Check it out:
Try a Hydraulic Drive Train: This Car of the Future Gets 75 MPG - Green Transportation - MOTHER EARTH NEWS
And:
Can an Inertial Storage Transmission Double Your Car's Mileage? - Green Transportation - MOTHER EARTH NEWS
I read these back in the 80s (I used to collect MEN, I have since become a bit less wide-eyed) and I thought we'd finally see it getting some traction when I read Ford was exploring the concept a few years ago:
Ford F-150 Used to Develop 40 MPG Hydraulic Hybrid Powertrain - PickupTrucks.com News
But nothing has come of it yet. Now buyers have cut their teeth on one kind of hybrid and the automakers have seen that we are willing to make a small extra investment if it means paring some out of our eventual fuel purchase. I like the hydro option in particular because it is, in my opinion, more sustainable than potentially toxic batteries. You would need to get your pressure vessels hydrotested periodically, however.
I also had high hopes for flywheel load leveling schemes, but I think I'm more afraid of those than I am of the pressure vessels. A disk burst in a flywheel energy storage device could release a catastrophic amount of energy, the space and weight it takes to build a containment vessel for it might negate the advantages of a flywheel.