Rolls-Royce concludes testing of plane technology to break electric speed record | Hacker News Reader

Rolls-Royce concludes testing of plane technology to break electric speed record

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90 pointslelf103 comments

Speed is comparatively easy for electric propulsion -- on land (Tesla) or air (here).

Range is quite another issue. A Cesna 172 has a 1,289 km range, four times the Rolls-Royce prototype, and carries more than just the piolot and fuel.

https://en.wikipedia.org/wiki/Cessna_172#Specifications_(172...

For both land and air, total payload is another hard limit. Tesla achieves admirable results for a passenger vehicle by using high energy/volume and energy/mass batteries integrated into the vehicle structure itself. Battery EV trucks have had range limits of measured in single-digit kilometers. Their fuel-based counterparts exceed four digits (1,000 km). Accellerating, decellerating, and managing mass costs joules.

The basic problem with electric aircraft vs. electric cars is that aircraft spend a much larger proportion of their journeys operating at high power levels.

A slow, economy cruise setting in a Cessna 172 is a 45% power setting - ~80 hp, and climbs will be at full power (180 hp).

On the other hand, you might only need ~20 hp to maintain 65mph on a level road in a sedan; and most people never use anything like the full power of their cars for more than seconds at a time.

That, and the fact that fuel-powered aircraft will shed half their takeoff weight during flight. Battery-powered aircraft either maintain constant mass or gain mass in the case of metal-air batteries, as energy is discharged.

Long-haul alt-energy aircraft utilise very low airspeed (low drag), extreme mass budgets (they're wonders of materials and structural engineering), and altitude profiles (potential energy0 to manage overall energy budgets and use.

>> fuel-powered aircraft will shed half their takeoff weight during flight.

While that is true of jets optimized for altitude, small piston aircraft (cessnas) do not burn nearly that much fuel on takeoff. Small aircraft can generally do a dozen or more takeoff/landing cycles on a single tank.

Fuel is burned throughout the flight, not just take-off (and initial climb to altitude), which is why airliners flying long-haul routes usually gain altitude as they progress. The Cessna 172 seems to have a fuel fraction of around 20% (an order of magnitude higher than a car), and airliners are much higher (~40%).[1][2]

[1] https://en.wikipedia.org/wiki/Cessna_172

[2] https://en.wikipedia.org/wiki/Fuel_fraction

A 2020 Toyota Camry --- a typical 4-door sedan --- has a curb weight of 3,340 lb and a 15.8 gallon fuel tank (95 lb), for a fuel-to-weight ratio of 2.8%.

Range is ~615 miles at 39 mpg highway.

Passenger and cargo capacity is another 1,500 lb on top of this, give or take.

This is far lower than any conventional aircraft, though a few ultralights might compare.

Even on a tiny little Cessna fuel is 1/8th of the weight, and fuel burn per hour in cruise is approx. 50lbs/hr.

And again with my Camry example, assuming 60 mph and 39 mpg, that's about 9 lb/hr fuel consumption.

The Cesna has a 140 mph cruise, a bit over double that of the Camry on highway, but that still means 20 lb at equivalent distance for the car, less than half the light plane.

Aircraft are fuel burners.

(Agreeing, just giving land-travel equivalent.)

I think this depends where you're driving. If you're at 130kmh in a 70hp subcompact, something perfectly common in Europe, you're pretty close to max power..

A 70hp compact can't maintain 130kmph on a climbing highway, only on a flat highway. I know this from experience - I had to pull into the slow lane with the trucks. Plus, a compact car at full throttle is really an unpleasant experience - the car wheezes at high volume and you can feel it's doing its best and it's not enough.

Agreed. Which demonstrates that cars often do run at 80-100% of their capacity for long periods of time

I don't think that demonstrates that at all, but even assuming it did: when they do so, they get diabolical gas mileage.

Top Gear did an amusing test where they had a Toyota Prius drive flat out around a track, with a BMW M3 (a larger, heavier sports sedan) keeping pace with it.

Over the course of the test, the Prius returned 14.3 miles per gallon; and the BMW M3 achieved 16.1 - despite the fact the BMW has a 4 liter turbocharged, six cylinder engine vs. the Prius 1.5 liter, four cylinder.

To be fair, that test way way above legal speeds. I don't think it's representative even of motorway performance.

I don't see your point however - what I'm saying is that at 130kmh (standard motorway speed in most of europe) a 70-90hp subcompact (by far the most common car in europe and the world in general) is running at above 80% max power (testified by the fact that as soon as there's a significant incline their speed will fall back to 110-120). So running long times under heavy engine load is quite normal for cars

>aircraft spend a much larger proportion of their journeys operating at high power levels.

Luckily the high power level is maintained at reliably high speed [0], ensuring airflow for cooling.

