Car Advice

How does an airbag work?

We all know airbags save lives and while things like autonomous emergency braking are grabbing headlines airbags are vital for occupant protection. But, how does an airbag work?

Airbags are a vital piece of automotive safety equipment and it’s impossible to imagine a modern vehicle being released without a whole swag of the things scattered around the cabin. But that wasn’t always the case.

Who invented the airbag?

Rough patents for an airbag released after contact with the vehicle’s bumper date back to the 1950s, but it wasn’t until US researcher, Allan Breed created a sensor and safety system that the airbag was born.

It was Mercedes-Benz that first introduced the airbag, right?

Wrong. The Mercedes-Benz S-Class released in 1981 offered an airbag as a cost-option but the headline grabber wasn’t the airbag but the fact that it combined with seatbelt pre-tensioning to reduce the force of contact of an occupant and the airbag.

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Indeed, airbag system but without the seatbelt restraint had been available in the 1970s after development breakthroughs (the use of the same sodium azide propellant used today) in the 1960s that allowed airbags to inflate without 30 milliseconds. These early airbags were offered by Ford, GM and Chrysler as cost-options but without the seatbelt restraint component they failed to gain popularity. Indeed, many car makers railed against safety advocates pushing for the further development of airbags and greater use.

Some advocated the use of airbags instead of seatbelts but seatbelt as a mandatory safety item won out around the world and stifled development of the airbag. It wasn’t until 1987 and the release of the Porsche 944 Turbo that an airbag and seatbelt restraint system were fitted as standard equipment. But it wasn’t until the 1990s that the fitment of airbags onto a wide range of vehicles became a thing.

Where are airbags hidden?

Depending on the number and types of airbags your vehicle is fitted with they’ll generally be hiding behind plastic flaps designed to flip up and out of the way when the airbag is deployed. You can usually tell where the airbag is via the SRS branding which stands for Supplemental Restraint System, the airbag was named SRS after it was accepted that seatbelts would become the primary safety feature of a vehicles with the airbag as support.

How is an airbag triggered?

Many believe it’s the impact that causes the deployment of an airbag but that’s not quite true. Rather, it’s a sudden and rapid deacceleration that triggers the deployment but that doesn’t mean slamming on the brakes will cause the airbags to deploy.

The sensors used to detect ‘an impact’ which causes the sudden and rapid deacceleration are called MEMS accelerometers. To keep it simple, there’s an element inside the sensor that measures the deceleration and then sends a signal to trigger the airbag. This type of sensor needs to be used because collisions don’t always occur in the same way they do in the controlled environment of a crash test lab and speeds are almost always either higher or lower than repeatable crash tests.

And the programming of airbag triggering sensors is becoming more and more complex as technology improves. These days, airbag deployment (depending on the vehicle – adaptive airbag deployment is usually something linked to premium cars with things like electric seats) speed can be adjusted based on things like the weight of the driver and passenger, seat position and speed of the impact/deacceleration.

Is it just air that inflates the airbag?

Very early airbags were filled with carbon dioxide but that was knocked on the hand because it couldn’t fill the bag fast enough. Now, airbags are inflated via sodium azide…once the sensor contacts the cylinder containing the sodium azide the electric charge detonates an ignition compound which causes the sodium azide to decompose and convert into sodium metal and nitrogen gas – it’s the gas that inflates the airbag. From the moment the sensor detects the deceleration to the complete inflation of the airbag takes just 30 milliseconds which is less time than it takes to say ’30 milliseconds’. The driver or passenger will have hit the now deflating airbag within 50 milliseconds.

It’s worth noting that depending on where you live, say, US or Europe or Australia, the size of the airbag, and force of inflation, in your vehicle can differ. Quite often, US-built cars have slightly bigger and more forceful airbags than those built in Europe.

What happens when the airbag deflates?

If you’ve spoken with anyone who’s been in a collision where the airbags have inflated they’ll tell you about the dust in the cabin. When the airbag inflates it immediately begins deflating and that’s what absorbs the driver/passengers impact.

As the airbag deflates, the gas escaping from the vents at the back of the airbag, once it encounters air, becomes sodium carbonate, or baking soda.


Isaac Bober

Isaac Bober

Isaac Bober was born in the shadow of Mount Panorama in Bathurst and, so, it was inevitable he’d fall into work as a motoring writer. He began his motoring career in 2000 reviewing commercial vehicles, before becoming editor of Caravan & Motorhome magazine. He then moved to MOTOR Magazine before going freelance and contributing to Overlander 4WD, 4×4 Australia, TopGear Australia, Men’s Health, Men’s Fitness, The Australian, CARSguide, and many more.