Marine Diesel Engines PDF

 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. ...

 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 M a rin e D iesel E n g in es Table O f Contents B y Deven Aranha Preface Acknowledgements © Shroff Publishers and Distributors Pvt. Ltd. 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 Concept of Internal Combustion Engines......................... 01 Stroke....................................................................................01 th is copyright n otice, m ay be rep ro d u ced o r u tiliz e d in any Mean Piston Speed............................................................02 form o r b y any m eans, electronic o r m echanical, including Advantages / Disadvantages of Diesel Engines 03 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 Classification of 1C. Engines............................................ 04 CONTENTS and retriev al system , w ith o u t th e w ritte n perm issio n o f the Otto, Diesel. Dual and Actual Cycles................................06 copyright ow ners, n o r exported, w ithout the w ritten perm ission 2 -Stroke C y c le.....................................................................09 o f th e p ublishers. 4 -Stroke C y c le................................................................... 12 2-Stroke vs. 4-Stroke Engines.................................... 16 CH A PTER 2 : First Edition : July 2004 EN GINE COMPONENTS Seventh Reprint: January 2013 Engine Structure...............................................L. 19 Top B racing..................................................... 20 Fatigue Failure.....................................................................21 ISBN 13: 978-81-7366-927-9 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. 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. Piston Rings : Compression Rings. Oil Scraper Rings 36 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 Failures. Running-in. Shapes. Coatings. 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 , CPR Rings. Antipolishing Ring, SIPWA. e-m ail: sp dorders@ shroffp u b lish ers.co m , P rin ted a t D eco ra B o o k Stuffing Box G lan d............................................................. 44 Prints Pvt. Ltd., M umbai. Lmer. Liner W ear.................................................................45 Lubricating Quills and Accumulator 48 H Marine Diesel Engines Marine Diesel Engines Cylinder Head C over.......................................................... 50 CH A PTER 4 : Exhaust V alve................................................................... 51 A IR COMPRESSORS Valve Springs....................................................................... 53 Isothermal Com pression................................... 103 Valve Rotators......................................................................55 Adiabatic Compression and the Compression C ycle.... 103 Variable Exhaust Closing (VEC) 56 Multistage Compression.................................... 104 Crankshaft.......................................................................... 58 Reciprocating,and Rotary Compressors....... *................ 104 Crankshaft Stresses 62 Volumetric Efficiency and Bumping Clearance............... 105 Crankshaft Deflections....................................................... 63 Compressor V alves............................... fan..................... 105 Chain Drive, Tightening and Inspection 64 Compressor F au lts.................. laSLari............................ 106 Chain Elongation.................................................................67 Camshaft Readjustment after Chain Tightening 68 C H A PTER 5 : Bearings Plain Bush Journal, Pivot Pad Journal 69 FU EL SYSTEM CONTENTS CONTENTS Mam Bearings................................................................. 71 F u e liy p e s.............................................................................109 Connecting Rod and its Bearings 72 Fuel Properties................................................................... 110 Fuel Specifications...................................... U6 Bottom End Failures and Bolt Design 74 Combustion Phases............................................................ 117 Crosshcad Bearings............................................................ 75 K nock........................................................................... ns Puncture Valve.....................................................................77 Factors Affecting Com busuon.......................................... 119 Engine Materials 78 Combustion Chamber and Piston Crown Designs........ 121 Compression R a tio............................................................ 121 CH A PTER 3 : Residual Heavy Fuel O ils................................................. 122 A IR SYSTEM Bunkering........................................................................... 123 Scavenging,..:..;;;......................;............;....u..i..:;.............. 81 Fuel Injectors................. 125 Uniflow, Reverse, Loop and Cross Scavenging............. 81 Injector ty p e s........................................................ 126 Injection Methods...................... 130 Gas Exchange Process.....S................................ 84 Fuel Pu m p s............................ 131 Supercharging......................................................................S5 Suction Valve Controlled P u m p..................................... 131 Constant Pressure and Pulse Turbocharging 86 Suction and Spill Controlled P um p.............................. 133 Series. Parallel Supercharging 89 Port Controlled Jerk P u m p............................................... 134 TVo-Stage Supercharging 91 Injection System s................................................................ 135 Single and Multiple TVbochargcr Systems 91 Variable Injection Timings (V1T)..................................... 