Patent Publication Number: US-2021179282-A1

Title: Aircraft hybrid propulsion system

Description:
The present disclosure concerns a parallel hybrid propulsion system for an aircraft and an aircraft comprising the propulsion system. 
     Parallel hybrid aircraft have been proposed, in which an internal combustion engine is combined with one or more electric motors to drive one or more propulsors. Parallel hybrid systems can be distinguished from so-called “serial hybrid” systems, in that in a parallel hybrid system, a mechanical connection is provided between the internal combustion engine and at least one propulsor, with at least one electric motor driving either the same propulsor as that driven by the internal combustion engine, or a further propulsor. 
     According to a first aspect there is provided an aircraft hybrid propulsion system comprising; 
     an internal combustion engine comprising a main drive shaft;
 
an electric machine comprising an electric machine rotor;
 
a propulsor mounted to a propulsor shaft; and
 
a clutch arrangement configured to selectively couple each of the gas turbine engine main drive shaft and electric machine rotor to the propulsor drive shaft;
 
wherein
 
the electric machine rotor is mounted coaxially with the main drive shaft; and
 
the clutch arrangement comprises a first overrunning clutch configured to couple the main drive shaft to the propulsor drive shaft, and a second overrunning clutch configured to couple the electric machine rotor to the propulsor drive shaft.
 
     Advantageously, a single propulsor can be driven by either or both of the internal combustion engine and the electric motor. Furthermore, in the event of a failure of either the electric motor or internal combustion engine, the propulsor can continue to be driven by the other input, in view of the clutch arrangement. 
     Each of the main shaft, propulsor shaft and machine rotor shaft may be coaxial with one another. 
     The main shaft may be provided radially inward of the electric machine rotor. Alternatively, the main shaft may be provided radially outward of the electric machine rotor. The propeller drive shaft may be located between the electric machine rotor and main shaft. 
     The propulsor shaft may be coupled to a radially outer race of the first overrunning clutch, and may be coupled to a radially inner race of the second overrunning clutch. Alternatively, the propulsor shaft may be coupled to a radially inner race of the first overrunning clutch, and may be coupled to a radially outer race of the second overrunning clutch. 
     The electric motor may comprise one of a permanent magnet motor and an induction motor. The inventors have found that the present invention is suitable for a wide variety of motor types, but it particularly suitable for permanent magnet motors. In the event of a permanent magnet motor failure, it is important that the electric motor does not continue to turn. By providing an overrunning clutch, the propulsor can continue to turn, while preventing the motor from turning. 
     The first overrunning clutch may be configured to couple the main engine shaft to the propulsor drive shaft when torque is applied by the main engine shaft to the propulsor drive shaft in a first direction relative to the propulsor drive shaft, and decouple the main engine shaft from the propulsor drive shaft when torque is applied in a second direction. The second overrunning clutch may be configured to couple the electric machine rotor to the propulsor drive shaft when torque is applied by the electric machine to the propulsor drive shaft in the first direction relative to the propulsor drive shaft, and decouple the electric machine from the propulsor drive shaft when torque is applied in the second direction. 
     The propulsor drive shaft may be coupled to the propulsor via a reduction gearbox. Advantageously, a relatively fast turning internal combustion engine and/or electric motor can be used in combination with a relatively slow turning propulsor. Such an arrangement provides for relatively efficient, compact electric motors and internal combustion engines, as well as an efficient propulsor. 
     The reduction gearbox may comprise an epicyclic gearbox. 
     The aircraft hybrid propulsion system may comprise one or more of an electric energy storage device and a generator configured to provide electrical power to the electric motor. The generator may be coupled to the internal combustion engine. 
     The internal combustion engine may comprise a gas turbine engine. The gas turbine engine may comprise a compressor coupled to a first turbine, and may comprise a second turbine. The second turbine may be de-coupled from an engine compressor. The first and/or second turbine may be coupled to one or both of the generator and the combining gearbox. 
     According to a second aspect there is provided an aircraft comprising the propulsion system of the first aspect. 
     The skilled person will appreciate that except where mutually exclusive, a feature described in relation to any one of the above aspects may be applied mutatis mutandis to any other aspect. Furthermore except where mutually exclusive any feature described herein may be applied to any aspect and/or combined with any other feature described herein. 
    
