Electric and hydraulic transmissions both offer power management, energy storage and flexibility. It is even possible to eliminate the mechanical drive train completely and have an in-wheel motor installed in the hub of each wheel.
These series systems are only feasible if all the transmission components have a high efficiency, also at part load and start up conditions. Furthermore the power density, especially for the wheel motors is a challenge.
Compared with electric series hybrids the Hydrid has an extreme high regenerative efficiency, more than double than that of electric hybrids. Furthermore challenges connected to hydraulic hybrids like weight, cost and packaging/NVH can now be met due to the new enabling floating cup technology.
http://www.innas.com/HD_EorH.html Hydraulic Hybrid - The Most Efficient Lowest Cost Hybrids
World’s First Full Series Hydraulic Hybrid Delivery Vehicle Prototyped in a UPS “Package Car.”
Through the use of innovative hydraulic hybrid technology EPA and its industry partners have been able to create the most efficient and cost-effective powertrain technology in the world. EPA has been able to improve city fuel economy of a UPS package car by 70% and reduce CO2 greenhouse gas emissions by 40%. The unique energy recovery technology used while braking reduces brake wear by 75%, increasing the net savings substantially.
A fleet owner operating one of these high efficiency vehicles would save up to 1,000 gallons of fuel each year.
What is even more astounding is that the additional cost for this technology manufactured in high volume has the potential to be less than 15% of the price of the base vehicle. At current fuel prices, this technology will pay for itself in two to three years.
The net lifetime savings over this vehicle's typical 20 year lifespan would be over $50,000.
Hydraulic Hybrid Vehicles (HHVs) rely on proven innovative technology which can be applied to light duty work trucks, SUVs and heavy duty urban vehicles such as city transit buses and garbage trucks.
http://www.epa.gov/otaq/technology/420f06043.htm