Everyone agrees that aircraft engines need to get greener. Does the answer lie in revolutionary propulsion systems, or will our future travels be powered by the same old jet engine running on renewable fuels? Metalworking World looks into the aero engines of the future.
The gas turbine jet engine has served the airline industry well for more than 50 years, with relatively few changes to the propulsion system. But with the aircraft industry facing increasingly stringent legislation on emissions and noise pollution as well as spiraling fuel costs, researchers are working to ratchet up the speed of technology development. Where is this development heading? Will the gas turbine keep being improved forever or will a radical new form of propulsion emerge to condemn the jet engine to flying history?
Aviation contributes about 2 percent of the CO2 emissions from fossil fuel use, and as demand for air travel rises, so do emissions. Meanwhile, airport nitrogen oxide (NOx) emissions from burning jet fuel, which cause acid rain and smog and cost society billions of dollars each year from illnesses and death, are expected to double before 2020. To counter this, in 2001 the European aviation industry set a target to reduce fuel consumption by 50 percent per passenger-kilometer by 2020 and NOx emissions from commercial aircraft by 80 percent by the same year.
In the short term, experts agree that progress in making engines cleaner and more efficient will take place via a series of small evolutions, rather than a revolution. Development projects at universities, research institutions and engine manufacturers are constantly coming up with minor tweaks to engines to improve their fuel efficiency, such as the Rolls-Royce-led PANACEA project in which Sandvik Coromant was a key partner (see separate article). By introducing new materials for engine components, the project will reduce fuel consumption by 0.3 to 0.5 percent, saving 1,300 pounds of CO2 every time an aircraft crosses the Atlantic.
Meanwhile, competing engine manufacturer GE Aviation recently announced a new propulsion system for business jets that draws on a combination of military and civilian technologies. Its Passport engines, which are under development and expected to enter full-scale testing in 2013, feature a higher pressure ratio and a compressor made of new – and unnamed – advanced materials. GE predicts that the engines will achieve 8 percent lower fuel consumption and considerably lower NOx emissions. “Passport is … the world’s first integrated propulsion system specifically designed for ultra-long-range, large-cabin business jets, giving customers the power to fly … more quietly and efficiently,” says Brad Mottier, vice president and general manager of GE’s Business & General Aviation organization.
Another GE development project, this one for regional jets, is expected to offer 15 percent better fuel efficiency than existing engines, again thanks to secret advanced materials, new cooling technologies and a new approach to combustors, the part of the engine where air is mixed with fuel and ignited.
Such small steps are undoubtedly a welcome improvement in the environmental performance of aircraft, but more significant leaps forward will not be easy to achieve. “There are a number of challenges,” says Tomas Grönstedt, associate professor in the department of applied mechanics at Chalmers University in Gothenburg, Sweden. “These include increasing difficulties in going to higher overall pressure ratio engines due to increasing cooling air temperatures, constraints imposed by developing new material technologies and detrimental weight and drag impact on ultra-high bypass ratio engines. We continuously need to find ways to develop new materials, and there is no guarantee that this can be done keeping the current pace of advancement.”
The technologies currently under development that are being hailed as plausible solutions in the medium term include the intercooled engine, the intercooled recuperated engine and the open rotor. “If realized, the intercooled recuperated engine and the intercooled engine are not expected to enter service until after 2020,” says Grönstedt. “I would probably put open rotor engines a little bit past 2020 as well if you consider entry-into-service dates.”
The pulse detonation engine, which has the potential to radically increase thermal efficiency, is one of the more exciting propulsion technologies being researched. However, Grönstedt says, there are still a number of important issues that need resolving. “For instance, you have to handle turbine cooling in an intermittent combustion environment,” he says. “Noise is also a challenge.”
But should these challenges be resolved, the environmental benefits will be considerable. “By combining the PDE with open rotor architectures, airframe developments such as blended wing bodies and reduced flight speeds, a 75 percent reduction in fuel burn per passenger-kilometer could be achieved by 2050 against the base year 2000,” says Grönstedt.
Such technologies would be a big step forward, but they are still variations on today’s internal combustion engines. So when will we be flying in “science fiction” solutions based on totally new and radical propulsion systems?
