Piston Ring Functions and Operation
Piston rings are metallic seals and have the function of sealing the combustion chamber from the crankcase and assuring the flow of heat from the piston to the cylinder. Other functions are to prevent the oil not needed for lubrication from passing from the crankcase to the combustion chamber and to provide a uniform oil film on the cylinder bore surface.
To achieve this the piston rings must be in contact with the cylinder wall and piston groove side. Radial contact is generally achieved by means of the inherent spring force of the ring. Fig. 1 shows the forces acting on the piston ring and Fig. 2 the most important terminology used.
Gas pressure in the combustion chamber augments both the radial and the axial contact in the piston ring groove, i.e. the action of the gas pressure increases the sealing capability of the piston ring. Axial contact can alternate between the top and bottom side of the groove owing to the interaction of gas, inertia and friction forces.
The operational characteristics of piston rings, i.e. faultless sealing against combustion gas and lubricating oil, are contingent on the engine design, the thermal and combustion-related dynamic loading, the cylinder design and finish, the piston, the lubricating oil used, the fuel and, most significantly, the quality and design of the rings themselves.
Besides service in the piston cylinder system of internal combustion engines, piston rings are also employed as metallic seals for rotating shafts and are used both as contracting and expanding seals .
Fig. 1: Forces Acting on the Piston Ring
Fig. 2: Piston Ring Nomenclature