How does a carburetor work? (2024)

by Chris Woodford. Last updated: August 22, 2022.

Fuel plus air equals motion—that's the basic science behind most of the vehiclesthat travel on land, over sea, or through the sky. Cars, trucks, andbuses turn fuel into power by mixing it with air and burning it inmetal cylinders inside their engines. Exactly how much fuel and airan engine needs varies from moment to moment, depending on how longit's been running, how fast you're going, and a variety of otherfactors. Modern engines use an electronically controlled systemcalled fuel injection to regulate the fuel-air mixture so it'sexactly right from the minute you turn the key to the time you switchthe engine off again when you reach your destination. But until theseclever gadgets were invented, virtually all engines relied oningenious 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'redesigned around a chemical reaction called combustion: whenyou burn fuel in air, you release heat energy and produce carbondioxide and water as waste products. To burn fuel efficiently, youhave to use plenty of air. That applies just as much to a car engineas to a candle, an outdoor campfire, or a coal orwood fire in someone's home.

With a campfire, you never really have toworry 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 (oreven a fuel-burning device like a gas central-heating furnace (boiler) toproduce 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 havejust enough atoms of oxygen to burn all your atoms of fuel, that's calleda stoichiometric mixture. (Stoichiometry is part of chemistry,the chemist's equivalent of making sure you have just enough of each ingredientbefore 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 itdoes 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 calledburning "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 toolittle; both are bad for the engine in different ways.

What is a carburetor?

Gasoline engines are designed to take in exactly the right amount of air sothe fuel burns properly, whether the engine is starting from cold orrunning hot at top speed. Getting the fuel-air mixture just right isthe job of a clever mechanical gadget called a carburetor: atube that allows air and fuel into the engine through valves, mixingthem together in different amounts to suit a wide range of differentdriving 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 carbonor hydrocarbons. So, technically, a carburetor is a device that saturates air (the gas) with fuel(the hydrocarbon).

Photo: Adjusting the manual carburetor "choke" (air intake valve) in a 1956 DeSoto Firedome engine. Photo by Laurie Pearson courtesy of US Marine Corps and DVIDS.

Who invented the carburetor?

Carburetors have been around since the late 19thcentury when they were first developed by automobile pioneer (andMercedes founder) Karl Benz (1844–1929). There were earlierattempts at "carbureting" in other ways. For example, the French engine pioneerJoseph Étienne Lenoir (1822–1900) originally used a rotating cylinderwith sponges attached that dipped into fuel as they turned around, lifting it out of its container and mixing it into the air as they did so. [1]

The diagram below, 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 fromhis 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 comingdown 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 theyburn to make power. Artwork from US Patent 382,585: Carburetor by Karl Benz. May 8, 1888, courtesy of US Patent and Trademark Office.

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How does a carburetor work?

Photo: A typical carburetor isn't much to look at! Photo by David Hoffman courtesy of US Navy and Wikimedia Commons.

Carburetors vary quite a bit in design and complexity. The simplest possible one isessentially a large vertical air pipe above the engine cylinders witha horizontal fuel pipe joined onto one side. As the air flows downthe pipe, it has to pass through a narrow kink in the middle, whichmakes it speed up and causes its pressure to fall. This kinkedsection is called a venturi. The falling pressure of the aircreates a sucking effect that draws air in through the fuel pipe atthe side.

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 howcan we adjust the air-fuel mixture? The carburetor has two swivelingvalves above and below the venturi. At the top, there's avalve called the choke that regulates how much air can flowin. If the choke is closed, less air flows down through the pipe and theventuri sucks in more fuel, so the engine gets a fuel-richmixture. That's handy when the engine is cold, first starting up, andrunning quite slowly. Beneath the venturi, there's a second valvecalled the throttle. The more the throttle is open, the moreair flows through the carburetor and the more fuel it drags in fromthe pipe to the side. With more fuel and air flowing in, the enginereleases more energy and makes more power and the car goes faster.That's why opening the throttle makes a car accelerate: it's theequivalent of blowing on a campfire to supply more oxygen and make itburn more quickly. The throttle is connected to the accelerator pedalin 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 afloat-feed chamber (a little tank with a float and valve inside it).As the chamber feeds fuel to the carburetor, thefuel level sinks, and the float falls with it. When the float drops below a certain level, it opens a valve allowing fuelinto 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. (Thefloat-feed chamber works a bit like a toilet, with the floateffectively doing the same job as the ballco*ck—the valve that helps a toilet refillwith 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:

1. Air flows into the top of the carburetor from the car's air intake, passing through a filter that cleans it of debris.
2. 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).
3. In the center of the tube, the air is forced through a narrow kink called a venturi. This makes it speed upand causes its pressure to drop.
4. The drop in air pressure creates suction on the fuel pipe (right), drawing in fuel (orange).
5. 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.
6. The mixture of air and fuel flows down into the cylinders.
7. Fuel (orange) is supplied from a mini-fuel tank called the float-feed chamber.
8. As the fuel level falls, a float in the chamber falls and opens a valve at the top.
9. 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.

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On this website

• Brakes
• Gasoline car engines
• Gears
• Diesel engines
• Wheels and axles

Books

For older readers

• Holley Carburetors: How to Rebuild by Mike Mavigran. CarTech, 2016.
• Rochester Carburetor Manual by Mike Stubblefield. Haynes, 1994.
• Weber Carburetors by Pat Braden. HP Books, 1988.

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).

Videos

• 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.

Articles

• Waving Goodbye to Carburetors, Nascar Prepares Switch to Fuel Injection by Paul Stenquist. The New York Times, July 20, 2011. How Nascar finally made the switch away from carburetors for the 2012 race season—and why it took so long.
• Technology; It's Goodbye, Carburetors by John Holus. The New York Times, October 22, 1981. An article from The Times archive heralds the arrival of fuel injection in the early 1980s.
• Ford's new variable venturi carburetor by E.F. Lindsley. Popular Science, August 1976. This old article from the Pop Sci archives has some great cutaway illustrations of the different types of venturi carburetors.

Patents

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.

1. ↑Gas and Petroleum Engines: A Practical Treatise on the Internal Combustion Engine by William Robinson. E. & F.N. Spon, 1890, p.175.