In this Article, we will Explain about
Piston Rings and Answers of all questions related to this topic.
What is Piston Rings ?
A piston rings is a split ring in a reciprocating engine, such as an internal combustion engine or steam engine, that fits into a groove on the exterior diameter of a piston.OrIt is a circular metal rings which fit in the grooves of wall of the pistons and ensure a snug fit of the piston inside the cylinder liner.OrIt is an expandable split ring used in Internal Combustion Engines to provide a seal between the piston and the wall of the cylinder liner.Or,It is an open-ended ring that fits the outer diameter of the piston into a groove.Or,It is a springy metal ring fitted in grooves of outer periphery of piston and used for sealing the gap between the piston and the cylinder liner.Or,It is a metallic split rings which is fitted in the outer circumference/diameter of the piston in the reciprocating engine like internal combustion engine or steam engine.
Piston rings is located between the pistons and cylinder on outer periphery/diameter of piston.Or we can sat thatIt is located in ring grooves on the piston outer circumference in which they are free to rotate, twist and undergo limited axial movements.
Functions of Piston Rings :-
The main functions is to seal the gases generated in the combustion inside the combustion chamber.It help s to transfer heat to the wall of the cylinder, and then to lubricate and scrape the oil out of it.Getting the quantity of oil right is vital. Too much oil will cause the engine to burn off during combustion, potentially causing your engine to produce blue smoke, and too little will ultimately cause the engine to seize.1.Maintain Gas compression pressure between piston and liner :-
To provide an effective reciprocating gas seal without scuffing with sufficient radial wall pressure.Or ,it is used to seal the Combustion Chamber (cylinder liner ) so that loss of gases to the crankcase is minimum.Or,
to seal off gases generated in the internal combustion process so that gas generated during combustion not leak into the opening b/w piston and cylinder.If gas seal leak then Engine not provide sufficient power and also increase fuel consumption.i.e Economically and environmtally ,it is not good.
Gas Seal Functions
Gas seal in IC means combustion chamber must be made as gas-tight as possible, so that the pressure produced by the combustion of gasses move the piston in the cylinder, which allows the crankshaft to rotate, making power available.This not only provide seal in combustion/expansion ,gas seal is important during intake, compression and exhaust stroke of piston.2.To avoid excessive pressure buildup in the crankcase causing crankcase explosion in the dry sump engine.
3.Heat transfer :-
Conduct heat to the liner wall from the Piston.( Improve the heat transfer from the piston to the wall of the cylinder. )
When combustion of gas takes place inside the combustion chamber, temperature inside chamber near piston crown is approximately 300℃.The engine can be damaged if heat accumulated and that’s why heat needed to release and done by rings.Explanation
Heat from transfer from grooves of piston to the rings and then cylinder liner and finally to the jacket water ( coolant ).4. To function as a scraper and oil control to prevent excessive consumption of oil that could result if lubricant is allowed to leak past into the combustion chamber.( Regulating the consumption of Cylinder engine oil by scraping oil back from the cylinder walls to the sump.)
Rings need some lubricating oil for lubrication of cylinder wall, but it is desirable to keep this quantity to a minimum. The rings work in a scraping manner, keeping the excess oil out of combustion chamber s. In this way, oil use is kept at an appropriate amount and also reduce the amount of harmful emissions.5. Maintaining adequate oil quantities between the piston and the cylinder wall.( Maintaining the proper amount of oil between the piston and the wall of the cylinder. )
A small amount of oil is poured over the pistons so that they travel smoothly, with little metal-to – metal friction. rings adjust the oil quantity/amount.6.Provide Support piston in the liner.
It prevent the piston from knocking on the wall of the cylinder.7.It did the work of minimising the wear and tear of the wall of cylinder liner by maintaining constant layer/film of the oil between the two surfaces.
Material for piston rings :-
pearlitic grey cast iron
It may include chromium, molybdenum vanadium,Titanium,Nickel and copper.In some engine spheroidal graphite iron is used ,which has greater hardness and tensile strength.Cross section :- cross section is a rectangular with small radii on all edges. This allows an oil wedge to build up on the outer surface and prevents sticking at the back of ring groove.The section may very adjacent to the ring butts.Construction :-Rings are machined either circular or to a xam shape from which they expand to form a circle at working temperatures and avoid port chipping.A radial cut is made and the shape of ring joint ends may be butt (vertical ), which gives a robust joint for top rings:Scarfed (diagonal ), giving better gas seal but robust;Or a form of lap or bayonet joint which gives a good gas seal but is more vulnerable to breakage and is only used in some lower rings.
Why piston rings clearance are necessary ?
