Numerical Study of the Formation of Soot Precursors during Low

Oct 17, 2014 - Guangxi Key Laboratory of Manufacturing System and Advanced ... Computational fluid dynamics (CFD) software was used to establish a ...
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Numerical Study of the Formation of Soot Precursors during LowTemperature Combustion of a n‑Butanol−Diesel Blend Xiaorong Zhou,†,‡ Mengtian Song,† Haozhong Huang,*,†,‡ Ruzhi Yang,† Mingxia Wang,† and Jiaxing Sheng† †

College of Mechanical Engineering, Guangxi University, Nanning, Guangxi 530004, People’s Republic of China Guangxi Key Laboratory of Manufacturing System and Advanced Manufacturing Technology, Nanning, Guangxi 530004, People’s Republic of China

‡

ABSTRACT: To study the soot precursor emission characteristics of a n-butanol−diesel blend during low-temperature combustion (LTC), a model of n-heptane, n-butanol, polycyclic aromatic hydrocarbons (PAHs), and toluene was chosen. Computational fluid dynamics (CFD) software was used to establish a three-dimenisonal (3D) numerical model and to couple it with the chemical kinetics mechanism. The factors studied on soot precursor production were n-butanol blending ratio, exhaust gas recirculation (EGR) rate, injection timing, and intake pressure. The results showed that the formation of four types of soot precursors benzene (A1), naphthalene (A2), phenanthrene (A3), and pyrene (A4) was delayed and the final amount of precursor produced decreased with increasing the n-butanol blending ratio. The delay in formation of these precursors and the reactions mostly occurred during the premixed combustion stage and with increasing the EGR rate. The final amount of A1 produced increased with the EGR rate; however, those of A2, A3, and A4 showed a decreasing trend after an initial increase, and the EGR rate corresponding to the peak decreased with increasing the n-butanol blend ratio. The final amounts of A2, A3, and A4 produced increased with increasing the intake pressure. Soot precursors A1, A2, A3, and A4 formed in advance during the injection timing advancing with the final amounts of A1 and A3 produced were less. The final amounts of A2, and A4 produced showed a decreasing trend after an initial increase.

1. INTRODUCTION Because of the serious environmental contamination and aggravating energy crisis, regulations regarding internal combustion engine emissions are becoming increasingly stringent. These require the engines to perform with a clean and high-efficient combustion. New and renewable fuels and advanced combustion methods are important ways to achieve this goal.1 In this regard, n-butanol is usually regarded as a representative of renewable fuel, and the low-temperature combustion (LTC) is considered as a new combustion mode. To realize LTC, a large volume of exhaust gas recirculation (EGR) should be introduced into the conventional diesel engine. This reduces the temperature inside the cylinder. At the same time, this also achieves low NOx and soot emissions. In addition, extending the ignition delay is a way to realize LTC, because it enables sufficient time to form a homogeneous mixture. Reducing the cetane number allows for extension of the ignition delay, which is possible by adding fuel with low-cetane and highoctane numbers.2,3 The studies by Miers et al.4 showed that, in comparison to pure diesel, a 40% proportion of n-butanol in diesel decreased the soot emissions by 80%. Research conducted by Chotwichien et al.5 showed that n-butanol−diesel is more suitable for a compression ignition engine than other alcohol fuels, such as methanol and ethanol. Currently, studies worldwide are mainly focusing on diesel engine soot emission under LTC using a n-butanol−diesel blend. The influence of the properties of a n-butanol−diesel blend on reducing soot emission was studied by Liu et al.6 The conclusions were that the influence of the boiling point of the blend was minor and that a minimum cetane number and the oxygen © 2014 American Chemical Society

