four stroke piston

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Chapter 8 Chapter 8 The 4 Stroke Medium Speed Trunk The 4 Stroke Medium Speed Trunk Engine Piston Engine Piston

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Page 1: four stroke piston

Chapter 8Chapter 8

The 4 Stroke Medium Speed The 4 Stroke Medium Speed TrunkTrunk

Engine PistonEngine Piston

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Pistons for medium speed 4 stroke engines are Pistons for medium speed 4 stroke engines are different in design from their crosshead engine different in design from their crosshead engine counterparts.counterparts.

The top end of the connecting rod swings about the The top end of the connecting rod swings about the piston or gudgeon pin located in the piston skirt.piston or gudgeon pin located in the piston skirt.

The piston skirt or piston trunk gives rise to the name The piston skirt or piston trunk gives rise to the name trunk piston. trunk piston.

The purpose of this skirt or trunk in four-stroke cycle The purpose of this skirt or trunk in four-stroke cycle engines is to act in a similar manner to a engines is to act in a similar manner to a crosshead. crosshead.

It takes the thrust caused by connecting-rod It takes the thrust caused by connecting-rod angularity and transmits it to the side of the angularity and transmits it to the side of the cylinder liner, in the same way as the crosshead cylinder liner, in the same way as the crosshead slipper transmits the thrust to the crosshead guide.slipper transmits the thrust to the crosshead guide.

With such engines, which are termed trunk-piston With such engines, which are termed trunk-piston engines, the engine height is considerably reduced engines, the engine height is considerably reduced compared with that of a crosshead engine of similar compared with that of a crosshead engine of similar power and speed. power and speed.

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The engine-manufacturing costs are also The engine-manufacturing costs are also reduced.reduced.

For engines which burn distillate fuels (MDO, For engines which burn distillate fuels (MDO, Gas Oil), the piston can be manufactured in Gas Oil), the piston can be manufactured in one piece either of spheroidal graphite cast one piece either of spheroidal graphite cast iron or a silicon aluminium alloy.iron or a silicon aluminium alloy.

Cast iron is generally used for smaller engines.Cast iron is generally used for smaller engines. It is strong, wear resistant, and has an It is strong, wear resistant, and has an

expansion rate similar to that of the liner, expansion rate similar to that of the liner, which means the clearance between piston which means the clearance between piston and liner remain constant throughout the and liner remain constant throughout the operating range of the engine. operating range of the engine.

Because cast iron is heavy, on larger engines Because cast iron is heavy, on larger engines the high inertial forces at change of direction the high inertial forces at change of direction between exhaust and inlet strokes lead to between exhaust and inlet strokes lead to increased bearing loading and stresses in increased bearing loading and stresses in the bottom end bolts.the bottom end bolts.

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Aluminium alloy is light, and therefore has low Aluminium alloy is light, and therefore has low inertia, reducing bearing loading.inertia, reducing bearing loading.

It work hardens to give good wearing It work hardens to give good wearing surfaces.surfaces.

It has a relatively high coefficient of It has a relatively high coefficient of expansion which mean there is increased expansion which mean there is increased risk of siezure if overheated, therefore larger risk of siezure if overheated, therefore larger working clearances when cold are required.working clearances when cold are required.

At low loads this increased clearance can lead At low loads this increased clearance can lead to piston slap.to piston slap.

Aluminium suffers from carbon build up which Aluminium suffers from carbon build up which could lead to burning of the piston crown. could lead to burning of the piston crown. (300°C). (300°C).

Ring grooves, especially the top ones, are Ring grooves, especially the top ones, are subject to high wear rates, necessitating subject to high wear rates, necessitating cast in ring inserts.cast in ring inserts.

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The pictures below illustrate an aluminium The pictures below illustrate an aluminium alloy piston from a Pielstick PC2 engine alloy piston from a Pielstick PC2 engine designed to burn distillate fuel.designed to burn distillate fuel.

It has a cast iron insert for the top It has a cast iron insert for the top compression ring and a cast in cooling coil compression ring and a cast in cooling coil to circulate the oil to cool the crown.to circulate the oil to cool the crown.

The crown of the piston is shaped to allow for The crown of the piston is shaped to allow for the open valves as the piston passes over the open valves as the piston passes over TDC on the exhaust stroke. TDC on the exhaust stroke.

This shaping also causes the air to move This shaping also causes the air to move rapidly toward the centre, which aids better rapidly toward the centre, which aids better mixing with the fuel, leading to complete mixing with the fuel, leading to complete combustion.combustion.

