Regulated and unregulated emissions from a diesel engine fueled ...

Atmospheric Environment 45 (2011) 2174e2181

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Regulated and unregulated emissions from a diesel engine fueled with diesel fuel blended with diethyl adipate Ruijun Zhu a, b, C.S. Cheung b, *, Zuohua Huang a, Xibin Wang a a b

School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an, PR China Department of Mechanical Engineering, The Hong Kong Polytechnic University, Hung Hom, AKowloon, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 2 July 2010 Received in revised form 22 January 2011 Accepted 28 January 2011

Experiments were carried out on a four-cylinder direct-injection diesel engine operating on Euro V diesel fuel blended with diethyl adipate (DEA). The blended fuels contain 8.1%, 16.4%, 25% and 33.8% by volume fraction of DEA, corresponding to 3%, 6%, 9% and 12% by mass of oxygen in the blends. The engine performance and exhaust gas emissions of the different fuels were investigated at five engine loads at a steady speed of 1800 rev/min. The results indicated an increase of brake specific fuel consumption and brake thermal efficiency when the engine was fueled with the blended fuels. In comparison with diesel fuel, the blended fuels resulted in an increase in hydrocarbon (HC) and carbon monoxide (CO), but a decrease in particulate mass concentrations. The nitrogen oxides (NOx) emission experienced a slight variation among the test fuels. In regard to the unregulated gaseous emissions, formaldehyde and acetaldehyde increased, while 1,3-butadiene, ethene, ethyne, propylene and BTX (benzene, toluene and xylene) in general decreased. A diesel oxidation catalyst (DOC) was found to reduce significantly most of the investigated unregulated pollutants when the exhaust gas temperature was sufficiently high. Ó 2011 Elsevier Ltd. All rights reserved.

Keywords: Euro V diesel fuel Diethyl adipate (DEA) Diesel engine Regulated emissions Unregulated emissions

1. Introduction The use of oxygenated fuels as alternative fuels or as additive in diesel fuel for diesel engine is beneficial to reducing both diesel fuel consumption and pollutant emission. It is commonly accepted that cleaner combustion can be achieved when a diesel engine is fueled with oxygenated fuels, especially in the reduction of particulate emissions (Miyamoto et al., 1998; Akasaka and Sakurai, 1997). However, various oxygenated fuels have different physical and chemical properties which might lead to different effects on engine performance and exhaust gas emissions. Among the many oxygenated fuels, diethyl adipate (DEA; C10H18O4) has rarely been investigated. DEA was screened and selected as one of eight oxygenates, out of a total of 71 oxygenates, for testing on advanced diesel engines (Natarajan et al., 2001). Actually, DEA can be mixed with diesel fuel at normal temperature and pressure. It has high oxygen content, low sooting tendency and suitable physico-chemical properties for application to diesel engine. Moreover, DEA is a colorless liquid which has relatively low toxicity, corrosivity and reactivity. It can be derived from the esterification of adipic acid and ethanol in the presence of

* Corresponding author. Tel.: þ86 852 2766 7819; fax: þ86 852 2365 4703. E-mail address: [email protected] (C.S. Cheung). 1352-2310/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2011.01.062

concentrated sulfuric acid, and is readily available in Chinese mainland. The cost of DEA is higher than that of diesel fuel at present. DEA has been screened and selected for investigation by Natarajan et al. (2001) based on several criteria, one of which is that the cost of DEA could be competitive when it is used in large quantity. Manuel et al. (2001) tested the eight oxygenates screened by Natarajan et al. (2001) and found that reduction of particulate emissions could be achieved by adding DEA to diesel fuel, however there could be an increase in NOx and CO emissions. Their test on DEA was carried at a single mode of 0.42 MPa at 2300 rev/min. Ren et al. (2007) studied the combustion and emission characteristics of diesel-DEA blends. On the emission side, they only measured smoke opacity and NOx. The literature shows that there is lack of comprehensive investigation on the emissions of a diesel engine fueled with dieselDEA blends, in particular the unregulated emissions from the diesel-DEA blends have not been investigated before. The present study is aimed to study the regulated and unregulated emissions of a diesel engine fueled with Euro V diesel fuel blended with different proportions of DEA under five engine loads at a steady speed of 1800 rev/min. The regulated emissions including HC, CO, NOx and particulate mass concentrations, while the unregulated emissions include formaldehyde, acetaldehyde, 1,3-butadiene, ethene, ethyne, propylene and BTX (benzene, toluene and xylene) most of which are air toxics.

