Diesel Power Plants

8/14/2019 Diesel Power Plants

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13Diesel Power Plants

131. IntroductionDiesel plants are more efficient than any other heat engine ofcomparable size. These plants are cheap by way of initial cost, can

be started and stopped quickly and can burn a wide range of fuels.A Diesel plant does not require any warming period; it need not bekept running for a long time before peaking up loads. As a resultthere is no standby losses. Another advantage of such a plant isthat it does not need large amount of water for cooling. A dieselplant can be commissioned in such a much little time comparedwith a hydro, steam or nuclear power station.In view of these advantages a Diesel station is suitable for

localities where fuel costs are low, where water supply is limited,where oil is cheaper than coal and where loads are of such magnitudesthat they can be handled by a plant of small capacity.Another means of generating electricity (i.e. hydro, thermal,nuclear) are rivals to Diesel plants and can be attractive undercertain conditions. Also a gas turbine plant for continuous powergeneration is superior to a diesel plant where fuel is very cheap (asat a refinery or where load factors are very poor).Not withstanding competition from its rivals a diesel plantprovides the most economical means of generating electricity onsman scale particularly where there is no convenient site for micro-hydroplants, cheap fuels are not available and load factors areconsiderably large.The important fields of applications of diesel engines are asrail road locomotives, ship propulsion, road building and farm

machinery, electric generators for small supply units for public,industrial and institutional purposes e.g. cinema halls, hospitals,municipalties etc. These are used in freight trucks, and buses.However, since diesel engines can make efficient use of fuels thatare cheaper than gasoline, they are being utilized increasingly inautomobiles.Diesel electric power plants have been chiefly used as peak

load and standby units, for the hydroelectric power plants. These


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554 POWEH PLANT TECHNOLOGYare used as emergency standby units which normally remain idleand are run only where there is a failure of the central station andwhere key industrial processes can not be interrupted to avoid financialloss.132. Diesel Engine : Working Principle and GeneralDescription

An internal combustion engine in which the fuel is ignited byinjecting it into air that has been heated to a high temperature byrapid compression; hence, diesel engines are also called compression-ignition engines. The concept of ignition compression was patentedby Rudolf Diesel in 1892, and first demonstrated in an engine, fiveyears latter. The compression ignition engine is a heat engine (i.e.one that converts heat partially into mechanical work) operating onan approximation to the idealized Diesel cycle in which combustionof the fuel, that is, the heat addition stage, occurs at essentiallyconstant pressure.

Diesel cycle. A repeated succession of operations (or cycle)representing the idealized behaviour of the working fluid in thediesel engine form of heat engine. The diesel cycle is illustrated anddescribed in Fig. (1321.) Following main events are taking place ina cycle.

b

tp

o

vFig. ]821. Di('se] cycle.

dCDa

Suction operation (oa) at constant pressure in which air issucked inside the cylinder from atmosphere at nearly atmosphericpressure.Adiabatic compression of the working fluid i.e., air (gas) alonga b ; the temperature and pressure are increased.


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DIESEL POWER PLANTS 555Heat addition along beat const?nt pressure; the gas temperatureand volume are increased.Adiabatic expansion along c d, work is done by the expandinggas, and, the temperature and pressure decrease.Heat removal (rejection) along d, a at constant volume; the

pressure and temperature decrease, and the gas is restored to itsinitial condition at a. Here cycle is completed.In the description each stage is assumed to have been completedbefore the next stage is initiated. However, in an actual engine

there is a gradual rather than a sharp transition from one stage tonext; hence the sharp points in the figure would actually be roundedoff. In a diesel engine (Fig. 1322), air is down into a cylinder whereFuel I Both valves ~injector close d

INTAKE STAGE-1 COMPRESSIONSTAGF-2 POWER STAGE-] EXHAUST STAGE-4

Fig. J;1'~'~' "'our stJokc dl(;~'-'I'-'''gllle.it is compressed adiabatically by the inward motion of the pistonand thereby heated (stage 1). Just prior to maximum compression,fuel is injected and it burns rapidly in the very hot compressed air;heat is thus added to the working fluid at essentially constant pressure(stage 2). The hot combustion gases expand adiabatically and indoing so push back the piston and mechanical work is done stage 3).At stage 4, exhaust valve opens and operation 4 heat rejection andthen exhaust takes place.

Following four strokes are taking place in one cycle.1. Intake or Suction. The piston moving downward (i.e., out

of the cylinder) draws air into the cylinder by way of the openintake valve. The exhaust valve is closed (operation oa).2. Compression. The intake valve is closed and piston moving

upward (i.e., into the cylinder) compresses the air. The pressure isincreased to about 35 to 40 atm. (35 to 4 MPa), and the airtemperature rises to 450 to 500C.


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556 POWER PLANT TECHNOLOGY3. Power. Just before the point of maximum compression, withboth valves closed, a spray of very small droplets of fuel is injectedinto the top of the cylinder. At the existing high temperature of the

air the fuel burns rapidly and produces extremely hot compressedgases. The gases expand and push back the piston ; this is thepower stroke in which mechanical work is done. Not all of this workis available, however since part is utilized in the other strokes,especially in the compression stroke.4. Exhaust. The piston moving upward pushes the some whatcooled gases out through the open exhaust valve.The network in a Diesel cycle in the difference between theworkdone by the working fluid in stages 2 and 3 and the work doneon the fluid in stage 1. The thermal efficiency (i.e. the fraction ofthe heat supplied in stage 2 that is converted into net mechanicalwork) is increased by increasing the temperature at c and bydecreasing that at d. An equivalent statement is that an increase inthe compression ratio (volume at a divided by volume at .b) anddecrease in the cut ofTratio (volume at c divided by the volume at b)

increase the thermal efficiency. The minimum value of the cut offratio is unity.Four Stroke and Two-Stroke Engines

Diesel engines like spark ignition engines can operate on fourstroke or two stroke cycle (A stroke is an in or an out motion of thepiston). In the four-stroke cycle there are two in and two out motion(i.e., two revolutions of the crankshaft) per cycle. However, only oneof these four strokes is a power stroke ; hence there is only onepower stroke for two rotation of the crankshaft. In the two strokeengine, on the other hand, there, are one in and one out operation(i.e., one rotation of the crank shaft) per cycle. Consequently thereis one power stroke in each rotation of the crankshaft.

