WO2008082334A1

WO2008082334A1 - A gas turbine engine, an aircraft provided therewith, and a method of controlling the operation of such an engine

Info

Publication number: WO2008082334A1
Application number: PCT/SE2006/001516
Authority: WO
Inventors: Anders Hedman; Lennart Brusved
Original assignee: Volvo Aero Corporation
Priority date: 2006-12-29
Filing date: 2006-12-29
Publication date: 2008-07-10

Abstract

A gas turbine engine with a power transmission device (2) comprising: a first turbine shaft (11) of said gas turbine engine (1); a power output shaft (24); a gear mechanism (14) arranged to provide a variable gear change between said first turbine shaft (11) and said power output shaft (24), said gear mechanism (14) being provided to change between a first gear reduction iLpacc1 and a second gear reduction iLpacc2, wherein iLpacc1 and iLpacc2 are different from 0. Said gear mechanism (14) comprises a step gear mechanism.

Description

A gas turbine engine, an aircraft provided therewith, and a method of controlling the operation of such an engine

TECHNICAL FIELD

The present invention relates to a gas turbine engine with a power transmission device comprising: a first turbine shaft of said gas turbine engine; a power output shaft; a gear mechanism arranged to provide a variable gear change between- said first turbine shaft and said power output shaft, said gear mechanism being provided to change between a first gear reduction ΪLP_acci and a second gear reduction ΪLPacc2, wherein ΪLP_acci and hp_acd are different from 0. In other words, both gears provide for an actual transmission of power.

The invention also relates to an aircraft provided with such a gas turbine engine for its propulsion, as well as a method of operating such an engine. However, it should be understood that the principles of the invention might be applicable also to other gas turbine engines than those used in aircraft, and that such applications also may fall within the scope of the invention.

In particular, but not necessarily, said gas turbine engine is a multiple spool or turbofan engine for the propulsion of an aircraft. Typically, it comprises a low pressure turbine shaft and a high pressure turbine shaft, said first turbine pressure shaft being the low pressure turbine shaft.

Typically, the engine is of a type that comprises an external unit such as an accessory gearbox which is drivingly connected with said power output shaft and to which power is to be delivered from said first turbine shaft via said gear mechanism and power output shaft during operation of said engine.

In particular, the invention is advantageous for engines by which said first turbine shaft is arranged so as to operate with a rotational speed varying within a predetermined range from riLPmin to riLP_max, wherein the ratio nLp_max/nip_min is of such dignity that it might be problematic for an efficient transfer of power from the turbine shaft to any external unit, such as an accessory gearbox and units connected therewith.

hi the following text "drivingly connected" is referred to as either a direct connection or a connection through any further component, such as a gear transmission or the like. Drivingly connected components may or may not have coinciding rotational axes. On the other hand "rotationally interconnected" will indicate a higher degree of interconnection wherein two components are directly interconnected and will rotate with the same rotational speed, preferably with coinciding rotational axes. A component which is rotationally interconnected with another component is also drivingly connected therewith.

When a certain component, like a shaft, is said to be drivingly connected with a specific member of a planetary gear disclosed herein, it will mean that the component is connected to the planetary gear through said member.

Furthermore, "gear reduction" will be referred to as the ratio of the speeds of two drivingly connected components. Such a ratio may have a value that is larger or smaller than unity, or be equal to unity for the special case of rotationally interconnected components.

BACKGROUND OF THE INVENTION

A portion of the power provided by aircraft turbo engines is used for powering the ancillary services both of the turbo engines themselves and of the aeroplane for which they provide propulsion.

At the time being, this power is mainly extracted on the shaft of the high pressure stage, the HP-shaft, of the twin-spool engine mechanically in order to drive the input shaft of an accessory casing or gear box. Different ancillary machines, such as generators and oil or fuel hydraulic pumps, are contained and driven in this accessory casing.

The need of electric power to be used for the operation of such ancillary machines will most probably drastically increase in the future. Therefore, it will be necessary to retrieve more power from the turbine engines. However, the high pressure rotor shaft that, by tradition, has been the shaft from which said power is retrieved, will not be able to deliver all of said power alone, due to the fact that a large mechanical power extraction has a negative effect on the operation of the high pressure spool as it is likely to cause pumping of the compressor, in particular when the engine is running at low speed.

Accordingly, the manufacturers of aeroplane engines are considering using also the low pressure rotor shaft, the LP-shaft, for the purpose of transferring power to the accessory gearbox. There is also suggested to retrieve power only from the LP-shaft.

However, the LP-shaft will, in most cases, operate with a rotational speed varying within a predetermined range from riL_Pmi_n to nιp_max, wherein the low minimum speed n_LPmi_n as well as the high value of the ratio nip_max/n_Lp_mi_n is not likely to suit the external units to be powered by the LP-shaft.

PRIOR ART

hi order to address the problem of the low minimum speed n_Lp_min and the high value of the ratio nip_maxlnLp_min of the LP-shaft, US 6561940 suggests a system in which there is provided a continuous change of the gear reduction between the LP-shaft and the power output shaft by means of a continuously variable transmission. Thereby, the external units powered by the LP-shaft may be driven with a constant rotational speed. However, such a system is relatively complicated. It is likely to be expensive, heavy, and space-demanding and to present a rather low efficiency or yield.

