Abstract:
A method improving acceleration efficiency of a gas generator of a gas turbine by reducing take-up of electric power, during transition phases, to conserve a sufficiently large surge margin for an operating line. The method increases acceleration/deceleration power of the gas generator by adjusting voltage of an electric network onboard the aircraft. After a phase of starting the gas turbine, voltage of the onboard network is adjusted by a voltage set value controlled by determining unballasting/ballasting status of a main electricity generation source of the onboard network. The status-determining is executed according to demand for taking power to be supplied for propulsion of the aircraft. The status-determining is followed by selecting a voltage set value from plural levels according to the determining the unballasting/ballasting status, and applying the selected set value to a control loop of the voltage supplied to the onboard network.

Description:
TECHNICAL FIELD 
       [0001]    The invention relates to a method for controlling the electrical generation applied to an aircraft gas turbine, more particularly in its transient phases of acceleration and deceleration. The invention applies, in particular, to a helicopter turboshaft engine capable of implementing such a method. 
         [0002]    The invention relates to the field of gas turbines, in particular turboshaft engines, turbojet engines or turboprops of aircrafts (helicopters, airplanes, other flying machines). 
         [0003]    An aircraft engine typically comprises a compressor-combustion chamber-turbine assembly forming a gas generator. In this gas generator, the fresh air is compressed by the rotation of the compressor and brought into the chamber where it is mixed with the fuel; then, after combustion, the hot gases are evacuated with a high kinetic energy and expanded into the turbine which extracts the rotary energy for driving the compressor via a high-pressure drive shaft (HP in an abbreviated form) or HP body. The excess of kinetic energy provides the energy for moving the aircraft, either directly for airplanes, through an exhaust nozzle, or indirectly for helicopters, via a new expansion into a free turbine linked to a drive shaft (downstream, through-going or via a shaft outside the gas generator). 
       STATE OF THE ART 
       [0004]    In the case of a helicopter turboshaft engine, a great part of the mechanical energy delivered by this drive shaft is supplied to the rotor drive mechanism and to the consumers (hydraulic pump, electrical equipments, air conditioning, rotor brake, etc.) via speed-reducing gearings or, more rationally, via an accessory gearbox (hereinafter AGB). An appreciable part of the mechanical power can be taken directly from the gas generator (electrical generation, air bleeds for cabin heating . . . ). 
         [0005]    In the phase for starting the turboshaft engine, a reversible electric source—also called starter-generator, in an abbreviated form SG, which is fed by a battery—works, in starter mode, as a motor for driving the compressor into rotation until the gas generator works autonomously. Then, in transient phases—takeoff, landing, hovering—or in steady-flight intermediate phases—cruise, low-height search—, the SG source takes kinetic energy from the gas generator so as to work in generator mode and feed the electricity-consuming equipments (controls, air conditioning, pump . . . ) of the on-board electric network. 
         [0006]    However, the bleeding of energy from the gas generator to feed the on-board network is disadvantageous in terms of surge margin for the working line, and strongly contributes to limiting the acceleration efficiency of the gas generator when the instantaneous mechanical bleed is not known to the regulation system. So that a sufficient surge margin is maintained, the compressor does not functions any more at an optimal compression ratio and the efficiency deteriorates, in particular the specific fuel consumption increases. This is particularly noticeable in engines that are controlled by gas generator speed variation (dng/dt). Besides, as the acceleration and deceleration requirements are higher and higher, the ensuing speed variation of the free turbine and rotor drive mechanism can have serious consequences. 
         [0007]    It is known from patent document FR 2 929 324 how to recover electric energy by coupling the SG source with the free turbine, the SG source being switched to generator mode after the starting phase. This solution requires a new equipment to be added, namely a dedicated switching flip-flop. This addition has an impact in terms of cost and mass, and requires a modification of the engine architecture. 
         [0008]    Besides, patent document FR 2 914 697 provides a system for assisting the transient phases by integrating an additional electric motor fed by a battery in order to supply kinetic energy to the gas generator. This solution has the same drawbacks. 
       DISCLOSURE OF THE INVENTION 
       [0009]    The invention aims at reducing the mechanical bleeds from the gas generator, in particular during the transient phases, so as to keep a sufficient surge margin making it possible to provide the acceleration efficiency of the gas generator without further power generation or need for additional sensors or actuators. To this end, the invention provides for increasing the acceleration/deceleration power of the gas generator by modulating the set point for regulating the voltage of the on-board electric network of the aircraft. 
