performance, combustion and emission characteristics … · to increased performance. therefore,...

PERFORMANCE,COMBUSTIONANDEMISSIONCHARACTERISTICSOFADIESELENGINEOPERATEDONDUALFUELMODEUSINGMETHYLESTERS‐COMPRESSED

NATURALGAS(CNG)ANDHCNG

1N.M.GIREESH,2N.R.BANAPURMATH,3V.S.YALIWAL,4R.S.HOSMATH,5P.G.TEWARI

1B.V.BCollegeofEngineeringandTechnology,Hubli,Karnataka,India,Email:[email protected]


3S.D.M.CollegeofEngineeringandTechnology,Dharwad,Karnataka,India,Email:[email protected]



ABSTRACT

Thispaperpresentstheperformance,combustionandemissioncharacteristicsofasinglecylinderfourstrokewatercooled5.2kWrunningat1500RPMdieselengineoperatedonCNG,Hydrogenblendedcompressednaturalgas,(HCNG)‐methylestersofHongeoil(H)andmethylestersof Jatrophaoil(J)combinations.Theperformanceofthebiodiesel‐HCNG fueledenginewasoptimizedintermsofcompressionratio,injectiontimingandexhaustgasrecirculation(EGR)andwascomparedwithbaseline diesel‐HCNG operation. The engine performancewas found to be betterwithincreased compression ratio, advancedinjectiontimingandappropriatepercentageofEGR(5%)forthetestedfuelcombinations.Comparedtodiesel‐HCNGbaselinefuel combination, themethyl esters of Honge and Jatropha oil‐HCNG operation resulted in overall poorer performance.Performanceofthedualfuelenginewasfurtherenhancedforoptimizedengineconditionsof17.5compressionratio,270bTDCinjectiontimingand5%EGR.TheCNGenrichedfuelandbiodieselcombinationsshowednearertodiesel‐HCNGcombinationperformanceintermsofbrakethermalefficiency,combustionparametersandemissionlevels.

Keywords:Methyl esters ofHonge oil (H),methyl esters of Jatropha oil (J),Hydrogen enriched Compressed natural gas(HCNG),Emissions,combustion,Exhaustgasrecirculation..

1. INTRODUCTION

Diesel engines are becoming more and more popularbecause of their higher brake thermal efficiency, power,reliability,anddurability.Hencedieselenginetechnologyplaysavitalroleintransportation,agriculturalandpowergeneration applications. Energy conservation with highefficiencyandlowemissionareimportantresearchtopicsforenginedesignanddevelopment. In Indiaandvariousother parts of the world, Biofuels such as biodiesel ofdifferentoriginandgaseousfuels likenaturalgas,biogasand producer gas have been explored as alternatives tofossil fuel in order to reduce the petroleum importburden.Oneof themostpromisingalternatives todieseland gasoline is natural gas. The natural gas is gainingmore popularity as vehicle fuel because it has higheroctaneand lower cetanenumber, lowerproduction cost,lower operating cost, and lower emissions factors. CNGdoesnotcontainanyharmfulcomponentssuchasleadorbenzene.Naturalgasisagaseousfossilfuel,consistingofvariousgasspeciesofethane,propane,butane.Themajorconstituent of natural gas is methane (75–98%)and itspropertiesareverysimilartothoseofmethane,whichisits primary constituent. Compared to gasoline or diesel,natural gas has a higher combustion enthalpy per unitmass,whichcomparestheenergydensitiesperunitmassandperunitvolume.Naturalgasisfocusedinthepresentworkasitsreservesarehigherandcanbeusedformanymore number of years [Korakianitis et al (2011)].Theenergyutilizationof CNG ismaximizedwhen it ismixed

with hydrogen, because its flame speed increases andleadstobetterandfastercombustion[KlausvonMitzlaff1998, Chandra et al 2011]. CNG and hydrogen are verystable at optimized compression ratioagainst knockingand can therefore be used in engines of highercompression ratio and thus, provides higher brakethermal efficiency and power output [Klaus vonMitzlaff1998].HCNGinductedpenetrates into theair,andmixeswith it and gets ignited by the liquid fuel injected andcombustion may proceed much faster withhydrogenaddition to CNG due to faster combustion properties ofthehydrogen.CNG isagreenhousegasandhasrelativelyhighleanflammabilitylimitofHCfuelmakesitdifficulttoachievestablecombustionnear theburningregime.CNGalwaysoperateunder leanburn conditionhence it leadsto increased performance. Therefore, excess air couldincreasetheratioofspecificheatsoftheburnedgasandimprove combustion efficiency. Furthermore, theknocking tendency is reduced because of lower cylindertemperature.The leanburnoperationnotonly improvestheperformancebutalsolowerstheHCandCOemissionlevels. Asthemaincomponentofnaturalgasismethaneit has higher self ignition temperature and lower flamepropagation speed. Therefore to enhance combustion oflean burn operation at faster rate, addition of smallamount of hydrogen to CNG is recommended[Kasiananthametal(2011)].

TheeffectofHCNGondieselenginecombustionhasbeeninvestigated by many researchers in the past decade

N.M. GIREESH et al. DATE OF PUBLICATION: JAN 14, 2015

ISSN: 2348-4098 VOL 3 ISSUE 1 JAN-FEB 2015

INTERNATIONAL JOURNAL OF SCIENCE, ENGINEERING AND TECHNOLOGY- www.ijset.in 223

[Kasianantham et al (2011), Saravanan and Nagarajan(2008), Saravanan and Nagarajan (2009), Das (2002),Gopal et al (1982), Qian et al (2011), Lee et al (2002),Banapurmath et al. (2014),Yi et al (1996)]. Severalresearchersobservedpoorutilizationof thegaseousfuelduringdual fuel operation at lowand intermediate load.This leads to poor engine performance and in higherconcentrations of carbonmonoxide emissions comparedto the respective values observed under normal dieseloperation.Athigherloadoperationimprovedgaseousfuelutilization, engine performance and carbon monoxideemissionswere observed, but they found inferior valuesofCOcomparedto therespectivevaluesobservedundernormal diesel operation [Papagiannakis et al(2004)].Increased CO and HC as well as decreasedparticulate have been reported in the literature[Papagiannakis et al (2007), Banapurmath et al (2014)].Several researchers have reported that addition ofhydrogen to CNG improves the combustion of CNGresulting in higher brake thermal efficiencies withsmoother combustion than a diesel engine and loweremission levels[Raman et al (1994),Orhan Akansu et al(2011),Gosal et al (2013)]. 20 to 30% higher efficiencyhas been reported with hydrogen addition[Gosal et al(2013)].Hydrogen has higher flame speed than naturalgas; therefore, theequivalence ratio ismuchhigher thanthe stoichiometric condition, the combustionofmethaneis not as stable as with a blend of H2‐CNG [Gosal et al(2013)]. Yi et al. [1996)] stated that brake thermalefficiencyofintakeportinjectionisclearlyhigherthanin‐cylinder injection at all equivalence ratios. Improvedefficiency has been reported with intake port injectioncomparedtoin‐cylinderinjectionatdifferentequivalenceratios [Yi et al. (1996)].Decreased combustion durationwith increased hydrogen blending fraction in CNG‐hydrogen blended gasoline engine has been reported byAndrea et al. [1998].Apostolescu and Chiriac [1996]showed that hydrogen addition during combustionreduced cyclic variation. Some investigators havereported that introduction of hydrogen into the dieselengine causes the energy release rate to increase at theearlystagesofcombustion,whichincreasestheindicatedthermalefficiency.Thisisalsothereasonfortheloweredexhaust temperature. According to them, for fixed H2supply at 50%,75%and100% load,H2 replaces13.4%,10.1% and 8.4% energy respectivelywith high diffusivespeedandhighenergyreleaserate[Jieetal(2013)].

