CARBURETORS AND THEIR FUEL
Although gasoline is inflammable in its liquid state, its combustion
is not sufficiently rapid to approach the _explosive_ point necessary
to render its energy available in the automobile engine cylinder. The
proper proportion of gasoline _vapor_ and air, however, forms a mixture
that is highly inflammable and that will be entirely consumed in the
engine cylinder under ordinary conditions within about one-twentieth
of a second after the formation of the spark. This rapid combustion
so nearly approaches the instantaneous action of an explosion that it
may be considered as such in all ordinary discussions of the gasoline
engine. Literally, however, the gasoline engine is not an _explosion_
motor, but rather is it an engine of the _internal combustion_ type. To
obtain this gasoline vapor in an easily-controlled form the carburetor
was designed as one of the most important adjuncts of the automobile.
The first form of carburetors, or "vaporizers," as they were called
then, employed a flat, woven lamp wick over which the gasoline flowed.
This spread the fuel out over a comparatively large surface and
rendered evaporation rapid and simple. The chamber containing this
wick was placed in the line of the intake pipe of the motor and was
connected with the cylinders on the descent of the pistons on the
suction stroke through the medium of the various inlet valves. In
a four-cycle motor, the piston acts as a suction pump on alternate
down-strokes and serves to draw the charge into the cylinder. This
suction created the necessary current of air to facilitate evaporation
of the gasoline on the wick, and by regulating the size of the
passages, the proper proportion of air and gasoline vapor could be
obtained.
The modern, high-speed automobile motor, with its varying demands upon
the carburetor, created the necessity for a more delicate, flexible,
and compact vaporizer than was to be found in the "lamp wick" type.
Consequently the wick was replaced by a small, slender, hollow tube
having a cone-shaped opening at its upper end through which the
gasoline from the feed pipe was made to pass. Fitting into the upper
end of this tube, and pointed to the same angle, was a cone-shaped
"needle" that could be moved in and out of the opening. If this needle
was unscrewed slightly so that it did not form a tight fit with the end
of the tube, a small ring would be formed through which the gasoline
must pass when sucked by the alternate down strokes of the pistons.
This tube and needle constitute, under various guises, the "needle
valve" with which practically every modern carburetor is equipped.
When the gasoline, rushing through the small tube, strikes the
restricted opening of the needle valve, it is broken up into a fine
spray which, under proper conditions, will become vaporized almost as
soon as it comes in contact with a current of air. This air current is
induced by the same pump-like effect of the pistons as that which sucks
the gasoline through the needle valve, and thus it occurs only when the
charge is desired in the cylinders.
But the carburetor is not merely to provide a compact device for
vaporizing the gasoline, for it must also furnish a means of regulating
the proportion of gas to air. Gasoline vapor is only highly inflammable
when mixed with the proper quantity of air, and if this proportion is
varied above one limit or below another, ignition of the charge will
not occur in the cylinders. In fact, the allowable variation in the
proportion of gasoline vapor to air is restricted between very narrow
limits, and should not change more than four or five per cent. from
one extreme to the other. The proportion of gasoline vapor to air by
weight is about one to eleven, although this will vary somewhat with
the different grades of fuels.
The point to be emphasized, however, is the fact that the proper
proportion of air to gasoline vapor, however it may vary with different
grades, should be kept constant at all speeds of the motor whenever
that particular grade of fuel is used. By volume, about 97½ per
cent. of the mixture should be air and the remainder gasoline vapor,
and it is the device that will the most nearly maintain this proportion
under all conditions of speed, temperature, and air pressure that will
prove to be the most delicate and flexible carburetor.
A carburetor may be adjusted for different motors, or for different
operating conditions of the same motor, by means of the needle valve.
The farther end of the slim rod on which the needle point is mounted
terminates in a thread and finger nut that projects through the shell
of the carburetor. By turning this nut in one direction, the needle
valve is screwed up toward the cone-shaped end of the tube and the
orifice through which the gasoline may pass is thus reduced in size.
This will decrease the amount of gasoline sprayed into the air passage
and will consequently change the composition of the mixture. This,
however, should not be confused with throttling the motor. When the
needle valve is tightened, the volume of the mixture passing to the
cylinders is the same, for it is only the proportion of gasoline vapor
in that mixture that is changed.
