more easily than water. A highly volatile liquid is one
that is considered to evaporate easily.
ATOMIZATION (fig. 4-17). Atomization is the
process of breaking up a liquid into tiny particles or
droplets. When a liquid is atomized, the droplets are all
exposed individually to the air. For this reason,
atomization greatly increases evaporation by increasing
the exposed surface area of the liquid.
Figure 1-17.Example of atomization.
the amount of molecules leaving the liquid for a given
time will increase, as the temperature increases.
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 the collision
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.
VOLATILITY. The term volatility refers to how
fast a liquid vaporizes. Some liquid vaporizes easily at
room temperature. Alcohol, for instance, vaporizes
The venturi effect (fig. 4-18) is used by the
carburetor to mix air with the gasoline. 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 the 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 is due to the downward intake
stroke of the piston, causing atmospheric pressure to
create an air flow through the throat. This air flow must
increase temporarily in speed, as it passes through the
venturi due to its deceased size. The increased speed of
air flow results in a corresponding decrease in pressure
within the venturi and at the end of the discharge nozzle.
This action permits the atmospheric pressure on the
surface of the gasoline in the float bowl to force the
gasoline out through the discharge nozzle. This
gasoline then sprays and atomizes in the passing air
flow to form the air-fuel mixture.
Figure 4-18.Venturi effect.