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Has anybody made an engine before that uses a modest heat source but cools with something along the lines of liquid nitrogen?

I'm not sure if it would work out as an energy-efficient exercise (how much energy does it take to liquefy nitrogen?) but it may be useful for compact engines where a high power output is desired.

Actually what got me thinking aobut this is a scooter I saw that I think was invented by dean cayman of Segway fame. It was a hybrid electric scooter that used a stirling engine to drive an alternator to charge a battery. The stirling engine was heated by an LPG bottle.

I remember a couple of years ago a friend of mine made a spa pool heated by a LPG fuelled barbeque - when the gas was running full noise the bottle froze over and got a layer of ice on it - so the gas is actually giving heating AND cooling at the same time - intersting huh?

Anybody have thoughts on this?

I've run my engine on dry ice before but that's nothing new. A bowl of water ice can run many of the production LTD models. I'm sure it's possible to "fuel" a stirling with liquid nitrogen but as you mention, I doubt it is cost effective or very "green" in terms of total energy consumed. A somewhat large engine could make use of the cooling effect of the propane bottle if it was fired (fueled) by propane I suppose. Could be a problem if it starts heating up the bottle though. The heat transfer ratio potential is over my head on this one. Interesting though.

I hadn't thought of this before, but I think its worth consideration.

I've worked with liquid nitrogen (LN2) and its very expensive. It is transported in a vacuumed double wall thermos container. A small amount of LN2 would make a large volume of cold gas, and coupled with a folding solar dish, it would certainly make powerful and tiny engine.

The Amish are well-known for not using electricity or cars. It is less well-known that thay use a lot of air-tools. The tools are powered from large tanks that store air at medium pressures, and are filled by windmill air-pumps.

This is similar to an air-conditioning cycle. Air is compressed and gets hot. After the tank air-cools a few hours, releasing the pressure then results in cold tools.

Stirlings work, and can generate electricity, but any added power can mean getting a good engine that is smaller.

I think LN2 will have a very tiny possible application, but for a remote engine, adding de-compressing gas (air?) cooling may prove valuable.

For a bench-top toy demonstrator, your idea of using cold propane on its way to the burner is very clever.