Effect of Oil Viscosity on Oil Film Thickness

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Hi Friends,
This is not another "Which is better, thin or thick oil?" thread. As there are tons of discussions about it in this forum
smile.gif

But i am a little confused, because Shell's research has different results compared with other researches. (Likely i have missed the point.)

Shell Research:
Quote:
The assumption that lower viscosity lubricants automatically give rise to thinner oil films in
key lubricated contacts in a gasoline engine is also open to question, particularly in the case
of piston rings. Laser Induced Fluorescence measurements have found that, in a Nissan
gasoline engine, the mid-stroke top ring oil film thickness was greater for an SAE-5W/20
lubricant than it was for an SAE-15W/40 lubricant. These effects were also observed in our
laboratory for monograde lubricants. Similar effects have been observed by S.L. Moore of
BP36. Figure 20 illustrates the observations. A qualitative explanation of such an effect
could be as follows : There are two routes by which lubricant reaches the top piston ring.
Route #1 (the “conventional” route) is that oil is left on the liner by the passage of the
preceding ring. The higher the oil viscosity, the larger will be the oil film thickness left on
the liner. Route #2 involves oil being transported to the top piston ring via the ring gaps
(such flows have been observed by Nakashima et al37), and this is thought to favour lower
viscosity lubricants. The precise balance between oil transported by the two routes will
determine whether the oil film thickness under the top ring is greater for a lower viscosity
oil or not.


However, another research shows that OFT is higher with 20w40 than 10w40
Quote:
Comparison of the performance of both SAE
10W40 and SAE 20W40, showed that the increase of
frictional power losses and oil film thickness in case of
SAE 20W40 was strongly related to its high viscosity
compared with SAE 10W40. The high viscosity of the
SAE 20W40 was substantially responsible for the
increase in shear stress in oil film, and could be a
strong reason for the increase in hydrodynamic friction
with the use of the SAE 20W40 oil.



Another research:
Quote:
The oils were compared with each other at different loads and speeds as depicted in
Figure 4.29 and Figure 4.30. Oils 026A, 001C, 004B and 001A which have similar
viscosity give similar OFT and oil AW004 which has the lowest viscosity of all oils
shows a smaller OFT, as expected. On the other hand, oil 097A has the highest OFT as
it has the highest viscosity.


Source:
https://priuschat.com/attachments/fuel-efficient-motor-oil-technical-article-pdf.11772/
https://www.researchgate.net/publication...bustion_engines
http://openaccess.city.ac.uk/11916/1/Oil Transport in Piston Ring Assemblies.pdf
 
I agree with Shell.

This

Quote:
The high viscosity of the SAE 20W40 was substantially responsible for the increase in shear stress in oil film, and could be a strong reason for the increase in hydrodynamic friction with the use of the SAE 20W40 oil.


is discussing the fact that higher viscosity oils result in higher hydrodynamic friction and shear stress, a different topic. The thicker the oil, the more HP you're putting into the oil to shear it and the higher the oil temp.

OFT and MOFT are highly dependent on base oil types and additives. I can can have a low viscosity finished oil show good OFT and MOST by the selection of different base oil viscosities, base oil types, and vII's used.

As for the third article, what was the Delta OFT between the two oills?

Furthermore, I doubt all three studies used the same type of engine or test equipment, loads, etc.
 
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The Shell quote gives two of the mechanisms potentially involved in the piston ring situation, and suggests that thicker oil films can develop in that situation with thinner oil because it gets past the rings in greater volume, i.e. there is more flow.

As I read it, this is to some extent a special case, and does not conflict with the general positive relationship between viscosity and film thickness, where the supply of oil is not limited.
 
Originally Posted By: NICAT

Shell Research:
Quote:
Laser Induced Fluorescence measurements have found ....



I would like to know a little more of how this testing method functions.
 
Originally Posted By: Silk
I've replaced far, far more rings than bearings or camshafts because of wear....


I was just going to say that rings tend to wear out more than most other lubricated parts.
 
Originally Posted By: Silk
I've replaced far, far more rings than bearings or camshafts because of wear....
Yer Lord of the Rings! Shot in NZ.
 
Originally Posted By: Ducked
The Shell quote gives two of the mechanisms potentially involved in the piston ring situation, and suggests that thicker oil films can develop in that situation with thinner (lower viscosity) oil because it gets past the rings in greater volume, i.e. there is more flow.

As I read it, this is to some extent a special case, and does not conflict with the general positive relationship between viscosity and film thickness, where the supply of oil is not limited.


I think you nailed it Ducked. (Slight add to above for clarity)

Quote:
Route #2 involves oil being transported to the top piston ring via the ring gaps, and this is thought to favour lower viscosity lubricants.

This is one of the special cases where flow is one of the limiting effects. For general engine lubrication, in a moderate / warm temperatures, the constant displacement oil pump gives the same flow for all reasonable viscosities. So normally flow is not a limitation. Here the small gap in the rings allows more low viscosity oil through, and given that the ring space is not saturated and can take more, then this does help.

Very interesting point.
 
Originally Posted By: Linctex


I would like to know a little more of how this testing method functions.


https://www.dantecdynamics.com/measurement-principles-of-planar-lif

Originally Posted By: andyd
Rings are not my primary concerns Bearings and cam lobes, then rings.
27.gif



My car has over 1.000.000 km on it, and both camshaft and crankshaft were in great condition during the rebuilt. Only replaced the rings.
Not sure about bearings, but they were not causing any symptoms
smile.gif
Just replaced them also, since they were cheap.
Cheapest mineral oil was used the whole time.
 
