How Cars Work
The Mix
CHAPTER TWO
Last chapter, we said the engine sucks in a mixture of air and gasoline, lights it on fire, and uses the explosion to push the piston down. That sentence skipped a real question. Where does the mixture come from?
There is no can of pre-mixed fuel-and-air sitting in the trunk. Cars don’t come with one. The mixture has to be made fresh — in the right amounts, hundreds of times every second, on its own — and dropped into each cylinder right when the cylinder needs it.
On an old car, the device that does this is called a carburetor. (Pronounced KAR-bur-ay-ter. Some people drop the middle and just say “carb.”) Before we get to it, we should know how the gasoline reaches it in the first place.
From the tank to the engine
Gasoline lives in a metal tank, usually mounted under the back of the car. When you go to a gas station, that’s what’s getting filled up. A typical tank holds twelve to twenty gallons — enough fuel for a few hundred miles.
But the engine is at the front of the car, ten or fifteen feet from the tank. The gasoline has to travel that distance. It does this through a thin metal pipe called a fuel line, which runs along the bottom of the car like a vein.
Pushing the fuel through the pipe is a fuel pump. On an old car, the fuel pump is a small mechanical device bolted to the engine. It works by being squeezed and released by a part of the engine that is already moving anyway. Every time the engine moves, the pump pumps a little gasoline forward toward the front. When the engine stops, the pump stops. Simple.
Modern cars use electric fuel pumps that can run even when the engine is off, but for our 1960s car, no battery, no electricity — just the engine pulling its own fuel forward.
Why mixing matters
Liquid gasoline, by itself, does not explode. A puddle of gasoline lit on fire just burns slowly, with a flame on the surface. There is no boom. To get the kind of fast, hard explosion that can push a piston down, the gasoline has to be in the form of tiny droplets suspended in air, like a mist. Think of bug spray, or perfume coming out of an old-fashioned perfume bottle, or fog from a fog machine. That is the form gasoline has to be in to burn explosively.
It also has to be mixed with air in the right proportion. Too much fuel and not enough air, and the mixture gets sooty and won’t ignite cleanly. Too much air and not enough fuel, and there’s nothing there to burn. The sweet spot is roughly fourteen parts of air to one part of fuel by weight. For every spoonful of gasoline, about fourteen spoonfuls of air. Off by much in either direction and the engine starts running badly or stops altogether.
So the carburetor has two jobs: turn liquid gasoline into mist, and mix that mist with air in the right ratio. Both at the same time, automatically, every cycle, on its own.