BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a control unit for two variable displacement compressors used for two independent air-conditioning systems. 
   2. Description of the Related Art 
   In the prior art, a variable displacement compressor of a regularly operative type for an air-conditioner of a vehicle, which is regularly operative by a power from an engine provided the engine is driven, is known from Japanese Unexamined Patent Publication (Kokai) No. 2000-220577. This compressor is capable of optionally changing the operating displacement in a range of approximately 0% to 100% by electric signals from an electronic control unit (ECU) in accordance with various operating conditions. When the air conditioning in a passenger compartment is necessary, the compressor is switched to an ON mode in which a coolant is compressed by the compressor based on the signal from ECU and discharged to an air-conditioning cycle. If the air conditioning in the passenger compartment is unnecessary, the compressor is switched to an OFF mode, which is a minimum displacement operation, and no coolant is discharged to the air conditioning cycle. 
   While sliding portions in the interior of the compressor usually become hot due to self-heating during the operation, in the case of the ON mode, the sliding portions are cooled because coolant at a low temperature is sucked from the air conditioning cycle into the compressor. On the other hand, in the case of the OFF mode, in which no air conditioning is carried out, as no coolant is sucked from the air conditioning cycle into the compressor, the interior sliding portions of the compressor are not cooled by the coolant. When the air is at a relatively low temperature, the compressor is usually driven in the OFF mode as the passenger requires no air conditioning. However, the interior temperature of the compressor does not rise to a dangerous value because the outer air temperature is low. Conversely, in an environment with a high air temperature, the compressor is driven in the ON mode because the air conditioning is usually necessary, and the compressor temperature does not rise to an extraordinary high value. 
   However, in a case of a large-sized vehicle such as a limousine having two air conditioning systems using two compressors, completely independent from each other, in which one compressor is used for the front seats and another compressor is used for the rear seats, the compressor for the rear seats may be operated in the OFF mode when the air temperature is high but there are no passengers in the rear seats. When the compressor is driven in the OFF mode at such a high air temperature, no cooling effect is obtainable from a returning coolant from the air conditioning cycle. Thus, the temperature of the compressor reaches a dangerous temperature due to self-heating to result in the seizing of the compressor in the worst case. 
   SUMMARY OF THE INVENTION 
   The present invention has been made to solve the above-mentioned problems in the prior art, and an object thereof is to provide a control unit for two variable displacement compressors directly coupled to a common external drive source and used for two completely independent air conditioning systems, which control unit is capable of preventing the compressor temperature from rising to a dangerous value by the self-heating due to the OFF mode operation of the compressor and from a resulting seizing of the compressor when the air temperature is at a predetermined value or higher. 
   According to the present invention, in an air-conditioner having at least two variable displacement compressors directly coupled to a common external drive source and used for at least two independent air conditioning cycles, provision is mode of a control unit for the variable displacement compressors which comprises detection means for detecting the air conditioning state of one of the two variable displacement compressors wherein, when the detection means detects a predetermined condition, both of the compressors are subjected to the variable displacement operation even though the only air conditioning operation of the other variable displacement compressor is necessary. 
   Thereby, irrespective of whether or not the air conditioning operation is necessary, when one of the air conditioning systems is operated, the other system is also operated so that seizing of the compressor due to self-heating of the compressor is avoided due to the cooling effect of the returning coolant. 
   In the present invention, the detection means is preferably means for detecting the air temperature, and the predetermined condition is that the air temperature is higher than a predetermined value. 
   The present invention may be more fully understood from the description of the preferred embodiments of the invention, as set forth below, together with the accompanying drawings. 


   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
       FIG. 1  is a sectional view of a swash plate type variable displacement compressor; 
       FIG. 2  illustrates an entire structure of an air conditioning system having two independent air conditioning cycles using two variable displacement compressors; and 
       FIG. 3  is a flow chart for controlling the control unit for the variable displacement compressor according to one embodiment of the present invention. 
   


   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The control unit for the variable displacement compressor according to the embodiment of the present invention will be described below with reference to the attached drawings.  FIG. 1  is a sectional view of a variable displacement compressor of a regularly operative type. The variable displacement compressor  100  of this type is adapted to be regularly operative by a power from an external drive source  8  provided the external drive source  8  such as an engine is in an ON mode. 
