Lubrication is essential for any kind of machinery onboard ships. The lubricant’s function in a Marine Engine is reduce friction, cool the internal parts, and assist in removing any debris or impurities.
There are a few main types of lubrication systems used:
- Hydrodynamic Lubrication: In this type of lubrication, the oil forms a continuous oil film of adequate thickness between the moving surfaces. The film is formed due to the motion of the moving parts and the self-generated pressure. For example, journal bearings of Main Engine have hydrodynamic lubrication. A film is formed between the main bearing and the journal of the crankshaft with the help of a wedge formed by the rotating shaft. Thrust bearings with a tilted pad design also have this type of lubrication as they form a converging wedge to obtain hydrodynamic lubrication.
- Hydrostatic Lubrication: Where oil film cannot be formed due to the motion of moving parts, the oil pressure has to be supplied externally. Such type of lubrication is known as Hydrostatic lubrication. For slow-moving heavy parts, their relative motion is not enough to provide self-generated pressure for lubrication and hence pressure is provided externally with the help of a pump. For example, many crosshead bearings design requires an additional crosshead lubrication pump to boost the pressure for crosshead bearing lubrication because the pressure cannot be self-generated.
- Boundary Lubrication: In this type, there is a thin film between two rubbing surfaces, which might have a surface contact. Boundary lubrication is used because of relatively slow speeds, high contact pressure and rough surfaces. For example, boundary lubrication in main engines occur during starting and stopping due to the above-mentioned conditions.
- Elastohydrodynamic Lubrication: In this type of lubrication, the lubricating film thickness considerably changes with elastic deformation of surfaces. This is seen in-line or at the point of contact between rolling or sliding surfaces, for example, rolling contact bearings and meshing gear teeth. Elastic deformation of metal occurs and there is an effect of high pressure on the lubricant.
The ability of an oil to react with an acidic reagent, which indicates the alkalinity, is expressed as TBN. It stands for Total Base Number. It should correspond to the sulphur percentage of fuel oil to neutralise the acidic effect of combustion. When high sulphur fuel oil is used for Main engines, a high TBN grade of cylinder oil needs to be used. When the main engine is a “change-over” to Low Sulphur Fuel Oil (LSFO) or Low Sulphur Marine Gas Oil (LSMGO), low TBN cylinder oil needs to be used.
Correct lubrication is essential for efficient engine operation, minimize lubricating oil costs, and optimize maintenance costs. It is essential that the cylinder lubricators are correctly set and that the correct cylinder lubricating oil is used for the fuel being burned.