Patent Publication Number: US-2004045307-A1

Title: Hybrid compressor system

Description:
BACKGROUND OF THE INVENTION  
       [0001] The present invention relates to a hybrid compressor system in use for a vehicle air conditioner.  
       [0002] In an idle stop control, an engine is automatically stopped when it is an idle state, that is, for example, when a vehicle stops running to wait for a traffic signal to change. For fuel saving, the idle stop control has been generalized recently. A hybrid compressor in a vehicle air conditioner utilizes not only an engine but also an electric motor as a drive source. Therefore, the air conditioner is capable of performing air conditioning even when the engine is in a stopped state. However, if the power of the electric motor for driving the compressor is requested to be substantially as large as that of the engine for driving the compressor, the electric motor becomes large-sized. Therefore, for example, when the electric motor is accommodated in a housing of the compressor, the compressor also becomes large-sized. Meanwhile, the displacement of the compressor per one rotation of a rotary shaft is set at a small value such that the compressor can be driven even by a small-sized electric motor. In this case, when the engine drives the compressor at a relatively low rotational speed, a large amount of discharged refrigerant by the compressor per a predetermined period cannot be ensured. Therefore, cooling performance is decreased.  
       [0003] In order to solve such a problem, a technique disclosed in Japanese Unexamined Patent Publication No. 11-93876 has been proposed. In the technique, a first drive part transmits power from an electric motor to a rotary shaft of a compressor for driving the rotary shaft of the compressor. A second drive part transmits power from the engine to the rotary shaft of the compressor for driving the rotary shaft of the compressor. A speed change mechanism that changes a rotational speed and transmits power to the rotary shaft is arranged at one of the first and second drive parts. Namely, for example, a speed reducing mechanism is arranged at the first drive part. Therefore, the compressor can be driven in a state that the displacement of the compressor per one rotation of the rotary shaft is set at a relatively large value and the rotational speed of the compressor is relatively low without a large-sized electric motor. Even when the engine drives the compressor at the relatively low rotational speed, the relatively large amount of the discharged refrigerant by the compressor per the predetermined period can be ensured. As a result, the cooling performance is ensured preferably.  
       [0004] Also, for example, a speed increasing mechanism is arranged at the second drive part. Therefore, the compressor can be driven in a state that the displacement of the compressor per one rotation of the rotary shaft is set at a relatively small value and a rotational speed of the compressor is relatively high. Driving torque for the compressor can become small. As a result, the electric motor can be miniaturized.  
       [0005] However, when the speed reducing mechanism is arranged at the first drive part, the speed reducing ratio of the speed reducing mechanism needs to be set at a relatively large value in order to drive the compressor with the relatively large displacement by the small-sized electric motor. When the speed increasing mechanism is arranged at the second drive part, the speed increasing ratio of the speed increasing mechanism needs to be set at a relatively large value, in order to ensure the relatively large amount of the discharged refrigerant per the predetermined period by driving the compressor with the relatively small displacement by the engine at the relatively low rotational speed.  
       [0006] When a transmission ratio (speed reducing ratio or speed increasing ratio) is set at a relatively large value, the speed change mechanism needs to be large-sized. Therefore, for example, when the speed change mechanism is accommodated in the housing, the compressor becomes large-sized. Namely, in the conventional technique, the miniaturization of the electric motor is difficult to be compatible with the miniaturization of the speed change mechanism.  
       [0007] In the hybrid compressor, when the relatively large amount of the discharged refrigerant by compressor per the predetermined period is required for cooling by driving the electric motor, the rotational speed of the electric motor requires to be high. Therefore, combined with the smallness of the electric motor, step out occurs in the electric motor, and the electric motor runs in an unstable manner.  
       [0008] The present invention provides a hybrid compressor system in which an electric motor is miniaturized and runs in a stable manner and in which a transmission mechanism that transmits power from an engine or the electric motor is miniaturized.  
       [0009] In accordance with the present invention, a hybrid compressor system in a vehicle air conditioner has a variable displacement hybrid compressor for compressing refrigerant, a drive source for driving a vehicle and an electric motor. The drive source for driving the vehicle is operatively connected to the compressor. The electric motor is operatively connected to the compressor, and the compressor is selectively driven by one of the drive source for driving the vehicle and the electric motor. The hybrid compressor system also has an air conditioning information detector for detecting information for air conditioning, a thermal load calculator and a controller. The air conditioning information detector is electrically connected to the air conditioning information detector and calculates a thermal load based on the detected information from the air conditioning information detector. The controller is electrically connected to the thermal load calculator. The controller compares the thermal load with a predetermined value. The controller changes displacement of the compressor to a first predetermined displacement value and selects the drive source for driving the vehicle as a drive source of the compressor when the thermal load is larger than the predetermined value. The controller changes the displacement of the compressor to a second predetermined displacement value and selects the electric motor as the drive source of the compressor when the thermal load is equal to, or smaller than the predetermined value. The second predetermined displacement value is smaller than the first predetermined displacement value.  
