Deven Aranha-Marine Diesel (Scanned)-1

By DEVENARANHA B.E. ( Mech.) Class I Engineer

S H R O FF P U B LIS H E R S & D IS TR IB U T O R S P V T. L TD .

Marine Diesel Engines

Table O f Contents

M a rin e D iesel E n g in es B y Deven Aranha

Preface Acknowledgements

© Shroff Publishers and Distributors Pvt. Ltd.

First Edition : July 2004 Seventh Reprint: January 2013 ISBN 13: 978-81-7366-927-9

P u b l i s h e d b y S h r o f f P u b l is h e r s a n d D is tr ib u to r s P v t. L td . C -1 0 3 , M ID C, T T C I n d u s tr ia l A re a , P a w a n e , N a v i M u m b a i 400 7 0 5 , T el: (91 2 2 ) 4 1 5 8 4 1 5 8 , F a x : (9 1 2 2 ) 4 1 5 8 4 1 4 1 , e-m ail: sp dorders@ shroffp u b lish ers.co m , P rin ted a t D eco ra B o o k Prints Pvt. Ltd., M umbai.

CONTENTS

CH A PTER 1 : INTERNAL COMBUSTION D IESEL ENGINES

A ll rig h ts re se rv e d . N o p a rt o f th e m aterial, p ro tected by th is copyright n otice, m ay be rep ro d u ced o r u tiliz e d in any form o r b y any m eans, electronic o r m echanical, including p h o to c o p y in g , r e c o rd in g , o r b y an y in fo rm a tio n sto ra g e and retriev al system , w ith o u t th e w ritte n perm issio n o f the copyright ow ners, n o r exported, w ithout the w ritten perm ission o f th e p ublishers.

Concept of Internal Combustion Engines......................... 01 Stroke....................................................................................01 Mean Piston Speed ............................................................02 Advantages / Disadvantages of Diesel Engines 03 Classification of 1C. Engines............................................ 04 Otto, Diesel. Dual and Actual Cycles................................06 2 -Stroke C y c le .....................................................................09 4 -Stroke C y c le ................................................................... 12 2-Stroke vs. 4-Stroke Engines .................................... 16 CH A PTER 2 : EN GINE COMPONENTS Engine Structure............................... ................L . 19 Top B racing..... ................................................ 20 Fatigue Failure.....................................................................21 Bedplate............................................................................... 22 Entabulature. A-Frame. Tie-Bolts and Pinching Screws 24 Holding Down Bolts and Chocks ...................................... 25 Resin, Resilient C hocks.... ......... 27 Piston : Water cooled. Oil cooled, Oros, Com posite.....29 2-Stroke versus 4-Stroke Pistons, Defects, Rotating Pistons. Piston Rings : Compression Rings. Oil Scraper Rings 36 Failures. Running-in. Shapes. Coatings. CPR Rings. Antipolishing Ring, SIPWA. Stuffing Box G lan d ............................................................. 44 Lmer. Liner W ear.................................................................45 Lubricating Quills and Accumulator 48

H

Marine Diesel Engines

■ Cylinder Head C over.......................................................... 50 Exhaust V alve..................................... .............................. 51 Valve Springs....................................................................... 53 Valve Rotators......................................................................55 Variable Exhaust Closing (VEC) 56 Crankshaft .......................................................................... 58 Crankshaft Stresses 62 Crankshaft Deflections....................................................... 63 Chain Drive, Tightening and Inspection 64 Chain Elongation.................................................................67 Camshaft Readjustment after Chain Tightening 68 Bearings Plain Bush Journal, Pivot Pad Journal 69 Mam Bearings................................................................. 71 Connecting Rod and its Bearings 72 Bottom End Failures and Bolt Design 74 Crosshcad Bearings............................................................ 75 Puncture Valve.....................................................................77 Engine Materials 78 CH A PTER 3 : A IR SYSTEM Scavenging,..:..;;;......................;............;....u..i..:;.............. 81 Uniflow, Reverse, Loop and Cross Scavenging............. 81 Gas Exchange Process.....S................................ 84 Supercharging......................................................................S5 Constant Pressure and Pulse Turbocharging 86 Series. Parallel Supercharging 89 TVo-Stage Supercharging 91 Single and Multiple TVbochargcr Systems 91 Power Take-In and Power Take-Off 92 Axial Flow Turbocharger 94 Uncooled Turbochargers 97 Surging................................................................................. 99 Compressor M a p .................................................................99

[ii]

CH A PTER 4 : A IR COMPRESSORS Isothermal Com pression................................... 103 Adiabatic Compression and the Compression C ycle.... 103 Multistage Compression .................................... 104 Reciprocating,and Rotary Compressors....... *................ 104 Volumetric Efficiency and Bumping Clearance ............... 105 Compressor V alves................. .............. fan..................... 105 Compressor F au lts.................. laSLari............................ 106

CONTENTS

CONTENTS

Marine Diesel Engines

C H A PTER 5 : FU EL SYSTEM F u e liy p e s.............................................................................109 Fuel Properties................................................................... 110 Fuel Specifications...................................... U6 Combustion Phases............................................................ 117 K nock........................................................................... ns Factors Affecting Com busuon.......................................... 119 Combustion Chamber and Piston Crown Designs ........ 121 Compression R a tio ............................................................ 121 Residual Heavy Fuel O ils................................................. 122 Bunkering ........................................................................... 123 Fuel Injectors................. 125 Injector ty p e s........................................................ 126 Injection Methods ...................... 130 Fuel Pu m p s............................ 131 Suction Valve Controlled P u m p ..................................... 131 Suction and Spill Controlled P um p.............................. 133 Port Controlled Jerk P u m p ............................................... 134 Injection System s................................................................ 135 Variable Injection Timings (V1T)..................................... 136 Fuel Quality Setting (FQ S)............................................... 140 Super-V IT and Conventional V1T.................................... 140 Fuel C a m .......................................................................... 146 High Pressure pipe sa fe ty............................. 147

[iii]

Marine Diesel Engines

Marine Diesel Engines

Start Air Interlocks.............................................................187 Slow Turning...................................................................... 188 Scavenge Air Limiter ................................................. 188 Firing Order of Cylinder................................................... 188 Reversing M ethods.......................................... 190 Loss Motion and Gain M otion........................................ 194 Running Direction Interlock ............................................ 195 Crash Manoeuvring ....................—................................... 195 Manoeuvring Flow C h a n ................................................. 197 Manoeuvring Diagram.......... 198 Bridge Control S y stem .......................................................202

CONTENTS

CONTENTS

CH A PTER 6 : LUBRICATION SYSTEM Friction and Friction Types...............................................149 Lubrication Types.............................................................. 151 Lube Oil Properties..................................... ...................... 152 Lube Oil Testing................................ ............................... 156 Microbial D egradation..................................................... 161 Cylinder Lubrication Types and System s........................162 Lubrication Pump U n it................................................... 166 Load Dependent Cylinder Lubrication..................... 167 Specific Cylinder Lube Oil Consum puon..................... 169 Frequency Controlled Electric Motor Lubricator.......... 169 Multilevel Cylinder Lubrication ............ 170 Crosshead Lubrication.................................. ....................171 C H A PT E R 7 : CO O LIN G SYSTEMS Function.............................................................................. 173 Bore Cooled Liners............................................................ 174 Load Dependent Liner Cooling....................................... 174 Piston Oil Cooling System.................................. Cooling Water TYeatment................................................... 175

175

CH A PTER 8 : STARTING , REVERSING AND MANOEUVRING Start System ....... ........................................ ...................... 177 Start Air Perio d.................................................. ............... 179 O verlap................................... ......... ............. ....................179 Start Air Receiver ...........................:............;................ 180 Start Air Pilot V alve.............. 182 Automatic Master Air Start Valve............................... .. 183 Start Air Cylinder Valve..................................................... 185 Start Air Distributor.... .................................. Start Air C a m ....................................................................... 187 [iv]

CH A PTER 9 : EN GINE STRESSES,V IBRATION AND DYNAMICS Forces Acting in a Single Cylinder E n g in e ......................205 Irregularity Factor............................................................. 207 Static and Dynamic Balancing........................................... 208 Primary and Secondary Imbalance —.................................209 Vibration D efinitions...................... ................................ 209 Torsional Crankshaft V ibration......................................... 211 Critical Speed ...................................... v.......................... 211 Barred Zone R ange............................................................. 212 Detuners and Dampers........................................................213 CH A PTER 10 : EN GINE OVERHAULS AND MAINTENANCE

186

Unit Decarbomsation................................ 215 Cylinder Head R em oval................................................. 216 Hydraulic Nut Removal ..................................................... 217 Exhaust Valve Rem oval...................................................... 218 Piston Removal. Inspection and Clearances 220 Piston Mounting................................-............................... 223 Liner Removal. Inspection and Calibration..................... 224 Main Bearing Removal .. ................... 225 M

Marine Diesel Engines

Crosshead Bearing R em oval..............................................227 Connecting Rod Bearing Rem oval.................................... 228 Crosshead Pin Removal...................................................... 229 Connecting Rod Removal................................................... 230 Thrust Bearing Pad Removal............................. 231 Bearing Clearances ...... 232 Fuel Pump Setting and A djustm ent...................................236 Fuel Pump Cut-out C hecks................................................. 238 Fuel Pump Cut-out............................................ 239 Fuel Pump L ead......... .....................------------------------- 239 4-Stroke Medium Speed Engine Fuel Pump Timings 241 Turbocharger Overhaul....................................................... 242 Turbocharger Out of Operation--------...-------- ------- ..... 243 Fuel Injector Overhaul............................. ....................... 244 Tie-Rod Tensioning.............................................................246 Air Compressor Overhaul .................................................. 249 Testing of Materials ........ 250 Heat T reatm ent............................ 250 CH A PTER 1 1 : EN GINE DESCRIPTIO N S AND SPECIFICATIONS Comparison of RD. RND and RTA Engines . . ...............253 RTA Engin es........................................................................ 254 RT-Flex Engines................................................................... 258 SM C E ngines....................................................................... 271 ME Engines........................ .........- ..... ............. ............... 278

L iner...................... ............ d lia u .J ...... ........ .................. 296 Cylinder Lubrication............. 297 P isto n ................................................................................... 297 Crosshead............................... _„...~.^...._J.i..L.................298 Engine Components......................................................... 298 CH A PTER 13 : EN GINE EMISSIONS

CONTENTS

CONTENTS

Marine Diesel Engines

Engine Emissions.............. ................ ................................301 SOx Effects and Remedy................... ..............................302 NOx Effects and Rem edy................................'.................302 Carbon Monoxide, Hydrocarbons, Particle Emission.... 304 S o o t..........................................................KiihillsU............305 Smoke and Opacity.................................:j.".......A............ 305 CH A PTER 1 4 : ENGINE PERFORM ANCE AND INDICATOR CARDS Engine Performance Definitions and Parameters...........307 Heat Balance Diagram 310 Power Ratings...................................................................... 310 Testing of Marine Engines ........................ ................. 311 Test Bed and Sea T rials......................................................312 Load Diagram and Propeller C u rv e ..................................314 Safety Margins .................................................................... 316 Indicator Diagrams and Analysis.................................... 318 Faults with Indicator Instruments...................................... 327

CH A PTER 12 : EN GIN E D EVELOPM ENTS

C H A PTER 15 : GOVERNORS AND CONTROL

Fuel Injection System ............................... - ......................291 Turbocharger System ........... - ........................... ................292 Scavenge System ............................................. - .............. , 296 Exhaust System .................................... - ............................296 Combustion Cham ber.......................................................... 296

Governor D efinitions................................ Mechanical G overnor............................... Hydraulic Governor with Compensation. Electric G overnor..................................... Governor Adjustments ............. - .............

[vi]

[vii]

329 331 331 333 334

Marine Diesel Engines

Marine Diesel Engines

Load Sharing and The Necessity o f D ro o p .....................335 Electronic Governor for Bridge C ontrol........................ 337

[viii]

ONTENTS

CONTENTS

C H A PT E R 16 : W ATCHKEEPING AND SAFETY Thlcing Over An Engine Room W atch............................345 Walk Through Checks of The Engine R oom ................. 345 Checks During The Engine Room Watch 350 Problems During The Engine Room W atch................... 351 Crankcase Explosion and Relief Valve............................ 351 Scavenge F ires.................................................................... 353 Oil Spill................................................................................354 Collision............................................................ ..................354 Flooding.............................................................. ,............. 355 G rounding.......................„ .v............................................... 355 Sudden O verspeeding........................................................ 355 Loss of Engine Pow er.............. ......................................... 356 Slack Tie-Rods................................................................... 356 Incorrect Fuel T imings........................................,............. 356 Engine Speed Fluctuation.................................................. 356 Funnel S p a rk s..................................................................... 357 Cylinder Relief Valve L iftin g ........ ..................................357 Reduced Compression Pressure .................................... 357 Smoky E x h au st.................................................................. 358 All Cylinders Exhaust Temperature Increase .............. 358 One Unit Exhaust Temperature R ise ................................359 Engine Speed D ro p s...........................................................359 One Unit Exhaust Temperature Drops.,...;)./...*...............359 Charge Air Pressure D ro p s................................................360 Engine Running Irregularly.............................. ............. 360 Jacket Water Pressure Fluctuation.................................... 360 Jacket Water Temperature Increase ................................ 360 Running Gear H o t.............................................................. 361 Engine Fails to Start on A ir ............................................. 361

Engine Turns on Air, Not on Fuel.......... ............................ 362 Engine Does Not Fue .......................................................... 362 Violent S tarting.....................................................................363 Engine Not Reversing....................................................... 364 Cracked Piston......................................................................364 Broken Piston Ring.............................................................. 365 Cracked L in e r.......................................................................365 Piston Running H ot......................................- ................... 365 Cracked Cylinder H e a d ...................................................... 366 Crankcase Inspection...........................................................366 Individual Piston Knocking at T D C .................................. 367 Bearing Temperature Increase............................................ 367 Lube Oil Sump Level R ising.............................................. 368 Automatic Stopping o f E n g in e ......................................... 368 Knocking in an Engine C y linder...................................... 368 Safeties in the Main Engine................................................ 369 Safeties in the Start Air S y stem ..........................................371 Leaky Start Air Valves.....----- ........---------- — ....... ......372 Start Air Line E xplosion......................................................373 Safeguard Against O vet speeding.................... 373 Bibliography

[ix]

PREFACE O v e r th e p a st d e c a d e , th e re h a v e b e e n s ig n ific a n t advances in the field o f m arine diesel engines.T he new m illen n iu m saw th e ad v en t o f a revolution in m arine engineering technology, w ith the introduction o f the latest ‘C a m sh a ft-le ss E le c tro n ic a lly C o n tro lle d In te llig e n t E ngine’ series. This b o o k h as been w ritten w ith a v iew to fulfilling the need o f m arine e ngineers to be in touch w ith up-to-date inform ation on present day engines, w hich h av e re p la c e d . the older series. In this age o f technological advancem ent, it is o f vital im portance that today’s m arine engineers keep abreast o f these developm ents and equip them selves w ith thorough know ledge o f the engines that they w ork on a reg u lar basis. A d istinctive feature o f this book is that th e text m atter is presented in ‘easy-to-understand’ point form , fo r the benefit o f m arine engineering students. B esides providing an in -d e p th u n d erstan d in g o f th e b a sic p rin cip les o f m arine diesel engines, th is book also g ives an insight into th e w orking o f m odern engines. T his b o o k w ill b e u seful to candidates appearing fo r the C ertificate o f C om petency exam inations.

Deven A ranha

CHAPTER 1

INTERNAL COMBUSTION DIESEL ENGINES Concept of Internal Combustion Engines Marine diesel engines are basically reciprocating engines using heavy fuel oil o r diesel oil in a Compression Ignition (C.I.) system. Unlike a Spark Ignition system w here a spark is used to ignite the fuel, a Compression Ignition system uses heat from compression to ignite the fuel in the combustion chamber. Fuel upon ignition in the combustion chamber gives a combustion force which pushes down the piston, i.e. work is done in the cylinder by combustive gases. This reciprocating motion o f the piston due to the combustive gas forces, is transformed into rotary motion o f the crankshaft. This is done by means o f the connecting rod and crank mechanism. Stroke (S)

Stroke is the distance covered by the piston between the top dead centre (TDC) and the bottom dead centre (BDC). Stroke = 2 ( Crank Radius)

Marine Diesel Engir

Internal Combustion Diesel Engines

M ean Piston Speed

Significance o f M ean Piston Speed The significance can be seen if we study the power equation. Pow er = Pm x (2 Sn) x A x n x constant. where, m ean piston speed = 2Sn Therefore, Pow er depends on M ean Piston Speed. Lim itations o f M ean Piston Speed The limitations of m ean piston speed are:

Vc = Volumeofcompressionchamber Va = Volume o f the cylinder Swept volume

Since,

= Volume swept by the piston from TDC to BDC = Vs = (Area) x length = (fi.D 2 ) S 4 Va= Vc + Vs .

Hence, C om pression R atio = M ean Piston Speed = =

= where,

2S

=

N

=

= Vc + Vs Vc Vc

1+Vis' Vc

(Piston distance in one revolution) x (R ate o f crankshaft rotation) 2§_n 60 Sr 30 Distance covered by the piston during one revolution. N um ber o f revolutions per second.

♦ The wear and life span o f the rotating and reciprocating parts due to friction; high temperatures and pressures; and lubrication conditions. ♦ Large forces due to rotating and reciprocating masses, w hich in turn give rise to stresses especially fluctuating stress; and moving parts due to inertia forces and dynamic forces. ♦ Gas exchange-scavenge period and efficiency: Higher the mean piston speed, greater will be the resistance to gas flow and exchange, when h ot exhaust gases have to be expelled and fresh air has to b e taken in. Advantages o f D iesel E ngines over S team E ngines ♦ High actual efficiency = Heat equivalent o f actual work done Total Heat generated in the engine ♦ Actual Efficiency, for steam engines for steam turbines for gas turbines for diesel Engines

= 12 to 18% = 2 2 to 32% = 2 5 to 36% = 36 to 42%

♦ High efficiency and recovery o f waste heat. 3

“ Marine Diesel Engines_____________________________________________

♦ H ighest use o f heat generated during combustion. ♦ Increased tim e period before refueling i.e. bunkering. ♦ Increased maneuvering abilities. ♦ Increased cargo carrying capacity since less space is required for the boiler, water storage, water consumption; and a smaller size o f engine in comparison to a steam plant and auxiliaries. ♦ Increased standby reliability. Disadvantages o f D iesel E ngines ♦ High inertia loads due to reciprocating and rotating masses. ♦ High capital cost, complicated design and construction. ♦ Pressures and temperatures are alw ays varying in the system. ♦ High lube oil costs in medium and high speed engines.

Internal Combustion Diesel Engines

4) Naturally A spirated o r Supercharged: In naturally aspirated engines, the piston itself sucks in air (e.g. 4-stroke engines) or is fed by a scavenge pum p (2-stroke engines). In supercharged engines, air under pressure is supplied to the cylinder which is pressurized externally by mechanical means o r an exhaustblower. 5) Compression Ignition (marine diesel engines) or Spark Ignition (carburetor a nd gas engines): In compression ignition, the fuel ignites with the air due to high temperature caused by compression of air. In spark ignition, an external electric spark is used for ignition. 6) Trunk type engines (4-stroke engines) o r Crosshead engines (2-stroke engines): In trunk type engines, the piston h as an extended skirt which acts as a guide. In crosshead engines, there is a crosshead which has shoes sliding over the crosshead guides.

♦ High idling speed o f crankshaft and irregular rotation.

7) Single o r M ulti cylinder: M odem m arine engines use 4 to 12 cylinders.

Classification of I. C. Engines

8) V ,W or X pattern o f arrangem ent o f the cylinders.

Classification can be done under various categories: 1) 2-stroke o r 4-stroke: Usually, 2-stroke is preferred for marine engine propulsion while 4-stroke is preferred for auxiliary diesel generation.

9) M ain Propulsion use (S hip’s propeller drive) o r A uxiliary engine use (power generation & auxiliaries).

2) Fuel used: Petroleum fuel ( gasoline, naphtha, kerosene, gas oil, diesel oil), heavy fuel ( m otor oil, b urner fuel), residual fuels, gaseous fuels (natural or producer gas) and mixed fuel (liquid fuel fo r starting combustion and gaseous fuel for running). 3) Single o r D ouble Acting: A single acting engine is one where the upper part o f the cylinder is used for combustion. A double acting engine is o ne w hich uses b o th the upper and low er part o f the cylinder alternatively, e.g. Opposed piston engines.

10) Low, M edium, a n d H igh Speed Low speed (100 to 350 rpm) M edium speed (350 to 750 rpm) High speed (750 to 2500 rpm). 11) M ean Piston Speed Low speed (4.5 m /s to 7 m/s) Medium speed (7 m /s to 10 m/s) High speed (10 m /s to 15 m/s). 12) Uni d irectional (sam e direction) or R eversible Engines using a reversing mechanism. 13) A head direction in clockwise or anti-clockwise direction.

Marine Diesel Engines

Internal Combustion Diesel Engines

Cycles

D ual Cycle

The important cycles are discussed below. Otto Cycle ( C onstant Volum e )

v Fig-2 4-5 Air Expanded Isentropically

0-1 Charging of Fresh Air (o Point 1 1-2 Air Compressed Isentropically 2-3 Heat Added at Constant Volume 3-4 Air Expanded Isentropically 4-1 Heat Rejected at Constant Volume.____________

A ctu a l Cycle T he A ctual C ycle is slightly different from the theoretical cycle in the following:

D iesel Cycle (C onstant P ressure)

i |

1

0-1 Charging of Fresh Air to Point 1 1-2 Air Compressed Isentropically 2-3 Heat Added at Constant Pressure 3-4 Air Expanded Isentropically 4-1 Heat Rejected at Constant Volume._____

1-2 Air Compressed Isentropically 3-4 Remaining Heat added at Constant Pressure 5-1 Heat Rejected at Constant Volume

♦ From 1 to 2, th e curve is sim ilar in the com pression stroke. ♦ From 2 to 3, com pression is n o t d o n e u n d e r c o n sta n t volume because the piston is already m oving during the stroke. It is n ot com pletely ad iab atic becau se o f heat transfer through the cylinder liner.

F ig - 5

♦ From 3 to 4, during expansion stroke, there is heat transfer.

Marine Diesel Engines

________________________

Internal Combustion Diesel Engines

j

♦ The heat transfer at this stage is varying, since some o f the fuel still bums in the expansion stroke. Even greater heat losses are involved owing to the unused energy lost by the compressed h ot gases, when the exhaust ports are uncovered o r exhaust valve opens before

♦ From 4 to 1, heat is rejected w ith changes in m ass flow, specific heat, low er pressures and temperatures. ♦ In the actual cycle, there are unavoidable thermal, hydraulic and mechanical losses.

the piston arrives. ♦ Action arising out o f reciprocating, rotating and robbing components also contribute to losses. ♦ Some energy is used to drive auxiliaries (lube oil pumps, jacket

♦ The air admitted into the cylinder thermally interacts with the hot cylinder liner and gases, and there is heat transfer. ♦ A certain am ount o f work is required to be done to overcome the resistance o f the inlet system through which the air is admitted.

water, scavenge pumps, etc). ♦ Cooling o f the liner is imperative to the cylinder, but this is also a source o f thermal loss.

♦ T he amount o f filling o f air into the cylinder depends on its temperature, speed and load o f the engine, engine construction and service conditions. ♦ Adiabatic compression is compression when there is no heat transfer with the surroundings. Thisisnotpossibleintheactualcycle. Here, there is heat transfer with the gases and the cylinder walls, which results in a change in pressure and temperature o f the compressed air.The area o f heat transfer is decreased as the piston moves upwards to TDC.

2 -S troke Cycle 2 S trokes

♦ The actual compression is a polytropic curve with a continuously varying exponent.

♦ Actual combustion overlaps the expansion stroke to some extent, due to the volume o f the cylinder space increasing. This leads to heat losses to the surroundings, impairing the effectiveness o f heat utilization in the cycle. ♦ Actual expansion is a poly tropic curve with a variable exponent.

8

2 strokes o f the piston Piston going u p + Piston going down O nce compression and once expansion 1 complete revolution gives 1 power stroke.

As the nam e im plies, the cycle is completed in two strokes o f the engine piston:

♦ It is more sim ilar to isothermal and adiabatic processes due to the high rate o f compression o f the air charge. ♦ The heatinput process is not ideal, since combustion o f fuel involves complicated physical and chemical changes with thermal losses in the final stage.

= = = =

'

(1) The Compression (Scavenging and Suction) stroke (2) The Power (Expansion and Exhaust) stroke.

These actual timings differ from engine to engine with respect to design and construction features such as stroke/bore ratio, engine rpm, engine rating, ratio o f connecting rod length to crank length, etc.

Internal Combustion Diesel Engines

Marine Diesel Engir,

An example o f 2-stroke valve timings are: Inlet (scavenge) opens Inlet closes Exhaust opens Exhaust closes Injection starts Injection ends

42 deg . before BDC 42 deg . after BDC 75 deg before BDC 60 deg after BDC 16 deg before TDC 20 deg after TDC.

Upstroke o f the Piston (Compression Stroke)

F ig -6

0

Scavenge ports are open

0-1 1

A ir is sucked in, which pushes o ut the residual exhaust gases Piston is at BDC

1-2

Completion o f scavenge process and filling with fresh air for combustion Scavenge ports are closed Post scavenging takes place Exhaust valve closes Compression o f air Fuel injection commences Fuel ignition commences, near TDC . Fuel injection and combustion completion Expansion o f the heat energy from combustion, being converted into work energy to push the piston downwards Exhaust valve opens

2 2-3 3 3- 4 4 5 6 6- 7 7 7-0

Blowdown o f exhaust gases seen as a sudden rapid pressure drop ontheP.V.diagram.

10

\ The scavenge and exhaust ports are uncovered and pressurized air is fed into the cylinder. This fresh air does the scavenge process i.e. it cleans the cylinder o f the exhaust gases from the previous cycle. The piston then travels upwards closing the exhaust and scavenge ports and starts compressing the air. A t the end o f the upward stroke, the a ir p ressu re in th e cy lin d er builds up to 32 to 4 5 b ar and correspondingly, it’s tem perature rises to 650 to 800 deg. C.

Internal Combustion Diesel Engines

Marine Diesel Engir,

Downstroke o f the Piston (Pow er Stroke)

I Inlet valve opens 1-2 Suction stroke 2 Inlet valve closes 2-3 Compression stroke 3 Injection begins 4 Injection ends 4-1 Expansion stroke 5 Exhaust valve opens 5-6 Exhaust stroke

W hen fuel is supplied by the injector to the hot com pressed air, it reaches its self ignition temperature and ignites. The combustion causes the expansion o f gases, which push the piston downwards towards BDC. The piston being pushed downwards by the combustion gases is doing work and hence, the stroke is called the Power or Expansion stroke. The exhaust ports are uncovered at approximately 4 0 to 75 degrees o f crank shaft rotation, ju s t before BDC. T his allows the exhaust gases to escape to the atm osphere and the pressure in the cylinder now falls to around 2 to 4 bar. The temperature is high due to the exhaust gases i.e. 250 to 500 deg. C. The exhaust ports are kept uncovered for approxim ately 118 to 130 deg. o f crank rotation. The scavenge ports are kept open for 100 to 140 deg. o f crank rotation.

4-Stroke Cycle 4 Strokes

= 4 strokes o f the Piston = 2 (Piston going up + Piston going down) = 2 complete revolutions give 1 pow er stroke. 12

An example o f 4-stroke valve timing i s : Inlet valve opens 20 deg. before TDC Inlet valve closes 60 deg. after BDC Injection begins 10 deg. before TDC Injection ends 12 deg. after TDC Exhaust opens 42 deg. before BDC Exhaust closes 60 deg. after TDC. A 4-Stroke engine operating cycle is completed in 4-strokes o f the piston. These a re : (1)

Suction (induction) stroke

(2)

Compression stroke

(3)

Power (expansion) stroke

(4)

Exhaust stroke.

13

Internal Combustion Diesel Engines

Marine Diesel Engines

(1)

Suction Stroke

compressed since inlet and exhaust valves are closed, and piston is m oving upwards from BDC to TDC. The air is pressurized to 32 to 4 5 bar and correspondingly, its temperature rises to 600 to 700 deg. C. The fuel is injected at the end o f the compression stroke at a fuel pressure o f 200 to 1500 bar, depending on the type of fuel. This fuel is injected in the form of an atomized fine spray, which m ixes with the high temperature air and self ignites. The fuel injection timing is around 10 to 35 degrees o f crank shaft rotation.

Fig-10 1

Exhaust value

9

2

Rocker Arm

10 Piston

3

Camshaft timing gear

11 Cylinder Liner

4

Camshaft Oil

12 Cylinder Head

5 6

13 Rocker Arm

Crankcase

14 Inlet valve

7

Crankshaft

15 Fuel Injector

8

Path o f crankpin

The piston is m oving downwards and a pressure difference between the cylinder pressure and the atm ospheric pressure is created above it. Atm ospheric air is sucked inside through the open inlet valve. The air adm ission is stopped w hen the inlet valve closes. The cylinder pressure is now approximately 0 .85 to 0.95 bar and the temperature 37 to 48 deg. C. (2) Com pression Stroke This stroke includes the compression o f air, mixing o f the fuel and air charge, and the start o f combustion. T h e air in the cylinder is now 14

F ig -ll

Connecting Rod

Optimum condition for fuel injection is when the fuel injection coincides with the peak air temperature in the cylinder for best combustion. At the end o f combustion, the pressure in the cylinder is 60 to 80 bar, and 1600 to 2000 deg. C. (3) Expansion Stroke (Pow er Stroke) In this stroke, work is done by the expansion of gases, to push die piston down to the crank pin through th e connecting rod, converting reciprocating linear motion o f the piston into a rotary motion o f the crank shaft, thereby turning the engine shaft. After expansion, the pressure and temperature decrease to 3.5 to 5 bar, at 750 to 900 deg. C.

i !

F i g - 12

Internal Combustion Diesel Engines

Marine Diesel Engines

(4) E xhaust Stroke W hen the piston nears BDC, the exhaust valve opens and the exhaust gases escape, since their pressure is more than the atmospheric pressure in the exhaust manifold. The exhaust gases are expelled and the piston now starts moving upw ards. T h e pressure o f the g ases now decreases fu rth e r to 1.1 to 1.2 bar, at a corresponding tem perature o f 430 to 530 deg. C.

2-Stroke versus 4-Stroke Engines ♦ The whole cycle ( suction, compression, expansion, and exhaust) is completed in tw o strokes o f the piston in a 2-stroke engine, as com pared to four strokes o f the piston in a 4-stroke engine. ♦ A comparison should only be m ade between operating cycles o f a 2-stroke engine and 4-stroke engine, having cylinders o f same geometrical dimensions and crankshaft speeds. Theoretically, the horsepower output o f a 2-stroke engine is twice that o f a 4-stroke engine. In actual practice, the output o f a 2-stroke engine is 1.5 to 1.8 tim es o f a 4-stroke engine. This is due to the actual operating cycle being only a fraction o f the total piston stroke, lasting between TDC and the instant o f uncovering the exhaust ports. ♦ At the start o f the compression stroke, there are higher pressures and tem peratures in a 2-stroke engine than in a 4-stroke engine (higher by 25 to 30%). This increase results in a 30 to 40% increase in the thermal load. Therefore, there are higher thermal stresses on the combustion chamber walls. 16

♦ There is m ore turning o f the crankshaft, since two idle strokes of the 4-stroke engine are n o t present in the 2-stroke engine. ♦ High speed 2-stroke engines are less efficient due to less volumetric efficiency. ♦ Fuel consumption is m ore in 2-stroke engines, since the engine works on the Otto Cycle principle. ♦ Unlike 4-stroke engines where there are two separate piston strokes for each o f these purposes, 2-stroke engines have much less time available for exhausting and scavenging. Hence in 2-stroke engines, some o f the combustion gases are left behind in the cylinder, which interfere with the normal cycle operations. Thus, 2-stroke engines appear to be less economical than 4-stroke. ♦ In the 2-stroke engine, tw o pow er strokes take place every two revolutions, while in the 4-stroke engine, only one power stroke takes place every two revolutions. ♦ 4-stroke trunk-piston engines have the advantage o f requiring less headroom than 2-stroke crosshead engines. ♦ Torque produced by a 2-stroke engine is less irregular than a 4stroke engine, due to the number o f operating cycles in a 2-stroke engine being twice that in a 4-stroke engine. ♦ The force applied to a piston o f a 2-stroke engine coincides with the axis o f the connecting rod at all times and never changes its I direction during the cycle.Therefore, dynamic loads coming on the | piston crowns in a 2-stroke engine are avoided unlike in a 4-stroke engine. ♦ In m arine applications, 2-stroke engines are used in low speed ■ high-powered diesel main propulsion, while 4-stroke engines are used in medium speed power generation. ♦ In m odem engines for main propulsion, fuel costs require cheaper | quality fuel to be used. This is possible in 2-stroke low-speed large

17

Marine Diesel Engines

crosshead diesel engines which have a very long stroke, aiding in m ore tim e for the scavenging- and exhaust process. Also, in 2-stroke crosshead engines, the cylinder space can be isolated from the crank case. This avoids the contamination o f the crank case oil due to the acidic residues entering the crank case, as in 4-stroke trunk-type engines. The total cost o f the expensive lube oil for slow 2-stroke engines is less than 4-stroke engines o f equivalent power.

