Patent Publication Number: US-8117858-B2

Title: Air conditioner

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is the National Stage of International Patent Application No. PCT/JP2007/057374, filed Apr. 2, 2007, which claims the benefit of Japanese Patent Application No. 2006-105754, filed Apr. 6, 2006, the disclosures of which are incorporated herein by reference in their entirety. 
     TECHNICAL FIELD 
     The present invention relates to an air conditioner capable of heating mode operation using high pressure hot gas in a refrigerant cycle. 
     BACKGROUND ART 
     Patent document 1 teaches a car air conditioner capable of auxiliary heating mode operation for supporting the heating capability of a water heater, wherein high pressure hot gas in a refrigerant cycle is led to an evaporator to heat air flowing through an air duct. ON/OFF operation of a compressor of the aforementioned air conditioner is controlled based on the detection signal of a pressure sensor for detecting pressure of the high pressure refrigerant in the refrigerant cycle. 
     Patent document 1: Japanese Patent Laid-Open Publication No. 5-223357 
     DISCLOSURE OF INVENTION 
     Problem to be Solved 
     [Object of the Invention] An object of the present invention is to provide an air conditioner whose operation mode is switchable between cooling mode and heating mode using highly pressurized hot gas in refrigerant cycle, wherein both a cooling mode operation for variably controlling the displacement of the variable displacement compressor, thereby controlling car interior cooling temperature to a predetermined level, and a heating mode operation for variably controlling the displacement of the variable displacement compressor, thereby controlling car interior heating temperature to a predetermined level, can be implemented. 
     Nowadays many cars have come to be equipped with air conditioners which comprise a variable displacement compressor provided with a control valve having a valve body, a pressure sensitive mechanism for sensing the lower pressure side pressure of a refrigerating cycle acting to force the valve body, and a solenoid for forcing the valve body based on an input electric current, wherein position of the control valve is controlled to vary the internal pressure of a control chamber, thereby variably controlling the displacement. In the air conditioner, the lower pressure side pressure of the refrigerant cycle is detected by the pressure sensitive mechanism of the variable displacement compressor, and the displacement of the variable displacement compressor is controlled to self-control the lower pressure side pressure of the refrigerant cycle to a predetermined level, thereby controlling the temperature of a car interior to a predetermined cooling level. Heating mode operation of a car air conditioner provided with a variable displacement compressor is possible by using the high pressure hot gas of the refrigerant cycle. However, a variable displacement compressor provided on a traditional car air conditioner is structured to variably control the displacement thereof to self-control the lower pressure side pressure of the refrigerant cycle to a predetermined level. Therefore, the traditional air conditioner cannot carry out heating mode operation in which the displacement of the variable displacement compressor is variably controlled to self-control the higher pressure side pressure of the refrigerant cycle to a predetermined level, thereby controlling a car interior temperature to a predetermined heating level. 
     An object of the present invention is to provide an air conditioner comprising a variable displacement compressor and a controller, wherein the variable displacement compressor comprises a control valve provided with a valve body, a pressure sensitive mechanism for sensing the lower pressure side pressure of a refrigerating cycle acting to force the valve body, and a solenoid for forcing the valve body based on an input electric current, position of the control valve is controlled to vary the internal pressure of a control chamber, thereby variably controlling displacement of the variable displacement compressor, and the controller controls the input electric current to the solenoid to control the position of the control valve, and wherein the operation mode of the air conditioner is switchable between cooling mode and heating mode using high pressure hot gas in the refrigerant cycle, and wherein the air conditioner can carry out a cooling mode operation for variably controlling the displacement of the variable displacement compressor to control a car interior temperature to a predetermined cooling level and a heating mode operation for variably controlling the displacement of the variable displacement compressor to control the car interior temperature to a predetermined heating level. 
     Means for Solving the Problem 
     In accordance with the present invention, there is provided an air conditioner comprising a variable displacement compressor and a controller, wherein the variable displacement compressor comprises a control valve provided with a valve body, a pressure sensitive mechanism for sensing the lower pressure side pressure of a refrigerating cycle acting to force the valve body and a solenoid for forcing the valve body based on an input electric current, position of the control valve is controlled to vary internal pressure of a control chamber, thereby variably controlling the displacement of the variable displacement compressor, and the controller controls the input electric current to the solenoid to control the position of the control valve, and wherein operation of the air conditioner is switchable between cooling mode and heating mode using highly pressurized hot gas in the refrigerant cycle, and wherein during the cooling mode operation the controller controls the input electric current to the solenoid to operate the control valve based on the lower pressure side pressure of the refrigerant cycle acting on the pressure sensitive mechanism and the quantity of the input electric current to the solenoid, and during the heating mode operation it controls the input electric current to the solenoid to operate the control valve based not on the lower pressure side pressure of the refrigerant cycle acting on the pressure sensitive mechanism but only on the quantity of the input electric current to the solenoid. 
