The aircraft they are using for this test looks like it has glider-like wings, which means it requires a lot less thrust to keep aloft, making a low torque and low power electric motor more able to work with it, at the compromise of lower speeds. A more traditional 2-4 seater aircraft capable of 100+ knots would require significantly more power.
The payload capacity of a general aviation aircraft fueled with power-rich gasoline is typically around 500 lbs. Unfortunately, the potential 30% savings in fuel weight (~65 lbs of fuel in a Piper Cherokee) would not be offset by the batteries (using the Prius' 100 lbs of batteries as a reference).
More weight = more power necessary to stay aloft = more power consumption from both the motor and engine.
I hope they can get the technology to work, but there's many more hurdles for a plane than there is for a car; however there have been electrically assisted gliders for some time, so the potential is certainly there.
Hybrid electric flight is a perfect fit for military UAV's. It enables longer flight times, with lighter loads due to higher efficiencies, maybe at reduced vibration(which can be a problem for imaging and reliability) due to better engine design(because of the smaller design range), and at critical times when total silence is needed , the UAV can run only on electricity and be quiet and almost without vibration.
And it won't be surprising to learn that some militaries already have such systems in use.
This article claims that the plane is more efficient, and that the batteries recharge in flight, but it doesn't say where that gain in efficiency comes from.
Hybrid cars work mainly by recovering energy from braking or going downhill to charge the batteries. Planes don't do that. Where do they recover energy?
> Hybrid cars work mainly by recovering energy from braking or going downhill to charge the batteries.
While hybrid cars do those things, they aren't the main source of the efficiency of parallel hybrids, parallel hybrids like the Prius work by keeping the engine at the most efficient throttle setting (which is much higher than what would normally be used for cruising) whenever it is running, and using excess power from the engine to recharge the batteries.
The article describes this aircraft as doing the same thing.
>> parallel hybrids like the Prius work by keeping the engine at the most efficient throttle setting
Parallel hybrids like the Prius, Fusion, etc... do benefit from having something like a CVT, but the biggest gain comes from recovering kinetic energy during braking. Notice that they all get better city mileage than highway! That's because cycles of stop/accelerate have a very low penalty when most of the energy is recovered, so the difference in fuel economy is dominated by drag which is worse at higher speeds.
Hybrid planes and boats make little sense. This one wins by using a smaller engine and supplementing it with an electric motor for takeoff.
Now give me a series hybrid twin-engine aircraft and you've got some possible redundancy/safety improvements, but not much in the way of fuel savings.
I think focusing on the city mileage is misguided in this case. Yes, hybrids get much better mileage in city driving than non-hybrids, but they also have substantially better highway mileage than non-hybrids. This is exactly because they have a smaller engine that can operate at a more efficient power point during highway cruising. Normal cars are ridiculously overpowered for cruising because they need to be able to accelerate. When the extra power comes from an electric motor, you don't need that. (It's not about the CVT, a non-hybrid can benefit from that, too.)
Hmm, I always thought that Atkinson cycle engines like they use in most hybrid cars[1] were, like "normal" Otto cycle engines, more efficient at low-ish RPMs, hence why cars tend to have low RPMs at freeway cruising speed. The figure in [2] is 2500-4000 RPM, which is definitely "normal" freeway RPM for most cars. [2] also talks about why engines like those used in most modern hybrids are not actually true Atkinson engines.
Otto engines tend to have the highest efficiency at low RPM but also at full throttle. Since car engines have to be designed to have much more power than necessary for cruising, it's not possible to use that operating point unless you use it in a (series) hybrid.
Thanks, that makes sense. One other question: what is "excess power from the engine" and how is it possible (2nd law) that using it to recharge the batteries is better than using it to move the plane directly?
Many planes have a "constant speed propeller", which allows the engine to be kept at its most efficient load, while adjusting the pitch of the propeller. Optimal use of power needed for climbing but otherwise left unused in straight and level flight is a solved problem.
How can you say the this approach is more efficient than one which keeps the engine at a constant load and varies propeller speed, for example? (it involves complicated fluid mechanics to figure it out)
This is a reason mathematicians loathe the overuse of "optimal" by engineers: in the real world there are often too much variables.
