26585218-notes-on-motor-03-oct-09

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Competency 6.1

6.1(1) Theoretical knowledge of construction & operation of MainMachinery?

General Construction of Engines

The diesel engine is now understood to be any reciprocating engine using fueloil and working on the compression ignition system. The marine diesel consistsessentially of a number of cylinders each fitted with water cooled liner in each ofwhich reciprocates a water or oil-cooled piston (or pistons). The piston (or pistons)transmit, through piston rods, guides and connecting rods, power to a crankshaftcrankpin. This rotates the crankshaft and so converts reciprocating motion into therotary motion needed to rotate a ships propeller. Many differences in design andconstruction exist in the main diesel engines in ships, but all work on the samefundamental principles.

Speed high speed, medium speed, slow speed

Usage automotive engines, locomotive engines, marine enginesOperation 2-stroke, 4-stroke, single acting, double actingCylinder arrangement horizontal, vertical, vee, radial

In the majority of British ships the main diesel engines are of the two-strokeopposed piston type and are un-supercharged. But the majority of present daymarine diesel engines are supercharged. The diesel engine compression ignitionsystem used for airless injection consists of compressing air in the cylinder liner to apressure which will give the air a temperature which will ignite the injected atomizedfuel oil. In marine engines this pressure must be such as to give consistent ignition atall working revolutions of the engine and under varying atmospheric conditions. To

accomplish compression ignition the working cycle of an engine may be two-stroke or four-stroke.

2 STROKE

In the two-stroke working cycle, the engine sucks in air, then compresses it,fuel is injected, combustion takes place, the gases expand and are exhausted all inone revolution of the crankshaft; that is, as the term two-stroke implies, during oneupward plus one downward stroke of the piston. The four-stroke cycle requires forthe same gas cycle two revolutions of the engine, that is, two upward strokes plustwo downward strokes of the piston.

Briefly, the different operations of the two-stroke cycle are completed in

sequence at about one third of each revolution. When the piston of a single-actingengine is at the bottom or end of its stroke both the exhaust and the air inlet areopen. At about one third of the upward stroke of the piston the exhaust and air inletsclose and as the piston moves upwards the air is compressed. Just before the top orend of the upward stroke of the piston the fuel injection begins. The piston continuesto travel to the end of its stroke during which the fuel injection continues andcombustion takes place, thus increasing the pressure in the cylinder. A few degreesafter top dead centre fuel injection ceases. During the downward stroke of the piston

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the gases expand and power is transmitted to the crankpin. At about two thirds ofthe downward stroke the exhaust opens and the gases are exhausted to theatmosphere. Continuing on the downward stroke, the piston uncovers the air inletthrough which air is admitted to the cylinder. After reaching the end of thedownward stroke the piston commences its upward stroke and the cycle is repeated.

4 STROKE

The different operations of the four-stroke cycle start with the piston of asingle-acting engine at the end of its upward stroke just about to commence itsdownward stroke. At this point the exhaust valve in the cylinder cover is not quiteclosed but the air inlet valve is open. As the piston moves down the exhaust valvecloses at about one third the downward stroke, but the air inlet valve deeps open andonly closes at the end of the stroke. During the upward stroke the air is compressedand near the end of this stroke fuel is injected. Combustion takes place andcontinues until the piston reaches the end of the upward stroke and for a shortdistance on the second downward stroke. As the piston continues on its second

downward stroke the gases expand until near the bottom of the stroke when theexhaust valve opens. The gases continue to be exhausted during the second upwardstroke of the piston and on reaching the top of the stroke the cycle is repeated.

Three principal types of machinery installation are to be found at sea today.The three layouts involve the use of direct coupled slow-speed diesel engines,medium-speed diesels with a gear box, and the steam turbine with a gear box drive tothe propeller.

A propeller, in order to operate efficiently, must rotate at a relatively lowspeed. Thus, regardless of the rotational speed of the prime mover, the propellershaft must rotate at about 80-100 rev/min. The slow speed diesel engine rotates at

this low speed and the crankshaft is thus directly coupled to the propeller shafting.The medium speed diesel engine operates in the range 250-750 rev/min and cannottherefore be directly coupled to the propeller shaft. A gear box is used to provide alow-speed drive for the propeller shaft. The steam turbine rotates at a very highspeed, in the order of 6000 rev/min. Again, a gearbox must be used to provide a low-speed drive for the propeller shaft.

Operation and Maintenance

The responsible of the marine engineer are rarely confined to the machineryspace. Usually all shipboard machinery, with the exception of radio equipment, ismaintained by the marine engineer. A broad based theoretical and practical trainingis therefore necessary for a marine engineer. He must be a mechanical, electrical,air conditioning, ventilation and refrigeration engineer, as the need arises. Unlike hisshore based opposite number in these occupation, he must also deal with thespecialized requirements of a floating platform in a most corrosive environment,Furthermore he must be self sufficient and capable of getting the job done with thefacilities at his disposal.

The modern ship is a complex collection of self-sustaining machinery providingthe facilities to support a small community for a considerable period of time. To



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simplify the understanding of all this equipment is the purpose of this book. Thisequipment is dealt with either as a complete system comprising small items orindividual larger items. In the latter case, especially, the choices are oftenconsiderable. Knowledge of machinery and equipment operation provides the basisfor effective maintenance, and the two are considered in turn in the following

chapters.6.1(2) Define a) Crosshead type diesel engines b) Trunk type diesel enginesc) Slow speed diesel engines d) Medium speed diesel engines e) High speeddiesel engines

There are two basic types of connecting a piston to a crankshaft;

(a) Crosshead construction-used by all slow speed two stroke enginemanufactures(b) Trunk piston construction- used in smaller four stroke engines

(a) Crosshead type diesel engines

The main difference between crosshead and trunk-piston type engines is themanner in which the transverse thrust from the piston and connecting-rod is taken upand the nature of the bearing assembly at the upper part of the connecting-rod.Crosshead engines have a piston-rod and trunk-piston engines do not.

The working parts of a crosshead engine consist of a piston head and rod,connected together. The crosshead block, pins and slippers form an assembly whichis attached to the lower part of the piston-rod. The slippers slide up and down withthe crosshead assembly in the engine guides. The crosshead assembly is connected tothe crankshaft through the crosshead bearings (top end bearings) and the connecting

rod bearing (big or bottom-end bearing). When the crank moves away from the topand bottom dead centre positions the connecting rod is at an angle to the line ofpiston stroke and, consequently, there is angularity. The down ward force exerted bythe piston together with the upward reaction from the connecting rod cause atransverse thrust to be set up (this can be shown with a triangle of forces). Thistransverse thrust is transmitted by the guide slippers on to the engine or cylinderguides. The transverse thrust is referred to as guide load.

There are fewer parts in trunk-piston engines. The working parts consist of thepiston, piston trunk, gudgeon bearing assembly and connecting-rod. The transversethrust or guide load is transmitted by the piston trunk or skirt on to the cylinder. Thefunction of the crosshead and piston trunk is to play a part in the conversion of the

reciprocating movement of the piston to the rotary motion of the crankshaft. Theyalso transmit the transverse loads, on to the fixed parts of the engine designed totake these loads.

Note: The guide load comprises the resultant of the piston-rod and connecting-rodloads caused by the cylinder pressures (static load) and the dynamic loads caused byinertia of the moving parts.

Advantages



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Crosshead type engines are able to develop much higher power at lowerrotational speeds than trunk-piston type engines, because the space available for thegudgeon bearing assembly. Trunk-piston type engines have the advantage ofrequiring less head room than crosshead engines. Their working parts are fewer innumber and much less costly to produce because their design lends itself to mass

production methods. The gudgeon bearing assembly is not particularly suited forhighly rated two-stroke engines unless special arrangements are made for itslubrication. Cheaper quality fuels may be used in crosshead engines as it is possibleto isolate the cylinder space from the crankcase, thus preventing acidic residuesentering the crankcase. The total cost for lubricants is less with crosshead enginesthan with trunk-piston engines of equivalent power.

(b) Trunk piston construction

The piston is directly attached to the connecting rod by a small end rotatingbearing. Side thrust is absorbed by extended skirts on piston. The main advantage isreduced engine height.