     When the control valve is operated during a cooling operation based on the lower pressure side pressure of the refrigerant cycle sensed by the pressure sensitive mechanism and the quantity of the input electric current to the solenoid to variably control the displacement of the variable displacement compressor, the lower pressure side pressure of the refrigerant cycle can be controlled to a predetermined level and the cooling temperature can be controlled to a predetermined level. On the other hand, when the control valve is operated during a heating operation not based on the lower pressure side pressure of the refrigerant cycle sensed by the pressure sensitive mechanism but only on the quantity of the input electric current to the solenoid, the higher pressure side pressure of the refrigerant cycle can be controlled to a predetermined level and heating temperature can be controlled to a predetermined level. 
     In accordance with a preferred embodiment of the present invention, the air conditioner further comprises a diode connected to the solenoid in parallel to form a flywheel circuit. The controller drives a switching element on and off at a predetermined cycle to control the ratio of ON/OFF, i.e., the duty ratio thereof, thereby controlling the quantity of the input electric current to the solenoid, drives the switching element during the cooling mode operation at a first cycle to obtain a smoothing effect of the electric current by the flywheel circuit, and drives the switching element during the heating mode operation at a second cycle lower than the first cycle so as not to obtain the smoothing effect of the electric current by the flywheel circuit. 
     When the switching element is driven during the cooling mode operation at a first cycle to obtain a smoothing effect of the electric current by the flywheel circuit and the duty ratio of the switching element is controlled, the input electric current to the solenoid can be controlled to control position of the control valve, the lower pressure side pressure of the refrigerant cycle can be self-controlled to a predetermined level, and cooling temperature can be controlled to a predetermined level. On the other hand, when the switching element is driven during the heating mode operation at a second cycle lower than the first cycle so as no to obtain the smoothing effect of the electric current by the flywheel circuit and the duty ratio of the switching element is controlled, the input electric current to the solenoid can be controlled to variably control the ratio of fully opened period and entirely closed period of the control valve, the higher pressure side pressure of the refrigerant cycle can be self-controlled to a predetermined level, and heating temperature can be controlled to a predetermined level. 
     In accordance with a preferred embodiment of the present invention, the controller comprises a sensor for detecting the higher pressure side refrigerant pressure of the refrigerant cycle or the higher pressure side refrigerant temperature of the refrigerant cycle, and the controller drives the switching element at the second cycle and varies the duty ratio to keep the detected pressure or the detected temperature in a predetermined range during the heating mode operation. 
     When the higher pressure side refrigerant pressure of the refrigerant cycle or the higher pressure side refrigerant temperature of the refrigerant cycle is controlled to a predetermined range during the heating mode operation, comfortable heating is achieved. 
     In accordance with a preferred embodiment of the present invention, the controller controls the duty ratio of the switching element to minimize the displacement of the compressor or stops the compressor when the detected pressure or the detected temperature rises to the upper limit beyond the predetermined range during the heating mode operation. 
     When the duty ratio of the switching element is controlled to minimize the displacement of the compressor or the compressor is stopped in a case where the higher pressure side pressure or the higher pressure side temperature of the refrigerant cycle rises to the upper limit beyond the predetermined range during the heating mode operation, the safety of the air conditioner is maintained. 
     In accordance with a preferred embodiment of the present invention, the controller decreases the duty ratio to a level lower than a predetermined level when the duty ratio is continuously kept higher than or equal to the predetermined level for a predetermined time during the heating mode operation. 
     In accordance with a preferred embodiment of the present invention, the controller controls the duty ratio to minimize the displacement of the compressor or stops the compressor when the duty ratio is continuously kept higher than or equal to a predetermined level for a predetermined time during the heating mode operation. 
     When the duty ratio is decreased to a level lower than a predetermined level or the displacement of the compressor is minimized or the compressor is stopped in a case where the duty ratio is continuously kept higher than or equal to the predetermined level for a predetermined time, temperature rise of the solenoid can be controlled within an appropriate range. 
     In accordance with a preferred embodiment of the present invention, the sensor for detecting the higher pressure side refrigerant pressure of the refrigerant cycle or the higher pressure side refrigerant temperature of the refrigerant cycle is located upstream of a refrigerant circuit switching valve for switching the operation mode between the cooling mode and the heating mode. 