   As shown in  FIG. 1 , a front housing  110  is coupled to a front end of a cylinder block  111 , and a rear housing  113  is fixedly coupled to a rear end of the cylinder block  111  via a plate member  115  such as a valve plate or a valve-forming plate. A rotary shaft  104  is supported for rotation by the front housing  110  and the cylinder block  111  defining a crank chamber  107 . A shock absorber  102  having a pulley  101  and a hub  103  is fastened by bolts or the like to the rotary shaft  104  projected outward from the crank chamber  107  through the shock absorber  102 . Power is transmitted from the external drive source such as a vehicle engine to the pulley  101  via a belt (not shown) or the like and further to the rotary shaft  104 . 
   A lug plate  105  is made integral with the rotary shaft  104  by, for example, press-fitting or others, and a swash plate  108  is supported by the rotary shaft  104  to be slidable in the axial direction thereof and tiltable thereto. A connecting piece  108   a  is fixed to the swash plate  108  and a guide pin  106  is integral with the connecting piece  108   a  by a press-fit or others. A guide hole  105   a  is formed in the lug plate  105 , and a head of the guide pin  106  is inserted into the guide hole  105   a  in a slidable manner. The swash plate  108  is tiltable in the axial direction of the rotary shaft  104  in association with the guide hole  105   a  and the guide pin  106  and is rotatable together with the rotary shaft  104 . 
   When a center portion of the swash plate  108  moves toward the cylinder block  111 , the inclination angle of the swash plate  108  increases. The maximum inclination angle of the swash plate  108  is limited by the contact of the lug plate  105  with the swash plate  108 . The minimum inclination angle of the swash plate  108  is limited by the contact of the swash plate  108  with a circlip  116  provided on the rotary shaft  104 . 
   In a plurality of cylinder bores  111   a  formed in the cylinder block  111 , pistons  112  are accommodated. The rotary motion of the swash plate  108  is converted to the forward and rearward reciprocation of the pistons  112 , whereby the respective piston  112  is slidable along the cylinder bore  111   a  forward and rearward. Accordingly, there are main sliding portions of this compressor  100  between the swash plate  108  and the shoe  109  and between the piston  112  and the bore  111   a.    
   In the rear housing  113 , a suction chamber  117  and a discharge chamber  118  are defined. In the plate member  115  interposed between the cylinder block  111  and the rear housing  118 , such as a valve sheet or a valve-forming plate, a suction valve and a discharge valve are formed. Accordingly, a gasous coolant in the suction chamber  117  pushes back the suction valve due to the returning motion of the piston  112  and flows into the cylinder bore  111   a . The gasous coolant thus flowing into the cylinder bore  111   a  pushes back the discharge valve due to the advancing motion of the piston to be discharged into the discharge chamber  118 . 
   In a pressure supplying path connecting the discharge chamber  118  to the crank chamber  107 , an electromagnetic type displacement control valve  114  is provided. This pressure supplying path is a path for supplying a coolant in the discharge chamber  118  which is a discharging pressure area to the crank chamber  107 . On a bellows  114   a  within the displacement control valve  114  which is a pressure-sensitive means, the pressure in the suction chamber  117  (a suction pressure) is applied. The suction pressure within the suction chamber  117  is influenced by a heat load. A valve body  114   b  is connected to the bellows  114   a  and opens or closes a valve hole  114   c . A spring force of a spring in the bellows  114   a  acts on the valve body  114   b  in a direction to open the valve hole  114   c . An electromagnetic drive force of a solenoid  114   d  in the displacement control valve  114  biases the valve body  114   b  to close the valve hole  114   c  against the spring force. The electric current supplied to the solenoid  114   d  is controlled by an electronic control unit (ECU)  7  as shown in FIG.  2 . 