Even better, aircraft by its very long-range nature will have much larger battery:engine ratio. For example this 2018 article [1] discusses airplane with 9x the Tesla's battery capacity and 3x the Tesla's engine power, in other words, its batteries would need to handle only 1/3rd electrical load as much at peak load (per unit). The lower power density[2] of the battery pack again provides for easier cooling; perhaps even passive cooling or air cooling instead of forced liquid cooling. Whereas Tesla's sustained performance is limited by battery pack thermals.

Lastly, it's worth noting the ICE aircraft engines are "overbuilt" as compared to the roadgoing ones, mostly for sake of reliability.

[0] aside of the engine run-up and initial acceleration, but both of those are short anyway

[1] https://news.ycombinator.com/item?id=18602771

[2] not to mix it up with energy density

Elon solution for this is to go much higher then a typical plane. This is much more possible with electric and helps with efficiency.

https://youtu.be/MBItc_QAUUM?t=2422

Getting higher is no easy feat, especially given the energy density of the current state of the art batteries.

Not to mention additional complexities going above 10,000 ft that adds weight: pressurised cabin (or oxygen) and anti-ice and de-ice systems and heating.

Exceeding 10,000 ft. is orthogonal to needing anti-/de-ice systems. In fact, you're often less likely to need them at the higher altitudes because it is too cold for airframe icing to occur. Counterintuitively, if it is too cold, you no longer have icing problems.

You need to pass all of the feet leading up to 10'000 feet though.

You can use the batteries as structure and they produce heat that you need to cool anyway. Seems like there is a lot you can do with that once you start optimizing everything around the battery.

I don't know how much weight a anti-ice/de-ice could add.

My point was only that once you think about it and redesign everything around the new constraints, you can come up with things that we don't yet think about.

The issue is that you do NEEd air in order to use the props. So there is a limit to how high you can go.

Air resistance vs not having enough air to push.

Basically - you can design the prop for high altitude, but then takeoff is gonna not be great. You could design a variable prop system (or even have 2 different ones?) but then weight / complexity.

https://www.hindawi.com/journals/ijae/2018/5782017/

If you go supersonic you get enough air in even when flying higher.

His plane is VTOL so the overpowered engines for supersonic will work well for liftoff.

>Accellerating, decellerating, and managing mass costs joules

Ideally, aside from parasitic losses, shouldn't decelerating pay joules?

Net it's a loss compared to constant velocity.

A few years ago an electric bus hypermiled to get some ludicrous range. Turns out the trial stripped all spare mass from the rig (seats, other interior), pumped tyres to munition-rated pressures, hugely oversized the battery, and held a constant speed of about 24 kph (15 mph).

https://www.latimes.com/business/autos/la-fi-hy-proterra-ran...

(The article, as with most, reveals no test conditions though does at least note "a Proterra spokeswoman said they won’t reveal the test bus speed".)

In production, range claims seem to be about 1/3 this.

Typical city busses stop every block or two, accelerating to about 40 kph, then braking, every 60 seconds or so.

Both mileage and wear and tear suffer immensely.

I'm not sure this is true. I've been told that, for example, the "city" MPG of hybrid cars with regenerative braking is higher than the "highway" MPG. This is because it simply costs energy to maintain speed in the face of hills, wind resistance, and parasitic/rolling losses. However in "city" driving much of the energy is reclaimed through braking.

A big part of this is wind resistance; as you know wind resistance increases with the square of the speed, so going 60MPH is going to cost quadruple what it costs to go 30MPH, and presumably "city" driving doesn't go much higher than 30 MPH.

> Battery EV trucks have had range limits of measured in single-digit kilometers.

I don't believe you. Find me an EV truck that any company has fielded in the past 20 years that has less than 10km range.

Also you're comparing a prototype to a mature technology. One might as well note that early automobiles had less range, lower speed, worse reliability, and higher cost than horses. And unlike horses, automobiles require infrastructure such as roads and gasoline stations. All of those things were true at the time, but horses couldn't be improved upon and automobiles could. Now we're in the same position with respect to gas vehicles vs electric vehicles.

Battery technology is improving at an impressive rate. Energy density has doubled since 2010. Price per kilowatt-hour is 1/9th what it was a decade ago.[1][2] These advances have made EVs competitive on the ground. It's quite likely that improvements will continue and EVs will become competitive in the air. EVs have lower refueling costs, lower maintenance costs, and are simpler designs (making them cheaper and safer). We already have battery-powered commercial drones. Human flight is more conservative when it comes to adopting new technologies, so it'll probably take longer for EV planes to become widespread. But long term? I would bet on EV planes replacing regional jets.

1. https://cleantechnica.com/2020/02/19/bloombergnef-lithium-io...

2. https://www.statista.com/statistics/883118/global-lithium-io...

