Energy Fuels 2010, 24, 2449–2454 Published on Web 03/11/2010
: DOI:10.1021/ef901543m
Effect of a Cetane Number (CN) Improver on Combustion and Emission Characteristics of a Compression-Ignition (CI) Engine Fueled with an Ethanol-Diesel Blend Shenghua Liu,* Zan Zhu, Zhijin Zhang, Guangxin Gao, and Yanju Wei School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an 710049, People’s Republic of China Received December 15, 2009. Revised Manuscript Received March 2, 2010
Ethanol-diesel blended fuel has the potential to reduce diesel engine exhaust smoke and particulate emission as well as partially solve the energy crisis. However, with the increase of the ethanol fraction of the blends, the engine combustion will be affected significantly because of the change of fuel properties, especially the ignition delay (τig). Generally, a cetane number (CN) improver should be added to improve the engine combustion. In this study, E30 (30 vol % of ethanol in the blend) was prepared. A CN improver was added at 0, 0.3, and 0.6% (in volume), respectively. With a two-cylinder diesel engine, the effects of a CN improver on the engine combustion and emissions were investigated. Experimental results show that, in comparison to the diesel engine, the brake thermal efficiency of the E30 engine is improved, the τig is prolonged, while the total combustion duration becomes shorter. With the increase of the CN improver fraction, the brake thermal efficiency, diffusive combustion phase, and total combustion duration all increase, while the τig decreases. The addition of ethanol and a CN improver in diesel has no effect on nitric oxides (NOx); however, the particulate matter (PM) and smoke emissions of the E30 engine decrease significantly, and they will be deteriorated with a high fraction blending of a CN improver.
of ethanol fumigation, a spray nozzle is needed to be installed at the intake port for the injection of ethanol. Moreover, the fuel injection strategy of the engine needs to be modified to adapt to different engine-operating conditions. Comparatively, the means of blending ethanol into diesel is more convenient because of less modification to the engine. However, two major aspects for the ethanol-diesel blends should be considered, the miscibility and the cetane number (CN) of the blends. The former problem can be easily solved by adding emulsifier or co-solvent. The CN of ethanol is around 8, while the CN of diesel is around 45; thus, mixing ethanol into diesel will result in the reduction of the CN of the fuel blend. The lower CN will prolong the ignition delay and, thus, affect the combustion and emissions of the engine. Although the retarded ignition can be partially recovered by optimizing the engine injection timing,8 it is better to add a CN improver into the blends because the fuel injection strategy does not need to be adjusted.9-11 Most studies focused on the ethanol fraction of less than 20%,2,4,9-12 and their results all prove that the reduction of
Introduction Researchers began to focus on alternative fuels since the global fuel crisis in the 1970s. Ethanol, as a kind of promising alternative fuels, has recently received considerable attention. As a kind of renewable energy, ethanol can be easily obtained from raw materials, such as sugar cane, molasses, cassava, sorghum, corn, barley, sugar beets, waste biomass materials, etc. Ethanol was first used in the diesel engine in the 1980s. Because ethanol is a kind of oxygenated fuel, the use of ethanol in diesel engines can reduce particulate matter (PM) and smoke emissions simultaneously.1-7 Generally, there are two ways to use ethanol in a diesel engine: fumigation6,7 and blending with diesel.1-4 By means *To whom correspondence should be addressed: School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an 710049, People’s Republic of China. E-mail:
[email protected]. (1) Ajav, E. A.; Singh, B.; 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, 357–365. (2) De Caro, P. S.; Mouloungui, Z.; Vaitilingom, G. Interest of combining an additive with diesel-ethanol blends for use in diesel engines. Fuel 2001, 80, 565–574. (3) Rakopoulos, D. C.; Rakopoulos, C. D.; Kakaras, E. C. Effects of ethanol-diesel fuel blends on the performance and exhaust emissions of heavy duty DI diesel engine. Energy Convers. Manage. 2008, 49, 3155– 3162. (4) Hwanam, K.; Byungchul, C. Effect of ethanol-diesel blend fuels on emission and particle size distribution in a common-rail direct injection diesel engine with warm-up catalytic converter. Renewable Energy 2008, 33, 2222–2228. (5) Alan, C. H.; Qin, Z; Peter, W. L. Ethanol-diesel fuel blends;A review. Bioresour. Technol. 2005, 96, 277–285. (6) Abu-Qudais, M.; Haddad, O.; Qudaisat, M. The effect of alcohol fumigation on diesel engine performance and emissions. Energy Convers. Manage. 2000, 41, 389–399. (7) Sahin, Z.; Durgun, O. Theoretical investigation of effects of light fuel fumigation on diesel engine performance and emissions. Energy Convers. Manage. 2007, 48, 1952–1964. r 2010 American Chemical Society
