Biodiesel emissions profile in modern diesel vehicles. Part 1: Effect of biodiesel origin on the criteria emissions

Science of the Total Environment 409 (2011) 1670–1676

Contents lists available at ScienceDirect

Science of the Total Environment j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s c i t o t e n v

Biodiesel emissions profile in modern diesel vehicles. Part 1: Effect of biodiesel origin on the criteria emissions Evangelos Bakeas a, Georgios Karavalakis b,⁎,1, Stamoulis Stournas b a

Laboratory of Analytical Chemistry, Chemistry Department, National and Kapodistrian University of Athens, Panepistimioupolis, 15771, Athens, Greece Laboratory of Fuels Technology and Lubricants, School of Chemical Engineering, National Technical University of Athens, 9 Iroon Polytechniou Str. Zografou Campus, 157 80, Athens, Greece

b

a r t i c l e

i n f o

Article history: Received 25 May 2010 Received in revised form 17 August 2010 Accepted 14 January 2011 Available online 12 February 2011 Keywords: Biodiesel Oxidized biodiesel Driving cycles Vehicles Exhaust emissions

a b s t r a c t This paper presents the regulated emissions profile of a Euro 4 compliant common rail passenger car, fuelled with low concentration biodiesel blends. Four biodiesels of different origin and quality blended with a typical automotive diesel fuel at proportions of 10, 20, and 30% v/v. Emission and fuel consumption measurements were conducted on a chassis dynamometer with constant volume sampling (CVS) technique, over the New European Driving Cycle (NEDC) and the real traffic-based Artemis driving cycles. Limited effects were observed on CO2 emissions, while fuel consumption marginally increased with biodiesel. PM, HC and CO emissions improved with the addition of biodiesel, with some exceptions. Some increases with biodiesel were observed over the NEDC, as a consequence of biodiesel characteristics and engine conditions. NOx emissions were increased with the use of biodiesel blends and positively correlated with fuel unsaturation levels. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Diesel engines are the main power source in heavy-duty trucks and buses, and their share is rapidly growing in passenger cars as well. More than half (53%) of new passenger cars in the European Union (EU) are sold with a diesel engine (ACEA, 2009). The increasing demands on transportation with middle distillate fuels have driven the EU to emphasize on the use of biodiesel as an alternative to petroleum diesel. Fatty acid methyl esters, commonly known as biodiesel, can be produced from any vegetable oil, animal fats and used frying oils via the transesterification reaction. The advantages of biodiesel include domestic origin, renewability, biodegradability, higher flash point, absence of sulfur and aromatic compounds, and inherent lubricity (Dunn, 2005). However, biodiesel has poor lowtemperature properties and its chemical nature makes it more susceptible to oxidation or autoxidation during long-term storage in comparison to petroleum diesel fuel (Knothe, 2007). Several studies have reported that the use of biodiesel has been shown to be effective at reducing most regulated exhaust emissions, such as PM, unburned hydrocarbons (HC), and carbon monoxide (CO) (Di et al., 2009; Durbin et al., 2000; Knothe et al., 2006; Szybist et al.,

⁎ Corresponding author at: University of California Riverside, Bourns College of Engineering, Center for Environmental Research and Technology, 1084 Columbia Ave, Riverside, 92507, USA. Tel.: + 1 9517815799; fax: + 1 9517815790. E-mail addresses: [email protected] (E. Bakeas), [email protected], [email protected] (G. Karavalakis). 1 Tel.: + 30 2107723213; fax: + 30 2107723163. 0048-9697/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.scitotenv.2011.01.024

2007). Only a few studies are available in the literature on modern diesel passenger cars, employing common-rail engine systems and exhaust aftertreatment technologies. Many of these studies may lead to results inconsistent to what is generally reported (Durbin et al., 2007; Fontaras et al., 2009; Karavalakis et al., 2009a; Luján et al., 2009). A number of authors have reported that the addition of biodiesel leads to increases in PM, CO and HC emissions over cycles such as the New European Driving Cycle (NEDC). The explanation given by the authors was that the higher viscosity and the lower volatility of biodiesel make especially difficult the fuel evaporation and atomization in the cold-start conditions, during the first phase of the cycle (Urban Driving Cycle-UDC). Another contributing factor may be the reduced efficiency of the oxidation catalyst when the engine is cold and the fact that the engine is tuned for regular automotive diesel fuel instead of biodiesel (Fontaras et al, 2010; Martini et al., 2007; Tinaut et al., 2005). An increase in NOx emissions is usually acknowledged, which is mainly attributed to certain physicochemical properties of biodiesel, fuel spray characteristics, engine type and condition, and engine operation. Several researchers have suggested that the higher oxygen availability in the combustion chamber when using biodiesel, could promote NO formation reactions (Cardone et al., 2002; Wu et al., 2009). Other authors indicate that NOx formation is highly dependent on the degree of unsaturation and cetane number of biodiesel (Fernando et al., 2006; McCormick et al., 2001). In general, higher degrees of saturation correlate with higher cetane numbers. Saturated biodiesel fuels (i.e. without double bonds) produce lower amounts of NOx than unsaturated fuels. Moreover, changes in physical properties

