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What is TBN and why is it important for diesel engines?

The value represented by TBN or Total Base Number according to ASTM D 974 is the amount of acid expressed in mg KOH / g (milligram of potassium hydroxide per gram of oil) required to neutralize all alkaline components present in engine lubricants of combustion. These alkaline components, also known as alkaline reserve. They perform an important function in diesel engines which is to neutralize the acid by-products generated during combustion due to the presence of sulfur (S) in the diesel. Recalling that the S-500 diesel has 500 ppm sulfur, while the S-10 diesel has only 10 ppm sulfur in its composition.

As part of this alkaline reserve in combustion engine lubricants are the detergent additives that basically have two functions: 1) Control the soot deposit on critical parts of the engine, such as rings and piston and 2) Neutralize the acid by-products from the combustion of diesel that could cause corrosion in the engine, especially in the bronzines.

What is TBN and why is it important for diesel engines?

Every diesel engine lubricant has a high initial TBN value (eg 10 mg KOH / g) which is consumed during the use of the lubricant and consequently causes its value to decrease as shown in the graph below:

How TBN is measured:

For new lubricants, the most commonly used test method is ASTM D 2896 which uses perchloric acid as the reagent. While for the sample of used lubricants, the most used method is the ASTM D 4739 that uses like reagent the hydrochloric acid, that is a weaker acid. The recommendation for the use of a weaker acid for used lubricants (ASTM D 4739) is given by the fact that the sample has wear metals that react in contact with a stronger acid, interfering in the final result of TBN.

Why does the TBN result reduce even with little time of use?

The initial TBN value should be sufficient to protect the engine against corrosion throughout the oil change interval recommended by the manufacturer, but for more modern performance level lubricants such as API CJ-4 and API CK-4 due to change (magnesium (Mg)), which is harmful to post-treatment systems equipped with a DPF filter, has been reduced or replaced with additives based on metallic salts such as calcium (Ca), magnesium (Mg) and Zinc (Zn) by non-metallic components.

This change, added to the difference between the acids used in ASTM D 2896 and ASTM D 4739, causes a difference in TBN result of approximately 2.0 mg KOH / g when using the ASTM D 4739 method. That is, a lubricant for diesel engine with initial TBN of 10 mg KOH / g reported by the ASTM D 2896 method in the lubricant supplier datasheet may have an initial TBN result of 8 mg KOH / g or even less when using the ASTM D 4739 method.

What is the minimum value for TBN in used lubricants?

The TBN value is one of the factors used to determine the optimum time for oil change. Because the dropping of the TBN can result in the loss of the lubricant’s ability to protect the engine against corrosion. Although TBN values above zero indicate that there is still alkaline reserve and corrosion protection still exists, the minimum and safe TBN value should be greater than or equal to 4.0 mg KOH / g. For values less than 4.0 mg KOH / g, oil change should be performed immediately; investigate the quality of the diesel used, confirming the sulfur content (S); check the oxidation rate of the oil caused mainly by high temperatures and long oil change intervals and consequently reduce the oil change interval to ensure protection to the engine.

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