CARBURETERS
A carbureter is a device which receives and mixes gasoline and air in
proper proportions, and in which a vapor is formed for gasoline engines.
The product of the carbureter is a mixture of gasoline vapor and air,
not a gas. A gas, as explained, is of such a character that it remains
fixed and will not stratify or condense.
Functions of a Carbureter.--The function of a carbureter is to supply
air and gasoline by means of its adjustable features so as to make the
best mixture. The proportions of air and gasoline will vary, but
generally the average is fifteen parts of air to one of gasoline vapor.
If there is too much gasoline, proportionately, a waste of fuel results,
as a great amount of soot is formed under those conditions. If there is
an excess of air the mixture, when ignited, will not have such a high
temperature, hence the expansive force is less, and the result is a
decrease of power.
While it is possible to get a rapid evaporation from gasoline by
heating it, experience has shown that it is more economical to keep the
gasoline cool, or at ordinary temperatures, provided the carbureter is
properly constructed, because the vapor, if heated, when drawn into the
engine, will be unduly expanded, and less fuel in that case is drawn in
at each charge, and less power results.
Rich Mixtures.--There are conditions under which rich mixtures are
advantageous. This is a mixture in which there is a larger percentage of
gasoline than is necessary for instantaneous combustion. For ordinary
uses such a mixture would not be economical.
At low speeds, however, or when carrying heavy loads, it is desirable,
for the reasons that at a slow speed the combustion is slower.
Rich mixtures are objectionable at high speeds because, as the
combustion is slow, incomplete combustion within the power stroke
results, the temperature of the gas at the end of the stroke is very
high, and this will seriously affect the exhaust valves. Furthermore,
there is likelihood of the gas continuing to burn after it is discharged
from the cylinder.
Lean Mixtures.--Such a mixture is one which has a less amount of
gasoline than is necessary to make a perfectly explosive compound. For
high speeds a lean mixture is desirable, principally because it burns
more rapidly than a rich mixture.
Types of Carbureters.--There are two distinct types of carbureters, one
which sprays the gasoline into a conduit through which air is passing,
and the other in which a large surface of gasoline is placed in the path
of the moving air column, which was originally used, but has been
absolutely replaced by the jet carbureters on account of their better
control features.
It will be remembered that reference was made to the manner in which
vaporization takes place, this term being used to designate that
tendency of all liquids to change into a gaseous state. All carbureters
are designed with the object of mechanically presenting the largest
possible area of oil to the air, so that the latter will become
impregnated with the vapor.
The Sprayer.--The best known type depends on dividing up the gasoline
into fine globules, by ejecting it from a small pipe or jet. The spray
thus formed is caught by the air column produced by the suction of the
engine pistons, and during its passage through the throttle and the
manifold, is in condition where a fair mixture of air and vapor is
formed, which will readily ignite.
The Surface Type.--This form of carbureter provides a pool of gasoline
with a large surface, within the shell, so arranged that as the air is
drawn past the pool it must come into contact with the oil, and thus
take up the necessary quantity of evaporated gasoline for charging the
air.
The _surface_ type has not been used to a large extent, but the
_sprayer_ is universally used, and of this kind there are many examples
of construction, each having some particular merit.
Governing a Carbureter.--It is a curious thing that one carbureter will
work admirably with one engine, and be entirely useless in another. This
is due to several factors, both in the engine design and in the
carbureter itself. The quality of mixture that an engine will take
depends on its speed. The suction of the pistons depends on the speed of
the engine.
If, at ordinary speed the carbureter gives a proper mixture, the throats
and passages through the pipes and manifold, as well as the valve which
discharges the gasoline, may be in a prime condition to do good work;
but when the pistons work at double speed the inrush of air may not
carry with it the proper amount of fuel; or, under those conditions, the
air may receive too great an amount of gasoline, proportionally.
The latter is usually the case, hence provision must be made for such a
contingency, and we shall therefore take up the various features
essential in the construction of the carbureter, so as to show what
steps have been taken to meet the problems arising from varying speeds,
differences in the character of the fuel, regulating the inflow and
mixture of gasoline and air, and adjustments.
