What’s the function&purpose of piston ring?

Generally the piston ring have 4 functions:

–Sealing

Preventing combustion chamber gas jumped into the crankcase,improving the thermal efficiency.

Leakage will not only reduce the power of the engine, but also make oil deterioration.Sealing is the first function of top ring.

–Controling oil

scraped off the excess oil of the cylinder wall,leaving a thin layer of film to ensure the normal lubrication of the cylinder& piston &ring.

–Transfering heat

Transfer the piston heat to the cylinder wall,70-80% of the piston top heat transmitted and scatered by the piston ring.

–Supporting 
The piston ring keeps the piston in the cylinder and ensure piston smooth movement ,preventing piston knocking the cylinder wall.