Dynamic / Kinematic relationship

[tstep]

disclaimer -- not an oil genius

Speaking strictly within the confines of viscosity (not addpacks, oil life, etc), for example, G-Oil 5w30 has the following characteristics:

-HTHS = 3.2-3.5 (probably closer to 3.2 but thicker than most 5w30s at operating temp regardless of actual value)
-KV100 = 9.9-10.0 (thinner than most 5w30, especially considering shear)
-KV40 = 54-55 (again thinner than most 5w30s)

Lets say the engine designer used a typical 5w30 of KV100=10.5, KV40=63, and HTHS=3.0 in consideraton of the engine design.

Over the long haul, Is there any downside to having a dynamic viscosity 10-20% thicker than considered in the engine design while at the same time having a kinematic viscosity 10-20% thinner than considered?

 

[A_Harman]

If the engine designer was worried about fuel economy, there would be a slight downside to the higher dynamic viscosity. If he was worried about oil film thickness, there would be an upside. But if he is designing an engine to run on 30-weight oil, he should be using the 2.9 cP minimum HTHS as the basis for his oil film calculations, and including some margin for shear.

 

[tstep]

Restating in a different scenario, such as the HTHS requirement of 3.5 or greater for some euro car brands.

If the min HTHS of 3.5 is achieved w/ some spare room for future shear (say German Castrol), does the 10-20% lower KV100 than other comparable oils that meet HTHS of 3.5 hinder GC performance in any manner?

 

[dparm]

HTHS / Dyn Viscosity @ 150C is a way to approximate shear resistance. It basically takes into account the relationship between the oil's kinematic viscosities and the HTHS.

It's a good indicator of a high quality oil. Renewable Lube 0w30, for example, has KV values of 58.0 & 11.3, yet offers HTHS of 3.8. Mobil 1 0w40 has KV values of 75.0 and 13.5 to obtain the same HTHS of 3.8!

However, as A_Harman and I have often speculated, this formula may not be quite as useful in certain oil blends (as the shear stability can reach or even surpass 1.0, suggesting it is shear-proof).

 

[bepperb]

IMO the ideal would be a HTHS a bit above what the engine requires and then the lowest KV possible.

 

[tstep]

Originally Posted By: dparm
HTHS / Dyn Viscosity @ 150C is a way to approximate shear resistance. It basically takes into account the relationship between the oil's kinematic viscosities and the HTHS.


Could you show me an example of this using oil values?

 

[tstep]

Originally Posted By: bepperb
IMO the ideal would be a HTHS a bit above what the engine requires and then the lowest KV possible.

I too draw this conclusion after reading countless threads in these forums.

Would the low KV affect anything? Ability of the oil to be carried by reciprocating parts - ie enough won't stick? Flowability thru the engine - ie too much like water laying in the bottom of the pan?

 

[dparm]

Originally Posted By: tstep
Originally Posted By: dparm
HTHS / Dyn Viscosity @ 150C is a way to approximate shear resistance. It basically takes into account the relationship between the oil's kinematic viscosities and the HTHS.


Could you show me an example of this using oil values?




In the case of Mobil 1 0w40...

3.8 / 4.422 = 0.8594

Here's a Google Documents calculator I whipped up a while ago. You can view but not edit it.

https://docs.google.com/spreadsheet/ccc?key=0AiRqQTiptSMpdG1QQUpPMnNWVWI1UXdtNkpDSVF0RVE

 

[dparm]

Originally Posted By: tstep
Originally Posted By: bepperb
IMO the ideal would be a HTHS a bit above what the engine requires and then the lowest KV possible.

I too draw this conclusion after reading countless threads in these forums.

Would the low KV affect anything? Ability of the oil to be carried by reciprocating parts - ie enough won't stick? Flowability thru the engine - ie too much like water laying in the bottom of the pan?



Not really. How long do you think the oil is 40C (104F) for? Hint: not long.

 

[dparm]

This calculator illustrates it well. Fill in some of the oils I linked to on my calculator up top:

http://www.widman.biz/English/Calculators/Graph.html

 

[A_Harman]

Originally Posted By: tstep
Restating in a different scenario, such as the HTHS requirement of 3.5 or greater for some euro car brands.

If the min HTHS of 3.5 is achieved w/ some spare room for future shear (say German Castrol), does the 10-20% lower KV100 than other comparable oils that meet HTHS of 3.5 hinder GC performance in any manner?


No. The best gauge of an oil's actual viscosity while it is under stress is HTHS. If an oil can achieve an HTHS of 3.5 while having a lower KV100 than another oil having 3.5 HTHS, the oil film thickness in a bearing would still be the same. The oil with lower KV would tend to drain back to the sump under the force of gravity slightly better.

 

[mechtech2]

This is why new G5 SN oil is biased for fuel economy, and the new Dexos is biased for a bit more robustness.

 

[tstep]

Thanks for all the explainations and links to the spreadsheets/calculators. Very enlightening, especially working thru the spreadsheet and really seeing the relationships between the various properties.