Patent Publication Number: US-10767567-B2

Title: Multi-spool gas turbine engine architecture

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from U.S. provisional patent application Nos. 62/363,956, filed Jul. 19, 2016, 62/363,955, filed Jul. 19, 2016; 62/363,952 filed Jul. 19, 2016; 62/363,949 filed Jul. 19, 2016; 62/363,947 filed Jul. 19, 2016 and U.S. application Ser. No. 15/266,321 filed Sep. 15, 2016, the entire contents of each of which are incorporated by reference herein. 
    
    
     TECHNICAL FIELD 
     The application relates generally to gas turbine engines and, more particularly, to a multi-spool engine architecture. 
     BACKGROUND OF THE ART 
     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 
     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. 
     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. 
     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 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. 
     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 
       Reference is now made to the accompanying figures in which: 
         FIG. 1  is a schematic cross-sectional view of a multi-spool gas turbine engine; 
         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 
         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 
       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 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. 
     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 . 
     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 . 
     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. 
     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. 
     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 . 
     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 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 . 
     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 . 
     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 . 
     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 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 . 
     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 . 
     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 . 
     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 . 
     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 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  42   a  or impeller and an axial compressor  42   b , 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 . 
     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. 
     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. 
     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 . 
     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. 
     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. 
     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 . 
     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 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 &amp; 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 . 
     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. 
     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. 
     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  68   a ,  68   b  in meshing engagement with an intermediate gear  69 . The first gear shaft  68   a  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  68   b  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 . 
     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 . 
     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. 
     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.