Effect of viscosity on hydrodynamic engine bearing

[Shannow]

actually
Effect of oil viscosity on hydrodynamic friction of engine bearings

but had to fit it in title bar.

http://www.substech.com/dokuwiki/doku.ph...engine_bearings

Quote:
The parameters of hydrodynamic lubrication of bearings [2] were theoretically calculated using software developed by King Engine Bearings. The software, called ENSIM™, is an advanced engine simulation module. It is capable of calculating the thermodynamic, dynamic, hydrodynamic and mechanical parameters of bearing operation. ENSIM™ is used for designing new bearings and for modifying the bearings of existing engines. For the purposes of this theme paper study, calculations of the parameters of hydrodynamic lubrication were made for high performance King CR 807XPN (connecting rod bearing).



Three oil grades, (0w5, 10W30, 10W60), modelled on one of their con-rod bearings, with 0.0004", 0.002", and 0.004" diametrical clearance, at a number of RPMs.

Calculations of Minimum Oil film thickness, power loss (frictional heating), and importantly, the ratio of pressure distributions across the bearing (easier leakage means peakier pressure profiles, and less even sharing of bearing loads)

Results shouldn't come as any great surprise, but there are quite a few myths perpetrated over the years.

Quote:
Conclusions

* The greater oil viscosity the greater minimum oil film thickness, power loss due friction and uniformity of oil film pressure.
* Effect of oil viscosity on min. oil film thickness, power loss and the Pmax/Pav ratio increases with an increase of bearing clearance.
* For the oil 0W5 the greatest value of oil film thickness is achieved at clearance of 0.0004” (1/5000 of the bearing diameter).
* For the oil 10W30 the greatest value of oil film thickness is achieved at clearance of 0.002” (1/1000 of the bearing diameter).
*For the oil 10W60 the greatest value of oil film thickness is achieved at clearance of 0.004” (1/500 of the bearing diameter).

 

[d00df00d]

Nice. Thanks for posting!

 

[Shannow]

Should be read in conjunction with this Shell Motorsport paper also.

http://www.eng.auburn.edu/~jacksr7/SAE2002013355.pdf

Quote:
The most significant physical property of a lubricant is viscosity. Lubricant viscosity is strongly dependent on temperature, shear rate, and pressure. These effects are often neglected in many simulations of engine components (it is often assumed that the viscosity of the lubricant is constant at the temperature of interest). However, if a realistic assessment of friction and oil film thickness in the contact is required, these effects need to be taken into account. Figure 1 shows the typical way in which lubricant viscosity varies with temperature and shear rate. Figure 1: Variation of viscosity with shear rate for a SAE-10W/50 lubricant Table 1 shows how the viscosity of different SAE viscosity grades differs.


and a version of this chart showing with a typical multigrade how shear rate affects viscosity (think of the cold end as LTHS)

Leap to the Conclusion

Quote:
CONCLUSION In this paper I have attempted to demonstrate that the physical properties of a lubricant are extremely important in determining both the minimum oil film thickness in key lubricated contacts, and the associated friction losses. It is important to use models for the piston assembly, the engine bearings, and the valve train, that include accurate information about how the lubricant viscosity varies with temperature, shear rate, and pressure. We have compared a conventional gasoline engine with a high performance Formula 1 engine, and shown that matching the lubricant to the engine can lead to significant reductions in engine friction, with consequent improvements in the available power to the wheels.


On the same topic as the first article, here's the bearing stuff...

Quote:
In addition to oil film thickness, it is also important to know what flow rate of oil is required to lubricate the bearings. If the flow rate is not sufficient, lubricant starvation can occur which may lead to catastrophic damage to the bearings.

 

[Kawiguy454]

One thing that must also be understood about such tests is that an engine has multiple bearings and that if any single bearing is over tolerance oil pressure drops not only in that bearing but in all bearings that have parallel oil supply passages. As the viscosity decreases so does the safety margin for all not just the marginal bearing.

 

[Kamele0N]

I am adding also this link to the chat....

http://www.sveacon.se/lectures/fe_bizol.pdf

 

[akela]

Interesting that the flow rate doesn't change that much with viscosity.

IIRC the flow has to be inversely proportional to the square root of viscosity, which kinda agrees with these numbers.

 

[JHZR2]

Some of this is relatively simply calculated. Granted, the simplistic versions aren't what turns into PhD dissertations, but a lot of the fundamentals can be better understood with a few (validated) design equations.

I took a good short course by Dr. Khonsari (LSU) a few years back, and we went through all the relationships and how they all play in the design equations.

This is a great book if you're interested:

https://www.amazon.com/Applied-Tribology-Bearing-Design-Lubrication/dp/0470057114

 

[Shannow]

At Uni, I had Shiggley, but being in imperial units made translation through an Australian engineering degree not the most fun. So I bought a set of Alternative Russian text from a guy called Orlov, which had the same stuff, in (Russian) metric, and looked at from a slightly different sngle.

There's not a bad one in this thread.

https://bobistheoilguy.com/forums/ubbthreads.php/topics/4225640/Bearing_Gold_Mine,_NASA_paper_#Post4225640


But as to the classical tables, like below, they are all based on Newtonian fluids, i.e. straight grades that don't have a shear/viscosity curve, and there's no real means of translating High Shear viscosity (easily) into them...

The second of the papers I linked took not only that, but the pressure/viscosity characteristics into account...pretty sure you need a machine for those calcs really.

 

[DeepFriar]

My ignorance is showing but does Table 7 say that at no point does the average friction power loss exceed one (1) horsepower? One horsepower = 750 watts +/-? I was just trying to draw inference to fuel economy loss due to higher viscosity.

 

[Shannow]

Per bearing.

