Why would you want an external combustion engine in a car?
The Sterling cycle is extremely efficient and can use all sorts of fuels, the only requirement is said fuel needs to be a liquid or gas.
The engines are not good at running at different speeds, that’s why development of them for cars was abandoned in the 1970’s.
“However, they are more efficient than most internal combustion engines”
https://en.wikipedia.org/wiki/Stirling_engine