Abstract:
A control system for a variable displacement compressor uses a mechanical valve to minimize energy consumption in an air conditioning system. The control system can also provide instantaneous indications to the vehicle controller of air conditioning power consumption to avoid engine loading. Controls are also used to contain oil within the compressor and to minimize its presence downstream of the compressor into the gas cooler and evaporator parts of the system.

Description:
FIELD OF THE INVENTION 
     This invention generally relates to variable displacement compressors for air conditioning systems in automobiles and trucks. Variable displacement compressors are used in air conditioning systems with clutchless and clutched compressors. 
     BACKGROUND OF THE INVENTION 
     Automotive air conditioning systems, like all air conditioning systems, are faced with a number of operating contradictions. These contradictions include a requirement to provide cooling, but not too much cooling, in the passenger compartment. There is a technical requirement to lubricate the refrigerant compressor, but not to foul downstream heat exchangers with the lubricant. In automotive systems, additionally, consumers expect instantaneous response in the passenger compartment to what may be a very large and very rapidly changing heat load. Of course, while the only power available is that supplied by the engine and the automotive battery, automotive consumers also expect that operation of the air conditioning system will not load the engine or cause any operating difficulty. Consumers also expect that the automotive air conditioning system will have low power consumption. 
     Traditional automotive air conditioning systems used a clutch, in which the air conditioning compressor was engaged or disengaged to provide power to the compressor and thus supply cooling to the passenger compartment. Of course, the on/off nature of this control provided slow response. The prior art tried to meet the needs described above in a variety of ways, principally by using a variable displacement compressor. In clutchless variable displacement compressors, the compressor is always on, i.e. always rotating, while the displacement of the compressor is determined by the angle at which a central swashplate is oriented to a number of pistons and cylinders in which refrigerant compression takes place. A narrow angle (perpendicular to a drive shaft) provides little compression, while steep angles (at some angle to the drive shaft) provide greater compression, depending on the angle selected. However, some present variable displacement compressors allow too much oil into the downstream air conditioning components, such as the gas cooler or condenser, and the evaporator, fouling their internal surfaces and reducing heat transfer to the passenger compartment. In addition, high loads on the compressors can load down engines, in extreme cases causing stalling in awkward situations. Finally, the response time for systems using variable displacement compressors can be long, resulting in longer cooling cycles and higher power consumption than necessary. What is needed is a control system that responds rapidly to air conditioning loads and minimizes oil contamination and energy consumption, without loading the engine or causing stalling. 
     SUMMARY 
     This invention meets these needs by providing an improved control system for an automotive air conditioning system. While the greatest advantage for the improved control system may be realized in a clutchless variable displacement compressor for an automotive air conditioning system, the control system may also be utilized in a variable displacement compressor having a clutch. 
     One aspect of the invention is a variable displacement compressor. The variable displacement compressor comprises a compressor housing having a crankcase chamber with a crankcase pressure, a suction chamber with a suction pressure, and a discharge chamber with a discharge pressure, the compressor also having a driveshaft, a swashplate connected to and driveable by the driveshaft, a plurality of pistons connected to the swashplate and reciprocating in a plurality of cylinders, wherein a displacement of the compressor is varied by the angle of the swashplate with the drive shaft. The compressor also comprises a three-way control valve having a valve body and a valve stem, at least one spring opposing motion of the valve stem, and three chambers in series for receiving three pressures from the variable displacement compressor, one chamber receiving a discharge pressure, one chamber receiving a crankcase pressure, and one chamber receiving an auxiliary pressure, wherein the control valve is operative to change the crankcase pressure and thereby change the displacement of the compressor. 
     Another aspect of the invention is a method of operating a variable displacement compressor. The method comprises controlling a displacement of the compressor with a three way valve using a discharge pressure, a crankcase pressure, and an auxiliary pressure, and adjusting the displacement with the three way valve based on a difference between the discharge pressure and the crankcase pressure. The method also comprises separating oil from a discharge line of the compressor; and routing the oil to a crankcase of the compressor. 
