Abstract:
An air conditioning system for a vehicle that is driven by a vehicular power source. The system has a compressor selectively operable by the vehicular power source and an electric motor. The electric motor is used as a drive force of the compressor when the vehicular power source is in a non-operative state. The compressor compresses refrigerant gas introduced into a suction chamber from an external refrigerant circuit. A displacement of the compressor is variable, based on a differential pressure. The compressor has a control valve that is disposed on a refrigerant passage communicating with the crank chamber. The control valve has a valve plunger for changing an opening size of the control valve to adjust pressure in the crank chamber. The air conditioning system comprises pressure sensing member, actuator, and controller. The pressure sensing member is disposed in the control valve and applies biasing force to the plunger based on pressure in the external circuit. The biasing force is applied to cancel change of the pressure in the external circuit. The actuator is disposed in the control valve and applies reverse force against the biasing force to the plunger. The plunger is moved to increase the displacement by the reverse force. The controller controls the actuator to increase the reverse force in steps by a magnitude that is small enough that the electric motor is able to stably drive the compressor.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to a vehicular air-conditioner that has a variable displacement compressor in a refrigerant circuit. The compressor is driven by a vehicular power source such as an internal combustion engine to compress refrigerant gas and is also driven by an electric motor to compress refrigerant gas when the internal combustion engine is not running.  
           [0002]    Recently, an idling stop system is becoming widely used to improve the fuel economy and for environmental protection. The idling stop system stops the engine when a vehicle is stopped at stoplights. A compressor, which has an electric motor as a drive source, has been proposed to enable the air conditioning of a passenger compartment while the engine is not running.  
           [0003]    An electric motor having the same driving performance as the engine is large and does not fit in the engine room. Therefore, the electric motor to be provided in the compressor needs to be small. However, a small electric motor may cause a power swing due to an excessive compressor torque, particularly at the activation of the motor. When the electric motor causes a power swing, the electric motor stops and might hinder the air conditioning performance.  
           [0004]    Japanese Laid-Open Patent Publication No. 10-236151 discloses such an air conditioning system. The disclosed air conditioning system employs a variable displacement compressor. Before actuating the compressor with an electric motor, the displacement of the compressor is minimized. Therefore, when the motor is activated, the compressor torque is small. Thus, the motor is reliably activated.  
           [0005]    However, even after the motor is activated, or in a case where the motor is unlikely to cause power swing as compared to when the motor is being activated, the displacement of the compressor is maintained at the minimum. To drive the motor in a stable manner at the same time as performing the air conditioning in a suitable manner, the displacement of the compressor needs to be changed (increased) within the range that can be managed by the output of a small motor.  
           [0006]    In a typical variable displacement compressor, the displacement is decreased when the pressure in a crank chamber is large and the displacement is increased when the pressure in the crank chamber is small. The compressor is provided with a control valve for adjusting the pressure in the crank chamber to vary the displacement. The control valve is, for example, located in a supply passage that connects the discharge chamber to the crank chamber. The control valve includes a bellows and an electromagnetic solenoid. The bellows moves a valve body in accordance with the pressure in the suction chamber (suction pressure). The electromagnetic solenoid applies force to the valve body based on external conditions, such as the temperature in the passenger compartment. The force that the solenoid applies to the valve body reflects the target value of the suction pressure (target suction pressure).  
           [0007]    An operation for decreasing the temperature in the passenger compartment will now be described. A controller computes the target suction pressure based on the information from several sensors to decrease the detected room temperature to a desired temperature. The controller commands a drive circuit to supply current to the electromagnetic solenoid based on the computed result to decrease the opening degree of the valve body. The commands of the controller constitute commands externally to the electromagnetic solenoid. When the opening degree of the valve body is decreased, the amount of refrigerant supplied to the crank chamber through the supply passage from the discharge chamber decreases. Accordingly the crank pressure is decreased, which increases the displacement of the compressor. As a result, the cooling performance is increased and the temperature in the passenger compartment decreases toward the desired temperature. Accordingly, the actual suction pressure decreases toward the target suction pressure.  
           [0008]    The bellows moves the valve body in accordance with the actual suction pressure such that the actual suction pressure seeks the target suction pressure. For example, when the suction pressure is greater than the target suction pressure, the bellows decreases the opening degree of the valve body. Therefore, as described above, the displacement of the compressor increases, which decreases the suction pressure toward the target suction pressure.  
           [0009]    To reliably activate the motor, the control valve maintains the valve body at the fully opened position when the solenoid is demagnetized, and the displacement of the compressor is minimized.  
