Integrated electrical power extraction for aircraft engines

An aircraft power system includes an aircraft engine having a plurality of engine spools, a plurality of AC generators driven by at least one spool, wherein each AC generator is independently operational to supply AC power to a respective AC power bus or a respective power electronics module, and at least one generator driven by a different engine spool, wherein the at least one generator is operational to supply additional AC power to each AC power bus in synchronization with each respective AC generator, or to supply additional AC power to each power electronics module in parallel with the AC power delivered to each power electronics module by the plurality of AC generators. The power electronics module(s) are operational to selectively provide either AC or DC power to desired distribution buses. Each AC generator and the at least one generator may be responsive to commands from an electrical power extraction (EPX) controller. At least one generator may be directly driven to provide variable frequency power.

BACKGROUND

The present invention is directed to electrical power extraction from aircraft engines, and more particularly to an electrical power generation system which enables efficient and dynamic production of electrical power from a turbofan engine.

Individual components of turbofan engines require different power parameters during operation. The fan rotational speed, for example, is limited to a degree by the tip velocity and, since the fan diameter is very large, rotational speed must be relatively low. The core compressor, on the other hand, because of its much smaller tip diameter, can be driven at a higher rotational speed. Therefore, separate high and low speed turbines with independent power transmitting devices are necessary for the fan and core compressor in aircraft gas turbine engines. Furthermore since a turbine is most efficient at higher rotational speeds, the lower speed turbine driving the fan requires additional stages to extract the necessary power.

Many new aircraft systems are designed to accommodate electrical loads that are greater than those on current aircraft systems. The electrical system specifications of commercial airliner designs currently being developed may demand significantly increased electrical power from current commercial airliners as loads transition from hydraulic and pneumatic in nature to electrical. Many mechanical actuators, for example, are being implemented using electric motors, replacing traditional, heavier hydraulic systems. These increased electrical demands require larger generators to supply the additional electrical power.

The increased electrical power demand must be derived from mechanical power extracted from the engines that propel the aircraft. When operating an aircraft engine at relatively low power levels, e.g., while idly descending from altitude, extracting this additional electrical power from the engine mechanical power may affect the ability to operate the engine to operate properly.

Traditionally, electrical power is extracted from a high-pressure (HP) engine spool in a gas turbine engine. The high and relatively constant operating speed of the HP engine spool, compared to that of the low-pressure (LP) engine spool, makes it an ideal source of mechanical power to drive the electrical generators connected to the engine. However, this added load placed on the HP engine core, can have a detrimental effect on engine performance under certain conditions.

Drawing power from elsewhere in the engine therefore is necessary in certain cases, and can also be advantageous, by extracting that power in a more fuel-efficient manner. The LP engine spool can provide this alternate source of power. Additionally, by selectively controlling the extraction of electrical power from either the HP or LP engine spools, as desired, benefits in engine performance can be realized.

Specific low-power fault-tolerant permanent magnetic (PM) generator technology installed within a turbofan engine is known. Although fault tolerant PM generators have not yet been used at primary power levels, such generators have been used at low power lever (i.e. less than 500 Watts). There has been however, no use of high power PM generators on turbo fan engines to date, for reasons of cost, complexity, and unproven technology, among other things.

Aircraft engine power extraction control systems are also known. Some of these systems include multiple generators controlled by a central control system. Although 400 Hz systems often run in parallel (aka, military transports and B747-400), variable frequency generators however (non-400 Hz), are not seamlessly paralleled or integrated with one another.

Generator arrangements including LP permanent magnet generators with some form of electrical control system are known in the prior art. None of these arrangements are known to be integrated with other sources (APU or HP driven).

Methods and systems directed to coordinating loads seen by each of the HP and LP generators are known to selectively switch or reduce the loads as the engine conditions permit. These methods and systems are not seamlessly integrated with an aircraft power system.

Multiple (i.e. three) spool engines having a generator on each shaft for supplying power to the engine itself, in addition to an airframe is also known in the art. This technology is not seamlessly integrated with an aircraft power system.

