Improving the Combustion and Emissions of Direct ... - ACS Publications

Energy & Fuels 2005, 19, 1879-1888

1879

Improving the Combustion and Emissions of Direct Injection Compression Ignition Engines Using Oxygenated Fuel Additives Combined with a Cetane Number Improver Xing-cai Lu¨,* Jian-guang Yang, Wu-gao Zhang, and Zhen Huang School of Mechanic and Power Engineering, Shanghai Jiaotong University, Shanghai, People’s Republic of China Received January 14, 2005. Revised Manuscript Received May 19, 2005

According to the fuel design concept, three oxygenated fuels including ethanol, dimethyl carbonate (DMC), and dimethoxy methane (DMM) were selected to mix with diesel fuel. Then, the effects of oxygen content and the cetane number of the blend fuels on diesel engine combustion and emissions were evaluated. The experiments were conducted on a four-cylinder high-speed diesel engine. The results show that, with the increase of the oxygen content in blend fuels, the ignition timing delays and the combustion duration shorten at different operating conditions, and the brake thermal efficiency improves from middle to full loads. The engine NOx emissions decreased at overall operating ranges when oxygenated fuels were added to the diesel fuel, and smoke emissions also improved for all oxygenated blend fuels except for ethanol-diesel blends at low and middle loads. In the case of the DMC-diesel blend fuel, when the oxygen content increased up to 15%, the smoke number decreased by 75%, and NOx emission improved by 15-20%. CO emissions of blend fuels decreased at large and full loads but deteriorated at lower and middle engine loads. HC emissions increase at overall operating ranges and increase much more with the increase of oxygenated fuel volume and decrease of the engine load. To solve this problem, the cetane number improver was added to the 15% ethanol-diesel blend fuel. As a result, the HC and CO emissions at overall operating ranges reduce substantially, and NOx emissions further reduce, but smoke emission maintains the same level when compared to the ethanol-diesel blend fuels without the cetane number improver. According to the heat release rate analysis, it can be found that the combustion characteristics of an ethanol-diesel blend fuel at large loads may be resumed to diesel fuel by the CN improver but that a large difference exists at lower loads.

Introduction Because of their excellent fuel efficiency, reliability, and durability, direct injection compression ignition engines have been widely used to power almost all highway trucks, urban buses, off-road vehicles, marine carriers, and industrial equipment. Unfortunately, they produce greater NOx emissions and particulate matter (PM) than spark-ignition engines. Therefore, it would be desirable to reduce emissions from diesel engines. However, because diesel combustion involves turbulent diffusion flames that produce locally rich low temperature and locally lean high-temperature regions,1,2 it is difficult to reduce both NOx and PM simultaneously and still maintain a high efficiency. There is a tradeoff relationship between NOx emission and PM emission. For example, advance of the fuel injection open angle shows positive effects on PM emission and engine economy but negative effects on NOx emission, while a * Corresponding author. Tel.: +86-21-64074085; fax: +86-21-64078095; e-mail: [email protected]. (1) Dec, J. E. A conceptual model of DI diesel combustion based on laser-sheet imaging; SAE 970873. (2) Dec, J. E.; Espey, C. Chemiluminescence imaging of auto ignition in a DI diesel engine; SAE 982685.

delay in the fuel injection open angle will improve the NOx emission but deteriorate the fuel consumption and lead to an increase of PM emission. According to this reason, a major obstacle faced by the engine researcher is to optimize the engine efficiency, PM, and NOx emissions simultaneously using modern technologies. On the other side, the global oil crisis in the 1970s triggered the need to develop alternative fuels to defend against the vulnerability of oil shortages. So, the exploration of clean alternative fuels for internal combustion engines is also a stringent research areas.3-7 Many oxygenated fuels, such as ethanol,8-10 methanol,11 DME (dimethy ether),12,13 DMC,14 DEE (diethyl ether),15 (3) Bailey, B.; Eberhart, J.; Goguen, S. Diethyl Ether (DEE) as a Renewable Diesel Fuel; SAE 972978. (4) Bertoli, C.; Del Giacomo, N.; Beatrice, C. Diesel Combustion Improvements by the Use of Oxygenated Synthetic Fuels; SAE 972972. (5) Beatrice, C.; Betroli, C.; D’Alessio, J. et al. Experimental Characterization of Combustion Behavior of New Diesel Fuels for Low Emission Engines. Combust. Sci. Technol. 1996, 120, 335-355. (6) Miyamoto, N.; Ogawa, H.; Nurun, M. D. et al. Smokeless, Low NOx, High Thermal Efficiency and Low Noise Diesel Combustion with Oxygenated Agents as Main Fuel; SAE 980506. (7) Bradley, L. E. Dimethyl ether and other oxygenated fuels for low emission diesel engine combustion, University of California, Berkeley, 1997.

