Some interesting graphs in that paper. Things that stand out to me.Oil is also crucial in the boundary lubrication regime, and unlike in the hydrodynamic lubrication regime, the viscosity becomes directly related to wear in the boundary lubrication regime, as opposed to having minimal or no effect on wear if it is above a certain minimum in the hydrodynamic lubrication regime.
Here are the figures in the paper on engine shear rates:
Shear rates in engines and implications for lubricant design | Semantic Scholar
By combining shear rate range data in engine components with measured viscosity shear rate curves on lubricants (at different temperatures), useful insights have been obtained on how the viscosity shear rate curve of a lubricant should be “designed” to give low friction (and hence improved fuel...www.semanticscholar.org
0° denotes TDC firing and 360° denotes TDC exhaust. There is a lot more load on the piston during firing. Minimum oil-film thickness (MOFT) for a given oil viscosity decreases with increasing load and decreasing speed (Stribeck curve).Some interesting graphs in that paper. Things that stand out to me.
1) Figure 2 - As we all know, MOFT in journal bearings increases as the viscosity increases. Viscosity headroom could matter.
2) Figure 3 - why is the film thickness 0 at 0 deg crank angle (TDC), but not 0 again at the next TDC at 360 deg? Is 0 deg TDC assuming a combustion event that takes the MOFT away on the top ring? Also, if the engine RPM is 2500 or higher the MOFT on the top ring is actually higher and about 2u and higher. Is a lot of idling bad for ring wear? ... could be.
3) Figures 4 and 6 - even though the peak shear rates are high for the rings and cam lobe/followers the duration is very short so the wear period per rev is also short. As mentioned before, when the shear rates are high which causes some added decrease in the HTFS and MOFT, the AF/AF additives have to kick in (film strength vs film thickness). And rings and cams/followers are made with materials and hardened to resist wear because they obviously have some metal-to-metal contact when in the boundary lubrication realm.
Yes, obviously if there is no motion the MOFT would essentially go to zero. Guess the load on the rings from the combustion is expanding the rings more against the cylinder walls is what that graph shows. It's not so much the load/force on the piston dome, but the high pressure getting behind and trying to expand the top ring against the cylinder wall.0° denotes TDC firing and 360° denotes TDC exhaust. There is a lot more load on the piston during firing. Oil-film thickness for a given oil viscosity decreases with increasing load and decreasing speed (Stribeck curve).
No, it has nothing to do with the combustion pressure expanding the rings if any such thing is happening at all.Yes, obviously if there is no motion the MOFT would essentially go to zero. Guess the load on the rings from the combustion is expanding the rings more against the cylinder walls is what that graph shows. It's not so much the load/force on the piston dome, but the high pressure getting behind and trying to expand the top ring against the cylinder wall.
The piston rings float in the piston ring grooves, so if there is a huge side force on the piston, the rings are not going to see any of that force because they float in the ring grooves. The force of the ring face on the cylinder walls (when there is no combustion) is a function of the ring spring tension/pressure. When a piston is at TDC, you can move the piston around within it's clearance to the cylinder, and the rings will remain against the cylinder walls at the force from the compressed ring spring tension action. What's going on between the piston skirt and cylinder wall is different than what's going on between the ring face and cylinder wall when there is no combustion pressure.No, it has nothing to do with the combustion pressure expanding the rings if any such thing is happening at all.
It's Newton's third law (action–reaction principle). When the piston applies a load on the oil film, there is an equal but opposite force applied by the oil film on the piston. As this force increases, the oil film gets thinner and eventually squeezed out. The same law also applies to the action–reaction forces between the piston and combustion gases. It is the same phenomenon in the bearings: when you tow, the load on the bearings will increase and the MOFT will get smaller. Obviously, no combustion gases are involved there.
Exactly what load on the top ring are you talking about? The load on the piston and connecting rod and small and big end bearings are from the combustion force pushing the piston down, but that motion doesn't cause the rings to push harder against the cylinder walls. The only thing making the radial force and load on the rings higher above the ring spring tension is the gas pressure in the cylinder pushing the ring face harder against the cylinder wall.The reason for the smaller MOFT at 0° firing vs. 360° TDC exhaust is not the ring expansion—it is the higher load. See Figure 2 in my link above for the con-rod bearings, where the same phenomenon occurs. It is more pronounced at the rings because the speed goes to zero and the geometry (eccentricity etc.) is less forgiving.
