TURBO ENGINE WITH RECUPERATED CYCLE

Description

Technical field of the invention

[0001] The present invention relates to a recovered cycle turboshaft engine for an aircraft. Technical background

[0002] Application FR2962487A1 on behalf of the applicant describes a first recovered cycle turboshaft engine architecture.

[0003] Such a turboshaft engine architecture includes, from front to back, an air intake, a compressor, a combustion chamber, an expansion turbine (linked turbine or high-pressure turbine), a power turbine (free turbine or low-pressure turbine), and finally an exhaust nozzle.

[0004] The compressor, combustion chamber and expansion turbine form a gas generator in which the compressor rotor is mechanically driven by the expansion turbine rotor via a drive shaft.

[0005] The power turbine is independent of the gas generator, and is intended to drive a power take-off (or power take-off) of the turboshaft engine to which, for example, the aircraft's propulsion system(s) are connected.

[0006] Air entering through the air intake is compressed by the compressor, then injected into the combustion chamber to be mixed with fuel. The air / fuel mixture is burned and expanded in the expansion turbine and then in the power turbine before being expelled from the turbocharger through the exhaust nozzle.

[0007] The turbocharger is said to be "recovered cycle" because the residual thermal energy of the exhaust gases is recovered via heat exchangers placed in the exhaust nozzle, then reinjected into the turbocharger to optimize its efficiency.

[0008] US20090277154A1 describes a second recovered cycle turboshaft engine architecture.

[0009] This second architecture is unique in that its combustion chamber is located at the rear end of the turboshaft engine. The expansion and power turbines are positioned axially between the compressor and the combustion chamber, and the exhaust gases are expelled from the turboshaft engine via two V-shaped nozzles positioned approximately in the middle of the engine. In this architecture, the air compressed by the compressor is routed to the rear into the combustion chamber through supply ducts arranged around the turbines, with the compressed air being diverted before entering the combustion chamber.

[0010] The thermal energy recovered by the heat exchangers is used here to preheat the compressed air coming out of the compressor before it enters the combustion chamber.

[0011] Such an architecture makes it possible to improve the performance of the turbocharger since the amount of fuel to be injected to reach operating temperatures is less than that required in the case of a conventional cycle turbocharger (i.e. a turbocharger in which the compressed air coming out of the compressor directly feeds the combustion chamber without being preheated).

[0012] The power take-off is driven by the power turbine via a reducer arranged axially between the compressor and the turbines.

[0013] Compared to the first architecture described above, this second architecture, described in US20090277154A1, has the advantage of crossing the airflow from the compressor, which is delivered to the combustion chamber via two supply ducts, with the hot gas flow from the two exhaust nozzles. It is therefore particularly well-suited to the integration of heat exchangers for the heat recovery cycle, while minimizing the required duct lengths and thus the mass and size of the heat recovery system.

[0014] However, the second architecture has drawbacks.

[0015] Indeed, firstly, the aforementioned reducer is generally associated with an accessory box which is intended to transmit mechanical power taken from the turbines to various accessories of the turbomotor such as a pump, an alternator-starter, an air / oil separator, etc.

[0016] The gearbox and accessory box are bulky, which significantly limits the available space around the compressor and turbines, and therefore the possibilities for installing the heat exchanger(s) intended for the recovery of residual thermal energy from the exhaust gases.

[0017] Secondly, the arrangement of the reducer almost in the middle of the turbomotor requires a significant offset of the power take-off to bypass in particular the compressor and the supply ducts, to the detriment of the mass and the balance (or mass distribution) of the turbomotor.

[0018] The objective of the present invention is therefore to provide a simple, effective, and economical solution to address the aforementioned drawbacks. Prior art also includes document RU2563079C1. Summary of the invention

[0019] The invention thus proposes a turboshaft engine for an aircraft, the turboshaft engine comprising: a gas generator comprising a compressor, a combustion chamber and an expansion turbine, the compressor and the expansion turbine extending along the same longitudinal axis X and being mechanically connected to each other, the combustion chamber being arranged axially at a rear end of the turbomachine; a power turbine arranged axially between the compressor and the expansion turbine, the power turbine driving a power take-off via a reduction gear; a heat exchanger comprising: a first circuit having an inlet connected to an outlet of the compressor, and an outlet connected to an inlet of the combustion chamber, and a second circuit having an inlet connected to an outlet of the power turbine, characterized in that the compressor comprises a first shaft driven in rotation by a second shaft of the expansion turbine via a transmission mechanism, said transmission mechanism and said reducer forming part of a transmission housing which is disposed axially at a front end of the turbomotor, so that the compressor is axially disposed between the transmission housing and the power turbine.

