Chet,
Thanks for posting this. I've worked on a similar set of ideas for years, using air and water as the working fluids, where the air is in an open cycle and the water is re-used over and over.
The front end is essentially an off-the-shelf piston compressor. Ambient air is drawn in and compressed, raising the temperature. This hot, compressed air is pushed into a small reactor chamber just off the piston head. Water is preheated by the input cylinder heat and then injected into the reactor chamber to flash to steam. Next, the valve opens and the compressed air and the expanding steam produce the power stroke in a "steam engine" type wet environment piston (like brass or bronze) with a shorter stroke so the water re-condenses right at the bottom of the stoke. Finally, the exhaust valve opens and the cool air and water are evacuated to be separated and the cycle starts again. The water is recycled but new ambient air is used with each stroke.
The machine is essentially a "solar powered steam engine" that works on the heat in the ambient air for 24/7 operation.
Chris Hunter's design, using LPG in a closed cycle, uses the same basic physics and this document looks to be the description of a real, working system. By using a single refrigerant in both its liquid and gaseous forms, it simplifies the engine design considerably. Is there any way to find out more? Pictures, diagrams, test results? There are people who would fund development of this technology.
I also discuss a variant of this idea published by Nikola Tesla in 1900 in a lecture I gave last summer titled Open System Thermodynamics.
Tesla called his machine the "Self-acting Engine." He believed that the "heat in the ambient air" was the largest, untapped energy source on the planet, and the engine that tapped that source would be the most important engine ever developed for the future of civilization.
Thanks for sharing.
Best regards,
Peter

But I don't know if it has COP > 1. It boiled two liters of water pretty fast. I am looking for a way to tune it to get the most out of it.