     In accordance with the aforementioned structure, the sensor for detecting the higher pressure side refrigerant pressure of the refrigerant cycle or the higher pressure side refrigerant temperature of the refrigerant cycle can be used not only in the cooling mode operation but also in the heating mode operation. Thus, the structure of the air conditioner is simplified. 
     In accordance with a preferred embodiment of the present invention, the air conditioner further comprises a check valve disposed in a discharge passage of the variable displacement compressor. The sensor for detecting the higher pressure side refrigerant pressure detects the pressure of the refrigerant upstream of the check valve. 
     The check valve disposed in a discharge passage of the variable displacement compressor prevents the higher pressure side refrigerant from backflowing into the idling variable displacement compressor during the stop period of the air conditioner and accumulating there as liquid. The sensor for detecting the higher pressure side refrigerant pressure detects the refrigerant pressure upstream of the check valve. Thus, abnormally high pressure in the discharge passage upstream of the check valve is promptly detected when the check valve fails and the safety of the air conditioner is maintained. 
     Effect of the Invention 
     In accordance with the air conditioner of the present invention, during the cooling mode operation, the control valve is operated based on the lower pressure side pressure of the refrigerant cycle detected by the pressure sensitive mechanism and the quantity of the input electric current to the solenoid to variably control the displacement of the variable displacement compressor, thereby controlling the lower pressure side pressure of the refrigerant cycle to a predetermined level and controlling the cooling temperature to a predetermined level. On the other hand, during the heating mode operation, the control valve is operated not based on the lower pressure side pressure of the refrigerant cycle detected by the pressure sensitive mechanism but only on the quantity of the input electric current to the solenoid to control the higher pressure side pressure of the refrigerant cycle to a predetermined level and control the heating temperature to a predetermined level. 
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Preferred embodiments of the present invention will be described. 
     First Embodiment 
     As shown in  FIG. 1 , a car air conditioner  1  comprises a first refrigerant circuit  10  (hereinafter called refrigerant circuit), a second refrigerant circuit  11  (hereinafter called hot gas bypass circuit), a first electromagnetic valve  12  and a second electromagnetic valve  13  for switching the refrigerant circuit between the refrigerant circuit  10  and the hot gas bypass circuit  11 . In the refrigerant circuit  10 , highly pressurized hot gas refrigerant discharged from a variable displacement compressor  100  passes through the first electromagnetic valve  12 , a condenser  14 , a receiver  15 , a check valve  16 , an expansion valve  17 , an evaporator  18  and an accumulator  19  serially in said order, and returns to the variable displacement compressor  100 . In the hot gas bypass circuit  11 , highly pressurized hot gas refrigerant discharged from the variable displacement compressor  100  passes through the second electromagnetic valve  13 , a fixed aperture  20 , the evaporator  18  and the accumulator  19  serially in said order, and returns to the variable displacement compressor  100 . 
     When the first electromagnetic valve  12  opens and the second electromagnetic valve  13  closes, the refrigerant circulates in the refrigerant circuit  10 . When the first electromagnetic valve  12  closes and the second electromagnetic valve  13  opens, the refrigerant circulates in the hot gas bypass circuit  11 . 
     When the refrigerant circulates in the refrigerant circuit  10 , the evaporator  18  operates as a heat exchanger for cooling, wherein cool gas-liquid two phase refrigerant entering through the expansion valve  17  evaporates to cool down the air passing through the evaporator  18 . When the refrigerant circulates in the hot gas bypass circuit  11 , the evaporator  18  operates as a heat exchanger for heating, i.e., an auxiliary heating apparatus, wherein hot refrigerant gas entering through the fixed aperture  20  heats up the air passing through the evaporator  18 . 
     As shown in  FIG. 2 , the variable displacement compressor  100  comprises a cylinder block  101  provided with a plurality of cylinder bores  101   a , a front housing  102  opposing one end of the cylinder block  101 , and a rear housing  104  opposing the other end of the cylinder block  101  with a valve plate  103  clamped between them. 
     The cylinder block  101  cooperates with the front housing  102  to define a crank chamber  105 . A driving shaft  106  extends across the crank chamber  105 . The driving shaft  106  passes through a swash plate  107 . The swash plate  107  is connected to a rotor  108  fixed to the driving shaft  106  through a link  109 . The driving shaft  106  supports the swash plate  107  variably at an inclination. A coil spring  110  is disposed between the rotor  108  and the swash plate  107  to force the swash plate  107  in a direction for decreasing the inclination. A coil spring  111  is also provided. The coil spring  111  and the coil spring  110  are disposed to face opposite surfaces of the swash plate  107 . The coil spring  111  forces the swash plate  107  in minimum inclination condition in the direction for increasing the inclination. 