Dual fuel engine using Hydrogen along with dieselinjectiontostudyperformanceofdualinjectionhydrogenfueledenginebyusingsolenoid in‐cylinder injectionandexternal fuel injection technique has been reported [Lee

et al. (2001)].Literatures on hydrogen fueled engineshowedthat,H2‐dieseldualfuelmodewith90%enrichedH2 gives higher efficiency, but cannot complete the loadrange beyond that due to knocking problems. Lee et al.[2001] suggested that in dual injection, the stability andmaximumpowercouldbeobtainedbydirect injectionofhydrogen. Karim (1996, 1996)investigated H2 and CH4blendedfueloperationsparkignitedengineusing100/0,90/10, 80/20, 70/30, 60/40, 50/50, 40/60, 30/70 and20/80CH4‐H2proportionsbyvaryingequivalenceratios.They investigated initiation speed (m/s), power outputdifference, indicated output efficiency, ignition lag,combustion duration (◦CA),maximum cylinder pressure,knocking regions in different proportions of H2 and CH4percentage, at different equivalence ratios and differentinjection timings (10◦; 20◦; 30◦bTDC). Increased poweroutput has been reportedwith increasing concentrationof hydrogen in the engine at 20◦bTDC and with theincreasing concentration of hydrogen in the engine, at30◦bTDC, power output decreased. If some amount ofhydrogenwas added to themethane as a fuel for the SIengine, performance characteristics of the engineincreaseddrastically.

There have been several studies addressing theperformance and emission characteristics of dieselengines operating on diesel‐CNG combinations. Whendieselengineisconvertedtorunondualfuel(DF)mode,wheregasisthemainfuelanddieselisthepilotfuel,thelevelsofemissionsforthistypeofengineismuchhigherthan that generated from regular diesel engines.Therefore, it is hardly to find less work that deals witheffectofenginevariablesinDFengines.Inviewofthis,anefforthasbeenmadetoenhancetheoverallperformanceof DF engine with reduced emission levels. Therefore,experimental investigationswere conducted on a single‐cylinder four‐stroke water‐cooled DI diesel engineoperated in DF mode with methyl estersHonge andJatropha oils and blend of CNG/H2 induction. In thepresentwork,theperformanceofthemethylestersHongeand Jatrophaoils‐HCNG fueledDF enginewas optimizedwith respect to compression ratio, injection timing andexhaustgasrecirculation(EGR)andcomparedwithbaseline diesel‐HCNG operation. Finally, results of methylestersofHongeandJatrophaoils‐HCNGDFoperationwascomparedwithbaselinedataofdiesel‐HCNGoperation.

2. FUELPROPERTIES

ThepropertiesofHongeoilandHOMEweredeterminedand are summarized in Table 1. Table 2 presents theproperties of the gaseous fuels, namely CNG and HCNG,respectively.

Table1:PropertiesofFuelsUsed

Sl.No. Properties Diesel MethylestersofHongeoil MethylestersofJatropha oil

1 ChemicalFormula C13H24 ‐‐‐‐ ‐‐‐‐

2 Density(kg/m3) 840 870 880

3 Calorificvalue(kJ/kg) 43,000 39800 38,010




4 Viscosityat40oC(cSt*) 2‐5 44.85 45.6

5 Flashpoint(oC) 75 210 167

6 CetaneNumber 45‐55 40 45

7 CarbonResidue(%) 0.1 0.66 ‐‐‐‐

8 Cloudpoint ‐2 ‐‐‐‐ 7

9 Pourpoint ‐5 ‐‐‐‐ 4

10 Carbonresidue 0.13 0.55 0.01

11 Molecularweight 181 227

12 Autoignitiontemperature(oC) 260 470

13 Ashcontent%bymass 0.57 0.01

14 Oxidationstability High Low Low

15 SulphurContent High No No

(*Centistokes)

Table2:PropertiesofCNG,andHCNG

Sl.No Properties CNG HCNG

1 DensityofLiquidat15oC,kg/m3 0.77 ‐‐‐

3 BoilingPoint,K 147K ‐‐‐

4 Lowercalorificvalue,kJ/kg 48000 47170

5 LimitsofFlammabilityinair,vol.% 5‐15 5‐35

6 AutoIgnitionTemp,K 813 825

7 TheoreticalMaxflameTemp,K 2148 2210

8 Flashpoint0C 124 ‐‐‐

9 Octanenumber 130 ‐‐‐

10 Burningvelocity,cm/sec 45 110

11 StoichiometricA/F,kgofair/kgoffuel 17:1 ‐‐‐‐

12 Flametemperature,0C ‐‐‐ 1927

13 Equivalenceratio 0.7‐40 0.5‐5.4

3. EXPERIMENTALSET‐UP

The experimental set‐up used for CNG and HCNG‐operateddual‐fuelenginesisshowninFigure1(a).Enginetests were conducted on a four‐stroke single cylinderwater‐cooled DI compression ignition engine with adisplacement volume of 662 cc, compression ratio of17.5:1anddevelopingpowerof5.2kWat1500 rev/min.Figure1 (b) showsexhaustgas recirculation (EGR)usedinthedualfuelengine.Theenginewasalwaysoperatedata rated speed of 1500 rev/min. The engine had aconventional fuel injection system. The injector opening

pressure (IOP) and the static injection timing (IT) was205 bar and 23°Before Top Dead Centre (bTDC)respectively as specified by the manufacturer. To studythe effect of different injection timings, static injectiontimings of 19°bTDC and 27° bTDC were adopted byvarying the lift length of the rod in the mechanical fuelpump apart from the manufacturer recommendedinjection timing. To study the effect of CR the optimuminjection timing was kept fixed and the CR was variedfrom15to17.5.Inaddition,tostudytheeffectofEGR,theoptimumITandCRwerekeptconstantandtheEGRwasvariedfrom5to20%instepsof5.Theengineisprovided




withagovernoranditmaintainsaconstantenginespeedatalltheloadsontheengine.Thegovernoroftheenginewas used to control the engine speed. The engine wasprovidedwithahemisphericalcombustionchamberwithoverhead valves operated through push rods. Cooling oftheenginewasaccomplishedbycirculatingwaterthroughthe jackets on the engine block and cylinder head. Apiezoelectric pressure transducer was mounted on thecylinder head surface to measure the cylinder pressure.Table3showsthespecificationoftheengineusedforthestudy. Exhaust gas analyzer andHartridge smokemeterwereusedtomeasureHC,CO,NOxandsmokeemissions.In the nextwork, arrangementwasmade to induct CNGandHCNGintotheinletmanifoldwasestablished.

4. RESULTSANDDISCUSSIONS

This section presents the results of experimentalinvestigations carried out on a diesel engine suitablymodifiedtooperateindualfuelmode.