[0020] Such a transmission mechanism allows the rotational motion initiated by the expansion turbine to be transmitted to the compressor, while also allowing its speed to be modified, thus enabling the compressor to operate within the desired range. For example, it is possible to increase the speed of the compressor's first shaft, thereby minimizing the number of compression stages required, thus reducing the axial size of the turboshaft engine.

[0021] This type of transmission housing arrangement (reducer and transmission mechanism) frees up space around the compressor and turbines for installing the heat exchanger. It is now possible to consider installing an annular heat exchanger around the X-axis.

[0022] Such an arrangement of the transmission housing also simplifies access to and operation of the power take-off.

[0023] This type of turbocharger is called a "recovered cycle" turbocharger because the residual thermal energy of the exhaust gases is recovered via the heat exchanger to preheat the compressed air exiting the compressor before it enters the combustion chamber. This reclaimed cycle architecture improves turbocharger performance since the amount of fuel injected to reach operating temperatures is less than that required in a conventional cycle turbocharger.

[0024] The turboshaft engine according to the invention may comprise one or more of the following features, taken individually or in combination with each other: the power turbine includes a third shaft, the first, second and third shafts being coaxial with said longitudinal axis X; the third shaft of the power turbine is arranged radially between the first and second shafts; said transmission mechanism has a transmission ratio greater than 1, so that the speed of the first shaft of the compressor is greater than the speed of the second shaft of the expansion turbine; said transmission mechanism is a gear mechanism; said transmission mechanism includes a first toothed wheel fixed to the first shaft of the compressor and a second toothed wheel fixed to the second shaft of the expansion turbine, said reducer including a third toothed wheel fixed to a third shaft of the power turbine, the third toothed wheel of the reducer being arranged axially between said first and second toothed wheels of the transmission mechanism;The transmission housing has, from front to back, the second gear, the third gear, and the first gear; said power take-off is coaxial or vertically aligned with said longitudinal axis X; the first compressor shaft is guided in rotation via a first bearing and a second bearing disposed in the transmission housing; said second expansion turbine shaft is guided in rotation via a third bearing disposed in the transmission housing, and a fourth bearing disposed either between the expansion and power turbines or at a rear end of said second shaft located on the side opposite the power turbine; said third power turbine shaft is guided in rotation via a fifth bearing disposed in the transmission housing, and a sixth bearing disposed axially between the compressor and the power turbine; the heat exchanger is annular around said longitudinal axis X;the heat exchanger is arranged at least partly around the power turbine; the heat exchanger is sectorized and comprises at least two sectors placed circumferentially end to end or circumferentially apart from each other, each sector of the heat exchanger comprising a sub-inlet of the first circuit connected to the outlet of the compressor; the heat exchanger is monobloc; the turboshaft engine comprises at least one first bypass duct having an inlet connected to the outlet of the compressor and an outlet connected to the inlet of the combustion chamber, so that the first bypass duct supplies the combustion chamber with compressed air exiting the compressor without passing through the heat exchanger;The turboshaft engine includes at least one second bypass duct having an inlet connected to the outlet of the power turbine and an outlet connected to the nozzle, so that the second bypass duct supplies the nozzle with exhaust gas exiting the power turbine without passing through the heat exchanger; the reduction gear is part of a transmission housing, the transmission housing comprising an accessory box for transmitting mechanical power taken from the compressor and / or the expansion turbine and / or the power turbine to various accessories of the turboshaft engine; the heat exchanger includes a retaining shield for the moving blades of the power turbine configured to contain said moving blades in the event of overspeed of the power turbine;The heat exchanger includes a sound-dampening layer. The present invention also relates to an aircraft, preferably a single-engine helicopter, comprising a turboshaft engine as described above. Brief description of the figures