136 Power Take-In and Power Take-Off 92 Fuel Quality Setting (FQ S)............................................... 140 Axial Flow Turbocharger 94 Super-V IT and Conventional V1T.................................... 140 Uncooled Turbochargers 97 Fuel C a m.......................................................................... 146 Surging................................................................................. 99 High Pressure pipe sa fe ty............................. 147 Compressor M a p.................................................................99 [ii] [iii] Marine Diesel Engines Marine Diesel Engines CH A PTER 6 : Start Air Interlocks.............................................................187 LUBRICATION SYSTEM Slow Turning...................................................................... 188 Friction and Friction Types...............................................149 Scavenge Air Limiter................................................. 188 Lubrication Types.............................................................. 151 Firing Order of Cylinder................................................... 188 Lube Oil Properties........................................................... 152 Reversing M ethods.......................................... 190 Lube Oil Testing............................................................... 156 Loss Motion and Gain M otion........................................ 194 Microbial D egradation..................................................... 161 Running Direction Interlock............................................ 195 Cylinder Lubrication Types and System s........................162 Crash Manoeuvring....................—................................... 195 Lubrication Pump U n it................................................... 166 Manoeuvring Flow C h a n................................................. 197 Load Dependent Cylinder Lubrication..................... 167 Manoeuvring Diagram.......... 198 Specific Cylinder Lube Oil Consum puon..................... 169 Bridge Control S y stem.......................................................202 CONTENTS CONTENTS Frequency Controlled Electric Motor Lubricator.......... 169 Multilevel Cylinder Lubrication............ 170 CHAPTER 9 : Crosshead Lubrication......................................................171 ENGINE STRESSES,VIBRATION AND DYNAMICS Forces Acting in a Single Cylinder E n g in e......................205 C H A PT E R 7 : Irregularity Factor............................................................. 207 COO LIN G SYSTEMS Static and Dynamic Balancing........................................... 208 Function.............................................................................. 173 Primary and Secondary Imbalance —.................................209 Bore Cooled Liners............................................................ 174 Vibration D efinitions...................................................... 209 Load Dependent Liner Cooling....................................... 174 Torsional Crankshaft V ibration......................................... 211 Piston Oil Cooling System.................................. 175 Critical Speed...................................... v.......................... 211 Cooling Water TYeatment................................................... 175 Barred Zone R ange............................................................. 212 Detuners and Dampers........................................................213 CH A PTER 8 : STARTING , REVERSING AND MANOEUVRING CH A PTER 10 : Start System..................................................................... 177 EN GINE OVERHAULS AND MAINTENANCE Start Air Perio d................................................................. 179 Unit Decarbomsation................................ 215 O verlap.............................................................................179 Cylinder Head R em oval................................................. 216 Start Air Receiver...........................:............;................ 180 Hydraulic Nut Removal..................................................... 217 Start Air Pilot V alve.............. 182 Exhaust Valve Rem oval...................................................... 218 Automatic Master Air Start Valve................................. 183 Piston Removal. Inspection and Clearances 220 Start Air Cylinder Valve..................................................... 185 Piston Mounting................................-............................... 223 Start Air Distributor...................................... 186 Liner Removal. Inspection and Calibration..................... 224 Start Air C a m....................................................................... 187 Main Bearing Removal..................... 225 [iv] M Marine Diesel Engines Marine Diesel Engines Crosshead Bearing R em oval..............................................227 L iner.................................. d lia u.J................................ 296 Connecting Rod Bearing Rem oval.................................... 228 Cylinder Lubrication............. 297 Crosshead Pin Removal...................................................... 