CHAPTER XICARBURETERS

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 acarbureted fuel. This is a wrong term. What is meant iscarbureted 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 ofpartsit 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 combustionis the desired object in the engine cylinder. The more nearly the vapor approachesan 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 distincttypes 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 thesprayingdevice.

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 thesurfacetype.

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-projectingtube 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.

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 flowsthrough 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.

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.

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 theamount 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.

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.

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 theforegoing 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.

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.

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