BLACK SPOTS ON CYLINDER LINERS OF 2-STROKE ENGINES

Occasionally patches of black deposits appear on the cylinder liner wall of 2-stroke engines. The black spots may appear anywhere on the cylinder liner running surface. New cylinder liners with still honing marks (plateau honing) present seem to have a stronger tendency for black lacquer formation.

Analysis shows the deposits mainly consist of calcium sulphate, some water is found, and some oxidation products from hydrocarbons. Calcium sulphate is commonly known as gypsum (CaSO4).

Calcium sulphate is formed due to neutralisation of condensed sulphuric acid by calcium carbonate from the detergent additive of the cylinder oil. Neutralization of condensed sulphuric acid is necessary to prevent corrosion. Under normal conditions the calcium sulphate is carried away by the continuous refreshment of the oil film on the cylinder liner.