Pressure vs Flow Regarding Wear Revisited

[AEHaas]

I finally got to review an article provide by wileyE on this thread:

http://theoildrop.server101.com/forums/showflat.php?Cat=0&Board=
UBB1&Number=926013&Searchpage=1&Main=924130&Word
s=http%3A%2F%2Fw3.wtb.tue.nl%2Ffileadmin%2Fwtb%2Fct-pdfs%2
FMaster_Theses%2Feindverslag_Edward_Slaats.pdf&topic=&Search=true#Post926013

(I broke the above link into several parts so as not to make too long a link. You have to make it into 1 line)

Here is the article: Oil Flow in Dynamically Loaded Plain Bearings, April 2007, Ing E.J.M. Slaats:

http://w3.wtb.tue.nl/fileadmin/wtb/ct-pdfs/Master_Theses/eindverslag_Edward_Slaats.pdf

I am still studying this bearing flow dynamics paper. But here are some initial observations:

They state that the oil hydrodynamic (HD) pressure is the sum of the oil feed pressure and the intra bearing HD pressure. Since the system feed pressure is so low compared to the HD pressure, the bearing Force is the same even with a system feed pressure of zero. This goes along with what I have said in the past. The oil pressure is not the determining factor for lubrication in a bearing. The effect is basically zero. The pressure only exists to move oil into the bearing as it is lost from the side of the bearing.

My comment - As long as the system pressure is 10 PSI for every 1,000 RPM then oil flow should be sufficient to keep the bearing wet in a properly working SI engine.

I cannot reproduce the equation here but see item 2.4.1. In order to keep the force, F, in the bearing a constant value while increasing the clearance, C, (more leakage of oil) you need to logarithmically increase the oil flow, Q. Note the multiple powers of the clearance, C, in the numerator. I also read this as a 1:1 ratio of force, F, to flow, Q. This is the best I can do to reproduce this equation:

Q Flow = nsides X FC/2u X (C/R)squared X 1/(B/D)squared X M

R is the bearing radius, B is bearing width, D is bearing diameter, M is the squeeze velocity, u the kinematic viscosity.

My point is that the oil pressure has nothing to do with lubrication or wear in a bearing.

Viscosity and film thickness are still important because of the spacing needed for dirt, soot and to keep the asperities separated. The paper did note that the combined roughness of the journal and bearing made oil film thickness a factor. But after the break in period, there was much less film thickness needed. They stated the roughness value for the bearing surface actually becomes zero so it is only the journal roughness that comes into play after this time period.

What goes against this is that many new cars are supplied with a thinner than specified oil grade than placed into the engine as OEM. Maybe they want a lesser film thickness than is needed during break in to help smooth the surfaces.

aehaas

 

[G-MAN]

Quote:

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What goes against this is that many new cars are supplied with a thinner than specified oil grade than placed into the engine as OEM. Maybe they want a lesser film thickness than is needed during break in to help smooth the surfaces.

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One of the things mentioned in the Toyota press releases for the new Lexus LS460 is that the bearing surfaces were micro-polished so that 0w20 oil could be used.

 

[mechtech2]

If you state that an engine needs 10 lbs for every 1000 RPM, how can you also state that oil pressure has nothing to do with lubrication or wear in a bearing?

 

[AEHaas]

Quote:

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If you state that an engine needs 10 lbs for every 1000 RPM, how can you also state that oil pressure has nothing to do with lubrication or wear in a bearing?

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The purpose of the oil pressure is to move oil into the bearing when rotation of the shaft causes an oil loss secondary to the clearance. There is also a need to spray oil into several locations. To keep the bearing supplied with oil there must be oil movement. In order to have movement there must be pressure from the pump or a vacuum at the bearing site. In actuality a vacuum is present in many bearings. Although this may provide the necessary fluid for a film thickness the other function of oil is inadequate. The other function is to cool bearings. More flow equals more cooling.

