Kan Zha

COMPARISON OF SOOT EVOLUTION USING HIGH-SPEED CMOS COLOR CAMERA AND TWO-COLOR THERMOMETRY IN AN OPTICAL DIESEL ENGINE FUELED WITH B20 BIODIESEL BLEND AND ULTRA-LOW SULFUR DIESEL

Biodiesel is a desirable alternative fuel for the diesel engine due to its low engine-out soot emission tendency. When blended with petroleum-based diesel fuels, soot emissions generally decrease in proportion to the volume fraction of biodiesel in the mixture. While comparisons of engine-out soot measurements between biodiesel blends and petroleum-based diesel have been widely reported, in-cylinder soot evolution has not been experimentally explored to the same extent. To elucidate the soot emission reduction mechanism of biodiesel, a single-cylinder optically-accessible diesel engine was used to compare the in-cylinder soot evolution when fueled with ultra-low sulfur diesel (ULSD) to that using a B20 biodiesel blend (20% vol/vol biodiesel ASTM D6751-03A). Soot temperature and KL factors are simultaneously determined using a novel two-color optical thermometry technique implemented with a high-speed CMOS color camera having wide-band Bayer filters. The crank-angle resolved data allows quantitative comparison of the rate of in-cylinder soot formation. High-speed spray images show that B20 has more splashing during spray wall impingement than ULSD, distributing rebounding fuel droplets over a thicker annular ring interior to the piston bowl periphery. The subsequent soot luminescence is observed by high-speed combustion imaging and soot temperature and KL factor measurements. B20 forms soot both at low KL magnitudes over large areas between fuel jets, and at high values among remnants of the fuel spray, along its axis and away from the bowl edge. In contrast, ULSD soot luminescence is observed exclusively as pool burning on the piston bowl surfaces resulting from fuel wall impingement. The soot KL factor evolution during B20 combustion indicates earlier and significantly greater soot formation than with ULSD. B20 combustion is also observed to have a greater soot oxidation rate which results in lower engine-out soot emissions. Measured soot temperatures near 1875K were similar for the two fuels for the duration of combustion. For both fuels, higher fuel injection pressure led to lower late-cycle soot KL levels. The trends of soot natural luminosity correlated well with the trends of soot KL factor, suggesting that relatively simple measurements of combustion luminosity may provide somewhat quantitative information about in-cylinder soot formation and oxidation. The apparent rate of heat release (ARHR) analysis under steady skip-fire conditions indicates that B20 combustion is less sensitive to wall temperature than that observed with ULSD due to a lesser degree of pool burning. B20 was found to have both a shorter ignition delay and shorter combustion duration than ULSD.Copyright

Investigation of Low-Temperature Combustion in an Optical Engine Fueled with Low Cetane Sasol JP-8 Fuel Using OH-PLIF and HCHO Chemiluminescence Imaging

ABSTRACT Low cetane JP-8 fuels have been identified as being difficult to use under conventional diesel operation. However, recent focus on low-temperature combustion (LTC) modes has led to an interest in distillate hydrocarbon fuels having high volatility and low autoignition tendency. An experimental study is performed to evaluate low-temperature combustion processes in a small-bore optically-accessible diesel engine operated in a partially-premixed combustion mode using low-cetane Sasol JP-8 fuel. This particular fuel has a cetane number of 25. Both single and dual injection strategies are tested. Since long ignition delay is a consequence of strong autoignition resistance, under the conditions examined, low cetane Sasol JP-8 combustion can only take place with a double injection strategy: one pilot injection event in the vicinity of exhaust TDC and one main injection event near firing TDC. In this work, the effects of autoignition properties are examined by comparing the behavior of a high cetane number JP-8 fuel with that of a low CN Sasol JP-8. The double injection strategy also served to reduce pressure rise rates during operation at light load (2 bar IMEP) conditions. Dual injection timing is optimized for peak IMEP, at which point simultaneous OH Planar Laser-Induced Fluorescence (OH-PLIF) and high-speed crank-angle-resolved HCHO chemiluminescence imaging are performed to analyze the partially-premixed combustion process. Fuel efficiency and engine-out emissions performance are also presented. In terms of IMEP, low CN Sasol JP-8 fuel is shown to be a satisfactory fuel for low-temperature combustion under light-load condition using the proper dual injection strategy. However, partially-premixed combustion operation in this work results in higher UHC emissions and lower fuel efficiency when compared with the high cetane JP-8 fuel.

