# Marine Diesel Engine Dual

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Constant volume cycle1-2 isentropic compression2-3 heat addition at constant volume3-4 isentropic expansion4-1 heat rejection at constant volume

Air standard thermal efficiency = 1-(1/r) 1 , r = comp. Ratio.

The working with reference to the P-V diagram and the T-S diagram is as follows;At the beginning of the cycle at the point 1 the cylinder is assumed to be full with acharge of fresh air.The point 1 is called the state point defining pressure and temperature of a certain volumeof air.

From 1 to 2 the air is compressed isentropically following the law PV =C.From 2 to 3 heat is added to the same mass of air at constant volume.

Point 3 represents maximum pressure and temperature in the cycle.From 3 to 4 air is expanded isentropically. From 4 to 1 heat is rejected at constantvolume. No rejection of the working substance is considered to have taken place.

Finally the same mass of air is brought back to its initial state at 1 and is ready to repeatthe cycle.For this cycle per unit mass of air the quantity of heat added Q a= C v ( T3 T 2 )C v is the specific heat of air at constant volume.Thermal efficiency th = Heat converted to work/ heat added.

=( Q a Q r ) / Q a = 1 {(T 4 T 1)/ (T 3 T2)}Using the relationship for perfect gas laws :T 2 / T 1= (V 1 / V 2) -1

= (r) -1.Since V 1 / V 2 = r, the compression ratio.

T 2 = T 1 x (r) -1

Again, T 3/ T 4 = ( V 4/ V 3 ) 1 = ( r ) 1 ,

since V 4 = V 1 and V 3 = V 2Substituting these values

th = 1 - T3 / (r) 1 - T2 / (r) 1

T3 T2= 1 {( 1/r ) 1 x (T3-T2)/ (T3-T2) = 1 ( 1/r ) 1 ..(1)

This equation is known as the air standard thermal efficiency of Otto cycle in terms of compression ratio and the properties of working substance ( ). The equation shows thatthe thermal efficiency depends on compression ratio for a given working fluid.

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Diesel cycle

T = temperature

S = Entropy

1-2 isentropic compression through comp ratio r = V1/V22-3 heat addition at constant pressure

3-4 isentropic expansion4-1 heat rejection at constant volumeair standard efficiency = 1 ( 1/r ) 1 { rc - 1 }

{ ( rc 1}where rc = V3/V2 , termed fuel cut-off ratio.

This is presented on P-V and T-S planes. Starting with the assumption as before, itconsists of an isentropic compression process from 1 to 2 through the compression ratio r = V1/V2.Addition of heat to the mass of air at constant pressure as the cycle passes from 2 to 3.At 3 heat supply is cut off and air is expanded isentropically.Rejection of heat at constant volume from 4 to 1 ; at 1 the substance regains its originalstate,i.e. pressure, volume and temperature.

Heat transferred to unit mass of air Qa = Cp ( T3 T2 ).Cp is the specific heat at constant pressure

And Heat rejected Qr = Cv ( T4 T1 )The thermal efficiency th = 1 {(Cv X T4 T1)/ (Cp / T3 T2)

= 1 1/ X ( T4 T1) / (T3 T2)Using the fundamental gas equationT2 = T1 ( r) -1

For the constant pressure process from 2-3,

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Mixed cycle of otto and diesel cycleHeat added partly at constant volume and partly at constant pressure.hence having advantages of both cycles.

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Equation (3) represents an expression for thermal efficiency of Dual cycle in terms of r,rc and rpIn this equation, if rp is substituted as 1, i.e. all the heat is supplied at constant pressure,then we have the efficiency equation for the Diesel cycle.When rc = I i.e. all the heat is supplied at constant volume then we have the thermalefficiency of constant volume cycle.

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Otto, Diesel and Dual Cycles comparedThe three air standard thermodynamic cycles can be compared for the samecompression ratio and heat input. The cycles are plotted on P-V and T-S planes. Sinceall the cycles have the same compression ratio, the compression line 1 to 2 is commonto all. The cycles then depart according to the mode of heat addition. 1, 2, 3, 4represents the Otto cycle; 1, 2, 3, 4 represents the Diesel cycle. It will be seen thatthe Dual cycle falls in between the two cycles and is represented by 1, 2, 2, 3, 4, 1,

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To satisfy the condition of equal heat input the areas under the T-S diagram for eachcycle must be the same.The general expression for efficiency is given by:

To satisfy the condition of equal heat input the areas under the T-S diagram for eachcycle must be the same.

The general expression for efficiency is given by:th= 1 (Qr/Qa)

Qa being the same for each cycle, that cycle which rejects the maximum heat is the leastefficient.The quantity of heat rejected Qr for Otto, Diesel and Dual cycles are represented by areasunder the curves 14, 14 and 14 respectively.This analysis reveals that the Otto cycle or the constant volume combustion gives thehighest economy as regards fuel consumption as it rejects minimum heat, but it gives ahigh maximum pressure as well.The Diesel cycle gives much less maximum pressure but least economy in fuel

consumption.The Dual cycle falls intermediate between the two. While the thermal efficiency is of utmost importance, the maximum pressure would limit the extent to which the gain can

be utilised in practice. The importance of the mixed cycle can now be realised in the lightof the above statement.

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The thermal efficiency of Diesel cycle decreases if r c is increased.The thermal efficiency of Dual cycle is increased if r p is increased.But the pressure rise associated with the increase is undesirable.It follows therefore that there is not much scope to manouvre for an increase of efficiency

by manipulating any of the quantities rc, rp, and .

An internal combustion engine is an air engine, hence y is constant.By suitably adjusting values of rc, rp, and the thermal efficiencies of Diesel and Dualcycles will approach but never reach that for the Otto cycle. The equation (1) shows thatif r can be increased indefinitely the efficiency will approach 1, i.e. 100%.

But a very high compression ratio cannot be used from practical considerations.A high compression ratio gives a very high peak pressure and temperature.The crankshaft and other members of the reciprocating engine mechanism are designed

to withstand the peak load.Hence too high a compression pressure would involve higher weight and cost of the

engine.

The mechanical load on bearing would be more and the engine components comprisingof the walls of the combustion chamber would have to bear a higher level of thermalstresses.

The upper limit of compression ratio is therefore fixed by the strength of the cylinder, the bearings and other parts whose stresses are determined by peak mechanical and thermalloading.Besides, increase in r in the lower range gives a proportionate gain in thermalefficiency.But in the higher range the gain becomes progressively less.Thus considering all aspects an optimum value of r is chosen.The large slow speed marine Diesel engines employ a value of.r in the neighbourhood of

12-14, medium speed engines can employ slightly higher value of r, about 16. A Diesellifeboat engine may have a value of r as 20 for good startability from cold.

Real cycle

Cont. line = actual curveDotted line = ideal curvex = compression loss.y = combustion loss.

