Particles emitted during the stops of an urban bus fuelled with ethanol–biodiesel–diesel blends

Urban Climate 2 (2012) 43–54

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Particles emitted during the stops of an urban bus fuelled with ethanol–biodiesel–diesel blends Octavio Armas ⇑, Arantzazu Gómez, Carmen Mata, Ángel Ramos Departamento de Mecánica Aplicada e Ingeniería de Proyectos, Universidad de Castilla-La Mancha, Av. Camilo José Cela s/n, 13071 Ciudad Real, Spain

a r t i c l e

i n f o

Article history: Received 24 July 2012 Revised 17 October 2012 Accepted 25 October 2012

Keywords: Diesel Ethanol Biodiesel Particle size distributions Urban bus

a b s t r a c t Urban buses with diesel engines produce NOx and particulate matter emissions thus affecting local city pollution. During stops, due to traffic lights, programmed stops, etc., pollutants are emitted close to people, being damaging to their health. This paper studies the effects on particle size distributions caused by the use of oxygenated fuels during stops of an urban bus. Fuels used are a blend of ethanol and diesel (denoted as ED) and a blend of ethanol, biodiesel and diesel (denoted as EBD), which are compared to a diesel fuel used as reference. Tests were carried out on a vehicle under real working conditions in the Spanish city of Seville. A Horiba 1300 on-board system (OBS) was used to measure operating conditions, and an Engine Exhaust Particle Sizer Spectrometer was used to determine particle size distributions. Results showed a reduction in the number concentration of accumulation mode when oxygenated fuels were used. However, they produced a slight increase in the nuclei mode, as well as a reduction in particle diameters from both modes when using EBD blends, being more significant when referring to accumulation mode. These findings may be considered negative for the oxygenated blends under these operating conditions. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction The increase of city pollution is partly due to growing number of light and heavy duty vehicles. Urban transportation services, such as bus and rail networks, are essential for controlling the number ⇑ Corresponding author. Tel.: +34 926295462; fax: +34 926295361. E-mail address: [email protected] (O. Armas). 2212-0955/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.uclim.2012.10.007

