Bearing Film Thickness

[TooSlick]

jay,

The kinematic viscosity @ 100C is obtained at a fairly low shear rate. If you look at the areas of the engine where the oil is being taxed the most, in the main bearings, between piston rings and cylinder walls and in the valvetrain area, it is being sheared at a pretty high rate. You also have some severe localized heating effects in these instances. I feel that HT/HS viscosity correlates much better to how the oil is functioning in an actual engine and to the oil pressure you will see with various formulations.

You can really spec oil on the basis of HT/HS viscosity and almost forget about the SAE grades.

TooSlick

 

[Jay]

quote:

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Originally posted by TooSlick:


...You can really spec oil on the basis of HT/HS viscosity and almost forget about the SAE grades.

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Got it. ACEA apparantly agrees since they group oils primarily according to HTHS.

 

[userfriendly]

The amount of oil flowing into the rod and main bearings has to remain at a high enough rate to maintain a film thick enough to "float" the journal so there is no contact between the bearing and the journal.
As the oil temperature and/or bearing speed rise, the flow rate away from the bearing increases.
In an automotive engine the top con-rod bearing and bottom main bearing is loaded during the power stroke.
The force on the bearings push the oil out of the clearance area.
If the oil is too thin, the pressure too high, or a combination of a lot of variables, all of the oil will be pushed out and the bearing will fail.
The oil supply to the bearing has to keep up with demand, which rises with RPM, journal size, and lubricant temperature.
Now if you can imagine if at 5,000 rpm with the oil temperature very high, there is just enough oil flow into the clearance area to prevent a bearing failure, the following will cause a engine failure:
1. A further increase in lubricant temperature, which increases the flow rate out of the bearing's clearance area.
2. Oil supply problems caused by pump cavitation , lubricant foaming, return to sump delay ect.
3. An increase in engine speed which pumps the lubricant out of the clearance area more times per second.
A high performance engine can be designed to:
1. Increase the oil supply to the bearing
2. Control the lubricant temperature
3. Control the rate the engine oil flows away from the bearing.
4. Design smaller journal sizes into the engines crankshaft.
5. Increase the clearance area of the journal.

Critical shear areas of an engine are mostly found in the valve train and thrust side of the piston.
Darn, the phone just rang and I have to go to work at 01:40 on a coal train..

 

[MolaKule]

quote:

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1. Increase the oil supply to the bearing
2. Control the lubricant temperature

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One and two are the same. Oil flow into and out of the bearing/journal keeps the bearing and journal from overheating. Lowering the oil temp also helps to maintain viscosity and keep a somewhat constant oil film.

In the days before forced lubrication, the only thing lubricating the bearing/journal was the natural pumping action of the oil wedge. But in those days you had large clearances, large bearing areas (to support the loads), and low rpms. In order to reduce bearing areas (lower bearing size to make a more compact engine) and increase rpms, you forced oil into the bearings to maintain a flow necessary to keep the bearings from self destructing. Forced oil also allowed a cleaner oil, since pressurized oil could then be filtered through a filtering medium.

BTW, love old trains. My dad once worked in the rail yards for Illinous Central Railroads, first rebuilding steam locomotives and then dismantling them to make way for the diesels.

I used to stand in awe in the shop as he would fit the large steam engine wheels to the axles and mate the connecting rods.

There was something about the sound of the "chug chug" of the steam locomotives that diesels just dont have.

[ September 27, 2003, 11:11 AM: Message edited by: MolaKule ]

 

[TC]

You guys MUST stop by the Transportation section of your nearest mega-bookstore and look for any books containing photographs by O. Winston Link. The guy took some incredibly awesome and nostalgic photos of steam choo-choo trains back in the late 50's as that rail era was fading. One such book I have with Link's photos is "Steam, Steel & Stars," readily found at Amazon.com. Some of his photos have been featured in New York's Museum of Modern Art -- they're that impressive.

 

[MolaKule]

Thanks TC. I love old machinery, especially steam locomotives and farm tractors.

 

[jason07-1zzfe]

Isn't 250F pretty high for a bearing input temp? That is basically the bulk temp as I don't see how it can heat up much just going through the pump through filter and to bearing.
So in real life the 20wt will provide about the same thickness as a 40wt in this test.

