Energy Fuels 2011, 25, 103–107 Published on Web 12/02/2010
: DOI:10.1021/ef101231k
Influence of Ethanol and Cetane Number (CN) Improver on the Ignition Delay of a Direct-Injection Diesel Engine Jie Liu,*,† Guangle Li,† and Shenghua Liu‡ † Engine Department, Shanghai Automotive Industry Corporation (SAIC) Motor Technical Center, 201 An Yan Road, Jiading District, Shanghai 201804, China, and ‡School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an 710049, China
Received September 11, 2010. Revised Manuscript Received November 19, 2010
As a kind of alternative energy, ethanol is widely and successfully used in the gasoline engine. However, when it is used in the diesel engine, a cetane number (CN) improver, such as isoamyl nitrite, is generally added in diesel/ethanol blends to compensate for the decrease of CN because of ethanol addition. A combined complicated mechanism for the autoignition of n-heptane/ethanol/isoamyl nitrite mixtures was validated against the shock-tube data and homogeneous charge compression ignition (HCCI) engine combustion experiment. The effects of ethanol and isoamyl nitrite addition on the ignition delay of diesel were studied, where n-heptane was used to simulate diesel. Numerical analyses showed that the addition of ethanol decreased the amount of OH radicals and, consequently, retarded the ignition. The reactive i-C5H11O and NO radicals generated in the isoamyl nitrite thermal decomposition process accelerated the low-temperature reactions and shortened the ignition delay.
their combustion mechanism is phenomenological and basic investigations are rare. To specify isoamyl nitrite decomposition and its effect on ignition of hydrocarbon (HC) fuels, numerical studies have been carried out, possible reaction paths are proposed and analyzed, and the computational results involving isoamyl nitrite are subject to interpretation in the present study. Dec5 developed a new conceptual model of diesel spray and combustion using an extensive set of laser-based diagnostic techniques, which improved the knowledge of the fundamentals of the combustion process. Higgins et al.6 linked the observable luminosity data with the pressure rise, which provided a better understanding about the events during the early mixing and combustion phasing in diesel combustion. These works have given researchers and modelers a verification of their detailed kinetic models. A new mechanism that combines Westbrook’s n-heptane oxidation model with Marinov’s ethanol oxidation model and includes the reactions of HC with NOx is established in this paper. Some numerical studies on the effect of ethanol and isoamyl nitrite addition on the ignition delay of diesel fuels have been performed, and the mechanisms are analyzed in this study. Possible reaction paths that are responsible for the effect of isoamyl nitrite on diesel ignition are proposed and analyzed. The computational model is mostly based on Dec’s experimental results. This one-dimensional numerical simulation is used to identify the ignition process but neglects the complex fluid mechanical processes. This approach costs less than a full simulation with complex chemistry in three spatial dimensions.
1. Introduction Fuel characteristics play a major role in the combustion engine. With the increasing knowledge of combustion and ignition properties, an efficient and systematic improvement of present and developing new combustion processes can be realized. As an oxygenated additive, ethanol is used to reduce smoke emissions in diesel engines. Because of its low cetane number (CN), it is also used as a reaction inhibitor in homogeneous charge compression ignition (HCCI) engines or as an octane improver in gasoline engines. In the diesel engine, because of the cooling effect and CN decrease as ethanol is introduced, the ignition delay period of the diesel/ ethanol blend fuel increases with the addition of the ethanol fraction.1-4 The chemical effect of ethanol addition is rarely mentioned. Various substances are added in diesel/ethanol blends to modify engine performance. However, most information about the effects of them is empirical. A better theoretical knowledge about the fuel chemistry will facilitate the choice of additives for a specific application. Although alkyl nitrites are widely used as diesel/ethanol blend additives, the research on *To whom correspondence should be addressed. E-mail: liuzhenjiang@ hotmail.com. (1) He, B.-Q.; Wang, J.-X.; Yan, X.-G.; Tian, X.; Chen, H. Study on combustion and emission characteristics of diesel engines using ethanol blended diesel fuels. Proceedings of the Society of Automotive Engineers (SAE) 2003 World Congress and Exhibition; Detroit, MI, March 3-6, 2003; SAE Tech. Pap. 2003-01-0762. (2) Lu, X.; Yang, J.; Zhang, W.; 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. (3) Li, W.; Ren, Y.; Wang, X.-B.; Miao, H.; Jiang, D.-M.; Huang, Z.-H. Combustion characteristics of a compression ignition engine fuelled with diesel-ethanol blends. Proc. Inst. Mech. Eng., Part D 2008, 222 (2), 265–274. (4) Ren, Y.; Huang, Z.-H.; Jiang, D.-M.; Li, W.; Liu, B.; Wang, X.-B. 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, 222 (6), 1077–1087. r 2010 American Chemical Society
(5) Dec, J. E. A conceptual model of DI diesel combustion based on laser-sheet imaging. Proceedings of the Society of Automotive Engineers (SAE) International Congress and Exposition; Detroit, MI, Feb 24-27, 1997; SAE Tech. Pap. 970873. (6) Higgins, B.; Siebers, D.; Aradi, A. Diesel-spray ignition and premixed-burn behavior. Proceedings of the Society of Automotive Engineers (SAE) 2000 World Congress; Detroit, MI, March 6-9, 2000; SAE Tech. Pap. 2000-01-0940.
