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PISTON

PISTON MATERIAL

LIGHT & STRONG, CONDUCT HEAT WELL, EXPAND ONLY SLIGHTLY, RESIST WEAR & LOW IN COST.

EARLIER MADE OF CAST IRON, TODAY ALLOY CAST IRON WHICH OFFERS GREATER HEAT RESISTANCE & BETTER WEARING QUALITIES.

ALUMINIUM ALLOY ARE LIGHT WEIGHT, USED IN HIGH SPEED ENGINES TO REDUCE INERTIA FORCE.

THEY EXPAND CONSIDERABLY WHEN HEATED.

COMPOSITE PISTONS : FORGED STEEL CROWNS & CAST IRON SKIRTS USED IN LARGE ENGINES.FOR STRENGTH & HEAT RESISTANCE IN UPPER SECTION, GOOD WEARING PROPERTIES OF CAST IRON IN THE LOWER SECTION.

CONSTRUCTION

THE PISTONWITH ITS RINGS , SEALS THE CYLINDER & TRANSMITS THE GAS PRESSURE TO THE CONNECTING ROD.

IT ABSORBS HEAT FROM GAS, WHICH SHOULD BE CARRIED AWAY.

IN LARGE TWO-STROKE ENGINES , PISTONS ARE COOLED BY OIL OR WATER.

TOP SECTION- CROWN, LOWER SECTION SKIRT.

SHAPE OF CROWN DEPENDS ON DESIGN OF COMBUSTION CHAMBER, CONVEX OR CONCAVE.

IT IS MADE THICK TO WITHYSTAND GAS PRESSURE, & PROVIDE GOOD PATH FOR HEAT FLOW.

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THE CROWN IS MADE WITH SLIGHT TAPER TO TAKE CARE OF EXPANSION, & CARRIES COMPRESSION RINGS.

SKIRT TAKES SIDE THRUST OF CONNECTING ROD & PREVENTS ROCKING OF PISTON , & CARRIES LOWER SET OF OIL CONTROL RINGS. THEY REMOVE EXCESS OIL FROM THE LINER.

PISTON SKIRT BECOMES THINNER ON THE LOWER SIDE. INTERIOR RIBS ARE USED TO STIFFEN THE SKIRT OF CAST PISTONS.

THE RUNNING CLEARANCE BETWEEN SKIRT & LINER MUST BE SMALL.

PISTON COOLING FOR TRUNK PISTON

MEDIUM IS LUB OIL.

LUB OIL IS FORCED THROUGH THE HOLLOW CONNECTING RODTO THE UPPER END OF THE ROD.

PART OF THE OIL LUBRICATES THE GUDGEON PIN BEARING, & REMAINDER PASSES THROUGH THE NOZZLE IN THE END OF THE ROD & SPRAYS AGAINST THE UNDERSIDE OF THE PISTON CROWN.

AFTER ABSORBING HEAT FROM THE METAL, THE OIL DRIPS DOWN THE PISTON & FALLS BACK TO THE CRANKCASE.

THE OIL CIRCULATES THROUGH AN ENCLOSED SPACE UNDER THE CROWN.

OIL IS CONDUCTED FROM THE CONNECTING ROD BOTTOM- END UP A PSSAGE INTO THE ROD & SO INTO THE PISTON PIN THROUGH PASSAGES IN THE PISTON PIN AND TO THE COOLING COIL IN PISTON.

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IT CIRCULATES THROUGH THE PISTON BOSS THEN PISTON PIN & RETURNS DOWN THE ROD BY A SIMILAR PASSAGE AS THAT OF SUPPLY.

PISTON FOR LARGE CROSSHEAD- TYPE MAIN ENGINE

PISTON IS CAST OF HEAT RESISTING ALLOY STEEL CONTAINING CHROMIUM & MOLYBDENUM TO MAINTAIN STRENGTH AT HIGH TEMP & RESIST CORROSION.

THE PISTON CROWN IS THIN TO ENSURE ADEQUATE COOLING & STRONG ENOUGH TO RESIST THE HIGH PRESSURE GAS LOAD.

