Abstract:
A vehicle air conditioner including a refrigerant circuit that incorporates a variable displacement compressor driven by an engine of the vehicle. The air conditioner includes a control valve for varying the displacement of the compressor and an ECU for controlling the control valve. The control valve has a bellows, a valve body, and a coil. The ECU varies the displacement of the compressor by energizing the coil to apply a force, which counters the movement of the bellows, to the valve body to move the valve body. This alters the moved amount of the bellows. The air conditioner control unit gradually changes the force applied to the valve body by the coil to adjust the pressure difference and vary the displacement of the compressor when the engine is running at an idle speed.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to an apparatus and method for controlling a vehicle air conditioner having a variable displacement compressor. 
     In the prior art, when an engine of a vehicle is idling, the activation of a variable displacement compressor, which is incorporated in an air conditioner of the vehicle, results in the execution of idle-up control, which increases the idle speed of the engine. The increase in the idle speed produces the torque required to drive the compressor and prevents the engine from stalling. Further, the increase in the idle speed enables the compressor to cope with high cooling loads. 
     There is a recent trend for decreasing the idle speed to improve fuel efficiency. However, the execution of the idle-up control when the compressor is activated decreases fuel efficiency. 
     The prior art idle-up control is always executed when the engine is idling and the compressor is activated. Accordingly, the idle speed fluctuates whenever the compressor is activated or deactivated. This increases the vibrations and noise of the vehicle. 
     Further, the increased amount of the idle speed during the idle-up control is determined presuming that the torque required to drive the compressor is maximal, that is, the displacement of the compressor is maximal. Accordingly, if, for example, the displacement of the compressor is small and the torque required to drive the compressor is low, the idle speed is increased in an unnecessary manner. This is not desirable from the viewpoint of fuel efficiency. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an apparatus and method for controlling the displacement of a variable displacement compressor in accordance with the engine idle speed. 
     To achieve the above object, the present invention provides a vehicle air conditioner including a refrigerant circuit that incorporates a variable displacement compressor driven by an engine of the vehicle. The air conditioner includes a control valve for varying the displacement of the compressor. An air conditioner control unit controls the control valve. The control valve includes a pressure sensing mechanism having a valve body and a pressure sensing member connected to the valve body and moved in accordance with a pressure difference between two pressure monitoring points located along the refrigerant circuit. The pressure difference corresponds to the displacement of the compressor and alters the moved amount of the pressure sensing member. A pressure difference adjusting actuator is controlled by the air conditioner control unit. The pressure difference adjusting actuator applies a force, which counters the movement of the pressure sensing member, to the valve body to move the valve body and alter the moved amount of the pressure sensing member. The pressure difference adjusting actuator further adjusting the force applied to the valve body to alter the moved amount of the pressure sensing member and vary the displacement of the compressor. The air conditioner control unit changes the force of the pressure difference adjusting actuator applied to the valve body to adjust the pressure difference and vary the displacement of the compressor. The changes in force when the engine is running at an idle speed is more gradual than when the engine is running at a speed other than the idle speed. 
     A further perspective of the present invention is a method for controlling a vehicle air conditioner including a refrigerant circuit that incorporates a variable displacement compressor driven by an engine of the vehicle, a pressure sensing mechanism, and a pressure difference adjusting actuator. The pressure sensing mechanism has a valve body and a pressure sensing member, which is connected to the valve body and moved in accordance with the pressure difference between two pressure monitoring points located along the refrigerant circuit. The pressure difference adjusting actuator applies a force, which counters the movement of the pressure sensing member, to the valve body to move the valve body, and changes the force applied to the valve body to alter the moved amount of the pressure sensing member, adjust the pressure difference, and vary the displacement. The method includes changing the force of the pressure difference adjusting actuator applied to the valve body to adjust the pressure difference and vary the displacement of the compressor when the engine is running at an idle speed. The changes in force when the engine is running at the idle speed are more gradual than when the engine is running at a speed other than the idle speed. 
     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 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 of a variable displacement compressor according to a preferred embodiment of the present invention; 
     FIG. 2 is a cross-sectional view showing a control valve incorporated in the compressor of FIG. 1; 
     FIG. 3 is a flowchart of a process executed by an engine ECU in the preferred embodiment; 
     FIG. 4 is a flowchart of a process executed by an air conditioner ECU in a normal state; 
     FIG. 5 is a flowchart of a process executed by the air conditioner electronic control unit (ECU) in an idle state; and 
     FIG. 6 is a cross-sectional view showing another control valve that may be incorporated in the compressor of FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A preferred embodiment of the present invention will now be discussed with reference to the drawings. 
     [Air Conditioner and Idle Speed Control Apparatus] 
     Referring to FIG. 1, an engine E, which is a drive source of a vehicle, includes an idle speed control valve (ISCV)  65 . When the engine E is idling, the ISCV  65  functions to adjust the amount of the intake air drawn into the engine E. 
