Note: Descriptions are shown in the official language in which they were submitted.
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EMERGENCY SHUT-DOWN DETECTION
SYSTEM FOR A GAS TURBINE
[001] This application claims the benefit of priority of U.S. provisional
application Serial No.,
62/173,400 filed on June 10, 2015, the disclosure of which is herein
incorporated by reference in
its entirety.
Background
[002] The present disclosure relates to a device and method for detecting an
emergency
condition for shutting down a gas turbine engine, and particular to a device
and method that
detects a shaft failure of the gas turbine engine.
[003] Gas turbine engines have been well known in the art for many years, and
are engines in
which a shaft is used to transmit the torque delivered by a turbine assembly
to a compressor
assembly. The compressor is used to pump a working fluid (typically air)
through the engine, a
combustion system (located between the compressor and turbine) is used to add
thermal energy
to the working fluid, the turbine assembly is used to extract work from the
working fluid to drive
the compression system, and the residual energy in the working fluid is used
to provide shaft
power (turboshaft, or turboprop) or motive thrust (turbojet or turbofan).
[004] Engines have been created which incorporate from one to any number of
shafts, typically
designated as single, dual, triple spool or multi-spool engines. Multi-spool
engine designs will
generally include two or three coaxial drive shafts. In a dual spool engine, a
low pressure
compressor is connected by a first coaxial drive shaft to a low pressure drive
turbine.
Downstream of the low pressure compressor assembly is a high pressure
compressor which is
connected to a second coaxial drive shaft which is driven by a high pressure
turbine assembly.
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[005] A similar arrangement is provided in a three spool engine design except
that there is now
a low, intermediate, and high pressure compressor assembly that are each
connected respectively
to low, intermediate, and high pressure turbine assemblies. In some
configurations, a final
turbine assembly may be used to drive an external load rather than driving a
second or third
compressor assembly, in which case the turbine assembly is often referred to
as the power
turbine.
[006] Whether the engine is the single, dual, or triple spool type, the drive
shafts must be
capable of rotating at thousands of rpm's for hours at a time, with
significant variations in
operating temperature, acceleration demands, centrifugal stress, axial stress,
etc. In extremely
rare instances during the life of an engine, situations can occur where the
load on a drive shaft
exceeds design limits which may result in failure of the affected shaft.
Failure or "decoupling"
of one or more of the engine shafts will generally occur suddenly and can lead
to an uncontained
failure of the released turbine assembly.
[007] When a gas turbine engine experiences a shaft failure between the
compressor and
turbine assemblies, the entire failure sequence may occur in less than one
second, and will result
in the sudden deceleration of the affected compression system, while the
turbine assembly
rotating components will experience an unregulated and rapid acceleration. The
rapid
acceleration of the turbine assembly poses the greatest hazard to the engine
and vehicle because
the increased rotational velocity may introduce forces on rotating assembly
that exceed the
mechanical strength limits of the assembly.
[008] When rapid acceleration occurs the turbine assembly will experience a
failure of the disk
or blade components leading to the release of high energy debris. Most gas
turbines are not
designed to contain such high energy release of failed hardware due to weight
and cost
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constraints. On a jet aircraft, an uncontained turbine disk or blade failure
could result in serious
damage to other engines or aircraft hardware and could result in loss of the
aircraft. Aviation
safety regulators mandate that in the event of a shaft break the gas turbine
must not release high
energy debris.
[009] Various attempts have been made to contain a component burst through the
engine
housing. In one such attempt, a solid containment ring formed of high strength
material, such as
a nickel cobalt alloy has been integrated into the outer engine housing to
circumferentially
surround the rotating components of the engine. Although such containment
rings have been
successful in containing fragmented components within the engine housing, they
add a
significant amount of additional weight to the engine, thus sacrificing fuel
economy and
passenger capacity. It is desirable to detect and act on a shaft failure event
quickly, to prevent
excessive stresses on the released turbine components.
