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Proceedings of ASME Turbo Expo 2019: Turbomachinery Technical Conference and Exposition GT2019 June 17-21, 2019, Phoenix, Arizona, USA
GT2019 - 91413
CONTROL TECHNOLOGY N EEDS FOR ELECTRIFIED AIRCRAFT PROPULSION SYSTEMS Donald L. Simon Joseph W. Connolly Dennis E. Culley NASA Glenn Research Center NASA Glenn Research Center NASA Glenn Research Center Cleveland, OH, USA Cleveland, OH, USA Cleveland, OH, USA ABSTRACT the form of several potential architectures as shown in Figure 1 Electrified aircraft propulsion (EAP) systems hold potential [ 3 , 4 ] . These EAP a rchitecture options include: for the reduction of aircraf t fuel burn, emissions , and noise.
Currently, NASA and other organizations are actively working All electric : Batteries provide the sole source of to identify and mature technologies necessary to bring EAP propulsive power.
designs to reality. This paper specifically focus es on the Hybrid electric : A combination of batteries and envisioned control technology challenges a ssociated with EAP combustion engines provide propulsive power. In designs that include gas turbine technology. Topics discussed p aralle l hybrid design s , a battery - powered motor and a include analytical tools for the dynamic modeling and analysis turbine engine are both mounted on a shaft that drives a of EAP systems, and control design strategies at the propulsion fan, so that either or both can provide propulsion . In and component levels. This includes integrated s upervisory series hybrid design s , only the electric motors are control facilitating the coordinated operation of turbine and mechanically connected to the fans; the gas turbine electrical components, control strategies that seek to minimize drives an electrical generator, which produces power to fuel consumption and lessen the challenges associated with drive the motors and/or charge batteries.
thermal management, and dynamic control to ensure engine Turboelectric: Combustion engines provide propulsive ope rability during system transients. These dynamic control power with all (full turboelectric) or some (partial strategies include innovative control approaches that either turboelectric) of the engine power output co nverted to extract or supply power to engine shafts dependent upon electricity .
operating phase, which may improve performance and reduced S eries/parallel partial hybrid system : H as one or more gas turbine engine weig ht. Finally, a discussion of control fans that can be driven directly by a gas turbine as well architecture design considerations to help alleviate the as other fans that are driven e xclusively by electrical propulsion/aircraft integration and certification challenges motors. T hese motors can be powered by a battery or associated with EAP systems is provided .
by a turbin e - driven generator .
I NTRODUCTION The NASA Aeronautics Research Mission Directorate Since the dawn of aviation, fossil fuel burni ng engines have strategic implementation plan outlines a vision to transition to served as the dominant source of aircraft propulsive thrust .
alternative propulsion and energy sources [ 5 ]. This includes a However, recent technological advances in batteries and range of electrified propulsion solutions including all - electric, electrical systems have enabled the exploration of alternative turboelectric, and hybrid electric designs. Several electrified designs that rely on the generation, storage, and transmissi on of aircraft concept vehicles have been proposed by NASA as shown electrical power for aircraft propulsion . The motivation to in Figure 2 . This includes fixed - wing aircraft design concepts consider electrified aircraft propulsion ( EAP ) designs is being such as the all - electric X - 57 M axwell [ 6 ], the Single-aisle d riven by aviation fuel burn, emission, noise , and cost reduction Turboelectric AiRCraft with Aft Boundary Layer propulsor goals [ 1 , 2 ] . EAP offers flexibility in storing and transmitt ing (STARC-ABL) [ 7 ], and the NX - 3 blended wing body with electrical power, which enable s aircraft designs that apply distributed turboelectric propulsion [ 8 ]. Also shown are advanced propulsion concepts such as distributed electric electrified rotorcraft vehicles proposed under NASA’s propulsion and boundary layer ingestion fans. EAP systems take Revolu tionary Vertical Lift Technology Project [ 9 , 10 ].
This material is declared a work of the U.S. Government and is not subject to copyright protection in the United States. Approved for public release; distribution is unlimited. 1 Parallel Hybrid All Electric Series Hybrid Battery Turbofan Turboshaft Motor(s) Electric Bus Electric Bus Motor Electric Bus Distributed Generator Fans Motor Battery Fuel 1 to Many Fan Fans Battery Motor Fuel Turboelectric Partial Turboelectric Series/Parallel Partial Hybrid Turbofan Turboshaft Turbofan Electric Bus Electric Bus Motor Motor Electric Bus Motor Distributed Distributed Distributed Fans Fans Generator Generator Generator Fans Fuel Fuel Fuel Fan Motor Motor Fan Battery Motor Figure 1 . Electrified a ircraft p ropulsion a rchitectures (from Refs. [ 3 , 4 ]) Fixed-Wing Aircraft N3-X X-57 Maxwell STARC-ABL Partial Turboelectric Distributed Turboelectric All Electric Rotorcraft Quadrotor Side-by-Side Helicopter Tiltwing All Electric Hybrid Electric Turboelectric Figure 2 . NASA electrified aircraft concept vehicles (from Refs. [ 6 - 10 ]) A multitude of EAP vehicle concepts are also being explored includes development of the control design tools and strategies in industry. Almo st 100 electrically propelled aircraft are in to ensure re liable and efficient operation of EAP systems, both development worldwide [ 11 ] . These are mostly all - electric under normal and anomalous operating scenarios . This paper designs targeting the general aviation and urban air mobility will specifically focus on the control technology challenges markets . EAP targeting larger commercial aircraft tend to be associated with the design and operation of EAP designs that turboelectric or hybrid electr ic designs. Examples include the include gas turbine tec hnology . Several of these challenges were E - Fan X series hybrid propulsion aircraft being developed by identified by the Commercial Aero - Propulsion Controls Airbus in partnership with Rolls - Royce and Siemens [ 12 ] and Working Group (CAPCWG), a consortium of NASA and United Zunum Aero’s regional airliner with hybrid electric propulsion States engine and aircraft manufacturers focused on identifying [ 13 ]. propulsion control and related technolog y development needs Multiple technology advances are r equired to enable EAP that are aligned with NASA’s Aeronautics Mission Dir ectorate implementation on next generation aircraft [ 4 ] . This includes Programs and Projects . The EAP control technology needs improvements in electrical motors and generators to achieve identified by CAPCWG in Ref. [ 14 ] are further expanded upon higher efficiency and specific power , technology to enable and discussed in this document.
