Restraint systems (Airbag Modules): Description and operation

Airbag Modules

During a frontal collision each airbag module deploys a gas filled bag to form a protective cushion between the front seat occupant and the steering wheel or fascia/windshield. The driver's airbag module is attached to the centre of the steering wheel. The passenger's airbag module is installed in the fascia, above the glovebox.

Each airbag module has a gas generator attached to a folded airbag installed in a housing. The driver's airbag has an inflated volume of 60 litres (2.12 ft³); the passenger's airbag has an inflated volume of 150 litres (5.30 ft³). The gas generator of the driver's airbag module is filled with a nitrocellulose based material; the gas generator of the passenger's airbag module is filled with a sodium azide based material.

The outlet of the gas generators incorporates a filter screen to prevent solid combustion by-products entering the airbag during deployment. An igniter (squib) in each generator provides an ignition source when triggered by a fire signal from the airbag DCU. A 2 pin connector provides the interface between the igniter and the vehicle wiring.

On the driver's airbag module, the housing is closed by a cover that forms the steering wheel centre pad; split lines are formed in the inner surface of the cover to direct the airbag through the required exit point during deployment. On the passenger's airbag module, the housing is closed by a trim panel profiled to match the fascia; a tethered deployment door forms an integral part of the trim panel.

Both airbag modules operate in the same way. On receipt of a fire signal from the airbag DCU, the igniter ignites the material in the gas generator. The burning material rapidly produces a large amount of nitrogen gas which passes through the filter screen into the airbag, forcing the airbag to unfold. On the driver's airbag module, the unfolding airbag ruptures the cover along the split lines; on the passenger's airbag module, the unfolding airbag breaks the deployment door fixings to the module housing and trim panel, and the deployment door lifts off the fascia (but remains tethered to the module housing). Once free of the housing the airbag inflates to its full extent. Vents in the airbag prevent excess pressure bursting the bag and, as soon as the material in the gas generator is exhausted, allows the airbag to instantly deflate.