WO2009067048A1

WO2009067048A1 - Gas turbine engine

Info

Publication number: WO2009067048A1
Application number: PCT/SE2007/001022
Authority: WO
Inventors: Anders Hedman; Lennart Brusved
Original assignee: Volvo Aero AB
Current assignee: GKN Aerospace Sweden AB
Priority date: 2007-11-20
Filing date: 2007-11-20
Publication date: 2009-05-28
Anticipated expiration: 2010-05-20

Abstract

The invention concerns a gas turbine engine (1 ), comprising a first turbine shaft (10), a second turbine shaft (11 ), and a power transmission device (2, 13, 14) for transmitting power from the first and second turbine shafts (10, 11 ) to a power output shaft (15) for driving auxiliary units, such as a generator and oil pumps. The invention is characterized in that the power transmission device (2, 13, 14) comprises a first clutch mechanism (24, 124, 224, 524, 624, 724) for drivingly connecting and disconnecting the first turbine shaft (10) to and from the power output shaft (15), and a second clutch mechanism (25, 125, 225, 525, 625, 725) for drivingly connecting and disconnecting the second turbine shaft (11 ) to and from the power output shaft (15). The invention also concerns an aircraft comprising a gas turbine engine (1 ) of the above type.

Description

TITLE

Gas turbine engine.

TECHNICAL FIELD

This invention relates to a gas turbine engine. In particular, the invention relates to driving of auxiliary units of a gas turbine engine arranged for propulsion of an aircraft.

BACKGROUND ART

Aircraft gas turbine engines Get engines, turbojet engines, turbofan engines etc.) are normally provided with a number of auxiliary units for operation of various hydraulic and electric systems. The power to these auxiliary units is taken from the gas turbine engine shafts, usually from the high pressure (HP) shaft. Power transmission from the turbine shaft to the auxiliary units is traditionally carried out by means of a mechanical system including bevel gears, shafts and an accessory gearbox.

Electricity requirements will increase drastically in the next generation of narrow-body airplanes compared to the airplanes used today and therefore there is a need to take off additional electric power from the aircraft engines. This is a problem because the HP-shaft is not capable of delivering enough power for driving the auxiliary units at certain operation conditions. To avoid this problem it has been proposed to use the low pressure (LP) shaft for power transmission. A problem with the LP-shaft is, however, its low rotational speed and its large speed range that do not suit conventional auxiliary units.

Auxiliary units, such as oil pumps and electricity generators, are normally adapted to operate within a certain speed range φ_aCc defined as the ratio between the upper and the lower limit of the speed range; φ_aCc = nac_cmax/n_ac_cmin. Typically, the auxiliary units are adapted to operate at a speed range of around 2, which normally corresponds well to the speed range of the HP-shaft; φ_HPac_c = n_Hp_maχ/n_HPmin * 17000/10000 = 1,7. The LP-shaft, on the other hand, typically has a much larger speed range; (p_LPacc = n_L.pmaχ/nι_Pmin ^« 4000/1000 = 4. Therefore, it is not possible to simply move the driving of the auxiliary units from the HP-shaft to the LP-shaft. Generally, re-design of the auxiliary units is not an option because of the time, effort and money required.

US 6561940 discloses a system including a variator that continuously adjusts the gear ratio between the LP-shaft and the auxiliary units such that the units can be driven with a constant rotational speed. Such a system is, however, rather complex which makes it heavy, costly and space demanding.

US 5103631 discloses a system where a power output is driven by the HP- and LP-shafts simultaneously via a centrally located differential gear. A drawback of this system is that the planet wheel of the differential gear is subjected to large centrifugal forces that may jeopardize its durability. A similar system is disclosed in US 7168913 but wherein the differential gear is located externally. A drawback of this latter system is that it requires many shafts and gear wheels making the system heavy and costly.

EP 1574687 discloses a system comprising inner bevel gears, tower shafts, outer bevel gears and lay shafts connecting each of the HP- and LP-shafts, via separate drivelines, to an auxiliary gearbox. Also this system requires many shafts and gears which makes the system heavy and costly. Further, a conventional auxiliary gearbox, adapted to be connected to only one input shaft, can not be used.

