lecture-diesel power plant

8/3/2019 Lecture-Diesel Power Plant

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Diesel power plant

1 Mussie T.

Introduction The oil engines and gas engines are called Internal

Combustion Engines. In IC engines fuels burn

ns e e eng ne an e ro uc s o com us on

form the working fluid that generates mechanical

power.

Whereas, in Gas Turbines the combustion occurs

in another chamber and hot working fluid

containing thermal energy is admitted in turbine.

Diesel engine is an internal combustion enginewhich uses diesel as fuel

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Diesel power plant

Diesel electric plants in the range of 2 to 50 MW capacity areused as central stations for small supply authorities and worksand the are universall ada ted to su lement h droelectricor thermal power stations where standby generating plants

are essential for starting from cold or under emergencyconditions

Diesel engine: is a heat engine which transforms the chemicalenergy of a fuel into thermal energy and utilizes thisthermal energy to perform useful work.

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Thermal

energy ofdiesel fuel

Mechanicalshaft work

Diesel Engine ElectricityGenerator

Engine ClassificationIC engines can be classified on the basis of different design

setups and operating conditions: Basic engine design: Reciprocating, rotary (wankel) Working cycle: Otto cycle (SI engine ), and diesel

cycle (CI engine) Number of strokes: four stroke and two stroke

(both SI and CI engines) Fuel supply and mixture preparation: carbureted

types, fuel supplied through carburetors and injectiontypes (fuel injected to inlet pots or inlet manifold and

ue n ecte nto t e cy n er ust e ore gn t on Method of Ignition: In SI engines battery or

magneto ignition Method of cooling: Water cooled or air cooled Cylinder arrangement: Inline, V, radial, opposed

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Operating Principles

In diesel engines, air is compressed separately and

mixed with the fuel at the time of combustion inthe engine cylinder.

In such an arrangement fuel can be injected into the

cylinder which contains compressed air at a higher

temperature than the self-ignition temperature of

the fuel.

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Such engines work on heavy liquid fuels. These

engines are called compression-ignition engines andthey work on a ideal cycle known as Diesel cycle

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How diesel engine works Diesel engine operates on a four stroke cycle:

A stroke is a single traverse of the cylinder by the pistonfrom TDC to BDC

1 revolution of crankshaft = 2 strokes of piston

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How diesel contd

1. Intake/suction stroke: The air is sucked in thecylinder by the piston sliding downward.

2. Compression stroke: The piston compresses the

air using work of the crankshaft.

3. Power stroke: In the upper dead-center, diesel fuel

is injected and the mixture ignites due to the high

temperature developed by high pressure. The

ressure of the burnin mixture ushes the iston

back into the cylinder: Work is performed.

4. Ejection/exhaust stroke: The burned exhaust isejected by the rising piston through a second valve.

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How diesel contd In the case of diesel engines, fuel is injected into the

combustion chamber towards the end of the compressionstroke

Diesel engines dont need

spark plug since the

temperature reached due

to compression exceeds

self-ignition temperature

The compression ratio

for diesel engines is high

ranging from 6 to 20.Fig. Diesel cycle

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Characteristics of Diesel engine

Has higher compression ratio; hence it has

efficiency.

Since the components need to withstand

the high pressure, diesel engines areheavierthan s ark i nition en ines.

As the fuel burns heterogeneously, dieselengine produce lower speeds.

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B=bore

S=stroke

Piston-Cylinder geometry of a

reciprocating engine

R=connecting rod length

a=crank offset

S=piston position

= crank angle

Vc = clearance volume

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Vd =displacement volume

TDC= top dead centre

BDC=bottom dead centre

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Thermodynamic analysis of IC engines

The three steps of Thermodynamic Analysis of ICEngines are

Idealized processes

Idealize working Fluid

Fuel-Air Cycle

Idealized Processes

Accurate Working Fluid Model

Actual Engine Cycle Accurate Models of Processes

Accurate Working Fluid Model11 Mussie T.