Now you're saying the rings expand ... how so?Moreover, any expansion of the rings would actually increase the minimum oil-film thickness (MOFT), not decrease it. That is because the eccentricity (wobbling) is reduced when the rings expand, and the MOFT is inversely proportional to the eccentricity (wobbling), as more wobbling squeezes out the oil film more.
Nevertheless, these are theoretical calculations, and I doubt that the authors bothered to factor the ring expansion into them.
What I am saying is that MOFT and the clearance between the cylinder wall and ring are different things. In a bearing, it looks like this:Now you're saying the rings expand ... how so?
I don't really know what you're trying to convey here. I know how a journal bearing operates, been discusding them for years on BITOG. You seem to think that I believe the oil film in a journal bearing and the oil film between the compression ring face and cylinder wall acts the same way - I don't. And the use of the term MOFT just doesn't mean inside a journal bearing. It can also mean the minimum oil film thickness between any moving parts/surfaces when the oil film varies in thickness due to operating conditions.What I am saying is that MOFT and the clearance between the cylinder wall and ring are different things. In a bearing, it looks like this:
Nobody said, including me, that there was a varying ring to wall clearance - how would that be possible? The rings are always in conract with the cylinder wall due to their spring tension. I think you're misinterpreting some of the things I'm saying for some reason.I doubt the authors assumed a varying ring–wall clearance.
Exactly what loads are you talking about? You don't think there is any ring expansion? Maybe you think "expansion" means something else in this case, like thermally (?).The effect of a smaller MOFT at combustion TDC vs. exhaust TDC is being caused by the load on the piston increasing by the combustion and the speed of the piston decreasing as it approaches the top dead center, not by the ring expansion.
The loads on the journal bearing caused by the rod/piston stroke motion inertia and the highest load from the combustion power stroke are in the bearing's radial direction, and causes the film thickness (MOFT) to vary by squeezing oil out the open sides of the bearing.A similar but less dramatic effect is seen at the con-rod bearings in the authors' figures.
What you're missing is the directional nature of these forces. The force due to the expansion of the ring is negligible in comparison to the force applied on top of the piston by the gas pressure multiplied by the sine etc. of the angle involved to get the resultant component. Here is the full analysis of the forces:I don't really know what you're trying to convey here. I know how a journal bearing operates, been discusding them for years on BITOG. You seem to think that I believe the oil film in a journal bearing and the oil film between the compression ring face and cylinder wall acts the same way - I don't. And the use of the term MOFT just doesn't mean inside a journal bearing. It can also mean the minimum oil film thickness between any moving parts/surfaces when the oil film varies in thickness due to operating conditions.
Nobody said, including me, that there was a varying ring to wall clearance - how would that be possible? The rings are always in conract with the cylinder wall due to their spring tension. I think you're misinterpreting some of the things I'm saying for some reason.
Exactly what loads are you talking about? You don't think there is any ring expansion? Maybe you think "expansion" means something else in this case, like thermally (?).
So you don't believe what the papers say about the high pressure combustion gas pushing the top ring outward (trying to increase its diameter) and forcing it harder against the cylinder wall? Just like in a journal bearing, a high load can reduce the film thickness by squeezing the oil out between the ring and wall - that's probably what the "squeeze effect" comment in the figure was referring to.
The loads on the journal bearing caused by the rod/piston stroke motion inertia and the highest load from the combustion power stroke are in the bearing's radial direction, and causes the film thickness (MOFT) to vary by squeezing oil out the open sides of the bearing.
The load on the piston rings from the same stroke motion inertia only acts in the direction perpendicular to the ring face. What causes the increased load on the ring face to wall at combustion TDC is the 1000s of PSI on the back side of the ring, as the many diagrams posted show. More force on the ring face is going to squeeze out oil and reduce the film thickness, especially because of how thin the ring face is.