           [0010]    However, the bellows of the control valve is automatically controlled based on the fluctuation of the suction pressure. Therefore, when the target suction pressure is changed by the solenoid, the movement of the bellows differs depending on the actual suction pressure at the time the target suction pressure is changed. The displacement of the compressor is changed in different manner based on the difference in the movement of the bellows.  
           [0011]    More specifically, the compressor is stopped when the power source of the compressor is switched from the engine to the motor. When the compressor is stopped, the suction pressure excessively increases. Thus, even though the target pressure is set relatively high by the solenoid after the motor is activated, the bellows rapidly decreases the opening degree of the valve body to decrease the excessive actual suction pressure to the target suction pressure. As a result, the displacement of the compressor rapidly and excessively increases, which hinders the reliable activation of the motor.  
           [0012]    The above described problem is caused not only in the case with the variable target suction pressure valve but in all types of control valves combining the pressure sensing mechanism and the electromagnetic actuator.  
         BRIEF SUMMARY OF THE INVENTION  
         [0013]    Accordingly, it is an objective of the present invention to provide a vehicular air-conditioner that stabilizes the operation of an electric motor and performs air conditioning in a suitable manner at the same time when actuating a variable displacement compressor by an electric motor.  
           [0014]    In order to achieve the above objective, the present invention provides an air conditioning system for a vehicle that is driven by a vehicle engine, said system has a compressor selectively operable by the vehicle engine and an electric motor outputting a force smaller than that of the vehicle engine, said electric motor being used as a drive force of the compressor when the vehicle engine is in a non-operative state, wherein said compressor compresses refrigerant gas introduced into a suction chamber from an external refrigerant circuit, wherein a displacement of the compressor is variable based on a differential pressure between the compression chamber and a crank chamber, wherein the compressor has a control valve that is disposed on a refrigerant passage communicating with the crank chamber, wherein said control valve has a valve plunger for changing an opening size of the control valve to adjust pressure in the crank chamber, said system comprising:  
           [0015]    pressure sensing member disposed in the control valve and applying biasing force to the plunger based on pressure in the external circuit, wherein the biasing force is applied to cancel change of the pressure in the external circuit;  
           [0016]    actuator disposed in the control valve and applying reverse force against the biasing force to the plunger, wherein the plunger is moved to increase the displacement by the reverse force; and  
           [0017]    controller for controlling the actuator to stepwise increase the reverse force by a magnitude at which the electric motor is able to stably drive the compressor.  
           [0018]    Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]    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:  
         [0020]    [0020]FIG. 1 is a cross-sectional view illustrating a swash plate type variable displacement compressor according to a preferred embodiment of the present invention;  
         [0021]    [0021]FIG. 2 is a cross-sectional view illustrating a control valve: and  
         [0022]    [0022]FIG. 3 is a flowchart showing an idling stop control of an air-conditioner ECU. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0023]    A vehicular air-conditioner according to a preferred embodiment of the present invention will now be described.  
         [0024]    As shown in FIG. 1, a swash plate type variable displacement compressor has a housing  11 . The housing  11  defines a crank chamber  12 . A drive shaft  13  is rotatably supported by the housing  11  and is located inside the crank chamber  12 . The drive shaft  13  is connected to and driven by an output shaft of a vehicular power source, which is an engine E in this embodiment, by a power transmission mechanism PT.  
         [0025]    The power transmission mechanism PT has a rotor  80 , which is rotatably supported by the housing  11 . A belt  81 , which is operably connected to the output shaft of the engine E, is wound about the peripheral surface of the rotor  80 . A hub  82  is secured to the end of the drive shaft  13  that projects from the housing  11 . A conventional one-way clutch  83  is located between the rotor  80  and the hub  82 .  
         [0026]    The power transmission mechanism PT has an electric motor  84  located inside the rotor  80 . The electric motor  84  includes a stator  84   a , which is secured to the housing  11 , and a rotor  84   b , which is secured to the hub  82  and surrounds the periphery of the stator  84   a . When the engine E is not running, an electronic control unit (ECU)  72  for an air-conditioner sends a command to a drive circuit  78 . Then, power is supplied to the stator  84   a  from the drive circuit  78  as required based on the command (see FIG. 2). When power is supplied from the drive circuit  78  to the stator  84   a , a rotational force is applied to the rotor  84   b . When the rotor  84   b  is rotated, the drive shaft  13  is rotated via the hub  82 . At this time, the one-way clutch  83  prevents power from being transmitted from the hub  82  to the rotor  80 . Thus, the force of the electric motor  84  is prevented from being transmitted to the engine E.  
