Effect of Multiple Injection Strategies on Combustion and Emission

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Effect of Multiple Injection Strategies on Combustion and Emission Characteristics in a Diesel Engine Su Han Park,† Hyung Jun Kim,‡ and Chang Sik Lee*,§ †

School of Mechanical Engineering, Chonnam National University, 77 Yongbong-ro, Buk-gu, Gwangju 61186, Repulic of Korea National Institute of Environmental Research, 42 Hwangyeong-ro, Seo-gu, Inchon 22689, Republic of Korea § School of Mechanical Engineering, Hanyang University, 222 Wangsimni-ro, Seongdong-gu, Seoul 04763, Republic of Korea ‡

ABSTRACT: This paper presents an experimental and numerical investigation on the spray and combustion characteristics in a common rail diesel engine with two different multiple injection strategies involving a fixed injection interval (double injection strategy) and a fixed second injection timing (split injection strategy). In this study, the double injection strategy had a fixed 10° interval with before top dead center (BTDC) 40°, 30°, and 20° of first injection timing. The split injection strategy consisted of a two-stage injection, in which the second injection timing was fixed at top dead center (TDC), followed by a first injection that varied in timing from BTDC 40−10°. A high-pressure chamber with a maximum pressure of up to 4 MPa was used to analyze the spray characteristics of the various injection strategies at high ambient pressures. Spray images were obtained using a spray visualization system to analyze the spray behavior with multiple injection conditions. The KIVA-3V code was coupled with a reduced n-heptane mechanism to analyze the combustion and emission characteristics. The numerical code was applied to calculate and analyze the effect of multiple injection strategies on the combustion and emission characteristics after the validation with the same experimental results [one single injection case and one multiple (split) injection case]. The spray behaviors exhibited different patterns according to the injection strategies, such as single injection and multiple injections, and injection timing. For double injection, the spray developed faster than that for split injection. The start of combustion was affected by the multiple injection timing, and for double injection, the heat release rate increased as the second injection timing was retarded. Furthermore, the combustion pressure and heat release rate that were calculated were consistent with the results obtained from the experiments with various injection strategies.

1. INTRODUCTION In general, diesel engines are used to operate large powered vehicles and vessels because these types of engines have a higher thermal efficiency and better fuel consumption rate than gasoline engines. In addition, various technologies have been developed for high-speed diesel engines for passenger cars. However, exhaust gases, including nitrogen oxides (NOx), hydrocarbon (HC), and particulate matter (PM), are produced in the cylinder during combustion, and the pollution as a result of these gases is an environmental concern. To address this problem, many attempts have been recently made to reduce the amount of harmful exhaust emissions that are generated. Researchers have investigated several types of reduction technologies for exhaust emissions, including high-pressure direct fuel injection with a common-rail system, homogeneous charge compression ignition (HCCI),1,2 where self-ignited combustion is achieved by compressing premixed intake air with fuel that is injected early, and after-treatment with a diesel particulate filter (DPF)3,4 or a diesel oxidation catalyst (DOC).5,6 The common-rail injection system uses a highpressure injector with a solenoid valve to control fuel injection at a high injection pressure. This system can administer multiple injections corresponding to the injection timing through accurate electrical control. When multiple injections are used, the ignition delay for the main injection decreases and combustion is rapidly hampered by a pilot injection. The multiple injection strategies divide the main injection into two to decrease fuel-rich regions in the cylinder because the © 2016 American Chemical Society

reaction zone between the air in the cylinder and the periphery of the combustion flame increases. Therefore, multiple injections are expected to reduce the amount of NOx emissions when compared to those generated with a single injection.7,8 Recent studies have carried out experiments and simulations to propose the effect that multiple injections have on a diesel engine. Roh et al.9 studied the effect of pilot injection and single injection on the combustion and emission reduction in a fourcylinder diesel engine. They reported that the maximum pressure at pilot injection was significantly decreased compared to that of a single injection case for three kinds of test fuels. This results mean that the application of the pilot injection strategy can decrease the engine combustion pressure and the rate of pressure rise in the cylinder. Li et al.10 investigated the effect of multiple injection strategies on the emission particle size. They applied three kinds of multiple injection strategies, such as single, pilot−main, and main−post injections. They reported that the pilot−main strategy resulted in a lot of particles and mass as a result of the increase of soot nuclei formation by the more diffusion combustion compared to single injection. Kokjohn et al.11 performed an experimental study and created a model of the combustion and emission characteristics with adaptive injection strategies in a diesel engine. Their results showed that the low pressure of the first Received: September 17, 2015 Revised: January 11, 2016 Published: January 11, 2016 810

