That had me guessing, too. We talk about "engine trouble" but we also have the"motor car". So, I did a search and found that now they are almost interchangeable. See what the MIT professor says here: http://engineering.mit.edu/ask/what%E2%80%99s-difference-between-motor-and-engine
I hope that helps a little. If anyone else wants to weigh in to clarify this please do.