         [0010]    More precisely, the object of the present invention is a method for generating electricity applied to an aircraft gas turbine and outputting into an on-board network after the phase for starting the gas turbine. In this method, the voltage of the on-board network is regulated by means of a voltage set point controlled by a step for determining the unloading/loading status of a main electricity-generating source of the on-board network according to the request for power bleed to be supplied to the propulsion of the aircraft, followed by a step for selecting a voltage set point among several levels according to the determination of the unloading/loading status, and by a step for applying the selected set point to a loop for regulating the voltage supplied to the on-board network. 
         [0011]    According to particular embodiments:
       a source of electricity remains activated in case of unloading of the main source in order to feed the on-board network, whereas this buffer source can be recharged when the main source is not being unloaded;   the unloading/loading status can be determined among three statuses, namely unloading, loading and stabilized, according to the speed variation of the gas generator and/or variations in the inclination of the rotor blades, commonly called “collective pitch”, in the case of a helicopter;   an unloading status can be determined when the speed variation of the gas generator is at least equal to an upper bound, taken between +2 and +5% per time unit, and/or when the collective pitch variation is higher than an upper bound by +10 to +30% of the full travel of the collective pitch per time unit in the case of a helicopter;   a loading status can be determined when the speed variation of the gas generator is lower than or equal to a lower bound taken between −2 and −5% per time unit and/or when the collective pitch variation is lower than an upper bound by +10 to +30% of the full travel of the collective pitch per time unit;   a stabilization status can be determined when the variation in speed or in the collective pitch are contained between ranges within the upper and lower bounds;   at least three levels of voltage set point are determined in a step for selecting the set point, in correspondence with the three statuses—unloading/loading/stabilized—determined in the preceding step: a median set-point level is selected when a stabilization status has been determined in the preceding step or when the state of the on-board network is defective; a low set-point level is selected when an unloading status has been determined in the preceding step; and an upper set-point level is selected when a loading status has been determined in the preceding step or when a stabilized status has been determined in order to recharge the buffer source of electricity, in particular further to an acceleration phase during an unloading status;   each voltage set point selection corresponds to definite flight phases: the median voltage set point corresponds to the steady-state intermediate flight phases; the unloading voltage set point, which is substantially lower than the median level set point, corresponds to the accelerations and takeoff of the transient phases; and the loading voltage set point, which is substantially higher than the median level set point, corresponds to the decelerations and landing of the transient phases;   the regulation voltage set point can be adjusted according to data relating to the state of work of the on-board network and/or to the charge of the buffer source;   the state of the on-board network is established by a state of failure or defect of the network, or a state of work or sensitivity of the controls of the consumers and accessories.       
 
         [0021]    The invention also relates to a helicopter turboshaft engine capable of implementing the above method. Such a turboshaft engine includes a gas generator which is capable of driving a rotor mechanism of a propulsion wing and is coupled with a reversible starter-generator capable of forming a source of kinetic energy for the gas generator or a source for supplying electricity to an on-board network. A numerical control unit includes a module for selecting the set point for generating the voltage of the on-board network. This module is capable of selecting a voltage set point from the data supplied by devices for measuring the speed of the gas generator or starter-generator, for measuring the position of the collective pitch of the propulsion wing and for measuring the voltage of the on-board network. The control unit is capable of applying the so selected voltage set point to the on-board network via a regulation loop which is coupled with a regulator. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]    Other aspects, characteristics and advantages of the invention will appear in the following non-restrictive description relative to particular embodiments in reference to the accompanying drawings which show respectively: 
           [0023]    in  FIG. 1 , a schematic view of a turboshaft engine fitted with the means capable of implementing the method according to the invention; 
           [0024]    in  FIG. 2 , an example of a curve of the variation, according to the time, of the speed of a gas generator of a turboshaft engine during various flight phases with a voltage-regulated on-board network according to the invention, and 
           [0025]    in  FIG. 3 , an example of a block diagram of means implemented according to the main steps of the method of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0026]    In reference to the schematic view of  FIG. 1 , a helicopter turboshaft engine “T” includes, according to an embodiment of the invention, a gas generator  1  and a free turbine  2  which, after a starting phase, is rotatively driven by a gas flow F g  with strong kinetic energy supplied by the gas generator  1 . 
