Hardly any two Diesel manufacturers follow the same design in the fuel valve, although there is some degree of similarity in a few designs, especially of the open-nozzle type.
American Diesel of the pioneer fuel valves was that of the American Diesel engine. This injection device was contained in a cast-iron housing, which was bolted to the side of the cylinder head, as shown in Fig. 112.
The needle valve is opened by the bracket lever shown, which, in turn, receives its motion from a reach-rod and cam, the latter being mounted on the engine camshaft. The closure of the needle valve is accomplished, as is the universal practice, by a spring, which has been compressed by the opening of the valve. housing of the fuel valve carries the needle valve, which is surrounded by the atomizer body. This atomizer is similar to Fig. 113. The fuel enters the fuel valve housing or cage through the passage A and flows along the recess between the atomizer and the valve body until it reaches the front or valve end of the atomizer.
The recess is enlarged, at this point, to include the entire circumfer ence of the atomizer. From this recess or ring a series of minute passages lead to the needle valve immediately above the valve seat. The injection air enters the housing at D and flows around the atomizer, behind the oil; it also fills the space about the needle valve stem. When the valve C opens, the flow of air around the stem results in a drop in pressure from the back of the valve toward the valve tip. The air pressure behind the oil charge, on account of the larger volume, remains practi cally constant and at a higher value than the pressure around the valve seat. This pressure difference forces the oil through the small ports E into the stream of high-velocity air rushing through the needle valve opening. The air thoroughly separates the oil into minute particles; this breaking-up process is further intensified by the flow of the oil and air through the openings in the nozzle-tip plate F betweeh the valve seat and the cylinder. The disadvantage of the valVe lies in the chilling effect of the air charge. The design allows a considerable part of the air to enter the cylinder ahead of the oil; since there is no resistance from an oil body present, this air attains a high velocity and, in expanding, lowers the temperature of the nozzle tip as well as exerts a similar effect upon the air which has been compressed in the cylinder.
The fuel valve is equipped with a by-pass valve of the screw-needle type; this valve is for the purpose of draining the valVe cage of any oil. In stopping the engine, the by-pass valve is opened and the oil flows through the valve back to the storage tank. The lack of fuel then causes the engine to stop. In starting the engine, the fuel pump is operated, by a hand crank, until oil appears at the by-pass overflow, indicating the fuel line is free from air. The by-pass valve is then closed.
Timing of Fuel Valve.—The fuel valve of the American Diesel engine should be timed as outlined in Fig. 79. The fuel valve cam has an adjustable tool-steel nose, the position of which can be varied to secure proper valve opening.
Adjustment and Repair. By-pass Valve.—The American Diesels in service have been operating a number of years, and conse quently frequent repairs are to be expected. One defect that will early develop is the leaking of the by-pass valve. The valve is of the needle type, and the corrosion of the point allows oil to seep at all times. This can be eliminated by redressing the point of the valve, followed by a regrinding.
Needle Valve.—The needle valve has a rounded end which seats on a bevel surface of the atomizer shell. Ordinarily the valve is made of toben bronze and wears rather rapidly. To regrind the valve, the end should be rounded and all rough spots removed. The valve can then be inserted into the atomizer, being coated with extra fine emery paste and ground to a proper contact with the seat. After a number of regrindings the valve stem becomes too short to afford any initial compression of the valve spring. If a washer is placed between the spring and cap, the spring will be compressed on assembling, and the valve made serviceable again. Since the bronze stems are rather expensive many plants use a cold rolled steel valve stem with the lower part of bronze screwed into the steel stem. In case wear shortens the stem beyond the ability of washers to cope with the spring compression, the bronze tip only need be replaced.