Combustion and Emission Characteristics of a Direct-Injection Diesel

1474

Energy & Fuels 2007, 21, 1474-1482

Combustion and Emission Characteristics of a Direct-Injection Diesel Engine Fueled with Diesel-Diethyl Adipate Blends Yi Ren, Zuohua Huang,* Haiyan Miao, Deming Jiang, Ke Zeng, Bing Liu, and Xibin Wang State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong UniVersity, Xi’an 710049, People’s Republic of China ReceiVed NoVember 2, 2006. ReVised Manuscript ReceiVed March 10, 2007

Combustion and emission characteristics of a direct-injection diesel engine fueled with diesel-diethyl adipate blends were investigated. The results show that the ignition delay and the amount of heat release in the main combustion duration increase. The diffusive combustion duration and total combustion duration decrease, while the amount of heat release in the diffusive combustion increases with the increase of the oxygen mass fraction in the blends. Both the maximum mean gas temperature and the duration of high gas temperature decrease with the oxygen mass fraction in the blends. For a specific engine load and engine speed, the center of the heat release curve moves close to the top-dead-center, and the brake specific fuel consumption (bsfc) increases with the increase of the fraction of diethyl adipate; however, the diesel equivalent bsfc decreases and the thermal efficiency increases with the increase of the oxygen mass fraction in the blends. For 100% engine load, the largest value of ηet is given in the range of engine speed from 1400 to 2000 rpm. The exhaust smoke concentration decreases and the exhaust NOx concentration gives a slight variation with the increase of the oxygen mass fraction in the blends. A flat NOx /smoke tradeoff curve is presented when operating on the diesel-diethyl adipate blends. The study shows that utilization of diesel-diethyl blends combining with postponing the fuel delivery advance angle can simultaneously decrease both smoke and NOx emissions.

1. Introduction The advantage of a diesel engine compared with a gasoline engine is the fuel economy benefits; however, the high NOx and smoke emissions still remain the main obstacles for the increasing application of diesel engines with the increasing concerns for environmental protection and implementation of more stringent exhaust gas regulations, thus further reduction in engine emissions becomes one of major tasks in engine development. However, it is difficult to simultaneously reduce NOx and smoke in the traditional diesel engine due to the tradeoff relationship between NOx and smoke. One promising approach to solve this problem is to use the oxygenated fuels or to add the oxygenated fuels to diesel to provide more oxygen during the combustion. In the application of pure oxygenated fuels, Fleisch et al.,1 Kapus et al.,2 and Sorenson et al.3 have studied dimethyl ether (DME) in the modified diesel engine, and their results showed that the engine could achieve ultralow emission prospects without fundamental change in combustion systems. Huang et al.4 investigated the combustion and emission characteristics in a compression ignition engine with DME and found that the DME engine has high thermal efficiency, short * Corresponding author. E-mail: [email protected]. (1) Fleisch, T.; McCarthy, C.; Basu, A. A new clean diesel technology: demonstration of ULEV emissions on a Navistar diesel engine fueled with dimethyl ether. SAE Trans. 1995, 104 (4), 42-53. (2) Kapus, P.; Ofner, H. Development of fuel injection equipment and combustion system for DI diesels operated on dimethyl ether. SAE Trans. 1995, 104 (4), 54-69. (3) Sorenson, S. C.; Mikkelsen, S. E. Performance and emissions of a 0.273 liter direct injection diesel engine fueled with neat dimethyl ether. SAE Trans. 1995, 104 (4), 80-90. (4) Huang, Z. H.; Wang, H. W.; Chen, H. Y. Study on combustion characteristics of a compression ignition engine fueled with dimethyl ether. Proc. Inst. Mech. Eng., Part D: J. Automobile Eng. 1999, 213 (D6), 647652.