Opposed piston enginesMainly built by Doxford and consisted of two opposing piston moving in a

common liner. Fuel injection occurred at the centre where the piston met.Construction is of the crosshead design with the upper piston connected to thecrankshaft via two side rods and transverse beam. Timing was approximately 180 oCexcept for a small angle of advance for exhaust timing.Advantages are as under:-

(i) Perfect primary balance by balancing

(aa) upper reciprocating masses and lower velocity side cranks against

(ab) lower reciprocating mass and the higher velocity centre crank

(i) No gas loading transverse to bed plate (normally via head and tie rods)on engine meaning that construction could be lighter

(c) Slow speed engine

Cylinder cut-out system. In the case of low loads, the traditional problem isfouling of the engine due to irregular injection and atomisation, leading toincomplete combustion. The irregular injection may be caused by jiggling of thegovernor, and/or play in the connections in the fuel pump rack control system. Theeffect in either case is that the fuel pumps, when operating so close to the minimuminjection amount, may sometimes just have enough index to inject fuel, at othertimes just not enough index to do so.

By the introduction of a system where approximately half of the cylinders arecut out at low speed, the injection into the remaining working cylinders is improvedconsiderably, giving more stable combustion and, consequently, stable running andkeeping particle emission in the low speed range at a minimum.To avoid that excessive amounts of cylinder lubricating oil are collected in cylindersthat are temporarily deactivated, the cutting out is made by turns between twogroups of cylinders in order to burn surplus lubricating oil and keep the same thermal



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(f) The initial cost of a geared installation may be 30 % less than that of theconventional two-stroke crosshead engine. The spare parts are not onlycheaper, but also easier to handle store or transported.

(g) As the engines are worked on clean distillate or light blended heavy oil,

the rate of wear of pistons, rings and liners are considerably less.While these are the advantages claimed, there are disadvantages which

would tend to limit the use of these engines in ship for main propulsionpurpose. The disadvantages are:-

(a)High lubricating oil consumption (1.2 gm per b.h.p hour as against 0.5gm per b.h.p hour in crosshead engines)

(b) Short service life of exhaust valves.(c) More maintenance work(d)High load on bearings needing more frequent attention and

replacement.

(e)Very high noise level.

Comparison of different speed engines

Slow speed(65/70-150 rpm)Advantages

(a) No gearing as engine can be directly coupled to an efficientpropeller.

(b) Can use 2 cycle as acceptable time is available for scavenging.

(c) Will operate on poor quality fuel (H.V.F. upto 3500 s)(d) Crankcase can be separated from the combustion oneusing a diaphragm around the piston rod. This reduces oilcontamination, lessens the risk of crankcase explosions, allowsunder piston augmenting of scavenge etc.

(e) Vibration frequency low producing less noise and fatigue.

(f) Parts are large with heavy scantlings hence reducing stress.

Disadvantages

(a) Size- The engine is very heavy and demands a largeheadroom to allow dismantling (withdrawal of piston androd)

(b) All engine parts are large and heavy demand liftingequipment for handling



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(c) The engine must be built ashore and then re-assembled inship. This may give problems due to poor alignment as theinitial fitting is carried out on a very rigid base.

6.1(3) Define a two stroke and a four stroke diesel engine and their

scavenging process. Also differentiate between: a) loop scavenging b)Uniflow scavenging.

Two Stroke engine

It is one in which the operating cycle is completed in two piston strokescorresponding to one crank shaft revolution. In two stroke engine, the piston issolidly connected to a piston rod which is attached to a crosshead bearing at theother end. The top end of the connecting rod is also joined to the crosshead bearing.Ports are arranged in the cylinder liner for air inlet and a valve in the cylinder headenables the release of exhaust gases. The incoming air is pressurized by a turbo-blower which is driven by the outgoing exhaust gases. The crank shaft is supported

within the engine bedplate by the main bearings. A-frames are mounted on thebedplate and house guides in which the crosshead travels up and down. Theentablature is mounted above the frames and is made up of the cylinders, cylinderheads and scavenges trunking.

Four Stroke engine

A four stroke engine is one in which the operating cycle iscompleted in four strokes of the piston. Depending on thedirection of piston travel and process taking place in the cylinderduring his travel, the strokes are referred to as suction(induction), compression, working (expansion) and exhauststrokes. The engine is made up of a piston which moves up anddown in cylinder which is covered at the top by a cylinder head.The fuel injector, though which fuel enters the cylinder, islocated in the cylinder head. The inlet and exhaust valves arealso housed in the cylinder head and held shut by springs. Thepiston is joined to the connecting rod by a gudgeon pin. thebottom end or big end of the connecting rod is joined to thecrankpin which forms part of the crankshaft. With this assemblythe linear up and down movement of the piston is converted intorotary movement of the crankshaft. The crankshaft is arrangedto drive through gears the camshaft, which either directly orthrough pushrods operates rocker arms which open the inlet andexhaust valves, The camshaft is timed to open the valves atthe correct point in the cycle. The crankshaft is surrounded bythe crankcase and the engine framework which supports the cylinders and houses thecrankshaft bearings. The cylinder and cylinder head are arranged with water-coolingpassages around them.

Comparison of two-stroke and four-stroke cycles



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The main difference between the two cycles is the power developed. The two-stroke cycle engine, with one working or power stroke every revolution, will,theoretically, develop twice the power of a four-stroke engine of the same sweptvolume. Inefficient scavenging however and other losses, reduce the poweradvantage to about 1.8 for a particular engine power the two-stroke engine will be

considerably lighter-an important consideration for ships. Nor does the tow-strokeengine require the complicated valve operating mechanism of the four-stroke. Thefour stroke engine however can operate efficiently at high speeds which offset itspower disadvantages; it also consumes less lubricating oil.


S No Two stroke engine Four stroke engine1. One revolution is one working

cycle and power producedTwo revolution is one working cycleand power produced.

2. Twice the power of a 4 strokeengine of the same swept volume

50% power of two stroke engine

3. Insufficient scavenging. Good scavenging

4. Engine is lighter & simple in design Heavier & complicated5. No complicated valve operating

mechanism.Valve operation.

6. Slow speed engine (115 RPM) Medium speed engine (720 RPM)7. No reduction gear to propeller

shaftReduction gear required

8. Fewer parts than 4 stroke More parts9. Mechanical efficiency is high

because no moving parts like Camfollowers, R/A, valves etc.,

Mechanical efficiency lower.

10. Easily reversible. No reversing mechanism

11. Excess lubrication Less lubrication

12. Uniform torque Hence lighter flywheel

Heavier fly wheel.

13. Used for Main Engine Used for Alternators and some timesfor M.E using gear box.

14. There is Cross Head/Piston Rod There is no Cross Head/Piston Rod15. Diaphragm divides combustion

space and crank case.There is no Diaphragm. Hencecombustion space and crank case areone unit.

16. Installation Dismantled from yardand shifted to the new vessel

Completely assembled enginedelivered to the vessel

17. More Head room and heavier liftingarrangement required.

Less Head room and lighter liftingar5rangement required

18. Less vibration More vibration due to high speed`19

Cumbersome maintenance due toheavier parts

Easy maintenance

20

Less noisy More noisy


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Scavenging

A basic part of the cycle of an internalcombustion engine is the supply of fresh air andremoval of exhaust gases. This is the gas exchangeprocess. Scavenging is the removal of exhaust gases

by blowing in fresh air.

Types of scavenging

The methods of charging have differed from oneengine design to another in slow speed diesel engines.The systems employed may be generally into two maingroups.

(a) Uniflow and

(b) Reverse flow.

Uniflow scavenging - In the uniflow system the scavenging air enters the cylinderform one end and leaves through the other. Air flows in streams with slight inducedrotational motion. Thecharge is not allowed tochange direction and henceintermixing is minimum.The scavenge efficiency isthe highest. The system isparticularly suitable in slowspeed engines with longstroke and large area of

escape for exhaust gases.It is achieved;

(i) by two pistons workingin one cylinder as inopposed piston engine. Inthis system the top pistoncontrols the exhaust portsand the bottom piston controls the inlet ports.

(ii) by a poppet valve arranged at the cylinder cover which provides a largeinstantaneous opening for exhaust gases to escape with sufficient rapidity so that thedesired pressure drop in the cylinder is created without turbulence at exhaust.

(iii) by an exhaust piston controlling the exhaust ports, while the air inlet ports arecovered and uncovered by the power piston



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Reversed flow scavenging The engines employinga reversed flow system of scavenging arestructurally simpler. Depending on the relativepositions of exhaust and air ports, the reversedflow systems are again divided into:

(i) Full loop scavenging with exhaust on topof air ports at the same side of engine. Themethod of loop scavenging is similar to thecross flow except the exhaust and scavenge

ports may be found on the same side.

(ii) Cross scavenging with scavenge ports facing the exhaust ports.

