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Fuel 86 (2007) 1765–1771

Performance and exhaust emissions in the use of biodieselin outboard diesel engines

S. Murillo, J.L. Mıguez, J. Porteiro *, E. Granada, J.C. Moran

E.T.S. Ingenieros Industriales, Universidad de Vigo, Campus Lagoas-Marcosende, 36200 Vigo, Spain

Received 21 September 2006; received in revised form 10 November 2006; accepted 20 November 2006Available online 20 December 2006

Abstract

The objective of this study is to compare the engine performance and emission results of biodiesel derived from used cooking oil whenapplied in different proportions in outboard engines. Results revealed that the use of biodiesel resulted in lower emissions of CO (up to12%) with an increase in emissions of NOx (up to 20%, except in one case which presented a slight reduction). Biodiesel also presented aslight increase in specific fuel consumption (lower than 11.4%) which may be acceptable considering the reduction in exhaust emissions.The experimental results proved that biodiesel alone or blended biodiesel can be used in compression ignition outboard engines, therebyproviding a viable alternative to diesel. Special attention should be paid to the use of biodiesel in boats operating on lakes and rivers andin sheltered bays, which are more vulnerable to pollution.� 2006 Elsevier Ltd. All rights reserved.

Keywords: Outboard engines; Biofuels; Exhaust emissions limits

1. Introduction

Outboard engine emissions have only recently been reg-ulated and they are classified as nonroad engines. Marineengines account for about 30% of nonroad engines, butas they tend to be concentrated in coastal areas (ports, rec-reational areas, lakes, rivers, etc.), the local levels of pollu-tants may become too concentrated.

Marine diesel engine manufacturers in the UnitedStates, Europe and Japan have all recognized the growingrole of biodiesel as a viable fuel component, and in mostcases, as a fully alternative fuel (100%) [1].

Exhaust emission limits have been set for carbon mon-oxide, hydrocarbons, nitrogen oxides and other pollutants.Exhaust emissions are measured in accordance with theharmonised standard ISO 8178-1:1996 [2]. All EU stateswere required to apply Directive 2003/44/EC [3] from 1

0016-2361/$ - see front matter � 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.fuel.2006.11.031

* Corresponding author. Tel.: +34 986812604.E-mail address: porteiro@uvigo.es (J. Porteiro).

January 2005; those measures came into force on 1 January2006, in the case of products covered by Article 1 (1a),compression ignition and four-stroke spark ignitionengines and, in the case of two-stroke spark ignitionengines, will come into force from 1 January 2007 – thusallowing two separate periods of transition. It should benoted that all existing outboard engines may continue tobe used under the EU 2006 emissions directive.

Biodiesel is a chemically modified alternative fuel for usein diesel engines without major modifications. It isobtained from vegetable oils and animal fats and it is pro-duced at a commercial scale by the transesterification ofvegetable oils with methanol or ethanol. The direct use ofalcohol as fuel causes several performance and corrosionproblems which can be solved through transesterification.

The given biodiesel contributes to reducing greenhouseeffect gas emissions when compared to conventional diesel[4–8], many researchers are addressing the suitability ofvegetable oil and its derivatives as fuel or as additives todiesel [9–13]. Considerable research has been undertakenrecently to understand the performance characteristics of

Table 1Test engine specifications

Engine code 8DFuel type DieselDisplacement (cm3) 945Bore/stroke (mm) 78/60Compression ratio 20.3:1Starting system Electrical with automatic compression releaseEngine type Four-stroke naturally aspiratedCooling type Liquid (sea water in open circuit)Number of cylinders 3Fuel supply system Inline pumpInjection type Direct injection mechanically controlledRated power (kW/HP) 18/24.6Rated speed (rpm) 3450Minimum speed (rpm) 950Engine position Vertical

1766 S. Murillo et al. / Fuel 86 (2007) 1765–1771

biodiesel-fuelled engines, as well as biodiesel productiontechnology [14,15]. Experiments on a direct injection tur-bocharged diesel engine have been reported by Salvatoreand Maddaleena [16], Choi et al. [17], Dorado [18]. It isalso important to note that most of the experiments con-ducted on biodiesel are mainly obtained from refined typeoils only.

