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StudentResource
SubjectB2-14
Propulsion
Copyright © 2008 Aviation Australia
Allrightsreserved.Nopartofthisdocumentmaybereproduced,transferred,sold,orotherwisedisposedof,withoutthewrittenpermissionofAviationAustralia.
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CONTENTS
Definitions iii
Study Resources iv
Introduction v
TurbineEngineFundamentals 14.1.1-1
EngineFuelSystems 14.1.2-1
EngineIndicationSystems 14.2-1
EngineStarting&IgnitionSystems 15.13-1
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DEFINITIONS
Define
Todescribethenatureorbasicqualitiesof.
Tostatetheprecisemeaningof(awordorsenseofaword).
State
Specifyinwordsorwriting.
Tosetforthinwords;declare.
Identify
Toestablishtheidentityof.
List
Itemise.
Describe
Representinwordsenablinghearerorreadertoformanideaofanobjectorprocess.
Totellthefacts,details,orparticularsofsomethingverballyorinwriting.
Explain
Makeknownindetail.
Offerreasonforcauseandeffect.
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STUDYRESOURCES
JeppesenSandersonTrainingProducts:
A&PTechnicianPowerplantTextbook.
AircraftGasTurbinePowerplantsTextbook.
AircraftTechnicalDictionaryThirdEdition
AircraftInstrumentsandIntergratedSystems.
FADECforPart-662ndEdition(www.totaltrainingsupport.com)
B2-14StudentHandout
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INTRODUCTION
Thepurposeofthissubjectistofamiliariseyouwithconstruction,components,operationandmaintenanceofgasturbineenginesandassociatedinstrumentandelectronoicfuelcontrol
systemsusedinaircraft.
Oncompletionofthefollowingtopicsyouwillbeableto:
Topic 14 1 1 Turbine Engine Fundamentals
StateNewton’slawsofmotion.
Definepotentialenergy,kineticenergyandBraytoncycle.
Definetherelationshipbetweenthefollowing:
Force
Work
Power
Energy
Velocity
Acceleration.
Definetheconstructionalarrangementandoperationofthefollowingenginetypes:
Turbojet
Turbofan
Identify thecomponentsofanddefine theoperationof the following turbopropandturbo-shaftenginesystems:
Gascoupled/freeturbineand
Gearcoupledturbine(Reductiongearbox).
Topic 14 1 2 Engine Fuel Systems
Identifyenginefuelsystemcomponentsanddescribesystemlay-outsandoperations.
Describetheoperationofenginefuelmeteringsystems.
Describetheoperationofelectronicenginecontrol(FADEC).
Topic 14 2 Engine Indication Systems
Identifycomponentsofthe followingengine indicationsystemsanddescribesystemoperation:
ExhaustGasTemperature(EGT);
TurbineTemperature(Interstage(ITT),Inlet(TIT/TGT));
EngineThrust;
EnginePressureRatio(EPR);
TurbineDischarge/JetPipePressure;
OilpressureandTemperature;
FuelpressureandFlow;
EngineSpeed;
VibrationMeasurement;
EngineTorque;
Power; ManifoldPressureand
PropellerSpeed.
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Topic 15 13 Engine Starting and Ignition Systems
Describecomponentsofenginestartsystemsandtheiroperation.
Describecomponentsofengineignitionsystemsandtheiroperation.
Interpretthesafetyprecautionstobeobservedwhenperformingmaintenanceonengineignitionsystems.
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TOPIC 14.1.1 TURBINE ENGINE FUNDAMENTALS
NEWTON’S FIRST LAW OF MOTION
Newton’sFirstLawmaybestatedas:“Abodywillremainatrestorcontinueitsuniformmotioninastraightlineuntilacteduponbyanexternalnetforce.”
Newton'sfirstlawofmotionisalsooftenreferredtoasthelawofinertia.
Thelargerthemass,thegreatertheinertia.
NEWTON’S SECOND LAW OF MOTION
Newton’sSecondLawofmotionstates:“Theaccelerationofabodyisdirectlyproportionaltotheforceappliedtoitandisinverselyproportionaltothemassofthebody.”
When a force acts on an object, giving it motion, it gains momentum. Once an object has
momentum,ittakesforcetohaltthemotion.
Force=MassxAcceleration,orF=MxA,where:F=Forceinpounds,M=Massinlbs./ft/sec.², A=Accelerationinft/sec.²
So,theforcedevelopedbyagasturbineengineisproportionalto:
themassofairflowingthroughtheengine;
theaccelerationgiventothatmassofair.
NEWTON’S THIRD LAW OF MOTION
Newton’sThirdLawofmotionstates:“Foreveryaction,thereisanequalandoppositereaction.”
“Equal”meansequalinsizeand“opposite”meansoppositeindirection.
Rocketsandreaction-jetthrustersrelyonNewton’sThirdLawofMotionfortheireffect
Theactionofexhaustgases leavingaturbojetengineproducea reactioncalledthrust.ThisisNewton’sthirdlawofmotioninrespectofgasturbines.
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FORCE
Forceisdefinedasthecapacitytodowork,orthetendencytoproducework.
Itis alsoa vectorquantity thattends to produceaccelerationof abodyin the direction ofitsapplication.Itcanbemeasuredinunitsofpounds.
Turbojetandturbofanenginesareratedinpoundsofthrust.
Theformulaforforceis:Force=PressurexArea,orF=PxA
Where:F=Forceinpounds
P=Pressureinpoundspersquareinch(psi)A=Areainsquareinches.
EXAMPLE:The pressureacross theopeningofa jet tailpipe (exhaustnozzle) is6psiaboveambientandtheopeningis300squareinches.Whatistheforcepresentinpounds?
F=PxA
F=6x300
F=1,800pounds
Theforcementionedhere ispresent inaddition to reactive thrust inmost gas turbineenginedesigns.This“pressurethrust”willbediscussedlaterinotherchapter.
WORK
Mechanicalworkispresentwhenaforceactingonabodycausesittomovethroughadistance.Workisdescribedasusefulmotion.Aforcecanactonanobjectvertically(oppositetheeffectofgravity),horizontally(90degreestotheeffectofgravity),orsomewhereinbetween.Aforcecanalsoactonanobjectinadownwarddirection,inwhichcaseitwouldbeassistedbygravity.Thetypicalunitsforworkare“inchpounds”and“footpounds”.
Theformulaforworkis:Work=ForcexDistance,orW=FxD
Where:W=Workinfootpounds;F=Forceinpounds;D=Distanceinfeet.
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Forinstance,liftingthesameobjectthesameverticalheightrequiresthesamework,nomatterthepath.
POWER
The definition ofworkmakes nomention of time.Whether it takes five seconds tomoveanobjectorfivehours,thesameamountofworkwouldbeaccomplished.Power,bycomparison,doestakethetimeintoaccount.Toliftatenpoundobject15feetoffthefloorinfivesecondsrequiressignificantlymorepowerthanto lift itin fivehours.Workperformedperunitof time ispower.Power ismeasuredinunitsoffootpoundspersecond,footpoundsperminute,ormilepoundsperhour.
Theformulaforpoweris:Power=ForcexDistanceFxD
Where:P=Powerinfootpoundsperminute;D=Distanceinfeet;t=Timeinminutes.
EXAMPLE:A2,500poundengineistobehoistedaheightof9feetintwominutes.Howmuchpowerisrequired?
P=(FxD)/t
P=(2,500x9)/2=11,250ft.lbs/mm.
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Inphysics, acceleration isdefined asa change invelocity with respect to time.Observe thatdistancetraveledisnotconsidered,onlylossorgainofvelocitywithtime.Thetypical(Imperial)units foracceleration are feet per second/second (fps/s) and miles per hour/second (mph/s).Feetpersecond/secondaresometimesreferredtoasfeetpersecondsquared(fps2).
HORSEPOWER
Horsepower isamore commonand usefulmeasure ofelectricalpower.Yearsago using the
multiplierof1.5timesastronghorse’sability todousefulwork,itwasdeterminedthat33,000poundsofweightliftedonefootinoneminutewouldbethestandardintheEnglishsystem.Ifpower is in foot pounds/minute, it can be divided by 33,000 to convert to horsepower.Mathematically,theunitsoffootpoundsperminutewillcanceleachotherout,leavingonlythenumber.Horsepowerdoesnothaveunits,sincehorsepoweristheunit.Ifpowerisbeingdealtwithinunitsoffootpoundspersecond,550istheconversionnumber.Ifpowerisinmilepoundsperhour,375istheconversionnumber.
Theformulaforconvertingtohorsepoweris:Hp=Power(inft.lbs/mm.)/33,000.
EXAMPLE:Howmuchhorsepowerisrequiredtohoista2,500poundengineaheightof9feetintwominutes(thepreviousexamplewhichrequired11,250ft.lbs./minofpower)?
Hp=Power/33,000=11,250/33000=0.34orapproximately1/3Hp
SPEED and VELOCITY
Velocitydealswithhowfaranobjectmoves,whatdirection itmoves,andhowlongittookittomovethatfar.
Velocity isexpressed inthesameunitsasspeed,typically feetper second(fps)ormilesperhour(mph).Thedifferenceisthatspeeddoesnothaveaparticulardirectionassociatedwithit.Velocityisidentifiedasbeingavectorquantity,whilespeedisascalarquantity.
Theformulaforvelocityis:
Velocity=Distance÷time,orV=D÷t
ACCELERATION
TheSIunit–metre/second2.
Theformulaforcalculatingaccelerationis:
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Theaccelerationrateduetogravity,whenanobjectisin freefallwithnodrag,is32.2feetpersecond/second.Whenanobjectacceleratesatthisrate,it isexperiencingwhat isknownasaforceof1“g”.
Ifwedivided theaccelerationrate for theexamplefighterairplaneby32.2,wewoulddiscoverhowmany“g”forcesitisexperiencing(132÷32.2=4.1g’s).
Negativeaccelerationiscalleddeceleration.
ENERGYEnergyisusedtoperformusefulwork. Inthegasturbineenginethismeansproducingmotionandheat.Thetwoformsofenergywhichbestdescribethepropulsivepowerofthejetenginearepotentialandkineticenergy.
Potential Energy
Energystoredbyanobjectbyvirtueof itsposition.Forexample, anobject raisedabove thegroundacquirespotentialenergyequaltotheworkdoneagainsttheforceofgravity;theenergyisreleasedaskineticenergywhenitfallsbacktotheground.Similarly,astretchedspringhas
stored potential energy that is releasedwhen the spring is returned to its unstretched state.Otherformsofpotentialenergyincludeelectricalpotentialenergy.
Chemicalenergyisausefulbutobsolescenttermfortheenergyavailablefromelementsandcompoundswhen they react,as ina combustionreaction. Inpreciseterminology, there isnosuchthingaschemicalenergy,sinceallenergyisstoredinmatteraseitherkineticenergyorpotentialenergy.
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Kinetic Energy
Theenergypossessedbyabodybecauseofitsmotion,equaltoonehalfthemassofthebodytimesthesquare of its speed,equal toonehalf themassof the bodytimes thesquareof itsspeed.
Formofenergythatanobjecthasbyreasonofitsmotion.Thekindofmotionmaybetranslation(motion along a path from one place to another), rotation about an axis, vibration, or anycombinationofmotions.Thetotalkineticenergyofabodyorsystemisequaltothesumofthekineticenergiesresultingfromeachtypeofmotion.
Thekineticenergyofanobjectdependson itsmassandvelocity.Forinstance, theamountofkineticenergyKEofanobjectintranslationalmotionisequaltoone-halftheproductofitsmass
mandthesquareofitsvelocityv,orKE=1/2mv².
Forexample,a500,000kgmassA380aircraftisflyingoverSydneyat250meterspersecond,whatisitskineticenergy?
KineticEnergy=½·500000·250²=15,625,000,000joules.
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BERNOULLI’S THEOREM
Bernoulli’sprincipledealswithpressureofgases.Pressurecanbechangedinthegasturbineenginebyaddingorremovingheat,changingthenumberofmoleculespresent,orchangingthevolumeinwhichthegasiscontained.
Bernoullidiscoveredthatairactsasanincompressiblefluidwouldactwhenflowingatsubsonicflowrates.
Theprincipleisstatedasfollows:“Whenafluidorgasissuppliedataconstantflowratethroughaduct,thesumofpressure(potential)energyandvelocity(kinetic)energyisconstant.”Inotherwords,whenstaticpressureincreases,velocity(ram)pressuredecreases.Orifstaticpressuredecreases, velocity (ram) pressure increases, meaning that velocity pressure will change inrelationtoanychangeinstaticpressure.
Ifairisflowingthroughastraightsectionofductingwhichthenchangestoadivergentshape,itskineticenergyintheaxialdirectionwilldecreaseastheairspreadsoutradially,and,asthetotal
energy at constant flow rate of the air is unchanged, the potential energy must increase inrelationtothekineticenergydecrease.
TherearemanyexampleswithinagasturbineengineoftheapplicationofBernoulli’sTheorem:
theairpassagesbetweenindividualbladesofacompressororturbine;
thediffusersectionofacentrifugalcompressor;
thecross-sectionalshapeofengineinletandexhaustducts;
theentiregasflowpaththroughtheengine.
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BERNOULLI’S HEOREM PRESSURE VELOCITY TEMP GRAPH
TheapplicationofBernoulli’sTheoreminatypicalsingle-spoolaxialflowturbo-jetengine.
Theanimationshows thechanges ofpressure,velocity, temperature (turbojet) duringgroundrun-up.
BRAYTON CYCLE
TheBraytoncycleisalsowidelyknownasa“constantpressurecycle”.Thereasonforthisisthatinthegas turbineengine,pressureis fairlyconstantacross thecombustionsectionasvolumeincreasesandgasvelocitiesincrease.
Combustiontakesplaceatconstantpressureingasturbineengines.
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The four continuousevents shown on thepressure- volumegraphare: Intake, compression,expansion(power),andexhaust.
Referringtothegraph,
AtoBindicatesairenteringtheengineatbelowambientpressureduetosuctionandincreasingvolumeduetothedivergentshapeoftheductinthedirectionofflow.
BtoCshowsairpressurereturningtoambientandvolumedecreasing.
CtoDshowscompressionoccurringasvolumeisdecreasing.
DtoEindicatesaslightdropinpressure,approximately3%,throughthecombustionsectionandanincreasingvolume.Thispressuredropoccursasa resultofcombustionheataddedandiscontrolled by the carefully sizedexhaust nozzleopening.Recall that there isabasicgaslawwhichstatesthatgaswill tendtoflowfromapointofhighpressuretoapointoflowpressure.Thepressuredropinthecombustorensuresthecorrectdirectionofgasflowthroughtheenginefromcompressortocombustor.Theairrushinginalsocoolsandprotectsthemetalbycentering
theflame.
EtoFshowsapressuredropresultingfromincreasingvelocityasthegasisacceleratedthroughtheturbinesection.
FtoGshowsthevolume(expansion)increasewhichcausesthisacceleration.Gcompletesthecycleasgaspressurereturnstoambient,orhigherthanambientatthenozzleifitischoked.
ENGINE STATIONS
Asystemofstandardstationnumberingmakes iteasier to findvarious locationsonandwithintheengine.
Numbers from 1 to 9 designate certain locations. For example, station 2 is always thecompressorinlet.
Inaddition tothestation numbers,prefixesareused toshowvariousparameters occurringatthesestationswithintheengine.
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Forexample,
TemperaturehastheprefixT.
o
Thetemperatureoccurringatstation5iscalledT5.
PressurehasaprefixPandcanbefurtherdividedinto:
o Pt–totalpressure;
o Ps–staticpressure.
Thestaticpressureatstation3isknownasPs3.
Engine Directional References
Forpurposesofidentifyingengineconstructionpoints,orcomponentandaccessoryplacement,
directionalreferencesareusedalongwithstationnumbers.Thesereferencesaredescribedasforwardattheengineinletandaftattheenginetailpipe,withastandard12hourclockorientation.Thetermsright-andleft-hand,clockwiseandcounterclockwise,applyasviewedfromtherearoftheenginelookingforwardtowardtheinlet.
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GAS TURBINE ENGINE TYPES
Gasturbineenginesareconsideredtobeoftwotypes:
a.
ThrustProducingEngines;
b. TorqueProducingEngines.
Thetwoclassificationsofthrustproducingturbineenginesare:a.Turbojet;b.Turbofan.
Thetwoclassificationsoftorqueproducingturbineenginesare:a.Turboprop;b.Turboshaft.
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TURBOJET ENGINE
The turbojet, as first patented by Sir Frank Whittle, had an impeller compressor, annularcombustor, and a single stage turbine. Today it ispossible tosee manyvarieties of turbojetenginedesigns,butthebasiccomponentsarestillthecompressor,combustor,andturbine.
