Diesel Engines Questions and Answers
A. J. Wharton, CEng, FlMarE
Foreword by Don Ewart, CEng, MIMarE, MRlNA
Editor-Fairplay International Shipping Journal
STANFORD MARITIME LONDON I - - - - - - -- I _ - --- PRADEEP@MSC
Stanford Maritime Limited Member Company of the George Philip
12-14 Long Acre London WC2E 9LP
First published 1975 Reprinted 1977
Q 1975 A. J . Wharton
Printed in Great Britain by J. W. Arrowsmith Limited Bristol
ISBN 0 540 07342 3
Foreword Written by experienced lecturers a t one of Britain's
leading marine engineering colleges, each book of this series is
concerned with a subject in the syllabus for the examination for
the Second Class Certificate of Competency. It is intended that the
books should supplement the standard text books by providing
engineers with numerous worked examples as well as easily
understood descriptions of equipment and methods of operation.
Extensive use is made of the question and answer technique and
specially selected illustrations enable the reader to understand
and remember important machinery details.
While the books form an important basis for pre-examination
study they may also be used for revision purposes by engineers
studying for the First Class Certificate of Competency.
Long experience in the operation of correspondence courses has
ensured that the authors treat their subjects in a conclse and
simple manner suitable for individual study-an ~mpor tan t feature
f o r enylncers studying at sea.
Preface This book is intended to provide some basic information
on marine diesel engines and their associated equipment used at
sea. while indicating the type of questions set in the motor
examination paper for the Department of Trade Second Class
Certificate of Competency for Marine Engineer Officers.
It is not intended to give model answers to be learned by rote
but to provide a foundation on which, together with his own
experience, the prospective candidate can produce suitable answers
in both written and oral examinations-where possible he should base
these on equipment in his own ship. At all times he should stress
safety in any operations described.
The Department of Trade examination paper lasts for three hours
during which time six questions must be attempted from the nine
set. A reasonable standard is required in both sketching and
written work. Sketches need not be to scale but should he in
proportion. unless in diagrammatic form.
Drawing instruments may be used. but these may slow sketching
which is quicker by freehand. Colours may be used provided they do
not confuse the completed sketch. The diagrams in this book are not
to scale and there will not be time in the examination to attempt
Questions tend to be concentrated upon l a y slow running,
two-stroke main engines since these are in the majority at sea.
With a number of principal engine manufacturers, a variety of
designs exist. N o one englne has been used in this book but
simplicity of diagrams, together with a wider fieldof coverage. is
the aim. Precise details of a particular engine may be obtained
from engine makers' handbooks.
A list of approved SI Units is given, together with principal
conversions. These units have been used in the book, although
kg/cm2 or atmospheres are still used at sea for pressure
measurement. It may be convenient to remember that kN/m2 - 100 =
kg/cm2 or atmospheres, approximately.
A. J. Wharton
Contents 1 Engine types 2 Cycles and timing 3 Gas exchange
processes 4 Engine parts 5 Operating systems 6 Control 7 Safety and
SI UNITS Mass = kilogramme (kg) Force = newton (N) Length =
metre (m) Pressure = newton/sq metre ( ~ / m ' ) Temperature =
degrees celcius ("1
CONVERSIONS 1 inch = 25.4mm = 0.025m 1 foot = 0.3048m 1 square
foot = 0.093m2 1 cubic foot = 0.078m 1 pound mass (Ib) = O.453L.g 1
UK ton (mass) = 10 1 6kg 1 short ton (mass) = 907kg 1 tonne mass =
lO0Okg 1 pound force (Ibf) = 4.45N 1 ton force (tonf) = 9-96kN l k
g = 9.81N
0.0011n = 0.025mm ( O F - 32) x ,' = "C llbf,ln'.= 6 8 9 5 ~ 1 ~
' = 6 . 8 9 5 k ~ / m ~ I kglcm- = 1 kplcm- 1 0 2 k ~ / m ~ 1 atmos
= 14.7lbf!ln- = 1 0 1 . 3 5 k ~ / m ~ 1 bar = 14.5lbfI1n- = 1 0 0 k
~ / m * Note: For approx~mate conversion of pressure unlts lOOkN/m
= 1 bar = 1 kg/cm2 = 1 atmos l tonf l~n ' = 1 5440kN/mZ =
15.44MN/m2 1 H P = 0.746kW
- h i n o reproduced by L i d &mission d Motor $hi@ Dorut '
louse, Stamford !bet, London SEl SLl& UK, from wlan.lt 4
wailable at an inclusive price of fS.00. %
Q. Give an outline sketch and briefly describe a large
single-acting two-stroke main engine using exhaust ports. What are
the advantages of this system and how does it affect the operating
cycle of the engine?
A. Fig. 1 shows a general cross-section of a large bore.
single-acting, two-stroke crosshead type engine using ported
F U E L V A I . V E \ T W O P A R T
/ ; ; / ' C Y L I N D E R C O V E R
W A T E R J A C K E T
, P I S T O N & S K I R T
P I S T O N R O D , - - G L A h D S
C A M S H A F T
L u 8 O I L T ? - - P I S T O N C O O L I N G C R C S S - E - Z
R E C P P I P E S
C R A I U K C A S F R E L I E F \ A L . : - ' F R A M E S
C R A N K S H A F T - W E B S /
B E D P L A T E
Ftg. 1 Large bore two-stroke engine
Such engines give high power and will operate on heavy fuel with
a high thermal efficiency. Cylinders are isolated from the
crankcase allowing selected oil to be used for liner lubrication
and preventing contamination of the crankcase. Cylinder liner and
piston ring wear is moderate, allowing extended periods between
The use of exhaust ports in the cylinder liner eliminates the
necessity for exhaust valves, together with their frequent
overhaul, maintenance of their operating gear, the engine power
absorbed in their drive and a more complex reversing system. The
piston crown uncovers ports to operate exhaust timing, giving rapid
opening of large area of ports.
Due to the positioning of ports, the cross-loop method of
scavenge is used and exhaust timing is symmetrical. This does lead
to some post-scavenge loss but an increase in the efficiency of the
turbo-charge system together with under piston charging offsets
This engine operates on the constant pressure turbo-charge
system with an electric driven auxiliary blower for use at reduced
engine speeds. Heavy gas loads from large bore engines are
transmitted directly to the bedplate by long tie bolts.
Cylinder liners are water jacketed with bore cooling passages to
reduce thermal stress adjacent to the combustion chamber and to
simplify construction. Cylinder covers are in two parts and are
water cooled. One centrally situated fuel injector is fitted to
each cylinder. These are operated by valve timed fuel pumps.
Pistons are water cooled by reciprocating pipes which are
completely isolated from the crankcase. A piston skirt is fitted to
prevent loss of scavenge air.
Crosshead bearings are directly supplied with oil from
articulating pipes, excess oil being passed to lubricate guides and
through the connecting rod to the bottom end bearings. Crosshead
design has flexibility of bearing supports, allowing bearings to
align with crosshead pins. Main bearings are directly lubricated
and have their keeps secured by jacking bolts from the engine
Direct reversing is cakried cut by the use of an oil operated
lost motion servo-motor fitted to the camshaft drive.
Slow engine speed allows it to be coupled directly to the
Q. State the advantages of the use of poppet exhaust valves in
large two-stroke engines. Give an outline sketch and describe a
main engine using this system.
A. The use of exhaust poppet valves in a two-stroke engine
allows use of the more efficient uniflow or through-scavenge
system. It also simplifies the cylinder liner construction, sealing
and cooling arrangements.
Exhaust ports adjacent to scavenge ports cause high thermal
stressing, increase liner wear and require bore cooling of exhaust
port bars. With poppet valves, all these difficulties are
eliminated. The edge of the piston crown is not subjected to such
severe thermal stressing from the rapid passage of hot exhaust gas
following closely by cool scavenge air.
Cylinder lubrication is a little easier with lower oil
consumption. Exhaust timing can be accurately controlled by cam
profiles and the post-
scavenge loss of scavenge air is reduced: it is unnecessary to
fit a piston skirt. A section of a poppet valve engine is shown in
Fig. 2. This engine has one large
poppet valve fitted to the centre of each cylinder cover. Each
valve is fitted in a separate water cooled housing for ease of
maintenance. Va!ve seats are stellite-faced
to reduce burnlns and snrrnilon \ -n l \ e\ a re operated from
the camshaft by push rods and rockers, tappet clearance b r ~ n g
allowed for thermal expansion.
Three fuel injectors are fitted to each c> llnder, equally
spaced around the cylinder cover with fuel spray patterns d~rected
clear of the exhaust valve.
The engine is of rigid construction with a deep bedplate and tie
bolts fitted for the cylinder gas load. Bearings are of large
diameters to reduce stress and bending, while improving
lubrication. Pistons are oil cooled, the oil being taken from a
common supply at the crosshead which it reaches by telescopic pipe.
Oil is passed up and down the piston rod through internal passages
and, after a thermometer pocket, the oil is returned to the
crankcase. The crosshead passes lubricating oil to the bottom end
bearings through passages in the connecting rod.
