volume 2 part 17 marine low speed diesel engines1

If a fresh air supply, at a pressure The Low Speed Diesel Engine above that of the exhaust gas, is directed into the cylinder it will remove theremaining exhaust gas and at the same time

MEP Series, Volume 2, Part 17

Marine Low Speed

Diesel Engines

by Dr Denis Griffiths

BEng (Hons), MSc, PhD, CEng, FIMarE

Published by The Institute of Marine Engineers

Published by the Institute of Marine Engineers

80 Coleman Street

London

EC2R 5BJ

Copyright ©2000 The Institute of Marine Engineers

A charity registered in England and Wales

Reg No 212992

All rights reserved No part of this publication may be reproduced or stored in

a retrieval system, or transmitted in any form or by any means, electronic,

mechanical, photocopying or otherwise, without the prior permission of the

copyright holders

A CIP catalogue record for this book is available from the British Library

ISBN 1-902536-33-9 paperback

Typeset in Palatino with Helvetica

Publishing Manager: JR Harris

Technical Graphics: Barbara Carew

Cover Design: Tma Mammoser

Printed by Hobbs The Printers Ltd

Contents

2.23 Combustion Air Supply

3.4 Power Take-In (PTI) Systems 103

For Victoria Elizabeth Ryan

my grand-daughterwith love

No book is ever the sole work of an author, many people assist in various ways,and the author would like to express his thanks to all the individuals andcompanies who have assisted

MAN B&W and Wiirtsilii-NSD have provided information to enable theauthor to complete the text and illustrations, without whose help this bookcould not have been produced Tony Woods at MAN B&W and David Brown

at Wiirtsilii-NSD, in particular, were most helpful in locating information andanswering particular queries

Over the years the Institute of Marine Engineers has published manyuseful books that have assisted marine engineers throughout the world, andthe author hopes this volume can be equally useful The Book AdvisoryCommittee has been responsible for selecting suitable books and authors, thework of that committee going unnoticed by most people, and these efforts areavailable for all to see in the high quality of books for which the Institute is wellknown The author would like to express his thanks to all members of the BookAdvisory Committee for the help they have given him and for the sterlingwork they have done on behalf of the Institute

Finally the author would like to express his thanks to Joli Harris, formerlythe Book Publications Manager, for her editorial help with this and otherbooks, and for the valuable service she has performed in maintaining the highstandard of the book publishing activities at the Institute

The Low Speed Diesel Engine

1 The Low Speed Diesel Engine

1.1 Definition of a Low Speed Diesel Engine

The term 'low speed' is not exact but in marine terms it is generally accepted

as an engine operating at a speed below about 200 revolutions per minute Inaddition to this, low speed engines are invariably of the crosshead type andoperate on the two-stroke cycle This has not always been the case, however,since its introduction the century of marine diesel engine evolution hasproduced an almost standard arrangement, something designers werelooking towards during the 1920s.At the beginning of the 21st century thereare still a number of different designs of low speed crosshead enginespowering merchant ships but many of these designs have not been built foralmost twenty years and can now be considered obsolete In order to ensurethis title meets the needs of current and future seagoing engineers only thosedesigns in production during the final decade of the 20th century will beconsidered in detail within this MEP

1.2 The Crosshead Engine

There are essentially two separate sections to any marine diesel engine, thecylinder in which power is developed and the crankcase where reciprocatingpower developed by the piston is translated into rotary power at thecrankshaft (Figure 1) This arrangement follows from the reciprocating steamengine developed in the latter part of the 19th century, where the cylinders aremounted above the crankcase A diaphragm separates the power cylindersfrom the crankcase preventing combustion products from the cylinders fromcontaminating the crankcase lubricating oil The diaphragm also acts as thebottom of the scavenge air trunking which surrounds the lower part of thecylinder liner A gland allows the piston rod to pass through the diaphragmwhile preserving the seal between scavenge trunking and crankcase Currentengines are of the single-acting type which means that combustion only takesplace in the portion of the cylinder above the piston, unlike the double-actingengine in which combustion would also take place below the piston requiring

a lower cylinder cover through which the piston rod had to pass

The Low Speed Diesel Engine

Translation from the reciprocating motion of the piston to the swinging action

of the connecting rod requires a bearing and this is provided by the crosshead,the upper end of the connecting rod is attached to the crosshead pin, bymeans of bearings, which is firmly bolted to the piston rod The lower end ofthe connecting rod is attached to the crank pin and as the engine operates theforces in the piston rod and connecting rod vary with time and the angularposition of the connecting rod, which changes as the crankshaft rotates Theangularity of the connecting rod causes a side thrust to be exerted at thecrankshaft and the crosshead Both of these side thrusts have to be resistedand while the crankshaft side thrust is dealt with readily by the crankshaftbearings, the crosshead side thrust has to be countered by the use of guides.Guide shoes, fitted to the crosshead, run in sets of guide bars which act toresist the side thrust due to the angularity of the connecting rod whichprevents the piston from rubbing against the cylinder liner wall The guidearrangement also accommodates the side thrust which occurs when the shiprolls, and keeps the piston aligned with the axis of the cylinder liner Fore andaft rubbing faces on guide shoes and guide bars accommodate thrust due topitching of the ship (Figure 2)

The Low Speed Diesel Engine

1.3.1 Astern Guides.

As shown in Figure 2 the connecting rod side thrust acts in opposite directions

during the power and compression stroke of the piston so it is necessary to

provide guides to accommodate both directional forces The force produced

during the downwards power stroke of the piston is greater than that

produced during the upwards compression stroke because the maximum

pressure during combustion is higher than that due to compression of the air

during the compression stroke (see Section 1.4, on the two-stroke cycle)

Under such circumstances it might be presumed that a smaller guide area

could be used for the guide acting during the compression stroke, however,

apart from the idea of using the same sized components there is a very good

reason for having identical guide shoes and guide bars The majority of low

speed engines are directly connected to the propeller shaft and are of the

reversible type, hence the engine rotational direction is changed so that the

ship's direction may be reversed When the engine operates in the reverse

(astern) direction the angle of the connecting rod during the power stroke of

the piston is opposite to that when turning in the normal (ahead) direction

and, hence, so is the direction of the force acting on the guide This means that

a large astern guide face is needed; even if the engine was not reversible

standard terminology refers to the two guide faces as ahead and astern

1.4 Two-Stroke Operating Cycle

(Figure3.)Power is produced in the cylinder of an engine by the combustion

of fuel oil, producing a high gas pressure which forces the piston downwards

In order to bum the oil sufficient air (oxygen) must be available in the cylinder

and that air must be at a very high temperature in order to ignite the fuel oil

when it is injected into the cylinder The diesel engine is a compression

ignition engine which means that compression of the air charge produces the

high temperature for ignition Following expansion of the hot combustion

gases during the power stroke of the piston the waste, or exhaust, gases must

be removed from the cylinder in order to allow the fresh air charge to enter

With a single-acting engine the two-stroke operating cycle allows for one

power stroke for every revolution of the crankshaft or two strokes of the

piston As the downward piston stroke produces the power and the upward

stroke achieves compression of the air charge it follows that removal of the

exhaust gas from the cylinder and its replacement by a fresh air charge must

take place while the piston is near the bottom of its stroke When the exhaust

passage from the cylinder is opened the waste exhaust gas will flow out but

there will still be exhaust gas in the cylinder If a fresh air supply, at a pressure

The Low Speed Diesel Engine

above that of the exhaust gas, is directed into the cylinder it will remove theremaining exhaust gas and at the same time provide the new air charge forcompression This process is known as scavenging Over the years a number

of different scavenging systems have been adopted but there are currentlyonly two in common use, 'Uniflow' employed by most engines of currentdesign, and 'Loop Scavenging' used by the Sulzer RND, RND-M and RLengines

(Figure4a.) Exhaust gas leaves the cylinder via a large exhaust valve located

in the cylinder cover and scavenge air enters through a number of ports cast

in the lower portion of the cylinder liner The valve is opened slightly beforethe scavenge ports are uncovered, this is known as the blowdown period, andthe cylinder pressure falls below the scavenge air pressure by time thescavenge ports are uncovered Exhaust valve opening and closing can beadjusted as the valve is actuated through a camshaft (see Chapter 2) but thescavenge ports are uncovered and covered by the piston so the scavengetiming is set and cannot be adjusted When the downwards moving pistonuncovers these ports air enters the cylinder and moves towards the lowerpressure region of the exhaust trunking, which effectively removes theremaining exhaust gas from the cylinder (this is the scavenging part of the gasexchange process) As the piston moves upwards during the compressionstroke the scavenge ports are covered thereby cutting off further air supply tothe cylinder, however, the exhaust valve is still in the process of closing and sosome of the air is lost from the cylinder (this is the post-scavenge period) This

5

The LowSpeed Diesel Engine

loss of air has to be accepted as part of the engine design as it is not possible to

close the exhaust valve instantaneously when the scavenge ports are covered,

nor is it advisable to begin closing the valve before the scavenge ports are

covered as that could reduce the effectiveness of the exhaust gas removal

process The amount of air loss during post-scavenging can be calculated and

an allowance made when determining the quantity of fuel to be injected

Figures4a and 4b UniflowScavengingand LoopholeScavenging

1.4.2 Loop Scavenging

(Figure 4b.) Sulzer engines constructed up until the 1980s employed Loop

Scavenging for the gas exchange process The RND, RND-M and RL engines

did not have exhaust valves The exhaust gas left the cylinder via ports cast in

the cylinder liner above and to one side of the scavenge air ports, and both

sets of ports were controlled by the piston As the piston moved downwards it

first uncovered the exhaust ports and cylinder pressure fell as exhaust gas

escaped By the time the scavenge ports were uncovered the cylinder pressure

was below the scavenge air pressure and incoming scavenge air forced out the

remaining exhaust gas Scavenge ports were angled and the upper sides of the

piston chamfered in order to direct the incoming scavenge air towards the top

of the cylinder so that the exhaust gas would be effectively removed The

piston controlled the opening and closing of the exhaust and scavenge ports

so that timing was symmetrical, giving identical blowdown and

post-scavenge periods For an earlier Sulzer engine, the RD type which employed a

pulse system of turbocharging, there was a large blowdown period and

consequently an equally large post-scavenge period during which a great deal

The LowSpeed Diesel Engine

of the air charge could be lost In order to overcome this problem Sulzer fitted

a rotary exhaust valve in the cylinder exhaust passage which would rotateand effectively close the exhaust passage even though the ports were stilluncovered The RD engine had a relatively short piston skirt which meantthat exhaust ports were uncovered during the compression stroke, and therotary exhaust valve effectively closed the exhaust passageway during thecompression stroke

