TBN

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TBN is just the number of mg of KOH per g of oil...KOH being a strong base. As KOH neutralizes acid the TBN drops. The higher the starting TBN the more acid can be neutralized. When TBN drops too low the oil no longer neutralizes acids which are a biproduct of combustion - mostly sulfuric acid generated from sulfur in fuel and H2O generated from combustion. When acids are not neutralized they can attack metal surfaces and corrosion occurs. Not too much more to it than that.
 
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Originally Posted by PWMDMD
TBN is just the number of mg of KOH per g of oil...KOH being a strong base. As KOH neutralizes acid the TBN drops. The higher the starting TBN the more acid can be neutralized. When TBN drops too low the oil no longer neutralizes acids which are a biproduct of combustion - mostly sulfuric acid generated from sulfur in fuel and H2O generated from combustion. When acids are not neutralized they can attack metal surfaces and corrosion occurs. Not too much more to it than that.

It's not quite as simple as that. Firstly, the "units" of TBN are mgKOH/g. However this is an equivalence because there isn't actually any KOH in the oil. KOH is used as a reference unit as it is a simple, strong base but the typical inorganic base in an oil is CaCO3. Then there is the organic base materials which also contribute to TBN.

In a road application there is generally very little sulfur in the fuel, so very little formation of sulfuric or sulfurous acids. There are weaker organic acids formed through combustion and oxidation processes though. Engines burning high sulfur fuel (ships, powergen etc) do need more protection against strong acid corrosive wear and consequently have much higher TBNs (anything up to 130 mgKOH/g).
 
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