 

[userfriendly]

I agree, at 250F there is very little difference in viscosity between an SAE 20 and an SAE 40 engine oil.
For high loads and or RPM with elevated engine oil temperatures, your engine better have a good oil pump to keep the rod and main bearings supplied.
The old 10 lbs min for every 1000 RPM (Chevy V-8 thing) comes to mind in those conditions.
edit: The new 4 stroke locomotives from GE chug.
There is a new GE 440HP V12 (instead of the old V16) yet to rattle the dishes in this neck of the woods.

[ November 01, 2003, 07:59 PM: Message edited by: userfriendly ]

 

[Dave]

quote:

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Originally posted by MolaKule:
From my reading of the four papers on this subject, it appears that the minimum weight of oil that can be used in modern engines is SAE 5 weight, and the maximum weight appears to be 40 weight. I believe this is why 5W30's and 10W30's have been the most popular oils for the last three decades; they appear to to be the median weights for all around service for oil temperatures at approximately 250 F bearing inlet temps. This may be another reason why 5W40 oils are becoming increasingly popular as well.

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Could you explain what effects the maximum weight oil that can be used in an engine? When oil viscosity gets too thick, what causes the lubrication layer to break down, for example in the bearings?

 

[Callisa]

quote:

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You can really spec oil on the basis of HT/HS viscosity and almost forget about the SAE grades.

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When this report was written in 1990, Exxon statet at the same time in a paper in Germany: "It's impossible to formulate a 0W-30 oil with a higher HTHS then 2,9. It is possible today.

The reason for that paper where damages on bearings due to temporary loss of viscosity in the bearings. This lead to the "invention" of HTHS.

Today, I would rather go with performance in engine tests, not HTHS.

The additve package does the (wear) performance of a modern oil, not the HTHS.

 

[MolaKule]

quote:

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Could you explain what effects the maximum weight oil that can be used in an engine? When oil viscosity gets too thick, what causes the lubrication layer to break down, for example in the bearings?

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The higher the oil viscosity, the thicker the oil film and the higher the HTHS, the lower the wear.

The problem with high viscosity fluids (>40 weight) is that unless they have a very wide VI (usually meaning full synthetic formulation), they will not flow well when cold. So you don't get the anti-friction lubricity you need when cold.

What you want is a multivis that flows well when cold but provides a thick oil film when hot; this is where I believe 5W40's will surface as a "Heavy-Duty" weight oil, much as did 10W40's in the 60-80's.

Now enter the EPA, where they are forcing light oils on the industry to reduce fuel consumption over a whole fleet.

If you can maintain a minimum oil film thickness of >1.0 um and minimum HTHS that doesn't go below 2.6 with a low viscosity oil, AND provide good AW protection, then this oil will become the Fuel Saving oil of the future.

[ August 26, 2004, 05:09 PM: Message edited by: MolaKule ]

 

[Callisa]

quote:

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The higher the oil viscosity, the thicker the oil film and the higher the HTHS, the lower the wear.

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I've seen the bearings in a Diesel engine, having run with 0W-20 oil on a Dyno. Diesel engines are most of the time more critical then Gasoline engines, due to higher pressures.

The bearings and especially the camshaft were perfect.

It's the performance that counts. Not HTHS.
You gain a little fuel economy with lower HTHS, that's why there is an industry trend over the last years towards lower viscosities.

Having higher HTHS does not mean automatically lower wear. The oils improved a lot since the invention of HTHS in the beginning of the '90's.

Stay curious.

 

[Ethan]

Molakule,

You wrote:

quote:

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From my reading of the four papers on this subject, it appears that the minimum weight of oil that can be used in modern engines is SAE 5 weight, and the maximum weight appears to be 40 weight.

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It doesn't say that 50 wt oils were tested, but could you speculate as to what results for the 50 wt. oil would be? Aside from improper cold flow and too much VII, at what point is too thick no longer too good?

Ethan

 

[MolaKule]

For 50 weights (and we'er talking about daily drivers and light truck vehicles here), the oil would probably produce a high viscous drag in the bearings which would cause localized heating. Eventually, the oil would thin, reducing the oil film in the bearing. In addition, an oil film need only be as thick as the lowest clearance.

