Patent Publication Number: US-2004045305-A1

Title: Air conditioner

Description:
BACKGROUND OF THE INVENTION  
       [0001] The present invention relates to an air conditioner with a refrigerant circuit including a variable displacement compressor and a control valve for adjusting an opening degree in connection with variation in displacement of the compressor.  
       [0002] Generally, target temperature of air that has just passed through an evaporator (target after-evaporator temperature) is determined based upon cooling load information, such as ambient temperature, temperature in a compartment of a vehicle and solar irradiance. Then, the displacement of the variable displacement compressor is adjusted by a feedback control based upon the target after-evaporator temperature and actual after-evaporator temperature detected by an evaporator sensor.  
       [0003] A variable displacement swash plate type compressor is widely used for an on-vehicle variable displacement compressor, and a displacement control mechanism for controlling the displacement of the compressor is provided for the compressor. With respect to a control valve of the displacement control mechanism, a position of a valve body is determined by a balance between force from a pressure sensing mechanism and force from an electromagnetic actuator so that pressure in a crank chamber is adjusted to determine the inclination angle of the swash plate, for example, as disclosed on page 8 through 11 and FIG. 3 in Unexamined Japanese Patent Publication No. 2001-173556.  
       [0004] Namely, the pressure sensing mechanism senses a pressure differential between two pressure monitoring points arranged in a refrigerant circuit by a pressure sensing member such as bellows and applies force based on the pressure differential to the valve body. The electromagnetic actuator strengthens and weakens the force applied to the pressure sensing member by an external control so that the set pressure differential between the two pressure monitoring points is optionally varied. The pressure differential governs internally mechanical motion of the pressure sensing mechanism. The external control of the electromagnetic actuator, that is, the variation in the set pressure differential of the control valve, is exerted based upon the target after-evaporator temperature and the detected after-evaporator temperature. In other words, when the detected after-evaporator temperature exceeds the target after-evaporator temperature, the set pressure differential is increased so that the displacement of the compressor increases. On the contrary, when the detected after-evaporator temperature is lower than the target after-evaporator temperature, the set pressure differential is reduced so that the displacement of the compressor reduces.  
       [0005] The pressure differential between the two monitoring points in the refrigerant circuit reflects the amount of refrigerant that flows in the refrigerant circuit. Accordingly, the amount of refrigerant that flows in the refrigerant circuit directly relates to load torque of the compressor, and the control valve directly controls the amount of refrigerant. For example, a computer for controlling a vehicle engine easily and properly estimates torque required for driving the compressor or an auxiliary machine based upon the set pressure differential (electrical signal) sent to the electromagnetic actuator of the control valve. As a result, the output of the engine is appropriately adjusted, and fuel consumption of the engine is reduced.  
       [0006] The electromagnetic actuator is capable of generating a small amount of electromagnetic force that can balance with a small amount of force based on the pressure differential between the two monitoring points. Accordingly, even if carbon dioxide is used as refrigerant, that is, even if pressure in the refrigerant circuit is much higher than the pressure when fluorocarbon is used as refrigerant, the enlarged electromagnetic actuator or the enlarged control valve is restrained. Namely, when the control valve of a variable set suction pressure type in which the pressure sensing mechanism operates based upon absolute value of the suction pressure requires to employ an especially large electromagnetic actuator that can generate a large amount of electromagnetic force balancing with a large amount of force based upon the suction pressure when the suction pressure increases due to the carbon dioxide refrigerant.  
       [0007] An unwanted feature is that the control valve detects the pressure differential that does not reflect thermal load of the evaporator and internally and autonomically adjusts the displacement of the compressor by the feedback control. Accordingly, the set pressure differential is changed by the external control based upon the variation in the detected after-evaporator temperature due to the variation in the thermal load of the evaporator. The variation in the after-evaporator temperature slowly responds to the variation in the thermal load of the evaporator. For example, even if the thermal load of the evaporator rapidly varies, the above control valve cannot rapidly vary the displacement of the compressor. As a result, it takes a long time that the after-evaporator temperature reaches the target after-evaporator temperature so that air-conditioning feeling is deteriorated. Therefore, there is a need for an air conditioner that provides an excellent air-conditioning feeling.  
       SUMMARY OF THE INVENTION  
       [0008] In accordance with the present invention, an air conditioner has a refrigerant circuit, a control valve, a detector, a first calculator, a suction pressure sensor and a compressor controller. The refrigerant circuit includes a variable displacement compressor. First and second pressure monitoring points are located in the refrigerant circuit. The control valve adjusts its opening degree so as to vary a displacement of the compressor. The control valve includes an actuator and a pressure sensing mechanism that has a pressure sensing member and a valve body. The pressure sensing member autonomically detects a pressure differential between the first and second pressure monitoring points. The valve body is operatively connected to the pressure sensing member. The pressure sensing member moves in response to variation of the pressure differential, whereby the valve body is moved to vary the displacement of the compressor so as to cancel the variation of the pressure differential. The actuator changes a set pressure differential in such a manner that force applied to the valve body is changed by an external command. The set pressure differential is a reference value of a motion for determining a position of the valve body by the pressure sensing mechanism. The detector detects cooling load information in the refrigerant circuit. The calculator calculates target pressure in a relatively low pressure region in the refrigerant circuit in response to the detected cooling load information. The suction pressure sensor detects actual pressure in the relatively low pressure region in the refrigerant circuit. The compressor controller controls the actuator to eliminate a differential between the calculated target pressure and the detected actual pressure.  
