I think what OP had in mind is... where should you plug the small circular intake?
I get it, it fits perfectly on the engine... and then, where do you plug the whole thing? I've been fiddling with the editor for the last 20 minutes before resorting to Google and ending up here. I've even tried to use the Swivel liquid fuel engine to propel my first plane (wasn't pretty, at least I called it the Bad Idea Mk1)
Still trying to figure out how to plug the intake, the engine and the fuel line all together with just the tech from the Aviation tech node.
Edit: I asked the same question on Reddit