increase d battery speci fic energy , and p ower electr on ics and The remaining sections of th is paper are organized as power distribution system technology to enable operation at follows. First, a comparison between the control architectures higher voltage levels at altitude. Advances in gas turbine required for conventional aircraft engines versus EAP designs is technology are needed to enable high levels of engine power given. This is followed by a discussion of the modeling and extraction or power addition . Another s ignificant challenge is control design tools needed for developing EAP control systems.
thermal management of the EAP system . Next, EAP control strategies are discussed. This includes a In addition to the technology challenges noted above, EAP discussion of the integrated control strategies required for also presents significant controls - related challenges. This coordinated operation of turbine and electrical components, and the potential control enhancements offered by the flexible nature calculate control commands sent to actuators installed on the of EAP designs. The paper then provides a discussion of the test engine. Fuel flow rate is the primary parameter adjusted to facilities required for EAP evaluation and maturation . The paper control engine thrust or torque output. Since engine thrust output concludes with a discussion of the control considerations related cannot be sensed directly, a feedback measurement correlated to to the certification of EAP systems along w ith a summary . thrust , such as fan speed or engine pressure ratio , is used to establish a closed - loop fuel control design. Additional engine actuators such as variable guide vanes and bleed valves are NOMENCLATURE open - loop scheduled by the EEC to e nsure engine operability.
AC Alternating current The EEC supplies engine parameters back to the aircraft for Commercial aero - propulsion controls working cockpit gauge displays and health and status information CAPCWG group purposes.
CP Contingency power Engine control systems must be robust to account for DAL Development assurance level engine - to - engine performance variations that natur ally exist.
DC Direct current Limit logic is applied to ensure that the engine does not EAP Electrified aircraft propulsion encounter operability issues such as surge or combustor blowout, ECS Environmental control system and that structural and temperature limits are not exceeded.
EEC Electronic engine control Additionally, the engine control plays an important function in FHA Functional hazard assessment HEIST Hybrid electric integrated systems testbed engine fault detection, isolation, and accommodation. This HIL Hardware - in - the - loop includes logic to diagnose and accommodate faults.
HPC High pressure compressor Accommodation actions may include switching to physically HPT High pressure turbine redundant hardware (e.g., computer channel, sensor, or actuator), IFPC Integrated flight and propulsion control commanding actuato rs to failsafe positions, or switching to LPC Low pre ssure compressor revisionary control modes in the event of a fault. The LPT Low pressure turbine conventional engine control architecture tends to be centralized MCP Maximum continuous power in its design, and the controller is certified along with the engine.
MP Maximum power NEAT NASA electrified aircraft testbed EAP control architecture s are application dependent, but in NPSS Numerical propulsion system simulation general EAP control systems are expected to be more distributed MEE More electric engine and more complex than their conventional engine control PLA Power lever angle counterparts. A notional EAP control architecture for a hybrid PROOSIS Propulsion object - orie nted simulation software electric propulsion system is shown i n Figure 3 b. Here, SFC Specific fuel consumption propulsive thrust is generated by gas turbine engines and an array Single - aisle turboelectric aircraft with aft STARC - ABL of distributed electrically driven fans. Electrical components, boundary layer propulsor including generators, batteries, power electronics, electrical TEEM Turbine electrified energy management buses and motors, are included to enable the generation and TLD Time - limited - dispatch delivery of electrical power to the distributed fans. EEC units Toolbox for the modeling and analysis of T - MATS thermodynamic systems control the operation of the gas turbines, while an electronic component controller regulates the operation of the generators, COMPARISON OF CONVENTIONAL VERSUS EAP battery, and distribut ed electrical motor driven fans. A CONTROL ARCHITECTURES supervisory controller is included to control operation of the An aircraft engine ’s control system plays a vital role in turbine and electrical subsystems, and it also serves as the ensuring the safe, reliable, and efficient operation of the engine communication interface between the aircraft and the propulsion throughout the aircraft’s operating e nvel ope , w hich includes system. Given the coupling betwee n turbine and electrical controlling the engine during transient operation . A comparison system operation, the supervisory controller plays a vital role in between a conventional aircraft propulsion control architecture coordinating the operation of both subsystems to optimize and an EAP control architecture is shown in Figure 3 . These two efficiency, reduce thermal management challenges, and maintain architectures will be fu rther discussed in the paragraphs below.
overall operating limits. As with the conve ntional engine control In the conventional aircraft engine control architecture architectures, the EAP design must be robust to performance shown in Figure 3 a , c ommunication between the aircraft and variations and system faults. Due to their diversity of each engine installed on the vehicle occurs through an Electronic components and coupled nature, EAP systems are expected to Engine Control (EEC) computer. The EEC is a dual - channel present more failure modes and also enable new system computer that receives thrust demands along with power and reco nfiguration options in response to faults. As such, fault bleed offtake requests from the aircraft. These aircraft requests, detection and accommodation logic embedded within the control along with engine sensed feedback measurements, are processed system is expected to play a vital role in supporting EAP system by control logic i mplemented within the EEC and used to certification requirements.
