How do airbags save lives? The engineering behind these lifesavers

Do you know exactly how they pop out?
Maia Mulko
Renault Laguna with its deployed airbags.Source: David Perez/Wikimedia Commons

An airbag can be defined as a gas-inflated cushion that serves as a safety system for some of a vehicle’s occupants. 

Nowadays, the airbag is one of the most used safety devices, along with seat belts. In fact, the Congress of the United States passed a law in 1991 that required automakers to install airbags on both sides of the front seats of their cars and light trucks. The law came into effect in 1998, although General Motors had already sold the first car with a passenger airbag in 1973, a special edition of the Oldsmobile Impala, fitted with an airbag for front-end accidents. Called the Air Cushion Restraint System, starting in 1975, driver-side airbags also became an option on full-sized Oldsmobiles, Cadillacs, and Buicks. In 1987, the Porsche 944 Turbo became the first car to come with driver and passenger airbags as standard equipment. 

How do airbags save lives? The engineering behind these lifesavers
A deflated airbag. Source: Paul Sableman/Wikimedia Commons

According to the National Highway Traffic Safety Administration (NHTSA), frontal airbags saved more than 50,000 lives from 1987 to 2017. 

While it’s true that the rapid deployment of the first generation of airbags could actually hurt people (especially children and those in low-speed crashes), modern sensors and other technologies have made airbag deployment much safer, and today there is no doubt that airbags can reduce the risk of fatalities during a car crash by at least 30%. 

What is an airbag?

An airbag is a safety device consisting of a cushion that inflates automatically in a fraction of a second during a collision, slowing the driver’s or passenger’s forward motion and preventing the impact of the vehicle’s occupants against the steering wheel, dashboard, or windscreen. It is considered a supplemental restraint system that complements the protection provided by the seat belt, but it doesn’t replace it (and it actually works better when it is used along with a seat belt). 

Airbags are installed into the dashboard to safeguard the front passenger and within the steering wheel boss to safeguard the driver. An airbag will remain “hidden” there until multiple sensors determine that it’s time to deploy it and rapidly trigger the ignition of the gas that inflates the bag. After that, deflation will automatically begin as the gas is released through small vents.

Types of airbags

  • Front airbags are the most common type of airbags. They deploy from the steering wheel or the dashboard and are meant to protect the driver and the front passenger against frontal or near frontal collisions. But given that these types of airbags don’t offer any protection against side collisions or roll-over crashes, side airbags were created.
  • Side airbags are airbags that deploy from the sides of a vehicle, usually from the door, the backrest of the seat, or the roof rail. There are side airbags that are designed to protect the head, others designed to protect the chest, and others that are a combination of both. The goal is to form a cushioned curtain between the car’s occupant and the striking vehicle, pole, tree, etc., or the floor in case of a roll-over crash. 
    How do airbags save lives? The engineering behind these lifesavers
    Front and side airbags in a Suzuki Alto. Source:Pineapple fez/Wikimedia Commons
  • Front-center airbags or far-side airbags inflate between both of the front seats of the car to prevent the driver and the front passenger from colliding with each other during a side collision or a roll-over crash. Front-center airbags were created by General Motors in 2013.
  • Rear airbags are situated in the back of the front seat. Unlike front airbags, rear airbags inflate quickly but partially so that they don’t “blow up” violently in the face of any children riding in the back seat. The risk of airbag-related injuries can be diminished if the deployment force is properly regulated.

    There are rear-center and rear-window airbags, too. These are meant to protect people in the backseat during a rear-end collision.
  • Knee airbags are airbags that are placed below the steering wheel. They deploy from the lower part of the dashboard in order to prevent leg injuries during a collision. Their effectiveness, however, has been questioned by the Insurance Institute for Highway Safety (IIHS). 

Who invented airbags in cars?

The airbags’ history began in 1952 when the American engineer John W. Hetrick filed a patent for a “safety cushion assembly for automotive vehicles.” Hetrick, who worked for the US Navy, is credited for inventing the airbag, although his creation didn’t pique the interest of car companies at the time. (German engineer Walter Linderer received a German patent in 1953 for his airbag, also based on a compressed air system.)

In fact, Hetrick’s airbag was pretty ineffective because it used compressed air to inflate, and compressed air didn’t induce inflation fast enough to protect a vehicle’s occupant during a real car accident. Moreover, the mechanism was triggered by spring or bumper impact, which was really imprecise. 

In 1967, an American engineer called Allen K. Breed created a ball-in-tube mechanism for crash detection that replaced Hetrick’s mechanism. The design consisted of an electromechanical sensor with a steel ball attached to a tube by a magnet. When activated, the system caused an explosion of sodium azide, which inflated the airbag much faster than compressed air.

The airbag industry is believed to have started with Allen K. Breed’s invention, although the actual implementation of airbags in cars didn’t happen until the 70s, and it was discontinued by American car makers within a few years, citing a lack of interest. In fact, Ford and GM spent years lobbying against airbag requirements, arguing that the devices were not viable, before beginning to offer them again around 1984.

How do airbags work?

Modern airbags work with a specific type of electronic control unit known as the Airbag Control Unit (ACU). 

The ACU monitors and processes the signals of several sensors, such as impact sensors, wheel speed sensors, brake pressure sensors, gyroscopes, accelerometers, etc., to determine if the airbag should be deployed at any given time. 

If the ACU detects a collision through these sensors,  it “tells” the initiator to ignite the solid propellant or chemical explosive that lies inside the airbag inflator. For example, most airbags use sodium azide (NaN3), potassium nitrate (KNO3), and silicon dioxide. When ignited, the sodium azide decomposes to produce nitrogen gas and sodium metal, which then reacts with the potassium nitrate to release more nitrogen. Both reactions are exothermic, producing large amounts of heat. The hot nitrogen then inflates the airbag. The entire process takes only around one-twenty-fifth of a second.

As the gas cools down and dissipates through vents, the airbag deflates.

What are airbags made of?

Airbags are mostly made of thin, woven nylon fabric, usually coated with a heat shield to keep them from catching fire during deployment. The fabric of an airbag can also be coated with corn starch or talcum powder to facilitate assembly and keep them pliable and lubricated before use. Airbag materials coated with silicone or urethane do not need extra heat shield coating.

The inflator unit, which contains a burning propellant to initiate the chemical reaction for inflation, is generally made of stainless steel or cast aluminum. The inflator unit also contains metal-foiled filters that seal the propellant into the inflator. These filters are usually also made of stainless steel, forming a wire mesh. 

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