Zero Friction Piston Ring question

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I learned this in college too. I once know the name of these zero friction piston rings but I have forgot. If I knew the name I could do Google search and learn more about them. College professor said, grooves are .020" wide and .020" deep. Pressure drop across the grooves make them act like real piston rings. I put these grooves on all my sterling engine pistons it is hard to tell how well they work sterling engines do seem to have more power and run better.

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  • #2
sophiecentaur
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I can't help you with a name but it's a really interesting idea. It puts me in mind of the sort of non-contacting microwave seal that's referred to as a Choke Flange. Waveguide flanges often use this method and also microwave oven doors. In the RF world, an open circuit gap is 'transformed' by a λ/4 depth slot into a short circuit across the actual join between lengths of guide. This cleverly removes the problem of a bad contact (dirt etc.) between the guide sections. I imagine that turbulence in the slots can produce a high pressure between the piston and wall which resists leakage.
The choke flange has a limited bandwidth of operation and I wonder whether the above design has an optimum operating rpm.
 
  • #4
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This is interesting but how can that company claim this is new technology and apply for a patent when this technology was known in 1970 when I was in college. It was probably known well before 1970 the college professor was teaching it and he had several models to show the design and how it works.

I have been building zero friction piston rings in all my model engines for 50 years. I am hoping to find technical information that explains details about the best size and shape of the ring grooves. I have tried, flat bottom grooves, round bottom grooves, pointed bottom grooves, 45 degree angle bottom grooves, concave & convex grooves, groove inside of grooves.

I can imagine if air that leaks into the groove circles around the bottom of the groove then comes back out pushing against the incoming air that will block the income air. I can imagine a groove bottom of 45 degree or 60 degree angle will send air bouncing back and forth off the walls of the groove this could block incoming air too. If the bottom of the groove is ruff as sand paper there would produce 1000s of eddy currents that could block air too. If the bottom of the groove is round then top of groove has a radius air that going down inside the groove circles around then comes back out radius at top shoots the back down the cylinder in the direction the air originally came from.

Imagine a shooting a bullet into a U shape pipe the bullet makes an 180 degree turn and goes back the way it came. Now imagine the U shape bent into a 270 degree pipe air goes in then come out then air is show down the cylinder in the direction it originally came from. But inertia won't allow air to make an exact 90 degree turn and go straight down to the bottom of the groove.

Deeper groove might work better. Imagine a shallow tear drop shape groove air goes down a 60 degree angle circles around the round side then returns the exact way it came from no eddies just a lot of back pressure to resist the incoming air. I know the more eddy currents that can be generated inside the grooves the better they work.
 
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  • #5
CWatters
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Not my field but can you be sure the groove is full of air and not oil?
 
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  • #6
sophiecentaur
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Not my field but can you be sure the groove is full of air and not oil?
The pressure is at the dry end (combustion chamber) which has no oil in. ? Air will move through easier than oil so any small net movement will be downwards.
@gary350 patents can be very flimsy. Many don’t even work.
 
  • #7
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I'm struggling with the 'eddy current' approach to analyzing this. It seems (to me) that significant gas flow would be required to produce the turbulence that this analysis assumes; significant gas flow is what these would (presumably) prevent.
Could friction with the cylinder wall produce a usefully large closed circulation within each 'ring?' I'm skeptical.
Could these just (in effect) be 'liquid rings,' with oil as the liquid? Do they work where the piston is in a dry sump?
 
  • #8
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I'm struggling with the 'eddy current' approach to analyzing this. It seems (to me) that significant gas flow would be required to produce the turbulence that this analysis assumes; significant gas flow is what these would (presumably) prevent.
Could friction with the cylinder wall produce a usefully large closed circulation within each 'ring?' I'm skeptical.
Could these just (in effect) be 'liquid rings,' with oil as the liquid? Do they work where the piston is in a dry sump?
You can not put much oil on a hot air engine the oil will get hot producing expanding gas that interferes with the performance of the engine. I sometimes put 1 drop of oil on the cylinder wall then wipe it over the entire cylinder wall then wipe off all the oil I can get off. This micro small amount of oil helps to prevent rust from humidity in the air. Graphite works better than oil on hot air engines. Hot air engines are a closed air volume system, air inside the engine needs to stay trapped inside the engine, oil inside the engine produces expanding gas engine will not run until the oil burns away.
 
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  • #9
CWatters
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Sorry I didn't notice you said Sterling Engine in your OP. Was thinking of petrol or diesel engines when I mentioned oil in the ring slots.
 
  • #10
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What physics is taking place inside the ring grooves of a sterling hot air engine?

Remember piston up stroke is a pressure cycle. Piston down stroke is a vacuum cycle.
 
  • #11
256bits
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  • #12
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Labyrinth Groove, rings, seals, that is it.
 
  • #13
sophiecentaur
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It seems (to me) that significant gas flow would be required to produce the turbulence that this analysis assumes; significant gas flow is what these would (presumably) prevent.
The same thing applies to the labyrinth seal in a microwave oven door. The Open Circuit condition only applies after things have settled down which is after a few cycles of the RF. Likewise, I would imagine, you need a finite flow of gas, initially, before the seal establishes itself. The turbulence has to build up.