       [0009] Alternatively, in accordance with the present invention, an air conditioner has a refrigerant circuit, a control valve, a detector, a first calculator, a surface temperature sensor and a compressor controller. The refrigerant circuit includes a variable displacement compressor and an evaporator. First and second pressure monitoring points are located in the refrigerant circuit. The control valve adjusts its opening degree so as to vary a displacement of the compressor. The control valve includes an actuator and a pressure sensing mechanism that has a pressure sensing member and a valve body. The pressure sensing member autonomically detects a pressure differential between the first and second pressure monitoring points. The valve body is operatively connected to the pressure sensing member. The pressure sensing member moves in response to variation of the pressure differential, whereby the valve body is moved to vary the displacement of the compressor so as to cancel the variation of the pressure differential. The actuator changes a set pressure differential in such a manner that force applied to the valve body is changed by an external command. The set pressure differential is a reference value of a motion for determining a position of the valve body by the pressure sensing mechanism. The detector detects cooling load information in the refrigerant circuit. The first calculator calculates target surface temperature on the evaporator in response to the detected cooling load information. The surface temperature sensor detects actual surface temperature on the evaporator. The compressor controller controls the actuator to direct a control target to eliminate a first differential between the calculated target surface temperature and the detected actual surface temperature.  
       [0010] 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  
     [0011] The features of the present invention that are believed to be novel are set forth with particularity in the appended claims. The invention together with objects and advantages thereof, may best be understood by reference to the following description of the presently preferred embodiments together with the accompanying drawings in which:  
     [0012]FIG. 1 is a longitudinal cross-sectional view of a variable displacement swash plate type compressor according to a preferred embodiment of the present invention;  
     [0013]FIG. 2 is a longitudinal cross-sectional view of a control valve of the compressor according to the preferred embodiment of the present invention; and  
     [0014]FIG. 3 is a flow chart of an air-conditioning control according to the preferred embodiment of the present invention.  
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     [0015] A preferred embodiment of the present invention will now be described with reference to FIGS. 1 through 3. The preferred embodiment applies the present invention to a vehicle air conditioner.  
     [0016] Now referring to FIG. 1, the diagram illustrates a longitudinal cross-sectional view of a variable displacement swash plate type compressor C according to the preferred embodiment of the present invention. A housing  11  of the compressor C defines a crank chamber or a swash plate chamber  12 . A drive shaft  13  is rotatably supported by the housing  11  and extends through the crank chamber  12 . The drive shaft  13  is operatively coupled to an internal combustion engine E or a drive source for traveling a vehicle through a power transmission mechanism PT.  
     [0017] The power transmission mechanism PT may be a clutch mechanism, such as an electromagnetic clutch, that is selective to transmit and disrupt power by an external electric control or may be a constantly transmitting clutchless mechanism, such as the combination of a belt and a pulley, that has no such clutch mechanism. Incidentally, a clutchless type power transmission mechanism is employed in the preferred embodiment.  
     [0018] A lug plate  14  is arranged in the crank chamber  12  and is fixedly connected to the drive shaft  13  so as to rotate integrally with. The crank chamber  12  accommodates a swash plate  15 . The swash plate  15  is supported by the drive shaft  13  so as to slide and incline relative to the drive shaft  13 . A hinge mechanism  16  is interposed between the lug plate  14  and the swash plate  15 . Accordingly, the swash plate  15  is coupled to the lug plate  14  through the hinge mechanism  16  so that it synchronously rotates with the lug plate  14  and the drive shaft  13  and inclines relative to the drive shaft  13 .  
     [0019] A plurality of cylinder bores  11  a (only one of them shown in the drawing) is defined in the housing  11 , and each of the cylinder bores  11   a  accommodates a single-headed piston  17  so as to reciprocate. Each of the pistons  17  engages the outer periphery of the swash plate  15  through a pair of shoes  18 . Accordingly, the rotation of the swash plate  15  in accordance with the rotation of the drive shaft  13  is converted to the reciprocation of the pistons  17  through the shoes  18 .  
     [0020] A compression chamber  20  is defined in the rear side of the cylinder bore  11   a  and is surrounded by the piston  17  and a valve port assembly  19  provided in the housing  11 . A suction chamber  21  and a discharge chamber  22  are defined in the rear side of the housing  11 .  
     [0021] Refrigerant gas in the suction chamber  21  is introduced into each compression chambers  20  through a suction port  23  by pushing aside a suction valve  24  as each piston  17  moves from its top dead center to its bottom dead center. The suction ports  23  and the suction valves  24  are formed in the valve port assembly  19 . The refrigerant gas introduced in the compression chamber  20  is compressed to a predetermined pressure value as the piston  17  moves from its bottom dead center to its top dead center. Then, the compressed refrigerant gas is discharged to the discharge chamber  22  through a discharge port  25  by pushing aside a discharge valve  26 .  
     [0022] Still referring to FIG. 1, a bleed passage  27  and a supply passage  28  are provided in the housing  11 . The bleed passage  27  interconnects the crank chamber  12  and the suction chamber  21 . The supply passage  28  interconnects the discharge chamber  22  and the crank chamber  12 . In the housing  11 , a control valve CV is arranged in the supply passage  28 .  