The two-stroke diesel engine are designed without valves andwith only two ports in the cylinder wall; the ports are opened andclosed when they are uncovered and covered, respectively, by themoving piston.

The advantage of a two-stroke cycle in providing a power strokefor each revolution of the engine crankshaft, rather than onepower stroke in two revolutions in a four stroke cycle, is outweighted in a spark ignition(gasoline) engine by the associatedpower losses. In two stroke diesel engines, however, especiallythose operating at low and medium speeds these losses aregreatly decreased. There is no loss of fuel through the exhaust


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DIESEL POWER PLANTS 557port because the fuel is not added until both ports are closed.Consequently, because of its design simplicity and increased powerfor a given engine speed, the two-stroke diesel engine is quite common,whereas the corresponding spark ignition engine has found onlylimited use.

Thus advantages of two stroke cycle over four stroke cycle are:more power output, less frictional loss per horse power, compactand simple mechanical design, no trouble from valves, lighter flywheel due to improved turning moment. But a two stroke engineoverheats on heavy loads and under light loads the running is erratic.Moreover, there is always a certain loss of fuel which escapes throughthe exhaust port before the compression. A four stroke engine iseconomical on lubricating oil and fuel consumption. Moreover, theengine cooling is simple and better as more time is available for theremoval of heat. Also, the combustion gases can be completely clearedfrom the cylinder. The arrangement of cylinders is also importantsince it effects the foundations, building space and maintenanceproblems. Vertical in line arrangement is most commonly used. Tomake the engine more compact, the cylinders may be arranged inV-shape. Two stroke radial diesel engines require minimum spaceand foundations.

Engines in the speed range of 200-1000 r.p.ro. are more common.Each cylinder is designed for around 75 kW and multi-cylinder engineshaving upto 16 cylinders; arranged vertically, are used for higheroutputs.Diesel Fuels. A diesel engine can use a wide variety of

fuels, ranging from natural gas to fairly heavy petroleum distillateoils which are cheaper than gasoline. High-speed diesel enginesuse lighter fuels than do those operating at lower speeds. Theheavier fuels require larger times to be injected and to vaporizeprior to combustion and hence are more suited to low speedengines.A mixture of liquid hydrocarbons used as fuel in diesel (C 1)engines. Diesel fuels are either various distillates obtained inpetroleum refinning operations or blends of such distillates withresidual ojl. The boiling range (200-360C) and specific gravity (082to 092 ; 40 to 20 API) are higher than for gasoline; diesel fuels arealso more viscous.An important criterion of diesel fuel is the ignition quality asindicated by the cetane number. The cetane numbers of diesel fuelsare usually in the range of 30 to 60. A high cetane number is

desirable for easy starting and smooth operation.


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558 POWER PLANT TECHNOLOGYIn practice, a short time, called the ignition delay, elapses betweenthe start of fuel injection and ignition in a diesel engine. The ignitiondelay is usually not more than a few thousandths of a second (i.e. afew miIli seconds), but a relatively long delay time, may beaccompanied by difficult starting f:-om cold and, rough and noisyoperation. The property of a diesel fuel that affects ignition delay isexpressed by the cetane number; an increase in the cetane numberdecreases the ignition delay, facilitates cold starting and makes theengine run more smoothly.

133.Diesel Eledric Plant Main ComponentsThe essential components of a Diese] Electric Plant are:(1) Engine.(2) Engine air intake system.(3) Engine fuel system.(4) Engine exhaust system.(5) Engine cooling system.(6) Engine lubrication system.(7) Engine starting system.The diesel engine and the auxiliary equipment as stated aboveare discussed in detail in the following paragraphs. A typical schematicarrangnment of the diesel plant installation i" shown in Fig. 1331..

Raw watc..'rpurnD


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DIESEL POWER PLANTS 5591. The diesel engine. This is the main component of theplant which develops power. Generally engine is coupled direetly tothe generator. Diesel engine may be a four stroke or a two stroke

engine. Four stroke engine is generally preferred as it has higherefficiency, lower specific fuel consumption and more effectivelubrication than a two stroke engine. Other things which may bespecified in diesel engines are: arrangement and number of cylindersused, simple aspiration or supercharging, efficiency and economicalfuel consumption.2. Engine air intake system. This includes air filters, ducts

and supercharger (an integral part of the engine). The system suppliesthe required quantity of air for combustion. Air requirements c"large diesel plants are considerable, around 4-8 m:!per kwh. Air isdrawn from outside the engine room and delivered to the intakemanifold through the air filters which remove the dust and othersuspended impurities from air. The purpose of the filter is to catchany air borne dirt as it otherwise may cause the wear and tear ofthe engine. The filter should be cleaned periodically. Filters may beof dry type (made up of cloth, felt, glass wool etc) or oil bath type. Inoil bath types filter the air is swept over or through a bath of oil inorder that the particles of dust get coated. The supercharger increasesthe pressure of air supplied to the engine so that it could develop anincreased power output. Superchargers are generally driven by theengine.

3. Fuel system. This include fuel storage tanks, fuel transferpumps, strainers, heaters and connecting pipe work. Fuel transferpumps are required to transfer fuel from delivery point to storagetanks and from storage tanks to engine. Strainers (filters) are neededto ensure clean fuel. Heaters for oil may be required especiallyduring winter.Fuel oil delivered to the power plant is received in storagetanks. Oil is pumped from storage tanks and supplied it to thesmaller day tanks from where it is supplied to engine as shown in

Fig. 13


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560 POWER PLANT TECHNOLOGYPump

Day tanl"J

Bulk storage

StrainersMeters

Fuel fromUnloading line

To EnginesFig. 1332. Fuel supply system for a diesel power plant.

In diesel engines atomized fuel is sprayed in the cylindersof the engine under pressure usually ranging from approximately100 to 120 kg/cm2 The two common fuel injection systems are theair injection and solid or air less injection. In the air injectionsystem, a multistage compressor is used to supply air at a pressureof approximately 60-80 kg/cm2 into the fuel nozzle. This systemis now rarely used. 'The fuel delivered to the nozzle by thefuel pump thus, discharged into the combustion chamber.The governing is effected by controlling the operation of thefuel pump.