content in the fuel were the main factors in reducing soot emission. The influence of different n-butanol blending ratios at different loads and at a fixed speed was studied by Oğuzhan.7 A study on the exhaust gas recirculation (EGR) rate, intake pressure, and injection strategy influence was conducted by Usman et al.8 They reported that the influence of the EGR rate on NOx and soot emissions was connected to intake pressure and injection strategy. Zhang et al.9 and Liu et al.10 studied the effect of the EGR rate at different intake pressures on the soot emission blend. The results showed that the emission increased with the increasing EGR rate at different intake pressures and presented a decreasing trend in the emission rate after an initial increase. The results also showed that the oxygen content in the fuel increased, the cetane number decreased, and the soot emission reduced with increasing the n-butanol blending ratio. The influence of injection timing, intake oxygen concentration, and injection pressure on soot emission of a n-butanol−diesel blend was studied by Gerardo et al.11 Their results showed that the emission increased with injection timing advances and decreased with the increasing intake oxygen concentration and injection pressure. The influence of different injecting conditions on nbutanol−diesel soot production was studied by Beatrice et al.12 Their results showed that reducing fuel mass in the main injection and using multiple injections, especially post-injection, would reduce soot emission. Yutaka et al.13 demonstrated LTC using EGR at engine intermediate speed and load. The results Received: June 19, 2014 Revised: September 24, 2014 Published: October 17, 2014 7149

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that they obtained were that soot emission increased when the EGR rate reached 40%. It was also reported that, in combination with delayed closing of the intake valve, thereby reducing the effective compression ratio, the NOx and soot emissions could be reduced. Kook et al.14 studied how LTC influences soot emission. Their conclusion was that, if the cylinder combustion temperature was below 1650 K, the combustion process avoided the NOx and soot generation region to a large extent, regardless of the equivalence ratio. To summarize, the major parts of research were mainly focused on the soot emission with a n-butanol−diesel blend, and very few addressed the mechanism of soot precursor formation. The mechanism of soot formation in diesel engines can be divided into gas reaction kinetics and solid particle dynamics mechanisms,15 which correspond to the phase of precursor formation and particle nucleation and growth, respectively.16 Under the conditions of high temperature and oxygen deficit, hydrocarbon pyrolysis forms a soot meteorological precursor, the polycyclic aromatic hydrocarbons (PAHs).17 They have strong carcinogenesis, induce organism variation, and thus, are a serious threat to human health.18 The PAHs are the key components of soot emission. Their chemical reactions and the quantity in air entrainment in diffusion flame significantly influence the soot formation.19 The prediction of soot emission must be established on the basis of accurate prediction of the soot precursor formed. Hence, it is essential to study the formation mechanism of the soot precursor. Golovitchev20 built a nheptane−toluene reaction model, but its chemical kinetic mechanism only involved benzene and naphthalene with not more than two benzene rings, which impose limitations to simulate the soot production process. Frenklach and Wang21 and Mauss et al.22 described the pollutant-forming process by detailed reaction mechanisms, which include fuel pyrolysis, the formation of the first benzene ring, and the combination of aromatic hydrocarbons. The studies by Xiao et al.23 showed that, with a decreasing temperature, the appearance of intermediate species about soot formation/oxidation was delayed and the time-integrated mass of C2H2, soot precursors, OH radicals, and soot was reduced. Bi et al.24 confirmed that C2H2, precursor, and soot mass increased with decreasing O2 but decreased at 12% O2. It is difficult to measure the real-time production and growth process of the soot precursor in the complex environment of cylinder combustion.25 Therefore, in this paper, a simplified chemical kinetics mechanism of n-heptane−toluene−n-butanol−PAH was adopted and an ESE module of the AVL FIRE software coupled with the chemical kinetics mechanism software CHEMKIN were used to study the influence of the n-butanol blending ratio, EGR rate, injection timing, and intake pressure on the formation of a n-butanol−diesel soot precursor in LTC.

accurately, the reaction mechanism of toluene was added to the above mechanism by Wang et al.26 The molar ratio of n-heptane and toluene was set to 8:2. Therefore, a n-heptane−toluene−n-butanol−PAH reducing mechanism was obtained. With reference to the soot precursor, only the first four rings were considered. The next reaction after the formation of A4 was considered to be the formation of the carbon nucleus. 2.2. Three-Dimenisonal (3D) Modeling. A 3D combustion chamber model was established on the basis of the geometric parameters and operating conditions of a six-cylinder turbocharged diesel engine.9 The ESE module of the AVL FIRE software coupled with chemical kinetics mechanism software CHEMKIN was applied for simulation.29 The parameters of the engine studied are shown in Table 1. Because the

2. ESTABLISHMENT AND VALIDATION OF THE COMPUTATIONAL FLUID DYNAMICS (CFD) MODEL FOR A N-BUTANOL−DIESEL BLEND

Figure 1. Mesh of the model for the engine combustion chamber.

