Oil evaporation rate questions

[OilUzer]

Hello to fellow experts, rookies and anywhere in between.

Few rookie questions:

Q1:
Is oil evaporation linear over its "lifetime"?

Q2:
What part of oil (e.g. Additives, vii, base oil, etc.) is more susceptible to heat? Basically what contributes more to higher Mr. Noack?

Can you have initial high noack or very high evaporation rate?
does the age of the car and/or quality of oil play a role in this?

Background info:
some of my older cars would quickly "burn" 1/3 to 1/2 qt. within the first 500-1000 miles after an oil change and then it would stop ... did not document the details but am pretty sure it happened and not my imiganation! Note: I typically don't research or use "premium" oil for older cars. I know I should. We need to take of the elderly! lol

My political speech:
OilUzer is going to ban the entry of certain oils into our market until we can figure out what the heck is going on!

 

[SuperSalad]

A1:
Evaporation rates generally start high and level off over time as less and less volatile material remains in the bulk of the oil due to said evaporation.

A2:
Base oil is most susceptible to evaporation.

Yes higher initial evaporation rate is typical.
Not necessarily the age of the car but how well the cooling system works will affect evaporation losses. Quality of oil can definitely affect evaporation losses.

 

[CR94]

Originally Posted By: MotoTribologist
A1:
Evaporation rates generally start high and level off over time as less and less volatile material remains in the bulk of the oil due to said evaporation.
...
That seems plausible. Does that imply problems related to oil volatility will be worse if one changes oil too frequently? (Assuming the same high-Noack oil ... )

Thanks!

 

[SuperSalad]

Originally Posted By: CR94
Originally Posted By: MotoTribologist
A1:
Evaporation rates generally start high and level off over time as less and less volatile material remains in the bulk of the oil due to said evaporation.
...
That seems plausible. Does that imply problems related to oil volatility will be worse if one changes oil too frequently? (Assuming the same high-Noack oil ... )

Thanks!

Well the only "problems" I can think of are oil consumption and thickening, so doing the changes would address both problems. Are there other problems related to oil volatility I'm not thinking of?

 

[Overfill]

Originally Posted By: MotoTribologist
A1:

A2:
Base oil is most susceptible to evaporation.



I understand there are a lot of components that comprise a base oil to any given product.
Q1
In the realm of petroleum base oils, is the grade of the oil a major driver to how much it evaporates? (i.e. thin bases evaporate more @xF/ thick bases not as much @xF)
Q2
How does other (non-petrol) base oils rank in evaporative tendency, compared against petroleum oils and each other?

 

[userfriendly]

Back in the day, engine oils were kept "in grade" by evaporation.

 

[SuperSalad]

Originally Posted By: Brian553


Originally Posted By: MotoTribologist
A1:

A2:
Base oil is most susceptible to evaporation.



I understand there are a lot of components that comprise a base oil to any given product.
Q1
In the realm of petroleum base oils, is the grade of the oil a major driver to how much it evaporates? (i.e. thin bases evaporate more @xF/ thick bases not as much @xF)
Q2
How does other (non-petrol) base oils rank in evaporative tendency, compared against petroleum oils and each other?

A1:
Generally, lighter (less viscous) base oils are more volatile than thicker (more viscous) base oils

A2:
If comparing similar viscosity oils synthetics don't guarantee better volatility. Some esters and PAOs are very volatile just like some petroleum base oils. Some are more uniform in their molecular size and have lower volatility. So there's no hard rule, and it's pretty much on a case-by-case basis for base oil's contribution to total volatility.

 

[CR94]

Originally Posted By: MotoTribologist
Originally Posted By: CR94
Originally Posted By: MotoTribologist
A1:
Evaporation rates generally start high and level off over time as less and less volatile material remains in the bulk of the oil due to said evaporation.
...
That seems plausible. Does that imply problems related to oil volatility will be worse if one changes oil too frequently? (Assuming the same high-Noack oil ... )

Well the only "problems" I can think of are oil consumption and thickening, so doing the changes would address both problems. Are there other problems related to oil volatility I'm not thinking of?
What about deposits on intake valves, and ring sticking, both of which we've seen blamed (rightly or not) on oil volatility (via PCV, aided and abetted by EGR)?

Thanks!

 

[JAG]

I completely agree with MotoTribologist. I used to perform carefully conducted motor oil volatility tests. Virgin oil is quite a bit more volatile at a high temp. than when it has been subjected to that temp. for a while. This was true even for synthetic motor oils.

