The Stirling engine seems like an odd-ball design when you’ve only encountered gas and Diesel engines. Even though the Stirling engine is piston based it doesn’t use valves. It has a sealed working fluid and no exhaust gas. Stirling engines aren’t very common, but are being produced commercially as cryocoolers and for use in solar farms. Here are some thorough plans for constructing a model Stirling engine from aluminum cans.