Throttling consists in restricting the size of the opening through
which the _mixture_ passes, and thus limits the amount of the
charge that reaches the cylinders at each suction stroke of the
piston. Throttling is used to reduce the power--and consequently the
speed--developed by the motor, while a decrease in the amount of
gasoline supplied to the air through the needle valve may serve to
increase the power through an improvement in the nature of the mixture.
Since the gasoline vapor, by volume, forms only about three per cent.
of the explosive mixture admitted to the cylinders, a slight variation
in the size of the needle valve opening will result in a marked change
in the composition of the charge and may make all the difference
between poor and perfect running of the motor. Consequently the needle
valve nut should be moved but the small fraction of a turn for each
adjustment. A motor which may refuse absolutely to run at one position
of the needle valve may give perfect results if the nut is unscrewed
but the eighth of a turn.
In view of the marked difference in the results obtained from the use
of mixtures that are "just right," and those which vary but a slight
percentage in the proportion of gasoline vapor to air, it may be well
to examine, superficially, the effects of "rich" and "weak" charges,
and therefrom to obtain a list of "symptoms" which may aid us to
diagnose motor trouble properly.
We all know that air--or oxygen--is required to support combustion.
"Snuffing" a candle is merely covering its end so that air cannot reach
the flame. For the same reason, gasoline in a covered tank cannot burn,
no matter how great the heat applied to it. The heat of the electric
spark in the cylinder, although intense, does not cover a sufficiently
large area to ignite any charge except that composed of the proper
proportion of gasoline vapor and air. If there is too much gasoline
vapor, making a "rich" mixture, there will not be sufficient air in
the charge to support the entire combustion of the gas, and the burning
will be slow--if it takes place at all. The same conditions will
prevail if there is an insufficient supply of air for a given quantity
of gasoline vapor, and consequently a rich mixture may be obtained
by reducing the air flow as well as by adding to the amount of gas
admitted to the mixing chamber.
A rich mixture will cause irregular explosions in the cylinders, and
will often emit a black, pungent smoke at the exhaust. The motor will
probably overheat easily, due to the slow-burning properties of the
mixture and the resulting fact that a large portion of the cylinder
walls uncovered by the pistons will be exposed to the flame. In some
instances, the cylinders will miss fire at regular intervals, thus
changing the synchronism of the impulses with a well-defined and
periodic "skip" in the sound of the explosions.
While these are by no means certain symptoms of a rich mixture,
the first test to be made should be to tighten the needle valve
adjustment slightly when the motor is running and to note any resulting
improvement in the regularity of the explosions. It may sometimes be
difficult to distinguish between the symptoms of a rich and a weak
mixture, but the readjustment of the needle valve as just described
will at least serve to locate the trouble or to eliminate one or the
other possibility from consideration.
When a mixture is "starved", or when there is an insufficient supply
of gasoline vapor to unite with the air admitted to the cylinders, the
charge will not be highly inflammable and may not be ignited by the
small spark formed at the plug. Even when ignition does take place, the
resulting power impulse will be weak because of the comparatively small
amount of pressure-producing gas in the mixture. The explosions may
occur regularly for a while, but there will be a marked decrease in the
power developed by the motor, and owing to the fact that weak mixtures
may be slow-burning, "back-firing" will often result in some engines to
which such a charge has been fed.
On the other hand, if a motor will run at all on a weak mixture, it
will produce better results than would be the case were the charge
too rich in gasoline vapor. Consequently the needle valve should be
closed as much as is consistent with smooth running of the motor, but
the moment a loss of power or irregular explosions occur, the mixture
should be enriched.
At low speeds of the motor, the pumping action of the pistons is not as
great as is the case at high revolutions, and consequently the suction
drawing the gasoline through the needle valve is diminished. For this
reason, the needle valve opening must be made larger or the air passage
restricted for slow speeds of the motor, and it was consequently
necessary, on the old, non-automatic vaporizers, to _increase_ the
gasoline supply whenever the revolutions of the motor were to be
reduced. The modern carburetor is sufficiently automatic in its action
to provide the proper mixture within wide ranges of speed change of the
motor, but even nowadays it is often found necessary to increase the
gasoline supply or to reduce the amount of air admitted to the intake
pipe whenever it is desired to throttle the motor down to a very low
number of revolutions per minute.