         [0027]    The one-way clutch  83  permits the power transmission from the rotor  80  to the hub  82 . Therefore, power from the engine E is transmitted to the drive shaft  13  via the rotor  80  and the hub  82 .  
         [0028]    A lug plate  14  is located in the crank chamber  12  and is secured to the drive shaft  13  to rotate integrally with the drive shaft  13 . A swash plate  15  is located in the crank chamber  12 . The swash plate  15  slides along the drive shaft  13  and inclines with respect to the axis of the drive shaft  13 . A hinge mechanism  16  is provided between the lug plate  14  and the swash plate  15 . The hinge mechanism  16  causes the swash plate  15  to rotate integrally with the lug plate  14  and the drive shaft  13  and to incline with respect to the drive shaft  13 .  
         [0029]    Cylinder bores  11   a  (only one shown) are formed in the housing  11 . A single headed piston  17  is reciprocally accommodated in each cylinder bore  11   a . Each piston  17  is coupled to the peripheral portion of the swash plate  15  by a pair of shoes  18 . Therefore, when the swash plate  15  rotates with the drive shaft  13 , the shoes  18  convert the rotation of the swash plate  15  into reciprocation of the pistons  17 .  
         [0030]    A valve plate assembly  19  is located in the rear portion of the housing  11 . A compression chamber  20  is defined in each cylinder bore  11   a  by the associated piston  17  and the valve plate assembly  19 . A suction chamber  21  and a discharge chamber  22  are defined in the rear portion of the housing  11 . The valve plate assembly  19  has suction ports  23 , suction valve flaps  24 , discharge ports  25 , and discharge valve flaps  26 . Each set of the suction port  23 , the suction valve flap  24 , the discharge port  25 , and the discharge valve flap  26  corresponds to one of the cylinder bores  11   a.    
         [0031]    When each piston  17  moves from the top dead center position to the bottom dead center position, refrigerant gas in the suction chamber  21  is drawn into the corresponding compression chamber  20  via the corresponding suction port  23  and suction valve flap  24 . When each piston  17  moves from the bottom dead center position to the top dead center position, refrigerant gas in the corresponding compression chamber  20  is compressed to a predetermined pressure and is discharged to the discharge chamber  22  via the corresponding discharge port  25  and discharge valve flap  26 .  
         [0032]    As shown in FIG. 1, a bleed passage  27  and a supply passage  28  are located in the housing  11 . The bleed passage  27  communicates the crank chamber  12  with the suction chamber  21 . The supply passage  28  communicates the discharge chamber  22  with the crank chamber  12 . A control valve CV is located in the supply passage  28  in the housing  11 .  
         [0033]    The opening degree of the control valve CV is adjusted to control the flow rate of highly pressurized gas supplied to the crank chamber  12  through the supply passage  28 . The pressure in the crank chamber  12  is determined by the ratio of the gas supplied to the crank chamber  12  through the supply passage  28  and the flow rate of refrigerant gas conducted out from the crank chamber  12  through the bleed passage  27 . As the pressure in the crank chamber  12  varies, the difference between the pressure in the crank chamber  12  and the pressure in the compression chamber  20  varies, which changes the inclination angle of the swash plate  15 . Accordingly, the stroke of each piston  17 , or the compressor displacement, is varied.  
         [0034]    For example, when the pressure in the crank chamber  12  decreases, the difference between the pressure in the crank chamber  12  and the pressure in the compression chamber  20  decreases. Accordingly, the inclination angle of the swash plate  15  increases, which increases the displacement of the compressor. A chain double-dashed line in FIG. 1, represents the maximum inclination angle of the swash plate  15 . In contrast, when the pressure in the crank chamber  12  increases, the difference between the pressure in the crank chamber  12  and the pressure in the compression chamber  20  increases. Accordingly, the inclination angle of the swash plate  15  decreases, which decreases the displacement of the compressor. A solid line in FIG. 1 represents the minimum inclination angle of the swash plate  15 . The minimum inclination angle is not equal to zero.  
         [0035]    As shown in FIG. 1, a refrigerant circuit, or a refrigeration cycle, of the vehicular air-conditioner includes the compressor and an external refrigerant circuit  30 . The external refrigerant circuit  30  includes a condenser  31 , an expansion valve  32 , and an evaporator  33 .  
