ASSISTANCE DEVICE FOR A FREE-TURBINE ENGINE OF AN AIRCRAFT HAVING AT LEAST TWO FREE-TURBINE ENGINES

Abstract

An assistance device for a free-turbine engine of an aircraft having at least two free-turbine engines, the device including an electrical starter machine for providing prolonged assistance to the gas generator of a first engine using energy produced by an electric generator machine driven by the second engine, the device further including at least one electricity storage member electrically connected to the electrical starter machine for providing a burst of assistance to the gas generator, wherein the electrical starter machine is powered by a first power converter enabling it to exchange energy with the storage member for providing the burst of assistance, and that transmits thereto the energy supplied by a second power converter for the prolonged assistance.

Claims

1. An assistance device for a free-turbine engine of an aircraft having at least two free-turbine engines, the device comprising an electrical starter machine and an electric generator machine, the electrical starter machine providing prolonged assistance to the gas generator of a first engine using energy produced by the electric generator machine driven by the second engine, the assistance device further comprising at least one electricity storage member electrically connected to said electrical starter machine for providing a burst of assistance to said gas generator, a first power converter and a second power converter, the electrical starter machine being powered by the first power converter to exchange energy with the storage member for providing the burst of assistance, and that transmits thereto the energy supplied by the second power converter for the prolonged assistance, the assistance device being characterized in that it further comprises a computer for cutting off the flow of fuel to the gas generator during a determined period during the prolonged assistance and for maintaining said gas generator at a reduced speed for facilitating re-ignition of said gas generator.

2. The assistance device according to claim 1, wherein a disconnector member enables the two converters to be electrically isolated from each other, the storage member remaining connected to the first converter.

3. The assistance device according to claim 1, wherein the first electrical machine is also a generator.

4. The assistance device according to claim 1, wherein the second electrical machine is driven by the gas generator of the second engine.

5. The assistance device according to claim 1, wherein a switch member enables the second converter to be connected to the electrical storage member.

6. The assistance device according to claim 1, wherein the second converter is powered by a generator electrical machine driven by the gas generator of a second engine of the aircraft.

7. The assistance device according to claim 1, wherein the electricity storage member can be used, where appropriate, for assisting in controlled acceleration or deceleration of said gas generator under twin-engined flight conditions.

8. The assistance device according to claim 1, including one storage element per engine in order to participate in burst accelerations of the gas generators of either of the engines.

9. The assistance device according to claim 1, wherein the computer maintains the flow of fuel to the gas generator for a determined period during prolonged assistance and it reduces the speed of the gas generator so as to minimize fuel consumption.

10. The assistance device according to claim 1, wherein the electricity storage member comprises a supercapacitor, a hybrid competitor, a lithium-ion battery, or a flywheel having an integrated DC/AC converter.

11. The assistance device according to claim 1, wherein the electricity storage member is recharged by taking energy from the gas generator of the first or second engine during periods in which said gas generator is supplied with fuel.

12. The aircraft having at least two free-turbine engines, and including an assistance device according to claim 1.

Description

LIST OF FIGURES

[0038] FIG. 1 is an overall diagram of a system of the invention, incorporated in the propulsion and electrical systems of the aircraft.

[0039] FIG. 2 is a more particular diagram of an embodiment of the invention.

[0040] FIGS. 3 to 9 each show a stage or a mode of use of the embodiment of FIG. 2.

[0041] FIG. 10 shows an alternative embodiment of the invention.

[0042] FIG. 11 shows another alternative embodiment of the invention.

[0043] FIGS. 12 and 13 show two other alternative embodiments of the invention.

DETAILED DESCRIPTION

[0044] With reference to FIG. 1, the general electrical architecture of an embodiment of the proposed system is as follows.

[0045] The generation of electricity on the aircraft is provided by at least two alternators ALT1 and ALT2 driven by the main gearbox MGB, typically three-stage type 115 VAC/400 Hz machines, although other rotary machines could be envisaged.

