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MTM 3202
Diesel propulsion systems
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Diesel propulsion systems
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INTERNAL COMBUSTION ENGINES
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Course Learning Objective:Course Learning Objective: FamiliarizeFamiliarize
students to the basic cycle and design featuresstudents to the basic cycle and design featuresof modern marine diesel enginesof modern marine diesel engines
Specific Objectives:Specific Objectives:
- Define the theory and principle of Internal Combustion Engine- Define the theory and principle of Internal Combustion Engine
- Describe basic operations of working cycle- Describe basic operations of working cycle
- Identify the engine timing diagram
- Describe differences and advantages of 2S & 4S
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Principle of I.C.E An internal combustion engine is one in
which the fuel is burnt within the engine ->usually of the reciprocating type.
It involve system where combustion of the
fuel and the conversion of the heat energyfrom combustion to mechanical energytakes place within the cylinders (ICE)
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I.C.E. CATEGORIES
Spark ignition engines use gaseous or volatiledistillate fuels -> work on a modified Ottocycle -> operate on the 2 or 4 stroke cycle.
Compression ignition engine use distillateliquid fuels -> work on either 2 or 4 strokecycle and normally designed to operate on thedual-combustion cycle (Otto and Diesel cycle)
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Otto cycle
In the Otto cycle the theoretical pressure
volume diagram is formed from : two constant
volume and two adiabatic processes.
The air in the cylinder is compressed
adiabatically.
Heat is added to the air at constant volume ->
Work is done during the adiabatic expansion
and -> then heat is rejected at constant volume
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Otto Cycle
A B : Adiabatic compression
B C : Heat received at constant volume (combustion)
C D : Adiabatic expansion
D A : Heat rejected at constant volume (exhaust)
P r
e s
s u
r e
(
P
a )
Volume (m3)V1 V2
P1
P2
A
B
C
D
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Otto Cycle
1. The induction stroke takes place at A. Although in theory thepressure should be the same as atmospheric, in practice it's ratherlower. The amount of petrol air mixture taken in can be increased by useof a supercharger.
2. A to B is the compression stroke. Both valves are closed. Thecompression is adiabatic, and no heat enters or leaves the cylinder.
3. Ignition occurs at C. The gases resulting from the ignition expandadiabatically, leading to the power stroke.
4. D to A the gas is cooled instantaneously.
5. At A the exhaust stroke occurs and the the gases are removed atconstant pressure to the atmosphere.
6. Strange as it may seem, the piston does half a revolution at A.Actually it's slightly in practice, as the the valve timing is more complex.
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Diesel cycle
In the diesel cycle the theoretical pressure-
volume diagram is formed from two adiabatic
operations, one constant-pressure and one
constant-volume operation. Air is compressed adiabatically, then heat is
added at constant pressure. Adiabatic expansion
takes place and then heat is rejected at constant
volume
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Diesel Cycle
A B : Adiabatic compressionB C : Heat received at constant pressure(combustion)
C D : Adiabatic expansion
D A : Heat rejected at constant volume
P r
e s
s u
r e
(
P
a )
Volume (m3)V1 V2
P
A
B C
D
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Diesel cycle
1. The induction stroke takes air in ideally at constant volume,pressure at temperature.
2. The compression stroke takes place from A to B. The airis compressed adiabatically to about 1/20 of its original
volume. It gets hot.
3. From B to C fuel is injected in atomised form. It burnssteadily so that the pressure on the piston is constant.
4. From C to D the power stroke moves the piston down asadiabatic expansion takes place.
5. D to A cooling and exhaust occurs.
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Dual cycle
In the dual cycle, air is compressed
adiabatically, then heat is added, partly in
a constant volume process and the
remainder in a constant pressure process.
Expansion takes place adiabatically and
then heat is rejected at constant volume
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P r
e s
s u
r e
(
P
a )
Volume (m3)V1 V2
P
A
B
C D
E
A B : Adiabatic compression
B C : Heat received at constant volume
C D : Heat received at constant pressure
D E : Adiabatic expansion
E A : Heat rejected at constant volume
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COMPRESSION IGNITION
ENGINE Compression ignition engine works on dual cycle
The fresh air enters each of the engine cylinders and iscompressed by the upward movement of the piston.
