Fuel Injection Pump: Operation

Cascade Overflow Valve 

Instead of using an electric supply pump, this fuel system uses a gear supply pump located inside the rear of the high pressure pump. The pump is driven by an eccentric on the end of the high pressure pump shaft. The gear pump draws fuel from the fuel tank through the fuel filter.

The pressurized outlet side of the gear pump provides pressurized fuel to a branched circuit internal to the high pressure pump flange, which supples both the fuel quantity solenoid and the cascade overflow valve. Because the gear pump increases fuel flow and pressure as the engine rpm increases, the pressure is regulated by the cascade overflow valve. The cascade overflow valve and gear supply pump are not serviced independently of the high pressure pump.

The cascade overflow valve has two functions:

1. Regulation of lubrication fuel to the internal moving parts of the high pressure pump
2. Regulation of the fuel pressure being supplied to the fuel quantity solenoid

The cascade valve has a machined center piece that has three drillings. One for overflow, one for lubrication and one for supply. The valve works in three stages based on the pressure entering the inlet of the valve.

Stage 1 

When the fuel pressure entering the tip of the cascade valve is between 0 and 3 bar (44 psi), the spring force is not overcome and fuel only flows through the center drilling. This drilling always allows fuel flow through to the pump center ring and lubricates the pump bushings and internal moving parts. This circuit also allows air to bleed during initial cranking and returns the air to the fuel tank. The cascade valve is only in stage one during cranking.

Stage 2 

When the fuel entering the cascade valve exceeds 3 bar (44 psi), but is less than 5 bar (73 psi), the center piece of the valve moves against the spring force aligning another passage for lubrication purposes. Stage 2 can be reached during cranking and initial start up.

Stage 3 

When fuel pressure exceeds 5 bar (73 psi), the center of the valve aligns with the overflow passage. This stage relieves the pressure into an overflow circuit that sends the fuel back to the inlet side of the gear pump which limits maximum fuel pressure to 5 bar (73 psi). Lubrication fuel also continues to flow though the other ports during this stage. Excess is sent back to the fuel tank through the return circuit.

High Pressure Pumping Plungers 

The fuel quantity solenoid supples three high pressure pumping chambers. The pumping chambers have one way inlet valves that allow fuel to flow into the chambers. The valves then close during compression of the fuel and cause the high pressure fuel to overcome a ball and angled seat outlet valve.

All three pumping chambers are tied together in one circuit internal to the pump and provide high pressure fuel up to 1600 bar (23,000 psi) through a steel line, to the fuel rail.

The pump is driven at 1 :1 engine speed and is not responsible for injection timing. The pump is only responsible for providing high pressure fuel while the ECM controls the injection timing.