Abstract:
An improved method of operation for a vehicle air conditioning system controls an inlet air blower motor and an air inlet mixing device to reduce compressor and blower motor power consumption and achieve performance improvements associated with cabin air recirculation while maintaining a predefined level of outside air flow and a predetermined humidity level in the inlet air mixture of the system. The overall air flow is determined by the speed of the blower motor and the speed of the vehicle, and the speed of the blower motor and the position of the inlet air mixing device are adjusted as a function of both the vehicle speed and the selected blower motor speed so that the predefined level of outside air flow is preserved regardless of the vehicle speed and the selected blower motor speed.

Description:
RELATED APPLICATIONS  
       [0001]    This is a continuation-in-part of co-pending U.S. patent application Ser. No. 09/546,278 (Attorney Docket No. DP-301708), filed Apr. 10, 2000, and assigned to the assignee of this application. 
     
    
     
       TECHNICAL FIELD  
         [0002]    This invention relates to a vehicle air conditioning system, and more particularly to a control for maintaining a predetermined level of outside air in a system capable of mixing outside air with recirculated air and having a controllable air conditioning blower motor.  
         BACKGROUND OF THE INVENTION  
         [0003]    A vehicle air conditioning system performs two primary functions: temperature regulation and dehumidification. After an initial cool-down period where the air inside the vehicle is relatively hot, these functions and the vehicle fuel economy can often be enhanced by drawing at least a portion of the air supplied to the cabin space from the cabin itself instead of from outside the cabin. The introduction of air drawn from the cabin generally reduces the enthalpy and moisture content of the inlet air mixture to be conditioned for re-delivery to the cabin. In manually controlled systems operated in a “recirculation” mode, the introduction of outside air is commonly accomplished through the use of a mechanical bleed device designed to maintain a designated proportion of outside air under a specific set of static circumstances (for example, at a given vehicle speed and blower motor setting). In other manually controlled systems and some automatically controlled systems, inlet air mixing is achieved with an inlet air mixing device, such as controlled door in an inlet air duct. In such cases, the inlet air mixture comprising predominantly outside air when the system is operated in an “outside air” mode, and predominantly air drawn from inside the cabin when the system is operated in a “recirculation” or “Max A/C” mode. In the manually controlled versions, the operator selects the desired mode, while in the automatically controlled versions, the selection is performed by a system controller based on various input parameters such as the desired cabin temperature, the measured cabin temperature, and so on. In any event, it is commonly recommended that the usage of cabin air recirculation be limited in order to more effectively purge odors, carbon-dioxide and smoke generated in the cabin, and in order to prevent intrusion of exhaust gases under certain conditions. Additionally, extended operation in the recirculation mode can tend to lower the relative humidity of the cabin air to an uncomfortable level.  
           [0004]    At higher vehicle speeds, inlet air mixing and total air flow is also influenced by the flow of air around the vehicle body. Specifically, the air flow creates a positive pressure where ventilation air enters the vehicle and a negative pressure where ventilation air exits the vehicle. This can force a significantly higher amount of outside air through the ventilation system than ordinarily desired by the vehicle occupants.  
         SUMMARY OF THE INVENTION  
         [0005]    The present invention is directed to an improved method for controlling inlet air mixing in a vehicle air conditioning system having an inlet air blower motor and an air inlet mixing device for admitting inlet air from outside and/or inside the vehicle cabin, wherein the blower motor and mixing device are controlled under predefined operating conditions to reduce compressor and blower motor power consumption and achieve performance improvements associated with cabin air recirculation while maintaining a predefined level of outside air flow in the inlet air mixture of the system. The total air delivered to the vehicle cabin is determined by the speed of the cabin air blower motor, the speed of the vehicle, and the position of the inlet air control door. Under high air conditioning load, the control of this invention adjusts the speed of the blower motor and the position of the inlet air mixing device as a function of both the vehicle speed and the selected blower motor speed so that the predefined level of outside air flow is preserved regardless of the vehicle speed and the selected blower motor speed. At low or no air conditioning load, when the position of the inlet air mixing device is set to provide only outside air flow, the control of this invention adjusts the speed of the blower motor to provide the desired air flow based upon the vehicle speed so that a predefined level of outside air is preserved regardless of vehicle speed and the selected blower motor speed. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]    [0006]FIG. 1 is a block diagram of a vehicle air conditioning system according to this invention, including a microprocessor-based control unit, an inlet air mixing device and an inlet air blower motor.  
