Preparation and Planning
While I'm not getting much project time in at the moment, I'm not wasting the time I have in proper preparation and planning.
The first working fire is about to be built - I've got sand arriving tomorrow that I'll need for the refractory mix and I need to buy a hole cutter to make the blast so I should be able to go out this weekend and if the weather holds I'll put it all together.
My first working fire is a foundry that I'll repurpose for some hot metalwork. It'll be fine until I can build something bigger, and will let me make cast brass components that I may well need soon. To this end I also caved in and ordered a graphite crucible online, I think I'm going to make a steel one when I can but having "That crucible problem solved" for now is going to mean I can get stuck in.
I'm going to use a cheap air blower from an outdoor ship and see how hot the fire gets. Long term I've got plans for a peltier effect module to power the fan from the fire. But the first step is putting a blower on there and getting a feel for how much air it needs to get a good burn.
Patio Paving
After making a few sketches and standing in the space, I'm pretty much sold on a patio space 15" wide and 18" deep, with two benches and a circular firepit around 3" in diameter. This second firepit will be mostly for heat and show, but I want to be able to cook on it so a good first project for the working fire will be making a rotary spit.
Practical Project
The rotary spit will start as a simple A-Frame and crank handle, but if I want to spend two hours cooking a chicken I'm going to need to automate that rotation and the blog comes full circle to Roger in Technology.
Presenting Peltier
The three engines I'm considering are all quite different in their construction and maintenance, and each with its pros and cons. The first is to use it as an excuse to make a peltier generator, which is a remarkably good project to start now, especially if I want to use it to power a fan later on.
The simple construction is a metal squarebar with a peltier device and heatsink on. The squarebar can be staked into the firepit at an angle, with the heatsink off to the side. And then I'll run an electric motor on a belt drive to turn the spit.
The peltier solution is nice, but it'll be difficult to get much current without a reasonably large array of peltier effect generators and a large heatsink. I don't know how much current I need to generate the torque to rotate a chicken. In fact, I don't even know how much torque you need to rotate a chicken so I could test this with some 9V or 4.5V DC motors and see if they are likely to be up to the task.
Piston Power
While the peltier might not work, the age of steam has me covered. I know I can build a steam engine with a boiler I can place in a cooking fire that will have much than enough power available - so specifically I can build a very small steam engine that will do the job.
It's a little fiddly to have a steam boiler in the fire with brass pipes leading to a steam regulator and crankshaft setup. My concern is that I have two unique units connected by brass pipes, and the pipes will bend and break.
The next option here is an external combustion Stirling engine. Relying on the expansion and compression of the working fluid, the stirling could probably be a single unit. Like the others it would sit on the edge of the firepit. Its probably harder to construct than the steam, but doesn't need as much pressure in the boiler and feels like a safer bet. I find the stirling a more interesting design, and it should be very easy to maintain and run - needing little tending compared to the steam that might need me to look after the boiler.
Practical problems
Whatever the build, I need something that is easy to store. Its not going to live in the elements all year round, so I want something I can easily carry from the firepit and stow away to cool once the cooking is done.
Thanks for reading everybody, I know this wasn't really an action report and I don't have any pictures. I'll get some pictures of the gear and the space before the build starts.
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