Free turbine turbo engine with electric machines to support a gas generator and a free turbine

Description

Technical Field

[0001] The present invention relates to the general field of aeronautical turbomachines and more particularly to the driving of equipment via the free turbine without compromising their operation when starting the turbomachine. Prior art

[0002] A free turbine helicopter engine generally comprises a gas generator and a free turbine driven in rotation by the gas flow generated by the gas generator, as well as a reversible electrical machine which can be coupled to the gas generator in particular to set the gas generator in rotation during a start-up phase of the engine.

[0003] Traditionally, a gas generator consists of at least one compressor and one turbine coupled in rotation. The operating principle is as follows: fresh air entering the turbine engine is compressed by the rotation of the compressor before being sent to a combustion chamber where it is mixed with fuel. The gases burned during combustion are then evacuated at high speed.

[0004] A first expansion then occurs in the gas generator turbine, during which the latter extracts the energy necessary to drive the compressor and the equipment. The gas generator turbine does not absorb all the kinetic energy of the burnt gases and the excess kinetic energy corresponds to the gas flow generated by the gas generator. The latter therefore provides kinetic energy to the free turbine so that a second expansion occurs in the free turbine which transforms this kinetic energy into mechanical energy in order to drive a receiving organ, such as the rotor of the helicopter.

[0005] During the start-up phase of the turbine engine, it is necessary to rotate the gas generator, i.e. to rotate the compressor coupled to the turbine as well as the various connected equipment. As mentioned in the preamble, this is precisely one of the roles of the reversible electric machine which is most often an electric motor capable of operating reversibly as an electric generator.

[0006] As illustrated in the figure 1which schematically presents a free turbine turbomachine according to the state of the art, for starting, an electric machine 1 in a motor operation drives the accessories, not shown, and the mechanical shaft 2 of the gas generator 3, until the rotation of the latter is maintained by the combustion of fuel. The shaft 8 of the free turbine 9 being mechanically decoupled from the shaft 2 of the gas generator, the electric machine 1 does not drive the shaft 8. The free turbine 9 is then driven only by the gas flow leaving the gas generator. The rotational drive of the compressor by the electric machine 1 operating as a motor in fact makes it possible to drive the equipment to supply the turbomachine with fuel and oil and to circulate air in the compressor 4 and therefore to bring compressed air into the combustion chamber 5 in order to initiate combustion.This combustion then produces the gas flow enabling the turbine 6 of the gas turbine 3 to rotate, after which the compressor 4 and the equipment are directly rotated by the turbine 6, which means that the gas generator 3 operates autonomously, indicating the end of the start-up phase of the turbine engine.

[0007] Furthermore, for a free turbine turbogenerator like the one shown in the figure 1 , a second electric machine 7 can be provided engaged on the shaft 8 of the free turbine 9 to meet the need for high power generation.

[0008] It is known that aircraft, in which such turboshaft engines are intended to be integrated, include numerous electrical components that need to be powered by electrical energy. For example, for a vertical takeoff and landing aircraft with electric propulsion, it is necessary to power all of the electric rotors with electrical energy.

[0009] On a turbomachine as illustrated in the figure 1 , equipment such as the fuel pump, and the oil pump are mechanically connected to the shaft 2 of the gas generator 3 via an accessory box.

[0010] Once the start-up phase is complete, it is known to use the electrical machine 1 if it is reversible in a generator operating mode to produce non-propulsive electrical energy (28V network for example) to supply electricity to the electrical equipment. The electrical machine 1 generates electrical energy by taking mechanical power from the shaft 2 of the gas generator 3, the rotational kinetic energy taken from the gas generator being transformed into electrical energy by said machine.

[0011] This electrical machine 1 can be non-reversible and consist of a simple starter, if the need for electrical generation does not exist.

[0012] On the figure 2The evolution, as a function of time and the possible configuration of the turbomachine, of the speed of shaft 2 of the gas generator 3 is illustrated in solid lines and of the speed of shaft 8 of the free turbine 9 in dotted lines. It can be seen that the evolution of the speed of the two shafts is independent. The point corresponding to the instant of exit from the start-up phase is also indicated.