premixed combustion, and fast diffusion combustion duration, and their work was to realize low noise, smokefree combustion. Kajitani et al.5 studied the DME engine by delaying the injection timing to realize both smoke and NOx emissions. Practically, adding some oxygenated compounds into diesel fuel to reduce engine emissions without modifying the engine design seems to be a more attractive method. Huang et al. tested gasoline-oxygenate blends in a spark-ignited engine and obtained a satisfactory result for emission reduction,6 and they also investigated the combustion and emission characteristics of diesel-oxygenated additives blends in a compression ignition engine.7-9 Murayama et al.10 studied the emissions and combustion of diesel-dimethyl carbonate (DMC) blends with exhaust (5) Kajitani, Z.; Chen, L.; Konno, M. Engine performance and exhaust characteristics of direct-injection diesel engine operated with DME. SAE Trans. 1997, 106 (4), 1568-1577. (6) Huang, Z.; Miao, H.; Zhou, L.; Jiang, D. Combustion characteristics and hydrocarbon emissions of a spark ignition engine fuelled with gasolineoxygenate blends. Proc. Inst. Mech. Eng., Part D: J. Automobile Eng. 2000, 214 (D3), 341-346. (7) Huang, Z.; Lu, H.; Jiang, D.; Zeng, K.; Liu, B.; Zhang, J.; Wang, X. Combustion behaviors of a compression-ignition engine fuelled with diesel/ methanol blends under various fuel delivery advance angles. Bioresour. Technol. 2004, 95, 331-341. (8) Huang, Z. H.; Lu, H. B.; Jiang, D. M.; Zeng, K.; Liu, B.; Zhang, J. Q.; Wang, X. B. Engine performance and emissions of a compression ignition engine operating on the diesel-methanol blends. Proc. Inst. Mech. Eng., Part D: J. Automobile Eng. 2004, 217 (D4), 435-447. (9) Huang, Z. H.; Jiang, D. M.; Zeng, K.; Liu, B.; Yang, Z. L. Combustion characteristics and heat release analysis of a DI compression ignition engine fueled with diesel-dimethyl carbonate blends. Proc. Inst. Mech. Eng., Part D: J. Automobile Eng. 2003, 217 (D7), 595-606. (10) Murayama, T.; Zheng, M.; Chikahisa, T. Simultaneous reduction of smoke and NOx from a DI diesel engine with EGR and dimethyl carbonate; SAE paper 952518, Society of Automotive Engineers: Warrendale, PA, 1995.

10.1021/ef060546s CCC: $37.00 © 2007 American Chemical Society Published on Web 04/19/2007

Diesel Engine Fueled with Diesel-Diethyl Adipate

Energy & Fuels, Vol. 21, No. 3, 2007 1475

Figure 1. Heat release rate of the fuel blends.

with diesel-methanol blends.12 Miyamoto et al.13 and Akasaka et al.14 also conducted research on diesel combustion improvement and emission reduction by the use of various types of the oxygenated fuel blends. In addition, the physical and chemical properties including toxicity and solubility with diesel fuel for various oxygenated compound have been described in the literature.15 Diethyl adipate (DEA) has a relatively high oxygen content and better intersolubility with diesel fuel at normal temperature and pressure; thus, it is regarded as a promising oxygenates additive for diesel/oxygenates blends. However, there is little information about behaviors of diesel engines fueled with the diesel-diethyl adipate blends; some preliminary studies revealed that the reduction of particulate emissions and toxic gas pollutants could be achieved by adding diethyl adipate into diesel fuel,16 but no information on heat release and combustion characteristics was reported. Thus, the combustion and emission

Figure 2. Ignition delay versus oxygen mass fraction in blended fuels.

gas recycle (EGR). Ajav et al.11 studied the diesel-ethanol blends for emission reduction, and Huang et al. investigated the engine performance and emissions of a diesel engine fueled (11) Ajav, E. A.; Singh, B.; Bhattacharya, T. K. Experimental study of some performance parameters of a constant speed stationary diesel engine using ethanol-diesel blend as fuel. Biomass Bioenergy 1999, 17, 357-365.

