[avidiax]

I think there are startups making a similar sort of engine for general aviation. It's good to see that there is development of the same idea for commercial aviation.

This isn't like a hybrid car. It's a parallel hybrid, where the gas engine is just big enough for efficient cruise at altitude, and the electric motor/generator provides extra power for takeoff and ascent (or go-around power), and then charges slowly during cruise if needed.

This means that the battery is quite small and light, having only enough charge to take off and get to altitude.

I suspect that this system probably improves safety as well, if architected properly. If one or both of the gas engines fail, so long as they are not seized, that electric motor can still provide some power for diversion.

[kspacewalk2]

>This isn't like a hybrid car. It's a parallel hybrid, where the gas engine is just big enough for efficient cruise at altitude, and the electric motor/generator provides extra power for takeoff and ascent (or go-around power), and then charges slowly during cruise if needed.

Isn't that exactly what hybrid cars (e.g. Prius) are? Extremely efficient gas engine for highway cruising, but insufficient for acceleration, which is aided by electric motors?

[Tade0]

The Prius uses a planetary gear set to blend power of the engine with that of the two motor-generators.

Both the engine and motors are used at all speeds. Particularly during highway acceleration the entire assembly rotates in the same direction.

[dgfl]

This is a great related watch if you have some time to kill: https://youtu.be/KnUFH5GX_fI

[delecti]

That's such a fantastic video. I never totally grasped why hybrids were so much more efficient, because my naive assumptions about how they worked were so simplistic. The real-time graphs he showed were excellent for making his points.

[cogman10]

TC is filled with these sorts of videos. If you have time to burn then they are basically all this quality. His interests are also just wildly all over the place. From Christmas lights to dishwashers to coffee machines you just don't know what the next video will be.

[chabska]

One thing I really don't like about this video, is that he was specifically describing the advantages of that specific van and drivetrain, that being Toyota Sienna, and he's not wrong at that, but then he made grandiose general statement like "Hybrids are simple" and "minivans are great".

No, quite a few other hybrids are just an electrical motor tacked on to an existing ICE drivetrain, and they are actually more complicated than a regular ICE car, with horrible reliability and repair cost. And other non-hybrid minivans are either seriously underpowered, or require a seriously big engine that eats fuel.

The complaints that people have against hybrids and minivans are valid, if they're not talking about Toyota hybrids or this specific Toyota minivan.

[to11mtm]

IDK if anyone really answered -this- question properly, so I will.

> Isn't that exactly what hybrid cars (e.g. Prius) are? Extremely efficient gas engine for highway cruising, but insufficient for acceleration, which is aided by electric motors?

Well, it kinda depends. Where Hybrids get the biggest MPG boost is in city driving. The stop and go traffic lets you use Regen braking and go quite some time without the engine kicking back on while still moving forward.

I still don't understand Honda's system enough to speak well on it, but I can speak to THS because it seems to be the cheapest to do and is most proven on the road.

The general parameters for a THS type system on a Toyota Prius or Rav4, or a Ford Maverick/Fusion/Escape is a 2.0L or 2.5L (at least in modern US examples) engine paired to a simple planetary gearset containing a power split device. It is a single speed (At least in the cheap configurations, however that simplicity is possibly close enough to be viable for air usage vs a reduction gear.) That's part of why they tend to have fairly large engines, the valve timing magic gives them at least a bit more HP to not be too bad on the highway.

> but insufficient for acceleration

Going back to the modern cases, the engine is typically sized large enough to give some acceleration even on the highway. Not always great but usually enough.

At least as far as the non-plug-in hybrids, the 0-35MPH can be surprisingly peppy.

The bigger magic (again, at least as far as THS) is it makes it easy to just run the engine at the 'most optimal RPM' for certain tasks, excess energy gets piped to the battery or back out through the system, this does also help reliability tho, because you can then design the reliability of the engine around certain RPM ranges...

[coderenegade]

This is mostly correct. The Prius is actually a series-parallel hybrid, as it maintains a direct mechanical link between the engine and the wheels (and the engine is capable of adding power to the wheels) but it also uses two electric motors to isolate the two degrees of freedom (engine rpm and wheel speed). This second part is the series bit: the wheels and the engine are, in principle, fully independent, because the motors can absorb or add power to the crankshaft and the driveshaft to hit a target rpm without changing the rpm of the other shaft. In practice, the Prius doesn't run at a constant rpm due to sizing constraints of the motors, as the mechanical linkage means that one motor has to be able to absorb the engine power and torque of the other motor, which is impossible across all conditions.