R. Zhu et al. / Atmospheric Environment 45 (2011) 2174e2181

2. Experimental setup The experimental system is shown in Fig. 1. The test engine is a naturally aspirated, water-cooled, 4-cylinder direct-injection ISUZU diesel engine. Specifications of the engine are given in Table 1. The engine was coupled with an eddy-current dynamometer and engine operation was controlled by the Ono Sokki diesel engine test system. An Engelhard CCX8772A diesel oxidation catalyst (DOC) was used for after-treatment of the exhaust gas. Euro V diesel fuel was used as a baseline fuel in this study. Blended fuels containing 8.1%, 16.4%, 25% and 33.8% by volume fraction of DEA, corresponding to 3%, 6%, 9% and 12% by mass of oxygen in the blends, were prepared for the experiments. The blended fuels are named as DEA8, DEA16, DEA25 and DEA34 to reflect the DEA volume fractions. The properties of Euro V diesel fuel, DEA and the four blended fuels are given in Table 2. Compared to the Euro V diesel fuel, the blended fuels have lower cetane number and lower calorific value. The gaseous species in the engine exhaust were measured using online exhaust gas analyzers. A heated flame ionization detector (HFID) was used for HC; a heated chemiluminescent analyzer (HCLA) for NOx/NO; and non-dispersive infra-red analyzers (NDIR) for CO and CO2; exhaust gas temperature was measured with K-type thermocouple. The gas analyzers were calibrated with standard gases and zero gas before each test. Unregulated emissions including formaldehyde, acetaldehyde, 1,3-butadiene, ethene, ethyne, propylene and BTX (benzene, toluene and xylene) were measured with an Airsense multi-component gas analyzer. The Airsense gas analyzer is an Ion Molecule Reaction mass spectrometer, which allows dynamic studies of gaseous emission in low concentration (Dearth, 1999; Villinger et al., 1993, 1996). Standard benzene, toluene, methanol and formaldehyde gases were used to calibrate the Airsense multi-component gas analyzer while the other unregulated gases were calibrated indirectly with information provided by the equipment supplier. Particulate mass concentration was measured with a tapered element oscillating microbalance (R&P TEOM 1105), in which the main sample flow rate was 1.5 l/min and the inlet temperature was held at 47 C. The exhaust gas from the engine was diluted with a Dekati mini-diluter before passing through the TEOM. The application of the Dekati mini-dilutor and the TEOM for particle measurement has been covered in the literature (Patashnick and Rupprecht, 1991; Wong et al., 2003). The dilution ratio was determined from the measured CO2 concentrations of background air, undiluted exhaust gas and diluted exhaust gas. The measured dilution ratio varied from 6.15 to 6.5 in this study. Experiments were performed at the rated torque speed of 1800 rev/min, and at engine loads of 28, 70, 130, 190 and 240 Nm, corresponding to the brake mean effective pressures of 0.08, 0.20,

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Table 1 Specifications of the test engine. Model

Isuzu 4HF1

Engine type Maximum power (kW/rev min1) Maximum torque (Nm/rev min1) Bore stroke (mm/mm) Displacement (cm3) Compression ratio Fuel injection timing (BTDC) Injection pump type Injection pressure (MPa) Injection nozzle

4-cylinder, in-line, natural-aspirated 88/3200 285/1800 112 110 4334 19.0:1 8 Bosch in-line type 18.1 Hole type (with 5 orifices)