[010] Because shaft failure indications may only be available for a short time
(fractions of a
second) before the turbine rotating components start to fragment, it is
evident that such a warning
protocol must be automated, preferably in the form of a control logic utilized
by a high-speed on
board processor. If a shaft failure can be detected quickly and the engine can
be shut down while
the engine is displaying the early warning signs of shaft failure, and before
any component
fragmentation occurs, the need for using heavy containment rings can be
eliminated. Additional
damage to the engine resulting from the high speed component fragmentation can
be eliminated
as well. In this fashion, the safety of the operational engine can be
significantly improved, the
potential for component fragmentation can be eliminated and the safety and
integrity of the
vehicle maintained.
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[011] As mentioned above, gas turbine engines (e.g. jet engines) include a
rotating shaft having
a compressor and/or a turbine assemblies mounted thereon and rotating
therewith. Excessive
axial movement of the shaft supporting the turbine and compressor assemblies
relative to the
static structures of the engine is considered to be abnormal and indicative of
engine failure (e.g.
shaft breakage). Detection of axial movement of the shaft relative to the
remainder of the engine
can therefore be used to detect engine failure and to activate a shut off of
the engine. If the shaft
linking the turbine to a compressor is broken, the loads on the turbine
assembly will act to push it
backwards (towards the engine exhaust). The compressor loads however will act
to push it
forward (towards the engine intake) even as the compressor rapidly
decelerates. The turbine
elements will increase rotational speed due to the loss of the compressor load
and the continued
availability of energy from the combustion system.
[012] Previous systems for detecting turbine shaft failure have relied on
continuity systems to
detect abnormal movement of a turbine assembly relative to the engine casing.
In those systems,
when the axial movement of the turbine assembly exceeds a minimum level, the
turbine
assembly breaks a continuity circuit and in so doing shuts off fuel flow to
the engine. In another
system, movement of the circuit breaking element relative to the shaft breaks
a circuit and
thereby produces a signal. In a further approach, an electro-optic sensor
senses unwanted or
abnormal axial movement of turbine blades or rotors of a gas turbine.
[013] Another form of a broken shaft detection system uses a detector assembly
mounted
downstream of a power turbine wheel of a gas turbine engine to detect rearward
axial motion of
the wheel indicative of a broken shaft event. The detector assembly has a
plunger positioned to
be axially displaced against a link connected in an electrical circuit which
may be broken when
the plunger is displaced thereby creating an open circuit that may be detected
by a detection and
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test element. This detection may be communicated to an over-speed circuit that
controls a shut-
off switch that interrupts fuel flow to the engine. In another approach, a
frangible sensor element
is cut by a separating tang mounted on and moving axially with a gas turbine
shaft when the
shaft fails. The systems employed in the prior art thus rely on the addition
of electrical and
mechanical components to the engine, which increases the weight and complexity
of the engine.
[014] It is very important to avoid false shaft failure detection events.
Typically a sensor
monitoring for shaft breakage is directly coupled to a fuel cut-off circuit to
automatically and
quickly shut off the engine when the shaft breaks. A false detection would
therefore lead to an
unwarranted shut down of the engine which by itself increases the threat to
the vehicle. For
systems relying upon electrical circuits, discrepancies in the performance of
the detection circuit
itself can trigger a false shaft break signal.
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Summary
[015] According to one aspect of the present disclosure a gas turbine engine
comprises at least
one compressor and combustor, a turbine coupled to the compressor by a spool,
and an electrical
compressor speed sensor located in front of the compressor (toward the
compressor inlet). The
speed sensor includes an electrical continuity circuit to provide built-in
test capability to verify
acceptable continuity of the electrical speed sensor. The engine further
comprises a controller
having a number of inputs for receiving at least one measured engine parameter
and a number of
outputs for transmitting control signals which regulate engine operation
variables. In one aspect,
the controller includes an input for receiving the speed signal and the
continuity signal, and is
configured to generate a signal to shut down engine operation in response to a
loss of both the
speed signal and the continuity signal. In one feature of the presently
disclosed system, shaft
failure will result in the loss of both the speed and continuity signals.
Multiple speed sensors
may be provided and the controller may be configured to generate an engine
shut down signal
only when there is a loss in both speed and continuity signals for all of the
speed sensors.
