cork as a displacer?

[alecb]

i was thinking cork would be a great displacer in a gamma diaphram engine

[Ian S C]

Cork could work OK, at least it would let you know if it got too hot, I don't like the smell of burnt cork. Balsa wood is another option, it could either be a solid block, or hollow, made of thin sheet. Ian S C

[jimlarsen]

I have thought of the thin sheet balsa method, like we used to do with model airplane wings. But I wonder if it would flex under pressure, or perhaps be too porous and become dead space in the engine. I am going to try it sometime.

Cork is worth a try. I agree with Ian that it depends on the temperatures you will be operating at. I think displacer shape also influences the material choices. Thin flat displacers need to be able to support their own weight. Cork is fragile when cut thin.

I am going to experiment with closed cell foam for my next LTD engine. My only difficulty is finding closed cell foam that can tolerate the heat.

[Ian S C]

Jim, you could prevent the flexing by putting a few vertical partions, sort of a honey comb effect, and maybe some sort of sealer like model aeroplane dope, that also strengthens the balsa without adding much weight. It would only be worth it if the displacer was say 1/2" thick or more. Ian S C

[TomOliver]

Paulownia might be superior to balsa for displacer construction. Balsa weighs about 10 lbs per cubic foot, paulownia about 18 per cubic foot (oak about 40, for comparison). However, the shear strength of paulownia 5,740 psi, more than twice balsa, 2,800. A 3/16 cut of paulownia gives the same strength as 3/8 of balsa, meaning about the same weight but occupying less volume.

Thermal conductivity is one of the lowest among woods and its flash temp (burning point) is the highest of any wood. Paulownia begins to burn at 420C (788F). Most woods begin to burn at about 220C/428F. If using Paulownia in a high temperature environment, it is better not to varnish the wood because the ignition temp of the varnish is lower than the ignition temp of the wood. (I am considering a paulownia plate displacer with reflective foil on the bottom as a heat shield/reflector because I anticipate heat in excess of its flash temp.)

The wood is soft, similar to balsa, and easily sawed or shaved and takes nails or screws without cracking. It does not take glue well (moisture resistant, does not absorb the glue well).

Paulownia is replacing balsa in the production of surfboards because of the natural moisture resistance of its cellular structure. Balsa core surfboards must be fiberglassed. Paulownia is simply sanded and waxed.

Do not expect to find paulownia in your local lumberyard. Probably have to order online. The cost is much less than balsa.