Various pilot-main injection strategies are investigated in a single-cylinder optical Diesel engine. It is observed that as the dwell between a single pilot and the main injection is decreased toward zero, combustion noise passes through a minimum, and a reduction of 3 dB is possible. The injection schedules employed in the engine are analyzed with a hydraulic injection analyzer to provide rate shapes for each of the dwells tested. Two distinct injection events are observed even at the shortest dwell tested, and various rate shaping effects are observed with the main injection event as the dwell is adjusted. High-speed elastic scattering imaging of liquid fuel is performed in the engine to examine initial spray penetration rates. These compare favorably to the measured rates of injection, thus providing evidence that rate shaping of the initial phase of the main injection occurs in the engine and that this rate shaping is consistent with the injection rate data. However, experimental evidence suggests that these changes are not responsible for the observed trend in combustion noise as dwell changes. The combination of thermodynamic data and rate of injection data supports the theory that the main injection interacts with the pilot mixture field and influences its heat release process, thus playing a role in decreasing combustion noise. The relative phasing of the pilot and main heat release may play a significant role in reducing combustion noise; further studies will focus on combustion phasing effects.

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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 petroleumbased 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.lvol. 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.

In this work, linear, non-linear and a generalized renormalization group (RNG) two-equation RANS turbulence models of the k-epsilon form were compared for the prediction of turbulent compressible flows in diesel engines. The objectoriented, multidimensional parallel code FRESCO, developed at the University of Wisconsin, was used to test the alternative models versus the standard k-epsilon model. Test cases featured the academic backward facing step and the impinging gas jet in a quiescent chamber. Diesel engine flows featured high-pressure spray injection in a constant volume vessel from the Engine Combustion Network (ECN), as well as intake flows in a high-swirl diesel engine. For the engine intake flows, a model of the Sandia National Laboratories 1.9L light-duty single cylinder optical engine was used. An extensive experimental campaign provided validation data in terms of ensemble averages of planar PIV measurements at different vertical locations in the combustion chamber, for different swirl ratio configurations during both the intake and the compression strokes. The generalized RNG k-epsilon model provided the best accuracy trade-off for both swirl and shear flows, thanks to the polynomial expansion of coefficients C1 and C2 in the RNG kepsilon model with an effective 'dimensionality' of the strain rate field. Similar performance was seen across linear and nonlinear RNG models, which achieves good prediction of incylinder swirl flows; however, they noticeably underpredict jet penetration in the case of high-pressure sprays, suggesting the additional computational cost and lower stability of the nonlinear model do not justify greater suitability for engine calculations.