One piece pistons are not suitable for burning One piece pistons are not suitable for burning

heavy residual fuels; the higher heavy residual fuels; the higher temperatures at which they burn reduce temperatures at which they burn reduce strength and resistance to corrosion.strength and resistance to corrosion.

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Pistons designed to burn residual fuels are Pistons designed to burn residual fuels are made in two pieces, known as composite made in two pieces, known as composite pistons. Composite Pistons with a crown of pistons. Composite Pistons with a crown of alloy steel containing chromium and alloy steel containing chromium and molybdenum and a skirt of cast iron or molybdenum and a skirt of cast iron or aluminium are generally the choice for highly aluminium are generally the choice for highly rated medium speed engines burning residual rated medium speed engines burning residual fuels. fuels.

The piston ring grooves in the crown can be The piston ring grooves in the crown can be induction hardened or chromium plated.induction hardened or chromium plated.

The composite design using aluminium alloy for The composite design using aluminium alloy for the skirt, gives a strong light structure, the skirt, gives a strong light structure, capable of resisting high temperatures, but capable of resisting high temperatures, but with reduced bearing loading.with reduced bearing loading.

As in the one piece design, skirt/liner As in the one piece design, skirt/liner clearances must be made larger than for cast clearances must be made larger than for cast iron skirts, which may lead to piston slap at iron skirts, which may lead to piston slap at low loads.low loads.

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The sketches below and opposite are of a The sketches below and opposite are of a composite piston of the type fitted to the MAN composite piston of the type fitted to the MAN B&W L58/64 engine. B&W L58/64 engine.

It has an alloy steel crown containing chromium It has an alloy steel crown containing chromium and molybdenum to give increased strength and molybdenum to give increased strength and resist corrosion at high temperatures. and resist corrosion at high temperatures. Suitable for burning residual fuels. Suitable for burning residual fuels.

The aluminium alloy skirt (subject to lower The aluminium alloy skirt (subject to lower thermal and mechanical loads) is light and has thermal and mechanical loads) is light and has low inertia, reducing stress reversal on bottom low inertia, reducing stress reversal on bottom end bolts.end bolts.

As with many pistons for large marine diesel As with many pistons for large marine diesel engines, the gudgeon pin floats in the piston engines, the gudgeon pin floats in the piston skirt, retained by cir clips and lubricated by the skirt, retained by cir clips and lubricated by the oil as it returns from the piston cooling space. oil as it returns from the piston cooling space.

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The oil control ring situated at the top of the The oil control ring situated at the top of the skirt scrapes any excess oil on the liner skirt scrapes any excess oil on the liner back to the crankcase.back to the crankcase.

The large diameter gudgeon (piston) pin The large diameter gudgeon (piston) pin and large bearing surface keeps bearing and large bearing surface keeps bearing loads to acceptable limits, eliminating loads to acceptable limits, eliminating edge loading due to bending of pin. edge loading due to bending of pin.

The oil is directed into the cooling space by The oil is directed into the cooling space by means of a spring loaded oil catcher which means of a spring loaded oil catcher which slides across the top of the conrod as it slides across the top of the conrod as it swings about the gudgeon pin.swings about the gudgeon pin.

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PISTON COOLINGPISTON COOLINGAll pistons, in effect, are cooled when heat All pistons, in effect, are cooled when heat

flows from the combustion side of the flows from the combustion side of the piston crown to anything in contact with it piston crown to anything in contact with it which is at a lower temperature.which is at a lower temperature.

In small engines the heat flows from the In small engines the heat flows from the upper part of the piston crown to the lower upper part of the piston crown to the lower side of the crown and the walls or sides of side of the crown and the walls or sides of the piston. the piston.

The heat is taken away partly through the The heat is taken away partly through the cylinder liner by conduction, and partly into cylinder liner by conduction, and partly into the air in the crank case by radiation and the air in the crank case by radiation and conduction.conduction.

Oil splashed off the bearings onto the Oil splashed off the bearings onto the underside of the piston removes a large underside of the piston removes a large amount of heat by conduction. amount of heat by conduction.

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The heat passing into the piston crown is The heat passing into the piston crown is dissipated at a rate such that the dissipated at a rate such that the temperature of the piston material does not temperature of the piston material does not rise to the point at which its strength is rise to the point at which its strength is reduced below allowable limits, or causes reduced below allowable limits, or causes breakdown in the lubrication of the piston breakdown in the lubrication of the piston and piston rings.and piston rings.