Ans :-To allow Movement & thermal expansion.Axial clearance :-to allow gas pressure to pass to and from the back of the Rings.circumferential clearancewhy circumference clearance at the ring joint is necessary ??to allow for thermal expansion but should not allow accessing prove past of gas.Both clearance vary with engine size and rating.
Axial clearance for large engine
0.4 mm for top rings.0.2 mm for lower rings.
Circumferential clearance for a large engine
0.5 % of cylinder diameter (moderate ratings )1.0 % of cylinder diameter (higher ratings )
What will happen if excessive wear of rings.
Excessive wear will reduce engine efficiency and may cause1.hot gas to blow past the rings.2.Removing lubricating3.causing ring distortion and breakage.
Types of piston rings.
There are mainly two types of piston rings.1.Compression rings2.Oil Control ringsGenerally,there is confusion among people regarding the number and types of rings.Let’s get first clear about the number of rings.
The Number of rings:-
Depending upon the requirements, the number of rings used on a piston varies. Earlier engines used two or four compression rings and one or two oil control rings.Nowadays, for modern Ic Engines having lower height , manufacturers limit the number of rings to three.Out of theseTwo – Compression rings.One – Oil Control rings.One in the top groove, the other in the second groove and an oil ring in the third groove. Often a scraper ring is installed as the second ring or lower compression ring.
Why we use Two or more Compression rings.
Due to the high-pressure difference between the combustion chamber and the crankcase, a minimum of two compression rings is necessary. This difference in pressure could be as high as 70 bar.If we use one ring ,then it cannot withstand such high pressure that’s why manufacturer uses two or more compression rings instead of one.The Increase in the number of rings also decrease the pressure b/w the rings and walls of the cylinder liner.It reduces wear and increase the life of the engine.
Types of Rings
1.compression ring :-
Locations :- It is generally placed at the top of the head of piston and side of the piston.Or,They are places immediately after below the head of the piston.Functions :-rings for sealing the gases above the piston and preventing gas leakage.It is called compression rings or pressure rings or sealing rings.Those most commonly used being of the rams bottom type.Materials :- This is ring made of cast – iron with a uniform section either square or rectangular inside of the pistol being hammered.Number of compression rings :-The number of compression rings depend on type of engines and it’s design but nowadays mostly two compression rings used.Cross-section :- The cross-section of compression rings is eitherA) Rectangular or,B) wedged shapeWith a tapered face.A thin bearing edge presents on tapered face which helps in reduce fiction and wearing and provide provide proper sealing.
Wiper rings :-
It is also known as Napier ring ,Scrapper ring or Back up compression rings.It is type of compression rings fitted just below the top compression rings.
It is fitted Next to Compression rings.Or we can say thatThis rings have a tapered face located in the ring groove between the compression ring and the oil control ring.
The wiper ring is used to 1.further seal the combustion chamber.It act as a support back up ring because Combustion gases that pass by the compression ring are stopped by the wiper ring. and2. to wipe the cylinder wall clean of excess oil and also did the work of smearing the oil on the wall of cylinder ensuring that compression rings are properly lubricated.Construction :-Most of the wiper rings have a taper angle face that is positioned towards the bottom to provide a wiping action as the piston moves towards the crankshaft.What will happen if wiper ring installed incorrectly ?If the wiper ring is improperly placed with a tapered angle nearest to the compression ring, this causes in excessive oil consumption. This is caused by wiper ring because it wiping the excess oil towards the combustion chamber.
2.Oil control rings :-
As the name suggests,It is used regulate the amount of oil on the wall of cylinder liner when piston goes up and down.
It may be fitted below the compression rings on the Piston, or alternatively to the skirt.Or, we can say that this is placed at the bottom of piston, from the crankcase side.Or ,in some engines it is installed immediately after the compression rings and above the piston bores.For larger engines ,one more control rings fitted on part of skirts.
The main functions of oil control rings is used to control the amount of lubricating oil used in IC engines.It also did the work of ensuring oil spreading evenly on the wall of the cylinder & scrap excess amount of oil off the wall and return it to crankcase.This ring do not allow lubricating oil to get past b/w the cylinder wall and piston.
Their cross-section is shaped with a downwards wedge or lip which spreads the oil on the upstroke of the Piston but scrapes off excess oil on the downstroke.There is holes or slot found in the radial centre of oil control rings.Rings, called drain holes which allow rapid removal of oil to the back the ring groove from where it can pass through the holes to the inside of the skirt and finally to the crankcase.
For improved life the scrapping edge may be chromium plated.