IT IS SHAPED TO ASSIST FLOW DIRECTION OF GASES DURING SCAVENGING, & IS SUPPORTED & FURTHER COOLED BY INTERNAL RIBS.

THE CYLINDRICAL WALL OF THE PISTON IS SHAPED INTERNALLY TO ENSURE COOLING BUT IS THICKENED TO ACCOMMODATE THE PISTON GROOVES .

THE EXTERNAL SHAPE IS TAPERED SLIGHTLY ABOVE THE TOP RING GROOVE TO ALLOW SOME DISTORTION DURING COMBUSTION CONDITIONS.

THERE ARE FIVE PISTON RING GROOVES EACH OF WHICH HAS ITS LOWER WEAR SURFACE CHROMIUM PLATED TO RESIST WEAR.

THE PISTON IS WATER COOLED INTERNALLY WITH FRESH WATER WHICH ENTERS & LEAVES THROUGH RECIPROCATING PIPES & GLANDS.

THE WATER OUTLET IN THE PISTON IS SET NEAR THE CROWN TO ENSURE THAT THE PISTON REMAINS FULL OF WATER AT ALL TIMES.

DRAINAGE CONNECTIONS ARE MADE FROM THE WATER GLANDS TO PREVENT ANY WATER LEAKAGE FROM ENTERING THE CYLINDER OR CRANKCASE.

THE PISTON COOLING SPACE IS CLOSED BY A BOLTED COVER FITTED WITH RUBBER SEAL RINGS TO PREVENT LEAKAGE.

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DRAINAGE CONNECTIONS ARE MADE FROM THE WATER GLANDS TO PREVENT ANY WATER LEAKAGE FROM ENTERING THE CYLINDER OR CRANKCASE.

THE PISTON COOLING SPACE IS CLOSED BY A BOLTED COVER FITTED WITH RUBBER SEAL RINGS TO PREVENT LEAKAGE.

RUBBER SEAL RINGS ARE ALSO FITTED AT THE ATTACHMENTS OF RECIPROCATING COOLING PIPES & BETWEEN PISTON & THE PISTON ROD FLANGE.

A SHORT CAST IRON PISTON SKIRT IS SECURED BETWEEN THE PISTON ROD FLANGE & THE UNDERSIDE OF THE PISTON A SPIGOT & RUBBER RING SEALING THIS JUNCTION.

THE SKIRT IS UNCOOLED & ACTS AS A GUIDE WITHIN THE LINER.

IT HAS TWO LEADED BRONZE WEARING RINGS CAULKED INTO GROOVES TO PREVENT POSSIBLE DAMAGE BETWEEN SKIRT & LINER.

THE LOWER EDGE OF THE SKIRT ALSO REDUCES LOSS OF SCAVENGE AIR TO EXHAUST PORTS.

FRESH WATER COOLING HAS THE ADVANTAGE OF GREATER THERMAL CAPACITY THAN OIL. IT MAY ALSO SUSTAIN HIGHER OUTLET TEMP.

INHIBITORS ARE NECESSARY TO PREVENT CORROSION IN THE SYSTEM & ADEQUATE VENTING MUST BE MAINTAINED.

LEAKAGE FROM SEALS IS A PROBLEM.

OIL HAS PROBLEMS OF DEPOSITS CAUSING CHOKING OF PASSAGES. BUT NO DANGER OF CONTAMINATION

PISTONManufacturing and materials

Materials Piston crowns attain a running temperature of about 450oC and in this zone there is a need for high strength and minimum distortion in order to maintain resistance to gas loads and maintain the attitude to the rings in relation to the liner. The heat flow path from the crown must be uniform otherwise thermal distortion will cause a non-circular piston resulting in reduced running clearance or even possible contact with the liner wall. In addition to this thermal stress they are also subject to compressive stress from combustion and compression loads, as well as inertial loads. Materials such as pearlitic, flake and spheroidal cast iron, alloy cast irons containing Nickel and chromium, and aluminium alloys may be used. The determining factor is the design criteria for the engine.

