Dual-voltage start-up method and system for aero-turbine engines with free turbine and single-shaft gas generator

Abstract

This invention relates to a starting method for an aircraft turbine engine in a twin-engine aircraft. The aircraft turbine engine has a free turbine and a single-shaft gas generator and includes two independent power grids. Each power grid is equipped with 28-volt batteries (BAT1, BAT2) that selectively power a starter generator (S / G1, S / G2). To ensure the turbine engine is started under the control of the turbine regulation computer (EECU), it is first started at 28 volts by connecting the two batteries in parallel, and then started at 56 volts by connecting the two batteries in series. Furthermore, to avoid excessive acceleration of the gas generator, the two batteries are connected in series only when the combustion chamber of the gas generator is ignited and the speed of the gas generator is greater than a predetermined speed threshold N1, so as to provide a positive acceleration margin at the point of maximum drag of the gas generator through the series connection of the batteries.

Description

Technical Field

[0001] This invention relates to the field of start-up control of twin-engine aircraft turbines, and more specifically, to a dual-voltage start-up method and system for an aircraft turbine having a free turbine and a single-shaft gas generator. Background Technology

[0002] Climate change is a major issue of concern to numerous legislative and regulatory bodies worldwide. In fact, countries have adopted, are adopting, or will adopt various carbon emission control measures. In particular, a highly challenging standard applies to both new and currently in-service aircraft, requiring technological solutions to ensure compliance with existing regulations. The civil aviation sector has been actively working to contribute to addressing climate change for many years.

[0003] Technological research and development has significantly improved the environmental performance of aircraft. The applicant considers influencing factors at every stage of design and development to obtain lower energy consumption and more environmentally friendly aviation components and products. The integration and use of these components and products in civil aviation will only produce a moderate environmental impact, aiming to improve the energy efficiency of aircraft.

[0004] Therefore, the applicant is committed to reducing climate impact by adopting sound development and manufacturing methods and processes that minimize greenhouse gas emissions and thus reduce the environmental footprint of related activities.

[0005] These ongoing research and development efforts focus on: next-generation aircraft turbines, aircraft weight reduction (especially through the use of materials and lighter avionics), the development of electric propulsion technologies, and aviation biofuels as an important complement to technological progress.

[0006] The starting process of medium-power turbines (engine shaft power typically between 1500 kW and 4500 kW) is known to be complex due to the significant drag torque of their gas generators—particularly stemming from high mechanical friction, high compressor pressure ratio, airflow, and power extraction from gas generator-driven accessories (especially oil and fuel pumps). Therefore, starting these turbines usually requires a pneumatic starter or a high-power, high-voltage electric starter (e.g., powered by a 115V AC / 400Hz power supply). In both cases, an auxiliary power unit (APU) pre-started on the aircraft must be used as the pneumatic or electrical source, which significantly increases the architectural complexity of the aircraft system, thereby increasing overall weight and cost (especially the procurement and overhaul costs of the APU).

[0007] To avoid using an APU to start such medium-power turbines, a generator known as a dual-shaft gas generator is employed. This generator comprises two independent coaxial compressor-turbine shafts and bearing housings supporting these shafts, typically referred to as the high-pressure (HP) shaft and the low-pressure (LP) shaft, respectively. In this case, the starting torque required to start such a turbine is comparable to that of a low-power single-shaft turbine, because the starter only needs to drive the high-pressure shaft of the gas generator.

[0008] Therefore, as Figure 26 As shown, if powered by a battery of sufficient capacity, a 28-volt brushed starter generator with a power generation rating of 12 kW / 400 A can provide a starting torque 90 sufficient to compensate for the moderate high-pressure shaft drag torque 92 inherent in dual-shaft architectures, especially at the critical speed (point A) where this drag torque reaches its maximum value (positive acceleration margin M2). On the other hand, when starting a single-shaft turbine of equivalent power, the greater drag torque 94 inherent in this architecture exceeds the starting torque provided by such a 12 kW / 400 A starter generator, especially under harsh environmental conditions (such as extremely low air, fuel, and lubricating oil temperatures, which cause the compressor and pump drag torques to reach their maximum), resulting in a negative acceleration margin M1 at the point of maximum drag torque (point B), thus preventing the turbine from starting across the desired entire starting range.