229 P isto n................................................................................... 297 Connecting Rod Removal................................................... 230 Crosshead............................... _„...~.^...._J.i..L.................298 Thrust Bearing Pad Removal............................. 231 Engine Components......................................................... 298 Bearing Clearances...... 232 Fuel Pump Setting and A djustm ent...................................236 CH A PTER 13 : Fuel Pump Cut-out C hecks................................................. 238 EN GINE EMISSIONS Fuel Pump Cut-out............................................ 239 Engine Emissions..............................................................301 Fuel Pump L ead..............................------------------------- 239 SOx Effects and Remedy.................................................302 4-Stroke Medium Speed Engine Fuel Pump Timings 241 NOx Effects and Rem edy................................'.................302 CONTENTS Turbocharger Overhaul....................................................... 242 Carbon Monoxide, Hydrocarbons, Particle Emission.... 304 CONTENTS Turbocharger Out of Operation--------...-------- -------..... 243 S o o t..........................................................KiihillsU............305 Fuel Injector Overhaul.................................................... 244 Smoke and Opacity.................................:j.".......A............ 305 Tie-Rod Tensioning.............................................................246 Air Compressor Overhaul.................................................. 249 CH A PTER 1 4 : Testing of Materials........ 250 ENGINE PERFORM ANCE AND INDICATOR CARDS Heat T reatm ent............................ 250 Engine Performance Definitions and Parameters...........307 Heat Balance Diagram 310 CH A PTER 1 1 : Power Ratings...................................................................... 310 EN GINE DESCRIPTIO N S AND SPECIFICATIONS Testing of Marine Engines......................................... 311 Comparison of RD. RND and RTA Engines.................253 Test Bed and Sea T rials......................................................312 RTA Engin es........................................................................ 254 Load Diagram and Propeller C u rv e..................................314 RT-Flex Engines................................................................... 258 Safety Margins.................................................................... 316 SM C E ngines....................................................................... 271 Indicator Diagrams and Analysis.................................... 318 ME Engines.................................-................................. 278 Faults with Indicator Instruments...................................... 327 CH A PTER 12 : C H A PTER 15 : EN GIN E D EVELOPM ENTS GOVERNORS AND CONTROL Fuel Injection System............................... -......................291 Governor D efinitions................................ 329 Turbocharger System........... -...........................................292 Mechanical G overnor............................... 331 Scavenge System............................................. -.............. , 296 Hydraulic Governor with Compensation. 331 Exhaust System.................................... -............................296 Electric G overnor..................................... 333 Combustion Cham ber.......................................................... 296 Governor Adjustments............. -............. 334 [vi] [vii] Marine Diesel Engines Load Sharing and The Necessity o f D ro o p.....................335 Engine Turns on Air, Not on Fuel..................................... 362 Electronic Governor for Bridge C ontrol........................ 337 Engine Does Not F ire.......................................................362 Violent Starting...................................................................363 C H A PT E R 16 : Engine Not Reversing.......................................................364 W ATCHKEEPING AND SAFETY Cracked Piston................................................................. 364 Broken Piston Ring.................................................. 365 Thlcing Over An Engine Room W atch............................345 Cracked L in e r...............................................-.................. 365 Walk Through Checks of The Engine R oom................. 345 Piston Running Hot........................................................... 365 Checks During The Engine Room Watch 350 Cracked Cylinder H e a d................................................... 366 Problems During The Engine Room W atch................... 351 Crankcase Inspection........................................................366 Crankcase Explosion and Relief Valve............................ 351 Individual Piston Knocking at T O C...............................367 Scavenge F ires.................................................................... 353 Bearing Temperature Increase.........................................367 CONTENTS Oil Spill................................................................................354 Lube Oil Sump Level R ising...........................................368 Collision..............................................................................354 Automatic Stopping of E n g in e........................................ 368 Flooding.............................................................. ,............. 355 G rounding.......................