Patent Publication Number: US-8112983-B2

Title: Gas turbine engine

Description:
This is a Division of application Ser. No. 11/898,591 filed Sep. 13, 2007, which claims the benefit of British Patent Application No. 0621074.4 filed Oct. 24, 2006. The disclosures of the prior applications are hereby incorporated by reference herein in their entirety. 
    
    
     The present invention relates to a gas turbine engine and particularly but not exclusively to an apparatus and method for starting a bypass turbofan gas turbine engine. 
     In order to start a gas turbine engine, for example, a bypass turbofan gas turbine engine, it is necessary to accelerate the high pressure (HP) spool to a speed high enough for sufficient air pressure and mass flow to be developed in the combustion chamber for fuel metered into the combustion chamber to be ignited. After ignition of the fuel, fuel flow is increased until the engine reaches idle speed. 
     In one starting arrangement, pressurised air is impinged onto the HP turbine blades to impart sufficient momentum for the turbine to rotate. This arrangement requires pressurised air to be independently generated, for example, by means of a dedicated auxiliary air compressor. 
     In another starting arrangement, the HP spool driven by an electric starter motor, which is positioned externally of the engine. The starter motor is connected to the HP spool through gears and a clutch mechanism. 
     The invention seeks to provide a starting arrangement for a gas turbine engine, which does not require the use of an external motor and gearing or an auxiliary compressor or other externally mounted starting device. 
     According to the present invention there is provided a gas turbine engine comprising an engine casing disposed around a low pressure spool, a high pressure spool and a combustion chamber; a bypass casing disposed around the engine casing, a bypass duct disposed between the engine casing and the bypass casing, a fan for supplying air to the bypass duct and a starter motor for rotating the fan on engine start-up, characterised in that the engine casing is provided with closable apertures which, when open, provide communication between a region of the bypass duct and the interior of the engine casing upstream of the combustion chamber. 
     Preferably a first closure means is provided to reduce the flow area of an outlet of the bypass duct or to substantially seal an outlet of the bypass duct between the bypass casing and the engine casing. 
     Preferably, a second closure means is disposed in the engine casing for allowing air to flow from the bypass duct through the aperture into the combustion chamber when the second closure means is in an open position and for sealing the aperture when the second closure means is in a closed position. 
     Preferably, the first closure means is disposed downstream of the second closure means. 
     The second closure means may be biased to a closed position in which the bypass duct is sealed from the combustion chamber. 
     Preferably, the second closure means is positioned to allow airflow passing through the bypass duct to flow into the engine at the upstream end of the combustion chamber of the engine. 
     When the electric starter motor rotates the LP spool, the airflow generated by a LP fan at the upstream end of the engine takes the path of least resistance, ie passes through the bypass duct. The airflow through the engine is therefore minimal. In order to maximise this core airflow, the first closure means seals the outlet of the bypass duct and the second closure means opens the bypass duct to the combustion chamber and turbines of the engine. 
     In one aspect of the invention the first closure means is configurable to allow airflow from the bypass duct through the aperture into the combustion chamber when the closure means is in a first position for starting; and to allow airflow through the outlet of the bypass duct and to seal the combustion chamber from the bypass duct when the closure means is in a second position for engine operation. 
     The closure means may have a single actuated member. 
     In all embodiments of the invention, the starter motor may operate as a generator when the engine is operating. 
     The engine may be a multi-spool bypass turbofan engine. 
     According to a further aspect of the present invention there is provided a method of starting a gas turbine engine comprising an engine casing disposed around at least one low pressure spool, a high pressure spool and a combustion chamber ( 24 ); a bypass casing ( 30 ) disposed around the engine casing ( 28 ), a bypass duct ( 32 ) disposed between the engine casing ( 28 ) and the bypass casing ( 30 ), characterised by directing airflow from the bypass duct ( 32 ) into the upstream end of a combustion chamber ( 24 ) of the engine ( 10 ) through at least one closeable aperture ( 31 ) in the engine casing ( 28 ). 
     Preferably, the method further comprises reducing the flow area of an outlet of the bypass duct or substantially sealing an outlet of the bypass duct, and allowing airflow passing through the bypass duct to be directed into the combustion chamber. 
     Ideally, in all embodiments of the invention, airflow can be directed initially into the engine onto the turbine blades without passing through the HP compressor. 
    
