Patent Description:
Some propulsion systems can be described as "hybrid propulsion systems" for deriving motive power, such as thrust, from one or more electrical power sources. For example, a hybrid aircraft may include a hybrid propulsion system that derives power (e.g., for producing thrust) from an electric generator and/or motor driven by a gas turbine engine. Some hybrid propulsion systems may use electrical machines, such as electric starter motors, to assist the gas turbine engine and generators to produce electricity. A hybrid propulsion system according to the prior art is disclosed in <CIT>.

In general, the disclosure is directed to a hybrid propulsion system including a motor-generator in place of a typical accessory gearbox of a two-stage turbine gas turbine engine. The motor-generator may be configured to provide shaft power to a first drive shaft of the first stage turbine via a second drive shaft of second stage turbine during start-up and derive electrical power from shaft work of the second drive shaft during normal operation. The hybrid propulsion system includes a clutch configured to operatively couple the second drive shaft to the first drive shaft during start-up and decouple the first drive shaft from the second drive shaft during normal operation.

According to claim <NUM>, the present disclosure is directed to a hybrid propulsion system including a plurality of propulsors, a first drive shaft, a second drive shaft, a gas turbine engine, a motor-generator, and a clutch. The gas turbine engine includes a first turbine stage operatively coupled to the first drive shaft and a second turbine stage operatively coupled to the second drive shaft. The motor-generator is operatively coupled to the second drive shaft and configured to generate electrical power to drive at least one propulsor of the plurality of propulsors and selectively drive the second shaft. The clutch is configured to operatively couple the second drive shaft to the first drive shaft.

Using the motor-generator, rather than a dedicated electric starter motor (or other starter system) and a dedicated generator, may reduce accessory drive mechanisms and, thereby, reduce the number of mechanical systems, reduce weight of the hybrid propulsion system, or both compared to other propulsion systems.

In some examples, the clutch may include an overrunning sprag clutch.

The overrunning sprag clutch may be relatively smaller and have relatively less weight than other clutch systems.

In some examples, the clutch may be configured to automatically couple the first drive shaft to the second drive shaft in response to a first rotational rate of the first drive shaft and automatically decouple the first drive shaft from the second drive shaft at a second rotational rate of the first drive shaft.

By automatically coupling the second drive shaft to the first drive shaft at the first rotational rate, the hybrid propulsion system may not require additional control systems to enable an operator to manually couple the second drive shaft to the first drive shaft.

Further, by automatically decoupling the first drive shaft from the second drive shaft at the second rotational rate, the hybrid propulsion system may not require additional control systems to detect when a RPM of the first drive shaft should begin to exceed a RPM of the second drive shaft, to enable an operator to manually decouple the first drive shaft from the second drive shaft, or both.

In some examples, the second rotational rate may be greater than the first rotational rate.

In some examples, the first rotational rate may include a plurality of first rotational rates defining a startup operation of the gas turbine engine.

In some examples, the second rotational rate may include a plurality of second rotational rates defining a steady-state operation of the gas turbine engine.

In some examples, the gas turbine engine may further include a compressor. The first shaft may be operatively coupled to the compressor.

In some examples, the motor-generator may be configured to generate between <NUM> kilowatts (kW) and about <NUM> kW of electrical power.

In some examples, the hybrid propulsion system may not include an accessory gearbox.

In some examples, the motor-generator may be directly mechanically coupled to the second drive shaft.

According to claim <NUM>, the present disclosure is directed to a method of starting a hybrid propulsion system including a plurality of propulsors, a gas turbine engine, a motor-generator, and a clutch. Each respective propulsor of the plurality of propulsors may include a respective electrical motor mechanically coupled to a respective propeller. The gas turbine engine includes a first turbine stage operatively coupled to a first drive shaft and a second turbine stage operatively coupled to a second drive shaft. The motor-generator may be directly mechanically coupled to the second drive shaft. The motor-generator is configured to generate electrical power to drive at least one motor of the plurality of propulsors during a steady-state operation of the gas turbine engine and selectively drive the second shaft during a start-up operation of the gas turbine engine. The clutch is coupled to the second drive shaft and configured to automatically engage the first drive shaft during the start-up operation and automatically disengage the first drive shaft during the steady-state operation.

