what is the lubrication system in an engine? and why do we use lubrication oil?

The lubrication circuit is one of the most important ones in the engine. The engine cannot run smoothly for more than a few minutes without the lubricating oil.

Whenever two metallic surfaces move over each other under direct contact dry or solid fraction is produced. This is due to the irregularities on the two surfaces interlocking each other. The dry friction thus created produces a lot of heat and results in wear of the metal surface.

  However, when a film of some lubricating oil is interposed between two surfaces so that the two are not in actual physical contact with each other, the only resistance to motion remains the resistance of the oil itself. This type of friction is termed as viscous or fluid friction. This state, from a lubrication point of view, is called hydrodynamic lubrication. It is observed in practice that the coefficients of friction are obtained in viscous friction.

 Thus it is the viscous friction that is most desirable and every effort is made to keep the viscous friction conditions in the running engine. It is because of this fact that utmost importance is attached to the lubrication circuit and it is recommended not to run the engine even for a while when the lubricating oil is not present in proper quantity in the engine.

A third type of friction, however, also exists which means greasy or boundary friction which state is also termed boundary lubrication from a lubricating viewpoint. In this, some lubricant is there but that is insufficient to cause the complete separation of the two surfaces which means only their high sport touch.

 In this, the friction coefficients are undoubtedly higher than in the case of viscous friction, but nevertheless, they are far less than the coefficients in dry friction. Boundary lubrication may exist in piston rings and valve trains.