GAS TURBINE ENGINE

Abstract

There is provided a gas turbine engine comprising a low pressure shaft and a high pressure shaft; wherein the low pressure shaft connects a fan to a fan drive turbine, and the high pressure shaft connects a high pressure turbine to a compressor section. The low pressure shaft and the high pressure shaft are arranged such that when operating at idle the idle shaft speed ratio is greater than 6.05. The idle shaft speed ratio is the ratio of the speed of the high pressure shaft to the speed of the low pressure shaft at idle conditions.

Claims

1. A gas turbine engine comprising a low pressure shaft and a high pressure shaft; wherein the low pressure shaft connects a fan to a fan drive turbine, and the high pressure shaft connects a high pressure turbine to a compressor section; wherein the low pressure shaft and the high pressure shaft are arranged such that when operating at idle, the idle shaft speed ratio is greater than 6.05; wherein the idle shaft speed ratio is the ratio of the speed of the high pressure shaft to the speed of the low pressure shaft at idle conditions.

2. The gas turbine engine according to claim 1, wherein the idle shaft speed ratio is greater than 6.44.

3. The gas turbine engine according to claim 1, wherein the idle shaft speed ratio is less than 9.0, and optionally less than 8.3.

4. The gas turbine engine according to claim 1, wherein fan is connected to the fan drive turbine through a power gearbox.

5. The gas turbine engine according to claim 4, wherein the power gearbox comprises a gear ratio greater than 3.1, and optionally less than 3.8, and further optionally between 3.2 and 3.7.

6. The gas turbine engine according to claim 1, wherein the low pressure shaft is supported on a thrust bearing, and wherein the thrust bearing is axially aligned with the low pressure compressor.

7. The gas turbine engine according to claim 1, wherein the low pressure shaft is supported on a thrust bearing, and wherein the thrust bearing is axially forward of the low pressure compressor, and axially forward or rearward of the power gearbox.

8. The gas turbine engine according to claim 1, wherein the flow of oil to one or more bearings and/or components is proportional to the speed of the high pressure shaft.

9. The gas turbine engine according to claim 1, wherein the idle shaft speed ratio is more than three times the cruise shaft speed ratio, and optionally wherein the idle shaft speed ratio is more than four times the cruise shaft speed ratio; and wherein the cruise shaft speed ratio is the ratio of the speed of the high pressure shaft to the speed of the low pressure shaft at cruise conditions.

10. The gas turbine engine according to claim 1, wherein the fan diameter is greater than 230 cm, and optionally less than 400 cm, and further optionally between 240 and 280 cm.

11. The gas turbine according to claim 1, wherein the gas turbine engine has a thrust greater than 180 kN, and optionally less than 400 kN, and further optionally between 200 kN and 300 kN.

12. The gas turbine according to claim 1, wherein the gas turbine engine has a specific thrust at cruise of less than 110 Nkg.sup.−1s, and optionally less than 90 Nkg.sup.−1s, and further optionally between 100 Nkg.sup.−1s and 60 Nkg.sup.−1s.

13. The gas turbine according to claim 1, wherein the fan comprises less than 20 fan blades.

14. The gas turbine according to claim 1, wherein the gas turbine engine has a bypass ratio greater than 12, and optionally less than 18 and further optionally between 13 and 16.

15. A method of operating a gas turbine engine at idle, wherein the gas turbine engine comprises a low pressure shaft and a high pressure shaft, and wherein the low pressure shaft connects a fan to a fan drive turbine, and the high pressure shaft connects a high pressure turbine to a compressor section; the method comprising: operating a first shaft at a first speed and a second shaft at a second speed such that the idle shaft speed ratio is greater than 6.05.

16. The method according to claim 15, wherein the idle shaft speed ratio is greater than 6.44, and optionally less than 9.0, and optionally less than 8.3.

17. A method of designing a gas turbine engine, wherein the gas turbine engine comprises a low pressure shaft and a high pressure shaft, and wherein the low pressure shaft connects a fan to a fan drive turbine, and the high pressure shaft connects a high pressure turbine to a compressor section; the method comprising: setting the speed of the low pressure shaft at idle and setting the speed of the high pressure shaft at idle such that the idle shaft speed ratio is greater than 6.05; and connecting the low pressure shaft to a low pressure shaft thrust bearing and connecting the high pressure shaft to a high pressure shaft thrust bearing.

18. The method according to claim 17, wherein the idle shaft speed ratio is greater than 6.44, and optionally less than 9.0, and optionally less than 8.3.

19. The method according to claim 17, wherein the low pressure shaft thrust bearing is axially aligned with the low pressure compressor.

20. The method according to claim 17, wherein fan is connected to the fan drive turbine through a power gearbox, wherein the low pressure shaft thrust bearing is axially forward of the low pressure compressor, and axially forward or rearward of the power gearbox.

Description

[0069] Embodiments will now be described by way of example only, with reference to the Figures, in which:

[0070] FIG. 1 is a sectional side view of a gas turbine engine;

[0071] FIG. 2 is a close up sectional side view of an upstream portion of a gas turbine engine;

[0072] FIG. 3 is a partially cut-away view of a gearbox for a gas turbine engine.