     The engine E has an output shaft, which is connected to a swash plate type variable displacement compressor  40  by means of a power transmission mechanism PT. The compressor  40  is included in a refrigerant circuit (refrigerating cycle). 
     Referring to FIG. 2, the vehicle is provided with an engine ECU  71 , which controls the ISCV  65 , and an air conditioner (A/C) ECU  72 . The engine ECU  71  and the A/C ECU  72  communicate with each other. The engine ECU  71  functions to control the idle speed and alter a target idle speed. The A/C ECU  72  functions to control the compressor  40  and also functions to change the target idle speed. 
     The engine ECU  71  is connected to a vehicle state detector  73 . The vehicle state detector  73  includes a vehicle velocity sensor  74 , an engine speed sensor  75 , and a throttle position sensor  76 . The vehicle velocity sensor  74  detects the traveling velocity of the vehicle. The engine speed sensor  75  detects the engine speed. The throttle position sensor  76  detects the angle of a throttle (not shown) that changes in accordance with the depressed amount of an acceleration pedal (not shown). 
     The A/C ECU  72  is connected to an A/C state detector  77 . The A/C state detector  77  includes an A/C switch  79 , a temperature setting device  80 , and a temperature sensor  81 , which generate signals provided to the A/C ECU  72 . The A/C switch  79  is used to activate and deactivate the air conditioner and generates a signal indicating whether the air conditioner is activated. The temperature setting device  80  is used to set a target temperature Te set  of the passenger compartment (not shown) and generates a signal indicating the target temperature Te set . The temperature sensor  81  detects the actual temperature Te t  of the passenger compartment and generates a signal indicating the detected temperature Te t . 
     [Compressor] 
     Referring to FIG. 1, the compressor  40  has a housing  11 . A crank chamber  12  is defined in the housing  11 . A drive shaft  13  is rotatably arranged in the crank chamber  12 . The drive shaft  13 , which is connected to the engine E by the power transmission mechanism PT, is rotated by the engine E. 
     In the preferred embodiment, the power transmission mechanism PT does not have a clutch mechanism. Thus, the power of the engine E is constantly transmitted to the compressor  40 . However, the power transmission mechanism PT may be provided with a clutch (e.g., electromagnetic clutch) that disconnects the compressor  40  from the engine E. 
     A lug plate  14  is fixed to the drive shaft  13  in the crank chamber  12  and rotates integrally with the drive shaft  13 . A swash plate  15  is accommodated in the crank chamber  12 . The swash plate  15  is supported so that it inclines as it moves along the drive shaft  13 . A hinge mechanism  16  is arranged between the lug plate  14  and the swash plate  15 . The hinge mechanism  16  enables the swash plate  15  to incline while rotating integrally with the lug plate  14  and the drive shaft  13 . 
     A plurality of cylinder bores  11   a  (only one shown in FIG. 1) are formed in the housing  11 . A piston  17  is reciprocally retained in each cylinder bore  11   a.  Each piston  17  is engaged with the peripheral portion of the swash plate  15  by a pair of shoes  18 . As the drive shaft  13  rotates, the shoes  18  convert the rotating motion of the swash plate  15  to the reciprocating motion of the piston  17 . 
     A valve plate  19  is arranged at the rear (toward the right as viewed in FIG. 1) of the cylinder bores  11   a.  A compression chamber  20  is defined in each cylinder bore  11   a  between the associated piston  17  and the valve plate  19 . A suction chamber  21  and a discharge chamber  22  are defined in the rear portion of the housing  11 . 
     As each piston  17  moves from its top dead center position to its bottom dead center position, refrigerant gas is drawn from the suction chamber  21  into the associated compression chamber  20  through a suction port  23  and a suction valve  24 , which are formed in the valve plate  19 . The refrigerant gas drawn into the compression chamber  20  is compressed to a predetermined pressure as the piston  17  moves from the bottom dead center position to the top dead center position. Then, the refrigerant gas is discharged into the discharge chamber  22  through a discharge port  25  and a discharge valve  26 , which are formed in the valve plate  19 . 
     [Displacement Control Mechanism of Compressor] 
     As shown in FIG. 1, a bleeding passage  27 , a first gas supplying passage  28   a,  and a second gas supplying passage  28   b  are provided in the housing  11 . The bleeding passage  27  connects the crank chamber  12  to the suction chamber  21 . The first and second gas supplying passages  28   a,    28   b  connect the discharge chamber to the crank chamber  12 . A control valve CV is arranged between the first and second gas supplying passages  28   a,    28   b  in the housing  11 . 
     An opened amount of the control valve CV is varied to adjust the amount of high pressure discharge gas sent into the crank chamber  12  through the first and second gas supplying passages  28   a,    28   b  and the amount of gas sent out from crank chamber  12  through the bleeding passage  27 . In other words, the control valve CV controls the balance between the gas amount sent into the crank chamber  12  and the gas amount sent out from the crank chamber  12  to determine the pressure of the crank chamber  12 . The pressure of the crank chamber  12  is changed to adjust the difference between the pressure of the crank chamber  12  and the pressure of the compression chambers  20 , which act on the pistons  17 . This changes the inclination of the swash plate  15 , alters the stroke of the pistons  17 , and varies the displacement of the compressor  40 . 