The compression causes the temperature and pressure ofthe fresh air to increase
Fuel injectors or fuel valve will supply the fuel oil in finespray when the piston is nearly at top dead centre
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The fuel will then be mixed with air (compressed) and
burn inside the cylinder when the piston is at TDC.
The expanding gases on top of the piston (completed
combustion) will push the piston moving it downward and
rotating the crankshaft .
The cycle will be repeated until the engine stops
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Cycle of Operations
Four strokes of CI engine are as
follows:-
Suction Stroke / Induction Stroke Compression Stroke
Explosion Stroke / Power Stroke
Exhaust Stroke
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SUCTION STROKE
In which the air is admitted to the
engine cylinder
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COMPRESSION STROKE
In which the charge of fresh air is
compressed by the piston, and
fuel is injected just before thepoint of maximum compression
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POWER STROKE
In which the air- fuel mixture isignited by the heat produced by
compression of air
The pressure rises due to fuel
combustion and pushes pistondownwards to drive the engine
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EXHAUST STROKE
Exhaust valve opens at the end of
power stroke
The expanded burnt gases are
exhausted / expelled from thecylinder
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The four strokes in duel cycle of CI engine
are completed in two revolutions of the
crankshaft.
There are thus two piston strokes in each
revolution of the crankshaft
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FOUR STROKE ENGINE
INLET VALVE CYLINDER HEAD FUEL INJECTOR
PISTON
CYLINDER
LINER
CRANKSHAFT
DIRECTION
CRANK PIN
INDUCTION STROKE / COMPRESSION STROKE
SCAVENGE STROKE
POWER / EXPANSION STROKE
EXHAUST VALVE
EXHAUST STROKE
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How strokes are executed
Strokes are executed by combination of
valves and gears
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SUCTION / INDUCTION STROKE
Piston draws air into cylinderduring downward movement orstroke through opened inlet valve.(suction effect)
Exhaust valve and fuel injector areclosed
At the end of the stroke (BDC) theinlet valve close, which inside thecylinder now full with fresh air.
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COMPRESSION STROKE
Stroke begins when the pistonstarts to move upward (fromBDC to TDC).
Inlet valve, exhaust valve andfuel injector remain closed.
The air which is trapped in thecylinder is now compressedrising in temperature
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POWER STROKE Before the piston reaches
TDC(approx.15 20o), thefuel injectors supply fuel oil ina fine spray(end approx. 15-20o after TDC)
The mixture (fuel oil and air)ignites and explodes whilethe piston crosses TDC
High pressure (expansion of
the gases) on top of thepiston push the pistondownward towards BDC
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EXHAUST STROKE
Stroke begins when the piston againstarts to move upward (from BDC toTDC) as in compression stroke,however only exhaust valves areopened.
The exhaust gases are expelled fromthe cylinder through the exhaust valveports.
At the end of the stroke (TDC), the
exhaust valve closes but inlet valve isopened starting the cycle once again
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Power produced
Power produced by a 4-stroke cycle engine in
kW is given as
2
PLANPower=
P= Mean effective pressure, kN/m2
L= Stroke length, m
A= Area of cylinder bore, m2
N= Revolution/second
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4 - STROKE CYCLE
1
7
1
2
3 4 5
6
8
9
10
PRESSURE
DIRECTION
PIST
ON
POSITION
1/7
2
3 45
6
8
9
10
4 - STROKE CYCLE
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1-2 Suction stroke ends 2-3 Compression stroke. Inlet valve closed and
piston moved upwards to compress the
trapped air (Temperature rises). 3-4-5 Fuel injector in operation. Combustion
occurs (mixture of compressed air and fuel) 5-6 Due to expansion of gases piston
moves downward. (Power stroke)
6-7-8 Exhaust stroke. Exhaust valve opens andpiston moves upward removing gases. 8-9-10 Overlapping period: both exhaust and inlet
valves are open. 10-1 Suction stroke piston moves downward.
