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.
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,
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.
- 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.
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.