DC generators on LP spools are known in the prior art for providing power to DC busses. These LP generators require a device such as an inverter to interface DC to an AC bus. These approaches disadvantageously require significant integration with an airframe power system, and are not seamlessly integrated with the airframe power system.

In view of the foregoing, it would be advantageous and beneficial to provide a system and method for selectively controlling the extraction of electrical power from either the HP or LP engine spools, as desired, to optimize benefits in engine performance. It would be further advantageous if this system and method were to be seamlessly integrated with an airframe power system.

BRIEF DESCRIPTION OF THE INVENTION

The present invention is directed to an aircraft engine comprising a plurality of engine spools in which a plurality of AC bus generators are driven by at least one engine spool, and further wherein each AC bus generator is independently operational to supply AC power to a respective AC power bus; and at least one generator driven by a different engine spool, wherein the at least one generator is operational to supply AC power to each AC power bus in synchronization with each respective AC bus generator. One or more of the AC bus generators and/or the at least one generator may be a directly driven generator that provides variable frequency power.

Each starter-generator and at least one generator may be responsive to commands from an electrical power extraction (EPX) controller.

A method of providing aircraft power according to one embodiment comprises 1) driving a plurality of AC generators via an aircraft engine spool such that each AC generator provides AC power to a respective AC bus, 2) driving at least one generator via a different aircraft engine spool to selectively provide additional AC power to each respective AC bus, and 3) controlling at least one generator to selectively synchronize the AC power generated via at least one generator with the AC power generated via the plurality of AC generators. The plurality of AC generators and/or the at least one generator may be directly driven to provide variable frequency power.

Another embodiment is directed to an aircraft engine comprising an aircraft engine and a plurality of AC generators driven by the aircraft engine, wherein each AC generator is independently operational to supply AC power to a desired power electronics module in response to commands from an electrical power extraction (EPX) controller; and at least one generator driven by the aircraft engine, wherein the at least one generator is operational to selectively supply additional AC power to each desired power electronics module in parallel with each respective AC generator in response to commands from the EPX controller.

Yet another embodiment is directed to an aircraft engine comprising an aircraft engine comprising a plurality of engine spools and a plurality of AC bus generators driven by at least one engine spool, wherein each AC bus generator is independently operational to supply AC power to a respective power electronics module; and at least one generator driven by a different engine spool, wherein the at least one generator is operational to supply additional AC power to each power electronics module in parallel with each respective AC generator.

Still another embodiment is directed to a method of providing aircraft power in which the method comprises 1) driving a plurality of generators via an aircraft engine spool such that each generator provides AC power to a respective power electronic system; 2) driving at least one generator via a different engine spool to selectively provide additional AC power to each power electronics system; and 3) controlling the at least one generator to selectively supply additional AC power to each power electronics system in parallel with each respective AC generator.

While the above-identified drawing figures set forth alternative embodiments, other embodiments of the present invention are also contemplated, as noted in the discussion. In all cases, this disclosure presents illustrated embodiments of the present invention by way of representation and not limitation. Numerous other modifications and embodiments can be devised by those skilled in the art which fall within the scope and spirit of the principles of this invention.

DETAILED DESCRIPTION

FIG. 1illustrates a wild frequency electrical power extraction system architecture10with an externally located LP generator16, according to one embodiment of the present invention. A key feature of the systems and methods described herein below is that these systems and methods enable the seamless integration of an LP generator with existing HP starter-generators, with transparency to an airframe power system (i.e., it is fully compatible with most commonly used aircraft power systems). Although some embodiments are described with reference to HP starter-generators, other embodiments may just as easily employ HP generators that do not provide a start function to achieve the desired results according to the principles discussed herein below.