         [0036]    A first pressure monitoring point P 1  is located in the discharge chamber  22 . A second pressure monitoring point P 2  is located in the refrigerant passage at a part that is spaced downstream from the first pressure monitoring point P 1  toward the condenser  31  by a predetermined distance. 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 flow rate of refrigerant in the refrigerant circuit. That is, when the flow rate of refrigerant in the refrigeration circuit increases, the pressure difference ΔPd (ΔPd=PdH−PdL) between the first pressure monitoring point P 1  and the second pressure monitoring point P 2  increases. In contrast, when the flow rate of refrigerant is decreased, the pressure difference ΔPd is decreased.  
         [0037]    The first pressure monitoring point P 1  is communicated with the control valve CV by a first pressure introduction passage  35 . The second pressure monitoring point P 2  is communicated with the control valve CV by a second pressure introduction passage  36  (see FIG. 2).  
         [0038]    A shutoff valve  34  is located in the refrigerant passage between the discharge chamber  22  and the condenser  31  of the refrigerant circuit. The shutoff valve  34  disconnects the refrigerant passage when the pressure in the discharge chamber  22  is lower than a predetermined value to stop the circulation of refrigerant via the external refrigerant circuit  30 . The shutoff valve  34  may be a differential valve, which mechanically detects the difference between the pressure at its upstream section and its downstream section and operates accordingly. The shutoff valve  34  may also be an electromagnetic valve, which is controlled by the air-conditioner ECU  72  in accordance with the detected value of a discharge pressure sensor (not shown).  
         [0039]    The shutoff valve  34  may also be a valve that mechanically operates in conjunction with the minimum inclination angle of the swash plate  15 .  
         [0040]    As shown in FIG. 2, the control valve CV has a valve housing  41 . The valve housing  41  defines a valve chamber  42 , a communication passage  43 , and a pressure sensing chamber  44 . A transmission rod  45  is located in the valve chamber  42  and the communication passage  43  and moves in the axial direction (vertically as viewed in FIG. 2).  
         [0041]    The communication passage  43  and the pressure sensing chamber  44  are disconnected by the upper end of the transmission rod  45 , which is inserted in the communication passage  43 . The valve chamber  42  is communicated with the crank chamber  12  by a downstream portion of the supply passage  28 . The communication passage  43  is communicated with the discharge chamber  22  by an upstream portion of the supply passage  28 . The valve chamber  42  and the communication passage  43  forms a part of the supply passage  28 .  
         [0042]    A valve body  46  is formed at the middle portion of the transmission rod  45 . The valve body  46  is arranged in the valve chamber  42 . A step, which is defined by the valve chamber  42  and the communication passage  43 , serves as a valve seat  47  and the communication passage  43  serves as a valve hole. When the transmission rod  45  moves from the lowermost position shown in FIG. 2 to the uppermost position, at which the valve body  46  contacts the valve seat  47 , the communication passage  43  is disconnected. That is, the valve body  46  of the transmission rod  45  functions as a valve body for adjusting the opening degree of the supply passage  28 .  
         [0043]    A pressure sensing member, which is a bellows  48  in this embodiment, is accommodated in the pressure sensing chamber  44 . The upper end of the bellows  48  is secured to the valve housing  41 . The upper end of the transmission rod  45  is inserted into the lower end of the bellows  48 . A first pressure chamber  49  and a second pressure chamber  50  are defined in the pressure sensing chamber  44  by the bellows  48 . The first pressure chamber  49  is the inner space of the bellows  48 . The second pressure chamber  50  is the outer space of the bellows  48 . The first pressure chamber  49  is exposed to the pressure PdH at the first pressure monitoring point P 1  via the first pressure introduction passage  35 . The second pressure chamber  50  is exposed to the pressure PdL at the second pressure monitoring point P 2  via the second pressure introduction passage  36 . The bellows  48  and the pressure sensing chamber  44  form a pressure sensing mechanism.  
         [0044]    An actuator for varying a target pressure, which is an electromagnetic actuator  51  in this embodiment, is located at the lower side of the valve housing  41 . The electromagnetic actuator  51  has a cup-shaped cylinder  52  at the center of the valve housing  41 . A stationary core  53  is inserted in the upper opening of the cylinder  52 . The stationary core  53  defines a plunger chamber  54  at the lowermost portion of the cylinder  52 .  
         [0045]    A plunger, or a movable core  56  is accommodated in the plunger chamber  54  to move in the axial direction. A guide hole  57  is formed through the stationary core  53  extending in the axial direction. The lower end of the transmission rod  45  is located in the guide hole  57  to move in the axial direction. The lower end of the transmission rod  45  contacts the upper end of the movable core  56  inside the plunger chamber  54 .  
