Abstract:
An energy-efficient air conditioning control method regulates the capacity of a variable capacity refrigerant compressor based on the compressor suction and discharge pressures and a measure of the ambient temperature. A target suction pressure is selected based on the ambient temperature and the sensed discharge pressure, and the capacity of the compressor is adjusted as required to attain the target suction pressure. In a first embodiment of the control method, the ambient temperature is used to select a target evaporator outlet air temperature, which is used along with the sensed discharge pressure to select the target suction pressure, while in a second embodiment of the control method, the target suction pressure is selected directly on the basis of the ambient temperature and the sensed discharge pressure.

Description:
PRIOR APPLICATION 
     This application claims the benefit of prior Provisional Patent Application Serial No. 60/378,849 filed May 8, 2002. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to an energy-efficient control method for a variable capacity refrigerant compressor of an air conditioning system. 
     BACKGROUND OF THE INVENTION 
     Variable capacity refrigerant compressors have been utilized in both manual and automatic vehicle air conditioning systems, primarily to reduce engine load disturbances associated with compressor clutch cycling. In a typical implementation, the compressor includes one or more pistons coupled to a tiltable wobble plate or swash plate, and a pneumatic or electromagnetic control valve for adjusting the pressure in a crankcase of the compressor to control the compressor capacity. The system control strategy usually involves adjusting the compressor capacity to maintain a predetermined low-side refrigerant condition (refrigerant suction pressure or evaporator outlet air temperature, for example) that provides maximum cooling without evaporator icing, and using a high-side pressure switch to disengage the compressor clutch if the refrigerant discharge pressure becomes too high. The inlet air may consist of outside air or recirculated cabin air, and the temperature of the discharge air is typically controlled by adjusting a mechanism (such as an air mix door) that reheats a portion of the conditioned air. 
     While the above-described control strategy is simple and reasonably effective, it has been recognized that the energy efficiency of the system could be significantly improved by increasing the usage of recirculated cabin air and reducing the compressor capacity in a way that provides adequate dehumidification while minimizing reheating of the conditioned air. However, since energy-efficient controls typically require a number of external sensors for measuring system and ambient parameters, development efforts have primarily been focused on high-end automatic climate control systems that usually include such sensors anyway. Accordingly, what is needed is a compressor capacity control method that provides energy-efficient operation at a low cost. 
     SUMMARY OF THE PRESENT INVENTION 
     The present invention is directed to an improved and energy-efficient control method for a variable capacity refrigerant compressor of an air conditioning system, where the control is based on the compressor suction and discharge pressures and a measure of the ambient temperature. According to the invention, a target suction pressure is selected based on the ambient temperature and the sensed discharge pressure, and the capacity of the compressor is adjusted as required to attain the target suction pressure. In a first embodiment, the ambient temperature is used to select a target evaporator outlet air temperature, which is used along with the sensed discharge pressure to select the target suction pressure, while in a second embodiment the target suction pressure is selected directly on the basis of the ambient temperature and the sensed discharge pressure. The control method is conveniently and cost-effectively carried out in an implementation where the compressor includes a capacity control valve with integral suction and discharge pressure sensors, and integral signal conditioning circuitry. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The present invention will now be described, by way of example, with reference to the accompanying drawings in which: 
     FIG. 1 is a diagram of a vehicle air conditioning system according to this invention, including a variable capacity refrigerant compressor, an electrically activated capacity control valve, and a microprocessor-based control unit. 
     FIG. 2 is a cross-sectional view of the capacity control valve of FIG.  1 . 
     FIG. 3 is a block diagram of a compressor capacity control method carried out by the control unit of FIG. 1 according to a first embodiment of this invention. 
