Atomization (Fig. 3)
Atomization means breaking the
liquid fuel into very small particles so that
it can readily vaporize and mix with air. With
the fuel broken into small particles, there is
more air contact. The more air contact - the better
the vaporization.
Atomization is accomplished in two ways:
1. Fig. 3A shows air being
bled into the fuel as it moves through the carburetor
passages. This causes a turbulence which breaks
the solid stream into smaller particles.
2. The main fuel nozzle (shown
in Fig. 3B) is located at the point of highest
air velocity,
(lowest pressure) so that the air actually tears
the fuel into a fine spray as it enters the air
stream.
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If #5 is on the intake stroke, the mixture will be
drawn sharply around the corner to #5 at right
angles to the original direction. The large
drops of gasoline won't make such a sharp turn
and will continue in their path to the rear
of the manifold where they will probably be
drawn into #6 on its intake stroke. Thus, #5
receives a leaner mixture and #6 receives a
richer mixture than originally entered the manifold.
To
compensate for these problems, manifolds are
tailored to the engines to minimize the sharp
corners and provide as smooth a flow as possible.
The carburetor's principal job in distribution
is to break up the fuel as finely as possible.
The carburetor's principal job in distribution
is to break up the fuel as finely as possible
and furnish a uniformly vaporized mixture to
the manifold.
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Distribution (Fig. 4)
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 mixture for cylinders 4, 5, and 6 will initially
travel towards the rear of the engine. |
CARBURETOR REQUIREMENTS
Flow Curve -- Air Flow vs. Air/Fuel Ratio
The carburetor flow curve is
a graphic description of the air and fuel requirements
of an engine in all ranges of operation. For economy,
sufficient air must be supplied to burn every
particle of fuel, and for power requirements,
additional fuel must be provided to use all the
available air. Under normal conditions, the air/fuel
ratio requirements for gasoline engines will vary,
from 8 pounds of air to 1 pound of fuel to 20
pounds of air to 1 pound of fuel. Economy mixtures
normally range from 14 to 17:1 while power mixtures
require 2 or 3 ratios richer.
A typical
flow curve (Fig. 5) shows the air flow scale in
pounds per minute across the bottom of the chart,
and mixture ratio scale up the side. At curb idle,
about .5 lb. of air per minute is flowing at an
air/fuel ratio of 10.5 to 1. This is the richest
point on the curve. Idle air/fuel ratios are always
rich due to low engine speed which creates low
air velocity and poor scavenging of exhaust gases
from the cylinder causing dilution of the fresh
charge. Also, vaporization and distribution in
the intake manifold, under |
