If you said tio maŝino… you would probably be understood, but confusing to a great many people since tio is used in place of a noun, tiu is used to identify which specific noun one is speaking of.
Similarly the phrase could be a couched as a question by changing one letter: Kiu maŝino estas motoro would be "which machine is a motor," suggesting a number of machines to choose from. But you still know that they are machines and that one of them, at least, is a motor. I cannot think of a way to make kio or tio actually work in this sentence.
Tio means something unknown and we ask "kio estas tio?". Tio works like a noon, alone.
Tiu means something known. If we recognize an foreign object as human, we will use Tiu. If "this" or "that" are before a noon, the object is known, we use: tiu maŝino, tiu objekto, tiu homo.