(12) Huang, Z. H.; Lu, H. B.; Jiang, D. M.; Zeng, K.; Liu, B.; Zhang, J. Q.; Wang, X. B. Engine performance and emissions of a compression ignition engine operating on the diesel-methanol blends. Proc. Inst. Mech. Eng., Part D: J. Automobile Eng. 2004, 218 (D4), 435-447. (13) Miyamoto, N.; Ogawa, H.; Obata, K. Improvements of diesel combustion and emissions by addition of oxygenated agents to diesel fuels: influence of properties of diesel fuels and kinds of oxygenated agents. JSAE ReV. 1998, 19 (2), 154-156. (14) Akasaka, Y.; Sakurai, Y. Effect of oxygenated fuel on exhaust emission from DI diesel engines. Trans. JSME, Ser. B 1996, 63 (609), 1833-1839. (15) Natarajan, M.; Frame, E. A.; Naegei, D. W.; Asmus, T.; Clark, W.; Garbak, J.; Gonzalez, M. A.; Liney, E.; Piel, W.; Wallace, J. P. Oxygenates for adVanced petroleum-based diesel fuels: Part 1. Screening and selection methodology for the oxygenates; SAE paper 2001-01-3631, Society of Automotive Engineers: Warrendale, PA, 2001. (16) Manuel, A.; Gonzalez, D.; Piel, W.; Asmus, T.; Clark, W.; Garbak, J.; Liney, E.; Natarajan, M.; Frame, E. A.; Naegeil, D. W.; Yost, D.; Wallace, J. P., III. Oxygenates screening for adVanced petroleum-based diesel fuels: Part 2. The effect of oxygenate blending compounds on exhaust emissions. SAE paper 2001-01-3632, Society of Automotive Engineers: Warrendale, PA, 2001.

1476 Energy & Fuels, Vol. 21, No. 3, 2007

Ren et al.

Table 1. Engine Specifications bore (mm) stroke (mm) displacement (cm3) compression ratio combustion chamber maximum torque (N‚m) injection pressure (MPa) rated power/speed nozzle hole diameter (mm) number of nozzle hole

100 115 903 18 ω type 50 20 9.4 kW/2000 rpm 0.3 4

exhaust NOx was measured by AVL DiGas 4000 light. The cylinder pressure was recorded with the Kistler type cylinder pressure sensor, and the data were recorded for every 0.1 crank angle. The crank angle was measured with the Kistler type crank angle sensor. Meanwhile the cylinder pressure and emissions were measured and analyzed under the same brake mean effective pressure (bmep) and engine speed, and combustion analysis was taken based on the cylinder pressure information. Furthermore, comparisons in combustion and emissions were conducted among these blends for clarifying the behaviors of the engine fueled with diesel-oxygenates blends versus oxygen mass fraction in the blended fuels.

Table 2. Fuel Properties of Diesel and Diethyl Adipate

chemical formula mole weight (g) density (g/cm3) lower heating value (MJ/kg) heat of evaporation (kJ/kg) boiling point (°C) C wt % H wt % O wt %

base fuel (diesel)

blended fuel (diethyl adipate)

C10.8H18.7 148.3 0.86 44 260 180-330 86 14 0

C10H18O4 202 0.9996 25.5 295.1 127 59.4 8.9 31.7

Table 3. Fuel Properties of the Diesel-DEA Blended Fuels

DEA in the blends vol % DEA in the blends wt % lower heating value (MJ/kg) heat of evaporation (kJ/kg) C wt % H wt % O wt %