“There have been several super-radical concepts floated for the year 2050, but I don’t see a really major game-changer at this time,” says Grönstedt, explaining that the gas turbine’s advantage is still its colossal power density. “You can produce a fantastic amount of thrust at a very low weight, and that is very hard to match. It might be that in the year 2020 we will still use them, but we won’t invest as much in development because we are close to their limits in thermal and propulsion efficiency.”
Perhaps there is no need to actually replace the internal combustion engine. “The way I see a greening of aviation is through radical improvements in energy efficiency and full implementation of the use of biofuels,” says Grönstedt. “Algae could provide a solution to produce the necessary amounts of biofuels without competing with food production. But this requires that current technical obstacles are overcome.”
New engines, newer fuels
Open rotor engines:
Also known as propfans and ultra-high bypass engines, open rotor engines offer the fuel economy of a turboprop with the speed and performance of a turbofan. Patented in 1979, open rotor engines have the potential to deliver fuel savings of around 30 percent, but they are noisier than other engine types.
Pulse detonation engines:
First considered more than 70 years ago, the pulse detonation engine (PDE) concept uses detonation waves to combust the fuel and oxidizer mixture. Instead of burning fuel, it explodes it. In theory, PDE could power aircraft to speeds of around Mach 5, although to date no practical engine has been put into production. Challenges to PDE are noise and vibration.
Intercooled recuperated engine:
The integration into the aircraft engine of an intercooler and a recuperator, or heat exchanger, makes it possible to recover heat from the hot exhausts to the combustion chamber and to decrease the temperature rise of the burner. This can contribute to fuel savings of around 30 percent and at the same time reduce NOx and noise levels.
Renewable fuels:
Jatropha, a weed-like plant that grows on barren land, is being hailed as a potential source of jet fuel. Several airlines have successfully tested the oil produced from jatropha seeds, which, it is claimed, offers greenhouse gas reductions of up to 60 percent compared with petroleum-based jet fuel. In June 2011 a Gulfstream G450 powered by equal amounts of traditional fuel and a camelina-based biofuel made the first-ever transatlantic biofuelpowered flight.
The use of biofuels will increase dramatically.
New cutting challenges every time
The heat is on for green improvement
Some of the figures and statistics around aero engines can be hard to grasp: Each wide-chord fan blade exerts a centrifugal force of around 70 tonnes, equivalent to the weight of a modern locomotive; each high-pressure turbine blade generates the same amount of power as a Formula 1 car; and the blades in the hot part of the engine have to operate at several hundred degrees hotter than the melting point of the material they are made from.
“You have some pretty ridiculous numbers,” says Steve Weston, application development specialist at Sandvik Coromant. “And when you are trying to increase temperatures further to increase efficiency and you are already above the melting point of the material, that is where the challenges are.”
Weston says that some of the next-generation materials that Sandvik Coromant is asked to work with by aero engine manufacturers are almost unbelievable in their structure and composition. “They turn up in our workshop sometimes looking like moon rock,” he says. “They are challenging to cut because of their properties related to high dynamic shear strength and have poor heat dissipation, which can cause a lot more wear on inserts. But if they tell us what they want and give us time, we will come up with a way of cutting it, guaranteed.”
One such project in which Sandvik Coromant was involved was the PANACEA project – which stands for processing of an advanced nickel alloy for critical engine applications. Partners included Rolls-Royce and the Advanced Manufacturing Research Centre at the University of Sheffield, where Sandvik Coromant is a Tier 1 partner.
The aim was to develop engine components – specifically gas turbine discs – with a “dual microstructure” to give different mechanical properties at the centre and at the edge of the disc. This will enable the engines to run about 50°C hotter, which will make the engine 1.5 percent more fuel efficient. PANACEA technology will save 0.6 tonnes of CO2 every time an aircraft crosses the Atlantic.
Sandvik Coromant was involved in the machining of this specialized material. “We machined the disc absolutely complete on one machine, which is something quite unique, because normally several different machine platforms would be needed to machine various component features,” says Weston. “But we managed to make new tools and develop new strategies to machine every surface of the disc.”
A scaled-up version is currently being produced for full-scale testing, and the disc could be saving fuel and emissions in regular service within two years.
Sandvik Coromant was a key partner in the Rolls-Royce-led PANACEA project.