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of vehicles which travel around a town. Although in some Spanish cities small fleets of electric or gas fuelled buses can be seen, most of them are reliant on diesel engines. Due to their large engine size, and often high load operating conditions, they produce significant amounts of fine and ultrafine particle emissions (Canagaratna et al., 2004; Phuleria et al., 2007). Several studies have shown that these particles are harmful to health (Somers et al., 2004; Vineis and Husgafvel-Pursiainen, 2005), particularly from toxicology and mutagenicity point of views (Bünger et al., 1998; Gangwar et al., 2012; Green and Armstrong, 2003; McClellan et al., 2012). Most of the particles by number currently emitted by diesel engines are smaller than 50 nm in diameter (usually called nuclei mode), while by mass particles mainly belong to the accumulation mode, with diameters between 50 and 1000 nm (Kittelson, 1998). Although particle concentrations emitted during vehicle stops (due to traffic lights, stops for leaving or getting passengers, stops produced by traffic retentions, etc.) are relatively low compared with other transient operation sequences, the emission in this operating mode occur very close to the people. Likewise, although most of the stop time is at constant relative fuel–air ratio due to engine idling operation, in most cases there are two additional time periods, one at the beginning and other at the end of the vehicle stop, where the ratio changes. These reasons have motivated this work. In order to reduce particulate matter emissions, total or partial substitution of diesel fuel for oxygenated fuels as biodiesel or ethanol is being studied. Between them, biodiesel is the most widely used. In fact, current Spanish regulations establish until 7% of biodiesel in commercial fuels (http:// www.boe.es/diario_boe/txt.php?id=BOE-A-2011-5937). However, it has some disadvantages in comparison with ethanol like transesterification of biodiesel is more complex and expensive than ethanol production or its oxygen content is three times lower, decreasing its potential for reducing particulate matter emissions. Two additional reasons for using ethanol–biodiesel–diesel blends are: (a) the highest fuel lubricity of ethanol–biodiesel–diesel blend (EBD) compared to both diesel fuel and ethanol– diesel blend (ED), which can improves the durability of the injection system (Armas et al., 2011a) and (b) the highest stability of EBD compared to diesel and ED fuels, which makes possible a long storage without separation of the blend compounds (Lapuerta et al., 2009). Therefore, nowadays the effects of ED and EBD blends are being widely studied both under stationary (Xing-cai et al., 2004; Zhu et al., 2010; Aydin and Ilkılıç, 2010; Pang et al., 2006; Pidol et al., 2012; Sayin, 2010; Shi et al., 2005, 2006) and under transient conditions (Barabás et al., 2010; Armas et al., 2007). Several authors have found important mass reductions in particulate matter (Zhu et al., 2010; Pang et al., 2006; Pidol et al., 2012; Shi et al., 2005, 2006) and reductions in particle number concentration (Kim and Choi, 2008; Di et al., 2009; Lapuerta et al., 2008a). Nevertheless, the reduction in concentration does not necessarily mean that the diameter of the particles emitted by the engine also decreases. Lapuerta et al. (2008a) showed that although the concentration and the particulate matter are reduced with the use of ethanol–diesel blends, the size distribution moves towards smaller diameters which are more harmful to health. Di et al. (2009) showed that although the distribution is shifted towards lower diameters, an increase in the number of ultrafine particles was not seen. They also found that efficiency of the ethanol blends in decreasing the particle concentration is greater at higher particle diameters. Due to difficulty in carrying out accurate tests with the available experimental techniques, the number of works published which include measurements of particle emissions with oxygenated fuels in vehicles is small (Armas et al., 2009, 2010; Randazzo and Sodré, 2011; Ahmed, 2001; Hansen Alan et al., 2005; McCormick and Parish, 2001; Merritt et al., 2005). Most of the studies show clear trends in reduction in both particulate matter and smoke opacity (Randazzo and Sodré, 2011; Ahmed, 2001; Hansen Alan et al., 2005; McCormick and Parish, 2001; Merritt et al., 2005). Nonetheless, not many studies have been carried out studying changes in particle concentration. Lee et al. (2009) studied the effect on particles produced by the use of an ethanol–diesel blend during the New European Driving Cycle (NEDC), obtaining 28% reduction in total particle concentration with respect to diesel over the cycle. Armas et al. (2010) studied the effect of emissions with an ethanol–diesel blend in several transient sequences and obtained a reduction in the concentration and diameter in the accumulation mode, together with an increase in the nuclei mode and the appearance of the nuclei mode at points of the operating cycle where it did not exist with diesel fuel.

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In addition, since ethanol has low miscibility in diesel fuel, an important factor influencing the use of ethanol–diesel blends in vehicles in real-life conditions is the difficulty of creating a homogeneous blend with an acceptable stability and without problems in the injection system due to the blend separation (Lapuerta et al., 2007). There are fewer difficulties using EBD blends in vehicles in real-life conditions, since the biodiesel acts as a stabilizing agent (Kwanchareon et al., 2007; Fernando and Hanna, 2004), as commented above. In the present research, the main purpose was to evaluate the effect on the particle size distributions of an urban bus operating under real working conditions during those time intervals when it is stopped in programmed stops, using ED and EBD mixtures. 2. Experimental set-up 2.1. Vehicle The test vehicle was a Renault EURO II urban bus, 12,700 kg weight, 10 years old and with 3,500,000 km at the moment of the experimental tests, without exhaust gas recirculation (EGR), diesel oxidation catalyst (DOC) and diesel particle filter (DPF). It was equipped with a turbocharged, 6cylinder, 7790 cm3 Iveco direct injection diesel engine. It had a maximum power of 186 kW at 2050 min1 and a maximum torque of 1100 Nm at 1000 min1. The engine was equipped with a hydraulic controlled linear injection pump. The engine was linked to a Voith automatic transmission. 2.2. Fuels tested A diesel fuel with low sulphur content was used as reference. Similarly, a mixture denoted as ED with a 7.7% in volume of ethanol in reference diesel fuel and a 0.62% of stabilizing additive was tested. The final test involved a ternary ethanol–biodiesel–diesel blend denoted as EBD with 10% in volume of ethanol in a B30 mixture (30% in volume of soybean methyl ester in reference fuel). Biodiesel molecules have polar end with ethanol affinity, which is also polar in nature. But they also have a nonpolar carbon chain with affinity to diesel fuel. Due to these biodiesel properties no stabilizing additive was required in the EBD mixture. Both oxygenated fuels were supplied by Abengoa Bioenergy New Technology Co. The ethanol content in the mixtures was chosen based on the results from previous studies (Lapuerta et al., 2009, 2007; Randazzo and Sodré, 2011). Table 1 shows the main characteristics or properties of the fuels tested in this work. Cetane number of diesel fuel was measured by the supplier (Repsol Co.). However, in case of blends, this number was calculated using the procedure presented by Lapuerta et al. (2009). 2.3. Instrumentation The bus was equipped with a Horiba 1300 on-board system (OBS) and a TSI Engine Exhaust Particle Spectrometer (EEPS™) model 3090. The OBS analyzer included sensors to determine the relative Table 1 Properties of the fuels tested.

