I've always found most of the nitinol heat engine designs uninspiring. Loops of wire going round a pulley but not doing much of anything, or weird contraptions dunking themselves back and forth in tubs of water.
These things all basically operate in the same way a Stirling engine operates, but none I've seen really look like engines.
I guess there's no real reason any of them should, but they should at least have some kind of power takeoff to get some useable torque with a revolving crankshaft or something to hook a generator to.
Anyway, I've been working on a few designs. One in the style of a walking beam Stirling with a displacer pushing hot air over a Nitinol spring. Might work, but air doesn't really transfer heat very well to Nitinol wire from what I've seen. The response is much slower than when using hot water.
Direct flame from a candle or lighter works pretty well though.
So I came up with this single Nitinol wire heat engine:
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When relaxed and cool, the flywheel could pull the wire out straight which would draw it into the heat of the flame.
Upon heating the wire would take the U shape, pulling itself out of the flame while simultaneously tugging on the flywheel.
The combination of cooling from work output and lack of heat would allow the nitinol to relax quickly so the momentum of the flywheel could straighten it out again, pulling it back over the flame in the process.
Nitinol is supposed to exert a force of something like 50 Tons per square inch. I don't know what that amounts to for just a 1 mm thick wire but should be something.
Basically instead of using heat to just expand air the heat or hot air is being used to expand or change the shape of a Nitinol wire.
The interesting thing about Nitinol is that you can tell it exactly HOW you want it to "expand" which opens up some interesting design possibilities.