You can't have constant load -- the power needed is dictated by the conditions. But what is known is that you want to operate at full throttle to be the most efficient, so the way to vary power is to vary RPM. (Propellers are also generally more efficient at low RPM, but I don't think that's what was referred to here.)
That's only true in a naturally-aspirated engine at high enough altitude that you can cruise at full throttle. If you have to close the throttle, you lose efficiency.
>Hybrid cars work mainly by recovering energy from braking or going downhill to charge the batteries.
I always thought that hybrids were more efficient because you can size the engine to the average power requirement, and not the peak requirement, thereby operating in a more efficient region most of the time, using the battery reserves for when you need peak power. And for some reason I was thinking that operating an engine at a fixed speed allowed for better engineering trade-offs.
The plane gets to run its engines at the most efficient speed full-time and handle the increased thrust requirements of takeoff and climbing using the electric motor. If the batteries and electric motor weigh less than the weight savings in the primary engine, then it's a big win all around.
"The petrol engine is optimally sized to provide the cruise power at its most efficient operating point, resulting in an improved fuel efficiency overall."
The fact that an airplane's speed and power delivery is much more steady state than a car's is the other big reason this has not been attempted until now.
Or you put solar panels on the wings. Solar planes that charge during the day and stay aloft on batteries at night, for aircraft (drones and such) that you want to keep in the air for a long time. Not a new idea: http://en.wikipedia.org/wiki/Electric_aircraft
The article is a little light on details, and does not cover how the batteries are charged in flight. Reading to the end, though, it appears that this was a technology demonstration designed only to show that an electric motor can power an aircraft engine (a prop, I assume? This would be more interesting if it were a jet aircraft.), and that the weight of the batteries is not a deal-breaker, as some had thought they would be. So I suppose the batteries weren't recharged.
So, in short, the term "hybrid" makes this a little misleading, since it conjures imagined of the hybrid technology in cars. But this was a hybrid in the sense that an aircraft engine was powered by both a fossil-fuel burning four-stroke engine, and an electric motor in the same flight.
Perhaps one day, though, light enough batteries would allow a fully electric commercial aircraft (though hampered by the lack of an equivalent to regenerative braking), or technologies could be developed to partially charge the batteries using, e.g., air brakes during landing, or solar power.
The accompanying video is much better than the article in that respect: it explains quite clearly the additional thrust from the electric power is most useful when the aircraft is climbing, and the rest of the time it can be recharged via the main engine.
Fully electric commercial aircraft are going to need to stretch electric engines as well as battery technology well beyond what they're presently capable of. The video is probably being kind when suggesting it's mere "decades" away.
>> Fully electric commercial aircraft are going to need to stretch electric engines as well as battery technology well beyond what they're presently capable of.
No, just the battery. Electric motors can reach 96 percent efficiency at the mid to high power levels you need in aircraft. I hit 95 percent for a motor and inverter combined with I was in EV development. It's all battery improvements from here.
How does that efficiency transfer to edge cases like accelerating 500,000lb of mass to Mach 0.75 at 30000ft through air though? That's pretty standard performance for a modern commercial aircraft with intercontinental capability. Turbofan engines might be pretty inefficient for many purposes, but excel at generating the thrust required for commercial aircraft, which are rather more demanding than lightweight piston-powered testbeds. Serious question: are there any existing electric propulsion systems remotely capable of the same performance characteristics as a turbofan even assuming a zero weight battery?
>> Serious question: are there any existing electric propulsion systems remotely capable of the same performance characteristics as a turbofan even assuming a zero weight battery?
Sorry about the late response, but NO. When I wrote that I forgot about power density. Electric motors are currently better than (or comparable to) ICE, but probably not close to a turbine.
The airline industry is essentially beholden to the price of fuel, right? If an airline figures out how to not need so much fuel, they could undercut and make a larger profit than everyone else.
Is there work on hybrid or electric passenger flight? Will there be?
