Doc: You mean the car is out of gas?
Marty: Yeah, but it's no big deal, we have Mr Fusion right?
Doc: Mr. Fusion powers the flux capacitor, but the cars internal combustion engine runs on normal unleaded fuel, always has...

So I got to thinking about something today, specifically the line above from Back To The Future 3 when the Doc realizes they have to come up with an alternative means of propelling the delorion.

Why didn't the doc just build a freaking electric motor? Surely he's got the parts to do it in his blacksmith shop. If Mr Fusion can output the requisite 1.21 gigawatts of juice needed for the time jump using a banana peal, a candy bar wrapper, and half a Budweiser, surely they can drop something denser in (like, say, blacksmithing iron) and eek out a few extra kilowatts to power an electric engine!

This just suddenly popped into my head tonight while I was driving home from Kat's house.