CARBURETERS
In considering carbureters it would be well to have an understanding of
what is meant by this term. It is the practice to call the vaporized
fuel from the carbureter, a gas; but this is a misnomer. It is not
a gas, but a vapor, being merely air which is charged with small
particles of gasoline.
CARBURETED AIR.--It has been frequently termed also a _carbureted
fuel_. This is a wrong term. What is meant is _carbureted air_, because
the air carries the fuel with it, and is impregnated with a carbon
charge.
COMPOSITION OF GASOLINE.--Gasoline contains, approximately, 82 per
cent. carbon, and 15 per cent. of hydrogen. This mixture of the two
fuel elements requires about two parts of oxygen to one part of the
gasoline, but as common air is only one-fifth oxygen and four-fifths
nitrogen, which does not aid in combustion, it is necessary to supply
five times the amount of air, which would mean at least fifteen parts
of air to one of the gasoline.
In speaking of _parts_ it must not be understood, that reference is
made to parts in a liquid form, but it is necessary for the gasoline to
be put into the form of a gas, and this gas becomes the measure from
which we determine the parts.
GASOLINE EXPANSION.--If a cubic inch of gasoline is converted into a
gas, it will occupy a space equal to about one cubic foot, which means
that it now has a volume, or bulk of 1728 cubic inches. Now, for every
1728 inches, there must be about 30,000 cubic inches of air, in order
to make a combustible fuel out of the mixture.
REQUIREMENTS OF A CARBURETER.--A carbureter is designed to do several
well-defined things: First; it must be able to comminute, or break up
the liquid fuel into infinitesimally small particles.
Second; it must be able to properly mingle the vapor thus produced.
Third; it should be so constructed that it will automatically check the
inflow of gasoline, and prevent flooding, or waste of the fuel.
EVAPORATION.--All liquids have the property known as vaporization, and
will change their form into a gaseous state at ordinary temperatures.
All solids will vaporize, if sufficient heat is applied. But at the
ordinary temperature, with which we have to deal, in considering the
use of carbureters, air is the factor which facilitates the process.
AIR SATURATION.--Gasoline, confined in a vessel, will vaporize up to a
point where it completely saturates the air contained therein, and then
ceases. If allowed to stand in the open air, it will, in time, entirely
evaporate. This is true of water, also.
It is well, in this connection, to observe another thing. If the same
quantity of liquid is placed in two separate vessels, one very tall,
with a small surface of air in contact with the two surfaces, and the
other vessel very shallow, so it has a large surface in contact with
air, the latter will produce the most speedy evaporation. This shows
that contact with air is the factor of the greatest importance in
making a vapor.
AIR CONTACT WITH GASOLINE.--The office of a carbureter is to provide
the proper amount of air to the liquid fuel,--that is, up to that point
where it can be utilized as a fuel to the best advantage. If a drop of
gasoline, in one case is broken up into five hundred tiny particles,
and in the other case into one thousand, it is obvious that in the
latter case the air comes into contact with double the surface of the
liquid than in the former case, hence will be so much more efficient,
for the following reason:
_Perfect combustion_ is the desired object in the engine cylinder. The
more nearly the vapor approaches an impalpable gas the quicker will it
ignite. Furthermore, the more intimate the air and the vapor are mixed
the better will be the explosion or combustion.
COMPRESSION.--The compression of the carbureted air in the engine
cylinder performs certain very important things: When any gas is
compressed the temperature is increased, the theory being that at each
compression to one-half its volume, the temperature is increased double
its former heat.
If, therefore, compression in a cylinder reaches, say, 90 pounds,
the heat set up is sufficient to instantaneously break up the small
globules of gasoline, and at the same time produce a more intimate
unity, which tends to make a more efficient mixture than would be
possible without the compression.
COMPRESSION AS A MIXING MEANS.--It will also be understood, that
compression permits the bringing together of a much larger amount of
fuel at each charge than would be possible without it, so that the
two factors, namely, the volatilizing action of the air, the mixing
of the air and vapor, and the compression, all serve to mix together
the elements which will produce an explosion when the proper heat is
finally applied.
CARBURETER TYPES.--There are two distinct types of carbureters, one
in which the gasoline is forced out through a very fine nozzle, and at
the ejecting point is mixed with a current of air which passes to the
engine cylinders, and this is designated as the _spraying_ device.
