Abstract:
An improved method for controlling inlet air mixing in a vehicle air conditioning system having an air inlet mixing device, wherein the mixing device is controlled under predefined operating conditions to improve vehicle fuel economy 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 overall air flow is determined by the speed of an inlet air blower motor, and the control is enabled under high thermal loading to adjust the inlet air mixing device as a function of the blower motor speed so that the predetermined level of outside air flow is preserved regardless of the blower motor speed.

Description:
TECHNICAL FIELD 
     This invention relates to a vehicle air conditioning system having an inlet air mixing device, and more particularly to a control method for the inlet air mixing device that improves system efficiency and performance. 
     BACKGROUND OF THE INVENTION 
     A vehicle air conditioning system performs two primary functions: temperature regulation and dehumidification. These functions and the vehicle fuel economy can usually be enhanced by drawing at least a portion of the inlet air from the cabin of the vehicle after an initial cool-down period of operation because the introduction of cabin air generally reduces the enthalpy of the inlet air mixture. In most manually controlled systems, introducing cabin air into the inlet air-stream is achieved with a mechanical bleed device designed to maintain a given proportion of cabin air and outside air. In some manually controlled systems and most automatically controlled systems, inlet air mixing is achieved with an inlet air mixing device (such as controlled door in an inlet duct), with the inlet air mixture comprising predominantly outside air in a normal mode and predominantly cabin air in a recirculation mode. In the manually controlled versions, the operator selects either the normal or recirculation 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 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. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to an improved method for controlling inlet air mixing in a vehicle air conditioning system having an air inlet mixing device for admitting inlet air from outside and/or inside the vehicle cabin, wherein the mixing device is controlled under predefined operating conditions to reduce compressor 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 overall air flow is determined by the speed of an inlet air blower motor, and the control of this invention is enabled under high thermal loading to adjust the inlet air mixing device as a function of the blower motor speed so that the predetermined level of outside air flow is preserved regardless of the blower motor speed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a vehicle air conditioning system according to this invention, including a microprocessor based control unit. 
     FIG. 2 is a psychrometric chart illustrating different possible operating modes of the air conditioning system of FIG.  1 . 
     FIG. 3 is a flowchart representing computer program instructions executed by the microprocessor based control unit of FIG. 1 in carrying out the control 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 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. 
     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 . 
     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 heater core  54  effectively bifurcates the outlet duct  52 , and a re-heat door  56  pivoted at a point  84  next to heater core  54  is adjustable as shown to control how much of the 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 re-heat 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 door  64  is pivoted at point  74 , and is adjustable as shown to switch the outlet air between the defrost and panel outlets  68 ,  70 , as indicated by arrows  76 ,  78 , respectively. The mode control door  66  is pivoted at point  80 , and is adjustable as shown to control airflow through the heater outlet  72 , as indicted by arrow  82 . 
     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, condenser outlet pressure COP, and the usual operator demand inputs, such as the desired temperature, and override controls for the speed of blower  42 . The condenser outlet pressure COP is detected by a pressure sensor  92  that is coupled to line  38  at the outlet of condenser  20  and that produces an electrical representation of the sensed pressure on line  94 . In an automatically controlled system such as illustrated in FIG. 1, the commanded blower motor speed CBMS is obtained from the control unit  90  itself, which either sets the speed in accordance with a base control or in accordance with an operator override of the base control. In a manually controlled system, CBMS is provided as input to control unit  90  based on the position of an operator manipulated blower motor speed selector switch (not shown). Alternately, of course, a speed sensor may be provided for measuring the actual speed of blower  42  or blower motor  43 . 
     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 cooling blower motor  34 , the 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 compressor appears on line  102 , the output signal FC for the condenser fan control appears on line  104 , and the output signal CBMS for the 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 actuator SM, which in turn, is mechanically coupled to door  44 . For simplicity, output signals and actuators for the air control doors  56 ,  64 ,  66  have been omitted. 
     According to the present invention, the control unit  90  regulates the position of inlet air control door  44  based on the speed of blower motor  43  so that the inlet air comprises only a predetermined amount of outside air regardless of the blower motor speed. In the preferred embodiment, the predetermined amount depends on the occupant capacity 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 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 air represents a relatively low percentage (30%) of the air supplied to the cabin. 
     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. For example, at a low blower, stabilized condition, outside air having a dry bulb temperature of 100° F. and a relative humidity of 40% is represented by the point A. 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 dew point temperature of the evaporator  24  is controlled 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%. 
