factors affecting biodiesel engine performance and exhaust emissions – part ii: experimental study

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Energy xxx (2014) 1e18

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Energy

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Factors affecting biodiesel engine performance and exhaustemissions e Part II: Experimental study

James Pullen, Khizer Saeed*

Low Carbon Energy Research Group, School of Computing, Engineering and Mathematics, University of Brighton, Lewes Road, Brighton BN2 4GJ, UnitedKingdom

a r t i c l e i n f o

Article history:Received 10 July 2012Received in revised form19 November 2013Accepted 9 February 2014Available online xxx

Keywords:BiodieselFAME (fatty acid Methyl esters)FAEE (fatty acid ethyl ester)TransesterificationCatalyst

* Corresponding author. Tel.: þ44 1273 642304; faxE-mail addresses: [email protected], khizers

http://dx.doi.org/10.1016/j.energy.2014.02.0340360-5442/� 2014 Published by Elsevier Ltd.

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a b s t r a c t

Effects of variation in biodiesel fuel properties on engine performance are not completely understoodand there is a need for further research. The effects of altering the feedstock oil and alcohol type used inbiodiesel production on engine performance (power, economy) and exhaust emissions (O2, CO (carbonmonoxide), CO2, NO) were investigated. Nine fatty acid methyl esters (Rapeseed, Sunflower, Palm, Corn,Soybean, Olive, Used Cooking Oil, Lard, Beef Tallow) and five fatty acid ethyl esters (Rapeseed, Soybean,Palm, Lard, Tallow) were studied. Compared to petro-diesel, biodiesel showed increased O2 emissionsand 43% lower CO emissions. No differences in CO2 levels between any of the fuels observed. NOemissions for different biodiesel types were higher, similar, or lower than petro-diesel. Increased bio-diesel unsaturation level correlated with higher NO levels. Oxidation of biodiesel resulted in relativelyhigher exhaust O2 and lower NO and CO emissions. Contaminants (vegetable oil, antioxidant, water) hadlittle effects on engine performance. Methyl esters performed no differently to ethyl esters, though ethylesters showed improved cold flow properties. No differences in performance (power, economy) wereobserved between any of the biodiesels tested. Fuel consumption increased w10% when running onbiodiesel and maximum engine power was slightly reduced, compared to petro-diesel.

� 2014 Published by Elsevier Ltd.

1. Introduction

Increasing awareness of the depletion of fossil fuel resources aswell as their negative environmental impacts has triggered interestin the potential benefits of ‘biofuels’ such as biodiesel, which is analternative fuel for diesel engines. Biodiesel is a drop-in replace-ment for petro-diesel that is biodegradable, less toxic and canreduce harmful tailpipe combustion emissions (CO2, CO (carbonmonoxide), UHC (unburned hydrocarbon) and PM (particulatematter)) relative to petro-diesel [1]. Biodiesel is miscible withpetro-diesel, compatible with fuel delivery infrastructure, has highflashpoint for safer handling, and can be used in standard dieselengines requiring no engine modification. It is an oxygenated,renewable fuel, that compared to petro-diesel usually has higherCetane number, and contains no sulphur or aromatic compounds[2]. Biodiesel also offers improved lubricity over certain low-sulphur petro-diesels [3] and thus can help reduce wear of en-gine components [4]. Running diesel-engine equipment on bio-diesel can be beneficial in terms of environmental impact andenergy security. In Europe, biodiesel should meet the fuel quality

: þ44 1273 [email protected] (K. Saeed).

J, Saeed K, Factors affectinx.doi.org/10.1016/j.energy.201

standa rd known as EN 14214 [5] to be approved for use in dieselengines; limits are imposed on a range of important fuel properties,including: purity, combustion properties and fuel stability.

Biodiesel is mainly produced mainly from plant-seed oils:Rapeseed in Europe, Soybean in USA, via the alkaline-basecatalysed chemical reaction known as transesterification, whichconverts TAG (triglyceride) and alcohol into FAAE (fatty acid alkylesters), forming glycerol as co-product. Using methanol (CH3OH)yields FAME (fatty acid methyl esters) or biodiesel. Trans-esterification is a sequence of three consecutive and reversible re-actions in which DG (di-acylgylcerols) and MG (mono-acylglycerols) are formed as intermediates, before finally liberatingglycerol. The reaction proceeds stepwise via DG and MG, with afatty acid alkyl ester being formed in each step [6,7]. In practice, thestarting oil/fat is heated and vigorously mixed with a solution ofbase-catalyst and methanol usually for around 1 h, under atmo-sphere and below methanol boiling point (65 �C). The product isallowed to settle before it is purified by decanting glycerol and finalwashing [8]. The transesterification reaction must be sufficientlycomplete and the FAME purified in order to meet fuel qualitystandards such as EN 14214 [5]. Previous studies have investigatedand proposed optimum conditions for based-catalysed trans-esterification of : 6:1 methanol:oil molar ratio, 1 %m/m NaOH

g biodiesel engine performance and exhaust emissions e Part II:4.02.034

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J. Pullen, K. Saeed / Energy xxx (2014) 1e182

catalyst (by weight of oil), 60 �C reaction temperature, 60 min re-action time, and >600 rpm stir speed [6,9-17]. For CH3ONa catalyst1.3%m/m was optimum [10]. These conditions are consistent withthose reported in The Biodiesel Handbook [4]. Ethanol-basedtransesterification was investigated by the present author [18]and optimum conditions found were, 5:1 ethanol:oil molar ratio,60 �C reaction temperature at 600 rpm stirring speed, using 1.7%m/m C2H5ONa (sodium ethoxide) catalyst by weight of oil.

The main purpose of transesterification is primarily to reduceviscosity of TAG to a value much closer to that of conventionalpetro-diesel. Without converting TAG to FAEE (fatty acid ethylester), the high viscosity of TAG results in poor engine operationalperformance and potential failure [19]. Though some engines canbe designed or modified to use un-transesterified TAG, the vastmajority of engines require lower-viscosity fuel [4].

Biodiesel fuel properties differ in certain crucial respectscompared to those of petro-diesel. Different physical properties ofbiodiesel alter the injected fuel-spray characteristics [20]. Higherbulk modulus and viscosity lead to injection and combustionadvance in pump-line-nozzle fuel systems [21,22], so that CN (ce-tane number) alone does not determine the crank angle ignitionpoint. Biodiesel is also more dense (by w6%), and is oxygenated(w11% wt) thus has proportionally lower energy content [23]. Thelevel of biodiesel unsaturation, which depends on the original fat/oil feedstock, is usually significantly greater than petro-diesel sothat biodiesel tends to be less resistant to autoxidation than pe-troleum diesel [24]. Biodiesel containing relatively higher levels ofunsaturated and short chain length fatty acid alkyl esters has lowerCN, and increased density. Biodiesel CN is usually higher than thatof petro-diesel but varies widely according to composition [25].Biodiesel contains usually no sulphur therefore produces no sul-phate emissions [26], and offers improved lubricity over certainlow-sulphur petro-diesels [25] which reduces engine componentfriction.

Effects of biodiesel on exhaust emissions levels are difficult toconclude generally, since levels reported depend on study param-eters (engine type, measurement technique, operating conditions,biodiesel feedstock/quality etc.) [20]. Contrasting reports areevident in the literature, however dominant trends have beenfound in most cases [20,27]. Many studies of biodiesel combustiondo not consider the effect of fuel composition variation which is adrawback [22], so that this would appear to be a key aspect in needof further study. Limited engine study data and conflicting reportsexist, for example, which compare the performance of methyl andethyl esters produced from the same feedstock, so that the effect ofalcohol type used in biodiesel production on engine performanceand emissions remains uncertain. Hence there is a need for furtherstudy of the effects of biodiesel fuel properties variation on engineperformance and exhaust emissions.

Variability in biodiesel properties can effects the emissions ofNOx (oxides of nitrogen) and oxides of Carbon (CO and CO2) from itsuse in engines. Part I of this study [28] presented a detailed reviewof literature investigating the effects of biodiesel on engine per-formance and emissions. The main points relevant to the presentexperimental study are discussed below.

