Abstract:
The present invention concerns a method for controlling the operation of an automotive HVAC system. The HVAC system includes at least a refrigerant compressor and a refrigerant evaporator. The method includes the steps of calculating an ambient air enthalpy value; comparing the calculated ambient air enthalpy value to at least one predetermined enthalpy value; and selectively changing the operation of the refrigerant compressor based on the comparison.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates generally to automotive HVAC systems and methods of operating such HVAC systems.  
         [0002]     Automotive HVAC systems are well known and are utilized for heating and cooling the passenger compartments of vehicles. Hybrid vehicles, which utilize a battery and an intermittently operated internal combustion engine for vehicle propulsion, have difficulty keeping the passenger compartment cool when the engine is off. When the engine is off, the HVAC compressor, typically run by a clutch connected to the engine, is also off and the temperature in the passenger compartment can rise quickly. Since the majority of prior art automotive HVAC compressors are mechanically coupled with the internal combustion engine through an accessory system such as a clutch or the like, turning off the engine suggests a deteriorated occupant comfort due to the fact that the refrigerant compressor is non-operational while the engine is off. To maintain a certain level of air conditioning performance, the engine must be restarted, which has then a negative impact on the fuel economy of the hybrid vehicle.  
         [0003]     A common prior art automotive HVAC control system utilizes temperature-based control wherein a temperature sensor monitors ambient temperature and sends electrical signal(s) to a HVAC control module. A control algorithm embedded into the control module compares the temperature reading with an established temperature criterion. Based upon the algorithm, a control action will be executed to either couple the compressor to or decouple it from the accessory drive by engaging or disengaging the compressor clutch.  
         [0004]     Basing the control of the HVAC system on air temperatures alone has raised concerns in recent development of mild hybrid vehicles. One concern includes deteriorated air conditioning performance in high humidity and medium temperature ambient conditions due to the tendency to cause a musty odor to emanate from the air conditioning outlets when the engine is off. Another concern is an excessively negative impact on fuel economy in low humidity and medium to high temperature, low temperature and high humidity, and medium temperature and low to medium humidity ambient conditions because the temperature-based control method keeps the engine running or starts the engine earlier than is desirable. Fundamentally, temperature-based control is a single variable control system, in which temperature is disadvantageously the sole variable used for making operational decisions. This is disadvantage is particularly undesirable when such temperature-based control is applied with a hybrid vehicle.  
         [0005]     It is desirable, therefore, to provide a more efficient control methodology for operating an automotive HVAC system, including hybrid vehicles.  
       SUMMARY OF THE INVENTION  
       [0006]     The present invention concerns a method for controlling the operation of an automotive HVAC system. The HVAC system includes at least a refrigerant compressor and a refrigerant evaporator. The method includes the steps of calculating an ambient air enthalpy value; comparing the calculated ambient air enthalpy value to at least one predetermined enthalpy value; and selectively changing the operation of the refrigerant compressor based on the comparison.  
         [0007]     The thermodynamic properties of air at atmospheric pressure can be defined by knowing two variables: its temperature and humidity. Basically, the level of total energy (temperature, relative humidity) contained in air and the solar load determines the need for air conditioning cooling capacity. The basic variables needed to execute the control algorithm include ambient temperature and ambient relative humidity, solar load, air temperature at the evaporator outlet, and engine and/or vehicle speed. The HVAC system in accordance with the present invention advantageously includes measurement devices to provide measured values for the ambient temperature and ambient relative humidity, the solar load, the air temperature at the evaporator outlet, and the engine and/or vehicle speed in order to determine the enthalpy of the ambient air.  