10.1021/ef0500179 CCC: $30.25 © 2005 American Chemical Society Published on Web 06/22/2005

1880

Energy & Fuels, Vol. 19, No. 5, 2005

and DMM,16-18 etc. have been researched as alternative fuel and/or additives to diesel fuel to reduce smoke emissions. But, the influence of fuel molecule and structure, oxygen content in blend fuels, and cetane number of blend fuels on engine performance and emissions were not expounded clearly. Ethanol is regarded as one of the promising alternative fuels or oxygen additives for diesel engines, with its advantages of renewable energy and high oxygen content. However, because of the difficulty in forming a stabilized ethanoldiesel blend fuel, it has not been used in commercial diesel engines. In recent years, with the development of technology, Akzo Nobel Surface Chemistry19 and the Lubrizol Corporation20 have developed and produced a low cost additive, which makes it possible to blend ethanol with diesel to obtain a stable and clear fuel. DMM can be produced from natural gas by gas-to-liquid technologies and can also be produced via the oxidation of DME or through cogeneration of DME and DMM from methanol. DMC is a byproduct of DME, with a very high oxygen content and low boiling point. Adding these oxygenated fuels to diesel fuel is an effective way to improve vaporization of the fuel. In this paper, from the standpoint of the fuel design concept, ethanol, DMC, and DMM were used to mix with diesel fuel, and then different kinds of oxygenated/diesel blend fuels were obtained. A four-cylinder high-speed direct injection diesel engine was employed to investigate the effects of oxygen content in the blend fuels on diesel engine combustion and emissions. Discussed in the last part of this paper is an experiment to evaluate the influence of the cetane number of blend fuels on (8) Lu¨, X.; Huang, Z.; Zhang, W.; Li, D. The Influence of Ethanol Additives on the Performance and Combustion Characteristics of Diesel Engines. Combust. Sci. Technol. 2004, 176 (8), 1309-1329. (9) Ajav, E. A.; Bachchan, S.; Bhattacharya, T. K. Experimental study of some performance parameters of a constant speed stationary diesel engine using ethanol-diesel blends as fuel. Biomass Bioenergy 1999, 17 (4), 357-365. (10) De Caro, P. S.; Mouloungui, Z.; Vaitilingom, G. et al. Interest of combining an additive with diesel-ethanol blends for use in diesel engines. Fuel 2001, 80 (4), 565-574. (11) Huang, Z.; Lu, H.; Jiang, D.; Zeng, K.; Liu, B.; Zhang, J.; Wang, X.; Performance and Emissions of a Compression Ignition Engine Fueled with Diesel/Oxygenate Blends for Various Fuel Delivery Advance Angles. Energy Fuels 2005, 19, 403-410. (12) Kono, M.; Kajitani, S.; Suzuki, Y. Unburned Emissions from DI Diesel Engine Operated with Dimethyl Ether; Proceedings of the 15th Internal Combustion Engine Symposium (International): Seoul, Korea, July 13-16, 1999; pp 69-74. (13) Song, J.; Huang, Z.; Qiao, X.; Wang, W. Performance of a controllable premixed combustion engine fueled with dimethyl ether. Energy Conversion Manag. 2004, 45, 2223-2232. (14) Tadashi, M.; Ming, Z.; Takemi, C.; Young-Taig, O. Simultaneous Reductions of Smoke and NOx from a DI Diesel Engine with EGR and Dimethyl Carbonate; SAE 952518 (15) Bailey, B.; Eberhart, J.; Goguen, S. Diethyl Ether (DEE) as a Renewable Diesel Fuel; SAE 972978. (16) Salo, J. K. An Advanced Diesel Fuels Test Program; SAE 200101-0150. (17) Kenney, T. E.; Gardner, T. P.; Low, S. S.; Eckstrom, J. C.; Wolf, L. R.; Salo, J. K.; Szymkowicz, P. Overall Results: Phase I Ad Hoc Diesel Fuel Test Program; SAE 2001-01-0151. (18) Hideyuki, O. M.; Nabi, N.; Masahiro, M. et al. Ultralow Emissions and High Performance Diesel Combustion with Dimethoxy Methane (DMM)sApplication of High EGR at Partial Loads and Stoichiometric Combustion at High Loads with a Three-way Catalyst. Proc. Automotive Technol. 2000, 31 (4), 17-22 (in Japanese). (19) Urban, L., E. Diesel in Europe: A new available fuel technology; The 14th International Symposium on Alcohol Fuels (ISAF XIV): Phuket, Thailand, November 2002 (20) Corkwell, K.; Akarapanjavit, N.; Srithammavong, P.; Schuetzle, D.; Han, W. The development of diesel/ethanol fuel blends for diesel vehicles: fuel formulation and prosperities; The 14th International Symposium on Alcohol Fuels (ISAF XIV): Phuket, Thailand, November 2002.