Most hybrids are actually parallel or series-parallel. Series hybrids are comparatively rare because the traction motor has to be sized for the full operational range of the vehicle. Their primary advantages are mechanical simplicity and packaging; even though the power electronics and motors are bigger, the lack of mechanical coupling means you can put them wherever is convenient.

[arijun]

The Prius is in series, or something like it.

[idontwantthis]

No it’s not. The majority of power comes from the engine. It drives the electric motor mechanically, using it as a transmission. It is not just charging the battery.

[arijun]

Sorry, I meant in the low speed, high acceleration regime (maybe easily confounded with takeoff?). There the engine will turn one motor to generate electricity, which will then power the second motor, like a series hybrid.

[cwillu]

No, that's just wrong: at no point is (say) a Toyota Hybrid burning gas to generate electricity to immediately turn another motor that actually moves the wheels.

[arijun]

What do you imagine happens if the battery is low and the car wants to start from a stop? The engine can't shift down to first so it would just stall out if you tried to use it to drive the driveshaft directly, and there's no other source of electricity to put into the motor driving the wheels.

[idontwantthis]

Hybrid cars are mostly parallel hybrids. Only the Chevy Volt comes to mind as a serial hybrid.

[mikepavone]

Chevy Volt was still a parallel hybrid. The gasoline engine was used for driving the wheels for highway cruise because it was more efficient. I think the range extender version of the BMW i3 was a pure serial hybrid though

[NobodyNada]

Honda's recent hybrids (CR-V, Civic, Accord) are like this too. Mostly series, but the engine can directly drive the wheels via a single overdrive gear for better cruising efficiency.

[rsynnott]

Some hybrid buses are also serial hybrids.

[projektfu]

The BMW i3.

[osamagirl69]

And infamously the ill fated Fisker Karma

[SoftTalker]

The fuel burn of take-off and climb substantially lightens the aircraft for cruise. Electric batteries have no such effect, you're carrying all that dead weight for the rest of the flight. This reduces the passenger or cargo capacity of the aircraft, which reduces potential revenue.

And what if you need two go-arounds?

[repiret]

Don’t conflate airplanes with rockets.

On an airplane, most of the energy in cruise is spent overcoming parasitic drag, not induced drag. It’s spent pushing the airmass out of the way as it moves forward, not creating lift to stay aloft.

For that reason, a change in weight does not significantly change cruise fuel usage.

Weight is still precious, but that’s because airplanes’ load are more often weight constrained than volume constrained, and capital and operating costs are such that you want to maximize the load.

[arijun]

> you're carrying all that dead weight for the rest of the flight

If you're recharging the batteries for extra go-arounds during landing, they are as dead weight as the fuel you would otherwise reserve for that purpose. And if you have 30% more efficient engines, meaning less fuel and smaller engines, it's possible you could come out ahead, weight-wise.

> what if you need two go-arounds

I assume that a go-around requires less sustained power output than a full climb from takeoff, so you will probably get more than one go-around anyway, and we don't know how much over-capacity they're designing for. In any case, any design will require tradeoffs in safety, and having more engine-out capabilities might improve safety enough to overcome the higher risk with go-arounds.

Not saying this project is will work out or that you're even wrong necessarily (this could be the equivalent of a concept car for Pratt & Whitney).

[vablings]

Due to various penalties, wind resistance. gear down and aircraft configuration. A go-around consumes a huge amount of fuel, not as much as climbing to cruise but its alot

[dmitrygr]

> I assume that a go-around requires less sustained power output than a full climb from takeoff,

No.

Source 1: PE = mgh

Source 2: am pilot

[fransje26]

So, for a Dash 8-100, at 13,000 kg, disregarding drag, engine efficiency, etc, to take-off and climb to 1000m and accelerate to 150 knots (77 m/s), you will need:

- 13000 * 9.81 * 1000 = 127.5 MJ, to reach your altitude

- 0.5 * 13000 * (77)^2 = 38.5 MJ, to accelerate to your climbing speed.

Total: 127 + 38.5 = 166 MJ, or about 46.11 kWh

For a go around, re-accelerating from 1.3 * stall speed (85 knots / 44 m/s) to your climbing speed, and going to your missed approach altitude of 1000 m, you will need:

- 13000 * 9.81 * 1000 = 127.5 MJ, to reach your altitude

- 0.5 * 13000 * (77^2 - 44^2) = 26 MJ to accelerate back to your climbing speed.

