PISTON RINGS

PISTON RINGS

Types: 
1. Compression rings
2. scrapper rings

Compression rings

• Fitted into grooves provided in the piston crown.
• serves to prevent blow-bys.

Higher speed engine uses less rings then low-speed engines because of the fact that higher piston speeds lessen the possibility of a blow-by.

Q. HOW SEALING EFFECT IS PROVIDED BY COMPRESSION RING?
A. due to the pressure of the gases entering the so-called back clearance b/w the piston and ring and partly due to ring tension.

Maximum gas pressure comes onto the first and second rings next to the combustion chamber.
Pressure decreases for the lower rings due to the throttling effect of the labyrinth formed by the back clearance.

TYPE OF LUBRICATION FOR COMPRESSION RING
operates under boundary lubrication

• To ensure ring tension and allow for the expansion of the rings as they become hot, joints are provided in the rings.
• Ring diameter in a free state is always larger then the bore.

SHAPE OF JOINT

1. Straight
2. Bevelled
3. Stepped

• Low speed diesel engines employ bevelled and stepped joints
• High speed diesel engines employ straight joint.

• Experiment proves that the piston rings rotates in the operation.
• On the other hand it is desirable that on 2-stroke engines the piston ring retain their original position so that no joint would appear against the ports in the cylinder liner and become trapped and broken when the engine is running.
• Special retainers are integrally made with the piston ring serve to secure this in place.