(8) Cenk, S. A.; Mustafa, C. Effects of injection timing on the engine performance and exhaust emissions of a dual-fuel diesel engine. Energy Convers. Manage. 2009, 50, 203–213. (9) Lu, X. C.; Yang, J. G.; Zhang, W. G. Effect of cetane number improver on heat release rate and emissions of high speed diesel engine fueled with ethanol-diesel blend fuel. Fuel 2004, 83, 2013–2020. (10) Lu, X. C.; Yang, J. G.; Zhang, W. G. Improving the combustion and emissions of direct injection compression ignition engines using oxygenated fuel additives combined with a cetane number improver. Energy Fuels 2005, 19, 1879–1888. (11) Ren, Y.; Huang, Z. H.; Jiang, D. M. Effects of the addition of ethanol and cetane number improver on the combustion and emission characteristics of a compression ignition engine. Proc. Inst. Mech. Eng., Part D 2008, 1077–1087. (12) Liu, J.; Liu, S. H.; Wei, Y. J.; Diesehol, C. I. Engine performances, regulated and nonregulated emissions characteristics. Energy Fuels 2010, 24, 828–833.
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Energy Fuels 2010, 24, 2449–2454
: DOI:10.1021/ef901543m
Liu et al. Table 2. Fuel Properties of Diesel and Ethanol 3
density (g/cm ) lower heating value (LHV, MJ/kg) heat of evaporation (kJ/kg) self-ignition temperature (°C) CN carbon (wt %) hydrogen (wt %) oxygen (wt %)
Figure 1. Molecular structural formula of EHN. Table 1. Specific Components of Fuels components and percentages (vol %) fuels
diesel ethanol emulsifier CN improver
diesel 100 E30 þ 0.0% CN improver 67 E30 þ 0.3% CN improver 66.7 E30 þ 0.6% CN improver 66.4
30 30 30
3 3 3
diesel
ethanol
EHN
0.86 42.5 260 200-220 45 87 12.6 0.4
0.79 26.78 854-904 636 8 52.2 13 34.8
1.635
43.4 1.4 11.6
Table 3. Engine Parameters engine type cylinder number bore (mm) stroke (mm) total displacement (cm3) combustion chamber compression ratio rated power (kW)/speed (rpm) maximum torque (N m)/speed (rpm) hole number diameter (mm) delivery timing (deg CA BTDC)
0.3 0.6
CN caused by blending ethanol can be recovered by adding a CN improver. On the basis of our previous studies, the higher ethanol fraction of 30% (E30) was applied in this paper. To deal with the separation of diesel and ethanol, an emulsifier was applied, tested under low temperatures, and fixed in a constant fraction. The CN improver was also added in different fractions as well. The effects of the CN improver content on the engine combustion and emission characteristics were investigated experimentally.
2102QB 2 in line 102 115 1880 ω 17.5:1 23/2800 120/1400 4 0.3 24
Results and Discussion Rate of Heat Release and Combustion Characteristics. According to the measured cylinder pressures, the rates of heat release of E30 and diesel can be calculated.13 On the basis of heat release rates, four typical engine combustion characteristics were analyzed, i.e., ignition delay (τig), premixed combustion duration (jpre), diffusive combustion duration (jdif), and total combustion duration (jtot). Figure 4 shows their definitions. The start and end of combustion are defined as the points of dQb/dj recovering to 0 and the accumulative heat release rate reaching 90%. τig and jtot are defined as the CA intervals between the points of fuel injection and the start and end of combustion, respectively. jtot contains jpre and jdif, which are divided by the first trough on the heat release rate curve. The cylinder pressure and heat release rate of different fuels are showed in Figure 5. In comparison to the diesel engine, the start of combustion and the occurrence of the maximum cylinder pressure are both retarded, while the maximum heat release rate is increased when the diesel engine is fueled with the E30 blend without a CN improver added. Because of the recovery of the CN for the E30 blend, the occurrences were advanced and the maximum heat release decreased with the increase of the CN improver in blend. However, there are still obvious differences on the pressure and the heat release rate curves for diesel and E30 blends in the range of this study. As can be seen in this figure, the CN improver of 0.6% cannot recover E30 burning processes to diesel mode, mainly because of the higher fraction of ethanol; i.e., EHN can only partially recover the CN of E30 blends to that of diesel. The comparisons of τig under different operating conditions are showed in Figure 6. τig of the diesel engine is about 12° CA, and it varies little with the increase of the engine load. It decreases only about 1° CA when the engine load increases from 0.1 to 0.5 MPa of brake mean effective pressure (BMEP). When the engine runs on the E30 blend without a CN improver, τig increases significantly, there is a retard of 6° CA at 0.1 MPa, and it decreases 3° CA under the higher engine load of 0.5 MPa.
