Abstract:
One embodiment relates to a method for preventing evaporator core freezing in an air conditioning system, comprising determining the occurrence of an evaporator core freezing condition for a first one of a plurality of evaporators. Determining the occurrence of an evaporator core freezing condition may be done without measuring the air out temperature first evaporator. The method also comprises adjusting an operating parameter of the air conditioning system to eliminate the core freezing condition.

Description:
BACKGROUND 
       [0001]    The present application relates generally to the field of vehicle air conditioning systems. In particular, the application relates to a method and system for rear evaporator freeze protection. 
         [0002]    In some automotive applications it is desirable to include a dual evaporator air conditioning system. Such systems allow for a more consistent temperature to be maintained throughout the cabin than a single evaporator system. Alternatively, these systems may be used to allow a different set temperatures for different areas of the occupant cabin. 
         [0003]    In automotive applications, it is generally preferred to utilize a fixed displacement compressor. While more expensive variable compressors can be used, they typically are not as reliable as fixed displacement compressors. Accordingly, the control of the compressor operation is usually binary in nature, in that the compressor motor is either engaged or it is not engaged. A clutch may be used to engage or disengage the compressor motor. 
         [0004]    One problem that may occur with automotive air conditioning systems is freezing of condensate water on the evaporator core. When the evaporator core freezes in this way, the evaporator core or other components of the system may be damaged. Also, the accumulation of ice on the evaporator core reduces cooling efficiency and constricts air flow. In dual evaporator systems both cores are susceptible to freezing and either may freeze independently of the other. In these systems, sensors (such as a front evaporator air out temperature sensor) may be used to detect freeze conditions for the front evaporator. While similar sensors may be used in connection with the rear evaporator, such configurations increase hardware costs. 
         [0005]    Various methods may be used to prevent rear evaporator core freezing without the expense of additional sensors. For example, a low performance evaporator may be utilized for the rear evaporator. In such systems, the rear evaporator is designed such that it will not reach temperatures below freezing. One drawback of these systems is lower cooling performance of the air conditioning system. Another drawback is lower efficiency that results in higher energy consumption by the system. 
         [0006]    Another alternative is to increase the rear suction line pressure drop. This also has drawbacks. This technique also results in lower cooling performance and inefficient operation. 
         [0007]    A third alternative is to increase the minimum airflow rate over the rear evaporator. However, such a configuration reduces a user&#39;s ability to control the system. This may result in user dissatisfaction as a result of not being able to select a lower air flow rate. 
         [0008]    While all of these methods may be capable of preventing rear evaporator core freezing, they all have drawbacks that limit system performance or controllability. Accordingly, there is a need for an improved method and/or system for the prevention of rear evaporator core freezing. There is also a need for a system that actively predicts and reacts to evaporator core freezing conditions. 
       SUMMARY OF THE INVENTION 
       [0009]    One embodiment relates to a method for preventing evaporator core freezing, comprising determining the occurrence of an evaporator core freezing condition for a first one of a plurality of evaporators in an air conditioning system. Determining the occurrence of an evaporator core freezing condition may be done without measuring the air out temperature for the first evaporator. The method also comprises adjusting an operating parameter, of the air conditioning system, e.g., by use of a controller, to eliminate the core freezing condition. 
         [0010]    Other embodiments relate to a method for preventing evaporator core freezing in a vehicle air conditioning system having a plurality of evaporators. One method comprised detecting an operating condition for a first evaporator. The method also comprises detecting an environmental condition for the vehicle interior, and determining the occurrence of an evaporator core freezing condition for a second evaporator without measuring the air out temperature for the second evaporator. The method further includes adjusting an operating parameter for the second evaporator, e.g., by use of a controller, to eliminate the core freezing condition. 
         [0011]    One preferred air conditioning system for a vehicle comprises a first evaporator core, a second evaporator core, and a temperature sensor for detecting the air out temperature for the first evaporator core. The system further comprises a processor configured to determine the occurrence of a freezing condition for the second evaporator core without an air out temperature measurement for the second evaporator core, and a controller. The controller includes controller logic configured to adjust an operating parameter to eliminate the freezing condition for the second evaporator core. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is a schematic view of a dual evaporator vehicle air conditioning system. 
           [0013]      FIG. 2  is a block diagram for an air conditioning control system. 
           [0014]      FIG. 3  is a correlation between calculated air out temperature and measured air out temperature. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0015]      FIG. 1  illustrates a dual evaporator air conditioning system  10 . In some embodiments, the system may be used in a vehicle such as an automobile, truck, airplane, rail car or other vehicle. The system generally includes a compressor  12 , a condenser  14 , first expansion device  16 , first evaporator  18 , first blower motor  20 , second expansion device  22 , second evaporator  24 , and second blower motor  26 . The components are arranged to provide a refrigeration loop with two evaporators in parallel. A vaporized refrigerant, such as R134a or other suitable refrigerant, may be compressed by compressor  12 . The compressed refrigerant is then condensed in condenser  14  that is cooled by outside air. A portion of the condensed refrigerant may then be routed to the first expansion device  16  and first evaporator  18 . Air from outside the vehicle or recirculated from the cabin may be blown over first evaporator  18  by first blower motor  20 . The resulting cool air stream is directed to the cabin e.g., passenger compartment, of the vehicle to cool a portion of the cabin. 
