Radu Florea

Chemiluminescence imaging of pre-injection reactions during engine starting

The thermal and chemical state of residual gas is known to influence the likelihood of autoignition, ignition delay and combustion phasing of the subsequent diesel engine cycle. To elucidate the role of residual gases in these processes, ultraviolet chemiluminescent reactions and their spectra are observed during the pre-injection, compression period in a dynamometer-driven, optically-accessible, diesel engine operated with a single fuel injection event. During a cold start sequence, while the engine is motored and fuel is injected without firing, the pre-injection chemiluminescence (PIC) intensity increases from cycle to cycle. This leads to a second mode of intermittent firing cycles which are observed to follow a higher intensity of PIC. In the third mode, decreased PIC intensity is measured in firing cycles that are preceded by partial misfires. In the fourth mode, firing is continuous, but with a high IMEP coefficient of variation (COV). Here, PIC intensity is found to strongly correlate with advanced combustion phasing. As firing continues, it is observed that COV, PIC intensity and the phasing correlation decrease. Upon fuel shutoff, PIC intensity decays with time. Spectral measurements confirm that reactions of low temperature combustion intermediates, including chemiluminescent formaldehyde (HCHO*) and CHO* comprise the observed PIC.

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.

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