| Definitions EVAPORATION is the changing of a
liquid to a vapor. The molecules of the liquid not being closely tied together are
constantly moving 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:
TEMPERATURE.
The rate of movement of the molecules increase with temperature. Because of this, 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 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.
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. |