[0013] For a turbine engine, with a free turbine and a conventional starting system using a reversible electric machine, the mechanical draw on the gas generator shaft used to generate electrical power by the reversible electric machine 1 in a generator operating mode affects the performance of the gas generator.

[0014] In fact, the variation, during the flight, of the mechanical power taken by the electrical machine 1 and the equipment on the gas generator results in a displacement of the operating line of the engine in the compressor field. This displacement corresponds to a margin for pumping which must be provided, which has the consequence of: to penalize the optimization of the engine operating line, by prohibiting the use of the compressor at an optimal pressure rate; to thereby degrade stabilized performance, with an impact on specific consumption.

[0015] With such a configuration, a solution to not draw mechanical power from the gas generator shaft in order to generate electricity consists of using an electric machine for the starting function with a clutch system, and another electric machine integral with the free turbine shaft for the generation function as illustrated in the figure 1 by the second electric machine 7, which is penalizing in terms of mass and cost for a classic configuration of turboshaft engine or turbogenerator. This architecture, however, could become necessary if the need to decouple the “non-propulsive” electrical start / generation function from the main “propulsive” electrical generation function is mandatory from an electrical architecture point of view (redundancy, type of network / storage available).

[0016] A known architecture allowing a free turbine turbogenerator to be started without the addition of a specific starter is disclosed in document FR 2 929 324. In the case where the need to decouple the functions does not exist, this technical solution makes it possible to reduce the total mass and the cost and to increase the reliability of the turbogenerator compared to a turbomachine comprising two electric machines as in the figure 1 The technical solution described in this document consists of a switching system using two freewheels to start the gas generator of a free turbine engine, then to generate non-propulsive electrical energy by taking mechanical energy from the free turbine shaft. The equipment remains driven via the gas generator and the accessory box.

[0017] This solution makes it possible in particular to improve the transient performance of the gas generator, by avoiding the disadvantages of drawing kinetic energy from the gas generator, and in particular the problem of the displacement of the engine operating line in the compressor field due to the variation, during flight, of the mechanical power drawn by the electric machine.

[0018] Document US2016 / 097328A1 discloses a turbomachine comprising a gas generator provided with a rotating mechanical shaft, a first reversible electric machine and a free turbine according to the state of the art.

[0019] In order to improve the performance of the free turbine turbomachine and equipment during the start-up phase, it would also be interesting to be able to drive all of the turbomachine's equipment via the free turbine. But since the performance of the fuel pump is associated with its rotational speed, the evolution of the speed of the free turbine after the start-up phase would not allow, or would strongly impact, the design of the entire fuel system to ensure good start-up of the turbomachine. Statement of the invention

[0020] To this end, the present invention proposes a free turbine turbomachine equipped with a plurality of propulsive and non-propulsive electric machines making it possible to improve the overall efficiency of the turbomachine while driving the equipment of the turbomachine independently in time and speed relative to the start-up of its gas generator.

[0021] In one subject of the invention, a turbomachine is proposed, in particular for a rotary wing aircraft, comprising a gas generator provided with a rotating mechanical shaft, a first reversible electrical machine, a free turbine driven in rotation by a gas flow generated by the gas generator, at least one accessory from among an oil pump and a fuel pump, an accessory box comprising a gear train configured to drive said at least one accessory, and a second electrical machine.

[0022] According to a technical characteristic of the invention, the second electric machine is reversible, the first electric machine is mechanically coupled to the gas generator, and the second electric machine and the accessory box are mechanically coupled to the free turbine, and the turbomachine is devoid of kinematic connection between the gear train of the accessory box (14) and the shaft (18) of the gas generator.

[0023] According to this architecture, the gas generator and the accessory box are thus devoid of any kinematic connection between them. In addition, the accessories, in whole or only part of them, are coupled to the free turbine to be driven solely by the free turbine once the latter has reached its autonomous operating regime. The autonomous operating regime of the free turbine corresponds to a regime at which the free turbine provides a requested power without any assistance, the only energy supplied to the free turbine being that of the gas flow generated by the gas generator.

[0024] Accessories coupled to the free turbine only are no longer linked to the rotation speed of the gas generator, which allows them to be driven at the right speed, and above all in relation to their needs and not to the needs of the gas generator. For example, the pumps can be driven before the gas generator to prime the fuel circuit and therefore fill the pipes and avoid a failed start.

