Piston rings for a larger stirling engine

[vederstein]

"Soon" I will be building the Myers Heinrici stirling engine casting kit I received for Christmas. I'm a bit intimidated by the piston on this engine. It's 1.500 diameter and the bore is about 4 inches.

http://www.myersengines.com/engines/Heinrici.htm

I've built a couple of small stirling engines in the past and most posts on this forum give advice on the smaller engines.

The plans call for a cast iron piston .0005 to .001 under the cast iron bore. I'm concerned I won't be able to control the cylinder and it may taper during machining or when heated up.

On a larger stirling engines like this, do piston rings make sense? If so, any suggestions (material, size, source of supply)? What about the displacer rod clearances, which goes directly through the center of the piston?

If anyone has built this engine, could you please let me in on your experiences?

Thanks,

 

[robcas631]

I've heard that graphite tape is good to use for piston rings.

 

[myrickman]

Do not be intimidated by the bore length. The engine only uses the top portion by the power piston. If you make the clearance small, you should be able to get away with an oring and a top ring-shaped plate on the top of the piston. As you tighten the screws on the ring, it compresses the o ring pushing it out adjusting the clearance creating a seal with minimal drag. A neoprene lip seal like in a pneumatic cylinder may work. It is all about minimizing drag. If you are friends with someone with a sunnen hone, you can get the bore and piston to fit where a ring may not be needed.

 

[Goldflash]

The biggest problems with model Stirling engines I have found is having friction at a minimum and
My concern with adding piston rings is that they may add more friction. The Power Cylinder/Piston should not get that hot so I wouldn't be to worried about expansion or distortion especially with cast iron piston running in a cast iron cylinder.
What I did on my cast iron piston on my home designed unit was to machine labyrinth grooves ( as per oil free compressor pistons ) to prevent air leakage past the piston. ( It also Slightly reduces contact area reducing friction )
If you do end up with a slightly tapered bore then this can be corrected by honing or lapping and then finishing the piston to size

With such tight clearances on the piston and cylinder make sure everything is clean before assembly as any dirt can cause some nasty scratches in the cylinder bore. I boil everything in hot soapy water to remove any abrasive particles from cast iron and this is common practice for large cast iron engine block and compressor cylinders after honing.

Next time I pull my Stirling engine apart I am going to get the power piston and the displacer piston teflon coated to see if this will reduce friction and increase performance

Good Luck
Goo

 

[idahoan]

No even on an engine this size rings would cause enough friction for the engine to keep the engine from running. I built the Essex hot air fan from Myers which has a piston close to 2" I also made a new piston for a Pakistani hot air fan that I restored. Both of these pistons I sent to High Performance Coatings and had them coated with their Moly graphite coating. This has a couple of bennifits first it is super slippery and second it has an affinity for oil; so the oil film will wet the piston. Both of these engines are great runners.

One other thing about the coating; you get a decent fit on your piston first and when you get it back it won't fit in the bore. then you can fine tune the fit with some fine sand paper and then plain white paper for the final polish.

Here is the link to HPC's web site.
http://www.hpcoatings.com.au/coatings-dryfilm.html



If the piston is machined properly to start with it shouldn't change enough to cause problems while running. I'm not sure about the Heinrici but the Essex had an aluminum cylinder casting; I bored mine out and pressed in a cast iron liner. I wasn't going to put that much work into an engine with an aluminum bore.

Orrin who checks in here occasionally built the Heinrici from Myers castings and I may add that it is one of the most beautiful examples you will ever find; maybe he will chime in with some encouragement on your project.

Please think about posting your build progress of this great engine.

Dave

 

[vederstein]

It's strange. I'm the Sr. Mech. design engineer for a leak test company and have designed many, many expanding seals. For some reason, I've never considered an adjustable expanding seal for a dynamic seal.

Looking at oring materials, I'm finding (from McMaster Carr):

Silicone isn't very tough, but is highly temperature tolerance (+400 deg F)
Buna-N has good toughness, but is only good for about 200 deg F
PTFE (Teflon) is very tough and high temp (+500 deg F) but is stiff.

Other seals are too exotic (read expensive) to put into a toy.

Considering the application, teflon is probably the best choice.

I'll probably (try) to make the piston as designed and another piston with your suggestion. Then I'll go with which ever works better. I may design the piston to include the displacer rod seal to work the same way.

Thanks for the suggestion.

(Anyone that has other ideas, please let me know. I'm just trying to figure out the best way to a successful running engine)

I only consider my engines a success if they run. If they look wonderful, but don't run, I've failed.