Aircraft Aircraft • Health and status information • Thrust demands • Cockpit gauge and indicator • Aircraft bleed and • Thrust demands • Engine health and information power offtake • Aircraft bleed status information demands Supervisory and power • Cockpit gauge and Controller offtake demands indicator information Electronic Engine Control Units Electrical Power Management Electronic Engine and Distribution Engine #1 Engine #2 Control Units EEC EEC Distributed Battery Management Engine #1 Engine #2 Electrically System EEC EEC Sensed Driven Fans Sensed Actuator Actuator Actuator Sensed feedback feedback feedback commands commands Motor commands measurements measurements Control Unit measurements Battery Generator Control Unit Pwr. Elec.
Control Unit Motor Power Motor Generator Engines Electronics Control Unit Control Unit Engine #2 Engine #1 Motor Generators a) Conventional aircraft propulsion b) Electrified aircraft propulsion control architecture control architecture Figure 3 . Comparison of conventional and electrified aircraft propulsion control architectures AIRCRAFT ENGINE CONT ROLS DEVELOPMENT MODELING AND CONTROL DESIGN TOOLS PROCESS AND APPLIED TOOLS Dynamic system modeling and computational analysis tools A high - level illustration of the aircraft engine controls are integral to the aircraft engine control development process.
development process and applied tools is shown in Figure 4 . Duri ng the development cycle of an engine, a non - linear physics - Here, a series of maturation steps are shown , each of increasing based model of the engine is created and used to design cost and complexity . Often , development iterations are needed to turbomachinery and evaluate system - level performance. Such make control system updates . The process begins by receiving models are complex , capturing the behavior and coupling of all in formation on the propulsion system design concept . This is engine components including the inlet, fan, compressors, typically obtained through system studies conducted to design combustor, turbine, and exhaust nozzle. Other design aspects of and size the propulsion system to match its intended aircraft the engine such as bypass ducts, cooling flows, bleed and mission. Given the propulsion system design concept , the control mechanical power off takes, and variable guide vanes are also developm ent process includes the steps of dynamic modeling, represented in these models.
control design, real - time simulation and hardware - in - the - loop The models may be either steady - state or dynamic , with (HIL) evaluation , engine testing, and flight testing. Certification steady - state models capturing the “on - design” performance of considerations are applied throughout this process to ensure that the engine and dynamic models enabling simulation of the “off - the desi gn c omplies with the airworthiness standards set forth by design” performance encountered by the engine during regulatory agencies. The upcoming sections will discuss the transients . Dynamic models ar e necessary for the design of tools, control design strategies, facilities, and certification engine control systems , which are tasked with ensuring the safe considerations related to EAP control system development . and reliable transient operation of the engine in response to varying thrust requests and flight conditions . The dynamics captured by these models are typically i n the 10’s of Hz range , Control System Maturation set by conventional fuel and variable geometry actuation systems Certification Considerations and the dominant spool inertias transient response . Higher Real-Time Engine System frequency dynamics such as blade flutter, compressor stall, or Control Simulation Testing, Dynamic Design Design and HIL Flight Modeling combustion instabilities are typically not in cluded in the models Concept Evaluation Testing used for control design, although the operating limits where these instabilities are expected to be encountered should be Development Iterations defined in the models .
Figure 4 . Aircraft engine control development process Vari ous modeling tools are available for constructing gas EAP CONTROL STRATEGI ES turbine engine models , such as the Numerical Propulsion System EAP systems will present a number of control design Simulation (NPSS) [ 15 ], GasTurb [ 16 ], Propulsion Object - challenges due to their complexity and integrated coupling .
Oriented Simulation Software ( PROOSIS ) [ 17 ], and the Toolbox However, they are also expected to enable exciting new for the Modeling and Analysis of Thermodynamic Systems opportunities when it comes to controls . The need for integrated (T - MATS) [ 18 ]. Enhancements to these tools are necessary to control design approaches is anticipated, with an emphasis on enable modeling of the electric machines, energy storage coordinated turbine and electrical system operation to optimize devices, and power management and distribution hardware efficiency and oper ability, while minimizing thermal found in EAP concepts . These enhanced modeling tools should management challenges . Potential EAP control strategies are also enable modeling of the relevant dynamic interaction further discussed in the subsections below.
