Do you think thick oil always protects rings better? Think cavitation!

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Originally Posted by TurboLuver
Is this theory only or was it proven in practice as well?

Yes, definitely both in theory or practice. There are a lot of experimental and theoretical studies on this and it's a current research topic. Ring - cylinder liner lubrication is extremely complicated. Nevertheless, rings are incredibly robust in most cars and conditions despite their extremely challenging lubrication.

It's not to say that thicker oil will definitely be worse but certainly going arbitrarily thick can cause problems. You need to keep transport (flow) and cavitation in mind when you think of ring lubrication. Rings are not flooded with oil at all times as the bearings and (1) oil starvation frequently happens and (2) some cavitation always happens.

Another study was also saying that the worst conditions are cold starts and warm idles, where the oil is very thick and/or the cylinder bore is warped because of the temperature. The old saying "Do not race a cold engine" also applies to the rings and liners.

Blow-by and tribological performance of piston ring pack during cold start and warm idle operations
https://link.springer.com/article/10.1007/s11431-016-6021-6


Abstract:

To reduce the fuel consumption of internal combustion engines, more attention has been paid to the tribological performance of the piston ring pack during the cold start and idle operations. In this research, a numerical model considering the cylinder liner deformation and the piston ring conformability is developed to predict the blow-by, lubrication, friction and wear of the piston ring pack under different operating conditions. The gas flow rate, inter-ring gas pressures, minimum oil film thickness, frictional force and wear load during cold start are calculated and compared with those during warm idle operating conditions. The results show that cylinder liner deformation and piston ring conformability together obviously affect blow-by and other tribological performance. Meanwhile, it is found that friction loss is larger during cold start than during warm idle operating conditions. However, the wear process is more severe during warm idle operation than during cold start. From this research, the blow-by and tribological performance of the piston ring pack during cold start and warm idle operations are understood more deeply.

Conclusions:

A numerical model that considers cylinder liner deformation
and piston ring conformability was developed to
predict the blow-by and tribological performance of the
piston ring pack under cold start and warm idle conditions.
The following conclusions were drawn as follows.
During the transient period of cold start, a higher engine
speed is helpful to form the oil film.
Compared with Case 2 of the cold start in flooded lubrication,
the thinner oil film of the piston ring pack in Case 1
of the cold start in starved lubrication conditions leads to a
larger frictional force and FMEP. The wear load of the piston
ring pack in Case 1 is also more severe than that in Case 2.
In Cases 1 and 2, for ring 2 the minimum oil film thickness
at the downstroke is much smaller than that at the upstroke.
The frictional force and FMEP at the downstroke
is larger than that at the upstroke.
Compared with cold idle conditions, the larger liner
deformation in the vicinity of TDC leads to the higher gas
flow rate and inter-ring gas pressures during warm idle conditions.
The frictional force is larger during cold idle conditions,
whereas the wear process of the piston ring pack is more
severe during the warm idle conditions. Because of the
uneven liner deformation and ring profile, the wear of the
piston rings during the warm idle conditions is uneven in
both the circumferential and axial directions.
Through this research, the blow-by and tribological performance
of the piston ring pack during the cold start and
warm idle operating conditions are understood more deeply.
In future research, engine tests should be performed to validate
the numerical model.

Another study:

Analysis of the Piston Ring/Liner Oil Film Development During Warm-Up for an SI-Engine
https://www.researchgate.net/public...elopment_During_Warm-Up_for_an_SI-Engine


Abstract:

A one-dimensional ring-pack lubrication model developed at MIT is applied to simulate
the oil film behavior during the warm-up period of a Kohler spark ignition engine. This is
done by making assumptions for the evolution of the oil temperatures during warm-up
and that the oil control ring during downstrokes is fully flooded. The ring-pack lubrication
model includes features such as three different lubrication regimes, i.e., pure hydrodynamic
lubrication, boundary lubrication and pure asperity contact, nonsteady wetting
of both inlet and outlet of the piston ring, capability to use all ring face profiles that can
be approximated by piece-wise polynomials, and, finally, the ability to model the rheology
of multigrade oils. Not surprisingly, the simulations show that by far the most important
parameter is the temperature dependence of the oil viscosity.

