CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of United Kingdom Patent Application No. 1910008.0 filed on Jul. 12, 2019, the disclosure of which is herein incorporated by reference in its entirety.

TECHNICAL FIELD

The present disclosure relates to electric machines of gas turbine engines, and gas turbine engines comprising electrical machines.

BACKGROUND

Aircraft gas turbine engines provide both propulsive and electric power for aircraft. Consequently, such engines comprise electric machines such as generators for providing electric power. Further electric machines in the form of motors configured to drive fluid pumps such as oil and fuel pumps may also be provided. The function of the generators and motors can be combined, such that, for instance, a generator may operate as a starter motor during starting.

As electrical power demands increase, and electrical motors are increasingly used to replace hydraulic and pneumatic motors in aircraft, the size of electric machines on aircraft is steadily increasing relative to the size of the gas turbine engines. Consequently, packaging of such machines within the limited space available is becoming increasingly difficult.

A conventional packaging arrangement for an electric machine for a gas turbine engine is shown inFIG. 1. A gas turbine engine1comprises a main engine compressor shaft2, which rotates when the engine is in operation. This is coupled to an offtake shaft3via bevel gears4. The offtake shaft3is in turn coupled to an angle drive shaft5through a second set of bevel gears6. Further gears such as spur gears may also optionally be included in the drive train. An electric machine7is coupled to the angle drive shaft5. Such an arrangement is heavy, complex, and inefficient, in view of the gearing.

Alternative arrangements have also been proposed, including so-called “embedded generators”, in which an electric machine is installed within an annular space radially inward of a compressor gas path, coaxially with the compressor. Such an arrangement provides for efficient packaging, but presents other difficulties.

The present invention seeks to provide an improved gas turbine engine having an embedded electric machine.

SUMMARY

According to a first aspect there is provided an aircraft gas turbine engine comprising: a main engine shaft; a main engine shaft bearing arrangement configured to rotatably support the main engine shaft; and an electric machine comprising a rotor and a stator; wherein the rotor is mounted to the main engine shaft and is rotatably supported by the main engine shaft bearing arrangement, and the stator is mounted to static structure of the gas turbine engine.

Consequently, the machine rotor is supported by the same bearings as the main engine shaft. Consequently, additional electric machine rotor bearings are not required, thereby reducing cost, weight and space requirements for the electrical machine. The arrangement also provides for additional advantages, as outlined below and in the detailed description.

The stator of the electric machine may be cantilevered from the static structure of the gas turbine engine. Consequently, the electric machine stator is mounted at one end only, with the opposite end being free-floating. In general, an axial forward or rearward end may be mounted, with the other of the axial rearward or forward end being free-floating. Consequently, a strong but flexible structure is provided to mount the stator, which allows for relative movement of the core engine casing in use, while maintaining a small and consistent clearance between the electric machine rotor and stator. On the other hand, if the stator were to be mounted at both ends, flexing of the engine static structure would result in flexing of the stator or its casing, which would result in large loads being applied to the stator, and/or inconsistent clearance between the rotor and stator.

The electric machine stator may be mounted by a mount member extending generally rearward from a gas turbine engine static structure. Alternatively, the machine stator may be mounted by a mount member extending generally forward from a gas turbine engine static structure. Advantageously, the electric machine stator mounting can be accessed from one axial end. Consequently, the electric machine stator can be removed and installed from the gas turbine engine without requiring access to both ends.

The main engine shaft may comprise a radially inner portion and a radially outer portion separated by an annular space, and joined at one end. The electric machine rotor may be mounted to the radially outer portion of the main engine shaft. Advantageously, some degree of flexibility may be provided for between the outer and inner portions of the engine shaft, which may allow for relative movement between the two, thereby ensuring minimal loads on the electric machine rotor due to vibration for example, and a consistent gap between the rotor and stator. Such an arrangement may also isolate torsional vibration from the electric machine rotor from the main engine shaft. Additionally, relative thermal expansion between the rotor and the main engine shaft to which the rotor is mounted is allowed for. Finally, by separating the two portions, the electric machine rotor shaft diameter can be set separately to the main engine shaft diameter, allowing for independent optimisation of these two parameters.

The radially inner and radially outer portions may be joined by a fusible connector. Advantageously, in the event of a failure of the electric machine, connection between the electric machine rotor and main engine shaft can be severed, thereby preventing damage to the gas turbine engine, and removing the need for an engine shut-down.

Alternatively, the electric machine rotor may be directly mounted to an external surface of the main engine shaft.

The main shaft bearing arrangement may comprise a first bearing and a second bearing. The first bearing may comprise a radial load bearing and the second bearing may comprise a thrust bearing.

