Resources > Air Bag Deployment - How Air Bags Work
Air Bag Deployment
Air Bag Crash Sensors
When you are in a crash, the air bag sensors in your car are the first components to detect the crash. In older cars, these sensors were simple electro-mechanical switches that responded to the speed change of your car. If that speed change happened quickly enough, a ball or roller would move and touch an electrical contact that would cause electricity to flow to the air bag modules. Newer vehicles use a similar concept, but the air bag sensors are usually electronic, allowing them to process the vehicle crash data to decide whether air bag deployment is warranted. To prevent unnecessary deployments, most air bag systems required at least two crash sensors to detect the crash before the air bags were allowed to deploy.
Air Bag Inflation
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. (These byproducts are what can cause severe reactions to a person’s respiratory system, particularly if the person has asthma.) In some vehicles, the sodium azide inflator was replaced with an inflator using pressurized gas, usually a combination of helium and argon, or with other chemicals. With either type of inflator, the gas from the inflator then fills the fabric air bag that was folded over the inflator.
As the inflator generates the gas that fills your air bag, it breaks out from behind its plastic cover on the steering wheel and dash, and inflates to its full 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 completely 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. Although the final inflated shape of the air bag sometimes looks like a pillow, virtually all air bags deploy very forcefully and can severely injure or even kill you if they interact with you before they are fully inflated. Our independent testing has documented that some air bags deploy at speeds well over 300 mph!
For frontal air bags, the process of sensing the crash and inflating the air bags is usually over in less than one-tenth (1/10) of a second! As the crash forces propel you forward into the air bag, it begins to absorb the energy by compressing, and by letting some of the gas out through the fabric or through specially-designed vent holes. This is why you may remember the distinct chemical odor of the inflation gas and remember seeing smoke in the car after an accident where your air bags deployed.
Side Air Bags and Rollover Air Bags
For side air bags and rollover air bags, the process is similar, but the parts and their locations are slightly different. 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 and usually smaller than the frontal air bags. Some of the older side impact air bags protected only the person’s chest, while the better ones protected the head, neck and chest. 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, neck and chest, and to reduce the risk of the consumer being ejected through an open or shattered window.
What I have described here is the general function of your typical air bag system. Because airbags are complex systems, a failure or defect in any one of the components can cause the entire air bag 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.