Study on the Mechanism of Soot Formation and Oxidization in a DI

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Study on the Mechanism of Soot Formation and Oxidization in a DI Diesel Engine Using Postinjection Xiuyong Shi,*,† Xinqi Qiao,† and Yuanyuan Zheng‡ School of Mechanical and Power Engineering, Key Laboratory for Power Machinery and Engineering of Ministry of Education, Shanghai Jiaotong UniVersity, Shanghai 200240, People’s Republic of China, and Department of CiVil Engineering, Shandong Jiaotong UniVersity, Jinan 250023, Shandong ProVince, People’s Republic of China ReceiVed NoVember 9, 2008. ReVised Manuscript ReceiVed March 12, 2009

The formation of particulate matter (PM or soot) emissions in diesel engines is of great interest, and the development of a predictive mechanism is an important part of understanding the production of these pollutants. In this paper, a soot-oxidization model is presented on the basis of previous soot-mechanism research, referred to as average-reaction-rate (ARR) model. The ARR soot-oxidization model was combined with the Hiroyasu soot-formation model to compose the H-ARR soot model. With the H-ARR soot model, the process of soot formation and oxidization was numerically modeled according to different conditions of fuel postinjection. Meanwhile, the impact factors and effect mechanism of fuel postinjection on soot oxidization were analyzed in combination with experimental data. Finally, it was demonstrated that the resulting H-ARR soot model could describe the real process of soot formation and oxidization in diesel engine, as well as the influence rule of fuel postinjection on soot oxidization. Especially, it was observed that the brake specific soot emission can be reduced to 30% at the best experimental point of fuel postinjection.

1. Introduction It is well known that diesel engine is a type of higher efficiency, lower fuel-consumption power machinery. However, the major problem with diesel engines is the high level of NOx and particulate-matter (PM) emissions. Currently, there is a worldwide concern over the PM emitted by diesel engines because of their damage to our environment and their adverse effects on people’s health. At the same time, diesel engines have been faced with increasingly stringent emission legislations. On the basis of these reasons, many methods have been proposed in order to meet the challenging standards, such as the fuelinjection pressure, the fuel-injection rate, the retardation of fuelinjection timing, the combustion-chamber design, the utilization of after-treatment devices, and so on. Among those engine exhaust emissions, it is particularly difficult to decrease the NOx and particulate emission simultaneously. This will typically lead to more NOx formation with a better mixing and faster combustion process which is employed in order to decrease soot formation. In this effort, the application of multidimensional models for diesel engine appears promising. Furthermore, soot formation is a persistent problem in the fossil-fuel combustion process. Over the past few years, several research groups have studied extensively the phenomena of detailed soot formation and have attempted to describe these phenomena, including soot-particle dynamics.1-4 Some numerical models have been developed to represent the physical and * Corresponding authors. Telephone: +86-21-34206381. Fax: +86-2134206139. E-mail: [email protected] (X. S.) and Telephone: +86-53180687903. Fax: +86-531-80687903. E-mail: [email protected] (Y. Z.). † Shanghai Jiaotong University. ‡ Shandong Jiaotong University. (1) Wang, H.; Frenklach, M. A detailed kinetic modeling study of aromatics formation in laminar premixed acetylene and ethylene flames. Combust. Flame 1997, 110, 173–221.