     FIG. 4 is a block diagram of a compressor capacity control method carried out by the control unit of FIG. 1 according to a second embodiment of this invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, the reference numeral  10  generally designates a vehicle air conditioning system, including a variable capacity refrigerant compressor  12  coupled to a drive pulley  14  via an electrically activated clutch  16 . The pulley  14  is coupled to a rotary shaft of the vehicle engine (not shown) via drive belt  18 , and the clutch  16  is selectively engaged or disengaged to turn the compressor  12  on or off, respectively. The compressor capacity control is effectuated by an electrically activated capacity control valve  17  mounted in the rear head of compressor  12 . In the illustrated embodiment, the compressor  12  has an internal bleed passage coupling its crankcase to the suction port  30 , and the capacity control valve  17  selectively opens and closes a passage between the crankcase and the discharge port  28  to control the crankcase pressure, and therefore, the compressor pumping capacity. As described below in reference to FIG. 2, the capacity control valve  17  additionally includes integral suction and discharge pressure sensors and signal conditioning circuitry coupled to the pressure sensors. 
     In addition to the compressor  12 , the system  10  includes a condenser  20 , an orifice tube  22 , an evaporator  24 , and an accumulator/dehydrator  26  arranged in order between the compressor discharge port  28  and suction port  30 . A cooling fan  32 , operated by an electric drive motor  34 , is controlled to provide supplemental air flow through the condenser  20  for removing heat from condenser  20 . The orifice tube  22  allows the cooled high pressure refrigerant in line  38  to expand in an isenthalpic process before passing through the evaporator  24 . The accumulator/dehydrator  26  separates low pressure gaseous and liquid refrigerant, directs a gaseous portion to the compressor suction port  30 , and acts as a reservoir for the reserve refrigerant charge. In an alternative system configuration, the orifice tube  22  is replaced with a thermostatic expansion valve (TXV); in this case, the accumulator/dehydrator  26  is omitted, and a receiver/drier (R/D) is inserted in line  38  upstream of the TXV to ensure that sub-cooled liquid refrigerant is supplied to the inlet of the TXV. 
     The evaporator  24  is formed as an array of finned refrigerant conducting tubes, and an air intake duct  40  disposed on one side of evaporator  24  houses an inlet air blower  42  driven by an electric blower motor  43  to force air past the evaporator tubes. The duct  40  is bifurcated upstream of the blower  42 , and an inlet air control door  44  is adjustable as shown to control inlet air mixing; depending on the door position, outside air may enter blower  42  through duct leg  44   a , and passenger compartment air may enter blower  42  through duct leg  44   b.    
     An air outlet duct  52  disposed on the downstream side of blower  42  and evaporator  24  houses a heater core  54  formed as an array of finned tubes that conduct engine coolant. A temperature control door  56  pivoted at a point near the heater core  54  is adjustable as shown to control what proportion of air exiting evaporator  24  must pass through the heater core  54 . The heated and un-heated air portions are mixed in a plenum portion  62  of outlet duct  52  downstream of heater core  54  and temperature control door  56 , and a pair of mode control doors  64 ,  66  are adjustable as shown to direct the mixed air through one or more outlets, including a defrost outlet  68 , a panel outlet  70 , and a heater outlet  72 . 
     In the illustrated embodiment, the compressor clutch  16 , the capacity control valve  17 , the condenser motor  34 , the blower motor  43 , the air inlet door  44 , the temperature control door  56 , and the mode control doors  64  and  66  are controlled by a microprocessor-based control unit  90 . For convenience, actuators for positioning the doors  44 ,  56 ,  64  and  66  have been omitted in FIG.  1 . As indicated in FIG. 1, the control is carried out in response to a number of input signals including the refrigerant suction and discharge pressure signals SP, DP developed by the sensors within capacity control valve  17 , the ambient air temperature AT, the in-car air temperature IC, the evaporator outlet air temperature EOAT, a set temperature TSET, and the air conditioning request status AC. The EOAT signal is developed by a temperature sensor  92  positioned on the evaporator  24  or in its outlet air stream, and the temperatures AT and IC are developed by suitably positioned temperature sensors  94  and  96 . The TSET and AC signals are provided by a driver interface panel (DIP)  98 , including a mechanism such as a knob for selecting a desired cabin temperature and a pair of switch inputs for activating normal or energy-efficient air conditioning. The desired cabin temperature is indicated by the TSET signal, and the air conditioning request (i.e., AC off, normal AC, or energy-efficient AC) is indicated by the AC signal. 
