Haven't been able to get more info on this company Energine, but its a Korean firm which has developed a new type of hybrid using a small two-stroke compressed air motor, and an electric engine powered by batteries.

I'm not sure, but it seems to work by using stored compressed air only to accelerate and climb hills, and uses the electric motor for everything else, including recharging the compressed air tank when it isn't driving the wheels.

Don't know if it also has a regenerative braking system, but using compressed air is apparently a more efficient way of capturing braking energy than batteries.

The article is about 2/3 down the page:
http://www.driversdrive.com/apr05/

So I wonder if they are using perfect rectifiers in regenerative braking systems?

hydraulic hybrid technology - No batteries nor electric needed

http://www.epa.gov/otaq/technology/recentdevelopments.htm

60-70% better fuel economy in laboratory tests
40% or more reduction in carbon dioxide, the primary greenhouse gas

http://www.epa.gov/otaq/technology/images/ups-truck.jpg

From the EPA:

==============================================
How it works
This innovative technology is simple. The main components in HHVs are:

The high pressure accumulator stores energy as a battery would in a hybrid electric vehicle using hydraulic fluid to compress nitrogen gas

http://www.epa.gov/otaq/technology/pumpdiagram.jpg

The low pressure reservoir stores the low pressure fluid after it has been used by the pump/motor


The rear drive pump/motor converts the pressure from the hydraulic fluid into rotating power for the wheels, and recovers braking energy which is stored in the high pressure accumulator


The engine pump/motor pressurizes and transfers hydraulic fluid to the rear drive pump/motor and/or high pressure accumulator


The hybrid controller monitors the driver's acceleration and braking, and commands the hybrid system components
There are three key design features enabling an HHV to provide maximum fuel efficiency:

Regenerative Braking. When stopping the vehicle, the hybrid controller uses the energy from the wheels by pumping fluid from the low pressure reservoir into the high pressure accumulator. When the vehicle starts accelerating, this stored energy is used to accelerate the vehicle. This process recovers and reuses over 70% of the energy normally wasted during braking.


Optimum Engine Control. The engine pump pressurizes and transfers fluid from the low pressure reservoir to the rear drive pump/motor, and under certain operating conditions, to the high pressure accumulator. In the full series hybrid design, there is no conventional transmission and driveshaft connecting the engine to the wheels freeing the engine to be operated at its maximum efficiency "sweet" spot to achieve optimum vehicle fuel economy.


Shutting Engine Off When Not Needed. The unique hybrid design not only allows the engine to be operated at its maximum efficiency, but also enables the engine to be completely shut off during certain stages of operation because it is activated by the controller only when it is needed. As a result, in stop and go urban city driving engine use is cut almost in half.
================================================

An article on the hydraulic hybrid from Popular Science :

http://www.popsci.com/popsci/automotivetech/e5690576b64fc010vgnvcm1000004eecbccdrcrd/2.html

Batteries continually self-discharge even when no load is being applied. The technology of mechanically containing gas pressure with no loss over long periods of time is very well developed and capable.

http://www.popsci.com/popsci/automotivetech/e5690576b64fc010vgnvcm1000004eecbccdrcrd/2.html

Excellent research Yale!
So I'm wondering why can't an electric version of this hydraulic design be made?

This would be a car whose wheels are driven entirely by hydraulic fluid, forced by compressed nitrogen, whose compression would be maintained by a small electric motor running off batteries.

The UPS truck someone else has posted is made by International (NAV on the stock exchange) Eaton makes the drive train (I THINK) See pictured at:
http://www.pressroom.ups.com/pressreleases/current/0,1088,4694,00.html

from where the following text is copied (Much more there also)

"....The EPA cited laboratory tests showing that the technology has the potential to dramatically improve the fuel economy of urban vehicles* used in applications such as package delivery, shuttle and transit buses and refuse pick-up.

The EPA estimated that when manufactured in high volume, the added costs of the hybrid components could be recouped in less than three years through lower fuel and brake maintenance costs.

In the series hydraulic hybrid diesel, a high-efficiency diesel engine is combined with a unique hydraulic propulsion system, replacing the conventional drive train and transmission. The vehicle uses hydraulic pumps and hydraulic storage tanks to store energy, similar to what is done with electric motors and batteries in hybrid electric vehicles. Fuel economy is increased in three ways: vehicle braking energy is recovered that normally is wasted; the engine is operated more efficiently; and the engine can be shut off when stopped or decelerating.

The diesel hydraulic hybrid truck is potentially eligible to qualify for a tax credit that is up to 40 percent of the incremental cost of the vehicle under a provision of the Energy Policy Act of 2005 for medium- and heavy-duty vehicles. ..."
----------------------------------
*Billy T note: This hybrid is a very good idea in development ONLY for very frequent start/stop and big vehicles like school buses, UPS, trash and mail trucks in central city service, traveling mainly on nearly level ground.

For a car in typical use, there is too little energy stored in compressed air tanks of any reasonable size and weight and much too much heat produced if the air is compressed greatly to try to store enough energy to climb back up even a small hill with the stored energy. Anyone who has pumped up even a bicycle tire with a hand pump should know that. What you can store without too much heat production in a reasonable weight truck is a good fraction of the energy the truck had when going roughly 40mph. If your car were going 60mph the air tanks it could carry would store less than half of the energy (recoverable energy, not heat), perhapse less than 20%, I do not know the number, but it is not worth it.

If I were to try to make a similar system for a car, I would at least consider a dual compression system (two different fluids, one air and the other an easily vaporized liquid). The heat produced as the air is compressed could then later assist the reconversion of the other liquid back into vapor to turn a turbin, but I suspect this is getting much to complex and costly.

Excellent research Yale!
So I'm wondering why can't an electric version of this hydraulic design be made?

This would be a car whose wheels are driven entirely by hydraulic fluid, forced by compressed nitrogen, whose compression would be maintained by a small electric motor running off batteries.

I think that the machinery to do the hydraulic version is a lot simpler.