As aviation propulsion goes green, new technologies emerge to reach clean flight.
By David Ison
Professor, Graduate School Northcentral University
The aerospace industry seems to be in constant flux, pushing to improve every aspect of its existence, from safety to manufacturing and efficiency.
Cost savings have always been high on the list of improvements to offset rising prices of industry essentials, such as fuel and labor.
Nevertheless, it is no longer just about money – carbon dioxide (CO2) emissions and climate change have come to the forefront of concern across the globe. In order to reduce costs and potential adverse effects on climate, the aviation industry has taken a multi-faceted approach to improvements.
Historical trends in powerplants
Aviation has always been obsessed with efficiency, primarily in speed and cost. After all, if improvements on the 12-hp, 170-lb, 30-mph engine of the Wright Flyer did not occur, aviation would not have had much utility.
Efficiencies manifested in various forms, from improved aerodynamics to lighter and stronger structures, and, of course, improved propulsion. As aircraft engines became more sophisticated, cruise speeds moved gradually upwards, increasing efficiency in terms of miles in exchange for fuel.
Yet by World War II, aircraft topped out on speed due to the aerodynamic limitations of propellers, ending somewhere around 300–400 kts. The next significant shift in efficiency came with the adoption of jet propulsion. Although early jets guzzled gas, they allowed aircraft to fly faster and higher, and had a larger capacity for passengers and cargo, yielding more profit per flight.
Not surprisingly, there was constant motivation to improve the fuel efficiency of jet engines. Migration from turbojets to turbofans reduced fuel consumption and emissions. According to the International Council on Clean Transportation, aircraft powerplant fuel consumption improved by 45% from 1968 to 2014, and has continued at an average of 1–2% annually since.
In the 2000s, efforts to improve turbine engine fuel efficiency ramped up due to rising fuel costs and concerns about environmental impacts. CFM’s Leading Edge Aviation Propulsion (LEAP) project was launched around 2005 with the goal of reducing fuel consumption by 15–20% while boosting the bypass ratio of its engines from 5:1 to 10:1.
The higher the bypass, the greater the efficiency. Pratt & Whitney also jumped into the efficiency race, first working on geared turbofan engines in the mid-1990s. Research into geared engines led to the eventual introduction of the PW1500G geared turbofan, with a bypass ratio of 12:1 and a 16–18% improvement in fuel efficiency.
In addition, enhanced efficiency engines have been introduced by other manufacturers, such as Rolls-Royce with its Trent 1000. Such new highly efficient engines have enabled Boeing 787s to fly longer-distance routes. Turboprop (TP) engines, although highly efficient for operation within their designed flight envelope, have also been tweaked to maximize performance.
For example, GE’s Catalyst engine is slowly being adopted as a retrofit option for existing aircraft and as original equipment in new production planes such as Textron’s Beechcraft Denali. The fuel efficiency of the Catalyst powerplant is about 20% better than its competitors.
Its power output 10% greater than similar engines, and time between overhauls is double that of most turboprops. Thanks to new materials used in the compressor and turbine sections, there is less wear and tear. Additive manufacturing (3D printing) and ceramics have made this possible.
However, the push for further engine improvements has only increased in recent years. A few new entrants have surfaced within these endeavors, which can be organized into novel propulsion engines, sustainable aviation fuel (SAF), and electric propulsion engines.
Novel propulsion engines
An old engine concept from the mid-1980s – the open rotor turbine engine – is back, although the original example of this design, the GE36 unducted fan (UDF), never went beyond testing.
The new proposed example is an unshrouded (uncowled) set of counter-rotating fans powered by a traditionally-cored turbine engine oriented in a pusher configuration, and boasting a reduction in emissions and fuel consumption of ~30%.
Although these engines are louder than turbofans, they still should comply with current noise regulations. Another example on the drawing board is the propulsive fuselage concept (PFC), which uses boundary layer ingestion to leverage the fuselage itself as a source of propulsive power.
The most common PFC setup embeds an engine in the aircraft’s tail with its inlet flush with the fuselage. It uses airflow over the fuselage as part of the inlet airflow for the rear engine. At the same time, the disturbed air that is a byproduct of fuselage boundary layer interaction (BLI) is “corrected” by the engine and discharged rearward.
This results in an 8.5% reduction in fuel burn. Other novel powerplant designs are in the works, such as those being developed under the auspices of Europe’s Clean Sky 2 program.
