How Cars Work
The Brain
PART TWO — How Cars Got Smart
CHAPTER SIX
Everything in Part 1 was a piece of metal pushing or pulling another piece of metal.
The carburetor mixed air and fuel because air rushing through a narrow throat created low pressure, which sucked fuel up through a hole. There was no decision in there. It was just shaped metal and physics.
The ignition system fired sparks at the right time because a rotating shaft inside the engine pushed two pieces of metal apart at exactly the right moment. Again — no decision. Just shaped metal and timing.
Mechanical solutions like these were clever, but each one could only do one thing. A carburetor can mix fuel and air well, but it cannot change the mixture if the air outside gets thinner. A distributor can fire sparks in order, but it cannot decide that this particular cylinder needs the spark a tiny bit later than the next one.
To make a car that can change its behavior — that can adjust to what is happening — you need something that can think.
That something is a computer.
Why a car needs a brain
Two things happened in the 1970s that pushed cars toward needing computers.
First, the air. American cities in the 1960s had terrible smog — yellow-brown haze, hard to see across, hard to breathe. A lot of it came from cars. Burning gasoline produces several different chemicals as exhaust, some of which are bad for the air. A perfectly mixed engine burns much more cleanly than a poorly mixed one. But a mechanical carburetor can’t always hit the perfect mix. It does its best, and its best isn’t quite good enough.
Second, the gas. In 1973, gasoline got expensive, fast. People wanted cars that could go farther on a gallon. Like emissions, fuel economy depends on burning the fuel-air mixture exactly right, every time, in every condition — and again, mechanical systems weren’t precise enough.
The fix was to replace the carburetor with fuel injection — small sprayers that squirt a metered amount of gasoline directly into the intake at exactly the right moment — and to control those sprayers with a computer that could measure conditions and adjust on the fly. The mechanical distributor got similar treatment: replaced by electronic spark control, where the computer decides exactly when each spark should fire.
By the early 1980s, every new American car had a small computer under the hood. Today, a typical car has dozens of them, each handling a different job.
The big one — the one that runs the engine — is called the engine control unit, or ECU. (Some cars call it an engine control module, or ECM. Same thing.) This chapter is about how the ECU works.