„.v............................................... 355 Sudden O verspeeding........................................................ 355 z Knocking in an Engine C y linder.....................................368 Safeties in the Main E ngine......................... 369 Safeties in the Start Air S y stem....................................... 371 Loss of Engine Pow er....................................................... 356 Slack Tie-Rods................................................................... 356 Incorrect Fuel T imings........................................,............. 356 H Leaky Start Air V alves..........................-.........................372 Start Air Line E xplosion...................................................373 Engine Speed Fluctuation.................................................. 356 Z Safeguard Against O vetspeeding.................................373 Funnel S p a rk s..................................................................... 357 Cylinder Relief Valve L iftin g..........................................357 O Bibliography 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 [viii]  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: ♦ The wear and life span o f the rotating and reciprocating parts due Vc = Volumeofcompressionchamber Va = Volume o f the cylinder to friction; high temperatures and pressures; and lubrication conditions. Swept volume = Volume swept by the piston from TDC to BDC ♦ Large forces due to rotating and reciprocating masses, w hich in = Vs = (Area) x length = (fi.D 2 ) S turn give rise to stresses especially fluctuating stress; and moving 4 parts due to inertia forces and dynamic forces. Since, Va= Vc + Vs. ♦ Gas exchange-scavenge period and efficiency: Higher the mean piston speed, greater will be the resistance to gas flow and Hence, C om pression R atio = = Vc + Vs 1+Vis' exchange, when h ot exhaust gases have to be expelled and fresh Vc Vc Vc air has to b e taken in. M ean Piston Speed = (Piston distance in one revolution) Advantages o f D iesel E ngines over S team E ngines x (R ate o f crankshaft rotation) ♦ High actual efficiency = Heat equivalent o f actual work done = 2§_n Total Heat generated in the engine 60 = Sr ♦ Actual Efficiency, 30 for steam engines = 12 to 18% where, 2S = Distance covered by the piston during for steam turbines = 2 2 to 32% one revolution. for gas turbines = 2 5 to 36% N = N um ber o f revolutions per second. for diesel Engines = 36 to 42% ♦ High efficiency and recovery o f waste heat. 3  “ Marine Diesel Engines_____________________________________________ Internal Combustion Diesel Engines ♦ H ighest use o f heat generated during combustion. 4) Naturally A spirated o r Supercharged: In naturally aspirated ♦ Increased tim e period before refueling i.e. bunkering. engines, the piston itself sucks in air (e.g. 4-stroke engines) or is ♦ Increased maneuvering abilities. fed by a scavenge pum p (2-stroke engines). In supercharged engines, air under pressure is supplied to the cylinder which is ♦ Increased cargo carrying capacity since less space is required for pressurized externally by mechanical means o r an exhaustblower. the boiler, water storage, water consumption; and a smaller size o f engine in comparison to a steam plant and auxiliaries. 5) Compression Ignition (marine diesel engines) or Spark Ignition ♦ Increased standby reliability. (carburetor a nd gas engines): In compression ignition, the fuel ignites with the air due to high temperature caused by compression Disadvantages o f D iesel E ngines of air. In spark ignition, an external electric spark is used for ignition. ♦ High inertia loads due to reciprocating and rotating masses. 6) Trunk type engines (4-stroke engines) o r Crosshead engines ♦ High capital cost, complicated design and construction. (2-stroke engines): In trunk type engines, the piston h as an extended skirt which acts as a guide. In crosshead engines, there ♦ Pressures and temperatures are alw ays varying in the system. is a crosshead which has shoes sliding over the crosshead guides. ♦ High lube oil costs in medium and high speed engines. ♦ 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: 9) M ain Propulsion use (S hip’s propeller drive) o r A uxiliary 1) 2-stroke o r 4-stroke: Usually, 2-stroke is preferred for marine engine use (power generation & auxiliaries). engine propulsion while 4-stroke is preferred for auxiliary diesel 10) Low, M edium, a n d H igh Speed generation. Low speed (100 to 350 rpm) 2) Fuel used: Petroleum fuel ( gasoline, naphtha, kerosene, gas oil, M edium speed (350 to 750 rpm) diesel oil), heavy fuel ( m otor oil, b urner fuel), residual fuels, High speed (750 to 2500 rpm). gaseous fuels (natural or producer gas) and mixed fuel (liquid fuel 11) M ean Piston Speed fo r starting combustion and gaseous fuel for running). Low speed (4.5 m /s to 7 m/s) Medium speed (7 m /s to 10 m/s) 3) Single o r D ouble Acting: A single acting engine is one where the High speed (10 m /s to 15 m/s). upper part o f the cylinder is used for combustion. A double acting 12) Uni d irectional (sam e direction) or R eversible Engines engine is o ne w hich uses b o th the upper and low er part o f the using a reversing mechanism. cylinder alternatively, e.g. Opposed piston engines. 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 1-2 Air Compressed Isentropically Fig-2 3-4 Remaining Heat added at Constant Pressure 0-1 Charging of Fresh Air (o Point 1 1-2 Air Compressed Isentropically 4-5 Air Expanded Isentropically 5-1 Heat Rejected at Constant Volume 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 D iesel Cycle (C onstant P ressure) in the following: ♦ From 1 to 2, th e curve is i 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 1 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 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 ♦ From 3 to 4, during expansion stroke, there is heat transfer. 