[016] In another aspect, the engine further comprises a pressure sensor
operable to generate a
pressure signal in response to a fluid pressure within the compressor, such as
the P30 pressure
(compressor exit pressure). The controller receives the pressure signal as an
input and is
configured to use that signal to decide whether a fast fuel shut-off is
necessary in the event of the
shaft failure event. If the pressure does not exceed a minimum pressure
threshold, even if the
shaft has failed an emergency shut-down is not required.
[017] In a further aspect, the controller may be optionally configured to
detect compressor
surge and to generate a signal indicating that a compressor surge has
occurred. The controller is
further configured to generate an engine shut-down signal when a surge signal
is received at the
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same time that the P30 pressure exceeds the minimum threshold, detected as
described above,
and a shaft failure is indicated. In this configuration, compressor surge is
an expected response
to a spool shaft failure. In this embodiment, the controller will only command
an engine shut-
down when four conditions are present ¨ a speed probe fault, a continuity
fault, the P30
compressor pressure exceeds a minimum pressure threshold and a compressor
surge is indicated.
Commanding engine shut down only when all four conditions are met
significantly reduces the
risk of false shaft failure detection.
[018] In a further aspect of the disclosure a method is provided for shutting
down a gas turbine
engine that comprises detecting a fault in a signal from a compressor speed
probe upstream of
the spool connecting a compressor to a turbine, detecting a fault in a
continuity signal for the
compressor speed probe, and commanding an engine shut down in response to the
detection of a
fault in both the speed probe signal and the continuity signal. In a further
feature, the method
comprises detecting the P30 pressure of the engine, and commanding the engine
shaft failure
induced shut down only when the P30 pressure signal exceeds a predetermined
threshold value
related to the disc burst threshold for the engine. In a further aspect, when
the P30 pressure
condition is met, the engine shut down may be commanded only if a compressor
surge condition
is also detected.
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Description of the Figures
[019] FIG. 1 is a schematic of a multi-stage gas turbine engine.
[020] FIG. 2 is an enlarged cross-sectional view showing the location of a
compressor speed
probe and compressor bearing in a gas turbine engine such as the engine shown
schematically in
FIG. 1.
[021] FIG. 3 is a logic diagram of a shaft failure detection and engine shut-
off condition using
the compressor speed probe and continuity circuit to detect the shaft failure.
[022] FIG. 4 is a logic diagram of a shaft failure detection and engine shut-
off condition using
the compressor speed probe and continuity circuit and the P30 pressure to
detect the shaft failure.
[023] FIG. 5 is a logic diagram of a shaft failure detection and engine shut-
off condition using
the compressor speed probe and continuity circuit, P30 pressure and compressor
surge to detect
the shaft failure.
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Detailed Description
[024] For the purposes of promoting an understanding of the principles of the
disclosure,
reference will now be made to the embodiments illustrated in the drawings and
described in the
following written specification. It is understood that no limitation to the
scope of the disclosure
is thereby intended. It is further understood that the present disclosure
includes any alterations
and modifications to the illustrated embodiments and includes further
applications of the
principles disclosed herein as would normally occur to one skilled in the art
to which this
disclosure pertains.
[025] The schematic representation of a three-spool gas turbine engine 10 is
shown in FIGS. 1-
2. In particular, the engine 10 includes in sequence a low pressure (LP) fan
12, compressor
components 13 including an intermediate pressure (IP) compressor 14, a high
pressure (HP)
compressor 16, a combustor 18, and turbine components 19 including a HP
turbine 20, an IF
turbine 22 and a LP turbine 23. The LP turbine 23 is coupled to a LP spool 24
that drives the LP
fan 12. It is noted that in some two-spool engines the LP fan may be driven by
the first
compressor component through a gearbox. In the three-spool engine shown in
FIG. 1, the IF
turbine 22 is coupled to an IP spool 25 that drives the EP compressor 14,
while the HP turbine 20
drives the HP compressor 16 through the HP spool 26.