There is thus still a need for improvements in the field of taking off power for auxiliary units from the shafts of aircraft gas turbine engines. DISCLOSURE OF INVENTION

The main object of this invention is to provide a gas turbine engine for an aircraft that exhibits improved properties with regard to driving of auxiliary units compared to conventional gas turbine engines. This object is achieved by the gas turbine engine defined by the technical features contained in independent claim 1. The dependent claims contain advantageous embodiments, further developments and variants of the invention.

The invention concerns a gas turbine engine, comprising a first turbine shaft, a second turbine shaft, a power output shaft suitable for driving an accessory gearbox, and a power transmission device for transmitting power from the first and second turbine shafts to the power output shaft. In the inventive gas turbine engine, the power transmission device comprises a first clutch mechanism for drivingly connecting and disconnecting the first turbine shaft to and from the power output shaft, and a second clutch mechanism for drivingly connecting and disconnecting the second turbine shaft to and from the power output shaft.

Such a design has the advantageous effect that it becomes possible to switch power source between the first and the second turbine shafts depending on the operating conditions since either of the first and the second turbine shaft can be used to drive the power output shaft independently of the other turbine shaft. For instance, if the first turbine shaft is an HP-shaft and the second turbine shaft is an LP-shaft, the LP-shaft can be used to drive the accessory gearbox, and thereby the auxiliary units, at low engine speeds, such as during idling and taxiing, whereas the HP-shaft can be used in most or all other operating conditions. By using the LP-shaft only in situations where the engine speed is low, it is possible to use only a portion of the large speed range φι_p_acc of the LP-shaft, which portion corresponds well to the suitable speed range φ_aCc of the auxiliary units. The present invention is thus in clear contrast to the systems disclosed in US 5103631 , US 7168913 and EP 1574687 where there is no possibility to disengage any of the turbine shafts from the power output shaft. Thus, there is no possibility to alternately let the turbine shafts one at a time drive the same power output shaft.

In an embodiment of the invention the power transmission device comprises a first bevel gear wheel concentrically arranged onto the first turbine shaft, a second bevel gear wheel concentrically arranged onto the second turbine shaft, a third bevel gear wheel arranged onto a shaft arrangement, and a fourth bevel gear wheel arranged onto the shaft arrangement, wherein the first and second bevel gear wheels are arranged to interact with the third and fourth bevel gear wheels, respectively, such as to allow a driving connection between the first and second turbine shafts and the shaft arrangement, and wherein the shaft arrangement is drivingly connected to the power output shaft. Such a design is simple and does not require much space.

In an embodiment of the invention the shaft arrangement comprises a first driving shaft provided with the third bevel gear wheel and a second driving shaft provided with the fourth bevel gear wheel, wherein the first and second clutch mechanisms are arranged to drivingly connect and disconnect the first and second driving shafts, respectively, to and from the power output shaft. Preferably, the first and second driving shafts are coaxially arranged. In such a design the clutch mechanisms can be arranged in a transfer gearbox outside the engine which allows for easy access for e.g. maintenance purposes.

In an embodiment of the invention the third and fourth bevel gear wheels are arranged onto a common driving shaft, wherein the first and second clutch mechanisms are arranged to drivingly connect and disconnect the first and second bevel gear wheels to and from the first and second turbine shafts, respectively. Preferably, the common driving shaft is drivingly connected to the power output shaft. In such a design the clutch mechanisms can be positioned in an internal gearbox inside a fan housing of the gas turbine engine. This way they become better protected from exposure to dirt and damage. Further, in some applications there is more space available inside the engine than outside. Moreover, in this embodiment only one driving shaft is needed between the internal gearbox and a transfer gearbox, which makes it possible to use a conventional transfer gearbox.

