Note: Descriptions are shown in the official language in which they were submitted.
A MULTI-SPOOL GAS TURBINE ENGINE ARCHITECTURE
TECHNICAL FIELD
[0001] The application relates generally to gas turbine engines and, more
particularly,
to a multi-spool engine architecture.
BACKGROUND OF THE ART
[0002] Gas turbine engines are subject to continued improvements. For
instance,
there is continuing need to improve the thermodynamic cycle performance of
engines
while providing for compact and lightweight engine installations.
SUMMARY
[0003] In one aspect, there is provided a multi-spool gas turbine engine
comprising: a
low pressure (LP) spool; a high pressure (HP) spool, the LP spool and the HP
spool
being independently rotatable about a central axis, the LP pressure spool
comprising an
LP compressor and an LP turbine, the HP spool comprising an HP turbine and an
HP
compressor; and an accessory gear box (AGB) mounted in axial series at one end
of
the engine, the LP compressor being axially positioned between the HP
compressor
and the AGB, the AGB being drivingly connected to the HP spool through the
center of
the LP compressor.
[0004] In another aspect, there is provided a multi-spool gas turbine engine
comprising: a low pressure (LP) spool; a high pressure (HP) spool; the LP
spool and
the HP spool being mounted for rotation about a central axis; the LP pressure
spool
comprising an LP compressor and an LP turbine , the HP spool comprising an HP
turbine and an HP compressor; and an accessory gear box (AGB) drivingly
connected
to the HP spool, the LP compressor being axially positioned between the HP
compressor and the AGB and drivingly connected to the LP turbine via a gear
train.
[0005] In a further aspect, there is provided a turboprop or turboshaft engine
comprising: an output drive shaft configured to drivingly engage a rotatable
load; a low
pressure (LP) spool comprising an LP turbine and an LP compressor, the output
drive
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shaft being drivingly connected to the LP turbine, the LP compressor being
drivingly
connected to the LP turbine via a first gear train; a high pressure (HP) spool
rotatable
independently of the LP spool, and an accessory gearbox (AGB) drivingly
connected to
the HP compressor, the LP compressor being positioned axially between the AGB
and
the HP compressor, and wherein the AGB has an input axis coaxial to a
centerline of
the LP compressor. =
[0006] In a still further aspect, there is provided a reverse flow gas turbine
engine,
comprising: an output drive shaft having a front end configurable to drivingly
engage a
rotatable load; a low pressure (LP) spool including an LP turbine drivingly
engaged to
the output drive shaft, and an LP compressor drivingly connected to the LP
turbine via a
gear train, the LP turbine disposed forward of the LP compressor relative to a
front end
of the output drive shaft; and a high pressure HP spool including an HP
turbine and an
HP compressor drivingly engaged to an HP shaft rotatable independently of the
LP
spool, the HP compressor disposed forward of the LP compressor and in fluid
communication therewith, and the HP turbine disposed aft of the LP turbine and
in fluid
communication therewith.
DESCRIPTION OF THE DRAWINGS
[0007] Reference is now made to the accompanying figures in which:
[0008] Fig. 1 is a schematic cross-sectional view of a multi-spool gas turbine
engine;
[0009] Fig. 2 is an enlarged cross-section of the engine shown in Fig. 1 and
illustrating a gear driven low pressure (LP) compressor and an axially mounted
accessory gearbox (AGB) driven through the center of the LP; and
[0010] Fig. 3 is an enlarged cross-section view similar to Fig. 2 but
illustrating another
possible gear arrangement between the LP shaft and the LP compressor.
DETAILED DESCRIPTION
[0011] Fig. 1 illustrates a gas turbine engine 10 of a type preferably
provided for use
in subsonic flight, generally comprising in serial flow communication an air
inlet 11, a
compressor section 12 for pressurizing the air from the air inlet 11, a
combustor 13 in
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which the compressed air is mixed with fuel and ignited for generating an
annular
stream of hot combustion gases, a turbine section 14 for extracting energy
from the
combustion gases, an exhaust outlet 15 through which the combustion gases exit
the
engine 10. The engine 10 further has a drive output shaft 16 having a front
end
configured to drive a rotatable load (not shown). The rotatable load can, for
instance,
take the form of a propeller or a rotor, such as a helicopter main rotor.
Depending on
the intended use, the engine 10 can be configured as a turboprop engine or a
turboshaft engine. Fig. 1 illustrates a turboprop configuration. The gas
turbine engine 10
has a centerline or longitudinal center axis 17 about which the compressor and
turbine
rotors rotate.