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: DOI:10.1021/ef101231k
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epoxide species, conjugated olefins, and β-decomposition products via three different reaction paths. QOOH ¼ OH þ QO ð7Þ
2. Model Description Conventional diesel contains hundreds of HC species, whose composition varies from suppliers and changes all of the time. It is difficult to build a chemical kinetic mechanism to model all of the species. For precise modeling studies, it is necessary to identify a well-characterized fuel as a surrogate for diesel fuel.7 n-Heptane is frequently used as a diesel surrogate because its cetane number is 56, which is typical of ordinary diesel fuels. The detailed reaction mechanism of n-heptane was well-developed by Curran et al.8 Correspondingly, the reaction mechanism of ethanol was published by Marinov.9 The NO reactions were from GRI-MECH 3.0.10 The coupled reactions of HC and NOx were from Anderlohr et al.11 and Glaude et al.12 On the basis of these previous studies, a complicated n-heptane oxidation model consisting of 2871 reactions and 612 species is developed in this paper. The oxidation reaction of the n-heptane is initiated via the bimolecular fuel reaction with molecular oxygen (reaction 1). RH þ O2 ¼ R þ HO2
In reaction 1, RH represents C7H16 and R represents the alkyl radical (C7H15) that is formed by abstracting a hydrogen atom from C7H16. When the temperature is below approximately 900 K, O2 direct addition on the alkyl radical is favored, shown in reaction 2, which leads the peroxy radical to undergo branching reactions. The heptylperoxide radical (RO2) isomerizes via internal hydrogen-atom abstraction to form the hydroperoxy-alkyl radical (QOOH), shown in reaction 3. The QOOH species reacts with O2 to form the peroxyalkylhydroperoxide radical (O2QOOH), shown in reaction 4. The O2QOOH isomerizes, releases OH, and forms the ketohydroperoxide (C7KET) species, shown in reaction 5. The subsequent decomposition of the C7KET molecule leads to the formation of a carbonyl radical and another OH radical and provides chain branching because it produces two radical species from one stable reactant. It is especially important that the formation and decomposition of the C7KET molecule produce two OH radicals, because these reactions are the principal source of radicals at these temperatures. Subsequently, the fuel is oxidized, which is the principal source of water, shown in reaction 6. On the whole, this low-temperature oxidation process is exothermic. R þ O2 ¼ ROO ð2Þ ð3Þ
QOOH þ O2 ¼ O2 QOOH
ð4Þ
O2 QOOH ¼ C7 KET þ OH
ð5Þ
RH þ OH ¼ R þ H2 O
ð6Þ
ð8Þ
QOOH ¼ OH þ olefin þ carbonyl
ð9Þ
These routes are at the expense of the reaction pathways through the C7KET species, which produce only one radical rather than two. Therefore, the increasing importance of these propagation channels leads to a lower reactivity of the system, which is observed as the negative temperature coefficient (NTC) region. As the temperature increases, the production of HO2 increases, which accelerates the rate of reactions 10 and 11. When the temperature approaches 1000 K, the decomposition of H2O2 is much slower than its production, which leads to a steady increase in the H2O2 concentration. RH þ HO2 ¼ R þ H2 O2 ð10Þ
ð1Þ
RO2 ¼ QOOH
QOOH ¼ HO2 þ olefin
HO2 þ HO2 ¼ H2 O2 þ O2
ð11Þ
When the temperature reaches approximately 1000 K (the critical temperature is dependent upon the pressure13-15), the following branching reaction becomes important: H2 O2 þ M ¼ 2OH þ M
ð12Þ
