by Chris Woodford. Last updated: January 17, 2020.
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!
Artwork: Carburetors in a nutshell: they add fuel (red) to air (blue) to make a mixture that's just right for burning in the cylinders. Modern car cylinders are fed more efficiently by fuel-injection systems, which use less fuel and make less pollution. But you'll still find carburetors on older car and motorcycle engines, and in the compact engines in lawnmowers and chainsaws.
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.
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.
Artwork: In theory, a car engine needs 14.7 times more air than fuel if the air-fuel mixture is to burn properly. This is called a stoichiometric mixture and it works out as 94 percent air and 6 percent fuel. In practice, the ratio may be different.
With a car engine, it's a bit more complex. If you have
just enough atoms of oxygen to burn all your atoms of fuel, that's called
a stoichiometric mixture. (Stoichiometry is part of chemistry,
the chemist's equivalent of making sure you have just enough of each ingredient
before you set about cooking from a recipe.) In the case of a car engine,
the ratio is usually around 14.7 parts of air to 1 part of fuel (though it
does vary depending on exactly what the fuel is made up of).
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." Having slightly too much air (a slightly lean mixture) will give better fuel economy, while having slightly too little (a slightly rich mixture) will give better performance. Having far too much air is just as bad as having far too
little; both are bad for the engine in different ways.
Photo: A typical carburetor isn't much to look at! Photo by David Hoffman courtesy of US Navy.
What is a carburetor?
"The carburetor is called the 'Heart' of the automobile, and it cannot be expected that the engine will act right, give the proper horse-power, or run smoothly if its 'heart' is not performing its functions properly."
Edward Cameron, The New York Times, 1910
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
Who invented the carburetor?
Carburetors have been around since the late 19th
century when they were first developed by automobile pioneer (and
Mercedes founder) Karl Benz (1844–1929).
This diagram, which I've colored to make it easier to follow, shows the original
Benz carburetor design from 1888; the basic working principle (explained in the box below) remains the same to this day.
Artwork: A very simplified diagram of Karl Benz's original carburetor from
his 1888 patent. Fuel from the tank (blue, D) enters what he called the generator (green, A)
underneath, where it evaporates. The fuel vapor passes up through the gray pipe and meets air coming
down the same pipe, which enters from the atmosphere through perforations at the top. The air and fuel mix in the red chamber (F), then pass through a valve (turqouise, G) into the cylinder H, where they
burn to make power. Artwork from US Patent 382,585: Carburetor by Karl Benz. May 8, 1888, courtesy of US Patent and Trademark Office.
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
Artwork: 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!)
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.
Find out more
On this website
For older readers
For younger readers
- Car Science by Richard Hammond. Dorling Kindersley, 2007. From the materials they're made from to the way they cut through the air, this book explains the science that makes cars go (ages 9–12).
- Carburetors—Explained: This video from Engineering Explained covers much the same ground as my article, but talks us through what's happening. It also covers carburetors that have a second venturi.
- Float Style Carburetors Explained by Pimpinpenz. A good clear overview of a float carburetor using a needle valve.
For more technical detail, check these out:
- US Patent 382,585: Carburetor by Karl Benz. May 8, 1888. The original fuel-air mixing device invented in the late 19th century by automobile pioneer Karl Benz.
- US Patent 1,520,261: Carburetor by George F. Ritter et al, Tillotson Manufacturing. December 23, 1924. A typical early 20th century carburetor.
- US Patent 1,938,497: Carburetor by Charles N. Pogue. December 5, 1933. This design aims to vaporize more of the fuel and provide more power to the engine.
- US Patent 4,501,709: Variable venturi carburetor by Tadahiro Yamamoto and Tadaki Oota, Nissan. February 26, 1985. In this more modern type of carburetor, the size of the venturi changes automatically to maintain a constant level of suction.