The other research studies the biodegradability and tox-icity of biodiesel in aquatic environments. It seems that thefirst research on this topic dates from the early 1970s, con-ducted by Novak and Kraus [19]. Since then, a great dealhas followed. After studying the biodegradability of severalkinds of biodiesel and their comparison with commercialdiesel and their blends, Zhang et al. [20] concluded thatbiodiesel is easily biodegradable in aquatic environmentsand has a higher biodegradability than commercial diesel.Tests performed by Cytoculture found that 37% of the ves-sels surveyed chose to use biodiesel for environmental rea-sons, 33% for mechanical reasons (normally related to thebetter lubricating properties of biodiesel), while 33% basedtheir decision on subjective reasons, such as safety upondirect contact with the skin and lower smoke level. VonWedel [1] also studied biodiesel toxicity in humans.

The emission properties of diesel engines operating onpure biodiesel and its blends with diesel have been pub-lished by several authors [6,9,10,12–19]. Most of them havereported substantial reductions in CO, HC and PM emis-sions along with higher NOx emissions in the exhaust.The more widely accepted notion is that the high oxygencontent of BD leads to more complete combustion andlower emissions. However, this higher oxygen contentalso leads to lower calorific values, which may result in sig-nificant power losses and the increase of specific fuel con-sumption. Furthermore, the direct application of purebiodiesel (BD) or pure methyl-ester (ME) as an alternativein conventional diesel engines has encountered severalother drawbacks due to the technical problems related toits higher density, viscosity, poor filtration and lowervolatility.

Engine performance with the different blends of dieseland biodiesel, as well as its emission characteristics,depends largely on the combustion chamber and injectornozzle design, injection pressure, air turbulence, air-fuelmixture quality, actual start of combustion and many othersingularities that make test results vary from one engine toanother. Furthermore, each engine parameter varies withspeed and load; emissions will therefore be studied in differ-ent operational points employing a specific ISO testmethod for determining overall emissions.

2. Experimental installation and methodology of the research

A four-stroke, direct injection and naturally aspiratedsingle cylinder diesel outboard engine has been tested inthis study. This kind of engine is usually used in fishingoperations by small craft or in pleasure boats. Details ofthe engine’s specifications are shown in Table 1.

The engine was coupled to a hydraulic dynamometer(Marine 2000) for torque measurement. Engine speed wasmeasured by a laser sensor. Fuel consumption was deter-mined by a calibrated load-cell installed under the fueltank. Inlet air flow was determined through fuel consump-tion, fuel composition and exhaust gas composition,according to ISO 8178 [3]. Each test was performedaccording to ISO 3046 [21] and the maximum estimateduncertainties were below the limits imposed by those stan-dards. Fig. 1 provides a diagram of the experimentalinstallation.

Exhaust gas composition was obtained by the extractionof a continuous sample of gas from the exhaust stream.The gas sample was passed through an unheated line, awater separator and a treatment line. The exhaust gaswas then fed into the gas analyzers. The sensors were cali-brated using certified standard gases. The exhaust gas anal-ysers used in this study were the following, which fulfil therequirements laid down by ISO for the determination ofeach species:

• Horiba Mexa 574GE NDIR (Non-Disperse Infra-Red)for the determination of CO2, CO, and O2 concentra-tion, as well as for estimating the air/fuel equivalenceratio (k).

• Horiba FIA-510 FID (Flame Ionization) for the deter-mination of THC.

• Horiba CLA-510SS CLD (Chemi-Luminescence) for thedetermination of NO and NO2 (NOx) emissions.

The output signal from each sensor was collected by adata acquisition board and sent to a PC for post-process-ing. Data were analysed and displayed using a speciallydeveloped customised program.

In accordance with ISO 8178, emissions for marineengines have to be tested in specific cycles. The exactcycle depends on the kind of engine, the kind of vesseland the kind of propeller. In the case of diesel out-board engines, the cycle to be applied is C3, as shown inTable 2.

Fig. 1. Experimental installation.

