Has this Nitinol engine technology simply just been forgotten or has it been deliberately supressed by the oil companies?
This Nickel and Titanium alloy has the ability to 'memorise' a shape. At a temperature of 500ºC you set the Nitinol into the required shape. While the metal is cooling to room temperature, the shape is ‘trained’ into the metal.
Once the 'memory' is set you can change the shape of the metal. Then by raising its temperature to the metals 'transitional' level the Nitinol will abruptly and with a force of up to 55 tons/square inch return to the memorised shape. Different Nickel and Titanium percentages in the alloy produce different transitional temperatures including room temperature.
In the 1970s and 1980s toys and even spectacle frames were made of Nitinol but all that seems to have been forgotten.
A Nitinol engine made with a low temperature differential could easily use a heat source like, solar, geothermal, friction or waste heat etc.