fuel 1

fuel 2

fuel 3

fuel 4

5 5.96 41.5 262.1 84.41 13.7 1.9

10 11.8 40.5 264.1 82.86 13.4 3.74

15 17.52 39.6 266.15 81.34 13.11 5.56

20 23.14 38.6 268.12 78.1 13.28 7.33

3. Results and Discussion 3.1. Combustion Characteristics. The ignition delay is defined as the time interval from the beginning of nozzle valve lifting to the beginning of rapid pressure rising; the main combustion duration is the time interval from the beginning of rapid pressure rising to the timing of 90% heat release accumulated, and the total combustion duration is the duration from the beginning of rapid pressure rising (it is regarded as the beginning time of the heat release) to the end of heat release. A thermodynamic model is used to calculate the thermodynamic parameters in this paper; the model neglects the leakage through the piston rings,16 thus the energy conservation in the cylinder is written as follows: dQB

dφ

-

dQW d(mu) dCV dT dV dV ) +p ) mCV + mT +p (1) dφ dφ dφ dφ dφ dφ

The gas-state equation is

pV ) mRT characteristics of diesel engines fueled with the diesel-diethyl adipate blends are worthy of investigatation. The study is expected to provide the information of engine combustion operating on diesel-oxygenate blends and guidance for engine optimization. On the basis of the authors’ previous study, the objectives of this study are to investigate the combustion and emission characteristics based on the heat release analysis for various fractions of DEA addition in diesel fuel at various fuel delivery advance angles, and we expect to extend the understanding of combustion and emission characteristics of compression ignition engines operating on diesel-DEA blends.

(2)

The variation of the gas-state equation with crank angle is given by

p

dp dT dV +V ) mR dφ dφ dφ

(3)

The heat release rate dQB/dφ can be derived from formulas 1 and 3 as follows:

Cp dV CVV dp dCV dQW dQB )p + + mT + dφ R dφ R dφ dφ dφ

(4)

Where, the heat transfer rate is given by 2. Test Engine and Properties of Fuel Blends In this study, diesel fuel is the base fuel while diethyl adipate (DEA) is used as the oxygenate additive. Four fractions of dieselDEA blends were investigated in the study, and the volume fractions of DEA in the blended fuels are 5%, 10%, 15%, and 20%, respectively. The above four fuel blends and pure diesel fuel were tested in a direct-injection diesel engine. For the test engine, the original fuel delivery advance angle is 25 CA BTDC (crank angle before top-dead-center), and the maximum value of torque is reached at 1400 rpm. The detailed specifications of the test engine are listed in Table 1. Fuel properties and the fractions of four blends are given in Tables 2 and 3. The tested diesel fuel was provided by China Petroleum & Chemical Corporation. The oxygen mass fraction in the fuel blends ranges from 1.9% to 7.33% as shown in Table 3. It can be seen that DEA has a higher oxygen content while the lower heating value is lower than those of pure diesel fuel. In the experiment, the beginning timing of nozzle valve lifting was measured by the needle lift detecting apparatus; smoke meter of FQD-201B was used to measure the exhaust smoke, and the (17) Heywood, J B. Internal Combustion Engine Fundamentals; McGrawHill Book Company: New York, 1988.

dQW ) hcA(T - TW) dφ

(5)

The heat transfer coefficient hc uses the Woschni’s correlation formula.16 Cp and CV are temperature-dependent parameters, and their formulas are given in ref 16. Figure 1 gives the heat release rate of the diesel-DEA blends. The results show that the initial combustion phase delays with the addition of DEA, due to the decrease of the cetane number of the blends with the addition of DEA, which has a lower cetane number. Moreover, the maximum rate of heat release increases with the increase of the DEA mass fraction in the blends, due to the increase of the amount of the fuel blends burned in the main combustion duration. Figure 2 illustrates the ignition delay versus oxygen mass fraction in the blends. As shown in Figure 2, the ignition delay increases with the increase of the oxygen mass fraction in the blends. This suggests that addition of DEA decreases the cetane number of the blended fuel, as DEA has a lower cetane number



Figure 3. Rapid burning duration and rapid burning heat release versus oxygen mass fraction in blended fuels.