     Another aspect of the invention is a variable displacement compressor. The variable displacement compressor comprises a compressor housing having a crankcase chamber with a crankcase pressure, a suction chamber with a suction pressure, and a discharge chamber with a discharge pressure, the compressor further comprising a driveshaft, a swashplate connected to and driveable by the driveshaft, a plurality of pistons connected to the swashplate and reciprocating in a plurality of cylinders, wherein a displacement of the compressor is varied by the angle of the swashplate with the drive shaft. The variable displacement compressor also comprises an oil separator in a discharge line of the compressor, and a four-way control valve having a valve body and a valve stem, at least one spring opposing motion of the valve stem, and four chambers in series for receiving an oil separator pressure, a discharge pressure, a crankcase pressure, and a suction pressure from the variable displacement compressor, with an orifice connecting the crankcase chamber with the suction chamber, wherein the control valve is operative to change the crankcase pressure and thereby change the displacement of the compressor. 
     Another aspect of the invention is a method of operating a variable displacement compressor. The method comprises controlling a displacement of the compressor with a four way valve having an orifice between two chambers of the valve, and adjusting the displacement using the four way valve, based on a difference between a discharge pressure and a crankcase pressure. The method also comprises separating oil from a discharge line of the compressor; and routing the oil to a crankcase of the compressor. 
     Other systems, methods, features, and advantages of the invention will be or will become apparent to one skilled in the art upon examination of the following figures and detailed description. All such additional systems, methods, features, and advantages are intended to be included within this description, within the scope of the invention, and protected by the accompanying claims. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       The invention may be better understood with reference to the following figures and detailed description. The components in the figures are not necessarily to scale, emphasis being placed upon illustrating the principles of the invention. Moreover, like reference numerals in the figures designate corresponding parts throughout the different views. 
         FIG. 1  depicts a cross-sectional view of a first embodiment having a four-way control valve. 
         FIG. 2  depicts cross-sectional view of a second embodiment having a four-way control valve. 
         FIGS. 3 and 4  depict a cross sectional view of an embodiment of a check valve useful in the present invention. 
         FIG. 5  is a closer cross-sectional view of a four-way valve for the first and second embodiments. 
         FIG. 6  is a block diagram of another embodiment showing connections of the variable displacement compressor to a four-way control valve. 
         FIG. 7  is a block diagram of another alternate embodiment of a control system. 
         FIGS. 8 and 9  depict cross sectional views of a three-way control valve of one embodiment. 
         FIGS. 10 and 11  are cross sectional views of further alternative embodiments of a compressor and control system. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  illustrates a variable displacement type compressor, generally indicated in the drawings as reference  10 . The compressor  10  includes a cylinder block  12 , a housing  14  that defines a crank chamber  16 , a drive shaft  18 , a swashplate  20 , a swashplate spring  22 , a rear housing  24 , at least one cylinder bore  26 , and at least one piston  28 . The rear housing  24  defines a suction chamber  30  and a discharge chamber  32 . There is a valve plate  44  that defines a suction port  34  and a discharge port  36  for each cylinder. The compressor comprises a plurality of pistons and cylinders, for example,  5  pistons and cylinders, or  6  pistons and cylinders. The drive shaft  18  is supported by the housing  14  such that a portion of the drive shaft  18  is disposed within the crank chamber  16 . The swashplate  20  is mounted on the drive shaft  18  such that it is contained within the crank chamber  16  and is tilted away from a plane perpendicular to the longitudinal axis of the drive shaft  18 . The degree to which the swashplate  20  is tilted away from the plane perpendicular to the longitudinal axis of the drive shaft  18  is indicated in the drawing as angle A. A spring  22  acts upon swashplate  20 . The cylinder block  12  defines the cylinder bore  26 . The piston  28  is disposed within the cylinder bore  26  such that the piston  28  can slide in and out of the bore  26 . This slideable movement of the piston  28  is possible, at least in part, due to the presence of a small clearance  38  between the interior surface  40  of the cylinder block  12  in the cylinder bore  26  and the exterior surface  42  of the piston  28 . The pistons  28  may be secured to the swashplate  20  by shoes  54 , which allow for movement of the swashplate relative to the pistons. 