Rolls-Royce, for example, is working on 2 engines which are due for adoption by 2025. The company’s Advance design implements a 3-shaft turbine architecture, adding a new high-pressure core, and boasting a bypass ratio of 11:1.
The Advance engine also has a fuel consumption and CO2 output 20% lower than conventional Trent 800 engines. The Ultrafan design, on the other hand, couples a 2-shaft Advance core to a gearbox that drives the fan section.
This engine uses 25% less fuel and emits 25% less CO2 than the Trent 800. Along similar lines, Safran is developing what it calls its Ultra-High-Bypass Ratio (UHBR) turbofan.
The engine is expected to have a bypass ratio of 15:1 or higher, leading to a 5–10% improvement in fuel economy versus LEAP engines, and 20–25% over pre-LEAP engines.
Additional engine architecture features can augment efficiency. Adding a “zero hub fan” – essentially where the fan blades are integrated to the spinner at the center of the fan (the spinner itself is part of the blading of the fan) – can add 2–4% fuel efficiency.
And adaptive and active flow control within the engine has been shown to potentially increase efficiency by 10–20%. Advanced combustor components adding 5–10% efficiency are also possible sources for improvement.
Manufacturing advances also provide means for further enhancing fuel economy. Engine components can be created using ceramic matrix composites (CMCs), comprising ceramic fibers injected with a ceramic resin, resulting in components that weigh 1/3 less but are more heat resistant than an equivalent metal part.
The use of CMC permits operating temperatures in excess of 2,400º F – higher than the melting point of some metals, which allows for higher engine thermal efficiencies.
Carbon fiber composite technologies have also allowed for huge fans with high bypass capabilities. It is expected that open fan designs could result in bypass ratios of 70:1 or higher.
Lastly, additive manufacturing allows for lighter and more durable components, both attributes helpful to operational and maintenance costs over the engine’s lifetime.
Sustainable aviation fuels
While a significant amount of effort has been put toward making the powerplants themselves better for the environment and the pocketbook, a rapidly evolving trend with similar goals has been taking place in the research and development of new aviation fuels.
Due to the adverse effects on the planet and ephemerality of fossil fuels, alternative sources of fuel have been sought for some time. Sustainable aviation fuels (SAF) are synthetic equivalents of kerosene/Jet-A derived from renewable and sustainable materials and methods.
SAF includes the subcategory of “biofuels” that rely on natural biological processes to create the fuel. SAF can be developed from plant or algae oils, grease, fats, organic waste, alcohols, and CO2.
There are several advantages to using SAF. One is that they reduce carbon life cycle emissions by approximately 80%. Another is that they have lower particulate matter and sulfur emissions. SAF has also been shown to augment fuel efficiency by 1.5–3% due to its higher energy density.
And, of course, SAF can be created as needed rather than relying on wells and conventional oil supplies. While SAF sounds like a panacea for aviation’s thirst for fuel, the current production capacity is at about 1% of global fuel needs.
This is expected to double by 2025, but SAF has a long way to go before it can serve as a viable replacement for jet fuel. While until recently only flights using a 50/50 blend of SAF/jet fuel had been explored, several promising assessments have taken place this year using 100% SAF.
In February, Rolls-Royce conducted a 100% SAF test using its Pearl 700 engine, which is slated for use by business jets. The Pearl 700 offers 5% higher efficiency and just shy of a 10% boost in takeoff thrust.
Furthermore, in March, Airbus conducted an exploratory flight using 100% SAF on an A350. The mission was a success, paving the way for further adoption of SAF. And United Airlines recently conducted a test flight using a Boeing 737 Max 8 using 100% SAF created from beets, sugarcane, and corn.
The flight departed Houston and stayed in the airport area as fuel performance metrics were verified. No problems were noted. The exciting thing about the fuels used during these tests is that they’re suitable “drop-in” substitutes for jet fuel, thus not requiring modifications of any type to the aircraft or its engines.
The ideal situation, of course, would be an aircraft that does not require any fuel and has zero emissions. Much like the movement toward hybrid and electric vehicles on the ground, aviation is also exploring electrically-powered aircraft in haste.
But the idea of electric aircraft is far from new. The first use of electric propulsion occurred in the late 1800s with an electrically-powered airship. This invention proved impractical due to limited battery capacity.
Sounds familiar? Eventually, more advanced batteries were invented, such as nickel-cadmium (NiCad). While NiCad was an improvement over its predecessors, early tests were limited to around 15 minutes of flight due to battery capacity.