4-1 Heat Rejected at Constant Volume._____  Marine Diesel Engines ________________________ j Internal Combustion Diesel Engines ♦ From 4 to 1, heat is rejected w ith changes in m ass flow, specific ♦ The heat transfer at this stage is varying, since some o f the fuel still heat, low er pressures and temperatures. bums in the expansion stroke. Even greater heat losses are involved owing to the unused energy lost by the compressed h ot gases, ♦ In the actual cycle, there are unavoidable thermal, hydraulic and mechanical losses. when the exhaust ports are uncovered o r exhaust valve opens before the piston arrives. ♦ The air admitted into the cylinder thermally interacts with the hot cylinder liner and gases, and there is heat transfer. ♦ Action arising out o f reciprocating, rotating and robbing components also contribute to losses. ♦ A certain am ount o f work is required to be done to overcome the ♦ Some energy is used to drive auxiliaries (lube oil pumps, jacket resistance o f the inlet system through which the air is admitted. water, scavenge pumps, etc). ♦ T he amount o f filling o f air into the cylinder depends on its ♦ Cooling o f the liner is imperative to the cylinder, but this is also a temperature, speed and load o f the engine, engine construction and service conditions. source o f thermal loss. ♦ Adiabatic compression is compression when there is no heat transfer with the surroundings. Thisisnotpossibleintheactualcycle. Here, 2 -S troke Cycle there is heat transfer with the gases and the cylinder walls, which 2 S trokes = 2 strokes o f the piston results in a change in pressure and temperature o f the compressed = Piston going u p + Piston going down air.The area o f heat transfer is decreased as the piston moves = O nce compression and once expansion upwards to TDC. = 1 complete revolution gives 1 power stroke. ♦ The actual compression is a polytropic curve with a continuously As the nam e im plies, the cycle is completed in two strokes o f the varying exponent. 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. (1) The Compression (Scavenging and Suction) stroke ♦ The heatinput process is not ideal, since combustion o f fuel involves ' (2) The Power (Expansion and Exhaust) stroke. complicated physical and chemical changes with thermal losses in the final stage. These actual timings differ from engine to engine with respect to design ♦ Actual combustion overlaps the expansion stroke to some extent, and construction features such as stroke/bore ratio, engine rpm, engine due to the volume o f the cylinder space increasing. This leads to rating, ratio o f connecting rod length to crank length, etc. 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  Internal Combustion Diesel Engines Marine Diesel Engir, An example o f 2-stroke valve timings are: Inlet (scavenge) opens 42 deg. before BDC Inlet closes 42 deg. after BDC Exhaust opens 75 deg before BDC Exhaust closes 60 deg after BDC Injection starts 16 deg before TDC Injection ends 20 deg after TDC. Upstroke o f the Piston (Compression Stroke) \ F ig -6 0 Scavenge ports are open 0-1 A ir is sucked in, which pushes o ut the residual exhaust gases 1 Piston is at BDC 1-2 Completion o f scavenge process and filling with fresh air for combustion 2 Scavenge ports are closed 2-3 Post scavenging takes place 3 Exhaust valve closes 3- 4 Compression o f air 4 Fuel injection commences 5 Fuel ignition commences, near TDC. The scavenge and exhaust ports are uncovered and pressurized air is 6 Fuel injection and combustion completion fed into the cylinder. This fresh air does the scavenge process i.e. it 6- 7 Expansion o f the heat energy from combustion, cleans the cylinder o f the exhaust gases from the previous cycle. The being converted into work energy to push the piston downwards piston then travels upwards closing the exhaust and scavenge ports 7 Exhaust valve opens and starts compressing the air. A t the end o f the upward stroke, the 7-0 Blowdown o f exhaust gases seen as a sudden rapid pressure drop a ir p ressu re in th e cy lin d er builds up to 32 to 4 5 b ar and ontheP.V.diagram. correspondingly, it’s tem perature rises to 650 to 800 deg. C. 10  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 An example o f 4-stroke valve timing i s : W hen fuel is supplied by the injector to the hot com pressed air, it Inlet valve opens 20 deg. before TDC reaches its self ignition temperature and ignites. The combustion causes Inlet valve closes 60 deg. after BDC the expansion o f gases, which push the piston downwards towards Injection begins 10 deg. before TDC BDC. The piston being pushed downwards by the combustion gases Injection ends 12 deg. after TDC is doing work and hence, the stroke is called the Power or Expansion Exhaust opens 42 deg. before BDC stroke. The exhaust ports are uncovered at approximately 4 0 to 75 Exhaust closes 60 deg. after TDC. degrees o f crank shaft rotation, ju s t before BDC. T his allows the A 4-Stroke engine operating cycle is completed in 4-strokes o f 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 piston. These a re : 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 (1) Suction (induction) stroke scavenge ports are kept open for 100 to 140 deg. o f crank rotation. (2) Compression stroke 4-Stroke Cycle (3) Power (expansion) stroke 4 Strokes = 4 strokes o f the Piston (4) Exhaust stroke. = 2 (Piston going up + Piston going down) = 2 complete revolutions give 1 pow er stroke. 