Piston ring construction

Piston ring construction

Piston rings are worn on the piston create a tightly movable connection between the piston and the cylinder. There are some kinds of piston rings: compression piston ring and oil piston ring.

The first provide the required compression by reducing the amount of gases that erupt from the combustion chamber into the crankcase and divert the heat from the piston head to the cylinders (usually it located in the ring groove to the piston head). Oil piston ring is used for prevents oil from the crankcase into the combustion chamber.

Piston rings are made of special cast aluminum alloy or steel. Piston ring has a rip, which names piston ring lock. It can be direct, oblique or stepped. The most common are piston rings with direct rip.

The diameter of the piston ring in the usual state is larger than the internal diameter of the cylinder, so the ring input to the groove of piston and

The diameter of the piston ring in the free state is larger than the internal diameter of the cylinder, so the input of the ring to the groove of piston do in compression state and after decompression it fits snugly to the inside surface of the cylinder. Gap in the piston ring lock give the ability to expand when it heated.

Piston rings scheme:

Piston ring construction
Piston ring construction

a)     cross-sectional shapes of compression piston rings and how they works; b) composite oil piston ring; c) engine piston head with piston rings; d) scheme of compression piston rings work;  e) scheme of oil piston rings work; 1.1 – rectangular ring; 1.2 – ring with a conical outer surface; 1.3 –
ring with chamfer on the inside; 1.4 – ring with recessed on the inside;

1 – piston ring of disk shape; 2 – axial dilator; 3 – radial expander;  4 – piston ring lock; 5 – compression piston rings; 6 – piston; 7 – hole in the groove of oil piston ring;  8 – cylinder; 9 – oil piston ring; 10 – slot in the ring; 11 – hole in the piston.

There are different forms of cross-sectional shapes compression rings.

The piston ring with the conical outer surface (a) is in contact with the cylinder not with the entire lateral surface, but only with a small edge and exerts considerable pressure on the walls of the cylinder. Such a ring is better to run-in to the cylinder, and the piston-cylinder connection will be better sealed.

A feature of rings with a chamfer is the fact that they are put on the piston and inserted into the cylinder; they are twisted to the center. Such piston rings abut the cylinder mirror with a sharp edge and work the same way as conical ones, but they provide greater tightness of the movable joint as a result of better contact with the end surfaces of the piston groove. Piston rings with chamfers and undercuts are placed on the piston so that the chamfers and undercuts are directed upwards, towards the cylinder head.

The first compression piston ring works in high-temperature conditions and high pressure, so it wears out faster. To improve wear resistance, it is subjected to porous chrome plating or molybdenum spraying. The chrome plating the first compression piston ring increases the durability of others piston rings.   

Oil piston rings is setting set on the piston below the compression piston rings. The main different oil piston rings from compression piston rings in that there are annular grooves, through-slots or oil holes on their outer surface.


Author: delfi

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