[0046] This architecture is advantageous in the context of single-engined low-cost cruising flight since it guarantees functional and organic independence between electricity generation and the operation of the turbine engines, thus making it possible to conserve a sufficient level of availability and redundancy for generating electricity during low-cost cruising flight, while one of the two engines is being maintained in a standby mode that is incompatible with taking any power from its gas generator.

[0047] Furthermore, this architecture is less penalizing for the operation of the engines than taking power from the gas generators of the engines, in particular in terms of impact on acceleration and specific consumption performance, insofar as the mechanical power corresponding to the electrical power consumed by the on-board network of the aircraft is taken from the free turbine and not from the gas generator.

[0048] ALT1 and ALT2 power the electricity network of the aircraft, other available energy sources for powering this network possibly being constituted by an on-board auxiliary power unit APU, one or more storage batteries, or indeed, when on the ground, a ground power unit.

[0049] The main gearbox MGB is driven by the engines GT1 and GT2. In this example they are free-turbine turboshaft engines. Each of them has a gas generator and a power turbine (free turbine) driving the MGB via a freewheel.

[0050] Each engine GT1 and GT2 has a respective rotary machine (respectively G/S1 and G/S2) suitable for operating as a starter and as a generator, and in the embodiment described mechanically connected to the gas generator of the engine via an accessory gearbox. In order to optimize the compactness and the weight of the device, it is preferred for G/S1 and G/S2 to present a machine architecture that is compatible with being driven at high speed by the gas generator, and thus without a rotor winding, such as for example and in non-exhaustive manner a brushless synchronous machine with permanent magnets, a variable reluctance machine, or an asynchronous machine. The two machines G/S1 and G/S2 are included in an independent electrical assembly 100 that operates independently of the electricity network of the aircraft.

[0051] With reference to FIG. 2, the independent electrical assembly 100 comprises the following subassemblies.

[0052] Firstly there are two buses, Bus No. 1 and Bus No. 2 operating with direct current (DC) and at high voltage (of the order of several hundreds of volts), which buses are independent of each other and of the on-board network.

[0053] The independent electrical assembly further comprises two reversible DC/AC static power converters SPC1 and SPC2 (e.g. of the two level inverter type or of some other type) that serve in particular to power and control the rotary machines G/S1 and G/S2 in torque and in speed. While the electrical machine is being driven by the gas generator, each of these converters is capable of operating as a controlled rectifier and of regulating the voltage of the corresponding bus.

[0054] The independent electrical assembly 100 further comprises electricity storage members S1 and S2 that are optimized for delivering short and intense discharges of power. By way of example, they may be supercapacitors, or hybrid capacitors (possibly fitted with their own control system), lithium ion (Li-ion) batteries fitted with their own battery management systems (BMSs) or a flywheel (with its loading/unloading AC/DC converter).

[0055] The independent electricity assembly 100 also includes an electrical disconnector member 120 of the electromechanical contactor type or of the solid sate power controller (SSPC) type serving to connect together the two DC buses (in parallel), and conversely to isolate electrically on one side the assembly S1, Bus 1, SPC1, G/S1, GT1 and on the other side the assembly S2, Bus 2, SPC2, G/S2, GT2.

[0056] With reference to FIG. 3, there follows a description of a stage of twin-engined flight.

[0057] Once both engines GT1 and GT2 have started, the two electrical machines G/S1 and G/S2 are driven by the gas generators of the engines GT1 and GT2 and they operate in generator mode, with the DC/AC converters being controlled as rectifiers in application of an appropriate current/voltage relationship for the purpose of recharging and/or maintaining the charge in the storage members S1 and S2. The disconnector member 120 is open circuit.

[0058] Since the storage members might possibly have been discharged while starting the engines on the ground, it may be necessary to wait for the storage members S1 and S2 to be charged once more to their nominal level prior to authorizing takeoff.

