Method and system for starting 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 having a free turbine and a single-spool gas generator, wherein, in order to ensure, under the control of a computer for controlling the turbine engine, the turbine engine is started only from a battery delivering a nominal DC voltage of 28 V, two electric machines coupled to the same accessory drive mechanically linked to the gas generator of the turbine engine and connected in parallel to the battery are actuated sequentially, the first electric machine being started with a starting torque enabling an increase in the speed of the gas generator with a given minimum acceleration and the second electric machine being started only after the ignition of the combustion chamber of the gas generator has been detected and only before a critical speed corresponding to the maximum resisting torque of the gas generator has been reached, the sum of the starting torques (40) produced by the two electric machines being sufficient to ensure, at all times, that the torque margin M at the maximum drag point B corresponding to the maximum resisting torque (42) of the generator is positive.

Description

[0001] Description

[0002] Title of the invention: Method and system for starting an aeronautical turbomachine with free turbine and single-body gas generator

[0003] Technical Field

[0004] The present invention relates to the field of controlling the start of aircraft turbomachines and it relates more particularly to a method and a system for starting an aeronautical turbomachine with free turbine and single-body gas generator.

[0005] Prior art

[0006] 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.

[0007] 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.

[0008] 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.

[0009] 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.

[0010] 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).

[0011] 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 6, 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 80 sufficient to compensate for the moderate drag torque 82 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 84 specific to this architecture may exceed the starting torque that such a 12kW / 400A starter-generator can provide, particularly under 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.

[0012] 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.

[0013] Thus, there is a need for a simpler starting system than those existing, i.e. one that does not require the addition of an APU or a double-body architecture, and which therefore provides, particularly in the context of re-engining, the least possible modification to an existing single-body architecture of a rotary or fixed-wing aircraft and whose main low-voltage on-board electrical network is supplied exclusively with 28VDC from a battery.

[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 medium-power aircraft turbomachines overcoming the aforementioned drawbacks.

[0016] This aim is achieved by a method for starting an aeronautical turbomachine with a free turbine and a single-body gas generator, characterized in that to ensure, under the control of a turbomachine regulation computer, the starting of the turbomachine solely from a battery delivering a nominal direct voltage of 28V, two electrical machines coupled to the same accessory box mechanically linked to the gas generator of the turbomachine and mounted in parallel on the battery are actuated sequentially, the first electrical machine being started under a starting torque allowing a rise in speed of the gas generator with a determined minimum acceleration, and the second electrical machine being started only after detection of the ignition of the combustion chamber of the gas generator and before reaching a critical speed corresponding to the maximum resistive torque of the gas generator,the sum of the starting torques developed by the two electrical machines being sufficient to guarantee in all circumstances that the torque margin at the point of maximum drag corresponding to the maximum resistive torque of the gas generator is positive.,

[0017] Thus, by this simple structure with two 28V brushed electric machines available as standard on shelves, the torque margin at the maximum drag point of the gas generator is guaranteed without resorting to an APU but with the only 28VDC battery supplying the aircraft's on-board electrical network.

[0018] Preferably, once the ignition of the combustion chamber of the gas generator has been detected, in order to start the second electrical machine before reaching the critical speed, it is further verified that the rotational speed of the gas generator NG is greater than a predetermined minimum speed NI or that the acceleration of the gas generator is less than a predefined acceleration threshold DN2.

[0019] Advantageously, the predetermined minimum speed is defined so as to ensure moderate acceleration of the gas generator in a preferential ignition window of the combustion chamber, and the predefined acceleration threshold DN2 calculated so as to avoid a phenomenon of stagnation of the gas generator in a speed zone corresponding to the maximum drag torque.

[0020] Preferably, the turbomachine control computer stops the first and second electric machines when, the maximum drag point having been exceeded, the speed of the gas generator of the turbomachine has reached a threshold from which the gas generator is capable of accelerating itself to idle speed.

[0021] The invention also relates to a system for starting an aeronautical turbomachine with a free turbine and a single-body gas generator, a turbomachine control computer controlling the starting of the turbomachine solely from a battery delivering a nominal direct voltage of 28V, characterized in that it comprises two electrical machines coupled to the same accessory box mechanically linked to the gas generator of the turbomachine and mounted in parallel on the battery, the turbomachine control computer being configured to sequentially actuate the two electrical machines, the first electrical machine being started under a starting torque allowing a rise in speed of the gas generator with a determined minimum acceleration,and the second electrical machine being started only after detection of ignition of the combustion chamber of the gas generator and before reaching a critical speed corresponding to the maximum resistive torque of the gas generator, the sum of the starting torques developed by the two electrical machines being sufficient to guarantee in all circumstances that the torque margin at the point of maximum drag corresponding to the maximum resistive torque of the gas generator is positive.,

[0022] According to the embodiment envisaged, the first and second electrical machines may be two identical 28V brush starter-generators each equipped with their own regulator box (GCU for “Generator Control Unit”) or the first electrical machine may be a 28V series starter and the second electrical machine a 28V brush starter-generator equipped with its own regulator box GCU. Preferably, each of the two electrical machines is respectively connected to the battery by an associated starter contactor.