Air-Standard Cycle Assumptions

Simplifications to the real cycle include:

Fixed amount of air (ideal gas) for working fluid

Combustion process not considered

Intake and exhaust processes not considered (There is Heat

addition and heat rejection source and Sink)

There is no heat losses from the system to the surrounding

Engine friction and heat losses not considered

All the processes that constitute the cycle are reversal

Specific heats independent of temperature

For Air Cp = 1.005 kJ/kg K Cv = 0.717kJ/kgK , = 1.4 M

= 29kg/kmol

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DIESEL CYCLE

In Diesel cycles, heat is

added at constant pressure

Ideal diesel cycle

Processes in diesel cycle:

1-2 Isentropic Compression

2-3 Con Pre. Heat Addition Fuel injection starts

13

-

4-1 ConVol. Heat Rejection

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Thermal efficiency of the Diesel cycle

th Dieselnet

in

out

in

W

Q

Q

Q, 1

- =, ,

the heat input is:

Q U P V V

Q Q mC T T mR T T

Q mC T T

net

net in v

in p

,

,

( )

( ) ( )

( )

23 23 2 3 2

23 3 2 3 2

3 2

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Thermal contd

Apply the first law closed system to process 4-1, V= constant

Thus, for constant specific heats

Q U

Q Q mC T T

Q mC T T mC T T

net

net out v

out v v

,

, ( )

( ) ( )

41 41

41 1 4

1 4 4 1

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Thermal contd

th DieselvC T T

C T T,

( )

( )

1 4 1

3 2

What is T3/T2 ?

PV PV P P3 3 2 2 3 2 where

k

T T T

T T T

( / )

( / )

1

1 1

1

1 4 1

2 3 2

T

T

V

Vrc

3 2

3

2

3

2

Where rc is called the cutoff ratio, defined as V3 /V2,and

is a measure of the duration of the heat addition at

constant pressure.Since the fuel is injected directly into the cylinder, thecutoff ratio can be related to the number of degrees thatthe crank rotated during the fuel injection into thecylinder.

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Thermal contd

PV

T

PV

TV V4 4

4

1 1

1

4 1 where

T

T

P

P

4

1

4

1

PV PV PV PV k k k k 1 1 2 2 4 4 3 3

and

4 1

Recall processes 1-2 and 3-4 are isentropic, so

Since V4 = V1 and P3 = P2, we divide the second equationby the first equation and obtain

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DIESEL CYCLE Efficiency

th Dieselk

T T T

T T T,

( / )

( /

1

1 1

1

1 4 1

c

k

c

k

c

k

c

k

T

T

r

r

r

r

k r

( )

( )

11 1

1

11 1

1

1

2

1

1

1111

1

c

k

c

kDieselr

r

kr

18

Note the term in the square bracket is always larger than unityso for the same compression ratio, r, the Diesel cycle has alowerthermal efficiency than the Otto cycleAlso note: diesel needs higher r compared to ignite

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Engine Performance Parameters

In the evaluation of engine performance, certainbasic parameters (called engine performance

operating conditions, design concepts andmodifications on these parameters are studied.

Engine performance is also characterized byconvenient graphical presentation called enginecharacteristic curves

ng ne c arac er s c curves are cons ruc e romthe data obtained during actual test runs of the

engine and are particularly useful in comparing theperformance of one engine with that of another

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Engine contd

The basic performance parameters are:

Power and Mechanical efficienc

Volumetric efficiency and Fuel-air ratio

Mean effective pressure and torque

Specific output

Specific fuel consumption

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Thermal efficiency and heat balance

Exhaust smoke and other emissions

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Power and Mechanical Efficiency

The main purpose of running an engine is mechanical

power

Defined as the rate of doing work and is equal to theproduct of force and linear velocity

The power developed by an engine by the output

shaft is called the Brake power (bp) and is give by:

Where, N= Engine speed, RPM

T= Torque, N-m

p

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Torque and power against engine speed

The speed at which peaktorque occurs is calledmaximum brake torque (MBT)

.Indicated power increases withspeed while brake powerincreases to a maximum andthen decreases. This is becausefriction power increases withengine speed to a higher powerand becomes dominant athigher speed

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Many modern IC engines havemaximum torque in the 200 to300 N-m range at enginespeeds usually around 4000 to6000 RPM

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Mechanical Efficiency

The total power developed by combustion of fuel inthe combustion chamber is, however, more than the bp

The difference between the ip and bp is the indication

of the power lost in the mechanical components of the

engine and forms the basis ofMechanical efficiency

( )m

bpMechanical efficiency

The difference between ip and bp is called friction

power (fp)

ip = fp + bp23 Mussie T.