Exhaust valve closed and inlet valve open. 1- the rest The cycle continues until the
engine stops
4 STROKE CYCLE
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Rotation
SUCTIONSTROKE
COMPRESSIONS
TROKE
POWERS
TROKE
EXHAUST
STROKE
Exh. v/v
opens
Inlet v/v
closes
Inlet v/v
opens
Fuel
injection
begins
Fuel
injection
ends
Exh. v/v
closes
FOUR STROKE TIMING DIAGRAM
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VALVE OVERLAPPING
It can be defined as the period when inlet and exhaustvalve were open at the same time.
E.g., Inlet valve opened before the piston reached TDC at
the end of exhaust stroke, say 20o before TDC.
Exhaust valve remained open and will be closed atcertain degree of the piston movement after TDC,say 20o after TDC.
By providing overlapping period on 4 stroke engine,the residual exhaust gases will be expelled effectively
with the rushing in of fresh air.
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VALVE OVERLAP
OVERLAPPING PERIOD
Inlet v/v
opens
Exh. v/v
closes
TDC
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2-Stroke cycle diesel engines
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Learning Objective:Learning Objective: Know the basic cycleKnow the basic cycle
and design features of modern marine dieseland design features of modern marine dieselenginesengines
Specific Objectives:Specific Objectives:
Describe the operation cycle process of a2-stroke diesel engine.
Identify the 2-stroke engine timing diagram
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The two stroke cycle is so called because
it takes two strokes of the piston or one
revolution of crank shaft to complete the
processes needed to convert the energyin the fuel into work.
TWO STROKE CYCLE
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Why 2-Stroke Cycle Engines
We know 4-stroke cycle engine gives only
one power stroke out of 4 strokes of the
piston or one power stroke in two
revolutions of the crank shaft.
This makes engines power to weight
ratio low mainly because three strokes
consume power against one which
produces
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2S
In the two stroke engine, cycle is completed in two strokes ofthe piston or one revolution of the crankshaft.
Thus out of 3 power consuming strokes of the 4-stroke cycletwo strokes are saved
Engine thus produces one power stroke in every revolution ofthe engine which is two times in comparison to 4-stroke cycle
This improves power to weight ratio of the engine and
reduces its size for same power.
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2-Stroke cycle is achieved by eliminating suction andexhaust strokes of the 4-stroke cycle
In order to eliminate suction and exhaust strokes, somespecial arrangements are required to be provided for:-
-.charging air into cylinder without suction from piston- Exhaust gases must be expelled out of the cylinderwithout assistance from piston
2S
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Inlet air
port
Exst portExst port
Power
stroke CompstrokePiston
PistonInlet air
port
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The crankshaft is revolvingclockwise and the piston is
moving up the cylinder,compressing the charge ofair.
Because energy is being
transferred into the air,pressure and temperatureincrease.
By the time the piston is near
the top of the cylinder(known as Top Dead Centeror TDC) the pressure is >100bar and the temperature >
500C
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Just before TDC fuel is injectedinto the cylinder by the fuel
injector.
The fuel is "atomised" into tinydroplets. Being very small, thesedroplets heat up very quickly andstart to burn as the piston passesover TDC.
The expanding gas from the fuelburning in the oxygen forces thepiston down the cylinder, turning
the crankshaft.
It is during this stroke that workenergy is being put into theengine; during the upward stroke
of the piston, the engine is having
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As the piston moves down thecylinder, the useful energy
from the burning fuel isexpended.
At about 110 after TDC theexhaust valve opens and thehot exhaust gas (consistingmostly of nitrogen, carbondioxide, water vapour andunused oxygen) begin to
leave the cylinder.
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At about 140 after TDC thepiston uncovers a set of ports
known as scavenge ports.
Pressurized air enters thecylinder via these ports and
pushes the remainingexhaust gas from thecylinder, "scavenging".
The piston now goes pastBDC and starts moving upthe cylinder, closing thescavenge ports. The exhaustvalve then closes and
compression begins.
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1 -2 Compression2 - 3 Fuel Injection
3 - 4 Power
4 - 5 Exhaust Blowdown
5 - 6 Scavenging
6 - 1 Post Scavenging
1. approx 110 BTDC
2. approx 10 BTDC
3. approx 12 ATDC
4. approx 110 ATDC
5. approx 140 ATDC
6. approx 140 BTDC
The two stroke cycle can also be illustrated ona timing diagram.
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1
2
3
4 5 6
7
8
PISTON
POSITION
PRESSURE1
2
3
4 5
6
7
8
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1-2 Scavenging period, both exhaust and inlet portsare open.
