A single, spring loaded, delivery valve (Figure 4-78) is located in the center of the DB2 pump rotor. It serves as a one way check valve to seal the pumping chamber from the injection line and also controls residual line pressure by providing volume unloading.
At the beginning of pumping, the delivery valve is displaced into its spring chamber until, at a valve lift qual to the dimension “ H” , fuel flows from the plungers to the discharge port. At the end of the pumping cycle, the plungers travel outward allowing the delivery valve to move toward its seat. As the valve closes and the retraction volume (Valve Area H) is removed from the delivery valve spring chamber, a negative pressure wave is propagated toward the nozzle. The negative pressure wave lowers the injection line pressure to allow rapid nozzle closure and prevents secondary injection.
The delivery valve operates in a bore in the center of the distributor rotor. The valve requires no seat — only a stop to limit travel. Sealing is accomplished by the close clearance between the valve and bore into which it fits. Since the same delivery valve performs the function of retraction for each injection line, the result is a smooth running engine at all loads and speed.
When injection starts, fuel pressure moves the delivery valve slightly out of its bore and adds the volume of its
displacement to the delivery valve spring chamber. Since the discharge port is already opened to a head outlet, the
retraction volume and plunger displacement volume are delivered under high pressure to the nozzle. Delivery ends
when the pressure on the plunger side of the delivery valve is quickly reduced, due to the cam rollers passing the
highest point on the cam lobe.
Following this, the rotor discharge port closes completely and a residual injection line pressure is maintained. Note that the delivery valve is only required to seal while the discharge port is opened. Once the port is closed, residual line pressures are maintained by the seal of the close fitting head and rotor. It is possible that the residual pressure may vary between injection lines.