GB2613631A

GB2613631A - Aircraft electric propulsion

Info

Publication number: GB2613631A
Application number: GB2117910.6A
Authority: GB
Inventors: Ahmed Shirazee Nabeel; Singh Grewal Narotam
Original assignee: Epropelled Ltd
Current assignee: Epropelled Ltd
Priority date: 2021-12-10
Filing date: 2021-12-10
Publication date: 2023-06-14
Also published as: GB202117910D0

Abstract

An electric propulsion system for aircraft comprises a propulsion fan 26 and a plurality of electrical machines 2) connected at respective positions to an elongate driveshaft 17 which is coupled to the fan. At least one of the electrical machines is operable as a motor, and at least one is operable as a generator. In some embodiments, the machines are operable as both a motor and a generator. Preferably, the machines are mounted in a duct, where the fan lies at one end, and airflow passes down the duct.

Description

Aircraft electric propulsion This invention related to the electric propulsion of aircraft and more particularly but not solely to an electrically powered device for providing propulsion for aircraft, and associated systems and methods.

The global aviation industry is one of the contributors to global warming and accounts for 2.5% of CO2 emissions and 3.5% of effective radiative forcing, which is a closer measure of its impact on global warming, as per studies. Most aircraft have propulsion systems powered by kerosene as fuel, which is converted to CO2 when burned. Such aircraft propulsion systems are inefficient and noisy i.e., the sound pressure level of a jet propulsion engine at 30m is 150dB.

Several companies are working towards solutions to overcome the above-mentioned disadvantages including replacing aircraft having propulsion systems which are powered by fossil fuels with aircraft which have hybrid-electric or purely electric propulsion systems. Advantages of such hybrid-electric and electric propulsion systems is that they emit less greenhouse gases, are quieter, and more efficient.

EP3647184 discloses an electric propulsion system for aircraft, the system comprising at least one propeller and at least one ducted fan, wherein each propeller is mounted on a respective duct of the ducted fan outer perimeter and is adapted to be driven by a first respective electric motor, and at least one ducted fan is mounted within the duct and is adapted to be driven by a second respective electric motor. In use, the electric motors are controlled to vary the rotational speed of the ducted fan and propeller and the pitch of the blade angle. This offers optimized efficiency, reduction of noise and minimisation of combined torque of the system. A problem with the system of EP3647184 is scalability and it does not reap the full benefits of electric propulsion systems in terms of efficiency and specific power density to propel single aisle or larger commercial passenger aircraft.

US2021/0119499 discloses an electric propulsion system for aircraft, the system comprising one or more motors each having a stator assembly including a stator yoke having a hollow cylindrical shape with a length and a diameter, wherein the length is measured along a longitudinal thrust direction of the aircraft and is greater than the diameter, stator teeth integral with the stator yoke, wherein individual stator teeth extend from an inner surface of the stator yoke toward a centre line of the hollow cylindrical shape, stator windings attached to a set of the stator teeth, the stator windings configured to provide magnetic flux using electrical power; and a rotor assembly inside the hollow cylindrical stator yoke, wherein the rotor assembly and the stator yoke are separated by an airgap, a shaft carrying the rotor assembly coaxially with the stator, wherein an end portion of the shaft extends along the longitudinal thrust direction past a peripheral edge of the stator assembly; and a support assembly contacting the end portion of the shaft, wherein the support assembly is configured to allow the shaft to rotate in place and provide support for the shaft along a direction perpendicular to the longitudinal thrust direction and against gravitational forces. In use, each shaft is coupled to a propeller and an electric battery is coupled to the or each electric motors to provide power. A disadvantage of systems that use electric propeller driven fans that rotate in an open space is that they become energy inefficient when the rotational speed of the tips of the propeller approaches the speed of sound. In addition, rotation of the propeller at speeds faster than the speed of sound must be avoided because it can cause shockwaves powerful enough to shatter them. Also, noise generated by propellers increases to levels which may exceed 100db, as they approach approximately 0.9 mach (1 mach = speed of sound).

With the foregoing in mind, we have now devised improvements relating to the electrical propulsion of aircraft.

In accordance with the present invention, as seen from first aspect, there is provided a propulsion device for aircraft, the device comprising a propulsion fan and a plurality of electrical machines connected to a driveshaft which is coupled to the fan, at least one of the machines being operable as an electrical motor for propelling the aircraft and at least one of the machines being operable as an electrical generator to convert kinetic energy and/or potential energy of the aircraft into electrical energy.