         [0027]    The free turbine  2  is mounted on a shaft  3  which transmits mechanical energy to a gearbox enabling speed interfacing with an accessory gearbox (AGB). This AGB  4  is capable of transmitting power to the drive mechanism  41  of the propulsion-wing rotor. 
         [0028]    As for it, the gas generator  1  drives a set of mechanical (fuel pumps . . . ) and electrical elements (alternators, starters . . . ) as well as—more and more because of the “all electric” trend—the controls of hydraulic (pumps . . . ), mechanical (rotor brake, etc.) or pneumatic equipments (compressors, air conditioning . . . ). The electrical generation occurs then via a reversible motor  7  in generator mode SG. As the SG is reversible, it enables the starting up by driving the gas generator, then the feeding with electric energy once the gas generator runs autonomously, since the SG is mechanically connected, without freewheel, with the gas generator shaft  13  (see the description hereinafter). 
         [0029]    The gas generator  1  comprises a compressor  11  and a turbine  12  which are mounted on a rotary shaft  13 , as well as a combustion chamber  14  formed between the compressor  11  and the turbine  12 . An air flow (arrow F 1 ), which comes from an air inlet  5 —formed in a casing  6 —and is compressed via the compressor  11 , is sent into the chamber  14  to be mixed with the fuel so as to produce, by combustion, the kinetic gas flow F g . This flow F expands while passing through the turbine  12 —whereby it rotatively drives the compressor  11  via the rotary shaft  13 —then the free turbine  2 , whereby it rotatively drives the drive shaft  3 . 
         [0030]    The transmission of the mechanical energy illustrated in  FIG. 1  is the type with front inlet gearbox with a reversing through-going drive shaft  3  which is co-linear with the rotary shaft  13 . Alternately, a front inlet gearbox with an outside reversing shaft or a rear inlet gearbox can be implemented without going beyond the scope of the invention. 
         [0031]    The turboshaft engine “T” also includes a reversible electric motor  7  capable of working as an electric generator to feed the electric controls  42  of the consumers and accessories of the on-board network  10 . This reversible electric motor constitutes a starter-generator source, in an abbreviated form: SG. As an equivalent, it is also possible to use a starter-generator of the type “direct current with brushes” or of the brushless-type, or else an alternator-starter. 
         [0032]    The motor  7  is coupled with the shaft  13  in order to rotatively drive the gas generator  1  during the starting phase. Whatever the flight phase may be, the SG remains connected to the gas generator and rotates at a speed which is proportional to that of the gas generator. 
         [0033]    The battery for feeding the SG when starting up or an auxiliary battery  8 , for example accumulators or a battery with a superconductive storage bobbin, also participates in the electrical generation. So, this battery  8  can supply a sufficiency of electric energy so as not to reduce the voltage of the on-board network  10  during acceleration phases while, as described hereinafter, the reversible motor  7  in generator mode is unloaded. The battery  8  is also appealed to in the starting phase to start the motor  7  in “motor” mode in order to drive the shaft  13  of the gas generator  1 . 
         [0034]    An accelerated recharging of the battery  8  is performed via the reversible motor  7  in generator mode, during the deceleration phases and, appropriately as explained hereafter, during the steady-state phases. 
         [0035]    So as to control the electrical generation of the on-board network  10  according to the various flight phases, a numerical control unit  9 , typically called FADEC (after the initials of “Full Authority Digital Engine Controller”), includes a module for selecting  19  a voltage set point to be applied to the on-board network  10 . The module  19  receives data supplied by various measuring devices and calculates its time variations in collaboration with the unit  9 : measurements of and variations in the speed of the gas generator, N G , and the position of the collective pitch of the rotor, X PC , as well as the measurement of the effective voltage U N  of the on-board network  10  so as to make sure of the follow-up of the set point. 
         [0036]    The control unit  9  issues a voltage set point for regulating the on-board network according to the flight phases. This set point is transmitted to the regulation loop  15 , which is going to control the power delivered by the SG accordingly and, actually, its torque taken from the gas generator. 
         [0037]    An example of a succession of flight phases of a helicopter is illustrated in  FIG. 2  by the flight profile  20 , wherein the gas generator speed N G  is expressed according to the time “t”. 
         [0038]    In the acceleration phases “B”, the ratio of bled power dedicated to the reversible motor in generator mode is strongly minimized, or even canceled, by the control unit according to the requirements: the reversible motor is then unloaded in order to supply the power to the propulsion-wing rotor. The requirements of the on-board network are then met by the buffer battery. In the steady-state phases “A”, “C” or “E”, the electrical requirements of the on-board network are met by the reversible motor. 
