Marine vol 1 part 3

As the piston reaches the bottom of the cylinder for the second timethe exhaust valve is opened and during the upward and fourth stroke thehOt spent gases are expelled through it.. If th

MARINE ENGINEERING PRACTICE

S H HENSHALL, C.Eng., F.I.Mar.E., F.I.Mech.E.

Published by The Institute of Marine Engineers

The Memorial Building

© 1973 Marine Management (Holdings) Ltd

A catalogue record for this publication is available from theBritish Library

ISBN 0 900976 09 8

All rights reserved No part of this publication may be reproduced,stored in a retrieval sytem, or transmitted in any form or by anymeans, electronic, mechanical, photocopying, recording or other-wise, without the prior permission of the publisher

Printed in the United Kingdom by Hobbs the Printers Ltd,

BruneI Road, Totton, Hampshire S040 3WX

Page

The author acknowledges with gratitude the assistance of his colleagues at Mirrlees Blackstone Limited, a Hawker Siddeley Company, and the permission of the Directors of that Company to publish this work His thanks are also due

to Bryce Berger Limited for their kindness in providing the figures illustrating fuel injection equipment, and to Turnbull Marine Design Co Ltd., for their help in drawing some of the original figures.

1 DEFINITION OF MEDIUM SPEED DIESEL

ENGINES

1.1 Diesel engines operate on either a four stroke or a two stroke cycle.The feature which distinguishes diesel engines from other reciprocatinginternal combustion engines is that the fuel is not introduced into thecylinder until just before the beginning of the expansion (or power) strokeand it is ignited by coming into contact with the air in the cylinder which

is very hot as the result of compression For this reason, they are sometimescalled compression ignition engines

1.2 Medium speed diesel engines may be defined as those having ratedspeeds within the range of 300 to 1000 rev / min

1

2 FOUR STROKE CYCLE DIESEL ENGINES

2.1 The essential parts of a four stroke cycle diesel engine are shown inFig 1, and in Fig 2 are depicted the events of each of the four strokes.Starting with the piston at the top of the cylinder, the induction stroke isperformed as the piston moves down During this stroke the inlet valveremains open and a charge of air is drawn into the cylinder The pistonthen returns to the top of the cylinder whilst both inlet and exhaust valvesremain closed and the charge of air is compressed The compression raisesthe temperature of the air and as the piston reaches the end of this stroke,

a controlled amount of fuel is injected into the cylinder in the form of a finespray On coming into contact with the hot air the fuel ignites causing arapid rise in pressure which drives the piston downwards on the expansionstroke As the piston reaches the bottom of the cylinder for the second timethe exhaust valve is opened and during the upward and fourth stroke thehOt spent gases are expelled through it

2.2 If the charge of air entering the cylinder during the induction stroke

is drawn in by the piston movement only without being assisted in any way,then the engine is said to be naturally aspirated

2.2.1 The power of any diesel engine is determined by the amount offuel that can be burnt in each cylinder per cycle and the speed at which itcan be run The rotational speed is limited by the forces arising from theinertia of the moving parts In the case of a naturally aspirated engine theamount of fuel that can be burnt is limited by the mass of air drawn intothe cylinder during the induction stroke Diesel fuel requires about 14'5times its own mass of air for complete combustion The time available forcombustion in the diesel cycle is very short; if only the chemically correctamount of air were provided, the fuel would not have time to burn com-pletely and, in practice, almost twice this quantity is found to be necessary.2.3 The cylinder can be charged with a greater mass of air by supplying

it under pressure The air is pressurized in a compressor, sometimes termed

a blower, and fed to the induction manifold of the engine The process iscalled pressure charging or supercharging

2

2.3.1 In some special cases the compressor is driven mechanically from theengine crankshaft More usually, some of the energy present in the exhaustgas is utilized by passing it through a turbine which is directly coupled to

a centrifugal compressor The compressor- and turbine together form a freerunning unit, separate from the engine, known as a turbocharger Figure

3 shows diagrammatically a turbocharger and a turbocharged engine.2.3.2 The pressure of the air in the inlet manifold to the cylinders of a

turbocharged engine is termed tile boost pressure In engines having a highboost pressure the air leaving the compressor is hot and it is beneficial andsometimes necessary to cool it as this assists in increasing the mass of airfilling the cylinders and in keeping the internal parts of the engine cool Theair is cooled by passing it through an intercooler.

FOUR STROKE CYCLE DIESEL ENGINES 5

2.3.3 The quantity of air provided by turbocharging is so great that theamount of fuel that can be burnt in each cylinder per cycle (and hence thepower) is not limited on this account but by the temperature which theexhaust valves, cylinder heads and pistons can withstand

3 TWO STROKE CYCLE DIESEL ENGINES

3.1 Two stroke cycle diesel engines take several forms

3.1.1 One form, which is illustrated in a simple manner in Fig 4, has a

TWO STROKE CYCLE DIESEL ENGINES 7valve in the cylinder head through which the exhaust gases leave thecylinder and ports around the lower part of the cylinder through whichthe air enters Its operation is shown diagrammatically in Fig 5.