(a):DualfuelenginewithCNG/Hydrogeninductionarrangement

(b) ExhaustgasrecirculationSystem

Figure1:OverallviewofExperimentalSetup

During the experimentation, the gas flow rate of HCNGwasmaintained constant (0.25 kg/hr) and engine speedwas maintained at 1500 rpm. In this study effect ofinjection timing, compression ratio and exhaust gastemperature (EGT) on the overall performance waspresented. A suitable carburetorwas developedwith 12holes having6mmorifices to ensure stoichiometric air‐gasmixturetobesuppliedtotheengine. The liquidfuelofmethylestersofHongeoil(H)andJatrophaoil(J)wereusedasinjectedfuels.

Table3SpecificationoftheCIengine

SlNo Parameters Specification

2 Type TV1(Kirloskermake)

3 Softwareused Enginesoft

4 Nozzleopeningpressure 200to225bar

5 Governortype Mechanicalcentrifugaltype

6 Noofcylinders Singlecylinder

7 Noofstrokes Fourstroke

8 Fuel H.S.Diesel

9 Ratedpower 5.2kW(7HPat1500RPM)

10 Cylinderdiameter(Bore) 0.0875m

11 Strokelength 0.11m

12 Compressionratio 17.5:1

AirMeasurementManometer:

13 Made MX201




14 Type U‐ Type

15 Range 100– 0– 100mm

Eddycurrentdynamometer:

16 Model AG– 10

17 Type Eddycurrent

18 Maximum 7.5(kWat1500to3000RPM)

19 Flow WatermustflowthroughDynamometerduringtheuse

20 Dynamometerarmlength 0.180m

21 Fuelmeasuringunit–Range 0to50ml

4.1 OPTIMIZATION OF INJECTION TIMING FORDIESEL‐HCNG,H‐CNG,J‐CNG,H‐HCNGANDJ‐HCNGDUALFUELOPERATION

TheeffectofITontheengineoperationofdiesel‐HCNG,H‐CNG/HCNG(methylesterofHongeoil‐CNG/HCNG)andJ‐CNG/HCNG (methyl esterof Jatropha oil ‐CNG/HCNG)fuelled dual fuel (DF) engine were investigated. Duringthe engine operation, compression ratio of 17.5, andmixing chamber venture having 6mmhole geometry intheinletmanifoldwasused. Theinjectornozzleopeningpressurewasmaintainedat230bar.BothCNGandHCNGflow rate of 0.5 kg/h was kept constant throughout theexperiment.

Figure 2 shows the brake thermal efficiency (BTE)variation with injection timing (IT) for diesel‐HCNG,H‐CNG/HCNG and J‐CNG/HCNG dual fuel operation usingdiesel, fuel combinations operated for 80% load.Advancing the IT from 19° to 27°bTDC, the BTE wasincreased. More time would be available for HCNG fuelburning and resulted in enhanced BTE. From resultsobtained, it is observed that advancing the pilot fuelinjection timing in DF system improves the engineperformance with smooth operation of the engine andreducestheignitiondelayleadingtohigherBTEbuttendsto incurslight increasedspecific fuelconsumption[Jieetal (2013)]. Maximum pressure and higher pressure riseratewith improved BTEwas achieved by advancing theIT.

Further, BTE for Diesel/biodiesel‐HCNG combinationresulted inhigherBTEcompared toCNGoperation.ThiscouldbeduetohighercalorificvalueofHCNGandflamevelocity compared to CNG. The presence of hydrogenallows the leanburn limit tobeextendedbecauseof thefast burn rate of hydrogen. The expansion of theflammabilitylimitinfluencesthereductioninlossbyhighcombustion temperature and heat transfer; hence, thebrakethermalefficiencywasimproved.Thefastburnrateofhydrogencauses thecombustionduration todecreasewhiletheheatreleaserateandexhaustNOxincreasewithan increase,percentageofhydrogen (Borgeset al. 1996;Cho andHe2008; Park et al. 2013). The lower viscosityand higher calorific value of the diesel along with theHCNG gaseous fuel combinations performed bettercompared to two injected biodiesels. For the two

biodiesels considered H‐HCNG combination resulted inslightly higher BTE compared to J‐HCNG and H/J‐CNGcombinations. The reasons could be the lower fatty acidcomposition of Honge oil methyl ester compared toJatrophaoilmethylester.BTEvaluesfordiesel‐HCNG,H‐HCNG, J‐HCNG,H‐CNGand J‐CNGDFoperation at 19, 23and27°bTDCinjectiontimingwerefoundtobe25.4,24.3,23.1,22.4%and21.8%respectivelyat80%load.

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H+HCNG D+HCNG J+HCNG H+CNG J+CNG

Speed: 1500, HCNG flow rate: 0.5kg/hr, CR: 17.5, IOP:230 bar,Injector:3 hole, 0.3mm Load:100%.

Bra

ke th

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al e

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ienc

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Injection timing, 0 CA

Figure2:EffectofinjectiontimingonBTE

Theemissioncharacteristicsof theengineare importantfromenvironmentalperspectives.Theemission fromtheengine reflects the quality of combustion taking placeinside the engine. The emission levels for the producergas operation under a dual‐fuel mode were measuredunder steady‐state conditions using calibratedinstruments.Thedifferentemissionparametervariationsduring the dual‐fuel mode of operation are discussedbelow.

Figure 3shows that smoke opacity decreased withadvanced IT for all fuel combinations in DF moderespectively. Better combustion prevailing inside theenginecylinderattribute tosmallersmoke levels.Highercalorific value of HCNG and lower C/H ratio of fuelcombinationareresponsibleforthistrend.Asengineloadincreases,thesmokeemissionsalsoincreasedslightlydueto decreased volumetric efficiency in DF operation. It isalsoobservedthatsmokeemissionofbiodiesel‐HCNGDF




operationdecreaseswithadvancingpilotinjectiontimingas the fuel combination burns more completely due tomaximumtimeavailableforcombustion.

In addition, the smoke levels for biodiesel‐CNGcombinationresultedinhighersmokelevelscomparedtoHCNGoperation.Also,gasesofCNGandHydrogenbeingcommon, properties of biodiesels injected resulted inhigher smoke opacity compared to diesel‐HCNGoperation.Itismainlyduetoheaviermolecularstructureand higher viscosity of the respective biodiesels whichmakes atomization difficult and leads to higher smokeemissions. The major advantage of DF engine usinghydrogen is that the smoke emissions were loweredcompared to CNG operation. This may be due to lesscarbon content and faster burning rates associatedwithclean burning characteristics of hydrogen compared toCNG.Thehigherburningvelocityand flametemperatureofHCNG leads tomorebetterburning compared toCNGduring thedual fueloperation. Smokevalues fordiesel‐HCNG,H‐HCNG,J‐HCNG,H‐CNGandJ‐CNGDFoperationat19,23and27°bTDCinjectiontimingwerefoundtobe56,65,67,69HSUand71HSUrespectivelyat80%load.

