Carburetors

Carburetors

Fuel plus air equals motion—that's the basic science behind most vehicles that travel on land, over the sea, or through the sky. Cars, trucks, and buses turn fuel into power by mixing it with air and burning it in metal cylinders inside their engines. Exactly how much fuel and air an engine needs varies from moment to moment, depending on how long it's been running, how fast you're going, and various other factors. Modern engines use an electronically controlled system called fuel injection to regulate the fuel-air mixture. Hence, it's exactly right from the minute you turn the key to the time you switch the engine off again when you reach your destination. But until these clever gadgets were invented, virtually all engines relied on ingenious air-fuel mixing devices called carburetors.

Gasoline engines are designed to take in precisely the right amount of air, so the fuel burns appropriately, whether the engine is starting from cold or running hot at top speed. Getting the fuel-air mixture just right is the job of a clever mechanical gadget called a carburetor: a tube that allows air and fuel into the engine through valves, mixing them in different amounts to suit a wide range of other driving conditions.

You might think "carburetor" is quite a weird word, but it comes from the verb "carburet." That's a chemical term meaning to enrich a gas by combining it with carbon or hydrocarbons. So, technically, a carburetor is a device that saturates air (the gas) with fuel (the hydrocarbon).

How It Works

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 airflow pulls in fuel to join it, which is 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. Opening the throttle produces 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. 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. 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.

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