[0025] Coupling all or part of the accessories to the free turbine rather than to the gas generator improves the overall efficiency of the turbomachine. Some accessories, preferably those requiring only low mechanical extraction energy, may however remain coupled to the gas generator, i.e. in direct or indirect engagement with the latter's shaft. An example of an eligible accessory may be a PMA (Permanent Magnet Alternator) type alternator which is used to power the engine computer and which may also integrate a device for measuring the speed of the gas generator shaft.

[0026] The architecture of the turbomachine according to the invention with its two electric machines thus makes it possible to initiate the rotation of both the accessories, the free turbine and that of the gas generator until the gas generator can operate autonomously, then to continue to drive the free turbine, and the accessories until the free turbine can also operate autonomously. The accessories engaged on the TL can thus be driven by the second electric machine in advance of phase with respect to the gas generator, which makes it possible to have a fuel and oil circuit in the best conditions to improve the starting performance of the gas generator and to improve the suction and self-priming performance of the fuel circuit with respect to the constraints of integration in the aircraft.

[0027] In a turbogenerator-type turbomachine application, the electrical machine engaged on the free turbine is sized to generate high electrical power levels, and can, when used in engine mode, easily deliver torque to drive the free turbine and accessories during the start-up phase.

[0028] Furthermore, the mechanical independence of the free turbine and the gas generator offers the possibility of achieving independent ventilation of the free turbine and the gas generator in order to thermally control the turbomachine when it is stopped.

[0029] Furthermore, the architecture according to the invention makes it possible, in the event of a break in the free turbine shaft during operation, to directly impact the drive of the accessories (including the fuel pump) which is provided via this shaft. Since the fuel pump is no longer driven, this has the direct effect of instantly shutting down the engine and therefore preventing the establishment of an overspeed of the free turbine. This architecture therefore has the effect of protecting the free turbine against an overspeed which could lead to a break in parts of the turbine under the effect of centrifugal forces. This architecture thus makes it possible to avoid the need for other overspeed protection devices such as the blade shedding device known under the English name "Blade Shedding", with a potential weight saving (shielding).

[0030] According to a first aspect of the turbomachine, the second electric machine can be mechanically coupled to the free turbine via the accessory box.

[0031] According to a second aspect of the turbomachine, the accessory box can be mechanically coupled to the free turbine via a freewheel.

[0032] This solution only prevents the free turbine from being driven by the second electric machine during the start-up phase, which reduces inertia and resistive torque.

[0033] This solution has the advantage of being able to be used also in a turboshaft configuration because the freewheel means that the rotor does not have to be driven.

[0034] The case of operation in blocked free turbine mode could be achieved by transiently controlling the second electrical machine in connection with the equipment.

[0035] According to a third aspect of the turbomachine, the turbomachine may further comprise a control unit for the second electric machine configured to operate the electric machine in engine mode from the start of the turbomachine and until one of the operating temperatures of the free turbine has exceeded a threshold from which the free turbine is autonomous.

[0036] The turbomachine may also comprise a control unit for the first electrical machine configured to operate the first electrical machine in engine mode from the start of the turbomachine or according to a performance criterion at the equipment levels (fuel pressure, etc.) until one of the mechanical parameters of the first mechanical shaft has exceeded a threshold from which the gas generator is autonomous, then in generator mode once said parameter of the first mechanical shaft has exceeded said threshold from which the gas generator is autonomous.

[0037] According to a fourth aspect of the turbomachine, the second electric machine is a reversible electric machine, and said control unit of the second electric machine is configured to operate the second electric machine in generator mode once said operating parameter of the free turbine has exceeded said threshold from which the free turbine is autonomous.

[0038] Once the turbine is free to rotate independently, the second electrical machine can operate in generator mode and thus provide electrical power to other elements.

[0039] According to a fifth aspect of the turbomachine, the control unit can be configured to operate the second electric machine in engine mode with a predetermined advance relative to the start of the gas generator driven by the first electric machine, so that the driving of an oil pump and a fuel pump by the second electric machine is carried out at rotation speeds making it possible to have a fuel circuit and an oil circuit each operating under optimal conditions for the start of the gas generator.