In an embodiment of the invention the first turbine shaft is connected to a first turbine adapted to operate at a first pressure and the second turbine shaft is connected to a second turbine adapted to operate at a second pressure, wherein the first pressure is higher than the second pressure. In a common biaxial aircraft gas turbine engine this means that the first turbine shaft is the

HP-shaft and that the second turbine shaft is the LP-shaft. In such an application the driving of the power output shaft can be shifted between the

HP- and LP-shafts in the advantageous way described above. In a gas turbine engine having an intermediate pressure (IP) shaft in addition to the

HP- and LP-shafts, it means that the first turbine shaft is the HP-shaft or the

IP-shaft and that the second turbine shaft is the IP-shaft or the LP-shaft. Similar advantages can be achieved also in such a case.

In an embodiment of the invention at least one of the first and second clutch mechanisms is of a dog clutch type. Such a type of clutch mechanism is simple and reliable.

In an embodiment of the invention at least one of the first and second clutch mechanisms is of a slip friction type. An advantage of such a type of clutch mechanism is that changing of gear, i.e. switching driving source between the two turbine shafts, can be made without interrupting the driving of the auxiliary units. In an embodiment of the invention at least one of the first and second clutch mechanisms is of a free-wheel type. An advantage of such a type of clutch mechanism is that switching driving source between the two turbine shafts can be made without interrupting the driving of the auxiliary units. A further advantage is that gear changing becomes simpler since only one clutch mechanism needs to be controlled. A further advantage is that a free-wheel mechanism reduces the wear.

In an embodiment of the invention each of the first and second clutch mechanisms is of the dog clutch type.

In an embodiment of the invention each of the first and second clutch mechanisms is of the slip friction type.

In an embodiment of the invention the first clutch mechanism is of the slip friction type and the second clutch mechanism is of a free-wheel type arranged to transmit power in a direction from the second turbine shaft to the power output shaft.

In an embodiment of the invention the gas turbine engine comprises a starter engine drivingly connected to the power output shaft, said starter engine being controllable such as to facilitate disengagement and engagement of the first and second clutch mechanisms. This is of particular interest when using a dog clutch since shifting gear with such a clutch involves torque-free disengagement and synchronized engagement.

The invention also concerns an aircraft comprising a gas turbine engine arranged for propulsion of the aircraft, wherein the gas turbine engine is of the above type. BRIEF DESCRIPTION OF DRAWINGS

In the description of the invention given below reference is made to the following figures, in which:

Figure 1 shows a schematic overview of an aircraft gas turbine engine provided with a power transmission device according to the invention,

Figure 2 shows, in a schematic view, a first preferred embodiment of the gas turbine power transmission device according to the invention,

Figure 3 shows, in a schematic view, a second preferred embodiment of the gas turbine power transmission device according to the invention,

Figure 4 shows, in a schematic view, a third preferred embodiment of the gas turbine power transmission device according to the invention,

Figure 5 shows, in a schematic view, a fourth preferred embodiment of the gas turbine power transmission device according to the invention,

Figure 6 shows, in a schematic view, a fifth preferred embodiment of the gas turbine power transmission device according to the invention, and

Figure 7 shows, in a schematic view, a sixth preferred embodiment of the gas turbine power transmission device according to the invention. EMBODIMENT(S) OF THE INVENTION

Figure 1 shows, in a schematic overview, an axial flow aircraft gas turbine engine 1 provided with a power transmission device 2, 13, 14 according to the invention. In general, the gas turbine engine 1 shown in figure 1 is of conventional construction and comprises, in axial flow series, an air intake 3, a low pressure compressor 4, a high pressure compressor 5, combustion equipment 6, a high pressure turbine 7, a low pressure turbine 8 and an exhaust outlet 9. During operation, the high pressure compressor 5 is driven by the high pressure turbine 7 via a first hollow shaft, the high pressure (HP) turbine shaft 10. Similarly, the low pressure compressor 4 is driven by the low pressure turbine 8 via a second hollow shaft, the low pressure (LP) turbine shaft 11 , which is coaxially disposed within the first turbine shaft 10.

The gas turbine engine 1 operates, in general, in a conventional manner whereby air drawn in through the air intake 3 is compressed by the low pressure compressor 4 before passing into the high pressure compressor 5 where it is further compressed. 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 outlet 9.

