Energy Fuels 2010, 24, 856–862 Published on Web 11/17/2009
: DOI:10.1021/ef900993g
Effect of the Cetane Number on the Combustion and Emissions of Diesel Engines by Chemical Kinetics Modeling Wenmiao Chen, Shijin Shuai,* and Jianxin Wang State Key Laboratory of Automotive Safety and Energy, Tsinghua University, Beijing 100084, China Received September 7, 2009. Revised Manuscript Received November 2, 2009
A seven-step chemical reaction mechanism of di-tertiary butyl peroxide (DTBP), a type of cetane number improver, was combined with a reduced diesel surrogate fuel mechanism to model the effect of the cetane number on diesel ignition. The DTBP concentration in diesel fuel was tuned up to modify the cetane number. A chemical reaction solver, SENKIN, was used to display the ignition delay of diesel fuel containing different DTBP concentrations. It was learned that DTBP advances the ignition timing of diesel fuel under different pressures, temperatures, and equivalence ratios. The effect of advancing becomes more evident with a higher DTBP concentration. The combined mechanism of diesel surrogate fuel with DTBP was coupled in KIVA-3 V2 code to model the effect of cetane numbers 50-64 on the ignition timing, in-cylinder pressure, rate of heat release, engine-out NOx, and soot emissions of a Euro 4 diesel engine under different conditions. The simulation results revealed a sound agreement with the experiments. For the engine equipped with a common rail multiple injection, the increase in the cetane number advances ignition for the pilot injection; it has a negligible effect on the combustion process for the main injection and post-injection. The engine-out NOx and soot emissions are almost unchanged.
proportional components as the diesel surrogate fuel cannot fully reflect the effects of cetane numbers. There are hardly any modeling studies focusing on the effects of the cetane number on the combustion and emission of diesel engines. Reitz et al.7 modeled the effects of the cetane number on the cold-start combustion process of a diesel engine; they applied an empirical method of adjusting the kinetic constants of the Shell model based on the cetane number of the diesel fuel. Di-tertiary butyl peroxide (DTBP) is a common fuel additive used as a cetane number enhancer. Griffiths et al.8,9 developed the DTBP oxidation mechanism using 27 species and 69 reactions based on combustion experiments. This was conducted in a closed mechanically stirred vessel and rapid compression machine. A number of researchers have since modeled the ignition advancing effects of DTBP on liquefied petroleum gas (LPG) engines10 and combustion in a gasoline homogeneous charge compression ignition (HCCI) engine11,12 by tapping the DTBP mechanism of Griffiths et al. A chemical reaction mechanism of DTBP was combined with the reduced diesel surrogate fuel chemical reaction mechanism13 developed from Golovitchev’s n-heptane and toluene mechanism.14 The DTBP dissociates at a low temperature to produce small radicals involved in the diesel surrogate fuel mechanism. The reduced diesel surrogate fuel
1. Introduction Three-dimensional computational fluid dynamics (CFD), coupled with chemical kinetic mechanisms to simulate an engine in-cylinder combustion process and emission formation, may be used to significantly shorten the development period of engines. The chemical compositions of real diesel fuel are too complex to be modeled with a detailed chemical mechanism. Therefore, researchers are focusing on the chemical kinetics model of diesel surrogate fuel to represent the real diesel chemical kinetics with limited individual components.1 Paraffins, naphthenes, and aromatics are the main components of real diesel fuel. Golovitchev et al.2-4 combined 70 mol % n-heptane with 30 