Piston ring - wikipedia Piston ring This article needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. ( August 2007 ) Horizontal steam engine. The piston is shown at D. The three piston rings prevent steam from escaping between chambers A and B. 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. Spring - loaded piston rings. A pair of piston rings mounted on a 47mm two - stroke cycle scooter piston. 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 main functions of piston rings in reciprocating engines are : Sealing the combustion chamber so that there is minimal loss of gases to the crank case. Improving heat transfer from the piston to the cylinder wall. Maintaining the proper quantity of the oil between the piston and the cylinder wall Regulating engine oil consumption by scraping oil from the cylinder walls back to the sump. 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. Contents 1 History 2 Automotive 3 Wear due to ring load on the bore 4 Fitting new piston rings 5 See also 6 References 7 External links History 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. Automotive 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 to 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. Most automotive pistons have three rings : The top two, while also controlling oil, are primarily for compression sealing ( compression rings ). Meanwhile, the lower ring is for controlling the supply of oil to the liner, which lubricates the piston skirt and the compression rings ( oil control rings ). At least two piston rings are found on most piston and cylinder combinations. Typical compression ring designs will have an essentially rectangular cross section or a keystone ( right angled trapezoidal ) cross section. The periphery will then have either a barrel profile ( for the top compression rings ) or a taper napier form ( for the second compression ring or scraper ring ). There are some taper - faced top rings as well, and on some old engines simple plain - faced rings were used. Oil control rings typically are of three types : single piece cast iron helical spring backed cast iron or steel multipiece steel The spring - backed oil rings and the cast iron oil rings have essentially the same range of peripheral forms, which consist of two scraping lands of various detailed form. The multipiece oil control rings usually consist of two rails or segments ( these are thin steel rings ) with a spacer - expander spring which keeps the two rails apart and provides the radial load. The piston might be a fairly loose fit in the cylinder. If it were a tight fit, it would expand as it got hot and might stick tight in the cylinder. If a piston sticks ( seizes ) it could cause serious damage to the engine. On the other hand, if there is too much clearance between the piston and cylinder walls, the ultimate result will be insufficient sealing of the piston rings against the cylinder walls, and thus much of the pressure from the burning gasoline vapour will leak past the piston ( a condition known as blow - by ) and into the crankcase. In such a situation, the push on the piston from combustion will be much less effective in delivering power. Wear due to ring load on the bore 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. In two - stroke engines, the port design is additionally critical to ring life. In modern motorcycle engines, various proprietary shapes are used to help maximize ring longevity. Examples include specialized port shapes such as tapered serrations and the use of various combinations of heat - treating and special coatings applied by PVD ( physical vapour deposition ) processes and other techniques. Typically, top ring and oil control rings will be coated with chromium, or Nitrided, possibly plasma sprayed or have a PVD ceramic coating. For enhanced scuff resistance and further improved wear, most modern diesel engines have top rings coated with a modified chromium coating known as CKS or GDC, a patent coating from Goetze which has aluminium oxide or diamond particles respectively included in the chrome surface. The lower oil control ring is designed to leave a lubricating oil film, a few micrometres thick on the bore, as the piston descends. Three piece oil rings, i.e. with two rails and one spacer, are used for four - stroke gasoline engines. Fitting new piston rings Piston ring, in different 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 ( there are several variations ) When fitting new piston rings or breaking them in within an engine, the end gap is a crucial measurement. In order that a