     System Controls 
     There is a solenoid valve  60 , comprising a stem  62  and two flow control elements  64 ,  66  fixed on the stem. The valve defines five chambers  68 ,  70 ,  72 ,  74 ,  76  for controlling the operation of the variable displacement compressor  10 . Passage  67  communicates P c  from chamber  74  to chamber  68 . In this embodiment, chambers  68  and  74  are thus at crankcase pressure, P c , while chamber  70  is at the pressure of an oil separator, P os , which will be described below. Chamber  72  is at discharge pressure, P d , and chamber  76  is at the compressor suction pressure, P 1 . Orifice  77 , about 0.4 mm to about 1.0 mm diameter, communicates between chambers  72  and  74 . In some embodiments, a pressure of refrigerant gas returning from the evaporator, P ev , may be used in place of P s . The solenoid has a coil  78  which receives power from an external power source. The solenoid valve also has springs  80  and  82  at opposite ends of the stem to balance the forces on the stem  62 . Spring  80  is larger (having a greater spring constant) than spring  82 , so that when there is no current to the coil  78 , spring  80  urges the stem upward. 
     As shown in  FIG. 1 , there is sufficient current to coil  78  so that it has been drawn downward, allowing communication between chamber  70  (P os ) and chamber  72  (P d ), and also between chamber  72  (P d ) and chamber  74  (P c ) and between chamber  74  (P c ) and chamber  76  (P s ). In this configuration, the swashplate will be at angle A, an intermediate position between its minimum angle (almost parallel to a plane perpendicular to the longitudinal axis of the drive shaft  18 ) and its greatest angle, which will vary according to the particular compressor used, but may be as great at 30 degrees. With this geometry, the control of valve  60  depends primarily on the difference between discharge pressure P d  and P s . Because P d  is much more active and variable than P s , this valve, and thus the compressor, is able to react more quickly to changes in cooling demand by the cooling load in the automobile or truck of which the compressor and air conditioning system is a part. This quick reaction is much faster, for instance, than a valve which connects the P c  and P s  chambers. 
     The compressor system may also include a control system  95 , including a microprocessor-based controller  96  and memory  97 , and signal-conditioning circuitry  99  that controls the current to the solenoid coil  78 . The microprocessor-based controller may include any useful controller, including PID or other types of controllers, and also desirably includes a pulse-width-modulation (PWM) routine for very quickly controlling the current to the solenoid. The controller may have a number of inputs/outputs  98 , which may include a temperature indication from the passenger compartment and may also indicate a relative humidity from the passenger compartment. The controller may control and also monitor the current to the solenoid by a current-reading device  94 , which may be internal or external to the controller. The solenoid current is proportional to the load on the compressor and the air conditioning system. In one embodiment, the control system  95  may send an indication of the solenoid current or solenoid valve position to the vehicle powertrain control module for indicating the load on the compressor, and thus on the vehicle, caused by the air conditioning system. 
     System Operation 
     When the swashplate is at its minimum angle, the pistons reciprocate to the least extent possible as the drive shaft rotates, compressing the smallest possible amount of refrigerant in the compressor, and using the least energy. When the swashplate is at its greatest angle, the pistons reciprocate up and down in their respective cylinders to the maximum extent, compressing much more refrigerant, and allowing the greatest air-conditioning effect. To achieve the greatest swashplate angle, the solenoid pulls the stem and flow control elements even further down in  FIG. 1 , so that flow control element  64  closes communication between chamber  72  (P d ) and chamber  74  (P c ). The desired amount of cooling by the air conditioning system and the compressor, and the degree of travel of the stem and flow control elements in valve  80 , correspond with the current needed for the coil  78 . In one embodiment, control system  95  and microprocessor-based controller  96  includes a pulse-width-modulation (PWM) routine for controlling the movement of solenoid valve  60 . In PWM routines, a current is switched on and off, typically at a varying frequency, to achieve a desired control output by means of very fast switching between on and off. For instance, a square-wave pattern with short “off” periods may be used with longer and longer “on” periods to simulate a sinusoidal current. When the current is on, the valve stem is pulled downward, and the valve closes. When the current is off, spring  80  overcomes the force of spring  82 , and the valve opens. This allows the valve to be very fast-acting and very responsive to the control signal, in this case, the difference between P d  and P s . 
