Energy Fuels 2010, 24, 828–833 Published on Web 10/21/2009
: DOI:10.1021/ef900930a
Diesehol CI Engine Performances, Regulated and Nonregulated Emissions Characteristics Liu Jie, Liu Shenghua,* Wei Yanju, Li Yi, Li Guangle, and He Hun School of Energy and Power Engineering, Xi’an Jiaotong University Xi’an, 710049, People’s Republic of China Received August 27, 2009. Revised Manuscript Received October 8, 2009
The blended fuel of diesel and ethanol is known as diesehol. In this paper, pure diesel and three kinds of diesehols, containing 10, 20, and 30% of ethanol by volume, were used to investigate the effects of ethanol/ diesel ratio on engine power, thermal efficiency and emissions, especially the exhaust ethanol and formaldehyde emissions. Experimental results indicate that with the increase of ethanol fraction in the fuel blends, the engine power is decreased to some extent, while the brake thermal efficiency improves slightly and the diesel equivalent BSFC decreases. CO emission increases at low load, while decreases at high load. NOx emission has almost no change and smoke emission decreases significantly at higher engine load conditions. The NOx/ smoke trade-off correlation has been improved obviously. A gas chromatograph is calibrated and used to measure the ethanol and formaldehyde emissions. Measurement indicates that ethanol emissions increase with the increase of its content in the blends and changes little with engine load; formaldehyde emission has a good linear relationship with the amount of cyclically supplied ethanol when the engine fueled with diesehol. Because the response of the flame ionization detector to ethanol is 1.7 times that of C1 and it has no response to formaldehyde, the total hydrocarbon emission of the engine is revised accordingly. unburned hydrocarbons are increased to some extent.7,8 The addition of ethanol could promote or inhibit the emissions of CO and NOx, depending on the engine type and operating conditions.9-11 However, the nonregulated emissions, such as formaldehyde and ethanol, are rarely mentioned. Formaldehyde (CH2O) can be the intermediate product of the combustion of diesel fuel12 and ethanol.13 It is toxic, allergenic, and carcinogenic. Formaldehyde is regarded as a probable human carcinogen by the U.S. Environmental Protection Agency. However, it is proven that hydrocarbon detector and flame ionization detector (FID) are less sensitive to CH2O.14 The CARB method 1004 (ASTM method D5197, U.S. EPA methods TO-11A and 8315) is commonly used to monitor aldehyde emissions from engines. However, except
1. Introduction Limited fossil fuel availability and air pollution have drawn increasing attention to the study of alternative fuels. There have been a number of studies on applying diesel fuel blended with oxygenates to reduce diesel engine emissions, especially the particulate matter (PM).1-4 Among them, ethanol has the most potential. It is renewable, and it can be made in massive amounts from very common crops such as sugar cane, potato, manioc, and corn. Moreover, ethanol is biodegradable without harmful effects on the environment. Since late 1990s, diesehol has been widely studied for use in diesel engines. The main purpose of this application is to reduce engine smoke and particulate matter (PM), and the results are positive and obvious. For instance, 15% ethanol in diesel could reduce PM emission by about 40%.5,6 The total
(7) Shih, L. K. Comparison of the effects of various fuel additives on the diesel engine emissions. SAE 1998, 982573. (8) Cole, R. L.; Poola, B.; Sekar, R.; Schaus, J. E.; Mcpartlin, P. Effects of ethanol additives on diesel particulate and NOx emissions. SAE 2001, 2001-01-1937. (9) Merritt, P. M.; Ulmet, V.; McCormick, R. Regulated and Unregulated Exhaust Emissions Comparison for Three Tier II Non-road Diesel Engines Operating on Ethanol-diesel Blends. SAE 2005, 2005-012139. (10) Li, D. G.; Huang, Z.; Lu, X. C.; Zhang, W. G.; Yang, J. G. Physico-Chemical Properties of Ethanol-Diesel Blend Fuel and Its Effect on Performance and Emissions of Diesel Engines. Renewable Energy 2005, 30, 967–976. (11) Ahmed, I. Oxygenated Diesel: Emissions and Performance Characteristics of Ethanol-diesel Blends in CI Engines. SAE 2001, 2001-01-2475. (12) Takada, K.; Yoshimura, F.; Ohga, Y.; Kusaka, J.; Daisho, Y. Experimental Study on Unregulated Emission Characteristics of Turbocharged DI Diesel Engine with Common Rail Fuel Injection System. SAE 2003, 2003-01-3158. (13) Taylor, P. H.; Shanbhag, S.; Dellinger, B. The High-temperature Pyrolysis and Oxidation of Methanol and Ethanol: Experimental Results and Comparison with Vehicle Emission Tests. SAE 1994, 941904. (14) Hunter, M. C.; Bartle, K. D.; Lewis, A. C.; McQuaid, J. B.; Myers, P.; Seakins, P. W. The Use of the Helium Ionization Detector for Gas Chromatographic Monitoring of Trace Atmospheric Components. J. High Resolution Chromatagr. 1998, 21, 75–80.
