Particle Number and Size Emissions from a Small Displacement

Article pubs.acs.org/IECR

Particle Number and Size Emissions from a Small Displacement Automotive Diesel Engine: Bioderived vs Conventional Fossil Fuels Federico Millo,† Davide Simone Vezza,† Theodoros Vlachos,† Andrea De Filippo,§ Claudio Ciaravino,§ Nunzio Russo,‡ and Debora Fino*,‡ †

Energy Department and ‡Department of Applied Science and Technology, Politecnico di Torino, Corso Duca degli Abruzzi 24, 10129 Torino, Italy § General Motors Powertrain Europe, Corso Castelfidardo 36, 10138 Torino, Italy ABSTRACT: Experimental work has been carried out on a small displacement Euro 5 automotive diesel engine fueled alternatively with ultralow sulfur diesel (ULSD) and with two blends (30% vol) of ULSD, with two different fatty acid methyl esters (FAME) obtained from rapeseed methyl ester (RME) and jatropha methyl ester (JME). The engine-out particulate matter (PM) emissions have been characterized in terms of number and mass size distributions; measurements were performed under different engine operating conditions that are representative of the New European Driving Cycle (NEDC), including cold start of the engine. No significant differences were detected in the particle numbers (PN) for the different fuels under steady-state operating conditions, while a moderate reduction in particle mass size distribution was observed for the biofuel blends. The effects on PN emissions due to shifts in the engine operating points on the calibration maps, caused by the different fuel characteristics, have been shown to be significantly larger than the effects due to the different combustion characteristics of the biofuel blends, thus highlighting the need for a specific adjustment of the engine calibration.

1. INTRODUCTION Although diesel emission engine regulations have been traditionally based on a gravimetric method for PM measurement, the upcoming Euro 5b/Euro6 regulation1 has introduced a number based approach in addition to the current mass based approach. The number and size distribution of a diesel engine are affected by the fuel characteristics.2,3 In recent years, studies conducted to comprehend how a fuel typology can affect the particle size distribution have increased, particularly since there has been a growing interest in biodiesel usage. This interest is mainly driven by the current legislation. European Directive 2009/28/EC depicts a new scenario concerning the share of energy from renewable sources in transport which will lead to the introduction of a share target, which should be achieved, by the European Union (EU) member states, by 2020 of 10% of the final energy consumption.4 In addition, future legislations will introduce mandatory limits, in terms of CO2 emissions, thus further fostering an increase in biofuel usage.5 A new generation of biodiesels is currently being explored in Asian countries. These biodiesels, which are sourced from nonedible seed oils, such as a wild plant named Jatropha Curcas, which can grow in arid, semiarid and wastelands, could be a viable solution for sustainable biodiesel production, since they lead to green cover of the wastelands.6 Despite the growing interest in biodiesel emissions, which is reflected by the large number of investigations and researches reported in literature, few studies have been carried out on last generation Euro 5 automotive engines,7 and most of the published works focus on different engine types, such as heavy duty engines, large displacement light duty engines, or naturally aspirated, single-cylinder engines,8−10 or they focus only on the total PN reduction due to biodiesel usage.11,12 © 2012 American Chemical Society

An extension of the investigations to modern engines and aftertreatment systems, which could include advanced combustion technologies13 and closed-loop combustion controls,14 therefore seems to be necessary to fully understand the effects of biodiesel usage and to avoid jeopardizing its potential emission benefits. The aim of this research is therefore to characterize the effects of two different biodiesels, sourced from rapeseed oil (RME) and jatropha oil (JME), and blended with Ultra-Low Sulfur Diesel (ULSD), on both number and mass based particulate matter emissions, at the engine outlet of a modern, small displacement, common-rail, Euro 5 automotive diesel engine, for different engine operating conditions.

2. EXPERIMENTAL SETUP AND TEST PROCEDURE 2.1. Engine and After-Treatment System. The experiments were carried out at the Politecnico di Torino on a modern small displacement, turbocharged, common-rail Euro 5 direct injection (DI) automotive diesel engine, one of the smallest engines on the market, considering unit displacement. It is equipped with a close-coupled after-treatment system, and features a diesel oxidation catalyst (DOC) and a catalyzed diesel particulate filter (DPF) integrated in a single canning. The engine was connected to an eddy current dynamometer, while engine fuel consumption was measured by means of an AVL 733S gravimetric fuel meter. Special Issue: Russo Issue Received: Revised: Accepted: Published: 7565

August 25, 2011 January 8, 2012 January 11, 2012 January 11, 2012 dx.doi.org/10.1021/ie201868z | Ind. Eng.Chem. Res. 2012, 51, 7565−7572