     The compressor has a number of passages to allow for communication of refrigerant pressure, and also for flow of refrigerant in, the compressor. Passage  46  communicates crankcase pressure P c  from the crankcase  16  to chamber  74  of the valve  60 . Passage  56  communicates suction pressure (P s ) to chamber  76  of the valve. Passage  58  communicates discharge pressure (P d ) from the discharge chamber to chamber  72  in valve  60 . In one embodiment, passage  58  may be a short passage from 1 to about 5 mm in diameter, preferably about 2-3 mm in diameter. Within the valve, chamber  68  communicates with chamber  74  and receives crankcase pressure (P c ) through optional passageway or piping  67 . Orifice  77  allows a flow of oil from chamber  72  at P d  to chamber  74  at P c , and to the crankcase itself. In addition, there may be a passage  85  from check valve  84  to crankcase  16 , and there may also be an additional passage  87  from the crankcase  16  to the suction chamber  30 . Passage  85  enables oil and refrigerant from the discharge to return to the crankcase. Passage  85  is from about 1 mm to about 5 mm, preferably 2 mm to 3 mm. Passage  87  allows flow between the crankcase and the suction. Passage  87  may be from 0.25 to 2 mm in diameter, preferably 0.8 mm. The passage itself may be long or may be as short as 2–4 mm. 
     Refrigerant compressed by the compressor leaves the discharge chamber  32  via check valve  84 . Piping  86  may convey the compressed refrigerant to an oil separator  88 , to prevent oil from entering the refrigeration system downstream of the oil separator  88 . Refrigerant leaves to a gas cooler or condenser (not shown) via plumbing  92  while oil is returned in oil return line  89  with flow control device  89   a . Flow control device  89   a  may be an orifice or may be an electronic valve. The oil return line desirably returns to the crankcase, where oil is needed to lubricate the working parts of the compressor, especially the pistons, cylinders, shoes and drive shaft. The check valve may also have an oil return line  91  with flow control device  91   a  to return oil to the crankcase. Either or both of the flow control devices  89   a  and  91   a  may be orifices or electronic valves, such as solenoid valves, that may be remotely opened or closed via controller  95 . 
     Second Embodiment 
       FIG. 2  depicts another embodiment of a variable displacement compressor  11 , which is similar to the embodiment of  FIG. 1 .  FIG. 2  is depicted with somewhat different arrangements of plumbing, and is also shown in a state in which the swashplate  20  is at its minimum angle. In this view, the swashplate in now almost vertical, and piston  28  and shoes  54  have moved to the left, revealing more of cylinder bore  26 . In this position, there will be little compression of refrigerant, but all the working components within the crankcase chamber still require energy from the vehicle engine as the drive shaft continues to turn, and lubrication to prevent wear on all the moving parts. In the embodiment shown in  FIG. 2 , the solenoid valve  60  is shown in the closed position, with flow control element  66  preventing communication between chamber  76  (P s ) and chamber  74  (P c ), and flow control element  64  preventing communication between chamber  72  (P d ) and chamber  70  (P os ). Orifice  77  allows a small pressure flow between P d  and P c . 
     There may be no current from the control system  95  to the solenoid coil  78 , and control system  95  may communicate this low load to the vehicle powertrain control module or to a vehicle controller. In this embodiment, the refrigerant leaves the discharge chamber  32  and is directed first to an oil separator  88  and then to a check valve  105  before leaving via plumbing  107  to the downstream air conditioning components, such as a gas cooler. The oil separated by the oil separator  88  may return via line  89  and flow control device  89   a  to the crankcase chamber  16 . Flow control device  89   a  may be an orifice or may be an electronic valve. Oil may also return to the crankcase from the check valve  105  via return line  101  and flow control device  103 , which may be an orifice or may be an electronic valve, such as a solenoid valve. The pressure in the oil separator may be communicated to the valve  69  via line  90 . 
