Menu Operational Theory Energy Released When a Fuel is Burnt Other Pages The Basics The 2 Stroke Engine The 4 Stroke Engine Operation Members The energy released by a unit quantity of fuel when it is burnt is known as its calorific value and is usually quoted in kJ/kg. The calorific value of a fuel depends on its make up. The constituents of fuel which release energy when they are burnt are Hydrogen, carbon and sulphur Hydrogen releases the most energy; 144000kJ/kg. Carbon releases 33700kJ/kg. Sulphur is the lowest, only releasing 9750kJ/kg. It should be evident therefore, that the less sulphur, and the more hydrogen a fuel contains, the greater its calorific value will be. For example, a typical marine heavy fuel oil may contain 85% carbon, 12% hydrogen and 3% sulphur by mass. . The higher calorific value (HCV) of the fuel will be (0.85 × 33700) + (0.12 × 144000) + (0.03 × 9750) = 46217.5kJ/kg. Not all the energy released can be utilised; when hydrogen burns, the water produced is as steam, and this takes energy from the burning fuel. This energy lost is known as the enthalpy of evaporation, and is 2442kJ/kg (of water produced). The useful energy in the fuel is known as it's Lower Calorific Value (LCV) and is calculated by subtracting the energy required to turn the water produced from burning the hydrogen into steam from the Higher Calorific Value (HCV) When a mass of Hydrogen is burnt, 9 times that mass is produced as water. So LCV = HCV (9 × mass of hydrogen/kg fuel × 2442) For the example above LCV = 46217.5 (9 × 0.12 × 2442) = 43580.14 kJ/kg Download the spreadsheet here and calculate the HCV & LCV of a fuel Engine Manufacturers quote their fuel consumption figures in g/kWh. Typical figures for a modern engine are between 165 and 170g/kWh. This gives a guide to the efficiency of the engine. Because the specific fuel consumption would vary depending on its calorific value (the lower the value, the more fuel must be burnt to produce a certain amount of power), engine manufacturers base their figures on a calorific value of 42700kJ/kg for example if a fuel with a lower calorific value of 42700kJ/kg is burnt then 1g will release 427kJ of useful energy and 170grams (0.17kg) will release 7259kJ of useful energy. If this is the energy released in one hour to provide 1kW of power for one hour then the energy released per second by the fuel is 7259 ÷ 3600 = 2.02kJ
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Theenergyreleasedbyaunitquantityof fuelwhen it isburnt isknownas
itscalorificvalueand isusuallyquoted
inkJ/kg.Thecalorificvalueofafueldependsonitsmakeup.TheconstituentsoffuelwhichreleaseenergywhentheyareburntareHydrogen,carbonandsulphurHydrogenreleasesthemostenergy144000kJ/kg.Carbonreleases33700kJ/kg.Sulphuristhelowest,onlyreleasing9750kJ/kg.Itshouldbeevidenttherefore,thatthelesssulphur,andthemorehydrogenafuelcontains,thegreateritscalorificvaluewillbe.Forexample,atypicalmarineheavyfueloilmaycontain85%carbon,12%hydrogenand3%sulphurbymass.
