Nitinol is a nickel/titanium alloy. This metal becomes pliable/loose at low/room temperature and immediately regain it's former form when exposed to warm temperatures. And it does so with more force than it needs to get it to take the initial shape. This opens possibilities to very efficient heat to motion conversions. As an added advantage, the more it is used the stronger the effect gets (the metal goes into it's former state with more force).
By the way, if you are interested in experimenting with Nitinol, you can purchase it on eBay.
Prize winning 1982 CNN report by science editor Kevin Sanders:
His follow-up articles on Nitinol in Science Digest are here:
UPDATE REPORTS & ARTICLES: (more below)
New Nitinol Heat Engine Design (PDF report)
Nitinol Heat engine blogs/discussions
Update article on Nitinol Engines by Ridgeway Banks:
Nitinol Research Documents Missing:
This Shape Memory Metal (Nitinol) engine is driven by hot water!
You may recall the famous Thermobile, created in 1985 by Frederick Wang. The Heatmobile is a similar design, although offered as a simple self-assembly kit. It is essentially a heat engine that demonstrates the conversion of heat into mechanical energy. It uses the unique property of Nitinol alloy called the "memory effect". Once Nitinol has been formed into shape at high temperature (about 600º C) and allowed to cool to room temperature, it can be easily deformed. However, when heated above a transition temperature (in this case about 70ºC to 80º C) the Nitinol object abruptly returns to its high-temperature shape with substantially more force than that required to deform it when cold.
Heatmobile uses a Nitinol wire formed into a closed loop that drives the two connected wheels. However the high temperature shape of the wire is straight. So when the bottom part of the wire loop is heated by the water, it tries to deform into a straight shape. This action provides a torque at the bottom wheel, and causes it to rotate. As the wire goes up around the top wheel, it is cooled by the air.
The Heatmobile is about 18cm high, exluding the rotor. If measured to the top of the rotor, it is about 24cm high.