The Mietz and Weiss Fuel Pump the fuel pump for the multi-cylinder engines is of the single-plunger type, it becomes necessary to employ some mechanism to direct the flow of oil to the particular cylinder whose crank is at the injection angle. This purpose is achieved by the distributor, as shown in Fig. 305. This consists of a body G, gears H, and distributing disk I. The disk is driven by bevel gears from the governor-shaft and has an oil passage through it. The coverplate J of the dis tributor is further provided with openings from which the oil lines run to each of the cylinder nozzles.
The oil from the injection pump is forced into the cavity K below the disk. From this chamber it flows through the hole in the disk and enters the oil line whose opening registers with the disk opening. It is to be observed that, as the disk rotates in synchronism with the engine, its port successively registers with the pipe line to the various cylinders. In this manner the oil is injected into the proper cylinder. The disk must have an oil tight seat on the distributor cover. If a leak develops here, the oil will be forced into the cylinders at all points in the stroke. To correct any cutting or uneven wear, the cover should be removed and the plate and cover ground together, using emery paste. It requires a great deal of care to prevent the plate from being ground to a concave surface. Figure 306 shows a diagram matical layout of the oil distribution.
The Mietz and Weiss engine operates at a low compression pressure, about 90 pounds, and the fuel is injected very early in 25 the compression stroke. Using kerosene, the injection can begin as late as 45 degrees ahead of rear dead-center. This allows the cut-off at full load to occur a little before dead-center. With heavier oils the injection, as the governor is usually set, begins when the piston is in mid-position, or 90 degrees ahead of dead center, and cut-off occurs at approximately 45 degrees before dead-center. This engine, by means of the steam and water injection, which absorbs much heat with little increase in tempera ture, carries a low compression temperature. Due to this pecul iar arrangement, the fuel can be vaporized very early in the compression stroke. Even though the oil be completely vapor ized and mixed with the air charge, the temperature does not reach the ignition point until the piston is virtually _at the end of the stroke.
Pump Regulators.—It will be noted that all the pumps dis cussed have some type of hand regulator. This is in the form of a lever or a threaded nut. By adjusting the regulator the pump plunger's stroke is lengthened or shortened, as the case may be. The regulation of the pump stroke affects the injection angle; in this way, by proper manipulation, the injection angle can be altered over quite a range. It is. not clear to many engi neers why this is true. While it is generally stated that the regulator shortens the pump stroke, in actual operation it merely acts as a stop to limit the return or suction stroke of the pump. The pump, on the power stroke, can be moved as far as the throw of the eccentric will allow. Thus the stroke of the plunger is limited to that percentage of the eccentric's throw which lies between its point of contact with the plunger at the beginning of the power and its extreme point of throw. The argument is often advanced, when due thought has not been given to the subject, that, if the return travel of the pump is shortened, then the eccentric, in order to maintain speed, will increase its throw. This increased throw, so it is claimed, will increase the pump plunger travel sufficiently to enable the correct amount of oil to enter the cylinder. Then, the argument continues, the events are back to the original state, and no change is made in pump stroke or angle of injection admission.
To correct this erroneous belief, attention is invited to Fig. 307. The circle E with its center at A represents the engine eccentric. The crank is represented by the line OF, which shows the position of the crank when the eccentric rod strikes the pump plunger, beginning the fuel injection. The pump stroke then is the distance S, starting at the point F, a degrees from dead-center and continuing to R, which is at dead-center. It is, of course, understood that the end of injection need not be dead-center. It may be at any angle with dead-center dependent on the angle which the line of shaft and eccentric centers made with the crank. In the diagram shown this angle is zero; consequently the end of injection is at dead-center.