     [0023] The adjustment of the opening degree of the control valve CV controls a balance between the amount of discharged gas into the crank chamber  12  through the supply passage  28  and the amount of refrigerant gas out of the crank chamber  12  through the bleed passage  27  so that pressure in the crank chamber  12  is determined. A pressure differential between the crank chamber  12  and the compression chambers  20  through the pistons  17  varies in response to variation in the pressure in the crank chamber  12 . Thus, the inclination angle of the swash plate  15  is varied, and the stroke of the pistons  17 , that is, the displacement of the compressor C is adjusted.  
     [0024] When the pressure in the crank chamber  12  is reduced, the inclination angle of the swash plate  15  increases so that the displacement of the compressor C increases. The two-dotted line of the swash plate  15  in FIG. 1 indicates a state where the lug plate  14  contacts the swash plate  15  to regulate its further inclination, that is, the swash plate  15  is at its maximum inclination angle. On the contrary, when the pressure in the crank chamber  12  is increased, the inclination angle of the swash plate  15  reduces so that the displacement of the compressor C reduces. The solid line of the swash plate  15  in FIG. 1 indicates a state where the swash plate  15  is at its minimum inclination angle.  
     [0025] Still referring to FIG. 1, the refrigerant circuit of the vehicle air conditioner includes the above described compressor C and an external refrigerant circuit  30 . The external refrigerant circuit  30  includes a condenser  31 , an expansion valve  32  and an evaporator  33 .  
     [0026] A first pressure monitoring point P 1  is located in the discharge chamber  22 . A second pressure monitoring point P 2  is located at a predetermined distance from the first pressure monitoring point P 1  toward the side of the condenser  31  (the downstream side) in a refrigerant passage. A differential between a pressure PdH at the first pressure monitoring point P 1  and a pressure PdL at the second pressure monitoring point P 2  reflects the flow rate of refrigerant in the refrigerant circuit. The first pressure monitoring point P 1  communicates with the control valve CV through a first pressure introducing passage  35 . The second pressure monitoring point P 2  communicates with the control valve CV through a second pressure introducing passage  36  (See FIG. 2).  
     [0027] Now referring to FIG. 2, the diagram illustrates a longitudinal cross-sectional view of the control valve CV according to the preferred embodiment of the present invention. A valve housing  41  of the control valve CV defines a valve chamber  42 , a communication passage  43  and a pressure sensing chamber  44 . A rod  45  is arranged in the valve chamber  42  and the communication passage  43  so as to move in its axial direction (the vertical direction in the drawing). The upper end of the rod  45  inserted in the communication passage  43  separates the communication passage  43  from the pressure sensing chamber  44 . The valve chamber  42  communicates with the discharge chamber  22  through the upstream portion of the supply passage  28 . The communication passage  43  communicates with the crank chamber  12  through the downstream portion of the supply passage  28 . The valve chamber  42  and the communication passage  43  constitute a portion of the supply passage  28 .  
     [0028] A valve body portion  46  is formed at the middle portion of the rod  45  and is located in the valve chamber  42 . A step at a boundary between the valve chamber  42  and the communication passage  43  forms a valve seat  47 , and the communication passage  43  serves as a kind of valve hole. As the rod  45  moves from the lowest position shown in FIG. 2 to the highest position where the valve body portion  46  seats the valve seat  47 , the communication passage  43  is shut. Namely, the valve body portion  46  of the rod  45  functions as a valve body for adjusting the opening degree of the supply passage  28 .  
     [0029] A pressure sensing mechanism includes a pressure sensing member  48  and the pressure sensing chamber  44 . The pressure sensing member or a bellows spring  48  is accommodated in the pressure sensing chamber  44 . The upper end of the pressure sensing member  48  is secured to the valve housing  41 . The upper end of the rod  45  is fitted into the lower end of the pressure sensing member  48 . The inside of the pressure sensing chamber  44  is separated into a first pressure chamber  49  and a second pressure chamber  50  by the pressure sensing member  48 , which forms a cylinder with an opening at one end. The first pressure chamber  49  and the second pressure chamber  50  are respectively defined inside and outside the pressure sensing member  48 . The pressure PdH at the first pressure monitoring point P 1  is applied to the first pressure chamber  49  through the first pressure introducing passage  35 . The pressure PdL at the second pressure monitoring point P 2  is applied to the second pressure chamber  50  through the second pressure introducing passage  36 .  
     [0030] An electromagnetic actuator  51  for changing set pressure differential is provided in the lower side of the valve housing  41 . The electromagnetic actuator  51  includes a plunger housing  52  in the middle of the valve housing  41 . The plunger housing  52  forms a cylinder with an opening at one end. A center post or a fixed core  53  is fixedly fitted at the opening on the upper side of the plunger housing  52 . A plunger chamber  54  is defined at the lower region in the plunger housing  52  by fitting the center post  53 .  
     [0031] A plunger or a movable core  56  is accommodated in the plunger housing  54  so as to move in its axial direction. A guide hole  57  extends through the middle of the center post  53  along its axial direction. The lower end of the rod  45  is located in the guide hole  57  so as to move in its axial direction. The lower end of the rod  45  contacts the upper end of the plunger  56  in the plunger chamber  54 .  
     [0032] A coil spring  60  is accommodated in the plunger chamber  54  between the bottom end of the plunger housing  52  and the plunger  56 . The coil spring  60  urges the plunger  56  toward the rod  45 . The rod  45  is urged toward the plunger  56  by the spring property of the pressure sensing member or the bellows spring  48 . Accordingly, the plunger  56  and the rod  45  regularly move upward and downward together. Incidentally, the bellows spring  48  has a greater spring force than the coil spring  60 .  