1.5 Turbocharging and Supercharging

As mentioned, a two-stroke cycle engine requires scavenge air to be supplied

at a pressure above that of the atmosphere in order to ensure that the exhaustgas is removed from the cylinder and a fresh air charge is available for thenext cycle This air can be supplied by a number of systems including enginedriven rotary blowers, engine driven reciprocating pumps or electricallydriven rotary blowers However, since the 1950s it has been the practice toemploy rotary compressors driven by single stage gas turbines which derivetheir energy from the engine exhaust gas Use of such gas has manyadvantages including an improvement in operating efficiency and an increase

in power available at the crankshaft as no power is removed to drive thescavenge pumps or blowers

Supercharging is a means of increasing engine power output by increasingthe air supply which enables more fuel to be burned A supercharger can beany device which increases the pressure of the combustion air supply abovethat which is normally required A supercharger can be a turbocharger but itcan also be any of the other devices which increase air pressure Aturbocharger can be a supercharger, but only if it supplies air to the engine at

a pressure just above that which is needed for effective scavenging The effect

of supercharging can be seen from the following equations:

pv=mRT whichgivesm = pv/RT

where: p = Cylinder pressure (N/m2)

v = Cylinder volume (m3)

R = Gas constant aIkg K)

T = Air temperature (K)(Celsius Temperature +273)The cylinder volume when the scavenge ports are covered is constant, and ifthe air temperature is kept constant then the mass of air varies directly withthe pressure Doubling the pressure will double the mass of air in the cylinderand, theoretically the fuel mass burnt may be doubled to allow for an increase

in power developed A critical factor in this is the air temperature which isdealt with in more detail in Section2.23.3 on Air Coolers

The Low Speed Diesel Engine

1.6 Engine Parameters

The size of an engine depends upon a number of factors and an important

feature is the power to be developed Naturally the number of cylinders will

govern the power of the engine but the bore of the cylinder and the length of

the stroke will determine the power developed in the cylinder

The standard equation governing a two-stroke cycle engine cylinder

power is as follows:

Power (Watts) = PmXAXLXnWhere:Pm =Mean cylinder pressure (N/m2)

A=Piston area (m2)

L=Length of piston stroke (m)

n=Revolutions per second (lis)

Note: Sometimes the acronym PLAN is used in order to aid recollection of the

cylinder power calculation.

The cylinder bore governs the value of 'A' and this can vary from 980mm for

large engines down to 260mm for the smallest engines During the final

decade of the 20thcentury brake mean effective pressures have increased from

about 18.0bar to 19.0bar and at the same time maximum cylinder pressures

have risen from about 135bar to 150bar

1.6.1 Stroke to Bore Ratio

Engines today have a stroke to bore ratio of between 2.5:1 and 4:1 depending

upon the application The piston speed for normal running is governed by the

rate at which combustion takes place and the expansion of the gas produced by

combustion of the fuel Maximum piston speed occurs during the middle of the

piston stroke and the effect of the expanding gas acting upon the piston should

be such to exert an even rotation of the crankshaft During the 1990scrosshead

engine designers adopted mean piston speeds of8.0mls or slightly above

1.6.2 Long Stroke Engines

By allowing the gas to expand further greater fuel efficiency is obtained

because more energy is obtained from the expanding gas, however, this

requires a longer piston stroke and that in turn results in a lower rotational

speed Slower rotation of the propeller generally increases propulsive

efficiency and long stroke versions of a particular engine will often be selected

for the propulsion of large bulk carriers due to the higher overall efficiency

Such an engine is, however, taller and wider than the short stroke version and

would require more engine room space, although the space available for the

main engine in a bulk carrier is generally not restrictive

8

The Low Speed Diesel Engine

1.6.3 Specific Fuel Oil Consumption (SFOC)The emphasis on engine design has been to improve reliability, ease ofmaintenance and fuel efficiency A Specific Fuel Oil Consumption (SFOC) of170g/kWh (125g/bhph), based upon full load brake power, is expected fromengines designed during the late 1990s

1.6.4 Engine DimensionsThe crank throw of a long stroke engine is longer than that of the normal orshort stroke counterpart (crank throw is half the piston stroke) and so a longstroke engine would require a wider and deeper bed plate in order toaccommodate the crankshaft To keep the angularity effects of the connectingrod within reasonable limits the length of the connecting rod is generallyincreased and these factors, together with the increased length of the cylinderliner, mean that a long stroke engine would tend to be much taller than anormal or short stroke engine of the same bore In practice the availability ofimproved bearing materials means that the loading on bearings can beincreased without adverse effect and so the connecting rod length of a longstroke engine can be reduced, which has the effect of reducing engine heightand width Engine length depends upon the bore and number of cylinders butengine designers try to get the centres of the cylinders as close as possible inorder to keep engine length to a minimum Cylinder liners are located in castcylinder blocks which provide the cooling water jacket and space has to beprovided to accommodate these, but it is not just the cylinders which dictateengine length, particularly for the smaller bore engines Bottom end and mainbearings must be of a defined length in order to provide for a large enoughbearing area to resist the load Crank webs must also be of sufficient width toprovide crankshaft rigidity The influence of these is significant with smallbore engines and can result in a longer engine than the bore alone mightindicate Size also influences weight and a large engine is also a heavy engine

9

Engine Construction

2 Engine Construction

2.1 Engine Structure

The engine structure must be sufficiently rigid to ensure the crankshaft does

not bend excessively when it is subject to cylinder peak pressures during

ignition In addition the guide's forces must be accommodated without

distortion and the frames must adequately support the cylinder block (called a

cylinder beam by Sulzer), air box and turbochargers The frames are mounted

on the upper face of the bedplate using small diameter frame feet bolts and the

cylinder block attaches to the upper face of the frames using similar fitted

bolts Pairs of tie rods, located at the main bearings, hold the entire structure in

compression but the small fitted bolts are still needed, particularly at the frame

feet, in order to accommodate the side thrust due to the reaction at the guides

Mating faces between the bedplate, frames and cylinder block are machined

While the structure must be strong enough and rigid enough to take the

loading it should also be as light as possible and careful design is needed to

ensure stresses remain within acceptable limits High varying stress can result

in fatigue failure of components Modem design achieves strength and

lightness by treating the structure as a composite and not as separate parts and

in that way each part not only serves its particular purpose but it also provides

strength and support to other parts of the structure

2.1.1 Fatigue Cracking

Fatigue may be defined as the formation and propagation of a crack under the

action of a varying tensile stress The stress must vary with time and it must

be tensile A compressive stress causes crack faces to be forced together and so

the crack cannot propagate, while a static stress does not produce the change

in the grain structure of the material which is essential for fatigue crack

propagation Fatigue cracks form and propagate at stresses way below that

which would produce tensile failure in the material but a common factor in

fatigue cracking is the presence of stress raisers These may be existing small

cracks, gas inclusions, slag inclusions, or sharp changes in section due to poor

design and damage The stress applied to a component may be well below

that which would cause a fatigue crack to develop or grow but around the

stress raiser the stress level rises considerably and it is at these locations that

fatigue cracks form As the crack develops the area resisting the stress is

gradually reduced and so the actual stress rises, resulting in an increased rate

of crack growth Eventually the remaining cross-sectional area of the material

is unable to resist the applied load and sudden failure occurs The fracture

Engine Construction

surface of a component which has failed in fatigue exhibits two distinctsurfaces, the shell-like or polished surface of the fatigue crack as it propagatesthrough the grains of the material and the brittle surface of final failure Inorder to reduce the risk of fatigue failure the loading on a component must bekept within defined limits, but care must also be exercised in order to restrictthe presence of stress raisers While it is difficult to know about the presence

of gas or slag inclusions unless X-ray or a similar technique is applied, andthese are prohibitively expensive, good design will avoid sharp changes insection and care during maintenance will prevent mechanical damage whichresults in stress raisers

(Figure5.)The bedplate supports the crankshaft and provides the foundationfor the remainder of the engine structure, consisting of longitudinal steelgirders joined by cast steel cross or transverse girders which support the mainbearings Fabrication is employed for bedplate construction and only thetransverse girders are cast The use of casting eliminates residual stresseswhich might be present if the girders were fabricated as residual stresses arealways present in welds (see below)

Engine Construction

If welding was used for the transverse girders the bedplate section wouldhave to be stress relieved, while the slow cooling and ageing of castingsgenerally avoids the presence of residual stresses, and the need for stressrelieving, which makes the use of castings a less expensive and more practicalsolution A bedplate should be rigid and in most engines built up to the 1980sthe longitudinal girders were of box section in order to ensure that rigidity.Current practice employs mono-wall or 'Gondola' type bedplates where thelongitudinal box girders have been replaced by single plate girders Because

of the absence of longitudinal box girders it may appear that rigidity has beenlost but this is not the case as the composite crankcase structure, comprisingbedplate and frame mounted above, acts as a single very deep box sectionwhich has high rigidity Such an arrangement is not only more rigid than theformer design but it also saves weight and engine width, particularlyimportant for the long stroke engine with its large crank throw Thelongitudinal girders have to be positioned clear of the rotating cranks andassociated bottom ends, therefore, the width of the engine is twice the crankthrow (plus clearance for the bottom end) plus twice the width of the boxgirder The throw of the crank is set by the engine stroke, although byreplacing the box girders with mono-wall girders the engine width, andweight, can be reduced without reducing the rigidity (Figure6.)

2.2.1 Residual Stress

Tensile residual stress when added to the tensile working stress in acomponent can result in levels of stress higher than anticipated which canresult in fatigue failure When two plates are welded together there is a run ofmolten metal at the joint and upon solidifying and cooling this metal tries tocontract, however, the plates being joined prevent complete contraction Theresult is that the former molten metal at the weld is subject to a tensile stressbecause it is prevented from contracting as much as it should while the platesbeing joined are subject to a compressive stress as the total force in the jointshould be zero If not relieved this residual stress will increase the risk offatigue failure especially if there are slag or gas inclusions in the weld metalwhich will act as stress raisers

12

Engine Construction

steel plates, and doors with seals are fitted at various openings to allow access

to the crankcase The frame carries the weight of the cylinder block and other

equipment mounted above, such as the exhaust system and the

turbochargers, yet it is also subject to other loading, particularly the reactions

at the guides A number of Sulzer engines employ jacking screws to keep the

main bearing caps in place and these act on the frame cross pieces to impose

an additional stress (see main bearings) Sides of the engine frame have a

number of openings and it is important that explosion relief doors are fitted at

each cylinder (see Section 3.1.4) Frame design aims to achieve a strong and

rigid structure with the lowest possible weight and minimum need for

machining The lower face, where the frame attaches to the bedplate, and the

upper face, where the cylinder block sits, must be machined, along with the

mounting points for the guide bars Within the structure provision must be

made for the camshaft drive system whether that be by chain or gears

Engine Construction

2.4 Cylinder Block

Smaller bore engines may have cylinder blocks cast in sections comprising up

to three cylinders but for larger engines individual cylinder blocks are castand bolted together to form a complete longitudinal structure Cast iron isemployed because of its ease of casting into the relatively complex shape andbecause the mechanical loading on the cylinder block is light Cylinder linersfit into the individual blocks and are held in place by the cylinder covers asthe cylinder cover studs screw into the upper face of the block The lower part

of the cylinder block generally acts as part of the scavenge space and alsocontains the diaphragm with a provision for holding the diaphragm gland Inthis case the lower part of the cylinder block has a sloping face which allowsfor easy draining of that space but also provides a 'dead space' between thescavenge space and the upper part of the crankcase This can act as aninsulating space in the event of a scavenge fire and so reduce the risk of acrankcase explosion resulting from a fire In some engines this space is watercooled The individual nature of the cylinder block castings mean thatindividual cylinders may be isolated and drained of cooling water duringscheduled or breakdown maintenance