Conclusions:

This is the first attempt to describe the oil film development
during the warm-up period for SI engines. A new temperature
profile of the cylinder liner has been proposed to make the calculations
possible. Interesting findings include:
Fig. 8 Average friction mean effective pressure for the ringpack
during the warm-up
Fig. 9 Cycle averaged ring-pack friction mean effective pressure
during the warm-up
• For the piston ring lubrication, only the high shear viscosity
is of importance. Multigrade oils effectively exhibit full shear
thinning behavior for this application.
• For all temperatures between 20°C and the warm condition,
all rings change lubrication regime near TDC to the mixed lubrication.
Thereby, asperity contact occurs during the whole
warm-up phase.
• The minimum oil film thickness between the oil control ring
and cylinder liner scales fairly well with the square root of the
viscosity. The thickness between other rings in the ring-pack does
not scale with viscosity in any simple manner. While the thickness
of the oil left on liner is coupled with the minimum oil film thickness
of the compression ring, no simple scaling is found.
• The cycle averaged ring-pack FMEP increases four to five
times at cold conditions ~20°C! compared to the warm condition
~100°C! for the baseline SAE 10W30 oil. By averaging the above-mentioned
FMEP over the whole warm-up phase, the average
warm-up FMEP ~20°C!100°C! is twice the warm FMEP ~100°!.
• The oil film left on the liner is important for modeling the
absorption/desorption mechanism of fuel hydrocarbons in the oil
film. The thickness of this oil layer is predicted to be smaller than
previously calculated and to be in the order of a half to two microns,
and vary dependent on the oil temperature. Furthermore,
the oil layer left on the liner is found to have a small, but still
significant, thickness in the region not overrun by the oil control
ring, i.e., the distance between TDC of the compression ring and
TDC of the oil control ring. The thickness is on the order of
0.2-0.5 mm, depending on the oil temperature. This finding is
important for a further study of the warm-up absorption/
desorption process to estimate the contribution from this source to
the engine-out unburned hydrocarbons.
The model of the oil film behavior during the warm-up has yet
to be verified by experiments, and will be the topic of subsequent
work.
 
Originally Posted by Gokhan


Another study:

Analysis of the Piston Ring/Liner Oil Film Development During Warm-Up for an SI-Engine
https://www.researchgate.net/public...elopment_During_Warm-Up_for_an_SI-Engine


Conclusions:

• For the piston ring lubrication, only the high shear viscosity
is of importance
. Multigrade oils effectively exhibit full shear
thinning behavior for this application.


So do they show like other studies that a higher HTHS oil will provide less ring wear?
 
Thicker oil does not always equal higher film strength. I've seen a number of credible tests showing that (for example) Castrol GTX 5w30 conventional has a stronger film than some of the name-brand synthetic 5w30. This doesn't mean the GTX handles high heat better, or flows better at low temperature than the synthetics, I'm just talking about oil film strength here.
 
Originally Posted by DGXR
Thicker oil does not always equal higher film strength. I've seen a number of credible tests showing that (for example) Castrol GTX 5w30 conventional has a stronger film than some of the name-brand synthetic 5w30. This doesn't mean the GTX handles high heat better, or flows better at low temperature than the synthetics, I'm just talking about oil film strength here.


What parameter are you using to rate the "film strength"?
 
Originally Posted by Gokhan
Abstract:
To reduce the fuel consumption of internal combustion engines, more attention has been paid to the tribological performance of the piston ring pack during the cold start and idle operations. In this research, a numerical model considering the cylinder liner deformation and the piston ring conformability is developed to predict the blow-by, lubrication, friction and wear of the piston ring pack under different operating conditions.

Another study:

Abstract:
A one-dimensional ring-pack lubrication model developed at MIT is applied to simulate
the oil film behavior during the warm-up period of a Kohler spark ignition engine.


Too add ... numerical models can be pretty far off unless they can be validated through actual empirical testing. Some of these complicated models have quite a few assumptions made in their development.
 
Originally Posted by ZeeOSix
So do they show like other studies that a higher HTHS oil will provide less ring wear?

Higher HTHSV could lead to less or more wear. Ring - cylinder liner lubrication is very complicated. Higher HTHSV decreases the oil transport, which could lead to oil starvation. It also increases the length of the cavitation region, which exposes more bare metal. On the other hand, higher HTHSV could increase the MOFT, which is good. You need to strike the right balance. Oils that are very thick when cold could be especially bad.
 
