Method and system for dual-voltage start of an aeronautical turbine engine having a free turbine and a single-spool gas generator

Abstract

The invention relates to a method for starting an aeronautical turbine engine of a twin-engine aircraft, the aeronautical turbine engine having a free turbine and a single-spool gas generator and comprising two independent electrical networks each being provided with a 28 V battery (BAT1, BAT2) selectively powering a starter-generator (S / G 1, S / G 2), wherein, in order to ensure, under the control of a computer for controlling the turbine engine (EECU), the turbine engine is started first with a voltage of 28 V by connecting the two batteries in parallel and then with a voltage of 56 V by connecting them in series, while the gas generator is prevented from accelerating at an excessive speed, the two batteries are connected in series only when the combustion chamber of the gas generator has been ignited and the speed of the gas generator is higher than a predetermined speed threshold NI, which makes it possible to provide, by the batteries being connected in series, a positive acceleration margin at the maximum drag point of the gas generator.

Description

[0001] METHOD AND SYSTEM FOR DUAL-VOLTAGE STARTING OF AN AERONAUTICAL TURBOMACHINE WITH FREE TURBINE AND SINGLE-BODY GAS GENERATOR

[0002] Technical Field

[0003] The present invention relates to the field of starting control of twin-engine aircraft turbomachines and more particularly concerns a method and a system for dual-voltage starting of an aeronautical turbomachine with free turbine and single-body gas generator.

[0004] Prior art

[0005] Climate change is a major concern for many legislative and regulatory bodies around the world. Indeed, various carbon emission restrictions have been, are being, or will be adopted by various states. In particular, an ambitious standard applies to both new aircraft types and those currently in operation, requiring the implementation of technological solutions to ensure their compliance with current regulations. Civil aviation has been mobilizing for several years now to contribute to the fight against climate change.

[0006] Technological research efforts have already led to very significant improvements in the environmental performance of aircraft. The Applicant takes into consideration the impact factors in all phases of design and development to obtain less energy-intensive, more environmentally friendly aeronautical components and products whose integration and use in civil aviation have moderate environmental impacts with the aim of improving the energy efficiency of these aircraft.

[0007] Consequently, the Applicant is constantly working to reduce its climate impact by using methods and operating virtuous development and manufacturing processes and minimizing greenhouse gas emissions to the minimum possible in order to reduce the environmental footprint of its activity.

[0008] This sustained research and development work focuses on new generations of aircraft turbomachines, the lightening of aircraft, particularly through the materials used and lighter on-board equipment, the development of the use of electrical technologies to ensure propulsion, and, as an essential complement to technological progress, aeronautical biofuels.

[0009] It is known that medium-power turbomachines (typically between 1500 and 4500 kW on the engine shaft) are complex to start because their gas generator has a significant drag torque, due in particular to significant mechanical friction and a high compressor pressure ratio, air flow rate and power draw due to the accessories driven by the gas generator (oil and fuel pumps in particular). Starting these turbomachines therefore generally requires either a pneumatic starter or a high-power high-voltage electric starter (for example powered by 115VAC / 400Hz).In both cases, the use of an auxiliary power generator (APU for "Auxiliary Power Unit") as a source of pneumatic or electrical power, on board the aircraft and previously started, is inevitable, which considerably complicates the architecture of the aircraft systems, therefore the overall mass and costs (in particular the costs of acquiring and overhauling the APU).

[0010] To avoid the use of an APU to start such a medium-power turbomachine, it is known to use a so-called double-spool gas generator, consisting of two separate coaxial compressor-turbine shafts and the bearing enclosures supporting these shafts, usually designated respectively HP (High Pressure) spool and LP (Low Pressure) spool. The starting torque required to start such a turbomachine is then equivalent to that of a low-power single-spool turbomachine, since the starter only has to drive the HP spool of the gas generator.Thus, and as shown in Figure 26, provided that it is powered by a battery of sufficient capacity, a 28V brushed starter-generator with a nominal power of 12kW / 400A in generation is capable of providing a starting torque 90 sufficient to compensate for the moderate drag torque 92 of the HP body characteristic of a double body architecture, in particular at the critical speed (point A) where this drag torque is maximum (positive acceleration margin M2).On the other hand, when starting an equivalent single-spool turbomachine, the higher drag torque 94 specific to this architecture exceeds the starting torque that such a 12kW / 400A starter-generator can provide, particularly in adverse ambient conditions (such as very low air, fuel and oil temperatures which maximize the resistive torques of the compressor and pumps), resulting in a negative acceleration margin Ml at the point of maximum drag torque (point B) and therefore the impossibility of starting the turbomachine in its entire desired starting range.