Rounded corners due to non-instantaneous valve operation.1- 2 suction2 - 4 compression,

(3 - 4 fuel injection, 3 - 5 combustion)5 6 expansion

6 1 exhaust.

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The I.C. engine cycle and the equivalent air standard cycle are somewhat similar. TheOtto cycle is taken for the comparison with the I.C. cycle as the principles are generally

the same for most IC engine cycles.With reference to figure, the actual compression curve gives a lower terminal pressureand temperature than the ideal curve ( shown dotted ). This is caused by heat transfer taking place, variable specific heats, a reduction in due to gas-air mixing, etc.Resulting compression is not adiabatic and the difference in vertical height is shown as x.

The actual combustion gives a lower temperature and pressure than the ideal due todissociation of molecules caused by high temperatures.These twofold effects can be regarded as a loss of peak height of (x+y) and a loweredexpansion line below the ideal adiabatic expansion line. The loss can be regarded asclearly as shown between the ideal adiabatic curve from maximum height (shown chain

dotted ) and the curve with initial point x + y lower (shown dotted ).The expansion is also not adiabatic. There is some heat recovery as molecule re-combination occurs but this is much less than the dissociation combustion heat loss in

practical effect.

The expansion is also much removed from adiabatic because of heat transfer taking placeand variation of specific heats for the hot gas products of combustion. The actualexpansion line is shown as a full line.

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The assumptions made at the beginning on ideal cycles plus what has been describedabove are considered, along with practical details such as rounding of corners due to non-instantaneous valve operation, etc. mean that the actual diagram appears as shown in thesketch.

Working Principle 4 stroke engine

Induction strokeCompression stroke at the end pr. = 35 bar, temp. = 540 CPower stroke temp = 1650 CExhaust stroke

1. INDUCTIONThe crankshaft is rotating clockwise and the piston ismoving down the cylinder. The inlet valve is open and afresh charge of air is being drawn or pushed into thecylinder by the turbocharger

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2. COMPRESSIONThe inlet valve has closed and the charge of air is beingcompressed by the piston as it moves up the cylinder.Because energy is being transferred into the air, its

pressure and temperature increase. By the time the pistonis approaching the top of the cylinder (known as TopDead Centre or TDC) the pressure is over 100 bar andthe temperature over 500C

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3. POWER:

Just before TDC fuel is injected into the cylinder by the fuel injector. The fuel is"atomised" into tiny droplets. Because they are very small these droplets heat up veryquickly and start to burn as the piston passes over TDC. The expanding gas from thefuel burning in the oxygen forces the piston down the cylinder, turning the crankshaft.It is during this stroke that work energy is being put into the engine; during the other 3 strokes of the piston, the engine is having to do the work

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4. EXHAUSTAs the piston approaches the bottom of the cylinder (known as Bottom Dead Centreor BDC) the exhaust valve starts to open. As the piston now moves up the cylinder,the hot gases (consisting mostly of nitrogen, carbon dioxide, water vapour and unusedoxygen) are expelled from the cylinder.As the Piston approaches TDC again the inlet valve starts to open and the cycle

repeats itself

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Four stroke timing diagram

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The working cyclesThe actual engine requires four strokes or two strokes of the piston to complete processessuch as compression, expansion, exhaust and induction.Accordingly the engines are distinguished as four-stroke and two-stroke engines.The working cycle of a four stroke engine is described with respect to indicator and valve

timing diagrams.

1-2 induction Stroke: Air is drawn into the cylinder at the pressure existing in the intakemanifold. The inlet valve closes after the end of the stroke.2-3 Compression Stroke : With both inlet and exhaust valves closed, the air iscompressed by the piston in the clearance space. The injection of fuel begins at a few

degrees before the T. D. C. The fuel is ignited by the high temperature produced at theend of compression and most of the heat is released at constant volume.

3-4 Expansion or working stroke: The gases expand until at the end of stroke when theexhaust valve opens. The exhaust is blown down in exhaust pipe and the pressure in thecylinder drops.4-1

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Exhaust Stroke : The remaining gases in the cylinder are forced out by the displacementof piston extending over a fill stroke.

Two stroke engineWorking Principle of 2 stroke engine - Ported type

1) Compression2) Fuel injection3) Power and exhaust4) cross scavenging

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2 stroke Timing diagram (ported

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2 stroke Timing diagram ( v/v engine)

Working Principle 2 stroke engine( valve)

a) scavenge port covered, exh v/v about to closeb) exh v/v closed compression on, fuel injectionc) combustion, expansiond) exh v/v about to open.e) exhausting and scavenging.

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The crankshaft is revolving clockwise and the piston is moving up the cylinder,compressing the charge of air. Because energy is being transferred into the air, its

pressure and temperature increase. By the time the piston is approaching the top of the cylinder (known as Top Dead Center or TDC) the pressure is over 100 bar and thetemperature over 500C

2. Just before TDC fuel is injected into the cylinder by the fuel injector. The fuelis "atomised" into tiny droplets. Because they are very small these droplets heatup very quickly and start to burn as the piston passes over TDC. The expandinggas from the fuel burning in the oxygen forces the piston down the cylinder,turning the crankshaft. It is during this stroke that work energy is being put intothe engine; during the upward stroke of the piston, the engine is having to do thework

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3. As the piston moves down the cylinder, the useful energy from the burning fuel isexpended. At about 110 after TDC the exhaust valve opens and the hot exhaust gas(consisting mostly of nitrogen, carbon dioxide, water vapour and unused oxygen)

begin to leave the cylinder

4. At about 140 after TDC the piston uncovers a set of ports known as scavenge ports. Pressurised air enters the cylinder via these ports and pushes the remainingexhaust gas from the cylinder in a process known as "scavenging".The piston now goes past Bottom Dead Centre and starts moving up the cylinder,closing off the scavenge ports. The exhaust valve then closes and compression begins

The two stroke cycle can also be illustrated on a timingdiagram.

1 -2 Compression 1. approx 110 BTDC2 - 3 Fuel Injection2. approx 10 BTDC3 - 4 Power3. approx 12 ATDC4 - 5 Exhaust Blowdown4. approx 110 ATDC5 - 6 Scavenging5. approx 140 ATDC6 - 1 Post Scavenging 6. approx 140 BTDC

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ENGINE CONSTRUCTION

BED PLATEOperational Information The Two Stroke Crosshead Engine

The Bedplate

The Bedplate is the foundation on which the 2 stroke engine is built. It must berigid enough to support the weight of the rest of the engine, and maintain thecrankshaft, which sits in the bearing housings in the transverse girders, inalignment. At the same time it must be flexible enough to hog and sag with thefoundation plate to which it is attached and which forms part of the shipsstructure.If the bedplate was too rigid, then as the hull flexed, the holding down bolts,which secure the engine into the ship would be likely to break, and there would

be a danger of the bedplate cracking.Basically the bedplate consists of two longitudinal girders which run the lengthof the engine. Connecting these longitudinal girders are the transverse girderswhich are positioned between each crankshaft throw, and either side of thethrust collar. Built into the transverse girders are the main bearing pockets forthe crankshaft to run in.