Density at 15 °C (kg/m3) Kinematic viscosity at 40 °C (cSt) Higher heating value (MJ/kg) Lower heating value (MJ/kg) %C (by weight) %H (by weight) %O (by weight) %H2O (by weight) %S (by weight) Stoichiometric relative fuel–air ratio Cetane number Vaporization enthalpy (kJ/kg)

Diesel

ED

EBD

844 2.72 45.54 42.61 86.13 13.87 0 0.0057 0.0034 1/14.67 52 242.82

831 2.41 43.82 40.86 83.63 13.82 2.55 0.024 0.0031 1/14.25 48.7 285

843 2.61 43.15 40.31 81.07 13.26 5.67 0.065 1/13.63 49.5 277.98

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fuel–air ratio and atmospheric conditions (temperature, pressure and relative humidity). The vehicle speed was measured with a Global Positioning System (GPS). The GPS used is part of the Horiba 1300 OBS. The EEPS was coupled to a rotating disk (TSI, model MD19-2E), as first dilution stage and this coupled to a thermal conditioner (TSI, model ASET15-1), with an evaporation tube, as second dilution stage in order to determine, during transient operation, the particle size distributions. The dilution factors and temperatures at both the rotating disk and thermal conditioner were defined based on previous studies (Armas et al., 2011b). The total dilution factor used was fitted to 217.55 with 6.12% of diffusion and thermophoresis losses. The temperatures of the rotating disc and the evaporation tube were held constant at 80 and 300 °C, respectively, throughout the test. The OBS and the EEPS sampling frequency was 1 Hz. Synchronization of the signals, recorded by each equipment, was achieved by a procedure described in Arregle et al. (2006) and used in Armas et al., 2011b. The overall particle concentration was synchronized with respect to the signal from the relative fuel–air ratio with the Horiba OBS during accelerations preceded by idling. This procedure was also used to check the synchronization of the signals at other points of the cycle. 2.4. Experimental procedures The route followed by the bus (hereafter called cycle) was about 13 km long (Route 34 of the urban buses in Seville) and takes approximately 1 h. Data collection for each fuel was carried out throughout the route with real passenger transportation at the same hour over the five working days of the week. Weather was similar during tests campaign in order to avoid any effect of environmental conditions as temperature or humidity they might have on particle emission. Moreover, for the same reason the process of measurement always took place with the vehicle’s air conditioning and heating switched off. Derived from the relative fuel–air ratio and vehicle speed profiles, those time intervals with zero vehicle velocity were selected from the data as shown in Fig. 1. Division of the cycle into sequences has been previously presented by Armas et al. (2011b, 2012). As an example, Fig. 2 shows the time percentage of stop sequences (denoted as I derived from idle operation, which accounts 21% of the total urban cycle tested) compared to the other sequences registered during a cycle. The time

0.6

25 Stop sequences

20

0.4 15

10 0.2

Vehicle velocity (km/h)

Relative fuel air ratio

Relative fuel air ratio Vehicle velocity

5

0

0 0

100

200

300

400

500

time (s) Fig. 1. Vehicle speed and relative fuel–air ratio from the first part of the bus route identifying the sequences studied.