[0025] The invention will be better understood and other details, features and advantages of the invention will become more apparent upon reading the following description, given by way of non-limiting example and with reference to the accompanying drawings in which: [ Fig.1 ] there figure 1 is a schematic view of a turboshaft engine according to the invention; [ Fig. 2 ] there figure 2 is a detailed view of a first variant of the embodiment; [ Fig.3 ] there figure 3 is a detailed view of a second embodiment variant; [ Fig. 4 ] there figure 4 is a detailed view of a third embodiment variant; [ Fig. 5 ] there figure 5is a detailed view of a fourth embodiment variant. Detailed description of the invention

[0026] On the figure 1 A turboshaft engine 1 for an aircraft 2 is schematically represented. Aircraft 2 is preferably a single-engine helicopter, and in particular a light single-engine helicopter whose maximum take-off weight (known by the English acronym MTOW for "Maximum Take-Off Weight") does not exceed 3175 kg (or 7000 lbs).

[0027] Turboshaft 1 comprises: a gas generator 3 comprising a compressor 4, a combustion chamber 5 and an expansion turbine 6, the compressor 4 and the expansion turbine 6 being mechanically connected to each other, the combustion chamber 5 being arranged axially at a rear end 7 of the turboshaft engine 1; a power turbine 8 arranged axially between the compressor 4 and the expansion turbine 6, the power turbine 8 driving in rotation a power take-off 9 via a reduction gear 10; a heat exchanger 11 comprising: a first circuit 12 having an inlet 13 connected to an outlet 14 of the compressor 4, and an outlet 15 connected to an inlet 16 of the combustion chamber 5, and a second circuit 17 having an inlet 18 connected to an outlet 19 of the power turbine 8.

[0028] According to the invention, the compressor 4 comprises a first shaft 22 driven in rotation by a second shaft 23 of the expansion turbine 6 via a transmission mechanism 24, said transmission mechanism 24 and said reducer 10 being part of a transmission housing 25 which is axially disposed at a front end 20 of the turbomotor 1, so that the compressor 4 is axially disposed between the transmission housing 25 and the power turbine 8.

[0029] Such a transmission mechanism 24 makes it possible to transmit the rotational movement initiated by the expansion turbine 6 to the compressor 4 while having the possibility of modifying its speed, so as to operate the compressor 4 in the desired operating range.

[0030] Such an arrangement of the transmission housing 25 makes it possible in particular to free up space around the compressor 4 and the turbines 6, 8 to install the heat exchanger 11, but also to simplify access to and drive of the power take-off 9.

[0031] Such a turboshaft engine architecture 1 is called a "recovered cycle" because the residual thermal energy of the exhaust gases is recovered via the heat exchanger 11 to heat the compressed air exiting the compressor 4 before it enters the combustion chamber 5. Such a recovered cycle architecture makes it possible to improve the performance of the turboshaft engine 1 since the amount of fuel to be injected to reach the operating temperatures is less than that required in the case of a conventional cycle turboshaft engine.

[0032] The turbomotor 1 is defined along a longitudinal axis X which corresponds to the axis of rotation of the shafts 22, 23, 26 respectively of the compressor 4 and the turbines 6, 8 of the turbomotor 1.

[0033] By convention, in the present application, the terms "front" and "rear" define the axial positions of the elements of the turbomotor 1 relative to each other, knowing that the transmission housing 25 is arranged axially at a front end 20 of the turbomotor 1, and the combustion chamber 5 is arranged axially at a rear end 7 of the turbomotor 1.

[0034] The term "axial" or "axially" means any direction parallel to the X axis of the turbomotor 1, and "radial" or "radially" means any direction perpendicular to the X axis of the turbomotor 1.

[0035] Similarly, by convention in this application, the terms "internal" and "external" associated with turboshaft 1 are defined radially with respect to the X-axis of turboshaft 1.

[0036] As illustrated on the figure 1 The compressor 4 is supplied with air via an air inlet 21 and includes a first shaft 22 that can rotate about the X-axis. The compressor 4 may include one or more compression stages, each stage being either axial or centrifugal. The rotors of each stage (wheel or impeller) are rotationally fixed to the first shaft 22.

[0037] Advantageously, the last stage of compressor 4 is a centrifugal stage.