Table 1. Parameters of the Engine Studied parameter

value

cylinder diameter (mm) stroke (mm) connecting rod length (mm) compression ratio displacement (cm3) intake valve closing (deg CA ATDC) exhaust valve opening (deg CA ATDC) nozzle holes nozzle diameter (mm) spray angle (deg) common rail pressure (MPa)

105 125 210 16:1 1081.8 −137 +125 7 0.17 155 160

seven-hole injector was placed symmetrically in the center of the combustion chamber, only 1/7 of the structure was simulated, to save time. The modules, species transport, general gas-phase reactions, and spray modules in AVL FIRE were used, with some models, such as k−ζ− f turbulence model, Nordin droplet impact model, walljet1 droplet impact model, WAVE drop breakage model, multicomponent evaporation model, and Han−Reitz model, wall heat-transfer model, etc. The radial dimension of the computational grid is 0.1 mm, as shown in Figure 1.

2.3. Model Validation. The model was validated under the following conditions: (a) engine speed of 1400 revolutions/min (maximum torque speed), (b) compression ratio of 16, (c) fuel injection of 47.25 mg/cycle, (d) injection timing of 6.2° crank angle (CA) before top dead center (BTDC), (e) EGR rate of 47%, and (f) intake pressure of 0.16 MPa. Pure diesel and a blend of 10% n-butanol− diesel were used to validate the model. Figure 2 shows that the simulation and test results of the cylinder pressure9 are in good agreement. The simulated heat release rate peak is higher than the test value. The main reason for this difference is that the calculated heat release rate of the model only considers the release of chemical energy and excludes the contribution of wall heat transfer, heat

2.1. Reaction Mechanism. A simplified reaction mechanism for nheptane−n-butanol−PAH proposed by Wang et al. was used in this paper.26 The mechanism for n-heptane−n-butanol−PAH proposed by Wang et al. contained 76 components and 349 elementary reactions, which were verified by the test data of the shock tube, constant volume combustion bomb, and engine bench test. Research carried out by Corcione et al.27 and Golovitchev et al.28 indicate that substitutes mixed with n-heptane and toluene would commendably simulate diesel combustion characteristics. A mixture of n-heptane and toluene was used as a representative for diesel in this simulation. To simulate the mixture of diesel and n-butanol more 7150

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Figure 2. Comparison of pressure inside the cylinder and the heat release rate results obtained from test and simulation. loss, and similar factors. Besides, the measurement method and error may have contributed to the deviation. The percentages of uncertainties of various measurements involved in the engine study have been shown in Table 2. total uncertainty = square root of {(uncertainty of load)2 + (uncertainty of speed)2 + (uncertainty of temperature)2 + (uncertainty of air flow rate)2 + (uncertainty of diesel fuel measurement)2 + (uncertainty of n‐butanol measurement)2 + (uncertainty of pressure pickup)2 + (uncertainty of crank angle encoder)2 }

Figure 3. Mass fraction of A1 at different proportions of n-butanol in the blend at 45% EGR rate.

= square root of {(0.2)2 + (0.1)2 + (0.15)2 + (1)2 + (1)2 + (1)2 + (0.1)2 + (0.2)2 } = 3.08%

Table 2. Uncertainties and Measurement Methods of Instruments Used in Engine Experimentation measurement

uncertainty (%)

measurement technique

load speed temperature air flow rate diesel fuel measurement n-butanol measurement pressure pickup crank angle encoder

±0.2 ±0.1 ±0.15 ±1 ±1 ±1 ±0.1 ±0.2

strain gauge type load cell magnetic pickup principle thermocouple orifice meter volumetric measurement volumetric measurement magnetic pickup principle magnetic pickup principle