between electrical and mechanical components, as well as the effects of modulating available actuators. Additionally, these Optimal Energy Management tools should model sources of heat generation and dissipation Focused on hybrid designs that combine output power from within the system for thermal management considerations . gas turbine engines and energy sto rage devices such as batteries , Tools are also needed to develop models representative of energy management refers to the integrated control task of off - nominal EAP system behavior. Simulating component scheduling how power is drawn from all available sources to performance variations due to variations in environmental supply the demanded power . For optimal efficiency throughout conditions, manufacturing tolerances or normal deterioration a mission , t he scheduling of this engine/ ene rgy storage device that components are expected to experience over their life time of power split should be done to minimize fuel burn while adhering use will allow control robustness to be assessed. Additionally, to operating constraints. Optimal energy management the simulation of EAP system faults will allow initial approaches are applied within the automotive industry to define development and evaluation of fault detection, isolation, and the power schedules implemented within the engine control accommodation logic, including the logic required to mitigate systems of hybrid automobiles [ 20 ]. This is done by seeking to funct ional hazards and enable certification. minimize a defined performance index, J , representative of the Most aircraft engine control designs are based on linear total fuel consumed during a reference mission of time T . Such control approaches . As such, tools for the automated generation a performance index can be calculated based on the integration of linear state - space models based on non - linear models are of a defined cost function , L (·) , as shown in the equation below needed . These l inear state - space m odels should be extractable at [ 20 ] : multiple operating points spanning the EAP system’s operating 𝑇 envelope, allowing them to be coupled together in a piecewise ( ) ( 1 ) 𝐽 = ∫ 𝐿 ( 𝑡 , 𝑢 𝑡 ) 𝑑𝑡 linear fashion [ 19 ] . R eal - time code generation capabilities are also desired to support real - time simu lation and hardware - in - the - loop evaluation of control s ystems (see Figure 4 ) . Control design Here , u ( t ) is the control vector provided by the supervisory tools to coordinate operation of the tur bine and electrical controller. T he cost function L (·) reflects instantaneous fuel flow subsystems will also be beneficial .
rate plus a ny penalt ies incurred for violation of operating Challenges include modeling EAP systems t o the proper constr aints. These can be hard or soft constraints and can include level of fidelity. While power electronics and power a variety of factors such as engine and electrical system operating management systems can have switching frequencies above the limits, battery charge/discharge rates and state - of - charge limits, 10 kHz range, there is a timescale tradeoff with model fidelity.
noise, emission, and thermal considerations. If desired, Eq. ( 1 ) The emphasis should be to develop tools that enable modeling of can be adapted to achieve goals beyond minimizing fuel t he electrical system to the proper level of fidelity. This includes consumption. This may include minimizing total energy modeling of control actuators and sensors plus capturing the consumption, mission cost, or other metrics of interest .
system response to transient changes in electrically driven To illustrate the application of energy management propulsors, dynamic balancing of electrical loads, and the strateg ies to aircraft hybrid electric propulsion designs, consider dynamic coupling between engines and the electrical system. As the notional specific fuel consumption (SFC) versus power with gas turbine models, these models should capture electrical output curve of a turboshaft engine at a given flight condition as system performance not only at the system design point, but at shown in Figure 5 . The green star denotes minimum or optimal “off - design” transient operation as well, spanning the operating SFC, which is the engine’s most efficient operating point.
envelope that the system will be required to function within. The Typically, a n engine is designed to operate close to this “design modeling of thermal loads over a mission is also important, as point” for a significant portion of its intended mission.
such information can be used to optimize the design of thermal Additional power se ttings of interest , w hich constrain available management systems. Functional operating limits of the system engine power output , are also shown . This includes: should also be included so that control protection logic can be incorporated to restrict operation to appropriate regions.
I dle : M inimum permitted power setting .
levels of power are demanded , perhaps during takeoff or hover Maximum Continuous Power (MCP) : M aximum amount of power that the engine can continuously o perations, both the engine and the battery are called upon to supply the required power (mode 4). Th ose instances when the provide without any time restriction s .
power demand is less than the engine’s optimal SFC power Maximum Po wer (MP) : The maximum power output setting provide an opportunity to run the engine at its optimal that the engine can provide for a finite amount of time SFC setting while using any excess engine generated power to (e.g., 5 minutes).
recharge the battery (mode 2). As losses occur during the Contingency Power (CP) : More common in two - engine mechanical to electrical power conversion process, not all of the helicopters, CP is a high power setting that an engine engine generated power can be converted into battery energy in may provide for a short time (e.g., 2.5 m inutes) during mode 2 . The range i n power demand transitioning between the contingency events such as the opposite engine optimal SFC power setting up to the power setting that marks the becoming inoperative.
start of mode 4 reflects a region of engine - only power generation with no battery charging or discharging occurring (mode 3 ) .
SFC vs. Power Output There are several aspec ts of a hybrid EAP control schedule Idle worth noting. First, having the mode 3 region as shown in Figure Optimal SFC 6 may only be practical for those engines where the optimal SFC Max Continuous Power Max Power power setting resides to the left of the MCP sett ing as for the Contingency Power example curve shown in Figure 5 . For engines where optimal SFC resides at or above MCP, it may make sense to omit mode 3 entirely and simply transition directly from mode 2 to mode 4.
Also, the trans ition between modes might be dependent on phase of flight or battery state of charge. While newer automobiles apply “start - stop” technology that shuts off their engine at Specific Fuel Consumption stoplights to save gas, it is unclear whether similar technology could ever be certif ied for application to aircraft engines in - Power Output flight. As, such it may only be practical to operate in all electric Figure 5 . Notional turboshaft SFC vs. power output curve mode (mode 1) during taxi operations at the airport.