E. Bakeas et al. / Science of the Total Environment 409 (2011) 1670–1676 Table 1 Technical specifications of the test vehicle. Engine type

Hyundai i-10 (CRDi VGT)

Fuel injection system Cylinders/valves Displacement (cm3) Maximum power (kW) Maximum torque (N m) Weight (kg)

Direct injection, common-rail 3/12 1120 75/4000 rpm 153/1900–2750 rpm 1127

such as viscosity, speed of sound, bulk modulus and density may also contribute to higher NOx levels (Szybist et al., 2005). Previous studies on the oxidation stability of biodiesel showed that certain properties, such as heating value, cetane number, acid value and viscosity can be significantly affected (Dunn, 2005; Knothe, 2007). Oxidative processes bring about increased viscosity as a result of condensation reactions involving double bonds, also leading to the formation of insolubles, gums and other impurities, which can potentially plug fuel filter, nozzle and injection pump of an engine (McCormick et al., 2007). Moreover, the acid and hydroperoxide formation may cause corrosion of fuel system components and hardening of elastomers (Lin and Chiu, 2009). Thus, biodiesel instability may not only lead to engine stalling or even breakdown, but may also impact engine emissions. The purpose of this study was to investigate the emissions profile of a modern passenger car representative of the current European fleet with biodiesels obtained from different sources, and to assess the effect of using oxidized biodiesel on the formation of criteria emissions.

2. Experimental 2.1. Test vehicle and fuels A 2007 model year Hyundai i-10, equipped with a common-rail direct injection diesel engine and meeting Euro 4 emission standards, was used in this study. CO, HC, and PM emissions for this vehicle were controlled by a diesel oxidation catalyst (DOC). All emission tests were performed with the vehicle in its original configuration. The technical specifications of the vehicle are listed in Table 1. Thirteen fuels were evaluated in this study. An ultra low sulfur diesel, meeting the current fuel quality requirements for diesel vehicles (EN 590:2009), was used as a reference and for the creation of blends with four types of biodiesels at proportions of 10, 20, and 30% by volume. The biodiesel fuels were a soy-based methyl ester produced from soybean oil blended with palm oil (SMEP), an animal

1671

fat methyl ester (AFME), a used frying oil methyl ester (UFOME), and an olive oil methyl ester (OME). It should be mentioned that both UFOME and OME were naturally oxidized fuels stored for a long period of time. It was found that sulfur content was slightly above the specification and more than three times higher than the specification EN 14214. The sulfur in used frying oils is suspected to originate from food residues cooked in the oil, while feedstocks from slaughterhouse waste fats as hairs usually contain sulfur. The neat biodiesels were analyzed according to the automotive FAME standard EN 14214 (Table 2), with the main quality properties of the diesel fuel and its blends are given in Table 3. Most of the fuel properties were found to agree with the specification limits, except for the water content of the OME blends which was found well above the maximum specification limit. The sulfur content for the AFME blends was also found above 10 μg g− 1, as well as the density values of AFME-30 and UFOME-30.

2.2. Driving cycles and measurement protocol In order to investigate the impact of biodiesel on the exhaust emissions and fuel consumption, the vehicle was driven on a chassis dynamometer over the certification NEDC and the non-legislated Artemis driving cycles. The Artemis cycles are distinguished into an urban (Urban), a rural (Road), and a motorway (Motorway) part, each representative of the corresponding driving condition. Compared to the certification test, the Artemis driving cycles exhibit more frequent speed variation and stronger accelerations. The speed vs. time profiles of the applied driving cycles can be found elsewhere (Fontaras et al., 2009). The daily measurement protocol started with the NEDC, which is a cold-start driving cycle. This comprises two parts; an urban part (UDC) where the engine starts from room temperature and an extraurban part (EUDC), which aims at testing the car at higher than urban speeds. The NEDC was then followed by the three Artemis cycles. This protocol was repeated twice per fuel blend, with two sets with the reference fuel were conducted at the beginning and end of the campaign. Prior to each measurement, the vehicle was conditioned for about 350–400 km before testing, whenever a fuel change was required.

