Cross head engines use cylinder oil to lubricate the piston, rings and liner.
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Cylinder Oil
Cylinder Oil is used on slow speed cross head type engines where the piston is separated from the crankcase. This type of engine is mainly used for marine installations where the very low crankshaft speed allows a direct drive for a large diameter propeller. A few examples are used for land based power generation.
The original function of a cylinder oil was to lubricate the piston ring/cylinder liner wall interface and provide a dynamic gas seal. At or around TDC, these requirements become crucial and very difficult to meet. The piston rings change from upward and downward motion with largely boundary lubrication conditions, whilst gas pressures and the piston ring face load is rapidly rising.
Higher viscosity oils offer a solution, providing film strengths under boundary conditions to both maintain the gas seal and separate the piston ring from the liner wall.
The use of heavier grades of residual fuel oil and an associated higher sulphur content has increased the acid neutralisation requirement of the cylinder oil. In some cylinder oils, the additive comprises over 30% of the product; in other words, a large amount of the lubricant is additive and not oil.
Super long-stroke engines and higher combustion pressures have increased the problems facing the cylinder lubricants. Engine builders are calling for liner wall temperature to be increased from an average of 160°C to 190°C in the interest of increased thermal efficiency and greater margins to avoid acid dew point temperatures. In order to maintain oil film conditions at these higher linear surface temperatures, heavier grades of cylinder oil having viscosity of 20-24cSt @ 100°C are now common. The drawback to this is a reduced tendency for oil to spread over the surface of the cylinder liner. However, at the elevated temperature of the cylinder liner, the viscosity difference between the top end of SAE 50 grade and lower end is minimised (approximately 2-3.5cSt @ 200°C) so the effect of increased viscosity is small.