         [0016]    In many applications it is advantageous to provide a second evaporator to allow cool air to be introduced to the cabin in more than one location. This allows for a more uniform cabin temperature than a single evaporator refrigerant loop could provide due to imperfect distribution of the cool air into the cabin. Alternatively, when coupled with additional control capabilities, a dual evaporator system can be used to provide multiple temperature zones in order to accommodate multiple occupants who may have different desired air temperatures. 
         [0017]    Accordingly, in the dual evaporator air conditioning system  10 , a portion of the condensed refrigerant is or may be directed to the second expansion device  22  and the second evaporator  24 . A second blower motor may be used to recirculate cabin air over second evaporator  24  to provide a second cool air stream. The expanded refrigerant portions from the first evaporator  18  and the second evaporator  24  may then be combined and directed to the compressor  12  to complete the refrigerant loop. In some embodiments one or more valves (i.e., valves  28  and  30 ) may be used to control the refrigerant flow to the first and second evaporators. In other embodiments, in particular some vehicle applications, valves  28  and  30  may be omitted. 
         [0018]    As previously stated, evaporator core freezing is a problem that should be avoided when operating an air conditioning system. One way to detect evaporator freezing is to measure the air out temperature from the evaporator core. If the air stream leaving an evaporator is at or slightly above 0° C. it is very likely that water vapor is condensing and freezing on the evaporator core. The maximum air out temperature that corresponds to evaporator core freezing will vary by system but will generally be above the freezing point of water as the air temperature will increase between the evaporator core and the outlet. This air out temperature that corresponds to freezing of the evaporator core is the critical freezing temperature for the evaporator core (T CR ). The resultant ice build up can place mechanical stresses on components of the evaporator and other equipment that can result in damage to the system. Also, the accumulation of ice on the evaporator core reduces cooling efficiency and constricts air flow. 
         [0019]    As shown in  FIG. 1 , system  10  also includes a controller  32 , and sensors  34 ,  36 , and  38 . The sensors may be coupled directly to a controller or indirectly through a BUS or other device. Sensor  34  is positioned in the cool air stream leaving first evaporator  18  and measures the temperature of the stream. Sensor  34  is coupled to controller  32 . Controller  32  may be configured to control a clutch  13  on compressor  12 . Controller  32  may then use a controller logic whereby controller  32  disengages the compressor motor when sensor  34  detects that the cool air stream leaving first evaporator  18  is at or below 0° C. 
         [0020]    Sensor  36  is positioned to detect the air temperature within the vehicle cabin and is coupled to controller  32 . Sensor  38  is configured to measure the air temperature outside the vehicle and is also coupled to controller  32 . Many vehicles include interior and exterior air temperature sensors that may be utilized without the cost of additional hardware. 
         [0021]    Controller  32  is also coupled to first blower motor  20  and second blower motor  26 . Controller  32  may be configured to monitor and control the voltage across the blower motors such that the controller can vary the blower motor speed by varying the voltage. 
         [0000]    While the system and method are especially useful to enhance the performance of systems using a fixed displacement compressor, it is similarly useful to enhance the performance of a system utilizing a variable compressor. 
         [0022]      FIG. 2  illustrates a control algorithm to detect and/or prevent freezing of a second evaporator without requiring a temperature sensor for measuring the temperature of the cool air stream leaving the second evaporator. Controller  32  obtains data from the sensors and motors to which controller  32  is coupled. In some embodiments, the data includes an air out temperature for the first evaporator (detected by sensor  34 ), a cabin air temperature (detected by sensor  36 ), and an outside air temperature (detected by sensor  38 ). Additionally, controller  32  may determine the voltage across each of first blower  20  and second blower  26 . 
         [0023]    A processor may include software that allows the processor to calculate a value that correlates to the occurrence of the second evaporator freezing. In some of these embodiments, the processor may be programmed with an equation to predict the air out temperature (T S ) of the second evaporator ( 24  in  FIG. 1 ). The calculated value is then compared to a predetermined value that corresponds to evaporator core freezing (i.e. the critical freezing temperature T CR ). In other embodiments, the processor may be configured to predict another value indicative of evaporator core freezing. One such calculation may be made using the air out temperature for the first evaporator, the cabin air temperature, an outside air temperature, first blower voltage, and the second blower voltage. These values may be correlated to the air out temperature for the second evaporator by a regression model. This allows the air out temperature for the second evaporator to be approximated without the addition of another temperature sensor. 
       EXAMPLE 
       [0024]    Table 1 includes data that was collected and used to develop a regression model. 
         [0000]    
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                   
                   