CHAPTER 2

ENGINE COMPONENTS ICngine Structure l( is the foundation o f the main engine. R equirem ents 1. Strength to resist fatigue failure. 2. Rigidity a) to allow for crankshaft stresses which can cause excess bending loads on the main bearings. It allows uniform loading on the main bearings. b) to control the structure’s natural frequency and keep it away from the engine’s natural frequency. The engine will therefore be designed to run above o r below the critical rpm. c) to allow for true alignment o f the piston and the running gear, so that no uneven loads fall over the crosshead guides, stuffing box and cylinder blocks. Engine S tructure’s Transverse Strength

'I'lieengine’s structural transverse strength is provided b y : ♦ The transverse girder being rigidly fixed to the longitudinal girders. It gives resistance to twisting.

18

19

Marine Diesel Engines

Engine Components

♦ T he transverse girder’s strength w hich allows for inertia and combustion forces through the main bearing. ♦ T he ‘A’ fram e which transmits the guide forces to the bed plate. ♦ The top bracing units which dampen the lateral structural vibrations. ♦ The cylinder block units which provide strength against transverse flexing. ♦ The tie bolts which put the structure under compressive stress and reduces the tendency to separate.

A m echanical top bracing consists o f shims 1 between two plates hydraulically fastened by a bolt 4. The bracing stiffening plates 2 are thereby attached to a strong support 3. E ngine Structure D efect Areas ♦ Below the main bearing due to bending stresses. ♦ A t any change o f sections, w here stress levels are concentrated e.g. crosshead guides and holding down sites. ♦ Bolt holes and welds due to shear stresses. ♦ Anchor points for top bracing units.

E n g in e S tru ctu re’s L o n g itu d in a l Strength The longitudinal strength is provided by: ♦ Each ‘A’ fram e u n it: This also reduces the chances o f fretting at bolted joints. ♦ Rigid attachment to the stiffened tank top. Closely spaced framing o f 750 m m is the requirement for the double bottom construction. ♦ Ranges attached to the top and bottom o f the longitudinal girder. ♦ Each cylinder block unit.

Top Bracing This is usually of mechanical or hydraulic type, fitted to the top part o f the engine to provide stiffening and support against tw is tin g f o r c e s fro m th e crosshead guide. Normally, these braces are fitted to only one side o f the engine e.g. the exhaust side.

E ngine Structural Cracks Cracks in the engine structure are usually caused by fatigue failure. Fatigue failures are discussed below.

Fatigue Failure It is the failure o f the material which has undergone fluctuating stresses. Each fluctuation causes minute amounts o f plastic strain. Fatigue cracks start at the point o f maximum concentration o f tensile or shear stress. The material fails at a point much below it’s elastic limit and therefore, there is no distortion o f surrounding material. Factors A ffectin g F atigue L ife ♦ Temperature: Increase in temperature lowers the endurance limit o f the material (usually, the endurance limit = 108 cycles, i.e. 48% o f UTS for steel). ♦ M ean stress levels. ♦ Combined tensile and shear stresses. ♦ Cyclic stress frequency.

Fig-14

20

21

Engine Components

Marine Diesel Engines

♦ Concentrated stress areas depending on the groove geometry and sensitivity. ♦ Sharp notches, surface finish, corrosion, direction o f grain structure and heat treatment o f the surface. F atigue F ailu re Causes ♦ Incorrect tension and maintenance o f holding down bolts, tie bolts and top bracing. ♦ W rong engine operation with respect to overload, imbalance o f engine firing loads and im balance o f rotating masses (e.g. piston removal). F ig -15

♦ Manufacturing defects and poor quality materials. ♦. Ineffective vibration dampening units.

1

♦ Cold cracks d ue to the presence o f dissolved hydrogen or high residual stress in the joint or a small triggering defect

2 3

Longitudinal girders, two in number, which1form the side walls and a set of transverse I-beams or box girders strengthened with stiffness. Transverse strength girders housing the main bearings. Lower part of the bedplate has flanges for seating onto the hull foundation.

Fatigue C rack D etection M ethods ♦ Visual inspection at the stress concentration points. ♦ D ye penetrant method. ♦ Non destructive testing. ♦ Magnetic particle inspection. ♦ Checking o f the tension o f the surrounding bolts.

Bedplate It is the base o f the engine which carries the other components o f the engine structure. Strength and stiffness are required for the bedplate to withstand the inertia loads o f moving parts, dead loads o f supported elements and forces from the firing cylinder gases.

22

Fig -16

M aterial fo r Bedplates ♦ Cast Iron (C .I.) absorbs and dampens vibration. ♦ M ild Steel (M .S.) plates or castings welded together are cheaper and lighter.

23

Marine Diesel Engines

Engine Components

E n ta b u la tu re , A -F ra m e , T ie B olts a n d P in c h in g Screw s

T ie Rods

The position o f the entabulature, A-frame and T-Bolts are shown in the figure.

Tie rods are bolts which keep the w hole engine structure under compression. They provide for fatigue strength. They also provide for proper running gear alignment which prevents fretting. They help to reduce the bending stress being transmitted to the transverse girder. Tie rods transmit the gas forces which act on the cylinder head. The firing pressure force o f the piston is directly transmitted to the main bearing and consequently to the engine frame through the tie rod support.

H olding D ow n Bolts a n d C hocks Holding down bolts along with chocks have the following functions: ♦ To provide a clamping force through friction between bedplate, chock and the ship’s structure in order to resist the propeller thrust. ♦ To provide stiffness to the engine. ♦ To position the engine within the ship’s structure. ♦ To provide good alignment o f the engine and transmission shafting and, hence equal load on all bearings.

A -F ra m e As the nam e im plies, these fram es are ‘A’ in shape to provide support to the cylinder block.

1 Protecting Cap 2 Spherical Nut

‘A’- frames are usually produced as a single unit, as this helps in stiffening o f th e e n g in e . A w e ld e d ‘A ’-fra m e contributes to 40% o f the en g in e’s structural stiffness. T he m aterial is fabricated steel plates.

3 Spherical Washer 4 Distance Pipe 5 Round Nut 6 Holding down Bolt Fig-19

Fig-18

24

Slack H olding D own Bolts They cause fretting between the bedplate, chock and the tank top. M isalignment o f the bedplate w ill occur i f these slack bolts are

25

Engine Components

Marine Diesel Engines

retightened. Stiffness o f the holding down arrangements is decreased, whilst vibration o f the engine and ship’s structure increases. Load on other chocks increase and this may also cause fretting in them. Holding down bolts may eventually shear in serious cases, although end-chocks are provided to prevent this shear failure. Recurrence o f slackness may increase, as the tension o f the bolt has now changed with respect to the whole holding down arrangement Torsional stresses will increase as an effect o f fretting and misalignment. There will be an imbalance of bearing loads.

Chocks

Resin Chocks

Fig-21

These are commonly used with the advantage o f less manpower skill and time. They are very useful for re-chocking repairs on fretted and uneven foundation plates. A dvantages ♦ Cheaper installation and less skill for installing. ♦ No dependence on correct hand-fitting. ♦ N on corrosive and chemical resistant. ♦ 100% contact on uneven surfaces. Disadvantages ♦ Maximum limit o f temperature is 80 deg. C. ♦ In case o f overstressing o f holding down bolts, the chocks may shatter and collapse. ♦ If incorrectly fitted, the chock life is decreased drastically. A pplication Procedure ♦ Calculation is to be m ade for the chock area and the bolt tension. ♦ Engineis to be aligned with shafting. ♦ Allowance for chock compression is 1/1000 o f chock thickness.

M ain chocks are usually fitted beneath the longitudinal frame. Side chocks are fitted in line w ith the m ain bearings. End chocks two in number, are fitted at the aft end o f the main engine. These are provided with ‘through-bolts’ so that they limit the forward motion o f the engine.

26

♦ Class.approval is to b e sanctioned. ♦ Clean the work area o f the engine frame and tank tops o f dirt and oil. ♦ All hull renewals and engine alignments should be complete.

27

Marine Diesel Engines

Engine Components

♦ Dam s are prepared using a m etal sheet and putty sealant to hold the chocking resin liquid. ♦ N o heavy w ork during the cure period. Cure period is around 18 to 36 hours, depending on ambient temperature. ♦ A m bient temperature should be from 20 to 25 deg. C. ♦ Lim it fo r chock thickness is 25 mm, o r else u se m ore steps.

engine specifications. The rubber element can take compression and also shear loads. They have in-built buffers to stop excessive movements in heavy sea conditions as well as stopping and starting. All m ounts are loaded to the sam e amount. The tolerance o f 2 mm is given for conical mounts. Using shims, one can further adjust these heights.

♦ Tighten the holding down bolts after the cure period is completed.

Piston

♦ T he hardness o f the.chock is checked.

Requirements

Resilient Chocks

♦ To withstand the mechanical stresses o f combustion gas pressure and inertia forces.

♦ These are normally used in case o f medium speed engines (e.g. 4stroke engines for power generation). Basically, they help to dampen the vibrations transmitted from the m edium speed engine to the tank top.

♦ To withstand the thermal stresses during combustion.

♦ 2-stroke main propulsion engines are heavy in weight and, therefore, have high rotational and static masses causing higher out-of-balance forces w hich preclude the use o f resilient chocks, whose design would also have to take into consideration the heavy weight o f the engine. ♦ 4-stroke engines for power generation plants are smaller and lighter in comparison. Therefore, they have lower out-of-balance forces, whose natural frequency w ill be from 6 to 25 Hz fo r400 to 1500 rpm speeds. The natural frequency o f the engine can be changed, but not the natural frequency o f the hull (2 to 5 Hz) or the bulkheads/ decks (10 to 15 Hz) or the stem (4 to 7 Hz). ♦ Resilient chocks consist o f a num ber o f flexible rubber vertical mounts used on under-slung engines. They have main mounts as well as side and end mounts. Since these are flexible mounts, the engine crank shaft center w ill m ove + /-1 m m and the top o f the engine approximately +/- 5 m m during start up, depending on the 28

Pistons are designed to ta ke into consideration the follow ing: ♦ The crown is directly exposed to heat and gas load and hence, has a tendency to deform. Hence, the material should not only be thick for mechanical strength, but also thin enough to minimize thermal stress. ♦ The cyclic loading causes the top and the sides o f the crown to flex which can lead to fatigue failure. ♦ The shape o f the combustion space also depends on the shape of the crow n. Concave or convex pistons are used. ♦ Wall thickness can be reduced with strength provided for by internal ribs o f radial or concentric designs. ♦ The topmost ring undergoes the brunt o f the direct flame and it is much higher in position than the others. ♦ The m aterial o f the crow n should take into consideration the working temperature, the hardness o f the ring groove landing areas, the corrosiveness o f the gas mixtures and the cooling o f the piston. ♦ A high top land helps in more effective lubrication and moving the ring pack to a cooler zone. 29

Marine Diesel Engines

Engine Components

Water Cooled Pistons Water cooled pistons (older designs) have internal support webs cast in the crown for mechanical strength, but are prone to thermal stress failures. Cooling is done by the ‘Cocktail Shaker effect’.

Oil Cooled Pistons 1. SHAKER

GEEl OH

2. JE T

F ig -

22

Oil cooled pistons employ a spray nozzle plate. Cooling oil (common to bearing lube oil) is fed through swinging arm links into the crosshead, which provides a ‘je t shaker-effect’ as the piston moves up and down. Increased cooling o f the crow n is provided by a number o f spray nozzles which direct the cooling oil into the blind bores o f the crown at all crank angles. W hen the piston is atT D C , the ‘shaker’ cooling effect o f the oil takes place. W hen the piston is going towards BDC, je t type cooling takes place. A dvantages o f B ore Cooled Pistons o ver C onventional Pistons ♦ Low er thermal stresses and strain. ♦ Problems involved in casting o f internal ribs are eliminated. ♦ Lower piston maximum temperature at the crown. ♦ Lower gas load stresses and better cooling efficiency.

30

1 Curve of maximum temperature of piston crow in conventional type piston 2 Curve of maximum temperature of piston crow in bore water cooled piston 3 Conventional internal support webs or ribs 4 Conventional piston 5 Self supporting bores 6 Bore water cooled piston.

Flow o f P iston C ooling Oil The flow is from the main bearing lube oil to the crosshead pin, then through slots in the piston rod. It then flows through the inlet oil pipe in the piston rod which leads to the cooling bores through spray nozzles in the spray plate. The oil then returns through the outlet piping in the piston rod into the crosshead pin, w here it emerges sideways to the engine sump through internal drains; and temperature and flow alarms. Piston M aterials Crown - Aluminium or cast steel (4-stroke). Crown - C ast chrome nickel molybdenum alloy steel (2-stroke). Skirt - Si-Aluminium alloy (4-stroke) or cast iron. Rod - Forged steel. 31

Marine Diesel Engines

Engine Components

Conventional Type

Oros Type

Mean Temp.

500 deg. C

409 deg. C

Max. Temp.

510 deg. C

420 deg. C

Cooling oil side Mean Temp.

197 deg. C

185 deg. C

209 deg. C

216 deg. C.

Gas side

Max.Temp.

Com posite Pistons Composite pistons (fig - 25) are those pistons that are made up o f ‘composite’ m aterials i.e. two o r m ore parts (crown, skirt, etc.) o f different materials. Medium speed engines use these pistons. The crown withstands the high cylinder pressure gas loads as well as it limits the inertia forces. Applications for heavy fuel oil use are suitable. They are o f self supporting type. Concave or convex crowns are used which have internal support. Gudgeon pins are free floating type at the operating temperature o f the piston. T he trunk or skirt is separate from the crown. Hence, the name trunk-type piston is given.

‘OROS’ Piston A new design employed by M AN B&W, which has the advantage of reduction in temperature and h eat load at the piston crown. The following is a table o f temperatures o f the piston at 100% load.

32

The trunk o r skirt provides the follow ing advantages: ♦ Better thermal conductivity. ♦ Reasonable strength. ♦ Alow relative mass in comparison with the crown to reduce piston weight. ♦ Better radial and vertical contact due to the elliptical barrel shape reducing the load during horizontal guide thrust. ♦ Better manufacturing reproducibility. ♦ Better resistance to scuffing. ♦ Better expansion cold clearances. ♦ Better thickness since density is relatively lower. ♦ Better skirt stiffness. 33

Engine Components Marine Diesel Engines

Piston Defects ♦ Deformation o r burning o f the crow n top surface due to direct impingement o f firing gas, poor injection or bad fuel.

1 Crown (Cast steel) 2 Skirt or trunk ( Al-Sf-Alloy or nodular C.I.) 3 Bearing (Lead bronze) • 4 Gudgeon pin (Carburised steel) 5 Keep plate 6 Connecting rod (Forged steel).

♦ Cracks on the internal or external surfaces due to built up thermal o r mechanical stresses. The reasons for these stresses are poor injection, bad fuel quality, poor cooling due to insufficient coolant or fouled cooling spaces, corroded material, poor lubrication, and bad operations like an overloaded engine. ♦ Scuffing due to overheating or poor lubrication.

Fig-25

♦ Worn ring grooves due to poor lubrication, overloaded or incorrect operation, poor combustion, worn liner o r piston rings, etc.

D ifferences B etw een 2-Stroke a n d 4-Stroke Pistons 2-Stroke Pistons

4-Stroke Pistons

(1) It is of crosshead type i.e. piston rod connected to the crosshead bearing both reciprocate along the axis of the piston.

It is of trunk type i.e. the skirt (no piston rod) is connected to the connecting rod by means of a gudgeon pin and bearing.

(2) The crosshead slipper transmits the connecting rod angularity thrust to the crosshead guides.

Trunk or ‘extension’ piece or extended ‘skirt’ takes the connecting rod angularity thrust and transmits it to the side of the cylinder liner.

(3) More height for same power and speed.

Less height for same power and speed.

(4) Higher engine manufacturing costs.

Lower engine manufacturing costs.

(5) It has compression type piston rings.

It has compression as well as oil scraper rings.

(6) More head room.

Headroom is limited.

(7) Usually, used in low speed engines.

Usually, used in medium speed engines.

♦ Cooling spaces deterioration due to corrosion; coking o r scale build up caused by poor cooling water treatment; or low oil coolant flow o r overheating. ♦ Fretting due to incorrecttensioning and assembly o f studs; damaged studs; o r overheating.

R otating Pistons These pistons are employed for medium speed 4-stroke engines. An example is the Sulzer Z40 series. Rotation of the piston is accomplished by using a spring loaded paw l and ratchet. It has the disadvantage of a high initial cost. It has the advantages o f lower specific bearing loads; low er risk o f edge loading; low er risk o f piston seizing; smaller clearances between piston and liner; lower vibration of cylinder wall due to lower piston slap; lower cavitation erosion; lower heat variation; more uniformity and distribution o f heat; improved spreading o f lube oil on the piston and the liner; and a symmetrical crown and skirt which reduces thermal stresses.

■35 34

Engine Components

Marine Diesel Engir,

P isto n R ings There are usually three to six compression rings and one o r two oil scraper rings. C om pression R ings Their purpose is to prevent blow-by. They should provide an effective seal o f the combustion cham ber space. The initial ‘compression’ o f the ring i.e. ring tension, puts a radial pressure onto the liner wall. Further sealing is provided by the gas pressure itself entering the ‘back clearance space’ between the piston and ring. They transfer a large portion o f heat from the piston to the cylinder liner, which in turn, has jacket cooling. H igh piston speeds require less compression rings, since there is a less possibility o f blow-by.

F ig -26

The figure show s the gas pressure ‘p ’ entering the back clearance spaces o f each compression ring and causing the ring sealing pressures p i , p2, p3, p4, p5 to provide a sealing effect by pushing the rings tightly against the liner. It uses the labyrinth principle o f decrease in pressure. Therefore, the gas pressure that is leaked in behind each

36

compression ring is successively decreased in steps with each ring, to equal the pressure which acts on the underside o f the piston. Hence, radial pressure changes with the position o f each compression ring. It is highest at the top. Oil Scraper Rings They are rings which elim inate the possible ingress o f oil into the combustion chamber. They are fitted lowermost o f the rings on the skirt in trunk type pistons. The oil is scraped by the rings w hilst the piston goes downwards, and is returned to the crank case via oil drains in the piston on the upstroke. The ring’s beveled side surfaces slide over the oil film without dragging them upwards. The figure shows the pumping action o f the compression rings when the liner bore o f trunk type pistons becomes over lubricated. W hen the piston is going down, the piston compression rings are pressed against the upper sides o f the ring grooves and oil enters the spaces below the rings. When the piston is traveling u p w a rd s, th e rin g presses upon the lower sid e s o f th e rin g g ro o v e s an d o il is fo rc e d th ro u g h the back and upper side clearances towards the combustion chamber.

37

Marine Diesel Engines

Engine Components

Piston Ring Failures (1) Collapse It is the ‘collapse’ i.e. inward push o f the ring against the piston body due to gas pressure build up against the ‘running face’ o f the ring. It is caused by the pressure build up against ring running face and liner wall due to reduced axial clearance; poor ring and groove sealing; rings not free to m ove in the groove; or poor lubrication on sealing surfaces.

In Fig. A, pressure P I decreases at the sam e rate as the cylinder pressure, while ring pressure P2 falls at a slower rate than the cylinder pressure. In Fig. B, when P 2 suddenly b e c o m e s m o re th a n P I , m ovem ent occurs sin ce P2 changes and this causes a flutter. In both figures, observe the first piston ring fluttering and moving up and down in its ow n place. F ig -29

(3) E xcess w ear T his is due to p oor clearances, corrosion, abrasion, scuffing or improper lubrication. (4) Jam m ed or sticking piston rings This is due, to the build up o f carbon deposits o r poor clearances.

In Fig. A, the reduced axial clearance reduces the gas pressure P I, building up behind the ring to form a reduced P2 ring pressure. In Fig. B , as P2 increases slowly, P I gets betw een the liner and the ring. In Fig. C, the ring collapses against the piston groove body. (2) Flutter Flutter is the oscillation m ovem ent o f the piston ring along its own plane. It is caused by a radially w orn ring leading to a reduction in radial areas, or pounding o f piston rings in the grooves when the piston changes its direction.

38

(5) Scuffing It is the overall damage on the sliding contact surfaces, caused by the formation o f local welds. These welds occur due to high local temperature (800 deg. C+), which hardens the base metal, forming hardened particles at that point. Scuffing depends o n : ♦ Oil film quantity, oil retention and countered rings to promote oil film generation. ♦ Rotating pistons moving around any o f the dry hot spots which are prone to welds. ♦ High temperatures due to poor sealing o r poor heat transfer by bore cooling. 39

Marine Diesel Engines

Engine Components

♦ Running-in o f new piston rings or liner. ♦ C orrect sc u ff resistan t m aterials used i.e. so ft copper or molybdenum for running in, and hard chromium or nitriding alloys for normal use.

Running-In I t consists o f : ♦ A purposeful w ear on the piston ring profile to m atch the liner surfaces for proper gas sealing and lubrication. W hen the liner is rough, the ring is not properly sealed, and a matching profile is required. ♦ A wear running-in coating layer is used which is meant to be worn out, thereby creating a correct profile o f the piston ring to match with the liner wall. ♦ The engine load is increased during the running-in period to promote increased wear o f the running-in layer. ♦ Lower TBN cylinder lube oil is used to provide corrosive wear of the rings. ♦ Fuel o f high sulphur content (m ore than 0.5% sulphur) is used to increase acid corrosive wear during the running-in period. ♦ Cylinder lube oil feed rate should be increased. P iston R in g M aterial The piston ring is made o f Cast Iron. ♦ Grey Cast Iron gives better wear and scuffing resistance. ♦ Nodular chromium-plated malleable Cast Iron gives better fatigue resistance. ♦ Carbidic malleable Cast Iron gives better fatigue and wear resistance. ♦ R.VK with AL-Bronze as a running-in coating.

40

Piston R in g M anufacture P ot Casting is done in oval pots o r by drum casting in static sand moulds; or by centrifugal casting. Machining is carried out in a camturning lathe and later, a gap is cut out or the ring is split. Tensioning is done by hammering the inner surface to induce residual stress o r by inserting a distance piece in a cut ring and heating in an oven to relieve stress. Piston R in g Shapes Different types o f piston rings have different cross sections, as shown in the figure. 1. P la in ty p e is sim p le and inexpensive. 2. Barrel faced chrome-plated c o o lin g ty p e. T he b a rre l en ab les b e tte r an d fa s te r bedding-in with liner profile. C h rom e-plating is a hard coating given for increased life. 3. M aidtypewheretheinnerlaid material (molybdenum o r electroplated chrome) provides scuff resistance, while the outer laid provides edge protection and oil 4. 5. 6. 7. 8.

control. Taper running face provides faster bedding-in. Stepped scraper provides oil scraping and gas sealing. Beveled undercut provides downward oil removal. Slotted oil passages for oil scraping. Conformable oil scraper for consistent oil control.

41

Marine Diesel Engines Engine Components

Piston R in g Coatings Wear resistance coatings ♦ Plasm a Coating (using a plasm a spraying m ethod where a gas mixture is directed through an electric arc generated between a tungsten electrode and a w ater cooled copper tube to create a ‘plasm a state’ at 10,000 to 15,000 deg. C). T his plasma state m elts and fuses any m etal, w ith gas m olecules and atoms disassociating.

Piston R in g L ife Ring wear rate (around 0.1 m m/1000 hrs) depends on: ♦ Fouling o f the turbocharger.

♦ Chrom e plating: It is a hard outer galvanic chrome layer. Double chrome plating is done on both sides o f the ring. This increases the w ear and corrosion resistance.

♦ Poor fuel injection or poor fuel quality.

♦ Tungsten carbide coating w hich gives a better wear resistance. R unning-in Coatings These are soft coatings such as copper, graphite or phosphate which are meant to wear quickly and give the ring a similar profile as the liner.

Controlled Pressure Relief (CPR) Rings In CPR type, the topmost ring has one double-lap ‘S ’ seal and six controlled pressure relief grooves cut across the face. This ensures even pressure distribution and decrease o f therm al load to the second piston ring as well as the liner. O ther piston rings have an A l-bronze coating and oblique cuts.

3l/.."j Fig-31

♦ Reduced scavenge air pressure due to m ore dirt in the ring pack area. ♦ Overloaded engines or excessive pressure rise. ♦ Poor clearances. ♦ Poor lubrication. ♦ Poor water shedding in scavenge air which produces water drops on the cylinder liner affecting lubrication and causing scuffing. ♦ Poor maintenance o f grooves or incorrect fitting o f rings. Piston Cleaning R in g It is the ring which is embedded in the top edge o f the liner just below the cylinder head level. Its purpose is to rem ove the excessive carboneous deposits at the top-land portion of the combustion chamber wall which would otherwise contaminate and affect lubrication. A n ti-P olishing R in g It is the ring 1 which reduces the polished effect of the liner w all, which is form ed due to the hard deposits from combustion in contact with the liner. Polishing is unwanted, since it does not allow oil film retention on the liner wall, and the oil passes over the ring pack portion to the combustion area when it is burnt and wasted. Polishing depends on oil feed rate, excessive peak pressures, ring and liner materials, and an increase in combustion hard products at liner-ring interface.

42 43

Fig - 32

Marine Diesel Engir.

Engine Components

SIP W A (S u lze r’s In tegrated Piston Wear A nalysis) It is a m ethod u sing a continuous online feedback m easurement o f the piston ring wear condition.The piston ring has incorporated a wear-band (shaded section). A s wear down of. the piston ring takes place, a corresponding wear down o f the copper wear-band takes place. A sensor in the cylinder liner senses the wear of the copper wear-band and transmits this signal to an online electronic unit, which records and prints any wear down, which can be used as a pre-warning.

s i

Piston Rod Stuffing Gland t

Casing in two parts

2

Spacer ring

3,5 Oil scraper rings 4

Sealing ring

6,8 Screws 9

Ring in two parts

10 Piston rod 11 O-ring

It is a seal between the scavenge spaces and the crankcase in the area o f the piston rod penetration. It seals the crankcase oil entering into the scavenge space, and scavenge deposits or cylinder oil entering the crankcase. It is m ade o f two sections. Each section consists o f segmented metal rings held against the piston rod by garter springs. Materials Housing - Cast iron or cast steel. Rings - Cast iron o r brass o r bronze or PTFE Lamellas - C ast iron or carbon. Stuffing B o x Problems ♦ Poor sealing caused by worn out rings, badly aligned ring joints, sticky rings, closed b utt joints, w eak springs, excessive axial clearance or scoring/wearing o f the piston rod. ♦ Consequences o f stuffing box not performing properly is a loss of crankcase oil, higher costs, contamination o f crankcase with scavenge deposits and unbumt cylinder oil. ♦ Indications o f poor stuffing box gland sealing: Crankcase oil contamination test giving poor results. A case o f no oil replenishment. Increasing TBN o r viscosity. Reduced piston cooling effect. Poor lubrication.

Liner

12 Locating pin

Fig-34 44

M anufacture Liners are usually sand cast (above 300 mm diameter size). They may be o f split type to avoid distortion o f bore shape due to non-uniform heat deformation. Split type is usually seen in 2-stroke engines, where there is a difference in liner temperature near the scavenge ports and exhaust valve region. Liners are press fitted into the respective bore o f the cylinder block. 45

Engine Components

Marine Diesel Engines

M aterial Cast Iron with alloys o f nickel, chromium, molybdenum, vanadium, copper and titanium is used. Cast Iron is chosen because its high strength; refined grain structure with inclusions o f alloys; smooth sliding surface due to graphite content for improved lubrication; porous surface which retains oil as well as exposes a fresh surface in case o f scuffing o r scoring; and wear and corrosion resistance. 1 2

Water guide jacket Exhaust valve seat

3

Cylinder head

4

Annular space in cylinder head

5

Lubricating quill

6 7

Upper lubrication grooves in liner Cooling bores

8

Sealing metal ring

9

Lower lubrication grooves in liner

Liner Wear There are three types o f liner wear. Corrosive Wear It is the wear on the liner surface due to low temperature corrosion of sulphur. Sulphur oxides in the gaseous state combine with water, which has formed due to the condensation or sweating, when the temperature is low. Thus, acids are formed which lead to corrosion. Remedy ♦ Increase liner wall temperature above the dew point o f the water acid mixture. ♦ Use o f an alkaline cylinder lube oil to neutralize the acid content at the liner wall. ♦ Use o f a low sulphur content fuel with a limit on the sulphur value. Abrasive Wear It is due to hard particles o f ash deposits and catalytic fines, which continuously cut, scratch and plough the liner surfaces. Friction or A dhesive Wear Mechanical friction wear is due to the piston ring friction on the liner wall. This wear takes place usually where the oil film has depleted or broken down.

10 Cooling water space 11 Cooling water 12 O-ring 13 Outer Jacket 14 Ring space devoid of water

Clover L e a f Wear It is the uneven wear in the shape o f a clover leaf on the liner surface in the radial mode.

15 Sealing ring 16 O-ring 17 Cylinder block

Reason Uneven distribution o f cylinder lube oil causes the depletion o f its TBN, before it has completely covered the liner surface. High corrosive wear occurs on the liner surface between oil injection points.

18 Cylinder liner 19 Scavenge ports 20 Piston underside scavenge space

46

47

Engine Components Marine Diesel Engir

areas) Horizontal Section of Cylinder Li

Effects In extreme cases, combustion gas blow-by takes place past the piston rings, or failure o f the liner can occur.

Lubricating Quills These are non-retum valves passing through the jacket water space, which supply cylinder lube oil under pressure to the liner surface.

1 2 3 4 5 6 7 8 9 10 11

Working piston Piston rings Cylinder liner Support ring Spring Accumulator piston Diaphragm Passage for lubricating quill Bush Filling pin Screw ______

12 13 14 15 16 17 18 19 20 21 22

Joint Flange Flange Lubricating quill Non-retum valve O-ring Set screw Oil space Lube oil inlet Jacket water space Lubricatingoil groovesin the cylinder.

L iner F ailure A reas

Lubricating Accumulator It is fitted at the outer end of the quill. It delivers oil through a non-retum ball valve, only when the cylinder pressure falls below the accumulator pressure. The accumulator is sealed against the oil space by a flexible diaphragm. This diaphragm is pressed downwards by the spring force. This builds up an oil pressure, which is somewhat higher than the charge air pressure o f the engine in the combustion cylinder. When the charge air pressure o f the engine o r the cylinder pressure falls below the accumulator pressure, oil flows into the cylinder. When the accumulator pressure is less than the cylinder pressure, the ball valve o f the accumulator closes. Iftheaccumulatorfails, oil delivery still continues, controlled by the cylinder lubrication pump’s delivery stroke.

48

Area1 Excessive, incorrect or uneven tightening of cylinder head studs causes cracks. Area2 Poor liner support shows hoop stress cracks. Area3 Upper ring area is prone to wear ridge circumferential cracks. Area4 Flame impingement region in the combustion space leads to star shaped cracks. Area5 Jacket water leaks at the lube oil quill piping causes star shaped cracks. Area6 Scavenge port areas due to scavenge fires or overloaded engine operation. Area7 Clover leafing wear near fuel injection points. 49

Marine Diesel Engir. Engine Components

Cylinder Head Cover The cylinder head is a cover for the cylinder liner and block, which also seals the combustion cham ber at the top. It sustains dynamic thermal and mechanical loads caused by the combustion pressure and temperature. It houses the exhaust value, fuel injectors, starting air valve, safety valve, indicator cock and cooling w ater passages. 1 2 2a 3 4 5 5a 6 7 8 9 10 11 12 13 R

Cylinder head Nut Cylinder head stud Cooling water outlet Leak oil outlet Exhaust valve cage Stud of exhaust valve Connection for the lubrication 6 Fuel injection valve 5 Starting valve Connection for hydraulic oil Indicator valve Relief valve Air inlet for valve spring Water guide jacket Eye screw

♦ Molybdenum Steel for elasticity and strength (0.3 % C, Mo 1.5%). ♦ Steel casting or forging o f deep section, single piece, bore cooled and machined at sealing faces. Cylinder H ead D efects ♦ Cracks due to thermal changes in the cooling water temperature; sudden overloading o r heating o f the engine; o r uneven incorrect tightening o f studs. ♦ Distortion due to temperature variations. ♦ Cooling space fouling due to poor water treatment; and scaleorsludge deposits. ♦ Corrosion on the low er side being exposed to the combustion chamber. ♦ Gas erosion and acidic corrosion due to leaking exhaust valve cage seats. E x h au st Valve

M aterials Requirements ♦ Good casting characteristics (Cast Iron is good, while Cast Steel is prone to defects). ♦ High strength, high thermal resistance and high corrosive resistance. Cylinder heads are m ade o f: ♦ Composite structure i.e. Grey Cast Iron which has a good tensile strength and casting characteristics.

