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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