Air quality sensor


  • Preface
  • Automatic control
  • Installation location
  • Health Benefits

Vehicles with an automatically controlled air conditioning system are often equipped with an air quality sensor. This sensor measures the quality of the air drawn in, even before it is drawn through the opening of the stove housing (and thus through the cabin filter).

The moment the air quality sensor detects an exceedance of the number of harmful gases in the outside air, the sensor sends a digital signal to the ECU of the air conditioning. This ECU then controls the recirculation valve in the heater housing: by closing the recirculation valve, no outside air enters the interior.

The drawing below shows the elements in the air quality sensor. The outside air flows into the sensor on the left. The air arrives at the sensor element through the filter. By means of an electrical resistance measurement, he “measures” the air pollution in the drawn-in outside air. The sensor electronics translate the resistance value into a digital signal and send this signal to the ECU. The sensor needs a power supply and ground to work. This makes the sensor a active sensor.

Automatic control:
The sensor starts working after the ignition is switched on. The ECU receives the sensor reading at all times, but does nothing with it when the automatic recirculation mode is turned off.

On the control panel of the interior ventilation (heating / air conditioning), in case a vehicle is equipped with such an air quality sensor, we find an automatic mode. In the following two figures, the button of the automatic recirculation mode is indicated by 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 lights are on, the signal from the air pollution sensor is not being used to operate the recirculation valve. It is therefore best to leave automatic mode (A) switched on at all times.

Smart Climate and Air Care:
On recent models from the VAG group, the automatic recirculation mode is no longer shown with a button or setting in the climate control menu. When “Auto mode” is activated in the “Classic Climate” menu, all sensors related to the climate control are used to keep the climate in the vehicle comfortable. We can think of the regulation 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 follows…

Installation location:
The air quality sensor is located near the inlet opening from the outside air to the interior. The sensor is often covered by plating above the parafan, as can be seen in the following image (bottom left) of a Volkswagen. After removing this plating, the sensor is visible.

We also sometimes find the sensor immediately after opening the hood in the intake area next to the cabin filter, as with the BMW in the image below right. The sensor is circled in red.

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 polluted substance. The working temperature of the sensor element is about 350 degrees Celsius. The measurement takes place similar to that we know from the lambda probe.

  • Low resistance: oxidizable gases such as carbon monoxide, hydrocarbons, sulfur compounds are present in the air drawn in;
  • High resistance: there are reducible gases such as nitrogen oxide in the intake air.

The following particles in the exhaust gases are detected by the sensor:

Gasoline engine exhaust:
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 translate 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.

Oxidizable gases
Reducible gases

The A/C control unit processes the air quality sensor signal and determines whether there is a reason to open or close the recirculation valve.

The following picture shows part of the stove housing, with the recirculation valve and its actuator at the top. In the condition shown, the recirculation damper has shut off the air supply from the outside (no more outside air can enter) and has opened the opening from the interior. The passenger compartment fan now draws in the air from the passenger compartment, blows the air through the evaporator and/or heater core to cool or heat the air, after which the air returns to the passenger compartment through the outlet openings. The air is then recirculated.

We do not want to do recirculation permanently: there is a great chance that unpleasant odors, tightness of the occupants and fogged windows will occur. In principle, we only want to use the recirculation valve to make the air conditioning as cold as possible (to cool the cooled air even more) or to prevent air from outside entering the interior.

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