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Smok

e op

acity

, HSU


Figure3:Effectofinjectiontimingonsmokeemissions

The HC variation with IT for diesel‐HCNG, H‐HCNG, J‐HCNG, H‐CNG and J‐CNG DF operation for 80% load isshowninFigure4.FromtheresultsitisobservedthatHCemission levels were decreased considerably withadvancingIT.Itcouldbemainlyduetobettercombustionoccurringinsidetheenginecylinderduetobetterburningof the fuel combinations. IncreasedBTE as advancing ITresults in more heat released during premixedcombustion is also responsible. Advancing the pilot fuelinjection timing leads to longer ignition delay andincreasedpremixedcombustionresults inreducedUBHCemissions. The longer ignition delay allows a full spraypenetrationleadingtobetterburningcausedbyincreasedflamepropagationhelpstocontroltheHCemissions.Thelarger premixed zone develops maximum combustiontemperaturesandthus,lowerstheHCemissions.

In addition, for the same injection timing, hydro‐carbonemissionswithbiodiesel inbothversionsof the gaseousfuel operation were higher compared to diesel. Lowervolatilityandcalorificvalueofbiodieselusedcomparedtodiesel is responsible for this trend. The lower BTEassociated with biodiesel operation could also be

responsible for this behavior. With advanced ITall fuelcombinations resulted in similar trends. Biodiesel beingcommon,thepropertiesofthetwogasesinductedresultsin the behavior shown and accordingly H‐HCNG and J‐HCNGoperationresultsinlowerHCemissionscomparedtoH‐CNG and J‐CNGoperation. This could be due to theCNG charge, which causes lean, homogeneous, low‐temperature combustion, resulting in less completecombustion [López et al (2009)]. This is because smallamount of pilot fuel cannot propagate longer distanceinside the cylinder to burn the whole premixed fuelmixture.Thoughtheoxidationofunburnedhydrocarbonsincreased due to burnt gas temperature, the filling ofunburned mixture in the crevice volumes withcombustiblemixtureduringtheeventofcompressionandignition will become a main source of HC emissions[Sahooetal2009].WhereasduringHCNGoperation,withadvancedIT,thehydrogencontent inCNGgivesastrongreduction of unburned hydrocarbon emission whichresultsinmorecompletecombustion[Das2002,Simioetal.2013].InadditionHCNGengineincreasestheH/Cratioof the fuel, which drastically reduces the carbon basedemissions.ThepresenceofhydrogeninCNG(HCNG)hashigher flame velocity and flame temperature results inbettercombustioncomparedtoCNG.HCemissionvaluesfordiesel‐HCNG,H‐HCNG,J‐HCNG,H‐CNGandJ‐CNGdual‐fuel operation at 19, 23 and 27°bTDC injection timingwere found to be 56, 65, 67, 69 HSU and 71 HSUrespectivelyat80%load.

TheCOvariationwith IT forDiesel‐CNG,H‐CNG,J‐CNG,H‐HCNGandJ‐HCNGandoperateddual fuelenginefor80%load is shown in Figure5. Incomplete combustion is themaincausefortheCOemissionlevels. Itmainlydependson the amount of air‐fuel ratio prevailing inside thecombustion chamber relative to stoichiometricproportions. Advancing the IT from 190 to 270bTDC, COemission considerably reduced. With larger injectionadvance,overallbettercombustionandtheactivityofthebetteroxidationreactionsreducetheCOemissions.Withadvancedinjectiontiming,comparativelybetteroxidationleads to reduced CO emissions with improved brakethermal efficiency. The CO emissions are found to behigher for biodiesel‐CNG/HCNG than diesel‐HCNG DFoperation. Results showed that lower CO levels for DFoperation with advanced injection timing [Jie et al(2013)].

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Hyd

roca

rbon

s, p

pm





Figure4:EffectofinjectiontimingonHCemissions

18 20 22 24 26 280.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50



Car

bon

mon

oxid

e, %

Injecion timing, 0 CA

Figure5:EffectofinjectiontimingonCOemissions

For the same injection timing, the premixed combustionof biodiesel‐CNG/HCNG results in less heat release rateleadingtohigherCOemissionscomparedtodiesel‐HCNGoperation. This could be attributed to incompletecombustion of biodiesels injected due to their pooratomization and impropermixing of fuels due to higherviscosity and density of biodiesel used.However for thesame fuel combinations, lowerCO emissions levelswereobserved forDFoperationwithHCNGcomparedtoCNG.Moreover, the combustion temperatures are higherwithHCNGfuelandtheenginerunshottertherebyfacilitatingbettercombustion.Also,biodieselsofHongeandJatrophaoil being common, the properties of the two gasesinducted results in the behavior shown and accordinglybiodesels–HCNGoperationresults in lowerCOemissionscompared to H‐CNG and J‐CNG operation. Hydrogenaddition toCNG improvescombustion resulting in lowerCO formations due to increased flame speed and bettercombustion. LowerCO emissionswith advanced IThavebeen reported. The increased injection timing beyond270bTDC tends to cause engine knock leading to poorperformanceoftheengine.COemissionvaluesfordiesel‐HCNG, H‐HCNG, J‐HCNG, H‐CNG and J‐CNG dual‐fueloperation at 19, 23 and 27°bTDC injection timing werefoundtobe56,65,67,69HSUand71HSUrespectivelyat80%load.

VariationofNOxemissionwithdifferentITforvariousfuelcombinationsat80%loadispresentedinFigure6.Severalinvestigators have reported higher NOx emissions withadvancedIT.Thecouldbeattributedtoimprovedair‐fuelmixing leading to better combustion and more heatrelease during premixed combustion phase. NOxemissions usually influenced by changes in adiabaticflame temperature and higher NOx formation withadvancedITareprobablycontrolledbyappropriateuseofEGR method. For diesel, biodiesel‐CNG/HCNG DFoperation it is observed thatNOx emissionswere higherfor the pilot injection timing of 27obTDC compared to19obTDC.NOx emissions increased with the increase ininjection timing due to the higher combustiontemperature in flame zone caused by advanced ignition,which results in higher maximum combustion pressure.

However it is observed that biodiesel‐HCNG resulted inhigherNOxlevelscomparedtobiodiesel‐CNGoperation.Itmay be due to presence of hydrogen in the fuelcombination.Moreover,hydrogenhashigherflamespeedandcalorificvalue.GenerallyNOxemissionsalsoexhibitatrade‐offrelationshipwithsmokeemissions.NOxemissionvalues for diesel‐HCNG, H‐HCNG, J‐HCNG, H‐CNG and J‐CNGdual‐fueloperationat19,23and27°bTDC injectiontimingwere found tobe56,65,67,69HSUand71HSUrespectivelyat80%load.

4.2 OPTIMIZATION OF COMPRESSION RATIO FORDIESEL‐HCNG, H‐CNG, J‐CNG, H‐HCNG AND J‐HCNGDUALFUELENGINEOPERATION

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Oxi

des

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en, p

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Figure6:EffectofinjectiontimingonNOxemissions

ThissectionprovidesthecompressionratioeffectontheDiesel‐HCNG,H‐CNG,J‐CNG,H‐HCNGandJ‐HCNGdualfuelengine operation for 80% load respectively. The enginewas operated at a constant gas flow rate of 0.5 kg/h,injectiontimingof27°bTDCwithmixingchamberventurihaving 6mm hole geometry in the inlet manifold. Theinjector nozzle opening pressurewasmaintained at 230barforbiodieselsused.Thecompressionratiowasvariedfrom15to17.5.