                   
                 Rear 
               
               
                   
                   
                 Front 
                 Rear 
                 Front 
                 Air Out 
               
               
                 Cabin Air 
                 Outside Air 
                 Blower 
                 Blower 
                 Evaporator 
                 Temp - 
               
               
                 Temperature 
                 Temperature 
                 Voltage 
                 Voltage 
                 Air Out 
                 Data 
               
               
                 [C.] 
                 [C.] 
                 [V] 
                 [V] 
                 Temp [C.] 
                 [C.] 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 23 
                 45 
                 4 
                 4 
                 8.3 
                 1.5 
               
               
                 11.5 
                 25 
                 12.5 
                 11.5 
                 4.4 
                 3.5 
               
               
                 12 
                 25 
                 8 
                 8 
                 2.06 
                 1.5 
               
               
                 6 
                 15 
                 12.5 
                 11.5 
                 −3.2 
                 −2.5 
               
               
                 7 
                 15 
                 8 
                 8 
                 −1.1 
                 −1.8 
               
               
                 6 
                 5 
                 8 
                 8 
                 −1.1 
                 −1.7 
               
               
                 15 
                 30 
                 4.2 
                 4.1 
                 4.2 
                 1.5 
               
               
                 14 
                 20 
                 4.2 
                 4.1 
                 2.6 
                 0.5 
               
               
                 7 
                 15 
                 8 
                 8 
                 −1.35 
                 −1.86 
               
               
                 12 
                 25 
                 8 
                 8 
                 1.76 
                 1.59 
               
               
                 20 
                 25 
                 7.1 
                 6.3 
                 3 
                 4 
               
               
                 15 
                 20 
                 11.5 
                 11.5 
                 4.6 
                 7 
               
               
                 13.5 
                 20 
                 7 
                 6.2 
                 2.3 
                 1.15 
               
               
                 13 
                 20 
                 7 
                 6.2 
                 1.8 
                 0.4 
               
