FUEL SYSTEM - Continued
As the plunger moves downward, under pressure of the injector rocker arm, some of the fuel under the plunger
moves into the supply chamber through the lower port until the port is covered by the lower end of the plunger.
The fuel below the plunger continues to move up through a central passage in the plunger into the fuel metering
recess and into the supply chamber through the upper port until that port is covered by the upper helix of the
plunger. With the upper and lower ports both covered, the remaining fuel trapped under the plunger is subjected
to increased pressure by the continued downward movement of the plunger.
When sufficient pressure is built up, it opens the flat check valve. The fuel in the check valve cage, spring cage,
tip passages, and tip fuel cavity is compressed until the pressure force acting upward on the needle valve is
sufficient to open the valve against the downward force of the valve spring. As soon as the needle valve lifts off of
its seat, the fuel is forced through the small orifices in the spray tip and atomized into the combustion chamber.
When the lower land of the plunger uncovers the lower port in the bushing, the fuel pressure below the plunger is
relieved and the valve spring closes the needle valve, ending injection.
A pressure relief passage has been provided in the spring cage to permit bleed-off of fuel leaking past the needle
pilot in the tip assembly.
A check valve, directly below the bushing, prevents leakage from the combustion chamber into the fuel injector in
case the valve is accidentally held open by a small particle of dirt. The injector plunger is then returned to its
original position by the injector follower spring.
On the return upward movement of the plunger, the high pressure cylinder within the bushing is again filled with
fuel oil through the ports. The constant circulation of fresh cool fuel through the injector renews the fuel supply in
the chamber, helps cool the injector and also effectively removes all traces of air which might otherwise
accumulate in the system and interfere with accurate metering of the fuel. The fuel injector outlet opening, through
which the excess fuel oil returns to the fuel return manifold and then back to the fuel tank, is directly adjacent to
the inlet opening.
Changing the position of the helices, by rotating the plunger, retards or advances the closing of the ports and the
beginning and ending of the injection period. At the same time, it increases or decreases the amount of fuel
injected into the cylinder. With the control rack pulled out all the way (no injection), the upper port is not closed by
the helix until after the lower port is uncovered. Consequently, with the rack in this position, all of the fuel is forced
back into the supply chamber and no injection of fuel takes place. With the control rack pushed all the way in
(full injection), the upper port is closed shortly after the lower port has been covered, thus producing a maximum
effective stroke and maximum injection. From this no injection position to full injection position
(full rack movement), the contour of the upper helix advances the closing of the ports and the beginning
The positive displacement gear-type fuel pump transfers fuel from the supply tank to the fuel injectors. The pump
circulates an excess supply of fuel through the injectors, which purges the air from the system and cools the
injectors. The unused portion of fuel returns to the fuel tank by means of a fuel return manifold and fuel return line.
The fuel pump cover and body are positioned by means of two dowels. The dowels aid in maintaining gear shaft
alignment. The mating surface of the pump body and cover are perfectly flat ground surfaces. No gasket is used
between the cover and body since the pump clearances are set up on the basis of metal-to-metal contact. A very
thin coat of sealant provides a seal against any minute irregularities in the mating surfaces. Cavities in the pump
cover accommodate the ends of the drive and driven shafts.