Hydraulic hybrids <link> are known for their simplicity, (relatively) low cost and very high regeneration efficiency. A pump loads up an (high pressure) accumulator, which is tapped to run an hydraulic motor, the spent fluid then flows into a (low pressure) reservoir, rinse and repeat.

The BMW TurboSteamer <link> brings to reality a long-held desire to harness otherwise-wasted heat from internal-combustion engines.

Research is underway for using supercritical carbon-dioxide in closed loop turbines <link>. At 30ºC, LCO2 enters the supercritical phase, its density decreasing dramatically as the temperature rises further. Supercritical fluids are very slippery.

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The idea then is to use liquid/supercritical CO2 as regular engine coolant and exhaust coolant (extracting thermal energy from same) to create a thermal-engine within the workings of an hydraulic hybrid system using sc-CO2 as working fluid.

We combine a regular engine cooling system...
engine -> coolant-pump -> radiator -> engine,

with a regenerating hydraulic system...
reservoir -> pump -> accumulator <-> motor -> reservoir,

creating...
reservoir -> pump -> engine/exhaust-cooling -> accumulator <-> motor -> radiator -> reservoir.

(note how the heating and cooling steps bracket the motor)

Pros (compared with regular hydraulic hybrid system and Turbosteamer)
- decreased complexity: the heat conversion is part of a full-featured hybrid
- increased effectiveness: sc-CO2 starts expanding at 30ºC, not 100ºC
- safe compared to steam
- the hydraulic-fluid/engine-coolant(/aircon charge) is environmentally inconspicuous and costs basically nothing.

Cons
- liquid CO2 means a base pressure of 74 atmospheres, meaning heavier construction of both the engine cooling system componentry (jackets, pipes, radiator) and the hydraulic system
- shared single point of failure for both components of the hybrid system.

Etc.
The system of course includes an electrically-operated CO2 sequesterer to top off, or fill from scratch. All components of the system are cross-valved so you can, for instance, run the engine without having to run the hydraulics, and perhaps have a chance of isolating a leak.

Regarding hydraulic hybrids in general, the system could/should be used for cheaper/simpler power-braking/steering, windows/doorlocks, winches, etc.
-- FlyingToaster, May 03 2014

http://en.wikipedia...ki/Hydraulic_hybrid [FlyingToaster, May 03 2014]

http://en.wikipedia.org/wiki/Turbosteamer [FlyingToaster, May 03 2014]

http://en.wikipedia...oxide#Working_fluid [FlyingToaster, May 03 2014]

Erm, how about just using water, in a "thermal engine" at say 1/5 of atmospheric pressure on the radiator water, and just a boring old closed cycle steam engine on the exhaust pipes?

<realises I am repeating myself here...it must be that kimchi>
-- not_morrison_rm, May 03 2014


^ You mean apart from the "Pros" section of the post ?

From the sole viewpoint as an hydraulic-fluid, CO2 might not have much going for it: even though it's probably much easier to pump through the lines due to superlow viscosity, it has to be kept at ~1,000psi or it's not a liquid/scf, meaning some components have to be more robust: there's not as much headroom as in a system which can theoretically use ambient (or less) as a base pressure.

On the other hand, from the viewpoint of integrating a heat-engine and hydraulic subsystem together, which is the meat of the idea, it completely blows away the Turbosteamer idea.

The fact that they're putting together a 550ºC sc-CO2 closed-loop turbine for electricity production would lead one to assume that its properties as a working fluid are comparable or superior to water in such an application, which is similar to this one.

Basically, if you have an hydraulic hybrid concept, you can add heat-recovery to it by running it through the engine and around the exhaust, replacing the existing engine-cooling system.
-- FlyingToaster, May 03 2014


There's quite a difference between water in its liquid and gaseous states, including a large energy cliff at the boiling point. The post uses a supercritical fluid with no chaotic transitional properties throughout the entire temperature range.

Apart from the base pressure considerations, it's rather less complicated than a steam engine.
-- FlyingToaster, May 03 2014



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