     For example, when the pressure of the crank chamber  12  decreases, the inclination of the swash plate  15  increases, and the displacement of the compressor  40  increases. The broken lines in FIG. 1 show the swash plate  15  arranged at a maximum inclination position. In this state, the swash plate  15  is in contact with the lug plate  14 . This restricts further inclination of the swash plate  15 . When the pressure of the crank chamber  12  increases and the inclination of the swash plate  15  decreases, the displacement of the compressor  40  decreases. The solid lines in FIG. 1 show the swash plate  15  arranged at a minimum inclination position. In this state, the swash plate  15  is inclined relatively to a plane perpendicular to the axis of the drive shaft  13  at an angle that is slightly greater than zero. 
     [Refrigerant Circuit] 
     Referring to FIG. 1, a refrigerant circuit (refrigerating cycle) of the vehicle air conditioner is formed by the compressor  40  and an external refrigerant circuit  30 . The external refrigerant circuit  30  includes a condenser  31 , an expansion valve  32 , and an evaporator  33 . 
     In the refrigerant circuit, a shutting valve  34  is arranged between the discharge chamber  22  of the compressor  40  and the condenser  31 . The shutting valve  34  shuts the passage between the discharge chamber  22  and the condenser  31  when the pressure of the discharge chamber  22  is lower than a predetermined value to stop circulating refrigerant through the external refrigerant circuit  30 . 
     The shutting valve  34  may be a differential valve that detects the difference between the pressure at its upstream side and the pressure at its downstream side and functions in accordance with the pressure difference. Alternatively, the shutting valve  34  may be an electromagnetic valve controlled by the A/C ECU  72  in accordance with the detection of a discharge pressure sensor (not shown). Further, the shutting valve  34  may be a valve that closes mechanically when the swash plate  15  is arranged at the minimum inclination position. 
     The refrigerant circuit includes a first pressure monitoring point P 1  and a second pressure monitoring point P 2 . The first pressure monitoring point P 1  is located in the discharge chamber P 1 . The second pressure monitoring point P 2  is arranged downstream of the first pressure monitoring point P 1 , or between the shutting valve  34  and the condenser  31 . The difference between the pressure PdH at the first pressure monitoring point P 1  and the pressure PdL at the second pressure monitoring point P 2  reflects the amount of refrigerant flowing through the refrigerant circuit. The first pressure monitoring point P 1  and the control valve CV are connected by a first pressure detection passage  35 . The second pressure monitoring point P 2  and the control valve CV are connected by a second pressure detection passage  36  (FIG.  2 ). 
     [Control Valve] 
     As shown in FIG. 2, the control valve CV has a valve housing  41  in which a valve chamber  42 , a communication passage  43 , and a pressure sensing chamber  44  are defined. A rod  45 , which is movable in its axial direction, is arranged in the valve chamber  42  and the communication passage  43 . The top portion of the rod  45 , which is inserted in the communication passage  43 , disconnects the communication passage  43  from the pressure sensing chamber  44 . The valve chamber  42  is connected to the discharge chamber  22  by the first gas supplying passage  28   a.  The communication passage  43  is connected to the crank chamber  12  through the second gas supplying passage  28   b.  The valve chamber  42  and the communication passage  43  are located between the first and second gas supplying passages  28   a,    28   b.    
     A valve body  46 , which is defined on the middle portion of the rod  45 , is arranged in the valve chamber  42 . A valve seat  47  is defined at the boundary between the valve chamber  42  and the communication passage  43 . The communication passage  43  functions as a valve hole. When the rod  45  moves upward from the state shown in FIG. 2 (lowermost position) to an uppermost position at which the valve body  46  is received by the valve seat  47 , the communication passage  43  is closed. In other words, the valve body  46  of the rod  45  functions to adjust the opened amount of the gas supplying passage  28 . 
     A pressure sensing member, or bellows  48 , is accommodated in the pressure sensing chamber  44 . The top of the bellows  48  is fixed to the valve housing  41 . The bottom of the bellows  48  is fixed to the top portion of the rod  45 . In the pressure sensing chamber  44 , the internal space of the bellows  48  defines a first pressure chamber  49  and the external space of the bellows  48  defines a second pressure chamber  50 . The pressure PdH at the first pressure monitoring point P 1  is communicated to the first pressure chamber  49  via the first pressure detection passage  35 . The pressure PdL at the second pressure monitoring point P 2  is communicated to the second pressure chamber  50  via the second pressure detection passage  36 . The valve body  46 , the bellows  48 , and the pressure sensing chamber  44  form a pressure sensing mechanism. 
