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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
2 Mussie T.
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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.
3 Mussie T.
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
4 Mussie T.
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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.
5
Such engines work on heavy liquid fuels. These
engines are called compression-ignition engines andthey work on a ideal cycle known as Diesel cycle
Mussie T.
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.
7 Mussie T.
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
8 Mussie T.
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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.
9 Mussie T.
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
10
Vd =displacement volume
TDC= top dead centre
BDC=bottom dead centre
Mussie T.
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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
Mussie T.
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
14 Mussie T.
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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
15 Mussie T.
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
17 Mussie T.
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
19 Mussie T.
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
20
Thermal efficiency and heat balance
Exhaust smoke and other emissions
Mussie T.
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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
21 Mussie T.
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
22
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
25Mussie T.
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
26
AF input of gasoline: 12-18, AF input of CI engine18-70
Equivalence ratio,
Mussie T.
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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
27 Mussie T.
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:
28
o Indicated mean effective pressure (imep)
o Brake mean effective pressure (bmep)
Mussie T.
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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
29 Mussie T.
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
31 Mussie T.
Mean contd
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Specific fuel consumption
m
33
( / )bsfc kg kw hrpb
Mussie T.
sfc contd
34 Mussie T.
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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
35 Mussie T.
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
37
engine. Also largerengines operate at
lower speeds whichreduces friction losses.
Mussie T.
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)
38
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
39
ue w c urns.
Combustion efficiency typically has values in the range0.95 to 0.98 when an engine is operating properly.
Mussie T.
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
41Mussie T.
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
43 Mussie T.
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
44 Mussie T.
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Emissions contd
Specific emission:
Emission index:
45 Mussie T.
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.
47
. o contro t me o n ect on.
5.To spray fuel into the cylinder in atomizedform for thorough mixing and burning.
Mussie T.
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
48 Mussie T.
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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
49 Mussie T.
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
52
to a m t ue tonozzles.
Mussie T.
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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
53 Mussie T.
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
54
plant. Storage of fuel for power plant is always
provided outside the main building.
Mussie T.
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General contd
55 Mussie T.
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
56
about 60-70% of heat produced during combustion of
fuel.
Mussie T.
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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
Mussie T.57
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
58
D. Heat Unaccounted for (Heat Lost Due to Friction,
Radiation etc.)
)( 34 TTegm
Mussie T.
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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%
59
(d) Heat unaccounted for
(by difference)
10%
Total 100% 100%
Mussie T.
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
60
.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
61
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.
Mussie T.
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.
62
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.
Mussie T.
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
64 Mussie T.