The propulsion device of the present invention includes a plurality of electrical machines which replace the central core of a conventional jet engine, in a manner which allows the system to be relatively easily retrofitted to conventional aircraft in place of jet engines or incorporated into new aircraft.

During take-off and ascent of an aircraft, the or each electrical machine operable as an electrical motor is energised to provide thrust. When the aircraft starts to descend, the or each electrical machine operable as a generator can feed power back into any electrical machine being operated as an electrical motor and/or into the on-board power source. This utilisation of power from the generator to the motor for propulsion system substantially reduces power requirements from external sources.

The present invention can provide electric propulsion system for larger passenger aircraft with zero carbon emissions. The propulsion device is energy-efficient and can have a specific power density of at least 25 kW/kg with the efficiency of the electric propulsion device being up to 99%.

The device may utilise electrical machines capable of rotating up to 20,000 rpm..

At least one of the machines may be selectively operable as either a generator or as a motor. The electrical machines can be independently operated as propulsion motors and/or generators based on the operating conditions of the aircraft. During take-off and ascent of an aircraft, peak power will be required and therefore all the electrical machines may be used as a propulsion motor. When the aircraft reaches cruising altitude some of the propulsion motors may reduce their power as less overall power is required during cruise mode. Alternatively, some of the machines may operate as generators for onboard power generation.

The machines may be mounted at respective positions disposed along the axis of the driveshaft.

The machines may be mounted in a duct which extends axially of the driveshaft, the propulsion fan being disposed at one end of the duct and adapted such, in use, that a proportion of the airflow through the fan is directed along the duct. The electrical machines are thus positioned such that a portion of the air from the propulsion fan is routed through the electrical machines to cool the machines and their drive control circuits and other electronics efficiently. The bulk of the volume of air passing the propulsion fan is used to generate thrust. The propulsion device and the electronics can also be liquid cooled.

A control device for each electrical machine may be mounted within the duct.

At least one arm may extend radially outwardly from the duct remote from said one end for supporting the electrical machines within the device.

One or more cables and/or ducts extend along an arm which extends between the electrical machines and a support structure. The arm may extend through an airflow passage of the device so that it acts as a heatsink for removing heat from the machines into an airflow generated by the propulsion fan.

Said one end of the duct may comprise a plurality of radially outwardly extending support members which are connected at their outer ends to a tubular cowl which surrounds the propulsion fan.

Said one end of the duct may be flared outwardly.

Also in accordance with the present invention, as seen from a second aspect, there is provided a propulsion system for aircraft, the system comprising at least one propulsion device as hereinbefore defined, a control unit for varying the speed of rotation of the at least one electric motor and for controlling the degree of electrical and/or mechanical energy output by the electrical machines of the device.

A power supply may be provided for energising the at least one electric motor and for storing energy generated by the at least one electrical generator.

Also in accordance with the present invention, as seen from a third aspect, there is provided an aircraft comprising a propulsion system as hereinbefore defined.

The method may comprise selectively controlling the at least one electric motor of the propulsion device to rotate the propulsion fan and propel the aircraft, selectively controlling the at least one generator of the propulsion device to convert kinetic energy and/or potential energy of the aircraft into electrical energy when the altitude of the aircraft is reduced and/or when the speed of the aircraft is reduced.

The electrical machines may be selectively controlled to operate as an electrical generator or as an electrical motor according to the selected operating conditions of the aircraft.

The electrical energy generated by the at least one electrical generator may be applied to the at least one electrical motor and/or to said power supply.

Embodiments of the present invention will now be described by way of example, is only with reference to the accompanying drawings, in which: Figure 1 is a perspective view of a passenger aircraft incorporating an electrical propulsion system in accordance with the present invention, with some parts being 10 shown cut away; Figure 2 is a longitudinal sectional view of a propulsion unit of the system of Figure 1; Figure 3 is a perspective view of internal components of the propulsion unit of Figure 2, with some parts being shown cut away; and Figures 4a to 4c are perspective cut away views of various embodiments of drive motors of the propulsion unit of Figure 2.

Referring to Figure 1 of the drawings, an aircraft 10 comprises a fuselage 11, a pair of oppositely directed wings 12, each carrying at least one propulsion unit 13, although it will be appreciated that propulsion unit can be mounted anywhere on the aircraft, such as on its tail, although wing-mounted units are preferred as they require less cabling. Each propulsion unit 13 comprises a housing or so-called nacelle 14 that is configured to encircle and contain a propulsion device 15 which provides the propulsive force to move the aircraft 10 forward in a forward direction F, which may also be referred to as the fore direction. A supporting pylon 16 is configured to securely mount the nacelle 14 and the propulsion device 15 to the wing 12 of the aircraft 10.