[Illustration: _Fig. 31. Carbureter._]
So many different types of carbureters have been devised, that it is
difficult to select one which typifies all the best elements of
construction.
In Fig. 31 we have shown a well known construction, and which will
illustrate the features of the sprayer type to good advantage. The body
of the device, represented by A, has a flange by means of which it is
secured to the pipe which carries the carbureted air to the engine. The
lower end of this tubular body is contracted, as shown at B, so as to
form what is called a venturi tube.
Exteriorly this contracted tube is threaded, as shown at C, so as to
receive thereon a threaded body D, the lower end of the body having an
enlarged disk-head E, integral therewith, and an upwardly-projecting
annular flange F is formed around this disk to receive and hold a
cylinder G, which constitutes the float and fuel chamber.
The upper end of this cylinder rests against a seat cast with the body
A, and packing rings are placed at the ends of the cylinder to prevent
the oil from leaking out. Within the tubular body D is a vertical tube
H, integral with the disk head E, and oil is supplied to this tube
through ducts I, which communicate with the chamber within the reservoir
G.
A drain cock is at the lower end of this tube, and an adjustable cap K
screws on the tubular stem of the drain tube, around which air is
admitted, the air passing upwardly through vertical ducts L, as shown,
and thus mixes with air at the contracted part of the venturi tube.
A ring-like float N is placed within the glass chamber, and this is
adapted to engage with the inner end of a lever N´, this lever being
pivoted at O, within a side extension P of the carbureter shell. The
inner end of this lever has a link hinged thereto, the lower end of
which serves as a needle valve to close the ejecting orifice of the tube
L.
The outer end of the lever N´ engages a shoulder on a
vertically-disposed needle valve Q, which has its point in the inlet
opening of the pipe R, through which gasoline is supplied to the glass
chamber. A spring T serves to keep the valve stem normally on its seat.
Directly opposite this chambered extension P is another extension U,
also cast with the shell, through which is a vertical stem V. This stem
carries a downwardly-opening valve W, that seats against a plug, and a
spring X below the valve, serves to keep it against its seat, unless
there should be an extraordinarily heavy pull or suction.
This is the auxiliary air inlet, and the lower spring is actuated only
when the engine is running at moderate speeds, but when running at high
speed and an additional quantity of air is required the upper spring Y
is compressed, and thus a much greater quantity of air is allowed to
pass in and mingle with the spray at the throttle valve Z.
The throttle valve is mounted in the discharge opening, and is
controlled by a lever on the outside of the carbureter.
The device operates as follows: Primary air enters the opening between
the cup K and the disk-head E, passing up into the space around the oil
tube H. As the spring T, around the needle valve Q, draws up the valve
from its seat, oil is permitted to flow in through the duct R and fill
the chamber, until the float engages with the inner end of the lever N,
and raises it, thus uncovering the ejecting end of the tube H, and at
the same time closing the inlet tube R.
The suction from the engine then draws air through the primary duct, as
stated, and also an additional quantity through the secondary source, by
way of the valve W, this valve being so regulated as to supply the
requisite quantity.
The auxiliary air source serves the purpose that means should be
provided to supply more than the ordinary amount of air, when running at
high speeds.
From the foregoing it will be observed that a carbureter must be so
constructed that it will perform a variety of work. These are: First,
Automatic means for filling the float chamber when the gasoline goes
below a certain level. Second, Cutting off the supply of gasoline.
Third, Providing a primary supply of gasoline for spraying purposes.
Fourth, Furnishing an auxiliary air supply. Fifth, Throttling means in
the discharge opening.
It is thus a most wonderful contrivance, and considering that all the
elements necessary to make it work satisfactorily are provided with
adjustable devices, it may be seen that to make it perform correctly
requires a perfect understanding of its various features.
Requirements in a Carbureter.--In view of the foregoing it might be well
to know how to select a carbureter that is ideal in its operation.
First. The adjustment of the auxiliary valve should be of such a
character that at the slowest speed the valve should not be lifted from
its seat.
Second. It must be so arranged that it is not difficult to change the
relative amount of air and gasoline.
Third. The floating chamber should be so arranged that the float will
act on the lever which lifts the valve of the injecting pipe, even
though the carbureter body should be tilted at an angle. This is
particularly important when the carbureter is used in automobiles.