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O. Armas et al. / Urban Climate 2 (2012) 43–54 950s 26.4%

DwF

D 310s 8.6%

1595s 44.3%

A I

Acceleration (A) Idle (I) Deceleration with fuel consumption (DwF)

745s 20.7%

Deceleration without fuel consumption (D) Fig. 2. Time distribution during a cycle.

for both acceleration (A) and deceleration with fuel consumption (DwF) sequences accounts 44% and 26% of the total cycle time respectively. The rest of the time is used for decelerating without fuel consumption (D). Fig. 3 shows, as example, the time distribution of the stop sequences with reference diesel fuel. A typical programmed stop lasts 20 s or longer, representing 61% of the total time used during the stops in a cycle. Therefore this sequence was chosen as a reference for later analysis. 3. Results and discussion 3.1. Fuel–air ratio profiles During the stops, although the vehicle velocity was zero, the engine operating conditions were variable at the beginning and at the end of the sequence, as registered by relative fuel–air ratio (Fr). Fig. 4 shows an illustrative scheme of the evolution of the relative fuel–air ratio profiles during the stop sequences when the reference diesel fuel was tested. Similar profiles were registered for ED and EBD. Fig. 4 shows that a stop sequence can be divided into three different zones. The first (zone 1), which lasts 6–8 s (tZ1), has a maximum relative fuel–air ratio at tZ1 6 2 s, and then subsequently declines. In

2s 3s 4s

1%2% 2% 5s 3% 6s 3%

2s

11s

3s

12s

4s

13s

5s

14s

9s

6s 7s

15s 16s

10s

8s 9s 10s

17s 18s 19s

7s 4%

8s 3% 2%

3%

<20s de61% 20s

> 20s

Fig. 3. Time distribution of stop sequences with reference fuel.

11-19s 11%

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Fig. 4. Evolution of the relative fuel–air ratio profiles during stop sequences.

this case the ratio mostly decreases from a value of 0.275–0.15. The second (zone 2), whose duration (tZ2) depends on the total time of the sequence (tt), is characterized by a constant relative fuel–air ratio 0.15. Finally, the third zone (zone 3) comprises the last 4 s (tZ3) of the sequence and is characterized by an increase in the relative fuel–air ratio up to values of around 0.25. Fig. 5 shows the time evolution of the mean relative fuel–air ratio of sequences with the three fuels tested. Mean profiles were determined from 10 sequences with the highest coincidence for each fuel, which showed similar ratios each other. This was done in order to guarantee equality in operating conditions, so that possible variations in pollutant emissions could be attributed exclusively to the fuel. This analytical procedure was described previously in Armas et al. (2011b). 3.2. Particle size distributions Fig. 6 shows particle size distributions derived from the 1st and 8th second of zone 1 for the three fuels used. Bimodality of the resulting distributions is the main characteristic observed which may be produced for different reasons. First, as the engine has neither DOC nor DPF, the hydrocarbon emissions may have contributed to the appearance of bimodality. The low temperature in the primary dilution system (80 °C in the walls of the rotating disk) could cause condensation and thereby the formation of small hydrocarbon droplets. Although the evaporation tube between the first and second dilution stages should prevent the condensation of hydrocarbons, it is probably too short to evaporate all the volatile compounds in the sample. Second, the decrease of the relative fuel–air ratio in zone 1 leads

Fig. 5. Mean relative fuel–air ratio profiles for the stop sequences with the three fuels tested.