With continued reference toFIG. 1, all of the HP starter-generators12,14, the LP generator16, and power electronics (PE)18,20associated with EPX system architecture10can be seen to be located externally to the engine22. The electrical power supplied by the LP generator16is then processed using power electronics18,20and synchronized to each of the independently-supplied (wild-frequency (WF) or constant-frequency (CF)) buses24,26. The LP generator16output28is divided between two identical channels30,32. An EPX controller34in communication with a full authority digital engine control (FADEC) unit36commands the LP power electronics18,20to pick up or shed AC bus loads38,40for the LP generator16. This combined approach enables the engine controller (FADEC) to selectively load either LP or HP engine spools, as desired, to yield improvements in fuel efficiency, transient performance, stall margins, or other parameters.

Alternatively, the EPX controller34can be designed to operate independently and without direct FADEC unit36supervision. By monitoring the frequency of the HP generator12outputs, it is possible to determine the operating point of the engine, and therefore schedule the HP/LP load sharing as desired.

Control of the electrical load amounts carried by each respective starter-generator/generator12,14,16can be achieved, for example, by modifying the output power angle of power electronics18,20relative to the fundamental HP starter-generator12,14electrical frequency. The present invention is not so limited however, and other control techniques can just as easily be applied, such as, but not limited to, voltage droop methods.

Division of the power electronics18,20into two parallel channels30,32also advantageously provides a measure of reliability for the LP generating system. The LP generator16, itself may be designed to fail safely under all operating conditions (including the most severe internal stator winding fault). It should be noted that engine starting through the HP starter-generators12,14can still be provided through another installed set of power electronics (not shown) that is independent of the EPX system10.

In summary explanation, an electrical power extraction system architecture10can be advantageously applied without significant modification to an existing airframe power system. The EPX system architecture10enables the seamless integration of an LP generator16with existing HP starter-generators12,14, with transparency to the airframe power system (i.e., it is fully compatible with most commonly used aircraft power systems).

FIG. 2illustrates a constant frequency electrical power extraction system architecture100with an externally located LP generator16, according to one embodiment of the present invention. The power electronics18,20in EPX system100are modified to provide the generation of constant-frequency AC power to an aircraft power system. A constant 400 Hz power system may be desirable from an airframer perspective, as it represents the standard that has been used in aircraft power systems for decades.

The power electronics18,20need only be modified by the addition of a passive rectifier to allow the output of the HP starter-generators12,14to be paralleled at DC links within power electronics18,20. A DC-to-AC output inverter stage within power electronics18,20is then used to synthesize constant-frequency AC power, thus avoiding the need to install an entirely independent set of power electronics for the HP starter-generators12,14alone. The rating of the power electronics18,20can advantageously also remain at levels less than the total installed generating capacity, as the specified power system loads may be less than that amount.

Many different AC-to-DC conversion schemes can be employed within power electronics18,20to allow the outputs of the HP starter-generators12,14and LP generator16to be paralleled at DC links within power electronics18,20. Further, eliminating DC-to-AC output inverter stages within power electronics18,20then will then allow DC power to be distributed to power buses124,126.

Since the rectifier used at the HP starter-generators12,14could also be active (controllable), the power electronics18,20could then also be used for HP engine starting. It should also be apparent that all other benefits described for the embodiment ofFIG. 1are retained in the embodiment ofFIG. 2.

FIG. 3illustrates a constant frequency electrical power extraction system architecture200with an internally located LP generator16, according to one embodiment of the present invention. The embodiment shown inFIG. 3is similar to the embodiment shown inFIG. 1, being modified only by placing the LP generator16at a location that is internal to the engine22. The present invention is not so limited however, and those skilled in the art will readily appreciate that numerous implementations of this internally installed LP generator16may be accommodated in accordance with the principles described herein. The embodiment shown inFIG. 3also retains all other benefits available when using the embodiment shown inFIG. 1and described herein before.

DC power distribution can be obtained simply by eliminating use of DC-to-AC power inverters within power electronics18,20, as described herein before with reference toFIG. 2. Buses124,126can then be implemented as either AC power busses or DC power busses, depending on the power electronics18,20topology.