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of chemicals, and Curran et al.24 developed a detailed n-heptane mechanism containing 570 species and 2520 reactions, while a smaller mechanism for n-heptane oxidation containing 179 species and 1642 reactions was derived from the detailed mechanism.25 However, this mechanism still contains too many species and reactions. Hence, the mechanism reduction methodology was used to derive a reduced Engine Research Center (ERC) n-heptane mechanism26 consisting of 36 species and 74 reactions from the detailed mechanism.26 The reduction in the ERC n-heptane mechanism was applied to KIVA-3V code to simulate the self-ignition and combustion processes of this study. The chemical reactions for diesel oxidation were computed using this code coupled with the CHEMKIN-II chemistry solver.27 On the other hand, the NOx formation process was modeled using a reduced Gas Research Institute (GRI) NO mechanism28 to predict the NOx emissions using the KIVA code in combination with the CHEMKIN-II chemistry solver. The original GRI NO mechanism is composed of 101 reactions among 22 species related to nitric oxides,29 and it is reduced into the reduced GRI mechanism with 4 species (N, NO, N2O, and NO2) and 9 reactions, which was then added to the fuel oxidation mechanism. The soot emission model in this study consisted of the Hiroyasu soot formation30 with the Nagle− Strickland-Constable oxidation model.31 Detailed calculation conditions for comparison between single and multiple injections according to the start of injection were illustrated in Table 2.

injection led to a reduction in the HC emission as a result of a reduction in the spray wall impingement and an improvement in fuel economy. Husberg et al.12 investigated the combustion characteristics in an optical engine with multiple advanced pilot injections, and these experimental results were compared to those obtained from calculation. They reported that NOx and CO emissions decreased with multiple advanced pilot injection and increased in comparison to those of single injection, respectively. In addition, soot emissions increased after the start of combustion for the main injection. In addition, many experimental and numerical studies have been conducted on the combustion and emission characteristics with various multiple injection strategies, such as by varying the injection timing, the interval time between the pilot and main injections, and quantity ratio under various operating engine conditions, to reduce the exhaust emissions in diesel engines.13−16 Thus, the objective of this study is to investigate the effect of multiple injection strategies on the combustion and emission characteristics in a single-cylinder diesel engine with a common-rail injection system. The detailed explanation and definition for multiple injection strategies are described in the part of an experimental procedure (section 3). A visualization system was used to analyze the spray characteristics of multiple injections with various injection timings in a high-pressure chamber. In addition, the numerical results for the combustion that had been obtained using the KIVA-3V code coupled with a reduced n-heptane mechanism were validated against the experimental results. These results were used to conduct a comparison and analysis of the combustion and emission characteristics under various multiple injection conditions.

3. EXPERIMENTAL SETUP AND PROCEDURE FOR VALIDATION

2. NUMERICAL FORMULATION The combustion and emission characteristics were analyzed according to various multiple injection conditions by conducting calculations using the KIVA-3V code, which has been extensively used to simulate the combustion process and formation of emissions in an engine.17,18 The Shell multi-step model proposed by Halstead et al.19 was applied to analyze the autoignition phenomenon of the diesel spray in the cylinder, and this model was expressed according to the composition of the generic reactions for HC fuel and the generic species, such as radical, intermediate species, and branching agents. The combustion model was combined with the autoignition model to analyze the overall combustion phenomena in the diesel engine as suggested by Kong et al.20 In this study, the switching temperature between the ignition and combustion models was set to 1100 K, and in the case of the local cell temperature being lower than 1100 K, the ignition model was activated for a low-temperature chemistry but the laminar and turbulent characteristic time scale model was used to simulate the hightemperature chemistry in the case where temperatures reached higher than 1100 K. The Kelvin−Helmholtz and Rayleigh−Taylor hybrid breakup model21 was applied to calculate the spray and atomization characteristics of the injected droplets, and the Renormalization Group (RNG) k−ε model22 and the spray-wall interaction model23 were used to calculate the turbulent flow in the cylinder and the impingement on the cylinder wall. The nheptane fuel is a diesel fuel substitution that consists of a large HC molecule with a molecular weight that is very close to that of aviation fuels. The Lawrence Livermore National Laboratory (LLNL) developed and validated the detailed chemical kinetic reaction mechanisms for the combustion of HC and other types