Additionally, if a battery ever reaches its fully charged state , the From a fuel efficiency standpoint, the ideal case is to always control is forced to t ransition out of battery charging (mode 2) run the engine at its minimum SFC operating point . But this is and into one of the other modes, regardless of whether excess not possible in conventional propulsion designs as the requested engine power is available. If such constraints are encountered , it propulsive power from the aircraft varies throughout a flight.
will be necessary for the engine to operate off of its maximum However, a paradigm shift occurs when considering hybrid efficiency point.
p ropulsion designs that include energy storage devices. In such architectures , it might be possible to run the gas turbine at its most efficient operating point for extended periods of time while using any excess engine power for battery charging and drawin g power from the battery when the requested power exceeds that which can be supplied by the gas turbine alone. Considering a hybrid architecture that permits a battery to be both discharged and recharged during flight , there are four possible system opera ting modes : 1) Engine off, Battery discharging 2) Engine on, Battery charging 3) Engine on, Battery energy level is static ( neither charging or discharging ) 4) Engine on, Battery discharging Given these operating modes, a simplified hybrid EAP control schedule is sh own in Figure 6 . Here, instantaneous power Figure 6 . Notional hybrid EAP power control schedule generated is shown versus instantaneous propulsive power demand ed . The dashed red vertical lines denote transition points Thermal Management in the control schedule where switching between t he four EAP system developers face significant thermal operating modes described above occurs . D uring regions of low management design challenges [ 4 ]. The need for inc reased levels power demand such as ground taxi, th e engine is turned off and of energy and power supplied by smaller and lighter components the battery supplies all demanded power (mode 1). When high are factors contributing to increased thermal loads. Advances in Transient Control Schedules and Limit Logic high temperature materials, the development of more efficient Aircraft engine control logic is responsible for ensuring the safe and reliable transient operation of the engine throughout its components, and innovations in passive and active th ermal operating envelope [ 21 ] . This includes protection logic to guard management systems are needed to effectively withstand and against exceedance of engine ope rational, structural or safety dissipate thermal loads. EAP supervisory controllers tasked with limits. The concept of transient control is illustrated in the coordinating the operation of the engine and power systems will compressor map shown in Figure 8 [ 21 ] . Here, the solid black play a key role in addressing thermal management issues. This line represents the steady - state operating li ne that the engine will includes scheduling engine and electrical system operation under follow over the range of power settings. During throttle varying levels of requested thrust and operating conditions. In transients, engine operation will move off the steady - state conventional aircraft engine control designs, thermal challenges operating line as denoted by the acceleration and deceleration are partially addressed by applying a maximum rated thrust trajectories shown in the figure. Also shown are se veral engine sch edule as shown in Figure 7 . This schedule reflects the rated operating limits . This includes the compressor surge line, the or maximum thrust that the engine can produce as a function of combustor blow - out limit, and the turbine temperature limit.
outside air temperature at a given flight condition [ 21 ] . At this During transient operation, t he engine controller regulates fuel flow to ensure that the engine does not exceed defined flight condition, m aximum thrust is const ant for outside air acceleration/deceleration rate schedules or defined engine temperatures below the temperature value where an engine’s operating limits.
turbine temperature limit is encountered. As outside air For conventional aircraft gas turbine engines, transient temperature increases beyond this value, maximum available control design accounts for approximately 75% of the total thrust is decreased to ensure that the maximum turbine control law design and development effort [ 21 ]. Given their temper ature limit is not exceeded. For EAP designs, analogous complexity and inherent coupling, EAP designs are expected to thrust and power scheduling logic is needed to provide present similar transient control challenges . Protection limits are temperature limit protection, although such logic is expected to expected to be necessary to ensure the health and life of electrical be more complex and will likely involve the need to coordinate components. This includes con trol limits placed on electric the modulation of available actuators (including engine bleed machine speed and torque levels, battery charge/discharge rates, and mechanical power offtakes) to satisfy multiple temperature overall power levels, and component operating temperatures.
limits simultaneously. Additionally, consideration needs to be given to the dynamic coupling between the electrical system and the turbomachinery .
In addition to implementing limits to guard against temperature exceedances, EAP engine and energy storage This is especially true given the fact that the electrical system dynamic response can potentially be much more rapid than that devices will als o need to provide the power to drive any thermal management active cooling systems. This task is closely related of the turbomachinery. As such, it is likely that speed limits and over - speed protection logic will be n ecessary for any electric to the optimal energy management task introduced in the previous subsection. In fact, thermal management constraints motor driven propulsors. Also, it is likely that acceleration and deceleration schedules will be needed to restrict how rapidly the can be directly considered within Eq. ( 1 ) giving rise to a combined optimal energy and thermal management problem. electrical system can respond during transient operation to ensure both electrical and turbomachin ery operability limits . As Here, the objective becomes scheduling the coordinated operation of engine and energy storage devices to ensure that all previously shown in Figure 3 b, a supervisory control to ensure proper integrated coordination between the engine s and the thermal limits are m aintained while simultaneously operating the system in the most fuel efficient manner. electrical power system is essential.
