Carburetor

I

Introduction

Carburetor, device that mixes fuel and air for burning in an internal-combustion engine. A carburetor atomizes (converts into a vapor of tiny droplets) liquid gasoline. An airflow carries the atomized gasoline to the engine’s cylinders, where the gas is ignited.

The carburetor has been part of internal-combustion engines since the beginning of the 20th century. In most passenger vehicles and light trucks built since 1985 the carburetor has been replaced by fuel injection, a more efficient, computer-controlled method of injecting fuel into the engine. Diesel engines, because of their design, have always used fuel injection instead of carburetors. Carburetors today are found only on older gasoline engines in cars and trucks. They are still used in some boat and aircraft engines, and in some sports vehicles, including personal watercraft, snowmobiles, and motorcycles. Other vehicles and machines that may have carburetors for their motors include tractors, snowblowers, lawn mowers, and chainsaws.

II

How It Works

The basic carburetor is built around a hollow tube called a throat, or barrel. Downward motion of the engine’s pistons creates a partial vacuum inside the cylinders that draws air into the carburetor’s throat and past a nozzle that sprays fuel. The mixture of air and fuel produced inside the carburetor is delivered to the cylinders for combustion.

A

Throttle Valve

A throttle valve at the base of the carburetor controls the amount of air pulled through the engine by the partial vacuum in the pistons. The driver opens the throttle valve by pressing down on the accelerator (gas pedal). As the valve opens wider, more air flows through the carburetor, delivering larger amounts of fuel to the engine. The driver closes the throttle valve by decreasing pressure on the gas pedal.

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B

Venturi

Carburetors pull fuel into the airflow using a principle called Bernoulli’s effect, named for Dutch-born Swiss scientist Daniel Bernoulli. Bernoulli discovered that pressure in a fluid decreases as its velocity increases. Italian physicist Giovanni Venturi (1746-1822) designed a specialized type of passageway for fluids based on Bernoulli’s effect. A carburetor has such a passageway, called a venturi, in its throat.

The venturi is a narrowing of the carburetor’s throat and makes the throat look a little like an hourglass—narrow in the middle and wider at the ends. Air rushing through the narrow part speeds up. At the same time, air pressure against the sides of the passageway decreases, creating a partial vacuum inside the throat. This partial vacuum draws fuel through the nozzle and into the air.