2.3. Exhaust sampling and emission analyzers Emission measurements were conducted following the European regulations (Directive 70/220/EEC and amendments). Gaseous and PM mass were sampled using a dilution tunnel with constant volume sampling (CVS) technique. A schematic of the sampling system and

Table 2 Physicochemical properties of the neat methyl esters. Property

SMEP

AFME

UFOME

OME

EN 14214 limits

Test method

Viscosity (40 °C, mm2 s− 1) Density (15 °C, g cm− 3) Flash point (°C) Sulfur content (μg g− 1) Water content (μg g− 1) CFPP (°C) Oxidation stability, 110 °C (hours) Iodine number Acid value (mgKOH g− 1) Monoglyceride content, % (m/m) Diglyceride content, % (m/m) Triglyceride content, % (m/m) Free glycerol, % (m/m) Total glycerol, % (m/m) Ester content, % (m/m) Linolenic acid methyl ester, % (m/m) Gross heating value, (cal g− 1)

4.2 0.885 N140 1 420 −2 6.3 127 0.7 0.67 0.16 0.05 0 0.20 97.7 6.3 9530

4.59 0.8752 N 140 34 760 10 8.1 52 0.28 0.41 0.16 0.11 0.018 0.234 91.9 0.74 9631

4.64 0.8825 172 11 661 −3 0 102 0.52 0.31 0.12 0.08 0.009 0.346 93 3.0 9415

5.33 0.8857 178 5 1874 0 0 79 1.47 0.23 0.11 0.10 0.014 0.251 90 0.44 9323

3.5–5.0 0.860–0.900 120 min 10 max 500 max − 5 max 6 min 120 max 0.50 max 0.80 max 0.20 max 0.20 max 0.02 max 0.25 max 96.5 min 12 max –

EN ISO 3104 EN ISO 12185 EN ISO 3679 EN ISO 20846 EN ISO 12937 EN 116 EN 14112 EN 14111 EN 14104 EN 14105 EN 14105 EN 14105 EN 14106 EN 14105 EN 14103 EN 14103 IP 12

1672

E. Bakeas et al. / Science of the Total Environment 409 (2011) 1670–1676

Table 3 Main quality properties of the diesel fuel and its blends with biodiesel. Property

Diesel

Viscosity (40 °C, mm2 s− 1) Density (15 °C, g cm− 3) Flash point (°C) Sulfur content (μg g− 1) Water content (μg g− 1) CFPP (°C) Distillation IBP 10 50 90 95 FBP Polycyclic aromatic hydrocarbons, % (m/m) Gross heating value (cal g− 1)

