News+Stories: In the current environment, the large engines used in aviation and on aircraft are not exactly seen as an eco-friendly way to travel. But there’s a lot happening in this particular field, isn’t there?
Robert Krewinkel: There really is. The EU wants us to develop climate-neutral aircraft engines by 2050, and we need to more than halve emissions by 2030. This is why we are currently conducting in-depth research into completely new fuels – but also into brand-new engine architectures. It’s an incredibly exciting time.
What alternative fuels are available in the aviation sector?
Krewinkel: At the moment, things are moving in two different directions. One is sustainable aviation fuels or SAFs: in other words, fuels from sustainable sources. They can be used with current infrastructure and, just like kerosene, they can be burned in existing aircraft engines. There are also some demonstration projects and the fuels are added in small quantities, but unfortunately they’re really expensive. And we’re not sure whether they can be produced on a sufficiently large scale to replace kerosene completely. What’s more, they still contain carbon. There are a few ideas about how this can be filtered and reused, but that’s very expensive as well. So with SAFs, flying would be significantly more pricey and we would need far more efficient engines.
Another focus is research into completely new fuels, including hydrogen and hydrogen carriers. But they require totally new types of engine and there is still lots of research to be done before they become a viable option. With conventional fuels, the flame is much slower and cooler than with hydrogen – this is why H2 combustion won’t work in our current engines without major adaptations. And we would require entirely new hydrogen infrastructure at airports. A way would have to be found to refuel aircraft, which calls for corresponding safety measures.
What ideas for new engines are being looked into at the moment?
Krewinkel: There are some really exciting ideas. We’re mainly collaborating with GE, General Electric, which is working on large open-fan engines. This type of engine uses large, open rotors without the casing that we’re familiar with today. These large fans are significantly more efficient than the “small” ones used today, but as the actual engines are set to become smaller, that means they’ll be less efficient. We need to optimise this and strike the right balance. And we also need to carry out research into the interplay between the new fan and the aircraft. So there’s still lots of basic work that needs to be done.
What about the drawbacks?
Krewinkel: At present, subsidies are mainly being paid for actual engine development, but far less is being channelled into basic research. That’s because we only have six years left to build these new engines under the EU’s timeline. But the public greatly underestimates how long you actually need to develop new engines. If I started today, development and the necessary approval process would take about ten years. The airlines can’t even start integrating the new technologies until after that. This isn’t just the case for wide-ranging adaptations. Small innovations also have to go through the process of obtaining approval.
Obviously, new developments like these are also extremely capital-intensive. And the engine manufacturers and aircraft makers need to pay these costs up front – without knowing whether the airlines will even accept the new aircraft or whether airports can be retrofitted. At the same time, airports won’t modify their infrastructure as long as they don’t have to. It’s a catch 22.
Aside from new technologies, modern-day aircraft are already extremely efficient. This is a question of costs, among other things. How much more can still be achieved on that front?
Krewinkel: With the current technologies, there’s not that much more that can be done. We’re already at the stage where efficiency levels are well over 90%. But we can take a look at other components. The cooling, for example – one third of the air that is drawn in is used to cool the engine. That could be reduced quite a lot. And completely new technologies would significantly enhance efficiency, too. In particular, the use of artificial intelligence would bring us a huge step further.
In your view, will it be possible to fly with a clear conscience some day?
Krewinkel: Yes, but I think flying will become more expensive. Honestly, I think everyone needs to take a minute to think about whether it’s really necessary to take a long-haul flight for a weekend away. In many cases there won’t be any other option, but we could be far more mindful. But even if aircraft could be made climate-friendly, manufacturing them would still generate CO2 emissions.
To be fair, though, it’s also worth pointing out that global air traffic accounts for 3% of emissions worldwide – that’s about the same as the emissions from data centres.
How do aircraft engines differ from those on ships, for instance?
Krewinkel: Above all, weight plays a very different role. With ships or land-based applications, weight is secondary. The large engines on the stern actually help to stabilise a ship. But it’s totally different with an aircraft. Every single gramme makes itself felt and needs to have a high performance density. This is why we use many exotic materials like titanium, which would be too expensive for other applications. We also need to take a far more critical view of aircraft emissions, because they are discharged very high up in the atmosphere.
What do you find particularly stimulating about this line of research?
Krewinkel: It’s just great to have knowledge that nobody has ever had before. Our systems are so complex that we’re discovering new things all the time. Or methods evolve, allowing us to gain new insights. Things that you saw five years ago make sense all of a sudden! But I still find passing this knowledge on to the students the most enjoyable part of all.
This research area is anchored in the Field of Expertise “Mobility & Production”, one of five strategic foci of TU Graz.
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