1-22. Fuel Flow. Fuel flows from the vehicle fuel tank to the fuel supply pump inlet. The fuel supply pump delivers fuel through the fuel filters to a tee installed in the fuel density compensator inlet port. From the tee, fuel flows to the density compensator and to the inlet of the hydraulic head. Fuel flows through the hydraulic head and the head outlets to the engine cylinders. Excess fuel from the hydraulic head passes through the overflow valve, which maintains a constant fuel pressure in the hydraulic head. The constant flow of fuel through the hydraulic head not only supplies fuel for injection but also serves as a coolant for the hydraulic head. A pressure regulator valve (JJ, fig. 1-16) in the density compensator maintains pressure at 20 to 21 psi. Excess fuel from the density compensator and hydraulic head flows back to the vehicle fuel tank. 1-23. Fuel Pumping and Distribution. a. The fuel injection pump camshaft is driven at engine crank- shaft speed. As the camshaft rotates, the tappet moves the fuel plunger (U, fig. 1-16) up and down. The quill shaft, driven off the camshaft, drives the plunger drive gear which rotates the fuel plunger. When the camshaft has rotated twice, the fuel plunger has rotated 360° and completed six fuel injection strokes. b. At the intake phase (fig. 1-24), the fuel plunger is on the base circle of the camshaft, the fuel plunger is down, and fuel enters and fills the pressure chamber. As the camshaft rotates, the fuel plunger closes off the inlet and outlet ports and is at the beginning of delivery phase (fig. 1-25). The fuel in the pressure chamber is pressurized and starts to open the fuel delivery valve. At the delivery phase (fig. 1-26) the fuel is under sufficient pressure to fully open the fuel delivery valve. This highly pressurized fuel then flows around the fuel plunger annulus, through the distributing slot, which is now alined with one of the six outlet passages, and out the outlet passage to the injector nozzle. At the end of delivery phase (fig. 1-27), after sufficient upward movement of the fuel plunger, the spill passages passes over the edge of the plunger sleeve. The fuel pressure then escapes down the plunger vertical fuel passage into the fuel sump which is at supply pressure. The reduction of pressure causes the fuel delivery valve to close. TM 9-2910-226-34 Figure 1-24. Fuel intake flow diagram. Figure 1-25. Beginning of fuel delivery flow diagram. 1-27