2006 the effect of rapeseed oil methyl ester on direct injection diesel engine performance and...

8/12/2019 2006 the Effect of Rapeseed Oil Methyl Ester on Direct Injection Diesel Engine Performance and Exhaust Emission

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The effect of rapeseed oil methyl ester on direct injectionDiesel engine performance and exhaust emissions

Gvidonas Labeckas *, Stasys Slavinskas

Department of Transport and Power Machinery, Lithuanian University of Agriculture, Student Street 15, P.O. Box LT-53067,

Kaunas Academy, Lithuania

Received 8 March 2005; accepted 11 September 2005Available online 20 October 2005

Abstract

This article presents the comparative bench testing results of a four stroke, four cylinder, direct injection, unmodified,naturally aspirated Diesel engine when operating on neat RME and its 5%, 10%, 20% and 35% blends with Diesel fuel. Thepurpose of this research is to examine the effects of RME inclusion in Diesel fuel on the brake specific fuel consumption(bsfc) of a high speed Diesel engine, its brake thermal efficiency, emission composition changes and smoke opacity of theexhausts.

The brake specific fuel consumption at maximum torque (273.5 g/kW h) and rated power (281 g/kW h) for RME ishigher by 18.7% and 23.2% relative to Diesel fuel. It is difficult to determine the RME concentration in Diesel fuel that

could be recognised as equally good for all loads and speeds. The maximum brake thermal efficiency varies from 0.356to 0.398 for RME and from 0.373 to 0.383 for Diesel fuel. The highest fuel energy content based economy (9.369.61 MJ/kW h) is achieved during operation on blend B10, whereas the lowest ones belong to B35 and neat RME.

The maximum NOxemissions increase proportionally with the mass percent of oxygen in the biofuel and engine speed,reaching the highest values at the speed of 2000 min1, the highest being 2132 ppm value for the B35 blend and 2107 ppmfor RME. The carbon monoxide, CO, emissions and visible smoke emerging from the biodiesel over all load and speedranges are lower by up to 51.6% and 13.5% to 60.3%, respectively. The carbon dioxide, CO2, emissions along with the fuelconsumption and gas temperature, are slightly higher for the B20 and B35 blends and neat RME. The emissions ofunburned hydrocarbons, HC, for all biofuels are low, ranging at 521 ppm levels. 2005 Elsevier Ltd. All rights reserved.

Keywords: Diesel engine; Rapeseed methyl ester; Performance; Emissions; Smoke

1. Introduction

The growing popularity of renewable fuels rests on the intention to create new opportunities for sustain-able multi-functional agriculture of rural development in a more market oriented non-food vegetable oils

0196-8904/$ - see front matter 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.enconman.2005.09.003

* Corresponding author. Tel.: +370 37 752 285; fax: +370 37 752 311.E-mail address:[email protected](G. Labeckas).

Energy Conversion and Management 47 (2006) 19541967

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production to support a flourishing country life and to address the urgent concern about ambient air pollu-tion, which has drastically increased during the last decades because of the growing number of heavy dutyroad-trains, powerful tractors, self propelled agricultural machines and personal cars. The Commission WhitePaper European policy predicts that by the year 2010, the CO2 emissions from transport will have risen toabout 1113 million tons annually with the main responsibility resting on road transport, which accounts

for 84% of the transport related CO2

emissions. Overall usage of biofuels as clean and renewable energy inroad transport, ferry lines, city buses, forestry and agricultural sectors with a special respect to the nationalparks is the only way to cope with the new challenges of tackling environmental problems, preserving thediversity of nature and protecting peoples health and wild life too.

Application of pure rapeseed oil as a cheap and convenient alternative in conventional Diesel engines hashardly been promoted because of some technical problems related to its high density, viscosity, poor filtrationand low volatility[14]. In Europe, one of the mostly popular renewable fuels is rapeseed methyl ester (RME)because its usage in Diesels contributes to a reduction of ambient air pollution by realization of the global CO2cycle. In Austria and Germany, neat biodiesel is widely used, whereas in France, Italy, Spain, Sweden, CzechRepublic and other European Countries up to 2530% RMEDiesel fuel (DF) blends are popular. Transeste-rification improves the technical properties of rapeseed oil so that it meets the requirements to be used in mod-ern Diesel engines. However, the effects of RME and its blends with fossil fuel on power output, specific fuel

consumption and exhaust emission have not been studied completely, and debates about what RME concen-tration in Diesel fuel would be recognised as the best solution continue among experts.