In large engines oil must be circulated to In large engines oil must be circulated to remove the larger amount of heat remove the larger amount of heat generated. generated.

This can be done by one of the following This can be done by one of the following methods:methods:

1. Some of the oil led up the connecting rod to 1. Some of the oil led up the connecting rod to the top end bearing is sprayed onto the the top end bearing is sprayed onto the underside of the piston crown.underside of the piston crown.

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2. Oil is circulated through a mild steel coiled tube 2. Oil is circulated through a mild steel coiled tube which is cast into position during manufacture which is cast into position during manufacture of the piston. Oil circulation is from the main of the piston. Oil circulation is from the main bearings, via passages drilled in the crankshaft bearings, via passages drilled in the crankshaft to bottom end bearings and then up a bore in to bottom end bearings and then up a bore in the connecting rod to a passage in the hollow the connecting rod to a passage in the hollow gudgeon pin. It passes to the coil from one end gudgeon pin. It passes to the coil from one end of the pin and returns through the other end.of the pin and returns through the other end.

3 Oil from the gudgeon pin passes into cooling 3 Oil from the gudgeon pin passes into cooling spaces within the piston crown, where cooling is spaces within the piston crown, where cooling is effected by the shaker method due to the effected by the shaker method due to the reciprocating action of the piston. The oil drains reciprocating action of the piston. The oil drains back through the skirt to the crankcase. back through the skirt to the crankcase. Because of the swinging motion of the Because of the swinging motion of the connecting rod, some form of catcher may be connecting rod, some form of catcher may be arranged to allow the piston crown to be arranged to allow the piston crown to be supplied continuously with oil.supplied continuously with oil.

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If cooling is not supplied in sufficient If cooling is not supplied in sufficient quantity or there is a build up of carbon on quantity or there is a build up of carbon on the underside of the piston crown which the underside of the piston crown which acts as an insulating layer, then the piston acts as an insulating layer, then the piston crown will overheat leading to weakening crown will overheat leading to weakening of the material, erosion, thermal stressing of the material, erosion, thermal stressing and possible cracking of the crown. and possible cracking of the crown. Overheating will cause more oil to Overheating will cause more oil to carbonise on the cooling surface carbonise on the cooling surface increasing the thickness of the insulating increasing the thickness of the insulating layer and exagerating the situation. Unlike layer and exagerating the situation. Unlike slow speed two stroke engines, no record slow speed two stroke engines, no record is usually taken of piston oil return is usually taken of piston oil return temperatures.temperatures.

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FLOATING GUDGEON PINSFLOATING GUDGEON PINSThe floating gudgeon pin is a common feature of The floating gudgeon pin is a common feature of

medium speed marine diesel engines. medium speed marine diesel engines. This allows better alignment of the small end This allows better alignment of the small end

bearing with decreased wear.bearing with decreased wear.The pin is a clearance fit in the skirt and The pin is a clearance fit in the skirt and

retained by circlips or an end plate. retained by circlips or an end plate. Because the bores in the skirt now act as Because the bores in the skirt now act as

bearing surfaces, oil is supplied from the con bearing surfaces, oil is supplied from the con rod top bearing or piston cooling supply.rod top bearing or piston cooling supply.

In the example shown, the pin is hollow with an In the example shown, the pin is hollow with an insert which allows distribution of oil to the top insert which allows distribution of oil to the top end bearing and to the skirt bearing surfaces.end bearing and to the skirt bearing surfaces.

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On a piston fitted with a floating gudgeon On a piston fitted with a floating gudgeon pin, the oil scraper ring will be fitted at the pin, the oil scraper ring will be fitted at the top of the skirt, to remove the excess oil top of the skirt, to remove the excess oil passing through the ends of the pin onto passing through the ends of the pin onto the liner wall.the liner wall.

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SKIRT LUBRICATIONSKIRT LUBRICATIONSkirt lubrication brings the lubricating oil Skirt lubrication brings the lubricating oil

through to the piston skirt via drilling through to the piston skirt via drilling Positioned above the gudgeon pin and Positioned above the gudgeon pin and below the oil scraper ring, the oil is below the oil scraper ring, the oil is distributed around the skirt by a distributed around the skirt by a circumferential groove. Ensures adequate circumferential groove. Ensures adequate lubrication of skirt and liner at side thrust lubrication of skirt and liner at side thrust transmission point and reduces the need transmission point and reduces the need for separate cylinder lubrication via the for separate cylinder lubrication via the liner.liner.