THE OBJECT OF THE INVENTION The object of the invention is to present a gas turbine engine with a power transmission device as initially defined that provides for a variable gear reduction between a turbine shaft and a unit driven thereby, and that will alleviate at least some of the above-mentioned disadvantages of said prior art.

Accordingly, it is an object of the invention to present a gas turbine engine with a power transmission device as initially defined, which is relatively uncomplicated as to its design, is relatively cost-efficient to produce, install and maintain, is of relatively low weight and which will present a rather high efficiency as to the transfer of power from a turbine shaft to any external unit.

It is also an object of the present invention to provide a method of controlling the operation of a multiple spool or turbofan engine for the propulsion of an aircraft, especially in connection to start up of the engine, but also during operation.

SUMMARY OF THE INVENTION

The object of the invention is achieved by means of the initially defined gas turbine engine, characterised in that said gear mechanism comprises a step gear mechanism.

According to one embodiment said first turbine shaft is arranged so as to operate with a rotational speed varying within a predetermined range from n_LPmi_n to nιp_max, wherein the gear mechanism is provided to change gear from said first gear reduction i∑Pacci to said second gear reduction iip_acc2 when the first turbine shaft reaches a predetermined rotational speed within said range. Accordingly, at low values of the rotational speed Π_LP of the first turbine shaft, the gear mechanism should provide a gear reduction that results in a reasonably high value of the rotational speed of the power output shaft and any external unit driven thereby. Thus, the gear reduction iLP_ac_c^Lp /n_acc should have a relatively low value. As the speed of the first turbine shaft increases a predetermined value, the gear reduction

be changed to a higher value. The stepwise change between the different gear reductions is to be controlled such that the rotational speed of any external unit driven by said turbine shaft is kept within an acceptable range.

According to one embodiment the transmission device comprises a power-shift device enabling a gear change by the gear mechanism under continuous transmission of power from said first turbine shaft to said power output shaft. Thereby, continuous powering of external units powered by said turbine shaft is provided for.

According to one embodiment said gear mechanism comprises a planetary gear. Preferably said gear mechanism comprises a planetary gear that comprises three interacting members comprising: first gearwheel; a second gearwheel; a planet carrier carrying at least one planetary wheel, wherein each of the gearwheels is in engagement with at least one of said at least one planetary wheel.

Preferably, said planetary gear presents a first operation mode corresponding to one of said gear reductions ΪLP_acc, hp_acc2 , and a second operation mode corresponding to the other of said gear reductions i∑pacc, iiPacc2-

According to one embodiment, in said first operation mode, one of said three interacting members is held in a non-rotating position. Preferably this is done by means of a brake device arranged so as to brake and hold one of said members when activated. The brake device may be supported by a housing that surrounds the gear mechanism. When one out of these members is held in a non-rotating position, there will be a gear reduction between the remaining two members.

In said second operation mode, two of said interacting members are rotationally interconnected. When two out of said three members are rotationally interconnected, the whole planetary gear will rotate like one single body, and all three interacting members will have the same rotational speed. Accordingly, one of said three members is to be operated on by said brake device, while two of the members are arranged so as to be rotationally interconnected, preferably by means of a suitable clutch mechanism. The member acted upon by the brake device may be any of the two members that are interconnected in the second mode, or may not be any of them. The turbine shaft may be drivingly connected or rotationally interconnected with any of the three interacting members, but, according to a preferred embodiment, it is arranged so as to be drivingly connected or rotationally interconnected with one of the two members that are rotationally interconnected in said second mode.

According to one embodiment, the above-mentioned power-shift device comprises said brake device. The function thereof will be described later.

According to one embodiment, said power output shaft is drivingly connected with said second of said three interacting members, and said first turbine shaft is drivingly connected with said third of said three interacting members.

According to one embodiment, said first one of said members is the first gearwheel, said second one of said members is the second gearwheel, and said third one of said members is the planet carrier.

According to one embodiment said first and second gearwheels are sun wheels in meshing engagement with said at least one planetary wheel through their outer periphery. The first operational mode will then correspond to said first gear reduction ΪL_Pacci and the second operational mode will correspond to said second gear reduction ΪLPa_cc2- The gear reduction of the first mode will be dependent on the pitch diameters of the gearwheels of the planetary gear. Preferably, in this case the pitch diameter ratio between said sun wheels is between 0.7 and 1.4.