The principal advantages of areversed flow scavenge system lies inits simplicity. There are however anumber of disadvantages which are

listed below:(a) There is a greater possibility

of intermixing between thecharge air with the exhaustgases. As a result the purity

of charge is less and charge temperature is higher.

(b) A sharp difference of temperature exists within a small area aroundscavenge and exhaust ports. Consequently the possibility of thermal cracksappearing at the bars and the chance of thermal distortion of the liner aregreater.

(c) The exhaust back pressure may rise due to narrowing of exhaust passage bydeposit of unburnt carbon. The scavenging of the cylinder will be adverselyaffected.

(d)The piston rings will wear out unevenly resulting in their being leakyearlier.



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6.1(4) Explain reasons for supercharging and indicate typical superchargingpressures.

Another method of scavenging is by a turbo-charger (or turbo-blower).This is a turbine-compressor; the turbine uses energy from the engine exhaust

gases and drives an air compressor which supplies air to the engine forscavenging and supercharging. The main structure of an exhaust gas turbo-charger may consist of four separate casings which can be bolted together invarious positions to suit different arrangements of engine ducting. Exhaustgases from the diesel engine enter the single stage gas turbine though thewater cooled cast iron inlet casing, expand in the nozzles thereby gaining invelocity, and pass through the turbine blades whilst driving the turbine rotor.The exhaust gases leave the turbine though the water cooled outlet casing andflow to the atmosphere, in some cases via a waste heat boiler. Combustion airfor the diesel engine enters the centrifugal air compressor through a silencer-filter. The air is compressed and delivered from the compressor to an aircooler and then to the engine cylinders.

In some engines a scavenge pump unit is formed by sealing the underpiston space from the crankcase and fitting appropriate valves. This unit isusually incorporated in series with a turbo-charger as a booster units, is usefulfor starting and slow running conditions when insufficient air is being deliveredby the turbo-charger.

Sometimes, electrically driven blowers are fitted for starting and slow runningof the engine then under full power conditions the air supply from the turbo-charger outstrips delivery from the electric blower which causes the latter to

be automatically shut off.

Supercharging

Supercharging is a process of charging air into the engine cylinder used toindicate that the weight of air supplied to the engine is increased, to burn more fuelto get more out put power of the engine.

At the beginning of compression stroke, the cylinder is full of air atatmospheric pressure. If the pressure at this point is increased aboveatmospheric pressure is called pressure charged of super charged.

Amount of air in naturally aspirated engines breath limited by area of inlet

passage & attainable air velocity.

Power developed by diesel engines depends on :

- Quantity of air in engine breadths per unit time.

- Proportion of air utilized

- Thermodynamic efficiency of cycle.



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Two methods increase quantity of air rotational speed of engine & increasingair density at intake

Supercharging increases Mean Indicated Pressure (MIP)

Improves scavenging and combustion and lower exhaust temperature.

Improves pressure a temperature of inlet air /scavenge air.

How are modern two-stroke and four stroke engines pressure charged?

Modern diesel engines are pressure-charged by utilizing the energy in theexhaust gases to drive a gas turbine connected to a rotary blower. The blowercompresses air so that it is delivered under pressure to the engine cylinder. Becausethe air is under pressure, a greater mass can be contained in the cylinder and so morefuel can be burnt per stroke, which increases the power developed. Engines pressurecharged in this way with exhaust gas turbo driven blowers as often referred to as

turbo-charged engines.Super-charging or Pressurecharging

When the piston of a normal oil engine is beginning the compression stroke, thecylinder should be full of air at atmospheric pressure. If means are adopted to causethe pressure at this p9oint of the cycle to be greater than that of the atmosphere theengine is said to be super charged or pressure charged. The two terms aresynonymous.

Turbo-charger

A modern exhaust gas turbo-blower is essentially a single stage impulse turbine

connected through a commonshaft to a centrifugal type airblower. The turbine and blowerare housed in a circular casingdivided into two separate spacesby a circular division plate, whichmay be watercooled orprotected heat insulation on theexhaust gas side. The section ofthe casing which houses theturbines is fitted with one or more

flanged exhaust-gas inlets, whichlead to nozzle-blade ringassembly. The exhaust gases passthrough this ring and air isdirected on to the turbine rotorblading. The gases enter themoving blades the turbine rotor at

high velocity. The passage of the gas through the rotor blades causes a change of



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direction in the gas flow, resulting in a change of momentum, which exerts a force onthe turbine blades. This force causes the rotor to revolve at high speed. The exhaustpasses from the rotor into a circular space connected to the exhaust gas outletbranch.

The air-blower casing is fitted

with filters at the air inlet to thecasing. The entry passages afterthese filters are usually fitted withsplitters to guide the air through thepassages and reduce the draughtlosses caused by a change in the air-flow direction. Sound absorbentmaterial is used to cover the insideof the air passages and the splitters,to reduce wind and blower noise.

6.1(5) Explain what areindicator diagrams and how following can be read or calculated from it: a)Peak pressure b) Compression pressure c) indicated horse power

Power Measurement

There are two possible measurement s of engine power; the indicatedpower and the shaft power. The indicated power is the power developedwithin the engine cylinder and can bemeasured by an engine indicator. The shaft

power is the power available at the outputshaft of the engine and can be measuredusing a torsionmeter or with a brake.

The engine indicator

An engine indicator is shown in Fig. Itis made up of a small piston of known sizewhich operates in a cylinder against aspecially calibrated spring. A magnifyinglinkage transfers the piston movement to adrum on which is mounted a piece of paperor card. The drum oscillates (movesbackwards and forwards) under the pull of acord. The cord is moved by a reciprocating(up and down) mechanism which isproportional to the engine piston movement in the cylinder. The stylus drawsout an indicator diagram which represents the gas pressure on the enginepiston at different points of the stroke, and the area of the indicaor diagram



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produced represents the power developed in the particular cylinder. Thepower can be measured knowing the sealing factors, spring calibration andsome basic engine details.

Torsionmeter

If the torque transmitted by a shaft is known, together with the angularvelocity, then the power can be measured, i.e.

shaft power = torque x angular velocity

The torque on a shaft can be found by measuring the shear stress or angle oftwist with a torsionmeter.

(a) Peak pressure

Combustion chamber pressure curve

Pressures and temperatures areshown on the sketches whereappropriate. The draw card is anextended scale picture of thecombustion process. In early marinepractice the indicator card was drawnby hand-hence the name. In modernpractice an out of phase (90degrees) cam could be provided

adjacent to the general indicatorcam. Incorrect combustion detailsshow readily on the draw card. Thereis not real marked differencebetween the diagrams for 2-stroke

and 4-stroke. In general the compression point on the draw card is more difficult todetect on the 2-stroke as the line is fairly continuous. There is no induction exhaustloop for the 4-stroke as the spring used in the indicator is too strong to discriminateon a pressure difference of say 1/3 bar only.

(b) Compression pressure

Compression curves

Compression diagram isalso given in the fig with thefuel shut off expansion andcompression should appear asone line. Errors would be due



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to a time lag in the drive or a faulty indicator cam setting or relative phase differencebetween camshaft and crankshaft. Normally such diagrams would only be necessaryon initial engine trials unless loss of compression or cam shaft on the engine wassuspected.



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(c) Brake horse powerThe brake horse power is calculated from the torque and the rev/min. The

brake men effective pressure (Pb)is obtained by analogy with the indicted horsepower

formula:

Pb = B.H.P x 3300 in imperial unitsL x A x N x nc

Where

Pb is in lb/in2

L is in ftA is in in2

N is rev / min

Pb = B.H.P x 4500 in metric units

L x A x N x ncWhere

Pb is in Kg/cm2

B.H.P is 10 metric horse powerL is in mA is in cm2

N is rev / min

Pb = Brake Power in S.I unitsL x A x N x ncx100

Where

Pb is in barsBrake powers is in kWL is in mA is in m2

N is rev / sec

In general the determination of brake horsepower is less open to errorthan is the determination of indicated horsepower. Brake mean effectivepressure is therefore commonly used as a basic measurement of load when

comparing the performance of engines.Brakespecific fuel consumption (B.S.F.C) is obtained by dividing the fuel

used in unit time by the b.h.p and the brake thermal efficiency is given by

100x3300 per cent, imperial unitsBSFCxCVxJ

or



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100x4500 per cent, metric unitsBSFCxCVxJ

or

3.6x106 per cent, S.I units

BSFCxCVxJ

Two stroke engine power diagrams

Two stroke cycle power card

Bottom deadcentre

1. scavenge portclosed

2. exhaust portshut-commenceof compression

3. fuel injection4. top dead centre

5. 7postcombustionexpansion

6. exhaust portopens

6.1(6) Describe a fuel injector and reasons for using high pressure fuel

FuelInjection

The function of the fuel injection system is to providethe right amount of fuel at the right moment and in suitablecondition for the combustion process. There must therefore besome form of measured fuel supply, a means of timing thedelivery and the atomization of the camshaft. This camshaftrotates at engine speed for a two-stroke engine and at halfengine speed for a four-stroke. There are two basic systems



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hydraulic operations. The most common system is the jerk pump: the other is thecommon rail.