    
     
       For a better understanding of the present invention, and to show more clearly how it may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which:— 
         FIG. 1  shows a schematic cross-sectional view through a multi-spool gas turbine engine in accordance with the invention in an initial stage in a starting procedure; 
         FIG. 2  shows a schematic cross-sectional view through the gas turbine engine of  FIG. 1  in a second stage in the starting procedure; 
         FIG. 3  shows a schematic cross-sectional view through the gas turbine engine of  FIG. 1  in a third stage in the starting procedure; 
         FIG. 4  shows a schematic cross-sectional view through an alternative embodiment of multi-spool gas turbine in an operating condition in accordance with the invention; and 
         FIG. 5  shows a schematic cross-sectional view through the engine of  FIG. 4  in a starting condition. 
     
    
    
     Referring firstly to  FIG. 1 , a first embodiment of multi-spool gas turbine engine is indicated generally at  10 . The engine  10  is conventional in that it comprises three spools, that is to say it includes a low pressure (LP) spool  12 , an intermediate pressure (IP) spool  13 , and a high pressure (HP) spool  14 . However the invention may equally be applied to any single spool, twin spool or multi spool engine arrangement. A fan  16  is mounted on the front or upstream end of the LP spool, IP booster stage blades  18  are mounted on the IP spool  13 , and a compressor  20  is mounted on the HP spool  14 . HP turbine blades  22  are connected to the compressor  20 , ie on the HP spool, IP turbine blades  21  are mounted on the IP spool  13  and LP turbine blades  23  are mounted on the LP spool. A combustion chamber  24  is downstream of the compressor  20 , but upstream of the turbine blades  22 ,  21 ,  23 . 
     An electrically driven starter motor  26  is mounted directly about the downstream end of the LP spool  12 , which may be axially extended for this purpose, within the nozzle of the engine. The starter motor  26  may alternatively be mounted directly about the upstream end of the LP spool  12 , within the nose cone of the engine. An engine casing  28  surrounds the IP booster stage blades  18 , the HP compressor  20 , the combustion chamber  24  and the turbine blades  22 ,  21 ,  23 . A bypass casing  30  surrounds the fan  16  and extends around and along the engine casing  28 , creating a substantially annular bypass duct  32  between the engine casing  28  and the bypass casing  30 . 
     A plurality of bypass duct closures  34 , two of which are shown, are provided in the bypass casing  30  equi-spaced around the casing  30  at the downstream end of the engine, proximate the outlet of the bypass duct  32 . The closures  34  are movable to seal the outlet of the bypass duct between the bypass casing  30  and the engine casing  28 . The closures  34  comprise a plurality of actuated flaps. The seal made may be a complete seal (that is to say does not permit any leakage to the outlet of the bypass duct  32 ) or a partial seal (that is to say, it permits a controlled leakage to the outlet of the bypass duct  32 ). Alternatively the closures  34  may be moveable to reduce the flow area of the bypass duct  32 . 
     A plurality of apertures  31  are disposed in the engine casing  28  at the upstream end of the combustion chamber  24 . Engine casing closures  36  are mounted inside the engine casing  28  to cover and seal the apertures  31 . When open, as shown in  FIG. 2 , the closures  36  allow air to pass through apertures  31  from the bypass duct  32  into the combustion chamber  24 . The engine casing closures  36  are spring loaded to a closed position as indicated in  FIG. 1 , which shows the engine in an operating condition. Although two apertures  31  and engine casing closures  36  are shown, a plurality are provided, equi-spaced around the engine casing  28 . 
