Total Base Number for Engine Oils

Corrosion inhibitors are added to counter acidic effects on metals. In engine oils, reserve alkalinity is included in the formulation to neutralise acids formed by combustion. This is reflected by the Total Base Number (TBN) of an engine oil.

The TBN value of an oil is calculated from the amount of acid that is required to counteract its basic characteristics.  The TBN is expressed as the Equivalent mass in milligrams (mg) of potassium hydroxide (KOH) per gram of the oil.

Pentane Insolubles or Soot Content.

The laboratory can also monitor the amount of detrimental soot contained in an engine oil by filtration of the material insoluble in a solvent called Pentane. This filtration is at 0.8 micron in size, on the basis that material less than 0.8 micron would not be likely to cause problems. The material removed is weighed and expressed as a percentage of the oil. Values below 0.35 % by weight are usually considered acceptable in the normal service interval for a diesel engine.

Levels of about 0.35 % by weight indicate a slight detrimental effect on the oil and gives evidence of “sooting” usually caused by poor ring seal. The root causes of these problems could be excessive periods of idle running, cold running, or fuel washing the oil seal away in cases of defective injectors.

If levels above 0.35 % by weight are experienced a definite problem has occurred and renders the oil unsuitable for further use and should be changed.

Fuel Dilution by Gas Chromatography

Fuel dilution in an engine oil can be caused by several factors. Determining the extent of the contamination by fuel by accurate means is essential for the effective monitoring of engine performance. Gas Chromatography can precisely determine the fuel dilution in a lubricant to as low as 0.2% v/v by separating and quantifying the actual fuel content.  Other methods employed in the past included approximation from flash point values to an accuracy of + or - 4%. In instances where the 2 stroke engine of the Detroit type are used, the 4% margin can be the difference between engine failure or not. This is due to excessive fuel in the oil which can have the effect of thinning it out to an unacceptable level.

Although indication of fuel dilution can be determined from viscosity values in some instances, “sooting”, another product of incomplete combustion of the fuel, can have a thickening effect of the oil and thereby disguise fuel dilution problems.