[0011] Furthermore, all other things being equal, a twin-spool turbomachine is significantly more complex from a mechanical point of view, more bulky, heavier and more expensive than a single-spool turbomachine of equivalent performance, in particular due to the two coaxial shafts which constitute its gas generator.

[0012] A simpler starting system than existing ones was therefore also proposed, i.e. not requiring the addition of an APU or a double-body architecture, and based on the use of two batteries with a nominal voltage of 28V connected first in parallel sharing the starting current and then in series to deliver a double voltage of 56V. Powering a brush starter-generator with two 28V batteries in series makes it possible to provide a significantly higher armature current and therefore a significantly higher mechanical torque capable of assisting the acceleration of the gas generator in the speed range where the drag torque of a single-body gas generator is too high for the starting torque obtained with a single 28V battery.

[0013] However, such a system is not able to manage the problem of the excessively rapid acceleration it provides during the gas generator ignition window, which is likely to degrade the combustion chamber ignition performance. On the other hand, supplying a brush starter with a double voltage of 56V (instead of 28V on a single battery) at initially zero speed causes, during the first moments of start-up, a very high torque which requires mechanically over-sizing the auxiliary transmission.

[0014] Statement of the invention

[0015] To this end, the invention is the result of technological research aimed at very significantly improving the performance of aircraft and, in this sense, contributes to reducing the environmental impact of these aircraft. For this purpose, the present invention therefore has as its main aim a method and a system for starting turbomachines of medium-power twin-engine aircraft overcoming the aforementioned drawbacks.

[0016] This aim is achieved by a method for starting an aeronautical turbomachine with free turbine and single-body gas generator of a twin-engine aircraft comprising two independent electrical networks each comprising a 28V battery selectively supplying a starter-generator, a turbomachine regulation computer controlling the starting of the turbomachine first at a nominal voltage of 28V by placing the two batteries in parallel and then at a nominal voltage of 56V by placing them in series while avoiding excessively rapid acceleration of the gas generator,characterized in that the turbomachine regulation computer is configured to only control the series connection of the two batteries once the gas generator combustion chamber is ignited and the speed of the gas generator is greater than a predetermined speed threshold NI making it possible to ensure, by this series connection, a positive acceleration margin at the point of maximum drag of the gas generator.,

[0017] Thus, excessively rapid acceleration during the ignition window can be avoided, ensuring optimal ignition conditions for the combustion chamber of the turbomachine. Advantageously, the series connection of the two batteries can also be controlled if, once the combustion chamber has been ignited and before the maximum drag point, the acceleration of the gas generator is below a predetermined acceleration threshold DN2, so as to avoid a risk of stagnation during startup.

[0018] Preferably, the speed threshold NI of the gas generator is between 10 and 25% of a nominal speed NTOP of the gas generator or the acceleration threshold DN2 of the gas generator is between 1 and 3% of a nominal speed NTOP / s of the gas generator.

[0019] Advantageously, the end of start threshold NCUTOFF corresponding to the speed from which the gas generator of the turbomachine is capable of accelerating by itself to idle speed, is between 50 and 60% of a nominal speed NTOP of the gas generator.

[0020] Preferably, in order to avoid a possible overlap of the electrical contactors resulting in short-circuiting a battery during reconfiguration, the series connection of the two batteries is preceded by a dead time lasting between 150 and 300 ms.