         [0008]     By introducing humidity as a second variable for controlling the operation of an automotive HVAC system, the HVAC system can be controlled based on the heat value or enthalpy contained in the air rather than on temperature alone. By doing so, the HVAC system will provide greater comfort to the vehicle passengers in high humidity, medium temperature conditions and increase the efficiency of the vehicle in medium humidity, low temperature, and low humidity, low temperature conditions. The HVAC system is also advantageously able to adjust the control of the refrigerant compressor based on measured solar load values and measured evaporator outlet temperature values. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]     The above, as well as other advantages of the present invention, will become readily apparent to those skilled in the art from the following detailed description of a preferred embodiment when considered in the light of the accompanying drawings in which:  
         [0010]      FIG. 1  is a schematic view of a HVAC system in accordance with the present invention;  
         [0011]      FIG. 2  is a schematic view of a psychrometric chart showing enthalpy zones in accordance with the present invention;  
         [0012]      FIG. 3  is a flowchart of a method for operating the HVAC system of  FIG. 1 ;  
         [0013]      FIG. 4  is a flowchart of an energy algorithm module of the flowchart shown in  FIG. 3 ; and  
         [0014]      FIG. 5  is a flowchart of a first temperature algorithm module of the flowchart shown in  FIG. 3 ; and  
         [0015]      FIG. 6  is a flowchart of a second temperature algorithm module of the flowchart shown in  FIG. 3   
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0016]     Referring now to  FIG. 1 , a HVAC system in accordance with the present invention is indicated generally at  10 . The HVAC system  10  is disposed in a vehicle, indicated schematically at  12 . The vehicle  12  may be a hybrid vehicle having an internal combustion engine  14  operating in conjunction with a battery (not shown) or a conventional vehicle having the internal combustion engine  14  only. The HVAC system  10  includes a HVAC module, indicated generally at  15 . The HVAC module  15  includes a HVAC air duct  16  and a blower  18  adapted to direct a flow of air in a direction indicated by an arrow  20  through the HVAC duct  16 . An evaporator  22  is located within the HVAC duct  16  downstream of the blower  18 . The evaporator  20  includes a refrigerant inlet  24  from and a refrigerant outlet  26  to a refrigerant circuit, indicated generally at  28 .  
         [0017]     The refrigerant circuit  28  includes a refrigerant compressor  30  that is preferably driven by the engine  14  through a clutch  32 . The compressor  30  may be a fixed displacement compressor or a variable displacement compressor, as will be appreciated by those skilled in the art. Alternatively, the compressor  30  is a variable displacement compressor that is driven by the engine but does not include a clutch, or is an electric-driven compressor. The refrigeration circuit  28  also includes a condenser  34 , a receiver/dryer  36 , and a thermostatic expansion valve  38  in fluid communication with the compressor  30  and the evaporator  20 . The thermostatic expansion valve  38  may be replaced by an orifice tube (not shown) or similar refrigerant expander. A refrigerant is contained in the refrigerant circuit  28  and so flows through the compressor  30 , the condenser  34 , the receiver/dryer  36 , the refrigerant inlet  24 , the evaporator  22 , and the refrigerant outlet  26 . The refrigerant is selectively circulated through the piping during operation of the HVAC system  10 , discussed in more detail below. A heater core  40  is disposed in the HVAC duct  16  downstream of the evaporator  20  and includes coolant inlet (not shown) from and a coolant outlet (not shown) to an engine cooling circuit (not shown) of the internal combustion engine  14 .  
         [0018]     A HVAC electronic control module  42  is also disposed in the vehicle body  12 . The HVAC control module  42  is in communication with a powertrain electronic control module  44  via a serial bus  46  or the like. The HVAC control module  42  and the powertrain control module  44  each may be a single microprocessor or a plurality of interconnected microprocessors. For example, the HVAC control module  42  and the powertrain control module  44  may be a single integrated HVAC and powertrain controller (not shown). Furthermore, the HVAC control module  42  and the powertrain control module  44  may be hardware, software, or any combination thereof as will be appreciated by those skilled in the art.  
         [0019]     A damper  48  is disposed in the HVAC duct  16  downstream of the evaporator  20  and adjacent the heater core  40 . The damper  48  includes an actuator  50 , such as an electric motor or the like, that is operable to selectively expose and block the heater core  40  to an air flow from the blower  18 . The actuator  50  is in communication with the HVAC control module  42 . When the actuator  50  moves the damper  42  to a first position  42   a , the air flowing from the blower  18  in the direction  20  bypasses the heater core  40 . When the actuator  50  moves the damper  42  to a second position  42   b , the air flowing from the blower  18  in the direction  20  flows through the heater core  40 .  