*To whom correspondence should be addressed. E-mail: shenghua@ mail.xjtu.edu.cn. (1) L€ u, X.-c.; Yang, J.-g.; Zhang, W.-g.; Huang, Z. 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 (5), 1879–1888. (2) Ghobadian, B.; Rahimi, H.; Nikbakht, A. M.; Najafi, G.; Yusaf, T. F. Diesel engine performance and exhaust emission analysis using waste cooking biodiesel fuel with an artificial neural network. Renewable Energy 2009, 34, 976–982. (3) Karabektas, Murat; Hosoz, Murat Performance and emission characteristics of a diesel engine using isobutanol-diesel fuel blends. Renewable Energy 2009, 34, 1554–1559. (4) Zhu, R.; Wang, X.; Miao, H.; Huang, Z.; Gao, J.; Jiang, D. Performance and Emission Characteristics of Diesel Engines Fueled with Diesel-Dimethoxymethane (DMM) Blends. Energy Fuels 2009, 23 (1), 286–293. (5) Kim, H.; Choi, B. 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. (6) Kass, M. D.; Thomas, J. F.; Storey, J. M.; Domingo, N.; Wade, J.; Kenreck, G. Emissions from a 5.9-Liter Diesel Engine Fueled with Ethanol Diesel Blends. SAE 2001, 2001-01-2018. r 2009 American Chemical Society
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: DOI:10.1021/ef900930a
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for the sampling time, it takes over 1 h for the operation of derivation, elution, extraction, and detection of the DNPH hydrazone. The complex sample pretreatment is not endurable, especially when a lot of testing points are needed. Over the past few years, a fast chromatographic method with the pulsed discharge helium ionization detector (PDHID) has been developed, which is capable of monitoring low levels of formaldehyde and with a simple sampling process in several minutes.15 The presence of PDHID makes it much easier to investigate engine-out CH2O emission characteristics over a wide range of engine speeds and torques. If diesel is regarded as n-heptane, it is expected that ethanol takes about 50% mole fraction in a fuel blend at 30% volume fraction. Therefore, the role of ethanol combustion in an engine becomes dominant. The change of the engine combustion mechanism by ethanol addition results in different characteristics of the engine emissions. As the response of ethanol on a flame ionization detector (FID) is 1.7 times that of the HCs and the HCs are evaluated by equivalent methane, the measured value of ethanol is less than its theoretically predicted value. And FID has no response on CH2O. The measurements of total HC (THC) will inevitably result in errors, especially at higher ethanol fraction where the situation gets even worse with an FID without special calibration. Moreover, the more the ethanol ratio in the diesehol, the higher the error will be. Most literature reports use an FID directly without taking this into consideration. In this study, a solvent additive was added to prevent ethanol and diesel from phase-separating. Pure diesel and three diesehols were prepared. The effects of ethanol/diesel ratio on engine power performance and emissions were investigated. The nonregulated emissions including formaldehyde and ethanol were detected by PDHID in selective ArPDHID mode to improve the response and selectivity by eliminating the interference of water and permanent gases.16
Table 1. Engine Specifications item
parameter
engine type combustion chamber bore(mm)stroke(mm) displacement (cm3) compression ratio rated power(kW)/Rated speed(rpm) max torque(N 3 m)/speed(rpm) nozzle hole numberdiameter(mm) fuel delivery angle
2102QB diesel engine ω type 102115 1880 17.5:1 23.1 /2800 120 /1400 40.3 25 °CA BTDC (before top dead center)
Figure 1. Engine power output.