Again - 15W-60 in my Volvo. How long have I got...should I start looking for another car next week....next year...10 years ?
 
Originally Posted by ZeeOSix
Originally Posted by DGXR
Thicker oil does not always equal higher film strength. I've seen a number of credible tests showing that (for example) Castrol GTX 5w30 conventional has a stronger film than some of the name-brand synthetic 5w30. This doesn't mean the GTX handles high heat better, or flows better at low temperature than the synthetics, I'm just talking about oil film strength here.

What parameter are you using to rate the "film strength"?

DGXR is right.

Ring - cylinder liner lubrication happens in the elastohydrodynamic lubrication regime for the most part. In the elastohydrodynamic lubrication regime, the oil is under great pressure (in the order of a gigapascal or 100,000 psi or more). The oil viscosity increases with the pressure, which is determined by the pressure - viscosity coefficient. As a result, the pressure - viscosity coefficient is an integral part of the calculations on the ring - cylinder liner lubrication.

Oil-film strength refers to the viscosity of the oil under pressure, which is determined by the viscosity at the ambient pressure and pressure - viscosity coefficient. The simplest form of the viscosity - pressure relation is:

nP = n0 x exp(alpha * P)

nP is the viscosity under pressure P; n0 is the viscosity under atmospheric pressure, and alpha is the pressure - viscosity coefficient. exp( ) is the exponential function.
 
Originally Posted by Silk
Again - 15W-60 in my Volvo. How long have I got...should I start looking for another car next week....next year...10 years ?

Impressive for a 23-year-old Volvo. How many kilometers do you have on it? How much oil is it consuming? Any problems?

Why do you need to run 15W-60? I would think anything beyond 15W-40 is unnecessary, especially in a commute car like a Volvo.
 
Originally Posted by Gokhan
Originally Posted by ZeeOSix
So do they show like other studies that a higher HTHS oil will provide less ring wear?

Higher HTHSV could lead to less or more wear. Ring - cylinder liner lubrication is very complicated. Higher HTHSV decreases the oil transport, which could lead to oil starvation. It also increases the length of the cavitation region, which exposes more bare metal. On the other hand, higher HTHSV could increase the MOFT, which is good. You need to strike the right balance. Oils that are very thick when cold could be especially bad.


So why the troll bait thread title...which dragged the usual in VERY fast ?

Again, it's NOT exposing bare metal...it's separation of the oil film, on already wetted surfaces...to claim so demonstrates that you don't grasp the topic at all.

The Min oil film thickness is still there (hmin), and the cavitated region is still kept above that.

(NOTE....the papers are SIMULATIONS...you can't claim bare metal on bare metal as a result...you simply can't, as it's not the case)
 
Originally Posted by Shannow
Again, it's NOT exposing bare metal...it's separation of the oil film, on already wetted surfaces...to claim so demonstrates that you don't grasp the topic at all.

The Min oil film thickness is still there (hmin), and the cavitated region is still kept above that.

(NOTE....the papers are SIMULATIONS...you can't claim bare metal on bare metal as a result...you simply can't, as it's not the case)

Yes, I know that there is some oil film around the cavitation. There is a more realistic sketch in the second paper. However, that's not the MOFT, which is defined for the hydrodynamic region. It's basically what you would get in boundary or mixed lubrication; so, the cavitation area accelerates wear if the asperities make contact.

Nothing that nothing is to scale in their sketch:

[Linked Image]


Their wording from the second paper (on cavity length) by K. Stadler et al.: "This means that the cavity produced in the outlet region can be entrained into the conjunction, and break the fluid film, which may lead to surface failure."

No, it's not simulation only. They have taken photographs of the cavitation in the second paper, which is both experimental and theoretical.
 
Originally Posted by Gokhan
No, it's not simulation only. They have taken photographs of the cavitation in the second paper, which is both experimental and theoretical.


But did they validate their model with empirical test data to compare outputs? Anyone can take photos of cavitation without doing a whole gambit of testing to verify a model.
 
Originally Posted by Gokhan
Originally Posted by ZeeOSix
Originally Posted by DGXR
Thicker oil does not always equal higher film strength. I've seen a number of credible tests showing that (for example) Castrol GTX 5w30 conventional has a stronger film than some of the name-brand synthetic 5w30. This doesn't mean the GTX handles high heat better, or flows better at low temperature than the synthetics, I'm just talking about oil film strength here.

