Need help with our gamma school project engine

[Ferraccio]

Hayman, Ian, my speak.
My hunch is excessive strength.
With small power, as it is, the strength "necessary" in terms of thickness measurement is of the order of 1 / 100 of an inch, for obvious reasons of workability thicknesses are best to consider the order of 1 / 16, or at least the minimum possible.
Why do I say this? When using a metal as a means of transmission of the heat transfer coefficient must see.
The heat transported through a wall comes from the wall material (material that is used) and inversely to the thickness of the wall itself, which means that it is four times as thick to use a material that conducts four times less, and therefore almost insulated, and This is important both for the hot-headed, and for the finned cooler cold.
A system of thermal insulation between the hot and cold parts, (such as a ring of glass) I think it can be highly useful, even if seems "the same ".... but it would run slower.
With powers of a few watts (from the structural point of view) the connecting rods and crankshaft should asserted profiles to "I" or "L" with a thickness of the order of 1 / 64 of an inch, for reasons of workability, without arriving at these values, however, reduce the thickness to few /16 of inch to said reciprocating masses (eccentric) to balance, which reduced the power play, become important.
Another factor, very important: the game between the piston and cylinder power: it must be of the order of a thousandth of a thousandth and half of an inch, with the cylinder placed with the vertical axis, the piston (clean of any traces of lubricant) must scroll down slowly and do not fall, or just - just held. Apart from a manual cleaning of delicate surfaces with abrasive flowing delicate (such as the polishing of silverware, then carefully removed with turpentine), do not practice "hand adjustments" which may do oval diameters.

[Ferraccio]

I apologize for the bad translation:
I mean: if the wall thickness is four times higher is as how to use a material that conducts four times less, halving the thickness is like double the conductivity of the metal.
Since the Stirling is an external combustion engine, or (with an external source of heat: the sun), the main problem is ever to bring "in" the heat, ...and extract "out" some other in the cooler.
Since the engine uses energy transfer with gas, avoid the heat by-pass, in the metal.
Given that the powers involved are very low, to reduce frictions, and do not lubricate the piston power, (lubricant are viscous).
Compliments however for your work!!
Ferraccio (bad Iron)

[Ferraccio]

cooler fins should be 1/16 "thick and 1/4" high, spaced with clearances of 1/8".

The reduction of the distance beetween displacer axis and power piston axis, reduce dead space (hole conducting gas is shorter).

Taking into account the picture of 1st of August:
I think this architecture:
By the support frame in shape of portal (twoo legs), and greatly reducing the dimension of the bush of displacer axis (1/4 "OD, of 3/4" long, but with internally two annular bearing-surfaces interposed by a groove -1/3-1/3-1/3-, what huge bush of brass?), you can pull approached, spaced 1/8 ", (or less) the cylinder piston from said bush .

If the two bodies (refrigerator, and motor cylinder) are embedded in the plate for a bit (only a bit) less than half its thickness, the two diameters (internal diameter of cooler and and diameter of power piston) are so rooms overlooked, (in part), and is just enough to break through a small overlooked daphragm between them, and the communication hole of the gas, so long, is no longer needed. You so have a large passage clearance, with no significant increase of dead spaces. Reduction of dead spaces, and huge reduction of losses to pass through the hole.

This solution may be (may be) is more hy tech, and I think admit to test the engine under heat by concentraded sun radiation.

[Ferraccio]

Sorry again:
I had not seen the engine data, but I guess I do not know the stroke, I think piston s. is the same as that for the displacer.
Improving the characteristics, the piston engine can be imagined engine 5/8" diameter.
The thickness of the cylinder wall must be of the order of 2 mm
Stirling engines operate well with very short strokes.

[Longboy]

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.

....no I heard you clearly and once again you do not want to do this. I see in the photo you put on a ring cap with a gasket between the cap and lead cooling fin. This is totally unecessary. You already burned up your gaskets so you experianced a percieved solution to a non existing problem as a novice builder. You want to promote heat transfer here not isolate it from the radiator. That is the point of cooling fins, to dissapate excessive heat from the torch. The isolation of this hot cylinder/ radiator assembly is at the point where it mounts to the frame........You have a runner here but a lot of heat input for a slow running engine. When you get it to run at that speed under a alcohol burner you solved some of its issues. In the video you are "cooking your engine", dirrecting the flame at the hot tube... and also at the radiator! Now you know why the gasket burned up. Take the ring cap off and direct the torch on the tip and away from the engine! With the same heat input, place some ice cubes on the radiator and see if the speed changes. If it doesn't, you most likely have enough isolation from the heat source back to the cold cylinder. One of those infared thermometers can show you the migration of heat thru the engine...... Looking good guys, you got your feet in the fire in the world of Stirling!