[0021] The invention also relates to a system for starting an aeronautical turbomachine with a free turbine and a single-body gas generator of a twin-engine aircraft comprising two independent electrical networks, each comprising a 28V battery selectively supplying a starter-generator, a turbomachine regulation computer controlling the starting of the turbomachine first at a nominal voltage of 28V by placing the two batteries in parallel and then at a nominal voltage of 56V by placing them in series while avoiding excessively rapid acceleration of the gas generator,characterized in that the turbomachine regulation computer is configured to only control the series connection of the two batteries once the combustion chamber of the gas generator is ignited and the speed of the gas generator is greater than a predetermined speed threshold NI making it possible to ensure, by this series connection, a positive acceleration margin at the maximum drag point of the gas generator. The starting system further comprises a ground socket intended to be connected to a 28V ground power unit in which the turbomachine regulation computer is further configured to supply a first starter-generator from the ground power unit and then by series connection of the ground power unit with one of the two batteries,once the combustion chamber is ignited and the speed of the gas generator is greater than a predetermined speed threshold NI, making it possible to ensure, through this series connection, a positive acceleration margin at the point of maximum drag of the gas generator.,

[0022] The invention finally relates to a rotary or fixed wing aircraft turbomachine comprising a starting system as mentioned above and the twin-engine aircraft incorporating it.

[0023] Brief description of the drawings

[0024] Other characteristics and advantages of the present invention will emerge from the description given below, with reference to the appended drawings which illustrate an exemplary embodiment thereof without any limiting character and in which:

[0025] [Fig. 1] Figure 1 schematically illustrates a starting system according to the invention applied to a single-spool aeronautical turbomachine,

[0026] [Fig. 2] Figure 2 shows an example of the electrical architecture of the starting system of Figure 1,

[0027] [Fig. 3-13] Figures 3 to 13 show the different stages of operation of the electrical architecture of Figure 2,

[0028] [Fig. 14] Figure 14 shows the variation curves of the starting torque and the drag torque as a function of the speed of the gas generator,

[0029] [Fig. 15] Figure 15 shows a control flowchart for the starting system of a single-body turbomachine according to the invention,

[0030] [Fig. 16-25] Figures 16 to 25 show the different stages of operation of the electrical architecture of Figure 2 from a ground outlet, and [Fig. 26] Figure 26 shows the variation curves of the starting and drag torques as a function of the speed of the gas generator for a single-spool turbomachine and a double-spool turbomachine.

[0031] Description of the embodiments

[0032] In multi-engine (and particularly twin-engine) architectures of rotary or fixed-wing aircraft, the 28VDC electrical network is already most of the time organized into at least two independent networks, one per generator and therefore one per engine, each including at least one 28V battery.

[0033] The invention is therefore based on the principle of reconfiguring two pre-existing 28V batteries during start-up, so as to supply the starter-generator (S / G) with a voltage of 28V (batteries in parallel) at the start of start-up, then 56V (batteries in series) in order to overcome the resistive torque at the point of maximum drag of the gas generator.

[0034] However, in order to minimize the starting torque at zero speed as well as the acceleration of the gas generator in the ignition window, as too rapid acceleration in this speed range can be detrimental to the ignition of the combustion chamber, it is advisable to only supply the starter-generator with 56V from a certain speed threshold, preferably when the combustion chamber is already ignited.

[0035] The invention therefore proposes a low-voltage electrical network architecture adapted to a twin-engine rotary or fixed-wing aircraft, including reconfiguration equipment (contactors), available voltage sources (batteries, ground socket, starter-generators) and associated with a strategy for managing this reconfiguration equipment, making it possible to supply the starter-generators of the free turbine and single-spool gas generator turbomachines first at a nominal voltage of 28V, then at a nominal voltage of 56V by placing two 28V batteries in series, avoiding excessively rapid acceleration in the ignition window.It should be noted for the remainder of the description that the voltages of 28V and 56V are nominal operating values, the no-load voltage of a 28V battery typically being likely to vary between 18V and 26V depending on its state of charge and its temperature, and the voltage actually applied to the terminals of the starter-generator being even lower due to voltage drops in the cables and contactors.

[0036] It should be noted that supplying a brushed starter-generator designed for a nominal voltage of 28V with 56V for the limited duration of starting a turbomachine - typically around twenty seconds - does not damage the latter, such a voltage of 56V remaining very significantly below the dielectric performance of the insulators, whether those of the windings of the rotating machine or the cables (see for example the aeronautical standards EN2282 and MIL-STD-704 which provide for transient overvoltages of the order of 50 to 60 Volts on 28V on-board networks).