13 12 Marine Diesel Engines Internal Combustion 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 Fig-10 injection timing is around 10 to 35 degrees o f crank shaft rotation. F ig -ll 1 Exhaust value 9 Connecting Rod 2 Rocker Arm 10 Piston Optimum condition for fuel injection is when the fuel injection coincides 3 Camshaft timing gear 11 Cylinder Liner with the peak air temperature in the cylinder for best combustion. At 4 Camshaft 12 Cylinder Head the end o f combustion, the pressure in the cylinder is 60 to 80 bar, 5 Oil 13 Rocker Arm and 1600 to 2000 deg. C. 6 Crankcase 14 Inlet valve 7 Crankshaft 15 Fuel Injector (3) Expansion Stroke (Pow er Stroke) 8 Path o f crankpin In this stroke, work is done by the expansion of gases, to push die piston down to the crank The piston is m oving downwards and a pressure difference between pin through th e connecting rod, converting the cylinder pressure and the atm ospheric pressure is created above reciprocating linear motion o f the piston into it. Atm ospheric air is sucked inside through the open inlet valve. The a rotary motion o f the crank shaft, thereby i air adm ission is stopped w hen the inlet valve closes. The cylinder turning the engine shaft. After expansion, the ! pressure is now approximately 0.85 to 0.95 bar and the temperature pressure and temperature decrease to 3.5 to 37 to 48 deg. C. 5 bar, at 750 to 900 deg. C. (2) Com pression Stroke This stroke includes the compression o f air, mixing o f the fuel and air F i g - 12 charge, and the start o f combustion. T h e air in the cylinder is now 14 Marine Diesel Engines Internal Combustion Diesel Engines (4) E xhaust Stroke ♦ There is m ore turning o f the crankshaft, since two idle strokes of W hen the piston nears BDC, the exhaust valve the 4-stroke engine are n o t present in the 2-stroke engine. opens and the exhaust gases escape, since their ♦ High speed 2-stroke engines are less efficient due to less volumetric pressure is more than the atmospheric pressure efficiency. in the exhaust manifold. The exhaust gases are ♦ Fuel consumption is m ore in 2-stroke engines, since the engine expelled and the piston now starts moving works on the Otto Cycle principle. upw ards. T h e pressure o f the g ases now ♦ Unlike 4-stroke engines where there are two separate piston strokes decreases fu rth e r to 1.1 to 1.2 bar, at a for each o f these purposes, 2-stroke engines have much less time corresponding tem perature o f 430 to 530 available for exhausting and scavenging. Hence in 2-stroke engines, deg. C. 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 2-Stroke versus 4-Stroke Engines revolutions, while in the 4-stroke engine, only one power stroke takes place every two revolutions. ♦ The whole cycle ( suction, compression, expansion, and exhaust) ♦ 4-stroke trunk-piston engines have the advantage o f requiring less is completed in tw o strokes o f the piston in a 2-stroke engine, as headroom than 2-stroke crosshead engines. com pared to four strokes o f the piston in a 4-stroke engine. ♦ Torque produced by a 2-stroke engine is less irregular than a 4- ♦ A comparison should only be m ade between operating cycles o f a stroke engine, due to the number o f operating cycles in a 2-stroke 2-stroke engine and 4-stroke engine, having cylinders o f same engine being twice that in a 4-stroke engine. geometrical dimensions and crankshaft speeds. Theoretically, the ♦ The force applied to a piston o f a 2-stroke engine coincides with horsepower output o f a 2-stroke engine is twice that o f a 4-stroke the axis o f the connecting rod at all times and never changes its engine. In actual practice, the output o f a 2-stroke engine is 1.5 to I direction during the cycle.Therefore, dynamic loads coming on the 1.8 tim es o f a 4-stroke engine. This is due to the actual operating | piston crowns in a 2-stroke engine are avoided unlike in a 4-stroke cycle being only a fraction o f the total piston stroke, lasting between engine. TDC and the instant o f uncovering the exhaust ports. ♦ In m arine applications, 2-stroke engines are used in low speed ♦ At the start o f the compression stroke, there are higher pressures high-powered diesel main propulsion, while 4-stroke engines are and tem peratures in a 2-stroke engine than in a 4-stroke engine used in medium speed power generation. (higher by 25 to 30%). This increase results in a 30 to 40% ♦ In m odem engines for main propulsion, fuel costs require cheaper increase in the thermal load. Therefore, there are higher thermal | quality fuel to be used. This is possible in 2-stroke low-speed large stresses on the combustion chamber walls. 16 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. CHAPTER 2 The total cost o f the expensive lube oil for slow 2-stroke engines is less than 4-stroke engines o f equivalent power. 