SMEP10

SMEP20

SMEP30

AFME10

AFME20

AFME30

OME10

OME20

OME30

UFOME10

UFOME20

UFOME30

EN 590 limits

3.25 3.30 0.8325 0.836

3.36 0.841

3.51 0.8469

3.29 0.8368

3.41 0.8408

3.55 0.8462

3.40 0.8368

3.49 0.8371

3.69 0.8377

3.33 0.8361

3.42 0.8409

3.53 0.846

68 4.7 45 − 10

69 5.3 55 −9

74 3 87 −8

77 2.4 127 −5

69 15 96 −7

73 17 149 −6

75 19 214 −3

69 4.5 212 −7

74 3.7 323 −6

76 3.4 529 −6

71 3.9 101 −8

73 4.3 142 −5

78 3.2 174 −4

2.00–4.50 0.820– 0.845 55 min 10 max 200 max + 5 max

197 238 279 336 368 373 2.7

205 235 283 339 365 374 –

205 238 293 342 355 367 –

199 231 300 349 354 357 –

176 204 283 349 354 357 –

194 215 293 339 353 355 –

176 208 300 338 348 349 –

203 231 282 340 359 363 –

169 215 299 343 361 362 –

183 231 302 345 346 354 –

204 208 281 337 355 360 –

210 232 289 343 351 352 –

207 239 303 340 350 353 –

11 max

10,655

10,348

10,915

10,209

10,245

10,458

10,254

10,044

10,411

10,382

10,188

11,068 10,704

detailed information regarding the emission analyzers are given in Karavalakis et al. (2009a). 3. Results and discussion 3.1. Normalized emissions and fuel consumption The following analysis was performed in order to better characterize the statistical significance of the experimental results. Emission results retrieved for each pollutant over each driving cycle were normalized against the average baseline emissions recorded for the particular pollutant over the same driving cycle. All driving cycles were considered in the analysis with the exception of NEDC when investigating HC and CO emissions, because of the cold-start effect on exhaust aftertreatment system, which creates a distinct case not directly comparable with the rest of the results. Normalization eliminated the driving cycle effect and provided a basis for a more global comparison of each feedstock's impact. Two tailed, paired ttests were performed between the baseline results and those of the various blends in order to calculate the statistical significance of the observations. NOx and PM emissions are of high interest when considering diesel engine emissions. Fig. 1(a) shows a trend towards higher NOx emissions with biodiesel blends. On average, emissions were increased by 2.06, 5.92, and 9.28% for B10, B20 and B30 respectively. When examining the results individually it appears that the application of AFME blends led to almost similar NOx emissions than those of diesel fuel. On the other hand, the unsaturated SMEP blends and the oxidized blends resulted in noticeable increases. As explained in the following section, the degree of saturation in the methyl ester is the parameter that correlates the most with NOx emissions. As shown in Fig. 1(b), PM reductions were −0.73, −3.42, and − 5.78% for B10, B20, and B30 respectively. It appeared that the application of B10 resulted in marginal differences, while the PM reduction potential was maximized at a blending ratio of 30% v/v. No particular trend was observed regarding the feedstock effect on PM emissions. However, the application of the oxidized blends systematically resulted in higher PM emissions than the other biodiesel blends, while the SMEP blends the lowest. The latter was as expected since there was seemingly a trade-off between the two: higher NOx and lower PM. Fig. 1(c–d) shows that HC and CO emissions presented similar evolution. The reductions in HC emissions were −3.43, −8.13, and

−12.73% for B10, B20, and B30 respectively. The use of the oxidized blends generally led to higher reductions when compared to diesel fuel and the other biodiesel blends. Regarding CO emissions the reductions were −7.48, − 16.98, and − 22.61% for B10, B20 and B30 respectively. It appeared that the use of SMEP led to higher reductions followed by the oxidized UFOME and OME blends. In all cases, the observed differences in HC and CO emissions were statistically significant. As shown in Fig. 1(e), the use of biodiesel resulted in limited CO2 increases. These were 0.38, 1.11, and 1.6% for B10, B20, and B30 respectively. For all four types of blends the impact was almost the same regardless of biodiesel source material. Fuel consumption was also increased by 2.38, 4.35, and 6.05% for B10, B20, and B30 respectively (Fig. 1f). It should be noted, that the application of the oxidized blends resulted in higher fuel consumption than the other biodiesel blends. 3.2. Regulated emissions, CO2 and fuel consumption Fig. 2(a) presents the NOx emissions for all fuel/cycle combinations. A trend towards higher NOx emissions over the NEDC was observed for all biodiesel blends except for the saturated blends of AFME. The 10% blend was a NOx neutral fuel relative to the baseline diesel, while AFME-20 and AFME-30 produced lower NOx emissions in the order of −7 and −13% respectively. However, the AFME blends resulted in higher NOx emissions over Artemis operation. On average, these increases were 2, 6 and 8% for AFME-10, AFME-20 and AFME-30 respectively. The unsaturated SMEP blends resulted in increases over the NEDC, which were in the order of 2, 10 and 16% for SMEP-10, SMEP-20 and SMEP-30 respectively. These results are in line with previous studies that have reported that with increasing saturation levels (lower iodine number) it is possible to get reductions in NOx emissions (Fernando et al., 2006; Karavalakis et al., 2009b; McCormick et al., 2001). It is widely accepted that iodine number is inversely related to cetane number (low iodine number correlates with high cetane number). The use of saturated fuels, which are usually characterized by a high cetane number, may lead to an advancement of combustion by shortening the ignition delay and thus lowering NOx formation (Ban-Weiss et al., 2007). However, the cetane number should not be considered as solely responsible for the NOx emissions profile of this study. It is possible that the higher oxygen availability in the combustion chamber when using biodiesel, could also promote the NO formation reactions (Lapuerta et al., 2008a).

