Airbags are usually described as a Supplemental Restraint System, or SRS, but in some countries, wearing seat belts is not mandatory in which case air bags then become the primary restraint mechanism, and they would need to trigger at lower speeds and be larger in volume. There are a number of different types of air bag, their size and location determined by the type of protection they offer.
The most common location for an airbag is in the center of the steering wheel. It protects the driver from frontal impacts. Airbags are also commonly fitted to the passenger side of dashes for the same reason. Side impact air bags are located in the sides of front seats to protect the occupants from side impacts. Curtain side air bags are located in the side edge of roof linings to protect the occupant s head from side impacts.
The air bag assembly consists of:
There are two different types of air bag triggering mechanisms - electrical and mechanical. Most air bags are triggered electrically, with a small electric current delivered from a remote SRS control unit. Mechanically activated systems use inertia to move a triggering pin. Regardless of the type the triggering mechanism, the air bag deploys due to simultaneous explosions occurring within the squib, the igniter, and the gas generator. All three of these are located in a metal housing attached to the back of the air bag assembly.
When the control unit determines the air bag should be deployed the electric current triggers the squib. The heat generated causes the igniter to burn, which in turn explodes the gas generator. The high-pressure nitrogen gas is produced and the air bag rapidly inflates. When the air bag assembly is mounted, it sits behind a pad which has a fracture line cast into the inner face. When the air bag deploys, the force of the generated gas causes the line to rupture, allowing the bag to fully inflate.
Mechanically deployed air bags do not have any electrical circuitry. The squib is ignited with a firing pin. Under severe deceleration, inertia causes a steel ball to release a firing pin into the squib. Once the squib has been triggered, the deployment process is identical to electrically triggered air bags. The air bag is fully inflated within three hundredths or 0.03 of a second, cushioning the head and upper chest of the occupant as it moves forward.
The air bag is made from nylon and is folded into the front face of the assembly. It is coated in cornflower, which acts as a lubricant during deployment. Holes are usually located in the rear face of the air bag to allow the nitrogen gas to escape. This deflates the air bag and provides a cushioned, rather than a hard surface, to help protect the occupant.