[0040] According to another aspect of the invention, there is provided an aircraft, such as a vertical take-off and landing aircraft, comprising at least one turbomachine as defined above. Brief description of the drawings

[0041] [ Fig. 1 ] There figure 1 , already described, is a simplified schematic view of a free turbine turbomachine according to the state of the art. Fig. 2 ] There figure 2 , already described, is a graphical representation of the evolution, as a function of time and the possible configuration of the turbogenerator, of the speed of the gas generator shaft and the speed of the free turbine shaft of the turbomachine of the figure 1 . [ Fig. 3 ] There figure 3 is a diagram of a free turbine turbomachine according to a first embodiment of the invention. Fig. 4 ] There figure 4 is a graphical representation of the evolution, as a function of time and the possible configuration of the turbogenerator, of the speed of the gas generator shaft and the speed of the free turbine shaft of the turbomachine of the figure 3 . [ Fig. 5 ] There Figure 5 is a diagram of a free turbine turbomachine according to a second embodiment of the invention. Fig. 6 ] There figure 6is a graphical representation of the evolution, as a function of time and the possible configuration of the turbogenerator, of the speed of the gas generator shaft and the speed of the free turbine shaft of the turbomachine of the Figure 5 . Description of the embodiments

[0042] On the figure 3 a free turbine turbomachine 10 according to a first embodiment of the invention is schematically presented.

[0043] The turbomachine 10 comprises a first reversible electric machine 11, a free turbine 12, a gas generator 13, an accessory box 14, a fuel pump 15, an oil pump 16, and a second reversible electric machine 19 also in this embodiment.

[0044] The free turbine 12 comprises a first shaft 17 and the gas generator 13 comprises a second shaft 18. The second shaft 18 of the gas generator 13 is connected to the first electrical machine 11.

[0045] The first electrical machine 11 is configured to operate in engine mode when the turbomachine 10 is started and thus to rotate the second shaft 18 of the gas generator 13 from start-up until the gas generator is autonomous. In other words, until an operating parameter of the gas generator 13 has exceeded an operating threshold of the gas generator 13 in autonomy.

[0046] In this first embodiment, the first shaft 17 of the free turbine 12 is mechanically connected without intermediary to the accessory box 14 permanently. The accessory box 14 is mechanically coupled to the second electrical machine 19, which thus makes it possible to have the turbine 12 coupled to the second electrical machine 19 during all the operating phases of the turbomachine 10, in particular during the start-up phase and the electrical generation phase.

[0047] Alternatively, a rotor shaft of the second electric machine 19 could be directly engaged on the first shaft 17 of the free turbine 12, for example with a meshing by straight or bevel gears, therefore without passing through a gear train of the accessory box 14. In other words, the kinematic connection between the second electric machine 19 and the free turbine 12 would not be made via the accessory box 14. Here too, the free turbine 12 would remain coupled to the second electric machine 19 during all the operating phases of the turbomachine.

[0048] The accessory box 14 is also mechanically coupled to the fuel pump 15, on the one hand, and to the oil pump 16, on the other hand. In other embodiments, other accessories may be mechanically coupled to the accessory box 14.

[0049] The turbomachine 10 further comprises a control unit 30 for the second electric machine 19. The control unit 30 is configured to control the second electric machine 19 in a motor mode during a start-up phase of the turbomachine 10 and to control it in a generator mode when the power available on the first shaft 17 of the free turbine 12 reaches a sufficient level to keep the equipment rotating and to ensure the proper operation of the GG.Since the power is the image of the autonomy of the free turbine 12, it can be integrated in the form of an operating parameter. The control unit 30 thus receives information relating to an operating parameter of the free turbine 12 and to the parameters of the second electrical machine 19, such as the rotation speed of the free turbine, of the electrical machine or the torque developed by the free turbine 12 or the electrical machine, and regularly compares it to a threshold corresponding to the value of the parameter from which the free turbine can be considered as operating autonomously.The second electrical machine 19 being no longer useful for driving the free turbine 12 and the rotating equipment, it can be placed in generator mode once the free turbine is detected as autonomous, to provide electrical energy (for example to power the engines of an electrically powered aircraft, or even to store it in a storage unit).