ring may be fitted into the `` grooves '' of the piston, it is not continuous but is broken at one point on its circumference. The ring gap may be checked by placing the ring into the bore / liner ( squared to bore ) and measuring it with a feeler gauge. The end gap should be within recommended limits for size of bore and intended `` load '' of engine. Because metals expand with a rise in temperature, too small a gap may result in overlapping or bending when used under hot running conditions ( racing, heavy loads, towing ) and, even at normal temperatures, an undersized ring gap may lead to ring - gap closure, ring breakage, bore damage and possible seizure of the piston. Too large a gap may give unacceptable compression and levels of blow - by gases or oil consumption. When piston gap is measured in a used bore, it may serve to indicate either excessive bore wear or ring wear, or a combination of both types of wear. ( Radial wear on the ring face reduces the effective thickness of the used / worn ring, thereby essentially decreasing the face circumference of the ring. This increases the size of the ring 's end gap. ) It is considered good practice to build a new engine with the ring gaps staggered around the circumference of the bore. This means that any escaping gas must negotiate a labyrinth before escaping past the rings. However, while the engine is running, the rings will tend to rotate around the piston and not remain in the position as fitted. Many rings will then stick in one spot at random and remain there for the life of the engine. For this reason, ring position during build can not be considered to be important, although most engine builders would feel uncomfortable assembling an engine with the gaps aligned. When fitting new rings to a used engine, special `` ridge dodger '' rings are sometimes used for the top compression ring, to improve compression and oil consumption without reboring the cylinder. These have a small step of iron removed from the top section to avoid making contact with any wear ridge at the top of the cylinder, which could break a conventional ring. These are not widely recommended, however, as they are usually not required and may give inferior oil consumption. A more acceptable method is to remove the wear ridge with a `` ridge reamer '' tool before lightly honing the bore to accept new rings. In fact, if the `` ridge '' is measured it will generally be apparent it is not really a ridge but a relatively local hollow caused by the top ring near the ring reversal point. The upper edge of this hollow will take the form of a `` ramp '' about 2mm long from the point of maximum wear to the point of zero wear. In this case, there is not actually any ridge to hit, so light honing may be all that is required. During engine assembly, a piston - ring compressor is used to evenly squeeze the rings long enough to slide the piston into the cylinder. Rings are not a very expensive part, but fitting new ones is very costly on most engines. This is because in order to fit them, the mechanic must dismantle a large part of the engine so labour costs are the major factor. However, on some simple engines, such as lightweight two - stroke engines found in motorcycles, piston ring replacement is generally straightforward and relatively quick. See also Obturator ring References Jump up ^ Piston Ring Handbook. Federal - Mogul Burscheid GmbH. August 2008. Jump up ^ Alex Weiss ( 2008 ). Bearings. Special Interest Model Books. Jump up ^ VDi - richtlinie 2840 Kohlenstoffschichten, Grundlagen, Schichttypen, Eigenschafte Jump up ^ `` Piston Ring Coating Reduces Gasoline Engine Friction '' ( PDF ). Federal - Mogul. Jump up ^ Dimitrios Dardalis, PhD ( February 2004 ). `` Rotating Liner Engine : A New Approach to Reduce Engine Friction and Increase Fuel Economy in Heavy Duty Engines '' ( PDF ). Austin, TX : RLE Technologies, Inc. ^ Jump up to : `` Wear - Resistant Peripheral Coatings ''. Federalmogul.com. Retrieved 2010 - 07 - 04. Jump up ^ `` Wear - Resistant Peripheral Coatings ''. Federalmogul.com. Retrieved 2010 - 07 - 04. Jump up ^ `` Wear - Resistant Peripheral Coatings ''. Federalmogul.com. Retrieved 2010 - 07 - 04. Jump up ^ `` Wear - Resistant Peripheral Coatings ''. Federalmogul.com. Retrieved 2010 - 07 - 04. External links Wikimedia Commons has media related to Piston rings. Tech Tips for Automotive Mechanics Federal - Mogul Piston Ring Handbook Piston Ring Museum Construction of Piston Rings Types of Ring Gaps for Piston and Seal Rings Glossary of Ring Gaps types hide Automotive engine Part of the Automobile