Only in the past few decades has the world woken up to the devastating effect that greenhouse gas emissions produced by the internal combustion engine are having on the environment.Carbon-based fuels in the form of petroleum and kerosene have continued to power most aero engines since the dawn of aviation, from the Wright Flyer on its historic first manned powered flight in 1903 to the big turbofans used by today’s A380s and Dreamliners. Commercial airlines around the world consume a staggering 370 billion liters of fuel a year according to the latest estimates by the National Research Council Canada, with air travel accounting for over two per cent of global carbon dioxide (CO2) emissions annually.
Commercial airlines around the world consume a staggering 370 billion liters of fuel a year
CO2 is a ‘greenhouse gas’, a byproduct of burning fossil fuels to produce power. It is released with exhaust gas emissions into the atmosphere and allows the sun’s rays to pass through it but, like a greenhouse, CO2 prevents the heat that the sunlight brings from escaping again – hence the moniker ‘greenhouse gases and the phenomenon called global warming.
The aerospace industry has set the ambitious target for the year 2050 to reduce its net greenhouse gas emissions by 50 per cent compared with 2005 levels – but at present it is not on track to achieve them.
However, these facts conceal a hive of activity in the development of the latest technologies that will help to reach this objective.
A step in the right direction
Of course, existing new technologies in the industry are already having a positive impact on reducing harmful emissions: high by-pass ratio (HBPR) turbofan engines like CFM International’s LEAP use less fuel, while winglets (vertical extensions of wingtips) improve the airflow around the wing, reducing fuel burn and increasing cruising range. Increased use of carbon fiber composite materials in airframe construction are displacing conventional metals and have made significant weight reductions, leading in turn to savings in fuel.
The need to reduce greenhouse gas emissions has been recognized and accepted by the global aerospace industry and this is only the beginning of a seismic shift towards greater sustainability. Uninterrupted development of advanced technologies on a broad front is needed to meet the target of reduced net emissions by 2050, encompassing engine design, future technologies like electrification, sustainable fuels, and airframe composite materials. There’s also a growing consumer awareness about the environmental impact of air travel that in helping drive this change of focus.
Modern HBPR turbofans like the Rolls-Royce Trent XWB are among the most efficient flying in the world today, while the company’s Trent 1000, in service on the Boeing 787 Dreamliner, has already undergone tests using 100 per cent sustainable aviation fuel (SAF) on Rolls-Royce’s flying testbed. But why stop at the tried and tested gas turbine?
Advanced technology for engines embraces several different areas of propulsion, like electrification. “In aviation, there are opportunities for hybrid-electric and all-electric applications, particularly in the urban air mobility (UAM), commuter and regional markets,” said Rolls-Royce. “Through our ACCEL project and with support from the UK government, we are aiming to build the world’s fastest all-electric plane.”
The company’s zero-emission Spirit of Innovation single-seat electric aircraft is powered by a unique 6,000-cell battery pack. It has a range of about 200 miles on a single charge and a target speed of at least 300mph (480kph).
“Rolls-Royce Electrical has achieved key product advances with a world speed record for our all-electric aircraft. The characteristics that ‘air-taxis’ require from batteries are very similar to what was developed for the Spirit of Innovation so that it could reach record-breaking speeds. Rolls-Royce will be using the technology from the ACCEL project and applying it to products for the market,” commented Rolls-Royce. “We also are working on an electric aircraft charging & service and airport infrastructure portfolio to complement our offering to the advanced air mobility markets.”
An innovative approach is being taken by the aircraft leasing sector to support the aviation industry in its transition to net zero carbon emissions. Timothy Eyre, Investment Director at London-based Montrose Global Aircraft Management, explained: “20,000 aircraft are currently suitable for retrofitting with electric/hybrid propulsion technology. Monte is seeking to pioneer the clean technology market and provide financing to support and facilitate the retrofitting of existing aircraft,” he said. “We are working with a number of global organizations that are pioneering the retrofitting of existing aircraft with zero emission propulsion technology.”
Nate Boelkins, vice president of electric power systems at United Technologies’ Collins Aerospace, said the company is “exploring potential hybrid-electric and all-electric solutions for rotorcraft and urban air mobility (UAM) platforms. Hybrid-electric and all-electric propulsion hold significant potential benefits for rotorcraft and UAM platforms, including reductions in noise, fuel consumption, carbon emissions, and operation and maintenance costs.”
The open rotor or ‘prop fan’ concept, which is an evolution of the existing HBPR turbofans, has been around since the 1980s. It tends to resurface when oil prices are high, so engine builders are dusting off their open rotor designs once again.
By eliminating the need for a fan duct in the conventional cased turbofan engine, open rotor engines can offer a high propulsion efficiency while saving 25 to 35 per cent in fuel consumption compared to most current turbofan engines in service, and a 10 to 20 per cent saving for the newer turbofans recently introduced into the market.