The FuelInjector

A typical fuel injector is shown in Fig. It can be seen to be two

basic parts, the nozzle and the nozzle holder or body. The highpressure fuel enters and travels down a passage in the body and theninto a passage in the nozzle, ending finally in a chamber surroundingthe needle valve. The needle valve is held closed on a mitred seat byan intermediate spindle and a spring in the injector body. The springpressure, and hence the injector opening pressure, can be set by acompression nut which acts on the spring. The nozzle and injectorbody are manufactured as a matching pair and are accurately groundto give a good oil seal. The two are joined by a nozzle nut.

The needle valve will open when the fuel pressure acting on theneedle valve tapered face exerts a sufficient force to overcome the

spring compression. The fuel then flows into alower chamber and is forced out through a series

of tiny holes. The small holes are sixed and arranged to atomize,or break into tiny drops, all of the fuel oil, which will then readilyburn. Once the injector pump or timing valve cuts off the highpressure fuel supply the needle valve will shut quickly under thespring compression force.

A priming or air venting arrangement is fitted to the fuelsupply passage. All injectors should be primed before starting anengine after any period of idleness. Fuel injectors on large slow

speed diesels are arranged with internal passages which arecirculated with cooling water.

Reasons for using high pressure fuel

Allslow speed two-stroke engines and many medium speed four-stroke enginesare now operated almost continuously on heavy fuel. A fuel circulating system istherefore necessary and this is usually arranged within the fuel injector. Duringinjection the high-pressure fuel will open the circulation valve for injection to takeplace. When the engine is stopped, the fuel booster pump supplies fuel which the

circulation valve directs around the injectorbody. Before fuel can be injected into a

cylinder the pressure must rise to the point atwhich the fuel valve lifts. The pressurerequired will depend on various factors, butwill be between 245 and 445 bar. In some ofthe latest generation of engines, the fuel injection pressures may go up to about 1000



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bar; in some medium speed engines using low-cost fuels, higher injection pressuresgoing up to about 1250 bar may be used.

Jerk pump system

In the jerk pump system of fuel injection a separate injector pump exists foreach cylinder. The injector pump is usually operated once every cycle by a cam onthe camshaft. The barrel and plunger of the injector pump are dimensioned to suitthe engine fuel requirements. Ports in the barrel and slots in the plunger oradjustable spill valves serve to regulate the fuel delivery (a more detailed explanationfollows). Each injector pump supplies the injector or injectors for one cylinder. Theneedle valve in the injector will lift at a pre-set pressure which ensures that the fuelwill atomize once it enters the cylinder.

Common rail system

The common rail system has one high pressure multiple plunger fuel pump.

The fuel is discharged into a manifold or rail which is maintained at high pressure.From this common rail fuel is supplied to all the injectors in the various cylinders.Between the rail and the injector or injectors for a particular cylinder is a timingvalve which determines the timing and extent of fuel delivery. Spill valves areconnected to the manifold or rail to release excess pressure and accumulator bottleswhich dampen out pump pressure pulses. The injectors in a common rail system areoften referred to as fuel valves.

Injector pump

The injector pump is operated by a cam which drives the plunger up and down.The timing of the injection can be altered by

raising or lowering the pump plunger inrelation to the cam. The pump has aconstant stroke and the amount of fueldelivered is regulated by rotating the pumpplunger which has a specially arranged helicalgroove cut into it.

The fuel is supplied to the pumpthrough ports or openings at B (fig). As theplunger moves down, fuel enters the cylinder.As the plunger moves up, the ports at B areclosed and the fuel is pressurized and

delivered to the injector nozzle at very highpressure. When the edge of the helix at Cuncovers the spill port D pressure is lost andfuel delivery to the injector stops. A non-return valve on the delivery side of thepump closes to stop fuel oil returning from the injector. Fuel will again be drawn inon the plunger down stroke and the process will be repeated.



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The plunger may be rotated in the cylinder by a rack and pinion arrangementon a sleeve which is keyed to the plunger. This will move the edge C up or down toreduce or increase the mount of fuel pumped into the cylinder. The rack is connectedto the throttle control or governor of the engine. This type of pump, with minorvariations, is used on many diesel engines.

Sulzer Type Fuel pump (Variable rare injection)

The Sulzer differs from the Bosch scroll pump in that it operates with a plainplunger, timing being effected by operation of valves. The cam, which is driven viagears by the crank shaft forces the plunger up the barrel thereby delivering fuel to

the injectors during the period that both suction valve and discharge valve is shut.The eccentric cam which alters the timing of spill is rotated via the fuel rack drivenfrom the governor. The eccentric cam altering the opening and closing of the suctionport, may be altered manually or driven off an engine management system to changethe beginning of injection.

COMBUSTION:

Combustion process of the fuel takes place in three distinct phases.

First phase of combustion Ignition delay period is the time-span betweencommencement of injection and the start of ignition.

The fuel emerges into the cylinder as small liquid particles, which aresurrounded by not compressed air. They receive heat from the air andmore volatile constituents of the fuel vaporize.

During the ignition delay period a large part of the fuel charge isprepared for combustion.



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During the ignition delay the injector continued to inject fuel and, if thishas build up a sufficient quantity, the rapid combustion and pressure risewill be quite violent, causing detonation and shock loading creating anoise termed diesel nock.

First phase of combustion Rapid / Uncontrolled combustion usually occurs just after

ignition of fuel vapor.

After ignition commences flame propagation proceeds very quickly in thefuel vapor or air mixture, accompanied by rapid temperature andpressure increase.

Towards the end of the rapid pressure rise a point is reached where therate of pressure rise falls away quickly, and the curve flattens outtowards the maximum pressure point.

The point where the rate of pressure rise changes near and approachingthe maximum pressure point is the end of the second phase of

combustion.Third phase of combustion Controlled combustion is regulated by the rate at whichfuel continues to delivered.

Shows only a mall pressure rise, as the rate is decreased due todownward movement of the piston.

The end of injection occurs approximately at or slightly beyond themaximum pressure point.

6.1(7) State typical value of a) Brake Thermal efficiency b) mechanicalefficiency c) break specific fuel consumption per hour

Brake thermal efficiency

It is the objective of the marine engineer to keep the injection sett8ings, theair flow, coolant temperature (not to mention the general mechanical condition ) atthose values which give the best fuel consumption for the power developed.

Thermal efficiency (Th) is the overall measure of performance. In absoluteterms it is equal to

Heat converted into useful workTotal heat supplied

As long as the units used agree it does not matter whether the heat or work is

expressed in pounds-feet, kilograms-meters, BTU, calories, kWh or joules. Therecommended units to use are now those of the SI system.

Heat converted into work per hour = NkWh

= 3600 NkJ

Where N = the power output in kW

Heat supplied = M x K



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Where M = mass of fuel used per hour in kg

And K = calorific value of the fuel in kJ/kg

Therefore Th = 3600 N M x K

It is now necessary to decide where the work is to be measured. If it is to bemeasured in the cylinders, as is usually done in slow-running machinery, by means ofan indicator (though electronic techniques now make this possible directly andreliably even in high speed engines), the work measured (and hence power) is thatindicated within the cylinder, and the calculation leads to the indicated thermalefficiency.

If the work is measured at the crankshaft output flange, it is net of friction,auxiliary drives, etc., and is what would be measured by a brake, whence the termbrake thermal efficiency. (Manufacturers in some countries do include as output thepower absorbed by essential auxiliary drives but the present editor considers this togive a misleading impression of the power available.

Additionally, the fuel is reckoned to have a higher (or gross) and a lower (ornet) calorific value, according to whether one calculates the heat recoverable if theexhaust products are cooled back to standard atmospheric conditions, or assessed atthe exhaust outlet. The essential difference is that in the latter case the waterproduced in combustion is released as steam and retains its latent heat ofvaporization. This is the more representative case and more desirable as water inthe exhaust flow is likely to be corrosive. Today the net or lower calorific value (LCV)is more widely used.