         [0046]    A spring  60  is accommodated between the inner bottom surface of the cylinder  52  and the movable core  56  in the plunger chamber  54 . The spring  60  urges the movable core  56  toward the transmission rod  45 . The transmission rod  45  is urged toward the movable core  56  by the bellows  48 . Therefore, the movable core  56  and the transmission rod  45  always move integrally in a vertical direction. The force of the bellows  48  is greater than the spring  60 .  
         [0047]    A coil  61  is wound about at least a part of the stationary core  53  and the movable core  56 . The air-conditioner ECU  72  sends a command to a drive circuit  73  based on, for example, an on-off switch, which is an air-conditioner switch  74  in this embodiment, a temperature adjuster  75  for setting the passenger compartment temperature, a temperature sensor  76  for detecting the passenger compartment temperature, a rotational speed sensor  77  for detecting the rotational speed Ne of the electric motor  84 , and a current sensor for detecting the current I applied to the stator  84   a  of the electric motor  84 . The drive circuit  73  supplies current to the coil  61  based on the command.  
         [0048]    The coil  61  generates an electromagnetic force that corresponds to the value of the current from the drive circuit  78  between the movable core  56  and the stationary core  53 . The electromagnetic force is transmitted to the transmission rod  45  by the movable core  56 . The current to the coil  61  is controlled by adjusting the applied voltage. In the preferred embodiment, the applied voltage is controlled by pulse-width modulation PWM. Therefore, the duty ratio Dt that the air-conditioner ECU  72  commands the drive circuit  73  to send to the coil  61  corresponds to a command value from the air-conditioner ECU  72  to the electromagnetic actuator  51 .  
         [0049]    In the control valve CV, the position of the transmission rod  45  and the opening degree of the valve body  46  are determined in the following manner.  
         [0050]    When no current is supplied to the coil  61 , or when the duty ratio Dt is zero percent, the bellows  48  positions the transmission rod  45  at the lowermost position shown in FIG. 2. Thus, the valve body  46  fully opens the communication passage  43 . Therefore, the pressure in the crank chamber  12  is maximized. At this time, the difference between the pressure in the crank chamber  12  and the pressure in the compression chambers  20  is great and the inclination angle of the swash plate  15  is minimized, which minimizes the displacement of the compressor.  
         [0051]    When no current is supplied to the coil  61 , the pressure sensing mechanism stops functioning automatically. This minimizes the displacement of the compressor regardless of the fluctuation of the pressure difference ΔPd.  
         [0052]    When the displacement of the compressor is minimum, the pressure acting on the shutoff valve  34  on the side facing the discharge chamber  22  is lower than a predetermined value and thus the shutoff valve  34  is closed. Therefore, the circulation of refrigerant via the external refrigerant circuit  30  is stopped. Thus, even when the compressor continues to compress refrigerant gas, air conditioning is not performed unnecessarily.  
         [0053]    When a current of a minimum duty ratio Dt (min), which is greater than 0%, is supplied to the coil  61  of the control valve CV, the resultant of the upward forces of the spring  60  and the electromagnetic force surpasses the downward force of the bellows  48 , which moves the transmission rod  45  upward. In this state, the resultant of the upward forces of the spring  60  and the electromagnetic force acts against the resultant of the force based on the pressure difference ΔPd and the downward force of the bellows  48 . The position of the valve body  46  of the transmission rod  45  relative to the valve seat  47  is determined such that upward and downward forces are balanced. Accordingly, the displacement of the compressor is adjusted.  
         [0054]    As described above, the target value (target pressure difference) of the pressure difference ΔPd is determined by the duty ratio Dt of current supplied to the coil  61 . The control valve CV automatically determines the position of the transmission rod  45  (the valve body  46 ) according to changes of the pressure difference ΔPd to maintain the target value of the pressure difference ΔPd.  
         [0055]    When the engine E is running, the air-conditioner ECU  72  calculates the duty ratio Dt based on the detected temperature from the temperature sensor  76  and the target temperature from the temperature adjuster  75  while the air-conditioner switch  74  is on. The air-conditioner ECU  72  sends the calculated value to the drive circuit  73 .  
         [0056]    For example, when the detected temperature is greater than the target temperature, the passenger compartment is hot and the thermal load is great. Therefore, the air-conditioner ECU  72  commands the drive circuit  73  to increase the duty ratio Dt. Accordingly, the opening degree of the control valve CV is decreased, which increases the displacement of the compressor. The increased compressor displacement lowers the temperature at the evaporator  33  and lowers the temperature in the passenger compartment.  
