Hybrid-Electric Aicraft Engine Targeting 30% Fuel Efficiency
rtx.com
rtx.com
FTA: “The project aims to demonstrate up to 30% improved fuel efficiency for a typical 250-nautical-mile regional turboprop mission”
30% improvement makes much more sense.
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.
And what if you need two go-arounds?
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.
Perhaps it's not all negative: the electric portion could give a pilot a bit more glide than the gas portion dies.
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).
No.
Source 1: PE = mgh
Source 2: am pilot
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.
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.
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.
Interesting, thanks.
> In our theoretical aircraft with batteries, mass is the same.
The fuel that's expended during cruise reduces the mass.
- 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
"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)
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.
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.
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?
There's a video linked elsewhere in this thread that explains how the toyota system works, it's worth watching as it clears up several common misconceptions about the system.
... and instead drive one of the motors, generating electricity to power the other motor to push the car forward.
I feel like that video you're mentioning did a poor job if it didn't describe one of the more important and clever aspects of the Prius's design.
Both the engine and motors are used at all speeds. Particularly during highway acceleration the entire assembly rotates in the same direction.
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.
> 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...
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.
Like a big-boy prius.
Solar cells are actually quite thin and could be almost like paint on the wings of aircraft. I wonder if the energy generated vs required is even ballpark. There is plenty of sun at 30,000 feet during the day.
Take a Boeing 777, it has a wingspan of about 60 meters, and I'll ballpark an average upper wing chord of about 7 meters, for a total upper flat area of about 420 square meters.
High quality modern but standard single sided solar panels can do about 220 watts per square meter is full sunlight (around 22% efficiency tested at a 1000w/m^2 irradiance).
So that is 92,400 watts at full power.
92kW is less than the peak power of a Nissan Leaf. 92kW is 123 horsepower.
The two GE turbofans of a 777 are generating something like 40-50 MW of shaft power during cruise. MW.... megawatts. 50-60,000 horsepower.
Plastering the wing surface of a commercial plane with solar panels would make up less than a quarter of a percent of the total power it uses to produce thrust at cruise, at best case with them fully-lit.
Fully solar sailplanes do exist (NASA's Helios prototypes are an example) but that isn't anything close to a 'normal' aircraft with any appreciable payload/passengers.
So, no need to wonder.
yeah, I was thinking of the solar planes. I also didn't know that "horsepower at cruising speed" was something you could look up. obvious now, thanks!
The real takeaway is that power is power and energy is energy and regardless of how it gets to do the 'pushing' of the air, if you want to use solar energy (power, at any given moment, not integrating over time) those are the numbers you are comparing. It's all just unit conversions, at least when you're attempting such napkin math. Anything more - taking into account the actual systems, losses, efficiencies, etc.. just makes it all worse, not better.
A cool exercise is - given the 777 wingspan I esitmate and the power output from solar of said size... what kind of current aircraft use piston-driven engines with similar horsepower? Assuming we had motor inverter electronics and an electric motor that was 100% efficient, you could imagine trying to build a similar weight aircraft of that size that has such a wingspan.
This is why you wind up with the only solar powered aircraft out there being superlight, high altitude craft with super high aspect wing surfaces- maximizing wing area to weight / lift capability such as: https://en.wikipedia.org/wiki/AeroVironment_Helios_Prototype (note the largest of those prototypes and the tiny amount of power output they were making. All that wing area and it would have barely been able to carry a single passenger as payload and cruise a day without battery power)
Planes aren't really like this, they're actually a lot closer to boats. They move from fixed operating point to fixed operating point, and the engine is optimized around this. There's a lot less fat to trim, because the engines already have a narrow operating band.
That's not to say that this won't work; Pratt obviously knows what they're doing. But it's probably not an accident they've gone for smaller turboprops. I actually think the biggest opportunity for hybrid propulsion is in smaller drones where you want high performance piston engines, because tip clearance becomes an issue for gas turbines. A turboprop might suffer similar performance challenges that would make a hybrid configuration make sense.
Edit: just saw a post below on the relevant patents. Looks like they're using hybridization to decouple the two shafts. Without the motors, they're coupled via the gas path. With the motors, you can navigate the maps of the two shafts independently. I'm guessing this lets them pick a combination of shaft states that saves fuel. Most of that is pretty uncontroversial, but I'm surprised the claimed fuel savings are that high. As I mentioned above, it's probably because it's a turboprop on a small aircraft.
There's an economy of scale with turbofans to some degree. Large high bypass turbofans are extremely efficient, particularly at their cruising mode.
Smaller regional aircraft have more of their fuel consumption dominated by takeoff - the cruising altitude portion of the flight is smaller in proportion.
