Historically, Total Base Number (TBN) has been perceived as a key indicator of remaining useful oil life in heavy-duty engine oils. While acid neutralization is still an important function of engine oil, changes in engine design and the move to Ultra Low Sulfur Diesel (ULSD) fuels have decreased the amount of acids produced in the engine and influenced oil formulations over the last decade. This article explains the differences in ASTM test methods for TBN, industry changes leading to the current categories, and the change in how TBN values may be interpreted with the new oils.
The Test Methods
ASTM D2896 is the test method most commonly used to measure TBN on new oils. Use of a very strong acid identifies both “hard” and “soft” TBN, giving the total alkalinity reserve of the sample. The value obtained from this test is the number reported on most technical data sheets.
ASTM D4739 is the test favored by oil analysis labs on used oil samples. Using a weaker acid, it only identifies alkalinity from metallic elements like calcium, magnesium, and zinc. These metals are often doing double-duty in the oil (calcium provides detergency and also acid neutralization; zinc in the popular anti-wear additive ZDDP also contributes to anti-oxidation). This test does not identify newer ashless (i.e. non-metallic) additives, and reported values will be lower versus ASTM D2896.
API CJ-4 to CK-4
When the industry updated from API CI-4 / CI-4+ to API CJ-4 oils, the new oil chemistry differed from those of previous service categories. To safeguard the effectiveness and service life of exhaust after-treatment devices, API CJ-4 limited sulfated ash to no more than 1%, and oils were formulated with lower levels of metallic additives and new ashless additives. This resulted in finished oils with lower TBN under the ASTM D4739 method versus the previous category.
As a result of increased levels of ashless anti-oxidants, many current CK-4 oils may reflect a higher initial TBN via ASTM D2896 than previous CJ-4 versions, but those same new oils will also likely reflect an even lower TBN via ASTM D4739, due to the decrease in over-based metallic detergents (which create ash when burned, leading to engine deposits). The table below illustrates the differences one might see in initial observed TBN values, depending on the API specification claimed and testing method used:
Volvo / Mack T-13 – New Industry Test for Oxidation Stability
If TBN is no longer the best measure of useful oil life with regard to oxidation stability, how do we know that the new oils are up to the task? Mack addressed this topic with the inclusion of their test protocol as part of the new API CK-4 standard as well as Volvo / Mack’s own proprietary VDS-4.5 specification.
This new test evaluates the candidate oil’s oxidation stability, nitration and resistance to bearing corrosion. CJ-4 technology generally cannot pass this test without a significant antioxidant boost, making the T-13 test a critical part of the new CK-4 standard, as well as setting the performance limits for Volvo / Mack’s VDS-4.5 specification. Passing this grueling test indicates a significant increase in oxidation protection even at lower TBN values by ASTM D4739.
Putting It All in Perspective
As additive chemistry has shifted, the standard TBN testing protocol simply doesn’t provide the same level of insight that it once did. When looking at used oil analysis reports, the full range of available data should be considered. If TBN appears to be low but all other criteria are good (low wear metals, corrosion control, viscosity / oxidation control), there is likely little reason to worry.