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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. H

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?

wtallis 12y ago
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.
GregBuchholz 12y ago
>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.
dragonwriter 12y ago
> 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.
- matt_morgan 12y ago
  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?
  - nsajko 12y ago
    Maybe to keep the plane at an optimal velocity?
  - brulez 12y ago
    Planes need extra power for takeoff and emergency scenarios like a twin on a single engine. It's possible that these requirements lead to engines which have a optimal power output above what would be required in straight & level flight.
    - bdamm 12y ago
      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.
      - lutorm 12y ago
        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.
      - darkmighty 12y ago
        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.
        12y ago
- seanp2k2 12y ago
  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. 1. http://en.m.wikipedia.org/wiki/Atkinson_cycle http://en.m.wikipedia.org/wiki/Atkinson_cycle 2. http://www.curbsideclassic.com/blog/the-atkinson-and-miller-cycle-engines-not-exactly-how-they-started-out-to-be/ http://www.curbsideclassic.com/blog/the-atkinson-and-miller-...
  - lutorm 12y ago
    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.
- phkahler 12y ago
  >> 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.
  - lutorm 12y ago
    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.)
paulyg 12y ago
It was towards the end of the article: "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.
tpurves 12y ago
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 http://en.wikipedia.org/wiki/Electric_aircraft

wtallis 12y ago

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.

GregBuchholz 12y ago

>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.

dragonwriter 12y ago

> 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.

matt_morgan 12y ago
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?
- nsajko 12y ago
  Maybe to keep the plane at an optimal velocity?
- brulez 12y ago
  Planes need extra power for takeoff and emergency scenarios like a twin on a single engine. It's possible that these requirements lead to engines which have a optimal power output above what would be required in straight & level flight.
  - bdamm 12y ago
    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.
    - lutorm 12y ago
      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.
    - darkmighty 12y ago
      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.
      - 12y ago
seanp2k2 12y ago
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. 1. http://en.m.wikipedia.org/wiki/Atkinson_cycle http://en.m.wikipedia.org/wiki/Atkinson_cycle 2. http://www.curbsideclassic.com/blog/the-atkinson-and-miller-cycle-engines-not-exactly-how-they-started-out-to-be/ http://www.curbsideclassic.com/blog/the-atkinson-and-miller-...
- lutorm 12y ago
  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.
phkahler 12y ago
>> 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.
- lutorm 12y ago
  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.)

paulyg 12y ago

It was towards the end of the article: "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.

tpurves 12y ago

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 http://en.wikipedia.org/wiki/Electric_aircraft