The main functions of the engine bedplate are as follows:The bedplate must be strong enough for providing rigid support for the mainbearings and crankshaft.

It is the main platform for accurately mounting other parts such as columns,frames and guides which support engine cylinders, entablature and all workingparts.In large engines, must withstand heavy fluctuating stresses from operation of theengine and also transmit the load over an area to the ships hull.Collect crankcase lubricating oil and return to drain tank for further use. The two types of bedplate in general use is:The Trestle Type- Require elevated seating.The Box Form or Flat Bottom Type- More popular with most engine

manufacturers since the engine can directly be bolted to tank- top.

Forces applied to the bedplates:Firing load from cylinders.Side thrust from guide faces.Unbalanced inertia forces in the running gear.Weight of engine structure & running gear.Torque reaction from propeller.Hull deflections due to hogging, sagging, racking.

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Vibration due to torque variations, shock loading.Thermal stresses due to atmospheric and lubricating oil temperature changes.Inertia & gyroscopic forces due to ship's movement in heavy seas.

In addition to withstanding forces due to the above causes,, the bedplate should

provide.An oil tight chamber to contain the oil splash & spray of the forced lubricatingoil system.A drainage grid to filter out large particles before they enter the oil sump ordrain tank.A housing for the thrust bearing.Having provided for all the above the bedplate should also be small & light tokeep the overall size and mass of the engine to a minimum.

Basic Structure:The bedplate consists of longitudinal and transverse girders as shown below

Longitudinal Girders may be single or double plate construction

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Box girders -A box girder is stronger and more rigid then I or H section girder of thesame c.s.a.

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On the small bore engines, the bedplate can be made from cast iron as a singlecasting. Larger engines have a fabricated bedplate. This means it is welded together from steel sections, steel castings and plate. The steel is to Classification Societyspecifications and is a low carbon steel with a maximum carbon content of 0.23%.Earlier fabricated bedplates had box section longitudinal girders and box sectionfabricated transverse girders. Problems were encountered with cracking of thetransverse girders, which increased as engine powers and crankshaft throws got larger

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Operational Information Holding Down and Chocking

The engine is mounted on resin or cast iron chocks and bolted to the hull usingholding down bolts.The engine must be securely fixed into the ship. As the engine turns the propeller, the

propeller tries to push or thrust the propeller shaft and engine crankshaft forward intothe ship. The thrust bearing which is situated at the aft end of the engine transmits thisthrust from the crankshaft to the bedplate.The bedplate is mounted on chocks and is securely bolted to the engine foundation

plate on which it sits and which forms part of the structure of the hull.

The Engine must also be lined up with the propeller shaft. If the engine output drivingflange was higher or lower, or to port or stbd of the propeller shaft, then it is easy tovisualise that trying to connect them would cause bending stresses to be set up.The engine must also be bolted to a flat surface. If the surface was uneven, then when the

bolts were tightened the bedplate would be distorted, which in turn would distort thecrankshaft, causing unacceptable stresses to be set up when the engine was running.Before the engine is bolted down it is supported on jacks whilst it is aligned with thetailshaft bearing. This can be done by stretching a wire above the tailshaft and crankshsft,and measuring the distance from the wire to the crankshaft bearing centres. Modernmethods use a laser.

When the bedplate is in perfect alignment, cast iron chocks are hand fitted betweenthe machined underside of the bedplate and machined spots on the foundation plate.This is a skilled task and 80% contact is the aim.

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Once the engine is supported by the chocks the jacks are removed and the holdingdown bolts are tightened using a hydraulic jack to stretch the bolts.

Holding down bolts should be checked regularly for tightness. If they are allowed tocome loose, then the mating surfaces will rub against each other and wear away in a

process known as fretting. If this continues and the bolts are subsequently tighteneddown, the bedplate (and main bearings) will be pulled out of alignment.

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Conventional Holdingdown bolt

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Side Chocking

Side chocks are fitted to prevent the engine from movingsideways due to the movement of the vessel or becauseof the sideways component of thrust from thereciprocating and rotating parts.The chock is welded to the foundation plate as shown, aliner is hand fitted on a 100:1 taper and then drivenhome.

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This is a side chocking arrangement, where after driving the liner home, locking

screws are hardened down as shown.

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End Chock (aft end of the engine only)

Resin Chocking

Steel chocking has the disadvantages that each block must be individually fitted, a timeconsuming process, and after fitting are susceptible to fretting and wear. Resin chocks are

poured and therefore are much quicker to apply. They form into the shape of theclearance and key into surface imperfections. This much reduces damage due to frettingand removes bending momemts on the holding down bolts.

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The disadvantage is that the resin creation must be precise and that it is less straightforward to replace in the event of damage of misaligenement.

Properties The material used for the rsin chocking is Class tested to ensure minimum

standards. A sample cured in the correct way is tested for the following;

The impact resistance Hardness. Compressive strength (stress at maximum load) and modulus of

elasticity. Water absorption. Oil absorption. Heat deflection temperature. Compressive creep Curing linear shrinkage. Flammability.

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ENGINE CONSTRUCTION

A FRAME

Frames were earlier made of cast iron and made hollow to reduce the weight.They were sandwiched between bedplate and cylinder block by tie bolts, which left themin compression.The frames were later fabricated from mild steel tube and plate. Guides (cast iron ) were

bolted on the frames.This arrangement used individual frames at each cross girder (of the bedplate) position.The spaces between the frames along the length of the engine are fitted with plates boltedto the frames.

This type of structure is strong transversely, but comparatively little flexiblelongitudinally. Heavy covers or longitudinal stiffness are to be used to make side covers

oil-tight.This would be a weak structure to withstand a crankcase explosion.Alignment of cylinder block to bedplate would vary under ship movement.

Longitudinal girder construction is the latest development for this part of the structure.These, with most engines, are prefabricated steel; they carry guide surfaces and areusually bolted to bedplate and cylinder blocks or entablature, the latter being used for air supply purposes, jacket and cylinder support,

Operational Information The Two Stroke Crosshead Engine The A Frames

Otherwise known as the A Frames. These carry the crosshead guides and support theengine entablature (the cylinder block). On older engines, the A frames were individually

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erected on the bedplate directly above the transverse girders. When boxed in with platingthey formed the crankcase. The trend nowadays is to build the frame box as a separatefabricated construction and then, after stress relieving and machining the mating surfaces,to mount it on the bedplate. This has the advantage of saving weight.

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Lowering the A frame onto the bedplate. A small amount of jointing compound isused to ensure an oil tight joint.

