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Total Base Number in Marine Engines
What typical TBN levels are found in marine engines? Starting with system oils used in the crankcase of a cross head engine, 6-9 TBN is common. The crankcase is protected from combustion products so there is little need to neutralise acids. Oil is required to keep the crankcase clean and to remove any deposits formed under the piston crown on engines with oil cooled pistons but the overall operating conditions are not arduous. This is also the only application where the TBN can increase. Hence scraper box drains containing very high TBN cylinder oil waste and acid contaminants should not be returned to the crankcase.
Moving up the TBN range are medium and high-speed diesel engines run on distillate fuels. Typical applications are ferries, fishing boats and military vessels. These oils will have a TBN of 8 - 15 or thereabouts. More engine cleanliness is required and there are some combustion products entering the crankcase. Fuel sulphur levels are low (<0.5%) thus the requirement to counter acidic corrosion is also low. These oils are sometimes also used to lubricate the auxiliary engines.
Moving up the TBN scale again there are medium speed diesel engines operated on residual fuel oils. Here the typical TBN could be between 20 and 40 depending on the engine type, load regime and fuel sulphur levels. These engines can be operated on fuel with up to 5% sulphur although 3.5% would be more typical. They are often run at high load factors, producing relatively large amounts of deposits for the oil to retain in suspension.
The traditional upper limit for TBN in medium speed engines was about 40 but most oil suppliers now offer a TBN up to around 55. The reasoning is simple; some modern engine designs (e.g. Wartsila) have very low oil consumption (< 0.5g/kW.h). Relatively little fresh top up oil is used and therefore the TBN can deplete rapidly as it does not benefit from replenishment by fresh oil. The life of the oil charge becomes limited by the TBN depletion rate and using a higher initial TBN will help to extend this. A typical depletion limit is when the TBN level in the sump reaches about 50% of that for the new oil. This is very easy to test for, either on board the vessel or by using an oil analysis service. The figure below illustrates the relationship between TBN depletion and oil consumption for realistic range of oil consumption rates.
Last on the list are the cylinder lubricants used in cross head engines. These oils are used once then either burned off or rejected through the scraper box and scavenge drains. High TBN (and also fast reaction rates for the additives) are required to efficiently neutralise acidic combustion compounds and protect the large exposed areas of the cylinder walls. Typical TBN levels are between 60 and 100 again dependent on the fuel sulphur level, engine design and load regime.
Moving up the TBN range are medium and high-speed diesel engines run on distillate fuels. Typical applications are ferries, fishing boats and military vessels. These oils will have a TBN of 8 - 15 or thereabouts. More engine cleanliness is required and there are some combustion products entering the crankcase. Fuel sulphur levels are low (<0.5%) thus the requirement to counter acidic corrosion is also low. These oils are sometimes also used to lubricate the auxiliary engines.
Moving up the TBN scale again there are medium speed diesel engines operated on residual fuel oils. Here the typical TBN could be between 20 and 40 depending on the engine type, load regime and fuel sulphur levels. These engines can be operated on fuel with up to 5% sulphur although 3.5% would be more typical. They are often run at high load factors, producing relatively large amounts of deposits for the oil to retain in suspension.
The traditional upper limit for TBN in medium speed engines was about 40 but most oil suppliers now offer a TBN up to around 55. The reasoning is simple; some modern engine designs (e.g. Wartsila) have very low oil consumption (< 0.5g/kW.h). Relatively little fresh top up oil is used and therefore the TBN can deplete rapidly as it does not benefit from replenishment by fresh oil. The life of the oil charge becomes limited by the TBN depletion rate and using a higher initial TBN will help to extend this. A typical depletion limit is when the TBN level in the sump reaches about 50% of that for the new oil. This is very easy to test for, either on board the vessel or by using an oil analysis service. The figure below illustrates the relationship between TBN depletion and oil consumption for realistic range of oil consumption rates.
Last on the list are the cylinder lubricants used in cross head engines. These oils are used once then either burned off or rejected through the scraper box and scavenge drains. High TBN (and also fast reaction rates for the additives) are required to efficiently neutralise acidic combustion compounds and protect the large exposed areas of the cylinder walls. Typical TBN levels are between 60 and 100 again dependent on the fuel sulphur level, engine design and load regime.
