Purpose of Piston Rings

Piston rings serve three important functions:

(a) They provide a seal between the piston and the cylinder wall to prevent the force of the exploding gases from leaking into the crankcase from the combustion chamber. This leakage is referred to as blow by.

Blow by is detrimental to engine performance because the force of the exploding gases will merely bypass the piston rather than push it down. It also contaminates the lubricating oil.

(b) They prevent the lubricating oil from bypassing the piston and getting into the combustion chamber from the crankcase.

(c) They provide a solid bridge to conduct the heat from the piston to the cylinder wall. About one third of the heat absorbed by the piston passes to the cylinder wall through the piston rings.

Piston rings are split to allow for installation and expansion, and they exert an outward pressure on the cylinder wall when installed. They fit into grooves that are cut into the piston, and are allowed to float freely in these grooves.

A properly formed piston ring, working in a cylinder that is within limits for roundness and size, will exert an even pressure and maintain a solid contact with the cylinder wall around its entire circumference.

Although piston rings have been made from many materials, cast iron has proved most satisfactory as it withstands heat, forms a good wearing surface, and retains a greater amount of its original elasticity after considerable use.

There are two basic classifications of piston rings.

a. The compression ring seals the force of the exploding mixture into the combustion chamber.

b. The Oil Control Ring. The oil control ring prevents the engine's lubrication oil from getting into the combustion chamber.

Piston rings are arranged on the pistons in three basic configurations. They are:

(a) The three ring piston has two compression rings near the head, followed by one oil control ring. This is the most common piston ring configuration.

(b) The four ring piston has three compression rings near the head, followed by one oil control ring. This configuration is common in diesel engines because they are more prone to blow by, due to the much higher pressures generated during the power stroke.

(c) The fourring piston has two compression rings near the head, followed by two oil control rings. The bottom oil control ring may be located above or below the piston pin.

This is not a very common configuration in current engine design. In addition to the configurations mentioned, there are some diesel engines that use five or more piston rings on each piston to control the higher operating pressures.

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