[0037] Figure 1 illustrates an example of a single-body architecture of a medium-power free-turbine aeronautical turbomachine equipped with its starting system and intended to be mounted in a twin-engine aircraft. The turbomachine 10 conventionally comprises a compressor 12, a combustion chamber 14, a high-pressure turbine 16 (the assembly forming the gas generator), and a free turbine 18 driving the main transmission shaft 20 of the turbomachine actuating the main propulsion unit 22 (helicopter rotor or propeller for example) via a main mechanical reduction gear 24. An accessory box 26, mechanically linked to the gas generator, drives in particular the oil pumps and those associated with the fuel injectors of the combustion chamber, and generally also comprises a reduction assembly connecting it to the starting system 28.

[0038] Figure 2 shows a diagram of the electrical architecture of the 28V electrical network of a twin-engine aircraft (fixed wing or rotary wing) equipped with two free turbine turbomachines and single-spool gas generator such as that illustrated previously and adapted to the implementation of the invention. Conventionally, there are two DC BUS 1 and DC BUS 2 electrical networks, each comprising a 28V brushed starter-generator S / G 1 and S / G 2 and a 28V battery BATI and BAT2, a PS ground socket for connecting the on-board network to a 28V ground power unit GPU (for "Ground power unit"), as well as various contactors (electromechanical or static) for reconfiguring the on-board network and whose operation will be described further:

[0039] • K51 / K52 starter contactors for connecting starter-generator 1 or 2 to the corresponding DC BUS 1 or DC BUS 2 network,

[0040] • K61 / K62 park contactors allowing the PS park socket to be connected to the DC BUS 1 and DC BUS 2 networks respectively,

[0041] • A K4 coupling contactor, often called a “bus tie contactor” in English, typically used to connect the DC BUS 1 network to the DC BUS 2 network, particularly in the event of a failure of a starter-generator or a turbomachine in flight,

[0042] • K21 / K22 bus contactors for connecting battery 1 or battery 2 to the DC BUS 1 and DC BUS 2 networks respectively,

[0043] • Kll / K12 authorization contactors, specific to the invention, allowing the negative polarity of each of the two batteries to be disconnected from the aircraft reference potential (OV) in order to authorize series connection, and

[0044] • K31 / K32 connection contactors, also specific to the invention, allowing the batteries to be connected in series.

[0045] The operating sequence allowing the successive starting of the two turbomachines from the two on-board batteries is now detailed in figures 3 to 13. In the remainder of the description, by "starter" we mean "starter-generator".

[0046] Figure 3 illustrates the initial state of this electrical architecture corresponding to the shutdown of the two turbomachines and in which the two contactors K11 and K12 are closed, so that the negative polarity of each of the two batteries is referenced to the reference potential of the aircraft. All the other contactors are open. Figure 4 illustrates the following step corresponding to a first part of the start-up of turbomachine No. 1. When the start-up of turbomachine No. 1 is requested, contactors K21, K22, K4 and K51 are closed so as to supply starter No. 1 with a nominal voltage of 28V supplied by the two batteries in parallel. Simultaneously, the regulation computer of turbomachine No. 1 controls the injection of fuel into the combustion chamber according to an appropriate start-up law and energizes the spark plugs, causing the ignition of the combustion chamber.

[0047] Figure 5 illustrates the next step corresponding to the reconfiguration of the on-board network. From the moment when the speed NG of the gas generator exceeds a certain speed threshold NI or when its acceleration dNG / dt decreases below a certain threshold DN2, the development of which is detailed below, the contactors K11 and K4 are open. Starter No. 1 is therefore temporarily no longer supplied.

[0048] Figure 6 illustrates the next step corresponding to the second part of the start-up of turbomachine No. 1. As soon as the opening of contactors K11 and K4 is confirmed (the confirmation can be obtained for example via the acquisition by the regulation system of an auxiliary contact for copying the position of the main contacts of K11 and K4), contactor K32 is closed causing the starter to be powered by the two 28V batteries BATI and BAT2 then connected in series, one of the two batteries, in this case BAT2, being referenced to the reference potential of the aircraft. The start-up of turbomachine No. 1 therefore continues with its starter No. 1 powered at a nominal voltage of 56V.