Theturbojetgetsitspropulsivepowerfromreactiontotheflowofhotgases.Airenterstheinletand its pressure is increased by the compressor. Fuel is added in the combustor and theexpansioncreatedbyheatforcestheturbinewheeltorotate.Theturbinesectioniscoupledtothecompressorsectionanddirectlydrivesit.Theenergyremainingdownstreamoftheturbineinthetailpipeacceleratesintotheatmosphereandcreatesthereactionwerefertoasthrust.
Theyhaverelativelyfewmovingpartsandcreatethrustbyacceleratingarelativelysmallmassofairwithalargeamountofacceleration.
Theyarelessefficientduetolossesfromnoiseandincompletecombustion.
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Engine Pressure Ratio
Whendiscussingaturbojetengineyoumustbefamiliarwiththetermenginepressureratio,or
EPR. An engine’s EPR is the ratio of the turbine discharge pressure to the engine inlet airpressure.EPRgaugereadingsareanindicationoftheamountofthrustbeingproducedforagivenpowerleversetting.Totalpressurepickups,orEPRprobes,measuretheairpressureattwopointsintheengine;oneEPRprobeislocatedat thecompressorinletandasecondEPRprobeislocatedjustaftofthelaststageturbineintheexhaustsection.EPRreadingsareoftenusedasverificationofpowersettingsfortake-off,climb,andcruise.EPRreadingsareaffectedbyandaredependentonpressurealtitudeandoutsideairtemperature(OAT).
TURBOFAN
Theturbofan,ineffect,isa ducted,multi-bladedpropellerdrivenbyagas turbineengine.Thisfanproducesapressureratioontheorderof2:1,ortwoatmospheresofcompression.Generally,
turbofanscontain20to40fixedpitchblades.
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Bycomparison,thefandiameterofaturbofanengineismuchlessthanthatofthepropelleronaturbopropengine,butitcontainsmanymorebladesandmovestheairwithagreatervelocityfromitsconvergentexhaustnozzle.
Turbofanhasmoreturbinestagesthanaturbojetinordertodrivethefanatthefrontorback.
Thereare:
Forwardfanengines
Aft-fanengines:doesn’tcontributetocompression.
Fan Bypass Ratio
ThepropulsiveefficiencyofaTurbofanengineismeasuredbyFanBypassRatio.
Fanbypassratioistheratioofthemassairflowwhichflowsthroughthefanduct,dividedbythemass airflowwhich flows through thecoreportion of theengine. Fan airflowpassesover theouterpartofthefanbladeandthenoutofthefanexhaustandbacktotheatmosphere.Coreengineairflowpassesovertheinnerpartofthefanbladesandisthencompressed,combusted,andexhaustedfromthehotexhaustduct.
Thefanorbypassairisnotusedforcombustionbutproducesthemajorityofthrust.
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Turbofanenginescanbe
Highbypass(4:1ormore)
Mediumbypass(2or3:1)
Lowbypass(1:1)
Mostturbofanengineshaveseparatelowpressureandhighpressurecompressorandturbinespools.
Generaloverviewofatypicalhighbypass-ratioturbofanengine(AdaptedfromPratt&Whitney).
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TURBOPROP
Betterpropulsiveefficiencyatlowspeedcomparedtoaturbojet,theextraturbinestagesareusedtodriveashaft.
Connectedtotheshaftisareductiongearboxandapropeller.
Thepropellermovesalargemassofairwitharelativelysmallamountofacceleration.
Turbopropenginesareveryfuelefficientatlowerairspeeds.
Thepropellerstartstobecomeaerodynamicallyinefficientathigherairspeeds.
TwomaintypesofTurbopropengines:
Fixedshaft(AlsocalledGearCoupledturbine);
Freeturbine.Thefixedturbineisconnecteddirectlytothecompressor,reductiongearbox,andpropellershaft,in another words, themain power shaft of a fixed shaft engine goes directly to a reductiongearboxwhichcandriveapropeller,forexample,GarrettTPE331fixedshaftturbopropengine.
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Free turbine turboprop engine
Alsocalledgascoupled.
Forexample,Pratt&WhitneyPT6freeturbineturbopropengine(Reverseflowcombustor).
Thefreeturbineisconnectedonlytothegearboxandpropellershaft.Thisisanindependentturbine that isnot connected to themain turbine.This arrangementallows thefree turbine toseek itsoptimumdesignspeedwhilecompressorspeedissetatitsdesignpoint(pointofbestcompression).
Someoftheadvantagesofthefreeturbineare:
1.Thepropellercanbeheldatverylowrpmduringtaxiing,withlownoiseandlowbladeerosion.
2.Theengineiseasiertostart,especiallyincoldweather.
3.Thepropelleranditsgearboxdonotdirectlytransmitvibrationsintothegasgenerator.
4.Arotorbrakecanbeusedtostoppropellermovementduringaircraftloadingwhenengineshutdownisnotdesired.
Disadvantage:Theenginedoesnothavetheinstantaneouspowerofreciprocatingengines.
TURBOSHAFT
Turboshaftengines are gasturbine engines that operate somethingother than apropeller bydeliveringpowerto ashaft.Turboshaftenginesaresimilar toturbopropengines,and insomeinstances,both use the same design. Like turboprops, turboshaftengines usealmost all theenergyintheexhaustgasestodriveanoutputshaft.Thepowermaybetakendirectlyfromtheengineturbine,ortheshaftmaybedrivenbyitsownfreeturbine.Likefreeturbinesinturbopropengines,afreeturbineinaturboshaftengineisnotmechanicallycoupledtotheengine’smainrotor shaft, soitmay operate at its ownspeed.Freeturbinedesigns areused extensively incurrentproductionmodelengines.
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Thepi
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ctureshowingisaGeneralElectricT-64Turboshaftengine.
Turboshaftenginesarefrequentllargecommercialaircraft.
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ENGINE COMPONENTS
Therearesevenbasicsectionswithineverygasturbineengine.Theyarethe
airinlet.
compressorsection.
combustionsection.
turbinesection.
exhaustsection.
accessorysection.
systemsnecessaryforstarting,lubrication,fuelsupply,andauxiliarypurposes,suchasanti-icing,cooling,andpressurization.
Additional terms you often hear include hot section and cold section. A turbine engine’s hotsection includesthecombustion,turbine,andexhaustsections.Thecoldsection,ontheotherhand,includestheairinletductandthecompressorsection.
Air Inlet Duct
Theairinlettoaturbineenginehasseveralfunctions,oneofwhichistorecoverasmuchofthetotalpressureofthefreeairstreamaspossibleanddeliverthispressuretothecompressor.Thisisknownasram recoveryorpressurerecovery. Inaddition torecoveringandmaintainingthepressure of the free airstream, many inlets are shaped to raise the air pressure aboveatmosphericpressure.
Another function of the air inlet is to provide auniform supply ofair to the compressor so thecompressor can operate efficiently. Furthermore, the inlet duct must cause as little drag aspossible.It takesonlyasmallobstructionto theairflowinsideaducttocauseaseverelossofefficiency.Ifaninletductistodeliveritsfullvolumeofairwithaminimumofturbulence,itmustbemaintainedasclosetoitsoriginalconditionaspossible.Therefore,anyrepairstoaninletduct
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mustretaintheduct’ssmoothaerodynamicshape.Tohelppreventdamageorcorrosiontoaninletduct,aninletcovershouldbeinstalledanytimetheengineisnotoperating.
FOREIGN OBJECT DAMAGE
Toensure the operatingefficiencyofan air inlet duct, periodic inspection for ForeignObjectDamage(FOD)andcorrosionisrequired.
Preventionofforeignobjectdamage(FOD)isatoppriorityamongturbineengineoperatorsandmanufacturers.
COMPRESSOR SECTION
Theprimaryfunctionofa compressoris toforceairintotheengineforsupportingcombustionandprovidingtheairnecessarytoproducethrust.
Onewayofmeasuringa compressor’s effectiveness is tocompare thestaticpressureof thecompressordischargewiththestaticairpressureattheinlet.Ifthedischargeairpressureis30
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timesgreater than theinletair pressure, that compressorhas acompressorpressure ratioof30:1.
The compressor section has also several secondary functions. For example, a compressorsuppliesbleedair tocoolthehotsectionandheatedair foranti-icing.In addition,compressorbleedair isusedforcabinpressurization,airconditioning,fuelsystemdeicing,andpneumaticenginestarting.
Therearetwobasictypesofcompressorsusedtoday:
thecentrifugalflowcompressor,and
theaxialflowcompressor.
Eachisnamedaccordingtothedirectiontheairflowsthroughthecompressor,andoneorbothmaybeusedinthesameengine.
CENTRIFUGAL FLOW COMPRESSORS
Thecentrifugalcompressor,sometimescalledaradialoutflowcompressor,isoneoftheearliestcompressordesignsand isstillused today insomesmallerenginesandauxiliarypowerunits(APU’s).
Centrifugalcompressorsconsistofanimpeller,adiffuser,andamanifold.
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AXIAL FLOW COMPRESSORS
Anaxialflowcompressorhastwomainelements,arotorandastator.Therotorconsistsofrowsof blades fixed on a rotating spindle. The angle and airfoil contour of the blades forces airrearwardinthesamemannerasapropeller.Thestatorvanes,ontheotherhand,arearrangedinfixedrowsbetweentherowsofrotorbladesandactasdiffusersateachstage,decreasingairvelocityandraisingpressure.
Eachconsecutiverowofrotorbladesandstatorvanesconstitutesapressurestage.Thenumberofstagesisdeterminedbytheamountofairandtotalpressureriserequired.
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DIFFUSER
Asairleavesanaxialflowcompressorandmovestowardthecombustionsection, itistravelingatspeedsupto500feetpersecond.Thisisfartoofasttosupportcombustion,thereforetheairvelocitymust be slowed significantly before it enters the combustion section. The divergentshapeofadiffuserslowscompressordischargewhile,atthesametime,increasingairpressuretoitshighestvalueintheengine.Thediffuserisusuallyaseparatesectionboltedtotherearofthecompressorcaseandaheadofthecombustionsection.
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COMBUSTION SECTION
A combustion section is typically located directly between the compressor diffuser and turbinesection.All combustion sections contain the samebasic elements: one or more combustionchambers(combustors),afuelinjectionsystem,anignitionsource,andafueldrainagesystem.
Thecombustionchamberorcombustorina turbineengine iswherethefuelandairaremixedandburned.Atypicalcombustorconsistsofanoutercasingwitha perforatedinnerliner.Theperforationsarevarioussizesandshapes,allhavingaspecificeffectontheflamepropagationwithintheliner.
Thefuelinjectionsystemmeterstheappropriateamountoffuelthroughthefuelnozzlesintothecombustors. Fuel nozzlesare located in the combustionchamber case or in thecompressoroutletelbows.Fuelisdeliveredthroughthenozzlesintothelinersinafinelyatomizedspraytoensurethoroughmixingwiththeincomingair.Thefinerthespray, themorerapidandefficientthecombustionprocessshouldbe.
Atypical ignitionsourceforgasturbineengines isthehigh-energycapacitordischargesystem,consistingofanexciterunit,twohigh-tensioncables,andtwosparkigniters.Thisignitionsystemproduces60to100sparksperminute,resultinginaballoffireattheigniterelectrodes.Someofthesesystemsproduceenoughenergytoshootsparksseveralinches,socaremustbetakentoavoidalethalshockduringmaintenancetests.
A fuel drainage system accomplishes the important task of draining the unburned fuel afterengine shutdown. Draining accumulated fuel reduces the possibility of exceeding tailpipe orturbine inlet temperature limitsdue toanengine fire after shutdown. Inaddition,draining theunburned fuel helps to prevent gum deposits in the fuel manifold, nozzles, and combustionchamberswhicharecausedbyfuelresidue.
Inordertoallowthecombustionsectiontomixtheincomingfuelandair,ignitethemixture,and
coolthecombustiongases,airflowthroughacombustorisdividedintoprimaryandsecondarypaths.Approximately25to35percentoftheincomingairisdesignatedasprimarywhile65to75percentbecomessecondary.Primary,orcombustionair,isdirectedinsidethelinerinthefrontendofacombustor.
Thesecondaryairflowinthecombustionsectionflowsatavelocityofseveralhundredfeetpersecondaroundthecombustor’speriphery.Thisflowofairformsacoolingairblanketonbothsides of the liner and centers the combustion flames so theydo not contact the liner.Somesecondary air is slowedandmetered into the combustor through the perforations in the linerwhereitensurescombustionofanyremainingunburnedfuel.Finally,secondaryairmixeswith
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theburnedgasesandcoolairtoprovideanevendistributionofenergytotheturbinenozzleatatemperaturethattheturbinesectioncanwithstand.
TURBINE SECTION
After the fuel/air mixture is burned in the combustor, its energymust be extracted. A turbinetransformsaportionofthekineticenergyinthehotexhaustgasesintomechanicalenergytodrivethecompressorandaccessories.
ThepictureshowingisaPW400094-InchFanEngine.
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Ina turbojetengine,the turbineabsorbsapproximately60 to80%of the totalpressureenergyfromtheexhaustgases.Theturbinesectionofaturbojetengine islocateddownstreamof thecombustionsectionandconsistsoffourbasicelements;acase,astator,ashroud,andarotor.
EXHAUST SECTION
Thedesignofaturbojetengineexhaustsectionexertstremendousinfluenceontheperformanceofanengine.Forexample,theshapeandsizeofanexhaustsectionanditscomponentsaffectthe temperature of theairentering the turbine, or turbine inlet temperature, themass airflowthrough the engine, and the velocity and pressure of the exhaust jet.Therefore,an exhaustsectiondeterminestosomeextenttheamountofthrustdeveloped.
A typical exhaust section extends from the rear of the turbine section to the point where theexhaust gases leave the engine. An exhaust section is comprised of several componentsincludingtheexhaustcone,exhaustductortailpipe,andexhaustnozzle.
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ACCESSORY SECTION
Theaccessorysection,oraccessorydrive,ofagasturbineengineisusedtopowerbothengineand aircraft accessories such as electric generators, hydraulic pumps, fuel pumps, and oilpumps. Secondary functions include acting as an oil reservoir, or sump, and housing theaccessorydrivegearsandreductiongears.
Theaccessorydrivelocationisselectedtokeeptheengineprofiletoaminimumforstreamlining.
Typicalplaceswhereanaccessorydriveislocatedincludetheengine’smidsection,orthefrontorrearoftheengine.
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B:January2008 2
ENGINE MOUNTS
Enginemount designand construction for gas turbine engines is relativelysimple.Sincegasturbineenginesproducelittletorque,theydonotneedheavilyconstructedmounts.Themountsdo,however,supporttheengineweightandallowfortransferofstressescreatedbytheenginetotheaircraftstructure.
Onatypicalwingmountedturbofanengine,theengineisattachedtotheaircraftbytwotofourmountingbrackets.However,becauseofinducedpropellerloads,aturbopropdevelopshighertorque loads, so engine mounts are proportionally heavier. By the same token, turboshaftenginesusedinhelicoptersareequippedwithstrongerandmorenumerousmountlocations.
-EndofthisTopic-
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TOPIC14.1.2:ENGINEFUELSYSTEMS
Fuel Control and Metering Systems
Gasturbineenginesconvertthelatentenergyof fuel intoheattoprovidetheenergyfortheoperationoftheengineandthrustfortheaircraft.
The function of the fuel system is to provide the engine with fuel, in a form suitable forcombustion and to control its flow to the required rates necessary for easy starting,accelerationandstablerunning,inallengineoperatingconditions.
Fuel System Layout
Foragasturbineenginetodeliverthepowerrequired,itneedsasystemthatsuppliesfuelinsufficientquantitiestoallowforvaryingconditions,altitudesandpowersettings.
Layoutofaircraftandenginefuelsystemsvarywiththetypeandsizeofaircraft,however,mostsystemsincludethefollowingcomponents:
Fueltank.
Boostpump.
Fuelflowtransmitter.
Lowpressureshutoffvalve.
Lowpressuretransmitter.
Fuelheater.
Fuelfilter.
Highpressurefuelpump.
Fuelcontrolunit.
Highpressureshutoffvalve.
Pressurisinganddumpvalve.
Fuelburners.
Fuelpressuredifferentialswitch.
