Ventilation system for a vehicle and method for controlling air circulation

Abstract

A ventilation system for a vehicle, the vehicle having a passenger compartment (10), the ventilation system comprising: an air conditioning system (9) for treating air, the air conditioning system (9) having an inlet-selective opening (32) to an air source (42, 44) from (a) inside the passenger compartment (10) and (b) outside the passenger compartment (10); a controller (60) comprising means (40, 50) for selecting the air source (42, 44) of an intake air admitted to the air conditioning system (9); an engine oil temperature sensor (82) comprising means for detecting a signal representing a vehicle engine oil temperature and for providing a vehicle engine oil temperature signal to the controller (60), the controller (60) selecting the intake air admitted to the air conditioning system (9) from inside the passenger compartment (10) when the vehicle engine oil temperature signal is within a predetermined vehicle engine oil temperature signal range.

Description

[0001] The present invention relates generally to vehicle ventilation systems and, more particularly, to the use of recirculated air in such systems.

[0002] Passenger comfort in an automobile is maintained by air conditioning systems that provide heating, cooling, and ventilation of the passenger compartment air. These systems are commonly referred to as HVAC systems. Originally, vehicle HVAC systems relied on fresh air outside the passenger compartment for heating, cooling, and ventilation. As air conditioning systems evolved, a recirculation mode was introduced to increase efficiency. Accordingly, typical automotive HVAC systems have two ventilation modes: fresh air mode and recirculation air mode.

[0003] Vehicle operators and passengers often select the air conditioning system's air intake mode without considering the system's optimal performance, which includes the stress on the vehicle's components. For example, selecting fresh air mode as the air source for the HVAC system in hot weather places greater strain on the system compressor. This can lead to high temperature and pressure at the compressor outlet, which can cause compressor or clutch damage or damage to other cooling system components. In addition, engine coolant temperatures and engine and transmission oil temperatures can exceed recommended values, which can degrade powertrain performance. On the other hand, there are also factors that can negatively impact the use of recirculation mode under certain circumstances. For example, using recirculation mode under certain environmental conditions can cause passenger compartment fogging.

[0004] From DE 10 2007 037 626 A1 a ventilation system for a vehicle is known, wherein the vehicle has a passenger compartment and the ventilation system comprises: an air conditioning system for treating air, wherein the air conditioning system has an inlet-selective opening to an air source from (a) inside the passenger compartment and (b) outside the passenger compartment, a controller comprising means for selecting the air source of an intake air admitted to the air conditioning system, a sensor comprising means for detecting a signal representing an operating temperature of an engine coolant and for providing an engine coolant temperature signal to the controller, wherein the controller selects the intake air admitted to the air conditioning system from inside the passenger compartment when the engine coolant temperature signal is within a predetermined engine coolant temperature signal range.

[0005] DE 10 2007 045 272 A1 and JP 2001-213 144 A each show a ventilation system for a vehicle, the vehicle having a passenger compartment, the ventilation system comprising: an air conditioning system for treating air, the air conditioning system having an inlet-selective opening to an air source from (a) inside the passenger compartment and (b) outside the passenger compartment, a controller comprising means for selecting the air source of inlet air admitted to the air conditioning system, a sensor comprising means for detecting a signal representing a vehicle engine speed and for providing the vehicle engine speed signal to the controller, the controller selecting the inlet air admitted to the air conditioning system from inside the passenger compartment when the vehicle engine speed signal is within a predetermined vehicle engine speed signal range.

[0006] It would be desirable to create an HVAC system in which the recirculation mode of a vehicle HVAC system is automatically selected under certain conditions to optimize passenger compartment comfort and vehicle component life.

[0007] According to the invention, vehicle ventilation systems and a method for controlling air circulation in such systems are provided with the features of the independent claims. Advantageous developments of the invention are described in the subclaims.