[0012] The gas turbine engine 10 has an axially extending central core which
defines
a gaspath 18 through which gases flow, as depicted by flow arrows in Fig. 1.
The
exemplary embodiment shown in Fig. 1 is a "reverse-flow" engine because gases
flow
through the gaspath 18 from the air inlet 11 at a rear portion thereof, to the
exhaust
outlet 15 at a front portion thereof. This is in contrast to "through-flow"
gas turbine
engines in which gases flow through the core of the engine from a front
portion to a rear
portion. The direction of the flow of gases through the gaspath 18 of the
engine 10
disclosed herein can be better appreciated by considering that the gases flow
through
the gaspath 18 in the same direction D as the one along which an aircraft
engine travels
during flight. Stated differently, gases flow through the engine 10 from a
rear end
thereof towards the output shaft 16.
[0013] It will thus be appreciated that the expressions "forward" and "aft"
used herein
refer to the relative disposition of components of the engine 10, in
correspondence to
the "forward" and "aft" directions of the engine 10 and aircraft including the
engine 10 as
defined with respect to the direction of travel. In the embodiment shown, a
component
of the engine 10 that is "forward" of another component is arranged within the
engine 10
such that it is located closer to output shaft 16 (e.g. closer to the
propeller in a
turboprop application). Similarly, a component of the engine 10 that is "aft"
of another
component is arranged within the engine 10 such that it is further away from
the output
shaft 16.
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[0014] Still referring to Fig. 1, the engine 10 has multiple spools which
perform
compression to pressurize the air received through the air inlet 11, and which
extract
energy from the combustion gases before they exit the gaspath 18 via the
exhaust
outlet 15. More particularly, the illustrated embodiment comprises a low
pressure (LP)
spool 20 and a high pressure (HP) spool 40 mounted for rotation about the
engine
central axis. The LP and HP spools 20, 40 are independently rotatable about
the central
axis 17. The term "spool" is herein intended to broadly refer to drivingly
connected
turbine and compressor rotors and is, thus, not limited to a compressor and
turbine
assembly on a single shaft. As will be seen hereinbelow, it also includes a
rotary
assembly with multiple shafts geared together.
[0015] The LP spool 20 includes at least one component to compress the air
that is
part of the compressor section 12, and at least one component to extract
energy from
the combustion gases that is part of the turbine section 14. More
particularly, the LP
spool 20 has a low pressure turbine 21, also known as a power turbine, which
may
include different number of stages (three stages in the illustrated
embodiment), and
which drives an LP compressor 22 (also referred to as a boost). The low
pressure
turbine 21 drives the low pressure compressor 22, thereby causing the LP
compressor
22 to pressurize incoming air from the air inlet 11. The LP compressor 22 is
disposed
just forward of the air inlet 11. Both the LP turbine 21 and the LP compressor
22 are
disposed along the center axis 17. In the depicted embodiment, both the LP
turbine 21
and the LP compressor 22 include rotatable components having an axis of
rotation that
is coaxial with the center axis 17. It is understood that they can each
include one or
more stages depending upon the desired engine thermodynamic cycle.
[0016] The LP turbine 21 is forward of the LP compressor 22. The LP turbine 21
is
also aft of the exhaust outlet 15. The LP compressor 22 is forward of the air
inlet 11.
This arrangement of the LP turbine 21 and the LP compressor 22 provides for a
reverse-flow engine 10 that has one or more LP compressor stages located at
the rear
of the engine 10, and which are driven by one or more low pressure turbine
stages
located at the front of the engine 10.
[0017] The LP spool 20 further comprises an LP shaft 23 (also known as a power
shaft) coaxial with the center axis 17 of the engine 10. The LP turbine 21 is
drivingly
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connected to the LP shaft 23. The LP shaft 23 allows the LP turbine 21 to
drive the LP
compressor 22 during operation of the engine 10. As will be discussed in
greater details
hereinbelow, the LP shaft 23 may be drivingly connected to the LP compressor
22 via a
gear train to allow the LP compressor 22 to run at a different rotational
speed from the
LP turbine 21. This can provide more flexibility in the selection of design
points for the
LP compressor 22 while at the same time allowing to drivingly connect an
axially
mounted accessory gear box (AGB) to the HP spool 40 centrally through the LP
compressor 22, thereby minimizing the engine envelope in a direction radial
from the
engine axis 17.
