and rubber-mounted engine, short lengths of
gasoline-resistant flexible tubing are used. Occa-
sionally, road vibration may loosen and break the lines,
or they may become pinched or flattened by flying
rocks. Such damage may interfere with the flow of the
ASf04009
fuel.
Figure 4-9.--Spray gun.
A certain amount of scale forms within the lines
and sometimes causes a stoppage. If this condition
If the evaporation is taking place in a closed
occurs, blow the lines clear with compressed air.
chamber, such as in a jar, evaporation of the liquid will
soon stop; the closed space above the liquid will soon
CARBURETORS
become "filled" with escaped molecules. These
escaped molecules are in motion, just as all other
The principles of carburetion are presented so you
molecules are moving. Some of the escaped molecules
may better understand the inner workings of a
will be knocked back into the liquid. This action
carburetor and how the other components of the fuel
balances the escapes so that for every molecule that
system function to provide a combustible mixture of
leaves the liquid, a molecule will re-enter the liquid. As
fuel and air to the engine cylinders.
a result, a state of balance exists and evaporation stops.
Air is composed of various gases, mostly nitrogen
Volatility refers to the ease with which a liquid
and oxygen. These gases are, in turn, made up of tiny
vaporizes. A highly volatile liquid evaporates very
particles called molecules. All substances, whether
rapidly. A liquid of low volatility evaporates slowly.
solid, liquid, or gas, are made up of molecules. In
Gasoline is a mixture of several different components
solids, such as ice or iron, the particles are held very
with varying degrees of volatility.
closely together so that they seem to have no motion. In
Atomization means breaking a liquid into very tiny
liquids, the molecules are not held together so tightly,
particles or globules. Atomization helps to turn a liquid
so they can move with respect to each other. In gases,
into a vapor. If a spoonful of gasoline is put into a pan, it
there is still less tendency for the molecules to hold
will take several seconds for it to evaporate, the length
together, and the molecules can move quite freely. The
of time depending on temperature, volatility, and
molecules of a gas are attracted to the earth by gravity
pressure. However, if this spoonful of gasoline is put
(weight). It is the combined weight of the countless
into an ordinary spray gun (fig. 4-9), the gasoline will
molecules in the air that makes up atmospheric
be broken into a fine mist when the gun is operated. The
pressure.
mist will turn almost instantly into vapor because a
When a liquid changes to a vapor, it has evaporated.
much greater area of the liquid is exposed to air when
When this happens, molecules of the liquid move from
the gasoline is atomized. Evaporation takes place from
the liquid into the air. As this continues, the liquid
all surfaces and increases with greater surface area.
disappears from its container and appears as vapor in
A venturi is an hourglass-shaped restriction. In the
the air. The rapidity of evaporation varies with a
carburetor (fig. 4-10), a venturi is placed in the air horn
number of factors. One of these is the volatility of the
AIR HORN
liquid. Others include temperature, total pressure above
NOZZLE
the liquid, the amount of liquid, and amount of liquid
ATMOSPHERIC
that has already evaporated into the air above the liquid.
PRESSURE
VENTURI
When a substance is heated, the molecules move
faster. This is true regardless of whether the molecules
are in a solid, liquid, or gas. In other words, the rate of
evaporation increases with increased temperature. This
FLOAT BOWL
action can be seen in the process of boiling water.
When there is little gas (atmospheric) pressure
above the liquid, the molecules can escape from the
liquid more easily than when the pressure is high. This
ASf04010
is to say that liquid in a partial vacuum will evaporate
more rapidly than the same liquid under pressure.
Figure 4-10.--Simplified carburetor.
4-7