The main advantages related to the usage of RME in Diesel engines are related to the high oxygen contentof the fatty acids, and therefore, more complete combustion and lower emissions of harmful species, such asparticulate matter (PM) and smoke, can be achieved [1]. As other renewable fuels, RME is biodegradable,non-toxic and sulphur free, and consequently, at heavy loads and high gas temperatures, no sulphates areformed and the PM emissions can be reduced by up to 24%[5]. Mixing of RME with Diesel fuel improvesthe lubricating properties of the fuel blends to compensate for the reduced sulphur content in Diesel fuel,which has been lowered from 2000 ppm to 200500 ppm in recent years [6]. According to Ref. [2], Ball OnCylinder or BOCLE test results showed that biodiesel, or at least soybean and rapeseed oil methyl esters, havesuperior lubricity when compared to conventional low sulphur Diesel fuels.

The emission characteristics of Diesel engines operating on neat RME and its blends with Diesel fuel havebeen reported in various research papers [2,8,1013]. In many investigations, reductions in CO, HC and PMemissions and smoke, along with higher NOx, in the exhausts have been determined. For the fully loadedJohn Deere 4276T engine, the nitrogen oxides increase by 11.6% for the yellow grease methyl ester and by13.1% for the soybean oil methyl ester, while the CO2 emissions increase by 1.2% and 1.8%, respectively,along with significantly lower CO (17.8% and 18.2%), unburned HC (46.3% and 42.5%) and smoke (SN0.38 and 0.41)[14]. However, RME mixing with Diesel fuel reduces the calorific value of the fuel blend thatmay result in engine power losses and increased brake specific fuel consumption (bsfc) [79]. Investigationsconducted on a Petter model ACI indirect injection, four stroke, single cylinder Diesel engine showed about11% increased fuel consumption and higher CO2concentrations for neat RME along with reduced CO emis-sions at higher loads[8].

Tests conducted in a John Deere 4276T four cylinder, four stroke, direct injection Diesel engine operated ontwo different soybean methyl esters, one of which had been deliberately oxidized, and with their 20% blendswith Diesel fuel proved that the smoke number, CO and HC were decreased by 8% to 63%, 2% to 29% and 3%to 60%, respectively, while the NOxemissions increased by 0.5% to 18%[15]. The authors determined that theNOxemissions are linearly correlated to the actual start of combustion and, hence, depend on the maximumcylinder gas pressure and temperature. Investigations with a four stroke, four cylinder, direct injection, turbo-charged John Deere 4045TF Diesel engine further indicate that for the same cetane number and the same startof combustion, NOx emissions are the same for soybean biodiesel and Diesel fuel [16].

Experimentations on a Farymann type 18 D air cooled 4.2 kW one cylinder Diesel engine[17]show that theexhaust gas of RME contains about 1033% more ozone precursors than Diesel fuel, however the emissions ofregulated compounds changed quite linearly with the blend composition. So, the well-known positive and neg-ative effects of RME vary according to the blend composition. The authors determined that if the gas temper-

ature in the cylinder remains almost stable, mixing Diesel fuel and RME in different proportions does not lead

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to significant changes in NOx emissions depending on the blend and came to the conclusion that from theemission point of view, no optimal blend can be found at all.

The total NOxemissions, as a sum of both harmful pollutants, NO and NO2, depend actually on the biofuelfeedstock, its chemical structure, oxidation rate, thermal stability and iodine number, i.e. the presence of dou-ble bonds, cetane number, its volatility, flammability and other properties[2]. Road tests on a two wheel drive

Dodge pickup truck with turbocharged Cummins B 5.9 L Diesel engine showed an NOx

increase with increas-ing iodine number in the biodiesel feedstock, but the other emissions, CO, HC and PM, were lower and werefound not to have been affected by iodine number[11]. Other tests[16]showed, for soybean biodiesel, the NOxemissions increased approximately 14% relative to Diesel fuel, but for yellow grease biodiesel, NOxincreasedonly 1%. Biodiesel emission levels depend also on engine design and test conditions, i.e. whether studies wereconducted on bench stand or chassis dynamometer [2], the route and rate of acceleration of vehicles[10],fuel physical and chemical properties[16,18], its oxidation rate, fuel injection timing advance and actual startof combustion[15,19]. It has been disclosed[6]that in all performance modes, RME leads to lower CO and tohigher NOxemissions than the fossil fuels, however no general tendency was found regarding HC emissions.

Neat RME, due to its lesser volatility, may decrease the premixed portion to be burned in the first stages ofthe process, diminishing the peak temperature of the burned gases and resulting in lower NO x, but on theother hand, its oxygenated nature and longer auto-ignition delay may not contribute to such preconditions.

Analysing a large number of test results of various vegetable oil based fuels and their blends with Diesel fuel,Graboski and McCormick[2] concluded that the emissions of NOxincrease significantly with the weight per-cent of oxygen in the biodiesel blend for both neat and blended fuels in both two and four stroke engines.Besides, the highest values of NOxmay not appear at all speeds and loads but in specific regions of the enginemap only. Similar dependencies of NOxemission and other harmful species on the total weight percent of oxy-gen present in biofuels and fuel injection timing advance have been monitored during Cummins L10E engines[12]and Series 60 DDC[13]tests.