[0059] Recharging or maintaining charge of the storage members S1 and S2 (in order to compensate for internal losses e.g. due to balancing cells in a pack of supercapacitors or a battery, or indeed due to friction in a flywheel) is performed in a manner that is independent from the on-board network by taking energy from the gas generators of the respective engines GT1 and GT2. Depending on their technology, compensation for losses in each storage member S1 and S2 may represent no more than a few tens of watts under steady conditions.

[0060] In this mode of operation, the DC buses are electrically isolated from each other and they operate independently of each other.

[0061] Furthermore, in a variant, sequential management of the recharging of the storage members is implemented using a hysteresis type relationship: the member is charged up to an energy threshold E1, and then recharging is inhibited (i.e. no more power is taken from the gas generator) until the energy stored dropsas a result of internal lossesto below a threshold E2 that is less than E1. With battery or hybrid capacitor type technologies involving active balancing circuits, quite long pauses are achieved between two recharging stages.

[0062] With reference to FIG. 4, in twin-engined flight, the energy stored in the members S1 and S2 is used to optimize the operation of the engines. The general idea is to provide a transient input of mechanical energy to the gas generator of one of the engines. Two modes of operation can then be envisaged, and they are described in detail below. They are described for operation while the disconnector member 120 is open circuit.

[0063] Firstly, it is possible to deliver a burst of assistance for accelerating the gas generator. In the event of pitch being increased quickly from a low speed, delivering mechanical power to the gas generator serves to improve its acceleration, and therefore increase the speed with which the engine delivers power to the free turbine, and consequently significantly decrease the transient drop in the speed of rotation of the rotor of the helicopter as occurs at the end of such a maneuver, thus increasing the safety margin of the crew.

[0064] This function provides a significant improvement to performance when the initial speed of the gas generator is slow. This mode of operation may be activated automatically at the request of the turbine computer when various criteria are satisfied, e.g. and in non-limiting manner: the system is available (no failure detected), the level of energy stored in the members S1 and S2 is sufficient, the engine is operating, the initial speed of the gas generator lies in a given range, and a rapid increase in power demand is detected as a result of the operating line of the engine approaching its limit for protection against surging.

[0065] The surge of assistance is deactivated when the operating line of the engine moves away from its limit for protection against surging, when the level of energy stored drops below a certain threshold, or when the speed exceeds a certain threshold.

[0066] Thereafter, it is possible to deliver a burst of assistance for decelerating the gas generator. In the event of a rapid reduction in pitch during which deceleration of the gas generator is limited by the anti-flameout relationship, it is proposed to increase the generation setpoint of the converters for a few instants so as to take off a large amount of power from the gas generators. As a result, it is possible to decelerate the gas generator more quickly, and thus increase the speed with which regulation of the engine decreases the flow rate of fuel injected into the combustion chamber and consequently decrease the amplitude of the transient increase in the speed of the rotor.

[0067] This mode of operation is activated automatically at the request of the turbine computer when various criteria are satisfied, for example and in non-limiting manner: the operating line comes close to the anti-flameout flow rate limit, and providing there is the capacity available for storing the energy that is taken from the gas generator while it is being decelerated.

[0068] In a variant, the capacity of the storage member is thus slightly overdimensioned in order to ensure under all circumstances that there is a margin for storing energy.

[0069] In alternative manner, a device may be added to the DC bus for dissipating the energy taken to decelerate the gas generator, such as for example an assembly made up of a resistance element and a braking chopper arm.

[0070] It is also possible to provide a burst of assistance of the power injection type. Mechanical power P.sub.mec is injected to the gas generator so as to obtain an increased effect on the free turbine. In certain zones of the flight envelope, it is possible to recover power K.P.sub.mec from the free turbine, and thus from the MGB of the helicopter with gain K greater than 1. It should be observed that when appropriate conditions are present, delivering assistance to the gas generator can thus be more efficient than delivering assistance that is equivalent except that it is injected directly to the free turbine or to the MGB. This mode of operation may be activated when the energy stored in the members S1 and S2 is sufficient, either in preventative manner on request of the crew, e.g. taking off with high load and/or at high altitude and/or at high temperature, or else automatically at the request of the engine computer in order to provide additional power for a short duration at the one engine inoperative (OEI) contingency rating, e.g. in the event of the speed of the free turbine dropping below a certain threshold or on detecting a loss of power from the other engine.