[0073] FIG. 1 illustrates a gas turbine engine 10 having a principal rotational axis 9. The engine 10 comprises an air intake 12 and a propulsive fan 23 that generates two airflows: a core airflow A and a bypass airflow B. The gas turbine engine 10 comprises a core 11 that receives the core airflow A. The engine core 11 comprises, in axial flow series, a low pressure compressor 14, a high-pressure compressor 15, combustion equipment 16, a high-pressure turbine 17, a low pressure turbine 19 and a core exhaust nozzle 20. A nacelle 21 surrounds the gas turbine engine 10 and defines a bypass duct 22 and a bypass exhaust nozzle 18. The bypass airflow B flows through the bypass duct 22. The fan 23 is attached to and driven by the low pressure turbine 19 via a low pressure shaft 26 and an epicyclic power gearbox 30.

[0074] In use, the core airflow A is accelerated and compressed by the low pressure compressor 14 and directed into the high pressure compressor 15 where further compression takes place. 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 is combusted. The resultant hot combustion products then expand through, and thereby drive, the high pressure and low pressure turbines 17, 19 before being exhausted through the nozzle 20 to provide some propulsive thrust. The high pressure turbine 17 drives the high pressure compressor 15 by a high pressure shaft 27. The fan 23 generally provides the majority of the propulsive thrust. The epicyclic gearbox 30 is a reduction gearbox.

[0075] The low pressure shaft 26 is supported on bearings, for example bearings 40 and 42. The high pressure shaft 27 is also supported on bearings, for example bearings 44.

[0076] At idle conditions the high pressure shaft 27 rotates at a faster speed than the low pressure shaft 26. Table 1 below shows the shaft speeds for three engine configurations.

TABLE-US-00001 TABLE 1 Shaft speeds for engines 1, 2 and 3 at idle and cruise Engine 3 Engine 1 Engine 2 (prior art) idle Low pressure shaft 1700 1300 3800 speed (rpm) High pressure shaft 10500 8500 6500 speed (rpm) cruise Low pressure shaft 8000 8000 8000 speed (rpm) High pressure shaft 16000 13000 12000 speed (rpm)

[0077] Both the high pressure shaft 27 and the low pressure shaft 26 are supported on a thrust bearing and a roller bearing. In FIG. 1 the low pressure shaft 26 has two thrust bearings 40 and 42, one either side (forward and rearward) of the geared arrangement 30. In other examples there may be only one thrust bearing on the low pressure shaft (for example either thrust bearing 40 or thrust bearing 42). In other examples the second thrust bearing 40 may be rearwards of the gear arrangement 30, for example connected to the fan by a shaft radially inside the gear arrangement 30. In other examples the first thrust bearing 42 may be forwards of the gear arrangement 30. The high pressure shaft 27 is supported on thrust bearing 44. The low pressure shaft 26 is supported on roller bearing 46 and the higher pressure shaft is supported on roller bearing 48. In the FIG. 1 example the thrust bearings are roller bearings, however in other examples the thrust bearings may be of different type that can react axial thrust, for example roller thrust bearings or needle thrust bearings.

[0078] The difference in loading of the bearings, between power operations and idle, is greater for the thrust bearings 40, 42, 44 compared to the roller bearings 46, 48. The thrust bearings 40 and 42 will be the highest loaded bearings, since they react the axial thrust from the fan 23 and the low pressure turbine 19. Power operations may be when the gas turbine is generating axial thrust for forward propulsion, for example at max take-off or cruise. The difference in loading of the bearings, between power operations and idle, is greater for the thrust bearings 40, 42 on the low pressure shaft 26 compared to the thrust bearings 40 on the high pressure shaft 27. By providing the claimed idle shaft speed ratio each bearing is operating in more optimal conditions at idle.

[0079] A power offtake (not shown) may take power from the high pressure shaft 27, for example to power an auxiliary gearbox, in a conventional manner. One or more pumps may be driven from the higher pressure shaft 27 in a conventional manner. Each bearing requires a supply of oil for heat management and lubrication. The pump(s) supply oil to the bearings. The one or more pump's output may be proportional to the speed of the high pressure shaft 27. The pump(s) may pump oil to one or more bearings, for example one or more of bearings 40, 42, 44, 46 or 48. Each of the one or more pumps may supply oil to one or more bearings, for example two, three, four or five bearings, or for example bearings in the same bearing chamber. For example in one arrangement, five pumps each provide oil to one bearing. In another arrangement, three pumps are present, whereby two of the pumps provide oil to two bearings and the third pump provide oil to one bearing. In another arrangement, one pump provides oil to all bearings.

[0080] The geared fan gas turbine engine 10 is shown in FIG. 2. The low pressure turbine 19 (see FIG. 1) drives the low pressure shaft 26, which is coupled to a sun wheel, or sun gear, 28 of the epicyclic gear arrangement 30. The low pressure shaft 26 is supported on bearings, for example bearings 40 and 42. Radially outwardly of the sun gear 28 and intermeshing therewith is a plurality of planet gears 32 that are coupled together by a planet carrier 34. The planet carrier 34 constrains the planet gears 32 to precess around the sun gear 28 in synchronicity whilst enabling each planet gear 32 to rotate about its own axis. The planet carrier 34 is coupled via linkages 36 to the fan 23 in order to drive its rotation about the engine axis 9. Radially outwardly of the planet gears 32 and intermeshing therewith is an annulus or ring gear 38 that is coupled, via linkages 40, to a stationary supporting structure 24.