Figure 4. Diffusive burning duration and diffusive burning heat release versus oxygen mass fraction in blended fuels.

than that of pure diesel fuel.15 The amount of heat release in main combustion duration and main combustion duration versus oxygen mass fraction in the blends are plotted in Figure 3. The figures reveal that the amount of heat release increases with the increase of the oxygen mass fraction in the blends, while

the main combustion duration gives a slight variation with the increase of the oxygen mass fraction in the blends. The results indicate that once the combustion initiates, the main combustion phase can proceed at the same speed. In the case of the behavior of heat release of the main combustion duration, two possible


Figure 5. Total combustion duration versus oxygen mass fraction in blended fuels.

Figure 6. Maximum mean gas temperature and duration above 1800 K of temperature versus oxygen mass fraction in the blends.

factors can be taken into account: one is the increase of the ignition delay of the blended fuel, leading to the increase of the combustible mixture available within the ignition delay, and another is the enrichment of oxygen to promote the combustion.

Ren et al.

Figure 7. Crank angle of the center of the heat release curve φc versus oxygen mass fraction in blended fuels.

Figure 4 shows the amount of heat release in the diffusive combustion phase and the diffusive combustion duration versus oxygen mass fraction in the blended fuels. It can be seen that the amount of heat release during the diffusive combustion period and the diffusive combustion duration shows a remarkable reduction with the increase of the oxygen mass fraction in the blends. This is reasonable since the addition of DEA increases the amount of heat release during the main combustion duration and decreases the amount of heat release in the subsequent diffusive combustion phase. Combustion improvement is realized for diffusive combustion, due to oxygen enrichment by adding the oxygenates additive. And, this behavior is favorable to the reduction of exhaust smoke. The total combustion duration versus oxygen mass fraction in the blended fuels is illustrated in Figure 5. As the lower heating value of the blends decreases with DEA addition, more fuel should be injected for diesel-DEA blends compared with diesel fuel, to get the same engine load (bmep).The total combustion duration decreases with the increase of the oxygen mass fraction in the blends (as shown in Figure 5), due to improvement of the diffusive combustion. Figure 6 shows the maximum mean gas temperature in the cylinder and the duration above 1800 K of temperature versus oxygen mass fraction in the blends. In this experiment, the temperature was calculated with a thermodynamic model based on the cylinder pressure.9 The results show that both the maximum mean gas temperature and the duration above 1800 K of temperature decrease with the increase of the oxygen mass fraction in the blends. The possible reason for this behavior is that the increase of the DEA addition would increase the combustion velocity and decrease the flame temperature. In addition, more blended fuel is injected into the cylinder per cycle compared with pure diesel fuel, and this would increase the mass of combustion products and the thermal capacity.



Figure 8. Brake specific fuel consumption (bsfc) versus oxygen mass fraction in blended fuels.

Figure 9. Diesel-equivalent bsfc (beq) versus oxygen mass fraction in blended fuels.

Figure 7 gives the crank angle of the center of the heat release curve (φc) versus oxygen mass fraction in the blends. The figure shows that the center of the heat release curve moves close to the top-dead-center (TDC) with the increase of the oxygen mass fraction in the blends, due to the increase of heat release in the

main combustion phase and the decrease in the diffusive combustion phase. Figure 8 plots the brake specific fuel consumption (bsfc) versus oxygen mass fraction in the blends. The results show that on the whole the bsfc of the blends increases with the


Figure 10. Effective thermal efficiency versus oxygen mass fraction in blended fuels.

Figure 11. Exhaust smoke concentration and its reduction rate versus oxygen mass fraction in blended fuels.

Ren et al.