[0071] With reference to FIG. 5, there can be seen a single-engined low-cost cruising stage of flight.

[0072] When the conditions for allowing this mode are present, which implies amongst other things that there is sufficient energy stored in the members S1 and/or S2, the avionics sends a go-to-standby instruction to the computer of the gas turbine (e.g. GT2 in the figure).

[0073] In a first variant as shown in FIG. 5, referred to as the super idle variant, the computer of GT2 reduces the flow rate of fuel and regulates the speed of the gas generator to a low setpoint value, enabling the power turbine to be desynchronized from the MGB (so the power delivered to the helicopter is then zero) and enabling fuel consumption to be low. Simultaneously, the electrical machine G/S2 and the associated converter SPC2 are inhibited so as to avoid taking power from the gas generator of GT2.

[0074] The electrical machine G/S1 and its converter SPC1 pass to generator mode (if they were not already there); the two DC buses are then electrically connected together by reconfiguring the disconnector member 120. Energy taken from the gas generator of GT1 is used for maintaining charge in the storage members S1 and S2: this charge-maintaining function may be performed continuously or else discontinuously and sequentially on each of the two members.

[0075] In a second variant, as shown in FIG. 6, the computer of the engine GT2 cuts off the flow of fuel and regulates the speed of the gas generator on a setpoint value. Since the combustion chamber of GT2 is extinguished, fuel consumption is zero and the free turbine becomes desynchronized with the MGB.

[0076] Simultaneously, the electrical machine G/S2 and the associated converter SPC2 pass into motor mode with the speed setpoint as defined by the regulator and corresponding to the ideal ignition window for the combustion chamber. The gas generator passes into autorotation, and after a few seconds, its speed stabilizes on this setpoint, the combustion chamber being extinguished. The electrical machine G/S1 and its converter SPC1 pass into generator mode, if they were not already there.

[0077] The two DC buses are electrically connected together by reconfiguring the disconnector member 120. Energy taken from the gas generator of the engine GT1 is used to maintain charge in the storage members S1 and S2 and to power the electrical machine G/S2 via the power converter SPC2. This aspect constitutes prolonged assistance to the gas generator of the engine GT2, and is referred to as turning mode.

[0078] In a variant shown in FIG. 7, the computer maintains the supply of fuel to the engine GT2, and it is provided with prolonged assistance in rotating its gas generator, on the same principle as that described with reference to FIG. 6. For this purpose, the computer regulates the speed of the gas generator to a setpoint value so as to optimize the operation of the turbine and so as to minimize fuel consumption. In such a mode of operation, referred to as assisted super idle, SPC2 and G/S2 operate in motor mode.

[0079] During these stages of operation, the electrical assembly 100 remains independent from the on-board network.

[0080] Single-engined low-cost cruising flight mode can be exited in two different ways. Firstly, with reference to FIG. 8, when restarting GT2 is not urgent, it is started at the request of the avionics using the normal procedure: initially, the two DC buses are electrically isolated from each other by reconfiguring the electrical disconnector member 120.

[0081] If the engine GT2 was initially on standby with its combustion chamber ignited (super idle or assisted super idle mode), the electrical machine G/S2 is controlled so as to deliver driving torque to provide a burst of assistance using energy stored in the storage member S2 to accelerate the gas generator.

[0082] Simultaneously, the computer of the engine GT2 increases the fuel flow rate in application of a predefined relationship. If the engine GT2 was initially on standby with the combustion chamber extinguished (turning mode), the computer initiates a starting sequence analogous to that described above except that the gas generator of the engine GT2 is already being driven in the ideal ignition window. When ignition of the combustion chamber is detected, the torque delivered by the electrical machine G/S2 is increased and the computer of the engine GT2 increases the flow rate of fuel in application of a predefined relationship. Either way, when the speed NG exceeds a sustainable threshold, the electrical assistance is disconnected and the engine GT2 accelerates by its own means up to flight speed.