       [0010] The present invention also provides a method for controlling a hybrid compressor system in use for a vehicle air conditioner. The hybrid compressor system has a variable displacement hybrid compressor and two drive sources including an engine for driving a vehicle and an electric motor. The hybrid compressor is selectively driven by one of the engine and the electric motor. The method includes the steps of switching on the air conditioner, detecting information for air conditioning, calculating a thermal load based on the detected information, comparing the thermal load with a predetermined value, selecting the drive source of the compressor based on the comparison, and changing displacement of the compressor based on the selection. 
     
    
    
     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 hybrid compressor of a first preferred embodiment according to the present invention;  
     [0013]FIG. 2 is a flow chart illustrating air conditioning control by an air conditioner ECU for the first preferred embodiment; and  
     [0014]FIG. 3 is a longitudinal cross-sectional view of a hybrid compressor of a second preferred embodiment according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     [0015] A first and second preferred embodiments according to the present invention will be described.  
     [0016] Now, the first preferred embodiment will be described. As shown in FIG. 1, a hybrid compressor C that constitutes a refrigerant cycle in a vehicle air conditioner has a housing  11  for compressing refrigerant. The left side and the right side of the drawing respectively correspond to the front side and the rear side in FIG. 1. An electric motor  21  and a compression unit  12  are accommodated in the housing  11 . A power transmission mechanism  22  is arranged at the front end of the housing  11  outside the housing  11 . The compression unit  12  is a scroll type and a variable displacement type. The power transmission mechanism  22  transmits power from an engine E (an internal combustion engine) as a drive source.  
     [0017] The compressor C is selectively driven by the engine E through the power transmission mechanism  22  and by the electric motor  21 . The air conditioner includes the electric motor  21 . When the engine E is in a stopped state, the compressor C is driven by the electric motor  21 . Therefore, the air conditioner is capable of continuously performing air conditioning even when the engine E is in the stopped state. The air conditioner in the present first preferred embodiment is suitable for an idle stop vehicle that performs an idle stop control and a hybrid vehicle.  
     [0018] A pulley shaft  13  and a compressor shaft  19  are rotatably supported in the housing  11  at the front side of the housing  11  and at the middle of the housing  11  respectively. The pulley shaft  13  and the compressor shaft  19  are arranged such that the axis of the pulley shaft  13  corresponds to the axis of the compressor shaft  19 . The compressor shaft  19  are inserted into the pulley shaft  13  through a bearing  56  at the position where the rear end of the pulley shaft  13  faces the front end of the compressor shaft  19 , so as to rotate relative to the pulley shaft  13 .  
     [0019] A speed increasing mechanism  23  that is constituted of a planetary gear mechanism is arranged between the pulley shaft  13  and the compressor shaft  19  in the housing  11 . The speed increasing mechanism  23  increases the rotational speed of the pulley shaft  13  and transmits power to the compressor shaft  19 . The speed increasing mechanism  23  has a known structure that includes a sun gear  45 , an internal gear  46 , a holder  47  and a plurality of planetary gears  48 . The sun gear  45  is secured to the compressor shaft  19  so as to rotate integrally with the compressor shaft  19 . The internal gear  46  is integrally formed with the housing  11 . The holder  47  is secured to the pulley shaft  13  so as to rotate integrally with the pulley shaft  13 . The planetary gear  48  is rotatably held by the holder  47  and is interposed between the sun gear  45  and the internal gear  46 .  
     [0020] A rotor  14  and a stator constitute the electric motor  21 . The rotor  14  is mounted on the compressor shaft  19  in the housing  11  so as to rotate integrally with the compressor shaft  19 . The rotor  14  is constituted of an iron core and a conductor fixed to the iron core, which is not shown. A plurality of stator cores  16  is fixed to the inner circumference of the housing  11  so as to surround the rotor  14 . A coil  15  is formed around each of the stator cores  16 . The stator core  16  and the coil  15  constitute the stator Namely, the electric motor  21  is a reluctance motor without a permanent magnet. The coil  15  is arranged so as to surround the speed increasing mechanism  23  at its front side. Namely, the speed increasing mechanism  23  is arranged inside the stator.  
