Clearances and oil shear.

[Sonataman]

If a certain weight oil was required for an engine or trans with clearances for that certain weight oil, would less oil shear occur than if you used a heavier oil and had it shear to a certain weight regardless of mileage, within reason of course?

The reason I ask is that it seems if you run a car 10k miles on the highway vs all short in town trips, the oil running the highway is subject to far less cold starts and therefore less time with 40+ cst viscosity.

 

[jrustles]

Good question.

There are a lot of variables to consider. I would imagine this to be a 5w30 vs 5w20 shearing contest?

Probably all depend on the type of VII used, how dependent the finished KV is on said VIIs (as in, how much change in visc occurs when the VII molecules get shredded and truncated), oxidative stablility of the finished product vs the rate of shear and external factors like dilution and soot loading. These are the things I'd consider trying to answer your question. One of the experts can definitely shed some light on this, someone with first hand blending experience.

Originally Posted By: Sonataman
The reason I ask is that it seems if you run a car 10k miles on the highway vs all short in town trips, the oil running the highway is subject to far less cold starts and therefore less time with 40+ cst viscosity.



Could you clarify what you mean by this?

 

[SuperDave456]

Quality oils of today don't suffer to much shear. Or at least it shouldn't.
If it does it typically is due to other factors.

Over a Long OCI you do need to watch out for Fuel Dilution, in the case of a Diesel Soot, and ultimately TBN and TAN.

 

[lockguy]

the clearance thing is interesting to ponder. Maybe some engines need very specific oil viscosity due to certain tolerances.

My ford service van uses the duratec engine where in europe the spec oil is 5w30, here in the states the spec oil is 5w-20. I have used both, can't really tell a difference in performance or efficiency.

I would imagine that the 5w30 would shear a little more being a thicker, higher viscosity oil, but the end result at the end of an oil cycle would still be a higher viscosity than the 5w-20 at the end of an equivalent run.

Ive heard the cold starts are hard on an oil but don't know exactly why. I guess because the oil is less fluid when cold, it tends to shear more.

 

[gregoron]

My theory is that thicker oil to what is originally specified for that engine would shear more as the tighter clearance would breakdown the VII more and thus cause more oil dilution and potential sludge.

However, using heavier oil may also lessen blowby gas going through the cylinders at idle. But you get less oil flow at startup or when cold.

There are a lot of factors to consider hence it's best to use what is specified, or thinner oil as an alternative.

 

[mechtech2]

Shearing is more from timing chains, oil pump gears, things like that.

 

[Sonataman]

Originally Posted By: jrustles
Good question.

There are a lot of variables to consider. I would imagine this to be a 5w30 vs 5w20 shearing contest?

Probably all depend on the type of VII used, how dependent the finished KV is on said VIIs (as in, how much change in visc occurs when the VII molecules get shredded and truncated), oxidative stablility of the finished product vs the rate of shear and external factors like dilution and soot loading. These are the things I'd consider trying to answer your question. One of the experts can definitely shed some light on this, someone with first hand blending experience.

Originally Posted By: Sonataman
The reason I ask is that it seems if you run a car 10k miles on the highway vs all short in town trips, the oil running the highway is subject to far less cold starts and therefore less time with 40+ cst viscosity.



Could you clarify what you mean by this?


I guess what I was trying to ask was when oil is thicker than the clearances call for if this is when the majority of shear occurs. Cold or hot.

Say when the oil is cold, or thicker than called for when hot, it seems like the oil is having to be squeezed to fit into where it should easily go.

The thing that got me wondering about this is when thicker oil runs hotter than thinner oil and also the fact that some oils in a shared trans (motorcycle) can be a little notchy shifting at first, but over time thin out and shift smoother.

Looking at all the UOA's, bike and car, it seems the UOA's where the short trip or cold start is at a minimum is when less shear occurs.

 

[MolaKule]

Quote:
I guess what I was trying to ask was when oil is thicker than the clearances call for if this is when the majority of shear occurs. Cold or hot.

Say when the oil is cold, or thicker than called for when hot, it seems like the oil is having to be squeezed to fit into where it should easily go.

The thing that got me wondering about this is when thicker oil runs hotter than thinner oil and also the fact that some oils in a shared trans (motorcycle) can be a little notchy shifting at first, but over time thin out and shift smoother.

Looking at all the UOA's, bike and car, it seems the UOA's where the short trip or cold start is at a minimum is when less shear occurs.


Well, your questions are vascilating over different topics.

You seem to be mixing the topic of oil/engine temperatures with shear.

Let's stick with the topic of PCMO's.

If I understand your question, the oil film will go in between the clearances regardless of viscosity. Thinner oil will flow better especially at start up.

The rule of thumb in journal bearing design is, the higher the
rpm and lower the load, the thinner the oil, and the lower the rpm, but for higher loads, the thicker oil is required to maintain the hydrodynamic film under load.

Read mine and other posts on engine oil heating versus oil viscosity. The higher the viscosity of the oil, the more energy it takes to move the oil and it will run hotter than for thinner oils, all other things being equal.

I don't know what you mean by, "called for."

As for shear, the most shear occurs at the piston ring cylinder interface. The second area of shear occurs at the camshaft. In journal bearings, shear will occur also depending on the load.