Finally, by the 1980s, lithium-ion batteries emerged, making things like Solar Impulse possible. This experimental solar-powered aircraft completed a 16-month round-the-world flight in 2016, staying aloft at around 30 mph on each leg of its journey on solar power alone.
The sub-par performance allowed by battery capacity has practically kept useful electric aircraft grounded until recently. Much of the recent work involving electric aircraft has centered around urban air mobility (UAM).
Think Uber Air and Amazon Prime Air. Several manufacturers have surfaced, each with their ideas of how best to design electric vertical takeoff and landing (eVTOL) vehicles. Some promising options include the CityAirbus multicopter and the Lilium Jet.
The CityAirbus multicopter is advertised to fly 4 passengers 80 nm at 120 kts, while the Lilium Jet looks more like a conventional aircraft than a VTOL type, and uses 36 ducted electric vectored thrust (DEVT) engines mounted to the wings and canard of the aircraft.
These fans are directed downward for VTOL and tilt backward for conventional flight. It can carry 7 occupants up to 135 nm at 150 kts. Recently, electric motors have been migrating into the conventional aircraft world as well.
In 2020, a company called magniX launched an electric-powered Cessna Grand Caravan equipped with a 750-hp electric motor fed by 2000 lb of lithium-ion batteries. This version of the Caravan was quieter and used around $6 worth of electricity versus the typical $300 of jet fuel needed for a similar outing.
Moreover, electric motors are lighter and simpler to maintain their internal combustion counterparts. MagniX is also working with Harbour Air, a Canadian seaplane airline, to convert its entire fleet to electric – something on which the industry will surely keep a close eye.
Because of the high power demands for larger and faster aircraft, designers have opted to bypass battery capacity limitations by combining old and new technologies through various hybrid aircraft designs.
Like a hybrid car, a hybrid aircraft uses electric and some other flavor of propulsion to get it and keep it aloft. As an experiment, Airbus retrofitted an old BAe 146 with a 2-megawatt electric motor that stood in place of one of the aircraft’s 4 engines while the remaining 3 engines were gas turbine types.
Even among hybrid aircraft, there are many configuration options. The parallel-hybrid simultaneously supplies propulsion via both electricity and/or fuel as needed. Series-hybrids create electricity via a generator attached to a fuel-supplied gas turbine or engine.
The electricity is what provides the energy used to power the electric fan or propeller. And series/parallel-hybrids are a mix of the aforementioned. Perhaps 1 of 3 powerplants is electric while the other is gas-operated (but also provides additional electricity for use).
A number of hybrid aircraft are in development. One promising example is the Zunum Aero, a series-hybrid that looks like a small business jet. It is slated to have a range of around 600 nm at 300 kts.
As has been noted, the primary obstacle to fully-electric flight is battery-weight-to-power ratio. Current batteries, such as lithium-ion types, are too heavy to pack enough juice on board to power even small aircraft to make them viable for anything beyond local, relatively slow flights.
Some novel ideas have been tried to get around this restriction, such as Eviation, an Israeli aviation company that designed an aircraft in which the battery is an integral part of the entire plane – under the floor, in the wings, and so on.
Still, the weight of the battery and the fire hazards associated with lithium-ion batteries are of concern. Thankfully, new battery types with higher capacities and that are safer to use are coming to fruition.
One promising example is the lithium-sulfur (Li-S) battery, which is lightweight and packs twice the energy density of a comparable lithium-ion type – a win-win for aircraft designers. It is estimated that Li-S batteries will be capable of providing 600 watt-hours per kg by 2025.
To put that in perspective, current lithium-ion batteries provide around 250 watt-hours/kg. Not surprisingly, Li-S will be used to power new aircraft, one example being the Texas Colt. Looking like a clone of a new Cessna 172, the Colt will offer 2 hours of flight time and a range of 200 nm.
Undoubtedly, both UAM and conventional aircraft will take advantage of these and other new battery types to leverage more practical aircraft designs.
As is apparent, there is lots of action in the aviation propulsion marketplace, with efforts focusing on improved efficiencies and lower environmental impacts. Transformations are being ushered in by driving forces such as UAM and elevated concerns about climate change.
The resultant multipronged approach, with better engines, electric and hybrid aircraft, and sustainable fuels, is an encouraging development to ensure continuous improvements within an industry that has increasingly come under the microscope for its negative impacts on the planet.
While it used to seem that some of these ideas would fizzle, much like the “everyone is going to have a flying car” claims, it appears that these changes are here to stay and soon to become mainstream.