UPS trucks have been using this tech for some time so its not some pipe dream.

This direction is already being heavily pursued by the Scuderi group (see http://en.wikipedia.org/wiki/Scuderi_Engine) in combination with their intruiging split-cycle engine design. This tech has huge potential to improve IC engine efficiencies. And they\'re well on track to commercialization with several major manufacturers. Good stuff!

Basically it\'s the same as the famous american Scuderi Engine. See youtube

I\'ve thought this before myself - compressing air and then releasing it is much more efficient that converting the electricity etc. I differ a little however in that I think of it more as an energy return system where the same special brakes that compress the air while decelerating also return the power to the wheels when acceleration is required. Getting the combined brake/air motor right would be the key.
Some Aussie company patented something a bit like this that the US army brought to use on their trucks etc.

\"while hydraulic systems store the energy as pressurized fluid.\" When you say pressurized fluid - are you implying that fluids cab be compressed? Air can be but fluids cannot. That is why water is used to hydrostatically test gas cylinders and air is used to force the water. The water can expand the metal and test it\'s ductility. Brake fluid works like a steel cable in liquid form because it can not be compressed.

\'hydraulic systems store the energy as pressurized fluid.\' WRONG.
Pressurized fluid in hydraulic systems cannot STORE energy, they only CONVEY it, the very principle of hydraulics is the use of an incompressible fluid in order to transmit power from the source of pressure to where effort is needed. What allows a hydraulic hybrid to work is use of an accumulator whereby pressurized fluid COMPRESSES GAS in a piston.
These and other concepts are explained in my report on Atlas Motor Works ducted blade rotary engine here;
http://atlasmotorworks.com/dev/docs/DBR-tech_Feasibility_Analysis.pdf
There is finally an alternative to pistons!

Don; Pressurized is not the same as compressed. All fluids can be pressurized, those that are gasses will compress proportionately, PV = nKT - liquids will compress very little. A hydraulic ram with a load on it will hold pressurized, but not compressed*, hydraulic fluid. *Well, just a little.

I think they are very much overrating the mileage gain of the system. Most regenerative braking contributes about 10% to overall mileage. And that\'s with systems that get 90% efficient energy recovery. The main ingredients to the Prius\' high mileage are low drag, and Atkinson cycle ICE. The Atkinson cycle engine gets about 50% better mileage than a comparable Otto cycle engine. The loss in torque from an Atkinson cycle is offset by the electric drive. So here we have a system that only get UP TO 48% recapture, and it\'s supposed to magically do 4 times better than a Prius? I really doubt it. Bench tests are one thing, real world tends to throw that stuff out the window.

To answer the comment by donwine about compressing fluids; The fluid itself is not compressed, but the pressurized fluid pushes against a piston which has on its other side a compressible innert gas such as nitrogen. This device is called an accumulator and has been used extensively on airplane controls and crane and other construction equipment controls and pressure equalizers or moderators. The pressures are much greater than those of a compressed air system. The accumulators I was familiar with operated with pressures greater than 1500 psi. and could go up around 10,000 psi I\'m not sure if this system would be of any value for propelling a vehicle because the volume of fluid involved is usually not as great as the volume of air as it uncompresses. Regards

ALSO FIAT IS WORKING ON AIR HIBRID SYSTEM