If we take the case of an engine producing a (brake) output of 10000 kW for anhour using 2000 kg of fuel per hour having an LCV of 42000 kJ/kg

(Brake) Th = 3600 x 10000 X 100 %2000 x 42000

= 42.9 % (based on LCV)

Mechanical efficiency

Mechanical efficiency = output at crankshaftoutput at cylinders

= bhp = kW (brake)ihp kW (indicated)

The brake power is normally measured with a high accuracy (98 percent or so)by coupling the engine to adynmometer at the builders works. If it is measued in theship by torsionmeter it is difficult to match this accuracy and, if the torsionmetercannot be installed between the output flange and the thrust blowck or the gear boxinput, additional losses have to be reckoned due to the friction entailed by thesecomponents. The indicated power can only be measured from diagrams where theseare feasible and they are also subject to significant measurement errors.



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Fortunately for our attempts to reckon the mechanical efficiency, test bedexperience shows that the friction torque (that is, in fact, all the losses reckoned toinfluence the difference between indicated and brake torque) is not very greatlyaffected by the engines torque output



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6.1(8) Indicate a heat balance diagrams of a diesel engine

6.1(9) Sketch a section through a) Piston showing cooling arrangement b)engine bed plate showing girders, main bearings and tie bolt housing.

Piston



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Bed Plate



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Tie bolt housing arrangement

Bearings

Lubrication system for bearing and guides, etc, should be simple and effective.If we consider the lubrication of a bottom end bearing, various routes are available,the object would be to choose that route which will be the most reliable, least

expensive and least complicated. We could supply the oil to main bearing and bymeans of holes drilled in the crankshaft convey the oil to the bottom end bearing.This method may be simple and satisfactory for a small engine but with a large dieselit presents machining and stress problems.

In one large type of diesel the journals and crankpins were drilled axially andradially, but to avoid drilling through the crank-web and the shrinkage surfaces the oilwas conveyed fromthe journal to the crank pin by pipes. A common arrangement,mainly adopted with engines having oil cooled pistons, is to supply the bottom endbearing with oil down a central hole in the connecting rod from the top end bearing.

With any of the bearings (expecting ball or roller) the main object is to provide

as far as possible a good hydrodynamic film of lubricant (i.e a continuous unbrokenfilm of oil separating the working surfaces). Those factors assisting hydrodynamiclubrication are:-

(a) Viscosity. If the oil viscosity is increased there is less likelihood of oilfilm break down. However, too high a viscosity increases viscous drag andpower loss.



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(b) Speed. Increasing the relative speed between the lubricatedsurfaces pumps oil into the clearance space more rapidly and helps promotehydrodynamic lubrication.

(c) Pressure. Increasing bearing load and hence pressure (load/area)breaks down the oil film. In design, if the load is increased area can beincreased by making the pin diameter larger-this will also increaserelative speed.

(d) Clearance. If bearing clearance is too great inertia forces lead tobearing knock. This impulsive loading results in pressure above normaland breakdown of the hydrodynamic layer.

Hydrodynamic lubrication should exist in main, bottom end and guide bearings.The top end bearing will have a variable condition, e.g. when at T.D.C relativevelocity between crosshead pin and bearing surface is zero and bearing pressure nearor at maximum.

6.1(10) List the normal operating pressures and temperatures in a dieselengine for a) exhaust gas b) inlet air c) circulating water inlet and outlet d)lubricating oil e) fuel

S No Description Pressure Temperature(a) Exhaust gas Max 300 mm

(water gauge)250 (ideal) 350oC (max)

(b) Inlet air 150-200 mm (watergauge)

-

(c) Piston cooling wateroutlet 2.0-2.8 kg/cm2

55-60o

C

(d) Cylinder cooling outlet 1.5 kg/cm2(approx) 60-65oC(e) Lubricating oil (before

the engine)1.5 -2.0 kg/cm2 40-45o C

(f) Fuel 1 1.5 kg/cm2 -



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6.1(11) Draw line diagrams of following systems: a) fuel oil b) lub. oil c)piston cooling d) jacket cooling e) fuel valve cooling

Jacket water cooling system



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Fuel valve cooling system

lub

oil

cooling system



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Jacket Water System

Electronic cylinder lubrication



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6.1(12) Sketch an air reservoir with its fittings and safety devices and statetypical air pressure

Regulations

(a) There must be at least two starting air receivers, the total capacity of

which will give 12 starts for a reversing engine or 6 starts fo a non-reversingengine with CPP.

(b) There must be at least two compressors

(c) In addition to these there must be a compressor which can be started byhand i.e. with a dead ship. Note: this is not necessary if one of the compressorsis run off the emergency switchboard

(d) (i) A relief valve must be fitted to the HP discharge and be sufficientsize to ensure that the pressure rise does not exceed 10% of the

working pressure when the compressor is running and the outletvalves on the bottle are shut.

(ii) A relief valve or bursting disc on the hp cooler casing in order toprotect the casing from overpressure in the event of cooler tube failureNote: Bursting discs are generally preferred because they fail and stayfailed giving complete protection. A relief valve will reseat when thecompressor is stopped allowing water to enter the air side.

(iii) A drain must be fitted at each stage

Diesel start air system

The components

of the air start systemare taken to includecompressors andstorage bottles inaddition to the engineair start arrangement.The minimum of towcompressors should bematched to thestarting airrequirements of the

engine. Thecompressor aftercoolers should beprotected by abursting disc. All highpressure lines in thesystem to be of soliddrawn pipe.



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Air Receivers

There must be a means of access to allow cleaning and inspection of internals.The internal surface should be protected by a coating which is flexible enough tomove when the metal distorts. Copal varnish is generally used because it has these

properties and will not easily oxidise. Usually precautions are taken the same as foran enclosed space when entering. Ventilation is required to the solvent fumes in thevarnish. Drains must be fitted in the lowest part of the receiver. Receivers must beprotected by means of a relief valve, if the relief valve can be isolated from thereceiver than a fusible plug or plugs must be fitted. These are usually fitted becausein the event of a fire near to the bottle they will fail and release the entire contentsof the bottle rapidly. A relief valve however will only release air down to its closingpressure which is set point less blowdown. If the structure of the bottle becomesweakened by the heat then its ability to withstand even the reduced pressure isweakened a possible rupture could occur.

The inlet and outlet valves are to be arranged to prevent direct flow throughthe bottle with insufficient residence time for moisture to precipitate. Valves are tobe of the slow opening type to prevent excessive pressure rises. All attachmentsshould be via a support plate

Safety devices

The automatic valve (Main air start block valve) prevents connection betweenthe air receiver and air start manifold unless actually in the process of starting. Thisminimises the risk of an explosion in the air manifold actually propagating back to theair receiver where a much more severe explosion is possible. Safety devices are

incorporated in the air start manifold in order to dissipate the energy of an explosionthus keeping its effects local. Such devices include flame traps, relief valves andbursting discs.

Relief valves are fitted on each cylinder to prevent excess pressure. Reliefvalves are also provided on the cooling water jackets. These come in action in theevent of air leaking into the jacket and creating excess pressure. Oil traps and draintraps are provided so that oil or water does not pass into the air receiver.Compressors may be fitted with decompression arrangement in which the LP suctionvalve is kept open. This is used while starting to keep the starting current withinlimits. It may also be provided on compressors, which keep working continuously withautomatic pumping arrangements. The compressors will stop pumping air when the

receiver is full and start pumping when the pressure drops. The decompression valveis loaded by air pressure from the receiver.

All receivers must be fitted with relief valves, which discharge outside theengine room. In the event of fire in the engine room, air should not get released intothe engine room and help the fire. Valve should open at 3% above the designpressure. Air filling line and starting air supply line should be separate. Drain isprovided on the receiver. If the drain valve is not at the lowest point, an internal



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pipe leading to the bottom must be fitted, for effective draining. Oil or water shouldnot pass into the starting air system. Air receivers may also be fitted with fusibleplugs, which melt in the event of fire in the vicinity.

Receiver capacity should be enough to provide air to start a reversible engine 12times or a non-reversible engine 6 times without recharging.

6.1(13) Explain the purpose of following: a) sheathing on high pressure fuelline b) lagging on hot surfaces c) guards over moving machinery.

(a) Lagging on hot surfaces

All steam machinery, equipment and pipes must be adequately lagged to reduceradiation loss and provide protection to the personnel working. It is to be done insuch a way that personnel working in the space should be comfortable enough toundertake any kind of repairs. The surface temperature should not exceed 60oC.Pressure gauges and thermometers must be provided as required.