[0009] Furthermore, all other things being equal, a twin-shaft turbine is mechanically far more complex, larger, heavier, and more expensive than a single-shaft turbine of equivalent performance, primarily because its gas generator consists of two coaxial shafts.

[0010] Therefore, a simpler starting system than existing systems has been proposed in the prior art. Instead of requiring an additional APU or a dual-shaft architecture, it is based on using two 28-volt batteries, which are first connected in parallel to share the starting current, and then in series to output double the voltage of 56 volts. Powering the brushed starter generator with two 28-volt batteries in series can provide significantly higher armature current, thereby providing sufficient mechanical torque to assist the gas generator acceleration over a range of speeds where the drag torque of a single-shaft gas generator is excessively high compared to the starting torque obtained with a single 28-volt battery.

[0011] However, such systems cannot address the issue of excessive acceleration provided during the gas generator ignition window, which can lead to decreased combustion chamber ignition performance. On the other hand, powering the brushed starter with a double voltage of 56 volts instead of the 28 volts of a single battery at initial zero speed generates extremely high torque during startup, necessitating an oversized design for the auxiliary drive system. Summary of the Invention

[0012] Therefore, this invention represents a technological development achievement aimed at significantly improving aircraft performance. From this perspective, this invention helps reduce the environmental impact of aircraft. Thus, the main objective of this invention is to provide a method and system for starting a turbine in a medium-power twin-engine aircraft, overcoming the shortcomings of the prior art.

[0013] This objective is achieved through a starting method for an aircraft turbine with a free turbine and a single-shaft gas generator for a twin-engine aircraft. The aircraft turbine includes two independent electrical grids, each containing a 28-volt battery selectively supplying power to a starter generator. A turbine regulation computer controls the turbine's starting, initially by connecting the two batteries in parallel at a rated voltage of 28 volts, and subsequently by connecting the two batteries in series at a rated voltage of 56 volts, while preventing excessive acceleration of the gas generator. The method is characterized in that the turbine regulation computer is configured to instruct the two batteries to be connected in series only when the gas generator's combustion chamber is ignited and the gas generator's rotational speed is greater than a predetermined speed threshold N1, thereby ensuring a positive acceleration margin at the gas generator's point of maximum drag.

[0014] This avoids excessive acceleration during the ignition window, thus ensuring optimal conditions for ignition in the turbine combustion chamber.

[0015] Advantageously, if the acceleration of the gas generator is less than a predetermined acceleration threshold DN2 after combustion chamber ignition but before reaching the point of maximum resistance, the command will connect the two batteries in series to avoid the risk of start-up stall.

[0016] Preferably, the speed threshold N1 of the gas generator is between 10% and 25% of the rated speed NTOP of the gas generator, or the acceleration threshold DN2 of the gas generator is between 1% and 3% of the rated speed NTOP / s of the gas generator.

[0017] Advantageously, the start-up and end-of-life threshold N CUTOFF (The speed at which the gas generator of the turbine can automatically accelerate to idle speed) is between 50% and 60% of the rated speed NTOP of the gas generator.

[0018] Preferably, to avoid battery short circuits caused by overlapping actions of the electrical contactors during reconfiguration, a dead time with a duration between 150 milliseconds and 300 milliseconds is provided before the series connection of the two batteries.

[0019] The present invention also relates to a starting system for an aircraft turbine with a free turbine and a single-shaft gas generator for a twin-engine aircraft. The aircraft turbine includes two independent electrical grids, each containing a 28-volt battery that selectively powers a starter generator. A turbine regulation computer controls the turbine's starting, initially by connecting the two batteries in parallel at a rated voltage of 28 volts, and subsequently by connecting the two batteries in series at a rated voltage of 56 volts, while preventing the gas generator from accelerating too quickly. The system is characterized in that the turbine regulation computer is configured to instruct the two batteries to be connected in series only when the combustion chamber of the gas generator is ignited and the speed of the gas generator is greater than a predetermined speed threshold N1, so as to ensure a positive acceleration margin at the point of maximum drag of the gas generator through the series connection.

