design limits for viscosity

[GMorg]

In regard to oil, I have read here and on bottles of oil to use a particular grade of oil in "engines designed to use" a particular grade of oil. I have found this statement curious. Everything has a range of acceptible values. I suppose that fact could be part of the origin of grades (my bet is batch variation though). I want to know just how narrow of a range of viscosities can an engine be designed to use. Can an engine even be disigned to perform optimally with an 8 cSt @ 100C oil and yet perform poorly or have excessive wear at 10.5 cSt? Similarly, is that range a normal distribution (ie, 8 +/- some number or percentage). Or, is the range very small in one direction and rather large in the other direction? Can it fall within a single grade?

I read that we should follow the recommendations of the guys that designed the engines. OK, I'll buy in to that for a moment. But meanwhile, what are their limits of design? Even engineers are not allowed to break the laws of physics. Can one design a main bearing, a ring/cyclinder, and a cam/lifter to perform optimally for the same viscosity oil. If so, and again, what is the size of the acceptible range of viscosities that surround the design target? How far from this optimum can one go before a measurable difference occurs in wear?

 

[Titan]

The answers to these questions are why I say following the manufacturer's recommendations, and not vary MUCH from those recommendations. If you DO vary from the recommendations, you're acting like you know more than the manufacturer. Oh, BTW, I'm not educated enough to know the answers to your questions, BUT, I am smart enough to know you are asking the right questions!

 

[MGBV8]

Halfords in the UK provide a guide to Oil choice, unless you own a Ford or a Classic (20W50) its xxW40. The xxW depends on FI

5w/30 part synthetic-suitable for the latest high efficiency engines including all modern Ford from 1998 on.

10w/40 part synthetic-suitable for most modern multivalve,turbo or fuel injected engines where synthetic protection is essential.

10w/40 enhanced mineral-suitable for most fuel injected engines.

15w/40 part synthetic-suitable for most non-fuel injected engines since 1980 where synthetic protection is preferred.

15w/40 enhanced mineral-suitable for most non-fuel injected engines since 1980.

http://www.halfords.com/webapp/wcs/...ectionURL?storeId=10001&REDIRECT_URL=/600.asp

 

[Mitch Alsup]

GMorg

Notice the caviat of "A fully warmed up engine with proper 'pop-off' regulation of the oil pressure, operated under 'normal' circumstances"

Thin oils give better startup 'wear'
Thick oils give better extreme conditions 'wear'

 

[GMorg]

For a large portion of the planet, it sounds as though it may not be possible to design an engine in which a second SAE grade would be innappropriate.

 

[GMorg]

So, my manual recomends only 5W30. All that I can conclude is whomever authored my manual offered only single recommendation. I think that either EPA required that recommendation based on CAFE or GM thinks that their U.S. customers are idiots and cannot deal with choices. I am prepared to flip a coin and accept either possibility.

 

[Rickey]

And the third choice is... the EPA thinks that all US drivers bunch of idiots ?

 

[G-MAN]

quote:

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Originally posted by GMorg:
So, my manual recomends only 5W30. All that I can conclude is whomever authored my manual offered only single recommendation.

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That's "whoever."

 

[Mitch Alsup]

quote:

---

I want to know just how narrow of a range of viscosities can an engine be designed to use. Can an engine even be disigned to perform optimally with an 8 cSt @ 100C oil and yet perform poorly or have excessive wear at 10.5 cSt?

---

A fully warmed up engine with proper 'pop-off' regulation of the oil pressure, operated under 'normal' circumstances will ahve a nice long life with oils across a factor of 4-8 in actual kinemetric viscocity.

One of these factors of 2-2.5 is designed in when the manufacture considers extreme service in hot conditions where the oil operates in the 280dF range and that 10cSt oil thins to ~4cSt.

The other factor of 2-2.5 is designed in when opposite conditions prevail with short trips in mid winter conditions wher the oil never gets fully warmed up and that 10cSt oil at 200dF never gets above 130dF and stays thicker than 30cSt.

So when an engine is 'designed' for 10cSt oil it is simultaneously being designed not to have excessive towing wear at 4cSt and not to have excessive startup wear at 30cSt.

 

[GMorg]

Mitch Alsup,

Based on your response, I would be left to conclude that engines designed for 5W30 could easily run all commercially available grades in warm climates. From a wear point of view, the engine was actually "designed" for everything on the market. Am I missing something here?

 

[Blue99]

quote:

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Can an engine even be disigned to perform optimally with an 8 cSt @ 100C oil and yet perform poorly or have excessive wear at 10.5 cSt?

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I seriously doubt whether an engine could survive if the required oil viscosity range was exactly 10.5 +/- 2 cSt.

And the primary reason is oil changes viscosity with temperature.

During operation in colder temperatures, the oil will be at levels higher than 20 cSt for 5-10 minutes or more as the crankcase oil is being brought up to temperature by the oil returning from the hot zones of the engine.