0.38, 0.55 and 0.70 MPa, respectively. Before each measurement, the engine was warmed up until the cooling water temperature reached 80e85 C while the lubricating oil temperature reached 103e117 C, depending on the engine load. All the gaseous emissions and particulate mass concentrations were continuously measured for 5 min at the exhaust tailpipe of the diesel engine and the average data were used for the analysis. Each test was repeated three times to ensure that the results are repeatable within the experimental uncertainties. The experimental uncertainty and standard errors in the measurements are shown in Table 3, which have been determined based on the method of Kline and McClintock (1953). 3. Results and discussion 3.1. Engine performance Table 4 shows the fuel consumption, the brake specific fuel consumption (BSFC) and the brake thermal efficiency (BTE) for each fuel at each engine load. In the experiments, the volumetric flow of the fuel was measured and then converted into the mass consumption rate based on the density of each fuel. Based on the mass consumption rate of the fuel, the lower heating value of the fuel and the engine torque, the BSFC and the BTE can be calculated. In general, there is a decrease in BSFC with increasing engine load from 0.08 to 0.55 MPa, while a slight increase at the highest engine load of 0.70 MPa. At 0.55 MPa, the BSFC of Euro V diesel fuel is 236 g/kW h, which increases to 246 g/kW h for DEA8 and 270 g/ kW h for DEA34. The higher BSFC for the blended fuels is mainly due to the lower calorific value of DEA in comparison with that of the diesel fuel, thus more fuel is needed to maintain the same power output when the blended fuel is in use. For each fuel, the BTE increases with engine load from 0.08 to 0.55 MPa, while it drops slightly at 0.70 MPa. Compared to Euro V diesel fuel, the BTE of the blended fuels increases slightly for the engine loads of 0.20e0.70 MPa, while at the engine load of 0.08 MPa, there is very minor difference between the different

Table 2 Properties of Euro V diesel fuel, DEA and blended fuels.

Fig. 1. Schematic diagram of experimental setup.

Property

Euro V

DEA volume fraction (%) DEA mass fraction (%) Lower heating value (MJ/kg) Density (kg/m3)@20 C Boiling point ( C) Cetane number Heat of evaporation (KJ/kg) C (wt.%) H (wt.%) O (wt.%)

e e 8.1 e e 9.5 42.5 25.5 40.9 840 1005 855 210e235 127 e >51 15 e 250e290 295.1 e 86 59.4 83.5 14 8.9 13.5 0 31.7 3

DEA

DEA8 DEA16 DEA25 DEA34 16.4 18.9 39.3 870 e e e 81.0 13.0 6

25.0 28.4 37.7 855 e e e 78.4 12.6 9

33.8 37.9 36.1 900 e e e 75.9 12.1 12

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Table 3 Experimental uncertainties and standard errors. Parameter

Uncertainty (95% confidence level) (%)

Parameter

Standard error (%)

THC NOx NO CO Unregulated gas

0.98 0.72 0.56 0.4 0.42e0.68

Mass fuel consumption PM concentration

1.7 2.1

fuels. The elevated BTE of the blended fuels can be attributed to the oxygen enrichment coupled with the lengthened ignition delay, which enhances the combustion process and promotes the premixed burning phase, as shown in Ren et al. (2007). 3.2. Regulated emissions Fig. 2. Effect of DEA and engine load on CO emission.

3.2.1. CO and HC emissions Fig. 2 shows the variation of CO emission among the different fuels. For each fuel, in general, the CO emission increases when the engine load is increased from 0.08 to 0.20 MPa, while it decreases with further increase of engine load. The increase of CO concentration up to 0.2 MPa is associated with increase of fuel burned at low in-cylinder gas temperature. At higher engine load, the increase of in-cylinder gas temperature favors the conversion of CO into CO2, leading to a decrease of CO emissions. The peak CO concentration at the engine load of 0.20 MPa is 325 ppm for the diesel fuel and 623 ppm for DEA34. Compared to the diesel fuel, the CO concentrations are increased by 8%, 16%, 32% and 64% on average of the different engine loads, for DEA8, DEA16, DEA25 and DEA34, respectively. CO is primarily an incomplete combustion product which is formed either in locally rich spray region or because of insufficient combustion temperature for its conversion to CO2. Compared to Euro V diesel fuel, DEA has much lower cetane number and lower calorific value, which can lead to respectively longer ignition delay Table 4 Fuel consumption and brake thermal efficiency. Load (MPa)

Fuel

BSFC (g/kW.h)

BTE (%)