     Check Valves 
       FIGS. 3 and 4  show details of the check valve  84  shown in  FIG. 1 . This check valve checks flow until the pressure, in this case discharge pressure, P d , reaches a certain level. The check valve may be tailored by selection of spring  113  to allow flow only when the pressure has reached the desired level. In this embodiment, the check valve may be installed within the walls of rear chamber  24 .  FIG. 3  depicts the check valve closed, while  FIG. 4  depicts the valve open, allowing refrigerant to flow via passage  86 . In  FIG. 3 , the valve is closed, with flow control element  111 , urged by spring  113 , preventing passage of refrigerant from the discharge chamber  32  through piping  86 . Even in this configuration, however, there may be a narrow passage or orifice  115  within the flow control element  111 , to allow condensed oil to flow through orifice  115  to oil return line  91 . Orifice  115  is desirably narrow, about 0.1 mm, but may range from about 0.1 mm to about 0.4 mm. 
       FIG. 4  depicts the check valve  84  in an open position, indicating that the discharge pressure of the refrigerant has reached a point sufficient to overcome the spring  113 , which is shown in a compressed state. Refrigerant can now freely pass through piping  86 . Check valve  84  or its flow control element  111  may also use O-rings as shown, or other sealing devices as needed, such as piston rings. 
     Solenoid Valves 
       FIG. 5  is a larger, cross-sectional view of a preferred embodiment of a solenoid valve  60  used in  FIGS. 1 and 2 . As stated above, the valve is a very fast acting solenoid valve, preferably controlled by a PWM routine using 400 Hz with the microprocessor-based controller  96 . The output of the controller is current to solenoid coil  78 . The current causes stem  62  to move up or down, along with its flow control elements  64  and  66 . The stem is also urged in one direction by a larger spring  80  and in an opposite direction, by smaller spring  82 . When there is no current to the coil, spring  80  with a larger spring constant is able to overcome spring  82  with a smaller spring constant and close the valve. 
     Within the valve are five chambers,  68 ,  70 ,  72 ,  74  and  76 . The chambers receive pressures as discussed above, and are separated by valve head  69  and valve body internal walls  71 ,  73 ,  75 . The internal walls have orifices as shown to allow passage of the stem  62  and also to allow pressure to communicate from one chamber to another. There is also a tube  67  to communicate P c  from chamber  74  to chamber  68 . The valve has orifices  90   a  for receiving an oil separator pressure,  58   a  for receiving a discharge pressure,  46   a  for receiving a crankcase pressure, and  56   a  for receiving a suction pressure. Valve head  69  is movable within the valve, urged downward by spring  82 , upward by spring  80 , and upward or down by stem  62 . The valve is shown in the maximum open position, coil  78  at the maximum current, with flow control element  64  as far down as possible, allowing pressure to pass from chamber  70  (P os ) to chamber  72  (P d ) and preventing passage from chamber  72  (P d ) to chamber  74  (P c ). With flow control element  66  also at its lowest position, there is the greatest communication possible between chambers  74  (P c ) and  76  (P s ). In this position, there will be the greatest possible difference between the suction pressure and the discharge pressure. This will push the swashplate to its maximum angle, and the pistons will reciprocate to the maximum extent, thus compressing as much refrigerant as possible for the air conditioning system. 