     [0021] The power transmission mechanism  22  includes a pulley  17  and an electromagnetic clutch (EC)  18 . The pulley  17  is rotatably supported by the housing  11  and transmits power from the engine E to compressor shaft  19  through the pulley shaft  13 . When the electromagnetic clutch  18  is switched on (energized), the electromagnetic clutch  18  permits power transmission from the pulley  17  to the pulley shaft  13 . When the electromagnetic clutch  18  is switched off (de-energized), the electromagnetic clutch  18  blocks the power transmission.  
     [0022] The compression unit  12  includes a fixed scroll member  20 , a crankshaft  19   a  and a movable scroll member  24 . The crankshaft  19   a  is secured to the rear end of the compressor shaft  19 . The movable scroll member  24  is supported by the crankshaft  19   a . A plurality of compression chambers  26  is defined by the fixed scroll member  20  and the movable scroll member  24 . A suction chamber  30  is defined in the housing  11  at the outer circumference of the movable scroll member  24 , and a discharge chamber  28  is defined in the housing  11  at the rear side of the housing  11 . The movable scroll member  24  orbits around an axis of the fixed scroll member  20  based on the rotation of the compressor shaft  19 . As the compression chamber  26 , which is defined between the fixed scroll member  20  and the movable scroll member  24 , is radially and inwardly moved in accordance with the orbital movement of the movable scroll member  24 , the volumes of the compression chamber  26  decreases. Therefore, the refrigerant gas in the compression chamber  26  is compressed to a predetermined pressure value. Then, the refrigerant gas in the compression chamber  26  near the center of the fixed scroll member  20  is discharged into the discharge chamber  28 .  
     [0023] A movable valve body chamber  31  is formed in the fixed scroll member  20 . Communication ports  32  and  36  are formed between the movable body chamber  31  and the compression chamber  26  in the fixed scroll member  20  so as to interconnect the compression chambers  26  with the movable valve body chamber  31 . The communication ports  32  are arranged in the fixed scroll member  20  at a position where the volume of the compression chamber  26  is, for example, 50% with respect to its maximum volume. The communication ports  36  are arranged in the fixed scroll member  20  at a position where the volume of the compression chamber  26  is, for example, 20% with respect to its maximum volume. A communication hole  33  is formed in the fixed scroll member  20  so as to interconnect the suction chamber  30  with the movable valve body chamber  31  through an intermediate chamber  33   a  and a communication passage, which is not shown.  
     [0024] A movable valve body  34  is movably accommodated in the movable valve body chamber  31 . The movable valve body  34  includes large diameter portions  34   a ,  34   b  and  34   c  and a small diameter portion  34   d . The large diameter portions  34   a  through  34   c  are arranged in a longitudinal direction of the movable valve body  34  (a vertical direction in FIG. 1). The outer diameters of the large diameter portions  34   a  through  34   c  are substantially as equal as the diameter of the inner circumference of the movable valve body chamber  31 . The small diameter portion  34   d  is arranged between the large diameter portions  34   a  and  34   b  and between the large diameter portions  34   b  and  34   c  so as to connect the large diameter portions  34   a  through  34   c . A coil spring  35  is arranged between the upper surface of the movable valve body chamber  31  and the large diameter portion  34   a  arranged at the upper end of the movable valve body  34  to press the movable valve body  34  downward.  
     [0025] A pressure control chamber  31   a  is defined between the lower surface of the movable valve body chamber  31  and the lower end of the movable valve body  34  in the movable valve body chamber  31 . The pressure control chamber  31   a  is interconnected with the discharge chamber  28  through a supply passage  37 . High-pressure refrigerant having a pressure substantially equal to a discharge pressure is supplied from the discharge chamber  28  into the pressure control chamber  31   a  through the supply passage  37 .  
     [0026] The pressure control chamber  31   a  is capable of being interconnected with the suction chamber  30  through a bleed passage  38 . A control valve (CV)  39  constituted of an electromagnetic valve is arranged on the bleed passage  38 . The control valve  39  is capable of interconnecting the pressure control chamber  31   a  with the suction chamber  30  through the bleed passage  38  and of blocking the interconnection. When the control valve  39  is switched on (energized), the control valve  39  blocks the interconnection between the pressure control chamber  31   a  and the suction chamber  30 . When the control valve  39  is switched off (de-energized), the control valve  39  interconnects the pressure control chamber  31   a  with the suction chamber  30  through the bleed passage  38 .  