The engine shown is turbo-charged on the pulse system, groups of
three cylinder exhausts being connected to each charger. An
auxiliary fan may be fitted for manoeuvring.
The camshaft is chain driven and operates the fuel and exhaust
timing cams. Negative (inverted) cams are used for fuel pumps to
enable these and exhaust valves to be reversed by 'gained motion'
obtained by a sun and planet gear system, while the engine rotates
on starting air.
Q. Sketch a general arrangement of an opposed piston two-stroke
slow running engine. State the advantages for this type of engine.
How is the cylinder liner attached?
A. Fig. 3 shows a single-acting two-stroke opposed piston main
engine. A number of advantages are obtained with this type of
engine. With two pistons in each cylinder, the engine has a greater
power per unit and consequently, for a given
U P P E R P I S T O N C O O L I N G W A T E R ,
U P P E R P I S T O N
3 P A R T - - ------ L I N E R
L O W E R P I S T O N - - - C O O L W G O I L
M A N V A L V E S C R O S S H E A D - -
3 E D P i A T E
Fig 3 Opposed plston englne
power, the number of cylinders and engine length may be reduced.
It should be pointed out, however, that the engine height is
The two pistons moving in opposite directions will give good
primary balance to the engine, and, since both pistons are
connected to the crankshaft, all gas load is transmitted by working
parts, eliminating the need for tie bolts and allowing lighter
construction of the engine bedplate.
The lower piston will uncover the scavenge ports while the upper
piston uncovers the exhaust ports and so the engine has the
advantages of rapid, large exhaust passage opening, with no exhaust
valves fitted and no cylinder cover required. This engine operates
on the uniflow scavenge principal.
The lower piston is oil cooled by telescopic pipe to the
crosshead. The upper piston, which operates closer to exhaust
temperatures, is water cooled, its telescopic connections being
clear of the crankcase.
During overhaul, removal of two pistons for each one unit opened
up gives economy in time.
The upper piston is connected to the crankshaft by side rods,
crossheads and guides and thus three times the bearings are used
per unit, with the maintenance required.
Engine crankshafts must also be of more complex design. By
advancing the cranks of the upper pistons compared with those for
lower pistons, the exhaust timing is adjusted to reduce the
post-scavenge loss while increasing the blow down period. This will
be less efficient. h o w e ~ e r . when the engine is reversed and
for this reason the angle of lead between cranks is I~mited. To
restore balance of engine, the stroke of the upper piston is
The engine ~llustrated in Fig. 3 is turbo-charged on the pulse
system with an auxiliary blower for manoeuvring.
The fuel system operates on the 'common rail' system with
camshaft operated, mechanical timing valves passing fuel to operate
two injectors per cylinder. This allows very simple, direct
reversing being controlled by air start distributor only. The fuel
pumps are connected to a chain driven crankshaft and discharge to
the common rail at high pressure.
The cylinder consists of three parts, the upper and lower
liners, each of which are bolted to a central, cast steel
combustion chamber. This chamber has cooling water passages and
contains radial pockets to accommodate two fuel injectors, an air
start and a relief valve. Cast steel jackets reinforce the liners
and allow water cooling. The lower jacket is attached to the engine
entablature and thus secures the cylinder. The upper liner jacket
contains the exhaust belt with bore cooled exhaust port bars. The
liners are free to expand both u p and down from the central
combustion space, rubber seal rings being fitted at the end of each
Q. Medium speed. four-stroke trunk piston engines may be used
for main propulsion. Compare this t lpe of englne with a large bore
slow running engine. What advantages are obtained from Vee
A. The main advantages of medium speed main engines are their
improved powerlweight and power/size ratio compared with large slow
running engines. They also tend to have a reduced initial cost for
The higher speed of these engines for main propeller drive will
require reduction gearing and flexible couplings. This can be used
to improve the flexibility of the system by gearing a number of
engines to a common drive or using twin screw
V A L V E G E A R / \ ~
Fig. 4 Med~urn speed trunk pston eng ln s
arrangements. Propeller sizes may also be reduced where the
ship's draught is limited.
The reduction in engine size allows smaller and more
conveniently spaced engine-rooms for the ship designer,
particularly with less head room. Reduction in weight will also
require less stiffening of the ship's structure.
Cylinder sizes are smaller than for slow running engines and
consequently more units will be required. but this is partly offset
by the increase in engine speed.
Four-stroke engines have the advantage of reduced thermal stress
due to averaging of temperatures during the cycle. This will reduce
cylinder liner and piston ring wear, permitting increased mean
piston speeds. Cylinder liners are of simple construction with no
ports. but cyl~nder covers become complex with the increase in the
number of valves required. Cylinders are robust and higher rates of
turbo- charging can be employed. giving an increase in power
Engine scavenging is positive and there is no scavenge trunking
or possibility of scavenge fires. With larger angles of valve
opening. the port size becomes less critical.
The engine speed may place some limits on the use of very high
viscosity fuels. Trunk piston design reduces engine height and the
number of working parts.
Difficulties of lubrication of crosshead bearing in two-stroke
engines are eliminated. Lubrication in these engines is usually to
the main bearings, oil then passes through oil holes in the
crankshaft to bottom end bearings, up the connecting rod bores to
gudgeon pins and thence to the piston cooling. Oil is returned
direct to the crankcase.
Improved quality of lubricating oils has largely overcome
corrosion difficulties in the crankcase of trunk piston engines.
Alkaline oil is used and this will also lubricate the cylinder
liners. Oil should be continuously purified to remove
contamination, but water washing may not be possible with
detergent-dispersent type oils. Oil consumption in these engines
tends to be higher than for corresponding slow running engines.
With smaller engine parts, inertia forces are reduced but there
will be more parts for maintenance. These tend to be lighter and
easier to handle and store. With improved engine design overhaul is
simplified. Bearings may be quickly renewed when worn. In
multi-engine systems, it may be possible to carry out maintenance
Vee engines are developed from medium or high speed trunk piston
engines. With moderate cylinder sizes it is possible to arrange two
banks of cylinders in Vee
V E E E N T A B L A T U R E
Fig. 5 Medium speed Vee engine
formation operating on a single crankshaft and mounted on a
common crankcase and bedplate.
Vee engines improve still further the power produced for reduced
size and weight.
The Vee configuration allows convenient systems for air and
exhaust trunking to turbo-chargers.
Two camshafts are used, one for each bank of cylinders; both are
driven from a common gear system from which other auxiliary drives,
such as pumps and air compressors, may be taken to make the engine
system self contained.
It will not be necessary to have air starting valves fitted to
every cylinder and these usually act on one bank only.
For a given number of cylinders the crankshaft will be reduced
in length but it must have increased strength to transmit higher
power output. Each crank must accommodate two bottom end bearings.
These may be placed 'side by side', articulated with a master
bearing on the crank and a slave bearing attached to the master.
Alternatively one bearing may be mounted on the outer circumference
of the other.
Cycles and Timing
Q. Describe, with the aid of diagrams, the operating cycles of
two-stroke cycle and four-stroke cycle compression ignition
engines. Enumerate the operations during the cycle.
A. As its name implies, a two-stroke cycle takes place in two
consecutive strokes of the engine piston, or one revolution of the
crankshaft. Thus each operation in the cycle is repeated during
every revolution of the engine. The two strokes of the cycle may be
termed: Compression stroke and Power or expansion stroke.
Operations take place in a fixed order and must occur when the
piston reaches a corresponding position in its stroke. These
positions are shown as volumes on an indicator diagram which
relates them with pressure within the cylinder. It is convenient to
express them in terms of angles of crank position measured from top
dead centre (TDC) or bottom dead centre(BDC) and they ma! be shown
as a circle on a timing diagram.
Actual timing may differ between engines due to construction and
design differences such as: ratio of connecting rod lengthlcrank
length, strokelbore ratio, engine speed. englne rating. etc. Fig. 6
shows a typical two-stroke cycle with the operations numbered.
Fig. 6 Two-stroke cycle
CYCLES AND TIMING
Completion of scavenge. Air is entering the cylinder, expelling
exhaust gas and recharging it for the next combustion. Scavenge and
exhaust are open. Post-scavenge. Scavenge ports have closed and
some air within the cylinder may leak to exhaust. In some engines 2
and 3 are made to coincide to eliminate leakage of air.
Compression. Exhaust has now closed and the air trapped within the
cylinder is compressed by the upstroke of the piston to raise its
temperature sufficiently to ignite the fuel. Fuel injection takes
place and combustion occurs causing a rapid rise in pressure. The
period for which this continues depends upon the fuel pump setting
and power to be produced. Expansion. Combustion completed, the hot
gases expand forcing the piston downwards and converting the heat
energy from combustion into work on the piston. Exhaust blow down.
Exhaust has opened allowing gas to pass to exhaust manifold, and
pressure drops rapidly in cylinder. Scavenge. Scavenge ports have
opened and air enters to expel the remaining exhaust gas.