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(a)

(b)

Fig. 6. Mean particle size distributions measured at the 1st (a) and 8th (b) seconds of zone 1.

to an increase in oxygen availability. This is accompanied with a reduction in the mean in-cylinder temperature, leading to an increase of the ignition delay. In consequence, the combustion process occurs far from heterogeneous mixture and diffusive flames (Payri and Desantes, 2011). These phenomena lead to a lower soot formation, characterized by very low particle concentration. Due to the decrease of the relative fuel–air ratio in zone 1, the use of fuel blends reduced the accumulation mode concentration. However, an opposite trend can be seen in the nuclei mode. Likewise, when comparing fuels, Fig. 6 shows how the oxygenated mixtures considerably reduce the concentrations in the accumulation mode (up to 50% with ED and 68% with EBD in the final second) although at the expense of an increase in the concentration of the nuclei mode (up to 65% with ED and 50% with EBD at 8th second). Fig. 7 shows the particle size distributions at the 5th second of zone 2 as example of the trend observed along this zone for the tested fuels. In this zone they produced similar shape trend since the relative fuel–air ratio, and consequently the emissions, were constant. In zone 2, when comparing particle size distributions, such as occurred in zone 1, bimodal distributions were observed independently of the fuel tested. Similar trends were observed when comparing the fuel effect on particle concentration. Concentrations of the accumulation mode were lower with the oxygenated fuels with respect to diesel because of the presence of oxygen in the compound and its lower aromatic fraction (Lapuerta et al., 2009). The greatest reduction (around 89%) was achieved with the EBD blend because of its higher oxygen content. In contrast, the nuclei mode particle concentrations derived from the oxygenated fuel blends were again higher than those produced by diesel fuel. ED blend led to a significant increase in concentration

Fig. 7. Mean particle size distributions taken at the 5th second of the sample time in zone 2.

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of the nuclei mode (60% with respect to diesel). This also occurred with EBD, although to a lesser extent (40%). Finally, Fig. 8 shows the particle size distributions at the 2nd and 4th seconds of zone 3 for the three fuels. In this zone, again bimodality is observed independently of the fuel tested. Only at tz3 = 2 s the EBD blend produced a small accumulation mode. 3.3. Statistical parameters of the size distributions Fig. 9 shows the concentration of the nuclei and accumulation modes separately. Each mode of the bimodal distribution was adjusted to a log-normal monomodal distribution by a process set out in Gómez, 2006. Total particle concentration (CT) and geometric mean diameter (GMD) per mode, zone and fuel tested were calculated following Eqs. (1 and 2) respectively, being Ci the particle concentration (#/cm3) measured for each registered diameter, FDtot the total dilution factor calculated in units of diluted gas per units of the raw exhaust gas (cm3dil/cm3exh) and di the particle diameter registered by each channel of the EEPS.

CT ¼

X C i  FDtot

ð1Þ

i

P  i C i ln di P GMD ¼ exp iCi

ð2Þ

As expected, the evolution of time profiles of particle concentration of the accumulation mode was always in correspondence with that observed by the relative fuel–air ratio in each studied zone (see Fig. 5). This behaviour was observed for all fuels. Concerning to particle concentrations of the accumulation mode (mainly composed by solid particles derived from the soot formation) the effect of fuel was clearly observed. Independently of the zone studied during the stops, ED and EBD blends generated lower total particle concentrations compared to diesel fuel. This reduction was proportional to oxygen content of the blends as shown in Table 1. In addition, their lower content of sulphur and aromatic compounds also allowed the reduction in soot particles as found by Lapuerta et al. (2008b). In zone 1, the most important reductions in concentration of the accumulation mode were registered at the end, being 50% with ED and 68% with EBD with respect to diesel fuel. In zone 2, the most important reductions were detected by using the EBD blend (around 89%). Unlike in the accumulation mode, the nuclei concentration increased as the relative fuel–air ratio in zone 1 decreased. Similarly, there is an increase in the concentration with ED and EBD with a negligible difference between them. This behaviour could be explained by the lower cetane number and vaporization enthalpy of fuel blends compared to diesel fuel. Focusing on zones 2 and 3, differences between sequences become almost undetectable unlike what was found in the accumulation mode.

(a)

(b)

Fig. 8. Mean particle size distributions measured at the 2nd (a) and 4th seconds (b) of the sample time in zone 3.

