Motor Oil and Cavitation, Starvation

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Let’s talk about cavitation.
I have been trying to convince people that oil that is at a temperature less than that of normal operation is bad. People often say that oil when at 75 F and 200 cS thick is good for their engine. I say that oil starvation occurs. This is by the process of cavitation. The oil, even at 75 F is too thick to move through the oil ways at speed.

The system is designed for an oil of 10 cS thickness. A vacuum occurs in the pump and oil is not pumped. The gears simply rotate without oil movement. Furthermore there is a lot of heat generated that occurs locally that is detrimental to the metal parts and to the oil in the area causing oxidation, thickening and sludge formation. Many state that the difference in oil thickness between 3 and 4 cS is a BIG difference. Why then is the difference between 3 and 200 or 300 not a logarithmic difference for those small oil ways?

Our nuclear submarines have the same problem. The propellers can actually rotate so fast that cavitation and heat generation occurs. When needing to travel at very high speeds (60 MPH) they must be at a certain depth (deeper equals cooler water) so that the water can cool the propellers. Otherwise they melt (and become ineffective for propulsion as well). There are maximum speeds for the given temperature they are operating at.

People only think temperatures well below freezing are a problem for oil but it is also ROOM temperatures that are bad. It is my feeling that oil below 50 C (122 F) cavitate at more than 1,000 RPM and hence cause the sever wear that occurs at start-up. Only oils that are below 15 - 20 cS can be moved without cavitation in automotive applications.

This is why I pay more attention to pour point and viscosities at 40 C than all other people involved in the oils discussions here.

aehaas
 
If oil starvation was occuring, wouldn't you see a loss of oil pressure? I think your spot on when it is like northern Canada in winter cold, but 75 degrees F?
dunno.gif
 
I do not think it is as extreme as you sugest. It is fairly easy to put some M1 15W50 or Castrol 20W50 in a small block and take the intake manifold off and have a look. Once you start cranking a massive volume will start flowing out the lifter gally/bore. If you remove a lifter you can really see this. I have run to many OHC Toyotas with the valve cover off to show people how an OHC engine works. Oil flow is tremendous even with Castrol GTX 20W50 in them! We are talking about 40-50 weight multi viscosity oils right?

I have never found an oil that did not flow well at 50F and above! We even have Austrailian brethren that run 25W70 and they do not apear to have excessively early wear out from this insanely thick oil! Even if cavitation is occureing unless it causes a loss of oil pressure below the minimum required I doubt you would ever see it show up as premature wear!

I do think that localized hot spots and slow flow could result in more varnish and sludge in key area's if the oil never reach's operateing temp. prior to shut down. I was taught that in order for a thick oil to be worth while it has to warm up enough to enter laminar flow in the bearing area. This is all old school thinking though.

I also think that Castrols recomendation of 10F as the safe limit for their 15W40 Blend right on the lable says a lot about this. If you consider modern 5W40's and 0W40's then we are talking an all together diferent ball of wax!
 
quote:

Answers:

1. The temperature for an HTHS measurement is done at 150C or 302 F, which is the average peak temperature likely to be encountered in a bearing.

2. The oil is mechanically sheared at a rate of 1 x10^6 shearing operations/second.

3. Minimum Oil Film Thickness measurements (MOFT) of operating engines did not correleate well with actual wear in service. A method was devised by which the oil temperature would be elevated to worst case and sheared to determine the optimum viscosity which better correlated with wear.

Three Exxon Researchers found that a minimum HTHS of about 2.8 mPA.s was the MINIMUM HTHS viscosity needed for normal wear, with the higher the HTHS being better for minimum wear. IN general, the higher the viscosity, the greater the HTHS.

For example, in a fleet of taxicabs using a GM 4.3L V6 engine, if the HTHS was 2.35, the startup film was 0.097um and 2.56 um at running; if the HTHS was 2.98, cP, the Startup oil film thickness was 1.231 um while the running film thickness was 3.22 um.

In Dynomometer wear tests using four GM 3.8L engines, the wear mass of a connecting rod bearing was as follows:

HTHS 2.1
mass loss (gm.) - 190

HTHS 3.2
mass loss (gm.) - 28

For "mains" bearings:


HTHS 2.1
mass loss (gm.) - 150

HTHS 3.2
mass loss (gm.) - 40

A jump in HTHS by about +1.5 results in approximately 1/5 the wear. Now this relationship is not linear and flattens as one nears a 40+ weight oil.

I should also mention that this test showed little differences in wear between a high quality 5W20 and a 10W30 for oils of close HTHS. For example, The average wear of one of the 3.8L V6's showed a total wear of the Connecting Rod bearings as 48.4 grams for the 5W20 verses 44.3 grams for the 10W30. For a 10W40 oil, the wear was 39 grams!!!

[Mola's comment: I think this test verifies my earlier comments that most daily driver engines can use any oil from a high quality 5W20 to a 15W40 fleet oil.]

Summary: It was found that HTHS correlates better with wear values found in actual oil analysis and actual tear-down measurements than does measuring the oil film thickness in situ.


I too think that viscosity at any given temp. while a factor is not the most important! I think that when compareing wear HTHS and MOFT are the most important things we consider wear rates with different viscositys. Redlines 5W20 has an HTHS of 3.3 while M1 0W30 has an HTHS of either 2.9 or 3.0. SO I would expect the Redline 5W20 to perform better so long as presure adn flow were within spec. If a vechile's oilpump can not maintain adequate flow and presure then the thin oil even with the higher HTHS is not going to do you any good!