     [0027] When the control valve  39  is energized, the high-pressure refrigerant in the pressure control chamber  31   a  does not flow out into the suction chamber  30  through the bleed passage  38 . Therefore, the pressure in the pressure control chamber  31   a  keeps high. The movable valve body  34  is pushed upward against the urging force of the coil spring  35  by the high pressure in the pressure control chamber  31   a  (a state as shown in FIG. 1). In this state, the large diameter portions  34   a through  34   c are located at positions corresponding to the openings of the communication ports  32  and  36  at the rear side. Therefore, the interconnection is blocked between the compression chambers  26  and the suction chamber  30  through a path including one of the communication ports  32  and  36 , the movable valve body chamber  31 , the communication hole  33  and the intermediate chamber  33   a . At the time, the compressed refrigerant gas in the compression chamber  26  does not flow out into the suction chamber  30  through the path. As a result, the amount of the discharged refrigerant by the compression unit  12  per one rotation of the compressor shaft  19 , or, the displacement of the compressor C, becomes the maximum, or a first predetermined displacement value.  
     [0028] On the other hand, when the control valve  39  is de-energized, the pressure control chamber  31   a  is interconnected with the suction chamber  30  through the bleed passage  38 . Therefore, the pressure in the pressure control chamber  31   a  becomes low. The urging force of the coil spring  35  presses the movable valve body  34  downward from the position of the movable valve body  34  shown in FIG. 1. In this state, the openings of the communication ports  32  and  36  except the communication port  32  at the lowest position are opened to the movable valve body chamber  31 . The compression chambers  26  corresponding to the communication ports  32  and  36 , whose openings are open to the movable valve body chamber  31 , are interconnected with the suction chamber  30  through the path. The refrigerant gas in the corresponding compression chambers  26  leaks into the suction chamber  30  through the path. As a result, the displacement of the compressor C becomes the minimum, or a second predetermined displacement value. Namely, the displacement of the compressor C is switched between the maximum and the minimum.  
     [0029] As a control system of the vehicle air conditioner, the vehicle air conditioner includes an air conditioner ECU (Electric Control Unit)  40  that is similar to a computer, and an air conditioning information detector  50 . The air conditioner ECU  40  includes a thermal load calculator  41  and a controller  42 . The thermal load calculator  41  is electrically connected to the controller  42 . The controller  42  is communicably connected to an engine ECU  60  that is similar to a computer and that controls the engine E.  
     [0030] The air conditioning information detector  50  includes an air conditioner switch (ACS)  51 , or an on-off switch for the vehicle air conditioner, a temperature-setting device  52  for setting a temperature in a vehicle compartment and a temperature sensor  53  for detecting the temperature in the vehicle compartment. The thermal load calculator  41  is electrically connected to the air conditioning information detector  50 . The air conditioning information detector  50  provides to the air conditioner ECU  40  information on an on-off state of the air conditioner switch  51 , a temperature Tr in the vehicle compartment and a set temperature Ts. The thermal load calculator  41  calculates a thermal load of the air conditioner based on the information from the air conditioning information detector  50 .  
     [0031] A residual quantity sensor  58  as a residual quantity detector is arranged at a battery B for the electric motor  21  and the other electric equipment. The residual quantity sensor  58  detects a residual quantity Br (quantity of accumulation of electricity) of the battery B and provides the residual quantity information Br of the battery B to the engine ECU  60 . The residual quantity Br of the battery B is provided from the engine ECU 60  to the controller  42 . The controller  42  compares the residual quantity of the battery B with a predetermined quantity. The engine ECU  60  performs an idle stop control in which the engine E is stopped while the vehicle stops running to wait for a traffic signal to change for fuel saving. The residual quantity Br of the battery B is one of parameters when the engine ECU  60  determines whether to perform the idle stop control. When the residual battery Br is lower than the predetermined quantity Bs, the idle stop control is stopped, even if other information meets requirements to continue the idle stop control. The engine ECU  60  restarts the engine E so as to recharge the battery B to the predetermined quantity Bs or more. The controller  42  controls the electromagnetic clutch  18 , the electric motor  21  and the control valve  39  based on the thermal load from the thermal load calculator  41 , the information from the air conditioning information detector  50  and the information from the engine ECU  60 .  
     [0032] The air conditioner ECU  40  (the thermal load calculator  41  and the controller  42 ) performs air conditioning control shown by a flow chart in FIG. 2 according to a pre-stored program. In a step S 101 , the on-off state of the air conditioner switch  51  is monitored until the air conditioner switch  51  is switched on. When the air conditioner switch  51  is switched on, the process proceeds to a step S 102 . In the step S 102 , the thermal load of the air conditioner is calculated, and it is judged whether the calculated thermal load is large or small. Specifically, the thermal load of the air conditioner is calculated by subtracting a set temperature Ts provided by the temperature sensor  53  from a detected temperature Tr provided by the temperature-setting device  52 . In the above judgment, it is judged whether the remainder from the subtraction, or the thermal load is larger than a predetermined value α. Namely, the controller  42  compares the thermal load with the predetermined value α (&gt;0). Hereafter, the controller  42  performs the air conditioning control.  