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Locally here in Spain a company called Cepsa ( major Spanish oil company ) makes a Mineral 20W-40 API SF/CF oil, it has quite high Zddp and is very cheap and i've been tempted to try it in the Capri for some time and see how it likes it compared to 20W-50
smile.gif
 
That's great 1 million KM on cheap mineral oil.

May I ask a few questions?
- what car / engine
- what oil viscosity (eg 15W40)
- what oil specification (eg API SL)
- what oil change interval (OCI)

Thanks in advance.
 
Originally Posted By: SR5
Originally Posted By: Ducked
The Shell quote gives two of the mechanisms potentially involved in the piston ring situation, and suggests that thicker oil films can develop in that situation with thinner (lower viscosity) oil because it gets past the rings in greater volume, i.e. there is more flow.

As I read it, this is to some extent a special case, and does not conflict with the general positive relationship between viscosity and film thickness, where the supply of oil is not limited.


I think you nailed it Ducked. (Slight add to above for clarity)

Quote:
Route #2 involves oil being transported to the top piston ring via the ring gaps, and this is thought to favour lower viscosity lubricants.

This is one of the special cases where flow is one of the limiting effects. For general engine lubrication, in a moderate / warm temperatures, the constant displacement oil pump gives the same flow for all reasonable viscosities. So normally flow is not a limitation. Here the small gap in the rings allows more low viscosity oil through, and given that the ring space is not saturated and can take more, then this does help.

Very interesting point.


Yeh, sorry. "Thinner oil" was definately ambiguous in that context.

Not happy about this.

Doesn't mesh with my "thicker is better" mantra and may contra-indicate my SAE40 / Delvac 15W40 blend.
 
But there's other Shell papers that show increased oil consumption with thicker oils, and the rings being less able to scrape them off.

Rings are the area where I've come across diametrically contradictory studies
 
It's pure guesswork on my part (and perhaps I should have read the source stuff before opening my big gob!) but I wonder if there might be some weird viscosity crossover thing at play here?

At 100°C thick oils are obviously thicker than thin ones. However the top-ring always sees the highest temperatures and what exactly these temperatures (in so far as they impact on the oil film) are is probably tricky to define. Remember that the top ring is the primary conduit of heat from the piston (and in particular the hot piston crown) to the bore so there's probably quite a large temperature gradient across what is a very thin film.

Anyway, with oils, as you ramp up the temperature, the viscosity of all oils will drop and the rate of drop will depend (very crudely) on the oil's VI. If you compare a thick oil to a thin oil, the magnitude of the DIFFERENCE in viscosity will also drop as things get hotter (they all sort of tend to convergence in absolute terms).

So if you impose a difference in VI on top of a diminishing absolute difference, do you reach a point (say 200°C, not an unrealistic temperature for the top ring oil film) when the thin oil becomes thicker than the nominally thicker oil?

I'm probably talking rubbish but it sounds nice!
 
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I've always thought oil gets pumped up the ring lands, there is much more area for it to do that than past the ring gap. The thinner the oil, the better it will pump. In my considerable experience with oil burning engines (everything I have ever owned was worn out when I got it.), I've never seen one use less oil with thinner oil, but certainly get reduced consumption with thicker oil. Maybe one day I'll get a worn out more modern vehicle to test the ''burns more oil the thicker it is'' theory. But for now, I don't beleive it.
 
Originally Posted By: SR5
That's great 1 million KM on cheap mineral oil.

May I ask a few questions?
- what car / engine
- what oil viscosity (eg 15W40)
- what oil specification (eg API SL)
- what oil change interval (OCI)

Thanks in advance.


- Daewoo Nexia, 4 Stroke GM SOHC, 1.5L, 8 Valve
- 10W40, 15W40.
- Didn't take care of the specification. Using Russian, Ukrainan oils. 8$ per 4Lt
- 5000 Km oil change interval.

Actually engine was running nice before i have it rebuilt. Performance was fine. Fuel consumption was fine. Just burning oil, 1Lt/2000km. And making minor noise due to clogged hydraulic lifters. (Not replaced the lifers, just cleaned them)
 
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Originally Posted By: Shannow
But there's other Shell papers that show increased oil consumption with thicker oils, and the rings being less able to scrape them off.

Rings are the area where I've come across diametrically contradictory studies


Sir, Any theory that explains how that is possible ?
Like, if viscosity is resistance to flow, doesn't it mean that higher viscosity oil should be more resistant to piston ring clearances (gaps) ?
 
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Originally Posted By: NICAT
Originally Posted By: Shannow
But there's other Shell papers that show increased oil consumption with thicker oils, and the rings being less able to scrape them off.

Rings are the area where I've come across diametrically contradictory studies


Sir, Any theory that explains how that is possible ?
Like, if viscosity is resistance to flow, doesn't it mean that higher viscosity oil should be more resistant to piston ring clearances (gaps) ?


The theory is that the rings "skip" over the thick oil film.

I'm not saying that I'm an advocate of the theory, and I personally have never seen thicker oil make things worse, but it gets trotted out regularly...well if you've been here long enough.
 
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