Consumption of one H2O2 radical produces two OH radicals; therefore, the OH concentration increases rapidly and consumes the rest of the fuel, followed by the rapid increase of the temperature, which leads to the ignition process. At high temperatures, the overall reaction pathway via β-scission of the alkyl radical R proceeds rapidly to an olefin and other species, with chain branching primarily because of the reaction as follows: H þ O2 ¼ OH þ O
ð13Þ
3. Model Validation The experiments on the ignition delay of n-heptane/air and ethanol/O2/Ar mixtures were carried out in a shock tube.9,16 The SENKIN code17 was used to check the autoignition of normal heptane and ethanol, assuming constant-volume, homogeneous, and adiabatic conditions behind the reflected shock wave. The calculation results and the experimental data are compared in Figures 1 and 2. The calculation results successfully capture the NTC behavior of n-heptane at the temperature between 750 and 1000 K. The ignition delay of ethanol decreases with the increase of the temperature. The maximum deviation between calculation results and experimental results is 23.8%. Figure 3 displays the effect of NO addition on the main-flame ignition delays in the HCCI engine.18 Experiments show that the main-flame ignition delay decreases sharply with the small addition of NO (up to 50 ppm) and changes slightly with higher
At intermediate temperatures (700-900 K), the decomposition reactions of QOOH increase, because the energy barrier of its formation is much easier to overcome, leading to the formation of (7) Flynn, P. F.; Durrett, R. P.; Hunter, G. L.; zur Loye, A. O.; Akinyemi, O. C.; Dec, J. E.; Westbrook, C.K. Diesel combustion: An integrated view combining laser diagnostics, chemical kinetics, and empirical validation. Proceedings of the Society of Automotive Engineers (SAE) International Congress and Exposition; Detroit, MI, March 1-4, 1999; SAE Tech. Pap. 1999-01-0509. (8) Curran, H. J.; Gaffuri, P.; Pitz, W. J.; Westbrook., C. K. A comprehensive modeling study of n-heptane oxidation. Combust. Flame 1998, 114, 149–177. (9) Marinov, N. M. A detailed chemical kinetic model for high temperature ethanol oxidation. Int. J. Chem. Kinet. 1999, 31, 183–220. (10) http://www.me.berkeley.edu/gri-mech/. (11) Anderlohr, J. M.; Bounaceur, R.; Pires Da Cruz, A.; BattinLeclerc, F. Combust. Flame 2009, 156 (2), 505–521. (12) Glaude, P. A.; Marinov, N.; Koshiishi, Y.; Matsunaga, N.; Hori, M. Energy Fuels 2005, 19 (5), 1839–1849.
(13) Baulch, D. L.; Cobos, C. J.; Cox, R. A.; Frank, P.; Hayman, G.; Just, Th.; Kerr, J. A.; Murrells, T.; Pilling, M. J.; Troe, J.; Walker, R. W.; Warnatz, J. J. Phys. Chem. 1994, 23, 847–1033. (14) Kappel, Ch.; Luther, K.; Troe, J. Phys. Chem. 2002, 44392–4398. (15) Westbrook, C. K. Proc. Combust. Inst. 2000, 28, 1563–1577. (16) Ciezki, H. K.; Adomeit, G. Shock-tube investigation of selfignition of n-heptane-air mixtures under engine relevant conditions. Combust. Flame 1993, 93, 421–433. (17) Lutz, A. E.; Kee, R. J.; Miller; J. A. SENKIN: A FORTRAN program for predicting homogeneous gas phase chemical kinetics with sensitivity analysis. Sandia National Laboratories Report SAND-878248; Sandia National Laboratories: Albuquerque, NM, 1987. (18) Dubreuil, A.; Foucher, F.; Mounaim-Rousselle, C.; Dayma, G.; Dagaut, P. Proc. Combust. Inst. 2007, 31, 2879–2886.
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: DOI:10.1021/ef101231k
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Figure 1. Comparison of model predictions (lines) with the experiments (symbols) for n-heptane ignition.16
Figure 4. OH mole fraction.
