Marine Engineering Series

Diesel Engines

A. J. Wharton

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Diesel Engines

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Diesel Engines
Questions and Answers
A.J. Wharton, CEng, FiMarE

Foreword by

W. D. Ewart, CEng, FiMarE, MRINA
Editor-in-Chief—Fairplay International Shipping Weekly

STANFORD

MARITIME

LONDON

Stanford Maritime Limited
Member Company of the George Philip Group
12-14 Long Acre London WC2E 9LP

First published 1975
Reprinted 1977, 1980

© 1975 A. J. Wharton

Printed in Great Britain by
J. W. Arrowsmith Limited Bristol

ISBN 0 540 07342 3

Foreword
Written by experienced lecturers at 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 concise and simple manner suitable for individual
study—an important feature for engineers studying at sea.

W. D. Ewart

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
be 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 such detail.
Questions tend to be concentrated upon large 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. No one engine has been used in this book
but simplicity of diagrams, together with a wider field of 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/cm’ or atmospheres are still used at
sea for pressure measurement.
It may be convenient to remember that

kN/m’* + 100 = kg/cm’ or atmospheres, approximately.
A. J. Wharton

Contents
1

Engine types

2

Cycles and timing

3

Gas exchange processes

19

4

Engine parts

29

5

Operating systems

Bie.

6

Control

(p

7

Safety and operation

WS

Index

87

SI UNITS
Mass

=kilogramme

Force

= newton

1
9

(kg)

Length
= metre
Pressure
= newton/sq metre (N/m’)
Temperature = degrees celcius (°C)

CONVERSIONS
i inch = 25-4mm = 0:025m
1 foot = 0-3048m

1 square foot = 0-093m7

‘1 cubic foot = 0-028m*

1 pound mass (Ib) = 0-453kg
1 UK ton (mass) = 1016kg
1 short ton (mass) = 907kg
1 tonne mass = 1000kg
1 pound force (Ibf) = 4-45N
1 ton force (tonf) = 9:96kN
1kg = 9-81N

0-001in = 0:025mm
(CF= 32) x 2=°C

Lbf/in* = 6895N/m? = 6-895kN/m?*
1kg/em? = 1kp/cm? = 102kN/m?
1 atmos = 14-7Ibf/in? = 101-35kKN/m?
1 bar = 14-5lbf/in? = 100kN/m?
Note: For approximate conversion of pressure
units

100kN/m* = 1 bar = 1kg/cm* = 1 atmos
1tonf/in? = 15440kN/m? = 15-44MN/m?
1HP = 0-746kW

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SECTION

1

Engine Types
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 exhaust timing.
FUEL

VALVE

WATER

PISTON

JACKET

&

SKIRT

PISTON
ROD
oe
Git ANOS

oe

CAMSHAFT

PISTON
RECITES

1EF

VALVE

T

COOLING
PiiPES

~~ FRAMES

CRANKSHAFT
22

WS.

BEDPLATE

Fig. 1 Large bore two-stroke engine

ENGINE TYPES

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 overhauls.
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.
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 frames.
Direct

reversing

is catried

out

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 propeller shafting.
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 postscavenge 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. Valve seats are stellite-faced

ENGINE TYPES

EXHAUST
—FUEL

VALVE

PUMP

-CAMSHAFT
Soa

tf

tps

SCAVENGE
PORTS

AIR
MANIFOLD

PISTON
GLA ND

——

ROD

CROSSHEAD

GUIDES

CRANKSHAFT
BEDPLATE

Fig. 2 Large bore two-stroke poppet valve engine

to reduce burning and corrosion. Valves are operated from the camshaft by push rods
and rockers, tappet clearance being allowed for thermal expansion.
Three fuel injectors are fitted to each cylinder, equally spaced around the cylinder
cover with fuel spray patterns directed 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 te the crankcase. The crosshead passes lubricating oil to the bottom end
bearings through passages in the connecting rod.

ENGINE TYPES

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 asun 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
UPPER
PISTON
COOLING
WATER

UPPER

PISTON

3-PART
LINER
LOWER PISTON
COOLING OIL

<a

a

ie
E

|

i

LOWER
PISTON

as

ei
|

SIDE ROD
CROSSHEAD

SCAVENGE
PORTS
GUID

og

RELIEF
VALVES

MAIN
CROSSHEAD

BEDPLATE

Fig. 3 Opposed piston engine

ENGINE TYPES

power, the number of cylinders and engine length may be reduced. It should be
pointed out, however, that the engine height is increased.
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, however, when the engine is reversed and for this reason the
angle of lead between cranks is limited. To restore balance of engine, the stroke of

the upper piston is reduced.
The engine illustrated 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 up and down from the central combustion space,
rubber seal rings being fitted at the end of each jacket.
Q. Medium speed, four-stroke trunk piston engines may be used for main
propulsion. Compare this type of engine with a large bore slow running engine. What
advantages are obtained from Vee engines?
A. The main advantages of medium speed main engines are their improved
power/weight and power/size ratio compared with large slow running engines. They
also tend to have a reduced initial cost for equivalent power.
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

ENGINE TYPES

VALVE

GEAR

r ma
“tt.
Soy
i
ay
—
|
_

AIR
T=

ral
ne |Sllflles

SLE
if
ke

‘e
SA
Acai

wanes

JACKET

CAMSHAFT

Wea
QE
GEN
REA

Fig. 4 Medium speed trunk piston engine

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 cylinder covers become complex with the increase in
the number of valves required. Cylinders are robust and higher rates of turbocharging can be employed, giving an increase in power developed.
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.

ENGINE TYPES

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 at
sea.
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
VEE

VALVE

GEAR

’

ENTABLATURE

/

PISTON

FUEL

Re

PUMP

CAMSHAFT

LINER

CAMSHAFT

MAIN
BEARING

TWO BEARINGS
SIDE BY SIDE *
ON CRANK
THROW

BEDPLATE

Fig. 5 Medium speed Vee engine

ENGINE TYPES

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.

SECTION

2

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 may 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 length/crank length, stroke/bore ratio,

engine speed, engine rating, etc. Fig. 6 shows a typical two-stroke cycle with the
operations numbered.

COMPRESSION

Fig. 6 Two-stroke cycle

4

5

CYCLES AND TIMING

1-2
2-3

3-4

4-5-6

1-etc.

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 (TDC).

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.

EXHAUST
ASPIRATION
COMPRESSION

9

Fig. 7 Four-stroke cycle

10

=

3

4

CYCLES

AND TIMING

Numbering these operations in sequence on the timing diagram:

1-2
2-3
3-4-5
5-6

6-7-8
8—9-10

10-1
l-etc.

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 pressure. 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 slow running large two-stroke diesel engine.
How are diagrams taken and what information can be gained from them? How is
power balancing carried out?
A. An indicator diagram can be obtained from a diesel engine during its operation,
by use of an engine indicator (Fig. 8). The diagram represents a pressure-volume
diagram taken from conditions within the engine cylinder. Similar diagrams are
taken for each cylinder of the engine.

HANDLE

i

AWS

BiSN
/|
SS
|

MOTION
GAS

PRESSURE

Fig. 8 Engine indicator

11

CYCLES

AND 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 compression
spring. Corresponding cylinder swept volume is recorded on a horizontal scale due to
rotation 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 may be used to calculate the power produced or the mean indicated pressure
(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
atmospheric line and the highest point on the diagram.

;

MAXIMUM
PRESSURE

|

aS
ATMOSPHERIC

LINE

Fig. 9 Power indicator diagram

4

|

COMPRESSION
PRESSURE

Fig. 10

Compression diagram

Compression diagram (Fig. 10). 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
12

CYCLES

AND TIMING

liner, faulty piston rings, insufficient scavenge air or leaky exhaust valve, any of which
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
with piston stroke. This illustrates more clearly the pressure changes during fuel
combustion. Fuel timing or injector faults may be detected from its shape.

MAXIMUM
PRESSURE

ATMOSPHERIC

Fig. 11

LINE

Out of phase diagram

COMPRESSION

ATMOSPHERIC

LINE

SCAVENGE
OPENS

Fig. 12

Light spring diagram

Light spring diagram (Fig. 12). Again similar to the power card and in phase with
the engine, but this diagram is taken with a light compression spring fitted to the
indicator showing pressure changes during exhaust and scavenge to an enlarged
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 diagrams, or MIP, from each cylinder.
Fuel pump 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 power produced
within that cylinder. Such adjustments may be carried out to obtain equal power
from each cylinder, in which case the area of each power card will be equal. Steady
running conditions must be maintained while power balancing is carried out.
13

CYCLES

AND TIMING

Exhaust temperatures should be noted during power balancing since a limiting
exhaust temperature may make complete balancing difficult.

Q. How can valve settings on a large engine be checked while the engine is
operating at sea and how is fuel injection timing ascertained?
A. While an engine is running at sea, valve or fuel injection timings can only be
checked by instrumentation. Incorrect timings will affect power output and exhaust
temperatures and, provided these 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).

ATMOSPHERIC

LINE

Fig. 13 Diagram showing early and late exhaust

EARLY

IGNITION

Fig. 14

LATE

IGNITION

Diagram showing 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.
14

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 oscilloscope 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 or its drive mechanism is at 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 remedies.

(a)
(b)
(c)
(d)

Afterburning
Early firing
Choked fuel valve
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 combustion. There will also be a higher gas pressure at blow down which will increase pulsing
in the exhaust manifold.
Afterburning is detected by loss in power, 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 a reduction 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 turbocharger 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 afterburning may be incorrect fuel pump timing, faulty fuel injector,
heavy fuel oil temperature too low, lack of scavenge air or poor compression.
-

“\

JOS NORMAL

Fig. 15 Diagram showing afterburning

i)

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 with a corresponding knock from the engine. Thermal efficiency of the cycle
will be increased and power will be raised but exhaust temperature will be reduced.

Fig. 16 Diagram showing early ignition

Early firing can be detected by the engine knock and can be confirmed by an
indicator diagram draw card, (Fig. 16) which shows a high peak pressure. It may be
caused by early injection, by incorrect fuel condition, overheated parts such as a hot
piston, or even a scavenge fire causing local heat.
Remedies include checking and correcting fuel pump timing, maintaining correct
fuel temperature, and cooling of working parts. If not corrected, the shock loads may
cause damage to, or failure of bearings.
(c) Choked fuel valve may be due to contamination in the fuel in which debris
may choke the small atomiser holes in the injector. Alternatively it may 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 temperature.

Fig. 17

16

Diagram showing choked fuel injector

CYCLES

Fig. 18

AND TIMING

Diagram showing leaking piston 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 cylinder 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 with correct clearances. Maintain cylinder lubrication and
avoid overload.

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 opportunity.
“Fuel injection equipment is particularly important and fuel pump settings,
clearances and timings must be checked and maintained during periods when the
engine is out of service. Fuel injectors 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 service. 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.
1 7/

CYCLES

AND TIMING

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 amounts of 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 cylinder.
As a check that operating conditions in each cylinder are normal, peak
compression pressures may be taken and compared in a similar manner.
Some further evidence of power balance can be taken from the corresponding
position of fuel pump racks and also from the comparison of exhaust temperatures.
Exhaust temperatures are unlikely to be equal in a multi-cylinder engine,
particularly when turbo-charged, but they will tend to follow a pattern. When this
relative pattern is established it can be assumed the engine is power balanced.
Pump adjustment is limited to prevent fuel delivery at stop setting.
ADJUSTMENT
BALANCES
GAS
PRESSURE

Za

K&
E

BALAN

OmMe
es
ty
es
Say
eee,
aeea
awe

PEAK
PRESSURE
SCALE
RBRLRRBURERT:
CAL

Fig. 19

Peak pressure indicator

Cooling water return from each unit should also be approximately equal in
temperature.
If an engine operates in an unbalanced condition, some bearings and running gear
may become overloaded, this may cause overheating and bearing failure. Overload
in cylinders may cause piston blow past, with the corresponding dangers of
overheated or seized pistons. Unbalance will also set up vibrations which, if
maintained for prolonged periods, will cause fatigue from the fluctuating stresses
induced. This may in turn lead to fatigue cracking of metal in bearings, fracture of
bearing studs or bolts, cracks in crankshaft 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, lubricating oil and turbo-charge pressures.
The exhaust should be clear of smoke and there should be no unusual noise or
vibration.

18

SECTION 3

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
cooling 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 allows only a very short period of time for scavenge to be
completed.
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 system.

EXHAUST

SCAVENGE
AIR

N
NNy
&
N=
Ny

N

S

SKIRT

ogZ
33
:
34
Hy4
Fig. 20

PISTON

Loop scavenge

19

GAS EXCHANGE

PROCESSES

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 power.
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 within 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).

affinity
‘a

aS

EXHAUST

SCAVENGE
AIR FROM
>
NON-RETURN
VALYV ES

WD

PISTON
EEE'
BERS
ALES

BEUEEBEa'
BBEEEY

SKIRT
LBBBaaBanuel

Fig. 21

20

Cross scavenge

GAS EXCHANGE

PROCESSES

EXHAUST

A}

ee

Vee

SZ
5

S*.

SeER

Zl

VITA
ea
tA
TPT

AIR

PISTON

a
ee
ee

Fig. 22

SCAVENGE

Uniflow scavenge

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 and2a
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 stroke/bore 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 piston rings in 2. In single-acting engines,
however, it will require the fitting of exhaust valves together with the necessary
operating gear and maintenance.
In all engines the scavenge trunking must be kept drained, must be regularly
#
inspected and maintained in a clean condition.

Q. What are the advantages of turbo-charging a two-stroke cycle main engine?
Describe and sketch such a system 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
21

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 engine.
5
A simple system is shown in Fig. 23, which consists of a turbo-charger and charge
air cooler.
TURBOCHARGER

TURBINE
/

EXHAUST
COMPRESSOR

VALVE

F
NG
%

NY
NY

NO

Ni
Ni

CHARGE
COOLER

7

AIR

NY
NY

NA
Ne

He|
iy

s

3
AIR

~~

SCAVENGE
PORTS

MANIFOLD

Fig. 23.

Turbo-charge system

Exhaust gas from the cylinder operates the gas turbine, giving up some of its heat
energy to do 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-char¢ger 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 it 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 may 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:
2

GAS EXCHANGE

PROCESSES

1. Fitting of engine driven scavenge pumps in series with the turbo-cha
rger. 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
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 lubricated?
A.

