Got a used larger hobby engine, start up prep?

Discussion on Stirling or "hot air" engines (all types)
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cbstirling2
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Joined: Sat Dec 17, 2016 9:35 pm

Got a used larger hobby engine, start up prep?

Post by cbstirling2 » Mon Jan 02, 2017 8:28 am

I recently acquired a gamma engine built apparently by a machinist. (It's being shipped to me.)
What should I do to prep the engine?
Was planning to clean up the old oil and whatnot with gun cleaner spray.
For the power piston bore I was planning to lubricated with dry graphite.
For the linkages was going to use a very light oil with PTFE in it (gun oil).
What do you recommend?
Pictures to follow.
When looking at the pictures, does anybody recognize the kit? Another question is do you think I could run it off of alcohol lamps? I suppose that it's best just to try it.

Without modifying the metal of the engine, do you think would be effective to add a water tank that would be pushed against the head of the power piston cylinder? With Dielectric grease for heat transfer?

It has 4 inch flywheels.
CBStirling2

cbstirling2
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Re: Got a used larger hobby engine, start up prep?

Post by cbstirling2 » Mon Jan 02, 2017 8:38 am

Here is a side view.
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CBStirling2

cbstirling2
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Re: Got a used larger hobby engine, start up prep?

Post by cbstirling2 » Mon Jan 02, 2017 8:41 am

Top views.
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CBStirling2

Trevor
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Re: Got a used larger hobby engine, start up prep?

Post by Trevor » Tue Jan 03, 2017 1:27 am

Nice engine - Personally I like to use methylated spirits as the burner, especially for small engines, however, yours appears to require a gas one to heat the end. As for the piston, graphite works real well or a very light oil if the metals are not alloy.
Trev

Ian S C
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Re: Got a used larger hobby engine, start up prep?

Post by Ian S C » Tue Jan 03, 2017 3:28 am

What is the piston and cylinder made of? This will dictate what lubrication is required there. You could use your oil on the displacer rod, con rod ends and main bearings.
I think a nicely made spirit burner would do well, you only need the gas if you want a bit more power out of it.
Rather than water cooling, how about a fan mounted to blow over the finned area, driven off the flywheel on the power cylinder side. You might have to turn a groove in the rim of the pulley.
Ian S C

cbstirling2
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Re: Got a used larger hobby engine, start up prep?

Post by cbstirling2 » Fri Jan 06, 2017 6:11 pm

On the power side, the cylinder is lined with a thin magnetic sleeve. (Steel I presume)
The piston itself is more difficult to determine. I think it is steel as well since a little magnet sticks as well.
On the inside of the piston there is lamp black.

Ian, what do you recommend for lubrication when steel on steel?
The power cylinder looks to be 3 thin tubes pressed together. About 1.5 mm thin steel pipe inner, thin brass tube and then a machined outer tube of aluminum with fins.

When I spin it, I would preferred to feel more compression.
But haven't ran it yet I'm waiting for a reply on lubrication recommendations first.
CBStirling2

cbstirling2
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Re: Got a used larger hobby engine, start up prep?

Post by cbstirling2 » Fri Jan 06, 2017 6:38 pm

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CBStirling2

cbstirling2
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Re: Got a used larger hobby engine, start up prep?

Post by cbstirling2 » Fri Jan 06, 2017 7:41 pm

Looks like the machinist made a steel cup and then screwed in a brass piece.
The crank shaft design is interesting. It is a steel bar about an inch diameter.

As before, I am curious to lubrication requirements
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CBStirling2

Ian S C
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Re: Got a used larger hobby engine, start up prep?

Post by Ian S C » Sat Jan 07, 2017 2:38 am

If both the piston and cylinder are steel, a very light oil can be used, I'v used sewing machine oil and kerosene 50:50. The piston is best with a number of narrow, shallow grooves around it this forms a seal (labyrinth seal).
If the cylinder is steel, and the piston cast iron, run it dry, it to should have the labyrinth seal.
The fat crankshaft is helping the fly wheel I suppose, although really it is just adding weight where it's not really needed. I suppose it is a colour distortion, but the shaft looks more like brass in the photo.
Ian S C

cbstirling2
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Re: Got a used larger hobby engine, start up prep?