     [0033] A coil  61  is wound outside the outer circumference of the plunger housing  52  and ranges from the center post  53  to the plunger  56 . The coil  61  is supplied with electric current from a drive circuit  78  in response to a command of an air conditioner ECU or a compressor controller  72  for controlling the air conditioner. Electromagnetic force (electromagnetic attraction) corresponding to the amount of electric current supplied from the drive circuit  78  to the coil  61  is generated between the plunger  56  and the center post  53 , and the electromagnetic force is transmitted to the rod  45  through the plunger  56 . Incidentally, the electric current supplied to the coil  61  is controlled by adjusting applied voltage. A pulse width modulation (PWM) control is employed to adjust the applied voltage.  
     [0034] The opening degree of the control valve CV or the position of the valve body portion  46  of the rod  45  is determined as follows.  
     [0035] Still referring to FIG. 2, when no current is supplied to the coil  61  (duty ratio Dt=0%), downward urging force of the bellows spring  48  dominantly determines the position of the rod  45 . Accordingly, the rod  45  is positioned at the lowest position so that the valve body portion  46  fully opens the communication passage  43 . Therefore, the pressure in the crank chamber  12  becomes maximum in accordance with the present condition, and the pressure differential between the crank chamber  12  and the compression chambers  20  through the pistons  17  becomes large. Then, the inclination angle of the swash plate  15  is minimum so that the displacement of the compressor C is minimum.  
     [0036] When the coil  61  is supplied with electric current that is greater than the minimum duty ratio Dt(min) in the effective range of the duty ratio (Dt(min)&gt;0%), the upward electromagnetic force and the urging force of the coil spring  60  exceed the downward urging force of the bellows spring  48  so that the rod  45  initiates to move upwardly. In this state, the upward electromagnetic force and the upward urging force of the coil spring  60  oppose downward pressing force based upon the pressure differential ΔPd (=PdH−PdL) and the downward urging force of the bellows spring  48 . Then, the position of the valve body portion  46  of the rod  45  is determined based upon a balance among the above upward and downward urging forces. Thus, the displacement of the compressor C is adjusted.  
     [0037] For example, when the rotational speed of the engine E slows down to reduce the flow rate of refrigerant gas in the refrigerant circuit, the downward urging force based upon the pressure differential ΔPd weakens so that the upward urging force at the moment cannot maintain the balance between the upward and downward urging forces that act on the rod  45 . Accordingly, the valve body portion  46  of the rod  45  moves upwardly to reduce the opening degree of the communication passage  43  so that the pressure in the crank chamber  12  tends to reduce. Therefore, the swash plate  15  inclines to increase its inclination angle, and the displacement of the compressor C increases. The increased displacement increases the flow rate of refrigerant gas in the refrigerant circuit so that the pressure differential ΔPd increases.  
     [0038] On the contrary, when the rotational speed of the engine E speeds up to increase the flow rate of refrigerant gas in the refrigerant circuit, the downward urging force based upon the pressure differential ΔPd strengthens so that the upward electromagnetic force at the moment cannot maintain the balance between the upward and downward urging forces that act on the rod  45 . Accordingly, the valve body portion  46  of the rod  45  moves downwardly to increase the opening degree of the communication passage  43  so that the pressure in the crank chamber  12  tends to increase. Therefore, the swash plate  15  inclines to reduce its inclination angle, and the displacement of the compressor C reduces. The reduced displacement reduces the flow rate of refrigerant gas in the refrigerant circuit so that the pressure differential ΔPd reduces.  
     [0039] Furthermore, when the duty ratio Dt supplied to the coil  61  is increased to strengthen the upward electromagnetic force, the force based upon the pressure differential ΔPd at the moment cannot maintain the balance between the upward and downward urging forces. Therefore, the valve body portion  46  of the rod  45  moves upwardly to reduce the opening degree of the communication passage  43  so that the displacement of the compressor C increases. As a result, the flow rate of refrigerant gas the refrigerant circuit increases, and the pressure differential ΔPd increases.  
     [0040] On the contrary, when the duty ratio Dt supplied to the coil  61  is reduced to weaken the upward electromagnetic force, the force based upon the pressure differential ΔPd at the moment cannot maintain the balance between the upward and downward urging forces. Therefore, the valve body portion  46  of the rod  45  moves downwardly to increase the opening degree of the communication passage  43  so that the displacement of the compressor C reduces. As a result, the flow rate of refrigerant gas the refrigerant circuit reduces, and the pressure differential ΔPd reduces.  
     [0041] In summary, the control valve CV internally determines the position of the valve body portion  46  of the rod  45  in response to the variation in the pressure differential ΔPd so as to maintain the set pressure differential (a target pressure differential) of the pressure differential ΔPd determined by the duty ratio Dt to the coil  61 . Additionally, the set pressure differential is externally changeable by adjusting the duty ratio Dt to the coil  61 .  
     [0042] Incidentally, the pressure in the crank chamber  12  is applied to the plunger chamber  54  through a clearance between the guide hole  57  and the rod  45 . Accordingly, the pressure in the plunger chamber  54  (the pressure in the crank chamber  12 ) is applied to the rod  45  to close the valve hole. Meanwhile, the pressure PdH in the discharge chamber  22  is applied to the upper end of the valve body portion  46 . Accordingly, force based upon a pressure differential between the pressure PdH in the discharge chamber  22  and the pressure in the crank chamber  12  also influences on the determination of the position of the rod  45 , in addition to the force based upon the pressure differential ΔPd and the force from the electromagnetic actuator  51 . Namely, with respect to the control valve CV, even if the duty ratio Dt supplied to the coil  61  does not change, when there is a differential between the pressure PdH in the discharge chamber  22  and the pressure in the crank chamber  12 , the set pressure differential varies a little.  