[Ian S C]

hayman, one unfortunate thing about this particular design is that there is no provision with this type of crankshaft to drive anything, if it had an extension, a little fan would make quite a difference.
What are you using for a power piston? If its still aluminium, now is the time to change to cast iron if possible, and steel for the cylinder. Ian S C
Maybe try with the sintered bronze cylinder with the CI piston .

[hayman]

Right now we have the stainless cap right on the alum fin radiator. We made a alcohol burner that gets the tip up to 580 degrees F but drops down to about 380-400 when the engine is cycling. With the alcohol it does not keep it running and the only success we have had was with the tip burning red under the mapp gas. As for the ice on the radiator we did try some compressed air across the fins when running and did not notice a increase in speed.

As for the power piston it is still alum and the cylinder sleeve is oilite bronze. I do not think we have any cast iron do you think stainless would be better for the piston?

Another thing we want to try is machining the wall thickness of the cap down more its around .015" I think we could get to .007" or more. And without the gasket we can take more material off the flange that way the cap holds less heat.

[hayman]

Also on the clearance between the displacer piston and sleeve we seem to get two inputs. Some people say 1 mm (.040") and others are saying you want it as tight as .015". We made ours to be a .040" all around, do you think we would notice a increase with a larger diameter displacer piston?

[Ian S C]

hayman, the .040" clearance is as close as I would go, it must not scrape along the cylinder, and .060" would not be too big. If you think you can go thinner, go for it.
Steel, mild or stainless. I think mild steel might be best, lap it in to the cylinder, and get a good polished finish on it. Put two or three shallow narrow groves on the skirt ( say .050" deep x .010" wide), these are labyrinth grooves, to retain oil, and act as a seal. Make the piston so that it will just slide through the dry cylinder (NO oil), and if the end is closed the piston should stop, then move very slowly.
Sometimes its a good idea to run the motor with an electric motor, with the cap of the displacer removed, a small motor with a rubber tier on the spindle, and run against the flywheel, 1/2 an hour and you may find the revs go up. As the motor gets time running it will get faster, you'v been lucky, there have been times when I'v ended up with a sore wrist from flicking the flywheel, then eventually it goes, and the more of them I build, the less work I have getting them going. Ian S C

[hayman]

here's something interesting. the engine runs better in a vertical position with the hot side of the displacer facing down. We are now getting it to run on a small alcohol flame.

[Ian S C]

Hayman, my answer to that is, at the corners of the rectangular plate drill and tap 4 holes for 4 long bolts say about 3/16" or 5 mm, make these "legs" long enough so that when it is stood up there is room for the burner under the hot end. Don't just sit the legs on a table and get it going, attach them to a board or some thing as a base, you won't need the vise after that. I like a vertical motor up the other way, thats OK if you use a gas burner, I have not worked out how to do it that way with meths. Ian S C

[Longboy]

here's something interesting. the engine runs better in a vertical position with the hot side of the displacer facing down. We are now getting it to run on a small alcohol flame.

Now you have gravity working for you. The pushrod gland is no longer supporting the weight of the displacer assembly. Most model stirlings you find will be horizontal construction.....may be for the following reason: in vertical engines, the radiator now plays a bigger part in heat rejection as all the waste heat from the flame rises up into the body of the model. Turn it around with the hot cylinder upside and run it. The trick here with an alchol flame is dirrecting the heat onto the hot cylinder then. Not an issue with a propane torch though. Note any changes in the overall temps of the model.

[Ian S C]

With a spirit burner, and the cooling system that you have, you are proberbly best to stick to hot end down, crankshaft up. While the other way is best with gas, its difficult with a wick burner to effectively heat the hot end with it on top. Ian S C

[markombi]

I have found that old stainless vacuum cleaner tubes are good for displacers you just have to braze the cap. Just looking at your photo the displacer cylinder looks too small in relation to the power cylinder. Most of my engines have at least 1.5 to 1 ratio and they 800 run to 1000 rpm. Your engine looks beautifully made though, good work. I am like Ian S C I tend to make my engines out of found parts and junk so sizes tend to be dictated by whatever I find.
Mark

[Ian S C]

hayman, what do you use for measuring the temperature, wish I could measure the hot end temp, most I can do is the cooling water temperature, or if I pull the displacer cylinder off and check the colour of the steel displacer I can see how blue it gets, dark at the hot end, fades out about 1/3 the way along.
Ian S C