Should Aero Engines be More Electric?
By Lixin Ren
A future sustainable aircraft that promises to be greener, quieter, smarter and more affordable has inspired many innovative engineering solutions to be developed in the aerospace sector. The most notable trends are the recent developments towards the “More Electric Aircraft” (MEA) and “More Electric Engine” (MEE). It was projected that a full “transition to an all-electric aircraft may still be many years in the future, but aircraft engineer have made breakthroughs which combine electrical and hydraulic power and systems that assist in producing thrust thus the evolutionary ‘more electric aircraft’ (MEA) and ‘more electric engine’(MEE) ” concepts have evolved .
In the MEA, electrical power is used for extracting and distributing the non-thrust power. Advances in power electronics technology, fault-tolerant electric machines, electrical power systems, and energy storage technologies have renewed the interest in MEA/MEE. This is potentially a game changing development for the aerospace industry, reflected in Boeing’s 787 “Dreamliner”, the Airbus’ A380 in the Civil Aerospace sector and the Lockheed-Martin’s F-35 Fighter in the Military Aerospace sector.
The salient difference between Traditional Aircraft and a MEA to an engine is the increasing level of electrical power extracted via the engine-mounted electric generators with the lesser bleed air and mechanical power transfer via accessory gearboxes from the engine to the airframe . It is due to more electrically-powered systems replacing or partly replacing the pneumatic, mechanical, and hydraulic systems. This could bring benefits of improved Mean-Time-Between-Failures (MTBF), dispatch reliability, maintainability, reduced complexity and reduced operating costs.
The Airbus A380 uses a mechanical engine starter but with a variable-frequency generator, electro-hydrostatic actuators, and electrically-actuated thrust reversers which together offer better reliability than their pneumatic or hydraulic equivalents in addition to weight savings .
The Boeing 787 “Dreamliner” incorporates variable-frequency starter/generators and relies on higher levels of electric power derived from engine driven generators to eliminate pneumatically driven environmental controls and cabin pressurization systems, simplified hydraulic systems, and electrically powered engine start and wing ice protection functions. The benefits include more efficient power generation, better fuel efficiency, less drag and noise, lower maintenance frequency, and costs .
Although both Boeing’s 787 “Dreamliner” and Airbus’ A380 are More Electric Aircraft, neither is fitted with More Electric Engines. By establishing electrical power as the main energy transmission medium in a MEA airframe, more electric architectures are also promising for engines. Advantages may be gained by integrating the more electric engine with the more electric airframe at the overall aircraft system level.
Aero engines at the ‘heart’ of an aircraft provide the thrust that powers flight. The evolution of today’s hugely powerful gas turbine engines has been an extraordinary story of continuous incremental improvements in design and technology over many decades . The concept of the More Electric Engine considers the electrification of the mechanically driven engine accessories such as pumps and valves for the transportation and control of oil, fuel and air around the engine. The engine could use electrical machines driving through the accessory gearbox and controlled by a generator control unit to start the engine and to generate electricity when the engine is in normal operation. The potential exists to completely eliminate the shaft driven mechanical accessory gearboxes and integrate embedded electrical generators into the engine, which aims at providing benefits in terms of fuel burn saving, increased reliability, reduced maintenance cost and emissions reduction.
While offering great opportunities, ‘more electric’ also poses significant technical and business challenges to the industry. The increased levels of electric power in a ‘more electric’ platform inevitably demands higher power generation capacity with increased power transfer from the engine to the aircraft. The tendency of using higher voltages, adoption of a variable frequency (360Hz~800Hz) instead of 400Hz constant frequency electrical power system and associated electrical power converters and distribution system, will require the extensive use of solid state devices. Power electronics have become a key enabling technology essential for any future ‘more electric’ platform, bringing with it a number of challenges for the design of high power density power converter electronics with high temperature capability whilst having volume and weight constraints, including its thermal management.
