Energy efficiency has never been a critical factor in automotive sector even now. But with the increase in fuel prices it is becoming more and more desicive.
Lot less development has taken place till now regarding improvement in efficiency till now…. In 1940 when Ford and Volkwagen were mass producing cars engine efficiency was in rane of 20%. In last alonst 70 years the efficiency has increased just to 35%. Still we are wasting 1/3 power that can be used.
Most engines work most efficiently at highest rated RPM. But most of the time they are running below its rated speed. Hence they work even more inefficiently.
An exciting new technology named HYDRAULIC HYBRIDS looks like a great option to store energy. The working principle is similar to electric hybrids. here the electric generator and motor are replaced by hydraulic motors and Pumps. Battery is replaced by High pressure accumulators, and all the wiring and accesories are replaced by hydraulic tubing and valves.
Now why go for Hydraulic circuitsn electric.
main reasons for using draulic nad electric are
Power to weight ratio of Hydraulic circuit is many times more than electric system.
Accumulators are much more reliable, sturdy and light in comparisn to batteries. Above that Batteries cannot work efficienlty at suddden charge and discharge cycles. Accumulators are degined for sudden operations and hence work at full efficiency at any cycle.
Hydraulic motors have high torque at low rpm. This ability makes it much superior to electric motors as electric motors follow an constant slope efficiency curve for rising RPM while that for hraulic motors efficiency is almost constant around 85%.
At the moment there arre 2 circuits in consideration
1 Series circuit: where engine will be completely detached from the drive system. Engine will be coupled to Pump. The pump will charge the acummulators which will run the motors. The same motor will act as pumps and regenate the accumulators at time of braking
2. Parallel cicult: where engine has the mechanical driving system as well as hydraulic coupling system which will be functional at higher loads and will be regenrated while braking. This has a bit lesser efficiency and higher weight in comparison to Series hydraulic system.
It is projected that the efficiency will be increased by as much at 60 -80% in urban transport and around 30-40 % on highways depending upon the circuit used. This will also have a positive impact on the torque (which will make any ordinary car a SPORTS CAR 
)
I believe this is a huge step to upgrade the automotive industry. I have some published papers with me which may intrest u. If anyone wants to re them just send a reply with ur mail address or mail me on manas.pat27@gmail.com.
Thank-u