[0008] Accordingly, a vehicle ventilation system includes an air inlet that can receive fresh air from outside a passenger compartment. Alternatively, the system can direct the air inlet to receive recirculated air from inside a passenger compartment. The system includes a controller for selecting the source of intake air. The controller uses a control strategy to select the recirculation mode to optimize vehicle passenger comfort and system longevity.

[0009] The system includes various sensors for providing input to the controller, which determines whether the system is instructed to use recirculated air. For example, sensors may provide input to the controller to determine engine oil temperature, transmission oil temperature, engine speed, throttle pedal position, evaporator temperature, head pressure, compressor flow, etc. The present invention enables optimization of the ventilation system.

[0010] They show: Fig. 1 is a perspective view of a portion of a passenger compartment of a vehicle having a ventilation system of the present invention. Fig. Figure 2 shows a schematic view of a portion of the ventilation system of the present invention. Fig. Figure 3 is a schematic illustrating various sensors that may be used with the present invention. Fig. 4a to 4d show a flow diagram illustrating a strategy of the present invention for controlling the use of recirculated air.

[0011] Control of air temperature, air flow, and humidity within an automobile is achieved by using various airflow control devices, such as air distribution mode dampers and temperature blend dampers, operated by multiple actuators, as is well known in the art. The actuators may include electric motors, vacuum or hydraulic regulators, etc.

[0012] Fig. 1 shows a portion of a passenger compartment 10 of a vehicle with an air conditioning system 9. The air conditioning system 9 includes typical heating and cooling elements such as a heater core, evaporator core, a fixed or variable displacement compressor, a condenser, a coolant tank, ducting, etc. The vehicle instrument panel 11 includes heating, ventilation, and air conditioning (HVAC) controls 12, including a cabin temperature control lever that controls the temperature or blend door position to select a desired temperature. The system controls 12 further include an air recirculation mode button 16 and an air distribution mode knob 18. The air distribution mode knob 18 is used to select the HVAC cabin outlet, for example, from the exhaust hoods 20, floor outlets 22, or windshield defrost vents 24, or a combination thereof.

[0013] Fig. 2 schematically shows the inlet section 30 of the air conditioning system 9. The air conditioning system 9 has an air conditioning inlet (inlet-selective opening) 32 that can receive fresh air from outside the passenger compartment 10 or recirculated air from inside the passenger compartment 10. An inlet flap 40 selectively admits intake air into the air conditioning system 9 from a fresh air inlet 42 or a recirculation air inlet 44. The fresh air inlet 42 is connected to air outside the passenger compartment 10, while the recirculation air inlet 44 is connected to the passenger compartment 10. The inlet flap is operated by an electric servomotor 50. Of course, the flap 40 can be operated by several other types of actuators, such as vacuum-operated, hydraulically operated, etc., as known in the art.Although only a single inlet flap 40 is shown here, multiple inlet flaps such as those within the scope of the present invention may be used.

[0014] The inlet flap 40 is in Fig. 2 in the recirculation mode position 46, in which the flap 40 blocks the fresh air inlet 42 and enables communication between the passenger compartment 10 via the recirculation air inlet 44 and the air conditioning inlet 32. The inlet flap 40 can be moved between the recirculation mode position 46 and a fresh air mode position 48, as shown in the phantom image of Fig. 2. In the fresh air mode position 48, the damper 40 blocks the recirculation air inlet 44 and allows communication of outside air via the fresh air inlet 42 with the air conditioning inlet 32. Of course, the damper 40 can be positioned between the fresh air and recirculation air modes.

[0015] The servomotor 50, which operates the intake door 40, is controlled by an electronic controller 60. The default position of the intake door 40 is the fresh air mode position 48. When a vehicle occupant manually selects the recirculation mode by selecting the air recirculation mode button 16, the controller 60 receives instructions to move the intake door 40 to the recirculation mode position 46 and to cause the servomotor to move the intake door 40 to the recirculation mode position 46. The controller 60 includes override strategy software that allows the controller 60 to generate signals to control the servomotor 50 and actuators, which, under certain conditions, move the intake door 40 from the fresh air mode position 48 to the recirculation mode position 46, as explained below.