Fourth. The valves should be in such position that they are readily
accessible for cleaning or for examination.
Fifth. The float should be so arranged that it is adjustable with
reference to the lever that it contacts with.
Sixth. A gauze strainer should be placed at the gasoline inlet, and it
is also advisable to have a similar strainer above the mixing chamber,
beyond the throttle.
Seventh. There should be no pockets at any point in the body to hold the
gasoline which might condense.
Eighth. The body of the carbureter should be so constructed that every
part is easily accessible, and draining means provided so that every
particle of gasoline can be withdrawn.
Ninth. Means for heating it, in case of cold weather.
Size of the Carbureter.--The proper size of a carbureter for an engine
has been the subject of considerable discussion and experimenting. If
its passages are too large, difficulty will be experienced in starting
the engine, because the pulling draft through the primary will not be
sufficient to make a spray that will unite with the air.
A carbureter too large will only waste fuel, even after the engine has
been cranked up so it will start.
If the carbureter is too small the engine will not develop its required
output of power. While it might work satisfactorily at low speeds it
would be entirely inefficient at high speeds.
Rule for Size of Carbureter.--In all cases the valve opening and
cylinder capacity in the engine should determine this. The size of the
opening of the carbureter outlet should be the same as that of the
engine valve, which is also the case where the carbureter supplies a
multi-cylinder, as there is only one valve open at the same time.
It was formerly the custom to use a carbureter for each cylinder but the
practice has been abandoned, because it is obvious that a single
carbureter will, owing to the continuous suction, supply a mixture of
more nearly uniform character than two or more, even though they should
supply the mixture to a common manifold.
The Throttle.--Much of the economy in running an engine depends on the
manipulation of the throttle. As an example, with a certain motor and
carbureter it will be found that for maximum speed the throttle should
be open about one-eighth of the way. The proper way, in starting the
engine, is to open the throttle fully half way, and to retard the
spark. As soon as the engine begins to run properly, the spark is
advanced and the throttle closed down to the required point.
The engine speed may always be maintained by the throttle under a
constant varying load, by adjusting the throttle valve. A rich mixture
may be obtained by throttling the primary air supply.
The throttle may also be a most effective means of economizing fuel when
the engine has a first class sparking device, as in that case the
throttle can be closed down to provide a very small opening.
Flooding.--One of the most prevalent troubles in carbureters is the
liability to flood. This is usually caused by foreign matter getting
under or in the float valve, so that it will not properly seat.
Sometimes the mere moving of the float will dislodge the particle.
Another cause of flooding is due, frequently, to an improperly-arranged
float, which, when the engine is inclined, will prevent improper seating
of the valve, and flooding follows.
The greatest care should be exercised in seeing that the gasoline supply
is free from all impurities when it is poured into the tank. To strain
it is the best precaution, and it pays to be particular in this respect.
It is surprising to see the smallest speck, either stop the flow
entirely, or produce an overflow, either of which will cause a world of
trouble.
Water is another element which has no place in a carbureter. An
indication of this is the irregular movement of the engine. The only
remedy is to stop and drain the carbureter. A few drops may cause all
the trouble.
[Illustration: _Fig. 32. Carbureter._]
Types of Carbureters.--In Fig. 32 we show another type of carbureter,
which is simple in construction, and has many desirable features. The
cylindrical body of the carbureter, A, has a downwardly-projecting
globular extension B, at one side of which is a flange C to secure it to
the pipe, and through this is the discharge opening D. This globular
extension serves as the mixing chamber.
Within the cylindrical shell is an upwardly-projecting circularly-formed
extension E, and the top or cap F of the cylindrical body A has a
downwardly-projecting cylindrical rim G which overlaps the lower
circular extension E, and it is so constructed that a very thin annular
slit H is thus formed between the two parts, through which fuel oil
flows from the float chamber I into the space around the central tube J
which passes down through the two circular extensions E, G.
This central tube J is designed for the auxiliary air supply. It extends
down to the globular base B, and has a valve K seated against its end.
The stem L of the valve is vertically-movable within an adjustable stem
M, and a helical spring N, capable of having its tension adjusted by the
stem M, bears upwardly against the valve so as to keep it normally
against the lower end of the tube J.