50

1 Cam to operate hydraulic pump 2 Hydraulic pump piston 3 ' Oil from crosshead system 4 Cooling water outlet 5 Air spring piston 6 Hydraulic piston 7 Hydraulic actuator 8 Non return valve 9 Cam shaft L.O. system 10 Air spring action area 11 Valve guide 12 Exhaust gas deflector 13 Rotator vanes 14 Replaceable valve seat 15 Exhaust valve 16 Hydraulic oil 17 Control air at 7 Bar. Fig-40

51

Engine Components

Marine Diesel Engines

Hydraulic E xhaust Valve Working Hydraulic pressure is provided by the cam operated hydraulic pump, to the hydraulic piston o f the hydraulic actuator. Lube oil from the camshaft system is used to actuate the hydraulic actuator to open the exhaust valve by m oving it downwards. Control air at 7 bar pressure is supplied to the air piston to use it as an air spring, w hich closes the exhaust valve when the pneumatic air force is greater than the hydraulic oil force. E x h a u st Valve Types They are usually poppet mushroom shaped valves. Opening and closing are done by m echanisms such as valve springs and push rod-rocker arm arrangements, o r hydraulic operation using camshaft lube oil pressure to open and spring air to close the valve. Large single valves have simpler valve construction, simpler cylinder head construction and easier valve operation. Small size multiple valves have lower inertia forces, lighter weight, better volumetric efficiency, low er tem perature o f valve materials, less distortion o f valve lid at operating temperature and a smaller valve lift. The exhaust valve consists o f the valve, valve stem, valve face,'valve seat, valve cage, valve rotator and valve gas deflector. Valve M aterials Requirements are creep resistance at high tem peratures; corrosion and oxidation resistance; w ear resistance; erosion resistance; machinability; high temperature strength; compatibility with valve guide materials; im pact resistance and surface hardness.

♦ Austenitic steel (Cr & N i 25 %) ♦ Si-Chrom e steel (3 Si, 9 C r). Valve Face A ‘Stellite’ layer is welded to provide superior hardness, corrosion resistance, good surface finish and high tem perature strength.This portion is subjected to very high temperatures and thermal and mechanical stresses. ‘Stellite’ : 2C, 50 Co, 20 Cr, 18 Mo, 10 Tungsten Valve Seats ‘Stellite’ coating, since seats are also prone to corrosion and erosion. Valve Cages C ast Iron provides easy m anufacture and compatibility w ith guide material. Valve Guide ‘Pearlite’ C ast Iron. Valve Springs They provide support to the valve in the cylinder head as w ell as provide a spring force to close the valve. Single Spring type is simple, has a low er natural frequency o f vibration and a reduced risk o f valve bounce. There is a buckling risk for long single springs, while large diameter springs have higher bending movements and stresses.

Valve ♦ Nickel based alloy (0.1 C, 0.1 Fe, 15 Cr, 1.0 Ti, 5 Al, 20 Co, 4 M o, rem ainder Ni) ♦ Precipitation hardened steel (0.5 C , 25 Cr, 5 N i, 3 M o)

52

53

Marine Diesel Engines

Engine Components

Series springs have less buckling and bending stresses, but their designs are complex. A n example is show n in Fig - 41. Springs are shown in series numbered 1 and 2. Parallel springs are employed to alter the natural frequency. There is no axial vibration, and less breakage due to resonance. The safety factor is increased in case o f the failure o f o ne spring. A n example is shown in Fig - 42. Springs are shown in parallel numbered 1 and 2. E x h a u st S e a t P rofile C hange D u rin g L o a d

Fig 1 shows the inner contact area when exhaust valve is not loaded. In closed position (Fig. A), the ‘Belleville’ washer disc is pushed against the body with slight force and disc spring is not deflected. W hen the valve opens (Fig. B), the ‘Belleville’ washer disc gets pushed against the body w ith a higher force. T his load is transferred to the balls, which causes the balls to be pushed to the deeper recesses and induce rotation. Relieving o f pressure when valve closes, causes the balls and the springs to return to the original position.

Fig 2 show s the effect o f thermal load on the exhaust valve seat.

Valve R otation B enefits There are less deposits on seat passages and sealing faces. Corrosion and erosion is reduced. O verheating o f a single spot is prevented as the valve is rotating. Temperatures o f the valve seat and sealing faces are reduced. Rotation is needed when burning heavy fuel oils.

Fig 3 shows the increased even loading seating area.

Fig-43

54

Rotating m ethods a r e : ♦ Rotating vanes e.g. used in hydraulically operated exhaust vfclves. ♦ Rotocaps e.g. mechanical rotators used in mechanical spring operated exhaust valves as in 4-stroke engines.

55

Marine Diesel Engines

Engine Components

Variable Exhaust Closing (VEC) VEC = Variable Exhaust Closing = Exhaust Valve closed earlier to increase the compression, and consequently, Pcom p and Pmax. W hen the exhaust valve is open, less amount o f compression is done by the piston. W hen the exhaust valve is closed earlier, the piston can start compression earlier, resulting in a longer period for compression. VEC is carried out during 70 to 85% M CR load.

E xhaust Valve Failures ♦ High temperature corrosion by molten salts (sodium and calcium sulphate); and compounds from the fuel due to sulphur, vanadium, sodium, and catalyst fines (sulphur oxides, vanadium oxides, sodium oxides, etc.). ♦ Erosion at the seat area and sealing faces. ♦ Dents and scratches caused by harder particles. ♦ Solid deposits o f molten salts causing leakage and cracks. ♦ O verheated spots due to after burning, p oor cooling, improper combustion or overload.

VEC Operation In case o f a hydraulically operated exhaust valve, some o f this hydraulic oil pressure for opening the valve is leaked off, when the valve is still in the open position. This results in the valve closing slightly when open, and the valve fully closing earlier.

♦ Reseating failures due to incorrect tappet clearances, incorrect expansion clearance, overheating, jamming in the guide, distortion o f valve or spindle, and creep failures. ♦ Mechanical impact loading due to banging, heavy seating, uneven surfaces o r hard deposits. ♦ Abrasive action by products fromfuel combustion orcylinderlubeoil. ♦ Fouling o f valve or valve passages which limit the air or exhaust gas flow rates. ♦ Valve mechanism failures o f springs or rotating mechanisms. ♦ Valve lift reduction. Leaky E x h a u st Valve It causes a high exhaust gas temperature and increased smoke. Pcomp and Pmax reduce. The turbocharger may surge.

Curve A curve at 100 % load Curve C curve without VEC at part load Curve B curve with VEC at pan load Point p shows earlier closing of valve.

F ouled In let Valve It causes a restriction in the air flow. Hence, scavengeefficiency reduces and thermal stresses increase. The exhaust passages get fouled as a result and there is more smoke from the exhaust.

56

57

Engine Components Marine Diesel Engines

F ouled E x h a u st Valve It causes a reduction in the exhaust gas flow; and fouling o f the exhaust passages, the turbocharger and the exhaust gas economizer. The scavenge efficiency decreases, while exhaust temperatures increase. Exhaust gas may leak back into the cylinder and get recycled.

P lsa d vantages IThe webs should have considerable strength to allow two shrunk fits. J I I nee there is a lack o f grain flow, there is no benefit o f the same.

I

Crankshaft The crankshaft is a very important and heavily stressed com ponent It is subjected to fluctuating loads due to the inertia forces o f rotating masses, combustion gas pressure loads and high bending and torsion loads. The crank angle fo r the angular arrangement o f each crank with respect to the other depends on the num ber o f strokes and cylinders o f the engine. Balanced weights are fitted to the webs to balance inertia forces o f rotating and gyrating masses. Types: (1) Fully Built (2) Semi-built (3) Solid single piece (4) Fully welded type. Fully B u ilt Up C rankshafts They have all parts separately manufactured by steel casting or forging, and then fully built up i.e. assembled using a shrink fit (1/600 o f pin diameter). Shrink fit is the friction between the pin and web sufficient enough to transmit the torque without stressing the pin and web. It is done by cooling the pin in liquid nitrogen rather than heating the web. Very few engines use fully built up crankshafts. It is only used on some very large slow speed engines. Advantages Their construction and design is simple; easy replacement o f damaged parts; easy handling and machining o f parts; any part o f the crankshaft can be repaired in sections i f dam ages take place; and m ost o f the machinery can be completed during the manufacturing stage itselfbefore assembly.

58

S em i B u ilt Up Cranshafts [ They are shrunk fit assemblies o f complete crank throws (one crank [ pin and web together) and separate journal pins. They are widely I used on slow speed 2-stroke engines and large 4-stroke medium speed engines.

Fig-47 1 One crank throw

2 Journal pin

59

Marine Diesel Engines

Engine Components

Advantages Each crank throw is forged by continuous grain method which maintains a path for the grain flow along the crank throw axis. H ence it can use the benefit o f grain flow. It has a better fatigue resistance, less shrink fits, smaller webs and a lighter shaft weight. Larger pin diameters can be used. Solid Sin g le Piece C rankshafts They are those crankshafts where the whole crank shaft is forged or cast as one single piece.

| 2 ' •I

Half crank throw Full crank throw Two half crank throws welded leaving a small gap at the mating faces Dummy piece backing. F ig-49

M aterials ♦ High carbon steel (0.35 to 0.45 C ) for slow speeds. ♦ High carbon steel with alloys for medium high speeds. ♦ Chromium, tungsten, nickel and m agnesium alloys are used in percentage o f 1.5 % each. C rankshaft Failures Fatigue and cyclic stress failures are mostly due to high frequency low loads or low frequency high loads. The areas o f crankshaft failures are: ♦ Shrink fit stress raisers at dowel pins o r keys.

Advantages It has a better fatigue resistance, lesser stresses, a sm aller and lighter shaft, continuous grain flow throughout shaft and no need for shrink fits. Balanced counter weights can be fitted as shown in the figure. Fully Welded C rankshafts They are full, h alf forged, o r cast crank throws joined to the journal pins by continuous feed narrow gap, submerged arc welding. Advantages Here, there are no shrink fits or restrictions on the pin diameter. Smaller and lighter shafts can be used.

60

♦ Any sharp changes in section where stresses get concentrated. ♦ Severe operating conditions and overload. ♦ Lube oil passages, holes and drilling sections. The radii o f the lube oil hole should be ample to reduce the stress concentration. ♦ Pin to web fillet section should have ample radii. ♦ Surface defects and sharp edges. ♦ Incorrect manufacture like slag inclusion and poor heat treatment. ♦ Torsional stresses giving a helical-shaped crack at 45 degrees to the axis o f the pin. ♦ Misalignment of main bearings.

61

Engine Components

Marine Diesel Engines

♦ Slippage o f shrink fits are seen when engine timings change over some part o f the engine only, with an increase in vibration at that section and a shift in the ‘m arkings’ em bossed at the pin/web interface. This slippage can be due to piston seizure; hydraulic lock in cylinder during starting; starting the engine with turning gears engaged (in case o f no interlock on smaller engines); bottom end bolt failure; etc. If minor slippage occurs, adjust timings and monitor. If m ajor slippage (greater than 4 degrees) occurs, then return to original position using hydraulic jacks, strong backs and liquid nitrogen. N o heating is to be done to avoid stresses. ♦ Corrosion fatigue due to lube oil turning acidic caused by lube oil contaminated by combustion products. ♦ Lubrication failures. ♦ Poor support from bedplate foundation and tie rods. C rankshaft Stresses 1. Variable combustion gas lo a d : The radial component causes the pin and webs to bend and twist. The tangential component causes . bending stress in webs and torsion stress in the journal. 2. Torsional vibration stress in w eb pins is due to the shaft being wound up under torsional load and unwound due to its own stiffness. 3. Axial vibration stress due to the repeated in-plane flexing o f webs and the reaction the intermittent propeller thrust. 4. M isalignment o f the main bearings leading to cyclic opening and closing o f the crank throw causing in-plane bending and tangential bending stresses. Misalignment can be caused by:

Crankshaft Deflections The crankshaft w ill deflect i.e. webs open and close as the engine turns, in the vertical as well as horizontal directions.

Fig-5 0

Fig-51

Closing o f crank throw is a negative reading as shown in Fig. 50-A. D eflection Procedure Place a dial gauge opposite the crank pin on the port side and set the pointer to zero as shown in Fig. 51 -C. Looking in the forward direction, read the dial gauge readings as shown Fig. 50-B.

(a) Wear or distortion o f the bedplate o r excessive bending o f the engine framework. e.g. grounding or incorrect cargo distribution. (b) Worn main bearings due to incorrect adjustments, overloading, vibration, or poor lubrication.

62

63

Engine Components

Marine Diesel Engines

Factors a ffecting D eflections ♦ A flexible shaft and not a stiff one is desirable. A stiff crank shaft is one where the crank shaft is stiff enough to support itself across a span including a low bearing i.e. the journal may not be sitting on the bearing. Check by using a feeler gauge o r jack the shaft onto the bearing.

away from the guide bar. T he lim it o f slackness is h alf to one chain pitchlink. Iftoo slack or too tight, adjust the chain tension. Adjustment is done for slackness o f 1 pitch length.

♦ Ambient temperature near the engine. ♦ Movements o f the ship as in rough weather. ♦ Incorrect load condition i.e. hogging or sagging.

Chain D rive Chain drive is used to transm it the power drive from the crankshaft to the camshaft. A n intermediate wheel (for fuel pum p and exhaust cam drives) serves as a guide, while an adjuster w heel serves to adjust the chain. The intermediate wheel may be connected to a separate chain for driving m otion to the lubricators, governor, air distributor, etc. 1 Fitting tool 4 Bush

2 Outer link plate 5 Roller

Tightening Procedure

8 9 A, B, C, D

Fig-55

■ 3 Pin

C hain Tightening Checking Tightness Turn the engine so as to slacken the longest free lengths o f the chain. A t the m iddle o f the longest face length o f the chain, pull the chain

64

Lock washers Thrust Spring Nuts

♦ The engine is turned so that slackness is on the sam e side as the tightener unit. ♦ Loosen nuts A , B, C and D. ♦ Tighten the nut C till the free length is reduced by the dimension as per the manufacturer’s guide book. ♦ Chain tightener bolt is moved and the chain is tightened.

65

Engine Components

Marine Diesel Engines

♦ Lightly tighten nut B against pivot shaft face, while checking that the spring is not further compressed, since compression reduces chain tension. ♦ Tighten nut A and lock w ith lock nut and tab washer. ♦ Tighten nut C until the spring thrust disc bears against the distance pipe o f the bolt. ♦ The spring is further compressed, but this tension is not transmitted to the chain on account o f the already tightened nuts A and B. ♦ W hen the thrust disc presses tightly against the distance pipe, the nut C is further tightened to m anufacturer’s dim ension setting ‘D-2’. ♦ Tighten lock n u t D, locking both nuts with tab washer. Chain Inspection Check chain teeth w ear at point 1, as shown in the figure. Place a short straight edge plate, cover the points A and B, and m easure w ear a t poin t 1. Scratches on teeth sides due to the side plates are normal. Check for cracks on the possibly defective rollers and side plates. Check for seizure. C heck the rollers run freely and links m ove freely on pin and bush. Check for one com plete revolution. Check bolt, screw and nut connections. C heck lube oil pipe fo r dam age a nd j e t nozzle for deformations. Check rubber track o f guide-ways for cracks.

C hain D rive A dvantages Easy timing adjustments are possible. Maximum flexibility exists for positioning the gap between driven equipment. Its cost is economical and very few spares are required. It has a very high drive efficiency (98 %) andean cope with a certain extent o f misalignment due to axial movement o f shafts. Chain Elongation Elongation or stretch o f the chain is due to the wear between pins and bushings, roller and sprocket wheel, and between bushing and rollers. Elongation changes the camshaft position with respect to the crankshaft Fuel and valve timings depend on the camshaft position and are altered due to chain elongation. Maximum elongation allowed is 2%. A t 1.5% elongation renew the chain. Elongation is checked o n a ‘taut’ chain by measuring the length o f a number o f links from pin centre to pin centre. It is the difference between measured length and new chain length. S lack Chain It results in excess strain during starting and reversing. There is a greater shock loading during normal running and retarding o f timings in both directions due to backlash, especially during maneuvering and load changes. Vibration iri addition to cyclic stresses may cause possible fatigue failure.

C hain M aterials (1) Link plates :C r-M o steel (2) Pin : Hardened steel (interference fit into outer link plate) (3) Rollers : Alloy steel

T ight C hain It results in overloading o f the chain wheel bearings. This gives rise to wear on rollers, links and bearings; and can cause cracking o f links.

66

67

Engine Components

Marine Diesel Engines

Camshaft Readjustment After Chain Tightening Readjustment o f the camshaft’s angular position will be required to be done, in case o f repeated chain tightening, as this causes the camshaft position to be altered with respect to the crankshaft. The limit is a 2 degrees increase in lead angle over the initial angular position.

A bearing in a marine diesel engine is required to support the journal; to float the journal so that there is no metal to metal contact; to transmit the load via the lubricant; and to reduce rotational friction. Material properties required are anti-friction resistant; running-in and grindingin ability; noncorrosive by lubricants; should not scratch o r score the journal; build up adhesive oil films under boundary lubrication; allow abrasive particles to b e embedded in it without m ajor functional disability; tensile and compressive strength; fatigue resistant; thermal conductivity; high melting point especially when running hot; load carrying capacity; and ductility. B earing M aterials (1) While Metal Bearings : Anti friction, tin-based, white metal alloys (called Babbitt) consist o f :

Turn crank throw o f No. 1 cylinder to TDC. Check camshaft angular position using the pin gauge and marking. Remove plug screws for hydraulic oil connection in the coupling flange. Mount snap-on hydraulic connectors and piping to the hydraulic pump. A pply hydraulic oil pressure to float the coupling ( coupling floats w hen oil seeps out along shaft below coupling flange). 1\im and adjust coupling with a special spanner and check position with pin gauge. Release oil pressure after finishing. W ait for 15 minutes before plugging oil holes so as to allow the coupling flange to set again.

68

Tin (Sn)

88 %

Antimony (Sb)

8%

Copper (Cu)

4%

Soft matrix to allow for small changes in alignment between bearing and journal. H ad wear resistant cubes to absorb and transmit load. To segregate and hold antimony cubes in a tin matrix.

(2) Thin Wall Shell Bearings: These bearings are usually of tri-metal type, having 3 main layers and a steel backing shell, 1“ layer (Flash)

2“ layer (Overlay) 3"1layer (Interlay) 4®layer (Lining) Shell (Bottom)

1 micron thickness of lead / tin for corrosion before installing bearing. 20 micron thick white metal. 5 micron thick nickel dam helps to reduce corrosion of the white metal 2“ layer. 1 mm thick lead / bronze. It is a steel backing shell for shape and support

69

Engine Components

an 7

K s h J o u r n a l B earing rotation o f the shaft,

B earing F aults a nd D efects

due to'Ved§e PreSSUreis f0im ed

♦ Abrasive wear due to fine scoring by hard particles and impurities in the lube oil.

d i v e r t bein^ draW nint° the o f th e' 1secticm b y the motion j0lirnal. T h is oil pressure separate^ the jo u rn a l and the

♦ Corrosive wear due to acidic lube oil. The lube oil becomes acidic due to oxidation, contamination from combustion products,'or water ingress.

^ W

. » p l a i n b ushtype,

Fig-58

load but s effeCtlTe’ remaining two-thirds canies negligible loss C{iti]i causes the oil film to shear. Ib is results in heat and friction

♦ Erosive wear due to cavitation. ♦ Adhesive wear due to galling, scoring or scuffing. In galling, the softer metal tears due to the adhesive force which is a reaction o f the rubbing metal surfaces. ♦ Fatigue failure cracks at areas o f stress concentration.

li'ews S * J m r n a ‘ Beari" g journal

ofjoP™3^

0Wn0“ ^ 1 t a

the plain bush is repiaced by a series o f

8 P,V0“

Advanta^ It is d e s i ^ ^ to geminate oil whirl. .

S

6 cap^citv and efficiency is

V,^- Theradial loadia

and n o tp ^ tbroughthaby oil films , ■, JUSt one oil film. It has a

th S i!» ort>'lo ,h t* ad,5' The“ tof 1 “

? 0Slty

y ,djusE load, the feed and the the oil- It allows for

to t e f a ? * 1' Of inisalignment due adiustinsttc>th leivolingj°umalp ad s, adjusting eoflheshaft

70

“ d t a t 0 “ t' S“ erato8itS

♦ Overheating due to poor lubrication supply or contaminated oil, misalignment, incorrect clearances, uneven load distribution, poor surface finish and overloading. ♦ M isalignment o f the bearing due to distorted bedplate, adjacent bearing failure, or imbalanced cylinder pressures. ♦ Incorrect clearances or incorrect tensioning o f bolts. ♦ Poor design, manufacture o r low strength. ♦ Housing dimensions n ot perfectly suitable fo r bearing shells, especially during replacement.

Bearings In the Engine The following bearings in the engine are discussed below. M ain Bearing Main Bearings are the bearings w hich support the crankshaft o f the engine. The lower shell part o f the bearings are cut into the transverse strength members o f the bedplate. The upper shell cap is held in place by special jack bolts o r secured by wasted studs. Thin shell babbitt 71

Engine Components Marine Diesel Engir. F ig -62

(white metal) with a steel back is used for the main bearing. Babbitt has a low fatigue strength and hence, pressures and temperatures are limited.

1 2 3 4 5 6 7

Hydraulic nut Top cover cap Wasted stud Upper bearing shell Crank shaft Lower bearing shell Bedplate transverse cylinder

Connecting Rod and Bearings Connecting rod is the rod connecting the top-end bearing (crosshead bearing in 2-stroke slow speed engines or the piston gudgeon bearing in 4 stroke m edium speed) and the bottom end bearing (crank pin bearing). Its purpose is to convert reciprocating motion o f the piston into rotary motion o f the crankshaft It is the most highly stressed component o f a diesel engine. It is subjected to ahigh purely compressive force. It links the piston rod and crosshead to the crankpin. 2-Stroke C onnecting R ods ( Slow S p e e d ) They are o f split type i.e. tw o halves for each small and big end bearings. This helps in easy fitting and repair. T he round m id section changes to a rectangular palm section at the bearing ends by means o f the elliptical fillet shape. A round section is cheaper to manufacture. Examples are shown in Fig - 61 and Fig - 62.

72

1 2,3 4,5 7 6 ,8 0 10

Top cover of top end Bearing shells of top end Hydraulic stud nut Bottom end cover Bearing shells of bottom end Crosshead pin at top end Crank pin at bottom end

4-Stroke C onnecting Rods (M edium Speed) In these engines, only the b ig end bearings are split, usually in an oblique direction to reduce the big-end width, lessen lo ad o n bolts and increase crankpin diameter. The top-end may be a bush type bearing. Rectangular o r I-sections, although more expensive to manufacture, are necessary to resist th e high transverse in ertia w hip loading, the gas loads, and to fulfil the weight to strength requirements. It is subjected to high compressive-low tensile stresses o f bending as well as axial type. It connects the crank pin directly to the piston gudgeon pin.

1 3 5

Bush bearing Lubricating oil passages Serrated edge

Top end Gudgeon pin Obliquely split bottom-end

F ig -61 73

Marine Diesel Engines

Engine Components

C onnecting R o d Failures In slow speed 2-stroke engines, failures occur in veiy few cases, except due to slight buckling, when starting the engine if oil or water has leaked into the cylinder space. In m edium and high speed 4-stroke engines, fatigue cracks or fractures can occur in high stress concentration areas. Thin walled steel back shell bearings have more possibilities to fail rather than white metal bearings. Transverse buckling is usually caused by crank pin bearing seizures. B ottom E n d Failures In 4-stroke engines, the bottom end o f the connecting rod is more susceptible to failure. The forces acting on bearings and bolts a re : 1. C onstantly fluctuating inertia loads from reciprocating parts swinging in a ‘whip’ motion. 2 . Tensile load caused b y the centrifugal forces o f th e m ass o f connecting rod and crankpin. 3. Shear force tending to separate the tw o halves o f the bearing housing. B ottom E n d B olt Design ♦ A pretension is given to the bolt while fitting. Incorrect pretension is the m ost im portant cause o f fatigue failure o f the bolt which is initiated at a mechanical defect. ♦ T he resilient material used fo r the bolt should be less stiff than the bearing housing. ♦ The diam eter o f the shank sections should b e sm aller than the threaded root portion so that this ensures greater stresses act at the shanks rather than the threaded portion. ♦ The yield o f threads is prevented by a portion o f the shank having a tight clearance in the hole bore. Here, the nut is tightened ‘square’ into the spot faced bearing housing.

74

♦ Large fillet radii are given, since fillets are stress concentration areas as there is a change in the cross-section. ♦ Resilience o f bolts is increased by designing the housing part as long as possible. Large E n d B olt D efects If the large end bolts are defective, then they should be discarded in case o f overspeed failure, piston seizure, exceeded tolerance, completed designated life, acidic lube oil corrosion and mechanical damage like cracks and fractures to the surfaces o f land faces.

Crosshead Bearing Unlike the main bearings, b ig e n d b e a rin g s an d camshaft bearings, where motion is only rotational, crosshead bearings have to ta k e in to account oscillatory motion at high sliding speeds.

I |

1 Rail 2 Shoe 3 Pin________________________ 4 Plate____________________ ______ |

In 2-stroke engines, a cyclic unidirectional combined gas and inertia load acts continuously on the bearing in a downward direction. Hence, the bottom half o f the crosshead bearings are more prone to wear. In 4-stroke engines, the bottom half has some load relief during the suction and exhaust stroke where the inertia force is greater than the gas force. Lubrication at this time is ideal. 75

Marine Diesel Engir

Engine Components

Crosshead Failures Crosshead bearing failures are due to poor lubrication; misalignment with running gear (piston and liner); white metal cracking; fatigue failure; squeezing o f w hite m etal causing partial blocking o f oil holes; overheating; corrosion; white metal quality; and reduced strength due to improper thickness or type. Insufficient or contaminated oil results in poor lubrication o f the bearing. A nother im portant aspect in crosshead failures is the crosshead pin surface finish.

Crosshead D evelopm ents ♦ Oil grooves are cut into the bearing surfaces and the guides to act as oil reservoirs. ♦ For crosshead design, the pin can be considered as a single beam supported at the ends. Applying load only in the middle o f the pin creates a bending movement. This condition can be corrected by increasing the pin stiffness by having a pin o f a larger diameter for the same length. There is better distribution o f load since a larger surface area is now available. P in stiffness can be increased by using a hollow pin for better section modulus. ♦ Use o f flexible bearing mounts as in RND engines. Here, the pin distortion is taken by the mounts and edge loading is reduced. ♦ A rigid support over the w hole pin area is used rather than the fork-end type in earlier engines. ♦ M ounting o f the piston rod on top o f the crosshead pin, so as to use the full length o f the bottom bearing. The bottom shell is of ‘continuous’ type. ♦ Superior surface finish o f the bearing and pin.This is done by accurately grinding and then ‘super-finishing’ i.e. polishing the pin w ithth e a id o fh o n e so n a la th e . T he load carrying-capacity o f a

76

‘super-finished’ bearing surface is twice that of a very fine-ground bearing surface. Surface finish is very important as not only is the crosshead bearing under a very heavy instantaneous firing load, it is also very difficult to supply and maintain the oil film. Surface finish and roughness o f ‘in-use’pins is the criteria for judging the crosshead bearing’s further use. ♦ A lignment o f crosshead is im proved by changes in design and manufacturing techniques. In fully welded design, only longitudinal adjustment is provided. ♦ Improvedbearing materials are used like white metal, tin-aluminium, tin-cadmium, etc. ♦ Bearing material thickness is reduced by bonding it to a lining and steel backing. This improves overall strength. Example: Thin shell tri-metal bearing.

Puncture Valve ♦ It is a device to positively stop the engine irrespective o f the rack position. ♦ It reduces the high pressure o f the fuel oil by connecting the high pressure side to the pump body, thereby stopping the injection of fuel. ♦ Engine stops and shut downs are carried out using the puncture valve. ♦ It allows fuel oil recirculation when the engine is stopped since oil pressure is not totally bypassed. ♦ It is operated by pneumatic air pressure. ♦ It is used in M AN B&W engines.

77

Marine Diesel Engines

Engine Components

E n g in e M a te ria ls 1 Exhaust Valve

CoatingofStellite(iftemperatureis lessthan500deg. Q or Nimonic (if temperature is greater than 500 deg. C)

Exhaust ValveSeat

Mo-Steel with Stellite coating

Exhaust Valve Cage

Pearlite Cast Iron

2 Cylinder Head Cover.

Lamellar Cast Iron

3 Piston Crown

Cast Steel

Skirt

Cast Iron

Rod

Forged Steel

Ring

Vermicular Cast Iron, RVK- C, R-C Spheroidal Cast Bon, IhrkAlloy, Tarkall-A, Tark-C

5 Tie Rod

Mild Steel

6 Entabulature

Cast Iron

7 Stuffing Box Rings

Bronze

8 Crosshead Bearing

Tin-Al-white metal thin shell bearing

9 Crosshead Guides 10 Connecting Rod

Mild or Medium Steel (U.T.S. 500MN/sq.m.)

11 Crank Pin Bearing

White metal bearing

12 Crankshaft Web

0.2 to 0.4 % Carbon Mild Steel

13 Main Bearing

Thin shell white metal bearing

14 Saddle

Cast Steel

15 Bed Plate

Forged Steel or Cast Iron

16 A-Frame

Forged Steel

Propeller

Nicalium, Al-Bronze, Mg-Bronze

Hull

Mild Steel or High Tensile Steel (20 to 30 mm).

Fig-65

78

79

CHAPTER 3

AIR SYSTEM Scavenging It is the process in a diesel engine, in which low pressure air is utilized to blow out the waste gases o f combustion i.e. scavenging, and refill th e cylinder with fresh pressurized air for the next compression stroke. The various types o f scavenging are described below. Uniflow Scavenging Uniflow, as the name suggests, is an air flow in the sam e direction. Low pressure air is allowed in at the bottom o f the cylinder w ith slight rotation and the exhaust gas is pushed out from the to p o f th e c y lin d e r. U n iflo w scav en g in g is req u ired in m odern engines to use the advantages o f slow speed and a long stroke (which in tu rn ,_ requires better scavenge efficiency to burn present day cheap heavy fuel oils).

Fig-66

Marine Diesel Engines

Air System

Advantages T he scavenge efficiency is the highest. There is n o exhaust and scavenge intermixing. W orking temperatures are reduced. Costly cylinder lube o il consum ption is reduced (0.3 gm /bhp/hr to 0 .6 gm/bhp/hr fo r crosshead type engines). Less residual exhaust gas remains in the cylinder after scavenging. T h e air loss during exhaust and scavenging is nil. It’s liner design is much simpler than other types and a shorter piston skirt can b e used. Thermal stresses are also m uch less as com pared to oth er scavenging methods. M eth o d s: 1.

2.

U sing a single poppet type exhaust valve at top o f the engine cylinder. T he large area at the exhaust valve allows speedy exhaust gas escape and improves scavenge efficiency. M ost m odem 2-stroke engines em ploy this method.

Advantages The design is simpler. There is no valve gear maintenance nor power consumption required for the same. D isadvantages Consumption o f expensive cylinder lube oil increases. Undesirable mixing o f scavenge and exhaust gases is increased. Scavenge efficiency is less. Exhaust back pressure m ay increase due to narrow ing dow n o f exhaust passages w ith carbon deposits. Chances o f cracks are possible due to therm al stresses a t the scavenge and exhaust ports area. T he tem perature variation between scavenge and exhaust ports is confined to a limited area in th e region o f the ports. Uneven w ear o f piston rings can cause leaks. Liner costs are more as the liner design is more complicated. It cannot use the advantage o f a modem engine’s increase in stroke bore ratio, which is why it is rarely used nowadays.

Loop Scavenging In loop scavenging, the flow o f air and gas is in a ‘loop’ path. The air inlet and exhaust ports are arranged on the sam e side o f the cylinder.

Opposed piston method. In opposed piston engines, one piston controls the air inlet ports (bottom piston), while the other controls the exhaust ports (top piston). Only outdated older engines like D oxford engines employed this method.

Loop scavenging is best for stroke-bore ratios o f less than 2:3, or else it is thermodynamically disadvantageous. Hence, m odem engines with high stroke-bore ratios do not use the loop type m ethod. Fig-68

82

Reverse F low Scavenging It consists o f L oop o r Cross scavenging systems.

F ig- 6 9

A ir System

Marine Diesel Engir.

closing precisely when fresh air has fully filled the cylinder and residual gases have been fully pushed out. Inter mixing o f fresh air w ith exhaust gases is n ot desirable at this stage, as it would contaminate the fresh air with exhaust and increase the fresh air temperature. However, the sweeping action o f the fresh air produces a cooling effect low ering the cylinder temperature.

Cross Scavenging In cross scavenging, the a ir and gas flow is in the ‘across’ path. i.e. air inlet and exhaust ports are situated on opposite sides o f the cylinder.

