Piston ring

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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. 47mm piston rings.jpg
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.

A piston ring is a metallic split ring that is attached to the outer diameter of a piston in an internal combustion engine or steam engine.

Contents

The main functions of piston rings in engines are:

  1. Sealing the combustion chamber so that there is minimal loss of gases to the crank case.
  2. Improving heat transfer from the piston to the cylinder wall.
  3. Maintaining the proper quantity of the oil between the piston and the cylinder wall
  4. Regulating engine oil consumption by scraping oil from the cylinder walls back to the sump. [1]

Most piston rings are made from cast iron or steel.

Design

Piston ring configurations:
A) Rectangular section
B) Barrel face
C) keystone
D) Torsional twist
E) Taper face
F) Dykes
Which also differ for the type of retainer band:
Y) Behind-band
X) Above or under-band
Or for the ends that work without firm stops:
K) Step
J) Oblique
W) Oblique with step Fasce eleastiche.svg
Piston ring configurations:
A) Rectangular section
B) Barrel face
C) keystone
D) Torsional twist
E) Taper face
F) Dykes
Which also differ for the type of retainer band:
Y) Behind-band
X) Above or under-band
Or for the ends that work without firm stops:
K) Step
J) Oblique
W) Oblique with step
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. Piston ring illustrated.png
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.

Piston rings are designed to seal the gap between the piston and the cylinder wall. [2] If this gap were too small, thermal expansion of the piston could mean the piston seizes in the cylinder, causing serious damage to the engine. On the other hand, a large gap would cause insufficient sealing of the piston rings against the cylinder walls, resulting in excessive blow-by (combustion gases entering the crankcase) and less pressure on the cylinder, reducing the power output of the engine.

The sliding motion of the piston ring inside the cylinder wall causes friction losses for the engine. The friction caused by piston rings is approximately 24% of the total mechanical friction losses for the engine. [3] [4] The design of the piston rings is therefore a compromise between minimising friction while achieving good sealing and an acceptable lifespan.

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 (such as in 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, reducing the effectiveness of the lubrication.

Rings are also sprung to increase the contact force and to maintain a close seal. The spring force is provided by either 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. [5] Rings 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.[ citation needed ]

Number of rings

Sealing is often achieved by multiple rings, each with their own function, using a metal-on-metal sliding contact. Most pistons have at least two piston rings per cylinder.

Automotive piston engines typically have three rings per cylinder. [6] The top two rings— known as compression rings— are primarily for sealing the combustion chamber. The 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. [7]

Ring construction

The compression rings in an automotive engine typically have a rectangular or keystone shaped cross-section. The upper compression ring typically has a barrel profile for the periphery, while the lower compression ring typically has a taper napier facing. Some engines also use a taper facing for the top ring, and simple plain-faced rings were used in the past.

Oil 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. Cast iron oil rings and rings with a helical spring backing have two scraping lands of various detailed form. On 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.

The gap in the piston ring compresses to a few thousandths of an inch when inside the cylinder bore. Ring gap shapes include square cut, angle cut, tite joint, step cut, hook step and mitre step. [8]

History

Steam engine with 3 piston rings at location D Horizontal steam engine.png
Steam engine with 3 piston rings at location D
Spring-loaded piston rings Piston rings.jpg
Spring-loaded piston rings

Early steam engines used a hemp packing to seal the combustion chamber, [9] which caused high frictional resistance and did not provide a very effective seal.

The first use of a piston ring in the cylinders of a steam engine appears in 1825 by Neil Snodgrass, a Glasgow engineer and mill-owner, for use in his own machines. This used springs to keep the seal steam-tight. From use within the mill this was experimented on the steamer "Caledonia" which plied the Gareloch. [10] [11]

The modern design of a metallic split-ring was invented by John Ramsbottom in the 1850s. Ramsbottom's initial design in 1852 was a circular shape, however these wore unevenly and were not successful. In 1854, a revised design was claimed to have a lifespan of up to 4,000 mi (6,437 km). [12] 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. The revised piston ring was manufactured to an out-of-round shape, so that it would exert even pressure once installed in the cylinder. An 1855 patent documented this change. The 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.

Engine wear

Piston 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. To 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. Coatings used in modern motorcycles include chromium, [13] nitride, [14] or ceramic coating made by plasma deposition [15] or physical vapour deposition (PVD). [16] [17] Most modern diesel engines have top rings coated with a modified chromium coating (known as CKS or GDC), [13] [ dead link ] which has aluminium oxide or diamond particles respectively included in the chrome surface.

In two-stroke engines, the port design is also an important factor for the lifespan of the piston rings.

See also

Related Research Articles

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A piston is a component of reciprocating engines, reciprocating pumps, gas compressors, hydraulic cylinders and pneumatic cylinders, among other similar mechanisms. It is the moving component that is contained by a cylinder and is made gas-tight by piston rings. In an engine, its purpose is to transfer force from expanding gas in the cylinder to the crankshaft via a piston rod and/or connecting rod. In a pump, the function is reversed and force is transferred from the crankshaft to the piston for the purpose of compressing or ejecting the fluid in the cylinder. In some engines, the piston also acts as a valve by covering and uncovering ports in the cylinder.

<span class="mw-page-title-main">Wankel engine</span> Combustion engine using an eccentric rotary design

The Wankel engine is a type of internal combustion engine using an eccentric rotary design to convert pressure into rotating motion.

