Sanity check: airtight displacer a bad idea?

[PKM]

I've made a displacer out of a roughly 70mmx70mm section cut from an aluminium drink can with flat steel sheet glued over either end. I made the cuts neat and flat, and used plenty of JB Weld to make the join, so I suspect the displacer is now pretty much completely airtight.

I know the displacer is not meant to be "open" to the cylinder airspace as it presents a very large deadspace if it is, but surely if it's completely sealed then the air inside will pressurise as the engine heats up and it will explode (or at least deform and seize the engine)?

I would intuitively guess the solution is a tiny pinhole that lets the air pressure inside slowly equalise as it heats and expands, but doesn't allow a large amount to flow in and out so doesn't act as a large deadspace. I believe most engines run like this anyway, the slight leakage around the displacer rod seal lets the inside of the engine equalise with atmospheric pressure (as one of the common troubleshooting tips I see is "does your engine stop or cog at the power piston BDC? If so it is probably pressurising and needs to leak a bit more".)

[jimlarsen]

I am in agreement with you. But this debate has occurred here before. You will find opinions on both sides of this issue. I have had an airtight displacer burst inside a pop-can engine. It made a loud bang when it went off. But the engine still runs even with the now fractured displacer.

If you want the maximum amount of efficiency from an engine you will make it air tight and strong enough to hold the pressure. But if you do decide to put a pinhole vent in the displacer, the engine will still run if everything else is in order.

This is a video of the engine I spoke of. I don't recall if this was taken before or after the displacer popped. But it still works today. http://www.youtube.com/16strings#p/u/0/1c95l78hspU

Jim Larsen
http://StirlingBuilder.com

[Murph]

Venting isn't an issue, if you feel you should vent it, go ahead.

There have been more than one Stirling engine made with a wire mesh spool or stacks of wire mesh washers being used as a displacer piston, that should answer your question in itself.ph

Murph

[Junkie]

Steel wire wool displacers work really well. I use them in all of my engines.