     The present invention is specifically directed to a control of the compressor capacity by suitable modulation of the capacity control valve  17 , and such control is described in detail below in reference to FIGS. 3 and 4. In other respects, the functionality of control unit  90  may be substantially conventional in nature. For example, the condenser motor  34  may be activated in response to the discharge pressure DP, the compressor clutch  16  is normally activated whenever air conditioning operation is requested, and the blower motor  43  and the doors  44 ,  56 ,  64  and  66  are controlled by an automatic climate control algorithm based on TSET, AT, IC and various measured and/or estimated parameters. The automatic climate control algorithm essentially computes the cooling requirement of the vehicle, and retrieves pre-programmed command settings for the blower speed, the air discharge mode, the cabin air recirculation level, and the air discharge temperature. The commanded air discharge mode is used to position the mode control doors  64  and  66 , the commanded blower speed is used to activate blower motor  43 , and the commanded cabin air recirculation level is used to position the air inlet control door  44 . The commanded air discharge temperature is compared with a measured discharge temperature (such as an air duct temperature) to produce an error signal that is used to control the temperature control door  56 . 
     Since the present invention is directed to an energy efficient control of the compressor capacity based on ambient temperature AT, there will typically be more reserve capacity than occurs in a conventional control where the capacity is regulated to maintain maximum cooling capability without evaporator icing. Consequently, the automatic climate control will automatically respond by commanding less re-heating by heater core  54  and a higher level of cabin air recirculation. However, if the driver requests normal air conditioning, or if the commanded air discharge temperature cannot be achieved during energy-efficient air conditioning (as may occur under conditions of high humidity), the control unit  90  regulates the compressor capacity to provide maximum cooling. Thus, the compressor  12  is operated in an energy-efficient mode if enabled by the driver, so long as it is possible to satisfy the driver set temperature TSET. 
     As mentioned above, the capacity control valve  17  is electrically controlled to selectively open and close a passage between the compressor crankcase and the discharge port  28  to control the compressor pumping capacity, and includes integral suction and discharge pressure sensors  142 ,  144  and signal conditioning circuitry coupled to the pressure sensors  142 ,  144 . Referring to FIG. 2, the capacity control valve  17  includes three ports  152 ,  154  and  156  that are respectively placed in communication with chambers containing the compressor suction, crankcase and discharge pressures. The crankcase and discharge ports  154  and  156  are formed in a pressure port  160 , with the discharge port  156  being defined by the inboard end of a central axial bore  162  passing through pressure port  160 . A screen  161  prevents any foreign matter from entering the discharge port  156 . The pressure port  160  is secured to a housing shell  164  by a weld  166 , and a plunger  168  partially disposed within the bore  162  is axially positioned such that its inboard end either opens or closes a portion of bore  162  that couples the crankcase and discharge ports  154  and  156 . The housing shell  164  encloses an electrically activated solenoid assembly  171  for positioning the plunger  168  within the bore  162 , including a spring  172  for biasing the plunger  168  to a retracted position as shown in which the plunger  168  engages the housing piece  184  and refrigerant is permitted to flow from the discharge port  156  to the crankcase port  154 . The solenoid assembly  171  includes a set of permanent magnets  174 ,  176  disposed between inner and outer pole pieces  178  and  180 , and a cup-shaped spool  182  carrying a movable coil  140 . The spool  182  is secured to an outboard portion of plunger  168 , and the housing piece  184  defines a cavity  186  outboard of the spool  182 . Activating the coil  140  produces a force that opposes the bias of spring  172  and moves the plunger  168  to an extended position (limited by the stop  196 ) in which its outboard end blocks the portion of bore  162  between discharge port  156  and crankcase port  154 . A central axial bore  168 b through plunger  168  couples the discharge port  156  to the cavity  186 , and a passage  210  in housing piece  184  couples the cavity  186  to the interior of discharge pressure sensor  144  so that the pressure sensor  144  measures the compressor discharge pressure. The passage  208  couples the suction port  152  to the interior of pressure sensor  142  so that the pressure sensor  142  measures the compressor suction pressure. Significantly, the opening of passage  210  is directly aligned with the plunger bore  168   b  so that the discharge pressure sensor  144  is in direct communication with discharge port  156  regardless of the position of plunger  168 . 