mol % toluene as a diesel surrogate fuel, displaying the same level of cetane number and other properties as real diesel fuel. The kinetic mechanism was coupled with three-dimensional CFD to display combustion and soot formation in diesel engines. In Golovitchev’s mechanism, solid carbon particles appeared through global reactions of gaseous precursors in the “graphitization” process. The cetane number of diesel fuel poses a significant effect on the ignition timing, combustion, and engine-out emissions of diesel engines.5,6 Using n-heptane or a mixture of fixed *To whom correspondence should be addressed. Telephone: þ86-1062772515. Fax: þ86-10-62785708. E-mail:
[email protected]. edu.cn. (1) Farrell, J. T.; Cernansky, N. P.; Dryer, F. L.; Friend, D.; Hergart, C.; Law, C. K.; McDavid, R. M.; Mueller, C. J.; Patel, A.; Pitsch, H. SAE Tech. Pap. 2007-01-0201, 2007. (2) Gustavsson, J.; Golovitchev, V. I. SAE Tech. Pap. 2003-01-1848, 2003. (3) Fredriksson, J.; Bergman, M.; Golovitchev, V. I.; Denbratt, I. SAE Tech. Pap. 2006-01-0449, 2006. (4) Golovitchev, V. I.; Calik, A. T.; Montorsi, L. SAE Tech. Pap. 2007-01-1838, 2007. (5) Lee, R.; Pedley, J.; Hobbs, C. SAE Tech. Pap. 982649, 1998. (6) Hochhauser, A. M. SAE Tech. Pap. 2009-01-1181, 2009. r 2009 American Chemical Society
(7) Ayoub, N. S.; Reitz, R. D. SAE Tech. Pap. 952425, 1995. (8) Griffiths, J. F.; Phillips, C. H. Combust. Flame 1990, 81, 304–316. (9) Griffiths, J. F.; Jiao, Q.; Kordylewski, W.; Schreiber, M.; Meyer, J.; Knoche, K. F. Combust. Flame 1993, 93, 303–315. (10) Lee, D.; Goto, S.; Honma, H.; Wakao, Y.; Mori, M. SAE Tech. Pap. 2000-01-0193, 2000. (11) Gong, X.; Johnson, R.; Miller, D. L.; Cernansky, N. P. SAE Tech. Pap. 2005-01-3740, 2005. (12) Gupta, A.; Miller, D. L.; Cernansky, N. P. SAE Tech. Pap. 200701-2002, 2007. (13) Chen, W.; Shuai, S.; Wang, J. Fuel 2009, 88, 1927–1936. (14) Golovitchev, V. I. Chalmers University of Technology, Gothenburg, Sweden, 2007 (http://www.tfd.chalmers.se/∼valeri/MECH.html).
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: DOI:10.1021/ef900993g
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cetane number. The cetane number of the used diesel surrogate fuel is supposed to be 50. Adding DTBP in diesel with mass concentrations of 0.06, 0.2, and 1.2% could raise the cetane number to 52, 56, and 64, respectively, based on experimental results on the effects of DTBP on the cetane number of diesel fuel.18-21 SENKIN22 was used to model the ignition delays of diesel fuels with different DTBP concentrations in the constantvolume zero-dimensional adiabatic combustion process under different pressures, temperatures, and equivalence ratios. The ignition delay was defined as the time during which the temperature of the mixture rose by 400 K. The ignition delays of diesel fuels with different DTBP concentrations under different initial temperatures and with an initial pressure of 4.1 MPa and equivalence ratios of 0.5, 1.0, and 2.0 are illustrated in Figure 2. DTBP advances the ignition timing of the diesel fuel under different conditions, and the effects become evident with a higher DTBP concentration. The effects of the cetane number on the ignition characteristics of diesel fuel can be described by the combination of the THv2 mechanism and the DTBP dissociation reactions.
Figure 1. Dissociation pathway for DTBP.
mechanism, combined with DTBP reactions, was subsequently coupled with the KIVA-3 V2 code. The DTBP concentration in diesel fuel was adjusted to modify the cetane number and model the effects of the cetane number (50-64) on the ignition timing, in-cylinder pressure, rate of heat release, NOx, and soot emissions of a common rail multipleinjection, Euro 4, Cummins ISDe4 diesel engine. The results were compared in the experiments.