Piston Rings Installation
Before installing rings first checks new ring or spare rings.The marking of new rings checked and compare it with old one for the same grade or position.If the old marking of the piston ring is wiped out, check the piston ring manual for exact size of rings so that it can be put in its proper groove.Clean the piston :-Thoroughly clean the pistons and extract any traces of carbon oil from the ring grooves. Remove the carbon using a twist drill and tap wrench from the oil return bores; clean the grooves without damaging the sides of the groove. Replace cracked pistons, collapsed and wornCylinder cleaning :- Remove all carbon residue from the top surface of the cylinder bore that shows no wear.rings inspection :-The groove must be cleaned properly to ensure that there is no carbon and sludge deposit in the groove. While cleaning, it should be kept in mind that some of the piston grooves are coated with a special protective film. They should not be damaged by the use of chipping or grinding tools.If a distance of 0.12 mm or more is measured between the new parallel-sided compression ring and the corresponding groove side, the piston shall be excessively worn and replaced.Groove gap. piston usabilityCylinder inspection :-The cylinder (top ring reversal bore wear) must be replaced if the cylinder wear is greater than 0.1 mm in petrol engines and 0.15 mm in diesel engines.
After cleaning the groove s properly,the rings is inserted in the respective grooves by using piston rings tools or itting tools.This tool use for expanding rings and it is fitted in the grooves by sliding it from the top of the piston head.Ensure that the ring is inserted, keeping the top marking on the upper side. Most piston rings are marked “TOP” or the surface stamped identification number is considered to be the top face if there is no specific marking.
Once the piston rings have been installed, it must be ensured that they can move freely. Rotate the joint ends of the piston rings by 120 °.
Piston rings expander
It is a special types of tool used to remove and install rings with minimum ring distortion.
The use of the ring expander will significantly reduce the missalignment of the ring point, which may be detrimental to good ring sealing and engine performance.
It is extremely important to use the ring expander tool appropriately as wrong use may damage the ring or it may hurt the operator as the ring is under constant tension.
In small 4 stroke engines, in case the tool is not available, the ring can be expanded by use of clothes or rags available in the engine room. Two rags are placed on each side of the ring ends and they are pulled so that rings can be expanded and inserted from the top of the piston.
Piston rings compressor tool
The piston ring compressor is a special tool that is specifically designed for compressing the piston rings when a piston is re-installed. This is done by opening the piston ring compressor enough so that the piston can slip into the opening.Then the rings compress by tightening the tool to make it snug around the piston. The piston ring compressor can be tightened by turning the adjusting arm clockwise and loosened by turning the adjusting arm clockwise while holding the release spring down as shown below.It is important to lubricate both the piston ring compressor and the oil bore of the cylinder before trying to push the piston down into the cylinder.
How do piston rings work
Earlier ,I have explained different types of rings and their uses.Working of rings means ,how these rings work their functions during process of combustion.The topmost groove of the piston consists of a compression ring, the main purpose of which is to seal any kind of leakage within the combustion chamber during the combustion process. As the air-fuel mixture is ignited, the combustion gas pressure is applied to the piston head, pushing the piston to the crankshaft.Pressurized gases pass through the gap between the wall of the cylinder and the piston and through the ring groove of the piston. During the combustion process, the force of high-pressure gases forces the piston ring against the cylinder liner rim, which helps to form an effective seal. This pressure on the piston ring is proportional to the combustion gas pressure.The next set of rings in the piston that are located below the compression ring and above the oil rings are called wiper rings.
They have a tapered face construction and are used to further seal the combustion chamber. As the name suggests, they help extract any residual oil and impurities from the liner wall. If some of the combustion gases could pass through the compression ring, these gases would be blocked by the wiper ring in good condition.The last set of rings are the oil rings that are found in the bottom grooves of the piston nearest to the crankcase. The main purpose of the oil ring is to scrape any excess oil out of the walls of the cylinder liner while the piston is in motion. Most of the oil that has been cleaned is directed back into the crankcase to the oil sump. These oil rings come with a spring mounted on the back of the 4-stroke engine to provide an additional push for cleaning the liner.
Marine Diesel Engine Piston Rings
4 – stroke Trunk type engines
The requirements of piston rings in 4-stroke type engines is different because the assembly of piston liner is directly open to the sump.
Thus,oil-scraper rings additionally needed in the piston ring – pack.Typically,the number of rings is 2-5 , depending upon the type and specifications of engine.Usually, 2-4 compression rings are equipped to seal the gases from the combustion chamber and 1-3 oil control rings are provided to prevent excess oil from entering the combustion chamber.For marine diesel engine
The number of compression rings :-4 to 5And oil control rings :- 1to 3One more oil control rings in the skirt below the gudgeon pin.But in automotive engine :-Compression rings :-2
Oil control rings :- 1
2stroke marine diesel engine piston Rings
Generally,The number of piston rings in 2 – stroke diesel engines is 5.
