Chevrolet Light Duty Truck 1973, Service Manual

Page: 433 / 979

E N G IN E FUEL 6 M - 9
ignition timing, and other engine design factors. The
optimum mixture can best be determined by operating
an engine on a dynamometer where power, speed, and
fuel consumption may be measured over a wide range of
conditions.
Tests made at speeds throughout the engine’s operating
range reveal that the air-fuel ratio for maximum power
remains nearly constant at all except low speeds (Fig.
11). Here, some slight enrichment is necessary due to the
effect of exhaust-gas dilution in the cylinder and the
poor mixing and distribution from the lower velocity air
flow into the engine.
Similarly, the leaner air-fuel ratios for maximum
economy at part throttle (Fig. 12) are found to be
essentially equal throughout most of the operating range.
Again, enrichment is necessary at low speeds and during
idling; for the latter, the air-fuel ratio usually falls in the
range from 11 -to-1 to 12.5-to-l. Richer mixtures are also
used at high operating speeds and loads, where more
power is required.
It is usually desirable to operate the engine so that the
best fuel mileage can be secured during normal cruising,
while maximum power should be available when the
throttle is opened for acceleration or top speed (Fig. 13).
Through this point, we have covered the engine’s general
requirements insofar as the need for both a vaporized
mixture and the proportions, or ratio, of the air-fuel
vapor for efficient combustion. This leaves only one
question which needs clarification before we end this
discussion of air-fuel requirements, that is, "Why are
richer fuel mixtures needed for cold starts "
COLD STARTS
Recall for a moment that the cigarette lighter could be
ignited easily indoors but after exposure to colder air,
the lighter failed to light. It was concluded that the
Fig. 13- Air-Fuel Flow- Complete Air
lighter failed to light because there was too little fuel
vapor or actually the air-fuel ratio was too lean to burn.
Preheating to increase fuel vaporization to correct the
air-fuel ratio is the answer in the case of the cigarette
lighter but this would be impractical from a time and
cost standpoint for automotive use.
We are all aware that a choke is used on carburetors to
restrict the air flow and thereby enrich the fuel mixture.
Because of this, many assume that a richer mixture is
actually burned during cold engine operation but this is
definitely not the case. Although the air-fuel ratio may
vary fuel is always burned at a 15:1 ratio. While this
may seem contrary to actual conditions, the key is the
term "vaporize." Since only vaporized fuel burns and
the vaporization rate is sharply reduced at lower
temperatures, a rich fuel charge must be admitted so that
the total resultant fuel vapor reaching the cylinders will
be rich enough to create a combustible mixture.
The extra fuel (that which did not vaporize) is
minimized, or in some engines is discharged from the
port unburned as waste during the exhaust cycle and
burned in the exhaust system. While this represents a
considerable waste of fuel, it is necessary to provide
combustible fuel mixtures until the engine is warmed up.
While the discussion of engine air-fuel requirements has
been brief, the following points should be remembered:
• Only vaporized gasoline will burn
• Air-Fuel ratios are referenced in terms of weight
• Vaporization rates reduce as temperature declines
• Gasoline always burns at a 15-to-l ratio; extra
fuel is not burned but lost as waste.
BASIC CARBURETOR SYSTEMS
The carburetor performs a comparatively simple job but
LIGHT DUTY TRUCK SERVICE MANUAL