Industrial & Engineering Chemistry Research

Article

more accurate information concerning the characteristics of the biofuels and to obtain at least rough estimates of the different blending ratios that could be investigated in the near future. 2.3. Engine Operating Points. The number and mass size distributions were evaluated at three different part-load operating points which were selected as representative, in terms of speed and load levels, of the New European Driving Cycle (NEDC) for the tested engine. The tested operating points in terms of engine speed and brake mean effective pressure (BMEP) are the following: • 1500 rpm/2 bar BMEP; • 2000 rpm/5 bar BMEP; • 2500 rpm/8 bar BMEP. When fueling the engine with the B30 blends, the decrease in the engine torque output, due to the lower heating value (LHV) of the blend, was compensated for by running the engine at the same accelerator pedal position used under diesel operation and increasing the main injection energizing time. In this way the simplest adjustment of the ECU calibration was simulated, while the same settings was maintained for the main calibration parameters, such as injection pressure, exhaust gas recirculation (EGR) rate, injection advance, etc. Although an increase in the accelerator pedal position could simulate a more realistic operation of the engine, when running with biodiesel blends without any specific ECU recalibration, a specific ECU calibration adjustment strategy was chosen to avoid the shift in the engine operating point on the calibration maps as much as possible, as this would cause a significant variation in the PN emissions due to different engine operating parameters, such as the injection pressure, EGR rate, etc. Finally, the number and mass size distributions were measured during the warm-up phase of the engine for the ULSD and RME (B30) fuels. After engine ignition, the engine was run at the selected warm-up operating point (1500 rpm at roughly 2 bar BMEP), and PN measurements were carried out every 5 °C increase in the coolant temperature, starting from 20 °C (typical engine cooling temperature measured after engine ignition) up to the final steady-state thermal regime (approximately 85 °C). 2.4. Measuring System. A TSI 3080 scanning mobility particle sizer (SMPS) model was used to record the particle size distributions of the exhaust PM under different engine operating conditions. This instrument is composed of the following: • a TSI 3080 electrostatic classifier with a Kr-85 Bipolar Charger, using Kripton as the ion source; • an inlet impactor 1035900 (0.071 cm); • a TSI 3081differential mobility analyzer; • a TSI 3025A condensation particle counter. The polydisperse aerosol flow was set to 1.5 Lpm for the steady-state operating points, and for the warm-up tests, with a sheath flow of 15 Lpm; using a 0.071-cm impactor nozzle, it was possible to cover a broad range of particle diameters (from 6 to 225 nm), namely those within the fine particle classification.17 These classes of nanoparticles are the most significant since they include both ultrafine particles and the smaller range of the accumulation mode, which represents the typical classes of nanoparticles emitted from automotive diesel engines.18 The size distributions were corrected for the multiple-charged particles produced in the neutralizer within the SMPS.

Exhaust gases were sampled at the engine outlet, downstream of the turbine, and measured through a Fisher-Rosemount NGA-2000 gas analyzer which provided CO, CO2, HC, NOx, and O2 measurements. Smoke detection, in terms of filter smoke number (FSN) was carried out by means of an AVL 415S smoke meter, for the purpose of repeatability checks (a detailed description of the test rig used during this research activity can be found in ref 15). During the tests all the engine control parameters were controlled by a PC, which was directly connected to the engine control unit (ECU): the test engine was also equipped with a closed-loop combustion control that was capable of maintaining the mass fraction burned (MFB50) crank angle at its optimal position under part load operating conditions. Moreover, the piezoresistive pressure transducers integrated in the glow plugs used for the closed-loop combustion control, were also used for the measurement of the in-cylinder pressure. The output from these transducers was first filtered, using a low-pass filter to reduce high frequency noise and to prevent aliasing errors and signal distortion, and then finally sampled by means of a 12-bit high-speed multichannel data acquisition board, coupled to a high resolution (0.4°) crank-angle encoder to ensure proper timing of the sampled data. One hundred consecutive engine cycles were recorded at each operating condition to obtain a wide statistical sample. Data acquisition and postprocessing were performed by means of Internal Combustion Engine (ICE) Analyzer,16 a suitable program developed for internal combustion engine indicating analysis. 2.2. Fuels. The experimental tests were performed using the three following fuels: • ULSD: a standard ultra-low sulfur diesel fuel that complies with EN590 (sulfur 60%), but it falls below 15% for d = 40 nm sizes. Because the shorter transfer line could not be used for the whole experimental activity, for safety reasons and room constraints, the long transfer line had to be selected, even though it affects particle concentrations below 40 nm, as shown in Figure 1 (the error bars, for sake of a good readability, were omitted). It is worth pointing out that the results (comparisons between the particle emissions from different fuel blends) were obtained using the same experimental setup and are therefore comparable; however, a comparison of the presented results with other studies should be made with care due to differences that could emerge because of the different sampling systems. 7567

dx.doi.org/10.1021/ie201868z | Ind. Eng.Chem. Res. 2012, 51, 7565−7572


Article

Figure 4. Soot−BSNOx trade-off at 2000 rpm and 5 bar BMEP.