BMW's Terberg YT202-EV electric tractor, tested~2015, comes to mind. Its route during trials was 2km, though the claimed range was 100 km. I strongly suspect the latter was slow and unloaded. Speed was restricted in any regard:

BMW and the SCHERM Group have put a massive 40-ton EV into service for a one-year pilot. The Terberg YT202-EV electric tractor will travel a 2 km route eight times a day between the SCHERM group logistics center and the BMW plant in Munich, transporting vehicle components such as shock absorbers, springs and steering systems.

The standard tractor has two batteries (112 kWh), and a third battery can be fitted for extended operations. It is equipped with a 138 kW, 720 N·m Siemens motor and an Allison 3000 transmission. Top speed is 25 mph.

The YT202-EV has a range of up to 100 kilometers (62 miles), theoretically enough for a full production day. BMW says it will be charged exclusively with renewable electricity, and will save 11.8 tons of CO2 annually compared to a legacy diesel truck.

https://chargedevs.com/newswire/bmw-tests-40-ton-electric-tr...

Unless they charged it at every stop, that's a 16 km drive between charges, not 2km. 112kWh is enough to run the 138kW motor for 48 minutes full-out. At 25mph, that's 32km, so driving without stops cannot be less than a 32km range, and is probably significantly more (trucks run closer to to full power while cruising than cars, but still not 100%).

I may have understated somewhat.

Will you grant double digit km range? Still two orders of magnitude lower than a fuel-based truck in commercial operation?

150 gallons * 7 mpg = 1,050 mi / 1,690 km.

There’s more than just increasing capacity to address the range issue. Further, two digits of difference will be overcome sooner or later.

Such as?

Petrochemical fuels afford a near unbeatable energy/volume and energy/mass density. The best theoretical battery might attain 1/10th this, practical batteries are nearer 1/100th, if that. A ~100 kWh battery holds the equivalent of 2 gallons / 8 litres of petrol or diesel.

Electrics have advantages of vastly more efficient potential-to-kinetic energy conversion (90%= rather than 30% for an ICE), a more efficient drivetrain (often in-wheel motors), and regenerative braking. Air and tyre drag are equivalent for both domains.

Capacity, energy management, and recharge access are all there is.

Two-order-of-magnitude improvements in any non-informational domain are both very hard-won and utterly transformational. Commercial passenger aircraft are ~2 orders of magnitude faster than walking. A semi-trailer has ~2 orders of magnitude more cargo capacity than a man with a wheelbarrow.

Getting two orders of magnitude out of long-haul road-based heavy-cargo EVs is a very tall order. Mains-fed electric trainsets would be a far more likely technology, and are extant and commercially proven

I wonder how long haul trucks compare to passenger vehicles for energy efficiency.

Diesel engines tend to be more efficient the larger they are, with marine engines hitting 50% in fixed speed applications.

You'd want to compare net capacity as ton miles/gallon. Preferably as cargo rather than gross weight, though I'm using the latter for convenience.

Passenger cars net about 90 or so, for a high-efficiency vehicle, gross vehicular weight, at full capacity.

Assuming 140,000 lb GVWR and 7 mpg, I compute 490 ton-mpg. That's roughly equivalent to numbers given for rail ("move one ton of freight one mile on a gallon of fuel), and is likely high, though 100-250 ton-mpg seems likely.

This is not net cargo efficiency, where that rail number may come from. But gives a sense for numbers.

One option is to have multiple vehicles and switch payloads or batteries.

As others noted, we’ve just started taking a serious interest in renovating rechargeable batteries for this use. Lots more can and will be done once the political will is there.

Possible, though greatly increasing capital costs, also complexity and coordination dynamics.

Even conservatively estimating 80% efficiency and 80% cruise speed, and estimating 25% peak power during cruise, that's:

    83.1 km = (112kWh x 80% x 25 mph x 1.6 km/mile x 80%) / (138 kW x 25%)

which is an armchair number plenty close to the claimed 100 kilometers. And if it only has to go 2 km eight times a day, it's going to use less than a third of its capacity.

Well, not in the last 20 years, but how about the last 80? From the wikipedia article on milk floats in the UK:

> In 1937 they produced a ride-on four wheeled vehicle, suitable for a payload of 8–10 cwt (410–510 kg) and with a range of around 35 miles (56 km)

Due to the way that power is delivered in an EV, the lack of a regular gearbox, and the non-linear nature of air resistance, EVs have ranges that increase the slower you go. At 20mph the range of a Tesla damn near doubles by some estimates. This is the opposite of how ICE cars work, which typically get better MPG at higher speeds due to gearbox design and variable efficiency based on engine RPM.

That’s probably why milk floats could go so far at the time despite obviously inferior battery technology; milk floats didn’t go very fast.

To this point I wonder how full autonomy will impact this. A truck that drives itself and doesn't have to stop (besides to charge) will easily make the case for a freight or autonomous lane on highways where the vehicles don't go above a certain speed (67mph or 100kph). I don't really care if it takes me 10 hours to go 500 miles if I watch a movie, go to sleep, an wake up in front of my hotel.

