Interesting use of CO2 by Delugeinc.com in their natural energy engine.
You can see a few different implementations here:
http://www.youtube.com/user/NaturalEnergyEngine#p/uTwo Cylinder Prototype:
[youtube]s1Jp-QzRbFM[/youtube]
Couple Ideas to improve on the concept.
1. Change the piston type accumulator to a bladder type accumulator.
Eliminates friction and reduces response time to < 25ms.
Closed Loop CO2 charged instead of traditional N2 charged.
Gain Benefit of Liquid CO2 Expansion Ratio 845:1 allowing more hydraulic fluid to be pressurized.
2. for the hot side implement an inductive heating element.
96+% efficient with all heat delivered directly to the CO2 side of the accumulator.
Precise electronic timing and control of pressurization of CO2.
The bladder system will also absorb all heat from the working hydraulic fluids and act as the oil cooler.
3. Use a circulating liquid cooling system for the cold side. Capture the cooling of each pressure release cycle.
pump the liquid between the different accumulators based on need/state of operation.
4. Increase system operating pressure and temperature to supercritical CO2 state. 32C 72ATM.
System should easily be able to operate in pressures up to 10000psi
Picture 3 Citroen style accumulator Spheres pumping hydraulic fluid.
Each at the different cycle states to provide continuous pressurized fluid flow.
Sphere 1. Releasing Pressure
Sphere 2. Charging Pressure
Sphere 3. Charged and waiting
Effectively you have a silent and powerful closed loop co2 thermal hydraulic compressor.
Size the accumulators to your desired flow rate.. ie 2GPM.
The fluid can be used to run an electric generator, pump or really anything.
This isn't overunity per se as it is a thermal process using the 300k ambient heat and taking advantage of some of the unique properties and efficiencies of supercritical state of CO2.
Instead of an open loop internal combustion engine you can utilize this closed loop co2 internal compression engine.
Obviously Deluge has already proved the concept and have production systems installed for desalination, power generation, cooling etc. but their implementation operates sub-critical at much lower pressures and on an industrial scale. I believe with these few improvements we can develop a small and safe suitcase sized appliance.