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carburetor(kär`byərā`tər, –bə–), part of a gasoline engine in which liquid fuel is converted into a vapor and mixed with a regulated amount of air for combustion in the cylinders. Land vehicles, boats, and light aircraft have a float carburetor, in which a float regulates the fuel level in a reservoir from which the fuel is sucked into the intake manifold at a restriction called a venturi. This venturi metering system controls the flow of a continuous pumped spray into the intake manifold downstream from the carburetor. When there is an individual spray for each cylinder and the injection is an intermittent, timed spurt, or is metered differently, the device is usually called a fuel injector, not a carburetor (see fuel injectionfuel injection,
system in an internal-combustion engine that delivers fuel or a fuel-air mixture to the cylinders by means of pressure from a pump. It was originally used in diesel engines because of diesel fuel's greater viscosity and the need to overcome the high pressure of
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a device used to meter the flow of fuel and to prepare a combustible mixture of liquid fuel and air for internal combustion engines with external mixing layouts. The process of preparing a combustible mixture is called carburetion. In order to achieve complete and rapid combustion and maximum heat release in the cylinder, the fuel must be mixed with air in a certain way. Preparation of the mixture consists of breaking down the liquid fuel into small droplets (atomizing), intensively mixing the fuel and air, and vaporizing the mixture. Atomization of fuel in a carburetor occurs when a thin jet of fuel is allowed to flow from an atomizer into a rapidly moving air stream. The air stream breaks up the fuel into small droplets which mix with the air and are conveyed through the intake manifold into the cylinders of the engine.
Carburetors can be classified into three groups with differing directions of air flow: downdraft (descending stream), updraft, and horizontal. Downdraft carburetors are used primarily in motor-vehicle engines. Carburetors with horizontal flow are used primarily in motorcycle, boat, and supercharged motor-vehicle engines.
A carburetor is connected to the intake manifold of an engine. During the intake stroke the piston moves away from the cylinder head, creating a vacuum within the cylinder that outside air rushes to fill. The air passes with great velocity through the mixing chamber, where it picks up the fuel. The amount of combustible mixture fed into the cylinder is regulated by the throttle valve. The simplest types of carburetors are not equipped to change the composition of the combustible mixture, although changes are required if the operating conditions of the engine are changed. To adapt to changes in operating conditions, carburetors are equipped with automatically controlled metering devices. The graph of the compositional changes of the combustible mixture fed to the engine as a function of air comsumption or as a function of engine load shows the operating characteristics of the carburetor. The adjustment and working condition of the carburetor greatly influence engine operation. Carburetors that are out of adjustment cause a deterioration of operating economy and engine performance and an increase in the toxicity of exhaust gases.
REFERENCESGribanov, V. I., and V. A. Orlov. Karbiuratory dvigatelei vnutrennego sgoraniia, 2nd ed. Leningrad, 1967.
Bleiz, N. G. Avtomobil’nye karbiuratory, benzonasosy, fil’try. Moscow, 1967.
B. A. KUROV
A device that controls the power output and fuel feed of internal combustion spark-ignition engines used for automotive, aircraft, and auxiliary services. Its duties include control of the engine power by the air throttle; metering, delivery, and mixing of fuel in the airstream; and graduating the fuel-air ratio according to engine requirements in starting, idling, and load and altitude changes. The fuel is usually gasoline or similar liquid hydrocarbon compounds, although some engines with a carburetor may also operate on a gaseous fuel such as propane or compressed natural gas. A carburetor may be classified as having either a fixed venturi, in which the diameter of the air opening ahead of the throttle valve remains constant, or a variable venturi, which changes area to meet the changing demand. See Automobile, Engine, Fuel system, Venturi tube
A simple updraft carburetor with a fixed venturi illustrates basic carburetor action (see illustration). Intake air charge, at full or reduced atmospheric pressure as controlled by the throttle, is drawn into the cylinder by the downward motion of the piston to mix with the unscavenged exhaust remaining in the cylinder from the previous combustion. A cylinder is most completely filled with the fuel-air mixture when no other cylinder is drawing in through the same intake passage at the same time. The fuel is usually metered through a calibrated orifice, or jet, at a differential pressure derived from the pressure drop in a venturi in the intake air passage.