Carburetor
Device that mixes air and fuel for gasoline engines.
A carburetor (also spelled carburettor or carburetter) mixes air with fuel for a gasoline internal combustion engine, controlling the blend that enters the cylinders. It typically uses the Venturi effect, Bernoulli’s principle, or a pitot tube in its main metering circuit to draw fuel into the intake air, with extra components adding more fuel or air under specific conditions. Since the 1990s, fuel injection has mostly replaced carburetors in cars and trucks, but they remain common in small engines (like lawnmowers, generators, and concrete mixers), motorcycles, smaller outboard engines, and piston-engine aircraft. Diesel engines have always used fuel injection, not carburetors, because they compress only air and inject fuel later in the compression stroke.
The word "carburetor" comes from the verb "carburet," meaning to charge with hydrocarbons; early motoring texts called the output "carburetted air"—air mixed with hydrocarbon vapor. This usage replaced an older eighteenth-century noun "carburet," which is now called "carbide." The device charges intake air with vapor from petrol, autogas, or similar fuels. American English spells it "carburetor," British English "carburettor." Colloquial abbreviations include "carb" in the UK and North America, and "carby" in Australia.
In operation, air enters the carburetor (often through an air cleaner), fuel is added inside, and the air-fuel mixture exits into the inlet manifold, then through inlet valves into the combustion chamber. Most engines use one carburetor shared by all cylinders, though some high-performance engines historically used multiple carburetors or throats. The simplest carburetors rely on Bernoulli’s principle: static pressure of flowing air at the fuel entry point drops at higher speeds compared to the pressure in the float chamber (vented to ambient air), and this pressure difference forces more fuel into the airstream. If the tube has constant diameter, the setup is simpler than in typical diagrams. Except for the accelerator pump, pressing the throttle pedal does not directly increase fuel flow; instead, it increases airflow, which draws more fuel into the mixture.
- Field
- Internal combustion engine fuel delivery
- Known for
- Mixing air and fuel using Bernoulli's principle or Venturi effect
- Replaced by
- Fuel injection (since the 1990s for cars and trucks)
- Still used in
- Small engines, motorcycles, outboard engines, piston-engine aircraft
Lore & Background
The term 'carburetor' is derived from the English verb carburet, meaning 'to charge with hydrocarbons'; in early motoring books the product of the carburetor is carburetted air, i.e. air charged with hydrocarbon vapour. This meaning superseded that from the late eighteenth century in which carburet was a noun for which we now use the word carbide. The name is spelled 'carburetor' in American English and 'carburettor' in British English, with colloquial abbreviations including 'carb' in the UK and North America or 'carby' in Australia.
Air from the atmosphere enters the carburetor (usually via an air cleaner), has fuel added within the carburetor, then the air fuel mixture leaves the carburetor and enters the inlet manifold, then through the inlet valve(s), and finally into the combustion chamber. Most engines use a single carburetor shared among all cylinders, though some high-performance engines historically had multiple carburetors and/or throats. The simplest carburetors work on Bernoulli's principle: the static pressure of the flowing air at the fuel entry point reduces at higher speeds compared with the pressure in the float chamber, with the pressure difference then forcing more fuel into the airstream.
The orientation of the carburetor is a key design consideration. Older engines used updraft carburetors, where the air enters from below and exits through the top. From the late 1930s, downdraft carburetors became more commonly used (especially in the United States), along with side draft carburetors (especially in Europe). Various fuel circuits—including the main metering circuit, idle circuit, off-idle circuit, power valve, and choke—are used to provide extra fuel or air in specific circumstances.
Reader's Guide
The carburetor was a central component of gasoline internal combustion engines for much of the 20th century, enabling the precise mixing of air and fuel for combustion. Its operating principle relies on Bernoulli's principle or the Venturi effect, where airflow creates a pressure drop that draws fuel into the airstream. The device includes multiple circuits—such as the main metering circuit, idle circuit, off-idle circuit, and power valve—to adjust fuel delivery under varying engine conditions, as well as a choke for cold starts. Since the 1990s, carburetors have been largely replaced by fuel injection in cars and trucks due to better efficiency and emissions control, but they remain in use for small engines (e.g., lawnmowers, generators, concrete mixers), motorcycles, smaller outboard engines, and piston-engine aircraft. Diesel engines have always used fuel injection instead of carburetors, as they do not compress an air-fuel mix. The carburetor's legacy lies in its foundational role in automotive history and its continued application in niche areas where simplicity and cost-effectiveness are valued.
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