(Figure8.) During the firing period in a cylinder there are downward forcesacting on the piston, however, equal forces act upwards on the cylinder coverand these are transmitted through the cylinder block, engine frame andbedplate transverse girder, resulting in tensile stresses in these components

In order to limit those tensile stresses which could result in fatigue failureengines are provided with tie rods, fitted in pairs at each of the main girders.These tie rods extend from the lower face of the bedplate to the upper face ofthe cylinder block and the compressive force they exert on these parts of thestructure ensures that any tensile stress which does occur during the peakcylinder pressure period will be well below that which could cause fatiguecracking It would be possible to keep tensile stress low by increasing thesection thickness of all parts of the structure but that would result in a veryheavy and expensive engine Tie rods provide strengthening just where it isneeded and so allow for a light engine but one which is able to develop highpower Tie rods pass through tubes welded into the frames and bracing isprovided at certain locations in these tubes to prevent transverse oscillationswhich could cause fatigue failure Where engine headroom is limited, tie rodsare made in sections with suitably located joints to make removal andrefitting easier

assists the scavenging process but is primarily intended to improve mixing of

the air and fuel during fuel injection, as the fuel is injected in the opposite

direction to the air rotation Liners may be made from grey cast iron but

cylinder liners for modem highly rated engines are made from a sophisticated

cast iron alloy which provides a high degree of ductility in order to resist

thermal stress and at the same time provide good wear resistance The upper

part of the liner close to the combustion zone is the critical region as it is

subject to high pressure and high thermal loadings There is conflict in how

these requirements are met, as to resist the high pressure a thick wall section is

needed but to provide effective cooling, which avoids high temperature, it is

necessary to have the cooling medium as close as possible to the heat source

In these circumstances 'bore cooling' is used as it allows for thick sections and

a cooling arrangement close to the heat source (see Section 2.6.2) The liner

surface is sometimes finished using wave grinding; the surface looks to have a

very fine finish but on a microscopic scale it is undulating to allow for rapid

running-in and to provide a key for lubricating oil which carries away the

metal particles from the running-in wear process

2.6.1 Thermal Stress

(Figure10.) Cooling presents problems as it also induces thermal stress due to

different rates of expansion across the section Consider a liner wall section

with temperature THon the combustion chamber side and Tc on the cooling

water side With a thin section most of the temperature change on the hot side

from the combustion gases to the actual material takes place in the fluid

whilst on the cold side most of the change takes place in the cooling water at

the interface with the metal For a thick section there is little temperature

change in the fluid at these interfaces so the actual temperature change in the

material is higher The larger temperature difference in the material means a

greater differential expansion, if the hot side was allowed to expand freely

This is not the case in an engine component such as a cylinder liner, cover or

piston as thermal expansion is restricted The hot side which wanted to

expand cannot expand as much as it wishes and is subject to a compressive

stress, while the cold side is subject to a tensile stress in order to balance the

compressive stress induced in the hot section Tensile stress is a problem

because when added to the tensile stress induced due to cylinder pressure it

can put the component at stress levels which will cause fatigue cracks to

develop and propagate Ideally the cooling medium should be placed as close

as possible to the source of the heat as that allows heat to be removed without

causing large temperature gradients and thermal stress The problem is that

high working pressures require thick section material in order to resist the

mechanical loading, which again makes bore cooling a solution as it allows

EngineConstruction

Figure10.ThermalStress

2.6.2 Thick Section Bore Cooled Liner(Figure 11.)The upper section of the liner close to the combustion zone ismade as thick as necessary to withstand the mechanical loading due to thecombustion gas pressure and a bore cooling system is used to keep thematerial temperature within acceptable limits Cooling bores, small diameterholes through which cooling water flows, are located in the thickened sectionclose to the hot inner surface of the liner These bore holes are angled to theaxis of the liner and pass from the water space at the lower part of thethickened section to meet radial bore holes near the top of the liner The radialholes take the cooling water to a collar which surrounds the upper part of the

liner In some cases, particularly with engines running slightly derated, it has

been found that cooling is too effective resulting in low liner temperatures

which cause increased liner and ring corrosive wear Provision of mid-stroke

insulation around the outside of the liner and insulated cooling bore inserts

enables the correct temperature to be achieved at all parts of the liner surface

Figure11.Arrangementof Thick SectionBoreCooledLiner

2.6.3 Liner for Deep Section Cylinder Cover

(Figure 12.) In some long stroke engines the liner is extended some distance

above the cylinder block which allows for a cylinder block of reasonable

height and weight A separate cooling jacket is provided for the upper part of

the liner and water bends take the cooling water from the upper jacket to the

cylinder cover which is extended downward to form the combustion

chamber This arrangement requires long cylinder head studs A further

adaptation of this is to be found in engines having high topland pistons (see

fig 23.) where the downwards extension of the cylinder cover is large and the

EngineConstruction

piston has to move some distance before the upper part of the liner is exposed

to the combustion gases This reduces the thermal loading on the upper part

of the liner and so allows an uncooled thick section to be used without anydetrimental effect on the cylinder lubricating film

Figure12.ExtendedLinerand Deep SectionCover

2.7 Cylinder Cover

(Figure 13.) Design of the cylinder cover will differ for uniflow scavengedengines and those employing loop scavenging because the former must haveprovision for a central exhaust valve Covers are steel forgings which employbore cooling passages and pockets for fuel injectors, air start valve and a reliefvalve An important aspect of cylinder cover design is the contribution itmakes to the shape of the combustion chamber The piston crown profile andunderside of the cover form the combustion chamber and the design of bothmust be such as to obtain the optimum shape to ensure complete fuelcombustion without impingement of the flame The greater the distance of thefuel injector nozzles from the piston crown the less likely it is thatimpingement will take place In some cases engine builder apply anti-

21

corrosion cladding to the cover downstream of the fuel injectors in order to

protect against hot corrosion and erosive attack The seal between cylinder

cover and the top of the liner is achieved with a sealing ring, generally made

from mild steel Different engine designers have their own ideas about

cylinder combustion design, but there is a move towards the torroidal

combustion chamber opposed to the spherical The torroid shape suits the

three fuel injectors which are used (the exhaust valve does not allow for a

centrally fitted fuel injector) and provides for effective combusti!=>ndue to the

intimate mixing of fuel and air The raised central part of the piston crown

provides for the combustion chamber shape which approaches the torroid

Curved surfaces are still used, however, as they provide for better strength

from the same section thickness compared with flat surfaces

Figure13.CombustionChamberShape

2.7.1 Exhaust Valve

(Figure 14.) Valves are of the caged type, the whole unit comprising water

cooled body, detachable seat, valve, and springing unit The single valve is

located in the centre of the cylinder cover and connected to it by means of four

or more studs and hydraulically tightened nuts The valve cage must form a

gas tight seal with the cylinder cover and this can be obtained with a sealing

EngineConstruction

Figure14.ExhaustValveand Cage: ring of mild steel or soft iron located between the exhaust valve cage and its

iteat in the cylinder cover pocket The valve cage is water cooled as part of the

I engine cooling system, water generally passing to the exhaust valve bodyfrom the cylinder cover by means of cooling bends

However, in some cases there is a direct connection between the water.pace in the cylinder cover and the seat region of the valve cage and it is

i important that new '0' rings are fitted when replacing an exhaust valve,: although seals should be replaced regularly Valve cages are made from cast

iiron as they are not subject to any excessive mechanical or thermal loading,

I,although a detachable seat is employed This is made from a material such as

Engine Construction

molybdenum steel which is able to resist the thermal loading and corrosive

environment and, being detachable, can be machined with relative ease

following removal from the cage The actual seat area generally incorporates

an insert, of Stellite or similar material, which improves the operating life of

the unit due to its wear and damage resistance Stellite is a hard alloy which

has very good corrosion and damage resistance at high temperatures It is,

however, expensive and should only be used for seat inserts which would

cause valve leakage should they become damaged A bronze bush fitted in the

valve body acts as a guide for the valve spindle

Valve Spindle

Nimonic is used extensively for valve spindles due to its wear and corrosion

resistance at elevated temperatures It is an expensive nickel based alloy

which has good corrosion and wear resistance at high temperatures There are

a number of different nimonic alloys, each having particular properties, and it

is essential that the correct alloy is used for the intended purpose Engine

builders now tend to use nimonic for the entire valve due to the high loading

on the valves In order to provide wear and corrosion resistance valve spindle

~ati are of Stellite or a similar material Material selection is important in

order to ensure optimum operating life when burning low quality residual

fuels with high levels of sulphur, vanadium and other damaging chemicals

Like the detachable seat of the valve cage the spindle seat is ground in a

special rig using a grindstone of approved quality The underside of the valve

disc, the part facing into the combustion chamber, is sometimes coated with a

material such as Incone1625 in order to reduce the rate of hot corrosion Valve

stems are coated with a layer of chrome in order to reduce wear on the part

which passes through the stem bushing Although this is effective there are

environmental reasons for not using chrome and alternative materials are

available Cermet is a recently introduced material which has no

environmental problems and has proved to have a greater wear resistance

than chrome The Cermet coating is sprayed onto the valve stem using the

HVOF (High Velocity Oxygen Fuelled) process Material for the valve spindle

sealing ring must be compatible with the Cermet coating Valve rotation in

service is applicable to engines of current design and this necessitates the

fitting of a 'Spinner' to the valve stem (see Section 2.7.1.5 for details of valve

rotation) Valves may generally be classed as one of the following three types:

a) Austenitic materials with Stellite facing on the seat and a coated stem for

Rocker Operation of Valves

For many years exhaust valves were actuated by means of a push-rod androcker unit but this arrangement suffered from many operational problems Inservice valves expand and this expansion has to be accounted for by setting aclearance between the top of the valve stem and the operating face of therocker, known as 'tappet clearance' If too little clearance was allowedexpansion of the valve would cause it to remain open when the rocker forcewas removed thus resulting in gas leakage, loss of compression, loss of powerand valve seat and face damage If too much clearance was allowed thiswould reduce valve lift slightly, cause a slight change in timing and producehammering at the valve and rocker faces which could result in wear Rockeroperation could also result in excessive wear between the valve stem and itsbushing in the valve body because of the side thrust exerted by the push rod

on the valve stem during opening (Note: the only time a truly axial force is

exerted on the valve stem is when the rocker arm and valve stem are at right angles.)