3. Modeling of Combustion and Emissions in the Diesel Engine
2. Chemical Reaction Mechanism
The effects of the cetane number on the combustion and emissions of a common rail multiple-injection Euro 4, Cummins ISDe4 diesel engine have been studied through engine bench experiments using seven diesel fuels with different fuel properties.23 The engine specifications are presented in Table 1, while the properties of the tested diesel fuel are outlined in Table 2. The THv2 mechanism, combined with DTBP reactions, was coupled with the KIVA-3 V2 code to model the effects of the cetane number on the combustion process and emissions of the Cummins ISDe4 diesel engine; the modeling results were compared to the experiments. The modeling results included the in-cylinder pressure, rate of heat release, engine-out NOx, and soot emissions of small-, mid-, and high-load conditions with an engine speed of 1500 revolutions/min. The engine operating conditions are displayed in Table 3, while the piston geometry and computational grid employed for the simulation are exhibited in Figure 3. A 45° sector mesh was used because the diesel injector featured eight equally spaced nozzle holes. The mesh was composed of approximately 18 000 computational cells at the bottom dead center (BDC), with about 2 mm cell size near the top dead center (TDC). 3.1. Reduced Diesel Surrogate Fuel Mechanism. The THv2 mechanism was coupled with the KIVA-3 V2 code to model the combustion process and emissions of the ISDe4 engine under small-, mid-, and high-load conditions, with an engine speed of 1500 revolutions/min. The results were compared to the engine test results of the number 0 diesel available in the Beijing local market. The latter had a cetane number of 50 (number 7 fuel in Table 2) to validate the accuracy and reliability of the THv2 mechanism in modeling the combustion process and emissions of the multiple-injection diesel engine. The comparison of the in-cylinder pressure and heat release rate of the ISDe4 engine between the modeling results of the experiments and the THv2 mechanisms under
A reduced reaction mechanism for diesel surrogate fuel (TH mechanism)13 with 60 species and 145 reactions was developed from Golovitchev’s n-heptane/toluene mechanism.14 This was achieved through analyzing and retaining the important reactions and species during n-heptane and toluene oxidation and soot formation processes. In the TH mechanism, a one-step C14H28 decomposing global reaction was developed and the mole ratio of n-heptane and toluene was maintained at 7:3. The kinetic constants of soot oxidation reactions in the TH mechanism were decreased to reduce the soot oxidation rate, and the new modified mechanism was referred to as the THv2 mechanism. The THv2 mechanism was coupled with the KIVA-3 V2 code to model diesel fuel combustion processes in the constant-volume combustion vessel15,16 and the optical diesel engine17 of Sandia. The predicted ignition delay, combustion process, and soot concentration matched well with the experiments, and the calculation time was roughly half of the Golovitchev mechanism. The detailed development and validation of the THv2 mechanism are available in ref 13. The extended Zeldovich mechanism with three NO reactions was combined with the THv2 mechanism to display NOx formation during the combustion process. DTBP is a common fuel additive used as a cetane number enhancer. Griffiths et al.8,9 developed the DTBP oxidation mechanism based on the results of experiments. DTBP dissociates at a low temperature to produce tert-butoxy radicals (CH3)3O, which quickly dissociate to form acetone and methyl radicals. The acetone is stable below 900 K and cannot contribute to the improvement of combustion characteristics.8 The methyl radical, meanwhile, is reactive and produces products, such as formaldehyde and hydrogen peroxide, which enhance the ignition.12 The dissociation pathway for DTBP is illustrated in Figure 1. DTBP influences the ignition character mainly through the methyl radical produced at low-temperature dissociation. Therefore, the seven reactions of DTBP dissociate to acetone, methyl, and other small radicals that were combined with the THv2 mechanism to form the diesel surrogate fuel mechanism with cetane number enhancer reactions. The DTBP produced small radicals following the reactions in the diesel surrogate fuel mechanism. The DTBP dissociation reactions are illustrated in the Appendix. The THv2 mechanism was combined with DTBP reactions to model the combustion process of diesel fuel. The DTBP concentration in diesel fuel was adjusted to arrive at a different
(18) Schwab, S. D.; Guinther, G. H.; Henly, T. J.; Miller, K. T. SAE Tech. Pap. 1999-01-1478, 1999. (19) Goodrich, B. E.; Mcduff, P. J.; Krupa, C. C.; Alvarez, L.; Williams, A. M.; DeJovine, J. M. SAE Tech. Pap. 982574, 1998. (20) Thompson, A. A.; Lambert, S. W.; Mulqueen, S. C. SAE Tech. Pap. 972901, 1997. (21) Nandi, M. K.; Jacobs, D. C. SAE Tech. Pap. 952368, 1995. (22) Lutz, A. E.; Kee, R. J.; Miller, J. A. Sandia Report, SAND 87-8248, UC-4, 1988. (23) Chen, W.; Wang, J.; Shuai, S.; Wu, F. SAE Tech. Pap. 2008-010638, 2008.
(15) Pickett, L. M.; Siebers, D. L.; Idicheria, C. A. SAE Tech. Pap. 2005-01-3843, 2005. (16) Pickett, L. M.; Siebers, D. L. Combust. Flame 2004, 138, 114–135. (17) Singh, S.; Reitz, R. D.; Musculus, M. P. B. SAE Tech. Pap. 200601-0055, 2006.