[0023] The invention finally relates to a rotary or fixed wing aircraft turbomachine comprising a starting system as mentioned above and the aircraft incorporating it.

[0024] Brief description of the drawings

[0025] 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:

[0026] [Fig. 1] Figure 1 schematically illustrates a starting system according to the invention applied to a single-spool aeronautical turbomachine,

[0027] [Fig. 2] Figure 2 shows a first variant of the electrical architecture of the starting system of Figure 1,

[0028] [Fig. 3] Figure 3 shows a second variant of the electrical architecture of the starting system of Figure 1,

[0029] [Fig. 4] Figure 4 shows the variation curves of the starting torque and the drag torque as a function of the speed of the gas generator,

[0030] [Fig.5] Figure 5 illustrates the different stages of the starting process of a single-body turbomachine according to the invention, and

[0031] [Fig. 6] Figure 6 shows the variation curves of the starting and drag torques as a function of the gas generator speed for a single-spool turbomachine and a double-spool turbomachine.

[0032] Description of the embodiments

[0033] The invention consists in proposing a system for starting a medium-power gas turbine (typically between 1500kW and 4500kW on the motor shaft) based on two conventional electrical machines (with brushes) coupled to the same accessory box and powered by 28VDC, thus making it possible to avoid the use of an auxiliary power unit, an expensive element and presenting significant installation constraints on an aircraft equipped with a propulsion turbomachine, helicopter turboshaft or aircraft turboprop and turbofan, of medium power with free turbine and single-spool gas generator, which constitutes the ideal solution from a mass and cost point of view for powering a rotary-wing or fixed-wing aircraft, equipped exclusively with a 28V DC on-board network.

[0034] Figure 1 illustrates an example of a single-body architecture of a medium-power free-turbine aeronautical turbomachine equipped with its starting system. 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 also comprises a reduction assembly connecting it to the starting system 28.

[0035] According to the invention, the starting system comprises two 28VDC brushed electrical machines 30, 32 powered from a battery and which will be activated sequentially to ensure the starting of the aeronautical turbomachine. This battery, which can be a single battery or a group of batteries delivering the nominal direct voltage of 28V, also provides power to the main on-board electrical network of the aircraft.

[0036] It should be noted for the remainder of the description that the 28V voltage is a nominal operating value, the no-load voltage of a 28V battery typically being likely to vary between 18V and 26V depending on its state of charge and 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.More specifically, it is planned to begin the start-up with a single electric machine, under a torque allowing to ensure a moderate increase in speed of the gas generator allowing to optimize the probabilities of ignition of the combustion chamber, the second electric machine being started only after detection of the ignition of the gas generator and before reaching a critical speed corresponding to the maximum resistive torque of the gas generator, the sum of the starting torques developed by the two electric machines being sufficient to guarantee in all circumstances that the torque margin at the critical point (i.e. at the maximum resistive torque corresponding to the point of maximum drag of the gas generator) is positive.

[0037] Figures 2 and 3 show two examples of possible embodiments for the electrical architecture of the starting system according to the invention in which, depending on the embodiment envisaged, the two 28VDC brushed electrical machines coupled to the accessory box 26 are respectively two identical starter-generators, or a starter-generator and a simple series starter (series excitation electrical machine).

[0038] Each electrical machine 30, 32; 30, 34 is equipped with its own starting contactor 30A, 32A, 34A which ensures its connection with a common 28VDC starting battery 36. Of course, if a battery common to the two electrical machines has been represented, it is also possible to envisage each electrical machine being powered by a separate battery, or that the battery is made up of two different batteries of the same nominal voltages connected in parallel during starting. The brush starter-generators are each equipped with an electronic regulation unit 30B, 32B (GCU for "Generator Control Unit"), whereas the simple series starter is without one (so that it cannot operate as a generator).

[0039] In Figure 2, the two electrical machines 30, 32 which are two identical starter-generators therefore have two power take-offs having the same reduction ratio. On the other hand, in Figure 3, the two electrical machines being different, the power take-off of the series starter 34 will therefore have a reduction ratio different from that of the starter-generator 30, since once the series starter is cut, it is separated from the gas generator and therefore does not have to be sized for the mechanical constraints linked to the nominal operating speed NG of the gas generator.

