"Where is our motor?"
Translation:Где наш мотор?
"Мотор" is a masculine noun, so it combines with наш. The Genitive is, to put it simple, a form of a noun. Sentences do not have a case.
You can use the Genitive to in structures like "a part of the motor" (часть мотора) or "the size of the motor" (размер мотора). These often correspond to "of" in English—or, indeed, a possessive structure ("the motor's size").