     An electromagnetic actuator (pressure difference adjusting actuator)  51  is arranged in the lower portion of the valve housing  41 . A cylindrical sleeve  52 , which has a closed bottom, extends through the center of the electromagnetic actuator  51 . A fixed core  53  is fitted in the sleeve  52 . A plunger chamber  54  is defined in the sleeve  52  below the fixed core  53 . 
     A plunger  56 , which is made of a magnetic material and axially movable, is retained in the plunger chamber  54 . A guide bore  57  extends axially through the center of the fixed core  53 . The lower portion of the rod  45 , which is axially movable, is arranged in the guide bore  57 . The bottom end of the rod  45  is engaged with the top end of the plunger  56  in the plunger chamber  54 . 
     A plunger spring  60  is retained in the plunger chamber  54  between the bottom surface of the sleeve  52  and the plunger  56 . The plunger spring  60  urges the plunger  56  toward the fixed core  53 . The elastic force of the bellows  48  urges the rod  45  toward the plunger  56 . Accordingly, the plunger  56  and the rod  45  always move upward and downward integrally. The force of the bellows  48  is stronger than the force of the plunger spring  60 . 
     A coil  61  is wound around the fixed core  53  and the plunger  56  on the peripheral surface of the sleeve  52 . The A/C ECU  72  instructs a drive circuit  78  to supply the coil  61  with power in accordance with the information provided from the A/C state detector  77 . 
     An electromagnetic force (electromagnetic attracting force), which corresponds to the amount of power supplied to the coil  61  by the drive circuit  78 , is produced between the plunger  56  and the fixed core  53 . The electromagnetic force attracts the plunger  56  toward the fixed core  53 . The voltage applied to the coil  61  is adjusted to control the amount of power supplied to the coil  61 . Pulse width control (pulse width modulation) is executed to adjust the applied voltage. 
     As shown in the state of FIG. 2, when the drive circuit  78  does not supply the coil  61  with power (duty ratio Dt=0%), the dominant force in the control valve CV is the downward urging force of the bellows  48 . Thus, the rod  45  is arranged at its lowermost position, and the valve body  46  completely opens the communication passage  43 . Thus, the pressure of the crank chamber  12  is increased to the highest value possible under the present circumstances. This increases the difference between the pressure of the crank chamber  12  and the pressure of the compression chambers  20  acting on the pistons  17 . In this state, the swash plate  15  is arranged at the minimum inclination position, and the displacement of the compressor  40  is minimal. 
     When the displacement of the compressor  40  is minimal, the shutting valve  34  closes since the pressure of the discharge chamber  22  is lower than the predetermined value. This stops circulating refrigerant through the external refrigerant circuit  30 . In this state, the compressor  40  continuously compresses refrigerant gas but the air conditioner does not cool the passenger compartment. In other words, the compressor  40  is substantially deactivated. 
     The inclination of the swash plate  15  is not zero when arranged at the minimum inclination position. Thus, even if the displacement of the compressor  40  is minimized, refrigerant gas is drawn into the compression chambers  20  from the suction chamber  21 , compressed, and then discharged from the compression chambers  20  into the discharge chamber  22 . Accordingly, an internal refrigerant circuit extending from the discharge chamber  22 , through the first and second gas supplying passages  28   a,    28   b,  the crank chamber  12 , the bleeding passage  27 , the suction chamber  21 , the compression chambers  20 , and back to the suction chamber  21  is formed in the compressor  40 . Refrigerant and lubricating oil, which is suspended in the refrigerant, circulates through the internal refrigerant circuit. Thus, lubricating oil remains in the compressor  40  and continues to lubricate moving parts (e.g., the swash plate  15  and the shoes  18 ) in a satisfactory state. 
     The drive circuit  78  controls a duty ratio Dt to adjust the power supplied to the coil  61 . The duty ratio Dt is variable within a predetermined range. When the drive circuit  78  supplies the coil  61  with power corresponding to the minimum duty ratio DT min  (Dt&gt;0%) or greater, an upward electromagnetic urging force is added to the force of the plunger spring  60 . Thus, the upward urging force overcomes the downward urging force of the bellows  48  and moves the rod  45  upward. In this state, the electromagnetic force, which is added to the upward urging force of the plunger spring  60 , counters the downward urging force that is produced by the pressure difference ΔPd between the first and second pressure monitoring points (PdH−PdL) and added to the force of the bellows  48 . The valve body  46  of the rod  45  is positioned relative to the valve seat  47  at a location where the upper and lower urging forces are balanced. This adjusts the displacement of the compressor  40 . In this state, the compressor  40  is activated and the compressed refrigerant gas is sent to the external refrigerant circuit  30 . 
     For example, when the engine speed Ne decreases, the flow rate of the refrigerant in the refrigerant circuit decreases the downward urging force produced by the pressure difference ΔPd. This upsets the balance between the upward and downward urging forces that was obtained with the electromagnetic force. Accordingly, the rod  45  (valve body  46 ) moves upward, decreases the opened amount of the communication passage  43 , and decreases the pressure of the crank chamber  12 . This moves the swash plate  15  toward the maximum inclination position and increases the displacement of the compressor  40 . The increase in the displacement of the compressor  40  increases the flow rate of the refrigerant in the refrigerant circuit. As a result, the pressure difference ΔPd increases. 