chemical processes of the soot formation in diesel engine. These models have improved in complexity and capability. A better understanding of the in-cylinder phenomena in a diesel engine is important for the development of NOx and PM reduction strategies. Yet, the in-cylinder phenomena are complicated because they include the liquid-fuel atomization, vaporization, ignition, and combustion processes, accompanied with their emission formation. Both in-cylinder computational fluid dynamics (CFD) simulations and advanced experimental diagnostics are being actively pursued in order to analyze the in-cylinder phenomena. Multidimensional models resolve the flow field spatially and temporally and include the submodels for their physical process. Such models have been proven to be useful in predicting the in-cylinder events. Computer models such as the KIVA-3V5 and the FIRE code6 are available to aid in a better understanding of engine performance and emissions. They are especially useful when coupled with experiments to identify combustion and emission trends and provide direction to engine designers and ultimately improve engine performance and efficiency. (2) Mauss, F.; Schafer, T.; Bockhorn, H. Inception and growth of soot particles in dependence on the surrounding gas-phase. Combust. Flame 1994, 99, 697–705. (3) Pitsch, H.; Barths, H.; Peters, N. Three-dimensional modeling of NOx and soot formation in DI-Diesel engine using detailed chemistry based on the interactive flamelet approach. SAE 1996, 962057. (4) Gopalakrishnan, V.; Abaham, J. Computed NO and soot distribution in turbulent transient jets under diesel conditions. Combust. Sci. Technol. 2004, 176, 603–661. (5) Song, Ch. K.; Reitz. R. D. Application of detailed chemistry and CFD for predicting direct injection HCCI engine combustion and emissions. Proceedings of the Combustion Institute 2002, 29, 663-669. (6) Wallesten J.; Lipatnikov A.; Chomiak J. Modeling of stratified combustion in a direct-ignition, spark-ignition engine accounting for complex chemistry. Proceedings of the Combustion Institute 2002, 29, 703709.

10.1021/ef800970b CCC: $40.75  2009 American Chemical Society Published on Web 04/10/2009

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Therefore, the objective of the present work is to investigate the nature of soot emissions from a DI diesel engine under different conditions of fuel postinjection mode, by using a hybrid turbulent transport controlled rate soot model (H-ARR). The impact of different fuel-injection parameters on the characteristic of emissions from engines is presented, such as the postinjection quantity and postinjection interval. The effects of different engine operating conditions mentioned above on the soot and gaseous pollutant are also analyzed. On the basis of the detailed insight provided by the simulation results into the in-cylinder phenomena of the governing process, the various aspects of the combustion and emission formation are also discussed. 2. Soot Model Generally, it is well accepted that the production of soot occurs in two main phases, soot formation and soot oxidization. These processes depend on the fuel composition, in-cylinder gas pressure, in-cylinder gas temperature, and local fuel and oxygen concentrations. Soot models proposed so far are classified in three categories, namely, empirical models, semiempirical models, and models with detailed chemistry. Although the detailed mechanism of soot formation and oxidization in diesel engines is unclear until now, several theories have been proposed to explain these processes. 2.1. Previous Soot Models. Khan et al.7 presented a correlation equation of soot formation based on high-pressure diesel engine data. As a result, they further assumed that the rate of soot formation was controlled entirely by the formation of soot particles, that is, the soot inception rate. Parameters in this model were local values of temperature, unburned hydrocarbon concentration, and equivalence ratio. Their proposed model did not take into account the complete soot-oxidization process but was just an empirical one. Tesner et al.8 presented a kinetic scheme later, leading to a wide use of soot-formation model. It was based on a chain-type progress, involving the radical nuclei formation and growing up to be soot particle. It is a semiempirical model, which has incorporated some aspects of the physical and chemical phenomena, as a correlation function of experimental data. Surovikin9 also improved Tesner’s soot-formation model by adding an intermediate step to describe the growth of the radical nuclei and their conversion. It was a detailed chemistry kinetic model including the detailed PAH kinetics and soot-particle dynamics. More importantly, the oxidization process was taken into account in this formulation. They consisted of three steps: (a) the formation of radical nuclei, (b) the growth of the nuclei to a critical diameter, which will become incipient particles with a physical surface, and (c) the growth of the incipient particles into carbon particles. Their rates of oxidization were expressed as follows, dn ) n0 + (f - gn)n - g0Nn dt

(1)

where n is number densities (concentration, N/m3), n0 is the spontaneous production rate, f is branching reaction coefficient, and g is termination reaction coefficient. (7) Khan, I. M.; Greeves, C.; Wang, C. H. T. Factors affecting smoke and gaseous emissions from Direct Injection Diesel Engine and method of calculation. SAE 1973, 730163. (8) Tesner, P. A.; Snegiriova, T. D.; Knorre, V. G. Kinetics of dispersed carbon formation. Combust. Flame 1971, 17, 253–260. (9) Surovikin, V. F. Analytical description of the processes of nucleusformation and growth of particles of carbon black in the thermal decomposition of aromatic hydrocarbons in the gas phase. Solid Fuel Chem. 1976, 10, 92–101.

