Abstract:
A gas turbine, in particular an aircraft engine and to a method for generating electrical energy in a gas turbine is provided. The gas turbine comprises at least on engine core ( 18 ), in which a shaft ( 19 ) produces a shaft output. The turbine is equipped with means that generate electrical energy both from the shaft output produced by the engine core ( 18 ) and from the compressed air that is dissipated by the engine core ( 18 ).

Description:
FIELD OF THE INVENTION 
     The present invention relates to a gas turbine and a method for generating electrical power in a gas turbine, and an aircraft engine in particular. 
     BACKGROUND 
     In addition to propulsion for moving the aircraft forward, aircraft engines, either civil aircraft engines or military aircraft engines, also generate power for supplying attachments or auxiliaries of the gas turbine or for supplying aircraft-related systems such as the air conditioning system, for example. For generating power for supplying the attachments or auxiliaries and the aircraft-related systems it is known from the related art to draw mechanical power from a core engine of the gas turbine which is used, for example, to drive pumps and generators. DE 41 31 713 C2 describes an aircraft engine, for example, in which shaft power is drawn from a core engine and this shaft power is supplied to auxiliaries. 
     In aircraft development, a definite trend can be observed to the effect that increasingly more electrical power is required in the aircraft. The reason for this is that hydraulically and pneumatically operated devices or power units in the aircraft are replaced by electrically operated devices and that more and more power is needed per aircraft seat. Therefore, the aircraft engines must provide more and more electrical power. 
     For generating electrical power it is known from the related art to couple the shaft of the core engine of a gas turbine to a generator so that the mechanical shaft power drawn at the shaft may be converted into electrical power. However, this type of supply or generation of electrical power has the disadvantage that a displacement of the operating characteristic curve of the gas turbine in the characteristics map of the high-pressure compressor toward the surge limit can be noted. The surge limit in the characteristics map of the high-pressure compressor delimits the stable operating range of the gas turbine from the unstable operating range of the gas turbine. In order to ensure stable operation over the entire operating range and thus the load range of the gas turbine, a certain surge limit margin must be maintained. The effect that a displacement of the operating characteristic curve toward the surge limit can be observed increases with decreasing performance of the gas turbine, i.e., instabilities may occur in the lower load range of the gas turbine, i.e., during partial load operation. 
     In order to ensure reliable operation of the gas turbine even in its partial load range under the above-mentioned aspects, the approach of the related art is to design the gas turbine, its core engine in particular, to have a greater surge limit margin. This results in a greater overall length of the high-pressure compressor of the core engine in particular, as well as in a greater number of stages, a greater number of blades, and thus in greater weight and higher costs overall. If, however, the high-pressure compressor of the core engine is not designed to have a greater surge limit margin, then, according to the related art, the only alternative remains to lower the operating characteristic curve of the gas turbine, the core engine in particular, to the extent that a sufficient surge limit margin is maintained even during partial load operation. However, this has the effect that during full load operation the efficiency optima can no longer be achieved, resulting in lower efficiency. 
     SUMMARY OF THE INVENTION 
     Based on the aforementioned, an object of the present invention is to create a novel gas turbine, a novel aircraft engine in particular, and a novel method for generating electrical power in a gas turbine, an aircraft engine in particular. 
     In accordance with the present invention, a gas turbine comprises a core engine including a high pressure compressor and a shaft connected thereto for driving said high speed compressor. An electrical power generator generates electrical power from the shaft and from compressed air drawn from the high-pressure compressor. 
     According to the present invention, the gas turbine has means which generate electrical power from the shaft power transmitted from the core engine and which generate electrical power from compressed air drawn from the core engine. 
     According to an advantageous refinement of the present invention, the means generate the electrical power in a high load range of the core engine exclusively from the mechanical shaft power drawn from the core engine. However, in a lower load range of the core engine, the means generate the electrical power from the mechanical shaft power drawn from the core engine and from the compressed air drawn from the core engine. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Without being limited to them, exemplary embodiments of the present invention are explained in greater detail on the basis of the drawing. 
