How Cars Work · Crash Prevention

When prevention fails

Sometimes the crash happens anyway. A driver doesn’t see a stopped car ahead in time. A pedestrian steps off a curb. A truck runs a red light.

A different system takes over. Airbags don’t try to prevent the crash. They make the crash less awful for the people inside.

Embedded in the front of the car are sensors that measure deceleration — how fast the car is losing speed. Most of the time the deceleration is small (your mom hits the brakes at a stoplight). But in a real crash, a car can go from 30 miles per hour to zero in about a tenth of a second. That is an enormous deceleration, and it doesn’t look anything like normal braking. The sensor recognizes it and signals the airbag computer.

The airbag computer makes one decision: deploy.

From that moment, things happen very fast. An electrical pulse fires a small igniter inside the airbag module. The igniter sets off a chemical reaction (more in the sidebar). The chemical reaction produces a huge amount of nitrogen gas in milliseconds. The gas inflates a folded fabric bag, which bursts out of the steering wheel or the dashboard.

About fifty milliseconds — one-twentieth of a second — pass between the sensor recognizing the crash and the bag being fully inflated. By the time the driver’s body, still moving forward at the original speed, would have hit the steering wheel, there is a fabric pillow there instead.

The airbag does not stay inflated for long. Within another tenth of a second, vents in the bag let the gas out, and the bag absorbs the impact as it deflates. The airbag isn’t there to push you back. It is there to absorb the energy of you flying forward. Once it has done that, it gets out of the way.

Watching the road

The newest safety systems do not just watch the car. They watch the world around it.

A camera mounted near the rearview mirror reads the lane lines on the highway. If the car drifts toward a line without the turn signal on, the camera notices, and the computer either nudges the steering wheel back or shakes the seat to wake the driver up. This is lane-keep assist.

A radar in the front bumper measures the distance to the car ahead, and how fast that distance is shrinking. If the closing speed is dangerous and the driver hasn’t started braking, the computer slams on the brakes itself. This is automatic emergency braking.

Sensors in the rear corners watch for cars in the next lane and light up a small icon in the side mirror when one is there. Blind-spot monitoring.

All of these are sense-decide-act loops, exactly like the ECU and the wheel-watching systems. The sensors are different — cameras and radar instead of magnetic wheel-speed sensors — but the architecture is the same. Sense, decide, act. Faster than a human can react.

Try It: Catch the Ruler

You will need:

• A 12-inch ruler

• A friend or parent

Stand up and hold your dominant hand out in front of you, with your thumb and index finger open about two inches apart, ready to pinch closed.

Have your friend hold the ruler vertically so the bottom end of it is just above your fingers, with the 0-inch mark at the bottom and the 12-inch mark at the top. Don’t touch it yet.

Without any warning, your friend drops the ruler. The instant you see it start to fall, snap your fingers shut to catch it.

Look at the number where you grabbed it. That tells you how far the ruler fell before you reacted. If you caught it at the 4-inch mark, your reaction took about 0.14 seconds. At the 6-inch mark, about 0.18 seconds. At the 8-inch mark, about 0.20 seconds. If the ruler hit the floor, you were too slow — about half a second. (Try again.)

Most people fall in the 0.15-to-0.25 second range. That is how long it takes a human to see something happen and start to do something about it.

Now compare. ABS makes its decisions in less than 0.01 seconds — about twenty times faster than you. By the time you noticed your wheels were locking up, ABS would have already pulsed the brakes twenty or thirty times, on each wheel independently. The airbag controller decides to deploy in about 0.015 seconds. Stability control measures the car’s rotation a hundred times per second.

These computers aren’t smart in the way you are. They cannot decide what to have for dinner, or argue with their parents, or read a book. But for one specific job — reacting faster than a human can react — they are wildly better than you, or your mom, or any driver who has ever lived.

Did You Know? Inside an airbag, before it fires, is a small canister of solid chemical, usually sodium azide. (Pronounced AY-zide.) An electrical pulse from the airbag computer ignites the chemical. The reaction is nearly instant. In a few thousandths of a second, the solid chemical decomposes into sodium metal and pure nitrogen gas — a lot of nitrogen gas, fast. Nitrogen makes up about 78% of the air we already breathe, so it is harmless. But producing that much of it that fast is, technically, a small explosion. The fabric airbag absorbs the explosion and turns it into a controlled inflation, and the bag is fully open in about one-twentieth of a second. Most of the dust people see when an airbag deploys is talc and cornstarch — engineers add a little of each to keep the folded bag from sticking to itself before it fires. It is harmless. But it does look like the bag is smoking, which freaks people out.

A car full of computers

So the modern car is full of small computers, all of them watching for different kinds of trouble. The ECU watches the engine. The braking computer watches the wheels for skidding. The stability computer watches the chassis for unwanted spinning. The crash computer watches for sudden deceleration. The lane camera watches the road lines. The radar watches the car ahead.

A typical modern car has, depending on how you count, somewhere between fifty and a hundred small computers built into it. Each one does its own job. None of them are very smart on their own.

But they need to talk to each other — to share what they see, to coordinate their actions.

How they do that is the next chapter.

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