     The driving shaft  106  extends out of the housing at one end through a boss  102   a  of the front housing  102  to be connected to a car engine not through an electromagnetic clutch but directly through a transmission. The car engine and the transmission are not shown in  FIG. 2 . A shaft seal  112  is disposed between the driving shaft  106  and the boss  102   a.    
     The driving shaft  106  is supported radially and longitudinally by bearings  113 ,  114 ,  115  and  116 . 
     Pistons  117  are inserted into the cylinder bores  101   a . Each piston  117  is provided with a concave  117   a  at one end. The concave  117   a  accommodates a pair of shoes  118  for clamping the outer periphery of the swash plate  107  to be slidable relative to the outer periphery of the swash plate  107 . Rotation of the driving shaft  106  is converted to reciprocal movement of the piston  117  through the swash plate  107  and the shoes  118 . 
     The rear housing  104  forms a suction chamber  119  and a discharge chamber  120 . The suction chamber  119  communicates with the cylinder bores  101   a  through communication holes  103   a  formed in the valve plate  103  and suction valves. The discharge chamber  120  communicates with the cylinder bores  101   a  through discharge valves and communication holes  103   b  formed in the valve plate  103 . The suction valves and the discharge valves are not shown in  FIG. 2 . The suction chamber  119  communicates with the accumulator  19  of the air conditioner  1  through a suction port  104   a  and a pipe. 
     A muffler  121  is disposed outside the cylinder block  101 . The muffler  121  is formed by a cylindrical wall  101   b  formed on the outer surface of the cylinder block  101  and a cover  122  having a cylindrical form closed at one end, independent of the cylinder block  101  and connected to the cylindrical wall  101   b  with a seal member inserted between them. A discharge port  122   a  is formed in the cover  122 . The discharge port  122   a  connects to the electromagnetic valves  12  and  13  of the air conditioner  1  through pipes. 
     A communication passage  123  is formed through the cylinder block  101 , the valve plate  103  and the rear housing  104  to communicate the muffler  121  with the discharge chamber  120 . The muffler  121  and the communication passage  123  cooperate to form a discharge passage extending between the discharge chamber  120  and the discharge port  122   a.    
     A refrigerant pressure sensor  124  for detecting refrigerant pressure in the discharge chamber  120  is fitted to the rear housing  104 . 
     A check valve  200  is disposed in the muffler  121  to open and close the upstream end of the muffler  121  connecting to the communication passage  123 . The check valve  200  closes the upstream end of the muffler  121  to shut down the discharge passage extending between the discharge chamber  120  and the discharge port  122   a  when the difference between the pressure acting on the front surface of a valve body and the pressure acting on the rear surface of the valve body is less than a predetermined level, while opening the upstream end of the muffler  121  to open the discharge passage when the difference between the pressure acting on the front surface of the valve body and the pressure acting on the rear surface of the valve body is larger than the predetermined level. 
     The front housing  102 , the cylinder block  101 , the valve plate  103  and the rear housing  104  are disposed adjacent to each other with gaskets inserted between them and assembled as a unitary body with a plurality of through bolts. 
     A displacement control valve  300  is fitted to the rear housing  104 . The displacement control valve  300  controls the aperture of a communication passage  125  extending between the discharge chamber  120  and the crank chamber  105  to control the flow rate of the discharging refrigerant gas passing into the crank chamber  105 . The refrigerant gas in the crank chamber  105  is passed into the suction chamber  119  through spaces between the bearings  115 ,  116  and the driving shaft  106 , a space  126  formed in the cylinder block  101  and an orifice hole  103   c  formed in the valve plate  103 . 
     The displacement control valve  300  can control the internal pressure of the crank chamber  105  to control the displacement of the variable displacement compressor  100 . The displacement control valve  300  controls the supply of electric current to a built-in solenoid based on an external control signal to control the displacement of the variable displacement compressor  100 , thereby keeping the internal pressure of the suction chamber  119  at a predetermined level. The displacement control valve  300  stops the supply of electric current to the built-in solenoid to mechanically open the communication passage  125 , thereby minimizing the displacement of the variable displacement compressor  100 . 
