Lubrication of all moving parts is essential for the smooth operation of an engine. To supply lubricating oil between the moving parts is termed as lubrication.

There are two stages of lubrication.

Fluid Lubrication. When there is a thin and continuous film of oil separating the moving surfaces, preventing metal to metal contact, it is called fluid lubrication.

Boundary Lubrication. Boundary Lubrication is an intermediate state between dry friction and fluid lubrication. It is basically a state where the lubricant fills the cavities, which exist between friction surfaces.