FIG. 4illustrates a constant frequency electrical power extraction system architecture300with dual internal HP and LP generators12,14,16, according to one embodiment of the present invention. The EPX system architecture300represents an extension of all three embodiments described herein before with reference toFIGS. 1-3, in that the LP generator16and HP generators12,14can not only be located within the engine22, but also in the form of a dual, integrated machine. This integrated machine comprises concentric layers including an inner rotor for HP/middle stator for HP and LP/outer rotor for LP. The present invention is not limited to use of a particular integrated machine architecture however. Numerous implementations of an internal LP generator can be employed to implement the embodiments described herein above, so long as the embodiments are implemented in accordance with the principles described herein before.

Again, DC power distribution can be obtained simply by eliminating use of DC-to-AC power inverters within power electronics18,20, as described herein before with reference toFIG. 2. Buses124,126can then be implemented as either AC power busses or DC power busses, depending on the power electronics18,20topology.

Electrical power extraction system architectures described with reference toFIGS. 1-4advantageously enable fuel saving entitlements through FADEC-controlled EPX. If engine design is specifically EPX-optimized, it can represent fuel savings beyond that achievable with presently known engines.

Installation of LP generation may enable a reduction in HP S/G generating capacity requirements. It also enables incremental machine implementations, starting with external LP generators, and provides a path to internal installation, such as described herein before with reference toFIGS. 3 and 4.

Installation of LP generation, as described above, also enables implementation of incremental power electronics, including WF, DC, and 400 Hz, as well as provision of power electronics for increased engine starting capabilities.

Since PM machines reach closest to power density and efficiency entitlement, PM technology can be spiraled to HP generators in future embodiments. Modular power electronics enable full physical integration with an engine.

Modular power electronics also enable integration with a power system that is transparent to an airframer. By allowing LP fans to windmill for power, downsizing or elimination of a ram air turbine emergency generating system may also be possible.

Transient engine performance can be influenced with selective HP/LP loading associated with embodiments described herein before with reference toFIGS. 1-4.

Thus, a complete electrical power generation system has been described that provides integrated, flexible power extraction within an engine to enable efficient and dynamic production of electrical power from a turbofan engine. The engine's low-pressure (LP) spool is coupled to an electrical generator having its output paralleled with the output of generators driven by the engine's high-pressure (HP) spool. Electrical paralleling of multiple generators is accomplished through robust power electronics, and optimally controlled through commands issued by the engine controller. The power from these combined sources can then be processed via power electronics to supply power of the type preferred by the airframer (e.g., WF, DC, or 400 Hz).

Hp-driven generators today are typically wound-field synchronous machines having electrical frequencies directly proportional to the engine core speed, as stated herein before. A future, LP-driven generator may be a robust, power-dense permanent magnet machine, in that it is designed to fail safely under all conditions. Power electronics described herein before are used to regulate the output voltage of this type of generator, and also perform the additional function of synchronizing to the electrical frequency of the HP-driven generator(s).

The electrical power extraction systems and methods described herein enable the seamless integration of an LP generator with existing HP starter-generators, with transparency to the airframe power system (i.e. it is fully compatible with most commonly used aircraft power systems).

The embodiments described herein also provide a system-level approach for an electrical power extraction system that enables seamless integration of additional power generating capabilities at the engine level. The result is transparent to the airframe's power system.

The electrical power extraction system architecture described herein before has been shown to allow the power extraction from an engine to be coordinated by an FADEC controller to yield increased fuel savings, engine operability, and dynamic performance, among other things. Alternatively, power extraction could be scheduled in a manner that operates in response to changes in engine power as observed by monitoring HP generator frequency.

Further, the electrical power extraction system architecture can be implemented in incremental stages, whether external or internal to the engine.

The electrical power extraction system architecture, as discussed herein before, can generate wild frequency, constant frequency power, or DC, as desired, and further includes inherent levels of redundancy that is reliable to help ensure safe operation in flight.