0.08

Euro V DEA8 DEA16 DEA25 DEA34

Mf (kg/h) 2.64 2.74 2.91 3.09 3.17

502 518 552 585 600

16.9 17 16.6 16.3 16.6

0.20


4.04 4.16 4.35 4.55 4.71

306 315 330 345 357

27.7 27.9 27.8 27.7 27.9

0.38


6.05 6.31 6.52 6.78 6.97

247 258 266 277 284

34.2 34.3 34.4 34.5 35.1

0.55


8.47 8.79 9.04 9.49 9.67

236 246 252 265 270

35.8 36.0 36.3 36.1 37.0

0.70


10.75 11.19 11.52 11.90 12.25

238 247 255 263 271

35.2 35.6 36.0 36.3 36.8

period and more fuel burned in the cylinder in order to maintain the same power output. Thus, with an increase of DEA in the blended fuel, there would be an increase in the fuel being decomposed beyond the main combustion duration and hence an increase in CO emission. At low engine load, the in-cylinder temperature is inherently low. The increase in fuel consumption and the slightly higher latent heat of evaporation of DEA might further reduce the in-cylinder temperature which favors the formation of CO. With increase in engine load, the in-cylinder temperature becomes higher associated with higher engine load, leading to a reduction in CO formed as well as a reduction in the difference in CO emission among the blended fuels. Manuel et al. (2001) also found increase of CO emission with increase of DEA in the blended fuel. The variation of HC concentration for each fuel is shown in Fig. 3. It is observed that HC emission peaks at 0.20 MPa. Compared to Euro V diesel fuel, HC concentrations are increased by 2%, 5%, 9% and 18% for DEA8, DEA16, DEA25 and DEA34 respectively, based on the different engine loads. The possible reason is that DEA results in longer ignition delay and increase of fuel consumption. Thus, part of the blended fuel might decompose behind the main combustion duration, leading to an increase in HC emission. 3.2.2. NOx emission Fig. 4 illustrates the variation of NOx emission with DEA and engine load. Obviously, NOx concentration increases with

Fig. 3. Effect of DEA and engine load on HC emission.


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Fig. 4. Effect of DEA and engine load on NOx emission.

Fig. 5. Effect of DEA and engine load on particulate mass concentration.

increasing engine load for each fuel because of the higher in-cylinder gas temperature at higher engine load. Regarding the effect of DEA, NOx concentration varies slightly among the test fuels. In general, DEA16 gives the highest NOx concentration while DEA8 gives the lowest. Compared to the diesel fuel, the NOx concentration of DEA16 is increased by 7% on average of the different engine loads, while the average NOx concentration for DEA8 is decreased by 7%. As for the NOx emissions of DEA25 and DEA34, they are almost unchanged from 0.08 to 0.55 MPa, whereas they decrease slightly at the engine load of 0.70 MPa in comparison with Euro V diesel fuel. Some researchers reported that oxygenate fuels could increase NOx emissions, while others obtained opposite results. Manuel et al. (2001) reported an increase of NOx emissions when a diesel engine was fueled with DEA-diesel blends, and attributed it to the higher in-cylinder gas temperature by using the oxygenated fuels in comparison with that of diesel oil. Ren et al. (2007) reported that NOx emissions fluctuated or changed very little at the same engine load with increasing DEA adding to diesel fuel. It is known that NOx formation is strongly related to the incylinder gas temperature as well as the oxygen content in the fuel, which are influenced by many parameters, such as fuel structure and injection timing. The lower cetane number of the blended fuel could result in longer ignition delay and hence more complete mixing of fuel vapors with air before ignition occurs, which will lead to lower PM emissions and higher NOx emissions. However, the larger amount of fuel to be evaporated could reduce the incylinder gas temperature to some extent. These conflicting effects on NOx emissions lead to the results obtained in this study.

fuel carbon were converted to CO in the rich premixed region, rather than to soot precursors. Secondly, the lower cetane number of DEA results in longer ignition delay and hence reduces the amount of fuel burned in the diffusion mode. Thirdly, the decrease of carbon content and fuel aromatics of the blended fuels could also lead to the reduction of PM. The above reasons may enhance each other, leading to a larger reduction in particulate mass concentration than the fraction of DEA in the blended fuel.