4.2.1BrakeThermalEfficiency

The variation of BTE with brake power for diesel‐HCNG,H‐CNG/HCNG (methyl esterof Honge oil ‐CNG/HCNG)andJ‐CNG/HCNG(methylesterofJatrophaoil‐CNG/HCNG)at 80% loadarepresented in Figure7. It isobservedthatBTEincreaseswithincreaseincompressionratio (CR) from 15 to 17.5. The increasing compressionratioallows fuel toburncompletely leading to improvedBTE. The combustion noise increaseswith increased CRgenerally due to higher auto‐ignition temperature of theCNG/HCNG.ForthesameCR,lowerBTEwasobservedforbiodiesel‐CNG/HCNGDFmode of operation compared todiesel‐HCNG combinations. This could be mainly due todifferences in the fuel properties and decreased auto‐ignition temperature of the mixed fuels (biodiesel–CNG/HCNG). Inaddition,H‐CNGand J‐CNGDFoperationresulted in lowerBTEcompared toH‐HCNGand J‐HCNGDFoperation.It couldbeattributed tobetter combustionoccurring inside the engine cylinder caused by theincreased flame speed due to addition of hydrogenleading to increased premixed and rapid combustion




phase.TheuseCNGinadualfueloperationresultslowerperformance compared to HCNG because CNGoperationmay require considerably little more time toensure the complete combustion. But, addition ofhydrogen in the CNG causes fast burning rate of thegaseousfuelalongwithinjectedfuelwhileincreaseintheheat release rate and exhaust NOx increase.The brakethermalefficienciesfordiesel‐HCNG,H‐HCNG,J‐HCNG,H‐CNG and J‐CNG dual‐fuel operation at 15, 16 and 17.5compression ratio were found to be 56, 65, 67, 69 HSUand71HSUrespectivelyat80%load.

15.0 15.5 16.0 16.5 17.0 17.5 18.00

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Speed: 1500 RPM, HCNG Flow rate: 0.5 kg/hr,IT: 27 deg. bTDC, IOP:230 barInjector: 3 hole, 0.3 mm Load:80%


Bra

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Compression ratio

Figure7:Effectofcompressionratioonbrakethermalefficiency

4.2.2SmokeOpacity

Figure 8shows increase in the CR decreased smokeemissionlevelswasobtained.It isawellknownfactthatsmoke emissions reduces remarkably in dual fueloperation.ThecombustiontemperatureandpressureareincreasedwhentheCRisincreased;resultedinimprovedcombustion,asthe flameproducedbytheburningofthefuels reaches the entire area of the cylinder. Similarobservations were recorded even for full load engineoperations.Hence,thelowersmokeemissionlevelswereobtained at a higher CR for the dual‐fuel operation.Experimental investigation showed higher smoke levelsfor biodiesel‐CNG/HCNG operation compared to diesel‐HCNG operation. The CNG/HCNG combustion need notproduce any particulates unlike the pilot injection ofbiodiesel fuel as they have heavier molecular structurebesideshavinghigherfreefattyacidsandviscosity.

However, for the same CR and injected fuels, drasticreduction in the smoke levels were observed for HCNGoperationcomparedtoCNG.Itcouldbeattributedtolesscarbon content and faster burning rates associatedwithclean burning characteristics of hydrogen compared toCNG.Thehigherburningvelocityand flametemperatureofHCNGleadstobetterburningcomparedtoCNGduringthe dual fuel operation. Lower smoke levelswere foundfortheH‐CNGoperationcomparedtoJ‐CNG/HCNGduetodifferences in the properties of injected fuels. Resultsindicatethatsmokeemissionsignificantlyaffectedbythetype of injected fuels instead gaseous fuel because allgaseous fuels are clean burning fuels having lower C/Hratio (Banapurmath et al 2009). The smoke emission

valuesobtainedattheCRsof15,16and17.5were68,61and54HSU,respectively,fortheH‐HCNG,J‐HCNG,H‐CNGandJ‐CNGdual‐fueloperationcomparedwiththevalueof32HSUatCRof17.5forthediesel‐HCNGoperation.

15.0 15.5 16.0 16.5 17.0 17.5 18.0

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e, H

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Figure8:Effectofcompressionratioonsmokeopacity

4.2.3HCandCOEmissions

Thevariationsinhydrocarbon(HC)andcarbonmonoxide(CO) emission levelswith respect to the varyingCRs forthe various fuel combinations at 80% load operating onDFmode are shown in Figures 9 and 10. Decreased HCemission levelswere observedwith increase in CR. ThiscouldbeduetothefactthatwhentheCRisincreased;thecombustion temperature and pressure are increased,resultinginimprovedcombustion.Thisistheresultoftheincreaseofburnedgas temperature thathelpstooxidizeefficiently the unburned hydrocarbons. The increasedbrake thermal efficiency due to complete combustiondecreased the HC and CO emissions with increase incompression ratio. With the increase of compressionratio, there is a sharp decrease of HC emissions underdualfueloperation.However,LowerHCandCOemissionswereobservedatCRof17.5comparedtotheoperationatCRof15and16.However,ata lowerCR, thecombustiontemperature is lower, leading to the freezing of theoxidation process. The combustion characteristic of theengine determines the variation of unburnedhydrocarbonsintheexhaustgases.

However, for the same CR and injected fuels, lower HCandCOemission levelswereobservedwithdiesel‐HCNGcompared to biodiesel‐CNG/HCNG operation. Thecomplete burning of the fuel combinations due toincreased flame propagation caused by the hydrogenadditionandathigherCRhelpsinburningtheentirefuelmixture, hence lower HC and CO emission levels wereobtained with HCNG operation compared with CNGoperation. The use CNG in a dual fuel operation resultsslightly higher exhaust compared toHCNG because CNGoperation may require considerably little more time toensure the complete combustion. ExperimentalinvestigationonvariousfuelcombinationsshowedlowerHC and CO emission levels for the H‐HCNG operationcompared to J‐CNG/HCNG operation. It could beattributed to differences in the type and properties ofinjected fuels. Lower viscosity and density and higher




calorific value of Honge oil methyl ester compared toJatropha oil methyl ester is also responsible for thisobservedtrend(Banapurmathetal2013).Fromresultsitisconcludedthatadditionofhydrogenand increasedCRlowers bothHC and CO levels in the exhaust because ofimprovedcombustionwithhigherBTEobserved.Reducedmixture temperature andcompleteburningofunburnedgaseousfuel leadsto lowerHCandCOformation indualfuel operation. This is the result of the improvement ofgaseous fuel utilization especially during the secondphaseofcombustion.TheHCemissionvaluesobtainedatthe CRs of 15, 16 and 17.5 were 68, 61 and 54 HSU,respectively, for the H‐HCNG, J‐HCNG, H‐CNG and J‐CNGdual‐fueloperationcomparedwiththevalueof32HSUatthe CR of 17.5 for the diesel‐HCNG operation. Similarly,COemissionvaluesobtainedattheCRsof15,16and17.5were68,61and54HSU,respectively, fortheH‐HCNG,J‐HCNG, H‐CNG and J‐CNG dual‐fuel operation comparedwiththevalueof32HSUattheCRof17.5forthediesel‐HCNGoperation.

