7-11
FOR TRAINING PURPOSES ONLY
LEARJET 20 SERIES PILOT TRAINING MANUAL
FlightSafety
international
FUEL SYSTEM
General
A hydromechanical engine fuel system provides
for automatic fuel metering, variable geometry
system sequencing and operation, and engine
overspeed control. Automatic compensation is
provided for such variables as ambient pres-
sure, temperature, and engine rpm throughout the
complete operating range of the engine.
Fuel Pump
The fuel pump is a combination centrifugal
and high-pressure positive displacement pump.
The centrifugal pump supplies fuel under pos-
itive pressure to the high-pressure pump and to
the pressurizing and drain valve. The high-
pressure positive displacement pump supplies
high-pressure fuel to the engine fuel controller,
the overspeed governor servo, and the jet pumps.
Fuel Controller
The fuel controller is mounted on and driven
by the fuel pump assembly and is divided into
a computing section and a metering section.
The computing section senses four parame-
ters: compressor inlet temperature, compres-
sor discharge pressure, engine rpm, and thrust
lever position.
The metering section meters fuel to the com-
bustion section to produce or maintain the de-
s i r e d t h r u s t . T h e f u e l c o n t r o l u n i t l i m i t s
maximum engine rpm to 101.2% while automat-
ically maintaining the desired power setting
as operating temperatures and pressures change.
It regulates fuel flow to simultaneously posi-
tion the variable inlet guide vanes and bleed
valves during transient and steady-state oper-
ation, and it provides a positive fuel shutoff.
Variable Geometry System
The variable geometry system includes actu-
ating, operating, and synchronizing linkage to
position the variable inlet guide vanes and the
engine compressor bleed valves.
The function of the variable geometry system
is to prevent compressor stalls and surging
when operating in a low power range and dur-
ing acceleration and deceleration.
The system includes bleed valves at the third,
fourth, and fifth stages of the compressor.
When these valves are moved toward open, a
volume of compressor air is dumped, thus un-
loading the compressor and maintaining a
safe stall-surge margin between compressor
inlet and outlet pressures.
The bleed valves operate together with vari-
able guide vanes located in the front frame.
The operating linkage is such that when the
bleed valves are fully open, the variable guide
vanes are at their fully closed position, re-
stricting air mass flow to the compressor inlet.
This is the position of the bleed valves and
guide vanes at low power setting and/or when
the engine is shut down.
A variable geometry servo in the fuel con-
troller directs high-pressure (unmetered) fuel
to the actuators for synchronous operation of
the bleed valves and guide vanes. A feedback
cable supplies bleed valve and guide vane po-
s i t i o n i n f o r m a t i o n t o t h e f u e l c o n t r o l l e r
through the variable geometry servo.
Overspeed Governor
The overspeed governor is a hydromechani-
cal, self-contained unit placed in series with the
main fuel control and is driven by the accessory
gearbox. Its purpose is to override the main
fuel control of the engine overspeeds. Under ab-
normal operating conditions all fuel passes
through the governor unhindered. However, if
an overspeed occurs (103.5 = .5% rpm), the
governor cuts in and maintains a constant en-
gine speed by metering the flow through it and
bypassing the excess fuel to the main fuel pump
inlet. The overspeed governor has a ground test
which limits engine rpm to 90.5 + 1% rpm until
placed back to normal operation.