[0038] The combustion chamber 5 is axially arranged at the rear end 7 of the turboshaft engine 1. The chamber 5 is supplied with compressed and heated air (via the heat exchanger 11), and with fuel via one or more injectors, depending on the combustion chamber technology used. The air / fuel mixture is burned by one or more ignition devices. The combustion chamber 5 may be a split-combustion chamber, a direct-flow chamber, or a reverse-flow chamber.

[0039] A combustion chamber with separate pots offers the advantage of low production costs, provided the number of injectors is reduced (it can be limited to a single injector), and of a compact design. This allows, for example, the integration of devices aimed at minimizing pollutant emissions at the rear of the chamber. Such a combustion chamber also has a reduced-sized casing, which minimizes the required cooling air and simplifies the implementation of Lean Premix Prevaporized (LPP) technology.

[0040] A direct-flow combustion chamber also has the advantage of being compact, which minimizes the required cooling air. The amount of cooling air needed for a direct-flow combustion chamber is less than that required for a reverse-flow combustion chamber.

[0041] A reverse flow combustion chamber (or return combustion chamber) has the advantage of minimizing the axial footprint of the turbomachine and freeing up a central space in which it is possible to integrate one or more elements such as a shaft guide bearing 23 of the expansion turbine 6.

[0042] The expansion turbine 6 (also called the high-pressure turbine) includes a second shaft 23 that rotates about the X-axis. The expansion turbine 6 may have one or more expansion stages, each stage being either axial or centripetal. The rotors of each stage (wheel or impeller) are rotationally fixed to the second shaft 23. The exhaust gases from the combustion chamber 5 are expanded in the expansion turbine 6 and then in the power turbine 8.

[0043] The power turbine 8 (also called a free turbine or low-pressure turbine) is independent of the gas generator 3 and includes a third shaft 26 that rotates about the X-axis. The power turbine 8 may have one or more expansion stages, each stage being either axial or centripetal. The rotors of each stage (wheel or impeller) are rotationally fixed to the third shaft 26. The exhaust gases exiting the power turbine 8 first pass through the heat exchanger 11 (specifically the second circuit) before being discharged into the external environment via an exhaust nozzle 27.

[0044] The power take-off 9 (also called the power take-off) is located at the output of the reduction gear 10 and drives, for example, one or more propellers of the aircraft 2, or an alternator-generator for electrical generation. When the aircraft 2 is a helicopter, the power take-off 9 can drive a main rotor via a main gearbox 28 (known by the acronym BTP) and a tail rotor (also known by the acronym RAC for tail rotor) via a tail gearbox (known by the acronym BTA) (not shown).

[0045] As illustrated on the figure 1 , the turboshaft engine 1 comprises, from front to back, the transmission housing 25 (reducer 10 and transmission mechanism 24), the compressor 4, the power turbine 8, the expansion turbine 6 and the combustion chamber 5.

[0046] The gas generator 3 forms the high-pressure body of the turbomotor 1, and the power turbine 8 and the reducer 10 form the low-pressure body of the turbomotor 1.

[0047] As indicated above, the heat exchanger 11 recovers the residual thermal energy from the exhaust gases to heat the compressed air exiting the compressor 4 before it enters the combustion chamber 5.

[0048] The heat exchanger 11 can be, for example, a tube heat exchanger, a plate heat exchanger, or a finned heat exchanger. The heat exchanger 11 can notably be manufactured using additive manufacturing.

[0049] The inlet 13 of the first circuit 12 of the heat exchanger 11 is connected to an outlet 14 of the compressor 4 via a front supply 29 comprising one or more conduits. The front supply 29 may include a radial diffuser and an axial diffuser (also called a rectifier), the outlet 14 of the compressor 4 being connected to an inlet of the radial diffuser and the inlet 13 of the first circuit 12 being connected to an outlet of the axial diffuser.

[0050] The outlet 15 of the first circuit 12 of the heat exchanger 11 is connected to an inlet 16 of the combustion chamber 5 via a rear feed 30 comprising one or more conduits. This rear feed 30 may include diffusion grids to control the Mach number and vortex of the flow supplying the combustion chamber 5 via its inlet 16.

[0051] The inlet 18 of the second circuit 17 of the heat exchanger 11 is connected to the outlet 19 of the power turbine 8 via one or more internal conduits of the exhaust nozzle 27.

