AviationBanter - View Single Post

**Michael Ash**

In rec.aviation.soaring wrote:
In rec.aviation.piloting Michael Ash wrote:
There is also an advantage which comes from only needing to size the
engine for cruise, not for acceleration, since you can suppliment the
smaller engine with the batteries during acceleration. Smaller engines are
generally more efficient than larger ones when putting out the same amount
of power.

If you are trying to say it takes less power to maintain speed than to
accelerate, yes that is true.

The advantage from the electric engine at cruise is that it uses zero
energy.

There were attempts to increase mileage of gas engines by turning off
uneeded cylinders at cruise. They didn't work that well and you still
had to move the pistons, the big crank, and all the rest of the stuff.

Right, so a hybrid is like that system, except that the undeeded cylinders
are replaced with an electric motor. Instead of, say, having six cylinders
and only running four during cruise, you only *have* four cylinders, and
then you suppliment them with the electric system during acceleration.
That way you aren't moving them around and you get better efficiency from
the smaller engine.

The other advantage is that the engine can stay in the engine's efficiency
band even when the RPM demanded of it is higher (acceleration) or lower
(initial start).

The transmission keeps the engine RPM within a limited range.

Hybrids have no effect on that.

The transmission *tries to* keep the engine RPM within a limited range,
but it doesn't always work.

My car's efficiency band appears to be around 1500-2000RPM since that's
where it stays most of the time. But if I floor it on the highway it'll
easily hit 6000RPM at a great loss of efficiency. On a hybrid that extra
power is going to come from the electrical system.

However, these also don't help nearly as much on aircraft as on cars. The
difference between acceleration and cruise power on an aircraft is much
less than in a car, and aircraft engines tend to spend most of their time
in the efficiency band anyway, especially if there's a constant-speed prop
affixed. The extra drag caused by the extra weight of the batteries and
the rest of the hybrid system would probably outweigh any efficiency gain.

It doesn't help at all on airplanes.

The advantage to hybrids is they get better gas mileage.

They do that by using the deceleration to charge batteries which recovers
some of the kinetic energy instead of using it all to heat the brake linings.

This is true but incomplete. Regenerative braking is *one of the ways*
hybrids get better mileage. They also get better mileage by using smaller
engines and running those engines in a more efficient than would be
possible with a direct-drive system. But the conclusion is the same in the
end: although the last two would help in an aircraft they would not help
nearly enough to make such a system worthwhile.

--
Michael Ash
Rogue Amoeba Software