The SATISFACTORY LUBRICATION OF Diesel engines presents some of the most difficult problems encountered by oil technologists. This is especially true of large marine engines, where, due to low speeds and high loads, it is difficult to establish fluid film lubrication. Cylinder lubrication is particularly difficult due to the high temperatures encountered. This problem is more difficult in two-stroke engines than in four-stroke engines as, in the former, there is no non-working stroke during which it is easier to form an oil film on the cylinder walls. Pressure-charged two-stroke engines are the most difficult of all to lubricate satisfactorily. The problem is aggravated in engines operating on residual fuel due to the high sulphur content increasing corrosive wear, and to the abrasive ash forming constituents present in such fuels. In addition, the contaminating influences of partially burnt products of combustion on the crankcase oil have to be considered. The ever-present risk of water leakage into the crankcase oil, either from condensation, or from leakage of the cooling system, influences and often restricts the use of otherwise beneficial additives.