Figure 12. Exhaust NOx concentration versus oxygen mass fraction in blended fuels.

increase of the oxygen mass fraction in the blends and that the bsfc of the blends with 5% DEA addition gives the smallest value. Two reasons are considered for this behavior: One is the decrease of the lower heating value of the blends by the DEA addition, and this increases the bsfc with the increase of the oxygen mass fraction in the blends, as more fuel should be injected to get the same bmep. Another is the improvement of the combustion, due to oxygen enrichment, and this helps to decrease the bsfc of the blends. The comprehensive influence gives the above experimental results. Under 100% engine load, the smallest value of the bsfc is presented at an engine speed between 1400 and 1500 rpm. And, the bsfc of the blends at θfd ) 25 CA BTDC is larger than that at θfd ) 21 CA BTDC. This suggests that delaying of the fuel delivery advance angle postpones the heat release and moves the center of the heat release curve away from TDC. The diesel equivalent bsfc (beq) versus oxygen mass fraction in the blends is illustrated in Figure 9. The diesel equivalent bsfc is defined by the formula beq ) [(Hu)blends/(Hu)diesel]be, the parameter removes the influence of the lower heating value. Figure 9 shows that the beq of the blended fuels decreases with the increase of the oxygen mass fraction in the blends, owing to the improvement of the combustion with the DEA addition. Similarly, beq of the blends at θfd ) 25 CA BTDC also gives a larger value than that at θfd ) 21 CA BTDC. The effective thermal efficiency ηet versus oxygen mass fraction in the blends is shown in Figure 10. The behavior of ηet is consistent with that of diesel equivalent bsfc, and this is reasonable since thermal efficiency and diesel-equivalent bsfc (beq) is related by the formula ηet ) 3.6 × 106/((Hu)dieselbeq). Generally speaking, thermal efficiency is more representative to reflect the fuel economy when operating on the oxygenated fuels. Similar to the behavior of diesel equivalent bsfc, the effective thermal efficiency shows a slight increase with DEA addition, and a

larger value at θfd ) 25 CA BTDC compared with that at θfd ) 21 CA BTDC is presented, under 100% engine load. The ηet gives the largest value at engine speeds between 1400 and 1500 rpm. Figure 11 shows the exhaust smoke concentration and its reduction rate versus oxygen mass fraction in the blends. The results show that the smoke concentration decreases with the increase of the oxygen mass fraction in the blends, and this behavior is more obvious at high engine load. As smoke is mainly produced during the diffusive combustion phase, so adding DEA can reduce engine smoke due to the improvement of the diffusive combustion and the promotion of postflame oxidation of smoke in the late expansion and exhaust processes. For a specified engine speed and bmep, a remarkable reduction rate in smoke is presented from diesel fuel to the blended fuel of 2% oxygen mass fraction. When the oxygen mass fraction in the blends is over 2%, a relatively slow increase in smoke reduction rate is demonstrated with the increase of oxygen mass fraction. Under 100% engine load, the smoke concentration gives a remarkable decrease with the DEA addition, and under the same engine speed and engine load, the reduction of the engine smoke with the increase of the oxygen mass fraction in the blends at θfd ) 21 CA BTDC gives a larger value as compared to that at θfd ) 25 CA BTDC, indicating that smoke the reduction rate is larger at a retarded fuel delivery advance angle. This can be explained by the following reasons: the delaying of the fuel delivery advance angle decreases the ignition delay and increases the fraction of diffusive combustion, and the enrichment of oxygen by oxygenate addition gives larger reduction in the case of more diffusive combustion. Figure 12 illustrates engine NOx emission versus oxygen mass fraction in the blends. It can be seen that for a specified engine speed, the engine NOx concentration increases with the increase of the engine load (bmep), due to the increase of the premixed


Ren et al.