(b) Guards over moving machinery

Appropriate guards preferably metallic, to be placed for appropriate machinerywith regard to moving parts in order to prevent any untoward accidents. Thefollowing to be kept in mind prior placing guards over moving machinery:-

(i) Personal safety

(ii) Rag pieces or jute should not get into the shaft of the moving machinery

(iii) No leakages/drains should fall on running shaft / rotor which lead to shortcircuit of the motor

(c) Sheathing on high pressure fuelIn all modern ships, sheathing on high pressure fuel is undertaken as a preventive

measure in order to avoid any untoward incidents like fire, in the engine room. Incase of leakage or burst in the high pressure fuel line, chances of scattering of fuelover engine hot surfaces may pose endanger to the hygiene of the compartment aswell as a potential hazard to the fire. A leak off line is provided along with highpressure fuel line to lead the drain to a separate storage space in case of any leakagethrough an alarm



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6.1(14) Explain the purpose of turning gear and interlocks

Turning gear

Diesel engines are fitted with many safety gadgets. Main diesel engine is fittedwith a turning gear, which turns the engineat very slow speed during maintenance,

warming up and shutting down. This gear must be disengaged before the engine canbe started on air. Compressed air used for starting has to pass through the turninggear interlock before it reaches the control valve.

Interlocks

The engine should not start in a wrong direction. Reversing systeminterlock and the wrong direction alarm ensure this. This interlock will blockthe starting air if the fuel pump cams, starting air distributor, exhaust valvedrive shaft are not in the position corresponding to the direction indicated bythe telegraph. These blocks are operated by lub oil pressure in sequence.Thus this interlock also incorporates the safety to prevent starting of the

engine if the lub oil pressure does not develop sufficient pressure. Lub oilpressure also controls a servo valve on the fuel oil line. If the lub oil systemfails and there is no pressure in the line, fuel to the engine will beautomatically cut off and the engine will come to a halt.

6.1(15) Describe how engine speed is varied and how overspeeds areprevented

(How is the speed of a main engine controlled? What types of device can beused to control speed?)

The speed of main engines is controlled primarily by the fuel-lever or fuel-wheel

setting. The fuel lever or wheel controls the fuel pump settings which in turn controlthe amount of fuel injected per working cycle into each cylinder. Provided the loadon an engine did not change the speed of the engine would remain constant for anyfuel lever setting. Unfortunately this condition occurs only in very smooth water; assoon as a ship starts to pitch the propeller rises and falls and the load on the enginechanges. If the speed of the engine were controlled only by the fuel-lever setting,the speed would rise and fall with the pitching of the vessel and the correspondingload changes.

Small changes in engine speed can be tolerated. But in bad weather when theship is pitching heavily it is possible for the propeller to come clear of, or nearly clear

of, the water, and in such circumstances the speed of the engine could risedangerously. A similar situation could arise if the propeller shaft fractured and /orthe propeller was lost.

In order to keep the engine speed within reasonable bounds in heavy weather, orin the event of shaft failure, a governor overrides the fuel lever and reduces the fuelinjected into the cylinders, so preventing the engine speed from rising further.



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There are four types of governor used for this purpose, the inertia type governor;the mechanical type governor with spring loaded sleeves and flyweights; themechanical hydraulic-type governor; and the electronic governor. The inertia-typegovernor was at one time used only on main engines, to limit the maximum speedwhen the engine was racing in heavy weather.

A propulsion engine directly driving (no clutch between engine and propeller) ascrew propeller will always have a load on the engine. As the speed of the engine isincreased the power demand on the engine follows the cube law. That is the power

given out by the engine varies as thecube of the speed. Within loadchanges where the engine does notover-speed, an engine driving agenerator will have a slight decrease inspeed as the load on the engineincreases, provided the voltage is keptat a constant value. The change will

be linear or follow the form of a graphwhere Y = MX+C. Fig shows the torquespeed relationship of an enginepropelling a ship. The relationship is

similar when an engine is driving a centrifugal pump or a centrifugal fan.

Overspeed trip

Overspeed trips are fitted on engines where the governor does not fail safe.Their function is to shut off the fuel supply to the cylinders in th4e event of the speed

of the engine rising to a dangerous level. They are always fitted on steam turboalternators or generators. The overspeed trip usually consists of a bolt with arelatively heavy head. The bolt is fitted at the forward end of the engine shaft whereno torque is transmitted. It is fitted in a space bored out across the diameter of theshaft. The bolt is held in place by a nut and supported by a spring. When the engineoverspeeds the centrifugal force exerted by the bolt head overcomes the support ofthe spring and flies outwards until restrained by the nut and compression of thespring. The bolt head in the thrown position strikes a trip lever which is also thrownand shuts off the engine fuel eventually bringing the engine to rest.

The means whereby the fuel is shut off varies with the make of engine. As somemotor ships have a steam turbo alternator it is worth mentioning that in many turbo

sets the overspeed trip often works through the lubricating oil low pressure cut-outand steam stop valve. In other cases the overspeed t4ip operates through a system oflevers and links to the turbine steam stop valve. When the overspeed trip operates,the pawl holding the steam stop valve open is released and a spring closes the stopvalve. The tripping speed can be adjusted by adding or removing thin spacingwashers under the spring so that its compression is altered.


Power or Torque (P)

p N3

Speed (N)


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A small permanent magnet alternator driven from the engine camshaft inconjunction with a rectifier can also be used with an electronic governor to preventthe engine racing. When, the engine speeds up, the voltage increases. At some levelit will operate the governor to reduce the fuel supply to the engine and prevent adangerous increase in engine speed.

6.1(16) Explain what is oil mist, when does it become dangerous and theworking and maintenance of an oil mist detector

Crankcase oil mist detector(Obscuration) (set point 2.5%L.E.L)

Oil mists can be readilydetected at concentrations well

below that required forexplosions, therefore automateddetection of these oil mists canbe an effective method ofpreventing explosions. Shownabove is the Graviner oil mistdetector. This is in common usein slow speed and high speedengines. The disadvantage ofthis type if system is that thereis a lag due to the time taken for

the sample to be drawn from theunit and for the rotary valve toreach that sample point. For this reason this type of oil mist detector is not commonlyused on higher speed engines.

Modern detectors often have the detection head mounted in the probe, theprobe is able to determine the condition of the crankcase and output an electricalsignal accordingly. The assembly consists of the following:-

(a) Extraction fan-draws the sample from the sample points through thereference and measuring tubes via non-return valves.

(b) Rotary valve-This valve is externally accessible and is so marked so as toindicate which sample point is on line. In the event on exceeding the set point,the valve automatically locks onto that point so giving a clear indication of thelocality of the fault condition.

(c) Reference tube-measures the average density of the mist within thecrankcase, as there will always be some mechanically generated mist.



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(d) Measuring tube- measures the opacity of the sample by means of aphotoelectric cell as with the measuring cell. To exclude variables in lamps asingle unit is used with beams directed down the tube by mirrors.

The photoelectric cell gives an output voltage proportional to the light falling

on it. In this way the opacity of the sample is measured, the voltages generated in thecell in the measuring and reference tubes are compared in an electronic circuit. Thedifference is compared to a potentiometer varied set point which if exceed initiatesan alarm circuit. The alarm circuit, dependant on installation, will generally declutchthe drive to the rotary valve, give an output signal to the engine room alarmmonitoring system and an output to the engine protection system causing it toslowdown.

The rotary valve also has a position marked 'O' at which air is supplied to bothtubes, and zero automatically (and manually if necessary) adjusted at each cycle. Inaddition at position 'L' an average sample of the crankcase is compared to air.

Crankcase oil mist detector (light scatter)The disadvantage of obscuration types is that they are generally slow to

operate and suffer from inaccuracies and false alarms caused by such things as a dirtylens. Light scatter do not suffer from these problems, are faster reacting and do notneed to set zero during engine operations.

The relationship between the lightlandings on the sensor is nearlyproportional to the oil mist densitytherefore the unit can be calibratedin mg/l. It is possible to have thesensor and a LED emitter in a singleunit which may be mounted on thecrankcase. Several of these can beplaced on the engine each with aunique address poled by a centralcontrol unit. The results of which maybe displayed on the control room.

Having these heads mounted on the engine removes the need for long sampletubes which add to the delay of mist detection. This makes the system much more

suitable for use with medium and high speed engines were otherwise detection wouldbe impossible.

Crankcase doors (non relieving)

The older type consisted of doors lightly held by retaining clamps or clips. Withdoors of this type a pressure of 0.5psi would give a permanent set of about 25mm, thedoors would be completely blown off by pressures of 2 to 3 psi. Modern large slow



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speed engines have two types of crankcase door, a large securely held heavy mildsteel square door which allows goodaccess for heavy maintenance. Asecond smaller round dished aluminiumdoor at around x-head height which

allows entry for inspection. Due to thecurved design the door is able towithstand pressures well above the setpoint for the relief doors.