According to one embodiment the engine comprises a second turbine shaft, and a clutch mechanism arranged to drivingly connect said second turbine shaft to said power output shaft. Typically the transmission device of the engine comprises one or more clutch mechanisms arranged to disconnect the first turbine shaft from the power output shaft when connecting the second turbine shaft to the power output shaft. Accordingly, power may be retrieved either from one or both of the first and the second turbine shafts and transmitted through the transmission device to the power output shaft. During start up of the engine, when power is transmitted in a reversed direction from a starter motor through said power output shaft, possibly through the gear mechanism, to the turbine shafts, one of the shafts may be disconnected by means of said clutch device or devices. Thereby, a more efficient start procedure may be achieved. According to the invention it is preferred that the first turbine shaft is a low pressure turbine shaft, or at leas a lower pressure turbine shaft, of the engine, while the second turbine shaft is a high pressure turbine shaft of the engine. It is preferred to start the engine by disconnecting the low pressure turbine shaft from the power output shaft and connecting the latter to the high pressure shaft, i.e. the second turbine shaft, which presents a lower torque and will be more easily and readily started. When the second turbine has been started, thereby indirectly starting the first turbine, it is suggested to disconnect the second turbine shaft from the power output shaft and to drivingly connect the latter with the first turbine shaft through the planetary gear. The transmission device according to the invention may then be used as suggested in order to provide for a variable gear reduction between the first turbine shaft and the power output shaft and any external elements connected to and driven by the latter, such as an accessory gearbox.

The invention also includes a method of controlling the operation of a gas turbine engine according to the invention, characterised in that said first turbine shaft is controlled so as to operate with a rotational speed varying within a predetermined range from nιp_min to nu>_max, wherein a gear reduction between the first turbine shaft and the power output shaft is changed in a stepwise manner from a first gear reduction Ϊ_LPa_cci to a second gear reduction iιp_acc2 when the first turbine shaft reaches a predetermined rotational speed within said range.

According to one embodiment, said stepwise change of gear reduction is performed under continuous delivery of power from the first turbine shaft to the power output shaft by means of a power-shift device. According to one embodiment, during start of the engine, power is delivered from a starter motor through said power output shaft to a second turbine shaft, while said first turbine shaft is disconnected from said power output shaft.

When the start procedure is ended, the first turbine shaft is connected to the power output shaft through the gear mechanism, while the second turbine shaft is disconnected from the power output shaft, and power is delivered from the first turbine shaft to the power output shaft.

Typically said first turbine shaft is a low pressure or at least lower pressure turbine shaft, normally operating in a first rotational speed range of approximately 1000 to approximately 4000 rpm, while the second pressure turbine is a high pressure turbine, normally operating in a second rotational speed range of approximately 10 000 to approximately 17 000 rpm. Deviations from the suggested rpm values may occur. However, the first speed range is at a lower level than the second speed range, and the ratio nL_PmaxlnLP_mm of the first range is larger than the corresponding ratio nH_PmωJn_HPmi_n of the second range, normally with a factor of 2 or more. According to one embodiment

> 2. According to a further embodiment

> 3. According to yet another embodiment

> 4.

Further features and advantages of the invention will be disclosed in the following detailed description of preferred embodiments and in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Hereinafter preferred embodiments of the invention will be described by way of example with reference to the accompanying drawings on which:

Fig. 1 is a schematic representation of a first embodiment of a gas turbine engine provided with a transmission device according to the invention, Fig. 2 is a schematic side view of a first embodiment of a transmission device of a gas turbine engine according to the invention,

Fig. 3 is a schematic side view of a second embodiment of a transmission device of a gas turbine engine according to the invention, and

Figs. 4-6 show three different clutch mechanism positions of the clutch mechanisms in the transmission device according to fig. 3.

DETAILED DESCRIPTION OF THE INVENTION

Fig. 1 is an oversight view of a gas turbine engine 1 according to the invention provided with a device 2 according to the invention. The gas turbine engine 1 shown in fig. 1 is of conventional construction and comprises, in axial flow series, an air intake 3 , low pressure compressor 4, high pressure compressor 5, combustion equipment 6, high pressure turbine 7, low pressure turbine 8 and an exhaust outlet 9. During operation, the high pressure compressor is driven by the high pressure turbine via a first hollow shaft, the high pressure turbine shaft 10. Similarly, the low pressure compressor is driven by the low pressure turbine via a second hollow shaft, the low pressure turbine shaft 11, which is coaxially disposed within the first hollow shaft 10.

The gas turbine 1 operates in the conventional manner whereby air drawn in through the air intake 3 is compressed by the low pressure compressor before passing into the high pressure compressor where it is compressed further. The compressed air then flows into the combustion equipment 6 where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through the high and low pressure turbines 7, 8 before being exhausted to the atmosphere through the exhaust nozzle 9.

The device 2 of the invention comprises a transmission that has as its main task to transfer power from the low pressure compressor rotor 4 and/or the high pressure compressor rotor 5 of the engine 1 to an accessory gearbox 12 during engine operation, said accessory gearbox 12 being located on the outside of the engine 1. A transfer gearbox 13 may also be provided through which power is transferred to the accessory gearbox 12. Typically, different ancillary machines, such as generators and oil or fuel hydraulic pumps, are driven through the accessory gearbox 12. However, during start up of the engine 1 , the power transfer direction is reversed, whereby power is transferred from a starter motor through the accessory gearbox 12 to the rotor of at least one of said compressors 4, 5 preferably to the high pressure compressor 5. Once, by definition, the engine 1 has been started and power maybe retrieved from the coaxial compressor or turbine shafts 10, 11, the device according to the invention is set in an operation mode by which power is transferred from the low pressure shaft 11 to the accessory gearbox 12 through said device 2, and the high pressure turbine shaft is disconnected in the sense that no power is transmitted from the latter to accessory gearbox 12.

