Piston ring

A piston ring is a split ring that fits into a groove on the outer diameter of a piston in a reciprocating engine such as an internal combustion engine or steam engine.

The three main functions of piston rings in reciprocating engines are :

The gap in the piston ring compresses to a few thousandths of an inch when inside the cylinder bore. Piston rings are a major factor in identifying if an engine is two stroke or four stroke. Three piston rings suggest that it is a four stroke engine while two piston rings suggest that it is a two stroke engine. Most piston rings are made of a very hard and somewhat brittle cast iron.

The split piston ring was invented by John Ramsbottom who reported the benefits to the Institution of Mechanical Engineers in 1854. It soon replaced the hemp packing hitherto used in steam engines. The use of piston rings at once dramatically reduced the frictional resistance, the leakage of steam, and the mass of the piston, leading to significant increases in power and efficiency and longer maintenance intervals.

Piston rings have been an area of considerable focus and development for internal combustion engines. The needs of diesel engines and small piston-ported two-stroke engines have been particularly difficult. Piston rings may account for a considerable proportion of the total friction in the engine, as much as 24%. This high friction is a result of the design compromises needed to achieve good sealing and long lifetime. Sealing is achieved by multiple rings, each with their own function, using a metal-on-metal sliding contact.

Rings are also sprung to increase this contact force and maintain a close seal, either by the stiffness of the ring itself or by a separate spring behind the seal ring. It is important that rings float freely in their grooves within the piston, so that they can stay in contact with the cylinder. Rings binding in the piston, usually due to a build-up of either combustion products or a breakdown of the lubricating oil is a common cause of failure, especially for diesel engines.

Lubrication of piston rings is difficult and has been a driving force to improvements in the quality of motor oil. The oil must survive high temperatures and harsh conditions with a high-speed sliding contact. Lubrication is particularly difficult as the rings have an oscillating motion rather than continuous rotation, as for a bearing journal. At the limits of piston movement, the ring stops and reverses direction. This disrupts the normal oil wedge effect of a hydrodynamic bearing, leading to pronounced wear and the formation of a 'step' in the cylinder bore around the height of the upper ring. Noting that some sleeve valve engines suffered far less from such wear, complex designs such as a rotating cylinder liner have been considered, just to address this problem.

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Wikipedia