Hydraulic Hybrid Bicycle Research | Clean Automotive Technology | US EPA
The weight of the accumulator would depend on its construction. Carbon fiber accumulators would save considerable weight.
The link above shows a HH bicycle. I am not saying it's practical but it certainly is do able.
My thoughts center around the early Insight I had and how it would be neat to find one that had a bad transmission or other components that made it too expensive to repair, and convert it to a HH.
With one of my drive designs in each front wheel and a carbon fiber accumulator replacing the battery and other components that would no longer be necessary, it would be interesting to compare it to the original designs efficiency.
The early Insight's battery only range was very limited, but they used the battery in the same way a HH would operate. Accumulator capacity would be minimal, at least half of the 14 gallons required to move something the weight of the Chrysler minivan.
Depending on the actual configuration, you could eliminate the transmission, axles, induction system, battery, battery modules, high power electrical system. and some other components I probably forgot.
I wonder what the mileage would compare to the 68 MPG I averaged in mine for over 20k miles, and I managed 70 MPG one day for 575 miles at average speeds of right at 55 MPH.
You could also toss the IC engine and use a battery to drive the primary pump, with no voltage conversion or modules, just an on off setup the kept the accumulator charged to a certain level, then drive the car and regenerate with the hydraulic accumulator and in wheel drives. Range would be limited like current electric designs, but it would be much more efficient in urban scenarios where there was a significant amount of regeneration opportunities.
regards
Mech