2-3 Scavenge stroke ends. Exhaust ports remain open to
ensure only fresh air remains in thecylinder.
3-4 Compression takes place. Both ports closed.
The air is then compressed by the upward movement ofthe piston.
4-5-6 Fuel injector is operational supplying fuel oil. 6-7 Due to expansion of gases, piston moves downward.
(Power stroke)
7-8 When piston crown/top ring passes the exhaust ports,exhaust begins
8-1 When the piston passes the inlet ports, Scavenging beginsand fresh air fills the cylinder, thus pushing the remaining exhaustgases out
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Rotation
Fuel
injection
begins
Fuel
injection
ends
SCAVENGE
COMPRESSION P
OWERSTRO
KE
EXHAUST
Scavenge
ports
open
Scavenge
ports
close
Exhaust
ports
open
Exhaust
ports
close
TWO STROKE TIMING DIAGRAM
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The 2 strokecrosshead engine
works on exactlythe same principleand cycle as the 2stroke trunk piston
engine.
http://www.marinediesels.info/2_stroke_crosshead_engine_access.htm -
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The disadvantages of the two stroketrunk piston engine are that:
It has a low overall height, lubricatingoil splashed up from the crankcase tolubricate the liner can find its way intothe scavenge space, causing foulingand a risk of fire.
There is also the likelihood of liner andpiston skirt wear, allowing air into thecrankcase. This can supply therequired oxygen for an explosionshould a hot spot develop.
The crankcase oil must have additiveswhich can cope with contaminationfrom products of combustion, and theacids formed during combustion due tothe sulphur in the fuel.
The majority of 2 stroke engines encountered at sea are of the "crosshead" type.In this type of engine the combustion space (formed by the cylinder liner piston
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In this type of engine the combustion space (formed by the cylinder liner, pistonand cylinder head), and the scavenge space are separated from the crankcase bythe diaphragm plate.
The piston rod is bolted to the piston and passes through a stuffing box mounted
in the diaphragm plate. The stuffing box provides a seal between the two spaces,stopping oil from being carried up to the scavenge space, and scavenge air leakinginto the crankcase.
The foot of the piston rod is bolted to the crosshead pin. The top end of theconnecting rod swings about the crosshead pin, as the downward load from theexpanding gas applies a turning force to the crankshaft.
To ensure that the crosshead reciprocates in alignment with the piston in thecylinder, guide shoes are attached either side of the crosshead pin. These shoesare lined with white metal, a bearing material and they reciprocate against thecrosshead guides, which are bolted to the frame of the engine. The crossheadguides are located in-between each cylinder.
Using the crosshead design of engine allows engines to be built with very longstrokes - which means the engine can burn a greater quantity of fuel/stroke anddevelop more power. The fuel used can be of a lower grade than that used in atrunk piston engine, with a higher sulphur content, whilst high alkalinity cylinderoils with a different specification to that of the crankcase oil are used to lubricate
the cylinder liner and piston rings and combat the effects of acid attack.
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SCAVENGING
To ensure a sufficient supply of fresh air forcombustion by removing all remaining exhaust gasesby blowing with these fresh air.
Supercharging is a large mass of air that is supplied tothe cylinder by blowing it in under pressure either byelectrically driven auxiliary blower or exhaust gasdriven turbocharger.
The flow path of the scavenge air is decided by theengine port shape and design and the exhaustarrangements.
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SCAVENGING PERIOD
It can be defined as a period when inlet and exhaustare open at the same time:
Remaining exhaust gas will be expelled from thecylinder through exhaust ports or exhaust valve (if
fitted).
Fresh air which has collected in the scavenge
manifold rush into the cylinder Scavenging period: Normally when piston is at
BDC,(or as per maker or engine design or the location of the portsitself)
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SCAVENGING METHODS
CROSS/DIRECT FLOW SCAVENGING
LOOP SCAVENGING
UNIFLOW SCAVENGING
C / f
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Cross/direct flow
scavenging
Exhaust
manifold
Scavenge
manifold
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Loop scavenging
Exhaust
manifold
Scavenge
manifold
2 stroke engines do not have exhaust
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2 stroke engines do not have exhaustvalves; With scavenge ports in the cylinderliner, they are fitted with exhaust portslocated just above the scavenge ports.