[0018] It is understood that the LP shaft 23 is not limited to the
configuration depicted
in Fig. 1. For instance, instead of being provided in the form of a one piece
through
shaft, it could be divided into serially interconnectable sections. Splines or
other suitable
connections could be provided between adjacent shaft sections to transfer
torque from
the LP turbine 21.
[0019] Still referring to Fig. 1, it can be appreciated that the LP shaft 23
also extends
axially forwardly from the LP turbine 21 for driving the output shaft 16. The
LP shaft 23
is drivingly connected to the output shaft 16 via a suitable reduction gear
box (RGB) 31.
A rotatable load, a propeller (not shown) according to the illustrated
example, is
connectable to a front end of the output shaft 16. In this way, the LP turbine
21 can be
used to drive the rotatable load (e.g. the propeller) at a reduced speed
relative to the
speed of the LP turbine 21. In such a configuration, during operation of the
engine 10,
the LP turbine 21 drives the rotatable load such that a rotational drive
produced by the
LP turbine 21 is transferred to the rotatable load via the LP shaft 23, the
RGB 31 and
the output shaft 16 coming out forwardly from the RGB 31. The rotatable load
can
therefore be any suitable component, or any combination of suitable
components, that
is capable of receiving the rotational drive from the LP turbine section 21.
[0020] The RGB 31 processes and outputs the rotational drive transferred
thereto
from the LP turbine 21 via the LP shaft 23 through known gear reduction
techniques.
The RGB 31 allows for the load (e.g. the propeller according to the
illustrated turboprop
example) to be driven at its optimal rotational speed, which is different from
the
rotational speed of the LP turbine 21. The RGB 31 is axially mounted at the
front end of
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the engine. The RGB 31 has an input and an output axis parallel (coaxial in
the
illustrated embodiment) to the central axis 17 of the engine 10.
[0021] In an alternate embodiment where the engine 10 is a turboshaft, the
rotational
load (which may include, but is not limited to, helicopter main rotor(s)
and/or tail rotor(s),
propeller(s) for a tilt-rotor aircraft, pump(s), generator(s), gas
compressor(s), marine
propeller(s), etc.) is driven by the LP turbine 21 via the RGB 31, or the RGB
31 may be
omitted such that the output of the engine 10 is provided directly by the LP
shaft 23.
[0022] The LP shaft 23 with the portions thereof extending forward and aft of
the LP
turbine 21 provides the engine 10 with bidirectional drive. Modularity
criteria for gas
turbine engines may require the use of distinct shaft sections in opposed
axial
directions from the LP turbine 21. The LP shaft sections may be directly or
indirectly
connected together. Alternately, the LP shaft 23 can be integral with a first
segment of
the LP shaft extending axially between the LP compressor 22 and the LP turbine
21,
and a second segment extending between the rotatable load and the LP turbine
21.
Whether the LP shaft 23 is integral or segmented, the LP turbine 21 provides
rotational
drive outputted at each end of the LP shaft 23.
[0023] In light of the preceding, it can be appreciated that the LP turbine 21
drives
both the rotatable load and the LP compressor 22. Furthermore, the rotatable
load,
when mounted to the engine 10, and the LP compressor 22 are disposed on
opposite
ends of the LP turbine 21. It can thus be appreciated that one or more low
pressure
turbine stages are used to drive elements in front of the LP turbine (e.g.
propeller, RGB
31, etc.) as well as to drive elements to the rear of the LP turbine (e.g. LP
compressor
22). This configuration of the LP turbine 21 allows it to simultaneously drive
the
rotatable load and the LP compressor 22.
[0024] Still referring to Fig. 1, the HP spool 40 has at least one component
to
compress the air that is part of the compressor section 12, and at least one
component
to extract energy from the combustion gases that is part of the turbine
section 14. The
HP spool 40 is also disposed along the center axis 17 and includes an HP
turbine 41
(also referred to as the compressor turbine) drivingly engaged (e.g. directly
connected)
to an HP compressor 42 by an HP shaft 43 rotating independently of the LP
shaft 23. In
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the illustrated embodiment, the HP shaft 43 is a hollow shaft which rotates
around the
LP shaft 23. That is the LP shaft 23 extends axially through the HP shaft 43.
Similarly to
the LP turbine 21 and the LP compressor 22, the HP turbine 41 and the HP
compressor
42 can each include one or more stages of rotors, depending upon the desired
engine
thermodynamic cycle, for example. In the depicted embodiment, the HP
compressor 42
includes a centrifugal compressor 42a or impeller and an axial compressor 42b,
both of
which are driven by the HP turbine 41. During operation of the engine 10,
torque is
transferred from HP turbine 41 to the HP compressor 42 via HP shaft 43.