...Ved

Do not be intimidated by the bore length. The engine only uses the top portion by the power piston. If you make the clearance small, you should be able to get away with an oring and a top ring-shaped plate on the top of the piston. As you tighten the screws on the ring, it compresses the o ring pushing it out adjusting the clearance creating a seal with minimal drag. A neoprene lip seal like in a pneumatic cylinder may work. It is all about minimizing drag. If you are friends with someone with a sunnen hone, you can get the bore and piston to fit where a ring may not be needed.

 

[deverett]

Graphite is often used for Stirling engine pistons. Your engine may be too large to consider this option, but it might be able to be used for the displacer rod bush.

One problem potentially solved!

Dave
The Emerald Isle

 

[Orrin]

Dave, I'm not sure my engine deserves your generous words; but, I appreciate them, anyhow. Thank you for making my day.

This engine would not run if it had any piston ring friction. Don't worry, you won't have any trouble getting your piston to fit close enough. It just takes a little time to lap it true.

Bore the cylinder. If you are lucky it won't have any taper; but, even if it is straight it is unlikely to have a smooth enough finish. If it is straight you can perhaps get by with honing the cylinder smooth with a brake cylinder hone. IMHO, however, it still won't give you a smooth enough finish.

I recommend you make an expanding lap and lap the cylinder. Lapping will remove any taper and it will produce a very smooth finish.

I wish I could point you to a Web page that gave a good explanation on how to lap a cylinder; but, every search I've made, so far, has come up with so much bum dope that I hesitate to direct you to any of them. For instance, I see frequent references to using toothpaste as a lapping compound. Well, if you have a cylinder with a thousandth of taper, you'll take eons to make it straight with toothpaste!

Some people seem to have good luck making a lap out of hardwood. That has never worked for me because wood compresses too easily. I make mine out of aluminum, others make theirs with an outer layer of copper.

Either way, the lap should be soft enough to allow the grit to embed into it. I start out with a grit of about 150 until I'm assured of a uniform diameter for the entire length of the cylider. Then, I wash all the grit from the lap and switch to something finer, say 300. I repeat at about 800 grit.

I usually make the length of my laps about twice bore diameter bore diameter. In other words, if the bore is 1.5-inches the lap will be about three inches.

Use your imagination in making a split lap. It needs to be a cylinder that you can expand with a couple of adjusting screws. You'll also have to make it with a shank that you can drive with a drill press or drill motor.

Be careful to not spend too much time at the ends of the cylinder or you'll bell-mouth them.

You might find Jerry Howell's tips to be useful; but, he addresses graphite pistons, only. I'm afraid that graphite will not have the strength for the Heinrici. The center displacer rod guide is threaded into the piston and I'm afraid graphite threads will strip or cause the cylinder to split.

http://www.model-engine-plans.com/engineeringtips/powerpistons.htm

I don't have an easy way to post pictures, right now, so if you have trouble visualizing a lap, send me a PM and I'll e-mail you a picture of one of mine.

You can see some pictures of my engine if you "Google" orrin heinrici

I see that the server that carries my pictures is down, now. Try again, later.

Regards,
Orrin

 

[vederstein]

I'm resurrecting this thread because it's related to the same engine.

I finally got the engine together and running. But there were some issues:

I did hone the cylinder and I thought I had a good cylinder/piston fit. The piston only slowly dropped though the cylinder when I covered the end. Unfortunately there still was too much leak and I think I may have had a slightly tapered bore.

So I pulled an o-ring trick from my knowledge of being a designer of leak test equipment:

I turned an o-ring groove into the piston with a fairly deep gland depth (.133 gland depth for a .139 thick o-ring). Then I tried the proper sized o-ring for that diameter seal...

Too much friction.

So I went to the next smaller o-ring. Running the calculations, stretching a smaller oring doesn't change the cross section diameter very much, so I could keep going down in size until I got a good seal with very little friction.

In the end I got the engine to run, but I have to put a propane torch on the hot end to get it to run and the hot end is red hot in the area where the torch is applied. So my question is thus:

How hot should the hot end of a Stirling be? Should burning charcoal under the hot end be sufficient for this type of engine?

Any tips to get this thing to run better would be helpful.

Thanks.

 

[Mechanicboy]

The piston ring or O-ring is robbing the power from the stirling engine due friction. If possible, use leather as sealant, it will give less friction and good sealing against leakage when the leather is well oiled in (not wet dripping oil from leather into the cylinder). I has used leather as piston sealant without problem in the stirling engine.