     On the other hand, when the engine speed Ne increases, the flow rate of the refrigerant in the refrigerant circuit increases the downward urging force produced by the pressure difference ΔPd. This upsets the balance between the upward and downward urging forces that was obtained with the electromagnetic force. Accordingly, the rod  45  (valve body  46 ) moves downward, increases the opened amount of the communication passage  43 , and increases the pressure of the crank chamber  12 . This moves the swash plate  15  toward the minimum inclination position and decreases the displacement of the compressor  40 . The decrease in the displacement of the compressor  40  decreases the flow rate of the refrigerant in the refrigerant circuit. As a result, the pressure difference ΔPd decreases. 
     Further, for example, when the duty ratio Dt of the coil  61  is increased to increase the upward electromagnetic force, this upsets the balance between the upward and downward urging forces that was obtained with the force produced in accordance with the pressure difference ΔPd. Thus, the rod  45  (valve body  46 ) moves upward, decreases the opened amount of the communication passage  43 , and increases the displacement of the compressor  40 . As a result, the flow rate of the refrigerant in the refrigerant circuit increases. This increases the pressure difference ΔPd. 
     When the duty ratio Dt of the coil  61  is decreased to decrease the upward electromagnetic force, this upsets the balance between the upward and downward urging forces that was obtained with the force produced in accordance with the pressure difference ΔPd. Thus, the rod  45  (valve body  46 ) moves downward, increases the opened amount of the communication passage  43 , and decreases the displacement of the compressor  40 . As a result, the flow rate of the refrigerant in the refrigerant circuit decreases. This decreases the pressure difference ΔPd. 
     Accordingly, the control valve CV automatically moves the rod  45  (valve body  46 ) when the pressure difference ΔPd fluctuates to maintain the pressure difference ΔPd at its target value, which is determined by the duty ratio Dt of the coil  61 . The pressure difference ΔPd may be adjusted by an external device that controls the duty ratio Dt of the coil  61 . 
     [Operation of the Engine ECU] 
     When the engine E is running, the engine ECU  71  executes the process illustrated in FIG.  3 . 
     In step S 301 , the engine ECU  71  determines whether the conditions for executing an idling state intake air amount control (hereafter simply referred to as idling control) are satisfied by referring to the information provided by the vehicle state detector  73 . For example, if the ECU  71  receives information indicating that the vehicle velocity is zero and that the throttle is completely closed from the vehicle state detector  73 , the ECU  71  determines that the conditions for executing the idling control are satisfied. 
     If the engine ECU  71  determines that the conditions for executing the idling control are not satisfied in step S 301 , the ECU  71  proceeds to step S 302  and informs the A/C ECU  72  that the idling control execution conditions are not satisfied. The engine ECU  71  then returns to step S 301  from step S 302  and repetitively monitors the idling control execution conditions. 
     If the engine ECU  71  determines that the conditions for executing the idling control are satisfied in step S 301 , the ECU  71  proceeds to step S 303  and informs the A/C ECU  72  that the idling control execution conditions are satisfied. The engine ECU  71  then proceeds from step S 303  to step S 304  and determines whether the A/C ECU  72  is generating an idle-up request. If the engine ECU  71  determines that an idle-up request is not being generated in step S 304 , the engine ECU  71  proceeds to step S 305  and sets a target idle speed Ne set  at a predetermined first value Ne set1  (e.g., 700 rpm). 
     If the engine ECU  71  determines that the A/C ECU  72  is generating an idle-up request, the engine ECU  71  proceeds to step S 306  and sets the target idle speed Ne set  at a predetermined second value Ne set2  (e.g., 900 rpm), which is greater than the first value Ne set1 . 
     The engine ECU  71  proceeds from step S 305  or step S 306  to step S 307  and executes idling control, which is known in the art. More specifically, the engine ECU  71  operates the ISCV  65  to increase or decrease the idle state intake air amount while referring to the information of the engine speed Ne from the vehicle state detector  73  so that the engine speed Ne matches the target idle speed Ne set1 . 
     [Operation of the A/C ECU] 
     Normal State 
     In a state in which the engine E is running normally and the engine ECU  71  informs the A/C ECU  72  that the idling control execution conditions are not satisfied, the A/C ECU  72  continues to execute the process illustrated in FIG. 4 until informed that the idling control execution conditions are satisfied. 
     In step S 101 , the A/C ECU  72  performs various initializations in accordance with an initialization program. For example, the A/C ECU  72  sets the duty ratio Dt of the control valve CV at an initial value of zero (i.e., the coil  61  not being supplied with power). 
     In step S 102 , the A/C ECU  72  checks whether the A/C switch  79  is turned on. If the A/C switch  79  is turned on, the A/C ECU  72  proceeds to step S 103  and sets the duty ratio Dt of the control valve CV at the minimum duty ratio Dt min . 