temperature is set to 770 K, because the chemical reactions start when the mixture reaches this temperature.7,19-21 The calculation is started at 12° crank angle (CA) before top dead center (BTDC). 5. Effect of Ethanol on the Ignition Delay For the ethanol oxidation process, the consumption of ethanol is initiated from the decomposition reactions.9 Then, some of the ethanol goes through hydrogen-atom abstraction reactions. C2 H5 OH þ X ¼ CH2 CH2 OH þ XH C2 H5 OH þ X ¼ CH3 CHOH þ XH C2 H5 OH þ X ¼ CH3 CH2 O þ XH
Figure 2. Comparison of model predictions (lines) with the experiments (symbols) for ethanol ignition delay time.9
ðX ¼ OH, O, H, HO2 , CH2 , etc:Þ However, when mixed with n-heptane, ethanol is mainly consumed through hydrogen-atom abstraction reactions especially by OH radicals (reactions 14-16). This may be the reason that is causing the delay of the ignition time. ð14Þ C2 H5 OH þ OH ¼ C2 H4 OH þ H2 O C2 H5 OH þ OH ¼ CH3 CHOH þ H2 O
ð15Þ
C2 H5 OH þ OH ¼ CH3 CH2 O þ H2 O
ð16Þ
Figures 4 and 5 show the calculated OH mole fractions and OH production rates of the E0, E10, E20, and E30 fuels (n-heptane/ ethanol blends containing 10, 20, and 30% of ethanol in the mole fractions). The OH mole fraction and the OH production rate show the same trend with the ethanol addition. Figure 6 shows the OH mole fractions produced in main reactions during the low-temperature period as a function of time. The OH mole fractions produced by particular reactions are plotted in a cumulative manner, so that the amount is
Figure 3. Comparison of model predictions (line) and experiments (symbols) for ignition delays of the main flame as function of added NO obtained in an HCCI engine.18
NO concentrations. The calculation results capture this behavior well, and the deviations are not obvious. Therefore, good agreement between calculation results and experimental data confirms the validity of this model.
(19) Curran, H. J.; Fisher, E. M.; Glaude, P.-A.; Layton, D. W.; Pitz, W. J.; Westbrook, C. K. Detailed chemical kinetic modeling of diesel combustion with oxygenated fuels. Proceedings of the Society of Automotive Engineers (SAE) 2001 World Congress; Detroit, MI, March 5-8, 2001; SAE Tech. Pap. 2001-01-0653. (20) Mueller, C. J.; Pitz, W. J.; Pickett, L. M.; Martin, G. C.; Siebers, D. L.; Westbrook, C. K. Effects of oxygenates on soot processes in DI diesel engines: Experiments and numerical simulations. SAE Tech. Pap. 2003-01-1791; Society of Automotive Engineers (SAE) International: Warrendale, PA, 2003. (21) Westbrook, C. K.; Pitz, W. J.; Curran, H. J. Chemical kinetic modeling study of the effects of oxygenated hydrocarbons on soot emissions from diesel engines. J. Phys. Chem. A 2006, 110 (21), 6912–6922.
4. Software and Computational Conditions The internal combustion engine model in SENKIN17 code was chosen in this study to analyze ignition delay in a directinjection (DI) diesel engine. In this calculation, the initial conditions are listed as follows. The fuel/air equivalence ratio is 3.0 for the base fuel, and the cylinder pressure is 4 MPa. The initial 105
Energy Fuels 2011, 25, 103–107
: DOI:10.1021/ef101231k
Liu et al.
Figure 8. Pressure and heat release rate. Figure 5. Production rate of OH radicals.
delay. The reason is that ethanol does not have low-temperature oxidation mechanisms. When ethanol consumes OH radicals, there are fewer OH radicals left to react with n-heptane through reaction 6 and fewer R radicals are produced, leading to the production of fewer OH radicals. OH radical consumption by ethanol decelerates the lowtemperature oxidation process, followed by the delay of the temperature increase. The calculated pressures and heat release rates of the diesel/ ethanol blends are plotted in Figure 8. It is obvious that the ignition is retarded as the ethanol mole fraction increases. 6. Effect of CN Improver on the Ignition Delay The O-N bond in the isoamyl nitrite molecule is labile, and thermal scission leads to the generation of i-C5H11O and NO radicals, shown in reaction 17. The reactions of i-C5H11O are included in the n-heptane ignition model, and the NO reactions are added to the coupled mechanism. The main reactions involved in the accelerating mechanism of isoamyl nitrite are shown in Table 1. Three E30 blends containing 0.3, 0.6, and 0.9% of isoamyl nitrite in the mole fraction, named E30A, E30B, and E30C, respectively, are used in this study. The effect of CN improver addition on the OH mole fraction is plotted in Figure 9. The increase of the OH mole fraction is advanced by the addition of the CN improver. The reactions of NO and i-C5H11O are the main reactions that produce the OH radicals at the initial time. In this paper, the kinetic model indicates that the main reactions involved in the accelerating mechanism of NO are reactions 18-21, which globally transform HO2 radicals into active hydroxyl radicals. NO radicals are regenerated during the NO2 reactions, which occurs via reactions 19-21. Specifically, nitrous acid (HONO) is formed via reaction 19 and further decomposes to the NO radical and OH radical. Therefore, the production of HONO accelerates the reaction rate. However, the decrease of the ignition delay caused by the NO mechanism is very small. Alkoxyl radicals are important intermediate species in the combustion process. These radicals go through several types of transformations: unimolecular decomposition, oxidation, isomerization, and addition to multiple bonds. Unimolecular decomposition is the main reaction. The decomposition of i-C5H11O radicals by carbon-carbon bond fission yields i-C4H9 and CH2O radicals, shown in reaction 22. Subsequently, the i-C4H9 radical reacts with O2 to produce butylperoxide and ketohydroperoxide. The decomposition of these
Figure 6. OH production fraction.
