Patent Description:
Turbofan engine manufacturers continue to seek further improvements to engine performance including improvements to thermal, transfer and propulsive efficiencies.

<CIT> discloses a gas turbine engine having spools arranged to drive two or more generators, including a main generator and an auxiliary generator. <CIT> discloses a system for extracting additional power from a low pressure turbine in a turbofan gas turbine propulsion engine, and allowing a starter-generator that is normally driven by a low pressure turbine to supply starting torque to the turbofan engine. <CIT> describes a gas turbine engine system having a low pressure shaft and high pressure shaft, and a continuously variable transmission, the output of which being adapted to rotate faster than its input, the input being connected to the low pressure shaft, and the output being connected to a generator. <CIT> discloses a gas turbine engine having a compressor section, an air tap, an auxiliary compressor, an electric motor, and first and second heat exchangers. <CIT> discloses a method for operating a gas turbine engine having a starter-electric generator driven by one of a plurality of shafts of the gas turbine engine.

From a first aspect of the invention, a turbofan engine is provided according to claim <NUM>.

In another embodiment according to any of the previous embodiments, the first generator and the second generator are electrically actuated to remove a load exerted on a corresponding one of the high spool and the low spool.

In another embodiment according to any of the previous embodiments, the first engine operating condition includes an idle operating condition where the high spool and the low spool are rotating within an idle speed range.

In another embodiment according to any of the previous embodiments, the second engine operating condition includes a non-idle operating condition where the high spool and the low spool are rotating at a speed above the idle speed range.

In another embodiment according to any of the previous embodiments, the turbofan engine further includes an accessory gear box that is coupled to the high spool through a first tower shaft and the first generator is driven by an output from the accessory gearbox.

In another embodiment according to any of the previous embodiments, the turbofan engine includes a second tower shaft that is coupled to the accessory gear box and the second generator is driven by an output of the accessory gearbox driven by the second tower shaft.

In another embodiment according to any of the previous embodiments, the second generator is disposed aft of a turbine section and coupled to the low spool shaft.

In another embodiment according to any of the previous embodiments, the second generator is disposed at least partially in a tail cone.

In another embodiment according to any of the previous embodiments, the first generator includes a motor/generator that is capable of driving the high spool for starting of the turbofan engine.

From a further aspect of the invention, a method of operating a turbofan engine as claimed in claim <NUM> is provided.

In another embodiment according to the previous embodiment, decoupling of the first generator and the second generator includes actuating a corresponding one of a first clutch and a second clutch to disengage rotational input from a corresponding one of the high spool and low spool.

In another embodiment according to any of the previous embodiments, decoupling of the first generator and the second generator includes electrically decoupling a corresponding one of the first generator and the second generator to not impart a load on a corresponding one of the high spool and the low spool.

In another embodiment according to any of the previous embodiments, the method further includes transitioning between generating power with the second generator in the idle engine operating condition to generating power with the first generator in the non-idle engine operating condition by overlapping power generation from both of the first generator and the second generator for a transition period.

In another embodiment according to any of the previous embodiments, the first generator is operated to generate electric power within a rotational speed range that corresponds to non-idle operation of the high spool and the second generator is operated within a rotational speed range to generate electric power within a rotation speed range that corresponds with idle operation of the low spool.

Although the different examples have the specific components shown in the illustrations, embodiments of this invention are not limited to those particular combinations.

<FIG> schematically illustrates a turbofan engine <NUM>. The turbofan engine <NUM> is disclosed herein as a two-spool turbofan that generally incorporates a fan section <NUM>, a compressor section <NUM>, a combustor section <NUM> and a turbine section <NUM>.

It should be understood that various bearing systems <NUM> at various locations may be provided, and the location of bearing systems <NUM> may be varied as appropriate to the application.

The inner shaft <NUM> is connected to a fan <NUM> through a speed change mechanism, which in exemplary turbofan engine <NUM> is illustrated as a geared architecture <NUM> to drive the fan <NUM> at a lower speed than the low speed spool <NUM>.

