A Sustainable Future for Commercial Aviation
We are very grateful to have received this guest blog, written by Kyra Thole-Wilson, a Senior Design Engineer at Pratt & Whitney. Kyra has spent the last 6 years designing, testing & assembling commercial and military jet engines. In this blog she explains how engine manufacturers currently try and eek out every ounce of efficiency from jet engines, and what leaps the industry will need to make to reduce its emissions all the way to zero. The views expressed in the blog are her own, and do not reflect those of her employer.
How engines are currently designed to be as efficient as possible, and what we need to do to take aviation to a Net Zero future
On New Years Day, 1914, the first commercial airline was born. The “St. Petersburg Tampa Air Boat Line” carrying one passenger 17 miles via air from St. Petersburg, FL to Tampa, FL in only 23 minutes, was rapid in comparison to the 11 and a half-hour train ride most travelers could take at the turn of the century. While it is probably no surprise that its tenure was short lived, its legacy remains in our modern world. Today, 23 minutes borders the shortest amount of time some major airlines allow for connections between flights, and air travel connects our world. Aviation allows us to conduct business spanning continents, travel the world with ease, and even fall in love with those across oceans, but it also harbors a challenge for humanity, these machines that have fascinated us for over a century, burn fossil fuels.
Today’s commercial gas turbine engine is significantly different than the 75-horsepower water cooled engine that took a single passenger across a Florida swamp in 1914. Engines that power commercial jets are incredibly precise efficient machines cooled by air rather than water, comprised of 5 major systems: a fan, a multistage compressor, a combustor, a multistage turbine, and exhaust. The compressor, combustor, and turbine are what is known as the engines core. As the air is funneled through the engine's core, each stage of the compressor gets smaller, and thereby squeezes the air enough to increase its pressure before it enters the combustor. The highly compressed air readily ignites with the fuel sprayed into the combustor, and the air expands. This rapid expansion of air spins the turbine at high speeds, which in turn rotates the shaft of the engine that keeps the fan and the compressor spinning. Essentially, after the combustor is lit when the engine is first turned on, the back of the core is propelling the front of the engine.
Unlike the engine in your car that supplies fuel when you press the accelerator, a jet engine is continuously spraying and igniting fuel to keep everything spinning, and the most commonly used energy source in today’s aircrafts is jet fuel (in some type of specific derivate depending on the conditions in which the plane is intended to fly.) These kerosene type fuels are composed of chemicals derived from petroleum, and while these fuels burn efficiently within the combustor to ensure proper engine conditions, their constant use for the duration of a flight not only contributes to global CO2 emissions, but the byproducts of jet fuel combustion is suspended in the atmosphere as a collection of ice crystals known as contrails, estimated to be contributing 1.7 times more than CO2 alone in warming our planet.
Jet engine technology has advanced enormously to help improve the process described above. Advancements have been made in trying to keep the engines' weight at a minimum using composite materials, and tightly controlled manufacturing processes, because the lighter the plane, the less fuel is needed to lift off the ground. Other more complex technologies are focused on higher efficiency combustors and turbines. The temperatures that the “hot section” components experience are above melting temperature for most advanced aerospace metals. To combat this, relatively ‘colder’ compressed air is bled off the compressor prior to being ignited and channeled to the internal cavities of the hot parts, where it exits small cooling holes to create a barrier of cold air surrounding the surfaces exposed to hot temperatures. This method is why these engines are denoted as ‘air cooled’, however air stolen to cool the hot parts is air the engine has used energy on but will not ignite to power the turbine. This siphoning of air reduces overall engine efficiency. Hot section technologies strive to reduce the amount of air required to cool parts, and thereby increase efficiency and decrease fuel consumption.
Engine systems are also designed to run fans and turbines at optimal speeds through various methods. Fans that run slower than turbines are more efficient at moving air and providing thrust, and newer engine technologies have been implemented to capitalize on these benefits.
While all these advancements are effective at reducing fuel consumption, their contributions are nowhere near what is required for net zero, more will be needed to reach this goal.
The currently proposed solutions are somewhat defined, yet complicated. Unlike cars that are suited for an electric future, an airplane is at constant war with weight, and a full fuel tank is lighter than enough batteries needed for a long-haul flight. Successful flights have been made with fully electric aircraft; however, these planes are much smaller in comparison to the commercial passenger aircraft. Hybrid-electric aircraft have potential, but their development has been in the making for years and there is still yet to be a commercially viable option in time to help reduce emissions. So, for now these solutions might be best left to an electric comeback for the St. Petersburg Tampa Air Boat Line. Other technologies proposed are hydrogen-powered aircraft and even fuel cell-powered planes, but these have yet to make advancements beyond small plane test flights.
The shortest pathway we have for large commercial air travel achieving goals by 2050 trends towards maintaining our current fleets of gas turbine engines and fueling them alternatively with Sustainable Aviation Fuels (SAFs).
SAFs cover a wide range of fuel options with varying degrees emissions reductions depending on their primary sources and whether they must be mixed with jet fuel. The components of these fuels can range from biomass to hydrogen derived fuels, but biomass is the most readily available option on the market today. SAFs made of biomass, which have the potential to produce 94% less CO2 emissions than traditional jet fuels (if pure SAFs are used) provide more energy dense efficient fueling for aircraft, and are derived from processes that produce much less emissions than traditional refinement.
Biomass can come from waste from industries such as forestry, municipal waste treatment and organic compounds such as algae and energy crops. Issues arise regarding the land usage required for enough energy crops to sustain the sustainable fuel market, and some farming practices are not yet sustainable. Additionally, these fuels are expensive in comparison to traditional petroleum-based jet fuel, and are merely just talked about as the future of sustainable aviation rather than widely used in practice, which begs the question: why would these fuels be the path forward for net zero aviation by 2050? The answer to that question lies in the business structure of how airlines buy from airframers (companies that make planes) and how airframers buy engines.
The pandemic did significant damage to the airline industry in many ways, but it also provided the opportune moment to replace aging aircraft, and subsequently aircraft engines with newer ones. Planes can be in service between 20 to 30 years, and in that time technology at jet engine manufacturers can be rapidly changing. The major engine manufacturers found it important to test their combustion processes with SAFs, even taking test flights powered by 100% SAFs to prove their latest combustion technology can successfully contribute to a cleaner world even before SAFs were touted as a possible solution for net-zero aviation. Airframers were also interested in choosing engines that maximized fuel efficiency and could use alternative fuels, to sell to their airline customers. While the latest sustainable fuels may be expensive for the airline to purchase for now, the cost pales in comparison to the cost and effort of replacing fleets worth of aircraft that are under service contracts with airframers and engine manufactures that were planned to span decades, in favor of technologies that have not yet reached the scale required for mass transit.
Like with many issues regarding a sustainable future, the solutions to our current dilemma of sustainable aviation are much more complex than finding a technical solution that we have not yet discovered. To establish a clean pathway for aviation, work needs to be done to ensure cost reductions for sustainable fuels, thereby incentivizing airlines to choose these cleaner fuels. Advancements in synthetic hydrogen-based fuels need to be made to reduce the demand for land usage for energy crops, and future hybrid and electric aircraft need to be pushed forward beyond demonstrated capability.