The solid/mechanical-injection systems are available in threetypes:1. The common rail system;2. The distributor-injection system; and3. The pump and pressure operated nozzle systems.The last is the most often used.Common rail injections. This method uses a multi-cylinderfuel pump to maintain Iiconstant high pressure in the fuel dischargeline which supplies fuel to all injector valves of the engine, these

valves being always under pump pressure. A typical common railinjection system for a diesel engine is shown in Fig. 1333. A high


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DIESEL PO)VER PLANTS

Pressurerelief andtiming value

Control lever

Spring loadedspray valveFuel lines to

other cy linders

561

Hrgh pressurerelief val ve

Pump drive

F":~:::m;, ~"tCOlled pre,,",etank ' ~-- pumpFig. 1333. A typical common rail injection system for a diesel engine.

Primarypump

Metering ~pressurepump

CamFig. 1334. Typical distributor system.

pressure header or 'common rail' is supplied by a single pump withbuilt in pressure regulation which adjusts pumping rate to maintainthe desired injection pressure. The function of the pressure reliefand timing valves is to regulate the 'injection time and amount.Spring loaded safety valve acts merely as a check. When injectionvalve lifts to admit high pressure fuel to spray valve, its needlerises against the spring when the pressure is vented to the atmosphere,the spring shuts the valve.

Distribution system. A typical distributor injection system is

shown in Figr1.4.It is also called nnit injector method, in which(r~f


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562 POWER PLANT TECHNOLOGYthe whole process of metering, pressurizing timing, and injectiontake place in a pump-cum-atomizer unit, called the unit injector,one such injector being used for one cylinder. The high pressurefuel pipes are eliminated and the device is fitted in the cylinderhead, actuated by a push rod and rocker arm in a way similar tothe operation of the overhead valve. In the distributor block, camoperated poppet valves feed fuel to the cylinders in proper firingorder by opening just before injection. Controlling a by pass valve inthe pump or in the pump discharge line or varying the time ofclosure of the fuel pump inlet valve generally provides the governingeffect.Pump injector method. A typical pump and pressure operatednozzle system is shown in Fig. 1335. In this system fuel nozzle isconnected to a separate injection pump. The measuring of the fuelcharge and control of the injection timing are done by the pumpitself. The delivery valve in the nozzle is actuated by fuel oil pressure.The atomizers or the injection valves which are spring loaded injectthe fuel into the combustion chamber in a fine spray.

InjectIonnozzles

Control rackHigh pressu~fuel lines

\\\\\ \h\\ ,',

Pump withindividualcyllnd/for each nozzle

Fig. 1335. Typical pump and pressure operated nozzcl system.4. Engine Exhaust System. The function of the exhaust systemis to discharge the engine exhaust to the atmosphere outside thebuilding. This includes silencers (mumer) and connecting ducts/ pipes.A good exhaust system should keep the noise at a low level, exhaustwell above the ground level to reduce the air pollution at breathinglevel and should isolate the engine vibrations from the building by

using a flexible selection of exhaust pipe. The exhaust pipe is providedwith a muffler to reduce pressure in the exhaust line and reducesthe noise level. A typical exhaust system is shown il'\ Fig. 1336.


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DIESEL POWER PLANTS

Exhauststack

563

Diesel engine

//.~

Fig. 1336. Typical exhaust system.The exhaust stack usually stands on the muffler top. As thetemperature of the exhaust gases is sufficiently high, heat of thesegases is utilized in heating oil or air supplied to the engine. Theheat of exhaust gases may also be recovered in waste-heat boilersfor steam generation.

5. Engine Cooling System. This includes coolantpumps, sprayponds, water treatment or filtration plant and connecting pipe work.The purpose of the cooling system is to carry heat from enginecylinder to keep the temperature of the cylinder within safe limits.The extra heat, not used for doing useful work, has to be removedfrom the engine, otherwise this extra heat may disintegrate thelubricating oil film on the cylinder walls and damage the cylinderliners, heads, walls, piston and rings. Small engines may be aircooled,but large stationary engines use water circulating in cylinderjacket with the help of a pump. The hot water is cooled in a spraypond and recirculated.

Cooling water must be controlled in temperature ; when toolow, the lube oil (lubricating oil) will not spread properly and willresult in cylinder and piston wear ; when too high, the lube oilburns. It is necessary to keep the exist temperature of the coolingwater around 70C. The coolingwater requirement of diesel engine(for 10Ctemperature rise) is around 2-4 litres per bhp per minute.It is possible to utilize the heat of exit coolingwater for heating oil


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564 POWER PLANT TECHNOLOGYor buildings. It is necessary to treat the make up water to removethe scale forming impurities, zeolite softener or lime or lime sodaash treatment is employed.There are three system for the recooling of water for continuousu~:

1. Open system or direct evaporation.2. Closed system including heat exchangers with a secondarywater circulation.3. Radiators.The simplest cooling system would need only a water source, apump and place of disposal of hot water. Usually, however the samewater is re~rculated by cooling it in devices such as radiators,evaporative coolers, cooling tower, spray pond etc.Fig. (1337) a, b, c and d shows the different methods of enginecooling.

Fins

Radiator

Cap

Wate~in--=t:.

~tladiator

Jacket

Cap

-

Cylinder

(0)

Waterin-~-

piston

Cylinder

(a) Direct air cooling (b) Indirect system (naturalcirculation)(c) Indirect cooling with forced (d) Non-circulating coolingcirculation of water system with water.

Fig. 1337. Different methods of engine cooling.


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DIESEL POWER PLANTS 565Direct air cooling method employs fins casted on the cylinderhead to increase the exposed surface of contact with air. Air forcooling the fins, may be obtained from a blower or fan driven by the

engine. Air movement relative to engine may be used to cool theengine as in case of motor cycle engine. The direct air cooling isemployed in small industrial engines, motor cycle engines and aircraft engines.'J:he indirect cooling system may use natural circulation(thermosiphon) or forced circulation of water. In the thermo-syphonmethod the change in the density of water due to change intemperature causes it to circulate in the system. As the' ~ater iscooled in the radiator is descends while the hot water in the jacketrises and flows to the radiator at the top. This system is simple ,butthe motive force producing circulation of water is small and canprovide only slow rate of circulation, necessiating larger coolingelements. Some times a water tank of sufficient capacity may beused instead of the radiator to provide thermo-siphon coolings.The forced circulation, indirect cooling system is most widely

used in large and medium sized units. Cold water is passed throughthe cylinder jacket with the help of a pump usually mounted on theengine frame and getting the power from the engine crank shaft.The hot-water is sent to a cooling device, such as, cooling tower ora spray pond, whence it is taken in again for circulation after beingcooled.Water cooling systems in stationary diesel plants are of two

types as shown in Fig. 1338.