     [0033] In the present embodiment, if the judgment is YES in the step S 102 , that is, if the thermal load is larger than the predetermined value α, the controller  42  always selects the engine E as a drive source of the compressor C, and air conditioning is conducted. Meanwhile, if the judgment is NO in the step S 102 , that is, if the thermal load is equal to, or smaller than the predetermined value α, under the condition that the engine E is in the stopped state, the controller  42  selects the electric motor  21  as the drive source of the compressor C, and the air conditioning is conducted. Also, if the judgment is NO in the step S 102 , under the condition that the engine E is in a running state and the residual quantity Br of the battery B is equal to, or larger than the predetermined quantity Bs, the controller  42  selects the electric motor  21  as the drive source of the compressor C, and the air conditioning is conducted. In short, in the present first preferred embodiment, the compressor C is driven by the electric motor  21  only when the thermal load is equal to, or smaller than the predetermined value α.  
     [0034] Namely, if the judgment is YES in the step S 102 , the process proceeds to a step S 103  where it is judged whether the engine E is in the running state based on information on a condition of the engine E (e.g. a rotational speed of the engine E and a running speed of the vehicle) that is provided by the engine ECU  60 . For example, when the vehicle is in an idle stop state, the judgment is NO in the step S 103 . In this state, the engine E cannot drive the compressor C. Therefore, the process proceeds to a step S 104  where the controller  42  instructs the engine ECU  60  to start the engine E. The engine ECU  60  starts the engine E according to the instruction from the controller  42 , and the vehicle is released from the idle stop state.  
     [0035] When the judgment is YES in the step S 103 , or when the process finishes in the step S 104 , the process proceeds to a step S 105 . In the step S 105 , the control valve  39  is energized, and the displacement of the compressor C becomes the maximum. In a step S 106 , the electromagnetic clutch  18  is energized, and power is transmitted from the engine E to the compressor C. Also, the speed increasing mechanism  23  is arranged in the compressor C. Accordingly, if the displacement of the compressor C is the maximum, enough displacement of the compressor C per a predetermined period is ensured even by the engine E in an idling state. The detected temperature Tr is decreased toward the set temperature Ts. When the detected temperature Tr approaches the set temperature Ts to some extents, the judgment is NO in the step S 102  next time. Namely, when the thermal load becomes equal to, or smaller than the predetermined value α, the judgment is No in the step S 102 .  
     [0036] If the judgment is NO in the step S 102 , it is judged whether the engine E is in the running state in a step S 107 . If the judgment is YES in the step S 107 , that is, if the engine E is in the running state, it is judged whether the residual quantity Br of the battery B that is provided by the engine ECU  60  is equal to, or larger than the predetermined quantity Bs in a step S 108 .  
     [0037] If the judgment is NO in the step S 108 , that is, if the residual quantity Br of the battery B is smaller than the predetermined quantity Bs, the running state of the engine E is probably the state that is released from the idle stop state only for recharging the battery B. At the time, the engine E is selected as the drive source of the compressor C.  
     [0038] Namely, the control valve  39  is de-energized in a step S 109 , and the displacement of the compressor C becomes the minimum. In a step S 110 , it is judged whether the detected temperature Tr from the temperature sensor  53  is larger than the set temperature Ts from the temperature-setting device  52 . In a step S 111 , it is judged whether the detected temperature Tr is smaller than the set temperature Ts. If the judgments are NO both in the steps S 110  and S 111 , the detected temperature Tr is equal to the set temperature Ts. Therefore, the state of the compressor C (run or stop), which affects the temperature in the vehicle compartment, or the state of the electromagnetic clutch  18  (on or off) is not changed.  
     [0039] On the other hand, if the judgment is YES in the step S 110 , the controller  42  instructs the electromagnetic clutch  18  to be energized so as to start the compressor C in a step S 112 . Since the compressor C is started, the temperature Tr in the vehicle compartment is decreased.  
     [0040] If the judgment is YES in the step S 111 , the controller  42  instructs the electromagnetic clutch  18  to be de-energized so as to stop the compressor C in a step S 113 . Since the compressor C is stopped, the temperature Tr in the vehicle compartment is increased.  
     [0041] As mentioned above, in the step S 112  and/or the step S 113 , the on-off control of the electromagnetic clutch  18  is performed. Even when the detected temperature Tr is different from the set temperature Ts, the temperature Tr in the vehicle compartment converges around the set temperature Ts by the on-off control soon.  
