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How to make an engine (eg. with CO2, N2, NH3 or other substance) where liquid is boiled to vapor by 0..30C air (or ground)?

In other words you put in heat (== it is a refrigerator or air conditioner) and it gives output shaft power.

A car driven by that engine would use the input airflow for heating (boiling) the matter, not taking away waste heat. Obviously, exhaust will be cooler than input.

Let's say we start with 1kg 6..10bar NH3  (CO2 with the lower pressure around 5.5 .. 6 bar seems more practical, but let's discuss with NH3 for now)

We let it do  isentropic adiabatic expansion (say, 5E work gained). Because of expansion it cools down. With a suitable expansion, it will partially condense. Let's say 200g condensates, 800g is still gas.

This mist is probably difficult (or time consuming) to separate inside the work cylinder (unless it is a TeslaTurbine).

So we can displace this gas/vapor mixture to a separator. We close sufficient amout of liquid into a section of a tube where it will boil from environmental heat (=air conditioning).
We need to compress the 800g gas only (4E work). If it were 1000g, we (the flywheel of the mainshaft) would need to do exactly the SAME amount of work (5E) as we gained by the adiabatic expansion (this reversibilty is the very nature of isentropic adiabatic expansion). But we only need to do 80% work, because 200g liquid is removed, and boiled to high pressure free by the environment.

NH3 will boil to 9.5bar at 22C
R134a would be suitable for a working prototype. It's AC technology. Perhaps some type of off-the shelf compressor could be used as piston engine. We need a few valves  to control the liquid and gas in the sections of the machine.


It might be theoretically possible to design with H2O, but H2O needs very little pressure (==huge expansion ratio) so that would be impractical. When choosing matter

  • around 40..100 bar operational pressure seems practical
  • we want a high percentage of the gas to condense (say 30..50%, not 5..10%)
  • low cost, environmentally friendly gas preferred


CO2 with the lower pressure around 5.5..8 bar seems quite good. Upper pressure point around 20..40bar (or perhaps 100 bar or higher?).

See CO2 phase-change diagram
Note: when the lowest pressure during the adiabatic expansion is kept above 5.2bar triple point, (eg. at 10 bar), dry-ice does not seem like a problem. Especially that with partial condensation, no part of the vapor / liquid will be colder than the boiling point.


Other refrigerants:


  • N2: too low boiling point
  • N2O: certain metals or organics might be the fuel, it might explode. Also very problematic above 34C.
  • R134a: too low pressure at 0..20C to give useful work (even lower than NH3).
  • NH3: could be good for prototyping. Boils at 9.5bar at 22c

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How to make an engine (eg. with CO2) where liquid is boiled to vapor by 0..30C air (or ground)?

Using adiabatic expansion, condensation (yes!), than separation of liquid and vapor.

The physics is very simple - yet mind-blowing.  It's definitely a breakthrough.
There are technological challenges of course. We need to integrate technologies that were used separately earlier. Perhaps piston implementation might be more suitable first, than with turbines.

If you'd like to cooperate, edit http://www.eleg.hu/tiki/LowBoilingPointSteamEngine.html 



Created by: cell. Last Modification: 2009-04-27 (Mon) 18:32:46 CEST by cell.