Directive 2003/30EC of the European Parliament and Council calls for Member States to assure a mini-mum proportion of biofuels and other renewable fuels for transport purposes on their markets by 31 Decem-ber 2010 shall be 5.75% on the basis of energy content. Lithuania, as a new Member State, shall monitor theeffect of the use of biofuels in Diesel blends above 5% by non-adapted vehicles and shall take measures to in-

sure compliance with the relevant Community legislation on emission standards too. Lithuania is seeking forexperience in biodiesel practical usage because this production is a quite new branch of the industry, which hasbeen started in May of last year.

The basic technical properties of Diesel fuel and RME that was brought from a new production plantRapsoila, Lithuania, are given in Table 1. The net heating value of the RME is 12.5% lower than thatof Diesel fuel, however this deficiency is compensated largely by its higher density (5.0%) and viscosity(59.9%). As was determined in a previous investigation[21], a higher viscosity of biofuel increases the volu-metric fuel delivery per stroke by about 2.6% because of the reduced internal leakages of the injection pump.Higher density and viscosity tend to compensate for the lower calorific value of RME, and therefore, the ac-tual energy content delivered per plungers active stroke may not be substantially lowered relative to that ofDiesel fuel. The elementary composition of RME includes 77.2% carbons, 11.9% hydrogen and 10.9% oxygen.Actual proportion in mass between the carbons and hydrogen of RME is 6.5 compared to 6.9 in Diesel, whichmay lead to more complete combustion of biofuels and, consequently, to higher engine performance efficiency.The low pour point and cold filter plugging point allow the use of RMEDiesel blends at low ambient tem-peratures of15 C.

The cetane number of RME is about the same as that of Diesel fuel, however its volatility is poorer so that,along with its 2.2 times higher flash point, 8 times higher water content and 25 times higher overall contam-ination, it may affect the auto-ignition delay, increase the amount of fuel premixed for rapid combustion andboost the cylinder gas temperature, creating preconditions for NOx formation. The engine performance onRME and its blends with Diesel fuel, as well as its emission characteristics, depend on the combustion cham-ber and injector nozzle design, needle valve opening pressure, airfuel mixture quality, actual start of combus-tion and heat release peculiarities. Therefore, the test results of different engines may vary substantially.Besides, most regulated emissions, including the total NOx, produced by biodiesel depend on the engine speed

and loading conditions[8,9].

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The agricultural sectors of Lithuania, Latvia and Estonia employ a lot of tractors and self propelled ma-chines that were produced in Russia and Belarus. Mainly, they have not been completely adapted for perfor-mance on biofuels. To date, only a few biodiesel tests have been conducted with these engines [3,4,9],consequently the absence of reliable test results retards overall usage of renewable fuels. The purpose of thisresearch was to investigate the influences of RME and its blends with Diesel fuel on the performance, effi-ciency and emission characteristics of one of such engines, like the Diesel D-243, which is widely used in 80

hp agricultural tractors Belarus. The objectives of this study may be stated as follows:

1. Determine the brake specific fuel consumption and fuel energy conversion efficiency when operating onneat RME, Diesel fuel and their blends over a wide range of loads and speeds.

2. Examine the emission composition changes including emissions of nitric oxide NO, nitrogen dioxides NO2,total NOx concentration, carbon monoxide CO, unburned hydrocarbons HC and smoke opacity of theexhausts when operating on RME, Diesel fuel and their blends over a wide range of loads and revolutionsper minute.

2. Experimental apparatus and methodology of the research

Tests have been conducted on a four stroke, four cylinder, water cooled, direct injection, naturally aspi-rated Diesel engine D-243 (59 kW) with splash volumeVl= 4.75 dm

3, bore 110 mm, stroke 125 mm, compres-sion ratio e = 16:1 and toroidal type compressionignition combustion chambers in the piston heads. The fuelwas delivered by an in line fuel injection pump through five holes injection nozzles with the initial fuel deliverystarting at 25 before top dead centre (BTDC). The needle valve lifting pressure for all injectors was set to17.5 0.5 MPa.