     Alternate Embodiments 
       FIG. 6  depicts an alternate combination of the compressor and controls. Compressor  130  and control valve  132  are connected as described above, with pressures from the compressor communicated to the valve by passages  137 ,  139  and  141 , respectively from the compressor suction chamber  136 , crankcase chamber  134 , and discharge chamber  138 . Passage  137  includes auxiliary passage  135  from the suction chamber. Valve  132  comprises coil  140 , stem  142  and flow control elements  142   a  and  142   b , as described above. Valve  132  also comprises chambers  143 ,  145 ,  147 ,  149  and  151 , the chambers separated by movable valve head  144  and valve body internal walls  146 ,  148 ,  150 . Tubing  67  communicates P c  from chamber  149  to chamber  143 . Passage  148   a  allows for a small flow from chamber  149  at P c , to chamber  147 , at P d . Control system  195  controls valve  132 . 
     In this embodiment, refrigerant leaves discharge chamber  138  via line  155  to check valve  152 . Check valve  152  may also be equipped with a return line  154  to return oil to the crankcase  134 . Line  154  may have a flow control device  153  to regulate the flow of return oil. Flow control device  153  may be an orifice or may be an electronic control valve controlled by control system  195 . After check valve  152 , the refrigerant may flow via line  157  to oil separator  158  and then to the refrigeration system via line  160 . In one embodiment line  160  is preferably tubing about 5 mm in diameter, but tubing of other diameters may also be used, so long as too great a pressure drop is not induced in conveying the hot, compressed gas from the compressor to the other components of the vehicle refrigeration system. 
     The oil separator may have an oil return line  156  and flow control device  156   a  to return oil to the compressor crankcase section  134 . Flow control device  156   a  may be an orifice or may be an electronic control valve controlled by control system  195 . 
     In one embodiment, the flow control device  156   a  is an orifice from about 0.1 mm to about 0.5 mm, preferably about 0.2 mm in diameter. P os  may be communicated to chamber  145  via tubing  159  with flow control device  159   a , which may be an orifice or may be an electronic control valve. In one embodiment, oil return line  156  is omitted and all oil from the oil separator  158  is returned via line  159 , preferably about 3 mm in diameter, to chamber  145  in valve  132 . In one embodiment, the oil return line  154  from check valve  152  is preferably about 3 mm in diameter; other diameter lines may be used. 
       FIG. 7  depicts another arrangement of lines for the compressor  130 , the oil separator  158  and the check valve  152 . In this embodiment, the discharge chamber  138  connects to the oil separator  158  via line  163 , the oil separator also having oil return line  167  with flow control device  167   a  to return oil to crankcase chamber  134 . After leaving the oil separator  158 , refrigerant flows to check valve  152  via line  161 , with an oil return line  165  to the oil separator. Refrigerant then leaves the check valve on its way to the downstream air conditioning equipment. The compressor, check valve, and oil separator of  FIG. 7 , as well as other configurations of a check valve, oil separator, and return line, may be used with three-way valves as well as four-way control valves. 
     Three-Way Control Valves 
     The above embodiments have dealt mostly with four-way control valves. Other embodiments may use three-way control valves. Three way control valves may be used, for example, if the above-mentioned pressures, P d  (discharge pressure), P c  (crankcase pressure), and P s  (supply pressure) are used to control the variable displacement of the compressor by controlling the angle of the swashplate or other controlling device, such as a wobbler plate. Three-way control valves may also be used if an auxiliary pressure is used to help control the pressures. An auxiliary pressure, P a  that has been found useful is one that results from a pressure drop from P d , the discharge pressure. In one embodiment using R134a, P d  is from about 5 to 20 bars (1 bar is 1 atmosphere of pressure), while P a  is from about 0.1 to about 1 bar below that of P d . In an embodiment using R134a, a pressure that has the requisite value for the auxiliary pressure may be obtained by tapping the discharge pressure after it has gone through the control valve and associated piping, and has dropped by about 0.5 bar to about 1 bar. In a system using CO 2 , P d  is from about 50 to 160 bars, while P a  is from about 0.1 to about 10 bars less than that of P d . In a CO 2  embodiment, a pressure that has the requisite value for the auxiliary pressure may be obtained by tapping the discharge pressure after it has gone through the control valve and associated piping and has dropped by about 0.1 bar to about 10 bars. 
