Fig. 26. Stromberg Model M Carburetor—Sectional View
Under given temperature and pressure the rate at which the combustible mixture will burn depends upon the ratio of air to vapor. This rate of burning is known as the rate of propagation, and it is apparent that it is desirable to obtain a mixture whose rate of propagation is a maximum, because the force of the explosion will depend upon the rapidity with which the entire mixture is completely ignited.
The limits of combustion of gasoline (.70 sp. gr.) can be taken approximately as follows: lower limit, 7 parts air (by weight) to 1 part gasoline, upper limit, 20 parts air to 1 part gasoline.
The Stromberg Plain Tube Model M Carburetor.—A plain tube carburetor is one in which both the air and the gasoline openings are fixed in size, and in which the gasoline is metered automatically, without the aid of moving parts by the suction of air velocity past the jets.
[Fig. 26] shows a longitudinal section of a type M plain tube carburetor, and shows the location of the gasoline when the motor is at rest. The various parts are indicated by names and arrows. An elementary requirement of a carburetor is that as a metering device it shall properly proportion the gasoline and air throughout the entire operating range.
Fig. 27. Stromberg Carburetor Model M—Air Bleeder Action
In the carburetor under discussion this mixture proportioning is properly maintained by the use of what is termed the air bleed jet. [Fig. 27] shows the principle of the action of the air bleeder. The gasoline leaves the float chamber, passes the point of the high speed adjusting needle, and rises through a vertical channel “B.” Air is taken in through the air bleeder “C,” and discharged into the gasoline channel before the latter reaches the jet holes in the small venturi tube “E.” The result is that the air thus taken in breaks up the flow of gasoline and produces a finely divided emulsion. Upon reaching the jet holes of the small venturi tube this emulsion is discharged into the high velocity air stream in the form of a finely divided mist. If the reader will recall how thoroughly a soap bubble divides itself when it bursts, he will readily appreciate how completely the air bleed jet will atomize the fuel.
Before explaining the operation of the accelerating well it is advisable to know the reason for its existence. Suppose we had a large tube such as the intake manifold of a motor through which air and particles of gasoline were flowing due to a certain suction at one end. What would be the result if we suddenly increased the suction? It would be this: Due to the fact that air is so much lighter than gasoline, the air would respond almost instantly to the increased suction and its flow would be accelerated very suddenly, whereas the particles of gasoline, owing to that characteristic known as inertia, would not respond so rapidly, and due to its heavier weight its flow would not accelerate as much as the air. This would mean that the air would rush ahead of the gasoline particles, and the proportion of air to gasoline would be greater until the inertia forces had been overcome and the gasoline particles responded completely to the increased suction. This very thing will take place in a carburetor unless provision is made for it. That is to say a sudden opening of the throttle will tend toward producing a very lean mixture at the motor due to the lagging of the gasoline explained above. A lean mixture at this time, when acceleration is desired, would obviously be detrimental to the result wanted. It is at this particular time that additional gasoline is most desirable in order to compensate for the lagging gasoline and maintain the proper mixture at the motor. In the Stromberg carburetor this is accomplished by means of the accelerating well shown in [Fig. 28]. The operation is as follows: The action is based upon the principle of the ordinary U tube. If a U tube contains a liquid, and if pressure is applied to one arm of the tube, or what is the same, if suction is applied to the other arm, it is self-evident that the level of the liquid will rise in the arm on which the suction is applied and will drop in the other arm. So it is in the construction of the accelerating well. Referring to the illustration, [Fig. 28], the space “F” forms the one arm of the U tube, and the space “B” the other arm. These spaces communicate with each other through the holes “G” thus forming a modified form of U tube.
