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Diesel power plant
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Introduction
The oil engines and gas engines are called Internal Combustion Engines. In IC engines fuels burn i id the inside th engine i andd the th products p d t off combustion b ti 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 engine which uses diesel as fuel
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Diesel power plant Diesel electric plants in the range of 2 to 50 MW capacity are used as central stations for small supply authorities and works and they are universally adapted to supplement hydroelectric or thermal power stations where standby generating plants are essential for starting from cold or under emergency conditions Diesel engine: is a heat engine which transforms the chemical energy of a fuel into thermal energy and utilizes this thermal energy to perform useful work.
Thermal energy of diesel fuel
Diesel Engine
Mechanical Generator shaft work
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Electricity
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Engine Classification IC 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 injection types (fuel injected to inlet pots or inlet manifold and f l injected fuel i j d into i the h cylinder li d just j before b f ignition) i ii ) 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
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Operating Principles In diesel engines, air is compressed separately and mixed with the fuel at the time of combustion in the 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. Such engines work on heavy liquid fuels. These engines are called compression-ignition engines and they work on a ideal cycle known as Diesel cycle
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How diesel engine works
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Diesel engine operates on a four stroke cycle: A stroke is a single traverse of the cylinder by the piston ((from TDC to BDC)) 1 revolution of crankshaft = 2 strokes of piston
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How diesel … cont’d 1. 2. 3.
4.
Intake/suction stroke: The air is sucked in the cylinder by the piston sliding downward. Compression stroke: The piston compresses the air using work of the crankshaft. 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 pressure of the burning mixture pushes the piston back into the cylinder: Work is performed. Ejection/exhaust stroke: The burned exhaust is ejected by the rising piston through a second valve.
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How diesel … cont’d In the case of diesel engines, fuel is injected into the combustion chamber towards the end of the compression stroke
Diesel engines don’t need spark plug since the temperature reached due to compression exceeds self-ignition temperature of diesel The compression ratio for diesel engines is high ranging from 6 to 20. 8
Fig. Diesel cycle Mussie T.
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Characteristics of Diesel engine
Has higher compression ratio; hence it has the potential to achieve higher thermal efficiency.
Since the components need to withstand the high pressure, diesel engines are heavier than spark p ignition g engines. g
As the fuel burns heterogeneously, diesel engine produce lower speeds.
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Piston-Cylinder geometry of a reciprocating engine B=bore S=stroke R=connecting rod length a=crank offset S=piston position θ= crank angle Vc = clearance volume Vd =displacement volume TDC= top dead centre BDC=bottom dead centre 10
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Thermodynamic analysis of IC engines The three steps of Thermodynamic Analysis of IC Engines are Ideal Gas Cycle (Air Standard Cycle) Idealized processes Idealize working Fluid Fuel-Air Cycle Idealized Processes Accurate A W Working k Fluid Fl d M Model d l Actual Engine Cycle Accurate Models of Processes
Accurate Working Fluid Model
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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
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For Air Cp = 1.005 kJ/kg K = 29kg/kmol
Cv = 0.717kJ/kgK , γ = 1.4 M Mussie T.
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Ideal diesel cycle
DIESEL CYCLE
In Diesel cycles, heat is added at constant pressure
Processes in diesel cycle: 1-2 Isentropic Compression 2-3 Con Pre. Heat Addition
Fuel injection starts
3 4 Isentropic Expansion 3-4
Actual diesel cycle
4-1 Con Vol. Heat Rejection
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Thermal efficiency of the Diesel cycle th , Diesel
Wnet Q 1 out Qin Qin
Apply the first law closed system to process 22-33, P = constant, constant the heat input is:
Qnet , 23 U 23 P2 (V3 V2 ) Qnet , 23 Qin mCv (T3 T2 ) mR ( T3 T2 ) Qin mC p (T3 T2 ) 14
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Thermal … cont’d Apply the first law closed system to process 4-1, V = constant
Thus, for constant specific heats
Qnet , 41 U 41 Qnet , 41 Qout mCv ( T1 T4 ) Qout mCv ( T1 T4 ) mCv ( T4 T1 ) 15
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Thermal … cont’d th , Diesel
C ( T T1 ) 1 v 4 C p ( T3 T2 ) 1
1 T1 ( T4 / T1 1) k T2 ( T3 / T2 1)
What is T3/T2 ?