         [0020]     The HVAC duct  16  extends to a passenger compartment, indicated schematically at  60 . A second damper  52  and a third damper  54  are disposed in the HVAC duct  16  downstream of the heater core  40 . The second damper  52  includes an actuator  56 , such as an electric motor or the like, and the third damper  54  includes an actuator  58 , such as an electric motor or the like. The actuators  56  and  58  are each in communication with the HVAC control module  42 . The dampers  52  and  54 , when moved by the respective actuators  56  and  58 , are operable to direct flow to various portions of the passenger compartment  60  of the vehicle body  12  such as, but not limited to, a floor outlet, a torso outlet, and a windshield outlet (not shown).  
         [0021]     A recirculation damper  62  is disposed between an outside or fresh air inlet  64  and a return inlet  66  from the passenger compartment  60  to supply air to the blower  18 . The damper  62  includes an actuator  68 , such as an electric motor or the like, that is operable to selectively expose and block the heater core  40  to an air flow from the blower  18 . The actuator  68  is in communication with the HVAC control module  42 . The recirculation damper  62  can move between a first position  62   a  and a second position  62   b . The recirculation damper  62  is operable to selectively provide only fresh air from the fresh air inlet  64  (when the actuator  68  has moved the damper  62  to a first position  62   a ), only recirculated air from the recirculation air inlet  66  (when the actuator  68  has moved the damper  62  to a second position  62   b ), or a mixture of fresh air and recirculated air to the blower  18 .  
         [0022]     An evaporator outlet temperature measurement device  70 , such as a temperature sensor, a thermistor measurement device, or the like, is disposed in the HVAC duct  16  downstream of the evaporator  20 . A plurality of duct temperature measurement devices  72 , such as temperature sensors or the like, is disposed in the HVAC duct  16  downstream of the heater core  40 . The measurement devices  70  and  72  are each in communication with the HVAC control module  42 .  
         [0023]     The HVAC control module  42  is connected to and in communication with a driver  74 , such as software or the like, for the compressor  30 , a solar load measurement device  76 , and an ambient temperature and ambient humidity measurement device  78 . Alternatively, the ambient temperature and ambient humidity measurement device  78  is a pair of measurement devices (not shown).  
         [0024]     The powertrain control module  44  is connected to and in communication with the engine  14  to obtain an engine speed value, indicated schematically at  80 , and a measurement device (not shown) to obtain a vehicle speed value, indicated schematically at  82 . The powertrain control module  44  is also connected to and in communication with a clutch drive  84  for the compressor  30 , a driver  86 , such as software or the like, for at least one cooling fan  87  adjacent to the condenser  34 , and a condenser outlet pressure measurement device  88 .  
         [0025]     The connections between the HVAC control module  42  and the powertrain control module  44  and the measurement devices  76 ,  78 ,  80 ,  82 , and  88 , are illustrative and a non-limiting example of control connections for the HVAC system  10 . The HVAC control module  42  and the powertrain control module  44  may be connected to all or none of the measurement devices  76 ,  78 ,  80 ,  82 , and  88 , as will be appreciated by those skilled in the art.  
         [0026]     Referring now to  FIG. 2 , an example of a psychrometric chart is indicated generally at  100 . Vertical lines in the chart  100  represent constant air temperature values. For example, a vertical line  102  represents a constant dry bulb temperature value equal to 32 degrees Celsius. Horizontal lines in the chart  100  represent constant air humidity ratio values. For example, a horizontal line  103  represents a constant humidity ratio value equal to 25. Elliptical lines in the chart  100  extending upwardly from left to right in the chart  100  represent constant air relative humidity values. A line  104  represents a constant relative humidity value equal to fifty percent (0.50) relative humidity. Generally straight lines in the chart  100  extending downwardly from left to right in the chart  100  represent constant air enthalpy values. A line  106  represents a constant enthalpy value equal to approximately 65 kJ/kg. The line  106  represents an upper target enthalpy zone having an upper range  106   a  and a lower range  106   b . A line  108  represents a lower target enthalpy zone having an upper range  108   a  and a lower range  108   b.    