[026] The gas turbine 10 includes an engine controller 32 that may be a
digital engine
controller having a memory and configured to execute program or software
instructions to
control the engine operating parameters. In particular, the controller 32
includes a number of
inputs for receiving at least one measured engine parameter, a processor for
generating control
signals for controlling engine operation variables and a number of outputs for
transmitting the
control signals to appropriate control components, such as a fuel system
metering valve or
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controller 35 operable to control fuel supplied to the combustor 18. Among the
inputs is an input
for receiving compressor speed data. In particular, the engine includes a
speed probe 30
positioned adjacent the HP compressor 16. In one embodiment, the speed probe
30 may be an
inductive device that contains a pick-up coil with a magnetized core 37
mounted in close
proximity to a phonic wheel 38 mounted on a spool shaft, such as spool 26. The
phonic wheel is
essentially a toothed wheel that rotates with the spool so that the passage of
each tooth by the
probe 30 causes a variation in magnetic flux to thereby generate an
alternating voltage in the
pick-up coil. This alternating voltage signal is sensed by the controller 32
and software or
firmware within the controller to convert this alternating voltage into a
shaft speed value. The
controller uses this shaft speed value in conjunction with other data
indicative of engine
performance to control the operation of the engine. For instance, the
controller 32 may be
operable to execute software or firmware to detect a turbine overspeed
condition when the shaft
speed value obtained from the speed probe signal exceeds a predetermined
stored value, and then
to command a fuel reduction to the combustor to thereby reduce the engine and
turbine speed.
[027] The speed probe 30 incorporates an electrical continuity circuit that is
used to verify the
integrity of the probe. In particular, the continuity circuit is configured to
provide a continuity
signal to the controller 32, and the controller is configured to continually
monitor the continuity
signal. When the continuity circuit is disrupted it fails to generate a
continuity signal. This
disruption is sensed by the controller which in turn generates an alarm or
fault signal indicative
of a failure of the speed sensing system. In speed probes incorporating a pick-
up coil, the pick-
up coil is integrated into the continuity circuit so that a loss of voltage
indication from the pick-
up coil is indicative of a break in the electrical continuity circuit. In this
condition, the failure of
the continuity signal is also indicative of a loss of speed signal because the
loss of electrical
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continuity in the pick-up coil produces an open circuit. Loss of continuity
and ultimately loss of
the shaft speed signal can be caused by a variety of events, many of which do
not require
shutting down the engine.
[028] In certain engines at least two independent speed probes 30 are provided
at the same axial
location relative to the phonic wheel 38 so that the probes read the same
rotational speed. Each
probe may communicate with the controller 32 or with its own associated
controller, so that the
continuity signal associated with each speed probe is continuously monitored.
The loss of
continuity of less than all of the speed probes is not interpreted by the
controllers as a shaft
failure. On the other hand, the simultaneous loss of continuity and shaft
speed signals from all of
the speed probes is highly indicative.
[029] In accordance with one aspect of the present disclosure, the existing
speed probe and
continuity circuit for a gas turbine engine is used to detect a shaft decouple
or shaft breakage in
the HP spool 26. It is known that in a shaft decouple condition the combustion
pressure forces
the HP turbine stage 20 rearward (i.e., toward the exhaust) and forces the HP
compressor 16
forward (i.e., toward the inlet and fan 12). The forward displacement of the
BP compressor can
be used to disrupt the integrity of the speed probe and/or continuity circuit.
As shown in FIG. 2,
the speed probe 30 may be incorporated into an HP compressor arrangement
having a bearing
assembly 40 that incorporates a spring arrangement between the HP compressor
and a stationary
housing element of the engine. As illustrated in FIG. 2, the bearing assembly
40 includes a
bearing 42 mounted on the second spool 26 and supporting an inner housing 44,
an outer housing
46 and a spring bar assembly 48 between the two housings. The speed probe(s)
30 is/are aligned
with the teeth of the phonic wheel 38 mounted on the shaft 26. The coil, core
and continuity
circuit of each speed probe are contained within a corresponding housing 36.
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[030] The spring bar assembly includes a plurality of circumferentially
disposed spring bars 49
that are configured to flex to absorb normal axial loads exerted on the
bearing assembly 40
during operation of the engine. Further details of the bearing assembly 40 and
the spring bar
assembly 48 are found by reference to U.S. Published Application No.