Lu¨ et al. Table 1. Engine Specifications cylinder number × bore (mm) × stroke (mm) displacement (l) compression ratio combustion chamber rated power (kW)/speed (rpm) maximum torque (N m)/speed (rpm) injection system injector open pressure (MPa) nozzle number × orifice diameter (mm) advanced angle of injector open

4 × 98 × 105 3.168 18.5:1 ω type 58.5/3400 185/2200-2600 pump-linenozzle system 19.5 5 × 0.24 7 °C ABTDC

diesel engine combustion by adding the cetane number improver to 15% ethanol-diesel blend fuels. Experimental Procedures The engine tests were carried out on a water-cooled, fourstroke, four-cylinder naturally aspirated high-speed direct injection diesel engine whose major specifications are given in Table 1. The engine was coupled to an electrical generator through which the load was applied by increasing the field voltage. During the experimental tests, the engine parameters including fuel injection timing, fuel supply rate, and injector open pressure were not changed for all test fuels at overall operating ranges. The effects of blend fuels on engine thermal efficiency, exhaust emissions, and combustion characteristics were studied. The AVL emission analyzer was used to measure the concentration of nitric oxides (NOx), total unburned hydrocarbon (HC), and carbon monoxide (CO). The smoke density was tested using a Bosch smoke meter. The cylinder pressure was obtained by a Kistler piezoelectric sensor type 6125A, and the output of the pressure transducer was amplified by a Kistler charge amplifier type 5015A and then converted to digital signals and recorded by a data acquisition apparatus (Yokogawa: GP-IB), which was used to calculate the rate of heat release and to analyze the combustion characteristics.

Test Fuels In this work, three oxygenated fuels including ethanol, DMC, and DMM were mixed with the diesel fuel in different volume contents. These blend fuels include 5, 10, 15, and 20% ethanol-diesel blends; 10, 20, and 30% DMC-diesel blends; and 10 and 20% DMM-diesel blends. Some common characteristics of these oxygenated fuels can be found as follows. (i) The low heating values of oxygenated fuels are lower than that of diesel; therefore, a larger fuel supply is needed to ensure the same engine power output. (ii) The cetane number of oxygenated fuels is lower than diesel fuel; therefore, a longer period of ignition delay is expected to be realized as compared with those of diesel operation. (iii) The oxygenated fuels contain about 34.8, 42.1, and 53.1% oxygen for ethanol, DMM, and DMC, respectively. Therefore, combustion-produced emissions such as CO, HC, CO2, smoke, and PM are expected to be lower than those of diesel operation and tolerate a higher EGR ratio to reduce NOx. (iv) The latent heat of evaporation of oxygenated fuels is higher than that of diesel, which is beneficial to the NOx reduction owing to the larger temperature drop of the mixture in the cylinder. (v) Oxygenated fuels have lower viscosities. This will lead to an improvement of the fuel spray and atomization.

