How are engine makers addressing the decarbonisation challenge?

Today every self-respecting aerospace engine manufacturing executive has a cool roadmap to hand, charting the pathway, over the coming two decades, towards a decarbonised air transport industry. The grand mission is to achieve net zero carbon emissions by 2050.

Important waypoints along the journey are a massive increase in the use of sustainable aviation fuel (SAF) and to make decisive progress on “disruptive technologies” such as hydrogen. These are vital ingredients in parallel with the ever-present desire to reduce fuel burn and deliver engines with improved efficiency.

As Eric Dalbiès, senior executive vice-president research & technology and innovation at Safran told the EU’s Clean Aviation Summit in March: “For the coming decade the focus is on ultra-efficient aircraft… educing fuel burn is a no-regret choice.”

The speed at which sustainability has rocketed to the very top of the aerospace agenda is astonishing. Equally remarkable is the fact it has taken place during a global pandemic that saw the air transport industry grind to a halt.

The world’s urgent need to tackle climate change may be argued to have been a welcome call to arms for aviation when it was at its lowest ebb.

Pre-pandemic, the air transport industry was in rude health, bathing in a decade of unbridled growth. However, at the end of the twenty-tens decade the industry was showing signs of significant growing pains. In 2019, FlightGlobal’s annual commercial engines report saw the manufacturers facing criticism over design issues and in-service problems. In addition, with bulging orderbooks, the engine makers were in a steep production ramp-up bringing strains on the supply chain and contributing to delivery delays.

DESIGN ISSUES

Each of the big three had significant issues in the years running up to the end of the decade. However, by 2019, Rolls-Royce was getting to grips with the premature turbine blade deterioration problems suffered by the Trent 1000 engine, which is an option for the Boeing 787 widebody. Similarly, Pratt & Whitney’s Geared Turbofan (GTF), the manufacturer’s big bet on returning to the narrowbody powerplant market, was, after three years in service, overcoming its early high-profile engine issues.

At GE Aviation, the story was different, but no less troublesome. Its GE9X engine was selected by Boeing as the sole powerplant choice for the 777X in 2013. Issues with components in the high-pressure compressor, which came to light in 2019, caused a re-design that delayed the first flight of the 777X. That was scheduled for 2019 with first deliveries to airlines of the initial 777-9 version in 2020. While GE has fixed the engine issue, Boeing itself now expects the aircraft to enter service in 2025 under the current certification timetable.

Despite these hiccups, the engine makers entered 2020 in good health, with record orderbooks and strong aftermarket revenue flows. That changed abruptly with the onset of the global pandemic in March that year. As revenues from the all-important service contracts, which are based on how much engines are flown, fell off a cliff, all the manufacturers shed staff, raised liquidity, and restructured their businesses to cut losses and preserve cash.

For example, in August 2020 Rolls-Royce announced that it would reduce the number of roles in its Civil Aerospace business by 8,000, about a third of the pre-Covid-19 total.

After two years of eye-watering losses and unprecedented business trauma, the recovery in air travel is very welcome as the engine majors seek a return to normality. Revenues are flowing again as flying hours rise and maintenance shop visits return.

“We’re currently planning for improved GE CFM departures, which we expect to average down about 10% off 2019 levels and total year shop visits to be up about 20% year-over-year,” explained Lawrence Culp, General Electric Company chairman during its earnings call in January 2022. “We expect revenue will increase more than 20% driven by strong worldwide shop visit growth and the ramp of Leap engine deliveries.”

As business revives, and the discipline of delivering hundreds of highly sophisticated engines annually returns, the strategic landscape has changed for the propulsion experts. The decarbonisation challenge has become an overwhelming obligation. Every manufacturer was deeply absorbed in R&D to find more efficient, lower emissions engines prior to the pandemic. Now that occupation has turned into an obsession. The manufacturers are quick to stress that despite the pandemic no-one cut back their expenditure on R&D to any great extent. The technologies under scrutiny include electric, hybrid-electric, and engines powered by liquid hydrogen or hydrogen fuel cells.