2007/0031078, published
on Feb. 8, 2007 in the name of Assignee Rolls-Royce, PLC, the entire
disclosure of which is
incorporated herein by reference, specifically Figs. 1-12 and the associated
written description in
Paras. 0025-0040.
[031] As shown in FIG. 2, the HP compressor speed probe(s) 30 is/are disposed
immediately
axially adjacent the outer housing 46. The spring bars 49 are configured to
fracture when the
bearing assembly is subjected to a predetermined axial force indicative axial
displacement of the
HP compressor 16 upon a failure of the HP spool 26. When the spring bars
fracture the bearing
assembly 40 translates axially forward with sufficient force to contact and
fracture or sever the
speed probe 30. The outer housing 46 may be provided with a protrusion 52 in
the form of a
knife edge, for instance, which is aligned with the speed probe 30 and
particularly adapted to
sever the probe.
[032] Upon a fracture of the HP spool 26 between the HP turbine 20 and the HP
compressor 16,
the HP compressor generates sufficient forward axial force to fracture the
spring bars 49 and to
propel the bearing assembly 40 forward against the speed probe. The bearing
assembly, and
particularly the outer housing 46 and/or protrusion 52 moves forward with
enough force to sever
each speed probe 30. The speed signal and continuity signals from each speed
probe to the
controller(s) 32 are simultaneously terminated resulting in speed and
continuity fault conditions.
The controller(s) 32 is continuously monitoring the speed and continuity
signals as part of the
normal operation of the engine. When the two signals for each speed probe are
no longer
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received by the controller(s) 32, software or firmware within the controller
immediately
generates a speed sensor fault signal and a continuity fault signal. When
these fault signals are
sensed, the controller immediately sends a signal to the fuel system 35 to
shut-off the fuel supply
to the affected engine.
[033] The controller(s) 32 may thus implement the AND-gate logic depicted in
FIG. 3 based
on receipt of both a speed probe fault signal and a continuity circuit fault
signal. The presence of
both faults in a single speed probe at the same time reduces the chance of
false detection of a
shaft failure scenario. The presence of both faults in multiple speed probes,
such as probes 1, 2,
...n depicted in FIG. 3, significantly reduces the chance of false detection.
For a typical two
speed probe configuration, four fault signals are required to initiate a fuel
shut-off command. A
time component can also be incorporated into the AND-gate test, namely that
the fault signals
arise within a predetermined time of each other. In other words, when a shaft
failure occurs, the
speed probe and continuity circuit of a given speed probe are immediately
compromised
together. Likewise, the time element may be implemented with respect to the
timing of speed
and continuity faults of multiple speed probes. Any appreciable delay between
the fault
conditions may not be indicative of a shaft failure but of some other
condition that does not
require an immediate engine shut-down.
[034] With an eye toward further minimization of false detection, the present
disclosure
contemplates using further data obtained or generated by the controller 32
during normal
operation of the engine. In particular, the controller 32 receives data from a
pressure sensor 33
downstream of the HP compressor, also known as the P30 pressure. The P30
pressure is used to
provide a fuel flow set point value to control a flow metering device or fuel
flow valve for
delivering fuel to the combustor 18. In one aspect of the present disclosure,
the engine controller
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32 is operable to compare the P30 pressure value to a minimum threshold
pressure that does not
achieve a requisite margin to disc burst speed. If the pressure value at the
exit of the HP
compressor is not above that threshold, then there is no risk of a turbine
failure due to an
overspeed condition even if the HP shaft is compromised. Since there are other
conditions in
which the P30 pressure may exceed the minimum threshold that do not
necessitate an engine
shut-down, the controller 32 implements the AND-gate logic shown in FIG. 4
that requires both
speed probe and continuity faults and P30 pressure above the minimum threshold
pressure. In
one aspect of the present disclosure, the engine controller 32 may implement
software commands
to obtain the P30 pressure value, compare it to a predetermined threshold
stored value and
generate a P30 fault signal if the sensed pressure exceeds the threshold. In
some controllers 32 a
P30 fault signal or fault bit may already exist in which case the controller
32 simply reads the
available P30 fault signal and feeds the signal value to the AND-gate shown in
FIG. 4.