In this study, two different experimental configurations, such as a spray visualization system and a single-cylinder diesel engine system, were used for the code validation. The spray visualization system was used to investigate the macroscopic characteristics of the spray behavior including the spray development process and the spray tip penetration for single and multiple injection strategies, as shown in Figure 1a. The spray development process was visualized for a six-hole injector from the bottom view using a high-speed camera (FASTCAM APS-RS, Photron) with two metal halide lamps (HVC-SL, Photron). The highspeed camera and test injector were synchronized using a digital delay/ pulse generator (model 555, Berkeley Nucleonics Corp.). The diesel fuel mass that was injected was controlled by the solenoid signal produced by the injector driver (TEMS, TDA-3200H). In addition, the injection pressure was fixed and stabilized by the common-rail injection system and two high-pressure pumps. The quantity of diesel fuel that was injected was fixed to 10 mg (for a single injection). Single and multiple injection strategies were applied to the spray experiments, and the multiple injection characteristics of the diesel fuel were analyzed by controlling the injection signal of the first injection from before top dead center (BTDC) 40° to 10° with a fixed main injection at top dead center (TDC). The combustion and emissions characteristics for investigating the effect of multiple injection strategies were measured and analyzed using a naturally aspirated single-cylinder diesel engine with a common-rail direct injection system. The single-cylinder engine has 84.5 mm bore, 75.0 mm stroke, displacement volume of 373.3 cm3, and a compression ratio of 17.8, and the detailed specifications and dimensions of the test engine are provided in Table 1. The experimental apparatus consisted of a test engine, a fuel injection system, a dynamometer with control systems, and a combustion analyzer, as shown in Figure 1b. The engine load and speed were controlled using a direct-current (DC) dynamometer (55 kW) system. The in-cylinder pressure was measured using a piezo-electric pressure transducer (6052B1, Kistler) coupled to a charge amplifier (5011B, Kistler). For each test case, the combustion pressure data were 811

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maintained at 70 ± 1 °C, and the injection pressure of test fuel was fixed at a constant pressure of 50 MPa. The detailed test conditions for experimental and numerical modeling are listed in Table 2.

Table 2. Test Conditions experimental conditions for validation engine speed (rpm) 1400 rpm injection pressure (MPa) 50 MPa injection quantity (mg) 10 mg single injection injection timing BTDC 10° injection quantity (mg) 5 mg + 5 mg multiple injection injection timing BTDC 30° + TDC numerical modeling conditions engine speed (rpm) injection pressure (MPa) injection quantity (mg) double injection injection timing multiple injections

injection quantity (mg) split injection

injection timing

1400 rpm 50 MPa 5 mg + 5 mg BTDC 40° BTDC 30° BTDC 20° 5 mg BTDC BTDC BTDC BTDC

+ + + +

40° 30° 20° 10°

BTDC 30° BTDC 20° BTDC 10° 5 mg + + + +

TDC TDC TDC TDC

Figure 2 shows the injection strategies for single and multiple injections. In this study, the multiple injections divided into double injection and split injection. In fact, the multiple injection applied to this study has the identical fuel injection quantity as 5 mg (single injection of 10 mg). The double injection means that two injections have the same injection quantity and the fixed interval between Figure 1. Experimental apparatus for spray, combustion, and emission measurements.

Table 1. Specifications of the Single-Cylinder Diesel Engine item

specification

engine type number of cylinders bore × stroke (mm) displacement volume (cm3) fuel injection system valve type compression ratio injector number of holes hole diameter (mm) spray angle (deg)

direct injection diesel engine 1 75.0 × 84.5 373.3 Bosch common rail DOHC 4 valves 17.8 6 0.128 156

measured during 1000 cycles and were acquired using a DAQ board (PCI-MIO-16E-1, National Instrument) with a sampling interval of 0.1° for the crank angle to ensure accurate ignition timing and phasing of the heat release. The in-cylinder pressure versus crank angle data were averaged to eliminate the effect of cycle to cycle variations, and thus, the rate of heat release (ROHR) was calculated to analyze the combustion characteristics for each of the test conditions. An injection pressure controller (TDA-1100, TEMS) maintained the fuel pressure of the common rail. An injector driver (TDA-3300, TEMS) synchronized with a crank angle sensor was used to control the injection timing and the injection mass, and all tests were conducted under a constant engine speed of 1400 rpm using a dynamometer with a constant engine speed mode. The coolant and oil temperatures were