Fig. 24 shows a section of a turbo-charger as fitted to large bore engines.
DIFFUSER
VGEUIHE

EXHAUST
YRINTH

_—- COOLING
SPACES

IMPELLER

Fig. 24

Turbo-charger

It consists of a single stage, axial flow exhaust gas-driven turbine mounted on a
common shaft with a centrifugal air 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
23

GAS EXCHANGE

PROCESSES

to 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 reduce noise.
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 or assists cooling of the
turbine disc. The seals prevent possible oil leakage into the turbine or 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 gases.
Two shaft bearings are fitted, one at each end of the shaft allowing accessibility
and cooling. End thrust is taken at the compressor bearing, the turbine bearing
allowing expansion adjustment.
Bearings may be either of plain sleeve type with copper lead bushes on hardened
steel shaft sleeves, or 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 intervals (say 8000 hours).
Lubrication of the bearings may be by various means. Ball and roller bearings
may be lubricated by self-contained gear type pumps operated from the shaft and
drawing oil directly from the independent bearing sump. Oil level must be
maintained in these sumps and the oil should be renewed at stated intervals.
Alternatively the bearings may be lubricated by external systems. Either by
connections from the engine lubricating system, through a fine filter, or alternatively
by an independent system consisting of pumps, cooler, filters, oil sump and alarms.
Updraught in the exhaust system may cause the turbo-charger to rotate while the
engine is out of service. Care must be taken that damage to the bearings is not caused
by lack of lubrication.
Independent systems allow a choice of turbine oils to be used. All lubrication
systems must maintain adequate lubrication when tilted to an angle of 15° in any
direction or a temporary tilt of 223°. Under normal conditions the turbo-charger
shaft should be horizontal.
Q. Why are coolers necessary after a turbo-charger? Sketch and describe such a
cooler. What are the effects if undercooling takes place?

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 between the turbo-charger and the engine inlet manifold.
It causes increased air density at lower induction temperature. The engine is
maintained at safe working temperatures and the lower compression temperature
reduces stress on piston rings, piston and liner.
24

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
SEA

IN

WATER

OUT

DIVISION

PLATE

| ETTEE: 5
FIXED
PLATE

AIR

TUBE

IN

oe

Dun
—-m
2>Or

MOVING
PLATE

TUBE

TUBE
PLATE

SEAL
RING
WATER
DETAIL

OF

SEAL

RING

BOX
DETAIL

OF
TO

FINS SOLDERED
TUBES

Fig. 25. Charge air cooler

25

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 occurred.
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 55°C 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 20/25°C 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 pressure drop indicates fouling of the air passages, while
increase in water pressure drop indicates fouling of water side.
When necessary, charge air coolers can be cleaned while out of service. Water
side may be cleaned by removing water boxes and brushing through tubes. If this
does not remove the scale, acid cleaning may be carried out, after which it must be

flushed through.
Air side may be blown clear with a compressed air or steam jet and specially

shaped brushes may be used. If these do not remove

the dirt, the stack may be

immersed in a hot degreasing fluid and then blown clear.

After cleaning, a pressure test of about 400kN/m? should be carried out on 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 less than
recommended values?

A. (a) Air inlet and exhaust valves in diesel engines are arranged to open inwards
in order to maintain positive closing under pressure in the cylinder and ensure their
non-return action. Gas pressure will act upon the area of the valve lid to hold it
against the seat and supplement the closing action of the springs. This positive
contact of valve and seat removes carbon or other deposits on the valve seat which, if
26

GAS EXCHANGE

PROCESSES

allowed to build up, cause blow-by of hot gases and burning of vatves. 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 maintaining valve
alignment. Valve damage may also be reduced by depositing hard corrosion resisting
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 530°C, 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 slowly, thereby avoiding local overheating.

SPRINGS

SSSR

SX

N:VA
YI

BF
BF
5
FF
SW

COOLING
WATER SPACE

aee
A
FEE
3
A
AT
at

EXHAUST

VALVE

CAMSHAFT

Fig. 26

Exhaust valve

27

GAS EXCHANGE

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
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
continues 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 clearances.
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.

28

SECTION

4

Engine Parts
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.

CYLINDER

COVER

JACKET
LINER

SEAL

RINGS

ENLARGED
VIEW
OF SEAL

Fig. 27

Cylinder liner

29

ENGINE PARTS

The thickness of the liner must give adequate strength to resist gas load but
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 at the top of its stroke. Water from the main cooling system enters
the jacket at its lower end and leaves at 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 at this position. Tie bolts pass from the top of the jacket
to the transverse members of the engine bedplate, these transmit the gas load and are
pre-stressed to maintain the jacket in compression.
The portion of the liner encased in the scavenge space has a row of scavenge ports
which are uncovered by the piston at the bottom of its stroke. The liner is free to
expand downwards, a water seal being made at 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 be detected here. A similar gland with two more seal rings is fitted where the liner
enters the scavenge trunk.
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 include copper rings to assist location of the liner.

Q. Describe how a new liner is fitted to a large two-stroke diesel engine. State any
checks made and the procedure when bringing the engine back into service.
A.

It willbe necessary to remove the old liner which may be carried 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 quills
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 or by applying oil pressure to
jacking nuts, the liner can be ‘started’ from its landings. Strongback nuts are followed
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 surfaces of jacket should be cleaned and inspected.
It is advisable that, after cleaning and close inspection, the new liner is gauged
before fitting. The liner should be tried in position without seal rings to ensure
clearance. The strongback, now inverted, 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 may be applied to the

30

ENGINE PARTS

JACKING

NUTS

STRONGBACK

EJ

p

sal

=

ws

SS

a

—=

JACKET

194
tr
peer

sear
eee
IS

(ja

Ff
rer

joodocnnt

(a

al
Cpr

SOS
SSS
SSS
SS
SASSSSSE
[IGS
SES
See
SES
SS
terre
i
auanunet
Eye
Sooo

CROSSBAR

7corer
BRubsessnseumeuaseces’
BEBeeed

BIST

Saaeneee’
5

Fig. 28 Changing a liner

sealing surfaces of the liner flange. The new liner may now be lowered carefully into
the jacket. Final landing may be carried out by nuts on cover studs drawing the
strongback down until the liner has landed securely 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 be carried out.
After the cylinder cover is hardened down, a water test must be carried out on the
jacket, lubricator points and seal rings.
For engines in which cylinder liners land on a head ring on the entablature, the
lifting strongback may be secured directly to the head ring and this, together with the
liher, may then be jacked from the entablature without the necessity for a crossbar.
Q. How is wear in a cylinder liner measured? Give causes of liner wear. What is the
effect of running an engine with more than the recommended maximum wear?
A. Wear in a cylinder liner is mainly due to friction, abrasion dnd corrosion. Each
of 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.
Sil

ENGINE

PARTS

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 dew-point.
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—all 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 effects.

Fig. 29 Liner gauging

32

ENGINE PARTS

CYLINDER

1

NUMBER

=

|

DATE

OATE OF
LAST GAUGING
HOURS SINCE
LAST GAUGING

DATE FITTED
TOTAL HOURS

POSITION

PANOS

FANOA

ites

—

H
MAX.

WEAR

OVALITY

MEAN

MAX, WEAR

WEAR

RATE

SINCE

WEAR

RATE

SINCE

NEW
Al

LAST GAUGING
REMARKS

Fig. 30 Record of wear

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 (Fig. 31).
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
thé 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 0-1mm 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. To allow running-in, cylinder lubrication should
be increased temporarily.

38

ENGINE PARTS

POSITION
OF TOP
PISTON
RING
Ye aa

EXHAUST
PORTS

SCAVENGE

PORTS

Fig. 31

Liner wear pattern

—e
WEAR
MAX.
MEAN

TIME

Fig. 32

Ports

must

be cleaned,

—————_

Liner wear rate

sharp edges removed,

lubricators

tested and a close

inspection made for 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 lubricating 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 scavenge fires.
34

ENGINE PARTS

Q.

How
(a)
(b)
What are

is cylinder liner lubrication carried out:
In large bore crosshead type engines
In trunk piston engines?
the effects of insufficient or excessive cylinder lubrication?

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 cylinder blowing back into the system. They may pass
through the jacket cooling space, in which case water seals must be fitted. These
should be tested periodically. The vertical position of the lubrication points will
depend upon the engine design. They should be clear of the combustion space with its
high pressures and temperatures but should also be clear of exhaust or scavenge ports
in the liner, since unused oil may 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 attempted.
Mechanical lubricators should be operated by hand before starting the engine
to ensure priming of the connections and injection of oil for the first engine movement.
The supply should be increased 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 to each liner, one set situated within the stroke of each piston. Due to the higher
operating temperature, it is usual to supply a relatively greater quantity of oil to the
upper or exhaust piston.
(b) In trunk piston engines some cylinder lubricators may be fitted as in
crosshead engines. Due to smaller cylinder bores, there are usually fewer points
around the liner. Since most of these engines are four strokes, there will be no ports in
the liner through which oil may be lost. The majority of the cylinder lubrication will
be effected by oil splashing from the crankcase into the open lower end of the liner.
This would cause excessive lubrication and oil scraper or control rings are fitted to the
piston skirt to reduce this and return excess oil to the crankcase. Used oil from the

35

ENGINE PARTS

CYLINDER

LUBRICATOR

QUILL

LigekVor
aloe

N.R.VALVE

\

IS =SAAN

LINER

SPLASH

LUBRICATION

Fig. 33

Liner lubrication

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 piston 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 maintenance.

36

ENGINE PARTS

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 crankcase system.
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 manoeuvring.
It may act as a support for telescopic pipes and glands for piston cooling water
connections.
The piston rod must pass through the diaphragm and a piston rod gland is fitted.

PISTON

USE

DIAPHRAGM

OIL

ROD

D—q——[***

R SEAL
Ss

Ts
UR
ay

VENT

Na

Nd
NZ Z

ee
OIL
RETURN
TO CRANKCASE

GARTER

Fig. 34

SPRINGS

Piston rod gland

Si

ENGINE PARTS

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 is cooling necessary and how is it carried out? Give the
advantages of oil or water cooling for pistons.

LIFTING

SEAL
RINGS

HOLES

:

se.
=<

nS N
CawN

aN |
J

A=!

Val ~ J

NWA

Gin

4

4

PIPES

‘

COOLING

SSS

BEARING
RINGS

tt ff

SY

S

Y

=

COMPRESSION
RINGS

mM A

ill

WATER

5

zie

I

H

H

"

nt

Hi

Hl

|

SKIRT

H

q

i
J

4
i

qf
87,
sf Ae

N
EXTENDED

LOCKING

PLATE

Fig. 35

38

Piston

NUTS

/

/

ENGINE PARTS

|

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 ribs.
/
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, a spigot
and rubber ring sealing this junction. The skirt is uncooled and acts as a guide within
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. It also limits thermal expansion to maintain correct clearances between piston and liner and between piston ring grooves and rings. Cooling
may be carried out by circulating either water or oil.
Fresh water cooling has the advantage of greater thermal capacity than oil; it may
also sustain higher outlet temperatures (up to 70°C) improving thermal efficiency.
Inhibitors are necessary to prevent corrosion in the system and adequate venting
must be maintained.
Water cooling has the disadvantage 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 (56°C) 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 contamination of
39

ENGINE PARTS

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 a period 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 piston.
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
INE
NE

ROOM

LIFTING
PLATE
SECURELY
BOLTED
TO PISTON
CROWN

a
a

EQ)

IQ

OST
SUSY
_ISy

a
teeere
OD
‘a

DE.
CLLR

AEE
Qe
=
C7
Te
TL
2LLLIII

N

y|
¥

GLAND
HOUSING

:
b

PROTECTING
SLEEVE

:

Fig. 36

40

Changing a piston

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. Locking devices on piston studs
are dismantled and nuts slackened hydraulically. Studs must be inspected and tested.
All surfaces must be cleaned and new seals fitted. After reassembly, a pressure test of
cooling spaces should be carried out.
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 replaced. A
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
the engine turning gear.
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 necessary and the reasons for these.
Give possible causes for defective piston rings in service and the effects on
operation of the engine.

A. Materials for diesel engine piston 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 groove clearances.
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.
41

ENGINE PARTS

:

LAR

CAM TURNED
RING

PW ee
RING

SECTION

PS

=a

Ake
YP)

jel

AXIAL

BUTT

itl aah
pottery er et
LN

CLEARANCE

re

erat}

a

p— LAP

ee
Fig. 37

Piston rings

Rings are machined either circular or to a cam shape from which they
expand to
form a circle at working temperatures and avoid port chipping.
A radial cut is made
and the shape of ring joint ends may be butt (vertical), which gives
a robust joint for
top rings; scarfed (diagonal), giving better gas seal but is less
robust; or a form of lap
or bayonet joint which gives a good gas seal but is more vulnerabl
e to breakage and is
only used in some lower rings (Fig. 37).
Piston compression rings are fitted in corresponding ring grooves
machined in the
piston. They will bear heavily on the lower surface, or land
of the groove during the
power stroke and these lands must remain true or rings will
distort. To reduce wear

on the groove lands, they may be chromium plated, heat
treated, or have wear rings
fitted.

42

ENGINE PARTS

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 depth of
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 higher here and
these rings will have higher wear rates than others lower in the piston.
Ring clearances are necessary to allow movement and thermal expansion. Axial
clearance in the groove must be gauged, this will allow gas pressure to pass to and
from the back of rings. This clearance will increase with wear and it must not be
allowed to taper. Clearances vary with engine size and rating but for a large engine
may be 0-4mm for the top ring to 0-2mm for lower rings.
Due to ring and groove wear being uneven, rings should not be moved from one
groove to another during overhaul. New rings should be fitted when clearance or
wear becomes excessive.
Circumferential clearance at the ring joint is necessary to allow for thermal
expansion but should not allow excessive blow past of gas. It is measured before
fitting by inserting the ring into the least worn (lower) part of the liner bore. This
clearance will also vary with the shape of joint used, engine size and rating, but for
large engines approximate figures may be from 0-5 per cent of cylinder diameter for
moderate ratings to 1-0 per cent for higher ratings. Circumferential wear of rings can
be measured by reduction in the width of the ring section and by increase in joint
clearance at the corresponding liner bore.
Excessive wear will allow rings to distort and eventually break and will reduce
engine efficiency.
It will reduce compression, causing poor combustion and will allow gas blow past
to cause increase in rate of wear of rings, grooves and liner. Lubrication will be
removed, piston side walls may be burned and liner damaged.
Q. Sketch and describe a piston for a four-stroke medium speed trunk piston
engine. State materials used and show how the piston is cooled. Give details of piston
rings fitted.
A. Fig. 38 shows a piston fitted to a medium speed four-stroke diesel engine. The
piston and skirt form a single casting of silicon aluminium alloy which is light but
work-hardens to give good wearing surfaces. The skirt acts as a guide to transmit side
thrust to the liner.
Piston crown and ring grooves are cooled by oil circulated through a mild steel
coiled tube which is cast into position during manufacture of the piston. Oil
circulation is from the main bearings, via passages drilled in the crankshaft to bottom
end bearings and then up a bore in the connecting rod to a passage in the hollow
gudgeon pin. It passes to the coil from one end of the pin and returns through the
other end. It is transmitted to the connecting rod and then returns to the crankcase.
No record is taken of piston return temperatures.
The piston crown is recessed to allow opening of pairs of inlet and exhaust valves
during the corresponding strokes.
43

ENGINE PARTS

OI1L SCRAPER
RIN

(7)

Cee

eee
Cee SSE
eae
(pak
eorna

eA

Ui}

"

== -------

—_

[Z

PRESSI
G GROOVES

suena

eSB
OIL

DRAINS

II

|

!