 

[DeepFriar]

Yes. So less than 8HP loss in a V8 from the big end at 7500RPM? Sorry, I don't mean to be a pain. I'm just getting the feeling I don't need to worry very much about my predilection for heavier oil.

 

[DeepFriar]

Would the main bearings perform more or less the same scaled up for size of course?

 

[Shannow]

Originally Posted By: DeepFriar
Would the main bearings perform more or less the same scaled up for size of course?


It depends...

Here's a couple of pics I particularly like from another paper...

Why do I particularly like them ?

because it shows the heat flows being generated IN the bearings, and moving FROM the bearing shells into the Rod big end, and into the block (and then onto the cooling system).

Popular myth/posit is that the flowing oil is carrying away heat from these components, when it is actually HEATING them...the heat that it carries out of the bearing is entirely it's own.

 

[zeng]

Thanks for posting.

Hmm..... trying to figure out what's being transpired, though.

 

[Solarent]

Quote:

Conclusions

* The greater oil viscosity the greater minimum oil film thickness, power loss due friction and uniformity of oil film pressure.
* Effect of oil viscosity on min. oil film thickness, power loss and the Pmax/Pav ratio increases with an increase of bearing clearance.
* For the oil 0W5 the greatest value of oil film thickness is achieved at clearance of 0.0004” (1/5000 of the bearing diameter).
* For the oil 10W30 the greatest value of oil film thickness is achieved at clearance of 0.002” (1/1000 of the bearing diameter).
*For the oil 10W60 the greatest value of oil film thickness is achieved at clearance of 0.004” (1/500 of the bearing diameter).


Yet another reason in my mind for the future of engine oils to be co-developed with equipment. New developments in oils allowing for reduced clearances which can contribute to engine downsizing and better fuel economy. This article only talks about the viscosity side of the equation. Most mechanical engineers I know don't even think about lubricants beyond that. It would be nice if they started thinking about how tribofilms could also impact their design parameters - ie planning to use a specific chemical set to prevent wear mechanisms on advanced metallurgy.

 

[Shannow]

Solarent,
while I agree, there's a gap between industrial mechanical engineering and automotive...e.g. a Shell paper acknowledges it perfectly.

http://www.eng.auburn.edu/~jacksr7/SAE2002013355.pdf

Quote:
The strategy for minimizing engine friction in a Formula 1 engine is fairly simple. If the engine has a lot of boundary friction, you would use a higher viscosity lubricant, with a friction modifier, whereas if the engine has less boundary friction, you would use a lower viscosity lubricant (again with a friction modifier). By looking at engine friction results for the conventional engine, it is possible to decrease engine friction by 10 or 20% by using the correct lubricant. There are a number of papers41-47 which the reader can refer to for more information on total engine friction calculations. To optimize power output from a high performance engine, it is necessary to choose a lubricant which gives the lowest possible friction. This entails choosing the lubricant viscosity which gives the lowest friction over the range of operating conditions appropriate for the engine, and choosing an optimum friction modifier for reducing friction in boundary lubricated contacts.


On my turbines, where the design life is 25 years/150,000 hours, and we usually run them for 50 years/350,000 hours, no engineer would elect to abandon full blown hydrodynamic lubrication for boundary/mixed.

Yes, dropping from ISO46 to ISO32 saves a mass of energy over the life of the machine, and that's where we went, but still hydrodynamic.

An automotive engine, going 200,000 miles (say 5,000 hours) when it will be scrapped, or a race engine lasting 10, there's obviously MUCH scope to explore tribofilms.

As I've explained previously (and been howled down many many times), the latter is an entirely fair compromise, saving the end user lifetime costs.

As an engineer, I take some umbrage in that an engineer will make these decisions based on design criteria...maintaining full blown hydrodynamic is an entirely rational decision...if made rationally.

 

[userfriendly]

I couldn't help notice that the flow rate @ 7500 rpm didn't change much with incremental grade changes to thinner engine oil.
I would assume then, that oil pressure from one grade to the next, would not change at all.

If I throw this discussion back to oil pressure vs viscosity, vs oil pump capacity and relief pressure setting, very little would change at high rpm with viscosity changes.
My take; The pump then, if properly sized for the engine regardless of viscosity, should be operating at relief pressure at high rpm.

 

[Shannow]

https://bobistheoilguy.com/forums/ubbthreads.php/topics/4227382/Re:_Couple_of_oiling_system_de#Post4227382

 

[ZeeOSix]

Originally Posted By: userfriendly
I couldn't help notice that the flow rate @ 7500 rpm didn't change much with incremental grade changes to thinner engine oil.
I would assume then, that oil pressure from one grade to the next, would not change at all.


That paper doesn't talk about oil pumps and the effect of supply pressure on bearing flow. They are just talking about the flow the journal bearing will "naturally" obtain due to rotational speed with those different viscosity oils.

You can see in Tables 7 and 8 that as the oil viscosity does down, the "natural" oil flow rate of the bearing goes up, and the minimum oil film thickness goes down. Happens at both the 2500 and 7500 RPM test point.

Originally Posted By: userfriendly
If I throw this discussion back to oil pressure vs viscosity, vs oil pump capacity and relief pressure setting, very little would change at high rpm with viscosity changes.
My take; The pump then, if properly sized for the engine regardless of viscosity, should be operating at relief pressure at high rpm.


If you start putting a higher viscosity oil through a PD pump feeding the oiling circuit, you will definitely see a higher oil system pressure reading. Oil pump design (volumetric output and relief pressure) has to take into account the expected oil viscosity used in the specific oiling system. As an extreme example, you really wouldn't want to run a 10W-60 in an oiling system designed for a 0W-20.

 

[Mitch Alsup]

It seems to me that really small bearing clearances are at least as good as thinner oils in saving power.

Did anyone notice a savings of 600W--this is less than 1HP.