effects of split injection, oxygen enriched air, and heavy egr rate on soot emissions in a diesel engine

Effects of Split Injection, Oxygen Enriched Air, and Heavy EGR Rate on Soot Emissions in a Diesel Engine Nguyen Le Duy Khai The Graduate School Sungkyunkwan University Department of Mechanical Engineering Effects of Split Injection, Oxygen Enriched Air, and Heavy EGR Rate on Soot Emissions in a Diesel Engine Nguyen Le Duy Khai A Dissertation Submitted to the Department of Mechanical Engineering and the Graduate School of Sungkyunkwan University in partial fulfillment of the requirements for the degree of Doctor of Philosophy June 2009 Approved by Professor Nakwon Sung Major Advisor Contents List of Tables ii List of Figures iii Nomenclature viii Chapter Introduction Chapter Soot Model Chapter Results and Discussion 3.1 General results 25 3.2 Effects of split injection 42 3.3 Effects of oxygen enriched air (OEA) and split injection 57 3.4 Effects of heavy EGR, OEA, and split injection 78 Chapter Conclusions 94 References 97 Abstract 106 - i - List of Tables Table Data for carbon 14 Table Rate of change of mass fraction of species 20 Table Engine specifications 23 Table Computational test conditions 23 Table Mass of species in the intake air 79 - ii - List of Figures Figure Schematic diagram of flame in diesel engine (Dec, 1995) Figure Schematic diagram of the soot model 12 Figure Saturation vapor pressure of carbon as function of temperature 15 Figure Comparison of net acetylene mass between Lee model and Fusco 21 model in case of single injection without EGR Figure Comparison of net soot mass between Lee model and Fusco model 22 in case of single injection without EGR Figure Schemes of split injection and single injection 24 Figure The computational meshes at 3oBTDC 24 Figure Comparison of calculated and measured cylinder pressure and heat 26 release rate in case of single injection without EGR Figure Comparison of calculated and measured cylinder pressure in case of 27 split injection Figure 10 Comparison of soot and NOx emissions between calculation and 28 experiment of single injection with different injection timing Figure 11 Comparison of NOx and soot emissions with various amount of fuel 29 injected during the first pulse of split injection without EGR Figure 12 Distributions of temperature and soot on the cutting plane at 30 4oATDC in case of single injection without EGR Figure 13 Variation in heat release rate 31 - iii - Figure 14 Variation in average gas temperature 32 Figure 15 Variations in mass of precursor formation, conversion into soot 33 particles, oxidation, and net precursor Figure 16 Results at 10oATDC 34 Figure 17 Variation in gas mass fraction with φ = 1.2-2.0 35 Figure 18 Variations in acetylene formation, oxidation, consumption, and net 36 acetylene Figure 19 Rate of acetylene formation, oxidation, and consumption at 37 10oATDC Figure 20 Rate of acetylene formation, oxidation and consumption at 38 90oATDC Figure 21 Distribution of acetylene and soot at 10oATDC 39 Figure 22 Variations in soot formation, soot oxidation and net soot 40 Figure 23 Summarized diagram of soot production 41 Figure 24 Comparison of heat release rate between single injection and split 42 injection Figure 25 Variation in average gas temperature 43 Figure 26 Variation in precursor formation, conversion into soot, and 44 oxidation Figure 27 Variations in precursor formation rate 45 Figure 28 Distribution of equivalence ratio, temperature and precursor 46 formation rate at 15oATDC - iv - Figure 29 Variations in rich mixture (φ = 1.2-2.0) mass fraction 47 Figure 30 Variations in net precursor 48 Figure 31 Variations in acetylene formation, oxidation and consumption 49 Figure 32 Variations in net acetylene 49 Figure 33 Variations in soot formation and oxidation 50 Figure 34 Distributions of temperature and soot formation rate at 90oATDC 51 Figure 35 Reaction rate of soot oxidation at 20o, 50o and 90oATDC 52 Figure 36 Variations in net soot 54 Figure 37 Variations in NOx 54 Figure 38 Diesel emissions of various schemes 55 Figure 39 Indicated power 56 Figure 40 Variations in burned fuel 57 Figure 41 Variations in average gas temperature 58 Figure 42 Distribution of equivalence ratio, temperature, and precursor 59 formation rate at 10oATDC Figure 43 Variations in rich mixture (φ = 1.2 - 2.0) mass fraction 60 Figure 44 Variations in precursor formation, conversion into soot, and 61 oxidation Figure 45 Variations in net precursor 61 Figure 46 Variations in acetylene formation, oxidation, and consumption 62 Figure 47 Reaction rate of acetylene formation, oxidation, and consumption 64 Figure 48 Reaction rate of acetylene consumption at 30oATDC 65 -v- Figure 49 Variations in mass of net acetylene 65 Figure 50 Variations in soot formation and oxidation 66 Figure 51 Reaction rate of soot formation and soot oxidation at 90oATDC 67 Figure 52 Variations in net soot 68 Figure 53 Variations in NOx 68 Figure 54 Variations in average gas temperature 69 Figure 55 Variations in burned fuel 70 Figure 56 Variations in net precursor 71 Figure 57 Variations in net acetylene 72 Figure 58 Reaction rates of acetylene formation, oxidation, and growth with 74 soot Figure 59 Variations in soot formation and oxidation 74 Figure 60 Distributions of soot formation rate and oxidation rate at 90oATDC 75 Figure 61 Variations in net soot 76 Figure 62 Trend of diesel emissions with various oxygen concentrations 