APPLICATION OF THE PRINCIPLES
A carburetor consists of an air passage, known as the throat, and a fuel chamber. The venturi is located in the throat. The fuel chamber is connected to the carburetor throat by a passage. One end of this passage is located at the bottom of the fuel chamber. The other end terminates in the narrowest part of the venturi, which is above the normal level of the fuel in the fuel chamber.
For the purpose of brevity and clarity, down-draft carburetor
illustrations will be used in the balance of this explanation. The
principles dealt with apply equally, however, to all carburetor types.
To illustrate the flow of fuel, let us assume that we have a carburetor sitting on a table with fuel in the fuel chamber at the level shown in Figure 8. The fuel chamber and the carburetor throat are open to atmospheric pressure.
Therefore, the level of the fuel in the passage will be the same as the level of the fuel in the fuel chamber because the pressure of the atmosphere is the same at both points.
Now, we will connect a simple pump to the opening in the fuel chamber. By operating the pump, the pressure can be increased in the space above the fuel. Assume that the pressure is increased to 15.7 pounds per square inch. Let us also assume that we have atmospheric pressure of 14.7 pounds per square inch in the throat. With the pressure at 14.7 pounds per square inch (atmospheric) in the carburetor throat, the higher pressure in the fuel chamber (15.7 pounds) will push fuel through the passage and out into the throat.
This difference in pressure is the principle that supplies fuel to the air stream when the carburetor is operating on the engine. (The figures denoting pressure are chosen arbitrarily and are only for the purpose .of illustrating a principle.)
The next step is to place the carburetor on the engine and start the engine. The pressure is reduced in the engine cylinder by the downward movement of the piston. This results in a flow of air through the carburetor. Air enters the throat at atmospheric pressure. As it passes through the venturi, the pressure is reduced. The fuel chamber is open to atmosphere; therefore, the pressure on the surface of the fuel is atmospheric.
Because the pressure in the venturi, adjacent to the connecting passage outlet, is lower than atmospheric, the fuel will then be pushed up and out the end of the passage into the air stream. The resulting mixture of fuel and air is burned in the engine and causes it to operate.
For continued operation it is necessary to maintain a constant supply of fuel. All carburetors are equipped with a float operated fuel valve mechanism to regulate and maintain this supply.
This much of a carburetor would permit operation of the engine only at a constant speed. To regulate engine speed, a throttle valve is incorporated in the throat of the carburetor.
The movement of the throttle valve controls the flow of air through the carburetor and varies the engine speed. When the throttle is in wide open position, there is a maximum flow of air. The result is a maximum pressure difference between the venturi and the fuel chamber. As the throttle is moved toward the closed position, there is a decrease in the volume and velocity of air through the venturi.
When the throttle valve is nearly closed the venturi will no longer develop sufficient pressure difference to move fuel through the main passage.
Therefore, a supplemental passage is incorporated in the carburetor for engine operation at idle or low speed. When the throttle valve is in the idle position, the low pressure in the engine manifold is transmitted through this idle passage. A lower pressure will then exist at the point where the idle fuel passage connects with the passage for idle air intake. Because of the difference in pressure then existing between the idle passage and the fuel chamber, fuel will be forced into the idle air stream. The resulting fuel-air mixture will be delivered through the idle passage to supply engine requirements for idle or slow speed operation.