     In step S 104 , the A/C ECU  72  determines whether the detected temperature Te t  of the temperature sensor  81  is greater than the target temperature Te set  set by the temperature setting device  80 . If the detected temperature Te t  is not greater than the target temperature Te set , the A/C ECU  72  proceeds to step S 105  and determines whether the detected temperature Te t  is less than the target temperature Te set . If the detected temperature Te t  is not less than the target temperature Te set , this indicates that the detected temperature Te t  is equal to the target temperature Te set . In such state, there is no need to change the duty ratio Dt. Thus, the A/C ECU  72  proceeds to step S 108  without instructing the drive circuit  78  to change the duty ratio Dt. 
     If the A/C ECU  72  determines that the detected temperature Te t  is greater than the target temperature Te set  in step S 104 , this indicates that the passenger compartment is hot and that the compressor  40  must operate under a large cooling load. The A/C ECU  72  thus proceeds to step S 106  and increases the duty ratio Dt by a predetermined grading amount ΔD and instructs the drive circuit  78  to change the duty ratio Dt to the corrected value (Dt+ΔD). This slightly decreases the opened amount of the control valve CV and increases the displacement of the compressor  40 . As a result, the amount of heat exchanged by the evaporator  33  increases and the temperature Te 1  decreases. 
     If the A/C ECU  72  determines that the detected temperature Te t  is less than the target temperature Te set  in step S 105 , this indicates that the temperature of the passenger compartment does not have to be decreased and that the cooling load applied to the compressor  40  is small. The A/C ECU  72  thus proceeds to step S 107  and decreases the duty ratio Dt by a predetermined grading amount ΔD and instructs the drive circuit  78  to change the duty ratio Dt to the corrected value (Dt−ΔD). This slightly increases the opened amount of the control valve CV and decreases the displacement of the compressor  40 . As a result, the amount of heat exchanged by the evaporator  33  decreases and the temperature Te 1  increases. 
     At step S 108 , the A/C ECU  72  determines whether the A/C switch  79  is turned off. If the A/C switch  79  is not turned off, the A/C ECU  72  returns to step S 104  and repeats the subsequent steps. If the A/C switch  79  is turned off, the A/C ECU  72  proceeds to step S 101 . This sets the duty ratio Dt of the power supplied to the coil  61  of the control valve CV at zero. In such state, the compressor  40  is substantially deactivated. 
     The correction of the duty ratio Dt in step S 106  and step S 107  and the automatic valve opening adjustment of the control valve CV gradually converge the detected temperature Te t  to the target temperature Te set . 
     Idle State 
     If the engine ECU  71  informs the A/C ECU  72  that the idling control execution conditions are satisfied when the engine E is running, the A/C ECU  72  continues to execute the process illustrated in FIG. 5 until informed that the idling control execution conditions are not satisfied. The engine ECU  71  uses the first value Ne set  as the target idle speed Ne set  when executing the idling control of the engine E. 
     In step S 201 , the A/C ECU  72  performs initialization in the same manner as in step S 101  of FIG.  4 . In step S 202 , the A/C ECU  79  checks whether the A/C switch  79  is turned on or off in the same manner as step S 102 . Further, when the A/C switch  79  is turned on, the A/C ECU  72  proceeds to step S 203  and sets the duty ratio Dt of the control valve CV at the minimum duty ratio Dt min  in the same manner as in step S 103 . Then, in steps S 204  and S 205 , the A/C ECU  72  determines the relationship between the detected temperature Te t  and the target temperature Te set  in the same manner as in steps S 104  and S 105 . 
     If the A/C ECU  72  determines that the detected temperature Te t  is greater than the target temperature Te set  in step S 204 , the A/C ECU  72  proceeds to step S 206  and increases the duty ratio Dt by a predetermined grading amount ΔD/10 and instructs the drive circuit  78  to change the duty ratio Dt to the corrected value (Dt+ΔD/10). If the A/C ECU  72  determines that the detected temperature Te t  is less than the target temperature Te set  in step S 205 , the A/C ECU  72  proceeds to step S 207  and decreases the duty ratio Dt by the predetermined grading amount ΔD/10 and instructs the drive circuit  78  to change the duty ratio Dt to the corrected value (Dt−ΔD/10). 
     Accordingly, in steps S 206  and S 207 , the grading amount used to change the duty ratio Dt when the engine E is idling is less than that used to change the duty ratio Dt when the engine E is running normally (in the preferred embodiment, one tenth). The grading amount ΔD/10 is set so that the duty ratio Dt increases from the minimum value Dt min  to the maximum value of the duty ratio range within about 5 to 15 seconds. 