Figure 2. Injection strategies for single and multiple injections. 812

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Energy & Fuels injections, which is 10° in this study. The split injection means that two injections have the same injection quantity and the fixed second injection timing to specific timing, which is TDC in this study.

4. RESULTS AND DISCUSSION 4.1. Macroscopic Spray Behaviors for Single and Multiple Injections. Before studying the deep combustion and emission characteristics of the multiple injection strategies, the spray development process and the spray tip penetration were measured and analyzed for single and multiple injections by analyzing an image of the spray obtained using a spray visualization system. Figure 3 shows the spray development

Figure 3. Spray development images for single and multiple injections according to the start of injection (Pinj = 50 MPa, and Pamb = 2MPa, at a constant volume chamber, tasoi means the time after the start of injection). Figure 4. Spray tip penetration of single and multiple injections (double injection and split injection) according to the start of injection (Pinj = 50 MPa, and Pamb = 2 MPa, at a constant volume chamber).

processes for the single and multiple injections under an ambient pressure of 2 MPa according to the start of injection. An elapsed time of 1.2 ms corresponds to 10° of the crank angle at 1400 rpm. The shape for the six injected fuel sprays shows similar development patterns, and it is believed that the droplets are atomized more actively at the spray tip area than in the center region because the spray image is blurred in the spray tip region. In addition, the spray image at an early injection stage of the single injection is similar to that of the first injections in multiple injections, but the single injection shows a higher visibility than that of the first injection of multiple injections from 3.6 ms after the start of the injection because the fuel mass is increased as a result of the increase in the injection duration and the mass flow rate. In the case of the multiple injection, the spray development of the multiple injection (interval time of 1.2 ms) progresses more than that of the first injection (interval time of above 2.4 ms) because the droplets in the multiple injection after the end of the first injection have a large residual momentum relative to that of the first injection. Figure 4 shows the spray tip penetration of the single and multiple injections according to the time after the start of injection. The spray tip penetration of the single and multiple injections increased rapidly in the early injection stage, and the multiple injections show a similar increasing pattern as the single injections. However, the spray tip penetration for the second injection of the multiple injection increased faster than that for the single injection as a result of the increase in momentum resulting from the combination of the velocities of

the first and second injections, as illustrated in Figure 4a. In the case of split injection, the increasing rate for the spray tip penetration for the first injection slowed from 2.0 ms after the start of the injection because the droplets after the end of injection lost internal momentum as a result of the ambient density of the chamber. The spray tip penetrations for the 3 s injections, according to the injection intervals, show a similar increasing pattern, as shown in Figure 4b. 4.2. Model Validation for Combustion of the Single and Multiple Injections. Figure 5 shows injection rate data for single and split injections and a comparison of the combustion characteristics, such as the in-cylinder combustion pressure and the normalized accumulated heat release rate (HRR), obtained from the experiment and the calculation to validate the model of the single and split injections. For validation, two cases of single injection (BTDC 10°) and split injection (BTDC 30° + TDC) were selected. To validate the calculation results, the injection rate data were applied to the numerical modeling, and it was measured from the injection rate meter based on Bosch’s principle. The injection rate data for the single and split injections are represented in Figure 5a. As shown in Figure 5b, the single injection combustion has one peak for the combustion pressure but split injection combustion showed two low peaks for the combustion pressure according to the number of injections. In addition, the increasing rate of combustion pressure in the split injection 813

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Figure 6. Calculated combustion pressure and rate of heat release of multiple injection strategies (double and split injections) according to the start of injection (Pinj = 50 MPa, and double injection quantity = 5 + 5 mg).