NO (nitric oxide) is predominantly produced inside the dieselengine cylinder along with small amounts of NO2(nitrogen dioxide)[29]. Total oxides of nitrogen (NO plus NO2) are typically measuredin exhaust gases with a chemiluminescence analyser [29]. How-ever, measurement of tailpipe emissions of NO alone is a goodapproximation e.g. using commercially available models of garagevehicle exhaust gas analyser [30].

Usually an increase in NOx is expected when running biodiesel[27,31], although this is not unanimously reported in the literature.The fundamental reasons for NOx emissions variation when

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switching to biodiesel are not all well understood. Excessive igni-tion delay (low CN) and poor combustion performance, or changesin fuel density and bulk modulus were proposed in the literature aspossible causes of high NOx.

Experimental studies had shown that NOx emissions vary ac-cording to biodiesel composition. The ‘degree of biodiesel satura-tion’ appears to affect NOx emissions. More saturated, longer chainlength esters increase biodiesel CN and tend to decrease NOx [26].NOx emissions increased with the number of double bonds in thefatty acid chain. While the experimental studies have shown theeffect of biodiesel molecular structure on NOx emissions, thefundamental reason this occurred was ‘open for speculation’.

It is also observed that injection and combustion advance, due tobiodiesel fuel properties, occurs in pump-line-nozzle fuel systemscausing significant increases in NOx emissions [21] [22]. However,trends reported in different studies still vary significantly accordingto the engine type and operating conditions. Other factors havebeen reported to affect biodiesel NOx emissions, but appear not tobe well evidenced in the literature. Increased flame temperaturedue to reduced soot levels and reduced soot radiative heat transfermay be related to NOx increases [20]. Biodiesel unsaturation,oxidative degradation of biodiesel, relative stoichiometry, heatingvalue, fuel spray characteristics, biodiesel type (methyl or ethylester), increased oxygen availability and levels of aromatics mayalso be factors influencing NOx emissions due to their direct effectson flame temperature [32]. There are some reports that higherresidual glycerine levels [26] and antioxidant additive levels [33,34]can also lower NOx. Understanding of the mechanisms which in-fluence biodiesel NOx is incomplete and there is a need for furtherresearch. The effect of biodiesel on NOx formation in common railengines, where injection advance is not an issue, also appears to bea key area for research.

The majority of relevant studies report a decrease in CO emis-sions for biodiesel, although levels rangewidely andwith operatingconditions. Reductions in CO for pure biodiesel reported in a reviewof relevant studies [20] included: 50%, 28e37%, 14%, and 22%, anddependent on operating conditions. The EPA [31] showed nearly a50% reduction in CO emissions with pure biodiesel. Factorsexplaining CO reduction observed with biodiesel [20] include: i)increased fuel oxygen content, which promotes more completecombustion, ii) increased biodiesel CN, reducing the probability offuel-rich zones formation and iii) advanced injection and com-bustion timing increasing combustion residence time. Severalstudies have concluded biodiesel origin/feedstock was a factoraffecting CO emissions. The EPA [31] showed greater CO reductionsfor biodiesel made from [more saturated] animal fats, as did severalother researches. One study showed lower CO as fatty acid chainlength increased. However other studies reported no differences.Oxidation of biodiesel was also reported to lower CO, linked tohigher CN of oxidised biodiesel [35].

The literature survey by Xue et al. [27] observed that a slight lossof maximum rated-engine power was usually reported for bio-diesel, due to reduced volumetric energy content, though this po-wer loss was often less than expected. Some studies even reportedno significant differences between pure biodiesel and petro-diesel,as well as synergistic blend effects with petro-diesel. The review ofstudies carried out by Lapuerta et al. [20], concluded that comparedto petro-diesel, biodiesel does not cause any loss of power unlessmaximum engine power is demanded, since at lower engine loadsan increased quantity of fuel is injected by the fuel system tocompensate for the reduced calorific value of biodiesel. Moststudies reported some decrease in rated power. However, thisdecrease was often lower than the reduction in biodiesel volu-metric heating value, compared to petro-diesel. Rated poweroutput reductions of w8% (the approximate loss of volumetric


J. Pullen, K. Saeed / Energy xxx (2014) 1e18 3

heating value relative to petro-diesel) would be expected for bio-diesel, though literature study results vary.

BSFC (brake specific fuel consumption) for biodiesel increasesaccording to the relative reduction in calorific value of the fuel on amass basis (MJ/kg) i.e. the lower calorific value of biodiesel iscompensated by higher fuel consumption. An indicator of the lossof heating value and thus the expected fuel consumption increase isthe oxygen content in the fuel. Good correlation between BSFCincrease with oxygen content was reported in the literature surveyby Lapuerta et al. [20]. The majority of studies surveyed [20]confirmed that BSFC increase was similar to the loss of heatingvalue (usually around 14% on a mass basis for pure biodiesel),although reported BSFC increases vary considerably. Oxidation ofbiodiesel reduces calorific value and BSFC as shown by Monyemand Van Gerpen [35].

Biodiesel fuel properties can degrade by autoxidation [24].Products of degradation are considered undesirable and are ex-pected to increase: levels of insoluble (polymeric) contaminants,fuel viscosity, acid value, CN, and reduce the fuel heating value.Studies have shown TBHQ (Tertiary-butylhydroquinone) is one ofthemost effective antioxidant additives that can be used to increasebiodiesel’s resistance to autoxidation. However, limited investiga-tion has been undertaken of the effects of such additives on com-bustion and emissions. Likewise, the effects on engine performanceand emissions of the fuel property changes that occur after bio-diesel has been significantly oxidised are relatively unconfirmed.

Ryu [36] investigated the effects of antioxidants on biodieselengine performance and exhaust emissions. TBHQ was the mosteffective antioxidant tested, which achieved the greatestimprovement in resistance to oxidation measured by Rancimat. Inengine tests the effect of increasing antioxidant dose was measuredon BSFC, smoke levels, UHC, and NOx emissions levels. It wasconcluded none of these parameters were influenced by addition ofantioxidant. Few studies investigating impacts of antioxidantaddition on biodiesel combustion performance have been carriedout so that further study is warranted.

Limited research has so far been conducted to determine theeffects of oxidised biodiesel on engine performance and emissions.One of few studies which has investigated these effects, was thatcarried out byMonyem and Van Gerpen [35]. SME (soybean methylester) was compared with No. 2 petro-diesel fuel and a 20% (B20)blend thereof. Oxidised and un-oxidised SME were also compared.Elevated Peroxide Value indicated significant oxidation. Resultsindicated higher BSFC for biodiesel (g/kWhr), where un-oxidisedbiodiesel BSFC was 13.8% higher than petro-diesel, and oxidisedbiodiesel BSFC was 15.1% higher. This was explained by the lowerenergy content (MJ/kg) of the respective biodiesel fuels, whereoxidation relatively reduced the energy content. Emissions resultsshowed that oxidised biodiesel produced significantly loweremissions of CO and HC, compared to the un-oxidised biodiesel. Thecause was reported possibly attributable to the increased oxygencontent of the biodiesels and higher CN. Smoke levels were alsoreduced when the engine was fuelled with biodiesel, where oxi-dised biodiesel showed the greatest reduction. However, noobvious difference in NOx was observed between oxidised and un-oxidised biodiesel. Higher bulk modulus of biodiesel causedadvanced injection timing which was at least partially responsiblefor the higher biodiesel NOx. Un-oxidised biodiesel CN (51.1) wasrelatively higher than the petro-diesel CN (47.4), and crucially theoxidised biodiesel CN was much higher (72.7). Usually, higher CN isassociated with lower NOx, as discussed in Part I [28]. Results [35]suggested biodiesel oxidation can reduce emissions of CO, HC andsmoke, with little effect on NOx levels.

