by Chris Woodford. Last updated: June 23, 2018.
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 driving conditions.
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.