Introduction:
Vehicles with automatically controlled air conditioning are often equipped with an air quality sensor. This sensor measures the quality of the intake air, even before it is drawn in through the opening of the heater housing (and thus through the cabin filter).
At the moment the air quality sensor detects that the amount of harmful gases in the outside air exceeds the limit, the sensor sends a digital signal to the ECU of the air conditioning system. This ECU then controls the recirculation flap in the heater housing: by closing the recirculation flap, no outside air can enter the interior anymore.
The drawing below shows the elements in the air quality sensor. On the left-hand side, outside air flows into the sensor. Via the filter, the air reaches the sensor element. By means of an electrical resistance measurement it “measures” the air pollution in the drawn-in outside air. The sensor electronics convert the resistance value into a digital signal and send this signal to the ECU. The sensor requires a supply voltage and ground in order to operate. This makes the sensor an active sensor.
Automatic control:
The sensor starts operating after the ignition is switched on. The ECU receives the sensor value at all times, but does nothing with it when the automatic recirculation mode is switched off.
On the control panel of the interior ventilation (heating / air conditioning), if a vehicle is equipped with such an air quality sensor, we find an automatic mode. In the following two images, the button for the automatic recirculation mode is indicated with green arrows.
With this recirculation button, the permanent recirculation mode can be switched on, the automatic recirculation mode, or it can be switched off permanently. If no indicator lights are on, the signal from the air pollution sensor is not used to control the recirculation flap. So it is best to always leave the automatic mode (A) switched on.
Smart Climate and Air Care:
In recent models from the VAG group, the automatic recirculation mode is no longer shown as a button or setting in the climate control menu. When the “Auto mode” in the “Classic Climate” menu is activated, all sensors related to the climate control are used to keep the climate in the vehicle comfortable. We can think here of the control of the different climate zones by means of temperature sensors in several places in the interior and the light intensity sensor. An image of the screen will follow…
Installation location:
The air quality sensor is located near the inlet opening of the outside air to the interior. The sensor is often covered by paneling above the plenum chamber, as can be seen in the following image (bottom left) of a Volkswagen. After removing this paneling, the sensor becomes visible.
We also sometimes find the sensor directly after opening the bonnet, in the intake area next to the cabin filter, as with the BMW in the image on the bottom right. The sensor is circled in red.
Operation:
An air quality sensor is a metal oxide semiconductor (MOS). The electrical conductivity changes under the influence of gas. From the change in electrical resistance, it is possible to deduce the presence and concentration of the contaminant. The operating temperature of the sensor element is approximately 350 degrees Celsius. The measurement takes place in a similar way to what we know from the lambda sensor.
- Low resistance: oxidizable gases are present in the intake air, such as carbon monoxide, hydrocarbons, sulfur compounds;
- High resistance: reducible gases such as nitrogen oxide are present in the intake air.
The following particles in the exhaust gases are detected by the sensor:
Exhaust gases from petrol engines:
CO – Carbon monoxide
C6H14 – Hexane
C6H6 – Benzene
C7H16 – n-heptane
Exhaust gases from diesel engines:
NOX – Nitrogen oxides
SO2 – Sulfur dioxide
H2S – Hydrogen sulfide
CS2 – Carbon disulfide
The sensor electronics convert the resistance value into a digital output signal. This signal is sent to the air conditioning control unit. The two images below show the resistance measurement of the sensor element.
The air conditioning control unit processes the air quality sensor signal and determines whether there is a reason to open or close the recirculation flap.
The following image shows part of the heater housing, with the recirculation flap and its actuator at the top. In the state shown, the recirculation flap has closed off the air supply from outside (no outside air can enter anymore) and has opened the opening from the interior. The interior fan now draws air from the interior, blows the air through the evaporator and / or heater radiator to cool or heat the air, after which the air returns to the interior through the outlets. The air has then been recirculated.
We do not want to recirculate permanently: there is a high risk of unpleasant odours, occupants feeling stuffy and fogged-up windows. In principle, we only want to use the recirculation flap to make the air conditioning as cold as possible (cooling the cooled air even further) or to prevent air from outside entering the interior.
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