Scavenging then continues for the next cycle. Position 1 represents
bottom of stroke (BDC). Position 5 represents top of stroke
Fig. 7 illustrates a four-stroke cycle. This takes place in four
strokes or two revolutions of the engine. These strokes may be
termed: Compression stroke, power or expansion stroke, exhaust
stroke and aspirating or induction stroke.
E X H A U S T
d S P l 9 4 T 0 % --
Fig. 7 Four-stroke cycle
CYCLES A N D TIMING
Numbering these operations in sequence on the timing
Completion of aspiration. Compression. Air inlet valve has
closed, air in cylinder is now compressed to raise its temperature
for combustion of fuel. Fuel injection. Combustion takes place with
corresponding rise in pres- sure. Period controlled by fuel pump
setting. Expansion. Combustion completed, gas pressure does work on
piston during downward stroke. Exhaust. Exhaust valve opened.
piston expels exhaust gas on upward stroke. Overlap. Air inlet
valve opened while exhaust remains open. The length of this is
increased in supercharged or high speed engines. Aspiration.
Exhaust valve closed, piston draws air into cylinder during
downward stroke. Aspiration continues for next cycle. 4 and 9 are
TDC positions. 1 and 7 are BDC positions
Q. Sketch indicator diagrams for a ilou running large two-stroke
diesel engine. How are diagrams taken and u hat ~nformation can he
gained from them? How is power balancing carried out'.'
A. An indicatc)r d~agram can be obtained from a diesel
engineduringitsoperation, b! use of an enpne ~ndlcator (Fig. S) .
The diagram represents a pressure-volume diagram taken from
cond~tions within the engine cylinder. Similar diagrams are taken
for each c)linder of the engine.
D R U M
P A R A L L
'- S P R I N G
E L L I N K
D L E
I G A S P R E S S U R E
Fig. 8 Engine indicator
I CYCLES A N 0 TIMING
The indicator cock for the chosen cylinder is first blown
through to clear it of carbon and the indicator is then connected
to it. The cord on the indicator drum is attached to some form of
engine stroke synchronising mechanism from the crosshead or a cam.
The cock is now opened and the indicator pen is held against the
card wrapped round the drum tracing a diagram for one cycle of the
engine. Pressure is recorded to a vertical scale according to the
stiffness of the indicator co_mp~es&n
II ---- --.-- . - m. Corresponding cylinder swept volume is
recordao? a-horizontal scale due to rotat~on of the drum by its
cord. By turning the indicator cock to a vent position, a
horizontal line representing atmospheric pressure is added to the
diagram. This can act as a pressure datum line.
Four types of indicator diagram can be obtained from a slow
running engine. Power card is taken with the indicator drum in
phase with piston movement (Fig.
9). The area within this diagram represents the work done during
the cycle to scale. This mav be used to calculate the Dower ~ r o d
u c e d or the mean indicated Dressure (MIP) for the cylinder.
Irregularities in the shape of the diagram will show operational
faults. Maximum or peak pressure may be measured to scale between
the a ~ r E 3 E e a n d the highest point on the diagram.
Fig. 9 Power ind~ca to r d ~ a g r a m
- - - - 7 i
C O M P R E S S I O N P R E S S U R E
Compression diagram (Fig. 1 0 ) . This is taken in a similar
manner to the power card but with the fuel shut off from the
cylinder. The height of this diagram shows maximum compression
pressure. If compression and expansion lines coincide, it shows
that the indicator is correctly synchronised with the engine.
Reduction in height of this diagram shows low compression which may
be due to a worn cylinder
CYCI ES AND TIMING
liner, faulty piston rings, insufficient scavenge air or leaky
exhaust valve, any of which v
will cause poor combustion. Draw card or out of phase diagram
(Fig. 11). Taken in a similar manner to the
power card, with fuel pump engaged but with the indicator drum
90" out of phase ki th piston stroke. This illustrates more clearly
the pressure changes during fuel combustion. Fuel timing or
injector faults may be detected from its shape.
~ T ? I O S P H E R I C L I N E
F I ~ 1 1 Out ( > f phase d ~ a g r a m
E X H A U S T - - - - -
.'o P E N S
A T M O S P H E R I C L I % E S C A V E N G E C l P E ' V S
Light spring diagram (Fis. 1 2 ). .Aga~n s ~ m ~ l a r to the
power card and in phase with the engine, but this d~agram I \ tahsn
u ~ t h a light compression spring fitted to the indicator showing
pressure changes during exhaust and scavenge to an enlarges
scale. It can be used to detect faults in these operations.
Provided the cycle operations are correct, power balancing of an
engine can be carried out by comparing power d~agrams, or MIP, from
Fuel pumD controls can be adjusted to increase or decrease the
quantity of fuel injected into each cylinder and this in turn will
raise or lower the p_oop_r_ocIu_ced within that cvlinder. Such
adiustments mav be carried out to obtain eaual Dower
from each cylinder, in which case the ea of each Dower card will
be equal. Steady running conditions must be maintained while power
balancing is carried out.
CYCLES AND TIMING
Exhaust temperatures should be noted during power balancing
since a limiting exhaust temperature may make complete balancing
Q. How can valve settings on a large engine be checked while the
engine is operating at sea and how is fuel injection timing
A. While an engine is running at sea, valve or fuel injection
timings can only be checked by instrumentation. Incorrect timings
will a 3 t power o u t p ~ t and exhaysf te-mperatures and,
provided thgse are normal, and no other irregularities occur, it
can be assumed that engine timing is correct.
Exhaust valve setting on a slow running engine can be checked by
means of a light spring indicator diagram. This will not give an
accurate timing check but by comparison with a normal diagram or
one taken during original engine trials, it may be seen if valve
opening is early or late (Fig. 13).
E A R L Y O P E \ ' k S
N O R M A L
L A T E O P E N I N G
E A R L Y I G 2 T L U \ ! L A T E I G N I T I O N
Fig. 1 1 Dia!:am ahowlng early and late injection
Fuel injection timing can be checked by means of power and draw
cards taken from the cylinder. The draw card particularly will
illustrate early or late fuel injection (Fig. 14). This does not
show the actual time injection commences but that at which ignition
takes place. Provided the fuel is in correct condition and the
injector operating normally, the time between injection and
ignition is almost constant. It may also be possible to obtain
needle lift diagrams from the fuel injectors, these give accurate
timing but not many engines have facilities for taking them.
CYCLES AND TIMING
Sophisticated equipment is available in which transducers can
record the pressure pulse within the fuel pipe and compare this
with the pressure in the cylinder. This equipment requires an
osci!loscope and is unlikely to be available in a normal ship's
engine-room. If apparent faults occur in one cylinder unit only,
they may be caused by defective equipment, incorrect adjustment or
wrong cam position for that unit alone. If, however, they occur in
all units, it would appear that the timing of the camshaft o r its
drive mechanism is a t fault.
Accurate timing checks, measurements of clearances, adjustments,
inspection and testing of parts can only be carried out while the
engine is out of service.
Q. Describe how you would detect the following faults during the
operation of a diesel engine. State possible causes and
(a) Afterburning ( b ) Early firing (c) Choked fuel valve (d)
Leaky piston rings.
A. (a) Afterburning refers to slow or late combustion of fuel
which takes place during the expansion stroke of the engine cycle.
It causes loss in power, since the fuel is not burned correctly to
transmit energy to the piston at the most effective part of the
stroke. Combustion may still be incomplete when exhaust takes
place, and the heat energy remaining. together with some unburned
fuel, will be lost. Exhaust gases will be at high temperature and
will contain black smoke from incomplete combus- tion. There will
also be a higher gas pressure at blow down which will increase
pulsing in the exhaust manifold.
Afterburning is detected by l_o&~ower. high exhaust
temperature with smoke. It is confirmed by taking an indicator
power card and draw card (Fig. 15). These show an increase in depth
of diagram towards end of expansion together with late or slow
ignition. Due to loss in engine power there will be ireduction in
engine efficiency due to afterburning. It may cause burning in the
exhaust valves and fouling of the exhaust system including
turbo-chargers. These in turn may give rise to surging of turbo-
charger and possibly fires in the uptakes. There will be a
corresponding reduction in scavenge efficiency and high cylinder
temperatures may make liner lubrication difficult.
Causes of afterburn~ng may be incorrect fpgldumg tlmi2gLfaulty
fuel injectt, - ----- heavy fuel oil temperature too l o ~ . lack
of scavenge alr or poor compression.
- - --- - -- -
Fig. 15 Diagram showing afterburning
CYCLES AND TIMING
Remedies are to correct fuel timing by adjusting the fuel pump,
fuel cam or clearance, change fuel injector, maintain fuel
temperature for correct viscosity, clean scavenge ports and
turbo-charger. It should be noted that low fuel temperature and
lack of scavenge air (apart from choked ports) will affect all
cylinders in the engine.
( b ) Early firing in a cylinder will cause a very high peak
pressure at about top centre of the piston stroke. This will cause
a heavy shock load to be transmitted to the bearings w ~ t h a
corresponding knock from the englne. Thermal effic~ency of t h z c
l e will be Increased and power will be ra~sed but exhaust
temper2fiire w~l l be reduced.