The block diagram in Figure 1.2-1 shows the fuel system layout of a typical gas turbineengine. At the lowest point of the fuel tank (1), an electrically driven boost pump (2)incorporatingameshfilterdeliverslowpressurefuelthroughfuelflowtransmitter(3)tothe
lowpressureshutoffcock(4)locatedontheenginefirewall.Fromtherefuelflowsthroughthelowpressuretransmitter(5)tothefuelheater(6)andontothefilter(7).Fuelisthendeliveredtothehighpressurepump(8)throughtheFCU(9)totheandhighpressureshutoffcock(10). Itthenflowstothepressurisinganddumpvalve(1.2)andontofuelmanifoldsandburners(12).
A fuel pressure differential switch (13) takes a pressure reading from near the fuel flowtransmitter(3) and frombetweenthe fuel filter (7) and highpressurepump(8) togiveanindicationthat the fuel filter isbecomingblockedbyiceorforeignmaterial inthefuel thusenablingthepilottoselectfuelheatingtoremoveicefromthefilter.
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Figure1.2-1
Fuel System Components
Fuel Flow Transmitter
Fuelflowmetersareusedinfuelsystemstoshowtheamountoffuelconsumedperhourbytheengine,thusallowingthepilottoaccuratelycalculatetheavailableflighttimeremaining. Asfuelflows throughthemeter,it spinsasmallturbinewheelandadigitalcircuit readsthenumberofrevolutionsinaspecifiedperiodandconvertsthistoafuelflowrate.
Low Pressure Shut Off Valve
Low pressure shut off valves on modern aircraft, normally mounted behind the enginefirewall,areusedtoisolatetheenginefuelsystemfromtheairframeincaseoffireorsystemmaintenance.Thetwocommontypesofshutoffvalvesare:
Motordrivengatevalve.
Solenoidoperatedvalve.
Motor Driven Gate Valve
Thisvalveshown inFigure1.2-2usesa reversibleelectricmotor linked toaslidingvalveassembly.Themotormovesthevalvegateinandoutofthepassagethroughwhichthefuelflows,thusshuttingofforturningonthefuelflow.
Figure1.2-2
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Solenoid Operated Valve
Asolenoidvalvehasanadvantageoveramotordrivenvalve,beingmuchquickertoopenorclose.ThevalveinFigure1.2-3isasolenoidoperated,poppettypevalve.Whenelectricalcurrentmomentarilyflowsthroughtheopeningsolenoidcoil,amagneticpullisexertedonthe
valvestemthatopensthevalve.Whenthestemriseshighenough,thespringloadedlockingplungerisforcedintothenotchinthevalvestem.Thisholdsthevalveopenuntilcurrentismomentarilydirected to the closing solenoid coil. Themagneticpull of this coil pulls thelockingplungeroutofthenotchinthevalvestem,thespringclosesthevalveandshutsofftheflowoffuel.
Figure1.2-3
High Pressure Shut Off Valve
Thehighpressure(HP)shutoffcockisavalvemountedinthefuelcontrolunit(FCU)andisusedtogiveadefiniteshutoffofthefuellinefromtheFCUtothefuelburnernozzles.
The HP cock may be connected directly to the engine power lever and operates frommaximumthrottle(HPcockopen)toidlethrottle(HPcockopen)thenthroughagatetocutoff(HPcockclosed).
However,onturbopropelleraircraftitisnormallyconnectedinconjunctionwiththepropeller
feather control lever to give amovement through gatesofengine run (HP cock open) toenginestop(HPcockclosed)thenpropellerfeather(HPcockclosed).
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Pressurising and dump valve
A fuelpressurisingand dumpvalve isnormallyrequiredonenginesusingduplextype fuelnozzles,todividethefuelflowintoprimaryandmainmanifolds,andtodrainfuelfromthese
manifoldsonshutdown.Pressurising Valve
Thefuelpressurisingvalvecontrolsthefuelflowsrequiredforstartingandaltitudeidling,allfuelpassesthroughtheprimarymanifold.Asfuelflowincreases,thevalvebeginstoopenthemainmanifolduntilatmaximumflowthemainmanifoldispassingapproximately90%ofthefuel.
Dump Valve
Thedumpvalvegivesthecapabilityto“dump”ordrainfuelfromthefuelmanifoldsaftershutdown.Manifolddumpingisaprocedurewhichsharplycutsoffcombustionandalsopreventsfuelboiling,orafterburning,asaresultofresidualengineheat.Thisboilingtendstoleave
soliddepositswhichcouldclogfinelycalibratedpassageways.Operation
The construction and operation of pressurisation and dump valves varies with differentmanufacturers, however, the following is a description of the operation of a typicalpressurisationanddumpvalve,showninFigure.1.2-4.
Whenthepowerleverisopened,apressuresignalfromthefuelcontrolunitmovesthedumpvalveagainstthespringpressureclosingthedumpportandopeningthepassagewaytothemanifolds.Ataspeedslightlyaboveidle,thefuelpressurewillbesufficienttoovercomethepressurisingvalvespringforce,andfuelwillalsoflowtothemainmanifold.
Onshutdownwhenthefuellever ismovedtoOFF,thepressuresignalholding thedump
portclosedandthefuelpassageopen,islost.Springpressureclosesthefuelpassageandopensthemanifoldstothefueldump,orreturnline.
Figure1.2-4
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Drain Valves
Thedrainvalvesareused for drainingfuel fromvariouscomponentsof the enginewhereaccumulated fuel is most likely to present operating problems. This valve is normallyoperatedbypressuredifferential.
Fuelaccumulatesinthebottomofthelowercombustionchamberfollowingshutdownorafalsestart.Whentheairpressureinthecombustionchamberreducestonearatmospheric,the valveopens and allows the accumulatedfuel todrainaway. It is imperative that thisvalveisingoodworkingorder,otherwiseahotstartduringthenextstartattempt,oranafterfireonshutdownislikelytooccur.
Low Pressure Transmitter
Foraircraftfittedwithmorethanonefueltank,itisdesirabletohaveameansofwarningthepilotthatfuelinthesupplyingtankisexhausted(ortheboostpumpisnotoperating)andthatthefuelselectormustbesettodrawfuelfromanothertank.Thelowfuelpressureswitchisheldopenbynormalfuelpressures,buttheswitchcloseswhenthepressurefalls.Thisturns
onthewarninglightinthecockpit.
Turbinepoweredaircraft thatoperateathigh altitudesandlowtemperatures forextendedperiodsoftimehavetheproblemofwatercondensingoutofthefuelandfreezingonthefuelfilters.Topreventthis,theseaircrafthaveafueltemperaturegaugeandorafilterdifferentialpressurewarninglightthatilluminateswheniceobstructsthefilter.
Thepurposeofthefuelheateristoprotectthefuelsystemfromiceformationandtothawicethatformsonthefuelfilterscreen.Thisisachievedbyusinghotairthathasbeenheatedbythecompressorsectionoftheengine.AfuelheaterisdepictedinFigure1.2-5.
Figure1.2-5
Fuel / Oil Cooler
Thefuel/oilcoolerisdesignedtocoolthehotenginelubricatingoilbyusingthefuelflowingtothe engine passing through a heat exchanger. A thermostatic valve controls the oil flowwhichmaybypasstheheatexchangerifnocoolingisrequired.
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Fuel Filter
Becausethehighpressurefuelpump,fuelcontrolunit,pressurisationvalve,dumpvalveandtheburnersaremanufacturedtoveryfinetolerancesandfittedwithmanysmallorifices,a
filterisinstalledtoprotectthefuelcontrolcomponentsfromcontaminates.Thefiltermustbecapableofremovingparticlesmeasuringassmallas10microns.
High Pressure Fuel Pump
Enginemountedfuelpumpsarerequiredtodeliveracontinuoussupplyoffuelattheproperpressureatalltimesduringoperationoftheaircraftengine.Thefuelpumpsmustbecapableofdeliveringmaximumneededflowathighpressuretoobtainsatisfactorynozzleatomisationandaccuratefuelregulation.Thetwocommontypesofenginedrivenfuelpumpsnormallyusedare:
Spurgear.
Pistontype.
Spur Gear
Gear type pumps have approximately straight line flow characteristics, whereas fuelrequirements fluctuate with flight or ambient air conditions. Hence a pump of adequatecapacityatallengineoperatingconditionswillhaveexcesscapacityovermostoftherangeofoperation. This isacharacteristicwhichrequiresthe use ofapressurerelief valve fordisposingofexcessfuel.AtypicalconstantdisplacementgearpumpisillustratedinFigure1.2-6.Thefuelentersthepumpattheimpellerwhichgivesaninitialpressureincreaseanddischargesfueltothetwohighpressuregearelements.Eachoftheseelementsdischargesfuelthroughacheckvalvetoacommondischargeport.Shearsectionsareincorporatedinthedrivesystemofeachelement.Thus,ifoneelementfails,theothercontinuestooperate.Thecheckvalvespreventcirculationthroughtheinoperativeunit.Oneelementiscapableof
supplyingsufficientfuelformoderateaircraftspeeds. Areliefvalveisincorporatedinthedischargeportofthepumptoallowfuelinexcessofthatrequiredbytheenginetoberecirculatedtoininletsideofthehighpressureelements.
Figure1.2-6
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Piston
The variable displacement pump (Figure 1.2-7) system differs from the constantdisplacementpumpsystem. Pumpdisplacement ischangedtomeet thevaryingfuelflowrequirements,thatis,theamountoffueldischargedfromthepumpcanbemadetovaryat
anyonespeed.Thisisduetotheinclinationofthecamplate,movementoftherotorimpartsareciprocatingmotiontotheplungers,thusproducingapumpingaction.Thestrokeoftheplungersisdeterminedbytheangleofinclinationonthecamplate.Thedegreeofinclinationisvariedbythemovementofaservopistonthatismechanicallylinkedtothecamplateandisbiasedbyspringstogivethefullstrokepositionoftheplungers.Thepistonissubjecttoservopressureon the spring sideand onthe otherside topumpdeliverypressure, thus,variations in the pressure difference across the servo piston cause it to move withcorrespondingvariationsofthecamplateangleandthereforepumpstroke.
Withavariableflowpump,thefuelcontrolunitcanautomaticallyandaccuratelyregulatethepumppressureanddeliverytotheengine.
Figure1.2-7
Fuel pressure differential switch
Thedifferentialpressureswitchisusedinthefuelsystemtodetectthepresenceoficingonthefuelfilterandilluminatesacockpitwarninglightwhenthepressuredifferentialreachesasetamount.
Afuelpressuredifferentialswitchtakesapressurereadingfromnearthefuelflowtransmitterandfrombetweenthefuelfilterandhighpressurepumptogiveanindicationthatthefuelfilterisbecomingblockedbyiceorforeignmaterialinthefuelthusenablingthepilottoselectfuelheatingtoremoveicefromthefilter.
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Fuel Control Units
Thecontrolofpower(orthrust)inagasturbineengineisaffectedbyregulatingthequantityoffuelinjectedintothecombustionchamber.If toomuchfuelissuppliedtothecombustion
chamber,theturbinesectionmaybedamagedbyexcessheat,thecompressormaystallorsurgebecauseofbackpressurefromthecombustionchambersorarichblowoutmayoccur. A rich blowout occurs when the mixture is to rich too burn. If too little fuel enters thecombustionchambersaleandieoutoccurs. Aleandieoutoccurswhen themixtureis toleantoburn.
Theusualmethodofvaryingthefuelflowtothecombustionchamberisviaafuelcontrolunit.Fuel control units operate using either, hydropneumatic, hydromechanical,electro-hydromechanicalorelectroniccontrolprinciples.
Hydromechanical.
Formanyyears themajorityof fuel controlunits havebeenhydromechanicalinoperation.Thismeans theiroperation iscontrolledboth byhydraulic (fuel)andmechanicalmeans to
controlthefuelflowtotheengine.
Hydropneumatic.
Thesefuelcontrolunitsuseengineairpressuresandmechanicalforcestooperateitsfuelschedulingmechanisms.
Electro-hydromechanical.
Latermodelgasturbineenginesarecontrolledbyelectronicfuelcontrolsystems.Theseareknown as electro-hydromechanical fuel control units. These systems use computers thatsenseinputstosetthehydromechanicalsectionofthefuelcontrolunitthatlimitsthefuelflowtotheengine.
Electronic.
Many modern engines, now use a computer or electronic device that controls the fuelmanagementsystem.Withthesecontrolsit ispossibletopressthestartbutton,thenmovethe throttle to maximum power, the engine control then regulates the engine to achievemaximumpowerwithoutexceedingRPM,acceleration,temperatureandpressure limitsofthatengine.
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Hydropneumatic Fuel Control
AsimpleRPMcontrolsystem,showninFigure1.2-8,provides:
RPMcontrol.
Accelerationanddecelerationcontrol.
Minimumandmaximumflowcontrol.
Ithasinputsof:
RPMcommand.
ActualRPM.
Inlettemperature.
Compressoroutletpressure.
InregardtoFigure1.2-8,thefuelpumpsuppliesmorefuelthanisrequiredandthebypassvalve returns excess back to the pump inlet. The bypass valve incorporates a pressureregulator to ensure the pressure differential across the metering valve is unaffected bymovementof themeteringvalve. Therefore, fuel flowiscontrolledonlybymetering valveposition.
RPM is the primary control parameter, and compressor discharge pressure and inlet airtemperaturearesecondaryparameters.Togethertheycontrolthemeteringvalveviaaservobellowsassembly.
“On speed” RPM ismaintained by the governor in conjunction with the governor bellowspressurePy. The flyweightsof thegovernorrespond toanRPMchangebyincreasingordecreasingtheopeningofthegovernorvalve,whichinturnaltersPyandthustheextensionofthegovernorbellows.ThebellowsassemblyopensthemeteringvalveslightlywhenthereisafallinRPMandclosesitslightlywhenthereisariseinRPM.
POWERLEVER
IDLE MAX
INLET AIR TEMPERATURESENSOR
DECELERATION BELLOWS GOVERNOR BELLOWS
MAXIMUM FLOW STOP
METERING VALVE
OP E N
C L O S E D
FUEL TO ATOMISERS
BYPASS ANDPRESSUREREGULATINGVALVE
MINIMUM FLOWSTOP ACCELERATION BELLOWS
PUMP
FUEL IN
COMPRESSOR OUTLET PRESSURE Pc
AIRFLOW
BI METALLIC
DISCS
RPM GOVERNOR
FLYWEIGHTS
SPEEDER SPRING
GOVERNOR VALVE
Py
Px
Figure1.2-8
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Figure1.2-9
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Foracceleration,aninputforcefromthepowerleverincreasescompressiononthespeederspring.Thismovestheflyweightsofthegovernorinwardsandclosesthegovernorvalve.Withthegovernorvalveclosed,accelerationcontrolpressure(Px)andgovernorpressure(Py)bothincreasewithcompressorpressure(Pc),causingthebellowsassemblytogradually
open the metering valve. System design ensures that increasing fuel flow matchesincreasingairflowthroughtheengineandthataccelerationtakesplacewithoutriskofstallorsurge.WhenthedesiredRPMisreached,thegovernoragainmaintains“onspeed”RPM.
Duringdeceleration,thereversesequenceoccurs.Therateofdecelerationiscontrolledbythedecelerationbellows,whichensuressmoothdecelerationwithouttheriskofflameout.
The bi-metaldiscsare a typicalmeans ofsensing inletduct temperature. TheycontrolameteringdevicewhichaffectspressuresPxandPy.Thisreducestheaccelerationrateunderhotconditions,preventingexcessiveturbinetemperatureandtheriskofcompressorstallorsurge.
Hydro-mechanical Fuel Control System
Hydro-mechanicalFCU’sunitusefloworpressurecontroltoregulatetheflowoffuel.Flow Control
Flowcontrolunitsregulatethefuelsystembybypassingexcessunwantedfuelbacktotheinletsideofthefuelpump.
PriortothestartbeingactivatedtheFCUisinthefollowingconditions:
Fuelshutoffvalveclosed.
Powerleveratidle.
Governorspeederspringisinanexpandedcondition.
Governorflyweightsinanunderspeedcondition.
Burnerandinletpressurebellowsaresensingbarometricpressureandthemultiplyinglinkageisinthedecreaseposition.
Differentialpressureregulatingvalvewillbeclosed.
Meteringvalveisheldofftheminimumflowstopbythebalancedspringpressuresofthegovernorandmainmeteringvalve.
ReferFigure1.2-9:
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Figure1.2-10
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Starting
After thestartbutton ispressedtheenginebeginsto rotate, the flyweightsin thegovernorbegintoopen,overcominginitialspeederspringtensionmovingtherollercageupwardsthus
reducingthemeteringvalveopening.The fuel pump pressurises the fuel system until the relief valve pressure in the pump isreached.
WhentheenginehasacceleratedbythestartertoasetRPM,orafteracertainperiod,thefuelshutoffvalveisopenedcausing:
Fueltoflowtotheburnerscausingadifferentialpressureacrossthemeteringvalve,thereforethedifferentialpressureregulatorsensesthedifferenceandbeginstoregulatethefuelpressure.