In spite of the uncertainties of the soot-formation and -oxidization mechanisms, there are some important processes which have been agreed upon by many researchers. Hiroyasu et al.10 developed a simple mechanism of soot model that was later applied for modeling the soot formation in diesel engines by Belardini. This model predicts the production of net soot ˙ s, by a single-step reaction between the soot mass mass, M ˙ sf, and the soot mass oxidization rate, M ˙ so, formation rate, M according to, ˙ sf - M ˙ so ˙s ) M M

(2)

The Arrhenius soot mass formation and oxidization rates can be obtained from, ˙ sf ) KfMFV M

(3)

˙ so ) KoMs M

(4)

The soot-formation and -oxidization rate coefficients, Kf and Ko, are a function of pressure and temperature and can be expressed as Kf ) Af P1/2 exp(-Ef /RT) Ko ) Xo2AoP

1.8

exp(-Eo /RT)

(5) (6)

where Af and Ao are pre-exponential constants, P is the gas pressure in-cylinder, Ef and Eo are activation energies, R is the ideal air constant, T is the temperature, and XO2 is the oxygen pressure fraction. The activation energies and Arrhenius pre-exponential constants were modified by Belardini et al.11 to fit into the baseline of engine data. This model, however, was found to give relatively low peak in-cylinder soot concentration. A more realistic prediction was obtained by using the Nagle and Strickland-Constable (NSC)12 oxidization model in replacement of eq 4. In this model, the carbon oxidization occurs in two mechanisms, the reaction rates of which depend on the surface chemistry involving more reactive A sites and the less reactive B sites, with the conversion of A to B. The chemical reactions are expressed as follows: A + O2 f A + 2CO

(7)

B + O2 f A + 2CO

(8)

AfB (9) According to the reaction correlation above, the coefficient of soot mass oxidization rate is then replaced by the NSC oxidization rate coefficient ˙ so ) M n

6Mc ˙ MM Fsds s T

(10)

where Mc and Ms are the molecular weight of carbon and soot, Fs and ds are the soot density and diameter, respectively, and ˙ T is the net reaction rate (g-atom carbon/sec · cm2). M Hiroyasu’s formation process, together with NSC oxidization process, is still used widely in phenomenological and numerical models of combustion systems including diesel engines. (10) Hiroyasu, H.; Kodota, T. Model for combustion and formation nitric oxide and soot in direct-injection diesel engine. SAE 1976, 760129. (11) Belardini, P.; Bertoli, C.; Ciajolo, A. Three-dimensional calculations of DI diesel engine combustion and comparison with in-cylinder sampling valve data. SAE 1992, 922225. (12) Nagle J.; Strickland-Constable R. F. Oxidization of Carbon between 1000∼2000 °C. Proceedings of the Fifth Conference on Carbon New York: Pergamon, 1962.

Soot Formation and Oxidization in a DI Diesel Engine

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2.2. Hybrid Turbulent Controlled Rate/Oxidization Rate Soot Model. The effects of turbulent mixing rate play a predominant role in the production of soot particles in diesel engine. Magnussen and Hjertager13 deduced from experiments on free diffusion flames that the soot was formed and contained in the turbulent eddies within the flame and that the burn-up of soot was related to the dissipation of the turbulent energy. In a DI diesel engine with strong air swirl, the mixing rate depends on the fuel-injection and air-swirl processes. The mixing process is controlled by the slower phenomenon, which is the air swirl. By comparing the typical time scales of soot formation and oxidization with those of turbulent flows in a diesel engine, Garo et al.14 concluded that the oxidization of soot particles was the process that would be largely influenced by the turbulent motions, whereas the nucleation and coalescent coagulation would not. The formation of chain will be influenced only to a certain extent. The Kolmogorov time scales are clearly smaller than the smallest eddies of the turbulent flow, Tt, and are likely to influence the coagulation. The time scales of soot-formation processes are critical for an incomplete oxidization. Soot is a type of solid-phase particle, and each stage of soot formation comprises complicated physical and chemical processes. The soot formation rate and the soot properties vary with its particle size, which is quite small (i.e., the particle diameter ranges from 10-5 to 10-6 mm) and is on the same order as the gas mean free path. Hence, the soot can be considered as acting as the gas-phase species. When the particles are produced in a combustion process, the soot is convected by the gas flow. It is assumed that no slip occurs between the soot and the fluid. Therefore, the spatial distribution of soot is a function of the gas flow. The soot-concentration transport equation is solved together with all other flow and spray equations as follows