[0016] With reference to Fig. 3, the air conditioning system 9 includes various sensors that detect the condition or characteristics of vehicle components, vehicle systems, or vehicle environments. The sensors provide input to the controller 60 in the form of signals representing various conditions or characteristics. For example, a voltage or current signal representing engine speed may be provided to the controller. Input of a condition or characteristic to the controller 60 herein means that the input may be in the form of a signal representing the condition or characteristic.

[0017] The controller 60 determines whether the input from the various sensors meets certain predetermined conditions. Predetermined conditions include the sensor input being within a predetermined range. Predetermined ranges may include a range at or below a predetermined value. Predetermined ranges and values, as mentioned herein, are determined by considering various factors, such as vehicle size and weight, engine size, final drive ratio, likely road conditions such as flat or sloped, likely climatic ambient air temperatures, engine cooling system, likely coolant system load, etc.

[0018] The sensors include a system "on" sensor 62 that determines whether the air conditioning system 9 is activated or "on." The sensor 62 provides a signal to the controller 60 when the air conditioning system 9 is activated.

[0019] An intake damper position sensor 64 provides signals to the controller 60 relating to whether the intake damper 40 is in the fresh air mode position 48. A hood distribution mode sensor 66 provides signals to the controller 60 relating to whether one of the exhaust hoods 20 is in use. This can be accomplished by determining whether the air distribution mode knob 18 is in any hood position.

[0020] An ambient air temperature sensor 68 measures the ambient air temperature outside the vehicle and provides ambient air temperature signals to the controller 60. The controller 60 determines whether the ambient air temperature is above a predetermined value.

[0021] The controller 60 also determines whether the likelihood of using recirculated air from within the passenger compartment will create fog in the passenger compartment. The controller 60 may use one or more different factors to make the determination, such as passenger compartment or cabin temperature, ambient air dew point or humidity, HVAC blower speed, HVAC outlet air temperature, etc. Various sensors provide signals to the controller 60 to make the fog probability determination. The cabin air temperature sensor 70 provides passenger compartment temperature information. The dew point or humidity sensor 72 provides cabin air humidity information to the controller 60. The blower motor voltage sensor 74 represents the blower speed and provides information representative of the blower speed to the controller 60.The HVAC outlet air sensor 76 provides the HVAC outlet temperature information to the controller 60.

[0022] A vehicle coolant temperature sensor 78 provides engine coolant temperature signals to the controller 60. The controller 60 determines whether the engine coolant temperature is at or above a predetermined engine coolant temperature value. A transmission oil temperature sensor 80 provides transmission oil temperature signals to the controller 60. The controller 60 determines whether the transmission oil temperature is at or above a predetermined transmission oil temperature value. An engine oil temperature sensor 82 provides engine oil temperature signals to the controller 60. The controller 60 may determine whether the engine oil temperature is at or above a predetermined engine oil temperature value.

[0023] An engine speed sensor 84, such as a tachometer, detects the vehicle engine speed and provides engine speed signals, such as revolutions per minute, to the controller 60. An engine speed timer 86 determines the time during which the engine speed is at or above a predetermined value and provides a signal to the controller 60 representing that time. A vehicle speed sensor 88, such as a vehicle speedometer, detects the vehicle speed and provides the vehicle speed signals to the controller 60.

[0024] An accelerator pedal position sensor 90 detects the accelerator pedal position and provides accelerator pedal position signals to the controller 60. A transmission gear sensor 92 detects the engaged vehicle transmission gear and provides transmission gear signals to the controller 60.

[0025] An evaporator temperature sensor 94 detects the temperature of the HVAC evaporator. A mix door position sensor 96 detects the position of the mix / temperature door (not shown). A head pressure sensor 98 detects the refrigerant system head pressure, while a compressor current sensor 99 detects the current applied to the variable displacement compressor motor. Each sensor 94, 96, 98, and 99 provides information in the form of signals to the controller 60.

