The injection pump is a single-plunger mechanical pump which meters and distributes fuel to the injectors in the correct firing order. The pump is driven by the camshaft spur belt or gear at one-half crankshaft speed. All moving parts inside the pump are lubricated by diesel fuel, so the pump is maintenance-free; diesel pumps operate reliably for a long time if clean fuel is used.
Idle speed, maximum speed, and injection timing can be adjusted with workshop equipment; the stop solenoid can be replaced, the fuel delivery valve body can be replaced, but any internal problem means replacement of the pump.
Fuel Delivery Valves
The fuel delivery valves on the injection pump help ensure that the injector will close quickly at the end of each injection. The injectors must close quickly in order to prevent fuel "dribble" which can cause pre-ignition and high exhaust emissions.
At the start of injection the delivery valve is lifted off of its seat and pressurized fuel flows to the injector:
At the end of injection the delivery valve moves to the closed position. A column of fuel is now trapped in the injection line:
The force of the spring will push the valve back further onto its seat and the trapped column of fuel now expands. The sudden drop in pressure caused by the expanding fuel, allows the injector to snap shut and eliminate and fuel "dribble":
The rotary-vane pump inside the injection pump draws fuel through the filter from the tank and supplies it to the distributor plunger.
The vane pump rotor is driven by the engine camshaft spur belt or gear. As the rotor spins, centrifugal force holds the vanes against the walls of the pressure chamber. The off-center or eccentric layout of the rotor and pressure chamber "squeezes" fuel trapped between the vanes and forces it out the delivery port.
Vane pump delivery pressure is between 3-7 bar depending on the engine speed and is controlled by the regulating valve.
Vane pump fuel pressure lubricates moving parts in the rest of the pump, supplies fuel to the distributor plunger for the injectors, and controls injection timing advance mechanism.
Injection pump manufacturers use a special test "bench" to set and check internal pump pressures. The vane pump and distributor plunger injection pressures cannot be checked easily with normal workshop equipment.
If clean fuel is used, diesel injection pumps operate reliably for a long time. Diesel pumps should not be disassembled or "adjusted." Normal shop work consists only of troubleshooting to determine whether a pump might need replacement.
Injection & Distribution
The injection pump driveshaft turns the vane pump, distributor plunger, and cam plate as a unit.
Springs hold the cam plate and distributor plunger against stationary rollers - in this way, the plunger also moves back and forth as it turns:
Whenever an intake port in the plunger is in line with the filling port in the pump body, fuel from the vane pump fills the pressure chamber.
As the plunger turns, the intake port is covered up so that fuel is trapped in the pressure chamber. Now, the cam plate and rollers push the plunger and pressurize the fuel to about 1800 psi.:
As the plunger continues to turn, the outlet port in the plunger lines up with the injection passage in the pump body, opening the check valve and supplying high-pressure fuel to the injector.
The injection ports in the pump are arranged so the injectors receive fuel in the cylinder firing sequence:
The amount of fuel injected is controlled by changing the injection cut-off point according to engine speed and load conditions.
The injection cut-off point is controlled by the position of a metering sleeve on the distributor plunger. The metering sleeve usually covers a relief port in the plunger. Uncovering the relief port stops injection.
The position of the metering sleeve is controlled by a linkage connected to a centrifugal governor and also the accelerator pedal.
Metering Fuel - Starting
When the engine is not running, the leaf spring presses the starting lever to the left so that the metering sleeve moves to the right.
The distributor plunger must move further before the relief port is exposed. Injection lasts longer so that more fuel is supplied during starting: