Ignition, soot formation, and end-of-combustion transients in diesel combustion under high-EGR conditions

The ignition, soot formation, and end of combustion transients of n-heptane and #2 diesel jets were investigated in an optically accessible constant-volume combustion vessel under high exhaust-gas recirculation (EGR) environments. A wide range of EGR levels were simulated by systematically decreasing the ambient oxygen concentration from 21 to 8 per cent, while holding other experimental conditions constant. Characteristics of the effect of EGR on the ignition transient include: development of a cool flame early after injection for all EGR levels, an increase in the premixed-burn (high-temperature combustion) ignition delay inversely proportional to ambient oxygen concentration, ([O2]−1), and lower apparent heat-release rates during the premixed-burn with increasing EGR. The timing of soot formation is strongly dependent upon EGR, and the time between ignition and the first soot formation increases with decreasing ambient oxygen concentration. Soot-forming fuel jets are shown to become soot-free at high-EGR conditions by reducing the injection duration to be less than the soot formation time, but longer than the ignition delay time (negative ignition dwell). While past studies show success in reducing soot formation when the injection duration is less than the ignition delay (positive ignition dwell), this result shows that high EGR can suppress soot formation even with negative ignition dwell, thereby permitting higher-load operation by using longer injection durations. At the end of injection, increasing EGR presents difficulties in completing combustion because of the lower ambient oxygen concentration. Despite eventually reaching the same pressure rise (i.e., combustion efficiency) more time is required for the higher EGR conditions to mix fuel with sufficient oxygen to complete combustion.

[1]  Charles J. Mueller,et al.  Investigation of Fuel Effects on Dilute, Mixing-Controlled Combustion in an Optical Direct-Injection Diesel Engine , 2007 .

[2]  Ingemar Denbratt,et al.  Low Soot, Low NOx in a Heavy Duty Diesel Engine Using High Levels of EGR , 2005 .

[3]  Sanghoon Kook,et al.  Visualization of Diesel Spray Penetration Cool-Flame Ignition High-Temperature Combustion and Soot Formation Using High-Speed Imaging. , 2009 .

[4]  Paul C. Miles,et al.  The Influence of Charge Dilution and Injection Timing on Low-Temperature Diesel Combustion and Emissions , 2005 .

[5]  Hajime Shimizu,et al.  Characteristics of Diesel Combustion in Low Oxygen Mixtures with Ultra-High EGR , 2006 .

[6]  Tie Li,et al.  Dependence of Ultra-High EGR and Low Temperature Diesel Combustion on Fuel Injection Conditions and Compression Ratio , 2006 .

[7]  Dennis L. Siebers,et al.  Soot in diesel fuel jets: effects of ambient temperature, ambient density, and injection pressure , 2004 .

[8]  Cherian A. Idicheria,et al.  Effect of EGR on diesel premixed-burn equivalence ratio , 2007 .

[9]  Caroline L. Genzale,et al.  Effect of Ignition Delay on In-Cylinder Soot Characteristics of a Heavy Duty Diesel Engine Operating at Low Temperature Conditions , 2009 .

[10]  A. G. Gaydon.  The spectroscopy of flames , 1957 .

[11]  M. Musculus,et al.  Multiple Simultaneous Optical Diagnostic Imaging of Early-Injection Low-Temperature Combustion in , 2006 .

[12]  Hua Zhao,et al.  Understanding the Effects of Recycled Burnt Gases on the Controlled Autoignition (CAI) Combustion in Four-Stroke Gasoline Engines , 2001 .

[13]  H. Kosaka,et al.  Investigation of early soot formation process in a diesel spray flame via excitation—emission matrix using a multi-wavelength laser source , 2008 .

[14]  Michael Weißbäck,et al.  Alternative combustion@@@Alternative Brennverfahren — Ein Ansatz für den zukünftigen Pkw-Dieselmotor: An approach for future HSDI diesel engines , 2003 .

[15]  Dennis L. Siebers,et al.  Diesel-Spray Ignition and Premixed-Burn Behavior , 2000 .

[16]  Dennis L. Siebers,et al.  Liquid-Phase Fuel Penetration in Diesel Sprays , 1998 .

[17]  Dennis L. Siebers,et al.  Relationship Between Ignition Processes and the Lift-Off Length of Diesel Fuel Jets , 2005 .

[18]  Cherian A. Idicheria,et al.  Soot Formation in Diesel Combustion under High-EGR Conditions , 2005 .

[19]  L. Pickett,et al.  Formaldehyde Visualization near Lift-off Location in a Diesel Jet. , 2006 .

[20]  John B. Heywood,et al.  Internal combustion engine fundamentals , 1988 .

[21]  Shuji Kimura,et al.  Ultra - Clean Combustion Technology Combining a Low - Temperature and Premixed Combustion Concept fo , 2001 .

[22]  K. Akihama,et al.  Mechanism of the Smokeless Rich Diesel Combustion by Reducing Temperature , 2001 .

[23]  Jerald A. Caton,et al.  Evaluation of the equivalence ratio-temperature region of diesel soot precursor formation using a two-stage Lagrangian model , 2006 .

[24]  Christoph Espey,et al.  Chemiluminescence Imaging of Autoignition in a DI Diesel Engine , 1998 .

[25]  G. Bruneaux,et al.  Combustion structure of free and wall-impinging diesel jets by simultaneous laser-induced fluorescence of formaldehyde, poly-aromatic hydrocarbons, and hydroxides , 2008 .