Need help with our gamma school project engine

[hayman]

Me and my partner are building a gamma type engine for a school project. The size of the engine is .500" dia power piston, .750" dia displacer with 2.25" length. The flywheel is made of bronze in 3" diameter. The inside of the displacer sleeve is .830" leaving approximately 1 mm on the sides. There is very little friction in the engine and it turns over on itself pretty well. However it will not run.... I do notice with the displacer sleeve made of out aluminum the heat travels to the fins pretty fast. We have some material that insulates heat and we are thinking of making the sleeves two parts with that in between the hot and cold side. Any more information just ask we need help. Thanks

I will try to get some pictures up next week.

[hayman]

oh and i forgot the shaft on the displacer has a oil lite bronze bushing and the power piston is also in a oil lite bronze bushing. This creates a good seal with little friction

[lvspiano]

Hello, Hayman, I'm new, but I have a little input. Are you using a coolant on top as well as heat on the bottom? Also, I don't think you are supposed to use oil because it can get in the machine and vaporize which is no good, but I don't think a tiny but will do anything.

[Longboy]

I'd like to see the photos. Your hot cylinder/sleeve is the wrong material. Use Pyrex or steel/ stainless steel here. With aluminum and brass you are DOA here! You want to heat up the air within....not cook the engine. The displacer clearance is fine. Give the displacer pushrod a couple drops WD40 at the bronze bushing. Air leaks and friction kills Stirling. I test for air leaks with a blow off nozzle off the air compressor at very low pressure setting thru the power cylinder. You should be able to also with a 1/2inch power cyl. bore. Your friction usually comes from the displacer rubbing the inside of hot cylinder and the pushrod thru the gland bushing.

[Ian S C]

hayman, If you can, make the displacer cylinder from stainless steel, .5 mm thick at the hot end is a good thickness, apart from alowing the heat to pass through easily, it restricts the flow of heat along its length. You did'nt say what you made the displacer from, it to should be stainless, and I'v used .0075" thick tubing with a bit of the same material TIG welded on the end to close it (the cylinder made the same way). Only thing is finding the tubes, then finding someone to do the welding (that thin is quite a challenge even for an expert). The other way is to carve it out of solid bar, I'v done it both ways. At the worst, I use mild steel, but if you get it too thin, it bulges, and on horizontal motors it sometimes droops, and fouls the displacer. Just as I said on another thread, don't use aluminium on high temp engines.
As far as the gland bearing goes, I think you would be better off with plain brass, or bronze, or cast iron. Oillight bushes are pourous, leaking air, and they need oil, which causes drag, which is a form of friction.
Do you have plain, or ball bearings on the crankshaft, and connecting rods?
What are you using for the power piston, and cylinder?

[hayman]

Both the displacer piston and sleeve are made from aluminum. We do have a piston machined from delrin that we was going to try to prevent thermal shorts but we thought that maybe the engine would get too hot. So it looks like the hot tip on the sleeve we need to machine out of steel...

[hayman]

We have a ball bearing on the crankshaft and all the connecting rods are just dowels.

[hayman]

If we made the hot side of the displacer sleeve out of stainless would we need to isolate the heat from traveling to the cooling fin alum side of the displacer sleeve?

[Ian S C]

hayman, you have a nice looking little motor there. A steel, or preferably cast iron cylinder, with a cast iron piston is about the best all metal solution, ie low coefficent of friction, Aluminium is about as bad as it gets unless it is hard anodised, there is also a teflon coating (just a few microns thick). The best available combination is a glass cylinder, with a graphite piston.
To mahe a hot cap for the motor, I would cut the existing hot end off flush with the end of the cooling fins. Make a new cap to replace it, with a flange on the open end. Drill 4 to 6 holes around this, and tap into the last (thick) fin, 3mm screws would be about right, or some thing similar, you may need a high temperature gasket , or jointing compound. This will take a good size meths burner, or a small gas flame, or even a large freznel lens. If you could find one, parabolic reflector, with the hot end through a hole in reflector would go very well, it could be mounted on a camera trypod, or similar.
Keep the crank pins well oiled. As far as the flywheel goes, it could be lighter, the only place that weight is required is at the rim, so if you drill say 6 large holes (think of wheels with spokes), this area between the hub, and the rim can also be thined down a bit too, 3/16" to 1/4" would be OK, leave the hub large enough to take the crank pins. Make the inside diameter of the rim about 2" diameter. These are just my ideas, but you should get a good working motor. Ian S C

[Longboy]

If we made the hot side of the displacer sleeve out of stainless would we need to isolate the heat from traveling to the cooling fin alum side of the displacer sleeve?

NO! Your cooling fins is your radiator. You want that in contact with the sleeve, not isolated from it. Lets clear up some terms here. There is no hot side or aluminum side of the sleeve of your engine. Its all a hot cylinder either side of frame. Think in terms of metal to metal contact where heat would migrate to the cold cylinder. Where you would isolate the sleeve/ radiator is at the attachment point to the frame / block. In your photo, this is totally bad...... a one piece aluminum sleeve & radiator that looks flush mounted to its frame. An O-ring (1/8 to 5/32in thickness) between the frame and sleeve/radiator works well here. A goove machined into either the frame or radiator on the inside to your screw mounting holes will locate the O-ring. This O-ring serves two purposes here. Helps slow heat migration to the frame and gives a positive seal at the sleeve preventing air leaks at the point of attachment. Now as for constructon technics. A stainless sleeve with a slip fitted aluminum radiator over sleeve bonded together at the back side where attached to block. Screw holes drilled and tapped into radiator to secure to frame and the O-ring on the inside diameter of mounting hole pattern.

[hayman]

longboy I think you misunderstood me. I want to isolate the stainless heating tip from transferring heat to our aluminum sleeve with the cooling fins. We have some paper gasket material you can make engine gaskets with, that should help prevent heat transfer.

We have some stainless in the shop so we are going to try a machine a new cap with about a .015" wall thickness.

[Ian S C]

hayman, I don't think you have too much to worry about with heat conduction,hot to cold, keep the stainles cap as thin as possable, this with restrick heat flow along the cylinder, the joint it's self is a heat barrier, more so if there is some sort of gasket. None of my motors have any heat insulation between the hot and cold end. Ian S C
You can see some of my motors in the gallery.

[hayman]

Well just finished machining a stainless cap with a .015" wall thickness. The engine still does not run....

Also I cut the weight of my 3" flywheel from 500grams to 285grams by removing some of the thickness in the center.

It may be in the bearing that is on the crankshaft/flywheel. Free spinning it by hand it only gets 1.5 to 2 rotating before it stops.

[hayman]

UPDATE!

We removed the bearing, took the plastic dust caps off and cleaned out the thick grease. We then applied a light oil... it runs. It is not fast however... We have to hit it with a lot of heat due to the paper gasket burning away and creating a leak.

http://youtu.be/qoNU2bDiOoc

[Ian S C]

hayman, congrats, you need a high temp jointing compound in there, but you found that. On your new hot cap I would thin the flangeto about 1/8" if you can, that will be less metal to retain heat.
As I build my motors, I have a small set of scales (for weighing food for some diet system), they go up to 500gms, and as I make a componentI weigh it, machine it weigh it, then see how much more I can take off, sort of a competion with myself, there are times when I go too far, but with the small amount of power, strength is not a great problem, ie., a cast iron piston will have a skirt .5 mm thick, and the crowm 3 mm thick, so the larger it gets, it comparatively gets lighter. Ian S C