Hot air engine displacer material

I want to make a regenerative displacer for a hot air engine, size approx 1-1/2" dia x 1-1/2" high. Heat source will hopefully be a tea light, but probably end up being a mini gas ring to make the engine work.

Materials available are:
1. Mild steel pipe which can be machined to size. Top and bottom caps - easy to silver solder in place.
2. 316 Stainless steel bar easy to silver solder top and bottom caps, but apart from the waste, can I machine a solid lump to leave about 10 thou wall thickness, I ask myself.
3. Brass shimstock which would have to be rolled and silver soldered, hoping I would not burn away the joint area when soldering it all together.
4. Aluminium which would have to have the top and bottom pieces JB Welded. Would the heat source, assuming the gas ring be too much for the JB Weld?
There could be a combination of materials for side and top/bottom pieces if advantageous.

So, the question is: which of the materials available to me would the knowledgeable use?

Dave
The Emerald Isle

It really depends on what you want to do with the engine. Mild steel is cheap and easy, it rusts of course but can be kept clean if only used for demonstration. Stainless is harder to work and does not conduct heat well, would not fancy boring it out. but in some designs s/s may be better but with a bottom made from a better conductor, like cooking pots. I'm making a 1/3 scale Robertson engine and using mild steel tube, but this can easily be dismantled for cleaning. Not much help I'm afraid.

Brass and ali will conduct too much heat. If you bore a bit of stainless that is larger than you need then push it onto a mandrel you can then skim down the outside without risk of it deforming. You can silver solder stainless with HT5 flux.

Have you been following the Robinson threads over on MEM? Graham Corry likes a crab meat tin for the Robinson but that is a bit larger dia.

Thanks for the replies, gents.

The engine in question is the Polly version of the Robinson hot air engine; they supply a brass spinning for the displacer piston. I have bored this out to give about 10 thou wall thickness but as you say JB, brass conducts too much heat, hence the search for a better solution. I may give the stainless a go using your suggestion. If I can't get success that way, I'll revert to the mild steel.

Being a member of HMEM, I don't visit MEM except in leap years. (Can't see many pics of projects there and there's no point in joining just to repeat the few posts I put on the other site.)

Dave
The Emerald Isle

Joining would allow you to see all the pics posted as attachments, you don't have to post even if you join.

There is also an ongoing thread about the Robinson X-Type, don't know if you still have your set of castings for that one but may be of interest. Also a Retlas build there.

Be wary of changing the established design on a Stirling engine. If, as you say, it is a regenerative displacer, it is supposed to hold a certain amount of heat from the departing hot air and then impart it to the incoming cool air on the next cycle. That may be why they used brass?

Arm your self with a 6" / 150 mm rule and head to the super market, tinned food / pet food areas. Keep clear of aluminium. In one of his articls in ME on building a hot air motor James G. Rizzo told of buying a type of soft drink in UK that was in a stainless steel can, and how he always tried to take a number of them back to Malta each time he went home..

Ian S C

Has anyone tried turning a thread in an insulating cylinder, then winding a single layer of wire into the thread, with a pitch such that the strands of wire don't quite touch each other? That would seem to be a simple way to get a regenerative displacer.

Edited By Mark Rand on 09/03/2018 10:26:05

The Robinson motor has a very shot displacer, length and diametre are about the same, and the hollow displace is packed in this case with steel wool. In the long type displacer the ratio of length to diametre is about 3 : 1 and the displacer (in smaller motors) the sides of the cylinder acts as a regenerator.

On the Robinson the displacer has holes in each end, so there is no pressure build up caused by heating so the metal can be even lighter than a sealed displacer, food cans are ideal, just no aluminium.

Ian S C

Edited By Ian S C on 10/03/2018 11:45:28

Following on from JB's suggestion, I turned up a new displacer piston from 316 stainless steel. I was aiming for 10 thou wall thickness, but it turned out to be more like 8 thou when finished and the top even less.

The original displacer weighed 93 gms and the new one was down to 34 gms - quite a saving, and hopefully the heat transfer will be a lot slower, enabling the Robinson to run for a while without external cooling.

In the end, I decided against a regenerative displacer, as this one seems to be a great improvement on the original brass one.

Dave
The Emerald Isle

Edited By steamdave on 01/04/2018 10:28:23

Looks good Dave, let us know how the engine runs with the new bits.

J

If you are interested in finding out the value of regeneration, you could make two displacers, one with and one without built in regenerator, could be quite interesting. That's the sort of thing that happens with my hot air engines.

Here is one of my flame lickers rigged with a Prony Brake to test the torque(had that motor out today, it refused to go!). There is 1 oz on the beam at 3".

Ian S C

Edited By Ian S C on 01/04/2018 12:14:50