     As shown in  FIG. 3 , the displacement control valve  300  comprises a bellows  303  disposed in a pressure sensitive chamber  302  formed in a valve housing  301 . The bellows  303  is provided with a vacuum inner space and a spring disposed in the inner space. The bellows  303  operates as a pressure sensitive member for receiving internal pressure of the inlet chamber  119  (hereinafter called inlet pressure) through a communication hole  301   a  and a communication passage  127 . The displacement control valve  300  comprises a valve body  304 . The valve body  304  is disposed in a valve chamber  312  formed in the valve housing  301  at one end portion to receive internal pressure of the crank chamber  105  (hereinafter called crank chamber pressure) and open and close a valve hole  305   a  disposed on the communication passage  125  between the discharge chamber  120  and the crank chamber  105 , slidably supported by a support hole  301   b  formed in the valve housing  301  at the other end portion, and connected to the bellows  303  at the other end. The displacement control valve  300  further comprises a valve seat forming member  305  provided with the valve hole  305   a  and a valve seat  305   b  and press fitted in an accommodation hole  301   c  formed in the valve housing  301 , a solenoid rod  304   a  formed integrally with the valve body  304 , a movable iron core  306  press fitted on one end of the solenoid rod  304   a , a fixed iron core  307  fitted on the solenoid rod  304   a  to oppose the movable iron core  306  at a predetermined distance, a spring  308  disposed between the fixed iron core  307  and the movable iron core  306  to force the movable iron core  306  in the opening direction of the valve body  304 , a cylindrical member  310  fitting on the fixed iron core  307  and the movable iron core  306  to be fixed to a solenoid case  309 , and an electromagnetic coil  311  surrounding the cylindrical member  310  and accommodated in the solenoid case  309 . 
     The pressure sensitive chamber  302  and the bellows  303  form a pressure sensitive mechanism  300 A for detecting the inlet pressure acting to force the valve body  304 . The solenoid rod  304   a , the movable iron core  306 , the fixed iron core  307 , the cylindrical member  310 , the electromagnetic coil  311  and the solenoid case  309  form a solenoid  300 B for forcing the valve body  304  based on the input electric current. The spring  308  forces the valve body  304  to open the valve hole  305   a  when the solenoid  300 B is demagnetized. 
     A communication hole  301   d  formed in the valve housing  301  at right angles to the valve hole  305   a  crosses the accommodation hole  301   c  and communicates with the discharge chamber  120  through the communication passage  125 . Therefore, the valve hole  305   a  communicates with the communication hole  301   d  through the accommodation hole  301   c . The other end of the valve body  304  connected to the bellows  303  is shut off from the accommodation hole  301   c . Therefore, the other end of the valve body  304  connected to the bellows  303  is shut off from the discharge chamber  120 . The valve chamber  312  communicates with the crank chamber  105  through a communication hole  301   e  and the communication passage  125 . The communication hole  301   d , the accommodation hole  301   c , the valve hole  305   a , the valve chamber  312  and the communication hole  301   e  form a part of the communication passage  125  between the discharge chamber  120  and the crank chamber  105 . 
     [Disclosure of the Invention] An air conditioner comprises a variable displacement compressor and a controller  400 . The variable displacement compressor comprises a control valve provided with a valve body, a pressure sensitive mechanism  300 A for sensing the lower pressure side pressure of a refrigerating cycle acting to force the valve body and a solenoid  300 B for forcing the valve body based on an input electric current, position of the control valve is controlled to vary internal pressure of a control chamber, thereby variably controlling the displacement of the variable displacement compressor. The controller  400  controls the input electric current to the solenoid  300 B to control the position of the control valve. Operation of the air conditioner is switchable between cooling mode and heating mode using highly pressurized hot gas in the refrigerant cycle. During the cooling mode operation, the controller  400  controls the input electric current to the solenoid  300 B to operate the control valve based on the lower pressure side pressure of the refrigerant acting on the pressure sensitive mechanism  300 A and the quantity of the input electric current to the solenoid  300 B, and during the heating mode operation it controls the input electric current to the solenoid  300 B to operate the control valve based not on the lower pressure side pressure of the refrigerant cycle acting on the pressure sensitive mechanism  300 A but only on the quantity of the input electric current to the solenoid  300 B. 
     The car air conditioner  1  comprises a controller  400 . 
     As shown in  FIG. 4 , the controller  400  is connected to an in-vehicle battery  500 . The in-vehicle battery  500  supplies the controller  400  with direct current electric power when the ignition switch of a car engine is turned ON. 
