Patent Publication Number: US-11041443-B2

Title: Multi-spool gas turbine engine architecture

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 15/351,818 filed Nov. 15, 2016 which claims priority from U.S. application Ser. No. 15/266,321 filed Sep. 15, 2016, the entire contents of which are incorporated by reference herein. 
    
    
     TECHNICAL FIELD 
     The application relates generally to gas turbine engines and, more particularly, to multi-spool gas turbine engines. 
     BACKGROUND OF THE ART 
     Gas turbine engines, particularly those which power aircraft, are often provided with accessories such as electrical generators, pumps and the like, which are required for operation of the engine and an associated aircraft. It is common practice to mechanically connect such accessories to the engine by means of an accessory gearbox which is itself mechanically connected to the rotational shaft of the engine and externally mounted to the engine casing in offset relationship with the engine centerline. It will be readily appreciated that the ease in which the accessories and the gearbox may be removed for repair and maintenance is largely a function of the manner in which the accessories and the gearbox are mounted on the engine which is determinative of the free space surrounding the accessories and gearbox available for the removal and reinstallation thereof for maintenance and servicing. 
     SUMMARY 
     In one aspect, there is provided a multi-spool gas turbine engine comprising: a low pressure (LP) spool and a high pressure (HP) spool rotatable independently of one another about a central axis, the LP spool having an LP compressor and an LP turbine, the HP spool having an HP turbine and an HP compressor; the engine further comprising an accessory gear box (AGB) and a tower shaft drivingly connecting the AGB to the HP spool, the AGB is disposed on the engine so that the central axis extends through the AGB, wherein the LP compressor is axially positioned between the HP compressor and the AGB. 
     In another 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 rotatable about an engine axis and including an LP turbine drivingly engaged to the output drive shaft, and an LP compressor drivingly connected to the LP turbine, 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 rotatable about the engine axis independently of the LP spool, the HP spool including an HP turbine and an HP compressor drivingly engaged to an HP shaft, 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; a tower shaft mechanically coupled to the high pressure spool; an accessory gearbox (AGB) disposed aft of the LP compressor, the engine axis extending through the AGB; and an AGB drive shaft having a first end mechanically coupled to the tower shaft and a second end mechanically coupled to the AGB. 
     In yet 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 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), the central axis extending through the AGB and the LP compressor being axially positioned between the HP compressor and the AGB, the AGB comprising first and second gear trains, the first gear train having a first drive input, the second gear drain having a second drive input, the first and second drive inputs being respectively drivingly connected to the HP and LP spools. 
     In a still further general 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; an accessory gear box (AGB), the central axis extending through the AGB and the LP compressor being axially positioned between the HP compressor and the AGB; and at least one accessory drivingly connected to the AGB, the at least one accessory being mounted on a side of the AGB and having an input axis oriented transversally with respect to the central axis. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       Reference is now made to the accompanying figures in which: 
         FIG. 1  is a schematic cross-sectional view of a gas turbine engine, according to an embodiment of the present disclosure; 
         FIG. 2  is a schematic rear end view of the engine shown in  FIG. 1  illustrating accessories side-mounted to an axially mounted accessory gearbox (AGB) of the engine; 
         FIG. 3  is a schematic cross-sectional view of the gas turbine engine but this time with axially mounted accessories; 
         FIG. 4  is a schematic rear end view of the engine shown in  FIG. 3  and illustrating the accessories projecting axially from a rear axially facing face of the AGB; 
         FIG. 5  is a schematic cross-sectional view of a gas turbine engine having an AGB including a dual gear train with a first drive input from the high pressure (HP) spool and a second drive input from the low pressure (LP) spool centrally through the LP compressor; and 
         FIG. 6  is a schematic cross-sectional view of the engine shown in  FIG. 5  and illustrating sided-mounted accessories projecting from both sides of the AGB. 
     
    
    
     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 gas turbine engine  10 . The engine  10  includes a propeller  16  which provides thrust for flight and taxiing. The gas turbine engine  10  has a longitudinal center axis  17 . 
