Engines and related systems make significant gains toward aviation’s environmental goals
Innovations in alternative fuels, propulsion technologies, aircraft design, and manufacturing techniques promise great performance, economy, and environmental compatibility.
By Don Van Dyke
ATP/Helo/CFII, F28, Bell 222.
Pro Pilot Canadian Technical Editor
Business and commercial aviation offer safe, secure, and flexible transportation in a network which connects people, commerce, and culture. The overarching challenge to ensure that related activities are environmentally sustainable drives the merging of technologies and innovation.
Atmospheric carbon dioxide (CO2) is a greenhouse gas that absorbs and radiates heat. Balancing both effects is critical to the environmental health of our planet.
Annually, a variety of activities – especially those involving burning fossil fuels for energy – release more CO2 into the atmosphere than natural processes (“sinks”) can absorb. This results in climate change, and it’s evidenced by record-high global temperatures, wildfires, and intense storms. Aviation is a conspicuous element of this process.
Emissions of nitrous oxide (NOX) and ultrafine particulates are also of critical concern, as is the formation of contrails and the impact of noise on communities adjacent to airports.
The United Nations Framework Convention on Climate Change (UNFCCC) reinforces the leadership of the International Civil Aviation Organization (ICAO) on issues related to international aviation and climate change.
In 2022, the 41st ICAO Assembly adopted a long-term global aspirational goal for international aviation of net-zero carbon emissions by 2050.
However, the Intergovernmental Panel on Climate Change (IPCC) notes that simply eliminating emissions will not be sufficient for the world to reach net-zero by 2050, and points out that removing and storing CO2 from the atmosphere is also necessary.
The International Air Transport Association (IATA) estimates that in 2050 the industry will require mitigation of 1.8 gigatons of carbon.
A potential scenario is that 65% of this will be abated through sustainable aviation fuels (SAF). New propulsion technology, such as electric and hydrogen, will mitigate another 13%, and efficiency improvements will account for a further 3%. The remainder could be dealt with through carbon capture and storage (11%) and offsets (8%).
Pathways to sustainability
Aircraft are particularly “hard to abate” since their emissions cannot be captured at source and depend on proactive decarbonization measures. ICAO Guidance on the Development of States’ Action Plan on CO2 Emissions Reduction Activities (Doc 9988) identifies measures related inter alia to propulsion such as:
• Retrofitting and upgrading existing aircraft (replacement of engines).
• Optimizing improvements in aircraft produced in the near- to mid-term (aerodynamic designs, engine technology).
• Adopting revolutionary new engine designs (electric, open rotor, etc).
• Making available and using SAF.
Selections from among the options available and the trajectory to reach climate goals will depend on what solutions are most cost- and operationally effective at any given time. It is here that the counsel of experts and specialists, including professional pilots, will be critically important.
Alternative powerplant concepts
Desirable aircraft engine attributes include reliability, output power, fuel efficiency, size, profile, maintainability, weight, and cost. Increasingly, environmental impact is an urgent consideration. Powerplant technologies currently under consideration include advanced gas turbine, all-electric, hybrid-electric, and hydrogen-electric designs.
Table 1 highlights the main characteristics of these competing aircraft propulsion concepts. And Table 2 presents a non-exhaustive list of advanced powerplants by original equipment manufacturer (OEM) and their primary aircraft application.
Gas turbine. OEMs are developing smarter, cleaner, and greener technologies to reduce the environmental impact of gas turbine engines. At Pratt & Whitney Canada (P&WC), the focus is on continuing significant improvements in thrust and fuel efficiency to reduce carbon. The high-bypass geared turbofan (GTF) achieves a 16% reduction in fuel burn by introducing a gear that enables the turbo machinery to optimize efficiency.
Similarly, General Electric (GE) has incorporated technology developed during design and testing of the Catalyst, a new 850- to 1600-shp centerline engine for single and twin turboprops (TPs).
Sustainability must also account for environmental effects of manufacturing and supply chain issues, which often continue to be problematic. Addressing this concern, GE 3D printed 12 titanium alloy parts that replace 855 components, reducing engine mass by 5% and avoiding supply chain obstacles.
All-electric. Conventionally-powered aircraft face increasing expense, owing mainly to carbon offsets and environmental penalty costs. All-electric engines are among the most promising technologies for the 1- to 19-place aircraft market. Longer battery life, lower electric motor maintenance cost, low noise during takeoff/landing, and decreased electricity prices have led to better economics over time.
Hybrid-electric. Rolls-Royce has developed a new turbogenerator that can power hybrid-electric flights using SAF or hydrogen, while Honeywell and GE have developed turbogenerators which can run on renewable diesel to power hybrid-electric aircraft.
Hydrogen-electric. ZeroAvia notes that with up to 30 times higher specific energy and lower cycling costs than Lithium-ion (Li-ion) batteries, hydrogen-electric powertrains represent a viable and scalable solution for zero-carbon-emission aviation.
Electric motor OEM magniX has partnered with US startup Universal Hydrogen (UH2) and hydrogen fuel-cell producer Plug Power to produce a 2-MW, zero-emission powertrain for retrofit into 40- to 60-seat regional aircraft, beginning with the de Havilland Dash 8 Q300.
Alternative fuels
Whereas fossil fuels add to the overall level of CO2 by emitting carbon that had previously been locked away, SAF recycles the CO2 which has been absorbed by the biomass used in the feedstock during the course of its life. Moreover, energy cost as a percentage of aircraft operating costs is growing. This motivates ongoing research into creating clean, renewable sources of energy, as well as promoting eco-friendly awareness and activities.
Sustainable aviation fuel. SAF is a liquid fuel that can be produced from a variety of sources (feedstock), including waste oil and fats, green and municipal waste, and non-food crops. It is sustainable precisely because the raw feedstock does not compete with food crops or water supplies, and does not promote deforestation.
Green jet fuel. Honeywell green jet fuel is chemically similar to fossil fuel, but it’s made from more sustainable alternatives, such as camelina, jatropha, algae, or animal fats.
Renewable diesel. A type of biofuel made from biomass or waste sources, renewable diesel, or hydrotreated vegetable oil (HVO), has a lower carbon footprint than fossil fuels.
Dual fuel combustion. Interest in hydrogen, particularly liquid hydrogen (LH2), as a potential alternative to SAF is growing. LH2 emits no CO2 during combustion and can be produced with near-zero carbon emissions if made using renewable electricity.
Conclusion
Demand for new aircraft will shift from fleet growth to accelerated replacement of older, less efficient aircraft. Sustainability will become a key consideration for aircraft owners wishing to reduce their carbon, waste, and noise emissions.
Notable investments include the Bombardier EcoJet blended wing research project to develop and mature enhancements in aerodynamics and propulsion. And GE Aerospace is investing $20 million in its Dayton OH Electrical Power Integrated Systems Center to meet demand for hybrid-electric engine testing.
Promising technologies include rechargeable lithium-sulfur (Li-S) batteries which may displace Li-ion cells because of their theoretically greater capacity, higher energy density, and lower cost.
Traditional engine development has reached a crossroads as focus on sustainability shifts the industry toward an all-electric, hybrid-electric, and hydrogen-electric future.
Don Van Dyke is professor of advanced aerospace topics at Chicoutimi College of Aviation – CQFA Montréal. He is an 18,000-hour TT pilot and instructor with extensive airline, business and charter experience on both airplanes and helicopters. A former IATA ops director, he has served on several ICAO panels. He is a Fellow of the Royal Aeronautical Society and is a flight operations expert on technical projects under UN administration.