Total: 127 + 26 = 153 MJ, or about 42.5 kWh

[usrusr]

Nice to see some numbers. So for the peak load situations, a Dash 8-100 would not require a battery bigger than that a short range BEV ("city", though in reality the short range BEV use case is more for the rural equivalent of stuff that would be walkable in a city setting). And that's even before considering the energy contributed by the fossil fuel engine.

"13000 * 9.81 * 1000 = 127.5 MJ, to reach your altitude"

Presumably quite a bit of that would be harvested back during the descent that follows. The conventional engine would still need some excess power (relative to cruise load) to fill the gap left by drag and imperfect circle efficiency of the electric motor/generator, but mass x altitude is stored energy, not lost. (I'm still talking about the "what if we need a second abort" of the root post)

[arijun]

You don't do a full climb after a go-around, so the heights are not equal, and the mass is less since you've expended fuel. You also retain some kinetic energy but I assume that is closer to a negligible effect.

Also, PE = mgh is probably an not a great formula for energy cost of takeoff/go-around, as there are probably large costs it ignores (gravity loss, less efficient engine use, maybe less efficient turbines?).

For your source 2 I have no rebuttal so will have to defer to you, but would ask for an explanation.

[dmitrygr]

> You don't do a full climb after a go-around,

an IFR missed approach can have you climb quite high, especially in areas with serious terrain. Example: https://aeronav.faa.gov/d-tpp/2607/00346IZLZ17R.PDF airport is at 4400 feet over sea level, but missed approach says: climb to 13,000. Also, some go arounds will lead you to have to divert to an alternate airport, getting there may require climbing high to clear terrain or gaining required engine efficiency to fly the distance.

> And the mass is less since you've expended fuel

In our theoretical aircraft with batteries, mass is the same.

> You also retain some kinetic energy but I assume that is closer to a negligible effect.

Negligible indeed.

[arijun]

> you climb quite high, especially in areas with serious terrain.

Interesting, thanks.

> In our theoretical aircraft with batteries, mass is the same.

The fuel that's expended during cruise reduces the mass.

[SoftTalker]

Not a pilot, but on approach for landing you bleed off a lot of energy. For a go-around you need to reverse your descent and build up enough energy to fly away again. Take-off/Go-around tends to be the same throttle setting, AFAIK. Of course it also depends on how early you decide to throw away the approach and go around. Doing it at 1000 feet is different from bouncing it off the runway.

[serf]

but the point they were making is that it inevitably takes less energy to get to a level flying state (in similar weather conditions) due to fuel consumption.

so, unless the pilot is fighting weather it would make sense that equal throttle levels and equal pitch plans in equal weather conditions would require less and less fuel burn until the tanks are empty.

[card_zero]

Time to invent regenerative air brakes, like fold-out windmills.

[dmitrygr]

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

[buildsjets]

And besides that, there is the looping parachute APU.

https://patents.google.com/patent/US9452721B2/en

[Melatonic]

Would imagine these are significantly more useful on heavier and faster aircraft - surely the weight and whatnot to retract move them is less worth it for smaller planes ?

[0cf8612b2e1e]

Energy density of liquid fuels cannot be beat by batteries, so this is not competitive if you are looking to maximize cargo. However, there are plenty of short haul flights: private jets, island hopping, regional routes where you need to move little mass.

[usrusr]

"And what if you need two go-arounds?"

Easy: you don't try the second landing approach before the battery is sufficiently recharged to contain enough energy for the second abort. Chances are this does not take any longer than going through the pattern anyways.

The saving is not just the dead weight of the bigger engine you'd need to do take-off, climb and abort without electric assist, it's also the fuel saved during cruise from running an engine that is completely designed for efficiency at cruise load instead of for some compromise between cruise efficiency and sufficient peak power for start and abort.

[xattt]

Exactly. A hybrid passenger car can tolerate unpredictable power output that may come with an auxiliary power setup that may or may not be available when stronger dynamics are called for.

A plane doesn’t have this luxury and needs predictable output. The fossil fuel engine either needs a sacrificial “overboost” mode for emergencies (at the cost of wear/long-term longevity), or has to be sized for full power at the ultimate cost of efficiency.

[Melatonic]

So theoretically if the electric motors fail (or battery is dead) an engine could be sized and designed smaller (for cruise efficiency) but have some sort of boost mode that still ensures safety ? At the cost of increased maintenance or wear or something if it must be used

[Tade0]

On anything but very short flights most of the fuel is spent on cruising.

[dmoy]

I would classify 290 miles as a very short flight, that's like 1-2 hours or something?

[hobonation]

Valid.

Perhaps it's not all negative: the electric portion could give a pilot a bit more glide than the gas portion dies.