system consists of a Gs-OxyPLOT capillary column (10 m 0.53 mm inner diameter 10 μm film, Agilent Technologies) and a pulsed discharge ionization detector (PDHID, model D4-I-SH17R, Valco Instrument) performing in Ar-PDHID mode.17,18 The engine was warmed up until the cooling water temperature varied from 80 to 85 °C while the lubricating oil temperature was over 60 °C, and then loaded to the test points. Each measurement was repeated three times, and the average data were used for analysis. If the deviation of the measurement data is beyond the predefined limit, the measurement should be conducted again to avoid measurement errors. At each mode of operation, the engine was allowed to run for a few minutes until the exhaust gas temperature, the cooling water temperature, and the lubricating oil temperature stabilized before data were measured. Before running the engine with a new blended fuel, it was allowed to use the new fuel to cleanout the remaining fuel from the pipeline of the engine to avoid the leftover interfering each other. In each tested condition, the variation of speed was controlled within (5 r/min, the controlled precision of torque was 0.5%. The fuel consumption uncertainty was less than 2%. The relative experimental error is less than 2% for the CO emission, 3% for the THC and NOx emissions.
2. Test Fuels Four kinds of fuels were prepared. They are commercial diesel, three diesehols containing 10, 20, and 30% of ethanol in volume, named to be E0, E10, E20, and E30, respectively. To prevent phase separation, 2%vol of normal octanol was doped in blends as the cosolvent. 3. Experiment Apparatus and Procedure A twin cylinder, water-cooled, four-stroke, direct-injection (DI) compression ignition (CI) engine was applied in this study. Its main specifications are listed in Table 1. The regulated emissions such as carbon monoxide (CO), total hydrocarbon (THC), and nitric oxides (NOx) were detected by MEXA7100DEGR analyzer (Horiba, Japan). Smoke was measured by AVL DiSmoke 4000. Nonregulated emissions CH2O and C2H5OH were detected by GC-2010 (Shimadzu, Japan). The GC detecting
4. Results In this paper, engine power was first studied under full load operating conditions. Fuel economy was compared, as well. For emission studies, experiments were executed at two engine speeds of 1400r/min and 1870r/min, which refers to ESC test engine speed nA and nB. The brake mean effective pressures (load) were set to be 0.1, 0.2, 0.3, 0.4, and 0.5 MPa. At each test condition, experiments were carried out with pure diesel and three diesehols. 4.1. Engine Performance. Figure 1 gives the full-load power output of the engine for the pure diesel and three diesehols. As shown in Figure 1, the engine output power decrease with the increase of the ethanol fraction.
(15) Hopkins, J. R.; Still, T.; Al-Haider, S.; Fisher, I. R.; Lewis, A. C.; Seakins, P. W. A Simplified Apparatus for Ambient Formaldehyde Detection via GC-pHID. Atmos. Environ. 2003, 37 (18), 2557–2565. (16) Dojahn, J. G.; Wentworth, W. E.; Stearns, S. D. Characterization of Formaldehyde By Gas Chromatography Using Multiple PulsedDischarge Photoionization Detectors and a Flame Ionization Detector. J. Chromatogr. Sci. 2001, 39, 54–58. (17) Wei, Y. J.; Liu, J.; Zhu, Z.; Li, G. L.; Liu, S. H. Trans. Chin. Soc. Intern. Combust. Engines 2008, 26, 533–537. (18) Wei, Yanju; Liu, Shenghua; Liu, Fangjie; Liu, Jie; Zhu, Zan; Li, Guangle Formaldehyde and Methanol Emissions from a Methanol/ Gasoline-Fueled Spark-Ignition (SI) Engine. Energy Fuels 2009, 23, 3313–3318.
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Figure 2. Diesel equivalent BSFC and thermal efficiency.
The decrease of E10 is slightly; however, the decrease of E20 and E30 is more obviously at medium and high speed region, about 6.5 and 9%, separately. The power decrease is mainly due to the heat reduction cyclically delivered to the engine, though the volume may be the same. The LHV of ethanol is about 63% of diesel, and its density is 0.79 g/cm3, lower than 0.84 g/cm3, the density of diesel. Figure 2 displays the diesel equivalent BSFC and effective thermal efficiency for pure diesel and three diesehols at different BMEP. The diesel equivalent BSFC is calculated using eq 1. ð1Þ beq ¼ ðHublends =Hudiesel Þbe
Figure 3. CO emissions.