[0020] The starting system also includes a ground connection for connecting a 28-volt ground power unit, wherein the turbine regulating computer is configured to: first, power the starter generator from the ground power unit, then power the ground power unit in series with one of the two batteries, and after ignition in the combustion chamber and the gas generator speed is greater than a predetermined speed threshold N1, ensure a positive acceleration margin at the point of maximum resistance of the gas generator through the series connection.

[0021] Finally, the present invention relates to a rotor or fixed-wing aircraft turbine including the above-described starting system, and a twin-engine aircraft integrating the turbine. Attached Figure Description

[0022] Other features and advantages of the invention will become apparent from the following description of a non-limiting embodiment of the invention with reference to the accompanying drawings, in which:

[0023] Figure 1 A schematic illustration shows a starting system according to the invention applied to an aircraft single-shaft turbine;

[0024] Figure 2 It shows Figure 1 An example of the electrical architecture of a startup system;

[0025] Figures 3 to 13 It shows Figure 2 Different operating steps of the electrical architecture;

[0026] Figure 14 The curves showing the changes in starting torque and resistance torque as a function of gas generator speed are shown.

[0027] Figure 15 A flowchart illustrating the operation of the starting system for a single-shaft turbine according to the present invention is shown.

[0028] Figures 16 to 25 It shows Figure 2The electrical architecture involves different operational steps starting from ground connections; and

[0029] Figure 26 The curves showing the changes in starting torque and drag torque of single-shaft and dual-shaft turbines with gas generator speed are presented. Detailed Implementation

[0030] In multi-engine (especially dual-engine) architectures of rotorcraft or fixed-wing aircraft, 28-volt DC power grids are typically organized into at least two independent grids, with one grid for each generator and thus one grid for each engine, and each grid contains at least one 28-volt battery.

[0031] Therefore, the principle of this invention is to reconfigure two pre-existing 28-volt batteries during startup so that the starter generator (S / G) is powered at 28 volts (batteries in parallel) at the beginning of startup, and then powered at 56 volts (batteries in series) to overcome the resistance torque at the point of maximum resistance of the gas generator.

[0032] However, in order to minimize the starting torque at zero speed and the acceleration of the gas generator within the ignition window (excessive acceleration within this speed range may adversely affect combustion chamber ignition), it is recommended to supply the starter generator with 56 volts only after a specific speed threshold is reached (preferably after combustion chamber ignition).

[0033] Therefore, this invention proposes a low-voltage power grid architecture suitable for rotorcraft or fixed-wing twin-engine aircraft, which includes a reconfiguration device (contactor), available voltage sources (batteries, ground connection, starter generator), and associated management strategies for the reconfiguration device to: firstly power the starter generator of the turbine with a free turbine and a single-shaft gas generator at a rated voltage of 28 volts, and then power it at a rated voltage of 56 volts by connecting two 28-volt batteries in series, while avoiding excessive acceleration within the ignition window.

[0034] It should be noted that in the following description, 28 volts and 56 volts are rated operating values. The open-circuit voltage of a 28-volt battery usually varies between 18 volts and 26 volts depending on its state of charge and temperature, and the actual voltage applied to the generator terminals will be lower due to voltage drops in the cables and contactors.

[0035] It should also be noted that supplying a brushed starter generator with a design rated voltage of 28 volts with 56 volts for the limited duration of turbine startup (typically about 20 seconds) will not damage it. This 56 volt voltage is still far below the dielectric properties of the insulators, whether they are the insulators of rotating motor windings or cables (for example, see aviation standards EN2282 and MIL-STD-704, which specify that the transient overvoltage of a 28-volt airborne electrical network is about 50 to 60 volts).

[0036] Figure 1 An example of a single-shaft architecture for a medium-power aerospace free-turbine turbine mounted on a twin-engine aircraft, equipped with the starting system of the present invention, is shown. The turbine 10 typically includes a compressor 12, a combustion chamber 14, a high-pressure turbine 16 (these components form a gas generator), and a free turbine 18, which drives the turbine's main drive shaft 20 via a main mechanical reduction gear 24, thereby driving the main propulsion unit 22 (e.g., a helicopter rotor or propeller). An accessory gearbox 26 is mechanically connected to the gas generator and primarily drives the pumps associated with the combustion chamber injectors and the lubricating oil pump. The accessory gearbox 26 typically also includes a reduction gear assembly that connects it to the starting system 28.

