Why not use 0W-20 in a diesel engine?

[Dad2leia]

Ok, correct my logic if it's wrong, but here it goes:

High rpms would require thinner oil as to make sure the oil film is in place under those conditions, so low rpms would require thicker oil so that the oil film is maintained so as the oil wouldn't just run off of the part needing the lubrication.

This might be an oversimplification statement of oil's job, but it makes sense, right?

 

[G-MAN]

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Ok, correct my logic if it's wrong, but here it goes:

High rpms would require thinner oil as to make sure the oil film is in place under those conditions, so low rpms would require thicker oil so that the oil film is maintained so as the oil wouldn't just run off of the part needing the lubrication.

This might be an oversimplification statement of oil's job, but it makes sense, right?

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I think you may be on to something. Big rig engines rarely see RPMs much over 2500 and I think most redline at something like 3000.

This is not to say, however, that the oil pump on these engines could not be redesigned to assure adequate flow with low vis oils at these low rpms. It would not surprise me if that is the next step in the evolution of big rig engines. We'll see a shift from 15w40 to 5w30 and eventually 5w20.

 

[Dad2leia]

On that note G-Man, I have to ask this question: Are the fuel efficiency requirements such that those thinner oils would be required or make that much of a difference in the oil based friction internally instead of sticking with the really robust and well formulated 40 weight oils?

It really does seem like fuel conservation is taking a front seat to every other concern and purpose of the lubricating oil.

 

[JAG]

From what I remember reading in my engineering book as recently as a few months ago, increasing flow rate through a journal bearing that already gets enough flow (so that flow in = flow out) does not increase the film thickness. The hydrodynamic wedge that forms when their is rotation is what keeps the parts separated which is based on viscosity, part geometry, RPM, and some other things but not flow rate.

 

[JAG]

The book I mentioned above is at home. But I dug out a book, Machinery's Handbook, just now. For journal bearings, it has a chart with RPM as x-axis and viscosity as y-axis. There are 4 curves on it that are nearly linear.
1. Mixed Film lubrication at heavy load
2. Mixed film lubrication at low load
3. Full film lubrication at heavy load (250 psi)
4. Full film lubrication at low load (100 psi)

Have to know what lubrication regime you have and how high the load is, then from the RPM and the right curve, you get the right viscosity. Y-axis goes from SAE 5 to SAE 50 and even to Grease (for 10 to 200 RPM).

It also says the bearings just need enough viscosity to provide protection. More viscosity above that is just wasted energy, but we already know that.

 

[G-MAN]

JAG, I think you're just getting at my point but in a more detailed way: namely, the current state of design for big rig diesels is such that the bearings and oil pumps are designed to operate optimally with XXw40 oils. Once the engine designers decide to change the paradigm, they will be able to effectively design bearings and oil pumps that will operate optimally on 5w30 or even 5w20 oils.

 

[JAG]

Agreed that it's possible to design it to run on thinner oils! I don't know if the rewards would justify the effort though. Big rigs use so much fuel fighting air drag that the oil drag penalty is so tiny in comparison. Combustion efficiency improvements that can be made may also outweigh oil drag considerations at this point. I think what could help save fuel is using smaller engines with higher boost pressures. But for engines that need instant gobs of just above idle RPM torque, that design would not be as good as what's currently used.

 

[Dad2leia]

Well, as you state JAG, we are seeing that in the VW requirements of 30 weight oils and such, and thanks for answering the question of if it is worth the effort.

It really doesn't sound like the gains would be enough to warrant designing a system to run on 20 and 30 weights, especially the Duramax, Cummins, and Powerstroke varieties, let alone the over the road big boys.

 

[Master ACiD]

i dont think its from a diesels low rpm and high torque because modern diesels are not low rpm slugs that they use to be. my vw diesel revs out to 5500rpm's before the limiter kicks in. my sisters old mercedes did the same rpm's.

 

[Jay]

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After reading the "Why do Euro cars use heavier oils post below" and the mention of diesel engines, I got to wondering: Why don't diesel's use 5-30 or even 0/5-20???

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They do.