The first bearing may be provided axially rearward of the electric machine rotor, and the second bearing may be provided axially forward of the electric machine rotor. Advantageously, the electric machine rotor is held rearward and forward, to minimise whirl in use, in order to maintain a minimal rotor-stator air gap in use, and to prevent contact between the rotor and stator.

The bearing arrangement may comprise one or more roller bearings, and may comprise one or more ball bearings.

The first or second bearing may comprise a double-row bearing. The double-row bearing may comprise a back-to-back bearing, in which the contact angles of each of the bearings of the double row bearing diverge inwardly. Advantageously, the bearing can carry thrust loads in both a forward and a reverse direction without skidding of the bearings in the event that the load is reversed.

The electric machine may comprise an electric motor configured to provide motive power to start the gas turbine engine in a starting mode, and may comprise a generator configured to generate electrical power when in a running mode.

The electric machine may comprise a radial flux electric machine. The stator may be provided radially outward or radially inward of the rotor.

The electric machine may comprise one of a permanent magnet electric machine and a switched reluctance electric machine. Permanent magnet machines have a high power density, and so enable still further reductions in weight, and increased packaging efficiency.

The main gas turbine engine shaft may be arranged to couple a turbine to a compressor to form a spool. The spool may further comprise one or more compressor or turbine drive arms, and may comprise one or more compressor or turbine discs. The drive arms may be configured to couple one or more discs to the shaft.

The gas turbine engine may comprise a high pressure spool comprising a high pressure compressor coupled to a high pressure turbine by a high pressure shaft.

The gas turbine engine may comprise a low pressure spool comprising a low pressure compressor coupled to a low pressure turbine by a low pressure shaft.

The electric machine rotor may be coupled to one of the high pressure shaft and the low pressure shaft, and may be coupled to the low pressure shaft via a reduction gearbox.

The low and high pressure shafts may be coaxial, and the low pressure shaft may be provided radially inward of the high pressure shaft along at least part of its length.

The gas turbine engine may comprise a fan coupled to a main engine spool.

The low pressure compressor may be located axially forwardly of the high pressure compressor.

The electric machine may be located forward of the high pressure compressor, and may be located rearward of the low pressure compressor.

Alternatively, the electric machine may be located axially forward of the low pressure compressor, or in a common axial plane with the low pressure compressor.

The electric machine stator may comprise a plurality of part circumferential machine stator segments comprising respective windings, which may be individually mounted to static structure of the gas turbine engine. Consequently, the electric machine stator can be removed from within the gas turbine engine core, without disassembling the whole engine.

Arrangements of the present disclosure may be particularly, although not exclusively, beneficial for fans that are driven via a gearbox. Accordingly, the gas turbine engine may comprise a gearbox that receives an input from one or more main engine shaft and outputs drive to the fan so as to drive the fan at a lower rotational speed than the core shaft. The input to the gearbox may be directly from the core shaft, or indirectly from the core shaft, for example via a spur shaft and/or gear. The core shaft may rigidly connect the turbine and the compressor, such that the turbine and compressor rotate at the same speed (with the fan rotating at a lower speed). The electric machine may be located axially between the gearbox and the low pressure compressor. Alternatively, the electric machine rotor may be mounted to one of the rotating components of the gearbox.

The gas turbine engine as described and/or claimed herein may have any suitable general architecture. For example, the gas turbine engine may have any desired number of shafts that connect turbines and compressors, for example one, two or three shafts.

The gearbox may be arranged to be driven by the core shaft that is configured to rotate (for example in use) at the lowest rotational speed (for example the first core shaft in the example above). For example, the gearbox may be arranged to be driven only by the core shaft that is configured to rotate (for example in use) at the lowest rotational speed (for example only be the first core shaft, and not the second core shaft, in the example above). Alternatively, the gearbox may be arranged to be driven by any one or more shafts, for example the first and/or second shafts in the example above.

According to a second aspect of the invention, there is provided a method of assembling an embedded electric machine in a gas turbine engine, the method comprising: mounting a stator of the electric machine to a static structure of the gas turbine engine; then inserting a gas turbine engine main shaft comprising an electric machine rotor into an annular space defined by the electric machine stator; the gas turbine engine main shaft being supported by at least one main engine shaft bearing during insertion.

Advantageously, the electric machine can be assembled within the core of the gas turbine engine, whilst being supported by one or more main gas turbine engine bearings. The bearings can react forces generated by interaction of the machine rotor and stator during insertion, thereby allowing for more straightforward assembly and disassembly of the gas turbine engine, without requiring additional tooling for support of the machine rotor during installation.

The skilled person will appreciate that except where mutually exclusive, a feature or parameter described in relation to any one of the above aspects may be applied to any other aspect. Furthermore, except where mutually exclusive, any feature or parameter described herein may be applied to any aspect and/or combined with any other feature or parameter described herein.