In some examples, the present disclosure is directed to method of operating a hybrid propulsion system. The method may include generating, by a motor-generator, shaft work to drive a second drive shaft. The second drive shaft may be operatively coupled to a gas turbine engine. The gas turbine engine may include a first turbine stage operatively coupled to a first drive shaft; and a second turbine stage operatively coupled to the second drive shaft. The method also may include engaging, by a clutch operatively coupled to the second drive shaft, the first drive shaft in response to a first rotational rate of the first drive shaft. The method also may include disengaging the clutch from the first drive shaft in response to a second rotational rate of the first drive shaft.

By automatically engaging the second drive shaft to the first drive shaft at the first rotational rate, the hybrid propulsion system may not require additional control systems to enable an operator to manually couple the second drive shaft to the first drive shaft.

Further, by automatically disengaging the first drive shaft from the second drive shaft at the second rotational rate, the hybrid propulsion system may not require additional control systems to detect when a RPM of the first drive shaft should begin to exceed a RPM of the second drive shaft, to enable an operator to manually decouple the first drive shaft from the second drive shaft, or both.

In some examples, the hybrid propulsion system may further include a plurality of propulsors. The method may further include, after disengaging the clutch, driving, by the second drive shaft, the motor-generator to generate electrical power to drive at least one propulsor of the plurality of propulsors.

In some examples, engaging the first drive shaft may include automatically engaging, by the clutch, the first drive shaft in response to the first rotational rate of the first drive shaft.

In some examples, disengaging the first drive shaft may include automatically disengaging the first drive shaft in response to the second rotational rate of the first drive shaft.

The present disclosure is directed to hybrid propulsion systems and techniques that include a plurality of propulsors, a first drive shaft, a second drive shaft, a gas turbine engine, a motor-generator, and a clutch. The gas turbine engine may include a first turbine stage operatively coupled to the first drive shaft and a second turbine stage operatively coupled to the second drive shaft. The motor-generator may be operatively coupled to the second drive shaft. By operatively coupling to the second drive shaft, the motor-generator may be configured to selectively drive the second shaft and generate electrical power to drive at least one propulsor of the plurality of propulsors. The clutch may be configured to operatively couple the second drive shaft to the first drive shaft.

The first turbine stage (N1) may include a gas producer turbine configured to drive a compressor of the gas turbine engine and, in some examples, self-sustaining systems of the hybrid propulsion system, such as oil pumps and/or fuel pumps. The second turbine stage (N2) may include a power turbine configured to drive the plurality of propulsors and, in some examples, accessories of the vehicle. In some examples, N2 may drive the motor-generator, which produces electricity to power the plurality of propulsors.

The gas turbine engine may include various operational modes, such as start-up, steady-state operation, and shutdown. During start-up, shaft work may be necessary to begin rotation of the compressor to deliver air to the combustion chamber of the gas turbine engine to prevent a hot start. In some examples, shaft work may be delivered from the motor-generator to N2. N2 may be operatively coupled via a clutch to N1 to begin rotation of the compressor. The shaft work may be used to increase the rotational speed, e.g., revolutions per minute (RPM), of N1. At a predetermined RPM of N1 (e.g., ignition RPM), fuel may be delivered to and ignited in a combustion chamber of the gas turbine engine. The ignition of fuel may result in expansion of gases in the combustion chamber sufficient to drive N1 and N2 to achieve self-sustaining combustion of steady-state operation. Before ignition, e.g., during start-up, N1 and N2 may rotate at the same or nearly the same RPM. After ignition, e.g., during steady-state operation, N1 may rotate faster than N2. N1 and N2 may be configured to operate at different rotational speed to drive different operational system of a vehicle. In some examples, N1 may rotate at up to about <NUM>,<NUM> RPM and N2 may rotate at up to about <NUM>,<NUM> RPM.

In some examples, the motor-generator may be positioned within an existing casing of a gas turbine engine. For example, the motor-generator may be configured to replace the power and accessory gearbox (the "accessory gearbox") of a turboshaft gas turbine engine, such as a gas turbine engine configured for use in a rotary-wing vehicle. Using a generator-motor, rather than a dedicated electric starter motor (or other starter system) and a dedicated generator, may reduce accessory drive mechanisms and, thereby, reduce the number of mechanical systems, reduce weight of the hybrid propulsion system, or both compared to other propulsion systems.