PV PV 3 3 2 2 where P3 P2 T3 T2 T3 V3 rc T2 V2
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, the cutoff ratio can be related to the number of degrees that the crank rotated during the fuel injection into the cylinder. 16
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Thermal … cont’d
What is T4/T1 ?
PV PV 4 4 1 1 where V4 V1 T4 T1 T4 P4 T1 P1
Recall processes 1-2 and 3-4 are isentropic, so k k k k PV and PV 1 1 PV 2 2 4 4 PV 3 3
Since V4 = V1 and P3 = P2, we divide the second equation by the first equation and obtain
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DIESEL CYCLE Efficiency th , Diesel 1
1 T1 (T4 / T1 1) k T2 (T3 / T2 1)
1 T1 rck 1 1 k T2 (rc 1) 1
1 r k 1
rck 1 k (rc 1)
Diesel
1 1 rck 1 1 k 1 r k rc 1
Note the term in the square bracket is always larger than unity so for the same compression ratio, r, the Diesel cycle has a lower thermal efficiency than the Otto cycle Also note: diesel needs higher r compared to ignite 18
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Engine Performance Parameters
In the evaluation of engine performance, certain basic parameters (called engine performance parameters) are chosen and the effect of various operating conditions, design concepts and modifications on these parameters are studied. Engine performance is also characterized by convenient graphical presentation called engine characteristic curves Engine characteristic curves are constructed from the data obtained during actual test runs of the engine and are particularly useful in comparing the performance of one engine with that of another
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Engine … cont’d
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The basic performance parameters are:
Power and Mechanical efficiencyy
Volumetric efficiency and Fuel-air ratio
Mean effective pressure and torque
Specific output
Specific fuel consumption
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 the product 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:
b p 2 N T Where, N= Engine speed, RPM T= Torque, N-m 21
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Torque and power against engine speed
The speed at which peak torque occurs is called maximum brake torque (MBT) or maximum best torque. torque Indicated power increases with speed while brake power increases to a maximum and then decreases. This is because friction power increases with engine speed to a higher power and becomes dominant at higher speed Many modern IC engines have maximum torque in the 200 to 300 N-m range at engine speeds usually around 4000 to 6000 RPM 22
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Mechanical Efficiency
The total power developed by combustion of fuel in the combustion chamber is, however, more than the bp and is called indicated power (ip) 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 of Mechanical efficiency
Mechanical efficiency(m )
bp ip
The difference between ip and bp is called friction power (fp)
ip = fp + bp 23
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Volumetric Efficiency
Ideally, a mass of air equal to the density of atmospheric air times the displacement volume of the cylinder should be ingested for each cycle. However, because of the short cycle time available and the flow restrictions presented by the air 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 the measure of the degree to which the engine fills its swept volume
It is defined as the ratio of the mass of air inducted into the engine cylinder during the suction stroke to the mass of the air corresponding to the swept volume of the engine at atmospheric pressure and temperature.