Super Charging or Pressure Charging Fig - 70

Gas Exchange Process In a diesel engine, the gas exchange process consists o f : 1. Blow D own o f E xhaust Gases It starts w hen e x hau st valv es o pen o r e x h au st ports are uncovered. Exhaust gases are ‘blown down’ rapidly into the manifold. They are helped by the sudden opening o f the exhaust valves o r ports. This advance in tim ing o f the opening o f the exhaust valve before th e inlet valve is called Exhaust Lead. T he end o f this blow dow n period is when the inlet ports are uncovered. T he cylinder pressure falls below the scavenge pressure after blow down. 2. Scavenging Since the cylin d er pressure is less than the scavenge box pressure, the fresh scavenging air pushes the residual gases out, the m om ent the scavenge ports open. 3. Post-Scavenging P ost o r A fter S cavenging p eriod is the com pletion o f the scavenge process and prevention o f any fresh air loss through the exhaust valve or ports. This depends o n the exhaust valve 84

Combustion and pow er depend on the am ount o f fuel and air supplied, since proper combustion requires a stoichiometric air fuel ratio o f 14 : 1. The am ount o f fuel to b e b urnt is limited by the ratio o f air that can be supplied. I f we increase the m ass o f air i.e. its density and pressure, w e can use m ore fuel for burning. Hence supercharging o r pressure charging o f the combustion air su p p lied allo w s m ore p o w er to b e dev elo p ed w ith p roper com bustion. Supercharging o r Turbocharging is the pressure charging o f air supplied to th e cylinder at th e beginning o f com pression. In 2-stroke m arine engines, in order to achieve correct combustion, good scavenging and effective cooling, thrice the am ount o f ideal com bustion air quantity is supplied. This is called Excess A ir for proper combustion. Advantages o f super o r pressure charging Power is increased for the sam e engine dimensions and piston speed. There is no appreciable increase in cylinder m aximum pressure. The initial costs are reduced, since a m ore powerful engine can have sm aller size, space and mass. It gives better reliab ility and cy linder operating conditions. T h ere is less m aintenance. F uel co n sum ption red u ces w h ile m echanical efficiency increases. Codling is im proved since a greater m ass o f

85

Marine Diesel Engines

A ir System

fresh cool air is supplied. T h ere is better utilization o f waste exhaust gas energy w hich can b e used to drive the turbochargers. S u p e r c h a r g in g M ethods 1) M echanical Supercharging using : ♦ A rotary air blow er driven by the diesel engine crankshaft. Here, som e indicated engine power is w asted in the drive. H ence there is less m echanical efficiency and m ore fuel consumption. It is inefficient at higher pressures. ♦ Scavenge Pumps which are o f engine driven reciprocating type. ♦ Under Piston Space Scavenging using under piston spaces to pum p the air. ♦ A uxiliary B low ers w hich are o f independently driven type. T hese are used m ostly in the first o r second stage o f a com bined supercharging system only as scavenge assistance. 2) Turbine Supercharging Turbochargers use w aste heat o f the exhaust gas to drive a turbine w hich in turn, drives a com pressor (blower) on the sam e shaft to supply pressurized air.

Turbocharging Types D ifferent types o f turbocharging m ethods are discussed below. C onstant P ressure Turbocharging In this type, exhaust gas from each cylinder is lead to a common ex h a u st m a n ifo ld w h ich th e n supp lies e x h a u s t g a s to the turbocharger at a ‘constant pressure’. The exhaust m anifold space is large enough for the volum e o f combined exhaust gases without any pressure rise. Hence, a constant pressure is available to the 86

turbine. However, the exhaust manifold should not be too big, as then there would be a longer tim e required for the desired exhaust pressure rise in it. T he exhaust gas flow into the manifold creates eddies which, in turn, dam p out any pressure waves or pulses. Work is n ot done when exhaust gas is throttled through the exhaust v a lv e in to th e la rg e m an ifo ld . W ork is done when exhaust gases expand through the turbine nozzle and blades w hich is seen as a thermodynamic drop i.e. an utilizatio n o f exhaust gas heat. 1 2 3 4 5 6 7

Exhaust manifold Turbine Compressor Aircooler Air receiver Engine piston Engine cylinder

Advantages o f constant pressure type It is m ore efficient. T he tu rbine operation is b etter w hen a c o n sta n t p re ssu re is a v a ila b le a t th e tu rb in e in let. B e tte r scavenging is p ossible at h igher loads. Exhaust-grouping is n ot required. It can use th e advantage o f m o d em ‘long stro k e ’ en gines, sin ce m ore tim e is av ailab le fo r ex p ansion in the com bustion cylinder itself. Hence, g reater use o f heat energy in the cylinder and low er exhaust tem peratures is possible. Since exhaust pressure p ulses are not used, m ore energy is available fo r re c o v e ry a t th e tu rb in e an d co m p re sso r. H e n ce, th e

87

Marine Diesel Engines

A ir System

com pressor o u tp u t is increased. T h ere is a greater utilization o f w aste exhaust energy used in m arine engines because the main engine runs a t a hig h er lo ad m o st o f th e tim e allow ing a constant load w ith less load changes. Disadvantages It cannot cope up at low or part loads. Here, the auxiliary electric blow ers supply air w hen the pressure falls below a preset value. D ue to the large exhaust m anifold, there is a very slow response to load changes. P ulse Turbocharging Pulse Turbocharging uses the pressure pulse w ave to expand the gas further a t the turbine nozzles and blades. E xhaust gas from each cylinder is directly lead to the turbine inlet. Here, pulses i.e. pressure waves are created, when the exhaust valve suddenly opens and exhaust is blown down into the exhaust piping o f smaller diameter, thereby pressurizing it. For m axim um usage o f the pulse, th e pulse should be as close to the turbine inlet. Work is d o n e b y th e ex h au st gas e x p a n d in g f u r th e r a t the turbine nozzle and blades.

-

VC '6

1 Turbine 2 Compressor

ar

3 Air cooler ' 4 Air receiver 5

Rotor

6

Cylinder

7

Exhaust Piping

The requirements o f efficient p u lse turbocharging are : ♦ A rapid opening o f the exhaust valve. ♦ Exhaust piping o f a large diameter, but m uch sm aller than the exhaust valve opening to allow for creation o f pulses. ♦ Exhaust piping to b e as near as possible to the turbine inlet to use the pulse effectively as well as prevent any pulse reflection. E xh a u st Grouping Exhaust grouping is necessary to prevent blow back o f one cylinder into another in pulse type turbocharging. Each exhaust pipe has a separate inlet to the turbine. Example: Three cylinders are coupled to one turbine, with a firing interval o f 120 deg. crank difference. Advantages It utilizes the high kinetic energy o f the exhaust gas i.e. unutilized energy from the combustion cylinder. It can w ort: effectively even at low loads. It has a good response to load changes. It is widely used in auxiliary pow er generators, w here load changes are frequent and longer periods o f low load operation is common.

Series 2-Stage Supercharging 1 2 3 4 5 6 7 8 9 A

Turbine Compressor Air cooler Air receiver Scavenge pump Scavenge ports Exhaust valve Exhaust manifold Air cooler & receiver. Single air inlet for series F ig- 7 3

88

Marine Diesel Engin

Here, there is only one air inlet. Supercharging is done in two stages in series.

1“ staSe : Air is compressed (e.g. by the turbocharger) and then cooled in an inter cooler and supplied to the inlet of the 2ndstage in series. 2nd sta g e :

A ir is further compressed (e.g. by a scavenge pum p or under piston spaces) and sent to an after cooler and then, to the scavenge air ports.

Parallel Supercharging A B 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

Separate air inlet to turbocharger Separate air inlet to under piston spaces Cylinder head Tie bolts Engine cylinder Piston Fuel injection pump Camshaft Engine frame Control hand wheel Bedplate Connecting rod Crosshead Piston rod Valve Air cooler Rotary exhaust valve Turbine Blower.

■Air System

Here, there are two separate air inlets. Supercharging is done in p arallel. S im u ltan eo u s d eliv ery o f a ir tak es p la c e from a turbocharger and the under piston space pum ping effect.

Two-Stage Supercharging Supercharging in two stages gives the advantage o f more efficiency and boost air pressure ratio, since w ork done in compressing the air is reduced. Inter cooling between stages helps the compression to approach isothermal conditions which reduce the work to be done in compressing the air.

Single Turbocharger Systems This type is usually used for constant pressure type turbo charging systems. Disadvantages It relies only on one turbocharger and there is no standby in case o f a failure, A larger capacity o f the turbocharger is required causing a slower response to load changes, since it will have a h ig h er in ertia force. Spare p arts replacem ent w ill be m ore expensive. Two Turbochargers System This type is usually used for pulse type turbo charging systems, since the pulse o f one cylinder may interfere w ith another cylinder. In case o f failure o f one turbocharger, engine pow er output is still sufficient although it is reduced. A t part loads, exhaust gas to one turbocharger can b e bypassed. In this case, although only one turbocharger is in use, there will b e an increase in air mass flow. It provides better flexibility at part load.

91

Marine Diesel Engines

A ir Sysler.

Pow er Take-In ( P T I ) It is a system w here pow er is ‘taken-in’ b y the m ain engine. The main engine has excess exhaust gas energy at full load i.e. in excess o f that required fo r scavenging and for the economizer. This excess energy can b e channeled b ack to the engine shaft to take-in and utilize this w aste exhaust gas energy. P art o f the exhaust gas can be led to a turbine w hich can supply energy to the propeller shaft th ro u g h g e a rin g . I t c a n b e u s e d o n ly in h ig h ly e ffic ie n t turbochargers, w here efficiency is greater than 64%. Pow er T a k e -O ff (PTO) It is a system w here pow er is ‘taken-off’ from the m ain engine. M ethod (1): H ere, exhaust gas is ‘taken-off’ from the exhaust m an ifo ld and is le d to driv e a tu rbine electrical generator.

Here, power is ‘taken o ff’ from the main engine shaft and supplied to an electric generator via a special ‘constant speed step-up g e a r’. This gear converts variable engine speed into a constant speed supply to the generator. PTO pow er can b e tapped from 42% pow er to overload. It reduces th e costs o f running, maintaining, spares requirements, and lube oil consumption o f additional diesel generators. M ethod (3): Excess scavenge air from the m ain engine air receiver can b e le d to su p p le m e n t th e a u x ilia ry d iesel generators, when the auxiliary diesel generators are running on heavy fuel oil at low loads. The main en g in e scav en g e air is led eith e r to th e d iesel alternator’s scavenge receiver or to it’s turbocharger compressor using nozzles.

M ethod (2): T u rb o ch arg er Types Basically, they a re o f two types based o n the flow : ♦ Axial Flow Here, a single stage im pulse reaction turbine drives a centrifugal compressor. Exhaust gas flow in and out o f the turbine blades is along the axis o f the shaft. This type is the m ost commonly used in m arine applications. ♦ Radial F low Here, the exhaust flow into the turbine blade is along the radial direction. The exhaust gas flows off the trailing edge o f the blade and the outlet is along the axis o f the rotor. It is used in small high speed engines.

92

93

Air System

Marine Diesel Engines

A x ia l F low Turbocharger T he figure shows an axial flow type o f turbocharger w ith details.

Construction On the sam e shaft is m ounted a single stage im pulse reaction turbine and a centrifugal compressor. The Turbine consists o f a gas inlet casing w ith a nozzle ring; a gas outlet casing; a turbine w heel forged integral w ith the shaft; blades that are fitted through side entry slots; and a provision for water cooling. In earlier designs, the casing was water-cooled, but m odem engines employ uncooled type turbochargers. The Compressor consists o f a volute casing w hich houses the impeller, inducer and diffuser. The inducer guides the air inlet flow smoothly into the eye o f the impeller. The im peller throws the air outwardly with a centrifugal force. T he diffuser at the discharge end converts the kinetic energy i.e. its velocity into pressure energy, and leads the air to the volute casing. The volute shaped casing decreases the velocity further and increases its pressure.

1 Volute casing

11 Lube oil sump

2

Stationary diffuser

12 Nozzle ring

3

Shaft protection sleeve

13 Exhaust gas inlet 14 Exhaust gas outlet

4 Bearing (turbine side) 5,6 Bearing lubrication from pump

15 Rotor shaft

7

Bearing (compressor side)

16 Inducer

8

Sealing air

17 Impeller 18 Labyrinth gland.

9 Air inlet

Bearings are o f b all and roller type combination o r o f journal sleeve type. Bearings are mounted in resilient type housings. These housings have lam inar springs w hich provide axial and radial damping as well as they do not allow the bearing surfaces to chatter o r flutter w hen stopped. Bearing Lubrication is integral o r separate. It also allows transfer o f heat. Roller B earings have the advantages o f less friction losses and m ore accurate alignment. The disadvantages are that they are more expensive; are prone to brinelling effect; and need higher grade lubrication and frequent changing. Sleeve Bearings : A lthough these bearings can run at higher temperatures, running at low loads create high friction.

10 Lube oil pump

• 94

95

A ir System

Marine Diesel Engir,

Seals : Labyrinth seals are used to prevent exhaust gas leaking into the air side and into the bearing housing. Sealing air from the air side is leaked o ff to cool and seal th e shaft. Binding w ir e : A binding w ire in sm all segments is loosely passed through holes o f four to six blades. In order to fasten this binding wire, it is w elded to the first blade o f that segment. It w orks on the principle o f centrifugal action, resulting in the loosely fitted wire touching the outside o f the blade holes at high speeds. This alters the frequency o f vibration and dam pens it. In auxiliary diesel generator engines, binding w ires are not necessary because they run at a constant rpm. Fir-Tree Blade R o o t: It provides better and more even distribution o f stress at the root portion w hich is prone to failures. There is less stress concentration at the joint o f the blade and the root. Side entry fitting provides improved balance and easy replacement.

Compressor Impeller, Volute Casing, D iffuser & Inducer : A lum inum alloy for light w eight strength and sm ooth surface finish.

Uncooled Dirbochargers M odern m arine engines use uncooled turbochargers, since the exhaust gas temperatures are relatively low er than earlier types. Instead o f wasting the heat energy by cooling through water cooled casings, this heat energy can b e recovered in the exhaust gas economizer. Thermal efficiency o f the overall plant increases. M ore heat is available at the exhaust gas econom izer inlet. Corrosion defects are avoided which w ere due to the sulphur products at low loads on the gas side o f w ater cooled casings. Further details are listed in the chapter on Engine Developments.

Damping wires are required w hich pass through the blades. These dam pen the low frequency blade vibrations. Locking o f the blade is needed in the axial direction and a tab washer m ay be used to secure the blade in place. M a teria ls

Turbine Wheel, N ozzle Ring, R otor Shaft a n d Blades : Nim onic 90 (Nickel-Chrom e alloy) (Ni 75% , CO 18%, Ti 3%, A1 2% , C r 2%) These have im pact resistance, strength, thermal stability and creep resistance at high temperatures o f continuous operation upto 650 deg. C.

Pt. A is the temperature of exhaust gas leaving the turbocharger in a water cooled Pi. B is the temperature of the exhaust gas leaving the turbocharger in an uncooled

Turbine Casin g :

Cast Iron w ith corrosion preventive plastic coatings in case of water cooled turbochargers.

Pt. B is much greater than Pt. A showing more heat available to the exhaust gas

96

97

Marine Diesel Engir. A ir System

Turbocharger Faults/Problems ♦ Fouling : T he intake filter gets fouled due to oil carryover or po o r com bustion at low loads w hich further leads to fouling o f turbine nozzle and blades. Fouled exhaust gas passages cause a hig h er back pressure. M etal erosion is caused by particles in the exhaust gas. D efective blow er bearing oil seals cause carryover o f oil to air side, thereby dirtying it. T he air cooler sea water and air side also get fouled and require constant cleaning. D am ping wires and blade roots get fouled during running. T he sealing air pipe to the com pressor labyrinth may b e blocked. Hence, oil o r vapour is sucked in through the labyrinth. ♦ Bearing fa u lts : These are d ue to overheating; vibration; poor lubrication feed o r quality; m isalignm ent; fouling imbalance; and poor sealing and erosion o f bearing m aterial, balls, or rollers due to contam inated particles in the lube oil. ♦ Resilient mounting failures : These are d ue to poor support or improperfitting. ♦ Vibration: It is caused due to loose foundation bolts; excitation from external sources; w ater ingress d ue to casing leaks; and poor com bustion operations. ♦ C orrosion : T h e a ir side gets corroded due to corrosive pollutants in the air intake area. T he gas side gets corroded due to sodium and vanadium sulphate from the exhaust gas turning acidic at low tem peratures and also d u e to poor combustion. The cooling w ater side gets corroded due to poor jacket w ater treatm ent Or poor sealing or cracks, w hich lead to exhaust gas leaking into w ater spaces.

Surging It is the phenomenon o f irregular pulsations due to a change in the m ass flow rate o f air w ith respect to its pressure ratio. First, we have to understand ‘mass flow rate o f air’ and ‘pressure ratio’. The figure shows the mass flow rate o f air and pressure ratio from a compressor (blower) through a damper. Incase ‘A ’, the damper is fully o p en , m ass flo w ra te is maximum, and pressure ratio is minimum. The mass o f air will flow easily w ithout any resistance from the damper.

CP —

*

O 3 —» z

b

CP .. — '...^ c

In case ‘B ’, the dam per is cow* throttled slightly. Resistance Fig-78 d u e to th e d a m p e r w ill increase. M ass flow rate decreases, pressure ratio increases.

In case 'C', the dam per is throttled significantly and suddenly. Resistance due to the dam per increases, mass flow rate is so low and pressure ratio is so high that the m ass flow breaks down. A t this breakdow n, the pressure pulsation is relieved backwards to the compressor. This phenomenon is called ‘surging’ , w here loud ‘gulps’, howling and banging sounds a re heard. Compressor M ap Characteristics T he C o m p resso r M ap show s th e co m p resso r p erform ance characteristics. Here, the effect o f changes in speed (i.e. constant speed lines at different percentages o f blow er rpm N) are shown w ith respect to the m ass flow rate an d pressure ratio o f air. Isentropic efficiency curves are shown for 80% , 75% , 70% and

98 99

Marine Diesel Engines

Air System

In Case A -

Normal flow through the impeller and diffuser is shown.

In Case B - The effect o f sudden speed changes cause incidence losses at the diffuser entry. Eddies are form ed in the diffuser. T his is the trigger for surging. In Case C - The eddies produce a turbulent choking effect at the diffuser w hich throttles the air flow like a damper. Sudden pressure changes due to this choking o r throttling effect cause a breakdown o f m ass flow. A back flow o f air now takes place from the scavenge m anifold at a higher pressure to the turbocharger compressor side at a low er pressure. The reverse flow pressure pulsations tend to drive the turbocharger in the opposite direction, and partly stall it. Fig-79

65% efficiency. Engine operation on the left side o f the surge line will bring about instability and surging. On the right side o f the surge line, although there are changes in operation, the change in the amount o f air flow is m atched o r balanced b y a proportionate change in pressure. A safety margin in the difference between the surge line and the main engine operating line is shown.

Sum m arizing, w e understand that if there is a pressure ratio decrease in the compressor, air flows in the reverse direction in a ‘sufge’, due to higher pressure at the scavenge m anifold than the compressor. Im m ediately after this surge o r reverse flow, the compressor recovers its pressure ratio and functions normally. T his is rep eated until a ir dem and is increased and stable conditions are achieved. However, during surging, air supply to the engine cylinders continues without any interruption. Surging Symptoms These are noises at the turbocharger, gulping air sounds at the compressor intake, repeated violent pressure fluctuations, sudden quick surges in scavenge pressure, and howling or banging noises.

Fig - 80 1 Impeller and Inducer of compressor wheel 2 Stationary diffuser.

100

101

Marine Diesel Engines

Surgin g Causes ♦ A ny factor which causes a change in air m ass flow rate. ♦ Excess fouling in the system like intake air filter, compressor o r turbine w heel, turbine blades, nozzle ring, exhaust gas economizer, or even a blockage o f air filters as in the case o f a cloth covering it.

CHAPTER 4

AIR COMPRESSORS

♦ Sudden load changes during maneuvering, rough seas, overloading, or crash astern conditions. ♦ The changes in engine rpm which cause vibration in the air flow rates. ♦ Fuel starvation; dirty fuel filter; and fuel system component defects lik e fa u lty fu e l p u m p , fu e l h ig h p r e s s u re p ip e dam age, or severely wrong timings.

Isotherm al Compression It is the compression o f a gas under constant temperature conditions.

Adiabatic Compression It is the compression o f a gas under constant enthalpy conditions.

Surgin g rem edy a n d action ♦ R educe engine speed w hich, in turn, reduces scavenge air pressure and there is less tendency o f rev erse flow from scavenge air m anifold to the turbocharger diffuser. ♦ D irty o r fouled com ponents to b e checked and cleaned. ♦ Proper m atching o f turbocharger to the engine w ith respect to the co m pressor m ap characteristics, co m pressor im peller, diffuser and n ozzle area design. ♦ Regular gas and air side w ashing o f turbocharger.

102

♦ There is no heat transfer to or from the gas through the cylinder walls. ♦ As seen in the figure, it is more advantageous to compress the gas isothermally (curve A), rather than adiabetically (curve B) as less work is done (shaded area) in isothermal compression. 1 1-2 2 2- 3 3 3- 4 4 4-

Suction and discharge valve shut Compression Discharge valve is open Discharge of pressurized air Discharge valve shut Re-expansion of residual air Suction valve is open 1 Intake of air.

103

Marine Diesel Engines

A ir Compressors

M u lti S tag e C om pression Compression done in stages has the advantage o f w ork saved by inter-cooling between stages. T he fig u re show s the actual compression (Curve C ) with inter c o o lin g A b e tw e e n sta g e s . Isothermal compression (Curve B ) is show n in ‘dash ’ lines. T he w ork saved is show n as the shaded area. C o m p resso r Types Reciprocating Com pressors ♦ In marine use, m osdy single crank, tandem piston reciprocating type compressors are used. ♦ The pressure ratio between the stages o f compression is limited by the final temperatures after compression. ♦ Reciprocating types can be easily arranged fo r multi-staging. ♦ These types provide better positive sealing. ♦ Valve maintenance is increased. Rotary Compressors ♦ These are either vane or screw type. ♦ They have a higher mass flow capacity. ♦ Each stage pressure rise is limited to 7 b ar due to leakages of the rotor. ♦ Proper lubrication o f the rotor is important for sealing as well as to prevent wear. ♦ It requires a high speed drive.

104

Volumetric E fficiency ♦ It is the ratio o f the volume o f air taken in during each stroke to the swept volume o f the cylinder. ♦ A loss in volumetric efficiency o f the compressor can be due to poor valve condition, dirty intake filter, increased bumping clearance, discharge line blocked, o r restrictions in the inter cooler. B um ping Clearance ♦ It is the clearance given to avoid the chance o f mechanical bumping o f the piston and the cylinder head cover. ♦ It is the distance between the top o f the piston and the cylinder cover when the piston isatT D C . ♦ It is approximately between 0.5% to 1% o f the cylinder bore. ♦ It is checked by placing a lead metal piece on the top o f the piston and then turning the compressor m anually to obtain a lead impression. ♦ It can be adjusted by placing additional shims between the cylinder head cover and cylinder block, or under the connecting rod.

Compressor Valves ♦ Mostly plate type valves are used. ♦ They have a low in ertia o f m oving parts and good flow characteristics. Valve M aterials Body - Steel (0.4 % C) with hardened seat area. Plates - Steel (Ni or C r or M o -A llo y ) Springs - Haldened alloy steel. Valve D efects ♦ Worn or damaged seats, plates or springs. ♦ D irt or lube oil deposits on the valve parts. ♦ Incorrect assembly. 105

A ir Compressors Marine Diesel Engines

♦ Worn or seized piston rings. ♦ Overheating caused by air leakage back to suction side (recycling) o r cooler problems. 2-Stage C o m p re sso r F aults First stage suction valve leakage causes loss o f air back to the suction filter side during compression. Hence, running tim e is increased w ith less air being delivered at every stroke, and the second stage suction pressure is reduced. 2. First stage delivery valve leakage causes loss o f air back to the first stage cylinder, instead o f delivering this air to the second stage. Hence, less fresh air can be drawn in during the next suction stroke. This recycling o f a part o f the air m eant to be delivered causes an increase in first stage and second stage temperatures. A ir delivery is thereby reduced. 3. S econd stag e suction v alve leakage causes second stage compressed air to leak into the second stage suction line between the tw o stages, increasing its pressure and temperature. The first stage shows increased delivery pressure since there is additional back pressure from the second stage air leaking back. Air delivery .capacity is reduced and the com pressor runs for a longer time. 4. Second stage delivery valve leakage causes the second stage delivery air to leak back to the cylinder during the second stage suction process. Hence, the second stage shows an increased suction pressure. Air suction and delivery o f the second stage is reduced and the compressor runs for a longer time with increased second stage temperatures.

♦ Increased bumping clearance due to w orn bearings. ♦ Blocked suction filters. 6.

' Low pressure safety valve blows due to second stage suction or delivery valve leaking back to the second stage suction line between the stages.

7.

High pressure safety valve blows in case the isolation stop valve in the compressor outlet delivery line is shut.

1.

5.

Compressor capacity reduces o r full pressure not achieved, is due to: ♦ Dirty, dam aged o r worn valves.

8.

Valves require frequent attention due t o : ♦

Overheating due to poor quality water circulation or air leaking into the water side in the cooler tubes. Impurities being sucked in when the suction filter is damaged. Too much moisture carried in the air. Check tightness o f gaskets between cylinder block and cover. Pressure test the cooler to 1.5 times its working pressure. 9. Overheating o r knocks in the crankcase caused b y : ♦ Defective bearings or blockage o f lubrication oil channels. ♦ Longitudinal bearing clearances of the crankshaft is not correct due to a bent piston rod o r an edge pressure on the bearing. 10. Overheated piston caused b y : ♦ Piston o r crosshead bearing being w rongly fitted. Inspect piston rings, crosshead bearing, cylinder lubrication, piston bumping clearance and side clearances. ♦ Ineffective cooling due to poor cooling water circulation, cooler leakage, cavitation, or an air lock in the cooling water. 11. Low lube oil pressure caused by low oil level, dirty oil filter, blocked oil piping or channels, and a defective oil pump or bearings. ♦ ♦

♦ O il coking on valves due to defective piston scraper rings. 107 106

Marine Diesel Engines

12. Blocked intake filter or suction : It can cause the discharge temperature to increase to the auto ignition point o f the lube oil. 13. Com pressor running unloaded, caused by a problem in the unloader: ♦ ♦ ♦

CHAPTER 5

Check timer relay o f electrical activation. Check all air piping to unloader. C h e c k u n lo a d e r p isto n o r 0 -rin g asse m b ly fo r dirt or stickiness.

FUELSYSTEM F uel Types Crude Oil is the source o f fuel from the earth. It is a viscous oily liquid, yellowish-green to dark black in appearance. It consists o f a complex mixture o f liquid hydrocarbons with organic compounds containing oxygen, nitrogen and sulphur. Petroleum products are obtained after straight-run vacuum distillation in a refinery. Distillation produces low boiling fractions, free o f unwanted by-products. Separation during distillation provides the following fuels at different temperatures: ♦

Petroleum ether

( 4 0 to 95 deg.C)

♦

Aviation gasolene

( 4 0 to 180 deg.C)

♦

M otor gasolene

( 40 to 200 deg.C)

♦

Naphtha

(120 to 240 deg.C)

♦

Turbine fuel

(150 to 315 deg.C)

♦

Diesel fuel

(190 to 350 deg.C)

♦

Gas oil

(230 to 360 deg.C)

♦

Burner fuel

(300 to 400 deg.C).

109

Marine Diesel Engines

Fuel System

M arine F uels These are pure distillate fuels o r their blends. They are low viscous diesel fuels and heavy residual fuels. ISO 8217 is the only standard for fuel specifications. To reduce costs in m odem engines, cheaper residual fuels are used.

Kinem atic Viscosity It is the ratio o f the dynamic viscosity and the density of the fluid at the same temperature. The units are Stokes, Centi Stokes, Saybolt Seconds, o r Redwood Seconds. 1 Stoke

Fuel Properties D ensity It is the ratio o f the m ass to the volum e o f the fuel. Units are kg/cub.m. Viscosity It is the frictional resistance betw een layers o f the fluid to resist a change in shape d ue to an applied force. It is the resistance to fluid flow due to shear resistance between adjacent layers in a moving fluid. Specific Viscosity It is the ratio o f the efflux tim e o f 200 cubic cm s o f fuel at 20 to 50 deg.C, and that o f 200 cubic cm s o f distilled w ater at 20 deg.C as measured by a viscometer with a 2.8 m m orifice. T he unit is ‘degree o f specific viscosity’. D ynam ic Viscosity It is the viscosity o f a fluid in a laminar stream lined flow containing layers spaced one centimeter apart, which require a tangential force o f one dyne per square centimeter to be moved at velocities differing by one centim eter per second. The unit o f dynamic viscosity is poise, centi-poise o r poiseulle. IP = 1 Poise = 0.1 N-Sec/sq.m . 1 cP = 1 Centi-Poise = 0.001 N-Sec/sq.m. 110

= 1 St = 0.0001

sq.m./sec

1 Centi Stoke = 1 cSt = 0.000001 sq.m./sec Viscosity Index It is the index of an oil which measures the change o f viscosity due to a change in temperature. It has no units. Carbon R esidue It is the tendency o f a fuel to form carbon residue deposits. Its unit is coke value which should not exceed 0.05 to 0.1 %. It affects piston rings, liner wear, plugging o f injectors, fouling o f gas passages, etc. The testing for carbon residue is done by Conradson Test or Micro Carbon Residue Test. Conradson Carbon R esidue It is the residue quantity o f carbon measured as a percentage o f the original mass o f the fuel, after carrying out the Conradson Test. S u lp h u r It is an undesirable corrosion-inhibiting constituent o f fuel. It forms sulphur dioxide which combines with water vapour at low temperature, resulting in the formation o f sulphuric acid.

ill

Fuel System

Marine Diesel Engines

F la sh P o in t It is the minim um temperature that an oil has to be heated, to produce sufficient volatile vapours capable o f ignition when in contact with an open flame. It is the main fire hazard classification o f oil. A ll diesel fuels on the ship should have a flash point greater than 66 deg.C. The two types o f flash points are open flash point and closed flash point. Closed F lash P oint It is the minim um temperature for enough flammable mixture to give a flash w hen a test lam p source o f ignition is introduced in a closed container. Closed flash point is measured in a Pensky-Martin closed tester where the outside atmosphere does not influence the oil vapours. O pen F la sh P oint Here, there is no lid on the container. Therefore no vapour is lost, but the temperature is sufficient to give a flash, when a test lamp source of ignition is introduced in an open container. O pen flash point is approxim ately 15 deg.C higher than closed flash point. F lash P o in t exam ples For temperatures above 15 d eg .C , the test used is the Pensky-Martin clo se d flash p o in t test, o r else th e A b el te s t is used. F lash point examples are: Less than 22 deg.C 22 to 66 deg.C Above 66 deg.C Diesel Oil Heavy Fuel Oil Lube oil Petrol

Gasolene, Benzene (dangerous liquids) Kerosene, Vapourising Oils. Oils safe for m arine use. 95 deg.C 100 deg.C 230 deg.C 17 deg.C

112

F ire Point It is the temperature that an oil has to be heated to produce sufficient volatile vapours, capable o f ignition by a flammable application and continuing to bum thereafter. It is approximately 40 deg.C higher than the closed flash point. S elf-Ignition P oint It is the minimum temperature at which a fuel is capable o f ignition on its ow n accord, without an external application o f heat or flame. It is used when the choosing the compression ratio to match the fuel grade. Pour Point It is the lowest temperature at which an oil ceases to flow, o r can be poured. It is important when considering storing, heating, pumping, wax crystallization, or solidification o f an oil. Calorific Value It is the amount of heat produced by complete combustion o f one unit mass o f fuel. For one kg burnt, diesel fuels have a high calorific value i.e 10,100 to 10,300 Kcal, while heavy residual fuels produce 9500 to 10,000 Kcal. It is used while measuring the thermal efficiency o f an engine. Cetane N um ber It is an index o f the ignition quality (ignition delay characteristics) o f the diesel fuel which defines the way combustion proceeds in the engine. It is determined by comparing the ignition quality o f a standard solution (which is a m ixture o f two hydrocarbons called cetane and alphamethyl naphthalene) with the ignition quality o f the fuel tested. It is the percentage of cetane contained in the standard solution which has an ignition delay equaling the ignition delay of the fuel tested. Cetane

113

Fuel Sigg”

Marine Diesel Engines

which has very good ignition quality is assigned the number o f ‘100’. Alpha-Methyl Naphthalene is assigned the number o f ‘O’, due to it’s poor ignition quality. The higher the cetane number, better is the fuel, shorter is the ignition delay, and easier is the starting o f combustion. The cetane num ber o f diesel fuels vary from 35 to 55. If the density increases, the cetane number also increases. Octane N u m b er It is a measure o f the knock rating o f the fuel combustion in the engine. Iso-Octane is assigned a num ber o f ‘ 100’, because o f its excellent anti-knock characteristics. H eptane is assigned a num ber o f ‘O’, because o f its poor antiknock characteristics. B etter the fuel, higher is the octane number. Specific Gravity It is used for denoting the weight o f the oil while handling o r storage. A sh It is the quantity o f inorganic incombustible impurities in the fuel. It mainly consists o f sand and metal oxides like vanadium or sodium. It causes abrasive wear. Vanadium It is an undesirable impurity in the fuel. During combustion o f fuel, vanadium products like vanadium pentoxide are formed, which are deposited on the surrounding surfaces. These deposits are highly corrosive above 700 deg.C.

compound eats into the metal surface, leaving the surface e x p o ^ to corrosion. Catalytic F ines A fter vacuum distillation, catalytic cracking is often carried ou . Catalytic cracking is done to crack the oil vapours by reheating W1 silica and alumina as catalysts. These catalysts are used in poW ®r form in an oil vapour. Some o f these catalysts break up to form a us known as catalytic fines. They cause abrasion wear in the engibesA ir/F u el Ratio The stoichiometric ratio for proper combustion is 14.5 kg air t° 8 fuel.The actual air ratio is 30 to 44 kg per 1 kg fuel. Excess air I3*101S 36.5 kg p er l k g fuel. O ther F u e l Im purities . Other impurities in the fuel include water, iron, phosphorus, e ’ lead, calcium, etc. Total S edim ent Test It m easures the stability o f the asphaltene phase o f the fuele sedim ent accumulates at the bottom o f the storage tank and l138 a very high asphaltness content. This affects filters and componei118Wax It is a residue formed due to high paraffinic content. It is soluble*11a petroleum oil base. It crystallizes at it’s cloud point which may ^ 38 high as 35 deg.C.

