All about Piston rings

Most of you would be knowing about piston rings. Many of you might also have have got a chance to see them. They are small but very important part of engine.Let's today discuss about them. 

Piston rings

Piston Rings

Piston rings are thin rings fitted on outer boundary of piston. There are thin grooves on piston surface and the rings are fitted in those grooves. The rings are not complete circle, they have a free joint for their fitment on piston. 

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Material of Piston Rings

The pistons rings are generally made of cast iron ,since cast iron is wear resistant. The rings continuously move  against cylinder walls, so they have very high chances of wear . That's why they have to be made from some wear resistant material. Apart from this cast iron can easily be coated or treated to improve its properties. The piston rings also transfer the heat from piston to cylinder block, that's why they need to be good conductor of heat also. Therefore sometimes they may be made from Aluminium alloys.

Function of piston rings

Piston and the Rings

The piston rings have many functions. The most important being that they provide a sealing between the combustion chamber and the crankcase. They prevent the combustion gases from leaking inside crankcase and the oil on cylinder walls from leaking inside the combustion chamber. This helps in maintaining pressure inside the combustion chamber and also prevent the degradation of engine oil by the combustion gases. The oil rings scrap the excess engine oil present on the cylinder walls and hence help in uniform lubrication of walls. This also reduce the chances of leaking of excess engine oil to the combustion chamber, which may generate a lot of pollutants on burning . The other major function is heat transfer. The temperature of the combustion chamber is very high since it continuously burns the fuel. That high temperature may lead to unwanted thermal expansion of piston causing the engine to be seized. That heat needs to to transfered to cylinder block, and this is done by the piston rings. They prevent the wear of piston by preventing its contact with cylinder block. They also helps the piston to reciprocate easily inside cylinder since their use decreases the surface in contact and hence reducing the contact  friction. In case of wear only the rings are to be replaced and the piston remains safe.

Types of Piston Rings

Three types of Piston rings

The four stroke engines generally contain three type of rings. The top two rings are called the compression rings, based on their function. The third one is oil rings. There ate two oil rings separated by a separator which is a zig-zag ring. The second one may also be called an intermediate rings, since it peforms the functions of both, the compression ring and the oil scrapper rings.

Arrangement of Piston Rings

Arranging piston rings on piston is a very important task. You need to be quite careful while fitting them.  A wrong fitment can lead to combustion gases leaking inside crankcase. The two oil rings and their separator are arranged at 120 degrees to each other. The separator is fitted such that it's  free joint is just below exhaust valve. The two oil rings are arranged making 120 degree angle to it. The intermediate ring (second compression ring) is fitted such that it's free joint is just above the free joint of separator ring.  Now the compression ring 1 is arranged such that it's free joint is 180 opposite to that of the intermediate ring.

This was my small effort to explain the functions of the Piston Rings. I hope you liked it. Please give your feedback in comment section.