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PISTONFor a modern slow speed engine steel forging or castings of nickel-chrome steel or molybdenum steel are common. The weight of the material is not normally a governing factor in this type of engine although resistance to thermal stress and distortion is. Efficient cooling is a required to ensure the piston retains sufficient strength to prevent distortion. For medium and high speed engines the weight of the material becomes important to reduce the stresses on the rotating parts. The high thermal conductivity of aluminium alloys allied to its low weight makes this an ideal material. To keep thermal stresses to a reasonable level cooling pipes may be cast into the crown, although this may be omitted on smaller engines. Where cooling is omitted, the crown is made thicker both for strength and to aid in the heat removal from the outer surface. Hard landings are inserted into the ring groves to keep wear rated down. Composite pistons may be used consisting of an cast alloy steel crown with an aluminium-alloy or cast iron body.

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PISTONAnnealing After casting or forging the component is formed of different material thicknesses. The thinner parts will cool more quickly thereby setting up internal stresses. Annealing removes or reduces these stress as well as refining the grain structure.

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PISTONWater CooledHigh specific heat capacity therefore removes more heat per unit volume Requires chemical conditioning treatment to prevent scaling Larger capacity cooling water pump or separate piston cooling pump and coolers although less so than with oil Special piping required to get coolant to and from piston without leak Coolant drains tank required to collect water if engine has to be drained. Pistons often of more complicated design Cooling pumps may be stopped more quickly after engine stopped

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PISTONOil Cooled Low specific heat capacity Does not require chemical treatment but requires increased separate and purification plant Larger capacity Lube oil pump, sump quantity and coolers No special means required and leakage not a problem with less risk of hammering and bubble impingement. Increased capacity sump tank required Thermal stresses in piston generally less in oil cooled pistonsLarge volumes of oil required to keep oxidation down and extended cooling period required after engine stopped to prevent coking of oil

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PISTONWear rings Wear rings are found on some slow speed engines employing loop or cross flow scavenging although they may be found in most designs. They are made of a low coefficient of friction material and serves two main purposes. To provide a rubbing surface and to prevent contact between the hot upper surfaces of the piston and the liner wall. In trunk piston engines wear rings to negate the distortion effect caused by the interference fit of the gudgeon pin . The ring may be inserted in two pieces into the groove then lightly caulked in with good clearance between the ends.

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PISTONB&W LMC oil cooled piston

The piston has a concave top. This is near self supporting and reduces the need for internal ribbing. It prevents the cyclic distortion of the top when under firing load. This distortion can lead to fatigue and cracking

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PISTONPistons may be cooled by oil or water. Oil has the advantage that it may be supplied simply from the lubrication system up the piston rod. Its disadvantage are that maximum temperatures is relatively low in order to avoid oxidised deposits which build up on the surfaces. In addition the heat capacity of oil is much lower than that of water thus a greater flow is required and so pumps and pipework must be larger. Also, if the bearing supply oil is used as is mainly the case a greater capacity sump is required with more oil in use. Water does not have these problems, but leakage into the crankcase can cause problems with the oil (such as Micro Biol-Degradation). The concave or dished piston profile is used for most pistons because it is stronger than the flat top for the same section thickness

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PISTONSulzer watercooled piston (RND)Increasing section thickness would result in higher thermal stress. Sulzer piston require a flat top because of the scavenging and exhaust flow arrangement (loop scavenging of RND etc). in order to avoid thicker sections internal support ribs are used.

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PISTONHowever these ribs cause problems in that coolant flow is restricted. The flow of water with an RD piston is directed to and from the piston by telescopic pipes. The outlet is positioned higher than the inlet within the cooling cavity and on the opposite side of the support rib in order to ensure positive circulation.

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PISTONWith highly rated engines overheating occurred in stagnant flow areas between the ribs and so a different form of cooling was required. The cocktail shaker effect has air as well as water in the cooling cavity as the piston reciprocates water washes over the entire inner surface of the piston just as in a cocktail shaker. Unfortunately air bubbles become trapped in the water and flow to outlet reducing the air content and removing the cocktail shaker effect. To avoid this problem air must be supplied to the piston some engine builders use air pumps feeding air to the inlet flow. The sulzer engine allows air to be drawn into the flow at a specially designed telescopic transfer system.