77 Figure 63 Effect of EGR ratio on soot emissions in case of single injection 78 Figure 64 Variations in temperature 80 Figure 65 Variations in NOx 81 Figure 66 Results at 10oATDC 82 Figure 67 Distributions of temperature at 20oATDC 83 Figure 68 Variations in rich-mixture mass percentage 83 Figure 69 Variations in precursor formation and net precursor 84 - vi - Figure 70 Variations in acetylene formation, oxidation and consumption 84 Figure 71 Variations in net acetylene 86 Figure 72 Variations in reaction rate of acetylene formation and growth with 87 soot Figure 73 Variations in soot formation and oxidation 88 Figure 74 Reaction rate of soot formation and oxidation at 90oATDC 89 Figure 75 Variations in net soot 90 Figure 76 Variations in NOx 91 Figure 77 Diesel emissions with different schemes 91 Figure 78 Indicated power 92 - vii - Chapter Introduction In the past, diesel engines were used mainly in heavy duty trucks and buses, however many passenger cars and SUVs have been changed over to diesel nowadays because diesel engines offer more benefits than gasoline engines Diesel engines are designed to have higher compression ratio which gives them more torque Another advantage of diesel engines is lower fuel consumption There is no doubt that diesel vehicles have been gained more appreciation from consumers and the number of diesel vehicles will be raised in the future As the number of diesel engines increases, reducing their emissions has become more important Nitrogen oxides (NOx) and soot emissions in diesel engines are the most challenging to reduce Nitrogen oxides are efficiently controlled in the combustion process by exhaust gas recirculation (EGR) However, a decrease in NOx is usually accompanied by an increase in soot An aftertreatment device such as a diesel particulate filter is needed to control soot emissions, however using this device requires additional cost, including equipment cost, operation cost, and maintenance cost A strategy to reduce NOx and soot simultaneously without after treatment devices is necessary; for example, a combination of split injection with oxygen enriched air (OEA) and EGR Split injection has been used as a method to reduce soot emissions in diesel engines In 1990, Schulte et al [1] showed the possibility of applying split injection with an electronic unit injector Split injection became more practical later with the development -1- Chapter Conclusions The modified Foster soot model, which was incorporated into the KIVA-3V code, is used to calculate the soot emissions in a diesel engine with a variety of engine techniques such as split injection, oxygen enriched air, heavy EGR, and combination of them In this model, soot is formed through a series of eight reactions including fuel pyrolysis to precursor and acetylene, inception of soot particles from precursors, surface growth of soot particles with acetylene, coagulation of soot particles, and oxidation of precursors, acetylene and soot particles Precursors, which are one of two products of fuel pyrolysis, are converted into initial soot particles due to the vapor-solid phase equilibrium Acetylene is another product of fuel pyrolysis Together with precursor, acetylene plays important role in control soot emissions When split injection is applied, a dwell time between two injections causes the decrease in local temperature and the fuel-rich area in the flame, which leads to the significant decrease in precursor formation Thus, the soot formation in split injection is decreased compared to single injection When fuel is injected by the second pulse, the downward movement of the piston during the expansion stroke sucks fuel to the bottom of the piston bowl, resulting in the increase in stoichiometric area in the cylinder This fact, together with the increase in temperature due to a secondary combustion of second injected fuel, leads to the increase in soot oxidation rate of split injection Net soot mass of split injection is lower than that of single injection in consequence of lower soot - 94 - formation and higher soot oxidation Compared to single injection, the soot emissions are reduced by 15% with split injection scheme of 75.8.25 The decrease in temperature during the early stage of combustion also results in a 40% decrease in NOx emissions of this case When the oxygen concentration in the intake air is increased, fuel is burned faster resulting in the increase in temperature and thus, the increase in soot formation rate during the early stage of combustion However, the soot formation rate is decreased during the later stage of combustion The reason is that the increase in temperature leads to the increase in consumed acetylene but does not enhance the acetylene formation As a result, net acetylene is reduced The concentration of net acetylene in the cylinder affects strongly the soot formation process and thus the soot emissions The lower concentration of acetylene results in the lower soot formation Besides, the soot oxidation rate is improved due to the increase in temperature Because of lower soot formation rate and higher soot oxidation rate, net soot mass of OEA case is decreased during the later stage of combustion Compared to single injection, the combination of split injection with 22% oxygen concentration in volume shows a 47% decrease in soot emissions with a penalty of an 18% increase in NOx emissions A combination of split injection with high EGR ratio