Table 2ISO C-3 performance and weighting factors

Mode number 1 2 3 4

Speed% 100 91 80 63Load% 100 75 50 25Weighting factor 0.20 0.50 0.15 0.15

S. Murillo et al. / Fuel 86 (2007) 1765–1771 1767

To obtain a baseline test, the engine was run first on die-sel fuel. A full load curve was obtained with the gradualincrease of speed from 1500 rpm to 3500 rpm. Althougha marine engine is not normally operated at full load inthe lower speed range, as the propeller power-curveincreases approximately cubically with speed, this testwas conducted in order to compare the overall perfor-mance of the engine running on the different blends andfuels. This performance test was conducted in accordancewith ISO 3046 [21] in the following sequence: pure diesel

Table 3Physical–chemical properties of the diesel and biodiesel used

Fuel Commercialdiesel

Commercialbiodiesel

Density (kg/m3) 830 883.7Lower heating value (MJ/kg) 43.0 37.2Cetane number 51.6 53.9Flash point (closed cup) (�C) 72 160Viscosity (cSt) [40 �C] 3.16 4.151Distillation T50 (�C) 277.7 325Final boiling point (�C) 376.7 345Cold filter plugging point (�C) �15 �5Sulphur content (wt%) <0.04 <0.04

(B-0), 10% biodiesel, 90% diesel (B-10), 30% biodiesel,70% diesel (BD-30), 50% biodiesel, 50% diesel (BD-50),and pure biodiesel (B-100). The properties of the pure fuelsused in this work are given in Table 3.

The emissions from each configuration were subse-quently tested as per ISO 3046 in conjunction with ISO8178 cycle C3.

3. Results and discussion

3.1. Engine performance

Fig. 2 shows the brake power output for all the perfor-mance tests carried out according to ISO 3046. The varia-tion in engine power at full load with different fuels can beverified. At rated speed (approx. 3450 rpm), the ratedpower measured is close to that declared by the manufac-turer, albeit slightly lower. As shown by the figure, increas-ing the amount of biodiesel in the fuel decreases enginepower. In the extreme case of pure biodiesel (BD-100),the power loss is higher than 1 kW, which accounts for7.14% of the rated power. In this case, when the speed ofthe engine is low, the power obtained is similar to BD-10and BD-30. If the speed is increased beyond 2500 rpm thenthe power obtained is much lower, with a loss of more than2 kW (12.5%) at rated speed. A further significant conclu-sion obtained from Fig. 2 is that when the engine is coupledto a specific propeller, the direct substitution of the fuel willlead to a reduction in propeller speed, and therefore to afurther reduction in the system’s output. In this case, the

6

8

10

12

14

16

18

20

1500 2000 2500 3000 3500 4000

N (rpm)

P (

kW)

BD-0

BD-10

BD-30

BD-50

BD-100

Propeller law

Fig. 2. Brake power output (kW) vs. engine speed (rpm) obtained duringperformance tests.

0.18

0.20

0.22

0.24

0.26

0.28

0.30

0.32

1500 2000 2500 3000 3500 4000

N (rpm)

Bra

ke t

her

mal

eff

icie

ncy

(%

)

BD-0

BD-10

BD-30

BD-50

BD-100

Fig. 3. Brake thermal efficiency (%) vs. engine speed (rpm) obtainedduring performance tests.

1768 S. Murillo et al. / Fuel 86 (2007) 1765–1771

real power loss of BD-100 will be 8.9%. If a more suitablepropeller were to be selected for each case, this furtherpower loss could be avoided. This aspect has not beenexplored in this paper, but should be kept in mind whenanalysing the performance curves of marine engines.

The display of the variation in brake-specific fuel con-sumption with speed for different fuels could be misleading.If different fuels are compared, it should be noted that theircalorific values decrease with the increase of biodiesel in theblend and also that the calorific value is lower than conven-tional diesel, for that reason, brake thermal efficiency isshown in Fig. 3 instead of specific fuel consumption. Asshown, only BD-100 presents a better brake thermal effi-ciency, whereas the other blends behave in a similar wayto conventional diesel, only slightly less efficiently. Theseresults concur with those obtained by Ramadhas et al.[12,13] who compared the performance of a diesel enginerunning on blends of conventional diesel and biodieselderived from rubber seed oil. Elsewhere, Raheman andPhadatare [22] recorded a slightly different behaviour. Inthis case, they obtained a maximum thermal efficiency withblends B-20 and B-40, whilst B-100 recorded a poorer per-formance. In all these cases, the differences were in generallower than 2% and should therefore be generally consid-ered as equal [23].

Several reasons have been identified for the influence ofbiodiesel on engine performance. Although there is a rela-tively high data scatter regarding the biodiesel cetaneindex, it is known that biodiesels are usually the same asconventional diesel in this aspect [23], so the influenceof biodiesel on engine performance is probably moreclosely related to the oxygenated nature of biodiesel, andalso to its higher viscosity and lower calorific value, whichhave a major bearing on spray formation and initialcombustion.