Surge Line Over-temp Zone Turbine Surge Turbine Temp Limit temperature limit Zone Acceleration Line Max Power Pressure Cruise Deceleration Ratio Line Constant Maximum Thrust Combustor Idle Speed Transition between Blow-Out Lines Limit flat rated thrust and Combustor Steady-State Blow-Out Zone flat rated temperature Operating Line Outside Air Temperature Corrected Mass Flow Figure 7 . Engine rated thrust schedule at a fixed flight Figure 8 . Compressor map indicating e ngine steady - state operation, transient operation , and operating limits condition Turbine Electrified Energy Management (TEEM) Reducing the amount of margin implies tha t engine design can Turbine Electrified Energy Management (TEEM) is a new be safely modified to achieve a number of benefits affecting control technology that addresses the occurrence of off - design performance and efficiency metrics. Those benefits generally engine operation that occurs during changes in engine power appear in the form of weight and volume reduction such as the setting or other momentary disruptions [ 22 ]. These operability elimination of compressor stages or elimination of s tability bleed issues represent potential risks to the engine that the control valves . It may also enable reduced off - incidence flow in system traditionally mitigates by limiti ng the rates of change of compressor blades leading to improvements in blade design for commanded variables, such as fuel flow rate . Temporary off - lower loss operation. Finally, optimizing the transient operability design operation is an expected natural response of the turbine of the turbine engine may impact electrified propulsio n system engine , but it is actually a symptom of an energy imbalance design by fully utilizing the engine as the most efficient means between rotating components of the engine that occurs during of converting fuel into power , thus minimizing the need for transient operation . Inertial energy stored in the rotating energy storage, which currently has a high weight penalty .
components and heat energy soaked into the mass of the engine contribute to this energy imbalance . Fuel Electric The TEEM technology focuses on counteracting the energy Electric LPC LPT HPC HPT Machine Machine imbalance inherent in gas turbine engine s during transients N2 through the use of electric machines applied to add /extract N1 mechanical power to /from the shafts of the engine. This enables the engine to operate close to its on - design condition during Energy Electric transients . Applying TEEM, the rotat ional velocities of engine Machine Storage components are matched to the flow condition s in the gas path, as estimated by the commanded fuel flow rate . This allows the Figure 9 . TEEM architecture flow incidence angle to match the design point incidence at a particular power setting . In theory, by ma tching the rotational speeds of the shafts with the instantaneous fuel flow rate that defines an engine flow condition, it is possible to maintain the steady - state operating line of the turbomachinery components , particularly the compressors. This is possi ble, even during transient maneuver s such as a change in power setting . The steady - state operating line is generally the most efficient operating condition for that component.
One potential implementation of TEEM is shown in the Figure 10 . Schematic of the N1/N2/Wf relationship during architecture given in Figure 9 . Here, electric machines are steady - state and acceleration/deceleration transients. The coupled to the rotating shafts of the turbine engine . Drawing black arrows indicate the desire to modify the shaft speeds power from an energy storage device, the electric machines are during transients to operate closer to the steady - state used to implement shaft speed control during the momentary operating line.
periods wher e the shaft response s would naturally lag behind commanded fuel flow rate due to their high moment s of inertia .
Novel Cycle and More Electric Engine Controls In addition to supplying mechanical shaft power , the electric Compared to conventional aircraft engines in service today, machines connected to the engine are also able to extract shaft the gas turbine engines included in EAP systems are expected to power that is conv erted to electricity and used to charge energy have fundame ntal differences, both in their design and control .
storage devices or drive other electric machines on the vehicle.
In addition to providing propulsive thrust, today’s c onventional Figure 10 illustrates the typical dynamic behavior that is aircraft gas turbine engines also supply bleed air for the aircraft’s observed for a dual spool engine during ac celeration and environmental control system (ECS) and mechanical power deceleration transients and the notional steady - state relationship offtake to ge nerate electricity for the aircraft. However, the that is to be maintained by TEEM for the low spool speed (N1), engine power extracted for these functions is only a small high spool speed (N2), and fuel flow rate (Wf). The objective is fraction of the total power output of the engine . Conversely, not necessarily to match the shaft speed to th e design point but some EAP designs will extract substantially higher percentages to maintain acceptable levels of stall margin during the transient of overall engine powe r. This is expected to necessitate the need condition. Generally, this requires a high impulsive power, but for novel engine cycle designs , such as variable fan and variable ideally it is not beyond the rating of the electric machine for its core nozzles to provide stability margin when electric power is original design purpose. In terms of e nergy storage capacity to extracted or applied to engine shafts [ 23 ]. With these variable drive the machine, it is modest due to the short duration of the cycle engines will come the need to apply control strategies to transient.
schedule operation of the engine and its variable geometry in The overall effect of TEEM is to reduce the amount of concert with the power extraction/addition demands placed upon transient stall margin required in the compressor system.
it [ 24 ] .
