What is the purpose of Lubrication in Engine? // Properties of lubrication || knowledge about automobile

What is the purpose of Lubrication in Engine?

 Lubrication is the admittance of oil between two surfaces having relative motion. The purpose of lubrication may be-

1. Reduce friction and wear between the parts having relative motion.

2. Cool the surfaces by carrying away heat generated due to friction.

3. Seal a space adjoining the surfaces such as piston rings and cylinder liner.

4. Clean the surface by carrying away the carbon and metal particles caused by wear.

5. Absorb shock between bearings and other parts and consequently reduce noise.

Properties of lubricants :

1. Viscosity

2.Flash point

3. Fire point

4. Cloud point

5. Pour point

6.Oiliness

7. Corrosion

8. Emulsification

9. Physical stability

10. Chemical stability

11. Neutralisation number

12. Adhesiveness

13. Film strength

14.Specific gravity.

1. Viscosity.

It is the ability of the oil to resist internal deformation due to mechanical stresses and hence it is a measure of the ability of the oil film to carry a load. A more viscous oilcan carry a greater load, but it will offer greater friction to sliding movement of the one bearing surface over the other. Viscosity varies with the temperature and hence if a surface to be lubricated is normally at high temperature it should be supplied with oil of a higher viscosity than would be suitable for, say journal bearings.

2. Flash point.

It is defined as the lowest temperature at which the lubricating oil will flash when a small flame is passed across its surface. The flash point of the oil should be sufficiently high so as to avoid flashing of oil vapours at the temperatures occurring in common use.High flash point oils are needed in air compressors.

3. Fire point.

It is the lowest temperature at which the oil burns continuously. The fire point also must be high in a lubricating oil, so that oil does not burn in service.

4. Cloud point.

When subject to low temperatures the oil changes from liquid stage to a plastic or solid state. In some cases the oil starts solidifying which makes it to appear cloudy. The temperature at which this takes place is called the cloud point.

5. Pour point.

Pour point is the lowest temperature at which the lubricating oil will pour.It is an indication of its ability to move at low temperatures. This property must be considered because of its effect on starting an engine in cold weather and on free circulation of oil through exterior feed pipes when pressure is not applied.

6. Oiliness.

This is the property which enables oil to spread over and adhere to the surface of the bearing. It is most important in boundary lubrication.

7. Corrosion. 

 A lubricant should not corrode the working parts and it must retain its properties even in the presence of foreign matter and additives.

8. Emulsification.

A lubricating oil, when mixed with water is emulsified and loses its lubricating property. The emulsification number is an index of the tendency of an oil to emulsify with water. 

9. Physical stability.

A lubricating oil must be stable physically at the lowest and highest temperatures between which the oil is to be used. At the lowest temperature there should not be any separation of solids, and at the highest temperature it should not vapourize beyond a certain limit.

10. Chemical stability.

A lubricating oil should also be stable chemically. There should not be any tendency for oxide formation.

11. Neutralisation number.

An oil may contain certain impurities that are not removed during refining. The neutralisation number test is a simple procedure to determine acidity or alkalinity of an oil. It is the weight in milligrams of potassium hydroxide required to neutralise the acid content of one gram of oil. 

12. Adhesiveness.

It is the property of lubricating oil due to which the oil particles stick with the metal surfaces. 

13. Film strength.

It is the property of a lubricating oil due to which the oil retains a thin film between the two surfaces even at high speed and load. The film does not break and the two surfaces do not come in direct contact. Adhesiveness and film strength cause the lubricant to enter the metal pores and cling to the surfaces of the bearings and journals keeping them wet when the journals are at rest and presenting metal to contact until the film of lubricant is built up.  

14. Specific gravity.

It is a measure of density of oil. It is an indication regarding the grade of lubricant by comparing one lubricant with other. It is determined by a hydro metre which flats in the oil, and the gravity is read on the scale of the hydrometer at the surface of the oil.