4.2.4NOxEmissions

The variations inNOx emission levelswithbrakepowerforvarious fuelcombinationsat80%loadarepresentedin Figure 11.It is observed thatNOx emissions increasedwith increase in compression ratios. The reason forincreasedNOxemissionswithincreasedCRisduetomoreheat released during premixed combustion phase. Thelower premixed combustion observed with biodieselscomparedtodieseliscompensatedbytheirhigheroxygenconcentrationwith CNG/HCNG induction. The formationofnitrogenoxidesisfavoredbybondedoxygenpresentinthebiodieselsusedandhighermixturetemperature.NOxemissions by biodiesel‐CNG/HCNG operation arerelativelylessercomparedtodiesel‐HCNGandarehigherfor high load conditions. This behavior of fuelcombination is mainly due to higher premixedcombustion phase observed diesel‐HCNG operation. UseofHCNG insteadofCNGfurther increases theNOx levelsduetoincreasedburningspeedof fuelcombination.Thisvalidates the idea of simultaneous control of smoke andNOx which can be achieved with biodiesel‐CNG/HCNGdualfuelcombustionexceptinthefullloadcase.ThermalNOx reduction in dual fuel combustion is a result ofreduced temperature of combustion caused by thebiodiesel injection. For the same compression ratio,exploring the causes of increasedNOx formation in dualfuel combustion at the full load case it is suggested thatthe NOx is increased greatly by increase in, in‐cylindertemperatures. At increasedCR and higher load, a higherengine temperature and increased turbulence inside thecombustionchamberenhancestheflamespeed,resultingintheincreaseofNOxformation.TheNOxemissionvaluesobtainedattheCRsof15,16and17.5were68,61and54ppm,respectively,fortheH‐HCNG,J‐HCNG,H‐CNGandJ‐CNGDFoperationcomparedwiththevalueof32ppmattheCRof17.5forthediesel‐HCNGoperation.

15.0 15.5 16.0 16.5 17.0 17.5 18.00

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Hyd

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rbon

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pm

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Figure9:EffectofcompressionratioonHCemissions

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Figure10:EffectofcompressionratioonCOemissions

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Figure11:EffectofcompressionratioonNOxemissions

4.3OPTIMIZATIONOFEXHAUSTGASRECIRCULATION(EGR)FORHCNG‐DIESEL,H‐CNGJ‐CNG,H‐HCNGANDJ‐HCNGDUALFUELENGINEOPERATION




This section provides the effect of exhaust gasrecirculation(EGR)ontheperformanceofdiesel,Hongeoilmethylester(H)andJatrophaoilmethylester(J)togetherwith CNG/HCNG inducted in DF mode of operation for80%loads.TheengineisoperatedataconstantITof270BTDC and CR of 17.5 with mixing chamber venture 2having 6mm orifice in the inlet manifold. The injectornozzleopeningpressurewasmaintainedat230bar.TheEGRamountwasvariedfrom0to20%instepsof5%forallabovefuelcombinationsused.FortheoptimizedITof27oBTDCandCRof17.5, itwasobservedthatH/J‐HCNGresultedintohigherlevelsofNOxcomparedtoH‐CNGandJ‐CNG.Therefore,inordertoaddressacceptableemissionlevelsofNOx frombiodiesel fueledDFengineoperation,furtherexperimentalinvestigationswerecarriedoutwithonlyH‐HCNGwithdifferentEGRratesandwerecomparedwithbaselinedataofdiesel‐HCNGoperation.

4.3.1BrakeThermalEfficiency

DecreasedBTEwasobservedwhenEGRpercentagewasincreased as shown in Figure 12. BTE reducessignificantly due to predominant dilution effect of EGRleavingmore exhaust gases in the combustion chamber.Dilution, chemical and thermal effects of EGR in adieselengine plays an important role and governs thecombustion characteristics of dual fuel engine operation(Jie et al 2013, López and Gomez 2009). The inlettemperature drastically increases when the EGR isintroduced.ThedecreaseinBTEmaybeattributedtothedestruction of the fuel burning rate caused by thereduction of the air excess ratio. In case of dual fueloperation, use of exhaust gas recirculation is not muchappreciated because already part of was replaced bygaseousfuel.ThereforeuseofEGRcansignificantlyaffecttheperformancenegatively.Also the chemical effectdueto the active free radicals present in the exhaust gastaking part in pre‐ignition reactions tends to enhancecombustionnegativelyandcausesadropinBTE(Mahlaetal2010).

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40 SPEED:1500 RPM; IT:270bTDC; C.R: 17.5;HCNG FLOW RATE: 0.5kg/hr;IOP(Biodiesel):230 barCNG FLOW RATE: 0.5kg/hr; Number ofInjector holes:3;Hole diameter:3mm: IOP(Diesel):205 bar

Bra

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ienc

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Load, %

D+HCNG H+HCNG+ 5% EGR H+HCNG+ 10% EGR H+HCNG+ 15% EGR H+HCNG+ 20% EGR

Figure12:EffectofEGRonbrakethermalefficiency

Experimental investigations showed diesel‐HCNGoperationresultedinhigherBTEcomparedtoH‐HCNGatall EGR rates. However, at lower EGR rates, H‐HCNG

performednearlybettercompared to theoperationwithhigher EGR rates (10, 15 and 20%). Therefore, from theresults,itisobservedthattherewasmarginaldecrementin BTE with lower EGR, while a further increased EGRbeyond10%results intoamore intensedeteriorationofengine efficiency. This specific deterioration could beascribed primarily to the increased delay period of thefuelcombinationused,whichaffectstheheatreleaseratenegatively, especially during premixed and rapidcombustion phase. The BTE values for the H‐HCNG DFoperation at the EGRof 5, 10, 15%and20%were 24.1,23.5, 22.5 and 20.1 % respectively, compared with thevalueof25.8%fortheD‐HCNGoperation.

4.3.2SmokeOpacity

Figures13indicatethatsmokeopacitywashigherincaseof EGR induction compared to that without EGR forHCNG‐diesel, H‐CNG, H‐CNG, H‐HCNG and H‐HCNGcombinations.UseofEGRhasanegativeeffectonsmokeemissions. The main reason for increased smoke is thereductionofengineair/fuel ratio supplied to theengine.Practically gaseous fuel produces no smoke, while itcontributes to theoxidationof the soot formed fromthecombustionoftheliquidfuel.Experimentalinvestigationsshowed lower smoke levels evenwith20%EGRrate fordiesel‐HCNG operation compared to H‐HCNG at all EGRrates.However,atlowerEGRrate,theH‐HCNGoperationresulted in slightly lower smoke levels compared to theoperation with higher EGR rates (10, 15 and 20%).Therefore, fromtheresults, it isobservedthattherewasmarginaldecrementinBTEatlowerloadandEGR,whileafurther increase of the EGR beyond 10% results to ahighersmokeemission.Thisspecific incrementinsmokelevels at higher EGR for biodiesel‐HCNG combinationcouldbeascribedprimarilyduetocombinedeffectofre‐circulated exhaust gas and incomplete combustion ofbiodiesel due to heavier molecular structure, higherviscosity and density, reduced air‐fuel ratio. Reducedcombustion temperature and soot oxidation is alsoresponsible for this observed trend. However, addinghydrogen toCNGslightlyreducessmokeemissionsofH‐HCNG dual fuel engine. This could be due to bettercombustion of the fuel combination and faster burningrates associated with clean burning characteristics ofhydrogencompared toCNG.Thehigherburningvelocityand flame temperature of HCNG leads to more betterburningcomparedtoCNGduringthedualfueloperation.ButuseofEGRmayreducenitricoxidebutmayincreaseparticulate emissions at high loads. Hence, there istradeoff between nitric oxide and smoke emission. It isessentialtouseaparticulatetraptoreducetheamountofunburned particulates with EGR operation, especiallywhen using vegetable oils. The smoke levels for the H‐HCNG DF operation at the EGR of 5, 10, 15% and 20%were67,75,74and78HSUrespectively,comparedwiththevalueof62HSUfortheD‐HCNGoperation.




