Diesel Engine Lift Pumps
A fuel supply, feed, transfer, or lift (the terms are interchangeable) pump supplies low-pressure fuel to the suction side of the injector pump or unit injectors. Most lift pumps are driven mechanically, although in recent years, electric drive has become popular. Pressure varies with 50 psi as a ballpark figure.
Stanadyne, Lucas/CAV and Bosch distributor-type injector pumps employ an integral vane-type lift pump as pictured back in Fig. 5-9 and subsequent drawings. The sliding vanes rotate in an eccentric housing with the outlet port on the periphery.
Gear-type lift pumps, similar to lube-oil pumps, develop pressure from the tooth mesh and usually incorporate a spring-loaded pressure-limiting valve.
Chrysler-Nissan SD22 and SD33 engines feed through a piston pump driven off the injector pump (Fig. 5-36). The operation of the pump is slightly unorthodox, in that fuel is present on both sides of the piston to eliminate air locks. As the piston retracts, fuel under it is expelled and the inlet check valve opens to admit fuel into the chamber above the piston (Fig. 5-37). As the piston rises, the check valve closes to permit the upper chamber to be pressurized. Fuel under the piston then reverses course to fill the void left by the rising piston. Near tdc the spring-loaded dischargeside valve opens. Spring tension determines pump pressure. The upper chamber can also be pressurized manually to bleed the system.
Small engines are often fitted with low-pressure diaphragm pumps such as the AC unit pictured in Fig. 5-38. The pump consists of a spring-loaded diaphragm activated by the engine camshaft. Inlet- and discharge-side check valves are often interchangeable.