Motor-car principles; the gasoline automobile

by Roger B. Whitman

Fig. 19.—Carburetor Principles.

The principal parts of a float-feed carburetor are the float chamber and the mixing chamber, the gasoline flowing from the supply tank to the float chamber, and from there to the mixing chamber, where it is combined with the air. The gasoline flows out of the float chamber through a small pipe, the end of which, called the spray nozzle, projects into the mixing chamber so that the current of air rushes past its tip (Fig. 19). When the inlet stroke is not being performed, and there is no air passing through the mixing chamber, the float in the float chamber keeps the gasoline at such a level that it stands just below the tip of the spray nozzle. In this condition the gasoline in both the float chamber and the spray nozzle is under atmospheric pressure; but when the pressure in the combustion space is reduced as the piston makes the inlet stroke, the pressure in the inlet pipe and mixing chamber is also reduced, and the gasoline will be forced out of the spray nozzle by the higher pressure in the float chamber. The passage in the tip of the spray nozzle is small, and the gasoline is broken up into fine drops as it spurts out, and in this condition is partly absorbed by the air and partly carried into the cylinder. By regulating the amount of gasoline that may pass out of the spray nozzle, it may be adjusted to the volume of air flowing through the mixing chamber, so that any desired proportion may be obtained.

If the engine were to be run at a constant speed, the relative pressures on the gasoline in the float chamber and spray nozzle, and the quantity of gasoline forced out of the spray nozzle, would remain in correct proportion to the volume of air; but as the engine of an automobile is run at greatly varying speeds, the pressure in the mixing chamber is not constant, but varies to correspond. If the piston makes fifty inlet strokes a minute, the reduction of the pressure in the mixing chamber is more gradual than would be the case with the piston making two hundred inlet strokes a minute. The more rapidly the pressure is reduced, the greater will be the effect of the unchanging atmospheric pressure in the float chamber, and the more gasoline will be forced out of the spray nozzle. This will result in the presence of too much gasoline in proportion to the air, giving a mixture that is too rich. It is therefore necessary to provide an arrangement by which the pressure in the mixing chamber will not be changed as the speed of the engine varies, and this is accomplished by the auxiliary air inlet, which is closed when the engine runs slowly, but opens to correspond with increasing speed (Fig. 19). The faster the engine runs, the more the auxiliary air inlet will open, to admit a correspondingly greater amount of air to prevent the pressure in the mixing chamber from being reduced below the point at which the required amount of gasoline is forced out of the spray nozzle.

The most rapid combination of the gasoline and air is secured by breaking the gasoline up into fine spray as it leaves the nozzle. In order to break the gasoline into fine particles, the tip of the spray nozzle is made with a fine opening, and often forms the seat for the gasoline adjusting valve, which is a needle-pointed rod that is screwed in or out to reduce or enlarge the opening; a further breaking up results from the placing of a metal cone, or the end of a rod, in such a position as to be struck by the gasoline as it flows out.

A carburetor with an auxiliary air inlet is provided with two points of adjustment, to control the flow of gasoline from the float chamber to the spray nozzle, and to govern the admission of the auxiliary air. The flow of gasoline must be just sufficient to carburet thoroughly the air passing through the mixing chamber when the engine runs at low speed, and the auxiliary air inlet must open to correspond with increasing speed, to admit sufficient air to keep the pressure reduced to proper proportions.

The auxiliary air inlet may be operated either by the reduced pressure that permits the atmospheric pressure to open a valve, or by the governor of the engine, which opens the inlet as the speed increases, and closes it on slowing down. The first of these two types, called an automatic carburetor, is in most general use for automobiles, and is sufficiently satisfactory, but is not as accurate as the mechanically controlled type, which acts independently of pressure, and exactly according to the speed of the engine.

Fig. 20.—Automatic Carburetors.

Carburetors may be divided into two types, according to design: those in which the float and mixing chambers are side by side, and those in which the mixing chamber passes through the center of the float chamber (Fig. 20). The former is along the lines of the first forms of float-feed carburetors, and the latter is of more recent design, its object being a more compact device, and one that is not affected by a change of level when the car is on a hill. Both have advantages and disadvantages, and the use of one as against the other is optional and a matter of opinion.

The carburetors illustrated are not of any particular makes, and are intended to show principles rather than construction.