TBN (Total Base Number)

Total base number, TBN, is the measure of an oil’s alkaline reserve in milligrams of potassium hydroxide equivalent per gram, the capacity to neutralize the acids combustion produces. In cylinder oils the alkalinity comes from overbased calcium or magnesium detergents that react with sulfuric acid from fuel sulfur to form calcium sulfate and water. The cylinder oil’s fresh TBN is matched to fuel sulfur, BN 25 to 40 for the 0.50 percent global cap and 70 to 100 for high-sulfur fuel burned with a scrubber. In system and trunk-piston oils, the used-oil TBN is tracked: a falling TBN warns the oil’s acid-neutralizing reserve is spent and a top-up or change is due.