Hydraulic Valve Actuation

(Figure 15.) In order to overcome the problems associated with rockeractuation of exhaust valves hydraulic actuation was introduced This basicallyrequires the fitting of a piston on the top of the valve stem, as the hydrauliccylinder in which the piston operates is connected to an actuator pump unit

by means of a high pressure sheathed pipe

Engine Construction

A pair of piston rings provide sealing in the open bottomed hydraulic

cylinder An oil bleed valve, located at the top of the hydraulic cylinder,

allows a controlled amount of oil to pass to the air cylinder, located

immediately below the hydraulic cylinder, to provide lubrication The

camshaft driven actuator pump delivers oil to the valve cylinder on the

upwards stroke of its piston causing the valve to open, so that on the

downwards stroke oil returns to the actuator pump cylinder As there is no

delivery valve in the discharge line it allows the valve to close under the

action of the closing mechanism, coil springs or air piston A relief valve is

located at the top of the pump cylinder and there is a make-up oil connection

to the pump body in order to ensure that there is always a full oil charge in the

system The make-up connection is located just above the top of the pump

piston when it is at the bottom of its stroke, and oil is supplied under pressure

from the cam-box lubrication system A puncture valve on the actuator pump

body may be actuated to release some pressure from the system and this has

the effect of delaying exhaust valve opening This is used when starting in the

astern direction to ensure the full effect of the starting air is gained

Sheathed Pipe

(Figure16.)In the event of leakage from the high pressure pipe connecting the

valve actuating pump and cylinder a fire could result from the hydraulic oil

spraying on the hot exhaust manifold In order to safeguard against such an

incident the HP pipe is surrounded by a braided steel pipe and there is a

narrow gap between the HP pipe and the braided pipe Failure of the HP pipe

will result in defective operation of the exhaust valve with consequent poor

cylinder performance which would be brought to the attention of the engineer

through the exhaust temperature Leaking oil then drains from the gap

between the HP and braided pipes, via small holes in the actuator pump body,

into the cam box

Valve Rotation

Impurities in residual fuel produce a number of damaging products when the

fuel is burned, including vanadium pentoxide, and these can cause damage to

the valve face and seat The vanadium pentoxide slag is highly corrosive,

while solidified slag and ash particles can be hammered into the sealing faces

when the valve closes If the valve is allowed to rotate slightly as it closes a

light grinding effect is produced at the faces and these deposits are removed

Another advantage of valve rotation is that local overheating is prevented

(such overheating could be the result of a defective fuel injector) Should there

be poor combustion due to a worn or damaged injector, flame impingement

could cause local overheating and even burning of the valve disc Rotation of

Engine Construction

the valve, however, means that a new section of valve disc is subjected to theheating at each stroke so that large temperature gradients are not present Amechanical device such as the Rotocap, fitted to medium speed engine exhaustvalves, is not practical for large low speed engines, nor would it provide forthe grinding effect when the valve is closing as the Rotocap produces a setangle of rotation when the valve is opening The spinning rotation effect isbrought about by the action of exhaust gas on vanes or spinners fitted to thevalve spindle As soon as the valve opens the escaping exhaust gas acts on thespinners but rotation is only produced if there is no other frictional contactbetween the valve stem and the body of the valve Such frictional contact exists

if coil springs are used and so these have been replaced by 'Air Springing'

Engine Construction

Air Spring Cylinder

(Figure17.) Some positive force is needed to ensure the valve closes after the

rocker or hydraulic opening force is removed and early systems employed coil

springs The strength of the closing spring system had to be sufficient to ensure

that the rocker push rod follower remained in contact with its cam at all times

in order to avoid impact damage Coil springs are subject to fatigue failure and

they also tended to cause rubbing wear at the contact faces at the valve body

and spring cover They also provide a frictional contact between the valve

body and valve stem which prevents valve rotation, so an alternative means of

closing the valve has to be used This alternative is 'Air Springing'

Figure 17 Exhaust Valve' Air Springing' Cylinder

A piston is fitted to the valve stem below the hydraulic cylinder and this

piston reciprocates in a cylinder, compressing the air in the cylinder when the

valve opens This compressed air acts on the piston to close the valve when

the opening hydraulic force is removed Air leakage from the cylinder is

made-up from an air supply system maintained at a pressure of 5 bar and a

non-return valve in the supply line prevents back flow during opening of the

valve The area of the piston is calculated to ensure the correct closing force on

the valve, the maximum pressure in the cylinder being governed by the 5 bar

28

Engine Construction

starting pressure and the stroke of the piston The space above the piston isvented in order to ensure that there is no opposing force during valve closingand lubrication of the cylinder wall is provided by controlled leakage fromthe hydraulic valve opening cylinder Sealing air to the gland contains oil mist

in order to limit wear at the seal and valve stem With no rocker or valvesprings visible some method has to be used to enable the engineer to checkthat the valve is lifting and rotating At the top of the casing is a spring loadedcheck rod which passes through a bush and has contact with a groove in thetop of the air piston As the piston reaches the top of its stroke it causes thecheck rod to lift, indicating that the valve is opening The action of valverotation produces further movement of the check rod indicating that the valve

is rotating as it reseats

Variable Exhaust Closing

(Figures 18and 19.) The setting of engine parameters such as exhaust valvetiming suits one set of conditions, usually full load, for optimum performancebut at lower loads fuel efficiency is not so good At reduced load an increasedcylinder air charge can be obtained if the exhaust valve is closed earlier Thatincreased air mass and pressure, compared with what they would have beenhad the valve closed later, together with an alteration in fuel injection timing,allow for a lower specific fuel oil consumption (SFOC)at the reduced load Thesystem is similar to the standard hydraulic valve actuation system save formodification of the actuator pump piston and incorporation of control lines.The actuator piston has a hole from the top connecting with a port at the pistonside This port lines up with a spill pipe when the piston is at the top of itsstroke The spill pipe is kept closed by a control valve, but opening of thatcontrol valve results in a rapid fall in the hydraulic pressure acting on the valvepiston and so the exhaust valve will close Control of the valve in the spill lineallows the exhaust valve to be closed earlier, reducing the opening period ofthe exhaust valve This causes the cylinder pressure to becomes higher than itwould otherwise have been, which is a factor influencing fuel efficiency

2.7.2 Cylinder Relief Valve(Figure20.) In order to protect the engine parts from overload in the event ofexcessive cylinder pressure, particularly the top end bearing, a cylinderpressure relief valve must be fitted This is a spring loaded valve which willlift when the pressure acting on the valve face exceeds a predetermined value

It takes very little release of gas to reduce cylinder pressure to a safe value, sothe cylinder relief valve does not have to be very large, but it is essential thatgas passageways are kept clear and the valve overhauled periodically eventhough it may never have lifted

Engine Construction

Figure18.Arrangements for Variable Exhaust Valve Closing

2.7.3 Indicator Cock

The indicator cock allows for connection of the engine indicator which is used

to assess cylinder performance but not all engines have indicator drive

equipment as standard fittings In many cases electronic systems are

employed to determine cylinder performance but an indicator cock is still

fitted in order to allow for cylinder pressure to be relieved when turning the

engine using the turning gear

Engine Construction

(Figure 21.) The piston is exposed to the heat and force of the combustion

gases and must be able to withstand the mechanical and thermal stress of

such conditions In addition it must seal with the liner in order to prevent gas

leakage which would result in loss of power and other problems Pistons are

of composite construction consisting of a crown, made from some heat

resisting alloy such as chrome-molybdenum steel, which is directly exposed

to the combustion flame, a rod which transmits the force acting on the piston

to the crosshead, and a skirt The steel piston rod is surface treated to

minimise friction between the rod and diaphragm gland thus allowing for

increased gland sealing ring contact pressure Generally the piston skirt is

provided with a rubbing ring of cast iron or bronze, which is slightly proud of

the skirt itself, and is intended to assist in the running-in of the liner Piston

rings, located in grooves, provide for a seal against the liner The number of

piston rings fitted is usually four or five Coolant flow to and from the piston

is via the hollow piston rod or telescopic pipes depending upon the cooling

Combustion gases acting on the upper piston ring impose thermalloading which can cause cracking As a way of reducing this one enginebuilder has introduced pistons with high top land, the distance from the top ofthe piston crown to the upper piston ring (Figure22.) This increased distancebetween the top of the piston crown and the upper ring means thattemperatures and pressures acting on the ring pack are lower which reducesthe rate at which the rings lose their tension and increases ring pack life andperformance The increased topland allows for a lowering of the interfacebetween the cover and the liner (see section on cylinder liners) therebyreducing the thermal loading on the upper part of the liner while increasingthe effectiveness of the cylinder oil film

2.8.2 Piston Cleaning Ring

(Figure 23.) This ring is sandwiched between the upper part of the liner and

the lower part of the cylinder cover, standing slightly proud of both Its

position is just above the top piston ring when the piston is at the top of its

stroke Its purpose is to scrape ash and carbon from the top land of the piston

thus preventing these deposits from making contact with the liner and so

removing the oil film from the liner The rubbing of the ash and carbon

deposits against the liner wall also have a polishing effect This polished

surface prevents the oil film from adhering to the liner effectively

-2.8.3 Piston Rings

(Figure 24.) It is essential that piston rings provide an effective seal with the

liner surface in order to ensure that correct compression is achieved, maximum

power is developed in the cylinder and there is no blowby into the scavenge

space The depth of the rings and associated groove must be such that the rings

will be adequately supported in the groove but at the same time there must be

sufficient space at the back of the groove to allow for a cushion of gas That gas

pressure also acts to force the ring into contact with the liner surface but the

rings must have sufficient spring to provide the initial seal with the liner

Groove clearance is needed in order to allow the gas access to the space behind

the ring However, if the clearance is too small there is a risk of ring jamming as

carbon deposits build up in the groove while excessive clearance allows rings

to twist resulting in jamming or breaking Butt clearance should be as small as

possible in order to restrict gas blowby but must also be large enough to

EngineConstruction

accommodate thermal expansion of the ring Butt joints are angled andadjacent rings have their angles facing in opposite directions For someengines the top ring is of the controlled pressure relief type, where a double lapjoint replaces the angle joint This type allows for almost constant pressuredrop across the ring irrespective of ring and liner wear, while with the anglejoint type the pressure drop increases with wear because the butt gapincreases Piston rings must retain their spring even when subject to hightemperatures and they must also resist thermal and mechanical cracking whenunder load Nodular cast iron provides sufficient strength and resistance tothermal cracking while also providing some self-lubricating properties.Surface coatings are often applied as an aid to running-in Copper and carboncoatings have been used and recently aluminium-bronze has been applied.These coatings wear rapidly to produce the optimum surface for rubbingagainst the liner, but do not need any special running-in procedure whichsaves time and money Ring grooves are usually chromium plated to produce

a smooth surface which will minimise wear Alternatively the upper and lowerfaces of the ring may be chromium plated as this provides a hard surface forimproved wear resistance Chromium plated rings must never be used inchromium plated grooves, however, as the plating layers will be tom away