( )

∂Ms ∂Ms ∂ 1 ∂ ˙s + (ViMs) ) Ds +M ∂t ∂xi F ∂xi ∂xi

(11)

where i ) 1, 2, 3, vi is the velocity in the direction i, and Ds is the effective soot diffusivity15 and can be determined as Ds )

kB T·Cc 3π µ·ds

(12)

where ds is generally an averaged soot-particle diameter, kB is Boltzmann’s constant, and µ is the viscosity. When the sootparticle diameter approaches the same order as the mean free path, λ, of the suspending fluid, the resisting force offered by the fluid is smaller than that predicted by the Stokes Law. In a realistic DI-diesel-engine combustion process, the oxidization reactions take place on the surfaces of soot particles once the soot particles are formed. The soot oxidization rate is proportional to the local concentration of soot and oxidant. The soot oxidization rate will be zero when one of those soot and oxidant is zero. In turbulent combustion, the soot particles are contained within the turbulent eddies. These soot particles will be burned up swiftly with the dissipation of these eddies in sootoxidization zones. Because the chemical reactions usually have (13) Magnussen B. F.; Hjertager B. H. On Mathematical Modeling of Turbulent Combustion with Special Emphasis on Soot Formation Combustion. Sixteenth Symposium (International) on Combustio; Combustion Institute: Pittsburg, 1976; pp 845-858. (14) Garo, A.; Said, R.; Borghi, R. Model of soot formation: Coupling of turbulence and soot chemistry. Soot formation in combustion; Springer Verlag: Berlin, 1994; pp 527-548. (15) Tennision P. J.; Gerojon, T. L.; Farrell P. V.; Reitz R. D. An experimental and numerical study from a common rail injection system for use in an HSDI diesel engine. SAE. 1998, 980810.

time scales which are short compared with those characteristics of the turbulent transport processes, the soot oxidization rate is determined by the rate of intermixing on a molecular scale of turbulent eddies (or the dissipation rate of turbulent eddies). Magnussen and Hjertager13 presented a model for soot formation from turbulent combustion. They used Tesner’s soot-formation model and thought that soot oxidization was determined by the quantity of oxygen and the dissipation rate of turbulent eddies. The hybrid soot-oxidization rate model can be expressed as

(

M MsSs ˙ so ) C ε min Ms, ox , M m k Ss MsSs + MfuSf

)

(13)

where C is a constant and usually is equal to 4, ε/k is the eddy turnover time that represents the turbulent time scale. Mox and Mfu are the mass of oxygen and fuel, respectively. Ss and Sf are stoichiometric ratios of soot-oxygen and fuel-oxygen. Magnussen’s model has been distinguished from other models, because the effects of turbulent motion have been taken into consideration in this soot model. As mentioned previously, the flexible fuel injection rate is one of those methods explored to improve exhaust-emission levels for diesel engine. Particularly, the fuel postinjection technique has a significant influence on soot emissions of diesel engine. In order to study the effects of fuel postinjection on soot emission and consider the effects of chemical kinetic reaction and turbulent mixing rate on soot oxidization, an average-reaction-rate (ARR) oxidization model is presented here. By the regular average between eqs 10 and 13, an average reaction rate oxidization equation is defined as j so ) R A

˙ so ˙ so M M n m ˙ so + M ˙ so M n m

(14)