Fig. 2 shows a first embodiment of a transmission device 2 of the gas turbine engine according to the invention. The transmission device 2 comprises a gear mechanism that comprises a planetary gear 14 that comprises: first gearwheel 15 rotationally interconnected with a first rotational shaft 16; a second gearwheel 17 rotationally interconnected with a second rotational shaft 18; and a plurality of planetary wheels 19. Each of the gearwheels 15, 17 is in meshing engagement with the planetary wheels 19. Each of the gearwheels 15, 17 is in meshing engagement with the planetary wheels 19 through corresponding peripheries. In this specific embodiment the gearwheels 15, 17 define coaxial sun wheels the outer periphery of which are in meshing engagement with said planetary wheels. As an alternative, both gearwheels could define ring wheels, the inner periphery of which would then have been in engagement with said planetary wheels. The planetary gear 14 also comprises a planet carrier 20 carrying said planetary wheels 19 and rotationally interconnected with a third rotational shaft, which, in this embodiment, is the low pressure turbine shaft 11.

The rotational shafts 16, 18, 11 of the planetary gear 14, are coaxial with the first turbine shaft 11. The first rotational shaft 16 is connected with a brake device 21, which is supported by a non-rotational stator part 22 of the engine 1. Upon activation of said brake device 21, the latter will hold the first rotational shaft 16 and the corresponding first gearwheel in a non-rotating position.

Through a bevel gear 23 the second rotational shaft 18 is drivingly connected with and in meshing engagement with a power output shaft 24, which, in its turn, is drivingly connected with the aforementioned accessory gearbox 12.

There is also provided a first clutch mechanism 25, which, upon activation thereof, is provided to rotationally interconnect the second rotational shaft 18 with the third rotational shaft 11, which, in this embodiment, is the low pressure turbine shaft 11. It should be understood that said third rotational shaft need not be the low pressure turbine shaft 11 itself, but might instead be drivingly connected or rotationally interconnected with the latter.

The high pressure turbine shaft 10 forms a second turbine shaft that, by means of a second clutch mechanism 26 may be drivingly connected with the power output shaft 24 through a bevel gear 27, upon activation of said second clutch mechanism.

Upon start of the engine 1, the brake device 21 as well as the first clutch mechanism 25 are inactivated, thereby providing for a freewheeling of the planetary gear 14 and no transmittal of power from the power output shaft 24 to the first turbine shaft 11. On the other hand, the second clutch mechanism 26 is activated, thereby providing for a transmittal of power from the power output shaft 24 to the second turbine shaft 10. The power output shaft 24 is, in its turn, drivingly connected to a starter motor (not shown) that drives said output shaft 24 during the start procedure. Accordingly, power will be transmitted from the starter motor through the power output shaft 24 to the second turbine shaft, which, in this case, is the high pressure turbine shaft 10. The high pressure compressor 5 and the associated high pressure turbine 7 will thus be accelerated until they are, as per definition started. Thereby, also the low pressure turbine will have been accelerated as a mere consequence of the acceleration of the high pressure turbine. However, it will operate in a lower range of its rotational speed range, which extends from niPmin to riLP_max- Now, power is to be retrieved from the first turbine shaft 11 and transmitted to the accessory gearbox through the transmission 2. Accordingly, the second clutch mechanism 26 is inactivated, thereby disconnecting the second turbine shaft 10 from the power output shaft 24. At low rotational speed of the first turbine shaft 11, a low gear reduction is requested that results in a relatively high rotational speed of the power output shaft 24. This is achieved by activating the brake device 21. Thereby, the first gearwheel 15 is locked in a non-rotating position, and the power will be transmitted through the second gearwheel 17 and the planet carrier 20 to the first turbine shaft. Since the second gearwheel 17 is a sun wheel, it will rotate at a higher speed than the planet carrier 20, thereby resulting in a required gear reduction. According to a preferred embodiment, the second gearwheel 17 and the planet carrier 20 are designed such that the rotational speed of the second gearwheel will be approximately twice the rotational speed of the planet carrier 20.This is a first mode of operation of the planetary gear 14, used for transfer of power from the first turbine shaft 11 to the power output shaft 10 at low rotational speed of the first turbine shaft, preferably up to the middle of the range n_LPmin to n_Lp_max-

As the speed of the first turbine 11 shaft increases, the rotational speed of the power output shaft 24 and the components of an accessory gearbox 12 connected therewith will increase to a corresponding degree. At a predetermined level of the rotational speed of the first turbine shaft 11, in order to avoid excessive rotational speeds in the accessory gearbox 12, the gear reduction is changed in a stepwise manner by a change of operation mode of the planetary gear 14. Accordingly, the brake device 21 is gradually inactivated, such that the rotational speed of the first gearwheel 15, or the shaft 16 rotationally interconnected therewith, is slowly starting to rotate. Thereby, the rotational speed of the second gearwheel 17 will, as a consequence of the design of the transmission device 2, decrease correspondingly. When the rotational speed of the second gearwheel 17, and the associated second rotational shaft 18, is approximately the same as that of the planet carrier 20 and its associated rotational shaft, which in this embodiment is the first turbine shaft 11 itself, the brake device 21 is fully inactivated and the first clutch device 25 is activated. Thereby, the planet carrier 20 and the second gearwheel 17 will rotate together as one single body, with one and the same rotational speed. Accordingly, a second a gear reduction is achieved, resulting in a lowering of the rotational speed of the power output shaft 24 and of any of the components connected therewith through the accessory gearbox 12. This is a second operational mode of the planetary gear 14 of the transmission device 2 of the gas turbine engine 1.