To obtain the baseline parameters, the engine was first operated on Diesel fuel grade C. Load characteris-tics were taken with a gradual increase of loads for crankshaft revolutions of 1400, 1600, 1800, 2000 and2200 min1. After all load characteristics were taken of the engine performance on Diesel fuel (DF), fourblends were prepared by a splash mixing technique, which consists of pouring the RME into a Diesel containerin the following proportions by volume: 95% DF and 5% RME (B5), 90% DF and 10% RME (B10), 80% DF

and 20% RME (B20) and 65% DF and 35% RME (B35), and similar experiments were conducted over the

Table 1Properties of Diesel fuel and RME

Property parameters Diesel fuel (grade C) RME

Chemical formula C13H24 C19H35.2O2Density at 15 C (g/cm3) 0.842 0.884Viscosity at 40 C (mm2/s) 2.94 4.70

Flash point, open cup (C) 68 >150Volatility, min 95% at C 360 366Flammability (C) 250 342Cold filter plugging point (C) 5 16Pour point (C) 0 10Cetane number 51.6 51.0Sulphur (mg/kg) 33


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same range of engine loads and revolutions per minute. Mixing Diesel fuel in given proportions with RME ledus to obtain fuel blends with various amounts of oxygen by mass as 0.925%, 1.45%, 2.5% and 4.075%. After-wards, the Diesel engine was fuelled with a neat RME that is known to have the highest mass percent, 10.9%,of oxygen, and the engine performance characteristics were taken again and the exhaust gas emissions mea-sured over the same test ranges.

The torque of the engine was measured with a three phase asynchronous 110 kW electrical AC dynamom-eter with a definition rate of 1 Nm. The engine load characteristics were taken with a gradual increase fromthe point that was close to zero up to its maximum value of 290310 Nm. This means that the effective powerof the engine at rated speed n= 2200 min1 had been changed from the minimum up to 110% of its ratedvalue.

According to specifications, the engine D-243 at fully open throttle should develop controlled torques thatat revolution frequencies of 1400, 1800 and 2200 min1, correspond to the external loads of bmep = 0.77, 0.76and 0.68 MPa. Mainly, at the given loading conditions, comparative analysis of the engine performance on thevarious RMEDiesel blends and their emission characteristics were performed.

The revolution frequency of the crankshaft was measured with the universal ferrite-dynamic stand tachom-eter TSFU-1 and its counter ITE-1 that guarantees an accuracy of 0.2%. The fuel mass consumption wasmeasured by weighing it on the electronic scale VLK-500, and the volumetric air consumption was determined

by means of the rotor type gas counter RG-400-1-1.5 installed at the air tank for reducing pressure pulsations.The amounts of carbon monoxide CO (ppm) and dioxide CO2 (vol%), nitric oxide NO (ppm), nitrogen

dioxide NO2(ppm) and the residual content of oxygen O2(vol%) in the exhaust were measured with the Testo33 gas analyser.

The amounts of unburned hydrocarbons HC (ppm vol) and residual oxygen O2(vol%), which were deter-mined afterwards, as well as the carbon monoxide CO (vol%) and dioxide CO2 (vol%) emissions, in the ex-haust gases were additionally checked with the TECHNOTEST Infrared Multigas Tank gas analysermodel 488 OIML.

The smoke opacity D (%) of the exhausts was measured with the Bosch device RTT 100/RTT 110 in I100% scale with 0.1% accuracy. The gas temperature in the exhaust manifold was measured with thechrome-aluminium thermocouple TChK-400U connected to the galvanometer MKD-50M.

3. Test results and analysis

Graphs of the brake specific fuel consumption (bsfc) in g/kW h as a function of load obtained during engineoperation on RME, Diesel fuel and their blends at speeds of 1400, 1800 and 2200 min1 have been superim-posed as shown inFig. 1. As is obvious from the figures, the bsfc values decrease gradually with the load tolevels that depend on the engine rotation speed and the biofuel used, remaining at the highest level for neatRME. At the speed 1400 min1 and fully opened throttle, the brake specific fuel consumption of 234.5 g/kW h was obtained for the Diesel fuel and 256.4 g/kW h for the RME. In spite of the different calorific values,the fuel blends B5 and B35 maintain, at equivalent loads, about the same bsfc as that of Diesel fuel, whereasthe B10 and B20 blends suggest the bsfc is lower by 3.2% and 1.7%, respectively. The lower bsfc can be related,reasonably, to the higher, 1.45% and 2.50%, amounts of oxygen present in the considered blends. Fuel basedoxygen, because of its indigenous property, accelerates reactions from within the extremely fuel rich spray pat-terns themselves, leading to more complete combustion at low speed.

At a higher speed of 1800 min1, the bsfc of the fully loaded engine for the B20 blend is the same as that forDiesel fuel, 230.4 g/kW h, whereas B35 and RME suggest the bsfc is higher by 8.5% and 18.7%, respectively.The bsfc of blend B5, which differs itself as having the lowest, 0.925%, amount of oxygen, appeared to benearly the same as that of blend B10 (222.0 g/kW h), both of them having lower bsfc by 3.6% relative to Dieselfuel. This result indicates a diminishing role of fuel oxygen at high speed and, hence, high airfuel mixtureturbulence intensity, allowing residual air borne oxygen to be available at the final combustion stages.