Figure 5. (a) Injection rate data and (b) comparison of combustion characteristics obtained from the experiment and the calculation for model validation.

case is smaller than that in the single injection case as a result of the difference of the ignition delay. On the other hand, one and two increasing points appeared in the accumulated HRR for the single and multiple injections, respectively. As seen in the calculated results, the combustion pressure and accumulated HRR were mostly consistent with the experimental results for both single and multiple (split) injections. However, there is a small difference in the accumulated heat release results for the multiple injection. It can be explained that this difference was contributed by the difference of the ambient conditions during injections. In the injection rate meter, the ambient conditions during injection events are uniform. However, the ambient conditions in the diesel engine during multiple injection events are varied. 4.3. Combustion Pressure and Heat Release Characteristics. Figure 6 shows the combustion pressure and the rate of heat release that were calculated for the double injection and split injection strategies as a function of the start of the injection timing. The peak combustion pressure for the cases with the multiple injection, which includes the double injection and the split injection, increased with the advanced first injection timing for two reasons. First, the combustion for the advanced first injection occurs near the cylinder head or crevice volume, and this adds to the combustion at the bowl regions. Second, the advanced first injection timing with a fixed injection interval induces an increase in the increase of ignition delay (refer to Figure 9), and the compressed air/fuel mixture is then instantly combusted. On the other side, the combustion pressure and rate of heat released at BTDC 20−10° shows a

two-stage combustion pattern because the temperature and pressure in the cylinder of the combustion are sufficiently high when the first spray has been injected. Therefore, the ignition delay for this case is shorter than that for the other cases. Figure 6b illustrates the effect of the start of the injection on the combustion pressure and rate of heat release calculated for the split injection combustion according to the crank angle. The combustion pressures for all split injection conditions have two peaks contrary to the combustion patterns of the double injection. A longer injection interval between the first and second injection of split injections, excluding the case of BTDC 10°−TDC, explains two peaks in combustion pressure. A long injection interval means that the second injection begins after the end of the combustion of the first injection. In addition, the second injection leads to a decrease in the temperature in the cylinder as a result of the evaporation of fuel. Also, the case with BTDC 30°−TDC shows a higher combustion pressure and rate of heat releases than the other cases because the spray injected at BTDC 30° impinged on the protrusion point in the cylinder. Therefore, the fuel sprays in two directions for the right head and bowl regions in the cylinder advanced, and this can efficiently use the air in the cylinder during combustion. The combustion temperature distribution for the double injection and the split injection according to the start of the injection at the crank angle of BTDC 6° is shown in Figure 7. In the double and split injections with the BTDC 40° for the start of the injection, combustion occurs at the right head and crevice regions of the cylinder because the early first injection 814

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Figure 7. Calculated combustion temperature distribution of single and multiple injections according to the start of injection (crank angle = BTDC 6°).

after impingement on the right cylinder head progresses toward the crevice volume of the cylinder. However, combustion happens along the bowl shape in the start of the injection after BTDC 30°. In the double injection, combustion occurs as a result of the short injection interval and the high-temperature region centered partially on the combustion chamber. In addition, this temperature distribution shows a pattern similar to that of a single injection. On the other hand, the temperature distribution for the split injection is obvious in the two hightemperature regions as the injection interval increases. In addition, the second injected spray is promptly kindled at the high pressure and temperature in the cylinder as a result of the combustion of the first injection. The high-temperature regions of split injection are said to have a wider distribution than those of the double injection. Figure 8 shows the normalized accumulated heat release characteristics, the ignition delay, and the combustion duration in the double injection strategy. The normalized heat release was calculated using the values for the rate of heat release and the maximum accumulated heat release. In this graph, CA10 and CA90 refer to the start and end of ignition at a crank angle of 10 and 90% of the maximum value in the accumulated heat release, respectively. The ignition delay can be calculated from the difference between the first injection timing and CA10, and the combustion duration can be calculated from the interval between CA10 and CA90. As shown in Figure 8a, the ignition timing (CA10) and the termination of combustion (CA90) advanced as the first injection timing advanced. The combustion process slowed when the first injection timing was retarded. A detailed analysis of the ignition delay and the duration of the combustion were described in Figure 8b. The ignition delay in Figure 8b decreased as the first injection timing was retarded, while the duration of the combustion increased. The reason for the long delay in the ignition for the case with BTDC 40−30° can be conjectured to be a result of the low in-cylinder temperature and pressure. The long ignition delay results in a well-mixed air/fuel mixture with good ignitable characteristics. Hence, the duration of the combustion is short. In general, the ignition delay and duration of the