Speaking at GE Aviation’s investor day in March, Mohamed Ali, vice-president of engineering, talked about the great strides made in breakthrough technologies and materials to enable next-generation engines like the GEnx and Leap. Ali highlighted the introduction of composite fan blades to replace metals on the GE90, the creation of highly durable ceramic components for Leap, and the invention of additive manufacturing technology to produce hitherto “impossible-to-make” lightweight parts.

FUTURE ADVANCES

“We are excited about building our arsenal of technologies for the future with sustainability as our north star,” said Ali. These advances will enable GE Aviation to reduce fuel burn by more than 20%, whether the fuel is kerosene, SAF or hydrogen, he added.

The key component of bringing GE Aviation’s roadmap to fruition is a batch of “breakthrough technology demonstrators” with ground and flight tests to show technology readiness this decade, said Ali. This is the critical timeline to meet the countback in years for the Airbus and Boeing ambitions for next-generation aircraft using “disruptive” technology in service from 2035.

GE has three demonstration programmes lined up. The first is a partnership with NASA and Boeing, with BAE Systems recently added to provide electricity management systems. Through NASA’s Electrified Powertrain Flight Demonstration (EPFD) project, GE will test hybrid electric configurations on a modified Saab 340B turboprop with GE CT7-9B engines to help prove out the technology. It says the programme will fly a full hybrid electric aircraft by the mid-2020s.

“Anybody can do a motor – a hybrid electric motor or an electric motor and test it on the ground,” said Ali. “Anybody can fly perhaps even up to 10,000ft. Above 10,000ft, high-voltage electric machines behave very differently. We are testing in collaboration with NASA at the NASA facility, a megawatt electric motor in a 40,000ft environment… and we believe we have the technology to enable that.”

In February, Airbus and CFM International announced one of the most significant moves by the aerospace majors to date on the hydrogen front. Airbus will use an A380 as the flight-test demonstrator for a future hydrogen-fuelled engine. The aim is for first flight by the end of 2026, says Sabine Klauke, chief technology officer of Airbus.

CFM will modify the combustor, fuel, and control system of a GE Passport turbofan to run on hydrogen. The engine was selected because of its physical size, advanced turbo machinery, and fuel flow capability. It will be mounted along the rear fuselage of the A380 testbed to allow engine emissions, including contrails, to be monitored separately from those of the engines powering the aircraft.

The physical property of hydrogen means it has many challenges to become a viable liquid fuel for either gas turbine combustion engines or to make electricity in a fuel cell, however many players in aviation believe it has a future role in the decarbonisation picture. “There are only a limited number of ways of getting to net zero emissions,” explains Arjan Hegeman, general manager advanced technologies at GE Aviation. “Hydrogen combustion does get to zero carbon emissions, so it is a logical thing to look at.”

COLLABORATIVE EFFORT

As the first ground testing on this programme begins this year, GE’s third demonstrator using technology called adaptive cycling is already running. This is being conducted in collaboration with the US Air Force to develop the XA100 adaptive cycle engine for the F-35 fighter. The latest phase of tests began in March at the USAF Arnold Engineering Development Complex in Tennessee.

“An adaptive cycle means the engine actually changes its geometry depending on which part of the mission it is in to maximize the fuel burn advantage for that mission,” said Ali. It has the potential to give the “best of both worlds”, switching between high thrust and efficiency with the promise of 10% more thrust and 25% better fuel efficiency compared to today’s engines.

According to Ali: “We are going to be taking all of these technologies and putting them in what we call the RISE demo, where RISE stands for Revolutionary Innovations for Sustainable Engines.” This demonstration programme, launched in June 2021 by CFM, is aimed at developing open fan powerplants that can be powered by 100% SAF or liquid hydrogen and including hybrid electric capability for the next generation of single-aisle aircraft from 2035. The target is to reduce fuel consumption and CO2 emissions by over 20% with a flight demonstration engine planned for mid-decade.