     The gas turbine engine  10  (sometimes referred to herein simply as “engine  10 ”) has a central core  18  defining a gas path through which gases flow as depicted by flow arrows in  FIG. 1 . The exemplified engine  10  is a “reverse-flow” engine  10  because gases flow through the core  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 core  18  of the engine  10  disclosed herein can be better appreciated by considering that the gases flow through the core  18  in the same direction D as the one along which the engine  10  travels during flight. Stated differently, gases flow through the engine  10  from a rear end thereof towards the propeller  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 the propeller  16 . 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 propeller  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 core  18  via the exhaust outlet  15 . According to the illustrated example, the engine  10  is provided in the form of a multi-spool engine having a low pressure (LP) spool  20  and a high pressure (HP) spool  40  independently rotatable about axis  17 . However, it is understood that a multi-spool engine could have more than two spools. 
     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  which extracts energy from the combustion gases, and which is drivingly engaged (e.g. directly connected) to an LP compressor  22  for pressurizing the air. The LP turbine  21  (also referred to as the power turbine) drives the LP compressor  22 , thereby causing the LP compressor  22  to pressurize the air. Both the LP turbine  21  and the LP compressor  22  are disposed along the axis  17 . In the depicted embodiment, both the LP turbine  21  and the LP compressor  22  are axial rotatable components having an axis of rotation that is coaxial with the center axis  17 . They can include one or more stages, depending upon the desired engine thermodynamic cycle, for example. 
     In the depicted embodiment, the LP spool  20  has a power shaft  23  which mechanically couples the LP turbine  21  and the LP compressor  22 , and extends axially between them. The shaft  23  is coaxial with the central axis  17  of the engine  10 . The shaft  23  allows the LP turbine  21  to drive the LP compressor  22  during operation of the engine  10 . The shaft  23  is not limited to the configuration depicted in  FIG. 1 , and can also mechanically couple the LP turbine  21  and the LP compressor  22  in any other suitable way provided that it transmits a rotational drive from the LP turbine  21  to the LP compressor  22 . For example, the shaft  23  can be combined with a geared LP compressor  22  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. 
     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 compressors located at the rear of the engine  10 , which are driven by one or more LP turbines located at the front of the engine  10 . 
     Still referring to  FIG. 1 , the engine  10  includes an output drive shaft  24 . The drive shaft  24  extends forwardly from the LP turbine  21  and is drivingly engaged thereto. In the illustrated example, the drive shaft  24  is distinct from the power shaft  23  and mechanically coupled thereto to be driven by the LP turbine  21 . In the depicted embodiment, the drive shaft  24  and the power shaft  23  are coaxial and interconnected.  FIG. 1  shows that the power and drive shafts  23 ,  24  are interconnected with a spline  25 . The spline  25 , which can include ridges or teeth on the drive shaft  24  that mesh with grooves in the power shaft  23  (or vice versa), allows for the transfer of torque between the drive shaft  24  and the power shaft  23 . In the depicted embodiment, the power shaft  23  lies at least partially within the drive shaft  24 , such that the spline  25  transfers the rotational drive or torque generated by the LP turbine  21  from the drive shaft  24  to the power shaft  23 . The spline  25  can operate so that the power shaft  23  and the drive shaft  24  rotate at the same rotational speed. Other mechanical techniques can also be used to interconnect the power and drive shafts  23 ,  24 . For example, the power and drive shafts  23 ,  24  can be interconnected by curvic coupling, pins, and interference fits. Other configurations of the drive shaft  24  and the power shaft  23  are also possible. For example, the drive shaft  24  and the power shaft  23  can be a single shaft driven by the LP turbine  21 . The drive shaft  24  therefore transfers the rotational output of the LP turbine  21  in a forward direction to drive another component. 
     A rotatable load, which in the embodiment shown includes the propeller  16 , is mountable to the engine  10 , and when mounted, is drivingly engaged (e.g. directly connected) to the LP turbine  21 , and is located forward 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. 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  21 , as now described. 
     In the embodiment shown, a reduction gearbox  31  (sometimes referred to herein simply as “RGB  31 ”) is mechanically coupled to a front end of the drive shaft  24 , which extends between the RGB  31  and the LP turbine  21 . The RGB  31  processes and outputs the rotational drive transferred thereto from the LP turbine  21  via the drive shaft  24  through known gear reduction techniques. The RGB  31  allows for the propeller  16  to be driven at its optimal rotational speed, which is different from the rotational speed of the LP turbine  21 . 
     The propeller  16  is mechanically coupled to the output of the RGB  31  via a propeller shaft  35 . The propeller shaft  35  allows the rotational drive outputted by the RGB  31  during operation of the engine  10  to be transferred to the propeller  16  to provide propulsion during flight. In an alternate embodiment where the engine  10  is a turboshaft, the propeller  16  is omitted and 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 propeller  16  and RGB  31  are omitted such that the output of the engine  10  is provided by the output drive shaft  24 . 
