Piston sealing

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whitakec
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Joined: Thu Aug 13, 2009 9:33 pm
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Piston sealing

Post by whitakec »

I am trying to make a Stirling engine. The main thing that is
troubling me is how to seal the piston. What I'm thinking is a tight
fitting metal ring like on an internal combustion engine, but I have
my doubts about this type of sealing for a Stirling engine. What is
the suggested method of sealing?
stan.hornbaker
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Piston sealing

Post by stan.hornbaker »

It is difficult to make a recommendation without more information as to the sizes of piston & cylinder etc. Metal rings increase friction and 'steal' power. "Vitron" might be suitable depending...
whitakec
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Response to Piston sealing

Post by whitakec »

I'm planning on making a small engine with about 1 in pistons. Now I haven't started making the engine yet. I know that I can make the pistons and everything else. I wanted to be sure that I could seal the pistons before I started. So most of the parameters are negotiable, I just know that it will be small. Also if there are any suggestions for air pressure that would be a help to. If you would like me to pin down numbers i can.
stan.hornbaker
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Response to Piston sealing

Post by stan.hornbaker »

You might find it worth your time to study the material posted at:



This web resource is intended to be a totally self-contained learning resource in Engineering Thermodynamics, independent of any textbook. It is designed to be suitable for a two course sequence for Mechanical Engineering majors, however it can be used in any format, including that of self-study.

Also look at what is possible in home build Stirling Engines at:



and check out all of the links to Stirling engines.

johnemack
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Response to Piston sealing

Post by johnemack »

Because most model hot air engines have little power they are much impaired by friction and leakage, so the piston seal is a worthy consideration. There are many factors, which can be followed till . . . ZZZzzzzz.

Three kinds of seal are regularly used, although their engineering is not often discussed in detail: piston ring(s), lip seals and lapped fits. Oiled lubrication is undesirable - in hot engines because it fouls the heat exchangers, in LTD engines because it causes too much drag for their puny power. Any wear debris will eventually foul the HE's, too.

For lowest piston friction use an interlever or a bellcrank to take the crank-conrod side load (on small-diameter slow bearings) and provide almost axial drive to the piston-conrod.

1. Piston rings can be of metal, plastic or carbon.

Few designers consider the pressure distribution around the section of the ring, especially along the leakage paths, yet that is what determines the ring friction. Basically, the gas pressure on the non-sealing faces drives the ring against the bore and against one face of the groove. Thus it is obvious that the section of the ring should be minimal, a point that is seldom emphasized. That further requires the outer corners of the ring and groove to be sharp.

Along the bore leakage path there is a pressure gradient of unknown form. Assuming it is linear for simplicity, that would halve the radial force due to gas pressure. Additionally, there needs to be a radial preload to effect the initial seal, so keep that small, too. For minimum leakage use a non-circular technique (from engine design sites) for ensuring even loading around the circumference and use the smallest gap the relative expansion will allow.

The leakage pressure gradient is influenced by any taper on the face of the ring. Also, with the “air wedge” aerostatic effect, friction may be reduced. For a power-producing piston, such taper can be induced by making the axial faces of the ring or the groove slightly conical so the ring bears near its inner diameter. The gas force then causes the ring section to cock over microscopically in its groove, in opposite directions for alternate strokes. (The working clearance is minute so a small rotation causes a significant wedge.)

There is another pressure gradient: across the face of the groove. If linear, that also halves the friction in the groove. That friction is important because it can reduce the radial pressure as the piston slaps across the bore or if the bore is tapered.

I have never seen it mentioned but the bore is best heated at one end to create the operating temperature difference while it is being lapped. It will then be tapered when cold but parallel when the engine is running.

For a metal ring, cast iron retains its spring to a high temperature and is self lubricating against a cast iron or SS bore, but the coefficient of friction, mu, is rather high.

If of Tufnol or plastic, choose a PTFE loaded variety for low mu. They will cause little wear debris. They will need metal preload springs (which can be either a non-round spring ring, a “garter” spring pushing outwards or a foil ring with many fingers. Another possibility, although I have never seen it, is a spring ring inside the plastic with tags which engage holes near the gap and is sprung to spread the ring. The ring is then in hoop compression and the radial pressure against the bore is even.) Some plastics (eg, PTFE) will stretch over the piston and can be compressed back into the groove. That obviates the ring gap.

Carbon or graphite rings are excellent but fragile.

2. Lip seals are usually made of Rulon, a PTFE loaded plastic. Sometimes they are self-energized against the bore but, because plastic creeps over time, they are best spread by, eg, a spring ring They do not need a gap.

Again, pressure distribution around the section should be considered. For automatic aerostatic wedging, to reduce friction, the bore of the seal might have slight tapers at each end (or at one end if the engine is pressurized).

Instead of a lip seal a U-seal can be used, where the base of the “U” bears against the bore. The legs of the “U” can then be coned outwards to seal lightly against the groove – but watch it, they can snap inwards like Bellville washers and you won’t know why they don’t seal. I know!

Both the face and the sides can bear grooves. These should reduce the leakage because the entry and exit losses of the leakage flow are said to exceed the pressure drop along a smooth path. Keep the grooves narrow.

3. For best results use graphite pistons lapped to the (parallel at temperature?) bore. They can carry a number of narrow grooves for reduced leakage. Choose materials for compatible expansion. Longish pistons that can just slide under their own weight have low friction and very low leakage. Airpot supply graphite pistons in glass cylinders.

Jester.
whitakec
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Response to Piston sealing

Post by whitakec »

Thank you for all of the great information. I think this should be
enough to get me on the right track.

Once again, Thank you.
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