EWEN

G

=

NNIASSIS

LELELEL ELIE EEL TY

|

PL

‘S|

PASSAGES

=

GUDGEON

PIN

Diasia Oe? Senta

ROD

OI1L PASSAGES
Fig. 38

IN ROD

Medium speed piston

There are four compression rings and two oil control or scraper rings. The top
ring groove being contained in a heat resisting, cast iron insert, which is also cast into
the piston during manufacture. This insert reduces the heavy wear and gives good
heat transfer from the top ring.
The gudgeon pin is floating and is free to rotate within the piston skirt. It has a
locating plate and oil seal at each end to restrict end movement and prevent oil
leakage between skirt and liner.
The piston and skirt are hollow and shaped to locate the connecting rod at the
gudgeon pin bearing.
In this particular piston, the top compression ring is chromium plated with a
serrated working edge to retain the lubricant. The other three compression rings may
be cast iron with bronze inserts, or cast iron with serration and copperplated. Either
type of ring gives a good gas seal with rapid bedding in.
The oil control rings remove excess oil from the liner, passing it through drain
holes to return it to the crankcase. Both rings have two scraping edges and may be
spring backed.
Clearances in compression rings will be considerably less than for large engines.
Axial clearances vary from 0-17mm for top ring to 0-06mm for bottom ring.
Circumferential ring gap is approximately 2mm on a bore of 400mm.
Some medium speed engines have pistons built up of an aluminium alloy skirt
with a cast steel piston crown bolted to it. This gives increased strength to the crown
44

ENGINE

PARTS

and reduces the effects of thermal expansion. Cooling spaces can be arranged
between the castings and an oil seal ring is fitted. The top ring grooves are included in
the crown for better wear resistance.
If both oil control rings are situated above the gudgeon pin, lubrication of the
skirt is increased and may reduce wear due to side thrust.
Q. State briefly the difficulties in lubrication of crosshead bearings
two-stroke engines. What methods are employed to overcome these?

in large

A. The crosshead of a large two-stroke engine is difficult to lubricate; due to the
cycle there is an almost continuous downward load of varying intensity on the
bearing tending to squeeze the oil film from the underside of the crosshead pins. This
load is due to gas pressure on the downward stroke followed by compression on the
upward stroke. With increase in engine bore and mean cylinder pressure, these loads
become extreme.
No oil wedge or fluid film may be built up in these bearings since there is little
relative movement between bearing surfaces due to the articulation of the connecting rod. Other methods must be used to distribute the oil for lubrication and cooling.
To ensure adequate flow of oil the lubrication system supplies directly to the
crosshead by means of telescopic or articulating pipes, excess oil is then passed to
guides and bottom end bearings.

Fig. 39

Crosshead bearing

45

ENGINE PARTS

To distribute the oil over the lower bearing surface, oil gutters are cut in an axial
direction in the bottom shell. These gutters are spaced closely enough to allow an oil
film to remain between them as the bearing swings. Gutters are connected to the oil
supply, they do not extend to the end of the bearing but small holes or grooves may be
cut to promote circulation of oil for cooling. Gutters must not be too great in number
since they reduce the effective bearing area. No gutters are cut in the top keeps of the
bearings.
The usual type of crosshead has two bearings and, with high gas loads, some
bending will occur. This will cause excessive load on the inner edge of each bearing
and fatigue failure may be caused.
To overcome these difficulties a number of different measures may be adopted.
Harder bearing metals may be used with crosshead pins ground and polished.
Bearings may be increased in diameter, this allows reduction in length and bending
and increases rubbing speeds.
Bearings may be mounted on flexible supports allowing them to align with the
pins when bending occurs and thus distribute the load more evenly.
Construction of the crosshead connection with the piston rod may be altered
allowing a continuous lower half bearing surface for the whole length of crosshead,
this gives increased bearing area, transmits piston load directly and reduces bending.
The top keeps of this bearing are unchanged.
A separate pad or bearing surface may be added between the normal bearings,
the underside of the crosshead being machined to engage with this to reduce bending.
This pad must be aligned and lubricated as the other bearings.
Special high pressure pumps may be added to inject oil at high pressure between
the lower bearing surfaces. These pumps can be automatically operated by the
relative movement of the connecting rod, they draw oil from the passage within the
rod and inject it into the oil grooves. This pressure wave occurs near BDC when
bearing load is low.
All of these measures have met with success in various engines.

Q. Describe how alignment of a diesel engine crankshaft may be checked and
recorded. Why are such tests made and what may be the results of misalignment?
A. Misalignment of an engine crankshaft may occur due to wear of main bearings
or from distortion of engine bedplate, transverse members or possibly from damage
to the supporting ship’s structure. This misalignment can be detected by measuring
deflections of crankshaft webs.
If misalignment exists the crankwebs will open and close slightly as the engine is
rotated; this is measured by means of a clock gauge. The gauge is fitted between
adjacent webs, opposite the crankpin at half the diameter from the shaft centre. A
spring extension rod will hold this in position.
The first measurement is taken with the engine just beyond bottom dead centre
position with the gauge close to the side of the connecting rod. It is usual to set the
gauge to zero at this point. The engine is now rotated by the turning gear and stopped
at each quarter turn where gauge readings are taken, as plus or minus values. The
final (fifth) reading being taken near bottom centre, with the connecting rod on the
opposite side of the gauge to the first reading. The first and last readings are averaged
to use as an approximation for bottom centre position. This procedure is repeated for
46

ENGINE PARTS

each unit in turn. Feeler gauges should be used to ascertain that the crankshaft has
not lifted or sprung in adjacent main bearings.
It may be advisable to rotate turning gear slightly in reverse direction when
stopping for readings, this will ensure free positioning of the cranks. This is
particularly so for units adjacent to the turning gear.

CRANK

Fig. 40

EQUIVALENT
VERTICAL

POSITIONS

Crankshaft deflections

A+E

5

DEFLECTION
A+eE

.

2

HORIZONTAL DEFLECTION

Fig. 41

Record of deflections

47

ENGINE

PARTS

All readings are recorded and these should be compared with previous values,
preferably with the ship in a similar load condition and at similar temperatures.
The total deflection vertically and horizontally is calculated for each crank. The
vertical total will be proportional to misalignment between the bearings due to wear
down. The horizontal total indicates side wear in the bearings.
_
By plotting all vertical deflections for the whole engine it is possible to obtain
information as to which main bearings are ‘high’ and which are ‘low’. This may be
assisted by bridge gauge readings from the bearings but these do not take possible
distortion of the bedplate into account.
Limiting values for maximum deflection are set by engine builders. These depend
upon the stiffness of crankshaft, engine stroke bore ratio, etc. They indicate the limits
to which misalignment may be permitted before remetalling of bearings and
realignment are necessary.
Deflection measurements are readily taken and they relate directly to misalignment. They should be taken at regular intervals or at other times when damage to
running gear in the crankcase, propeller, shafting, or ship’s structure is suspected.
Excessive misalignment will cause bending of the crankshaft and webs with
fluctuating and alternating stresses causing fatigue and possibility of shaft failure. It
will set up vibrations and cause damage to main bearings.

Q. Sketch and describe the construction of the bedplate for a large slow running
diesel engine. How is the bedplate secured and what inspections should be carried
out? How is the gas load transmitted from cylinders to the bedplate?
A. Fig. 42 shows a bedplate for a large crosshead type of main diesel engine. It is
fabricated from steel parts welded together.

SEATING
TIEBOLT

HOLES

Fig. 42

48

Engine bedplate

ENGINE

PARTS

The bedplate must act as the main strength part of the engine section, providing
rigid support for the main bearings and crankshaft. It is also a platform on which the
other components such as columns, frames and guides may accurately be mounted to
support engine cylinders, entablature and all working parts.
In large engines the bedplate must withstand heavy fluctuating stresses from
operation of the engine and must transmit engine loads over an area to the ship’s
Structure. It is also used to collect crankcase lubricating oil, returning it to the drain
tank for further use.
The bedplate consists of two main longitudinal box structures. These have
stiffening members and webs for additional rigidity. Lightening and access holes are
cut, with compensating sleeves, to maintain lightness with strength. Transverse
members or girders join the two boxes. In the design shown they are of cast steel
designed to withstand fluctuating stresses while supporting the main bearings and tie
bolts. They are fitted between each throw of the crankshaft, as close as the design
allows and are secured by substantial butt welds to complete the rigid structure of the
bedplate.

HYDRAULIC
NUTS

CYLINDER
COVER

TIEBOLTS
ENTABLATURE

FRAMES

BEDPLATE

Fig. 43

Tie bolts

49

ENGINE

PARTS

All welding in the bedplate must be to a high standard and is carefully controlled.
It must be stress relieved, shot blasted and tested for flaws. All plate edges must be
correctly prepared and double butt welds used where possible. Plates of different
thickness should not be butt welded together. Bedplate flanges are machined for
landing on support chocks and for assembly of other parts.
In most single-acting engines, apart from opposed piston engines, the main gas
loads from the cylinders are transmitted by long tie bolts. Two bolts are fitted to each
transverse girder and they pass up through the casting, through tubes constructed in
the engine frames and through the entablature or cylinder jackets where locking nuts
are fitted. They are hydraulically tightened to pre-stress the structure, maintaining
the engine columns in compression. Tie bolt centres should be as close to the
crankshaft axis as possible to reduce bending stress on the girders and to prevent
unbalanced loads being transmitted to the welds (Fig. 43).
The complete bedplate is landed on the ship’s structure which is specially
stiffened to support it. Main bearing centres are carefully aligned with the centre line
of propeller shafting. Engine packing chocks are now accurately machined and fitted
to support the engine. Holding down bolts secure the bedplate to the ship’s structure.
Brackets are welded to the tank tops at the ends of each box structure with
packing chocks to transmit longitudinal thrust from the engine; similar brackets are
welded at positions of transverse girders to transmit any side thrust.
Inspections will be internal inspection of stressed parts, particularly the transverse girders, for fatigue cracks.
Tie bolts should be checked for tightness and flaws. Holding down bolts and
engine chocks must be kept tight.

Q.

Why is a chain used for camshaft drive of a diesel engine? Give the causes for

loss of tension, what effect has this and how may it be compensated for? Give details

of the maintenance required.

A. A roller chain may be used to drive the engine camshaft from the crankshaft,
irrespective of the distance between centres. It forms a flexible drive, robust but light
in weight with very small friction loss, it is small in width which adds little extra length
to the engine. It can accommodate a number of additional driven wheels rotating at
different speeds or even in opposite directions, and can be used to operate engine
driven pumps, blowers, etc.
A roller chain consists of alternate pin links and roller links. Each pin link consists
of two pins riveted between two side plates. A roller link consists of two rollers free to
rotate on two bushings which are a press fit into two side plates. The pins fit within the
bushings of two adjacent roller links. (See Fig. 44). Alternate links are all riveted
together to form an endless chain of correct length.
Chains have a very high factor of safety to prevent stretching and any loss in
tension in the chain will be due to wear while in use. Wear takes place between pins
and bushings, between bushings and rollers and between rollers and sprocket wheel
teeth.
Wear in wheel teeth flanks can be checked by a profile gauge while wear in the
chain is measured by its extension.
While in place, the extension and loss in tension is indicated by the transverse play
in the chain span between wheels on the slack side of the drive. The chain is adjusted
to allow a limited transverse movement approximately equal to one link pitch.
50

ENGINE

PARTS

BUSHING

PINLINK

Fig. 44

RIVETED

PIN

eens

ROLLER

LINK

Camshaft chain

To measure the extension, the total length over a given number of links on the
longest span, when drawn tight, is measured and the increase is calculated as a
percentage of the original length of an equal number of links. A chain should be
renewed when its extension approaches 2 per cent. For a camshaft drive its life
should also be limited to fifteen years, irrespective of wear.
In addition to the driver (crankshaft wheel) and driven (camshaft wheel), a chain
system will contain jockey and idler wheels. These may be used to alter the direction
of run of the chain and reduce unsupported lengths of chain. At least one jockey
wheel has the position of its centre adjustable. It can usually move about a pivot to
adjust the tension in the chain. Adjustment may be made by a screw or hydraulic jack
arrangement or a spring loaded pulley. The adjustable wheel is usually located on the
slack side of the chain when going in ahead direction.
Adjustment is usually limited to the length of two chain links and when this has
_ been taken up, the chain must be shortened by removing two links. To carry this out,
_ two adjacent pin links are removed with their common roller link. The ends of the
chain are then brought together and joined by riveting in a new pin link. On no
account should an old pin link be riveted a second time, it is always necessary to use a
new link.
After tensioning adjustment has been completed, the timing of the camshaft must
be checked. If this is incorrect, the camshaft flange is slackened and adjusted to
synchronise the cam timings with the crankshaft position. This may be checked either
by putting one engine unit on top centre position or from corresponding marks on the
flywheel. After re-positioning the camshaft, the flange is again keyed and bolted
together.
When in use, the chain is lubricated by means of oil sprayers which direct a
continuous stream of lubricating oil into the chain bearings. This oil will lubricate and
cool the bearings and will cushion fluctuating forces.

