Hybrid Electric Airliners Will Cut Emissions—and Noise

Electric propulsion will develop in the air as it did on the ground, by initially using batteries to assist a fuel-burning generator


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Editor’s note: On 31 May 2018 a Magnus eFusion electric airplane equipped with a Siemens propulsion system crashed in Hungary, killing the pilot mentioned in this article, János B. (as his name has been given in the Hungarian press) and a passenger. Siemens says it is working with authorities to determine the cause. For more, see the post online at “Prototype Electric Plane Built by Siemens and Magnus Aircraft Crashes in Hungary, Killing Both on Board.”

I am sitting in the cockpit of one of the most extraordinary airplanes in the world. It’s a two-seat light plane called the eFusion, made by Magnus Aircraft, but fitted with an electric motor by Siemens, a huge company not known for aviation. I pull my feet clear of the control pedals just before the pilot turns the thing on.

The propeller immediately spins into transparency. And yet it’s so quiet that we could easily converse without headsets. That’s the first sign that this plane is powered by electricity.

We start rolling down the little runway, in a grassy field outside of Budapest. It’s 10 o’clock on a sunny morning, and there are farms off in the distance. Suddenly we lurch into the sky and begin climbing steeply, shrinking the farmhouses into cuteness. A cow down below doesn’t even look up. That quick-off-the-mark acceleration, a kind of aeronautical equivalent to the “Ludicrous” mode of the Tesla Model S, is another sign of electric operation. You get all the performance the motor can give, and you get it instantly.

Then we swoop, swerve, and soar, leaving my belly behind. Not bad for a merely semi-aerobatic plane, as Gergely György Balázs, head of Siemens’s Budapest research outfit, described it to me, somewhat apologetically, just before I climbed in. (Lucky me—the pilot of the fully aerobatic model was away on business.)

After 15 exhilarating minutes, the battery is down by half, to less than 10 kilowatt-hours, and it’s already time to land. That’s the final sign of electric propulsion. For although today’s lithium-ion batteries—racks of which are stowed just ahead of the cockpit—store far more energy than they could just a few years ago, they can’t come close to a tank of gasoline. So, for years to come, all-electric planes will be limited to short hops, mostly between neighborhoods rather than between cities.

Aviation is the source of between 2 and 3 percent of global greenhouse gas emissions. But the sector’s effective overall emissions are believed to be greater because a great deal of the emissions are in the stratosphere. Aviation’s share of overall emissions is expected to climb sharply over the next couple of decades as air travel increases and emissions from other sources—notably electricity generation and passenger cars—decline.

In 2016, 23 countries agreed to limit the carbon emissions of their commercial aircraft beginning in 2020, according to standards set by the International Civil Aviation Organization (ICAO), an agency of the United Nations. So researchers all over the world are working hard to find ways to do it.

But how can electric planes help, if they are limited to ridiculously short hops? Because they are seen as a critical step in a grand technological evolution in aviation that will recapitulate the migration, just beginning now, of the automotive industry from combustion engines to electric motors. In perhaps 15 years, hybrid passenger aircraft that combine electric and combustion power trains will begin serving in short- and medium-haul routes. Hybrids burn fuel, but they do so frugally.

“We can make a huge impact at the scale of a small battery-powered trainer because there the physics isn’t working against us,” says George Bye, CEO of Bye Aerospace, who is partnering with Siemens to supply his company’s planned electric trainer with motors. “But for the higher speeds and masses that you need for airliners, you have to go to a hybrid configuration. The industry is working hard on that.”

Hybrids are needed, for now, because although aviation fuel gives 12,500 watt-hours per kilogram, lithium-ion batteries give only 160 Wh/kg, including the weight of packaging and everything else you need to make the battery safe.

To get hybrids up in the air will require lots of technological advances. These advances will come, of course, from R&D programs aimed at building hybrid planes. But they’ll also come out of efforts—such as Siemens’s—to introduce electric trainer aircraft and, perhaps most of all, to build an envisioned industry of intra-urban air taxis whisking people around in what amount to overgrown drones. Siemens itself is working with Airbus Helicopters on one such all-electric project, called CityAirbus. Airbus also has another, parallel effort, called Vahana, under way at its subsidiary in Silicon Valley. And there are many other startups, including China’s Ehang, which provided the first public demonstration of a passenger flight early this year, when an engineer took to the skies in the company’s octocopter.

In aviation, most hybrid designs depend on the series architecture, in which the fuel-burning engine—either an internal-combustion design or a turbine—drives a generator that powers electric motors, which spin the props and also charge the batteries. In this scheme, the batteries provide the burst of power needed to take off, a stratagem that lets technicians tune the fuel-burning engine to run at its ideal rate. Those massive jet engines you see hanging from the wings of your plane are fully engaged only during takeoff; at other times, they’re basically just idling and weighing down the aircraft.

