Hacker News Reader

Comment by quanto | Hacker News Reader

The web page claims that the electric motor can output 2000 kW at 10 kW/kg.

For comparison, Europrop TP400, a modern turboprop engine, has

8200 kW 1938 kg (= 4.1 kW/kg)

So, although the electric motor is smaller by a factor of 4, its power density is about 2.5 times higher. The web page claims that the motor design is scalable up to 4000 kW, which is great.

The fastest turboprop airplane was TU-114, a Soviet airliner in the 50's that flew almost as fast as modern jet airliners. TU-114 uses four engines (Kuznetsov NK-12) that are the most powerful turboprop engines ever in production (11000 kW at 2900 kg). We need more powerful electric motors to emulate this performance..

View on HN

Big claims for sure, but it looks like they're tied in with ARPA-E and a major airline. So, it's not some backwoods mechanics making bold claims about their infinite energy machine that runs on their special brew of moonshine.

https://mediacentre.easyjet.com/story/14628/easyjet-partner-...

Not surprising, the electric motor has some fairly high quality energy flowing in but the gas motor has to act as a heat engine as well, converting localized heat into higher quality energy as it goes.

That means a comparison of power densities between electric and internal combustion motors aren't entirely fair. But it also means that electrical motors have a big advantage in that it's a lot easier to add more of them. It looks like aircraft with many propellers along the length of the wing might have real advantages over single propellor aircraft and that's only something that's practical with electric motors.

> The web page claims that the electric motor can output 2000 kW at 10 kW/kg. > For comparison, Europrop TP400... > 8200 kW 1938 kg (= 4.1 kW/kg)

FROM SITE:

>2 MW - 2x improvement in []size[] over megawatt scale motors being demonstrated in the aerospace industry >10kW/kg specific power - a 2x improvement compared to available aircraft propulsion motors.

To make a fare comparison, you need to also compare the volumetric power densities (i.e. power delivered within a volume envelope vice just under a given mass threshold).

This is true for both energy and power densities - and it drives me bananas that these metrics aren't clearly specified for every single energy storage and conversion device.

Why does this matter? Because increasing a vehicle engine volume demands more supporting structure (frame), which generally drives up overall vehicle mass (hence cost of construction) and if the vehicle is operating in some fluid (air or water) the resistance to motion increases which drives up your installed energy and power for the same speed.

Imagine if you increase your car's mass pwr density by 2X, but doing so grows your engine compartment volume by 2X. Was that a wash? You can't tell until you eveluate the construction cost, energy storage and mpg impacts of having a larger car frame. Same is true with any vehicle.

Mass (specific) density is important, but volumetric density is too. Both need to be compared for a more thorough apples:apples comparison.

Why not make a ducted engine? Just replace the fuel burning part of a jet engine with this motor?

Why do you need a duct?

Jet engines achieve higher speeds and efficiencies.

No, not efficiencies. Jet engines have lower efficiencies than propellers up to near 0.8 speeds of sound, and terrible ones at low speeds, and low altitudes.

Last generation airliners have designed cruise speeds below 0.8 speeds of sound.

So, a propeller can still be a viable option at that speed, it will just have to be enormous.

If propellers are more efficient, then why do airlines use jet engines? Noise? Reliability?

There's a reason jet engines completely dominate the market for commercial airliners. Would making the propellers electric change that?

Because jet engines have better power to weight ratio than cylinder engines.

The comparison talked about here is not between jet and piston engines, it’s about turbofans and turboprops.

> then why do airlines use jet engines? Noise? Reliability?

- Noise, yes. EU effectively bans turboprops by noise limits

- Reliability, yes. Gearboxes needs frequent, and expensive overhauls. That's acceptable on smaller regional planes, but not airliners costing tens of megabucks.

- Size. There is an upper limit on propeller blade size after which aeroelastic effects cannot be overcome by any known material.

I would argue then if you can't practically build a propeller to replace a jet engine on an airliner, that being more efficient in theory is meaningless.

I'll refine my initial claim to say jet engines are more efficient in many practical uses and configurations. Would you agree with that?

Would agree partially.

5-6 seat abreast planes can be switched to powerful turboprops for practical use.

Widebody airliners, not at the current level of technology.

Greater parasitic drag from the duct?

The compression effect of the jet engine blades is what ignites the fuel. I don't think that part is replaceable, it's what makes a jet engine a jet engine.

The hot gasses from the fuel burn produce thrust, and also sends rotational force back into the turbine blades to keep the compression effect going.

> The compression effect of the jet engine blades is what ignites the fuel

Not exactly, the igniter ignites the fuel initially, and after a short while the combustion becomes self-sustaining (i.e. new fuel is evaporated and heated above its autoignition temperature by all the residual heat around it). The igniters are turned off after a while and are only turned on again under certain conditions (high risk of flameout for a variety of reasons).

Also the "hot gasses from the fuel burn produce thrust" part has slowly become more and more incorrect, because nowadays in airliners we use what's called high-bypass turbofans where the primary purpose of the hot engine core is not to generate thrust, but to drive a turbine connected to the huge fan in the front, which sends most of the air AROUND the core ("high-bypass") in order to create thrust. The core does produce some thrust, but the air coming out of it is pretty fast and creates more noise, so we try not to have too much of that.

