The air bag or Supplemental Restraint System (SRS) is a safety device designed to be used in conjunction with the seat belt. Its purpose is to help protect the driver in an impact exceeding a certain set limit. The system consists of the air bag module(s), impact sensors, a clockspring and a dedicated air bag control module.
The air bag is a fabric bag or balloon with an explosive inflator unit attached. The system employs impact sensors and a safing sensor, as well as an inflator circuit and control module.
When the control unit receives the sensor signals, power is supplied to the inflator circuit, either from the battery or backup system. A small heater causes a chemical reaction in the igniter; the non-toxic gas from the chemical mixture expands very rapidly (in milliseconds), filling the bag and forcing it through the cover pad. Since all this is happening very rapidly, the expanding bag should reach the occupant before he/she reaches the steering wheel/dashboard during a frontal collision. The chemical reaction is complete by the time the air bag is fully inflated; as the occupant hits the bag, the gas is allowed to escape slowly through vents in the back of the bag.