It is a good discussion. However, there's one factor that has not been mentioned yet... pressure-viscosity coefficient.
The blowby pressure behind the ring won't be uniform. There will be a larger pressure (ring pushed in more) on the thrust side and a smaller pressure (the ring pushed out more) on the antithrust side. When I made the calculation in post # 96, I neglected this, but to get the lateral force, you need to find the difference between the pressures on the thrust and antithrust sides. This will result in a lateral con-rod force smaller than the 2,600 N I calculated—perhaps about half that large.Wanted to comment on this. Even if there is that much combustion pressure behind the top ring making it expand with that much force, it's still possible for the piston to tilt enough (if the piston skirt to wall clearance is excessive) for the area of the ring pack to rub on the cylinder. it also depends a lot on the piston design. That means the ring(s) would have to be totally flush with the piston surface at that point.
I think the part you're missing is that even when there is a large force pushing the top compression ring outward from combustion gas pressure, the ring pack can still float in the piston grooves. If the lateral force on the piston due to the stroke acceleration is great enough, and the piston to wall clearance is also great enough, the piston can still move laterally in the thrust direction regardless of how hard the rings are pushing against the cylinder wall. The rings don't "lock on" to the ring grooves in any case and prevent the piston from rocking back and forth in the thrust directions.
If you watched the video I posted in post #90, that is explained. Also the photo of the piston in post #90, along with the pistons shown in the video at time 7:05, you can see the area above the top ring has wear, so it obviously had to be rocking enough to contact the cylinder wall, and the top ring had to be completely pushed flush with the piston surface in order for that wear to happen.
As that video also explained, it's the piston skirt to cylinder wall clearance and the exact design of the piston that are the main factors that control piston tilting in the bore. If the clearance is large, and/or the piston skirt is very short (typically used in high performance engines), then the piston could certainly tilt enough to rub the top ring area on the cylinder wall. Look at this piston, the skirt is very short, and therefore if the piston to bore clearance isn't pretty tight, it's going to want to tilt much easier than a piston with a much longer shirt. Piston and ring design is truly engineering intensive, as well as an art.
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Here are a couple of other examples of what I'm talking about. The pressure behind the top ring didn't ensure the area above the top ring didn't tilt and rub on the wall due to the lateral force applied by the connective rod. The rings still fully float in their grooves regardless of how much combustion pressure is behind them.
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I agree ... I'm always up for a civil technical discussion, and it does also become a learning experience because we have to go dig up all the technical references to debate viewpoints.
I do believe the shear rates in the Shell paper when they assume super small MOFT. But I also understand that their peak shear rate numbers are most likely not going to be seen very often if at all on a normally driven street car that lives between 2,000 and 3,000 RPM 98% of the time. For the track guys, racers and heavy towers (much hotter oil temps), obviously they need to bump the viscosity up to get that needed higher HTFS, even if the higher viscosity grade oil has a larger HTHS to HTFS reduction due to VIIs and high shear rates.
I think it would actually depend on how large the ring gap is, and where the ring gap is located with respect to the thrust direction of the piston. Rings can and do rotated some in their groove. It would also depend on the clearance between the top of the ring and the top surface of the ring groove. I'll use this figure again which shows the blow-by going down through the ring gap, and through the top gap in the ring groove around the ring circumference.The blowby pressure behind the ring won't be uniform.
Not sure how you're seeing the statement in bold. Even if the pressure distribution behind the top ring is not uniform, the ring never loses contact with the bore wall. There might be an uneven ring radial force distribution which causes the force of the ring face to be different around the circumference of the ring, but it's not causing the ring to push out more in any direction. Look at the figure above - they have shown that with the different colors on the top ring. There is less pressure and force at the ring gap because gas is leaking past that area and trying to pressurize the backside of the 2nd compression ring.There will be a larger pressure (ring pushed in more) on the thrust side and a smaller pressure (the ring pushed out more) on the antithrust side. When I made the calculation in post # 96, I neglected this, but to get the lateral force, you need to find the difference between the pressures on the thrust and antithrust sides. This will result in a lateral con-rod force smaller than the 2,600 N I calculated—perhaps about half that large.
I think it would actually depend on how large the ring gap is, and where the ring gap is located with respect to the thrust direction of the piston. Rings can and do rotated some in their groove. It would also depend on the clearance between the top of the ring and the top surface of the ring groove. I'll use this figure again which shows the blow-by going down through the ring gap, and through the top gap in the ring groove around the ring circumference.
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But in any respect, the ring is always in contact with the bore wall regardless if there is zero or 1000s of PSI behind the ring. The rings never loses contact with the bore wall, it's just the force at which the ring face contacts the wall that changes based on the gas pressure behind them. So think of the rings as always in contact with the bore wall throughout the piston strokes, and depending on the piston design, piston to bore clearance (ie, piston tilt capability), and how much piston acceleration side inertia force there is (ie, effected by power level and RPM), that will determine just how much the piston tilts and is able to contact the bore wall in the ring pack area. The rings don't move with respect to the bore wall in the ring's radial plane - they float in the grooves - but the piston can tilt in the thrust direction around the rings. Imagine the rings are stationary in the radial plane, and the piston can move fore/aft from thrust in the bore while the rings never move in the radial plane.
Not sure how you're seeing the statement in bold. Even if the pressure distribution behind the top ring is not uniform, the ring never loses contact with the bore wall. There might be an uneven ring radial force distribution which causes the force of the ring face to be different around the circumference of the ring, but it's not causing the ring to push out more in any direction. Look at the figure above - they have shown that with the different colors on the top ring. There is less pressure and force at the ring gap because gas is leaking past that area and trying to pressurize the backside of the 2nd compression ring.