Figure 8. Combustion characteristics of double injection.

combustion are inversely proportional to each other, as shown in Figure 8b. These combustion and ignition characteristics are strongly related to the normalized accumulated heat release characteristics from Figure 8a. Figure 9 shows the normalized accumulated heat release characteristics, the ignition delay, and the combustion duration for the split injection strategy. Figure 9 shows three steps in the heat release graphs. The first small step is a result of the lowtemperature reaction, and the second and third steps are due to the high-temperature reactions. The retardation of the first injection timing caused an increase in the combustion heat because the ignition timing for the first injection is strongly related to the injection timing.32 The most retarded first injection timing has a most significant effect on the combustion of the main injection fuel, and then it showed the shortest ignition delay, as shown in Figure 9b, because the in-cylinder temperature and pressure are very high for this retarded injection timing condition. In addition, the split injection conditions exhibited a decrease in the duration of the combustion with the ignition delay when the first injection timing was retarded. This is the reason why the retardation in the injection timing induced an approach for both injection events to TDC, which means that ignition and combustion proceeded under high-temperature and high-pressure conditions. The exhaust emissions are predicted according to the multiple injection conditions, and the emission characteristics were analyzed using various calculation models, such as the 815

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Figure 9. Combustion characteristics of split injection.

Figure 10. ISNOx and ISsoot emission characteristics of multiple injection strategies.

reduced n-heptane mechanism, GRI NO mechanism, and phenomenological soot model, as previously described in section 2. The NOx, soot, CO, and HC emissions calculated for the double injection strategy with a fixed injection interval (10°) are shown in Figures 10 and 11. For the NOx emissions, the cases with the multiple injection exhibit more NOx emissions than the case with the single injection because the advance in the injected fuel and the second fuel spray form richfuel regions and rapid combustion occurs at these regions. Therefore, the NOx emissions with a high temperature increase as a result of the advanced combustion in the cylinder. However, the NOx emissions for the split injection are lower than those of the single injection. The reason for this is that the rapid combustion is prevented by forming a homogeneous air− fuel mixture with a longer injection interval. This result suggests that the pilot injection method is more efficient than the multiple injection to reduce the NOx emissions. In addition, the advance in the first injection timing that keeps the injection interval caused a decrease in indicated specific (IS)NOx and uniform ISsoot emissions. The decrease in the NO emissions is related to the distribution of the in-cylinder temperature. The early injection (BTDC 40−30°) formed a more uniform mixture distribution as a result of the long delay in the ignition. For this reason, the in-cylinder temperature distribution showed a more homogeneous and relatively lower value than that of the other cases. In the case of ISsoot, all of the test conditions exhibited very low values below 0.01 g/kWh as a

result of active oxidation of the reaction of soot. Hence, the difference among the test conditions is insignificant. Figure 11 shows the ISCO and ISHC emission characteristics. The HC and CO emissions are large as a result of the first injection of BTDC 40° in the multiple injection that flowed into the crevice volume. The HC emissions for the split injection have a somewhat larger amount than that for the double injection as a result of the wall wetting of the fuel by the second injection at the TDC. These results indicate that multiple injections with start of ignition (SOI) after BTDC 30° exhibit similar or lower HC and CO emissions when compared to that with single injection.

5. CONCLUSION This study presented the results obtained from an experimental and numerical investigation of the combustion and emission characteristics with multiple injection strategies in a commonrail diesel engine. The spray characteristics for multiple injections under various injection timings were measured using a visualization system. In addition, the combustion and emission characteristics that were calculated according to various multiple injection conditions were compared and analyzed using the KIVA-3V code. The conclusions are summarized as follows: (1) With regard to the spray characteristics, the spray development progressed further for the double injection than for the split injection because droplets 816

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +82-2-2220-0427. Fax: +82-2-2281-5286. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was financially supported by the Chonnam National University (2015) and was also supported by the Basic Science Research Program (2014R1A1A2057805) and the Basic Research Laboratory Program (2015R1A4A1041746) through the National Research Foundation of Korea (NRF) funded by the Ministry of Education.

