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March 11, 2009 > TechKnow Talk: When Hot Air Can Save Your Life

TechKnow Talk: When Hot Air Can Save Your Life

The light is green and as you drive into the intersection you check your watch, mentally calculating your arrival time. Suddenly, another car darts through the red light right in front of you. You stomp on the brake, but realize it is too late. The squeal of tires is replaced by a sickening crunch of metal and a shower of breaking glass. Though briefly stunned, you are able to force your buckled door open far enough to slip through. Only then, looking back into the car, do you notice the limp, white balloon hanging from your steering wheel.

It's possible the airbag has saved you from serious injury. When a vehicle stops suddenly, passengers continue moving until they make contact with something inside the vehicle to stop them. The purpose of an airbag is to reduce a passenger's velocity to zero as gradually as possible, distributing the force across as much of the body as possible and shielding the passenger from contacting hard objects such as the steering wheel, the dashboard, or the windshield.

In general, there are two types of airbags: frontal and side. Frontal airbags are located in the steering wheel for the driver, and in the dashboard on the passenger side. These are intended to reduce injuries from collisions to the front of the vehicle. Side airbags come in several types, and may deploy from the side of the seat or from the door. These protect passengers in side impact or rollover situations.

Despite various styles, shapes, and purposes, all airbags work in a similar way. Vehicles built in recent years contain sensors that monitor factors such as speed, acceleration, braking force, passenger positions, seat belt use, etc. and feed this information to an Airbag Control Unit (ACU), sometimes called an Airbag Control Module. This device analyzes the input and determines when to deploy one or more airbags.

Based on an evaluation of the various factors associated with the accident, sufficient force to trigger deployment of a frontal airbag will typically occur in a "brick wall" collision of about 10-15 miles per hour or more, or an impact with a parked car at more than about 20-25 miles per hour. Upon sensing an impact, the ACU also sends a signal to remove any slack from the seat belts.

When an airbag assembly receives the signal to deploy, it ignites a small canister of solid propellant. This is a chemical mixture that burns tremendously quickly, producing a large quantity of nitrogen gas. The gas expands the airbag, pushing it outward into the passenger compartment. This entire process happens so quickly it is essentially an explosion. The time required to fully deploy an airbag is less than one-tenth of a second.

The airbag itself is a strong nylon bag perforated with tiny holes around the sides. The nitrogen gas exhausts through these holes as it absorbs the force of the passenger's body, cushioning the impact. Thus the bag is deflated almost as quickly as it was inflated. By the way, nitrogen is a harmless gas, the most common element in the air we breathe.

Frontal airbags on the passenger side of the vehicle are larger than those on the driver's side, as they have a larger space to fill. Side air bags come in a variety of shapes and sizes, and are intended to protect the passenger from contact with the door or window. Side airbags designed to deploy in a rollover event remain inflated for several seconds, as such as accident takes longer to unfold. These also protect against secondary impacts following the original collision.

After several experiments with frontal airbags dating back to the 1970s, they became common in cars in the mid-1990s. Since 1999, both driver and passenger frontal airbags have been required by U.S. law. Side airbags are not legally mandated, though more than 90% of 2009 model cars offer them as either standard or optional equipment. They are less common in pickup trucks.

As an interesting aside, the Honda Gold Wing touring motorcycle has offered an airbag since the 2006 model year. Deployment is triggered by a sensor in the front forks, inflating the bag into the space between the rider and the windscreen. While the rider is still likely to be thrown from the bike, the intent is to reduce his or her velocity and force of impact with the road surface or other vehicles.

Airbag technology is still relatively new, and additional advances are forthcoming. One of the most active areas of development is the ACU. These electronic brains are becoming increasingly sophisticated, as is the network of sensors that feeds them information. The algorithms necessary to analyze the data and make complex decisions in mere milliseconds are continually under refinement by automakers.

In addition, airbag design is evolving to reduce injuries associated with the airbags themselves. There has been considerable public debate regarding the perceived dangers associated with airbags, including the potential for burns and injuries to the head and neck. In part to address this concern, airbags with the ability to deploy at a variety of speeds and forces are in development, to most effectively react to specific impact and passenger circumstances.

While airbags have caused injury, it is important to weigh this risk against their value in preventing and lessening the severity of injuries. The National Highway Traffic Safety Administration (NHTSA) has estimated approximately 300 deaths are attributable to airbag deployments. The majority of these involved children and pre-1999 airbag designs. Most victims were not wearing seatbelts. By comparison, various studies estimate 7000 to 30,000 people are alive because of airbags. A frontal airbag reduces the risk of fatality in a front-end collision by about 30%.

The nitrogen gas is very hot when it inflates the airbag, and though it cools very quickly, it is possible it may cause burns to the arms, hands, or face as it exits the bag. However, the greatest danger arises when a passenger is too close to the explosive deployment. A frontal airbag may deploy at 200 miles per hour. A safe distance is considered 10 inches. Thus, a driver should position his or her seat to maintain this minimum separation from the steering wheel.

Properly using seatbelts is also essential to gaining the full advantage of an airbag and limiting the risk of injury. The ACU removes slack from the seatbelt as it detects the impact, ensuring the passenger is well supported and constrained in an ideal posture to receive maximum benefit from the airbag.

Unrestrained and very young children are at greatest risk from an airbag. Children in child seats should never be positioned so as to place the child's head near the dashboard. Children should ride in the rear seat whenever possible, in seat belts or child seats as appropriate. If a child must be located in the front seat, ensure they are properly restrained and move the seat as far back as feasible.

Rather than quoting more statistics, the TechKnow Guy would like to emphasize that if you are properly positioned and belted in, airbags are far more likely to prevent injury than to cause it, and may even save your life. For more information on airbag safety, see the vehicle owner's manual or contact the NHTSA.

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