     [0043] Still referring to FIG. 2, the information detector  77  includes an air conditioner switch or an A/C switch  79 , a temperature setting device  80 , a compartment temperature sensor  81  for detecting temperature in a vehicle compartment, an ambient temperature sensor  82  for detecting ambient temperature, a solar irradiance sensor  85 , a suction pressure sensor  83  and an evaporator sensor  84 .  
     [0044] The A/C switch  79  is an ON-OFF switch of the air conditioner. The temperature setting device  80  is a device by which a passenger sets temperature in the vehicle compartment (set temperature Tset). The compartment temperature sensor  81  is a device for detecting temperature Tr in the vehicle compartment. The ambient temperature sensor  82  is a device for detecting ambient temperature Tam. The solar irradiance sensor  85  is a device for detecting solar irradiance Ts. The suction pressure sensor  83  is a device for detecting a pressure Ps(x) in a relatively low pressure region in the refrigerant circuit, such as a suction pressure region (for example, the suction chamber  21 , the inside of a conduit near the relatively low pressure region of the external refrigerant circuit  30  and an adjacent outlet of refrigerant gas in the evaporator  33 ). The evaporator sensor  84  is a device for detecting temperature Te(x) of air that is just passed through the evaporator  33 .  
     [0045] Particularly, a cooling load information detector includes the temperature setting device  80 , the compartment temperature sensor  81 , the ambient temperature sensor  82  and the solar irradiance sensor  85 . The cooling load information detector detects cooling load information in the refrigerant circuit, such as the set temperature Tset, the compartment temperature Tr, the ambient temperature Tam and the solar irradiance Ts.  
     [0046] The air conditioner ECU  72  adjusts the duty ratio Dt of the control valve CV, that is, the set pressure differential of the control valve CV, in response to the information detected by the information detector  77 . Incidentally, The air conditioner ECU  72  not only controls the control valve CV but also, for example, controls air quantity by a conventional manner for adjusting the rotational speed of a blower motor (not shown) in response to the information detected by the information detector  77 .  
     [0047] Now referring to FIG. 3, the diagram illustrates a flow chart of an air-conditioning control according to the preferred embodiment of the present invention. When the engine E is started, the air conditioner ECU  72  exerts various initialization in accordance with an initial program at a step  101  (S 101 ). For example, the air conditioner ECU  72  sets “0” as an initial value to the duty ratio Dt of the control valve CV (Namely, no electric current is supplied to the coil  61 ). The ON/OFF state of the A/C switch  79  is observed until it is turned on at S 102 . When the A/C switch  79  is turned on, the air conditioner ECU adjusts the duty ratio Dt of the control valve CV to the minimum duty ratio Dt(min) at S 103  so as to start up the internally mechanical control function of the control valve CV (a function for maintaining the set pressure differential).  
     [0048] Required blowing temperature Ta0 of the air conditioner is calculated at S 104  based upon the cooling load information (Tset, Tr, Tam and Ts) that is sent from the temperature setting device  80 , the compartment temperature sensor  81 , the ambient temperature sensor  82  and the solar irradiance sensor  85 . The air conditioner ECU  72  serves as a calculator for calculating the target after-evaporator temperature at S 105  and calculates the target after-evaporator temperature Te(set) from the calculated required blowing temperature Ta0 with reference to map data that are previously memorized. The air conditioner ECU  72  compares the calculated target after-evaporator temperature Te(set) with the after-evaporator temperature Te(x) detected by the evaporator sensor  84  and judges whether or not a differential between Te(set) and Te(x) is equal to or less than a predetermined value (for example, 2 degrees centigrade) at S 106 .  
     [0049] When the judgment of S 106  is false, that is, when the differential between Te(set) and Te(x) exceeds the predetermined value, the air conditioner ECU  72  revises the duty ratio Dt of the control valve CV so as to change the target value to the suction pressure Ps(x) detected by the suction pressure sensor  83 .  
     [0050] Namely, the air conditioner ECU  72  serves as a calculator for calculating a target suction pressure at S 107  and calculates a target suction pressure Ps(set) from the target after-evaporator temperature Te(set) calculated at S 105  with reference to map data that are previously memorized. The air conditioner ECU  72  judges whether or not the suction pressure Ps(x) detected by the suction pressure sensor  83  is greater than the calculated target suction pressure Ps(set) at S 108 . When the judgment of S 108  is false, the air conditioner ECU  72  judges whether or not the detected suction pressure Ps(x) is smaller than the target suction pressure Ps(set). When the judgment of S 109  is also false, the detected suction pressure Ps(x) is equal to the target suction pressure Ps(set).  
     [0051] Thereby, even if the air conditioner ECU  72  does not change the duty ratio Dt of the control valve CV, it soon judges the differential between the target after-evaporator temperature Te(set) and the detected after-evaporator temperature Te(x) becomes within the predetermined value and switches a process to S 116  without sending a command to change the duty ratio Dt to the drive circuit  78 . Namely, as the duty ratio Dt of the control valve CV is changed, the suction pressure Ps(x) varies at first, and then the after-evaporator temperature Te(x) varies at a certain interval from the variation of the suction pressure Ps(x).  
