{"history":[],"article":{"id":"ce732032-c227-4397-8292-3e657fd5db10","title":"Piston ring","slug":"piston-ring","content":":::summary\n:::\n\n# \nA **[piston](https://bliptext.com/articles/piston) ring** is a metallic split ring that is attached to the outer diameter of a [piston](https://bliptext.com/articles/piston) in an [internal combustion engine](https://bliptext.com/articles/internal-combustion-engine) or [steam engine](https://bliptext.com/articles/steam-engine).\nThe main functions of piston rings in engines are:\nMost piston rings are made from [cast iron](https://bliptext.com/articles/cast-iron) or [steel](https://bliptext.com/articles/steel).\n# [Two piston rings mounted on a two-stroke engine piston. The ring gap for the bottom ring is visible in the centre of the image. | upload.wikimedia.org/wikipedia/commons/1/12/47mm///piston///rings.jpg]\n\n# Design\nPiston rings are designed to seal the gap between the piston and the cylinder wall.\nIf this gap were too small, thermal expansion of the piston could mean the piston seizes in the cylinder, causing serious damage to the engine.\nOn the other hand, a large gap would cause insufficient sealing of the piston rings against the cylinder walls, resulting in excessive [blow-by](https://bliptext.com/articles/blow-by) (combustion gases entering the crankcase) and less pressure on the piston, reducing the power output of the engine.\nThe sliding motion of the piston ring inside the cylinder wall causes friction losses for the engine.\nThe friction caused by piston rings is approximately 24% of the total mechanical friction losses for the engine.\nThe design of the piston rings is therefore a compromise between minimising friction while achieving good sealing and an acceptable lifespan.\nLubrication of piston rings is difficult and has been a driving force to improvements in the quality of [motor oil](https://bliptext.com/articles/motor-oil).\nThe oil must survive high temperatures and harsh conditions with a high-speed sliding contact.\nLubrication is particularly difficult as the rings have an oscillating motion rather than continuous rotation (such as in a bearing journal).\nAt the limits of piston movement, the ring stops and reverses direction.\nThis disrupts the normal oil wedge effect of a [hydrodynamic bearing](https://bliptext.com/articles/hydrodynamic-bearing), reducing the effectiveness of the lubrication.\nRings are also sprung to increase the contact force and to maintain a close seal.\nThe spring force is provided by either the stiffness of the ring itself or by a separate spring behind the seal ring.\nIt is important that rings float freely in their grooves within the piston, so that they can stay in contact with the cylinder.\nRings binding in the piston, usually due to a build-up of either combustion products or a breakdown of the lubricating oil, can cause engine failure and is a common cause of failure for diesel engines.\n# [Piston ring configurations:
\nA) Rectangular section
\nB) Barrel face
\nC) keystone
\nD) Torsional twist
\nE) Taper face
\nF) Dykes
\nWhich also differ for the type of retainer band:
\nY) Behind-band
\nX) Above or under-band
\nOr for the ends that work without firm stops:
\nK) Step
\nJ) Oblique
\nW) Oblique with step | upload.wikimedia.org/wikipedia/commons/9/98/Fasce///eleastiche.svg]\n# [Piston ring. The piston ring (PR) is a split band pressed against the wall of the cylinder by springs (S) mounted in the inner \"junk ring\" (JR). The tongue (T) maintains the seal as the ring expands and splits apart. | upload.wikimedia.org/wikipedia/commons/5/56/piston///ring///illustrated.png]\n\n## Number of rings\nSealing is often achieved by multiple rings, each with their own function, using a metal-on-metal sliding contact.\nMost pistons have at least two piston rings per cylinder.\nAutomotive piston engines typically have three rings per cylinder.\nThe top two rings—known as compression rings—are primarily for sealing the combustion chamber.\nThe bottom ring—known as the oil control ring—is primarily for controlling the supply of oil to the cylinder wall, in order to lubricate the piston skirt and the oil control rings.\n\n## Ring construction\nThe compression rings in an automotive engine typically have a rectangular or [keystone](https://bliptext.com/articles/keystone-architecture) shaped cross-section.\nThe upper compression ring typically has a barrel profile for the periphery, while the lower compression ring typically has a taper napier facing.\nSome engines also use a taper facing for the top ring, and simple plain-faced rings were used in the past.\nOil control rings are typically made from either a single piece of cast iron, multiple pieces of steel, or steel/iron with a helical spring backing to create the tension required for a close seal.\nCast iron oil rings and rings with a helical spring backing have two scraping lands of various detailed form.\nOn the other hand, multi-piece steel oil control rings usually consist of two thin steel rings (called rails) with a spacer-expander spring between them to keep the two rails apart and provide radial pressure.\nThe gap in the piston ring compresses to a few thousandths of an inch when inside the cylinder bore.\nRing gap shapes include square cut, angle cut, tite joint, step cut, hook step and mitre step.