This point of view is further supported by data obtained for the engine runs under full load at the ratedspeed 2200 min1, where only blends B5 and B10 maintain their bsfc lower by 1.5% relative to Diesel fuel(228 g/kW h). The higher RME concentration in blends B20 and B35, as well as the use of neat RME, suggest

the bsfc is higher by 8.8%, 14.0% and 23.2%, respectively.



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The higher fuel consumption of the B20 and B35 blends and RME can be related primarily to the lower, on

average by 12.5%, net heating value of RME (Table 1). However, this is probably not the only reason thatleads to the higher biofuels consumption in grams per unit energy developed. At the engine rated speed,the bsfc values for the B20 and B35 blends have, on an average, been increased by 0.42% for every 1% increasein the RME inclusion in the Diesel fuel. That is 3-fold higher relative to the effect given in the test reported inRef.[1]. The rapid increase in bsfc for highly concentrated RMEDiesel blends could possibly be reduced oreliminated after investigation of the effect of fuel injection timing advance, needle valve opening pressure andthe spray parameters [15,19].

For further analysis, the brake thermal efficiencies as a function of load for the Diesel engine operating onRME, Diesel fuel and their blends have been superimposed as shown in Fig. 2. At the rated speed of2200 min1, during engine transition from light to heavy load operation, the brake thermal efficiency increases

Fig. 1. The brake specific fuel consumption (bsfc) as a function of engine load (bmep) at various rotation speeds (n).

Fig. 2. The brake thermal efficiency as a function of engine load (bmep).



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with load up to bmep = 0.68 MPa, reaching the top efficiency values of 0.375, 0.380 and 0.378 for Diesel fueland the B5 and B10 blends, respectively. Here are seen, probably, the most visible disadvantages of the B20and B35 blends. Although those thermal efficiencies increase smoothly with load, they tend to converge to theone obtained under Diesel operation on neat RME with lower top efficiency values averaged at 0.355. Thelatter indicate that higher than 10 vol% of RME in Diesel fuel lowers the fuel energy conversion efficiency

for the biofuel.In order to learn more about this situation, it was decided to test for the speed dependencies of the max-imum brake thermal efficiency values as a function of percentage of RME premixed into Diesel fuel. As it fol-lows from the analysis of the curves in Fig. 3, the maximum values of the brake thermal efficiency increasewith RME concentration in the blend up to a degree that at every particular speed, tends to be a bit different.For common Belarus engines like this one, the best solution that is more or less acceptable for all speedswould be blend B10, which maintains the maximum thermal efficiency values ranging from 0.378 to 0.398.Test results indicate, evidently, that the fuel energy conversion efficiency starts to decline rapidly when themass percent of RME in Diesel fuel exceeds 10% by volume, and this result is true for all loads and speedstested. During engine operation on the B20 and higher blends, its performance declines to the lowermost effi-ciency levels and hardly responds to treatment by further RME addition in the Diesel fuel.

Test results indicate that when the mass percent of fuel oxygen exceeds 1.45% the oxygen loses its positive

influence on the fuel energy conversion efficiency in this particular engine. New evidence related to possiblecauses of such behaviour must await further investigations, but the worsening of the process can be linkedto the blends structure and its physical properties, changes in injection timing advance, fuel evaporationand the start of combustion [16,19]. Because of the different structures of RME, its evaporation proceedsat much high temperatures, ranging between 325 and 366 C than those of Diesel fuel (210360C). The high-er flash point, poor volatility and flammability may have an influence on the auto-ignition and combustionprocesses of biofuels. This is especially important at high speeds where the extent of evaporation and the com-bustion processes actually play a key role. Because the operating conditions of the unmodified Diesel enginecould have been not optimal for all RMEDiesel blends at all loads and speeds, the fuel energy conversionefficiency in our tests appeared as rather being dependent on the content of biofuel premixed into the Dieselfuel[2].

As one can see inFig. 4, the emission of nitric monoxide NO increases gradually with load, reaching themaximum values of 19242066 ppm for RME and 18231925 ppm for Diesel fuel at high loads. When oper-ating at the low speed of 1400 min1, the minimum NO emissions at adequate loads were maintained by theB5 (0.925% oxygen) blend. This blend suggests NO emissions lower by 3.8% to 17.0% relative to Diesel fuel.Biofuels with higher oxygen contents, 1.45% to 4.075%, and neat biodiesel produce higher or a bit lower NOemissions, depending on the load. In the maximum torque regime of 1800 min1, the NO emissions for all thebiofuel blends average from 3.5% to 15.3% higher than those of Diesel fuel, whereas at the rated speed of2200 min1, only the B5 and B10 blends suggest NO emissions lower by 22.5% to 7.2% and 11.2% to 7.6%,respectively, at adequate loads.