Post by cbstirling2 » Sun Jan 08, 2017 12:02 am



I got the engine, is in dire need of a tune-up as the oil was Gummy and one of the bearing blocks was rubbing a bit.
CBStirling2

cbstirling2
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Re: Got a used larger hobby engine, start up prep?

Post by cbstirling2 » Sun Jan 08, 2017 12:20 am

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To clean up the engine ended up taking a lot of it apart.
One thing that I learned is that would never build an engine this way.
The crankshaft assembly in my opinion is way too heavy.
A shared crank with a 90 deg offset would be far simpler and more performance.
Also the displacer lacks any sort of a generator. I noticed the displacer shaft had a lot of stiction. Are used 1000 grit sandpaper to try to remedy that a bit. Surprisingly on this engine the displacer seems to get more problems than the power piston itself. This experience has led me to believe that alphas are probably more simple especially if graphite piistons are to be used.

Interestingly, there's another engine at the builder built that looks somewhat similar however that was water cooled. I suspect the water version works a lot better. One of the things that I ran into was that after a while it slowed down and stopped because it got too warm.

I think that if I was going to build one, I would use graphite pistons for both the hot and the cold side. Since pistons of graphite on the two signs then it makes sense to alpha version and according to my research alphas seem to produce more power.. Also I'd include a water jacket, and that would at least limit the cold side to 212 degrees Fahrenheit and most likely colder than that.
CBStirling2

Ian S C
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Re: Got a used larger hobby engine, start up prep?

Post by Ian S C » Sun Jan 08, 2017 2:42 am

The power piston looks ok with the grooves around it (good start). the gland for the displacer rod can be a problem, specially on a horizontal motor. Making it as long as possible reduces the leakage. If you are polishing the displacer rod, do it length wise.
The hollow cylindrical displacer, made to the usual formula of length = three times the diameter actually works as a regenerator.
Ian S C

cbstirling2
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Re: Got a used larger hobby engine, start up prep?

Post by cbstirling2 » Sun Jan 08, 2017 8:23 am

Ian, thank you for your insight.

I see several things to try:
1. Run the engine vertical as a test.
1a. I suspect it would be best to have the displacer dangle rather than be on top. Although if I make a new stand I'd have to redesign the propane nozzle.
2. Remove a flywheel. (Easy to slide off.) They are heavy.
3. I don't have a lathe but I wonder if I could make a better displacer/regenerator.
3a. First displacer Idea: Make 2 or 3 hollow cans but thermally isolate the areas with a gaskets.
3b: 2nd displacer idea: make displacer out off stainless wool or wire like so [#|#] where the # is the wool, | is a thermal insulator. This would be a great heat exchanger but may increase dead space too much.
4. Lighten the very thick crankshaft by disassembling and drilling weight reduction holes close to the conrods. The issue here would be how much weight does one remove by holes? Could be actually very difficult however I think the current arrangement isn't weighted very well.
5. Make a little fan as Ian suggested before.



Is there a tacho (light style) thats inexpensive?
CBStirling2

Ian S C
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Re: Got a used larger hobby engine, start up prep?

Post by Ian S C » Mon Jan 09, 2017 1:33 am

To fire the motor with a spirit burner, it would be worth making a bit of a furnace, just a square open ended box with a hole in one side for the hot cap, and at the bottom, probably at the back another opening for the burner, and to allow a good air supply. This will keep the flame to the back end, and shelter it from breezes.
My tacho cost about $NZ90, but they are advertised on line for about $ 30.
Without a lathe I would leave that crankshaft alone. It is possible to make a crankshaft without a lathe, but you must be able to drill holes in reasonably thick steel accurately.
Ian S C

Ian S C
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Re: Got a used larger hobby engine, start up prep?

Post by Ian S C » Mon Jan 09, 2017 5:31 am

Here's a rough sketch of what I mean about the crankshaft, some 1/4"/6 mm steel would be ideal for the three crank cheeks, a bit of steel rod full length for the main shaft that will fit the bearings. Two shorter bits for the crank pins. Some shaft lock adhesive, Loctite. Drill the hole through the end of the outer cheeks and the bottom corner of the right angle center cheek, then match up the side cheeks and drill them while located by a pin through the first hole.
Ian S C
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