         [0027]     A first, high enthalpy, zone in the chart  100  indicated generally at  110 . The high enthalpy zone  110  is the region on the chart  100  where the enthalpy values are all greater than the value of the upper target enthalpy zone  106 , wherein the air has a high enthalpy value. A second, medium enthalpy, zone in the chart  100  is indicated generally at  112 . The medium enthalpy zone  112  the region on the chart  100  where the enthalpy values are all greater than the value of the lower target enthalpy zone  108  and less than the value of the upper target enthalpy zone  106 , wherein the air has a medium enthalpy value. A third, low enthalpy, zone in the chart  100  is indicated generally at  114 . The low enthalpy zone  114  the region on the chart  100  where the enthalpy values are all less than the value of the lower target enthalpy zone  108 , wherein the air has a low enthalpy value.  
         [0028]     The corresponding values of temperature, humidity ratio, relative humidity, and enthalpy in the chart  100  can be stored as a matrix or a lookup table in, for example, a ROM chip, as stored memory in the control modules  42  or  44  or the like for easy access by the control modules  42  or  44  during operation of the HVAC system  10 , discussed in more detail below. The values represented by the lines  106  and  108 , and the zones  110 ,  112 , and  114  are illustrative only and a non-limiting example of control values for the HVAC system  10 .  
         [0029]     During operation of the HVAC system  10 , the ambient temperature and ambient humidity measurement device  78  provides an ambient temperature value T a  and an ambient humidity value, φ, to the HVAC control module  42 . From the measured T a  and the φ values, a saturation humidity ratio W s  is calculated by the following equation: 
 
 W   s =(1.8·10 −3 +3.79329·10 −4   ·T   a )−(4.39116·10 −6   ·T   a   2 )+(5.93915·10 −7   ·T   a   3 )   (Equation 1), 
        where 0° C.≦T a ≦50° C.        
 
         [0031]     After W s  is calculated in Equation 1, the humidity ratio, W, in  FIG. 2 , can by calculated by the following equation: 
 
 W=W   s ·φ/{1+( 1−φ·W   s /0.62198)}  (Equation 2) 
 
         [0032]     After the humidity ratio, W, is calculated in Equation 2, an enthalpy value, h, in  FIG. 2 , can be calculated by the following equation: 
 
 h= 1.006·T a   +W ·(2501+1.805 ·T   a )  (Equation 3). 
 
 Alternatively, the HVAC control module  42  can determine the enthalpy value h by ascertaining or looking up a given enthalpy value based on two calculated variables (T a  and W) from the chart  100  stored in the lookup table or matrix. 
 
         [0033]     Equations 1, 2, and 3, are calculated by the HVAC control module  42  after receiving the measured T a  value and the measured φ value, from the ambient temperature and ambient humidity measurement device  78 . After the values for W and h are determined, the HVAC control module  42  compares the calculated values to the predetermined values for humidity ratio and enthalpy from the chart  100  stored in the matrix or the lookup table to determine the current operation and current zone  110 ,  112 , or  114  of the HVAC system  10 .  