[0037] Figure 2 An electrical architecture diagram of a 28-volt power grid for a twin-engine aircraft (fixed-wing or rotary-wing) is shown. This aircraft is equipped with two free-turbine turbines and a single-shaft gas generator, for example, as previously described and suitable for embodiments of the present invention. Typically, there are: two DC power grids, BUS1 and BUS2, each containing 28-volt brushed starter generators S / G1 and S / G2 and 28-volt batteries BAT1 and BAT2; a ground connection PS for connecting the airborne power grid to a 28-volt ground power unit (GPU); and various (electromechanical or static) contactors for reconfiguring the airborne power grid, the operation of which will be detailed below.

[0038] • Starting contactors K51 / K52 are used to connect starter generator 1 or 2 to the corresponding DC power grid BUS1 or BUS2;

[0039] • Ground contactors K61 / K62 are used to connect the ground connection PS to the DC power grids BUS1 and BUS2 respectively;

[0040] • Coupling contactor K4 (commonly referred to as "busbar connection contactor") is mainly used to connect DC grid BUS1 to DC grid BUS2 when the generator is started in flight or when the turbine fails.

[0041] • Bus contactors K21 / K22 are used to connect battery 1 or battery 2 to DC power grids BUS1 and BUS2 respectively;

[0042] • Authorized contactors K11 / K12 (unique to this invention) are used to disconnect the negative terminals of the two batteries from the aircraft reference potential (0 volts) so that the authorized batteries are connected in series;

[0043] • Connecting contactors K31 / K32 (also unique to this invention) are used to connect two batteries in series.

[0044] The following will combine Figures 3 to 13 The procedure for sequentially starting the two turbines using two onboard batteries is described in detail. In the following description, "starter" refers to "generator starter".

[0045] Figure 3 The diagram shows the initial state of the electrical architecture, corresponding to the state where both turbines are shut down. In this state, contactors K11 and K12 are closed, causing the negative terminals of both batteries to reference the aircraft's reference potential. All other contactors are open.

[0046] Figure 4 The next step is shown, corresponding to the first stage of starting turbine 1. When turbine 1 is requested to be started, contactors K21, K22, K4, and K51 close so that the 28-volt rated voltage provided by the two parallel batteries powers starter 1. Simultaneously, the regulating computer of turbine 1 manipulates the injection of fuel into the combustion chamber according to the appropriate starting pattern and energizes the spark plugs to ignite the combustion chamber.

[0047] Figure 5 The next step, corresponding to the reconfiguration of the onboard electrical grid, is shown. When the gas generator's speed NG exceeds a specific speed threshold N1 or its acceleration dNG / dt drops below a specific threshold DN2 (the basis for which will be detailed below), contactors K11 and K4 disconnect. Therefore, starter 1 is temporarily de-energized.

[0048] Figure 6 The next step is shown, corresponding to the second stage of starting turbine 1. Once it is confirmed that contactors K11 and K4 are open (e.g., the regulating system obtains this confirmation by acquiring feedback from the auxiliary contactors of the positions of the main contactors K11 and K4), contactor K32 closes, allowing power to be supplied to the starter via two 28-volt batteries, BAT1 and BAT2, connected in series. At this point, one of the two batteries (BAT2 in this example) references the aircraft's reference potential. Thus, starting turbine 1 continues, with its starter 1 powered by a 56-volt rated voltage.

[0049] Figure 7 The next step is shown, corresponding to the end of startup and the process of the gas generator of turbine 1 automatically accelerating to idle speed. When the starter cut-off speed threshold N is reached... CUTOFF At this time, contactor K32 disconnects. Under the action of the gas expansion in the combustion chamber, the gas generator continues to accelerate using its own power.