         [0007]    [0007]FIG. 2 is a psychrometric chart illustrating different possible operating modes of the air conditioning system of FIG. 1.  
         [0008]    [0008]FIG. 3 is a flowchart illustrating a software routine executed by the microprocessor-based control unit of FIG. 1 in carrying out the control of this invention.  
         [0009]    [0009]FIG. 4 is a flowchart detailing a portion of the flowchart of FIG. 3 that updates control signals for the inlet air mixing device and the inlet air blower motor of FIG. 1. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0010]    Referring to FIG. 1, the reference numeral  10  generally designates a vehicle air conditioning system, including a refrigerant compressor  12  coupled to a drive pulley  14  via an electrically activated clutch  16 . In the illustrated embodiment, the compressor  12  has a variable stroke for adjusting its capacity, and includes a stroke control valve  17  that is electrically activated to effect capacity control. 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 system  10  further 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.  
         [0011]    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  pivoted at point  46  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  as indicated by arrow  48 , and passenger compartment air may enter blower  42  through duct leg  44   b  as indicated by arrow  50 .  
         [0012]    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. The outlet duct  52  is bifurcated with the heater core  54  disposed in one air stream of duct  52 . A temperature control door  56  pivoted at a point  84  near the heater core  54  is adjustable as shown to control what proportion of air must pass through the heater core  54 . Air passing through heater core  54  is indicated by the arrow  58 , while air by-passing the heater core  54  is indicated by the arrow  60 . The heated and un-heated air portions are mixed in a plenum portion  62  of outlet duct  52  downstream of temperature control door  56 , and a pair of mode control doors  64 ,  66  direct the mixed air through one or more outlets, including a defrost outlet  68 , a panel outlet  70 , and a heater outlet  72 . The mode control doors  64  and  66 , pivoted at points  74  and  80 , respectively, are adjustable as shown to switch the outlet air between various combinations of defrost outlet  68 , panel outlets  70  and heater outlet  72 , as indicated by arrows  76 ,  78  and  82 , respectively.  
         [0013]    The system  10  is controlled by the microprocessor-based control unit  90  based on various inputs. In the illustrated embodiment, such inputs include: passenger compartment air temperature PCAT, vehicle speed VS, outside air temperature OAT, and the usual operator demand inputs, such as the desired cabin temperature, and override controls for the speed of inlet air blower motor  43 . In an automatically controlled system such as illustrated in FIG. 1, the selected blower motor speed SBMS is obtained from the control unit  90  itself, which either sets SBMS in accordance with a base control or in accordance with an operator override of the base control. In a manually controlled system, SBMS is provided as input to control unit  90  based on the position of an operator manipulated blower motor speed selector switch (not shown).  
         [0014]    In response to the above-mentioned inputs, the control unit  90  develops output signals for controlling the compressor clutch  16 , the capacity control valve  17 , the condenser blower motor  34 , the inlet air blower motor  43 , and the air control doors  44 ,  56 ,  64  and  66 . In FIG. 1, the output signal CL for the clutch  16  appears on line  100 , the output signal STROKE for the stroke control valve  17  appears on line  102 , the output signal FC for the condenser blower motor  34  appears on line  104 , and the blower motor speed signal BMSS for the controlling the speed of inlet air blower motor  43  appears on line  106 . Finally, the output signal IACD for positioning the inlet air control door  44  appears on line  108 , and is applied as an input to an actuator such as stepper motor SM that is mechanically coupled to door  44 . For simplicity, output signals and actuators for the air control doors  56 ,  64 ,  66  have been omitted from FIG. 1.  
         [0015]    According to the present invention, the control unit  90  regulates the speed of inlet air blower motor  43  and the position of inlet air control door  44  based on SBMS and VS so that the inlet air comprises only a predetermined amount of outside air for any combination of SBMS and VS. In the preferred embodiment, the predetermined amount depends on the number of occupants of the vehicle in which the system  10  is installed. A generally accepted guideline is that at least 15 cubic-feet-per-minute (CFM) of outside air should be provided for each of the vehicle occupants. For example, the predetermined amount of outside air may be set to 90 CFM for a, six-passenger vehicle. At the lowest blower motor speed (120 CFM, for example), the predetermined amount of outside air represents a relatively high percentage (75%) of the air supplied to the cabin, whereas at the highest blower motor speed (300 CFM, for example), the predetermined amount of outside air represents a relatively low percentage (30%) of the air supplied to the cabin. When the system is operated in the “recirculation” or “Max A/C” modes, the percent of outside air increases as vehicle speed increases. When the system is operated in the “outside air” mode, the total volume of air increases as the vehicle speed increases.  
