Recently I've gotten into making model Stirling (hot air) engines. Stirling engines are essentially external combustion engines. They are sealed systems. The technology is about 200 years old, but is still being developed for uses such as power generation in space.
The engine requires only a temperature gradient to run. Here is a link to an explanation of how it works.
Here is a model of a 2 cylinder Stirling engine I built:
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