(a) Open or single circuit cooling system.Heatexchanger

Jacket waterpump pump(b) Closed or dou.hle circuit system.

Fig. 1338. Water cooling systems for stationalY diesel plants.


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DIESEL POWER PLANTS 567(3) Filtering, filtration through strainers and filters of absorbentand non-absorbent type. The effectively remove small amounts ofimpurities but are costly for large amounts.(4) Chemical-reclaiming. This uses a combination of heatactivated clays.Modern lube oils have additives to act as oxidation inhibitors,foam reducing agents, pour point depressants, and other agents.Dopes and additives may be used in oils to refresh them.7. Engine StarUng system. Because of the high compression

pressure, even a small diesel engine in a power plant can not bestarted by hand cranking. The various methods used for startingare:(1) Compressed air starting for medium and large capacitystationary and mobile units,(2) Electric-motor starting for small high-speed gasoline anddiesel engine, and(3) Auxiliary-engine starting for medium capacity mobile units.Compressed air system is mostly used for starting diesel enginesin power plants. Compressed air, from air tank, at about 20 timesatmospheric pressure is admitted to a few of the engine cylindersmaking them work like reciprocating air motors to turn the engineshaft. Compressed air causes the engine crankshaft assembly torotate. Fuel is admitted to the remaining cylinders and ignites inthe normal way causing the engine to start. Gradually the enginegains momentum and by supplying fuel the engines will start running.(2) Electrical starting system. Includes electric motor whichdrives a pinion which is engaged a toothed rim on engine flywheel.A small electric generator driven from the engine supplies electricfor the motor. Storage battery (12 to 36 volts) may also be used tosupply power to the electric motor for small plants. As soon as the

engine is started, electric motor disengages automatically.(3) The use of auxiliary engine usually petrol driven. Inthis method a small petrol engine is connected to the main enginethrough clutch and gear arrangements. The clutch is first disengagedand the auxiliary engine started by hand, or by a self starter motor.When it has warmed up and runs at normal speed the drive gear isengaged through the clutch, and the main engine is thus cranked.

Automaticany disengagement of clutch takes place after the mainengine has started.


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568 POWER PLANT TECHNOLOGY134. Method of Starting and Stopping Engines

The actual process of starting may differ from engine to engine,but there are certain common steps in the process which are asfollows:

(1) If air starting system is employed, the pressure of the airshould be checked and the air system inspected for possible leakage.Air should not leak into the cylinders.The storage battery should be checked if electric motor is usedfor starting. Periodic checking of battery is also required.(2) Check for fuel, lube oil and cooling water as prescribed bythe manufactures is necessary before starting engine.(3) There should be no load on the engine at starting anddecompression device is used.(4) The engine is run at slow speed for a few minutes, andthe various systems such as fuel, lubricating oil system etc. are

again checked.(5) The speed ofthe engine should be gradually increased tillit synchronises with the bus bars.(6) Then the generator is connected to the bus bar when it isin synchronism and the speed is increased till it begins to share theload as desired.The engine should not be stopped abruptly prescribed procedure

should be followed. The methods normally used are:1. Stopping fuel supply.2. Stopping the action of injection pump.3. Keeping the exhaust valve open.4. Shutting off air supply.Anyone of the above methods can be employed. If an engine is

to be stopped, its speed should be reduced gradually until practicallyno power is delivered by the alternator. Then the unit should bedisconnected from the bus and engine allowed to idle for a fewminutes. It should then be stopped in conformity with the instructionsdetailed by the manufactures. Flow of coolant and lubricating oilthrough the engine should be maintained for sometime after sloppingthe engine.l,3S. Diesel Plant Efficiency and Heat BalanceThe power developed in the cylinder or at the piston is necessarily


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DIESEL POWER PLANTSgreater than that at the crank shaft due to engine losses, thus

IHP = BHP + Engine losses

569

The indicated horse power (IHP) of a diesel engine is computedon the basis of the indicator diagram. The workdone in the enginecylinder per cycle equals the net area of the indicator diagram (i.e.area of positive loop less area of negative loop). From the area ofindicator diagram it is possible to find an average gas pressurewhich while acting on piston throughout one stroke would accountfor the net work done. This pressure is called indicated mean effectivepressure (i me p). The indicated mean effective pressure is calculatedby finding the mean height of the indicator diagram, and multiplyingit by the spring constant of the indicator spring. The work done onthe piston in each working stroke is calculated from the mean effectivepressure the area of the piston and length of the stroke. Thus indicatedhorse power -

IHP Pm LAn(MKS) = 4500where Pm = Mean effective pressure in kg/sq. cm.

L = Stroke or the piston in metre.A = Area of the piston in sq. em.n = Number of working strokes or number ofexplosions occurring in the cylinder per min.

N (r.p.m.)n= 2 for four stroke engineIn S.I. units, indicated power

= 100 x pm x L x A x n kWwhere Pm = Mean effective pressure expressed in bar.

A = Area if the piston in sq. m.n = Number of working strokes/secs.

BHP or Brake horse power is defined as the net poweravailable at the crank shaft. It is measured on the brake drumof a dynamometer, which gives it the name of brake horsepower (bhp).

Brake horse power BHP (MKS)_ 2 nNT- 4500

where T = torque in kg. m.


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570 POWER PLANT TECHNOLOGY(It resist the motion of the brake drum of the dynamometer)

N = Brake drum speed in rpmIf W is the net load in kg applied on the brake drum and R isthe radius of brake drum in metre then

T= W.RIn S.l. units brake power = 2 r:.!!T kJ/sec.When '1' is expressed in kilo-newton-meter. The difference ofIHP and BHP is called FHP. It is utilised in overcoming fricdonalresistance of rotating and sliding parts of the engine.

