>> 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.

1. http://en.m.wikipedia.org/wiki/Atkinson_cycle 2. http://www.curbsideclassic.com/blog/the-atkinson-and-miller-...

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?

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.

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.

Maybe to keep the plane at an optimal velocity?