2.8.4 Piston Cooling

Cooling of the piston is essential in order to maintain strength and oil is themedium used for most pistons of current design, although water is moreeffective as a coolant due to its higher specific heat capacity and highermaximum allowed temperature The advantage of oil over water is that anyleakage from the flow system into the crankcase does not causecontamination Engines employing uniflow scavenging tend to have lowerpiston cooling requirements than loop scavenged engines because only theincoming scavenge air flows over the piston crown thus extracting some heat

As can be seen from figure 21, a convenient route for supplying and removingthe cooling oil is by way of the piston rod, an internal pipe allowing forseparation of the inlet and outlet flows Oil is directed to the crossheadthrough a telescopic pipe The oil required for piston cooling flows up thepipe in the hollow piston rod to the cooling cavities in the piston crown Theoutlet is via the annular space formed by the inlet pipe in the hollow pistonrod It is important that the oil flow is substantial in order to ensure that thepiston crown temperature remains within designed limits and the oiltemperature does not rise to a high value where carbon and other deposits canform on the cooling surfaces High oil temperatures also result in rapidoxidation of the oil and consequent deterioration in its lubricating properties

Engine Construction

piston cooling space, the cavity not being completely filled with coolant but

containing air As the piston reciprocates the coolant in the piston cooling

space lags behind the piston movement and is effectively washed in and out

of the blind cooling bores, removing heat from even the most inaccessible

parts On the upward stroke of the piston the coolant lies at the bottom of the

cooling cavity and so there is no cooling effect on the crown This was not a

particular problem with water cooling as on the next downwards stroke of the

piston the water would extract the heat and the high temperature would not

have any detrimental effect With a change to oil cooling there were potential

problems due to the lower maximum temperature allowed for the oil in order

to avoid the formation of deposits in the cooling space There is also a desire

to keep the piston crown temperature low in order to avoid burning Cooling

in the bores is achieved through the use of spray nozzles which direct oil into

these spaces This forced cooling oil supply also directs air into the piston

cooling space and this maintains the 'Cocktail Shaker' effect

2.8.5 Piston Rod

The surface is hardened in order to resist frictional wear when passing

through the diaphragm gland Attachment of the piston rod to the crosshead

pin is by means of a rectangular palm end and two or four holding bolts This

method has been adopted to leave the lower face of the crosshead pin clear

and so allow for a continuous bearing Because of this palm end it means that

the diaphragm gland must be removed from the engine whenever the piston

is lifted Earlier engines had piston rods bolted through the crosshead pin as

this allowed the piston assembly to be removed without disturbing the

crosshead, however, the connecting rod had to have a forked top end (later

Sulzer engines still have this arrangement)

2.9 Piston Rod Gland

(Figure 26.) The diaphragm gland consists of two sets of rings, held in a split

steel housing, which are in contact with the piston rod The upper set of rings

provide a seal for the scavenge air space preventing the passage of air, waste

cylinder oil and carbon into the crankcase where it could contaminate the

lubricating oil The lower set of rings scrape oil from the piston rod back into

the crankcase Each complete ring is formed from three or four segments

although some sealing rings consist of two rings, one on top of the other with

the butt joints staggered Scraper rings are generally fitted with lamella inserts

which do the actual scraping of the oil from the rod Garter springs, located in

grooves machined in the outer faces of the segments, hold the rings against

the piston rod surface

Engine Construction

Clearance between the rings and the housing grooves is as small as possible to

ensure correct sealing Between the sealing and scraper ring there is a 'dead

space' in the housing with a drain connection allowing the effectiveness of the

gland to be assessed (in some cases this 'dead space' is provided at the upper

scraper ring groove) Scraper ring grooves are provided with passageways

through which oil scraped from the piston rod may be returned to the

crankcase It is important that the correct butt clearances are provided for the

segments as when wear takes place the butts move together, unlike piston

rings where the butt clearance increases with wear, and if the butts make

contact then the rings no longer touch the piston rod

(Figure 27.) The crosshead translates reciprocating action of the piston into

semi-rotary motion at the connecting rod, therefore, a bearing is essential In

addition it accommodates guide shoes which act on guide bars mounted on

the engine frame and transmit the side thrust due to the angularity of the

connecting rod to the engine structure Connections to the crosshead provide

for the supply of lubricating oil to the bearing and guide shoes through holes

in the pin, oil also passes down the connecting rod to the bottom end bearing

and up the piston rod to act as a piston coolant The hardened crosshead pin

has a very fine surface finish and provides a full width surface for the bearing

Keep plates ensure that the crosshead pin remains positioned on the

connecting rod

2.10.1 Crosshead Bearings

Crosshead pins are of as large a diameter as possible in order to give a large

bearing surface to keep the bearing pressure at an acceptable level Maximum

load comes on the crosshead bearing during the compression and firing

periods and it is at this time that there is little relative movement between the

pin and bearing, therefore, no hydrodynamic oil wedge can form In order to

ensure the pin and bearing are kept apart at this critical time some engine

builders provide a separate high pressure lubrication system for the

crosshead This hydrostatic lubrication gives a sufficient bearing lift during

this crucial period The large diameter pin does not bend readily when under

firing load and the continuous bearing further assists in avoiding edge

pressure With earlier crosshead pins supported on two bearings it was

possible for pins to deform under load and make contact with the edges of

the bearings

40

EngineConstruction

2.10.2 Guides

Guide shoes float on cylindrical extensions to the crosshead pin, the float

allowing for a limited degree of rotation of the shoes relative to the pin, the

rotation being limited by stop screws or similar This float ensures that an oil

wedge can form between the shoe and guide bar when turning ahead and

astern Fore and aft alignment of the crosshead is adjusted by means of the

shims located between the inner edges of the guide shoes and their respective

guide strips The guide strips project from the shoes and contact the sides of

the guide bars The guide bars are firmly mounted in the engine frame

Rubbing faces of the guide shoes are lined with cast-on white metal

2.11 Connecting Rod

Forged steel connecting rods may have palm ends at top and bottom in order

to accommodate the top and bottom end bearing arrangements or they may

be shaped so as to form the bearing seats Much depends upon the size of

engine and length of connecting rod As already discussed the shorter the

connecting rod the greater the angularity and the larger the side thrust at the

guides However, long connecting rods result in tall engines A hole in the

connecting rod allows for the passage of lubricating oil from the crosshead to

the bottom end bearing

2.12 Bearings

(Figures 28 and 29.) White metal lined bearings are applicable to main,

bottom end and crosshead duties Such bearings are of the shell type, either

thin shell or thick shell depending upon the area of application In both cases

a steel backing shell is lined with a layer of white metal Thick shell bearings

are used for main bearing duties and the edges of the steel backing shells are

provided with lips which allow for correct location within the bearing keeps

Thin shell bearings have no edge lips and the layer of white metal is much

thinner in comparison with the thick shell type The quality of the running

surface of the thin shell bearing is much higher than the thick shell type as

cooling is easier to control during casting, allowing for a finer grained

structure A more rapid and even cooling of the thin shell bearing gives a finer

grained structure which is consistent throughout the product The thin layer

of white metal, particularly if it is of fine grain, allows increased load to be

carried compared with a thick cast layer The main increase in bearing loads

has taken place at the crosshead and crankpin (bottom-end) bearings and so it

is these which tend to be of the thin shell type, while thick shell bearings can

still be found undertaking main bearing duties

EngineConstruction

Figure28.Thickand Thin Shell Main Bearings2.12.1 Bearing Materials

Although white metal is used extensively for crosshead engine bearings due

to its relative ease of casting, cost and its tolerance of lubricant impurities, itcan present problems under certain circumstances The tin in the white metalcan oxidise and the tin-oxide which forms on the surface of the bearing isvery hard and brittle; it is also thicker than the original tin from which itforms and this can reduce the bearing clearance The main problem with tin-oxide, however, is that if the layer breaks-up fragments of the oxide canbecome embedded in the white metal and act to severely damage the pin byscoring Some white metal bearings have been provided with lead overlays

to assist with the running-in, but the practice is no longer common As analternative to white metal Sn40AI (tin-aluminium) has been used because it is

a much stronger material, yet such bearings tend to be applied to the smallerbore engines where space restrictions require smaller and more highlyloaded bearings

Figure29.Thin Shell Crankpin and CrossheadBearings

2.12.2 Bearing Design

Shell bearings are manufactured under strictly controlled conditions to ensure

the quality of the product is high and there is little likelihood of impurities in

the white metal layer In some cases a layer of copper or bronze is put on the

steel shell to improve adhesion of the white metaL Where bronze is used it is

the bronze layer which actually takes the load and this is the main reason for

its use For thin shell bearings steel backing shells are about 5-lOmm thick,

depending upon the size of bearing and the layer of white metal is about 2mm

thick Full hydrodynamic operation does not exist at the crosshead bearing

due to the fact that there is not complete rotation and so care must be taken to

ensure that a complete oil film always exists between pin and bearing The use

of oil grooves assists in spreading the oil to all parts of the bearing from the

central supply oil way The run out from the oil groove is in the form of a

wedge which spreads the oil across the bearing surface All bearings are

provided with an oil way which distributes the oil circumferentially from the

EngineConstruction

supply point, which is in the top of the main bearing shell and in the lowercrankpin bearing shell A problem has existed in the lining-up of top andbottom bearing shells because any slight misalignment between the two shellscreates an oil scraping edge For thick shell bearings the fitting of verticalguide pins between the upper and lower shells, together with bore relief(chamfering) at the joint, avoids the problem With thin shell bearings it is notpossible to fit guide pins but bore relief is used as a way of overcoming themisalignment problem with its consequent oil scraping edge

2.12.3 Main Bearing Caps(Figure30.) Main bearing caps have generally been held in place using studsand nuts but this arrangement occupies space alongside the bearing on eachside, which requires the tie rods to be positioned some distance from thecentre of the crankshaft When the engine is under load the bedplatetransverse girders may be likened to simply supported beams, with thecrankshaft the centre load and the tie rods the supports The further the tierods are placed from the centre of the crankshaft the larger the bendingmoment will be, and the greater the stress induced in the transverse girder Inorder to overcome the problem one engine builder employed wide bearingcaps with four studs, a slot being cut in each side of the bearing cap holdinglug to accommodate the tie rods Holes for the retaining studs were located inthe bearing cap holding lugs fore and aft of the tie rods

