What are they and why are they important ?
TAN oil analysis
A common misconception is that a TAN oil analysis is used to determine the acidic strength of an oil. Actually, TAN oil testing is used to find out the amount of acidic components present within the oil, i.e. the acidic concentration. To put this into context, a single molecule of animal fat would give the same TAN reading as a single molecule of hydrochloric acid, even though hydrochloric acid is by far the most corrosive of the two. Indeed, the acid present within a synthetic turbo oil is about the same strength as household vinegar!
TAN Oil analysis is crucial to maintaining the mechanical integrity of equipment and to prevent internal damage to components. An oil's TAN will increase with the passage of time or if exposed to high running temperatures - the oil becomes oxidised (high temperatures cause oil molecules react with the oxygen within the air). Oxidation severely affects an oil's ability to protect internal components and can also affect the viscosity.
In synthetic turbo oils, hydrolysis (a chemical reaction involving water) can also cause an increase in the TAN, especially when the oil is subject to heat.
The TAN is defined as the weight (in milligrams) of a standard base (e.g. potassium hydroxide, KOH) that's required to neutralise all of the acidic components within the oil. Its unit is mg KOHg-1 (milligrams of KOH per gram).
An initial decrease in TAN is no cause for concern - some of the lighter acid compounds present within the oil when it was manufactured will evaporate away which will in turn reduce the TAN.
TBN oil analysis
Oils are continually exposed to acidic compounds which cause the oil to turn more acidic. This is particularly true of crankcase oils. In an attempt to combat this problem, manufacturers give the oil a 'reserve alkalinity' which is designed to 'cancel out' any acidity which forms in the oil during use. The TBN determines how effective the battle will be against any acids formed during the combustion process. A higher TBN means the oil has more reserve alkalinity available which can be used to reduce the corrosive effects of acids.
A low TBN can also reduce the detergency of an oil and can therefore lead to fouling within the crankcase.
As a general rule of thumb, if the TBN is measured at 2.0mg KOH g-1 or less, or if it's 50% of the virgin oil TBN, the oil is considered unfit for engine protection and there is a risk that corrosion could take place. The use of a high sulphur fuel will decrease the TBN at a faster rate due to the increased formation of sulphuric acid.