ring to groove clearance, minimum?

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MadBill
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Re: ring to groove clearance, minimum?

Post by MadBill »

Barry_R wrote:We could always muddy the waters further by noting that the top ring also needs to seal during the intake stroke where there is no "combustion pressure" helping things out. Might be some risks/rewards/opportunities there for various gas port designs...along with some minimum cross section and tension considerations.
That's probably why applying crankcase vacuum to an engine usually increases the V.E...
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Re: ring to groove clearance, minimum?

Post by David Redszus »

MadBill wrote:
Barry_R wrote:We could always muddy the waters further by noting that the top ring also needs to seal during the intake stroke where there is no "combustion pressure" helping things out. Might be some risks/rewards/opportunities there for various gas port designs...along with some minimum cross section and tension considerations.
That's probably why applying crankcase vacuum to an engine usually increases the V.E...
Compression gas pressure and combustion gas pressure provide the necessary force to seal the rings against the cylinder wall and ring groove lower surface. When the radial clearance between the upper ring land and cylinder wall is reduced due to piston thermal expansion, or deposits, compression gas pressure can no longer adequately seal the rings; supplemental gas ports can provide the necessary pressure paths to ensure ring sealing during the expansion stroke.

But, during the induction stroke, gas ports will have the opposite effect. The reduced induction pressure will move the ring from the wall surface towards the back of the ring groove. It will also provide a flow path around the back of the ring and ring bottom sealing surface. This must be overcome by increased ring tension against the cylinder wall. It also makes the second ring serve a usefull purpose. Typically the induction pressure depression is not more than 8 psi, and could be much less with boosted engines so that ring withdrawal is much less significant.

Ring motion does indeed cause wear in the ring grooves. First the rings will rotate as the move vertically due to uneven friction to the cylinder wall. Second the rings will move radially due to cylinder taper and gas pressure. And, finally the rings will twist due to ring face friction and piston vertical motion. This is where ring phosphating, which retains lubricant becomes most useful. Also useful is the anodizing of ring grooves to prevent micro welding.

Ernst Mahle is reported to have said: "A piston is a piece of metal, important to an engine, that has the unique ability to keep brilliant engineers very humble." Well said.
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Re: ring to groove clearance, minimum?

Post by MadBill »

David Redszus wrote:
MadBill wrote:
Barry_R wrote:We could always muddy the waters further by noting that the top ring also needs to seal during the intake stroke where there is no "combustion pressure" helping things out. Might be some risks/rewards/opportunities there for various gas port designs...along with some minimum cross section and tension considerations.
That's probably why applying crankcase vacuum to an engine usually increases the V.E...
...but, during the induction stroke, gas ports will have the opposite effect. The reduced induction pressure will move the ring from the wall surface towards the back of the ring groove. It will also provide a flow path around the back of the ring and ring bottom sealing surface. This must be overcome by increased ring tension against the cylinder wall. It also makes the second ring serve a usefull purpose. Typically the induction pressure depression is not more than 8 psi, and could be much less with boosted engines so that ring withdrawal is much less significant....
Yes, that was my point re one effect of pan vacuum: evacuating the crankcase to say minus 8-10 psig means that on the induction stroke there is still a positive pressure differential sealing the top ring against the bore.
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Re: ring to groove clearance, minimum?

Post by piston guy »

I have seen gas ports ( lateral) on the bottom of the second ring groove. So someone has explored the idea already. The second ring's design limits the amount of crankcase pressure the top ring sees and I seriously doubt the minimal pressure the top ring "might" see is enough to overcome the tension of the ring itself ( unless you are using rings like Warp Speed does). Having a "vacuum condition" in the crankcase allows the rotating assembly to move more freely than it does in a "pressurized" condition. To "me" that's where some of the power comes from as once the vacuum goes beyond a specific amount , the power doesn't increase any further.
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Re: ring to groove clearance, minimum?

Post by Barry_R »

piston guy wrote:Having a "vacuum condition" in the crankcase allows the rotating assembly to move more freely than it does in a "pressurized" condition. To "me" that's where some of the power comes from as once the vacuum goes beyond a specific amount , the power doesn't increase any further.
Just guessing with zero data to back it up - but I'd be willing to bet that the benefits of vacuum are almost entirely ring related. Controlling/reducing oil spray onto cylinder walls in the crankcase allows lower oil ring tension - increasing power. Controlling inter-ring pressure stabilizes the ring pack, improving and speeding seal on the intake stroke and increasing power. Controlling gas flow direction and reducing oil on the walls will also prevent/reduce oil migration into the chamber, providing a better & less contaminated mixture & allowing more aggressive tuning.

I do not think the vacuum - or pressure - in the pan means as much to the rotating assembly. Any pressure (or vacuum) a given piston sees moving in one direction would seem to be largely offset by a sister piston heading the other way.
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Re: ring to groove clearance, minimum?

Post by Kevin Johnson »

piston guy wrote:Having a "vacuum condition" in the crankcase allows the rotating assembly to move more freely than it does in a "pressurized" condition. To "me" that's where some of the power comes from as once the vacuum goes beyond a specific amount , the power doesn't increase any further.
There is turbulence within the windage cloud that is reduced and otherwise generates a parasitic loss. Even with a simple flywheel this occurs. Here is a patent from 1908 that addresses this issue: http://www.google.com/patents/US957243

A larger source of parasitic loss occurs when oil droplets become small enough so as to allow their entrainment within the wind currents. Einstein's Tea Leaf paradox explains how particles that are heavier than the air molecules can be drawn back into the center of rotation where friction occurs from impact.

When sufficient atmosphere is removed from the crankcase the transport mechanism for oil droplet entrainment deteriorates. Conversely, if blowby pressurizes the crankcase the resulting windage and entrainment losses are magnified.

However, a faux atmosphere of small droplets can experience multiple impacts even in full vacuum. There are many high speed videos showing coronal ejection patterns from high speed droplet impacts on surfaces. Many people have witnessed tropical downpours so heavy that water is visibly "bouncing" back up from the ground.

As the atmosphere in the crankcase diminishes, rivulet and droplet ejection patterns begin to revert to "discs" in three space following tangential paths. This leads to a higher probability of droplets impacting on the thrust and anti-thrust areas of the cylinder bore. Unfortunately this is orthogonal or at right angles to the sides of the cylinder bore where piston pin oiling is typically effected by guiding removed oil to the ends of the pin where capillary pumping action will draw it in. So one area of the rings circumference has a heavier load than perhaps designed for whilst another area receives less than was anticipated. The transition appears to be at depression levels greater than 15 inches.
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