 

[Orrin]

How hot should the hot end of a Stirling be?

It depends upon the amount of work it is doing. I have several Stirling engines that will run on the heat from a birthday cake candle.

If red heat is necessary to run an "idling" Stirling, there is far too much friction.

Orrin

 

[Mechanicboy]

If red heat is necessary to run an "idling" Stirling, there is far too much friction.

Orrin

Or bad design of the stirling engine due loss of temperature..

 

[bacon9023e]

Hello I am currently a senior in a machining program at my local trade school Making Sterling engine for a senior project I originally came to this post looking for the best way to seal my engine using O rings but any advice would be helpful

 

[fcheslop]

I use ptfe packing in the pistons of larger engines as in 2 to 5inch bore my last 2 inch bore engine will run on one 1/4 dia wick meths fired as can be seen on the wee video
I know others have used the old style of graphited rope for packing

 

[propclock]

I agree, I use Gore tek , that is an expanded form of ptfe aka teflon.
I cut it into strips rather than trying to make a ring. Just cut a strip
the width of your groove and angle cut the ends . Also graphite
helps even with PTFE. Friction less and air tight are the goal.
I have a Denny that I run a lot at shows. Pumps water all day.

 

[BobsModels]

I used graphite string (ie packing) in my Rider with a 2" cylinder. It has been in the engine since 2015. I personally think it has less friction than PTFE. It gets lubricated with silicone oil at the start of each day. Reason for silicone oil is it will not form a coating of carbon(insulator) on inside the bottom of the hot tube like a carbon based oil will. The engine runs about 80 hours a year at different shows. It still has original string I put in and engine runs real nice and slow.

Bob

 

[Rocket Man]

The smaller the bore diameter the more critical the piston clearance becomes. .0005 cylinder bore clearance is good but .0002 is better. You can not have piston rings this engine does not produce enough power for piston ring drag. Labyrinth Grooves act like zero friction rings. Labyrinth Grooves should be .020" wide and .020" deep .150" apart as many as you can get on the piston. Another thing that kills the power on that engine is the heavy cast iron piston and heavy displacer cylinder. Remove all the extra metal you can on that cast iron piston. Clearance hole through piston for displacer needs .0005 clearance or less. I built the same engine years ago I don't remember displacer diameter or length. Most people don't realize a lighter weight displacer requires less power for the engine to move it back & forth. Air trapped in the cylinder has to move back & forth top to bottom very fast if clearance between displacer cylinder & cylinder is too small air pressure builds up on each side of the displacer this requires more horse power for the engine to pump air. I built a displacer with a tomato paste can. Drill holes in both ends of tomato paste can then wash out the tomato paste and silver solder can to the displacer rod. The can is very light weight. A much lighter displacer can be made by solver soldering 2 very thin .030" metal flat washers to the displacer rod space between flash washers = length of your displacer then wrap 1 layer of aluminum foil around the 2 washers to form a displacer piston shape. This aluminum foil displace is so light it uses very little power from the engine to move it back & forth. I sold my original kit engine then built several engine from scratch. If you replace the solid displacer rod with steel tubing you reduce the weight more and the engine runs better & faster. You can build a dynamotor Horse Power tester with a small 1" permanent magnet electric motor use it like a generator to see how much power your engine is producing. I like the challenge of improving engine efficiency so engine runs faster and produces more power.

Here is my 2" bore engine with Labyrinth Groove, 0002" cylinder bore clearance, aluminum foil displacer, hollow displacer rod, light weight aluminum piston, it runs 900 RPMs fairly easy, add a little bit more heat it runs 1200 RPMs. The engine turns a cooling fan. A 1/16" O-ring is used like a fan belt 10 to 1 ratio.

 

[Longboy]

The plans call for a cast iron piston .0005 to .001 under the cast iron bore. I'm concerned I won't be able to control the cylinder and it may taper during machining or when heated up.
On a larger stirling engines like this, do piston rings make sense?
Thanks,

Maybe you can overbore the block to receive a DOM thin wall liner. That cures your threat of a tapered bore. The engine doesn't care what material the piston is made from and aluminum in a steel cyl. works. Nor does it care if you reduce the bore from plan as a practice run for your cylindrical skills! As for a piston ring in a Stirling? It may be in the plan but I would do without first and make a dual diameter piston to keep the seal and reduce the friction.

 

[vederstein]

It's strange how old threads get revived. I started this thing in 2011.

At one point in history, the engine actually did run, a bit clanky though. No so much anymore...