     A similar cabin temperature and relative humidity level is achieved, but with reduced energy consumption, by adjusting the inlet air control door  44  as a function of blower motor speed, 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 is lower 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. 
     FIG. 3 depicts a flow diagram representative of computer program instructions executed by the control unit  90  for carrying out the above-described control in the context of a system in which the compressor capacity is adjusted based on various inputs including the condenser outlet pressure COP. The block  110  is first executed to obtain the previous position command DOOR_POS(old) for the inlet air control door  44  and the commanded blower motor speed CBMS. 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 inlet air mixture control of this invention is not enabled, and the block  114  is executed to set the new position command DOOR_POS(new) for door  44  to AUTO, a position dictated by an automatic climate control algorithm carried out by control unit  90 . 
     If block  112  is answered in the affirmative, the block  116  is executed to determine if the air conditioning load is high. In the illustrated embodiment where the condenser outlet pressure COP is measured for compressor control purposes, the air conditioning load is determined by comparing COP to a reference pressure COP_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 temperature, or compressor outlet pressure or temperature. In any event, the comparison should include some hysteresis to ensure that the inlet air control does not influence the comparison. In the illustrated embodiment, the reference COP_REF is initialized at a relatively high value, such as 120 PSIG, and if COP exceeds this value (indicating high air conditioning load), the blocks  122  and  124  are executed to select an appropriate value for DOOR_POS(new) and to set COP_REF to a lower value, such as  90  PSIG. If the load is subsequently reduced, and COP falls to the lower value, the blocks  118  and  120  are executed to set DOOR_POS(new) to full outside air, and to restore COP_REF to the high value (120 PSIG). 
     In an alternate implementation, the load-based determination of block  116  may be replaced with a comparison of the enthalpy of the outside air relative to the cabin air. In such an implementation, DOOR_POS(new) is set to full outside air if the cabin air has a higher enthalpy than the outside air, whereas DOOR_POS(new) is determined by table look-up in accordance with this invention if the outside air has a higher enthalpy than the cabin air. Such an implementation requires knowledge of the temperature and relative humidity of both the cabin air and the outside air. 
     As indicated at block  122 , the value of DOOR_POS(new) when the inlet air mixture control is enabled may be determined by table look-up as a function of the commanded blower motor speed CBMS (or the measured blower motor speed, as mentioned above). The table values may be determined empirically based on measured air flow through the ducts  44   a ,  44   b  at different blower motor speeds, so that the retrieved door position DOOR_POS(new) obtained from the table will result in an inlet air mixture comprising a predetermined amount (flow) of outside air, as explained above. 
     The block  126  is then executed to determine if the count of an INLET CONTROL TIMER exceeds a reference count REF. If not, block  128  is executed to retain the current door position (i.e., DOOR_POS(new) is set equal to DOOR_POS(old)), and the block  130  increments the INLET CONTROL TIMER. Once the count of the INLET CONTROL TIMER exceeds the reference REF, the block  132  resets the INLET CONTROL TIMER to zero and the blocks  134 - 144  determine an appropriate value for DOOR_POS(new). Thus, the INLET CONTROL TIMER limits the updating of the door position during inlet air mixture control to a desired maximum rate, such as one step per second. 
     The block  134  determines the requested change DOOR_POS in door position according to the difference [DOOR_POS(new)−DOOR_POS(old)]. If DOOR_POS indicates that the door is to be moved in a positive direction (determined by convention) by an amount at least as great as an actuator step in that direction MOTOR_STEP_POS, as determined at block  136 , the block  138  sets DOOR_POS(new) equal to the sum [DOOR_POS(old )+MOTOR_STEP_POS]. On the other hand, if DOOR_POS indicates that the door is to be moved in the opposite (negative) direction by an amount at least as great as an actuator step in that direction MOTOR_STEP_NEG, as determined at block  140 , the block  142  sets DOOR_POS(new) equal to the difference [DOOR_POS(old )−MOTOR_STEP_NEG]. If DOOR_POS is less than the minimum step size of actuator  109 , block  144  is executed to retain the current door position by setting DOOR_POS(new) equal to DOOR_POS(old). And in any event, the block  130  is then executed as described above to increment INLET CONTROL TIMER, completing the routine. 
     Thus, the control unit  90  gradually adjusts the position of inlet air control door  44  under conditions of high air conditioning load to increase the amount of recirculated cabin air in the inlet air mixture, while retaining a predetermined amount of outside air, 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 configured differently than shown in FIG. 1; for example, systems having a fixed displacement compressor, or utilizing a different capacity control methodology. 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.