     A three way control valve using P d  and P a , and also using P c , is depicted in  FIGS. 8 and 9 . Three-way control valve  200  is similar in some respects to the four-way control valve described above, but is less complicated. Three-way control valve  200  has a coil  201 , stem  202  with flow control elements  204  and  206 , a first strong spring  207 , second spring  209 , and an internal spring  208 . Valve body internal walls  215 ,  217  have orifices to allow passage of stem  202  and also pressures from chambers  214 ,  216 , and  218 . Valve  200  receives pressures from orifices  222  (P c ),  224  (P a ), and  226  (P d ). Internal spring  208  may be used as an auxiliary spring in balancing the forces that move valve stem  202  in controlling the valve. Placed between fixed internal wall  215  and movable wall  213 , spring  208  may sometimes act to oppose the motion of stem  202  and sometimes act to reinforce the motion of stem  202 , depending on the force applied by coil  201  and springs  207 ,  209 . 
     In communicating pressures from the compressor to the control valve, tubing may be used, or channels internal to the compressor may be used to connect directly to the valve. Thus, discharge pressure may connect from the discharge chamber of the compressor to chamber  216  via orifice  226  and tubing  225 . Tubing  225  is desirably large enough to communicate P d  without an appreciable drop in pressure. An auxiliary pressure P a  may result if tubing  225  and orifice  224 , communicating between discharge pressure P d  and chamber  214 , have diameters small enough to restrict flow and to induce a small pressure drop. Tubing having a diameter of preferably 3–4 mm is sufficient for this purpose. Other tubing having a diameter from about 1–5 mm may also be used. 
       FIG. 8  depicts the valve in maximum open position, with maximum current to coil  201 , and stem  202  and flow control elements  204 ,  206  in their furthest upward positions, overcoming the force of strong spring  207 . Flow control element  204  prevents flow between chambers  218  and  216 , while flow control element  206  allows maximum flow or pressure equalization between chambers  216  and  214 . In this embodiment, this position minimizes the difference between P a  and P d , and prevents communication between P c  and P d , thus allowing for maximum compressing of refrigerant in the compressor.  FIG. 9  depicts the same valve  200 , now in the off position. In this position, coil  201  receives the minimum or no current. Strong spring  207  overcomes spring  209 , forcing stem  202  downward in  FIG. 9 , and allowing communication between chambers  216  and  218 , but not between chambers  214  and  216 . This allows for the minimum possible compression, and tends to equalize the discharge and crankcase pressures, thus moving the swashplate to a position nearly perpendicular to the longitudinal axis of the drive shaft, and parallel or nearly parallel to a plane perpendicular to the longitudinal axis of the drive shaft. In the three-way valve depicted in  FIGS. 8 and 9 , there may be a small passage between chambers  216  and  218 , from about 0.05 mm to about 0.6 mm. The passage is provided as either a passage  227  in chamber wall  217  (see  FIG. 8 ) or a passage  205  in flow control element  205  ( FIG. 9 ). Passages  227  or  205  allow oil from the compressor discharge to return to the crankcase. 
     With respect to the operation of the solenoid valves in  FIGS. 8 and 9 , the pressure difference across chamber wall  217  is the discharge pressure P d  minus the crankcase pressure, P c . These two pressures are inversely related. When cooling demand is high, P d  will be high, P a  will be low, and P c  will be low, and P c  will be very close to P s  When cooling demand is low, P d  may be vented to the crankcase through the control valve raising P c , while P a  will drop only little from P d . In this case, therefore, P c  will be high and P a  may be low. In other embodiments, the three-way control valve may use the three chambers for P d , P c , and P d . Springs may be designed with specific spring constants for the pressures and pressure ranges used. It will be appreciated that there are many other ways to use the three-way control valves depicted in  FIGS. 8 and 9 . For instance, one alternate embodiment may use the three chambers, in order, for P s , P c  and P d , with a single control element to regulate, as desired, the orifice between the chamber with P s  and the chamber with P c , or the orifice between the chamber with P c  and the chamber with P d . The source of the discharge pressure may be the oil separator return line, with the oil return running through the valve, through the P c  chamber, and returning oil to the crankcase. In a preferred embodiment, there may be a small orifice, from about 0.05 mm to about 0.6 mm, between the chamber with P d  and the chamber with P c . 
