Venturi Principle; Distribution - Chevrolet Light Duty Truck 1973 Service Manual

VENTURI PRINCIPLE (FIG. 8)
To obtain a greater pressure drop at the tip of the nozzle
to cause the fuel to flow, the principle of increasing the
air velocity to create a low pressure area is used. A
device called the "venturi" is used to increase the air
velocity and lower the pressure at the discharge nozzle.
The increased pressure differential between that of the
fuel bowl and at the carburetor throat increases fuel flow
sufficiently, at a given air flow, so that the resulting air-
fuel proportions result in a combustible mixture.
For example, a carburetor with a 1 1/2" pipe will supply
the volume of air required for a given displacement
engine. However, the pressure drop within the carburetor
throat is insufficient to cause enough fuel to flow into the
carburetor at the desired speed to create a combustible
fuel mixture due to the large weight difference between
air and gasoline. By necking down the inner diameter of
the carburetor throat into a venturi, the air flow is forced
to speed up at the restriction area, thus further reducing
air pressure and increasing fuel flow proportionately to
achieve the desired mixture.
To be most effective, the venturi must be designed for a
certain curvature and length. The venturi design can be
tailored to provide fuel flow under any condition of air
flow. However, a small venturi may restrict high speed
engine operation and a large venturi will not provide
enough pressure differential for low speed operation. The
production venturi size is usually a compromise to
provide adequate low and high speed operation.
The carburetor discharge nozzle is located in the center
of the venturi throat to take advantage of the maximum
ATMOSPHERIC
'Low Pressure)
Fig. 8 -Venturi Principle
pressure drop and promote atomization of the fuel. The
large venturi, cast in the carburetor bore, is called the
primary or main venturi.
Most carburetors use a primary and one or most boost
venturis. The boost venturi is usually located over the
primary with its discharge end in the low pressure area
of the primary. The purpose of the boost venturi is to
further lower the pressure at the nozzle. Additional boost
venturis may be used for finer control of pressure drop
but at high speed they tend to restrict air flow to the
engine.
Because actual venturi size is a compromise, two- and
four-barrel carburetors are used where requirements are
extreme. A two-barrel carburetor allows use of smaller
venturi for improved low speed operation yet gives
relatively good high speed operation due to the larger
throttle area provided by the two throttle valves. The
primary side of a four-barrel carburetor is designed
much the same as the two-barrel carburetor with small
venturi for low speed economy. The secondary side of a
four-barrel carburetor uses large bores and venturi for
extremely good high speed breathing. The secondary
side of the four-barrel carburetor operates only at high
degree primary throttle openings or when performance
is required.

DISTRIBUTION

For good combustion and smooth engine operation, the
air and fuel must be thoroughly and uniformly mixed,
delivered in equal quantities to each cylinder and evenly
distributed within the combustion chamber.
Good distribution requires good vaporization. A gaseous
mixture will travel much more easily around corners in
the manifold and engine, while liquid particles, being
relatively heavy, will try to continue in one direction and
will hit the walls of the manifold or travel on to another
cylinder.
As an example, consider a six cylinder engine with the
carburetor mounted at the center of the manifold. The
DISTRIBUTIO N
Fig. 9-Distribution
E N G I N E FUEL 6 M - 7
INTAKE
STROKE
LIGHT DUTY TRUCK SERVICE MANUAL