1. When the flow of oil is reduced to the point that parts cooling is inefficient. Thick oil flows slower than thinner oil. The slower the oil flow, the less heat carried off by the oil. This also means (as in 2.) that the oil temp is going to rise locally; hot spots will develop. The oil may actually boil and evaporate at these spots.

2. When larger fractions of engine horsepower are turned into heating the oil instead of going to the rear wheels.

In bearings, whether sliding (pistons and liners) or rotating (as in journals), the two main requirements are friction reduction and cooling. If hot spots develop, the oil film thins considerably. So there is tradeoff between max oil film and cooling efficiencies.

This rule of thumb still applies:
Thick oils for low rpm, highly-loaded systems, and large clearances. Thinner oil for high rpm, lower loaded systems, and those with tighter clearances.

 

[userfriendly]

This thread took about 30 minutes to read. Very captivating.
One member stated that an oil could be classified by its HTHS value.
That was 11 years ago, and we are seeing more of HTHS being referred to than ever before.
Honest Engine predicted that 5W40 would become a standard HDMO in the future. Also true.
And, an ester blended in with full synthetic (PAOs were the "synthetic" in topic)will cause it to act non-newtonian.

It was not stated, but everyone knew about rod angles and other engine basics in the thread.

Crank journal finish from a high polish to matte is a change from 2003 in some engines.

What else has changed since this topic was first posted?

Does everything talked about still hold true?

 

[Jetronic]

how would the hths viscosity differ between

a pure mineral
a pure PAO
a pure POE oil

of the same kv100 (4-ish, a thick 5w as that's the minimum the test engine could run).

I know pure single group oils are utopian, but just wondering. I'd expect a HTHS of no more than 1.5?

 

[MolaKule]

Hmmm.

I think the esters would beat all the others in a 4 cSt@100C fluid.

As for the esters, it would depend on the type.

I would think a pentaerythritol ester would have the highest HTHS but I haven't seen any data on that ester alone.

Besides, most HTHS testing is done on a formulated lubricant using a mix of various base oil viscosities and various base oil types with a PI package.

In fact, I am not sure HTHS testing for any of those base oils by themselves would be meaningful.

 

[Shannow]

Given that for the mineral oils (and I expect the PAOs), they are Newtonian, their KV150 should be the same as their HTHS.

Multigrades the KV150 is higher then the HTHS, like the KV100 is higher than the HTHS100 (see the supertech VOA thread).

Spectrasyn 4
http://exxonmobilchemical.ides.com/en-US/ds244622/SpectraSyn%E2%84%A2%204.aspx?I=30156&U=0

Has a VI of 126, and an HTHS of 1.46.
A mineral will obviously be a little bit less than that, but the VI of 100 to 126 isn't going to make a wild difference.

As Molakule suggests, the parameter is of limited use in discussion without being a formulated oil

 

[MolaKule]

Originally Posted By: used_0il
This thread took about 30 minutes to read. Very captivating.
One member stated that an oil could be classified by its HTHS value.
That was 11 years ago, and we are seeing more of HTHS being referred to than ever before.
Honest Engine predicted that 5W40 would become a standard HDMO in the future. Also true.
And, an ester blended in with full synthetic (PAOs were the "synthetic" in topic)will cause it to act non-newtonian.

It was not stated, but everyone knew about rod angles and other engine basics in the thread.

Crank journal finish from a high polish to matte is a change from 2003 in some engines.

What else has changed since this topic was first posted?

Does everything talked about still hold true?


If you are referring to post #530360, the physics has not changed.

What has changed are better additives and base oils for lubricants, better finishes and anti-friction coatings for engine internals, and more accurate engine controls such as coolant temperature control.

 

[userfriendly]

"The higher the rpm, the thicker the film from any
of the mono-grade oils".
(From post #530360)

I think I know now why you said that, for an engine
oil to act like a multi-grade, it must contain VIIs.

Are the following statements true?

The thicker the oil film, the lower the shear rate
and fluid friction.

A mono-grade oil would heat up less than a VII containing
multi-grade oil during it's duty cycle in the bearing space.

VIIs were fully activated at 120C, but lost viscosity due
to temporary shear under stress.

The Illinois Central was purchased by www.CN.ca