Engine Construction

An alternative solution was to employ bearing cap jacking screws which hold

the cap from above, the screws being supported at their upper ends by a

transverse girder in the engine frame Although satisfactory in operation these

jacking screws imposed additional upwards force on the frames and for

modem highly rated engines it was considered unsuitable A replacement

uses conventional style bearing caps held in place by means of elastic holding

down studs of relatively small diameter which still allow tie rods to be placed

close to the centre of the crankshaft

2.13 Crankshaft

The crankshaft is at the heart of the engine and transmits power from the

cylinders to the propeller shaft Strength and rigidity are essential features of

any crankshaft design but it must also be compact as, to a great extent, the

crankshaft has an influence upon engine length, particularly with smaller

bore engines Each engine unit shares two main journals with neighbouring

units and has a crankpin and two webs The length of the crankpin must be

such that a large bearing area is provided together with a relatively long oil

path from the oil supply point at the centre of the crankpin bearing to the

edges Increasing the diameter of the crankpin allows for a reduction in length

without reducing the bottom end bearing area and similar arguments apply to

the sizing of journals Webs must transmit the force from the cylinder to the

journals and they are dimensioned accordingly, the aftermost set of webs

transmitting the entire engine load In addition, for semi-built crankshafts the

webs must be large enough to accommodate the shrinkage stress Loads

acting on a crankshaft vary with time and rules governing their construction

are based upon a combination of theory and experience

2.13.1 Semi-built Crankshaft

(Figure 31.)Forged steel elements comprising crankpin and adjacent webs are

joined to forged journal pins by means of shrink fits The shrinkage allowance

is carefully chosen to ensure a strong interference fit without large shrinkage

stresses which could cause overload when the working stress is added In

order to reduce the weight of the crankshaft all unnecessary material is

removed and that means that journal pins and crankpins are counterbored as

the material at the centre of a shaft takes very little load (compare the torsion

theory for solid and hollow shafts) Similarly webs are tapered and chamfered

as far as possible and there is little excess web material above the crankpin In

order to avoid stress problems associated with the machining of oil holes

modem crankshafts do not contain oil passageways, lubricant for the bottom

end or crankpin bearing passing down the centre of the connecting rod from

the crosshead

Engine Construction

Engine Construction

2.13.2 Welded crankshaft

(Figure32.) The welded crankshaft concept, patented by MAN-B&W,has cast

steel units comprising web, half crankpin and half journal pin These are welded

together at the journal and crank pins to form a complete crankshaft, and welds

are of the narrow gap submerged arc type The welds are subsequently heat

treated to remove residual stress and then ground to the required finish This

form of construction is lighter than the semi built type as no additional material

has to be provided in order to accommodate the shrink fit Strict classification

society rules apply to the construction of all crankshafts and the manufacture of

welded crankshafts is particularly restrictive because of the difficulties

involved with the welding process and risk of stress raising inclusions

Figure32.WeldedCrankshaft2.13.3 Axial Damper

(Figure 33.)Axial or longitudinal vibration is excited by the radial forces due

to cylinder operation and the axial forces from propeller rotation (torsional

vibration also causes axial deflection of the crankshaft) Axial vibration not

only induces additional stresses in the crankshaft, it also imposes extra

loading on the thrust block and can result in additional vibration of the ship's

structure In order to restrict axial vibration amplitude an axial detuner or

vibration damper is fitted when there is a possibility of high vibration

amplitudes existing This is located at the free end of the crankshaft and is

designed to move the natural frequency of the axial vibration to a value above

that obtained at the normal maximum operating speed It essentially consists

of a piston or collar on the crankshaft, this piston being located in a cylinder

Engine Construction

which is attached to the forward end of the foremost main bearing Seals areprovided between the cylinder and the crankshaft and the cylinder and theedge of the piston The cylinder chambers formed each side of the piston have

no connection apart from via a throttle valve which can be adjusted to limitthe transfer of oil between the two chambers which produces the vibrationdamping effec1

2.13.4 Thrust Block

In order to save on engine weight the thrust block is normally located within

the engine itself and the thrust collar usually serves as the gear or chain drive

wheel for the camshaft drive A standard pad type thrust bearing is used and

this is generally located at the after part of the engine It is important that the

engine structure is strengthened in way of the thrust housing and that the

engine holding down arrangements are suitable for transmission of the thrust

2.14 Camshaft

The camshaft activates fuel injection pumps and exhaust valves by means of

cams which are shrunk onto the camshaft but which can be individually

adjusted by hydraulic pressure For a chain drive system attachment of the

camshaft to the drive sprocket or gear wheel is by means of an interference fit

coupling which may be slackened hydraulically in order to adjust the timing

due to chain stretch Camshafts are built up in sections and each section is for

one cylinder or a pair of cylinders as this allows for easier replacement of

cams should anyone become damaged Flanged couplings are used to join the

sections of camshaft together The camshaft is carried in underslung bearings

which have only one shell, the lower one; thin shell bearings are used This

arrangement allows sections of the camshaft to be removed readily if

necessary The flanges can be uncoupled, the lower cover of the bearing casing

removed (the bearing is usually located in the housing containing fuel pump

and exhaust valve drives), the bearing keep removed, and the section of

camshaft lowered It is only necessary to fit a lower bearing shell because the

loading is always in a downwards direction Shell type bearings have to be

used due to the impact nature of the loading, as the oil wedge which forms

between the camshaft and bearing dampens the effects of the impact

2.14.1 Camshaft Drive

Camshafts are used to actuate exhaust valves, fuel pumps, cylinder

lubricators and starting air distributors, as well as certain other devices, and it

is important that the rotation of the camshaft matches that of the engine

crankshaft Recently engine designers have developed engines which do not

use a normal camshaft but almost all of the engines currently in service

employ camshafts which are driven from the crankshaft by either a chain or

gear wheel system

EngineConstruction

2.14.2 Camshaft Chain Drive

(Figure 34.) In order to transmit sufficient power to drive the camshaft amultiple chain is generally required and many larger engines are fitted withdouble or triple chains These are all linked together but each have their ownsprocket wheels For engines with up to six cylinders the chain drive isusually fitted at the after end but for more than six cylinders it is usuallyplaced at the centre of the crankshaft

Figure34.CamshaftChain DriveApart from its relative simplicity the chain drive has an advantage in that it is

no more complex to have a long chain than a short one thus the camshaft can

be placed high in the engine close to the cylinder covers This means that shorthydraulic connections to fuel injectors and exhaust valve actuators can beused thereby minimising timing errors due to the elasticity of the hydraulicpipe systems Chains are less susceptible to damage by foreign particles thangear wheels but they do suffer from wear at the pins and bushes resulting in

Engine Construction

chain extension, commonly referred to as 'stretch' It is important for the

chain to be correctly tensioned to avoid damage to the chain links and the

sprocket teeth A slack chain caused by 'stretch' must have its tension

adjusted when required In many cases spring loaded automatic tensioners

are provided but these need to be checked to ensure that they are performing

to expectation Chain 'stretch' will also cause incorrect timing which is the

main disadvantage of camshaft drive chains Even if the correct tension has

been restored it is necessary to periodically check the timing of the camshaft

against the crankshaft using the gauges provided Should adjustment be

necessary this is usually achieved by slackening the connection between the

camshaft drive sprocket and the camshaft, rotating the camshaft to the correct

position and then retightening the drive connection Long free lengths of

chain are guided by rubber clad guide bars and lubrication is provided by

spray pipes situated at all sprocket wheels and guide bars A small chain

drive from an intermediate sprocket is often fitted to drive the cylinder

lubricators and the mechanical engine governor, if fitted

2.14.3 Camshaft Gear Dive

(Figure 35.) Gear wheels do not suffer from 'stretch' and so there is no need

for adjustment but they do tend to be more costly than chain drives Although

cost could be reduced by placing the camshaft lower this would increase the

length of the hydraulic connections, so the gain from having shorter high

pressure oil pipes generally outweighs the lower cost of fewer or smaller gear

wheels Gear wheels are thinner than an equivalent power chain system and

that can reduce engine length Although 'stretch' is not a problem with gear

wheel drives the teeth are still susceptible to damage from foreign particles

and poor lubrication

Direct solid injection of fuel is used with each cylinder having its own fuel

pump and the number of injectors employed depends on the size of cylinder

bore Atomisation of the fuel oil at the injectors depends upon the viscosity of

the oil and in order to obtain the correct viscosity heating is required Hot fuel

oil is taken from storage tanks and pumped through a final heater at the

engine before passing to the injection pumps The final heater is controlled by

a viscosity measuring device fitted in the fuel line before the injection pumps

In order to restrict heat loss all pipes must be adequately lagged Because of

the high operating temperature of fuel pumps and injectors, an allowance for

thermal expansion must be made in the initial design, and if the system is to

operate on relatively cold gas oil when manoeuvring it has to be expected that

52

Engine Construction

the higher clearances will result in some leakage Fuel systems are sealed andpressurised in order to prevent 'gassing-up' of fuel injection pumps due toliberation of the more volatile elements or water in the oil which is heated totemperatures up to 150°C.4

Figure 35 Camshaft Gear Wheel Drive Arrangement

4. (For detailed information about fuel see A Practical Guide to Marine Fuel Oil Handling by Chris Leigh-Jones, Volume 3, Part 19

in the MEP series, published by the Institute of MarineEngineers )

53

2.15.1 Helical Control Fuel Pump

(Figure36.)Delivery of fuel from this type of pump commences when the top

of the plunger covers the spill port in the barrel and delivery ceases when the

helical groove, cut in the side of the plunger uncovers that same port Spill

occurs because pressure oil in the space above the plunger gains access to the

helical groove via a vertical slot cut in the side of the plunger; an alternative

route is via a vertical hole drilled down the centre of the plunger and a radial

hole from the helical groove to meet that vertical hole

EngineConstruction

Helical control means that the side of the plunger containing the helicalgroove is subject to full injection pressure which can produce a high sidethrust forcing the plunger against the pump barrel and increasing wear Asolution is to provide two identical sets of grooves diametrically oppositeeach other which balances the pressure forces When the vertical groove is inline with the spill port no oil pressure is produced when the plunger movesupwards as oil can simply flow to spill (this is the no load position of thepump) Rotating the plunger so the spill port is covered by the side of theplunger above the helix results in fuel delivery until the spill port isuncovered Delivery commences as soon as the top of the plunger covers thespill port Pressurised oil will flow down the vertical groove and occupy theannular space below the helix but it will remain pressurised as it cannotescape Delivery of fuel continues until the helix uncovers the spill portallowing the pressurised oil in the annular space, and in the space above theplunger top, to spill Immediately this happens delivery of oil to the injectorceases although the plunger will still be moving upwards The length of theeffective stroke, and hence the amount of fuel delivered, is changed byrotating the plunger and this is done by means of the control rack linked tothe governor A spring loaded suction valve is located at the top of the pumpbarrel, fuel oil being supplied under pressure from the fuel circulating pump.The delivery valve is at the top of the pump cover and above this a puncturevalve is connected The puncture valve is actuated by the control system when

an engine stop is signalled or during an emergency condition, such as a fueloil leakage alarm, in the event of high pressure fuel pipe failure Under suchcircumstances compressed air is applied to the puncture valve piston whichforces a pin to open a valve diverting high pressure oil flow from the pumpdelivery line to the spill chamber

Helical Control Variable Injection Timing Pump

(Figure 37.) With a helical control fuel pump the quantity of fuel delivered,and timing of the end of injection, is varied by rotating the pump plunger inorder to alter the point in the plunger stroke at which spill occurs To makethis type of pump into a Variable Injection Timing (VIT)pump it is necessary

to provide a means by which the commencement of fuel injection can bevaried As injection commences when the top of the plunger covers the spillport the solution is to provide some means whereby the spill port can beraised or lowered relative to the plunger The fitting of an insert within thebarrel achieves this as the barrel insert, containing the spill port, is capable oflimited vertical movement At the base of the barrel insert is a coarse threadwhich engages with a threaded nut, called a timing guide, held in the pumpbody Rotation of the nut causes the barrel insert to move vertically as the nut