.Thehighercalorificvalue(HCV)ofthefuelwillbe(0.8533700)+(0.12144000)+(0.039750)=46217.5kJ/kg.Notalltheenergyreleasedcanbeutilisedwhenhydrogenburns,thewaterproducedisassteam,andthistakesenergyfromtheburningfuel.Thisenergylostisknownastheenthalpyofevaporation,andis2442kJ/kg(ofwaterproduced).Theusefulenergyinthefuelisknownasit'sLowerCalorificValue(LCV)andiscalculatedbysubtractingtheenergyrequiredtoturnthewaterproducedfromburningthehydrogenintosteamfromtheHigherCalorificValue(HCV)WhenamassofHydrogenisburnt,9timesthatmassisproducedaswater.SoLCV=HCV(9massofhydrogen/kgfuel2442)FortheexampleaboveLCV=46217.5(90.122442)=43580.14kJ/kgDownloadthespreadsheethereandcalculatetheHCV&LCVofafuelEngineManufacturersquotetheirfuelconsumptionfiguresing/kWh.Typicalfiguresforamodernenginearebetween165and170g/kWh.Thisgivesaguidetotheefficiencyoftheengine.Because
thespecific fuelconsumptionwouldvarydependingon itscalorificvalue
(the lower thevalue, themore fuelmustbeburnt toproducea certain
amountofpower), enginemanufacturersbase their figures on a
calorific
valueof42700kJ/kgforexampleifafuelwithalowercalorificvalueof42700kJ/kgisburntthen1gwillrelease427kJofusefulenergyand170grams
(0.17kg)will release7259kJof useful energy. If this is the energy
released in onehour to
provide1kWofpowerforonehourthentheenergyreleasedpersecondbythefuelis72593600=2.02kJ
Thismeans that foreverykWofpowerproducedat theoutputshaft,
theenginemustproduce2.02kJenergy from
thefuel,makingitjustunder50%efficient.Thecalorificvalueofafuelcanbedeterminedwithoutknowingitspreciseratioofcarbonhydrogenandsulphur.Todothis,apieceofequipmentcalledabombcalorimeterisused.
Thedesignof a typical bomb calorimeter is shown in
thepicture.Asmallsampleoffuel(usuallylessthan1gram)isplaced in a
crucible inside a steel pressurevessel called abomb. The steel bomb
is placed inside a container
filledwithwater,whichiskeptataconstanttemperaturerelativetotheentirecalorimeterbyuseofaheaterandastirrer.Thetemperatureof
thewater ismonitoredwith a thermometerfitted with a magnifying
eyepiece, which allows
accuratereadingstobetaken.Heatlossesareminimizedbyinsertinganair
spacebetween thebucketandanexterior insulatingjacket. Slots at the
top of the steel bomb allow ignitionwires and an oxygen supply to
enter the vessel, both ofwhich are critical in starting the
chemical reaction. Thebomb is pressurised with oxygen to 20 bar to
ensurecomplete combustion of the fuelWhen an electric
currentpassesthroughtheignitioncoil,thefuelisignited.Theheatreleasedfromthesampleisabsorbedbythewaterandthebomb,
which results in an increase in temperature.
Theincreaseintemperatureismeasuredandthecalorificvalueofthefuelcanbecalculatedasshownbelow.
Heatreleasedbyburningfuel(Q)=massofwater(m)specificheatcapacityofwater(C)changeintemperature(T)Thespecificheatcapacityisdefinedasthequantityofenergyrequiredtoraise1kgofthesubstancethrough1C,andforwateris4.2kJ/kgCThe
heat energy absorbed by the bomb and the water container must also
be taken into account. To simplify
thecalculationthemanufactureroftheequipmentgivesawaterequivalentofthebombandcontainerwhichisaddedtothemassofwatertoallowfortheheatenergyabsorbedbythem.TheequationthereforebecomesQ=(massofwater+waterequivalent)4.2TForexample:Afuelsampleof0.7graisesthetemperatureof2kgwater+bombandcontainerwithawaterequivalentof0.5kgby3C.Calculatethecalorificvalueofthefuel.Q=2.54.23=31.5kJ.This
is the heat energy released by 0.7 grams of fuel. Therefore the
heat released by 1kg fuel = 31.5 0.0007
=45000kJ/kgThehighercalorificvalue(HCV)ofthefuelistherefore45000kJ/kgThisisthehighercalorificvaluebecausethehydrogenburnstowater(intheformofsteam).Thissteamthencondensesontheinsidesurfacesofthebombgivingupitsheatenergy.Tocalculatethelowercalorificvalue(LCV)theamountofwaterproducedbyburningthefuelsamplemustbemeasured.Thewaterproducedbyburning1kgoffuelcanthenbecalculated.andifthisfigureismultipliedby2442(theenthalpyofevaporation)andsubtractedfromtheHCV.TheLCVwillbefound.Forexample:Afterthecombustionofthesampleabove,thebombisopenedandfoundtocontain0.75gofwater.LCV=HCV{(0.750.7)2442}=450002616.4=42383.6kJ/kgThespreadsheetallowsyoutoinputdataandviewtheresultsoftheexperiment.