[0026] Fig. 4a through 4d depict a preferred embodiment of a flowchart for a strategy 100 used by the controller 60 to bypass the fresh air mode of the air conditioning system 9. Of course, the present invention encompasses many variations and modifications of the disclosed preferred embodiment. As used in the flowchart, "TBD" means a value or amount that must be determined based on various factors, such as vehicle size and weight, engine size, final drive ratio, likely road conditions such as flat or banked, likely climatic ambient air temperatures, engine cooling system, likely coolant system load, etc. While the flowchart of the disclosed preferred embodiment indicates that the respective TBD values ​​must be met or exceeded, the present invention contemplates that ranges may be preferred for certain sensor signals.Such ranges include, for example, a higher amount than a TBD value, a lower amount than a TBD value, etc.

[0027] Fig. Figure 4a shows a series of system conditions or preconditions that must be met in the disclosed preferred embodiment before the controller 60 takes any action. The first step 101 is determining whether the air conditioning system 9 is "on." This determination is made by the controller 60 using an input signal from the system "on" sensor 62. Of course, if the system is not "on," the controller 60 takes no action.

[0028] If the system input signal from sensor 62 indicates that the air conditioning system 9 is "on," the next step 103 for the controller 60 is to determine from the input signal from the inlet door position sensor 64 whether the inlet door 40 is in the fresh air mode position 48, i.e., whether the fresh air inlet 42 is communicating with the air conditioning inlet 32. If the fresh air inlet 42 is not open to the air conditioning inlet 32, i.e., the inlet door 40 is in the recirculation mode position 46, in which recirculated air from the passenger compartment is used by the air conditioning system 9, the controller 60 proceeds to step 112 and takes no action. Step 112 does not deactivate the strategy but redirects control to step 101 to start reconsideration of the strategy from the beginning.

[0029] In the next step 105, the controller 60 determines whether the air conditioning system 9 is using air distribution modes such as the exhaust hoods 20. If the signal from the sensor 66 indicates that only other vents are being used, for example, floor vents or windshield defrost vents 24, the controller 60 takes no action. Typically, the floor vents are not used unless the air conditioning system 9 is providing cool air to the vehicle occupants. Fresh air is also preferred when the system is using the windshield defrost or defog mode. This concept can also be applied to air distribution modes other than the exhaust hood modes if the need arises in the future.

[0030] At step 107, the controller 60 receives an outside ambient air temperature signal from the sensor 68 and determines whether the ambient air temperature signal is above a predetermined value, in which case the controller 60 takes no action.

[0031] At step 109, the controller 60 estimates the probability of fog in the passenger compartment if recirculated air from the air conditioning system is being used. If the probability estimate is at or above a predetermined value, the controller 60 takes no action. Fog probability estimation techniques are known in the art. The controller 60 determines the fog probability using signals provided by one or more sensor inputs, such as the cabin temperature sensor 70, the dew point sensor 72, the blower motor voltage sensor 74, the blend door position sensor 96, the outlet air temperature sensor 76, the vehicle coolant temperature sensor 78, and the vehicle speed sensor 88. Additional data may be used to determine the fog probability, such as solar load determined by a glass temperature sensor or an instrument panel temperature sensor.

[0032] Provided that the preconditional general conditions of Fig. 4a are met, the controller 60 then makes the determinations in steps 111, 113 and 115 of Fig. 4b. The input from the vehicle coolant temperature sensor 78 is used by the controller 60 to determine whether the engine coolant is at or above a predetermined temperature. The input from the transmission oil sensor 80 is used by the controller 60 to determine whether the transmission oil is at or above a predetermined temperature. The input from the engine oil temperature sensor 82 is used by the controller 60 to determine whether the engine oil is at or above a predetermined temperature.