Direct Injection Compression Ignition Engines

Energy & Fuels, Vol. 19, No. 5, 2005 1881

Figure 1. Heat release curves of the blend fuels at selected operating conditions (3400 rpm). (a and b) DMC-diesel blend fuels and (c and d) ethanol-diesel blend fuels.

Effect of Blend Fuels on Engine Combustion and Emissions Combustion Characteristics of Blend Fuels. According to the recorded in-cylinder gas pressure, the heat release rate of the combustion process can be obtained. Figure 1 shows the heat release curve of DMC-diesel blends and ethanol-diesel blends at selected engine loads. It is can be found from the figure that, with the introduction of oxygenated fuels in the diesel fuel, the ignition timing and the crank angle corresponding to the maximum heat release rate of different oxygenated blend fuels delay not only at low engine loads but also at large engine loads. But, the maximum heat release rate exhibits another tendency, a decrease with the increase of the ethanol and DMC volume at low engine loads but an increase with the increase of the DMC and ethanol addition in blends at large engine loads. The ignition timing, combustion duration, and incylinder maximum gas temperature of blended fuels as a function of oxygenated fuel volume at different engine loads are shown in Figure 2. The ignition timing is the beginning of heat release. The total combustion duration is the time interval from the beginning of the heat release to the end of the heat release. For the case of engine speed at 3400 rpm, the ignition timing of all kinds of blend fuels delays with the increase of the ethanol, DMC, and DMM volumes from lower to higher engine loads as compared to the neat diesel fuel. Particularly, the ignition timing retards substantially at lower engine loads (such as BMEP ) 0.208 MPa) when the oxygenated fuels volume is in excess of 15%. The ignition delay can be attributed to the decrease of the cetane number of the blend fuels.

The combustion duration shows the reverse tendency as compared to the ignition timing. For a specific engine load, the combustion duration obviously shortens with the increase of the oxygenated fuel volume for ethanol, DMC, and DMM-diesel blend fuels. Also, for a specific oxygenated fuel volume, the combustion duration shortens with the increase of the engine load. Since the ignition timing delays with the oxygenated fuel addition, more homogeneous air/fuel mixtures were formed during the ignition delay period, and this leads to a quickly premixed combustion. Moreover, it can be seen from the heat release curves that the heat release ended nearly at the same crank angle. As a result, at the same engine load, the combustion duration shortens when more oxygenated fuel was added to the diesel fuel. For a fixed engine load, the oxygenated fuel volume plays little effect on the maximum gas temperature. Since the low heating values of ethanol, DMC, and DMM are lower than that of diesel, to keep the same engine power, more blend fuels should be supplied to the cylinder, while more oxygenated fuel in blend fuels leads to a delay of ignition timing as compared to the diesel fuel. On the basis of two factors, the oxygenated fuel volume shows a moderated effect on the maximum gas temperature. For specific blend fuels, the maximum gas temperature increases substantially with an increase of the engine load. Brake Thermal Efficiency. Figure 3 displays the effects of oxygen mass content in blended fuels on engine brake thermal efficiency. Under a large engine load (Figure 3c,d), the thermal efficiency improves with an increase of oxygen content. In the case of BMEP at 0.612 MPa, the thermal efficiency of neat diesel fuel is

1882


Lu¨ et al.

