Abstract:
A method of operating a gas turbine engine having a turbine and a compressor connected via a shaft, a main fuel supply line for supplying fuel to a combustor that is positioned to release expanding hot gases to the turbine, the engine further including a starter/generator connected to the shaft via a gearbox assembly, the method is characterised the step of during engine start up fuel is circulated in a re-circulating fuel circuit positioned on the main fuel supply line and which a first fuel/oil heat exchanger, for cooling the oil, and a fuel accumulator.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application is entitled to the benefit of British Patent Application No. GB 0707319.0 filed on Apr. 17, 2008. 
       FIELD OF THE INVENTION 
       [0002]    The present invention relates to a method and apparatus for cooling a starter/generator of a gas turbine engine at start-up. 
       BACKGROUND OF THE INVENTION 
       [0003]    One conventional means of starting a modern turbofan engine is via a compressed air starter where no dedicated cooling of such an air starter is required, as the heat rejection from such an air starter is low. 
         [0004]    Another conventional method of starting more electric gas turbine engines is via an electric driven starter. Here the starter is a variable frequency starter/generator that has a built-in pump for pumping oil through the starter to a dedicated closed circulating oil cooling circuit to transfer the heat generated from the starter to the oil in the oil circuit. The heated oil is then cooled by a flow of fuel via a heat exchanger. The fuel is pumped to the heat exchanger and returned to a fuel tank housed within the aircraft (conventionally known as a fuel return to tank cooling system). This method relies on the fuel mass in the aircraft&#39;s fuel tank acting as an external thermal sink to remove heat from the VFSG oil circuit. Because there is a finite amount of fuel, this system is limited to the amount of heat that it can dissipate. In addition, there are problems in the fuel balance within aircraft fuel tanks and fuel tank contamination. 
         [0005]    In principle, the thermal mass of the fluids and metals in the cooling circuits could be used as a simple static thermal sink, however, this method is of limited use due to the limited thermal capacity in the cooling circuits. Increasing the fuel volume in the loop would increase the thermal sink capacity, which means a large amount of fuel being stored on engine, and hence, adversely increases the weight of the engine and causes an unnecessary fire hazard for the engine. 
         [0006]    Therefore it is an object of the present invention to provide an improved cooling system for the starter/generator that is not hazardous and is acceptable for engine certification requirements. 
       SUMMARY OF THE INVENTION 
       [0007]    In accordance with the present invention there is provided a as turbine engine comprising a turbine and a compressor connected via a shaft, a main fuel supply line for supplying fuel to a combustor that is positioned to release expanding hot gases to the turbine, the engine further comprises a first fuel and oil heat exchanger associated with an engine starter/generator that is connected to the shaft via a gearbox assembly, the engine is characterised by a re-circulating fuel circuit positioned on the main fuel supply line and comprising a second fuel/oil heat exchanger and a fuel accumulator, the re-circulating fuel circuit cools the oil in the starter/generator via the first fuel and oil heat exchanger. 
         [0008]    Preferably, the second fuel/oil heat exchanger is fluidly connected to an engine oil circuit. 
         [0009]    Preferably, the first fuel/oil heater exchanger is fluidly connected to the starter/generator. 
         [0010]    Preferably, the circuit comprises a fuel filter and which may be positioned between the second fuel/oil heat exchanger and the combustor. 
         [0011]    Preferably, the circuit comprises a shut off valve that is provided to stop fuel return flow during normal engine running. 
         [0012]    In another aspect of the present invention there is provided a method of operating a gas turbine engine comprising a turbine and a compressor connected via a shaft, a main fuel supply line for supplying fuel to a combustor that is positioned to release expanding hot gases to the turbine, the engine further comprises a starter/generator connected to the shaft via a gearbox assembly, the method is characterised the step of during engine start up fuel is circulated in a re-circulating fuel circuit positioned on the main fuel supply line and which comprises a first fuel/oil heat exchanger, for cooling the oil in the starter/generator oil circuit, and a fuel accumulator. 
         [0013]    Preferably, a further step comprises supplying the cooled fuel to the starter/generator for cooling. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  is a schematic section of part of a ducted fan gas turbine engine incorporating the present invention; 
           [0015]      FIG. 2  is a schematic layout of a re-circulated fuel circuit used for cooling the engine&#39;s starter/generator. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0016]      FIG. 1  shows a ducted fan gas turbine engine generally indicated at  10  and comprising three main rotational shafts, however, the present invention is equally applicable to an engine having any number of shafts. This engine  10  has a principal and rotational axis  11 . The engine  10  comprises a core engine  9  having, in axial flow series, an air intake  12 , a propulsive fan  13 , an intermediate pressure compressor  14 , a high-pressure compressor  15 , combustion equipment  16 , a high-pressure turbine  17 , and intermediate pressure turbine  18 , a low-pressure turbine  19  and a core exhaust nozzle  20 . A nacelle  21  generally surrounds the engine  10  and defines both the intake  12  and a bypass exhaust nozzle  22 . 
