Charles Alcock AIN Online
Engine Makers Explore Several Paths to Decarbonization
February 18, 2024
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  • With aircraft engine makers at the vanguard of efforts to decarbonize air transport, the next 10 years or so will see the biggest breakthroughs in propulsion technology since the dawn of the jet age. The big three turbofan manufacturers­—CFM International, Pratt & Whitney, and Rolls-Royce—are investing heavily in multistage work that starts with standardizing the use of sustainable aviation fuel (SAF) and progresses towards electric and hydrogen propulsion.

    Pratt & Whitney has begun pursuing several paths to introducing cleaner and more efficient engines between now and 2050. According to Frank Preli, the U.S. group’s vice president for advanced propulsion and materials technologies, the objective centers not just on cutting carbon dioxide, but other damaging emissions as well, including nitrogen oxides (NOx).

    In the near term, the company remains focused on making the GTF Advantage version of its geared turbofan available for the A320neo airliner. The engine would deliver an additional 1 percent in fuel efficiency compared with the current GTF, increasing to 17 percent its performance margin over earlier engines like the V2500.

    RTX Group Pushes for Hybrid-Electric Advances

    Beyond that, Pratt & Whitney, which is part of the RTX technology group, continues work on a hybrid-electric technology demonstrator it aims to start flight testing early next year in Canada. The powertrain will combine a 1-megawatt electric motor developed by sister company Collins with a 1-megawatt engine to achieve a 30 percent improvement in energy performance on a mission basis. Last month, Pratt & Whitney announced the development of a mobile charging unit to support the 1,500-volt batteries that Swiss firm H55 has committed for the project.

    Under a separate project called SWITCH, which is supported by the European Union’s Clean Aviation initiative, RTX is developing a hybrid-electric propulsion system for Pratt & Whitney’s geared turbofan family. This could be used for future narrowbody airliners.

    “We are talking about achieving five percent of the thrust capability using electric motors on each of the engines,” Preli told AIN. “This will enable us to alter the cycle by taking energy from one or both [of the electric motors or generators], which gives us very interesting capabilities so that we don’t have to completely rely on aerodynamic or thermodynamic cycles. The electric power can be used for taxiing, for instance, and to manage surge margins in the engine as you can separately change the high compressor and low compressor rotational speeds using the two motor generators.”

    The Grid, a new Collins-operated R&D center in Rockford, Illinois, plays a key role in the work. As an indicator of the scale of progress being made, Preli explained that the 1-megawatt generator developed for the RTX Hybrid Electric Flight Demonstrator will deliver four times the power and twice the voltage, with half the heat loss and weight as the 250-kilowatt generator developed a decade ago to provide the electrical power for Boeing’s 787 widebody. 

    While not excluding the possibility of hydrogen’s contribution, Pratt & Whitney views that mode of propulsion as a longer-term play in air transport. “We don’t expect hydrogen to be widely available for some time, and [to develop hydrogen propulsion systems] there are a lot of basic technologies that need to be developed first,” Preli said. “There are ongoing studies with airframers for hydrogen propulsion concepts, and we’re working very closely on the integration [of hydrogen engines and the aircraft].”

    Pratt & Whitney is also looking at two concepts to address different types of emissions from engine combustors, using water reclamation and steam injection in a hydrogen engine that could cut NOx emissions by 80 percent. “You can also get higher efficiency with the water cycle because you can operate the engine at a higher operating pressure ratio without an increase in temperature, which would limit materials durability,” Preli explained.

    CFM Banks on Rise Engine to Lower Carbon

    By mid-2025, CFM expects to start flight testing a technology demonstrator for its Rise open fan engine. The joint venture between GE Aviation and Safran targets at least a 20 percent reduction in fuel burn and carbon dioxide emissions compared with today’s Leap engines in time to support new narrowbody airliners from the likes of Airbus and Boeing.

    In January, Safran Aircraft Engines and France’s Onera aerospace research agency began wind tunnel tests with the EcoEngine one-fifth scale open fan demonstrator. The partners aim to mature the aerodynamics and acoustics of the fan module by exposing it to real-world wind speeds and validating the fan blade design.

    Since February 2022, CFM and Airbus have worked on a hydrogen demonstration program with the aim of flight testing a propulsion system on an A380 by the middle of this decade. This work contributes to Airbus’s ZeroE program, which centers on the development of a hydrogen-powered airliner able to carry around 200 people up to 2,000 nm by 2035.

    The demonstrator will involve the modification of a GE Passport business jet engine to run on hydrogen. The design calls for mounting the engine on the rear fuselage of a flying testbed.

    Rolls-Royce Expects UltraFan to Cut Fuel Burn

    At its Derby, UK, headquarters, Rolls-Royce continues testing the UltraFan technology it expects will deliver a 25 percent reduction in fuel burn when integrated with its Trent family of turbofans. The manufacturer plans to introduce a family of two-shaft, three-shaft, direct-drive, and geared propulsion systems ranging in thrust from 25,000 and 110,000 pounds for narrowbody and widebody airliners that could enter service in the 2030s. Working as part of the European Union-backed Clean Aviation program, the engine company expects to expand bypass ratios to improve the energy efficiency of turbofans through changes in architecture, such as shortening inlets.

    The group’s Bristol facility also is working on what it calls micro-hybridization, integrating an electric starter generator with a smaller engine. The approach involves making use of electrical power during some phases of flight to supplement improvements in energy efficiency.

    In September 2023, Rolls-Royce and partners conducted a ground test in which they ran a Pearl 700 business jet engine on 100 percent hydrogen fuel. The work, supported by the UK’s Loughborough University and German aerospace research agency DLR, forms part of longer-term efforts to develop a hydrogen combustion engine for future narrowbody aircraft.

    In late November, Rolls-Royce announced its intention to seek a buyer for the electrical division involved in developing battery-electric propulsion systems for new aircraft such as the Vertical Aerospace eVTOL model.