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For a winter break science experiment for my 7-y-o I thought we would build a simple lamina flow engine similar to this: http://www.youtube.com/watch?v=ZjgcsW0O ... er&list=UL

Ours is even simpler: http://lh6.ggpht.com/_thDShfGzeUg/TQ9-G ... AG0066.jpg

However it does not work :(

The water will push up the tube away from the heat but never starts oscillating. There must be some ratio of air to water and heat but I can't seem to find that information anywhere after two days of searching. Any tips on a sure fire setup? My test tube is 125 X 15 mm and I have several size tubes. I also have some 1/2" copper tubing. I would like to keep the idea of using a candle.

I don't think your engine is going to work, because it doesn't have anything to store momentum (the job done by a flywheel in a mechanical rotary engine).

The running engine contains two elements absent from yours- the first is simply a large volume of water. The water will act as an inertial mass which keeps the air in the engine moving in the same direction for a short while. A very small mass would presumably result in a much faster oscillation (all else being equal) and so probably a much shorter movement.

The other feature is a (presumably sealed) bottle. I don't know enough fluid dynamics to say for sure exactly what this is doing but I suspect it "buffers" pressure changes, allowing pressure to build up without immediately pushing out the water piston. I'm not familiar with this type of engine but it looks in some ways similar to a water hammer pump, which you can look up on Wikipedia. For an example, imagine you have a mouthpiece connected to a rubber balloon and a flat 100 foot hose full of water. If you blow a quick puff of air into the mouthpiece, the balloon will immediately inflate, then the increased pressure will start to push water out of the hose as the balloon deflates. Alternatively, if you hook an elastic cord to a full shopping cart and pull on the end, the elastic will stretch to "buffer" the force before the trolley starts moving.

I suspect without these two features your engine won't run. The water is likely to be vital, but is easy to add. If that still doesn't work you could try a larger diameter tube, and adding the sealed bottle as a pressure vessel.

Velos, try looking up Fluidyne Engine developed by Dr Colin West. Ian S C