[0052] The outlet 33 of the second circuit 17 of the heat exchanger 11 is connected to the external environment via one or more external conduits of the exhaust nozzle 27.

[0053] Advantageously, as illustrated on the figure 1 The heat exchanger 11 is annular around the X axis. Such a configuration is possible due to the positioning of the transmission housing 25 at the front end 20 of the turboshaft engine 1.

[0054] Advantageously, the heat exchanger 11 is axisymmetric with respect to the X axis, so as to obtain a balanced turboshaft engine.

[0055] Advantageously, the heat exchanger 11 is disposed at least partly around the compressor 4 and / or the power turbine 8 and / or the expansion turbine 6, and preferably at least partly around the power turbine 8.

[0056] According to the embodiment illustrated on the figure 1 The heat exchanger 11 is annular and extends continuously around the X-axis. In such a configuration, the heat exchanger 11 can be a single piece (or a single unit), and preferably manufactured using additive manufacturing (e.g., selective powder bed fusion). Additive manufacturing has the advantage of being able to produce complex shapes.

[0057] The heat exchanger 11 can be divided into sectors and comprise at least two sectors 31 placed circumferentially end-to-end or circumferentially separated from each other, each sector 31 of the heat exchanger 11 comprising a sub-inlet 32 ​​of the first circuit 12 connected to the outlet 14 of the compressor 4. The first circuit 12 can be subdivided into a plurality of sub-first circuits, each sector 31 potentially comprising one sub-first circuit. When the upstream supply 29 comprises a radial diffuser and an axial diffuser, each sub-inlet 32 ​​of the first circuit 12 can be connected to a sub-outlet of the axial diffuser. A multi-outlet axial diffuser (also called a "pipe diffuser") can be used in this configuration. Each sector 31 can be a single piece (or a single unit), and preferably manufactured using additive manufacturing (e.g., selective powder bed fusion).The advantage of having a sectorized heat exchanger 11 is to simplify maintenance and allow the replacement of a defective sector 31 independently of the others.

[0058] According to the first variant of the embodiment illustrated on the figure 2 , the heat exchanger 11 is sectorized and comprises an annular row of twelve sectors 31 placed circumferentially end to end, each sector 31 of the heat exchanger 11 comprising a sub-inlet 32 ​​of the first circuit 12 connected to the outlet 14 of the compressor 4.

[0059] According to the second embodiment illustrated on the figure 3 , the heat exchanger 11 is sectorized and comprises six sectors 31 distributed regularly around the X axis, two successive sectors 31 being circumferentially distant from each other, each sector 31 of the heat exchanger 11 comprising a sub-input 32 of the first circuit 12 connected to the output 14 of the compressor 4.

[0060] Advantageously, the turboshaft engine 1 includes at least one first bypass duct 34 having an inlet connected to the outlet 14 of the compressor 4 and an outlet connected to the inlet 16 of the combustion chamber 5, so that the first bypass duct 34 directly supplies the combustion chamber 5 with compressed air coming out of the compressor 4 without going through the heat exchanger 11 (or by bypassing the exchanger).

[0061] The turboshaft engine 1 can obviously include several first conduits 34 distributed or not regularly around the X axis, the first conduits 34 being able to have identical or distinct dimensional and geometric characteristics.

[0062] The first duct(s) 34 are used, in particular, to supply the combustion chamber 5 during deceleration phases, when it is necessary to send the coolest possible air to the high-pressure turbine and thus avoid being negatively impacted by the heat stored in the heat exchanger. The need for deceleration is crucial for the maneuverability of a helicopter's turboshaft engine(s); otherwise, there is a risk of over-revving the main rotor. The first duct(s) 34 can be fitted with valves to control the flow of air and the supply rate.

[0063] According to the embodiment illustrated on the figure 1The turboshaft engine 1 includes a single first bypass conduit 34 which is arranged around the heat exchanger 11. When the heat exchanger 11 is sectorized, one or more first conduits 34 can pass through the exchanger 11 via the circumferential space defined between two successive sectors 31.

[0064] Advantageously, the turboshaft engine 1 includes at least one second bypass duct 35 having an inlet connected to the outlet 19 of the power turbine 8 and an outlet connected to the nozzle 27, so that the second bypass duct 35 directly supplies the nozzle 27 with exhaust gas exiting the power turbine 8 without passing through the heat exchanger 11 (or by bypassing the exchanger).