         [0034]     Referring now to  FIG. 3 , a flowchart of a method of operating the HVAC system  10  of  FIG. 1  for a hybrid vehicle (not shown) is indicated generally at  200 . The method  200  begins at a step  201  and proceeds to a step  202  where input values, such as values from the engine  14  including an engine status (not shown) or the engine speed  80 , ambient temperature and ambient relative humidity from the ambient temperature and ambient humidity measurement device  78 , evaporator outlet temperature from the evaporator outlet temperature measurement device  70 , the vehicle speed  82 , and a solar load value from the solar load measurement device  76  are input into one or both of the HVAC control module  42  and the powertrain control module  44  of  FIG. 1 . In a step  204 , the state of the HVAC system  10  is determined, such as an economical (ECO) mode or not. If the HVAC system  10  is in an ECO mode, the method  200  moves to a step  206 , where it determines if the air conditioning mode is on or off. If the air conditioning is not on, the method  200  returns to the step  202 . If the HVAC system  10  in the step  204  is determined not to be in an ECO mode, the method proceeds to a step  208 , where it determines if the air conditioning mode is on or off, similar to the step  206 . If the air conditioning is not on, a command is sent to an engine, such as the engine  14  of  FIG. 1 , in a step  209  to turn the engine  14  off, such as by sending a command from the powertrain control module  44  to disengage the clutch drive  84  for the compressor  30 , after which the method returns to the step  202 . If the air conditioning is on, the method  200  proceeds to a step  212 , where it is determined if the air conditioning is in automatic mode. If the air conditioning is not in automatic mode, the method proceeds to a step  216 , where the engine  14  is turned on and after which the method returns to the step  202 . Similarly, if method  200  in the step  206  determines the air conditioning is on, the method procees to a step  210 , where it is determined if the air conditioning is in automatic mode. If the air conditioning is not in automatic mode, the method proceeds to a step  214 , where the engine  14  is turned on and after which the method returns to the step  202 .  
         [0035]     After each of the steps  210  and  212 , if the air conditioning is determined to be in an automatic mode, the method proceeds to a step  218  of an energy algorithm module, indicated generally at  219 . Referring now to  FIG. 4 , after the step  218 , the module  219  proceeds to a step  220 , wherein the values read in the step  202  are utilized to compute a value for Ws, W, and h, utilizing equation 1, equation 2, and equation 3, respectively. After the values for Ws, W, and h are computed in the step  220 , the solar load value (α) read in the step  202  is compared to a first predetermined solar load value, a, stored in the HVAC control module  42  or the powertrain control module  44  in a step  222 . If the measured solar load value (α) is greater than a in the step  222 , the module  219  proceeds to a step  224 , where an upper enthalpy setpoint or control value (h h ) is set equal to h1 and a lower enthalpy setpoint or control value (h L ) is set equal to h4, discussed in more detail below. If the measured solar load value (α) is less than a in the step  222 , the measured solar load value (α) is compared, in a step  226 , to a second predetermined solar load value, b, stored in the HVAC control module  42  or the powertrain control module  44 . If the measured solar load value (α) is less than b in the step  226 , the module  219  proceeds to a step  228 , where the upper enthalpy control value (h h ) is set equal to h3 and the lower enthalpy control value (h L ) is set equal to h6, discussed in more detail below. If the measured solar load value (α) is greater than b in the step  226 , the module  219  proceeds to a step  230 , where the upper enthalpy control value (h h ) is set equal to h2 and the lower enthalpy control value (h L ) is set equal to h5.  
         [0036]     The h3 and h6 values correspond to a high solar load value, the h2 and h5 values correspond to a normal solar load value, and the h1 and h4 values correspond to a low solar load value. As an illustrative and nonlimiting example, the h1 value corresponds to the lower range line  106   b  of the upper target enthalpy zone  106  in  FIG. 2 . The h2 value corresponds to the upper target enthalpy zone line  106  in  FIG. 2 . The h3 value corresponds to the upper target enthalpy zone line  106   a  in  FIG. 2 . The h4 value corresponds to the lower range line  108   b  of the lower target enthalpy zone  108  in  FIG. 2 . The h5 value corresponds to the upper target enthalpy zone line  108  in  FIG. 2 . The h6 value corresponds to the upper target enthalpy zone line  108   a  in  FIG. 2 .  