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SPEED:1500 RPM; IT:270bTDC; C.R: 17.5;HCNG FLOW RATE: 0.5kg/hr;IOP(Biodiesel):230 barCNG FLOW RATE: 0.5kg/hr; Number ofInjector holes:3;Hole diameter:3mm: IOP(Diesel):205 bar

Smok

e op

acity

, HSU

Load, %

Figure13:EffectofEGRonsmokeopacity

4.3.3HCandCOemissions

Figure 14 and 15 shows the variation of HC and COemissions with EGR induction. As EGR percentageincreased, the oxygen concentration in the charge andtemperatureofcombustionproductsbothdecreased.Alsotherewaslesspercentageofoxygenduetoreplacementofair by HCNG gaseous fuel. All these factors negativelyaffect the combustion of fuel combination used. Theincrease in HC and CO emission was observed with theincrease in EGR percentage. This was due to poorcombustion resulting inside the combustion chamber.With the EGR introduction, the combustion degradationlowersthecombustiontemperatureandoxygen(Mahlaetal 2010). Decreased air‐fuel ratio associated withincreased EGR percentage and reduced combustiontemperatureresultedinincompletefuelcombustion.Itisobservedthatdiesel‐HCNGoperationresultedinlowerHCand CO emission levels compared to H‐HCNG operation.The decreased air–fuel ratio associated with higherviscosity, EGR induction and reduced combustiontemperatureresultingincompletefuelcombustionarethemainreasons forsuchobserved trend.Alsovariations inthe specificheat of gasnegatively affect the combustion.Therefore, insufficient oxygen is available for bettercombustion and the lower combustion temperatureprevailing in the engine cylinder leads to comparativelyhigherHCandCOemissions.The smoke levels for theH‐HCNG dual fuel operation at the EGR of 5, 10, 15% and20%were66,77,76,and80ppmrespectively,comparedwith the value of 61 ppm for the D‐HCNG operation.Similarly carbon monoxide levels for the H‐HCNG DFoperation at the EGR were of 0.15, 0.21, 0.2 % and0.205%and0.11%respectively,comparedwiththevalueof61ppmfortheD‐HCNGoperation.

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Load, %

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rbon

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Figure14:EffectofEGRonHCemissions

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SPEED:1500 RPM; IT:270bTDC C.R: 17.5; IOP(Biodiesel):230 barHCNG flow rate:0.5kg/hr CNG FLOW RATE: 0.5kg/hr Number ofInjector holes:3Hole diameter:3mm IOP(Diesel):205 bar

Figure15:EffectofEGRonCOemissions

4.3.4NOxemissions

Figure 16 shows reduction in NOx emission levels withincreased EGR percentage. EGR induction decreases thecombustion temperature of products due to higherspecificheat capacityaswell as oxygenconcentrationofthe charge and hence lowerNOxwas observed (Jie et al2013, Banapurmath et al 2009). Diesel‐HCNG DFoperation resulted in higher NOx levels compared to H‐HCNGoperation.AtlowerEGRratesandwithsubstitutionofHCNG,theheatreleaserateresultedinhigheremissionofNOxlevels.HoweverathigherEGRrates,pilotquantitybeinginjectedsmallerduringdualfueloperation,reducesthe flame speed and flame cannot travel far enough toburn the entire mixture and leads to lesser NOx levels.EGR appears to reduce the adiabatic flame temperaturethereby combustion temperature, resulting reduced NOxemissionlevels.ThereforeNOxconcentrationdecreasesasCI engine inlet air flow is diluted at a constant fuellingrate. The NOx emission levels for the H‐HCNG DFoperation at the EGR of 5, 10, 15% and 20%were 736,




806, 850 and 945 ppm respectively, comparedwith thevalueof678ppmfortheD‐HCNGoperation..

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SPEED:1500 RPM; IT:270bTDC, C.R: 17.5; IOP(Biodiesel):230 bar, HCNG flow rate:0.5kg/hr CNG FLOW RATE: 0.5kg/hr, Number ofInjector holes:3Hole diameter:3mm, IOP(Diesel):205 bar

Figure16:EffectofEGRonNOxemissions

4.4COMBUSTIONCHARACTERISTICS

Figure 17 to 20 shows the effect of variation of brakepower on ignition delay, cylinder peak pressure,combustion duration, In‐cylinder pressure variation andheat release rate fordifferentmodesof operation. InDFoperationignitiondelaymainlydependsofonthenatureof primary inducted fuel and its concentration in themixture that is admitted in to the engine cylinder. Thelengthof the ignitiondelay alsodependson themixturetemperature during compression, energy release duringpre‐ignition, heat transfer to the surroundings and thecontributionofresidualgases(Selimetal2008,Mahlaetal2010).

The variations of ignition delay for Diesel‐HCNG and H‐HCNG operation at different EGR rates with respect todifferent loads is presented in Figure 17. The ignitiondelay is calculated based on the static injection timingusing pressure crank angle history for 100 cycles. Theignitiondelay forH‐HCNGatdifferentEGRratesshowedlonger ignitiondelay compared todiesel‐HCNGdual fueloperation. However, diesel‐HCNG, showed shorterignitiondelaysthanHongebiodiesel‐HCNGDFoperation.IgnitiondelayincreasedwithincreaseinEGRratescausedbymixturedilutionandslowburningratesobservedwithfuelcombinations.Itmayalsobeduetoslowmixingratewith gaseous fuel operation as they have higher octanenumberandselfignitiontemperature.However,H‐HCNGoperation with 5 percent EGT showed comparativelylower ignition delay due to burning velocity of HCNGbeingcomparativelyhigherduringcombustioncomparedtotheDFoperationathigherEGRrates.IgnitiondelayinH‐HCNGDFmodeofoperationwith5, 10,15%and20%EGRrateswerefoundtobe17.8,18.8,19.2and20.56deg.CAcomparedto15.2deg.CAfordiesel‐HCNGoperationat80%loadrespectively.

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A b

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Figure17:VariationofignitiondelaywithECRflowrate

VariationsofcombustiondurationforDiesel‐HCNGandH‐HCNG operation at different EGR rates with respect todifferent loads are presented in Figure 18. Thecombustion duration was calculated based on theduration between the start of combustion and 90%cumulative heat release. The combustion duration wasincreasedwithincreaseintheloadforalltheselectedEGRrates.DuringH‐HCNGoperation,thecombustiondurationincreases with increase in the EGR rates. This could bedue to their slow mixing rate caused by the mixturedilation.Basicallygaseous fuelsarehavinghigheroctanenumber and self ignition temperature causes slowercombustion. Improper air–fuel mixing observed withbiodieselsalongwith longer time forgasburningresultsin incomplete combustion. However, H‐HCNG with 5%EGR showed lower combustion duration as hydrogenpresent in CNG leads to faster combustion and mixturedilution at lower EGR rate is less dominating. Higherflamevelocity,highercalorificvalueandfastburningrateof hydrogen in CNG (HCNG) causes the combustionduration to decrease while the heat release rate andexhaustNOxproductionincreasewithhydrogenaddition.IgnitiondelayinH‐HCNGDFmodeofoperationwith5,10,15% and 20%EGR rateswere found to be 38.1, 42.02,45.0and46,045 deg. CA compared to 38.12 deg. CA fordiesel‐HCNGoperationat80%loadrespectively.