However, while the motor-generator may be operatively coupled to N2 via direct mechanical coupling to the second drive shaft, e.g., to generate electrical power from shaft work of N2, the motor-generator may not be directly coupled to N1 or the first drive shaft. For example, the additional gearing and/or additional accessories to enable the motor-generator to couple to both N1 and N2 may be complex and/or lessen the benefits of using a generator-motor, rather than a dedicated electric starter motor and a dedicated generator. Thus, the hybrid propulsion system may include a clutch configured to operatively couple the second drive shaft to the first drive shaft.

For example, during start-up, the motor-generator (coupled to N2 via the second drive shaft) may produce shaft work to drive the second shaft. The clutch may operatively couple the second drive shaft to the first drive shaft. In some examples, the clutch may include an overrunning clutch. For example, the clutch may include an overrunning sprag clutch. An overrunning sprag clutch may be relatively smaller and have relatively less weight than other clutch systems. The clutch may be coupled to the second drive shaft and configured to engage the first drive shaft only when the first and second drive shafts are rotating at the same speed. For example, when second drive shaft is being driven by the motor-generator during start-up, e.g., at a first rotational speed between substantially zero RPM to the ignition RPM, the clutch may automatically couple the second drive shaft to the first drive shaft. By automatically coupling the second drive shaft to the first drive shaft at a first rotational speed, the hybrid propulsion system may not require additional control systems to enable an operator to manually couple the second drive shaft to the first drive shaft.

The clutch may be configured to disengage from the first drive shaft when the first driven shaft rotates faster than the second drive shaft. For example, when the expansion of hot combustion gases causes N1 to drive (and accelerate) the rotation of the first drive shaft after ignition, e.g., at a second rotational speed greater than the ignition RPM, the clutch may automatically decouple the first drive shaft from the second drive shaft. Because N2 remains operatively coupled to the motor-generator, the motor-generator may be configured to generate electrical power to drive at least one propulsor of the plurality of propulsors during steady-state operation of the gas turbine engine. By automatically decoupling the first drive shaft from the second drive shaft at a second rotational speed, the hybrid propulsion system may not require additional control systems to detect when the RPM of the first drive shaft should begin to exceed the RPM of the second drive shaft, to enable an operator to manually decouple the first drive shaft from the second drive shaft, or both. In this way, the clutch may engage the first drive shaft and the second drive shaft during start-up and disengage the first drive shaft and the second drive shaft during steady-state operation.

<FIG> is a conceptual diagram illustrating an example system <NUM> (also referred to herein as "vehicle <NUM>" although non-vehicle examples of system <NUM>, such as power stations, and the like, are also possible). In some examples, vehicle <NUM> includes an aircraft. In other examples, vehicle <NUM> may include any type of gas turbine engine-powered vehicle, including one or more types of air vehicles; land vehicles, including but not limited to, tracked and/or wheeled vehicles; marine vehicles, including but not limited to surface vessels, submarines, and/or semi-submersibles; amphibious vehicles; or any combination of one or more types of air, land, and marine vehicles. Vehicle <NUM> may be manned, semiautonomous, or autonomous.

In examples in which vehicle <NUM> includes an aircraft, vehicle <NUM> may include a fuselage <NUM>, wings <NUM>, an empennage <NUM>, and multiple hybrid propulsion systems 18A and 18B (collectively, "hybrid propulsion systems <NUM>"). In other examples, vehicle <NUM> may include a single hybrid propulsion system <NUM> or more than two hybrid propulsion systems <NUM>.

In some examples, hybrid propulsion systems <NUM> may include one or more gas turbine engines 20A and 20B (collectively, "gas turbine engines <NUM>"), one or more motor-generators 22A and 22B (collectively, "motor-generators <NUM>"), and one or more propulsors 24A and 24B (collectively, "propulsors <NUM>"). In some examples, gas turbine engines <NUM> may be configured to generate shaft work that is used by motor-generators <NUM> to produce electricity to power propulsors <NUM>.

Vehicle <NUM> may include one or more gas turbine auxiliary power units (APUs). An APU may include, for example, a secondary gas turbine engine, a piston engine, a hybrid engine, or the like. Propulsors <NUM> may include fans, rotary wings, propellers, or the like. Although illustrated in <FIG> as collocated in respective wings <NUM> of vehicle <NUM>, in some examples, any one or more of gas turbine engines <NUM>, motor-generators <NUM>, and/or propulsors <NUM> may be located in other portions of vehicle <NUM>. For example, gas turbine engines <NUM> and/or motor-generators may be located in fuselage <NUM> and propulsors <NUM> may be located at wings <NUM> and/or empennage <NUM>.

