There are a few ways oxidation, nitration and sulphation can be measured, but the most common way is by FTIR analysis where infra-red light is shone through the sample and the level of absorbance / transmittance (i.e. not absorbed) of the light at various wavelengths is measured. Absorbance’s at various wavelengths suggest presence of different bond types. For instance measuring oxidation we look for the carbonyl (C=O bond) peak, which is present in organic acids. However, this does have interferences such as ester based additives giving an appearance of oxidation even when it is not present.
The most commonly used methods used include JOAP (USA military method which measures area under a peak e.g. 1670 to 1800 for oxidation) and DIN 51453 (the preferred method for most German based engine manufacturers – measuring height at a specific wavelength e.g. 1710 for oxidation, but subtracting the new oil value to baseline the value). This means with the DIN 51453 values such as oxidation can be N/A if no reference is available as you cannot baseline the value if nothing to baseline against. It also means that oxidation values can also be negative if the sample tested has a lower oxidation peak than the new oil being used to baseline. A slightly negative value e.g. -3 or -5 if the oil is fairly new is quite possible as this can be down to normal batch variation in the product additives and also reproducibility of the method. However, vey low negative values suggest a new reference sample is required from the IBC / barrel to confirm the lab are base-lining against the latest/same regional formulation of that product. Although negative values are discussed first, they are quite rare, and diagnostically, it is actually the high oxidation values that are significant as they can indicate deterioration of the oil, which are discussed in the rest of this article.
Example FTIR trace (Blue is new oil, red is used oil). Shows transmittance (so peaks point downwards)
Oxidation – high temperatures and oxygen (in air) increase the rate of oxidation of lubricants where the oxygen chemically bonds with the oil to form organic acids (measured by Acid Number) that can cause corrosion in machinery. Excessive oxidation can result in oil thickening, sludges and varnishes in machinery.
Nitration – This is typically only starts at higher loads, and at prolonged temperatures >70C as the bonds of nitrogen (N2) in air requires approximately twice as much energy to break as the bonds of Oxygen (O2) in air. The rate of nitration formation increases up to a peak of ~130OC, however, at extremely high temperatures (>150OC) nitration products break down leading to a reduction in nitration formation (see graph above)
It is heavily influenced by the air/fuel ratio, reduces when crankcase ventilation and oxygen improves. Like oxidation this can result in oil thickening and sludge formation. Additionally, when combined with water this can form nitric acid causing corrosion to components.
Sulphation– Sulphur present in fuel or engine oil during combustion can combine with water produced during the combustion process to form sulphuric acid (battery acid) which can cause corrosion to engines. Engine oil lubricants are designed with an specialist antacid additives (Base Number) to neutralise the acids (Acid Number).