3.2.3. Particulate mass concentration The effect of DEA and engine load on particulate mass concentration is shown in Fig. 5. The particulate mass concentration increases with engine load but decreases with an increase of DEA in the fuel. Compared to the Euro V diesel fuel, the particulate mass concentrations are reduced by 19%, 33%, 55% and 65% for DEA8, DEA16, DEA25 and DEA34, respectively, based on the different engine loads. Several reasons might be responsible for the particulate reduction. Firstly, the oxygen in the DEA molecule could improve the air/fuel ratio, especially for those regions at the core of the fuel spray, leading to more complete combustion and promoting the oxidation of the soot formed. Flynn et al. (1999) developed a combustion kinetic model by using mixtures of nheptane and methanol or dimethyl ether as fuel. They found that the soot precursors decreased significantly when increasing the oxygen content in the fuel, and conclude that larger fractions of the

3.3. Unregulated emissions 3.3.1. Formaldehyde and acetaldehyde emissions Aldehydes in vehicle exhaust mainly come from the incomplete combustion of saturated aliphatic hydrocarbons (Magnusson and Nilsson, 2002) or oxygenated compounds in the fuel (Takada et al., 2003). Among which, formaldehyde and acetaldehyde are the two major aldehyde species in the exhaust from motor vehicles. The variation of formaldehyde is shown in Fig. 6. In general, there is only a slight variation for each fuel with increasing engine load. Takada et al. (2003) found that, for a diesel engine operating on diesel fuel, formaldehyde was emitted more under the lower loads and higher engine speed conditions due to the low temperature and the super lean regions in the cylinder. Pang et al. (2006) conducted tests on ULSD blended with biodiesel and ethanol. They concluded that the total carbonyls emission decreased from low

Fig. 6. Effect of DEA and engine load on formaldehyde emission.

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engine load to medium engine load but the total emission increased gradually at high engine load. The formaldehyde and acetaldehyde emissions showed similar trend with minimum emissions occurring at medium load. In our case, the engine load has little effect on formaldehyde emissions. It is possible that for DEA-diesel blend, the decrease in formaldehyde emissions with engine load is balanced by the increase of DEA in the fuel with an increase in engine load. It is found that the formaldehyde emissions of the blended fuels increase evidently with an increase of DEA in the blends. Compared to the Euro V diesel fuel, the formaldehyde emissions are increased by 22%, 30%, 41% and 46% on average of the different engine loads for DEA8, DEA16, DEA25 and DEA34, respectively. Zhang et al. (2008) measured the formaldehyde emissions on a diesel engine fueled with DME, while Chao et al. (2000) investigated the effect of methanol on the emissions of carbonyl compounds generated from the diesel engine. They concluded that the higher formaldehyde emission was attributed to the oxygen content in the fuel. Moreover, Zervas (2008) found that formaldehyde emission increased with a decrease of H/C ratio in the fuel, thus we would expect a slight increase of formaldehyde emissions with increasing DEA in the blends. Furthermore, DEA is derived from the esterification of adipic acid and ethanol in the presence of concentrated sulfuric acid. Arapaki et al. (2007) and da Silva and Pereira (2008) suggested that formaldehyde could be produced by the free glycerol which is formed during the esterification process. It seems that the formation of formaldehyde is related to many factors. The acetaldehyde emission for each fuel decreases generally from the engine load of 0.08e0.55 MPa, while increases slightly at 0.70 MPa, as shown in Fig. 7. He et al. (2003b) reported similar emission trend, and suggested that the low engine loads lead to low oxidation rate of acetaldehyde due to the low combustion and exhaust temperature, leading to higher acetaldehyde emission. In comparison with Euro V diesel fuel, the acetaldehyde emission of the blended fuels is increased by 22%, 53%, 68% and 98% on average of the different engine loads, for DEA8, DEA16, DEA25 and DEA34, respectively. Many previous publications reported an increase of acetaldehyde emission when the diesel engine was operated with diesel-oxygenate fuel blends. Merritt et al. (2005) indicated an increase of acetaldehyde emissions with the addition of ethanol in diesel and suggested that ethanol was the main precursor of acetaldehyde in vehicular emission. Arapaki et al. (2007) found that acetaldehyde emission increased significantly when a diesel engine was fueled with biodiesel-diesel blends, in comparison with the

Fig. 7. Effect of DEA and engine load on acetaldehyde emission.

