In-Line Injection Pump

Some diesel engines use in-line fuel injection pumps to meter and pressurize the fuel Figure 54-18. The basic principle is for a plunger to act on a column of fuel to lift an injector needle off its seat. A pumping element and delivery valve for each cylinder are located inside the pump Figure 54-19.

The element has a barrel, with a mated plunger inside. Their accurate fit and highly polished finish ensure minimal fuel leakage past them without requiring additional seals. The barrel usually has two holes, or ports, called the inlet, or fill, port and the spill port. The ports are located in the injection pump fuel gallery. The gallery contains filtered fuel from the low-pressure supply system.

At the top of the barrel are a delivery valve, a delivery valve holder, and the high-pressure steel pipe to carry fuel to each cylinder Figure 54-20. When the fuel pressure rises, the delivery valve is lifted off its seat. When the plunger covers the fill and spill ports, fuel is pressurized and flows to the injector. When injection ceases, the pressure below the delivery valve drops to gallery, or supply, pressure.

FIGURE 54-18
An in-line fuel injection pump.
FIGURE 54-19
Fuel injection pumping element.
FIGURE 54-20
A delivery valve, delivery valve spring, and delivery valve holder at the top of a plunger barrel.

To meter the fuel, the upper end of the plunger has a vertical groove, extending from its top to an annular groove. The top edge of this annular groove is cut in a helix shape, also called the control edge Figure 54-21. Rotating the plunger controls the duration of the stroke for which the spill port is covered. This is called the effective stroke. It determines how much fuel is delivered to the injector Figure 54-22.

A short effective stroke means a small amount of fuel is injected. A longer effective stroke lets more fuel be delivered. To stop the engine, the vertical groove on the plunger is aligned with the spill port. This alignment stops pressure in the barrel from rising because the fuel can pass through the groove and exit through the spill port.

The plunger is rotated by a control sleeve, a rack, and a pinion. Moving the rack rotates the pinion, the control sleeve, and then the plunger. The rack’s movement is controlled by the governor.

FIGURE 54-21
A pumping plunger with a helical control edge.
FIGURE 54-22
The effective stroke is the duration of the plunger movement after the spill port is covered.

The stroke of the plunger is accomplished by a camshaft, cam follower, and spring, which move the plunger in a reciprocating motion. When the plunger is below the ports, fuel from the gallery enters the barrel above the plunger, thus ensuring that the barrel is full of fuel. As the camshaft rotates, the plunger is pushed past the ports, which traps and pressurizes the fuel.

Moving the plunger farther raises the pressure of the fuel. The increased pressure forces the fuel out past the delivery valve, along the fuel line to the injector. Fuel flows to the injector until the control edge of the helix uncovers the spill port. The pressurized fuel above the plunger then moves down the vertical groove, to the annular groove, and into the spill port.

The delivery valve prevents fuel from leaking from the pipe back into the element. It reduces pressure in the fuel line to ensure there is no dribbling by the injector. This pressure reduction is accomplished by a relief plunger and a conical face, which is held against its matching seat by the delivery valve spring. The relief plunger on the valve is a close fit inside the bore of the delivery valve seat.

The delivery valve operates as follows: Fuel pressure above the delivery valve forces the valve toward its seat bore, sealing the area above and below the delivery valve. Further movement of the delivery valve toward its seat increases the volume in the injector pipe and reduces the pressure therein. This drop in pressure causes the injector needle to snap shut, helping to prevent fuel dribble from the injector. The conical face of the delivery valve then contacts the seat, further sealing the plunger from the injector pipe.