Freight should be done more by rail. It’s way more efficient to ship by rail, and easier to electrify. Our heavy usage of semi trucks is partially due to the hidden subsidy that semi trucks get via road maintenance, where semi trucks don’t pay anywhere near enough gas tax to offset the damage they do. EV trucks will be worse.

Generally speaking, the solution for car troubles is to reduce the scope that cars and trucks are used to solve transit problems. Road damage due to semi trucks? Build more freight rail! Inner city congestion? Add a subway! Long commutes? Build more housing, and if that doesn’t work add dedicated commuter rails. Too much air traffic? Offset local flights onto high speed rail to free up room for long distance flights.

https://en.wikipedia.org/wiki/Milk_float

The Nikola One has a range of 0km (unless it's powered only by gravity)

> I would bet on EV planes replacing regional jets

Energy density of batteries are around 1/40th of that of kerosene (0.9 mj/kg vs 44mj/kg) so we are talking 44x the energy density we need to make up for to get perfect parity. From what I can find I’ve seen 35% thermal efficiency for modern airliners and I believe 95% for electric motors. Ok so this means we can reduce the density gap required for parity to just under ~ 44/2.5 so ~18/19 mj/kg? (I’m doing this in my head so it’s rough). Rough calls show we need batteries with 20x the capacity to replace what we have right now with electric alternatives. While the gains in battery densities have been impressive we need to keep these gains in mind that we need to be making much bigger gains for this to become a reality. Personally I don’t see it being feasible for a very long time given the progress and I believe we’re better off taking fewer unnecessary flights than hoping on some miracle battery breakthrough

(Edited for clarity)

Your calculations are correct, but you've baked a few other assumptions into them.

- Fuel has no structural strength. Batteries are rigid and can double as parts of the airframe, saving weight.

- Combustion engines are complex and expensive. This precludes mounting a dozen tiny engines on a plane and integrating them at just the right spots. Electric fans are cheaper and easier to integrate into the airframe, reducing drag. This also helps with engine-out capability. If you have two engines and one fails, you've lost half your thrust. If you have 36 engines and one fails, you've lost 3% of your thrust.

- Air becomes less dense with altitude. This reduces drag, but it also reduces the power of combustion engines. Electric aircraft don't need oxygen, so their power remains the same at altitude. There are losses due to the fan having to rotate more to move the same mass of air, but in general, electric aircraft become more efficient at altitude.

- Electric fans respond much faster to throttle inputs and are easier to gimbal than combustion propellors or jets. This allows designs with reduced or even eliminated control surfaces. The lack of rudder and elevator reduces drag, allowing for greater range.

That last trick might sound insane for human-rated aircraft, but there are already flying prototypes that use this approach.[1][2]

1. https://www.youtube.com/watch?v=5ukmS9ZJm40

2. https://www.youtube.com/watch?v=ZH7DSFRCqDQ

I wonder if you could significantly charge the batteries from the energy of a vibrating wind. Or (and this is out there) from atmospheric electricity

You can definitely significantly charge batteries from sun - see https://en.wikipedia.org/wiki/NASA_Pathfinder - but it's at the moment far from a convenient commercial solution.

Did we forget that electric vehicles were some of the first sold in 1900's? Now imagine what battery tech we would have if a hundred years of development went into battery technology?

Be patient, we'll figure something awesome out.

Here is Elon idea to 'fix' this issue. Some interesting ideas.

https://youtu.be/MBItc_QAUUM?t=2422

- Going higher

- Using gravitational energy on the second half/landing part of the journey. Like a car going down hill.

- Making use of the higher power density of the engines

- (Doesn't mention this, but using batteries as structural elements)

I also think that this:

"Prandtl Wing Minimum Drag Update" - Al Bowers (Chief Scientist at NASA) would be a key technology

https://www.youtube.com/watch?v=bCwtcDNB15E

A new type of wing and maybe more importantly a new type if turbine fan.

So basically Elon has suggested doing all the things they already do today to maximize fuel savings in ICE planes...

Note that airplane fuel has significantly higher power density than an electric battery (~40x as much, see https://www.theverge.com/2018/8/14/17686706/electric-airplan...), so no EV will ever be able to achieve the same range as an ICE plane, ton-for-ton.

> So basically Elon has suggested doing all the things they already do today to maximize fuel savings in ICE planes...

No. If you think that you didn't get his point.

- Fuel is not structural in a plane. Only fuel tanks.

- ICE are only optimally efficent at a particular height.

- ICE can not convert gravitational energy into fuel.

- ICE vehicles have a lower power density and are bigger. Making it harder to gimbals them for VTOL. This is a cause of huge issues with military VTOL.