The automatic action of the ordinary carburetor is obtained by
increasing the air supply at higher speeds of the motor. Consequently
the motorist will realize that whenever the needle valve is to be set,
such regulation should be made when the motor is well throttled, for
if an ample gasoline supply is obtained at low speeds, the mixture
will certainly be sufficiently rich at increased revolutions. If, on
the other hand, the carburetor should be set to supply a proper mixture
at high speeds, the mixture would be impoverished when the motor is
throttled, and irregular running would result.
The air for the operation of the motor at ordinary speeds is supplied
through a fixed opening in the carburetor connected with the chamber
into which the gasoline spray is introduced. In addition to this, most
carburetors are supplied with an "auxiliary air opening" which serves
to furnish the additional air necessary for the mixture at high speeds
of the motor. The fixed opening, being restricted in size, cannot
admit the increased quantity of air demanded by the higher speeds
of the motor. The auxiliary opening is provided with some form of
automatic valve which may consist either of a series of ball "checks,"
a spring-actuated "mushroom valve," or a series of special valves, each
of which opens at successively increased speeds of the motor.
All of these devices operate on the same principle, however, and
allow the increased suction of the motor to add to the size of the
air passage automatically--either by the farther opening of a
single valve, or by the successive opening of different valves. Some
carburetors are provided with an adjustment by means of which the
"delicacy," or ease of opening, of the auxiliary air valve may be
regulated. This may be done by means of a nut and screw which will
increase or decrease the tension of the controlling spring. If this
spring is set with a high tension, the auxiliary valve will act only
when the motor is exerting great suction, or at fast speeds.
The regulation of the auxiliary valve is an adjustment that should be
made only after the needle valve has been set properly for slow speeds
of the motor. When this condition is obtained, the throttle should be
opened and the further adjustment of the carburetor for high speeds of
the motor should then be made through the auxiliary air valve. In other
words, the needle valve should be set so that the motor runs properly
at low speeds, while the adjustment of the auxiliary air valve should
be made only to secure smooth operation at a high number of revolutions.
It is not to be understood that less gasoline is actually required at
high speeds of the motor because the supply often needs to be cut down
at the needle valve under these conditions. The actual amount required
at high speeds is, of course, greater than is the case at slow, on
account of the greater number of explosions in the former instance. But
the suction of the motor generally increases the gasoline flow beyond
the demands of the cylinders at high speeds, and it is for this reason
that the automatic auxiliary air supply is provided to furnish the
additional air required to support combustion. In fact, at heavy loads,
when the total amount of gasoline consumed must be great, a secondary
jet of fuel is brought into action in some carburetors. This is known
as the "multiple-jet" type and is found on some of the large engines
that must possess a speed and power variation between wide ranges. The
action of these various jets is entirely automatic and is dependent
upon the speed and fuel requirements of the motor.
Were the gasoline fed directly from the fuel tank to the needle valve
of the carburetor it is evident that the rate of flow of the liquid
would depend, to a large extent, upon the amount in the tank and upon
the position of the car. This would cause each charge to differ in the
proportion of gasoline vapor to air, and it is hardly probable that the
motor could be run at all under such conditions. In order that the
pistons may suck the gasoline from a level that does not vary with
the amount of fuel in the tank or the position of the car, a separate
compartment is provided in the carburetor. This is known as the "float
chamber," and it is from this compartment that the gasoline passes
through the needle valve into the vaporizing or mixing chamber.
A cork or hollow metal float is placed in this float chamber and is
mounted on a lever connected with a valve located at the end of the
gasoline feed pipe. As the gasoline is admitted to the chamber, the
float rises and closes the valve controlling the flow of fuel. As the
gasoline is sucked through the needle valve from the float chamber,
the float in the latter lowers, and the fuel is again admitted by
the opening of the above-described valve. The float and valve are
exceedingly delicate in their operation and the gasoline is thus kept
at a constant level in the chamber under all conditions of the car and
tank.