But , number also depends upon the size and types of engines.For e.g an RTflex 35 will have a very short skirt and is equipped with three piston rings, but an RTA engine may have 5 piston rings.The combustion chamber of a 2 stroke marine diesel engine is a large space generating a tremendous amount of heat and stresses. The upper rings of the piston are directly in contact with the combustion chamber, hence they need better protection and coating to tackle the thermal stress and provide proper sealing.
Piston Rings Coatings
Wear resistance coatingsPlasma Coating (using a plasma spraying method where a gas
mixture is directed through an electric arc generated between a tungsten electrode and a water cooled copper tube to create a plasma state” at 10,000 to 15,000 deg. C). This plasma state melts and fuses any metal, with gas molecules and atoms
disassociating.Chrome plating: It is a hard outer galvanic chrome layer. Double
chrome plating is done an bath sides of the ring. This increases the
wear and corrosion resistance.Tungsten carbide coating which gives a better wear resistance.Running-in Coatings
These are soft coatings such as copper, graphite or phosphate which
are meant to wear quickly and give the ring a similar profile as the liner.For large two stroke marine diesel engine , many new designed for piston rings were introduced.Some of important designed discussed below :-Main engine :-
CPR (controlled pressure relief ) Rings :-
The topmost rings is controlled pressure (cpr ) relief rings.
This rings have six controlled pressure relief grooves (oblique hallow grooves ) cut across the face.This ensures even pressure distribution and decrease of thermal load to the second ring as well as the liner.It has s type joints at the rings end or we can say that ring has one double lap s-seal.A new design has also recently been introduced, a modified version of the CPR rings known as CPR Port on Plane rings (CPR POP). In this types of rings, the only modification done is that they placed the position of grooves in the lower side of rings,as it was noted that the wear of the CPR ring grooves on the running side was faster than usual.
Second or Intermediate Rings:
The other rings have an oblique cut at the ends of rings. All the piston rings are aluminium-coated on the outer surface for helping in running-in.
Wartsila engine :-
In the Wartsila 2 stroke engine, the piston ring grooves on the surface of the piston are hardened for excellent wear resistance. The top piston ring (also known as the Gas Tight (GT) ring in Wartsila) has overlapping ends to avoid gas leakage with an asymmetrical barrel shape. They are chromium-ceramic (CC) coated with a running-in coating (RC).
Piston Cleaning Ring
It is the ring which is embedded in the top edge of the liner just below
the cylinder head level. Its purpose is to remove the excessive carboneous deposits at the top-land portion of the combustion chamber wall which would otherwise contaminate and affect lubrication.
It is the ring which reduces the polished effect of the liner wall, which is formed due to the hard deposits from combustion in contact with the liner.
Polishing is unwanted, since it does not allow oil film retention on the liner wall, and the oil passes over the ring pack portion to the combustion area
when it is burnt and wasted. Polishing depends on oil feed rate, excessive peak pressures, ting and liner materials, and an increase in combustion hard products at liner-ring interface.
Piston rings failure
Inside the combustion chamber,there is tremendous amount of temperature and pressure available during combustion.If there is unusual change in pressure and temperature inside combustion chamber, it causes problem in performance of rings I.e (efficiency of engine )The reason of unusual change in combustion chamber is1.detonation and pinging of the fuel from the leaky injector or when the fuel is mixed with dirty air.2.Contaminated fuel oil or incorrect grade of cylinder oil would also affect the performance of the ring. As the ring starts to wear down , their ability to seal combustion gases will become apparent.3.Bad fuel or cylinder oil quality, poor combustion process, incorrect fuel timing, worn liner etc. are the normal cause of piston rings worn out. The most common indication or sign of a worn ring is gas passing through the crankcase or under the piston region known as blowby.4.Sticky ring due to carbon or sludge deposit and breaking or crack on the ring can result due to wear down.
Life span of piston rings
All machines have a fixed periods for overhauling and replacement.So,rings have also fixed time.But The life of a rings entirely depends on the
1. ring type,
2. the size of the engine it is fitted on,
3. and the operational condition of the ring and the liner.For a large 2-stroke diesel engines having approximately Bore diameter 900 mm has life up to 24,000 hours.And small engines having bore diameter 500 mm has life up to 16,000 hours.The rings of Four stroke marine auxillary diesel engines having medium high speed have less life than 2 stroke engine.The avg. lifespan of a marine four stroke high-speed engine is approximately 8000 hours, post which renewal is required.