Figure 4. A1 final emission at different proportions of n-butanol in the blend.

volume. These blends are denoted as B0, B10, B20, and B30, respectively. The influence of n-butanol blending on the production of the soot precursor at the EGR rate of 45% was studied. The formation of the soot precursor primarily depends upon the formation of the benzene ring. First, hydrocarbon fuel generates a variety of small intermediary radical molecules, such as CH3, C2H2, C3H3, C4Hx, etc., and then there are two main reaction pathways for the formation of the first benzene ring: by the cyclization of C4Hx and acetylene and by the combination of oxygen-poor propyne.30 The formation rule is shown in Figure 3. The precursor A1 decreases after an initial increase with the crank angle in the entire combustion process. It has a prominent peak. This is mainly because the cracking reaction occurs throughout the fuel, generating small molecules of intermediary radicals and

3. SIMULATION RESULTS The influences of the EGR rate, n-butanol blending ratio, injection timing, and intake pressure on the production of a nbutanol−diesel soot precursor under LTC are evaluated. The production of four types of soot precursors, namely, benzene (A1), naphthalene (A2), phenanthrene (A3), and pyrene (A4) were considered. Simulation was conducted from intake valve closing (IVC) to exhaust valve opening (EVO); namely, from 583° to 845° CA (top center compression corresponds to 720° CA). 3.1. Influence of the n-Butanol Fraction in the Blend on the Formation of the Soot Precursor. The proportions of nbutanol blended in diesel tested were 0, 10, 20, and 30% by 7151

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Figure 5. Mass fraction of A2 at different proportions of n-butanol in the blend at 45% EGR rate.

Figure 9. A3 final production at different proportions of n-butanol in the blend.

Figure 6. A2 final production at different proportions of n-butanol in the blend.

Figure 10. Mass fraction of A4 at different proportions of n-butanol in the blend.

Figure 11. A4 final production at different proportions of n-butanol in the blend.

Figure 7. Temperature inside the cylinder at different proportions of nbutanol in the blend at 45% EGR rate.

Figure 12. Pressure inside the cylinder and mass fraction of A1 at different EGR rates and with 10% n-butanol in the blend.

Figure 8. Mass fraction of A3 at different proportions of n-butanol in the blend at 45% EGR rate.

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Figure 13. A1 final emission at different proportions of n-butanol in the blend and EGR rates.

Figure 17. A3 emission with different proportions of n-butanol in the blend and EGR rates.

Figure 14. Pressure inside the cylinder and mass fraction of A2 at different EGR rates and with 10% n-butanol in the blend.

Figure 18. Pressure inside the cylinder and mass fraction of A4 at different EGR rates with 10% n-butanol blend.

Figure 15. A2 final emission with different proportions of n-butanol in the blend and EGR rates.

Figure 19. A4 final emission with different proportions of n-butanol in the blend and EGR rates.

then forming multiple benzene and phenyl groups by the cyclization and composition in the early stages of combustion. According to the mechanism of dehydrogenation and acetylene,31 the amount of A1 produced keeps increasing and forms multiple-loop precursors, resulting in the decrease of A1 as the combustion progresses. The formation of A1 is delayed with increasing the proportion of n-butanol in the blend. The cetane number of diesel is below that of n-butanol. The cetane number of the blend reduces, ignition delay prolongs, and the pyrolysis delays with the addition of n-butanol to diesel, and all of these result in A1 formation. The peak of A1 with B0 (pure diesel) is higher than the other blends. This is due to the oxygen content in pure diesel being lower than that in other blends. This leads to the increase in benzene production among propinyl combination and, therefore, the increase in A1 formation.

Figure 16. Pressure inside the cylinder and mass fraction of A3 at different EGR rates with 10% n-butanol blend.

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Figure 20. Emission of each precursor component under different intake pressures.

