keith, yes, it overwhelms CO2. But water is a positive feedback loop (more heat, more water vapour), we should be careful of tickling that particular loop (although increased cloud increases albido).
BTW, the climate change facility I work at emits 36,000 tonnes of CO2 a day, and 100,000 tonnes of water vapour (cooling towers). Not to mention the 6GW of waste heat, and the 2.3GW of electricity that ultimately ends up as heat that must be radiated out into space.
As to the 6 stroke engine, I can see how a liquid to vapour transition could give a little more oomph from a given charge. I don't think that recompressing the expanded working medium is the sensible way of going about it 'though.