Graboski et al. [26] in contrast found that oxidised biodiesel hadno significant effect on emissions. A different study carried out by


Yamane et al. [37] observed different trends, where un-oxidisedand oxidised SME biodiesels were tested in a single cylinder dieselengine. Results showed as the degree of oxidation of the biodieseltest fuel increased, ignition delay was decreased (CN wasincreased). Measurements of in-cylinder conditions showed earlierstart times of heat release (shorter ignition times) for the oxidisedfuel. Fuel oxidation reduced the amount of pre-mixture formation,and reduced the rate of heat release during the initial combustionphase.

However, NOx emissions were clearly higher for the heavilyoxidised biodiesel. Achieving high temperature earlier should alsopromote combustion of CO, and this was also clearly observed.Oxidised biodiesel showed lower CO emissions, slightly lower BSFC,and lower smoke density. Results suggested that despite its higherviscosity, oxidised biodiesel can improve combustion performance(except for a measured NOx increase); suggesting that the chemicalproperties of the fuel (presence of hydroperoxides and organicacids), surpasses the negative effects due to high viscosity and poorspray formation.

Relatively few data are available on the impact of biodieseloxidation on the emissions fromdiesel engines [19]. Further studiesare needed to evaluate the effects of biodiesel oxidation, whichcauses variation in engine performance and emissions.

2. Experimental objectives

In light of the review of literature review in Part I and abovediscussion, limited study data and conflicting reports existregarding the effect of several fuel quality parameters, which arenot well understood and in need of further research. Experimentalobjectives were to examine the effect of the following parameterson engine performance and emissions:

1. Contaminants: residual triglyceride and antioxidant (TBHQ)levels. The effect of water contamination also appears to beunreported in the literature. Biodiesel samples (otherwiseidentical) were deliberately contaminated and compared.

2. Oxidised biodiesel fuel. A biodiesel sample was deliberatelyoxidised by prolonged exposure to elevated temperature and airbubbling. The altered fuel properties were measured and theoxidised fuel was compared to the original un-oxidised sample.

3. The alcohol type used in biodiesel production (ethanol versusmethanol). Methyl and ethyl esters were produced from thesame oil/fat feedstock and compared.

4. The oil/fat feedstock type and hence the biodiesel fatty acidprofile and degree of unsaturation, relative to the other fuelproperty changes (1. to 3.). A range of biodiesel test fuels wereprepared for comparison using different vegetable oils and an-imal fats.

3. Experimental procedure

3.1. Manufacture of test fuels

A consistent base-catalysed transesterification and purificationmethod was used for production of each BTF (biodiesel test fuel). Abespoke 10 L batch size ‘bio-refinery’ (see Fig.1) was constructed bythe author for this purpose, consisting of the following componentparts:

Reactor (20-litre stainless steel); hotplate for indirectly heatingreactants; circulation pump for reactant mixing (Model: Marco 164060-UP6); methoxide (alcohol/catalyst mixture) dosing system;decanting vessels (HDPE tanks); stainless steel and Nalgene� clear


Fig. 1. ‘Bio-refinery’ apparatus used for preparation of biodiesel test fuels (10 L batchsize).


PVC pipe work; ball valves; dry-wash system containing PurolitePD206 resin (5 L); particle trap filters (>10micron); Bund/spill tray.

A bespoke system, offered several advantages: compact size,improved safety and cost, e.g. the hotplate heating element couldnot be directly exposed to inflammable vapours, and the reactor lidretained toxic vapours. The following procedure was used to makeeach BTF:

1. The oil/fat was weighed (9 kg � 0.01) and heated (60 �C).2. Catalyst and alcohol were weighed using a balance (5 kg) ac-

curate to ( � 0.1 g) and were mixed in a Winchester bottle usinga magnetic stirrer.

3. Reactants (oil, alcohol, catalyst) were mixed using pump circu-lation for 60 min.

4. Glycerol was decanted from the product; after w30 min andagain after settling overnight.

5. Crude biodiesel was ‘dry-washed’ by filtering through Puroliteresin until pH neutral; adsorbing residual contaminants, asperformed by Berrios and Skelton [8].

6. Dry-washed esters were further washed with approx. twice thevolume of warm tap water, removing any residue impurities e.g.excess alcohol.

7. Effluent water was decanted and esters were dried by heating(80 �C) and application of a fan over the fuel circulating in thereactor e evaporating residual moisture.

8. Water content was reduced (<500 ppm) and checked using aKFC (Karl Fischer Coulometer), supplied by Metrohm Ltd.

9. Biodiesel was fine filtered (10 micron) before being stored in aHDPE container.

Seven different FAMEs were prepared from vegetable oils:Rapeseed, Soybean, Corn, Olive, Sunflower, Palm, UCO (UsedCooking Oil), and two from animal fats: Lard, Beef Tallow. Vegetableoils/fats were purchased from a local catering supplier. UCO wasobtained from a local biodiesel manufacturer.

Materials used for FAME production were: sodium methoxide(CH3ONa) solution 30 wt % pure in methanol (Acros Organics), andanalytical grade Methanol (>99.5% purity), purchased from FisherScientific Ltd. Reaction conditions were: 1.35%m/m CH3ONa cata-lyst (by weight of oil), 6:1 methanol:oil molar ratio, 60 �C reactiontemperature, 60 min reaction time, 20 L/min circulating pumpflow-rate.

Five ethyl ester (FAEE) fuels were also prepared respectivelyfrom Rapeseed, Soybean, Palm, Lard, and Tallow. The catalystused was sodium ethoxide (C2H5ONa) powder (>96% purity, Acros


Organics). High purity bio-ethanol (1942 ppm water content) wasused, obtained from a local biodiesel manufacturer. Reaction con-ditions were: 1.7%m/m C2H5ONa catalyst, 5:1 ethanol:oil molarratio, 60 �C reaction temperature, 60 min reaction time, 20 L/mincirculating pump flow-rate. It was necessary to mix warm water(w10%vol) at stage 4 above, in order to trigger phase separation ofglycerol.

For each BTF, 10 L was prepared. Except Rapeseed FAME where40 L was made, homogenised and split into 6 aliquots: (1 � 10 L,BTF#1) and (5 � 6 L, BTF#2-6). BTF#1 was left pure. BTF#2 wascontaminated with Rapeseed oil (20%vol). BTF#3 was contami-nated with TBHQ (5000 mg/kg), as was BTF#4 (10,000 mg/kg).TBHQ was weighed and dissolved in biodiesel by stirring. BTF#5was deliberately oxidised by prolonged exposure to air and heat;achieved by circulation of the sample in the bio-refinery reactor at80 �C and aeration using an aquarium air pump and a fan for around12 h (3 exposure times of 4 h on consecutive days). BTF#6 wascontaminated with water (w4000 mg/kg) and water contentchecked using a KFC.

3.2. Measurement of biodiesel properties

Fuel properties were measured according to EN 14214 [5] testmethods. Ester content, fatty acid composition; linolenic acidcontent and content of FAME with �4 double bonds [%m/m] weremeasured by GC (gas chromatography) standard test method (BSEN 14103, [38]) using a DB-WAX capillary column coated with apolyethylene glycol stationary phase of length 30 m, internaldiameter 0.32 mm, film thickness 0.25 mm. OS (oxidation stability)was measured by the Rancimat method [h], (method BS EN 14112,[39]) using an 873 Biodiesel Rancimat instrument, supplied byMetrohm Ltd., Herisau/Switzerland. Kinematic Viscosity at 40 �Cwas measured by glass capillary viscometer [mm2/s], (method ENISO 3104, [40]) in a temperature controlled water-bath: TCB-7 MkII, Poulten Selfe & Lee Ltd. Acid Value was measured by titrationwith potassium hydroxide solution [mg KOH/g], (EN 14104, [41])using an 809 Titrando, auto-titrator, supplied by Metrohm Ltd.,Herisau/Switzerland. Iodine value was measured by titration usingsodium thiosulfate solution [g I2/100 g], (EN 14111, [42]) 809Titrando, auto-titrator. Density at 15 �C was measured by glasshydrometer, (EN ISO 3675, [43]), Fisher Scientific Ltd. Flash point,by closed-cup flash point test [�C], (EN ISO 3679, [44]), using aSetaflash series 3 closed cup tester, Stanhope-Seta Ltd. Watercontent, by titration with a KFC [mg/kg], (method EN ISO 12937,[45]), supplied by Metrohm Ltd., Herisau/Switzerland. Cold flowproperties, cloud point (method BS ISO 3015) and pour point (BSISO3016), were recorded by observing the temperature of samplesheld in a stirred glass jar placed in a salted-ice bath (�12 �C). Un-fortunately due to resource limitations it was not possible tomeasure other important fuel properties, including: cetane num-ber, calorific value, lubricity, surface tension, sulphated ash, carbonresidue, sulphur and trace metals content.