   ECU  7  supplies the electric current to the solenoid  114   d  when a switch for operating an air conditioning system is in an ON state, and stops the electric current when the switch is in an OFF state. An electric signal from ECU  20 , which becomes a control current for the solenoid  114   d , is determined by processing, in the ECU  7 , an air conditioning environment such as a passenger compartment temperature, a solar radiation or an outer air temperature; a condition for operating an air conditioner such as an operating switch, an air conditioner operative mode or a set temperature; and a vehicle environment such as an engine rotational speed, or an opening degree of accelerator. An opening degree of the displacement control valve  114  is determined by a balance between an electromagnetic drive force generated from the solenoid  114   d , a spring force and a bias of the bellows. Accordingly, the displacement control valve  114  carries out the control for generating a suction pressure in correspondence to the current value supplied to the solenoid  114   d.    
   As the current value supplied to the solenoid  114   d  becomes higher, the opening degree of the displacement control valve  114  becomes smaller to reduce an amount of coolant supplied from the discharge chamber  118  to the crank chamber  107 . As the coolant in the crank chamber  107  flows to the suction chamber  117  via a pressure-release path, the interior pressure in the crank chamber  107  is lowered. Accordingly, an inclination angle of a swash plate  108  becomes larger to increase a discharged amount of the coolant. The increase of the discharging amount results in the lowering of the suction pressure. When the current value supplied to the solenoid  114   d  becomes lower, the opening degree of the displacement control valve  114  becomes larger to increase the amount of the coolant supplied from the discharge chamber  118  to the crank chamber  107 . Accordingly, the interior pressure of the crank chamber  107  rises to decrease the inclination angle of the swash plate  108  and reduce the discharge amount of the coolant. The reduction of the discharge amount results in the increase of the suction pressure. 
   If the current value supplied to the solenoid  114   d  becomes zero, that is, when the compressor  100  is operated in an OFF mode, the opening degree of the displacement control valve  114  is maximum and the inclination angle of the swash plate is minimum. When the inclination angle of the swash plate  108  becomes minimum, the coolant suction path is closed to interrupt the circulation of the coolant through an external coolant circuit, whereby the cooling of the passenger compartment is not carried out. When the current is supplied again to the solenoid  114   d , the opening degree of the valve becomes smaller to lower the pressure in the crank chamber  107 , and the inclination angle of the swash plate  108  increases from the minimum value. As the inclination angle of the swash plate  108  increases from the minimum value, the suction path is opened and the coolant flows from the suction path to the suction chamber  117 , whereby the circulation of the coolant through the external coolant circuit is started again to carry out the cooling of the passenger compartment. 
     FIG. 2  illustrates an entire structure of two completely independent air conditioning systems using two variable displacement compressors. These first and second compressors  1  and  11  are connected via belts or others (not shown) to the external drive source  8  such as an engine from which power is transmitted. 
   The first air conditioning system forms an air conditioning cycle for circulating a hot and high pressure gaseous coolant discharged from the first compressor  1  through an external coolant circuit sequentially consisting of a first condenser  2 , a first receiver  3 , a first expansion valve  4  and first evaporator  5 , and returning to the first compressor  1 . 
   The second air conditioning system forms an air conditioning cycle for circulating a hot and high pressure gaseous coolant discharged from the second compressor  11  through an external coolant circuit sequentially consisting of a second condenser  12 , a second receiver  13 , a second expansion valve  14  and second evaporator  15 , and returning to the second compressor  11 . As the function of the coolant is well-known in the air conditioning cycle, the explanation thereof will be eliminated. 
   The first electromagnetic displacement control valve  6  in the first compressor  1  and the second electromagnetic displacement control valve  16  in the second compressor  11  are respectively controlled by electric signals  9 ,  19  from the electronic control unit (ECU)  7 . The air conditioning environment such as the air temperature, the condition for operating (setting) the air conditioner such as an operating switch or a set temperature and the vehicle environment such as an engine rotational speed are input into ECU  7  and processed therein to output electric signals  9 ,  19  to the displacement control valves  6 ,  16 , respectively. 
