Originally Posted By: JAG
The paper did not test or discuss cleaning of deposits.
Every base oil tested was of a significantly different chemical make-up. That strongly effects their performance in almost any performance test. Viscosities at 100C we’re either around 9.7 for two of them, 5.0 for three of them, 4.2 for one, and 5.9 for none. That is a poor set to make conclusions about the effect of viscosity at 100C in deposit formation due to both of those realities. That was not one of the main objectives of the paper, but because they were doing a lot of regression analyses, they apparently figured that they might as well do it also do it on viscosity. If it had been a main focus they would have chosen the base oil(s) of the same chemical make-up(s), and varied the viscosity over a sufficiently large range (ideally at least as large as 4 cST to 10 cSt).
The base oils did not contain additives. The oils that underwent less processing contain various sulfur and nitrogen compounds that came from the crude oil and survived the processing. These compounds act as antioxidants that improves their performance in oxidation tests. PAO has essentially none of these compounds. Additionally, different base oil types respond differently to antioxidants added by formulators. The heavily processed base oils tend to respond more strongly to adding antioxidant additives than do the lesser processed ones. Fully formulated oils we buy of course also contain a lot of other additive types besides antioxidants, which are chosen specifically for the type(s) of base oils they will be mixed with. Testing pure base oils informs scientists and their understanding for the sake of understanding, but the results should not be used by us to make big statements about fully formulated oils.
Deposit tests in which the oil thickness is thin and oil vapors are removed from the test will give a large advantage to the more volatile oils. If an oil molecule is not present on the heated surface, it cannot possibly leave a deposit on it. That is why the oil that 100% evaporated away had 0 deposits and largely why the oils that evaporated the least had the most deposits. One must think carefully of what parts in an engine are similar to a test like this. There is certainly no complete or largely complete analog to it.
Those hydrogenated aromatic base oils performed relatively well in those tests. They had low deposits and the highest temperature of onset of oxidation in the PDSC test. They have too many negative traits so they are typically purposely kept out of our lubricants to a large extent. PAO performed relatively poorly due to a combination of its low volatility, having no natural antioxidant, and the nature of its chemical make-up.
Spot on!
Especially "Testing pure base oils informs scientists and their understanding for the sake of understanding, but the results should not be used by us to make big statements about fully formulated oils."
The paper did not test or discuss cleaning of deposits.
Every base oil tested was of a significantly different chemical make-up. That strongly effects their performance in almost any performance test. Viscosities at 100C we’re either around 9.7 for two of them, 5.0 for three of them, 4.2 for one, and 5.9 for none. That is a poor set to make conclusions about the effect of viscosity at 100C in deposit formation due to both of those realities. That was not one of the main objectives of the paper, but because they were doing a lot of regression analyses, they apparently figured that they might as well do it also do it on viscosity. If it had been a main focus they would have chosen the base oil(s) of the same chemical make-up(s), and varied the viscosity over a sufficiently large range (ideally at least as large as 4 cST to 10 cSt).
The base oils did not contain additives. The oils that underwent less processing contain various sulfur and nitrogen compounds that came from the crude oil and survived the processing. These compounds act as antioxidants that improves their performance in oxidation tests. PAO has essentially none of these compounds. Additionally, different base oil types respond differently to antioxidants added by formulators. The heavily processed base oils tend to respond more strongly to adding antioxidant additives than do the lesser processed ones. Fully formulated oils we buy of course also contain a lot of other additive types besides antioxidants, which are chosen specifically for the type(s) of base oils they will be mixed with. Testing pure base oils informs scientists and their understanding for the sake of understanding, but the results should not be used by us to make big statements about fully formulated oils.
Deposit tests in which the oil thickness is thin and oil vapors are removed from the test will give a large advantage to the more volatile oils. If an oil molecule is not present on the heated surface, it cannot possibly leave a deposit on it. That is why the oil that 100% evaporated away had 0 deposits and largely why the oils that evaporated the least had the most deposits. One must think carefully of what parts in an engine are similar to a test like this. There is certainly no complete or largely complete analog to it.
Those hydrogenated aromatic base oils performed relatively well in those tests. They had low deposits and the highest temperature of onset of oxidation in the PDSC test. They have too many negative traits so they are typically purposely kept out of our lubricants to a large extent. PAO performed relatively poorly due to a combination of its low volatility, having no natural antioxidant, and the nature of its chemical make-up.
Spot on!
Especially "Testing pure base oils informs scientists and their understanding for the sake of understanding, but the results should not be used by us to make big statements about fully formulated oils."