         FIG. 1  shows a schematic representation of a characteristics map of a high-pressure compressor of a gas turbine, namely a core engine of the gas turbine; 
         FIG. 2  shows a block diagram for clarifying a first embodiment of the present invention, and 
         FIG. 3  shows a block diagram for clarifying a second embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention is subsequently described in greater detail with reference to  FIGS. 1 through 3 . 
       FIG. 1  shows a characteristics map of a high-pressure compressor of a core engine of a gas turbine. In the diagram of  FIG. 1 , π is the compression ratio or the pressure ratio of the high-pressure compressor, {dot over (m)} is the mass flow through the high-pressure compressor, T is the temperature, ρ is the pressure inside the high-pressure compressor, and n is the rotational speed of same. The reference numeral  11  indicates lines in characteristics map  10  in which the ratio n√{square root over (T)} is constant. Moreover, in characteristics map  10  according to  FIG. 1 , the reference numeral  12  indicates the surge limit of the high-pressure compressor of the core engine. 
     In the case in which the gas turbine is exclusively used for generating propulsion for the aircraft, except for the normal attachments—such as the fuel pump and the oil pump—no further shaft power of the high-pressure compressor or the core engine is drawn and the high-pressure compressor of the gas turbine is operated with the operating characteristic curve which is indicated in  FIG. 1  using the reference numeral  13 . For operating characteristic curve  13  there is a sufficient margin from surge limit  12  over the entire characteristics map of the high-pressure compressor. However, if mechanical shaft power is drawn at the shaft of the high-pressure compressor, the operating characteristic curve in the characteristics map is displaced toward the surge limit; an operating characteristic curve of the high-pressure compressor when shaft power is drawn is indicated in  FIG. 1  using the reference numeral  14 . 
     It is apparent in  FIG. 1  that when additional power is drawn, e.g., for driving electric units, the effect of the displacement of the operating characteristic curve toward surge limit  12  increases while the power of the high-pressure compressor decreases. In particular in the lower load range of the high-pressure compressor and thus the core engine, instabilities in the operation of the high-pressure compressor have to be expected when mechanical shaft power is drawn. 
     Within the scope of the present invention, a gas turbine and a method for generating and removing electrical power in a gas turbine are proposed, with the aid of which the displacement, described in connection with  FIG. 1 , of operating characteristic curve  13  in the direction of operating characteristic curve  14  may be avoided. 
     Before preferred exemplary embodiments of the present invention are described below in greater detail with reference to  FIGS. 2 and 3 , it should be pointed out that, according to the present invention, mechanical shaft power is drawn from the core engine and this drawn shaft power is converted into electrical power and that compressed air is also drawn from the core engine and the pneumatic energy contained in the compressed air is also converted into electrical power. It is therefore the object of the present invention to generate the electrical power in a high load range of the core engine exclusively from the mechanical power drawn. In contrast, the necessary electrical power is generated in a lower load range from the mechanical shaft power drawn and from the pneumatic energy contained in the compressed air. Due to the withdrawal of compressed air in the lower load range of the high-pressure compressor or the core engine, the operating characteristic curve of the high-pressure compressor may be influenced in such a way that a sufficient margin from surge limit  12  is maintained in the lower load range. 
     The reference numeral  15  in  FIG. 1  indicates an operating characteristic curve of the high-pressure compressor which is established when the present invention is used. Switching between the two states, which are to be differentiated in principle, takes place in a middle section  16  of operating characteristic curve  15 , the electrical power being generated in a first state exclusively by drawing mechanical power and the electrical power also being generated in a second state from the pneumatic energy contained in the drawn, compressed air. 
       FIG. 2  shows a highly schematic representation of a first preferred exemplary embodiment of a gas turbine according to the present invention.  FIG. 2  shows a high-pressure compressor  17  of a core engine  18  including a shaft  19  of high-pressure compressor  17 . The mechanical power of shaft  19  is picked up at shaft  19  of high-pressure compressor  17  of core engine  18  via a gear  20  and is transferred to a generator  21  which generates electrical power from the mechanical power. Within the scope of the present invention, compressed air is drawn from the high-pressure compressor  17  via a controllable valve  22 . The compressed air is supplied to an air turbine  23 , air turbine  23  generating mechanical power from the pneumatic energy contained in the compressed air and drives a shaft  24 . Shaft  24  is connected to a second generator  26  via a second gear  25 . Second generator  26  ultimately converts the pneumatic energy contained in the compressed air, after its conversion into mechanical power by air turbine  23 , into electrical power. 