     In the example above, the chamber with P s  was used for sensing only. An equivalent is to use a two-way valve, without a chamber for P s , and with appropriate compensation from springs or with appropriate input from the control system, in which the oil returns through the control valve. In one embodiment, there may be a narrow orifice from the oil-return or P d  chamber to the P c  chamber, the orifice as stated above, from about 0.05 mm to about 0.6 mm. It may also be possible to instead place the orifice in the control element that seals the control orifice, as depicted in  FIG. 4 , such that there is always at least a narrow orifice for oil to return from the oil return line to the crankcase chamber through the valve. 
     Embodiments with P a  and a Three-Way Valve 
       FIGS. 10 and 11  are cross sectional views of a compressor  240  using a three-way control valve  200 .  FIG. 10  depicts compressor  240  with upper housing  248   a  and lower housing  248   b , control valve  200 , and controller  290 , as described above in the description for controller  95 . The compressor has a drive sheave  242 , drive shaft  244 , swash plate  246 , shown at a minimum angle to the drive shaft, and valve plate  250 , defining a crankcase chamber  252 , suction chamber  254  and discharge chamber  256 . After refrigerant leaves the discharge chamber and goes to the downstream refrigeration system (not shown), the refrigerant returns from the evaporator at a relatively low pressure, the pressure of the evaporator, to suction port  258 . There may also be a passage  262  with a control orifice  263  between suction chamber  254  and crankcase chamber  252 . 
     In the embodiment of  FIG. 10 , flow from the suction port  258  to the suction chamber  254  is governed by a suction shut-off valve  280  with upstream chamber  282  in compressor lower housing  248   b . Suction shut-off valve  280  is shown in the closed position, preventing low-pressure refrigerant from passing from suction port  258  to suction chamber  254 . To prevent oil starvation, there may also be a small passage or orifice  284  in shut-off valve  280  allowing small amounts of oil to flow from the control valve discharge port, through plumbing  270  to valve  280 , and to suction chamber  254 . This passage may be from about 0.05 to about 0.6 mm in diameter, preferably about 0.1 to about 0.15 mm. The valve  280  may also have a spring  286  urging the valve closed and a second spring  287  on the opposite side urging the valve open. Spring  286  preferably has a spring constant slightly higher than the spring constant of spring  287 , biasing the valve  280  closed. 
     Line  268  communicates P s  to suction port  258 . Line  270  communicates P a  to upstream chamber  282  of shut-off valve  280 , thus controlling the position of valve  280 . Shut-off valve  280  will thus be biased closed by spring  286  and P s , with spring  287  and P a  opposed, tending to open valve  280 . In the embodiment of  FIG. 10 , valve  200  is open, allowing pressure equalization between P d  and P c , and tending to push the swashplate  246  to a minimum angle, and thus a minimum flow, in  FIG. 10 . 
     In  FIG. 11 , there is more demand for air conditioning, and movement of the internal components has occurred. The position of the valve  200  is close to that depicted in  FIG. 8 , with no communication between chambers  216  and  218 . The discharge pressure is not communicated to the crankcase, but rather is used fully for cooling. As a result, P d  increases, while P a  decreases, overcoming the force of spring  286 . Shut-off valve  280  in  FIG. 11  moves upward, allowing communication between suction port  258  and suction chamber  254 . There will now be a much greater difference between the suction and discharge pressures, and the swashplate will move to a greater angle to the drive shaft of the compressor. 
     Various embodiments of the invention have been described and illustrated. However, the description and illustrations are by way of example only. Other embodiments and implementations are possible within the scope of this invention and will be apparent to those of ordinary skill in the art. Therefore, the invention is not limited to the specific details, representative embodiments, and illustrated examples in this description. Accordingly, the invention is not to be restricted except in light as necessitated by the accompanying claims and their equivalents.