3.3. Engine test setup and procedure

The diesel engine test bed was a 3 cylinders, air-cooled tractorengine made by Deutz AG [50]. The tractor model was a FendtFarmer 250v. Engine specifications are presented in Table 1. The fuelinjection system comprised of a traditional inline diesel injectionpump (Bosch), fed by a diaphragm supply pump from the fuel tankvia a fuel filter. A return fuel line sent unused fuel from the injectorsback to the fuel tank (Table 2).

Fig. 2 shows a schematic of the test bed setup with the FendtFarmer 250v tractor, which was coupled by a drive-shaft to thedynamometer (model: Sigma 5, made by N J Froment Ltd, Stamford,


Table 3Test engine specifications [4].

Make & model Deutz F3L912Type 3 Cylinders, in line, air cooled,

naturally aspirated.Fuel injection system Direct injection, in-line injection pump

Table 1Comparison of biodiesel and petro-diesel fuel properties obtained from the literature.

Reference Units Rao Pulagala, Vasanta et al., 2009 Bannister, Hawley et al., 2009

Property Petro-diesel SSOME RBOME Petro-diesel RME

Density @ 15 �C kg/m3 820 870 870 833 883Kinematic viscosity @40 �C mm2/s 2.2 3.9 4.5 2.75 4.56Flash point �C 66 158 153 65 182Cetane number 48 52 57 52.8 49.5Cold filter plugging point �C e e e �18 �20Net calorific value MJ/kg 42.5 37.6 38.6 42.6 40.0Sulphur content mg/kg 0.3 0.0 0.0 7.0 1.8Carbon content %m/m 86.3 77.3 77.8 86.2 77.1Hydrogen content %m/m 12.5 11.8 11.8 13.8 12.2Oxygen content %m/m 0.3 10.0 9.4 0.0 10.7Acid Value mg KOH/g e - e 0.2 0.18Bulk modulus MPa 1475 1800 1800 - e


UK). A laptop PC (not shown) running Sigma Dynatest� softwarewas used to control the dynamometer and log test data. TheDynatest software logged readings of drive-shaft speed (rpm),Torque (Nm) and Power (kW), and facilitated comparison of testdata. Dynamometer control modes used were: ‘Automatic’ and‘Direct’. In ‘Automatic’ or ‘auto-test’ control mode, the tractor en-gine was accelerated to maximum speed and then the dynamom-eter automatically gradually increased the load (the appliedbraking force) until the engine speed was reduced to 40% of nom-inal, before then removing the load in a controlled manner. As thetest progressed, drive-shaft speed, torque and power were loggedand respective torque and power curves were plotted by the soft-ware. In ‘Direct’ mode, applied braking torque (Nm) could becontrolled manually from the software by adjusting a set-point.

An OIML Class 1 compliant automotive emissions analyser(Sykes Pickavant Ltd, model: SP9550) was used to measure exhaustemissions: CO2(carbon dioxide), CO (carbon monoxide), O2 (oxy-gen), and NO (nitrogen monoxide). The analyser probe wasattached to the tractor exhaust pipe (Fig. 2). The unit was equippedwith a rpm sensor which detected engine speed from fluctuationsof voltage produced by the alternator across the engine batteryterminals. The emissions analyser was connected to a laptop PCrunning P9550 data logging software (Autocal Ltd, Leicester, UK),which logged emissions and engine speed data once every secondto aMicrosoft Excel spreadsheet data file. The SP9550 analyser usedNDIR (Non-Dispersive Infra Red) measurement principle to mea-sure Carbon Monoxide (CO %vol) and Carbon Dioxide (CO2 %vol).Oxygen (O2 %vol) was measured by electro-chemical cell. NO (ppm)was also measured by electro-chemical cell.

NDIR measurement used the principle that gas molecules ofinterest absorb infra-red electro-magnetic radiation at discretewavelengths, dependent on the molecular species (as described byNorris of EMSTEC Ltd, in a report prepared for the UK’s Departmentfor Transport [30]). For example, absorption energy bands (cm�1)for key species in automotive emissions are: CO (2143), CO2 (2349),NO (1876), NO2 (1617), water vapour (1595), (3750) [30]. Somespecies have unique absorption energies, though exceptions areNO2 and water vapour which both have absorbencies around 1600(cm�1). Thus, IR absorbance is a possible technique for the detec-tion of CO, CO2, and NO. NDIR absorption technique can measure

Table 2Fuel properties for oxidised and un-oxidised SME, reported by Yamane et al. [3].

Un-oxidised(fuel#1)

Oxidised(fuel#2)

Heavily oxidised(fuel#3)

Treatment time (h) 0 12 24PV (meq/kg) 18 567 760KV (mm2/s@30 �C) 5.2 6.4 11.2AV (mg KOH/g) 0 1 3.5


NO, but not NO2 due to interference from water vapour. In NDIRmeasurement, selected wavelengths are transmitted through thegas sample, which the target species will absorb. NDIR is thetechnique used to measure exhaust gases in the majority of garagevehicle exhaust 4-gas analysers [30].

Oxygenmeasurement by the SP9550 utilised an electrochemicalcell, where oxygen diffusing into the sensor was reduced to hy-droxyl ions at the cathode, which in turn oxidised the anode,generating current proportional to the amount of oxygenconsumed. The SP9550 measured NO (and not NO2) by electro-chemical cell. In principle, the electrochemical cell oxidised NO andthe reaction generated electrical current of a magnitude propor-tional to the mass flow-rate of NO [30].

Fuel was delivered to the tractor engine from a surrogate fueltank (a 10 L HDPE aspirator bottle equipped with an isolating fueltap) which was supported on a set of digital weighing scales(�0.01 kg). BSFC was calculated (g/kWhshaft) according to equation(4.1) from measurements of the mass of fuel consumed (at least0.5 kg) over a period of time (between 5 and 10min) measured by astopwatch (�1 s), whilst maintaining steady output PTOP shaftpower (kW).

BSFC ¼ mfuelPshaft � t

(4.1)

where: mfuel was the mass of fuel (grams) consumed over themeasured time t (h), and Pshaft was the steady PTO shaft power(kW).

The test procedure for each BTF was as follows:

1. The surrogate fuel tank, connecting fuel lines and tractor’sfuel filter were first emptied, and then filled with test fuel.The tractor fuel system was purged (engine off) using the

with mechanical centrifugal governor,five-hole-nozzle fuel injectors.

Displacement 2.827 LStroke 120 mmCylinder bore 100 mmCompression ratio 19:1Rated power 38 kW at 2350 rpmBrake mean effective pressure

(at rated power)6.84 bar

Peak torque 176 Nm at 1450 rpm


Table 4Fuel properties of Biodiesel Test Fuels measured according to EN 14214 test methods.

