DIESEL ENGINE SPEED,
Diesel Engine Fundamentals
FUEL CONTROLS, AND PROTECTION
The motion of the injector rocker arm (not shown) is transmitted to the plunger by the injector
follower which bears against the follower spring. As the plunger moves downward under
pressure of the injector rocker arm, a portion of the fuel trapped under the plunger is displaced
into the supply chamber through the lower port until the port is closed off by the lower end of
the plunger. The fuel trapped below the plunger is then forced up through the central bore of the
plunger and back out the upper port until the upper port is closed off by the downward motion
of the plunger. With the upper and lower ports both closed off, the remaining fuel under the
plunger is subjected to an increase in pressure by the downward motion of the plunger.
When sufficient pressure has built up, the injector valve is lifted off its seat and the fuel is forced
through small orifices in the spray tip and atomized into the combustion chamber. A check
valve, mounted in the spray tip, prevents air in the combustion chamber from flowing back into
the fuel injector. The plunger is then returned back to its original position by the injector
On the return upward movement of the plunger, the high pressure cylinder within the bushing is
again filled with fresh fuel oil through the ports. The constant circulation of fresh, cool fuel
through the injector renews the fuel supply in the chamber and helps cool the injector. The fuel
flow also effectively removes all traces of air that might otherwise accumulate in the system.
The fuel injector outlet opening, through which the excess fuel returns to the fuel return manifold
and then back to the fuel tank, is adjacent to the inlet opening and contains a filter element
exactly the same as the one on the fuel inlet side.
In addition to the reciprocating motion of the plunger, the plunger can be rotated during operation
around its axis by the gear which meshes with the fuel rack. For metering the fuel, an upper
helix and a lower helix are machined in the lower part of the plunger. The relation of the helices
to the two ports in the injector bushing changes with the rotation of the plunger.
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. Figure 27 illustrates the various plunger
positions from NO LOAD to FULL LOAD. With the control rack pulled 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 fuel injection. From this no-injection position to the
full-injection position (full rack movement), the contour of the upper helix advances the closing
of the ports and the beginning of injection.