As shown in Figure 2, it decreases slightly with increase in the ethanol fraction owing to the improvement in combustion. The effective thermal efficiency is in an inverse ratio to the diesel equivalent BSFC as indicated in eq 2, ηet ¼ 3:6 106 =ðHudiesel beq Þ
of ethanol causing lower combustion temperature and results in a lower oxidation rate of CO. However, at high load, the CO emission is moderately decreased, which would contribute to be more complete combustion by reducing the equivalence ratio of the fuel rich region. 4.2.1.2. NOx Emissions. Figure 4 shows NOx emission characteristics of the pure diesel and three diesehols with the change of engine load. It can be seen that NOx emissions is almost not changed with the addition of ethanol. Only at high loads, there is a slightly increase. Ethanol affects NOx formation in two aspects. First, ethanol provides oxygen component and improves local air/fuel ratio, and therefore it promotes NOx formation. Second, ethanol reduces the combustion temperature because of its higher evaporation latent heat, and therefore it suppresses NOx formation. The interaction of these two aspects makes the NOx emissions changing little. 4.2.1.3. Smoke Emissions. Figure 5 shows the smoke emission versus brake mean effective pressure for four tested fuels. It is seen that smoke is mainly produced at high load condition. The addition of ethanol will reduce the equivalence ratio of the fuel rich regions and suppress soot formation in the combustion chamber. 4.2.1.4. NOx/Smoke Correlation. The relationship between NOx and smoke at various load conditions are plotted in Figure 6. It is shown that the traditional trade off correlation is broken, the curve is relatively flat. The trade-off relationship between NOx and smoke has been improved obviously. 4.2.2. Nonregulated Emissions. Formaldehyde is an important intermediate product of hydrocarbon (HC) and oxygenated hydrocarbon oxidation. If diesel is regarded as n-heptane, it is expected that ethanol takes about 50% mole
ð2Þ
so they must reflect the same phenomenon. As shown in Figure 2, thermal efficiency is improved slightly with increase in the ethanol fraction in the blended fuels. Considering both power output and fuel economy, without modification to the engine, 10% percent of ethanol addition to diesel fuel is proposed to be the best choice. If the fuel inject rate was adjust and with the assistant of cetane number improver to obtain the same power output as running on pure diesel for the diesehols, 30% percent of ethanol addition to diesel fuel is the best choice. 4.2. Engine Emissions. When ethanol is used as fuel, unburned ethanol will emit, and it impacts the formaldehyde emission as the change of fuel oxidation mechanism. Although the two emissions are not restricted in vehicle emission regulations in many countries, it is significant to investigate their emission characteristics. In this paper, the emission characters of C2H5OH and CH2O are presented. Moreover, THC emissions were revised by the C2H5OH and CH2O emissions. The regulated emissions are also presented. 4.2.1. Regulated Emissions. 4.2.1.1. CO Emissions. Figure 3 shows the CO emission of the pure diesel and three diesehols at different operation conditions. CO emission is the product of incomplete combustion and it is controlled primarily by the fuel/ air equivalence ratio and temperature. As seen in Figure 3, with the addition of ethanol, at low engine load, CO emission increase remarkably, it can be explained by the fact that higher latent heat 830
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: DOI:10.1021/ef900930a
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Figure 4. NOx emissions.
Figure 6. NOx-smoke trade-off relationship.
Figure 7. C2H5OH emission.
change of fuel oxidation mechanism results in different characteristics of the engine emissions, more ethanol and formaldehyde will be emitted. 4.2.2.1. Ethanol Emissions. Figure 7 shows the ethanol emission concentration in the exhaust gas. In general, ethanol emissions increase with the increase of its content in the blends and changes little with load, also shown in Figure 8. And ethanol emission only comes from the unburned ethanol, since it is detected only in the cases of E10, E20, and E30 fuels. Unburned ethanol will exist at too lean or too rich region of the spray. There are several other factors that influence the emission of unburned ethanol. First, at higher engine load the combustion temperature is higher, ethanol will be better postflame oxidized. Second, at a given load, the
Figure 5. Smoke emissions.
fraction in E30 fuel. Therefore, the role of ethanol combustion in an engine becomes increasingly important. The 831
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: DOI:10.1021/ef900930a
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Figure 8. Relationship between the ethanol emission and cycle supplied ethanol.
Figure 10. Relationship between the CH2O emission and cycle supplied ethanol.
Figure 9. CH2O emission.