Soot Evolution With Cyclic Crank-Angle-Resolved Two-Color Thermometry in an Optical Diesel Engine Fueled With Biodiesel Blend and ULSD

Biodiesel is a desirable alternative fuel for the diesel engine due to its low engine-out soot emission tendency. When blended with petroleum-based diesel fuels, soot emissions generally decrease in proportion to the volume fraction of biodiesel in the mixture. While comparisons of engine-out soot measurements between biodiesel blends and petroleum-based diesel have been widely reported, in-cylinder soot evolution has not been experimentally explored to the same extent. To elucidate the soot emission reduction mechanism of biodiesel, a single-cylinder optically-accessible diesel engine was used to compare the in-cylinder soot evolution when fueled with ultra-low sulfur diesel (ULSD) to that using a B20 biodiesel blend (20% vol./vol. biodiesel ASTM D6751-03A). Soot temperature and KL factors are simultaneously determined using a novel two-color optical thermometry technique implemented with a high-speed CMOS color camera having wide-band Bayer filters. The crank-angle resolved data allows quantitative comparison of the rate of in-cylinder soot formation. High-speed spray images show that B20 has more splashing during spray wall impingement than ULSD, distributing rebounding fuel droplets over a thicker annular ring interior to the piston bowl periphery. The subsequent soot luminescence is observed by high-speed combustion imaging and soot temperature and KL factor measurements. B20 forms soot both at low KL magnitudes over large areas between fuel jets, and at high values among remnants of the fuel spray, along its axis and away from the bowl edge. In contrast, ULSD soot luminescence is observed exclusively as pool burning on the piston bowl surfaces resulting from spray wall impingement. The soot KL factor evolution during B20 combustion indicates earlier and significantly greater soot formation than with ULSD. B20 combustion is also observed to have a greater soot oxidation rate, which results in lower late-cycle soot emissions. For both fuels, higher fuel injection pressure led to lower late-cycle soot KL levels. The apparent rate of heat release (ARHR) analysis under steady skip-fire conditions indicates that B20 combustion is less sensitive to wall temperature than that observed with ULSD due to a lesser degree of pool burning. B20 was found to have both a shorter ignition delay and shorter combustion duration than ULSD.

Simultaneous High-Speed Two-Color Thermometry and Laser-Induced Incandescence Soot Measurement in a Small-Bore Optical Engine Fueled With JP-8

In-cylinder soot measurements obtained with a high-speed two-color method are compared to those simultaneously determined by the laser-induced incandescence (LII) technique in a single-cylinder, optically-accessible diesel engine fueled with JP-8. A double injection strategy was chosen to reduce pressure rise rates during operation at light load (2 bar IMEP) conditions. Injection timing was optimized for peak efficiency, at which point sufficient soot was produced to provide ample signal for both optical diagnostic techniques. Application of the two-color method to a highspeed CMOS camera allows the crank-angle-resolved observation of soot temperature and soot optical depth (KL) evolution, while LII provides soot volume fraction distribution at a known axial location in the cylinder independent of combustion gas temperature. Comparison of soot KL and LII signal at various stages of combustion shows high spatiallyaveraged correlation of the two signals near TDC. The degree of correlation decreases as the piston bowl descends and the line-of-sight soot KL value increasingly includes soot volumes not in the path of the laser sheet, the location of which is fixed 6.5 mm below the fire deck. The correlation between the two parameters again increases during the late cycle, indicating that in the later phases of combustion soot occurs in the squish zone above the piston bowl. Spatial cross-correlation of the two signals is weak, but increases in the highly luminous period immediately following heat release and illustrating a high degree of soot stratification. Soot KL and temperature evolution over a cycle are presented, which show no indication of being affected by the LII laser fluence.

A CFD Study of the Effect of HCHO Addition on Autoignition and Combustion

Experimental and theoretical research [1] targeted towards the effect of formaldehyde on combustion has identified its OH-scavenging role and recent data suggests it plays an important role in combustion instability observed during engine cold-starting. This effect was further studied using a CFD approach and the kinetic inhibiting effect of formaldehyde on the combustion process has been found to be enhanced by thermal/diffusion effects.Copyright