When changing gear by going from the second operational mode to the first operational mode, the above-mentioned measures are taken in a reversed order. However, the brake device 21 should be gradually activated in order to lower the rotational speed gradually until the point when the clutch 25 can be inactivated.

Fig. 3 shows an alternative embodiment of the transmission device 2 of the gas turbine engine 1 of the invention by which the rotational axis of the planetary gear 28 is displaced in relation to the rotational axis of the first turbine shaft 11. In contrast to the first embodiment of fig. 1, the planetary gear is provided at the outside of an engine housing 29 that encloses the turbines 7, 8 and the compressors 4, 5. Thereby, the planetary gear 28 is more easily accessible for purposes such as maintenance.

Likewise to the previous embodiment, the planetary gear 28 of this embodiment comprises: a first gearwheel 30 rotationally interconnected with a first rotational shaft 31 ; a second gearwheel 32 rotationally interconnected with a second rotational shaft 33; and a plurality of planetary wheels 34. Each of the gearwheels 30, 32 is in meshing engagement with the planetary wheels 34. Each of the gearwheels 30, 32 is in meshing engagement with the planetary wheels 34 through corresponding peripheries. In this specific embodiment the gearwheels 30, 32 define coaxial sun wheels the outer periphery of which are in meshing engagement with said planetary wheels 34. As an alternative, both gearwheels could define ring wheels, the inner periphery of which would then have been in engagement with said planetary wheels. The planetary gear 28 also comprises a planet carrier 35 carrying said planetary wheels 34 and rotationally interconnected with a third rotational shaft 36.

There is also provided an intermediate shaft 37 that is drivingly connected with the first turbine shaft 11 through a first bevel gear 38. A clutch mechanism 39 is provided for the purpose of, upon activation thereof, rotationally interconnect the intermediate shaft 37 with the one of said rotational shafts of the planetary gear 28, more precisely the third rotational shaft 36 in this case.

There is also provided a second clutch mechanism 40 for the purpose of, upon activation thereof, rotationally interconnect the second rotational shaft 33 with the third rotational shaft 36.

A power output shaft 41 is drivingly connected with the second rotational shaft 33 through a second bevel gear 42, and is also drivingly connected with the components of an accessory gearbox 12.

The second turbine shaft 10, i.e. the high pressure turbine shaft, is connected to a second intermediate shaft 43, through a third bevel gear 44. This second intermediate shaft 34 is coaxial with the first intermediate shaft 37 and extends inside the latter, which in this specific embodiment is of tubular shape. The second intermediate shaft 43 drivingly connects the second turbine shaft 10 with the power output shaft 41. In order to enable a connection, more precisely a rotational interconnection, of the second intermediate shaft 43 with one of said rotational shafts of planetary gear 28, in this case the third rotational shaft 36 thereof, there is provided a third clutch mechanism 45. Upon activation of said third clutch mechanism 45, the second intermediate shaft 43 is rotationally interconnected with the third rotational shaft 36. It should be noted that the first and third clutch mechanisms 39 and 45 might as well be regarded as a double-acting clutch mechanism which connects one of said interacting members, in this case the planet carrier 35 through the third shaft 36, with either the first intermediate shaft 37 or the second intermediate shaft 43. Likewise to the first embodiment there is provided a brake device 46 supported by a non-rotational part of the engine 1 or an aircraft carrying said engine. Upon activation of the brake device 46, it holds the first gearwheel 30 and its associated rotational shaft 31 in a non-rotating position.

Apart from the constructional differences between the first and second embodiments, the basic principles of the functions of the transmission devices 2 of the two embodiments are the same. During a start up of the engine with the second embodiment of the transmission device 2, the third clutch mechanism 45 and the brake device 46 are activated, while the first and second clutch mechanisms 39, 40 are inactivated (see fig. 4). Thereby, power is transmitted from a starter motor (not shown) to the second turbine shaft 10 through the power output shaft 41, the second rotational shaft 33, the third power output shaft 36 and the second intermediate shaft 43. The planetary gear will reduce the rotational speed of the power output shaft 41 to the second intermediate shaft 43 thanks to the transmittal of power through the second gearwheel 32 and the planet carrier 35.

When the gas turbine engine 1 has been started, and the rotational speed of the first turbine shaft 11 is in the lower part of its range, in which the first operational mode of the planetary gear 28 is to be applied and power is to be transmitted in a reverse direction from the first turbine shaft 11 to the power output shaft 41, the third clutch mechanism 45 is inactivated, the first clutch 39 is activated and the brake device 46 is activated (see fig. 5). Thereby, the second gearwheel 32 and second rotational shaft 33 will rotate with a higher rotational speed than the third rotational shaft 36.

