Viscosity for Liquid Lubricants

Viscosity measurements of new and used oil characterise the lubricant as to its grade.  Viscosity grades are listed as SAE or ISO.

The thickness of an oil is graded and calculated as the Viscosity in mm2/s (Centistokes). ISO oils are tested at 40⁰C. SAE oils are tested at 100⁰C.

The Viscosity Index of the lubricant is a calculated value based on the viscosity values at 40⁰C and 100⁰C. Again, like the viscosity value itself, the VI can be used to characterise or confirm the identity of a lubricant as mono-grade or multi-grade.

Oxidation

Lubricants will oxidise when exposed to air or products of combustion in engine oils. The oxidation level  can be determined using infra-red signatures of the lubricant and any increase in oxidation from the “new oil” value  is a measure of how the oil is standing up to the harsh environment in which it must operate. The smaller the number quoted in the report, the lower the amount of oxidation. Conversely a high oxidation level will indicate the likelihood of the oil thickening and eventual failure of the lubricated component due to a lack of effective lubrication. In applications where the lubricant has only minimal exposure to air  such as sealed gear compartments and hydraulic systems, the oxidation level would not be expected to increase to the same extent as occurs in engine lubrication. As such, the lubricant life is generally longer in these compartments than in engines. Oxidation preventing additives, called oxidation inhibitors or anti-oxidants, are generally incorporated into most formulations to counteract the effect that oxygen and heat, the major cause of the oxidation, have on the lubricant.

Nitration

A major component of air is the gas Nitrogen. In extreme cases, it can react with the lubricant and oxygen to produce an effect called Nitration. In compartments such as gear boxes or hydraulic systems, the nitration effect would be minimal since the exposure to air and high heat (>300 deg C) is rarely encountered. However, in the combustion process in engines, the temperatures exceed 600 degrees C. Oxygen, Nitrogen, fuel and lubricating oil combine to form nitration products including nitrogen oxides which by and large are exhausted to atmosphere. Some can however, find its way past the rings and into the crankcase. Once in the crankcase the nitration product will combine with soot, oxidation and sulphation  products. The nature of the soot (carbon formed by incomplete combustion of the fuel) is such that nitrogen oxides and nitration products are absorbed and retained in the sump oil. Again, as in the case of oxidation, the infra-red signature of the lubricant shows the extent of presence of nitration. As would be expected, the value for a new oil is low and would reflect the relatively small amount of  nitrogen based products formulated into the lubricant as anti-oxidants. As the soot content of the used oil increases, so does the nitration level.