Vanadium a n d S odium When both these impurities are presentinaNa:Varatio o f 1:3, vanadium pentoxide w hich is formed com bines w ith sodium to a form a very hard compound whose m elting point is around 630 deg.C. This

Calculated Carbon A rom acity Index (CCAI) It is a rating o f the fuel which indicates ignition quality, because is?utl0n directly depends on the aromatic content in the fuel. AromaticS 31:6

114

115

Fuel System

Marine Diesel Engines

compact benzene ring structures present in the fuel w hich affect the ease o f w hich a hydrocarbon fuel m olecule can bum . A low CCAI rating means better ignition, better fuel quality and less ignition delay. Low ratings are upto a CCAI ratio o f 850. High ratings are from 850 to 950, and 870 is the lim it for its use. It does not affect ignition in modem 2-stroke low speed marine engines, but it mostly affects ignition in medium speed engines.

Fuel system line diagram

Fuel Specifications Given below are the m aximum limits for Heavy Fuel Oil and Marine Diesel Oil: Heavy Fuel Oil (1) Density at 15 deg.C

991 kg/cub.m.

Marine Diesel Oil 840 to 920 kg/cub.m.

(2) Knematic Viscosity at 50 deg.C

700 cSt

at 40 deg.C (3) Sulphur

Combustion Phases 14cSt

5%

2%

(4) Conradson Carbon Residue 10 %

There are 4 phases in the combustion process: 1. Injection delay

2.5%

(Micro Carbon Residue) (5) Ash

0.2%

0.02 %

(6) Water

1 to 2 %

0.25%

(7) CCAI

880

(8) Sodium

lOOmg/kg

(9) Vanadium

600mg/kg

(10) Aluminium + Silicon

80mg/kg

(11) Sediment

0.1 %m/m

116

117

Marine Diesel Engines

Fuel System

1. Injection delay o r la g : It is the tim e delay between the closing of the spill ports/ valve and the opening o f the fuel injector. It depends on the pressure rise in the fuel pump and the pressure ,in the injector line. 2. Ignition delay o r lag : It is the tim e delay betw een the start o f injection and the start o f combustion. Factors affecting ignition delay are a rise in scavenge air o r cooling w ater temperatures, retarded fuel injection tim ing, ignition quality o f fuel, low load and low speeds. 3. Combustion o f the already injected fuel and fuel still beinginjected: ' Ignition delay directly affects the combustion in this phase. In case o f a large ignition delay, a large pressure rise can cause a diesel ‘knock’, 4. A fter b urning: I t is the burning o f fuel after injection is finished. Afterburning is considerable in case o f a large ignition delay, since heat is now given out in the expansion stroke and cannot be utilized efficiently. K nock It is the phenom enon o f a high sudden pressure and temperature rise due to the detonation o f fuel. It sends heavy shock waves, an increased flam e front speed, an increase in noise and vibration and a shock loading to engine components like bearings, piston rings, cylinder, etc. In case o f a ‘knocking’ sound, check whether it is a mechanical or a fuel knock by cutting out the fuel. M echanical knock is due to worn out bearings; broken or loose com ponents; o r an excessive play

118

between the piston and the liner (worn rings or a worn liner). Diesel knocking depends o n engine speed, load, com pression ratio, turbulence, mixture strength, fuel characteristics, ignition delay, injection timings, cetane number and octane number.

Factors Affecting Combustion A to m isa tio n It is the breakup o f the liquid fuel into a m inute vapour mist, so that these fuel vapour particles possess a very high surface area to self ignite with hot compressed air. Atomisation depends on the small orifices o f the injector; the pressure difference betw een the fuel line and cylinder; and the temperature, mass flow rate and viscosity of the fuel. If too much atomization takes place, then very small particles will not have enough kinetic energy to go through the whole combustion space. They w ill gather near the injector due to resistance from the dense compressed air. Hence, they will be starved during combustion and afterburning will take place. If too little atomization takes place, larger particles will possess m ore kinetic energy and g et deposited on the liner wall. This causes after burning and poor combustion. Carbon deposits w ill be seen on the liner walls, the side o f the piston crown and the piston rings. P enetration It is the distance traveled by the fuel particles into the combustion space before ignition takes place. A fuel je t should penetrate well into the combustion space without im pingement onto the liner or piston crown. Normally, penetration is up to 60% o f the liner bore for liquid fuel, w ith only fuel vapour being allowed to im pinge on liner wall. Penetration depends o n nozzle diameter size, length o f nozzle hole, fuel particle size and atomisation. 119

Fuel System

M arine Diesel Engines

F u e l D istribution

Fuelshouldbedistributedevenlythroughoutthecombustionspace withoutoverlapping,forgoodcombustiontotakeplace.

m b u stio n C ham ber a n d Piston Crown Designs pious designs o f the combustion space cham bers with respect to Mon crown shape are shown in the figure.

S w ir l It is the motion given to the incoming air charge entering the combustion space. T his is done by the shape o f the com bustion space and the direction o f entry o f the air charge. T urb u le n c e It is a factor that has already been designed during manufacture and can only be influenced by fouling o f inlet ports or exhaust ports; and scavenge or exhaust pressures. It is given to improve the air fuel mixing. It is done by giving a swirl to the intake air by means o f the inlet valve passag e shape o r angle; changing the size o f scavenge ports; the positioning and alignment o f the fuel injectors; the burning o f fuel; and the squish from the piston shape. A ir F u e l M ixing T h e fuel is injected into the cylinder at a velocity o f 150 to 500 m/s form ing a cone-shaped spray with a greater density at the center. Its penetration length depends o n th e injection pressure i.e. 120 to 500 kg/sq.cm for slow speed engines. To ensure proper combustion especially during overloaded conditions or poor air-fuel intermixtures, excess air is provided.

Com pression Ratio It is the ratio o f the volum e o f air at the start and the finish o f the compression stroke. For compression ignition engines, it is 12.5 to 13.5. Loss o f compression is due to poor sealing or excess clearance volume. The causes are w orn piston rings; w orn liner; or excess

E xcess A ir C oefficient is the ratio o f the actual am ount o f air to the theoretical am ount required to bum 1 kg o f fuel. On diesel engines, it varies between 1.3 and 2.2 to achieve complete combustion.

Im pingem ent W hen there is less atomisation o f the fuel, the fuel particles are larger. They travel with a higher velocity and get deposited on the liner and piston crown. This impingement is undesirable as it causes burning at

It

bearing clearances.

that area.

120

121

Marine Diesel Engines

Fuel System

F u e l com bustion is also in flu en ced by ♦ Scavenge air pressure, tem perature, and charge air quality depending on the scavenging method. ♦ Exhaust gas back pressure due to fouling o f exhaust passages which also affect combustion and proper scavenging. ♦ Fuel parameters i.e. its tem perature at the inlet to the engine, its viscosity, its ignition quality, its fuel ratings and its injection timings. ♦ Fuel pum ping faults due to fuel pum p internal wear; injector conditions affecting the m axim um pressure delivered; injection delay; fuel particle size and penetration.

Residual Heavy Fuel Oils M arine engines use cheaper heavy residual fuels fo r constant MCR operations and low viscous diesel fuels fo r starting, maneuvering, running-up and stopping. Heavy fuel oil is the residual fraction o f a crude oil source after all other distillation products are extracted in a refinery. It is also a mixture with lighter distillate fraction oils. In modem engines, due to escalated fuel oil prices, residual heavy fuel oils are used to cut on costs. Undesirable properties o f the heavy fuel oil are: high viscosity, increased sulphur, ash, sodium, vanadium, salts, water, solid particles and sediments. The harm ful effects o f these contents have been discussed earlier. R esidual F u e l Treatm ent In order to use residual heavy fuel oil fo r the engine, the oil has to be treated to reduce the problem s faced w ith these im purities. The following treatment is carried out: 1.

Lim iting the im purities w hen purchasing o r bunkering the oil. Lim its fo r each property and param eters are laid dow n by ISO 8217 (1996).

122

2. Separation o f water and sludge in settling and service tanks. The settling tanks and service tanks have heating coils and bottom collection space to rem ove sludge and water. M axim um temperature o f the settling o r service tank must be 15 deg.Cbelow the flash point o f the fuel, b ut not m ore than 9 0 deg.C or else, volatile vapours may form creating an explosive hazard. 3. Filtration is done w ith filters to rem ove sediments and particle impurities; These are commonly fitted (a) at the outlet o f die storage bunker tank i.e. at the inlet to the transfer pum p known as cold filters; and (b) at the inlet to the supply pum ps after the heaters known as hot filters. 4. A mixing tank or column to gradually mix heavy fuel oil and diesel oil during change over operations. It also serves the purpose o f venting and degasification o f trapped air and gases. 5. Purification in centrifugal separators to rem ove w ater and som e amount o f sediment. 6. Heating to reduce viscosity. 7. Usage o f a cylinder lube oil TBN having a high alkalinity to neutralize acids formed due to sulphur content; and maintaining a low cooling water temperature.

Bunkering Bunkering is done to replenish fuel and lube oil supplies required for mnning the main propulsion plant and auxiliaries. A bunker plan is first drawn up. T his is a written procedure detailing all pipelines and sequence o f events. It describes in detail the quantities to be filled in each tank as well as the rate. The Chief Engineer is directly in-charge and is required to personally supervise all operations. To assist him, another engineer and an assistant are designated. Before starting, fire fighting equipm ent and spillage gear are to b e positioned and kept

123

Fuel System

Marine Diesel Engines

ready. Com m unications betw een ship and bunker barge is to be checked. D rainage scuppers leading to slop tanks on deck, which can be filled in case o f a large oil spill are to be checked that they are open. An air operated pum p to transfer oil in em ergency is set up. Hoses and seals are to bfe checked at the connections. Smoking is not allowed. N o oil transfers during bunkering is permitted. Explosionproof tools and lam ps to be used. A breathing apparatus is to be provided in case o f poisonous gas hazard. A fuel sample is to be taken by a standard approved method. This is then sent for testing (FOB AS). Initially, oil is supplied at a very low rate. All lines and valves are checked for leaks and whether the correct quantities are being received in th e d e sig n ated tan k s. O th er tan k s are also so unded as a precautionary measure in case of leaking valves. The bunker line valves should be open and set under the Chief Engineer’s supervision. After bunkering, once the fuel quality and quantity are acceptable, then only will the Chief Engineer sign the receipt forms.

Optimum Injection ♦ Injection o f the fuel is best o r optim um if injection is done immediately after maximum combustion pressure is achieved and injection supply is very rapid at this point. ♦ Injection tim e is only 20 degrees o f crank angle a t full load, but maximum firing load is reached only in the latter half o f this period i.e. latter h a lf o f the injection period. Therefore, w e m ust inject m ore fuel towards the end o f injection after the m aximum firing pressure is reached and supply this remaining fuel as fast as possible. ♦ It is best achieved in the Intelligent Electronically controlled engine series ( RT-flex or M E engines) for different load conditions.

124

Fuel Injectors The fuel injector valve consists o f the valve body, valve head, union nut and atomizer nozzle. In the valve body, there is the thrust spindle, thrust spring, thrust foot and valve unit. 1 2 3 4 •5 6 7 8 9 10 11 12 13 14

O-Ring Fuel Valve Head O-Ring Locking Pin Thrust Foot O-Ring Thrust Spindle Fuel Valve Unit Union Nut Spring Atomiser' Nozzle Valve Body Locating Pin O-Ring

Injector F unctions a n d R equirem ents ♦ It should inject and disperse the fuel evenly into the engine cylinder in a finely atomised spray. ♦ The size, position and orientation o f the injector nozzle has the function o f creating a fine atomized spray with good penetration. ♦ The injector also serves as a non-retum valve not allowing any combustion space gas back into the fuel system. ♦ It should not open till a preset pressure is built up. ♦ A t the start o f injection, the droplet size should not be too large as this will encourage ‘slow burning’.

125

Fuel System

Marine Diesel Engines

♦ T he valve opening should b e prom pt to prevent pressure loss through throttling, during the opening process. ♦ It should provide cooling o f the valve whilst in use which prevents softening o f the valve and seat, as well as reduces expansion o f the trapped fuel in the ‘sac’ area. Injector Types ♦ Cam-operated o r Hydraulic-operated types. In m arine use, mostly hydraulic operated type is used. ♦ Open or Closed valve type: Open injectors dispense with a valve between the fuel line and the combustion chamber, while as closed type do not do so. Open type are n o t used in m odem m arine engines because they suffer from after-dripping o f fuel after the injection stroke. Hydraulically O perated F u e l Valve ♦ In this type, the operation o f opening and closing o f the fuel valve is perform ed hydraulically by the fuel pressure delivered by the fuel pum p to the fuel valve. Valve opening is initiated by an oil pressure shock wave in the oil contained in the high pressure fuel piping. T he shock wave is caused by a sudden very high pressure increase. This high pressure increase is d ue to the increasing acceleration o f the fuel pum p plunger and the fuel cam. This accelerated fuel causes a shock wave when the inlet port or suction valve is closed during pump delivery. ♦ Fuel pressure from the fuel pum ps act on the needle. The needle opens inwardly. T he needle is loaded by a thrust plate, a spring and a screw ed spindle. T he thrust plate serves the function o f limiting the needle lift

126

♦ W hen the fuel oil pressure force overcomes the spring force, the needle lifts. Oil pressure acts on the annular area at the end o f the valve spindle where it is machined to a smaller diameter than the spindle diameter. A fter opening, the lift exposes the full cross sectional area o f the spindle for quick opening. ♦ Prompt and rapid opening is achieved during opening, because an extra effective area o f the needle seat is exposed for fuel oil and pressure to act upon after initial lifting o f the needle. ♦ Coolant is circulated through the space around the bottom o f the nozzle cooling oil flow. Passages are drilled in the valve body to the top. ♦ Leakages o f the valve component faces will be seen in the spring space vent hole. ♦ Atomiser holes vary from a diameter o f 0.075 m m to 1 mm. ♦ The valve lift is around 1 m m to 1.5 mm.

Fuel Injector Faults Improper Cooling ♦ Too m uch cooling causes sulphur corrosion o f the tip due to the injector tip temperature falling below the condensation temperature. This is not seen on m odem engines and was only experienced on older engines. Acorroded atomizing nozzle tip will alter the spray penetration, atomisation and pattern. Water condensation takes place at temperatures below 110 deg.C, allowing sulphur oxides from the fuel to turn into acids. ♦ Too less cooling causes softening o f the valve and seat, and allows expansion o f trapped fuel in the ‘sac’ area. This causes carbon trumpets on the tip, poor combustion and smoky exhausts. A t temperatures above 180 deg.C, the fuel starts cracking into particles which clog the nozzle. 127

Fuel System

Marine Diesel Engines

Dribbling Nozzle A dribbling nozzle will result in fuel-burning at the nozzle tip which is seen as carbon trumpets. Dribbling nozzles are a result o f poor seating o f the fuel valve, which in turn, is caused by the impurities in the fuel causing abrasive w ear to the seat surfaces; poor cooling; increased banging o f the valve needle; poor maintenance and overhaul; and wrong spring pressures. Carbon trumpets adversely affect combustion since they influence the spray pattern o f the fuel. This leads to smoky exhaust, higher exhaust temperatures, poor combustion and loss o f power. Wrong Spring Pressure The spring pressure directly influences the size o f the fuel particles. Low er spring pressure leads to the valve opening and closing a t a lower pressure. W hen the injector opens at a lower pressure, larger fuel particles are formed and these larger fuel particles bum ineffectively, resulting in a reduced cylinder pressure and smoky exhaust. The causes of a wrong spring pressure are incorrect overhauling; fatigued material; or extended life o f the spring. Nozzle hole diameter, depth a nd num ber This will influence the penetration, atomization and overall combustion. Nozzle holes may be choked due to fuel impurities, carbon trumpet formation, burning o f ‘sac’ area, trapped fuel and prolonged running o f engine at low loads. T he length o f the nozzle hole is usually thrice the size o f the diameter o f the hole.

Here, hot oil is circulated when the injector is n ot injecting. O nce the fuel pressure a t the beginning o f the fuel pum p pressure stroke, increases to more than 8 bar, the recirculation line is closed. During re-circulation, 2 to 8 bar pressu rised fu el o il flow s through the center bore in the valve body to a hole in the thrust spindle; then to the thrust piece to a circulation hole at the slide top; and out o f the valve housing through an outlet pipe. R e- n c irc u la tio n sto p s w hen oil Fig-87 pressure exceeds approximately 10 bar. T h is in c re a se in pressure above 10 bar overcomes the slide valve spring pressure. The slide pushes the thrust piece, thereby closing the circulation holes and fuel oil now passes further down to the space above the valve spindle seat.

U n-Cooled Injectors These are used in m odem engines using residual heavy fuel during maneuvering operations. In order to run on residual heavy oil during maneuvering, un-cooled injectors are used on latest engines e.g. MAN B&W.

Injection System Requirem ents ♦ The fuel injection system consists o f the fuel injector, fuel pump and metering control. ♦ It should supply a finely atomized spray w ith correct penetration and even distribution into the combustion chamber. ♦ The quantity o f fuel is to be metered and the same amount is to be supplied to each cylinder to obtain equal pow er and balancing of all units.

128

129

Marine Diesel Engines

Fuel System

♦ Correct timing and quantity o f injection corresponding to different stages in the com bustion cycle is important. T his is required to efficiently utilize the heat and energy o f combustion and have the correct cylinder pressure rise to control combustion. ♦ Prom pt and rapid opening and closing o f the injector is very important

Types of Injection Methods The main types are Blast Injection o r Solid Injection methods. In blast injection, fuel is blow n into the cylinder by an air ‘blast’. In solid or ‘airless’ or ‘mechanical’ injection, fuel is forced into the cylinder through a fuel valve by a high fuel pum p pressure i.e. by the ‘solid’ fuel. So lid Injection System s There are 3 commonly used ty p es: 1. Common rail injection system. 2. Gas compression injection system. 3. Individual unit injection system. 1.

Common R ail Injection System It consists o f fuel pumps, distribution blocks, accumulators, a com m on piping o r ‘rail’, and cam shaft operated spring loaded injectors. T he fuel pumps supply oil pressure to a common pipingor rail which is connected to an accumulator to damp out pressure fluctuations. The common rail then supplies the fuel injectors through a fuel timing valve whose opening and closing is camshaft operated. I t is an outdated system, used earlier in D oxford P and J-type engines. However, the latest camshaft-less RT-Flex and M E engines employ a type o f com m on rail system. D etails are discussed under the engine description chapter.

130

2. Gas Compression Injection System In this type, combustion gas pressure from the main engine combustion chamber is led to drive the fuel pump piston in the fuel pump chamber. Hence, a camshaft is not required to drive the fuel pump. T iming o f injection is done by m eans o f a timing valve operated by an oscillating lever and eccentric fulcrum. M odem marine engines do not use this type o f injection. 3. Individual Unit Injection System In this type, an individual fuel pump and injector, meter and supply fuel for combustion in the engine cylinder. Timing is carried out by means o f a camshaft drive to the fuel pump plunger. The governor linkage also influences the fuel pump rack control. Governor input is common to all units, but the rack on each pump can be adjusted to compensate for internal pump leakage. M ost m arine engines use this injection system.

Fuel Pumps The function o f the fuel pump is to control the quantity and timing of the fuel injected into the combustion space and to provide the high fuel pressure required to hydraulically operate the fuel injector. M ost commonly used fuel pumps in marine engines are discussed below. Suction Valve Controlled P um p This ‘variable beginning constant end’ type pum p uses a push rod to operate the pump suction valve, w hich in turn, controls quantity and tim ing o f fuel injected. It was used on older Sulzer RD engines upto the mid 1960s.

131

Fuel System

Marine Diesel Engines

1 2 3 4 5 6 7

peak combustion pressures and thermal efficiency. At low loads, charge air pressure is lower and with this system, combustion firing pressures dec rease even further. Cheaper fuels imply longer ignition delay which add to the already delayed ignition problems o f these pumps.

Plunger Roller Cam Governor Control Lever Eccentric Rocker Arm Push Rod Barrel

Working During the downward stroke o f the plunger, the barrel is filled with fuel oil since the suction valve is open. D uring the upstroke, alth o u g h th e p re ssu re starts z increasing, no fuel is delivered till the suction valve closes. Hence, the ‘beginning’ o f delivery can be ‘varied’, depending on the suction valve closing ‘early o r late’. After the suction valve is closed and the pressure built up is sufficient to. lift the delivery valve, delivery commences. Hence, the ‘end’ is ‘constant’. Raising or lowering the suction valve is used to alter the closing o f the suction valve earlier or later, thereby changing the fuel timings. A dvantages . Volumetric efficiency is improved and constant. A djustment is easy w hich enables geometrically correct delivery. The plunger design is sim ple w ithout h elix edge w ear and it has a longer life. Easy maintenance, lapping, grinding and replacem ent o f suction valve is possible. Disadvantages It is m ore expensive than the jerk helix type pump. The fuel timing is not ideal for all load changes. At a low rpm, most o f the fuel is delivered after TDC. This delayed ‘later’ injection causes a drop in maximum 132

Suction a n d Spill Controlled F u el Pump It is a ‘constant beginning variable end" type pump, in which the suction valve is not connected to the governor and hence a ‘constant beginning’ is achieved, while the spill valve connected to th e governor controls the end o f ignition i.e. a ‘variable end’. It is used in Sulzer RND onward designs.

Working During the downward stroke o f the plunger, fuel flows and fills the barrel since the suction valve is open. D uring the upw ard stroke, the plunger and the spill valve push rod rise, but the suction valve push rod goes down, which closes due to the delivery pressure. Delivery now takes place once the suction valve is closed and the delivery valve opens at its preset pressure. The

133

Fuel System

Marine Diesel Engines

plunger is still moving up, along with the spill valve push rod and after the clearance is passed, the spill valve is lifted to open. This shows th atth e ‘end’ o finjectionis ‘variable’, depending on the opening o f the spill valve. The spill valve opening depends on the governor input and corresponds to the engine load. The suction valve opening depends on the length o f the push rod and the eccentric shaft position. It is initially set and is not variable with the load. Advantages as com pared to ‘Variable Beginning’Pumps B etter peak pressures and better therm al efficiencies are possible. Fuel is injected at a ‘constant beginning’ i.e. at the same crank angle. Hence, at low revolutions, fuel would be injected earlier than required and this would balance the ‘longer ignition delay’ period required by cheaper fuels. D isadvantages T he w hole quantity o f fuel is delivered before T D C even at low revolutions. This m ay result in ‘knocking’ effects.

the top edge o f the plunger covers the suction ports and the pressure is greater than the delivery valve setting. End o f injection is variable and is controlled by the helical edge uncovering the spill port. (This can be varied by moving a rack and pinion mechanism which rotates the plunger and helix). The spill port spills fuel back to the suction side. ;

Port-C ontrolled H elix J e rk P um p It is commonly used in M AN B&W engines as well as 4-stroke engines.

Advantages The port and helix control does not require the use o f suction or spill valves. It is more reliable and most commonly used.

Working During the downward stroke, the pump barrel fills up w ith oil through the suction port w hich is uncovered as in fig. 91-A. During the upward stroke, the plunger covers the suction and spill ports as in fig. 91 -B . The beginning o f injection is constant and is achieved by the fuel pressure rising above the spring loaded delivery valve preset pressure. The delivery ends when the helical edge uncovers the spill port as in fig. 91-C. Beginning of injection is initially set and constant. It starts when

134

Pilot Injection System Pilot injection can be done by three w ay s: ♦ A Jerk pump is used which has a cam with two lobes, instead o f a delivery valve. The first cam lobe opens the valve at a Iowa: pressure e.g. 75 bar, and injects a small pilot charge which has a long delay period. This pilot ignition heats up the combustion space so that the main charge bums well. The second cam lobe opens the valve at a higher pressure e.g. 415 bar, and injects the main charge. This reduces ignition delay for the main charge and gives a slower rate o f pressure rise. The chance o f ‘knocking’ is reduced. It was used on outdated Polar 2-stroke medium speed engines. 135

Fuel System

Marine Diesel Engines

♦ Pilot ignition by means of a double injection profile jeik pump which will give two injection pulses. ♦ Pilot ignition by means o f an electronic control o f the injector. Tw in Injection System In this type, two injectors are used i.e. the pilot and the main injector. It is used on Wartsila Vasa-46 engines. It minimizes ignition delay and knock. The engine can run on low loads for unlimited periods. It allows high viscous fuels (380 cS t at 50 deg.C) and highly aromatic fuels (low cetane no., but CCAI not high) to be burnt more efficiently. The pilot injector injects a constant volume for different loads. Atomisation in the pilot injector is better due to finer nozzle holes. Tw in F u e l P u m p B arrel System In this type, tw o fuel pumps in parallel supply fuel to the same injector. One pump plunger controls the beginning o f injection, whilst the other controls the termination o f in je c tio n . T hey achieve m uch higher pressures than that which can be achieved by a single fuel pump. This system is used on Wartsila’s largest m edium speed engine. Electronic In jectio n Control It is used on latest engines by Wartsila-Sulzer and M AN B&W. Here, the engine rpm, crank angle position, etc. are fed into a microprocessor which gives an output signal to the injection pumps. More details are listed in the engine description chapter.

Variable Injection Timings (VIT) VTT

= Variable Timings = Variable beginning and Variable end o f injection.

136

Reasons fo r VIT M o d ern e n g in e s (slo w Hpeed, h ig h p re s s u re ch arged types) lo se too much combustion pressures and tem peratures a t low lo a d s a n d s p e e d s. T h e Mater’ delayed ignition, as in the case o f constant end types, led to low er peak p re ssu re s a n d lo w e r f*-INJECTION — efficiency at low loads. With Fig-90 costs o f fu el increasing, cheaper highly viscous residual fuels are now used which have longer ignition delays, lower peak pressures, delayed combustion, higher exhaust temperatures and higher fuel consumption. Latest engines have a high stroke-bore ratio i.e. super long stroke for m ore power output and run at a lower rpm. In 1978, Sulzer introduced the V IT on the ‘RLA’ engines mainly to allow better combustion and maximum pressure at low er loads (75% load) while using residual fuel. VIT - Sulzer Engines It is a type o f fuel pump control which allows the engine to achieve the designed maximum combustion pressure a t a range o f 7 5% to 100% power. It is done by varying the injection timings to maintain higher combustion pressures a t reduced loads. VIT Advantages The thermal efficiency and combustion efficiency are improved, while Specific Fuel Oil Consumption (SFOC) is reduced i.e. a reduction of 7. gm /kw hr in Sulzer engines.There is no dark sm oky exhaust; less 137

Fuel System

Marine Diesel Engines

therm al stresses; im proved N O x emissions; im proved temperature control for preventing corrosion; and the strength o f parts like the crankshaft is betterutilized. The fuel oil consumption directly depends on the expansion ratio and thermal efficiency. Expansion ratio

=

Ratio o f the m aximum combustion pressure to the pressure at the commencement of exhaust blowdown. H eat added

In normal engines, Pmax is achieved only at full load power, whilst in VIT, Pm ax is achieved at low er loads. A t low er loads, there will be less fuel consum ption b ut an increase in Pmax. This leads to an improved expansion ratio; improved utilization with higher Pmax at lower loads; and im proved therm al efficiency. Therefore, SFOC is reduced. VIT M ethod In suction and spill valve controlled pumps, injection timings can be varied by raising or lowering the position of the suction and spill valves. Raising or lowering o f the suction and spill valve positions are done by changing the position o f the eccentric. Raising the valve implies earlier timings, while lowering the valve im plies later timings. The suction valve controls the beginning o f ignition i.e. the timing of injection, while the spill valve controls the end o f injection i.e. the quantity o f fuel. Advancing Here, the suction valve is ‘low ered’. Hence, injection commences earlier. This results in more fuel quantity being delivered, since earlier injection gives m ore injection tim e and m ore fuel is delivered. To maintain the same fuel quantity, the spill valve is ‘raised’ to give earlier

138

end o f injection i.e. decreasing the amount o f fuel delivered. Hence, quantity o f the fuel delivered does not increase. Advancing = Suction valve lowered + Spill valve raised. Retarding This procedure is just the opposite of advancing. Retarding = S uction v alv e ra ise d + Spill valve lowered.

139

Marine Diesel Engines

Fuel System

F u e l Quality Settin g (FQS) It is a m anually adjustable lever w hose setting can b e changed to com pensate fo r various fuel qualities. T he ‘FQ S’ angle is a user parameter setting in the engine control and can be adjusted within the range o f- 2 t o + 2 degrees. T he governor output shaft is connected to the V IT control and superimposed on the ‘FQ S’ linkage.

Super VIT It is a VIT m ethod used on B& W ’s larger L/K/S-M C engines. Super VIT = A djustable Timings + A djustable Break Point

It is a means o f automatically varying the commencement o f injection In order to maintain the maximum combustion pressure (MCR) Pmax constant, o ver a range o f 85% to 100% full load. T he break point normally at 85 % load is a pre-specified part load above w hich the maximum combustion pressure is maintained constant. Super V IT is used on larger L/K/S - M C engines. The Super V IT mechanism consists o f a jerk type pump with double thread, a V IT regulation lever, a V IT position servo, a control air signal, a position servo unit with input from the governor, a FQS lever and a regulating shaft. Super V IT M ethod In this m eth o d , the jerk type fuel pump does n ot have a profile i.e. n o extra oblique-cut on the plunger. The vertical position o f the pum p barrel is raised or lowered to change the commencement of injection by a rack and pinion mechanism and a double thread. 1. Upper threads control the suction ports i.e. commencement o f injection by changing the vertical position o f the pump barrel with respect to the plunger. 2. Lower threads control the spill port i.e. the fuel quantity and end o f injection by rotating the helix scroll o f the plunger with respect to the spill port. Fig-94

140

141

Marine Diesel Engines

Fuel Syster.

The VTT-rack setting is controlled according to the engine load via th e regulating shaft and th e governor. T h e V IT rack setting position is done by m eans o f a control air signal supply which pushes the VTTra c k 'ppositio rack o sitio n servo. The control air position sensor valve gets its input from the governor, the FQS lever and th e regulating shaft.

lo w Load Operation Here, the V IT system is out o f operation. As shown in the figure (at *ero load), the beam is fully lifted and control air pressure is ‘O’. Delayed injection takes place. Increasing Load As the load increases, the V IT is still zro (delayed injection) till point I. Control air pressure at point I is now 0.5 B ar and the beam A has made contact with the sensor pickup.

x

v

/

1 /

VIT Start Pt, Cornair0,5 Bat

.

1 i '

{ ,

Fuel index I (Quantify)

VIT-index , (VIT control pressure!

Break-Point 100% 85%

142

Run-up till 85% Load From point I to point n , the control pressure increases further making the V IT position servo change the V IT index setting. The timing is now advanced. A t 85% Load A t Point n , Pmax is achieved early due to the advancing from point I to point II. T he beam A touches the supporting points. The sensor pickup is fully depressed.

^0AD

143

Fuel System

Marine Diesel Engines

85% to 100% L o a d A bove point n , the beam A rotates around the support. C ontrol air pressure causes the VIT-rack position servo to ‘retard’ the injection timing in order to maintain Pmax constant at this range.

retard timings. Collective adjustment is done to compensate for two main reasons, which are (a) different fuel qualities, and (b) worn fuel pumps. Break P oint and Pm ax Adjustment This is carried out in case the fuel cams have been moved. Break point values a re : Fixed pitch propeller M K I engines = 78% load Fixed pitch propeller M K II engines = 85% load Controllable pitch propellers = 85% to 90 % load New engines will set the break point 2 to 3 % higher to compensate for an excessive pressure jum p from Pcomp to Pmax, as the engine becomes older. Non-Return Throttle Valve This valve is fitted in the control air line between the position-sensor valve and the position servo. It prevents excessive combustion pressure during sudden reduction o f load in the upper load range i.e. above the break point e.g. in rough weather. It prevents rapid fuel rack oscillations from being transmitted to the VIT-rack i.e. for a stable V IT rack in case o f slight governor jiggling.