Experimental Setup Fuel Blends Preparation. Commercial China 0 diesel was used as the base fuel. Anhydrous ethanol (99.5% purity) was first mixed with an emulsifier (octanol), and then, the mixture was blended with commercial diesel. At last, the CN improver was added. The contents of E30 fuel blends used in this study are listed in Table 1. The CN improver was 2-ethylhexyl nitrate (EHN, C10H4Cl3NO2), and its molecular structural formula is shown is Figure 1. In addition, the properties of the components are listed in Table 2. Test Apparatus and Procedure. The experiments were conducted on a four-stroke, water-cooled, direct-injection diesel engine without any retrofits. Its major specifications are listed in Table 3. The engine test bench is schematically shown in Figure 2. The cylinder pressure was sensed by a water-cooled piezoelectric pressure transducer (Kistler 7061B), and the signals were amplified by a charge amplifier (Kistler 5011). The signals of the crankshaft position and the piston top dead center (TDC) were obtained from an optical rotary encoder (Kistler 2013B). The injection timing was measured with a needle valve lift sensor. Therefore, the cylinder pressure and fuel injection timing can be recorded and dealt as a function of the crank angle (CA). The above three signals were recorded by a data acquisition apparatus (YOKOGAWA DL750). Meanwhile, the exhaust emissions of nitric oxides (NOx), hydrocarbon (HC), and carbon monoxide (CO) were measured online by the exhaust gas analyzer (Horiba MEXA 7100DEGR). The exhaust smoke was detected by AVL DiSmoke 4000, and PM was tested by Horiba MDLT-1302TMA. The fuel consumption was measured by an electronic balance (ES-20KA), with a sensitivity of 0.1 g. In this experiment, the engine test operating points were selected according to the European steady-state 13-mode test cycle (ESC), which are shown in Figure 3. To analyze the engine combustion and emission characteristics varying with the engine load, tests were conducted under load characteristics at speeds of 2340 revolutions per min (rpm) particularly. Besides, to improve measurement accuracy, the variation of speed in each tested condition was controlled within 5 rpm and the controlled precision of torque was 0.5%.
(13) Heywood, J. B. Internal Combustion Engine Fundamentals; McGraw-Hill: New York, 1988.
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: DOI:10.1021/ef901543m
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Figure 2. Schematic diagram of the engine test bench.
Figure 3. ESC 13-mode test cycle of the ethanol-diesel engine.
The addition of a CN improver will increase the CN of the blends. The blending of EHN will draw the τig of the E30 engine close to the diesel engine. The difference of τig between diesel and the E30 blend is reduced by adding the CN improver into the E30 blend. When a high engine load (pme = 0.5 MPa) is taken as an example, the difference decreases from 3.86 to 1.53 with the increase of the EHN content from 0 to 0.6%. On the other hand, the difference of τig cannot be continuously reduced by the blending of the CN improver.14,15 When the high engine load is taken as an example again, the decrement of τig for E30 blends with the CN improver fraction from 0 to 0.3% is about twice that from 0.3 to 0.6%, as can be seen in Figure 6. The combustion durations are illustrated in Figure 7. With the increase of the engine load, more fuel will be injected and burned, it takes a longer time to complete the combustion processes, but the increase is not so great. In addition, when E30 fuel is used under the same load conditions, jtot decreases greatly. It shows that a little
Figure 4. Definition of combustion parameters.
