How airbags work

The design is conceptually simple; a central "Airbag control unit" (ACU) (a specific type of ecu) monitors a number of related sensors within the vehicle, including accelerometers, impact sensors, side (door) pressure sensors, wheel speed sensors, gyroscopes, brake pressure sensors and seat occupancy sensors. The bag itself and its inflation mechanism is concealed within the steering wheel boss (for the driver), or the dashboard (for the front passenger), behind plastic flaps or doors which are designed to "tear open" under the force of the bag inflating. Once the requisite 'threshold' has been reached or exceeded, the airbag control unit will trigger the ignition of a gas generator propellant to rapidly inflate a fabric bag. As the vehicle occupant collides with and squeezes the bag, the gas escapes in a controlled manner through small vent holes. The airbag's volume and the size of the vents in the bag are tailored to each vehicle type, to spread out the deceleration of (and thus force experienced by) the occupant over time and over the occupant's body, compared to a seat belt alone.

The signals from the various sensors are fed into the Airbag control unit, which determines from them the angle of impact, the severity, or force of the crash, along with other variables. Depending on the result of these calculations, the ACU may also deploy various additional restraint devices, such as seat belt pre-tensioners, and/or airbags (including frontal bags for driver and front passenger, along with seat-mounted side bags, and "curtain" airbags which cover the side glass). Each restraint device is typically activated with one or more pyrotechnic devices, commonly called an initiator or electric match. The electric match, which consists of an electrical conductor wrapped in a combustible material, activates with a current pulse between 1 to 3 amperes in less than 2 milliseconds. When the conductor becomes hot enough, it ignites the combustible material, which initiates the gas generator. In a seat belt pre-tensioner, this hot gas is used to drive a piston that pulls the slack out of the seat belt. In an airbag, the initiator is used to ignite solid propellant inside the airbag inflator. The burning propellant generates inert gas which rapidly inflates the airbag in approximately 20 to 30 milliseconds. An airbag must inflate quickly in order to be fully inflated by the time the forward-traveling occupant reaches its outer surface. Typically, the decision to deploy an airbag in a frontal crash is made within 15 to 30 milliseconds after the onset of the crash, and both the driver and passenger airbags are fully inflated within approximately 60-80 milliseconds after the first moment of vehicle contact. If an airbag deploys too late or too slowly, the risk of occupant injury from contact with the inflating airbag may increase. Since more distance typically exists between the passenger and the instrument panel, the passenger airbag is larger and requires more gas to fill it.

Older airbag systems contained a mixture of sodium azide (NaN₃), KNO₃, and SiO₂. A typical driver-side airbag contains approximately 50-80 g of NaN₃, with the larger passenger-side airbag containing about 250 g. Within about 40 milliseconds of impact, all these components react in three separate reactions that produce nitrogen gas. The reactions, in order, are as follows.

(1) 2 NaN₃ → 2 Na + 3 N₂ (g)

(2) 10 Na + 2 KNO₃ → K₂O + 5 Na₂O + N₂ (g)

(3) K₂O + Na₂O + 2 SiO₂ → K₂O₃Si + Na₂O₃Si (silicate glass)

The first reaction is the decomposition of NaN₃ under high temperature conditions using an electric impulse. This impulse generates to 300 °C temperatures required for the decomposition of the NaN₃ which produces Na metal and N₂ gas. Since Na metal is highly reactive, the KNO₃ and SiO₂ react and remove it, in turn producing more N₂ gas. The second reaction shows just that. The reason that KNO₃ is used rather than something like NaNO₃ is because it is less hygroscopic. It is very important that the materials used in this reaction are not hygroscopic because absorbed moisture can de-sensitize the system and cause the reaction to fail. The final reaction is used to eliminate the K₂O and Na₂O produced in the previous reactions because the first-period metal oxides are highly reactive. These products react with SiO₂ to produce a silicate glass which is a harmless and stable compound.

According to a patent, the particle size of the sodium azide, potassium nitrate, and silicon dioxide are important. The NaN₃ and KNO₃ must be between 10 and 20 , while the SiO₂ must be between 5 and 10 µm.

There has been a recent effort to find alternative compounds that can be used in airbags which have less toxic byproducts. In a journal article by Akiyoshi et. Al., it was found that for the reaction of the Sr complex nitrate, (Sr(NH₂NHCONHNH₂)∙(NO₃)₂ of carbohydrazide (SrCDH) with various oxidizing agents resulted in the evolution of N₂ and CO₂ gases. Using KBrO₃ as the oxidizing agent resulted in the most vigorous reaction as well as the lowest initial temperature of reaction. The N₂ and CO₂ gases evolved made up 99% of all gases evolved. Nearly all the starting materials won’t decompose until reaching temperatures of 500 °C or higher so this could be a viable option as an air bag gas generator. In a patent containing another plausible alternative to NaN₃ driven airbags, the gas generating materials involved the use of guanidine nitrate 5-amino tetrazole, bitetrazole dehydrate,  nitroimidazole, and basic copper nitrate. It was found that these non-azide reagents allowed for a less toxic, lower combustion temperature reaction and more easily disposable air bag inflation system.

Front airbags normally do not protect the occupants during side, rear, or rollover accidents. Since airbags deploy only once and deflate quickly after the initial impact, they will not be beneficial during a subsequent collision. Safety belts help reduce the risk of injury in many types of crashes. They help to properly position occupants to maximize the airbag's benefits and they help restrain occupants during the initial and any following collisions.

In vehicles equipped with a rollover sensing system, accelerometers and gyroscopes are used to sense the onset of a rollover event. If a rollover event is determined to be imminent, side curtain airbags are deployed to help protect the occupant from contact with the side of the vehicle interior, and also to help prevent occupant ejection as the vehicle rolls over.