Carburetors vary quite a bit in design and complexity. The simplest possible one is essentially a large vertical air pipe above the engine cylinders with a horizontal fuel pipe joined onto one side. As the air flows down the pipe, it has to pass through a narrow kink in the middle, which makes it speed up and causes its pressure to fall. This kinked section is called a venturi. The falling pressure of the air creates a sucking effect that draws air in through the fuel pipe at the side.
The venturi effect: When a fluid flows into a narrower space, its speed increases but its pressure drops. This explains why wind whistles between buildings and why canal boats, drifting parallel to one another, are often pushed together. It’s an example of the law of conservation of energy: if the pressure didn’t drop, the fluid would be gaining extra energy as it flowed into the narrow section, which would violate one of the most basic laws of physics.
The air flow pulls in fuel to join it, which is just what we need, but how can we adjust the air-fuel mixture? The carburetor has two swiveling valves above and below the venturi. At the top, there’s a valve called the choke that regulates how much air can flow in. If the choke is closed, less air flows down through the pipe and the venturi sucks in more fuel, so the engine gets a fuel-rich mixture. That’s handy when the engine is cold, first starting up, and running quite slowly. Beneath the venturi, there’s a second valve called the throttle. The more the throttle is open, the more air flows through the carburetor and the more fuel it drags in from the pipe to the side. With more fuel and air flowing in, the engine releases more energy and makes more power and the car goes faster. That’s why opening the throttle makes a car accelerate: it’s the equivalent of blowing on a campfire to supply more oxygen and make it burn more quickly. The throttle is connected to the accelerator pedal in a car or the throttle on the handlebar of a motorcycle.
The fuel inlet to a carburetor is slightly more complex than we’ve described it so far. Attached to the fuel pipe there’s a kind of mini fuel tank called a float-feed chamber (a little tank with a float and valve inside it). As the chamber feeds fuel to the carburetor, the fuel level sinks, and the float falls with it. When the float drops below a certain level, it opens a valve allowing fuel into the chamber to refill it from the main gas tank. Once the chamber is full, the float rises, closes the valve, and the fuel feed switches off again. (The float-feed chamber works a bit like a toilet, with the float effectively doing the same job as the ballcock—the valve that helps a toilet refill with just the right amount of water after you flush. What do car engines and toilets have in common? More than you might have thought!)
How a carburetor works.
In summary, then, here’s how it all works:
Air flows into the top of the carburetor from the car’s air intake, passing through a filter that cleans it of debris.
When the engine is first started, the choke (blue) can be set so it almost blocks the top of the pipe to reduce the amount of air coming in (increasing the fuel content of the mixture entering the cylinders).
In the center of the tube, the air is forced through a narrow kink called a venturi. This makes it speed up and causes its pressure to drop.
The drop in air pressure creates suction on the fuel pipe (right), drawing in fuel (orange).
The throttle (green) is a valve that swivels to open or close the pipe. When the throttle is open, more air and fuel flows to the cylinders so the engine produces more power and the car goes faster.
The mixture of air and fuel flows down into the cylinders.
Fuel (orange) is supplied from a mini-fuel tank called the float-feed chamber.
As the fuel level falls, a float in the chamber falls and opens a valve at the top.
When the valve opens, more fuel flows in to replenish the chamber from the main gas tank. This makes the float rise and close the valve again.