Syl

ENGINE PARTS

Maintenance

will consist of periodic examination

to check wear of the chain,

chain wheel teeth and wheel bearings. Free any seized rollers, check alignment of
wheels by examining for bright marks on side plates, and examine for possible fatigue
cracks.
Excessive tension on a chain will cause high loads and possible damage, while
excessive slack will cause vibrations, add to cyclic stresses and possibly to fatigue
failure.
When renewing a chain, it must be ascertained that wheels are in correct
alignment and that relative positions of wheels and timing are correct.
Where heavy loads are transmitted, duplex or triplex chains may be used. These
consist of two or three identical matched chains. Any adjustment carried out to one
of such a set must be carried out equally to the others.

32

SECTION

5

Operating Systems
Q. Describe with a line diagram a fuel system suitable for a large main engine to
operate on heavy fuel.

A. Fig. 45 shows a fuel oil system for a large slow running engine. This system
allows the engine to operate on heavy oil and it may be manoeuvred on either heavy
or diesel oil.

H.O.TANKS

'CENTRIFUGES

§

VISCOSITY
REGULATOR

lt Sal
Dd

(-)

Py

PA

(-)

><J

3

Fig. 45

Fuel oil system

To operate on heavy oil the fuel system to the engine must be well lagged and
steam trace heating may be used to maintain oil at sufficient temperature for correct
viscosity at injectors. There must be arrangements to circulate heated oil through the
system when warming through preparatory to sailing or when the engine is stopped.
By using diesel oil for manoeuvring the engine, the circulating procedure is
reduced to a minimum and the system is left clear of heavy oil, which becomes more
viscous as it cools. This oil may be difficult to heat and recirculate when cold and may
make maintenance of the fuel system more difficult.
During normal operation on heavy oil at sea, oil is taken from the storage tanks,
and is heated and purified and discharged to service tanks. There are usually two such
tanks fitted and they are used alternately, tank suctions being changed over each day.
53

OPERATING

SYSTEMS

One tank will be in use while the other is being filled. These tanks are heated toa
moderate temperature and self-closing drain valves are fitted to remove any water or
sludge which may settle out.
Oil is drawn from the service tank by a primary or surcharge pump and discharged
at low pressure to the fuel oil heater. A viscosity regulator is fitted at the heater
discharge through which the oil will pass. This will automatically control the
temperature of oil fuel leaving the heater to maintain its viscosity within close limits.
The oil is then discharged through a fine strainer to the main engine operated fuel
pump suctions. A pressure regulating valve is fitted in the system and excess oil is
returned, either to the heavy oil service tanks, to a balancing tank or primary pump
suction. The pressure control valve may also be opened to allow circulation of hot oil
when engine is stopped.
A diesel oil fuel tank is included in the system with its discharge to the primary
pump suctions through a change over valve. By operating this valve the engine may
be operated on diesel oil. Change over should be very gradual to allow temperatures
in the system to stabilise. During this period, the excess oil will return to either the
heavy oil service tanks or a balance tank.
The primary pumps should be in duplicate and a by pass must be fitted to the
viscosity regulator. A relief valve on the primary pumps will return excess pressure to
the system.
Various safety devices must be included in the system with alarms to detect loss in
oil pressure, low tank level, etc. Quick closing valves, which can be closed from
positions outside the machinery space, must be fitted to all tanks and the engine main
inlet. Primary pumps must have remote cut-out switches for emergency.
There must be arrangements for venting and draining the system, cleaning
strainers, etc., but utmost care must be taken. Drain trays and save-alls, where fitted,

must be kept clean, all joints must be kept tight with safeguards to prevent possibility
of hot oil spraying on to-heated surfaces.
Oil contained in tanks with open drains should not normally exceed a temperature of 51°C or, if this is lower, may not exceed a temperature 20°C below the flashpoint of the oil.
Q. State why heavy fuel oil must be heated before burning in the cylinders of a
diesel engine. Sketch and describe a system which will automatically control the
heating of the fuel.
A. When burning heavy fuel oil in a diesel engine it is necessary to reduce the high
viscosity of the fuel to a value at which correct atomisation can take place in the fuel
injectors. This will allow correct mixing and burning of the fuel for efficient
combustion.
The viscosity of a fuel may be reduced by raising its temperature and it is passed
through a heater to do this. Automatic control of the heater may either be regulated
to maintain a constant temperature or to measure and control the viscosity.
It is possible that oils of varying properties are contained within a ship’s bunker
tanks or even one tank, when its contents are from a number of different sources.

Consequently it is preferable that the actual viscosity of the fuel is controlled within
close limits. In most cases this is regulated between 50-70 seconds Redwood No: 1 at
38°C (100°F).
The Viscotherm shown in Fig. 46 is an instrument which measures the fuel oil
viscosity at the heater discharge and regulates the heater temperature to control this.
54

OPERATING

SYSTEMS

It consists of a small gear type pump rotated at a slow but constant speed (40rpm). It
is fitted within the fuel supply close to the heater discharge. The pump draws fuel
from the system at a controlled rate and discharges it through a capillary tube.
The form of flow within a capillary tube at corresponding speeds is such that the
pressure difference between each end of the tube is directly proportional to the
viscosity of the oil flowing through it.

THERMOMETER

VISCOSITY

CAPILLIARY

Fig. 46

SCALE

TUBE

Viscosity regulator

Pressures at the corresponding points are measured with Bourdon tubes and
compared to read as viscosity. In addition these pressures are fed to a differential
pressure transmitter which can automatically operate the heater control to maintain
fuel viscosity within close limits.
All parts of the instrument are of stainless steel. In the event of a failure of this
control, an oil by-pass valve is included in the system and the temperature may then
be controlled by hand or by a thermostat.

Q. State the indications of good combustion in a slow running diesel engine. Write
notes on each of the following:
(a) Viscosity
(b) Atomisation
(c) Penetration

(d)

Turbulence.

A. The general indications of good combustion are similar in any operating diesel
engine. They are: aclear exhaust, power produced and exhaust temperatures normal
for the throttle setting. There should be no uneven running, knocking from cylinders
or the fuel system.
(a) Viscosity, or resistance to flow, in a fuel oil is important when considering
combustion. The viscosity of fuel must be low enough to ensure correct atomisation
at the fuel injector. Since viscosity reduces as temperature is increased, it will be
25)

OPERATING SYSTEMS

necessary to heat heavy fuel oil to reduce its viscosity to about 70 seconds Redwood
No: 1 at 38°C (100°F) before atomisation for combustion.
(b) Atomisation is the splitting up of the fuel into very small droplets. It is
carried out by the fuel injector forcing fuel at high pressure through small atomiser
holes. The size of the droplet will depend upon the size of holes and the pressure
difference between fuel pump discharge and that of the compressed air in the
combustion chamber. Droplet size may vary during the injection period.
Small oil droplets are emitted into the high temperature dense air. They have a
high surface/mass ratio and are readily heated, evaporated and mixed with the
surrounding air to form a combustible mixture. The time required for this to occur is
termed the ignition delay period.
(c) Penetration refers to the distance the oil droplets travel into the combustion
space before mixing with the air and igniting. This will depend upon droplet size
(atomisation), velocity leaving the injector and the conditions within the combustion

ATOMISATION

/

/

y

/
a

/

v oO

/

TURBULENCE
TO SWIRL

Fig. 47

56

Combustion

DUE

OPERATING

SYSTEMS

chamber. It is desirable that fuel should penetrate into the whole of the combustion
space for good mixing but droplets should not impinge on the internal surfaces before
burning. The number of atomiser holes and their position will decide the spray
pattern.
(d) Turbulence is the movement of compressed air and fuel within the
combustion space before combustion occurs. This movement may have several
causes. Swirl is imparted to the air during its entry at scavenge ports. It may be further
agitated by the fuel spray pattern and the shape and movement of the piston crown.
Turbulence will improve the mixing of fuel and air for effective and rapid
combustion. It is particularly desirable for rapid combustion of heavy fuels in
medium or higher speed engines.
Fig. 47 illustrates these processes.
Q. Sketch and describe a hydraulically operated fuel injector for a diesel engine.
State the maintenance required and defects which may occur.
A. Fig. 48 shows a section through a hydraulically operated fuel injector as fitted to
a large two-stroke diesel engine. The general design is similar for most engines and
consists of a spring loaded non-return needle valve operated hydraulically by a fuel
pressure wave from the fuel pump to discharge fuel at high pressure through an
atomiser.

NEEOLE

VALVE

ATOMISER
HOLES

Fig. 48(a) Fuel injector needle valve

Sy

OPERATING

SYSTEMS

SS

i
Ss

FUEL

RLS

INLET

a

7

(CN

NY

N

N Ni

=

iZEEE ==,

=

N Ni

Kyy

‘
NN

NN)
NN
Ni

ZLLLLL

VALVE

BODY

N)

NN

Nh

OIL

PASSAGE

N NT

SPRING

NAN Wy
NA

NN

NN
NN
NN

NN
NN

Nie |
NAT

i

NANA

'

"

NI
‘
NK: ays

N ¥ 4

vecoue_
vor N
NUT

YEN

SY

NUN
HONG:

PRL)
: N\ NZ
G

NAIA WY

INTERMEDIATE

OSPINDLE
DOWEL

NOZZLE

ass

QR \
NAS

INDER
R

Fig. 48

|

Fuel injector

The main components are the valve body or nozzle holder to which the nozzle or
atomiser is secured by a retaining nut. The complete injector is inserted in the fuel
valve pocket of the engine cylinder. The retaining nut has a ground surface to make a
gas tight face joint at the landing in the pocket to seal off the combustion chamber.
The valve is secured in position by studs and nuts.
The ideal position for the fuel injector is in the centre of the engine cylinder cover.
This allows a symmetrical fuel spray pattern in the combustion chamber. This
position may be altered in some engine designs due to the position of exhaust valves,
or in an opposed piston engine where a number of valves are fitted, displaced
circumferentially around the combustion chamber.
The valve body contains the spring, compression nut and an intermediate spindle.
There is a passage to convey fuel oil to the atomiser and in most cases there will also
be cooling passages.
The lower end of the body is ground flat with a hardened surface accurately
lapped to form an oil pressure tight face joint with the atomiser.
58

OPERATING

SYSTEMS

The atomiser has its top surface similarly ground and lapped to match with the
body. This face will also include matching passages for fuel oil and cooling and a
dowel is fitted to ensure these are aligned.
The needle valve is lapped into the bore of the atomiser. These are accurately
matched and must be kept as one unit. At the base of the atomiser there are two
chambers as shown in Fig. 48(a). The upper chamber is charged with fuel oil from the
fuel pump and it is sealed by the needle valve. At the lower end the mitre seat is
accurately ground and lapped to form a positive oil seal while at the top of the
chamber the larger diameter of the needle valve above its shoulder also forms an
effective oil seal.
The lower chamber, which is also sealed by the mitre seat of the needle valve, has

a number of small atomiser holes of correct size and pattern to atomise and distribute
the fuel spray into the combustion chamber.
Injector spring compression is adjusted under test and a compression ring fitted.
It is set to allow the needle valve to lift or open at a predetermined fuel pressure. The
intermediate spindle conveys the spring compression to the needle valve and may be
arranged to limit its lift.
The valve will open when the pressure from the fuel pump acting on the shoulder
of the needle valve overcomes the spring compression. As the needle valve lifts, oil
flows to the lower chamber in the atomiser. The additional area of the needle mitre
now subjected to pressure causes the needle to lift rapidly allowing fuel at high
pressure to pass through the atomiser holes into the combustion chamber.
When the fuel pump cuts off pressure, the valve will close under spring
compression. Since the full area of the needle is now exposed to pressure, closing will
occur at a pressure lower than that at which it opened.
The action of the needle valve must be rapid and positive with no oil leakage.
When burning heavy fuel, injectors will require cooling. This may be carried out by
circulating water or oil through cooling passage which should be as close as possible
to the injector tip. An independent cooling system is used due to the possibility of
contamination from fuel leaks. If the cooling temperature is too high, carbon will
form on the valve and, if it is too low, external corrosion may be caused.

A fine edge strainer may be fitted at the fuel inlet and a priming or venting plug is
fitted to the fuel passage. Valve should be primed before commencement of the
voyage.
Fuel injectors must be overhauled at regular intervals to ensure correct operation
and combustion. The injector spring must be screwed back before slackening the
retaining nut. Parts are cleaned, inspected and renewed if necessary. Lapped surfaces
must be free of damage and correctly aligned, springs inspected for distortion,
atomiser holes must be clear and unworn.
After assembly the injector is tested with a test pump. Operating pressure and
fuel spray pattern are checked and there must be no leakages.
Defects in injectors, while in use, may be choking due to dirt in the fuel or carbon
building up at the atomiser. A leaking needle valve will cause secondary burning and
this will reduce combustion efficiency. Cooling faults have already been listed.

Q. Sketch and describe a fuel pump as fitted to a diesel engine. How does it regulate
the timing and quantity of fuel supplied and how is it adjusted?
A. Fig. 49 shows a Jerk type fuel pump. This type of pump with small variations is
used in many diesel engines.
59

OPERATING

SYSTEMS

FUEL

N.R.

FUER

DISCHARGE

VALVE

Neem

PLUNGER

RACK

SPRING

—

CAM
FOLLOWER

BARREL

.FUEL

CAM

Fig. 49

Fuel pump

The pump consists of a cam operated, single acting plunger of fixed stroke.
Helical springs are fitted to return the plunger on its down stroke and to maintain
contact of follower on the cam.
No timing valves are required and fuel delivery commences at a fixed point on the
up stroke when the top edge of the plunger blanks off the suction port. A helix or
scroll is machined on the plunger and delivery of fuel ceases on the up stroke, when
the curved surface of the helix uncovers the suction port. This allows fuel pressure
above the plunger to fall to the suction pressure through a vertical slot or hole.
The quantity of fuel delivered is regulated by the vertical length of the helix where
it is in line with the suction port. This setting may be altered by rotating the plunger.
A rack is fitted to the pump to engage with a pinion machined on the outside of a
sleeve. The sleeve fits over the plunger and has slots engaging with keys. In this way
the plunger may be rotated by movements of the rack (Fig. 50).
The fuel cam is designed to raise the plunger at the rate required to build up fuel
pressure and maintain this for the corresponding period to operate the fuel injector.
Since the pump only discharges on its up stroke, only one flank of the cam operates
the timing. The trailing flank of the cam returns the plunger to the bottom of its
stroke to allow the chamber to refill.
Timing is controlled by the relative angular position of the cam peak to the
crankshaft. It can be adjusted by moving the cam with respect to the shaft. Further
adjustment is made by raising or lowering the pump plunger with respect to its follower. Raising the plunger will make cut off of the fuel suction port and
corresponding fuel injection early, while lowering the plunger will make these later.