Within the aviation industry there is an ongoing trend to Electric Integrated Systems Testbed (HEIST) [ 31 ] and the NASA transition to more e lectric engine (MEE) designs [ 25 , 26 ] . This Electr ic Aircraft Testbed (NEAT) [ 32 ]. The NEAT facility, replaces aircraft engine mechanical and pneumatic driven shown in Figure 11 , is located at the NASA Glenn Research accessories with electrical - mechanical actuators . This includes Center Plum Brook Station . NEAT is a reconfigurable testbed apparatus such as accessory gear box - driven fuel and oil pumps, developed to enable end - to - end development and testing of engine bleed off - takes for hea t exchangers, ECS , and anti - ice ful l - sca le electric aircraft powertrain s .
systems. Replacing these accessories with electrically - driven systems will help to reduce weight and improve overall engine efficiency. A primary advantage of electrically actuated systems is that their operation can be sche duled in accordance to the required demand as opposed to today’s gearbox driven systems that must operate at the speed dictated by the rotation of the engine. This results in the need for bypass circuits to absorb excess flow, which is inefficient. MEE des igns also seek to replace pneumatic or fluid driven actuators with electrical - Figure 11 . NASA Electric Aircraft Testbed (NEAT) facility mechanical designs. The readily available source of electricity offered by EAP systems is expected to further accelerate the T he focus of testing to date at NEAT is subscale testing of transition to MEE designs in the future. With this wi ll come the the powertrain of the STARC - ABL concept aircraft , a single - associated control design needs to optimally schedule the aisle turboelectric aircraft with an aft boundary layer propulsor operation of the electrical - mechanical actuators and sy stems [5] . A depiction of the STARC - ABL aircraft along with an inherent in these designs.
overview diagram showing the STARC - ABL turbomachinery and electrical system interconnections implemented at the N EAT Integrated Flight and Propulsion Control facility is provided in Figure 12 . STARC - ABL consists of two Unlike conventional flight control strategies, which wing - mounted turbofan engines and a tail fan propulsor driven pri marily view the engine as an actuator for adjusting thrust, by electric motors. Pilot power lever angle (PLA) commands Integrated Flight and Propulsion Control (IFPC) considers specify the requested thrust output from the turbofans and the tail fan. In addition to producing thrust, the two turbofan engines a lso control of the vehicle and its propulsion system in a coordinated supply mechanical offtake power delivered to generators to fashion. This includes modulation of engine thrust output to produce electricity. A lternating current (AC) from the generator s perform veh icle flight control functions. IFPC has been applied travels through rectifier s to transport the power over direct in short takeoff and vertical landing aircraft applications [ 27 , 28 ].
current (DC) bus es . M otor controller s command i nverter s to T he feasibility of performing flight control of multi - engine deliver the commanded current at the appropriate voltage and fixed - wing aircraft through the modulation of engine throttles frequency, to generate the necessary torque at the tail fan motors was also dem onstrated under the Propulsion Controlled Aircraft to achieve the desired tail fan speed . The inv erter/motor project led by NASA during the 1990’s [ 29 ]. Given their control lers also provide information to the generator s so that the distributed propulsion nature, EAP vehicle designs are well corresponding amount of torque load from each turbofan is suited for IFPC. Coordinated modulation of the thrust output of commanded to extract the required power.
an array of distributed pr opulsors strategically positioned on the aircraft allows basic flight control maneuvers such as turns, climbs, and descents to be performed. This can eliminate or reduce the size of flight control surfaces reducing overall vehicle weight and cost. IFPC dev elopment for EAP vehicles will require a combined effort between flight and propulsion controls Turbofan 1 Command Fan Speed Control engineers. Key control issues are to ensure that the propulsors (NEAT – Motor) can efficiently provide the range and dynamic response in thrust Torque Load on LP needed for flight control, whil e adhering to all operational limits Inverter/ Generator Bus Rectifier Motor 1 Control Tail Fan and constraints.
Command Pilot (NEAT – PLA Generators) Inverter/ Rectifier Bus Motor 2 FACILITIES TO ENABLE EAP TEST AND MATURATION Generator Control Torque Load on LP The development of EAP systems and components will Corrected Fan Speed Command Turbofan 2 Torque Command from Tail Fan Inverter/Control require corresponding facilities to test and mature the Fan Speed Control Command Torque on Tail Fan Spool technologies . This includes facilities to perform testing of (NEAT – Motor) DC Bus Current and Voltage megawatt - class EAP designs [ 30 ]. To help address this need, Figure 12 . STARC - ABL (top) and o verview diagram of NASA created testbeds to enable testing of EAP systems and STARC - ABL turbomachinery and electrical system their associated technologies . Examples include the Hybrid interconnections implemented at NEAT facility (bottom) In addition to serving as a testbed for powertrain technology, Test facilities are also needed to test EAP electrical components NEAT also provides the capability to perform real - time and full - scale EAP designs, including the control and operation hardw are - in - the - loop testing with emulated turbofan engines to of these systems in flight test environments.
enable initial evaluation of turbomachinery and electrical component integration challenges as well as control approaches CERTIFICATION CONSID ERATIONS REGARDING EAP to coordinate their operation. For the STARC - ABL CONTROL DESIGN configuration, this is acco mplished by developing nonlinear Established aerospace practices define guidelines for the dynamic real - time models of the turbofan engines and the tail development of civil aircraft and systems [ 34 ] and for fan. These models are developed using available turbomachinery conducting safety assessments on these systems [ 35 ]. This modeling tools [ 15 , 18 ]. Prior to imple mentation of these real - includes guidelines for onboard electronic hardware and time models at NEAT, a simple power flow model of the software , such as that included in control systems [ 36 , 37 , 38 ].