    
     
       An embodiments will now be described by way of example only, with reference to the Figures, in which: 
         FIG. 1  is a plan view of an aircraft comprising a parallel hybrid propulsion system; 
         FIG. 2  is a schematic diagram of a parallel hybrid propulsion system for the aircraft of  FIG. 1 ; 
         FIG. 3  is a schematic front view of part of a clutch arrangement of the hybrid propulsion system of  FIG. 2 ; and 
         FIG. 4  is a schematic side view of the clutch arrangement of  FIG. 3 . 
     
    
    
     With reference to  FIG. 1 , an aircraft  1  is shown. The aircraft  1  is of conventional configuration, having a fuselage  2 , wings  3 , tail  4  and a pair of propulsion systems  5 . One of the propulsion systems  5  is shown in detail in  FIG. 2 . 
       FIG. 2  shows the propulsion system  5  schematically. The propulsion system  5  includes an internal combustion engine in the form of a gas turbine engine  10 . The gas turbine engine  10  comprises, in axial flow series, a compressor  14 , combustion equipment  16  and high and low-pressure turbines  18 ,  20 . 
     The gas turbine engine  10  works in the conventional manner so that air flows through the compressor  14  where it is compressed, before delivering that air to the combustion equipment  16 , where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through, and thereby drive the turbines  18 ,  20  before being exhausted through a nozzle. The high  18  and low-pressure turbines  20  drive respectively the compressor  14  and a propulsor  12  in the form of a propeller or fan, each by suitable interconnecting main engine shaft  22 ,  24 , which rotate about an axis of rotation X. The low-pressure shaft  24  is coupled to the propulsor  12  via a clutch arrangement  50  and an optional reduction gearbox  36 . The gearbox  36  could be of any suitable type, such as an epicyclic gearbox. 
     Other gas turbine engines to which the present disclosure may be applied may have alternative configurations. By way of example such engines may have an alternative number of interconnecting shafts (e.g. three) and/or an alternative number of compressors and/or turbines. Further, the engine may comprise a gearbox provided in the drive train from a turbine to a compressor and/or fan. 
     The propulsion system  5  further comprises one or more electrical machines driving the propulsor  12 . In particular, the system  5  comprises an electric motor  28 . The motor  28  is of a conventional type, such as an induction or permanent magnet electric machine, and is configured to drive a propulsor such as the fan  12 . In the present embodiment, the motor  28  comprises a permanent magnet AC motor, and is coupled to the fan  12  via a motor shaft  56 , clutch arrangement  50 , and reduction gearbox  36 , which will be described in further detail later. 
     The electric motor  28  is shown in further detail in  FIG. 4 . The motor  28  is of conventional construction, comprising a stator  70  comprising a plurality of stator coils (not shown), which are energised in use to produce a rotating magnetic field. This rotating magnetic field crosses an air gap to link with a magnetic field produced by permanent magnets of a rotor  78 . Consequently, the motor  28  acts as a motor when the stators  72  are energised. On the other hand, where the rotor  78  is rotated, the rotor permanent magnets produce a rotating magnetic field, which links with the stator windings  72  to produce an electric current, and so may act as a generator under some circumstances. 
     The electric motor  28  is positioned such that the stator  70  and rotor  78  surround a main engine shaft, which in this case comprises the low-pressure shaft  24 . The motor  28  is of an “in-runner” type, in which the rotor  78  is provided radially inward of the stator  70 . The rotor  78  is coupled to the low-pressure shaft  24  via the clutch arrangement  50 , and is configured with an axis of rotation which is co-axial with the axis of rotation X of the low-pressure turbine  24 . 
     The electric motor  28  is coupled to an energy storage device  30  in the form of one or more of a chemical battery, fuel cell, and capacitor, which provides the electric motor  28  with electrical power during operation. In some cases, multiple energy storages systems, which may be of different types (chemical battery, fuel cell etc) may be provided for each propulsion system  5 . In other cases, a common energy storage device  30  may be provided for multiple propulsion systems. 
     The propulsion system further comprises a generator  32 , which is electrically coupled to one or both of the motor  28  and the energy storage device  30 , such that additional electrical energy can be provided in operation. The generator  32  is typically driven by the low-pressure shaft  24  of the gas turbine engine  10 . 