[0050] Figure 8The next step is shown, which corresponds to the process of starter 1 switching to generator mode. When turbine 1 completes startup and reaches ground idle speed (at which point power can be drawn from its gas generator), contactor K11 closes, and starter 1 can charge battery BAT1.

[0051] After turbine 1 is started, the same operation must now be performed on turbine 2.

[0052] therefore, Figure 9 The next step is shown, corresponding to the first stage of turbine 2 startup. When turbine 2 is requested to start, contactors K4 and K52 close to supply starter 2 with a 28-volt rated voltage, provided by the two parallel batteries and starter 1, which subsequently operates in parallel as a generator (typically, a function called "cross-start" integrated in the generator control unit (GCU) limits the current supplied by starter 1 to its rated generating current, for example, approximately 400 amps for a 12 kW generator / starter). Simultaneously, turbine 2's regulating computer manipulates fuel injection into the combustion chamber according to the appropriate starting pattern and energizes the spark plugs, igniting the combustion chamber.

[0053] Figure 10 The next step is shown, which involves reconfiguring the onboard electrical grid. When the gas generator's speed NG exceeds a specific speed threshold N1 or its acceleration dNG / dt drops below a specific threshold (the basis for which will be detailed below), contactors K4 and K12 disconnect. Therefore, starter 2 is temporarily de-energized.

[0054] Figure 11 The next step is shown, corresponding to the second stage of starting turbine 2. Once it is confirmed that contactors K12 and K4 are open, contactor K31 closes, causing the two 28-volt batteries BAT1 and BAT2, connected in series, to power the starter. Battery BAT1 and starter 1 remain connected in parallel and reference the aircraft's reference potential. Therefore, starting turbine 2 continues, powering starter 2 with a rated voltage of 56 volts.

[0055] Figure 12 The next step is shown, corresponding to the end of the start-up and the process of the gas generator of turbine 2 automatically accelerating to idle speed. When the starter cut-off speed threshold N is reached... CUTOFF At this point, contactor K31 disconnects, and the gas generator of turbine 2 continues to accelerate using its own power. From this point on, the two DC power grids, BUS1 and BUS2, are isolated from each other.

[0056] Figure 13The final step is shown, which switches starter 2 to generator mode. Once turbine 2 has started and reached ground idle (at which point power can be drawn from its gas generator), contactor K12 closes, and starter 2, acting as a generator, can charge battery BAT2.

[0057] At this point, the onboard electrical grid is configured to its rated operating state. Both turbines are started, and each starter, operating as a generator, supplies power to its respective 28-volt grid (in particular, it can charge the corresponding battery). The two DC grids, BUS1 and BUS2, are isolated from each other by contactors K4, K31, and K32, which are in the open position.

[0058] Figure 14 The curve shape of the starting torque obtained by this device as a function of the gas generator NG speed is shown. It should be noted that this is in accordance with the reconfiguration corresponding to batteries BAT 1 and BAT 2 (in... Figure 5 and Figure 10 During the dead time T of the switch from parallel to series connection, the starting torque briefly drops to zero because the starter stops supplying power for a short period. In fact, this dead time T is crucial for confirming that contactors K4 and K11 (for turbine 1) or K4 and K12 (for turbine 2) have been disconnected before the battery reconfiguration (contactor K32 or K31 closing) to avoid potential overlaps that could lead to battery short circuits, which must be absolutely avoided. Physically, for electromechanical contactors suitable for high starting currents, this dead time (confirmation of disconnection + closing time) is typically 150 to 300 milliseconds. Considering the high mechanical inertia of a single-shaft gas generator in a medium-power turbine, the corresponding speed drop is still less than 1% of the gas generator's rated speed NTOP, which typically corresponds to the turbine's speed at maximum extraction power (extraction power). This ensures no risk of combustion chamber flameout and does not significantly adversely affect the turbine's starting performance.

[0059] As can be seen from the start-up curve, during contactor closure, the initial starting torque at zero speed (point A) is limited by the supply voltage corresponding to the parallel connection of batteries BAT1 and BAT2 (i.e., a reduced voltage of up to 28 volts), and the combustion chamber ignites when the starter supplies power at the reduced voltage (battery in parallel). At this time, the acceleration of the gas generator remains moderate, thus being under favorable conditions.

