Air Flow Around Engine Piston Rings

The design of the shaft you connect the piston to will dictate what you need to do to center the piston.

If rings are expected to carry the weight of piston and transfer it to liner, as indicated the rings will get stuck and may not float.
Like in bigger compressors, piston should be supported by wrist/rider ring/arrangement and rings should be independent to float. Still the free float is subject to carbon coating, sticky oil slurry etc.

Further...

The available running clearance would concentrate only at the top portion of the ring . This would leads a greater force on the top half of the ring compared to the bottom half (pressure X area). Together with the self-weight of ring this force will always ensure a rub at the bottom. So availing the suggested chamfering at the bottom portion (or may be for 120 degree) of the ring sound logical.

Mike, "The piston has continuous circular rings of ceramic material. The cylinder also has a ceramic sleeve."

Laughing Jaguar, "One piece ceramic rings have expansion rates too. A one piece ceramic ring in a bore as suggested will have no place to grow and will, under the resulting thermal expansion, exert pressure until the loads cause the ring to fail."

Will it be possible to have the dimensions/fits of continuous circular One piece ceramic rings and ceramic sleeve such a way their expansion matches? Rings expand as well the sleeve expands with sliding clearance is not reduced objectionably?

Free piston engine, as in friction free piston/ring-wall interface?

Friction free will come at a cost, you will loose your compression pressure down the sides of the piston, past the rings between the walls and the rings.

The rings are there to lessen the surface contact area of the piston to sidewall interface, if you wish to have zero friction between the moving part and the wall, then there is no need for rings, but also what will stop the piston from its sideways movement? if your running the piston-conrod-crank approach, then the piston will have lateral forces from the conrod-piston angles (greatest when the crank is at 90 degrees to the running plane of the piston) this is another area where you will get piston to sidewall friction.

I think a free piston engine have better features to keep into some kind of self-alignment:did you first asked how a common engine gets so when the piston is pulled or pushed by outlines forces?I should adecuate the lower part of piston bellow the rings grooves, for sliding in the cilinder surface,then,i should use common metallic rings those you can get in all sizes in any shop (same for pistons).Commonly people thinks they know how a piston ring works:They don't(commonly).Really different than hidraulics plastics seals:learn about this because none of the clearences are chosen by taste:may be you can get "engineering design" by P.Ordov in case you want make piston and rings by youself.Watch rings grooves are made in different material than piston,you know why?.In every case you must try avoid any air flow thru the rings:the only airflow leakage is because the metallic rings are open to keep some pressure by its elasticity against the cilinder until the pressure inside the cilinder do that job thanks to some clearance

You will get more "blow-by" in a cold engine than in a warm engine, as the rings have not heated and expanded to almost close the gap.

If the piston ring gap is too small (new piston rings and have not taken time to properly gap them) will expand under combustion chamber temperatures, collide, then bind between the piston and wall, and either wear the ring/wall interface, or shatter.

The metallic rings needs be open to put them in their grooves in first place; some of them have some enough stiffness and others needs an spring in the internal side;is true temperature determines a minimal gap...I don't know what to call the primary reason..

Some rings I have seen are like a spring, but without space in the crossing coils, much like a "Thread Saver" or "Heli-coil" these don't run a gap as the "ring" is a coil of wire.

The ones with a "Internal spring" are the oil control rings with spacer (if this is what your talking about)

http://www.mooretool.com/

Hi,

something similar to your idea is realised in the weight relieve of the vertical guide of their ultraprecison turning and grinding machines.

But this is operated only at room temperature and with very slow velocity and leakage is existing and depending on gap.

In reality you will need a piston ring that is working as an "air-bearing" or fluid-bearing.

If there is enough oil on the cylinder-liner to fill a gap then this may be a fluid-bearing

if there is no oil this may be a gas-bearing.

Centering can be done by the structure that has to be machined (few micrometer depth) on the circumference of the ring. This will act as a radial-bearing with some considerable stiffness. But no tilt stiffness unless two these rings with some distance.

I doubt that this is possible as piston rings - tolerances are likely to be too stringent.

May be possible in a totally ceramic piston. (No rings).

Biggest problem is the thermal expansion to be isotropic and not changing too fast.

RHABE

Have you given any thought to how your experimental assembly will be able to center it self while on any other than the compression stroke?

