I revisited this old Chevron Oronite reference about TBN retention and Ca vs. Mg detergents.
It studies oil life and wear in diesel engines. TBN measures the amount of acid-neutralizing detergents (Ca- or Mg-based) in the oil and TAN measures the amount of acid in the oil. Here is what I gathered from the paper, even though this is subject to ongoing research and debate:
(1) Neither TBN nor TAN alone can tell you the condition of the oil.
(2) TBN retention -- how slowly TBN decreases during service -- has no importance.
(3) Oil life or the ability of an oil to provide protection is determined by the difference of TBN from TAN = TBN - TAN. If TBN - TAN becomes negative, oil can no longer protect against wear and corrosion, the problem becoming worse and worse with a more negative number. The more positive TBN - TAN, the better the wear protection is.
(4) Initial TBN (fresh-oil TBN) is very important. Higher the initial TBN, the better it is. With a higher initial TBN, it will take the increasing TAN longer to reach the decreasing TBN.
(5) In general, Mg-based detergents are bad because they can't neutralize certain kinds of acids. Ca-based detergents are good in neutralizing virtually all kinds of acids. With Mg-based detergents, it could be misleading as TBN doesn't go down as rapidly but TAN increases more rapidly, as the Mg-based detergents aren't fully used in neutralizing the acids. Mg-based detergents also fight and oxidize the base oil.
(4) Acids are not only produced through combustion (NO_x and SO_x from the burning of the fuel) but also through the oxidization of the base oil. Therefore, higher the quality of the base oil, the better it is. PAO is better than Group III, and Group III is better than Group II for producing less acids in the oil.
(5) Wear metals increase with the acids, especially certain wear metals.
For me this is very interesting because I had thought that if TBN was above 2.0 or 3.0, it was fine. On the contrary, that's not the case at all. In fact, I worry even more now with the modern engine oils having smaller and smaller initial TBNs.
PS: Here is another good reference on oil life, also talking about other elements.
It studies oil life and wear in diesel engines. TBN measures the amount of acid-neutralizing detergents (Ca- or Mg-based) in the oil and TAN measures the amount of acid in the oil. Here is what I gathered from the paper, even though this is subject to ongoing research and debate:
(1) Neither TBN nor TAN alone can tell you the condition of the oil.
(2) TBN retention -- how slowly TBN decreases during service -- has no importance.
(3) Oil life or the ability of an oil to provide protection is determined by the difference of TBN from TAN = TBN - TAN. If TBN - TAN becomes negative, oil can no longer protect against wear and corrosion, the problem becoming worse and worse with a more negative number. The more positive TBN - TAN, the better the wear protection is.
(4) Initial TBN (fresh-oil TBN) is very important. Higher the initial TBN, the better it is. With a higher initial TBN, it will take the increasing TAN longer to reach the decreasing TBN.
(5) In general, Mg-based detergents are bad because they can't neutralize certain kinds of acids. Ca-based detergents are good in neutralizing virtually all kinds of acids. With Mg-based detergents, it could be misleading as TBN doesn't go down as rapidly but TAN increases more rapidly, as the Mg-based detergents aren't fully used in neutralizing the acids. Mg-based detergents also fight and oxidize the base oil.
(4) Acids are not only produced through combustion (NO_x and SO_x from the burning of the fuel) but also through the oxidization of the base oil. Therefore, higher the quality of the base oil, the better it is. PAO is better than Group III, and Group III is better than Group II for producing less acids in the oil.
(5) Wear metals increase with the acids, especially certain wear metals.
For me this is very interesting because I had thought that if TBN was above 2.0 or 3.0, it was fine. On the contrary, that's not the case at all. In fact, I worry even more now with the modern engine oils having smaller and smaller initial TBNs.
PS: Here is another good reference on oil life, also talking about other elements.