A power transmission device, which comprises an internal gearbox 2, a shaft arrangement 14 and a transfer gearbox 13, is arranged to transmit power from the first and second turbine shafts 10, 11 to a power output shaft 15 that is drivingly connected to an accessory gearbox 12 that in turn drives various auxiliary units (not shown).

Figure 2 shows, in a schematic view, a first preferred embodiment of the gas turbine power transmission device according to the invention. The internal gearbox 2 is positioned inside a fan housing (not shown) of the gas turbine engine 1. The internal gearbox 2 comprises a first bevel gear wheel 26 concentrically arranged onto the HP-shaft 10, a second bevel gear wheel 27 concentrically arranged onto the LP-shaft 11 , a third bevel gear wheel 28 arranged onto a first driving shaft 14a and a fourth bevel gear wheel 29 arranged onto a second driving shaft 14b. The first and second driving shafts 14a, 14b are coaxially arranged and form together the shaft arrangement 14 shown in figure 1.

The first bevel gear wheel 26 and the third bevel gear wheel 28 form a first bevel gear 21 for establishing a direct driving connection between the HP- shaft 10 and the first driving shaft 14a. Similarly, the second bevel gear wheel 27 and the fourth bevel gear wheel 29 form a second bevel gear 22 for establishing a direct driving connection between the LP-shaft 11 and the second driving shaft 14b.

The transfer gearbox 13, which receives the first and second driving shafts 14a, 14b, comprises a fifth bevel gear wheel 30 that, via a third bevel gear

23, is directly connected to a sixth bevel gear wheel 31 that is arranged onto the power output shaft 15 that drives the accessory gearbox 12. Moreover, a first clutch mechanism 24 is provided for connecting/disconnecting the first driving shaft 14a to/from the fifth bevel gear wheel 30 and a second clutch mechanism 25 is provided for connecting/disconnecting the second driving shaft 14b to/from the fifth bevel gearwheel 30.

By engaging the first clutch mechanism 24, the HP-shaft 10 and the power output shaft 15 become drivingly connected via the first bevel gear 21 , the first driving shaft 14a and the third bevel gear 23. Consequently, in this mode the auxiliary units are drivingly connected to the HP-shaft 10. In the following, this mode is occasionally denoted HP-mode. Similarly, by engaging the second clutch mechanism 25 the LP-shaft 11 and the power output shaft 15 become drivingly connected via the second bevel gear 22, the second driving shaft 14b and the third bevel gear 23. Consequently, in this mode the auxiliary units are drivingly connected to the LP-shaft 11. Accordingly, this is in the following occasionally denoted LP-mode.

The HP-mode is primarily used for starting the gas turbine engine 1 and during most of the time when flying the aircraft. Since the HP-mode is used most of the time it is an advantage if as few teeth as possible are engaged in the driveline in order to keep the efficiency at a high level.

The LP-mode is primarily used during idling and taxiing of the aircraft when the engine speed is low. By using the LP-shaft only in situations where the engine speed is low, it is possible to use only a portion of the large speed range cpLP_acc of the LP-shaft, which portion corresponds well to the suitable speed range φ_aCc of the auxiliary units.

The clutch mechanisms 24, 25 shown in figure 2 are of a type usually called dog clutch, which means that switching between HP-mode and LP-mode can normally not be carried out without interrupting the power transmission. Engaging/disengaging the clutch mechanisms 24, 25 has some principles in common with the procedure used for changing gear in an automatic step gear box of a road vehicle wherein the engine is controlled such as to achieve torque-free disengagement and synchronized engagement. However, in the present application the engine can not be controlled in the same way.

Normally, the group of auxiliary units includes a starter motor, which could be a separate unit or the same unit as the generator. To start the gas turbine engine 1 the first clutch mechanism 24 should be engaged, i.e. the power transmission device should be set in HP-mode, so that the starter motor can drive the HP-shaft 10. When the gas turbine engine 1 has started the starter motor is controlled such that the first clutch mechanism 24 becomes torque- free and thereby possible to disengage. The starter motor is further controlled such as to synchronize the rotation speed of the fifth bevel gear wheel 30 with that of the second driving shaft 14b as to allow engagement of the second clutch mechanism 25, i.e. the power transmission unit runs in the LP- mode.