GE, which had developed a single open rotor concept, combined this with French engine builder Safran’s open rotor know-how to produce an open rotor with simplified operation. In June 2021, GE launched the CFM RISE (Revolutionary Innovation for Sustainable Engines) program targeting 20 per cent lower emissions (GE and Safran together are CFM International). Led by a joint GE/Safran engineering team, the program will span some 15 years and include open fan architecture (open rotor) and hybrid-electric capability, with engines that could enter service by the mid-2030s.
The biggest issue for the open rotor concept has been one of excessive rotor noise, which is something that GE/Safran claim to have conquered. “Improvements in material technology and digital modelling have enabled the partners to eliminate the significant size, weight and noise penalties inherent in those earlier designs,” said Arjan Hegeman at GE Aviation.
RISE engines will be ‘fuel agnostic’ and developed in jet fuel/SAF and hydrogen-burning versions, with the potential to achieve at least a 20 per cent additional improvement in fuel efficiency compared with today’s state-of-the-art single-aisle aircraft engines.
UltraFan – the world’s largest fan engine
Rolls-Royce is working on its own open rotor engine concept, but the company’s focus is on its Advance 3 and UltraFan developments. The Advance program will also play a part in future UltraFan (set to become the world’s largest aircraft engine and an important step forward on improvement of the gas turbine) and possible open rotor developments. Work began in 2021 on building the UltraFan demonstrator, intended to power narrow and widebody aircraft, delivering a 25 per cent fuel efficiency and lower emissions compared to the Trent. “The first full demonstrator test engine is now well advanced in assembly and will test at sea level in Derby, England, around the middle of 2022,” said a company spokesperson. It will run on 100 per cent SAF and has been designed to produce fewer emissions all round.
Sustainable Aviation Fuel (SAF)
Sustainable Aviation Fuel (SAF) will play a crucial role in meeting the aviation industry’s greenhouse gas emissions targets, but it should be considered as one of several essential gadgets in the toolchest to help achieve this goal. Balkiz Sarihan, Head of Urban Air Mobility (UAM) Strategy Execution and Partnerships at Airbus, reinforces this important point: “We must use all the gears. Not one lever alone will get us to decarbonization.”
While development is ongoing of electrification and hydrogen technologies as long-term alternatives to fossil fuels, SAF is a low-emission near-term answer to the decarbonization of air travel. SAF is produced from feedstocks like cooking oil and other non-palm waste oils from animals or plants, as well as solid waste from homes and businesses, such as packaging, paper, textiles, and food scraps that would usually go to landfill or incineration – Air bp’s SAF is currently made from used cooking oil and animal waste fat.
SAF can be mixed at up to 50 per cent with traditional jet fuel and the blend is then re-certified as Jet A or Jet A-1. It has the potential to provide a carbon reduction of up to 80 per cent compared to the traditional jet fuel it replaces, and it can be used in the same way – a so-called ‘drop-in’ fuel.
A full ‘drop-in’ option describes an alternative fuel that is completely interchangeable and compatible with a particular conventional fuel – in this case petroleum-derived, which means no changes are needed in the fueling infrastructure or for an aero engine wanting to use SAF. At the time of writing, however, SAF is only certified for blends with conventional jet fuel of up to 50 per cent.
In 2008, Virgin Atlantic became the first airline to conduct a ‘bio-fuel’ test flight using a commercial aircraft when it flew a Boeing 747-400 from London-Heathrow to Amsterdam-Schipol. More recently, Rolls-Royce demonstrated that current engines for large civil and business jets can operate with 100 per cent SAF as a full ‘drop-in’ option, laying the foundation for moving this type of fuel towards certification. An Airbus A380 test aircraft took off from Toulouse-Blagnac airport on 22 March 2022 and in a flight lasting about three hours, one Rolls-Royce Trent engine was operating on 100 per cent SAF. Pratt & Whitney announced on 15 March 2022 that it had successfully tested its new GTF (Geared Turbo Fan) Advantage engine configuration with 100 per cent SAF, with entry into service expected in 2024.
On 9 November 2021, an Airbus H225 performed the first ever helicopter flight with 100 per cent SAF powering one of the Safran Makila 2 engines. “While all Airbus helicopters are certified to fly with up to a 50 per cent blend of SAF mixed with kerosene, it is our company’s ambition to have its helicopters certified to fly with 100 per cent SAF within the decade,” commented Stefan Thomé of Airbus Helicopters.
The H225 test helicopter flew from Marignane with an unblended SAF derived from used cooking oil, provided by TotalEnergies, which offers a net 90 per cent CO2 reduction compared to regular jet fuel.
Safran Helicopter Engines and Bell Textron Inc have also announced that they are collaborating on an initiative to explore technical performance and economic impacts of SAF on the Arrius 2R-powered Bell 505, while the Bell 525 has trialled SAF in its flight test program.