At the bottom dead centre the exhaust valve and the inlet ports areboth open at the same time The pressure of the air in the inlet manifold

is arranged to be higher than the pressure in the exhaust manifold so that

on entering the cylinder it sweeps out the exhaust gases and fills the cylinderwith a fresh charge This process of displacing the spent gases in the cylinder

by the incoming air is termed scavenging

As the piston rises it cuts off the inlet ports and the exhaust valve

is arranged to close at the same time Compression of the charge of air takesplace and fuel is injected into the hot compressed air as top dead centre isapproached Combustion occurs and the piston is driven downwards on theexpansion stroke Towards the end of this stroke the exhaust valve is openedallowing the exhaust gases to escape and the pressure in the cylinder fallbelow that of the air manifold Shortly afterwards, as the inlet ports areopened by the downward moving piston, the scavenging air enters anddisplaces the remaining exhaust gas in preparation for the next cycle.3.1.2 Another form of two stroke cycle engine, known as the opposedpiston type, is shown in Fig 6 In operation it is very similar to the type justdescribed, the chief difference being that the exhaust gases leave the cylindervia ports controlled by the upper piston instead of through a valve

The two pistons may have the same length of stroke or the pistoncontrolling the exhaust ports may have a shorter stroke than the one con-trolling the inlet ports Power is obtained from both pistons and a variety

of mechanisms is available to connect them together: the two mostcommonly used are separate crankshafts geared together, as shown inFig 7, and side connecting rods for the upper piston operated from extra

8

TWO STROKE CYCLE DIESEL ENGINES 9

cranks formed on the single crankshaft, as shown in Fig 8 The piston controlling the exhaust ports is usually given a few degrees lead over the piston controlling the inlet ports in order to allow the cylinder pressure to blow down before the scavenge air enters.

FIG. 8.-0pposed piston engine with side rods.

3.1.3 In the two stroke engines described so far the scavenging is carried out from one end of the cylinder to the other This is termed uniflow scavenging Another arrangement appears in Fig 9, where inlet and exhaust ports are both at the same end of the cylinder The path taken by the scavenging air leads to the term "loop scavenge" being used to describe this form of two stroke cycle engine The events of the cycle are exactly the same

as for the other forms The loop scavenge engine is mechanically simpler than the other types but because one piston controls both inlet and exhaust ports,

it is usually necessary to provide light, quick-acting non-return valves in the inlet port to prevent backflow of exhaust gas before the cylinder has blown down to a pressure lower than that in: the scavenge air manifold.

3.2 All two stroke cycle diesel engines require the scavenge air to be at a pressure above that in the exhaust manifold and a scavenge blower is necessary to provide this slightly pressurized air Scavenge blowers may be reciprocating air pumps driven directly from the crankshaft or running gear,

or routs blowers or centrifugal compressors driven indirectly from the

10 MARINE ENGINEERING PRACfICE

crankshaft by chain drives or gearing The volume of air displaced by thescavenge air blower is a little in excess of the swept volume of the cylinders

in order to ensure that scavenging is complete

3.3 Two stroke cycle engines can be turbocharged in the same way as fourstroke cycle engines In most cases a scavenge blower, mechanically driven,

is retained to assist the turbocharger The flow of air may all pass throughthe compressor of the turbocharger and then through the scavenge blower

in series, or part of it may pass through the compressor and part throughthe scavenge blower in parallel In the latter case the size of the blowerrequired is smaller but it has to be capable of dealing with the full boostpressure ratio Crosshead type engines sometimes use the undersides of theirpistons as scavenge blowers to assist a turbocharged cycle

4 CONSTRUCTION

4.1 CYLINDER AND CRANKCASE

The cylinders of medium speed diesel engines are water cooled Thebore of each cylinder is formed in a liner which can be replaced when wornout and which is surrounded by a cooling water jacket The water is in directcontact with the outer surface of the liner as a result of which it is termed a

"wet" liner The most usual arrangement is for a number of cylinder liners

to be enclosed in one cast iron casing to form a cylinder block The coolingwater jacket is common to all the cylinders but there are often dividingwalls to ensure that each cylinder receives the right amount and flow ofwater Doors are provided on the cylinder casing; through which the waterspaces may be cleaned and inspected when overhauling the engine

In an alternative construction each cylinder is enclosed by a separatewater jacket which is approximately cylindrical in shape The cylinderunits so formed are carried in a frame which is the major structuralfeature of the engine As this frame does not come into contact with thewater, any danger of corrosion is minimized and some designs takeadvantage of this feature to use welded steel for its construction

The top of each cylinder is closed by a cylinder head or cover helddown by studs and nuts to the cylinder block or engine frame and making

a gas tight seal between the head and liner

The complete cylinder block is supported above the crankcase by astructure which may take one of several forms A classical design for avertical in-line engine is shown in Fig 10 The crankshaft is carried inbearings formed in a bedplate On the bedplate is mounted a casting termedthe column which forms the crankcase and supports the cylinder block