Switching from LP-mode into HP-mode, for instance at take-off, is carried out in a reversed order compared to what is described above: using the starter motor for making the second clutch mechanism 25 torque-free, disengaging the second clutch mechanism 25, synchronizing the fifth bevel gear wheel 30 with the first driving shaft 14a and engaging the first clutch mechanism 24 allowing the HP-shaft 10 to drive the accessory gearbox 12.

Normally, it is an advantage if changing of gear, i.e. switching between HP- mode and LP-mode, can be made without interrupting the driving of the auxiliary units. This can be achieved by using slipping clutch mechanisms or a combination of slipping clutch and free-wheel mechanisms. In the field of gear boxes for road vehicles, such gear shifting is often denoted power-shift.

Figures 3 and 4 show embodiments of the invention that include such clutch arrangements. Except for the clutch mechanisms, the components of figures

3 and 4 correspond to what is shown in figure 2.

In figure 3, the first and second clutch mechanisms 124, 125 are of a slip friction type. Changing gear between HP-mode and LP-mode can with these clutch mechanisms 124, 125 be performed without power interruption in the power transmission device because the two clutch mechanisms 124, 125 interact during gear changing. The first clutch mechanism 124 does not lock completely until the second clutch mechanism 125 has been fully released, and vice versa. This way, a gradual transmission of the driving power from one clutch mechanism 124, 125 to the other, and thereby from one turbine shaft 10, 11 to the other, is achieved.

By choosing a suitable gearing for the first bevel gear 21 and the second bevel gear 22, it is possible to reduce the difference in rotational speed in the first and second clutch mechanisms 124, 125. This can be used to make the gear shifting faster and to decrease the need for synchronizing and/or slipping. This is valid for all embodiments in figures 1-7.

In figure 4, the first clutch mechanism 224 is of a slip friction type, whereas the second clutch mechanism 225 is of a free-wheel type that only transmits power in one direction, which direction is indicated by an arrow. Thus, the second driving shaft 14b is connected to the third bevel gear 23 via the freewheel mechanism 225. Power can be transmitted from the LP-shaft 11 , via the second driving shaft 14b, to the power output shaft 15, but power can not be transmitted in the opposite direction. In similarity with what is described above, the LP-shaft 11 drives, via the free-wheel mechanism 225, the auxiliary units during e.g. idling and taxiing. In this mode, the first slipping clutch mechanism 224 is completely disengaged so that no power is transmitted.

To shift into the HP-mode, the first clutch mechanism 224 is gradually tightened. This will gradually move the source of the power transmission from the LP-shaft 11 to the HP-shaft 10 until the rotational speed of the fifth bevel gear wheel 30 exceeds that of the second driving shaft 14b. At this stage, all transmittance of power occurs between the third bevel gear 23 and the first driving shaft 14a.

An advantage of this embodiment is that gear changing becomes simpler since only one clutch mechanism, i.e. the first clutch mechanism 224, needs to be controlled. A further advantage is that most of the operation time, i.e. when the HP-shaft 10 drives the accessory gearbox 12, no slip clutch needs to be in a slipping mode since a free-wheel mechanism is used instead. This reduces the wear.

To shift into the LP-mode, the first clutch mechanism 224 is gradually released.

Figures 5-7 show embodiments of the invention where the main difference compared to the embodiments shown in figures 2-4 is that the clutch mechanisms are located in the internal gearbox 2 positioned inside the fan housing (not shown) of the gas turbine engine 1 , instead of being located in the transfer gearbox 13. This way they become better protected from exposure to dirt and damage. Further, in some applications there is more space available inside the engine than outside. Moreover, in these embodiments only one driving shaft is needed between the internal gearbox 2 and the transfer gearbox 13. Thus, these embodiments allow the use of a conventional transfer gearbox 13.