Oncecombustioncommences,theenginebeginstoaccelerate,theburnerpressureincreasescausingtheburnerpressurebellowstomovethemultiplyinglinkageto
beginopeningthemainmeteringvalvethroughtherollercage. AstheengineacceleratestowardsidleRPM,thespeedgovernorandpressurebellowsbegintoregulatemeteringvalveopeningcommencinggovernedoperationatidlespeed.
ReferFigure1.2-10:
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Figure1.2-1.2
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Governed or Steady Operation
Duringgovernedorsteadyoperation,considerthatthepowerleverissetatacertainpositionandnotchanged.Aftertheenginespeedisset,theengineissubjecttocertainoperatingvariablessuchasaircraftspeedandaltitudetowhichitmustreact.
Ifanaircraft isincreasingspeedordescending itwill increases inletramair pressureandmassairflow.Alternativelyanaircraftthatisinaclimbandorslowingwilldecreaseinletairpressureandmassairflow.
Theinletandburnerpressurebellowssensethesechangesandmovesthemultiplyingleverinanappropriatedirectiontomaintainthefuelmixtureratio.Atthesametime,theenginespeedgovernorreactstoanyspeedvariations,movingthepilotservorodvalvetoreturntheenginetoasteadygovernedstate.
ReferFigure1.2-1.2:
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Figure1.2-12
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Acceleration
Movementofthepowerleverinanincreasedirection,causesthespringcaptoslidedownthepilotservovalverodandcompresstheflyweightspeederspring.
In doing so, the spring base pushes down and forces the flyweights in at the top to anunderspeedcondition,movingthepilotvalverodinadownwardsdirection.
Thepilotservovalvefunctionstoslowthemovementofthepilotservocontrolrodpreventingsuddenfuelratiochangesbyusingitsfluiddisplacedtoptobottomasarestrictor.
Whenthepilotvalverodmovesdown,therollerwillmovedowntheinclineplaneandtotheleft. As itmoves left, the roller will force themeteringvalve to the left against its spring,allowingincreasedfuelflowtotheengine.
Asfuelflowincreasesthedifferentialpressurevalvewill senseadecreaseddifferentialandclosetomaintainthedifferential.Withincreasedfuelflow,theenginewillspeedupanddrivethefuelcontrolshaft faster,as theenginespeed increases theburnerpressure increaseswhich expands the burner pressurebellows thatmoves the multiplying linkage to the left
furtherincreasingthefuelflow.
The new flyweight force will come to equilibrium with the speeder spring force as theflyweightsreturntowardanuprightposition.
Theyarenowinpositiontoactatthenextspeedchange.
ReferFigure1.2-12:
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Figure1.2-13
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Deceleration
Movementofthepowerleverinadecreasedirection,causesthespringcaptoslideupthepilotservovalverodandreleasepressureontheflyweightspeederspring. Indoingso,thespringbasemovesupandtheflyweightsmovetoanoverspeedcondition,movingthepilot
valverodinaupwardsdirection.
Thepilotservovalvefunctionstoslowthemovementofthepilotservocontrolrodpreventingsuddenfuelratiochangesbyusingitsfluiddisplacedtoptobottomasarestrictor.
Whenthepilotvalverodmovesup,themeteringvalvespringwillforcethemeteringvalveand the roller to the right as it moves up the inclineplane, allowing less fuel flow to theengine.
With decreased fuel flow, the differential pressure valve senses the increased differentialacross themeteringvalveandopens tomaintain the differential and the enginewill slowdownanddrivethefuelcontrolshaftslower,thisslowingoftheenginedecreasestheburnerpressure which through the bellows moves the multiplying linkage to the right further
decreasingthefuelflow. As the new flyweight force comes into equilibrium with the speeder spring force, theflyweightsreturntowardanuprightposition.
Theyarenowinpositiontoactatthenextspeedchange.
ReferFigure1.2-13:
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Figure1.2-14
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Shut Down
Priorto shutdowntheenginemustbeallowedtostabiliseatidleforaperiodtoensureagradual cooling of the turbine and scavenging of propeller control oil in turbo propellerengines.Onasimplifiedfuelcontrolunit,shutdowntakesthefollowingprocedure(Figure
1.2-14):
Withtheengineatidlegovernedspeed,thefuelshutoffvalveisclosed.
Whentheshutoffvalveisclosed,therewillbenofuelflowtogiveadifferentialfuelpressure,thusclosingthedifferentialpressureregulatingvalvecausingthefuelpumppressurereliefvalvetocontrolmaximumfuelpressure.
Oncecombustionceases,theenginespeedwillbegintodecreasesendingthegovernorintoanunderspeedcondition,atthesametimetheburnerpressurewilldecreasemovingthemultiplyinglinkagetoclosethemeteringvalve.
ReferFigure1.2-14:
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Figure1.2-15
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Hydropneumatic Fuel Control
Hydro-pneumatic fuel control units rely heavily on compressor discharge pressures tomaintainthecorrectairfuelratio.AcommonsystemisshownatFigure1.2-15.
Fuelissuppliedtothefuelcontrolunitatpumppressure(P1)whichisappliedtotheentrancetothemeteringvalve.Themeteringvalve,inconjunctionwiththemeteringheadregulatorvalvesystem,servestoestablishfuelflow.
Thefuelpressureimmediatelydownstreamofthemeteringheadbecomes(P2).Thebypassvalve maintains a constant fuel pressure differential (P1-P2) across the metering valveassuringthatfuelflowisafunctionofthemeteringheadorificeonly.
Operation of Control
Unmeteredfuelpressure(P1)issuppliedtotheFCUbythefuelpump
Thedifferentialmeteringheadregulatormaintainsaconstantpressuredropacrossthemeteringhead(P2).Ensuringconstantflow.
Fuelbypassedbacktopumpinletbecomes(Po)
Theairsectionisoperatedbycompressordischargeair(Pc).
Whenmodifiedthisairbecomes(Px&Py)whichacttopositionthemeteringvalve.
Tt2 Sensor
TheTt2 sensor acts tovaryPxbleed inlinewithvaryingair densityatidlepositionsthuspreventingidlestallproblemsthroughoverorunderfuelling. Thiscircuitlosesit’sauthorityabovetheidleposition.
When the Power Lever is Advanced
Theflyweightsdroopin,thespeederspringforcebeinggreaterthantheflyweightforce.
ThegovernorvalveclosesoffthePybleed.
Theenrichmentvalvemovestowardsclosed,reducingPcairflow(notasmuchairpressureisrequiredwhenPybleedsareclosed).
Px&Pypressuresequaliseonthesurfaceofthegovernor.
Pxaircontractstheaccelerationbellowsandthegovernorbellowsrodisforceddownward.Thediaphragmallowsthismovement.
Thetorquetuberotatescounterclockwiseandthemainmeteringvalvemovesto
open. TheflyweightsmoveoutwardsasenginespeedincreasesandthegovernorvalveopenstobleedPyair.
The enrichment valve re-opens and Px air increases over the Py value
ReducedPyvalueallowsthegovernorbellowsandrodtomoveuptoanewstabilisedposition.
Themeteringvalveresumesanewpositionthroughtheactionofthetorquerodassembly.
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When the Power Lever is Retarded
Theflyweightsmoveoutwards-speederspringforcebeinglessthanflyweightforceduetohighengineRPM
ThegovernorvalveopensdumpingPyair.Thebackupvalveisalsodepressed,dumpingadditionalPyair
Theenrichmentvalveopens,allowingincreasedPxairflow
Pxairexpandsthegovernoranddecelerationbellowstoit’sstop
Thegovernorrodalsomovesupandthemainmeteringvalvemovestowardsclose.
Pxairdecreaseswithenginespeeddecreasebuttheaccelerationbellowsholdsthegovernorrodup.
Asenginespeedslows,theflyweightsmovebackin,closingthePybleedatthegovernorvalveandthebackupvalve
TheenrichmentvalvemovestowardsclosedandPyairincreasesinrelationtothePxvalue
Thedecelerationbellowsmovesdownward.Themeteringvalvemovesslightlyopentoproduceastabilisedfuelflow
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Electro-hydromechanical Control System Operation
Electro-hydromechanical fuelcontrol systemsaresometimes referred to aselectronic fuelcontrolsbecausethemajorityofthesystemismadeupofelectroniccircuits.Becauseofthe
needtopreciselycontrolmanyfunctionsintheoperationofmodernhighbypassturbofanengines, electronicenginecontrolsystemshavebeendeveloped. Thesesystemsprolongengine life, save fuel, improve reliability, reduce crew workload and reduce maintenancecosts.Twotypesofelectronicenginecontrol(EEC)systemsinuseare:
SupervisoryElectronicEngineControl.
FullAuthorityElectronicEngineControl.
Supervisory Electronic Engine Control System
Essentially the supervisory electronic engine control system is a electronic device whichreceives information from various engine parameters and then limits the fuel flow to thehydromechanicalfuelcontrolandengine.
As can be seen in Figure 1.2-16 the control amplifier receives a signal from turbine gastemperature(TGT)andtwocompressorspeedsignals(N1andN2).
Thiscontrol,worksasahydromechanicalunituntilnearfullpower,whentheelectroniccircuitstarts to function as a fuel limiting device to control maximum TGT and, N1 and N2compressorspeeds.
Thepressureregulatorinthisinstallation,regulatesthefuelpressureatthefuelpumpratherthanthefuelcontrolunit.Nearfullpower,whenpredeterminedTGTandcompressorspeedvaluesarereached,thepressureregulatorreducesfuelflowtothespraynozzlesbyreturningincreasingamountsoffueltothefuelpumpinlet.
Thefuelflowregulator in thiscontrolactsasahydromechanical control, receiving signals
from high speed compressor (N3), gas path pressure (P1, P2 and P4) and power leverpositiontoregulatefuelflowtotheengine.
Figure1.2-16
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Full Authority Electronic Engine Control System
Fullauthorityelectronicfuelcontrolunitsuseanelectronicdevicethatsensesvariousinputsfromtheengineandpilottodeterminehowmuchfuelshouldbedeliveredtothefuelnozzles.
The full authority electronic engine control system performs all functions necessary tooperateaturbofanengineefficientlyandsafelyduringalloperatingconditionsfromstartuptoshutdown.
Benefitsofusingelectronicenginecontrolarereducedcrewworkload, increasedreliability,improvedreliability,andreducedfuelconsumption.
Flight crewworkloadis decreasedbecause thepilotutilisestheEPRgaugeto set enginethrustcorrectly.TheEECwillautomaticallyaccelerateordeceleratetheengineto theEPRlevelwithout thepilothaving tomonitor theenginegauges. Reducedfuel consumption isattainedbecausetheEECcontrolstheengineoperatingparameterssothatmaximumthrustisobtainedfortheamountoffuelconsumed.
Engine trimming iseliminatedby the use of full authorityEEC, as the engine fuel control
systemhasfaultsensing,selftestingandcorrectingfeaturesdesignedintotheEECgreatlyincrease the reliability and maintainability of the system. The only adjustments that arecarriedoutbythemaintainerisspecificgravityandidleRPM.
TheEECisprovidedwithfeedbackviavalvesandactuatorsfittedwithdualsensors.
Theelectroniccomputermayhavemanyinputsandoutputsincluding:
N1 Fanspeed.
N2 Intermediatepressurecompressorspeed.
N3 Highpressurecompressorspeed.
Tt2 Inlettotaltemperature.
Tt8 Highpressureturbineinlettemperature.
Pt2 Inlettotalpressure.
28V DC Inletpower.
PMG PermanentmagnetACpower.
PLA Powerleverangle.
IGV AInletguidevaneangle.
Ps6 Highpressurecompressordischargestaticpressure.
Wf Fuelflow. ACC Activeclearancecontrol(compressorandturbineblade.Coolingair suppliedbyfanair).
EPR Enginepressureratio.
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Toprovideahighdegreeofreliability,FADECsystemsaredesignedwithseveralredundantanddedicatedsubsystems.AnEECconsistsoftworedundantchannels(AandBchannels)that send and receive data. Each channel consists of its own processor, power supply,memory,sensors,andactuators.Inaddition,anyonechannelcantakeinformationfromthe
otherchannel.Thisway,theEECcanstilloperateevenifseveralfaultsexist.Asasecondbackupshouldbothchannelsfail,theactuatorsarespringloadedtoafailsafepositionsothefuel flow will go to minimum. If both channels are serviceable, the Active channel willalternatewitheachenginestart.TheotherchannelisinStandbymode.Powermanagementcontrolstheenginethrustlevelsbymeansof throttleleverinputs.Itusesfanspeed(N1)asthethrustsettingparameter.
As shown in Figure 1.2-17, the full authority electronic engine control receives data fromvarious areas, then analyses the data and sends commands to position the Inlet GuideVanesandschedulefuelflowthroughthehydro-mechanicalsectionofthefuelcontrolunit.
Figure1.2-17
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Fuel System Maintenance
Properandregularinspectionofaircraftfuelsystemsiscriticaltothesafeoperationoftheaircraft. The failure ofa componentmay result inanengine failure due to insufficient orexcessfuelbeingthedeliveredtotheengineorcouldresultinafireinoraroundtheengine.
While the following text providesgeneral informationconcerning engine fuel systems, themanufacturers specific guidelinesmust be followed when performingmaintenanceof gasturbineenginefuelsystems.
Routine maintenance of gas turbine engine fuel systems include the following genericinspectionsandoperations:
Allfuellinesforleaks,chafedorfrayedwallsandcontactwithothercomponents.
Cleaningandinspectingfuelfilters.
FCUcontrolsforserviceabilityandrigging.
Lowandhighpressureshutoffvalvesforoperationandsealing.
Drainvalvesforoperationandsealing.
Pressurisinganddumpvalvesforoperationandsealing.
Fuelheatersforleaksandoperation.
Oilcoolersforleaksandoperation.
Pressuresensinglinesforrestrictions.
OnenginesfittedwithEEC,performselftestandanalysisofthecomputersystem
Bleedingofanytrappedairinsystemsthatarenotselfbleedingafterdisturbinganyfuelsystemcomponent.
Regularengineperformancechecks.
Anymaintenancethatmaybedeemednecessaryfollowinginspectionoftheenginefuelorrelatedsystems.
Fuel System Faults Table 1)
Fuel systems, Fuel Pumps and Fuel Control Units can cause a wide variety of enginemalfunctions someofwhichmay be difficult toanalyse. A thorough understandingof thesystemanditscomponentsisessentialif thetechnicianhopestoresolvetheproblemsofaparticularsystemeffectively.
The following chart lists some common problems encountered with fuel systems andsuggestsgenericremedies.
Technicians should analyse the type of system on which they are working and becomefamiliarwiththeoperationofthefuelcontrolandothercomponentsusedinthatsystem.
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INDICATION POSSIBLE CAUSE REMEDY
Enginemotorsoverbut
doesnotstart.
Improper rigging of shut off
valve.
Checkandre-rigairframeand
enginelinkages. Cloggedoricedfuelfilters. Clean.
Malfunctioningfuelpump. Checkandorreplace.
Malfunctioningfuelcontrol. Replacecontrol.
Pressurising and dump valvestuckopen.
Replacevalve.
Enginestarts,butwillnotacceleratetocorrect
speed.
Insufficientfuelsupplytocontrolunit.
Check fuel system to ensureall valves are open andpumpsareoperative.
Fuel control main meteringvalvesticking.
Flush system. Replacecontrol.
Fuel control bypass valvestickingopen.
Flush system. Replacecontrol.
Drainvalvestuckopen. Replacedrainvalve.
Starting fuel enrichmentpressure switch setting toohigh.
Replacepressureswitch.
Control has entrapped airpreventingproperoperation.
Bleed control as permaintenancemanual.
EGTtoolowduringstart.
Accelerationcam in fuelcontrolincorrectlyadjusted.
Re-trimasrequired.
EGTtoohighduringstart.
Fuel control bypass valvestickingclosed.
Flush system. Replacecontrol.
Fuel control acceleration camincorrectlyadjusted.
Replacecontrol.
Defectivefuelnozzle. Replacenozzlewith a knownserviceableitem.
Fuelcontrolthermostatfailure. Replacecontrol.
Pressurisation and dump valvewitheithervalvepartiallyopen.
Replace pressurisation anddumpvalve.
EnginehashighEGTattargetenginepressureratiofortakeoff.
Engineoutoftrim. Re-trimasrequired.
Enginerumblesduringstartandatlowpowercruiseconditions.
Pressurising and drain valvemalfunction.
Replace pressurising anddrainvalve.