There are other advantages. By distributing power by wire, the hybrid design lets you put the propellers exactly where you want them, without having to organize everything around the placement of enormous engines. Some hybrid designs envision putting the propeller at the back of the plane, or even on top of the vertical stabilizer.

Two major consortia are working on hybrids. In Europe, Airbus has teamed up with Siemens and Rolls-Royce in an alliance that is separate from the CityAirbus endeavor. In the United States, Boeing and JetBlue are part of a rival project managed by the startup Zunum Aero, based in Kirkland, Wash. Both consortia are talking about getting hybrids into the air by the early 2020s.

The Airbus group plans to begin with a modified version of an existing plane, the 100-seat British Aerospace 146, in which one of the four wing-mounted nacelles holds not an engine but a 2-megawatt electric motor. It will draw power from a generator spun by a small gas turbine housed in the fuselage (and thus offering little air resistance). If the electrical system fails, the three conventionally powered propellers could fly the plane safely. Airbus reportedly is preparing a hybrid for possible demonstration at next year’s International Paris Air Show.

The U.S. consortium has released almost nothing about its plans. In August 2017, GE Aviation produced a white paper outlining the substantial work it said it was doing on hybrid-electric motor-generators. In one ground-based experiment, GE Aviation used a motor rated at 1 MW to turn a 3.3-meter (11-⁠foot) propeller. In another experiment, it used the compressors of a GE F110 jet engine to power a generator rated at 1 MW; meanwhile, the engine continued to produce thrust.

Though detailed information from both consortia is scarce, interviews make it clear that they are focusing on improvements in four technological categories: battery capacity, motor and generator weight, power electronics efficiency, and airframe materials and design. In the European consortium, for example, Siemens specializes in the motor, the generator, and the electronics. In addition, the company has modified some small airplanes to create all-electric designs, in the belief that it can optimize the parts together only with the plane they’ll be installed in.

“We are gaining experience with the complete system of electric propulsion, with everything that’s between the pilot and the propeller,” says Frank Anton, head of Siemens’s eAircraft department. “The only way you can learn this is by flying the technologies.”

Electric motors can be relatively small and light, opening up many options. You can mount a bunch of small props on the wings and swivel them to help with a short takeoff. NASA is even investigating a design that would place a bank of little propellers all along the wing, directing the flow of air over the control surfaces as needed, to improve the lift-to-drag ratio. The result: shorter, thinner wings.

“Separate the generation from the propulsion,” Anton says, “and all of a sudden you can have all sorts of applications that use vectorized thrust.”

The key challenge of cutting the weight of an electric power train depends on two things. First, the energy density of the batteries must rise, which will be an incremental process, at least until today’s lithium-ion batteries give way to some entirely new technology, like metal-air batteries. And second, the power density of the motor and the engine-generator system that runs it must also go up. That’s Siemens’s specialty.

In the nose of the fully aerobatic Siemens plane sits the company’s SP260D aviation motor, which at 50 kg (110 pounds) and 260 kilowatts of power reaches a whopping 5:2 kilowatt-to-kilogram ratio. (The semiaerobatic plane has about the same ratio, but it’s only a little more than half the size.) The Extra first flew in public in 2016, at the Dinslaken field, in Germany; in 2017 it set a record for electric flight by topping 340 kilometers per hour (211 miles per hour). And Siemens engineers are working hard now to raise the motor’s power density still further.

At the Budapest research center, Balázs walks me over to a lab bench, where he hands me part of a motor that’s been cut in half. It’s a slice of a stator—the stable part around which the rotor revolves—and embedded in its sawed-off face are the rectangular cross sections of copper windings, which fit together like so many bricks on a wall. That rectilinearity is key to achieving high power levels—it prevents any air gaps that might interfere with the conduction of heat from the wires to the liquid-cooled housing. You have to wick away that heat or the wires’ insulation will break down, and then short out.

“We need a much more homogeneous heat exchange than we could get from round wire, and we also hope the electrical isolation will be better—which matters in an aviation motor,” Balázs declares. Siemens had the wire specially made at Furukawa Electric Co., a Japanese supplier.

The engineers here are the ones doing such workaday research, shaving weight off by the gram. That handcrafty approach makes these hand-built gems more costly than any Rolex. When I heft something to gauge its weight, Balázs flinches visibly. Carefully, I put it back.

In a few years, he tells me, thousands of these motors will be made every year for those air taxis, which Siemens and all its air-taxi rivals predict will swarm over cities like locusts. That’s when unit costs for the motors will drop, and probably to levels well below today’s comparable internal-combustion engine, with its hundreds of parts and countless complex mechanical interactions.

Gram-by-gram work gives way, eventually, to a revolutionary improvement. A big one came in the early 1980s, when General Motors and Sumitomo Special Metals separately introduced superpowerful neodymium magnets in motors. For the next game changer, look to machines whose electromagnets are wound with superconducting wire.