diesel fuel, and stated that carbonyl compounds were mostly influenced by the oxygen content in the fuel. It is believed that acetaldehyde is the incomplete combustion product of paraffin, and the decreasing aromatic content may increase the paraffin content. Thus, the acetaldehyde emission will increase with the reduction of fuel aromatics (Petit and Montagne, 1993). Thus, increasing DEA in the blended fuel could lead to increase of acetaldehyde emission. 3.3.2. Ethene, ethyne, propylene and 1,3-butadiene emissions In this study, besides formaldehyde and acetaldehyde, concentrations of other unregulated hydrocarbons, including ethene (C2H4), ethyne (C2H2), propylene (C3H6) and 1,3-butadiene, were measured because of their high reactivity and toxicity. It is believed that ethene, ethyne and propylene are the products of thermal pyrolysis which generally lead to the formation of the polycyclic aromatic hydrocarbons (PAH) that are soot precursor (Flynn et al., 1999; Slude and Wagner, 2006). The effect of engine load and DEA on the concentrations of C2H4, C2H2 and C3H6 in the exhaust gas is shown in Figs. 8e10, respectively. In general, the concentrations of C2H4 and C3H6 for each fuel decrease with an increase of engine load from 0.08 to 0.55 MPa, whereas they increase evidently with further increase of engine load. However, the C2H2 concentration increases slightly from 0.08 to 0.02 MPa and decreases with further increase in engine load. It is supposed that the emissions of C2H4, C2H2 and C3H6 are strongly related to the combustion temperature and fuel/air ratio. At high engine load, the higher combustion temperature contributes to the oxidization of the pyrolytic products while the higher fuel/air ratio may lead to an increase of pyrolytic products. After the addition of DEA, the emissions of C2H4, C2H2 and C3H6 reduce significantly from medium to high engine loads, while at lower engine loads of 0.08 and 0.20 MPa, there is a little fluctuation among the different blended fuels. Compared to Euro V diesel fuel, the C2H4 emission is reduced by 16%, 18%, 20% and 22% on average of the different engine loads for DEA8, DEA16, DEA25 and DEA34, respectively. The corresponding reductions are 11%, 12%, 17% and 24% for C2H2; and 16%, 27%, 33% and 41% for C3H6. Flynn et al. (1999) developed a chemical kinetic and mixing model to study the combustion process and found that the addition of oxygenated fuels including methanol and dimethyl ether could reduce the emissions of C2H4 and C2H2. According to their explanation, methanol and dimethyl ether contain no CeC bonds and cannot then produce high levels of these species. Moreover, the oxygen content in the fuel improves the diffusive combustion which also results in the decrease of C2H4, C2H2 and C3H6 emissions.

Fig. 8. Effect of DEA and engine load on ethene emission.


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Fig. 9. Effect of DEA and engine load on ethyne emission.

Fig. 11. Effect of DEA and engine load on 1,3-butadiene emission.

1,3-butadiene could be converted to genotoxic products through photochemical reaction in the presence of nitrogen oxides (Victorin and Margareta, 1998). For these reasons, 1,3-butadiene has been classified as a genotoxic carcinogen by the United States Environmental Protection Agency (Dollard et al., 2001). Fig. 11 shows the emission characteristics of 1,3-butadiene. It is observed that 1,3-butadiene emission for each fuel decreases gradually from the engine load of 0.08e0.55 MPa, whereas it increases with further increase of engine load. Takada et al. (2003) concluded that 1,3butadiene was emitted easily under high exhaust temperature condition and in the area with low fuel equivalence ratio. At lower engine loads, the exhaust gas temperature is not high, but the fuel equivalence ratio is low, resulting in the larger 1,3-butadiene emission. At higher engine loads, the high exhaust gas temperature leads to higher 1,3-butadiene emissions. In this study, the lowest 1,3-butadiene emission occurs at 0.55 MPa which is about 67% of full load. So the results in this study are in line with those in the literature. As an oxygenate additive, DEA also has an influence on the 1,3-butadiene emission. Compared to the Euro V diesel fuel, 1,3butadiene emissions are decreased by 12%, 19%, 25% and 31% on average, based on the different engine loads, for DEA8, DEA16, DEA25 and DEA34, respectively. Merritt et al. (2005) studied the 1,3-butadiene emissions on three diesel engines fueled with dieselethanol blends, and found that 1,3-butadiene emissions all decreased. Flynn et al. (1999) found that addition of methanol