As seen in Fig. 3, the brake thermal efficiency of theengine fueled with biodiesel is higher than that of diesel,while its blends are observed to have a lower efficiency thandiesel. We have seen this behaviour in other tests and insome published works [12,13,22]. We think that the causes

may be related to the atomization of the blends duringinjection and/or with the stability of the mixtures of fuelsduring storage, pumping and injection.

3.2. Pollutant emissions

Results concerning the main pollutant emissions duringtests are shown in this section. Each mode of operation pre-scribed by the ISO cycle has a specific weight in the overallperformance, which is given by its weighting factor. Theseoperating modes and their corresponding weighting factorare related to a hypothetical marine use of the engine cou-pled to a fixed-pitch propeller. Each operating mode isdefined through the fraction of the rated speed and thefraction of the rated load that must be used to obtain fullpower (100%) and the partial loads of 75%, 50% and 25%.

The mass flow rate for each component is obtained fromthe concentration data for each one in the exhaust gas foreach performance mode and from the value for specific fuelconsumption. This mass flow rate is weighted for eachperformance mode and measured power according to thefollowing expression:

MGASi ðg=kW hÞ ¼Pn

i¼1MGASi ðg=hÞ � W FiPni¼1P i ðkWÞ � W Fi

where the value of the weighting factors (W FiÞ gives greateror lesser importance to the emissions in each performancemode, giving rise to a typical cycle used in marine applica-tions. Thus, a unique value is obtained for specific emis-sions that will be the value subject to emission limitsimposed by the different organizations or committees.

3.3. Carbon monoxide emission

The emission characteristics of biodiesel are of specialinterest regarding compliance with environmental stan-dards. Fig. 4 shows the CO emissions of the different fuelsat each point of test cycle C-3. At full load, the CO emis-sions of diesel are the highest (15.2 g/kW h), with the other

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18

100%75%50%25%

Load (%)

CO

(g

/kW

h)

BD-0

BD-10

BD-30

BD-50

BD-100

Fig. 4. CO emissions (ppm) vs. engine load during C-3 cycle according toISO 8178.

0

2

4

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100%75%50%25%Load (%)

NO

x (g

/kW

h)

BD-0

BD-10

BD-30

BD-50

BD-100

Fig. 5. NOx emissions (g/kW h) vs. engine load during C-3 cycleaccording to ISO 8178.

S. Murillo et al. / Fuel 86 (2007) 1765–1771 1769

fuels recording lower emissions: BD-10 (12.8 g/kW h), BD-30 (11.7 g/kW h), BD-50 (10.7 g/kW h), and BD-100(11.4 g/kW h). At full load, the oxygen content of theblends and pure biofuels seems to play an important rolein reducing CO emissions. At partial loads, CO emissionsof diesel fuel are comparable to or even lower than allthe other fuels. At very low load (25%), emissions of COfrom the conventional diesel fuelled engine are higher thanthe others. These results agree with those reported by Choiet al. [17] and Dorado et al. [18].

The most accepted explanation for this behaviour is thatat full load the oxygen content of the different biofuelshelps to complete the oxidation of carbonaceous speciesduring combustion. At partial load, there is enough oxygenavailable for complete combustion anyway. CO emissionsare high at low load and low speed (25%), probably dueto the lower gas temperatures inside the cylinder which pre-vent CO being converted to CO2. In this case, CO emis-sions of biodiesel blends are lower than conventionaldiesel, probably due to their lower carbon content.

3.4. NOx emissions

The mechanism of NOx formation from atmosphericnitrogen has been studied extensively and it is accepted thatit is highly dependent upon temperature, due to the highactivation energy needed for the reactions involved. Hencethe most significant factor that causes NOx formation is

Table 4Weighted values obtained according to ISO 8178 cycle C-3

Specific fuel consumption(g/kW h)