The false thinking I am trying to dispel is that many people say:

"I have very good, high pressure, therefore I have the best lubrication and wear protection"

The thinking is that there will be less wear because there is more pressure. Wear and pressure are unrelated. It is the flow that counts.

aehaas

 

[AEHaas]

More:

They infer that wear (of main bearings) does not occur under normal operating conditions. Wear occurs when the load and oil sump temperatures are high and even at high loads under low RPM situations. They repeat that wear is also a function of the start up period.

They state that when the hydrodynamic pressure increases under high engine loads the oil viscosity increases. This would of course contrast to the shear thinning of oil with bearing velocity. Oil viscosity is not only shear but also pressure dependent. They say film thickness increases with increassing peak pressures because of the Elasto-hydrodynamic behavior of the oil.

It seems they study only the effects of hydrodynamic lubrication. When the film thickness decreases, the additive packages of the finished oil lubricant come into play. They do not go into this aspect of lubrication and wear protection.

What is not explained is why there is start up wear. They state that as long as the film thickness is 3 times the roughness there is no wear. Yet the oil is very thick at start up (relative to operating oil temperatures) and there is the highest level of wear. This was not addressed in the paper.

aehaas

 

[Tempest]

If my understanding of how an engine works is correct, it takes time for the pump to build pressure (a matter of seconds, but still time) so there is no or little oil on the bearing surfaces until the oil pressure builds. Dry surfaces = wear.

 

[AEHaas]

"there is no or little oil on the bearing surfaces until the oil pressure build"

Oil is present in all journal bearings. Wear immediately after start up is not the issue. The wear from start up from 75 F to 200 F oil temperature is start up wear. Start up wear lasts 20 to 30 minutes and the problem is not fully understood although it is being studied.

aehaas

 

[bruce381]

would think most wear is before oil is hot for additive reactions and most wear is from sliding surfaces BOT bearings.
bruce

 

[bruce381]

ment NOT

 

[RI_RS4]

Quote:

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My comment - As long as the system pressure is 10 PSI for every 1,000 RPM then oil flow should be sufficient to keep the bearing wet in a properly working SI engine.

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AE, one question. Why? I've seen this "rule of thumb" floated around the internet. What is it's basis in engineering?

 

[rriddle3]

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Start up wear lasts 20 to 30 minutes and the problem is not fully understood although it is being studied.
aehaas

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After 100 years you would think this answer would be known. I sometimes wonder if we don't overthink questions. We still don't know precisely why aspirin works like it does, but we just accept it.

 

[AEHaas]

"What is it's basis in engineering? "

It is a rule of thumb that is many years old. The principal is that some pressure is needed to make sure oil is delivered to the bearing at all times. If the bearing clearance is too large from wear as in an older engine then a thicker oil would be needed. The viscosity chosen is wear dependent and self correcting with this formula.

More pressure than needed to deliver oil to the bearing is a waste from parasitic losses. You need the viscosity that your engine and driving habits dictate. This is the lesson in my chapters.

aehaas

 

[RI_RS4]

AE, yes, again what is it's basis in engineering? Why not 5 psi per 1K rpm, or 15 or 7? And what happens in 8K rpm engines. Do they require 80 psi?

As an engineer myself, I hate rules of thumb. In electrical engineering they usually prove to be incorrect when subjected to scrutiny. In these cases, the original assumptions often prove to be either false, no longer operative, or just poor approximations. I just recently researched the origin of formulas and tables for the heat rise of copper traces on printed circuit boards, which have been published for years. It turns out that the accepted wisdom, which had been codified in specifications, was just plain wrong, due to misunderstanding of the original National Bureau of Standards research in the 50's.

It's one thing to say that an engine requires proper oil pressure to lubricate the bearings. It's quite another to authoritatively say that it requires 10 PSI/1000 rpm. Just because this particular "rule of thumb" has been repeated ad infinitum for years does not make it so.