change of τig will result in a great reduction of jdif, especially under low load and without EHN addition. It can also be seen in this figure, jpre increases to some extant, while jdif decreases significantly. It proves that most fuel is burned during the same time length of the premixed burning phase because of the several degree CA increase of τig;16-18 i.e., ethanol can improve spray atomization and combustion obviously. In this way, as can be seen in Figure 5, the peak of the heat release rate of the E30 operating conditions was greatly elevated in comparison to that of diesel and decreased with the increase of the CN improver fraction. Specific Fuel Consumption. Because of a smaller LHV of ethanol, the LHV of the E30 blend is about 88.90% diesel. The equivalent brake-specific fuel consumption (BSFCeq) is
(14) Higgins, B.; Siebers, D.; Mueller, C. Effects of an ignitionenhancing, diesel-fuel additive on diesel-spray evaporation, mixing, ignition, and combustion. 21th International Symposium on Combustion, The Combustion Institute, Pittsburgh, PA, 1998; pp 1873-1880. (15) Hartmann, M.; Tian, K.; Hofrath, C. Experiments and modeling of ignition delay times, flame structure and intermediate species of EHN-doped stoichiometric n-heptane/air combustion. Proc. Combust. Inst. 2009, 32, 197–204.
(16) Saxena, P.; Williams, F. A. Numerical and experimental studies of ethanol flames. Proc. Combust. Inst. 2007, 31, 1149–1156. (17) Liao, S. Y.; Jiang, D. M.; Huang, Z. H. Determination of the laminar burning velocities for mixtures of ethanol and air at elevated temperatures. Appl. Therm. Eng. 2007, 27, 374–380. (18) Parag, S.; Raghavan, V. Experimental investigation of burning rates of pure ethanol and ethanol blended fuels. Combust. Flame 2009, 156, 997–1005.
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: DOI:10.1021/ef901543m
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Figure 5. Comparison of cylinder pressures and heat release rates.
Figure 6. Ignition delay for different fuels.
Figure 7. Combustion duration for different fuels.
used here for convenience to evaluate the fuel economy and thermal efficiency of the engine and is defined as follows: beq ¼ ðHuE30 =Hudiesel Þbe
duration and diffusive combustion duration, owing to the prolonging of τig and oxygen enrichment, respectively. In addition, the equivalent BSFC for the E30 blend will be further reduced with the increase of the CN improver fraction as well. With the addition of the CN improver, the combustion for the E30 blend will be enhanced because the occurrences of the start of combustion (SOC) and the in-cylinder peak pressure are both close to the top-dead center (TDC), which can be seen in Figure 5. Therefore, it
ð1Þ
where be is the BSFC, beq is the equivalent BSFC referring to the E30 blend, and HuE30 and Hudiesel are the LHVs of the E30 blend and diesel, respectively. It is can be seen from Figure 8 that the equivalent BSFC of the E30 engine is reduced in comparison to the diesel engine, i.e., because of the enhancement of the premixed combustion 2452
Energy Fuels 2010, 24, 2449–2454
: DOI:10.1021/ef901543m
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Figure 8. Equivalent BSFC for all fuels.
Figure 10. Exhaust smoke characteristics at 2340 rpm.
Figure 9. NOx emission characteristics at 2340 rpm. Figure 11. CO emission characteristics at 2340 rpm.