Similarly, remarkable challenges are still ahead for developing the high capacity, highly efficient electric machine and associated thermal management system for aerospace applications which can deliver higher performance in terms of efficiency, torque and speed within the normal volume, weight and cost constraints. Innovative technologies, for example switched reluctance machines, permanent magnet machine and active magnetic bearing technologies for an electric machine, appeared to have the potential for the application in the demanding physical environments in an aero engine.
Given that the aerospace industry is highly regulated, before any concept turns into a product for a commercially operated aircraft, significant validation and verification effort has to be directed into demonstration and test activities. As a result it may take many years commercialize a promising concept into a viable product. Notwithstanding, the technical challenges for each individual component, the overall system integration challenge must not be underestimated. This must be properly addressed so that the system impact of the individual component to the engine can be dealt with.
Recent progress made in the hybrid electric propulsion may ultimately lead to the development of a fully electric propulsion, or “All Electric Engine” (AEE) concept which could have ultra-high capacity battery systems and high density electric motors . We may eventually see the development of the all-electric aircraft just as the reality as the electric car has come to bear.
 A380: More Electric’ Aircraft, http://www.aviationtoday.com/av/commercial/A380-More-Electric-Aircraft_12874.html
“More Electric Engines for More Electric Aircraft”, Eddie Orr, IET Transport Newsletter, June 2013
 “The A380 Flight Control Electrohydrostatic Actuators, Achievements and Lessons Learnt”, Dominique van den Bossche, ICAS 2006.
 “787 Electrical System”, The Boeing Company.
 “The Jet Engine”, Rolls-Royce plc, 6th Edition 2010
 “Flying with Siemens Integrated Drive System”, Paris Air Show, le Bourget, June 2013
Dr Lixin Ren received a BEng (1st Class) in 1992 from North China Electric Power University (NCEPU) in Thermal Power Engineering and Automation, qualified as a MPhil in Electric Power System and Automation in 1995 from Electric Power Research Institute (EPRI), Beijing, China and is awarded a Doctor of Philosophy degree from the School of Electrical and Electronic Engineering at Queen’s University Belfast, United Kingdom in 2003 for his work in ‘Nonlinear Identification and Control of a Turbo generators using multiple models’.
He is currently a senior specialist engineer in system architecture and integration with application in aerospace, marine and energy and nuclear systems at Electric Power and Control Systems (EPACS), Rolls Royce plc, England.
Prior to join Rolls Royce plc., he had an extensive research and industrial track record and had worked in a number of Institution and companies including Power Plant Automation Department, EPRI China/Beijing DianYan ZhiShen Control Technology Ltd. (1992~1999), Queen’s University, Belfast (1999~2003), the Robert Gordon University (2003~2005), Mitsui Babcock Energy Ltd.(2005~2008), and Doosan Power Systems Ltd (2008~2012) where he held positions in various capacity including systems and control engineer, team leader, project manager, research fellow, lead EC&I engineer. This had provided him with rich experiences and a breadth of knowledge/skills portable across industrial sectors and enabled the high level thinking and system thinking in complex systems/programmes involving multiple disciplines. His expertise and area of interest includes thermal power generation, electric power and energy systems, power plant automation, emission control system (NOx reduction and Carbon Capture), intelligent system and control, control & instrumentation, electrical power systems, electric machine/drive and power electronics and transportation electrification, smart grid technology etc.
A member of Institute of Engineering and Technology (IET) in United Kingdom and a member of Institute of Electrical and Electronic Engineers, USA affiliated with IEEE Power and Energy System Society, Control System Society, Communication Society and in the past Power Electronics Society, and Computer Society.
He has contributed to the IET Power Generation Control Conference in 2007 and 2008 where he has both helped the conference programs as an industrial committee member and delivered a speech on the need of design guidelines for UK power generation control and group discussion as a panellist. He is serving as an Editor of the IEEE Transportation Electrification Newsletter and has been a reviewer for a number of international journals and conferences including IEEE Transaction on Energy Conversion, IEEE Transaction on Power Systems, IEEE Transaction on Industrial Electronics, and IEEE Transaction on Power Electronics etc.