PRINCIPLES GASOLINE/DIESEL FUEL SYSTEMS - OD1620 - LESSON 2/TASK 2
(g) When the control rack is pushed in fully, the upper port is
closed shortly after the lower port has been covered, thus producing a full
effective stroke and maximum injection.
(h) From the no-delivery to the full-delivery positions of the
control rack, the contour of the helix advances the closing of the ports and
the beginning of injection.
(i) On the downward travel of the plunger, the metered amount of
fuel is forced through the center passage of the valve assembly, through the
check valve, and against the spray tip valve. When sufficient fuel pressure
is built up, the spray tip valve is forced off its seat and fuel is
discharged through the hole-type injector nozzle. The check valve prevents
air leakage from the combustion chamber into the fuel system should the
spray tip valve not seat properly.
(j) On the return upward movement of the plunger, the high-
pressure cylinder is again filled with oil through the ports. The constant
circulation of fuel through the injectors back through the return helps to
maintain an even operating temperature in the injector, which would
otherwise tend to run very hot due to extreme pressures.
Constant
circulation also helps to remove all traces of air from the system.
The
amount of fuel circulated through the injector is in excess of maximum
needs, thus ensuring sufficient fuel for all conditions.
f. Pressure-Timed (PT) Injection System
(1) Overall System Operation (figure 52 on the following page). The
pressure-timed injection system has a metering system based on the principle
that the volume of liquid flow is proportional to the fluid pressure, the
time allowed to flow, and the size of the passage the liquid flows through.
The operation of the system is as follows:
(a) A fuel tank with a vented filler cap stores the fuel supply.
(b) Fuel is supplied from the tank to the pressure-timed gear
(PTG) pump through the delivery line. An in-line filter is placed in series
in the line to trap foreign matter and moisture.
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