Specification EN 14214:2008 þ A1# Rapeseed FAME Fatty acid methyl esters Fatty acid ethyl esters

Test Method Units Min Max BTF#1 BTF#2VO

BTF#35k

BTF#410k

BTF#5OX

BTF#6wet

Soya Corn Olive Sun Palm UCO Lard Tallow Rape Soya Palm Lard Tallow

Density@ 15 �C

EN ISO3675

kg/m3 860 900 882 889 882 882 894 881 882 879 876 882 872 882 872 867 873 874 873 865 864

Viscosity@ 40 �C

EN ISO3104

mm2/s 3.5 5 4.44 6.32 4.46 4.45 5.31 4.41 4.79 4.40 4.61 4.64 4.57 5.28 4.67 4.82 4.72 5.00 4.89 4.80 4.90

Flash point ISO 3679 �C 101 175 165 165 161 158 180 171 176 174 172 176 178 174 171 180 185 172 171 176Water content EN ISO

12937mg/kg 500 644 609 456 583 850 4110 490 470 480 541 510 604 565 494 653 568 544 474 443

Oxidationstability@ 110 �C

EN 14112 h 6 0.9 0.9 9.8 29.5 0.0 1.2 2.9 5.9 1.7 2.7 22.9 6.2 1.4 1.7 6.0 4.4 0.9 1.1 1.5

Acid value EN 14104 mg KOH/g 0.5 0.42 0.43 0.42 0.45 3.60 0.44 0.39 0.38 0.47 0.42 0.31 0.39 0.42 0.39 0.35 0.42 0.37 0.46 0.29Cloud point BS ISO

3015

�C Report �2 �4 �2 �2 �3 6 �1 �2 8 6 12 �3 10 15 �3 �4 19 12 14

Pour point BS ISO3016 �C �5 �7 �5 �5 �5 �4 �7 �4 �6 �11 10 �9 7 12 �12 �11 0 6 10Ester content EN 14103 %m/m 96.5 95.6 74.9 95.6 95.9 85.8 94.5 94.6 97.3 98.1 94.5 97.7 88.7 95.5 90.2 98.7 95.9 95.7 97.8 93.4Polyunsaturated

ester content(�4 doublebonds)

EN 14103 %m/m 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

Oleic (18:1) acidester

EN 14103 %m/m e 35.5 35.8 35.2 35.2 37.9 35.7 61.3 38.2 67.4 32.0 42.8 47.6 44.4 43.6 35.6 61.2 42.8 44.2 43.5

Linoleic (18:2)acid ester

EN 14103 %m/m e 43.6 43.2 43.2 43.2 38.7 43.6 18.0 46.1 15.7 51.6 11.4 31.2 8.7 1.6 43.2 18.1 11.4 8.6 1.6

Linolenic (18:3)acid ester

EN 14103 %m/m 12 5.4 5.3 5.4 5.4 4.0 5.4 7.4 0.8 0.6 1.2 1.4 4.6 2.0 0.2 5.4 7.4 1.4 2.0 0.2

Other estercontent

EN 14103 %m/m e 15.5 15.7 16.2 16.2 19.4 15.3 13.3 14.9 16.3 15.2 44.4 16.6 44.9 54.6 15.8 13.3 44.4 45.2 54.7

Iodine value EN 14111 g I2/100 g 120 120 119 119 119 110 120 103 114 86 119 60 106 61 49 114 98 57 58 47Oxidisability

index (OX)e e 55 55 55 55 47 55 34 48 18 55 15 41 14 3 55 34 15 14 3

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(2014),http://dx.doi.org/10.1016/j.energy.2014.02.034

Fig. 2. Schematic of the test bed setup.


tractor’s diaphragm fuel supply pump, which could bemanually lever operated (but was otherwise crank drivenwith the engine on).

2. The engine was started, allowed to idle and the fuel lineconnections were checked.

3. The PTO shaft clutch on the tractor was engaged to transmitdrive to the dynamometer.

4. The engine was run at high idle to warm up. The engine wasthen accelerated to maximum speed and then dynamometer‘auto-test’ was started (approximately 5 min duration),which produced a characteristic power curve (kW vs. PTOshaft rpm).

5. Replicate auto-tests were performed until two identical po-wer curves were obtained e thus ensuring the test wasperformed at optimum engine operating temperature.

6. With the load removed, the engine was allowed to idle for5 min before carrying out emissions and BSFCmeasurements.

7. The dynamometer was changed to ‘direct’ test mode, whereapplied braking torque (Nm) was controlled manually fromthe Dynatest software. Load was increased to the required

Table 5Peak power outputs measured by dynamometer auto-test.

Peak poweroutput (kW)

Figure reference

13a 13b

Petro-diesel 33.3 33.3Average all biodiesels 31.8 32%Power loss 4.5 3.9Highest biodiesel 32.6 32

BTF#3 Lard MEOther biodiesels 31.9 31.9

BTF#2 VO BTF#6 wet31.7BTF#4 10k31.6BTF#5 OX31.1BTF#1

1. Rao Pulagala, V., R.P. Vasanta, and S. Rao Nittala. Effects of key properties of biodieseInjection Systems for IC Engines. 2009. One Bridcage Walk, Westminster, London: IMech2. Bannister, C.D., et al., The impact of biodiesel blend ratio on vehicle performance and em3. Yamane, K. et al., Oxidation stability of biodiesel and its effects on diesel combustion and4. DEUTZ. 912/913. The engine for agricultural equipment - technical specifications. [cited 17deutz-912-agricultural-specs.pdf.


set-point, in turn either: 0, 120, 230, 350 or 470 Nm (0, 20%,40%, 60% and 80% of maximum respectively). Engine speedwas adjusted throughout to a constant 1940 rpm.

8. With torque (Nm) and engine speed (rpm) readings correct &steady, the SP9550 probe was attached to the tractor’sexhaust pipe and emissions data logging was started.

9. The mass of fuel in the surrogate tank was recorded at 2 minintervals until w0.5 kg had been consumed (5e10 minduration), and then emissions data logging was stopped.

10. The load was increased to the next higher load set-point(stage 7.). Stages 8 and 9 were then repeated.

11. Once emissions and fuel mass consumption data was ac-quired at all five load set-points, the engine was allowed toidle and cool before being switched off.

12. The above procedure was repeated for each BTF.

PTO shaft load set-points were (0, 120, 230, 350 or 470 Nm) at aconstant engine speed of 1940 rpm, or 500 rpm PTO shaft speed(after transmission). Respective values for PTO shaft power (setpoint values recorded by the dynamometer) were: 0.5, 6.3, 12.0,18.3 and 24.6 kW. Corresponding values for Brake Mean EffectivePressure (BMEP) can be calculated from the engine displacementand speed according to equation (5.2), as described by Stone [51].

PbrakeVd � N*

¼ BMEP (4.2)

where units of BMEP are in (N/m2) or (Pa)

Pbrake ¼ Measured brake engine power output (Watts)Vd ¼ Engine displaced volume, the total cylinder capacity (m3)

For a 4 stroke engine: N* ¼ number of cycles per second percylinder ¼ engine speed (rev/s)/2.

The Deutz test engine was a 4 stroke engine of 2.872 L totalcylinder capacity (Table 1). Values for BMEP are calculated forrespective load set points in Table 5. It is noted that BMEP values aremeasured at the tractor’s PTO drive-shaft, and not at the engineflywheel, thus are inclusive of associated transmission losses.

Ambient conditions have a significant influence on engine per-formance and emissions. Humidity, for example, has a large influ-ence on NOx emissions, which can be related to variation in intake

13c 13d 13e 13f

33.3 33 33.3 32.632 32.1 32.9 32.13.9 2.7 1.2 1.532.2 32.2 32.9 32.7Sun ME Palm ME Rape EE Soy ME32.2 32 32.9 32.2UCOME Palm EE BTF#1 Corn ME31.9 32.8 32Tallow EE Olive ME Soy EE31.7 31.4Tallow ME Lard EE

l fuels on injection, combustion, and emission characteristics of a DI-CI engine. inE.issions. Proceedings of IMechE: J. Automobile Engineering, 2009. 224: p. 405-421.emissions characteristics. Int. J. Engine Res., 2007. 8./1/13]; Available from: http://www.deutzpartsdirect.com/Documents/deutz-specs/


http://www.deutzpartsdirect.com/Documents/deutz-specs/deutz-912-agricultural-specs.pdf



air temperature profile [26]. Ideally, all engine tests would there-fore have been carried out under identical ambient conditions (airtemperature, pressure, humidity), however this wasn’t possible.Data was collected for the 19 different biodiesel test fuels overseveral days. At the start of each day of testing, the above testprocedure was carried out first using petro-diesel (drawn from thesame tank of fuel). The data obtained using petro-diesel served as adatum, showing relative changes in engine performance andemissions that occurred when running different types of biodiesellater on the same day.