Figure 5 shows, in a schematic view, a fourth preferred embodiment of the gas turbine power transmission device according to the invention. In similarity with the embodiments described above, the first and second bevel gear wheels 526, 527 are concentrically arranged onto the HP-shaft 10 and LP- shaft 11, respectively. In this case, however, the first clutch mechanism 524 is positioned in association with the first bevel gear wheel 526 for drivingly connecting/disconnecting the first bevel gear wheel 526 to/from the HP-shaft

10, whereas the second clutch mechanism 525 is positioned in association with the second bevel gear wheel 527 for drivingly connecting/disconnecting the second bevel gear wheel 527 to/from the LP-shaft 11. The third bevel gear wheel 528 and the fourth bevel gear wheel 529 are in this case arranged onto a common driving shaft 14.

The first bevel gear wheel 526 and the third bevel gear wheel 528 form the first bevel gear 21 for establishing a direct driving connection between the HP-shaft 10 and the common driving shaft 14. Similarly, the second bevel gear wheel 527 and the fourth bevel gear wheel 529 form the second bevel gear 22 for establishing a direct driving connection between the LP-shaft 11 and the common driving shaft 14.

The transfer gearbox 13, which receives the common driving shaft 14, comprises, in similarity with the embodiments described above, the fifth bevel gear wheel 530 that, via the third bevel gear 23, is directly connected to the sixth bevel gear wheel 31 that is arranged onto the power output shaft 15 that drives the accessory gearbox 12. In the embodiment shown in figure 5, there are, however, no clutch mechanisms arranged in the transfer gearbox 13. In this case the fifth bevel gear wheel 530 is arranged directly onto the common driving shaft 14.

By engaging the first clutch mechanism 524, the HP-shaft 10 and the power output shaft 15 become drivingly connected via the first bevel gear 21 , the common driving shaft 14 and the third bevel gear 23. Consequently, in this mode the auxiliary units are drivingly connected to the HP-shaft 10 (HP- mode). Similarly, by engaging the second clutch mechanism 525 the LP-shaft 11 and the power output shaft 15 become drivingly connected via the second bevel gear 22, the common driving shaft 14 and the third bevel gear 23. Consequently, in this mode the auxiliary units are drivingly connected to the LP-shaft 11 (LP-mode).

The clutch mechanisms 524, 525 shown in figure 5 are of the same type as shown in figure 2, i.e. a type usually called dog clutch. Engaging/disengaging the clutch mechanisms 524, 525 is performed in a similar manner as described in relation to figure 2. Also the starting procedure is similar.

In figure 6, the first and second clutch mechanisms 624, 625 are of a slip friction type, similar to what is shown in figure 3. Changing gear between HP- mode and LP-mode can with these clutch mechanisms 624, 625 be performed without power interruption in the power transmission device because the two clutch mechanisms 624, 625 interact during gear changing. The first clutch mechanism 624 does not lock completely until the second clutch mechanism 625 has been fully released, and vice versa. This way, a gradual transmission of the driving power from one clutch mechanism 624, 625 to the other, and thereby from one turbine shaft 10, 11 to the other, is achieved.

In figure 7, the first clutch mechanism 724 is of a slip friction type, whereas the second clutch mechanism 725 is of a free-wheel type that only transmits power in one direction, which direction is indicated by an arrow. This is principally similar to what is shown in figure 4. Thus, the second bevel gear wheel 527 can be connected to the LP-shaft 11 via the free-wheel mechanism 725. Power can be transmitted from the LP-shaft 11, via the common driving shaft 14, to the power output shaft 15, but power can not be transmitted in the opposite direction. In similarity with what is described above, the LP-shaft 11 drives, via the free-wheel mechanism 725, the auxiliary units during e.g. idling and taxiing. In this mode, the first slipping clutch mechanism 724 is completely disengaged so that no power is transmitted.

To shift into the HP-mode, the first clutch mechanism 724 is gradually tightened. This will gradually move the source of the power transmission from the LP-shaft 11 , that goes via the second bevel gear 22, to the HP-shaft 10, that goes via the first bevel gear 21 , until the rotational speed of the common driving shaft 14 becomes sufficient for rotating the fourth bevel gear wheel 529 faster than it is rotated by the second bevel gear wheel 527. At this stage, all transmittance of power occurs between the first bevel gear wheel 526 and the common driving shaft 14 (and further to the accessory gearbox 12).