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Fig. 9. Average particle concentrations determined during the stop sequences by zone.

It would appear; therefore, that concentration of the nuclei mode is independent of the relative fuel– air ratio, suggesting that it is formed by volatile matter which has condensed, as was noted previously. Fig. 10 shows geometric mean diameter of each mode. It can be seen that in zone 1 there is a slight rising trend to the mean diameter of the accumulation mode as the relative fuel–air ratio decreases for diesel and ED. The opposite trend was observed with EBD. The increase in oxygen content of the fuel led to a decrease in mean diameter. Regarding to zones 2 and 3, there was a decrease in geometric mean diameter as the oxygen content of the fuel increased, being much steeper for EBD. It can also

Fig. 10. Geometric mean diameter determined during the stop sequences by zone.

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be seen that, although with ED and diesel mean diameter is slightly reduced in the last section of zone 3, the opposite occurs with EBD, as was also seen in zone 1. Finally, the diameter of the nuclei mode in zones 1 and 2 remained constant for all fuels, around 10 nm and slightly lower for EBD in zone 2. Only at the end of zone 3 there was a slight increase independently of the fuel tested although with higher dispersion than in the other zones. 4. Conclusions This paper studied the effect of two blends (ethanol–diesel and ethanol–biodiesel–diesel) in comparison with diesel fuel on particle size distributions emitted by an IVECO EURO II urban bus during the stops in service. The main conclusions reached are as follows: – Under real conditions of emission measurement the sequences in which the vehicle was stopped do not involve a constant relative fuel–air ratio nor a steady state, since both at the beginning and the end of the sequence the ratio is variable. This is due to different factors: (a) the operation of the fuel injection control, (b) how the vehicle is driven and (c) the operation of the gear box control. – In zone 2, close to stationary conditions, different levels of the relative fuel–air ratio depending on the working state of auxiliary systems of the vehicle (air compressor, engine fan) were found. In zone 3 the different values of the relative fuel–air ratio may be produced by a different driving style or by the working state of the vehicle parts, as in zone 2. – In zones 2 and 3 the sequences with greater relative fuel–air ratios and the same fuel give a higher concentration of the accumulation mode, while there is no difference in the nuclei mode. – At the same relative fuel–air ratio the particle concentration of the accumulation mode is reduced when ED and EBD are used as fuel, although the reductions are larger with EBD because of its greater oxygen content. By zones, the reductions were greater in zone 2 (the zone closest to a stationary state). – The nuclei concentration increased with the oxygenated fuels, and the increase is greater with ED than with EBD. – The mean diameters of the accumulation mode decrease with the use of oxygenated fuels, and the difference is greater with EBD. The largest differences were obtained in zone 1. – The mean diameters of the nuclei mode slightly decrease with EBD in zone 2. In the others zones, however, the mean diameters were similar independently of the fuel tested. Based on these conclusions, the use of ED blends on urban buses during stops in real-world driving conditions carries out an increase of those smallest particles which are harmful to human health. However, disadvantages of ED can be made up for using EBD blends. Acknowledgments The authors wish to thank the CENIT programme and the University of Castilla – La Mancha for the financial support to the research projects I+DEA and DISTAPAR, respectively. Abengoa Bioenergy and TUSSAM in Seville, Spain, also acknowledge for their technical support. The authors also wish to thank the financial support to Angel Ramos, received from Castilla-La Mancha Community through the project COMBALT 2. References Ahmed, I., 2001. ‘‘Oxygenated Diesel: Emissions and Performance Characteristics of Ethanol–Diesel Blends in CI Engines’’. SAE paper 2001-01-2475. Armas, O., Cárdenas, M.D., Mata, C., 2007. ‘‘Smoke Opacity and NOx Emissions from a Bioethanol–Diesel Blend during Engine Transient Operation’’. SAE paper 2007-24-0131. Armas, O., Gómez, A., Mata, C., 2009. ‘‘Effect of a bioethanol–diesel blend on particle size distributions produced by a city bus during an acceleration sequence’’. 238th American Chemical Society National Meeting, Washington, DC (USA).

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