4. Conclusions

Figure 13. Relationship between NOx and smoke of the blended fuels.

combustion and combustion temperature. For a specified engine load (bmep), NOx shows a slight decrease or is unchangeable with the increases of the oxygen mass fraction in the blends. The increase of the premixed combustion is favorable to the increase of NOx, while the increase in the A/F ratio, the decrease of the maximum mean gas temperature, and the decrease of the duration of high gas temperature of diesel-DEA blends are favorable to the decrease of NOx. The final result is the comprehensive influence of the two factors. The results also reveal that under 100% engine load, NOx is decreased with the increase of the engine speed. This suggests that, with the increase of the engine speed, the A/F ratio moves away from the theoretical A/F ratio due to the decrease of the volumetric efficiency and the duration of high gas temperature decreases with the increase of the engine speed. Both behaviors help to decrease NOx. For the same engine speed and engine load, NOx decreases obviously with delaying the fuel delivery advance angle. In traditional diesel engine combustion, the delaying of fuel delivery advance angle can effectively decrease the engine NOx, but simultaneously increase the engine smoke. However, for diesel-DEA blend combustion, smoke and NOx can simultaneously be decreased by using oxygenates additives and postponing the fuel delivery advance angle. The relationship between NOx and smoke of the blends is shown in Figure 13. It can be found that under the same fuel delivery advance angle, unlike the engine operating with pure diesel fuel, there exists a flat tradeoff curve between NOx and smoke when operating diesel-DEA blends. At the same engine speed and engine load, for the blends with the large DEA fraction, both NOx concentration and smoke concentration under smaller fuel delivery advance angle (θfd ) 21 CA BTDC) have a lower value than those of the original engine (θfd ) 25 CA BTDC) fueled with pure diesel fuel.

The combustion and emission characteristics of a compression-ignition engine fueled with diesel-diethyl adipate blends were investigated, and the main results are summarized as follows: (1) Ignition delay increases with the increase of the diethyl adipate fraction due to the decrease of the cetane number of the blends. The main combustion duration and the amount of heat release in the main combustion duration increase, while the diffusive combustion duration and the amount of heat release in the diffusive combustion duration decrease with the increase of the oxygen mass fraction in the blends. Both the maximum mean gas temperature and the duration of high gas temperature decrease with the oxygen mass fraction in the blends. (2) The center of the heat release curve moves close to TDC with the increase of the oxygen mass fraction in blended fuels. The brake specific fuel consumption (bsfc) increases and the diesel equivalent brake specific fuel consumption decreases with the increase of the diethyl adipate fraction. (3) Effective thermal efficiency ηet decreases with a delayed fuel delivery advance angle. Under 100% engine load, a high value of ηet is maintained at engine speeds from 1400 to 2000 rpm. (4) The smoke concentration decreases with the increase of the oxygen mass fraction in the blends, and the reduction of the smoke using diesel-diethyl adipate blends produces a large effect with postponing the fuel delivery advance angle. (5) A flat NOx/smoke tradeoff curve is presented when operating on the diesel-diethyl adipate blends. Utilization of diesel-diethyl blends combined with postponing the fuel delivery advance angle can simultaneously decrease both smoke and NOx emissions. Acknowledgment. This study was supported by the National Natural Science Fund of China (50576070, 50323001, 50521604) and the Doctoral Foundation of Xi’an Jiaotong University (DFXJTU2005-04). The authors acknowledge the teachers and students of Xi’an Jiaotong University for their help with the experiment. The authors also express their thanks to the colleagues of Xi’an Jiaotong University for their helpful comments and advice during the manuscript preparation.

Notation ATDC ) after top-dead-center beq ) diesel equivalent bsfc (g/kW‚h) bmep ) brake mean effective pressure (MPa) BTDC ) before top-dead-center bsfc ) brake specific fuel consumption (g/kW‚h) C wt % ) mass fraction of carbon in fuel blend dQB/dφ ) heat release rate with crank angle (kJ/CA) Hu ) lower heating value (MJ/kg) H wt % ) mass fraction of hydrogen in fuel blend m ) mass of cylinder gases (kg) O wt % ) mass fraction of oxygen in fuel blend TDC ) top-dead-center φc ) crank angle of the center of the heat release curve (CA deg ATDC) θfd ) fuel delivery advance angle (CA BTDC) ηet ) effective thermal efficiency EF060546S

Combustion and Emission Characteristics of a Direct-Injection Diesel

Recommend Documents