When one is trying to recover the energy of a moving vehicle or
object slowing down one has a problem.  The energy is proportional
to the square of the axle RPM so at first there is a lot of energy recovery
and braking effect. As the vehicle slows down the braking  energy rapidly
falls off. If a generator is directly connected to the wheel the braking
effect of the generator and energy recovery will fall off directly
proportional to the RPM. This is not what is required to stop as quick as
possible. The conventional brakes must be applied I suspect unless some way
is found to rapidly increase the load on the generator such as changing gear
ratios and/or by electronic means.  The same thing can be said about
spinning up a flywheel to store the braking energy. This device, invented in
the early seventeen hundreds, might be used to speed up the flywheel or
generator as the vehicle comes to a stop
then is reset to wait for the next stop.

http://www.sailtexas.com/long.html
http://www.antique-pocket-watch.com/sinclair-harding.html

It will be very interesting next year when KERS is implemented in Formula 1
to see how the various teams achieve storing braking energy in a flywheel.

Paul Lamar

Not certain I understand fully, but seems vaguely like a CVT to me.

Flywheel -> CVT -> flywheel would be a good system for recovering
vehicle KE. Nissan might have a leg up on that one.

-Chris