     Therefore, in comparison to when the engine E is running normally, the duty ratio Dt is gradually changed by a smaller amount when the engine E is idling. In other words, more time is required to change the duty ratio Dt to a certain value. Thus, the displacement of the compressor  40  varies in a gradual manner, and the torque required to drive the compressor  40  changes in a gradual manner. As a result, the engine ECU  71  responds properly to fluctuations of the engine speed Ne, which is caused by changes in the torque of the compressor  40 , when performing idling control. This prevents the difference between the engine speed E and the target idle speed Ne set  from becoming large and destabilizing the idling state of the engine E. 
     The A/C ECU  72  proceeds from step S 206  to step S 208  to determine whether the duty ratio Dt of the control valve CV is greater than a predetermined threshold value Dt ref . The threshold value Dt ref  corresponds to the pressure difference ΔPd required for the compressor  40  to obtain its maximum displacement in a state in which the engine speed Ne is equal to the first target idle speed Ne set1 . 
     Accordingly, when the duty ratio Dt is not greater than the predetermined threshold value Dt ref  in step S 208 , the necessary flow rate of the refrigerant in the refrigerant circuit may be obtained by increasing the displacement of the compressor  40  even if the engine speed Ne is equal to the first target idle speed Ne set1 . In other words, the refrigerant flow rate may be increased without increasing the engine speed Ne when the engine E is idling. In step S 209 , the A/C ECU  72  thus informs the engine ECU  71  that there is no need to execute the idle-up control. Hence, the engine ECU  71  performs idling control using the first target idle speed Ne set1  (refer to step S 305  of FIG.  3 ). 
     When the duty ratio Dt is greater than the predetermined threshold value Dt ref  in step S 208 , the necessary flow rate of the refrigerant in the refrigerant circuit cannot be obtained even if the displacement of the compressor  40  is increased as long as the engine speed Ne is equal to the first target idle speed Ne set1 . In step S 210 , the A/C ECU  72  thus requests the engine ECU  71  to execute the idle-up control. Hence, the engine ECU  71  performs idling control using the second target idle speed Ne set2  (refer to step S 306  of FIG.  3 ). 
     The A/C ECU  72  proceeds from step S 205 , S 207 , S 209 , or S 210  to step S 211  to determine whether the A/C switch  79  is turned off. If the A/C switch  79  is not turned off, the A/C ECU  72  returns to step S 204  and changes the duty ratio Dt based on the relationship between the target temperature Te set  and the detected temperature Te t . 
     In step S 211 , if the A/C ECU  72  determines that the A/C switch  79  is turned off, the A/C ECU  72  proceeds to step S 212  and determines whether the duty ratio Dt is greater than the minimum duty ratio Dt min . If the duty ratio Dt is not greater than the minimum duty ratio Dt min , the A/C ECU  72  returns to step S 201  to set the duty ratio Dt to zero and substantially deactivate the compressor  40 . The torque required to drive the compressor  40  is small as long as the duty ratio Dt is less than or equal to the minimum duty ratio Dt min . Thus, deactivation of the compressor  40  subtly affects the engine speed Ne since the torque required to drive the compressor  40  is minimized. 
     If the duty ratio Dt is greater than the minimum duty ratio Dt min  in step S 212 , the A/C ECU  72  proceeds to step S 213 . In step S 213 , the A/C ECU  72  decreases the duty ratio Dt by the predetermined grading amount ΔD/10 and instructs the drive circuit  78  to change the duty ratio Dt to the corrected value (Dt−ΔD/10). The A/C ECU  72  therefore gradually decreases the duty ratio Dt by repeating step S 213  even if the duty ratio Dt is greater than the minimum duty ratio Dt min  by a significant amount. This gradually decreases the displacement of the compressor  40  and gradually decreases the torque required to drive the compressor  40 . The gradual torque decrease enables the engine ECU  71  to stabilize the idle speed as it executes the idling control. This prevents a sudden torque decrease from increasing the engine speed Ne in a sudden manner (a state referred to as racing) when the engine E is idling. 
     The preferred embodiment has the advantages described below. 
     (1) When the engine E is idling, the displacement of the compressor  40  gradually increases and decreases. Thus, the resulting change of the torque required to drive the compressor is gradual enough that the engine ECU  71  can stabilize the engine E through the idling control. Accordingly, the engine E continues to idle stably such that the engine E does not stall or race. As a result, the engine speed E may be decreased when the engine E is idling. In other words, the target idle speed Ne set  may be easily be set at a low value. 
     (2) If the A/C switch  79  is turned off when the engine E is idling, the displacement of the compressor  40  is gradually decreased before the compressor  40  is deactivated. Since the compressor  40  is not suddenly deactivated, the engine E is prevented from racing without having to execute a special control (e.g., decreasing the target idle speed Ne set ) when the A/C switch  79  is turned off. 
     (3) The target idle speed of the engine E is increased only when cooling is required even if the compressor  40  is activated. This reduces the vibrations and noise generating from the vehicle that would result from frequent changing of the target idle speed Ne set . Further, if the target idle speed Ne set  were to be increased even though cooling were not necessary, this would decrease the fuel efficiency of the vehicle. The present invention avoids such circumstance. 