We consider the steady state condition in many situations and get the 'right' answer; take the Feedback equations in electronics, for example.
 
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  • #14
The answer to the question of a seal, whether it be for a waveguide or a cylinder is whether it should be with or without friction? This applies to piston rings as well as waveguides.

Back in ‘84 I wrote an article for a U.S. Army Signal Corps Magazine entitled, “Friction is Your Friend” dealing with just this subject.

A technician had replaced sintered bronze gears on high speed impact line printers with iron gears. (Yes, he was the one with the large F-250 in the parking lot full of Toyota’s and Diesel VW Rabbits.). His reasoning was the gear was running extremely hot and must have been ready to fail. This was done on 3 such devices that printed over 20 pages a minute very noisily.

A week later the first one failed. Within 2 days the other two followed. There were virtually no teeth on these gears. The soft bronze gears had been on the printers for over 10 years. Just got a 10th of one drop of oil every month. The iron gears never heated up, they ran cool...and failed. Not enough friction.

At a microscopic level - think electron microscope. The surface has immense peaks and valleys like the Canadian Rockies. As a gear turns, the points of the mountaintops see immense pressure of several tons and temperatures of many of thousand degrees melting the metal occurs in a millisecond or less, with cooling just as quick. There must be enough residual heat from friction in the surface to allow this cooling to occur slowly enough that when the two surfaces are pulled away from each other after being welded together they pull apart in a fluidic matter rather than sustain mortal damage. If there is damage it will accumulate as wear quickly.

This is the reason a car has a thermostat - to bring the engine up to an operating temperature to allow friction to work in your favor.

A frictionless engine will win an award for efficiency but will not last from now until next Tuesday if it is under any load. - Hal Jordan
 
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  • #15
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20 years ago I had a paper that explains how Labyrinth Grooves act like zero friction rings on a piston but lost it. Best I remember escaping gas around the piston encounter a torturous path that spin in a circle around each ring grove. It is like a spinning tornado in a O-Ring shape on each ring. A piston with 12 grooves acts like 12 zero friction O-ring seals. Here is a video of a 2" bore stirling hot air engine that I built 20 years ago. Cylinder is cast iron and piston is light weight 6061 T6 aluminum with Labyrinth grooves. The bottom end of this hot air engine is about 900 degrees F. As the engine runs the engine turns a fan that cools the top part of the engine. If I had a laser thermometer I could learn the exact temperature this engine runs. There is probably a temperature different of 300 to 400 degree from top to bottom. The temperature different is what makes the engine run. The displace piston inside moves trapped air inside the engine from top to bottom back to top over and over 1000 times per minute. When 600 degree air at top moved to bottom it heats up 900 degree air expands the power piston is pushed up. When 900 degree air is pushed to the top cooled to 600 degrees vacuum sucks power piston down. Labyrinth groove differently work engine runs about 10% faster with the grooves on the piston vs no grooves on the piston. I built several engines each one a different design and more efficient. The engine with the largest 10" fan blade makes clanking noise cleanance is zero at the top of the stroke running fast the piston bangs against the top of the engine. If movement of the pistons pushed 100% of the air from top to bottom engine produces much more power vs an engine that only moves 90% of the air from top to bottom. Engine speed will change by changing how hot the bottom is heated or how much cooling top of the engine gets.



 
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  • #16
256bits
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The engine with the largest 10" fan blade makes clanking noise cleanance is zero at the top of the stroke running fast the piston bangs against the top of the engine.
Can that be fixed? it is kind of clanky.
 
  • #17
Baluncore
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I once know the name of these zero friction piston rings but I have forgot.
The parallel rings are not a true labyrinth seal because there is no opposite half fixed to the cylinder. A labyrinth seal functions by reducing the flow velocity, by increasing the path length.

The fine parallel ring grooves, or “flow control grooves” actually function as a flow regulator, where pressure is progressively dropped between the reservoir grooves, as the fluid flows down the cascade. The electrical equivalent is a ladder of series resistors and parallel capacitors.

Flow control grooves can only be used where there is no wear of the surfaces, such as in hydraulic or pneumatic applications where fire or combustion products are absent.

You will see parallel rings being used in proportional hydraulic valves, where the flow increases as the steps are progressively exposed in the valve. The electrical analogy is a series variable resistor.
 
  • #18
Baluncore
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The flow control ridges and grooves in a piston slide along the cylinder, while the equivalent labyrinth seals used in gas turbines rotate. They both have gas pressure appearing across the ridges.

3/4 volumes of DTIC “Labyrinth Seal Analysis”, are available from https://archive.org
Volume 1. Development of a Navier-Stokes Analysis for Labyrinth Seals.
Volume 2. User's Manual for the Navier-Stokes Analysis for Labyrinth Seals computer code.
Volume 3. Analytical and Experimental Development of a Design Model for Labyrinth Seals.
Volume 4. User's Manual for the Design Model computer code. NOT AVAILABLE ?
 
  • #19
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Can that be fixed? it is kind of clanky.
If I take cylinder apart then machine off .002" of metal that should stop the banging noise.
 
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  • #20
Baluncore
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If I take cylinder apart then machine off .002" of metal that should stop the noise.
Check for wear in the bearings. It may be over 2'''
 

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