In some examples, vehicle <NUM> may be any fixed-wing aircraft. Vehicle <NUM> may employ any number of wings <NUM>. Empennage <NUM> may employ a single or multiple flight control surfaces. In some examples, vehicle <NUM> may be a rotary-wing aircraft or a combination rotary-wing/ fixed-wing aircraft. In some examples, hybrid propulsion systems <NUM> may include a distributed propulsion system. For example, propulsors <NUM> may be distributed on any suitable surface of vehicle <NUM>, such as on any suitable portion of fuselage <NUM>, wings <NUM>, and empennage <NUM>. In some examples, propulsors <NUM> of a distributed propulsion system may be configured to ingest air at a boundary layer of vehicle <NUM>. The boundary layer may include the region of turbulent (or laminar) flow as vehicle <NUM> moves through a fluid (e.g., air or water). In some examples, gas turbine engine <NUM> may include a two-stage turbine gas turbine engine modified to include a motor-generator.

<FIG> is a conceptual diagram illustrating an example hybrid propulsion system <NUM> including a two-stage turbine gas turbine engine <NUM> ("engine <NUM>"). In some examples, engine <NUM> may include a turbo shaft gas turbine engine. Engine <NUM> may include first turbine stage <NUM> and second turbine stage <NUM>. In some examples, each of first turbine stage <NUM> and second turbine stage <NUM> may include one or more turbine wheels, such as, for example, two turbine wheels. In this way, each of first turbine stage <NUM> and second turbine stage <NUM> may include multiple turbine stages. First turbine stage <NUM> may be operatively coupled to first drive shaft <NUM>. For example, first turbine stage <NUM> may be directly mechanically coupled to first drive shaft <NUM> or mechanically coupled to first drive shaft <NUM> via one or more intermediate mechanical couplings, such as a collar or a mechanical gearing. In this way, first turbine stage <NUM> may rotate at the same RPM as first drive shaft <NUM> (or a ratio thereof). Similarly, second turbine stage <NUM> may be operatively coupled to second drive shaft <NUM>. In some examples, first drive shaft <NUM> may be coaxially positioned within a hollow shaft of second drive shaft <NUM> along a common axis. In other examples, second drive shaft <NUM> may be coaxially positioned within a hollow shaft of first drive shaft <NUM>. First turbine stage <NUM> and second turbine stage <NUM> include a plurality of turbine blades and a plurality of stator blades. In some examples, the plurality of turbine blades and the plurality of stator blades of each of the first turbine stage <NUM> and second turbine stage <NUM> are configured to rotate first drive shaft <NUM> and second drive shaft <NUM> in the same direction.

First drive shaft <NUM> may be operatively coupled to compressor <NUM> via a spur adapter gear shaft <NUM>. Compressor <NUM> may include one or more rotors configured to ingest and compress air to deliver to the combustion system fluidly coupled to first turbine <NUM> and second turbine <NUM> (e.g., such that fluid may flow from compressor <NUM> to the combustion system to first turbine <NUM> and second turbine <NUM>). In some examples, the combustion system may include a combustion liner (not shown) that encloses a continuous combustion process. In other examples, combustion system may include other combustion processes, such as, for example, a wave rotor combustion system, a rotary valve combustion system, a pulse detonation combustion system, or a slinger combustion system, and may employ deflagration and/or detonation combustion processes. The plurality of turbine blades and the plurality of stator blades of each of the first turbine stage <NUM> and second turbine stage <NUM> are configured to derive shaft work from a flow of combustion gas from the combustion system. In this way, first drive shaft <NUM> may be configured to rotate compressor <NUM> at the same RPM as first turbine stage <NUM>.

Second drive shaft <NUM> may be operatively coupled to generator drive shaft <NUM> via second turbine stage gear <NUM> and generator draft shaft gear <NUM>. Generator drive shaft <NUM> may be operatively coupled to generators <NUM>.

Generator <NUM> may be configured to generate electrical power from generator drive shaft <NUM>. Generator <NUM> may supply electrical power via electrical system <NUM> to plurality of propulsors 234A, 234B, and 234N (collectively, propulsors <NUM>). Electrical system <NUM> may include suitable electrical transmission lines, converters, inverters, power sources (e.g., batteries), or the like configured to transmit and/or store electrical energy such that generator <NUM> may power propulsors <NUM>.