[0065] The turboshaft engine 1 can obviously include several secondary conduits 35 distributed regularly or not around the X axis, the secondary conduits 35 being able to have identical or distinct dimensional and geometric characteristics.

[0066] The second duct(s) 35 are used, in particular, to expel exhaust gases more quickly while minimizing pressure losses, thereby increasing the turboshaft engine's power output. This additional power is especially useful when the turboshaft engine is operating under high acceleration, such as during takeoff, where it must rapidly reach Maximum Takeoff Power (MTOP). The second duct(s) 35 can be fitted with valves to control the flow of exhaust gases and the exhaust flow rate.

[0067] According to the embodiment illustrated on the figure 1, the turboshaft engine 1 includes a single second bypass conduit 35 which extends radially outwards from the outlet 19 of the power turbine 8. When the heat exchanger 11 is sectorized, one or more second conduits 35 may pass through the exchanger 11 via the circumferential space defined between two successive sectors 31.

[0068] Advantageously, the transmission housing 25 includes an accessory gearbox 36 for transmitting mechanical power from the compressor 4 and / or the expansion turbine 6 and / or the power turbine 8 to various accessories of the turboshaft engine 1. These accessories include, for example, a pump, a starter-alternator, an air / oil separator, etc. Positioning the accessory gearbox 36 at the front end 20 of the turboshaft engine 1 optimizes the arrangement of the accessories relative to one another and maximizes their number.

[0069] Advantageously, the heat exchanger 11 includes a retaining shield 37 for the moving blades of the power turbine 8 configured to contain the moving blades in the event of overspeed of the power turbine 8. The blades of the power turbine 8 each include a frangible section which is configured to break when the power turbine 8 is overspeeding; these frangible sections form a protective device (called in English "blade shedding") allowing the rotor of the power turbine 8 to be stopped in the event of overspeed.

[0070] The retention shield 37 could be configured to also contain the movable blades of the expansion turbine 6 in the event of overspeed of the expansion turbine 6.

[0071] The presence of the heat exchanger 11 forms an obstacle to the noise produced by the turbomotor, the exchanger 11 thus acting as an acoustic attenuator.

[0072] Advantageously, as illustrated on the figure 1 The heat exchanger 11 may further include a sound attenuation layer 38. This sound attenuation layer 38 may, for example, include a honeycomb structure. Advantageously, as illustrated in the figure 1 , the first, second and third trees 22, 23, 26 are coaxial with the X axis.

[0073] As illustrated on the figure 1 , the third shaft 26 of the power turbine 8 is arranged radially between the first and second shafts 22, 23.

[0074] As indicated above, according to the invention, the first shaft 22 of the compressor 4 is driven in rotation by the second shaft 23 of the expansion turbine 6 via a transmission mechanism 24.

[0075] The transmission mechanism 24 can have a fixed or variable transmission ratio, this transmission ratio being different from 1, namely either less than 1 or greater than 1.

[0076] When the transmission ratio is less than 1, the transmission mechanism 24 is a reducer (or multiplier), so that the speed of the first shaft 22 of the compressor 4 is less than that of the second shaft 23 of the expansion turbine 6.

[0077] When the transmission ratio is greater than 1, the transmission mechanism 24 is a multiplier, so that the speed of the first shaft 22 of the compressor 4 is greater than that of the second shaft 23 of the expansion turbine 6.

[0078] Advantageously, as illustrated on the figure 1, the transmission mechanism 24 has a fixed transmission ratio which is greater than 1, so that the speed of the first shaft 22 of the compressor 4 is greater than the speed of the second shaft 23 of the expansion turbine 6.

[0079] This configuration allows for finding the best compromise between the speed of compressor 4 and the speed of expansion turbine 6, thereby maximizing the efficiency of both components. Advantageously, as illustrated in the figure 1 The transmission mechanism 24 is a gear mechanism (or gear train). As indicated above, this gear mechanism 24 has a fixed transmission ratio greater than 1, so that the speed of the first shaft 22 of the compressor 4 is greater than the speed of the second shaft 23 of the expansion turbine 6.

