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I'd like to power a small fan (~15cm diameter) on top of my woodstove to move the hot air about so I need an engine that isn't going to melt.

For the power cylinder, I'm using a cast iron brake cylinder from a Ford Mondeo - the inner diameter is about 20mm. It's about 50mm long. I'm intending to use one of the original pistons (they're aluminium with a hardened steel insert - seriously hard, even after annealing my carbide lathe tools struggled!)

For the displacer cylinder, I've got a length of stainless exhaust pipe, about 60mm inner diameter by 80mm long - I think this is probably too long - please advise how long the stroke needs to be

To make the ends of the cylinder I've turned some circular grooves into some squares of 3mm aluminium plate - these will have some high temperature silicone and will be bolted together at the corners (if that makes sense)

Not sure how big and tight to make the displacer piston, however. Was thinking about using aluminium but with the thermal expansion it would need quite loose tolerances

I've got a lathe and a reasonable selection of other tools but no welding or milling machine available (sadly!)

Hi Spandit, I'v got two of them, the first one I made is a Ringbom type, but the displacer bouncing up and down without the control of a crankshaft made too much noise, ti also was a bit too powerful, the torque of the fan causes the whole motor to turn slowly on the stove top.
Next I made a Gamma motor. The displacer cylinder is about 35 mm dia X 150 mm long, its chrome plated mild steel, with a bit of 3 mm steel welded in the bottom. The displacer is made of thin walled stainless steel with the hot end TIG welded, and an aluminium plug glued in the cold end. You could braze the hot end in.The displacer has a stroke of 20 mm.
The power piston is made of cast iron, as light as possible, the power cylinder is steel 35 mm bore, the inner cylinder from a automotive shock absorber, the stroke is 16 mm.
The base of the motor is a 140 mm dia disc of 8 mm hot rolled steel plate with a hole in it for the displacer hot end to fit in. the cold end is a block of aluminium, the bearings are mounted on top of pillars above the cylinders, and it drives a 30 mm dia three blade fan. A bit more about the displacer; About 1 mm - 2 mm clearance at the sides,and 2 mm or so at top and bottom. If no stainless don't worry, mild steel is good, get soom tube and mount it on a mandrel and turn it down untill its about .25 mm or less, if you use aluminium you might have a melt down, also there is too much heat conduction to the cold end, with my displacer the only cooling required is the fan, its set up to blow the air back over the motor. Ian S C

Thanks, Ian. Sounds like your displacer cylinder is long and thin rather than short and fat like a lot of LTD engines I've seen.

Think I've got some thin steel tube in the garage that might do the trick. Not sure how I'll make ends for it but you've given me some starting points. Any pictures of your engine I could look at?

Robin

Sorry no pics, I don't think of stove tops as low temp, low temp for me is boiling water, and the stove top would be 3 or 4 times that at a guess, and a bit more when really stoked up. There is a motor on the net some where that uses the glass from a gas lanten as the displacer cylinder, can't remember where it is, but I seem to remember it gave quite a bit of info about builing it. You don't have to use the glass, metal would do, with the glass you can see the displacer going up and down. With the glass cylinder, the top and bottom are held in place with 4 long bolts, and that method could be used with a metal cylinder. I think the bolts are insulated from the metal at the top / cold end, and if using a metal cylinder an insulating gasket would help to slow heat transfer, and that may be quite a good way to go. Ian S C

Ian S C wrote:Sorry no pics, I don't think of stove tops as low temp

I didn't intend to imply that but reading my post I can see where the ambiguity lay. I notice that LTD engines seems to be very squat whereas your HTD engine has a longer stroke.

Still looking for a suitable displacer - none of the cans I have are the right diameter unless 3mm clearance is not too much! Oddly enough, I've got a large piece of steel bar that's the right diameter but there is no way in hell I'm going to start trying to hollow that out (and what a waste it would be too!)

LTD motors tend to have a ratio of up to 20:1displacer to power cylinder, where as HTD motors are about 1.5:1. You'll proberbly be OK with the 3 mm clearance, I'v had ones like that.
I'v carved a displacer cylinder out of a bit of 2 1/2" dia X 3" long 316 grade stainless steel, that was 20 years ago, and its still going strong, it better at the price of it.
The long displacer, length three times the diameter, or even a bit more works as a regenerator to a small extent.
One way to keep friction down is to use a fairly short stroke. what I do is set the stroke that I'm going to use for either the power, or displacer, then work out the volume of the cylinder, then work out what stroke will give the correct ratio on the other cylinder. Its dead easy if both cylinders are the same size, just make the displacer stroke half as long again over the power cylinder. With your brake cylinders i would not go to a stroke longer than 25 mm, and some where about 12 mm to 16 mm would be what I would use. With an open ended cylinder the piston should slide through quite freely under its own weight, but if you then close one end it should take quite a number of seconds. This is with no oil on the piston, try and get some cast iron to make the piston, failling that steel is the next best, aluminium has a high coefficient of friction, and expansion.
As for boring out big steel bars, I'v got a reasonably large lathe, but with a smaller machine it gets most tedious. Ian S C