Fig. 1. (a–f): Average normalized emissions and fuel consumption with the reference diesel fuel and the B10, B20 and B30 biodiesel blends. Error bars correspond to the maximum and minimum values recorded. NEDC results were not considered in the case of HC and CO emissions.

E. Bakeas et al. / Science of the Total Environment 409 (2011) 1670–1676

1673

1674

E. Bakeas et al. / Science of the Total Environment 409 (2011) 1670–1676

a

b 0.075

1

Diesel AFME-10 AFME-20 AFME-30 SMEP-10 SMEP-20 SMEP-30 OME-10 OME-20 OME-30 UFOME-10 UFOME-20 UFOME-30

0.0625

0.75

PM emissions, g km-1

NOx emissions, g km-1

0.875

0.625

0.5 0.375

Euro 4 Limit

0.25

0.05

0.0375

0.025

Euro 4 Limit

0.0125 0.125

0

0 NEDC

c

. Urban

Art

d rt. Roa

A

rway rt. Moto

A

d

0.16

an

Art. Ro

an

Art. Ro

NEDC

Art. Urb

NEDC

Art. Urb

torway

ad

Art. Mo

ad

Art. Mo

0.5 0.45

0.14

CO emissions, g km-1

HC emissions, g km-1

0.4 0.12 0.1

0.08 0.06

0.04

0.35

0.3 0.25

0.2 0.15

0.1 0.02

0.05

0

0 NEDC

an Art. Urb

ad Art. Ro

ay

torw Art. Mo

torway

Fig. 2. (a–d): Emissions of NOx (a), PM (b), HC (c), and CO (d) for the tested fuels over the NEDC and Artemis driving cycles.

In general, the oxidized biodiesel blends resulted in higher NOx emissions than diesel fuel and the other biodiesel blends. The highest emissions over the NEDC were found with the use of OME-30 and UFOME-30 and were 20 and 22% respectively, while the higher average emissions over the Artemis cycles were 12 and 8% respectively. The higher NOx emissions with the OME and UFOME blends may be attributed to the higher oxygen content of the methyl ester, which resulted from the presence of primary and secondary oxidation products. Although the exact oxygen content was not determined, this observation can be supported by the lower energy content of OME and UFOME when compared to the other biodiesel fuels. Despite the fact that UFOME had a relatively low number of, or no, double bonds, due to the breakdown of unsaturated fatty acids during thermal stressing, the formation of NOx may be favored by the presence of hydroperoxides (oxidation products). It is possible that during combustion these species caused higher levels of certain hydrocarbon radicals in the fuel-rich zone of the diesel spray, which resulted in increased formation of prompt NOx. Moreover, a different mechanism of increasing NOx may be associated with glycerol presence, as concentrations of glycerol have been associated with changes in the ignition delay and subsequent NOx increases (Hamasaki et al., 2004). Additionally, NOx increases may be favored by the higher viscosity of both oxidized blends when compared to AFME and SMEP blends (Zhu et al., 2010). It should be noted, that in some cases NOx emissions with biodiesel were found to be above the Euro 4 standards (0.25 g km− 1). This

result is consistent with the idea that the engine was initially tuned over the NEDC with regular automotive diesel instead of diesel/ biodiesel blends. Significantly higher levels of NOx emissions were found over the Artemis cycles when compared to those of the NEDC. However, an adverse effect of biodiesel on NOx emissions was still observed during these driving modes. In general, NOx emissions are usually higher during transient operation (Peterson et al., 2000). This is attributed to the increased load and speed for the Artemis cycles, which increases the combustion temperatures and thus favors the NOx formation (Tsolakis et al., 2007), as opposed to the smooth acceleration profile of the NEDC, where the engine uses only a very small area of its operating range (Zervas and Bikas, 2008). PM emissions produced discordant results, as shown in Fig. 2(b). The use of AFME and SMEP blends reduced PM emissions over the NEDC, while some increases were observed when oxidized blends were tested. The highest reductions were achieved with the use of AFME-30 and SMEP-30, which both were −13%. During Artemis operation the average reductions for AFME-30 and SMEP-30 were −11 and −12% respectively. It should be noted, that PM emissions for all the evaluated fuels were found to be below the Euro 4 limits. The reduction in PM emissions with biodiesel was mainly attributed to the presence of oxygen in the ester molecule and the absence of aromatic compounds and sulfur (Dwivedi et al., 2006). On the other hand, the PM increase may be due to the combined effect of fuel type and engine operating conditions. In general, the use of biodiesel results in a sharp increase in the soluble organic fraction, which is mainly formed by