Engine Construction

is restrained from moving vertically by the pump body The nut is rotated by a

rack and pinion arrangement similar to that employed for rotating the pump

plunger Raising the barrel insert delays covering of the spill port and retards

fuel injection while lowering the barrel insert allows the spill port to be

covered earlier in the plunger stroke which advances injection timing

Engine Construction

2.15.2 Valve Control VIT Pump

(Figure 38.) The valve type fuel pump employs a suction valve, whichregulates the commencement of fuel injection, and a spill valve, whichcontrols the end of injection Only Sulzer engines use valve type fuel pumps

Both valves are actuated by the pump plunger via levers As the plunger

moves upwards at the beginning of its stroke the suction valve lever allows

the suction valve to close as the fulcrum of the lever is situated between the

plunger connection and the suction valve push rod By rotating the eccentric,

which acts as the lever fulcrum, it is possible to vary the position in the

plunger stroke at which the suction valve closes, and hence vary the

beginning of fuel injection At some further point in the plunger stroke the

spill valve lever acts to lift the spill valve thus bringing fuel injection to an

end The fulcrum for the spill valve lever is positioned at the end of the lever

Again it is possible to rotate the spill valve lever eccentric and hence change

the point in the plunger stroke at which injection ceases, which allows the

quantity of fuel injected to be varied Both eccentrics can be regulated from

the engine governor thus the timing and quantity of fuel injected may be

varied to suit particular circumstances Screwed linkages on the individual

eccentric control rods allow for fine adjustment of individual fuel pumps

With this type of pump variable injection timing (VIT)and fuel quality setting

(FQS) may be achieved by means of a variable linkage between the governor

and the fuel pump VIT is adjusted by means of a cam and FQS is adjusted

with a screw to achieve optimum firing pressure depending upon the fuel

ignition quality

2.16 Reversing Systems

With crosshead engines there are two main systems which need consideration

when an engine is to be reversed, namely the fuel timing and exhaust valve

timing One solution is to drive each system from its own camshaft and fit

each camshaft with its own reversing servo-motor which gives the necessary

camshaft timing for running in the opposite direction This is unnecessarily

complex and also expensive and a simpler alternative is to drive all systems

from the same camshaft with a single servo-motor giving rotation for

reversing The problem here, however, is that each of the systems usually

require a different amount of rotation to bring its cams into position for

running in the opposite direction This is possible if the cams can be designed

to suit that same amount of camshaft rotation to the new position for running

in the opposite direction The exhaust valve must have an opening period, a

definite full open period and a closing period, therefore, the shape of the cam

is set If the same cam is to be used for ahead and astern operation the cam

must be rotated to the new position exactly for running in the opposite

direction The fuel pump cam is not so sensitive as it is only the rising portion

of the cam which produces fuel injection Once injection has ceased the

plunger may rest at the top of its stroke for any length of time provided that

EngineConstruction

there is a downwards stroke of the plunger ready for the next upwards stroke.The dwell period at the top of the fuel cam may be adjusted in order to suitthe amount of camshaft rotation devised for the exhaust valve cam Fuel camsare usually symmetrical which means that the falling portion is the sameshape as the rising portion, and the falling portion becomes the rising portionwhen running in reverse If the air distributor is also to be driven by the samecamshaft it may require a compromise on astern timings in order to allow allfunctions to be driven from the same camshaft

2.16.1 Fuel Pump Cam Roller Reversal

(Figure 39.) Some engines are fitted with a separate air distributor camshaft,driven by the main camshaft, with separate ahead and astern cams whichmeans the air distributor can be reversed by moving its camshaft axially tobring the desired cam into operation The main camshaft, therefore, only has

to deal with the fuel pumps and exhaust valves If the camshaft is not moved

to a new position when reversing no servo-motor is needed, which simplifiesthe camshaft system considerably By making the exhaust valve camssymmetrical the exhaust valve operation is satisfied, yet the fuel pumps must

be dealt with as they need to be retimed for running in the reverse direction.Reversing of the fuel pumps is achieved by moving the cam follower relative

to the cam An air cylinder, connected to the top of the reversing link, isactuated in order to reverse the fuel pump and this causes the cam follower to

be moved which repositions the fuel pump plunger Drive for the fuel pumpplunger is from the cam, through the follower to the reversing link, and then

to the guide and finally the pump plunger The reversing link is self lockingwhen in either the ahead or astern position therefore no external force isneeded once the link is in the desired position

2.17 High Pressure (HP) Fuel Oil Pipes

A sheathed pipe system is employed, similar to the exhaust valve HP pipe,and the passageway between the HP pipe and the outer steel wire armouredsheathing are provided with a drain bore in the fuel pump cover

High pressure pipe drains from all fuel pumps are connected to a commontank which incorporates a level switch The drain tank has an overflow pipewhich has a small bore allowing slight leakages to be drained without thelevel in the tank rising abnormally In the event of a HP pipe failure theamount of oil would not be able to escape through the small bore and the

Engine Construction

rising level in the tank would activate the float alarm An alternative system is

to have the drain pipe from each fuel pump connect with its own diaphragm

valve which would operate in the event of a large leak Operation of the

diaphragm valve would set off the alarm and also activate the puncture valve

on the fuel pump which would stop the oil flow in the HP pipe for the

or three are fitted to produce effective combustion with an even heating effect

Figure40.Fuel InjectorsThe setting of the spring may be adjusted in order to change the pressure atwhich the valve will lift It should be noted that the maximum fuel oilpressure at the injector will be higher than the valve lifting pressure when theengine is operating under load It is the maximum pressure, together with thesize of the atomiser hole(s) which dictates the size of the fuel droplets Thevalve, or spindle, lifts when the pressure force due to the fuel oil acting uponits angled face exceeds the force due to the spring This allows fuel oil into thefuel sac in the nozzle and from there the fuel passes through the fine sprayholes into the cylinder The sac ensures that the flow of oil to the nozzle sprayholes is even and regular, as steady flow is important to the atomising effect ofthe spray holes The nozzle tip with its fuel sac also means that the actualvalve is kept away from the heat of the combustion chamber and at less risk ofoverheating damage High nozzle temperatures can cause problems when

burning residual fuels due to the amount of fuel in the sac This fuel may

vaporise resulting in carbon deposits which then seriously impair the spray

pattern and combustion Carbon trumpets may form around the nozzle tip

and these have a detrimental effect on the spray pattern Overheating of fuel

in the sac can result in the combustion flame forming too close to the nozzle

tip with consequent higher tip temperatures and burning of the nozzle and

region of the cylinder head around the injector One engine builder has

designed an injector with a reduced volume fuel sac and the actual fuel

volume is only about 15 per cent of that found in a conventional injector The

reduction in sac volume is achieved by having a slide inside the fuel nozzle

(this slide is an extension of the actual spindle or valve) While reducing the

volume of the sac the slide does change the pattern of flow from the valve seat

area to the spray holes

EngineConstruction

2.19.1 Injector Cooling

(Figure42.) Cooling of the injector reduces operating temperature and ensuresthat there is less risk of seizure Cooling of the nozzle tip also minimises therisk of carbon forming A major problem with cooling is that of leakage,particularly leakage of the high pressure fuel into the cooling system, but if thefuel is also the coolant there is no such problem While it may appear strange

to be cooling with hot fuel oil the circulating fuel oil does extract heat and theinjector would certainly become much hotter if the fuel was not circulating

Engine Construction

The electrically driven fuel oil circulating pump forces hot oil through the fuel

injection pump and injector body causing the fuel to pass through small

passages in the lower part of the spindle, flowing upwards through the valve

body to the outlet pipe in the side of the valve head The space around the

tapered valve face on the spindle is also filled with oil but the pressure is

insufficient to overcome the force of the circulating spring and cause the spindle

to lift At the beginning of the fuel pump delivery stroke the fuel oil pressure

rises to a value which allows the force on the angled spindle face to overcome

the circulating spring force and the spindle rises to shut off the small circulation

passageway in the spindle guide Full fuel pressure from the injection pump

can now build-up causing the needle valve (spindle) to lift and bring about fuel

injection The whole process of closing the circulation passageway occurs very

quickly and so the timing of fuel injection is not affected

2.19.2 Atomisation

Very small droplets of fuel burn quickly and the aim of atomisation is to

produce droplets small enough to burn rapidly yet large enough to penetrate

into the cylinder in order to mix with the available air If droplets are too large

they will penetrate too well and will impact with the cylinder wall and piston

crown If the fuel is not burning when it reaches the cylinder wall it is scraped

off during the next piston stroke, but if it is burning it will damage the liner

and piston The combined effect of fuel pressure and nozzle hole diameter

produces fuel droplets and the combination must be correct in order to obtain

the ideal droplet size Fuel oil is forced through the nozzle holes and the

velocity of the jets depends upon the pressure and diameter of the holes

though which the oil is forced Viscosity influences the pressure and so it can

be said that viscosity has an influence upon the atomisation The higher the

pressure and the smaller the nozzle hole diameter, the higher the fuel jet

velocity will be Atomisation of the fuel occurs due to friction between the high

velocity fuel oil jet and the compressed air in the cylinder The compression

pressure is set by the compression ratio of the engine and is constant, therefore,

the fuel droplet diameter will be governed by the velocity of the fuel jet leaving

the nozzle holes, which in turn is governed by the fuel pressure and hole

diameter In service nozzle holes erode and the larger area results in large

droplets which burn more slowly If the engine operates at low speeds for

prolonged periods there can be performance problems as the low speed means

lower fuel pressure and hence larger droplets In order to overcome this

problem 'slow steaming' nozzles are sometimes fitted which have a smaller

diameter or fewer holes The use of such nozzles improves combustion thereby

reducing SFOC and the build-up of carbon deposits in the exhaust system

Engine Construction

2.19.3 Combustion

Fuel is not injected immediately when the fuel pump plunger begins to lift.There is a delay period caused by compression of the fuel in the high pressurepipe and the expansion of that pipe This differs slightly between slow andfull speeds as the fuel pressure is lower when running slowly compared withthe maximum fuel line pressure which is reached at full speed The shorter thehigh pressure pipe is, the shorter the delay will be This is a prime reason forhaving high camshafts with fuel pumps close to the cylinder cover When thefuel pressure has risen to a high enough pressure the injector needle valve willopen and fuel will be injected into the cylinder If the engine is operating athigh load fuel pressure will rise above the injector lifting pressure and it is thispressure which governs the fuel droplet size - not the lifting pressure Fueldoes not burn immediately as injection commences, there is an ignition lagperiod when the fuel droplets are being heated and the chemical reaction ofcombustion is beginning to take place The duration of the ignition lag period