         [0017]    The gas turbine engine  10  works in the conventional manner so that air entering the intake  11  is accelerated by the fan  13  to produce two air flows: a first air flow into the intermediate pressure compressor  14  and a second air flow which passes through a bypass duct  23  to provide propulsive thrust. The intermediate pressure compressor  14  compresses the air flow directed into it before delivering that air to the high pressure compressor  15  where further compression takes place. 
         [0018]    The compressed air exhausted from the high-pressure compressor  15  is directed into the combustion equipment  16  where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through, and thereby drive the high, intermediate and low-pressure turbines  17 ,  18 ,  19  before being exhausted through the nozzle  20  to provide additional propulsive thrust. The high, intermediate and low-pressure turbines  17 ,  18 ,  19  respectively drive the high and intermediate pressure compressors  15 ,  14  and the fan  13  by suitable interconnecting shafts. 
         [0019]    The fan  13  is circumferentially surrounded by a structural member in the form of a fan casing  26 , which is supported by an annular array of outlet guide vanes  27 . 
         [0020]    The engine  10  further comprises a gearbox/generator assembly  28  used for engine start up and for generating electricity once the engine has been started and working in convention fashion. The starter/generator is a variable frequency starter/generator (VFSG) as known in the art. The generated electricity is used for engine and associated aircraft electrical accessories as well known in the art. The gearbox/generator assembly  28  is drivingly connected to the high-pressure shaft  24 , however, in other embodiments may be driven by any one or more of the shafts. In this embodiment, the gearbox/generator assembly  28  comprises an internal gearbox  29  connecting a first drive shaft  30  to the high-pressure shaft  24 , an intermediate gearbox  31  connecting the first drive shaft  30  to a second drive shaft  32  and an external gearbox  33  drivingly connected to the second drive shaft  32 . The external gearbox  33  is drivingly connected to a starter/generator  34  that is capable of the aforesaid engine operation. The starter/generator  34  and external gearbox  33  are housed within the nacelle  21 , but may be positioned on the core engine, as opposed to the fan casing. The first drive shaft  30 , intermediate gearbox  31  and the second drive shaft  32  are housed within a bypass duct splitter fairing  40 . 
         [0021]    Referring now to  FIG. 2  the engine  10  further comprises a re-circulated fuel circuit  42  used for cooling the starter/generator  34 . The re-circulated fuel circuit  42  comprises a pump  44 , a first fuel and oil heat exchanger  56  (FOHE), a second fuel and oil heat exchanger  46  and a fuel accumulator  48 . The recirculated fuel circuit  42  is situated on the main fuel line  50  between the main aircraft fuel tank  52 , usually housed within the aircraft&#39;s fuselage or wing, and a fuel injector or the combustion equipment  16 . Note that the recirculation fuel circuit  42  is part of the engine  10  and not the aircraft. 
         [0022]    Inclusion of certain engine units in the fuel circuit is to increase the thermal capacity, e.g., the addition of a second FOHE  46  and filter  60 . 
         [0023]    The second FOHE  46  is fluidly connected to an engine oil circuit  54  and the first FOHE  56  is fluidly connected to a starter/generator  34 . The oil in the starter circuit is self-contained; it is not connected to the engine oil system. A small oil sump (not shown) is provided in the starter/generator  34 . 
         [0024]    A filter  60  is positioned between the second FOHE  46  and the combustor  16 . A shut off valve  62  is positioned downstream of the fuel accumulator  48  to stop fuel return flow during normal engine running. 
         [0025]    In use, the circuit  42  re-circulates fuel via the pump  44 , through the variable frequency starter/generator FOHE  56 . The FOHE  56  dissipates heat from the VFSG  34 , which is in contrast to the conventional practice of sending the fuel back to aircraft fuel tank  52 . 
         [0026]    The accumulator  48 , situated on a fuel return line  58  is used to control the volume of fuel in the circuit  42  and increases the fuel&#39;s thermal capacity to remove the limitations of the conventional static thermal sink (i.e. the main fuel tank  52  in a conventional engine/aircraft). The accumulator  48  will be filled with fuel before or during the engine start. When the engine has successfully achieved a start, the fuel in the accumulator  48  will be emptied and sent back to the engine fuel line to be consumed by the engine combustor  16  during engine running. The accumulator could be a dedicated unit or a modified existing unit on engine, e.g., the fuel drain tank. A mechanical means or a modified existing unit on engine, such as the ejector pump in fuel drain tank, may be employed to change the effective volume of the accumulator  48 . The mechanical means, such as a piston, may be powered by electrical, pneumatic or hydraulic devices or stored mechanical energy, such as a spring, could be used for this purpose. 
         [0027]    Advantageously, the present invention removes the need of returning fuel back to aircraft fuel tank  52  during engine start up and hence there is no disturbance on fuel balance in aircraft&#39;s fuel tanks  52  and there are no fuel contamination issues. 
         [0028]    Addition of the fuel accumulator  48  removes the limitation of the aircraft fuel tank as a static heat sink. 
         [0029]    The accumulator  48  minimises the amount of fuel stored on engine  10 . The accumulator is either evacuated by such means as an ejector pump, where the accumulated fuel is transferred into the main fuel lines and is consumed in the engine combustor or has its effective volume reduced mechanically to expel the contents into the fuel system, also to be consumed in the engine combustor.