When the first turbine shaft 11 is accelerated and reaches a predetermined rotational speed, the gear reduction between the first turbine shaft 11 and the power output shaft 41 is changed by a change of operational mode of the planetary gear 28. Thereby, likewise to the first embodiment, the brake device 46 is gradually inactivated, resulting in a gradual acceleration of the first gearwheel 30 and first rotational shaft 31, as well as a corresponding gradual deceleration of the second gearwheel 32 and its rotational shaft 33. When the latter has reached a rotational speed approximately the same as the third shaft 36, said second and third rotational shafts 33, 36 are rotationally interconnected by activation of the second clutch mechanism 40 (see fig. 6), and the brake device 46 is fully inactivated. As a result the planetary gear 28 will rotate as one single body with the same rotational speed as the first intermediate shaft 37.

When changing gear by going from the second operational mode to the first operational mode, the above-mentioned measures are taken in a reversed order.

According to an alternative embodiment, not shown, the first and third clutch mechanisms 39, 45 of the above-described second embodiment could be located between the first bevel gear 38 and the second bevel gear 44, and be arranged as a double-acting clutch mechanism. As a result the first and second intermediate shafts 37, 43 could be arranged as one single shaft extending from said third clutch mechanism to the planetary gear 28. The double-acting clutch mechanism should then be arranged so as to connect said intermediate shaft to the first turbine shaft 11 while disconnecting said intermediate shaft from the second turbine shaft 10 and vice versa, depending on operation (start or post-start).

It should be understood that the above description of preferred embodiments has been made in order exemplify the invention, and that alternative solutions will be obvious for a person skilled in the art, however without departing from the scope of the invention as defined in the appended claims supported by the description and the drawings.

For example, the above-clutch mechanisms for the rotational interconnection and disconnection of parts of the transmission of the engine may be either of the tooth clutch type, as indicated in figs. 2 and 3, or a free-wheel mechanism of a kind known per se. Free-wheel mechanisms have the advantage of not requiring any control equipment. For example, in the first embodiment, the second tooth clutch mechanism 26 may be replaced by a free-wheel clutch that permits the second turbine shaft 10 to rotate at a higher rotational speed than the concentric gearwheel in the second bevel gear 27. Upon start, this clutch will force the second turbine shaft 10 to rotate with said gearwheel. Likewise, the first tooth clutch 25 could be replaced by a free-wheel clutch that permits the conical concentric gearwheel in the first bevel gear 23 to rotate faster than the first turbine shaft 11 (during the first operational mode). When the brake device 21 is gradually inactivated the free-wheel clutch will force said gearwheel to rotate with the first turbine shaft, resulting in the second operational mode.

Correspondingly, for the second embodiment, the third clutch mechanism 45, shown as a tooth clutch, could be replaced by a pair of free-wheel clutches, one of which permits the second intermediate shaft 43 to rotate faster than the third rotational shaft 36. Upon start, such a clutch would force the intermediate shaft 43 to rotate with the third rotational shaft 36. The other free-wheel clutch would permit the third rotational shaft 36 to rotate faster than the first intermediate shaft 37, which is the case upon start. When the power transmission direction is reversed, this clutch will force the third shaft 36 to rotate with the first intermediate shaft 37. Likewise, the second tooth clutch 40 shown in fig. 2 could be replaced by a free-wheel clutch that would permit the second rotational shaft 33 to rotate at higher speed than the third rotational shaft 36 (which is the case upon start and during first operational mode). When the brake device 46 is gradually activated, the free-wheel clutch will force the second rotational shaft 36 to rotate together with the third rotational shaft 37.

Other planetary gear designs and arrangements than those described above may be conceived. If, for example, the gearwheels are ring wheels instead of sun wheels, the second mode of operation will be used for low rotational speed of the first turbine shaft, and the first mode of operation will be used for high rotational speed of the turbine shaft, since the second mode will then result in lower rotational speed of the second rotational shaft than that of the third rotational shaft. Such an alternative solution might be considered, for example if there is required a further counterforce against the centrifugal force of the planetary wheels. In the application, when the power output shaft or any turbine shaft or intermediate shaft is said to be drivingly connected to a specific member of the planetary gear, it is to be understood that the power train goes from said shaft into said planetary gear through that specific member.

"Disconnection" as referred to in the text, indicates that power is not admitted to be transmitted in a certain direction in the power train, either due to a physical disconnection of two parts, such as by the action of a tooth clutch mechanism, or due to the effect of a free-wheel clutch.

Claims

PATENT CLAIMS

1. A gas turbine engine (1) with a power transmission device (2) comprising:

- a first turbine shaft (11) of said gas turbine engine (1), - a power output shaft (24, 41),

- a gear mechanism (14, 28) arranged to provide a variable gear change between said first turbine shaft (11) and said power output shaft (24, 41), said gear mechanism (14, 28) being provided to change between a first gear reduction ΪL_Pacci and a second gear reduction hpacd, wherein ΪLPacci and iiPacci are different from 0, characterised in that - said gear mechanism (14, 28) comprises a step gear mechanism.