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NOMENCLATURE ATDC = after top dead center (deg) BTDC = before top dead center (deg) TDC = top dead center (deg) HRR = heat release rate (J/deg) tasoi = time after the start of injection Pinj = injection pressure (MPa) Pamb = ambient pressure (MPa) SOI = start of ignition CA10 = crank angle at 10% of the cumulative heat release CA90 = crank angle at 90% of the cumulative heat release IS = indicated specific (g/kWh) REFERENCES

(1) Vinayagam, N.; Nagarajan, G. Experimental study of performance and emission characteristics of DEE-assisted minimally processed ethanol fueled HCCI engine. Int. J. Auto Tech-Kor 2014, 15 (4), 517− 523. (2) Dec, J. E. Advanced compression-ignition enginesUnderstanding the in-cylinder processes. Proc. Combust. Inst. 2009, 32, 2727−2742. (3) Shi, Y. X.; Cai, Y. X.; Li, X. H.; Chen, Y. Y.; Ding, D. W.; Tang, W. Mechanism and method of DPF regeration by oxygen radical generated by NTP technology. Int. J. Auto Tech-Kor 2014, 15 (6), 871−876. (4) Kang, W.; Choi, B.; Kim, H. Characteristics of the simultaneous removal of PM and NOx using CuNb−ZSM-5 coated on diesel particulate filter. J. Ind. Eng. Chem. 2013, 19, 1406−1412. (5) Kabir, M. N.; Alginahi, Y.; Islam, K. Simulation of oxidation catalyst converter for after-treatment in diesel engines. Int. J. Auto Tech-Kor 2015, 16 (2), 193−199. (6) Seo, C.; Bae, J. Y. Effect of hollow silica support on the reduction performance of harmful gases of 3Pt−-2MgO−3ZrO2−2CeO2 for DOC. J. Ind. Eng. Chem. 2014, 20 (5), 3053−3060. (7) Badami, M.; Mallamo, F.; Millo, F.; Rossi, E. E. Influence of Multiple injection strategies on emissions, combustion noise and BSFC of a DI common rail diesel engine. SAE Tech. Pap. Ser. 2002, DOI: 10.4271/2002-01-0503. (8) Park, S. H.; Yoon, S. Injection strategy for simultaneous reduction of NOx and soot emissions using two-stage injection in DME fueled engine. Appl. Energy 2015, 143, 262−270. (9) Roh, H. G.; Lee, D.; Lee, C. S. Impact of DME-biodiesel, dieselbiodiesel and diesel fuels on the combustion and emission reduction characteristics of a CI engine according to pilot and single injection strategies. J. Energy Inst. 2015, 88 (4), 376−385. (10) Li, X.; Guan, C.; Luo, Y.; Huang, Z. Effect of multiple-injection strategies on diesel engine exhaust particle size and nanostructure. J. Aerosol Sci. 2015, 89, 69−76. (11) Kokjohn, S. L.; Swor, T. A.; Andrie, M. J.; Reitz, R. D. Experiments and modeling of adaptive injection strategies (AIS) in low

Figure 11. ISCO and ISHC emission characteristics of multiple injection strategies.

in the double injection after the end of the first injection have a large residual momentum. The penetration of the spray tip for multiple injections shows a similar increasing pattern for the single injection, but the spray tip penetration of the multiple injection increased faster than that of the single injection. In the case of the split injection, the increasing rate of the spray tip penetration for the first injection slowed from 2.0 ms after the start of the injection. (2) With regard to the combustion characteristics, combustion for the multiple injection occurs at a time as a result of the short injection interval and the hightemperature regions that are centered on the partially combustion chamber. However, the temperature distribution in the split injection obviously appears for the two hightemperature regions as the injection interval increases. In addition, the combustion pressure for the multiple injection is higher than that for the single injection, and the combustion pressures for all split injection conditions have two peaks, contrary to the combustion patterns of single and double injections. (3) With regard to the emission characteristics, the case of the split injection method was found to be more efficient than that of the double injection in reducing the NOx emissions. However, the soot emission from the split injection increases more than that for the double injection. Also, multiple injections with SOI after BTDC 30° show similar or lower HC and CO emissions compared to those of the single injection. 817

DOI: 10.1021/acs.energyfuels.5b02121 Energy Fuels 2016, 30, 810−818

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DOI: 10.1021/acs.energyfuels.5b02121 Energy Fuels 2016, 30, 810−818