Figure 2. Ignition timing, combustion duration, and maximum gas temperature of different blend fuels as a function of oxygenated fuel volume in blend fuels at different engine loads (3400 rpm). (a-c) Ethanol-diesel blend fuels; (d-f) DMC-diesel blend fuels; and (g-i) DMM-diesel blend fuels.

about 30%. When the 20% ethanol-diesel fuel and 20% DMM-diesel fuel were added to the diesel engine, the thermal efficiency increased up to 30.9 and 31.6%, respectively. Particularly, when 30% DMC-diesel blends, which contain 16% oxygen in fuel, were used in the diesel engine, the thermal efficiency further increased up to 32.6%, while the oxygen content plays a negligible effect on the thermal efficiency at lower engine loads (Figure 3a,b). The improvement of brake thermal efficiency at large engine loads can be attributed to the following reasons. First, the combustion process is more complete in the fuel-rich zone due to the oxygen content in blended fuels. Second, the evaporation properties of blended fuels may be improved because the boiling point and viscosity of the oxygenated fuels are lower than that of neat diesel fuel. Third, the ignition delay is prolonged for the blended fuels due to the lower cetane number of the oxygenated fuel. These reasons lead to the fuel/air mixture becoming more homogeneous. As a result, the combustion efficiency was enhanced. At low and middle engine loads, there is almost no fuel-rich zone because of the higher excess air/fuel ration, and the lean fuel/ air mixture and poor ignitibility always lead to incomplete combustion (this can be seen from the HC and CO emission levels). So, oxygen contents in blended fuels play a moderate effect on the combustion efficiency at low engine loads.

Emission Characteristics. Figure 4 gives the effect of the oxygen content in blended fuels on the Bosch smoke number of the diesel engine. For all kinds of DMCdiesel and DMM-diesel blended fuels, the smoke emissions decrease with an increase of oxygenated fuel volume at overall operating ranges, and the smoke emission is reduced much more for large engine loads. For the 30% DMC-diesel blend, in which the oxygen mass content reaches 16%, the smoke number was reduced to about 75% when compared to neat diesel fuel. It can be found from the figure that the ethanol content in blended fuel does have a negative effect on smoke emission at low and medium engine loads, while at large engine loads, the smoke emission also improved with the ethanol additives. The influence of oxygen content in blends on NOx emissions is shown in Figure 5. It can be seen that the NOx emissions of the diesel engine using oxygenateddiesel blended fuel decrease for large and lower engine loads. For the diesel engine, it is difficult to reduce the NOx and smoke emission simultaneously using traditional technologies. But, using the fuel design concept, the NOx and smoke emission reduce simultaneously by reformulating the fuel physical-chemical parameters. NOx emissions from internal combustion engines are dominated by two factors: the maximum gas temperature and the time interval of the high temperature. For the same engine BMEP, the maximum gas temper-



Figure 3. Effect of oxygen mass content in blends on brake thermal efficiency (n ) 3400 rpm).

Figure 4. Effect of oxygen mass content in blends on Bosch smoke number (n ) 3400 rpm).

ature maintains the same order, regardless of the oxygen content in blended fuel (Figure 2c,f,i), but the combustion duration shortens with an increase of the oxygenated fuel volume (Figure 2b,e,h). As a result, the NOx emissions decrease for all blended fuels at overall operating ranges.

Figure 6 illustrates the CO emissions of diesel engines using oxygenated-diesel blended fuels. CO emissions from internal combustion engines are controlled primarily by the fuel/air equivalence ratio. CO is a result of incomplete combustion in intermediate temperature regions where the OH radical concentration becomes

1884


Lu¨ et al.

Figure 5. Effect of oxygen mass content in blends on NOx emission (n ) 3400 rpm).

Figure 6. CO emissions of oxygenated-diesel blend fuels (n ) 3400 rpm).

significantly diminished, resulting in less conversion of CO to CO2. For fuel-rich mixtures, the CO concentrations increase steadily with an increase of the equivalence ratio.21 Figure 6d shows that, with an increase of the oxygen content in blended fuel, the oxygen-fuel ratio in the overall combustion chamber increases, and

then the CO emissions decrease linearally. This can be explained by the enrichment of oxygen owing to oxygen addition, as increasing the proportion of oxygen will promote the complete combustion of the fuel/air mixture within the cylinder and further oxidation of CO during the engine exhaust process.