2. A gas turbine engine (1) according to claim 1, characterised in that said first turbine shaft (11) is arranged so as to operate with a rotational speed varying within a predetermined range from ri_LPm_m to riLP_max, wherein the gear mechanism (14, 28) is provided to change gear from said first gear reduction hp_acci to said second gear reduction iu>_aCc2 when the first turbine shaft (11) reaches a predetermined rotational speed within said range.

3. A gas turbine according to claim 1 or 2, characterised in that it comprises a power-shift device enabling a gear change by the gear mechanism (14, 28) under continuous transmission of power from said first turbine shaft (11) to said power output shaft (24, 41).

4. A gas turbine engine (1) according to claim 3, characterised in that said power- shift device comprises a brake device (21, 46) provided so as to connect a rotational part (16, 31) of the gear mechanism (14, 28) with a non-rotational part (22, 47) of the engine.

5. A gas turbine engine (1) according to claim 4, characterised in that said non- rotational part (22, 47) comprises a housing enclosing said gear mechanism (14, 28).

6. A gas turbine engine (1) according to any one of claims 1-5, characterised in that said gear mechanism (14, 28) comprises a planetary gear.

7. A gas turbine engine (1) according to claim 6, characterised in that said gear mechanism (14, 28) comprises a planetary gear that comprises three interacting members comprising -a first gearwheel (15, 30), -a second gearwheel (17, 32),

-a planet carrier (20, 35) carrying at least one planetary wheel (19, 34), -wherein each of the gearwheels (15, 17; 30, 32) is in engagement with at least one of said at least one planetary wheel (19, 34).

8. A gas turbine engine (1) according to claim 7, characterised in that said planetary gear (14, 28) presents a first operation mode corresponding to one of said gear reductions hp_acc, iwa_cci , and a second operation mode corresponding to the other of said gear reductions ΪLPacc, iu>acc2.

9. A gas turbine engine (1) according to claim 8, characterised in that, in said first operation mode, one (15, 30) of said three interacting members (15, 17, 20; 30, 32, 35) is held in a non-rotating position.

10. A gas turbine engine (1) according to any one of claims 7-9, characterised in that it comprises a brake device (21, 46) for braking and holding one (15, 30) of said three interacting members (15, 17, 20; 30, 32, 35) in a non-rotating position.

11. A gas turbine engine (1) according to any one of claims 8-10, characterised in that, in said second operation mode, two (17, 20; 32, 35) of said three interacting members (15, 17, 20; 30, 32, 35) are rotationally interconnected.

12. A gas turbine engine (1) according to claim 11, characterised in that it comprises a clutch mechanism (25, 40) arranged to rotationally interconnect and disconnect said two members (17, 20; 32, 35) for the activation and inactivation respectively of said second operation mode.

13. A gas turbine (1) according to any one of claims 10 and 12, characterised in that said brake device (21, 46) is arranged to act upon a first of said three interacting members (15, 17, 20; 30, 32, 35), and that said clutch mechanism is arranged to rotationally interconnect a second and a third of said three interacting members (15, 17, 20; 30, 32, 35).

14. A gas turbine according to any one of claims 10-13, characterised in that said brake device (21, 46) is arranged to act upon said first gearwheel (15, 30).

15. A gas turbine according to any one of claims 7-14, characterised in that said power output shaft (24, 41) is drivingly connected with one of said three interacting members (15, 17, 20; 30, 32, 35) and that said first turbine shaft (11) is drivingly connected with another one of said three interacting members (15, 17, 20; 30, 32, 35).

16. A gas turbine according to any one of claims 13-15, characterised in that said power output shaft (24, 41) is drivingly connected with said second of said three interacting members (15, 17, 20; 30, 32, 35), and that said first turbine shaft (11) is drivingly connected with said third of said three interacting members (15, 17, 20; 30, 32, 35).

17. A gas turbine according to claim 16, characterised in that said first one of said members (15, 17, 20; 30, 32, 35) is the first gearwheel (15, 30), said second one of said members (15, 17, 20; 30, 32, 35) is the second gearwheel (17, 32) and said third one of said members (15, 17, 20; 30, 32, 35) is the planet carrier (20, 35).

18. A gas turbine according to any one of claims 7-17, characterised in that said first and second gearwheels (15, 17; 30, 32) are sun wheels in meshing engagement with said at least one planetary wheel (19, 34) through their outer periphery.

19. A gas turbine engine (1) according to any one of claims 7-18, characterised in that said first turbine shaft (11) is coaxial with said planetary gear.

20. A gas turbine engine (1) according to any one of claims 7-19, characterised in that said first turbine shaft (11) is rotationally interconnected with one of said three interacting members (15, 17, 20; 30, 32, 35).

21. A gas turbine engine (1) according to any one of claims 7-20, characterised in that said first turbine shaft (11) is rotationally interconnected with said planet carrier

(20).

22. A gas turbine engine (1) according to any one of claims 15-21, characterised in that said first turbine shaft (11) is coaxial with the one of said three interacting members (15, 17, 20) with which it is connected.

23. A gas turbine engine (1) according to any one of claims 7-21, characterised in that the rotational axis of said planetary gear (28) is offset in relation to the rotational axis of the first turbine shaft (11).