Fuelcontrolmalfunction. Replacefuelcontrol.
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INDICATION POSSIBLE CAUSE REMEDY
EngineRPM“hangs-up”
duringstart.
Lowambienttemperatures. If hang-up is due to low
ambient temperature, engineusually can be started byturning on fuel boosterpumporbypositioningstartlevertorun earlier in the startingcycle.
Engineunabletoobtaintakeoffpower.
Incorrectcontrolrigging. Check or re-rig engine andairframe.
Partiallycloggedfuelfilters. Cleanfilters.
Incorrectfuelpumppressure. adjust pressure or replacepump.
Incorrect control outputpressure.
Re-rigorreplacecontrol.
Highfueltemperature. Valvestuckopeninfuelheater. Replacefuelheater.
Highfuelconsumption. Fuelsystemleak. Repairasrequired.
Dump valve stuck partiallyopen.
Replace pressurisation anddumpvalve.
Lackofthrottleresponsefrommaximumcontinuous.
Fuelcontrolunitinternalfailure. Replacecontrolunit.
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Factors Controlling FCU Performance
TheFCUmust sense the variousoperating and environmental parameters toenable it tosupplytheenginefuelinthecorrectquantities.ParametersthatdirectlyeffecttheFCUare:
Powerleverangle.
RPM.
Airtemperature.
Airpressure.
Burnerpressure.
Fueldensity.
Power Lever Angle
The power leverangle is the pilotsmain controlover the engine. The power leverangle
schedules the fuel required to the enginewithout taking into account the otheroperatingparameters.
RPM
Tobeabletoproducethevaryingpowersrequired,theenginemustbeabletooperateatdifferentspeeds.RPMissensedsothattheFCUcanprovidetheappropriatefuelflowfortheRPMatwhichtheengineisoperating.
Air Pressure
Asexplainedin Fundamentals,air pressurehasadirectrelationshipwith theairdensityormassie.ifweweretotakeasealedballoonofairfromsealevel,toabout16500,feettheballoonwouldhaveexpandedtotwiceitssize,wewouldthenhavehalveditspressure.
For a turbineengine, an increase inaltitude / a decrease inair pressure, will reduce theweightofthetotalairmassthatwillflowthroughtheengineatagivenRPM.
Air Temperature
As explained in Trade Fundamentals, air temperature has a direct relationship with airdensity, ie. an increase in temperaturewill give an increase in volume. Therefore for aturbineengine,anincreaseinairtemperaturewillreducetheweightofthetotalairmassthatwill flow throughthe engineatagivenRPM, requiring the FCU toreduce the fuel flow tomaintainthecombustionprocess.AscanbeseeninFigure1.2-18,temperaturehasaneffectontheengineperformance.
Figure1.2-18
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Burner Pressure
Staticpressureinthecombustionchamberisausefulmeasureofmassairflow.Ifthemassairflowisknown,theair/fuelratiocanbemorecarefullycontrolled.Asaircraftusebleedairfrom the engine compressor to provide various services, it is imperative that the burner
pressureisknowntoprovideanaccuratefuelregulationwhentheseservicesarebeingused.
Combustion chamber pressure and inlet pressure acting through bellows and leverassembliescangiveaccuratecontrolofthefuelbeingintroducedintotheenginetocontroltheair/fuelmixture.
Fuel Density
As the different types of fuels that may be used in gas turbine engines have differentdensitiesorspecificgravities,thefuelcontrolunitneedsamethodofbeingabletoadjustforthevariousflowsthatoccurifdifferentfuelsareused.
Variationofthefueldifferentialpressurevalvespringtensioncanbeusedtochangethefuelflowtoaccommodatefordifferentfuel’sspecificgravity.
Specificgravityadjustment,shownatFigure1.2-19,isameansofresettingthetensiononthedifferentialpressureregulatorvalvespringwithinthefuelcontrolwhenanalternatefuelisused.
Figure1.2-19
Fuel control unit components
Toadjusttovaryingconditionsandthrustrequirements,anFCUhasdifferentcomponents
fittedthatreacttoensurecombustioniskeptwithinallowablelimits.Thesecomponentsare:
Speedgovernors.
Differentialpressureregulator.
Accelerationgovernors.
Pressuresensors.
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Speed governors
The FCU’sspeedgovernor(s) senseengine speedand act tomaintain the desiredRPM.Whendifferentloadsareapplied,flyweightsandaspeederspringoperate leversorbleed
controlstoadjustthemeteringvalveopeningallowingtheenginetomaintainthesetRPM.Differential Pressure Regulator
Thepressure regulatingvalvediaphragm isexposedononeside topumpoutlet pressureandontheothersidetothecombinedeffectofthrottlevalvedischargepressureandaspringforcepresettomaintainthedesiredpressuredropacrossthethrottlevalve.Withaconstantpressuredropacrossthethrottlevalve,flowthroughthethrottlevalvewillbeproportionaltoitsorificearea.Onflowcontrolsystems,anyexcessfuelabovethatrequiredtomaintaintheset pressuredifferential isbypassedback tothe inletsideofthe fuelpump. Onpressurecontrol systems, the pressure differential regulator controls the piston pump swash plateangle,thereforecontrollingpressureandflow.
Acceleration limiters
Precisecontroloffuelflowisnecessaryforgoodaccelerationresponsewithoutriskofturbineovertemperatureandcompressorstallorsurge.
Thefuelcontrolunitmustalsopreventoverrichmixturesduringacceleration,andoverleanmixturesduringdeceleration,asbothcancauseflameout.Theformerisrarelyaproblembecausethemaximumturbinetemperatureoccursbeforetherichlimitisreached.
Largeengineswiththeirhighinertiarotatingpartsaremoredifficulttoaccelerateandcontrolthansmallerengines.Theyusuallyhavecomplexaccelerationcontrolsystemswhichreactto RPM, inlet temperature, inlet pressure and compressor discharge pressure. Theseparameterscontrolthepositionoftheaccelerationcam,whichinturncontrolsthefuelflowtoallow the maximum acceleration rate (the rate varies with temperature, RPM, and
compressorpressureratio).Simpler acceleration control systems can be used on smaller engines, because theseengines have low inertia rotating parts, which naturally gives them a good accelerationresponse.
Pressure Sensors
ThepressuresensorsofanFCUaresubjecttoinletandcompressoroutletairpressuresandacttoeffectthemeteringvalveopeningthereforecontrollingfuelflow.
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Part 66 Subject B2-14 Propulsion
B:January2008 Revisi 1
AA Form TO-19
B2-14.1.2EngineFuelSystemsIssue on Page34of34
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TOPIC14.2:ENGINEINDICATIONSYSTEMS
Althoughengine installationsmaydiffer,dependingonthetypeofbothaircraftandengine,gasturbineengineoperationisusuallycontrolledbyobservingsomeoralloftheinstruments.
Engineindicationsaredividedintothreegroups.Theseare:
Performanceinstruments.
Conditioninstruments.
Warningsystems.
Performance Instruments
Performance instruments allow the operator, at a glance, to monitor the output orperformanceoftheengine.Thisisdonebycheckingthethrustonturbojetenginesorthe
horsepowerforturboprops.Thetwomainperformanceinstrumentsare:
EnginePressureRatio(EPR).
Torque.
Engine Pressure Ratio EPR)
Enginepressureratioisameasureofthethrustbeingdevelopedbytheengine.WhenEPRis measured the ratio is usually that of turbine discharge pressure to compressor inletpressure,however,onafanenginetheratiomaybethatofturbinedischargepressureandfanoutletpressuretocompressorinletpressure.
Suitablypositionedpitottubessensethepressuresappropriatetothetypeofindicationbeingtakenfromtheengine.ThesepitottubesareeitherdirectlyconnectedtotheindicatorortoapressuretransmitterwhichsendsanelectricalsignaltotheindicatorasshowninFigure2.1.
Figure2-1.
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EPR reflects the difference between the compressor inletpressureand turbine dischargepressure ie. the amount of work the engine is doing on the air. For example, an EPRindication of 2.4 means that the turbine discharge pressure is 2.4 times greater thancompressorinletpressure.
An example of how, when planning a flight, pilots use ambient temperature and apredetermined “Takeoff Thrust SettingCurve” tocalculate the EPR required for takeoff isillustratedinFigure2.2.
Figure2-2.
Withtheambientairtemperatureoftheairfield20°CandusingthegraphabovethepilotwillcalculatetheE.P.R.requiredfortakeofftobe2.6.
Thisfigureiswhattheengineshoulddevelopwhenoperatingat,ornear,fullthrottle.
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Torque
Enginetorqueisusedto indicatethepowerdevelopedbyaturbo-propellerengine,andthe
indicatorisknownasatorquemeterasdepictedinFigure2.3.
Figure2-3.
Theenginetorque,orturningmoment,istransmittedthroughthereductiongearboxtothepropeller.Thetorquemetersystemistheprimaryperformanceinstrumentforturbo-propellerengines.
An explanation of two different types of torquemeter systems is covered in the followingparagraphs.Theseare:
Hydraulictorqueindicatorsystem.
Torqueshaftindicationsystem.
Hydraulic Torque Indicator System
TheHydraulic Torque Indication system indicates torquebymeasuring hydraulic pressurecreatedbyatorquemetersystem.Thetorquemetersystemformspartofareductiongearassembly between theenginedriveshaftand thepropellershaft. The construction ofthesystemdependsonthetypeofengine,butallarebasedonthesameprincipleofoperation.
Thedriveshaftfromtheenginesuppliesatorquetothereductiongearassembly.Thisdrivestheplanetgearsaroundinthesamedirectionbutatafractionoftheenginespeed.Astheplanetgearsrotate,thepropellerrotatesaswell.
Thepropellerconvertsthisrotationforceintothrust.Todothistherotationofthepropellerisresistedduetoaerodynamicforces.Thisresistancecausestheplanetgearstotransferaportionofthetorquetothestationaryringgear.Figure2.4showshowthisoccurs.
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Figure2-4.
AsshowninFigure2.5theringgearmovementis resistedbypistonsworkinginhydrauliccylinderssecuredtothegearboxcasing.Oilissuppliedtothecylindersfromaspecialpumpandisallowedtodrainviaacalibratedbleedline.
Theoilissubjectedtoapressurewhichisproportionaltothetorqueorloadwhichisappliedtothepropellershaft.Thisoilpressureissensedbyabourdontubewhichiscoupledtoasynchrotransmitter.
Asimplesynchroindicatorinthecockpitdisplaysthetorqueinformation.
Figure2-5.
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Torque Shaft Indication System
Anothermethodofobtaininganindicationof torqueisbymeasuringtheamountoftwistinashaftcalledatorqueshaft.
The torque shaftconnects the engine to the propeller reduction gearbox. Ahollow shaft,called a reference shaft, ismounted so that it formsa sleeve around the torque shaftasshowninthecutawaydiagraminFigure2.6.
Figure2-6.
Figure6.6showsthatthetorqueshaftisconnectedtoboththeengineandthegearbox.Theenginerotatesandthepropellerisdraggedthroughtheair.Thepropellerwilllagslightly,thiscausesthetorqueshafttotwistslightly.
Thereferenceshaftisnotsubjectedtoanytorqueasitisonlyconnectedtotheengine.
Ontheendofbothshaftsisagear,calledanexciterwheel.Amagneticpick-upassemblyismounteddirectlyaboveeachexciterwheelasshowninFigure2.7.
Figure2-7.
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Astheshaftsrotate,theteethoftheexciterwheelspassbythemagneticpick-upassemblies.Eachtoothcausesapulsetobegeneratedbyitspick-upassembly.
When the engine isnot delivering anypowerto the gearbox, the teethonthe torque andreferenceshaftswillbealignedasshowninFigure2.8.
Figure2-8.
Whentheengineisdeliveringpowertothegearbox,thetorqueshaftwillbesubjectedtoatorquethatwillcauseittotwistslightly.Thisresultsintheteethonthetorqueshaftbecomingmisaligned with the teeth on the reference shaft (remember the reference shaft is notconnectedtoanythingandthereforewillnottwist)asshowninFigure2.9.
Figure2-9.
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Thissystemhas two othercomponents,aphase detector, anda torque indicator. Figure2.10showsacompletesystem.
Figure2-10.
Thesignalsfrombothpick-upassembliesarefedtothephasedetector.
The phase detector calculates the difference between the two signals and generates anoutputthatrepresentsthetorquethatisbeingmeasured.
Theoutputfromthephasedetectorisusedtodriveapointerinthetorqueindicator.
Condition Instruments
Condition instruments show the operator how hard the engine isworking to produce the
powerseenontheperformanceindicators.
Engineconditioninstrumentsinclude:
Gastemperature.
Fuelflow.
Compressorspeed.
Oiltemperature.
Oilpressure.
Inletairtemperature.
Enginevibration.
For free turbine engines, engine rpm is broken down into free turbine rpm (Nf) and gasgeneratorrpm(Ng).Forturbojetengines,enginerpmisbrokendownintolowpressurespoolrpm(N1),andhighpressurespoolrpm(N2).
Therelationshipbetweeninstrumentindicationsisaveryimportantguidetoenginecondition,efficiencyandperformance.Forinstance,iftorqueoilpressureorenginepressureratioislower than normal for a particular combination of turbine temperature, fuel flow, rpm, airtemperature, aircraft altitude and airspeed, then a loss of engine performance can besuspected.
Byanalysing instrument indications, flight crews andmaintenancepersonnel can forecast
troubleandtakepreventativeactionbeforeamajormalfunctiondevelops.Thisisknownas"trendmonitoring".
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Pressure Sensors
Figure2.11showsthecommonbasicdevicesusedforsensingpressure.Theyareusedtoactuatefluidicvalves,indicatorpointers,switchesandelectricalsignaltransmittersincontrol
andinstrumentapplications.
Theymaybedesigned,calibratedandconnectedtosense
Absolutepressure-thepressureabovethezeroofacompletevacuum.
Gaugepressure-thepressureaboveorbelowthe'ambient'(surrounding.atmosphereor,
Differentialpressure-thedifferencebetweentwopressures.
A flexible 'diaphragm'separating two chambers,as in (a), is sensitive to the difference inpressureeachsideofit.Thediaphragmdeflectsintothechamberwiththelowerpressure.Itisusually corrugated to increase its movement. If the chamber onone side isvented to
atmosphere,diaphragmdeflectiondependsonthegaugepressureintheotherchamber.Insomeapplicationsthepressuresinbothchambersmaydifferfromatmosphericpressureandfromeachother.
The'capsules'in(b)and(c)aremadefrompairsofdiaphragmsjoinedattheiredges.Apairofdiaphragmsformedintoacapsuleismoresensitivethanasinglediaphragmofthesamearea,thicknessandmaterial.
Sensitivitycanbefurtherincreasedbystackingcapsulesasin(c).Theamountacapsuleexpandsorcontractsdependsonthedifferenceinpressurebetweentheinsideandoutsidesurfaces. In(b) and (c) the capsulescouldbe 'plumbed' todifferential pressureor gaugepressure. Evacuating the capsules and then sealing them makes them sensitive to theabsolute pressure on their outside surfaces. They are then called 'aneroid' (without air)
capsules.'Bellows'liketheonein(d)arecylinderswithcorrugatedsidesthatallowthemtobereadilylengthenedwheninsidepressureishigherthanoutsidepressure,ortoshortenwhenoutsidepressureishigherthaninsidepressure.Theymaybeplumbedtosensegaugeordifferentialpressuresortheycanbeevacuatedandsealedtomakethemsensitivetoabsolutepressureontheirexternalsurface.
'Bourdontubes'arecurvedandhaveanovalcross-sectionasshownin(e).Pressureappliedtotheinsideofthetubetendstochangethecross-sectionfromovaltoround.Thiscausesthe tube tostraighten resulting inanoutwardsmovementof its freeend. Thereare also'helical'and'spiraltubes'asin(f)and(g)thatgivegreateroutputmovement.
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Thepressuresensorsillustratedandvariantsofthemareusedinmanyaircraftsystemsandcomponentsincluding
Enginefuelmeteringsystems.
Engineairsystems.
Indicatinginstrumentsthatmeasurealtitude,airspeed,MachNo.,verticalspeed;oilfuelandgaspressures;andtemperature.
Figure2-11.
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Pressure Indicator
A pressure capsule similar to (B) at Figure 2.11 is shown at Figure 2.12 installed in aninstrumentcase.Throughport"B"pressureissuppliedtotheinsideofthecapsule.Throughport "A" the case of' the indicator is vented to atmosphere or "ambient" pressure. Aspressure increasesaboveambient,thecapsuleexpandsandthroughtheleverandrockingshaft,thesectorgearismoved.Thepiniongearnowrotatesthepointeragainstthetensionofahairspring.Theindicatorwillreadinpoundstothesquareinchorthemetricequivalent.Thisreadingwillbe"gaugepressure”andwillvaryduetopressurechangesinsideoroutsidethecapsule.