Equation 14 is suitable for soot oxidization model controlled by chemical kinetic reaction and/or turbulent mixing rate simultaneously. The Hiroyasu soot-formation model together with NSC sootoxidization model forms the existing soot model, named H-NSC soot model. The Hiroyasu soot-formation model is combined with the presented ARR soot oxidization model to compose current soot model, called H-ARR soot model. In this study, the process of soot formation and oxidization was simulated with the two different soot models. The tendencies of soot formation and oxidization versus variation of fuel postinjection quantity and interval were analyzed in combination with experimental data. 3. Other Computational Models In the present study, the multidimensional combustion and emission characteristics of a heavy-duty DI diesel engine have been simulated by using an improved FIRE code, which solves the unsteady, compressible, and turbulent-reacting flows on finite-volume grids. This code is based on the modified turbulence, fuel spray, ignition and combustion, and NOx emission submodels. These improved submodels have been adequately described in the literature15-17 and hence are only briefly described here. (16) Kong, S. C.; Han, Z.; Reitz, R. D. The development and application of a diesel ignition and combustion model for multidimensional engine simulation. SAE 1995, 950278. (17) Rultand, C. J.; Ayouob, N.; Han, Z.; Hampson, G.; Kong, S. C.; Reitz, R. D. Diesel engine model development and experiments. SAE 1995, 951200.

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3.1. Turbulence Model. The RNG k-ε turbulence model has been used to simulate the variable-density turbulent flow within the engine combustion chamber. The modified RNG k-ε model18 is similar to the standard k-ε model, except that an extra term is added into the dissipation (epsilon) equation, in which the changes of the mean strain rate and compressibility of the flow are accounted for. The RNG k-ε turbulence model predicts a shorter spray penetration and increases the mixing processes compared the standard k-ε turbulence model. 3.2. Spray Model. The wave breakup model (also referred to as Kelvin-Helmholtz breakup model) has been applied to simulate the primary breakup of fuel spray. This model assumes that the aerodynamic instabilities are responsible for the liquidfuel breakup within the dense core region. The wave- or K-Hbreakup model has been combined with the so-called RayleighTaylor(R-T) breakup model in order to estimate the disintegration of the blobs into secondary fuel droplets. The R-T breakup model describes the development of instabilities on a liquid-gas interface subject to the strong normal acceleration pointed toward the gas phase. The changing rate of fuel-drop size is related to the frequency and wavelength of the fastest growing surface wave. The exponential growth of the perturbation of wavelength, Λ, with the maximum growth rate, Ω, dominates the whole breakup process. The fastest growing perturbation ultimately leads to the breakup phenomena.19 3.3. Ignition and Turbulent Combustion Model. The reaction mechanism used for simulating the hydrocarbon autoignition was the low-temperature flame chemistry of the SHELL model. This model is well suited for the simulation of autoignition phenomena in diesel engine. The reaction scheme consists of a multistep chemical kinetics and chain-propagation mechanism containing two routes, namely, the formation of branching agents and additional chain-terminating reactions. The laminar and turbulent characteristics time model was used to simulate the diesel engine combustion process.12 For the hightemperature flame chemistry combustion model based on the characteristic time scale, the changing rate of partial density of species i due to the conversion from one chemical species to another can be determined by Yi - Y*i dYi )dt τc

(15)

where Yi is the mass fraction of species i, Yi* is the instantaneous local thermodynamic equilibrium value of Yi, and Tc is the characteristic time for the achievement of such equilibrium condition. The characteristic time, Tc, can be formulated as Tc ) Tl + fτt

(16)

Table 1. Test-Engine Specifications parameter

value

bore stroke displacement compression ratio shape of combustion chamber

126 mm 130 mm 9.726 L 17 ω shape in the bottom of a bowl-in-piston configuration 213kw @ 2200 rpm 0.2 mm 8

rated power and speed nozzle hole diameter number of nozzle holes

in the energy equation are computed. Because the total chemical time scale includes the turbulent time scale, the effects of turbulence on mean reaction rate are then accounted for. 3.4. NOx Formation Model. In the present study, the extended Zeldovich thermal mechanism was used for modeling NOx emission in the diesel engine. Other sources of NOx formation are neglected. It consists of the following mechanisms: k1