     The drive shaft  24  extending forward of the LP turbine  21  and the power shaft  23  extending aft of the LP turbine  21  provide the engine  10  with bidirectional drive. Modularity criteria for gas turbine engines may require the use of distinct shafts  23 ,  24  that are directly or indirectly connected together. Alternately, the power shaft  23  and the drive shaft  24  can be integral with one another, with a first segment of the integral output shaft extending 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 power shaft  23  is integral with the drive shaft  24  or distinct therefrom, the LP turbine  21  provides rotational drive outputted at each end of the power 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 axially on opposite ends of the LP turbine  21 . It can thus be appreciated that one or more low pressure turbines are used to drive elements in front of the low pressure turbines (e.g. propeller  16 , RGB  31 , etc.) as well as to drive elements to the rear of the low pressure turbines (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 , if desired. As will be discussed in greater detail below, this arrangement of the rotatable load, the LP turbine  21 , and the LP compressor  22  can contribute to improving the thermodynamic efficiency of the engine  10 . 
     Still referring to  FIG. 1 , the HP spool  40  with 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 axis  17  and includes an HP turbine  41  drivingly engaged (e.g. directly connected) to a high pressure compressor  42  by an HP shaft  43  rotating independently of the power shaft  23 . Similarly to the LP turbine  21  and the LP compressor  22 , the HP turbine  41  and the HP compressor  42  can include various stages of axial rotary components. 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 , the HP turbine  41  drives the HP compressor  42 . 
     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 . 
     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 “T 4 ”) 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 when compared to the HP turbine  41 . The LP turbine  21  is sometimes referred to as a “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 engine  10  shown in  FIG. 1  is thus a “two-spool” engine  10 . 
     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 spool  40  includes a high pressure shaft  43  extending between the HP compressor  42  and the HP turbine section  41 . The high pressure shaft  43  is coaxial with the power shaft  23  and rotatable relative thereto. The relative rotation between the high pressure shaft  43  and the power 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 power shaft  23  using bearings or the like. In the depicted embodiment, the power shaft  23  is at least partially disposed within the HP shaft  43 . 
     The split compressor arrangement also allows bleed air to be drawn from between the HP compressor  42  and the LP compressor  22 . More particularly, in the embodiment of  FIG. 1 , the engine  10  includes an inter-stage bleed  44  port or valve that is aft of the HP compressor  42  and forward of the LP compressor  22 , which may provide for increased flexibility in the available bleed pressures. In a particular embodiment, the bleed pressure design point of the inter-stage bleed  44  is selected based on the pressure ratio of the LP compressor  22 , which runs independently from the HP compressor  42 . For operability, variable inlet guide vanes (VIGV) and variable guide vanes (VGV) can be used on the LP compressor  22  and at the entry of the HP compressor  42 , together with the inter-stage bleed  44 . 
     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 output 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  is axially aft of the air inlet  11 . More particularly, in the illustrated embodiment, the AGB  50  is mounted centrally relative to the engine axis  17  at the rear end of the engine  10 . As can be best appreciated from  FIG. 2 , by axially mounting the AGB  50  in series with the LP and HP spools  20  and  40  instead of side-mounting the AGB, the AGB  50  may be substantially accommodated with the engine envelope as schematically represented by circle C in  FIG. 2 . In this way, the engine  10  can be packaged as “straighter cylinder” engine, which may be advantageous in some installations. The in-line or axial mounting of the AGB instead of conventional side-mounting configuration allows minimizing the diameter of the engine envelope. It also allows to simplify the design of the AGB (cost, weight) compared to conventional side-mounted AGBs. 
     It is understood that the in-line mounting of the AGB  50  is not strictly limited to a coaxial or centralized mounting of the AGB  50  as shown in  FIG. 1 . For instance, the engine axis  17  could extend through the AGB  50  but be offset from the center thereof. The AGB  50  would nevertheless be located axially aft of the LP compressor  22  and the air inlet  11  along the axis  17  and be mounted to an axially facing surface of the engine. 