[0049] Figure 7 illustrates the next step corresponding to the end of the start and the autonomous acceleration of the gas generator of turbomachine No. 1 to idle. When the NCUTOFF starter cut-off speed threshold is reached, contactor K32 is opened. The gas generator continues its acceleration under its own power under the effect of the expansion of the gases in the combustion chamber.

[0050] Figure 8 illustrates the next step corresponding to the switching of starter No. 1 to generation mode. When turbomachine No. 1a has completed its start and has reached the ground idle speed from which it is possible to draw power from its gas generator, contactor K11 is closed and starter No. 1 can recharge the BATI battery.

[0051] With turbomachine No. 1 started, the same must now be done with turbomachine No. 2.

[0052] Figure 9 therefore illustrates the following step corresponding to the first part of the start-up of turbomachine No. 2. When the start-up of turbomachine No. 2 is requested, contactors K4 and K52 are closed so as to supply starter No. 2 with a nominal voltage of 28V supplied by the two batteries in parallel as well as by starter No. 1 then operating in parallel as a generator (generally, a so-called "cross-start" function integrated into the GCU makes it possible to limit the current supplied by starter No. 1 to the level of its nominal current in generation, i.e. around 400A in the case of a 12kW generator / starter). Simultaneously, the regulation computer of turbomachine No. 2 controls the injection of fuel into the combustion chamber according to an appropriate start-up law and energizes the spark plugs causing the ignition of the combustion chamber.

[0053] Figure 10 illustrates the next step in reconfiguring the on-board network. From the moment the gas generator speed NG exceeds a certain speed threshold NI or when its acceleration dNG / dt decreases below a certain threshold, the development of which is detailed below, contactors K4 and K12 are open. Starter No. 2 is therefore temporarily no longer supplied.

[0054] Figure 11 illustrates the next step corresponding to the second part of the start-up of turbomachine No. 2. As soon as the opening of contactors K12 and K4 is confirmed, contactor K31 is closed allowing the starter to be powered by the two 28V batteries BATI and BAT2 then connected in series, the BATI battery as well as starter No. 1 being always connected in parallel and referenced to the reference potential of the aircraft. The start-up of turbomachine No. 2 therefore continues with starter No. 2 powered at a nominal voltage of 56V.

[0055] Figure 12 illustrates the next step corresponding to the end of the start and the autonomous acceleration of the gas generator of turbomachine No. 2 to idle. When the NCUTOFF starter cut-off speed threshold is reached, contactor K31 is opened and the gas generator of turbomachine No. 2 continues its acceleration under its own power. The two electrical networks DC BUS 1 and DC BUS 2 are from this moment isolated from each other.

[0056] Figure 13 illustrates the final step of switching starter No. 2 into generation mode. When turbomachine No. 2 has completed its start and reached the ground idle speed from which it is possible to draw power from its gas generator, contactor K12 is closed and starter No. 2 acting as a generator can recharge battery BAT2.

[0057] The on-board network is now configured in its nominal operating state. The two turbomachines are started, each starter operating as a generator supplies its own 28V network and can in particular recharge the corresponding battery, the two networks DC BUS 1 and DC BUS 2 being isolated from each other by contactors K4, K31 and K32 in the open position.

[0058] The shape of the starting torque curve as a function of the speed of the NG gas generator obtained by this device is illustrated in Figure 14. It can be noted that during the dead time T corresponding to the reconfiguration of the batteries BAT 1 and BAT2 (change from the parallel connection to the series connection of Figures 5 and 10), the starter torque briefly falls to zero since for a short period of time, the starter is no longer powered. Indeed, this dead time T is essential to confirm the opening of the contactors K4 and K11 (case of the start of turbomachine No. 1) or K4 and K12 (case of the start of turbomachine No. 2) before the reconfiguration of the batteries in series (closing of the contactor K32 or K31), in order to avoid a possible overlap leading to short-circuiting a battery, an event which must be absolutely avoided.Physically, with electromechanical contactors adapted to high starting currents, the duration of this dead time (opening confirmation + closing delay) is typically of the order of 150 to 300 ms. Given the high mechanical inertia of the single-spool gas generator of a medium-power turbomachine, the corresponding speed drop remains less than 1% of the nominal rotation speed NTOP of the gas generator typically corresponding to the maximum power of the turbomachine at take-off (in English "Take-Off Power"), which guarantees the absence of risk of extinction of the chamber and does not perceptibly penalize the starting performance of the turbomachine.

