the valve seat  44 . Thus, the valve  40 A is completely closed and the flow of refrigerant gas in the bleeding passage from the crank chamber  3  to the suction chamber  8  is stopped. This causes the blowby gas to increase the crank pressure Pc and move the drive plate  17  to the minimum inclination position. 
     The advantages of the first embodiment will now be described. 
     The valve element  46  is kept in contact with the valve seat  44  and is unaffected by the crank pressure PC and the suction pressure Ps when the coil  67  of the solenoid  60  is not energized. Since the control valve  40 A remains closed when the air-conditioner switch  37  is OFF or when the cooling load is small, the crank pressure Pc increases and holds the drive plate  17  at the minimum inclination position. Thus, the compressor can perform minimum displacement operation continuously. Accordingly, the control valve  40 A is optimal for employment in a clutchless type variable displacement compressor such as that shown in FIG.  1 . 
     In the control valve  40 A, the effective area A of the bellows  52  is equal to the opening area B. This causes the current I flowing through the coal to directly determine the suction pressure Ps. Therefore, the target suction pressure Pset may be selected from a range that corresponds to the controllable range of the current I (I min  to I max ). Accordingly, the target suction pressure Pset can be selected from a relatively wide range when controlling the control valve  40 A. 
     [Second Embodiment] 
     A control valve  40 B according to a second embodiment of the present invention will now be described with reference to FIG.  3 . The valve element, the solenoid rod, and the movable core employed in the control valve  40 B of FIG. 3 differ from those of the control valve  40 A of FIG.  2 . 
     In the control valve  40 A of FIG. 2, the valve element  46  and the solenoid rod  65  are separate, and the solenoid rod  65  and the movable core  64  are integrally joined with each other. However, in the control valve  40 B of FIG. 3, a valve element  46   a  and a solenoid rod  46   b  are integrally formed, and the movable core  64  is separate from the rod  46   b.    
     The control valve  40 B of the second embodiment has the same advantages as the control valve  40 A of the first embodiment. 
     [Third Embodiment] 
     A control valve  40 C according to the present invention will now be described with reference to FIG.  4 . Although the control valve  40 C includes a valve mechanism  42  and a solenoid  60  like the control valve  40 A of FIG. 2, the structure of the valve mechanism  42  differs from that of the control valve  40 A. 
     In the control valve  40 C of FIG. 4 the valve mechanism  42  includes a valve housing  41 , which is defined by a main body  41   a,  a generally cylindrical first cover  41   b  located a above the main body  41   a,  and a cap-like second cover  41   c  located above the first cover  41   b.  The valve housing  41  houses a valve chamber  43 . A valve seat  44  extends along the wall of the middle portion of the valve chamber  43 . An upper region (crank chamber side region)  43   a  is defined above the valve seat  44  in the valve chamber  43 , and a lower region (suction chamber side region)  43   b  is defined below the valve seat  44  in the valve chamber  43 . 
     An entrance port  48  extends through the peripheral wall of the second cover  41   c  from the upper region  43   a  of the valve chamber  43 . A passage  50  extending through the compressor is connected with the entrance port  48 . The passage  50  connects the upper region  43   a  to the crank chamber  3 . An exit port  49  extends through the peripheral wall of the main body  41   a.  A passage  51  extending through the compressor is connected with the exit port  49 . The passage  51  connects the lower region  43   b  to the suction chamber  8 . Accordingly, a bleeding passage is defined between the crank chamber  3  and the suction chamber  8  by the passage  50 , the entrance port  48 , the valve chamber  43 , the exit port  49 , and the passage  51 . 
     A valve element  46  is retained in the upper region  43   a  of the valve chamber  43 . The valve element  46  is movable in the axial direction (vertical direction of the control valve  40 C) toward or away from the valve seat  44 . When the valve element  46  contacts the valve seat  44 , the valve element  46  closes the valve port  45  and disconnects the upper region  43   a  from the lower region  43   b.  The valve element  46  is cylindrical but has an upper step and a lower step. A spring  47  is held between the lower step and a step formed on the inner wall of the first cover  41   b.  The spring  47  constantly biases the valve element  46  toward the valve seat  44  (i.e., in a direction closing the valve port  45 ). 