The other form of construction depends for carbureting the air on
exposing a large body of the gasoline to a passing blast of air, and is
called the _surface_ type.
THE SPRAYING CARBURETER.--As most cars now use the spraying system,
that type will be considered first. There is no special form of nozzle
required to eject the fuel, and the distinctive features of the various
designs has been to produce positive and regular feed and to assure the
proper mixture at all times during the operation of the engine.
DISSECTING THE CARBURETER.--For the purpose of making each particular
part of a carbureter clear and distinct, let us build up one, so that
special attention may be directed to the various operative elements.
A cored cylindrical casting A, Fig. 78, is provided, which has a large
opening in its lower end that is closed by a plug B. This plug has
an upwardly-extending tubular projection B´. The upper end of the
cylinder has a cap C, open centrally, and having an opening formed by a
downwardly-projecting tube D, and this has a contracted throat as at E.
THE MIXING CHAMBER.--The exterior of the downwardly-projecting cap
tube, is turned up true, and fits into the tubular extension B´. The
particular feature of this sketch is to show the adjustment of the
needle valve which admits the gasoline, and the relative position of
the float.
[Illustration: Fig. 78. Carbureter Float and Needle.]
THE FLOAT CHAMBER.--The circularly-formed chamber G, within which the
float operates, contains the liquid fuel. The inner end of the plug B
has a cross duct I, and centrally is an upwardly-projecting tubular
extension J, the bore being flaring, as shown, and in this the needle
valve K rests and is made adjustable at its upper threaded end.
When the needle valve is raised, gasoline flows through the duct I
upwardly past the flaring orifice, in J, and air is permitted to flow
in through the openings I around the central tube J, so that the air
and gasoline meet above the upper end of the tube.
THE VENTURI TUBE.--The inwardly-projecting part E constitutes what is
called a venturi tube, the upwardly-rushing air between the contracted
opening formed around the tube at this point being such that when
the two fluids meet and spread out in the enlarged opening above,
the particles of gasoline are not only broken up minutely, but are
intimately mixed with the air.
[Illustration: Fig. 79. Carbureter Inlet Valve.]
THE INLET VALVE.--Now if this chamber G has at one side an extension,
like L, Fig. 79, means may be provided for adding a valve to be
controlled by the float. Within the extension is an upwardly-moving
needle valve M, which is designed to close the duct which leads from
the gasoline supply.
Between the valve and the float is the fulcrum O, of a lever N, the
short end of which engages with the upper end of the valve and the long
end rests on the float H, as shown. The movement of the float above the
predetermined point has the effect of seating the needle valve M, thus
cutting off the inflow of gasoline until that in the chamber G is drawn
out so that the float descends and again admits a fresh supply.
[Illustration: Fig. 80. Carbureter Discharge Port.]
Thus far we have the fuel oil control, together with the manner in
which the primary air supply is introduced. We shall now go a step
further, and illustrate the mixing chamber, discharge and throttle.
THE THROTTLE VALVE.--Referring to Fig. 80 it will be seen that directly
above the venturi tube described, is a space O. This is the mixing
chamber, which has an outlet P to the left, which connects with the
engine cylinders.
Within this tube is a throttle valve Q, operated by the throttle lever
on the steering wheel of the car. It is simply a disk which fits into
the interior of the conduit and is adapted to be turned by a stem R, on
which it is mounted.
While the lower inlets K are designed to supply the primary air for
carburetion, it is found necessary to admit a secondary supply, and
this should be taken into the mixing chamber directly instead of
passing the tube which conveys the oil.
THE SECONDARY AIR SUPPLY.--The particular reasons for thus admitting
the air may be explained as follows: When the engine draws in a supply
of carbureted air, more or less of a vacuum is brought about in the
mixing chamber O. The faster the engine runs the richer will the
mixture become, because the additional suction draws in an increasing
quantity of gasoline, but the throat of the tube does not change, and
the requisite, proportionate quantity of air does not follow, so that
the mixture has too much fuel for the air.
AUTOMATIC ADMISSION OF SECONDARY AIR.--If the engine should be speeded
up so twice the amount of oil is drawn into the mixing chamber, the
additional suction will not, at the same time, draw in twice the amount
of air.