E. Bakeas et al. / Science of the Total Environment 409 (2011) 1670–1676

4. Conclusions In this study, the impact of biodiesel source material and type on the regulated emissions and fuel consumption of a modern common-

a

250

CO2 emissions, g km-1

225

200 175

150

Diesel AFME-10 AFME-20 AFME-30 SMEP-10 SMEP-20 SMEP-30 OME-10 OME-20 OME-30 UFOME-10 UFOME-20 UFOME-30

125

100 75

50 25

0

b

ad

an

Art. Ro

an

Art. Ro

NEDC

Art. Urb

NEDC

Art. Urb

torway

Art. Mo

10 9

Fuel consumption, L 100km-1

higher molecular weight unburned hydrocarbons condensate on the particles surface (Cheung et al., 2009; Lapuerta et al., 2008b; Martini et al., 2007). It was therefore possible that the presence of cyclic acids and polymerization products in the oxidized blends did not fully oxidize inside the catalyst during the cold-start start-up phase of the NEDC. An unexpected increase was found over the Artemis Urban cycle with the oxidized blends. The highest increases were 25 and 47% for UFOME-20 and UFOME-30 respectively and may also be related to the partial oxidation of degradation products in the catalyst. However, the use of biodiesel led to some reductions over the high speed and load Artemis Road and Motorway cycles, which could be related to better combustion efficiency at high loads and to the better effectiveness of fuel-borne oxygen to reduce PM (Zervas and Bikas, 2008). Fig. 2(c) and (d) shows the HC and CO emissions, respectively, for all fuel/cycle combinations. The addition of biodiesel blends led to reductions in HC emissions over the NEDC and the Artemis driving cycles. The highest reductions over the NEDC were − 10 and −12% for AFME-30 and OME-30 respectively, while the highest average reduction over the Artemis cycles were −12 and −19% for OME-30 and UFOME-30 respectively. These reductions could be a consequence of the oxygen content in the methyl ester, which leads to a more complete combustion. Moreover, the presence of peroxides and hydroperoxides formed during the biodiesel oxidation process may result in lower HC emissions (Monyem and Van Gerpen, 2001). However, some increases in HC emissions were observed with biodiesel over the NEDC. In particular, the use of SMEP-10 resulted in a sharp increase (20%) over the NEDC and constantly led to higher HC emissions over the Artemis cycles (4% on average). In addition, the use of UFOME blend resulted in limited increases over the NEDC. This result may be due to the lower volatility of the biodiesel blends and the cold-start effect during the UDC operation (Fontaras et al., 2009; Lapuerta et al., 2008a). Similar to HC, CO emissions improved with the use of biodiesel blends, especially over the Artemis cycles. On average, the highest reductions during Artemis operation were − 19, −28, −22 and −22% for AFME-30, SMEP-30, OME-30 and UFOME-30 respectively. Limited increases were found over the NEDC (ranged between 1 and 3%), which can be attributed to the poor catalyst efficiency during the UDC phase. It was also found that the use of the oxidized blends led to higher CO emission levels, when compared to the other biodiesel blends. As explained in a previous study conducted by Hamasaki et al. (2001), this trend could be caused by a higher hydroperoxide concentration since they participate in CHO, HCHO and CO formation reactions. It is noteworthy, that the emission levels of both pollutants decreased as the mean load and speed of the driving cycle increased. Fig. 3(a) and (b) presents results for CO2 emissions and fuel consumption. The use of biodiesel, independent of its origin, showed a trend towards slightly elevated CO2 emission levels over all driving conditions. It is therefore difficult to draw a solid conclusion regarding the potential effect of biodiesel source material on CO2 emissions. Fuel consumption followed similar patterns to CO2 and increased with biodiesel. This increase was approximately proportional to the difference in energy content of the biodiesel blends. Since engine fuelling systems operate on a volumetric basis, proportional consumption increases should occur. The use of oxidized biodiesel blends also led to consistently higher fuel consumption over all driving cycles, than the reference diesel and the other biodiesel blends, which can be ascribed to the lower heating value of these fuels. The latter was possibly related to the presence of oxidation products and thus to the higher amount of oxygen in both UFOME and OME.

