Automotive Electronics

Airbag Inflators

Basic Description

The airbag inflator is one of the many components of an airbag system. The main purpose of an airbag is to slow the passenger's forward motion as evenly as possible immediately after a collision. One of the biggest challenges in early efforts to adapt airbags for use in cars was their high price and the technical difficulties involved in storing and releasing the compressed gas. These difficulties included providing sufficient room for gas storage in the car, maintaining the high pressure of the gas during the lifetime of the car, and producing quick and reliable expansion of the bag without causing injuries. These requirements suggested the need to develop an airbag design based on a chemical reaction that would produce nitrogen gas to inflate the airbag. The first automotive airbags developed in the 1970s employed a solid propellant. These airbag inflation systems reacted sodium azide (NaN3) with potassium nitrate (KNO3) in order to produce nitrogen gas. Hot blasts of nitrogen gas inflated the airbag. Because sodium azide is extremely toxic, these chemicals were widely phased out during the 1990s in favor of more efficient, less expensive, and less toxic alternatives. The alternative propellants may incorporate, for example, a combination of nitroguanidine, phase-stabilized ammonium nitrate (NH4NO3) or other nonmetallic oxidizers, and a nitrogen-rich fuel other than azide (e.g. tetrazoles, triazoles, and their salts).

Airbag Inflation

As illustrated in the above figure, when a crash is sensed, the control unit sends an electrical signal to the inflator. The chemical reaction is initiated by the igniter, generating primarily nitrogen gas to fill the airbag causing it to deploy through the module cover. Due to rapid speed change of the vehicles involved in a crash, airbags must inflate rapidly to decrease the risk of occupant injuries by protecting them from hitting the vehicle interior. From the beginning of the crash, the entire deployment process is about 50 milliseconds - faster than the blink of an eye (about 200 milliseconds). Tiny holes in the bag allow the gas to disperse, absorbing the force of the impact on the occupant.

Multi Stage Inflators:

To reduce injury as a result of airbag inflation, advanced technology inflators have been developed with the capability to change the inflation pressure. This helps control the force of deployment based on the intensity of the crash, the size of the occupant, and their position in the vehicle. These inflators consist of two chambers with dual initiators. Thus, by firing either one or both of the initiators, the force of the inflation can be controlled.

Autoliv, Daicel, Joyson Safety Systems, ZF
For More Information
[1] How Airbags Work, Marshall Brain,, Apr. 1, 2000.
[2] Airbag, Wikipedia.
[3] Airbags: How Do They Work?, YouTube, YouTube, Nov. 26, 2021.