Actions in the event of Oil Mistdetection

The consequences of a crankcaseexplosion are extremely serious and thegreatest possible caution in the actions

taken should be exercised. Should the oil mist detector activate an alarm condition,then personnel should take steps to ascertain if the fault is real. They should initiallyassume that it is, the bridge should be informed and the engines slowed if the oil mistdetector has not already done so. Should the bridge require maneuverability, and it isessential that the engine be operated then consideration of evacuation of the engineroom should be made. Otherwise the engine should be stopped and turned on gearuntil cooled.

The Graviner Oil Mist detector indicates via markings on the rotary valve whichsample point has the high readings. By inspection of the Graviner, and by viewingcrankcase (or thrust, gear case) bearing readings it is possible to ascertain whether a

fault condition exists. Under no circumstances should any aperture be opened untilthe engine has sufficiently cooled, this is taken as normal operating temperatures asan explosion cannot occur when no part has a temperature above 270'C (Cool flametemperature). Once cooled the engine can be opened and ventilated (the crankcaseis an enclosed space). An inspection should be made to locate the hot spot, theengine should not be run until the fault has been rectified.

6.1(17) State approx. exhaust gas temperatures at a) discharge fromcylinder b) at inlet to and outlet from exhaust gas turbine

(a) 250-350oC

(b) 60-70o

C



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6.1(18) Define Turbocharger Surging and action to be taken when itoccurs.

Surging

Takes place if the air mass delivered by the blower falls at a faster rate than

the air pressure of delivery. With all blowers it is possible to produce a graph showingthe effect. Surging gives an unpleasant noise. The initial action in order to prevent ablower surging is to reduce engine load. Blower efficiency is highest closer to thesurge line and so if a high efficiency is demanded there is little leeway againstsurging. In practice the fitting of blowers is a compromise between reasonable blowerefficiency and an acceptable degree of safeguard against surging.

Surging is acondition whereby

an imbalance indemand and supplyof air from theturbochargercauses a rapiddeceleration. Thisis accompanied bya loud barking noiseand vibration. Itwas not uncommon

on pulse systems in heavy weather; it is less prevalent in modern constant pressure

designs but may begin due to reasons explained later.

The normal characteristic of a turbocharger running at constant speed is one ofreducing possible pressure ratio for increasing air flow demands. This characteristic isexaggerated when frictional losses are taken into account. As described above frommaximum efficiency the air leaving the compressor wheel should enter the inducer atan optimal angle. Failure to do so leads to losses and a characteristic shown. It shouldbe noted that this shows a relationship at a specific instant of Turbocharger speed. Itwould be possible to plot many lines of constant speed on the graph. The point atwhich surging occurs could be plotted for each and a surge line drawn. Moving theplant operating line towards the surge line can lead to an increase in turbocharger

efficiency.



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The stable operating point is at A though which passes the respective engine

operating line ( this line indicates the relationship the engine requires between Airflow and pressure), the unstable point leading to surging is at B.If the air flow through the turbocharger reduces The effect would be a decrease inpressure at the receiver. However the pressure ratio of the turbocharger (running atconstant speed) would Increase. The effect of this is to return the system back to itsstable point A. For an engine operating on the line passing through B then theeffects of a reduced air flow will be a corresponding reduction in compressor pressureratio. The engine however requires increased air flow which the turbocharger cannotsupply and the result is surging. Theoretically this effect begins where the constantpressure line is flat.

Conditions leading to Surging

Turbochargers are generally specified in relation to set ambient operatingconditions and then matched to engine load requirements. Deviation away from thisdue to such things as changes in ambient conditions and changes in engine speed/loadrelationship has to be taken into account. It is very unusual for a modernturbocharger to such. However, surging may begin after several years of stableoperations. Some possible reasons are as follows:-

o For multi blower installations surging can occur due to a difference inmaintenance of cleaning causing one or more to operate at pressure

ratio's above its capabilityo Similarly difference in blower component wear, this is particularly true

for such things as increased blade tip clearanceo change in engine speed/ load relationship- say due to hull foulingo cylinder power imbalanceo faulty injectors or timingo dirty air filtero dirty air cooler (air side)



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o High air cooler cooling water temperatureo dirty turbine nozzle ringo deposits on blades or impellero damage to bladeso It is also possible that components downstream from the blower exhaust

such as a fouled exhaust gas boiler can also lead to surging

6.1(18) Define scavenge fire and action to be taken when it occurs.

Scavenge fires

Carbonized lubrication oil, unburned fuel oil and carbon from theresidual products of the combustion spaces area accumulated in the scavengespaces with the funning of the engine. Under certain faulty running conditionof the engine, these may ignite causing a fire in the4 enclosed scavenge space,

known as scavenge fire.The underside of a two stroke enginepiston is frequently utilized as an airpump either to supply air or to boostup pressure of air at the intake ofengine cylinder. The space isenclosed for this purpose and is also incommunication with the scavenge airbox common to all cylinders. In thisspace there is normal accumulation of

carbonised cylinder lubricating oil,unburnt fuel and carbon from theresidual products of combustion. Thedry carbonaceous deposits at thelower part may ignite under certainfaulty operating condition initiating afire in the enclosed scavenge air box.This is scavenge fire. Such a conditionmay arise due to the following

circumstances occurring either singly or in combustion.

(a) A prolonged blow-by owing to ineffective sealing of combustionspace, Leaky piston ring, sticky ring, broken ring, badly worn-out liner,scoring and scuffing at the liner surface, faulty lubrication (quality,quantity or timing), insufficient ring axial clearance are some of theprobable causes listed which will adversely affect proper sealing of thecombustion space.



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(b) An overheated piston arising out of continued slow burning in thecylinder. Faulty atomization, faulty fuel pump timing, low compression,ineffective sealing, failure of coolant circulation due to scale formation,generation of frictional heat are some of the reasons for which thepiston may accumulate higher temperature level in its body. The heat

will be dissipated to the underside causing heating of the air space.(c) A blow back of the exhaust gases through the scavenge ports. Sucha blow back is possible due to a rise in exhaust back-pressure.Carbonizing of parts, fouling of grid before turbine inlet, fouling ofturbine blades, chocking of silencer etc. are some of the reasons listed.

DetectionA scavenge fire is likely to be initiated first in the space at the

immediate vicinity of the piston. It should be detected at the stage. Ascavenge fire will manifest itself by the following indications:

(a) An increase in the exhaust temperature of the affected cylinder asthe cylinder is not receiving fresh air.(b) A drop in revolutions of the engine as the power generation in theaffected cylinder is less.(c) Black smoke with exhaust.(d) Discharge of spark, flame of smoke through drain cocks fromscavenge air box.(e) Evidence of local overheating of scavenge air box.(f) Visible evidence of fire if a transparent window is provided(g) Cooling outlet temperatures of the affected cylinder will indicate a

rise.(h) Rise of pressure and temperature of air, in the air box below thepiston. A temperature rise will be sensed and signaled by a transducer ifprovided which is amplified to energize an alarm circuit.

(i) As the fire spreads there will be more smoke in exhaust andfurther drop in revolutions or sign of overloading.

Actions to be taken (Preventive actions)If the fire is localized, the affected cylinder will have to be isolated.

Prompt action should extinguish the fire.

(a) The fuel pump plunger of the affected cylinder is lifted up and thefuel inlet valve to the pump should be shut.

(b) The speed of engine is to be reduced(c) The coolant flow rate through the piston and jacket can be

increased(d) The rate of lubrication in the affected cylinder may be increased(e) Drains are to be shut to prevent a blow of sparks in the engine room



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(e) Nobody should be standing near the scavenge space relief doorsAfter the fire is extinguished an investigation should follow to ascertain

the cause of fire. Crankcase or scavenge doors must not be opened until theengine is sufficiently cooled.

If the fire spreads(a) Inform the bridge and stop the engine(b) the scavenge air duct before the engine is flapped and thesmothering gas charged(c) cooling and bearing circulation is maintained till the engine iscooled.After the engine is sufficiently cooled down, it is necessary to open the

scavenge boxes of all cylinders for cleaning of deposits and burnt products.The cause of fire should be ascertained and rectified. Some of the probableplaces where damage may occur as a result of fire are:

(a) the piston rod stuffing box gland(b) the piston rod and cylinder liner surfaces(c) the alignment of piston and straightness of piston rod(d) scoring or crack on liner(e) the diaphragm below the piston for detection of crack(f) the tie-rods near the fire should be retensioned.