If bearing clearances or flow orifices only worked with a 10 cSt oil, how could an engine survive without a pre-heater to get the crankcase oil to the correct viscosity/temperature?

 

[Curious Kid]

Perhaps giving the one recommendation of 5w30 is thought the "sweet spot" by the manufacturer for the region and for the "intended" use of the vehicle by it's design layout right off the assembly line for the average masses. I sense vehicles as a comodity more and more, whether from the new skin on and old frame to less technical and sometimes over simplified glovebox manuals. Got problems...take it to the dealer for service.

- volvo dealer noted that most money was made in service not sales, so there's a special interest as well.

Interesting questions that I think would require great detail to satisfy I sense - bearing surface materials and type, oil and additive properties, etc.

 

[427Z06]

quote:

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Originally posted by GMorg:
...I think that either EPA required that recommendation based on CAFE...

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The EPA doesn't require any manufacturer to use any particular oil. They do require the manufacturer to recommend the oil the manufacturer chooses to use to meet EPA testing requirements.

 

[GMorg]

427Z06, I agree with your explaination. I didn't realize that my effort had been too brief.

As for Curious Kid's "sweet spot" explaination, I would further elaborate to say that the sweet spot is the oil that covers the masses regardless of region or use. For example, I would not recommed 'round year usage of 20W50 in my GM in Quebec, but I bet it would be fine from a wear perspective in Houston.

 

[another Todd]

So maybe we should use the widest spread oil we could find (I'm in so. cal) say a 5W-50 for me and a 5W-40 or 0W-40 in cooler climates.

 

[Titan]

It's not that simple, Todd. Higher spreads in viscosity can mean more Viscosity Index Improvers, or Pour Point Depressants. Those things shear to some extent, and they aren't necessarily lubricants anyway...so, you would probably be better served to use a oil minimially packed with either VII's or PPD's, that is specified by your manufacturer for the conditions you will be operating your vehicle.

 

[hominid7]

Quote:

---

So, my manual recomends only 5W30. All that I can conclude is whomever authored my manual offered only single recommendation. I think that either EPA required that recommendation based on CAFE or GM thinks that their U.S. customers are idiots and cannot deal with choices. I am prepared to flip a coin and accept either possibility.

---

I believe that a car maker has to recommend the type of oil they use when obtaining the gas mileage printed on the window sticker. They can of course list alternatives that are allowable for various temperatures.

My car is a 2001 but has an engine that has been in production since 1993 and you can bet the viscosity rec's have changed over the years. There was an oil pump change in 1997 i beleive, and after that 5w30 was allowed year-round by Ford.

My also has a specific recommendation NOT to use 5w20. So that tells me that SAE30 is the minimum viscosity allowed in the engine under operating temperatures. That would also tell me that one step high (SAE40) would not hurt anything.

I'd be hesitant to use 20w50 although some with my same engine have and so far their engine has not seized as a result.

My previous car listed every oil made in the owners manual from single viscosities to multi viscosities. However it only allowed 5w20 and 5w30 up to 20*F, allowed 10w30 and 10w40 up to 50*F and 20w50 up to 104*F. Over 104 and they recommended single weight SAE40 or SAE50. Granted this was a 1989 owners manual and a lot has changed since then.

Todays DOHC engines require a lot more top end lubrication than did SOHC or flat engines of yester-year. In addition thin rings, limited displacement blocks with large bores with multiple oil jets all need an oil that can flow FAST at startup, especially when it's cold outside.

A few years back 10w30 was seen as the overal universal oil for passenger cars and today i think we could safely say that 5w30 is as close to universal as we have now in this country. With the exception being that 5w40 will work in *most* engines and is used big-time overseas.

 

[MGBV8ROADSTER]

Many groups have accurately estimated the minimum oil-film thickness in an elastohydrodynamic contact. However, estimating the friction coefficient has proved much more problematic due to the large variation of viscosity with pressure, which is not always accurately known. Recently, Laurence Scales at Shell has shown that it is worth investing the effort to find accurate relationships between viscosity, temperature, pressure and shear rate because they allow both the minimum oil-film thickness and the friction coefficient to be estimated. To do this, however, requires at least eight parameters to characterize the lubricant.
Where's the rub?
Figure 5

But by combining models for the plain bearings, the piston assembly and the valve train, researchers have found that they can model the lubrication conditions in a complete internal combustion engine. Since the bearings and piston rings are lubricated predominantly in the hydrodynamic regime, a lubricant with a lower viscosity should lead to a thinner oil film and thus lower friction. However, the valve train operates in the mixed-boundary lubrication regime, which means that lower friction can only be obtained with thicker lubricants. Engine-friction models enable us to study the trade-off between viscosity and friction, and therefore select the optimum lubricant to improve a vehicle's fuel economy (figure 5).
http://physicsweb.org/articles/world/15/2/7