With the USA recently committing to net-zero emissions from aviation by 2050, the country’s top turbofan manufacturers are each pursuing multi-path strategies aimed at improving engine efficiency.
Those paths involve maturing several technologies simultaneously, with the goal of bringing various advances together into a new powerplant for narrowbody aircraft in 10 or 15 years.
It is no easy task. The path forward remains uncertain; the net-zero goal aspirational. But the engine makers insist they are committed to delivering substantial efficiency improvements.
“We are going to push every lever of technology,” says GE Aviation vice-president of engineering Mohamed Ali. “We want to make a step change.”
“We don’t see a single solution. We see multiple solutions,” says Michael Winter, senior fellow for advanced technology at Pratt & Whitney, a division of Raytheon Technologies.
Winter notes that other industries are getting cleaner fast, and that, without change, the aviation industry’s share of human-caused carbon dioxide output could surge from about 2.5% today to 18-20% in the coming decades. “This is a call to action,” he says.
On 9 November, the US government came in line with other countries in committing its aviation sector to achieving net-zero carbon output by 2050. Notably, the US government intends for industry to get there by converting exclusively to burning sustainable aviation fuel, the US Federal Aviation Administration (FAA) said in an accompanying report.
But the plan also assumes industry will develop aircraft that are 30% more efficient than today’s types, including new narrowbodies coming to market in 2035, and new widebodies in 2040.
The US government is supporting that work through NASA’s newly formed Sustainable Flight National Partnership, under which the agency has already awarded technology development contracts to GE Aviation, P&W and others.
While the engine makers are evaluating further-off technologies like hydrogen propulsion, their efforts focus largely on making traditional gs turbine engines about 20% more efficient. (That seems a smart move, as the FAA’s report predicts hydrogen and all-electric power for large aircraft will not be feasible for decades.)
Even a 20% improvement is asking a lot. But it is possible, says Ali. “That’s why we exist as engineers.”
The year 2021 might be called the “Year of Sustainability”, with seemingly every major aerospace player touting green goals.
In June, GE and partner Safran made a splash in announcing that their CFM International joint venture had launched a technology development programme called Revolutionary Innovation for Sustainable Engines (RISE).
Through RISE CFM aims to develop a narrowbody aircraft engine that is 20% more efficient, for service entry in the mid-2030s. Combined with aircraft improvements, efficiency gains could total 30%.
The RISE team is travelling three technological roads: developing an open-rotor engine (also called a propfan), a smaller engine core, and hybrid-electric technologies. Some 1,000 employees will be working on RISE by year-end, says Ali.
But the task is not a simple one: issues to overcome include blade-loss containment and noise.
GE has a long history of open-rotor development, having pursued such technology through its “unducted turbofan” (UDF) programme decades ago. It completed in-flight UDF demonstrations using a Boeing 727 in 1986 and a McDonnell Douglas MD-80 in 1987.
But to enable the UDF to operate at jet-like airspeeds, engineers gave the 1980s demonstrator contra-rotating fans, Ali says. Also, to meet noise standards, GE tweaked the design in ways that compromised efficiency.
“Even today, the UDF programme is still remembered within the community because it was a revolutionary concept in aero propulsion, and because the flight-test demonstrator was very loud,” a GE report from 2013 says.
This time it is different, Ali says. “Advances in computational aerodynamics” will enable GE to develop an open-rotor engine with a single rotating fan positioned ahead of fixed blades that meets noise and performance requirements, he says. “The concerns around noise are solvable.”
CFM intends to begin testing an open-rotor demonstrator around mid-decade, followed by flight tests.
At the same time, the RISE team aims to shrink the size of its next narrowbody engine’s core – to be the same size as cores in today’s business jet engines, says Ali. The smaller core will run hotter and at greater air pressures, yielding improved thermal efficiency. The design can also allow for increased bypass ratios.
To make it work, designers must use new materials capable of withstanding more heat. GE is evaluating advanced coatings and components made from ceramic matrix composites (CMCs).
“When the core runs hotter, the whole cycle becomes more efficient and consumes less fuel,” Ali says. “We need the advance cooling technologies and also the advanced materials.”
P&W has a similar effort under way to improve thermal efficiency. “We are focused on going to higher temperatures, smaller cores, using new materials… such as ceramic matrix composites,” says Winter.
The engine maker is also working to increase its next turbofan’s bypass ratio. Current narrowbody engines have ratios of about 12:1. P&W has run wind-tunnel evaluations of an 18:1 bypass ratio and has tested “full engines” with 15:1 ratios, Winter says. “Those technologies will be ready for the next generation.”
Both companies are advancing such technologies under NASA’s Hybrid Thermally Efficient Core (HyTEC) programme, under which GE recently won contracts valued at $12.2 million, while P&W won $6.6 million.
Through HyTEC, NASA aims to develop a 15:1 bypass engine that burns 5-10% less fuel, for 2030s service entry. It also wants to demonstrate that 10-20% of the engine’s power can be extracted as electricity – to power aircraft systems. The agency hopes to begin HyTEC ground demonstrations around 2026.
With corporate sister Collins Aerospace, P&W is also developing a hybrid-electric system composed of a 1MW motor. In partnership with the Canadian government, the companies aim to equip a De Havilland Canada Dash 8-100 turboprop with the system, and to conduct flight tests as soon as 2024. P&W forecasts efficiency gains of up to 30% for the Dash 8.
“We are also bringing these same components, but system-engineered in a new way… to a single-aisle [aircraft],” Winter says.
He foresees equipping a future turbofan with “a megawatt motor-starter-generator on the core of the engine, and another megawatt motor-generator on the lower-spool of the engine”.
“That’s worth about 5% efficiency,” Winter says.
GE is likewise developing a “megawatt-class hybrid-electric propulsion system”. Around mid-decade, it plans to perform ground and flight tests of the system using a modified Saab 340B with GE CT7-9B turboshaft engines.
“GE will systematically mature an integrated hybrid-electric powertrain to demonstrate flight readiness for single-aisle aircraft”, it said in October.
“We are very bullish on it,” Ali says of hybrid electric. “To make it safe, you have to go through a significant technology maturation programme.”
The engine maker recently won $179 million in NASA contracts to develop hybrid-electric technology as part of NASA’s Electric Powertrain Flight Demonstration (EPFD) programme. That project aims to “mature and transition integrated [electrified aircraft propulsion] technologies… for introduction into the US fleet no later than 2035”, the agency says.
“Different components of the system are being demonstrated separately, but when this becomes an engine programme… they are all going to converge and work together,” says Ali. “We are committed to exercise the full scale of every one of these technologies.”