> EV will ever be able to achieve the same range as an ICE plane, ton-for-ton

And they don't have to to beat ICE in the market.

1) If you want to get that technical, like fuel, battery cells aren't structural either. The battery casings like the fuel tank shells, are the structural components that can be used as structural elements. And on some planes, they are used in that manner for weight-saving.

2) Irrelevant. They're still more efficient than EV planes at any height, so comparing the relative efficiency of ICE to itself is pointless.

3) Neither can EV planes, unless someone has discovered how to turn gravity into electricity without water and a massive turbine.

4) ICE vehicles have higher power density (see previous Verge link, also https://www.topspeed.com/cars/warp-coils-seem-closer-to-real...). It's not even close. 43x-100x the power density of batteries, depending on the type of fuel and the type of battery. ICE vehicles are bigger because they are used to carry more things and people: hundreds of people or hundreds of tons of cargo on trips that can go almost halfway across the world without stopping. In contrast, an EV plane can currently carry itself, a pilot and a passenger, for almost long enough to get from LA to Fresno. (No EV currently on the market can even make the trip from LA to SF.)

And they don't have to to beat ICE in the market.

You're right. EVs will never be able to compete at the ranged market; they'll be limited to short-range hops. This means they won't be a viable option for the corporate/personal jet market beyond limited hobbyist use.

1) The can is a necessary element and they can be structural. This is a fact.

2) No, that is just flat out wrong. EV are more efficent, its not close. And they don't have 1 hight where they have the best efficency.

3) Yes they can. Have you ever driven an EV?

Its seem that you are just otherly clueless and seem to simply deny the existens of proven technology so there is no point in this argument.

I mean, this plane is a sprinter. If you want range, use sailplane techniques and with similar chemistry as for terrestrial lithium ion you can get >1000km range with multiple passengers and speed just as high as high speed rail (or higher). Eviation Alice and other concepts show how this could be done. Just a matter of time and a LOT of effort.

Battery EV trucks (lots of prototypes and maybe even low volume production going on, from Tesla to Daimler) regularly get multiple hundred kilometers of range and 1000km isn’t at all unreasonable although 500 miles is probably the sweet spot in the near term to avoid reducing available payload (any greater range has diminishing returns due to regulatory requirements for rest stops). And that’s without advanced chemistries that you can get now at low volumes like metal anode or lithium-sulfur which double the specific energy and allow double the range (to say nothing of lithium-air, which is easily over a decade away from practical use but would eliminate the disparity in useful specific energy between battery-electric and hydrocarbon combustion in almost all cases—rocketry and munitions being the major exception).

EDIT: Daimler has delivered some 300-400km range electric Freightliner semis for customer testing already: https://electrek.co/2020/03/04/daimler-electric-freightliner...

I get what you are saying, but it's going to be hard to compete with the speed of a jet engine, with propellers or ducted fans.

Unless I'm mistaken? Can ducted fans match the speed of a jet engine?

Most of the propulsion of a modern turbofan engine comes from the fan rather than the jet turbine so ducted fans can match the speed of a turbofan.

> Battery EV trucks have had range limits of measured in single-digit kilometers

Battery powered trucks that you can buy today from the likes of Scania/Daimler has a range of 200+ km. Not some prototype but currently in production for delivery next year.

Sure, it's not 1000 km, but that single digit km is off by two magnitudes.

Is it possible to assist the plane during its take off phase? Maybe with a electric wire that supplies energy and detaches. I know it’s messy. Or maybe using something like balloon to gain altitude and then engines to cruise? Or a special battery bank that gets depleted during take off and detached + a recovery mechanism?

I was just htinkin the same thing. Having a huge battery pack attached and then parachuted off after takeoff in a designated spot could work really well. Or even like a "plane assister" that just stayed attached for takeoff and then dropped off and landed back at the airport to re-charge / help the next plane.

I think it's all moot tho. Airbus seems to be going hydrogen for their next planes to have 0 emission.

The hydrogen project was basically started to take advantage of a grant from the French government, there are many doubts as to its viability.

The major issue with Hydrogen is usually containment (it just leaks through almost everything).

Do you have any links to the doubting of it's viability?

Aviation Week has had some coverage on the matter, their podcast is pretty good: https://aviationweek.com/aerospace/podcast-airbus-ambitious-...

Electric motors in the wheels might help on the takeoff roll during initial acceleration to rotation speed (100-150 mph). Perhaps also regenerative braking on landing. And taxiing.

Well, they do it on aircraft carriers, so technically speaking, it's possible.

Electric rail along the runway while taking off?

I find it interesting that Rolls-Royce Motor Cars is developing this, as opposed to Rolls-Royce plc - the company that works more closely with aviation.