60

OPERATING SYSTEMS

al

LEI
LLAVIL2

we,

rsa

LL
LIL
VLISLLI
GT
I A, FETILILICSES
Ib=
QT

Fig. S50

Fuel pump control

Alternatively the fuel pump casing itself may be lowered or raised on its mounting
to give the corresponding effect. The plunger must maintain sufficient clearance at
the top of its stroke.
Oil supply to the pump suction is by means of a continuously operating supply or
surcharge pump, this causes flooding of the fuel pump chamber as soon as the suction
port is uncovered by the plunger.
In some pumps, a non-return spring loaded discharge valve is fitted. This is
arranged to reduce pressure on its discharge side as it closes, ensuring positive seating

of the fuel injector needle and reducing cavitation within the pump.
A priming or vent plug is fitted to the discharge.
Possible faults in a fuel pump include wear of parts, such as plunger and barrel.
These surfaces are a close fit, the small clearance allowing some leakage to lubricate
the plunger. Larger clearances are necessary for the higher temperatures when
burning heavy fuel. Care must be taken that no leakage from the pumps can enter the
cafnshaft lubrication system as this may lead to fuel contamination of the crankcase
system.

Other faults may be erosion of suction port or plunger surface.
Cam timing and relative position of plunger should be checked periodically.
Q. Describe, with the aid of a line diagram, a lubricating oil system for a large
crosshead type diesel engine. Indicate the tests and operational procedures that are
carried out to keep the oil in optimum condition.

A.
ad

Fig. 51 shows a lubricating oil system for a large main engine. Pressure pumps,

strainers and fine filters are in duplicate, one set being used while the other acts as

stand by. Fine filters should be of a type which are capable of being cleaned without
interruption of the oil flow.

61

OPERATING

SYSTEMS

SALT

WATER
FROM
STORAGE
TANK

DISTRIBUTION

BRANCHES

pe

Panne

HEATER

Y
L\

V
><

ray

ENGINE
SEYETE RS

CENTRIFUGE

DRAIN

Fig. 51

TANK

Lubricating oil system

Lubricating oil pressure pumps draw from the main engine drain tank through
suction strainers. The tank suction should be clear of the lowest point to avoid intake
of any water or sludge present. Pumps discharge under pressure through fine filters to
the cooler, this pressure ensures that there is no possibility of salt water leaking into
the lubricating system in the event of a fault in the cooler.
From the cooler the oil passes to the engine supply connections. Regulating
valves and restriction plates control the distribution of oil and its pressure to various
bearings and parts of the engine. Used oil drains to the crankcase and then through
strainers to the drain tank by gravity. The oil return to the drain tank should be
remote from the pump suction and the discharge should be submerged to avoid
danger from crankcase explosions and to reduce aeration.
The oil drain tank is usually in the ship’s double bottom but must be surrounded
by cofferdams to prevent possible contamination. It should be fitted with an air vent,
level gauge and sounding pipe. The level gauge should be centrally mounted to avoid
fluctuations due to rolling or trim. Interior surfaces of the tanks may be coated to
prevent formation of rust, particularly on the top surface.
The lubricating oil system must be fitted with alarms arranged to give warning of
loss in oil pressure or level of oil drain tank. Temperature alarms may also be fitted.
Pressure relief valves are usually fitted at pump discharges, these must return back to
the oil system.
A purifier system is fitted to the oil drain tank. The purifier suction pump drawing
from the lowest point in the tank and after purification returning oil to a position
adjacent to the main lubricating oil pump suction.

Purification should be carried out continuously at sea, with a slow throughput to
ensure correct separation. The oil may be heated to assist this and water washing may
62

OPERATING

SYSTEMS

be applied to remove acids formed. The treatment carried out will depend upon
recommendations of the oil suppliers.
Whilst in port, batch purification may be carried out.
Oil will gradually deteriorate in use; the rate of deterioration will depend upon
many variables such as temperature, rate of circulation, contamination and treatment applied.
Temperatures should be kept moderate and contamination kept to a minimum.
Some indication of the oil condition can be ascertained by inspection of the purifier
sludge discharge.
At regular intervals, oil samples should be drawn from the system and sent for
analysis. The detailed analysis will give information of the oil condition and the
presence of unwanted elements may diagnose engine faults.
A number of simple tests may be carried out aboard ship to show the general
condition of the oil. These include a flash-point test for fuel contamination, settling of
oil to detect sludge or water present; colour titration tests to show water present,
acidity or alkalinity, filtration, simple viscosity tests and in certain cases blotter spot
tests.

Variations in oil level in the drain tank should be investigated immediately.
Consumption of oil with regular make up of fresh oil will help to maintain quality of
oil in the system.
Q. Enumerate the fittings to the shell of a main engine starting air receiver and
explain their purpose. Show how they are attached to the shell and state the
precautions that should be observed when filling, emptying, opening up, cleaning and
applying protective compounds to the internal surfaces.

A. The usual fittings required to a main starting air receiver are as follows: Safety
valve, pressure gauge connection, drain valve, filling valve, discharge to engine air
starting main, air to ship’s whistle, air to auxiliary systems, air to pneumatic control
system, etc.
Although each connection may be made separately to the shell of the receiver, it
may be preferred to fit a valve manifold with only one common connection to the
shell.
The safety valve must be set to relieve the receiver of excessive pressure rise and
must have sufficient area to prevent any accumulation of pressure from any reason.
The pressure gauge connection must be in direct communication with the internal
pressure irrespective of other valves being open or closed. The drain will remove the
contents from the lowest point in the receiver and should be of sufficient size to
prevent likelihood of choking. If it is possible to isolate the safety valve from the
receiver a fusible plug must be fitted to release pressure and discharge the receiver in
the event of heating due to a fire in the vicinity. Such heating would of course raise the
air pressure within the vessel. The fusible plug should have a melting temperature of
150°C and a connection must be made to discharge the contents in a safe manner.
All stop valves should be slow opening valves to prevent sudden build-up of
pressure when connecting any pipe system.
The filling connection from the air compressors must be independent of all
others. Further auxiliary systems may be taken from the auxiliary connection: these
will require stop valves, reducing valves and relief valves as necessary.
Two starting air receivers are required. They are of welded construction,
cylindrical in shape with dished ends. Each air receiver should have sufficient

63

OPERATING

SYSTEMS

capacity to supply the statutory number of engine starts without recharging. They
must meet all the regulations for construction of pressure vessels, and require
compensation where strength has been removed for openings in the shell. An
acceptable welded attachment for fitting of valve manifold, etc., is shown in Fig. 52.

COMPENSATING
RING

SHELLPLATE

;

Fig. 52

Air manifold fitting

MANHOLE

MANIFOLD

FOR

VALVES

SHELL

Fig. 53

WELD

Air receiver

A manhole must be fitted for access to allow internal inspection and cleaning.
This should either be fitted to the cylindrical shell with its minor axis arranged
longitudinally or else situated within the dished end. A flat internal joint face is made
for the manhole door gasket.
Care must be taken when filling the receiver that the air supplied is free of oil or
moisture and is not of excessive temperature. Drains are fitted on the filling lines and
these should be open when not under pressure; they are closed after the start of
filling, opened periodically during filling and opened again when filling is complete.
Similarly drains are fitted and used in discharge lines.

64

OPERATING

SYSTEMS

The drain on the receiver should be ‘blown’ periodically and particularly before
using air from the receiver. Discharge from the drain should be observed to give an
indication of conditions within the receiver and also the possible carry over from
compressor discharge. Discharge valves should be opened slowly to prevent shock
_ waves in pipelines, and drains should be open while this takes place. When air is
being used from the receiver, the pressure should not be allowed to fall too low in
_ case an emergency should arise. In most starting systems one receiver is kept closed
at full pressure while the other is in use.
When opening up the receiver care must be taken that all valves are closed and
that internal pressure is completely discharged before opening the manhole door.
This door is opened inwards and, if undue force is required, it should be confirmed

that no pressure remains in the receiver.
Before entering the receiver its interior must be well ventilated and at least two
closed stop valves should be situated between the receiver and any pipeline under
pressure.
Internal cleaning must be thorough and care taken to avoid debris entering
connections. It is usual to overhaul all fittings while cleaning takes place. Careful
inspection must be made for possible internal corrosion.
Before applying protective compound all surfaces must be clean and dry and
' connections plugged. Many compounds give off noxious fumes and great care must
_ be taken in confined spaces, protective clothing and goggles should be worn and a
second person outside the manhole must keep observation in case the person within
the receiver is overcome. The compound must be allowed to dry thoroughly, plugs
must be removed and all connections cleared before boxing up the receiver and
recharging.
» Q. Describe the operation of overhauling a two-stage reciprocating air com' pressor. Make reference to the methods employed to ensure correct clearances on
_ bearings and cylinders. Give details of the necessary tests before the compressor is
put back into service and the precautions to be observed when putting it back into
service.
A. Before overhaul the maker’s handbook should be read and any special
_ instructions carried out.
Ensure that all air receivers are fully charged and that remaining air compressors
are operational. The compressor for overhaul is now isolated. Electric switches are
off and fuses or circuit breakers are out. Discharge valve and all cooling water
connections are shut.
The precise order for dismantling will depend upon compressor design and only a
general list will be given. All parts should be cleaned upon removal and carefully
inspected for signs of wear, damage or mal-operation.
Crankcase is opened and drained, oil strainer and internals cleaned. Lubricating
pump overhauled and clearances checked. Cooling water spaces are drained,
opened, cleaned and inspected. Corrosion fittings are renewed if necessary. Cooling
water pump, glands and bearing are overhauled, water strainer cleaned.
Air suction filter is cleaned, suction and discharge valves for each stage are
dismantled, valves, cages, springs and seats are checked and renewed as necessary.
When dismantling or boxing up valves, ensure that set bolts for locating valve cages,
etc., are slackened back while cover nuts are tightened. Inspect valve pockets for
signs of excess oil carry-over.

65

OPERATING

SYSTEMS

2nd.STAGE
VALVES

1st. STAGE
VALVES

—

aS

INTERCOOLER
HEADER

SS
oa
H _———s
<

i

h

Hossa

he

AY
HH
9 Yeas SIS
HA
ay
ha

DRAIN

o

6)

NN

4)

SSS

AR
PISTON
(Cast

q

H

it
NNN

l|ron)

d

NI

ram

1st. STAGE

PISTON(Aluminiun

a
Ny
Ms

o

CRANKSHAFT

NN

\

NN

as
FiYAW)
hE Ese
=<

WATER

ho _P

Rh

PUMP
Ol1L

PUMP

AY
STRAINER

Fig. 54

Air compressor

Remove cylinder covers, dismantle bottom end bearings and draw pistons with
connecting rods. Clean, inspect and gauge piston, compression and oil control rings
and renew rings if necessary. Clean, inspect and gauge cylinder liners. Remove
gudgeon pins after checking clearances. Examine bearing surfaces, bearings should
be renewed if excessively worn.
Overhaul bottom end bearings, check clearances and journal surfaces. These
bearings are usually of the ‘thin wall’ type and should be renewed if unduly worn. Oil
holes and passages must be checked and bearing shells must have a good nip in the
keeps. Clearances are taken by inserting feeler gauges or by jacking the bearings and
measuring lift by a clock gauge.
Main bearings are dismantled, inspected and wear down checked. Shaft may be
turned by hand to check alignment. All lubrication connections must be clear and
pipes secured. If a drive chain is fitted for pumps, this should be inspected. Before
boxing up cylinders and crankcase, the piston end clearances must be checked at top
dead centre positions.
Intercooler and aftercooler must be opened up. Water spaces should be cleaned
and examined for corrosion and relief valve or bursting disc checked. Air side is
opened up, cleaned and inspected and tubes cleaned internally. Drain pockets
cleaned. Drains, relief valves and discharge valve should be overhauled.
If a cylinder mechanical lubricator is fitted, this should also be cleaned and

overhauled together with the lubricator points in the cylinder liner.
Holding down bolts and shaft coupling bolts should be checked for tightness. It is
assumed that the compressor motor will also have been overhauled.
66

OPERATING

SYSTEMS

After boxing up all parts, cooling water may be applied. Ensure adequate venting
and that all parts are completely filled. Check for water leaks. Recharge crankcase
with fresh oil up to the required level. The compressor should now be barred over or
turned by hand with drains open. If this is satisfactory, connections can be remade
and the compressor run for ten minutes with drains open. Stop and feel round all
bearings and coolers. If satisfactory restart compressor, check drains for excess oil,
check all temperatures and pressures together with electrical load. Drain thoroughly
before returning to service. Check all pressure gauges when discharge pressure has
built up. Check vents from water spaces. Jf automatic controls are fitted, these must

be tested and put into service. Check automatic pressure or temperature cut-outs or
alarms, together with remote reading pressure gauges.
Q. Why are intercoolers fitted to multi-stage air compressors? What attention do
they require, and what is the effect of operating with fouled intercoolers? Why are
bursting discs fitted?
A. Two-stage reciprocating air compressors are generally used to supply main
engine air starting systems, these being adequate for the pressure range required.
The use of multi-staging will give mechanical improvements of compressor balance,
a reduction in size and mass, robust construction of smaller high pressure parts and
reduction

in clearance

volume

losses. The

main

advantage,

however,

is that an

intercooler can be fitted between stages.
Air discharged from the first stage of compression is passed through the
intercooler where it is cooled at constant pressure, causing a reduction in temperature and volume.

