Friday, September 11, 2015

Carburetion vs forced air

Some really interesting reading this morning on strategies for mixing air and gas to the right ratio.  Based on last night's experiments, this will be key to getting this to work, and I've seen gas/air mixing as critical to almost every prototype I've worked on.

Basically, you need about 96% air and 4% propane to get a good burn.  In a confined space, like the sheath tube I'm using, getting enough air in there requires an active strategy.  One strategy is to just pump air into the sheath.  This is the simplest thing, and probably what I'll try first: drill some holes in the side of the sheath, and hook up an air pump.  It's inexact, but it should work.  If I go to a pressurized air tank, I can get higher flows, and use a needle valve to tune the ratio.

But an interesting alternative is to build a carburetor that pre-mixes the propane and air.  A carburetor basically uses the Venturi effect to draw air into the stream actively before the gas reaches the outlet.  This is a good example of such a design, for a propane burner that can burn much hotter than just burning raw propane:

The writing is unfortunately pretty bad, but the gist is that the teeny hole in the middle pipe lets out propane, and it draws air in through the giant hole in the reducer.  There's not much in the way of tuning, but it works pretty well apparently, possibly just based on the specific hole sizes used.  Using a design like this in the shuttle would be complicated; it's not clear how I would adapt this.  But it's a starting point for some thinking on how to make this work.

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