It should be understood, however, that the above parameters are only exemplary of one embodiment of a geared architecture engine and that the present disclosure is applicable to other gas turbine engines including direct drive turbofans.

The flight condition of <NUM> Mach and <NUM>,<NUM> ft (<NUM>,<NUM> meters), with the engine at its best fuel consumption may be referred to as bucket cruise Thrust Specific Fuel Consumption ('TSFC') and provides a parameter of lbm of fuel being burned divided by lbf of thrust the engine produces at that minimum point.

The example engine <NUM> includes a first generator <NUM> and a second generator <NUM> for producing electric power that is provided to an engine power distribution system <NUM> for operation of engine systems <NUM> and aircraft systems <NUM>. The first generator <NUM> is powered through a coupling to the high speed spool <NUM>. The second generator <NUM> is powered through a coupling to the low speed spool <NUM>. A controller <NUM> directs operation of the first generator <NUM> and the second generator <NUM> based on current engine operating conditions. At low power conditions such as during engine idle operating conditions, loads placed on the high speed spool <NUM> for power extraction are relative large in proportion to the overall load demands. Accordingly, the proportionally larger loads are of a sufficient level to influence the size of high pressure compressor <NUM> and the high pressure turbine <NUM> to compensate for stability concerns. Moreover, the increased size and capacity can impact overall engine efficiency due the required added energy to mitigate stability.

Extracting power from the low speed spool <NUM> during all engine operating conditions potentially requires additional devices and or components to compensate for the wide variation in operational speed ranges encountered during engine operation. The wide range of variations can be addressed by utilizing a larger generator that is sized to produce minimum rated power at low idle speeds. However, such a larger generator is then significantly oversized for the remainder of the engine operating cycle. Alternatively, a variable transmission could be utilized. However, such transmissions complicate operation and add weight to the point of exhausting any benefit.

The example engine <NUM> provides the first generator <NUM> that is sized and configured to provide efficient power generation at non-idle engine operating speeds. The second generator is sized and configured to provide efficient power generation at lower engine speeds such as are encountered during engine idle operation. The different sizing of the generators <NUM>, <NUM> may allow spools <NUM>, <NUM> to be designed differently to have more efficient engine operating points. Moreover, different sized generators <NUM>, <NUM> provides for placement of a load on the spool <NUM>, <NUM> best suited to power the generator <NUM>, <NUM> for a given engine operating condition. Additionally, the tailored sizing of the generators <NUM>, <NUM> to a specific engine operation range may avoid a need for speed compensating devices.

The sizing of the first generator <NUM> and the second generator <NUM> refers to the capacity to produce electric power at the relative speeds of each spool <NUM>, <NUM>. The sizing may include different lengths for each of the first generator <NUM> and the second generator <NUM>. The sizing may include different numbers of winding and/or winding configurations. Additionally, the first generator <NUM> and the second generator <NUM> may be of the same configuration but be driven by different gear systems to provide the desired speed to generate electric power.

It should be appreciated that the controller <NUM> can be part of an overall engine controller or an individual controller that controls production of electric power. The example controller <NUM> relates to a device and system for performing necessary computing or calculation operations. This system may be specially constructed for this purpose, or it may comprise at least a general-purpose computer selectively activated or reconfigured by a computer program stored in the computer. The computing system can also consist of a network of (different) processors. A computer program and also data required for its execution may be stored in a computer readable storage medium, such as, but is not limited to, any type of disk including floppy disks, optical disks, CD-ROMs, magnetic-optical disks, read-only memories (ROMs), random access memories (RAMS), EPROMs, EEPROMs, magnetic or optical cards, application specific integrated circuits (ASICs), or any type of media suitable for storing electronic instructions, and each coupled to a computer system bus. Furthermore, the computer referred to may include a single processor or may be architectures employing multiple processor designs for increased computing capability.

Referring to <FIG>, the example engine <NUM> is shown schematically with the first generator <NUM> and the second generator <NUM>. An accessory gearbox <NUM> is provided and mounted about an exterior of the engine <NUM>. In one disclosed example, the accessory gearbox <NUM> is mounted to the engine static structure <NUM>.

