Lubricating Oil in Marine Disel Engines: What are the Properties of Lub Oil used in Ships?

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We have been studying about lubricating oil system used in marine diesel engines in past several articles and learnt about crankcase lubrication system, cylinder lubrication system and so forth. In this article we will learn about some of the basic properties of the lubricating oil used in these systems and ways to take care of the same.

Properties and Checks

  • Oiliness – this refers to the property of the oil to stick around to the surfaces and is useful in situations such as those involving journal bearings.

  • Stability – the oil is said to be more stable if it can withstand relatively higher temperatures without cracking and this is useful in conditions of cylinder lubrication

  • Alkalinity – the lubricating oil used in corrosive conditions such as lubricating of cylinder liners is mixed with certain additives to make it alkaline. This helps to protect the liner surface by neutralizing against the effects of attack by acidic residues of combustion

  • Detergency – additives are mixed with lubricating oil in order to reduce the deposit formation tendency at important positions and this helps the system to be kept clean.

  • Volatility – if the flash point of the oil is too low it might ignite with high temperature and therefore it should normally be above 200 degrees Celsius.

  • Viscosity – this refers to the property which determines the ease of flow of the oil between small clearance spaces. It should neither be too low nor too high for proper lubrication to take place. If the viscosity is too low the oil will not provide proper boundary lubrication and if it is too less it will not flow properly in all the places required, hence the need to maintain optimum viscosity.

As the oil is used to lubricate the engine its properties deteriorate over a period of time due to addition of impurities which could include un-burnt fuel, water, acid, suspended particles and so forth. Purification and filtration of the oil is carried out during its use but still there is a limit to which the same oil can be used and at a certain stage it will need to be replenished.

Since lubricating oil performs a useful task and undue deterioration in its properties could damage the engine components, a close watch needs to be kept on the quality of the oil. Various checks include physical examination, blotting paper test, water content test and so on. Lubricating oil test kits are available on board ships which are used by duty engineers to check for the quality of the oil. Apart from that the oil should be renewed after a specified number of running hours depending on the exact engine in question.