     Various kinds of command signals are sent to the controller  400  from a mode selector switch  401  for selecting an air condition mode between a cooling mode using the refrigerant circuit  10  and an auxiliary heating mode using the hot gas bypass circuit  11 , a temperature setting switch  402  for setting interior temperature at a desired level, an air conditioner switch  403  for starting and stopping the variable displacement compressor  100 , a flow rate selector switch  404  for selecting flow rate of the fan of the evaporator  18 , etc. Various kinds of detection signals are sent to the controller  400  from an interior air temperature sensor  405  for detecting interior air temperature, an outside air temperature sensor  406  for detecting outside air temperature, a solar radiation sensor  407  for detecting interior solar radiation, an evaporator temperature sensor  408  for detecting temperature of the air just after passing through the evaporator  18 , an engine cooling water temperature sensor  409  for detecting temperature of engine cooling water flowing into a hot-water heater and the refrigerant pressure sensor  124  for detecting the internal pressure of the discharge chamber  120  (hereinafter called discharge pressure) of the variable displacement compressor  100 . 
     The controller  400  supplies control electric power to an air mix door, a blowout opening selector door, an internal air and external air selector door, a blower motor of the condenser  14 , a blower motor of the evaporator  18 , the first electromagnetic valve  12 , the second electromagnetic valve  13  and the electromagnetic coil  311  of the control valve  300 . 
     The electric power supply line for the electromagnetic coil  311  forms a flywheel circuit  411  with a diode  410  being disposed in parallel to the electromagnetic coil  311 . The electric power supply line for the electromagnetic coil  311  is grounded at the trailing end. An electric current sensor  412  is disposed to detect electric current flowing in the flywheel circuit  411 . The detection signal of the electric current sensor  412  is sent to the controller  400 . 
     The electric power is supplied to the electromagnetic coil  311  through a switching element not shown in  FIG. 4 . The quantity of the electric current supplied to the electromagnetic coil  311  is controlled by a pulse width modulation system (PWM control system), wherein the switching element is driven ON/OFF at a predetermined frequency, with the ratio of ON/OFF, i.e., the duty ratio, being varied. 
     Operation of the car air conditioner  1  will be described. 
     When the ignition switch of the car engine is switched ON to start the car engine, driving power is transmitted to the variable displacement compressor  100  directly connected to the car engine, and the in-vehicle battery  500  supplies the controller  400  with direct current electric power. 
     When the mode selector switch  401  selects the cooling mode operation, the controller  400  opens the first electromagnetic valve  12  and closes the second electromagnetic valve  13  to make the refrigerant circuit  10  ready for operation. 
     When the controller  400  judges based on the command signals from the switches and the detection signals from the sensors that conditions for starting the compressor  100  are fulfilled, the controller  400  drives the switching element ON/OFF at 400 Hz frequency. When the frequency range is 400 Hz or so, the electric current flowing in the electromagnetic coil  311  does not rapidly increase due to inductance of the electromagnetic coil  311  even if the switching element is driven ON and the switching element is driven OFF before the electric current becomes maximum. On the other hand, the electric current returns to the electromagnetic coil  311  due to the diode  410  even if the switching element is driven OFF and the switching element is driven ON before the electric current becomes zero. As a result, smoothed direct electric current circulates in the flywheel circuit  411  as shown in  FIG. 5 . When the duty ratio is variably controlled, quantity of the smoothed direct electric current circulating in the flywheel circuit  411  and flowing in the electromagnetic coil  311  is variably controlled. Therefore, when the frequency range is 400 Hz or so, the control valve  300  of the variable displacement compressor  100  operates as a closing valve for operating based on the inlet pressure acting on the pressure sensitive mechanism  300 A and the electric current flowing in the solenoid  300 B. In this situation, the control valve  300  has a control characteristic indicated by formula (1) in  FIG. 6 . Therefore, it is possible to vary the input electric current, thereby variably controlling the displacement and the inlet pressure as shown in  FIG. 7 . The control valve  300  has an inlet pressure control characteristic substantially not based on the discharge pressure Pd because Sv is only a little larger than Sr in the formula (1). 
     The controller  400  determines a target air temperature so as to control the temperature of the air at the exit of the evaporator  18  at a predetermined level based on the command signals from the switches and the detection signals from the sensors. The controller  400  compares the air temperature detected by the evaporator temperature sensor  408  with the target temperature to determine a target control electric current based on the difference between them. The controller  400  compares the detection signal from the electric current sensor  412  with the target control electric current to adjust the duty ratio of the switching element based on the difference between them, thereby adjusting the electric current flowing in the electromagnetic coil  311 . The controller  400  feedback controls the displacement of the variable displacement compressor  100  so as to make the electric current flowing in the electromagnetic coil  311  equal to the target control electric current, or make the inlet pressure equal to a target inlet pressure, or finally make the air temperature detected by the evaporator temperature sensor  408  equal to the target air temperature. 