     [0052] The air conditioner ECU  72  judges whether or not the A/C switch  79  is turned off at S 116 . When the judgment of S 116  is false, the air conditioner ECU  72  switches a process to S 104 . On the contrary, when the judgment of S 116  is true, the air conditioner ECU  72  switches a process to S 101  so that the control valve CV is in a non-energized state. Thus, the displacement of the compressor C becomes minimum.  
     [0053] When the judgment of S 108  is true, the thermal load on the evaporator  33  is regarded to be relatively large so that the air conditioner ECU  72  increases the duty ratio Dt by the unit quantity of ΔD at S 110  and commands the drive circuit  78  to change the duty ratio Dt to a revised duty ratio (Dt+ΔD). Accordingly, the opening degree of the control valve CV reduces a little so that the displacement of the compressor C increases. Then, the heat removal performance rises at the evaporator  33 , and not only the suction pressure Ps(x) but also the after-evaporator temperature Te(x) tends to reduce.  
     [0054] When the judgment of S 109  is true, the thermal load on the evaporator  33  is regarded to be relatively small so that the air conditioner ECU  72  reduces the duty ratio Dt by the unit quantity of ΔD at S 111  and commands the drive circuit  78  to change the duty ratio Dt to a revised duty ratio (Dt−ΔD). Accordingly, the opening degree of the control valve CV increases a little so that the displacement of the compressor C reduces. Then, the heat removal performance falls at the evaporator  33 , and not only the suction pressure Ps(x) but also the after-evaporator temperature Te(x) tends to increase. Additionally, the air conditioner ECU  72  switches S 110  and S 111  to S 116 .  
     [0055] As described above, S 110  and/or S 111  directs a control target to eliminate the differential between the detected suction pressure Ps(x) and the target suction pressure Ps(set). Even if the differential between the detected after-evaporator temperature Te(x) and the target after-evaporator temperature Te(set) largely exceeds the predetermined value (for example, 2 degrees centigrade), the differential is rapidly lessened in such a manner that the duty ratio Dt is revised at S 110  and/or S 111 . Accordingly, as coupled with the internally mechanical adjustment of the opening degree of the control valve CV, the differential between the detected after-evaporator temperature Te(x) and the target after-evaporator temperature Te(set) rapidly fits within the predetermined value.  
     [0056] When the differential between the detected after-evaporator temperature Te(x) and the target after-evaporator temperature Te(set) is within the predetermined value by a process for revising the duty ratio Dt at S 110  and/or S 111 , the judgment of S 106  is true. When the judgment of S 106  is true, a process for revising the duty ratio Dt of the control valve CV is directed to eliminate the differential between the detected after-evaporator temperature Te(x) and the target after-evaporator temperature Te(set).  
     [0057] Namely, the air conditioner ECU  72  judges whether or not the after-evaporator temperature Te(x) detected by the evaporator sensor  84  is greater than the calculated target after-evaporator temperature Te(set) at S 112 . When the judgment of S 112  is false, the air conditioner ECU  72  judges whether or not the detected after-evaporator temperature Te(x) is smaller than the target after-evaporator temperature Te(set) at S 113 . When the judgment of S 113  is also false, the detected after-evaporator temperature Te(x) is equal to the target after-evaporator temperature Te(set) so that the duty ratio Dt need not be changed for varying cooling performance. Therefore, the air conditioner ECU  72  switches a process to S 116  without sending a command for changing the duty ratio Dt to the drive circuit  78 .  
     [0058] When the judgment of S 112  is true, the thermal load on the evaporator  33  is regarded to be relatively large so that the air conditioner ECU  72  increases the duty ratio Dt by the unit quantity of ΔD at S 114  and commands the drive circuit  78  to change the duty ratio Dt to a revised duty ratio (Dt+ΔD). Accordingly, the opening degree of the control valve CV reduces a little so that the displacement of the compressor C increases. Then, the heat removal performance rises at the evaporator  33 , and the after-evaporator temperature Te(x) tends to reduce.  
     [0059] When the judgment of S 113  is true, the thermal load on the evaporator  33  is regarded to be relatively small so that the air conditioner ECU  72  reduces the duty ratio Dt by the unit quantity of ΔD at S 115  and commands the drive circuit  78  to change the duty ratio Dt to a revised duty ratio (Dt−ΔD). Accordingly, the opening degree of the control valve CV increases a little so that the displacement of the compressor C reduces. Then, the heat removal performance falls at the evaporator  33 , and the after-evaporator temperature Te(x) tends to increase. Additionally, the air conditioner ECU  72  switches S 114  and S 115  to S 116 .  
     [0060] As described above, S 114  and/or S 115  directs a control target to eliminate the differential between the detected after-evaporator temperature Te(x) and the target after-evaporator temperature Te(set). Even if the differential between the detected after-evaporator temperature Te(x) and the target after-evaporator temperature Te(set) exceeds the predetermined value, the differential is gradually optimized in such a manner that the duty ratio Dt is revised at S 114  and/or S 115 . Accordingly, as coupled with the internally mechanical adjustment of the opening degree of the control valve CV, the detected after-evaporator temperature Te(x) converges in high accuracy around the target after-evaporator temperature Te(set).  
     [0061] According to the preferred embodiment, the following advantageous effects are obtained.  