The stem upon which the float of some carburetors is mounted is
sometimes threaded and provided with a nut by means of which the float
may be raised or lowered. This furnishes an adjustment for varying the
level in the float chamber and determining at what point the flow
of gasoline shall be cut off by the automatic valve. The float is
supposedly properly regulated when the carburetor leaves the factory,
but the stem may become bent or the carburetor may be applied to a
motor other than that for which it was originally designed. In either
of these events, it may be found necessary to raise or lower the float
before the proper level of gasoline can be maintained in the chamber.
If the float is too high on its stem, the gasoline control valve may
not be operated until the fuel overflows in its chamber. This is known
as a "flooded" carburetor and produces a rich mixture which will
ultimately prevent the proper operation of the motor. Turning down
the gasoline supply at the needle valve will not remedy this, for the
fuel will reach the vaporizing chamber by another route. A flooded
carburetor often gives trouble, and while it may be remedied easily,
the amateur may experience difficulty in locating its source.
As soon as it is discovered that a carburetor has become flooded, the
needle valve should be tightened so that no gasoline can pass through
it, and the motor should then be cranked. This will serve to evaporate
the excess gasoline in the float chamber and reduce the level to the
point at which it will not overflow. The exact number of turns and
fractions of turns through which the needle valve nut was moved should
have been noted in order that the valve may be reset to its original
position after the surplus fuel has been "cranked out."
A float that is set too low on its stem will close the fuel supply
valve before a sufficient amount of the fuel has flowed into the
chamber, and will form a "lean" mixture at high speeds of the
motor--even though the needle valve should be opened wide. The obvious
remedy for such a condition is to raise the float until the gasoline
will be maintained at the proper level. If there is no nut and screw
adjustment by which the float may be raised, the arm to which it is
attached, and which is connected with the valve, may be bent slightly.
But the motorist should not "jump at conclusions" and assume that the
float is improperly set the moment the carburetor begins to flood or
the motor appears to "starve" at high speed. The first condition may be
caused by a piece of dirt or other foreign matter that may have become
lodged on the valve seat and prevented the valve from closing when
the gasoline reached the proper level in the float chamber. This will
produce exactly the same results as will a high float and is a trouble
that will more often occur in the average carburetor.
The difficulty may generally be remedied easily by draining the
gasoline from the float chamber after the valve in the main supply
pipe has been turned off. The offending foreign matter will generally
be carried with the gasoline as the latter is drained, and the valve
in the feed pipe may again be opened as soon as the drain cock is
shut off. If this fails to remedy matters, it is probable that the
difficulty lies with the float.
A clogged gasoline pipe or dirty strainer will produce the same effect
on the operation of the motor as will a float that is set too low on
its stem. When the motor seems to starve at high speed, and it is
evident that there is sufficient gasoline in the tank, the union should
be disconnected at the point where the feed pipe joins the carburetor.
If there appears to be an ample flow through this pipe when the main
valve is opened, it is probable that the stoppage has occurred in the
strainer. If the flow through the main feed pipe is not free, however,
it is possible that the vent hole in the filler cap on the tank has
become stopped or that the latter has been screwed down too tightly. In
the gravity feed systems, some method must be provided to allow the air
to flow into the tank to replace the gasoline fed to the carburetor. If
there is no hole in the filler cap, the latter should not be screwed
down so tightly that an airtight joint will be formed.
Probably the simplest method of determining whether the trouble lies
in a low float is to prime the carburetor and to observe the ease with
which this can be done and its effect upon the engine. Nearly every
carburetor is provided with a "flushing" or "priming" pin by means of
which the float can be depressed so that the gasoline chamber will
be filled rapidly to a point above its normal level. This is useful
in starting, as the desired rich mixture is quickly obtained without
an undue amount of cranking. If the carburetor flushes easily, it
is evident that there is no serious stoppage in the pipe. If this
easy flushing is followed by good running on the part of the motor,
and if this, in turn, is succeeded by gradually-diminishing impulses
indicating a weakening mixture, it is quite evident that the float is
preventing the flow of the gasoline at the proper time.