As GE, Safran and CFM look to the future with a suite of technology showcases, the hard work of building, certifying, and servicing its current products is constant. During the production slowdown induced by the pandemic, the focus was on improving the production process to reduce costs and key part losses on engine manufacturing. This has resulted in a 40% reduction in the cost of making a Leap engine since it was introduced in 2016 as the partners make “progress” towards Leap breakeven by 2025.

Then there is the challenge of raising production rates to meet the output demands of Airbus and Boeing. In early May, Airbus said it aims to reach a rate of 75 A320s monthly in 2025. According to Olivier Andries, speaking at the GE Investor Day: “In the short term, we are focusing on executing the ramp-up, the new ramp-up of the Leap on a trajectory to deliver more than 2,000 engines by 2023.” GE is also preparing for the ramp up of the GE9X that powers the delayed 777-9.

Pratt & Whitney ticked off a noteworthy milestone in late April with the delivery of the 1,000 Airbus A320neo powered by its GTF. The customer was European low-cost player Wizz Air, which now has 54 GTF-powered examples. The GTF is fast becoming the cornerstone engine in P&W’s portfolio and is its key development platform for next generation powerplants. The company says it has 10,000 orders and commitments for the GTF with 80 customers.

P&W’s roadmap to net zero has three core themes: smarter technology; clean fuels; and greener business, says Webb. Under the smarter technology banner come projects such as the Hybrid Thermally Efficient Core (HyTEC). P&W was selected by NASA in October 2021 for HyTEC to develop advanced high pressure turbine technologies for next generation single aisle aircraft.

These include ceramic matrix composite (CMC) materials capable of operating at higher temperatures than current CMCs, environmental barrier coatings, and advanced cooling and aerodynamic approaches that will enable new component designs and efficiencies, according to P&W.

HyTEC is part of NASA’s Sustainable Flight National Partnership, which is intended to enable breakthrough innovations and help accomplish the industry’s decarbonisation goals.

Using a cocktail of advanced fan technologies, new core development, increased use of hybrid electric to augment the engine, and more efficient propulsion-airframe integration, P&W will build the future GTF, the firm’s first chief sustainability officer Graham Webb, explained at the Sustainable Skies World Summit, hosted by Farnborough International Airshow in early April.

In December, P&W launched the GTF Advantage configuration, the next iteration of the engine, which has technology enhancements throughout the core. After completing a year of ground and flight testing it will be available for A320neo family aircraft from January 2024 offering greater thrust and a 1% increase in fuel efficiency.

P&W’s key demonstrator in the hybrid field is work led by Pratt & Whitney Canada which is partnering with De Havilland to equip a Dash 8-100 turboprop with a hybrid electric propulsion system. P&W’s roadmap sees hybrid electric technology coming into service from 2030.

Flight tests with the Dash 8 are scheduled to start in 2024 with ground tests slated for this year, said Webb. The target is a 30% reduction in fuel burn and CO2 emissions compared to today’s turboprops. A key partner in this project is P&W’s Raytheon Technologies stablemate Collins Aerospace which is providing the electric motor and controller.

P&W says this project will provide technology and component learnings with direct read across to larger applications. Although it has not announced a specific programme yet to augment the GTF with hybrid electric power it is in the plan. The thinking is to marry a 1 MW electric motor with the GTF to “enhance the flight operations” of a single-aisle airliner, said Webb. With a maximum power output around 20 MW, the GTF-scale hybrid-electric propulsion system would be an order of magnitude larger than the 2 MW of power being delivered for the Dash 8 project.

P&W’s roadmap, which is naturally in sync with the Airbus vision of a zero emissions aircraft in service, sees it ready to field a hydrogen-fuelled engine for a 100 plus seat airliner from 2035. Like its competitors, the company is accelerating its hydrogen research.