     A controller  34  is provided, which is configured to control at least the motor  28  and energy storage device  30 , to control the torque provided by the motor  28 , and the charging/discharging of the energy storage device  30 . The controller  34  may also be configured to control operation of the generator  32 , to control electrical power produced by the generator  32 . 
     As briefly mentioned, the gas turbine engine  10  and electric motor  28  are each coupled to the propulsor  12  via a clutch arrangement  50  comprising respective first and second overrunning clutches  52 ,  54  (also known as “freewheel” clutches). 
       FIG. 3  shows the clutch arrangement  50  in further detail. The clutch arrangement  50  comprises a first clutch in the form of a first overrunning clutch  52 . The first clutch  52  comprises an inner race  58 , which is coupled to the low-pressure shaft  24 . An outer race  60  is provided, which is coupled to a fan shaft  62 , which is in turn coupled to the fan  12  via the reduction gearbox  36 . 
     The outer race  60  of the first overrunning clutch  52  also forms an inner race of the second overrunning clutch  54 . The second overrunning clutch  54  further comprises an outer race  64 , which is coupled to the motor shaft  56  and is provided radially outward of the outer race  60  of the first clutch  52 . 
     Consequently, the propulsor  12  is coupled to both the main engine shaft  24  and electric machine  28  via the first and second overrunning clutches  52 ,  54  respectively. Each of the shafts  24 ,  56 ,  62  is co-axial, and the arrangement is radially nested, with the main engine shaft  24  being coupled to a radially innermost race  58 , the outer race  60  of the first clutch  54  being provided radially outward and coupled to the propulsor shaft  62 , and the outer race  64  of the second clutch  54  being provided at the radially outermost position, and coupled to the motor shaft  56 . It will be understood that the shafts  24 ,  56  could instead be reversed, with the shaft  56  being nested inside the shaft  24 . 
     The overrunning clutches  50 ,  52  are typically in the form of freewheel clutches, although other overrunning clutch types are known. The freewheel clutches  52 ,  54  comprises an input comprising the low-pressure shaft  24  of the gas turbine engine  10  and the motor shaft  56  respectively. An output shaft in the form of the propulsor shaft  62  is provided, which is coupled to the radially outer race of the first clutch  52 , which forms the radially inner race of the second clutch  54 . Between the respective races  58 ,  60  and  60 ,  64 , are two sets of rollers  66 , which engage against respective outer races  60 ,  64  when they are relatively rotated in one direction, and disengage when rotated in an opposite direction. 
     The first clutch  52  is arranged such that the races  58 ,  60 , and so the shafts  24 ,  62  are locked together to transfer torque when torque is applied by the shaft  24  to the inner race  58  in a first direction, and when the speed of the input shaft  24  is equal to the speed of the output shaft  62 . On the other hand, shafts  62 ,  24  rotate freely relative to one another, such that no torque is transmitted back to the input shaft  24 , when the shaft  62  rotates at a higher speed in the first direction than the input shaft  24 , or where the input shaft  24  rotates in a direction opposite to the first direction. Consequently, torque is transferred from the input shaft  24  to the shaft  62  only, and not the other way around. Similarly, the second clutch  54  is arranged such that the shafts  56 ,  62  are locked together to transfer torque when torque is applied by the shaft  56  to the outer race  60  in the first direction, and when the speed of the input shaft  56  is equal to the speed of the output shaft  62 . On the other hand, shafts  56 ,  62  rotate freely relative to one another, such that no torque is transmitted back to the input shaft  56 , when the shaft  62  rotates at a higher speed in the first direction than the input shaft  24 , or where the input shaft  24  rotates in a direction opposite to the first direction. Consequently, the fan  12  may be powered by either or both of the gas turbine engine  10  via the low-pressure shaft, and the motor  28 . 
     Since the shaft  56  is coupled to the outer race  64  of the second freewheel clutch  54 , and the shaft  24  is coupled to the inner race  58  of the first freewheel clutch  52 , the freewheel clutches  52 ,  54  are arranged “backwards” relative to one another. Consequently, rotation of the shaft  24  at a higher speed than the shaft  56  will causes the shafts  24 ,  62  to lock and rotate together, and also cause the shafts  56 ,  62  to unlock, to rotate independently of one another. Conversely, rotation of the shaft  56  at a higher speed than the shaft  24  will causes the shafts  56 ,  62  to lock and rotate together, and also cause the shafts  24 ,  62  to unlock, to rotate independently of one another 
