lecture 4 - piston rings

7/23/2019 Lecture 4 - Piston Rings

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Piston Rings

Functions and Operating Principle

- sealing the combustion chamber to the crankcase (compressionrings)- sealing the crankcase to the combustion chamber (oil control rings)- regulating the lubricating oil at the cylinder face- heat conduction from the piston to the cylinder wall- Piston rings must have a split at one point (the ring gap). The gapclearance of theinstalled ring must be small (leakage point), however it may never bezero (seize).


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- The gas pressure provides the sealing by forcing the compression ringstowards thecylinder liner- dditionally, the piston rings are pressed against the cylinder wall bymechanical pre-stress to guarantee sealing during starting and to avoid

vibration in case of high speeds.- !uring a combustion cycle, the contact of the piston ring changes backand forth from the lower and upper ring-groove (mass forces, gas forces,frictional forces).-The mass of the sealing rings should not be too high, due to the potentialfor ring vibrations.

These vibrations can result, if, shortly before combustion top dead centre,the piston ring mass force is higher than the resultant from gas andfrictional force. This condition causes the ring to lift prematurely from thelower groove side and reduces the gaspressure" subse#uently the piston ring can lift o$ of the cylinder wall.

- Piston rings generally rotate during engine operation.


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Piston Ring Designs

- %ompression rings typically have a rectangular cross-section.

- &ings that di$er from this simple form, such as the conical taper-face and the inside bevel, can improve the run-in process by

minimizing the initial contact area.


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- 'n case of coke formation the full keystone ring (double trapezoid ring)

is mainly intended to avoid ring clogging in the trapezoid groove.

- The stepped ring (nose ring) not only provides a sealing e$ect but alsoa distinct oil control e$ect.

- il control rings generally have a circular groove in the centre of their

cylindrical sliding surface which are used to lead the controlled oil to thebottom of the ring groove.- To increase the pressure force of the oil control rings (desirable due tomissing gas pressure support), several types of supporting springs areused" such asepanders, link springs (for the T-*e ring), and special springs (for

spiral-typeepanders).


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Piston Ring Materials and Production

The main re#uirements of piston ring materials are+

- good running properties (low wear, low friction)- high elasticity and mechanical strength- easy production

- These re#uirements are well met by grey cast iron, and is almosteclusively used as

a ring material.- teel is used only if the mechanical strength re#uirements areetreme, even though its elasticity is worse and a running surface mustbe applied due to the poor running properties.


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- enerally, the sliding surface of the upper piston ring, which is subect

to the highestdegree of wear, is hard-chromium-plated. The chromium-plated ringsurface is typicallyground to a crown shape

- 'f protection against stains from baking and amming of the ring isre#uired, the piston ring running surface is /lled or coated withmolybdenum, which has a high melting point (0102 deg %) and good

thermal conductivity.


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Typical Piston Ring Defects

'f the piston rings are not properly designed or are operated under invalid

conditions,the following defects can occur+

lecture 4 - piston rings

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