-Even more interesting given that Rolls-Royce plc has a number of boffins at a subsidiary in Trondheim, Norway working on exotic PM motor designs, including for aviation. (The company is called SmartMotor, was purchased by what was then Rolls-Royce Marine, then was retained under the RR umbrella when the rest of the Marine division was sold to Kongsberg a couple of years ago.

That appears to be an error in the article. Rolls-Royce plc has a press release on this, not Rolls-Royce Motor Cars: https://www.rolls-royce.com/media/press-releases/2020/24-09-...

Thanks, I didn't notice the link to the press release at the bottom of the article.

Is there any world where a series hybrid jet engine makes sense the way that series hybrid diesel engines used for trains does?

It seems like weight and displacement could be more of a concern. And turbines run at higher temperatures than diesel engines...

Well, Airbus had an event[0] last week about their hydrogen-hybrid passenger planes. And there the main power would come from adapted jet engines that combust hydrogen but the hybrid aspect would be fuel cell sourced electrical motor that would give some additional boost for some phases of the flight.

[0] - https://www.airbus.com/innovation/zero-emission/hydrogen/zer...

Yes. Or at least the concept/idea is being explored. From [1] "The hybrid version would generate electric power through a turbine within the plane. That power would be used to turn the fan blades of the single electric turbofan engine."

[2] Uses a slightly different approach "The propeller is powered by an electric motor with 65 kilowatts of continuous output. The electricity is supplied through a generator by a small Wankel engine that consumes little fuel"

[1]https://phys.org/news/2017-11-airbus-rolls-royce-siemens-hyb... [2] https://phys.org/news/2013-07-electric-hybrid-aircraft.html

The Wankel engine one is interesting. It seems the only way any of these make sense is with a new type of engine. Makes no sense to put a turbine in a plane just to run an electric motor, since it's more efficient to just use the turbine directly.

Agreed re the concept of [1]. Since the link in [2] was from 2013 I was curious to see if there were any updated PR releases but couldn't find any. I did stumble across [3] which seems to be the successor which uses a 3 cylinder diesel engine as the power source

[3] https://wiki2.org/en/Siemens-FlyEco_Magnus_eFusion

My recollection is that the problem with Wankels is the emissions, which they are claiming this system still reduces.

I wonder what a different engine option would do.

That Rolls Royce proposal seems to be based on the fact that airplanes draw power from the turbines and/or and auxiliary power unit.

They're driving one of the fans off of power parasitized from the other turbines. Technically that's a hybrid, but I was thinking more of a self-contained unit.

By singling our trains I’m thinking you are referring to diesel electric which is not the same as hybrid. On a train the generator to motor link it to act as essentially a clutch/gearbox allowing theoretic full torque at 0 rpm.

Most diesel-electric trains are not hybrids, they have no batteries and therefore always get power from the single source, the diesel engine. There have been attempts to make them hybrid by putting a car in that contains a lot of batteries for say storing power down a mountain to use going back up the next mountain rather than converting the power to heat in the dynamic braking grid. My understanding is so far it is not worth the cost or complexity for the minimal savings.

Diesel-electric is actually less efficient than a mechanical coupling at steady speed due to double conversion losses which is about 80% efficient vs high 90's for gears through standard mechanical transmission. Trains use the an electric drivetrain because they need precise traction control with huge torque to get the train moving, a mechanical transmission with torque converters, clutches and gears would be difficult to route power and wear out too easily due to the low speed lugging. Once up to cruising speed however it would be more efficient.

This is why there are no hybrid semis or even diesel electric semi's, standard mechanical transmission are more efficient and hybrids only have significant gains in stop and go traffic where as for mostly highway travel the hybrid would normally disengage and simply be dead weight in a parallel hybrid or simply a efficiency drain in a series hybrid. This is also why most hybrid cars are not series, they mechanically couple the engine to wheel for efficiency once up to speed.

Planes again would have little gains from being a hybrid as they would spend most of their time at cruise running the combustion engine and a straight mechanical connection from engine to prop is much more efficient and lighter than any electric double conversion scheme.

Electric planes might make sense because there is no double conversion or engine in the plane itself, but you still have the issue of energy density with batteries.

Excellent and well-written answer. I hadn't considered the dead weight of the generator. It's more proof that questions with seemingly simple answers rarely exist in the real world. As a friend of mine (a data analyst) likes to say: "the real world is messy".

Yes most people don't seem to realize the hybrid system tends to just shut down at highway speeds. The reason a hybrid car has good highway mileage is because it has skinny tries and good aerodynamics, if you ripped out the hybrid gear its would get even better highway mileage due to less weight.

There is another aspect specifically of gas hybrids they typically use Atkinson cycle engines to increase efficiency but have reduced torque. This makes it ideal for a hybrid setup as the electric motor can make up for the lack of torque off the line.