Considerhowthisindicatorcouldbeadaptedtoread(a)airspeed,(b)altitude.
Figure2-12.
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Oil Pressure Warning
General
Theoilpressurewarningsystemprovidesindication inthe flightcompartmentwhenengine
oilpressureisbelowapredeterminedsetting,orscavengefilterdifferentialpressureisaboveapredeterminedsettingasshownatFigure2.13.
Oil Pressure Light
Theoilpressurewarninglightprovidesanindicationwhenengineoilpressuredropsbelowspecifiedlimits,orscavengefilterdifferentialishigh.Fourlights,oneforeachengine,arelocatedonthePilot'sCentreInstrumentPanel.
Low Oil Pressure Warning Switch
Thelowoilpressurewarningswitchismountedonanadapterassemblyontherearfaceofthehighspeedexternalgearbox.Theswitchconsistsbasicallyofametalbodythathousesanelectricalswitchandconnector,andapressuresensingbellowstowhichoilissupplied
throughtwoholesinthemountingbase.Feedoilissuppliedtotheinsideofthebellowsandreturnoilissuppliedtothechambersurroundingthebellows.Expansionofthebellowsisopposedbyasnapactionspring,whichpreventsthebellowsfromactuatingtheswitchuntilapredeterminedoilpressuredifferentialisreached.
Adecreaseinfeedoilpressureoranincreaseinreturnoilpressurewillcontractthebellowsand,atthepredetermineddifferentialpressure,actuatetheswitchtocompletethecircuit tothewarninglight.Thisdifferentialpressureissetat19-23PSIDforincreasingpressuresand20-16P510fordecreasingpressures.
Filter Pressure Differential Switch
The Filter Switch is mounted on the same assembly as the low oil pressure switch, itspurposeistoprovideawarninglightearthifthefilterisblockedbeyondacceptablelimits.Its
twopipelinesareconnectedonetothefilterinletandonetothefilteroutlet.If thepressuredifferencebetweenthesetwopointsexceedsnormalvaluestheswitchclosesandcompletesthecircuitforthewarninglight.
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Figure2-13.
Oil Pressure Indicating System
General
Theoilpressure indicatingsystemprovidesvisual indicationon the flight engineer's lowerinstrument panel of oil supply pressure and main filter differential pressure. Systemcomponents for each engine consist of a supply pressure transmitter, a main oil filterpressuredifferentialtransmitter,andadualindicatingoilpressure/differentialindicator.
Oil Pressure Transmitter
The oil supply pressure transmitter basically consists of a cylindrical case, housing, twoidenticalstatorwindingssurroundanarmaturecarried-onacentralspindle,whichlocatestoacapsulestack.Twoholesinabaseplatealignwithholesinthemountingfacesothatone
connectstomain feedoilpressure,which is routed toachambersurroundingthe capsulestack,andonetoreturnoilpressure,whichisroutedtotheinsideofthecapsulestack.
Variation in the differential oil pressure causes the capsule stack to expand or contract,impartinglinearmovementtothespindleandarmature.Theresultantchangeininductanceofthestatorwindingsandthereforetheratioofcurrenttotheindicatorcircuitisshownasanincreasedordecreasedindicatorreading.
Oil Filter Pressure Differential Transmitter
Theoilpressurefilterinlettransmitterismountedontheoilpressurefilteratthefrontfaceofthehigh-speedexternalgearbox. Itissimilartotheoilpressuretransmitter,butitscapsulestacksensesoilpressureintotheoilfilterandoutoftheoilfilter.
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Temperature Indicating Capillary
Temperature Sensors
Thisbasictypeof"temperaturesensor"reliesontheexpansionandcontractionofliquidsandgases.
A 'Capillary' (small bore tube) type consisting of a temperature sensing bulb, a movingelement suchasa bourdon tube,andaconnectingcapillary tubeall completely filledwithmercury or alcohol. Changes of temperature vary the volume of the liquid. This in turncauses the bourdon tube to straighten with increasing temperature, or to curl more withdecreasingtemperature.
Thiscouldalsobea 'vapourpressuretemperaturesensor'. It issimilar to the expandingliquidtypedescribedaboveexceptthatatnormaldaytemperaturethebulbispartlyfilledwithavolatileliquid,andtherestofthesystemisfilledwithvapourfromthatliquid.Theamountof vaporisation and hence the pressure and bourdon tube movement varies with the
temperatureatthebulb.Thistypeoftemperaturesensorissuitedtoaircraftapplicationsbecausethesensingbulbcanberemotelylocatedfromtheindicator.It isusedasanengineoiltemperatureindicatoronmanylightaircraft.
Figure2-14.
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BI-Metal Temperature Sensor
Figure2.15illustratestheactionof'stripand'disc'typesof'bimetallictemperaturesensor'.Twometalsofhigh(brass)andlow(invar)temperaturecoefficientsarebondedtogether.Atsomedatumtemperaturethestripat (a)isstraight. Ifthestripisheatedthebrassexpands
morethantheinvartocauseittocurlasat(b).Ifthestripiscooledthebrasscontractsmorethantheinvartocauseittocurltheoppositewayasat(c).
Discshapedbimetallicsensorsarecommoninapplicationsrequiringasnapaction.Whenheated,aslightly domedbimetallicdiscwillsuddenlysnapacross tobeingdomedon theoppositeside.See(d)and(c).
Bimetallictemperaturesensorsareused:
Intemperatureindicators.
Astemperaturecompensatorsandcorrectorsinvariousinstrumentsandmechanisms.
Tooperateswitchcontactsincircuitbreakers,firedetectors,thermostatsandtimers.
Figure2-15.
Resistance Bulb Temperature Sensors
Theresistancewire,whichistheessentialfeatureoftheresistancebulb,restsinthespiralgroovesofaninsulatingmaterialandiscoveredwithametalshield,whichconductsheattoand from it very quickly. (See below) This metal shield must be able to withstand thecorroding influenceofengineoilsathigh temperatures, thehigh flash temperatures in thecarburettorofabackfiringengine,andthedeterioratinginfluenceoftheatmosphere.Eventhough the resistance bulb is covered with a metal shell and substantial insulation, itrespondstochangesintemperatureveryrapidly.Thissensitivityisimportantbecausethemembersof the flight crewarenot interestedinpasttemperatures; theywant toknowthesituationattheexactsecondthattheinstrumentisread.
Theactionofaresistancebulbmaybeunderstoodbystudyingthegraphbelow.Itwillbe
notedthattheincreaseinresistanceofatemperaturebulbisalmostlinearwithrespecttotemperaturechanges.
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A typical application of the resistance bulb temperature sensor is the engine oil tempindicatingsystem.Whenthebulbisconnectedtoasuitablecircuit,suchasaratiometerorwheatstonebridge,itwillindicateariseinmeterreadinginalinearfashionasitheatsup.The bulbis immersed intheengineoil and anelectricalconnectorplugconnects it tothe
indicator,whichissuppliedwith28vd.c.Becauseofthepositivetemperatureco-efficientofresistance,anopencircuitwillcauseafullscalehighreadingandashortedbulbwillreadfullscalelow.
Ratiometer Temperature Indicator
Aschematiccircuit illustratinghowa resistancebulb is connectedina ratiometercircuit isshownbelowatFigure2.16.Notethatthevoltagefurnishedbyabatteryisdividedbetweenthecircuitsofthetwocoilsby the fixedresistor inonesideandtheresistancebulb intheother. The series and shunt resistancesshown are for the purposeofcompensationandadjustment. Itisobviousfromthecircuitthatthecurrentthroughthetwosidesofthecircuitwillbeequalonlywhentheresistanceofthetemperaturebulbisequaltotheresistanceofthe fixed resistor. At this point themoving coils assume positions in fields of equal flux
density,asshown.Anychangeintheresistanceoftheresistancebulbwillcausetheratioofthecurrentstochangeandthecoilstoshifttoanotherposition.
Ratiometerthermometersmaybeusedforavarietyoftemperatureindications,amongwhicharethoseofinletairinajetengine,freeair,andengineoil.
Becausethepointerismovedbytheratioofcurrentinthetwocoilsthesystemdoesnothaveerrorsduetovariationsinsupplyvoltage.
Figure2-16.
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Thermocouple Instrument Systems
Oeofthemaincharacteristicsandadvantagesofthermocouple-typetemperaturemeasuringinstruments is their complete independence of the electrical system of the aircraft.Thermocouple-type instruments are used to measure cylinder head temperature (CHT),
turbineinlettemperature(TIT),andexhaustgastemperature(EGT)onreciprocatingengine-poweredaircraft.Onturbineengine-poweredaircrafttheyareusedtomeasuretheexhaustgastemperature(EGT),turbineinlettemperature(TIT),orintermediateturbinetemperature(ITT). Regardlessof the parameter theymeasure, these instruments work on the sameprinciple.
AsshownatFigure2.17,whenthejunctionofwiresmadeoftwodissimilarmetalsisheated,currentwillflowfromthejunctionthroughoneofthewires,throughthecoilofthemeasuringinstrument,andbacktothejunction.Theamountofthiscurrentisdeterminedbytwofactors:by the resistance of the circuit and by the temperature difference between the hot, ormeasuringjunction,andthecold,orreferencejunction.
Figure2-17.
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Exhaust Gas Temperature
The complete EGT system for a turbine engine consists of: the probes that sense thetemperatureof theexhaustgas,theharnessthatsurroundstheenginetailpipeandservesasconnectionforalloftheprobes,extensionwiresthatcarrythecurrentfromtheprobesinto
thecockpit,resistorstoadjusttheresistanceof thethermocouplestothevaluerequiredforthesystem,andthe indicating instrument in theaircraft instrumentpanel. Theprobesaremounted in the tail pipeandare connected inparallelso that theiroutput isaveragedasshownatFigure2.18.
Some EGT systems use as their indicator a special form of direct current measuringD’Arsonvalmetermovement very similar to theonesued inreciprocatingenginesystems.ButsomeoftheothersystemsfeedtheoutputofthethermocouplesintoanelectroniccircuitwheretheDCvoltagefromthethermocouplesisconvertedintopulsatingDCwhichisfedintoaservo-typeinstrument.Thistypeofindicatorcangivethepiloteitherananalogoradigitalreadoutand,inmanyinstances,bothtypes.
Figure2-18.
EGT Thermocouple Probes and Harness
Probes
Eachoftheeightprobescontainstwochromel-alumelthermocouplejunctionsencasedinaswaged stainless steel housing insulated with magnesium oxide. The junctions are atdifferentimmersiondepthswithaprotectivesleevedrilledtoprovidepositivegascirculation.
Theprobesareinstalledusingatwo-boltmountingflangeattachedtoamountingbossontheengine. The probes are permanently connected into pairs using a steel tube that alsoencasestheelectricalleads.
TheeightthermocoupleprobesareconnectedinparallelandtheindicatorreadstheaverageoftheE.M.F.generatedineach.
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Tachometers RPM)
Non-electrical tachometers
Almostallofthesmallgeneralaviationaircraftusenon-electricalmagnetic-dragtachometers.ThemechanismintheseinstrumentsisthesameasthatusedinanautomobilespeedometerandisshownatFigure2.19.
Analuminiumcupfitscloseoverthespinningmagnetbutitdoesnottouchit.Asthemagnetspins,itslinesoffluxcutacrossthealuminiumcupandinducesavoltageinit.Thisvoltagecauses current (eddycurrent) toflow inthealuminium,and thiseddycurrentproducesitsownmagneticfieldthatopposesthefieldthatcausedit.Thetwofieldsproduceatorquethatrotatesthedragcupagainsttherestraintofacalibratedhairspring.Thefasterthemagnetspins,thegreatertheeddycurrentandthegreateritsmagneticfield,andthemoredragcupwillberotated.Thedragcupissupported,inabrassbushingbyasteelshaftWhentheengine is not running, the restraining hairspring holds the drag cup over so the pointerindicateszeroRPMonthedial
Figure2-19.
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Electrical RPM Indication
Adirect-drivea.c.generatorisatFigure2.20,therotorbeingeithertwo-poleortwelve-pole,anddrivenviaasquare-endedshaft.Thetwopolegeneratorisutilisedinconjunctionwithathree-phase synchronous motor type of indicator, while the twelve-pole generator, which
produces a single-phase output at a high frequency is utilised in conjunction withcounter/pointerindicators,andalsoforsupplyingsignalstoenginecontrolunits.
Atypicalindicator,shown in(B)consistsof twointerconnectedelements:a drivingelementandaneddy-current-dragspeed-indicatingelement.
Letusconsiderfirstthedrivingelement.Thisis,infact,asynchronousmotorhavingastar-connectedthree-phasestatorwindingandarotorrevolvingontwoballbearings.Therotorisofcompositeconstruction,embodyinginonepartsoft-ironlaminations,andintheotherpartalaminatedtwo-polepermanentmagnet.
Analuminiumdiscseparates the twoparts,anda seriesof longitudinalcopperbarspassthrough the rotor forming a squirrel-cage. The purpose of constructing the rotor in this
manner is tocombine' theself-startingandhigh torquepropertiesofasquirrel-cagemotorwiththeself-synchronouspropertiesassociatedwithapermanent-magnettypeofmotor.
Thespeed-indicatingelementconsistsofacylindricalpermanent-magnetrotorinsertedintoadrumsothatasmallairgapisleftbetweentheperipheryofthemagnetanddrum.Ametalcup,calledadragcup,ismountedonashaftandissupportedinjewelledbearingssoastoreducefrictionalforcesinsuchawaythatitfitsoverthemagnetrotortoreducetheairgaptoaminimum.
Acalibratedhairspringisattachedatoneendofthedrag-cupshaft,andat theotherendtothemechanismframe.Atthefrontendofthedrag-cupshaftageartrainiscoupledtotwoconcentricallymountedpointers;asmalloneindicatinghundredsandalargeoneindicatingthousandsofrev./min.
Figure2-20.a
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Sectionalview(Figure2.20b)ofatypicalsynchronousmotortypetachometerindicator:
Figure2-20b
1.
CantileverShaft
2.
TerminalBlockAssembly3. RearballBearing
4. MagneticCupAssembly
5. DragElementAssembly
6. SmallPointSpindleandGear
7.
OuterSpindlebearing
8.
BearingLockingTag
9.
IntermediateGear
10.
bearingplate11. HairspringAnchorTag
12. InnerSpindleBearing
13. Frontballbearing
14. Rotorand
15.
Stator
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System Operation
As the generator rotor is driven round inside its stator, the poles sweeppast each statorwindinginsuccessionsothatthreewavesorphasesofalternatinge.m.f.aregenerated,the
wavesbeing120ºCapart(seebelow).Themagnitudeofthee.m.f.inducedbythemagnetdependsonthestrengthofthemagnetandthenumberofturnsonthephasecoilsasshownatFigure2.21.
Furthermore,aseachcoilispassedbyapairofrotorpoles,theinducede.m.f.completesonecycleatafrequencydeterminedbytherotationalspeedoftherotor.'Therefore,rotorspeedandfrequencyaredirectlyproportional,andsincetherotorisdrivenbytheengineatsomefixedratiothenthefrequencyofinducede.m.f.isameasureoftheenginespeed.
Thegeneratore.m.f'saresuppliedtothecorrespondingphasecoilsoftheindicatorstatortoproducecurrentsofamagnitudeanddirectiondependentonthee.m.f.'s.Thedistributionofstatorcurrentsproducesaresultantmagneticfieldwhichrotatesataspeeddependentonthegeneratorfrequency.
As the field rotates it cuts through the copper bars of the squirrel-cage rotor, inducing acurrent inthemwhich, inturnsetsup amagnetic fieldaroundeachbar. The reaction ofthesefieldswiththemainrotatingfieldproducesa torqueontherotorcausingit torotateinthesamedirectionasthemainfieldandatthesamespeed.
Astherotorrotatesitdrivesthepermanentmagnetofthespeed-indicatingunit,andbecauseofrelativemotionbetweenthemagnet and thedrag-cupeddycurrents are induced inthelatter. Thesecurrentscreate amagnetic fieldwhich reacts with the permanentmagneticfield,andsincethereisalwaysatendencytoopposethecreationofinducedcurrents(Lenz'slaw),thetorquereactionofthefieldscausesthedrag-cuptobecontinuouslyrotatedinthesamedirectionasthemagnet.