The variation in A1 final production with different proportions of n-butanol in the blend is shown in Figure 4. When the proportion of n-butanol is lower than 20%, A1 increases with the increasing n-butanol, but this trend is not distinctive. This is because of the low combustion temperature and the A1 conversion rate slowing with the n-butanol proportion increasing. The occurrence of misfire was also observed when

the proportion of n-butanol was 30%, which caused the A1 value to be zero. The mass fraction and final emission of A2 at different proportions of n-butanol in the blend at 45% EGR rate are shown in Figures 5 and 6, respectively. In Figure 5, the mass fraction of A2 shows a decreasing trend after an initial increase with the crank angle. The conversion of A1 to A2 speeds up with A1 7154

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generated by pyrolysis of the fuel. Increasing the EGR rate delays the cracking reaction and formation of A1. For each EGR rate, the mass fraction peak of A1 could be identified in the premixed combustion phase. In the diffusion combustion phase, the mass fraction of A1 tends to be stable, confirming that the formation and oxidation of A1 tend to balance and keep A1 production almost unchanged. Figure 13 shows A1 final emission contrast at different proportions of n-butanol in the blend and EGR rates. With B30, misfiring of the cylinder was observed for EGR rates higher than 45%, and with B20, misfiring of the cylinder was observed for an EGR rate of 55%. Emission of A1 was close to zero as a result. Besides, A1 increases with the EGR rate for different proportions of n-butanol in the blend. The reason for this increase is that, at high EGR rates, recycling exhaust gas replaces air in the cylinder and reduces the oxygen concentration, leaving A1 without being oxidized and, finally, causing emission of A1 to increase with the rising of the EGR rate. The variation of the pressure inside the cylinder and mass fraction of A2 with the crank angle at different EGR rates are shown in Figure 14. Apparently, the formation of A2 is delayed with increasing the EGR rate, and its peak occurs before that of the maximum pressure in the cylinder. Also, the A2 peak is noticed in the premixed combustion phase. The reason is that fuel cracking mainly occurs in the premixed combustion phase, with the formation of a large number of C2H2 and C3H3 free radicals and other small size molecules, leading to the increase in the formation of A1. The formation of A2 increases as well. Eventually, A2 is formed more rapidly, and this makes the peak appear. Figure 15 shows A2 final emission at different proportions of n-butanol and EGR rates. The formation of A2 shows first an increasing and then a decreasing trend as the EGR rate increased. However, the blend B30 is an exception from this trend. This is because the oxygen concentration is reduced with the EGR rate, and it aids the formation of A2. On the other hand, the temperature inside the cylinder and, thus, the oxidation of A2 are reduced. All of these factors cause the mass of A2 to increase. However, at higher EGR rates, the ignition delay is retarded and the combustion temperature is decreased. Consequently, the production of A2 is reduced. Figure 16 shows the variation of the pressure inside the cylinder and mass fraction of A3 at different EGR rates and for 10% n-butanol blend. The formation of A3 is also retarded with increasing the EGR rate. The order of magnitude of A3 mass fractions is much less than those of A1 and A2. The peaks of A3 appear in the premixed combustion phase and diminish in the diffusion combustion phase. Figure 17 shows A3 final emission with different proportions of n-butanol and EGR rates. It presents first an increasing and then decreasing trend. With increasing the EGR rate, the oxygen concentration and combustion temperature inside the cylinder are reduced, causing the oxidation of A3 to decrease and its emission to increase. At higher EGR rates, ignition delay of the blends defers, during which more fuel and air are mixed, resulting in a lower combustion temperature and eventually less A3 production. In addition, the EGR rate relative to the A3 peak becomes smaller with increasing the n-butanol blending ratio, which shows that increasing the proportion of n-butanol to lower A3 production helps in reducing the dependence upon the EGR rate and avoiding excessive soot peak. Figure 18 shows the pressure inside the cylinder and mass fraction of A4 at different EGR rates and for 10% n-butanol blend. The formation of A4 is delayed with increasing the EGR rate. This

Figure 21. Average temperature inside the cylinder under different intake pressures.