Figure 2. Engine-out number and mass size distributions for different fuel blends at 1500 rpm and 2 bar BMEP.



The large increase in particle number measured with JME could, at least in part, be attributed to a slight change in the EGR rate, which is likely to produce a dramatic change in soot emissions due to the particular setting of the engine calibration point on the soot−NOx trade-off, as shown in Figure 4, and which was highlighted in a previous research activity.15 Therefore, the extremely important effects on PM emissions that can be attributed to shifts in the engine operating points on the calibration maps highlight the need for a specific adjustment of the engine calibration on the basis of the fuel characteristics (e.g., for the lower LHV of biofuel blends) in order to avoid jeopardizing the potential emission benefits of biofuels. Finally, with reference to Figure 5, the change in fuel at the high speed-high load operating point (2500 rpm, 8 bar BMEP) does not lead to any appreciable variations in number of

showing peak values at the diameter of about 50 and 100 nm, respectively. Figure 3 reports the results for the medium-speed, mediumload engine operating condition (2000 rpm, 5 bar BMEP). Again in this case, RME does not show any considerable variations in number distribution, compared to diesel fuel, while an appreciable reduction can be observed in the mass size distribution, with a peak diameter at about 50 and 100 nm, respectively, for both fuels; conversely, at this engine operating condition, JME leads to a remarkable increase in PN, as well as in particle mass, with almost doubled peak distribution values compared to diesel fuel, while an appreciable reduction in number and mass can be also observed for particles below 10 nm. 7568



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particles emitted for either RME or JME, with a peak distribution diameter at about 50 nm; as far as mass size distribution is concerned, a moderate reduction can be observed when switching to B30 blends, with the peak diameter at about 100 nm, which is slightly lower than for diesel. These results clearly show that the effects on PN emissions related to the use of different fuels can mainly be attributed to changes in the engine operating conditions, as the JME case shown in Figure 3 points out. On the contrary, the effects of the fuel characteristics seem to be quite modest since the significant PM reductions that could be expected with biofuels, due to the absence of soot promoters (such as aromatic hydrocarbons) in the biofuel molecule and to the increased local oxygen availability during the combustion process (thanks to the oxygen content of the biofuel molecules), seem to be almost completely canceled out by the engine recalibration (i.e., by the increase in the injected fuel mass per cycle to compensate for the lower LHV of the biofuels, which leads to an almost identical overall relative air− fuel ratio). 3.2. Warm-Up Tests. Figure 6 shows the engine cooling temperature variation during the warm-up test; the warm-up

an easier repeatability of the test. This procedure caused a slight shift in the operating point on the engine calibration map for the B30 blend. The particle number and mass size distributions obtained from the warm-up tests are reported in Figures 7−10.

Figure 7. Comparison of the ULSD and B30 engine-out particle number and mass size distributions at 1500 rpm, 2 bar BMEP, and a 25 °C coolant temperature.

Figure 6. Engine cooling temperature as a function of time during the warm-up tests.

phase lasted about 1 h at the selected operating point, due to the fact that the cooling system volume of the test rig was artificially increased compared to the standard system. This modification allowed measurements to be made of the particle distribution using the previously described SMPS equipment, due to the small variation in cooling temperature during each of the SMPS scans (each scan lasted approximately 2 min). The engine operating point for the warm-up test was set, just after engine ignition, to 1500 rpm and 2 bar BMEP which was obtained with a slight increase in the accelerator pedal position for RME (B30) to obtain the same initial engine operating point, in order to compensate for the lower B30 blend LHV; the initial value of the accelerator pedal position was not modified anymore during the whole warm-up in order to allow

Figure 8. Comparison of the ULSD and B30 engine-out particle number and mass size distributions at 1500 rpm, 2 bar BMEP, and a 45 °C cooling temperature.

