Above: Kittiwake TBN test for oils TBN 0-55.
Put simply, the governing factor for the rate of decay of TBN is the fuel sulphur level and the initial sump volume. The governing factor for the stable final TBN level is the oil consumption rate and the fuel sulphur level.
Info Centre >> Fuel and Lube Oil Technical Manual >> 7. Lube Oil - Test Results >> 7.1 Key Condition Indicators for Lube Oils >> 7.1.5 Total Base Number (TBN)
Total Base Number (TBN) test for diesel engine lubricants
This TBN test is only relevant to diesel engine lubricants. It is not relevant for gear oils or hydraulic oils. Alkaline additives are present to neutralise acids derived from both combustion (mainly strong sulphuric and nitric acids) and those weaker, organic acids resulting from oxidation of the oil as occurs during ageing.
TBN is a measurement of the capacity of engine oil for neutralising strong acids from combustion of fuel oil. It is not a measure of how alkaline an oil is (the alkalinity is more akin to soapiness than strong alkali) but it instead measures the alkaline reserve of the oil or its ability to neutralise acids.
The TBN of trunk piston diesel engines (high and medium speed) will fall due to exposure to combustion products but generally reaches a stable level as consumption of TBN by neutralisation is matched with replenishment by fresh oil top-up. TBN of system oils in large 2 stroke cross head type engines may rise due to contamination of the oil with very high TBN cylinder oil draining, via the stuffing boxes or from top-up with incorrect oil grades.
A drop of around 50% of fresh oil TBN indicates that the oil is almost at the end of its useful life. Another useful indicator is a minimum TBN equal to 7 times the sulphur content of the fuel in use. Oil suppliers usually recommend a change or partial replacement at this level in order to optimise the acid neutralising properties of the oil. This recommendation is mostly based on engine manufacturer’s advice.
Low TBN reserves provide insufficient neutralisation capacity leading to corrosion of engine components particularly around the piston ring pack, piston ring lands and top end bearing. Fouling of the engine internals and under piston cooling ways may also increase.
TBN in gas-fuelled engines is often achieved using a very low ash additive pack. Additives are often based on automotive practice using magnesium in preference to calcium salts. Low ash properties are specified, as hot ash on combustion components can result in pre-ignition of the gas during the induction or compression strokes. TBN in these applications can fall very rapidly due to high operating temperatures and, if using land fill gas, contaminants in the fuel itself.
Rapid TBN decrease is caused by:
| Oil TBN Typical Application | |||||||||||||||||||
5 |
System oils, slow speed cross head engines. High speed diesel and gas engines run on automotive diesel. Fuel sulphur <0.2%. Automotive diesel engines. | ||||||||||||||||||
10 |
High speed diesel engines, distillate fuels sulphur levels <0.5%, gas oil, F76 etc. | ||||||||||||||||||
15 |
High and medium speed diesel fuel DBM (DMC max.) | ||||||||||||||||||
20 |
Medium speed trunk piston engines. Fuel sulphur <2% (DMC max.). Possible blended fuel in auxiliary engines. | ||||||||||||||||||
30 |
Medium speed trunk piston engines, any fuel. | ||||||||||||||||||
45 - 55 |
Medium speed trunk piston engines especially those with smaller sump capacity or low oil consumption, any fuel. | ||||||||||||||||||
>60 - 100 |
Cylinder Oils, slow speed cross head engines. | ||||||||||||||||||