Cylinder lubrication (BN)

Cylinder lubrication BN, the base number of the cylinder oil, measures the oil’s alkaline reserve in milligrams of potassium hydroxide equivalent per gram, set to neutralize the sulfuric acid formed when fuel sulfur burns and condenses on the liner wall. Acid attack scales with fuel sulfur, so high-sulfur fuel needs high-BN oil, typically BN 70 to 100, while 0.50 percent sulfur fuel under the MARPOL Annex VI global cap runs BN 25 to 40. Over-basing a low-sulfur fuel leaves unreacted calcium carbonate as hard deposits; under-basing a high-sulfur fuel lets cold corrosion pit the liner. Feed rate and BN are matched to fuel sulfur to keep iron in the scrape-down oil within limits.