     In order to start the engine  10 , initially the bypass duct closures  34  and engine casing closures  36  are in the positions shown in  FIG. 1 , ie with the engine casing closures  36  closed and the bypass duct closures  34  open. The starter motor  26  operates to drive the LP spool, causing the fan  16  to push air through the engine. When the LP spool reaches a sufficient speed, the bypass duct closures  34  are moved to the position as shown in  FIG. 2  such that they close, or partially close, the outlet from the bypass duct  32 . As a result, air pressure in the bypass duct  32  between the fan and the engine casing closures  36  increases, causing the engine casing closures  36  to be opened against the spring force, allowing air to be directed from the bypass duct  32  through the apertures  31  into the upstream end of the combustion chamber  24  and through the turbines  22 ,  21 ,  23 . The majority of air reaching the combustion chamber  24  has bypassed the high pressure compressor  20  by flowing through the bypass duct  32  and the apertures  31 . The air travelling through the combustion chamber  24  impinges on the HP turbine with enough momentum to begin initial rotation of the HP spool. 
     The HP spool  14  begins to accelerate and draws more air in through the mouth of the engine core. This drawing in of more air results in continued acceleration of the HP spool  14  and raises the pressure in the combustion chamber  24 . This pressure rise continues until the pressure reaches a level where it equals that of the air pressure in the bypass duct  32 . By this point, the pressure difference across the engine casing closures  36  will have reduced to the extent that the engine casing closures close under the action of the springs against the engine casing  28 . This produces the required seal to accommodate the continued pressure rise associated with the increasing speed of the HP spool  14 . The bypass duct closures  34  are configured to prevent the build up of pressure in the bypass duct  32 . That is to say they may remain closed, as shown in  FIG. 3 , or may be partially open, or may oscillate between being fully open, partially open and/or closed as required to prevent a pressure build up which results in aerodynamic instabilities that affect the operation of the fan  16 . 
     If the bypass duct closures  34  were now fully opened, the high pressure spool  14  may begin to slow down. Therefore, the bypass duct closures  34  remain closed, or at least partially closed, until successful ignition of fuel air mixture in the combustion chamber  24  can be achieved, or even longer to ensure that idle speed can be reached in the same way as with a conventional starting arrangement after ignition. Thereafter, the bypass duct closures  34  are opened and the engine  10  operates in the condition shown in  FIG. 1 . 
     In an alternative embodiment to that described above in relation to  FIGS. 1 to 3 , the closures  34 , 36  are activated and controlled hydraulically, pneumatically, electrically or by some other such suitable method. 
     Referring now to  FIGS. 4 and 5 , a second embodiment of a multi spool gas turbine engine is indicated at  40 . Common references numerals have been used to designate parts in common with the first embodiment described. In this embodiment, the bypass duct closures  34  and engine casing closures  36  of the previous embodiment described are integrated into actuated closures  42 , each comprising a single flap  44 . A plurality of these flaps  44  are hinged to the engine casing  28  and are disposed equi-spaced about the bypass duct  32 . When activated, for example either hydraulically, pneumatically or electrically, the flap  44  of each closure  42  moves between a normal operating position, in which it covers the aperture  31  in the engine casing  28  near the combustion chamber  28  and leaves the bypass duct  32  unobstructed, and a starting position ( FIG. 5 ), in which the aperture  31  is exposed allowing air flow into the combustion chamber  24  and substantially closing the bypass duct  32  at a position downstream of the aperture  31 . 
     The engine  40  is started in the same way as the engine  10 , save that once the LP spool  12  has reached a sufficient speed with the closures  42  in the starting position, the closures are closed for ignition, ie the combustion chamber  24  is sealed from the bypass duct  32 . 
     The invention is intended to include any physical arrangement for substantially sealing the outlet of the bypass duct  32  and allowing air flow into the combustion chamber  24  and through the turbine blades  22 ,  23  of the engine. When operating, the starter motor  26  functions as a generator. 
     In use on a jet aircraft, the invention also gives the advantage of providing means for varying core and bypass mixing areas by operation of the closures for optimising mixing throughout the flight envelope.