In February, it was awarded a US Department of Energy project called the Hydrogen Steam Injected, Inter-Cooled Turbine Engine (HySIITE). It is described as a revolutionary hydrogen combustion system that uses water vapour recovered from the exhaust stream to increase engine efficiency promising a reduction fuel consumption for next generation narrowbody airliners of 35% compared to the GTF. There are certainly challenges with delivering on liquid hydrogen as a fuel for aviation, but Webb sees the opportunity too.

FURTHER STUDY

“There is a lot more work and study to be done but we look at hydrogen as a promising fuel,” said Webb.

While hydrogen fuel grabs many headlines, the industry’s drive to increase SAF use is critical to achieve net zero. In March, P&W tested the GTF Advantage configuration with 100% SAF in what it describes as a key milestone toward 100% operation of GTF-powered aircraft with these fuels. Today SAF is approved in blends of up to 50% with regular kerosene.

The three big themes in the Rolls-Royce strategic drive to net zero are a step change in the efficiency of gas turbines, leading SAF demonstrators and adoption, and developing third generation technologies, the company’s chief technology officer Grazia Vittadini explained at the Clean Aviation Summit. “Engines are at the core of the decarbonisation challenge… and are the most impactful,” she said.

Of the engine majors, Rolls-Royce is exploring the widest range of potential power and propulsion technologies and applications, from small propeller aircraft and advanced air mobility vehicles, right up to widebody airliners and large business jet aircraft.

All-electric power will be viable for smaller aircraft with short-range requirements and is a relatively mature technology, says Rolls-Royce. In November, it flew the “Spirit of Innovation”, a high-speed demonstrator that set two new world speed records for an all-electric aircraft. This effort was part of the UK Government’s Accelerating the Electrification of Flight project and the advanced battery and propulsion technology developed has applications for the advanced air mobility market, says Rolls-Royce.

On the electric front it is also the engine partner with Italy’s Tecnam on the 11-seat P-Volt utility aircraft. It will feature two Rolls-Royce electric powerplants of 320kW each with Norwegian regional airline Wideroe set to take the first examples in 2026.

Rolls-Royce has created a division dedicated to furthering its efforts in the electric engine space. In addition to its work with Tecnam, another key programme for Rolls-Royce Electrical is providing the technology to power Vertical Aerospace’s 4-seat VX4 vertical take-off and landing vehicle. This is slated to be certified in 2024 with service entry soon after.

Rolls-Royce’s research into potential propulsion pathways encompasses hybrid electric, hydrogen fuel cells, and gas turbines burning hydrogen, with likely applications up to regional and narrowbody aircraft. As the aircraft move up in size, from narrowbody and to widebody territory, the gas turbine is the clear favourite. “There is still life in the gas turbine. Whether fuelled by kerosene, SAF or hydrogen, we need to invest in the basic efficiency of the gas turbine,” Alan Newby, director of aerospace technology & future programmes at Rolls-Royce, said at Clean Aviation.

The key demonstrator in the Rolls-Royce gas turbine world is its UltraFan engine design. UltraFan features a new engine architecture, material, and a new power gearbox to prove “all the building blocks that will go into a next generation engine”. Engine UF001 is now being built and will be tested with 100% SAF this year, said Newby. Rolls-Royce says the engine will be available in the second half of the 2020s and will be 25% more fuel efficient than a first-generation Trent powerplant. Although initially sized for a widebody jet, UltraFan would be scalable for narrowbody aircraft.

As this article describes, the engineers at the engine manufacturers are busier than ever. Vittadini summed it up well describing the technologies being explored as a “buffet” from which the engine makers will choose the tastiest to meet the low-emission aircraft applications coming down the line. “Revolutionary breakthroughs are required for aviation,” she said. “There is quite a series of daunting challenges.”

By the end of this decade, we will know which buffet items have the most promise for these pioneering aircraft. The biggest unknown is whether “disruptive” technologies like hydrogen will be one of the ingredients or whether super-efficient gas turbines burning SAF will be the right choice.