Georjean Watson, age 13, of Haggerstown, Md., for her question:
HOW DOES A CARBURETOR WORK?
A carburetor is that part of a gasoline engine that provides the mixture of gasoline and air that is necessary for an engine to work.
A carburetor must mix the gasoline with about 15 times its weight in air for the engine to run smoothly at all speeds.
A driver of an automobile controls the engine speed by increasing or reducing the flow of the fuel mixture.
The simplest carburetor is a metal tube. Air enters at one end. Gasoline is mixed with air in the middle. Gasoline air vapor is pushed from the other end of the carburetor into the engine.
The float chamber of the carburetor stores a small amount of gasoline that is pumped to it from the main gasoline tank. When the chamber is full, a float resting on top of the gasoline closes a valve in the fuel line. When gasoline is used by the engine, the float drops. This opens the valve in the fuel line and lets more gasoline flow into the chamber. The float keeps a steady level of gasoline in the carburetor.
The air cleaner is placed at the top of the carburetor to remove dust from the air. Dust would cause wear to the engine. Atmospheric pressure pushes the air through the filter.
The inside of the carburetor narrows to a small size, then widens out again. Its shape is called a venturi. The venturi increases the speed of the air rushing through and lowers its pressure. The higher air pressure in the float chamber then forces gasoline through the nozzle into the venturi. The air picks up the gasoline and turns it into a vapor.
The intake manifold is fastened to the opposite end of the carburetor tube from the air cleaner and it connects the carburetor with the engine cylinders.
The throttle valve controls the engine speed by letting more or less of the fuel mixture enter the intake manifold. The driver presses the accelerator pedal to open the throttle valve.
The choke valve looks like the throttle valve. But it controls the amount of air entering the carburetor. When it partly closes the carburetor tube, more gasoline and less air flow into the engine. Choking the carburetor makes it easier for the spark plugs to ignite the gasoline air mixture when the engine is cold.
The choke valve on many automobiles is controlled automatically by the temperature of the engine.
Carburetors are called updraft or downdraft according to their position. If the carburetor is below the intake manifold, it is updraft. If placed above, it is downdraft.
Fuel injection is a system for squirting fuel into the cylinders of gasoline and diesel engines. It replaces the carburetor when used on gasoline engines. With fuel injection, a pump forces fuel under high pressure to a nozzle at each cylinder.
On most gasoline engines that use fuel injection, the nozzles spray the fuel into an intake port or chamber near each cylinder. There the fuel mixes with air before a valve opens to admit the mixture into the cylinder.