Oil Rheology III - What's the Lubricant?

[MolaKule]

An SAE paper once made this conclusion:

"Thus, it is plausable that the observed fuel economy through ________ lubricant use is primarily due to friction reduction in the piston and ring package."


Fill in the blank.

[ September 23, 2003, 01:41 PM: Message edited by: MolaKule ]

 

[Pablo]

synthetic

 

[jason07-1zzfe]

solid-film

 

[MolaKule]

Hint:

Think in terms of grade type (not weights) of engine oils.

 

[jason07-1zzfe]

multi-grade

 

[Jelly]

Multi-weight lubricants...oil is temporarily sheared down to its "smaller" weight, thus the smaller fluid "wedge" between rings and wall equals less hydrodynamic friction.

Lets see if I'm right...

 

[Jelly]

quote:

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Originally posted by Jason Troxell:
multi-grade

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Ooooops...looks like Jason beat me to the punch.

 

[userfriendly]

Friction modified lubricants would improve engine efficiency where an oil film is present.

[ September 24, 2003, 02:58 AM: Message edited by: userfriendly ]

 

[MolaKule]

Monograde oils build up larger oil "dams" above and below the rings, thus increasing viscous friction. The larger the volume of oil to be moved, the greater the amount of energy expended in overcoming this wall of oil.

Multigrade oils show smaller oil dams above and below the rings which results in less viscous friction, thus a fuel savings increase results with multigrade engine oils.

 

[userfriendly]

Yea, and if you believe that I have a nice bridge for sale.

 

[MolaKule]

Honest Injun!

The SAE paper by Hoult, et. al., was entitled, "Direct Observation of the Friction Reduction of Multigrade Lubricants."

Here is the Abstract:

"The oil film thickness between the top ring and liner was observed using laser flourescence(LF). Five different commercial lubricants, two single [monogrades] grades and three multigrades, were studied at two azimuthal, midstroke locations for five speed/load combinations in a small IDI diesel engine. Cavitation was never observed. The lubricant always separates tangent to the ring surface. The rheology of the oil flow under the ring is consistent with a non-Newtonian viscosity without elasticity. The difference between lubricant type (single or multigrade) corresponds to differences in inlet and outlet conditions. Using an analytical model together with the measured oil distributions, calculations demonstrate a difference in friction between single and multigrade lubricants. The multigrade lubricants have lower friction coefficient, consistent with improvements in fuel economy reported in the literature."

The tests were done using a diesel engine and using laser flouresnce techniques, a calibrated signal measures the film thickness as the ring passes over a quartz window in the liner.

Some excerpts from the study:

"Second, there is a clear separation between both multi and single grades, the single grades showing 1) higher friction, 2) a greater wetted inlet height and length, 3) higher upstream film heights. There is a 30% maximum difference in friction betwee single and multigrade oils."

"Multigrade oils wet the ring less than single [monograde] oils. There is a clear separation of the multi versus single grades according to friction values. The data shows a maximum top ring friction reduction of 20% through multigrade use for the same viscosity, speed and load.
If half of all friction in the vehicle is generated by the ring pack, with one quarter of this generated in the top ring, we estimate a maximum total friction reduction of 1.3% due to multi versus single grade use for the top ring alone. A further friction reduction is expected in the rest of the ring pack too. This result is consistent with industry data which demonstrates a 2-4% savings in overall economy through multigrade use."

"Thus it is plausable that the observed fuel economy through multigrade lubricant use is primarily due to friction reduction in the piston and ring pack."

[ September 25, 2003, 10:57 AM: Message edited by: MolaKule ]

 

[TC]

I don't want to tweek anybody by going off-topic here, but I had a question regarding an eariler "Q of the Day" entitled "Bearing Film Thickness," and thought that posting on this latest topic might ensure it got answered. And by the way, thanks for all the great insight we're seeing in these topics, MolaKule (and others).

In the "Bearing Film Thickness" string, MolaKule stated that the industry test determined that with a bearing/journal clearance of 20 microns, the measured oil film thickness varied from 3.8 to 10 microns, depending on the oil weight and other factors. So the OIL FILM was that thickness, but I wanted to verify that they were also referring to the actual METAL-TO-METAL CLEARANCE as well (bearing to journal), and that there wasn't some level of "float" (if that's the proper term). What might this all suggest for particle filtration needs? Look for filtration smaller than 3.8 microns? Over 10 microns? Since the best full-flow media readily found at your local parts store (Mobil 1, PureOne, Wix, etc.) mention intended filtration down to the 10 micron level, how does this interplay with with test's 3.8-10 micron measurements?

I thought these questions would present a natural extension to the oil film measurements, with some practical recommendations hopefully resulting from same on what level of filtration we should look for. Thanks!

 

[userfriendly]

"Honest engine" is politically correct.
Now originally I had the same answer as the other guys, then I went back and changed it, just to have a little fun. I can't help it if the troll in me takes over more often than not.
I read the MIT test before, and I think they used the term "multi-grade" instead of friction modified.
The engine oils are friction modified by virtue of them being mulit-grades, not the other way around. (chicken/egg)
Would a mono grade that had a high enough natural un-improved VI to pass an xxW cold flow test qualify as a multi-grade?
Would a mono grade that had FM additives also qualify?
If so, then my answer is not wrong. I worded my ansewer with care and caution.