         [0057]    In contrast, when the detected temperature is less than the target temperature, the passenger compartment is cold and the thermal load is small. Therefore, the air-conditioner ECU  72  commands the drive circuit  73  to decrease the duty ratio Dt. Accordingly, the opening degree of the control valve CV is increased, which decreases the displacement of the compressor. The decreased compressor displacement lowers the heat reduction performance of the evaporator  33  and raises the temperature in the passenger compartment.  
         [0058]    The air-conditioner ECU  72  executes computation according to the following flowchart of FIG. 3 when certain conditions are satisfied. The conditions include that the air-conditioner switch  74  is on and that the air-conditioner ECU  72  has received information from an engine ECU  91  that the engine E is determined to be stopped in the process of the idling stop control. The engine ECU  91  is a computer for controlling start, stop, and output of the engine E. The engine ECU  91  is connected to the air-conditioner ECU  72  (see FIG. 2).  
         [0059]    As shown in FIG. 3, when receiving information from the engine ECU  91  that the engine E is determined to be stopped while the air-conditioner switch  74  is on, the air-conditioner ECU  72  proceeds to step S 201 . In step S 201 , the air-conditioner ECU  72  gives zero to the duty ratio Dt sent to the drive circuit  73 . Thus, no current is supplied to the coil  61 , which minimizes the displacement of the compressor. The compressor displacement is minimized before the engine E is stopped. In step S 202 , the air-conditioner ECU  72  stands by until it receives a signal from the engine ECU  91  representing that the engine E is stopped. The air-conditioner ECU  72  determines that the engine E is stopped when the rotational speed information of the engine E sent from the engine ECU  91  is zero.  
         [0060]    If it is determined that the engine E is stopped in step S 202 , the air-conditioner ECU  72  proceeds to step S 203 . In step S 203 , the air-conditioner ECU  72  sends a command to the drive circuit  78  to activate the electric motor  84 . The drive circuit  78  activates the electric motor  84  at a substantially constant rotational speed, or at a first threshold value Ne(set 1). When the electric motor  84  is accelerated and reaches the first threshold value Ne(set 1), the electric motor  84  is operated in a stable manner. In step S 204 , it is determined whether the electric motor  84  has reached a predetermined rotational speed, which is the first threshold value Ne(set 1), and has shifted to a stable operation state based on a signal from the rotational speed sensor  77 .  
         [0061]    If it is determined that the rotational speed Ne(t) of the electric motor  84  has shifted to the stable operation state in step S 204 , the air-conditioner ECU  72  proceeds to step S 205 . In step S 205 , the air-conditioner ECU  72  sends a minimum duty ratio Dt (min) to the drive circuit  73  and actuates an automatic control function (target pressure difference maintaining function) in the control valve CV. Then, the air-conditioner ECU  72  proceeds to step S 206 . In step S 206 , the air-conditioner ECU  72  increases the duty ratio Dt by a unit quantity ΔD and commands the drive circuit  73  to change the duty ratio Dt to the modified value (DT+ΔD).  
         [0062]    Therefore, the opening degree of the control valve CV slightly decreases, which slightly suppresses the pressure increase in the crank chamber  12 . Accordingly, the displacement of the compressor is slightly increased, thereby slightly increasing the compressor torque.  
         [0063]    In step S 207 , the air-conditioner ECU  72  determines whether the rotational speed Ne detected by the rotational speed sensor  77  is less than the second threshold value Ne(set 2)(Ne(set 2)&lt;Ne(set 1)) and the current value I detected by the current sensor  79  is greater than a first threshold value I (set 1). The rotational speed Ne of the electric motor  84  and the current value I each correlate with the compressor torque applied to the electric motor  84 . That is, if the rotational speed Ne is less than the second threshold value Ne(set 2) and the current value I is greater than the first threshold value I(set 1), the compressor torque can become excessive for the electric motor  84  to operate in a suitable manner.  
         [0064]    If the decision outcome of step S 207  is negative, it is determined that the capacity of the electric motor  84  is enough for the current compressor torque and the air-conditioner ECU  72  proceeds to step S 208 . In step S 208 , it is determined whether the duty ratio Dt has reached a predetermined value Dt(set). The predetermined value Dt(set) represents the target pressure difference of the control valve CV. The target pressure difference is the pressure difference ΔPd obtained when the displacement of the compressor is at the middle of the minimum value and the maximum value. The middle displacement is determined such that, although depending on other conditions, the compressor torque is substantially at the upper limit within the range of the capacity of the electric motor  84 .  
