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Written by Marshall Brain, HowStuffWorks.com, MCT
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Tuesday, 10 April 2007 |
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Imagine for a moment that you are driving your car. It's a beautiful day, and you are driving down the freeway at 60 mph. The windows are down; the radio is playing. You have not a care in the world.
The funny thing is that when you are sitting in that position, you are positioned right in front of an explosion that is waiting to happen. There is a computer in your car dedicated to that explosion — its single goal in life is to set that explosion off. And the computer is listening to a tiny sensor that in many cars is no bigger than a pea. When that sensor decides it is time for the explosion to happen, the computer will hear its tiny signal. In less than a millisecond the explosion will start. When the explosion happens, it is powerful enough to kill people, but for most people it is not deadly. Instead, it saves their life.
Welcome to the world of airbags. An airbag is an explosion designed to protect rather than to destroy.
The reason why your airbag is poised and ready to explode out of your steering wheel is because an accident can happen at any moment. For example, a car driving the other direction on the freeway could suddenly veer across the median and run into you head-on.
At the moment of a head-on collision, your airbag has about 5 hundredths of a second to react. During that time, your body is going to decelerate from 60 mph to 0 mph. The goal of the airbag is to help that deceleration happen as gently as possible.
The first thing that detects the crash is an accelerometer. An accelerometer is simply a device that can sense when something is rapidly accelerating or decelerating. Many cars use a tiny micro-machine etched out of silicon to detect the crash. But an accelerometer could be as simple as a steel ball attached to a magnet at one end of a small tube. During a crash, the ball breaks free from the magnet and hits two metal contacts to complete a circuit. The airbag control computer gets a signal from the accelerometer. Most cars have four or five accelerometers, and the airbag computer wants more than one of them to send a signal to be sure it is not a false alarm. Once the computer is satisfied that this is the real deal, it starts the fireworks.
What happens next is rather amazing. Inside your steering wheel is a bag made out of a fabric like nylon. There is also a metal canister called an inflator that contains sodium azide. Sodium azide is a highly reactive chemical that is very much like the fuel in a solid rocket engine. The signal from the computer fires an igniter, which lights the sodium azide. The sodim azide reacts incredibly quickly and it produces a huge cloud of hot nitrogen exhaust gas. That gas goes straight into the nylon bag. The bag inflates in a few milliseconds, with the front of the bag bursting out of the center of the steering wheel at 200 mph. This 200 mph piece of fabric is heading straight for your face.
The reason why it is heading for your face is because, if you think about it, your head is the most important thing to protect in this collision. Your torso is held in place by the seatbelt, but your head, attached to your spindly little neck, weighs as much as a bowling ball. This bowling ball contains your brain. Your brain is pretty fragile and it is still moving forward at 60 mph. The idea behind an airbag is to slam the piece of nylon, backed by a jet of nitrogen gas, into your head at 200 mph. That way, your head slows down to zero MPH as gently as possible. The airbag also takes a big load off your neck. Without an airbag, your neck has to take all the stress of bringing your bowling ball head to a stop.
So think about this the next time you are driving down the road. Sitting in front of you is your own personal explosion, connected to its own explosion-control computer. Around your car there are a number of hidden collision-sensing accelerometers. All of this equipment is sitting there quietly, waiting to bring your bowling ball head to a stop in 5 hundredths of a second, should the need ever arise.
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(Looking for more? For more on this or the scoop on other fascinating topics, go to HowStuffWorks.com. Contact Marshall Brain, founder of HowStuffWorks, at
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