Gamma-Type Wood-Stove Striling Design & Construct

[JohnPain]

So, after a lot of reading, here is my first post to this forum, so - Hello all!

I`ve only constructed one sort of a tin can engine a time ago, but it was a lot of fun.
Now I want to construct something for practical use. A stirling-fan for my wood stove...
I have some time left before winter comes :-)

But, it should not sit on top of the stove, as there are ceramics on it, so I am planing to put it
on the exhaust tube which goes out on top and then after 90 degrees into the wall.
So it will be a sort of hot-temperature-stirling. Could not measure temperatures so far, as it is
still summer here in germany :-) (e.g. something like 200 degrees celsius on top of tube?)

Design: (to be done - will be edited)

Something like the Vulcan stove - if you perhaps know it.

-gamma type
-displacer chamber at bottom
-power piston and chamber sitting on top of it
-connection through plate on top
-top and bottom plates massive copper like 10cm x 10cm (4mm thick or so - also for a nice look)


Displacer cylinder:

I have planned to use polished stainless steel for this, diameter 38mm outside and 35mm inside
suggestions? Would aluminium be better for this?

Reading the forum i figured out, that i should choose as lenght for the piston three times of the diameter. So this would be something like 96mm-105mm (depends on my piston material and dia)

Displacer piston:

I am still unsure.

Maybe stainless steel tube. But I have no machines to produce one :-(

I wanted to use some sort of massive material to drill 3 round holes in it through full length to stuff stainless steel in it. This should do it for a regenerator. Or would a small gap of 1mm between piston and wall be enough?

Suggestions for massive, light, drillable, temperature tolerant material?

Maybe use some sort of metal foam, but I have not found a supplier so far (maybe still too heavy as a displacer?)


Working cylinder and piston:

AirPot....
I have found a dealer here in germany. I will order if i know the sizes i need. Maybe 16mm or 24mm inner diameter (2k160 or 2k240). For me a cheap and very good solution.....


Compression Ratio / right stroke for the pistons:

Am I right that the right compression ratio depends on the temperature difference of the stirling? I have read something like 1,5 : 1 would be best for high temperature. Or better something like 2:1 ?? Stroke Displacer 1/3 of diameter?


Still planing, but I`ll keep you posted.
Greetings,

JohnPain

[jesterthought]

JohnPain, I’d use thin stainless for the displacer - and for the cylinder. Thick sections and aluminium conduct heat to the cold end where it is difficult to remove. Some cold ends warm up and the engine slows – even petering out. Avoid clamp screws that pass from hot to cold plates.

If the displacer is radially clear of the cylinder, neither needs be particularly round. So why not use wrapped shim? Fill the displacer with wire wool for best regeneration.

Can you make large bearing plates from aluminium to act as cooling fins? Use baffles to direct the rising warm air away from the cold end, and induce fresh-air cooling.

The power cylinder can be of stainless, brass, aluminium or glass, provided it is parallel and round and smooth. Its temperature isn’t high so expansion isn’t a problem. I prefer graphite pistons. (If you can find a re-winder, they usually have old motor brushes.) But the fit is critical. There must be no friction and no leakage – or as near as you can get. I find friction is worse than a slight leakage. The piston must drop through under its own weight – but only just. The longer the piston the greater the leakage resistance. Have a connection on the engine so you can pressurize it under water to assess leaks. It is also useful for balancing the engine without the effect of compression.

Your first engine will have shown you how much you’ll need to experiment!

Jester.

[Ian S C]

John Pain, I,v built two of these, the first a Ringbom type (the displacer is free, ie., not connected to the crankshaft)this one is a bit too noisy. The second one is a Gamma type. The displacer cylinder is made from a Chrome plated mild steel tube from a vacuum cleaner extension, I welded a bit of 6 mm steel plate in the hot end,(just used my stick welder with 1.5 mm rod). The hot end sitting on the stove is supported by a 120 mm x 8 mm mild steel discwith a hole bored in the middle, and the cylinder jambed into this. The cold end of the cylinder is made of aluminium alloy, the bearings are supported on top of this by steel supports, these reduce the conduction of heat to the bearings. The fan is set to blow the air over the motor, and no cooling fins atre required, you can hold onto the aluminium cylinder head with your hand. The power cylinder is made of steel ( the tube from inside an automotive shock absorber). The piston is made from cast iron, its 35 mm in diameter x 20 mm long, the inside is bored out to 33 mm x 17 mm, a 6 mm thread is in the center of the crown of the piston, and the fitting for the connecting rod is fitted to this, I use needle rollers out of old bearings for this, they are about 2.5 mm x 10 mm.
I used 8 mm skate board bearings for the power crank, and main bearings, the displacerwas a bit of a problem, the crank is in the middle of the shaft, so I used a split, plain bronze bearing.
Almost forgot, the displacer is made of stainless steel, very thin. I got a profesional welder to TIG an end onto this, and the cold end is aluminium glued in, and a bit of stainless tube 5 mm dia forms the rod at that end, it passes through a brass bearing about 20 mm long. The fan is made from wood.
Just did a wee trial today, using my 2.5 cc motor (see photo in my gallery),I used a small (5 mm dia) wick burner, fitted a 6" dia 4 blade metal fan, and that set up gave an adiquit breese, so a motor that size would be quite OK on a stove top. Standing on end, the whole thing would be less than 12" high, although it might need a bigger fan to keep the speed down! Ian S C
Ian S C

[greekjohnson01]

Wood stove efficiency varies wildly from stove to stove and yet is poorly understood in terms of what it means for heating your home with wood. Open fireplaces typically operate at about 10% efficiency, basic wood stoves between around 30% and 50% efficiency and modern high performance stoves can reach as high as 90% efficiency.

http://www.wood-orangery.co.uk/

[Ian S C]

There is a photo in my gallery of my stove top fan, thats a 6" rule next to it. Ian S C