     As is apparent in  FIG. 2 , gear  20  assigned to first generator  21  and second gear  25  assigned to second generator  26  are connectable via a clutch  27 . Clutch  27  is controllable and either decouples gears  20  and  25  from one another or couples them together. A freewheel  28  is integrated into shaft  24  which is driven by air turbine  23 . 
     In an upper load range of the high-pressure compressor, in which the electrical power is generated exclusively by drawing mechanical power of shaft  19  of high-pressure compressor  17  according to the present invention, both gears  20  and  25  are coupled to one another via clutch  27 , valve  22  is closed, and shaft  24  is decoupled from second gear  25  via freewheel  28 . In this state, first generator  21  and second generator  26  are exclusively driven by shaft  19  of high-pressure compressor  17  and both generators  21  and  26  convert the drawn mechanical power into electrical power. In contrast, in a lower load range of high-pressure compressor  17 , clutch  27  is disengaged and both gears  20  and  25  as well as both generators  21  and  26  are decoupled from one another. Valve  22  is open and compressed air is drawn from high-pressure compressor  17  and supplied to air turbine  23 . Freewheel  28  couples shaft  24  to second gear  25  so that the mechanical power generated by air turbine  23  from the compressed air can be transferred to second generator  26  for generating electrical power. In the lower load range, generator  21  is driven, according to the exemplary embodiment in  FIG. 1 , by shaft  19  of high-pressure compressor  17  via gear  20  and generator  26  is driven by air turbine  23 , to which the compressed air drawn is supplied via gear  25 . 
     Switching between these two states of high-pressure compressor  17  takes place via control means  29 . In the exemplary embodiment shown, control means  29  is designed as an ECU (energy control unit). Valve  22 , clutch  27 , and both generators  21  and  26  are controllable via control means  29  as it is indicated by arrows  30  in  FIG. 2 . Switching between the two operating states for generating electrical power takes place either on the basis of criteria stored in control means  29  or on the basis of measured values  31  which are conveyed to control means  29 . Measured values  31  may be, for example, the measured compression ratio π, measured rotational speeds n, or measured temperatures T. Criteria on the basis of which switching between the two operating states or connection or disconnection of air turbine  23  takes place for generating electrical power by withdrawing compressed air from high-pressure compressor  17  may be calculated from the measured values in control means  29 . 
       FIG. 3  shows a second exemplary embodiment of the present invention.  FIG. 3  again shows a high-pressure compressor  32  of a core engine  33  having a shaft  34 , mechanical power being drawn from shaft  34  via a gear  35  and applied to a generator  36  or multiple generators for generating electrical power. In the exemplary embodiment in  FIG. 3 , compressed air may also be drawn from high-pressure compressor  32  via a controllable valve  37 , the compressed air being supplied to an air turbine  38  of the engine, also known as an engine starter. Air turbine  38  may also be used as a starting device. Air turbine  38  or the starter in turn converts the energy contained in the compressed air into mechanical power and drives a shaft using this mechanical power. Via freewheel  39 , the shaft driven by air turbine  38  may be coupled to gear  35  or be decoupled therefrom. The approach in connection with the exemplary embodiment shown in  FIG. 3  is that during partial load operation of high-pressure compressor  32  the compressed air is supplied to air turbine  38  via valve  37 . Valve  37  is controllable via control means  29 . If an input speed of air turbine  38  is higher than a rotational speed of a shaft on which air turbine  38  is situated, freewheel  39  engages and transfers the generated mechanical power to gear  35  and thus ultimately to generator  36  for generating electrical power. 
     Both exemplary embodiments have in common that compressed air is drawn from the high-pressure compressor in the lower load range and that electrical power is generated from the energy contained in the compressed air. Due to the withdrawal of compressed air, the operating characteristic curve of the high-pressure compressor is influenceable in such a way that the operating characteristic curve moves away from the surge limit and an adequate surge limit margin may be maintained even in the lower load range of the high-pressure compressor.