Notice there are basically no diesel hybrids, this is because a diesel engine is more efficient than a Atkinson cycle gas and has even better torque than a Otto gas but they weigh more and cost more, so it makes little sense to add more weight and cost to a diesel drivetrain that is already very efficient and has plenty of torque. City buses are the exception due to the extreme stop and go low speed duty cycle that can really take advantage of a hybrid drivetrain.

You can have Atkinson cycle with diesel, so 1) do they, and 2) how much does that offset the transmission losses?

I have never heard of a Diesel-Atkinson and technically Diesel is a cycle, so if its a Atkinson burning diesel fuel its no longer a Diesel engine although the terms get fuzzy between the fuel and cycle.

> "they typically use Atkinson cycle engines"

Only the early-model Toyota Prius used Atkinson cycle engines, up until 2003 or so. You'd be hard pressed to find a modern hybrid using Atkinson cycle. Any modern Prius uses conventional engines with variable valve timing.

Well they use the variable valve timing to operate in a Atkinson cycle but can change valve timing back towards Otto.

Most still seem to refer to these engines as Atkinson cycle even Toyota themselves: "The Prius retains its 1.8-liter VVT-i equipped Atkinson cycle petrol engine (2ZR-FXE)"

https://global.toyota/en/detail/9827044

https://en.wikipedia.org/wiki/Toyota_ZR_engine#2ZR-FXE

> You'd be hard pressed to find a modern hybrid using Atkinson cycle.

Kia Niro?

https://futuresuvs.com/2021-kia-niro/

"The centerpiece of the 2021 KIA Niro PHEV is a 1.6-liter Atkinson cycle four-cylinder petrol unit."

> Notice there are basically no diesel hybrids, this is because a diesel engine is more efficient than...

Thx for the nice explanations.

About diesel engines I read long time back that it's as well because they need to be "hot" in order to be able to filter their exhaust fumes (don't know how to say this correctly - the thing that is done by the catalyzer in the exhaust pipe) => as a hybrid engine is meant to be switched off quite often, the catalyzer of a diesel engine would have to be kept very hot by the battery, reducing therefore range/effectiveness/etc... of the hybrid system.

Hard to say with modern diesels but I am sure it does contribute to the cost benefit, DPF filters will need more regeneration (and fuel use) if the exhaust temps are not high enough which is typical of diesel used for short trips vs longer highway trips.

The term that you are looking for is "wet stacking", and is due to the fact that diesels utilize the heat and pressure in the pistons and cylinder walls to induce combustion (there is no spark plug). That being said, one could inject the bare minimum to "idle" the motor and maintain this temperature, losing a bit of efficiency from completely stopping the motor entirely.

There are a few diesel hybrid prototypes, if memory serves me there was a two stroke diesel hybrid motorcycle called the eCycle that got something like 160mpg.

> Yes most people don't seem to realize the hybrid system tends to just shut down at highway speeds.

There's still an advantage to having a hybrid system onboard even at highway speeds. The continuously-variable ratio of the hybrid transmission allows increased freedom of the ECU to select {instantaneous gasoline engine RPM, instantaneous gasoline engine torque}, which permits operation in more favorable parts of the BSFC map more often.

Furthermore, the battery/motor-generators allow the ECU to temporarily run the gasoline engine at lower or higher output (without loss of traction power and without waste) than the power required at the wheels, further expanding the area of the BSFC map that the gasoline engine can operate at.

It is true that mechanical-electrical-mechanical conversion is more lossy than an all-mechanical system, but topologies such as the Toyota hybrid system allow for significant power transfer without electrical conversion; and spending significantly more time in more efficient regions of the BSFC map makes up for the conversion losses incurred.

Also, hybrids tend to have electrically powered coolant pumps and electrically powered air conditioning compressors, allowing for more aggressive fuel cut off (on a downhill section of the highway, say).

CVT transmissions are not only available on hybrids although the Prius CVT is a thing of beauty utilizing the electric motors with planetary gears for continuous ratio.

None of the other things you mention have more than negligible effect on the highway at cruise speeds where the combustion engine will be providing all the power to run all systems either directly or indirectly at typically ideal rpm most of the time. Also electrical cooling pumps and power steering etc again are not tied to hybrids and many non hybrids are going that route. A/C again does not need a hybrid but would exceed the capability of a standard alternator and direct mechanical compressor drive with a clutch is not that inefficient, again that would mostly be for stop and go urban driving at steady highway speed it probably more efficient to run mechanically off the engine.

A small diesel car and Atkinson hybrid will have similar highway mileage mainly due to the combustion efficiency of the cycles not the hybrid systems. However the hybrid systems allow a Atkinson gasoline engine to have acceptable performance in the city while the diesel doesn't need it. An Otto gas car will be behind but the cost saving can buy a lot of gasoline.

Battery-electric trains (not hybrids) are certainly a promising emerging technology, however. Countries like the UK and Germany are looking to replace diesel trains with battery-electric on non-electrified branch lines:

https://www.railwaygazette.com/battery-traction-agreement-si...