FHP = IHP - BHPThe ratio of BHP and IHP is known as mechanical efficiency,

TIm BHPTIm = DfP

In power plants which operates at constant speed, the mechanicalefficiency increases with increasing power output.Engine losses (FHP) that occur are pumping losses of tbe engine,windage loss at flywheel, mechanical losses in the bearings andpower required to drive the auxiliaries fitted on the engine. Theselosses may amount to 10 to 30% of the internal pOWH developed bythe engine.The efficiency of conversion of the heat energy of fuel into workis known as indicated thermal efficiency. It is the ratio of heatequivalent of IHP per minute to heat energy supplied in fuel perminute.

whereIHP x 4500

TltCMKS) = W X Cu x J .,. (131)W = Weight of the fuel supplied in kg per minuteCv = Lower calorific value of fuel oil ir. kcallkgJ = Joules equivalent = 427

Tli (S.L unit) = kWwhere kW is indicated power

W is expressed in kg/sec.and Cu in kJ/kg

... (132)


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DIESEL POWER PLANTS 571Brake thermal efficiency or overall efficiency is the ratio ofheat equivalent ofBHP per minute and heat energy supplied throughthe fuel per minute.

or7Jb(MKS) =

7Jb (S.l.) =

BHP x 4500Wx Cu xJBrake power

Wx Cv ... (133)where brake power is in kW.

W is in kg/sec. and Cv in kJ/kg. The value of this efficiency fordiesel engine varies from 30 to 40%.The specific fuel consumption (SFC) of a diesel engine is animportant parameter of engine performance and is calculated tojudge the economy in production with a particular engine, using aparticular fuel. It is defined as the amount of fuel burnt per b.h.p.!brake power per hour.

Heat Balance Account. A study of heat distribution in agiven engine will give sufficient indication as to how efficiently theengine is working and the general distribution of heat in an I.C.engine as shown qelow.

1-Heat lost tosurroundingmedium (air)by radiationetc.

JHeat rejectedthroughexhaust gasesr-~-tHeat in water Heat in

dry exhaustgases

JHeat rejectedto coolingwater

Heat supplied, to the en6rine (i.e. heat in fuel)1rHeat convertedinto workIHPr-Lj,B.H.P. Mechanicallosses

An account of the heat energy produced in the combustionchamber is maintained. The quantity of heat supplied to the engineis the product of the weight of the fuel and its calorific value. Inorder to draw up a heat balance sheet or account for a diesel enginecylinder, the engine should be tested over a period of time underconditions of constant load and speed. The following items arecalculated per unit oftime :

1. Heat supplied through the fuel,2. Heat equivalent of output produced,3. Heat lost to cooling water,


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572 POWER PLANT TECHNOLOGY4. Heat lost through exhaust gases,5. Heat lost due to radiation and other reasons, which cannot be directly measured. This is obtained as difference of

heat input and heat output as determined through items2 to 4 above.The following quantities must be measured for the period oftest:(1) Fuel consumption, (2) IHP or indicated power, (3) BHP orbrake power ; (4) quantity of water circulated for cooling ; (5)temperature of cooling water before entering and after leaving the

engine cylinder; and (6) quantity of exhaust gases and its temperature.All the measurements should be taken at regular interval oftime. Tests are conducted from no load to full load at various speedsarid results are tabulated at each speed. The unit of time may be anhour or a minute.Heat balance account may be drawn up as follows:

Heat Balance Sheet(In Kcal per min or in joules)

Heat input Kcalorpercent-eat expenditureKcaljoulesgeer minute or joules00%(i) Heat equivalent of BHP or brakeoolerii) Heat lost to jacketooling water(iii) Heat losto gases(iv) Heatlosty

It being noted that frictional power or FHP is not included asa separate item in balance sheet because most of the FHR willreappear as heat injected cooling water, exhaust gas etc.The energy produced by combustion of fuel in an engine is notfully utilized for the production of power. Maximum thermal efficiencyfor a small diesel engine may be about 30% and for large engines itmay be upto 40%. There are wide variations in the relative proportionsof the losses depending upon the type, size and operating conditionsof the engine under consideration.


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DIESEL POWER PLANTS 573A typical heat balance sheet at full load for Diesel engine eel)is as follows:Us~ul output -40%Heat lost to cooling water 30%Heat lost in exhaust gases 24%Heat lost in friction, radiation etc. 6%

Total 100%Example 13.5.1.During a test on a single cylinder oil engine,

250 mm bore, 600 mm stroke, working in 4 stroke cycle, the followingobservations were made. -Duration of test = 1hr.Area of indicator diagram = 451 cm2Length of the indicator diagram = 71 cmSpring index = 830 kgf/ cm2 / cm of compressionLoad on hydraulic dynamometer = 100 kgHydraulic dynamometer constant = 700Fuel consumptio,: per min. = 01867 kgCalorific value offuel used = 10,000 Kcal/kgMass of cooling water = 17 kg / mill,Inlet temperature of cooling water = 20COutlet temperature of cooling water = 45CTemperature of the exhaust gases = 400CWeight of dry exhaust gases -= 550 kg /mill,Room temperature = 25CSpecific heat of exhaust gases = 024Determine mechanical efficiency and draw up heat balance sheetin Kcal /min.Solution. Heat supplied by the fuel per min.

= WC"= 01867 x 10,000 = 1867 Kcal

Mean effective pressure PmArea of the indicator diagram x Spring no.

= Length of the card451 x 831 = 5.278 kgfJcm271


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:;:

574 POWER PLANT TECHNOLOGYpm LANIHP::: 45005-278 x 600 x!E. X 252 X 3501000 4 2 :;:60.504500

BHP:;: ~N (Hydraulic dynamometer)_ 100 x 350 _ 50- 700 -

Mechanical efficiency::: ~N:5011m::: 60.5 :::081::: 81% Ans.

Heat equivalent to BHP::: BHP;o 6324 (IHP::: 6324 Kcal/hr) :;:527 Kcal/min.

Heat lost in cooling water::: 17 x (45 - 20) :::425 Kcal/min.Heat carried by dry exhaust gases:;: m Cpg (tge - to)::: 550 x 024 x (400 - 25) :::495 Rcal/min.