---- - I - - ---- -- - -*
F I ~ 16 D~agram 5how1ng early ignltlon
Early fir~ng can be detected by the englne knock and can be
confirmed by an ~ n d ~ c a t o r d~agram draw card. (Ftg 16) wh~ch
shows a h ~ g h peak pressure. It may be caused by early Injection,
by ~ncorrect fuel cond~t~on , overheated parts suckas a-ho_t
-- - -
p~ston, or even a scavenge firgcaaussnglgca! heat - Remed~es
~nclude checklng and correct~ng fuel pump tlmlng. malntalnrng
corres! fueltemperature, and cool~ng of m o r n g p r t s i 7 n o t
corrected the shock loads may cause damage to, or fa~lure of
( c ) Choked fuel valve may 6 e x e to contamlnatlon In the fuel
In whlch d e b r ~ s may choke the small atomlser holes In the
Injector Alternat~\el\ ~t ma\ be caused by a leaky injector
allowing hot gas to blow back Into the Injector causing carbon to
form and choke the injector. Overheating of injector nozzle may
also cause build-up of carbon. There will be a loss in engine
power. There will probably be hammering in the fuel pipes between
fuel pump and injector and this may lead to rupture of fuel pipe. A
choked valve can be confirmed by indicator diagram power and draw
cards (Fig. 17), and reduced exhaust temperature.
The remedy is to change the fuel injector, clean the fuel system
and ensure correct centrifuging and filtering of fuel, and maintain
correct fuel valve cooling tempera- ture.
Fig. 17 Diagram showlng choked fuel lnjector
CYCLE-S A N D TIMING
Flg. 18 Diagram showing leaking plrton rings
( d ) Leaky piston rings are detected by poor combustion
together with blow past of hot combustion gases. There will be a
loss in engine power with the possibility of afterburning with the
corresponding high exhaust temperature and smoke. It will cause
high rate of cylinder liner wear due to poor lubrication, and may
cause scavenge fires due to fouling of scavenge spaces. There is
also risk of a seized piston due to local overheating. There will
be low compression and consequently poor combustion. A compression
diagram will show this (Fig. 18).
It may be caused by excessive cylinder liner wear; lack of
cylinder lubrication; worn, broken, stuck or poorly maintained
piston rings; worn piston ring groove landings allowing rings to
cant and jam: carbon jamming rings in grooves. It will be
aggravated if the engine is overloaded.
The remedy is to gauge c>l~nder liner and renew if necessary;
overhaul piston; clean ring grooves and gauge them: machine or fit
new groove inserts as necessary, and renew piston rings nith
correct clearances. Maintain cylinder lubrication and avoid
Q. Explain how individual cylinder powers in a medium speed
engine can be balanced. What is the effect of operating for long
periods under unbalanced conditions? How may a watchkeeper
ascertain when conditions are normal?
A. The accurate measurement of power from individual cylinders
in medium or high speed engines is difficult in practice. A number
of assumptions have to be made regarding the operating efficiency
of the engine. In order that these assumptions can be justified. it
is particularly important that regular and correct maintenance is
carried out on the engine and that any deviation from normal
running is noted, investigated and corrected at the earliest
Fuel injection equipment 15 particularly important and fuel pump
settings, clearances and timings must he checked and maintained
during periods when the engine is out of service. Fuel lnjectorc
must be changed regularly, cleaned and tested to ensure trouble
free operation. The injector is the most likely part of the system
to be subject to faults in servlce. A fault in one injector will
cause loss in power in the affected cylinder but may also mean that
other cylinders are subjected to overloading as the engine governor
attempts to maintain normal total power or speed.
With higher speed engines it is impracticable to take indicator
diagrams due to accelerations of the indicator drum mechanism. The
pressure scale may still be used and the indicator will produce a
vertical line, the height of which represents the peak pressure in
the cylinder during the cycle. Similar lines may be drawn for each
cylinder and an atmospheric line can be added by hand movement of
the indicator drum with pressure cock vented.
Alternatively a peak pressure indicator may be connected to the
indicator cock of each cylinder and used to measure peak pressure
in the cylinders (Fig. 19).
Assuming that the cyclic operations of the engine are normal,
the amountsof fuel burned and power produced are proportional to
the maximum (peak) pressure in the cycle. Thus a measure of engine
balance can be carried out while the engine is in service by
adjusting fuel pump settings to give equal peak pressure in each
As a check that operating conditions in each cylinder are
normal, peak compression pressures may be taken and compared in a
Some further evidence of power balance can be taken from the
corresponding position of fuel pump racks and also from the
comparison of exhaust temperatu~es.
Exhaust temperatures are unlikely to be equal in a
multi-cylinder engine, particularly when turbo-charged. but they
will tend to follow a pattern. Whenthis relative pattern is
established it can be as5umed the engine is power%alanced. Pump
adjustment is limited to prevent fuel delivery at stop setting.
P O I N T E R I N D I C A T E S P R E S S U R E B A L A N C
1 3 . J S T ' v l E N T R A L ; A S D ' t S S J R E
P E A K P R E S S U R E ' S C A L E
L O ' v ' L E C T T O , ' . 3 3 A T 3 R C O C Y
Cooling water return from each unlt ,hould also he approximately
equal in temperature.
If an engine operatei in an unbalanced condit~on. qome hearings
and running gear may become o\erloaded: thiq ma! cauw o\erheat~ng
and bear~ng failure. Overload in cylinders ma\ cause piston b l o ~
pazt. with the orr responding dangers of overheated or seized
pistons. Unbalance s i l l also iet up vibrations which, if
maintained for prolonged periods. \\111 cause fatigue from the
fluctuating stresses induced. This may in turn lead to fat~gue
crackins of metal in bearings, fracture of bearing studs or bolts,
cracks in crank5haft and bedplate and slackening or failure of
holding-down bolts. A watchkeeper may ascertain that running
conditions are normal by observation of the relevant temperatures
and pressures, particularly exhaust and cooling return
temperatures. iubricating oil and turbo-charge pressures. The
exhaust should be clear of smoke and there should be no unusual
noise or vibration.
I Gas Exchange Processes
Q. Describe with the aid of sketches the methods of scavenging
employed in large two-stroke diesel engines. Why is scavenging
necessary and how is it obtained?
A. Scavenging of an internal combustion engine consists of the
removal of exhaust gas from the cylinder after combustion and its
replenishment with air for subsequent combustion.
Efficient scavenging is necessary for good combustion and it is
required for the very first working cycle of the engine. The
passage of scavenge air will also assist cobling of the cylinder
and piston. Two-stroke engines rely upon a charge of scavenge air
under slight pressure sweeping through the cylinder and expelling
the exhaust gas in front of it. This process must take place while
both scavenge and exhaust connections are open and the piston is
near the bottom of the cylinder. Even in slow running engines. this
allow5 only a very short period of time for scavenge to be
Some mixing of air and gas will occur but this must be kept to a
minimum and scavenging can be improved by supplying a volume of air
in excess of the cylinder volume, the excess passing to the exhaust
P I S T O N S K I R
Fig. 20 Loop scavenge
GAS EXCHANGE PROCESSFS
Air must be supplied at a higher pressure than that in the
exhaust manifold and this may be obtained in a number of ways.
Reciprocating scavenge pumps or rotary blowers driven from the main
engine may be used. These will of course absorb some engine
The usual method in modern engines is to use exhaust gas driven
turbo-chargers which do not consume useful engine power; the air
will be cooled before reaching the scavenge ports. Combinations of
scavenge pumps and turbo-chargers may also be used.
Scavenge air enters through ports near the bottom of the
cylinder liner when these are uncovered by the piston crown near
the bottom of its travel. It will continue to enter until the
piston again covers these ports on its upward stroke. The
directional flow of scavenge air within the cylinder is decided by
the engine design and exhaust arrangements.
There are three basic methods of scavenging w~thin the cylinder.
1 . Loop scavenge in which air passes over the piston crown and
rises to form a loop within the cylinder, expelling gas through
exhaust ports cut in the same side of the liner above the scavenge
ports (Fig. 20). 2. Cross scavenge where scavenge air is directed
upwards. passing under the cylinder cover and down the opposite
side, expelling gas through exhaust ports on that side (Fig. 21).
3. Uniflow or through-scavenge in which scavenge air passes
straight up through the length of the cylinder forcing the exhaust
through ports and valves at the top of the cylinder (Fig. 22).
S C A \ E ' b S E 4 9 F R C " \st 4 E - b a V 4 L \ E S
X H A U S T
- P S T d N
(;AS FXCHANGE PROCESSES
E X H A U S T
t , - - E X H A U S T
V A L V E 0 P E N
S C A V E N G E A I R
S C A V E N G E A 1 R
P I S T O N
In all these cases the swirl or direction of the air is assisted
by the port edges being angled in one or two planes and by the
shape of the piston crown. In cases 1 and 2 a piston skirt or
exhaust timing valve will be necessary to prevent scavenge air
leaking to exhaust while the piston is at the top of its stroke.