[0050] The first electric machine 11 is controlled in generator mode when the gas generator 13 reaches a speed threshold identified as its autonomy threshold.

[0051] The first electrical machine 11 can be controlled in generator mode (end of start) before the second electrical machine 19 can also be controlled on its side (end of assistance).

[0052] On the figure 4is graphically represented the evolution, as a function of time and the possible configuration of the turbomachine, of the speed of the second shaft 18 of the gas generator 13 in solid lines and, in dotted lines, of the speed of the first shaft 17 of the free turbine 12 of the turbomachine 10 of the figure 3 .

[0053] Comparing the graph of the figure 2 and that of the figure 4, we see that the architecture of the turbomachine 10 according to the invention allows a faster increase in speed of the free turbine 12 until a first speed level P1 is reached. This first level corresponds to the assistance by the second electric machine 19 with control of the speed thereof, and is optimized for the proper functioning of the equipment or accessories for the start-up phase, such as the fuel pump 15 and the oil pump 16 in particular. Then, after this first level, stopping the control of the speed of the second electric machine allows a further increase in the speed of the free turbine 12, generated by the increase in the air flow delivered by the gas generator 13, until the free turbine 12 reaches a second speed level P2 corresponding to a nominal operating speed of the second electric machine 19 used in generator mode.The gear ratios of the gear trains of the accessory box 14 will be provided so that the nominal operating speed of the free turbine 12 corresponds to the nominal operating speeds of the second electrical machine 19 and of the other equipment or accessories driven by the accessory box.

[0054] On the Figure 5 a free turbine turbomachine 10 according to a second embodiment of the invention is schematically presented.

[0055] The turbomachine 10 of the second embodiment illustrated in the Figure 5 differs from the turbomachine 10 of the first embodiment illustrated in the figure 3only in that it comprises a coupling device 20 configured to be able to switch between an engagement configuration and a disengagement configuration. The coupling device 20 may comprise, for example, a freewheel, coupled between the free turbine 12 and the gears of the accessory box 14 to which the second electric machine 19 and other equipment or accessories are connected. In this second embodiment, the free turbine 12 is therefore mechanically coupled to the second electric machine 19 via the coupling device 20 and the accessory box 14.

[0056] Advantageously, the coupling device 20 can be passive, as is the case for a freewheel, however a controlled coupling device remains possible, for example a clutch. In the case of a freewheel, the latter is configured to disengage when the speed of the terminal of the freewheel connected to the accessory box exceeds the speed of the terminal of the freewheel connected to the shaft 17 of the free turbine 12. Thus, as long as the second electric machine 19 is controlled in a motor mode during a start-up phase of the turbomachine 10, the freewheel 20 is disengaged, such that the free turbine 12 is not driven by the second electric machine 19.

[0057] The increase in speed of the free turbine therefore evolves independently and more slowly than that of the second electric machine 19, since the free turbine is driven only by the air flow delivered by the gas generator 13. Conversely, when the speed of the free turbine reaches a certain threshold for which the free turbine is capable of driving the second electric machine 19 and the accessories via the accessory box 14, the freewheel 20 is engaged, the speeds at its two terminals then being the same.

[0058] During a start-up phase of the turbomachine, the second electric machine 19 is faced with less inertia and resistive torque, compared to the solution without freewheel described previously. If the turbomachine according to the invention is used in a turbogenerator configuration, for example to provide electrical energy used by an electrically powered aircraft, this solution has the advantage during the start-up phase of having to draw less from the electrical storage (batteries) of the aircraft to power the second electric machine 19. A reduction in the capacity and therefore the mass of the aircraft batteries is then conceivable.

[0059] The coupling device 20 of this solution is particularly advantageous in the context of the use of the turbomachine according to the invention in a turboshaft configuration, for example for a helicopter whose main rotor is driven by the shaft 17 of the free turbine via a gearbox. Whether this coupling device 20 is in the form of a freewheel or a piloted clutch, this makes it possible not to have to drive the main rotor of the helicopter during the start-up phase. The second electrical machine 19 can therefore be sized solely as a function of the electrical energy that it must produce in generator mode, and the drive of the accessories that it must provide in motor mode via the accessory gearbox during the start-up phase.