Although shown as including "n" propulsors, in other examples, propulsors <NUM> to fewer propulsors, such as one or two propulsor. Each propulsor of propulsors <NUM> may be mechanically coupled to a respective motor of plurality of motors 236A, 236B, and 236N (collectively, motors <NUM>) configured to convert electrical power produced by generator <NUM> into mechanical power, e.g., shaft work. The shaft work produced by motors <NUM> may be delivered to respective propellers of plurality of propellers 238A, 238B, and 238N (collectively propellers <NUM>). In some examples, propellers <NUM> may include other configurations, such as fans or rotary-wings.

In some examples, spur adapter gear shaft <NUM> may be coupled to a first turbine stage bearing <NUM>. First turbine stage bearing <NUM> may be configured to stabilize an axial position of spur adapter gear shaft <NUM> relative to second drive shaft <NUM>. In some examples, first drive shaft, e.g., via spur adapter gear shaft <NUM>, may be operatively coupled by first turbine stage gear <NUM> to accessory gear box <NUM>. Accessory gear box <NUM> may be positioned axial near a center of engine <NUM>. In some examples, accessory gearbox <NUM> may be operatively coupled to a starter (not shown), such an electric starter motor, an air-starter turbine, an auxiliary power unit, or the like. In this way, accessory gearbox <NUM> may be configured to rotate first drive shaft <NUM> during startup of engine <NUM>. In some examples, a gas turbine engine may include a motor-generator in place of an accessory gearbox.

<FIG> is a conceptual diagram illustrating an example hybrid propulsion system <NUM> including a two-stage turbine gas turbine engine <NUM> ("engine <NUM>") that includes a motor-generator <NUM>. Engine <NUM> may be the same as or substantially similar to engine <NUM> except for the differences described herein. For example, engine <NUM> may include first turbine stage <NUM> and second turbine stage <NUM>. First turbine stage <NUM> may be operatively coupled to first drive shaft <NUM>. First drive shaft <NUM> may be operatively coupled to compressor <NUM> via a spur adapter gear shaft <NUM>. Second turbine stage <NUM> may be operatively coupled to second drive shaft <NUM>. Spur adapter gear shaft <NUM> may be coupled to a first turbine stage bearing <NUM>, which may be configured to stabilize an axial position of spur adapter gear shaft <NUM> relative to second drive shaft <NUM>.

Rather than an accessory gearbox (e.g., accessory gearbox <NUM>), engine <NUM> includes motor-generator <NUM>. Motor-generator <NUM> may be operatively coupled to second drive shaft <NUM>. For example, motor-generator <NUM> may be directly mechanically coupled to second drive shaft <NUM> or mechanically coupled to second drive shaft <NUM> via one or more intermediate mechanical couplings, such as a collar or a mechanical gearing. Motor-generator <NUM> may be configured to be positioned in the space of accessory gearbox <NUM>, e.g., near a center of engine <NUM>, which may limit the size of motor-generator, the type of mechanical coupling to operatively couple motor-generator <NUM> to second drive shaft <NUM>, or both. In some examples, motor-generator <NUM> is configured to convert mechanical power to electrical power and to convert electrical power to mechanical power. For example, motor-generator <NUM> may be configured to derive electrical power from shaft work of second drive shaft <NUM>, e.g., during steady-state operation, and deliver shaft power to second drive shaft <NUM>, e.g., during start-up. In some examples, motor-generator may be configured to generate between <NUM> kilowatts (kW) and about <NUM> kW of electrical power.

Motor-generator <NUM> may supply electrical power via electrical system <NUM> to plurality of propulsors 334A, 334B, and 334N (collectively, propulsors <NUM>). Electrical system <NUM> may include suitable electrical transmission lines, converters, inverters, power sources (e.g., batteries), or the like configured to transmit and/or store electrical energy such that motor-generator <NUM> may power propulsors <NUM>. Although shown as including "n" propulsors, in other examples, motor-generator <NUM> may supply electrical power to fewer propulsors, such as one or two propulsor. Each propulsor of propulsors <NUM> may be mechanically coupled to a respective motor of plurality of motors 336A, 336B, and 336N (collectively, motors <NUM>) configured to convert electrical power produced by motor-generator <NUM> into mechanical power, e.g., shaft work. The shaft work produced by motors <NUM> may be delivered to respective propellers of plurality of propellers 338A, 338B, and 338N (collectively propellers <NUM>). In some examples, propellers <NUM> may include other configurations, such as fans or rotary-wings.

