Air Bag Systems: Description and Operation

Fig. 1:

In the event of a frontal impact, the vehicle body will deform, absorbing the energy of the impact. The crash pulse is transmitted through the steering column to the steering wheel and inflator.

If the pulse is of sufficient strength and duration, the sensing mass moves forward and releases the D shaft. The D-shaft releases the firing pin which, under spring pressure, travels forward and ignites the primer. The primer reacts with a propellant to produce nitrogen gas.

The air bag fills with nitrogen gas and expands, splitting open the module cover. The air bag continues to expand until it is fully inflated.

Meanwhile, the driver travels forward until contact is made with the fully inflated air bag. The driver is decelerated by the air bag, which is supported by the steering wheel.

As the driver travels into the air bag, the nitrogen gas is expelled at a controlled rate through a vent in the rear of the bag, thus absorbing impact energy.

Simultaneously, the steering column collapses and the steering wheel and steering column brackets deform, absorbing impact energy until the driver comes to rest.

Air Bag System Operation - Typical Time Elapse Sequence During 30 mph Frontal Impact Into Solid Barrier

0.000:

Start of impact.
Time elapsed 00 milliseconds

0.020:

Crash pulse reaches air bag sensor. Driver begins to travel forward. Sensor triggers firing pin. Nitrogen gas is produced by primer/propellant. Airbag starts to inflate. Module cover splits open.
Time elapsed 20 milliseconds.

0.040:

Airbag continues to inflate. Airbag is fully inflated at 40 milliseconds.

0.060:

Driver continues to travel forward into the airbag; nitrogen is exhausted through the vent hole and simultaneously, the steering column collapses, absorbing impact energy.
Time elapsed 60 milliseconds.

0.080:

Driver at foremost position during impact. Time elapsed 80 milliseconds. Deformation of the vehicle is complete at approximately 110 milliseconds. Driver starts to rebound from the deflating airbag to fully seated position.

0.170:

Air bag fully deflated. All nitrogen expelled. Time elapsed 170 milliseconds.