The auxiliary air, therefore, passes down centrally through the tube J,
while the primary air supply passes through openings O, surrounding the
tube J, downwardly past the slitted opening H, and thence to the
discharge port D.
Surrounding the tubular projections E, G, and within the float chamber
I, is the float P. This is designed to strike the bifurcated ends of a
lever Q, which is hinged near its outer end, as at R, and has its short
projecting end resting beneath the collar of a vertical needle valve S.
This needle valve is vertically placed within a chambered extension T at
the side of the shell A, and its lower end rests within the opening of
the inlet U which supplies the gasoline to the chamber I. The upper end
of the valve stem passes through a plug V, through which is a vent hole
W.
A spring X is used between the plug and the collar on the lower end of
the needle valve, so that the valve is kept on its seat thereby, unless
the gasoline in the chamber should fall so low as to cause the float to
rest on the inner end of the lever Q, when the needle valve would be
unseated thereby.
All the parts of this device seem to be accessible, and it is presented
as an example of construction that seems to meet pretty nearly all of
the ideal requirements of a device for furnishing a perfect admixture.
Surface Carbureter.--This type of carbureter also requires a float but
does not have secondary air inlet mechanism. It has one striking
advantage over the sprayer system, in the particular that the suction
of the engine is not depended upon to draw the gasoline from the float
chamber. It is much more sensitive to adjustment in the float level and
needle valve than the other type.
[Illustration: _Fig. 33. Surface Carbureter._]
The diagram, Fig. 33, shows a body A, somewhat bowl-shaped, with a
chambered extension, B, at one side, at the lower side of which is the
fuel inlet duct C. Directly above this duct the upper wall of the
extension has a plug D, the lower end of which carries therein the upper
end of a vertically-movable needle valve, E, the lower end of the valve
resting within the duct C.
A float F within the bowl-shaped body is secured at one side to a lever
G, which is hinged at a point near the needle valve E, and the short end
of this lever connects with this needle valve in such a manner that as
the float moves upwardly the valve is seated, and when the level of the
fuel oil falls below a certain point the needle is lifted from its seat,
and oil is permitted to flow into the float chamber.
The cap H of the float chamber has cast therewith a U-shaped tube, the
inlet end I being horizontally-disposed, while the discharge end J is
vertical. Directly above the lowest part of the bend in this tube, the
vertical dimension of the tube is contracted by a downwardly-projecting
wall K, so as to form a narrow throat L.
Below this contracted point, the U-shaped tube has integral therewith a
downwardly-projecting stem M, the lower end of which passes through an
opening in the float chamber, and is threaded, so as to receive a nut,
by means of which the cap H may be firmly fixed to the float chamber.
This stem M has a vertical duct N, which communicates with the float
chamber, and is provided with a drain plug O. Alongside of this duct is
a tube P which extends up into the U-shaped tube and is open at its
lower end so that the level of the gasoline within the bent tube cannot
extend above the end of this drain tube P.
An adjustable valve stem Q passes through one side of the bent tube, the
lower end being pointed and adapted to regulate the inflow of gasoline
through the duct N, and into the U-shaped tube.
A throttle valve R is placed in the discharge end of the U-shaped tube,
which is susceptible of regulation by means of a lever S. The diagram
shows the gasoline within the U-shaped tube, so that it is on a level
with the gasoline in the float chamber.
In operation a sufficient amount of gasoline is permitted to enter the
float chamber so that a pool is formed in the bottom of the U-shaped
tube. When suction takes place the air rushes through the tube, at I,
down beneath the wall K, and in doing so it sweeps past the surface of
the pool at that point, absorbing a greater or less amount of the vapor.
In order to adjust the device so that a smaller amount of the liquid
fuel will be exposed, the carbureter is adjusted so it will close the
needle valve before the level of the liquid is so high, and thereby a
less surface of oil is formed within the U-shaped tube.
It is obvious that this type of carbureter, owing to the absence of the
secondary air-supply mechanism, can be readily regulated and all
adjustments made while running, while for automobile uses the lever S,
which controls the throttle, can be connected up with a dash-board
control.