Piston-ring

Chapter 1 0Chapter 1 0

Piston RingsPiston Rings

Piston Rings perform Three main tasks:Piston Rings perform Three main tasks:. They seal the combustion space.. They seal the combustion space.. They spread the LO up and down the liner.. They spread the LO up and down the liner.. They transfer heat from the piston to the . They transfer heat from the piston to the

linerlinerPiston rings are manufactured using the pot Piston rings are manufactured using the pot

casting method, producing a short cylinder casting method, producing a short cylinder of oval cross section from which the rings of oval cross section from which the rings are machined. are machined.

This produces a balanced and homogenous This produces a balanced and homogenous casting around the entire circumference of casting around the entire circumference of the ring.the ring.

MaterialsMaterialsGenerally speaking the piston ring must be Generally speaking the piston ring must be

harder than the liner in which it runs. Rings harder than the liner in which it runs. Rings are cast and machined from grey cast iron.are cast and machined from grey cast iron.

This may include some additions such as This may include some additions such as chromium, molybdenum, vanadium, chromium, molybdenum, vanadium, titanium, nickel and copper.titanium, nickel and copper.

In some engines nodular or spheroidal In some engines nodular or spheroidal graphite iron is used, which has greater graphite iron is used, which has greater hardness and tensile strength, as has hardness and tensile strength, as has vermicular or compact graphite cast iron, vermicular or compact graphite cast iron, the use of which is becoming more the use of which is becoming more widespread. widespread.

Alloying elements such as titanium and Alloying elements such as titanium and vanadium form hard insoluble carbides vanadium form hard insoluble carbides within the matrix.within the matrix.

ManufactureManufactureThe primary sealing effect of the piston ring The primary sealing effect of the piston ring

is achieved by the inherent tension of the is achieved by the inherent tension of the ring which makes it contact the cylinder ring which makes it contact the cylinder wall around its whole circumference.wall around its whole circumference.

Rings in early engines were hammered Rings in early engines were hammered around their circumference to induce a around their circumference to induce a tension in a circular ring. tension in a circular ring.

Two methods are used today; the thermally Two methods are used today; the thermally tensioned ring and the oval pot cam tensioned ring and the oval pot cam turning method. turning method.

The cheaper method employed mainly for The cheaper method employed mainly for small engines is the “Thermally “ tensioned small engines is the “Thermally “ tensioned Ring. In this method the ring is machined Ring. In this method the ring is machined from a circular pot to the cylinder diameter. from a circular pot to the cylinder diameter.

A gap is cut in the ring into which a distance A gap is cut in the ring into which a distance piece is inserted, thus expanding the ring. piece is inserted, thus expanding the ring.

The ring and distance piece is then placed in The ring and distance piece is then placed in , an oven to relieve the induced stresses. , an oven to relieve the induced stresses.

This method suffers from the disadvantage This method suffers from the disadvantage the rings lose their tension in the heat of the rings lose their tension in the heat of he engine.he engine.

The Oval Pot Cam Turning Method is more The Oval Pot Cam Turning Method is more expensive but produces a ring which retains expensive but produces a ring which retains its tension when working in the engine. its tension when working in the engine.

The rings are machined from the cast oval The rings are machined from the cast oval pots in a cam turning lathe. pots in a cam turning lathe.

The lathe is equipped with an interchangeable The lathe is equipped with an interchangeable copying cam , which controls the cutting copying cam , which controls the cutting tools. By changing the shape or ovality of tools. By changing the shape or ovality of the cam, the circumferential pressure the cam, the circumferential pressure distribution around the ring can be altered.distribution around the ring can be altered.

When the pot has been machined and the When the pot has been machined and the ring parted off, the gap is cut in the ring (7 - ring parted off, the gap is cut in the ring (7 - 13% dia) and the ring ground on its flanks 13% dia) and the ring ground on its flanks to ensure good sealing in the piston ring to ensure good sealing in the piston ring grooves.grooves.

Plating of RingsPlating of RingsPiston rings need to combine strength with wear Piston rings need to combine strength with wear

resistance. resistance. To enhance the wear resistant properties the To enhance the wear resistant properties the

rings can be plated with a layer of different rings can be plated with a layer of different material.material.

Chrome plating is the most common form of Chrome plating is the most common form of surface treatment. surface treatment.

It is used on the running surface of compression It is used on the running surface of compression rings and on the flanks (landing surfaces). rings and on the flanks (landing surfaces).

It is also used on oil scraper rings. It is also used on oil scraper rings. Chromium plating combines good wear Chromium plating combines good wear

resistance with low friction and corrosion resistance with low friction and corrosion resistance. resistance.