Volumetric Efficiency Ideally, a mass of air equal to the density of atmospheric air

times the displacement volume of the cylinder should beingested for each cycle. However, because of the short cycleme ava a e an e ow res r c ons presen e y e a r

cleaner, carburetor (if any), intake manifold, and intake valve(s),less than this ideal amount of air enters the cylinder.

Volumetric efficiency of an engine is an indication of themeasure of the degree to which the engine fills its sweptvolume

It is defined as the ratio of the mass of air inducted into the

corresponding to the swept volume of the engine atatmospheric pressure and temperature.

It also shows the degree of completeness with which thecylinder is re-charged with fresh combustible mixture

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Volumetric. Contd

Volumetric efficiency,

Standard values of surrounding air pressure and temperature can be usedto determine density of air

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Air fuel ratio Energy input to an engine comes from combustion of a

hydrocarbon fuel.

Air is used to su l the ox en needed for this chemical reaction

For combustion reaction to occur, the proper relative amounts of

air (oxygen) and fuel must be present

Air-fuel ratio (AF) and fuel-air ratio (FA) are parameters used todescribe the mixture ratio

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AF input of gasoline: 12-18, AF input of CI engine18-70

Equivalence ratio,

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Mean effective pressure

It is known that pressure in the cylinder of an engine iscontinuously changing during the cycle.

hypothetical pressure which is thought to be acting

on the piston throughout the power stroke

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Mean contdmep is a good parameter to compare engines for

design or outputs because it is independent of

eng ne s ze an or spee

If torque is used for comparison, a larger engine

looks better

If power is used for comparison, the engine withhigher speed looks better

Classified in to two:

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o Indicated mean effective pressure (imep)

o Brake mean effective pressure (bmep)

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Mean contd

Indicated Mean Effective pressure (imep)

The net area on the p-V trace or indicator diagram from anengine is the indicated work done by the gas on the piston.

The imep is a measure of the indicated work output perunit swept volume, in a form independent of the size and

number of cylinders in the engine and engine speed.

The pressure in the cylinder initially increases during the

expansion stroke due to the heat addition from the fuel, andthen decreases due to the volume increase

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Mean contd

Brake Mean Effective Pressure (bmep)

The external shaft work per unit displacement

volume done by the engine

It is the average pressure that results in the same

amount of indicated or brake work produced by

the engine

N

where, N= number of working strokes per revolution

* 60Pb bmep LA

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bmep contd

In a more general way

60 b

Where: n shows number of working strokes per revolutionor minuteThus, n=N/2 for 4-stroke and N for 2-strokeK=number of cylinder

( )m ep p a L A n K

For two-stroke, there are N working strokes for N

revolutions For four-stroke, there are N/2 working strokes for N

revolutions of the engine

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Mean contd

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Specific fuel consumption

m

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( / )bsfc kg kw hrpb

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sfc contd

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Brake specific fuel consumption

bsfc decreases as engine speed increases, reaches a minimum, and thenincreases at high speeds.

Fuel consumption increases at high speed because of greater frictionlosses. At low engine speed, the longer time per cycle allows more heatloss and fuel consumption goes up

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Bsfc contd bsfc is minimum at a slightly lean condition, increasing

with both rich and leaner mixture

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Generally, average fuelconsumption is less

Bsfc contd

with larger engines.One reason for this isless heat loss due tothe higher volume tosurface area ratio ofthe combustionchamber in a large

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engine. Also largerengines operate at

lower speeds whichreduces friction losses.

