Environmental need drives aviation’s great engine race

The cocktail of possible engine technologies and fuels that are going to propel an industry desperate to prove it can decarbonise becomes more extraordinary by the day.

The ingredients in this cocktail are bewildering, encompassing next-generation turbines, sustainable and hydrogen aviation fuels, hybrid, electric, fuel cells, and an intoxicating mixture of all of them.

For the engine makers researching a raft of technical solutions, a discipline that constantly absorbs millions of dollars, this is their day job. However, the task of developing the next generation of engines has arrived at the worst moment financially, with balance sheets swamped by the pandemic-induced crisis.

Nobody, however, has been tempted to take a hatchet to research and development budgets. Andy Geer, chief engineer, UltraFan at Rolls-Royce, acknowledges the challenge: “Our R&D strategy is very much unchanged by the events of the last 12 months, the only thing that has changed is the need for it to be delivered in perhaps the most cost-constrained environment that can be imagined.”

Perversely, the crisis could offer industry more clarity on its priorities. Government bailouts and stimulus packages are inexorably connected with accelerating environmental progress: what some dub the “green recovery”.

The manufacturers are aligning themselves accordingly. “Essentially the engine game has not changed, but it is the speed with which these technologies are going to be adopted because the industry has really changed,” believes Arjan Hegeman, general manager of advanced technologies at GE Aviation.

“In the past 12 months, our strategy has not changed, but rather has been confirmed by the ongoing public discussions, which are driving us to continue to intensively pursue our goals,” says Stefan Weber, senior vice-president, engineering and technology for MTU Aero Engines.

Central to these goals is the all-consuming and unaltered drive for better performance. In the old days, the target was obvious: a better turbofan than the previous generation, bringing double-digit fuel burn efficiency gains for airliners to bring lower costs.

And they are good at it. For example, CFM International brought in a 20% improvement with the CFM56, while its successor, the Leap, has delivered another 15%, explains Jerome Bonini, vice-president research and technologies at Safran Aircraft Engines.

But when will the next step-change be required? Guillaume Faury, Airbus chief executive, offered clarity in March during a Eurocontrol webinar, saying that a 2035 entry-into-service date for a next-generation aircraft “makes sense”. A formal launch would take place in 2027-2028, giving industry a lead time of five to seven years to mature the technology, he said.

Faury has also been enthusiastic that a future jet will feature hydrogen fuel in some form. Airbus is pushing hydrogen because it offers the most significant impact on carbon emissions reduction.

STARTING POINT

So, what will the next-generation engine be? Hydrogen is firmly in the sights of all manufacturers, but whether the fuel is Jet A-1, sustainable aviation fuel (SAF) or hydrogen, for all, the turbine engine is the starting point.

“What is obvious is that, for the foreseeable future, there will be no alternative to the aero gas-turbine – in further optimised form – as a propulsion system for large commercial aircraft,” says Weber.

“We have spent $10 billion on a technology that was moving in a direction we knew we had to go,” says Geoff Hunt, senior vice-president engineering & technology at Pratt & Whitney, describing the US manufacturer’s bet on its geared turbofan (GTF). “We have established what I think is a benchmark architecture that provides the flexibility to adjust as the industry looks towards climate change alignment.”

As a matter of course, P&W expects to be able to continue the trend of annual fuel efficiency improvements of 1% on average as programmes mature. “Continuing to improve on the GTF architecture is very much the near-term drive and that is consistent with the long-term strategy if you are going into SAF or hydrogen fuel or hybrid-electric,” says Hunt. The introduction of these fuels and technologies “will all base off that architecture”, he adds.

“The first pillar of our strategy is to define and build an ultra-efficient engine architecture,” says Bonini. Safran is working on different architectures, for example the work it has conducted into open rotor design, in preparation for the aircraft design the airframers will chose.

Like P&W with the GTF, CFM joint venture partners GE and Safran have a modern-generation engine, in their case the Leap, as a baseline propulsive system architecture.

Alongside this work there is heavy investment in lightweight, high-performance materials, and sophisticated cooling technologies, as the trend towards higher engine pressures and temperatures continues. “All these new technologies will work their way into the engine,” says Hegeman.

The conviction that a next-generation turbine is an essential pillar for “decarbonising aviation” is why R-R remains committed to its UltraFan programme, says Geer. “We plan to have that engine available around the turn of the decade and as it is a scaleable technology, it is suitable for both narrowbody and widebody new aircraft programmes,” he adds.

For its part, “MTU is currently concentrating on the WET [Water-Enhanced Turbofan] engine,” says Weber. This employs a heat exchanger to use the energy from the engine’s exhaust gas stream to generate additional power.

Whatever mix of architecture, materials or fuel is in the cocktail, the engine technologists interviewed by FlightGlobal all agree the target in efficiency terms for the next-generation engine is 20% and more.

“This generation needs to provide a major step change in efficiency to, firstly, reduce emissions by simply burning less fuel and, secondly, to enable alternative fuels spanning from SAF to electric to hydrogen by providing longer range through its increased efficiency and, as such, overcoming some of the disadvantages of alternative fuels in supply constraints and/or associated weight increases [batteries] or airframe drag increases from large storage needs [hydrogen],” Hegeman says.

The use of liquid hydrogen fuel, for example, will require an aircraft shape and size like the Airbus ZEROe blended-wing body design, because hydrogen is about four times the volume of jet fuel.

NEW FUELS TO BURN

Another clear strategic pillar to achieve decarbonisation is to develop engines that run 100% on low-carbon fuels, such as SAF and synthetic fuels, or liquid hydrogen, which would be zero-carbon, says Bonini.