A first tower shaft <NUM> is coupled to the outer shaft <NUM> of the high speed spool <NUM>. The first tower shaft <NUM> is coupled through a geared interface <NUM> to the outer shaft <NUM> in a known manner. The tower shaft <NUM> powers a portion of the accessory gearbox <NUM> that is drives the first generator <NUM>. A second tower shaft <NUM> is coupled through a second geared interface <NUM> to the inner shaft <NUM> of the low speed spool <NUM>. The geared interfaces <NUM>, <NUM> may be of any configuration and arrangement within the contemplated scope of this disclosure.

A first clutch <NUM> provides for the decoupling of the first generator <NUM> from rotational input from the high speed spool <NUM>.

A second clutch <NUM> provides for decoupling of the second generator from rotational input from the low speed spool <NUM>. Operation of the clutches <NUM>, <NUM> is provided by the controller <NUM>. The controller <NUM> directs operation of the clutches <NUM>, <NUM> to unload a corresponding spool <NUM>, <NUM> depending on current engine operating conditions.

In a first engine operating condition, the engine is idling and rotation of the low spool, indicated at <NUM> is low and within a targeted speed range that corresponds with engine idling operation. The second generator <NUM> is sized and configured to operate within the targeted engine idling operation speed range. In a second engine operating condition, the engine is operating at higher non-idle speeds such as those encountered during takeoff, climb and cruise engine operating conditions.

The first generator <NUM> is sized and configured to operate efficiently during rotation indicated at <NUM> of the high speed spool <NUM> at the higher non-idle engine operating speeds. The first generator <NUM> may comprises a motor/generator that is capable of driving the high speed spool <NUM> for starting purposes.

It should be appreciated, that sizing and configuring of the first and second generators <NUM>, <NUM> as used in the context of this disclosure includes electrically sizing the generators to provide the desired electric power output at the targeted speed ranges. Moreover, such sizing and configuring also includes a gear ratio for each generator <NUM>, <NUM> to that provides for operation of the generator <NUM>, <NUM> given the input speeds of the targeted ranges.

The second tower shaft <NUM> is schematically shown as being coupled to the second generator <NUM> through the same accessory gearbox <NUM> as the first generator <NUM>. In such a configuration, the accessory gearbox <NUM> may include gear systems to separately drive each of the first and second generators <NUM>, <NUM>. It should be also understood that a separate accessory gearbox could be utilized for each of the generators <NUM>, <NUM> and is within the scope of this disclosure.

Referring to <FIG>, another engine embodiment is schematically shown with a second generator <NUM> disposed aft of the low pressure turbine <NUM>. In this example embodiment, the second generator <NUM> is directly coupled to the low speed shaft <NUM>. Additionally, the second generator <NUM> is disposed within a tail cone <NUM> of the engine to provide some protection from hot exhaust gases exiting the turbine <NUM>.

In the example embodiment shown in <FIG>, the first generator <NUM> is coupled through the accessory gearbox <NUM> to the high speed spool <NUM>. A clutch is not provide because the controller <NUM> may direct operation to electrically decouple operation of the first generator <NUM>. However, it will be appreciated that a clutch may also be provided. The electrical decoupling removes any significant load imparted on the high speed spool <NUM> by reducing or removing excitation current and/or through other electrical circuitry. Similarly, the second generator <NUM> is also decoupled through the use of electrical circuitry rather than (or in addition to) a mechanical decoupling. The second generator <NUM> may also be decoupled through the use of a clutch.

It should be appreciated, that disengaging of the first and second generators <NUM>, <NUM> such that load is removed from the corresponding spool <NUM>, <NUM> by other approaches is within the scope and contemplation of this disclosure. Accordingly, any way of removing loads on the corresponding spools <NUM>, <NUM> when not generating electric power can be utilized and is within the scope and contemplation of this disclosure.