Figure 7. OH consumption fraction.
represented as the distance between the lower and upper lines around the region with the reaction name. During the lowtemperature oxidation period, the reactions that produce OH radicals are mainly four types. They are all from the lowtemperature reactions of n-heptane. The OH mole fractions consumed by n-heptane and ethanol are shown in Figure 7. The mole fraction of OH radicals consumed by ethanol is increased, and the mole fraction of other OH radicals consumed by n-heptane is decreased, with the addition of ethanol. It is obvious that OH radical consumption by ethanol slows the global OH production rate, leading to a longer ignition 106
Energy Fuels 2011, 25, 103–107
: DOI:10.1021/ef101231k
Liu et al.
Table 1. Rate Expressions for Main Reactions in Isoamyl Nitrite Oxidation number
reaction
A
17 18 19 20 21 22 23 24 25 26 27 28
i-C5H11O-NO = i-C5H11O þ NO NO þ HO2 = NO2 þ OH NO2 þ HO2 = HONO þ O2 2HONO = NO þ NO2 þ H2O HONO þ M = NO þ OH þ M i-C5H11O = i-C4H9 þ CH2O i-C4H9 þ O2 = i-C4H9O2 i-C4H9O2 = i-C4H8OOH i-C4H8OOH = i-C4H8O þ OH i-C4H8OOH þ O2 = i-C4H8OOH-O2 i-C4H8OOH-O2 = i-C4KETI þ OH i-C4KETI-i = CH2O þ C2H5CO þ OH
2.000 10 2.11 1012 3.65 1013 1.02 1013 2.00 1013 1.717 1017 2.260 1012 7.500 1010 4.000 1011 2.260 1012 2.500 1010 1.500 1016 16
n
E
citation
0.00 0.00 0.00 0.0 0.0 -1.27 0.00 0.00 0.00 0.00 0.00 0.00
169.0 -480 8000.0 8540.0 0.0 2.018 104 0.00 2.440 104 2.200 104 0.00 2.140 104 4.200 104
NIST Gri3.0 Glaudy Glaudy Glaudy Curran Curran Curran Curran Curran Curran Curran
Figure 11. Ignition delay.
Figure 9. OH mole fraction.
On the basis of the analysis above, it is obvious that the formation rate of the alkyl radical from the decomposition of alkyl nitrite is much faster than the formation of the alkyl radical from the spontaneous oxidation of n-heptane, which promotes the low-temperature oxidation process and reduces the ignition delay. This modeling calculation only provides a possible mechanism by which isoamyl nitrite may improve spontaneous ignition. Further refinement of the reaction scheme for the low-temperature oxidation of isoamyl nitrite is a necessary prerequisite for further understanding. 7. Conclusions (1) The addition of ethanol has a chemical effect on the ignition delay. The chemical kinetic model indicates that ethanol consumes OH radicals through the hydrogen-atom absorption reactions, leading to a reduction in OH radicals during the lowtemperature oxidation period. This may delay the low-temperature oxidation and, consequently, delay the high-temperature oxidation. (2) The role of isoamyl nitrite addition is to accelerate the low-temperature oxidation process. The decomposition of isoamyl nitrite forms two reactive radicals: NO and i-C5H11O, and both of them can produce OH radicals during the lowtemperature reaction process. Moreover, the reactions of i-C5H11O release heat, which makes it more effective at reducing the ignition delay. OH radical production through reactions of NO and i-C5H11O accelerates the low-temperature reactions of n-heptane and reduces the ignition delay.
Figure 10. Pressure and heat release rate.
species produces OH radicals, as shown in reactions 23-28. This low-temperature oxidation process of i-C4H9 not only produces OH radicals but is also mildly exothermic; therefore, it is more efficient than the NO reactions in reducing the ignition delay. Figure 10 shows the calculated pressure and heat release rate of the E30 fuel with different mole fractions of the CN improver. It is obvious that the pressure rise and heat release are both advanced with the addition of the CN improver, which results in the reduction of the ignition delay, as shown in Figure 11.
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