     A bellows  52  is arranged in the upper region  43   a  of the valve chamber  43 . The effective area A of the bellows  52  is equal to the opening area B of the lower region  43   b  (A=B). As shown in FIG. 4, the upper end of the bellows  52  is engaged with an indentation formed in the top part of the second cover  41   c.  A spring  54  is arranged between the lower end of the bellows  52  and the upper step of the valve element  46 . The bellows  52  is pressed against the second cover  41   c  and is held between the second cover  41   c  and the valve element  46 . Thus, the upper end of the bellows  52  is fixed, and the lower end of the bellows  52  is movable. 
     The interior of the bellows  52  is in a vacuum, or is depressurized, and accommodates a spring  52   a.  The spring  52   a  biases the lower movable end of the bellows  52  axially toward the valve element  46 . Refrigerant gas is drawn into the upper region  43   a  of the valve chamber  43  through the passage  50  and the entrance port  48 . Thus, the bellows  52  expands and presses against the valve element  46  or contracts and separates from the valve element  46  depending on the crank pressure Pc. The opening size of the control valve  40 C (i.e., the opening size of the bleeding passage) is adjusted in accordance with the location of the valve element  46  in the valve chamber  43 . The pressure of the suction chamber  8  (suction pressure Ps) is applied to the lower region  43   b  of the valve chamber  43 . 
     The control valve  40 C, which is used in the compressor of FIG. 1, functions in the same manner as the control valve  40 A of the first embodiment. If the air-conditioner switch  37  is ON when the engine E is running, the computer  38  energizes the coil  67  to adjust the opening size of the control valve  40 C. This determines the inclination of the drive plate  17 , the compressor displacement, and the suction pressure Ps. The spring  54  functions as part of the bellows  52 . Thus, the downward biasing force f 0  of the bellows  52  includes the force of the springs  54  and  52   a.  Accordingly, equations (1) to (3) are also applied to the control valve  40 C of FIG.  4 . Thus, the suction pressure Ps is determined by the current I that energizes the coil  67  without influence from the crank pressure Pc. 
     If the air-conditioner switch  37  is OFF when the engine E is running or if the cooling load is small when the air-conditioner switch is ON, the computer  38  stops the flow of current to the coil  67 . This permits the spring  66  to move the movable core  64  and the solenoid rod  65  downward and separates the upper end of the solenoid rod  65  from the valve element  46 . As a result, the biasing force f 1  of the spring  47  and the biasing force (Pc−Ps) B produced by the differential pressure between the upper and lower regions  43   a,    43   b  of the valve chamber  43  are applied to the valve element  46 , which causes the valve element  46  to contact the valve seat  44 . The crank pressure Pc does not act to move the valve element  46  in a direction opening the control valve  40 C. Thus, the control valve  40 C is fully closed which prevents the flow of refrigerant gas through the bleeding passage from the crank chamber  3  to the suction chamber  8 . As a result, blowby gas increases the crank pressure Pc and moves the drive plate  17  toward the minimum inclination position. Accordingly, the control valve  40 C of FIG. 4 has the same advantages as the control valve  40 A of FIG.  2 . 
     [Fourth Embodiment] 
     A control valve  40 D according to a fourth embodiment of the present invention will now be described with reference to FIG.  5 . The valve body, the solenoid rod, and the movable core differ from those of the control valve  40 C of FIG.  4 . 
     In the control valve  40 C of FIG. 4, the valve element  46  and the solenoid rod  65  are separate, and the solenoid rod  65  and the movable core  64  are integrally joined with each other. However, in the control valve  40 D of FIG. 5, a valve element  46   a  and a solenoid rod  46   b  are integrally formed. Furthermore, the solenoid rod  46   b  and the movable core  64  are separate as in the embodiment of FIG.  3 . 
     Although the structure of the control valve  40 D differs from that of the control valve  40 C, the control valves  40 C,  40 D have substantially the same advantages. 
     It should be apparent to those skilled in the art that the present invention may be embodied in many other specific forms without departing from the spirit or scope of the invention. Particularly, it should be understood that the present invention may be embodied in the following forms. 
     A bellows  52  is employed in each of the above embodiments. However, the bellows  52  may be replaced by a diaphragm. 
     Each of the control valves  40 A- 40 D may be employed in a compressor that uses a clutch to transmit the power of an external drive source to the compressor. 
     The present invention may be employed in a compressor that uses a swash plate or an inclined cam plate as the drive plate. 
     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.