With modern base oils and additives, shearing of the oil has been greatly reduced .

 

[jrustles]

Originally Posted By: MolaKule
The rule of thumb in journal bearing design is, the higher the
rpm and lower the load, the thinner the oil, and the lower the rpm, but for higher loads, the thicker oil is required to maintain the hydrodynamic film under load.

This should become one of the Oil Commandments! It would really help with the fellas that believe in using 20w50 in their base Civics and Corollas; or 0w20 in their Volkswagen 2.0T

 

[MolaKule]

See Section 12.3 and 12.4 of this KSU paper regarding Petroff's Equation, of which the lubricant's viscosity is generally determined for journal bearings:


http://faculty.ksu.edu.sa/essam/Documents/ME301chapter12.pdf

Determining fluid film verses viscosity verses load for chains, cam lobes, and other items is more complex.

 

[Doug Hillary]

Hi,

From Molakule;

"As for shear, the most shear occurs at the piston ring cylinder interface. The second area of shear occurs at the camshaft. In journal bearings, shear will occur also depending on the load."

How true! BMC (Austin, Morris etc.) had a very practical lesson about permanent lubricant shearing - in 1959 - they introduced the Mini! Sharing a common engine/gearbox lubricant, even the expecially developed new viscosity 20W-50 (by Duckhams) had great difficulty going out to 1k. Castrol's 20W-50 was no answer either. The internal loadings were simply too great!!!!

Imagine how Dealerships who commonly used 20W-20, 20W-30 and perhaps 10W-30 lubricants coped with their Customer's problems when these typical lubricants were used in the Minis!!! And, the mechanical issues resulting..............

Today engine Manufacturer's anticipate the level of permnanent shear within their programmed OCIs.

The ACEA has gone a long way in understanding these issues and dissipating their knowledge and findings

Thanks again MoleKule..............

 

[Trav]

Originally Posted By: Doug Hillary
Today engine Manufacturer's anticipate the level of permnanent shear within their programmed OCIs.



I think they may have believed they had this down to a science but it seems now it may be a little more complex as we see now with GM lowering their OLM interval due to chain issues.

 

[MolaKule]

I have not seen any data as to the root cause of the timing chain issue.

It could be a number of factors:

1. Too high a tension

2. too much tensioner vibration

3. chain material and or manufacture.

4. harmonic vibration in the balancer system, etc.

So let's not be too quick to blame the oil.

 

[Trav]

Okay but why are the reprogramming the OLM? Fuel dilution on DI engines?

 

[Doug Hillary]

Hi,
Trav - We will probably never really know the reasons for the OLM reprogramming. Operation characteristics/environment are one thing and component manufacturing quality control/durability testing another - part of the matrix of possibilities

This is all a part of the process in Auto Engineering and Manufacture as you know doubt know

The Chinese are in the throes of learning this too with the VW Group recalling nearly 400000 vehicles with DSG gearbox issues there. Some no doubt caused by local manufacturing and quality control processes

That is largely why the British Auto Industry was doomed to fail from the early-mid 1950s

 

[Coprolite]

If it is a timing chain metallurgy issue, it might be mitigated by higher TBN and lower TAN. Shot in the dark.

 

[jrustles]

Originally Posted By: Doug Hillary
Hi,

From Molakule;

"As for shear, the most shear occurs at the piston ring cylinder interface. The second area of shear occurs at the camshaft. In journal bearings, shear will occur also depending on the load."

How true! BMC (Austin, Morris etc.) had a very practical lesson about permanent lubricant shearing - in 1959 - they introduced the Mini! Sharing a common engine/gearbox lubricant, even the expecially developed new viscosity 20W-50 (by Duckhams) had great difficulty going out to 1k. Castrol's 20W-50 was no answer either. The internal loadings were simply too great!!!!

Imagine how Dealerships who commonly used 20W-20, 20W-30 and perhaps 10W-30 lubricants coped with their Customer's problems when these typical lubricants were used in the Minis!!! And, the mechanical issues resulting..............

Today engine Manufacturer's anticipate the level of permnanent shear within their programmed OCIs.

The ACEA has gone a long way in understanding these issues and dissipating their knowledge and findings

Thanks again MoleKule..............


Yes, thank you for that link Mola

Doug HIllary, that is an interesting note about the "shared and sheared" oil problems of the past. I can't imagine how badly those primitive multigrades must have gotten shredded in such an environment. It seems our modern problems might not be far away, with the balance shaft gears to worry about, cam gears, lengths of hardened steel chains and cogs. Longer strokes, higher piston speeds can't be helping either, and the engineers know this. Thus long standing attempts at ever reduced ring tensions and thicknesses. Any wonder why some modern engines still consume oil like ye olde tymes? It seems that after all of our advancements in engine building and oil chemistry, that we still demand more and more of the lubricant. Yet we keep advancing.

But to extrapolate on OPs question, I'm curious about when a VII is more likely to be mechanically sheared; hot or cold? When the molecule is bunched up or when it's sprawled out. Also what kind of geometry have VIIs advanced to? Like from 'coil' shaped, to 'star' shaped. There's got to be far more types than that. Kind of like the advancement between regular moly esters and the "tri-nuclear" type we keep reading about