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When the frames are aligned on the bed plate they are secured together by drillingand reaming and using fitted bolts.

Cracking in A frames can occur leading to misalignment and excessive wear of therunning gear. Cracks can start from welds, sharp changes in section and wherestrengthening stringers are terminated sharply. Repairs can involve cutting the crack out, grinding and rewelding. The danger is that after repair there may still bemisalignment.

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Frame with Guides

GUIDES IN THE CROSSHEAD TYPE ENGINEThese guides are fitted to crosshead engines and are vertical sliding bearings which locateand maintain alignment of the crosshead over the whole length of engine stroke.They are subjected to fluctuating load from the transverse components of the connectingrod reaction.Guide bars or surfaces are secured to the frame adjacent to the unit and have either cast

iron or steel bearing surfaces.Guide slippers (or shoes) are attached to the ends of the crossheads and may be free toarticulate: they are white metal lined with oil grooves lubricated from the crosshead.

Guide clearances must be checked periodically and should not exceed0.7 mm for a largeengine.Excess clearance will cause noise, wear on bearings and glands, uneven loads and

fatigue.There are two major forms of guide / guide way : the 2-faced guides and the four faced

guides are there as shown.

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Cross head guidesCross head guides Fi tted to cross head engines only.Vertical sliding bearings locates and maintain alignment of the cross headduring entire stroke.Subjected to fluctuating loads from conn. rod reaction

Guide bars are secured to the frame adjacent to the units.material CI or steelthere are 2 forms of guides:-

2 faced guide ( M.A.N ENGINE)4 faced guide (B&W, Sulzer)

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TIE-BOLTS

The entablature, A-frames and bedplate are held together by long tie-bolts that transmitthe combustion gases from the tops of the cylinder down to the bedplate cross-members.The tie-bolts are hydraulically tightened to pre-stress the structure, maintaining theengine structures in compression. Bracing screws are located at the length of the bolts toreduce the vibrations.The firing load from the cylinder covers is transferred through the bottom studs to thecylinder beams. The beam transfers the load through the tie-bolt nuts and the tie-bolts tothe bedplate cross girders.

Operational Information The Two Stroke Crosshead Engine The Tie Bolts or Tie Rods

To understand the importance of the role played by the tie bolts or tie rods, it is necessaryto appreciate what is happening inside the cylinder of the engine. When the piston is just after top dead centre the pressure inside the cylinder can rise ashigh as 140 bar (14000kN/m2).This acts downwards through the piston rod and con-rod, pushing the crankshaft down

into the bearing pockets.At the same time, the pressure acts upwards, trying to lift the cylinder cover.

The cylinder head studs screwed into the entablature prevent this happening and so thisupward acting force tries to lift the entablature from the frames and the frames from the

bedplate, putting the fitted location bolts into tension.As the piston moves down the cylinder the pressure in the cylinder falls, and then risesagain as the piston changes direction and moves upwards on the compression stroke.

This means that the fitted bolts are under are cyclic stress. Because they are not designedto withstand such stresses they would soon fail with disastrous consequences.

To hold the bedplate , frames and entablature firmly together in compression, and totransmit the firing forces back to the bedplate, long tie bolts are fitted through thesethree components and then tightened hydraulically. To prevent excessive bendingmoments in the transverse girders, the tie bolts are positioned as close to the centre of the crankshaft as possible. Because the tie bolts are so close to the crankshaft, someengines employ jack bolts to hold the crankshaft main bearing cap in position insteadof conventional studs and nuts.

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Operating the engine with loose tiebolts will cause the fitted bolts holding the bedplate, frame and entablature in alignment to stretch and break.The machined mating surfaces will rub together, corrode and wear away (this isknown as fretting).Once this has happened the alignment of the engine running gear will be destroyed.Loose tie bolts will also cause the transverse girders to bend which could lead to

cracking, and main bearing misalignment.Once fretting between the mating surfaces has occurred, then tightening of the tie

bolts will pull the engine out of alignment.The crosshead guides, the cylinder liner, and the stuffing box will no longer be in

line and excessive wear will occur.Because the tie bolts will no longer be pulled down squarely they will be subject to

forces which may lead to them breaking.If fretting has occurred, then the only solution is to remove the entablature or/andframe and machine the fretted mating surfaces (a very costly exercise).

Tie bolts can break in service.To reduce the risk of this happening they must be checked for tightness; notovertightened; and the engine not overloaded.If a breakage does occur, this is not disastrous, as the engine can be operated withcare for a limited period (the load on the engine may have to be reduced).

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The position of the fracture will dictate how the broken pieces are removed.

Tie-bolt centers should be as close to the crankshaft as possible to reduce bendingstresses on the girdles and to prevent unbalanced loads being transmitted into the

welds. Tie-bolt should be checked for tightness and flaws.If any of the bolts were slack, the cylinder beam would flex and lift at the location.Landing faces of the tie-bolt upper and lower nuts, landing faces of the cylinder beamon the frame would fret and machined faces would eventually be destroyed. The

bracing bolts would also be slackened.

Sulzer - Jackbolts

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In Sulzer Engines, instead of bolts and nuts, Jackbolts are used for tigtening themain bearing. By this arrangement, the tie rods are brought as close as possible tothe crankshaft centreline, which helps to reduce the bending stress in the crossgirders of the bedplate.

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The MC -C engine with its twin tie bolts is an exception, starting at the fwd end andworking aft. If the engine is fitted with bearing jacking bolts, then these must beslackened before tightening the tie bolts. Any pinch bolts fitted must also beslackened off

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CYLINDER Liner and Jacket (Entablature)

The structure above the bedplate and the frame to which the cylinders are attached isknown as the entablature.In 2-stroke engines, it is generally of box form.

The entablature is the name given to the cylinder block which incorporates the scavengeair space and the cooling water spaces.It forms the housing to take the cylinder liner and is made of cast iron.

castings are either for individual cylinders which after machining on the mating surfacesare bolted together to form the cylinder beam, or they may be cast in multi - cylinder units, which are then bolted together.The underside of the cylinder beam is machined and then it is aligned on the A framesand fastened in position using fitted bolts

It is important to remember that the fitted bolts used to bolt the entablature, A frames andBedplate together are for alignment and location purposes only.They are not designed to resist the firing forces which will tend to separate the threecomponents.This is the job of the tie bolts.

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In the photograph opposite, the liners can be seen in place in the entablature. Note alsothe diaphragm plate and the stuffing box housing

Entablature Mounted On A Frame With Liners In Place

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The engine frame of a modern 4 stroke medium speed diesel can be produced as a singlecasting or fabricated from cast steel sections and steel plates welded together.With this design, there is no separate bedplate, frame and entablature as with a 2 strokeslow speed engine.

The photograph shows the frame of an engine with the liners and crankshaft in place.

