High sulphur fuel oil burns to produce oxides of sulphur (SOx) during combustion. In the presence of water, these SOx form sulphuric acid which causes corrosion in the engine; different levels of sulphur in the fuel oil contribute to varying levels of acidity.
One function of the cylinder oil is to protect the engine from acidic corrosion. This is achieved by the alkalinity of the cylinder oil, defined by its base number (BN) and its feed rate. Too much alkalinity however, will result in the formation of excessive abrasive deposits on the piston crown top lands, ultimately leading to increased liner wear and scuffing.