Boeing agrees. It has set a target of 2030 for all its commercial aircraft to be certified to fly on 100% SAF. Today, the maximum is a 50:50 Jet A-1/SAF mix. There is work on fuel standards and the technical changes needed to run turbines at 100% SAF, but it is not a tough nut to crack and 2030 or probably earlier is achievable.

The prospect of burning liquid hydrogen in aero engines is more captivating. The first thing the engine makers point out is that hydrogen is not new to them. P&W ran a hydrogen engine in the 1950s, while GE ran one in the 1960s. Russian scientists flew a modified Tupolev Tu-154 using hydrogen fuel in the late 1980s.

“We understand the challenges and the opportunities of hydrogen. From an engine manufacturer’s perspective, we do not see we are the long pole in the tent to get into a hydrogen solution at the aircraft level,” says Hunt.

“Burning hydrogen inside an engine is not the issue, getting the hydrogen into the right conditions to be used is the challenge,” explains Bonini. In February, Safran announced the Hyperion study, where it will work with space launch vehicle maker Ariane and Airbus on turbines that burn liquid hydrogen.

Yes, the combustor will be different, as is the control system, but the biggest challenge is not how the fuel is burnt, it is upstream and integrating it into the aircraft design, says Hegeman.

Questions on storing the bulky hydrogen in the airframe and the infrastructure needed to deliver it to an aircraft are harder to answer. “The bigger challenge, similar to SAF, is the need to scale it up. And the need will be for green hydrogen,” explains Hegeman, referring to hydrogen produced using renewable power generation.

The view is that liquid hydrogen will be an attractive longer-term option with advances in current turbine technology and the use of SAF both providing near-term benefits.

“An ultra-efficient engine is still the key to the future – whatever fuel you burn,” says Bonini. Embedding electrical systems in such engines to create a hybrid configuration will deliver further levels of efficiency and represents the third strategic pillar of decarbonisation.

The target is to make hybrid variants of current models, like P&W’s GTF family and CFM’s Leap, where electrical power is generated to help provide or save energy in the engine. The aim of this work, says Weber, is “the most complete electrification of the drive train possible in order to be as emission-free as possible in flight”.

Many believe the concerted hybrid research efforts in play will yield benefits in the short- to mid-term. “A hybrid-electric variant of GTF is certainly something that we are studying,” says Hunt. Although it did last year slow Project 804, an effort to develop a hybrid-electric engine for a regional aircraft, starting with a De Havilland Canada Dash 8-100, this work has a “lot of merit” and is ongoing, he says. However, a flight demonstration planned for 2022 will slip.

At P&W, there is a benefit having sister Raytheon Technologies company Collins Aerospace in the family. “We work extremely closely with Collins, which has very strong capability in the electrical systems area,” says Hunt.

GE has been working on a basket of hybrid research efforts for over a decade, including a project in 2016 where 1MW of electrical power was siphoned off from a military F110 engine while also generating thrust. This much electrical energy could power a small, six- to 10-seat aircraft.

The years of foundational work spent on such research gives GE confidence it will have the answers. “We are running all of these systems towards a flight of a full hybrid propulsion system,” says Hegeman. “We are very far along that journey, with a ground demonstration of a fully built-up engine and flight demonstration of a twin aircraft planned within a couple of years.”

The hybrid technologies under development will have applications in turbines powering jetliners in the 100+ seat class, and there are dozens of smaller aircraft being touted for cargo, regional airline, and urban air mobility opportunities. Further down the size class there are many working on all-electric engine applications.

Another technology with promise, which MTU advocates for the longer-term, is the conversion of hydrogen into electricity with the help of fuel cells. It is studying this technology with the DLR German Aerospace Center. “These technologies could go into series production even before 2040,” believes Weber.

THE RIGHT TIME?

Ultimately, all the engine makers say they will respond with a powerplant when asked. “There isn’t any user knocking on our door today. When they do, we are going to have a product,” says Hegeman.

Today, the product opportunities appear to centre on a new narrowbody to succeed the Airbus A320 and Boeing 737 Max families, while Boeing chief executive David Calhoun has hinted that it might still develop an aircraft with around 270 seats for the mid-market segment.

Airbus has been the most progressive and aggressive, clearly signalling a desire to speed up moves to low or zero-emission aircraft, while Boeing has committed to its 2030 target for using 100% SAF on all its commercial aircraft.

With their existing positions on the A320 and 737, CFM and P&W appear well-placed, while R-R is relying on its UltraFan to force its way into the reckoning. “This is a new generation of gas turbine, with the capability to grow and adapt to become an entire engine family, designed for service around the turn of the decade, offering a 25% efficiency improvement compared with our first generation of Trent engine,” says Geer.

As Bonini explains, a new class of low-carbon engine will bring millions of dollars and years of research to fruition: “This is a great challenge, a huge effort, which is why we need the construction of other technologies, including SAF, hydrogen and hybridisation, to achieve the decarbonisation goals we have.”

Hegeman agrees. “I don’t think there is one single solution that is going to work. All these different technologies are going to be necessary across the whole spectrum of commercial aircraft,” he says.

With its open rotor technology, Safran claims it has already demonstrated a 15% fuel efficiency gain compared to the Leap. Moving away from a nacelle as an open rotor does is one of the options airframers will ponder as they deliberate aircraft configurations.

Perhaps an open rotor design is one of the answers. Perhaps not. Today, it is impossible to judge which technologies, in which order, or in what blend, will flourish. The manufacturers are obliged to spend big across a variety of bets, uncertain as ever if their strategy will be the one that ultimately succeeds.

The flag has truly dropped on this urgent technology race to discover a new low- or zero-carbon aviation engine nirvana. Without doubt, it is a contest the engine manufacturers are relishing. Their tradition says they almost always come up with the answer. They are convinced they will this time too.