Physical and chemical stability of lubricating oil.

In simple language, the viscosity may be considered as the resistance of the lubricating oil to flow. It is this property alone due to which the two bearing surfaces are kept apart, which means hydrodynamic lubricating is maintained.

The viscosity of the lubricating oil should be just sufficient to ensure hydrodynamic lubrication. A higher value than this would be of no use since it would involve higher power losses due to the increased oil resistance.

            The viscosity of the lubricating oil, at the time of starting the engine, should be low, otherwise, if the viscosity is very high, the engine may not start/ on the other hand with the engine running, the oil viscosity decreases due to increase of temperature, which is contrary to what is desirable, since at all operating temperatures, the viscosity should not fall below the minimum value required to maintain hydrodynamic lubrication.

 Therefore, the viscosity should ideally remain the same at all temperatures. The relative change of viscosity with temperature is called the viscosity index. However, the viscosity of all the oil does vary with temperature and, therefore, the oil with minimum variation is preferred. Viscosity is the most important property of lubricants and they are mostly selected based on their viscosity and temperature coefficient of viscosity.

Physical stability

The lubricating oil must be stable physically at the lowest and the highest temperatures encountered in practice. There should not be any separation of solids at the lower temperatures and at higher temperatures it should not vaporize beyond a certain limit.

Chemical stability

At higher temperatures, the oil should remain chemically stable. There should not be any tendency for oxide formation; many of the oxidation products being sticky substances clog the lines and cause faulty piston rings and valve action.

            The oil should also not decompose at high temperatures to form carbon. Carbon thus formed lowers ring efficiency, thus reducing engine compression. The spark plugs and the valves also don’t function efficiently due to the sticking of carbon particles.