On the technical datasheet of engine oil, you can find a lot of data. Indications of viscosity at hot and cold temperatures, density but also TBN, Total Base Number. Behind this acronym, there is the following cold and complicated definition:
“Total Base Number (TBN) is the quantity of acid, expressed in terms of the equivalent number of milligrams of potassium hydroxide that is required to neutralize all basic constituents present in 1 gram of sample (ASTM Designation D 974)”.
However, this is rather abstract and do not give clearly an indication of what TBN in an engine does or how much is needed to get an effective engine protection.
In all the engine, you are burning fuel and as a side reaction, you have some reaction with oxygen, called oxidation. This is a chain reaction that cannot be avoided, and it is accelerated under hot and oxygen-rich environment like a combustion chamber. Oxidation leads to the formation of damaging acids. To measure the quantity of acid, there are different techniques that give the concentration expressed under the acronym TAN: Total Acid Number. One of the main element leading to the formation of strong acids is the presence of sulphur in fuel: the more sulphur, the more acids.
The presence of Sulphur in fuel depends mainly on the local legislation, as shown in the following world map for diesel as example.
To neutralize them, you need a base. Again, to get the concentration, it is measured and expressed as TBN: it shows how much bases are needed to neutralize a certain quantity of acid in the oil. Engine oil formulators can make lubricants with up to a TBN of 70 for the use of some marine engines operating with fuels up to 5% of sulphur, 50 000 part per million (ppm). As a comparison, diesel in Europe have less than 15 ppm of sulphur. Therefore, the requirement of TBN is much lower, around 8-9 for a passenger car Engine oil for example.
One of the best way to determine when to do the oil drain is to follow up with analyses, by measuring TAN and TBN.
TAN will increase overtime, and as the bases are used, the TBN will decrease. When the TAN is higher than the TBN, it is time to change the oil. Ideally, it should be done before the crossing point.
If there is more sulphur in the fuel, the TAN will increase faster, then the bases will be depleted at a more rapid pace. Thus, the drain will have to occur sooner. Therefore, it is sometimes difficult to give an accurate drain interval valid for every countries. Most of the OEM developed vehicles running on low sulphur fuel and give then recommendation of oil drain interval based on a level of 15ppm. You cannot use the same recommendation in countries with a level sometimes 1000 times higher.
TBN is an important part in the engine oil. In countries with high sulphur in fuel, make sur you have a have a strong TBN level to protect your engine against corrosion the best way possible!