         [0065]    If the decision outcome of step S 208  is negative, the air-conditioner ECU  72  proceeds to step S 206 . In step S 206 , the duty ratio Dt is increased by the unit quantity ΔD until the duty ratio Dt reaches the predetermined value Dt(set). The value of the unit quantity ΔD is predetermined such that the unit quantity ΔD must be added several times to reach the predetermined value Dt(set) from the minimum duty ratio Dt(set) in step S 206 . If the decision outcome of step S 208  is positive, or the duty ratio Dt has reached the predetermined value Dt(set), the air-conditioner ECU  72  proceeds to step S 207 . In step S 207 , the duty ratio Dt is maintained at the predetermined value Dt(set).  
         [0066]    That is, the air-conditioner ECU  72  drives the compressor at the upper limit within the range of the capacity of the electric motor  84 , which is determined based on the second threshold value Ne(set 2) of the motor speed and the first threshold value I(set 1) of the current, regardless of the target temperature of the temperature adjuster  75  and the temperature detected by the temperature sensor  76 . This is because the size of the electric motor  84  is restricted since the electric motor  84  is incorporated in the power transmission mechanism PT, the dimension of which is restricted by the pulley ratio with respect to the engine E. That is, the output of the small electric motor  84  is less than that of the engine E and thus the capacity for driving the compressor is also designed to be smaller than that of the engine E. The electric motor  84  is constantly driven at the upper limit of its capacity to drive the compressor.  
         [0067]    If the decision outcome of step S 207  is positive, it is determined that the compressor torque is excessive and the electric motor  84  tends to operate unstably, such as causing power swing, and the air-conditioner ECU  72  proceeds to step S 209 . In step S 209 , the air-conditioner ECU  72  decreases the duty ratio Dt by the unit quantity ΔD. The air-conditioner ECU  72  then commands the drive circuit  73  to change the duty ratio Dt to the modified value (Dt−ΔD). Therefore, the opening degree of the control valve CV is slightly increased, which increases the pressure in the crank chamber  12 . Accordingly, the displacement of the compressor is slightly decreased, which slightly lowers the compressor torque.  
         [0068]    After step S 209 , the air-conditioner ECU  72  proceeds to step S 210 . In step S 210 , it is determined whether the rotational speed Ne of the electric motor  84  is less than the third threshold value Ne(set 3)(Ne(set 3)&lt;Ne(set 2)) and the current value I detected by the current sensor  79  is greater than the second threshold value I(set 2)(I(set  2 )&gt;I(set 1)). If it is determined that the rotational speed Ne is less than the third threshold value Ne(set 3) and the current value I is greater than the second threshold value I(set 2), the compressor torque is excessive for the capacity of the electric motor  84 . Therefore, if the compressor is kept driven by the electric motor  84  in this state, the electric motor  84  is most likely to cause power swing.  
         [0069]    If the decision outcome of step S 210  is negative, the air-conditioner ECU  72  returns to step S 207 . In contrast, if the decision outcome of step S 210  is positive, the air-conditioner ECU  72  proceeds to step S 211 . In step S 211 , the air-conditioner ECU  72  gives zero to the duty ratio Dt sent to the drive circuit  73 . Thus, no current is applied to the coil  61 , which minimizes the displacement of the compressor. Then, the air-conditioner ECU  72  proceeds to step S 212 . In step S 212 , the air-conditioner ECU  72  commands the drive circuit  78  to stop the electric motor  84 . Then, the air-conditioner ECU  72  returns to step S 203 . In step S 203 , the electric motor  84  is reactivated. That is, if the electric motor  84  is driven unstably due to power swing for a long time, the air-conditioner is adversely affected. Thus, air-conditioner ECU  72  stops and reactivates the electric motor  84  to promptly stabilize the operation of the electric motor  84 .  
         [0070]    The present invention provides the following advantages.  
         [0071]    According to the above control, during idling stop, the duty ratio Dt is increased to the target value Dt (set) by adding the unit quantity ΔD several times (S 206  of FIG. 3). Therefore, it takes a long time from when the electric motor  84  is activated till the duty ratio Dt is increased to the target value Dt(set). That is, the duty ratio Dt is gradually increased, or increased in steps. As a result, when the target pressure difference is changed to actuate the bellows  48 , increasing of the pressure difference ΔPd is not delayed greatly. Thus, the difference between the target pressure difference and the pressure difference ΔPd is prevented from increasing excessively.  
