Desirable Properties of a Lubricant

The lubricant used must have some properties for the 3uccessful performance of the engine. The properties required for a good lubricant used in I.C. engine are listed below.

1. Viscosity. It is a measure of fluid resistance to flow and the unit used for the viscosity of the oil is poise. This property is most important property of lubricating oil because it determines how efficiently the oil film separates the moving surfaces from each other and prevents them rubbing directly on each other.

The value of the viscosity of lubricant used should lie within a certain range because lower viscosity will cause seizing of the rubbing surfaces therefore viscosity of the lubricant should be selected in such a way that it should not go down below a certain minimum value at the highest temperature at which the parts are likely to operate. The higher viscosity than this will be always safe but it gives higher coefficient of friction which causes greater power loss.

Flash Point: Flash point of the lubricant is the temperature at which it forms vapours and produces combustible mixture with air. The high flash point is always desirable because low flash point will allow the lubricating oil to burn and deposit the carbon on the different moving parts. The minimum flash point of lubricating oil used in I.C. engine varies from 200 to 250°C.

Pour Point. Pour point of the lubricating oil is the temperature below which oil will cease to flow in the pipe line under controlled test conditions. Low pour points are always recommended as its flow will start even when the engine is started in cold weather.

Carbon Residue. The carbon percentage in lubricating oil should be as minimum as possible because its burning forms the carbon deposits on piston head, piston rings and combustion chamber walls and increases the running action.

Neutralisation:. volume expansion  The lubricating oil should be neither acidic nor alkaline otherwise it will have corrosive action on the parts of the engine. The acidity of the oil is generally given by noting the neu'ralization number of an oil. The neutralization numbers of different lubricating oils are given in different codes.