\n\n# History\nEarly steam engines used a hemp packing to seal the combustion chamber, which caused high frictional resistance and did not provide a very effective seal.\nThe first use of a piston ring in the cylinders of a steam engine appears in 1825 by [Neil Snodgrass](https://bliptext.com/articles/neil-snodgrass), a Glasgow engineer and mill-owner, for use in his own machines.\nThis used springs to keep the seal steam-tight.\nFrom use within the mill this was experimented on the steamer [\"Caledonia\"](https://bliptext.com/articles/the-caledonia-paddle-steamer) which plied the [Gareloch](https://bliptext.com/articles/gareloch).\nThe modern design of a metallic split-ring was invented by [John Ramsbottom](https://bliptext.com/articles/john-ramsbottom-engineer) in the 1850s.\nRamsbottom's initial design in 1852 was a circular shape, however these wore unevenly and were not successful.\nIn 1854, a revised design was claimed to have a lifespan of up to 4000 mi. This was based on the discovery that a perfectly round (prior to installation) ring with a split in it does not exert an even pressure on the cylinder walls once installed.\nThe revised piston ring was manufactured to an out-of-round shape, so that it would exert even pressure once installed in the cylinder.\nAn 1855 patent documented this change.\nThe switch to metallic piston rings 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.\n# [Steam engine with 3 piston rings at location D | upload.wikimedia.org/wikipedia/commons/3/3c/horizontal///steam///engine.png]\n# [Spring-loaded piston rings | upload.wikimedia.org/wikipedia/commons/9/93/Piston///rings.jpg]\n\n# Engine wear\nPiston rings are subject to wear as they move up and down the cylinder bore, due to their own inherent load and due to the gas load acting on the ring.\nTo minimize this, they are made of wear-resistant materials, such as cast iron and steel, and are coated or treated to enhance the wear resistance.\nCoatings used in modern motorcycles include [chromium](https://bliptext.com/articles/chromium), [nitride](https://bliptext.com/articles/nitride), or ceramic coating made by [plasma deposition](https://bliptext.com/articles/plasma-enhanced-chemical-vapor-deposition) or [physical vapour deposition](https://bliptext.com/articles/physical-vapor-deposition) (PVD).\nMost modern [diesel engines](https://bliptext.com/articles/diesel-engine) have top rings coated with a modified chromium coating (known as CKS or GDC), which has aluminium oxide or diamond particles respectively included in the chrome surface.\nIn two-stroke engines, the port design is also an important factor for the lifespan of the piston rings.","current_revision":null,"created_by":"kACnuQmH5k9Nr5VNVEpVeg7XpvUmMctD","created_at":"2025-01-25T22:28:03.037Z","updated_at":"2025-01-25T22:28:03.037Z","search_vector":"'piston':1 'ring':2","search_content":" \n \n\n# \nA **[piston](https://bliptext.com/articles/piston) ring** is a metallic split ring that is attached to the outer diameter of a [piston](https://bliptext.com/articles/piston) in an [internal combustion engine](https://bliptext.com/articles/internal-combustion-engine) or [steam engine](https://bliptext.com/articles/steam-engine).\nThe main functions of piston rings in engines are:\nMost piston rings are made from [cast iron](https://bliptext.com/articles/cast-iron) or [steel](https://bliptext.com/articles/steel).\n# [Two piston rings mounted on a two-stroke engine piston. The ring gap for the bottom ring is visible in the centre of the image. | upload.wikimedia.org/wikipedia/commons/1/12/47mm///piston///rings.jpg]\n\n# Design\nPiston rings are designed to seal the gap between the piston and the cylinder wall.\nIf this gap were too small, thermal expansion of the piston could mean the piston seizes in the cylinder, causing serious damage to the engine.\nOn the other hand, a large gap would cause insufficient sealing of the piston rings against the cylinder walls, resulting in excessive [blow-by](https://bliptext.com/articles/blow-by) (combustion gases entering the crankcase) and less pressure on the piston, reducing the power output of the engine.\nThe sliding motion of the piston ring inside the cylinder wall causes friction losses for the engine.\nThe friction caused by piston rings is approximately 24% of the total mechanical friction losses for the engine.\nThe design of the piston rings is therefore a compromise between minimising friction while achieving good sealing and an acceptable lifespan.\nLubrication of piston rings is difficult and has been a driving force to improvements in the quality of [motor oil](https://bliptext.com/articles/motor-oil).\nThe oil must survive high temperatures and harsh conditions with a high-speed sliding contact.\nLubrication is particularly difficult as the rings have an oscillating motion rather than continuous rotation (such as in a bearing journal).\nAt the limits of piston movement, the ring stops and reverses direction.\nThis disrupts the normal oil wedge effect of a [hydrodynamic bearing](https://bliptext.com/articles/hydrodynamic-bearing), reducing the effectiveness of the lubrication.\nRings are also sprung to increase the contact force and to maintain a close seal.\nThe spring force is provided by either the stiffness of the ring itself or by a separate spring behind the seal ring.\nIt is important that rings float freely in their grooves within the piston, so that they can stay in contact with the cylinder.