performance on different materials for displacer

[ogrekill]

Hi,
Currently I study on performance on different materials for displacers. Anybody here got test using different types of displacer such as steel wool, polystyrene or others and how is the performance in term of power?
Thanks

[Ian S C]

ogrekill,asking to assess the performance of steel wool and polystyrene is like asking what's the length of a bit o string, it all depends on what type of engine you propose. Most of my engines use a cylindrical stainless displacer. I tryed one engine with a stainless steel wool displacer, but the performance dropped about 50%, and I have used aluminium and suffered melt downs, and in that case an improvement in performance when I changed to a stainless displacer. The only place for plastic is in LTD motors, and Balsa is another thing that can be used here. Ian S C

[ogrekill]

Sorry about that, my question maybe giving wrong direction. Mine main concern is the difference of the displacer such as design criteria, advantages and disadvantages for certain displacer. For example why using polystyrene as the displacer material

[Chriske]

ogrekill,

For example a polystyrene displacer is not to be used in an Alpha type Stirling engine because of the heat. Alpha engines needs lots of heat to run, in lost of these engines you'll need a gas-torch to heat the cylinder. Very often the displacer cylinder is heated until it's red-hot, no environment for a polystyrene displacer I'm afraid. These Alpha engines will perform very well and well built they deliver lots of power.

A second example, in a LTD engine you want the engine to run very light using only a very small flame or even just the warmth of your hand. In that case you'll need the displacer to be as light as possible. Polystyrene or balsa will do the trick in this case. These engines are only built to be used as display or as a toy.

Every type of engine has it's specs.

Chris

[ogrekill]

Mine engine is LTD Stirling engine. Besides lightweight, what other criteria is concern when designing the displacer? How about porosity and stiffness of the material?

[Chriske]

Making a LTD displacer you have to watch the relation between cylinder and its displacer but you'll have a rather loose tolerance compared with a Alpha or Gamma engine. A few tenths of a millimeter error will not harm a LTD engine. But in case of a Alpha make its displacer a few tenths of a millimeter to large or to small and it will not run at all. Alpha engines have very tight tolerances..!

About porosity, best is to make all displacer out of solid material if you want them to perform well.
In case of a LTD you could use steel wool as Ian proposed. Some have experimented with success using other materials. I once saw a displacer partly made of cooking hood filter. And look how it outperforms all other LTD's.... brilliant..!
http://www.youtube.com/watch?v=CdX7hwjhgA4
Never tried it myself.

So you see, lots to experiment I'd say...

Chris

[Coco Mbella]

hi orgkill,

i use wet floral foam, aluminium foil case and steel wool for my LTD Stiling Engine displacers. wet floral foam is lightweight, easy to machine and is able to resist higher temperatures than styrofoam (approximately 140 °C). currently i am building a new engine and i will try to replace the Steel wool with either copper wool or aluminium stripes.

http://www.youtube.com/watch?v=zjCKQHdObWQ
http://www.youtube.com/watch?v=Oe430aaEb_U

greetings from germany

Coco

[Chriske]

Coco,

Using wet floral foam, you're in the wrong department I think...
This is not the steam-engine forum but Stirling-engine forum if I'm not mistaken...

Just kidding, couldn't resist...

Chris

[Hopper]

Chris, that cooking hood filter engine really motors!
Anyone know a source of small diameter graphite like he uses for the con rod bearings etc? I see plenty on eBay but all half inch diameter and upwards. Stuff is so damn messy to machine I would rather start with smaller stock if possible.
Will the graphite/carbon rod that runs down the middle of a dry cell battery work, or is that a different material?

[Ian S C]

You can try it Hopper, but I think its too hard, don't know about its lubricating properties. I'v got a couple of carbon rods from the big telephone cells, I seem to think they are about 1/2" dia, and 4 or 5" long, might dig one out and see what happens with it, might be a day or two, but I think they may be softer, only thing is, I don't think that size battery is made now (its almost as big as the first cell phones/bricks). Ian S C

[Hopper]

Yeah, I remember those big old round batteries with two threaded terminals on top. The staple of backyard science projects for boys of several generations. Just googling around, it appears that most of today's large square six volt batteries actually contain 32 AA cell batteries. So its a cheap way to buy a lot of AA batteries but no source of larger carbon rod any more.

[Ian S C]

Hopper thats interesting about the 6V bateries, they used to have 4 cells a little longer than a D cell, and I think thats what they still have here, must have a look, bought some little button cells today, $NZ 3 for 30, I only wanted one. I think they used the big ones for the buzz box in the model T Ford, as well as telephones, I imagine that until cell phones arived in Austrailia, there would be places where the old crank phones would have been used, we had one in the 1960s in a country area not far from Auckland NZ. I think thats enough OT. Ian S C

[Ian S C]

For what its worth, the carbon rod from batteries is machinable, the only ones I had to try with were the telephone battery rods, and some AA battery rods, the AA rods are a much finer grade of graphite and might be useful as bearings--4 mm dia., is a bit small for a piston, the phone rods are 5/8" dia x 5 1/4" long and are a bit course in the grain, but may be OK, only thing is Union Carbide don't make 'em any more Ian S C