[0040] Indeed, the power take-off (or power take-off corresponding to the transmission shaft) of the series starter on the accessory box 26 is generally equipped with a freewheel, in order to mechanically separate the rotor of the electric machine from the accessory box when the series starter is switched off. This makes it possible to optimize the maximum speed and therefore the dimensioning of this series starter, which does not have to support the drive by the gas generator at its nominal operating speed, but only the cut-off speed of the series starter which is generally of the order of 50 to 60% of the nominal rotation speed NG of the shaft of the gas generator of the turbomachine. Due to the presence of this freewheel, it is preferable for the series starter to be energized first, and the starter-generator second.Indeed, if the series starter is energized second while the gas generator is already rotating under the effect of the drive by the starter-generator, its rotor, initially at a standstill because it is decoupled from the accessory box by the freewheel, will accelerate extremely quickly (because it is idle) until the freewheel suddenly resynchronizes with the accessory box, which could damage it.

[0041] Figure 4 illustrates the variation curves of the starting torque 40 of the starting system and the resistive or drag torque 42 of the gas generator as a function of the rotational speed NG of the main shaft of the turbomachine. In a first part (I) of these curves 40, 42, only one electrical machine is energized by closing its associated starting contactor. Given the resistive torque and the high inertia of the single-body gas generator, the increase in speed is relatively slow, which is favorable to the ignition of the combustion chamber of the turbomachine. On the other hand, the initial torque at power-up under zero speed (point A) is not higher than for a standard double-body configuration, which avoids mechanically oversizing the accessory box.Then, when in a second part (II) of curve 42, the drag torque begins to increase sharply and the starting torque of the first electric machine risks being insufficient to continue accelerating the gas generator, the second electric machine is energized by closing its associated starting contactor, so as to provide the additional torque necessary to ensure a positive acceleration margin M at the point of maximum drag, located around 30 to 40% of NG (point B). A third part (III) then begins, where the starting and drag torques decrease and if the start of the turbomachine is successful, the two electric machines are cut off simultaneously around 50 to 60% of NG, the gas generator continuing to accelerate by its own means under the control of the turbomachine control computer (EECU), until reaching its nominal idle speed.

[0042] A control flowchart for the electrical machines is illustrated in Figure 5. The turbomachine being initially stopped in a first step 50, a start order for the turbomachine is given by the pilot in a step 52 so that the turbomachine control computer (EECU) simultaneously commands in a following step 54 the energization of starter No. 1 (via the closing of the associated contactor), the injection of fuel according to an appropriate start law, and the energization of the spark plugs. As the gas generator increases in speed, air is admitted into the combustion chamber, the sparks from the spark plugs ignite the air-fuel mixture and the combustion chamber ignites.When the ignition of the chamber has been detected by the EECU in a new step 56, which is usually done by noting the increase in temperature of the combustion gases at the turbine inlet (TIT for “Turbine Inlet Temperature”), it is checked in a step 58 whether the speed NG of the turbomachine is greater than a predetermined speed NI.This NI threshold is a compromise between, on the one hand, a sufficiently low value allowing, under the most penalizing conditions (engine and oil impregnated at very low temperature, maximum air density, lowest starter supply voltage), to ensure that the energization of starter No. 2 will allow the combined starter torque of the two electrical machines to cross the point of maximum drag torque of the gas generator, and on the other hand, a sufficiently high value allowing to remain as long as possible in the ideal ignition window of the combustion chamber with only starter No. 1 energized, so as to minimize the acceleration of the gas generator in this speed range and thus optimize the ignition conditions.An alternative condition may consist, in this step 58, in directly checking whether the acceleration of the gas generator dNG / dt becomes lower than an acceleration threshold DN2, meaning that the margin between the engine torque supplied by starter no. 1 on the one hand and the resistive torque of the gas generator on approaching point B on the other hand becomes insufficient to continue to accelerate the gas generator (phenomenon known as stagnation, potentially damaging to the turbomachine).

[0043] When at least one or the other of these two conditions is verified, the energization of starter No. 2 is then commanded in a following step 60 by closing the associated contactor. The added torque of the two starters makes it possible to maintain a positive acceleration margin when the drag torque of the gas generator is at its highest. When in a new step 62 the speed NG of the turbomachine reaches the end of start speed NCUTOFF generally of the order of 50 to 60% of NG, that is to say the one where the turbomachine under the effect of the expansion of the combustion gases in the turbine of the gas generator is capable of accelerating by itself to the idle speed, the EECU, in a step 64, turns off the spark plugs, opens the contactors and thus deactivates the two starters, the turbomachine then being considered as started in the final step 66.Once idle speed is reached, the starter-generator(s) can be switched to electrical generation mode and supply power to the aircraft's 28VDC on-board electrical network, in particular to recharge the battery used to supply the two electrical machines with the energy needed for starting.