Results data from each ‘auto-test’ (power/torque curves) wereexported from the Dynatest software to Microsoft excel wheremultiple curves could be overlaid in order to observe any qualitativedifference in engine performance. A new emissions data logspreadsheet was created for each test (at each load set-point).Steady-state emissions values were calculated as the mean (time-averaged) value that was recorded to the log file, from the point inthefile atwhich themeasured value had stabilised to a steady value.

4. Results and discussion

4.1. Fuel properties

Table 3 shows the fuel properties measured for all BTFs. All fuelsmet EN 14214 requirements on density, viscosity and flashpointwith the exception of 3 fuels (BTF#2, BTF#5 and UCOME).

Density of BTF#2 was increased due to the contaminant vege-table oil, and BTF#5 was increased due to oxidation. Densities ofbiodiesels made from the more saturated feedstocks (Palm, Lard,Tallow) were slightly lower. Ethyl ester density was also slightlylower than the corresponding methyl ester, except for Palm wheredensity was similar.

Viscosity measurements were in general fairly consistent acrossall BTFs tested, with the exception of BTF#2 and UCOME, wherelower ester content (higher TAG content) was the reason for higherviscosity. Viscosity of BTF#5 was increased due to oxidation, dis-cussed further below.

Flash points were significantly higher than the EN limit, indi-cating removal of residue alcohol. Antioxidant addition (BTF#4)and oxidation (BTF#5) appeared to lower flashpoint. Little differ-ence was evident between FAME types, or the corresponding ethylesters.

Water content of most samples met or was close to the EN limit(<500 ppm). Water was deliberately added to contaminate BTF#6.

Oxidation stability (OS) of the biodiesels varied widely. Mostfailed to meet the EN 14214 requirement (>6 h). Contaminatingsamples with vegetable oil (BTF#2) and water (BTF#6) had littleeffect on OS. Antioxidant mixed with biodiesel increased OS ofBTF#3 and #4 which showed much higher OS (>6 h) after TBHQwas added. The OS of oxidised BTF#5 was completely diminished.Comparing different FAME types and corresponding ethyl esters, notrends were obvious in OS values, which probably varied accordingto level of residual antioxidant carried over from the parentfeedstock.

Acid value (AV) of all test fuels was less than 0.5 (mg KOH/g),indicating low free fatty acid levels, except for oxidised sampleBTF#5. No variation was evident with contaminants, or accordingto FAME or FAEE type.

Cold flow properties of each BTF were assessed bymeasurementof CP (cloud point) and PP (pour point). CP was the temperature atwhich the least soluble biodiesel component crystallised from so-lution; PP was the temperature at which biodiesel ceased to pourand solidified. Contaminant vegetable oil (BTF#2) appeared toslightly improve cold flow properties, whilst adding water (BTF#6)worsened cold flow. Neither oxidation nor TBHQ had any effect.


Palm, Lard and Tallow biodiesels contained higher concentrationsof high-melting point saturated long-chain fatty acids and there-fore tended to have poorer cold flow properties. For the Palm, Lard,and Tallow FAMEs, CP and PP were �7 �C. These biodiesels wouldbe clearly problematic fuels even in temperate climates. In contrast,themore unsaturated biodiesels e.g. Sunflower, Soybean, Rapeseed,Corn and Olive, tended to yield fuel with much lower CP and PP.Interestingly, ethyl esters recorded noticeably improved cold flowproperties (lower PP) relative to the corresponding methyl esters.Rapeseed ethyl ester PP was 7 �C lower than the methyl ester.Likewise for the corresponding ethyl ester, Soybean PP was (6 �C)lower, Palm (10 �C), Lard (1 �C), and Tallow (2 �C) lower. The reasonfor this was not clear, though it maybe that the larger ethyl moietyof FAEE better inhibits close packing of molecules, since it is knownthat wax crystallisation is initiated by “close packing” of molecules.Factors that disrupt or inhibit close packing of highly orderedmolecules will decrease CP and PP temperatures, such as intro-duction of a double bond into the FA chain, where the cis-configuration of unsaturation provides better low temperaturetest performance than trans [25].

Ester Content measurements indicated that most of the testfuels met, or were close to meeting the EN 14214 requirement(>96.5%m/m), except for BTF#2 which was contaminated with oil,and also BTF#5 which was oxidised. The ester content was other-wise indicative of the completeness of transesterification of thestarting oil/fat to alkyl esters. Oxidation products (e.g. short chainacids and polymeric species) are chemically different to their esterprecursors with altered characteristic retention times, hence GCanalysis showed reduced ester content for BTF#5. Ester content ofUCO FAME was relatively lower due to increased FFA (free fattyacid) content of the starting UCO feedstock. The AV of the UCO was5.31 mg KOH/g. In comparison AV was consistently <0.4 mg KOH/gfor the other feedstock oils. FFAs present in the UCO reacted withthe catalyst forming soap and water; reducing catalyst concentra-tion, and inhibiting the reaction. Ester contents of BTFs made fromanimal fats were also relatively low e it is thought this was prob-ably due to solid particles of fat remaining in the reaction mixturei.e. resulting in reduced reactant mixing, since AV of the startingfats was less than 1 mg KOH/g.

Fatty Acid (FA) composition results showed levels of unsatu-rated Oleic (18:1), Linoleic (18:2), and Linolenic (18:3) acid content(%m/m) for each test fuel. Identities are abbreviated (X:Y) where Xdenotes the fatty acid moiety carbon chain length, and Y is thenumber of double bonds present in the fatty acid chain. Theremaining ester content (shown as ‘Other Ester content’ in Table 3)was predominantly saturated fatty acids (X:0), though low levels ofmono-unsaturated Myristoleic (14:1) and Palmitoleic (16:1) acidswere detected present in the animal fats, and trace levels of mono-unsaturated Erucate (22:1) was detected in Rapeseed, Olive andUCO. Wide variation in fatty acid composition was measured withvariation of feedstock oil type. Soybean and Rapeseed biodieselsshowed the highest levels of Linolenate (18:3). Rapeseed, Corn, andSunflower contained mostly 18:2, while Soybean, Olive and UCOcontained relatively more 18:1. Rapeseed, Soybean, Corn, Olive andSunflower were dominated by unsaturated FA, whilst Palm, Tallowand Lard contained relatively less unsaturated components. Noneof the biodiesels tested contained any FAwith�4 double bonds. It isnoted that this EN 14214 specification serves to exclude highlyoxidatively-unstable oils, such as fish oils, as biodiesel feedstocks.

Iodine Value (IV) of Rapeseed FAME (BTF# 1-4 and 6) only justmet the EN limit (<120), and so did Sunflower FAME. IV is a stabilityindexmeasuring levels of unsaturation in organic compounds, suchas FAME. The IV is defined as the mass of iodine (grams) that can beformally added to 100 g of the sample, measured according to thestandard test method EN 14111. It is an indicator of the number of



carbon double bonds present in the sample; the higher the IV, thehigher the number of double bonds [46]. Iodine Value is one of theoldest and most common methods for determining the level ofunsaturation in a fatty oil or ester [47].

Oxidation of BTF#5 lowered the IV by consuming and relativelydiminishing the polyunsaturated fatty acid content. The otherbiodiesels which showed lower IV contained relatively higherlevels of saturated fats (e.g. Palm, Lard and Tallow). The FAEEsamples showed slightly lower IV relative to the correspondingmethyl esters, due to the slightly higher molecular weight of ethylester molecules.

Oxidisability (OX) stability index was calculated for each testfuel using the equation reported by McCormick et al. [48]. OXpredicted a slightly different relative order of susceptibility tooxidation for Palm and Lard biodiesels compared to the IV, becausethe IV stability index treats all double bonds as being equallyreactive whereas OX does not.