I suspect that there are also other considerations, besides flow, that might require even higher oil pressure. One example might be hydraulically actuated lifters, tensioners ... ect.

I'm not trying to be argumentative here. You are just repeating something that has been repeated over and over again, without the original justification, and assuming it is absolutely correct. Besides lubricant flow, there may be hydraulic, thermal, or other considerations in play that we do not totally understand, not being the designers of a particular engine.

 

[TallPaul]

I find it hard to believe start up wear lasts 20 to 30 minutes. I would think in summer, start up wear lasts little longer than for oil pressure to come up, or several seconds. In winter, I could see start up wear lasting minutes and on a 0F morning with 10w40, perhaps 5, maybe 10 minutes at most.

If start up wear really lasted 20+ minutes, then there are a lot of cars out there that almost always are in start up wear and still last a long, long time. Therefore, startup wear is insignificant. Well it is insignificant anyway. Engines routinely last well over 150 to 250 thousand miles when not abused.

Probably worse than start up wear is operating the engine with a cold block. Yeah, really it is probably the cold block and its out of proportion dimensions under severe cold that contributes more to wear, especially in the cylinder.

 

[Tempest]

Quote:

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Start up wear lasts 20 to 30 minutes

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Must have missed that one, got a link?

 

[RI_RS4]

Quote:

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Quote:

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Start up wear lasts 20 to 30 minutes

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Must have missed that one, got a link?

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AEHaas Post on Startup wear

 

[Steve S]

Quote:

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"What is it's basis in engineering? "

It is a rule of thumb that is many years old. The principal is that some pressure is needed to make sure oil is delivered to the bearing at all times. If the bearing clearance is too large from wear as in an older engine then a thicker oil would be needed. The viscosity chosen is wear dependent and self correcting with this formula.

More pressure than needed to deliver oil to the bearing is a waste from parasitic losses. You need the viscosity that your engine and driving habits dictate. This is the lesson in my chapters.

aehaas

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Pressure is the resistance to flow,so the rule of thumb is that with the ballpark pressure there should be enough flow. Take an oil pump and hook up a drill motor and pump oil ,lots of oil will move but there will be little if any pressure.Put the pump into the motor then there will be a restriction then pressure. Take an engine with minimal bearing clearances and measure the oil pressures . Take the same engine and increase the bearing clearances to the looser end or more of the manufactures tolerances and the oil pressure will drop

 

[Dad2leia]

Question though: If the start up wear is indeed until the oil gets up to optimal lubrication temperature, how low of a 40C cST number could one go, and still keep the SAE grade desired, as the measured temperature for the grading is at 100C? Is there a way to calculate the grade based on the lower temperature vs. the higher temperature reading?

 

[Auto-Union]

"Viscosity and film thickness are still important because of the spacing needed for dirt, soot and to keep the asperities separated. The paper did note that the combined roughness of the journal and bearing made oil film thickness a factor. But after the break in period, there was much less film thickness needed. They stated the roughness value for the bearing surface actually becomes zero so it is only the journal roughness that comes into play after this time period.

What goes against this is that many new cars are supplied with a thinner than specified oil grade than placed into the engine as OEM. Maybe they want a lesser film thickness than is needed during break in to help smooth the surfaces"

This was what I was thinking as I put higher visc into my last 2 new cars for break-in. I then will step down the visc over a few changes.

 

[Steve S]

Quote:

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Question though: If the start up wear is indeed until the oil gets up to optimal lubrication temperature, how low of a 40C cST number could one go, and still keep the SAE grade desired, as the measured temperature for the grading is at 100C? Is there a way to calculate the grade based on the lower temperature vs. the higher temperature reading?

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The metals are expanding the clearances are looser in a cold engine when the engine reaches steady state operating temps the pistons are round the heat activated antiwear additives are at their best.There is less blow by the air feul mixture is leaner than startup etc. It is not only the oil.