Air Bags In Automobiles

AIR BAGS IN AUTOMOBILES

Air bags are proven and effective safety devices. Between 1990 and 2000 inclusive, air bags saved about 300 lives in India. The number of lives saved increases each year, as air bags become more common in vehicles on our roads.

However, the number of lives saved is not the whole story. Air bags are particularly effective in preventing life-threatening and debilitating head and chest injuries. A study of real-world crashes conducted by the U.S. National Highway Traffic Safety Administration (NHTSA) found that the combination of seat belts and air bags is 75% effective in preventing serious head injuries and 66% effective in preventing serious chest injuries. That means 75 of every 100 people who would have suffered a serious head injury in a crash, and 66 out of 100 people who would have suffered chest injuries, were spared that fate because they wore seat belts and their vehicle had air bags.

The one fact that is common to all who died as a result of the air bag is NOT their height, weight, sex, or age. Rather, it is the fact that they were too close to the air bag when it started to deploy.

The automobile industry started in the late 1950's to research airbags and soon discovered that there were many more difficulties in the development of an airbag than anyone had expected. Crash tests showed that for an airbag to be useful as a protective device, the bag must deploy and inflate within 40 milliseconds. The system must also be able to detect the difference between a severe crash and a minor fender-bender. These technological difficulties helped lead to the 30-year span between the first patent and the common availability of airbags.

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 collision impact. 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 the airbag'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 inhibit 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 an electrical circuit, initiating the process of inflating the airbag.

Once the electrical circuit has been turned on by the sensor, a pellet of sodium azide (NaN3) is ignited. A rapid reaction occurs, generating nitrogen gas (N2). This gas fills a nylon or polyamide bag at a velocity of 150 to 250 miles per hour. This process, from the initial impact of the crash to full inflation of the airbags, takes only about 40 milliseconds

Air bags are inflatable cushions built into the steering wheel or dashboard, designed to rapidly expand in a frontal crash.. Bang. Your car hits something in front, decelerating fast enough to trigger the air bag sensors. The sensors turn a switch that energizes a wire, sending electricity into a heating element in the propellant, causing it to oxidize rapidly. This chemical reaction produces a gas that quickly fills the cloth air bag. As the gas expands, it cools considerably, as predicted by Charles' law. As soon as the bag fully inflates, it starts deflating, cushioning the impact. In less than 1/20 of a second, the bag inflates.

The Basics

Ñ"ж 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.

Ñ"ж 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.

Ñ"ж The air bag's inflation system reacts sodium azide (NaN3) with potassium nitrate (KNO3) to produce nitrogen gas. Hot blasts of the nitrogen inflate the air bag.

The inflation system is not unlike a solid rocket booster. The air bag 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! Even though the whole process happens in only one-twenty-fifth of a second, the additional time is enough to help prevent serious injury.

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