The first thing I did was to load the barbecue grill with scrap wood (non-treated!) to get some good hot coals. The grill's bottom vents are completely open.
Here are some large pieces of aluminum from a super old vacuum cleaner I found one day. The entire vacuum body was made from aluminum!
Next I piled some more wood on the aluminum. It is all stacked in a "log cabin" style for maximum natural air flow so there is minimal smoke.
Get a good bonfire going. A watering can or hose is handy to have nearby incase of uncontrolled fire/rapid change in wind direction (or, skip the watering can if you enjoy living on the edge...).
The metal reaches the point of "hot shortness" (just below melting temperature when aluminum crumbles easily) in only a few minutes. If you don't pay attention it'll actually reach melting temperature and start dripping down through the barbecue grill (I was entranced by the fire...). Thicker pieces will take more time, these pieces had 3/8" thick wall and took less than 10 minutes. Breaking up parts with oil in or on them (like an entire lawnmower engine may create significant smoke (so beware if you have nearby neigbors)!
I grabbed the metal from the fire with pliers (it was so weak it was crumbling as I was trying to pick it up) and only gentle taps from a hammer were needed to break it down.
There are a lot of coals left over which are good for another round of melting (I still have two 15 pound aluminum car wheels to break down) if you have a charcoal fueled furnace this is a great time to fire it up and make some ingots with the now broken scrap. Or... grill something for lunch (unless there's junk left in the coals).