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Has anyone ever tried using the old dehydration of sugar reaction to produce a somewhat porous carbon displacer piston? I am considering this because I am a cheap skate with not a lot of time on my hands. For those who don't do chemistry, I have a video link. Basically, concentrated sulfuric acid + sugar, just like in high school chemistry class.

http://www.youtube.com/watch?v=nqDHwd9rG0s

After making the carbon and rinsing it off, I have found out it holds up well to a torch if the mix was right. When it is forming, it can be shaped. (not too safe with hot acid on the surface, though) When it cools off, it is quite stable. Trap between two washers on a threaded rod, and you have it. Just a thought from a newbie.

An interesting thought,
but would be delicate and brittle, would be hard to control porosity and density.
What kind of engine is this for? What heat source, working fluid are you using.
Concept might work for an LTD engine, but in my opinion it would be a lot of work setting up molds and dealing with safety precautions. Also if there were any residual sulfuric acid or sulfur compounds and moisture got in it would spell doom for metal parts. The reaction between sulfuric acid and iron was once used to make hydrogen for balloons and blimps. the iron was turned to rust and a lot of heat was generated. the reaction with aluminum, if I recall my lab experience, was very similar and even more violent.
As for a high temperature difference engine if air or other oxygen containing mixture were used as a working fluid I worry the carbon (read this as:charcoal) might catch fire if the temperature got high enough.

if you are still set on this method and have the resources and tools and time to do it right, however, I encourage you to try, as long as you are willing to take the risk of ruining an engine. Most of the best devices in the world were not an accident, but the result of lengthy trial and error, I might be wrong.

I will be the first to admit that there was a sugary residue that you could smell. Hitting it with a torch didn't free up any leftover sulfuric acid because I soaked it in water for 24hrs, (despite it trying to float) then baked it in the oven to remove moisture. Smelled like I was cooking sugar cookies. Density could be controlled with a weighted lid inside the reaction vessel. Vary the weight, vary the density. I was thinking of using it in a low tech beta stirling which is still only on paper at this point.
The leftover sugar residue would be something to worry about, though. Nitrogen or helium gas would have to be the working fluid. The cylinder sleeve I am using will be cast iron, so reactions will be minimal, as sulfuric acid only pickles cast iron. My collection of cylinder sleeves from ebay is going to be useful, if I can help it.
Another use for the carbon is filler material for a sheet metal piston in a pressurized engine, for wall support.

Had a look for graphite specs on the web, no prices, a fine grained one JC4, next; Poco Graphite grade2 or 3 or AF5. Then I found Graphtec GR060 from Aerocon Systems. Or go to a foundry and see if they have electrode ends of EDM grade graphite, I think thats where I'll look, I know one or two places around this area that may have some, also a place that makes carbon brushes for motors and generators. Ian S C

Oh, don't worry about me, I have enough graphite to shut down a nuclear reactor. Got mine from ebay under glassblowing supplies. They use large flat slabs of graphite to rest the hot glass on to prevent contamination while working it. Get a hole saw, and you have a piston. www.graphitestore.com is a nice one also, cheaper than poco.
Its just that I am cheap and will always look for more than one way to do the job.

Thats great, the local glass works closed 20 years ago, but there are a number of foundies, and a factory that makes electric motor brushes, but I'll stick to cast iron for pistons for now. Ian S C