P. hp.

POLYTROPIC
Pyvn=C

#
w
«
=

7)
7)
a
[od
a

ISOTHERMAL
PV=C

P. Ip.

VOLUME

Fig. 55

P.V. Diagram for intercooler

67

OPERATING

SYSTEMS

The gain in efficiency can be illustrated on a pressure-volume diagram (Fig. 55)
on which it can be seen that isothermal compression (at constant temperature) causes
least area of diagram and therefore least power absorbed. Returning the compression line to this curve between stages will reduce power absorbed in second stage by
the shaded area shown.
A main consideration, however, is the reduction in air temperature. This makes

lubrication of later stage liners possible and generally adds to the safety of the system.
Intercoolers return the air to its original temperature and, due to an increase in its
density, later stages may be of reduced volume.
Intercoolers are fitted with pockets and drain valves which allow removal of
moisture condensed during cooling together with any excess lubricating oil mist
carried over in the air.
An aftercooler is fitted after the final stage of compression. This does not improve
the compressor efficiency but aids removal of moisture and oil and reduces air
temperature before passing it to the air receiver.
An intercooler usually consists of copper tubes through which the air passes.
These are expanded into brass tube plates and header, which allows for expansion.
Cast iron casing permits circulation with cooling water.
A relief valve should be fitted to the air connection and a bursting disc fitted to the
water casing. The bursting disc will relieve excess pressure in the water casing in the
event of failure of an air tube.

\

|

SSS

f}

Ort

OUT

{

{

|by

4

4

t

AIR

i

Hi

BURSTING
DISC

i
Y

4

J
4

DRA

}

VAL
FIT

{

|

WATER
IN

“ H C)

dS

Fig. 56

68

Intercooler

OPERATING

SYSTEMS

Little attention is required by intercoolers when in use. Correct temperatures and
adequate water circulation must be maintained and air drains opened when starting
or stopping the compressor and at frequent intervals while operating.
During overhaul, coolers must be cleaned internally and checked for corrosion.
The effect of operating with fouled intercoolers will be to raise the air temperature, this will cause lubrication difficulties in the compressor, efficiency will rapidly
fall off and there may be the risk of an explosion caused by overheated parts in the air
system.
Q. Describe, with the aid of a line diagram, an air starting system for a large diesel
main engine. Enumerate the safety devices fitted.
A. Fig. 57 is a line diagram of a manually operated air starting system for a large,
slow running main engine.

AIR

RECEIVER
vse
RESSOR

AUTOMATIC
VALVE

AIR STA
MANIFO Taon

CY EIN DER

ALR

START

Fig. 57

VALVES

Air starting system

Starting air is supplied at correct pressure from the air receivers through a slow
opening stop valve. This air should be free of moisture and oil. Air passes to the
control or pilot valve. This is a quick opening valve, spring loaded to return it to the
closed position after use. Interlocks or blocking devices may be fitted to prevent
operation of this valve in the event of turning gear being engaged, direction controls
incorrectly set, fuel control wrongly positioned or failure of essential engine systems.
As the control valve is operated, it causes the automatic or remote operating
valve to open allowing main starting air to pass to the air start valve manifold. This
automatic valve should incorporate a non-return valve.
69

OPERATING

SYSTEMS

Compressed air is also passed to the distributor or timing valves. These are
synchronised with the engine position in order to pass air to operate each cylinder air
start valve in the correct order and timing as the engine rotates. Timing is controlled
by cams or gear drive from the engine camshaft.
The cylinder air start valves are normally held closed by a.compression spring
together with cylinder pressure acting over the valve lid. Air from the manifold
enters these valves where it forms a pressure balance between the underside of the
valve lid and a balance piston of equal area on the valve spindle. Consequently this
does not cause the valve to open.
Cylinder valves are opened when operating air, transmitted from the distributor,
applies pressure to the larger operating piston on the valve spindle. As the valve is
forced open, starting air from the manifold enters the cylinder applying pressure on
the piston and causing the engine to rotate in the corresponding direction. To close
the cylinder valves the connection from the distributor is opened to atmosphere,
allowing the spring to close the valve and return the operating piston.
Safety devices required for each starting air system for reversing engines will
include a non-return valve, a relief valve fitted to the air manifold designed to relieve
rapidly any pressure build-up. A flame arrester, bursting cap or disc must be fitted to
each air start valve connection.
Drain valves must be fitted to keep the pipelines free of accumulations of oil,
additional drains being fitted to any low lying pipes.
Q. Describe, with the use of a line diagram, a cooling system for the pistons and
jackets of a slow speed large diesel engine. Explain how expansion of the parts is
accommodated without leakage.
A. Fig. 58 is a line diagram for a cooling system for fresh water cooling of pistons
and jackets of a large, slow two-stroke engine. Two separate circuits are used, the
cylinder jacket system including the cylinder covers and turbo-chargers. The piston
cooling system is separate to prevent any possibility of contamination from piston
cooling glands.
The jacket cooling system is a closed circuit. Water passing from the engine
returns through a cooler to the pump and then to the engine. (In some systems the
cooler is between pump and engine). A header or expansion tank is placed at a
reasonable height to allow venting of the system. This has connections from the
engine discharge and to the pump suction line.
A heater is included with by pass to warm the engine when necessary.
Water enters at the lower end of the jackets, passing up to connections from the
top of these to the cylinder covers and then to exhaust valve cages, if these are fitted.
Some water is taken from the discharge and passed through the turbo-charger
turbine cooling spaces, returning to the main discharge after a baffle which creates a
pressure difference in the line. An air separator is fitted in the return and this
discharges any air to the expansion tank. Other venting connections are taken to the
expansion tank from the top of the engine system, from the turbo-chargers and
cooler.
The piston cooling system pump draws from a supply tank passing water to the
piston cooler and then to the engine piston cooling connections. The return from
these flows by gravity to the supply tank. Vents are fitted at high points adjacent to
the piston cooling glands. Arrangements may also be included for the return of any
leakage from the glands. This must first pass through an oil separator and inspection
tank.

70

OPERATING

AUR

VENTS

_-EXPANSION

SYSTEMS

TANK

4
AIR

SEPARATOR

a4

2

STRAINER

SEA

WATER

COOLING

PISTON

WATER

PUMPS

COOLER

ENGINE

PISTON
WATER

STEAM

COOLING
TA NK

Fig. 58

COWL

Water cooling system

A steam coil is fitted in the piston cooling water supply tank for preparing the
engine for sea.
|
All fresh water coolers are circulated with salt water and have by-pass valves
| fitted. Fresh water pressure should always be greater than that of salt water to
prevent the possibility of salt water entering the engine system.
Both jacket and piston cooling systems must have alarms fitted to give warning of
loss in pressure, high or low tank level or, in some cases, excess temperature.
Pressure relief valves must also be fitted.
The main expansion allowances required are: cylinder liners which are firmly
secured at their top flanges with ample room to expand freely at their lower ends.
Rubber seal rings or other soft packings are fitted into grooves cut in the liner,
allowing a watertight joint with freedom to expand.
The tube stacks in heat exchangers will also require expansion arrangements.
One end of the tube stack is secured and the other is free to move, while a compressed
‘O’ ring prevents leakage.
In some engine jacket cooling systems, an emergency connection may be made to
circulate the system with salt water. In the event of such an extreme emergency,
spectacle blanks must be changed at inlet and discharge to circulate the system, the
cooler will not be used. Temperatures in the system must then be restricted to a
maximum of about 45°C to avoid excessive deposits and scale formation. This must
only be a temporary measure and at the first opportunity the system must be well
flushed and cleaned or de-scaled as necessary. Spectacle blanks must be replaced and
the system returned to normal fresh water conditions.
aA

SECTION

6

Control
Q. How may the reversing of a large two-stroke engine be carried out? Sketch a
reversing system and describe its operation. What is the purpose of ‘lost motion’?
A. Reversing an engine is carried out by altering the timing of valves and fuel
pumps to cause them to start the engine in the opposite direction and then continue
its operating cycle in this direction.
In a two-stroke exhaust ported engine, both scavenge and exhaust port timings
will be symmetrical about bottom dead centre and these will be identical when the
engine is reversed. In such engines, only the air start and fuel timings will require
adjustment. If both are operated from the engine camshaft, their cam profiles may be
designed to give the same retiming angle for reversing. Retiming is then carried out
by altering the position of the camshaft relative to the crankshaft of the engine. A
servo-motor may be fitted to the camshaft drive mechanism to do this.
Both air start distributor and fuel pumps will require ‘lost motion’ for reversing.
This means that the camshaft will lose motion, or be retarded, through a given angle
when the engine is operating in the reverse direction. The lost motion can be carried
out while the engine is at rest, the servo-motor being operated by the engine
reversing controls.
Interlocks will be necessary to ensure the lost motion is completed before
applying starting air.
AIR

FUEL

START

DISTRIBUTOR

PUMP

Fig. 59

72

Reversing system

CONTROL

A lost motion servo-motor is shown in Fig. 59.
After lost motion has been carried out, the air start cam will cause the distributor

to apply starting air to a cylinder, in which the position of the piston will rotate the
engine in the reverse direction. The cam will also now turn in the reverse direction,
operating the valves in reverse firing order.
Fuel pump cams will now operate injection timing on their reverse cam flanks
which are now correctly timed.
In the line diagram shown, reversing is carried out by means of an oil operated
lost motion clutch. Oil is passed under pressure to the corresponding spaces causing
the servo-motor to rotate through the required angle (98° in the case); the fuel pump
and air start cams will now operate the engine in the reverse direction.
If exhaust timing or poppet valves are fitted, they will also require retiming. This
can be carried out by fitting a second servo-motor to the exhaust valve drive.

Q. Sketch and describe a governor suitable for an auxiliary diesel engine. Why is it
fitted and what maintenance does it require?
A. Fig. 60 shows a mechanical hydraulic governor which may be fitted to control
the speed of an auxiliary diesel within close limits, while allowing a wide range in

FUEL

ag

Ss

SS

\

BALLHEAD

SPRING

"|!

.

1

Saree

FLYWEIGHT

|

"Wy

ie

“

Hy

Polis

GB

nt

“3

la

SA

VALVE

SPEED

ADJUSTMENT

i

if

Zolly

Ait

Naes NA
Whe

|

Sle

itera

SSSs3

PISTON

CONTROL

eS

\d\ =

us

fa

Ne

NS

ae

i}
i}

|

i |
1

VZZAN
ae
|
GNA

Fig. 60

“

INAIAAN

——
“~ RETURN
SPRING

ie
skiize

“~POWER

PISTON

Engine governor

power output according to demand. It may be used to control electric generators
where fluctuations in demand occur.
The governor will automatically alter the engine fuel pump settings to regulate
W3}

CONTROL

power output and return speed to the set value. It should ‘fail safe’, so that the engine
is stopped in the event of a failure.
By using a hydraulic system to operate the fuel control, greater power can be
applied from the governor without loss in the sensitivity. Oil pressure is either taken
from the engine lubrication system or from a separate gear pump on the governor
drive.
The governor incorporates two systems. The mechanical ball head which senses
any change in the speed of the engine, and the hydraulic piston valve and power
piston which operates the fuel pump control setting to give the required change in
power output.

The ball head consists of two identical, eccentrically pivoted, flyweights mounted
on opposite sides of a rotating sleeve. Speed of rotation is proportional to engine
speed and may be by direct drive from the cam shaft or through a step-up gear to
increase governor speed and sensitivity.
A crank on each flyweight bears on a sliding compression plate, which is held in
equilibrium between forces from the flyweight, and a compressive force from
compression of the helical governor spring.
If the engine speed slows, the centrifugal force on the flyweights decreases
allowing the moving sleeve to be lowered by the spring. If the engine speed increases,
the centrifugal force increases and flyweights raise the sleeve, compressing the
spring.
The sleeve is connected to a hydraulic piston valve and consequently any speed
alteration automatically causes a corresponding change in sleeve and piston valve
positions. The piston valve controls oil pressure to operate the governor power
piston.
If the engine speed slows, oil pressure passes to raise the power piston which is
linked to increase the fuel setting. The piston is spring loaded and, when the engine
speed increases, the oil pressure is released allowing the spring to force the piston
down and decrease the fuel setting. In this way, fuel is shut off from the engine in the
event of a failure in oil supply.
Linkage from the power piston will also operate a governor speed droop lever.
This is pivoted to alter the ball head spring compression thus resetting the piston
valve position in order to stabilise the governor action.
With this type of governor a slight change in engine speed occurs with any change
in load. This can be corrected by the speed adjustment control shown, which moves
the droop lever pivot and corresponding governor setting.
Maintenance of this type of governor is required to maintain the working parts in
a free condition without undue wear. Check the governor springs, maintain pressure,
oil tightness and cleanliness of hydraulic system and, if a separate oil system is used,
replenish this as required.

74

SECTION

7

Safety and Operation
Q. Describe how a crankcase explosion may occur and state the possible causes.
What safeguards may be used to reduce the risk of this occurring or of reducing
subsequent damage? List the procedures which should be taken if such conditions
occur.
A. The cause of a crankcase explosion is a ‘hot spot’ or overheated part within or
adjacent to the crankcase of an operating engine. Under normal running conditions
the air in a crankcase will contain oil droplets formed by lubricating oil splashing
from the bearings and impinging on moving surfaces. This mixture will not readily
burn or explode.
Crankcase lubricating oil should normally have a high closed flash-point (above
200°C) and this must be maintained in order to reduce risk of explosions. The most
common cause of lowering the flash-point is contamination with fuel oil.
Local ‘hot spots’ may arise due to overheating of bearings, piston rod gland,
timing chain, hot combustion gas or sparks from piston blow past in engines where no
diaphragm is fitted, or from fires in spaces adjacent to the crankcase, such as scavenge
trunks, etc.

2

Such sources can be eliminated by proper maintenance, correct lubrication and
oil condition, cleanliness and by avoiding overloading the engine. The general use of
white metal bearing materials which have moderate softening and melting temperatures will also help to avoid a rapid rise in temperature.
If a ‘hot spot’ exists, some oil will come into contact with it and will be evaporated.
The evaporated oil circulates to cooler parts of the crankcase and there it condenses
to form a white mist consisting of finely divided oil particles well mixed with air.
This mist is combustible within certain concentrations.
If the mist should now circulate back to the hot spot in such concentrations it will
be ignited and a primary- or minor-crankcase explosion will occur. This explosion
causes a flame front and pressure wave to accelerate through the crankcase,
evaporating further oil droplets in its path. The pressure shock wave may build up
sufficiently by the time it reaches the crankcase casing to rupture crankcase doors or
panels, unless otherwise relieved. In this event, the low pressure area following will
draw air back into the crankcase where it will mix with evaporated and burning oil to
cause a secondary- or major-explosion. This explosion will be of such intensity as to
cause widespread damage. It may start fires in the vicinity and injure personnel.
If the conditions of a hot spot do arise within the crankcase, a watchkeeper may
detect them by irregular running, by engine noise, by increase in temperatures, by
smell and by the appearance of the dense white oil mist.
Detection by instruments may be by temperature sensitive probes within the
crankcase, situated near the bearing oil returns, or more commonly by the use of a
crankcase mist detector (see Fig. 61). This operates visual and audible alarms in the

gi

SAFETY

AND OPERATION

event of a white mist being formed at well below the concentrations required for
explosive conditions.