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Material.Cast iron is generally regarded as a suitable material for construction of diesel enginecylinder liner. In order to improve strength and induce specific desirable properties suchas strength and surface properties, cast iron is alloyed with the inclusion of smallquantities of nickel, chromium, molybdenum, vanadium, copper etc. Such inclusionsrefine the grain structure of the material. The total percentages of alloying inclusionsshould not exceed beyond 5%.

Good quality Pearlitic Grey Cast Iron consist of the following alloying material:Carbon: 3 to 3.4%. Its graphite flakes assist lubrication.Silicon: 1 to 2.0%. Improves fluidity and graphite formation.Manganese: 0.6 - 0.8%Phosphorous: 0.5% maximum. Reduces porosityVanadium: 0.15%. Refines grain structureTitanium: 0.05%. Improves strength

SpecificationUltimate tensile strength: 200 Mn/mm 2.Ultimate bending strength: 520 Mn/mm 2.

Ultimate compressive strength: 900 Mn/mm 2.Brinell Hardness: 180 - 220 HB.Ductility: 1 to 5% Elongation.Reasons for using Cast Iron:Can be cast in to intricate shapes.Has good wear resistance:Due to large surface of irregular shaped graphite flakes.Due to semi-porous surface holding oil pockets.

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Possesses good thermal conductivity.Damps out vibrations due to rapid combustion.Cheap material.

The cylinder liner forms the cylindrical space in which the piston reciprocates. Thereasons for manufacturing the liner separately from the cylinder block (jacket) in which itis located are as follows;The liner can be manufactured using a superior material to the cylinder block. While thecylinder block is made from a grey cast iron, the liner is manufactured from a cast ironalloyed with chromium, vanadium and molybdenum. (cast iron contains graphite, alubricant. The alloying elements help resist corrosion and improve the wear resistance athigh temperatures.).The cylinder liner will wear with use, and therefore may have to be replaced. Thecylinder jacket lasts the life of the engine.At working temperature, the liner is a lot hotter than the jacket. The liner will expandmore and is free to expand diametrically and lengthwise. If they were cast as one piece,then unacceptable thermal stresses would be set up, causing fracture of the material.Less risk of defects. The more complex the casting, the more difficult to produce a

homogenous casting with low residual stressesThe Liner will get tend to get very hot during engine operation as the heat energy fromthe burning fuel is transferred to the cylinder wall. So that the temperature can be keptwithin acceptable limits the liner is cooled.

The liner must be gauged regularly to establish the wear rate and check that it is withinmanufacturers tolerances. The wear rate for a medium speed liner should be below

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0.015mm/1000hrs. Excessive wear is caused by lack of lubrication, impurities in fuel air or Lubricating oil, bad combustion and acid attack.

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The diagram shows a cylinder liner from an older Sulzer RTA engine.The liner is cooled for most of its length using a water guide ring inserted into the

entablature.Bore cooling brings the cooling water close to the liner surface, before being transferred

to the cylinder head by the guide jacketProblems were experienced, especially on the long stroke engines with cold corrosiondue to overcooling towards the lower end of the liner.

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To counteract this, the outside of the liner was coated in an insulating material called"Haramaki", and inserts placed in the cooling bores to reduce the flow rate.

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The MAN B&W 2 stroke engine also utilises bore cooling on the large engines In theseengines, the bores are not tangential, but are blind holes drilled close to the liner surfaceas shown Steel tubes are inserted into these bores, almost to the end of the blind holes andthe cooling water passes up the tubes and overflows down the bores, thus giving acooling flow

The water then passes through transition pipes to the cylinder headThe smaller MAN B&W engines use a cooling water jacket external to the engineentablature to contain the cooling water On some versions there is a small amount of cooling in the entablature, on others, the cooling is completely external

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On some large bore, long stroke engines it was found that the undercooling further downthe liner was taking place.Why is this a problem?Well, the hydrogen in the fuel combines with the oxygen and burns to form water.

Normally this is in the form of steam, but if it is cooled it will condense on the liner surface and wash away the lube oil film.

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Fuels also contain sulphur.This burns in the oxygen and the products combine with the water to form sulphuric acid.If this condenses on the liner surface (below 140) then corrosion can take place.Once the oil film has been destroyed then wear will take place at an alarming rate.One solution is to insulate the outside of the liner so that there was a reduction in the

cooling effect.On The latest engines the liner is only cooled at the very top.

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Cylinder lubrication: Because the cylinder is separate from the crankcase there is nosplash lubrication as on a trunk piston engine.Oil is supplied through drillings in the liner.Grooves machined in the liner from the injection points spread the oil circumferentiallyaround the liner and the piston rings assist in spreading the oil up and down the length of the liner.The oil is of a high alkalinity which combats the acid attack from the sulphur in the fuel.

The latest engines time the injection of oil using a computer which has inputs from thecrankshaft position, engine load and engine speed. The correct quantity of oil can beinjected by opening valves from a pressurized system, just as the piston ring pack is

passing the injection point.

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Gauging a liner is carried out for two reasons: To establish the wear rate of theliner, and to predict if and when the liner will require changing.

Although on a 2 stroke engine the condition of the liner can be established byinspection through the scavenge ports (evidence of blowby, scuffing etc.), theliner is gauged during the routine unit overhaul (15000 hrs), or if the unit has to

be opened up for any reason

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The relative speed of the piston is low, and so the lubrication is only boundary.Because of these factors wear at the top of a liner increases to a maximum a fewcentimetres below the position of the top ring at TDC, and then decreases as the ring

pressure and liner wall temperature decreases and the piston speed increases building upa hydrodynamic film between liner and ring surfaces.Then as the piston slows down and the rings pass over the port bars, the wear willincrease due to boundary lubrication, a reduction in surface area, and oil being blown outinto the scavenge space.

A liner is gauged by measuring the diameter of the liner at fixed points down its length. Itis measured from port to stbd (athwartships) and fwd to aft. An internal micrometer isused because of its accuracy (within 0.01mm).To ensure that the liner is always measured in the same place, so that accuratecomparisons may be made, a flat bar is hung down the side of the liner with holes drilledthrough where the measurements are to be taken.

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Measurements are taken at more frequent intervals at the top of the liner where wear rateis expected to be highest.To ensure accuracy, the micrometer gauge is checked against a standard, and the liner and micrometer should be at ambient temperature. If the temperature is higher then acorrection factor can be applied. To ensure micrometer and liner are at the sametemperature, lay the micrometer on the entablature for a few minutes before starting.The readings can be recorded in tabular form, and from the data obtained the wear rate/1000 hours can be calculated. Wear rate varies, but on a large 2 stroke crossheadengine ideally should be about 0.05mm/1000 hours. On a medium speed trunk pistonengine where the procedure for gauging is similar, the wear rate is around 0.015mm/1000hours.

