How Airbags Work.

For years, the trusty seat belt provided the sole form of passive restraint in our cars. There were debates about their safety, especially relating to children, but over time, much of the country adopted mandatory seat-belt laws. Statistics have shown that the use of seat belts has saved thousands of lives that might have been lost in collisions.

Ð'­ Like seat belts, the concept of the airbag -- a soft pillow to land against in a crash -- has been around for many years. The first patent on an inflatable crash-landing device for airplanes was filed during World War II. In the 1980s, the first commercial airbags appeared in automobiles.

Since model year 1998, all new cars sold in the United States have been required to have airbags on both driver and passenger sides. (Light trucks came under the rule in 1999.) To date, statistics show that airbags reduce the risk of dying in a direct frontal crash by about 30 percent. Then came seat-mounted and door-mounted side airbags. Today, some cars go far beyond having dual airbags to having six or even eight airbags. Having evoked some of the same controversy that surrounded seat-belt use in its early years, airbags are the subject of serious government and industry research and tests.

In this article, you'll learn about the science behind the airbag, how the device works, what its problems are and where the technology goes from here.

Laws of Motion

Before looking at specifics, let's review our knowledge of the laws of motion. First, we know that moving objects have momentum (the product of the mass and the velocity of an object). Unless an outside force acts on an object, the object will continue to move at its present speed and direction. Cars consist of several objects, including the vehicle itself, loose objects in the car and, of course, passengers. If these objects are not restrained, they will continue moving at whatever speed the car is traveling at, even if the car is stopped by a collision.

Stopping an object's momentum requires force acting over a period of time. When a car crashes, the force required to stop an object is very great because the car's momentum has changed instantly while the passengers' has not -- there is not much time to work with. The goal of any supplemental restraint system is to help stop the passenger while doing as little damage to him or her as possible.

What an airbag wants to do is to slow the passenger's speed to zero with little or no damage. The constraints that it has to work within are huge. The airbag has the space between the passenger and the steering wheel or dashboard and a fraction of a second to work with. Even that tiny amount of space and time is valuable, however, if the system can slow the passenger evenly rather than forcing an abrupt halt to his or her motion.

Airbag Inflation

The goal of an airbag is to slow the passenger's forward motion as evenly as possible in a fraction of a second. There are three parts to an airbag that help to accomplish this feat:

* The bag itself is made of a thin, nylon fabric, which is folded into the steering wheel or dashboard or, more recently, the seat or door.

* The sensor is the device that tells the bag to inflate. Inflation happens when there is a collision force equal to running into a brick wall at 10 to 15 miles per hour (16 to 24 km per hour). A mechanical switch is flipped when there is a mass shift that closes an electrical contact, telling the sensors that a crash has occurred. The sensors receive information from an accelerometer built into a microchip.

* The airbag's inflation system reacts sodium azide (NaN3) with potassium nitrate (KNO3) to produce nitrogen gas. Hot blasts of the nitrogen inflate the airbag.

The airbag and inflation system stored in the steering wheel.

The airbag and inflation system stored in the steering wheel. See more airbag images.

Early efforts to adapt the airbag for use in cars bumped up against prohibitive prices and technical hurdles involving the storage and release of compressed gas. Researchers wondered:

* If there was enough room in a car for a gas canister

* Whether the gas would remain contained at high pressure for the life of the car

* How the bag could be made to expand quickly and reliably at a variety of operating temperatures and without emitting an ear-splitting bang

They needed a way to set off a chemical reaction that would produce the nitrogen that would inflate the bag. Small solid-propellant inflators came to the rescue in the 1970s.

The inflation system uses a solid propellant and an igniter.

The inflation system uses a solid propellant and an igniter.

The inflation system is not unlike a solid rocket booster (see How Rocket Engines Work for details). The airbag system ignites a solid propellant, which burns extremely rapidly to create a large volume of gas to inflate the bag. The bag then literally bursts from its storage site at up to 200 mph (322 kph) -- faster than the blink of an eye! A second later, the gas quickly dissipates through tiny holes in the bag, thus deflating the bag so you can move.

Even though the whole process happens in only one-twenty-fifth of a second, the additional time is enough to help prevent serious injury. The powdery substance released from the airbag, by the way, is regular cornstarch or talcum powder, which is used by the airbag manufacturers to keep the bags pliable and lubricated while they're in storage.

Airbag Safety Concerns

Since the early days of auto airbags, experts have cautioned that airbags are to be used in tandem with seat belts. Seat belts were still completely necessary because airbags worked only in front-end collisions occurring at more than 10 mph (6 kph). Only seat belts could help in side swipes and crashes (although side-mounted airbags are becoming more common now), rear-end collisions and secondary impacts. Even as the technology advances, airbags still are only effective when used with a lap/shoulder seat belt.

It didn't take long to learn that the force of an airbag can hurt those who are too close to it. Researchers have determined that the risk zone for driver airbags is the first 2 to 3 inches (5