     [0042] If the judgment is NO in the step S 107 , that is, if the engine E is in the stopped state, the electric motor  21  is selected as the drive source of the compressor C. Or if the judgment is YES in the step S 108 , that is, if the engine E is in the running state and the residual quantity Br of the battery B is equal to, or larger than the predetermined quantity Bs, the electric motor  21  is selected as the drive source of the compressor C. Namely, the control valve  39  is de-energized in a step S 114 . Therefore, the displacement of the compressor C becomes the minimum. The electromagnetic clutch  18  is de-energized in a step S 115 , and the controller  42  instructs the electric motor  21  to start in a step S 116 . When the displacement of the compressor C becomes the minimum, driving torque for driving the compressor C becomes the minimum.  
     [0043] In a step S 117 , it is judged whether the detected temperature Tr from the temperature sensor  53  is larger than the set temperature Ts from the temperature-setting device  52 . In a step S 118 , it is judged whether the detected temperature Tr is smaller than the set temperature Ts. If the judgments are NO both in the steps S 117  and S 118 , the detected temperature Tr is equal to the set temperature Ts. Namely, the displacement of the compressor C is at a suitable value per the predetermined period. Therefore, the rotational speed of the electric motor  21  (RSEM) is not changed for changing the displacement of the compressor C. Then, the process according to the flow chart in FIG. 2 returns.  
     [0044] On the other hand, if the judgment is YES in the step S 117 , air-conditioning by the air conditioner is not enough, that is, the displacement of the compressor C is not large enough. Therefore, in a step S 119 , the controller  42  instructs the electric motor  21  to increase the rotational speed of the electric motor  21  by a predetermined value. The rotational speed of the compressor C increases, and the displacement of the compressor C per the predetermined period increases. As the displacement of the compressor C per the predetermined period increases, the temperature in the vehicle compartment decreases.  
     [0045] If the judgment is YES in the step S 118 , the air-conditioning by the air conditioner is excessively cooled, that is, the displacement of the compressor C is too large. Therefore, in a step S 120 , the controller  42  instructs the electric motor  21  to decrease the rotational speed of the electric motor  21  by a predetermined value. The rotational speed of the compressor C decreases, and the displacement of the compressor C per the predetermined period decreases. As the displacement of the compressor C per the predetermined period decreases, the temperature in the vehicle compartment increases.  
     [0046] As mentioned above, in the step S 119  and/or in the step S 120 , the rotational speed of the electric motor  21  is changed. Namely, when the electric motor  21  drives the compressor C, the rotational speed of the electric motor  21  is controlled in accordance with the thermal load. Even when the detected temperature Tr is different from the set temperature Ts, the rotational speed of the electric motor  21 , or the displacement of the compressor C per the predetermined period gradually has been controlled to be close to the desired value according to the above control for the electric motor  21 . As a result, the detected temperature Tr converges into the set temperature Ts.  
     [0047] As mentioned above, in the present embodiment, when the judgment is NO in the step S 103 , the controller  42  instructs the engine ECU  60  to start the engine E in the step S 104 . The engine ECU  60  starts the engine E, and the vehicle is released from the idle stop state. In this case, when the judgment is NO in the step S 102  later, the controller  42  dose not instruct the engine ECU  60  to run the engine E. Therefore, the engine ECU  60  stops the engine E if the other requirements for the idle stop are met.  
     [0048] Following effects are obtained in the present embodiment.  
     [0049] (1-1) When the calculated thermal load is larger than the predetermined value α, the controller  42  changes the displacement of the compressor C per one rotation to the maximum and selects the engine E as the drive source of the compressor C. Even when the rotational speed of the compressor C is low, a relatively large amount of discharge refrigerant by the compressor C per the predetermined period can be ensured. Enough cooling performance can be ensured, and the air conditioning is steadily performed. Therefore, as mentioned in the present first preferred embodiment, even when the speed increasing mechanism  23  that increases the rotational speed obtained from the engine E is arranged, the speed increasing ratio of the speed increasing mechanism  23  can be set at a relatively small value. The speed increasing mechanism  23  can be miniaturized. Also, a transmission mechanism that transmits the power from the engine E can be miniaturized.  
     [0050] (1-2) When the thermal load calculated by the thermal load calculator  41  is equal to, or smaller than the predetermined value α, the controller  42  changes the displacement of the compressor C per one rotation to the minimum and selects the electric motor  21  as the drive source of the compressor C. The driving torque for driving compressor C with the minimum displacement is small. Therefore, the electric motor  21  can be miniaturized without a speed reducing mechanism that decreases the rotational speed obtained from the electric motor  23 . When the thermal load is equal to, or smaller than the predetermined value α, the rotational speed of the electric motor  21  does not increase relatively. As a result, the electric motor  21  can run in a stable manner.  