The plating must be of high quality to operate The plating must be of high quality to operate under the arduous conditions of load and under the arduous conditions of load and temperature without peeling, breaking away or temperature without peeling, breaking away or softening.softening.

Plasma coating provides a running surface Plasma coating provides a running surface with better properties than chrome with better properties than chrome plating.plating.

A gas mixture is directed through an A gas mixture is directed through an electric arc generated between a tungsten electric arc generated between a tungsten electrode and a water cooled copper tube. electrode and a water cooled copper tube.

Very high temperatures are generated, the Very high temperatures are generated, the gas molecules atoms dissociate and into gas molecules atoms dissociate and into this plasma state metal, carbides, or this plasma state metal, carbides, or ceramics are sprayed as a fine powder, ceramics are sprayed as a fine powder, these melt, and coat the ring surface.these melt, and coat the ring surface.

Chromium plated, and plasma sprayed Chromium plated, and plasma sprayed coatings are limited in thickness and the coatings are limited in thickness and the strength of the bonding to the parent strength of the bonding to the parent metal.metal.

A method of laser treatment which hardens A method of laser treatment which hardens the running surface has been developed, the running surface has been developed, which produces a wear resistant outside which produces a wear resistant outside layer several times thicker than layer several times thicker than conventional coatings. conventional coatings.

The treated zone has a complex material The treated zone has a complex material matrix with high hardness and good matrix with high hardness and good sliding properties.sliding properties.

Soft Coatings for Running InSoft Coatings for Running In

Copper Plating is plated directly on to the base Copper Plating is plated directly on to the base material of the ring or on to the chromium material of the ring or on to the chromium plated layer. plated layer.

The plating is very thin and has a life long The plating is very thin and has a life long enough to allow bedding in of the ring whilst enough to allow bedding in of the ring whilst running in.running in.

Plain rings can be ferrox or phosphate coated. Plain rings can be ferrox or phosphate coated.

This creates a thin protective layer on the This creates a thin protective layer on the piston ring which absorbs oil and aids piston ring which absorbs oil and aids protection of the ring and liner during the first protection of the ring and liner during the first hours of use.hours of use.

Plasma rings can be graphite coated to aid with Plasma rings can be graphite coated to aid with running in.running in.

Piston Ring Sealing ActionPiston Ring Sealing ActionThe action by which the rings seal the gas in The action by which the rings seal the gas in

the cylinder is seen in the diagram opposite.the cylinder is seen in the diagram opposite. The pressure of the gas in the clearance The pressure of the gas in the clearance

spaces forces the ring down on to the side of spaces forces the ring down on to the side of the groove and outwards on to the cylinder the groove and outwards on to the cylinder wall. wall.

Contact at these faces must be gas tight Contact at these faces must be gas tight demanding smooth mating faces all the way demanding smooth mating faces all the way round the ring. round the ring.

To make it possible to assemble the rings on To make it possible to assemble the rings on the piston and to enable them to conform to the piston and to enable them to conform to the cylinder bore they have the familiar split the cylinder bore they have the familiar split or gap. or gap.

This gap provides a leakage path for the This gap provides a leakage path for the combustion gas so that one piston ring combustion gas so that one piston ring alone is insufficient to seal adequately. alone is insufficient to seal adequately.

Usually 4 or 5 piston rings are fitted. Usually 4 or 5 piston rings are fitted.

The top ring bears the brunt of the sealing The top ring bears the brunt of the sealing task, it sustains the greatest pressure drop task, it sustains the greatest pressure drop across it and it operates at the highest across it and it operates at the highest temperature.temperature.

Piston Ring GapsPiston Ring Gaps

The shape of ring joint ends may be butt The shape of ring joint ends may be butt (vertical), which gives a simple robust joint (vertical), which gives a simple robust joint scarfed (diagonal), giving better gas seal scarfed (diagonal), giving better gas seal but is less robust or a form of lap or but is less robust or a form of lap or bayonet joint which gives a good gas seal bayonet joint which gives a good gas seal but is more vulnerable to breakage.but is more vulnerable to breakage.

MAN B&W use a double lap seal on their top MAN B&W use a double lap seal on their top ring on the MC two stroke engines. ring on the MC two stroke engines.

The ring has small gaps machined into it The ring has small gaps machined into it known as CPR gaps. known as CPR gaps.

These gaps allow a small amount of gas to These gaps allow a small amount of gas to pass, reducing the force exerted between pass, reducing the force exerted between ring and liner. ring and liner.

This leakage of gas is then sealed by the This leakage of gas is then sealed by the second and subsequent rings, which have second and subsequent rings, which have scarfed joints alternating in direction. scarfed joints alternating in direction.

