Well they did use compressed air and compressed springs at the turn of the century. We are kind of going back to that where there are hundreds of car companies. Better for the enthusiast. But in this case they are compressing fluids. This kit the Colorado company Hydraulic Hybrid Systems is coming out with starts at $12,900. It is for gas/CNG Chevy 2500. A Ford F250 is the next one.
According to
George Constantinesco hydraulics and electricity operate in similar ways a unified theory of quantum mechanics. What he discovered was Sonics or compressing liquids. That is not what the above kit does.
He found hydraulics and electricity behaved in the same way and power could be transfered using 3 phase systems.