The auxiliary air valves are generally suction operated, opening progressively as the suction increases from higher speed or some other cause. If these valves could be made to have no inertia, they would follow the suction exactly and their action would be ideal. But the fact is that they open and close late. Added to these faults, is that there is no way of operating them that is not subject to variation. If springs are used they will change in strength. A weight is effected by vibrations of the car, as is also a mercury bath with a float, though to a less extent. In fact numerous methods have been experimented with and all found lacking in some respect.
There are many different makes of carburetors in use on commercial car engines at present. All carburetors which are used at the present time work by controlling the flow of gasoline in proportion to the air demand, some attempting this by raising the gasoline needle valve with the increased demands of the motor, such as the Schebler and Breeze carburetors; and others accomplishing this indirectly through various air regulations and auxiliary valves, as in the Kingston, Stromberg and others.
Some carburetors operate automatically, while others are so arranged that they may operate both automatically and mechanically.
This principle of automatic carburetion, which is employed in the majority of modern carburetors, may be outlined as follows:
A correct mixture having been obtained for the minimum suction on which the motor is capable of running, is compensated by the introduction of additional air at a rate varying automatically with the suction. The mechanical operation is obtained by connecting the butterfly valve, which controls the admission of gases from the carburetor, with the butterfly valve in the main air opening, the automatic operation of the auxiliary valve being retained.
During the past year there has been a tendency to provide dashboard adjustments, so that the mixture may be instantly varied throughout the entire range for heavier or lighter work, or because of other changing conditions. This is generally accomplished by varying the pressure of the auxiliary air valve spring. They are quite an advantage, as it is claimed that an automatic carburetor cannot adapt itself to changes in gasoline density, or in humidity, or to the gradual warming up of an engine when started from cold.
It would require too much space to illustrate all the various makes of carburetors used on the various commercial car engines, so the writer will present a few illustrations, which are explanatory of the various types discussed in this article, such as the raised needle valve type, the indirect type, and automatic and mechanically operated types.
Fig. 42 is a sectional view of the Model "L" Schebler carburetor, which is of the raised needle type, and is so designed that the amount of fuel entering the motor is automatically controlled by means of a raised needle seating in the jet, working automatically with the throttle. The float and its chamber surround the main air supply in which the jet is located. The jet opening is controlled by a needle, which permits of a variable opening as the throttle is opened or closed, being operated by a cam on the throttle lever. The auxiliary air valve is controlled by the suction of the motor, while the mixing chamber is water jacketed to apply heat to assist in vaporization. The main air supply can also be connected with a drum around the exhaust manifold for supplying warm air.
Fig. 43 is an illustration of the Kingston carburetor, which is provided with an adjustable jet surrounded by the float and chamber. The main air intake communicates with the mixing chamber, while the auxiliary air enters through five circular openings arranged in a semi-circle above the mixing chamber, and controlled by floating balls. These balls are so arranged that they cannot become displaced. They operate automatically, gradually lifting from their seats as the motor suction increases. The air passing the openings guarded by the balls has an unrestricted passage into the mixing chamber, and then to the motor. The main air intake is fitted with a butterfly throttle, so that the amount of air can be reduced for starting, around the nozzle of the jet, is a well, which becomes partially filled with gasoline while the motor is idle. This is brought about automatically by the level of the gasoline in the float chamber being slightly higher than the top of the jet. This gives a rich mixture for starting, and as long as the motor is running the gasoline is drawn through the jet into the intake manifold, the well remaining dry.
Some of the features of the Holley carburetor, illustrated in Fig. 44, are concentric float, adjustable jet, supplementary stand-pipe for starting and slow running and absence of air valves. Gasoline enters and passes through a strainer A and passages H into the jet. This jet is controlled by a milled screw, so that the opening can be varied at will. Gasoline passes into the jet M, which is in the shape of a venturi, or double-ended cone, and also into the standpipe J, to a level determined by the float. This stand pipe leads to the edge of the butterfly throttle, and when the latter is closed, the suction of the motor allows gasoline to be drawn up into the stand-pipe, past the plug A", and into the manifold. After the motor has been started, and the throttle opened, the gasoline is drawn through the main jet M, mixing with the air that enters from below, through the conduit N.
Fig. 42. Sectional View of Schebler Carburetor.
Fig. 43. Section of Kingston Carburetor.
Fig. 44. Holley Carburator in Section.
Fig. 45 illustrates the Pierce-Arrow automatic carburetor, with concentric float and adjustable jet. The gasoline supply from the tank passes through a fine gauze strainer, preventing water and dirt from entering the float chamber. The main air supply is taken through the lower inlet, and, coming from the proximity of the exhaust pipe, is warm, and passes up around the spray nozzle A. The auxiliary air is taken through two reed valves, which are controlled by flat springs. When the engine runs slowly, both auxiliary valves remain on their seat, and as the engine runs faster, the more intense suction opens the lighter reed valve, admitting air above the spray nozzle. A further increase in engine speed opens the heavier reed valve, permitting still more air to enter.
Fig. 45. Section of Pierce-Arrow Carburetor.