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Tell me if this is correct . A stirling engine isnt literally a heat engine. It is a heat differential engine. If you had the right working fluid and ambient temp. was zero c.,you could use this as your heat source. You dont have to add heat you can pull heat from the cold side. If you used liquid nitrogen to cool the cold side the engine would work.

As far as I know, any thing above zero Kelvin/ -273*C is heat, many people think of hot as feeling warm or hot to our selves. My low temp motor, if sat on ice will run backwards, or with the ice on top , run forewards, I say it's definately a heat engine. Ian S C

Ian SC;
I totally agree with your dedinition of heat,but almost all heat engines I know of ,you have to add heat to make them work. You cant add ice to a steam engine and make it work.Cold wont turn over a diesel or a gas engine.Its just a unique property of the stirling engine in our enviornment.Sure its a heat engine.You just dont need to add heat like other engines .You can add cold and get it to run.That makes it special. Thats what I meant by stating ,is it a heat engine? You dont need heat to run it.

Rather than heat differential, temperature differential might describe it better. Thus you will always have a 'hot' and 'cold' side whatever your definition of heat.

All engines are difference engines. In a strict sense, there's no such thing as cold, just different levels of heat. It doesn’t matter if you roll a boulder from a ten foot high hill or into a ten foot deep hole, the same potential moves from place to place. For harvesting energy the difference is what's useful, not the levels. The liquid nitrogen in your example isn't the energy source; it's a lower level for the boulder to roll down to. Another example of using ambient temperature as the heat source would be the little model steam engines that run on dry ice. Bumpkin

Bumpkin;
There is no such thing as cold and there is no such thing as absolute zero. There is always a little movement to cause heat. But just as reference it could be said 2000 kelvin is cold if you compare it to 3000 kelvin.Thanks for the note on the cold steam engine or dead steam as some called it. 1000psi. on dry ice.
Doggle; I agree temperature differential make more sense.
The only point I was trying to make is there arent to many engines you can add cold to and make them work.The heat source is always what is on the hot side.No magic just unique for stirling. The rankine cycle should also work with subtle heat differences with maybe something like freon.I dont care if Iam right or wrong I just want to learn and share.

When you add ice to a Stirling engine you are not adding cold, just extracting heat. This allows more heat to be added from the atmosphere to the hot end.
But you are right I think, no other engine can be made to run by taking heat away from it.
On the other hand there ain't no free lunch. It takes energy to extract that heat from the Stirling, eg running the refrigerator to make the ice cube to add to the engine. So by adding an ice cube, you are indirectly adding energy to the equation.

You could say that the heat from the condenser coils on the back of the refrigerator is being added to the Stirling engine when you take the ice cube out of the fridge, put it on the Stirling engine cold end and allow heat from the atmoshphere to enter the hot end. That atmosphere was heated by the fridge that extracted that heat from the water to turn it into ice.

Hopper;
There certainly is no free lunch in thermodynamics but there is a clear perception of one. When I would gather wood after a big storm and burned it in my woodstove it seemed free. Your right of course it really wasnt.I dont believe there are any other heat engines when turned mechanically become refrigerators.Iam just singing the praises of the stirling engine because its so versatile. It can also be an acoustic engine.What other heat engine could be made out of pop cans and a coat hanger?

This is true.