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Specific Energy Consumption

Brake Specific Energy Consumption (bsec)

Is the energy used by the engine to produce unit

power

It can be calculated as:

Bsec (KJ/kw-h)=bsfc * calorific value (KJ/Kg)

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sec *b bsfc LCV

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Combustion Efficiency

The time available for the combustion process of an

engine cycle is very brief, and not all fuel moleculesmay find an oxygen molecule with which to combine,

or the local temperature may not favor a reaction.

Consequently, a small fraction of fuel does not react

and exits with the exhaust flow. A combustionefficiency is definedto account for the fraction of

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ue w c urns.

Combustion efficiency typically has values in the range0.95 to 0.98 when an engine is operating properly.

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Thermal Efficiency

Thermal efficiency of an engine is defined

chemical energy input released by

combustion of the fuel.

It may be based on brake or indicated

output

It is the true indication of the efficiency

with which the thermodynamic input isconverted into mechanical work

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Brake thermal Efficiency,

The ratio of the energy in the brake power

b

to t e ue consumpt on

*100%*

b

f

p b

m L V C

This assumes 100% combustion efficiency

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Indicated thermal Efficiency,

The ratio of the energy in the brake

i

power to the fuel consumption

* 1 0 0%*

i

f

p i

m L V C

This assumes 100% combustion efficiency

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Engine efficiency Characteristic curves

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Emissions and Exhaust Smoke The four main engine exhaust emissions which must be

controlled are oxides of nitrogen (NOx), carbonmonoxide CO h drocarbons HC and solidparticulates (part).

Two common methods of measuring the amounts ofthese pollutants are specific emissions (SE) and theemissions index (EI). Specific emissions typically has unitsof gm/kW-hr, while the emissions index has units of

.

With increasing emphasis on air pollution control, allefforts are being made to keep them minimum

Smoke is an indication of incomplete combustion

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Emissions contd

Specific emission:

Emission index:

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Fuel System of Diesel Power Plant

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Essential functions of a fuel

injection system

1. To deliver oil from the storage to the fuelinjector.

2. To raise the fuel pressure to the level requiredfor atomization.

3. To measure and control the amount of fuel

admitted in each cycle.

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. o contro t me o n ect on.

5.To spray fuel into the cylinder in atomizedform for thorough mixing and burning.

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Fuel injection system

The fuel-injection system is the most vital

com onent in the workin of CI en ine

The engine performance is greatly dependent onthe effectiveness of the fuel injection system

The purpose of carburetion and injection is the

same

,

while in injection the fuel is speed is greater than

the air speed

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Fuel contd

The injection is CI engine is by a nozzle with largepressure differential across the nozzle orifice

e cy in er pressure at injection is typica y in t erange of 50 to 100 atm

Fuel injection pressure in the range of 200 to 1700atmare used depending on the engine size and type ofcombustion system employed

These large pressure differences across the injectornozz e are require so t at t e injecte iqui ue jetwill enter the chamber at sufficiently high velocity to:

Atomize into small sized droplets

Traverse the combustion chamber in the limited time

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Diesel Injector system need to make Accurate metering

Timing the injection

the desired heat release pattern

Proper atomization of the fuel

Proper spray pattern

Uniform distribution of fuel throughout the CC

Proper distribution in multi-cylinder engines

Injection timing should change to suite the engine speedand load requirements

Weight and size of the injector should be minimum

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Components of Fuel InjectionInjection Pump

Objective is to deliver accurately metered quantityof fuel under high pressure

enera y wo ypes o n ec on pumps:

Jerk Type

Works with a reciprocating plunger inside a

barrel

The plunger is driven by a cam shaft

Distributor Type

Contains single pumping element and the fuel isdischarged to each cylinder by means of a rotor

Small size and high weight51 Mussie T.

Components contd

Distributor system

The fuel is metered at a central point i.e., the pump thatpressurizes, meters and times the injection.

From here, the fuel

is distributed tocylinders in correctfiring order by camoperated poppetvalves, which open

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to a m t ue tonozzles.

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Components contdInjection Nozzle

Nozzle is that part of an injector which the

qu ue s spraye nto t e com ust onchamber

Should fulfill the following:

Atomization

Injection pressure

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General layout of diesel power plant

Generally the units are placed in parallel lines

In an lant some s ace is alwa s rovided forfurther expansion.