series Basic terminology Bore Compression ratio Crank Cylinder Dead centre Diesel engine Dry sump Engine balance Engine configuration Engine displacement Engine knocking Firing order Hydrolock Petrol engine Power band Redline Spark - ignition engine Stroke Stroke ratio Wet sump Main components Connecting rod Crankcase Crankpin Crankshaft Crossplane Cylinder bank Cylinder block Cylinder head ( crossflow, reverse - flow ) Flywheel Head gasket Hypereutectic piston Main bearing Piston Piston ring Starter ring gear Sump Valvetrain Cam Cam follower Camshaft Desmodromic valve Hydraulic tappet Multi-valve Overhead camshaft Overhead valve Pneumatic valve springs Poppet valve Pushrod Rocker arm Sleeve valve Tappet Timing belt Timing mark Valve float Variable valve timing Aspiration Air filter Blowoff valve Boost controller Butterfly valve Centrifugal - type supercharger Cold air intake Dump valve Electronic throttle control Forced induction Inlet manifold Intake Intercooler Manifold vacuum Naturally aspirated engine Ram - air intake Scroll - type supercharger Short ram air intake Supercharger Throttle Throttle body Turbocharger Twin - turbo Variable - geometry turbocharger Variable - length intake manifold Warm air intake Fuel system Carburetor Common rail Direct injection Fuel filter Fuel injection Fuel pump Fuel tank Gasoline direct injection Indirect injection Injection pump Lean - burn Stratified charge engine Turbo fuel stratified injection Unit injector Ignition Contact breaker Magneto Distributor Electrical ballast High tension leads Ignition coil Spark plug Wasted spark Electrics and engine management Air -- fuel ratio meter Alternator Automatic Performance Control Car battery ( lead -- acid battery ) Crankshaft position sensor Dynamo Drive by wire Electronic control unit Engine control unit Engine coolant temperature sensor Glow plug Idle air control actuator MAP sensor Mass flow sensor Oxygen sensor Starter motor Throttle position sensor Exhaust system Automobile emissions control Catalytic converter Diesel particulate filter Exhaust manifold Glasspack Muffler Engine cooling Air cooling Antifreeze ( ethylene glycol ) Core plug Electric fan Fan belt Radiator Thermostat Water cooling Viscous fan ( fan clutch ) Other components Balance shaft Block heater Combustion chamber Cylinder head porting Gasket Motor oil Oil filter Oil pump Oil sludge PCV valve Seal Synthetic oil Underdrive pulleys Portal Aircraft piston engine components, systems and terminology Piston engines Mechanical components Camshaft Connecting rod Crankpin Crankshaft Cylinder Cylinder head Gudgeon pin Hydraulic tappet Main bearing Obturator ring Oil pump Piston Piston ring Poppet valve Pushrod Rocker arm Sleeve valve Tappet Electrical components Alternator Capacitor discharge ignition Dual ignition Electronic fuel injection Generator Ignition system Magneto Spark plug Starter Terminology Air - cooled Aircraft engine starting Bore Compression ratio Dead centre Engine displacement Four - stroke engine Horsepower Ignition timing Manifold pressure Mean effective pressure Naturally aspirated Monosoupape Overhead camshaft Overhead valve engine Rotary engine Shock cooling Stroke Time between overhauls Two - stroke engine Valve timing Volumetric efficiency Propellers Components Propeller governor Propeller speed reduction unit Spinner Terminology Autofeather Blade pitch Constant - speed Contra - rotating Counter-rotating Scimitar Single - blade Variable - pitch Engine instruments Annunciator panel EFIS EICAS Flight data recorder Glass cockpit Hobbs meter Tachometer Engine controls Carburetor heat Throttle Fuel and induction system Avgas Carburetor Fuel injection Gascolator Inlet manifold Intercooler Pressure carburetor Supercharger Turbocharger Updraft carburetor Other systems Auxiliary power unit Coffman starter Hydraulic system Ice protection system Recoil start NDL : 00569120 Retrieved from `` https://en.wikipedia.org/w/index.php?title=Piston_ring&oldid=831122975 '' Categories : Pistons Seals ( mechanical ) Engine technology Hidden categories : Articles needing additional references from August 2007 All articles needing additional references Wikipedia articles with NDL identifiers Talk Contents About Wikipedia Беларуская བོད ་ ཡིག Čeština Dansk Deutsch Español Français 한국어 Italiano Nederlands 日本 語 Polski Română Русский Slovenčina Српски / srpski Svenska Türkçe Українська 11 more Edit links This page was last edited on 18 March 2018, at 21 : 40 ( UTC ). 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Question: the minimum number of rings in a piston is
Answer: two