1675

8 7

6 5

4 3

2 1

0 ad

torway

Art. Mo

Fig. 3. (a–b): CO2 emissions (a) and fuel consumption (b) for all fuel/cycle combinations.

rail passenger car was examined. The findings of this investigation revealed that the application of low concentration biodiesel blends led to statistically significant reductions in HC and CO emissions. The highest reductions were observed with the use of the oxidized blends, which may be ascribed to the presence of hydroperoxides and the higher oxygen content. In most cases, the use of biodiesel favorably influenced PM emissions. The highest statistically significant reductions were achieved with AFME and SMEP blends, while some increases were found with the application of the oxidized blends. No particular trend was observed regarding the feedstock effect on PM emissions. In some cases, marginal increases in HC, CO, and PM emissions were observed over the legislated NEDC, which may be attributed to certain physicochemical characteristics of biodiesel and to the cold-start effect. The use of biodiesel adversely affected NOx emissions over most driving conditions. The highest average emissions were observed with the oxidized blends, whereas the lowest with the saturated AFME blends. It should be noted, that in most cases the blends of B20 and B30 resulted in statistically significant higher NOx emissions. NOx emissions were found to be affected by biodiesel origin and quality. With increasing unsaturation of the methyl ester, higher NOx emissions were obtained. Moreover, total glycerol content in the fuel seemed to negatively influence NOx emissions. The use of the

1676

E. Bakeas et al. / Science of the Total Environment 409 (2011) 1670–1676

oxidized blends also led to higher NOx emissions when compared to diesel fuel and the other biodiesel blends. These results raise concerns on the impact of low quality biodiesel fuels on certain emissions and ultimately on urban air quality. The use of biodiesel had limited effects on CO2 emissions, which is an indication that the engine efficiency did not drop, especially under low speed–low load conditions. Fuel consumption increased with the increase in biodiesel content. The highest fuel consumption was observed with the use of the oxidized blends due to the lower energy content compared with diesel fuel and the other biodiesel blends. Acknowledgement This paper is dedicated to the memory of Professor Stamos Stournas. References ACEA. Available online at www.acea.be 2009. Ban-Weiss GA, Chen JY, Buchholz BA, Dibble RW. A numerical investigation into the anomalous slight NOx increase when burning biodiesel; a new (old) theory. Fuel Process Technol 2007;88:659–67. Cardone M, Prati MV, Seggiani M, Senatore A, Vitolo S. Brassica carinata as an alternative oil crop for the production of biodiesel in Italy: engine performance and regulated and unregulated exhaust emissions. Environ Sci Technol 2002;36:4656–62. Cheung CS, Zhu L, Huang Z. Regulated and unregulated emissions from a diesel engine fueled with biodiesel and biodiesel blended with methanol. Atmos Environ 2009;43:4865–72. Di Y, Cheung CS, Huang Z. Experimental investigation on regulated and unregulated emissions of a diesel engine fueled with ultra-low sulfur diesel fuel blended with biodiesel from waste cooking oil. Sci Total Environ 2009;407:835–46. Dunn O. Effect of antioxidants on the oxidative stability of methyl soyate (biodiesel). Fuel Process Technol 2005;86:1071–85. Durbin TD, Collins JR, Norbeck JM, Smith MR. Effects of biodiesel, biodiesel blends, and a synthetic diesel on emissions from light heavy-duty diesel vehicles. Environ Sci Technol 2000;34:349–55. Durbin TD, Cocker III DR, Sawant AA, Johnson K, Miller JW, Holden BB, et al. Regulated emissions from biodiesel fuels from on/off-road applications. Atmos Environ 2007;41:5647–58. Dwivedi D, Agarwal AK, Sharma M. Particulate emission characterization of a biodiesel vs diesel-fuelled compression ignition transport engine: a comparative study. Atmos Environ 2006;40:5586–95. Fernando S, Hall C, Jha S. NOx reduction from biodiesel fuels. Energy Fuels 2006;20: 376–82. Fontaras G, Karavalakis G, Kousoulidou M, Tzamkiozis T, Ntziachristos L, Bakeas E, et al. Effects of biodiesel on passenger car fuel consumption, regulated and nonregulated pollutant emissions over legislated and real-world driving cycles. Fuel 2009;88:1608–17. Fontaras G, Kousoulidou M, Karavalakis G, Tzamkiozis T, Pistikopoulos P, Ntziachristos L, et al. Effects of low concentration biodiesel blend application on modern