     The pressure sensors  142 ,  144  are retained with respect to the housing piece  184  by a spacer element  200 . The sensors  142 ,  144  are preferably conventional stainless steel pressure sensors, each having a diaphragm that is subject to flexure due to the pressure differential across it, although other types of pressure sensors could alternatively be used. The mechanical strain associated with the flexure is detected by a piezo-resistor circuit (not depicted) formed on the outboard surface of respective sensor diaphragm, and flexible conductors  216 ,  218  couple the respective piezo-resistor circuits to bond pads  220 ,  222  formed on a circuit board  202 . A connector  224  is secured to the outboard end of housing piece  184  by swaging for example, and a set of terminals  230 ,  232  passing through the connector  224  are soldered to the bond pads  220 ,  222 . An O-ring  234  compressed between the connector  224  and the housing piece  184  seals the enclosed area  236  from environmental contaminants, and also isolates the area  236  from barometric pressure. Accordingly, the pressures measured by the sensors  142  and  144  can be calibrated to indicate the absolute pressure of the refrigerant in the respective suction and discharge passages  208  and  210 , as opposed to a gauge pressure that varies with ambient or barometric pressure. 
     FIG. 3 depicts a compressor capacity control method according to a first embodiment of this invention. Referring to FIG. 3, the blocks  240 ,  242  and  244  develop an evaporator outlet air temperature target EOAT_TAR on line  246 . The block  240  is a look-up table that develops energy-efficient evaporator outlet air temperature values as a function of ambient air temperature AT on line  248 , whereas a pre-programmed evaporator outlet air temperature set point EOAT_SP for override operation is provided on line  250 . The lines  248  and  250  are supplied as inputs to selector switch  242 , which supplies one of the inputs to line  246 . In energy-efficient operation, the selector switch  242  supplies the output of block  240  to line  246  as indicated in FIG. 3; in an override condition, the selector switch is activated to supply EOAT_SP to line  246 . Activation of selector switch  242  is controlled by the override logic block  244 , which is responsive to automatic climate control parameters including the air conditioning request status AC, the discharge temperature error DTE (i.e., the difference between the commanded and measured discharge air temperatures), the commanded cabin air recirculation level (RECIRC), and the position TDPOS of the temperature control door  56 . If the inputs indicate that the driver is requesting normal AC, or that the driver is requesting energy efficient AC, but the set temperature TSET cannot be achieved after a given time delay, the override logic block  244  activates the selector switch  242  via line  252  to set EOAT_TAR equal to EOAT_SP. An inability to achieve TSET is detected when there is a persistent discharge temperature error (DTE) with full cabin air recirculation and no re-heating of the conditioned air. 
     The summing junction  262  forms a difference between the selected EOAT_TAR value and the measured EOAT value to form a temperature error EOAT_ERR. The temperature error EOAT_ERR is supplied as an input to PID (proportional-plus-integral-plus-differential) block  264 , which forms a suction pressure control signal SP_PID for reducing EOAT_ERR. For example, if EOAT_ERR indicates that EOAT is higher than EOAT_TAR, SP_PID will tend to reduce in value, which requests the system  10  to produce a lower suction pressure for increased cooling of the conditioned air. 
