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is further limited by the amount of air that the fuel
vapor is absorbed by. Combustible mixtures in an engine
are limited by the following proportions, or ratios, of air
to gasoline: eight parts air to one part gasoline is the
richest mixture that will fire regularly and a mixture of
18 1/2 parts air to one part gasoline is the leanest
mixture that will fire without missing in an engine.
Mixtures leaner than 18.5:1 tend to cause misfire. From
an automotive standpoint these ratios represent the
mixture extremes that an engine can tolerate, but these
limits do not provide the two conditions most sought.
The most desired ratios are a mixture that will produce
the most power per pound of gasoline and a mixture that
will provide the best economy or most miles per pound
of fuel with the least exhaust emissions.
For those who may be wondering why we are speaking
in terms of "pounds" of fuel instead of our more usual
gallon measure, let's take a moment for clarification.
Actually the reason we speak in terms of pounds rather
than gallons is so that the ratio, or air-fuel mixture
proportion, terms can be smaller numbers. For example,
for most efficient (and economical) combustion, 9000
gallons of air are required to burn one gallon of
gasoline, hence an air-fuel ratio of 9000:1 by volume.
Obviously, proportions with such numerical differences
would be difficult to comprehend and extremely awkward
to work with for experimentation and design.
By using weight as the base, let's make the same
comparison (Fig. 3). A gallon of gasoline weighs about
six pounds whereas 100 gallons of air are needed to
produce one pound of air. Converting our volume air-
fuel ratio to weight, we find that 9000 gallons of air
weighs 90 pounds (100 gals, equals 1 lb., therefore 9000
divided by 10090 lbs.) and one gallon of gasoline weighs
6 pounds. 90 divided by 6 equals 15 therefore we arrive
at an air-fuel ratio of 15:1 by weight.
METERING
Good combustion demands a correct mixture ratio
between fuel and air. To release all possible energy by
combustion, the right amount of fuel must be mixed with
a given amount of air. The metering job of the
carburetor is to furnish the proper air-fuel ratio for all
conditions, so that the engine operation will neither be
too lean for power requirements nor too rich for
economy (Fig. 4), while still meeting prime requirements
of low emissions.
AIR MEASUREMENT
Air flow through a pipe will create a pressure drop
within the pipe proportional to the speed of the air.
Thus, for a given pipe size, the pressure difference
between the outside air and the pressure within the pipe
provides a direct means of measuring the amount of air
flowing through the pipe.
The simplest means of measuring the pressure difference
between atmosphere and the pressure inside the pipe, is
to insert a U-tube partially filled with water into the pipe
as illustrated (Fig. 5). Since one end is open to
atmosphere, the water level will rise on the low pressure
(or pipe side) because water will flow toward an area of
lower pressure. Actually, the water is being pushed into a
state of balance by the weight of the air at the end of the
U-tube open to atmosphere vacuum. A common example
of the force exerted by the weight of air is a drinking
straw (Fig. 6). As you suck on a straw, the air within the
straw is removed, thus creating a low pressure or
vacuum. The weight, or pressure, of the air on the
surface of the liquid in the glass then forces the liquid
up the straw to fill the void. The important thing to
E N G I N E FUEL
6 M - 5
LIGHT DUTY TRUCK SERVICE MANUAL
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