               
                 20 
                 45 
                 4 
                 4 
                 6.9 
                 1.34 
               
               
                 13 
                 25 
                 11.5 
                 12.5 
                 4.6 
                 3.9 
               
               
                 6 
                 15 
                 11.5 
                 12.5 
                 −3.2 
                 −2.5 
               
               
                 6 
                 5 
                 8 
                 8 
                 −1.2 
                 −0.5 
               
               
                   
               
             
          
         
       
     
         [0025]    It has been found that the air out temperature for the second evaporator (T s ) may be approximated by use of the equation: 
         [0000]        T   s   =X+A T   C   +B T   O   +C V   F   +D V   R   +E T   F   Equation 1 
         [0000]    where 
         [0026]    T C  is the cabin air temperature 
         [0027]    T 0  is the outside air temperature 
         [0028]    V F  is the first blower motor voltage 
         [0029]    V S  is the second blower motor voltage 
         [0030]    T F  is the first evaporator air out temperature 
         [0031]    Values for the parameters (A, B, C, D, E, and X) can be determined by conventional linear regression methods. While the equation has an infinite number of solutions for the parameters, typical values will be in the ranges shown in Table 2 depending on the vehicle and typical operating conditions. While the values and ranges shown in Table 2 are typical, other values and ranges may be used. 
         [0000]    
       
         
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                   
                   
                 Range of Values for a 
               
               
                 Parameter 
                 Value Chosen 
                 Typical A/C System 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Intercept 
                 X 
                 −4.887 
                 −10 to 10 
               
               
                 Cabin Temperature 
                 A 
                 0.394 
                 −2 to 2 
               
               
                 Outside Temperature 
                 B 
                 −0.190 
                 −2 to 2 
               
               
                 Front Voltage 
                 C 
                 0.281 
                 −2 to 2 
               
               
                 Rear Voltage 
                 D 
                 0.194 
                 −2 to 2 
               
               
                 Front Temperature 
                 E 
                 0.711 
                 −2 to 2 
               
               
                   
               
             
          
         
       
     
         [0032]      FIG. 3  illustrates the correlation between the air out temperature for the second evaporator (T S ) predicted by use of Equation 1 where A, B, C, D, E, and X are given the chosen value from Table 2. As can be seen from  FIG. 3 , the predicted value correlates sufficiently well with the measured value. 
         [0033]    After the air out temperature for the second evaporator is predicted, the controller may determine if the value (T S ) is at or below 0° C. If T S  is at or below 0° C., the controller will take corrective action. In some embodiments, the controller may increase the voltage to the second blower motor to increase the heat transferred to the second evaporator and thus raise the air out temperature for the second evaporator. In other embodiments, the controller may disengage the compressor motor to allow the second evaporator temperature to rise. Other suitable corrective actions, including decreasing the flow rate of refrigerant to the second evaporator (e.g. in a system including optional valves  28  and  30  shown in  FIG. 1 ), or any other suitable action may be taken. 
         [0034]    The controller will continually sample the data collected by the sensors and recalculate T S . This may be done at any suitable sampling rate. In some embodiments, sensor readings may be sampled at a rate of about once every second. When T S  reaches a temperature above 0° C. (or another preset temperature), the air conditioning system will be returned to normal operation. The controller may also include a hysteresis loop to avoid over compensation by the controller. 
         [0035]    Although the foregoing has been described with reference to exemplary embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope thereof. For example, although different example embodiments may have been described as including one or more features providing one or more benefits, it is contemplated that the described features may be interchanged with one another or alternatively be combined with one another in the described example embodiments or in other alternative embodiments. The present subject matter described with reference to the example embodiments and set forth in the following claims is manifestly intended to be as broad as possible. For example, unless specifically otherwise noted, the claims reciting a single particular element also encompass a plurality of such particular elements. Many other changes and modifications may be made to the present invention without departing from the spirit thereof. The scope of these and other changes will become apparent from the appended claims. The steps of the methods described herein may be varied, and carried out in different sequences.