Basic Marine Engineering for Maritime Students

8/11/2019 Basic Marine Engineering for Maritime Students

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Course


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OBJECTIVES Accurately identify the

structural parts of a dieselengine

Correctly state the function ofeach part of a diesel engine

Explain in detail the cycle ofoperation in a two-strokeengine

Explain in detail the cycle ofoperation in a four-strokeengine; and

Explain in detail the basicdifferences between the four-stroke and the two-strokediesel engine


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Section 1 of Chapter 1


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Diesel engines are

widely used as stationary

power sources for electrical

generation units, pumping

stations, refrigeration

facilities and factories.

Heavy construction

equipment, locomotives,

commercial trucks andsome large pickups are

powered by diesels.


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Inventor of the dieselengine. It is a type of internalcombustion engine in whichheat caused by aircompression ignites the fuel.

At the instant fuel is injectedinto a diesel enginescombustion chambers, the airinside is hot enough to ignitethe fuel on contact. Dieselengines, therefore, do not

need spark plugs, which arerequired to ignite the air-fuelmixture in gasoline engines.


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TWO-STROKE ENGINE FOUR-STROKE ENGINE


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The two stroke Dieselengine does not mix fuel or oilwith the combustion air. Thecrankshaft bearings are lubricatedfrom pressurized oil in the sameway as a four stroke engine.

The two stroke cycle is socalled because it takes twostrokes of the piston to completethe processes needed to convertthe energy in the fuel into work.Because the engine is

reciprocating, this means that thepiston must move up and downthe cylinder, and therefore thecrankshaft must revolve once.


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It allows the engine to burn Heavy Fuel

Oil (HFO) efficiently.

This slow speed, around 100 rpm allows a

direct coupling of the propellers shaft to

the crankshaft, eliminating the need for

gearing and such.

It has less complicated design, NOINTAKE VALVES which reduces the

possibility of things failing.


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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 over100 bar and the temperature over

500C


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Just before TDC fuel is

injected into the cylinder by the fuel

injector. The fuel is "atomized" into

tiny droplets. Because they are very

small these droplets heat up very

quickly and start to burn as the pistonpasses over TDC. The expanding gas

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

into the engine; during the upwardstroke of the piston, the engine is

having to do the work.


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As the pistonmoves down the cylinder,the useful energy from theburning fuel is expended.

At about 110 after TDCthe exhaust valve opensand the hot exhaustgas (consisting mostly ofnitrogen, carbon dioxide,

water vapor and unusedoxygen) begin to leavethe cylinder.


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At about 140 afterTDC the piston uncovers a setof ports known as scavengeports. Pressurized air entersthe cylinder via these ports

and pushes the remainingexhaust gas from the cylinderin a process known as"scavenging".

The piston now goespast Bottom Dead Centre and

starts moving up the cylinder,closing off the scavenge ports.The exhaust valve then closesand compression begins


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The piston continues tomove down and just prior touncovering the air intake ports nearthe bottom of the cylinder, theexhaust valve opens, releasing thepressure inside the cylinder. Theexhaust gases at this stage are about600C. The continued downward travelof the piston uncovers the inlet ports,where positively charged airs, 30 to70kPa, provides a scavenging effectto drive exhaust gases out throughthe exhaust valve. This action notonly clears the cylinder of spent

gases but also cools the cylinder. Thecycle begins a new once the pistonreaches the bottom dead center(BDC).


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1 -2Compression

2 - 3 FuelInjection

3 - 4 Power

4 - 5 ExhaustBlow down

5 - 6

Scavenging 6 - 1 Post

Scavenging

1. approx.110 BTDC

2. approx. 10BTDC

3. approx. 12ATDC

4. approx.110 ATDC

5. approx.140 ATDC

6. approx.140 BTDC


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The four stroke cycle isso called because it takes fourstrokes of the piston to completethe processes needed to convertthe energy in the fuel into work.Because the engine is

reciprocating, this means that thepiston must move up and downthe cylinder twice, and thereforethe crankshaft must revolve twice.