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: DOI:10.1021/ef900993g
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Figure 2. Effects of DTBP on the ignition characteristics of diesel fuel. Table 1. Cummins ISDe4 Diesel Engine Specifications engine model type cylinder number displacement (L) bore stroke (mm) connecting rod length (mm) combustion chamber swirl ratio compression ratio injection system number of holes spray pattern included angle rail pressure (bar) nozzle orifice diameter (mm) IVC (°CA ATDC) EVO (°CA ATDC) rated power (kW)/speed (revolutions min-1) maximum toque (N m)/speed (revolutions min-1) after treatment
Table 2. Properties of the Tested Diesel Fuel
Cummins ISDe4 140 turbo-charging, intercooling, direct injection 4 4.5 107 124 192 Ω 1.2 17.5 common rail 8 143° 1600 0.168 -155 120 103/2500
T90 (°C) FBP (°C) sulfur (ppm) cetane number aromatics (%, v/v) density at 20 °C (kg/m3) a
1
2
3
4
5
6
7
355 365 190 51.9 10 835.0
318 343 42 64.3 5.8 809.8
318 344 111 64.8 6.1 809.8
300 340 214 65.1 5.4 810.0
330 342 510 47.9 11 842.5
355 365 520 55.9 11 832.2
334 347 45 50.1 a 838.4
No test data.
Table 3. Engine Operating Conditions engine speed (revolutions/min) torque (N m) BMEP (MPa) engine load (%) intake pressure (kPa) intake temperature (°C) injection quantity (mg) O2 concentration (vol %)
550/1500 urea SCR
small load
mid load
high load
1500 200 0.56 35 133 80 40 21
1500 400 1.12 70 171 88 70 21
1500 560 1.56 100 206 94 100 21
On the basis of Figure 4, the THv2 mechanism predicted that the ignition delay of multiple injections, in-cylinder pressure, and heat release rate matched well with the experimental results. The predicted ignition timing of the pilot injection was delayed in the small-load condition as compared to the experiment, and the ignition timing matched the experiments well under the mid- and high-load conditions.
different conditions are presented in Figure 4. The experiments and modeling results of the engine-out NOx and soot under the three conditions are compared in Figure 5. The engine multiple-injection strategies are undisclosed for proprietary reasons; thus, only the approximate sharp of the fuel supply is presented in Figure 4. 858
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On the basis of Figure 5, the THv2 mechanism predicted that engine-out NOx and soot have a trend similar to the experiments, while the quantitative predictions were slightly higher than the experiments. The THv2 diesel surrogate fuel mechanism could be effectively employed to model the combustion process and emissions of the multiple-injection diesel engine. 3.2. Modeling of Cetane Number Effects. The THv2 mechanism, combined with DTBP reactions, was coupled with the KIVA-3 V2 code to model the effects of the cetane number on the in-cylinder pressure, rate of heat release, engine-out NOx, and soot emissions of the Cummins ISDe4
Figure 3. Engine computational grid.
Figure 4. In-cylinder pressure and heat release rate for THv2 mechanism modeling and experimental results.
Figure 5. Engine-out NOx and soot emissions for THv2 mechanism modeling and experimental results.
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: DOI:10.1021/ef900993g
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Figure 6. Cetane number effects on the in-cylinder pressure and heat release rate for modeling and experimental results.
release.23 On the basis of Figure 6, the cetane number increased with a higher DTBP concentration in the diesel fuel. This advanced the ignition of the pilot injection, but the cetane number had a slight effect on the combustion process of the main and post-injections compared to the ISDe4 engine using the common rail multiple-injection technology. The modeling results of the effects of the cetane number on the combustion processes of the ISDe4 engine under different conditions matched well with the experiments. Raising the cetane number from 50 to 52, 56, and 64 caused the ignition timing to advance the pilot injection, as illustrated in Figure 7. On the basis of Figure 7, the predicted
diesel engine under small-, mid-, and high-load conditions with an engine speed of 1500 revolutions/min. The cetane number of diesel fuels was raised by adding DTBP in diesel with different mass concentrations. The comparison of the in-cylinder pressure and heat release rate of the ISDe4 engine between the modeling results of experiments and the THv2 mechanisms under different conditions are presented in Figure 6, using diesel fuels with cetane numbers of 50, 52, 56, and 64. The experimental results demonstrated that a higher cetane number could advance the ignition of the pilot injection and had a slight effect on the combustion duration and shape of the heat 860
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: DOI:10.1021/ef900993g
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ignition timing advancement exhibited a trend similar to the experiments. The effects of the increase of the cetane number on the pilot injection ignition timing advancement of the multiple-injection engine could be described by the combination of the THv2 mechanism and the DTBP dissociation reactions. The results of the advanced ignition timing with the increase in the cetane number from 50 exhibited almost the same quantitative values with experiments under the small- and high-load conditions. The modeling ignition advancement is slightly higher as compared to the experiment under the mid-load condition. The results also show that the chemical mechanism calculated appearance timing
of OH during the combustion process was found to advance with the increase in the cetane number as the DTBP concentration also increases. This may be the reason behind the advancement of the pilot injection ignition. Figure 8 presents the OH concentration during the combustion process of the 1500 revolutions/min, mid-load condition, using diesel fuels with different cetane numbers. The experiments and modeling results of the effects of the cetane number on the ISDe4 engine-out NOx and soot under 1500 revolutions/min, three different load conditions are compared in Figure 9. The diesel fuels used for the engine bench test exhibited a change in other fuel properties with the increase in the cetane number, because the cetane number of engine bench test fuels was not completely decoupled with
Figure 7. Effects of the cetane number on the ignition timing advancement of the pilot injection.