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The cylinder liner is cast separately from the main cylinder frame for the same reasons asgiven for the 2 stroke engine which are:The liner can be manufactured using a superior material to the cylinder block. While thecylinder block is made from a grey cast iron, the liner is manufactured from a nodular cast iron alloyed with chromium, vanadium and molybdenum. (cast iron containsgraphite, a lubricant. The alloying elements help resist corrosion and improve the wear resistance at high temperatures.)The cylinder liner will wear with use, and therefore may have to be replaced. Thecylinder jacket lasts the life of the engine.At working temperature, the liner is a lot hotter than the jacket. The liner will expandmore and is free to expand diametrically and lengthwise. If they were cast as one piece,then unacceptable thermal stresses would be set up, causing fracture of the material.Less risk of defects. The more complex the casting, the more difficult to produce ahomogenous casting with low residual stresses.

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Modern liners employ bore cooling at the top of the liner where the pressure stress is highand therefore the liner wall thickness has to be increased. This brings the cooling water close to the liner surface to keep the liner wall temperature within acceptable limits sothat there is not a breakdown in lubrication or excessive thermal stressing. Although theliner is splash lubricated from the revolving crankshaft, cylinder lubricators may be

provided on the larger engines.

On the example shown opposite, the lubricator drillings are bored from the bottom of theliner circumferentially around the liner wall. Another set of holes are drilled to meet upwith these vertically bored holes at the point where the oil is required at the liner surface.Other engines may utilise axial drillings as in a two stroke engine.

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Note that the liner opposite is fitted with a fireband. This is sometimes known as anantipolishing ring. It is slightly smaller in diameter than the liner, and its purpose is toremove the carbon which builds up on the piston above the top ring. If this carbon isallowed to build up it will eventually rub against the liner wall, polishing it anddestroying its oil retention properties

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Cylinder cover

The cylinder head forms the third and last component of the combustion chamber.Its main function is to close the end of the cylinder and seal in the gases as theyundergo a cycle involving extreme pressure and temperature. Stresses from theseextreme gas pressure and temperature may lead to cracks.

Cylinder heads in four-stoke engines have to accommodate valves and passages for theintroduction of air and the exit of exhaust gases. Valves found in four stroke engineswould be:IntakeExhaustFuel injector Relief valveIndicator cock Air starting valve

Those found in two stroke engines are:Large exhaust valveFuel injector Relief valveIndicator cock Air starting valve. There are no inlet valves. Loop and cross scavenging two-strokeengines need not accommodate any exhaust valves as they are not required.

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Cylinder heads for 4 stroke engines are of a complex design. They have to house the inletand exhaust valves, the fuel injector, the air start valve, relief valve and indicator cock.The passages for the inlet air and exhaust gas are incorporated, as are the cooling water

passages and spaces.

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Earlier engines were often fitted with two part cylinder cover.CAST IRON IS NOT SUITABLE FOR MODERN TWO STROKE VENGINESThe cylinder cover must be able to with stand gas loads with tends to deform its shape.Cast iron is not good at with standing bending stresses.

Hence steel is used with bore cooling.

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VALVE AND VALVE GEARS

Valve Gear:It designates the combination of all parts, including the various valves, which control the

admission of air charge and the discharge of exhaust gases in four stroke engines, thedischarge of exhaust gases in some two stroke engines (uniflow scavenging type), theadmission of fuel in air- injection and some mechanical-injection engines, and theadmission of compressed air for starting most of the larger engines.

Valve Actuating Gear:It designates the combination of those parts only which operate or actuate the variousintake, exhaust, fuel and air-starter valves, open and close them at the proper moment inrespect to the position of the piston and crankpin, and hold them open during the requiredtime.

Valve Timing Gear:It designates the combination of those parts only which affect and control the moment of opening and closing of the valves with respect to crank and piston position. These partsinclude cams, camshaft and camshaft drive. The valve gears of diesel engines varyconsiderably in their construction, depending on type, speed, and size of the engines.

C.13.VALVES AND VALVE GEARS.Valves - Valves are used to cover / uncover the passage of flow .Valve Gears to produce action on valves - combination of parts, including valves

which controls the operation of above.In all 4 stroke engines admission of air charge, discharge of exhaust Gas and in many

2 stroke engines discharge of exh. Gas.Basic drive - c/shaft drives cam shaft by gears or chain.Cams on the camshaft lifts push rod, transmits the action to rocker arm to operate the

valves for mechanical drive. for hydraulic drive the cam drives a hyd. Actuator, theoil in turn moves the valve by a piston.As soon as the closing side of the cam moves under the transmitting mechanism the

valve spring starts to return the valve to its seat( closed)

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Valve operating gear

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Valve Requirementto get fresh air into engine and exhaust gas outExh v/v opening (size) is as big as possible for 2 stroke engine exh open for short

duration, so to reduce back pressureInlet v/v opening (size) more important in 4 st. engines to reduce pumping loss and

also increase volumetric efficiency.Some 4 st engines have 2 inlet and 2 exh v/vs. for space arrangement, less v/v

opening, cooler valves.valve construction

. Mushroom- shaped poppet type. Head and stem as one pieceseating edge beveled at a 45* / 30* angleInlet v/v cooler - carbon or low alloy steelExh v/v hotter silicon-chromium steel ( nickal, chromium)v/v moves in a removable guide fitted in cylinder head.Springs holds the valves firmly against the seat.

valve construction

Head of the valve is cooled -conducts heat to seat in cyl.head (water clg). Theseat is a removable seat fitted in cyl head with cooling arrangement.The clearance between valve and guide due to excess wear overheating of

valve, carbon forms and sticky, excess oil consumption.To make valve and seat faces wear resistance, valve and seat faces are

hardened with cobalt-chromium-tungsten (stelite).Seat rings of wear resistantmaterial are also used, in some casesvalve cages to make valve seat removal easier, valve and seat as one unit and

fitted on cyl head. cage may be separately cooled.Some exh v/vs rotated a slight amount each revolution to keep the valve clean

(carbon deposits) and ensure even wear between v/v and seat.Timing gear

Responsible for actuating the valves at right time with respect to c/shaft (Pistonposition)In 4 st engine the camshaft speed is half the c/shaft speed.Chain drive and gear drive.

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Two different sized springs are fitted to aid positive closing of the valves. Thereason for fitting two springs are that if one fails, the other will prevent the valvedropping down into the cylinder. The two springs have different vibrationcharacteristics, so the incidence of resonance is reduced. (resonance is where twoitems vibrate at the same frequency thus the amplitude of the vibration isamplified.)

Caged Exhaust valve

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Valve cage

Camshafts in 4 st engines carries the cams for inlet valve, exh valve & fuel pump. in 2 st exh v/v type engines carries the exh cams & fuel pump cams. Additionally may carry cams for air starting operation and other aux.

