Clean and quiet: Low Noise Engines

Emissions caused by travel and transportation are a hot button issue. This also applies to the aviation world, where there have been immense leaps forward in developing more environmentally propulsion systems. In addition to saving resources, these should also have a positive effect on people’s lives by reducing noise emissions.

© Wolfram Schroll

24 June 2016 was a day that will change the future of aviation: at the small Dinslaken/Schwarze Heide airfield (EDLD), the Walter Extras aerobatic aircraft, Extra 330 LE that was made at the very same airfield, took off, powered by an electric motor. While the noise of the propeller is normally drowned out by the throbbing of the engine during take-off, all that could be heard this time was a quiet whirring sound. The motor, which was specifically developed for this application by Siemens promised record-breaking performance: a power output of 260 kW from a weight of just 50 kg! Engineers refer to this relationship as the “power-to-weight ratio” and a value of 5 kW/kg of motor mass had previously seemed out of reach.

The electrification of aviation begins with the general aviation

Space-saving and highly efficient - SP260D electric motor with corresponding converter / Extra 330 LE D-EPWR © Frank Martini

Against this backdrop, it was entirely reasonable that Dr Frank Anton, head of Siemens e-Aircraft, should therefore refer to a milestone in the history of aviation and, because these motors can be connected in series, it is even conceivable that they could be used to power regional aircraft carrying up to 100 passengers.

Lean construction and aerodynamic benefits

As electric motors have a much smaller cross-sectional area than turbine engines, their engine housings cover much less of the propeller’s radius. This means that the propellers can provide propulsion much more efficiently. The breakneck pace of development is shown by the collaboration agreement that the electronics group entered with the aircraft manufacturer, Airbus, in the same year. Together with the engine manufacturer, Rolls Royce, the aim is to demonstrate by 2020 that hybrid electric power is feasible for flights with up to 100 passengers. A BAe 146 will serve as the development platform.

For larger, longer-range passenger jets however, turbine power will remain essential for the foreseeable future. In this case, it is not only the aircraft‘s engines that generate noise; noise also comes from behind the jet, where the hot stream of gases from the combustion chamber and the surrounding current accelerated by the turbine meet with the cooler ambient air.

Chevron nozzles and geared turbines

For this reason, it is increasingly common for the trailing edges of the engine nacelles to be equipped with a saw tooth profile that causes different air currents to mix more effectively. Additionally, the incorporation of geared turbo fans into modern turbines helps them to run more efficiently and more quietly by optimising the rotational speed of the various components of the engines to generate a more favourable bypass ratio. Moreover, a honeycomb structure is incorporated into the frontal intakes of the Airbus A380’s engines to minimise the generation of turbulence, which would otherwise be perceived as noise on the ground.

Aerodynamic improvements to aircraft themselves, through surface coatings, winglets or a grooved “shark skin” surface, help to achieve more efficient flight with lower fuel consumption and reduced noise levels. Noise-reducing honeycomb structures, chevrons that are modelled on the wings of an owl and optimised surfaces that resemble the skin of a shark are just some of the developments inspired by nature and the science of bionics, which examines natural mechanisms and makes them feasible for production. This approach has been a striking success: the German Aviation Association (BDL) reports findings that show a 30 Decibel reduction in aircraft noise over the last 60 years – equivalent to no less than 88 per cent!

The serrated shape at the rear edges reduces noise caused by air turbulence

Chavrons of a Boeing 747-8i © Lufthansa FRA CI/C Bildarchiv

New approach procedures and noise reduction

In addition, the German aerospace body, the Deutsche Zentrum für Luft- und Raumfahrt (DLR) began to examine the effects of modified approach procedures and their effects on aircraft noise. Lufthansa now hopes to introduce one of these procedures, the Low Noise Augmentation System (LNAS) into its Airbus A320 fleet for long-term testing. LNAS is an assistance system that supports pilots in finding the optimal way to reduce noise and save fuel during their descent. Of course, this approach only helps local residents around airports, but work is also underway to benefit the ears of aircraft crews and passengers. For instance, the Hamburg Centre for Applied Aviation Research (ZAL), whose shareholders include the city of Hamburg and a number of universities as well as DLR, Lufthansa Technik and Airbus Operations, operates Europe’s largest acoustic laboratory, and with its Acoustic Flight Lab Demonstrator, ZAL can test the acoustics of new cabin concepts and put noise-reducing materials to the test.