[0080] The 24-gear mechanism may include one or more reduction stages. The 24-gear mechanism may also include one or more epicyclic gear trains. Epicyclic gear trains have the advantage of being able to achieve high reduction or multiplication ratios while remaining compact.

[0081] More specifically, as illustrated on the figure 1 The gear mechanism 24 includes a first gear 39 fixed to the first shaft 22 of the compressor 4 and a second gear 40 fixed to the second shaft 23 of the expansion turbine 6. The reducer 10 includes a third gear 41 fixed to the third shaft 26 of the power turbine 8. The third gear 41 of the reducer 10 is arranged axially between the first and second gears 39, 40 of the transmission mechanism 24.

[0082] As illustrated on the figure 1, the transmission housing 25 has, from front to back, the second toothed wheel 40, the third toothed wheel 41 and the first toothed wheel 39.

[0083] More specifically, according to the embodiment illustrated on the figure 1The second gear 40 is centered on the X-axis and is the driving gear. The second gear 40 is rotationally connected to a front end of the second shaft 23. The second gear 40 meshes with a first intermediate gear 42. The first intermediate gear 42 is the driven gear and is fixed to an intermediate shaft 43 that is free to rotate about an axis A, which is radially offset from the X-axis and parallel to the X-axis. The transmission mechanism 24 further includes a second intermediate gear 44 that meshes with the first gear 39. The second intermediate gear 44 is the driving gear and is fixed to the intermediate shaft 43. The first gear 39 is centered on the X-axis and is the driven gear. The gears of the transmission mechanism 24 have external contact. The first gear 39 has fewer teeth than the second intermediate gear 44.The second gear 40 has more teeth than the first intermediate gear 42. The first and second gears 39, 40 rotate in the same direction, while the intermediate shaft 43 rotates in the opposite direction. The various gears 39, 40, 42, 44 of the gear mechanism 24 allow the desired transmission ratio to be obtained. The first gear 39 can either be machined from the material along with the first shaft 22, or be mounted onto the first shaft 22 and rotationally connected to it by means such as splines or shrink fittings. According to the embodiment illustrated in the figure... figure 1 The third gear 41 is centered on the X-axis and is the driving (or driving) gear. The third gear 41 is rotationally linked to a front end of the third shaft 26.

[0084] The third gear 41 is meshed with a third intermediate gear 45. The third intermediate gear 45 is driven (or received) and fixed to the drive 9 which is movable around an axis B which is offset radially with respect to the X axis and parallel to the X axis.

[0085] According to the third embodiment illustrated on the figure 4 The third intermediate gear 45 is fixed to a second intermediate shaft 46, which is movable about axis B and is radially offset from axis X and parallel to axis X. The reducer 10 further includes a fourth intermediate gear 47, which meshes with an output gear 48. The fourth intermediate gear 47 is the driving gear and is fixed to the second intermediate shaft 46. The output gear 48 is the driven gear and is fixed to the power take-off 9. The power take-off 9 is coaxial or vertically aligned with axis X.

[0086] According to the embodiment illustrated on the figure 1 , the first shaft 22 of the compressor 4 is guided in rotation via a first bearing 49 and a second bearing 50 arranged in the transmission housing 25.

[0087] According to the embodiment illustrated on the figure 1 , the second shaft 23 of the expansion turbine 6 is guided in rotation via a third bearing 51 disposed in the transmission housing 25, and a fourth bearing 52 disposed at a rear end of the second shaft 23 located on the opposite side to the power turbine 8. Such an arrangement is used in particular when the combustion chamber 5 has reverse flow, which makes it possible to avoid the placement of servicing between the turbines 6, 8, to the benefit of the axial size of the turboshaft engine.

[0088] According to a fourth embodiment illustrated on the figure 5, the fourth bearing 52 for guiding the second shaft 23 is arranged between the expansion and power turbines 6, 8. Such an arrangement of the fourth bearing 52 is used in particular when the combustion chamber 5 does not have a free central space for the installation of a bearing.

[0089] According to the embodiment illustrated on the figure 1 , the third shaft 26 of the power turbine 8 is guided in rotation via a fifth bearing 53 disposed in the transmission housing 25, and a sixth bearing 54 disposed axially between the compressor 4 and the power turbine 8.