Again, an advantage of this embodiment is that gear changing becomes simpler since only one clutch mechanism, i.e. the first clutch mechanism 724, needs to be controlled. A further advantage is that most of the operation time, i.e. when the HP-shaft 10 drives the accessory gearbox 12, no slip clutch needs to be in a slipping mode since a free-wheel mechanism is used instead. This reduces the wear.

The procedure for starting the gas turbine engine 1 is the same for all the embodiments shown in figures 2-7, i.e. the first clutch mechanism 24, 124, 224, 524, 624, 724 is fully engaged so that the starter motor, via the accessory gearbox 12, can drive the HP-shaft 10. The gas turbine engine 1 further comprises a control unit (not shown) for controlling the controllable clutch mechanisms. Such controlling is known as such in the field of vehicle transmission systems and can, using the information disclosed in this document, be modified to the application described here. The control unit also controls e.g. the starter motor, both with regard to starting and synchronizing.

To avoid accidental engagement of both clutch arrangements at the same time, the gas turbine engine 1 is preferably provided with position sensors, rotational speed sensors and/or pressure sensors connected to the control unit.

Main advantages of the invention can be summarized as follows:

- Reduced weight and cost since no or only a few additional components (generators, shafts, bevel gears etc.) for power transmission from the LP- shaft are required. Additionally, conventional shafts, transfer gearboxes and accessory gearboxes can be used.

- An increased amount of power can be transmitted to the auxiliary units during certain operation conditions because the invention allows the LP-shaft to be used as a power source instead of the HP-shaft.

- Lower weight and power losses compared to a system using a variator for adjusting the gear ratio.

- The number of additional components is low compared to a system transmitting power only from the HP-shaft.

With the expression "clutch mechanism" is meant in general terms a device that in operation drivingly connects and disconnects two rotating parts. A free-wheel clutch mechanism is sometimes called "unidirectional clutch" or "one-way clutch".

With expressions like "driving connection" and "drivingly connecting/disconnecting" is meant that a certain component mechanically can drive another component. This could be indirectly, e.g. via various gears and shafts, or directly.

At a certain fixed rotational speed ratio between the turbine shafts 10, 11 it is possible to let both the turbine shafts 10, 11 drive the power output shaft 15 simultaneously. This can be used when using a dog clutch, such as in the embodiments shown in figures 2 and 5, to avoid interrupting the power transmission. In other situations such simultaneous driving should normally be avoided.

The invention is not limited by the embodiments described above but can be modified in various ways within the scope of the claims. For instance, the power transmission device (2, 13, 14) may comprise further driving shafts fastened to each other or connected to each other via further gears.

The accessory gearbox 12 is not necessary for the invention but is of course very useful if more than one auxiliary unit is to be driven by the power output shaft 15.

If the clutch mechanisms are contained in the internal gearbox 2 it is possible to dispense with the transfer gearbox 13 and let the common driving shaft 14 form the power output shaft 15. However, the transfer gearbox 13 normally makes it possible to position the auxiliary units in a more compact manner.

In a gas turbine engine having an intermediate pressure (IP) shaft in addition to the HP- and LP-shafts, the invention can in principle be applied to either two of the shafts. Preferably, the invention is applied to at least the HP-shaft in such a gas turbine engine to simplify the start of the engine.

In certain applications it may be useful to further combine the embodiments such that a dog clutch mechanism (figures 2 and 5) is combined with a slip friction type (figures 3 and 4) or a free-wheel type (figures 4 and 7).

Claims

1. Gas turbine engine (1 ), comprising

- a first turbine shaft (10),

- a second turbine shaft (11), and - a power transmission device (2, 13, 14) for transmitting power from the first and second turbine shafts (10, 11) to a power output shaft (15) for driving auxiliary units, such as a generator and oil pumps, c h a r a c t e r i z e d i n that the power transmission device (2, 13, 14) comprises - a first clutch mechanism (24, 124, 224, 524, 624, 724) for drivingly connecting and disconnecting the first turbine shaft (10) to and from the power output shaft (15), and

- a second clutch mechanism (25, 125, 225, 525, 625, 725) for drivingly connecting and disconnecting the second turbine shaft (11 ) to and from the power output shaft (15).

