If an implementation of a patent exists, and that patent expires, it is impossible for that implementation to infringe another patent for the same thing, because any other patent that applies would have been invalid when issued, because of the previous patent.
Eh, not necessarily, I've heard that one thing patent owners do is keep the thing ticking over by patenting something else sufficiently close to the original that they overlap and sufficiently different that they can be argued as distinct. Sure it's infringing - but it's infringing their own stuff. So, not invalid.
Note: as to whether any particular tactic like this stands up in court or not, you'll find out if you fight it in court. You and your huge heap of cash, that is. Don't have a huge heap of cash? Then the question is moot, isn't it, peasant?
Not necessarily. You could patent an improved engine. 19 years later, I could patent an improved alloy that someone might use in your engine. Your patent expires in year 20, but they still infringe if they make your engine with my alloy for the next 20 years.