     When the mode selector switch  401  selects the auxiliary heating mode operation, the controller  400  closes the first electromagnetic valve  12  and opens the second electromagnetic valve  13  to make the hot gas bypass circuit  11  ready for operation. 
     When the controller  400  judges based on the command signals from the switches and the detection signals from the sensors that conditions for starting the compressor  100  are fulfilled, the controller  400  drives the switching element ON/OFF at 10 Hz frequency. When the frequency range is 10 Hz or so, the electric current increases to the maximum current decided by the voltage of the in-vehicle battery  500  and the resistance of the electromagnetic coil  311  after the switching element is driven ON. As a result, the electromagnetic force of the solenoid  300 B becomes maximum and the valve body  304  of the control valve  300  moves in the closing direction regardless of the level of the inlet pressure acting on the bellows  303 . Thereafter, when the switching element is driven OFF, the electric current decreases to zero. As a result, the solenoid  300 B is demagnetized and the valve body  304  is forced by the spring  308  to move in the opening direction regardless of the level of the inlet pressure acting on the bellows  303 . Thus, when the frequency range is 10 Hz or so, the control valve  300  operates as a two position ON/OFF valve and a duty controlled ON/OFF valve. 
     When the control valve  300  operates as a duty controlled ON/OFF valve, the ratio of open period to closed period varies depending on the duty ratio. When the duty ratio is 0%, the control valve  300  is always fully open to make the displacement of the variable displacement compressor  100  minimum. When the duty ratio is 100%, the control valve  300  is always fully closed to make the displacement of the variable displacement compressor  100  maximum. Therefore, the displacement of the variable displacement compressor  100  can be variably controlled between the minimum level and the maximum level by variably controlling the duty ratio between 0% and 100%. 
     The controller  400  determines a target discharge pressure so as to control the discharge pressure of the variable displacement compressor  100  at a predetermined level based on the command signals from the switches and the detection signals from the sensors. The controller  400  compares the pressure detected by the pressure sensor  124  with the target discharge pressure to adjust the duty ratio of the switching element based on the difference between them, thereby adjusting the ratio between the fully open period of the control valve  300  and the fully closed period of the control valve  300 . The controller  400  feedback controls the displacement of the variable displacement compressor  100  so as to make the pressure detected by the pressure sensor  124  equal to the target discharge pressure. As a result, the discharge pressure of the variable displacement compressor  100  is controlled to a predetermined level to control the temperature of the air at the exit of the evaporator  18  to a predetermined level. 
     Control flow of the air conditioner  1  during the auxiliary heating mode operation will be described with reference to  FIG. 8 . The control valve  300  is driven under a condition of solenoid driving frequency=10 Hz and initial duty ratio=DT0. When the discharge pressure Pd detected by the pressure sensor  124  is Pd1&lt;Pd&lt;Pd2, the current duty ratio is kept to keep the current displacement. When the Pd is Pd1&gt;Pd, the control valve  300  is driven at a duty ratio increased by a predetermined quantity ΔPd to increase the displacement, thereby increasing the discharge pressure. When the Pd is Pd&gt;Pd2, the control valve  300  is driven at a duty ratio decreased by a predetermined quantity ΔPd to decrease the displacement, thereby decreasing the discharge pressure. As a result, the discharge pressure Pd is kept in the range Pd1&lt;Pd&lt;Pd2, the temperature of the air at the exit of the evaporator  18  is kept in a predetermined range, and comfortable interior heating of the car is maintained. 
     The pressure sensor  124  can be used both in the cooling mode operation and in the heating mode operation because it is located upstream of the first electromagnetic valve  12  and the second electromagnetic valve  13 . As a result, the structure of the air conditioner  1  is simplified. 
     The pressure sensor  124  can promptly detect abnormally high pressure in the discharge passage upstream of the check valve  200  when the check valve  200  does not open due to failure because the pressure sensor  124  is located upstream of the check valve  200 . Thus, the safety of the air conditioner is maintained. 
     Second Embodiment 
     A protector may be provided to reduce the duty ratio to 0%, thereby demagnetizing the solenoid  300 B to minimize the displacement of the variable displacement compressor  100  when Pd rises to Pd3(Pd3&gt;&gt;Pd2) beyond the range Pd1&lt;Pd&lt;Pd2. This maintains the safety of the air conditioner  1 . 