         [0039]    The battery can thus be recharged in these steady-state phases, in particular for a limited duration after an acceleration phase. In a phase of deceleration “D” or landing “F”, the bleed ratio dedicated to the reversible motor in generator mode is maximized and the battery can also be recharged. 
         [0040]    In reference to  FIG. 3 , here is now the description of a block diagram which illustrates the different steps for activating the electrical generation of the on-board network during the above succession of flight phases “A” to “F”. These steps follow a starting phase  50  in which the control unit  9  starts the motor  7  fed by the battery  8 . The gas generator  1  is then driven according to a known starting up until the gas generator  1  works in a self-contained way. At the end of this starting phase, the control unit  9  switches the reversible motor  7  to generator mode. 
         [0041]    In a first step  100  for selecting the unloading/loading status of the reversible motor  7  in generator mode, the request for power bleed P PREL  to be supplied to the propulsion rotor is assessed by the control unit  9  according to the flight conditions. 
         [0042]    So, according to an example applied to the above turboshaft engine, the request for bleed P PREL  is assessed by the control unit  9  according to the variations in speed dN G /dt of the gas generator  1  and the collective pitch dX PC /dt from sensors  30 , by data transmission to the selection module  19 . The transmission can be made by means of any appropriate wiring or by radio waves via adapted transmitting-receiving antennae  25  ( FIG. 1 ). 
         [0043]    The step for selecting the status  100  of the motor  7  in generator mode is determined among three statuses: an unloading status E DEL  corresponding to the acceleration phases; a loading status E LES  corresponding to the deceleration phases; and a stabilized status E STAB , corresponding to the steady-state phases; according to reference values of dN G /dt and dX PC /dt. In the example:
       the unloading status E DEL  is determined when the speed variation dN G /dt is at least equal to +3% per time unit or when the collective pitch variation dX PC /dt is higher than +20% of the full travel X PC  per time unit;   the loading status E LES  is determined when the speed variation dN G /dt is lower than or equal to −3% or when the collective pitch variation dX PC /dt is lower than −20% per time unit;   the stabilized status E STAB  is determined when:       
 
         [0000]      −1%&lt; dN   G   /dt&lt;+ 1% and −10%&lt; dX   PC   /dt&lt;+ 10%
 
         [0047]    To each one of these statuses E DEL , E LES  and E STAB  corresponds, in a step for selecting a voltage set point  200 , a voltage set-point level CT which is managed by the module  19  among three voltage set point levels U B , U H  and U M  to be applied to the on-board network:
       the lower set-point level U B , +24 volts in the example, when an unloading status E DEL  has been determined in a phase of acceleration of the gas generator;   the upper set-point level U H , +30 volts in the example, when a loading status E LES  has been selected in a phase of deceleration, or when a stabilized status E STAB  has been determined—in order to recharge the buffer battery for a limited duration, for example a few seconds, according to its state of charge (SOC) which is monitored by a dedicated device—in particular when the stabilized status follows an acceleration phase;   the median set-point level U M , +28 volts in the example, when a stabilized status E STAB  has been determined in the preceding step or when network malfunctions DON (i.e. “Defects On Network”) have been detected by dedicated sensors on the on-board network—or by the reading of its supply voltage by the FADEC—and transmitted to the voltage selection step  200 : network failure, state of the on-board network, electric controls, sensitive or defective consumers, etc.       
 
         [0051]    The so selected voltage level CT is applied, in an application step  300 , to the input of a loop for regulating  15  the voltage applied across the on-board network  10 , the loop being coupled with a regulator  16 . 
         [0052]    The effective voltage U N  of the on-board network  10 , the one which was brought into the loop  15  so that it is compared with the voltage set point CT, is measured and transmitted to the control unit  9 . In case an unloading status E DEL  of the main source  7  has been determined, the voltage is supplied by the battery  8 : the control unit  9  verifies then whether this voltage is sufficient for the network and, in case of insufficiency, whether a loading of the main source is possible, at least for a short duration. 
         [0053]    The invention is not limited by the described and represented examples. It is for example possible to define more than three voltage set point levels for the on-board network by differentiating sub-levels, corresponding to various acceleration levels of the power generator, or other transient phases (hovering flight, low-altitude search over the sea, etc.). Besides, it is possible to use several main reversible sources and/or several secondary sources of electricity.