In addition to the flush pin found on carburetors, many are provided
with other devices to render starting easy. It is well known that a
"high-test" gasoline, such as a 76, will vaporize more easily than will
one of a lower degree of specific gravity. Also, every motorist has
had impressed upon him the fact that heat aids in the vaporization of
gasoline. If we try to start a motor on a cold morning with a low-grade
gasoline, such as the 60- or 62-degree fuel now generally obtained, we
know that a rag dipped in hot water and wound around the carburetor
will help matters.
To enable low grades of fuel to be properly vaporized under all
running conditions, many carburetors are provided with a water jacket
surrounding the vaporizing chamber. This jacket is connected with the
cooling system of the motor, and the hot water surrounding the chamber
so warms the interior that vaporization is greatly facilitated. Some of
these systems are provided with a shut-off cock by means of which the
carburetor may be operated with hot water in the jackets, or not, as
desired.
Other carburetors employ a jacket surrounding the exhaust pipe of the
motor and connected with the vaporizing chamber. The air is heated by
the hot exhaust pipe as it is sucked into the carburetor, and this also
facilitates the vaporization of the fuel. Some carburetors are provided
with both jacket systems, while others have neither, but whatever
design is installed, the best results will be obtained if cold air is
used after the motor is once started. Cold air is more "concentrated"
and contains a greater amount of oxygen per cubic foot than does air
that has been expanded by heat, and consequently many carburetors are
provided with a means of turning off the hot air after the motor is
started.
The higher the degree of specific gravity of a fuel on the Baumè scale,
the more volatile will it be, and consequently a 68° gasoline will
vaporize more easily and give more power than will a 60° or 62° fuel.
72° gasoline is often used in races, but the average motorist does
not get better than 64°--and he is sometimes lucky to obtain fuel of
that specific gravity. A hydrometer, or specific gravity tester, is a
convenient instrument for the average motorist to own, and with it he
may tell exactly what grade of fuel he is paying for. The Baumè scale,
by which all gasoline is tested, reads in degrees, and the specific
gravity is obtained by observing the depth to which the hydrometer
sinks in the liquid. This instrument resembles somewhat a glass
thermometer, and is so graduated that the deeper it sinks in a liquid,
the higher will be the reading on its scale.
Water in the fuel is an annoyance that is often encountered by the
automobilist and the motor boatman, and this will make its presence
known by causing the motor to skip when all adjustments and connections
seem to be in perfect condition. Water is much heavier than gasoline
and has no affinity for it, and consequently, as it sinks to the
bottom of the tank, a few drops in a large amount of gasoline will
cause trouble by passing out through the needle valve at intermittent
intervals and forming an unexplosive mixture.
The presence of the water in the fuel may be detected easily without
the use of a hydrometer by drawing some gasoline from the bottom of
the tank into a tin or white-enameled cup. If water is present, it may
be seen in the form of small globules in the bottom of the cup. If the
contents of the cup are poured over a flat surface so that the liquid
may be allowed to spread, the gasoline will be seen to cover a large
surface and evaporate quickly, while the water will seem to remain
in the globules unevaporated for some time after the gasoline has
disappeared. This latter test will sometimes show the presence of water
when none can be discerned in the bottom of the cup before the contents
are poured out on the flat surface.
The practice of "doping" the fuel tank by adding to the gasoline ether
or some other highly volatile liquid is not to be recommended to the
average motorist. A few ounces of ether or chloroform added to the fuel
will form a more volatile and consequently more powerful mixture, but
unless the greatest care is taken, the motor is liable to be completely
ruined by such a procedure. Numerous cases are on record in which
cylinder heads have been blown off or castings cracked by the force of
some of the explosions when too much "dope" has found its way into the
mixture.
Although the average motor gasoline obtainable nowadays is hardly all
that could be desired as automobile fuel, a little care taken when
filling the tank will eliminate many of the carburetor annoyances to
which many cars seem to be subject. The cap of the tank should never
be taken off when the air is filled with particles of dust that are
liable to find their way into the fuel, and care should be taken to
see that no pieces of the rubber or leather washer or packing drop into
the gasoline when the cap is removed. Foreign matter and water that may
be in the gasoline when purchased may be removed by straining the fuel
through a chamois skin placed inside of the funnel through which the
tank is filled.