Total Base Number (TBN)
TBN is a property usually associated with engine oils. It is defined as the oils' ability to neutralize acid. The higher the TBN, the more acid it is able to neutralize. This quality is also referred to as alkaline reserve and is directly proportional to the amount of active detergent contained in the oil. New engine oils typically possess TBN's from 5.0 to 15.0, depending on manufacturer and intended service. As the oil is used, it becomes contaminated with acids and the TBN drops. Generally, TBN levels below 3.0 are considered too low and indicate that the oil should be changed. One cause of TBN depletion relates to the use of low quality, high Sulfur fuel. During the combustion process, this Sulfur turns to Sulfuric acid and in turn, accelerates TBN depletion. As discussed previously, overheating and over-extended drain intervals can cause oil oxidation. The products of oxidation are acidic and will cause the TBN to drop.
Total Acid Number (TAN)
TAN is a property typically associated with industrial oils. It is defined as the amount of acid and acid-like material in the oil. Oxidation and nitration resins make up the majority of this material. As the oil is used, acidic components build up in the lubricant causing the TAN number to increase. A high TAN number represents the potential for accelerated rust, corrosion and oxidation and is a signal that the oil should be replaced. Critical TAN numbers are dependent on oil type. Typically R&O and light duty oils have a maximum TAN of 2 while anti-wear and EP oils may have maximum levels of 3 to 4.
TAN - TBN Ratios in Engine Oils
TAN by itself is of limited value in determining oil condition of an engine oil due to the fact that it represents a combination of different chemical characteristics. The acid-like nature of anti-wear additives found in most modern engine oils cause a high initial TAN. Greatest benefit is derived from the TAN by comparing it to the TBN. TAN increases in service as TBN decreases. The point at which these two numbers meet has been indicated as the maximum useful oil change interval for that type of engine in that type of service. Studies have shown that when TAN exceeds TBN, engine wear accelerates at abnormally high rates.