Heat input Kcaleat expenditureKcalercentage per min.86700%eat equivalent ofBHP27823of fuel Heat in coolingwater 425277gases495700 473230

Example 1352. The following dqta related to a test on a fourstroke four cylinder diesel engine plant which has a cylinder bore of35 cm and a piston stroke of 45 cm. Speed is 300 rpm.Net brake load ....921 kg (8550 Newton),Indicated mean effective pressure ...75 kg / cm2 (735 N / sq cm)Effective radius of the brake drum :;:092 mFuel consumption per hour :;:75 kg


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DIESEL POWER PLANTS 575

Solution.

Calorific value of fuel = 10,000 Kcal (41868 kJ / kg)Air consumption per min = 30 kgQuantity of jacket cooling water = 80 kg / minRise in temperature of cooling water = 35CExhaust gas temperature 450CRoom air temperature = 25CSpecific heat of exhaust gases = 028 Kcal / kg. K (J17 kg / kg. K)Calculate - (i) Mechanical efficiency,

(ii) Indicated thermal efficiency,(iii) Brake thermal efficiency(iv) Specific fuel consumption, and(v) The plant heat balance.

(MKS System) :(.: 4 cylinder engine)pm [aN4500

45 1C ( )2 300= 4x75x 100 x"4 35 ~ ._~~ =4329IHP=

Brake horse power

=

BHP = 21C NT = 21C 300 x 921 x 092 - 3544500 4500 -Mechanical efficiency = ::2~ = 0822

= 822%. Ans.Indicated thermal efficiency = IHP x 4500W x Cu x J

4329 x 45007560 x 10,000 x 427

= 365% Ans.

=Heat equivalent of BHPHeat supplied in fuelSpecific fuel consumption = 37554= 0219 kg/BHP

= 75 x 10,000 = 12500 Kcal/min60354 x 4,500427

. = 3730 kcal/min. (2832%)~


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576 POWER PLANT TECHNOLOGYHeat given to jacket cooling water = 80 x 35

= 280Q kcal/min (224%)Weight of exhaust gases per minute= Air consumption per min. + Fuelconsumption per min.= 30 + ~~ = 3125 kg/min

Heat carried away by exhaust gases = 3125 x 028 x (450 - 25)= 3718 kcal/min = 2975%

Unaccounted = 12500 - (3730 + 2800 + 3718) = 2252 = 195%[SI unit] :

Indicated power = 4 (100 pm LA n) kW

= 4 x 100 x 7.5 x 45 x Tr x (..QQ..)2 X ~00 4 100 2 x 60= 324 kW.Brake power = 2 TrNT

= 2 Tr300 (8550 x 092) Nm /min= 2Tr300 x 8550 x 092 _ 267 k60 x 1000 - w

Mechanical efficiency = ~~~ = 822% Ans.Indicated thermal efficiency

_ kW _ 324 x 60 x 60- W x Cu -' 75 x 41868= 365% Ans.

Specific fuel consumption_ 75- 324= 0231 kg/kW.

Heat supplied in fuelW x Cu 75 x 41868= 60 = 60

= 52335 kJ/min


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DIESEL POWER PLANTS 577

21 mm27kNlm'/mm

14 litres400 rpm.75000 newton

Heat equivalent to brake power_ Brake power- 1,000_ 2,. x 300 x 8550 x 092- 1,000= 14821 kJ/min

Heat to cooling water = 80 x 4186 x 35= 11721 kJ/min

Heat carried away be exhaust gases= Weight of the exhaust gases x Specific heatx Temperature rise= 3125 x 117 x (450 - 25)(= 15539kJ/min

Heat unaccounted by difference = 10254Heat Balance SheetHeat input kJ/minHeat expenditureJ/min per min.233500%Heat equivalent4821832 of brake powerof fuel Heat in cooling11721240Heat carried

15539975Heat unaccounted

10254943.

Total100%

Example 1353. During a test on a four stroke cycle Dieselengine the following data and results were obtained:Mean height of the indicator diagramIndicator spring numberSwept volume of the cylinderSpeed of the engineEffective brake load


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578 POWER PLANT TECHNOLOGYEffective brake radius 07 metreFuel consumption 72 kg / hrCalorific value of the fuel 44,000 kJ / kgCooling water circulation 540 kg / hrRise in temperature of circulating water, 33C, specific heat ofwater, 418 kJ / kg Ie.Energy to exhaust gases, 336 kJ / sec.Draw up an overall energy balance in kJ / sec. and as a percentage.Also determine the mechanical efficiency.Solution: Indicated mean effective pressure

= 27 x 21= 567 kN/m2

Indicated power is given by[_ PmZ a h kW- 60

As the engine works on four stroke cycle principle and it issingle cylinder the number of working cycles per min will be 4g0 =200. I x A is equal the swept volume of the cylinder, which 14litresi.e. 0014 cubic metre.

=

=

:. Indicated power

Brake power

Mechanical efficiency

[_ 567 x ().014 x 200 x 103- 60= 26200 watts = 262 kW

B = 2nNT60_ 2n 400 x (W-8) R- 60

2,314,400,750, ()'760

= 2200 watts = 22kWBrake power

indicated power22= - =0842&2

i.e. 84% Ans.Heat from fuel = mass of the fuel x calorific value

72= 60 60 x 44000 = 88 kJ/sec .,


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DIESEL ~OWER PLANTSHeat to brake power ::: 22 kJ/sec.

Energy to circuJ 'lting water = 6~~~0 x 418 x 33::: 207 kJ/sec.Heat energy to exhaust gases = 336 kJ/sec.(given)Heat energy unaccounted i.e. to surroundings etc.

::: 88 - (22 + 207 + 306)= 117 kJ/sec.