N2 + O 798 NO + N

(17)

k2

k3

N + O2 798 NO + O

(18)

k4

k5

N + OH 798 NO + H

(19)

k6

Because the characteristic time of thermal NO formation is much longer than the characteristic physical and chemical time scales in the reaction front, it is possible to treat the thermal NO formation decoupled from the main combustion process. The reaction rate coefficients for eqs 17-19 can be obtained in the literature.20 The concentrations of O, OH, O2, H, and N can be approximated by a thermodynamic equilibrium assumption. 4. Test Engine and Computation Conditions A six-cylinder, four-stroke, water-cooled, turbocharged diesel engine was employed in the test. The detailed specifications are shown in Table 1. In this study, the fuel quantity and postinjection interval are controlled by a universal ECU, which is synchronized with an engine encoder and various sensors. According to the objective of this work, an experimental scheme was planed to conduct the test with various fuel quantity ∆m and postinjection interval ∆t, which are listed in detail in Table 2. The test engine was fully instrumented for temperature, pressure, and exhaust-emission measurements. Because the engine piston is of axial symmetry, the fuel injector is located at the center of the cylinder, and its eight orifices are equally dispersed; one-eighth of the cylinder domain (45° sector) was then simulated as shown in Figure 1. The geometry corresponds closely to that of the tested engine.

where f is the delay coefficient for the controlling role of turbulence effects, Tl is the laminar time scale, which is derived from the correlated one-step reaction rate through a single droplet autoignition experiment, and τt is the turnover time of turbulent eddy. Tc is assumed to be the same for the seven species considered, namely, fuel, O2, N2, CO, CO2, H2, and H2O, in order to predict each temperature accurately at the thermodynamic equilibrium. Among these seven species, six reactive species are accounted for in order to solve the instantaneous local thermodynamic equilibrium values Yi*. By using this combustion model, the chemical source term in the species continuity equation and the chemical heat release

5.1. Validation of Soot Model. In order to prove the presented H-ARR soot model, three cases were selected to simulate and validate with experiment data in this paper, including (1) single injection by using H-ARR model and (2) main + postinjection by using H-NSC model (∆t ) 1200 µs ∆m ) 5 mg/cyc), and (3) Main + postinjection by using H-ARR model (∆t ) 1200 µs ∆m ) 5 mg/cyc).

(18) Han, Z.; Reitz, R. D. Turbulence modeling of internal combustion engines using RNG k-e models. J. Comb.Sci. Tech. 1995, 106, 267–280. (19) Reitz, R. D. Modeling atomization process in high-pressure vaporizing sprays. J. Atomization Spray Tech. 1987, 3, 309–337.

(20) Gui, B. C.; Chan, T. L.; Leung, C. W.; Xiao, J.; Wang, H. W.; Zhao, L. B. Modeling study on the combustion and emissions characteristics of a light-duty DI diesel engine fueled with dimethyl ether(DME) using a detailed chemical kinetics mechanism. SAE 2004, 01-183.

5. Results and Discussion



Figure 3. Rate of heat release of combustion.

Figure 1. Calculation mesh of combustion chamber at TDC.

Figure 4. Temperature history in-cylinder.

pressure versus crank angle data are analyzed under the framework of the first law of thermodynamics to obtain the cylinder temperature and the rate of heat release. The cylinder pressure was measured by a pressure transducer (Kistler model 6125A), and the charge output from this transducer was measured by an analyzer (AVL Digas 4000). Because of the limitation of space, the experiment results discussed below are about the case of single injection and one of the conditions with postinjection (∆t ) 1200 µs ∆m ) 5 mg/cyc). Figure 3 shows the rate of heat release (RoHR) curves of combustion in diesel engine with single fuel injection, compared to that with fuel postinjection. The in-cylinder temperature history versus crank angle is also listed in Figure 4. First of all, it can be seen from Figure 3 that the RoHR curve goes up again, as a result of the heat released by postinjected-fuel burning. This phenomenon leads to the temperature rise in the later stage of the combustion period (Figure 4). Therefore, the time of soot oxidization becomes longer, resulting in further oxidization of the soot produced earlier. Accordingly, the final soot mass production can be found to be lower than that from experiment results of soot emission. To explore deeply the mechanism of soot oxidization at the condition of fuel postinjections, the simulated results by CFD code with H-ARR soot model are provided and compared Figures 5-8. The results were obtained at the piston position of 15 °CA after combustion TDC, in the cases of fuel single injection and of postinjection. As mentioned in Section 2.2, the Magnussen oxidization formula introduced in H-ARR soot model is proportional to ε/k. Accordingly, it can be seen from Figure 5 that the turbulent dissipation rate in-cylinder is increased by postinjection motion. The higher dissipation rate is located at the interface between produced soot and the residual oxygen in-cylinder, which accelerates the soot oxidization. This phenomenon also can be demonstrated in Figure 6; the soot oxidization rate with postinjection becomes higher obviously (the maximum goes