Figure 7. HC emissions of oxygenated-diesel blend fuels (n ) 3400 rpm).

At lower and medium engine loads, because of the reduction of the cetane number of blend fuels, the main combustion event occurred during the expanding stroke so that the maximum combustion temperature was very low, CO could not be oxidized to CO2 completely, and the engine-out CO emissions could not be further oxidized in the exhaust system for the lower exhaust gas temperature. Additionally, CO emissions of ethanol-diesel blends increase much more than DMMdiesel blends and DMC-diesel blends. Figure 7 shows the effects of oxygenated fuels on HC emissions. It is found that HC emissions increase with oxygen content both at large and lower engine loads. HC is mainly due to the chain termination during the chain propagation. Four possible emissions form the mechanism of HCCI combustion: flame quenching at the combustion chamber wall; the filling of crevice volumes with unburned mixture; absorption and adsorption of fuel vapor into an oil layer on the cylinder wall; partial burn; and misfire. Because of the lower boiling point and lower cetane number of the oxygenated fuel, when the blended fuels were injected into the cylinder, the oxygenated fuel in the blends disperse to the crevice volumes and dead volume of the combustion chamber, and these fuels are discharged from the cylinder during the expanding stroke. Effect of Cetane Number Improver on Combustion and Emissions It can be seen from the previous results that the engine brake thermal efficiency, smoke emission, and (21) Heywood, J. B. Internal combustion engine fundamentals; McGraw-Hill Book Company: New York, 1988.

NOx emissions improved when oxygenated-diesel fuel was used. But the CO emissions and HC emission increased for blend fuels. This is mainly due to the reduction of the cetane number of the blend fuels. On the basis of this reason, the author discussed the effect of the cetane number of blend fuels on engine performance and emission as follows. The 15% ethanol-diesel blend fuels with (0.2 and 0.4 vol %) and without the cetane number improver were used and compared to the diesel fuel. Emission Characteristics. Figure 8 illustrates the engine emissions versus BMEP for diesel fuel and various ethanol-diesel blend fuels. From Figure 8a, it can be found that the ethanol additive and the cetane number improver have little effect on smoke emissions from low to medium load but that smoke emissions reduced remarkably at large and full loads. In the case of NOx emissions, both ethanol additive and cetane number improver had positive effects at overall engine loads. For E15-D without CN improver, CO emissions increased remarkably at lower and medium loads, but it is worth noting that that the increasing degree of CO emission decreases with the increase of the CN improver addition. It is well-known that the combustion temperature within the cylinder shows an important impact on CO emissions. With the introduction of the cetane number improver in the ethanol-diesel blend fuels, the combustion phase gradually achieved diesel fuel, and then CO levels were similar to that of diesel fuel. From Figure 8d, it can be found that the HC emissions have a moderate increase for E15-D without the CN improver and E15-D with 0.2% CN improver at

1886


Lu¨ et al.

Figure 8. Effect of cetane number improver on CO and HC emissions (3400 rpm).

Figure 9. In-cylinder gas pressure and rate of heat release for ethanol-diesel blends and pure diesel fuel.

overall engine operating conditions. But, it is very interesting to note that the HC emissions for E15-D with 0.4% CN improver are similar to that of diesel fuel. From the previous analysis, the cetane number improver has a positive effect on CO and HC emissions at overall operating ranges. We can conclude that it is

necessary to add the cetane number improver in blend fuels when oxygenated fuels with lower cetane numbers were used. Combustion Characteristics. The in-cylinder pressures of the diesel engine fueled with ethanol-diesel blends and diesel fuel at the same operating conditions are



