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We had studied about the cylinder lubrication system used in large 2-stroke marine diesel engines used for the main propulsion of the ship propeller. This system is kept separate from the main lubricating system because of a different set of requirements involving high temperatures, pressures and a corrosive environment, as we saw earlier. Now we will take a look at the main lubricating oil system of the marine diesel engines which supplies lubricating oil to crankcase bearings and other external parts of the engine.
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Crankcase Lubricating Oil System
The system consists of a big reservoir at the bottom of the engine known as the oil sump which contains a sufficient quantity of lubricating oil all the time. There is a level indicator which should be frequently checked and oil should be replenished if required. Also routine lubricating oil tests are carried out to test the various properties of oil to see if it requires to be changed otherwise it can be changed after the specified interval of time.
There are normally two parallel pumps which take suction from the lubricating oil sump and usually only one pump is used at a time while the other is rested. This also provides time for repair if anything goes wrong with one pump and the other one can be used in the meantime. There are also lubricating oil filters in the line which should be periodically cleaned and replaced as and when required.
The oil from the sump gets distributed into various places and finally drains back into the sump after fulfilling its purpose of lubrication and cooling. Obviously the oil also gets contaminated during this cycle as it also gathers many impurities on its way. The system of taking care of this is different depending on whether we are talking about smaller 4 stroke engines which are used for power generation on big ships or the main propulsion engine. In case of large engines there is normally a separate circuit from the sump which purifies the oil continuously by passing it through a lubricating oil purifier and passes it back to the lubricating oil sump. In case of smaller engines the oil is simply replenished frequently and changed at the required intervals. The lubricating oil purifier or a centrifuge is a subject matter of separate discussion and we will take it up in a different article later on.
In case of large engines, there is an oil cooler which cools the oil as it returns back to the sump after absorbing heat which keeps the oil at the required temperature. The cooler is located in such as position that it can be easily accessed for cleaning at regular intervals. The bulk of fresh lubricating oil is stored in separate tank known as storage tank on the ship and whenever fresh oil is required for replenishment or renewal, it is taken from this tank.