[0059] It can be noted on the starting curve that the initial starting torque at zero speed (point A), when the contactors are closed, is limited by the supply voltage corresponding to the parallel connection of the BATI and BAT2 batteries, i.e. a reduced voltage (at most equal to 28V) and that the ignition of the combustion chamber takes place when the starter is supplied with reduced voltage (batteries in parallel), while the acceleration of the gas generator remains moderate, therefore in favorable conditions.

[0060] It can also be noted that, after the series reconfiguration of the two batteries, the starter torque becomes sufficient to guarantee a positive acceleration margin M at the maximum drag point (point B). At the same time, the fact that the turbomachine is already rotating and therefore the starter provides a non-zero back electromotive force (fce.m.) makes it possible to limit the amplitude of the armature current and therefore the starter torque during the reconfiguration of the two batteries in series (point P) to a level significantly lower than that which would be obtained if the starter was powered up with the two batteries in series from the start of the start, at speed and therefore at zero fce.m. (point P'), thus avoiding mechanical oversizing of the accessory box and the auxiliary transmission.

[0061] Figure 15 shows a control flowchart for the start-up sequence of turbomachine No. 1 from the point of view of controlling the various contactors.

[0062] In an initial step 50, contactors K11 and K12 are initially closed, in order to reference the negative polarity of each battery to the reference potential of the device. Turbomachine No. 1 is therefore stopped (step 52).

[0063] Once the start order for turbomachine No. 1 has been given by the pilot in a step 54, the turbomachine control computer (EECU) in a following step 56 simultaneously commands the closing of contactors K21, K22, K4 and K51, so as to supply starter No. 1 with the two 28V batteries BATI and BAT2 in parallel, the fuel injection according to an appropriate start law and the energization of the spark plugs.

[0064] As the gas generator increases in speed, air is admitted into the combustion chamber, the sparks from the spark plugs ignite the mixture and the combustion chamber ignites, resulting, in a following step 58, in the detection of the ignition of the chamber by the EECU (which is done for example by noting the increase in temperature TET for “Turbine Inlet Temperature” or T45 of the combustion gases).

[0065] The next step 60 consists of detecting at least one of the following two conditions necessary for the series connection of the BATI and BAT2 batteries:

[0066] 1) NG greater than a fixed NI threshold, speed threshold from which it is considered that the ignition of the combustion chamber is sufficiently stabilized so that the acceleration of the gas generator following the connection of the two batteries in series no longer risks blowing out the flame and extinguishing the combustion chamber, which would have the consequence of interrupting the start. This NI threshold can be set at a value corresponding to the upper limit of the ignition window, for example between 10 and 25% of NTOP (essentially a function of the combustion chamber technology),

[0067] 2) Or dNG / dt lower than a fixed DN2 threshold, indicating that the acceleration margin between the starter torque supplied at 28V and the drag torque of the gas generator becomes too low, with the risk of stagnation at start-up (inability of the gas generator to accelerate although the chamber is lit) and damage by overtemperature of the turbomachine. This minimum acceleration criterion of the gas generator can be set at a value of the order of 1 to 3% of NTOP / s.

[0068] The series connection of the two batteries thus achieved then makes it possible to ensure a positive acceleration margin when the speed of the NG gas generator approaches the maximum resistive torque zone, while protecting the turbomachine against the risks of stagnation at start-up and extinction of the combustion chamber.

[0069] When at least one of these two conditions is met, the opening of the contactors K11 and K4 is then commanded in a new step 62, and after a step 64 of confirmation of the effective opening of the contactors K11 and K4 (carried out for example, in the case of electromechanical contactors, via the rereading of position feedback contacts of the main contacts), the closing of the contactor K32 is commanded in a step 66. When, in a step 68, the speed of the gas generator reaches the end of start threshold NCUTOFF corresponding to the speed from which the gas generator of the turbomachine is capable of accelerating by itself to the idle speed, generally of the order of 50 to 60% of NTOP, the EECU, in a step 70, commands the extinction of the spark plugs and the opening of the contactor K32 so as to cut off starter no. 1.