Battery-electric trams are also a thing:

https://www.railtech.com/infrastructure/2019/12/12/first-uk-...

Yes similar to planes its an energy density and cost issue for the batteries although weight is less of a concern. I would imagine you could do some interesting things too like have the train run through an electrified section to charge itself and then run through a non-electrified section on batteries.

That's basically the plan for the UK network going forward, electrification has been ludicrously expensive so most of our rail network (in miles) is serviced by diesel trains, even some of the major lines have had full electrification indefinitely delayed.

It also allows un-electrified or inactive railway lines to be incorporated into urban metros much more easily and we're doing that as well.

Yeah much higher bypass ratio. Current turbofans bypass ratio is limited by the fan diameter. With a series hybrid you can power multiple fans from one engine. Or more likely engines. So you could design a plane with two engines and four fans.

For a reaction engines thrust is proportional to mass flow X delta V. And power is proportional to mass_flow X delta V squared. You can see why high bypass turbofans are more efficient. More thrust/less power.

Couple of other benefits. Much safer in an engine out condition because you can power your fans off the remaining engine. With a battery hybrid you could power the fans for a limited time with all engines out.

Probably less throttle lag with an battery hybrid. Throttle lag is what kills people during take off and landing in bad weather. Wind shear causes the airspeed to drop, pilot needs more power but the engines take seconds to respond.

Take off and landing under battery power --> way less noise.

Electric propulsion is very cool in and of itself.

If the goal is to fly without adding CO2, there are other approaches as well, though.

One could synthesize fuel usable by nornal existing airplanes from atmospheric CO2. This would require a lot of CO2-free energy though.

There is an ongoing project [1] by US NAVY to turn seawater into jet fuel. This way we would not need to burn fossil fuels and would not need to throw away much of existing aviation technology.

[1] https://www.eurekalert.org/pub_releases/2020-07/uor-lch07152...

Wouldn't converting seawater into jet fuel effectively just have us contribute to global warming even more aggressively by converting CO2 sequestered in the ocean to atmospheric CO2 (+ huge energy consumption for the intermediary step of jet fuel)? I'm sure I must be missing something there.

If source of energy is low CO2, such as nuclear, then losses dont contribute to CO2 increase.

As for taking CO2 from the ocean, it is done for this project as it is military for airplane carriers. CO2 can be had from atmosphere as well, with more energy cost.

I see how you're thinking about it. In your experience does this chemical process transfer from seawater to atmosphere? I'd love to learn about the details of this process based on your expertise. I've only taken public high school chemistry & it's been a while so I consider myself here a total novice.

From a logical perspective though, to me this has to be a net production of energy reaction though since otherwise it wouldn't be extending the operational time (an external power source would be needed & thus you may as well just use that power source to power your ship).

My guess would have been that the seawater -> CO2 + Hydrogen conversion is the part that produces the energy needed for the rest of the process as I would think CO2 -> CO1 -> liquidcarbons requires energy somewhere or is at best roughly neutral with a mix of +/- steps. I could easily be wrong there though in that assumption (again, don't know these reactions).

If that is actually a correct guess, I would have expect CO2 density differential between air & seawater to be a critical component. I then checked CO2 for density and water on Google [1]: 1.98 kg/m^3 in air, 997 kg/m^3 in water (didn't know these densities in advance of making my hypothesis).

This is a wild guess on all fronts, but given the magnitudes involved & how much fuel it requires to power ships (i.e. how much CO2 you'd need to extract from seawater to turn it into liquidcarbons to power a ship), that disparity in density would seem crucial to being able to produce the fuel at all.

Even without that, from a physical aspect, I would think the combination of density & how long the process takes could be the next limiting factors even if it did map. The 1000x disparity in density would mean that you could need 1000x the volume to produce the same amount of liquidcarbon fuel. Now of course there are differences between the use-cases that will alter the needs but I don't have an intuition if it would increase or decrease net, and if it were to decreae, would it be sufficient to make the disparity manageable. Would love your thoughts on this nuance.

From fuel efficiency and aerodynamics perspective, CELERA 500L is a great advancement that could possibly also be converted to electric in the future - https://www.ottoaviation.com/

Electric planes will replace car and bus trips before they replace gas planes. Short range, on demand, point to point, cheap travel could really be a game changer. Think what you used to do as a two hour drive- battery electric planes will be able to do that, a distance that today isn’t economical because of the high cost of jet and turboprop maintenance and fuel, and the need for runways. All those commenting- “but the energy density” are right, but don’t miss the forest for the trees. The sky is an underutilized asset.

I hope not, we've already exploited too many "underutilized assets".

Doubt it's Roll Royce Motor cars, more likely the other Rolls Royce that builds aircraft engines, related but not the same.

https://news.ycombinator.com/item?id=24631093

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