Also, excessive gas in the crankcase will promote big problems. On larger engines, this is the cause of most crank case explosions

Unless the piston/cylinder are oriented vertically, gravity will cause piston(rings) contact with the bottom of the cylinder. The only manner to avoid this inevitability is to supply a buffer fluid-either liquid or gas- to the bottom section of the piston to 'float' it off the cylinder wall. This would require an external pressure source for the buffer fluid, which could be introduced along the bottom of the cylinder through either small holes or slits. A ceramic cylinder might be suitable for etching/laser perforation to 'leak' the buffer fluid into the desired location, with circumferential collection grooves past the piston ring travel limits to channel the buffer fluid out of the cylinder. While this kind of arrangement WILL work, it requires a bit of experimentation to get the geometry and flows right. Stable operation would greatly simplify the control, while varying loads/speed will make control problematic. One could use a Hall Effect probe at the likely contact surface to detect incipient contact and increase buffer fluid flow. Good Luck

I understand that gravity could cause the floating ring to settle to the lowest point, the bottom. Using some sort of fluid buffer, if I understand you right, would end up being consumed during combustion? Did I mention that combustion occurs on both sides of the piston! The whole reason to avoid ring/cylinder contact is to do away with lubrication the would make the engine ' dirty' and not acceptable by todays standards.

I just had some thoughts, what if the rings themselves were fashioned differently so as to compensate for gravity, say, chamfering on the lower portion and not on the top, or having an external air source to bleed in the piston itself so as to lift the inner circumference of the top side of the ring. I guess if the engine was used where different attitudes were involved regulating this air flow could be a challenge. I have been steering away from firmly fastening the ceramic ring in place believing that because of the different expansion rates between aluminum, the intended main material of the engine, and ceramics ( alumina) could cause ring cracking with failure. My understanding of ceramic material is that it is hard but brittle. If one could deal with the aluminum piston expansion while maintaining the centering of the ceramic ring things could work.

My origional thought was that in nature things want to come to a state of equalibrium, if these rings were light in weight do you think the force of gravity would exceed that of the very high pressures that would tend to equally space the ring within the cylinder. This progect might just be one of those cases that experimentation would give us the answer.

A.The floating ring will not settle at the lowest point as there are high radial and tilting forces from dynamics and combustion.

B. To avoid contact would be best, no doubt.

C. If you think about rings I would try anodised aluminum.

D. Try to think about ringless machines. Is there a possibility to have radial gaps of 10 to 20 µm gap-width that are changing by temperature not more than 10 (20)%? This will require very accurate machining.

RHABE

I'm guessing you don't have plans for using a six pac carb setup like MOPAR did many years ago. hee hee!

Is there any chance of using extremely small amounts of lubrication instead of none? Zero clearance is a lot different than extremely close tolerance. That looks like a tall order. The efficiency of oil lubrication for sealing of the cylinder would be hard to overcome.

Sounds like you have this idea very well thought out.

"gravity will cause piston(rings) contact with the bottom of the cylinder"

The radial forces from slight form deviations are much higher than gravity force, so the gas-bearing has to have load capability and stiffness that is bigger than these radial forces.

"This would require an external pressure source"

External pressure is one possibility but the advent of the herringbone bearing (Muiderman in late 1960ies) and the step bearing (Mitchell) changed this situation. These types are self-pumping bearings that need some velocity to act. There have been many solutions with gas, liquids or grease as "lubricant". Most modern hard disc drives have oil lubricated bearings of the herringbone type.

RHABE

Just how will you extract any power if it has combustion on both sides of the piston?

What will push the piston on the compression stroke?

Better find a good supplier of Virtual Piston Rods, Virtual Crankshafts, Magnets, Mirrors, and Smoke!

You will also need a Quantum Computer for the controller!

Sorry Bill, but I think you have missed the mark. This engine that I'm modelling mine after did run and very well I might add. I guess part of the lack of your understanding is that I'm not at liberty to divulge too much in the way of detail without comprimising what I'm doing. Energy or power comes in many forms, namely in my case the production of high velocity high temp and high pressure gasses, no ' smoke or mirrors required ' !, just imagination and persistence!

Would this be similar to the "Stirling engine" I think the principle your talking about is similar, they use them in some Submarines for quietness.