     [0051] (1-3) When the thermal load calculated by the thermal load calculator  41  is equal to, or smaller than the predetermined value α and when the engine E is in the running state, the controller  42  selects the engine E as the drive source of the compressor C. Therefore, for example, the residual quantity Br of the battery B for the other electric equipment (e.g. a headlamp) as well as the electric motor  21  is not decreased at a relatively large degree. The operation of the electric equipment other than the electric motor  21  can be ensured.  
     [0052] (1-4) When the thermal load calculated by the thermal load calculator  41  is equal to, or smaller than the predetermined value α and when the engine E is in the running state, the controller  42  selects the engine E as the drive source of the compressor C and changes the displacement of the compressor C to the minimum. The electromagnetic clutch  18  is not frequently switched on and off due to the minimum displacement of the compressor C. Therefore, deterioration of drivability of the vehicle due to the shock caused by the on-off control action of the electromagnetic clutch  18  can be suppressed.  
     [0053] (1-5) When the thermal load calculated by the thermal load calculator  41  is equal to, or smaller than the predetermined value α, when the engine E is in the running state, and when the residual quantity Br of the Battery B is smaller than the predetermined quantity Bs, the controller  42  selects the engine E as the drive source of the compressor C. Therefore, since the electric motor  21  is in the stopped state, the engine E is not prevented from recharging the battery B due to the consumption of the power of the battery B by the electric motor  21 . The residual quantity Br of the battery B can be steadily increased. Therefore, the operation of the electric equipment other than the electric motor  21  can be ensured further.  
     [0054] (1-6) The compressor C includes the speed increasing mechanism  23  that increases the rotational speed from the power transmission mechanism  22  and transmits the power to the compression unit  12 . Therefore, the electric motor  21  can be miniaturized further. The speed increasing mechanism  23  is arranged in the housing  11  of the compressor C. As mentioned above, the speed increasing mechanism  23  may be small-sized. Therefore, even though the speed increasing mechanism  23  is accommodated in the housing  11 , the compressor C is not large-sized. Since the speed increasing mechanism  23  is accommodated in the housing  11 , the compressor C and the speed increasing mechanism  23  can be handled together. Therefore, when the vehicle air conditioner is installed in the vehicle, the compressor C is easy. to handle with the speed increasing mechanism  23 .  
     [0055] (1-7) The electric motor  21  is arranged in the housing  11  of the compressor C. As mentioned above, the electric motor  21  may be small-sized. Therefore, even though the electric motor  21  is accommodated in the housing  11 , the compressor C is not large-sized. Since the electric motor  21  is accommodated in the housing  11 , the compressor C and the electric motor  21  can be handled together. Therefore, when the vehicle air conditioner is installed in the vehicle, the compressor C is easy to handle with the electric motor  21 .  
     [0056] The speed increasing mechanism  23  is arranged inside the stator of the electric motor  21 . Therefore, space in the housing  11  is saved by a part where the speed increasing mechanism  23  overlaps with the stator in an axial direction, and the compressor C does not need to be lengthened by the length of the part in the axial direction. Such an arrangement prevents the compressor C from increasing its size in the axial direction. Namely, since the speed increasing mechanism  23  may be small-sized as mentioned above, the speed increasing mechanism  23  is easily arranged inside the stator in the present preferred embodiment.  
     [0057] (1-8) The compressor C is a scroll type, and the displacement of the compressor C is switched between the maximum and the minimum. For example, the energy efficiency of a scroll type compressor is better than that of a piston type compressor. Also, the displacement of the compressor C is switched between the maximum and the minimum by simple control, or on-off control of the control valve  39 .  
     [0058] Furthermore, the electric motor  21  does not include a permanent magnet. When the engine E drives the compressor C, the rotor  14  of the electric motor  21  is rotated. Electromotive force is not generated in the electric motor  21  even when the rotor  14  of the electric motor  21  is rotated. Therefore, power loss of the engine E, which is caused by the generation of the electromotive force, is avoided.  
     [0059] Next, the second preferred embodiment will be described. In the second preferred embodiment, the only difference between the first and second preferred embodiments will be described. The same reference numerals denote substantially identical elements as those in the first preferred embodiment.  
     [0060] As shown in FIG. 3, in the second preferred embodiment, the speed increasing mechanism  23  is deleted form the above-mentioned first preferred embodiment. The pulley shaft  13  and the compressor shaft  19  (hereafter a shaft  13 ) are integrally formed so as to rotate integrally and so as to have the same axis. The rotor  14  is supported by the shaft  13  through a bearing  57  so as to rotate relative to the shaft  13 . A speed reducing mechanism  65  is arranged between the rotor  14  of the electric motor  21  and the shaft  13 . The speed reducing mechanism  65  decreases the rotational speed obtained from the electric motor  21  and transmits the power to the shaft  13 .  