formation. This results in the increasing production of A2 with the combustion. Subsequent to the burning period, the rate of conversion of A1 to A2 keeps reducing with decreasing the A1 concentration until a balance in the reversible reaction is reached. The formation of A2 is delayed with the increasing proportion of n-butanol in the blend. This is because the cetane number of the n-butanol blend decreases with increasing the proportion of nbutanol and leads to delayed combustion and delay in A2 formation. The peak of A2 mass fractions reduces continuously with increasing the proportion of n-butanol in the blend. Figure 6 shows that the final amount of A2 emission decreases with the increasing the proportion of n-butanol at 45% EGR rate. This is because, with the increasing proportion of n-butanol in the blend, the ignition delay is prolonged and the average combustion temperature decreases (Figure 7), resulting in the decreasing production of A2. Figures 8 and 9 show the variation of the mass fraction and final amount of A3 emission with different proportions of nbutanol in the blend at 45% EGR rate, respectively. Figure 8 shows that the formation of A3 is delayed with increasing the proportion of n-butanol in the blend. Because the formation of A3 depends upon the formation of A1 and A2, the variation in the time of the A3 formation with the proportion of n-butanol agrees with those of A1 and A2. With increasing the proportion of nbutanol in the blend, both the mass fraction and final amount of emission of A3 decrease distinctly. The variation of A4 with the proportion of n-butanol in the blend is shown in Figures 10 and 11. The mass fraction of A4 decreases sharply with increasing the proportion of n-butanol in the blend, and its formation is delayed. The final production of A4 decreases with the increasing proportion of n-butanol, particularly for the blends from B0 to B10. The reason for this trend is that the ignition delays caused by the blends are long enough to mix with air and for the average combustion temperature inside the cylinder to decrease, resulting in the decreased production of A4 as the proportion of n-butanol in the blend increases from 0 to 10%. However, the influence of the ignition delay of the blends on the production of A4 decreases, resulting in nearly the same amount of production of A4 when the proportion of n-butanol increases from 10 to 30%. 3.2. Effects of the EGR Rate on the Production of Soot Precursors. Figure 12 shows the variation of in-cylinder pressure and mass fraction of A1 with the crank angle for 10% n-butanol in the blend at three different EGR rates. The formation of soot precursor A1 is delayed with the increase in the EGR rate. The reason for this trend is that small molecules, such as C2H2 and C3H3, which play a crucial role in A1 formation, are 7155

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Figure 22. Emission of each component of precursor at different injection timings.

emission with different proportions of n-butanol in the blend and EGR rates. It basically shows first an increasing and then a decreasing trend. With the increasing EGR rate, the oxygen content inside the cylinder decreases and the combustion

time delay is slightly behind the ignition time of the blends, because high EGR rates may lead to long ignition delays and low combustion temperatures. As a result with increasing the EGR rate, A4 mass fraction decreases. Figure 19 shows A4 final 7156

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Figure 23. Pressure and average temperature inside the cylinder at different injection timings.

rate are studied, and the results obtained are summarized. On the basis of the previous evaluations with injection timing of −6.2° CA after top dead center (ATDC), other injection timings, namely, −4.2°, −6.2°, −8.2°, and −10.2° CA ATDC are studied as well. The variation of mass fractions of soot precursors A1, A2, A3, and A4 with the crank angle is shown in panels a−d of Figure 22. With the advancement of fuel injection timing, the emission timing of the products also advances. The reason is that, with the advance of injection timing, both pressure and average temperature inside the cylinder rise (as shown in Figure 23) and maximum temperatures and pressures move forward; therefore, the fuel cracking moves forward, which finally leads to the earlier formation of A1, A2, A3, and A4. The variation of final emissions of A1, A2, A3, and A4 with the injection timing is shown in panels e−h of Figure 22. It can be seen that A1 and A3 decrease with the advance of fuel injection timing. This is because the ignition delay and air−fuel mixing time are prolonged, making preferable conditions for combustion and resulting in reduction of A1 and A3. The production of A2 and A4 presents first an increasing and then a decreasing trend. This is because, although the earlier injection timing makes better combustion and reduces the emission of the precursors, the injection delay, on the other hand, reduces the combustion temperature and the soot precursor production. Therefore, A2 and A4 productions with late injection timing are less than those produced with early injection timing.