     Since the suction pressure control signal SP_PID produced by PID block  264  may become too low from a systems perspective when EOAT_ERR is large, the block  266  sets the suction pressure target SP_TAR on line  268  to the greater (MAX) of SP_PID and a limit value SP_LMT determined by the blocks  270 - 276 . The limit value SP_LMT serves to limit the compressor discharge pressure, and is determined based on the compressor speed CS, the measured discharge pressure DP, and optionally an externally supplied discharge pressure limit value EXT_LMT. The compressor speed CS may be determined based on the engine speed ES and the known drive pulley ratio. The block  270  is a table of discharge pressure limit values LMT as a function of compressor speed CS, and the block  272  sets the discharge pressure limit DP_LMT on line  278  equal to the lesser (MIN) of LMT and EXT_LMT. The external limit EXT_LMT may be developed, for example, by a powertrain or engine controller for purposes of limiting the engine load imposed by system  10  during vehicle acceleration. The block  274  forms a difference between DP_LMT and the measured discharge pressure DP to form a discharge pressure error term DP_ERR on line  280 . The discharge pressure error DP_ERR is supplied as an input to PID block  276 , which forms a corresponding suction pressure control signal SP_LMT on line  282  for reducing DP_ERR whenever DP exceeds DP_LMT. Specifically, if DP exceeds DP_LMT, the output of PID block  276  will tend to increase and dominate suction pressure target SP_TAR on line  268 , which will tend to drive the compressor discharge pressure downward. 
     The summing junction  284  forms a difference between the suction pressure target value SP_TAR and the measured suction pressure SP to form the suction pressure error SP_ERR on line  286 . The suction pressure error SP_ERR is supplied as an input to PID block  288 , which forms a PWM duty cycle control signal DC on line  290  for reducing SP_ERR. For example, if SP_ERR indicates that SP is higher than SP_TAR, DC will tend to increase in value to increase the compressor pumping capacity. However, the output of PID block  288  is subject to limitation based on the compressor speed CS, as indicated by blocks  292  and  294 , with the limited duty cycle command on line  296  being supplied to a coil driver (CD)  298  for the capacity control valve coil  140 . 
     The PID blocks  264 ,  276  and  288  preferably each incorporate an anti-wind-up mechanism to limit the integral component of their output when the duty cycle output of PID block  288  on line  290  approaches 0% or 100%. Allowing the PID outputs to increase further under such conditions is undesirable, as it would tend to saturate the entire control and degrade the control performance. And of course, the PID functions could be replaced with other known control strategies, such as fuzzy logic or neural-network controls. 
     FIG. 4 depicts a compressor capacity control method according to a second embodiment of this invention. Referring to FIG. 3, the blocks  350 ,  352  and  354  develop a suction pressure target SP_TAR on line  356 . The block  350  is a look-up table that develops energy-efficient suction pressure values as a function of ambient air temperature AT on line  358 , whereas a pre-programmed suction pressure set point SP_SP for override operation is provided on line  360 . The lines  358  and  360  are supplied as inputs to selector switch  352 , which supplies one of the inputs to line  356 . In energy-efficient operation, the selector switch  352  supplies the output of block  350  to line  356  as indicated in FIG. 4; in an override condition, the selector switch  352  is activated to supply SP_SP to line  356 . Activation of selector switch  352  is controlled by the override logic block  354 , which is responsive to automatic climate control parameters including the air conditioning request status AC, the discharge temperature error DTE (i.e., the difference between the commanded and measured discharge air temperatures), the commanded cabin air recirculation level (RECIRC), and the position TDPOS of the temperature control door  56 . If the inputs indicate that the driver is requesting normal AC, or that the driver is requesting energy efficient AC, but the set temperature TSET cannot be achieved after a given time delay, the override logic block  354  activates the selector switch  352  via line  362  to set SP_TAR equal to SP_SP. An inability to achieve TSET is detected when there is a persistent discharge temperature error (DTE) with full cabin air recirculation and no re-heating of the conditioned air. 