The four strokes of thepiston are known as the induction

stroke, the compression stroke,the power stroke, and the exhauststroke. Students sometimesremember this as "suck, squeeze,bang, blow."


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INDUCTION: The

crankshaft is rotating

clockwise and the piston is

moving down the cylinder.

The inlet valve is open anda fresh charge of air is being

drawn or pushed into the

cylinder by the turbocharger


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COMPRESSION: Theinlet valve has closed and thecharge of air is beingcompressed by the piston as itmoves up the cylinder.Because energy is beingtransferred into the air, itspressure and temperatureincrease. By the time thepiston is approaching the top

of the cylinder (known as TopDead Centre or TDC) thepressure is over 100 bar andthe temperature over 500C


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POWER: Just beforeTDC fuel is injected into thecylinder by the fuel injector.The fuel is "atomized" into tinydroplets. Because they are verysmall these droplets heat up very

quickly and start to burn as thepiston passes over TDC. Theexpanding gas from the fuelburning in the oxygen forces thepiston down the cylinder, turningthe crankshaft. It is during this

stroke that work energy is beingput into the engine; during theother 3 strokes of the piston, theengine is having to do the work.


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EXHAUST:As thepiston approaches the bottomof the cylinder (known asBottom Dead Centre or BDC)the exhaust valve starts to

open. As the piston nowmoves up the cylinder, the hotgases (consisting mostly ofnitrogen, carbon dioxide, watervapor and unusedoxygen) are expelled from

the cylinder.As the Pistonapproaches TDC again theinlet valve starts to open andthe cycle repeats itself.


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Intake stroke points 1

to 2

Compression Stroke

points 2 to 3 Fuel injection stage

points 3 to 4

Which begins at about 10

degrees before TDC and

ends about 35 to 40

degrees after TDC.

Power Stroke points 4 to

5


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The Bedplate is the foundation onwhich the 2 stroke engine is built. It must berigid enough to support the weight of the rest

of the engine, and maintain the crankshaft,which sits in the bearing housings in thetransverse girders, in alignment. At the sametime it must be flexible enough to hog and

sag with the foundation plate to which it isattached and which forms part of the shipsstructure.


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BEDPLATE FRAME MAN B&W BEDPLATE


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The Frames are otherwise known as the AFrames. These carry the crosshead guides andsupport the engine entablature (the cylinder block).On older engines, the A frames were individually

erected on the bedplate directly above thetransverse girders. When boxed in with plating theyformed the crankcase. The trend nowadays is tobuild the frame box as a separate fabricatedconstruction and then, after stress relieving and

machining the mating surfaces, to mount it on thebedplate. This has the advantage of saving weight.


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The entablature is the name given to

the cylinder block which incorporates the

scavenge air space and the cooling water

spaces. It forms the housing to take thecylinder liner and is made of cast iron.


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To hold the bedplate , frames and entablaturefirmly together in compression, and to transmit thefiring forces back to the bedplate, long tie bolts arefitted through these three components and then

tightened hydraulically. To prevent excessivebending moments in the transverse girders, the tiebolts are positioned as close to the center of thecrankshaft as possible. Because the tie bolts are soclose to the crankshaft, some engines employ jack

bolts to hold the crankshaft main bearing cap inposition instead of conventional studs and nuts.


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The crankshafts on the large modern 2 strokecrosshead engines can weigh over 300 tonnes. Theyare too big to make as a single unit and so areconstructed by joining together individual forgings.

On older engines the so called fully built method wasused. This consisted of forging separate webs,crankpins and main journals. The crankpins and

journals were machined and matching holes bored inthe webs, which were slightly smaller in diameter.

The webs were heated up and the crankpins andjournals fitted into the holes (which due to the heathad expanded in size).


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A semi built crankshaft in the lathe


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The Connecting Rod is fitted between thecrosshead and the crankshaft. It transmits thefiring force, and together with the crankshaftconverts the reciprocating motion to a rotary

motion. Made from drop forged steel, on theolder engines the bottom of the con rodterminates in a flange known as a Marine Palmwhich is bolted to the split bottom end

(Crankpin) bearing, whilst at the top anotherflange is formed on which is bolted the twocrosshead bearings.