A motor-generator would lose almost no energy as waste heat if it had superconducting windings—a dream that could be entertained only after the advent of high-temperature superconductors. These ceramic materials superconduct at –135 °C, some 100 °C “warmer” than the original metal superconductors did. So, rather than cooling them with liquid helium, at a bare whisper above absolute zero, designers can rely on liquid nitrogen.

Siemens has been working on this concept for nearly two decades. It originally planned to put superconducting motors aboard ships at sea, where space and weight are at a premium. Even so, the company’s current version of the machine (used as a generator) is a piece of furniture bigger than a grown man. So the company’s engineers are now miniaturizing the machine for use in aviation. Its immediate power-density goal is 10 watts per gram. Siemens won’t show me this stuff, just a picture of the bigger supercooled machine with a diagram of the future aviation version superimposed on it. The drawing is maybe a tenth the size of the image.

Other companies are also on the hunt. GE Aviation is working on cryogenically cooled machines for NASA, but about this GE won’t say much either. All these companies are tight-lipped; perhaps they’re loath to tip their hands, or maybe they haven’t got much yet to show. In any case, NASA estimates that passenger planes using cryogenic systems of 30 MW and up won’t be ready to fly until the mid-2030s.

To take full advantage of such a superconducting motor—and, in a hybrid system, a superconducting generator as well—you’d want superconducting power inverters, too. NASA has a contract with GE to produce one that can handle 19 kW/kg at 99 percent efficiency.

Integrating the motor into a hybrid design—probably using a gas turbine to spin the generator—is still in the works. Siemens engineers are first modeling everything in silico, an interactive simulation I’m shown just a glimpse of, on a computer screen. And the glimpse I’m given is of the current iteration, of a normally cooled machine. “It’s a series hybrid, and this tells us how the power distribution will work,” Balázs tells me.

Today gas-turbine generation is used most commonly as backup power for the grid, where the weight of the components matters not at all. However, plenty of military aircraft now flying pull electrical power from turbines powered either by the jet engine’s compressors or from airflow produced by the forward motion of the plane.

It may seem like a great deal of work to save just a few kilos, but every bit counts. A kilo saved on the motor gives you precious extra kilos for the batteries. When United Airlines recently began printing its in-flight magazine on lighter-weight paper, saving 28 grams (1 ounce) per issue or about 5 kg (11 pounds) per flight, it calculated the move would save the company 640,000 liters of fuel every year, worth US $290,000.

That’s why new airliners, such as the Boeing 787, include so much carbon-fiber reinforced polymers. So does the Magnus eFusion: One guy is all it takes to pull the plane out of a hangar.

Okay, let’s fast-forward to the finished product. It’s a dozen years from now, and airlines are operating hybrid planes that are so quiet they can fly at night over cities. Thanks to their swiveling props, they can take off from shorter runways, perhaps even a runway right inside a city. They save energy because they’re lighter and more efficient. That means they cost less to run and therefore to own—the inverse of the situation with commercial aircraft today, where operating costs dwarf the purchase price.

One caveat: In the midterm, a decade or so hence, mere hybrids will be only slightly “greener” than conventional aircraft. The big improvement will come after experience and economies of scale with hybrids allow the industry to make the transition to pure-electric planes, perhaps in the 2030s. “We see energy savings of from 4 to 20 percent” through hybridization, says Otto Olaf, head of sales and business development in Siemens’s Munich office. “If we fully electrify the plane, there are even bigger savings.”

Just as compelling to the airlines is the associated reduction in the greenhouse gas emissions. “The European Union’s Flight Path 2050 aims to reduce emissions by a factor of more than 2,” says Siemens's Anton, “but by then passenger travel is expected to double, so we’ll need at least a fourfold improvement.”

It isn’t clear just how these numbers are to be calculated. The easy way is to compare exhaust emissions against passenger-miles. The more honest way is to estimate “well to wake” effects, taking into account the expected source of any electricity that might be generated on the ground and stored in batteries for later use in the air. This calculation would also have to account for how much energy is used to make the batteries, the motors, the ultralight carbon-composite airframe parts, and all the rest.

That same EU program also aims to reduce aircraft noise by half by 2050. And that, it turns out, is the greatest motive of the airline industry right now. Just to get around restrictions on nighttime flights, airlines sometimes spend money to muffle their older, louder planes, a job called “hush kitting.”

“That was the biggest surprise when Siemens started talking to airlines,” Anton says. “I was always mentioning quiet operation as the third thing, after energy and emissions. Now it’s the first thing.”

This wouldn’t be the first green technology to succeed for reasons having little to do with global warming. People bought the hybrid Prius to save on gas; they buy the Tesla to out-accelerate a Porsche. Airlines will buy hybrid-electric airplanes for their quiet operation, and lower greenhouse gas emissions will come almost as a side effect. But they will come.

This article appears in the June 2018 print issue as “The Prius of the Sky.”