What parameter are you using to rate the "film strength"?

DGXR is right.

Ring - cylinder liner lubrication happens in the elastohydrodynamic lubrication regime for the most part. In the elastohydrodynamic lubrication regime, the oil is under great pressure (in the order of a gigapascal or 100,000 psi or more). The oil viscosity increases with the pressure, which is determined by the pressure - viscosity coefficient. As a result, the pressure - viscosity coefficient is an integral part of the calculations on the ring - cylinder liner lubrication.

Oil-film strength refers to the viscosity of the oil under pressure, which is determined by the viscosity at the ambient pressure and pressure - viscosity coefficient. The simplest form of the viscosity - pressure relation is:

nP = n0 x exp(alpha * P)

nP is the viscosity under pressure P; n0 is the viscosity under atmospheric pressure, and alpha is the pressure - viscosity coefficient. exp( ) is the exponential function.


And just how is the film strength accurately measured to give a valid rating?

And please nobody bring up the 540RAT test rig.
grin2.gif
 
Originally Posted by ZeeOSix
Originally Posted by Gokhan
No, it's not simulation only. They have taken photographs of the cavitation in the second paper, which is both experimental and theoretical.
But did they validate their model with empirical test data to compare outputs? Anyone can take photos of cavitation without doing a whole gambit of testing to verify a model.

Originally Posted by ZeeOSix
And just how is the film strength accurately measured to give a valid rating?

And please nobody bring up the 540RAT test rig.
grin2.gif


Yes, here is the data for the experiment vs. theory without (Figure 16) and with (Figure 17) the correction for the oil-film strength (pressure - viscosity coefficient alpha, Table 1):

[Linked Image]
 
GokhanWhy do you need to run 15W-60? I would think anything beyond 15W-40 is unnecessary said:
It has a leaking rear main, something I won't do on a $500 car. It does no commuting, generally short runs and longer trips. The car will expire for other reasons than the oil is too thick, that part is not a worry to me at all.
 
Gokhan - that's all good and dandy ... but where's the data that shows a correlation between:

1) The cavitation level vs oil viscosity.

2) The cavitation level vs ring wear rate.

To support these statements:
Originally Posted by Gokhan
This means you need to strike the right balance between cavitation and oil-film thickness. Going thicker won't necessarily reduce ring wear. Cold engines could be even more problematic for thicker oil.

So, it's something to think about before you go arbitrarily thick to reduce ring wear.


All I see is cavitation measurement results.
 
Also ... this question was never addressed.

Originally Posted by ZeeOSix
And just how is the film strength accurately measured to give a valid rating?
 
Originally Posted by ZeeOSix
Gokhan - that's all good and dandy ... but where's the data that shows a correlation between:

1) The cavitation level vs oil viscosity.

2) The cavitation level vs ring wear rate.

To support these statements:
Originally Posted by Gokhan
This means you need to strike the right balance between cavitation and oil-film thickness. Going thicker won't necessarily reduce ring wear. Cold engines could be even more problematic for thicker oil.

So, it's something to think about before you go arbitrarily thick to reduce ring wear.


All I see is cavitation measurement results.

They didn't study wear -- only cavitation. That's a whole different study that would require bench or field tests of real engines and there are way too many variables (of engine type, oil type, operating conditions, and so on). Moreover, any quantitative real-engine wear measurement is very difficult to conduct and assess. You will have to live with my 1985 Corolla UOAs.
wink.gif


This is the cavitation - viscosity relationship:

[Linked Image]

[Linked Image]

[Linked Image]
 
Originally Posted by ZeeOSix
Also ... this question was never addressed.
Originally Posted by ZeeOSix
And just how is the film strength accurately measured to give a valid rating?

The pressure - viscosity coefficient (PVC) denoted by alpha can be measured in various ways. As far as I remember, the standard way of measuring the pressure - viscosity coefficient is to measure the elastohydrodynamic (EHD) oil-film thickness as a function of the pressure and then use the established formulas to calculate the PVC (alpha) from the oil-film thickness.
 
The question was with regard to "film strength".

So your thread title mentions ring wear, but neither of the papers offered that as a suggestion or conclusion ?
 
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