[0070] The gas generator then continues to accelerate by itself in the next step 72. Once the idle speed is reached, the starter of turbomachine no. 1 can be switched to electrical generation mode in order to supply the 28V DC BUS 1 on-board network and recharge the BATI battery.

[0071] The flowchart of the starting sequence of turbomachine No. 2 is of course similar. The start-up operates in a similar manner by following the same steps and in particular using the same conditions based on the speed or acceleration measurement of the gas generator to trigger the transition from the parallel connection to the series connection of the BATI and BAT2 batteries to power starter No. 2.

[0072] It will be noted that at any time (steps 74, 76 or 78), the pilot can issue a command to stop the turbomachine, initiating two new steps in which the contactor K32 is open (step 80) and where the EECU cuts the fuel injection, turns off the spark plugs, opens the contactors and thus deactivates the two starters (step 82), the gas generator finding itself in an autorotation regime in the following step 84 before the turbomachine returns to its initial stop position of step 52 when the NG speed becomes zero (step 86). Furthermore, the diagram in Figure 2 is also compatible with a ground start on a ground power unit (GPU), the main advantage being to save the charge of the on-board batteries. The start sequence is illustrated in Figures 16 to 25.

[0073] Figure 16 illustrates the initial state of the electrical architecture, which is identical to the battery start corresponding to the shutdown of the two turbomachines, and in which the two contactors K11 and K12 are closed, so that the negative polarity of each of the two batteries is referenced to the reference potential of the aircraft. All the other contactors are open.

[0074] Figure 17 illustrates the next step corresponding to the first part of the start of turbomachine No. 1. When the start of turbomachine No. 1 is requested, contactors K62, K4 and K51 are closed so as to supply starter No. 1 with a nominal voltage of 28V supplied by the GPU. Simultaneously, the turbomachine control computer (EECU) of turbomachine No. 1 controls the injection of fuel into the combustion chamber according to an appropriate start law and energizes the spark plugs igniting the combustion chamber.

[0075] Figure 18 illustrates the on-board electrical network reconfiguration step. From the speed threshold NI or acceleration DN2 defined above, contactors K4 and K11 are open. Starter No. 1 is therefore temporarily no longer powered.

[0076] Figure 19 illustrates the next step corresponding to the second part of the start-up of turbomachine No. 1. As soon as the opening of contactors K4 and K11 is confirmed, contactors K32 and K21 are closed and the starter is then powered by the GPU and the 28V BATI battery in series, the GPU being referenced to the reference potential of the aircraft. The start-up of turbomachine No. 1 therefore continues with its starter powered at a nominal voltage of 56V.

[0077] Figure 20 illustrates the next step corresponding to the end of the start and the autonomous acceleration of the gas generator of turbomachine No. 1 to idle. When the NCUTOFF starter cut-off speed threshold is reached, contactors K21, K32, K51 and K62 are opened. The gas generator of turbomachine No. 1 continues its acceleration under its own power.

[0078] Figure 21 illustrates the step corresponding to the first part of the start of turbomachine No. 2. When the start of turbomachine No. 2 is requested, contactors K61, K4 and K52 are closed so as to supply starter No. 2 with a nominal voltage of 28V supplied by the GPU, while K11 is closed to reference the 28V battery BATI to aircraft ground again. Simultaneously, the EECU of turbomachine No. 2 controls the injection of fuel into the combustion chamber according to an appropriate start law and energizes the spark plugs causing the ignition of the combustion chamber.

[0079] Figure 22 corresponds to the reconfiguration of the on-board electrical network. From the speed threshold NI or acceleration DN2 defined above, contactors K4 and K12 are open. Starter No. 2 is therefore temporarily no longer powered.

[0080] Figure 23 illustrates the step corresponding to the second part of the start-up of turbomachine No. 2. As soon as the opening of contactors K4 and K12 is confirmed, contactors K31 and K22 are closed and starter No. 2 is then powered by the GPU and the 28V battery BAT2 in series, the GPU being referenced to the reference potential of the aircraft. The start-up of turbomachine No. 2 therefore continues with starter No. 2 powered at a nominal voltage of 56V.

[0081] Figure 24 illustrates the next step corresponding to the end of the start and the autonomous acceleration of the gas generator of turbomachine No. 2 to idle. When the NCUTOFF starter cut-off speed threshold is reached, contactors K22, K31, K52 and K61 are opened, the two electrical networks DC BUS 1 and DC BUS 2 being from this moment isolated from each other. The gas generator then continues its acceleration under its own power.