24. A gas turbine engine (1) according to claim 23, characterised in that said first turbine shaft (11) is drivingly connected with one of said three interacting members (15, 17, 20; 30, 32, 35) of the planetary gear via an intermediate shaft (37).

25. A gas turbine engine (1) according to claim 24, characterised in that said first turbine shaft (11) is drivingly connected to said planet carrier (35) of the planetary gear via an intermediate shaft (37).

26. A gas turbine engine (1) according to any one of claims 1-25, characterised in that it comprises a second turbine shaft (10), and a clutch mechanism (26, 45) arranged to drivingly connect said second turbine shaft (10) to said power output shaft (24, 41).

27. A gas turbine engine (1) according to claim 7 and 26, characterised in that said clutch mechanism (45) is arranged to drivingly connect said second turbine shaft (10) with said power output shaft (41) by drivingly connecting said second turbine shaft (10) with one of said three interacting members (15, 17, 20; 30, 32, 35) of the planetary gear (28).

28. A gas turbine engine (1) according to claim 27, characterised in that said clutch mechanism (45) is arranged to drivingly connect said second turbine shaft (10) to said planet carrier (35) of the planetary gear (28).

29. A gas turbine engine (1) according to claim 27 or 28, characterised in that said clutch mechanism (45) is arranged to drivingly connect said second turbine shaft (10) to said one of said three interacting members (15, 17, 20; 30, 32, 35) via an intermediate shaft (43).

30. A gas turbine engine (1) according to claim 29, characterised in that said clutch mechanism (45) is arranged to rotationally interconnect said second intermediate shaft (43) with said one of said three interacting members (15, 17, 20; 30, 32, 35).

31. A gas turbine engine (1) according to any one of claims 26-30, characterised in that it comprises one or more clutch mechanisms (25, 39) arranged to disconnect the first turbine shaft (11) from the power output shaft (24, 41) upon connection of the second turbine shaft (10) to the power output shaft (24, 41).

32. A gas turbine engine (1) according to any one of claims 25 and 29-31, characterised in that said intermediate shaft (37) provided for the connection of first turbine shaft (11) to the planetary gear (28) and said intermediate shaft (43) provided for the connection of the second turbine (10) shaft to the planetary gear (28) are coaxially arranged, one (37) of said intermediate shafts (37, 43) being of generally tubular shape and surrounding the other (43) of said intermediate shafts (37, 43).

33. A gas turbine engine (1) according to any one of claims 1-32, characterised in that ni_PmJri_LPmm > 2.

34. A gas turbine engine (1) according to any one of claims 1-33, characterised in that n_LpmJn_Lpmin > 3.

35. A gas turbine engine (1) according to any one of claims 1-34, characterised in that n_LPmJnLPmin > 4.

36. A gas turbine engine (1) according to any one of claims 1-35, characterised in that it comprises a low pressure turbine shaft (11) and a high pressure turbine shaft (10), and that said first turbine pressure shaft is the low pressure turbine shaft (11).

37. A gas turbine engine (1) according to any one of claims 26-35, characterised in that it comprises a low pressure turbine shaft (11) and a high pressure turbine shaft

(10), and that said second turbine pressure shaft is the high pressure turbine shaft (10).

38. A gas turbine engine (1) according to any one of claims 1-37, characterised in that it comprises an accessory gearbox (12) which is drivingly connected with said power output shaft (24, 41) and to which power is to be delivered from said first turbine shaft (11) via said gear mechanism (14, 28) and power output shaft (24, 41) during operation of said engine (1).

39. A gas turbine engine (1) according to any one of claims 1-38, characterised in that it comprises a starter motor which is connected to said power output shaft (24, 41) and from which power is to be delivered through the power output shaft (24, 41) to a turbine shaft (10) during start of the engine (1).

40. A gas turbine engine (1) according to any one of claims 1-39, characterised in that it is a multiple spool or turbofan engine for the propulsion of an aircraft.

41. An aircraft, characterised in that it comprises a gas turbine engine (1) according to any one of claims 1-40.

42. A method of controlling the operation of a gas turbine engine (1) according to claim any one of claims 1-40, characterised in that said first turbine shaft (11) is controlled so as to operate with a rotational speed varying within a predetermined range from riLPmi_n to nιp_max, wherein a gear reduction between the first turbine shaft (11) and the power output shaft (24, 41) is changed in stepwise manner from a first gear reduction ΪLPacci to a second gear reduction irj>_aCc2 when the first turbine shaft (11) reaches a predetermined rotational speed within said range.

43. A method according to claim 42, characterised in that said stepwise change of gear reduction is performed under continuous delivery of power from the first turbine shaft (11) to the power output shaft (24, 41) by means of a power-shift device.

44. A method according to claim 42 or 43, characterised in that, during start of the engine (1), power is delivered from a starter motor through said power output shaft (24, 41) to a second turbine shaft (10), while said first turbine shaft (11) is disconnected from said power output shaft (24, 41).

45. A method according to claim 44, characterised in that, when the start procedure is ended, the first turbine shaft (11) is connected to the power output shaft (24, 41) * through the gear mechanism (14, 28), while the second turbine shaft (10) is disconnected from the power output shaft (24, 41), and power is delivered from the first turbine shaft (11) to the power output shaft (24, 41).