Figure 8. Effects of the cetane number on OH.
Figure 9. Effects of the cetane number on engine-out soot and NOx emissions for modeling and experimental results.
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23
other fuel properties. Therefore, the experimental results are linearly regressed to display a more obvious trend of emissions changing with the cetane number. The modeling results revealed that the increase of the cetane number from 50 to 64 had a slight effect on the engine-out NOx and soot emissions of the Euro 4, Cummins ISDe4 diesel engine, and the modeling results displayed a trend similar to the experiments.
concentration in diesel fuels were adjusted to arrive at different cetane numbers (50-64) and model their effects on the ignition timing, in-cylinder pressure, rate of heat release, engine-out NOx, and soot emissions of a Euro 4 diesel engine. The modeling results matched well with those of the experiments. This mechanism could be effectively used to model the effects of the cetane number on the combustion process and emissions of diesel engines. For the common rail multipleinjection, Euro 4 diesel engine, the increase in the cetane number advanced the ignition of the pilot injection and exhibited a slight effect on the combustion process of the main and post-injections, with engine-out NOx, and soot emissions remaining constant.
4. Conclusion A chemical reaction mechanism of DTBP dissociation at low temperature to produce small radicals with seven reactions was combined with a reduced diesel surrogate fuel mechanism to model the effects of the cetane number on diesel ignition characteristics. SENKIN was used to model the ignition delays of diesel fuels with different DTBP concentrations. DTBP advanced the ignition timing of diesel fuel under different conditions, and the effects became more evident with a higher DTBP concentration. The effects of the cetane number on ignition characteristics of diesel fuel could be described by the combination of the THv2 reduced diesel surrogate fuel mechanism and the DTBP dissociation reactions. The THv2 mechanism was coupled with the KIVA-3 V2 code to model the combustion process and emissions of a Cummins ISDe4 Euro 4 diesel engine under different conditions. The predicted ignition delay of multiple injections, incylinder pressure, and heat release rate matched well with the experiments. The THv2 mechanism predicted that engine-out NOx and soot have similar trend and quantitative values as the experiments. The THv2 mechanism, combined with DTBP reactions, was coupled with the KIVA-3 V2 code. The DTBP
Table A1. DTBP Low-Temperature Dissociation Kinetic Mechanism, with the Reaction Rate k = ATβ exp(-Ea/RT), Following the Arrhenius Formula number 1 2 3 4 5 6 7
reaction C8H18O2 = 2C4H9O C4H9O = CH3 þ C3H6O C3H6O þ O2 = C3H5O þ HO2 C3H6O þ OH = C3H5O þ H2O C3H6O þ O = C3H5O þ OH C3H5O þ O2 = CH2CO þ CH2 þ HO2 C3H5O = CH2CO þ CH3
A (cm, mol, s)
β
Ea (cal/mol)
2.00 1021 5.00 1015 1.00 1012 4.00 1012 3.00 1013 1.00 1012
0.0 0.0 0.0 0.0 0.0 0.0
36308.0 10988.0 49650.0 2661.0 6315.0 6019.0
1.00 1019 0.0
71496.0
Acknowledgment. The authors extend their appreciation to Professor Rolf D. Reitz of the Engine Research Center of the University of Wisconsin—Madison for supplying the KIVA-3 V2 code. The authors likewise acknowledge the assistance extended by the Cummins Company, which supplied the specifications of the ISDe4 diesel engine, fuel injection system, and fuel multipleinjection strategies.
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