This system prevents localised overheating This system prevents localised overheating from blowby which would occur should the from blowby which would occur should the top ring have a normal gap.top ring have a normal gap.

Circumferential and Axial Clearance.Circumferential and Axial Clearance.Ring clearances are necessary to allow Ring clearances are necessary to allow

movement and thermal expansion. movement and thermal expansion. Axial clearance is the clearance between Axial clearance is the clearance between

ring and groove. ring and groove. Axial clearance in the groove must be Axial clearance in the groove must be

gauged, this will allow gas pressure to gauged, this will allow gas pressure to pass to and from the back of rings. pass to and from the back of rings.

This clearance will increase with wear and it This clearance will increase with wear and it must not be allowed to taper.must not be allowed to taper.

Clearances vary with engine size and rating Clearances vary with engine size and rating but for a large engine may be 0.4mmbut for a large engine may be 0.4mm

for the top ring to 0.2mm for lower rings. for the top ring to 0.2mm for lower rings.

Clearances are allowed to double on aClearances are allowed to double on a

slow speed two stroke before machining the slow speed two stroke before machining the groove oversize, and fitting oversize rings. groove oversize, and fitting oversize rings. Excessive clearances will lead to poor Excessive clearances will lead to poor sealing and a pumping action on four sealing and a pumping action on four stroke engines which will lead to high oil stroke engines which will lead to high oil consumption.consumption.

Circumferential or butt clearance is measured Circumferential or butt clearance is measured by placing the ring in an unworn part ofby placing the ring in an unworn part of

the liner and measuring the ring gap. Too large the liner and measuring the ring gap. Too large a gap will lead to excessive blow by and a gap will lead to excessive blow by and localised overheating. If the ring gap is too localised overheating. If the ring gap is too small, the result could be a jammed or broken small, the result could be a jammed or broken ring when the piston and rings come up to ring when the piston and rings come up to working temperature.working temperature.

This clearance will vary with the shape of joint This clearance will vary with the shape of joint used, engine size and rating, but for large used, engine size and rating, but for large engines approximate figures may be from 0.5 engines approximate figures may be from 0.5 per cent of cylinder diameter for moderate per cent of cylinder diameter for moderate ratings to 1 .0 per cent for higher ratings. ratings to 1 .0 per cent for higher ratings. Circumferential wear of rings can be Circumferential wear of rings can be measured by reduction in the width of the measured by reduction in the width of the ring section and by increase in joint clearance ring section and by increase in joint clearance at the corresponding liner bore.at the corresponding liner bore.

Ring profiles and Running InRing profiles and Running InThe cross-section of piston rings is The cross-section of piston rings is

rectangular with small radii on all edges. rectangular with small radii on all edges. This allows an oil wedge to build up on the This allows an oil wedge to build up on the

outer surface and prevents sticking at the outer surface and prevents sticking at the back of the ring groove.back of the ring groove.

During running in the ring will tilt as shown, During running in the ring will tilt as shown, leading to a high edge pressure between leading to a high edge pressure between ring and liner, which will tend to scrape ring and liner, which will tend to scrape the oil film off the liner. This is one reason the oil film off the liner. This is one reason why engine load should be reduced during why engine load should be reduced during running in periods. If the engine is running in periods. If the engine is overloaded, blowby can occur burning off overloaded, blowby can occur burning off the oil film and leading to microseizure.the oil film and leading to microseizure.

If the ring has a barrel shaped profile, then If the ring has a barrel shaped profile, then the pressure between ring and liner is the pressure between ring and liner is reduced as shown. reduced as shown.

This reduces the likelihood of scraping off This reduces the likelihood of scraping off the oil film.the oil film.

After running in, the ring will take up the After running in, the ring will take up the profile shown opposite, with maximum profile shown opposite, with maximum surface contact via an oil film.surface contact via an oil film.

Blowby occurs when the combustion gases Blowby occurs when the combustion gases leak past the rings causing overheating, leak past the rings causing overheating, loss of power and destruction of the LO film.loss of power and destruction of the LO film.

Inspection of Rings Through Scavenge Inspection of Rings Through Scavenge PortsPorts

Condition of the piston rings can be assessed Condition of the piston rings can be assessed by inspecting them through the scavenge by inspecting them through the scavenge ports. Look for evidence of microseizure ports. Look for evidence of microseizure (vertical lines) abrasion (scoring) and (vertical lines) abrasion (scoring) and breakage at ring ends.breakage at ring ends.

The condition of the rings and liner can be The condition of the rings and liner can be assessed by doing a compression test on assessed by doing a compression test on the engine. the engine.