Brake therma

BD-0 273.0 0.314BD-10 289.5 0.298BD-30 304.5 0.287BD-50 315.8 0.281BD-100 331.5 0.280M-E 332.8 0.279

high combustion temperatures. Nitrogen oxides (NOx)emissions during test cycle C-3 are shown in Fig. 5, wherean overall decrease can be observed as speed and load areincreased. This is probably due to the increase in turbu-lence inside the cylinder, which may contribute to a fastercombustion and to lower residence time of the species inthe high temperature zones. As shown, almost every bio-fuel produced higher amounts of NOx than conventionaldiesel, which has been well documented [23,24]. It has beenproposed that certain injection systems suffer an unex-pected advance of fuel injection timing caused by thehigher bulk modulus of compressibility in the fuel blendscontaining biodiesel. This increases the speed of sound,causing a faster transference of the pressure wave fromthe injection pump to the nozzle, thereby advancing theneedle lift. It is well known that advancing injection timingcauses an increase in NOx emissions [25,26].

It must be taken into account that outboard engines donot usually employ EGR systems, due to the compactnessof their exhaust system that makes exhaust recirculationextremely difficult to implement under the hood. For thisreason, NOx emissions from outboard engines are normallyhigher than from other engines equipped with EGR systems.

3.5. Overall emissions

The overall values obtained through weighting of thedifferent modes of operation are shown in Table 4. Fig. 6

l efficiency CO emissions(g/kW h)

NOx emissions(g/kW h)

10.7 (1.000) 7.4 (1.000)10.5 (0.981) 7.1 (0.959)9.8 (0.916) 7.5 (1.014)9.7 (0.907) 8.0 (1.081)9.6 (0.897) 8.6 (1.162)

10.0 (0.935) 8.9 (1.203)

9.0

9.2

9.4

9.6

9.8

10.0

10.2

10.4

10.6

10.8

11.0

B-0 B-10 B-20 B-30 B.50 B-100

CO

(g

/kW

h)

0

1

2

3

4

5

6

7

8

9

NO

x (g

/kW

h)

CO

NOx

Fig. 6. Overall values obtained according to ISO 8178 cycle C-3.

1770 S. Murillo et al. / Fuel 86 (2007) 1765–1771

shows their comparison in terms of CO and NOx emis-sions, where the effect of biodiesel in the blend can beclearly seen. As reported by other authors, biodiesel mayreduce the engine’s overall brake thermal efficiency dueto its poorer performance in the low-mid speed range.Overall CO emissions were up to 10% lower when biodieselis blended with conventional diesel or used alone. It shouldbe noted that emissions are measured in g/kW h, thereforethe engine’s thermal efficiency is also affecting.

NOx overall emissions increase when the amount of bio-diesel is increased. If a 50/50 blend of diesel with biodieselis used, then an 8% increase in NOx emission could beexpected. If biodiesel was used alone, then emissions couldrise by 16%.

4. Conclusions

A four-stroke diesel outboard engine running on con-ventional diesel, conventional diesel blended with certainamounts of biodiesel (10%, 30% and 50%), and pure bio-diesel has been tested and proved to be an environmentalfriendly alternative to conventional fuels. A small reduc-tion of power of less than 5% has been observed for BD-10 and BD-30, with this reduction approaching 8% in thecases of BD-50 and BD-100.

It has been extensively reported that the use of biodieselreduces CO emissions. In this paper, CO emissions werereduced by the use of biodiesel between 3% and 10%, whichis below much of the data published by other references. Ithas been pointed out that biodiesel impact on emissionsvaries depending on the type of biodiesel (sunflower, soy-bean, rapeseed, animal fats. . .) and on the type of conven-tional diesel to which the biodiesel is added [27]. This,added to the fact that engine operating conditions and pre-cise features also have a strong influence on the impact ofbiodiesel, makes each study very specific.

Emissions of NOx appear to increase with increasingbiodiesel concentration in the blend. Indeed, emissions ofpure biodiesel are 16% higher than conventional diesel.These values are consistent with the data published by

other researchers [23–27]. It is known that certain aspectsof the engine can be corrected to reduce NOx emissions;however this possibility has not been explored in thispaper.

This paper has shown that the biodiesel can be usedblended with conventional diesel or even alone as a fuelin marine outboard engines without major modificationsto the engine, revealing that the increases in NOx emissionsare tolerable given the other benefits implied in the use ofbiodiesel. The use of a renewable fuel is particularly attrac-tive from an environmental perspective. The use of biodie-sel as partial diesel substitute can also help from a socialstandpoint, as it can boost the agricultural economy andreduce uncertainty on fuel sources.

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