         [0016]    The psychrometric chart of FIG. 2 illustrates the significance of the above-described control. The chart depicts the absolute humidity of air as a function of dry bulb temperature, with the curved broken lines representing lines of constant relative humidity, and the straight broken lines representing lines of constant enthalpy. The various data points A, A′, B, B′, C and D represent the condition of air outside the vehicle, at various points in the ducts  40 ,  52 , and in the passenger compartment. The point A represents a traditional system at a low blower, stabilized condition, with a dry bulb outside air temperature of 100° F., at 40% relative humidity. As the air passes through the evaporator  24 , its dry bulb temperature decreases with no change in absolute humidity until the relative humidity rises to 100%, as depicted by the line segment A-B. As the air is further cooled, water vapor condenses on the surface of evaporator  24 , with the relative humidity remaining at 100%. Under a given set of conditions, the control unit  90  regulates the compressor stroke to control the dew point temperature of evaporator  24  to approximately 38° F., so that air at the evaporator outlet is represented by the point C. Then, the air is re-heated by the heater core  54  so that the air temperature in the passenger compartment has a dry bulb temperature of 72° F., as represented by the point D. As the air is re-heated, its absolute humidity remains the same, but its relative humidity drops, as indicated by the line segment C-D, providing a cabin relative humidity of approximately 30%.  
         [0017]    In accordance with the present invention, a similar cabin temperature and relative humidity level is achieved, but with reduced energy consumption, by adjusting the blower motor speed and the position of inlet air control door  44  as a function of SBMS and VS, as described above. In this illustration, the outside air constitutes approximately 70% of the inlet air mixture, and is represented by the point A′. Significantly, the enthalpy, temperature, dew point and absolute humidity of the inlet air mixture are all decreased due to the influence of the cabin air; as a result, the net work performed by the compressor  12  to drop the temperature and humidity to the level designated by the point C is substantially reduced, as indicated by the difference in enthalpy between point A (42.6 BTU/LB) and point A′ (37.5 BTU/LB). When the cooling capacity of the system  10  is limited (due to low compressor speed, for example), the passenger comfort is also improved because the inlet air mixture can be cooled and de-humidified to lower levels than outside air alone.  
         [0018]    [0018]FIG. 3 depicts a flow diagram representative of computer program instructions executed by the control unit  90  for determining appropriate control settings DOOR_POS and CBMS for the inlet air control door  44  and the inlet air blower motor  43 . The parameter DOOR_POS is used to schedule the output IACD on line  108 , and the parameter CBMS is used to schedule the output BMSS on line  106 . The block  110  is first executed to obtain the previous position command DOOR_POS(old) for the inlet air control door  44  and the previous speed command CBMS(old) for blower motor  43 . The block  112  then determines if the compressor  12  is running (that is, whether clutch  16  is engaged) and the system  10  is operating in a panel discharge mode, as opposed to a defrost mode, for example. If not, the control of this invention is not enabled, and the block  114  is executed to set DOOR_POS to AUTO (a position dictated by an automatic climate control algorithm carried out by control unit  90 ), and to set CBMS equal to the selected blower motor speed SBMS.  
         [0019]    If block  112  is answered in the affirmative, the block  116  is executed to determine if the air conditioning load is high. The outside air temperature OAT is measured for control purposes, and the air conditioning load is determined by comparing OAT to a reference temperature OAT_REF. In other systems, an equivalent indication of high load may be obtained based on another load-indicative parameter, such as incoming air enthalpy, condenser outlet pressure or temperature, or compressor outlet pressure or temperature. In the illustrated embodiment, the reference OAT_REF is initialized at a relatively high value, such as 80 degrees F., and if OAT exceeds this value (indicating high air conditioning load), the blocks  122 ,  123  and  124  are executed to select new or target values DOOR_POS(new) and CBMS(new) for inlet air door  44  and inlet air blower motor  43 , and to set OAT_REF to a lower value, such as 75 degrees F. If the load is subsequently reduced, and OAT falls to the lower value, the blocks  118 ,  119  and  120  are executed to set DOOR_POS(new) to full outside air, to set CBMS(new) to the commanded blower motor speed in outside air mode CBMSOA, and to restore OAT_REF to the high value (80 degrees F.). As indicated at block  119 , CBMSOA may be determined by table look-up as a function of the selected blower motor speed SBMS and the vehicle speed VS.  