Also sufficient space should provide formaintenance of diesel engine.

Proper ventilation is also provided in power

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plant. Storage of fuel for power plant is always

provided outside the main building.

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General contd

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Heat balance sheet Heat balance sheet is a useful method to watch the

performance of the diesel power plant.

e str ut on o t e eat mparte to an eng ne s

called its heat balance.

The heat balance of an engine depends on a number of

factors among which load is primary importance.

The heat balance of an internal combustion engine shows

that the coolin water and exhaust ases carr awa

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about 60-70% of heat produced during combustion of

fuel.

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Heat contd

In order to draw the heat balance sheet of Diesel

eng ne, t e eng ne s run at constant oa an

constant speed and the indicator diagram is drawn

with the help of indicator

Preparation of heat balance sheet gives us an ideaabout the amount of energy wasted in various parts

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losses so incurred

Heat contdThe energy supplied to Diesel engine in the form of fuel

input is usually broken into:

. n ca e power

B. Heat Rejected to Cooling Water

C. Heat Carried Away by Exhaust Gases

)( 21 TTcwm

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D. Heat Unaccounted for (Heat Lost Due to Friction,

Radiation etc.)

)( 34 TTegm

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Item Head unitskcal or kJ

Percent Typicalvalues

Heat contd

(a) Heat absorbed by I.H.P. 30%

(b) Heat rejected to coolingwater

30%

(c) Heat carried away byexhaust ases

26%

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(d) Heat unaccounted for

(by difference)

10%

Total 100% 100%

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Diesel plant operation To ensure most economical operation of diesel engines of

different sizes when working together and sharing load, it isnecessary that they should carry the same percentage oftheir full load capacity at all times as the fuel consumptionwould be lowest in this condition.

In order to get good performance of a diesel power plant thefollowing points should be taken care of:

1. It is necessary to maintain the cooling temperaturewithin the prescribed range and use of very cold water

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.suspended impurities and suitably treated to be scale andcorrosion free. If the ambient temperature approaches freezingpoint, the cooling water should be drained out of the engine when itis kept idle.

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Diesel contd

2. During operation the lubrication system should workeffectively and requisite pressure and temperature maintained.The engine oil should be of the correct specifications and shouldbe in a fit. Condition to lubricate the different parts. A watch maybe kept on the consumption of lubricating oil as this gives anindication of the true internal condition of the engine.

3. The engine should he periodically run even when not required tobe used and should not be allowed to stand idle for morethan 7 days.

4. Air litter, oil filters and fuel filters should be periodically

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serv ce or rep ace as recommen e y t e manu acturersor if found in an unsatisfactory condition upon inspection.

5. Periodical checking of engine compression and firing pressuresand also exhaust temperatures should be made.

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Advantages of diesel power plants1. Very simple design also simple installation.

2. Limited cooling water requirement.

. .

4. Low fuel cost.

5. Quickly started and put on load.

6. Smaller storage is needed for the fuel.

7. Layout of power plant is quite simple.

8. There is no problem of ash handling.

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9. Less supervision required.10. For small capacity, diesel power plant is more efficient as

compared to steam power plant.

11. They can respond to varying loads without any difficulty

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Disadvantages of diesel power plants

1. High Maintenance and operating cost.

2. Fuel cost is more costl , es eciall non-oilproducing countries like Ethiopia.

3. The plant cost per kW is comparatively more.

4. The life of diesel power plant is small due tohigh maintenance.

5. Noise is a serious problem in diesel power

63

p ant.

6. Diesel power plant cannot be constructed forlarge scale.

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Application of diesel engines in power field Peak load plant: they can be easily started or stopped at a

short notice to meet the peak demand

o e an : can e eas y transporte on tra ers

Standby unit: can supply when short fall in power occurs

Emergency plant: can generate power for vital units like

hospitals or key industrial plants during power interruption

Nursery station: in the absence of main grid, a diesel plant

. Starting stations: can be used to run aouxiliaries (like FD and

ID fans, etc) for starting a large steam power plant

Central stations: as central stations where demand is small

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lecture-diesel power plant

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