Carburetor is a part of an internal combustion engine that delivers a mixture of fuel and air for burning. Carburetors were widely used in automobile engines for many years. By the 1980’s, however, an electronically controlled device called a fuel injector had replaced the carburetor in most automobiles (see Fuel injection ). Carburetors are still used in smaller, less expensive engines, such as those of motorcycles and lawn mowers.
The main part of a carburetor is an hourglass-shaped device called a venturi tube. The piston motion of an engine draws air through the venturi tube, and then through a pipe called the intake manifold. As the air enters the narrow part of the venturi, its speed increases and its pressure decreases because of a law of physics called Bernoulli’s principle.
A tube called the fuel nozzle is located in the narrow part of the venturi tube. There, the low pressure creates a vacuum that draws fuel from a fuel reservoir through the nozzle and into the venturi. The fuel reservoir is a small fuel storage tank. Its fuel level is maintained by a flow from the fuel tank through a fuel line. The fuel mixes with the air as it leaves the carburetor and travels through the intake manifold and into the combustion chamber, where the mixture of air and fuel burns. Heat from the engine vaporizes part of the liquid gasoline in the manifold and the rest in the combustion chamber.
A carburetor adjusts naturally to the fuel flow for different operating conditions. For example, an engine operating at a high speed requires a high fuel flow rate. As the air flow through the carburetor increases, the lower pressure at the nozzle results in a higher fuel flow from the fuel reservoir.
A disc called a throttle valve controls the quantity of fuel-air mixture leaving the venturi and entering the intake manifold. The throttle can be adjusted to fit the needs of the operating device. For example, a person using a lawn mower may use a control to open the throttle as wide as possible when mowing an extremely thick lawn.
The choke valve, a disc at the other end of the venturi, controls the amount of air that enters the carburetor. In some engines, the choke is controlled automatically by the temperature of the engine. A cold engine requires more fuel and less air to start than a warm engine does. Thus, when the engine is cold, the choke is partially closed, reducing the amount of air flowing through the carburetor.
