3. THE
VENTURI-Figure 3
Since
the operating conditions of an automotive engine
are subject to constant change, the carburetor
must automatically adjust the fuel flow for these
changes. The venturi in the carburetor provides
a force which controls the fuel discharge in the
normal cruising speed range.
The venturi is a specially designed
restriction which causes air to momentarily increase
its speed while passing through it. This creates
a drop in air pressure, commonly called vacuum,
in the venturi. As the speed of the air flow in
the carburetor increases with an increase in engine
speed, the vacuum in the venturi becomes correspondingly
greater. This vacuum in the venturi is utilized
to pull the required amount of fuel from the fuel
supply in the carburetor, as will be described
later.
The relationship
of air pressure to velocity can be illustrated
by a simple experiment. Hold the edge of a sheet
of paper to your lower lip, allowing the rest
of the sheet to hang limp. Blow across the top
of the paper -- you will
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notice that it rises. Air
in motion over the top of the paper exerts less
pressure than the normal atmospheric pressure
of the stationary air under the paper, and the
difference in pressure moves the paper upward.
You will also notice that, as the speed of the
air is increased, the pressure above the paper
is decreased correspondingly, moving the paper
still higher. Differences in air pressure similar
to the example given here provide the basic force
for carburetor operation.
4. THE MAIN METERING
SYSTEM
Figure 4
The main metering system, which
provides fuel for cruising speeds is a continuous
passage from the float chamber to the main discharge
nozzle. The main discharge nozzle is located at
the center of the venturi in the area of the greatest
vacuum. The float chamber, containing fuel, is
vented to the carburetor air inlet, where the
air pressure is practically atmospheric, being
only slightly affected by the restriction of the
air cleaner. The air pressure on the fuel in the
float chamber is greater than the air pressure
in the venturi. This difference in pressure forces
fuel through the main metering system. The fuel
flows from the float chamber through the main
metering jet and into the bottom of the main well.
It is drawn from the main well through an angle
channel and is discharged from the main discharge
nozzle.
The main
metering system delivers an economical mixture
of approximately one part of fuel to sixteen parts
of air by weight. These proportions may vary slightly
for different engines due to their design, but
the figures are satisfactory for purposes of illustration.
The metering, or measuring, of the fuel flow is
accomplished primarily by the main metering jet,
the smallest fuel restriction in the main metering
system. |