The NOACK volatility test is severe enough in terms of the temp. and test duration that it’s testing the average evaporation rate well past the phase where the oil was virgin and of extra high volatility. That is, it captures the long term volatility, not the virgin volatility. I’ve read that it is so severe that thermo-oxidative effects occur during the test. Based on the fact that non-synthetic oils tend to have a wider distribution of base oil molecule sizes and that the smaller ones are dramatically more volatile than the rest, that virgin non-synthetic oils will tend to be significantly more volatile than their NOACK volatilities indicate, relative to that same comparison for a synthetic oil.

 

[OilUzer]

Based on what you guys are saying, virgin oil is more volatile or the evaporation rate is much higher at first and it then levels off!

What if the more volatile stuff that are evaporating quickly are very useful stuff (additives?) that the engine needs?

Wouldn't it be helpful if the Noack test was also measuring the evaporation rate (amount per time) as well as just the amount?

Let's say oil X lost %90 of it's noack number in the first 5 minutes of the test and oil Y lost it more uniformly (e.g %8.3 of noack per each 5 minutes of the test) ...
Is that information usefull?
I would prefer oil Y over X!

Noack Volatility Test (Wikipedia):
In this test, a sample is heated at 250 °C for 60 minutes with a constant flow of air over it. The weight fraction lost is the result for the Noack volatility test.

 

[SuperSalad]

Originally Posted By: OilUzer
Based on what you guys are saying, virgin oil is more volatile or the evaporation rate is much higher at first and it then levels off!

What if the more volatile stuff that are evaporating quickly are very useful stuff (additives?) that the engine needs?

Wouldn't it be helpful if the Noack test was also measuring the evaporation rate (amount per time) as well as just the amount?

Let's say oil X lost %90 of it's noack number in the first 5 minutes of the test and oil Y lost it more uniformly (e.g %8.3 of noack per each 5 minutes of the test) ...
Is that information usefull?
I would prefer oil Y over X!

Noack Volatility Test (Wikipedia):
In this test, a sample is heated at 250 °C for 60 minutes with a constant flow of air over it. The weight fraction lost is the result for the Noack volatility test.

Volatile additives are rarely if ever used for engine oil. A highly volatile additive would be a poor choice for an application where heat is present and the formulators would be quite poor at their job if they chose them.

The Noack is considered an expected total loss during a normal use interval, though it will most certainly vary in real world applications. So the time factor you are thinking of is irrelevant in any practical sense. Yes you could collect that information for the evaporation rate, but in the end, if you evaporate 8% of the oil over 1 week of use or 2 weeks of use, will that make any difference during a 3-6 month change interval? You still have 92% of the "optimal" oil volume (and still well above the lower limit) for 84% or more of your service interval.

 

[weasley]

There are other tests that can be run to determine distillation profiles, but these are not commonly used for specification testing. What you need for a spec test is something quick, repeatable, reliable and relevant to the performance at hand. As MotoTribologist points out, the Noack test is designed to give a best estimate of the total 'normal' volatility of an oil over a typical service life. In reality a motor oil does not spend any extended period of time at 250°C, so the test is pretty much an accelerated situation, to get to a result fast.

 

[userfriendly]

The mono-grades I use; SAE 30 249C, SAE 40 263C flash points.

 

[ZeeOSix]

Originally Posted By: OilUzer
Let's say oil X lost %90 of it's noack number in the first 5 minutes of the test and oil Y lost it more uniformly (e.g %8.3 of noack per each 5 minutes of the test) ...
Is that information usefull?
I would prefer oil Y over X!

Noack Volatility Test (Wikipedia):
In this test, a sample is heated at 250 °C for 60 minutes with a constant flow of air over it. The weight fraction lost is the result for the Noack volatility test.


I'd say the test finds the oils that are going to evaporate quickly in the beginning and level off. If an oil was super volitile, by the end of the hour test it would have a horrible Noack rating. Bottom line is the tests shows comparatively what oils will typically be more volitile in actual use.

The acceptible Noack limit spec is set to 15%. You probably won't see much difference in "burn off" in real world use between say a Noack of 10% and 8%, but might between 15% and 8%. An oil that measures say 8% in the extreme Noack test may not lose anything close to 8% in real world use.

If an engine is using oil, it would be hard to determine how much was due to burn off volitility, or from getting past rings and valve guide seals. Switching to an oil with a very low Noack would tell more if the oil use rate decreased noticable with the new oil use.