         [0072]    As described above, the opening degree of the valve body  46  is prevented from rapidly and excessively increasing when increasing the pressure difference ΔPd to the target pressure difference. This prevents the compressor displacement from being rapidly and excessively increased. Accordingly, the compressor torque is prevented from increasing to a level that the capacity of the electric motor  84  cannot manage. Thus, even when the compressor displacement is increased, the electric motor  84  is unlikely to cause power swing. As a result, the operation of the electric motor  84  is stabilized and air conditioning is performed in a suitable manner at the same time.  
         [0073]    According to the idling stop control, the duty ratio Dt is changed from the minimum value Dt(min) to the target value Dt(set) more slowly than in a case where the duty ratio Dt is changed from the minimum value Dt(min) to the target value Dt(set) in accordance with the cooling load while the engine E is running. Changing of the duty ratio Dt to the target value Dt(set) is slower even when the duty ratio Dt is not changed step by step going through step S 209  (decreasing process of the duty ratio Dt).  
         [0074]    The air-conditioner ECU  72  gradually increases the duty ratio Dt at least immediately after the electric motor  84  is activated. Since the compressor torque is likely to become excessive, or overshoot, immediately after the electric motor  84  is started, it is particularly important to gradually increase the duty ratio Dt immediately after the electric motor  84  is activated to operate the electric motor  84  in a stable manner.  
         [0075]    The air-conditioner ECU  72  minimizes the compressor displacement before activating the electric motor  84  when driving the compressor by the electric motor  84 . Therefore, the electric motor  84  is activated in a stable manner without causing power swing. Thus, reliability of the air-conditioner is improved.  
         [0076]    The air-conditioner ECU  72  restricts increasing of the duty ratio Dt such that the compressor torque does not become excessive when actuating the compressor by the electric motor  84  (step S 208  in FIG. 3). Therefore, the compressor is operated at the upper limit of the capacity of the electric motor  84 . As a result, the operation of the electric motor  84  is stabilized and air conditioning is performed in a suitable manner at the same time.  
         [0077]    If the compressor torque is excessive when driving the compressor by the electric motor  84 , the air-conditioner ECU  72  decreases the duty ratio Dt (step S 207  (positive) and step S 209  in FIG. 3). Therefore, the compressor torque is reliably suppressed within the range that the capacity of the electric motor  84  can manage. As a result, the operation of the electric motor  84  is stabilized and air conditioning is performed in a suitable manner at the same time.  
         [0078]    If the electric motor  84  causes power swing, the air-conditioner ECU  72  stops and reactivates the electric motor  84 . Therefore, the electric motor  84  is prevented from being operated unstably for a long time. The air-conditioner is prevented from being adversely affected by the unstable operation.  
         [0079]    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 invention may be embodied in the following forms.  
         [0080]    A torque sensor for directly sensing the compressor torque may be provided. In this case, determinations in step S 207  and S 210  are performed based on the information detected by the torque sensor. Thus, the compressor torque is directly obtained. As a result, the operation of the electric motor  84  is stabilized and air conditioning is performed in a suitable manner at the same time.  
         [0081]    Determinations in step S 207  and/or S 210  in FIG. 3 may be performed in accordance with one of the rotational speed Ne of the electric motor  84  and the current value I. In this case, the computing load of the air-conditioner ECU  72  is reduced.  
         [0082]    The target value Dt(set) in step S 208  of FIG. 3 may be determined by feedback control based on the information obtained from the relationship between the air temperature just downstream of the evaporator  33  and the compressor torque, which are measured in advance.  
         [0083]    For example, an electric motor having higher performance than the electric motor  84  of the preferred embodiment may be used. In this case, the target value Dt(set) is varied in accordance with the cooling load in step S 208  of FIG. 3. This improves the performance of air-conditioning.  
         [0084]    The first pressure monitoring point P 1  may be located at the suction pressure zone between the evaporator  33  and the suction chamber  21  and the second pressure monitoring point P 2  may be located downstream of the first pressure monitoring point P 1  in the suction pressure zone.  
         [0085]    A variable target suction pressure valve may be used instead of the control valve CV.  
         [0086]    The control valve CV may be located in the bleed passage  27  instead of the supply passage  28 . In this case, the pressure in the crank chamber  12  is adjusted by the opening degree of the bleed passage  27 .  
         [0087]    The present invention may be embodied in a wobble type variable displacement compressor.  
         [0088]    The present invention need not be embodied in a vehicular air-conditioner for conditioning a passenger compartment. For example, the present invention may be embodied in a vehicular air-conditioner for conditioning inside a freezer car or a refrigeration car.  
         [0089]    Therefore, the present examples and embodiments are to be considered as illustrative and not restrictive and the invention is not to be limited to the details given herein, but may be modified within the scope and equivalence of the appended claims.