In many designs the forces produced by the reaction of the cylinder heads

to the gas pressure in the cylinders are transmitted from the top ofthe cylinder block directly to the crankshaft main bearings by throughbolts which pass through all the separate components of the structure, thusmaintaining them in compression and ensuring that all major tensile loadsare carried by these steel members

Smaller engines can be made to 'be more rigid than large ones and maynot have through bolts In addition, because the castings are not so bigand heavy, the column and cylinder block may be made in one piece.For Vee-form engines the top of the column is shaped to give two

12 MARINE ENGINEERING PRACTICE

sloping surfaces on which the cylinder blocks are placed at the correctrelative angle to each other Through bolts passing all the way from the top

of the cylinder blocks to the main bearing housings are very difficult toaccommodate in Vee engines without restricting access to the main bearings

and most designs break the continuity at the top of the bearing arch, seeFig II

Doors are formed in the columns of the both in-line and Vee engines

to give maintenance access to the crankshaft main and connecting rod largeend bearings

In another form of construction, used for both in-line and Vee engines.

the sides of the bedplate are carried upwards to form the crankcase andincorporate the crankcase doors The top of this V-shaped member may beclosed by the block of the in-line engine or combined blocks of the Veeform engine or there may be separate structural members to carry thecylinder blocks

CONSTRUCTION 13

An alternative method of supporting the crankshaft can be seen inFig 12 This is the underslung form which has been used for high speedengines for many years and which is now finding increasing popularity inmedium speed engine designs Through bolts can also be incorporated in

this design, but whether they are or not the reactions are transmitted moredirectly between cylinder heads and main bearings with the result that thebedplate being no longer a stressed member is replaced by a light sump

4.2 CRANKSHAFTS, MAIN BEARINGS AND SHAFT ALIGNMENT

4.2.1 The crankshafts of medium speed engines are almost invariably solidforged from a single piece of steel The type of steel is chosen for itsstrength, resistance to fatigue and hardness of bearing surface The cranks

of a multi-throw shaft are set at angles to each other giving a "firing order"for the engine This firing order is chosen primarily to give the smoothesttorque and the best possible mechanical balance but considerations of main

14 MARINE ENGINEERING PRACfICE

bearing loads exhaust arrangement suitable for turbocharging and torsional vibration may also be taken into account.

Although the crankshafts of medium diesel engines appear to be robust they rely on the support of the main bearings to develop their full strength When a crankshaft has to be handled outside the engine, it should be

carefully supported in a manner which will avoid imposing high bending moments on it In the engine it is essential to ensure that the bearings carrying it are in good alignment.

4.2.2 The main bearing shells are made of steel with a lining of bearing metal which may be white metal, copper-lead or aluminium-tin alloy; they often have a thin flash of lead or indium to provide a layer giving protection against corrosion They are held in position and shape by the bore of the bed plate or frame The external circumference of a pair of main bearing shells is slightly larger than that of the bore of the housing which receives them The difference is termed "nip" When the assembly is bolted up the result is the equivalent of an interference fit between the shells and the housing The alignment of the main bearings is a matter of ensuring that the main structure of the engine is properly lined up, this is dealt with in Section 6.4

A few engines have structures which are sufficiently rigid to be mounted

on four points only, more usually pairs of holding down bolts and chocks are situated in line with each main bearing Some manufacturers provide facings on the bedplate or frame for use in conjunction with optical align-

CONSTRUCTION 15

ment equipment, but whether this is used or not readings of crankshaft deflexion are usually taken as a measure of the accuracy which is achicved Misalignment of the main bearings causes bending of the crankshaft which can be detected hy measuring the deflexion of the webs as described in Scction 6.4.

At some point along its length the crankshaft must be located axially.

If the crankshaft is coupled rigidly to a shaft connected to a gearbox or

a fluid coupling which has a thrust bearing in it then this rigid connection will form the axial location for the crankshaft However if the shaft is coupled to a mechanical coupling which is free to take up any axial position then the crankshaft will be located axially somewhere inside the engine Such axial location is not intended to accommodate large axial thrust It

is achieved by allowing limited clearance at the sides of one particular journal bearing Flanges (in some instances crank webs) on each side of the one journal in question bear against flanges or rings on the side of the bearing shell, cap or housing see Fig 13 Engines are designed so that this location can be included or not according to the requirements of the shaft

system It is important that only one location of this nature exists on an} one shaft assembly and it is very important that such a location should not exist on the same shaft assembly as a thrust bearing.