Individual Adjustm ents These adjustm ents can be done at the individual pum ps to balance ‘Pm ax’ for all the engine cylinder units. (Pmax adjusted + o r - 3 Bar). Adjustment is done by m oving the position servo at each VIT-rack, or by adjusting the threaded connection betw een the position servo and the V IT control shaft (sim ilar to balancing the fuel racks).

Conventional VIT (B & W Engines)

Collective (O verall) Adjustments These adjustments aredonefor theengine as awholeunit, and common to all fuel pum ps. A djustm ent is done by adjusting screws on the position sensor unit which alters the control air pressure to advance or

144

F ig -100

145

Marine Diesel Engir,

Fuel System

It is the mechanism fo r varying the ignition timings used on smaller GB, L35M C and L42M C engines. Here, the break point is fixed in relation to the pum p index and not adjustable as in super-VTT. The fuel pum p plunger is profiled i.e. it has an extra oblique-cut. V IT conventional = Adjustable timings + Fixed Break Point

Fuel Cam

1

Base Circle It is the smallest circle o f the cam profile which acts as the base o f the cam.

2 Cam A ngle It is the angle o f the cam for which the follower is lifted.

A cam is a m eans o f providing the required m otion to its follow er in order to operate the opening and closing o f valves, o r regulate the timings o f a fuel pump.

1 2 3 4 5 6

Cam Types Regular, irregular, internal, external, inverse, single lobe, multi-lobe, etc.

A ngle o f D w ell It is the peak section o f the cam profile during which the follower is resting, although it is in a lifted position.The angle o f dwell is designed to take into consideration the fo llo w ing: checking o f the plunger clearance; allowing the exhaust cam to be fitted on the same camshaft in case o f reversible 2-stroke engines; and smooth filling and spill o f fuel without pressure changes.

Spill valve push rod Suction valve push rod Roller follower Base circle Fuel Cam Camshaft

K g - 101

Cam shaft Drive Cams are mounted on a camshaft, which in turn is driven by the engine crankshaft through chain drive or gear drive.

F u el C am requirem ents At the beginning o f the injection stroke, a high amount o f acceleration is desirable, b ut with a smooth transition to prevent shocks. During the injection stroke, constant velocity should be maintained without any pressure drop when the fuel valve opens. A t the end of the injection stroke, sharp deceleration is required to snap shut the fuel valve, but smoothly in order to avoid bouncing.

High Pressure Pipe Safety C am Profile It is the shape or curvature o f the w orking surface o f the cam which drives the follower with arequired motion to regulate the timings o f a fuel pump.

146

It is the very high pressure line between the fuel pum p and the fuel injector, which is subject to pressure shock waves and vibration. It is an im portant fire hazard because pressurized oil leaking from it can spray over numerous hot surfaces o f the engine and cause a fire. 147

Marine Diesel Engines

Protection and m onitoring o f the high pressure fuel line is a class requirement, especially for UM S ships.

CHAPTER 6

LUBRICATION SYSTEM F unction o f Lubrication It reduces friction, prevents excessive wear o f rubbing on surfaces, provides corrosion protection, removes some frictional heat, helps in cooling, and prevents accumulation o f unwanted deposits.

This high pressure fuel line has a protective double skin sheathing. It also has a leak o fflin e from the space between the pipe and the outer sheath. This line is led to a leak off tank which monitors leakage and gives o ff an alarm if the leakage is in excess. In case o f minor leakages, there is a small leak o ff hole connection which directly drains to the main overflow tank.

148

149

Marine Diesel Engines

Lubrication System

E ngine L u b e O il A pplications The following components o f the engine require lubrication: cylinder liner, piston, crankcase, bearings, centrifugal purifiers, camshaft gear or chain drive, exhaust valve actuation, crosshead guides, turbocharger bearings, power generators and pow er take-in/out units. Lubrication Feed Types are: full force feed lubrication for bearings, splash lubrication, combination lubrication, and m etered lubrication by a force feed lubricator.

Friction It is a rubbing force set up betw een surfaces in contact with each other due to relative motion between them. It depends on the normal load on the rubbing surfaces, the surface finish and the rate o f relative displacement. It causes wear and loss o f power because s.ome o f the power is used as w ork to overcom e the frictional force. Work done by frictional forces gets converted to heat energy, resulting in overheating o f the parts, which may lead to fusing or seizure in extreme conditions. Lubrication reduces this friction and wear. It also provides cooling and removal o f any impurities o r products o f wear.

Friction Types D ry F riction It is caused when solid surfaces move relative to each other without any lubricant between them. It is totally undesirable and leads to serious breakdowns. Boundary Lubrication F ailure Friction It is the friction caused when the lube oil film separating the surfaces in contact is destroyed and dry friction patches appear. Examples a re : (1) The lubrication between the piston compression rings and the liner. (2) The lubrication in the small end bearing o f the connecting rod at the start and stopping o f the engine. (3) The lubrication in bearings running at a very low rotational speed or a high unit load. C om plete L ubrication Friction This is the type o f friction caused when the moving surfaces in contact are separated by an adequate thickness o f lubricant.

Types of Lubrication 1. H ydrodynam ic Lubrication It is also called full fluid film lubrication. It is the lubrication between moving surfaces which are separated by a continuous unbroken oil film o f adequate thickness. Oil pressure is self generated due to the motion o f the moving surfaces. Exam ple: A journal bearing with perfect lubrication due to the oil wedge formed by the rotating shaft. . 2. H ydrostatic L ubrication It is similar to hydrodynamic lubrication except that the oil pressure is supplied by an external source. It is seen in slow-moving heavily loaded components, w here sufficient oil pressure cannot be

150

151

Marine Diesel Engines

Lubrication System

generated due to its relative motion and hence, external oil pressure from a pum p is required. 3. B oundary L ubrication It is a thin film lubrication which exists between the robbing surfaces so that full fluid film is not achieved and some degree o f dry patches occur with metal to metal contact. It is usually seen in cases o f very high relative movement between the rubbing surfaces. 4. Elasto-hydrodynam ic L ubrication It is also called “squeeze film lubrication”. It is the effect o f elastic deform ation o f the m etals and the effect o f high pressure on the lubricant Examples: Rolling contact bearings or meshing gear teeth. Here, contact is a nominal point or line contact. Lubrication depends on : Oil quantity, quality, viscosity, oiliness, dynamic coefficient o f friction, speed o f motion, load, surface finish and uninterrupted oil supply.

Lube Oil Properties Viscosity It determines the resistance o f oil internal cohesive forces and promotes setting up o f certain conditions for the friction between the moving surfaces. Lower or higher viscosity oils are both unacceptable. Viscosity depends on the temperature. C oking Capacity o r Carbon R esidue It is the tendency to form carbon residues while burning at elevated temperatures. High carbon residue causes gumming o f piston rings preventing their movement in the grooves.

152

Sedim ents These are grit particles formed due to wear and carbon. Their maximum allow able content is 1.5%. They cause clogged oil filters and purification problems. Corrosiveness » It is the tendency o f the oil to oxidize due to the presence o f oxygen in high temperature gaseous surroundings. The organic acidic products are very hazardous on lead bearing metals. Base N um ber It is the most important property o f lube oil for cylinder lubrication in an engine. It is the capacity o f the oil to neutralize the sulphuric compounds which are formed, especially in modem engines burning sulphur rich residual fuel. N eutralisation Value It is the measurement o f the acidity or alkalinity o f the oil. Total A c id N um ber (TAN) It is the measure o f the combined organic acids due to oxidation o f the oil, and the inorganic acids due to contamination by the acidic products o f combustion. Strong A cid N u m b er (SAN) It is the m easure o f the inorganic acids w hich are form ed due to contamination by the acidic products. Total B ase N um ber (TB N ) It is the measure o f the alkalinity o f alkaline oils. Example: TBN = 70 mg KOH/g for crosshead engines T B N = 3 0 mg KOH/g for trunk engines.

153

Lubrication System

Marine Diesel Engines

The difference is because trunk engines use the same oil for cylinder liner and crank case lubrication.

improve this property o f dispersing these harmful deposits. Additives are metallic based sulphonates o r phenates.

F lash P oint It is a m easure o f the tendency o f the oil vapours to ignite. It is an important consideration especially in case o f the crankcase oil getting contaminated with fuel leaks.

D e-E m ulsivity It is the property o f the oil to separate from water in a non-miscible emulsion. Exam ple: Water ingress into the lube oil requires the water to be separate (not miscible), so that the water can be removed.

P o u r P oint It is considered when the operation o f the engine component is at low temperatures. It m ay have to be preheated, if the oil is to be handled at temperatures exceeding the pour point by 15 deg.C o r less.

F oam ing It is the undesirable phenomenon of the oil mixing with air resulting in cavitation and overheating.

D ynam ic C o efficient o f F riction It is the ratio o f the tangential force to the norm al force required to overcome friction. O iliness It is the tendency o f the oil to adhere o r w et the m oving surfaces. A n ti- O xidation It is the tendency to resist oxidation. Additives are used to improve this property. Exam ples: Amines or organo-metallic additives. C racking Stability It is the property o f the oil to be stable and resist cracking a t high temperatures. Cracking is the breakdown o f molecules into smaller sizes at high temperatures.

L ube oil additives These are substances added to the mineral based lube oil to enhance and improve specifically required properties. Examples are: Anti foam agents, pour point depressants, extrem e pressure agents, viscosity index enhancers, anti-wear agents, dispersants, detergents, antioxidants and rust inhibitors. C loud P oint It is the temperature at w hich a cloud forms, due to w ax crystal formation at low temperatures. Example: Paraffin-base oils. Water Content Water reduces the viscosity and therefore, reduces the load carrying capacity o f the oil. Sea w ater ingress containing high salt content increases the acidity and leads to corrosion o f metal. W ater reacts with the additives blended in the oil and nullifies their effect.

D etergency a n d Dispersancy It is the tendency to colloidally suspend, disperse and wash away any harmful combustion products in the oil. Harmful deposits build up in the piston ring p ack area. Additives a re usually added to the oil to

L ube O il D eterioration It is due to a reduction in viscosity, TBN, flash point or dispersancy; and an increase in oxidation, water content or sediments.

154

155

Marine Diesel Engines

Lubrication System

Lube Oil Testing O n board testing as w ell as shore testing is carried out regularly to m onitor lube oil condition, deterioration and w hether oil is to be rejected. Crank case oil is changed after 10,000 running hours in low speed engines, and 5,000 to 10,000 running hours in medium speed engines. Oil samples are taken every 500 running hours in low speed engines and every 150 running hours in m edium speed engines. A detailed sample-taking and testing procedure is outlined. Sample points are usually before or after the filter or the pump. These points are marked and are to be the same for all samples in order to maintain a standard. A testing file or record book is maintained to monitor and compare results. Excessive lube oil consumption is also monitored and the cause is to b e ascertained in every case. Company specified standard testing kits are available on board fo r testing purposes. The aim o f testing is to m onitor deterioration o f oil, am ount of contamination, oil consumption, replenishment, condition/wear of lubricated machinery, further use o f oil o r oil rejection. I f the tests show satisfactory results, the oil can be used further and it need not be replaced as per running hours. Hence, a saving in costs is achieved. Good lube oil monitoring helps m aintain the m achinery in good condition, gives a warning in case o f deterioration, and lengthens time between overhauls and surveys.

Onboard Lube Oil Tests

10 m l oil sample and 10 m l R eagent N are m ixed and placed in a testing unit cup. 10 ml reactive reagent T is added and the testing unit cup sealed and properly mixed. The resultant pressure rise in compared with a chart according to the type o f oil used. Water C ontent Test The water content is ascertained by measuring the ‘resultant pressure rise’ o f a test mixture. 5 ml oil sample and 15 ml petroleum reagent A (a paraffin o r toluene) are mixed in the test unit cup. A standard amount in a sealed satchet o f reagent B (calcium carbide) is added and the mixture sealed and shaken thoroughly. The chemical reaction takes place between water in the oil and the reagent calcium carbide to form acetylene gas which gives a resultant pressure rise. Water Crackle Test It is done by heating 10 drops o f oil in an aluminium foil container over a flame. A crackling sound confirms the presence o f water in lube oil. Viscosity Test Viscosity is usually measured using a flowstick comparator method. The relative flow rate is measured between a new oil and the used oil. 3 ml new oil and 3 m l used oil at the same temperature are placed in the flowstick reservoir respectively. The flowstick is tilted allowing both the oils to flow through separate channels. When the new oil has reached the reference mark, the position o f the used oil is checked. Markings on the flowstick give the conditions o f the oil.

T B N Test The TB N valve is ascertained by measuring the ‘resultant pressure rise’ o f a test mixture. The chemical reaction is that o f the alkaline lube oil additive (calcium) with the reagent T.

A lka lin ity Test A ‘pH ’ paper indicator can be used to check the reserve alkalinity in the oil sample.

156

157

Marine Diesel Engines

Lubrication System

Fla sh Point This can be done if a Pensky-Martens apparatus is available on board. The flash point will change if there is a fuel oil leak into the lube oil.

Spectro-Analysis This test determines the contamination by metal and additives. The following metals can be found by this test:

Sp o t Test It shows the am ount o f insoluble particles in the oil. A standard oil sample is taken and mixed thoroughly. A spot o f oil is dropped on a special test ‘blotter’ paper and allowed to dry. A fter a few hours, the spot is com pared with the standard spot reference. Sea water content It tests the chlorine content o f the oil sample. 5 m l oil sample and 5 ml distilled w ater are m ixed and the w ater separates. 3 to 5 drops of mercuric thiocyanate and an iron salt are added to 1 m l o f the water from the earlier mixture. Chlorine ions react to form a reddish orange mixture o f chlorom ercurate and ferric thiocyanate. T his colour is compared to a scale chart calibrated from 0 to 300 ppm.

Shore Testing Standard samples are sent ashore for testing at regular intervals e.g. every three months. T he sample point should be marked and taken at the same point every time. The sample is to be taken when engine is running at norm al speed, so that oil is circulated. It is taken at the closest supply point into the engine. B efore collecting the sample, drain the line. T he sam ple is taken at a very slow rate i.e. decanted over 5 minutes. T he sample container label should have the following details: ship’s nam e, date, oil purpose and equipment, running hours oil type and sample point location. Samples are not to be taken from purifier lines, sumps or drain cocks. Shore testing involves the following tests:

158

Ti n. Lead, Copper, Aluminium - from bushes or bearings. Vanadium - from heavy fuel oil contamination. Sodium - from sea water salt ingress, HFO contamination. Chrome - from piston rings. Iron - from lubricated moving parts of the engine like piston crown, liner, camshaft, etc. M ethod Spectro-Analysis is done by Plasma Atomic Emission procedure for particles o f 10 micron (or less) in size. The quantity o f these particles can be determined by a particle quantifier which gauges the quantity in terms o f ‘PR index’. Separation o f the particles is done by a rotary particle depositor. Flash P oint Test It is done by using the Pensky M artens standard apparatus. The test sample is slowly heated in a closed apparatus at a constant rate and an external flame is introduced at different temperature intervals through an open shutter. For new lube oils, flash point should b e at least 220 deg.C. Base N um ber Oil sample + (Anhydrous chloro benzene + Glacial acid) is titrated w ith ( perchloric acid + glacial acid). Accurate titration is done by using an electrical potential bridge arrangement which gives a current reading proportional to the titrating rate. 159

Lubrication System Marine Diesel Engines

Kinematic Viscosity It is done by measuring the time required for a specific quantity o f oil at a certain temperature to flow under a fixed gravitational head in a capillary. This tim e m easurem ent is directly proportional to the kinematic viscosity. D ensity It is measured by m eans o f a glass hydrom eter with its temperature controlled. It is an im portant param eter when choosing the correct size gravity disc in a centrifuge. Insoluble C ontent It is a m easurement o f the Pentane or Toluene insolubles. ♦ For Pentane insolubles: A mixture o f the oil sample and pentane is centrifuged. It is decanted and the precipitate washed with pentane twice. The dried weight gives the pentane insolubles i.e. insolubles due to wear, carbon or dirt particles. ♦ For Toluene insolubles: Amixture o f the oil sample and pentane is centrifuged. It is decanted and the precipitate w ashed o ff with pentane twice. It is then washed once with a toluene alcohol solution, and again with toluene. The dried weight gives the toluene insolubles i.e. dirt and inorganic particles. Water C ontent It can be m easured by the distillation method. O il is heated under reflux w ith a water-immiscible solvent. The condensed water is separated from the solvent in a trap. M icro B iolo g ica l Test This test is only carried out if the lube oil is suspected o f microbial degradation. Anutritive gel is applied over a glass slide and immersed

160

in the oil sample. It is allowed to incubate for 12 hours. Bacteria manifests itself by red spots on the slide which is then compared with a reference guide.

Microbial Degradation of Lube Oil It is the degradation that takes place due to microorganisms thriving in the lube oil. M icro-organisms are bacteria, yeasts o r moulds. They require phosphorous, nitrogen, carbon and water. They require water to grow in the beginning, but later they can self-sustain themselves at 20 to 40 deg.C in stagnant conditions. The danger is that they multiply at a very rapid rate i.e. double in size and divide into two every half hour. Once the aerobic bacteria have consumed the dissolved oxygen, the sulphate reducing bacteria is activated. I b is bacteria attacks the metal and forms hydrogen sulphide. It results in corrosion o f steel. The properties o f the lube o il and its additives are also affected, enhancing corrosion and reducing the load bearing capacity. Acids are formed which cause corrosion especially at oxygen depleted zones. This microbial degradation is mostly seen in distillate fuels and not residual fuels. Indications Rotten egg smells, sliminess o f the oil in the crankcase painted surfaces, increased acidity and water content, filter choking more frequently, poor heat exchanger performance, black staining o f white metal bearings and corrosion o f exposed steel surfaces. Prevention Crankcase water content to be weekly monitored and within limits. Lube oil bearing surfaces, exposed steelwork and crankcase painted surfaces is to be visually inspected during every crankcase inspection. Regular circulation o f oil is to b e carried o ut by pum ps to avoid stagnant conditions. Lube oil temperature at the purifier is to be at

161

Lubrication System

Marine Diesel Engines

least 75 deg.C as the bacteria perish above 70 deg.C. Purification and re-circulation o f crankcase oil is to be continued even when the engine is stopped at port. Regular testing at various sample points is to be done. Inspection o f sludge from purifiers o r choked filters also indicates any degradation o f lube oil. Treatment U se o f biocides or fungicides is carried out. Heating and continuous purification above 75 deg.C is done and the entire sump to be purified within a period o f 12 hours. H eating is done to a temperature of 80 deg.C, but not exceeding the supplier’s lim it. This kills the bacteria. M anual cleaning o f the sump, filters and pipelines is carried out. Replenishm ent o f the sum p oil is done in case the lube oil is badly -infected.

Cylinder Lubrication R equirem ents ♦ to provide a lube oil film at the liner and the piston ring surface ♦ to separate the surfaces and reduce friction between them ♦ to neutralize the combustion and acidic products especially due to sulphur content in the fuel providing corrosion protection. ♦ to disperse the carbon particles w hich tend to accum ulate at the piston rings. ♦ to help in the sealing o f the piston ring to the liner surface. ♦ to bum without leaving hard deposits. ♦ to cater to the problem s associated w ith cheap residual fuel and running-in requirements ♦ to provide the correct feed rate i.e. quantity per feed ♦ to lubricate and neutralize the combustion products under different load conditions ♦ to inject the lube oil at the correct timing for optimum use o f cylinder lube oil.

162

Cylinder Oil Types Crosshead Engines Cylinder oil has aTBN value o f 70 mg KOH/g and a SAE 50 viscosity. Crankcase oil has a TBN value o f between 5 and 30, and a SAE 30 viscosity. Trunk-Type Engines Cylinder oil has a TBN value o f 30 mg KOH/g and a SA E 30,40,or 50 viscosity. The difference in the oil is because trunk-type engines use the same oil for the crankcase as well as cylinder lubrication, while crosshead type engines use separate oils. C rosshead engines use higher TB N oil because only a limited small consumable quantity is injected into the cylinder. In Trunk engines, a great amount o f oil is present. Hence, TBN level required is lower. T B N versus S u lp h u r Selection Selection o f T BN is done with respect to sulphur content to ensure low wear rates o f cylinder liner. Sulphur Content in Fuel Less than 0.25 % 0.25 to 1.0% 1.0 to 3 .0 % Above 3.5 %

TBN Value 10 m g KOH/g 10 to 20 m g KOH/g 70 m g KOH/g M ore than 70 m g KOH/g

O ptim um C ylinder L ube O il Injection The best timing for lube oil injection into the cylinder liner is between the top two piston rings, when the piston is on its upward stroke. The correct feed rate would be judged during overhauls o f the engine, if the piston rings are slightly damp and rings move freely in the grooves without much accumulation o f deposits. Another indication is the liner

163

Marine Diesel Engirt*

Lubrication System

w ear rates w hich should be less than 0.1 mm /1000 running hours. 1 T he oil feed quantity depends on the type and specifications o f the 1 lube oil, the quality and sulphur content o f the fuel, and the engine 1 loading conditions. O il feed rates range from 0.3 to 0.8 gm/bhp/hr.

Cylinder Lubrication Systems T he tw o im portant systems used in m odem engines a re : 1. Accumulation and Quill S y stem -S ulzer engines 2. Cylinder lubricator units pumping to orifices in the liner -M A N B & W engines. A c c u m u la to r a n d Q u ill System This system is used on Sulzer Engines. It consists o f a lubricator pump ' supplying oil pressure to a quill fitted with an accumulator.

1 Accumulator cylinder 2 Spring , 3 Accumulator piston 4 Cap nut 5 Diaphragm 6 Accumulator casing 7 Cap nut H Backing screw 9 Copper sealing rings 10 Cylinder liner II Lubricating quill

12 13 14 15 16 17 IS 19 20 21 22

Passage for lubricating quill Filling pin Steel ball Non-return valve housing Flange ring Screw Support ring Flange Joint Protecting bush O-ring

In this system , the accumulator gets charged by the lubricator pump for every 10 to 15 revolutions. This oil under pressure is stored in the accum ulator and enters the cylinder whenever the cylinder pressure falls below the accumulator oil pressure. The cylinder pressure is less than the accumulator twice for every revolution, (a) when the piston is moving down in its expansion stroke, and (b) when the piston is moving up, as th e piston rings pass the feed grooves.

^ - K _____ ;.^ .| X -p r r ^ EXPN.

--------------

If B0C CRANK ANGLE — F ig -106

In the figure, the shaded portion shows lubrication while the cylinder pressure falls below the accumulator oil pressure ( A - A ), with respect to the crank angle. 164

165

Lubrication System

Marine Diesel Engines

Q uills Quills are non-retura valves fitted at the liner oil grooves by screwing into the liner. They help to dam pen the pressure pulsations in the supply line; prevent cylinder combustion gases or products entering back into the oil line; and provide storage o f pressurized oil in the accumulator section. Direct contact with the quill and cooling water is prevented by a sealing pipe which allows easy removal o f the quill. Lubricator P u m p Unit This lubrication pumping unit gets a rotary drive from the driving shaft by means o f a gear and ratchet mechanism. This rotational drive is converted into reciprocating motion o f the lubricator plunger. Checking the pumping action can be done through the sight glass which shows a steel ball lifted and pushed up when the oil is pumped. Acylindrical oil non-flow alarm is also fitted. T he oil feed ratio can be adjusted for different load conditions. In modem engines, the lubricator pump drive is by a frequency controlled electric m otor which varies with the load changes i.e. i t is load-dependent. S om e m odem units have a pre­ lubrication, post-lubrication and emergency lubrication option by a switch in the control room , which starts an electric m otor for the lubricator drive. This is during slow turning o f the engine for one complete revolution. Manual cranking o f the lubricator is also possible. Lubricator Units

engine speed, load index and LCD signals. It sends an ‘on’ signal for lubrication to the solenoid valve to control the oil injection. The computer sends an ‘off’ signal to the solenoid valve to allow the oil back to the return line. The feed rate is adjustable by adjusting the interval between injection i.e. every 5* and 6threvolution. More details on this system is given in the chapter on Engine Descriptions. Advantages Lower lube oil consumption, lower wear rates o f the liner, increased time between overhauls; and better timing and utilization o f the expensive cylinder lube oil is possible. In case o f failure o f the solenoid valve o r transducer, the other lubricator automatically changes to maximum setting. If the air pressure fails, the standby pum p will automatically start. The computer unit too has a backup computer to ensure lubrication is continued.

Load Dependent Cylinder Lubrication Modem engines employ load dependent cylinder lubrication where the am ount o f cylinder lube oil to each lubricating point can b e individually adjusted and controlled as per the load changes, via the remote control system. The specific oil feed rate increases with the decreasing engine load. For example, at 20% engine load, the specific cylinder oil amount will

system used in M AN B&W engines. Here, a high pressure lubricator pump supplies oil to an injector to inject a fixed volume into the engine cylinder once in 4 revolutions. Acomputer control unit gets input from

also be 25% more than at 100% engine load. The desired increase in the specific liibe oil quantity can be programmed in the control unit. Whenever there is a sudden load increase or a load fluctuation o f the engine, correspondingly the cylinderlube oil flow rate will be increased automatically. The input signal for the oil increase is initiated from the

166

167

One o f the latest types o f lubrication systems is the Alpha lubrication

Marine Diesel Engines

Lubrication System

Specific Cylinder Lube Oil Consumption According to power, Specific cylinder lube oil consumption in g/kw-hr o r g/bhp-hr. - Cylinder lube oil consumption in kg/hr x 1000 Effective engine pow er in kw or bhp According to fuel consumption, Specific cylinder lube oil consumption in g/kw-hr or g/bhp-hr «=K _ x Assumed S.F.O.C. for the engine in g/kw -hr or g/bhp-hr

1000 w here K in kg/t =

Cvl. lube oil consumption in kg per 24 hrs Fuel oil consumption in tons per 24 hrs

F requency Controlled Electric M otor L ubricator M ost m odern engines use this ty p e o f lu b ric a to r driv e fo r load-dependent cylinder lubrication. load indicator transmitter. This input signal from the load indicator transmitter is sent to the rem ote control unit, w hich sends an output signal to change the speed o f the frequency-controlled electric motor drive to the lubricator. Below 20 % load, the oil feed rate is not reduced anym ore i.e. the speed o f the electric m otor rem ains constant. In ‘emergency lubrication’ mode i.e. when the normal cylinder lubrication control fails, the cylinder lubrication can be adjusted m anually by adjusting the knob on the lubricator. In this m ode, the regulation of lube oil quantity is no more load-dependent, b u t independent o f the engine load. The remote control signals the electric m otor to run with its nominal frequency. 168

169

Marine Diesel Engir,

Lubrication System

Multi Level Cylinder Lubrication

( 'rosshead Lubrication

In this type, cylinder lube oil is injected into the liner through quills at different levels (usually 2 levels).

K W p t-|

The position o f quills can be one o f the following: 1. A t 10% stroke from TD C : In this case, although the cylinder lube oil feed rate is m ore, there is poor circum ferential spreading due to oil flow breaking down at high temperatures.

|

2. A t 20% stroke from TD C : In this case, lu b rication is m ost effective especially for a single level o f quills. 3. Combination o f a ‘no groove’ row o f quills at 20 % stroke from TDC, and a ‘continuous groove’ row o f quills at 30 % stroke from TDC. 4. A bove the exhaust ports, in case o f loop scavenging engines. F ig - 109

Usually, quills are 250 m m apart from each other around the liner bore. Grooves are angled downwards. T he combustion gas pressure differential across the rings assist in pushing the oil downwards in the groove. T he disadvantage o f grooves is that they increase the area into w hich oil flows. H ence the velocity and pressure o f the oil decreases, thereby reducing its spreadability.

170

1 2 3 4 5 6

Piston rod stuffing gland box Crosshead bearing Crosshead guides Crosshead pin Lube oil articulated arm Lube oil inlet

E C ro ssh e a d L u b r ic a tio n \ Difficulties The requirements for effective lubrication are pumping action, tunple o f oil feed supply and an oil film creation strong enough to separate m etal surfaces. Pumping action o f acomponent to p ro d u ce o il p ressu re is difficult in the crosshead, since th e c ro s s h e a d m o tio n is oscillatory with a high sliding velocity. The speed o f rubbing is not sufficient to supply ample o il fe e d , n o r to p ro m o te p u m p in g a c tio n . U n lik e F ig - 110 4-stroke engines, there is no load relief in 2-stroke engines which would allow oil feed to be supplied and the bearing lubricated. Rupture o f the thin oil film w hich separates the rubbing surfaces is caused by cyclic unidirectional loads during firing, in large super charged 2-stroke engines.

171

Marine Diesel Engines

Crosshead L ub rica tio n M ethods M ethod (1) A s shown in the figure, o il is supplied at a m uch higher pressure (16 bar in RTA engines). H ere, the generation o f high oil pressure is done by hydrodynam ic m eans . A s the oil under pressure is now confined to the small clearance area, its elasticity comes into play which assists in maintaining the oil film for the momentarily instantaneous loading. This is called Elasto-Hydrodynamic lubrication. Oil supply is the sam e as bearing lubrication oil, whose main pressure is now boosted to 16 b a r and supplied via the lube oil articulated arm. A s shown in the figure, th ere is a second lube oil supply inlet for oil supply to the crosshead system in case o f crosshead pum p failure. M ethod (2) Providing a hydrostatic oil lift of the crosshead pin through hydraulic pil pumps.

172

CHAPTER 7

COOLING SYSTEMS Function of the cooling system The function o f the cooling system of a marine diesel engine is to cool down the engine components, the lubricating oil and the scavenging air to a point where optimum operating conditions are achieved. Cooling is required for the piston, cylinder head, cylinder liner, exhaust valves, turbochargers, injectors, etc. According to the heat balance chart, only a fraction o f the heat liberated by the engine is converted into useful work, the rest being wasted within the exhaust into the atm osphere o r absorbed by the engine components in contact w ith the hot combustion gases. T he loss o f heat energy to the cooling w ater is 20% at the cylinder head, 10% through the piston and 5 to 8% through the exhaust manifold and turbocharger. Trouble-free functioning is essential for the cooling system, not only during running o f the engine, b ut also during warm ing up before starting and manoeuvring conditions. Lack o f cooling causes non uniform heating o f the components inducing thermal stresses. An overheated piston or liner causes evaporation and burning o f cylinder lube oil and deposition o f lacquer and carbon. This deprives the piston rings o f elasticity and causes failures due to sticking o f rings. Under-cooling

173

Cooling System Marine Diesel Engines

reduces the cylinder clearance causing distortion and scuffing o f the piston and liner. Thermal stresses trigger off cracks in the piston crown which lead to combustion blow-by. Pistons were earlier water-cooled using telescopic pipes, b ut m odem engines use oil as the cooling medium. B ore Cooled L iners Bore-cooled liners provide intensive cooling at the working surface and also retain the strength o f the liner. Bores are drilled at a tangential angle or cooling pipes are inserted during the casting process. Insulated tubes are used in the bore holes to manipulate the desired control o f the cooling required at various sections. T he liner temperature should b e within 150 to 220 deg.C. Over-cooling o r under-cooling causes problems and is undesirable. Piston ring region temperature is limited to 220 deg.C, otherwise ring lubrication is adversely affected. This is achieved by bore cooling as well as keeping a high top land where the position o f die top piston ring is much below the hot crown top surface.

Load Dependent Liner Cooling In this system, the liner cooling rate is varied with respect to the load on the engine. In order to achieve less cooling, som e o f the cooling water flow is by-passed away from the liner to maintain the liner wall temperatures w hen load decreases. Maintaining the liner temperature above the dew point has the advantage o f preventing cold sulphur corrosion. T he m ass flow rate o f cooling w ater is reduced when the load decreases. Latest developments in liner material and lubrication allows a m ajority o f the liner portions to go w ithout cooling. The minimum cooling required is achieved from the scavenge air entering the lower section o f the liner. T he maximum admissible temperature fluctuations for cooling water outlet temperature is + /-2 d e g .C for constant load, and + / - 4 deg.C during load changes. T his avoids

undue tension in the combustion chamber parts especially in the liner and cylinder head region.

Piston Oil Cooling System Oil is preferred in modem engines for cooling o f the piston due to the absence o f water corrosion, or scaling, o r w ater leaks into the crankcase; simpler designs o f glands; and the absence o f telescopic pipes. The sam e oil and pressure can be used from the main lube oil system, thereby avoiding the necessity o f separate piston cooling pumps. Oil has the drawbacks o f coking at high temperatures; a reduced specific heat capacity compared to water; and a larger lube oil system size required in order to allow air release.