[ September 26, 2003, 06:36 AM: Message edited by: userfriendly ]

 

[Pablo]

From UF:

quote:

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The engine oils are friction modified by virtue of them being mulit-grades, not the other way around. (chicken/egg)
Would a mono grade that had a high enough natural un-improved VI to pass an xxW cold flow test qualify as a multi-grade?
Would a mono grade that had FM additives also qualify?
If so, then my answer is not wrong. I worded my ansewer with care and caution.

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Sure and if the question was phrased that way synthetic would be a correct answer as well...but his question has quote marks

 

[userfriendly]

I've put out that synthetic question before with out much success.
Do synthetics due to their inherant and natural, (as in the nature of the beast, not 'occuring in nature), high VI and multi-grade capabilities, as the defination is a viscosity at three or more temperatures, qualify as multi-grades in the above SAE paper? Friction reduction, meaning viscosity friction, not mechanical friction.
From friction modified engine oils is correct, not multi-grade. (ansewer? duh!)

[ September 26, 2003, 07:45 AM: Message edited by: userfriendly ]

 

[jason07-1zzfe]

Thanks userfriendly...That was exactly my point I was trying to make in the bearing film thickness thread.

Yes they are all multi-grade in that they have higher VI and thus thin less with increasing temperature.
BUT I believe that in the original sense of the word, before it was possible to make a multigrade without VII, that it is a 'fake' viscosity.
This is shown in the friction reduction seen here as the VII temporarily shears back to create less drag on the rings.
And I believe the reason some notice a hit in mpg with RL 10w30 over others because it has no VII. It has been stated that other syns do use a small amount and this may be one reason.
That is why I was trying to say 'multigrade' is no longer sufficient info. I would imagine RL 10w30 would not seem to be a 'multigrade' in this ring test...even thought it is.
Yes the multigrade showed thicker film than straight grade in the bearings...but was it measured at the highest load and shear rate that the bearing will see?? I would imagine not. Thus why I said it is a 'fake' viscosity.
So yes multigrade still means higher VI...but it does not indicate all of the other things that it used to.

 

[jason07-1zzfe]

Thanks for clarifying, Injun.

 

[userfriendly]

I'm gonna' hafta' read wulimaster's book Grasshoppa'.
Now could synthetics be considered friction modified and multi-grade, without additives such as FMs, pour point depressants, and VI improvers because of the nature of the product?
Due to the reduced molecular bonding, the synthetic base oils are said to have less of than conventionals, answer the following question:
How does the change in a fluids natural shear rate from one type of base oil to the next
affect viscosity in flow numbers, say in SUS, and how an engine, not an injun, precieves those shear rates in actual operation, say around the ring package/cylinder bore interface, as a friction modified multi-grade or an un improved mono-grade?

 

[MolaKule]

""Honest engine" is politically correct." Yes, but I am a REAL Injun and I am NOT politically correct.

The paper was making a comparison between petroleum monogrades and multigrades, not between mineral oils (conventional oil) and synthetics. In this paper, synthetics did NOT qualify according to the standards of the day, because most people, even petroleum engineers and some chemists, were still ignorant of synthetics and their capabilities. After this paper came out, it was determined that VII's did indeed assist in the total reduction of friction. In the days of highly VII modified oils, the VII's acted as an added synthetic (because it really is a synthetic fluid that acts Non-Newtonian) to the base oil.

A synthetic monograde with a large VI can indeed qualify as a multigrade today. Redline is a typical example.


[BTW, the clearances discussed in another thread were defined as the inner bearing bore dimension to outer radius dimension of the journal.]

While the term "multigrade" may muddy the waters somewhat with respect to synthetics, until the term is modified or redefined, we are stuck with it. Remember, it's the American PETROLEUM Institute (API), not Consumers Interest Association or Institute.

 

[MolaKule]

quote:

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I'm gonna' hafta' read wulimaster's book Grasshoppa'.

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After these questions, me too!

Seriously, good questions.

quote:

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Now could synthetics be considered friction modified and multi-grade, without additives such as FMs, pour point depressants, and VI improvers because of the nature of the product?

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Yes, because synthetics have naturally high VI and produce lower friction coefficients because of their inherent lubricity; all due to their molecular structure.

quote:

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Due to the reduced molecular bonding,

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Not sure what you mean here?

quote:

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...the synthetic base oils are said to have less of than conventionals, answer the following question:
How does the change in a fluids natural shear rate from one type of base oil to the next
affect viscosity in flow numbers

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Just to clarify, viscosity is resistance to flow with kinematic viscosity measured in centistokes, and volumetric flow of bulk oil is measured in G.P.M. In synthetics, the higher resistance to shear means that the base oil shears less at higher temps and higher mechanical shear, maintaining a more constant and consistent viscosity during shearing. So instead of a 30 weight shearing down to a 10 weight, it would more likely shear to only a 25 weight, for example.

quote:

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..., say in SUS, and how an engine, not an injun, precieves those shear rates in actual operation, say around the ring package/cylinder bore interface, as a friction modified multi-grade or an un improved mono-grade?

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The ring/liner would perceive the lubricating fluid as the tenacious film of an improved multigrade less the VII's; i.e., the fluid is now a Newtonian fluid highly resistant to shear.

[ September 27, 2003, 12:21 AM: Message edited by: MolaKule ]