Operations. construction forged as one piece including the cams or separate cams

keyed on a shaft. In large engines camshaft in sections, with cams eitherintegral or keyed / keyless fitting.

camshaft is supported by bearings plain bush or split sleeve.Pushrods

Generally tubes to reduce weight.

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The lower end contacts the follower which carries a roller( tappet roller)running on the cam.

The upper end is fitted with a cup.The end of the rocker arm (fitted with atappet bolt- end rounded shape)) fits into the cup.

Camshaft

There are several different methods of manufacturing camshafts for medium speed 4stroke marine diesel engines. On the smaller engines, the camshaft may be a singleforging complete with cams.Alternatively the camshaft can be built up in single cylinder elements, each element madeup of the fuel, inlet, and exhaust cam on a section of the camshaft with a flange on each

end.So that the element can be used on any unit in the engine, the number of holes for fitted bolts in the flanges must be sufficient to allow the cam to be timed for any unit on theengine.

For example, on a six cylinder engine, the flanges must have 6 equi spaced holes or amultiple thereof. The cams must be hard enough to resist the wear and abrasion due to

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impurities in the lub. oil, yet they must be tough enough to resist shattering due to shock loading. The cams are therefore surface hardened using the nitriding process.

On the larger engines it is usual to manufacture the camshaft and cams separately. Thenitrided alloy steel cams are then shrunk on to the steel shaft using heat or hydraulicmeans. Because the cams are fitted progressively onto the shaft, if the bores in the camswere all the same diameter, it would be very difficult, if not impossible, to fit the firstcams all the way along the length of the shaft to the correct position. To overcome this

problem the camshaft is stepped, with the largest diameters at the end which has the camsfitted first. The larger bored cams fit easily over the small diameter steps till they reachthe correct position on the camshaft.

Keys are not generally used to locate the cams as they would act as stress raisers.Most medium speed engines are unidirectional (i.e they only rotate one way). This is

because they either are driving an alternator, or because if they are used as direct main propulsion they tend to be driving a controllable pitch propeller. In the case where theengine is reversing, then the camshaft has two sets of cams, one for ahead operation, andone for astern.

To reverse the direction of the engine, pressure oil is led to one side of a hydraulic pistonwhich is coupled to the camshaft. The whole camshaft is moved axially and the camfollowers slide up or down ramps which connect the ahead and astern cams.

The camshaft is either chain or gear driven from the crankshaft. Because the engine is afour stroke, the camshaft will rotate at half the speed of the crankshaft. (the valves andfuel pump will only operate once for every two revolutions of the crankshaft).In a case where the cams are shrunk on the camshaft, if a cam becomes damaged and hasto be replaced, then it can be cut off using a cutter grinder. Care must be exercised not todamage the camshaft or adjacent cams during the operation

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The replacement cam is fitted in two halves which is then bolted on the camshaftin the correct position and the timing rechecked

CUT SECTION OF CAM

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Rocker ArmsTo actuate the valves in the cyl head via cam,cam follower and pushrod tappets.RA moves at an angle to vertical also some horizontal thrust-on valve stem, causes

wear on guide.Attachment to head by stanchion bolted.Swings on steel fulcrum pin or pivot / needle bearing.

Contact to v/v stem by roller / screw.(Tappet)Tappet clearance provided on the valve side to take care of wear and expansion to ensure v/v closes firmly.lubrication of fulcrum and contact points done.

Springsserves to close valves, made of highly tempered steel wire wound in a spiral coil.to prevent bouncing the spring is maintained in compression all time.

Valve Clearances

To allow thermal expansion. To be adjusted regularly due to wear.Clearance is required between valve stem and RA, when follower is on base of the

cam (v/v closed). If not the v/v will remain partly open.If more v/v will open late and close early , reduces the lift (stroke), and causes noise.If less open early and close late, increases the lift of the valve.It may prevent the

valve from closing completely as it expands

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To set valve clearance a feeler gauge is used in conjunction with tappet adjustment tomanufacturer specification.

Rocker or Tappet Clearances

Rocker or Tappet clearances refer to the clearance between the top of the valvespindle and the rocker arm. It is to ensure that the valve closes properly when itexpands as it gets to operating temperature. Clearances are set according tomanufacturers instructions, but usually done with the engine cold, and with thepush rod follower on the base circle of the cam. (one way of ensuring this is toturn the unit being adjusted to TDC on the power stroke.)

Hydraulically actuated Exhaust valve

No need for RAValve opens by hyd oil pressurethe actuating gear is equipped with a locking device to retain the roller guide in its

top position so that exh valve can be kept out of operation.

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Crankshaft

Function to convert reciprocating motion of piston to that of rotary motion at theoutput shaft.Consists of journals, crank webs and crankpin (conn rod journal)Two types single piece (4 stroke)and shrunk fit type (2 stroke,large).crank throw distance from c/l of main journal to c/l of crank pin equel to half of

engine stroke.Counter weights added to webs opp.to crankpins, improves the balancing of engine

and relieves the load on main bearing.Tyes of Crankshaft

Fully built webs are shrunk on to the main journal and crankpin large marinediesel engine.Semibuilt webs and crankpin as one unit shrunk on to journal large and medium

speed marine diesel engineSolid forged one piece, either cast or forged high speed diesel engine

Stresses in Crankshaft

The crankpin is like a builtin beam with a distributed load along its length thatvaries with crank position. Each web is like a cantilever beam subjected to bending& twisting. Journals would be principally subjected to twisting.

1.Bending causes tensile & compressive stresses.2.Twisting causes shear stress.3.Duto shrinkage of the web onto the journals, compressive stresses are set up in

journals & tensile hoop stresses in the webs.

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The force that occur in a vertical diesel engine crankshaft are as follows:i) Static weight of engine components (moving).ii) Alternating forces produced by varying gas pressure.iii) Inertia forces of the moving parts.iv) Centrifugal force at crank.v) The crank-web is subjected to tensile, compressive and shear stresses. Shear in way of

journal.

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MATERIALS.In the case of large marine diesel engine the type of shaft generally favored is the cast or forged steel semi-built, a typical analysis, method of construction and testing would be asfollows:Material analysis: Cast steel

Element. Percentage.Carbon 0.2Silicon 0.32.Manganese 0.7Phosphorus 0.01Sulfur 0.015Remainder iron.

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The Two Stroke Crosshead Engine The Crankshaft

The crankshafts on the large modern 2 stroke crosshead engines can weigh over 300tonnes.They are too big to make as a single unit and so are constructed by joining together

individual forgings.On older engines the so called fully built method was used.This consisted of forging separate webs, crankpins and main journals.The crankpins and journals were machined and matching holes bored in the webs,which were slightly smaller in diameter.