[0060] It is also worth noting that after the two batteries are reconfigured in series, the starting torque is sufficient to ensure a positive acceleration margin M at the point of maximum resistance (point B). Simultaneously, the turbine is already rotating, and therefore the starter generates a non-zero back electromotive force (EMF). This results in the armature current amplitude (and consequently the starting torque) being limited to a much lower amplitude during the reconfiguration of the two batteries in series (point P) than it would be at the initial startup when the starter is powered by the two series batteries from zero speed (i.e., zero back EMF) (point P'). This avoids the need for an overly large mechanical design of the accessory gearbox and auxiliary drivetrain.

[0061] Figure 15 The flowchart shows the operation of the turbine No. 1 start-up sequence from the perspective of controlling various contactors.

[0062] In the initial step 50, contactors K11 and K12 are initially closed to reference the negative terminal of each battery to the aircraft's reference potential. Therefore, turbine 1 stops (step 52).

[0063] In step 54, after the pilot issues the command to start turbine 1, in subsequent step 56, the turbine regulation computer (EECU) simultaneously commands contactors K21, K22, K4 and K51 to close so that starter 1 is powered by two parallel 28-volt batteries BAT1 and BAT2, fuel is injected according to the appropriate starting pattern, and the spark plugs are energized.

[0064] As the gas generator speed increases, air enters the combustion chamber, and the spark generated by the spark plug ignites the mixture and the combustion chamber is ignited. In subsequent step 58, the EECU detects the combustion chamber ignition (e.g., by monitoring the rise of the TIT temperature of the "turbo inlet temperature" or the gas temperature T45).

[0065] The next step 60 includes: detecting at least one of the following two conditions required for batteries BAT1 and BAT2 to be connected in series:

[0066] 1) When NG is greater than a fixed threshold N1 (speed threshold), the combustion chamber ignition is considered sufficiently stable, ensuring that the acceleration of the gas generator after the two batteries are connected in series no longer carries the risk of flame extinguishing or combustion chamber extinguishing (which would lead to start-up interruption). This threshold N1 can be set to a value corresponding to the upper limit of the ignition window, for example, between 10% and 25% of NTOP (mainly depending on the combustion chamber technology);

[0067] 2) If dNG / dt is less than the fixed threshold DN2, this indicates that the acceleration margin between the starter torque powered by 28 volts and the gas generator drag torque is becoming too small, posing a risk of start-up stall (i.e., the combustion chamber is ignited but the gas generator still cannot accelerate) and turbine damage due to overheating. The minimum acceleration standard for the gas generator can be set to a value of 1% to 3% of NTOP / sec.

[0068] The series connection of the two batteries ensures positive acceleration margin when the gas generator NG speed is close to the region of maximum drag torque, while protecting the turbine from the risk of start-up stall and combustion chamber shutdown.

[0069] When at least one of the above two conditions is met, in the new step 62, the command contactors K11 and K4 are disconnected, and after step 64 confirming that contactors K11 and K4 have been effectively disconnected (e.g., for electromechanical contactors, this can be confirmed by rereading the feedback contact of the main contactor position), in step 66, the command contactor K32 is closed. In step 68, when the speed of the gas generator reaches the start-up end threshold N... CUTOFF (This threshold corresponds to the time when the turbine's gas generator can automatically accelerate to idle speed, typically 50% to 60% of NTOP). In step 70, the EECU commands the spark plugs to shut off and disconnects contactor K32 to cut off the power supply to starter 1.

[0070] Subsequently, in the next step 72, the gas generator continues to accelerate automatically. Once the idle speed is reached, the starter of turbine 1 can switch to generator mode to power the 28-volt DC onboard grid BUS1 and charge battery BAT 1.

[0071] The flowchart for the start-up sequence of turbine 2 is naturally similar to that described above. Its start-up process follows the same steps, particularly by using the same conditions based on the speed or acceleration measurement of the gas generator to trigger batteries BAT 1 and BAT 2 to switch from parallel connection to series connection, thereby powering starter 2.