Heat Balance Sheet

579

Heat in kJ /sec.Heat expenditureJ/sec. per see800Heat energy to 225% brake powerHeat in cooling20735

Heat energy to33682

Heat energy to11733Total

100%

136.Building and Plan Layout for Diesel Power PlantSince diesel stations have small capacity and only a fewauxiliaries, the design of the building in their case is simplerectangular blocks to accommodate the engine generator sets. The

usual arrangement adopted is to place the engine and alternator ona large cQncrete block which may be reinforced, if necessary. Thefoundation should be made on sub-soil which is firm and solid, andthe design should provide for absorption of vibrations so that theseare not transmitted to the building or to the surrounding structures.In general, the recommendations of the manufacturers in this regardshould be followed.Sometimes oil tanks may be located underground. There isneed to plan and provide proper ventilation ~also in cold countriesthe question of heating should be carefully considered. For


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580 POWER PLANT TECHNOLOGYrequirement of floor area the approximate dimensions of equipmentshould be known. The most common arrangement for diesel enginesis with parallel centre lines with some room left for extension in thefuture. Spacing necessary between two units, the distance betweentheir centre lines, the distance between the centre line of the endunit and the wall and the distance between the head and of theengine and the wall and the generator end and wall, should becarefully considered. The location of the switch-board, station auxiliarytransformers, battery room, fuel oil tank, compressed air cylinderbottle for engine starting, compressors, lubricating oil circuits andcooling arrangements for cylinder jackets and suction and exhaustarrangements for the engines should also be given due consideration.

Fig. 1361 shows the layout of a medium size diesel powerstation. Generally the units are placed parallel to each other so thatWater cooling pumps

StorageandshopWashRoom

Switchboard

Off Ice

Hall

.-_.: =,pace for future~u Unil no 4 ._"

1_-__U_~_;t=~;2~~~=JWatercoolingtanks

Oil storagetanks

Fig. 1361. Layout of a diesel engine power plant.the electrical connections from alternators to the control board andair ducts and exhaust pipes are short. There should be sufficientspace between the sets for carrying out repair and maintenance anddismantling the sets, if necessary. The air intakes and filters aswell as the exhaust mullers are located outside the building or maybe separated from the main engine room by a partition w'al1. Goodnatural lighting and ventilation lilhould be provided in the engineroom and it may be necessary to provide forced air ventilation.Adequate space for storage of oil and for a repair shop, as well as,for an office' should be provided close to the main engine room. Bulk. storage of oil may be made out of doors. Arrangement for c()olingthe water required for cylinder cooling can be located near, preferablyout side the building.


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IIIl~SEL POWER PLANTS 581137.Maintenance of Diesel Power Plant

Plant maintenance depends on various factors. It is usual tomaintain a correct record of instrument readings and condition ofoperation at regular intervals, say every half hour. Such recordsform log sheets. For proper plant maintenance various temperatures,pressures electric load etc. have to be checked periodically.Maintenance includes cleaning of fuel oil from dirt and other impuri tiesby means of filters. Filters may have power element, or cloth orfibre or a combination of cloth and fibre-when filter element becomeschoke it should be replaced by a new one. Dirt in fuel oil ruins thefine lap offuel iQjection pumps and plugs the iQjection nozzle orifice.The temperature and flow of coolant, lubricating oil and exhaustgases should be checked at regular intervals.

The specific fuel consumption of diesel engines in almost constantfrom halfload to full load and is around 035 litres per kWh output.The specific fuel consumption increases sharply if the load is lessthan half load and is such it is not advisable to operate dieselengines at less than half load.138. Super Charging

Increasing the air consumption permits greater quantities of fuelto be added and results in a greater potential output. The powerdeveloped by an I.e. engine depends upon the effective burning offuel in the cylinder. The greater the fuel burnt, the greater is theengine power. If a greater quantity of air is supplied to an engine, itwould develop more power for the same size, so it is desirable thatthe engine takes in the greatest possible mass of air. Thus the methodof increasing the air capacity\ of an engine is termed super charging.In supercharging the supply of air is pumped into the cylinder at apressure greater than the atmospheric, usual rap.ge being 028 to 14kglcm2 The apparatus used to increase the air density is known assupercharger. It is raerely a compressor which proVides a denser chargeto the engine, thereby enabling the consumption of a greater massofthe charge with the same total pistOn displacement. For groundinstallation, it is used to produce a gain in the power output of theengine. For air craft installations, in addition to producing a gain inpower output at sea level, it also enables the engine to maintain ahigher power output as altitude is increased.

During the process of compressing air or charge, the superchargeproduces the following effects:(1) Provides better mixing of air fuel mixture. The turbulent

effect created by the supercharger assists in additional mixing' of


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582 POWER PLANT TECHNOLOGYthe fuel and air particles. The arrangement of certain types ofsuperchargers, particularly the centrifugal type, also encouragesmore even distribution of the charge the cylinders.(2) The temperature of the charge is raised as it is compressed,resulting in a higher temperature within the cylinders. This is partiaIlybeneficial in that it helps to produce better vaporization of the fuel,but deterimental in that ittends to lessen the density of the charge.The increase in temperature of the charge also affects the detonationof the fuel. Super charging t.ends to increase the possibility ofdetonation in a S.I. engine and lessen the possibility in a C.!. engine.

(3) Power is required to drive the supercharger. This is usuallytaken from the engine and thereby removes, from over-all engineoutput, some of the gain in power obtained through supercharging.There are three basic types of compressors that may be used assuperchargers, namely the positive displacement type, centrifugalflow type and the axial flow type.Positive displacement superchargers ,nay be fmther divided intothe piston and cylinder, the rotary, and the 'screw' types. In thepiston and cylinder arrangement, a piston compresses air in a cylinderin much the same manner as it,compresses the air in a C.I. engine.In the rotary type, the air may be compressed by a meshing 'gear'arrangement (exempIifiedby a Roots, blower), or by a rotating vaneelement. These are illustrated in Fig. 1381 (a) and (b). In both of

(a) Rotary (Roots) blower

IInlet ~ I\-.\,

\'-\---_} Outlet,,

(b) Vane blowerFig. 1381. 'Schematic diagrams of two positive displacementtypes of compressor.


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DIESEL POWER PLANTS 583these rotary types, a volume of our is taken from the intake anddischarged at the outlet end. The air is compressed as it is forcedagainst the higher pressures at the outlet side of the compressor.The 'Screw' arrangement traps air between the intermeshing helicalshaped 'gears' and forces it axially toward the outlet end. The 'gears'are in some cases designed so that the volume of the pocket ofentrapped air is reduced as it proceeds through the compressoraxially thus producing compression of the air. Positive displacementsuperchargers are used with many reciprocatingengines in stationaryplants, vehicles, and marine installations. The piston and cylinderarrangement is generally limited to use on large, low speed C.!.engines.The centrifugal compressor is widely used as the superchargerfor reciprocating engines, as wen as the compressor for gas turbines.It is found in both stationary plants and in the power plantsfor vehicles. It is almost exclusively used as the superchargerwith reciprocating power plants for aircraft, because it isrelatively light and compact, and produces continuous flow-ratherthan pulsating flow as in some positive displacement types.The centrifugal type consists of an impeller which rotates in ahousing at a high speed, maximum speed used being of the orderof 16000 to 30000 r.p.m. They have high capacity for small sizeand low weight, and are suitable for automobiles or aircraftengines.

