Why is TBN depletion non-linear?

[fdcg27]

If TBN is a measure of an oil's ability to neutralize acids, then why should the rate of depletion decline as the OCI progresses?
Put simply, if TBN has declined by, say, 80% in 4K miles, shouldn't it reach zero in another 1K?
I've sometimes posted in the UOA forum that oils with fairly low residual TBNs were not suited for longer drains, while others have replied that the oils would remain pretty stable at these low TBN levels for longer than a linear rate of depletion would indicate.
If this is so, then why?

 

[Donald]

No answers but some interesting reading:

http://www.machinerylubrication.com/Read/2170/oil-drain-interval-tan-tbn

 

[simple_gifts]

I asked something related years ago and some knowledgeable people weighed in.

http://www.bobistheoilguy.com/forums/ubbthreads.php?ubb=showflat&Number=198472

May have nothing to do with it, but logarithmic scales appear to be "linear" but the phenomena they are measuring are not. Richter scale comes to mind. One "step" up on the Richter scale indicates and increase of 10 for energy released in an earthquake.

 

[SubLGT]

Originally Posted By: Donald
No answers but some interesting reading:

http://www.machinerylubrication.com/Read/2170/oil-drain-interval-tan-tbn


Good article. They are recommending changing oil when TBN is 65% depleted.

"The following table represents TBN/TAN test results across a fleet of more than 450 pick-up/delivery trucks. TAN remains fairly steady until the TBN depletes from 12 to about 6. The two meet at between 50 and 65 percent depletion of the TBN. The TAN is significantly higher than the TBN after the TBN has reached 65 percent depletion, which indicates that the oil’s ability to neutralize acids has dropped significantly. The oil should be changed to prevent corrosive engine wear from occurring."

 

[4ever4d]

I'm not an extended drain person but this is very interesting. In theory it seems as though this crossover point could shorten the oil life, especially in some of the 10k and over oci's followed by members here. Without a series of trending uoa's one would not know. Starting with an engine clean would be curcial to optimal results. Is it not true that most labs have an upcharge for TAN testing? It seems as though the TBN is fairly linear until after the crossover point at that time TBN drops rapidly if i read correctly. I would like to see this thread develope further.

 

[JHZR2]

Ever do an equivalence point titration where the liquid changed red, in your high school chemistry class???

 

[dave1251]

Originally Posted By: fdcg27
If TBN is a measure of an oil's ability to neutralize acids, then why should the rate of depletion decline as the OCI progresses?
Put simply, if TBN has declined by, say, 80% in 4K miles, shouldn't it reach zero in another 1K?
I've sometimes posted in the UOA forum that oils with fairly low residual TBNs were not suited for longer drains, while others have replied that the oils would remain pretty stable at these low TBN levels for longer than a linear rate of depletion would indicate.
If this is so, then why?


The best short answer I can give you the total chemistry of the PCMO along with the amount of combustion, heat, and water will determine the TBN depletion. Different chemistry reacts differently with the acids thus retarding the TBN depletion at different rates. Some oils you will see thru UOA's the first couple of thousands of miles the TBN drops like a rock in water. Then from there the TBN depletion is very slow. Others have a very consistent TBN depletion.
Now to accurately determine the ROI of a oil from a TBN retention stand point you would have to run in your engine right to the 1.0 TBN point of your OCI's most people for good reason do not "feel" this is a good idea this quite frankly scares them, also from a testing point this could be cost prohibitive, so for the average novice "oil nerd" will not test a oil to the breaking point to determine the TBN depletion of a particular oil value under their driving and engine conditions. Also there is a good chance when someone finally goes thru all this trouble of figuring and testing all this out the blender could change the chemistry of the PCMO.

 

[fdcg27]

Hmm,
Interesting piece.
But if this were the case, wouldn't we expect to see a lot more metal in UOAs where TBN is run down to well below 65% of its starting value?

 

[dave1251]

Why would you see that? Just because there is less of a base that does not mean that it will not effectively neutralize the acid.

 

[fdcg27]

Because according to the article Donald offered the link to, after about 65% TBN depletion, TAN increases rapidly and will have a higher numeric value than TBN.
The article did mention a CJ-4 oil, so I assume they were talking about diesels.
It may be that the conclusions reached by the writer were in error.
We certainly see a number of good UOAs here with TBN well below 65% of its starting value.

 

[Jim Allen]

By "good UOAs" do you mean only metals?
By a long shot, most UOAs here don't have TAN, even if they have TBN listed (TBN is shown on less than 65 percent of the UOAs here by my rough count). Also, we don't often see the starting TBN or the starting TBN as measured by the lab doing the final TBN, which is the most accurate way to find that 65%, quoted. Unless you can compare that TAN and TBN, after comparing the starting TBN, and relate that to wear metals, it's pretty tough to tell anything from anything.

 

[fdcg27]

When I wrote of good UOAs, I was thinking of metals.
I don't think that even half of the UOAs posted here have TBN, which I consider one of the most valuable parts of a UOA.
When you can buy You are right in that we don't often have both starting TBN and used TBN from the same lab.
How comparable do you think the TBN listed on the Product Data Sheet for an oil is to the TBN as tested by any given lab, since this data is pretty easy to get?
Would it vary by much?
Still, the article suggested 65% TBN depletion as a rule of thumb condemnation limit.
This may or may not be valid, but is certainly worth discussing.
OTOH, for the user who has no clue as to starting TBN and doesn't get TBN in a UOA, then this has no meaning.