Cooling Water Treatment The cooling water used for engine cooling should be properly treated with an approved cooling water inhibitor and alkaline agents to avoid corrosion attack, sludge formation and scale deposits. The following treatments are u se d : 1. Sodium Nitrite or Sodium Borate They are safe for handling, non-toxic, not dangerous if over-dosed and contain a pH buffer to provide protection against acidic corrosion. They form a thin passive oxide surface layer on the metal. Sodium Borate is used when the material to be protected involves zinc or soft solder material. 2. Chromates It is not preferred since it is highly toxic and unsafe during handling and disposal. It is an anodic inhibitor, so pitting is caused if its concentration is low. It is not to be used if the engine jacket water is used for evaporator heating. 175

174

Marine Diesel Engines

3.

Soluble Emulsion Oil It is not preferred due to foaming problems, bacterial contamination, disposal problems, and no control over the film thickness. It forms a greasy film on the metal surface and prevents corrosion.

CHAPTER 8

STARTING, REVERSING AND MANOEUVRING Starting System M arine diesel engines are started and reversed w ith the aid of compressed air at a pressure o f around 30 kg/sq.cm. Pressurised starting air is supplied from air compressors and stored in two air bottle cylinders. Starting Torque The starting torque is achieved by the compressed air acting on the top o f the piston to push it down. This reciprocating motion o f the piston is converted into a torque at the crank shaft. T he am ount o f starting torque required is the amount o f torque needed to rotate the crankshaft at a speed that will produce the desired self ignition temperature to ignite the fuel in the cylinders. Starting is in three ste p s : ♦ Cranking the engine by compressed air to produce sufficient starting torque until some o f the cylinders fire. ♦ Picking up the combustion cycle on fuel w ithout the engine’s misfires.

177

Marine Diesel Engines Starting, Reversing and Manoeuvring

♦ Acceleration to a speed in accordance with the fuel injection pump setting. The tim e period w hich elapses before the engine is under its own pow er after being cranked by com pressed air is between 2 to 8 seconds. During this period, the engine running is irregular, combustion improper and exhaust is smoky. The irregular running is because some o f the cylinders misfire initially, while the engine speed increases in jerks as each cylinder fires one after the other. Start A ir Tim ing The start air timing position should consider that the engine is started in either direction. The best tim ing considering a reversible engine would be when the start air is admitted at TDC, to utilize the positive starting torque from the beginning o f the stroke. In practice, starting air is admitted slightly before TDC in order to take care o f the time lag for pilot valve activation, start air valve opening and full pressure availability to produce the desired starting torque. The start air should be admitted after the firing dead center to provide a positive torque in the correct direction at the start o f the working stroke. Id e a l F irin g S p eed It is the ideal speed o f rotation o f the crankshaft created by ‘compressed starting air’ cranking to compress the ‘combustion air’ in the cylinder to a temperature sufficient enough to self ignite the fuel when injected. Usually, the speed is achieved at 8 to 12% o f the MCR speed. F irin g In terval It is determined by dividing the number o f degrees in the engine cycle by the number o f cylinder units o f the engine. Example: Fo r a 3 cylinder 2 stroke engine, firing interval = 360 / 3 = 120 deg.

Start A ir Period It is the minimum cranking period plus an overlap period to provide sufficient starting torque to start the engine in any direction at any position. I t depends on the exhaust valve opening, as the start air should shut before the exhaust valve opens, or else the pressurized compressed starting air is wasted as it willjust be blown out o f the unit through the exhaust valve. In 2-stroke pulse turbocharged engines, the exhaust valve normally opens at 65 degrees before BDC or 115 degrees after TDC. This gives a maximum starting air angle o f 115 degrees.

Overlap Overlap is a period when two (or more) cylinder units are receiving starting air, where one unit is ‘phasing out’ while the other is ‘phasing into’ the start air period. It is essential to satisfy the requirement that the engine be started in any crank position. Overlap is reduced in case there are m ore number o f cylinder unite, but necessary for engines with less units to assist the starting torque for cranking. Overlap ensures that at every crank angle position, there is sufficient air turning moment to enable positive starting. It depends on the start air period, exhaust timings and the number o f cylinders. M inim um N u m b er o f Units fo r Overlap 1.

3 cylinder engine (2 stroke): Firing interval = 360 deg = 3 unite

120 degrees

Since maximum start air period is 115 degrees, no overlap'is possible. For overlap to occur in this case, the start angle should be greater than 120 degrees which is not possible.

178 179

Starting, Reversing and Manoeuvring

Marine Diesel Engines

2.

4 cylinder engine (2 stroke): Firing interval = 360 deg = 90 degrees 4 units If the start air period is 115 degrees, then Total Overlap Period

= Startairperiod-Firinginterval = 115 deg - (360 / 4) deg. = 115 deg - 90 deg = 25 deg.

T he firing sequence is 1 - 4 - 3 - 2.

Start Air System Components Start A ir C om pressors Two or more start air compressors are to be provided having sufficient capacity to pressurize both the start air bottles to the working pressure from the atmosphere pressure in one hour. Start A ir Receiver For reversible engines, tw o air bottles o f equal capacity are required, sufficient for 12 cold starts o f the engine (w ithout simultaneous replenishm ent by the start air compressors) in' alternate ahead and astern directions respectively. F or non-reversible engines, 6 starts are sufficient

Air Receiver Capacity = (Total air mass in receiver at m aximum pressure) - (Air mass in receiver at minimum start pressure) where, Total air mass in the receiver at maximum pressure = 12 starts x 2 x Total displacement volum e to give the required air mass per start. A ir Bottle Description The start air bottle is of welded steel type with the following components: ♦ A relief valve to limit accumulation o f pressure upto 10% with the compressor filling the bottle and the outlet valves closed. ♦ A fusible plug, in case the relief valve can be isolated. The fusible plug vents directly o ut o f the engine room to atmosphere via a separate piping, in case o f an excessively high engine room temperature (engine room fires). Usually, the melting pointofthe fusible plug is 150 deg.C. ♦ Outlet valves o f slow opening type to avoid sudden pressure surges in the start air lines. The main stop valve provided allows for manual isolation o f the entire start air system during overhaul. ♦ Manhole door for internal inspection. ♦ Drain valves to drain water from the air bottle receiver from the lowest point in the receiver without choking.

The capacity o f the air bottles are designed according to the swept volume o f the engine cylinders, the specified number o f cold starts (6 o r 12) and the a ir required per start. Usually, this air requirement is 10 to 12 litres per litre o f swept volume for cold engine starting and 5 to 8 litres for a warm ed up engine.

Start A ir Receiver Inspection Inspection is done when there is adequate tim e during w hich the air bottle will not be required. The air bottle is isolated and,all valves lashed and tagged with notices. The air botde is de-pressurized through the drain valve and then checked through another opening like the pressure gauge connection in case the drain line gets clogged. The m anhole door is opened and ventilation for the interior is provided.

180

181

Starting, Reversing and Manoeuvring

Marine Diesel Engines

Visual inspection is to be carried out for the interior coating and paint. A horoscope can be used where access is not possible. Inspection is carried o u t a t stress concentration areas like w elding seams, penetrations, drain holes, support points, sludge collection area, condensation areas, v alve connection openings, etc. The internal corrosion prevention coating layer is to b e inspected. In case o f deterioration, a coat o f Copal Varnish can be applied after properly preparing the internal surface to be coated. The fitting connections for draining valves are to b e cleaned. T he relief valve is to b e tested hydraulically to the stamped working pressure and checked for lifting in actual service after fitting back. In case o f serious deterioration e.g. severe corrosion o r pitting, the receiver can b e de-rated along with the compressor settings and relief valves to provide fo r a low er safer capacity. Start A ir Pilot Valve

1 To and from cylinder air start valve 2 Venting to atmosphere 3 From automatic valve to pilot valve 4 Spring to lift roller off the cam 5 Cam 6 Clearance between roller and cam.

It is operated by the start air lever or button in the control room. It’s function is to operate the opening and closing o f the automatic start valve and to operate the air distributor by loading up the distributor slide valve.

182

♦ In the figure, the pilot start air valve is shown shut since the spring lifts the roller off the cam. ♦ On starting the engine, the automatic valve sends air to the pilot valve which pushes the roller onto the cam. As the cam turns, the negative peak comes into play allowing air to pass through, to the automatic starting valve piston causing it to open. The shutting of the valve happens when the roller comes onto the idle surface o f the cam.

Automatic Master Air Start Valve Function ♦ To act as a stop valve which supplies o r shuts off main starting air into the main start air line at the engine cylinders only during the starting period. ♦ To act as a non-return valve preventing any blow back o f combustion gases in case an air start valve leaks back, and also preventing a flam e by u se o f a flam e trap incorporated in the assem bly. ♦ To shut off starting air supply automatically to the start air line ahead o f the stop valve, once the engine is on fuel or when the engine is shut down, thereby saving on air consumption and providing additional safety. Types They are classified into two types on the basis o f the operating principle: 1. Unbalanced type, w here the valve is opened due to relieving the pilot piston o f air pressure. 2. Balanced type, w here the valve is opened due to an air pressure applied to the pilot piston.

183

Marine Diesel Engines

1 2 3 4 5 6 7 A B

Starting, Reversing and Manoeuvring

Tapping for pilot valve Vent, when valve is shut Valve Spring pushing the pilot piston down Pilot piston Pilot air to open valve Valve body Start air inlet Start air outlet to start air line at cylinders.

A ir Start Valve An air start valve is fitted to each cylinder head o f the engine and is operated by the starting air distributor control valve. It is operated by the start air lever o r button in the control room. It’s function is to operate the opening and closing o f the automatic start valve and to operate the air distributor by loading up the distributor slide valve. 1 2 3 4 5 6 7 8 9 10 11 A B M

Nut Cover Intermediate ring Casing Casing o-ring Cylinder head Self-locking nut Pilot piston Valve spindle Allen screw Spring Piston rings Control valve Air gap

T h e f ig u r e sh o w s a balanced type automatic m a ste r sta rt a ir valve, which is more reliable than the unbalanced type. It consists o f the valve closed by the downward force o f the spring pressure along w ith a ir p re s s u re ‘A’. W hen the starting lever is s h ifte d to ‘S T A R T ’ position, the pilot air valve o p e n s a n d s e n d s a ir pressure to the space ‘a’. T h e upw ard force d ue to this air pressure on die pilot piston is greater than the Fig-112 downward force and the valve opens. A s soon as the engine is on fuel o r shut down, the pilot valve closes, stopping air pressure supply to the pilot piston o f the automatic valve, thereby shutting it. A ir is then vented via vent pipe connection ‘2’.

The start a ir v alve is shut compression spring force acting on the pilo t piston. I f th e cylinder pressure is higher than the starting air pressure, the valve cannot open. Hence, blow back o f combustion gas into the starting air manifold is avoided. The start air valve is opened pneumatically by air supplied from the respective start air control valve. This air pressure acts on the pilot piston causing it to overcome the spring force and open the valve.

184

185

Starting, Reversing and Manoeuvring

Marine Diesel Engines

Start A ir D istributor Function ♦ To admit pilot air to operate the cylinder air start valves with proper timing and sequence for starting in ahead and astern directions. ♦ To vent the low er cham ber o f the cylinder start air valves, which are not being supplied with starting air.

♦ During reversing, the distributor cam is also turned by the same angle. ♦ During running, the distributor piston valve is kept off the cam with the help o f a return spring, with start air supply being shut off. On pushing the starting lever, air is supplied to the distributor which pushes all the respective control valves onto the cam. ♦ The distributor sends pilot air in a proper sequence to each cylinder air start valve until the minimum cranking rpm is reached, after which start air admission is stopped and fuel is injected to self-ignite. Start A ir C am The start air cam is usually o f inverse type as it has the following advantages: ♦ Wear is reduced on the cam working edge because the roller is off the cam during normal running, as there is a definite clearance between them, when the engine is running. This ensures that the air distributor functions correctly inspite o f the spring failure. ♦ It allows more flexibility to position the control valve of the distributor so that it does not touch the cam when the engine is running.

♦ The distributor is driven by a cam connected to the fuel camshaft, which provides the correct sequence o f starting. Starting control valves are radially fitted around the distributor cam. 186

Starting Interlocks ♦ Thesearemechanicallinkagesordevices which willnotallowfurther operation until they receive an input signal that the predetermined conditions are fulfilled. ♦ The following interlocks are placed in the starting system : . (1) Turning gear is disengaged (2) Complete reversing is achieved (3) Correct running direction is done (4) Lube oil pressure is sufficient (5) Spring air pressure is sufficient (6) Auxiliary blower is o n ‘auto’. 187

Marine Diesel Engines

Starling, Reversing and Manoeuvring

Slow T urning ♦ Its function is to avoid fluid lock in case o f fluid accumulation in the combustion chamber, during engine stand stills for long periods (sim ilarto ‘blow through’). ♦ This is a ‘m ode’ o f the engine control system w here the engine is turned slowly fo r one complete revolution at a slow speed o f 5 to 8 rp m .

For 2-stroke the firing interval is 360 / Z, and for 4-stroke it is 720 /Z , where Z is the number o f cylinders.

♦ During m anouevring, while the engine is on B ridge Control, the ‘slow turning’ m ode automatically starts, if there is no telegraph movement for 30 minutes. ♦ In order to achieve slow turning, the flow o f start air to the engine is limited. Scavenge A ir L im iter ♦ It is a means o f governor control o f the fuel released depending on the availability o f scavenge air in the desired ratio required for good combustion. ♦ It is im portant w hile increasing the engine speed so that a proportional amount o f fuel is released as the scavenge air pressure increases. ♦ The scavenge air limiter can be over-ridden, in case o f failed start attempts so as to provide a better chance for starting with more fuel available. This is done by sending a false scavenge air pressure signal to the governor from the control air line. F irin g Order o f Cylinders ♦ The purpose o f a firing order is to relieve the crankshaft journals between adjacent cylinders from excessive loads, unavoidable if these cylinder loads would fire in succession. ♦ It provides better and regular crankshaft rotation w hen firing in equal intervals.

188

R eversing Requirements Repositioning o f the following cams are required for the correct firing sequence according to the reverse direction: 1.

Fuel Cam

2. A ir Distributor Cam

189

3. Exhaust Cam.

Marine Diesel Engines

Starling, Reversing and Manoeuvring

F irin g Order Reversed T he firing order sequence in the reverse direction can be as follows:

RND Engine Reversing Fuel and Start air distributor cam s get repositioned by a common hydraulic servomotor, which turns the camshaft by 98 degrees in the opposite direction relative to the crankshaft.

6-Cylinder 2-stroke en g in e: Ahead Firing O rder 1-5-3-4-2-6 Astern Firing O rder 1-6-2-4-3-5 Fig-105

Reversing Methods (A)

Camshaft is rotated with respect to crankshaft Exam ple: R D & RNDEngines

(B)

Camshaft is stationary but cams are turned Example: RTAengines

(C) (D)

Camshaft is displaced in the axial direction Exam ple: 4-stroke engines Shift in the contact position o f the fuel pump roller Exam ple: SM C engines.

3

2

Fig-116

r~i Gear train 2 To interlock systems | 3 To/fromreversing control valve._______ ._______________________ 1

RD Engine Reversing 1.

Fuel and Start air distributor cams get repositioned by a common hydraulic servomotor which turns the camshaft by 98 degrees in the opposite direction relative to the crankshaft Here, the engine is stationary and the camshaft physically rotates by 98 degrees.

2. Exhaust rotary valve cams get repositioned by another hydraulic servomotor connected to the camshaft drive, which turns the rotary valve cam by 160 degrees in the opposite direction. In RD engines, since rotary exhaust valves are used, the tim ing is asymmetric about BDC and repositioning o f exhaust cams is required.

190

RTA Engine Reversing In these engines, the fuel, air and exhaust cams are fitted on the main camshaft. Hence the camshaft cannot be repositioned, as this will not provide the correct repositioning o f all three types o f cams i.e. fuel, air and exhaust cams. Hence, the solution is to reposition only the cams, whilst the camshaft is stationary. 1.

Fuel Cams are turned by 70 degrees in the opposite direction while the camshaft is stationary. The cam s are m ounted on a reversing servomotor, which is mounted on the main camshaft. One servomotor is used to reposition two fuel cams.

191

2.

Start A ir Distributor Cams are tu rn e d b y 98 d e g re e s in the opposite direction b y a separate servomotor, while the camshaft is stationary.

98* Fig-118

3.

2 1 Fuel Cam 3 Oil ‘in’ for ahead direction.

3

Exhaust cams are symmetrical about BDC (since exhaust valves are used and not exhaust rotary valves). Hence, no repositioning is required. Exhaust cams are on the same shaft as the fuel cams.

Fig-117

2 Oil drained ‘out’ for astern direction

MC Engine Reversing

The control air pressure is ‘nil’ during reversal as it is connected to the side o f the flap w here pressure to relieve is acting. This control air pressure can be used as a signal to cut off fu e l.

1. Air Distributor The engine drives a rotary disc (distributor) which can be turned by the reversing angle by means o f areversing pneumatic cylinder. 2. Fuel Cam The fuel pump roller (not the cam) is shifted by a pneumatic cylinder. Fuel cam is o f inverse type. Each fuel pump roller has an individual pneumatic cylinder. During reversing, the cylinder gets pressurized pneumatically and moves the pump roller position. After completion, the cylinder is depressurized and vented. T he rollers are o f self­ locking type in their end position. The shift o f all fuel pump rollers take place during the first revolution o f the engine while still on air. After shifting o f rollers is done, this end position o f the rollers is sensed by limit switches w hich gives an indication in the control room that reversal has taken place.

192

193

RTA Reversing S ervom otor f o r F uel Cam It is a mechanism to turn and reposition cams for the reversal sequence o f firing. As shown in the figure, each reversing servomotor has three pipe connections: a) for sending oil pressure ‘in’ for ahead direction. b)

for draining oil ‘out’ for astern direction.

c)

for control pressure, which gets pressurized only when the flap is in the end position.

Marine Diesel Engines

Starting, Reversing and Manoeuvring

Gain M otion It is the gain in motion caused due to the camshaft turning in the same direction as the required direction when the engine is being reversed. It is used in B & W engines. Governor Booster It serves the purpose to boost the hydraulic pressure required for the governor to push the fuel racks when starting.

Running Direction Interlock It is an interlock w hich prevents admission o f fuel to the engine, if the running direction o f the engine does not m atch with the telegraph lever. It is fitted at the forward end o f the fuel pumps.

1 Fork lever 2 Angle of rotation.

3.

Exhaust cam s are symmetrical about B D C and are on the same camshaft as the fuel cams. Repositioning is not required for exhaust cams.

Crash Manoeuvring Crash manoeuvring is the application o f brake air, whilst the engine is still turning in the opposite direction.

L o st M otion It is the loss in motion caused due to the camshaft turning opposite to the required direction when the engine is being reversed. It is used in Sulzer engines.

In B & W engines ♦ Acknowledge the bridge request for reversal o f direction. ♦ The start air cam gets reversed due to telegraph acknowledgment. However, the fuel is cu t off by the running direction interlock, since telegraph is opposite to the turning direction o f the engine.

194

195

Starling, Reversing and Manoeuvring

Marine Diesel Engines

Manoeuvring Flow Chart

♦ Now put the fuel lever at ‘O’ setting. ♦ W hen rpm reduces to 20% to 40% M CR rpm, put the fuel lever to minimum start setting. ♦ Astern rpm is m uch less than the ahead rpm as the engine is tremendously overloaded due to increased propeller slip. ♦ Start air becom es braking air because the start air cam reversal allows air supply inforastem timings, when theengine is still moving with ahead timings.

♦ ♦ ♦

Control is from bridge, engine control room, or local manoeuvring stand. Safety interlock and pressure conditions are met. Only in emergency conditions, safety devices can be overridden.

♦ ♦ ♦ ♦ ♦ ♦

Telegraph lever is put to ahead or astern. Reversing of cams takes place. Camshaft is in end position (either in ahead or astern). Running direction interlock senses that correct reversal is completed. Fuel lever is set to minimum setting. Start button pressed or starting lever put to ‘start position'._______

♦ ♦ ♦

Turning gear interlock check is done. Pilot valve opens automatic valve and distributor control valves. Automatic valve sends start air to cylinder start air valves.

♦ ♦ ♦

Engine turns on air to the minimum firing speed. Minimum fuel is injected and cylinders fire. Start air is shut off.

♦ ♦ ♦

Engine speed is gradually increased. Critical speed is overridden. Engine speed is brought upto MCR revolutions and parameters checked.

\ \ 1 I

Marine Diesel Engines

Manoeuvring

F ig - 121

Starting, Reversing and Manoeuvring

Reversing servomotor

I

Running direction interlock

3

Fuel ptimp

5

Automatic valve

6

Governor

7

Start- air distributor cam

8

Start air distributor

9

Fuel pump cam being turned by

10

Air cam being turned by the reversing servomotor

1i Turning gear interlock

12

Air start bottle

13 Control slide valve

14

Starting lever interlock block valve

15 Engine room telegraph lever

16

Starting lever

17 Fuel cut out servomotor

18

Pilot air valve

19 Oil pressure supply at 6 bar

20

Automatic oil and water low pressure cut out

21 Fuel speed setting lever

22

Load indicator

Cylinder start air valve

23 Reversing control valve

Starting, reversing and manoeuvring are explained with reference to a RND manoeuvring diagram. M edium s a r e : ♦ Start air at 30 bar pressure is supplied from the start air bottle when the main bottle isolating valve is opened. Start air reaches the automatic valve (in closed position) and the pilot valve through the turning gear interlock block valve. ♦ Lube oil a t 6 bar pressure.

198

199

Marine Diesel Engines

Starting Telegraph lever action to fr e e sta rt lever The Bridge gives a telegraph order w hich is acknowledged with the telegraph lever 15 in the engine control room. T he telegraph lever sets th e required running direction b y turning the reversing control valve 23 to either ahead V, stop U o r astern R positions via linkage J. Lube oil at 6 bar pressure 19 now passes through the reversing valve to th e cam shaft reversing servom otor o il passages 2 and turns the camshaft. Only w hen the cam shaft has reached its end position, the running direction interlock 1 w ill allow oil pressure to the starting lever blocking device 14 vialineA . T h isfreesu p the starting lever 16 for movement. Freeing up o f fu e l lever Simultaneously with the above operation, the lube oil pressure supply goes along line B to the slide valve 13 and then to the fuel cut out cylinder 17 to free up the fuel control linkage along line C, so as to take up the position as per the load indicator setting 22, w hich is set up by the fuel lever 21. This freeing up o f the fuel lever assumes that the safety cut out pressures are met. Safety c u t o ut device A safety cut out device 20 is set to ensure that the lube oil, jacket cooling and piston cooling w ater pressures are above the pre­ determined setting. ♦ In case any o f the pressures are n o t upto the values set, then the slide valve 13 moves down due to a decrease in pressure at line D. This causes the slide valve to vent the fuel cut out cylinder, thereby bringing the fuel rack back to zero through line C. ♦ In an emergency, the automatic cut-out devices can be overridden as in the case o f reduced pressures.

Starting, Reversing and Manoeuvring

Starting operation ♦ Start lever 16 is put to ‘start’ position. ♦ This leverage raises the pilot air valve 18 opening it. ♦ Pilot air now passes to open the automatic valve 5 through line E by venting its underside and also to the start air distributor 8 control valves along line F to force them onto the cam 7. ♦ The start air distributor cam 7has already been positioned for the firing sequence by the reversing servomotor turning the camshaft in either ahead or astern end positions 10. ♦ Pilot air passes through the air distributor and goes to open the cylinder start air valve 4 via line G i.e. to the top o f the cylinder start air valve piston to push it down. The underside o f the cylinder start air valve piston is vented via line H. ♦ Starting air from the automatic valve is admitted to the engine cylinders, after each cylinder start air v alve is opened b y the distributor in the correct sequence via line 1. ♦ The fuel lever 21 is already set to around 3.5 setting.The engine turns on air and then fires on fuel. ♦ Once the engine starts, the starting lever 16 is released to its normal position by a spring fitted. This action m akes the leverage to lower the pilot valve 18, thereby shutting it and shutting pilot air to the distributor 8 and the automatic valve 5. Start air is now shut and the air in the start air manifold line is relieved through small leakage points in the starting air valves. Reversing operation ♦ The telegraph lever 15 is brought back from ‘ahead’ to ‘stop’ position. ♦ The fuel lever 21 is brought back to minimum setting around 3.5, so as to prevent excessive fuel injection when the engine is restarted.

Marine Diesel Engines

Starting, Reversing and Manoeuvring

♦ Bringing the telegraph lever 15 to ‘Stop’, puts the reversing control valve 23 to stop position U via linkage J. This relieves the oil pressure supply from the reversing control valve 23 to the reversing servom otor 2. T his pressure drop causes the slide valve 13 to m ove down, thereby bringing back the fuel cut-out cylinder 17 to cut fuel injection. ♦ Telegraph lever 15 is put to ‘astern’, thereby pushing the reversing control valve 23 to astern V position via link J. T he oil pressure from the reversing control valve 23 is supplied to the reversing servomotor 2 to turn the camshaft to astern position. O h reaching its end position, the running direction interlock will allow oil pressure to the starting lever blocking device 14 via line A, to free up the starting lever 16 for movement. ♦ The start lever 1 6 is now put to ‘start’ position and the starting sequence is repeated as per the starting operation described earlier.

Bridge Control System B ridge C ontrol U nit It consists o f the follow ing: 1. A telegraph lever handle for ahead / astern movement w ith speed positions like dead slow, slow, half ahead, full ahead and navigational full ahead. 2. A speed sensing u nit getting a signal directly from the engine flywheel.

5. Bridge control solenoid system in the engine control room. 6. A larm unit for alarms like low start air pressure remote system failure. Bridge Control Procedure ♦ O nce the engine is blown through and tested on fuel, controls are handed ova: to the bridge by pressing a button in the ‘engine control room’, which must be acknowledged on the bridge. ♦ Starting w ill be blocked, in case any o f the pre-set conditions are not met, such as: starting air pressure low, turning gear engaged, lube oil pressure low, cooling water pressure low, reversing running direction interlock, etc. ♦ Starting operation is the same as the engine control room starting sequence. ♦ In case o f a failed start attempt, start air will be automatically kept on. ♦ Three to four starts are allowed in case o f start failures, after which a false scavenge air pressure from the control air line is supplied to the scavenge air limiter, so that m ore fuel can be injected for a better start attempt. ♦ Start air is always kept open in the engine room even after the engine is full away. ♦ O nce the engine is started, the speed is increased as per the bridge telegraph lever position.

3. A control unit on the bridge.

♦ Speeds w ith each speed range can be varied by pressing a button or a fine setting knob.

4. A load programme unit either on the bridge or in the engine control room.

♦ Automaticjumping over the critical speed range (around 8 to 12% o f the M CR speed) is done by releasing m ore fuel.

202

203

♦ In case o f a ny deviation in critical parameters, the engine is autom atically slowed down or stopped. ♦ Emergency manoeuvring is possible by overriding the safety devicesJ

CHAPTER 9

ENGINE STRESSES, VIBRATION AND DYNAMICS

In a single cylinder engine; during the expansion stroke, a force is applied onto the piston due to the gas pressure and an inertia force of the reciprocating parts. While the former varies with the crank angle,

205

Engine Stresses, Vibration and Dynamics

Marine Diesel Engines

the latter equaling the product o f the acceleration o f the parts and their m ass varies directly w ith crank sh aft speed. T h e m ass o f the reciprocating parts equals the m ass o f the piston assembly and 30-40 % o f the mass o f the connecting rod. The resultant o f these forces, referred to as the motive force P is applied to the centre o f the piston pin and transmitted to the crankshaft through the connecting rod. T he motive force is resolved into tw o components N and S. The norm al com ponent force N presses the piston against the cylinder liner in a trunk-type engine o r it presses the shoe against the corresponding guide in a crosshead engine. This force, varying in both direction and magnitude, produces a recurrent piston thrust against the opposite sides of the cylinder liner. It also gives rise to an overturning m om ent about an arm equal to the distance betw een the axis o f the piston pin and the crankshaft axis. T he moment opposing the direction o f the crankshaft rotation is taken up by the bolts holding dow n the engine to the bedplate. The second component force S is brought down the line o f its action and applied to the crank pin center. I t can be resolved into two com ponents : a force T tangential to the crankpin and a force Z coinciding w ith the crankpin radius. T he force T produces a torque w hich varies w ith the crank angle from a m axim um to a minimum within a certain period. This torque causes the crankshaft to rotate irregularly. T he force Z bends the crankpin and creates wear in the bearing. In a multi-cylinder engine, the crankshaft is set to rotate by the torques produced by all the cylinders in succession. It w ill operate m ore regularly than the crankshaft o f a single cylinder engine. However, the torques w ill not coincide in time, because the cranks are arranged at certain angles to each other, rather than in the same plane. This implies 206

that the recurrence o f torque alterations increases directly w ith the number o f cylinders and the irregularity o f the crankshaft rotation decreases. The continuously changing engine torque is compared with the moment caused by the force resisting the crankshaft rotation. The torque exceeds the moment at the instance o f cylinder firing and is less than the moment during the intermissions. Hence, the two conditions are extra torque and torque deficiency, causing ‘irregularity’ in crankshaft rotation. Irregularity Factor It is the ratio o f the difference between the maximum and minimum angular velocities o f the crankshaft and the m ean angular velocity throughout a cycle o f torque alterations. F lyw heel A flywheel is fitted to the aft end o f the crankshaft to help reduce the irregularity o f crankshaft rotation. It is an accumulator which stores the energy o f the gyrating masses when there is extra torque, and supplying the stored energy during torque deficiency. Increasing the number of engine cylinders also decreases the irregularity o f crankshaft rotation. Exam ple: Adiesel engine with more than 12 cylinders does not require a flywheel. Static Loads These are loads caused by the weights o f the engine components and the bolt loads. D ynam ic Loads These are loads caused by the cylinder gas fluctuating pressure and inertia loads o f the reciprocating and rotating masses.

207

Marine Diesel Engines

Engine Stresses, Vibration and Dynamics

Static B ala n cin g ♦ It implies that the shaft is stationary or stops at a different position, if rotated w hen supported between centres. ♦ The sum o f all moments taken about its centre o f rotation should be zero at any angular position. ♦ It is done by placing counter weights to balance the moments so that their sum becomes zero.

Prim ary a n d Secondary Im balance

D ynam ic B ala n cin g Although a shaft may be statically balanced, imbalance is caused while it is rotating, due to rotating and reciprocating masses producing inertia forces, couples and moments. Dynamic balancing is balancing o f the unbalanced inertia forces together with their moments. A n inertia force is set up due to the translating (reciprocating) masses o f the connecting rod-crank m echanism , and due to unbalanced gyrating (rotating) masses. Both forces cause foundation vibration. The forces due to translating (reciprocating) masses o f the connecting rod-crank mechanism tend to either tear the engine off the foundation o r to press it against the foundation, depending on the direction o f action. The unbalanced gyrating (rotating) masses act along the crank web and are constant at any angle on the crankshaft at a given engine speed. They tend to shift the engine off the foundation or overturn it. M om ents caused by these tw o inertia fo rc e s : ♦ The gyrating (rotating) masses cause moments to act in the vertical and horizontal planes. ♦ T he translating (reciprocating) masses cause moments only in the vertical plane.

208

♦ Prim ary and secondary forces are set up due to the inertia force caused by reciprocating masses. ♦ The variation in these forces are in the form o f a sine w ave of simple harmonic motion. ♦ Considering one revolution of360degrees, the variation o f primary . forces (Curve 1) and secondary forces (Curve 2) is shown. Vibration ♦ It is the oscillation caused due to a disturbing force. ♦ It can be longitudinal, axial, transverse or torsional. E ngine Vibration Causes -♦ Constantly changing firing pressures.

209

Marine Diesel Engines

Engine Stresses, Vibration and Dynamics

♦ U n balanced forces, couples and m om ents due to reciprocating and rotating masses. ♦ Pulsations d ue to gas forces including exhaust gases. ♦ Guide fo rc e moments.

Resonance ♦ It is the coincidence o f the frequency o f the natural vibration and the frequency o f the forced vibration. ♦ It results in vibration, local overheating and overstressing o f the shafting.

♦ Axial fo rces due to in-plane bending o f crank webs. ♦ Torsional vibration caused by varying torque and propeller thrust. A m p litu d e It is the m ax im u m displacem ent o f vibration from the point o f equilibrium. N ode It is the point i n the vibrating system at which the amplitude o f vibration is zero. Order o f V ibration It is the n um ber o f vibration ‘cycles’ in one revolution o f the engine. Vibration M o d e It is designated by the number o f nodes in a system. N a tu ra l V ib ra tio n It is the vibration caused by the elastic forces o f the crankshaft material and the inertia o f its masses in the absence o f external forces. F orced V ibration It is the vibration o f the crankshaft and the shafting coupled to it, which is induced by a variable engine torque.

210

Vibrations D uring Starting ♦ Balanced engines tend to vibrate during starting, and gradually the vibrations die out as more cylinders develop their ow n power. ♦ This is due to interm ittent fuel delivery and m isfiring o f some cylinders giving rise to unbalanced inertia forces and moments. After a while, the combustion pressures in the cylinders level up and the imbalance is reduced.

