How Your Car’s ‘Smell Sensor’ Works

Car enthusiasts, particularly those that dabble in keeping older, more experienced vehicles alive, all know that feeling. You’re out on the highway, and you smell something oily, or maybe you smell raw fuel. You’re trying to figure out whether or not that’s coming from your little project, or someone else’s. But what if your car stopped you smelling those nasty things entirely? Well, some cars have a special sensor (which I’m just going to call the “smell sensor” for headline purposes) designed to do just that. Today we’re going to learn about how it works, in fine detail. I even cut one open to look inside!

The sensor goes by a lot of different names. Fundamentally, these are sensors that are able to detect the presence of certain gases in the air. However, automakers like to call them different things. SEAT calls it an air quality sensor, while Volvo calls it the Interior Air Quality Sensor because that sounds even better. However, it’s actually a bit of a misnomer, as we’ll find out shortly, because the sensor actually detects gases outside the cabin. BMW, one of the first companies to implement this technology, calls them AUC sensors. This stands for Automatische Umluft Control in German, which translates to automatic recirculation control in English.  Thanks to BMW’s highly descriptive nomenclature, that tips us off as to how these sensors help keep a car smelling fresh inside.

At their heart, these sensors are charged with detecting the presence of certain gases in the air. Typically, they detect carbon monoxide, nitrogen oxides, and hydrocarbons. The former two are typical products of the combustion process, while unburnt fuels or smoky oil vapors would be the prime example of hydrocarbons in the air out on the road. If you’ve ever driven past a badly-running diesel or a car with a nasty oil leak, you’ve probably suffered the hot, unpleasant smell indicative of a vehicle working its way towards the crusher.

The sensor is usually located somewhere in the front of the vehicle under the hood, and is thusly used to control the HVAC system to avoid these nasty gases making their way into the cabin. These sensors are typically only fitted to vehicles with higher-end automatic HVAC control systems. In duly equipped vehicles, the air quality sensor feature is enabled via setting the recirculation control to automatic or just generally putting the HVAC in full auto mode. Then, when the sensor detects something unpleasant outside the car, the HVAC system will automatically shut flaps to enter recirculation mode so the nasty air doesn’t enter the vehicle.

So far, so simple. But this is The Autopian, where we like to dive a little deeper. Thus, I grabbed one of these sensors and cut it open to try to find out what lurks inside. I selected an AUC sensor used on my 2008 BMW 320D, part number 64119240180. According to the plastic housing, the device was manufactured by Paragon AG, a company founded in 1988 which has long specialized in the production of these sensors. BMW was one of the first automakers to fit an air quality sensor to its vehicles, first doing so in 1989.

Taking apart the sensor was no easy feat, as the part designed to survive in the rough and tumble automotive environments. The components inside aren’t just placed inside a plastic housing, they were overmolded with layers of rubbery plastic goop. This serves to protect the internal components from knocks and vibrations, but makes disassembly very difficult for the ardent and curious engineer. In any case, perseverance and a good pair of side-cutters eventually got me a look at what lay inside

In any case, the sensor fundamentally consists of a small sensor element (see below), which lives inside the front of the plastic housing behind the air inlet holes and a simple filter screen. The sensor element is connected to a small electronic printed circuit board, which hosts the components that condition the sensor signal and convert it to something the vehicle’s ECU can readily understand. Depending on the vehicle, Paragon sensors typically communicate with an ECU via PWM signals, LIN bus, or Klimabus connection.

The sensor element itself is what actually measures the concentration of certain gases in the air. Just looking at it won’t tell you how it works, but a great deal of research will. Paragon’s most popular sensors detect carbon monoxide, nitrogen oxides, and various hydrocarbons such as those in automotive fuels using metal oxide sensing elements which have the benefit of being able to detect gases in low concentrations. These sensors are also very responsive, meaning they can pick up the presence of a gas quite quickly. They’re known for their good recovery time, which refers to the sensor’s ability to react in a timely fashion when the concentration of a given gas has decreased.

It all comes down to chemical reactions that happen on the surface of the metal oxide semiconductor sensing element. If you’re confident in your semiconductor science and university-level chemistry, you can dive into the literature, else, enjoy my simplified explanation here.  The metal oxide tends to have oxygen molecules adsorbed, or stuck, on its surface. Those oxygen atoms immobilize electrons in the conduction band in the surface region of the metal oxide. If that sounds too confusing and complex, here’s a simpler explanation: Oxygen on the surface of the sensor locks up some electrons in the metal oxide. This limits the conductivity of the material to a certain baseline level in regular air.

Left, oxygen molecules becoming “adsorbed”, or stuck on the surface of a metal oxide sensor, trapping electrons. Right, a carbon monoxide molecule claims an oxygen atom, becoming carbon dioxide and freeing a trapped electron. Credit: Metal Oxide Gas Sensors: Sensitivity and Influencing Factors, Wang, Yin, et al. 2010When gases come into contact with the sensing element, they react with the trapped oxygen atoms, and change the conductivity of the sensor. Chemists will tell you that so-called “reducing” gases like carbon monoxide will react with the oxygen atoms to become carbon dioxide. This removes the oxygen atoms from the sensor surface, and their removal leads to an increase in conductivity. This is because the oxygen atoms that are taken away are no longer present to keep the electrons immobilized. By contrast, “oxidizing” gases like nitrogen oxides will effectively donate more oxygen atoms to the sensor surface, immobilizing more electrons and reducing conductivity.

Long story short, certain gases that react with the oxygen on the metal oxide sensor either reduce or increase how conductive the sensor element is. High levels of carbon monoxide, nitrogen oxides, or hydrocarbons can thus be determined by the sensor’s conductivity. When the conductivity shifts up or down from the regular baseline point of clean air, it’s clear there’s an elevated level of the relevant gases that react with the sensor.

The electronics on the sensor measure the conductivity of the sensor element, and process this into a value that can be fed to the vehicle’s HVAC control unit. When elevated levels of the pollutant gases are detected, the HVAC unit shuts the flaps to configure the system for recirculating interior air.

In many cases, the onboard electronics on the sensor are also responsible for regulating its temperature. The performance and conductivity of the metal oxide sensor element can change with temperature, which could make it difficult to get reliable results regarding the presence or absence of pollutants in different conditions. To avoid this problem, a small heating element and temperature sensor is typically included as part of the sensor to keep the element in its ideal operating temperature range.

 

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These metal oxide gas sensors can be made relatively affordably, which has seen their rollout across more and more vehicles in recent years. Further development in this space is continuing, with companies like Paragon rolling out new types of sensor for picking up particulate pollution and other pollutants. Multi-gas sensors with multiple sensing elements are also becoming more common, using different sensor materials to individually quantify the presence of certain gases.

Historically, these sensors have been a fairly obscure feature. However, their use become more relevant in recent years as customers grow concerned about air quality and its impacts on human health. Notably, Polestar has put the feature front and center in the Polestar 2, with an entire air quality analysis feature available on its infotainment screen, featuring particulate and pollutant analysis.

If you’ve been driving a modern car with a high-end HVAC system, maybe you’ve passively noticed that the dodgy trucks and smoke-belching diesels on the roads don’t seem to smell as bad as they used to. Or, perhaps you never noticed anything at all, as there were no nasty smells to ensnare your nostrils. In any case, it’s worth a tip of the hat to the engineers and scientists that developed these affordable air quality sensors that keep our cars smelling fresh.

Image credits: Lewin Day, Metal Oxide Gas Sensors: Sensitivity and Influencing Factors, Wang, Yin, et al. 2010