     The resistance of the electromagnetic coil  311  is set at 10Ω or less at room temperature so as to widen the controllable range of the inlet pressure. In the auxiliary heating mode operation, the electric current is liable to be continuously applied to the electromagnetic coil  311  for a long time. Therefore, the temperature of the solenoid  300 B is liable to rise, thereby causing rapid deterioration of the solenoid  300 B. When a predetermined duty ratio is kept for a predetermined time in the heating mode operation, the duty ratio can be decreased to a level lower than the predetermined level prior to a control for achieving higher pressure, or the duty ratio can be decreased to 0% to minimize the displacement of the variable displacement compressor  100 , thereby preventing the deterioration of the solenoid  300 B. 
     The variable displacement compressor  100  can be connected to the car engine through an electromagnetic clutch. In this case, the electromagnetic clutch can be cut OFF to stop the variable displacement compressor  100 , thereby maintaining the safety of the air conditioner  1  when Pd rises to Pd3(Pd3&gt;&gt;Pd2) beyond the range Pd1&lt;Pd&lt;Pd2 in the auxiliary heating mode operation, or the electromagnetic clutch can be cut OFF to stop the variable displacement compressor  100 , thereby preventing the deterioration of the solenoid  300 B when a predetermined duty ratio is kept for a predetermined time in the auxiliary heating mode operation. 
     A temperature sensor for detecting temperature of the refrigerant in the discharge chamber  120  can be disposed instead of the pressure sensor  124  to duty control the control valve  300 , thereby keeping the temperature Td of the discharging refrigerant in a range Td1&lt;Td&lt;Td2 in the auxiliary heating mode operation. In this case, a protector may be provided to reduce the duty ratio to 0%, thereby demagnetizing the solenoid  300 B to minimize the displacement of the variable displacement compressor  100  when Td rises to Td3(Td3&gt;&gt;Td2) beyond the range Td1&lt;Td&lt;Td2. This maintains the safety of the air conditioner  1 . In a case where the variable displacement compressor  100  is connected to the car engine through an electromagnetic clutch, the electromagnetic clutch can be cut OFF to stop the variable displacement compressor  100  when Td rises to Td3(Td3&gt;&gt;Td2) beyond the range Td1&lt;Td&lt;Td2 in the auxiliary heating mode operation. This maintains the safety of the air conditioner  1 . 
     INDUSTRIAL APPLICABILITY 
     The present invention can be used for the following air conditioners. 
     1. An air conditioner comprising a variable displacement compressor provided with a control valve having a pressure sensitive mechanism operating based on the pressure difference between the pressure at a point located lower pressure side and the pressure at a point located higher pressure side.
 
2. An air conditioner comprising a variable displacement compressor driven by a motor.
 
3. An air conditioner comprising a variable displacement compressor of scroll type, vane type or wobble plate type.
 
4. An air conditioner using CO2 or R152a instead of R134a as refrigerant.
 
5. An air conditioner having a heat pump type heating mode operation.
 
6. An air conditioner other than a car air conditioner.
 
7. An air conditioner comprising not the pressure sensor  124  but instead a temperature sensor for detecting the higher pressure side refrigerant temperature or surface temperature of the evaporator  18 .
 
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram of an air conditioner in accordance with a preferred embodiment of the present invention. 
       FIG. 2  is a sectional view of a variable displacement compressor provided on the air conditioner in accordance with a preferred embodiment of the present invention. 
       FIG. 3  is a structural view of a displacement control valve of a variable displacement compressor provided on the air conditioner in accordance with a preferred embodiment of the present invention. (a) is a general sectional view, (b) is a fragmentary enlarged sectional view at the closed condition at and (c) is a fragmentary enlarged sectional view without a valve body. 
       FIG. 4  is a block diagram of a controller provided on the air conditioner in accordance with a preferred embodiment of the present invention. 
       FIG. 5  is a graph showing the electric current controlled by pulse-width modulation system and flowing in the electromagnetic coil of the control valve of  FIG. 3 . 
       FIG. 6  is a view showing a control characteristic formula of the displacement control valve of  FIG. 3 . 
       FIG. 7  is a diagram showing a control characteristic of the displacement control valve of  FIG. 3 . 
       FIG. 8  is a view showing a control flow of the air conditioner in accordance with a preferred embodiment of the present invention. 
     BRIEF DESCRIPTION OF THE REFERENCE NUMERALS 
     
         
         
           
               1  Air conditioner 
               12  First electromagnetic valve 
               13  Second electromagnetic valve 
               14  Condenser 
               18  Evaporator 
               100  Variable displacement compressor 
               124  Pressure sensor 
               200  Check valve 
               300  Displacement control valve 
               311  Electromagnetic coil 
               400  Controller 
               411  Flywheel circuit 
               500  In-vehicle battery