When doing something experimental, do it on something small to start with. Then scale up when you've got it working.
It's a boost-only motor, it doesn't/can't harvest energy on descent.
The patented solution (transient smoothing under auto-throttle control) puts electric motors on both the low spool and high spool, then uses a power-splitting algorithm to route high-frequency thrust changes to the electric motors while keeping fuel flow nearly constant on the thermal engine (turbine). The turbine cruises at a steady operating point with tight compressor/turbine clearances and the electric motor smooths out the spikes that are normally there with turbulence and load changes. Benefits: lower fuel burn, longer turbine life (fewer blade-rub risks from speed variation), and smoother ride quality since the auto-throttle bandwidth improves. This setup (based on the various cutaways and photos so far) seems to be only a single 1MW motor so it only runs on the low spool but can still help modulate the turbine decently in the same way it does in the Koenigsegg Regera's hybrid electric setup, that removes the need for a flywheel because the electric motor can smooth out the gas motor's inherent lumpiness.
Also disclosed in a previous press release [2], it's only a 200kWh battery so at 1MW peak boost (cited load during takeoff/ascent) it would only run for ~10-15 minutes at the beginning of the flight.
Seems most of the savings are due in part to not using as much fuel during takeoff (~20% of a 1-hour flight's fuel) but also in large part to the under-sizing and optimization of the thermal turbine to keep it running in it's peak efficiency zone for more of the flight (~10% of a 1-hour flight's fuel).
Curious how the safety margins work here - if the battery is depleted on takeoff (aborted takeoff) or there's an issue that requires descent-then-reascent, if the batteries can't be replenished in-flight there could be a power deficit in that window where you'd normally have 2+2MW of gas turbine power for the plane and now you only have 1+1mw of gas turbine power.
[1] https://patents.google.com/patent/US20250296689A1/en
[2] https://www.aerospacetestinginternational.com/news/h55-deliv...
I do wonder if prop engines can act as "windmills" (similar to turbine engines, which often in accidents still have been found to provide a bit of hydraulic power), which means regenerative braking could be used instead of speedbrakes.
A catapult launch can't help with the next 30,000 feet of climbing, though, while the impossible umbilical might.
I cringe every time a jet plane has to spool up, burning god knows how many gallons of fuel, just to go 15mph on the runway.
Or maybe you keep your downsized turbine motor running closer to peak efficiency even during landing and increase the mechanical energy harvesting to your APU or alternator thing or whatever to control speed and use that to charge a small supercapacitor. In the event of an aborted landing the supercapacitor could provide a boost of thrust to your electric motors. This would keep wear on the battery lower and also allow it to dump energy at its peak efficiency
Sorry but what the hell are you talking about? Airliners are heavy, very aerodynamic, very easy to overspeed and they regularly actively brake on descent!
Unless there are other constraints on the approach, the point of the entire descent phase is to have the engines IDLING the whole time. The FMC actually calculates the descent profile backwards, starting from the earliest known constraint (e.g. a certain point at 5000 ft) and extending that back into the air accounting for the expected weight and drag of the plane. The descent phase often begins more than a hundred kilometers from the destination. Remember how they tell you in the cabin that the plane is starting to descent and you should put your laptop away etc? That's when the engines go to idle and if everything goes right, they won't spool up again until just dozens of second before touchdown. The goal of the entire industry is to have that happen as often as possible, on as many airports as possible.
If the plane is a bit heavier or less draggy than expected, or if the approach requires a steeper descent at some point, the airplane will calculate all that (in advance!) and let the pilot know (e.g. a DRAG REQUIRED message) and will require spoiler deployment. This is very common.
Also I swear there was a company replacing a push pull type plane with a hybrid ?
There are SOME planes where you want to avoid the "no thrust" situation because their implementation of "no thrust" can lead to weird aerodynamics or prop pitch behavior, or can stress the engine.
My assertion is that there are no turboprop plans that can maintain a 3° glideslope at normal approach speed without thrust from the engines. If you think there is such a plane, name the type, and show me a citation from it's POH to support the claim.
> My assertion is that there are no turboprop plans that can maintain a 3° glideslope at normal approach speed without thrust from the engines
These are very different claims, in every dimension. Yes, the final approach is intentionally very shallow, is a minor part of the entire descent phase and the plane flies it in a very "dirty" configuration (gear down, flaps). Yes, the prop on turboprops causes a ton of drag. Are we going to mention the fact that a 737 or an A320 (the most common airliners in the world) are NOT turboprops, they both have a glide ratio comparable to trainer gliders and are fast as fuck?
I’m talking about turbo props because the article’s hybrid technology is specifically for turbo props.
My challenge stands: find me a turbo prop that would be happy at idle thrust for either approach or cruise descent.