Referring to Figures 2 and 3 of the drawings, the nacelle 14 is generally tubular and comprises a central through axis which extends substantially in the forward direction F of the aircraft 10. The propulsion device 15 is mounted inside the nacelle 14, such that an elongate driveshaft 17 thereof extends along the central through axis of the nacelle 14. The fore end of the propulsion device 15 comprises a tubular cowl 18 which is releasably secured to the pylon 16. A plurality of fins 19 extend radially inwardly from the cowl 18 to a centrally mounted neck 20 in the form of a truncated cone having fore and aft open ends, the fore end of the neck 20 having a diameter which is greater than the diameter of its aft end. A gearbox 21 is supported inside the neck 20 by struts 27 which extend radially inwardly from the neck 20. The fore end of the elongate shaft 17 extends rearwardly from the gearbox 21 towards the aft end of the propulsion device 15. A fan 26 is rotatably mounted inside the nacelle 14 at the fore end thereof, the fan 26 having a central boss which coupled to the gearbox 21.

The propulsion device 15 further comprises a plurality of electrical machines 22 which are arranged to convert electrical energy into mechanical energy and vice versa. In the embodiment shown, there are 8 electrical machines 22 which may or may not be of identical construction, the machines 22 being mounted side-by-side at respective positions along the axis of the elongate shaft 17. Each electrical machine 22 comprises a rotor which is fixed to the shaft 17 and a stator which is fixed to an elongate tubular body 23 which extends rearwardly from the aft end of the neck 20. A plurality of fins 24 extend radially outwardly from the aft end of the body 23, the outer end of the fins 24 being secured to pylon 16 via a metal brace (not shown) or the fins 24 are secured to a main outer duct of the engine which extends rearwardly from the front cowl 18, the duct then being bolted to the pylon 16 in the same manner as a conventional jet engine. 20 Each electrical machine 22 also comprises its own electronic solid-state drive circuit at its radially outer portion, although a shared solid-state drive circuit may alternatively be used. A control circuit (not shown) is arranged to selectively operate the electrical machines 22 as a propulsion motor, a generator or combination of according to the operating conditions of the aircraft 10. The neck 20 at the fore end of the body 23 acts as an air collector behind the fan 26, so that in use a high airflow is channelled along the elongate tubular body 23 in which the electrical machines 22 are mounted act, so as to cool the electrical machines 22 and their drive circuits 25. It will be appreciated that each electrical machine 22 may have its own control unit as necessary.

The fins 24 are positioned in the main airflow behind the fan 26 and act to dissipate heat from the electrical machines 22 into the airflow generated by the propulsion device 15. The fins 24 may comprise means such as heat pipes in order to improve heat extraction from the electrical machines 22. Further, the fins 19 and 24 are profiled to offer efficient airflow from the main fan to the exit openings at the aft of the propulsion unit 13. It is envisaged that most propulsion units will only require air-cooling for the propulsion system. However, larger propulsion units may require forced liquid cooling. All necessary cabling including power cables and control signal cables and any liquid cooling channels may pass through the rear cooling fins 24.

In use, the tubular nacelle 14 and the cowl 28 defined by the internal wall of the tubular nacelle 14 streamlines air flowing through the fan 26, which allows the tips of the blades of the fan 26 to reach supersonic speeds without structural damage. The airflow between the cowl 28 and the elongate tubular body 23 is called ultra-high bypass air B, which produces all of the thrust for the aircraft 10. For example, where the requirement of the propulsion system is to produce 10 MW of power for a thrust equivalent to approximately 120 kN, two of these electric propulsion devices, as shown in FIG.1 would be able to power a short to medium haul commercial aircraft of the kind having seating for approximately 130 to 140 passengers.

In use, a central control unit 33 controls the solid-state drive circuits 25 of the electrical machines 22 to cause them to start rotating in synchronisation with each other. The machines 22 may be powered up with varying speed and torque profiles to achieve maximum efficiency. For example, the machines 22 may be controlled to operate anywhere in the range of 1 rpm to 100,000 rpm or more.

