How an Air Bag System Works
by Taras S. Rudnitsky Air Bag Attorney and Former Air Bag Engineer
When your car, truck, SUV or van is involved in an accident, the air bag sensors are the first components to detect the crash. In earlier vehicles, these sensors were basic switches that responded to changes in velocity as the vehicle slowed down during the crash. Once two sensors “closed” to confirm that a crash was taking place, electrical current was allowed to flow to the air bag modules. In newer vehicles, electronic sensors measure the deceleration (negative acceleration) of the vehicle, process it mathematically through a computer algorithm, and then compare the measured values to the values stored inside it from crash testing. If the measured values indicate the crash is more severe than the stored crash tests, the control module would allow electrical current to flow to the air bag modules. Once the electrical current is flowing to the air bag modules, it heats up a “squib” within the inflator that has a small filament inside a container of chemically explosive or flammable material. Once the filament gets hot enough, the chemicals begin to burn. This burning sets off a larger reaction of a chemical called sodium azide within the inflator, which rapidly produces nitrogen gas, along with numerous byproducts. In some vehicles, the sodium azide inflator was replaced with an inflator using pressurized gas, usually a combination of helium and argon. With either type of inflator, the gas from the inflator then fills the fabric air bag that was folded over the inflator. As the gas fills the air bag, it increases in size, eventually breaking out from behind its plastic cover, and inflating to its maximum size. Driver air bags are generally shaped like a round pancake just larger than the diameter of the steering wheel, and are normally about 12 to 20 inches thick when filled. Passenger air bags are generally about 2 to 3 feet wide, and fill the space between the passenger and the dash or windshield. Thus, passenger air bags are usually 2 to 4 times larger than driver air bags, and require a more forceful inflator to fill that larger size in the same amount of time. For frontal air bags, the process of sensing the crash and inflating the air bags is usually over in less than one-tenth of a second. As the forces of the crash propel the consumer forward into the air bag, it begins to absorb the energy by compressing and letting some of the gas out through the fabric or specially-designed vent holes. This explains why many consumers who have been involved in an accident during which air bags have deployed remember the distinct chemical odor of the inflation gas and recall seeing smoke in the car.
For side air bags and rollover air bags, the process is similar. A sensor in the side structure of the car, or sometimes inside the front door, detects the rapid deceleration from the side or the vehicle beginning to rotate upwards during a rollover crash. Electrical current is then sent to the side air bags or to the rollover air bags (depending on the type of crash), which causes those air bags to deploy. Although the chemicals and gases may be different than for front air bags, the inflation process is very similar. When a side air bag deploys, it breaks out either from the side of the seat nearest the door, or from behind a plastic trim panel on the side of the car. They are much flatter than the frontal air bags and sometimes smaller (some older side air bags protected only the person’s chest, while others protected both the chest and head). Canopy or rollover air bags deploy from overhead and are also fairly flat, but extend along the side of the vehicle to protect the head and chest and reduce the risk of the consumer being ejected through an open or shattered window. What I have described here is the general function of a typical air bag system. The air bag system is fairly complex. Therefore, a failure or defect in any one of the components can cause the entire system to malfunction. In some cases, air bags have deployed in very minor crashes (or even when there is no crash), which can lead to devastating results. In other cases, due to improper design and testing, air bags have failed to deploy in head-on frontal collisions. Some of these airbag non deployments have occurred even when the closing speed has been over 60 miles per hour.