Referring to <FIG>, with continued reference to <FIG> and <FIG>, an example engine operation profile <NUM> for different stages of aircraft operation is schematically shown. In a first engine operating condition indicated at <NUM>, the engine <NUM> is idling and both the low spool <NUM> and high spool <NUM> are rotating at relatively low speeds. The disclosed idle speed range corresponds with operation of the engine <NUM> during ground operations and can vary based on specific engine configurations within the scope and contemplation of this disclosure. In this engine idling operating condition, the first generator <NUM> is decoupled from the high speed spool <NUM> to remove imparted load. The decoupling of the first generator <NUM> may be accomplished electrically or mechanically by actuation of the first clutch <NUM>. The second generator <NUM> is coupled to the low speed spool <NUM> and generates electric power that is supplied to the aircraft and engine systems.

As engine speeds increase during takeoff and climb operation, a transition between the second generator and the first generator is performed. In the transition period shown schematically at <NUM>, the engine speed increases to a speed corresponding with efficient performance of the first generator <NUM>. Operation of the second generator <NUM> is tapered off and finally discontinued. Tapering can be implemented either mechanical by way of a clutch or electrically by removing gradually removing an electric load. An overlap period during the transition where both generators <NUM>, <NUM> provides continuous power supply to the engine and aircraft systems.

After the transition period <NUM>, the first generator <NUM> produces electric power during a non-idling operational period shown at <NUM>. The second generator <NUM> is decoupled such that additional load for power generation is removed from the low speed spool <NUM>. The second generator <NUM> is turned off at the higher rotational speed of the low speed spool <NUM> to limit or avoid excessive voltage or exceedance of mechanical speed limitations when run above the lower deign speeds with a generator optimized for idle speeds.

The proportion of the total load on the high speed spool <NUM> during the operating period <NUM> imparted by the first generator <NUM> is low and therefore does not impart substantial performance debits.

As the rotation of the high and low speed spools <NUM>, <NUM> slows such as during descent, landing and taxing after landing indicated at <NUM> and <NUM>, the controller <NUM> directs transition back to power generation by the second generator <NUM>. The first generator <NUM> is decoupled from the high speed spool <NUM> to remove load to improve engine efficiency.

Accordingly, the disclosed selective operation of the high speed spool powered generator and the low speed spool generator provides power extraction at idle conditions where the maximum fuel burn benefit is achieved, without the addition of a complex transmission to correct the wide low spool speed range. Moreover, because both the high speed spool powered generator and the low speed spool powered generator are being used at targeted speeds, each is designed in a weight and size-efficient manner without speed-correction mechanisms.

Claim 1:
A turbofan engine (<NUM>), comprising:
a high spool (<NUM>) including a high pressure compressor (<NUM>) coupled to a high pressure turbine (<NUM>) through a high spool shaft (<NUM>);
a low spool (<NUM>) including a low pressure compressor (<NUM>) coupled to a low pressure turbine (<NUM>) through a low spool shaft (<NUM>);
a first generator (<NUM>) coupled to the high spool shaft (<NUM>);
a first clutch (<NUM>) operable to selectively decouple the first generator (<NUM>) from the high spool (<NUM>) when in a first engine operating condition;
a second generator (<NUM>) coupled to the low spool shaft (<NUM>), the second generator (<NUM>) generating electric power in a first engine operating condition and the first generator (<NUM>) generating power in a second engine operating condition,
a second clutch (<NUM>) operable to selectively decouple the second generator (<NUM>) from the low spool (<NUM>) when in the second engine operating condition;
a controller (<NUM>) that directs operation of the first generator (<NUM>), the first clutch (<NUM>), the second generator (<NUM>), and the second clutch (<NUM>), wherein in the second operating condition, the controller (<NUM>) operates the second clutch (<NUM>) and the first generator (<NUM>) to generate electric power within a rotational speed range corresponding to non-idle operation of the high spool (<NUM>) and in the first engine operating condition where the first clutch (<NUM>) decouples the first generator (<NUM>) from the high spool (<NUM>), the second generator (<NUM>) is operated within a rotational speed range to generate electric power within a rotation speed range corresponding with idle operation of the low spool (<NUM>).