4.3 CONNECTIN<i RODS

4.3 I With a few exceptions medium speed diesel engines have trunk pistons with the result that pistons and connecting rods have to be fitted together before being assembled into the cylinder The methods of assembly and overhaul tend to influence the design of the large end For one or two types of engine, the piston and rod cap be withdrawn downwards from the cylinder into the crankcase and then out through the crankcase door However, this design tends to result in a high engine and therefore the piston and rod is more usually withdrawn upwards This means that in the case

of engines having cylinder blocks, the rod must be small enough to pass through the bore of the cylinder In the case of engines having separate

16 MARINE ENGINEERING PRACTICE

waterjackets, the whole cylinder assembly with the piston and rod insidemay be removed as a single unit; the aperture in the frame for the water-jacket being much larger than the cylinder bore gives much more roomfor the connecting rod large end to pass

Connecting rod large ends are basically either of fixed centre or

"marine" type design These two basic designs are shown in Fig 14 In the

latter design, the large end bearing is separate from the rod which has apalm end A distance piece, with ground faces, known as a compressionplate may be interposed between the rod and the bearing housing, its thick-ness being chosen to ensure the correct compression ratio The fixed centrerod does not have this adjustment and relies on accuracy of manufacture toensure correct clearances

The fixed centre design of rod may have the large end split obliquely, asshown in Fig ]5, to accommodate a larger bearing, whilst presenting anarrow profile that will pass through the cylinder bore The cap may even

be in two pieces leaving only a small part of the bearing housing on therod The faces between the separate piece of such designs are usuallyprovided with serrations to take the §hear force across them The marinetype rod may take the form shown in Fig 16, the small palm permittingthe rod to pass through the cylinder bore With this design the large endbearing housing can be dismantled separately

The design of the large end bearing is similar to that of the main ing The steel shells, of relatively thin wall section, have a lining of bearing

bear-17

18 MARINE ENGINEFRING PRAcnCE

metal; white metal copper-lead or tin-aluminium, and a thin flashing of lead or indium to provide an anti-corrosion layer A typical shell is shown

in Fig 17 It has tags for location and oil grooves round the centre of its axial length which correspond with grooves in the bearing housing through which oil is transmitted up the rod to lubricate the small end and to cool the

piston These oil grooves effectively cut the thin wall shells in two and the two halves so formed arc held together by bridges The shells depend

on the accuracy of the bore to hold them in a truly circular shape Accordingly it is essential that the large end bolts are tightened correctly to the designed pull as under- or over-tightening will result in the housing bore departing from its true circular shape.

The large end bolts are very important components and are carefully designed to carry the high fatigue loads demanded of them It is, therefore essential to treat them carefully and to avoid any damage to their finely finished surfaces Raised collars on the bolts tit the reamed holes in the housings to prevent lateral movement of these parts Correct tightening

is necessary to ensure satisfactory performance of both bolts and housings and is obtained by the use of stretch gauges to measure the extension given

to the bolts, or by using a torque wrench, or by means of special hydraulic tightening equipment.

A method recently developed by one manufacturer is to make the bolts with a central drilling along their length in which can be placed an electric heating element The bolts are raised to a controlled pre-determined tem- perature, the nuts run down to hand tightness and the bolts allowed to cool.

As they contract they take up the correct tightening load.

For Vee-form engines the rods for corresponding cylinders in each

CONSTRUClION 19bank operate on the same crankpin and three arrangements of connectingrod large end are in use The rods may be side by side the design of largeend being the same as for a in-line engine, or they may be in the sameplane with large end bearings of fork and blade design as in Fig 18, orarticulated design as in Fig 19.

The small (top) end bearing is a bush having an interference fit in theeye bored in the rod The bush may be of bronze or other hard bearingmetal or it may be a composite structure of steel with a bearing metal lining.Some engines have stepped small end bearings to provide greater

bearing areas in the more heavily loaded direction In the case of two

stroke engines the loads on the small end are vertically downwards on tothe rod the whole of the time and some manufacturers employ a bearingextending over nearly the whole length of the gudgeon pin, but only halfway round (see Fig 20)

The shank of the rod usually has a bore throughout its length whichconducts oil from the large end to the small end for lubrication and to theinside of the piston for cooling

20

CONSTRUCTION 21

4.4 CYLINDER LINERS

4.4.1 Cylinder liners are made from close grained cast iron For four stroke cycle engines they are simple cylindrical shapes flanged at the top end to provide location and a means of securing them in the cylinder block or to the water jacket Immediately below this flange there is often a joint ring which may be of copper or in some designs of a heat resisting rubber The lower end is fitted with rubber rings to form a seal for the bottom of the water space As well as stopping water leaks into the crankcase these rubber rings may be arranged also to prevent oil from the crankcase entering the water jackets A "tell-tale" leak off hole is often provided between upper and lower rings to ensure that water passing the one or oil passing the other runs to the outside of the engine and draws attention to the need for renewal

of the joint rings without contaminating the other fluid.

The upper part of the liner bore where the top piston ring reaches the top

of its travel suffers the greatest wear This is because at this point the ring comes to rest and reverses its direction of motion and it is difficult to maintain an adequate film of oil between the surface of the ring and liner Also the gas pressure is highest when the ring is in this position forcing it hard against the liner and of course the top of the liner is hot from the repeated combustion cycles which tends to dry off any oil there is L\dditionally, tiny particles of carbonaceous matter are formed by the combustion processes, some of them may be abrasive and over a period of time an accumulation builds up in the groove around the ring leading to wear promoting conditions.