The electrical machines 22 are coupled to the gearbox 21, which may have a ratio of 20:1 for example, such that the fan 26 coupled to the gearbox 21 rotates at a speed which is 20 times less than that of the electrical machines 22. The high gear ratio allows the size of the electrical machine 22 to be small and the specific power density to be high e.g. 25 kW/Kg. The solid-state drive circuits 25 are operated with as high voltage as possible e.g. 3,000 volts. The high voltage allows manageable currents in the machine windings of the electrical machines 22. A problem with using high voltage is that the level of the dielectric breakdown voltage in the windings decreases dramatically at high altitudes such as 40,000 feet, which means that distances between all supply conductors, must be increased or the conductors wrapped in a material to increase the dielectric strength to avoid electric short-circuits. This applies to all electrical conductors, such as those of the electric machines 22, the solid-state drive circuits 25 and of all other electronics onboard the aircraft 10.

During incidents such as bird strikes or hailstorms, any debris or other material passing through the fan 26 and into the elongate tubular body 23 will not have a detrimental impact on the electrical machines 22 due to the fact that the air gap between the rotor and stator of each machine 22 is large (typically 3mm) and the windings of the machines 22 are protected, for example using a special potting compound, which does not fracture/break on debris impact. Any debris or other material will pass straight through the machines 22 because they have an open structure and do not comprise any endcaps. This arrangement also allows air to flow freely through the electrical machines 22 in a direction which extends axially of the driveshaft 17.

The present system is more reliable than a traditional jet propulsion system. Hence the life of the ultra-high bypass electric propulsion system is comparatively high. The windings of the electrical machines 22 can be concentric, distributed or any other type to reduce the active material usage. The high voltage windings of the electrical machines are insulated to withstand 10 of 1000s of voltage to prevent short-circuit.

The weight of the windings of the electrical machines 22 can be reduced by having an aluminium inner lining through a copper tube. A cooling fluid may be passed through the centre of the tube to assist with cooling the electrical machines 22. Additionally, the heat generated by the electrical machines 22 can be transferred via the tubes into the aircraft for heating purposes. The heated fluid from the electrical machines 22 can also be fed to the fuselage 11 and wings 12 of the aircraft to prevent ice formation.

The preferred kind of electrical machines 22 can be permanent magnet machines including but not limited to an inner rotor, outer rotor, embedded, IPM machines, axial flux machines, hybrid switch reluctance machines with or without hard magnetic materials. The solid-state drive circuits 25 can be provided in two independent chambers for redundancy purposes.

Referring to Figure 4a of the drawings, in one embodiment each electrical machine 22 can be a permanent magnet inner rotor machine with a wound annular stator 29a surrounding a rotor 30a embedded with magnets. The solid-state drive circuits 25 are positioned radially outwardly of the stator 29a. The driveshaft 17 extends through the centre of the rotor 30a and a plurality of circumferentially-spaced air channels 31a extend axially through the rotor 30a so as to allow cooling air from the neck 20 to flow through the rotor. Also, as mentioned previously, cooling air from the neck 20 can flow through the large air gap between the rotor 30a and stator 29a of each machine 22.

Referring to Figure 4b of the drawings, there is shown an alternative embodiment of electrical machine 22 and like parts to those of Figure 4a are given like reference numerals with the suffix b. In this embodiment, the electrical machine 22 is a permanent magnet outer rotor machine, in which the rotor 30b is annular and surrounds only on one side of the inner wound stator 29b. The inner wound stator 29b is connected to the inner surface of the elongated tubular body 23. The solid-state drive circuits 25b are attached to the inner surface of the elongated tubular body 23 so as to clear the rotating rotor 30b with a substantial airgap of 10mm or more.

Referring to Figure 4c of the drawings, there is shown another embodiment of electrical machine 22 and like parts to those of Figure 4a are given like reference numerals with the suffix c. In this embodiment, the electrical machine 22 is an axial flux machine, in which the stator 29c and rotor 30c are disposed axially of each other on the shaft 17.

The start-up sequence of a propulsion system in accordance with the present invention is very simple compared with that of a traditional jet propulsion system. In the present invention, all of the electrical machines 22 are instructed to power up via the central control unit 33 disposed in the cockpit of the aircraft 10, to provide torque to rotate the fan 26 of the or each electric propulsion unit 13. The rotational speed of the fan 26 is controlled via an engine throttle lever (not shown) disposed in the cockpit of the aircraft 10, which sends signal to the central control unit 33, to control the thrust applied by the or each electric propulsion unit 13 to the aircraft 10.

