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TBN determines how effective the control of acids formed will be during the combustion process. The higher the TBN, the more effective it is in suspending wear-causing contaminants and reducing the corrosive effects of acids over an extended period of time. The associated measurement ASTM D2896 and ASTM D4739-06 generally range from 6-80mg KOH/g in modern lubricants, 7-10mg for general automotive use and 10-15 for Diesel operations.
Marine grade lubricants generally will run from 15-50mgKOH/g, but can be as high as 70 or 80mg KOH/g. This high level is designed to allow a longer operating period between lubricant changes under harsh operating conditions. When the TBN is measured at 2mg KOH/g or less the lubricant is considered inadequate for engine protection, and is at risk for allowing corrosion to take place. Fuels containing a higher amount of sulfur will decrease the TBN sooner due to the increased formation of sulfuric acid.
In other words when we buy new engine oil, it has a 'base reserve' built in to the additive package, which is designed to neutralize the acids as they are produced. As with all acid base reactions, the 'base reserve' is used up in the process of neutralizing these acids.
This Base reserve is called the Total Base Number which is a measure of the level of BASE in the oil and is determined by measuring the amount of Potassium Hydroxide in mg taken to neutralize the base reserve in 1 gram of oil (mgKOH/gm).
In the average engine oil, the starting TBN is usually around 6 to 9, however if we are looking to extended drain interval oils, we need to increase the initial TBN so we don't allow the reserve to be depleted before the oil is drained. In some oils, a starting TBN 15 is not un-common and a depletion level should never get below TBN 3, which is a signal to change the oil.
TBN is also a useful tool in assessing an engines combustion efficiency, if rapid depletion is observed. If an engine's combustion is inefficient, un-burned diesel will enter the oil as blow-by, forming acids and using up TBN reserves. Exhaust emissions will often be visible as excessive smoke which in extreme cases will form 'wet' soot with large soot particles on the exhaust which will have a detrimental effect on engine wear.
Have your TBN checked periodically in your engine to monitor combustion efficiency.
Remember when practicing extended drain intervals in engines, always have your TBN checked, which with viscosity is used as a drain indicator.