The airborne special missions sector is moving towards lower emissions and sustainability through the use of SAF. UK-based Capital Air Ambulance made its first air ambulance transfer flight from Bristol Airport to collect a patient from Carcassonne, France, on 24 March 2021 using an SAF blend in its King Air 2000. In Austria, ÖAMTC air rescue is working with Safran, Airbus and Air bp in trialing the use of SAF in its air ambulance helicopters, while German air ambulance provider ADAC Luftrettung is extending use of SAF to its fleet of helicopters powered by Pratt & Whitney Canada (P&WC) engines.
Canadian operator Blackcomb Helicopters has purchased the equivalent amount in carbon offsets to mitigate its greenhouse gas emissions during each hour of flight from its helicopters. One carbon credit represents a measured reduction of one metric tonne of CO2 or its equivalent in other greenhouse gases.
However, there is a downside to SAF, and that is it’s currently more costly to produce than traditional fossil jet fuel, but it is hoped that as the technology ripens it will become more efficient and less costly for customers.
eFuels – a climate-neutral option
eFuels or electro-fuels are produced in a process called ‘Power-to-X’ (PtX) using electricity from renewable energy sources, water and CO2 from the air, and have an important role to play in the transition to net zero. Unlike conventional fuels, they do not release additional CO2, but are climate-neutral and compatible with today’s internal combustion engines. e-fuels such as e-hydrogen, e-methane, e-methanol, e-diesel or e-ammonia can be converted into electricity and propulsive power. Rolls-Royce stated: “We are currently working with customers, industry partners and scientific institutions on various initiatives to examine the entire PtX eco-system – from electricity provision to fuel production and finally its use in various applications.”
Hydrogen – a more efficient long-term solution?
Hydrogen could be the best long-term solution to the environmental concerns associated with carbon-based fuels used in most aircraft, but again, like SAF, cost is currently a barrier to its widespread use. Political willpower is needed to drive investments in time and money into developing the technology.
Hydrogen is unique as an energy carrier and can be created from water and electricity, allowing it to be produced from clean, renewable energy sources. Hydrogen fuel cell aircraft would be completely free from pollutant exhaust emissions, including particulates, oxides of nitrogen, carbon monoxide, and eliminating CO2 and reducing nitrogen oxide (NOx) emissions by 80 per cent.
In shorter-range segments hydrogen power will offer greater possibilities
The world’s big aero engine manufacturers – CFM, GE, Pratt & Whitney and Rolls-Royce – are all committing to the development of hydrogen combustion engines.
Rolls-Royce is developing a range of hydrogen products based on fuel cells and hydrogen combustion engines: “Hydrogen combustion or fuel cells offer a route to lower/zero emissions, however, there are still many technical challenges to be overcome for use of hydrogen in aviation. While these challenges look to be overcome, we don’t expect such solutions to be commercially viable before the 2030s,” commented Rolls-Royce.
The mechanics of making a turbofan run on hydrogen is relatively straightforward and represents the most practicable way to reduce air transport emissions of greenhouse gases. However, storage and transportation of hydrogen is more complex than that required for fossil fuels, which is where some of the cost and range penalties lie. Challenges with onboard storage of hydrogen and the limited power density of fuel cells means that for long-range flight, SAF driving gas turbines will probably remain the most likely solution for the time being. However, in shorter-range segments hydrogen power will offer greater possibilities.
In February 2022, Airbus and CFM International signed a partnership agreement to collaborate on a hydrogen demonstration program that will take flight around the middle of this decade. Airbus unveiled three concept aircraft that would rely on hydrogen as a primary power source and would ‘help us explore and mature the design and layout of the world’s first climate-neutral, zero-emission commercial aircraft, which we aim to put into service by 2035,’ said CEO Guillaume Faury. “This is the most significant step undertaken at Airbus to usher in a new era of hydrogen-powered flight since the unveiling of our ZEROe concepts [hydrogen-fueled zero emission aircraft] back in September 2020,” said Sabine Klauke, Airbus Chief Technical Officer.
In the same month as Airbus and CFM made their announcement, Pratt & Whitney reported that it had been selected by the US government to develop high-efficiency hydrogen engines for use in commercial aviation. The US$4 million Hydrogen Steam Injected, Inter‐Cooled Turbine Engine (HySIITE) project using liquid hydrogen combustion and water vapor recovery aims to eliminate carbon emissions and reduce nitrous oxide (NOx) inflight emissions by up to 80 per cent for commercial single-aisle aircraft. Geoff Hunt, Senior Vice President of Engineering and Technology at Pratt & Whitney said, “This truly is an exciting opportunity to start developing the key technologies that could bring the industry’s first hydrogen steam injected, inter-cooled engine from concept to reality.”
Sustainability in aviation is continued in next month’s issue with a look at how composite materials and smarter operating practices are contributing to a reduced carbon footprint.