[0072] It should be noted that at any time (steps 74, 76, or 78), the pilot can issue a turbine shutdown command, triggering two new steps: disconnecting contactor K32 (step 80), and in which the EECU cuts off fuel injection, shuts off the spark plugs, and disconnects all contactors, thereby deactivating the two starters (step 82). In the subsequent step 84, the gas generator enters autopilot speed, and then, when the NG speed drops to zero (step 86), the turbine returns to the initial shutdown position of step 52.

[0073] also, Figure 2The diagram also applies to ground-based startup on a ground power unit (GPU), with the main advantage of conserving onboard battery power. The startup sequence is as follows: Figures 16 to 25 As shown.

[0074] Figure 16 The initial state of the electrical architecture is shown, which is the same as the battery startup corresponding to the shutdown of the two turbines. At this state, both contactors K11 and K12 are closed, so that the negative terminals of both batteries are referenced to the aircraft's reference potential. All other contactors are open.

[0075] Figure 17 The next step is shown, corresponding to the first stage of starting turbine 1. When turbine 1 is requested to start, contactors K62, K4, and K51 close to supply power to starter 1 at a rated voltage of 28 volts provided by the GPU. Simultaneously, the turbine 1 regulation computer (EECU) manipulates fuel injection into the combustion chamber according to the appropriate starting pattern and energizes the spark plugs to ignite the combustion chamber.

[0076] Figure 18 The reconfiguration steps for the airborne electrical system are shown. From the aforementioned defined speed threshold N1 or acceleration threshold DN2, contactors K4 and K11 are disconnected. Therefore, starter 1 is temporarily de-energized.

[0077] Figure 19 The next step is shown, corresponding to the second stage of turbine 1's startup. Once it is confirmed that contactors K4 and K11 are open, contactors K32 and K21 close, at which point the starter is powered by the GPU and the 28-volt battery BAT1 in series, with the GPU referencing the aircraft's reference potential. Therefore, turbine 1's startup continues, with its starter powered at a rated voltage of 56 volts.

[0078] Figure 20 The next step is shown, corresponding to the end of startup and the process of the gas generator of turbine 1 automatically accelerating to idle speed. When the starter cut-off speed threshold N is reached... CUTOFF At this time, contactors K21, K32, K51, and K62 are disconnected. The gas generator of turbine 1 continues to accelerate under its own power.

[0079] Figure 21 The next step is shown, corresponding to the first stage of starting turbine 2. When turbine 2 is requested to start, contactors K61, K4, and K52 close to power starter 2 with the 28-volt rated voltage provided by the GPU. Simultaneously, K11 closes to rereference the 28-volt battery BAT1 to the aircraft's potential. At the same time, turbine 2's EECU manipulates fuel injection into the combustion chamber according to the appropriate starting protocol and energizes the spark plugs to ignite the combustion chamber.

[0080] Figure 22 Corresponding to the reconfiguration of the airborne electrical grid, contactors K4 and K12 disconnect from the aforementioned defined speed threshold N1 or acceleration threshold DN2. Therefore, starter 2 is temporarily no longer supplying power.

[0081] Figure 23 The steps corresponding to the second stage of starting turbine 2 are shown. Once it is confirmed that contactors K4 and K12 have opened, contactors K31 and K22 close, and then starter 2 is powered in series by the GPU and the 28-volt battery BAT2, with the GPU referencing the aircraft's reference potential. Thus, starting turbine 2 continues, with its starter 2 powered at a rated voltage of 56 volts.

[0082] Figure 24 The next step is shown, corresponding to the end of the start-up process and the automatic acceleration of the gas generator of turbine 2 to idle speed. When the starter cut-off speed threshold N is reached... CUTOFF At this point, contactors K22, K31, K52, and K61 disconnect, isolating the two DC power grids BUS1 and BUS2 from each other. Subsequently, the gas generator continues to accelerate using its own power.

[0083] Figure 25 The final step, which is performed after the turbine reaches idle speed, is shown, including switching the two starters to generator mode and charging the two batteries by closing contactors K51, K21, K12, K22 and K52.

