Quantcast Section II. Principles of Carburetion

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TM 9-8000
Figure 4-20 Dry Type Air Filter
Section II. Principles of Carburetion
c. Closed Chamber. As evaporation takes place in a
4-8. Composition of Air. Air is composed of various
closed container, the space above the liquid will reach a
gases, mostly nitrogen and oxygen (78 percent nitrogen
point of saturation. When this happens, every molecule
and 21 percent oxygen by volume). These gases, as are
of liquid that enters the air will cause another airborne
all substances, are made of tiny particles called
molecule of liquid to fall back.
molecules.  In the air surrounding the earth, as In all
gases, the molecules are able to move quite freely in
d. Volatility. The term volatility refers to how fast a
relation to each other. The molecules of air are attracted
liquid vaporizes. Some liquids vaporize easily at room
to the earth by gravity, creating the atmosphere (para 2-
temperature.  Alcohol, for instance, vaporizes more
22a).
The weight of the air molecules creates
easily than water. A highly volatile liquid is one that is
atmospheric pressure (para 2-22b).
considered to evaporate easily.
4-9.  Evaporation.  Evaporation is the changing of a
e.  Atomization (Fig. 4-21).  Atomization is the
liquid to a vapor. The molecules of the liquid, not being
process of breaking up a liquid into tiny globules or
closely tied together, are constantly moving about among
droplets. When a liquid is atomized, the droplets are all
themselves.  Any molecule that moves upward with
exposed individually to the air.
For this reason,
sufficient speed will jump out of the liquid and into the air.
atomization greatly increases evaporation by increasing
This process will cause the liquid to evaporate over a
the exposed surface area of the liquid.
period of time. The rate of evaporation is dependent on
the following:
4-10. Venturi Effect (Fig. 4-22). Venturi effect is used
a.  Temperature.  The rate of movement of the
by the carburetor to mix gasoline with air.  The basic
carburetor has an hourglass-shaped tube called a throat.
molecules Increases with temperature. Because of this,
The most constricted part of the throat is called the
the amount of molecules leaving the liquid for a given
venturi. A tube called a discharge nozzle Is positioned in
time will increase as the temperature Increases.
the venturi.  The discharge nozzle is connected to a
reservoir of gasoline called the float bowl. The negative
b. Atmospheric Pressure. As atmospheric pressure
pressure that exists in the combustion chamber because
Increases, the amount of air molecules present over the
of the downward intake stroke of the piston causes
liquid also increases.  The Increased presence of air
atmospheric pressure to create an airflow through the
molecules will slow the rate of evaporation.  This is
carburetor throat. This airflow must Increase temporarily
because the molecules of liquid will have more air
in speed as it passes through the venturi, due to its
molecules to collide with. In many cases, they will fall
decreased size.
back into the liquid after collision.
TA233398
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