         [0037]     After setting the h h  and h L  values, the module  219  returns to the method  200  in either of step  232  (non-ECO mode) or a step  262  (ECO mode). If the HVAC system  10  is in the non-ECO mode, in the step  232 , the calculated enthalpy value (h) calculated in the step  220  is compared to the upper enthalpy control value (h h ) set in the step  224 ,  226 , or  228 . If the calculated enthalpy value h is greater than the upper enthalpy control value h h , this means that the ambient air is in the zone  110  of  FIG. 2  wherein operation of the air conditioning system is required to maintain the passenger compartment at a desired temperature and the method proceeds to a step  234 , where it determines whether the engine  14  is on or off, such as by receiving an input value from the engine speed  80 . If the engine  14  is on, the method  200  returns to the step  202  and, if the engine  14  is off, a request to turn the engine  14  on is made in a step  236 , such as by sending a command from the powertrain control module  44  to engage the clutch drive  84  for the compressor  30 , after which the method  200  returns to the step  202 . If, in the step  232 , the calculated enthalpy value h is less than the upper enthalpy control value h h , the method proceeds to a step  238 , where the calculated enthalpy value is compared to the lower enthalpy control value (h L ) set in the step  224 ,  226 , or  228 . If the calculated enthalpy value h is less than the lower enthalpy control value h L , this means that the ambient air is in the zone  114  of  FIG. 2 , wherein operation of the air conditioning is not required to maintain the passenger compartment at the desired temperature and the method proceeds to a step  240 , where a request to turn the engine  14  off is made, such as by sending a command from the powertrain control module  44  to disengage the clutch drive  84  for the compressor  30 , after which the method  200  returns to the step  202 . If the calculated enthalpy value h is greater than the lower enthalpy control value h L , this means that the ambient air is in the zone  112  of  FIG. 2 , and the method proceeds to a step  242  of a temperature algorithm module, indicated generally at  243  and best seen in  FIG. 5 , to determine whether or not operation of the air conditioning will be required to maintain the passenger compartment at the desired temperature.  
         [0038]     Referring now to  FIG. 5 , in the temperature algorithm module  243 , the evaporator outlet temperature, T, measured by the evaporator outlet temperature measurement device  70  in the step  202  is compared to a first predetermined temperature value, A, stored in the HVAC control module  42  or the powertrain control module  44  in a step  244 . If the evaporator outlet temperature T in the step  244  is less than A, this means that air conditioning is not required and the module  243  proceeds to a step  246 , where it is determined if the engine  14  is on, such as by receiving an input value from the engine speed  80 . If the engine  14  in the step  246  is not on, the module  243  returns to the method  200  at the step  256 . If the engine  14  in the step  246  is on, the module  243  proceeds to a step  248 , where a request to turn the engine  14  off is made, such as by sending a command from the powertrain control module  44  to disengage the clutch drive  84  for the compressor  30 , after which the module returns to the method  200  at the step  256 . If the evaporator outlet temperature T in the step  244  is greater than A, the module  243  proceeds to a step  250 , where the evaporator outlet temperature T is compared to a second predetermined temperature value, B. If the evaporator outlet temperature T in the step  250  is less than B, this means that air conditioning is not required and the module  243  returns to the method  200  at the step  256 . If the evaporator outlet temperature T in the step  250  is greater than B, this means that air conditioning is required and the module  243  proceeds to a step  252  to determine if the engine  14  is on, such as by receiving an input value from the engine speed  80 . If the engine  14  is on, the module  243  returns to the method  200  at the step  256  and, if the engine  14  is not on in the step  252 , the module  243  proceeds to a step  254 , where a request to turn the engine  14  on is made, after which the module  243  returns to the method  200  at the step  256 . After the step  256 , the method  200  returns to the step  202 .  