STARC - ABL electrical system was developed and interfaced Starting with an initial concept, the development process of an with the turbomachinery models to create a full - system aircraft/system readies the concept for implementation [ 34 ] . T he simulation . The purpose was to facilitate control studies of the aircraft development process includes defining aircraft p ropulsion system with representative electrical component functions, allocating those functions to aircraft systems, models [ 33 ]. developing the system architecture, applying requirements, and After initial development and validation , the system implementation. As these development steps are turbomachinery models w ere implemented in real - time conducted, several additional processes integral to ensuring computer systems and interfaced with the STARC - ABL system safety, requirements validation, and process assurance powertrain at NEAT. Mo del outputs we re used to drive electric are happening concurrently in a coordinated, iterative fashion.
motors included in the NEAT facility to emulate the turbofan This includes a system safety assessment that consists of a produced torque supplied to the electric generators. This enable d functional hazard assessment (FHA) conducted to identify all real - time testing of the STARC - ABL propulsion system using potential failure conditions of each function, and classify those emulated turbomachinery and actual electric a l system failures according to the severity of their effects on the aircraft component hardware . A short 15 minute example flight profile or its occupants . The more severe a function’s failure condition consisting of a takeoff and climb phase, a cruise phase at 10,000 classification, the greater the development assurance level ft with a generator transient, and a descent phase was run . While ( DAL ) required for the function .
much shorter than typical aircraft flight profiles, th e example Typical engine functions considered during the system profile did allow evaluation of the system response throughout development and safety assessment process may include thrust various phases of flight . The commanded and actual tail fan modulation, thrust reverser c ontrol , communication of engine motor speed for this example flight profile is shown in Figure 13 . health and status information to the aircraft , and containment of The results in dicate that the tail fan motor speed tracks the engine failures to ensure passenger safety. A combination of commanded value well. protective strategies are applied to ensure that engine functions have safety levels in accordanc e w ith their DAL requirements.
These strategies may include defined maintenance and overhaul s chedules , containment systems to prevent uncontained failures , over - speed protection logic, and fail - safe design concepts leveraging system redundancy. The engine control system plays a significant role in assuring engine fail - safe operation .
Typically, the EEC is a redundant dual - channel design with built - in - test and monitoring capability for potential faults in processors , sensors, or actua tors. In the event of a system fault, logic within the EEC is designed to automatically detect and mitigate the anomaly . Mitigation actions may include reverting to physically redundant controls hardware, commanding Takeoff Begin Idle Cruise and Climb Descent actuators to failsafe positions, or reve rting to reversionary control modes that allow the engine to function safely , although perhaps at a reduced performance level .
Today, aircraft engines and their control systems receive type certificate approval as a stand - alone system to signify their Figure 13 . NEAT example flight profile tail fan spool air worthiness . However, the complex coupling and distributed rotational speed results with real - time turbomachinery nature of EAP designs are expected to place added challenges on simulation and 500 k W scaled powertrain hardware the certification of these systems . FHA’s are needed to identify and assign DAL’s to all propulsion system functions . It is NEAT and the STARC - ABL evaluation described above is expected t hat r edundancy within the EAP architecture will be just one example of the required EAP test facilities and the type required to assure th at the propulsion system can still deliver of controls development testing that can occur in these facilities.
propulsive thrust or torque in the event of a failure. As with installed on the aircraft as well as how EAP health and status conventional engine designs, t he EAP control system is expected information is conveyed back to the flight crew .
to play a significant role in assuring that EAP systems comply with the airworthiness standards set forth by regulatory agencies.
SUMMARY This includes fault detection and mitigation logic, reversionary Electrified aircraft propulsion systems hold potential for control modes , and contingency control modes to respond to reducing aircraft emissions, noise, and fuel burn. Several EAP system faults . The reconfiguration flexibility of EAP technology barriers must be addressed to bring these designs to architectures may allow multiple acceptable control mitigation fruition, including development of the controls technology responses for an individual fault , thus enabling optimal control required for EAP . Given their complexity, distributed nature, and reconfiguration based on c urrent mission objectives .
the inherent coupling between turbomachinery and electrical Additionally, as with conventional engine EEC designs, the systems, EAP system control designs are expected to provide application of Time - Limited - Dispatch ( TLD ) concepts for EAP new challenges. This paper discussed several control technology control system s is anticipated. TLD is a concept where a needs to enable EAP. These include modeling tools for creating redundant system is allowed to operate for a predetermined integrated turbine and electrical system models for control length of time with faults present in the elements of a redundant de sign and evaluation, control architectures and control companion system, before repairs are required [ 39 ] . This strategies for EAP systems, test facilities for the development of requires appropriate fault detection and fail - safe mitigation logic EAP systems and controls, and fault detection and mitigation to be included in the EAP control system. functions included within control logic to enable EAP Other propulsion - related func tions that must be considered certification . Deve lopment of the control technologies necessary during the development of aircraft equipped with EAP are the for EAP systems will require a concerted effort by NA SA and propulsion flight deck controls and displays. In conventional the aerospace community.
designs, individual throttle levers and cockpit gauges are available for each engine. For EAP designs with multiple ACKNOWLEDGMENTS distributed propulsors, flight crew control and monitoring of This work was conducted under the NASA Advanced Air each individual propulsor may be untenable and increase the Vehicles Program, Advanced Air Transport Technology Proj ect .
likelihood of human error . Therefore, c onsiderations must be The authors wish to thank members of the Commercial Aero - given to the format in which thrust commands are delivered to Propulsion Controls Working Group for their feedback on the the EAP system and then distributed to multiple propu lsors EAP control n eeds captured in this docume nt.
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