reduced 1,3-butadiene emissions and attributed this to the same reasons resulting in the reduction of C2H4 and C2H2. In addition, the increase of NO2 associated with the use of oxygenated fuel may also lead to the removal of 1,3-butadiene (Dollard et al., 2001). 3.3.3. Benzene, toluene and xylene emissions Benzene, toluene and xylene, referred to as BTX in this paper, are air toxics and have been identified as carcinogenic, mutagenic and teratogenic (Krahl et al., 2002). They are partly emitted from motor vehicles (Schauer et al., 2002). The effect of DEA on the brake specific emissions of benzene, toluene and xylene is shown in Table 5, for the engine loads of 0.08, 0.38 and 0.70 MPa. It is observed that the benzene, toluene and xylene emissions decrease with an increase of engine load for each fuel. Takada et al. (2003) observed large benzene emissions under lower engine loads, and concluded that the lower in-cylinder gas temperature at low load is the condition for emitting more benzene. Some other literatures also reported higher benzene emissions at lower engine loads (Cheung et al., 2008, 2009). Compared to Euro V diesel fuel, benzene emissions of the blended fuels increase gradually at the low engine load of 0.08 MPa but decrease slightly at the higher engine loads. After adding DEA to the diesel fuel, the higher brake specific fuel consumption may lead to a drop in in-cylinder gas temperature and hence a slight increase of benzene emissions, especially at low engine load (Di et al., 2009;

Table 5 Benzene, toluene and xylene emissions at various engine loads.

Fig. 10. Effect of DEA and engine load on propylene emission.

Load (MPa)

Fuel

C6H6 (mg/kW.h)

C7H8 (mg/kW.h)

C8H10 (mg/kW.h)

0.08


51.0 50.7 51.8 52.4 63.6

31.0 30.1 30.6 30.9 30.0

51.1 44.7 41.9 37.5 37.1

0.38


15.9 13.4 13.2 13.0 10.5

6.3 6.2 6.2 6.2 6.0

10.5 8.4 7.7 6.8 6.7

0.70


6.8 5.5 5.6 4.5 4.0

3.9 3.5 3.3 3.2 3.0

5.0 4.1 3.5 3.0 2.5

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Takada et al., 2003). At higher engine load, the high in-cylinder temperature is favorable for the reduction of benzene. Moreover, oxygen enrichment of DEA may promote the oxidization of benzene, which contributes to decrease in the benzene emission. According to Inal and Senkan (2002), unsaturated hydrocarbons in the fuel has the propensity to form aromatic and PAH species. Since DEA is free of unsaturated hydrocarbons, therefore, it has the effect of reducing the aromatics precursors, leading to the reduction of aromatics and PAH species. Thus at higher engine loads, there is a drop in benzene emission. Compared to Euro V diesel fuel, there is a slight decrease of toluene emissions for the blended fuels, the average reduction increases from 2.2% for DEA8 to 16% for DEA34, based on the different engine loads. However, the reduction of xylene emissions is higher, reaching 16% for DEA8 and 38% for DEA34. Nelson et al. (2008) suggested that benzene and toluene are formed largely from fuel fragments produced in the initial oxidative pyrolysis of the fuel. So the toluene emission is reduced for the same reasons stated for the reduction of benzene emission. However, xylene emission is strongly related to the aromatic content of the fuel. The aromatic free DEA will reduce the xylene emissions to some extent. The overall BTX emissions are obtained by adding up the above three emissions. Compared to the Euro V diesel fuel, the BTX emissions of the blended fuels are reduced by 2e9%,14e28% and 17e39%, corresponding to the engine loads of 0.08, 0.38 and 0.70 MPa, respectively, indicating a higher reduction of BTX emissions with increasing engine load. The literatures also show reduction of BTX with oxygenate fuel. Turrio-Baldassarri et al. (2004) measured the unregulated emissions using biodiesel-diesel blends and found that the BTX emissions decreased by 51.3%. Correa and Arbilla (2006) conducted tests on a diesel engine fueled with biodiesel-diesel blends. For B20, the reduction of the benzene, toluene and xylene were about 23%, 25% and 20% respectively. However, Krahl et al. (1996) showed a decrease in the level of BTX emissions (about 60%) when the engine was fueled with RME, with respect to the diesel oil, but an increase (about 135%) with the rapeseed oil. Although some of the non-regulated hydrocarbons decrease slightly, formaldehyde and acetaldehyde increase evidently at the same time which might contributes to the increase of THC. In addition, other toxic hydrocarbon compounds, such as n-hexane and naphthalene, were not measured in this study due to the limitation of experimental facilities available. 3.3.4. Effect of DOC on exhaust emissions The exhaust gaseous emission after the DOC is evaluated at the engine loads of 0.38, 0.55 and 0.70 MPa to ensure that the exhaust

