# Liquid piston (fluidyne) Stirling engine



## mklotz (Sep 18, 2007)

I want to build a fluidyne Stirling pump so I decided that it might be a good idea to see if I could get a liquid piston Stirling to operate.

The prototype is butt-ugly







to the point that it's difficult to see how it operates.  The glue used to secure the plastic blocks that form the engine, in combination with the heat from the resistor, caused the plastic to craze.

The back view shows the leads from the resistor but still isn't terribly revealing.






This diagram






will explain the operation.

Plastic blocks (crosshatched) cemented between two plastic sheets form a compartment which is partially filled with water.  A volume of air is trapped by the blocks and the water.  On the 'hot side' of the engine, a 22 ohm, 5 watt power resistor (labeled R in the diagram) is immersed in the water.  Its leads project through the back of the engine (via holes sealed with silicone cement).

When the resistor is heated by an external 13 VAC transformer, the entrapped air is heated and expands, pushing the water in the main cavity down and causing the water in the channel labeled A to rise.

The air cools when it contacts the water in the far right channel.  The water level falls to the level shown and the whole process repeats as long as power is supplied to the resistor.

The net effect is that the water column at A oscillates up and down.  Not at all spectacular visually but the clever mind will note that this oscillation can be harnessed to pump water.

Imagine channel A connected to a bog-simple pump - a length of tube with two ball check valves and the connecting T between these two valves.  As the water pressure in the A channel oscillates, so will the water pressure between the two valves oscillate with the result that water is pumped.

I took the design for this prototype from an article in Scientific American's "Amateur Scientist" column (which means the plans are copywrite protected).

It's fascinating to watch this thing, ugly as it is.  With no moving parts (save a quantity of water), the water in the output tube magically rises and falls.

Next step is to build a larger version using pipe and add the pump.


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## compound driver (Sep 18, 2007)

HI
Now im impressed! I read a bit about this some time ago but didnt think it was in any way practical but if you can get it to pump on all be it a limited basis id love to see it working.

This group gets more interesting by the day!

Cheers and thanks for a real interesting one before bed.


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## mklotz (Sep 18, 2007)

Oh, it will work.  The link below will take you to a video of one in action.

[ame]http://www.youtube.com/watch?v=J6SjIGWb4zM[/ame]

Although the device shown looks wildly different from my prototype, it is functionally and thermodynamically identical - except mine is smaller and lacks the pump.


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## Bogstandard (Sep 19, 2007)

Marv,
That is very impressive.
Do you think that there is enough power in the water displacement to drive a diaphragm? If so could it turn a flywheel or move a small beam?

John


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## mklotz (Sep 19, 2007)

John,

I can't answer your question directly because I've never measured or calculated the power output.

Regardless, the engine I built is aimed at a pumping application so it probably isn't the most suitable design to adapt to a flywheel.

There are fluidyne designs aimed at directly producing mechanical motion, such as the seesaw engine...

http://www.bekkoame.ne.jp/~khirata/seesaw/indexe.htm

but a much more direct approach is the Rota-Cola engine,

http://www.rotarystirlingengines.com/rotacola.htm

which uses the changing air pressure to move liquid from one can to another thus producing a mechanical imbalance that leads to rotation.  (Probably has a speed best described as glacial.)

I think that trying to get conventional rotary motion out of such a low-specific power design is, in a practical sense, a waste of time.  (Though I empathize with your desire to do it simply to see if it can be done.)  

The fluidyne pump is probably miserably inefficient too, if one works the numbers.  However, it's advantage lies in third world or remote area applications where one has low grade, cheap heat available (e.g., solar or campfire) and can afford to trade off the time needed to pump a given volume against the simplicity of materials and construction.

Speaking of campfires and Stirlings...  I've heard that small (conventional) Stirling engine powered generators were air-dropped to the French Resistance during WWII so they could power their clandestine radio transmitters from the campfires in their woodland hideouts.  I've never been able to find a reference to corroborate that but it makes a good story and highlights the utility of an external combustion engine that runs off any convenient source of heat.


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## Bogstandard (Sep 19, 2007)

Many thanks for the info Marv.
It was out of pure interest if it had ever been converted to a mechanical motion that I asked the question.
With reference to your portable generator, I think it was Stuart Turner who produced a small clandestine generator set that used twigs etc for powering the boiler, but I think it was a standard piston engine that was used. Ricardo also produced a small wood burning generator that was designed for charging batteries in the field, but was rather heavy at about 100lbs. Here is one for sale, go towards the bottom of the page
http://www.prestonservices.co.uk/generators.htm

John


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