(5) For metering purposes, a recess with an
maintain an even operating temperature in the injector,
upper helix and a lower helix or a straight cutoff is
which would otherwise tend to run very hot due to
machined into the lower end of the plunger. The relation
extreme pressures. Constant circulation also helps to
of this upper helix and lower cutoff to the two ports
remove all traces of air from the system. The amount of
changes with the rotation of the plunger. As the plunger
fuel circulated through the injector is in excess of
moves downward, the fuel in the high-pressure cylinder
maximum needs, thus ensuring sufficient fuel for all
or bushing is first displaced through the ports back into
the supply chamber until the lower edge of the plunger
closes the lower port. The remaining oil is then forced
Pressure-Timed (PT) Injection System.
upward through the center passage in the plunger into
the recess between the upper helix and the lower cutoff,
a. Overall System Operation (Fig. 5-16). The
from which it can flow back into the supply chamber until
pressure-timed injection system has a metering system
the helix closes the upper port.
The rotation of the
that is based on the principle that the volume of liquid
plunger, by movement of the rack, changes the position
flow is proportional to the fluid pressure, the time allowed
of the helix in relation to the ports. This will advance or
to flow, and the size of the passage the liquid flows
retard the closing of the ports and the beginning and
through. The operation of the system is as follows:
ending of the injection period.
This will result in a
regulation of the volume of the fuel charge that is
(1) A fuel tank with a vented filler cap stores
injected into the cylinder.
the fuel supply.
(6) When the control rack is pulled out
(2) Fuel is supplied from the tank to the
completely, the upper port is not closed by the helix until
pressure-timed gear (PTG) pump through the delivery
after the lower port is uncovered. This means that all
line. An in-line filter is placed in series in the line to trap
the fuel in the high-pressure cylinder bypasses back to
foreign matter and moisture.
the fuel supply and no fuel is injected into the combustion
(3) A return line from the PTG pump to the fuel
tank is provided to bleed off excess fuel so that operating
(7) When the control rack is pushed in fully, the
pressures can be regulated.
upper port is closed shortly after the lower port has been
covered, thus producing a full effective stroke and
(4) The PTG pump (para 5-18b) delivers
controlled amounts of fuel to the pressure-timed delivery
(8) From the no-delivery to the full-delivery
positions of the control rack, the contour of the helix
(5) Delivery of fuel to the PTD injectors is
advances the closing of the ports and the beginning of
through a common-rail type delivery line.
(9) On the downward travel of the plunger, the
(6) A common-rail type return line connects
metered amount of fuel is forced through the center
the PTD injectors to the fuel tank so that excess fuel may
passage of the valve assembly, through the check valve,
be diverted back to the fuel tank.
and against the spray tip valve. When sufficient fuel
pressure is built up, the spray tip valve is forced off of its
b. PTG Injection Pump (Fig. 5-17). The PTG
seat and fuel is discharged through the hole-type injector
pump is driven directly by the engine at a one-to- one
nozzle (para 5-14).
The check valve prevents air
speed ratio. The pump contains four main components.
leakage from the combustion chamber into the fuel
These four components and their respective operations
system in the event that the spray tip valve does not seat
are as follows:
(1) The gear-type pump (para 5-23) draws fuel
(10) On the return upward movement of the
from the supply tanks and forces it through the pump
plunger, the high-pressure cylinder is again filled with oil
filter screen to the governor. It is driven by the pump
through the ports.
The constant circulation of fuel
main shaft and picks up and delivers fuel
through the injectors back through the return helps to