However, this rotationof the drag-cup is restricted by the calibrated hairspring in such a
mannerthatthecupwillmovetoapositionatwhichtheeddy-current-dragtorqueisbalancedbythetensionofthespring.Theresultingmovementofthedrag-cupshaftandgeartrainthuspositionsthepointersoverthedialtoindicatetheenginespeedprevailingatthatinstant.
Figure2-21.
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Autosyn Instruments
An Autosyn system activates indicators in the cockpit without using excessively longmechanicallinkagesortubing.Theindicationispickedupbythetransmitterneartheengine,oratsomeotherremotepoint,andissentbyelectricalmeanstotheindicatorinthecockpit.
AnAutosynsynchrohastheappearanceofasmallsynchronousmotor.Forthisreason,theword"synchro"hasbecomesynonymouswiththisandothersimilarsystems.IntheAutosynsystem,onesynchroisemployedasatransmitterandanotherasanindicator.
A schematic diagramof an Autosyn system is shownbelow. The system is basicallyanadaptationof theself-synchronousmotor principle,whereby twowidely, separatedmotorsoperateinexactsynchronism;thatis,therotorofonemotorspinsatthesamespeedastherotorof theother.WhenthisprincipleisappliedtotheAutosynsystem,however,therotorsneitherspinnorproducepower.InsteadtherotorsofthetwoconnectedAutosynunitscomeinto coincidencewhen theyareenergisedbyanalternatingelectriccurrent,andthereaftertherotorofthefirstAutosynmovesonlythedistancenecessarytomatchanymovementoftherotorofasecondautosyn,nomatterhowslightthatmovementmaybe.
ItmustbeunderstoodthatthetransmitterandindicatorofAutosynunitsareessentiallyalike,bothinelectricalcharacteristicsandinconstruction.Eachhasarotorandastator.Whena-cpowerisappliedandarotorisenergised,thetransformeractionbetweentherotorandstatorcausesthreedistinctvoltagestobeinducedintherotorrelativetothestator.Foreachtinychange in the position of the rotor, a new and completely different combination of threevoltagesininduced.
When two Autosynsare connected asshown atFigure 2.22,and the rotors ofboth unitsoccupyexactlythesamepositions relativeto their respectivestators,bothsetsof inducedvoltagesare equaland opposite. For this reason, nocurrent flows in the interconnectedleads,withthe result thatbothrotorsremainstationary. Ontheotherhand,whenthetworotorsdonotcoincideinposition,thecombinationofvoltagesofonestatorisnotlikethatof
theother,androtationtakesplace,continuinguntiltherotorsareinidenticalpositions.Theinducedvoltagesarethenequalandopposite,andsothereisnocurrentflowinanyofthethreeconductors;hencetherotorswillbeinstationaryandidenticalpositions.
AnAutosynsystemmaybeused forawidevarietyof indicationsonanairplane. Amongthesearemanifoldpressure,oilpressure,rpm(tachometer),remotecompassindication,percentofpower,andfuelpressure.
Figure2
Figure2-22.
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Flow Sensors and Indicators
Themain application on aircraft is to sense the rate of fuel flow to the engines. ThoseillustratedatFigure2.23(a)to(e)andvariationsofthemareusedforthispurpose.
Withthe'taperedtube'typeat(a)thefloatiscarriedtoaheightintheverticaltubewhereitsweight equals the upward forceon it caused by the flowing fluid. Because the float isarestrictioninthetubeadifferentialpressureiscreatedacrossit.Foranygivenrateofflowthedifferentialpressure,andthereforetheupwardsforceonthefloat,willvarywiththecross-sectionalareaoftherestrictedpatharoundthefloat.
Therestrictionisgreatestatthebottomwherethetubeisnarrowest.Asthefloatisforcedupthewidening tubethereis lessrestriction, sotheupwardforceon the floatreducesuntilitequalstheweightofthefloat.Thisequalityoccurshigherorlowerinthetubedependingonwhetherthe{lowisincreasedordecreased.
Figure2-23.
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Vibration Analysis
Intheearlyseventies,aLockheedTri-StarwithRolls-RoyceRB211enginescrashedduetodisintegrationofoneoftheengines,killingmanypeople.Theaccidentinvestigationshowedthattheenginedisintegratedafteralubricationproblemwiththenumber1bearing,whichled
toseparation of the fan from the engine. Engine parts then flew away and damaged thefuselage.
Theinvestigationalsoshowedthattheaccidentcouldhavebeenavoidedifthepilothadhadanenginevibration indication.Thiswouldhaveshownthe vibration increasingdue to thelubrication problembuildingup. So the pilotcouldhave shut down the engine beforeanydamagecouldhappen.
Followingthis,theUSAFAAdeclaredenginevibrationmonitoringsystemsmandatoryfortheTristarandlaterforallaircraftwithenginesbiggerthanacertaindiameter.
Asexplainedbefore, the first functionofanenginevibrationmonitoringsystem(EVM)istogivethepilotacontinuousindicationofthevibrationleveloftheenginestoallowhimtotake
appropriatemeasuresifthevibrationreachesadangerouslevel.Forthisreason,everyenginevibration-monitoringunitconditionssomecombinationofrotorout-of-balance vibrationdata forcockpitdisplay.According to theaircraftandengine type,thesedataareselectedandconditioneddifferently.Typicaldisplaysmayinclude:
FanvibrationandLowPressureTurbine(LPT)vibration.
Fanvibration,LPTvibrationandoverallenginevibrationlevel(olderconcept)
Onevibrationindicationonly,computedasthemaximumlevelmeasured,eitherFanorLPTvibrations.
Togiveanunmistakablewarningtothepilotincaseofproblems,theEVMusuallymonitorsthevibrationlevelsforexceedingacertainalertthresholdandactivatesacockpitwarningin
caseofexceedance.
Apartfromcatastrophicevents,theout-of-balancevibrationlevelofanengineusuallyshowsamoreorlesssteadyincreaseovertimeduetomechanicalwear(birdstrikes,friction,etc...).Sincethetendencyofthevibrationevolutionovertimeissteady,itisquiteeasytopredictthetimewhenthevibrationwillreachacertainvibrationlevel,e.g.themaintenancealertlevel.This allowsmaintenancepersonnel toanticipatemaintenanceactionsandtoplan them inadvance.
For this reason, the vibrationdatafromtheEVMareusuallysent totheAircraftConditionMonitoringSystem(ACMS),orsimilarequipment,fromwheretheyareused,alongwithotherengineparameters,asinputtotheEngineConditionMonitoring(ECM)system.
Figure2-24.(Vibrationevolutionduetowearandcatastrophicevent.)
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TheairborneEVMsystemutilizepiezo-electrictransducers(accelerometers)tosenseenginevibration.Thechargesignalgeneratedby theaccelerometersis thenwiredthroughspeciallow-noisecablingtotheEVMU.Thislow-noisecablingisindispensableduetotheextremelysmallamplitudeofthechargesignal,leadingtoahighsusceptibilitytonoise.
ThesignalprocessingisprovidedbytheEVMU,whichextractstherelevantinformationfromthetotalvibrationsignalprovidedbytheaccelerometers.
Piezoelectricaccelerometersaremountedatrightanglestotheturbineshaft.Thecrystalwilloscillatewithapredeterminedelectricalinput.Thisoscillationismonitored.
Whenenginevibrationoccurs,thisalterstheoscillationfrequencyproducedbythecrystal,astheincreaseinvibrationwillcompressthecrystal.Asthevibrationincreases,sodoesthecompressionofthecrystal.Thisproducesachangeinthesignaloutputfromthecrystalanditisthischangeinfrequencythatisdetectedbythesignalconditionerandsenttotheindicator.
Thepiezo-electricaccelerometersproduceachargeoutputofverysmallamplitudewhichisdirectlyproportionaltotheaccelerationofthevibration,appliedtothem.Theirsensitivityis
expressedthereforeintermsofpico-Coulombs.Thesesensitivitiesarelimitedbythepiezo-electricmaterialssuitableforuseinthehostileengineenvironment.
Themaincharacteristicsoftheaccelerometersare:
Verylinearresponsebetweenapprox5Hzandatleast3kHz.
Veryhighreliabilityduetonomovingparts.
Resonanceandthereforehighamplificationofthevibrationat10to20kHz.
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Table2.1showstypicalareasmonitoredforvibration.
Component Frequency Area Description
StructuralVibration
1toabout15Hz Erraticvibrationorconstantcausedbytheaircraftstructure(wings,fuselage.)
AerodynamicVibration
5toabout40Hz Erraticvibrationcausedbytransientaerodynamicphenomena(turbulences,shockwaves...)intheengineinlet,betweentheengineandfuselage,etc.
RotorImbalance 10to250Hz Vibrationcausedbytheimbalanceoftheenginerotors(highpressure,lowpressureshaft).Showsinthespectrumassteadyvibrationpeaks.
AccessoryVibration
80to500Hz Vibrationcausedbyrotatingaccessories(pumps,etc...)drivenbyN2.Showsinthespectrumassteadyvibrationpeaks.
BladePassingVibration
300to10’000Hzandmore
Erraticvibrationcausedbytheperiodicmechanicalloadvariationsontherotorbladesinducedbytheir
passinginfrontofthestatorblades.
1/FNoise 0toabout20Hz Noisetypicalofwornelectricalcontacts(e.g.oxidized)leadingtoinstablecontactresistance.
BroadBandNoise Anyfrequency Noisetypicalofcontactproblems(e.g.looseconnections)leadingtobrutalinterruptionsofcontact.
Table2.1
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TOPIC15.13:ENGINESTARTINGANDIGNITIONSYSTEMS
Starting Systems
Electric Starters
Electricstartersarenotinverycommonuseonaircraftenginesbecauseoftheirexcessiveweight,althoughwhenusedasacombinationstarter-generator,theyprovideaweightsavingthat makes them feasible for use on small engines. Electric starters are, however, incommonuseonauxiliaryandgroundpowerunits.
Operation
Atypicalstartermotor,showninFigure15.13-1,isa12or24voltseries-woundmotor,whichdevelopshighstartingtorque.Thetorqueofthemotoristransmittedthroughreductiongearstotheclutch.Thisactionactuatesahelicallysplinedshaft,movingthestarterjawoutwardtoengage the engine cranking jaw before the starter jaw begins to rotate. After the enginereachesapredeterminedspeed,thestartermotorwillautomaticallydisengage.
Figure15.13-1.
Othertypesofelectricstartersnormallycontainanautomaticreleaseclutchmechanismtodisengage the starter drive from the engine drive when the engine has reached selfsustainingspeed,asdepictedinFigure15.13-2,adetailedbreakdownoftheclutchanditsoperationiscoveredintheensuingtextandFigure15.13-3.
Figure15.13-2.
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Theclutchmechanismalsoprovidesanover-torqueprotectiontoprotecttheenginedrive.Atapproximately130inlboftorque,smallclutchplatesinsidetheclutchslip
andactasafrictionclutch.Thissettingisadjustable.
Duringstarting,thefrictionclutchisdesignedtoslipuntilengineandstarterspeedincreasetodeveloplessthan the slip torque setting. It is important that the slip torque tensionbecorrectlysettoavoiddamagetotheenginedriveratchet,orslowandhot(hung)starts.
Anotherfunctionoftheclutchassemblyistoprovidean“overrunning”clutch.Thisconsistsofa pawl and ratchet assembly that contains three pawls that are spring loaded into thedisengageposition.
When the starter isenergised, inertia causes the pawls tomove inwardsandengage theratchetgearonthestarterdriveshaftasillustratedinFigure15.13-3.
Theinertiausedispresentbecausethepawlcageassembly,whichfloatsintheoverrunningclutchhousing,triestoremainstationarywhenthestarterarmaturetriestodrivetheclutchhousingaround.
The overrunning clutch housing overcomes the disengage springs and forces the pawlsinward.
Whentheengineacceleratesuptoapproximatelyselfsustainingspeed,itisturningfasterthanthestartermotorandthepawlsslipoutofthetaperedslotsoftheenginedrivegear,anddisengageundertheinfluenceofthedisengagesprings.
Thisoverrunningfeaturepreventstheenginefromdrivingthestartertoselfdestructspeed.Typically,startercircuitsdonotcontainfusesorcircuitbreakers.Thereasonisthatinitialmotorcurrent(serieswoundDCmotor)canbeexcessive.
Figure15.13-3.
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Starter-Generators
Starter-generators, illustrated in Figure 15.13-4, are most commonly found on private tomedium sized jets.Thesestartingsystemsuse astartermotor todrive the engine duringstarting. After the engine has reached a self-sustaining speed, it then operates as a
generatortosupplytheelectricalsystempower.Thestartergeneratorsimplyhasasheardrivesplinethatispermanentlyengagedintheengine.
Starter-generator units are desirable from an economical standpoint, because one unitperformsthefunctionsofbothstarterandgenerator.Alsothetotalweightofstartingsystemcomponentsisreducedandfewerpartsarerequired.
Figure15.13-4.
Pneumatic Starters
Pneumaticstartingisthemethodmostcommonlyusedoncommercialandmilitaryjetenginepoweredaircraft.Ithasmanyadvantagesoverothersystemsinthatitislightweight,simple
andeconomicaltooperate.Apneumaticstartermaytransmititspowerthroughareductiongearandclutchtothestarteroutputshaftwhichisconnectedtotheengine.AtypicalairstarterisshowninFigure15.13-5.
Figure15.13-5.
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Thestarterturbineisrotatedbyhighvolumelowpressureairtakenfromanexternalgroundsupply,anauxiliarypowerunit(APU)orbleedairfromarunningengine.Theairsupplytothestarteriscontrolledbyanelectricallyoperated,controlandpressureregulatingvalveasshowninFigure 15.13-6. This valve isoperatedwhenan engine start isselectedand is
automaticallyclosedatapredeterminedstarterspeed.
Figure15.13-6.
Thestarterclutchalsoautomaticallydisengagesastheengineacceleratesuptoidlespeed,andtherotationofthestarterceases.AtypicalairstartingsystemisshowninFigure15.13-7.
C R O S S F E E D F R O MR U N N I N G E N G I N E
A IR F R A M E P Y L O N
G R O U N DS T A R T S U P P L
A U X IL IA R YP O W E R U N I T (A
A IR C O N T R O L VA LV E
E N G I N E A I R S T A R T E R E X T E R N A L G E A R B O XE X H A U S T A IR
H I G H V O L U M EL O W P R E S S U R E
Figure15.13-7.
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Operation
Thepressureregulating/shutoffvalve,showninFigure15.13-8,consistsoftwosub-assemblies:
Thepressure-regulatingvalve,whichcontainsabutterfly-typevalve. Thepressure-regulatingvalvecontrol,whichcontainsasolenoidthatisusedtostoptheactionofthecontrolcrankinthe“off”position.
Theoperationoftheairstarter(Figure15.13-8)proceedsasfollows:
Turnonthestarterswitch.Thisenergisestheregulatingvalvesolenoidwhichretractsandallowsthecontrolcranktorotatetothe“open”position.
Thecontrolcrankisrotatedbythecontrolrodspringmovingthecontrolrodagainsttheclosedendofthebellows.
Sincetheregulatingvalveisclosedanddownstreampressureisnegligible,thebellowscanbefullyextendedbythebellowsspring.
Asthecrankrotatestotheopenposition,itcausesthepilotvalverodtoopenthepilotvalveallowingupstreamairtoflowintotheservopistonchamber.
Thedrainsideofthepilotvalve,whichbleedstheservochambertoatmosphere,isnowclosedbythepilotvalverodandtheservopistonmovestowardspositionB.
Thislinearmotionoftheservopistonittranslatedtorotarymotionofthevalveshaft.
Thisinturnopenstheregulatingvalve.
Astheregulatingvalveopens,downstreampressureincreasesandisbledbacktothebellowsthroughthepressure-sensingline.Thiscompressesthebellows.
Thecompressionofthebellowsmovesthecontrolrod. Thisturnsthecontrolcrankandmovesthepilotrodgraduallyawayfromtheservochambertoventtheairtoatmosphere.
Figure15.13-8.
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Whendownstreampressurereachesapresetvalue,theamountofairflowingintotheservochamberequalstheamountofairbeingbledtoatmosphereandthesystemisinastateofequilibrium.
Whentheregulatingvalveisopen,theregulatedairpassingthroughtheinlethousingofthe
starterimpingesontheturbineinthestartermotor,showninFigure15.13-5.
Astheturbineturns,thegeartrainisactivatedandtheinboardclutchgear,whichisthreadedonto a helical screw, moves forward as it rotates and its jaw teeth engage those of theoutboardclutchgeartodrivetheoutputshaftofthestarter.
When engine startingspeed is reached, a set of flyweights ina centrifugal cutout switchactuatesaplungerwhichbreaks the ground circuit of the regulating valvesolenoid. Thiscutoutswitchislocatedintheexternalgearbox.
