Does Your Engine Oil Base Number Drop? Watch out!


Why Does An Oil’s Base Number Drop?

Engine Oil

Marine Diesel engine lubricating oils have a wide range of TBN and the ECA type fuels have brought in even more varieties.  The properties of a trunk piston lubricant will differ a lot when compared to that of a cross head engine.  In this write-up we will investigate why and how the base number of a lubricant drops.

What is Base Number?

Base number (BN) is a property used to monitor combustion engine oils.  It is defined as the oil’s ability to neutralize acids that are formed as a by-product of combustion, chemical reactions and oil degradation.

Typically, the higher the BN the more acid it will be able to neutralize.  New crosshead engine oils usually have a BN range of 5 to 7, whereas the new trunk piston engine oils usually have a BN ranging from 8 to 40 depending upon the fuel used.  As oil is used in service it gets contaminated with acids, causing the base number to drop over time.  The rate of the drop is determined by the amount of acids introduced to the system.

Why & How BN of a Crosshead engine drops?

New Crankcase Oils have a TBN of 5-8.  Therefore, TBN depletion is a factor in the monitoring of used oil analysis.  TBN in the used oil should not be allowed to fall below 50% of the new value.  A drop of 50% in BN for a crosshead engine oil is a clear indication that something is abnormal and needs immediate action.  This is mainly due to the fact that the crankcase is separated from the combustion space, unlike trunk piston engines.

Probable causes for the change in TBN:

  • Contamination with water or additive depletion.
  • Over usage or the extended lifetime of oil without regular replenishment.
  • Oxidation of the lubricant due to high temperatures.
  • Contamination with cylinder oil drains.

An Increase in BN is most likely due to contamination with high TBN cylinder oil, either through stuffing box leaks or contamination with fresh cylinder oil.  This can be confirmed by an increase in calcium values in the used lubricant analysis reports.  Viswa Lab carries out used lubricant as well as under piston drain oil (scrape down oil) analysis that accurately determines the cylinder condition and wear rate.

Recommended Actions:

A TBN value below the minimum allowable level can normally be corrected by taking the following action.

  • On engines with a small oil charge – change the oil.
  • On engines with a large oil charge – change only a portion of the oil to bring the charge within limits.

An increase in BN has to be carefully monitored, and this increase is equally detrimental for engine operation.  An increase in BN is usually accompanied by an increase in Viscosity.  As you very well know an increase in viscosity has many disadvantages, including but not limited to, increased fuel consumption and heat generation.

Drop in BN for a Trunk Piston Engine:

The most common reasons for a drop in the base number are related to low-quality fuel and oil oxidation.  During combustion, a low-quality fuel with high sulfur content can produce sulfuric acid, which attacks the oil and causes a drop in the base number.  Oil oxidation as a result of the engine overheating or an attempt to extend the oil drain interval is another reason you may see a drop in the BN.  Mechanically, fuel could be entering the crankcase and causing dilution.  This will also result in the rate of BN decay to increase.  As the acidic combustion products mix with the lubricating oil, the BN drops.  Further, Viswa Lab has witnessed and reported that engines which use distillate fuels predominantly suffer either or all of the below points. 

  1. Drop in BN
  2. Drop in Viscosity
  3. Drop in Flash Point

Have you experienced any such rare scenario of a sudden drop in BN of oil or similar kind of incidents? Do share your experience.

Write to us at and we will publish it under your name!


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