[0025] In the illustrated reverse flow engine configuration, the HP turbine 41
is aft of
the LP turbine 21, and forward of the combustor 13. The HP compressor 42 is
aft of the
combustor 13, and forward of the LP compressor 22. From this arrangement of
the HP
turbine 41 and the HP compressor 42, it can be appreciated that during
operation of the
engine 10, the LP compressor 22 driven by the LP turbine 21 feeds pressurized
air to
the HP compressor 42. Therefore, the pressurized air flow produced by the LP
compressor 22 is provided to the HP compressor 42 and contributes to the work
of both
the LP turbine 21 and the HP turbine 41. This arrangement provides for a
boosted
reverse flow engine.
[0026] It can thus be appreciated that the presence of the above-described LP
and
HP spools 20, 40 provides the engine 10 with a "split compressor" arrangement.
More
particularly, some of the work required to compress the incoming air is
transferred from
the HP compressor 42 to the LP compressor 22. In other words, some of the
compression work is transferred from the HP turbine 41 to the more efficient
LP turbine
21. This transfer of work may contribute to higher pressure ratios while
maintaining a
relatively small number of rotors. In a particular embodiment, higher pressure
ratios
allow for higher power density, better engine specific fuel consumption (SFC),
and a
lower turbine inlet temperature (sometimes referred to as "T4") for a given
power.
These factors can contribute to a lower overall weight for the engine 10. The
transfer of
compression work from the HP compressor 42 to the LP compressor 22 contrasts
with
some conventional reverse-flow engines, in which the high pressure compressor
(and
thus the high pressure turbine) perform all of the compression work.
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[0027] In light of the preceding, it can be appreciated that the LP turbine 21
is the
"low-speed" and "low pressure" turbine section when compared to the HP turbine
41.
The LP turbine 21 is sometimes referred to as the "power turbine". The turbine
rotors of
the HP turbine 41 spin at a higher rotational speed than the turbine rotors of
the LP
turbine 21 given the closer proximity of the HP turbine 41 to the outlet of
the combustor
13. Consequently, the compressor rotors of the HP compressor 42 may rotate at
a
higher rotational speed than the compressor rotors of the LP compressor 22.
[0028] The HP turbine 41 and the HP compressor 42 can have any suitable
mechanical arrangement to achieve the above-described split compressor
functionality.
For example, and as shown in Fig. 1, the HP shaft 43 extends concentrically
about the
LP shaft 23 and is independently rotatable relative thereto. The relative
rotation
between the HP shaft 43 and the LP shaft 23 allow the shafts 23, 43 to rotate
at
different rotational speeds, thereby allowing the HP compressor 42 and the LP
compressor 22 to rotate at different rotational speeds. The HP shaft 43 can be
mechanically supported by the LP shaft 23 using bearings or the like.
[0029] Still referring to the embodiment shown in Fig. 1, the engine 10 also
includes
an accessory gearbox (AGB) 50. The AGB 50 receives a rotational input from the
HP
spool 40 and, in turn, drives accessories (e.g. fuel pump, starter-generator,
oil pump,
scavenge pump, etc.) that contribute to the functionality of the engine 10.
The AGB 50
can be designed with side-facing accessories, top-facing accessories, or rear-
facing
accessories depending on the installation needs.
[0030] According to the illustrated embodiment, the AGB 50 is concentrically
mounted
axially aft of the LP compressor 22 as an axial extension of the engine
envelope. The
axial positioning of the AGB 50 allows minimizing the overall radial envelope
of the
engine as compared to a split compressor or boosted engine having the AGB
mounted
on a side of the engine and connected to the HP spool via a tower shaft. In
the
illustrated embodiment, the AGB is accommodated within the envelope of the
engine in
a plane normal to the central axis 17.
[0031] In the illustrated embodiment, the AGB input drive axis is coaxial to
the LP
compressor centerline and the engine central axis 17. By so aligning the input
axis of
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the AGB 50 relative to the LP compressor centerline, the drive input to the
AGB 50 can
be provided centrally through the center of the LP compressor 22, thereby
eliminating
the need for a tower shaft and an externally mounted gear arrangement.