Case 3 tends to give the highest scavenge efficiency with the least
mixing of air and gas. It may also be used with greater strokelbore
ratio and in opposed piston engines. It avoids the difficulty of
high temperature gradient between adjacent scavenge and exhaust
ports in 1, or the temperature difference on opposite sides of
plston rings in 2. In single-actingengines, however. i t will
require the f i t t in of euhau\t \a l \es together with the
necessary operating gear and malntenancr.
In all engine. the tca\engr trunk~ng must be kept drained. must
be regularly inspected and m a ~ n t a ~ n e d In a clean
Q. What are the ad\antages of turbo-charging a two-stroke cycle
main engine'? Describe and sketch such '1 s!3tem and explain
arrangements made for manoeuvring.
A. Early two-stroke engines used main engine driven scavenge
pumps or blowers to supply scavenge air at low pressure, but these
absorbed power from the engine output. With the development of
modern exhaust gas driven turbo-charger systems, an adequate supply
of air can be obtained, not only for scavenging the engine but also
for pressure charging.
All the power required to operate the turbo-chargers has been
recovered from waste heat in the exhaust gases. The efficiency of
the system is increased by fitting a
GAS EXCHANGE PROCESSES
charge air cooler after the compressor. This will cool the air
at constant pressure, increasing its density before supplying it
for compression in the engine cylinders.
The mass of air per cycle can now be increased and the quantity
of fuel injected can be raised to give a corresponding increase in
engine output. It will also increase the thermal efficiency of the
A simple system is shown inFig. 23, which consists of a
turbo-charger and charge air cooler.
T U R B I N E T U R B O C H A R G E R ,/
E X H A U S T V P L V E
A V E N G E R T S
Exhaust gas from the cylinder operates the gas turbine, giving
up some of its heat energy to d o so. The turbine drives a directly
coupled air compressor, which draws air from the atmosphere.
compresses it and then cools it in the charge air cooler before
supplying it to the engine through scavenge ports.
A correctly matched turbo-charger is self regulating under
normal conditions and the supply of exhaust gas energy will be
matched by a corresponding demand for scavenge air. The
turbo-charger must also be matched to the engine to establish
stable operation under normal conditions.
When manoeuvring a two-stroke engine, scavenge air is required
for the first cycle. A turbo-charger. however. cannot build up
speed, compress air and supply ~t to the engine until there is some
build-up of exhaust energy. Consequently there will be a time lag
between demand for scavenge air and its supply. This lag ma! also
occur when rapid changes are made in engine output. Due to losses.
the turbo-charger alone may be unable to supply sufficient air to
operate the engine efficiently at low speeds and some alternative
air supply must be added.
Alternative methods may be:
GAS EXCHANGE PROCESSES
1. Fitting of engine driven scavenge pumps in series with the
turbo-charger. These take little power at full speed but supply a
positive quantity of scavenge air at low speeds. 2. Use of under
piston spaces to act as scavenge pumps. These will also be in
series with the turbo-charger; they improve scavenging but absorb
engine power. 3. Use of an auxiliary driven compressor to supply
additional air to the air manifold. This method has greater economy
since the compressor is only used when
I required. It is compact, requires little maintenance, uses
little power and may be controlled automatically by the air
pressure in the scavenge trunk. Q. Sketch and describe a
turbo-charger suitable for use with large bore engines. Give
materials used and describe bearing arrangements. How are these
A. Fig. 24 shows a section of a turbo-charger as fitted to large
T H R U S T B E A R I N G
It consists of a single stage. axial flow exhaust gas-driven
turbine mounted on a common shaft with a centrifugal alr
compressor. The turbine has a nozzle ring followed by a rotating
disc with a single row of moving turbine blades. These blades are
attached to the disc by fir-tree shaped roots and they have free
room to expand when heated. Binding wires are fitted to the blades
to reduce vibrations. Blades and nozzles are manufactured of heat
resisting steel or nickel alloy. The turbine casing is in two
parts, both of cast iron with adequate water cooling spaces. There
are inspection and cleaning covers to these spaces which are
circulated with fresh water from the engine cooling system. A drain
valve is fitted in the exhaust gas space.
The air compressor consists of a radial flow impeller disc
together with an inducer, both are of aluminium alloy. The impeller
discharges air through a diffuser
GAS EXCHANGE PROCESSES
t o a volute casing. Compressor casing, also in two parts, is of
cast aluminium and is uncooled. Air is drawn from the engine-room
atmosphere through inlet filters which may be removed for cleaning.
Air inlets are streamlined and fitted with insulation internally to
Turbine nozzle ring, air diffuser, impeller and inducer will be
replaceable to allow matching between turbine and compressor and
between turbo-charger and the engine to which it is fitted
Two labyrinth seals are fitted to the shaft, one between thrust
bearing and air compressor and the other between turbine and
bearing. They are sealed with air under pressure from the
compressor discharge through internal passages and restriction
plugs. Air from glands passes to the atmosphere o r assists cooling
of the turbine disc. T h e seals prevent possible oil leakage into
the turbine o r compressor or exhaust gas into the corresponding
bearing oil. Some air will pass down the back of the impeller
through a labyrinth arrangement. This air is then passed along the
turbine shaft assisting cooling and leaves with the turbine exhaust
Two shaft bearings are fitted, one at each end of the shaft
allowing accessibility and cooling. E n d thrust is taken a t the
compressor bearins. the turbine bearing allowing expansion
Bearings may be either of plain sleeve type with copper lead
bushes on hardened steel shaft sleeves, o r alternatively ball and
roller bearings may be used. Ball and roller bearings reduce
friction drag but are susceptible to vibrations and fatigue both
when running and also from externally generated vibrations when not
in use. They must be fitted in resilient mountings which use
springs and oil damping in both axial and radial directions. These
bearings also have a fixed fatigue life and they must be renewed at
stated lnter\als (sa! 8000 hours).
Lubrication of the bearinpi ma! be by various means. Ball and
roller bearings may be lubricated b! \elf-conta~ned gear t!pe pumps
operated from the shaft and drawing oil directl! from the
~ndepsndcn t bearing sump. Oil level must be maintained In the5e
iump\ and the 011 \houlJ be renewed at stated intervals.
Alternativel:, the bearlngr. ma! be lubricated b! external
s\stems. Either by connections from the engine lubricating !.item
cons~sting of pumpling takes place3
A. In modern two-stroke turbo-charged engines a charge air
cooler is necessary. Compression will raise the air temperature and
a charge air cooler is fitted to reduce the temperature of the air
betneen the turbo-charger and the engine inlet manifold. I t causes
increased air density at l one r induction temperature. The engine
is maintained a t safe working temperatures and the lower
compression temperature reduces stress on piston rings, piston and
GAS EXCHANGE PROCESSES
Increased density will raise scavenge efficiency and allow a
greater mass of air to be compressed, more fuel may now be burned
giving an increase in power.
Fig. 25 shows a section of a charge air cooler. The air makes a
single pass through the cooler and, for efficient cooling, its
velocity should be low and cooling area large. This is obtained by
making the air inlet connection divergent; the outlet is convergent
to restore air velocity after cooling.
Condensation of moisture in the compressed air will occur during
cooling and a drain is fitted to the outlet side air casing to
allow this condensation to be removed. A moisture eliminator may
also be fitted to remove entrained water droplets from the
S E A W A T E R
1 N O U T
I t D I V I S I O N P L A T E
W A T E R T U B
\ F I X E D T U B E
A I R DL' A I R I N
F I N S
R l N G -. -
M O V I N G T U B E P L A T E
S E A L R l N G
W A T E R B O X
D E T A I L O F S E A L R I N G D E T A I L O F F I N S S O L D
E R E D T O T U B E S
Fig. 25 Charge air cooler
GAS EXCHANGE PROCESSES
air stream. The drain should be kept open and its discharge
noted. This will also indicate if a cooling water leak has
The cooler consists of a tube stack of aluminium brass tubes
rolled and solder-bonded into two brass tube plates. Cast iron
water boxes are attached to tube plates and allow salt water
circulation within the tubes to make two passes. One tube plate is
secured to the casing while the other is free to move axially as
thermal expansion occurs. The air seal is maintained by means of a
fitted rubber joint ring. An air vent is fitted to the top water
box to remove air which may have been released from the salt water
system. Corrosion plugs may be fitted within the water space.
Thin copper fins are solder-bonded to the outside of the cooler
tubes, the air will pass between these plates, which greatly
increase the area of heat transfer. The cooler is completed by two
side plates of mild steel or aluminium alloy.
Temperatures and pressures are recorded at each inlet and
discharge. Discharge air temperature should not exceed 55C since
engine temperatures-notably the exhaust temperatures-will increase,
with loss in efficiency due to reduction in air density.
Undercooling is cooling the air below its dew-point at the
corresponding pressure. The temperature should not be taken below
20125C or excessive condensation may occur.
Excess water carried into the engine cylinders will promote
corrosion and wear and may remove cylinder liner lubrication.