The ULSD engine-out number distribution has a bimodal shape at a cooling temperature of 25 °C (Figure 7), with peak distribution diameters at about 20 and 50 nm, while the mass size distribution shows a peak diameter of about 100 nm; on the other hand, the RME (B30) number and the mass size distributions also show a bimodal shape with a large peak in engine-out PN emitted at 20 nm. 7569



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shapes for ULSD, as well as for RME (B30), from a bimodal to an unimodal shape, with an increase in the number and mass peak distribution diameters from 55 to 60 nm and from 100 to 105 nm, respectively. The number and mass size distributions of both fuels gradually settle to a final unimodal steady distribution, for the increase in the cooling temperature up to the final steady-state thermal regime of the engine (85 °C, Figure 10), no significant differences can be observed between ULSD and RME (B30). Further considerations on the nature of the emitted particles for both ULSD and RME (B30) fuels could also be made comparing the HC emission trends with the PN trends for two selected particle sizes, namely 20 and 60 nm, as well as for the total particle number, as a function of the cooling temperature, as shown in Figure 11. It can clearly be noticed that for both

Figure 9. Comparison of the ULSD and B30 engine-out particle number and mass size distributions at 1500 rpm, 2 bar BMEP, and a 65 °C coolant temperature.

Figure 11. ULSD and B30 engine-out HC emissions and particle numbers for 20- and 60-nm diameters at 1500 rpm, 2 bar BMEP as a function of the cooling temperature.

fuels the number of larger particles emitted in the 60-nm diameter class tends to increase with an increasing cooling temperature, largely prevailing over the smaller particles for higher coolant temperatures than 45 °C. Conversely, while the number of smaller particles in the 20-nm diameter class slightly decreases from 25 to 45 °C and then remains almost constant for ULSD, a dramatic decrease in the smaller number of particles is evident for the RME as the cooling temperature increases from 25 to 45 °C. These data, along with a similar HC vs temperature trend, suggest that most of the smaller particles measured at a low engine cooling temperature with RME (B30) are probably related to semivolatile components, a result that is different from what was reported in previous studies in literature.26,27 The engine-out PN of the 60-nm diameter class for both fuels shows an increase for rises in the engine cooling temperature of 25 to 65 °C, with a final PN decrease of 65 to 85 °C; the EGR levels during the warm-up phase, which are shown in Figure 12, varied according to the calibrated warm-up

Figure 10. Comparison of the ULSD and B30 engine-out particle number and mass size distributions comparison at 1500 rpm, 2 bar BMEP, and a 85 °C coolant temperature.

The presence of the 20-nm peak diameter on the engine-out number distributions for both fuels is most likely due to the nucleation of a portion of HC into semivolatile particles, especially taking into account the very narrow distillation temperature range of the pure RME distillation curve (generally in a range of between 330 and 360 °C).24,25 Nevertheless, further investigations concerning the nature of these nanoparticle formations under cold start, which could not be performed with the current experimental setup, would be needed to establish whether these particles are semivolatile or solid. An increase in cooling temperature (Figures 8 and 9) determines a change in the number and mass size distribution 7570



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AUTHOR INFORMATION

Corresponding Author

*Telephone: +39-011-0904710. Fax: +39-011-0904699. E-mail: [email protected].

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ACKNOWLEDGMENTS This research was financially supported by General Motors Powertrain Europe, whose valuable support is gratefully acknowledged by the authors.

■ Figure 12. EGR rates measured during engine warm-up for B0 and B30.

control strategy, which explains, at least in part, the PN trend for the 60-nm diameter class shown in Figure 11. Finally, it should be pointed out that remarks on the particle nature, especially for sub-30-nm particles, are made here solely on the basis of the HC trends from the warm-up tests; a more detailed investigation should be carried out to analyze the 2−60-nm particle range.

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4. CONCLUSIONS PN and mass emissions measurements have been carried out on a small displacement Euro 5 automotive diesel engine alternatively fueled with a standard low-sulfur diesel fuel (EN590) and with two blends (30% vol) of ULSD and two different fatty acid methyl esters (FAME) obtained from rapeseed methyl ester (RME) and jatropha methyl ester (JME), respectively. The engine-out PM emissions for each fuel were characterized in terms of particle number and mass size distributions by means of a two-stage dilution system coupled to a scanning mobility particle sizer (SMPS). Whereas no significant differences in terms of particle number were generally detected at engine-out for the different fuels under steady-state operating conditions, a moderate reduction could be observed in particle mass size distribution for the biofuels. Furthermore, the effects on PM emissions, due to shifts in the engine operating points on the calibration maps caused by the different fuel characteristics (i.e., by the lower LHV of the biofuel blends) were shown to be extremely important and significantly larger than the effects due to the different combustion characteristics of the biofuel blends, thus highlighting the need for a specific adjustment of the engine calibration. A significantly different behavior was highlighted between ULSD and RME (B30) during engine cold starts, with considerably higher particle number emissions from RME at lower cooling temperatures, especially in the nanoparticle range (

Particle Number and Size Emissions from a Small Displacement

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