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 A m echanical top bracing consists o f shims 1 between two plates combustion forces through the main bearing. hydraulically fastened by a bolt 4. The bracing stiffening plates 2 are ♦ T he ‘A’ fram e which transmits the guide forces to the bed plate. thereby attached to a strong support 3. ♦ The top bracing units which dampen the lateral structural vibrations. E ngine Structure D efect Areas ♦ The cylinder block units which provide strength against transverse ♦ Below the main bearing due to bending stresses. flexing. ♦ A t any change o f sections, w here stress levels are concentrated ♦ The tie bolts which put the structure under compressive stress and e.g. crosshead guides and holding down sites. reduces the tendency to separate. ♦ 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: E ngine Structural Cracks ♦ Each ‘A’ fram e u n it: This also reduces the chances o f fretting at Cracks in the engine structure are usually caused by fatigue failure. bolted joints. Fatigue failures are discussed below. ♦ Rigid attachment to the stiffened tank top. Closely spaced framing Fatigue Failure o f 750 m m is the requirement for the double bottom construction. It is the failure o f the material which has undergone fluctuating stresses. ♦ Ranges attached to the top and bottom o f the longitudinal girder. Each fluctuation causes minute amounts o f plastic strain. Fatigue cracks ♦ Each cylinder block unit. 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, Top Bracing there is no distortion o f surrounding material. This is usually of mechanical or Factors A ffectin g F atigue L ife hydraulic type, fitted to the top ♦ Temperature: Increase in temperature lowers the endurance limit part o f the engine to provide o f the material (usually, the endurance limit = 108 cycles, i.e. 48% stiffening and support against o f UTS for steel). tw is tin g f o r c e s fro m th e ♦ M ean stress levels. crosshead guide. Normally, ♦ Combined tensile and shear stresses. these braces are fitted to only one side o f the engine e.g. the ♦ Cyclic stress frequency. exhaust side. Fig-14 20 21 Marine Diesel Engines Engine Components ♦ 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 Longitudinal girders, two in number, which1form the side walls and a set of transverse I-beams or box girders strengthened with stiffness. ♦ Cold cracks d ue to the presence o f dissolved hydrogen or high 2 Transverse strength girders housing the main bearings. residual stress in the joint or a small triggering defect 3 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 Fig -16 engine structure. Strength and stiffness are required for the bedplate M aterial fo r Bedplates to withstand the inertia loads o f moving parts, dead loads o f supported ♦ Cast Iron (C.I.) absorbs and dampens vibration. elements and forces from the firing cylinder gases. ♦ M ild Steel (M.S.) plates or castings welded together are cheaper and lighter. 22 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 Tie rods are bolts which keep the w hole engine structure under the figure. 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 A -F ra m e and, hence equal load on all bearings. As the nam e im plies, these fram es are ‘A’ in shape to provide support to the 1 Protecting Cap cylinder block. 2 Spherical Nut ‘A’- frames are usually produced as a 3 Spherical Washer single unit, as this helps in stiffening o f 4 Distance Pipe th e e n g in e. A w e ld e d ‘A ’-fra m e 5 Round Nut contributes to 40% o f the en g in e’s 6 Holding down Bolt structural stiffness. T he m aterial is Fig-19 fabricated steel plates. Slack H olding D own Bolts They cause fretting between the bedplate, chock and the tank top. Fig-18 M isalignment o f the bedplate w ill occur i f these slack bolts are 24 25  Engine Components Marine Diesel Engines retightened. Stiffness o f the holding down arrangements is decreased, Resin Chocks 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 Fig-21 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 These are commonly used with the advantage o f less manpower skill as an effect o f fretting and misalignment. There will be an imbalance of and time. They are very useful for re-chocking repairs on fretted and bearing loads. uneven foundation plates. Chocks 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 ♦ Class.approval is to b e sanctioned. chocks are fitted in line w ith the m ain bearings. End chocks two in ♦ Clean the work area o f the engine frame and tank tops o f dirt and oil. 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. ♦ All hull renewals and engine alignments should be complete. 27 26 Marine Diesel Engines Engine Components ♦ Dam s are prepared using a m etal sheet and putty sealant to hold engine specifications. The rubber element can take compression the chocking resin liquid. and also shear loads. They have in-built buffers to stop excessive ♦ N o heavy w ork during the cure period. Cure period is around 18 movements in heavy sea conditions as well as stopping and starting. to 36 hours, depending on ambient temperature. All m ounts are loaded to the sam e amount. The tolerance o f 2 ♦ A m bient temperature should be from 20 to 25 deg. C. mm is given for conical mounts. Using shims, one can further adjust these heights. ♦ Lim it fo r chock thickness is 25 mm, o r else u se m ore steps. ♦ Tighten the holding down bolts after the cure period is completed. Piston ♦ T he hardness o f the.chock is checked. Requirements ♦ To withstand the mechanical stresses o f combustion gas pressure Resilient Chocks and inertia forces. ♦ These are normally used in case o f medium speed engines (e.g. 4- ♦ To withstand the thermal stresses during combustion. stroke engines for power generation). Basically, they help to dampen the vibrations transmitted from the m edium speed engine to the Pistons are designed to ta ke into consideration the follow ing: tank top. ♦ 2-stroke main propulsion engines are heavy in weight and, therefore, ♦ The crown is directly exposed to heat and gas load and hence, has have high rotational and static masses causing higher out-of-balance a tendency to deform. Hence, the material should not only be forces w hich preclude the use o f resilient chocks, whose design thick for mechanical strength, but also thin enough to minimize would also have to take into consideration the heavy weight o f the thermal stress. engine. ♦ The cyclic loading causes the top and the sides o f the crown to flex which can lead to fatigue failure. ♦ 4-stroke engines for power generation plants are smaller and lighter ♦ The shape o f the combustion space also depends on the shape of in comparison. Therefore, they have lower out-of-balance forces, the crow n. Concave or convex pistons are used. whose natural frequency w ill be from 6 to 25 Hz fo r400 to 1500 ♦ Wall thickness can be reduced with strength provided for by internal rpm speeds. The natural frequency o f the engine can be changed, ribs o f radial or concentric designs. 