[0081] Note that the terms “low pressure turbine” and “low pressure compressor” as used herein may be taken to mean the lowest pressure turbine stages and lowest pressure compressor stages (i.e. not including the fan 23) respectively and/or the turbine and compressor stages that are connected together by the interconnecting shaft 26 with the lowest rotational speed in the engine (i.e. not including the gearbox output shaft that drives the fan 23). In some literature, the “low pressure turbine” and “low pressure compressor” referred to herein may alternatively be known as the “intermediate pressure turbine” and “intermediate pressure compressor”. Where such alternative nomenclature is used, the fan 23 may be referred to as a first, or lowest pressure, compression stage.

[0082] The epicyclic gearbox 30 is shown by way of example in greater detail in FIG. 3. Each of the sun gear 28, planet gears 32 and ring gear 38 comprise teeth about their periphery to intermesh with the other gears. However, for clarity only exemplary portions of the teeth are illustrated in FIG. 3. There are four planet gears 32 illustrated, although it will be apparent to the skilled reader that more or fewer planet gears 32 may be provided within the scope of the claimed invention. Practical applications of a planetary epicyclic gearbox 30 generally comprise at least three planet gears 32.

[0083] The epicyclic gearbox 30 illustrated by way of example in FIGS. 2 and 3 is of the planetary type, in that the planet carrier 34 is coupled to an output shaft via linkages 36, with the ring gear 38 fixed. However, any other suitable type of epicyclic gearbox 30 may be used. By way of further example, the epicyclic gearbox 30 may be a star arrangement, in which the planet carrier 34 is held fixed, with the ring (or annulus) gear 38 allowed to rotate. In such an arrangement the fan 23 is driven by the ring gear 38. By way of further alternative example, the gearbox 30 may be a differential gearbox in which the ring gear 38 and the planet carrier 34 are both allowed to rotate.

[0084] It will be appreciated that the arrangement shown in FIGS. 2 and 3 is by way of example only, and various alternatives are within the scope of the present disclosure. Purely by way of example, any suitable arrangement may be used for locating the gearbox 30 in the engine 10 and/or for connecting the gearbox 30 to the engine 10. By way of further example, the connections (such as the linkages 36, 40 in the FIG. 2 example) between the gearbox 30 and other parts of the engine 10 (such as the input shaft 26, the output shaft and the fixed structure 24) may have any desired degree of stiffness or flexibility. By way of further example, any suitable arrangement of the bearings between rotating and stationary parts of the engine (for example between the input and output shafts from the gearbox and the fixed structures, such as the gearbox casing) may be used, and the disclosure is not limited to the exemplary arrangement of FIG. 2. For example, where the gearbox 30 has a star arrangement (described above), the skilled person would readily understand that the arrangement of output and support linkages and bearing locations would typically be different to that shown by way of example in FIG. 2.

[0085] Accordingly, the present disclosure extends to a gas turbine engine having any arrangement of gearbox styles (for example star or planetary), support structures, input and output shaft arrangement, and bearing locations.

[0086] Optionally, the gearbox may drive additional and/or alternative components (e.g. the intermediate pressure compressor and/or a booster compressor).

[0087] Other gas turbine engines to which the present disclosure may be applied may have alternative configurations. For example, such engines may have an alternative number of compressors and/or turbines and/or an alternative number of interconnecting shafts. By way of further example, the gas turbine engine shown in FIG. 1 has a split flow nozzle 18, 20 meaning that the flow through the bypass duct 22 has its own nozzle 18 that is separate to and radially outside the core engine nozzle 20. However, this is not limiting, and any aspect of the present disclosure may also apply to engines in which the flow through the bypass duct 22 and the flow through the core 11 are mixed, or combined, before (or upstream of) a single nozzle, which may be referred to as a mixed flow nozzle. One or both nozzles (whether mixed or split flow) may have a fixed or variable area. Whilst the described example relates to a turbofan engine, the disclosure may apply, for example, to any type of gas turbine engine, such as an open rotor (in which the fan stage is not surrounded by a nacelle) or turboprop engine, for example. In some arrangements, the gas turbine engine 10 may not comprise a gearbox 30.

[0088] The geometry of the gas turbine engine 10, and components thereof, is defined by a conventional axis system, comprising an axial direction (which is aligned with the rotational axis 9), a radial direction (in the bottom-to-top direction in FIG. 1), and a circumferential direction (perpendicular to the page in the FIG. 1 view). The axial, radial and circumferential directions are mutually perpendicular.

[0089] It will be understood that the invention is not limited to the embodiments above-described and various modifications and improvements can be made without departing from the concepts described herein. Except where mutually exclusive, any of the features may be employed separately or in combination with any other features and the disclosure extends to and includes all combinations and sub-combinations of one or more features described herein.