[0082] Figure 25 illustrates the final step performed once the turbomachine has reached its idle speed, and consisting of switching the two starters to generator mode and recharging the two batteries by closing the contactors K51, K21, K12, K22 and K52. The invention thus has numerous advantages compared to a starting system requiring an APU or a turbomachine architecture with a double-spool gas generator:

[0083] • It only requires standard equipment available on shelves (28VDC brushed starter-generators, series starter, high current contactors),

[0084] • It is fully compatible with a 28VDC on-board electrical network provided that two 28V starter batteries of sufficient capacity are available, • It avoids oversizing the accessory box, the starter being powered with the two batteries in series only above a certain speed threshold,

[0085] • It allows to start a medium power turbomachine with single body gas generator and high drag torque, by limiting the torque in the ignition window of the combustion chamber, and

[0086] • The associated software modifications for managing the boot sequence are simple and therefore technically low risk in terms of development.

Claims

Claims

1. Method for starting an aeronautical turbomachine with free turbine and single-spool gas generator of a twin-engine aircraft comprising two independent electrical networks each comprising a 28V battery (BAT 1, BAT2) selectively supplying a starter-generator (S / G 1, S / G 2), characterized in that to ensure, under the control of a turbomachine regulation computer (EECU), the starting of the turbomachine first under a nominal voltage of 28V by placing the two batteries in parallel and then under a nominal voltage of 56V by placing them in series while avoiding excessively rapid acceleration of the gas generator, the placing in series of the two batteries is only commanded once the combustion chamber is ignited and the speed of the gas generator is greater than a predetermined speed threshold NI making it possible to ensure, by this placing in series, a positive acceleration margin at the point of maximum drag of the gas generator.

2. A starting method according to claim 1, wherein the speed threshold NI of the gas generator is between 10 and 25% of a nominal speed NTOP of the gas generator.

3. A starting method according to claim 1, wherein the series connection of the two batteries is further controlled if, once the combustion chamber is ignited and before the point of maximum drag, the acceleration of the gas generator is less than a predetermined acceleration threshold DN2, so as to avoid a risk of stagnation of the start.

4. A starting method according to claim 3, wherein the acceleration threshold DN2 of the gas generator is between 1 and 3% of a nominal speed NTOP / s of the gas generator.

5. A starting method according to claim 1, wherein, in order to avoid a possible overlap of electrical contactors resulting in short-circuiting a battery during reconfiguration, the setting in series of the two batteries is preceded by a dead time whose duration is between 150 and 300ms.

6. System for starting an aeronautical turbomachine with free turbine and single-body gas generator of a twin-engine aircraft comprising two independent electrical networks each comprising a 28V battery (BAT 1, BAT2) selectively supplying a starter-generator (S / G 1, S / G 2), a turbomachine regulation computer (EECU) controlling the starting of the turbomachine (10) first under a nominal voltage of 28V by placing the two batteries in parallel and then under a nominal voltage of 56V by placing them in series while avoiding excessively rapid acceleration of the gas generator,characterized in that the turbomachine regulation computer is configured to only control the series connection of the two batteries once the combustion chamber of the gas generator is ignited and the speed of the gas generator is greater than a predetermined speed threshold NI making it possible to ensure, by this series connection, a positive acceleration margin at the maximum drag point of the gas generator.,

7. A starting system according to claim 6, further comprising a ground socket (PS) intended to be connected to a 28V ground power unit (GPU).

8. Starter system according to claim 7, in which the turbomachine regulation computer is further configured to power a first starter-generator (S / G 1; S / G 2) from the ground power group then by putting the ground power group in series with one (BATI; BAT 2) of the two batteries, once the combustion chamber is ignited and the speed of the gas generator is greater than a predetermined speed threshold NI making it possible to ensure by this putting in series a positive acceleration margin at the point of maximum drag of the gas generator.

9. Twin-engine rotary or fixed-wing aircraft turbomachine comprising a starting system according to any one of claims 6 to 8.

10. Twin-engine rotary or fixed-wing aircraft comprising two turbomachines according to claim 9.

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