The carburetor (carburettor in British English, "carb" for short) is a device which mixes air and fuel for an internal-combustion engine. Carburetors are still found in small engines and in older or specialized automobiles such as those designed for stock car racing. However, most cars built since the early 1980's use fuel injection instead of carburetion.
Most carbureted (as opposed to fuel-injected) engines have a single carburetor, though some, primarily higher performance engines, can have multiple carburetors, each feeding a set of cylinders. Small propeller-driven flat airplane engines have the carburetor below the engine.
Some carburetors have more than one venturi, or "barrel"; this is to accommodate the higher air flow rate on larger-displacement engines. Multi-barrel carburetors can have primary and secondary barrels, the latter opening only when the engine is working hard. A 4-barrel carburetor often has two primary and two secondary, for example. The reason for this is that a big carburetor, optimised for high flow rates, is inefficient at lower rates; such a primary/secondary arrangement attempts to be the best of both worlds.
When the engine is cold, a much higher ratio of fuel to air is required. A mechanical (or more commonly automatic electrical) choke closes the butterfly valve, a larger version of the throttle plate.
Too much fuel in the fuel-air mixture is referred to as too "rich"; not enough fuel is too "lean". The "mixture" is normally controlled by adjustable screws on an automotive carburetor, or a pilot-operated lever on a propeller aircraft (since mixture is altitude-dependent).
In diesel engines, and most modern gasoline (petrol) engines, the carburetor has been replaced by fuel injection which mixes air and fuel and injects the mixture directly into each cylinder.
The carburetor was invented by the Hungarian engineer Donát Bánki in 1893.
Parts of a Carburetor:
When you press the accelerator ("gas pedal"), the throttle plate (a circular flap which limits the amount of air entering the carb) opens, fuel enters the carb through the accelerator pump, and flows into the bowl, where a float measures a fixed volume of fuel ready for use. The movement of the pistons (when the intake valves are open), causes air to be drawn into the carburetor (through the air cleaner). Air enters the neck of the carburetor and flows through the venturi, where the narrowed passage causes the air to speed up - lowering pressure - drawing fuel into the venturi from the bowl through the jets. The venturi also causes a tornado-like airflow to mix the air and fuel. The air-fuel mixture then exits through the intake manifold, through the intake valves into the cylinders.
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