     According to the illustrated embodiment, the AGB is drivingly connected to the HP spool  40 . To get around the LP compressor  22 , which is physically disposed between the HP compressor and the AGB, an HP offset drive may be used. The HP offset drive may include a tower shaft  51  that is mechanically coupled to a rear of the HP shaft  43  and driven thereby. The tower shaft extends from the HP spool  40  in a direction away from the engine axis  17  for connection with an accessory gear box drive shaft  52  having a first geared end  52 A mechanically coupled to the tower shaft  51 , and a second geared end  52 B mechanically coupled to the AGB  50 . As can be appreciated from  FIG. 1 , the AGB drive shaft  52  has a main axial component parallel to the engine axis  17  to bridging the tower shaft to the AGB  50 . 
     In the depicted embodiment, the accessory gearbox drive shaft  52  extends across the air inlet  11 . This configuration of the accessory gearbox drive shaft  52  can take different forms. For example, it can be located outside the air inlet  11 , or may be placed within the air inlet  11  along a strut of the air inlet  11 . It can thus be appreciated that the second end  52 B of the accessory gearbox drive shaft  52  meshes with an input gear of the AGB  50  to drive the AGB  50  across the air inlet  11 . 
     During operation of the engine  10 , the high pressure shaft  43  transmits a rotational drive to the tower shaft  51 , which, in turn, drives the accessory gearbox drive shaft  52  to thereby drive the accessories A ( FIG. 2 ) connected to the AGB outputs. As shown in  FIG. 2 , the accessories A are mounted on opposed lateral sides of the AGB  50  with their respective input axes AA transversal to the engine axis  17 . The side-mounting of the accessories A on the in-line mounted AGB  50  facilitates access to the accessories A during on-wing maintenance operations. It also contributes to reduce the engine overall length. It may also simplify cooling line routing for some accessories, such as the starter and the generator (accessories closer to engine cowling). 
     As shown in  FIGS. 3 and 4 , the accessories A could also be mounted on the rear axially facing side of the AGB  50  with respective input axes of the accessories extending parallel to the engine axis  17 . Such an in-line mounting of the AGB and the accessories may be suitable in some applications. 
     As shown in  FIGS. 5 and 6 , the AGB  50 ″ may have more than one drive input. For instance, in addition to the drive input provided by the HP spool  40  via the tower shaft  51  and the AGB drive shaft  52 , the power or LP shaft  23  could be extended aft of the LP compressor  22  to provide a second drive input to the AGB  50 ″, thereby providing a secondary drive input to the AGB along the engine axis  17 . The provision of a second driving source for the AGB allows reducing the load on the HP spool  40 . Indeed, the load required to drive the AGB could be shared by both the LP and the HP spools  20 ,  40 . The HP spool  40  and the LP spool  20  can be used to jointly drive a single gear train or to provide independent drive to individual gear trains of a multi-gear train arrangement. 
     For instance, according to the embodiment shown in  FIGS. 5 and 6 , the AGB  50 ″ is a dual gear train comprising first and second gear trains  50   a ″ and  50   b ″. The first gear train  50   a ″ is drivingly connected to the HP spool  40  via tower shaft  51  and the AGB input shaft  52 . The second gear train  50   b ″ is drivingly connected to the LP spool  20  via the LP shaft  23 . The first and second gear trains  50   a ″ and  50   b ″ can be respectively drivingly connected to first and second groups of accessories A. For instance, the first gear train  50   a ″, which is driven by the HP spool  40 , may be drivingly connected to the main accessories, such as the starter, the fuel control unit, and the oil pump. The second gear train  50   b ″, which is driven by the LP spool  20 , may be drivingly connected to secondary accessories, such as an air vacuum pump and an electric generator. The first and second gear trains  50   a ″,  50   b ″ are provided with an output connection for the associated accessories. 
     As shown in  FIG. 6 , the accessories can be side-mounted to the AGB  50 ″ as described herein above with respect to  FIGS. 1 and 2 . However, the accessories could also be axially mounted as shown in  FIGS. 3 and 4 . Alternatively, some of the accessories A could be side-mounted while others are axially mounted. Various accessories mounting combinations are contemplated irrespective of the AGB configuration (single gear train or dual gear train). 
     The accessories could also be selectively driven by one or both of the HP and LP spools  40 ,  20 . A clutch or the like could be provided to effectively drivingly connect the LP spool  20  and the HP spool  40  to the AGB  50  or  50 ″. Also, accessories could be driven by the HP offset drive shaft arrangement only or some could be driven by either the HP or LP spools  40 ,  20 . 
     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.