6.1(20) Construction and operation of Turbocharger

Turbochargers

For Combustion of a fuel, an adequate quantity of air is required. For aTurbocharger system capacity should be sufficient to ensure that the air demand ismet when the turbocharger is not at its optimum. In a four stroke diesel engine,this air is induced during a down stroke in one of the two engine cycles per powerstroke. The exhaust gasses are removed by the preceding upstroke. For a twostroke no such cycle for scavenging and air replenishment exists. Instead, air underpressure is supplied at the end of the power stroke providing a new charge of airand removing the exhaust gasses. The period allowed for scavenging is limited asthe longer the exhaust port or valve remains open so the shorter the travel of pistonis available for compression. The greater the mass of air that can be supplied, themore efficient the scavenging process will be, and also the greater mass of air will

be available for the combustion of an equally greater mass of fuel. The mass of airis increased by increasing the pressure at which it is supplied. Pressure chargingcan be obtained by a number of means including scavenge pumps, chain drivenrotary blowers and exhaust gas driven blowers.

Exhaust gas driven blowers or Turbochargers make use of gas in the cylinderwhich theoretically could be expanded further, the power that would be developedcould be used for driving an engine driven scavenge pump. In practice it is more



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efficient to use this exhaust gas in the turbocharger as further expansion of the gaswould require an increased stroke. Increased stroke would mean increased engineheight with problems of crankshaft construction, cylinder lubrication and effectivescavenging coming into play. The work that could be extracted from this low pressuregas would be limited and more efficiently extracted in a rotary machine.

Construction

Axial



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Centrifugal

Centrifugal turbochargers aregenerally cheaper to produce than

axial flow. In addition for smallersized radial units the effects of bladeleakage are less important They arevery common in automotive systemswhere they are suited to themanufacture of large volumes ofstandard design. Axial flow may beselected even when there arecentrifugal alternatives as it is bettersuited to individual modifications andis able to operate better on heavy

fuels.

A turbocharger is made basically in two linked parts, the gas side and the air side.

The gas side is made out of cast iron, is in tow parts and is generally watercooled. The turbine inlet casing carries the nozzle blade shroud ring and forms thebearing housing. The turbine outlet casing forms the main part of the blower whichincludes the mountings. In addition it forms a shroud for the shaft and contains bledair passageways for supplying air to the labyrinths seals.

Compressor

The air side casing is also intwo parts but is made of aluminium alloy. The inlet casingmay be arranged to draw air formthe engine room or from the deck,both methods via a filter andsilencer arrangement. Theadvantage of drawing air formoutside the engine room is that itwill tend to be cooler and lesshumid. An advantage of drawing

from the engine room would besimpler ducting arrangements andthat the engine room tends to beslightly pressurized.

The main parts of the Compressor are the Compressor wheel (made up from aseparate Inducer and Impeller on larger designs), the diffuser, and the air inlet andoutlet casing.



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With the wheel rotating a unit of air massing the compressor wheel experiencescircumferential velocity (v)at its distance from the wheel centre line (radius r). Aradial velocity is experienced of value v2/r which causes it to move radially outwards.The unit of air leaves the compressor with a resultant velocity the angle of incidenceof which should, by careful design, match the inducer inlet angle. This leads to

maximum compressor efficiency. The effects of frictional losses, whether due tosurface imperfections or fouling of the compressor wheel will result in changing theangle of incidence and thus a drop in efficiency

Surging

Takes place if the air mass delivered by the blower falls at a faster rate thanthe air pressure of delivery. With all blowers it is possible to produce a graph showingthe effect. Surging gives an unpleasant noise. The initial action in order to prevent ablower surging is to reduce engine load. Blower efficiency is highest closer to thesurge line and so if a high efficiency is demanded there is little leeway againstsurging. In practice the fitting of blowers is a compromise between reasonable blower

efficiency and an acceptable degree of safeguard against surging.

Surging is a condition whereby an imbalance in demand and supply of air fromthe turbocharger causes a rapid deceleration. This is accompanied by a loud barkingnoise and vibration. It was not uncommon on pulse systems in heavy weather; it is lessprevalent in modern constant pressure designs but may begin due to reasonsexplained later.

Rotor

This may again be thought of two parts; the gas side and shaft and the air

compressor side. They are usually made of two materials. The advantage of makingthe compressor end of a lighter aluminium alloy material rather than using the samematerial throughout, is that it reduces the total mass of the rotor , is more easily castinto intricate shapes, and the rotational inertia is reduced.

Must be capable of maintaining strength at high temperatures so material isusually a chromium steel. The impeller is made of an aluminium alloy and for largercompressors may have a separate inducer section at the eye. Whatever the form ofconstruction is undertaken it must preserve the rotor balance and that means refittingin the same position after removal from the rotor.

Blades

The blades shown above are twisted and tapered toallow for the increased blade velocity with increasedradius. Blades must be capable of withstanding the highexhaust temperatures and also the highly corrosive



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environment of the exhaust gas. Stainless steel is frequently used. The blades aremade lose fit for the following reasons;

o To allow for thermal expansiono To prevent force fit stress adding to centrifugal stress (stops the

material 'yielding')o Help dampen vibrations from the gas pulses as the blades pass the

nozzles (especially when partly or wholly blocked)

Bearings

Most main engine turbochargers are water cooled in order to keeptemperatures reasonable. On the most modern of turbochargers this cooling water hasbeen reduced in quantity to that is required for cooling the bearings. The spacebetween the compressor and turbine is filled with insulation material. There are somesmaller blower designs which by design can be cooled by air flow. As no cooling jacket

is required it is convenient do place the bearings in between the turbine andcompressor wheels. This allow for better rotor support.

Plain white metal bearings may be used; these have an indefinite life butrequire lube oil to be supplied at pressure. They also require a header system tosupply oil in the event of the main supply pump failure. A common system is bysupplying from the main engine lube oil system via a header system similar to thatemployed with steam turbines.

Plain bearing Lube oil system

Care should be taken to ensure

that the bearings are adequatelyprotected when the engine isstopped as the blower is liable toturn due to natural draught.Locking the blower, isolating theblower from the scavenge belt byuse of a slide valve, putting coversover the blower suction orcontinuation of supply of lube oilafter engine stoppage may beused.

Ball or Roller bearings require good amount of lubrication and may be suppliedby means of a shaft driven gear pump from an integral sump. The gear pump isoperated by rotation of the rotator. Ball and roller bearings have a definite life andmust be changed on running hours bases, typically every 15,000 Hrs. Forturbochargers fitted with plain bearings a double-sided thrust is fitted at both ends.This takes the form of a collar on the rotor acting on white metalled 'Mitchell' typesegments. Double-sided thrusts are fitted to locate the turbine during rolling and



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pitching. Generous oil quantities are supplied to bearings in order to allow for coolingas well as lubrication

Labyrinth Seals

These are provided at each ends of therotor and between the turbine andcompressor and serve to prevent thepassage of exhaust gas and also toprevent oil laden air being drawn intothe eye of the impeller from the bearing.Oil seals in the form of thrower platesare also fitted at the bearings to preventthe passage of oil along the shaft.Labyrinth seals consist of projections onthe rotor which almost touch the casing.

Principle of the Labyrinth Gland

The leakage of steam is reduced by theuse of labyrinths; these provide atorturous path for the gas to follow toexit the turbine reducing the pressureacross a series of fine clearances. Withinthe cavity where the flow is turbulent,

the velocity of the gas is increased withan associated drop in pressure. Thekinetic energy is the dissipated by the

change in direction, turbulence and eddy currents.



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introduced detecting changes in current flow. For increased current, that is anincreased electrical load, the governor can act to supply increased fuel before theengine has begun to slow.

Overspeed trip

Overspeed trips are fitted on engines where the governor does not fail safe.Their function is to shut off the fuel supply to the cylinders in th4e event of the speedof the engine rising to a dangerous level. They are always fitted on steam turboalternators or generators. The overspeed trip usually consists of a bolt with arelatively heavy head. The bolt is fitted at the forward end of the engine shaft whereno torque is transmitted. It is fitted in a space bored out across the diameter of theshaft. The bolt is held in place by a nut and supported by a spring. When the engineoverspeeds the centrifugal force exerted by the bolt head overcomes the support ofthe spring and flies outwards until restrained by the nut and compression of thespring. The bolt head in the thrown p

Documents

26585218-notes-on-motor-03-oct-09