is independent of the engine and depends upon the ignition quality of thefuel The higher the ignition quality is the shorter the lag period will be andthe lower the ignition quality is the longer the lag period will be As soon asthe flame appears the fuel in the cylinder will burn rapidly, as will any fuelinjected after that time It is important that the engineer knows about theinfluence of fuel ignition quality on ignition and that they take steps to adjustthe engine accordingly The action which can be taken depends upon thefacilities available at the fuel pumps If the pumps have a Fuel Quality Setting(FQS) then this must be adjusted in order to compensate for any change infuel ignition quality Moving to a lower ignition quality fuel without makingany change to the ignition timing will result in delayed ignition and latecombustion, which can cause afterburning and damage to the cylinders andexhaust valve as well as a loss of power Changing to a higher ignition qualityfuel without adjusting the timing will result in early ignition, high peakcylinder pressure and possible damage to bearings - particularly thecrosshead bearing For Sulzer engines having pumps fitted with FQS and VIT,the Variable Injection Timing (VIT)function is load dependant and is adjusted

in order to reduce fuel consumption

2.20 Governors

A governor is basically a device for controlling the speed of the engine Itmust have some part which senses the actual engine speed, a section whichcompares this speed with a value set by the operator, and an output systemthat adjusts the fuel supply to the cylinders in order to restore the engine

speed to the set value whenever a speed change has taken place The simplest

arrangement is to employ rotating weights which move inwards if there is a

speed reduction and outwards if the speed increases Spring loaded levers

attached to the weights allow for vertical movement of a linkage which in

turn adjusts the fuel pumps This is known as a 'Ballhead arrangement' and

only allows for fuel control at one engine speed unless the spring tension is

changed Engines currently designed generally have the governor situated

between the speed control station and the fuel pumps Movement of the

control is not used to adjust the fuel pumps directly but to change the

governor spring setting In this way the governor has control over the fuel

pumps at all speed settings and not just at full speed

2.20.1 Servo-Governors

The actual force exerted by a Ballhead depends on the mass of the rotating

weights and their rotational speed, and the force required to adjust the fuel

linkage of a large engine is considerable This would necessitate large mass

weights or a speed increase system in order to give the necessary force, both

of which tend to take additional power from the engine while direct control in

this manner would also increase frictional losses in the system These

problems are avoided if a servo system is employed and the Ballhead is only

required to move the small pilot valve of the servo unit

Hydraulic power can provide the force to move the fuel linkage and the

hydraulic pump is conveniently located within the governor casing (the

pump drive is from the same shaft which rotates the Ballhead) A reduction in

engine speed due to load increase would result in the Ballhead weights

moving inwards, lowering the servo pilot valve which would allow oil to flow

to the servo-piston causing it to rise and increase fuel to the engine

In order to allow for stability there has to be some feedback from the fuel

linkage otherwise 'hunting' would occur as the servo piston would alternate

between full fuel and no fuel Feedback is generally by means of a linkage

between the servo output and pilot valve shaft and it has the effect of

introducing Speed Droop to the governor

Speed Droop is the characteristic which provides the motive force to

produce any adjustment of the fuel linkage and it also introduces stability

Essentially it means that there is one speed for each engine load or fuel

setting The larger the value of Speed Droop the greater the force will be

trying to make a fuel adjustment, but there will also be a greater difference

between controlled engine speeds at, say, 50 per cent engine load and full

load A governor of this type will not control to a single speed

EngineConstruction

2.20.2 Isochronous GovernorThe Isochronous governor adjusts the engine, within its capabilities and that

of the fuel system, to the same speed whatever the load There are transientspeed changes but the final speed after adjustment is always that set by theoperator Various settings on the governor dictate how readily it will respond

to a load change but it has to be borne in mind that there will always be aslight lag between an engine load change, and hence a speed change, and thefuel adjustment to restore the engine speed to its original value Rapidresponse must be paid for by the risk of control overshoot which results in anincrease in the time taken for the actual speed to return to a steady state value.2.20.3 Electro-Hydraulic Governor

(Figure43) The Electro-Hydraulic Governor has a speed sensing unit whichsends an electrical signal to the governor hydraulic actuator unit The speedsensing unit is simply a camshaft driven generator which produces a signalwith a strength proportional to the speed of the engine This signal is rectifiedand used to energise an electromagnet, known as a torque motor, in thegovernor actuator

Engine Construction

Movement of the torque motor beam varies the clearance between the beam

and nozzle at the top of the pressure regulator and this varies the rate of oil

escape from the system In some cases it is also possible to have a BalIhead

back-up system which can provide control should the electrical part of the

system fail The arrangement is essentially two governors in one, both using

the same hydraulic oil supply and servo output systems Changeover from

one mode to the other is by means of the control valve In the BalIhead or

mechanical mode oil from the accumulators are routed through the control

valve to the Ballhead pilot valve while in Electrical mode oil is routed to the

actuator pilot valve then through the BalIhead pilot valve system When in

electrical mode it is necessary to raise the BalIhead speed setting above the

electrical speed setting in order to prevent the BalIhead system operating

2.20.4 Electrical Governor Actuator Pilot Valve

(Figure 44.) The heart of the Electrical Governor system is the Actuator Pilot

Valve as this regulates the oil supply to the servo cylinder in order to adjust

the fuel The servo piston is spring loaded and if the control oil supply is

removed the spring forces the servo piston to shut off fuel to the engine

Engine Construction

Movement of the servo piston against the spring force increases the fuelsupply thus the Actuator Pilot Valve must regulate the oil pressure acting onthe servo piston Oil is supplied to the Actuator Pilot Valve from the pressurepump, and the accumulators maintain the pressure of the supply at apredetermined value The position of the Pilot Valve plunger in its housing isdictated by the oil pressures acting on the different parts of the valve Themain downwards force on the Pilot Valve plunger, which causes a reduction

in oil supply to the servo piston, is varied by the amount of oil leakingthrough the nozzle at the top of the Pilot Valve That leak-off rate dependsupon the clearance between the top of the nozzle and the end of the TorqueMotor beam If the engine speed reduces, the Torque Motor Beam rises whichincreases the clearance and allows for greater leakage and a reduction inpressure acting on the top of the Pilot Valve plunger This valve consequentlyrises and allows for increased oil flow to the servo piston which then increasesfuel to the engine At the same time the Pilot Valve plunger rises due to thereduction in pressure on its upper face and the rate of leak-off through thenozzle is reduced

2.20.5 Emergency Manual Control

In the event of governor malfunction it is necessary to provide for manualcontrol of the engine and to achieve this the governor must be disengagedfrom the fuel control linkage and that linkage attached to the manual controlconsole Engine designers have different ways of achieving this and thesimplest system is to remove the pin connecting the fuel linkage to thegovernor and use it to attach the fuel linkage to the emergency control.Whatever the system available it is important that all parts are checkedperiodically and lubricated in order to ensure they will be operable if needed.This is particularly important with linkages as wear at pivot points results inlost motion in the system and defective control

2.20.6 Overspeed Trip

Although an engine may be fitted with an electrical or electro-hydraulicgovernor which has rapid response there is always a risk of serious enginedamage if the load is suddenly removed In order to safeguard against such aneventuality engines are fitted with overspeed trips This device acts to shut-offthe fuel supply completely until the cause of the problem is investigated andthe overspeed trip reset The device is usually driven by the camshaft andgenerally consists of a spring loaded mass which moves outwards due tocentrifugal force when the engine speed rises above a certain level The masswill then actuate a switch or similar system which shuts off the fuel completely

Engine Construction

2.20.7 Safety Cutouts

Failure of the lubricating oil supply or cooling water system can result in

serious engine damage and it is important that the load on the engine is

reduced immediately any failure occurs In the event of such failure it is

necessary to override the governor system and shut off fuel to the engine If

fuel pumps are fitted with puncture valves, these are activated to provide

immediate fuel shut-off

2.21 Lubrication Systems

Lubrication of all contacting moving parts is essential in order to reduce

friction and wear as well as carry away heat Oils employed for cylinder

lubrication also provide additional services in that they neutralise the acid

products of combustion due to the burning of fuels containing sulphur There

are essentially two distinct engine lubrication systems, that for the crankcase

and associated areas such as the camshaft system and that for the cylinders In

the crankshaft system the oil is circulated and reused but cylinder lubrication

is a total loss system and new lubricant has to be continuously supplied

2.21.1 Crankcase Lubrication

Crankcase Lubrication takes place under pressure and oil is supplied to main,

bottom end and top end bearings, the thrust bearing, crosshead guide shoes,

and the camshaft chain drive or gear drive system Man B&W engines were

provided with a separate cam box lubrication system in order to prevent fuel

oil leaking from the fuel pumps This practice has now been ceased following

the redesign of the fuel pump drive system Turbochargers usually have their

own bearing lubrication system which is self contained and has no connection

with the crankcase system As most engines employ oil cooled pistons the

crankcase oil must also act as a coolant Oil from all crankcase bearings and

the pistons is directed to the bottom of the crankcase where the drains tank is

located A pump suction is located at the lowest part of the drains tank, the

suction being covered by a strainer plate, and the lube oil circulating pump

directs the oil through a filter to the cooler where the temperature is reduced

to a value which would suit the engine load Piston cooling requirement is an

important factor in deciding the degree of cooling needed in order to avoid

carbon deposits forming in the piston cooling spaces and to prevent oxidation

of the oil After the cooler the oil may again pass through a filter to the

lubricating oil main where it is distributed to individual cylinder units The

supply connection is usually to each crosshead for distribution to the top end

bearing, guide shoes, piston (for cooling) and bottom end bearing; the main

Engine Construction

bearings, chain/ gear drive and thrust bearings have their own supply pipes

In some engines the crosshead bearing pressure is boosted and thisnecessitates an additional pump and pipe system to supply the crossheadbearings A centrifuge system is connected to the main engine lube oil drainstank which allows for purification of the lubricant As part of the lubricationsystem the centrifuge may also be connected to the renovating tank in order

to maintain the condition of the oil

2.21.2 System Lubricant Properties

The actual properties required of any lubricant depend upon the duties it isexpected to perform and the following are a guide:

Dispersancy

This enables the oil to hold contaminants in suspension which allowsimpurities to be transported to the filters Good dispersancy prevents sludgedepositing at the bottom of the drains tank

Detergency

The use of detergents in the oil helps remove deposits from surfaces whichkeeps surfaces such as the piston undercrown clean The dispersants in the oilthen keep the deposits in suspension

Corrosion Protection

Water in the oil, and fuel and combustion related acids, can give rise to corrosion

in the system and the addition of additives can safeguard against corrosion

Anti-Oxidation Properties

At elevated temperatures, and in the presence of air, oil will oxidise readilyand result in a loss of lubrication properties and the formation of sludges.Contact with the atmosphere cannot be avoided and as one of the functions ofthe oil is to carry heat away from the bearings and pistons a temperatureincrease is to be expected Anti-oxidation additives reduce the rate at whichoxidation will take place