The webs were heated up and the crankpins and journals fitted into the holes(which due to the heat had expanded in size). As the webs cooled down, so thediameter of the bored holes would try and shrink back to their original size. Indoing so, the crankpins and journals would be gripped tightly enough to stop thembeing able to slip when the engine was being operated normally.

Today, crankshafts for large 2 stroke crosshead engines are of the semi builttype. In this method of construction the crankshaft "throws" consisting of two

webs and the crankpin are made from a single forging of a 0.4% carbon steel.The webs are bored to take the separately forged and machined main journalswhich are fitted into the webs using the shrink fitting method described above.The shrink fit allowance is between 1/570 and 1/660 of the diameter.The advantages of this method of construction is that by making the two websand crankpin from a single forging the grain flow in the steel follows the webround into the crankpin and back down the other web.

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Because the crankpin and webs are a single forging, the webs can be reduced inthickness and a hole is sometimes bored through the crankpin as shown,reducing the weight without compromising strength.

Built up Crankshaft Manufacture

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Crankshaft views

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THE WELDED CRANKSHAFT

The welded crankshaft was developed in the 1980s. It was made up of a series of

forgings each comprising of half a main journal, web, crankpin, second web, andhalf a main journal. These forgings were then welded together using asubmerged arc welding process to form the crankshaft. After welding the

journals were stress relieved and machined. As well as having the advantage of continuous grain flow, the webs could be made thinner (no shrink fit toaccommodate), leading to a lighter shorter crankshaft.Why aren't all crankshafts produced by this method? Cost! It was veryexpensive and only about twenty crankshafts were produced by this method.They have performed very well in service however.

All Welded C/shaft

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Crankshaft and bearings

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In the hydraulic forging press the crankshaft throws and flanges are formed.

The crankshaft is locally heated to a white heat where the webs are desired to beformed. The crankshaft is then compressed axially to form the start of the webs

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The forgings are then machined, stress relieved, and the radii at the change of sectioncold rolled.If the crankshafts are to be surface hardened they are made of a steel alloy known asnitralloy (a steel containing 1.5%Cr, 1% Al and 0.2% Mo)The crankshaft is heated to 500C in ammonia gas for up to 4 days. The nitrogen

dissociates from the ammonia gas and combines with the chromium and aluminium toform hard nitrates at the surface. The molybdenum refines the grain structure at thestill tough core.

At the change of section between journal and web and web and crankpin, fillet radiiare machined so there is not a sharp corner to act as a stress raiser. These radii arecold rolled to remove machining marks, harden the surface and to induce a residualcompressive stress, again to increase fatigue resistance.Re-entrant fillets are sometimes employed; This allows for a shorter crankshaftwithout compromising on bearing length.

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Oil Holes in Crankshafts.

Unlike the crankshafts for slow speed 2 stroke crosshead engines, which lubricatethe bottom ends by sending the oil DOWN the con rod from the crosshead, the

crankshaft for the medium speed trunk piston engine must have holes drilled in itso that oil can travel from the main bearing journals to the crankpin and then UPthe con rod to lubricate the piston pin and cool the piston. If the surface finish of the holes is not good, then cracks can start from the flaws.

At the exit points on the crankpin, the holes must be smoothly radiused. So thatthe crankshaft strength is not compromised the holes should be positionedhorizontally when the crank is at TDC.

Crankweb Formation from Round Rod

Web Formation

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Crank Formation

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bearings - circular sections are most suitable for this purpose.Stress and load concentration is reduced at the ends of the rod by increasing the areathrough a tapered section, having generous fillets. Solid ends provide a rigid platform for the top end-bearings and gives good support to the bottom end bearing. This essentiallyused for thin shell bearings, to prevent fretting between the back of the shell and itshousing.Accurate and uniform pre-tensioning of the bottom end bolts is necessary to:1. Reduce the risk of fretting between palm and housing.2. Eliminate bending moments on the bolts (caused by uneven tightening, resulting instress concentration in the root of the thread.3. Reduce the range of stress fluctuation, which is a major factor in fatigue failure (themaximum stress way be increased but, the fluctuation range is reduced).4. Provide the correct nip to the thin shell bearings (to prevent fretting on the blocking

piece and fatigue-crazy cracking on the bearing surface).For lower power engines a forked top end arrangement has been used which allows topend bearing to be integral with the connecting rod and provide access to the piston rodnut.With increased power the greater flexibility of this design resulted in:1. Misalignment between top-end pins and bearings resulting in edge loading (due to loadacting between the forks, producing a bending moment).2. Cracking at root of the fork due to repeated flexing resulting in fatigue.The bottom end bearing is located on the palm by a spigot through which the oil passes.This helps torelieve the bolts of shear forces imposed by the transverse loads.

Failures:Usually due to abrupt stopping the engine or breakage of bottom end bolts.Cracks may develop:1. Around the edges of the boltholes.2. On the underside of the foot running across the line of fillet run out (particularly if compression plates are fitted).Materials for Connecting Rod:Forged steel: Carbon: 0.30 0.50% (Normalised).

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U.T.S: 500 700 N/mm2.Forgings should have a fine grain structure. It should be free from coarse non-metalicinclusions and segregations especially in highly stressed areas.

Connecting Rod Bolts:

Important Designing Considerations:? Well-formed fillet between bolt head and shank. There should be a proper chamfer atthe mouth hole.? There should be smooth radii wherever there is a change in diameter.? Surface of the bolt should be given a high degree of finish.? It would be beneficial to reduce the diameter of bolt shank less than the core diameter atthe bottom of the thread (about 10% less).? Bolt material should have adequate strength and high resilience.? It would be ideal to make the bolt of uniform cross-sectional area but it is necessary tohave certain parts of shank enlarged in diameter for the fitting portions.Bolt Material:

Low alloy steel (alloy content < 5%).U.T.S: 750 to 1100 N/mm2.Tightening of Bolts:Tightening of important bolts such as these should not be left to chance.Following methods are in use:1. Applying the desired preload by means of hydraulic cylinder and following up nut.2. Measuring the extension of the bolt with a micrometer device whilst the bolt istightened.3. Hand tightening lightly, and then turning up the nut through a predetermined andcalculated angle with respect to the bolt.4. Using torque spanner, e.g. a spanner which reads the torque or set to give way at a

predetermined torque.Methods (1) & (2) as mentioned above are most accurate. Method (3) is good if boltstiffness is knownand calculation is accurate.Torque spanners (method 4) are useful for small medium sized bolts; care has to be takenas regardsLubrication.Failure in Bolts:Failure is essentially due to fatigue.Factors contributing to failures are as follows:1. Stress concentrations at bolt heads, change of section, surface finish etc.2. Over stretching of bolt.3. Uneven tightening.4. Inadequate pretension.5. Improper seating of nut or bolt head causing bending stresses.6. Corrosive attack in the form of bending.

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Conn

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• 8/2/2019 Marine Diesel Engine Dual

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