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Figure18:VariationofcombustiondurationwithEGRflowrate

Figure 19 shows peak pressure variation with differentEGR rates for Diesel‐HCNG and H‐HCNG operation athigher loads of engine operation. The peak pressuredependsonthecombustionrateandthathowmuchfuelis taking part in rapid combustion period. Theuncontrolled combustion phase is governed by theignition delay period and by the mixture preparation.Higher cylinder pressure obtained with diesel‐HCNGcompared toH‐HCNGoperation because the higher heatreleaseratewasobservedduringrapidcombustionphasefor diesel‐HCNGoperation. Lower cylinder pressurewasobservedwithincreaseintheEGRrates(morethan10%EGR) forH‐HCNGoperation. It couldbe attributed to anincreaseintheignitiondelayperiodoffuelcombinationsathigherEGRrates.HenceitisseenthatignitiondelayoftheH‐HCNGwassignificantly increasedwithan increaseof EGR rates. This increase in ignition delay period ismainly due to impropermixing of the fuel combination,mixture dilution and lowering of compressiontemperature.However, apartof theairwas replacedbygaseous fuels which in turn changes the chargemixturespecificheatreducedtheoxygenconcentrationinthefuelmixture and slower burning rate of fuel combination athigher EGR rates is also responsible for this trend[Papagiannakisetal(2007)].

Combined effect of poor mixture preparation, longerignition delay, lower calorific value and lower adiabaticflame temperature and slow burning nature of the H‐HCNG compared to diesel‐HCNG resulted in lower peakpressureandmaximumrateofpressurerise.Asthedieseland HCNG are having higher calorific value and longerignition delay compared toH‐HCNG combination resultsintohigherpeakpressure(Figure.19).Combinedeffectoflonger ignition delay, lower calorific value and adiabaticflame temperature and slow burning nature of the H‐HCNG compared to diesel‐HCNG resulted in lower peakpressureandmaximumrateofpressurerise.Poormixingof biodiesel with air due to their higher viscosity andlower volatility for biodiesel‐HCNG operation at all EGRrates are also responsible for this observed trend.However, the higher flame velocity, calorific value andslightly increased ignition delay of HCNG during DFoperationat5%EGRrate leads to increasedcombustionduring rapid combustion phase compared to the DFoperationat10,15and20%EGR.However,athigherEGRrates,thepresenceofexhaustgasinthefuelcombinationleadstolowercylinderpressure.HenceH‐HCNGwith5%EGRresultsintohigherpeakpressureandmaximumrateof pressure rise. Accordingly higher peak pressure andheat release rate were observed for biodiesel‐HCNGoperation due to the higher flame velocity and fastburning of hydrogen content in presence of CNG. PeakpressureinH‐HCNGDFmodeofoperationwith5,10,15%and20%EGRrateswerefoundtobe61,58,56and54barcompared to 66bar for diesel‐HCNG operation at 80%loadrespectively.

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SPEED:1500 RPM; IT:270bTDC, C.R: 17.5; IOP(Biodiesel):230 bar, HCNG flow rate:0.5kg/hr CNG FLOW RATE: 0.5kg/hr, Number ofInjector holes:3Hole diameter:3mm, IOP(Diesel):205 bar

Figure19:VariationofpeakpressurewithECRflowrate

Figure20presentsheatreleaseratefordiesel‐HCNGandH‐HCNGDFmodeof operation at 80% load and variousEGRrates.H‐HCNGoperationatallEGRratesledtolesserfuel being prepared for rapid combustion after theignition delay. More burning occurring in the diffusionphase rather than in thepremixedphasewithbiodiesel‐HCNGwasobservedatallEGRrates.SignificantlyhighercombustionratesduringthelaterstageswithH‐HCNGledto higher exhaust temperatures and lower thermalefficiency.However,hydrogenadditiontoCNGduringH‐HCNG operation with 5% EGR showed improvement inheat release rate compared to dual fuel operation at 10,15and20%EGRrates.

Figure20:Heatreleaseratediagramat80%loadanddifferentECRflowrate

5. CONCLUSIONS

From theexhaustive studyon thedual fuel engineusingmethyl estersofHonge, Jatrophaoils andHCNGgaseousfuelthefollowingconclusionsweremade.

1. Dual fuel engines significantly reduce smoke,particulate and NOx emissions when compared totheircounterpartbiodieseloperateddieselengines.

2. BiodieselsofmethylestersofHongeandJatrophaoilcanprovidepartial substitution to dieselwhenused




in dual fuel mode along with CNG, HCNG gaseousfuels.

3. HCNG operated dual fuel engines besides usingrenewable fuels of methyl esters of Honge andJatropha oilcan provide partial substitution torenewable fuels and leads to lower emissions of HCandCOemissionswhencomparedtoCNGoperation.

4. AddinghydrogentoCNGsignificantlyimprovesbrakethermal efficiency and reduces emissions of smoke,HC,COemissionsexceptNOx.AtanIT27oBTDCandCRof17.5,theBTEfortheH‐HCNGDFoperationwithEGRof5,10,15%and20%were24.1,23.5,22.5and20.1% respectively, compared to 25.8% for the D‐HCNGoperation.HCNGenablesengineoperationwithleanerfuelairmixture,resultingintoloweremissionsofHCandCOandhigherNOxemissions.

5. Increased EGR rates increases smoke, HC and COemissions and the brake thermal efficiency reduceswhileNOxreducessignificantly.

6. Higher flame speed of hydrogen in HCNG results inreduced ignition delay, combustion duration whilethe peak pressure and heat release rates bothincreased.

Ontheclosure,itcanbeconcludedthathigherpercentageof hydrogen in CNG can affect the engine performancefavorably but limit the engine operation with severeknockespeciallyathigher loadsofengineoperation.Theproblem of knocking can be effectively addressed byinjecting HCNG into the intake manifold when the inletvalve opens to avoid pre‐ignition. Advancing the IT ofpilot biodiesel fuel and increased CR can improve theperformanceofsuchDFengines.

Current research work in diesel engines using gaseousfuels(LPG,CNGandHCNG)insinglemodeorwithliquidfuels in DF mode make use of port or manifold gasinjection systems and facilitate to overcome thedrawbacksofthebackfireandpre‐ignitionwhichisproneto occur in carbureted engines. This can be eliminatedwith proper gas injection timing using low pressure gasinjector operated by a suitable Electronic Control Unit(ECU). The continued work by the authors will addressthe drawbacks of gas induction system using venturereplacedbyanECU forCNG/HCNG injection indual fuelmodesofengineoperation.

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performance, combustion and emission characteristics … · to increased performance. therefore,...

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