Air at very low temperatures will also cause thermal shock when
in contact with high temperature liners and pistons.
Some measure of cooler efficiency can be ascertained from the
difference between air discharge temperature and cooling water
inlet temperature under normal running conditions. a rise in this
indicates fouling of the cooler.
An increase in the air pre\\ure drop indicates fouling of the
air passages, while increase in water pressure drop indicate\
fouling of water side.
When necessary. charge air cooler\ can he cleaned while out o f
service. Water side may be cleaned by remo\ ins water bo le \ and
bru\hing through tubes. If this does not remove the scale. acid
cleaning ma! be carr~ed out. after which it must be flushed
Air side may be blown clear \\ith a cornpre55ed alr or steam jet
and specially shaped brushes may be used. If these do nor remove
the dirt. the stack may be immersed in a hot degreasing fluid and
then blown clear.
After cleaning, a pressure test of about 300kN/m2 should be
carried out o n the water spaces.
Q. With regard to a four-stroke diesel engine, explain why: (a)
Air inlet and exhaust valves open inwards (b) Some valves are
cooled while others are not (c) Tappet clearances are necessary in
valve operating gear.
What are the consequences of having clearances in (c) greater or
G A S E X C H AXGE PROCEShF,
allowed to build up, cause blow-by of hot gases and burning of
valves. Some protection of the valve seat is given by the valve lid
during combustion in the cylinder.
Valve springs and the operating mechanism can be of moderate
proportions, reducing inertia of parts and power demand from the
engine. In order to facilitate overhaul of the valves without
removing the cylinder cover, valves together with their springs,
etc., may be fitted in separate cages.
(b) Cooling of exhaust valves will prolong the useful life of
valves, seats and bushes. It will maintain temperatures low enough
to prevent burning and rapid wear and also allow lubrication of the
spindle bushes, reducing wear and maintainingvalve alignment. Valve
damage may also be reduced by depositing hard corrosion
I materials such as stellite on seats, in way of bushes and
tappets. When burning heavy fuels containing vanadium and sodium
compounds, valve temperatures must be kept below 530C. above which
deposits and corrosion may occur.
Cooling is carried out by circulating the valve cage and seat
with fresh water. In some cases the valve itself may be cooled by
cooling passages with flexible external connections. Valves and
seats should be made of materials which readily conduct heat from
the valve lid. Valve cages must be a good fit in the cylinder cover
in order to transfer heat to the cover.
When exhaust valves are not central in the combustion chamber.
heating will not be symmetrical on the valve lid and an automatic
rotating device may be fitted causing the valve to rotate
slo\vl>. thereby avoiding local overheating.
E X H A U S T V A L V E C A M S H A F T
F I ~ . 26 Exhaust valve
GAS E.XCHANGE PROCESSES
Air inlet valves do not require additional cooling since their
mean temperature is much lower due to the passage of cool air
through the valves during each cycle. These 4 valves operate under
less arduous conditions than exhaust valves and the period between
their maintenance is longer.
(c) Tappet clearances are necessary to allow for thermal
expansion of the valve spindle length at working temperature and to
ensure that positive closing of the valve ~mntinues as it wears or
seats during use. Clearances should normally be set while the
engine is cold and the cam follower is off the cam peak. Wear of
the valve gear will tend to increase cleara~ices.
Excessive tappet clearance will cause the valve to open late and
close early in the cycle and will reduce the maximum lift of the
valve. It will also cause noise, and eventually damage, from the
impact of working surfaces.
Insufficient clearance will cause the valve to open early and
close late with increased maximum lift. In extreme cases, it may
prevent the valve from closing completely as it expands, or beds
in. This, in turn, will cause hot gases to blow past valve faces,
causing burning of valve, low compression. etc.
Q. Sketch and describe a cylinder liner suitable for a large
two-stroke main engine. Show how it is secured and how expansion is
allowed. Why is cooling necessary?
A. The cylinder liner shown in Fig. 27 is for a large two-stroke
poppet valve engine. The liner is manufactured from pearlitic grey
cast iron containing vanadium and titanium; these refine the
structure giving increased strength and wear resistance while
reducing corrosion from sulphur present in the fuel.
C Y L I N D E R C O V E R
L I N E R
L U B R c 4 - 3 H O L E S
S C A V E N G E P O R T S
Fig. 27 Cylinder liner
The thickness of the liner must give adequate strength to resist
gas load hut thickness is limited by the necessity to maintain
cooling and limit thermal stress.
Cooling is carried out by the circulation of fresh water within
a cast iron cylinder jacket into which the liner is fitted. Cooling
water space on the outside of the liner extends from just above the
scavenge space up to the position of the top piston ring when the
piston is a t the top of its stroke. Water from the main cooling
system enters the jacket at its lower end and leaves a t the top
from where it passes to cylinder covers. Cleaning and inspection
covers are fitted to the jacket.
The cylinder cover, which lands on the top of the liner, is
secured to the jacket by a number of cover studs and these ensure a
watertight joint between liner, flange and jacket; the liner being
fixed a t this position. Tie bolts pass from the top of the jacket
t o the transverse members of the engine bedplate, these transmit
the gas load and are pre-stressed to maintain the jacket in
T h e portion of the liner encased in the scavenge space has a
row of scavenge ports which are uncovered by the piston a t the
bottom of its stroke. The liner is free to expand downwards, a
water seal being made a t the lower end of the jacket by two
silicone rubber rings fitted within grooves machined in the liner.
There is an access space between the jacket and scavenge trunk and
any water leakage from the gland can b e detected here. A similar
gland with two more seal rings is fitted where the liner enters the
Lubricator connections to the liner are positioned within the
access space. Cooling of the liner is necessary to reduce thermal
stress within the material. It
will also limit thermal expansion in order to maintain clearance
of piston. The reduction in surface temperature of the liner will
allow adequate lubrication of this surface, ensuring gas seal and
reduced liner and piston ring wear.
In engines with exhaust ports. further seals must be made in the
jackets because of these. Such seals will then ~ncludt. copper
rings to assist location of the liner.
Q. Describe how a new liner is fitted to ,i large tuo- \ t roke
dieqel engine. State any checks made and the procedure \ \hen
br~ngirig the e n g n e back Into \er\Ice.
A. It will be necessary to remove the old l ~ n e r \\ hich ma!.
be c a r r ~ e d out as follows: Cylinder cover together with
valves. operating gear and connections must first be
removed and landed safely. Piston and rod are then removed. The
cylinder jacket is now drained and cylinder lubricator connecting
removed. A strongback is now fitted to span the top of the liner
and is supported on the
jacket at each end. Long bolts pass through the strongback to a
crossbar fitted at the lower end of the liner (Fig. 28).
By tightening the nuts on top of the strongback o r by applying
oil p r m u r e to jacking nuts. the liner can be 'started' from
its landings. Strongback nuts are follcnved down to grip the liner
axially between strongback and crossbar. The crane is attached and
it may now be lifted clear.
Cooling spaces and landing wrfaces of jacket \hould bc cleaned
and ~nspected. I t is advisable that. after cleaning and close
inspection. the ne\\ liner 1s gauged
before fitting. The liner should be trled in position without
seal rings to ensure clearance. The strongback. now ~nver t ed .
and the crossbar should have been fitted to the new liner.
Correct rubber seal rings are fitted in the appropriate grooves
and may be smeared with a lubricant such as tallow: jointing
compound mav be applied to the
sealing surfaces of the 11ner flange. The new liner may now be
lowered carefully into the jacket. Final l a n d ~ n s ma! be
carried out by nuts on cover studs drawing the strongback down
until the liner has landed securety on its joint faces. Care must
be taken to align the liner circumferentially with the markings on
the jacket; this will locate correct positioning for lubricator
connections, etc. In some engine liners the ports will differ
around the circumference.
The new liner should be regauged after final landing to check
any distortion. These gaugings should be recorded to assist later
estimates of liner wear, etc. to be made. Lubricator connections
are refitted and lubricators tested.
New piston rings should be fitted with a new liner and all
running-in procedures of reduced fuel and increased lubrication
should he carried out.
After the cylinder co\ er is hardened down. a water test must be
carried out on the jacket, lubricator point\ and \eal r l n p .
For engines in nhich c\llndrr liners land on a head ring on the
entablature, the lifting strongback ma! he iscured d~rectly to the
head ring and this, together with the liner, may then be lacked
from the entablature without the necessity for a crossbar.
Q. How is wear in 'I c\ l~nder I~nsr nien~ured" Give causes of
llner wear. What is the effect of running an enpnz n ~ t h more
than the recommended maximum wear?
A. Wear in a cylinder liner iz mainly due to friction, abrasion
and corrosion. Each o f these may have a number of causes.
Frictional wear takes place between the sliding surface of
cylinder liner and piston rings. This will depend upon the
materials involved, surface conditions, efficiency of cylinder
lubrication, piston speed. loading of engine with corresponding
pressures and temperatures, maintenance of piston rings, combustion
efficiency and contamination of air or fuel.
