Mild steel body stirling engine ?

[icebird]

Hi there,

I tried building a stirling engine similar to the Tin Can Stirling Engine, but using some thick mild steel mig-welded together. I figured it'd be easier.

Here is the kinda finished engine (I broke the crankshaft holder shortly after testing it).


Unfortunately, it doesn't work :(
The piston is causing a lot of friction, partly because the pipe isn't very smooth/straight, and partly because the connecting rod gets stuck. Also, the "pushing angle" of the piston/connecting rod is far from straight (see the schematic), which doesn't help.



Now ... I'm not even sure steel is a suitable material, due to its low thermal conductivity. The bottom of the displacer cylinder is 0.8 mm thick (0.3 inch, 20 gauge), and the sides are 5 mm thick (1/4 inch).

When I designed this, I figured the cold part could be left as is and heat would leave by itself. Having seen the heat required to get through the bottom of the cylinder, I'm not sure of this anymore. Any hint on this?

Finally here are some of the various parts. Note that the top of the displacer cylinder is sealed with silicon. You can also see the piston before I butchered it, making things even worse.



So here I am, a bit desesperate on the future of this engine. If any of you could provide me with indication that steel is suitable for the task, I could try replacing the power piston assembly with an old brake cylinder from a car.
The cooling problem, in case it is indeed one, remains to be adressed though.

Any comment is appreciated!

[Tom Booth]

A few impressions...

It looks like your power cylinder and piston may be disproportionately large as compared with the displacer.

I've seen similar engines with about the same size displacer cylinder but smaller piston and power cylinder.

If the little tea candle is your heat source, I would suspect that this would not be adequate for such a big engine and such heavy metal. The steel is probably absorbing nearly all the heat and the air inside is probably not heating up much at all. All that heavy metal is holding the heat and little if any is being conducted through to the air inside. Not enough anyway.

I would suspect that even a propane torch would have trouble providing enough heat to get through the relatively heavy steel.

It might help to insulate the sides of the bottom half of your displacer cylinder - something like a cement jacket - so that whatever heat does make it through is not simply dissipated to the outside air as fast as to the inside.

The aluminum displacer will also likely absorb some heat from the air once it does get in, though how much ever gets that far ?

But I would guess that the main problem is the steel absorbing and conducting away most of your heat before it ever reaches the inside of your displacer chamber so as to heat up the air inside.

Ideally, the sides of your displacer chamber should be of some non-heat conducting material to prevent the heat from migrating from the hot bottom end to the top end which you want to keep cool, but perhaps if you could weld a copper bottom on the displacer cylinder instead of the steel bottom, this might conduct heat in and through to the air faster - along with insulating the sides - since less heat would reach the steel and the steel would conduct whatever heat does reach it from the copper perhaps more slowly than the copper conducts the heat through to the air... you might at least get the thing running... still, you would probably need something greater for a heat source.

Then, of course, there are the other problems you have already mentioned.

If that candle has been your only heat source, I think that the first thing I would try would just be to get a bigger hotter flame from something like a propane torch. This won't eliminate the other problems but it may get some heat in fast enough to at least get it running, as is.

[icebird]

Thanks a lot for your answer.

I was afraid of being out of luck, but I might just give it another try then. If propane ain't enough, oxyacetylene might do the trick!

I'll let you know either way.

[Junkie]

Is the can on the right your displacer ? If so it doesn't look tall enough it should be about 2/3s of the height of the cylinder it runs in.

[icebird]

Yes it is. 2/3? Oh well, it's about 1/3 ... Thanks for the tip!