This necessitates a provision whereby the secondary air shall be
admitted automatically only at times when the suction exceeds the
normal requirement, or to prevent too rich a mixture, which is
explained by reference to Fig. 80.
[Illustration: Fig. 81. Carbureter Secondary Air Inlet.]
The extension S, on the right side of the shell, has an opening T,
with a seat to receive a weighted valve, like a ball U, preferably
reinforced by a spring V, which is capable of having its pressure on
the seat regulated by an adjusting screw W.
It will be obvious, therefore, that during the normal action of the
engine suction, no air will enter the duct T; but when an undue vacuum
exists in the chamber O, the ball valve U is raised, and additional
air is supplied to the carbureted air within the chamber.
[Illustration: Fig. 82. Complete Carbureter.]
CARBURETER ADJUSTMENT.--Each of these four elements has some
particular method of adjustment, as will be more particularly noticed
in the completely assembled carbureter, made up of the foregoing
illustrations, in which the details are refined and shown as actually
made in one of the well known types of carbureters.
Fig. 82 shows the different parts arranged in a practical manner, in
which the regulating arm for controlling the throttle, as well as the
secondary air supply and the gasoline inlets are capable of being
adjusted by special means.
SPECIAL POINTS CONCERNING CARBURETERS.--A rich mixture is undesirable,
except in the case of heavy loads and at slow speed, for various
reasons. It does not burn quickly, or explode as readily as a lean one,
and owing to the slow combustion the temperature in the engine cylinder
remains high to the end of the stroke.
THIN MIXTURES.--On the other hand, a thin mixture will compress better
and burn with greater facility, and at the same time heat the cylinder
less than the rich mixture, to say nothing of the saving in fuel. It
has long been recognized that a carbureter will not act uniformly with
all engines. Some have better compression than others, and some have
more efficient sparking means. This has a bearing on the character of
the fuel delivered to the cylinders.
SPEEDS AND MIXTURES.--There is also a wide difference in the
performances of engines at high and at low speeds, as to the quality of
the mixtures required, so it will be seen that a carbureter which is
capable of being controlled for all emergencies, is the one to select.
Above all, the structure should be such that the valves can be easily
taken out for inspection and repairs. It is impossible to prevent grit
from finding its way into the gasoline, and it is astonishing how the
smallest piece of fiber, finding a lodgment in a valve, will disarrange
the entire power system.
SURFACE CARBURETER.--These devices depend on presenting as large an
area of gasoline as possible, and then conducting the air flow over the
surface so as to take up the volatile hydro-carbon.
THE FLOAT.--Such devices also require a float to regulate the inflow of
fuel, and the distinctive feature of construction depends on increasing
or decreasing the area so exposed to the moving air column.
Fig. 83 shows a well-known type of this character which is a
combination spray and surface carbureter. A U-shaped tube A, with the
air inlet at B, and discharge at C, has a butterfly valve D in its
latter end. Below the U-shaped bend, is a reservoir E to contain a
float F, vertically-movable around a central stem G which is part of
and projects down from the U-shaped tube.
[Illustration: Fig. 83. Surface Carbureter.]
Through this stem G is a duct H, the lower end of which communicates
with the gasoline reservoir, or float chamber, and the upper end has a
small orifice leading to the U-shaped tube. A valve stem I is adapted
to regulate the inflow of gasoline through the duct.
THE GASOLINE INLET.--At one side of the reservoir is an extension J,
within which is a vertically disposed needle valve K, seated in the
duct I, by way of which gasoline is admitted. A lever M, pivoted at N,
has one end attached to the float F, and the other end is in engagement
with the needle valve K.
The float is so arranged as to permit the gasoline to flow up into the
U-shaped tube A, and form a small pool of the fuel before it closes the
needle valve K.
SECURING SURFACE FOR AIR CONTACT.--Directly above the oil inlet duct
H, the U-shaped tube is contracted by a downwardly-projecting wall
P, the object being to compel all the passing air to intimately come
into contact with the gasoline pool, and thus take up as much vapor as
possible.
In this arrangement the suction of the engine does not draw up the
gasoline from the reservoir, but all the energy is expended in moving
air through the tube, and past the contracted throat.
In starting the engine the float is momentarily depressed by the pin Q,
and a drain duct R is provided to prevent flooding of the tube A.