     [0062] (1) The air conditioner ECU  72  revises the duty ratio Dt of the control valve CV so as to direct a control target to eliminate the differential between the detected suction pressure Ps(x) and the target suction pressure Ps(set). The suction pressure Ps(x) is physical quantity that responds to the variation of the thermal load on the evaporator  33  more rapidly than the after-evaporator temperature Te(x). Accordingly, for example, the displacement of the compressor C is rapidly varied in response to the rapid variation of the thermal load on the evaporator  33  due to the rapid variation of the rotational speed of the blower motor (air quantity). As a result, the after-evaporator temperature Te(x) rapidly approaches the target after-evaporator temperature Te(set) so that air conditioning feeling becomes satisfactory.  
     [0063] (2) The control valve CV is configured to subtly vary the set pressure differential when the differential between the pressure PdH in the discharge chamber  22  and the pressure in the crank chamber  12  differs, even if the duty ratio Dt supplied to the coil  61  are the same. Accordingly, in a conventional manner, for example, even if the rotational speed of the engine E (the compressor C) rapidly varies due to rapid acceleration of a vehicle and the like, that is, even if the flow rate of refrigerant gas in the refrigerant circuit rapidly varies, an external control in response to the variation of the detected after-evaporator temperature Te(x) due to the above rapid variation changes a set pressure differential to deal with the above rapid variation. Namely, the process for revising the duty ratio Dt of the control valve CV directs a control target to eliminate the differential between the detected after-evaporator temperature Te(x) and the target after-evaporator temperature Te(set). The above revising process still has the same problem as the prior art mentioned in the background of the invention when the rotational speed of the engine E rapidly varies. Namely, it takes a relatively long time that the after-evaporator temperature Te(x) approaches the target after-evaporator temperature Te(set). Thereby, air conditioning feeling is deteriorated.  
     [0064] However, the air conditioner ECU  72  in the preferred embodiment directs a control target to eliminate the differential between the detected suction pressure Ps(x) and the target suction pressure Ps(set) and revises the duty ratio Dt of the control valve CV. The suction pressure Ps(x) is physical quantity that responds to the variation of the rotational speed of the engine E more quickly than, for example, the after-evaporator temperature Te(x). Accordingly, the displacement of the compressor C is quickly varied in response to the rapid variation of the rotational speed of the engine E, and the after-evaporator temperature Te(x) quickly approaches the target after-evaporator temperature Te(set). As a result, even if the rotational speed of the engine E rapidly varies, air conditioning feeling is satisfactory.  
     [0065] (3) When the differential between the target after-evaporator temperature Te(set) and the detected after-evaporator temperature Te(x) is relatively small, the air conditioner ECU  72  directs a control target to eliminate the detected after-evaporator temperature Te(x) and the target after-evaporator temperature Te(set) and revises the duty ratio Dt of the control valve CV. Accordingly, the detected after-evaporator temperature Te(x) converges in high accuracy around the target after-evaporator temperature Te(set) so that air conditioning feeling is further improved.  
     [0066] The present invention is not limited to the embodiment described above but may be modified into the following alternative embodiments.  
     [0067] In alternative embodiments to those of the above preferred embodiment, referring to FIG. 2, the suction pressure sensor  83  is changed to a surface temperature sensor  86  for detecting surface temperature T ST  (temperature of a heat exchanging fin) of the evaporator  33 . Additionally, a portion of the process for revising the duty ratio Dt of the control valve CV by the air conditioner ECU  72 , particularly, the several steps (S 107  through S 111 ) in the flow chart in FIG. 3 are changed to steps (S 107 ′, S 108 ′, S 109 ′, S 110 , and S 111 ) as follows.  
     [0068] Now referring to FIG. 4, the diagram illustrates a potion of flow chart that is modified from that of FIG. 3. The air conditioner ECU  72  serves as a calculator for calculating target surface temperature T ST (set) at S 107 ′ and calculates the target surface temperature T ST (set) from the target after-evaporator temperature Te(set) calculated at S 105  with reference to map data that are previously memorized. The air conditioner ECU  72  judges whether or not the surface temperature T ST (x) detected by the surface temperature sensor  86  is greater than the calculated target surface temperature T ST (set) at S 108 ′. When the judgment of S 108 ′ is false, the air conditioner ECU  72  judges whether or not the detected surface temperature T ST (x) is smaller than the target surface temperature T ST (set) at S 109 ′. When the judgment of S 109 ′ is also false, the detected surface temperature T ST (x) is equal to the target surface temperature T ST (set).  
     [0069] Thereby, the air conditioner ECU  72  soon judges the differential between the target after-evaporator temperature Te(set) and the detected after-evaporator temperature Te(x) is within the predetermined value (for example, 2 degrees centigrade) without changing the duty ratio Dt of the control valve CV and switches a process to S 116  without commanding the drive circuit  78  to change the duty ratio Dt. Namely, as the duty ratio Dt of the control valve CV is changed, the surface temperature T ST (x) of the evaporator  33  varies at first. Then, the after-evaporator temperature Te(x) varies at a certain interval from the variation of the surface temperature T ST (x).  
     [0070] When the judgment of S 108 ′ is true, the thermal load on the evaporator  33  is regarded to be relatively large so that the air conditioner ECU  72  increases the duty ratio Dt by the unit quantity of ΔD at S 110  and commands the drive circuit  78  to change the duty ratio Dt to a revised duty ratio (Dt+ΔD). Accordingly, the opening degree of the control valve CV reduces a little so that the displacement of the compressor C increases. Then, the heat removal performance rises at the evaporator  33 , and the surface temperature T ST (x) of the evaporator  33  and the after-evaporator temperature Te(x) tend to reduce.  
