In carburetors with a side mixing chamber the float is usually a metal box, the joints of which are as far as possible proof against leakage. Guides prevent it from having any but an up-and-down motion, and in thus moving it controls the gasoline inlet valve by a rod attached to it, or by a separate valve stem. In this carburetor a separate valve stem is used, and it is moved through the action of a rocker arm controlled by the float. This construction is usual when the gasoline enters the float chamber from the bottom, the flow being permitted or checked by a needle valve. When the level in the float chamber drops as the gasoline runs out of the spray nozzle, the float sinks, and lifts the valve point from its seat. This admits gasoline from the supply tank, and the float in rising on it depresses the valve point, shutting off the flow. The gasoline adjusting valve is in the passage between the float and mixing chambers. The main or initial air inlet is in the side, the air being drawn through the mixing chamber, and past the spray nozzle. The auxiliary air inlet is a simple valve, opening inward, and held against its seat by a coil spring, the tension of which is adjustable. The pressure in the mixing chamber being reduced in accordance with the increasing speed of the engine, the valve is opened more and more as the atmospheric pressure against the outer surface of the valve overcomes the tension of the spring.
In carburetors with central mixing chamber the float is ring- or horseshoe-shaped, and usually made of cork, well varnished to prevent the absorption of the gasoline. The air enters at the bottom, passing directly to the mixing chamber. The auxiliary air inlet is at the top, and is of the arrangement already described. The mixture passes out at the side, and may be controlled by a throttle, which may be a damper arrangement, as shown, or other device by which the quantity passing to the combustion space is at the will of the operator.
Fig. 21.—Mechanically Controlled Carburetor.
The float and mixing chambers of a mechanically operated carburetor are the same as in the side-float chamber type, the difference being in the control of the auxiliary air inlet (Fig. 21). This consists of a tube attached to the mixture outlet, within it sliding another tube moved by the governor as that expands or contracts with the speed of the engine. There are openings in the sides of both tubes, but when the sliding tube is at its inmost position these are not in line, and consequently are closed. When the governor acts with increased engine speed, the sliding tube is drawn out, and one or more openings come into line, air entering through them to the mixing chamber. The faster the engine runs, the larger become the openings, and in consequence the greater is the amount of air that they admit. The illustration shows the ball type of gasoline valve, the ball on the end of the valve stem being drawn against its seat as the float rises.
Fig. 22.—Float Feed Carburetor with Gravity Gasoline Feed. 1, Filling cap; 2, tank; 3, gasoline valve; 5, trap and strainer; 6, carburetor; 7, float chamber; 8, float; 9, float valve; 10, spray nozzle; 11, gasoline adjustment; 12, air adjustment; 13, throttle; 14, mixing chamber; 15, initial air inlet; 16, primer.
While these types are in practically universal use for automobile engines, there are other methods by which the proportions of the mixture may be maintained. In one form the gasoline drops on a funnel made of fine wire gauze, which is placed in the mixing chamber in such a manner that the air in entering passes through it. The liquid forms a film over the gauze, and is picked up by the air, as it is in a condition that permits it to evaporate rapidly. In surface carburetors air is forced through the gasoline tank, or through an absorbent material soaked with gasoline, and becomes thoroughly saturated. This mixture is then thinned with pure air until the desired proportion is obtained, when it passes to the combustion space. The objections to these forms arise from the clogging of the parts with the impurities present in gasoline, and while they give excellent results when new, they deteriorate rapidly and present such resistance to the flow of the air current that they become useless.
There are two methods of supplying the carburetor with gasoline. Of these the most usual is the gravity feed, in which the tank is placed at a higher level than the carburetor, so that the gasoline flows down to it. The tank is usually placed under the seat, and the piping so arranged that the carburetor is the lowest point of the system. This method of feeding is satisfactory if the tank can be placed sufficiently above the carburetor to have the flow unaffected by an ordinary hill, but if it is not so placed, a steep ascent may tilt the car to such an extent that the carburetor is above the level of the gasoline in the tank, in which case the flow of course ceases.