[0090] Advantageously, the bearings are roller bearings. The gears and bearings are lubricated with a liquid lubricant such as oil. Generally, each gear 39-48 can either be machined from the same material as the corresponding shaft, or mounted onto the corresponding shaft and rotationally connected to it by coupling means such as splines or shrink fittings. The advantage of using coupling means is that the gears can be easily replaced individually when they are worn and need replacing.

Claims

1. A turboshaft engine (1) for an aircraft (2), the turboshaft engine (1) comprising: - a gas generator (3) comprising a compressor (4), a combustion chamber (5) and an expansion turbine (6), the compressor (4) and the expansion turbine (6) extending along a common longitudinal axis (X) and being mechanically connected to one another, the combustion chamber (5) being arranged axially at a rear end (7) of the turboshaft engine (1); - a power turbine (8) arranged axially between the compressor (4) and the expansion turbine (6), the power turbine (8) driving a power take-off (9) in rotation via a reduction gear (10); - a heat exchanger (11) comprising: - a first circuit (12) comprising an inlet (13) connected to an outlet (14) of the compressor (4), and an outlet (15) connected to an inlet (16) of the combustion chamber (5), and - a second circuit (17) comprising an inlet (18) connected to an outlet (19) of the power turbine (8), characterised in that the compressor (4) comprises a first shaft (22) driven in rotation by a second shaft (23) of the expansion turbine (6) via a transmission mechanism (24), said transmission mechanism (24) and said reduction gear (10) forming part of a transmission casing (25) which is arranged axially at a front end (20) of the turboshaft engine (1), so that the compressor (4) is arranged axially between the transmission casing (25) and the power turbine (8).

2. The turboshaft engine (1) according to claim 1, characterised in that the power turbine (8) comprises a third shaft (26), the first, second and third shafts (22, 23, 26) being coaxial with said longitudinal axis (X).

3. The turboshaft engine (1) according to claim 2, characterised in that the third shaft (26) of the power turbine (8) is arranged radially between the first and second shafts (22, 23).

4. The turboshaft engine (1) according to one of the preceding claims, characterised in that said transmission mechanism (24) has a transmission ratio greater than 1, so that the speed of the first shaft (22) of the compressor (4) is greater than the speed of the second shaft (23) of the expansion turbine (6).

5. The turboshaft engine (1) according to one of the preceding claims, characterised in that said transmission mechanism (24) is a gear mechanism.

6. The turboshaft engine (1) according to claim 5, characterised in that said transmission mechanism (24) comprises a first toothed wheel (39) secured to the first shaft (22) of the compressor (4) and a second toothed wheel (40) secured to the second shaft (23) of the expansion turbine (6), said reduction gear (10) comprising a third toothed wheel (41) secured to a third shaft (26) of the power turbine (8), the third toothed wheel (41) of the reduction gear (10) being arranged axially between said first and second toothed wheels (39, 40) of the transmission mechanism (24).

7. The turboshaft engine (1) according to claim 6, characterised in that the transmission casing (25) has, from front to back, the second toothed wheel (40), the third toothed wheel (41) and the first toothed wheel (39).

8. The turboshaft engine (1) according to one of the preceding claims, characterised in that said power take-off (9) is coaxial or vertically aligned with said longitudinal axis (X).

9. The turboshaft engine (1) according to one of the preceding claims, characterised in that the first shaft (22) of the compressor (4) is guided in rotation via a first bearing (49) and a second bearing (50) arranged in the transmission casing (25).

10. The turboshaft engine (1) according to one of the preceding claims, characterised in that said second shaft (23) of the expansion turbine (6) is guided in rotation via a third bearing (51) arranged in the transmission casing (25), and a fourth bearing (52) arranged either between the expansion and power turbines (6, 8) or at a rear end of said second shaft (23) located on the opposite side to the power turbine (8).

11. The turboshaft engine (1) according to one of claims 2 to 10, characterised in that said third shaft (26) of the power turbine (8) is guided in rotation via a fifth bearing (53) arranged in the transmission casing (25), and a sixth bearing (54) arranged axially between the compressor (4) and the power turbine (8).

12. An aircraft (2), preferably a single-engine helicopter, comprising a turboshaft engine (1) according to one of the preceding claims.