\nRings binding in the piston, usually due to a build-up of either combustion products or a breakdown of the lubricating oil, can cause engine failure and is a common cause of failure for diesel engines.\n# [Piston ring configurations: \nA) Rectangular section \nB) Barrel face \nC) keystone \nD) Torsional twist \nE) Taper face \nF) Dykes \nWhich also differ for the type of retainer band: \nY) Behind-band \nX) Above or under-band \nOr for the ends that work without firm stops: \nK) Step \nJ) Oblique \nW) Oblique with step | upload.wikimedia.org/wikipedia/commons/9/98/Fasce///eleastiche.svg]\n# [Piston ring. The piston ring (PR) is a split band pressed against the wall of the cylinder by springs (S) mounted in the inner \"junk ring\" (JR). The tongue (T) maintains the seal as the ring expands and splits apart. | upload.wikimedia.org/wikipedia/commons/5/56/piston///ring///illustrated.png]\n\n## Number of rings\nSealing is often achieved by multiple rings, each with their own function, using a metal-on-metal sliding contact.\nMost pistons have at least two piston rings per cylinder.\nAutomotive piston engines typically have three rings per cylinder.\nThe top two rings—known as compression rings—are primarily for sealing the combustion chamber.\nThe bottom ring—known as the oil control ring—is primarily for controlling the supply of oil to the cylinder wall, in order to lubricate the piston skirt and the oil control rings.\n\n## Ring construction\nThe compression rings in an automotive engine typically have a rectangular or [keystone](https://bliptext.com/articles/keystone-architecture) shaped cross-section.\nThe upper compression ring typically has a barrel profile for the periphery, while the lower compression ring typically has a taper napier facing.\nSome engines also use a taper facing for the top ring, and simple plain-faced rings were used in the past.\nOil control rings are typically made from either a single piece of cast iron, multiple pieces of steel, or steel/iron with a helical spring backing to create the tension required for a close seal.\nCast iron oil rings and rings with a helical spring backing have two scraping lands of various detailed form.\nOn the other hand, multi-piece steel oil control rings usually consist of two thin steel rings (called rails) with a spacer-expander spring between them to keep the two rails apart and provide radial pressure.\nThe gap in the piston ring compresses to a few thousandths of an inch when inside the cylinder bore.\nRing gap shapes include square cut, angle cut, tite joint, step cut, hook step and mitre step.\n\n# History\nEarly steam engines used a hemp packing to seal the combustion chamber, which caused high frictional resistance and did not provide a very effective seal.\nThe first use of a piston ring in the cylinders of a steam engine appears in 1825 by [Neil Snodgrass](https://bliptext.com/articles/neil-snodgrass), a Glasgow engineer and mill-owner, for use in his own machines.\nThis used springs to keep the seal steam-tight.\nFrom use within the mill this was experimented on the steamer [\"Caledonia\"](https://bliptext.com/articles/the-caledonia-paddle-steamer) which plied the [Gareloch](https://bliptext.com/articles/gareloch).\nThe modern design of a metallic split-ring was invented by [John Ramsbottom](https://bliptext.com/articles/john-ramsbottom-engineer) in the 1850s.\nRamsbottom's initial design in 1852 was a circular shape, however these wore unevenly and were not successful.\nIn 1854, a revised design was claimed to have a lifespan of up to 4000 mi. This was based on the discovery that a perfectly round (prior to installation) ring with a split in it does not exert an even pressure on the cylinder walls once installed.\nThe revised piston ring was manufactured to an out-of-round shape, so that it would exert even pressure once installed in the cylinder.\nAn 1855 patent documented this change.\nThe switch to metallic piston rings 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.\n# [Steam engine with 3 piston rings at location D | upload.wikimedia.org/wikipedia/commons/3/3c/horizontal///steam///engine.png]\n# [Spring-loaded piston rings | upload.wikimedia.org/wikipedia/commons/9/93/Piston///rings.jpg]\n\n# Engine wear\nPiston rings are subject to wear as they move up and down the cylinder bore, due to their own inherent load and due to the gas load acting on the ring.\nTo minimize this, they are made of wear-resistant materials, such as cast iron and steel, and are coated or treated to enhance the wear resistance.\nCoatings used in modern motorcycles include [chromium](https://bliptext.com/articles/chromium), [nitride](https://bliptext.com/articles/nitride), or ceramic coating made by [plasma deposition](https://bliptext.com/articles/plasma-enhanced-chemical-vapor-deposition) or [physical vapour deposition](https://bliptext.com/articles/physical-vapor-deposition) (PVD).\nMost modern [diesel engines](https://bliptext.com/articles/diesel-engine) have top rings coated with a modified chromium coating (known as CKS or GDC), which has aluminium oxide or diamond particles respectively included in the chrome surface.\nIn two-stroke engines, the port design is also an important factor for the lifespan of the piston rings.","revisionCount":0},"hasMore":false,"nextCursor":null}