[0044] It will be noted that at any time (steps 68 or 70), the pilot can issue a command to stop the turbomachine, initiating a new step 72 in which the EECU cuts the fuel injection, turns off the spark plugs, opens the contactors and thus deactivates the two starters, the gas generator finding itself in an autorotation regime in the following step 74 before the turbomachine returns to its initial stop position of step 50 when the NG speed becomes zero. The invention thus has numerous advantages compared to a starting system requiring an APU or a double-spool architecture:

[0045] • It only requires standard equipment available on the shelf (28VDC brushed generators, series starter, high current contactors), • It is fully compatible with a 28VDC on-board electrical network (provided you have one or more starter batteries of sufficient capacity),

[0046] • It avoids oversizing the accessory box for the torque that the two starters would provide when closing the starter contactor if they were energized simultaneously,

[0047] • 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,

[0048] • In the two-generator-starter variant, it provides redundancy of the 28VDC electrical generation, and

[0049] • 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-body gas generator, characterized in that to ensure, under the control of a turbomachine regulation computer, the starting of the turbomachine (10) solely from a battery (36) delivering a nominal direct voltage of 28V, two electric machines (30, 32;30, 34) coupled to the same accessory box (26) mechanically linked to the gas generator of the turbomachine and mounted in parallel on the battery are actuated sequentially, the first electric machine being started under a starting torque allowing a rise in speed of the gas generator with a determined minimum acceleration, and the second electric machine being started only after detection of the ignition of the combustion chamber (14) of the gas generator and before reaching a critical speed corresponding to the maximum resistive torque of the gas generator, the sum of the starting torques developed by the two electric machines being sufficient to guarantee in all circumstances that the torque margin at the maximum drag point corresponding to the maximum resistive torque of the gas generator is positive.;

2. A starting method according to claim 1, wherein, once the ignition of the combustion chamber (14) of the gas generator has been detected, to start the second electrical machine before reaching the critical speed, it is further verified that the rotational speed of the gas generator is greater than a predetermined minimum speed NI or that the acceleration of the gas generator is less than a predefined acceleration threshold DN2.

3. A starting method according to claim 2, wherein the predetermined minimum speed NI is defined so as to ensure moderate acceleration of the gas generator in a preferential ignition window of the combustion chamber, and the predefined acceleration threshold DN2 calculated to avoid a phenomenon of stagnation of the gas generator in a speed zone corresponding to the maximum drag torque.

4. A starting method according to any one of claims 1 to 3, in which the turbomachine control computer stops the first and second electrical machines when, the maximum drag point having been exceeded, the speed of the gas generator of the turbomachine has reached a threshold from which the gas generator is capable of accelerating by itself to idle speed.

5. System for starting an aeronautical turbomachine with free turbine and single-body gas generator, a turbomachine regulation computer controlling the starting of the turbomachine (10) solely from a battery (36) delivering a nominal direct voltage of 28V, characterized in that it comprises two electrical machines (30, 32;30, 34) coupled to the same accessory box (26) mechanically linked to the gas generator of the turbomachine and mounted in parallel on the battery, the regulation computer of the turbomachine being configured to sequentially actuate the two electric machines, the first electric machine being started under a starting torque allowing a rise in speed of the gas generator with a determined minimum acceleration, and the second electric machine being started only after detection of the ignition of the combustion chamber (14) of the gas generator and before reaching a critical speed corresponding to the maximum resistive torque of the gas generator, the sum of the starting torques developed by the two electric machines being sufficient to guarantee in all circumstances that the torque margin at the maximum drag point corresponding to the maximum resistive torque of the gas generator is positive.;

6. A starting system according to claim 5, wherein the first and second electrical machines are two identical 28V brushed starter-generators (30, 32) each equipped with their own regulation box (GCU - 30B, 32B).

7. A starting system according to claim 5, wherein the first electrical machine is a 28V series starter (34) and the second electrical machine is a 28V brushed starter-generator (30) equipped with its regulating box (GCU - 30B).

8. A starting system according to claim 5, wherein each of the two electrical machines is respectively connected to the battery (36) by an associated starting contactor (30A, 32A; 34A).

9. A rotary or fixed wing aircraft turbomachine comprising a starting system according to any one of claims 5 to 8.

10. Rotary or fixed wing aircraft comprising at least one turbomachine according to claim 9.

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