         [0039]     Referring again to  FIG. 3 , if the HVAC system  10  is in the ECO mode, in the step  262  the calculated enthalpy value (h) calculated in the step  220  is compared to the upper enthalpy control value (h h ) set in the step  224 ,  226 , or  228 . If the calculated enthalpy value h is greater than the upper enthalpy control value h h , this means that the ambient air is in the zone  110  of  FIG. 2  wherein operation of the air conditioning system is required to maintain the passenger compartment at the desired temperature and the method proceeds to a step  264 , where it determines whether the engine  14  is on or off, such as by receiving an input value from the engine speed  80 . If the engine  14  is on, the method  200  returns to the step  202  and, if the engine  14  is off, a request to turn the engine  14  on is made in a step  266 , such as by sending a command from the powertrain control module  44  to engage the clutch drive  84  for the compressor  30 , after which the method  200  returns to the step  202 . If, in the step  262 , the calculated enthalpy value h is less than the upper enthalpy control value h h , the method proceeds to a step  268 , where the calculated enthalpy value is compared to the lower enthalpy control value (h L ) set in the step  224 ,  226 , or  228 . If the calculated enthalpy value h is less than the lower enthalpy control value h L , this means that the ambient air is in the zone  114  of  FIG. 2 , wherein operation of the air conditioning is not required to maintain the passenger compartment at the desired temperature and the method proceeds to a step  270 , where a request to turn the engine  14  off is made, such as by sending a command from the powertrain control module  44  to disengage the clutch drive  84  for the compressor  30 , after which the method  200  returns to the step  202 . If the calculated enthalpy value h is greater than the lower enthalpy control value h L , this means that the ambient air is in the zone  112  of  FIG. 2 , and the method proceeds to a step  272  of a temperature algorithm module, indicated generally at  273  and best seen in  FIG. 6  determine whether or not operation of the air conditioning will be required to maintain the passenger compartment at the desired temperature.  
         [0040]     Referring now to  FIG. 6 , in the temperature algorithm module  273 , the evaporator outlet temperature, T, measured by the evaporator outlet temperature measurement device  70  in the step  202  is compared to a first predetermined temperature value, A, stored in the HVAC control module  42  or the powertrain control module  44  in a step  244 . If the evaporator outlet temperature T in the step  274  is less than A, this means that air conditioning is not required and the module  273  proceeds to a step  276 , where it is determined if the engine  14  is on, such as by receiving an input value from the engine speed  80 . If the engine  14  in the step  276  is not on, the module  273  returns to the method  200  at the step  275 . If the engine  14  in the step  276  is on, the module  273  proceeds to a step  278 , where a request to turn the engine  14  off is made, after which, the module returns to the method  200  at the step  275 . If the evaporator outlet temperature T in the step  274  is greater than A, the module  243  proceeds to a step  280 , where the evaporator outlet temperature T is compared to a third predetermined temperature value, C. If the evaporator outlet temperature T in the step  280  is less than C, this means that air conditioning is not required and the module  273  returns to the method  200  at the step  275 . If the evaporator outlet temperature T in the step  280  is greater than C, this means that air conditioning is required and the module  273  proceeds to a step  282  to determine if the engine  14  is on, such as by receiving an input value from the engine speed  80 . If the engine  14  is on, the module  273  returns to the method  200  at the step  275  and, if the engine  14  is not on in the step  282 , the module  273  proceeds to a step  284 , where a request to turn the engine  14  on is made, such as by sending a command from the powertrain control module  44  to engage the clutch drive  84  for the compressor  30 , after which the module  243  returns to the method  200  at the step  275 . After the step  275 , the method  200  returns to the step  202 .  
         [0041]     The predetermined solar load values, a and b in  FIG. 4 , and the predetermined temperature values, A, B, and C in  FIGS. 5 and 6  are preferably editable or changeable values in the control modules  42  or  44 , depending on the configuration of HVAC system  10 . For example, the predetermined temperature values A, B, and C in  FIGS. 5 and 6  may be set by the occupants of the vehicle at a HVAC system user interface (not shown), such as a climate control cluster in the instrument panel or the like, during use of the vehicle  12 . Moreover, while the method  200  has been described in the context of a hybrid vehicle wherein the engine  14  is turned on or off in the various steps  209 ,  214 ,  216 ,  236 ,  248 ,  254 ,  266 ,  278 , and  284 , these steps could be performed wherein the operation of a refrigerant compressor, such as the compressor  30  in  FIG. 1 , is selectively changed in these steps, such as by engaging or disengaging the clutch  32  of the compressor or varying the output of a variable displacement compressor, depending on the measured ambient temperature and humidity values and resulting calculated ambient enthalpy values.  
         [0042]     In accordance with the provisions of the patent statutes, the present invention has been described in what is considered to represent its preferred embodiment. However, it should be noted that the invention can be practiced otherwise than as specifically illustrated and described without departing from its spirit or scope.