4.2. Emissions data and engine performance

Fig. 3 (aef) shows oxygen levels (%volume) that were measuredin exhaust emissions at discrete PTO shaft load points, corre-sponding to BMEP values (0.11, 1.36, 2.58, 3.94, 5.30 bar). Eachfigure (aef) respectively shows data recorded on the same day. Apolynomial trend-line is fitted to the data points recorded forpetro-diesel (dashed line) which distinguishes petro-diesel resultsfrom those for the biodiesels. It is noted that oxygen emissions arenot harmful or regulated, but are of interest since values indicatethe level of excess oxygen (equivalence ratio). Results indicatedslightly elevated oxygen levels when running biodiesel, relative topetro-diesel, though the difference was less noticeable in Fig. 3 (f).Oxidised biodiesel (BTF#5 OX) recorded the highest exhaust oxy-gen levels (Fig. 3a) e suggesting the oxygenation level of the fuelitself (the bound oxygen in the fuel) may have been the cause.Oxygen is presentwithin the alkyl ester structure of biodiesel as thedouble bonded carbonyl oxygen and the single bonded carboxyloxygen (w11%wt). Higher levels of fuel-bound oxygen would befound in oxidised biodiesel, where fatty acid hydroperoxide (ROOH)has formed at unsaturation sites by the process known as autoxi-dation [24]. In contrast, petro-diesel is expected to contain little orno oxygen [21] [23],.

Fig. 4 (aef) shows measured carbon monoxide (CO) levels (%vol). At the highest load point, results consistently indicated lowerCO levels when running biodiesel relative to petro-diesel, thoughdifferences were somewhat less obvious at lower loads. At thehighest load point, average CO levels were reduced on respectivedays by (46%, 55%, 52%, 47%, 32%, 23%) making the overall averagereduction 43%. These results agreed with the dominant trend re-ported in the literature, which was that biodiesel decreases COemissions, depending on operating conditions. At the highest loadpoint, vegetable oil contamination (BTF#2) increased CO slightly.TBHQ added to biodiesel (BTF#3 and 4) appeared to have noobvious effect on CO emissions. Fig. 4 (bef) suggests there was littledifference in CO emissions between different FAME types and alsothe corresponding ethyl esters. Oxidised biodiesel (BTF#5 OX)recorded lower exhaust CO levels at all loads, relative to petro-diesel and to four other un-oxidised biodiesel samples from thesame batch (Fig. 4a). This result agreed with previous studies[35,36] that also observed CO reductions for oxidised biodiesel,despite its higher viscosity, suggesting the presence of hydroper-oxides (increased oxygen content) improves oxygen availability andpromotes complete combustion; surpassing the negative effects ofhigh viscosity and poor spray formation.

As reviewed above (5.1.3), the cause of reduced CO emissionsthat are typically observed with biodiesel, is often identified to be


the fuel oxygen content, and thus leaner combustion stoichiometry(improved oxygen availability), which promotes complete com-bustion of the fuel. Increased biodiesel CN also reduces the prob-ability of fuel-rich zones formation, and advanced injection andcombustion timing occurs (due to fuel compressibility effects inpump-line-nozzle fuel systems) which increases combustion resi-dence time. Another factor is the flame temperature (increasedlocally for biodiesel as observed by Joonho Jeon et al. [52]) whichmay promote more complete combustion and reduced CO emis-sions. Although the greater surface tension of biodiesel is expectedto lead to slower vaporisation and less complete combustion, whichwould likely cause increased CO. This effect may be suppressed byfuel oxygenation. Biodiesel fatty acid composition is reported toaffect CO emissions, where less volatile, longer chains reduce theprobability of complete vaporisation and combustion, therebyincreasing CO emissions. However results (Fig. 4) showed noobvious dependence of CO emissions on feedstock type.

Fig. 5 (aef) indicates there was little obvious differencemeasured in carbon dioxide (CO2) levels (%vol) when runningbiodiesel, relative to petro-diesel. These results are not discussedfurther.

Fig. 6 (aef) shows levels of NO (ppm) recorded for each BTF.Some biodiesels recorded similar or slightly higher NO levelscompared to petro-diesel, whereas certain others recorded lowerNO levels. At higher loads, oxidised biodiesel (BTF#5 OX) recordedobviously lower NO levels, relative to petro-diesel and to four otherun-oxidised biodiesels (Fig. 6a). Fuel oxidation therefore reducedNO emissions. This result contrasts with results of previous authors[35,37] who observed, similar and increased NOx levels for oxidisedbiodiesel.

Levels of NO were essentially unchanged after contaminatingsamples with vegetable oil, TBHQ and water (Fig. 6a). Resultstentatively agreed with Ryu [36] who also observed no effect ofTBHQ antioxidant on NOx emissions. Though at the highest loadpoint, NO was lowered (by w100 ppm) for BTF#4. This differencemay have been significant but further investigation is needed.There were no obvious differences in NO emissions for methyl andethyl esters made from the same oil/fat feedstock, which agreedwith results obtained by Graboski et al. [26].

Relatively lower NO levels were observed for biodiesels whichcontained relatively less unsaturated fatty acid components: Palm(Fig. 6 d), Tallow (Fig. 6c) and Lard (Fig. 6b). Biodiesels dominatedby unsaturated fatty acid esters showed consistently similar orslightly higher NO levels compared to Petro-diesel, which were:Rapeseed (Fig. 6 a, b, e), Sunflower (Fig. 6c), Soybean, Corn (Fig. 6f),and Olive FAME (Fig. 6e). Fig. 7a shows NO levels measured at eachengine load, plotted as a function of the fuel’s iodine value (for alltest fuels except BTF#5). Linear best fit lines all showed positivegradients (ranging from 0.3 to 0.7), indicating a trend of increasingNO levels with higher biodiesel unsaturation level. This trend ismore obvious in Fig. 7 (b to f) which show DNO calculated ac-cording to equation (5.1), i.e. DNO is determined for each biodieselrelative to values measured the same day for petro-diesel.

DNOðppmÞ ¼ NOPetrodiesel �NOBTF (5.1)

Fig. 7 (be f) indicates the more saturated biodiesels of lower IV(40e80 range) produced relatively lower NO levels. This is consis-tent with trends reported by Graboski et al. [26], which observedbrake-specific NO levels that increased linearly with biodiesel IV.Linear trend lines fitted to the data (Fig. 7 bef) all showed positivegradients (ranging from 0.6 to 1.9) which increased with load,indicating differences in NO levels were amplified as loadincreased. Oxidised biodiesel (BTF#5) clearly produced lower NOrelative to other biodiesels of similar IV. As well as the degree of


Fig. 3. Oxygen levels (%volume) measured in exhaust emissions with increasing BMEP (0.11, 1.36, 2.58, 3.94, 5.30 bar).


biodiesel unsaturation, the degree of biodiesel oxidation was asignificant factor causing variation in NO emissions.

Biodiesel which contains longer, more saturated fatty acidchains has higher CN (shorter ignition delay) which reduces NOx

emissions, by reducing the amount of fuel involved with premixedcombustion, causing maximum heat release and max temperatureto occur later. Thus shorter, more unsaturated fatty acid chains


produce higher NOx emissions, compared to more saturated bio-diesel fuels. In pump-line-nozzle fuel systems, injection and hencecombustion commences earlier with biodiesel fuel of higher vis-cosity and bulk modulus, which is the primary mechanism causingsignificant increases in NOx emissions, compared to petro-diesel.

High glycerine content and antioxidant additives have beenproposed to affect NOx emissions, but the present study observed


Fig. 4. Carbon monoxide levels (%volume) increasing BMEP (0.11, 1.36, 2.58, 3.94, 5.30 bar).


no effect of TBHQ or high TAG content on engine performance andemissions. Methyl and ethyl esters produced from the same feed-stock also produced similar NOx emissions, so that alcohol typeshowed no effects.

Other factors highlighted in the literature that influence NOx

emissions, include: adiabatic flame temperature, level of soot for-mation, oxygen availability, fuel spray characteristics, the engine


design. A complete understanding of associated NOx formationmechanisms for biodiesel has yet to be determined, so that morestudy is called for.