Whenthegroundcircuitisbrokenandthesolenoidisde-energised,thepilotvalveis forcedback tothe "off"positionopening the servochamber toatmosphere(seeFigure 15.13-9).Thisactionallowstheactuatorspringtomovetheregulatingvalvetothe"closed"position.
Tokeepleakagetoaminimuminthe"off"position,thepilotvalveincorporatesaninnercapwhichsealsofftheupstreampressuretotheservoandtheservochamberbleedpassage.
Figure15.13-9.
Somegasturbineenginesarenotfittedwithstartermotors,butuseanairimpingementontotheturbinebladesasameansofrotatingtheengineasdepictedinFigure15.13-10.Theairforthissystemissuppliedfromanexternalsource,orfromanenginethatisoperating.Theairisdirectedthroughnon-returnvalvesandnozzlesontotheturbineblades.
Figure15.13-10.
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Hydraulic Starters
Hydraulicstartersareusedforstartingsomesmalljetengines.Inmostapplications,oneoftheenginemountedhydraulicpumpsisutilisedandiscalledapump/starter,althoughotherapplications may use a separate hydraulic motor. Methods of transmitting the torque
producedtotheenginemayvary,butatypicalsystemwouldincludeareductiongearandclutchassembly.
Operation
Powertorotatethestarterisprovidedbyhydraulicpressurefromagroundsupplyunit,oranaircraftaccumulator,andistransmittedtotheenginethroughthereductiongearandclutch.Thestartingsystemiscontrolledbyanelectriccircuitthatalso,insomeinstances,operateshydraulicvalvessothatoncompletionofthestartingcyclethepumpfunctionsasanormalhydraulicpump.Ahydraulicstarterissimilartoahydraulicmotorwiththefluiddrivingthegearinthestarter.
Starting Sequence
Two separate systems are required to ensure that a gas turbine engine will startsatisfactorily:
Rotationofthecompressor.
Ignitionofthefuel/airmix.
Tohelpensurethattheenginecomesonspeedquicklyandwithoutdamage,itisnecessarytocontrolthesequenceofeventsduringagasturbineenginestartingcycle.
Theexactsequenceofthestartingprocedureisimportant,becausetheremustbesufficientairflowthroughtheenginetosupportcombustionatthetimethefuel/airmixtureisignited.The fuel ratewill not besufficient toaccelerateuntilafterself sustainingspeedhas beenattainedandafailuretocorrectlysequencethestartingeventswillpreventtheenginefrom
reachingthisspeed.
Theusualsequenceofeventsduringanenginestartare:
Selectstart(ignitionon).
Highpressurefuelon.
Lightup.
SelfsustainingRPM.
Startercircuitcancelled.
IdleRPMstabilised.
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Illustrated in Figure 15.13-11 is a graphical representation of RPM and TGT during acorrectlysequencedstart.
Figure15.13-11.
For easeofmaintenance itmustbepossible tomotorover theenginewithout the ignitionsequenceinitiating,andoperatetheignitionsystemwithoutrotatingthestartermotorforinflightrelightingoftheengineintheeventofaflameout.
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IGNITION SYSTEMS
System Types
Theretwocommonclassificationsofjetengineignitionsystems.Theseare:
Lowtension(DCvoltage).
Hightension(ACvoltage).
Both low and high tension systems are in general use on todays aircraft. Low tensionsystemsaredesignedtousedirectcurrent(DC)andhightensionsystemsaredesignedtousealternatingcurrent(AC)asinputpower.DCoperatedsystemsreceivetheirpowerfromthe battery bus, and AC systems are powered from the aircraft AC bus. Although theoperatingvoltagesofthesystemsaredifferent,bothsystemscontainsimilarcomponentsasillustratedbyFigure15.13-12.
Figure15.13-12.
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Allignitionsystemscanbegroupedintooneoftwotypesofsystems.Theseare:
Intermittentdutycycle.
Continuousdutycycle.
Thetermdutycyclereferstothetimelimitplacedontheoperationoftheignitionsystembythemanufacturertopreventdamagetoitscomponents.Intermittent duty cycletypesdrawsufficiently high amounts of current to causeoverheatingwithin their units if operated forextendedperiods.Forthisreasontheyhavearestricteddutycyclebasedonoperatingtime,followedbyacoolingoffperiod.Forexample,twominutes“on”,threeminutes“off”(cooling).
Continuous dutytypeshavelongdutycyclesorinsomecasesnolimitsatall.Thatistheycanbeincontinuousoperation.
Intermittent Duty Cycle
Intermittentdutycycleignitionsystemscanonlybeusedforshortperiodsandonlyusuallyduring ground starting. Once the engine has reached self sustaining RPM, the ignition
systemisturnedoff.Someaircraftprovideforadditionaluseoftheleftorrightplugfromthemainsystematfulltransformercapacity(fullpower)asrequiredbut forlimitedperiodsonly,eg. take off. These time periodsarescheduledby thepilots andcan select ignition onwhenevertheywish.
Onotherintermittentdutycycletypeignitionsystems,alowtension,continuousdutycircuitisincorporatedwithin one of the transformerunits. Thisallows low powerdischarge tooneigniterplug(whichagaincanbeselectedbythepilot).ThissystemcanbeoperatedforaslongasthereisaneedforselfrelightcapabilityinflightasshowninFigure15.13-13
Figure15.13-13.
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Continuous Duty Cycle
Ifacontinuousdutycyclemainignition system is installed, full ignitioncanbeselected tobothoreitherplugatthepilotsdiscretion.Duringcriticalflightmanoeuvres(eg.takeoffandlanding),thepilotmayselectbothigniterplugstogiveinstantaneousrelight.Atnormalhigherlevel flight, one igniter plug is selected as a short delay in relighting the engine will notendangertheaircraftorcrew.
Ignition System Components
Gas turbineenginesare typically equippedwith adual highenergy ignitionsystem. TheprinciplecomponentsofadualsystemareshowninFigure15.13-14anddescribedonthefollowingpages.
Figure15.13-14.
Ignition and Relight Switches
The ignition and relight switches are located in the aircraft cabin, usually close to thethrottles. They connect bus voltage to the ignition relayand HEIUs (highenergy ignitionunits).
Ignition Relay
Whenenergised,theignitionrelaysupplieselectricalpowertothehighenergyignitionunits.Itiscontainedinacontrolboxwhichisusuallylocatedinanequipmentcompartmentinthe
enginenacelle.
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High Tension Ignition Leads
Thehightension(HT)ignitionleadsarelocatedontheaircraftengine,connectedbetweentheHEIUsandtheigniterplugs.TheyconductthehighvoltagefromHEIUstotheigniters.
High Energy Ignition Units HEIUs)
TheHEIUsdevelopthehighvoltagenecessaryforengineignition.Inadualignitionsystemtherearealwaystwounitsfittedtoeachengine.AnigniterplugisconnectedtoeachHEIU.
Types
TheignitionsystemcanbesuppliedwitheitherACorDCvoltage,dependingonthetypeofHEIUsfitted.ADCtypeHEIUcontainsatremblermechanism(coveredlaterinthistopic)oratransistorcircuit,whileanACtypeHEIUcontainsa transformer. Inanycase, thebasicoperationissimilarforeachofthesetypes.
HEIUsareratedin ‘joules’(one jouleequalsonewattper second). Theyaredesigned toproduceoutputswhichmayvaryaccordingtorequirementsandaregenerallyclassifiedaseither:
Highjoule(twelvejoule).
Lowjoule(threetosixjoule.
Althoughmany engines are fitted with high joule HEIUs, low joule units are sufficient fornormal starting requirements. The high joule units are requiredwhere it is necessary torelighttheengineathighaltitudes.
Undernormalflightconditions,theHEIUsareturnedOFFaftertheengineshavestarted.Butduring take-off where ice, heavy rain or snow exists, the HEIUs may be operatedcontinuouslytogiveanimmediaterelightshouldanengineflame-outoccur.Thiscontinuous
operationisusuallyperformedby lowjouleHEIUs,aspersistentoperationofthehighjouleunitsmayreducethelifeoftheigniterplugs.
Tosuitallengineoperatingconditions,acombinedsystemhasbeendevelopedwhereoneHEIUemitsahighoutputtooneigniterplug,andthesecondunitsuppliesalowvalueoutputtothesecondigniter.
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Construction
Asmentionedearlier,thebasicoperationofthedifferenttypesofHEIUsissimilar,sowewilllimitourdiscussiontotheDCtrembleroperatedHEIUshowninFigure15.13-15.Itcontainsthefollowingcomponents:
Induction Coil: consists of primary and secondary windings.
Trembler Mechanism: consistsofa capacitor and a set of contactswhich vibrate rapidly,openingandclosingtheprimarycircuitoftheinductioncoil.
Reservoir Capacitor:chargesup,thendischarges,supplyingtheHEIU’shighvoltageoutput.
Glass Sealed Discharge Gap:comprisestwometalliccontacts,separatedbyanairgap,allencapsulatedwithinasealedglasstube.
High Voltage Rectifier: convertstheoutputoftheinductioncoiltoDCtochargethereservoircapacitor.
Choke: aninductorwhichextendsthetimetakenforthereservoircapacitortodischarge.
Figure15.13-15.
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Operation
Figure15.13-16showsasimplifiedschematicdiagramofaDCtrembleroperatedHEIU.
Thisunitoperateswhen28VDCissuppliedtotheprimarywindingoftheinductioncoilandthetremblercontacts.Thetremblercontactsvibraterapidly,openingandclosingtheprimarycircuit,inducingavoltageintothesecondarywinding.Theresultinghighvoltageoutputisthenrectifiedbythehighvoltagerectifierandsuppliedtochargethereservoircapacitor.
Thereservoircapacitorisrepeatedlychargedinthiswayuntilitsstoredvoltageisequaltothe breakdown value of the sealed discharge gap. The reservoir capacitor will thendischargeacrossthegap,throughthechokeandsupplytheigniterplugwiththehightensionvoltagerequiredtoignitetheair/fuelmixtureintheenginecombustionchamber.Thechoke(inductor)extendsthedurationofthedischarge.
Thenormalsparkrateofatypicaljetengineignitionsystemisbetween60and100sparksperminute.
Figure15.13-16.
Theenergystoredinthereservoircapacitorispotentiallylethal. Forthisreason,dischargeresistorsareconnectedacrossthecapacitortoensurethatanychargeonthecapacitorisdissipatedwithinapproximatelyoneminuteofthesystembeingswitchedoff.
Thesafetyresistorsenabletheunittooperatewithoutdamagetotheunitifthehightensionleadisdisconnectedandisolated.
Igniter Plugs
Duetothemuchhigherintensityspark,igniterplugsforjetenginesdifferconsiderablyfromspark plugs used in reciprocating engines. They are normally constructed fromnickel-chromiumalloywiththethreadsbeingsilverplatedtopreventseizing.Thehotendofthe igniter plug is generally air cooled to keep it between 500-600o F cooler than thesurroundinggastemperatures.Coolingairispulledinwardthroughthecoolingholesintheflametube,andovertheendoftheigniter,bythepressuredifferentialbetweentheprimaryandsecondarycombustorairflow.
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Low Tension Igniters
Igniterplugsforlowtensionsystemsarereferredtoastheselfionisingorshuntedgaptype.Thefiringendcontainsasemi-conductivematerialwhichinitiallyprovidesapathbetweenthecentreelectrodeandthegroundelectrode.Astheinitialcurrentflows,thesemi-conductorreachesanincandescentstate(glowswhitehot).Thisheatingissufficienttoionisetheairgapandthemaincurrentflowtakesthispathtothegroundelectrode.AtypicallowtensionigniterisillustratedinFigure15.13-17.
Figure15.13-17.
High Tension Igniters
Hightensionigniterplugs(orannulargapplugs)operatewithasimilarprincipletoanormalsparkplug. Thehigh tensioncurrentpassingthroughtheplug initiallycauses theair gapbetweentheelectrodestobeionised.Thisionisationoftheairgapallowsthehighintensitysparktoflowbetweenthecentreandgroundelectrodes,(ReferFigure15.13-18).
Figure15.13-18.
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Many types of igniter plugs are available, as shown in Figure 15.13-19. Only one willnormally suit the needs of a particular engine. Care must be taken to ensure themanufacturersrecommendedigniterplugisused.
Figure15.13-19.
Ignition System Operation
Aschematicdiagramofabasic jetengineignitionsystem is illustratedinFigure15.13-20.
Forsimplicity,onlyoneHEIUandoneigniterplugisshown.
Figure15.13-20.
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Whenstartingtheengine,oncethestartermotorhasbeenengagedandtheengine’srotatingassemblybeginstoincreaseinspeed,theaircrewwillclosetheignitionswitch.28VDCwillnowbesuppliedtotheignitionrelay.Onceenergised,theignitionrelaywillsupplyvoltagetothe‘ignitionon’lightandtotheinputoftheHEIU.Thehighlevel,pulsatingDCoutputvoltage
fromtheHEIUwillbeconducted,viathehightensionignitionlead,tosupplytheigniterplug. Atthisstage,ifthestartermotorhasincreasedtheengine’sspeedsufficientlytocorrectlymixtheairandfuelsuppliedtotheengine,ignitionwilloccur.Oncetheair/fuelmixturehasbeenignited, the flame spreads rapidly through the engine combustion chambers; thus thecombustionisselfsustaining,andtheignitionsystemcanbeswitchedoff.Onmanyaircraft,a timer relay is employed to automatically shut down the ignition system after apredeterminedtime.
Engine Relight
Ifaflameoutoccurswhilsttheaircraftisinflight,theenginewillcontinuetorotateduetotheflowofairthroughthecompressor.Tore-ignitetheair/fuelmixtureintheenginecombustionchamber,only asource of ignition isnecessary. This isachieved by the selection of the
‘relight’switch.Withthisswitchclosed,28VDCwillbesupplieddirectlytotheHEIU.
In some cases however, a low joule HEIU is fitted and operated continuously, providingautomaticrelight.
Testing, Inspection and Maintenance
Maintenanceof the turbineengine ignition systemconsistsprimarilyof inspection, testing,troubleshooting,removalandinstallation.Thefollowinginstructionsaretypicalexamplesofinspectionproceduresthatyoumayberequiredtoperform.
IMPORTANT
Prior to performing maintenance on an ignition system, always consult the relevant technical
publication for all applicable safety precautions, maintenance procedures and specifications.
Igniter Plugs
The igniter plugsare inspected visually for burning and erosion of the electrode or shell,crackingoftheceramicinsulator,anddamagetothethreads,orflange.Ifdamageisvisible,theignitershouldbediscarded.
HT Ignition Leads
Theignitionleadsarecleanedwithanapprovedsolventandinspectedforwornorburned
areas,deepcuts,frayingandgeneraldeterioration.
Theignitionleadsconnectorsarevisuallyinspectedfordamagedthreads,corrosion,crackedinsulators,andbentorbrokenconnectorpins.
Thecontinuityoftheleadsischeckedwithamultimeterandinsulationpropertiescheckedwith a meggar in accordance with specifications laid down in the relevant technicalpublication.
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Operational Test
SomeaircraftservicingmayrequireanoperationaltestoftheignitionsystemtochecktheserviceabilityoftheHEIUs,HTleadsandigniterplugs. Inthistest,theenginestarter-motorisdisabledsotheenginewillnotrotate,preventingenginestart.
Whenthe‘battery’andengine‘relight’switchesareclosed,sparkingfromtheigniterplugswill be clearly audible. This enables assessment of the ignition system’s serviceability. Anothermethodistosimplystarttheengine.
Safety Precautions
Theterm“HIGHENERGY”infersthatalethalchargeispresentandturbineengineignitionsystems require special maintenance and handling. The manufacturers instructions andenginemaintenancemanualsshouldbe fully understoodandfollowedwhen handlinganycomponentofajetengineignitionsystem.
Sometypicalprecautionsareasfollows:
WARNING
Ensure that the ignition switch is turned off before performing any maintenance on the
system.
To remove an igniter plug, disconnect the transformer input lead, wait the time
prescribe by the manufacturer usually 1-5 mins), then disconnect the igniter lead and
ground the centre electrode to the engine. The igniter plug is now safe to remove.
Exercise great caution in handling damaged transformer units. Some contain
radioactive material, eg. cesium-barium 137).
Unserviceable igniter plugs containing aluminium oxide and beryllium oxide, a toxic
insulating material, should be disposed of properly.
Before a firing test of igniters is performed, the fitter must ensure that the combustion
chamber is not fuel wetted, as a fire or explosion could occur.
Do not energise the system for troubleshooting if the igniter plugs are removed.
Serious overheating of the transformers can result.