     (4) The threshold value Dt ref , which is used to determine whether to increase the target idle speed of the engine E (step S 208  of FIG.  5 ), corresponds to the pressure difference ΔPd required for the compressor  40  to obtain its maximum displacement in a state in which the engine speed Ne is equal to the first target idle speed Ne set1 . Thus, the idle speed is increased only when necessary to increase the cooling capacity. 
     (5) The control valve CV of the above embodiment functions to change the pressure difference between two pressure monitoring points. However, a control valve that functions to change the suction pressure may be used instead. Such control valve, for example, includes a pressure sensing mechanism, which mechanically detects the suction pressure and moves a valve body to absorb fluctuations of the detected suction pressure, and a pressure difference adjusting actuator, which varies the suction pressure used by the pressure sensing mechanism to position the valve body. 
     When employing such control valve, which sets a target suction pressure to vary the displacement, the displacement of the compressor  40  may not vary gradually even when the target suction value is changed. For example, if the amount of heat exchanged by the evaporator  33  were to be large and the actual suction pressure were to be significantly greater than the target suction pressure, the displacement of the compressor  40  would become maximal soon after the A/C switch  79  is turned on even if the pressure difference adjusting actuator gradually changes the force applied to the pressure sensing mechanism. Thus, to prevent the engine E from stalling due to a sudden increase in the torque required to drive the compressor  40  when the engine E is idling, idle-up control must be executed when the A/C switch  79  is turned on. 
     However, in the present invention, the A/C ECU  72  does not use the suction pressure, which is affected by the amount of heat exchanged by the evaporator  33 , to control the displacement of the compressor. The A/C ECU  72  feedback controls the displacement of the compressor  40  based on the pressure difference ΔPd between the two pressure monitoring points P 1 , P 2  that reflect the flow rate of the refrigerant in the refrigerant circuit. Accordingly, the duty ratio Dt of the control valve CV is gradually changed to gradually vary the displacement of the compressor regardless of the amount of heat exchanged by the evaporator  33 . Thus, the idle speed of the engine E may also be decreased with such structure. In other words, the target idle speed Ne set  may easily be set at a low value. 
     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. 
     Referring to FIG. 6, the compressor  40  may employ a control valve CV 2 , which incorporates a movable partition  90  to serve as the pressure sensing member in lieu of the bellows  48 . In this case, the pressure PdH at the first pressure monitoring point P 1  is applied to one side of the partition  90 , and the pressure PdL at the second pressure monitoring point P 2  is applied to the other side of the partition  90 . The partition  90  moves in accordance with the difference between the pressures PdH and PdL and functions in accordance with the bellows  48  of the preferred embodiment. 
     In the process performed by the A/C ECU  72  when the engine E is idling, the grading amount of the duty ratio in steps S 206  and S 207  (refer to FIG. 5) may be equal to the grading value ΔD of the duty ratio Dt used in steps S 106  and S 107  (refer to FIG. 4) when the engine E is running normally. In this case, a step for delaying the time from when the A/C ECU  72  performs step S 206  to when the A/C ECU  72  performs step S 207  is included between the steps S 206  and S 207 . This changes the duty ratio Dt more gradually in comparison to when the engine W is running normally. 
     A clutch mechanism electrically controlled by an external device to selectively connect and disconnect the drive source (engine E) and the compressor  40 , such as an electromagnetic clutch, may be used as the power transmission mechanism PT. 
     The first pressure monitoring point P 1  may be located in a suction pressure region defined between the evaporator  33  and the suction chamber  21 , and the second pressure monitoring point P 2  may be located in the same suction pressure region downstream of the first pressure monitoring point P 1 . 
     The first pressure monitoring point P 1  may be located in a discharge pressure region defined between the discharge chamber  22  and the condenser  31 , and the second pressure monitoring point P 2  may be located in a suction pressure region. 
     The first pressure monitoring point P 1  may be located in the discharge pressure region, and the second pressure monitoring point P 2  may be located in the crank chamber  12 . 
     Alternatively, the second pressure monitoring point P 2  may be located in the crank chamber  12 , and the first pressure monitoring point P 1  may be located in the section pressure region. In other words, one of the pressure monitoring points P 1  and P 2  may be located in the crank chamber  12 , which defines an intermediate pressure region. 
     The communication passage  43  may be connected to the discharge chamber  22  through the first gas supplying passage  28   a,  and the valve chamber  42  may be connected to the crank chamber  12  through the second gas supplying passage  28   b.  This decreases the pressure difference between the communication passage  43  and the second pressure chamber  50 , which is adjacent to the communication passage  43 . As a result, pressure leakage between the communication passage  43  and the second pressure chamber  50  is reduced and the compressor displacement is controlled with high accuracy. 
     A control valve connected to the bleeding passage  27  instead of the gas supplying passages  28   a,    28   b  may be employed in lieu of the control valve to adjust the opening of the bleeding passage  27  and control the pressure of the crank chamber  12 . 
     The variable displacement compressor  40  may be of a type that uses a wobble type swash 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.