Fig. 23.—Pressure Feed Gasoline System. 1, Filling cap; 2, tank; 3, pressure pipe; 4 and 5, check and relief valves; 6, hand pump; 7, check valve; 8, pressure gauge; 9, feed pipe; 10, carburetor.
The pressure feed, which has been adopted on all high-grade cars, operates through the maintenance of pressure in the supply tank, the gasoline being forced out without regard to gravity. The tank is tight, so that the pressure cannot escape, and is connected by a long pipe of small diameter either with the combustion space of one of the cylinders or with the exhaust pipe, so that the pressure of the burned gases is maintained in it. As the pressure cannot exist until the engine is running, a hand air pump is usually provided, by which a sufficient pressure may be produced in the tank to force out enough gasoline for starting. It is necessary to use as long a pressure pipe as possible, in order to prevent the possibility of flame passing through it to the supply of fuel, a long pipe, exposed to the air, cooling the gases to such an extent that they cannot ignite the gasoline.
The pressure pipe is always fitted with a check and relief valve, which acts as a safety valve in preventing the pressure in the tank from reaching a point at which the joints might be strained, and also retains the pressure which would otherwise escape when the engine stops. In some cars an auxiliary gasoline tank is provided on the dash, being fed by pressure from the main tank, and from which gasoline flows to the carburetor by gravity. The short distance of this tank from the carburetor and its elevation prevent the possibility of the flow being stopped by any tilting of the car short of an upset.
Fig. 23A.—Types of Float Valves.
Because of the liability of the presence of water in the gasoline, as well as dirt and grit, the gasoline line should be fitted with a strainer, or trap. This may be in any position, but it is usual to have it close to the carburetor, if not built into it. The simplest strainer consists of a number of thicknesses of fine wire gauze, so arranged that it may be easily taken out for cleaning. This will separate the dirt from the gasoline, and water may be caught in a trap, which is a pocket where the water, being heavier than the gasoline, may settle and be drawn off.
CARBURETOR PARTS
Fig. 23B.—Types of Auxiliary Air Inlets.
Practically all the carburetors on the market are combinations of a few forms of float valves, auxiliary air inlets, and spray nozzles. In addition to the forms shown in Figs. 20 and 21, the most usual float valves may be seen in Fig. 23A. In the first two types shown in this diagram, the float valve stems are separate from the floats, and are sufficiently heavy to shut off the flow of gasoline by their weight. In the third type, the gasoline enters the float chamber from the top, and as the valve stem is attached to the float, the rising of the float results in the shutting off of the gasoline. The fourth type is in use on carburetors with central mixing chambers, the float being hinged to one wall of the float chamber. The loose valve stem is supported by the hinge, and rises to a seat in the valve when the gasoline is at the proper depth on the float chamber.