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Connecting Rods

on the later engines are

produced as a single

drop forgingincorporating the top

half of the crankpin

bearing housing and the

bottom half of the solidcrosshead pin bearing

housing.


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The crosshead pin connects the piston

rod to the connecting rod. On either side of

the crosshead pin are mounted the

crosshead slippers. The slippers run up anddown in the crosshead guides as the piston

and rod are reciprocating and prevent the

top of the connecting rod from movingsideways.


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The crosshead bearing is

difficult to lubricate effectively. Because

the top of the connecting rod swings about

the pin and changes direction each time

the piston reaches mid stroke, the relative

speed between bearing and pin at mid

stroke is zero, accelerates to a maximumas the piston approaches top or bottom

dead center and then decelerates back to

zero again as the piston approaches mid

stroke and the con rod changes direction.

This means that hydrodynamic lubrication,

where the pin is separated from the

bearing by a wedge of oil only occurs over

part of the swing; i.e when the relative

speed between the two components is

high enough.


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The cylinder liner forms the cylindrical space in which the piston reciprocates. The reasons

for manufacturing the liner separately from the cylinder block (jacket) in which it is located are as

follows;

The liner can be manufactured using a superior material to the cylinder block. While the

cylinder block is made from a grey cast iron, the liner is manufactured from a cast iron alloyed with

chromium, vanadium and molybdenum. (cast iron contains graphite, 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. The cylinder

jacket lasts the life of the engine.

At working temperature, the liner is a lot hotter than the jacket. The liner will expand more

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 stresses.

The Liner will get tend to get very hot during engine operation as the heat energy from the

burning fuel is transferred to the cylinder wall. So that the temperature can be kept within acceptable

limits the liner is cooled.


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The Piston comprises of two pieces; the crownand the skirt. The crown is subject to the hightemperatures in the combustion space and thesurface is liable to be eroded/burnt away. For this

reason the material from which the crown is mademust be able to maintain its strength and resistcorrosion at high temperatures. Steel, alloyed withchromium and molybdenum is used, and somepistons have a special alloy welded onto the hottest

part of the crown to try and reduce the erosioncaused by the burning fuel. The crown also carriesthe 4 or 5 piston ring grooves which may be chromeplated.


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THIS PHOTO SHOWS THENOZZLE PLATE AND

NOZZLES

THIS PHOTO SHOWS THEUNDERSIDE OF THE

PISTON CROWN


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Hydraulic exhaust valve


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GEARED DRIVE CHAIN DRIVE


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Fuel has to be injected into the engine at ahigh pressure so that it atomizes correctly.Injection takes place over a short period oftime and this period of time must be

accurately controlled; late or early injectionwill lead to a lack of power and damage to theengine. Because the timing of injection iscrucial, cams mounted on the camshaft,

which is driven by the crankshaft are used tooperate the fuel pumps, one of which isprovided for each cylinder.


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MAN B&W VIT PRINCIPLE FUEL PUMP


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MAN B&W INJECTOR


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Before then the pressurized airneeded to "scavenge" thecylinders of the exhaust gasesand supply the charge of air forthe next combustion cycle wasprovided by mechanically

driven compressors (RootsBlowers), or by using the spaceunder the piston as areciprocating compressor(Under Piston Scavenging).This of course meant that theengine was supplying the work

to compress the air, whichmeant that the useful workobtained from the engine wasdecreased by this amount. Turbochargers on a Sulzer RTA96


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Exhaust valves aresubject to arduousconditions, and requireregular overhaul. To aidthis, exhaust valves are

often fitted in separatecages. This allows theexhaust valve to bechanged and overhauledwithout removing thecylinder head. The cageshave water coolingpassages connected tothe cylinder head coolingwater.


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Basic Marine Engineering for Maritime Students

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