Crankcase explosion relief valves (see Fig. 62) must be fitted to all but the
smallest crankcases. These valves open automatically at low pressures allowing
pressure of the primary or minor explosion to be dissipated and preventing possible
rupture of the casing. The valves instantly close when the pressure drops and thus
prevent the ingress of air, and eliminate the possibility of a major explosion. Valves
are fitted with wire gauze to prevent the emission of flames and may have external
deflectors to aim hot gases in directions where they will do least damage.
In the event of a hot spot or explosive condition being detected, the engine must
immediately be slowed and should be stopped as soon as possible to allow the
overheated parts to cool down. It may be advisable to operate the engine turning
gear, with indicator cocks open, to prevent seizure of overheated parts.
Personnel should avoid the vicinity of relief valves.
On no account must the crankcase be opened until the parts have cooled, as such
action may allow the ingress of air and precipitate a major explosion. When the parts
have cooled, inspection and maintenance or repairs may be carried out.
Sub-division of crankcases will prevent any build-up of high velocities and
pressures of flame propagation through the crankcase from a primary explosion.
Crankcase doors should be of robust construction to prevent rupture. Any
internal crankcase lighting must be flameproof. Vent pipes fitted to crankcases
should not be too large; they must be led to a safe place, remote from the engine and
fitted with gauze. Oil drain connections from the engine must extend below the oil
level in the sump. There must be no common communication between the crankcases
of two engines.
In some cases, inert gas flooding systems may be fitted to crankcases. If these are
used, after cooling, the crankcase must be well ventilated before personnel may enter
for inspection.
Q. Sketch and describe a crankcase mist detector. State how it operates and how
samples are taken from the engine crankcase.
A. Fig. 61 is a diagrammatic view of a Graviner oil mist detector which may be
fitted to monitor samples taken continuously from the crankcase of a diesel engine.
Such a device will detect the presence of oil mist at concentrations well below the
level at which crankcase explosions may occur. This gives warning in time to allow
P
Cc

MIRRORS

MEASURING

TUBE

EXHAUST

SELIECT OR
(VAPOU Bits
SAMPLE
PO

Fig. 61

76

Crankcase mist detector

SAFETY

AND OPERATION

avoiding action to slow the engine and prevent either serious bearing damage or an
explosion.
The detector consists basically of two parallel tubes of equal size, each having a
photo-electric cell fitted at one end. Photo-electric cells are light sensitive and
generate an electric current directly proportional to the intensity of the light falling
on their surface. Lenses are fitted to seal the ends of each tube but allow light to pass.
Two identical beams of light from a common lamp are reflected by mirrors to pass
along the tubes on to the cells which are then in electrical balance.
One tube is sealed to contain clean air and is termed the reference tube. The other,
termed the measuring tube, has connections through which samples of the vapour
content of the engine crankcase are drawn by means of an electrically driven
extractor fan.
In the event of a concentration of oil mist being present in the sample, light will be
obscured before reaching the cell of the measuring tube. Electrical balance between
the two cells will be disturbed and an alarm will be operated.
Sampling points should be fitted to each cylinder crankcase and their connections
are brought to a rotating selector valve which is driven from the fan motor. This
repeatedly connects each sampling point to the measuring tube in sequence.
In the event of oil mist being detected the rotator stops to indicate which sampling
point is concerned. The instrument must be reset before the alarm ceases and
sampling will recommence its sequence.
Sampling connections should not exceed 12-5 metres in length and must slope to
ensure positive drainage of oil; they must avoid any loops which could fill with oil.
The extractor fan is of very small dimensions and after testing, the samples are
exhausted to atmosphere.
The detector should be tested daily and the sensitivity checked. The lenses and
mirrors should be cleaned periodically. In this model the total mist concentration is
measured with respect to clean air.
An alternative model draws samples through both reference and measuring
tubes.
A mixture from all cylinder crankcases is passed through the reference tube while
comparison is made with samples from each cylinder crankcase and also from the
atmosphere in turn. In this manner a general sample of all cylinders is compared with
the normal atmosphere and each individual sample is compared against the average.
Q.

Sketch and describe a crankcase explosion relief valve, list the materials used

and any maintenance required.
A. Fig. 62 shows a crankcase explosion relief valve which may be fitted to a diesel
engine crankcase. It consists of a light spring loaded non-return disc valve of simple
construction.
The valve disc is manufactured of aluminium alloy which reduces its mass and the
inertia to be overcome when opening or closing valves rapidly. The large diameter
spring will give sensitivity and allow the valve to float. The absence of a valve spindle
eliminates the risk of the valve sticking.
The valve landing must make a gas and oil tight seal when closed and a non-stick
oil and heat resisting rubber ring is fitted to the disc face.
An external aluminium valve cover secures the valve spring and acts as a deflector
to direct any gas emitted over an arc of 120°, this arc being aimed in the direction
where the hot gas can do the least damage. Inside the crankcase there is a dome
WI

SAFETY AND OPERATION

CRANKCASE

a

PPI 2222212

LIGHT
SPRING

A Lk kb

Oo | L

Ww E TF Ti E D

GAUZE

\
Sey
iM]

M
oe

4iy4

:.

DEFLECTOR

Z
A

VALVE

ay
SEAL

Fig. 62

RING

Explosion relief valve

shaped flame trap made of several layers of woven, mild steel wire gauze. This
projects into the crankcase where it will become wetted with oil mist or splash from
adjacent bearings. When wet with oil in this manner the gauze dissipates greater heat
and becomes more effective as a flame trap. Free area of the gauze must at least be
equal to the area of the open valve.
The valve assembly is secured to an aperture cut in the crankcase by a number of
coverstuds and distance spacers; these act as guides for the valve disc.
The valve spring is designed to allow the valve to open under an internal pressure
of approximately 5kKN/m’ and will close automatically when pressure has been
relieved.
Regulations demand that for engines of over 300mm bore, one crankcase relief
valve of approved design is fitted to each crankcase and chain case. The combined
area of the relief valves should be not less than 115cm’ per cubic metre of crankcase
volume. The free area of each valve is not to be less than 45cm’.
For smaller engines a reduction in the size and number of valves is allowed.
These regulations also apply to the crankcases of large air compressors, etc.
Crankcase doors should be robust to prevent damage or rupture before relief
valves operate to relieve pressure.
Valves will require little maintenance but should be tested periodically by hand;
the spring should be inspected and the gauze cleaned.
78

SAFETY AND OPERATION

Q. How are scavenge fires caused in two-stroke diesel engines? How are such fires
detected and what procedures should be carried out after detection?
Describe the precautions that should be taken before bringing the engine back to
full power. What safety devices may be fitted to scavenge trunks?
A. Ascavenge fire may be caused by the ignition of unburned oil and carbon which
has been blown from the engine cylinder into the scavenge spaces. This may include
unburned fuel or cylinder lubricating oil and may be due to incorrect combustion
caused by a defective injector, faulty fuel pump timing, incorrect fuel condition, lack
of scavenge air, partially choked exhaust, low compression, afterburning, by
operating the engine at overload conditions, or due to defective piston rings, badly
worn cylinder liner, or by wrongly timed, or excessive cylinder lubrication.
The oil will build up in scavenge spaces where it will become carbonised by
further heating and will then reach a condition in which it can burn in the presence of
air. It may be ignited by hot gases and burning particles from blow past of piston
rings.
Indications of a scavenge fire are loss in power and irregular running of the
engine, high exhaust temperatures of corresponding units, smoke in exhaust gas,
high local temperature in scavenge trunk, surging of turbo-charger and sparks and
smoke emitted from scavenge drains.
If a fire is detected, engine speed must be reduced, fuel shut off from the affected
units, cylinder lubrication increased and scavenge drains shut.
In the case of a minor fire this may shortly burn out and conditions will gradually
return to normal. The affected units should be run on reduced power until inspection
of the scavenge trunking and overhaul of the cylinder and piston can be carried out at
the earliest opportunity.
Should a fire persist, if there is a risk of the fire extending or if the scavenge trunk
is adjacent to the crankcase with risk of a ‘hot spot’ developing, the engine must be
stopped, normal cooling maintained and it may be advisable to engage turning gear
to prevent seizure. If air discharge flaps are fitted these should be closed.
Fire extinguishing medium should be applied through fittings in the scavenge
trunk, these may inject carbon dioxide, dry powder or steam smothering.
Measures must be taken to avoid the spread of the fire and, if necessary, cooling
may be applied to the outer surfaces. On no account must the scavenge trunk be
opened up while it is still hot or danger of an explosion may arise.
Personnel should avoid standing close to relief valves.
After extinguishing the fire and cooling down, the scavenge trunking and
» scavenge and exhaust ports should be cleaned and the trunking together with
cylinder liner and water seal, piston, piston rings, piston skirt, piston rod and gland
must be inspected. Tightness of tie bolts should be checked before restarting the
engine. Fire extinguishers should be recharged at the first opportunity and faults,
diagnosed as having caused the fire, must be rectified.
To prevent scavenge fires good maintenance and correct adjustment must be
carried out. Scavenge trunking must be periodically inspected and cleaned and any
build-up of contamination noted and the appropriate remedy applied.
Scavenge drains should be blown regularly and any passage of oil from them
noted.
Safety devices fitted to scavenge systems may include an electrical temperature
sensing device fitted within the trunking. This will automatically sound an alarm in
the event of an excessive rise in local temperature (above 200°C).
TES:

SAFETY

AND OPERATION

Pressure relief valves consisting of self-closing spring loaded valves are fitted and
should be examined and tested periodically. Other safety devices could include the
scavenge drains and fittings or a fixed fire-extinguishing system. With such a system
maintenance will include inspection and cleaning of injection spreaders, weighing of
gas containers and rotation of dry powder containers to maintain powder free
flowing.
The alarm systems should be tested daily.
When any fire occurs in the engine spaces, the fire alarm should be sounded and
assistance or advice summoned. Cleanliness in these spaces will help to prevent fires
spreading.
Q. State the conditions which may lead to an explosion in the starting air pipe lines
of a large compression ignition main engine. What precautions are adopted to
prevent or minimise such an explosion?
A. The most likely causes of explosions in air start high pressure pipe lines are a
leaky cylinder air start non-return valve or such a valve sticking in the open position
while the engine is being manoeuvred.
Under normal! operation, some lubricating oil mist may be discharged from the
air compressor to the air start system. This oil may be from compressor cylinder
lubrication, faulty oil scraper rings or may be drawn in through the air suction with
contaminated engine-room atmosphere. The oil discharged is kept to a minimum by
draining the aftercooler, air receiver and starting system. Oil will deposit as a thin
moist film over internal pipe surfaces but is not readily combustible.
If a cylinder non-return valve should leak while the engine is in operation, some
hot gas, possibly with unburned fuel and cylinder lubricating oil, may be blown
through the valve to the adjacent air manifold. With further heating from the leaky
valve, this, together with the already deposited oil film, will carbonise and form
incandescent carbon. In the event of starting air now being applied to the system,
while still hot, the high pressure compressed air coming into contact with the burning
carbon may cause an explosion.
Such an explosion will cause a flame to pass rapidly back through the air start pipe
system, evaporating the deposited oil film and igniting it in the presence of air. Very
high velocities and shock waves are generated which may rupture pipes and fittings.
Alternatively, if excessive oil has entered the air start system, a mixture of air and
oil droplets may be discharged through the open cylinder non-return valve during
starting. This spray may ignite due to high temperatures in the cylinder causing a
flame to pass back through the still open valve to the air manifold.
To prevent an explosion, air start valves must be correctly maintained and
lubricated to ensure correct timing and free movement with positive closing. Oil in
the system must be kept to a minimum, pipe lines must be drained and cleaned
internally when necessary and oil discharge from air compressors must be kept toa
minimum by good maintenance.
To minimise the effects of such explosions, the air start manifold to each cylinder
valve must be fitted with a flame trap and ample relief valves; bursting caps or discs
must be fitted to relieve excessive pressure and an isolating non-return valve fitted to
the system.
A leaking air start valve can be detected while the engine is operating by the local
overheating of the pipe adjacent to the valve. If this should occur, the engine should
be stopped at the first opportunity and the valve replaced. Asa temporary measure, a
blank flange may be fitted to the air manifold connection to isolate this valve, but
80

SAFETY

AND OPERATION

such action may cause a ‘dead spot’ in the engine revolution at which—due to
inoperation of the valve—the engine may not immediately be started and the bridge
must be warned of this.
When the air start system is not in use it must be shut down and all drains should
be opened. Drains are closed after air pressure has been put on the system. Air start
valves should be lubricated, where such fittings are present, before the start of a
voyage or during a long voyage without use. Cylinder non-return valves may be
tested for tightness while in port by shutting isolating valves on the control air
connections and applying air to the air start manifold. Escape
of air through open
indicator cocks will detect a leaking valve.
In the event of overheating the discharge from the air compressor to the filling
line an explosion would be possible between the compressor and the air reservoir.
Overheating may be caused by failure of compressor intercooler and circulating
water. In this event the high temperature within the high pressure stage will make
operation of the compressor and its cylinder lubrication difficult.
Excess discharge temperature is detected either by an alarm system or a fusible
plug which will melt at 121°C to give warning.
In some engines it is possible to gag an air start valve. It must be stressed that
gagging does not stop the valve leaking and should not be adopted.