Obviously, wear will be reduced if the top part of the liner is kept ably cool Liners are sometimes specially shaped to fit the jacket in a way that promotes cooling without sacrificing strength Several designs of high output engines have cylinder liners with deep flanges in which a large number of small bore passages are formed carrying the coolant close to the cylinder bore whilst retaining a very strong structure (see Fig, 21).

reason-Cylinder liners for two stroke engines have ports about midway along their length for the admission of air, and for the exhaust in the case of loop scavenge engines The water jacket does not usually extend below the port belt The sealing rings are therefore situated just above the ports and are designed to prevent scavenge air (and exhaust in the case of loop scavenge engines) entering the water spaces as well as water escaping from the jackets They are made of specially formulated rubber, highly resistant to heat and oil An additional sealing ring of similar material is located below the ports to prevent the pressurized scavenge air blowing through to the nankcase.

The bores of cylinder liners when new have a specially prepared surface designed to aid the running in of the piston rings and the liner It is slightly rough in order to retain the oil and to promote rapid wear in As the rings run in a glazed surface is produced which resists wear When the piston

22 MARINE ENGINEERING PRACTICE

rings are renewed the glaze on the liner bore should be broken by honing

in order to provide a surface suitable for rapid running-in In order to

provide a harder wearing surface, some designs of liners have a deposit of

chromium plate on their bores This chromium plate is not the usual shinyvariety used for decorative purposes but is of a porous nature to provide

an oil retaining surface

4.5 PISTONS

4.5.1 The piston is a pot shaped component as shown in Fig 22, thecrown is fairly thick with the cylindrical parts tapering to a thinner section.The combustion chamber is enclosed between the cylinder head and thepiston and most of it is contained in the top of the piston For this purposethe top surface of the piston crown may be bowl shapd or toroidal shaped

as shown in Fig 23 The rim of the piston may contain cut-out portions toaccommodate the valves when th.ey op~n The cooling of the piston iscarried out by circulating lubricating oil across the underside of the crown

and inside the ring belt or through specially shaped passages In Fig 24,

a number of ways of carrying out this cooling are shown, the simplest means

is by splash or spray on the underside of the piston crown A more complex

CONSTRUCTION 23design involves a chamber specially constructed for the oil to circulatethrough In some designs this chamber takes the form of a coil cast into thematerial of the piston and conducting the heat away from the piston ringregion In another design the chamber is open and constructed so that theoil splashes about violently This design is termed the "cocktailshaker"; themotion of the oil providing extremely good heat transfer The pistons of

24 MARINE ENGINEERING PRACTICE

high output engines are often made in two pieces bolted together givingthe designer an opportunity to form precisely shaped cooling chambersbelow the crown and behind the ring belt

Pistons may be made in aluminium alloy in order to keep the weightdown for balancing purposes at high speed In other designs where thenecessity for light weight is not so important, they are made wholly of castiron The high output two piece design is usually used in conjunction with aheat resistant steel crown bolted to a skirt portion which may be of lightalloy or of cast iron

Pistons for two stroke engines are usually somewhat longer than thosefor four stroke engines as the skirt has to cover the ports when the piston is

at the top of the stroke

The gudgeon pin is carried in bosses in the piston skirt which areattached to the load carrying portion of the crown by strongly ribbedstructures Generally the gudgeon pin is fully floating in both connecting rodsmall end and the piston bosses It is located endwise, either by pads fixed

to the piston or by means of circlips entered into small grooves at eachend of the piston boss bores In two stroke engines where these bores runacross the ports it is often the case that end pads are used to fill in the ends

of the bores and locate the pin axially at the same time

CONSTRUCTIOr-; 25

In order to seal the gases in the top of the cylinder and prevent theirleakage down the sides of the piston, piston rings are used operating ingrooves turned in the piston crown The action by which the rings seal thegas in the cylinder is shown in Fig 25 The pressure of the gas in the

clearance spaces forces the ring down on to the side of the groove andoutwards on to the cylinder wall Contact between these surfaces must begas tight demanding smooth mating faces all the way round the ring Tomake it possible to assemble the rings on the piston and to enable them toconform to the cylinder bore they have the familiar split or gap This gapprovides a leakage path for the combustion gas so that one piston ring alone

is insufficient to seal adequately A few diesel engines employ just twocompression rings but usually there are three or four Sometimes the lowestring in the pack performs the dual function of gas sealing and oil control.The top ring bears the brunt of the sealing task it sustains the greatestpressure drop across it and it operates at the highest temperature

It is clearly desirable that the ring gap should be as small as possibleand it is equally important that it should never close completely If thebutts of the ring were to make contact with each other during operationthe face would be forced into heavy contact with the cylinder bore andsevere scuffing would ensue Axial or side clearance of the ring in its groove

is essential to ensure that it is free to move relative to the piston in order tomaintain contact with the cylinder bore This clearance is best kept to theminimum that will serve this purpose Notes on checking these clearancesand limits for renewal are given in Section 6.8