Fuel cost does have a huge impact, but up-front capital costs and maintenance costs are also a big part of the picture. Which is why there are still a lot of planes in major airlines' fleets that are not as fuel-efficient as the latest models. Airlines are slowly turning them over, but the capital costs for a new plane are very high [1], and they also have to fit any new models into their maintenance plans.
Aircraft finance is readily available though, and at current prices airlines' annual capital cost per aircraft (amortised over a 20 year lifespan, and often on a lease for 5 years or less) is about half the annual fuel bill if that aircraft is in regular operation. Whilst there are certainly savings to be had from sharing existing spares pools and pilot and engineering type-ratings across a large fleet[1] which make some optimisations unprofitable, a drastic improvement in fuel economy would see Western fleets improved as fast as the new aircraft could be delivered in an era where 1% per annum is the improvement rate the engine OEMs strive for. The real obstacle would be actually delivering workable technology and getting it through the certification process.
By which time we might all be flying delta-winged contraptions with open rotors.
Or more likely, similarly-shaped aluminium tubes with turbofan engines that have improved an awful lot over several decades.
[1]which is why Northwest/Delta were able to get away with running thirsty but long paid-for DC-9s that had been obsolete for nearly a couple of decades until recently.
I suspect there is not much to be gained for transport aircraft, because they cruise at high power settings (people want to get there, and turbine engines are really inefficient at low power settings).
I've been fascinated with the idea of electric airplanes after hearing Elon Musk discuss the benefit of being able to fly at a much higher altitude and speed.
> I am nothing close to a mechanical enginer, but I always thought that hybrid car engines use the wasted kinetic energy from things like breaking to recharge their batteries.
You probably mean to say "braking" -- recovering energy from breaking to recharge the battery would be an interesting feature, but not particularly useful for most operation -- but, in any case, while regenerative braking is a feature common to hybrids, its not the only thing they do. A big advantage is running the engine at its most efficient setting whenever it is running at all -- this is usually higher than typical "cruising" requirements, even with the smaller engine used by hybrids -- and using any surplus power to charge the batteries.
Piston engines were already replaced by something more efficient: jet engine. I dont buy that batteries are 'light enough'. More likely we will see modern jet-turboprops even in tiny aircrafts. Or perhaps some sort of catapult for faster take-off.
Turbines are excellent for reliability and power-to-weight ratio specifications. They are worse than pistons for specific fuel consumption overall, way worse for specific fuel consumption at low altitudes, and capital cost specifications.
Which is more important in a given application depends of course, but small turbines are terribly inefficient compared to pistons (or even to large turbines), so I wouldn't expect to see your prediction come to pass.
I don't fully get it. How come
an airplane can recharge its battery in flight? I was thinking it will need to use breaking energy at some point but airplanes rarely slow down compared to cars.
>> I don't fully get it. How come an airplane can recharge its battery in flight?
In a normal plane takeoff is at full power, while cruise is at 75 percent power. So build a smaller engine that can achieve what the big one did at 80 percent and supplement takeoff with the electric motor. Now during cruise you can use the extra 5 percent to recharge the battery. Smaller engine = possibly better efficiency and weight.
Think of hybrid technology replacing transmissions. You're no longer dependent on mechanical linkage for efficiency. Diesel locomotives have used this advantage for 60+ years (generator runs at optimal speed, tied to electric motors; you'd destroy any transmission with the torque required to pull a freight train).
The aircraft they are using for this test looks like it has glider-like wings, which means it requires a lot less thrust to keep aloft, making a low torque and low power electric motor more able to work with it, at the compromise of lower speeds. A more traditional 2-4 seater aircraft capable of 100+ knots would require significantly more power.
The payload capacity of a general aviation aircraft fueled with power-rich gasoline is typically around 500 lbs. Unfortunately, the potential 30% savings in fuel weight (~65 lbs of fuel in a Piper Cherokee) would not be offset by the batteries (using the Prius' 100 lbs of batteries as a reference).
More weight = more power necessary to stay aloft = more power consumption from both the motor and engine.
I hope they can get the technology to work, but there's many more hurdles for a plane than there is for a car; however there have been electrically assisted gliders for some time, so the potential is certainly there.