The only thing that "pushes the ring in more" with respect to the piston circumference is the piston tilting fore/aft in the bore as the rings float. If the piston tilts all the way towards the bore, then the ring face on that side is pushed into the ring groove more, and the ring 180 deg from there is poking out little more (it has too) because the ring is always in contact with the bore around its entire circumference.
Again, the ring will always be in contact with the bore wall around it's whole circumference (assuming of course everything is perfectly round and true), so if the piston tilts and contacts the bore wall on one side, then there will be larger gap between the piston surface above the ring pack and the bore wall.
Have you ever put a piston on TDC and take your hand and rock the piston fore and aft in the bore? Example video below. The piston moves as much as the piston to bore clearance allows, and the rings never lose contact with the bore ... they float. That's the whole key to understanding how the piston and rings are not locked together and act independently as the piston goes through the strokes in the bore. Lots of videos showing this.
I have to disagree. My last change of “warren” synthetic blend 5w30 made my daily drivers old 302 Ford pickup loud and rough instantly and it’s still noticeable weeks later. I actually considered changing it out it’s so annoying.It’s the good feel of fresh oil. Nothing more.
I think I see what you're saying. Yes, if there is an uneven force distribution on the ring face (as seen in Figure 1 above) due to uneven gas pressure, and the film thickness is smaller where the ring face force is smaller, then the ring will be moved inward in it's groove a few micrometers more. Dang, I know we like spitting hairs on BITOG, but this ranks up there pretty high.Right, I never disputed that the ring never lost contact with the bore—albeit, the actual clearance will be different on the thrust and antithrust sides, depending on how much the oil film is being squeezed at either side.
What I meant by the ring being pushed in was only in the relative sense. Of course, if you consider the engine block as the stationary reference frame, then the piston is being pushed out on the thrust side, but if you consider the piston as the stationary reference frame, then the ring is being pushed in (with respect to the piston) on the thrust side.
That oil runs fine in lots of other things I use it in. Just seems that my warn out truck doesn’t like it.I would write to warren they may offer you a refund or something for your troubles.
This is very complicated stuff, probably beyond the expertise of anyone except those doing research specifically in this field. That said, I will conclude with a reference to an MIT master's thesis, in case someone wants to dig deeper into the rings, blowby, oil, and lubrication:I think I see what you're saying. Yes, if there is an uneven force distribution on the ring face (as seen in Figure 1 above) due to uneven gas pressure, and the film thickness is smaller where the ring face force is smaller, then the ring will be moved inward in it's groove a few micrometers more. Dang, I know we like spitting hairs on BITOG, but this ranks up there pretty high.
Thing is, the radius of the ring face and the radius of the bore over the piston's stroke distance is probably way beyond a few microns from perfect round. Once it all breaks in, the fit variance would be a little better, but probably still not perfect. So assuming the ring face force was exactly the same around the entire ring circumference, if you could measure the oil film thickness at every square micrometer between the ring face and bore surface over the entire swept area of the ring, it would most likely vary by quite a few microns.
Warn out engine calls for formula shellThat oil runs fine in lots of other things I use it in. Just seems that my warn out truck doesn’t like it.
That paper looks to be modeling everything but the oil film on the ring face, and instead the gas and oil dynamics going on between the rings and ring grooves ... but it's sill interesting, and a whole other part of what's going on in the ring pack.This is very complicated stuff, probably beyond the expertise of anyone except those doing research specifically in this field. That said, I will conclude with a reference to an MIT master's thesis, in case someone wants to dig deeper into the rings, blowby, oil, and lubrication:
Are there any other factors that effect the pressure-viscosity coefficient besides the base oil and oil temperature?Now let's throw in the factor of pressure-viscosity coefficient. PAO has ~9% greater film thickness @ 100°C compared to group I-III and close to the same at 80°C. Down at 50°C though, the mineral oil is upwards of 20% greater film thickness than PAO.
So, throw all of that in the mix. Oh, and don't forget the POE and AN having their own effects.
Oil temperature on the top compression ring will be well above 100C on the daily driver. Not trying to make an argument here.Are there any other factors that effect the pressure-viscosity coefficient besides the base oil and oil temperature?
What about PAO and the comparitive pressure-viscosity coefficient at temperatures well above 100C, like seen in track use?
Yes, Figure 4 in post #114 above shows the OFT on the top ring as a function of oil temperature in the cylinder.Oil temperature on the top compression ring will be well above 100C on the daily driver. Not trying to make an argument here.
Hermann, to your point. my Pontiac 'dealer' put 15W40 Quarker State in everything! i questioned the service manager once('90's) and he said the engines 'run' quieter. #2 my BMW 'dealer' put 20W50 Valvoline Racing in everything. i questioned the service manager('90's) and he said 'they had better experiences with the 20W50 racing(castrol 10W60 in M's). i had a pont. GP 3.4L '91. i decided to try an off brand of oil , AGIP SINT 10W40. and i was surprised by how much quieter the engine ran. at some point manufacturers required their dealers to utilize 'their' Branded motor oil IF dealers want warranty support...i don't know what my BMW dealer has in their over head lube lines these days. i bring my oil(5w40 pennz) and have them to install it(have it noted on work order)'10BMW X5M..note; REDLINE ESTER AND MOTUL 300V OILS run quieter vs most others.Again I trust my senses as much as measurements. Always have Always will
Example:
I think my old Carver amp sounds better than equipment of the same era, which had 1/50th of the measured Total Harmonic Distortion. Again I was talking about a a personal observation, not a measurement.