As the piston uncovers the exhaust ports onthe power stroke, the exhaust gas starts toleave the cylinder.
When the scavenge ports are uncovered,scavenge air loops around the cylinder andpushes the remaining exhaust gas out ofthe cylinder.
This type of engine is known as a loop
scavenged engine. Note that the pistonskirt is much longer than that for a uniflowscavenged engine. This is because the skirthas to seal the scavenge and exhaust portswhen the piston is at TDC.
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TWO STROKE ADVANTAGES
Compactness in relation to the power output. Not requiredto increase brake mean effective pressure or the engine
speed to increase rating.
(High bmep increases the stresses on engine components,
greater rate of cylinder wear, whilst the alternative of higherspeed, valve flutter may become a serious problem)
Each out-stroke being a working stroke gives more even
turning for the same number of cranks, consequently a
lighter flywheel may be employed. The reversing operation of rotation is simplified since there
is less valve gear to contend with.
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OTHER ADVANTAGES
Fewer moving parts and lower maintenance Lower specific fuel consumption
No gear loss
Simplicity in construction Longer life time
Higher reliability (product)
Low lubricating oil consumption Better ability to burn low quality fuel oil
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Good volumetric efficiency, good combustioncharacteristic and positive exhaust scavenging.
The thermal and mechanical efficiencies are
slightly better than 2S engine. Only half the quantity of the heat generated in
the cylinders has to be dealt within a given time,so that efficient lubrication of the piston and
cooling of the cylinder is more easilyaccomplished.
FOUR STROKE ADVANTAGES
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Lower initial cost for equivalent power Ease ofinstallation
Lowerweight per unit power
Saving in weight and engine room length
Increased cargo capacity
Free choice ofpropeller speed through
gearing
Suitable forelectrical powertake off
OTHER ADVANTAGES
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Supercharging/Turbocharging
Process of pushing a higher pressure air
charge into the cylinder greater than
atmospheric pressure, so that extra mass
of air can be delivered into cylinder to burnmore fuel and produce extra power.
Turbocharging can increase power output
of engine by 60%
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Turbocharging
Very effective pressure charging.
Utilizes 20% of waste heat in exhaust gas
which contains 35% of fuel heat.
How?
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By increasing mass of air in cylinder, more fuel can beburned and correspondingly power output will be
increased
Various methods can be adopted:Electrically powered auxiliary blower
Utilization of heat energy from exhaust gas todrive a single stage impulse turbine directly coupled toa simple blower (free running unit) called exhaust gasturbocharger
Turbocharger utilizes free energy of exhaustgases and hence improves efficiency of the engine
Typical heat balance of an
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Typical heat balance of an
engineUseful Output (Brake Power) 34%
Cooling Loss 30%
Exhaust Loss 26%
Friction, Radiation, etc. 10%
-------
Total Heat Input100%
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TurbochargerSystem
Advantages
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Advantages
Increased power for an engine of the same size
OR reduction in size for an engine with thesame power output.
Reduced specific fuel oil consumption ->
mechanical, thermal and scavenge efficienciesare improved due to less cylinders, greater airsupply and use of exhaust gasses.
Thermal loading is reduced due to shorter moreefficient burning period for the fuel leading toless exacting cylinder conditions.
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Risk
Crankcase explosion
Scavenge fire
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Design consideration
Types of fuel and fuel oil system design Types of lubricating oil and lubricating oil systems
Cooling systems
Waste heat utilization systems
Intake and exhaust valve systems Starting air systems
Instrumentation system
Control and automation system
Installation items
Safety features
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Summary
Principle of ICE
Theoretical Cycles
Basic principle of operations of working cycle
Cycle & Timing Diagram
Principles of Scavenging & Arrangements
Advantages of 2S & 4S
Structural differences
Overlap of Inlet & Exhaust
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References
Introduction to Marine Engineering,
Marine Engineering , Roy L. Harrington, SNAME, 198
El-Hawary, F. (2001). Ocean Engineering Handbook. CRC
Press, UK.
Calder, Nigel (2007): Marine diesel engine: maintenance,
troubleshooting and repair.