Performance of aircraft can be achieved by enhancing the efficiency of combustion engines and simultaneously exploring electric and hybrid propulsion systems
The global aerospace industry is being influenced by environmental issues and operating cost. The key areas of focus in 2019 were development of new propulsion and autonomous systems. The need to preserve the environment and follow regulations while boosting the performance of aircraft engines, is a key area. Aircraft engines need to produce more power, consume less fuel with lower noise and emission levels. This can be achieved by enhancing the efficiency of combustion engines and simultaneously exploring electric and hybrid propulsion systems. Considering that large number of drones are beginning to fly over populated areas, the aero-acoustics of these engines will also be a design focus. Research and development in propulsion technology will shorten engine development cycle, reduce weight, increase performance, reduce fuel consumption, enhance reliability, reduced 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 and improved reliability. The next generation engine technologies will involve engine-airframe integration, improved materials and material-processing techniques, advances in turbo-machine and combustion technology and improved utilisation of Computational Fluid Dynamics (CFD) in engine design procedures. 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.
There is clearly an urgency to address the problems of emissions and noise abatement through technological innovations. The two widely used aircraft today - the Boeing 737 and the Airbus A320, have shown that newer models of the same aircraft not only carry more passengers, but also burn 23 per cent less fuel. Emissions of CO2, H2O, O2 and N2 are functions of engine fuel burn efficiency. The action areas are lightweight low pressure systems for turbofans, including composite fan blades and high efficiency low pressure turbine; advanced engine externals and installations including novel noise attenuation; high efficiency Low Pressure (LP) spool technology while further advancing high speed turbine design; option of an aggressive mid-turbine inter-duct; high efficiency and lightweight compressor and turbine and low emission combustion chamber for next generation rotary-craft engine.
ENGINE DESIGN CONSIDERATIONS
Ultra-high bypass turbofans, open rotor engines, use of alternative fuels, relocating engines on the body of the aircraft to deflect engine noise 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 has beenachieved through the manned ‘Solar Impulse’ and the unmanned NASA ‘Pathfinder’ aircraft.
THERE IS CLEARLY AN URGENCY TO ADDRESS THE PROBLEMS OF EMISSIONS AND NOISE ABATEMENT THROUGH TECHNOLOGICAL INNOVATIONS
HEAT RECOVERY CONCEPTS
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 is wasted as 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.
NEXT GENERATION INOVATIONS
Developed under the US Department of Defense’s Adaptive Versatile Engine Technology and Adaptive Engine Technology Development programmes, is the GE Adaptive Cycle Engine (ACE). Unlike traditional engines with fixed airflow, the GE ACE is a variable cycle engine that will automatically alternate between a high-thrust mode for maximum power and a high-efficiency mode for optimum fuel savings. ACE is designed to increase thrust by up to 20 per cent, improve fuel consumption by 25 per cent to extend range by over 30 per cent and provide significantly more aircraft heat dissipation capacity. These adaptive features are coupled with an additional stream of cooling air to improve fuel efficiency and dissipate aircraft heat load. Incorporating both heat-resistant materials and additive manufactured components in the Pulse Detonation Engine (PDE), gives it the potential to radically increase thermal efficiency.
LARGE JET ENGINES - POWER AND EFFICIENCY
Early 2018, General Electric (GE) completed its first flight test of the world’s largest jet engine GE9X for the new long-haul Boeing 777X due to take to the skies in 2020. The engine has approximately the same diameter as the fuselage of a Boeing 737 and houses a 134-inch-diameter front fan. The 100,000-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 127,500 lbs of thrust.
Growth of computer technology and the microelectronics revolution allowed full-authority electronic digital controls on aircraft engines. Next stage was the active controls at component or sub-component level within the compressor, gas turbines and bearings. The smart engine has 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 features. 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.
ADVANCED REAL-TIME DIAGNOSTICS
Real time analysis is being used to drive 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 time 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 AS MAINTENANCE TOOLS
Drones, along 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. Drone-based 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.
BIG DATA PREDICTIVE MAINTENANCE
The big data revolution and information derived from it, will soon allow maintenance companies to amass the correct parts and technicians to undertake 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.
There are challenges and opportunities for more-electric aircraft of 787 or A350 class. Hydraulic and pneumatic systems such as those for actuation or air conditioning, are already being replaced by electrical systems to save weight and improve reliability. In 2011, the Boeing 787 was the first large passenger aircraft to use electricity rather than enginebleed air, to power the cabin air conditioning system. It also featured electrically actuated brakes and an electric de-icing system. Each 787 can produce around 1,000kVA for its onboard systems, markedly more than previous-generation models. Onboard power storage has also grown significantly.
Airbus believes that it would require 40MW of power for the takeoff phase, dropping to 20MW during cruise. Airbus, along with partners Rolls-Royce and Siemens, is developing its E-Fan X hybridelectric demonstrator, which should fly in 2020. It will replace one of the four engines on a BAe 146 regional jet with a 2MW electric motor which will be powered by electricity generated by a modified Rolls-Royce turbo-shaft engine mounted in the aft fuselage. Boeing will initially develop an electrically powered 10-seater aircraft. New market entrants such as Wright Electric have the ambition of bringing to market an electrically powered 180-seat short-haul aircraft by 2027. Roland Berger hopes that battery energy storage density of 400-450Wh/kg may be reached by the mid-2020s vis-a-vis jet fuel which has the energy storage density of around 12kWh/kg. Hybridelectric 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.
URBAN AIR MOBILITY (UAM)
Electric power could find early application in UAM. Daimler-backed Volocopter and Chinese startup Ehang have already demonstrated their aircraft in Dubai where the government plans to have a proof-of-concept up and flying soon. Dozens of programmes are evolving including Uber as well as Google founder Larry Page, electric ground vehicles such as Workhorse and more traditional rotary-wing aircraft manufacturers such as Bell and Airbus Helicopters.
By 2020, engine manufacturer GE Aviation estimates that it will be producing 100,000 individual components via 3D printing. MRO organisations will also benefit from the additive manufacturing revolution. Rather than maintaining costly inventories of spare parts, maintenance providers will, in theory, will be able to 3D print components as required. However, there remain issues of capital costs to set up such a capability and the time taken to print parts. Airbus has enabled smallbatch manufacturing that is quicker and produces components that are around 15 per cent lighter than earlier versions.
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. Both new engines have been deployed on different versions of Airbus A320neo. Efforts to introduce bio-fuels to power jet engines are on. Airbus/Rolls-Royce hybrid electric with gas-turbine engine will allow peak power for takeoff and climb while for 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. This will comprise harnessing the benefits of connectivity and big data to drive predictive maintenance, changes to technology embedded into aircraft, the coming revolution in full-electric or hybrid-electric power and other disruptors like additive manufacturing.