[0033] If any of the temperatures of sensors 78, 80, or 82 provided by the data are at or above their respective predetermined temperatures, the controller 60 proceeds to step 110 to electronically override the system controls 12 to switch the system from a fresh air mode to a recirculation air mode by moving the inlet door 40 from the fresh air mode position 48, in which the air conditioning inlet 32 ​​is open to the fresh air inlet 42, to the recirculation mode position 46, in which the air conditioning inlet 32 ​​is open to the recirculation air inlet 44, so that the air is recirculated. In addition to optimizing the air conditioning system 9, the powertrain condition strategy also mitigates powertrain cooling problems that occur during high load conditions, such as when towing a trailer or with high loads caused by long road grades, without reducing engine power under certain conditions.

[0034] If none of steps 111, 113 or 115 from Fig. 4b results in the controller 60 directing the system controls 12 to move the inlet flap 40 to the return mode position 46, step 117 of Fig. 4c. The input from the engine speed sensor 84 is used by the controller 60 to determine whether the engine speed is at or above a predetermined engine speed value. If the engine speed is at or above the predetermined value, the controller 60 then considers the input from sensor 86 to determine whether the engine speed has been at or above the predetermined engine speed value for at least a predetermined time. By considering the time the engine speed was at or above the predetermined engine speed value, the controller 60 can ensure that the engine speed was not increased due to a momentary acceleration of the vehicle. The controller 60 delays action until the sensor 86 indicates that the engine speed has been at or above a predetermined engine speed for a predetermined time, such as 30 seconds.

[0035] If the engine speed has been at or above the predetermined engine speed value for at least the predetermined time, the controller 60 next considers steps 121, 123, and 125. At step 121, the input from the vehicle speed sensor 88 is used by the controller 60 to determine whether the vehicle speed is at or below a predetermined vehicle speed value. At step 123, the input from the accelerator pedal position sensor 90 is used to determine whether the accelerator pedal position is at or above a predetermined accelerator pedal position value, which may be a pedal angle or a certain percentage of the full range of accelerator pedal travel. At step 125, the transmission gear sensor 92 provides the input to the controller 60 as to which transmission gear is engaged, and the controller 60 determines whether the engaged transmission gear is above a predetermined transmission gear value.If the controller 60 determines that each of the conditions of step 121, 123, or 125 has been met, the controller 60 proceeds to step 110 to bypass the system controls 12 and switch the air conditioner to the recirculation mode, as previously described.

[0036] If the engine speed value is below the predetermined engine speed value at step 117, or if the engine speed has not been at or above the predetermined engine speed value for the predetermined time or more as determined at step 119, the next step 127 is Fig.4d. At step 127, the input from the HVAC evaporator temperature sensor 94 is used by the controller 60 to determine whether the evaporator temperature is at or above a predetermined evaporator temperature value. If not, the controller 60 proceeds to step 112 and takes no action to bypass the system controls 12. The controller 60 simply recycles steps 101 to continuously check whether any of the system logic parameters have been met, as previously described.

[0037] If, at step 127, the controller determines that the evaporator temperature is above the predetermined evaporator temperature value, the controller 60 then considers the factors of steps 129, 131, 133, and 135 to determine whether the system controls 12 should be bypassed to place the air conditioning system 9 in recirculation mode. At step 129, the controller 60 uses the input from the blend door position sensor 96 to determine whether the blend door position is below a predetermined air blend door value. The predetermined blend door value may be a position of the blend door as a percentage of the extent to which it is moved from the full "cool" position to the full "heat" position. If the blend door position is below a predetermined blend door value, the controller 60 bypasses the system controls 12 to place the air conditioning system 9 in recirculation mode, as previously described.

[0038] At step 131, the controller 60 uses the input from the head pressure sensor 98 to determine whether the refrigerant outlet pressure is at or above a predetermined head pressure value. At step 133, the controller 60 uses the input from the blower motor voltage sensor 74 to determine whether the blower speed is at or above a predetermined blower speed value. At step 135, the controller 60 uses the input from the compressor current sensor 99 to determine whether the compressor current is at or above a predetermined compressor current value. If the controller 60 determines that each of the inputs from step 131, 133, or 135 is at or above its corresponding predetermined value, the controller 60 proceeds to step 110 to bypass the system controls 12 and switch the air conditioning system 9 to the recirculation mode, as previously described.

