A Stirling engine may look like a steam engine, but it is not: The cylinder is a closed system, and the way external heat is converted to work is a cycle of gas expansion when it heats up, and compression when it cools down again. It is a very simple process, and does not require much in the way of engineering.
This inspired a very skilled mechanical engineer to build one small enough to fit on the end of a pencil! I can only watch this in awe, because I don't have the skill or the tools to do this....