Pump Valves.—To insure close regulation, the cut-off valve seat must be in good condition. Even though it is rather un handy to reach, at least monthly the valve should be removed and, if the seat is not smooth, it should be reground. To remove the valve, the overflow-pipe joint can be broken and the check-valve cage lifted.
Timing Fuel Pump.—The plunger eccentric .is keyed to the camshaft, and no change of the timing is necessary.
In timing the cut-off valve, the valve should just start to leave the seat as the pump plunger commences its downward stroke when the governor is at rest and the collar in its lowest position. This is the full-load position. The governor springs should then be removed and the weights moved to their maximum outward positions, which raises the collar to its highest point. The cut off valve should now be seated on the plunger during the entire stroke of the plunger, with the exception of about 5 i o o o inch. This is the no-load position of the valve. An adjusting lever and lock C are placed on the bell-crank outside of the casing. This allows the timing of the valve to be altered to conform to the desired setting. To stop the engine this lever is moved to lower the cut-off valve.
National Transit 1918 Diesel.—The later National Transit Diesels are supplied with a fuel pump that, in form, resembles the pump discussed above but which functions on an entirely different principle. This pump appears in Fig. 146. In this design the pump plunger is actuated by a rocker and cam in place of the former's eccentric and strap. The plunger P is hollow and carries at its top the cut-off valve V. This valve has a tri angular shank which extends down into the hollow plunger, and the valve is controlled by the fingered or forked lever E. This lever is moved by the governor and limits the downward travel of the valve V. As the plunger is forced downward on in suction stroke by the spring S, the oil enters the pump body at the open ing A and, passing through the ports B, enters the hollow plunger. Since the lever E has engaged the valve V at some point on-the downward stroke of the plunger P, this valve is open and the oil fills the cavity J. On the upward stroke of the plunger the oil displaced flows back through the open cut-off valve V, through the ports B and suction opening A, to the source of supply. As the plunger continues its travel, it comes in contact with the valve V, which thereupon seats at D. The continued travel of the plunger entails a further displacement of oil, which lifts the dis charge valve H and is forced to the fuel injection nozzle. It will be observed that, while with the pump in Fig. 145 on no load the cut-off valve was closed during the entire plunger stroke, with this pun-1p the cut-off or suction valve V is opened during the entire plunger stroke when the governor is in the no-load posi tion. This change in design eliminates the suction valve and the overflow valve while improving the fuel-measuring accuracy of the pump.
Standard Fuel Oil Diesel Engine Fuel pump of this two-cycle Diesel engine is illustrated in Fig. 147. The fuel pump is a plain barrel B, having a plunger directly under the influence of a Rites Inertia governor. The barrel and plunger A is the air injection control. Both the suction and discharge openings have two sets of wing poppet valves. The pump plunger is pivoted to a bell-crank which, in turn, is driven by the eccentric rod. The eccentric-rod side of the crank has a worm screw adjustment whereby the stroke of the pump, for any given position of the governor weight arm, may be altered. Lowering the rod end gives a greater stroke to the pump plunger, and raising it, a lessened stroke. In timing the engine a good prac tical plan to set the pump stroke for no-load conditions is to first lower the eccentric until the engine speeds up above normal. Then the eccentric rod should be raised by the screw until, at a slight overspeed, the plunger movement is zero. This posi tion of the eccentric end should give sufficient stroke to the plunger at the full-load position of the governor.
In stopping the engine the lock screw C is loosened, cutting out the pump. Since the engine is two-cycle, the pump works against a much greater pressure than exists with a four-stroke cycle engine, even though an open nozzle is employed on both engines. For this reason the wear on the pump valves is greater, and regrinding is more frequent. The engineer must give special attention to the plunger stuffing-box, as a small leakage effects the engine's regulation.
General.—After the valves of any design of fuel pump have been replaced, after grinding, etc., the discharge line should be disconnected from the injection valve, if no by-pass valve is attached. The should then be primed. This will effect the escape of any air that might be trapped in the pump cylinder.
In case the engine, on starting, fails to fire on any particular cylinder, the pump is probably air-bound, and the lines must be freed before oil will enter the fuel valve. At times the fuel-line check valve leaks, allowing air to flow down the pipe to the fuel pump. This will prevent the pump from delivering any oil.
Valve Grinding.—In grinding any poppet-type valve powdered glass, or emery flour, and vaseline make the best compound. The operator must exercise judgment in the amount of pressure exerted on the valve as it is rotated. If the pressure is excessive, the compound will groove the valve faces.
When a discharge valve or suction valve is under suspicion, the best method of ascertaining if it actually leaks is to disconnect the discharge line and place on the coupling a high-pressure gage. The engine can be turned over until the pump discharge pressure registers the usual value, 900 lbs. with closed nozzle and about 200 lbs. with open nozzle. The engine should then be stopped and the discharge pressure noted. If it falls, it can be taken as an indication of a leaky valve.