[0060] On the figure 6is graphically represented the evolution, as a function of time and the possible configuration of the turbomachine, of the Nmel speed of the second electric machine 19, in thin line, of the Ng speed of the second shaft 18 of the gas generator 13 in strong line and, in dotted lines, of the Ntl speed of the first shaft 17 of the free turbine 12 of the turbomachine 10 of the Figure 5 .

[0061] Comparing the graph of the figure 2 and that of the figure 6 , we see that the increase in speed of the free turbine 12, Ntl, is identical to that of the figure 2 for the same speed increase, Ng, of the gas generator 13, but on the other hand, the speed increase Nmel of the second electric machine 19 is similar to the speed increase Ntl of the free turbine 12 of the figure 4, which makes it possible to quickly operate equipment or accessories, such as the fuel pump 15 and the oil pump 16 in particular. Once the speed of the free turbine 12 has reached that of the second electric machine 19, at the reduction ratio if there is an indirect drive between the first shaft 17 and the rotor of the second electric machine 19 via gears of the accessory box 14, the motor mode of the second electric machine 19 is stopped, the free turbine 12 then being able to drive the accessories 15 and 16.

[0062] The free turbine turbomachine according to the present invention thus makes it possible to optimize the mass, cost and reliability of the switching system and therefore of the turbomachine. The use of a first electric machine for the gas generator on the one hand and a second electric machine for the free turbine on the other hand, by mechanically decoupling the free turbine and the gas generator, makes it possible, thanks to the configuration of the invention, to be able to drive the accessories via the free turbine in order to improve the overall efficiency of the turbomachine while retaining the possibility of driving the accessories independently of the gas generator during the start-up phase.

Claims

1. A turbomachine (10), particularly for a rotary-wing aircraft, including a gas generator (13) provided with a rotary mechanical shaft (18), a first reversible electric machine (11), a power turbine (12) rotationally driven by a stream of gas generated by the gas generator (13), at least one accessory (15, 16) from among an oil pump and a fuel pump, an accessory gearbox (14) comprising a gear train configured to drive said at least one accessory (15, 16), and a second electric machine (19), characterized in that the second electric machine (19) is reversible, the first electric machine (11) is mechanically coupled to the gas generator (13), the accessory gearbox (14) and the second electric machine (19) are mechanically coupled to the power turbine (12), and the turbomachine (10) is devoid of any kinematic coupling between the gear train of the accessory gearbox (14) and the shaft (18) of the gas generator (13), the turbomachine (10) further comprising a unit (30) for controlling the second electric machine (19), and, during the start-up of the turbomachine (10), the control unit (30) being configured to make the electric machine (19) operate in motor mode with a predetermined advance with respect to the start-up of the gas generator (13) driven by the first electrical machine (11), such that the driving of an oil pump (16) and a fuel pump (15) by the second electric machine (19) is done at rotation ratings making it possible to have a fuel circuit and an oil circuit each operating under optimal conditions for the start-up of the gas generator (13).

2. The turbomachine (10) as claimed in claim 1, wherein the second electric machine (19) is mechanically coupled to the power turbine (12) via the accessory gearbox (14).

3. The turbomachine (10) as claimed in one of claims 1 or 2, wherein the accessory gearbox (14) is mechanically coupled to the power turbine (12) via a coupling device (20).

4. The turbomachine (10) as claimed in claim 3, wherein the coupling device (20) comprises a free wheel.

5. The turbomachine (10) as claimed in one of claims 1 to 4, wherein the control unit (30) is configured to make the second electrical machine (19) operate in motor mode from the start-up of the turbomachine (10) and until one of the operating parameters of the power turbine (12) has exceeded a threshold above which the power turbine (12) is self-sustaining.

6. The turbomachine (10) as claimed in claim 5, wherein the second electric machine (19) is a reversible electric machine, and said unit (30) for controlling the second electric machine (19) is configured to make the second electric machine (19) operate in generator mode once said operating parameter of the power turbine (12) has exceeded said threshold above which the power turbine (12) is self-sustaining.

7. An aircraft comprising at least one turbomachine as claimed in one of claims 1 to 6.