by Chris Woodford. Last updated: August 13, 2014.
Fuel plus air equals motion—that's the basic science behind most of the vehicles
that travel on land, over 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 a variety of other
factors. Modern engines use an electronically controlled system
called fuel injection to regulate the fuel-air mixture so 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 (spelled
"carburettor" in some countries and often shortened to just "carb"). What are they and how do they work? Let's take a closer look!
Photo: Older car engines like this one, from a 1970s Jaguar sports car, used cylinders fed by carburetors. Modern car cylinders are fed more efficiently by fuel-injection systems, which use less fuel and make less
How engines burn fuel
Engines are mechanical things, but
they're chemical things too: they're
designed around a chemical reaction called combustion: when
you burn fuel in air, you release heat energy and produce carbon
dioxide and water as waste products. To burn fuel efficiently, you
have to use plenty of air. That applies just as much to a car engine
as to a candle, an outdoor campfire, or a coal or
wood fire in someone's home.
Photo: A candle mixes wax fuel with air from its surroundings. With too little air, the flame goes out; with too much, the flame will roar and burn blue. A car engine burns fuel in a similar way. Getting its air supply just right is more tricky—and more critical.
With a campfire, you never really have to
worry about having too much or too little air. With fires burning indoors, air is in shorter supply and
far more important. Having too little oxygen will cause an indoor fire (or
even a fuel-burning device like a gas central-heating furnace (boiler)) to
produce dangerous air pollution, including toxic
carbon monoxide gas.
With a car engine, having too much air is just as bad as having too
little. Too much air and not enough fuel means an engine burns
"lean," while having too much fuel and not enough air is called
burning "rich"; both are bad for the engine in different ways.
What is a carburetor?
That's why gasoline engines are designed to take in exactly the right amount of air so
the fuel burns properly, 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 together in different amounts to suit a wide range of different
driving conditions. Carburetors have been around since the late 19th
century when they were first developed by automobile pioneer (and
Mercedes founder) Karl Benz (1844–1929).
Photo: A typical carburetor isn't much to look at! Photo by David Hoffman courtesy of US Navy.
How does a carburetor work?
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 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!)
In summary, then, here's how it all works:
- Air flows into the top of the carburetor from the car's air intake.
- 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.