[0039] If none of steps 129, 131, 133, and 135 cause the controller 60 to switch the system to the feedback mode, the controller 60 proceeds to step 112 and takes no action other than reconsidering the system logic starting at step 101.

[0040] The software of controller 60 includes built-in hysteresis to prevent the intake flap 40 from continuously "chattering" or moving. For each predetermined value mentioned herein that controller 60 uses to determine whether action is required, there is a hysteresis value that must be reached before controller 60 changes its decision, as is known in the art. For example, if a predetermined engine coolant temperature of 100 degrees Celsius has been reached and controller 60 switches air conditioning system 9 to recirculation mode, the control software may allow the temperature to drop below the predetermined temperature, for example, to 95 degrees Celsius, before switching air conditioning system 9 back to fresh air mode.

[0041] In summary, the controller 60 bypasses the fresh air intake mode and switches the input of the air conditioning system 9 to a recirculation mode in the event that certain general conditions or preconditions have been met and one of powertrain temperature, driving or air conditioning control conditions has been met.

[0042] Although a preferred embodiment of the invention has been described in detail, those skilled in the art to which this invention belongs will recognize various alternative designs and embodiments for practicing the invention as defined in the following claims.

Claims

[1] A ventilation system for a vehicle, the vehicle having a passenger compartment (10), the ventilation system comprising: an air conditioning system (9) for treating air, the air conditioning system (9) having an inlet-selective opening (32) to an air source (42, 44) from (a) inside the passenger compartment (10) and (b) outside the passenger compartment (10), a controller (60) comprising means (40, 50) for selecting the air source (42, 44) of an intake air admitted to the air conditioning system (9), an engine oil temperature sensor (82) comprising means for detecting a signal representing a vehicle engine oil temperature and for providing a vehicle engine oil temperature signal to the controller (60), the controller (60) selecting the intake air admitted to the air conditioning system (9) from inside the passenger compartment (10) when the vehicle engine oil temperature signal is within a predetermined vehicle engine oil temperature signal range. [2] The ventilation system of claim 1, further comprising a transmission oil temperature sensor (80) comprising means for detecting a signal representing a vehicle transmission oil temperature and providing the transmission oil temperature signal to the controller (60), wherein the controller (60) selects the intake air admitted to the air conditioning system (9) from within the passenger compartment (10) when the transmission oil temperature signal is within a predetermined transmission oil temperature signal range or the vehicle engine oil temperature signal is within a predetermined vehicle engine oil temperature signal range. [3] The ventilation system of claim 2, further comprising a vehicle coolant temperature sensor (78) comprising means for detecting a signal representative of an engine coolant temperature and for providing an engine coolant temperature signal to the controller (60), wherein the controller (60) selects the intake air admitted to the air conditioning system (9) from within the passenger compartment (10) when, first, the transmission oil temperature signal is within a predetermined transmission oil temperature signal range, second, the vehicle engine oil temperature signal is within a predetermined vehicle engine oil temperature signal range, or third, the engine coolant temperature signal is within a predetermined engine coolant temperature signal range. [4] Ventilation system according to claim 3, further comprising means for detecting a condition indicating a probability of fog in the passenger compartment (10) when the air source (42, 44) of the air conditioning system (9) is air from within the passenger compartment (10) and for providing the probability of fog in the passenger compartment (10) to the controller (60), and an evaporator temperature sensor (94) comprising means for detecting a signal representing an evaporator temperature and for providing the evaporator temperature signal to the controller (60), wherein the controller (60) selects the inlet air admitted to the air conditioning system (9) from within the passenger compartment (10) when the evaporator temperature signal is within a predetermined evaporator temperature signal range and when the probability of fog in the passenger compartment (10) is below a predetermined value, and the means for detecting the condition,which indicates a probability of fog in the passenger compartment (10), one or more of a cabin temperature sensor (70), a dew point sensor (72), a blower motor voltage sensor (74), a blend door position sensor (96), an outlet air temperature sensor (76), the vehicle coolant temperature sensor (78), a vehicle speed sensor (88), a glass temperature sensor and an instrument panel temperature sensor. [5] A ventilation system for a vehicle, the vehicle having a passenger compartment (10), the ventilation system comprising: an air conditioning system (9) for treating air, the air conditioning system (9) having an inlet-selective opening (32) to an air source (42, 44) from (a) inside a passenger compartment (10) and (b) outside a passenger compartment (10), a controller (60) comprising means (40, 50) for selecting the air source (42, 44) of the inlet air admitted to the air conditioning system (9), an evaporator temperature sensor (94) comprising means for detecting a signal representing an evaporator temperature and for providing the evaporator temperature signal to the controller (60), and means for detecting a condition indicative of a likelihood of fog in the passenger compartment (10) when the air source (42,44) of the air conditioning system (9) is air from within the passenger compartment (10) and for providing the probability of fog in the passenger compartment (10) to the controller (60), wherein, the controller (60) selects the intake air admitted to the air conditioning system (9) from within the passenger compartment (10) when, firstly, the evaporator temperature signal is within a predetermined evaporator temperature signal range and, secondly, the probability of fog in the passenger compartment (10) is below a predetermined value and the means for detecting the condition indicative of a likelihood of fog in the passenger compartment (10) comprises one or more of a cabin temperature sensor (70), a dew point sensor (72), a blower motor voltage sensor (74), a blend door position sensor (96), an outlet air temperature sensor (76), a vehicle coolant temperature sensor (78), a vehicle speed sensor (88), a glass temperature sensor, and an instrument panel temperature sensor. [6] The ventilation system of claim 5, further comprising the mixing door position sensor (96) for providing an air mixing door position signal to the controller (60), wherein the controller (60) selects the intake air admitted to the air conditioning system (9) from within the passenger compartment (10) when, further third, the air mixing door position signal is within a predetermined air mixing door position signal range. [7] A ventilation system according to claim 5, further comprising a head pressure sensor (98) comprising means for a signal representing an air conditioning head pressure and for providing the head pressure signal to the controller (60), wherein the controller (60) selects the intake air admitted to the air conditioning system (9) from within the passenger compartment (10) when further third the head pressure signal is within a predetermined head pressure signal range. [8] A ventilation system according to claim 5, further comprising a second sensor comprising means for detecting a signal representing an air conditioning fan speed and for providing the fan speed signal to the controller (60), wherein the controller (60) selects the intake air admitted to the air conditioning system (9) from within the passenger compartment (10) when further third the fan speed signal is within a predetermined fan speed signal range. [9] The ventilation system of claim 5, further comprising a compressor current sensor (99) comprising means for detecting a signal representing a current used by the air conditioning compressor and providing the compressor current signal to the controller (60), wherein the controller (60) selects the intake air admitted to the air conditioning system (9) from within the passenger compartment (10) when, further third, the compressor current signal is within a predetermined compressor current signal range. [10] A ventilation system according to claim 5, further comprising an ambient air temperature sensor (68) comprising means for sensing an ambient air temperature and providing the ambient air temperature to the controller (60), wherein the controller (60) selects the intake air admitted to the air conditioning system (9) from within the passenger compartment (10) when, further third, the ambient temperature is above a predetermined ambient temperature value. [11] A method for controlling air circulation in a ventilation system for a vehicle, comprising the steps of: manually selecting an air intake mode from one of a fresh air mode and a recirculation air mode; sensing a vehicle engine oil temperature; and overriding the manually selected air intake mode by automatically selecting the recirculation air mode when the vehicle engine oil temperature is above a predetermined temperature.

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