Yup, the charcoal is an old idea. We called them "Cummins' pre-heaters". We cut the bottom 6" out of a 55 gallon drum, knocked a couple of holes in the side for draft, loaded it up with charcoal, set her on fire, slapped the lid on it and slid her under the engine.

Couldn't use it on Detroits. Too much leaked oil, errh, auxillary rust deterrent.