[0084] Compared to start-up systems that require an APU or a turbine architecture employing a dual-shaft gas generator, this invention offers numerous advantages:

[0085] Only standard off-the-shelf hardware is required (28V DC brushed starter generator, series starter, high-current contactor);

[0086] • Fully compatible with an onboard DC power grid of 28 volts and equipped with two 28-volt starting batteries of sufficient capacity;

[0087] • Avoid designing the accessory gearbox to be too large, because the starter is only powered by the two batteries in series once a certain speed threshold is exceeded;

[0088] • By limiting the torque within the combustion chamber ignition window, a medium-power turbine with a single-shaft gas generator and high drag torque can be started;

[0089] The software used to manage the startup order is easy to modify, and therefore carries low technical risk at the development level.

Claims

1. A method for starting an aircraft turbine with a free turbine and a single-shaft gas generator for a twin-engine aircraft, said aircraft turbine comprising two independent electrical grids, each of said electrical grids containing 28-volt batteries (BAT1, BAT2) selectively supplying power to starter generators (S / G1, S / G2), characterized in that, To ensure turbine startup under the control of the turbine regulation computer (EECU), the turbine is first started at a rated voltage of 28 volts by connecting the two batteries in parallel, and then at a rated voltage of 56 volts by connecting the two batteries in series. To prevent the gas generator from accelerating too quickly, the two batteries are only instructed to be connected in series when the combustion chamber is ignited and the speed of the gas generator is greater than a predetermined speed threshold N1. This series connection ensures a positive acceleration margin at the point of maximum resistance of the gas generator.

2. The startup method according to claim 1, wherein, The speed threshold N1 of the gas generator is between 10% and 25% of the rated speed NTOP of the gas generator.

3. The startup method according to claim 1, wherein, If the acceleration of the gas generator is less than a predetermined acceleration threshold DN2 after the combustion chamber is ignited but before the maximum resistance point is reached, a further instruction is given to connect the two batteries in series to avoid the risk of startup stall.

4. The startup method according to claim 3, wherein, The acceleration threshold DN2 of the gas generator is between 1% and 3% of the rated rotational speed of the gas generator per second (%NTOP / s), where the acceleration threshold represents the rate of change of the gas generator rotational speed relative to time, expressed as a percentage of the rated rotational speed NTOP per second.

5. The startup method according to claim 1, wherein, To avoid battery short circuits caused by overlapping operation of the electrical contactors during reconfiguration, a dead time with a duration between 150 milliseconds and 300 milliseconds is provided before connecting the two batteries in series.

6. A starting system for an aircraft turbine with a free turbine and a single-shaft gas generator for a twin-engine aircraft, the aircraft turbine comprising two independent power grids, each of the power grids containing 28-volt batteries (BAT1, BAT2) selectively supplying power to starter generators (S / G1, S / G2), a turbine regulation computer (EECU) controlling the starting of the turbine (10), first by connecting the two batteries in parallel at a rated voltage of 28 volts, and subsequently by connecting the two batteries in series at a rated voltage of 56 volts, and preventing the gas generator from accelerating too quickly, characterized in that, The turbine regulating computer is configured to instruct the two batteries to be connected in series only when the combustion chamber of the gas generator is ignited and the speed of the gas generator is greater than a predetermined speed threshold N1, so as to ensure a positive acceleration margin at the point of maximum resistance of the gas generator through the series connection.

7. The startup system of claim 6 further includes a ground connection (PS) for connecting to a 28-volt ground power unit (GPU).

8. The startup system according to claim 7, wherein, The turbine regulation computer is also configured to: firstly, supply power to the starter generator (S / G1; S / G2) by the ground power unit, and then supply power to the ground power unit in series with one of the two batteries (BAT1; BAT2) after the combustion chamber is ignited and the speed of the gas generator is greater than a predetermined speed threshold N1, and ensure a positive acceleration margin at the point of maximum resistance of the gas generator through the series connection.

9. A turbine for a rotorcraft or fixed-wing twin-engine aircraft, comprising: The startup system according to any one of claims 6 to 8.

10. A rotorcraft or fixed-wing twin-engine aircraft, comprising: Two turbines as described in claim 9.

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