Figure 2. Soot history of formation and oxidization process in-cylinder.

In Figure 2, the diagram on the left top describes different fuel-injection modes, involving single injection and multi injection (main + postinjection). The soot history of formation and oxidization process in-cylinder are shown by the curves. Furthermore, the two points are test results of soot emission at the condition of single-injection and multi-injection modes. Obviously, the tendency of soot-formation stage with H-NSC model is nearly similar to that obtained with H-ARR model, which can be seen from curves 2 and 3. However, the difference takes place in the later soot-oxidization process, where the oxidization rate is raised when using H-ARR model. As for the H-ARR model in the process of soot producing, the trend of soot oxidization is nearly the same in the cases of single injection and with postinjection, which can be seen in curves 1 and 3. Finally, the net soot production can be found to be in good agreement with experimental results. For the same case, the simulated result of soot with postinjection is 0.041 g/kW · h, whereas the test result is 0.043 g/kW · h. Therefore, it can be concluded from the above analysis that the H-ARR model predicts the soot oxidization rate better than the original H-NSC model. 5.2. Combustion Parameters in-Cylinder. In order to understand the effects of fuel postinjection on soot oxidization, several behaviors in-cylinder are discussed in this part, such as the temperature in-cylinder, turbulent dissipation rate, oxygen concentration, soot oxidized time, and so on. Here, the cylinder

Table 2. Experimental Scheme of Different Fuel-Injection Modes fuel injection mode single injection main + postinjection

injection start angle/ATDC -5 °CA -5 °CA

main injection duration/°CA

postinjection interval ∆t/µs

postinjection quantity ∆m/(mg/cyc)

20 15

0 800,1200,2000

0 1.0,3.0,5.0,7.0

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Figure 5. Comparison of turbulence dissipation rate.

Figure 6. Comparison of soot oxidization rate.

Figure 7. Comparison of gas-flow velocity in-cylinder.

from 50.13 to 61.34, increased by 22%). Meanwhile, the higher oxidization rate has a wide distribution, which means that the oxidization area of soot produced before is enlarged. Consequently, it can be considered that the turbulent flow motion incylinder has a strong influence on the soot oxidization. This phenomenon should be focused on in the future to gain insight into in-cylinder soot producing mechanism in diesel engine. In Figure 7, it can be found that the fuel postinjection brought faster velocity of air flow in-cylinder. The maximum velocity of air flow changed from 62.70 to 81.14 m/s, increased by 30%. Furthermore, the fuel postinjection enhanced the mixing motion between air and unburned fuel, increasing the surrounding air usage. When the fuel spray jet was injecting in the cylinder, the ambient air was entrained in and then would be transported to the zone of combustion production. So, the soot produced earlier would be enclosed by an oxygen-enriched atmosphere, and hence, the oxidization of soot was accelerated. Accordingly, it can be concluded that the disturbing effect of postinjection spray was very important to boost the soot oxidization.