Figure 11. THC emissions.
ignition will be delayed with the increase of ethanol content in the fuel blends due to the higher latent heat and lower cetane number, there may be more ethanol too lean mixed with air and consequently results in an increase in the unburned ethanol. These two effects control the engine ethanol emission. 4.2.2.2. Formaldehyde Emissions. According to the ethanol and primary reference fuels oxidation mechanism studies,19-21 the processes of formaldehyde formation and oxidation are influenced by the temperature, the residence time and concentration of hydrocarbons in the burned gases. Figure 9 shows the formaldehyde emission versus brake mean effective pressure for the four types of fuels. To the low ethanol fraction fuel, formaldehyde emission was less affected by the addition of ethanol. When 30% ethanol was added, ethanol combustion becomes dominated, and then formaldehyde emission increases. However, formaldehyde emission increases with fuel delivery quantity, and especially it has a good linear relationship with the amount of cyclically supplied ethanol, which is shown in Figure 10.
4.2.2.3. THC Emissions. Figure 11 illustrates the THC emission at the speed of 1870r/min measured by an FID. HC emissions increase with the increase of ethanol content, and the engine load has little effect on HC emission. It is known that FID response differs from the kind of HC, especially if the HC coexists with oxygen. The response of C2H5OH is 1.7 times that of CH4 and has no response on CH2O. So the real value of THC emission must be revised by the data measured by both the Horiba device and the GC device. THC emission should be THCreal ¼ THCHoriba þ 0:3CC2 H5 OH þ CCH2 O There are two primary paths by which fuel can escape from the normal combustion process unburned: the fuel-air mixture is too lean or too rich. The over lean mixture oxidation path is believed to be the most important source of HC emission from CI engines. When fuel was injected into the cylinder, the amount of fuel that is mixed leaner than the lean combustion limit increase rapidly with time.22 The magnitude of the unburned HC from these over lean regions depends on the amount of fuel injected during the ignition delay and the mixing rate with air. Because the boiling point and viscosity of the ethanol are lower than that of diesel fuel, the evaporation properties of the blended fuel were improved. The ignition delay is also increased with the addition
(19) Marinov, N. M A Detailed Chemical Kinetic Model for High Temperature Ethanol Oxidation. Int. J. Chem. Kinet 1999, 31, 183–220. (20) Chaos, M.; Kazakov, A.; Zhao, Z. W.; Dryer, F. L. a HighTemperature Chemical Kinetic Model for Primary Reference Fuels. Int. J. Chem. Kinet. 2007, 39, 399–414. (21) Curran, H. J.; Pitz, W. J.; Westbrook, C. K.; Callahan, C. V.; Dryer, F. L. Oxidation of Automotive Primary Reference Fuels at Elevated Pressures. In 27th Symposium (International) on Combustion, 1998; pp 379387
(22) Heywood, J. B. Internal Combustion Engine Fundamentals; McGraw-Hill Book Company: New York, 1988.
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5. Conclusions The effects of ethanol on performance and emissions of a 2-cylinder DI compression ignition engine fuelled with pure diesel and three diesehols have been investigated, and compared to the baseline diesel fuel. The main results can be obtained as follows: (1) The maximum engine power output is decreased with the increase of ethanol proportion in the blended fuels, and is more obvious at high loads. For E20 and E30 blended fuels, the decrease is around 6.5% and 9.0%, for E10 fuel the decrease is slightly; (2) The BSFC increases when diesehol is applied, but the diesel equivalent BSFC decreased. The brake thermal efficiency improved slightly with the addition of ethanol; (3) With the addition of ethanol, CO emission increase at low load, however, it decreases at high load; (4) NOx emissions are almost not changed with the addition of ethanol. Only at high loads, there is a slightly increase. Smoke is mainly produced at high load condition and with the addition of ethanol, smoke emission decrease significantly. The traditional NOx/ smoke trade off correlation is broken; (5) The C2H5OH emissions increase with the increase of its content in the blends and changes little with load, while CH2O emission is linear to the amount of cyclic supplied fuel ethanol; (6) THC emissions increase with the increase of ethanol content, and the engine load has little effect on HC emission. With the revision of the THC emissions, the THC emissions are all increased, and the difference between E20 and E30 is more obvious.
Figure 12. Revised THC emissions.
of ethanol, and thus the HC emission increases as ethanol is added. As shown in Figure 11, the increase of THC emission form E20 to E30 fuel is slightly. That is because when ethanol fraction increases, the ethanol and formaldehyde emissions increase, as illustrated in Figures 7 and 9. The error between the measured value and the real value of THC increases. In order to minimize this error, the THC emissions were corrected by the data of ethanol and formaldehyde emissions. Figure 12 shows the revised THC emission. The difference between E20 and E30 is more obvious, and the THC emissions are all increased with the revision.
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