During take-off, all of the electrical machines 22 act as propulsion motors. During cruising, the power of the propulsion motors 22 is reduced to match the required cruising thrust, whereupon some may also act as generators. During descent, some of the propulsion motors 22 act as generators and feed the generated power into the remaining propulsion motors 22 which act to maintain a suitable airspeed to keep the aircraft in the air. In this manner, the amount of power drawn from the on-board power supply is substantially reduced. The on-board power supply may be battery in the form of fuel cells. During steeper descent, conventional aircraft use airbrakes to control the speed and slow the aircraft down. In order to harness this lost energy, more of the propulsion motors 22 act as generators during steep descents and landings, saving additional kinetic and/or potential energy. In this manner, the kinetic and/or potential energy of the aircraft 10 is converted into electrical energy and very little or no energy is lost. The generated energy is fed back to the power supply for storage and subsequent use. Excessive power can also be fed into supercapacitors, which can then later be used as a power source.

In view of the foregoing mode of operation, it will be appreciated that a propulsion system in accordance with the present invention enables a substantial amount of kinetic and/or potential energy to be harnessed, thereby substantially reducing the carbon emissions of aircraft and helping to save the ozone layer and our planet.

The present invention thus provides an electric propulsion device for aircraft, the device comprising a propulsion fan and a plurality of electrical machines connected at respective positions to an elongate driveshaft which is coupled to the fan. Each electrical machine is selectively operable as either an electrical motor for propelling the aircraft or as a generator for converting kinetic and/or potential energy of the aircraft into electrical energy. All of the electrical machines can operate as a propulsion motor during take-off and ascent to meet the power demands of a typical commercial aircraft.

During descent of the aircraft, some of the electrical machines operate as a generator to generate electrical energy which is either used to assist with propulsion or fed to a storage device. In this manner, the electric propulsion device has an extremely high efficiency. Furthermore, the device is simpler in construction than a conventional jet engine and is thus far more reliable with substantially increased duty cycles, meaning less maintenance and downtime of the device. The device is also substantially lighter than conventional jet engines thereby further enhancing the efficiency of aircraft fitted with such devices.

Claims

CLAIMS1 A propulsion device for aircraft, the device comprising a propulsion fan and a plurality of electrical machines connected to a driveshaft which is coupled to the fan, at least one of the machines being operable as an electrical motor for propelling the aircraft and at least one of the machines being operable as an electrical generator to convert kinetic and/or potential energy of the aircraft into electrical energy.
2. A propulsion device as claimed in claim 1, in which at least one of the machines is selectively operable as a generator or as a motor.
3. A propulsion device as claimed in claim 1 or claim 2, in which the machines are mounted at respective positions disposed along the axis of the driveshaft.
4 A propulsion device as claimed in claim 3, in which the machines are mounted in a duct which extends axially of the driveshaft, the propulsion fan being disposed at one end of the duct and adapted such, in use, that a proportion of the airflow through the fan is directed along the duct.
5. A propulsion device as claimed in claim 4, in which a control device for each electrical machine is mounted within the duct.
6 A propulsion device as claimed in claim 3 or claim 4, in which at least one arm extends radially outwardly from the duct remote from said one end for supporting the electrical machines within the device.
7 A propulsion device as claimed in claim 6, in which one or more cables and/or ducts extend along an arm between the electrical machines and a support structure.
8 A propulsion device as claimed in any of claims 3 to 7, in which said one end of the duct comprises a plurality of radially outwardly extending support members which are connected at their outer ends to a brace and/or a pylon.
9. A propulsion device as claimed in any of claims 3 to 8, in which said one end of the duct is flared outwardly.
A propulsion system for aircraft, the system comprising at least one propulsion device as claimed in any preceding claim, a control unit for varying the speed of rotation of the at least one electric motor and for controlling the degree of electrical and/or mechanical energy output by the electrical machines of the device.
11. A propulsion system as claimed in claim 10, comprising a power supply for energising the at least one electric motor and for storing energy generated by the at least one electrical generator.
12. An aircraft comprising a propulsion system as claimed in claim 10 or claim 11.
13 A method of operating the aircraft of claim 12, the method comprising selectively controlling the at least one electric motor of the propulsion device to rotate the propulsion fan and propel the aircraft, selectively controlling the at least one generator of the propulsion device to convert kinetic and/or potential energy of the aircraft into electrical energy when the altitude of the aircraft is reduced and/or when the speed of the aircraft is reduced.
14. A method as claimed in claim 13, in which the electrical machines are selectively controlled to operate as an electrical generator or as an electrical motor according to the selected operating conditions of the aircraft.
15. A method as claimed in claim 13 or claim 14, in which the electrical energy generated by the at least one electrical generator is applied to the at least one electrical motor and/or to said power supply.
16. A propulsion device as claimed in claim 4, in which the machines are open and comprise openings which extend axially of the duct.