In some examples, motor-generator <NUM> may be configured to power other electrical systems of a vehicle (e.g., vehicle <NUM>). The other electrical systems may include, for example, electrical power distribution systems, power conversion systems, power electronics, digital electronics, and environmental control systems. For example, motor-generator <NUM> may be electrically coupled to, and configured to store electrical energy in, a power source, such as one or more batteries.

Additionally, engine <NUM> includes clutch <NUM> configured to operatively couple second drive shaft <NUM> to first drive shaft <NUM>. In some examples, clutch <NUM> may include an overrunning clutch. For example, clutch <NUM> may include an overrunning sprag clutch. In examples in which clutch <NUM> includes an overrunning sprag clutch, clutch <NUM> may be relatively smaller and relatively less weight than other clutch systems. Clutch <NUM> is coupled to second drive shaft <NUM>. Clutch <NUM> may be positioned adjacent to first turbine stage bearing <NUM> to enable lubricant and/or cooling fluid to be delivered to clutch <NUM>, to improve rotor dynamics of engine <NUM>, or both. Clutch <NUM> may be configured to engage first drive shaft <NUM> when first drive shaft <NUM> and second drive shaft <NUM> are rotating at the same speed. For example, when second drive shaft <NUM> is being driven by motor-generator <NUM> during start-up, e.g., at a first rotational speed, clutch <NUM> may automatically couple second drive shaft <NUM> to first drive shaft <NUM>. The first rotational speed may include any RPM between substantially zero RPM, such as when second drive shaft <NUM> first begins to rotate, and the ignition RPM. In examples in which clutch <NUM> includes an overrunning sprag clutch, clutch <NUM> may include a plurality of sprags, each sprag urged by a respective spring member of a plurality spring member against first drive shaft <NUM>. The sprags may be oriented such that the plurality of sprags engage first drive shaft <NUM> when second drive shaft <NUM> is rotated in a first rotational direction. By automatically coupling the second drive shaft to the first drive shaft at a first rotational speed, the hybrid propulsion system may not require additional control systems to enable an operator to manually couple the second drive shaft to the first drive shaft.

Clutch <NUM> may be configured to disengage from first drive shaft <NUM> when the first driven shaft rotates faster than second drive shaft <NUM>. For example, the plurality of sprags may not engage first drive shaft <NUM> when second drive shaft <NUM> rotates in a second rotational direction, opposite the first rotational direction, or rotates more slowly than first drive shaft <NUM>. In some examples, when the expansion of hot combustion gases causes first turbine <NUM> to drive (and accelerate) the rotation of first drive shaft <NUM> after ignition, e.g., at a second rotational speed greater than the ignition RPM, clutch <NUM> may automatically decouple first drive shaft <NUM> from second drive shaft <NUM>. Because N2 remains operatively coupled to motor-generator <NUM>, motor-generator <NUM> may be configured to generate electrical power to drive at least one propulsor of propulsors <NUM> during steady-state operation of engine <NUM>. By automatically decoupling first drive shaft <NUM> from second drive shaft <NUM> to at a first rotational speed, hybrid propulsion system <NUM> may not require additional control systems to detect when the RPM of first drive shaft <NUM> exceeds the RPM of second drive shaft <NUM>, to enable an operator to manually decouple first drive shaft <NUM> from second drive shaft <NUM>, or both. In this way, clutch <NUM> may engage first drive shaft <NUM> with second drive shaft <NUM> during start-up and disengage first drive shaft <NUM> from second drive shaft <NUM> during steady-state operation.

<FIG> is a plot illustrating start-up of an example hybrid propulsion system <NUM> including mechanical decoupling of first drive shaft <NUM> from second drive shaft <NUM> using clutch <NUM>. As illustrated in <FIG>, during start-up, when motor-generator <NUM> provides shaft work to second drive shaft <NUM> which is coupled to first drive shaft <NUM> by clutch <NUM>, first drive shaft <NUM> (e.g., N1) and second drive shaft <NUM> (e.g., N2) may rotate at the same or nearly the same RPM from an initial RPM <NUM> to an ignition RPM <NUM>, as indicated by line <NUM>. In some examples, initial RPM <NUM> may include an RPM of about zero RPM. In some examples, the ignition RPM <NUM> may include an RPM sufficient to enable compressor <NUM> to deliver enough air to the combustion chamber of engine <NUM> to prevent a hot start. At or near ignition RPM <NUM>, as indicated by vertical line <NUM>, clutch <NUM> may disengage second drive shaft <NUM> from first drive shaft <NUM>. This may allow the RPM of first turbine stage <NUM> to increase to a steady-state operational RPM, as indicated by line <NUM>. As indicated by line <NUM>, the RPM of second turbine stage <NUM> may also increase to a slower steady-state operational RPM, albeit at a slower rate compared to first turbine stage <NUM>. The hybrid propulsion systems describe herein may be used to startup a gas turbine engine using any suitable technique.