     [0071] When the judgment of S 109 ′ is true, the thermal load on the evaporator  33  is regarded to be relatively small so that the air conditioner ECU  72  reduces the duty ratio Dt by the unit quantity of ΔD at S 111  and commands the drive circuit  78  to change the duty ratio Dt to a revised duty ratio (Dt−ΔD). Accordingly, the opening degree of the control valve CV increases a little so that the displacement of the compressor C reduces. Then, the heat removal performance falls at the evaporator  33 , and the surface temperature T ST (x) of the evaporator  33  and the after-evaporator temperature Te(x) tend to increase.  
     [0072] The surface temperature T ST (x) of the evaporator  33  is physical quantity that responds to the variation of the thermal load on the evaporator  33  more quickly than the after-evaporator temperature Te(x). Accordingly, the same advantageous effects to those mentioned in the paragraphs (1) through (3) of the preferred embodiment are obtained.  
     [0073] In the above preferred embodiment, when the differential between the detected after-evaporator temperature Te(x) and the target after-evaporator temperature Te(set) is within the predetermined value, the air conditioner ECU  72  directs a control target to eliminate the differential between the detected after-evaporator temperature Te(x) and the target after-evaporator temperature Te(set) and revises the duty ratio Dt of the control valve CV. Furthermore, when the differential between the detected after-evaporator temperature Te(x) and the target after-evaporator temperature Te(set) exceeds the predetermined value, the air conditioner ECU  72  directs a control target to eliminate the differential between the detected suction pressure Ps(x) and the target suction pressure Ps(set) and revises the duty ratio Dt of the control valve CV. In alternative embodiments to those of the above preferred embodiment, irrespective of the differential between the target after-evaporator temperature Te(set) and the detected after-evaporator temperature Te(x), the air conditioner ECU  72  directs a control target to eliminate the differential between the detected suction pressure Ps(x) and the target suction pressure Ps(set) and revises the duty ratio Dt of the control valve CV. Namely, for example, S 106  and S 112  through S 115  are omitted from the flow chart in FIG. 3 in the above preferred embodiment. Even so, when the rotational speed of the engine E or the thermal load on the evaporator  33  rapidly varies, the displacement of the compressor C is quickly varied so that air conditioning feeling is satisfactory.  
     [0074] In alternative embodiments to those of the above preferred embodiment, referring to FIG. 5, the diagram illustrates a portion of flow chart that is modified from that of FIG. 3. The control target in connection with the process for revising the duty ratio Dt(x) of the control valve CV is changed in response to large and small of the set pressure differential of the control valve CV, that is, large and small of the duty ratio Dt(x) supplied to the coil  61 . Namely, when the duty ratio Dt(x) of the control valve CV is within the predetermined value Dt(set), that is, when the flow rate of refrigerant gas in the refrigerant circuit is controlled in a relatively large flow rate range, the air conditioner ECU  72  directs a control target to eliminate the differential between the detected after-evaporator temperature Te(x) and the target after-evaporator temperature Te(set) and revises the duty ratio Dt(x) of the control valve CV. On the contrary, when the duty ratio Dt(x) is less than the predetermined value Dt(set), that is, when the flow rate of refrigerant gas in the refrigerant circuit is controlled in a relatively small flow rate range, the air conditioner ECU  72  directs a control target to eliminate the differential between the detected suction pressure Ps(x) and the target suction pressure Ps(set) and revises the duty ratio Dt(x) of the control valve CV.  
     [0075] Thereby, a control of the flow rate in the refrigerant circuit is stable in a relatively small flow rate range so that air conditioning feeling is satisfactory. Namely, the control valve CV is configured to detect the pressure differential ΔPd between the pressure monitoring points in the refrigerant circuit and internally and autonomically exerts a feedback control of the displacement of the compressor C. Accordingly, when the flow rate of refrigerant gas in the refrigerant circuit is relatively small, the variation of the pressure differential ΔPd in response to the variation of the flow rate of refrigerant gas is relatively small (not clear) so that the internally mechanical control of the control valve CV does not properly function. As a result, when the air conditioner ECU  72  directs a control target to eliminate the differential between the detected after-evaporator temperature Te(x) and the target after-evaporator temperature Te(set) for revising the duty ratio Dt(x) of the control valve CV, the flow rate control of the refrigerant circuit in a relatively small flow rate range becomes unstable due to a slow response of the detected after-evaporator temperature Te(x) in response to the revising of the duty ratio Dt(x).  
     [0076] In alternative embodiments to those of the above preferred embodiment, referring to FIG. 1, a first pressure monitoring point P 1 ′ is located at a suction pressure region between the evaporator  33  and the suction pressure chamber  21  including the evaporator  33  and the suction pressure chamber  21  in the refrigerant circuit, while a second pressure monitoring point P 2 ′ is located downstream to the first pressure monitoring point P 1 ′ in the same suction pressure region.  
     [0077] In alternative embodiments to those of the above preferred embodiment, the control valve CV employs a bleed side control valve that adjusts the pressure in the crank chamber  12  by adjusting the opening degree of the bleed passage  27  instead of the supply passage  28 .  
     [0078] In alternative embodiments to those of the above preferred embodiment, the variable displacement compressor employs a wobble type.  
     [0079] 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 of the appended claims.