Fig. 3. Dependencies of the maximum brake thermal efficiency on percentage of RME premixed into Diesel fuel at various engine rotation

speeds.



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According to theory [20], the higher is the temperature of the burned gases, the higher the rate of NOemission can be expected. In the case of the Stanadyne fuel injection pump, one of the reasons for higherNOxemissions for biodiesel is related to the timing advance that in such cases, automatically occurs to obtainhigher volumetric delivery per stroke[19]. In contrast to a distributor pump, an in line one compensates for thelower calorific value of RME by altering the end of the plunger s active stroke. This means that in our tests, theactual start of fuel injection could have been changed because of interference of the fuel physical properties.

Only such properties as the bulk modulus, density and viscosity remain as reasons for the relevant NOxvari-ations. As determined in Ref.[18], the density, speed of sound and the isentropic bulk modulus change linearlywith biodiesel blend percentage, which lets us expect the actual timing advance to be a bit earlier.

Emissions of nitrogen dioxide NO2tend to increase slightly with load, except in the rated speed regime. It isdifficult to determine clear NO2 changing tendencies due to the narrow variation interval, however, for allloads and speeds, the lowest NO2emissions, 2836 ppm, were measured for the B5 blend, and the biggest ones,4960 ppm, were obtained for the Diesel fuel. Other biofuels produce NO2emissions within appointed inter-vals converging at the rated power to about a common 42 ppm level. Experiments with a single cylinder, directinjection Diesel engine indicate that the bsfc, CO, PM and smoke density are higher, but the maximum CO2and NO2 emissions may be lower for biofuels than for Diesel fuel [7]. Lower NO2 emissions have beenobtained, probably, because of the oxygenated nature of biofuels that stimulates NO2 conversion back to

NO[20].

Fig. 4. The nitric monoxide NO and nitrogen dioxide NO2emissions as a function of engine load (bmep) at various speeds (n).



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In order to get a full scale view of the total nitrogen oxides behaviour, the maximum NOx values weresuperimposed as a function of mass percent of fuel oxygen and engine speed as shown in Fig. 5. Analysisof the test results shows, that the maximum NOx emissions increase proportionally with the mass percentof oxygen in the RMEDiesel blends. During transition from the conventional fuel (0.4% oxygen) to theB35 blend (4.075% oxygen), the maximum NOx values increase from 1873 to 2132 ppm, or by 5.0% to

9.1%, depending on the speed, which is in good agreement with test results reported by many researchers[2,1113]. It is interesting to note that the NOxemissions continue to increase with the amount of fuel oxygenin spite of deterioration of combustion efficiency (Fig. 2) for higher than B10 (1.45% oxygen) fuel blends.Thus, in this research, like it was determined in other tests [15], the flame temperature changes alone cannotadequately explain the higher NOx levels for oxygenated fuel blends, supporting the priority of the oxygenbeing contained in the fuel itself, which makes extra oxygen available for combustion in fuel rich zones, caus-ing the NOx emissions to rise.

The total NOxemission for RME with the highest (10.9%) percent of fuel oxygen appeared to be a little bitlower relative to that measured for the B35 blend. One of the reasons for the lower NO x emissions for neatRME can be related to its indigenous feature of not containing any aromatic compounds. According toRef. [2], because of the non-aromatic structure of biodiesel, one can expect lower emissions of PM andNOxtoo. Other possible reasons for the reduced NOxemissions from RME can be related to its slower evap-

oration and lower calorific value that may also contribute through relevant gas temperature changes. In gen-eral, however, because of the effect of fuel oxygen, the NOxemissions for RME remain on average higher by3.7% relative to Diesel fuel.

The maximum NOx emissions increase with revolutions up to 2000 min1, reaching the top value of

1983 ppm for Diesel fuel and 2132 ppm for the B35 blend. When the maximum values of NO x have beenreached, the nitrogen emissions start to decrease gradually for the rated speed. As it reflects on the rearmostcolumns ofFig. 5, the NOx emissions from biofuels at the rated speed are lower relative to their previouslyobtained readings at 2000 min1, which correlates well with the lower fuel energy conversion efficiency andlower gas temperature at the considered speed.

The carbon monoxide CO emissions as a function of load (bmep) for biofuels are presented inFig. 6. Inspite of the overall fuel lean mixture of k 6.58.0, typical for the easy load operation, poor atomisation

and uneven distribution of small portions of fuel across the combustion chamber, along with a low gas tem-perature, may lead to local oxygen deficiency and incomplete combustion. That could be the answer as to whyCO emissions tend to increase for the easy loaded engine. In such circumstances, the CO emission of the B10

Fig. 5. The maximum of the total NOxemissions as a function of mass percent of oxygen (%) in RMEDiesel blends and engine rotation

speed (n).