         [0020]    As indicated at blocks  122 - 123 , DOOR_POS(new) and CBMS(new) under high air conditioning load are determined by table look-up as a function of the selected blower motor speed SBMS and the vehicle speed VS. The table values may be determined empirically based on measured air flow through the ducts  44   a,    44   b  under various combinations of SBMS and VS, so that the values of DOOR_POS(new) and CBMS(new) obtained from the look-up tables will result in an inlet air mixture comprising a predetermined amount (flow) of outside air, as explained above.  
         [0021]    Once DOOR_POS(new) and CBMS(new) have been determined, the blocks  126 ,  128 ,  130 ,  132 ,  134 ,  136  are executed to carry out required changes in inlet air door position and blower motor speed at a controlled rate. The block  126  determines if the count of an INLET CONTROL TIMER exceeds a reference count REF. If not, blocks  128 ,  130  and  132  are executed to retain the current door position and blower motor speeds (i.e., DOOR_POS is set equal to DOOR_POS(old), and CBMS is set equal to CBMS(old)), and to increment the INLET CONTROL TIMER. Once the count of the INLET CONTROL TIMER exceeds REF, the block  134  resets the INLET CONTROL TIMER to zero, the block  136  updates DOOR_POS and CBMS, and the block  132  increments the INLET CONTROL TIMER. Thus, the INLET CONTROL TIMER limits the updating of the inlet air door position and inlet air blower motor speed during inlet air mixture control to a desired maximum rate, such as one unit of adjustment per second.  
         [0022]    [0022]FIG. 4 illustrates block  136  of FIG. 3 in further detail. Referring to FIG. 4, the block  150  determines the required changes in blower motor speed and inlet air door position, the blocks  152 ,  154 ,  156 ,  158 ,  160  update CBMS, and the blocks  162 ,  164 ,  166 ,  168 ,  170  update DOOR_POS. The required change ΔCBMS in blower motor speed is determined according to the difference [CBMS(new)−CBMS(old)], and the required change ΔDOOR_POS in inlet air door position is determined according to the difference [DOOR_POS(new)−DOOR_POS(old)]. In the illustrated embodiment, both the blower motor  43  and the inlet air door  44  are controlled in step-wise fashion. In the case of blower motor  43 , for example, there are a predetermined number of speed settings (sixteen, for example), each associated with a corresponding blower motor speed signal BMSS. If block  152  determines that ACBMS is positive, the block  160  increments the speed setting by setting CBMS equal to the sum (CBMS(old)+1). If ACBMS is negative, blocks  152  and  154  will be answered in the negative, and the block  156  decrements the speed setting by setting CBMS equal to (CBMS(old)−1). If ΔCBMS=0, the block  158  retains the current speed setting by setting CMBS equal to CBMS(old). Similarly, if block  162  determines that ΔDOOR_POS is positive by an amount at least as great as an actuator step in that direction MOTOR_STEP_POS, the block  170  sets DOOR_POS equal to the sum [DOOR_POS(old)+MOTOR_STEP_POS]. On the other hand, if ΔDOOR_POS is negative by an amount at least as great as an actuator step in that direction MOTOR_STEP_NEG, as determined at block  164 , the block  166  sets DOOR_POS equal to [DOOR_POS(old )−MOTOR_STEP_NEG]. If ΔDOOR_POS is less than the minimum step size of actuator SM, block  168  is executed to retain the current door position by setting DOOR_POS equal to DOOR_POS(old).  
         [0023]    Thus, the control unit  90  gradually adjusts the speed of blower motor  43  and the position of inlet air control door  44  under conditions of high or low air conditioning load to increase the amount of recirculated cabin air in the inlet air mixture, while retaining a predetermined amount of outside air regardless of the selected blower motor speed and the vehicle speed, thereby improving both the efficiency and performance of the air conditioning system  10 . While described in reference to the illustrated embodiment, it is expected that various modifications in addition to those mentioned above will occur to those skilled in the art. For example, the control of this invention may be applied to air conditioning systems having a fixed displacement compressor, other expansion devices, or utilizing a different capacity control methodology. Also, blower motor voltage, power or current, or an anemometer, could be used instead of blower motor speed as an indicator of the desired air flow rate, and the control could also be compensated for the mode and temperature door positions. Thus, it will be understood that methods incorporating these and other modifications may fall within the scope of this invention, which is defined by the appended claims.