Fig. 8 (aef) shows BSFCmeasured at the drive-shaft (g/kWh). Nodifferences were obvious in the data between different biodieseltypes. Results did however indicate a clear increase in BSFC whenrunning biodiesel, relative to petro-diesel.


Fig. 5. Carbon dioxide levels (%volume) increasing BMEP (0.11, 1.36, 2.58, 3.94, 5.30 bar).


For each biodiesel data point (Fig. 8 aef), the percentage in-crease in BSFC was calculated relative to the result measured forpetro-diesel at the same load, on the same day. The overall averagepercentage increase in BSFC when running biodiesel was 9.3%.Standard deviation for a total of 80 observations was 4.7%, indi-cating that the percentage increases measured in BSFC varied quite


significantly, probably due to the varying heating value of differentcomposition biodiesels, and also due to error (6%) associated withthe instrumentation and method used for calculating BSFC (see3.3). In summary, the data certainly indicated an increase in BSFCwhen running biodiesel of approximately 10%, compared to resultsfor petro-diesel.


Fig. 6. Nitrogen monoxide (NO) levels (ppm) increasing BMEP (0.11, 1.36, 2.58, 3.94, 5.30 bar).


As reviewed in the literature, BSFC is expected to increase ac-cording to the relative loss of heating value on a mass basis (MJ/kg).An indicator of heating value loss and thus the expected BSFC in-crease is the oxygen content of biodiesel, which is typically 11%wt[25]. BSFC increase was similar to the loss of heating value, around14% on a mass basis for pure biodiesel in studies reviewed in theliterature by Lapuerta et al. [20]. Results obtained in the presentstudy therefore indicated an increase in BSFC that was slightlylower than might be anticipated from literature values. However,


without direct measurement of fuel heating values, it was notpossible to confirm whether increases in BSFC were in line withreductions in heating value, although this is expected to be the case.

Biodiesel heating value varies with fatty acid composition; asfatty acid chain length increases (for a constant unsaturation level)the mass fraction of oxygen decreases, so the heating value (MJ/kg)increases. Unsaturation level also has a strong influence uponheating value, as discussed for example by Ramirez et al. [49].Compared to saturated esters, unsaturated esters have lower


Fig. 7. NOx (ppm) measured with Varying BMEP as a function of BTF Iodine Value.


energy content by mass (MJ/kg) so BSFC shall be higher. However,no trends in BSFC with unsaturation level or density were obviousin the data obtained in the present study.

More unsaturated esters have lower energy content bymass, butthis is compensated by relatively increased density and volumetricenergy content [25]. Because fuel injection pumps meter fuel byvolume, not by mass, a greater mass of fuel is injected according tobiodiesel’s relatively higher density, thus compensating for lowerheating value on a mass basis. Therefore the fuel’s volumetric


energy content (MJ/litre) is the more significant comparator interms of impact on engine power output.

Fig. 9 (aef) show results of dynamometer ‘auto-tests’ whichobtained a characteristic power curve (kW vs. PTO drive-shaft rpm)for each test fuel. Each Fig. 9 (aef) respectively shows data recordedon the same day. Replicate auto-tests were performed for each testfuel until two power curves closely matched. The curve thatshowed (very slightly) higher peak power output was selected asrepresentative of the optimum fuel performance and is plotted


Fig. 8. BSFC (g/kWhshaft) measured at steady load (0, 120, 230, 350 or 470 NM), at constant engine speed (1940 rpm).


(Fig. 9). Results clearly showed that the engine produced morepower when running petro-diesel, although the difference was lessnoticeable in Fig. 9 (e) and (f). No differences were obvious betweendifferent biodiesel types that were compared on the same day.Contaminants (vegetable oil, TBHQ, water) and oxidation had noapparent effect on output power (Fig. 9a). Trends in power levelswere neither apparent with biodiesel feedstock type (oil/fat,alcohol).

Table 4 shows peak power output values recorded for all of thetest fuels. Petro-diesel recorded 33.3 kW on four test days, and onthe other two days peak power was slightly lower, probably due to


changing atmospheric conditions, making the average 33.1 kWover6 days. Considering results for all 20 biodiesel auto-tests, peak po-wer output ranged between 31.1 and 32.9 kW. The overall averagewas 32.1 kW, and standard deviation was 0.48. The average peakpower loss compared to petro-diesel was therefore 1 kW (3% loss).

According to Lapuerta et al. [20], rated power output reductionofw8% is expected for biodiesel compared to petro-diesel, which istypically the loss of volumetric heating value. However, smallerexpected volumetric heating value reductions (5e6%) were re-ported by Hoekman et al. [25]. The actual difference in volumetricenergy content between petro-diesel and biodiesel clearly depends


Fig. 9. Characteristic power curve for each BTF measured on the same day.


on the exact petro-diesel and biodiesel composition. Lapuerta et al.[20] observed many studies found the loss of power running bio-diesel was less than expected. The present study also found this.

Factors reviewed in the literature that could explain power re-covery include: higher biodiesel viscosity, bulk modulus and CN


causing combustion advance leading to higher peak pressure andoutput power, enhanced fuel spray penetration giving better fuel-air mixing, and improved fuel lubricity reducing frictional losses.Other factors may be the increased oxygen content, lower stoi-chiometric air:fuel ratio, and lower final boiling point of biodiesel,



which lead to relatively improved combustion efficiency (consis-tent with lower CO emissions).

5. Conclusions

1. Fuel property measurements showed more saturated methylesters recorded lower density, as did ethyl esters. Oxidation ledto increased density, viscosity and acid value, and lowered theflashpoint, iodine value, and ester content. Most biodieselsshowed lowOS (<6 h), but adding TBHQ recovered OS. Cold flowtemperatures were worse for more saturated feedstocks butwere relatively improved for ethyl esters. Contaminant vege-table oil slightly improved cold flow, whilst water did theopposite. TBHQ and oxidation had no obvious effect on coldflow.

2. In engine tests, exhaust oxygen levels were increased whenrunning biodiesel. Oxidised fuel showed the highest exhaustoxygen levels, possibly due to fuel-bound hydro-peroxides.

3. At the highest engine load point, biodiesel CO emissions werelower than levels recorded for petro-diesel by on average 43%.At low loads differences in CO were less obvious. Oxidised bio-diesel recorded noticeably lower CO at all engine loads, whichhas also been observed by previous studies. Biodiesel contami-nants (vegetable oil, TBHQ, water) and variation of the feedstocktype (fat/oil, alcohol) had no obvious effect on CO.

4. Biodiesel NO levels were either: slightly higher than, similar to,or clearly lower than levels recorded for petro-diesel, whereincreased unsaturation (higher IV) correlated with higher NOlevels, and this trend was amplified with engine load. Theseobservations were consistent with previous studies. Contami-nants (vegetable oil, TBHQ, water) and feedstock alcohol type(methanol vs. ethanol) showed no observable effects on NOemissions, whereas oxidationwas a significant factore oxidisedfuel recorded noticeably lower NO levels. This effect of oxidationseen on NO contrasts with the results of previous studies.

5. BSFC for biodiesel was clearly increased (w10%) compared topetro-diesel, but no differences were obvious between differentbiodiesels.

6. Average peak power loss running biodiesel was 3%, which wasless than expected. Power recovery may be attributable to fac-tors such as: combustion advance, improved fuel lubricity andfuel-air mixing, which may lead to improved combustion effi-ciency (consistent with lower CO emissions). Biodiesel fuelproperties variation with fatty acid profile, feedstock alcoholtype, contaminants (vegetable oil, TBHQ, water) and oxidation,had no obvious effect on power output.

7. Comparison of methyl esters with ethyl esters (made from thesame fat/oil), suggested there was no difference in performance,apart from ethyl esters improved cold flow properties. A quali-tative difference was noticed while carrying out engine testing,which was a distinctly (in the opinion of the author) improvedodour of ethyl ester exhaust emissions.

Acknowledgements

Authors wish to express thanks to Rye Biofuels Ltd (UK) and theEngineering and Physical Sciences Research Council (EPSRC) of theUnited Kingdom for their support.

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