Piston rings have to be made of a material which is compatible withbeing rubbed up and down the cylinder bore without scuffing Flake graphitecast iron usually alloyed with manganese, chromium and up to 1 per centmolybdenum is used to provide the necessary strength Almost invariably

26 MARINE ENGINEERING PRACfICE

the top ring is chromium plated on the cylindrical face; the obvious tion to this being when chromium plated cylinder bores are used

excep-A number of developments are aimed at providing rings capable ofbeing run in quickly Figure 26(a) shows an inlaid ring in which the

chromium plate is slightly below the surface of cast iron edges which rapidlywear to a shape conforming to the cylinder bore Another type is shown inFig 26(b) where thin bronze inserts are let into the surfaces of a chromiumplated ring Again, initially these are slightly proud and designed to run inrapidly A different approach is illustrated in Fig 26(c) The face of the ring

is very slightly tapered so that initial contact is made at one edge; this ringrapidly beds to form a band over which contact with the bore is established

As running in progresses, the band widens until contact is completed overthe full width of the ring The engine is usually ready to accept full loadlong before this final stage of contact over the full width is reached Whenassembling taper faced rings of this kind on a piston they should be placedwith the contacting edge at the bottom or high lubricating oil consumptionwill result The taper on the face is very slight and it is difficult to distin-guish, so that ring manufacturers usually mark such rings to show whichside should go uppermost

In light alloy pistons, axial wear of the grooves may be troublesomeand is often countered by the use of ferrous inserts which embrace the top

or the top and second grooves and which are cast in during manufacture.Piston rings for two stroke engines are often located in a manner whichprevents the ring rotating round the piston This is necessary where the portsare sufficiently wide for the horns of the rings to snag in them should they

CONSTRUCTION 27come opposite to them A design of pegged ring is shown in Fig 27 Some-times top rings are made of taper section as shown in Fig 28 in order toprovide a ring which does not easily stick in its groove The use of this type

of ring is almost entirely confined to two stroke cycle engines

The piston has to be well lubricated for it to slide easily in the cylinderliner whilst carrying the side thrusts imposed upon it On the other hand

an excess of oil will flood the compression rings and will become burnt andoxidized; this results in an increase in both lubricating oil consumption andundesirable carbonaceous deposits in the piston ring grooves The flow ofoil to the top rings is controlled by the operation of the lower rings Theseoil control rings are placed either at the bottom of the skirt or just belowthe compression rings The simplest oil control ring is a plain ring bevelled

on the face to provide a narrow edge in contact with the cylinder bore.This type of ring is termed a single edged oil control ring and its form isshown in Fig 29(a) When the ring is pushed upwards the bevelled facerides up over the film of oil on the cylinder wall The wedge of oil that is

formed easily generates sufficient film pressure to spring the ring inwardswhilst on the downward stroke this influence is completely absent and theoil film is scraped down from the bore This action leads to the description

"oil scraper rings" which is often used A more severe form is shown inFig 29(b), this is termed a hooked scraper ring The object of this design

is to maintain more accurately the axial dimension of the face and hencethe wall pressure as wear takes place

28 MARINE ENGINEERING PRACTICE

Unlike the compression rings the oil control rings do not have high gas pressure to force them against the cylinder wall and have to rely on their own strength to generate the wall pressure necessary to keep the oil film to the required thickness The wall pressure of a simple ring is restricted by the radial depth of the ring which is controlled by the need for it to be

capable of passing over the piston diameter when sprung outwards to its limit, in ordcr that it may be assembled in its groove Various designs of spring backed rings are aimed at providing higher wall pressures, some

of these are shown in Fig 29(c) This type of ring has an important tage that the ring itself is radially thin and flexible and can conform to a worn or distorted bore that is no longer circular.

advan-The design of the piston will have embodied in it escape routes for the unwanted oil A stepped or bevelled space is often provided immediately below the ring in which the oil can collect: this is shown in Fig 30 Drain holes convey the oil from this space and also from the back of the groove.

4.6 CYLINDER HEADS

4.6.1 Each cylinder of a medium speed four stroke cycle engine is closed

by separate cylinder head which-carries the injector and the valves It is secured by studs which hold it down to the cylinder block or waterjacket these studs carry the firing loads and at the same time provide the forces which hold together the seal between the head and the liner There is a metallic joint, a copper or soft iron ring, interposed to make the seal.