[0083] It should be observed that an analogous sequence can be used for starting the engine while the helicopter is on the ground, before takeoff, except that the gas generator of each engine is initially fully stopped. The engines are usually started sequentially, one after the other. Once both engines have started, and before takeoff, the storage members S1 and S2 are recharged using the procedure describe above (see FIG. 3).

[0084] With reference to FIG. 9, under certain conditions of single-engined low-cost flight, the crew might require power from the engine GT2 quickly: this can happen for example in the event of power being lost from the engine GT1, or in the event of an unexpected power need requiring power from both engines and thus justifying a quick exit from the single-engined mode (avoiding an obstacle, etc.). Under such circumstances, restarting is performed using the emergency procedure, the purpose being to cause the engine GT2 to reach its flight speed or even its OEI rating in a short period of time.

[0085] Initially, the two DC buses are electrically isolated by reconfiguring the electrical disconnect member 120. If the engine GT2 was initially on standby with its combustion chamber ignited (super idle or assisted super idle mode), then the electrical machine G/S2 is operated so as to deliver driving torque in order to provide a burst of assistance in accelerating the gas generator, this assistance being at a level that is substantially higher than for the normal restarting procedure. Simultaneously, the computer of the engine GT2 increases the fuel flow rate in application of a predefined relationship, likewise optimized for fast restarting of the turbine.

[0086] If the engine GT2 was initially on standby with the combustion chamber extinguished (turning mode), the computer triggers ignition of the combustion chamber, with this operation being made easier by the fact that the gas generator is already being driven in rotation in the ideal ignition window. Thereafter, as above, the computer proceeds to request a burst of electrical assistance for accelerating the gas generator and it increases the flow rate of fuel in application of a predefined relationship, likewise optimized for fast restarting of the turbine.

[0087] In both situations, the burst of electrical assistance to the gas generator may be extended beyond the starter cut-off speed threshold used in the normal starting procedure in order to minimize the time taken by the engine to accelerate to its flight or OEI rating.

[0088] Once the helicopter is on the ground, before switching off the turbines, it may be advisable to recharge the storage members of the electrical hybridizing device so that they are ready for subsequent starting. This procedure may be performed during the required passage to the ground idle speed used for balancing temperatures in the engines before stopping them.

[0089] A variant is described with reference to FIG. 10.

[0090] The independent electrical assembly 101 is similar to the independent electrical assembly 100 as described above, but the storage members S1 and S2 are replaced by a single storage member S. By way of example, it is dimensioned so as to be capable of emergency starting a single engine. The advantage is then a saving of almost two in terms of weight and compactness. In recharging mode (twin-engined operation), one of the two power converters SPC1 or SPC2 is specified as the master by the supervisor computer and is in charge of recharging the storage member S. The reconfiguration member 121 enables the storage member S to be connected to the converter SPC2 and enables the assembly S-SPC2-G/S2 to be electrically isolated from the assembly SPC1-G/S1, or on the contrary, it enables the storage member S to be connected to the converter SPC1 and the assembly S-SPC1-G/S1 to be electrically isolated from the assembly SPC2-G/S2 for the stages of charging the member S with one or the other of the engines or of providing a burst of assistance to an engine.

[0091] The reconfiguration member 121 is also capable of keeping the assembly SPC2(G/S2)-SPC1(G/S1) electrically connected together for single-engined stages of flight involving powering one of the electrical machines operating as a motor by the other electrical machine operating as a generator (turning mode and assisted super idle mode).

[0092] Another variant is described with reference to FIG. 11.

