The Fuel System:
Principles of Carburetion
Composition of Air
Air is composed of various gases, mostly nitrogen and oxygen (78 percent nitrogen and 21 percent oxygen by volume). These gases are composed of tiny particles called molecules as are all substances. In the air surrounding the earth, the molecules are able to move quite freely in relation to each other as in all gases. The molecules of air are attracted to the earth by gravity, creating the atmosphere. The weight of the air molecules creates atmospheric pressure.
Evaporation
Evaporation is the changing of a liquid to a vapor. The molecules of the liquid, not being closely tied together, are constantly moving about among themselves. Any molecule that moves upward with sufficient speed will jump out of the liquid and into the air. This process will cause the liquid to evaporate over a period of time. The rate of evaporation is dependent on the following:
(1) Temperature. The rate of movement of the molecules increases with temperature. Because of this, the amount of molecules leaving the liquid in a given time will increase as the temperature increases.
(2) Atmospheric Pressure. As atmospheric pressure increases, the amount of air molecules present over the liquid also increases. The increased presence of air molecules will slow the rate of evaporation. This is because the molecules of liquid will have more air molecules to collide with. In many cases, they will fall back into the liquid after collision.
(3) Closed Chamber. As evaporation takes place in a closed container, the space above the liquid will reach a point of saturation. When this happens, every molecule of liquid that enters the air will cause another airborne molecule of liquid to fall back.
(4) Volatility. The term volatility refers to how fast a liquid vaporizes. Alcohol, for instance, vaporizes more easily than water. Some liquids vaporize easily at room temperature. A highly volatile liquid is one that is considered to evaporate easily.
Venturi Effect
A venturi effect is used by the carburetor to mix gasoline with air. The basic carburetor has an hourglass shaped tube called a throat. The most constricted part of the throat is called the venturi. A tube called a discharge nozzle is positioned in the venturi. The discharge nozzle is connected to a reservoir of gasoline, called the float bowl.
The negative pressure that exists in the combustion chamber, because of the downward intake stroke of the piston, causes atmospheric pressure to create an airflow through the carburetor throat. This airflow must increase temporarily in speed as it passes through the venturi, due to its decreased size.
FIGURE 10. VENTURI EFFECT.
The increased speed of the airflow will also result in a corresponding decrease in pressure within the venturi and at the end of the discharge nozzle. When this occurs, atmospheric pressure will push gasoline through the discharge nozzle and into the carburetor throat, where it will mix with the intake airflow.
A Basic Carburetor
The ideal state for the fuel to be in when it reaches the cylinder is to be vaporized completely. Good intake manifold design will help to vaporize the fuel, but the carburetor must properly atomize the fuel beforehand. Atomization of the fuel occurs as it is drawn into the venturi. As the fuel comes out of the discharge nozzle, it is broken into tiny droplets which enter the airflow.
To ensure that there is a high degree of atomization, a tiny hole called an air bleed is used to allow air to mix with the fuel in the discharge tube. The fuel is then further atomized as it enters the venturi. To ensure proper fuel flow, a secondary venturi or a venturi booster may be used. It will further decrease the pressure at the discharge nozzle.
Air-Fuel Ratio
The proportions of an air-fuel mixture are expressed in terms of the air-fuel ratio. It is the relationship by weight of the mixture. An example of how this is expressed would be:
Air-Fuel Ratio = 12:1.
In this air-fuel mixture, the air would be 12 times as heavy as the fuel. The operational range of air-fuel ratios in the average gasoline engine are from approximately 9:1 to approximately 17:1. Air-fuel ratios on the lower end (less air) are considered to be rich mixtures; the air-fuel ratios at the higher end (more air) are considered to be lean mixtures.
A gasoline engine, propelling a vehicle at a steady speed, operates on an air-fuel ratio of approximately 15:1. Considering that gasoline weighs approximately 640 times as much as air, it can be seen that a gasoline engine consumes a tremendous amount of air. If, in fact, the air-fuel ratio was considered by volume rather than weight, it would be seen that a gasoline engine operating on an air-fuel ratio of 15:1 consumes approximately 9600 gallons of air for every gallon of gasoline. |