2. Gas turbine engine (1) according to claim 1 , c h a r a c t e r i z e d i n that the power transmission device (2, 13, 14) comprises - a first bevel gear wheel (26, 526) concentrically arranged onto the first turbine shaft (10),

- a second bevel gear wheel (27, 527) concentrically arranged onto the second turbine shaft (11),

- a third bevel gear wheel (28, 528) arranged onto a shaft arrangement (14, 14a), and

- a fourth bevel gear wheel (29, 529) arranged onto the shaft arrangement (14, 14b), wherein the first and second bevel gear wheels (26, 526, 27, 527) are arranged to interact with the third and fourth bevel gear wheels (28, 528, 29, 529), respectively, such as to allow a driving connection between the first and second turbine shafts (10, 11 ) and the shaft arrangement (14, 14a, 14b), and wherein the shaft arrangement (14, 14a, 14b) is drivingly connected to the power output shaft (15).

3. Gas turbine engine (1) according to claim 2, characterized in that the shaft arrangement (14) comprises a first driving shaft (14a) provided with the third bevel gearwheel (28) and a second driving shaft (14b) provided with the fourth bevel gear wheel (29), wherein the first and second clutch mechanisms (24, 124, 224, 25, 125, 225) are arranged to drivingly connect and disconnect the first and second driving shafts (14a, 14b), respectively, to and from the power output shaft (15).

4. Gas turbine engine (1 ) according to claim 3, characterized in that the first and second driving shafts (14a, 14b) are coaxially arranged.

5. Gas turbine engine (1 ) according to claim 2, characterized in that the third and fourth bevel gear wheels (528, 529) are arranged onto a common driving shaft (14), wherein the first and second clutch mechanisms (524, 624, 724, 525, 625, 725) are arranged to drivingly connect and disconnect the first and second bevel gear wheels (526, 527) to and from the first and second turbine shafts (10, 11), respectively.

6. Gas turbine engine (1) according to claim 5, characterized in that the common driving shaft (14) is drivingly connected to the power output shaft (15).

7. Gas turbine engine (1 ) according to claim 5, characterized in that the common driving shaft (14) forms the power output shaft (15).

8. Gas turbine engine (1) according to anyone of the above claims, characterized in that the first turbine shaft (10) is connected to a first turbine (7) adapted to operate at a first pressure and that the second turbine shaft (11 ) is connected to a second turbine (8) adapted to operate at a second pressure, wherein the first pressure is higher than the second pressure.

9. Gas turbine engine (1) according to anyone of the above claims, characterized in that at least one of the first and second clutch mechanisms (24, 124, 224, 524, 624, 724, 25, 125, 225, 525, 625, 725) is of a dog clutch type.

10. Gas turbine engine (1) according to anyone of the above claims, characterized in that at least one of the first and second clutch mechanisms (24, 124, 224, 524, 624, 724, 25, 125, 225, 525, 625, 725) is of a slip friction type.

11. Gas turbine engine (1 ) according to anyone of the above claims, characterized in that at least one of the first and second clutch mechanisms (24, 124, 224, 524, 624, 724, 25, 125, 225, 525, 625, 725) is of a free-wheel type.

12. Gas turbine engine (1) according to claims 8 and 10, characterized in that the first clutch mechanism (124, 224, 624, 724) is of the slip friction type.

13. Gas turbine engine (1) according to claim 12, characterized in that the second clutch mechanism (125, 625) is of the slip friction type.

14. Gas turbine engine (1) according to claims 11 and 12, characterized in that the second clutch mechanism (225, 725) is of a free-wheel type arranged to transmit power in a direction from the second turbine shaft (11) to the power output shaft (15).

15. Gas turbine engine (1) according to anyone of the above claims, characterized in that the gas turbine engine (1) comprises a starter engine drivingly connected to the power output shaft (15), said starter engine being controllable such as to facilitate disengagement and engagement of the first and second clutch mechanisms.

16. Aircraft comprising a gas turbine engine (1) arranged for propulsion of the aircraft, characterized in that the gas turbine engine (1) is configured according to anyone of the above claims.