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and higher blends, including neat RME, are reduced up to two times, ranging between 300 and 600 ppm withmore rapid increase only for heavy loads.

When the engine load reaches a certain bmep (=0.72 MPa), the CO emissions start to increase more rap-idly. At fully opened throttle, the biggest CO emission of 1150 ppm was measured for Diesel fuel, and the low-est of 557 ppm was obtained for blend B10. The CO emissions for blends B5, B20, B35 and neat biodiesel were633, 695, 580 and 811 ppm, respectively. Reduced CO emissions were maintained, probably, thanks to theoxygen inherently present in biofuels, which differs as having the highly acceptable feature of being righton the spot, facilitating the combustion of big fuel portions. In contrast to air borne oxygen, the fuel basedoxygen accelerates the combustion process from within the fuel rich spray patterns themselves. This advantageis especially beneficial at heavy loads when the air-to-fuel equivalence ratios in local oxygen deficiency spraycores have been diminished far beyond the critical 1.6 point.

At high revolutions and high radial turbulence intensity in the toroidal chamber, the mixing of the fuel richportions with ambient air should improve, but on the other hand, the duration of the combustion process ex-pressed in units of time becomes limited too, which results in only a slight CO emission decrease with Dieselspeed. Anyway, at high speed, the positive effects of RME and its blends on CO emissions remains, without

undergoing significant changes.The effect of biofuels on the smoke opacity remains similar to that described above because both emissions

are known as primary forecasters related to the engines performance efficiency. According to Ref.[20], soot isan unavoidable combustion by product produced by the pyrolysis reaction at high gas temperatures of about10002000 K. It appears even at sufficient amounts of air to burn the fuel completely because combustion pro-ceeds through dissociation of hydrocarbons with the presence of intermediate products, such as acetyleneC2H2 and polycyclic aromatic hydrocarbons, which are known as precursors of soot formation. The smokeopacity depends finally on both the balance of soot particles formation and their oxidation rate, as well ason the presence of volatile or soluble unburned organic compounds (PM) suspended in the exhausts [22].

As is obvious from analysis of the curves inFig. 7, when operating at light to moderate loads, the smokeopacity is as low as 310%, however, during transition to heavy loads, it increases rapidly. At the same airfuel

Fig. 6. Carbon monoxide emissions as a function of load (bmep).

Fig. 7. Smoke opacity of Diesel exhausts as a function of load (bmep).



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mixture composition of k= 1.6, the engine develops the bmep = 0.713 MPa for Diesel fuel and thebmep = 0.656 MPa for RME with the corresponding smoke levels of 46% and 31.4%. The other biofuels, pre-sented in ascending fuel oxygen concentration order, suggest smoke of 39.8%, 36.0%, 35.2% and 38.4%,respectively. Visibly lighter smoke, along with drastically reduced CO emission and a bit higher NOxconcen-tration, was obtained because of the positive effect of the fuel oxygen, whose role is evident at critical loading

conditions.As is seen inFig. 8, the emission of unburned hydrocarbons HC for all fuels and speeds is negligibly small,521 ppm, increasing slightly with load and proportion of fuel injected. It is quite difficult to determine anyreliable dependencies, however, the HC emissions for biofuels proceed in the graphs at a bit lower levels thanthat of Diesel fuel.

The temperatures of the exhaust gases of the fully loaded engine operating at rated speed and constantk= 1.6 appear as high as 495 C for the Diesel fuel and 500 C for RME. The other biofuels indicate similargas temperatures, ranging between 494 and 503 C, i.e. close to that of conventional Diesel fuel. The high tem-perature of the burned gases prevented condensation of the heaviest hydrocarbons in the sampling line, sug-gesting proper conditions for HC emission analysis [22].

Emissions of carbon dioxide CO2 increase with load proportionally to the fuel consumption in mass(Fig. 9). It seems pretty clear that the amounts of CO2 and the gas temperatures have been divided at high

loads into two groups. The higher exhaust temperatures, along with the higher CO 2 emissions from B20,B35 and RME correlate reasonably well with the observed stratification in the brake thermal efficiency curvesinFig. 2. The higher CO2emissions are a direct indication of the slightly higher fuel consumed for blends B20,

Fig. 8. Hydrocarbons emissions in Diesel exhausts as a function of load (bmep).

Fig. 9. Dependencies of exhaust gas temperatures and carbon dioxide emissions on engine load (bmep).



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B35 and neat RME. In the case of biodiesel, the higher carbon dioxide emission should cause less concernbecause, during the forthcoming summer, the ultimate sun energy will utilise a major part of the CO2, produc-ing new hydrocarbons by raising oil crops and releasing oxygen to be freed to the open blue sky, enjoyingNatures recovery.