<FIG> is a flow diagram illustrating an example operations performed to start-up an example hybrid propulsion system using a starter motor-generator. The operations illustrated in <FIG> are described with reference to hybrid propulsion systems <NUM>, although a person of ordinary skill in the art will appreciate that similar operations may be used to start-up a gas turbine engine of a vehicle system, such as system <NUM>, and that hybrid propulsion system <NUM> may be started using different operations.

In operation, motor-generator <NUM> may generate shaft work to drive second drive shaft <NUM> (<NUM>). As discussed above, second drive shaft <NUM> is operatively coupled to gas turbine engine <NUM>. For example, gas turbine engine includes first turbine stage <NUM> operatively coupled to first drive shaft <NUM> and second turbine stage <NUM> operatively coupled to second drive shaft <NUM>. In some examples, motor-generator <NUM> may be directly mechanically coupled to second drive shaft <NUM>. For example, motor-generator <NUM> may be configured to replace an accessory gearbox of gas turbine engine <NUM> to be positioned to directly coupled to second drive shaft <NUM>.

Clutch <NUM> may engage first drive shaft <NUM> in response to a first rotational rate of first drive shaft <NUM> (<NUM>). For example, the first rotational rate of first drive shaft <NUM> may include a plurality of first rotational rates defining a startup operation of gas turbine engine <NUM>, as discussed above. In some examples, engaging first drive shaft <NUM> may include automatically engaging, by clutch <NUM>, first drive shaft <NUM> in response to the first rotational rate of first drive shaft <NUM>. As discussed above, clutch <NUM> may include an overrunning clutch, such as an overrunning sprag clutch. In this way, the technique may include automatically coupling motor-generator <NUM> to first drive shaft <NUM> via second drive shaft <NUM> and clutch <NUM> to rotate first turbine stage <NUM> and compressor <NUM>.

Clutch <NUM> may disengage from first drive shaft <NUM> in response to a second rotational rate of first drive shaft <NUM> (<NUM>). For example, the second rotational rate may include a plurality of second rotational rates defining a steady-state operation of gas turbine engine <NUM>, as discussed above. In some examples, disengaging first drive shaft <NUM> comprises automatically disengaging first drive shaft <NUM> in response to the second rotational rate of first drive shaft <NUM>.

In some examples, the technique may include, before disengaging clutch <NUM>, burning a fuel in gas turbine engine <NUM>, e.g., in a combustion chamber of gas turbine engine <NUM>, to drive rotation of first turbine stage <NUM>. For example, as discussed above, at an ignition RPM, fuel may be delivered to and ignited in the combustion chamber of gas turbine engine <NUM>. Combustion gases from the combustion chamber may drive the rotation of first turbine stage <NUM> and second turbine stage <NUM> to facilitate a continuous combustion process, or other combustion process, as described above, to achieve a steady-state operation.

Claim 1:
A hybrid propulsion system (<NUM>) comprising:
a plurality of propulsors (<NUM>);
a first drive shaft (<NUM>);
a second drive shaft (<NUM>);
a gas turbine engine (<NUM>) comprising:
a first turbine stage (<NUM>) operatively coupled to the first drive shaft (<NUM>); and
a second turbine stage (<NUM>) operatively coupled to the second drive shaft (<NUM>);
a motor-generator (<NUM>) operatively coupled to the second drive shaft (<NUM>) and configured to:
generate, during a steady-state operation of the gas turbine engine, electrical power to drive at least one propulsor of the plurality of propulsors (<NUM>); and
selectively drive the second shaft (<NUM>); and
a clutch (<NUM>) characterised in that it is configured to automatically engage, during a start-up operation of the gas turbine engine, the second drive shaft (<NUM>) to the first drive shaft (<NUM>), and automatically disengage, during the steady state operation of the gas turbine engine, the second drive shaft (<NUM>) from the first drive shaft (<NUM>).