CONSTRUCTION 29The studs range in number from four to ten per cylinder head according

to the design of the engine To make a satisfactory joint and to ensure thatthe studs are not subject to excessive fatigue loads it is essential to tightenthem evenly and to the correct tension If they are tightened manually then asystem of tightening each stud a small amount in turn following a sequence

to bring the heads down at all sides evenly must be followed The tension

at final tightening should be ensured by careful use of a torque wrench orbetter still by observing the stretch of each stud

The stretch of the studs can be controlled by measuring the anglethrough which the nuts are turned Some manufacturers provide hydraulictightening gear by which all the studs for one cylinder head can be stretched

at the time The nuts are then brought down to hand tightness and thehydraulic pressure released Another method is the controlled heating of thestuds by electrical heating elements in the form of rods which can be inserted

in holes drilled down the axis of each of the studs The studs are raised intemperature to a predetermined figure, the nuts brought to hand tightnessand the studs allowed to cool; the contraction of the studs resulting in evenand correct tension

The water spaces in the cylinder head are connected to those in thecylinder jacket The connexions between the two components may be inthe form of a single connecting piece fitted after the head, liner and jackethave been bolted together, or there may be several points of transfer aroundthe liner of which the joints are made at the same time as the gas joint Thislatter type of water connexion is often in the form of soft rubber sealswhich can compress easily to ensure adequate sealing for the water withoutinterfering with the higher joint pressures of the gas seal

The cylinder head is fitted with doors giving access to the water spaces

30 MARINE ENGINEERING PRACTICE

for cleaning and inspection at major overhauls The cooling water outlet

is usually situated at the top of the head in a position which avoids air becoming trapped in the water space, some designs, particularly on Vee engines, incorporate venting passages to ensure freedom from trapped air These should be checked at overhaul times to ensure that they do not become blocked.

In four stroke and two stroke loop scavenge engines the injector is placed centrally in the cylinder head In two stroke engines with exhaust valves in the head it is frequently placed to one side, the nozzle being arranged to spray the fuel tangentially It is enclosed where it passes through the water spaces by either a boss cast into the cylinder head or by a thin wall tube of steel, brass or copper screwed or pressed in.

The cylinder head has to accommodate the air inlet and exhaust valves the air starter valve and the relief valve together with the appropriate passages for the air and exhaust gas all of which are surrounded by the water spaces.

4.7 INLET AND EXHAUST VALVES

4.7.1 The exhaust valves open against the pressure within the cylinder at the end of the working stroke This pressure is considerably higher than that against which the inlet valves have to open Furthermore, the pressure of the exhaust gases assists, once the valve is open, in expelling the gases through the open valve Because of these considerations it is not unusual to find that the exhaust valves are designed to be of smaller diameter than the inlet valves Being smaller also assists with keeping them cool which is important as exhaust valves often give rise to thermal problems Both the inlet valves and the exhaust valves may seat against the flame plate of the cylinder head These valve seats become damaged during operation and from time to time they have to be reconditioned by grinding in the valves This

is required much more often in the case of the exhaust valves because of operation at higher temperatures and because the gases flowing through may contain particles of carbonaceous matter which occasionally get trapped under the valve seat causing pitting Engines operating on heavy fuel are particularly prone to deterioration of the exhaust valve seat The products

of combustion are likely to form deposits which can be corrosive in nature and which cling to the valve seats Portions of these deposits can crack oft' allowing the combustion gases to blow through the space that is left and cause erosion of the material of the valve To assist in frequent recondition- ing of exhaust valve seats it is not uncommon on engines designed to run on heavy fuel for the exhaust valves fa be enclosed in separate cages which can be removed from the cylinder head without the need for dismantling the whole component On large medium speed engines it is sometimes the case that both exhaust and air valves are enclosed in cages which arc separate from the head As the majority of such engines have four valves

CONSTRUCTION 31

in the head, two inlet and two exhaust, the use of cages considerablylightens the task of reconditioning valves

The life of an exhaust valve between reconditioning can be extended

if the thermal loads to which it is subjected can be evenly distributedaround the valve This is accomplished by rotating the valves slowly as theengine is working Valve rotators which carry out this movement have atype of ratcheting mechanism which indexes the valve round a small amountevery time it is operated by the rocker gear

The condition of the exhaust valve is influenced greatly by the perature at which it operates To reduce its temperature the cage is cooled inthe upper part near the guide and in some instances round the seat region

tem-as well Figure 31 shows the fine -ptem-assages which conduct water down to

an annular passage immediately behind the valve seat and back again

An alternative system is to cool the valve head itself This is shown

in Fig 32 A fitting at the top of the valve receives water via a flexibleconnexion and passes it to a central tube which takes it right down to the

32 MARINE ENGINEERING PRACTICE

valve head where it circulates and then returns up the annulus formedbetween the central tube and the interior of the valve stem It then passesoutside the valve through another passage in the fitting and another flexibleconneXlOn

4.7.2 Cylinder heads are fitted with relief valves in order to draw attention

to any abnormally high firing pressure As illustrated in Fig 33 the principle

of these valves is that of a spring loaded non-return valve; the setting of thespring being such that the pressure required to open the valve is a fewhundred pounds per square inch above the normal firing pressure Pro-vision is also made in the cylinder head to connect an indicator formeasurement of cylinder pressures Frequently this passage is combinedwith that of the relief valve

4.7.3 The starting air valve is also accommodated in the cylinder head.This is a non-return valve which will admit the compressed air requiredfor starting purposes but which prevents the high pressures which occurinside the cylinder during normal operation getting back into the startingair system

4.8 CAM, CAMSHAH AND CAMSHAFf DRIVE

4.8.1 The camshaft of a four stroke cycle engine rotates at half the speed

CONSTRUCTION 33

of the crankshaft That of a two stroke engine rotates at the same speed

as the crankshaft In both cases the camshaft is driven at the appropriatespeed directly from the crankshaft Usually a train of gears is employedfor this purpose but on some engines it may be by a chain drive Fourstroke engines requiring the two to one ratio usually employ a compoundtrain in order to obtain a compact system of gears

FIG. 33.-Cylinder relief valve.