deteriorates, resulting in higher A4 production. With higher EGR rates, the ignition delay of the blends is prolonged, during which more fuel and air are mixed, eventually resulting in the decrease of A4 emission. 3.3. Effects of Inlet Pressure on the Production of Soot Precursors. The formation of diesel soot precursors is closely related to the combustion process. The combustion depends upon the condition of air inside the cylinder. Therefore, the air intake status affects the formation of soot precursors. The same inlet temperature and higher intake pressure make the air denser. The air under such conditions when charged into the cylinder will improve fuel economy and emissions. This section mainly focuses on effects of different inlet pressures on the production of soot precursors. The simulation was performed under the operating conditions of (a) engine speed of 1400 revolutions/min (speed with maximum torque), (b) compression ratio of 16, (c) fuel injection of 47.25 mg/cycle, (d) injection timing of 6.2° CA BTDC, (e) EGR rate of 45%, and (f) 10% n-butanol and 90% diesel. Effects of different inlet pressures, namely, 0.13, 0.16, 0.19, and 0.22 MPa, were evaluated. It can be seen from panels a−d of Figure 20 that, with the increase in the inlet pressure, the formation of all soot precursors advanced. This is due to earlier ignition timing and fuel pyrolysis caused by the higher inlet pressure, which promoted A1 formation. This also helped in the formation of A2, A3, and A4 which depend upon A1 formation. The peak of each component also increased with the increase in the inlet pressure. In panels e− h of Figure 20, except for A1, the final production of A2, A3, and A4 increased as the inlet pressure rises. This is because the low inlet pressure leads to longer ignition delays and poor air−fuel mix, making the production of soot precursor components decrease. Injection quantity remains the same when the inlet pressure increases, and the excess air coefficient increases as a result, which leads to lower combustion temperatures inside the cylinder. As shown in Figure 21, the amount of oxidized soot precursor components are reduced at lower combustion temperatures, thereby the emission of each component is accordingly increased. 3.4. Effects of Injection Timing on the Formation of the Soot Precursor. For LTC of diesel, control of fuel injection is an important strategy. It is known from the characteristics of the fuel injection, atomization, and ignition process that air−fuel mixing before ignition is of great importance for combustion and reducing emissions as well as soot precursor production. In this section, the effects of different injection timings on the production of soot precursors in LTC with B10 at 45% EGR

4. CONCLUSION The results obtained from the simulations and analysis presented in the previous sections can be summarized as follows: (1) With increasing the proportion of n-butanol in the blend, the formation of precursors A1, A2, A3, and A4 is delayed. The final production of A1 increases with increasing the proportion of nbutanol as long as the proportion of n-butanol in the blend is less than 20%. The final productions of A2, A3, and A4 decrease with the increasing the proportion of n-butanol in the blend. (2) The soot precursors are mainly formed in the premixed combustion phase. With increasing the EGR rates, the formation of A1, A2, A3, and A4 is delayed; the final production of A1 increases, while those of A2, A3, and A4 first increase and subsequently decrease, showing prominent peaks. (3) The EGR rate related to the peak of A3 diminishes with the increase in the proportion of n-butanol in the blend, showing that increasing n-butanol in the blend can reduce the dependency upon EGR while avoiding excessive soot peak at the same time. (4) With increasing the intake pressure, the formation of all soot precursor components analyzed in this 7157

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study advances. Except for A1, final productions of all other components increase as the inlet pressure increases. (5) Earlier injection timing leads to earlier formation of the soot precursor components. Emissions of A1 and A3 decrease with early injection timing, while those of A2 and A4 show first an increasing and, subsequently, a decreasing trend.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: 86-15507812553. Fax: 86-0771-3232294. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant 51076033), the Guangxi Natural Science Foundation (2014GXNSFGA118005), the Foundation of Guangxi Education Department (2013YB006), and the Guangxi Key Laboratory of Manufacturing System and Advanced Manufacturing Technology (12-071-11S05). The authors thank Dr. Quanchang Zhang for providing experimental data.

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dx.doi.org/10.1021/ef501370u | Energy Fuels 2014, 28, 7149−7158