The axial flow compressor consistsofsevefl~lstages ofaltematingfixed and moving blades which compress the air as it moves axiallyalong the compressor. While it is seldom used to superchargereciprocating engines, it is widely used as the compressor unit ofgas turbines.

The power required to drive the supercharges increases rapidlyif the discharge pressure is increases, and the increased in poweroutput as a result of supercharging is not proportional to increasein fuel rate. For maximum advantage the supercharger blower maybe coupled to an exhaust turbine and by driven by the velocity ofexhaust gases. Such a combination is known as 'Turbo-charger' andis used in many heavy diesel engines. Thus the waste energy of theflue gases in utilized in improving the engine output. Manifold air.pressure is automatically increased varying with engine load andspeed. Air flow into the turbo-charged engine may be about doublethat of a naturally aspirated engine ofthe somedisplacement,rotatingat the same speed. More air makes it possible to bum more fuel,and this results in greater engine power.


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584 POWER PLANT TECHNOLOGY13.9.Advantages and Disadvantages of Diesel Power Plants

The diesel power plants have got several advantages over othertypes of power plants. They are as listed below:1. They can be easily located at the load centre withoutcausing pollution in the environment.

2. Handling of fuel (oil) is easier. Smaller storage is neededfor the fuel, and there is no refuse to be disposed off.3. The size of the plant is comparatively small for the samecapacity, which results in reduced cost of foundations and buildings.4. Diesel power plants maintain their high operating efficiencyirrespecti ve of load.5. They can be easily started from cold conditions, and canbe put on full load.6. ,No standby loses.7. Cooling water requirement is limited and also quantity ofmake-up water required for this plant is much less than other plants

of same capacity.8. The operation of the plant IS easy and less number oflabour is needed to operate it, sa-economy in labour is there.9. There is less fire hazard.10. Plant is compact and light.11. Maintenance charges are less as the auxiliary plant isalso small in size.12. Thermal efficiency of a diesel power station is always higherthan that of a steam plant of equivalent size.13. The plant layout is very simple. Installation and

commissioning of a diesel engine plant does not take much time.14. Total cost of the plant per kW of the installed capacity isabout 20 to 30% less than that of a steam plant of same size.15. In contrast with a steam plant, the diesel efficiency fallsoff very little with use.16. They maintain their high operating efficiency irrespectiveof load.

Disadvantages(1) The capacity of the plant in this case is limited, largecapacity units as available in case of steam power plants, are usuallynot possible. '


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DIESEL POWER PLANTS 585(2) Diesel oil is costly.(3) Cost oflubrication is also high.(4) This type of plant does not work satisfactorly under overloadconditions for longer time.(5) Noise from the exhaust is also a problem.

Questions131. Draw a neat sketch of a diesel power plant showing all the systems.132. What are 4-stroke and 2-stroke cycles and what are the advantages anddisadvantages of each? Describe the action of each cycle.133. What are the different methods of fuel injection used in diesel plant?Which method is commonly used in large capacity diesel plant andwhy?134. What are the ways of cooling employed in modern diesel engines? Whatprecautions should be taken to ensure that cooling is satisfactory.135. What are the different ways of starting diesel engine, and what is thefield of application of each? What precautions should be taken before anengine is started?136. Why the supercharging is necessary in diesel plant? What are themethods used for supercharging the ~iesel engine?137. What are the advantages and disadvantages of diesel power plant ascompared to other power plants.

Objective Type Questions131. Reciprocating motion of the piston is converted into a rotaryone by

(a) Crankshaft (b) Connecting rod(c) Gudgeon pin (d) Crank web.

132. Compression ratio of an I.C. engine is the ratio of(a) ,.Total,:r~l,urne (b) Total VO!~I?e

(d) none of the above.c) Clearance VolumeTotal Volume133. For importing power crank webs are provided

(b) balancinga) energy storage(c) force.

134. In the case of diesel engine, the pressure at the end ofcompression is in the range of(a) 7-8 kg/cm2(c) 35-40 kg/cm2 (b) 20-25 kg/cm2(d) 50-60 kg/cm2


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586 POWER PLANT TECHNOLOGY135. Maximum temperature which is developed in the cylinder ofdiesel engine is of the order of

(a) 1000--1500C~) 2000--2500C (b) 1500--2000C(d) 2500--3000C.

(b) 20-400

136. Most high speed diesel engines work on(a) Diesel cycle (b) Dual combustion cycle(c) Camot cycle.

137. The cetane number of diesel fuels are usually in the range of(a) 10--200(c) 30 to 60.

138. In Diesel cycle(a) Compression ratio and expansion ratio are equal(b) Compression ratio is greater than expansion ratio(c) Compression ratio is less than expansion ratio.

139. In multicylinder engines a particular sequence in the firingorder is necessary(a) to operate the ignition system smoothly(b) to obtain uniform turning moment(c) to provide the best engine performance.

1310. State the following sentence whether true (T) or false (F).(i) In C.!. engines combustion is initiated by producing aspark in the combustion chamber just after the end ofthe compression stroke.(ii) A four stroke engine is less efficient as compared to atwo stroke one.(iii) In C.!. Engines mixing of air and fuel is achieved uptosome extent through an injector.(iv) In coil ignition system a better spark of uniformintensity is achieved at all speeds.(v) Magneto ignition system is more reliable and compactas compared to coil ignition system.(vi) Water cooling is more efficient than air cooling. )(vii) Cooling should be adequate and even excessive coolingof the engine is undesirable.(viii) Air cooling is used for large mobile I.C. engines.


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DIESEL POWER PLANTS 5871311. In a diesel engine the heat lost to the cooling water is about

(a)30% (b)0%0%.

Answers2. (b). (b). (c). (c). (c). (b). (c)i)F (ii) Fiii) Tiv) Tv) Tvii) Tviii) F1. (a).

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Diesel Power Plants