is indicated from this figure that EHN can enhance the brake thermal efficiency of the E30 engine at middle and high engine loads and with a slight increase. Emissions. To display the relationship between exhaust emissions and the CN improver fraction, the emission characteristics varying with the engine load at the speed of 2340 rpm were investigated. The emission results of the ESC test are exhibited as well. As seen from Figure 9, the NOx emission was rarely affected by the substitution of ethanol and the CN improver for diesel. When ethanol is blended, τig is prolonged, which will result in the increase of premixed combustion. This factor has a positive effect on the formation of NOx. While on the other hand, the in-cylinder combustion temperature would be decreased because of the high vaporization latent heat of ethanol, which would suppress the formation of NOx. The two opposite effects compete and finally make the NOx emission from the E30 engine similar to that from diesel. Further, the adding of the CN improver shows nearly no influence on the NOx emission for the E30 blend. Although the decrease of the maximum heat release rate and the increase of jtot with the blending of the CN improver would reduce the maximum combustion temperature, the reduction is so small that there is nearly no change to the NOx emission. Engine smoke strongly depends upon jdif. When ethanol is added to diesel, it enhances spray atomization and
evaporation. As a result, the diffusive combustion duration is greatly decreased, which is shown in Figure 8. Ethanol is an oxygenate component; it itself does not give out PM or a PM precursor when burning. It can decrease the smoke and PM formation and emission. However, the engine smoke increases with the increase of the CN improver fraction according to the prolonged jdif. As seen in Figure 10, the smoke emission is almost as high as that of diesel when the content of the CN improver reaches 0.6%. Therefore, the highest fraction of CN should be less than 0.6% to avoid the increase of smoke or PM. CO and HC are both incomplete combustion products of fuels. As seen in Figures 11 and 12, both CO and HC emissions decrease with the increasing engine load and increase over a high-load condition. In comparison to dieseloperating conditions, CO is higher than that of diesel under lower and middle-load conditions and lower than that of diesel at higher load conditions, while HC is higher at the two ends. When ethanol is blended, ethanol is easy to be evaporated and forms more and a leaner mixture during the longer ignition delay, resulting in more emissions of CO and HC. Moreover, the further oxidation of CO will be enhanced because of the increase of the oxygen proportion when compared to the diesel engine. That results in the lower CO emission for E30 blends under high-load conditions. The CN 2453
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: DOI:10.1021/ef901543m
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Conclusions The effects of EHN on the combustion, fuel economy, and emissions were investigated on an E30-fueled engine. The following conclusions can be drawn: (1) Because of the enhancement of the mixture formation and combustion, the equivalent diesel BSFC decreases when ethanol is added in diesel and the brake thermal efficiency improves slightly with the increase of the CN improver fraction in blends. (2) τig of E30 without the CN improver is prolonged, and the total combustion duration becomes shorter than pure diesel conditions. The addition of the CN improver will shorten τig and advance the start of combustion and the peak cylinder pressure while slightly prolonging the total combustion duration. (3) Both CO and HC emissions decrease with the increase of engine loads and then increase over high-load conditions with the engine fueled with the E30 fuel blends. The CN improver has little influence on them. Ethanol and the CN improver have significant effects on PM but have an ignorable effect on NOx emissions. (4) When the CN improver fraction in blends is up to 0.6%, there is not so much engine combustion improvement but PM and smoke emissions will be increased greatly, especially under high-load conditions. Experiments show that a 0.3% CN improver is better for E30 engine combustion and emissions. In this case, HC and NOx emissions are at the same level of diesel operation, while the reduction of CO and PM emissions can reach 23.7 and 62.6%.
Figure 12. HC emission characteristics at 2340 rpm. Table 4. Emission Results for Fuels under the ESC 13-Mode Test Cycle (g kW-1 h-1) fuel
CO
HC
NOx
PM
diesel E30 þ 0.0% CN improver E30 þ 0.3% CN improver E30 þ 0.6% CN improver Euro III
5.326 4.159 4.061 4.015 2.1
2.305 3.116 2.839 2.671 0.66
10.859 10.892 10.883 10.792 5.0
0.548 0.201 0.205 0.452 0.10
additive, EHN, just shows a weak influence on CO and HC emissions and will slightly reduce CO and HC emissions simultaneously. The averaged CO, HC, NOx, and PM emissions for four kinds of fuels are listed in Table 4 under the ESC 13-mode test cycle. The results show that ethanol gives positive effects on the reduction of CO and PM emissions, negative effects on the HC emission, and nearly no effects on the NOx emission. The addition of EHN to E30 blends can reduce CO and HC emissions, provide no influence on the NOx emission, and increase the PM emission. The adding fraction of EHN should be no more than 0.6%. By comprehensive consideration of the relationship between the CN improver fraction and CO, HC, NOx, PM, and smoke emissions level of the E30 engine, the CN improver fraction of 0.3% is thought to be the best selection for the E30 blend. In this case, the reduction of CO and PM emissions of the E30 engine is about 23.7 and 62.6%, respectively, although HC and NOx emissions are at the same level of diesel operation.
Acknowledgment. This study was supported by the National Natural Science Fund of China (50876088).
Nomenclature BSFC = brake-specific fuel consumption BMEP = brake mean effective pressure CI = compression ignition CN = cetane number LHV = lower heating value τig = ignition delay jpre = premixed combustion duration jdif = diffusive combustion duration jtot = total combustion duration E30 = blend of 30% ethanol in volume with diesel ESC = European stationary cycle NOx = nitric oxides PM = particulate matter CO = carbon monoxide HC = hydrocarbon
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