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Overview of How Airbags Work

Timing is crucial in the airbag's ability to save lives in a head-on collision. An airbag must be able to deploy in a matter of milliseconds from the initial collisionimpact. It must also be prevented from deploying when there is no collision. Hence, the first component of the airbag system is a sensor that can detect head-on collisions and immediately trigger theairbag's deployment. One of the simplest designs employed for the crash sensor is a steel ball that slides inside a smooth bore. The ball is held in place by a permanent magnet or by a stiff spring,which inhibits the ball's motion when the car drives over bumps or potholes. However, when the car decelerates very quickly, as in a head-on crash, the ball suddenly moves forward and turns on anelectrical circuit, initiating the process of inflating the airbag. Once the sensor has turned on the electrical circuit, a pellet of sodium azide (NaN3) is ignited. A rapid reaction occurs, generatingnitrogen gas (N2). This gas fills a nylon or polyamide bag such that the front face of the bag travels at a velocity of 150 to 250 miles per hour. This process, from the initial impact of the crash tofull inflation of the airbag, takes only about 40 milliseconds. Ideally, the body of the driver or passenger should not hit the airbag while it is still inflating. In order for the airbag to cushionthe head and torso with air for maximum protection, the airbag must begin to deflate (i.e., decrease its internal pressure) by the time the body hits it.
Otherwise, the high internal pressure of theairbag would create a surface as hard as stone—not the protective cushion you would want to crash into!

What about the Gas Used to Fill the Airbag?

Chemical Reactions Used to Generate the GasInside the airbag is a gas generator containing a mixture of NaN3, KNO3, and SiO2. When the car undergoes a head-on collision, a series of three chemical reactions occur inside the gas generator....