And the core itself doesn't really need to burn fuel, you could rip the combustion chamber out and put a nuclear-reactor-fed heat exchanger there and it would work the same.

Or a beefy electric motor that would drive the fan and also utilize its waste heat to imitate a very-high-bypass turbofan.

Thanks for the detailed explanation. I am not connected to the aerospace industry.

In the context of retrofitting an electric motor where the fossil fuel combustion happens today, it still doesn't seem possible.

> Or a beefy electric motor that would drive the fan

Isn't that what Weight is doing with their motor? Motor attached to fan blades to produce thrust?

The compression is the easiest part to emulate. The hard part is the enormous amount of heat you need to generate and quickly transmit into that compressed fluid. Combustion is great on both accounts for that, but you could do it electrically as well given a sufficient power supply.

> The hard part is the enormous amount of heat you need to generate and quickly transmit into that compressed fluid.

Without a combustible fuel in the picture, isn't it more efficient for the kinetic energy of an electric motor to directly spin fan blades or turboprop propellers, vs converting that energy to heat?

Maybe! There are certainly some operational regimes where it would be less efficient to spin props than it would be to shoot out hot gas, but they might be places where an electric aircraft wouldn't ever operate.

> although the electric motor is smaller by a factor of 4, its power density is about 2.5 times higher

Is there any standard way of calculating this with the battery / full tank?

I think it’s most appropriate to compare vs “tanked for a given/defined range (including reserves)” rather than fully tanked. Airliners are rarely flown with full tanks, because that’s rarely needed. (Tanks are often sized for maximum ferry missions rather than maximum cabin load missions.)

See my comment to answer this question. Volumetric power density impacts are neglected in the OP's 'back of the napkin' estimate comparison. As you suspect, you need to account for the motor size and the specific application to see if this motor is a benefit to you.

Complicating matters is the fact that the OEM does a poor job of presenting these metrics up front on the landing page.

>TU-114 uses four engines (Kuznetsov NK-12) that are the most powerful turboprop engines ever in production (11000 kW at 2900 kg).

Interesting detail about the NK-12 (according to Wikipedia):

The design that eventually became the NK-12 turboprop was developed after World War II by a team of Soviet scientists and deported German engineers under Ferdinand Brandner, who had worked for Junkers previously; ... Thus, the NK-12 design evolved from late-war German turboprop studies.

Do longer/larger prop blades translate to better efficacy?

I wonder if we would be better off with array of smaller electric motors rather than pursuing 1:1 replacement of ICE engines.

all of those options have been studied for years; how many motors, what they weigh, where they are mounted, blades on the prop, rpm on the prop, any gear reduction. there is a room of aircraft designers that will show you why the current design is the "sweet spot", and the technical reasons that drove them there.

my point being this was all analyzed ahead of time, in modeling. no-one just said, "lets go with 4 foot props"..

Electric motors are more compact, need less cooling, work well at all altitudes, have likely more mounting options, can spin fast, offer different power curve, probably easier to service and diagnose.

I think there's enough of differentiating factors that could move the "sweet spot" when compared to ICE motors.

I think lots of little motors are the approach lilium is taking. https://lilium.com/ I’m a firmware engineer not an aviation engineer so I have no idea what the efficiency would be.

This NASA project seems to be pursuing that line if thinking

https://www.nasa.gov/specials/X57/

Depends on flight speeds and prop rotation (tip speed specifically), but the propellers have for a long time had a 90+% efficiency already. Ideally, a long and narrow one-bladed propeller would have best performance, somewhat similar to sailplane's wing. For practical reasons (ease of balancing and limited diameter) we use two or more shorter blades, even if it causes minor efficiency loss.

A compounding concern is that of a propeller tip speed - when it approaches speed of sound, the physics of airflow become complex and it's very hard to maintain efficiency - and near impossible to maintain reasonable noise levels. This necessitates keeping blades of high speed propellers rather short. The Tu-114 was exceptionally loud for a plane, both due to propeller speed concerns, and also due to contra-rotating propellers inherently causing increased turbulence.

There is a good four-part series on propeller theory and practice:

https://www.youtube.com/watch?v=SgoPPg8oVt8&list=PLD2EcpzcvT...

If all else is equal (in reality it isn’t), you always get more efficiency by increasing blade length.

You want to maximise the momentum of the air your moving in any unit time for a fixed energy cost. If you double blade area you double both momentum and energy, if you halve the blade speed you halve the momentum and quarter the energy, so if you quadrupled blade length while halving the speed you get double the force for the same power.

This is of course a gross oversimplification, but it is why the human-powered helicopter a while back had such massive blades: https://en.wikipedia.org/wiki/AeroVelo_Atlas

the original article shows ten electric engines on their planned plane.

A question to ask... is it the engine weight only, or engine + its gearbox?

It's very easy to make very high power to ratio electric engine, if RPMs, and voltages can be arbitrarily high.

Reply on news.ycombinator.com