 

[SubLGT]

Polaris Labs advises:
"..............Comparing the used oil’s TBN to its starting TBN when
new, will determine how much longer the oil can be used without causing any
damage to the engine. When TBN has dropped to 35% of the starting TBN, the
oil should be changed. NOTE: In order to compare apples to apples and to set
accurate condemning levels, ASTM D4739 should be used to determine the TBN
of both new and used oils. ..............."

http://www.polarislabs.com/pdf/when-to-extend-recommended-drain-intervals.pdf

 

[Donald]

Can't one find the specs or a VOA for the starting TBN?

Polaris has a paper out suggesting to get a starting UOA. Which I assume means change the oil and run it enough to mix up the new oil all over the engine and then pull a sample. A little on the expensive side for a car, but peanuts for the diesel engine of a ocean liner.

 

[modularv8]

The chemistry of an acid/base neutralization reaction is based on equivalency, protons exchanged for electrons. Without getting to technical, if you have ever done an acid-base titration which is essentially what is happening in your engine when combustion forming acids are being neutralized by the motor oil's base, the titration curve will look like this (scroll down to graphs). At some point along the curve the rate at which the base (TBN) is used up by the acid (TAN) will increase (TBN decreases rapidly) till it reaches an inflection point (equivalency) where TBN &TAN are in equilibrium. Beyond this point, the TBN continues to decrease rapidly in short span of time/miles, then starts to stabilize but not much life left in the oil.

You want to change your oil ideally just beyond the equivalency point. This is difficult to determine in practice because alot of variables impact how much acid is being generated in your engine.

BTW, Nitric and Sulfuric Acid which are the two main acids produced in an engine are both strong acids. Also, a UOA showing a TBN remaining that is beyond the 60% mentioned in the Polaris article does not mean no corrosion is occurring in your in engine despite the comments made by some UOA reports. It is best to change you oil before reaching this relative low level of TBN.

 

[Doug Hillary]

Hello,
A number of Posters have indicated that such things as chemistry being factors that influence TBN depletion in a nonlinear manner. In a practical sense top up rates and engine design and/or condition are amongst other things that also influence this situation

My testing of heavy duty high speed (HDHS) diesel engines over many millions of kms showed these results with both mineral and synthetic HDEOs having an initial TBN of 11 (D4739). The engine Manufacturer’s TBN condemnation point (D4739) was 1

Lowest TBN at OC = 0.30 (at 100kkms (62kmiles)). TAN was 6.7 at this point

Average TBN at OCI = 2.64 (Average OCI 90kkms (56kmiles).

Average TAN was 6.7

Lowest TBN at OC = 2.05

Highest TBN at OC = 3.6

Lowest TAN at OC = 6.13

Highest TAN at OC = 7

Average top up was 1 litre/6kkms (3.7kmiles).This means that an average of 15 litres was introduced into an oil pan holding 38 litres during an OCI

The lowest TBN (0.1 at a TAN of 8.5) and the lowest TAN (5.34) were both recorded with the mineral lubricant

In one case the TBN held at 0.5 for 10kkms (6kmiles) and the TAN rose from 6.1 to 6.8

In another case the TBN rose from 2 to 2.1 over 20kkms (12kmiles) and the TAN reduced from 7.2 to 5

So practical knowledge of the product, the Lab and the testing regime used all play vital roles in real life. There was never any linear relationship shown in either an increase in the TAN or depletion in the TBN. UOAs were done at regular intervals within the OCI

In the end the TBN and TAN are but two of the factors in a UOA that show the suitability of the lubricant for further use

 

[modularv8]

Originally Posted By: simple_gifts
I asked something related years ago and some knowledgeable people weighed in.

http://www.bobistheoilguy.com/forums/ubbthreads.php?ubb=showflat&Number=198472

May have nothing to do with it, but logarithmic scales appear to be "linear" but the phenomena they are measuring are not. Richter scale comes to mind. One "step" up on the Richter scale indicates and increase of 10 for energy released in an earthquake.


The answer to your question in that other thread is that the base does not exist as a simple base like NaOH. Rather it is a detergent base or soap (organic salt or other) that is "activated" according to the pH around it. It will exchange an electron for a proton (acid). Therefore, by itself it does not attack the metallic surfaces within the engine. Also passivators (Boron, etc) may be used to protect non-ferrous surfaces (aluminum) in certain formulations (ie, diesel/high detergent).

 

[SubLGT]

Originally Posted By: Doug Hillary
...........The engine Manufacturer’s TBN condemnation point (D4739) was 1 .............


Doug, did you ever follow the rule that oil was changed at 65% TBN depletion, or 50% depletion?

According to an application note at Bently Tribology, the TBN "alarm limit" for Caterpillar and Detroit Diesel is 1.0mg KOH/g, and for Cummins it is 2.0mg KOH/g, or 50% depletion, or when TAN = TBN (presumably depending on the Cummins engine model)

http://bentlytribology.com/publications/appnotes/app24.php

 

[Doug Hillary]

Hi,
SubLGT - Yes, the TBN limit of 1 was a parameter used. Soot and/or Fe condemnation levels were typically always reached prior to the TBN limit. TBN was replaced as a parameter by substituting a max TAN

When reading UOAs it is the mattrix of parameters that matter and occasionally one or the other may temporarily exceed their limit without problems occuring

It is wise to be careful of "spikes" which are usually a Lab error

 

[dparm]

IMO you need to be taking into account the TAN-TBN ratio, which means you also need to know the virgin TAN (rarely published).