Translation:Η μηχανή μου.
Ok, yes, "μηχανάκι" should have been accepted and I've just added it to the list of correct translations. However, you should be aware that the hints are like a dictionary. They may give a variety of definitons but not all of them are correct in each sentence. Play it safe and always choose the first one unless you are sure.
Thanks. It's probably a bit ofa crutch but if there are multiple translations I tend to use the one with the least alternate meanings, so between μηχανή (engine/motor/motorbike) and μηχανάκι I tend to use μηχανή=engine and μηχανάκι=motorbike.