I belive that TooSlick's poor results with 5W20 were do to flow and presure issues in a system not designed for 5W20. DC had to re-clearance their G-Rotor oil pump on the 4.7 and 3.7 from it's origanal spec. whena at the last minute they decided to make 5W30 the prefered grade instead of 10W30. The rest of the engine remained the same.
 
aehaas, your diatribe above is riddled with false premises, incorrect conclusions and a lack of understanding of several physical/chemical processes. I'd correct them for you, but then, I'd want a percentage when you publish your book.

Although, I have to admit, it's a pretty slick idea to have everyone here help edit all the errors out of your copy so you might actually end up with something worth publishing in the end.
 
I like the syringe analogy.

I found the cavitation and temperature dependence of submarine propulsion, amoung other things, very interesting. I had dinner Sunday with a.....
I guess I really cannot say. The person has submarine knowledge, only some that I was able to be told. We talked about rheology and of all things - nut and bolt science - all very important in this person's line of work.

aehaas
 
quote:

A vacuum occurs in the pump and oil is not pumped. The gears simply rotate without oil movement.

Well, yes, but...

This viscosity probably is more in the several thousand cSt range.

I think many of us, with industrial hydraulics experience, realize that even a simple gear pump can handle a wide variety of temperature related viscosities without cavitating, if the lift for prime is not excessive.

I agree with 427Z06, some of these theories seem to be laid out more as "trial ballon concepts" rather than from a factual basis.

Makes for good discussions, as long as we all are on the same page.
smile.gif
 
quote:

Originally posted by AEHaas:
I found the cavitation and temperature dependence of submarine propulsion, amoung other things, very interesting. I had dinner Sunday with a.....
I guess I really cannot say. The person has submarine knowledge, only some that I was able to be told. We talked about rheology and of all things - nut and bolt science - all very important in this person's line of work.


Yep...and I have a buddy who knows where all the Aliens are stashed in Area 51.
rolleyes.gif
 
quote:

Yep...and I have a buddy who knows where all the Aliens are stashed in Area 51.

YOU SAID YOU WOULD NEVER MENTION THAT!!!!!!

Gosh ..why did I trust you??
pat.gif


quote:

The person has submarine knowledge,

One old friend was an ex-submariner out of the nuke program. Oddly enough his degree was in marine biology ..but apparently he was of the "right stuff" to work for an engineering firm that did mods on Limrick Nuke plant (as well as many others). We never talked of his naval experience other than his distractions when on liberty. In fact ..beyond a few talks on principles of steam ..we didn't talk much about nukes either. Maybe containment embrittlement due to neutron bombardment...
 
I don't have any knowledge about subs, but besides the temperature at depth I would think the pressure would have an impact on cavatation. As far as its application to oil pumps, I would think there is a wide variation in pumps and it is possible that some pumps could be affected, but there is a large difference in volutility between oil and water. If you ran the engine at a very high speed (WOT) when the oil was cold, maybe. I don't know the relationship between engine speed and oil pump speed, they are driven by a gear off of the crank??, but at what speed. A very fine oil pickup screen would have an impact too. Somewhere I read that Hondas used a very fine screen, don't remember if that was on this site...
dunno.gif
 
I believe the best response when faced with potentially severe cavitation is daily use of an ADA approved dentifrice...

PS When thinking about cavitation problems with a submarine screw, think increased depth = increased water pressure...
 
Cavitation does not cause bulk heating of the sort that would melt impellers etc.

A cavitation bubble is a tiny tiny bubble, which as pointed out previously forms when the local pressure drops below the vapour pressure in a locality (pull apart a big industrial pump, and you'll see very localised cavitation.

When the bubble collapses, it can create extraordinary pressures (and temperatures) locally which can blow a small (read tiny) chunk of metal out of the impeller/casing pipe whatever.

Repeat this a billion times, and noticeable amounts of metal go missing.

Yep, increasing pressure (not reducing temperature) by going deeper reduces cavitation, that's why NPSH is critical to pumps in industrial (and military) applications.

Interesting that temperatures can get so high that cavitation bubbles can emit light under certain conditions (and maybe allow fusion reactions to occur).
 
quote:

I dont understand that at all!!

Think of a large syringe trying to suck up 50 weight that you left in the freezer over night. If there was any air in the the syringe ..or if the O ring seal wasn't strong enough, the piston may move ..but may either have a vacuum in the chamber..or air entering from around an imperfect seal. No liquid would be drawn into the syringe other than that which is induced due to vacuum. If you move on to a check valve (reed valve) system and have the piston of the syringe reciprocate, you would end up with a piston travelling up and down in its bore with a varying air bubble in it and no flow being produced.

This is sorta the concept trying to be relayed here.


I wasn't aware of the heat aspect of high speed propellers due to cavitation. I was aware that screw wear was a byproduct of cavitation ..that the slamming of the rejoining resulted in erosion of the vane.


I don't think that this is a major issue ..even below 75°F (or 30°F for that matter). Oil pumps don't appear to be cavitating to any consequence in long term wear whether they use 20 weight or 40 weight (or for our OZ friends, 60 or 70 weight).

Now if you want to assure that you suffer less from this effect ..then sure ..you will be moved closer to "perfection" ...but I think it's like trying to achieve the speed of light when you're already at .999LS. Achieving half the distance to the goal line just doesn't get you much progress above where you're already at.
 
quote:

Originally posted by AEHaas:
I like the syringe analogy.

I found the cavitation and temperature dependence of submarine propulsion, amoung other things, very interesting. I had dinner Sunday with a.....
I guess I really cannot say.
aehaas


With a......alien? Must'a been, as all the nukiepoos, sonar girls,and other folk who either thrill to the kill, or hide with pride around these parts have titles and job descriptions. I'm quite sure the classifed stuff stayed that way, so you can spare the hyperbole.
 
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