Corrosion occurs mainly in engines burning heavy fuels,
particularly with high sulphur content. It is caused by acids
formed during combustion and these must be neutralized by the use
of alkaline type cylinder oil. Sulphuric acid corrosion may be
caused in the lower part of the liner if the jacket cooling water
temperature is too low. This may allow moisture to condense in the
cylinder, forming sulphuric acid. It can be prevented by
maintaining jacket temperatures above the corresponding
General corrosion may also be increased if the charge air cooler
is undercooled, allowing condensed water droplets to be carried
into the cylinder with scavenge air.
Abrasion may take place from the products of mechanical wear,
corrosion and combustion-ali of which form hard particles.
Cylinder liners should be gauged internally at fixed intervals
during cylinder overhaul (6000-8000 hours) to measure accurately
the increase in cylinder bore. Continuous records of gaugings
should be kept for each cylinder.
The liner must be cleaned and inspected. General appearance of
the surface may show whether lubrication has been adequate.
The liner is now gauged with a micrometer and extension bar
which has been calibrated against a master gauge. The liner should
preferably be cold. but if this is not possible, the gauge must be
at the same temperature as the liner to cancel expansion
Fig. 29 Ltner gauging
I F I ~ 30 Record of w e a l Gaugings are taken at a number of
vertical positions (6 to 8) over the area swept
by piston rings. Readings are taken in fore and aft and in
athwart-ships directions. To ensure readings are taken at
corresponding points. a template may be used. Gauging figures are
noted as total wear from original and mean rate of wear since the
last recording was made.
The pattern of wear over the length of the liner will differ
according to engine type but in single-acting, two-stroke engines,
it tends to be greatest at the top of the stroke adjacent to the
combustion space where pressure and temperature are greatest. This
reduces towards the lower end of the stroke, but will increase in
way of exhaust and scavenge ports where relative pressure on port
bars is increased and blow past may remove lubricating oil film
The rate of liner wear varies over the life of the liner. It is
high during the initial running-in period after which it should
reduce to an almost constant rate for most of i the useful life of
the liner. Finally the wear rate will progressively increase as
wear becomes excessive (Fig. 32) .
Normal wear rates differ but an approximate figure of O.lmm per
1000 hours is usually acceptable. Wear rates will be increased if
the engine is over-loaded. Maximum wear before renewal is usually
limited to 0.6-0.8 per cent of original diameter. The time when
this figure will be reached can be anticipated from wear records,
and advance ordering of replacements can be made.
After gauging, any ridges on the liner should be ground off.
These may be evident at the top of the ring travel and at port
bars. Ridges may be due to broken piston rings or where the piston
has been raised to readjust compression. If new piston rings are
fitted, the liner should be de-glazed. T o allow running-in,
cylinder lubrication should be increased temporarily.
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P I S T O N R I N G
Fig. 3 1 Liner wear pattern
T I M E -
Fig 12 Liner wear rate
Ports must be cleaned. ,harp edges removed, lubricators tested
and a close inspection made f o r possible cracks before putting
back into service.
Chromium plated liners may be used to reduce wear rates. These
have deposits of porous chrome which retain lubrication while
reducing wear and corrosion. Liner life may be extended in this way
but the initial cost of the liner is increased.
If the cylinder is operated with excessive wear. the rate of
wear will rapidly increase. Gas blow past may remove lubr~cating
oil film. piston rings may distort and break. piston slap may cause
scuffing. Compression may be reduced causing incorrect combustion
with fouling of the exhaust system. carbon may be formed at exhaust
ports, and oil blown into the scavenge will increase the risk of
Q. How is cylinder liner lubrication carried out: (a) In large
bore crosshead type engines (b) In trunk piston engines?
What are the effects of insufficient or excessive cylinder
A. Cylinders liners require adequate lubrication in order to
reduce piston ring friction and wear. The oil film also acts as a
gas seal between liner and rings and as a corrosion inhibitor.
(a) In large crosshead type engines the cylinder is isolated
from the crankcase and a separate cylinder lubrication system is
fitted which supplies a measured quantity of oil to each liner.
Special highly alkaline cylinder oils are used when burning
heavy fuel and since these properties are expended in use, any oil
recovered from drains must not be used further.
Oil is injected through a number of holes drilled in the liner,
there are usually six or eight of these, displaced
circumferentially around the liner at a chosen vertical position
within the piston stroke. Oil is supplied by pressure pulse from
mechanical lubricators driven from the engine camshaft and
regulated to deliver at the required rate. Lubricator quills are
connected to the oil holes, these contain non-return valves to
prevent hot gases from the cylindcl- blowing back Into the system.
They may pass through the jacket cooling space. in ~vhich case
water seals must be fitted. These should be tested periodicall!.
The ~er t i ca l position of the lubrication points will depend
upon the cnglne design. The! should be clear of the combustion
space with its high pressure5 and temperatures but should also be
clear of exhaust or scavenge ports in the liner. since unused oil
ma) be lost through these.
Distribution of oil around the liner circumference may be aided
by oil gutters adjacent to the lubricator points and angled
downwards to assist flow by gravity, while reducing piston ring
chipping effect. Some engines are fitted with an oil spreading ring
in the piston. Lubricating oil is spread over the length of the
liner by the piston rings during their stroke.
Ideally lubricators should be timed to inject oil between the
piston rings as they pass. In practice. due to the elasticity of
the system. this accuracy is difficult to achieve. while out of
phase timing may cause a greater loss of oil to be swept into the
ports. Consequently. timing of lubricators is not generally
Mechanical lubricatorz should be operated by hand before
starting the engine to ensure prlmlng of the connection\ and
injection of oil for the first engine move- ment.
The supply should he ~ncrcdwd during running-in periods for new
cylinder liners or new piston rings.
In opposed piston or exhaust piston engines. two sets of
lubricator points are fitted toeach liner. one
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C Y L I N D E R L U B R I C A T O R Q U I L L
I N L E T
N . R . V A L V E
-1 S P L A S H L U B R I C A T 1 9 \
cylinders will also drain into the crankcase and consequently a
common oil is used for both cylinders and crankcase.
Operation of an engine with insufficient cylinder lubrication
will cause high wear rates to liner and pisto2 rings. Corrosion of
the liner may increase when burning heavy fuel. Loss of oil seal to
the piston rings will cause blow past of hot gases causing local
overheating, rapid breakdown of surfaces and possibly piston
seizure. In trunk piston engines there is also danger of a
crankcase explosion. Excessive lubrication will cause carbon
deposits. piston rings sticking in grooves allowing possible
breakage or blow past. There will be fouling of the exhaust system
including turbo-charger and contamination of scavenge spaces.
Q. How are cylinders isolated from the crankcase in large
crosshead type engines, and why is this desirable? Describe the
necessary maintenance and give effects of incorrect
L -- - - -- - - -- -- -- --
I A. In large crosshead type diesel engines a diaphragm is
fitted to isolate the lower end of the cylinder from the crankcase.
This is desirable to prevent contamination of the crankcase
lubricating oil with residue from combustion, corrosion, wear, used
cylinder lubricating oil and exhaust gases which may be blown past
the piston rings. The use of a diaphragm allows the choice of a
separate special oil for cylinder lubrication which. when burning
heavy fuel, requires high alkalinity and other properties not
compatible with its use in the crankcase. Mineral type oils with
better lubricating and cooling properties may then be used for the
When burning heavy fuel in engines not fitted with diaphragms,
it is found that contamination of the crankcase oil causes
corrosion of white metal bearing surfaces during operation. and
corrosion of steel journals while the engine is stationary. This
leads to destruction of fine bearing surfaces, bearing failure.
choking of oil passages, etc.
The diaphragm in some engines is used to seal off the scavenge
space and by the addition of non-return valves is used for under
piston scavenging to assist the turbo-charging, particularly during
It may act as a support for telescopic pipes and glands for
piston cooling water i I connections.
! The piston rod must pass through the diaphragm and a piston
rod gland is fitted.
Fig. 34 Piston rod gland
This consists of inward sealing metallic packing and oil scraper
rings. Each ring consists of three or four segments which are a
good fit on to the piston rod surface and are held by a coiled
garter spring. There must be sufficient circumferential butt
clearance between segments.
The gland consists of two sections, and rings in the upper
section act as seal rings for scavenge pressure and scrape off any
residue or dirt from the piston rod on its downward stroke. This
contaminated oil residue should be conveyed to the sludge tank.
The lower section rings act as oil control rings and scrape off
excess crankcase oil from the piston rod during the upward stroke.
This oil is returned via drains to the crankcase system.
A void or vent space is left between the two sections and the
drain from this should be inspected regularly to ascertain the
efficiency of the gland.
Maintenance of the gland consists of maintaining correct
clearances of ring segments in circumferential (butt), axial, and
radial directions; checking tension of garter springs and keeping
drains and vents clear. Particular care must be taken during
removal of piston and rod to prevent damage to the gland.