We have an industrial Caterpillar emergency generator at work. It's a quad-turbocharged V-12 rated at 1600kw (2100hp). 15w-40 is the preferred weight, but 0w-20 can be used if ambient temperatures don't exceed 40degF. Thousands of similar engines work in artic environments where 0w-20 may be needed to get them started.

 

[1sttruck]

"High rpms would require thinner oil as to make sure the oil film is in place under those conditions..."

I don't know what they run now, but when I rode them a lot of motorcycles used 40 weight oils, bikes that would hit 9k rpm and higher. These days sport bikes seem to commonly be 12k rpm and higher.

 

[GeorgeCLS]

One of the five attributes for an engine oil is "cushioning". In a gasoline engine the ignition process is a "burn"; the gas/air mixture is ignited by a spark source and a "burn" process takes place. A nice, smooth application of power to the connecting and main rod bearings.

In a diesel, the process of combusion is a violent "explosion". Therefore the connecting rods need all the "cushioning" protection possible to prevent bearing wear and even damage.

The biggest single advancement in diesel engine life came about 20 years ago when diesels transitioned from a straight 30W diesel engine oil to 15W040 multi-viscosity oils. On highway class 8 engine life when from around 300,000 miles between overhauls to sometimes approaching 1,000,000 miles without having to change main and connecting rod bearings..
i.e. the affect of having that 40W cushion enabled main and connecting rod bearings to achieve a much lower wear rate.
I had seen connecting rod bearings literally crushed as a result of 30W lubricant failure... More than once..
I have never seen this with the use of a 40W diesel engine oil. (in many, many diesel engine tear downs)

The only reason manufacturers are approving 30W diesel engine oil is for paper CAFE fuel economies. If longevity is one's primary concern, a high quality XXW-40W is the formulation for that goal.
George Morrison, STLE CLS

 

[teven7492]

GEORGECLS there's no such thing as 30w... its SAE-30.

 

[Master ACiD]

youre getting into semantics. why not just take georges info?

 

[JAG]

Good points George. I forgot about the shock loads in diesels.

 

[Dad2leia]

So it is not so much an rpm issue as it is an internal stress issue from the differences in combustion processes between gasoline and diesel fuels, right?

How much of a factor does HTHS figure into this equation? And can a shear stable 10w40 work, or will it shear too quickly due to the 30 point difference in viscosity levels?

 

[bruce381]

george is correct also hydrodynamic wedge formation is more reliant on oil vis and crank rotation than oil flow or presure so a lighter oil to form a wedge at "slow" speeds will need a larger smaller clearence which maybe limited in a diesel due to punding and other harmonics.
bruce

 

[Tornado Red]

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In a diesel, the process of combusion is a violent "explosion". Therefore the connecting rods need all the "cushioning" protection possible to prevent bearing wear and even damage.

The biggest single advancement in diesel engine life came about 20 years ago when diesels transitioned from a straight 30W diesel engine oil to 15W040 multi-viscosity oils. On highway class 8 engine life when from around 300,000 miles between overhauls to sometimes approaching 1,000,000 miles without having to change main and connecting rod bearings..
i.e. the affect of having that 40W cushion enabled main and connecting rod bearings to achieve a much lower wear rate.
(snip)
The only reason manufacturers are approving 30W diesel engine oil is for paper CAFE fuel economies. If longevity is one's primary concern, a high quality XXW-40W is the formulation for that goal.
George Morrison, STLE CLS

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I assume that, since the "explosion" is the same in the diesel engine of a Class 8 truck or a 3-cylinder VW Polo TDI, your recommendation of a 40-weight oil would apply to either extreme?

 

[G-MAN]

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george is correct also hydrodynamic wedge formation is more reliant on oil vis and crank rotation than oil flow or presure so a lighter oil to form a wedge at "slow" speeds will need a larger smaller clearence which maybe limited in a diesel due to punding and other harmonics.
bruce

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A "larger smaller clearance"?

 

[bruce381]

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george is correct also hydrodynamic wedge formation is more reliant on oil vis and crank rotation than oil flow or presure so a lighter oil to form a wedge at "slow" speeds will need a larger smaller clearence which maybe limited in a diesel due to punding and other harmonics.
bruce

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A "larger smaller clearance"?

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Duh covered my Butt with that one LOL

a SMALLER bearing clearence is what I ment.
bruce