In order to maintain adequate fuel burning conditions that are necessary for strict analysis of the effect of

biofuels on the engine performance, it was decided to calculate and gather the main parameters all together forthe same air-to-fuel mixture composition, which could be recognised as typical for many loaded engines. Thecolumns in Fig. 10 reflect in more detail the percentage changes in the engine load (bmep), brake specificenergy consumption (bsec), the total emissions of NOx, CO and smoke opacity at constant overall air-to-fuelequivalence ratio ofk = 1.6 for three speed ranges and the five biofuels relative to ordinary Diesel fuel with thecorresponding data denoted at each set of parameter columns. With such approach, the energy contents accu-mulated by the various air fuel mixtures remain nearly the same, 1.7641.752 MJ/kg, suggesting proper pre-conditions for accurate analysis.

As is obvious from both the bmep and bsec columns, the blends B5 and B10 at all speeds suggest the changein power output by up to 1.8% higher and that in the brake specific fuel energy consumption by up to 4.2%lower relative to ordinary Diesel fuel. It becomes clear that only low concentration RME blends could be

Fig. 10. Dependency in percentage changes of engine performance parameters on RME concentration in Diesel blends at three speed

ranges and constant overall air-to-fuel equivalence ratio 1.6.



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recognised as potential candidates to be certificated for full scale usage in unmodified engines. The higher con-centration blends B20, B35 and neat RME have proved themselves as efficient alternatives to be used mainly atlow revolutions and heavy loads. At higher speeds, the engine performance on blends B20, B35 and RMEundergoes significant changes, leading to power output on average 8.78.0% lower and bsec 4.76.4% higher.

At all speeds, the emissions of CO for all the biofuels remain up to 51.6% lower and the smoke opacity on

average 13.560.3% lower relative to Diesel fuel. The better utilisation of cylinder air indicates the lower resid-ual oxygen in the exhaust manifold, 8.19.2 vol% for biodiesel versus 11.812.9 vol% for ordinary fuel. Thelower CO emission and reduced smoke correlate well with the slightly higher (up to 8.8%) NO x emitted bybiodiesel (B35), thus remaining in good harmony with the test results of many other researchers.

4. Conclusions

1. The brake specific fuel consumption at maximum torque (273.5 g/kW h) and rated power (281 g/kW h) forRME is higher by 18.7% and 23.2% relative to Diesel fuel. It is difficult to determine the rate of RME inclu-sion in the Diesel fuel that could be recognised as optimum for use at all loads and speeds. At fully openedthrottle and low speed, the bsfc is lower by 3.2% and 1.7% for the B10 and B20 blends. At moderate rev-olutions, it is lower by 3.5% for both the B5 and B10 blends, whereas at rated power, the B5 blend suggests

slightly better (1.5%) fuel economy.2. The fuel energy conversion efficiency depends actually on both the RME inclusion percent in the Diesel fuel

and the engine performance conditions. The maximum brake thermal efficiency values vary between 0.3560.398 for RME and 0.3730.383 for Diesel fuel. At all speeds and loads, the highest fuel energy contentbased economy (9.369.61 MJ/kW h) is achieved during operation on blend B10, whereas the lowest onebelongs to the B35 blend and neat RME (9.2010.40 MJ/kW h).

3. The maximum NOxemissions increase proportionally with the mass percent of oxygen in the biofuel andengine speed, at 2000 min1 reaching the highest, 2132 ppm, value for the B35 blend and 2107 ppm forRME. At the rated speed, the total NOx emissions for all fuels are slightly lower, ranging from1885 ppm (DF) to 2051 ppm (B35), which correlates well with the lower brake thermal efficiency at this par-ticular speed.

4. The carbon monoxide CO emissions and visible smoke emerging from the biodiesel at constant air-to-fuelequivalence ratio of k= 1.6 over all loads and speeds are lowered by up to 51.6% and 13.5% to 60.3%,respectively. The carbon dioxide CO2 emissions, along with the fuel consumption and gas temperature,are slightly higher for the B20 and B35 blends and neat RME. The emission of unburned hydrocarbonsHC for all fuels is low, 521 ppm, showing slightly milder values for RME and its blends with Diesel fuel.

The test results indicate that due to the quite comparable cost and the real advantages in terms of perfor-mance efficiency and environmentally friendly emissions, up to 10% biofuel blends could be regarded as pri-mary candidates to be put on stream for full scale usage in unmodified Diesel engines.

Acknowledgements

The authors would like to thank the authority of a new RME production plant Rapsoila Ltd., Mazeikiairegion, Lithuania, for assistance in obtaining the necessary amounts of RME and its quality parameters.

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