Alternative propulsion concepts and lower noise levels: these are also issues that have concerned both the automotive industry and its supply chain for many years. It is hardly surprising, therefore, that there are parallels in the development of new propulsion techniques, and parallels to the Extra 330 LE aircraft can be found in well-known and long-established models such as the Opel Ampera-e and the Tesla range, which exclusively use pure battery power. Hybrid-electric power, which makes use of combustion engines and electric motors, is also familiar from both industries. However, there is another combination showing promise for the future, combining an electric motor with a fuel cell! While this solution is a longstanding development in the automotive industry, the first example of the technology in an aviation is much more recent: the HY4. The maiden flight of this four-seater aircraft, which was developed by DLR, the Slovenian aircraft manufacturer Pipstrel, and Stuttgart Airport, took place in September two years ago, while fuel cell-powered cars like the Honda FCX have been around since the 1990s.


Generating electricity using the Redox reaction © Max-Planck-Gesellschaft

Ground traffic

Fuel cells have even made inroads into the public transport market. In 2005, 30 hydrogen-powered Mercedes Citaro buses were in use in Reykjavik, Iceland. Due to the country’s abundance of renewable energy sources, such as geysers and waterfalls, Iceland was the ideal test bed for this technology, as the large amounts of energy used to electrolyse the gas were not a problem there. However, the time taken to build a fuel station network and the constant improvements in battery technology mean that the future seems less promising for hydrogen-powered cars than for purely battery-powered cars. However, the situation is changing gradually: in Hamburg, the local public transport network is working to make the switch to fuel cells. The Hanseatic port has four hydrogen filling stations of its own, while the abundance of wind energy in Schleswig-Holstein provides ample electricity to produce the hydrogen that is needed.

On the other hand, hydrogen hybrid developments are mainly being pursued in Asia. Following the iX35 FCEV, Hyundai introduced the Nexo at the start of the year, while Toyota, which already makes a hydrogen-powered car in the form of the Mirai, wants to introduce fuel cells to the mass market and is currently testing a hydrogen-powered heavy truck in Los Angeles. In this vehicle, at least, there should be plenty of space for adequately-sized hydrogen tanks. Meanwhile, Mercedes has ten e-Actros trucks under test with selected customers and MAN is also opting to offer e-trucks for urban goods deliveries. Both German manufacturers have so far chosen to rely on lithium-ion batteries as a power source for their electric vehicles.

Whisper-quiet asphalt and silent tyres

However, regardless of whether a vehicle is powered by batteries, hybrid electric power, an internal combustion engine or fuel cells, one thing is constant when you hit the road – noise comes from more than just the engine. In modern cars, it is expected that the rolling noise of the tyres will drown out the sound of the engine or other mechanical sources of noise. Alongside ever-better internal insulation for the passengers, manufacturers have been attempting to reduce noise emissions for the rest of the environment for many years. One approach to solving this problem is known as whisper asphalt, which is an open-pore street surface that uses hollow cavities to reduce noise and which was developed as far back as the 1980s.

Tyre manufacturers also want to play their part in reducing noise, with leading manufacturers having incorporated a special plastic foam into the inner tubes of their tyres for several years. They dampen air vibrations, which are generated inside the tyre as it rolls, helping to reduce noise – especially inside the car.

Although humanity’s numbers grow from one year to the next – and traffic levels grow with them – our world is becoming quieter than ever, both on the land and in the sea, and the development of new propulsion technologies has played a significant role in that respect.

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