and high oxygen concentration is the best solution to reduce diesel emissions Split injection helps reducing soot emissions The high EGR ratio causes a lower temperature at the beginning of combustion process, which results in low NOx emissions The high oxygen concentration increases temperature during the later stage of combustion, thus compensates for the drawback of EGR on soot emissions and keeps soot emissions at - 95 - low level In this study, the split injection scheme of 75.8.25 with 23% O2 in volume and 30% EGR ratio reduces soot emissions by 45%, with NOx emissions is remained as the same level as in single injection In conclusion, this study shows that with an appropriate use of combined engine technologies such as split injection, high OEA and high EGR, the soot emissions of a diesel engine can be reduced significantly without penalty in NOx emissions - 96 - References Schulte, H., Duernholz, M., and Wuebbeke, K., “The Contribution of the Fuel Injection System to Meeting Future Demands on Truck Diesel Engines,” SAE paper No 900822, 1990 Miyaki, M., Fujisawa, H., Masuda, A., and Yamamoto, Y., “Development of New Electronically Controlled Fuel Injection System ECD-U2 for Diesel Engines,” SAE paper No 910252, 1991 Stumpp, G and Ricco, M., “Common Rail – an Attractive Fuel Injection System for Passenger Car DI Diesel Engines,” SAE paper No 960870, 1996 Bianchi, G M., Pelloni, P., Corcione, F E., and Luppino, F., “Numerical Analysis of Passenger Car HSDI Diesel Engines with the 2nd Generation of Common Rail Injection Systems: The Effect of Multiple Injections on Emissions,” SAE paper No 2001-01-1068, 2001 Nehmer, D A and Reitz, R D., “Measurement of the Effect of Injection Rate and Spilt Injections on Diesel Engine Soot and NOx Emissions,” SAE paper No 940668, 1994 Tow, T C., Pierpont, D A., and Reitz, R D., “Reducing Particulate and NOx Emissions by Using Multiple Injections in a Heavy Duty D.I Diesel Engine,” SAE paper No 940897, 1994 Han, Z., Uludogan, A., Hampson, G J., and Reitz, R D., “Mechanism of Soot and NOx Emission Reduction Using Multiple-Injection in a Diesel - 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104 - Turbulent Combustion with Special Emphasison Soot Formation and Combustion,” The 16th Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, pp 719-729, 1977 61 Rakopoulos, C.D., Hountalas, D.T., Zannis, T.C., and Levendis, Y.A., “Operation and Environmental Evaluation of Diesel Engines Burning Oxygen-Enriched Intake Air or Oxygen-Enriched Fuels: A Review,” SAE paper No 2004-01-2924, 2004 - 105 - ABSTRACT Effects of Split Injection, Oxygen Enriched Air and Heavy EGR Ratio on Soot Emissions in a Diesel Engine Nguyen Le Duy Khai Department of Mechanical Engineering Sungkyunkwan University The effects of split injection, oxygen enriched air, and heavy exhaust gas recirculation (EGR) on soot emissions in a direct injection diesel engine are studied using the KIVA3V code When split injection is applied, a dwell time between two injections causes the decrease in local temperature and the fuel-rich area in the flame, which leads to the significantly decrease in precursor formation Thus, the soot formation in split injection is decreased compared to single injection The second injection of fuel into a cylinder results in two separate stoichiometric zones which help increase soot oxidation As a result of soot formation decrease and soot oxidation increase, the soot emissions are decreased When oxygen enriched air is applied together with split injection, a higher concentration of oxygen results in more burned fuel and higher temperature in the - 106 - cylinder The increase in temperature promotes the growth reaction of acetylene with soot, however it does not enhance the acetylene formation during the second injection of fuel As more acetylene is consumed in the growth reaction with soot, the concentration of acetylene in the cylinder is decreased, which leads to a decrease in soot formation With an increase in soot oxidation caused by split injection and a more decrease in soot formation caused by oxygen enrichment, the soot emissions are decreased significantly, with a small penalty in NOx emissions To take the full advantages of oxygen enriched air in reduction of soot and of EGR in reduction of NOx, a combination of split injection with high concentration of oxygen and high EGR ratio is used This combination shows the best results in terms of diesel emissions In this study, the split injection scheme of 75.8.25 with an oxygen concentration of 23% in volume and an EGR ratio of 30% shows a 45% reduction in soot emissions with the same NOx emissions as in single injection This study shows that with a suitable combination of engine technologies, the soot emissions of a diesel engine can be reduced significantly without penalty in NOx emissions - 107 - .. .Effects of Split Injection, Oxygen Enriched Air, and Heavy EGR Rate on Soot Emissions in a Diesel Engine Nguyen Le Duy Khai A Dissertation Submitted to the Department of Mechanical Engineering... 59 Variations in soot formation and oxidation 74 Figure 60 Distributions of soot formation rate and oxidation rate at 90oATDC 75 Figure 61 Variations in net soot 76 Figure 62 Trend of diesel emissions. .. Figure 73 Variations in soot formation and oxidation 88 Figure 74 Reaction rate of soot formation and oxidation at 90oATDC 89 Figure 75 Variations in net soot 90 Figure 76 Variations in NOx 91