Chevrolet Light Duty Truck 1973, Service Manual

Page: 432 / 979

6 M - 8 E N G IN E FUEL
mixture for cylinder 4, 5, and 6 will initially travel
towards the rear of the engine.
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 (Fig. 9). 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 and furnish a uniformly vaporized
mixture to the manifold.
FUEL-AIR REQUIREMENTS
(Fig. 10)
As previously mentioned, we know that without regard
for efficiency, an air-fuel mixture within the range of
8-to-l and 18 1/2-to-l must be provided for an
automobile engine to run. More practically the air-fuel
ratio should be utilized which would allow peak power
output, minimum emissions, and peak fuel economy.
Unfortunately, no single air-fuel ratio can provide both
the above peak conditions. Tests prove the best power
output is obtained using a 12.5-13.5:1 mixture whereas
best fuel economy results using a 15.0-16.0:1 mixture.
Since an automobile engine must be able to provide both
requirements, obviously no compromise fuel ratio would
be satisfactory. Therefore a carburetor must be able to
quickly match varying engine requirements with the best
possible fuel mixture to meet the demand. This means
that fuel ratios must be provided not only for the
demand but caused by light speed variations and
changing engine load conditions such as a moderate
grade.
One of the reasons that the air-fuel ratio must be varied
is because of imperfect conditions within the combustion
chamber. Exhaust gases remaining in the cylinder dilute
the fresh charge. The fuel and air are not perfectly
mixed, miniature droplets of unvaporized fuel being
carried along by the mixture of air and evaporated fuel.
The intake manifold itself does not deliver exactly equal
air-fuel mixtures to all cylinders. The air-fuel ratio must
therefore be adjusted, depending on such factors as
engine speed and whether power or economy is desired.
POWER VS. ECONOMY
If maximum power is desired, it is necessary to burn all
the oxygen in the air, since the power-production ability
of an engine is limited by the amount of air it can take
in. Additional fuel must therefore be added to insure
that each molecule of oxygen combined with the
necessary fuel. Thus, the mixture required for maximum
power usually falls in the range from 12 to 13.5-to-l,
being "richer" in gasoline than the theoretical ratio.
On the other hand, for maximum economy and least
emissions, it is desirable to burn all of the fuel, in order
to extract as much energy as possible with minimum
residual. Because of the imperfect combustion condi­
tions, additional air is required to insure that each
molecule of fuel can be combined with the necessary
oxygen. As a result, the actual air-fuel ratio for
maximum economy tends to be somewhat "leaner" in
gasoline than that calculated for chemically perfect
combustion.
The best air-fuel ratio for any given operating condition
varies from engine to engine due to differences in
manifolding, combustion chamber design, valve timing,
P(
D W E R-W IDE O P E N THRO TTI F
-— IDLE E N G IN E SPEED H IG H —
Fig. 10-Air-Fuel Flow Fig. 11 -Air-Fuel Flow -Full Throttle
LIGHT DUTY TRUCK SERVICE MANUAL