passenger cars. Part 1: feedstock impact on regulated pollutants, fuel consumption and particle emissions. Environ Pollut 2010;158:1451–60. Hamasaki K, Kinoshita E, Tajima H, Takasaki K, Morita D. Combustion characteristics of diesel engines with waste vegetable oil methyl ester. The 5th international symposium on diagnostics and modeling of combustion in internal combustion engines (COMODIA); 2001. Hamasaki K, Kinoshita E, Tajima H. Effects of water droplet size and crude glycerin content as an emulsifier on diesel combustion with emulsified biodiesel. Trans Jpn Soc Mech Eng 2004;70:2907–13. Karavalakis G, Stournas S, Bakeas E. Effects of diesel/biodiesel blends on regulated and unregulated pollutants from a passenger vehicle operated over the European and the Athens driving cycles. Atmos Environ 2009a;43:1745–52. Karavalakis G, Stournas S, Bakeas E. Light vehicle regulated and unregulated emissions from different biodiesels. Sci Total Environ 2009b;407:3338–46. Knothe G. Some aspects of biodiesel oxidative stability. Fuel Process Technol 2007;88: 669–77. Knothe G, Sharp CA, Ryan TW. Exhaust emissions of biodiesel, petrodiesel, neat methyl esters, and alkanes in a new technology engine. Energy Fuels 2006;20:403–8. Lapuerta M, Armas O, Fernandez JR. Effect of biodiesel fuels on diesel engine emissions. Prog Energy Combust Sci 2008a;34:198–223. Lapuerta M, Rodríguez-Fernández J, Agudelo JR. Diesel particulate emissions from used cooking oil biodiesel. Bioresour Technol 2008b;99:731–40. Lin CY, Chiu CC. Effects of oxidation during long-term storage of the fuel properties of palm oil-based biodiesel. Energy Fuels 2009;23:3285–9. Luján JM, Bermúdez V, Tormos B, Pla B. Comparative analysis of a DI diesel engine fuelled with biodiesel blends during the European MVEG-A cycle: performance and emissions (II). Biomass Bioenergy 2009;33:948–56. Martini G, Astorga C, Farfaletti A. Effect of biodiesel fuels on pollutant emissions from EURO 3 LD diesel vehicles. Transport and Air Quality Unit; Institute for Environment and Sustainability, EC-Joint Research Centre; 2007. McCormick RL, Graboski MS, Alleman TL, Herring AM, Tyson KS. Impact of biodiesel source material and chemical structure on emissions of criteria pollutants from a heavy-duty engine. Environ Sci Techol 2001;35:1742–7. McCormick RL, Ratcliff M, Moens L, Lawrence L. Several factors affecting the stability of biodiesel in standard accelerated tests. Fuel Process Technol 2007;88:651–7. Monyem A, Van Gerpen JH. The effect of biodiesel oxidation on engine performance and emissions. Biomass Bioenergy 2001;20:317–25. Peterson C, Taberski J, Thompson J, Chase C. The effect of biodiesel feedstock on regulated emissions in chassis dynamometer tests of a pickup truck. Trans ASAE 2000;43:1371–81. Szybist JP, Boehman AL, Taylor JD, McCormick RL. Evaluation of formulation strategies to eliminate the biodiesel NOx effect. Fuel Process Technol 2005;86:1109–26. Szybist JP, Song J, Alam M, Boehman AL. Biodiesel combustion, emissions and emission control. Fuel Process Technol 2007;88:679–91. Tinaut FV, Melgar A, Briceno Y, Horrillo A. Performance of vegetable derived fuels in diesel engine vehicles. Int Congr Combust Sci. Poland: PTNSS Kongres; 2005. Tsolakis A, Megaritis A, Wyszynski ML, Theinnoi K. Engine performance and emissions of a diesel engine operating on diesel-RME (rapeseed methyl ester) blends with EGR (exhaust gas recirculation). Energy 2007;32:2072–80. Wu F, Wang J, Chen W, Shuai S. A study on emission performance of a diesel engine fueled with five typical methyl ester biodiesels. Atmos Environ 2009;43:1481–5. Zervas E, Bikas G. Impact of the driving cycle on the NOx and particulate matter exhaust emissions of diesel passenger cars. Energy Fuels 2008;22:1707–13. Zhu L, Zhang W, Liu W, Huang Z. Experimental study on particulate and NOx emissions of a diesel engine fuelled with ultra low sulphur diesel, RME-diesel blends and PME-diesel blends. Sci Total Environ 2010;408:1050–8.