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PISTONThe telescopic arrangement is designed to prevent leakage and allows air to be drawn into the coolant flow to maintain the cocktail shaker effect. Consider the inlet telescopic, a double nozzle unit is fitted to the top of the standpipe. Small holes allow connection from the main seal to the space between the nozzles. Water flowing through the lower nozzle is subject to pressure reduction and a velocity increase. The space between the nozzles is therefore at a lower pressure than other parts of the system. Any water which leaks past the main seal is drawn through the radial holes into the low pressure region and hence back into the coolant flow.

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PISTONThe pumping action of the telescopic draws air past the lower seal and this is also drawn through the radial holes into the coolant flow. This maintains the air quantity in the piston and so maintains the cocktail shaker effect. The sulzer water cooled piston differs from that of the Oil cooled variety by the method it uses for distributing the cooling medium.In this case the piston is not continually flooded but instead contains a level governed by the outlet weir. Cooling of the crown occurs during change of direction at the top of the stroke by so called 'Cocktail shaker' action.

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PISTONComposite pistonsWith medium speed and higher speed engines considerable inertia forces are placed on the conn rod and bearings as the piston changes direction at the ends of the stroke. The amount of force is a factor of the speed and rotating mass. To reduce this force whilst maintaining the same engine speed it is necessary to reduce this rotating mass.

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PISTONAluminium, with its lower density than steel is used when alloyed with silicon for extra strength. Even alloyed the aluminium has less mechanical strength than the steel, therefore damage is possible due to gas pressure acting on crown and piston rings. The piston could deform sufficiently to prevent proper operation of the rings in their grooves. Some engine manufacturers fit cast iron inserts into the grooves but more generally the piston is made in two parts with a cast steel crown containing two grooves.

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PISTONAluminium has a better coefficient of heat transfer than steel thus overheating is not a problem. Its lower coefficient of friction avoids the problems of fitting bushes for the gudgeon pin, thus a floating gudgeon pin may be used. The higher coefficient of expansion could lead to the need for greater piston/liner clearance. However, as the main body is not subject to the high temperatures of combustion this expansion is not a problem.

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PISTONSulzer rotating piston

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PISTONThis piston rotates as it reciprocates. The rotation being brought about by the swing of the con rod. This causes two spring loaded palls located in the spherical top end to oscillate. These palls engage with a toothed rim which is connected to the piston by means of a compensating spring. As the conrod swings the palls act on the toothed rim causing it, and hence the piston, to rotate. The amount of rotation is limited to one tooth pitch every engine rev and the action is similar to that of a ratchet mechanism.

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PISTONThe advantage of this is that local overheating of the piston or the liner due to blow past is prevented.Running in characteristics are improved and liner wear are improved. There is a better spread of oil brought about by the piston rotation. A spherical top end is required but this provides better support for the piston which does not distort as much as one fitted with a gudgeon pin.Piston to liner clearance may therefore be reduced.

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PISTONTransfer of gas loads from crown to piston rodIs usually transmitted from the reinforced crown to the piston rod by internal mechanism avoiding possible distortion of the ring belt. The tops of pistons are made dome shaped or have strong internal ribbing. Thermal distortion of Piston

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PISTONAnti-Polishing ringsHigh topland ( the 1st piston ring is positioned well below the upper surface of the piston) with associated reduced ring heat load has given better ring pack performance by improving working conditions for the cylinder lube oil.

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PISTONThe disadvantage of this system is that a coke build up can occur above the piston which leads to 'bore polishing'. This polishing reduces the ability of the cylinder lube oil to 'key' into the liner therefore increased cylinder lube oil consumption/increased liner wear can result. To combat this piston cleaning rings are incorporated into the liner. These slightly reduce the bore removing the deposits.

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PISTONModern DesignThe top piston ring is moved further down the piston. This allows the crown to enter deeper into the crown reducing temperature and pressure on the liner. The top piston ring is a 'Controlled Pressure relief' (CPR) ring. This design has several oblique shallow grooves in the piston ring face allowing some gas presure to pass through to the 2nd ring thereby reducing load on the top ring. To reduce blowpast an 'S' type joint is formed n the ring ends

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