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Civil Aero Engines — Review of Trends

Aircraft engines need to produce more power while consuming less fuel, produce less noise and reduce emission levels

Issue: 11-2021By Air Marshal Anil Chopra (Retd)Photo(s): By Safran Group, CFM
The CFM RISE (Revolutionary Innovation for Sust ainable Engines) programme targets more than 20 per cent lower emissi ons and will demonstr ate a range of new, disruptive technologies like open fan architecture and hybrid electric capability for future engines

The civil aerospace industry is primarily being influenced by environmental issues and cost of operations. The key areas of attention are propulsion and autonomous systems. Top engine-makers are unveiling new technologies to cut CO2 emissions and yet boost the performance of aircraft engines. Aircraft engines need to produce more power while consuming less fuel, produce less noise and reduce emission levels. These performance parameters can be achieved by enhancing the efficiency of combustion engines and simultaneously exploring electric and hybrid propulsion systems. Considering that large number of drones and Urban Air Mobility (UAM) systems are beginning to fly over populated areas, the aero-acoustics of these engines will also be a design focus. Benefits of research and technology in propulsion will shorten engine development cycle, reduce engine weight, increase engine performance, reduce engine fuel consumption, enhance reliability, reduce emissions and noise, increase component life and reduce maintenance requirements.


Past three generations of gas turbine engines have incorporated increased turbine inlet temperature, increased compressor pressure ratio, increased bypass ratio, improved fan and nacelle performance, reduction of noise and emissions as well as improved reliability. The new engine technologies will involve engine-airframe integration, new and improved materials and material processing techniques, advances in turbo-machine technology, progress in combustion technology and vastly improved utilisation of Computational Fluid Dynamics (CFD) in engine design procedures. The carbon-fibre blades allowed high-bypass jet engines that helped in developing efficient long-haul jets such as the Boeing 777 and the Boeing 787 Dreamliner that could use just two engines rather than four. Novel technologies such as “smart engines” and the use of magnetic bearings will change the course of engine development. Additive manufacturing offers lighter, cheaper and quick-to-manufacture parts which will cut assembly costs and time, simplify maintenance and save on fuel.


Reduced emissions and noise abatement has been possible through technological innovations. Newer models of the two most-widely used aircraft today - the Boeing 737 and the Airbus A320, not only carry more passengers, but also burn 23 per cent less fuel, through much better fuel burn efficiency. Lightweight low pressure turbofans using composite fan blades, high efficiency low pressure turbine, advanced engine externals and installations including novel noise attenuation, advancing high speed turbine design, aggressive mid-turbine interductand even a low emission combustion chamber are being developed for next generation rotary-craft engine. Ceramic Matrix Composites (CMC) have one-third the weight of steel; but can withstand temperatures as high as 2,400 degrees fahrenheit, beyond the melting point of many advanced metallic super-alloys, thus improving the engine’s thermal efficiency. 3D-printed components, hybrid-electric systems, advanced heat-transfer circuits are other breakthrough technologies.


Ultra-high bypass turbofans, open rotor engines, use of alternative fuels, relocating engines on the body of the aircraft such that engine noise is deflected upwards are some design considerations. Blended wing-body as in X-48B aircraft prototype and advanced electrical power technologies are being experimented with. Improvement in performance can be achieved by moving from a component-based design to a fully integrated design by including wing, tail, belly fairing, pylon, engine, and high-lift devices into the solution. Electric engines using lithium polymer batteries and solar-powered manned aircraft designed to fly both day and night without the need for fuel, are already under development. Solar electric propulsion is also being evaluated and developed by NASA using the unmanned ‘Pathfinder’ aircraft.


Two new engine concepts currently under investigation, include the ‘Combined Brayton Cycle Aero Engine’ and ‘Multi-Fuel Hybrid Engine’. Currently, over 50 per cent of the energy gets ejected as waste heat. A heat exchanger integrated in a turbofan core can convert recovered heat into useful power which can be used for onboard systems or to power an electrically driven fan to produce auxiliary thrust. A dual combustion chamber, wherein the high temperature generated in the first stage, allows ignition-less combustion in the inter stage, thus reducing CO2 and NOx emissions. Cryogenic bleed air cooling can enhance the engine’s thermodynamic efficiency.


The US Department of Defence’s Adaptive Versatile Engine Technology (ADVENT) and Adaptive Engine Technology Development (AETD) programmes and the GE Adaptive Cycle Engine (ACE) are areas of action. Unlike traditional fixed airflow engines, the variable cycle engines automatically alternate between a high-thrust mode for maximum power and a high-efficiency mode for optimum fuel savings. Incorporating heat-resistant materials and additive manufactured components in the Pulse Detonation Engine (PDE), gives it the potential to radically increase thermal efficiency. These adaptive features also have additional stream of cooling air to improve fuel efficiency and dissipate aircraft heat load. Such engines will also increase combat aircraft engine thrust by up to 20 per cent and improve fuel consumption by 25 per cent to extend range by more than 30 per cent.


Early 2018, General Electric (GE) completed its first flight test of the world’s largest jet engine GE9X being built for the new long-haul Boeing 777X. The engine has approximately the same diameter as the fuselage of a Boeing 737 and houses a 134-inchdiameter front fan. The engine certification was completed in September 2020. The 1,00,000-lbs thrust engine will be the most fuel-efficient engine the company has ever produced. The world record still belongs to the engine’s GE predecessor, the GE90-115B, which generated 1,27,500 lbs of thrust.