Either take compression pressures when the Either take compression pressures when the engine is running, or do a compression test engine is running, or do a compression test when the engine is stopped by turning each when the engine is stopped by turning each unit to TDC on compression stroke and unit to TDC on compression stroke and connecting an air line to the indicator cock. connecting an air line to the indicator cock.

Listen in crankcase (on 4 stroke engine) for Listen in crankcase (on 4 stroke engine) for air blowing past rings.air blowing past rings.

Causes of Ring BreakageCauses of Ring BreakageInsufficient piston-ring and groove clearance. Insufficient piston-ring and groove clearance. This may cause the ring to jam in the groove This may cause the ring to jam in the groove

when the engine comes up to working when the engine comes up to working temperature and allow blow past.temperature and allow blow past.

Insufficient lubrication, causing the ring to jam Insufficient lubrication, causing the ring to jam in the groove due to deposits of carbon. in the groove due to deposits of carbon.

Sufficient lubricating oil should be supplied to Sufficient lubricating oil should be supplied to keep ring grooves clean.keep ring grooves clean.

Large amounts of wear in cylinder liners.Large amounts of wear in cylinder liners.Excessive diametral clearance between the Excessive diametral clearance between the

piston and cylinder.piston and cylinder.Excessive wear on piston-ring landing face in Excessive wear on piston-ring landing face in

the piston-ring groovethe piston-ring groove

Ring gap too small. Ring gap too small. This usually leads to ring breakage, but in This usually leads to ring breakage, but in

the ultimate could lead to disastrous or the ultimate could lead to disastrous or very serious consequences.very serious consequences.

Incorrect preparation of ends of piston ring Incorrect preparation of ends of piston ring adjacent to gap, causing breakage as ring adjacent to gap, causing breakage as ring passes over port barspasses over port bars

Wear on the port bars relative to cylinder-Wear on the port bars relative to cylinder-liner working surface.liner working surface.

The ring expands and contracts as it passes The ring expands and contracts as it passes over the ports, causing stress induced over the ports, causing stress induced fatigue.fatigue.

Oil Scraper RingsOil Scraper RingsIn trunk-piston type engines with splash-In trunk-piston type engines with splash-

lubricated piston and cylinders there is lubricated piston and cylinders there is always some difficulty in controlling the always some difficulty in controlling the amount lubricant passing up the cylinder to amount lubricant passing up the cylinder to lubricate the piston. lubricate the piston.

Some of this lubricant is splashed from the Some of this lubricant is splashed from the crankcase but a large amount may come crankcase but a large amount may come from the gudgeon pin by leakage through from the gudgeon pin by leakage through the fastening of pin in the piston skirt.the fastening of pin in the piston skirt.

In such engines the scraper rings are usually In such engines the scraper rings are usually fitted in the piston skirt and scrape the fitted in the piston skirt and scrape the lubricant down the liner.lubricant down the liner.

The ring is then fitted sharp-edge down wards The ring is then fitted sharp-edge down wards and bevel uppermost, therefore scraping and bevel uppermost, therefore scraping down wards .down wards .

Drain holes are radially drilled through the Drain holes are radially drilled through the skirt so that excess lubricant passes skirt so that excess lubricant passes through the drain holes back into the through the drain holes back into the crankcase.crankcase.

Scraper rings have many variations on three Scraper rings have many variations on three basic forms which are the bevel type, the basic forms which are the bevel type, the hook type, which might also be called a hook type, which might also be called a self sharpening variations of the bevel self sharpening variations of the bevel typetype

and the twin or double-edge type.and the twin or double-edge type.

Bevel-tpe scraper rings start with a basic Bevel-tpe scraper rings start with a basic square or rectangular section in which the square or rectangular section in which the radial width is greater than the vertical or radial width is greater than the vertical or axial height. The angle of the bevel from axial height. The angle of the bevel from the cylinder wall between 70deg the cylinder wall between 70deg and75deg.and75deg.

The section of a hooked scraper ring is its The section of a hooked scraper ring is its name implies hook shaped. name implies hook shaped.

It is similar in shape to bevel edge ring but It is similar in shape to bevel edge ring but has a additional hook- shaped relief on has a additional hook- shaped relief on the lower flat-face surface of the ring the lower flat-face surface of the ring opposite to the bevel’opposite to the bevel’

. Double-faced rings are rectangular in . Double-faced rings are rectangular in section with a peripheral “U” section slot section with a peripheral “U” section slot in the outer circumferential face of the in the outer circumferential face of the ring. ring.

Two scraping lands formed by the slot make Two scraping lands formed by the slot make contact with the cylinder liner surface, and contact with the cylinder liner surface, and drain holes or vents are made through the drain holes or vents are made through the ring across the thin section at the bottom ring across the thin section at the bottom of the>U= shaped slot.of the>U= shaped slot.