However,
unlike conventional reverse flow engines (like the well-known PT6 engine
manufactured
by Pratt & Whitney Canada), which do not include a compressor boost, the
presence of
the LP compressor 22 axially between the HP compressor 42 and the AGB 50
physically interferes with the connection of the AGB 50 with the HP spool 40.
In the
illustrated embodiment, this particular problem is overcome by passing the
input drive
shaft 52 of the AGB 50 centrally through the LP compressor 22. As best shown
in Fig.
2, the AGB input shaft 52 extends along the engine central axis 17 through the
central
bore of the LP compressor 22. A first gear train 54 is provided for drivingly
connecting
the AGB input shaft 52 to the HP compressor 42. In the illustrated embodiment,
the first
gear train 54 comprises a geared shaft 56 having a first gear 58 in meshing
engagement with a corresponding gear 60 at a distal end of the AGB drive shaft
52 and
a second gear 62 in meshing engagement with a corresponding gear 64 at the
rear end
of the HP shaft 43 or HP compressor 42. To physically permit this gear drive
connection
between the AGB input shaft 52 and the HP spool 40 through the center of the
LP
compressor 22, a discontinuity between the LP shaft 23 and the LP compressor
22 is
provided and the LP shaft 23 is drivingly connected to the LP compressor 22
via a
second gear train 66. Indeed, if the LP shaft 23 was to extend continuously to
the LP
compressor 22, the AGB input shaft 52 could not be geared to the geared shaft
56,
which is disposed radially outwardly relative to the LP shaft 23.
[0032] According to the illustrated embodiment, the second gear train 66
comprises a
geared shaft 68 comprising a first gear 70 in meshing engagement with a
corresponding gear 72 at the rear end of the LP shaft 23 and a second gear 74
in
meshing engagement with a corresponding gear 76 on a hub portion projecting
axially
forwardly from the LP compressor 22. As mentioned herein above, the gear
connection
between the LP turbine 21 and the LP compressor 22 is also advantageous in
that it
allows to drive the LP compressor at a different speed than the LP turbine. It
can thus
allow for overall thermodynamic cycle performance improvement.
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[0033] According to the illustrated embodiment, the first and second gear
trains 54
and 66 are contained in a central cavity 80 radially inwardly of the gaspath
18 axially
between the HP and LP compressors 42 and 22. The central cavity 80 is bounded
by
the compressor inner gaspath wall 82. This provides for a compact arrangement.
[0034] It is understood that the first and second gear trains 54, 66 could
adopt various
configurations. The configuration illustrated in Figs. 1 and 2 is given for
illustrative
purposes only. For instance, as shown in Fig. 3, the gear train 66' between
the LP shaft
23 and the LP compressor 22 could comprise additional gears to provide a
desired gear
ratio between the LP turbine 21 and the LP compressor 22. According to the
illustrated
alternative, the gear train 66' comprises first and second geared shafts 68a,
68b in
meshing engagement with an intermediate gear 69. The first gear shaft 68a is
provided
with a first gear 70' at a first end thereof in meshing engagement with a gear
72' at the
rear end of the LP shaft 23 and a second gear 74' in meshing engagement with
the
intermediate gear 69. The second gear shaft 68b has a first gear 71 in meshing
engagement with the intermediate gear 69 and a second gear 73 in meshing
engagement with a gear 76' on the LP compressor 22.
[0035] Also according to a non-illustrated alternative, the HP shaft 43 could
extend
centrally through the LP compressor 22 directly into the AGB 50. In this
embodiment,
the AGB input shaft 52 could be viewed as part of the HP shaft 43. The end of
the HP
shaft 43 would carry a gear in meshing engagement with a corresponding gear at
the
AGB input end. According to such an embodiment, the LP shaft 23 could also be
extended axially rearwardly through the LP compressor 22 and the gear train
between
the LP shaft 23 and the LP compressor 22 could be provided within the AGB 50
aft of
the LP compressor 22.
[0036] It can thus be appreciated that at least some of the embodiments of the
engine 10 disclosed herein provide a mechanical architecture of turbomachinery
that
allows for a split compressor system in a compact PT6 type configuration. Such
a split
compressor engine in a reverse flow or through flow configuration may be used
for
aircraft nose installations, as well as for wing installations.
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[0037] The above description is meant to be exemplary only, and one skilled in
the art
will recognize that changes may be made to the embodiments described without
departing from the scope of the invention disclosed. Modifications which fall
within the
scope of the present invention will be apparent to those skilled in the art,
in light of a
review of this disclosure, and such modifications are intended to fall within
the
appended claims.
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