Computer technology and the microelectronics revolution allowed full-authority electronic digital controls on aircraft engines. Next stage was the active controls at the component or sub-component level within the compressor, gas turbines and bearings. The smart engine has a huge magnitude of computational power. It incorporates real-time feedback control within the device. Active suppression of fan and compressor surge and stall, active combustor monitoring control, magnetic bearings control and active noise controls are some of the areas. Magnetic bearings suspend the rotating members in magnetic fields, eliminating friction and lubrication requirements. Specific advantages over rolling contact bearings include elimination of the lubrication system, active damping of shaft dynamics and vibration, greatly increased temperature capability up to 800°C and large increases of load capability.


Real time analysis is being used to drive faster and better decisions by processing the data as it comes in. The Internet of Things (IoT) helps achieve this. Flight data is tracked in real time and it helps making minor changes to flight plans and aircraft speed to reduce flight times and fuel consumption, improve engine efficiency, reduce maintenance time and costs between flights and also the ‘Life Cycle Cost’. This can result in revolutionising flight efficiency and profitability.


Drones combined with improved imaging technology are increasingly being used for aircraft/engine maintenance. They can be used to detect surface damage, such as from lightning or bird strikes. It reduces time and frees technicians for other tasks. Dronebased mobile 3D scanners can be used automated non-destructive scanning. Drones can also enter confined spaces within engines and difficult to access parts without having to strip the engine.


The big data revolution and Information Technology now allows maintenance companies to amass the correct parts and technicians to make any repairs as soon as an aircraft lands. This certainly holds promise for increased safety and enhanced operational efficiency by cutting aircraft-on-ground time, which is estimated to cost the industry $62 billion annually. Even a five per cent reduction in unplanned maintenance events could save the industry up to $656 million per year.


Airbus, along with partners Rolls-Royce and Siemens, is developing its E-Fan X hybrid electric demonstrator, which had to be postponed due Covid. Boeing will initially develop an electrically powered ten-seater aircraft. New market entrants such as Wright Electric have the ambition of bringing into the market an electrically powered 180-seat short-haul aircraft by 2027. Roland Berger hopes that battery energy storage density of 400-450Wh/kg and near 1,000 Wh/lis reached in next two to three years. Remember, the jet fuel has the energy storage density of around 12kWh/kg. Hybrid-electric system would initially be heavier than the fossil fuel-based propulsion system. To compensate, it would need to reduce the airframe mass by around 20 per cent.


GE Aviation, a leading jet engines manufacturer recently produced its 1,00,000th 3D printed fuel nozzle tip for its CFM LEAP engine. Technologies are being up-scaled and maintaining production quality. MRO organisations will also benefit from the additive manufacturing revolution. Spares on demand will save from costly inventories of spare parts. Capital costs to set up such a capability still remain an issue.


The CFM RISE programme will take the next generation of single-aisle aircr aft to a new level of fuel efficiency and reduced emissions

GE Aviation and Safran have launched a bold technology development programme targeting more than 20 per cent lower fuel consumption and CO2 emissions compared to today’s engines. The CFM RISE (Revolutionary Innovation for Sustainable Engines) programme will demonstrate and mature a range of new, disruptive technologies for future engines that could enter service by the mid-2030s. The programme includes open fan architecture, hybrid electric capability, demonstrator ground and flight tests around middle of decade. It plans 100 per cent sustainable aviation fuel, and hydrogen capability. The companies also signed an agreement extending the CFM International 50/50 partnership to the year 2050, declaring their intent to lead the way for more sustainable aviation in line with the industry’s commitment to halve CO2 emissions by 2050. Through the RISE technology demonstration programme, they plan to reinvent the future of flight, bringing an advanced suite of revolutionary technologies to market that will take the next generation of single-aisle aircraft to a new level of fuel efficiency and reduced emissions. They also embrace the sustainability imperative. Deliver for the future is the mantra. Plan is to accelerate efforts to reduce our impact on the environment. Their LEAP engine already reduces emissions by 15 per cent compared to previous generation engines.

MRO organisations will also benefit from the additive manufacturing revolution

Central to the programme is state-of-the-art propulsive efficiency for the engine, including developing an open fan architecture. This is a key enabler to achieving significantly improved fuel efficiency while delivering the same speed and cabin experience as current single-aisle aircraft. The programme will also use hybrid electric capability to optimise engine efficiency while enabling electrification of many aircraft systems.


Technology is already delivering an impressive one per cent per annum saving on fuel burn. Pratt & Whitney says its new engines will use an internal gearbox to lower the speed of the fan saving 20 per cent on fuel consumption. CFM International introduced advanced engine called the Leap, using lightweight composite materials which could achieve similar improvements without a radical break from existing technology. Efforts to introduce biofuels to power jet engines are on. Airbus/Rolls-Royce hybrid electric with gas-turbine engine will allow peak power for take-off and climb while for the descent, the engine is shut down and the electric fans recover. Research is on for plasma jet engines that will use electricity to generate electro-magnetic fields instead of fuel by compressing and exciting argon gas into a plasma similar to that inside a fusion reactor. New technologies will bring change, challenge and opportunity, too. This will comprise harnessing the benefits of connectivity and big data to drive predictive maintenance, changes to technology embedded onto aircraft, the coming revolution in full-electric or hybrid-electric power and other disruptors such as additive manufacturing.