Torsional Crankshaft Vibration ♦ The engine crankshaft, its flywheel gears and the different elements o f the propeller shafting form an elastic system, incapable o f being absolutely stiff. ♦ Application o f a torque to the crankshaft causes it to ‘twist’ within elastic limits. Removal or reduction o f the torque causes the crankshaft to twist o r untwist in the opposite direction. This state will recur, for the crankshaft will be urged by the elastic forces of its material and the inertia forces o f its masses to vibrate at a certain frequency. ♦ Torsional vibration is the relative vibration o f the masses o f the elastic system causing it to twist and untwist.

Critical Speed ♦ It is the crankshaft speed at which resonance m ay occur. ♦ There may be more than one critical speed range for an engine.

211

Marine Diesel Engines

Engine Stresses, Vibration and Dynamics

♦ It manifests itself by a shaxp increase in the amplitude o f torsional shaft vibration. ♦ Critical speed can be measured by a torsiograph, which automatically records the torsional vibration on a paper tape.

Barred Zone Range ♦ It is a range o f operational speed which is ‘barred’ i.e. overridden. This is a critical speed range w hich m ust be passed as soon as possible. ♦ U nder B ridge control, the B ridge control u n it program m e autom atically increases the speed setting so that m ore fuel can enable the engine to cross over this speed range as fast a possible.

2. Axial vibration due to in-plane bending o f crank webs can be countered by fitting an axial vibration damper at the free end o f the crankshaft. 3. Torsional vibration due to varying torque and propeller thrusts is countered by detuning or damping. 4. Vibration due to guide force moments is countered by detuning, by using top bracing to increase the stiffness.

Detuners They are frequency control devices used to change the frequency of the system. Examples:

♦ It is specified for a given engine. ♦ The means o f avoiding these resonant frequencies is to adjust the speed o f the engine or the mass o f the flywheel or the engine firing order. ♦ T he m ost effective means o f reducing the amplitude o f torsional vibration is the sectionalizing o f the shafting and interposing special couplings between the sections. ♦ A nother m ethod is to use vibration absorbers which are fitted to the crankshaft to dissipate the energy o f vibration in a given range o f engine speeds. R eduction o f E n g in e Vibration 1. T he vibrations d ue to reciprocating and rotating masses can be countered by com pensating masses rotating at the engine speed for first order frequency, and tw ice the engine speed for second order frequency. T hese com pensators o r balancers can be positioned in the chain drive.

212

1. Top bracing supporting the engine: The bracing increases the stiffness and raises the natural frequency beyond the operating range. 2. Flexible couplings: These couplings sectionalise the system. T he flexible element absorbs part o f the vibrational energy and hence, decreases its amplitude. The flexible element can be either rubber o r a spring element 3. Hydraulic oil-filled mechanical detuners: Here, the oil gets passed to and fro past the springs, causing detuning as well as damping.

Dampers These are devices which absorb part o f the vibrational energy. Examples: 1. Rubber damper using the elasticity o f rubber to absorb part o f the vibrational energy. 213

Marine Diesel Engines

2.

Viscous damper using a viscous silicone fluid. It is made up o f tw o masses i.e. a light outer casing and a heavy inner ring. The inner heavy ring rotates a t a lesser speed than the light outer ring separated by viscous silicone fluid. This heavier ring is driven by the viscous shear o f th e silicone. T he energy required fo r the viscous shear (relative oscillating motion) is provided from the vibration energy, thus giving a damping effect.

CHAPTER 10

ENGINE OVERHAULS AND MAINTENANCE Unit Decarbonisation Safety P recautions to be o b served: ♦ The port authorities are to be informed that immobilization o f the engine is to take place. ♦ In case o f turning the propeller, propeller clearance is to be taken from the Bridge. ♦ Spare parts, tools, lifting devices, gaskets, 0-rings, hydraulic jacks, special tools, gauges, operational crane, etc. are to be kept ready. ♦ Engine is to be isolated: A t Finished W ith Engines (FWE), bring the telegraph lever and fuel lever to zero. Take over the controls from the Bridge to the ECR. Stop pumps and shut valves for fuel, exhaust valve air, start air, lube oil and jacket water systems. Use ‘Do not operate’ tags and signs, or lash valves. Engage die turning gear. Usually die turning gear is engaged and run for a few revolutions before stopping the lube oil pumps. Drain the jacket water for that unit.

214

215

Marine Diesel Engines

Engine Overhauls and M aintenance

Cylinder Head Removal Tools required Hydraulic tensioning device, suspension lifting device and special eye bolt screws. Procedure ♦ Remove the cooling waterpiping for the exhaust valve; high pressure fuel oil pipes to the injectors; air piping to the cylinder start air valve; lube oil hydraulic pipe fo r exhaust valve actuation; drain pipe between exhaust valve and hydraulic actuator; and exhaust valve bellow.

Hydraulic n ut removal ♦ H ydraulic pressure is used to elongate the stud. The n ut is then opened by a turn, by a tommy bar inserted into holes on the side of the n u t Hydraulic pressure is then released and the nut unscrewed easily. ♦ Hydraulic pressure can b e supplied to one point as shown in the fig-124 and vented before applying full pressure. Example shown is as per a ‘LG F’ engine. ♦ Other engines use a hydraulic tensioning device consisting o f a pump and a single flexible hose branching out to each nut itself. Example shown in fig-125 is as per a ‘RTA’ engine.

♦ Clean the threads o f the cylinder h ead studs after rem oving the stud caps. Place the hydraulic device to remove the hydraulic nuts on the cylinder head studs.The hydraulic pressure to b e applied by the hydraulic pump is given in the manufacturer’s manual. Example:

600 bar pressure fo r RTA engines; 700 b ar pressure for L G F engines.

Fig-125

F ig - 124

216

1 3 5 7 9

Stud Pin Hydraulic nut piston Sealing ring Oil pressure inlet

2 4 6 8

217

Nut Vent screw Hydraulic n at cylinder Hole to insert tommy bar

Engine Overhauls and M aintenance

Marine Diesel Engir,

► Once the hydraulic nuts are removed, lifting eye bolts are screwed on to lift the cylinder head cover (along with the small water jacket) by the crane. ►Land the cylinder cover onto wooden blocks placed on the platform floor plates. • D iscard the sealin g m etal gask et between the cylinder cover and liner. ’ R em ove the mountings and clean the cylinder head cover. ■ Lap the fuel, start air and exhaust valve bores. ’ Use new seal rings and cooling w ater connection gaskets while assembling back. ■ A fter assem bling, air supply to the exhaust valve is opened first so that the exhaust valve spring air closes the exhaust valve, after which camshaft lube oil pum p is started.

Exhaust Valve Removal ♦ The procedure is similar to cylinder head removal. Only the exhaust valve can be rem oved while the head is still in place. ♦ T he necessary exhaust valve piping connections like hydraulic actuation pipe, exhaust bellow and expansion piece are removed. ♦ The hydraulic nuts which secure the exhaust valve to the cylinder head are removed. ♦ W ith the help o f a suspension device, eye bolts and the engine room crane, the valve is rem oved and placed on woodenblocks.

218

219

Marine Diesel Engines

Engine Overhauls and Maintenance

Piston Removal

Piston Withdrawal ♦ Rem ove the piston and land it in the space provided, through the engine room platforms. Supporting devices in two halves are provided for the purpose. ♦ A rubber sheet or a w ooden board is placed over the crosshead to protect it from dirt falling from the top.

1. Cleaning o f the liner top and the p iston crown hole threads After the cylinder head is removed, clean the carbon deposits from the upper p art o f the liner. Clean the lifting holes in the piston crown top. Tap the threads o f the holes in the crown to enable the fixing o f the lifting tool. Fitthe lifting tool into the threaded holes o f the piston crown.

Piston Inspection ♦ Check the crown surface for any traces o f fuel, water o r cracks. ♦ The piston crown is cleaned and the bum -away on the surface is checked with the help o f a template. For cracks, use a simple white chalk test o r dye penetrant test. Mok. permiuible burn-away ♦ The ring area and liner surface * pi,,on ,af> should be seen as slightly damp with lube oil to confirm whether cylinder lubrication is correct. ♦ R e m o v e th e rin g s w ith th e expander tool.

2. Removal o f the piston rod palm n ut The piston rod palm nut is removed hydraulically. T he w eight o f the piston is now taken b y the engine room crane. 3. S e p a r a te th e c r o s s h e a d bearing Turn the engine with the turning gear and lower the crosshead bearing so that it is separated and clear from the piston rod. In some engines, the stuffing box is taken out along with the piston, whilst in other engines it is taken out after th e p iston is removed.

FITTING OF TENSION SPRING

Fig-128 F ig - 129

220

221

Marine Diesel Engines

Engine Overhauls and Maintenance

♦ Clean the grooves and measure the groove / ring clearances. The groove inner com ers should be cleaned o f deposits. Piston R in g Clearances

Wear rate =

(1) R ing gap o r b u tt clearance It is taken w here the liner is least worn, usually at the lower part, or in a new liner. T he used ring is inserted into the liner and the ring gap (or butt clearance) is taken ,by m aking an impression o f the gap on a paper. (2) Groove axial clearance It is taken using a feeler gauge inserted horizontally in the gap between the top o f the ring and the groove.

R ing w ear x 1000 Running hrs.

w here Pi = 3.14

Piston Mounting ♦ The rings are fitted correctly by checking the ‘top’ m arking on each ring. ♦ Coat the piston ring, piston rod, and liner with lube oil; and mount the lifting tool. ♦ U se new 0-rings on the outside o f the stuffing box and smear a coat o f lube oil. ♦ Remove the protective rubber sheet for crosshead protection. ♦ Remove the stuffing box hole cover. ♦ Mount the piston guide ring piece (bell mouth) and lower the piston with the crane. ♦ The piston rod foot is to be guided into the stuffing box opening. ♦ Lower the complete piston in the liner leaving a gap between the guide ring and the lifting tool. ♦ Turn the engine with the turning gear to put the piston rod centre hole into the crosshead bearing section. ♦ Remove the guide ring and the lifting tool. ♦ Tighten the piston rod screws and the stuffing box screws.

222

223

Marine Diesel Engir.

Engine Overhauls and Maintenance

L in e r Removal ♦ D rain the ja c k e t w ater fro m the cylinder unit after isolating it ♦ Remove the cylinder head, piston and stuffing box. ♦ Remove tw o screws which locate the liner on the support ring. ♦ Remove the quills, protecting devices and oil connections. ♦ Lower the beam tool 1 from the top and fasten it w ith screws 2 at the bottom o f the liner. ♦ Turn the engine to TD C and place a support piece 5 along with a hydraulic jack 4 on to the crosshead pin 3. . ♦ A bridge lifting tool d ism ounted on the top o f the liner 7 with the help of screws 8. ♦ Ja c k up slig h tly w ith h y d rau lic pressure and check that the two 0rings are detached and liner is loose. ♦ Pull the liner out w ith the help o f the crane. L iner Inspection Check and clean the corrosion layer o f the jacket.U se new 0-rings when fitting back. Lubricate guide areas with lube oil.Clean landing faces and quill holes. W hen using a new liner, the protection coating layer should not be scraped out. Remove the coating with diesel oil to prevent any damage o f the surface. Check the cylinder liner lubrication after fitting of the quills

224

L in e r Calibration ♦ Once the cylinder head and piston are removed, the liner is cleaned before calibration. ♦ A straight edge tool 1 is supplied to p rovide th e points a t w hich the measuring gauge is put.

Main Bearing Removal

Fig-134

Example Sulzer RTA: Upper H a lf ♦ Turn the engine so that the respective crank web is approximately horizontal. ♦ Disconnect the lube oil pipes at 6. ♦ Som e engines have ja c k b olts 2 securing the top half o f the bearing, while other engines have thrust bolts or wasted stud bolts. Slacken them hydraulically and remove the nuts. ♦ L ift the top cover vertically w ith a lifting tackle 6, wire slings and a chain block. ♦ Now take the top cover outside the crankcase horizontally with another lifting tackle, wire sling and chain block. ♦ Fit an eyebolt 3 on the top half bearing 4 and take it out. 225

Fig-135

Marine Diesel Engines

Engine Overhauls and M aintenance

The figure 136 show s the removal o f the main bearing top cover 1, upper bearing shell 2 and low er bearing shell 3 as in a B& W engine.

Crosshead Bearing Removal The crosshead bearing is the same as the connecting rod top end bearing. Example given is as per ‘RTA’ engines. First, take the crosshead clearances. 1.

Bottom H a lf ♦ T h e en g in e is tu rn e d so th a t th e respective crank w eb is parallel to the bedplate separating face. ♦ M ount the support cross-piece 2 and jacks 3 below the adjoining crank 4. Jack up 6 the crankshaft by 0.1 to 0.15 m m (m ax 0.2 mm). Check the lift with a dial gauge 1. ♦ The shim s 8 are rem oved and a rope support piece 9 is fitted. ♦ A steel rope 7 is passed around the low er shell 5 and pulled out with a rope pulley.

Suspend the lube oil articulate arm ♦ Loosen the screws o f the lube oil articulate linkage arm. ♦ M ount the suspending tool. ♦ Turn the engine to TDC to suspend the arm.

Fig-138

2. Suspend the piston ♦ Turn the engine to allow access to the piston rod screws and remove them hydraulically. ♦ To suspend the piston, first turn the engine to TDC to take the piston up. Fit two eyebolts to either side o f the piston rod foot, and suspend w ith two chain blocks to the hook provisions at the top comer o f the crankcase (port and starboard). ♦ Take the crosshead down with the turning gear so that the piston is suspended (hanging) b y the tw o chain block attachments. 3. Remove the con-rod top end upper h a lf cover w ith shell ♦ Rem ove the four hydraulic nuts which secure the top end upper half cover. ♦ M ount the lifting attachment to the top cover o f the con-rod. Fig- 139

226

227

Engine Overhauls and Maintenance

Marine Diesel Er\gir,

Top h a lf o f th e bottom e n d bearing ♦ Take the bottom end bearing section out with the help o f chain blocks and wire slings. ♦ Suspend th e crosshead w ith guide supports o r retaining pins o r lifting tackles, etc. (as explained earlier in crosshead bearing removal) as shown

♦ Using tw o chain blocks and tw o eye bolts, rem ove the upper h alf cover to inspect the shell. . Suspend the crosshead ♦ Take the crosshead up towards TDC. ♦ Secure the crosshead by fitting 4 nos guide supports (or by lifting tackles in some engines o r retaining pins).

F ig - 140

S u p p o r t th e c o n -r o d a n d tu r n th e e n g in e to in s p e c t the bottom h a lf bearing ♦ T he con-rod is to be supported on either side by chain blocks. ♦ By turning the engine shaft with the turning gear, the bottom half can be inspected.

in 2. ♦ Turn the engine till the top half o f the bo tto m end b earin g is c le a r fo r inspection as shown in 3.

Crosshead Pin Removal T his is very rarely done, except in case o f dam age to the crosshead pin. A brief rem oval procedure is described below.

Connecting Rod Bearing Removal The con-rod bottom end bearing is the same as the crank pin bearing. Bottom h a lf o f the bottom end bearing ♦ The crank case doors are opened for access. ♦ T\im the engine to TDC. ♦ Support the low er half o f the bottom end bearing with chain blocks, tackles, wire slings, etc. as shown in 1. ♦ Remove the securing nuts hydraulically. ♦ Lower the bottom h alf w ith a chain, block.

228

Fig-142

229

Engine Overhauls and Maintenance

Marine Diesel Engines

♦ Remove the working piston, crosshead lubrication toggle lever and crosshead b e a rin g to p c o v e r e x p o sin g th e crosshead pin top side. ♦ Mount a special lifting plate 2 onto the crosshead pin and take its weight with the engine room crane 1. ♦ Secure the co n -ro d and raise the crosshead head pin. ♦ Remove the guide rails (fuel pump side) leading to the neighbouring cylinder, both guide shoes and the m iddle piece 3 on each side o f the pin. Fig -141 ♦ The crosshead pin can now be removed from the middle piece.

Thrust B earing Transm ission The thrust transmission is from the engine crankshaft to the thrust collar to the thrust pads to the thrust block housing to the bedplate to the holding down bolts to the foundation plate and to the ship’s hull.

Thrust Bearing Pad Removal

Connecting Rod Removal The connecting rod can b e removed, e ven w ithout rem oving the working piston and crosshead pin. ♦ Remove top-end and bottom-end bearing covers as described in earlier procedures. ♦ Suspend the crosshead with retaining pins 1 or guide supports. ♦ Remove the con-rod w ith chain blocks and wire slings 2 as shown.

♦ Remove the top bearing cover 1.

♦ The crosshead pin m ust be carefully wrapped for protection.

♦ Insert a ‘turning out’ device at the gear wheel.

♦ Remove the retainer 2 and its screws. ♦ Turn the crankshaft so that an eyebolt can be screwed into a pad 3 w hich can be lifted and removed one b y one. Fig-145

♦ All pads are numbered.

231

Marine Diesel Engines

Engine Overhauls and Maintenance

Bearing Clearances

Crosshead B earing Clearances

The following table gives an approximate idea o f clearance values: Bearing

Clearance Value

Main bearing Crank pin bearing (Conrod bottom end) Crosshead bearing (Conrod top end)

0.3 to 0.4 mm 0.4 to 0.6 mm Pin and Shoe Shoe and Rail Plate and Rail 0 .5 to 1.0m m 0.1 to 0.2 mm

Thrust bearing Camshaft bearing

Procedures for taking Clearances M ain B earing Clearances M ethod 1 A fter rem oving the bearing top cover and shell, a special ‘Bridge’ is placed. The clearance is taken by placing a feeler gauge between the bridge gauge and the journal.

M ethod 2 The bearing lube oil pipe and insert are removed, and a special feeler g auge is in serted to take the reading.

232

- 0 .1 to 0.3 mm - 0.4 mm - 1.5 mm (m ax 2 mm) ♦ The crank pin should stand in a horizontal position 90 degrees towards the fuel pump side. Hence, the crosshead is automatically pressed by the con-rod against the rail surfaces on the exhaust side and the clearance is taken on the fuel pump side. ♦ The crosshead bears on one side fully. However, clearances are to be taken on both exhaust and fuel pump sides. One side should give a ‘zero’ value or else, the piston is not aligned or the liner is worn. 1. Pin a nd Shoe The radial clearance between the crosshead ‘pin’ and ‘shoe’ is very difficult to measure when the pin is fitted in the engine. It can only be taken by measuring the pin outside diameter and the shoe inner diameter by a micrometer. 2. Shoe a nd Rail It is m easured w ith a long feeler gauge inserted at the top and bottom o f each guide shoe.

233

Engine Overhauls and M aintenance

Marine Diesel Engines

3.

Plate a n d Rail The complete crosshead m ust be pressed axially to one side with suitable hardwood edges or sim ilar aids. This side pressure should be exerted onto the shoe and not the pin. Clearances are taken with a feeler gauge.

M ethod 2 (Example : Sulzer engines) The total displacement which results from pushing the crankshaft axially both ways until it touches the thrust pads 1 in ahead and astern is measured w ith a ‘clock gauge’. It is com p ared w ith the engine manual guide. Incase o f increase, there could be possible wear of thrust pads.

M ethod 1 (Exam ple: B & W engines): ♦ Turn the engine so that the aftermost crank is at BDC. This ensures that the thrust bearing collar rests on the forward (foremost) thrust bearing pads. Hence, the value 'B ’ = 0 .

Example: Axial clearance fl = 0.8 to 1.3 M ax im u m f l v alu e due to w ear = 2.5 mm

♦ A feeler gauge is inserted at ‘A’ between the side o f the aftermost bearing and the crank throw.

Connecting Rod Clearances

♦ Maximum thickness o f gauge entering ‘A’ should be 2 mm. ♦ If the gauge entering ‘A’is less than [2 m m - (B + C)], then clearance is within limits.

These are taken with feeler gauges at the crosshead pin (top end) and the crank pin (bottom end).

♦ I f th e g a u g e e n te r in g ‘A’ is e q u a l to o r g r e a te r than [2 m m - (B + C)], then clearance is more than the limit.

I F ig -152

II

♦ Clearance is 0.5 to 1.0 m m for new engines and its m aximum value is 2 mm.

234

\ r

235

Engine Overhauls and Maintenance

Marine Diesel Engines

F u e l P u m p Setting /A d ju stm e n ts

Initializing Spill Valve an d the Plunger

It is carried out in suction and spill type fuel pumps. Example: Sulzer engines

♦ R o ta te th e en g in e in th e astern direction.

%

♦ Cam roller to be on the base. It's p u rp o se : ♦ To check if the fuel pum p setting is correct for the injection timings. ♦ To compare w ith the original data f o r :

♦ Fit dial gauges 2 with 1 mm pretension over the spill valve (now closed) and plunger. ♦ Set both gauges to ‘O’.

(1) Idle Stroke = ‘a’ in mm. (2) Beginning o f injection angle, before or after TDC. (3) Total injection stroke = ‘b ’ in mm. (4) End o f injection angle, after TDC. (5) Effective plunger stroke = b - a . C h e c k in g B e g in n in g o f In je c tio n i.e. Closing o f Suction Valve

Procedure Initializing Suction Valve D ial Gauge ♦ Rotate the engine in ahead direction.

♦ R o ta te th e en g in e in th e ahead direction till die suction valve gauge 3 shows 0.02 mm.

♦ C am roller to b e o n the peak.

♦ Note the plunger gauge 4 reading= ‘a’.

♦ Fit dial gauge 7 with 1 m m pretension over the suction valve (now closed) and set to ‘O’.

♦ Also note the flywheel angle.

F ig - 153

236

F ig -155

Engine Overhauls and M aintenance

Marine Diesel Engir

Checking E n d o f Injection' i.e. Opening o f the S pill Valve 4 Rotate the engine in ahead direction till the spill valve gauge 5 show s 0.02 mm. ♦ Note the plunger gauge reading = ‘b \ ♦ Also note the flywheel angle. ♦ Plunger stroke = ‘b-a’. 4 Carry out cut-out checks.

Fuel Pump Cut-Out ♦ W hen the cut-out lever at th e fuel pum ps is turned by 180 degrees, the mechanism lifts the rollers from the cams. H en ce, th ere are no Fig-157 plunger movements. ♦ W hen the fuel pump is cut out by hand, the clearance between the rollers o f the plunger and the cam m ust have at least 0.5 mm clearance.

Fuel Pump Lead

Fuel Pump Cut Out Checks and Zero Setting Checks 1. A t zero position o f the governor, the load indicator and cut-out servomotor should coincide for ‘zero’ fuel injection. 2. W hen the governor is tripped by hand, the suction valves o f the fuel pum p should be lifted by at least 6 mm. 3. When the governor and speed adjusting lever is at ‘zero’, the fuel pum p eccentric shaft should also be at zero. 4. W hen the fuel pum p is manually cut-out, the clearance between the cam and rollers should be at least 0.5 mm. 5. A t zero setting shield position, the suction and spill valves must never be closed at the same tim e i.e. when one is open, the other is closed.

238

It is carried out in jerk type fuel pumps e.g. ‘B& W ’ engines. ♦ It is the distance that the plunger top is lifted above the upper cut-off holes in the barrel, w hen the unit’s piston is at TDC. 4 F u e l p u m p le a d = Y = X + D5 4 D5 is a correction factor. It is the distance between the plunger top and upper cut­ o ff h o les top, w hen the p lunger top reaches the exact position at which light can be seen through the low er cut-off holes in the barrel and plunger.

Fig-158

239

Marine Diesel Engines

Engine Overhauls and M aintenance

3. 4. 5. 6.

Adjust the measuring tool dial gauge to ‘zero’. Turn the engine ahead till the engine piston is at TDC. Note the dial gauge reading = ‘X ’. Fuel pum p lead = Y = X + D 5.

4 - Stroke Medium Speed Engine Fuel Pump Timings In a 4-stroke engine, the fuel camshaft rotates at half the speed o f the crankshaft. Hence, during the two revolutions o f the crankshaft, injection takes place only once. In order to m ake sure that it is the injection stroke, check the fuel cam.

Preparation Turn the unit to TDC, shut the fuel oil inlet and drain from the bottom. Disconnect the air pipe to the puncture valve. Remove the protection cover and the puncture valve. R em ove the erosion plugs from the pump housing. R em ove the connecting pin and disconnect the VIT index arm. Pull out the VIT index arm to ‘zero’index. A lign the cross bore in the plunger with the lower cut-off holes in the barrel. Put the fuel oil index to 21.5 or 93.5. Verify the alignment by shining a torch through the put-off h o le s. Procedure 1.

2.

Turn the engine ahead till the upper edge ofthe plunger reaches the exact position at which light can be seen through the ‘lower cut-off holes’ in the barrel and plunger. Mount the measuring tool so that it touches lightly against the top o f th e plunger.

240

♦ Open the cam case doors to see the fuel cams. ♦ During injection stroke, the roller will not be on the base circle o f the cam. ♦ Turn the flywheel to the angle specified by the manufacturerforfuel delivery commencement. ♦ Check th e jerk type fuel pum p w indow marking. ♦ The start o f delivery should coincide with the top m ark 1. ♦ Turn only in one direction o r else, there will be an error due to play. Fig-160

TUrbocharger Overhaul Compressor End ♦ Remove air filter. ♦ Drain lube oil. ♦ Remove the bearing space cover. ♦ Check the true run ‘B 1’ o f the nipple with a dial gauge. ♦ Rem ove the nipple. ♦ Check the true run ‘B 2’ o f the oil slinger with the dial gauge. 241

Marine Diesel Engines

Engine Overhauls and M aintenance

Turbocharger Out of Operation

Fig-161 Remove the cap nut and the locking washer. Measure dimension K. Remove oil slinger using an extractor and holding device. M easure K1 and K2. K1 is measured at the same place as ‘K ’ w hile pushing the rotor towards the compressor. ♦ K 2 is m easured at the same place as ‘K ’ w hile pulling the rotor towards the turbine. ♦ Remove the bearing using the extractor screwed to the inner bearing bush. ♦ ♦ ♦ ♦ ♦

Turbine End ♦ Similarly, remove the turbine-side bearing also. ♦ Using a special pipe and an eye bolt screwed to the shaft, the rotor can be removed. ♦ The clearances K, K1 and K 2 are compared during disassembly and assembly. ♦ Check the labyrinth seal, binding wire, blades, pitting on the shaft, casing nozzle ring damage and corrosion. ♦ Clearance L = K - K l , and M = K 2 - K .

242

Case] : In case one turbocharger is damaged ♦ The following measures are to be taken in case o f one o r m ore turbochargers are still in operation. The engine can still be run at low rpm and with less power. ♦ T he charge air pressure, tem perature, turbocharger rpm, firing pressure, etc. are to b e monitored. ♦ R em ove th e expansion piece betw een turbocharger and exhaust manifold and fit the flanges A and B. ♦ F it a b la n k fla n g e C b e tw e e n th e turbocharger air outlet and diffusor. ♦ Isolate the turbocharger cooling system. Stop th e lube oil supply only if the turbochargers are provided with external lubrication system. ♦ Block the rotor o f the defective turbocharger. Case2: In case all turbochargers are damaged ♦ B lock rotor and stop lube oil supply from external lubrication. ♦ Open all covers D o n the charge air receiver. ♦ Open and remove cover E o n the auxiliary blower. ♦ Start the auxiliary blower and put in use. ♦ M onitor exhaust temperatures before the turbine, exhaust smoke, charge air pressure, turbocharger speed, firing pressures, etc. Run the engine at a reduced rpm. 243

Marine Diesel Engines

Engine Overhauls and M aintenance

Fuel Injector Overhaul A fuel injector is checked and overhauled for the following: ♦ Condition o f the valve spindle (sticky, etc.). ♦ Opening pressure o f the valve. ♦ Functioning o f the slide valve. ♦ Oil tightness o f valve seat between valve spindle and spindle guide. ♦ Direction and spray o f fuel jet. ♦ Slackness o f the needle. Overhaul

♦ The nozzle holes are cleaned and cleared with special needle drills o f diameter size 0.025 m m smaller than the nozzle. ♦ A test plug gauge is used to ascertain whether the hole is still proper. If the test plug enters the hole, then it should be discarded. The test plug is 10% larger in size than the normal spray hole size.

♦ The fuel valve is disassembled by unscrewing the union nut with a tommy bar or a spanner, while retaining the valve in a vice with softjaws.

♦ The needle should not be too slack in the nozzle. Test it by leaving it to fall into the nozzle. It should go down smoothly and slowly.

♦ Clean and examine all parts.

♦ Atomisation into a fine spray is checked by quick pumping movement of the test machine handle.

♦ Lapping o r grinding o f seating surfaces by grinding mandrels is done manually or by a slow speed drill if required.

♦ Needles and nozzles are a pair and are to be replaced together.

♦ The direction o f the spray is checked at its opening pressure. Here, the oil spray jet direction can be seen through a transparent control screen. ♦ The correct functioning of the valve is checked by testing the opening and closing pressures o f the spindle guide. Apply and oil pressure to the valve to a value o f 50 kg/sq.cm below the opening pressure. This means that the pressure should not be raised above approximately 200 kg/sq.cm, following which it w ill fall relatively slowly towards zero. A t around 8-10 kg/ sq.cm, when the return oil passage has been re-established, the pressure should fall abruptly.

244

Engine Overhauls and Maintenance

Marine Diesel Engines

0-R in g Check

♦ If there is clearance, tighten the nut w ith the round tommy bar. ♦ Release hydraulic press, apply non-acidic grease to the threads

Raise the pressure slowly so that the return oil connection is n o t closed, until oil flow s out o f ‘A’. Then plug the outlet hole, raise the pressure to 100 kg /sq .cm , and m aintain it at this level for a moment to see that o-ring ‘B ’ seals tight.

Checks D uring L oosening a nd Tightening

and cap the nut.

♦ Pinching or clamping screws should be removed. ♦ If the tie-rods are newly tightened, then the wasted studs or jack bolts o f the main bearings also have to be checked for correct pre­ tensioning. ♦ Tightening is done in the correct sequence.

Checking Pre-Tension of the Tie Rods

Tie R o d T ensioning M ethods

This is done to check if the tension is correct for already tightened tie rods. If tensioning is incorrect, then there will be fretting which may permanently misalign the affected components. If fretting is already present, then even correct tensioning over fretted tie-rods will cause misalignment. The only remedy is corrective machining. Pretensioning C heck P rocedure Exam ple: (Sulzer RTA) ♦ Remove the thread protection caps and clean contact face o f the intermediate ring. ♦ Screw both pre-tensioning jacks onto the two tie-rods lying opposite each other, until the hydraulic jack cylinder rests on the intermediate ring o f the n u t ♦ Slightly slacken vent screws o f the hydraulic jack. ♦ C onnect operate and vent the high pressure oil pump. ♦ Operate the oil pum p till 100 M pa pressure is obtained and maintain this pressure. ♦ U se a feeler gauge inserted into the slot, to check that there is no clearance between tie rod nut and intermediate ring o f the nut. 246

M ethod (1)

.

Example: (Sulzer RTA) ♦ Slacken the m ain bearing w asted studs o r ja c k bolts, i f initial tensioning is to be done for new fittings. ♦ Slacken the pinching or clamping screws. ♦ Attach a hydraulic pumping unit to opposite nuts.

247

Engine Overhauls and M aintenance

Marine Diesel Engir.

♦ Follow the correct tightening sequence starting from m id­ engine. ♦ Raise the hydraulic pressure to 350 bar. ♦ W ith the round bar, tighten the nuts as per tightening sequence. ♦ Raise the hydraulic pressure to 600 bar. ♦ M easure the elongation o f the tie-rod and com pare w ith the reference manual values. ♦ Tighten all bolts at 600 bar. ♦ Check w ith a feeler gauge that there is no clearance between n u t an d in te rm e d ia te rin g washer. ♦ R e -tig h te n th e p in c h in g o r clamping screws, so that it just nips (touches) the tie-bolt.

M ethod (2) Exam ple o f B & W ‘M C ’ Engines ♦ Ensure pinching or clamping screws are slack. ♦ Attach and operate the hydraulic pum ping unit to 700 Bar, starting in the correct tightening sequence. ♦ Tighten the nut with the round tom m y bar. ■

248

Air Compressor Overhaul ♦ Before disassembly, record all temperatures; pressures; and starting and running current parameters; as a reference for later comparison. ♦ Spare parts and tools to b e kept ready. ♦ Compressor to be properly isolated and tagged. ♦ Disassemble the compressor. ♦ Check the piston condition, piston ring clearances, liner wear, gudgeon p in surface and w ear in the outer diameter, crankshaft bearings, oil seals, crankcase lube oil condition and renewal, lube oil strainer, float switches, lubricators for cylinder lubrication, valves, unloaders, pressure testing o f inter and after coolers, cooling pump safety devices like bursting disc, relief valve testing, alarms and cut outs, automatic drain valves, etc.

249

Engine Overhauls and M aintenance

Marine Diesel Engir,

Testing of Materials

Tempering

D estructive Tests

Normalising: HeatingtoUppercrit. temp

+ (30to40deg.Q+

1. Tensile Test is done to test the strength and ductility. The specimen is elongated an