     Since the target suction pressure SP_TAR on line  356  may become too low from a systems perspective when the ambient air temperature AT is high, the block  366  sets the suction pressure target SP_TAR on line  368  to the greater (MAX) of the value on line  354  and a limit value SP_LMT on line  382  determined by the blocks  370 - 376 . The limit value SP_LMT serves to limit the compressor discharge pressure, and is determined based on the compressor speed CS, the measured discharge pressure DP, and optionally an externally supplied discharge pressure limit value EXT_LMT. The block  370  is a table of discharge pressure limit values LMT as a function of compressor speed CS, and the block  372  sets the discharge pressure limit DP_LMT on line  378  equal to the lesser (MIN) of LMT and EXT_LMT. As indicated above, the external limit EXT_LMT may be developed, for example, by a powertrain or engine controller for purposes of limiting the engine load imposed by system  10  during vehicle acceleration. The block  374  forms a difference between DP_LMT and the measured discharge pressure DP to form a discharge pressure error term DP_ERR on line  380 . The discharge pressure error DP_ERR is supplied as an input to PID block  376 , which forms a corresponding suction pressure control signal SP_LMT on line  382  for reducing DP_ERR whenever DP exceeds DP_LMT. Specifically, if DP exceeds DP_LMT, the output of PID block  376  will tend to increase and dominate suction pressure target SP_TAR on line  368 , which will tend to drive the compressor discharge pressure downward. 
     The summing junction  384  forms a difference between the suction pressure target value SP_TAR and the measured suction pressure SP to form the suction pressure error SP_ERR on line  386 . The suction pressure error SP_ERR is supplied as an input to PID block  388 , which forms a PWM duty cycle control signal DC on line  390  for reducing SP_ERR. For example, if SP_ERR indicates that SP is higher than SP_TAR, DC will tend to increase in value to increase the compressor pumping capacity. However, the output of PID block  388  is subject to limitation based on the compressor speed CS, as indicated by blocks  392  and  394 , with the limited duty cycle command on line  396  being supplied to a coil driver (CD)  398  for the capacity control valve coil  140 . 
     As with the embodiment of FIG. 3, the PID blocks  376  and  388  preferably each incorporate an anti-wind-up mechanism to limit the integral component of their output when the duty cycle output of PID block  388  on line  390  approaches 0% or 100%. Allowing the PID outputs to increase further under such conditions is undesirable, as it would tend to saturate the entire control and degrade the control performance. And of course, the PID functions could be replaced with other known control strategies, such as fuzzy logic or neural-network controls. 
     In summary, this invention provides an energy-efficient control method for a vehicle air conditioning system including an electrically variable capacity refrigerant compressor. The system can be configured as an automatic climate control as described in the illustrated embodiment, or as a manually controlled system in which the driver manipulates interface panel knobs and/or levers to position the doors  44 ,  56 ,  64 ,  66  and to control the blower speed. Additionally, the capacity control valve  17  may be configured to provide less or more functionality than shown; for example, the pressure transducers may be provided externally, or at least some of the functionality of the control unit  90  may be performed by control circuitry resident within the control valve  17 . Also, the refrigerant pressures may be estimated or indirectly determined based on measured temperatures, if desired. Moreover, the control methods of this invention are also applicable to air conditioning systems where the compressor is driven by an electric motor (in which case, the compressor capacity is adjusted by changing the motor speed), and to clutchless systems where the pulley  14  is rigidly coupled to the compressor drive shaft (in which case, the compressor is effectively turned off by reducing its capacity to a minimum value). Many other variations are also possible, and it should be recognized that control methods incorporating such modifications may fall within the intended scope of this invention, which is defined by the appended claims.