Sean Bradshaw, Engineering Fellow, Sustainable Propulsion, on net-zero carbon emissions aims.

Aviation is committed to net-zero carbon emissions by 2050. Propulsion systems will clearly play the major part in achieving that aim and Pratt & Whitney is at the forefront of industry innovation.

The core of the company’s work is its GTF engine family. GTF engines are 16-20% more efficient than the engines that they replace, a considerable step forward in sustainability as more efficient engines mean lower emissions. The GTF Advantage engine for the A320neo family will build on this leadership while also delivering greater thrust.

Nevertheless, other factors must come in to play if air transport is to be carbon neutral.

In the short term, the goal is flying on 100% sustainable aviation fuels (SAF). There are seven SAF blends certified for use on aircraft, some up to a 50% blend with normal jet fuel, with more SAF technical pathways on the cards.

For Pratt & Whitney, readiness is the key. To this end, they have already tested the GTF Advantage engine on 100% SAF. “There was no change to thrust or operability and no issues with any of the hardware in the engine,” says Sean Bradshaw, Engineering Fellow, Sustainable Propulsion, Pratt & Whitney. “All results were as expected, which indicates that GTF engines will be fully compatible with all 100% SAF approved by ASTM International.”

Over 16 years of testing SAF, Bradshaw reports, Pratt & Whitney has not identified any impact on hardware or changed engine maintenance requirements, even at levels beyond the approved blends. One hundred percent SAF also contains about 2% more energy per kg compared with conventional kerosene, equating to a 2% reduction in fuel burn and associated emissions.

But SAF represent just 0.1% of global aviation fuel use. The expectation is to reach 2% by 2025 and 5% by 2030. The goal in the United States is to double this 2030 figure.

“The technology works,” says Bradshaw. “SAF are now an economic decision.”

Synthetic fuels—most notably electrofuel or power-to-liquid (PtL) fuel—will complement SAF efforts to scale the production of SAF produced from biological feedstocks. PtL is a mixture of captured CO2 and hydrogen. A hydrogen molecule is produced using green technologies and combined with CO2 through a carbon capture process. This generates drop-in liquid hydrocarbon fuel.

The Air Transport Action Group’s Waypoint 2050 says such fuels will be critical to the industry achieving its net zero target by 2050.

Hybrid-electric engines

Beyond SAF, the next step in sustainability is likely to be hybrid-electric engines, which could be operating on single-aisle aircraft like the Airbus A320 family by the mid-2030s.

Pratt & Whitney is working with De Havilland Canada to demonstrate the technology on a Dash 8. The project includes an advanced electric motor and controller from Collins Aerospace. The expectation is a significant improvement in aircraft efficiency across all phases of flight. Reductions of up to 30% in fuel burn and CO2 emissions compared with a modern regional turboprop are anticipated. Pratt & Whitney is targeting ground testing in 2022, with a flight test scheduled for 2024.

One of the most interesting aspects of the project is increased hybridization to a 50/50 mix. Combined with a number of electric initiatives in general aviation, it means the industry is constantly developing the potential of the technology.

Bradshaw doesn’t see this progressing to fully electric single-aisle aircraft any time soon, however. “I don’t think we should expect to see a fully electrified A320 flying across the Atlantic,” he says. “The energy density of a battery relative to jet fuel is far too low. There simply isn’t enough power for an aircraft to travel any significant distance.”

Hydrogen

The big hope, therefore, is hydrogen. “The more we look at it, the more potential we see,” says Bradshaw. “There is no CO2 in the tailpipe, no particulates to affect air quality, and it may even mitigate the impact of contrails, although more research will be needed.”

With an engine designed to run on the appropriate thermodynamic cycle, hydrogen offers a 30-35% efficiency improvement.

Pratt & Whitney has been selected by the US Department of Energy (DoE) to develop hydrogen-fueled propulsion technology for commercial aviation, as part of its Advanced Research Projects Agency-Energy (ARPA-E).

The work is known as the HySIITE (Hydrogen Steam Injected, Inter-Cooled Turbine Engine) project. Cryogenically stored liquid hydrogen is heated by an exhaust recovery system before it enters the combustion chamber. This creates water vapor, which is converted to steam and reinjected back into the chamber. The approach not only boosts power but also reduces NOx emissions up to 80%.

Also, because the hydrogen is stored at cold temperatures, less cooling of the engine is necessary, making it more efficient.

There are challenges. Most obvious is the need for global, green hydrogen infrastructure. Most hydrogen is derived from fossil fuel, but it needs to originate from hydro, solar, or wind power. Its distribution is also troublesome as hydrogen requires more power to transport due to its low density. This low density also means its storage on aircraft requires three or four times more volume than normal jet fuel. And as the lightest gas in the universe, hydrogen also has a tendency to leak.

“There is a lot of work to be done but we can overcome these challenges,” says Bradshaw. “[We] will continue to work with the industry to advocate for the necessary infrastructure changes and to ensure hydrogen propulsion is a safe, efficient alternative to liquid hydrocarbons by the mid-2030s.”

Conclusion

There is no silver bullet to solve aviation’s sustainability challenges. “Rather, it will be a plethora of solutions, representing everything from small steps to giant leaps,” he concludes. “That makes it a very exciting space to work in.

“Whatever the future of sustainability holds, Pratt & Whitney will be leading the way.”

Sean Bradshaw, Engineering Fellow, Pratt & Whitney

Images supplied by Pratt & Whitney

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