The great majority of medium speed diesel engines intended for mainpropulsion purposes are made in direct reversing form Astern runninginvolves carrying out the events of the cycle in the reverse order It is clearthat this is easier to do on the two stroke engine where one cycle involves onerotation of the crankshaft and one rotation of the camshaft The cams areusually made symmetrical so that their rising and falling characteristicsare the same and can be interchanged in function when the engine isrunning in reverse To prepare for reverse operation involves only a smallrotation of the camshaft relative to the crankshaft With four stroke enginesthe altering of the camshafts is a 'little more complex as the sequence ofoperation of inlet and exhaust cams has to be interchanged The usualmethod is to slide the camshaft axially bringing into use a different set ofcams for operation in the reverse direction In some engines a mechanism

34 MARINE ENGINEERING PRACTICE

is incorporated which lifts the followers clear of the cams whilst they are moved axially Tn others the cams are provided with ramps at the side of the profile to enable this sliding operation to take place.

Sliding a long camshaft on a multi-cylinder engine requires considerable effort It is usually carried out by hydraulic or pneumatic means or a com- bination of both Pneumatic control of hydraulic rams is common for moving the camshafts into position.

4.8.2 Cams and Followers

Cams are shaped to open the valves and operate the fuel pump at the appropriate times in the cycle and to carry out this opening and closing smoothly Followers may be of lever form or of tappet form In both cases

rollers are fitted to run on the cam surface Sliding followers are not favoured in medium speed engines because the loads and running speeds are high and wear would be excessive.

4.9 VALVE OPERATING GEAR

4.9.1 The motion of the cam follower lever or tappet is transferred to the valve by push rods and rocker levers; Fig 34 shows a typical layout An adjusting screw is provided at one end of the rocker lever by means of which the clearance between the rocker lever and the valve cam can be adjusted.

FIG. 34.-Valve operating gear.

Some clearance is essential for satisfactory operation of the valve The valve must be held on its seat by the gas pressure in the cylinder during the

CONSTRUCTION 35

periods when it is to remain closed If there were no clearance, the valve could be propped open slightly allowing gas to leak through the seat and cause dcmmge On the other hand, the clearance must not be excessive because the valve would then be hammered by the rocker gear when being lifted at! the seat and would again hammer on to its seat when the rocker gear left it behind during the re-seating movement causing damage to the valve and shortening its life.

The valves and components forming the valve operating gear are almost certain to expand by different amounts with the result that when the engine is warm the clearance between the valve rockers and the valves is different from that when the engine is cold In settin3 the clearance allow- ance must be made for this fact and careful note taken that the clearance

is set in accordance with the temperature of the engine Some makers give cold clearance others give clearance appropriate to the engine at running temperature.

4.10 AIR AND EXHAUST SYSTEMS AND TURflOCHARGERS

4.10.1 Modern medium speed diesel engines are turbocharged The exhaust gas is led from the cylinder heads to the turbine entry casing by means of exhaust manifold pipes An exhaust pipe may have one individual cylinder connected to it or it may carry the exhausts from groups of two

or three cylinders The grouping and arrangement of these exhaust pipe" and their connexion to the turbine entry casing depends on the number of cylinders of the engine and the firing order The connexions to the turbine entry casings are to separate passages which keep the various exhaust streams separate right up to the nozzle ring inside the turbine The operation

of the turbocharger will be understood from a study of Fig 3.

4.10.2 The exhaust gases entering the casing are led up to the nozzle ring which is of annular form containing a number of stationary blades which direct the flow of the gas on to the moving blades of the turbine wheel The blades of the turbine wheel are individual members attached to the disc at their roots by specially shaped fixing Their surfaces are curved

to extract the maximum amount of energy from the exhaust gases and impart it to the turbine shaft After passing through the turbine the exhaust gas is at a pressure almost down to atmosphere It is conducted away from the turbocharger through the outlet casing and via exhaust pipes to the exhaust silencer, then out into the open air through the stack.

4.10.3 The turbine of the turbocharger drives a centrifugal compressor The impeller of the compressor is mounted on the same shaft as the turbine wheel It consists of a disc on the side of which are a number of radial vanes which diminish in thickness as they approach the periphery At the centre they are formed to gather the air which enters at the "eye" of the impeller.