[035] In order to further reduce the chance of a false shaft failure
detection, the present
disclosure further contemplates the use of compressor surge information that
is already being
monitored by the engine controller 32. Surge is a phenomenon that arises in
the compressor of
the gas turbine engine in which the compressor pumping characteristic is
compromised and can
no longer maintain the adverse pressure gradient across parts of or the entire
assembly. Normal
operation of the compressor depends on factors such as the pressure ratio,
i.e., the ratio of the
outlet pressure to the inlet pressure, and the mass flow through the
compressor. If the pressure
ratio is too high for the current mass flow, the compressor may start to
stall, which can then lead
to reverse mass flow through the compressor due to the higher outlet pressure.
This condition is
known as surge and can result in a loss of thrust/power, excessive vibration
and even damage to
the engine. The engine controller 32 monitors the compressor pressure ratio
and the mass flow
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through the compressor. If the engine controller, detects a potential surge
condition the engine
controller activates bleed valves in the compressor in some engines to bleed
some air from the
core airflow through the compressor to ultimately reduce the pressure ratio.
In other engines, the
engine controller 32 may adjust compressor inlet guide vanes to change the
compressor pressure =
characteristics.
[036] The engine controller 32 constantly monitors the pressure 34 at the exit
plane of the
compressor to anticipate a potential surge condition. The controller thus
receives the sensor data,
calculates the compressor operating characteristics (e.g., pressure ratio and
mass flow,
continuously compares these values to stored threshold values indicative of a
surge condition,
generates a surge byte or other signal if a surge condition is detected, and
activates the surge
recovery protocol. In accordance with the present disclosure, if a speed probe
fault and
continuity fault condition is sensed indicative of a shaft failure, then the
engine controller 32
queries the surge data. If a surge byte or signal has been generated, the
controller 32 determines
that a shaft failure has occurred and issues an emergency fuel shut-off
command and reports the
event to a cockpit display. For the highest degree of certainty that an actual
shaft failure has
been detected, or put another way, for the lowest probability of a false
detection, the controller
32 may only query the surge data if the P30 threshold has also been exceeded.
As illustrated in
FIG. 5, each of the conditions ¨ speed probe fault, continuity fault, P30
threshold and surge ¨ is
fed to an AND logic gate so each condition must exist before the controller 32
senses a shaft
failure. The combined probabilities of a "false positive" for the four
conditions ultimately can
lead to an overall probability of false detection of shaft failure that is
negligible, on the order of
one in a billion. Under certain circumstances, any one fault can be indicative
of a shaft failure,
but the risks of a false positive leading to engine shut down require a much
higher degree of
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certainty, particularly since an improper engine shut-down can itself be cause
for a negative
event.
[037] It can be appreciated that the shaft failure detection protocols
described above can all be
implemented within the existing engine controller 32 as software or firmware
commands. The
fault signals used in the shaft failure detection disclosed herein are all
available and accessed by
the controller 32 for use in controlling the operation and performance of the
engine, either as a
fault bit or as a data value. The shaft failure fault detection protocols
disclosed herein can thus
be implemented as software instructions executed by the controller to read the
respective fault
bits and/or data and perform the AND-gate logic illustrated in FIGS. 3-5. For
some controllers
the fault signals are fault bits that are either "0" when no fault condition
exists or "1" when a
fault exists. In this instance, the controller may incorporate a digital AND-
gate that receives the
fault bytes and generates a "1" only when all the fault bits are "1". The
controller 32 may
continuously monitor the output of the AND-gate and then issue a fuel shut-off
command when
the AND-gate output is "1". With either approach, the only modification
required to an existing
gas turbine engine is in the engine controller 43, optimally only requiring a
software change or
addition to the controller.
[038] The present disclosure should be considered as illustrative and not
restrictive in character.
It is understood that only certain embodiments have been presented and that
all changes,
modifications and further applications that come within the spirit of the
disclosure are desired to
be protected.
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