     As a consequence of the above arrangement, the shaft  62  will be driven by the shaft  24 ,  56  having the highest rotational speed relative to the other shaft  56 ,  24 . Consequently, one shaft  56 ,  24  can be driven to provide for rotation of the output shaft  62 , without causing rotation of the other shaft  24 ,  56 . 
     For example, in a first operational mode, the gas turbine engine  10  is used to drive the propulsor  12  alone. In this mode, the windings  72  of the motor  28  are not energised with electrical power from either the generator  32  or the batteries  30 . In this case, the first freewheel clutch  52  engages in view of the torque input from the low-pressure shaft  24 . Consequently, rotation of the low pressure shaft  24  drives the propulsor  12  via the reduction gearbox  36 . However, in this mode, the second clutch  54  disengages, in view of the rotation of the inner race relative to the shaft  56 , and the non-rotation of the motor shaft  62 . Consequently, the electric machine  28  is not driven. This is particularly advantageous where the electric motor  28  comprises a permanent magnet motor, since rotation of the rotor  78  would normally generate electrical current. This may not be desirable, as this will reduce the amount of power available to drive the propulsor  12 , and will also generate additional heat. This mode of operation is also useful where the motor  28  has suffered a fault. Consequently, the motor  28  does not have to be of a “fault tolerant” type, which may reduce cost and weight. 
     In a second operational mode, the motor  28  is used to drive the propulsor  12  alone. In this mode, the gas turbine engine  10  is shut-down. In this case, the second freewheel clutch  54  engages in view of the torque input from the motor  28 , and the load imposed by the propulsor  12 . Consequently, rotation of the motor  28  drives the propulsor  12  via the reduction gearbox  36 . However, in this mode, the first clutch  52  disengages, in view of the rotation of the shaft  62 , and the non-rotation of the low pressure shaft  24 . Consequently, the gas turbine engine  10  is not driven. This mode of operation allows the gas turbine engine  10  to be shut-down in flight. Since gas turbine engines are only efficient when operated at high power, this may reduce overall fuel usage. Furthermore, the propulsor  12  can continue to be driven by the electric motor  28  in the event of a failure of the gas turbine engine  10 , even where the gas turbine engine is not able to rotate due to the failure. Again, this increases safety, and improves operational flexibility. 
     In a third operational mode, the gas turbine engine  10  and the motor are used to drive the propulsor  12  together. In this mode, the windings  72  of the motor  28  are energised with electrical power from either the generator  32  or the batteries  30 , and the gas turbine engine  10  is also operated to generate torque. In this case, both clutches  52 ,  54  engage in view of the torque input from the low pressure shaft  24  and the motor  28 , which causes shafts  24 ,  56  to rotate at the same speed. Consequently, rotation of the low pressure shaft  24  and the motor  28  drive the propulsor  12  via the reduction gearbox  36 . In this mode, load is shared between the gas turbine engine  10  and motor  28 . This mode is particularly advantageous for operation at high power levels, such as take-off, where increased power is required. 
     Consequently, the present arrangement describes a lightweight, reliable aircraft propulsion system, which is flexible, efficient, and tolerant of failures. The system allows for provision of an electric machine that is co-axial with the main gas turbine engine shaft, while providing for automatic connection and disconnection of the gas turbine engine and electric machine under various circumstances. Such a system is compact. No additional actuation is required, which makes the system lightweight and reliable. 
     Except where mutually exclusive, any of the features may be employed separately or in combination with any other features and the disclosure extends to and includes all combinations and sub-combinations of one or more features described herein. 
     It will be understood that various modifications could be made to the system, without departing from the scope of the invention, as laid out in the claims. 
     For instance, the gas turbine engine could take a different form. For instance, the gas turbine engine could comprise additional compressors, which may be driven by one or more of the shafts, or by additional, independently rotatable shafts. Different forms of overrunning clutches could be used. The reduction gearbox could be omitted, as could the generator, with the electric machine being supplied with electric power from an energy storage device alone. 
     Different electric machine types could be employed, such as induction motors. Similarly, the electric machine could take a different physical form, being an “out runner”, in which the electric machine rotor is provided radially outward of the stator, or the electric machine could comprise an axial flux machine.