Q. Sketch and describe a cylinder relief valve for a diesel engine cylinder and state
what maintenance it requires. Give the circumstances under which such a valve may
operate.
A. Fig. 63 shows a section of a cylinder relief valve as fitted to the cylinder cover of
a large, slow running poppet valve engine.
The valve is of stainless steel with a mitre seat and it is loaded by the compression
of a helical spring. The spring keep is screwed hard down into the valve housing and
spring compression is adjusted by the thickness of the adjusting ring at its lower end.
The lower end of the valve spindle lands on the spindle seating disc, allowing the
valve to align itself on its seat. Valve lift is limited by a collar at the upper end of the
spindle.
The valve will protect the cylinder against excessive internal pressure and it
should be set to lift at not more than 20 per cent above the maximum designed
pressure for the cylinder. The discharge from the valve should be directed in such a
way as to be harmless to personnel.
* Valve maintenance will mainly consist of cleaning and inspection and should be
carried out at similar intervals to cylinder overhaul. The valve and seat must be
examined and ground in if necessary; the spring checked for warping and its free
length measured; the adjusting ring should be changed if necessary, and the valve
pressure tested. This can be carried out by an adapter fitted to the fuel injector test
ump.
: When a valve lifts in service it indicates incorrect conditions within the cylinder.
These must be ascertained and correcting action taken; the relief valve should also be

inspected at the first opportunity.
The most likely occasion on which relief valves may lift are starting, slow running
or manoeuvring of the engine. It may be due to one of several causes. Excess pressure
in the cylinder, due to the leakage of fuel into the cylinder during priming of the fuel
system or due to a leaking fuel injector, will cause the valve to lift during the first
81

SAFETY AND OPERATION

SPRING

VALVE

HOUSING

KEEP

=m
SPRING

VALVE

SPINDLE

VENT

HOLES

VALVE

PINDLE
!

VALVE

Fig. 63

SEATING

SEAT

Cylinder relief valve

revolution; there may be violent ignition due to the engine turning too slowly at first
firing.
Lifting may be due to incorrect fuel pump setting or timing, the air start valve
stuck open or wrongly timed; from these it can be seen that a faulty cam
may also
cause some of these faults or it may be the relief valve itself which is at fault.
A number of relief valves may lift if the camshaft drive is incorrectly timed,
chain
breakage, etc. occurs or if the reversing mechanism is incorrectly set or there
is a
governor fault, particularly if the engine tends to race in heavy seas.
In some engines, when manoeuvring, it is possible to set the controls to
reverse,
and then apply starting air, before the engine has come to rest. In such
an event,
excessive compression pressures will be caused and cylinder relief valves
will lift.
82

SAFETY AND OPERATION

In the event of a large leakage of water or oil into the cylinder while the engine is
stopped, the area of the relief valve may be insufficient to safeguard the cylinder upon
starting, and damage may be caused to the engine. To protect against this, the engine
should first be rotated slowly with indicator cocks open.
Q. What are the causes of black smoke from an engine exhaust?
defective unit be ascertained and what action should be taken?

How

can a

A. Black smoke in an engine exhaust indicates incomplete combustion of fuel and
unless the reason is obvious, efforts must be made to ascertain whether one particular
unit or a number of units are causing the smoke.
A particular unit making smoke may be detected by shutting fuel off from each
cylinder in turn, while observing the exhaust condition. This method will be
inconclusive if more than one unit is causing smoke. On some engines, test cocks are
fitted to the exhaust branches from each unit. By opening these in turn, the exhaust
gas from each unit can be sampled. The gas may be allowed to blow on to a piece of
wet rag or damp paper on which a deposit will show from units causing smoke. For
smoke from a single unit, various combustion or injection faults may be the cause.
These may include: incorrect fuel pump timing, faulty fuel injector, lack of scavenge
air or low compression. Fuel timing may be due to incorrect fuel pump setting
(usually late injection to cause smoke), worn fuel pump plunger, worn, slack or
wrongly positioned fuel cam. Injector faults may be a worn atomiser, partially
choked injector, leaky injector or wrongly set spring. Lack of scavenge air may be
caused by carbon in ports for a two-stroke or a faulty inlet valve for a four-stroke
engine. Low compression may be due to faulty piston rings, worn cylinder liner or
leaking inlet or exhaust valve. Further assistance in diagnosing the fault can be
obtained by taking indicator diagrams from the cylinder, e.g. draw card for fuel
combustion faults, compression card to show low compression or light spring
diagram to detect choked scavenge, etc. Exhaust temperature may also indicate the
nature of the fault such as afterburning, overloading, etc. If all or a number of units
are producing smoke, possible reasons are: a scavenge fire, this will also affect
exhaust temperatures and it should be checked first. Incorrect fuel condition, with
heavy fuel: this may be caused by too low a temperature and too high a viscosity at
the injectors. Insufficient scavenge air: this may be due to fouling of the turbocharger, which would probably be detected due to surging. Fouling of charge air
cooler, dirty intake filters or partly restricted exhaust system. This would also include
leakage of valves in the scavenge pumps or under piston charging. Overheating of the
“injector tips, with heavy fuel; this would cause carbon build up which would interfere
with the combustion pattern. Overloading of the engine or when changes in the
engine controls are made; smoke is then due to low air pressure caused by lag or
delay in supply from the turbo-charger. This must accelerate as additional exhaust
energy is supplied before it can satisfy the demand from the engine for a greater mass
of scavenge air.
Remedial action will depend upon the fault. For scavenge fire see page 79. Faulty
fuel injectors may be replaced, fuel condition corrected, valve cooling temperature
lowered, a fouled turbo-charger may be water washed, filters may be cleaned and
overload may be reduced. A fire in the silencer or exhaust uptake will also generate
black smoke. This will have little effect on the running of the engine. Such a fire may
be caused by unburned carbon carried over from incomplete combustion in the
engine. By keeping the engine running, an inert gas effect is obtained from the
83

SAFETY

AND OPERATION

exhaust. External surfaces in the vicinity of the fire should be cooled and care taken
that combustible material is removed.
Q.

Give
(a)
(b)
(c)

possible causes for the following and state how they may be overcome:
fluctuations in level of oil sump
excessive noise from crankcase
choked exhaust port.

A. (a) Fluctuation in the level of the oil sump or drain tank may be due to a
number of causes. This may occur to a limited extent due to movement of the
ship—either pitching or rolling—this is particularly noticeable if the oil level gauge is
not situated at the centre of the tank. The reliability of the level gauge should also be
checked since float or transmission equipment may be faulty. A check must be
carried out to ascertain that no valves have been incorrectly opened or closed,
allowing oil to enter or leave the system.
If the oil level has risen it is possible that there is a leak into the system, this may
be water from leaks in the cooling system, such as glands or seal rings, allowing
contamination of the crankcase from cylinder jackets, water cooled pistons, etc.
There may be a leak at the oil cooler although the oil pressure should be greater than
that of sea water to prevent this. A test for water will indicate such a leak.
Although fuel contamination could occur under certain circumstances, it is
unlikely to be at sucha rate as to be noticed at sump level. A flash-point test on the oil
will ascertain if fuel is present. Choking of the oil filters or failure of the pumps may
allow a level rise but this will be evident from the pressures within the system and
perhaps pressure alarms.
If the oil level has fallen there may be an oil leak in the system and this must be
searched for at all pumps, filters, pipes, joints and glands, at the crankcase and the
drain tank. The water seal in the oil purifier together with the purifier pump should
also be checked. Choking of oil drain grids within the crankcase will cause the oil
level to build up there and not return to the drain tank. The possibility of the pressure
relief valves allowing oil to leak from the system should be examined.
In trunk piston engines, failure of oil scraper rings may case the consumption of
lubricating oil to be high. This should be evident by high exhaust temperatures,
smoke and blow past from jammed piston rings. It may cause danger of fires or an
explosion.
(b) Excessive noise from a crankcase may be caused by slack or worn chain or
camshaft gear drive. Knocking from slack bearings, particularly guides, due to wear,
misalignment, slackening or breakage of bolts, studs or other fixtures. Engine
overload or early ignition will transmit heavy forces to bearings. Piston cooling noises
such as water hammer or from slack glands. Lubrication faults from insufficient oil
pressure, loss of lubricating properties of oil, failure of oil pipes or choking of oil
passages.
In trunk piston engines additional noise may be from piston ring blow past or
from overheated piston or liner. Any change or sudden increase in the noise pattern
must be investigated and the necessary adjustments carried out.
(c) Choked exhaust ports are caused by incorrect combustion and this in turn
may be due to incorrect fuel condition, defective fuel injection or incorrect fuel pump
timing. Other causes are lack of scavenge air from fouled turbo-charger or charge air
cooler, high back pressure in exhaust system from choking in exhaust grids,
turbo-charger, silencer, etc. It may also be caused by excessive cylinder lubrication.
84

SAFETY

—$—
ATMOSPHERIC

Fig. 64

AND

OPERATION

LINE

Diagram showing choked exhaust

It becomes evident by a loss in power and possibly surging of the turbo-charger with
smoke at the exhaust. An indicator diagram will show a high exhaust pressure within
the cylinder.
The remedy will be to remove the carbon, carry out necessary cleaning, and
adjustments or renewals to improve the combustion.
Q.

How are the
(a) Leaking
(b) Leaking
(c) Leaking
What precautions

following engine defects detected:
exhaust valve
air starting valve
fuel injector?
are necessary and how are repairs carried out?

A. (a) A leaking exhaust valve can be detected during operation by high local
exhaust temperature with possibly noise and smoke. The compression pressure and
peak firing pressure during the cycle will be reduced.
If allowed to continue, damage to the exhaust valve and seat will increase due to
burning by high velocity, very hot gases. Partly burned fuel may pass to exhaust grids,
turbo-chargers, silencers and uptakes causing fouling, loss in efficiency, surging of
turbo-charger, uptake fires or explosions under certain circumstances.
The valve must be changed at the first opportunity and, until then, fuel should be
shut off that cylinder. To prevent such leakages, exhaust valves must be changed and
overhauled at regular intervals and valve tappet clearances checked. Excessive
powers must be avoided and combustion efficiency maintained.
“
(b) A leaking air start valve is detected by an increase in local temperature of
valve and air start pipe adjacent to it. Build-up of carbon due to leakage may
eventually cause an air starting line explosion when the engine has been stopped and
re-started while hot.
The engine should be stopped and the defective valve changed, alternatively, the
air start line to that valve may be blanked but warning must then be passed to the
bridge of possible starting difficulties due to the loss of starting for that cylinder.
(c) A leaking fuel injector can be detected by a loss in power in the affected
cylinder together with smoke at exhaust and high exhaust temperature. There may
also be a knock or pressure wave in the injection system. An indicator diagram taken
from this cylinder will show fluctuations of pressure during the expansion process due
to secondary burning of fuel leaking from the valve (Fig. 65). A higher expansion line
at the exhaust opening indicates afterburning (see Fig. 15).

85

SAFETY

AND OPERATION

Fig. 65

Diagram showing leaking injector

Loss in power will be due to incorrect combustion since the fuel pressure wave
from the fuel pump must refill the space left by fuel leaked from the injector; this
may also cause the knock. Hot gas from the cylinder may blow back into the injector
tip forming carbon and choking the atomiser. Carbon may also form on the outside of
the atomiser due to burning of the dribble of fuel. Afterburning causes the high
exhaust temperature, and partly burned fuel may now pass into the exhaust system
causing fouling and loss in efficiency of the turbo-charger, coking of exhaust ports
and grids, etc.
The injector should be changed and tested. To reduce the possibility of leaking
injectors, good maintenance of fuel pumps and injectors should be carried out with
routine changing of the injectors. Fuel must be purified and filtered and must be
supplied at the correct temperature. Fuel injector nozzle cooling must also be
maintained at the correct temperature.
In an engine with mechanically timed fuel, a leaky valve will allow an excess of
fuel to enter the cylinder causing overloading, knocking, high exhaust temperature
and possibly lifting of the cylinder relief valve.

86

Index
Afterburning
15
Air compressor
65
Arr inlet valve
26
Air starting system
69, 85
Atomisation
55
Balancing, power

17

injector

79

16, 57, 85

penetration

pump
system

50

Charge air cooler

Diagram, compression
indicator

11, 12

“light spring

13

12

out of phase
13
Diaphragm
36
Draw card
13
Engine indicator

55

59
53

22,24

Combustion
55
Compression diagram
12
Cooiing systems
70
Cooling, jacket
70
valves
26
Crankcase, explosion
75
relief valve
77
Crankshaft alignment
46
Crosshead bearing 45
Cycle, four-stroke
9
two-stroke
9
Cylinder, jacket 29
liner 29
lubrication
35
wearof
31
relief valve
81

Governor

73

Gudgeon pin

44

Indicator diagram
11, 12, 13
Indicator, peak pressure
18
Inlet valve
26
Intercooler
67

Jacket cooling 70
Jerk pump
59
Light spring diagram
13
Liner, chromium-plated
34
cylinder
29
Lost motion

2, 72

Lubrication, sump level
system
61

8&4

Mean indicated pressure
12
Medium speed engines
5
Mist detector
75, 76
Needle valve

57

Opposed piston engine
Out of phase diagram

4
13

11

Engine, four-stroke

Peak pressure
12, 18
indicator
18

5

medium speed
5
opposed piston
4
trunk piston 5
two-stroke
-Vieee”
O57
Exhaust ports

Fires, scavenge

80

Four-stroke cycle 9
Four-stroke engine
5
Fuel, injection
55

Bedplate
48
Blow-past
17
Bursting disc 67
Camshaft chain

Exhaust valve
2, 26, 85
Explosion, air starting line
crankcase
75

Penetration (fuel)
Piston, cooling

1, 2,4
1, 84

Exhaust temperature

18

large 38
medium speed
rings 41
rod gland
37

55

38, 70

43

87

INDEX

Power card
12
Pressure, mean indicated

Pump, jerk

59

Relief valve, crankcase

cylinder
Reversing

12

77

Two-stroke engine

81
72

receiver
63
system
69
Surging, turbo-chargers

Tappet clearance
26
Tie bolts 2, 30, 49
Trunk piston engine
5

88

1, 2,4

Undercooling
24, 26
Under-piston charging

Scavenge fires 79
Scavenge systems
19
Seal rings
30,71
Smoke, causes of 83
Starting air, explosion

Turbo-charger
22, 23
surging of 79
system
22Turbulence
55
Two-stroke cycle 9

80

79

2

Valves, cooling of 26
crankcase relief
77
exhaust
2, 26, 85
inlet
26
needle
57

timings
14
‘Vee’ engines
Viscosity
55
meter

5,7

54

Wear of cylinder liner

31, 34

a

XN

u

Drawing reproduced by kind permission of The Motor Ship,
Dorset House, Stamford Street, London SE1 9LU, UK,
from whom it is available.

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Marine Engineering Series
Marine Boilers: Questions & Answers

Steam Turbines and Gearing: Questions & Answers
Diesel Engines: Questions & Answers
Mathematics: Questions & Answers
Heat Engines: Questions & Answers
Engineering Science: Questions & Answers
Feed Water Systems and Treatment
Notes on Instrumentation and Control
General Engineering Knowledge

ISBN

0540 073423