Thin oil 1

[PLeon]

This thread could have been started in the lubricant forum but it is more about engine design.

Almost all new car makers require really thin oils for their engines. This maybe because the oils and their additives are much better today or maybe because engines are manufactured with much tighter tolerance/clearance and larger diameter bearings (are they?)

So if new engines are designed to run on really thin oil, that means that new engines don't like or don't start in really cold weather? What's true, tight engines or thin oil fashion?

 

[Fabrico]

Tighter tolerances, better additives, improved fuel economy.

 

[PLeon]

Is there a (manufacturing) reason that new engines can't run on thick oil? I was under the impression that if an engine can't run on 15W50 because it's too thick, that engine can't run at all.

 

[Fabrico]

No, there is a survival reason. It would run fine on straight 30wt in most climates. 15W50 might work in hot climates, but would be asking for trouble.

What's the point? Proper oil has always been important; it's just a little more critical these days.

 

[rmw]

It's done to help meet CAFE standards.

rmw

 

[PLeon]

I wanted to know if engines today have for example different bearings that raise the same oil pressure (same rpm) with 5W30 as old engines did with 20W50. I know that a 5W30 may outperform a 20W50 in a 4 ball wear test, the reason I am asking is cold weather performance. If new engines are built tighter so they run fine on 5W30, what happens when it gets really cold? No oil flow?

 

[kenvlach]

Correct. That's why they need dogsleds in Alaska & Canada, and those in the other northern states hibernate or go to Florida in the winter.

 

[automotivebreath]

It takes less power to pump thin oil

 

[Knap]

With the mention of CAFE its intersting to note that thicker oil still tends to be specified in UK compared to US for similar engine, and the comments from Penrire preferring thicker oils.

Advantages of thin oils are well made in this article

http://63.240.161.99/motoroil/

Perhaps a 0W100 would suit everyone!

 

[EHudson]

Not so much CAFE as EPA. When cranking, the engine is sending raw fuel down the exhaust pipe so the quicker it lights off the lower the emissions #'s. The thin oil is mainly for faster starts. There is a slight reduction is viscous friction which helps economy some.

Do I give up film thickness for those benefits in my car? No thanks, 30W please.

Pumps are bigger and bearing clearances are controlled more closely to compensate for the thin oil in new engine designs. Hot idle is the real test for those things.

 

[Mimbrogno]

Isn't there another reason why they are moving to thin oil? Even though you my be maintaining the same oil pressure, doesn't more thin oil move through engine than thicker oil? I may be mistaken, but I thought part of the reason for the new thin oils is that with thin oil, you can have more volume flow through the engine than before, and with more volume, you can carry away more heat. By keeping parts cooler, they can run them harder with higher temperatures, so they burn fuel cleaner with a little more efficiency and power.

I know one thing, even if you can put heavier oil in a thin oiled engine, you never want to use a thinner oil than specified!!

Matthew Imbrogno
Mechanical Vollenteer - Arizona Railway Museum

 

[patprimmer]

Oil pumps are positive displacement. They will pump the same volume whether thin or thick. The only difference will be the pressure obtained. The increased pressure will require more power to pump. It might be wasted if it forces the pressure relief valve to open.

The guys who designed the engine should have matched the pump size to bearing clearance and oil viscosity.

I would expect that it is still relevant to adjust viscosity to ambient temperatures. Any advice to the contrary might be due to passing required tests under controlled conditions that do not reflect the true use variations in conditions.



Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[ivymike]

hmm.. what happens to the mass flow of oil thru the bearings if the prv is open?

 

[patprimmer]

It goes out the window of course, however it should not open when at operating temperature with the "CORRECT" weight of oil. Of course it will open a lot if to viscous an oil is used at low temperatures. That will rob the bearings of cooling oil flow.

Sorry if I presumed that point and missed explaining it.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[drwebb]

Oil can fail two ways when an engine gets cold; a) Too thick to start, in which case the engine is fine but the motorist stranded, and b) Too thick to pump, in which case the motorist is fine but the engine is starved. There are separate viscosity tests to control for each; a) cold cranking simulator and b) mini rotary viscometer.

Keep in mind that low temperature grades and operating temperature grades are different. At cold temperature it is the W grade that is relevant. Back when 20W's were recommended in passenger cars, OEMs usually told you to switch to a 10W for cold weather. Now most OEMs make it easier on motorists by recommending a single multigrade for year-round use- after all its hard enough to get the general public to observe correct change intervals anyway.

A 5W oil is about 11 times thinner (8% as thick) than a 20W oil at -15 C, but a 30 weight is less than twice as thick as a 50 weight at 100 C. So the cold temperature difference is huge.

Modern engine designs are actually easier to start in cold weather than older ones; this led the SAE to slightly ease the specifications for the CCS test for engine oils in 1999.