You have an accumulator - more or less a pressurized tank. When braking a
hydraulic pump adds fluid under pressure to the tank. You recover the energy
by running the high pressure fluid through a hydraulic motor (which could be
the pump running in reverse).
OK, I was too simple. They have an accumulator like in an automatic
transmission (sort of). You have a piston, or membrane separating the fluid
from the gas. The gas side is pre-pressurized. As you pump in fluid into the
liquid side, the liquid side expands and the gas on the other side of the
divide (membrane or piston) is compressed further. The pressure from the gas
side pressurizes the liquid (hydraulic fluid). You can release the
pressurized fluid to drive a pump. Instead of pressurized gas, you can also
use a mechanical spring, but this give you less storage capacity.
Have you ever seen one of the "water rockets" kids can buy (see
http://www.cas.unt.edu/~klittler/demo_room/mech_demos/1N22_20.html )? They
use pressurized gas over a liquid to propel the rocket. Just imagine the
same set-up except that the rocket exhaust is used to drive a pump or
turbine. In an automotive application you need a membrane or piston to
seperate the fluid from the gas, but the principal is the same.
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