   When the air conditioning system is operated in the ON mode by using the above-structured variable displacement compressors  1 ,  11  and  100 ; that is, when the variable displacement control valves  6 ,  16  and  114  are driven by ECU  7  to increase the inclination angle of the swash plate  108  in the respective compressor from the minimum value so that the coolant circulates the external coolant circuit, sliding portions in the compressor, for example, between the swash plate  108  and a shoe  109  or between a piston  112  and a cylinder bore  111   a  are heated by the sliding motion. However, this heat generation is cooled by the coolant returning from the air conditioning cycle. On the other hand, in the OFF mode (the minimum displacement operation) of the compressors  1 ,  11  and  100  in which no air conditioning operation is necessary, that is, when the displacement control valves  6 ,  16 ,  114  are not driven and the inclination angle of the swash plate  108  is a minimum to interrupt the circulation of the coolant through the external coolant circuit, the effect for cooling the heated sliding portions is not obtainable by the coolant returning from the air conditioning cycle. However, as the outer air temperature is low in an environment requiring no air conditioning, the compressor does not reach a dangerous zone in which the sliding portions are seized by self-heating. 
   However, as shown in  FIG. 2 , in a case of a large-sized vehicle having two independent air conditioning cycles, a first air conditioning system for the front seats and a second air conditioning system for the rear seats may be mounted to the vehicle. When the air temperature is relatively high and there is no passenger other than a driver in the vehicle, the first air conditioning system must be operated as the driver needs the air conditioning, whereby the first compressor  1  is operated in the ON mode. That is, the electric signal  9  is sent from ECU  7  to the first electromagnetic type displacement control valve  6  and the first compressor  1  carries out the variable displacement operation to circulate the coolant through the external coolant circuit thereof. Therefore, while the sliding portions of the first compressor  1  are cooled by the returning coolant in the first air conditioning system, the second air-conditioning system for the rear seats is not operative as there are no passengers and the second compressor  11  is operated in the OFF mode. That is, no electric signal  19  is sent from ECU  7  to the second electromagnetic type displacement control valve  16  and the second compressor  11  carries out the minimum displacement operation, in which no coolant is supplied to the external coolant circuit thereof. Therefore, even though the air temperature is high, the second compressor  11  is not cooled by the coolant returning from the air conditioning cycle and is forced to be driven at a high temperature due to self-heating, whereby the sliding portions of the second compressor  11  may reach to a high temperature state and may be in an oil-film broken state to result in seizing. 
   To solve such a problem, the control unit for the variable displacement compressor according to one embodiment of the present invention controls the operation of the air conditioning system in accordance with a control flow shown in FIG.  3 . For example, if a driver is a sole passenger of the vehicle, to operate the first air conditioning system for the front seats, the first electromagnetic type displacement control valve  6  is initially driven by the electric signal  9  from ECU  7  to drive the first compressor  1  in the ON mode at step S 1 . Then, at step S 2 , it is determined by ECU  7  whether or not the air temperature is higher than a predetermined value T 0 . If the answer is affirmative, the routine proceeds to step S 3  at which the electric signal  19  is sent from ECU  7  to the second electromagnetic displacement control valve  16  which is driven thereby to operate the second compressor  11  in the ON mode, whereby the second air conditioning system for the rear seats is operated. Contrarily, if the answer is negative, the routine proceeds to step S 4 , at which the second air conditioning system for the rear seat is not operated and the second compressor  11  is maintained in the OFF mode. 
   In such a manner, according to the present invention, as a control system in which, when the outer air temperature is higher than the predetermined temperature T 0  and the driver uses the air conditioning system, the second air conditioning system for the rear seats is operated by issuing the electric signal  19  from ECU  7  irrespective of whether or not there are passengers in the rear seats so that the second compressor  11  is operated in the ON mode, is adopted, it is possible to avoid the seizing of the sliding portions of the compressor and maintain the reliability thereof. 
   While the present invention has been described by reference to specific embodiments chosen for the purposes of illustration, it should be apparent that numerous modifications could be made thereto by those skilled in the art without departing from the basic concept and scope of the invention.
A control unit for variable displacement compressors is used for two independent air conditioning cycles, wherein two compressors 1, 11 driven by a common external drive source 8 include electromagnetic displacement control valves 6, 16, respectively. By controlling these control valves by ECU 7, the displacement of the compressor is variable. When the air temperature is higher than a predetermined temperature T 0, both of the compressors are subjected to the variable displacement operation even though one of them is unnecessary for the air conditioning operation. On the contrary, if the outer air temperature is lower than the predetermined temperature, only one compressor, needed for the air conditioning operation, is subjected to the variable displacement operation, while the other compressor is subjected to the minimum displacement operation.