It also shows the degree of completeness with which the cylinder is re-charged with fresh combustible mixture
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Volumetric…. Cont’d
Volumetric efficiency,
Standard values of surrounding air pressure and temperature can be used to 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 supply pp y the oxygen yg 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 to describe the mixture ratio
AF input of gasoline: 12-18, AF input of CI engine18-70 Equivalence ratio, 26
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Mean effective pressure It is known that pressure in the cylinder of an engine is continuously changing during the cycle. Mean Effective pressure(mep) is defined as a hypothetical pressure which is thought to be acting on the piston throughout the power stroke
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Mean … cont’d mep is a good parameter to compare engines for design or outputs because it is independent of engine i size i and/or d/ speed d If torque is used for comparison, a larger engine looks better If power is used for comparison, the engine with higher speed looks better Classified in to two: o Indicated mean effective pressure (imep) o Brake mean effective pressure (bmep) 28
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Mean … cont’d Indicated Mean Effective pressure (imep)
The net area on the pp-V trace or indicator diagram g from an engine is the indicated work done by the gas on the piston. The imep is a measure of the indicated work output per unit 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, and then decreases due to the volume increase
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Mean … cont’d Brake Mean Effective Pressure (bmep) The external shaft work per unit displacement volume l d done by b the h engine It is the average pressure that results in the same amount of indicated or brake work produced by the engine
Pb bmep b * LA
N 60
where, N= number of working strokes per revolution 30
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bmep … cont’d In a more general way b m ep ( kkp a )
60 pb LAnK
Where: n shows number of working strokes per revolution or minute Thus, n=N/2 for 4-stroke and N for 2-stroke K=number of cylinder
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 … cont’d
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Specific fuel consumption
b f (kg bsfc k / kw k hr h )
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m f pb
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sfc … cont’d
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Brake specific fuel consumption
bsfc decreases as engine speed increases, reaches a minimum, and then increases at high speeds. Fuel consumption increases at high speed because of greater friction losses. At low engine speed, the longer time per cycle allows more heat loss and fuel consumption goes up
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Bsfc … cont’d
bsfc is minimum at a slightly lean condition, increasing with both rich and leaner mixture
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Bsfc … cont’d
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Generally, average fuel consumption is less withh larger l engines. One reason for this is less heat loss due to the higher volume to surface area ratio of the combustion chamber in a large engine. Also larger engines operate at lower speeds which reduces 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)
b sec bsfc * LCV 38
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Combustion Efficiency The time available for the combustion process of an engine cycle is very brief, and not all fuel molecules may 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 combustion efficiency is defined to account for the fraction of f l which fuel hi h bburns. Combustion efficiency typically has values in the range 0.95 to 0.98 when an engine is operating properly.
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Thermal Efficiency Thermal efficiency of an engine is defined as the ratio of the output to that of the 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 is converted into mechanical work
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Brake thermal Efficiency,b
The ratio of the energy in the brake power to the h fuel f l consumption i
b
pb *100% m f * L V C
This assumes 100% combustion efficiency Mussie T.
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Indicated thermal Efficiency, i
The ratio of the energy in the brake power to the fuel consumption
i
pi *100% m f * L V C
This assumes 100% combustion efficiency 42
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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), carbon monoxide (CO), hydrocarbons (HC), and solid particulates (part).
Two common methods of measuring the amounts of these pollutants are specific emissions (SE) and the emissions index (EI). Specific emissions typically has units of gm/kW-hr, while the emissions index has units of emissions flow per fuel flow. flow
With increasing emphasis on air pollution control, all efforts are being made to keep them minimum
Smoke is an indication of incomplete combustion
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Emissions … cont’d 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 fuel injector. 2. To raise the fuel pressure to the level required for atomization. 3. To measure and control the amount of fuel admitted in each cycle. 4T 4. To controll time i off injection. i j i 5.To spray fuel into the cylinder in atomized form for thorough mixing and burning. 47
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Fuel injection system
The fuel-injection system is the most vital component p in the workingg of CI engine g The engine performance is greatly dependent on the effectiveness of the fuel injection system The purpose of carburetion and injection is the same In carburetor carburetor, air speed is greater than fuel speed while in injection the fuel is speed is greater than the air speed 48
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Fuel … cont’d The injection is CI engine is by a nozzle with large pressure differential across the nozzle orifice Th cylinder The li d pressure at injection i j i iis typically i ll iin the h range of 50 to 100 atm Fuel injection pressure in the range of 200 to 1700atm are used depending on the engine size and type of combustion system employed These large pressure differences across the injector nozzle l are required i d so that h the h iinjected j d liquid li id fuel f l jjet will enter the chamber at sufficiently high velocity to:
Atomize
into small sized droplets Traverse the combustion chamber in the limited time 49
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Diesel Injector system need to make
Accurate metering Timing the injection The rate of the injection should be such that it results in 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 speed and load requirements Weight and size of the injector should be minimum 50
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Components of Fuel Injection Injection Pump
Objective is to deliver accurately metered quantity of fuel under high pressure G Generally ll ttwo ttypes off iinjection j ti 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 is discharged to each cylinder by means of a rotor Small size and high weight 51
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Components … cont’d Distributor system
The fuel is metered at a central point i.e., the pump that pressurizes, meters and times the injection. From here, the fuel is distributed to cylinders in correct firing order by cam operated poppet valves, which open to admit d i fuel f l to nozzles.