[0093] In this example, the electrical assembly 102 is not independent from the on-board network. Electrical connection between electrical machine G/S1 and the converter SPC1 takes place via the on-board network. There is only one electrical storage member S and it is dedicated to providing a burst of assistance to the engine GT2 via the converter SPC2 and the electrical machine G/S2. It may be charged by the converter and the electrical machine G/S1 or by the converter SPC2 and the electrical machine G/S2, in particular as a function of the position of the reconfiguration member 122. The engine GT1 is not put into a standby mode during low-cost cruising flight. In contrast, the engine GT2 may be put into a standby mode with its combustion chamber ignited (assisted super idle mode) or with its combustion chamber extinguished (turning mode), with the electrical energy needed for prolonged assistance to the gas generator then coming from the engine GT1 via G/S1, SPC1, SPC2, and G/S2, or via ALT1, SPC1, SPC2, and G/S2 (see ALT1 in association with FIG. 1). In this second example, G/S1 may be replaced by a simple, non-controlled starter.

[0094] The reconfiguration member 122 enables the storage member S to be connected to the converter SPC2 and enables the assembly S-SPC2-G/S2 to be electrically isolated from the assembly SPC1 and on-board network, or on the contrary enables the storage member S to be connected to the converter SPC1 and the assembly S-SPC1-G/S1 to be electrically isolated from the assembly SPC2-G/S2 for the stages of charging the member S or of providing a burst of assistance to an engine.

[0095] Another variant is described with reference to FIG. 12. The engine GT1 has an accessory board including motion takeoffs for two electrical machines, specifically a starter electrical machine D1 and a generator electrical machine G1. The machine D1, which is used for normal starting of the engine GT1, is powered by the on-board network, while the machine G1 is connected to the converter SPC1. The remainder of the electrical circuit is similar to that of FIG. 9. The single storage member S is dedicated to providing a burst of assistance to the engine GT2.

[0096] The reconfiguration member 123 enables the storage member S to be connected to the converter SPC2 and enables the assembly S-SPC2-G/S2 to be electrically isolated from the assembly SPC1-G1, or on the contrary it enables the storage member S to be connected to the converter SPC1 and the assembly S-SPC1-G/S1 to be electrically isolated from the assembly SPC2-G/S2-GT2 for the stages of charging the member S by one or other of the engines or of providing a burst of assistance to the engine GT2.

[0097] The reconfiguration member 123 is also capable of keeping the assembly SPC2(G/S2)-SPC1(G1) connected together for the stages of flight that involve powering the electrical machine G/S2 by the electrical machine G1.

[0098] The assembly constituted by the elements G1, SPC1, 123, S, SPC2, and G/D2 is an independent electrical assembly referenced 103. It is independent from the on-board network.

[0099] Another variant is described with reference to FIG. 13.

[0100] The electrical assembly 104 comprises a converter SPC1 connected to the on-board network.

[0101] It also comprises a converter SPC2 connected to switch members 130 for connecting it either to the electrical machine G/S1 connected to the engine GT1, or to the electrical machine G/S2 connected to the engine GT2. The two electrical machines G/S1 and G/S2 must not both be connected at the same time to the converter SPC2.

[0102] A configuration member 124 also serves to connect the sole storage member S to the converter SPC1 in order to be charged by the on-board network, or to the converter SPC2 to provide a burst of assistance to one of the engines GT1 and GT2, as a function of the position of the switch member 130. The storage member S can also be connected to both converters SPC1 and SPC2 simultaneously.

[0103] The reconfiguration member 104 enables the converter SPC1 to be connected via the switch member 130 to the converter SPC2 in order to provide prolonged assistance to the gas generator of the engine connected to the converter SPC2 (in particular in standby mode with the combustion chamber extinguished while the gas generator is maintained in the preferred ignition window, i.e. in super idle mode, and in standby mode with the chamber ignited, i.e. in assisted super idle mode).

[0104] It should be recalled that the on-board network is powered by one or more alternators that are driven indirectly or directly by at least one of the engines GT1 or GT2, and that when one or the other of them is extinguished, it is necessarily the other one that provides power in prolonged manner to the on-board network.

[0105] The invention is not limited to the embodiments described, but extends to all variants within the ambit of the scope of the claims.