This is a great discussion!
You guys are really helping me visualize what we're trying to do here, and at what scale. Yes, Greg/Michael, that is what I meant. The efficiency that I'm looking for is relatively large amounts of high quality char and low smoke. That's what I've been getting. The flame goes very high up into the chimney, and burns the volatile gases. There is no air that can come in from the sides. The char is very light weight, and makes a musical tone when it hits something. I do try to make the wood super dry for that reason, and I try to cut them into approximately equal lengths. I have learned over time that I don't want unburned wood and I don't want ash at the end. By timing it just as the flames die down almost to the char, it turns out really well each time.
Great collaboration on the design and visual, Jan and Chris. When I visualized a cone, I didn't get it. When it looks like a cylinder it makes much more sense to me.