I was ruminating over A Connecticut Yankee in King Arthur's Court. In the book, a trained American engineer goes back in time and creates guns. The book itself predates tanks, but one could imagine shoddy homemade technicals rolling around in medieval England.
Question is: is that possible? Let's imagine we have the full support of an entire kingdom, and we're working purely from memory. We still have bad quality steel, primitive tools, and no CAD. Can we make an engine at all, even a terribly leaky one?