The failure happened inside an operating engine. Most fire detectors are IR detectors. Do you have an innovative design for a fire detector, versus an overtemperature detector (typically a thermocouple), that can distinguish between the fire and hot air that is supposed to be inside of it, and the fire/hot air that isn't?
Usually a car engine which is burning oil is pretty obvious to the naked eye - there is blue smoke coming out of the tailpipe.