<span class="mw-page-title-main">Two-stroke engine</span> Internal combustion engine type

A two-strokeengine is a type of internal combustion engine that completes a power cycle with two strokes of the piston during one power cycle, this power cycle being completed in one revolution of the crankshaft. A four-stroke engine requires four strokes of the piston to complete a power cycle during two crankshaft revolutions. In a two-stroke engine, the end of the combustion stroke and the beginning of the compression stroke happen simultaneously, with the intake and exhaust functions occurring at the same time.

<span class="mw-page-title-main">Lubrication</span> The presence of a material to reduce friction between two surfaces.

Lubrication is the process or technique of using a lubricant to reduce friction and wear and tear in a contact between two surfaces. The study of lubrication is a discipline in the field of tribology.

<span class="mw-page-title-main">Connecting rod</span> Piston engine component which connects the piston to the crankshaft

A connecting rod, also called a 'con rod', is the part of a piston engine which connects the piston to the crankshaft. Together with the crank, the connecting rod converts the reciprocating motion of the piston into the rotation of the crankshaft. The connecting rod is required to transmit the compressive and tensile forces from the piston. In its most common form, in an internal combustion engine, it allows pivoting on the piston end and rotation on the shaft end.

In a reciprocating engine, the cylinder is the space in which a piston travels.

Engine braking occurs when the retarding forces within an engine are used to slow down a motor vehicle, as opposed to using additional external braking mechanisms such as friction brakes or magnetic brakes.

<span class="mw-page-title-main">Galling</span> Form of wear caused by adhesion between sliding surfaces

Galling is a form of wear caused by adhesion between sliding surfaces. When a material galls, some of it is pulled with the contacting surface, especially if there is a large amount of force compressing the surfaces together. Galling is caused by a combination of friction and adhesion between the surfaces, followed by slipping and tearing of crystal structure beneath the surface. This will generally leave some material stuck or even friction welded to the adjacent surface, whereas the galled material may appear gouged with balled-up or torn lumps of material stuck to its surface.

<span class="mw-page-title-main">Crankcase</span> Crankshaft housing in reciprocating combustion engines

In a piston engine, the crankcase is the housing that surrounds the crankshaft. In most modern engines, the crankcase is integrated into the engine block.

<span class="mw-page-title-main">Bourke engine</span> Type of internal combustion engine

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<span class="mw-page-title-main">Hydraulic cylinder</span> Mechanical tool for applying force

A hydraulic cylinder is a mechanical actuator that is used to give a unidirectional force through a unidirectional stroke. It has many applications, notably in construction equipment, manufacturing machinery, elevators, and civil engineering.

<span class="mw-page-title-main">Head gasket</span> Gasket that sits between the engine block and cylinder head(s) in an internal combustion engine

In an internal combustion engine, a head gasket provides the seal between the engine block and cylinder head(s).

<span class="mw-page-title-main">Oil pump (internal combustion engine)</span> Internal combustion engine part that circulates engine oil under pressure

The oil pump is an internal combustion engine part that circulates engine oil under pressure to the rotating bearings, the sliding pistons and the camshaft of the engine. This lubricates the bearings, allows the use of higher-capacity fluid bearings and also assists in cooling the engine.

Crankcase dilution is a phenomenon of internal combustion engines in which unburned diesel or gasoline accumulates in the crankcase. Excessively rich fuel mixture or incomplete combustion allows a certain amount of fuel to pass down between the pistons and cylinder walls and dilute the engine oil. It is more common in situations where fuel is injected at a very high pressure, such as in a direct-injected diesel engine.

Deglazing is a process by which the surface of an engine cylinder is roughened to create friction between the moving parts and allow engine oil to grip the sides of the cylinder.

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<span class="mw-page-title-main">Single- and double-acting cylinders</span> Classification of reciprocating engine cylinders

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A Wills Ring or Cooper Ring is a form of all-metallic O-ring seal. They are used for extremely arduous service, such as sealing the head gasket of high performance piston engines.

<span class="mw-page-title-main">Internal combustion engine</span> Engine in which the combustion of a fuel occurs with an oxidizer in a combustion chamber

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Abradable Powder Coatings (APC), also known as Additive/Abradable Powder Coatings (AAPCs), are a class of abradable materials that are applied using standard powder coating technology. They are used for clearance control, sealing and lubrication over a wide range of temperatures in oiled and dry devices. Application of the coatings is inexpensive and environmentally friendly.

References

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  5. "Piston Rings". www.globmaritime.com. Archived from the original on 2 June 2014.
  6. "Factors influencing oil control in a spark ignited gasoline engine". www.hastingsmfg.com. Archived from the original on 31 March 2008.
  7. "Piston & Piston Ring Assembly". www.riken.co.jp. Archived from the original on 19 August 2002.
  8. "Custom Ring Gaps". www.precisionrings.com. Archived from the original on 6 July 2017.
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  10. "Neil Snodgrass' Cigar". 14 May 2017.
  11. Glasgow Herald (mewspaper) 5 February 1849: Snodgrass's obituary
  12. "Proceedings Institution of Mechanical Engineers to 1859". www.steamindex.com. Retrieved 4 December 2019.
  13. 1 2 "Wear-Resistant Peripheral Coatings". www.federalmogul.com. Archived from the original on 10 August 2012. Retrieved 4 July 2010.
  14. "Wear-Resistant Peripheral Coatings". www.federalmogul.com. Archived from the original on 10 August 2012. Retrieved 4 July 2010.
  15. "Wear-Resistant Peripheral Coatings". www.federalmogul.com. Archived from the original on 10 August 2012. Retrieved 4 July 2010.
  16. "Wear-Resistant Peripheral Coatings". www.federalmogul.com. Archived from the original on 10 August 2012. Retrieved 4 July 2010.
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