c. The power turbine governor lever schedules the
governor requirements. The power turbine governor,
in turn, schedules the gas producer speed to a changed
power output to maintain output shaft speed.
d. Fuel flow for engine control depends on com-
pressor discharge pressure (Pc), engine speed (gas
producerN1 and/or power turbineN2), and lever
angle. Fuel flow is a function of Pc as sensed in the
fuel control. Variations of fuel flow schedules are ob-
tained by modulating the Pc to Px and Py pressures in
the control through the action of a bleed-down circuit
actuated by the governors. (See figure 1-4.)
1-12. Gas Producer Fuel Control.
a. The gas producer fuel control has a bypass
valve, metering valve, acceleration bellows, governing
and enrichment bellows, manually operated cutoff
valve, maximum pressure relief valve, torque tube
seal and lever assembly, and a start derichment valve.
The maximum pressure relief valve protects the
system from excessive fuel pressure.
b. Fuel enters the control from the engine fuel
pump and filter assembly and is delivered to the
metering valve. The bypass valve maintains a cons-
tant pressure differential across the metering valve.
Also, excess fuel is bypassed to the fuel pump and
filter assembly through an external line connecting
the pump bypass inlet to the bypass outlet port of the
gas producer fuel control.
c. The metering valve is operated by lever action
through movement of the governor and acceleration
bellows, Metering valve area depends on valve travel,
Before light -off and acceleration, the metering valve is
set at a pre-determined open position by the accelera-
tionbellows under the influence of ambient pressure
(Pc at zero rpm).
d. The start derichment valve is open during
light-off and acceleration to a set Pc. The open derich-
ment valve vents Py pressure to atmosphere. Venting
Py allows the governor bellows to move the metering
valve against the minimum flow stop. At minimum
flow the metering valve provides the required lean fuel
schedule after light-off. As compressor rpm increases,
the derichment valve is closed by Pc acting on the
derichment bellows. When the derichment valve is
closed, control of the metering valve is returned to the
normal operating schedule.
e. During acceleration, the Px and Py pressures
are equal to the modified compressor discharge
pressure (Pc) up to the point where the speed enrich-
ment orifice is opened by flyweight action. Opening
the speed enrichment orifice bleeds Px pressure whil
Py remains at a value equal to Pc. Under the influence
of the Py minus Px pressure drop across the governor
bellows, the metering valve moves toward the max
imum flow stop where it increases fuel flow.
f. Gas producer speed is controlled by the gas
producer fuel control governor. A set of flyweights
operate the governor lever which controls the governor
bellows (Py) bleed at the governing orifice. Flyweight
operation of the governor lever is opposed by a
variable spring load. The spring force is established
by the throttle lever acting on a spring scheduling
cam. Opening the governing orifice bleeds Py pressure
and allows Px pressure to control the governor
bellows. The Px influence on the bellows moves the
metering valve toward minimum flow and at a posi-
tion where metered flow is at steady state re-
g. The governor reset assembly in the gas
producer fuel control limits or governs power turbine
speed. Control of the reset assembly is derived from
the power turbine governor. The power turbine gover-
nor also provides quick responding overspeed protec-
t ion by bleeding governor servo (Py) pressure from the
gas producer fuel control.
1-13. Power Turbine Governor.
a. Power turbine speed is scheduled by the power
turbine governor lever and the power turbine speed
scheduling cam. The cam sets a governor spring load
which opposes a flyweight output. As the desired
speed is approached, the flyweights operating against
the governor spring move a link to open the power tur-
bine governor orifice. The flyweights also open the
overspeed bleed (Py) orifice but at a higher speed
than the regular governor (Pg) orifice.
b. The governor orifice is downstream of a bleed
supplied by a regulated air pressure (Pr). Opening the
orifice results in a reduced pressure downstream of the
bleed (Pg) as an inverse function of increasing speed.
Regulated pressure (Pr) and governing pressure (Pg)
are applied to opposite sides of a diaphragm in the
governor reset section of the gas producer fuel control,
The force generated by Pr minus Pg across the
diaphragm acts on the gas producer power output link
through the governor reset rod. This force
supplements the weight force in the gas producer fuel
governor to reset (reduce) the gas producer speed. Gas
producer speed cannot exceed the gas producer fuel
governor setting. The Pr minus Pg diaphragm is
preloaded for establishing the active Pr minus Pg
range. Pr pressure is supplied from engine Pc pressure
by an air regulator valve.