Incorrect, or lack of, maintenance may lead to contamination of
scavenge space with oil, loss of scavenge air, contamination of
crankcase oil and damage. with possible overheating of the piston
rod, leading to danger of a hot spot within the crankcase and risk
of eventual explosion.
Q. Sketch and describe a piston for a large crosshead type main
engine. State materials used. Why 1s cooling necessary and how is
it carried out? Give the advantages of oil or water cooling for
= I : , 3 5 3 1 " T S
L O C K I N G P L A T E
A. The piston shown in Fig. 35 is for a large two-stroke,
crosshead type main engine. The piston is cast of heat resisting
alloy steel containing chromium and molybdenum to maintain strength
at high temperatures and resist corrosion. The use of such material
allows the top of the piston crown to be thin enough to ensure
adequate cooling while strong enough to resist the high pressure
gas load. It is shaped to assist flow direction of gases during
scavenging, and is supported and further cooled, by internal
The cylindrical wall of the piston is shaped internally to
ensure cooling but is thickened to accommodate the piston ring
grooves. The external shape is tapered slightly above the top ring
groove to allow some distortion during combustion conditions.
There are five piston ring grooves, each of which has its lower
wear surface chromium plated to resist wear.
The piston is water cooled internally with fresh water which
enters and leaves through reciprocating pipes and glands. The water
outlet in the piston is set near the crown to ensure that the
piston remains full of water at all times. Drainage connections are
made from the water glands to prevent any water leakage from
entering the cylinder or crankcase.
The piston cooling space is closed by a bolted cover fitted with
rubber seal rings to prevent leakage. Rubber seal rings are also
fitted at the attachments of reciprocating cooling pipes and
between piston and the piston rod flange. A short cast iron piston
skirt is secured between the piston rod flange and the underside of
the piston, aspigot and rubber ring sealing this junction. The
skirt is uncooled and acts as a guidewithin the liner. It has two
leaded bronze bearing rings caulked into grooves to prevent
possible damage between skirt and liner. The lower edge of the
skirt also reduces loss of scavenge air to exhaust ports.
The piston rod is of forged steel and its top flange is attached
to the piston by a number of long piston studs. These are fitted
with distance washers to improve their resilience. Nuts are locked
after hydraulic tightening. The lower end of the rod is stepped to
pass through the crosshead, the piston rod nut being hydraulically
tightened and then a locking device is fitted. A locating dowel is
fitted and the lower end of the rod is counterbored to improve
resilience of the screw thread.
Cooling of a piston is necessary to remove excess heat from
combustion and to limit thermal stressing. I t also limits thermal
expansion to maintain correct clear- ances between piston and l ~ n
e r and between piston ring grooves and rings. Cooling may be
carried out b! circulat~ng e ~ t h e r Lvatsr or oil.
Fresh water cooling has the ad\ antage of greater thermal
capacity than oil; it may also sustain higher outlet temperatures
(up to 70C) improving thermal efficiency. Inhibitors are necessary
to pre\ent corrosion in the system and adequate venting must be
Water cooling has the disad\.antage of requiring flexible
connections and glands to convey it to and from the piston: these
glands require maintenance and there is the possibility of
crankcase contamination from gland leakage in some engines.
Oil cooling has lower thermal capacity and a lower temperature
limit (56C) to prevent carbon or lacquer forming on hot surfaces,
with consequent loss in heat transfer or choking of passages. This
may require an increased through-put. The system may form part of
the crankcase lubrication system using the same oil and a common
connection to crosshead and piston cooling may be employed. Simple
glands may be used for piston connections and there is no danger of
ENGINE PARTS P ",
crankcase. There may be some deterioration in oil condition due
to the thermal " - - cycling.
In either system periodic cleaning and inspection of internal
cooling surfaces must be carried out.
When operating cooling systems the rate of circulation should be
maintained to keep the system flooded. Necessary adjustments can be
made to temperatures.
Cooling of jackets and pistons must be maintained for aperiod
after the engine is stopped in order to allow gradual reduction in
temperatures and thermal stresses.
Q. Describe the operations required for removal of a piston and
rod from a crosshead type engine. State what inspections and
adjustments should be made. A. Before removing the piston it will
of course first be necessary to remove the cylinder cover. valves
and connections. If carbon has been deposited around the top of the
liner, this should be cleaned off before attempting to lift the
Engine turning gear should be used to turn the piston to top
dead centre position. Screw threads in lifting holes in the piston
crown must be cleaned and the piston lifting plate or brackets
securely bolted into place; the crane is then attached. After
removal of its locking device, the piston rod nut or nuts securing
it to the crosshead are slackened hydraulically and removed.
The weight of the piston and rod is taken on the crane and then
turning gear is used to turn the engine and lower the crosshead
clear of the piston rod. Care must be
E N G I N E 9 0 0 M C R A N E
Fig. 36 Changing a piston
1 . ENGINE PARTS taken to protect threads from damage. The
piston and rod may now be lifted by the crane. In some engines, the
piston rod gland is freed from the diaphragm and lifted with the
rod, in others special devices prevent damage as the rod is lifted.
Where piston cooling connections are fitted, care must be taken to
avoid damage or misalignment.
When piston and rod are clear of the liner they may be landed
for inspection, removal of rings and cleaning.
The piston should be inspected externally for wear, corrosion
and cracks. The crown should be gauged to reveal any burning or
corrosion. Careful examination should be made for fatigue cracks.
Piston ring clearances are gauged and then the rings removed by
means of the retractor or tensioner. Ring grooves are cleaned,
examined and gauged for wear or taper, particular care being taken
with flatness of landing surfaces.
Internal cooling spaces of the piston should be cleaned and
surfaces inspected. This will require removal of the piston rod and
seals. Lockingdevices on piston studs are dismantled and nuts
slackened hydraulically. Studs must be inspected and tested.
r All surfaces must be cleaned and new seals fitted. After
reassembly, a pressure test of cooling spaces should be carried
The piston rod should be inspected for wear and cracks,
particularly at its connection with the crosshead.
If new piston rings are to be fitted. they must be gauged for
gap in the unworn part of the liner. and axial clearance and free
movement in grooves must be checked. Ring
. gaps should be spaced at 180' in alternate grooves. After
cleaning and gauging of liner. etc.. the piston and rod may be
guide ring is placed at the top of the liner to assist entry and
all necessary surfaces are ?lubricated. During lowering, equal care
must be taken of threads, glands and cooling systems. The maker's
instructions should be carried out, particularly regarding
slackening and tightening of all bolts and studs. Locking devices
must be fitted correctly.
The utmost safety must be observed at all times. Lifting gear
must be kept in good order and all parts made secure. Air starting
system must be shut off before engaging I the engine turning
I Q. What materials are used in the manufacture of piston rings
for large slow running diesel engines'? How are these fitted to the
pistons? State the clearances that are necessarq and the reason\
Give possible causes for defectite piston rings in service and
the effects on operation of the engine.
A. Materials for diesel engine plston rings must have good
strength, elasticity and wear resistance with low friction. and
must maintain these properties at high working temperatures. They
must resist corrosion. readily transfer heat and have thermal
expansion compatible with the piston in order to maintain ring
For large two-stroke engines. most rings are cast and machined
from pearlitic grey cast iron. This may include some additions such
as chromium, molybdenum, vanadium, titanium, nickel and copper. In
some engines spheroidal graphite iron is used, which has greater
hardness and tensile strength.
The cross-section of piston rings is rectangular with small
radii on all edges. This allows an oil wedge to build up on the
outer surface and prevents sticking at the back of the ring groove.
The section may vary adjacent to the ring butts.
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C I R C U M F E R E N T I A L O R B U T T C L E A R A N C E
6 1 R I N G S E C T I O N - - _ _ _ --
F I ~ 37 Piston rlngs
Rings are machined either circular or to a cam shape from which
they expand to form a circle a t working temperatures and avoid
port chipping. A radial cut is made and the shape of ring joint
ends may be butt (vertical). which gi\es a robust joint for top
rings; scarfed (diagonal). giving better gas seal but i. less
robust: or a formof lap or bayonet joint which gibes a good gas
seal but is more \ ulnerable to breakage and is only used in some
lower rings (Fig. 3 7 ) .
Piston compression rings are fitted In corresponding ring
grooves machined in the piston. They will bear heavily on the
lo~ver surface. or land of the groove during the power stroke and
these lands must remain true or rings will distort. T o reduce wear
on the groove lands, they may be chromium plated, heat treated, o r
have wear rings fitted.
Rings are sprung outwards to pass over the piston and released
into the grooves which have been oiled. The rings should then float
freely within the whole depthof the grooves. A special retractor
should be used to give even bending moment without twisting the
ring section. Excessive bending should be avoided.
In use piston rings convey heat to the cylinder liner and act as
a gas seal between this and the piston. They must be free to follow
the liner surface irrespective of transverse movement and will
build up an oil wedge on the liner, reducing wear and spreading the
cylinder lubrication. The outward pressure is initially due to
elasticity in the compressed ring but is increased by gas pressure
which acts on the back of the ring. This pressure is greatest in
the top rings, the temperature is also