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). ♦ The topmost ring undergoes the brunt o f the direct flame and it is much higher in position than the others. ♦ Resilient chocks consist o f a num ber o f flexible rubber vertical ♦ The m aterial o f the crow n should take into consideration the mounts used on under-slung engines. They have main mounts as working temperature, the hardness o f the ring groove landing areas, well as side and end mounts. Since these are flexible mounts, the the corrosiveness o f the gas mixtures and the cooling o f the piston. engine crank shaft center w ill m ove + /-1 m m and the top o f the ♦ A high top land helps in more effective lubrication and moving the engine approximately +/- 5 m m during start up, depending on the ring pack to a cooler zone. 28 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 2. JE T 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 GEEl OH F ig - 22 5 Self supporting bores Oil cooled pistons employ a spray nozzle plate. Cooling oil (common 6 Bore water cooled piston. 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 Flow o f P iston C ooling Oil nozzles which direct the cooling oil into the blind bores o f the crown at The flow is from the main bearing lube oil to the crosshead pin, then all crank angles. W hen the piston is atT D C , the ‘shaker’ cooling through slots in the piston rod. It then flows through the inlet oil pipe effect o f the oil takes place. W hen the piston is going towards BDC, in the piston rod which leads to the cooling bores through spray nozzles je t type cooling takes place. 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 A dvantages o f B ore Cooled Pistons o ver C onventional Pistons engine sump through internal drains; and temperature and flow alarms. ♦ Low er thermal stresses and strain. Piston M aterials ♦ Problems involved in casting o f internal ribs are eliminated. Crown - Aluminium or cast steel (4-stroke). ♦ Lower piston maximum temperature at the crown. Crown - C ast chrome nickel molybdenum alloy steel (2-stroke). ♦ Lower gas load stresses and better cooling efficiency. Skirt - Si-Aluminium alloy (4-stroke) or cast iron. Rod - Forged steel. 30 31 Marine Diesel Engines Engine Components Conventional Type Oros Type Gas side 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 Max.Temp. 209 deg. C 216 deg. C. 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. 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. ‘OROS’ Piston ♦ Better resistance to scuffing. A new design employed by M AN B&W, which has the advantage of ♦ Better expansion cold clearances. reduction in tem perature and h eat load at the piston crown. The ♦ Better thickness since density is relatively lower. following is a table o f temperatures o f the piston at 100% load. ♦ Better skirt stiffness. 32 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 ♦ Cracks on the internal or external surfaces due to built up thermal or nodular C.I.) o r mechanical stresses. The reasons for these stresses are poor 3 Bearing (Lead bronze) injection, bad fuel quality, poor cooling due to insufficient coolant 4 Gudgeon pin (Carburised steel) or fouled cooling spaces, corroded material, poor lubrication, and 5 Keep plate bad operations like an overloaded engine. 6 Connecting rod (Forged steel). ♦ 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 ♦ Cooling spaces deterioration due to corrosion; coking o r scale build up caused by poor cooling water treatment; or low oil coolant 2-Stroke Pistons 4-Stroke Pistons flow o r overheating. ♦ Fretting due to incorrecttensioning and assembly o f studs; damaged (1) It is of crosshead type i.e. piston It is of trunk type i.e. the skirt rod connected to the crosshead (no piston rod) is connected to studs; o r overheating. bearing both reciprocate along the the connecting rod by means of axis of the piston. a gudgeon pin and bearing. R otating Pistons (2) The crosshead slipper transmits the Trunk or ‘extension’ piece or These pistons are employed for medium speed 4-stroke engines. An connecting rod angularity thrust to extended ‘skirt’ takes the connecting the crosshead guides. rod angularity thrust and transmits example is the Sulzer Z40 series. Rotation of the piston is accomplished it to the side of the cylinder liner. by using a spring loaded paw l and ratchet. It has the disadvantage of (3) More height for same power and Less height for same power and a high initial cost. It has the advantages o f lower specific bearing loads; speed. speed. 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 (4) Higher engine manufacturing costs. Lower engine manufacturing costs. due to lower piston slap; lower cavitation erosion; lower heat variation; (5) It has compression type piston rings. It has compression as well as oil scraper more uniformity and distri