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Components … cont’d Injection Nozzle
Nozzle is that part of an injector which the li id fuel liquid f l is i sprayed d into i the h combustion b i chamber Should fulfill the following: Atomization Distribution Injection pressure
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General layout of diesel power plant Generally the units are placed in parallel lines In any plant some space is always provided for further expansion. Also sufficient space should provide for maintenance of diesel engine. Proper ventilation is also provided in power plant. Storage of fuel for power plant is always provided outside the main building.
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General … cont’d
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Heat balance sheet
Heat balance sheet is a useful method to watch the performance of the diesel power plant. Th distribution The di t ib ti off th the hheatt iimparted t d tto an engine i iis 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 cooling water and exhaust gases carry away about 60-70% of heat produced during combustion of fuel.
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Heat … cont’d In order to draw the heat balance sheet of Diesel engine, i the h engine i iis run at constant load l d and d constant speed and the indicator diagram is drawn with the help of indicator Preparation of heat balance sheet gives us an idea about the amount of energy wasted in various parts and allows us to think of methods to reduce the losses so incurred
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Heat … cont’d The energy supplied to Diesel engine in the form of fuel input is usually broken into: A A. B.
Indicated power Heat Rejected to Cooling Water
mcw (T 1 T 2 ) C.
Heat Carried Away by Exhaust Gases
meg (T 4 T 3) D.
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Heat Unaccounted for (Heat Lost Due to Friction, Radiation etc.) Mussie T.
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Heat … cont’d Item
Head units kcal or kJ
Heat in fuel supplied (a) Heat absorbed by I.H.P.
Percent Typical values 100% 100% 30%
(b) Heat rejected to cooling water (c) Heat carried away by exhaust ggases
30%
(d) Heat unaccounted for (by difference)
10%
Total 59
26%
100%
100%
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Diesel plant operation
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To ensure most economical operation of diesel engines of different sizes when working together and sharing load, it is necessary that they should carry the same percentage of their full load capacity at all times as the fuel consumption would be lowest in this condition. In order to get good performance of a diesel power plant the following points should be taken care of: It is necessary to maintain the cooling temperature within the prescribed range and use of very cold water should be avoided. avoided The cooling water should be free from suspended impurities and suitably treated to be scale and corrosion free. If the ambient temperature approaches freezing point, the cooling water should be drained out of the engine when it is kept idle. 60
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Diesel … cont’d During operation the lubrication system should work effectively and requisite pressure and temperature maintained. The engine oil should be of the correct specifications and should be in a fit. Condition to lubricate the different parts. A watch may be kept on the consumption of lubricating oil as this gives an indication of the true internal condition of the engine. The engine should he periodically run even when not required to be used and should not be allowed to stand idle for more than 7 days. Air litter, oil filters and fuel filters should be periodically serviced i d or replaced l d as recommended d d by b the h manufacturers f or if found in an unsatisfactory condition upon inspection. Periodical checking of engine compression and firing pressures and also exhaust temperatures should be made.
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3.
4.
5.
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Advantages of diesel power plants 1.Very simple design also simple installation. 2. Limited cooling water requirement. 3 Standby losses are less as compared to other Power plants. 3. plants 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. 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 62
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Disadvantages of diesel power plants 1. High Maintenance and operating cost. 2. Fuel cost is more costly, y, especially p y non-oil producing countries like Ethiopia. 3. The plant cost per kW is comparatively more. 4. The life of diesel power plant is small due to high maintenance. 5. Noise is a serious problem in diesel power plant. l 6. Diesel power plant cannot be constructed for large scale. 63
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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 M bil pl Mobile plant: t can be b easily il transported t t d on trailers t il 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 can be installed to supply power in a small town. town 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
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