Can you use what you know (or can quickly look up) about how an automobile engine works, to see what problems solid paraffin wax would hold. You can probably find several things wrong, (and if you want to talk costs, you can look that up too), and use them all to write a good discussion your teaher will like. Keep some in the back of your head too, so you can write them on the exam, when no one else has heard them yet, for bonus points.