4. Conclusions The present study focuses on investigating the engine performance, regulated emissions and unregulated emissions of a directinjection diesel engine operating on Euro V diesel fuel and four DEA-diesel blended fuels. Compared to Euro V diesel fuel, the blended fuels lead to an increase in the brake specific fuel consumption, mainly due to the lower calorific value of DEA. However, the oxygen enrichment of the blended fuel contributes to more complete combustion, leading to a slight increase in brake thermal efficiency at high engine loads. Regarding the regulated gaseous emissions, in general, DEA generates higher CO and HC emissions, while NOx emissions experience fluctuation with an increasing percentage of DEA in the blends. As expected, the particulate mass concentrations are reduced with the addition of DEA in the fuel. For the unregulated gaseous emissions, formaldehyde and acetaldehyde emissions increase evidently with the increase of DEA in the fuel, which is mainly due to the fuel structure and oxygen content in the fuel. On the other hand, ethene, ethyne, propylene and 1,3-butadiene emissions decrease when the engine is fueled with the blended fuels, due to the improved combustion by the use of oxygenated fuel. In comparison with diesel fuel, emissions of BTX (benzene, toluene and xylene) in general decrease. Among which, the reduction of benzene and toluene can be attributed to the oxygen content in the blends, while the reduction of xylene is due to the aromatic free DEA. The diesel oxidation catalyst can effectively reduce most of the unregulated emissions investigated. Thus the combined use of DEA-diesel fuel and DOC can improve both regulated and unregulated engine emissions, except probably NOx and formaldehyde emissions. Acknowledgements

Table 6 Reduction ratio (%) of exhaust gaseous emissions after DOC. Load (MPa)

Fuel

C2H4O

C4H6

C2H4

C3H6

C2H2

C6H6

C7H8

C8H10

0.38


84.2 80.1 73.3 61.9 57.8

95.6 97.3 96.5 94.0 94.4

98.3 98.3 98.2 97.9 97.9

95.9 95.1 93.7 92.0 91.7

99.0 98.9 98.8 98.7 98.3

>99 >99 >99 >99 >99

55.0 65.0 68.1 71.0 80.0

55.6 52.8 48.6 47.6 53.4

0.55


82.3 80.8 78.5 75.1 71.9

93.1 96.6 96.3 89.7 88.9

96.9 96.6 96.2 95.2 94.7

94.8 94.4 93.9 92.4 90.5

98.1 97.9 97.7 96.8 96.4

>99 >99 >99 >99 >99

65.0 65.0 81.7 74.4 88.0

53.7 49.3 52.9 45.8 50.4


85.1 88.0 89.4 89.9 90.2

97.2 97.7 96.8 96.0 94.3

98.3 98.2 98.0 97.6 97.0

97.8 97.7 97.0 96.5 95.8

98.2 97.9 97.7 97.4 97.3

>99 >99 >99 >99 >99

78.9 76.9 72.1 80.0 89.3

54.4 53.9 46.2 50.3 41.6

0.70

gas temperature is sufficiently high for effective DOC operation. The effect of DOC on CO, HC, NOx and particulate mass reduction can be found in the literature. This part focuses on the unregulated emissions. Table 6 indicates a reduction of more than 90% in ethene, ethyne, propylene and 1,3-butadiene emissions. For the aldehydes, formaldehyde reduction is insignificant and hence not shown in the table; while acetaldehyde reduction varies from 60% to 90%. As for the BTX emissions, benzene is almost completely eliminated, while the reductions of toluene and xylene range from 40% to 89% at different engine loads. It seems that the DOC can effectively reduce both the regulated and unregulated pollutants, and hence, lower exhaust emissions could be expected by the use of diesel-DEA blends in combination with the DOC.

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