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How Do Airbags Work?

Niel Stender

A look at how airbags work with background on their development, mechanics, and varieties of airbags outside the traditional driver and passenger models.

The Ever-Important Airbag

Crash test showing a deployed airbag
Crash test showing a deployed airbag

Since 1998, passenger vehicles and light trucks produced for the US market have been equipped with at least a driver and front-passenger airbag. More recently, vehicles can be had with upwards of 10 airbags throughout the interior and exterior. They are the unsung heroes of automotive safety operating behind the scenes, or trim panels in this case. That may have you asking, “How do airbags work?”

An Advancement in Vehicle Safety

The first patent for an airbag was awarded to American John Hetrick in 1952. Using compressed air and springs, he was ahead of his time and the patent expired before airbags were widely adopted. Fast forward to 1987 and Porsche made them standard equipment on the 944. By 1998, US government regulations required airbags on most vehicles.

So, How Do Airbags Work?

Driver putting on seatbelt
Driver putting on seatbelt

Vehicle airbags are supplemental restraints, operating in concert with seat belts. Seat belts keep occupants in position during a crash while the airbag provides a cushion for preventing or limiting the impact of a passenger with the vehicle interior. The basic components of an airbag are a nylon bag, inflation system and airbag control unit (ACU). It is all folded up and remains stored in its given location, ready to inflate in less than a second in the event of a crash.

In the US, airbags are designed to deploy when a crash is detected that creates the force equivalent to hitting a brick wall at 14 mph. A system of vehicle sensors are monitored for the signal that a crash has occurred. They can include accelerometers, gyroscopes, and sensors for impact, pressure, wheel speed, and seat occupancy. When activated, they provide details to the ACU like collision type, angle, and severity. At this point, the airbag begins deployment.

Steering wheel airbag inflator
Steering wheel airbag inflator

The inflation system starts by heating the “electrical match” which is a conductor surrounded by solid propellant. The heat ignites the propellant and a chemical reaction, typically composed of sodium azide, potassium nitrate and silicon dioxide, creates nitrogen gas. That gas rapidly fills the nylon bag to the point that it bursts from storage to provide an occupant cushion. This whole process happens faster than you can blink.

Upon occupant impact with the bag, it “catches” them and immediately starts deflating as the gas escapes through vent holes in the bag. This is normally accompanied by the release of dust-like particles, which are typically cornstarch or talcum powder. This powdery mixture is designed to keep the bag pliable while it is in storage. It can cause minor eye and throat irritation if the passenger remains in the vehicle with the windows closed post-deployment.

How Airbags Have Gotten Better

Steering wheel that has an airbag within it
Steering wheel that has an airbag within it

Modern, advanced airbags have implemented algorithms to adjust triggering based on crash circumstances. Taking into account factors like occupant weight and posture, seatbelt use, and distance from the airbag, they can modify how the deployment happens. Multi-stage inflators can deploy less forcefully in moderate crashes versus very severe impacts. These adaptive airbag systems can also adjust the pressure within the airbag to increase or reduce the force exerted on passengers.

Standard front airbags are located within the steering wheel and behind the passenger side dashboard. These areas operate like doors, designed to tear open under the force of the airbag being inflated very rapidly. These are intended to restrain occupants in the event of a frontal or head-on collision. Of course, accidents can happen from any side of the car at a variety of angles. So, airbags have evolved to provide a broader safety net.

While not required equipment, side curtain airbags have become more commonplace. Typically, they are mounted up high and run the length of the front and back door. Upon deployment, they create one long inflated curtain. The idea is to protect passengers’ heads from hitting the vehicle pillar during a side or angled, overlap impact. Side torso airbags may also be used to specifically protect the pelvic and lower abdomen region of occupants.

Driver airbag and passenger curtain airbag
Driver airbag and passenger curtain airbag

A variation of the curtain style is a rollover curtain airbag. If the vehicle rolls, this type of airbag will cover a larger percentage of the windows and remain inflated longer. Intended to keep occupant heads cushioned, rollover airbags also serve to prevent ejection from the vehicle. SUVs and pickups are more likely to have these as their higher center of gravity makes them more prone to rolling during a crash.

Seat belt airbags, which reside within the belt, are designed to distribute force across the bag and avoid rib cage or chest injuries. For compact cars, a rear shield airbag can be used to inflate across the back and protect occupants’ heads in the event they are rear-ended. Volvo offers an exterior, pedestrian focused airbag. When deployed, hard areas like the A-pillars and hood edges are covered by airbags to limit the severity of injuries in pedestrian accidents.

Keep Your Airbag Safe and It’ll Keep You Safe

Steering wheel with cover removed
Steering wheel with cover removed

An important note about vehicle airbags is that they are designed to stay energized even if the battery connection to the ACU is severed during an accident. Using capacitors, this ensures the airbags continue to work properly under these conditions. However, if your vehicle requires maintenance involving the steering wheel, the same thing applies. Typically, the airbag module will maintain power for up to 30 minutes after disconnecting the battery. So it is important to work with a trained professional when it comes to service of this nature.

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Niel Stender
Niel Stender

Niel got his start in the car world as a vehicle dynamics engineer with SVT at Ford. He still has dreams of running figure eights on the wet pad in an S197 GT500. These days, when not fending off karate kicks from the 3 tiny humans that run his household, he can be found on St. Louis backroads pretending to be Ken Block in his 6-speed Mazda 6.

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