Airbags: How do they work

by Lisa Cardy

AIRBAGS are relatively new and very effective car safety devices, but how exactly do they work

When a car crashes into another object anything inside continues moving at the same speed as the car was at the point of impact. Although seat belts are very important for stopping people going through wind screens many crash victims sustain serious head injuries from colliding with the steering wheel or dashboard. An airbag slows a driver or passenger down to virtually zero before they hit the steering wheel.

You may be surprised to learn that chemistry plays a central role in inflating the bag when a car crashes. Airbags are not inflated from some compressed gas source, but rather from the products of a chemical reaction. The chemical at the heart of the airbag reaction is called sodium azide, or NaN3.

Under normal circumstances, sodium azide is quite stable. But when heated it breaks down and produces nitrogen gas. Only 10 grams of sodium azide are needed to make 67 litres of nitrogen gas, enough to inflate a normal airbag.

This very effective chemical reaction is set off by sensors at the front of the car. Inflation happens when a collision force equal to running into a brick wall at 10 to 15 miles per hour (mph) occurs. An electric signal is then sent to the canister that contains the sodium azide and it detonates a small amount of an igniter compound. The heat from this ignition starts the decomposition of the sodium azide and the production of nitrogen gas, which fills the airbag.

It only takes 0.03 seconds to inflate an airbag. The bag then literally bursts from its storage site at up to 200 mph. And in just 0.05 seconds the driver hits the airbag and its deflation absorbs the forward moving energy of the occupant safely.

The bag itself is made of a thin nylon fabric. Originally the bag was stored in the steering wheel or dashboard but more recent cars store the bag in the seat or door.

The workings of another piece of modern technology unravelled. We, at The AnswerBank, would love to hear from you if there's something you'd like to know about, to ask about how something works, just click here.