Historically, Total Base Number (TBN) has been 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 also resulted in generally weaker acids produced. Explained below are 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.
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 acid neutralization; zinc in the popular antiwear 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.
CI-4+ to 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 percent versus the previous generation at 1.5 percent, and oils were formulated with lower levels of metallic additives and new ashless additives. This resulted in finished oils with lower TBN under both ASTM test methods but also a bigger spread in values between the two test methods.
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). Like they did moving from CI-4+ to CJ-4, the spread between the values returned by the two test methods increased. The table above illustrates the differences one might see in initial observed TBN values depending on the API specification claimed and testing method used.
Note that the values shown using ASTM D4739 have steadily decreased over three generations of oil categories. This has resulted in a spread between the values of the test methods that is now twice as big as it was during CJ-4 and three times bigger than during the CI-4 category.
TEST OF 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 its test protocol as part of the new API CK-4 standard as well as Mack/Volvo’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 Mack/Volvo’s VDS-4.5 specification. Passing this grueling test indicates a significant increase in oxidation protection even at lower starting TBN values by ASTM D4739. End of test oil analysis on passing oils frequently shows TBN values between 1 to 1.5, well below where prior conventional wisdom would have told you to drain the oil. This is more evidence of the disconnect between what TBN measures versus the actual performance provided.
Previous guidance from Detroit Diesel advised draining the oil when the TBN as measured by ASTM D4739 reached one-third of the starting TBN or 3 mg KOH/g, whichever came first. This standard became a quick and easy rule of thumb for many oil analysis laboratories and many fleet maintenance managers. Detroit Diesel recently released its 2018 Service Bulletin (DDC-SVC-BRO-0001, Table 10—“Single Sample Used-Oil Analysis Warning Limits”) in which it has removed TBN limits entirely from the used oil analysis parameters. Discussions with other key engine builders has revealed that others have noticed that used oil TBN values are often lower than ever before, sometimes lower than 1 mg KOH/g, yet without any additional indication of adverse oil condition. In time, we expect other engine builders to follow Detroit Diesel’s lead and either update or eliminate their TBN guidelines.
WHAT TO MONITOR
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. Oxidation/Nitration values around 25 units should start to get your attention, and a condemning limit between 35 to 40 is reasonable. This measure should also be paired with the viscosity trend to determine when oxidation is about to accelerate.
RELEVANT OR RELIC?
As additive chemistry has shifted, standard used oil TBN testing protocol simply doesn’t provide the same level of insight it once did. In the last 20 years much has changed—diesel fuel has dropped from 500 parts per million sulfur to a max of 15 ppm, fewer and weaker acids are found in used oil, engine designs have evolved, and oil chemistry has improved dramatically. While TBN was once an easy and effective way to predict remaining oil life, it no longer bears that relevance.
ABOUT THE AUTHOR
Tony Negri is the commercial product manager at Phillips 66. Find out more, visit www.phillips66lubricants.com.