Piston Design

[carbide_burner]

Hi there all! I'm having a go at building a Laminar Flow engine as a first project. I made the cylinder of high grade aluminium. Bore 24mm, total length 51mm. I'm making the piston of Vesconite, and aim for a 25mm stroke. What is a good length for the piston? Is there maybe a rule of thumb to determine the length of a piston? Obviously in a stirling type engine the piston would have to be long enough to ensure a gas tight seal, but also not too long to introduce unnecessary drag.

Just for interest sake: has anyone used this combination of materials successfully?

Thanks for any suggestions!!

 

[Twinsquirrel]

Hi CB,

I have a lamina currently on the back burner and can say that if you are not building from plans the best thing you can do is experiment, experiment, experiment. Lamina's seem to be very finicky more so than LTD Stirling engines even. Use a piece of 6061 aluminium for your cylinder and make the bore longer than you think you will require so that you have some room to play with the stroke, don't bother with cooling fins for now. If your design allows make the restrictor a separate piece so that you can move it within the tube and start small with the bore for the restrictor, you will probably find that the bore you end up with is around 6mm. To test hold all the parts in clamps/vices until you have the dimensions that work.


All those bits are very quick to machine up so once you have the measurements for a working design you can remake if necessary.

I don't have any experience with Vesconite in fact I had never heard of it before but after looking it up it seems to be ideal for this application. For my experiments with that bore my piston (graphite) was 18mm long and sealed very well after lapping.

Hope this helps

David

 

[carbide_burner]

Thanks for the advice. I made the piston last night. The Vesconite is lovely to machine, and was dead easy to get to size with a good finish. Only thing is that it seems to expand quite a bit when heated. I machined it taking my time and taking quite a number of small cuts. I stopped when it was a nice tight fit, and accidentally moved the material in the chuck when I pulled the cylinder off after a trial fit. I was planning to hone the cylinder to get the fit just right, as setting the piston up to remove 0.0005 ?!? milimeter was going to be tough!

This morning I tried the piston again in the cylinder, and it almost feels too loose!! I will bore the piston out tonight, part it off, and then decide on it's fate.

The graphite available locally is in my opinion way overpriced. (or is my opinion wrong?).
Does it sound right that graphite is waaayyy more expensive than brass???

If the Vesconite piston doesn't work, then what? Aluminium or brass? (in the Aluminium cylinder)

'later.

 

[Stan]

A brass piston works well in an aluminum cylinder. Do a search on ebay for EDM graphite. You might have to wait a while for a good deal but they come up. I recently got a 2"x4"x13" block (over 10 pounds) for less than $30.00.

 

[carbide_burner]

Thanks Stan!
How sensitive is an Aluminium cylinder/Brass piston to temperature variations? I am busy honing my cylinder bigger and bigger to achieve an acceptable fit when the assy is warm. It still feels like it seals o.k. when the thing is cooled down, but I don't kow how warm it will get if the engine is run for a long time, and maybe I'm wasting my time? ??? I've got plent patience, so I'll still try and make it work before scrapping the vesconite piston. Then I'll make a brass piston.....by wich time my patience will probably be low ;D

Cheers!

 

[Stan]

Any time you use dissimilar materials you will have a difference in coefficient of expansion. This is bad news if the piston grows larger than the cylinder. Aluminum grows more than most other materials that we work with so makes good cylinders. My best running engine is a fire eater with a 1" brass piston in aluminum cylinder. My worst running engine is an I/C with cast iron piston is a steel cylinder. I am sure there are lots of theories to explain that but I just go on to the next project.

One thing to keep in mind is that we are making model engines that are frequently displayed, stopped or running. They are not built to develop max horsepower for long periods of time with major design criteria involving temperature rise and expansion of materials. I/C engines will hopefully run when we build them. Almost no cooling is needed on the engine with no load and short run times. The question comes down to whether you enjoy the machining aspect or you are trying to prove theoretical physics.

 

[carbide_burner]

O.K. so i've given up on the Vesconite piston idea. The smallest amount of heat makes it grow. It is almost unbelievable to feel the fit getting tighter and tighter while you play with it. Gee wizz, sounds funny....

In any case: People talk about graphite pistons all the time, but what about carbon? Like the type they use to manufacture brushes for generator slip rings? I can probably lay my hands on some of that!! My brother is a mechanical engineer at a power station (6 X 600Mw generators), and he said the brushes gets scrapped when there is still a huge chunck left of it. - AND it should'nt be a problem to get some out the gate!

Heading to GOOGLE now to investigate carbon vs. graphite :-\ or I'll rather just wait for a reply :big:

Cheers!

 

[ksouers]

Dare I say it?

Graphite = Carbon.

It's the same thing. ;D

I've never bought any, but I'm sure you could get it in different densities and hardness. Some varieties may be referred to as carbon.

Carbon has many flavors, depending on how the molecules are arranged and how many carbon atoms are in the molecule. You get the whole range from simple charcoal to diamonds and buckyballs.

 

[Stan]

Motor and generator brushes come in many different compositions. A hard dry carbon brush would quickly erode an aluminum cylinder. Slip ring brushes have a fairly high percentage of graphite added which makes them feel quite slippy in your fingers. Like unidentified metal, you take your chances but many times, 'free' makes the decision.

After buying a block of EDM graphite on ebay, I can't imagine using anything else for pistons, where graphite is acceptable.