In addition to fuel spray jet disturbing effects, another impact factor for fuel postinjection reducing soot emission is the local temperature rise, which was caused by the injected fuel burning. When the combustion flame of postinjected fuel flowed and spread, the ambient air temperature raised consequentially. This resulted in a variation of the air density gradient, which enhanced the fuel/air mixture diffusion. All of these were beneficial for the produced soot oxidization. In Figure 8, the cloud represents the temperature distribution, and the contour shows the soot mass fraction. It can be observed that the maximum of soot mass fraction distribution in-cylinder was 1.093% in the case of single injection, whereas it was reduced to 0.514% in the case of postinjection. Therefore, it can be concluded that the in-cylinder temperature in the later stage of combustion duration had a significant effect on the soot oxidization. 5.3. Experimental Results of Different Fuel-Injection Modes. In this study, the experiments have been done with several different fuel-injection modes, and the smoke opacity



Figure 8. Comparison of temperature and soot distribution in-cylinder.

only in combustion and expansion strokes, because all the oxidization reactions have to stop if the temperature in-cylinder decreases dramatically when the exhaust valve opens. A certain amount of fuel inducted in the later combustion phase enhances the disturbance of fuel/air mixture in-cylinder. Their burning again raises the temperature in-cylinder additionally, which promotes the oxidization of the incomplete combustion production. Because of all these aspects, the soot, HC, and CO mass emission can be reduced remarkably. Figure 9. Experimental results of soot emitted under conditions of different fuel injections.

was measured by a smoke meter (AVL 439). According to smoke value measured, the brake specific soot emission (BSSE, g/kw · h) was obtained by the method of soot-emission conversion formula provided in the literature.21 The converted BSSE experiment results are listed in Figure 9. It can be observed from Figure 9 that the soot mass emission decreases dramatically for fuel postinjections. Also, it can be found that the brake specific soot emission decreases with the postinjection quantity increasing. The soot emission has a sharp drop, in particular when the postinjection quantity increased from 3 mg/cyc to 5 mg/cyc. Moreover, in the case of postinjection (∆m ) 7 mg/cyc and ∆t ) 2000 µs), the brake specific soot emission is reduced by 70% compared with that of single injection. Regarding this phenomenon, several reasons are proposed here. In diesel engine, the soot formation mainly takes place at the beginning of the diffusive combustion phase. At that time, the fuel spray has been separated by entrainment air flow, and the fuel drop is surrounded by high-temperature production left by premixed combustion. Meanwhile, the highertemperature air and later mixing disturbance will accelerate the oxidization of soot, and the agglomerate time of carbon particles can be shortened. However, the soot formed in the later combustion phase is be difficult to oxidize for two reasons. First, it is close to the end of the combustion period, and second, the temperature decreases rapidly in expansion stroke. In the same manner, the soot produced during the main combustion phase will not be oxidized easily for the lower temperature in-cylinder. It is well known that the soot oxidization can be carried through (21) Christian R.; Knopf F.; Jasmek A. eta1. A New Method for the Filter Smoke Number Measurement with Improved Sensitivity. Germen: MTZ 54, 1993.

6. Conclusions (1) On the basis of previous research on soot mechanism, an ARR soot oxidization model was presented here and programmed into CFD code with Hiroyasu soot formation model. The combustion and emission processes were simulated in a DI diesel engine by using this H-ARR soot model. In comparison with experimental data, a good agreement has been achieved for the modeling results. (2) The mechanism of soot reduction was studied in details by using fuel postinjection, the impact factors of which included temperature variation in-cylinder, oxygen concentration, disturbance caused by fuel spray jet, turbulent motion, and so on. Finally, it was concluded that the turbulence caused by postinjection was not negligible in soot oxidization process. (3) Also, the experiments were conducted under different conditions of fuel postinjection quantity and interval. Through the analysis of experimental results, it can be found that the fuel postinjection had a positive effect on the soot reduction to a large extent. From the soot emission point of view, the soot production can be decreased to 30% under the best experimental conditions. Acknowledgment. This work was supported by the Research Fund for the Doctoral Program of Higher Education (Grant 20050248013). The authors acknowledge the students of Shanghai Jiaotong University for their help with the experiment. The authors also express their thanks to the colleagues of Shanghai Jiaotong University for their helpful comments and advice during the manuscript preparation. Note Added after ASAP Publication. There were errors in Figure 2 in the version of this paper published ASAP on April 10, 2009; the corrected version published ASAP April 15, 2009. EF800970B

Study on the Mechanism of Soot Formation and Oxidization in a DI

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