     [0061] The speed reducing mechanism  65  has a known structure that includes a sun gear  66 , an internal gear  67 , a holder  68  and a plurality of planetary gears  69 . The sun gear  66  can rotate integrally with the rotor  14  and can rotate relative to the shaft  13 . The internal gear  67  is integrally formed with the housing  11 . The holder  68  is mounted on the shaft  13  so as to rotate integrally with the shaft  13 . The planetary gear  69  is rotatably held by the holder  68  and is interposed between the sun gear  66  and the internal gear  67 .  
     [0062] The compressor C includes the speed reducing mechanism  65 . Therefore, even if the minimum displacement of the compressor C is set at a value that is larger than the above-mentioned first preferred embodiment, the electric motor  21  does not need to be large-sized. The ratio between the minimum and maximum displacements of the compressor C is limited. Since the minimum displacement of the compressor C can be set at the value that is larger than the above-mentioned first preferred embodiment, the maximum displacement of the compressor can be also set at a relatively larger value. Therefore, even when the rotational speed of the engine E is low, a relatively large amount of the discharged refrigerant by the compressor C per the predetermined period, which is substantially as large as the above-mentioned first preferred embodiment, can be ensured without the speed increasing mechanism  23 .  
     [0063] Following effects are obtained in the present embodiment.  
     [0064] (2-1) When the thermal load is larger than the predetermined value α, the controller  42  changes the displacement of the compressor C per one rotation to the maximum and selects the engine E as the drive source of the compressor C. Therefore, even when the rotational speed of the engine E is low, the relatively large amount of the discharged refrigerant by the compressor C per the predetermined period can be ensured. A speed increasing mechanism that increases the rotational speed obtained from the engine E is unnecessary.  
     [0065] (2-2) When the thermal load is equal to, or smaller than the predetermined value α, the controller  42  changes the displacement of the compressor C to the minimum and selects the electric motor  21  as the drive source of the compressor C. The driving torque for driving compressor C with the minimum displacement is small. Therefore, even though the speed reducing mechanism  65 , which decreases the rotational speed obtained from the electric motor  21 , is arranged, the speed reducing ratio may be relatively small. The speed reducing mechanism  65  can be miniaturized, and also a transmission mechanism that transmits the power from the electric motor  21  can be miniaturized. The miniaturization of the speed reducing mechanism  65  and the transmission mechanism can be compatible with the miniaturization of the electric motor  21 . Furthermore, when the thermal load is equal to, or smaller than the predetermined value α, the rotational speed of the electric motor  21  is not increased relatively. Therefore, the electric motor  21  runs in the stable manner. The same advantageous effects are obtained as mentioned in paragraphs (1-3) through (1-8) according to the first preferred embodiment.  
     [0066] Following alternative embodiments may be practiced.  
     [0067] The speed increasing mechanism  23  may be deleted in the above-mentioned first preferred embodiment. The speed reducing mechanism  65  may be deleted in the above-mentioned second preferred embodiment.  
     [0068] In each preferred embodiment, the compression unit  12  has a structure for switching between the two displacement values including the maximum and the minimum. The compression unit  12  may have a structure for switching among three, four, five, six, or seven displacement values. In this case, the maximum displacement value may be considered the first predetermined displacement value, and the others may be considered the second predetermined displacement value. Also, the minimum displacement value may be considered the second predetermined displacement value, and the others may be considered the first predetermined displacement value. Further, the displacements values that are equal to, or larger than a certain value may be considered the first predetermined displacement value, and the other displacement values that are smaller than the certain value may be considered the second predetermined displacement value.  
     [0069] In the above embodiment, it is judged whether the thermal load of the air conditioner is large by comparing the detected temperature Tr with the set temperature Ts. It may be judged by referring to a refrigerant pressure (a suction pressure) at an outlet side (a suction chamber side) of an evaporator that constitutes the refrigerant cycle. It may also be judged by referring to the temperature at the outlet of the evaporator.  
     [0070] In the above embodiment, when the electric motor  21  drives the compressor C, the electric motor  21  is started after the electromagnetic clutch  18  is de-energized. The electromagnetic clutch  18  may be de-energized after the electric motor  21  is started.  
     [0071] The electric motor  21  may be a SR motor (a switched reluctance motor), or a VR motor (a variable reluctance motor). The electric motor  21  may be also an induction motor without a permanent magnet. Since starting torque of a reluctance motor is larger than that of an inductance motor, the reluctance motor is more advantageous than the inductance motor for ensuring starting torque.  
     [0072] The electric motor  21  may include a permanent magnet. The scroll type compression unit  12  is utilized in the above preferred embodiment. A piston type variable displacement compression unit may be utilized. A vehicle that does not perform an idle stop control may be utilized as the vehicle.  
     [0073] 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.