Figure 10. Ignition timing and combustion duration for different fuels (n ) 3400 rpm). Table 2. Properties of Diesel and Three Oxygenated Fuels chemical formula mole weight (g) density (g/mL) boiling point (°C) lower heating value (MJ/kg) latent heat of evaporation (kJ/kg) (20 °C) cetane number auto ignition (°C) surface tension (mN/m) viscosity (mm2/s) oxygen content (wt %) carbon content (wt %)

ethanol

DMM

DMC

diesel

C2H5OH 46.1 0.794 78 27 856 180 42.5 290 45 235 32.9 3.44 0 87.4

shown in Figure 9a. The heat release rate at the same operating points of different ethanol-diesel blend fuels and neat diesel fuel are shown in Figure 9b. It is clear from Figure 9a that the ignition time retards and that the maximum gas pressure decreases when the diesel engine is fueled with ethanol-diesel blends but that the ignition delay may be shortened with the increase of the CN improver volume in the ethanol-diesel blends. From the heat release rate at a specific BMEP at 3400 rpm, it can be found that the premixed combustion for all ethanol-diesel blends is prolonged when compared to neat diesel fuel but that the premixed combustion gradually shortened with the increase of CN improver volume in blend fuels. It is suggested that the change in ignition delay is responsible for this behavior. Since the ethanol fuel has an extremely low cetane number, which results in a lower cetane number for ethanol-diesel blends, the cetane number for blend fuels may be resumed with the increase in the CN improver volume in blends. To obtain a detailed understanding of the effects of the CN improver on combustion characteristics, the basic combustion parameters are obtained on the basis of the heat release analysis, and the results are shown in Figure 10. Figure 10 shows the ignition timing and total combustion duration for different ethanol-diesel blend fuels and diesel fuel. Some features can be found as follows: (i) the ignition timing gradually advances with the increase of the engine BMEP for all fuels. That is to say, the ignition delay decreases with the increase of engine load; (ii) for a specific BMEP, the ignition timing of all ethanol-diesel blends retards when compared to the diesel fuel; and (iii) the ignition timing of ethanol-

35.6 0.63 53.3 40

diesel blends can be resumed to neat diesel fuel at large loads by the cetane number improver, but there still exists a large difference at lower loads. One explanation for this is that the large evaporation heat of ethanol results in a higher temperature reduction at lower loads, and the in-cylinder gas temperature at the end of the compression stroke is very low. That is to say, at lower loads, the ignition delay of ethanol-diesel blends cannot be resumed only through the CN improver. The relationship between total combustion duration and engine load for different fuels is given in Figure 2b. Obviously, the total combustion duration of ethanoldiesel blends is shortened when compared to diesel fuel and shows a remarkable change with an increase in the CN improver addition. For a specific BMEP, the total combustion duration increases and gradually closes to diesel fuels when the CN improver was added to the blend fuels. It is suggested that the change in ignition delay is responsible for this behavior. Although the beginning of the heat release rate retards for ethanoldiesel blend fuels, the end of heat release remains at almost the same crank angle regardless of the CN improver volume addition to the ethanol-diesel blend fuel. Conclusions (i) The ignition timing of the blend fuels is delayed with the addition of oxygenated fuel at different operating conditions, the combustion duration shortens for all blend fuels, and the brake thermal efficiency improves from middle to full loads with an increase of oxygen content in blend fuels.

1888


(ii) The engine NOx emissions decrease at overall operating ranges when oxygenated fuels were added to the diesel fuel, and smoke emissions also improved for all oxygenated blend fuels except for ethanol-diesel blends at low and middle loads. CO emissions of blend fuels decreased at large and full loads but deteriorated at lower and middle engine loads. HC emission increased at overall operating ranges and increased much more with the increase of oxygenated fuel volume and decrease of the engine load. (iii) The cetane number of the blend fuels plays an important role on diesel engine performance and emissions. When the cetane number improver was added to

Lu¨ et al.

ethanol-diesel blends, the CO and HC emissions at overall operating ranges improved. The combustion characteristics of blend fuel at large loads may be resumed to diesel fuel by a CN improver, but a large difference exists at lower loads. Acknowledgment. This work was supported by the National Basic Research Program (Grant 2001CB209208) and the Key Project of the National Nature Science Foundation (Grant 50136040). EF0500179

Improving the Combustion and Emissions of Direct ... - ACS Publications

Recommend Documents