The 's signifies possession. You add it when you want to stress that the second noun is the property of the first one. Without the 's, it's a more generic version.
For example, if you bring a friend to your garage, and he sees a bunch of bolts and screws lying on a workbench and asks "What're those?" then you can say "They are engine parts", and you would mean that they are things that go into engines, but they don't belong to any engine specifically, or it doesn't matter which engine they belong to. They might just be some parts you have for a later repair.
On the other hand, you could also say "Those are the engine's parts", which would indicate that they came from a specific engine and they still belong to it. Probably because you're repairing an engine and you removed them from it for cleaning, and they will be put back in later.
So whether to use the 's or not is usually part of the context; do you want to say that "the parts" belong to "the engine" or do you want to say that "the parts" are for "an engine"?
(same with the others, although they are less likely to come up.)