The function of the fuel injector is to deliver finely atomized fuel under high pressure to the combustion chamber of the engine. All component parts of the injector are carried in nozzle holder. The main part of the injector is the nozzle comprising nozzle body and nozzle needle valve. The nozzle body and the needle valve are fabricated from alloy steel. They are thoroughly machined and have high surface hardness necessary for operation in conditions of high temperatures and elevated pressures. The bore in the nozzle body and the nozzle needle valve are lapped to a close tolerance and are a matched set, so that neither the nozzle body nor the needle valve may be replaced individually. The needle valve is pressed against a conical seat in the nozzle body by the spring acting through the intermediary of the stem. The spring pressure, hence injection pressure, is adjusted by the adjusting screw. The adjusting screw is screwed in the bottom of the injector spring cap nut which in turn is screwed in the nozzle holder. A lock nut is used to prevent the adjusting screw from unscrewing spontaneously. The screw is covered by the nozzle holder cap nut provided with a threaded hole to connect the leak-off pipe through which the leak-off fuel (used to lubricate the nozzle valve) filling the pressure spring and adjusting screw area is returned to the fuel tank or the secondary fuel filter.
In operation, fuel from the injection pump enters the pressure chamber (gallery) in the nozzle body through a supply passage and a high-pressure pipe. When the fuel pressure in the pressure chamber becomes so high that the force acting on the pressure taper of the needle valve from below exceeds the set spring force on the stem, the needle valve lifts off its seat and comes to rest with its upper shoulder against the face of the nozzle holder. Fuel is then forced out of the nozzle spray holes into the combustion chamber in a spray pattern which depends on the type of nozzle used.
After the injection of fuel has been ended, the fuel delivery from the injection pump ceases, the pressure in the pressure chamber of the nozzle drops instantly, and the pressure spring snaps the needle valve onto its seat, preventing unpressurized fuel from leaving the nozzle.
The fuel injector is installed in a brass injector tube, or sleeve, which is fitted in a hole in the cylinder head, and is held in place by a special clamp.
Low-pressure fuel lines use brass pipes or thin-walled steel pipes provided with a corrosion-resistant coating. Some engines make use of PVC tubing for the purpose.
High-pressure fuel lines connecting the fuel-injection pump with the injectors are steel pipes 2mm inside diameter and 7mm in outside diameter. The external surfaces of the pipes are oxidized for corrosion protection. The ends of the pipes are upset by a special device to form tapered male seats for connecting the pipes to the fuel-injection pump discharge and injector inlet fittings. The pipes should be fitted well to their respective fittings, so that the union nuts may be screwed on the fittings by hand. The nuts are finally tightened with a wrench.
Before installing them on the engine, all fuel pipes should be thoroughly washed in diesel fuel and then blown through with compressed air.
The fuel-injection pump serves the purpose of delivering, under high pressure, accurately timed and metered quantities of fuel to each cylinder of an engine, in accordance with the engine speed and load.
The pumping element consists of a plunger-and-barrel as-sembly, a spring, a roller tappet, the cam of the injection pump camshaft. And the delivery valve with its body.
The plunger and barrel assembly is the heart of the pumping element. It comprises a plunger reciprocating in the close-fitting barrel. The plunger and the barrel are tab ricated from alloy steel and heat treated to have a high hardness, for they must withstand high fuel pressures during operation. On the delivery stroke of the plunger, the fuel must lubricate the rubbing surfaces of the pumping element, but at the same time, it must not leak past the plunger from the upper to the lower side of the barrel. Therefore, these ports are lapped together in pairs to a very close tolerance (0. 001 to 0. 002 mm) and must not be replaced individually.
The fuel injection pump barrel is a center and a cylinder with somewhat thickened upper portion. In the thickened part of the barrel, there are two opposite side openings located at different heights. The upper opening, the inlet port, serves to fill the barrel space above the plunger with fuel. The lower opening the spill port, is used to bypass fuel to the inlet side of the pump. When the barrel is mounted in the pump housing, both ports are open to the U-shaped fuel manifold of the pump.
The upper part of the plunger has a center-aged a cross-drilled hole and a fuel cut-off (spill) spiral groove, called the helix, the center-drilled hole and the helix communicating with each other via the cross-drilled hole. The helix makes it possible to vary the delivery of the fuel-injection pump without changing the actual stroke of the pump plunger. An annular groove in the center part of the plunger serves for uniform distribution of fuel over the barrel face, the fuel in this case playing the part of lubricating oil.
On the lower part of the plunger, there are two flanges and a circular recess. The flanges slide in the slots of the control sleeve which carries a gear segment meshing with the control (fuel) rack of the pump. The circular recess is used for hooking the lower seat of the plunger spring to the plunger. The spring serves to return the plunger-its lower position on the intake stroke.
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