Publications

Laser-induced incandescence (LII) of soot has commonly been implemented using Nd:YAG harmonics at 532 or 1064 nm. Recent atmospheric-pressure flame studies have shown that significant C2 and C3 fluorescence interference can arise at fluences as low as 0.2 J/cm2 at 532 nm and 1064 nm. This paper explores LII fluorescence interference in a low-temperature combustion (LTC) diesel engine. Results show that the spectral and spatial distributions of LII for mature soot are similar at 532 and 1064 nm. Closer to the onset of soot formation, however, the 532-nm LII spectrum has strong blue-shifted broadband emission compared to 1064-nm LII, but without any clear evidence of C2 or C3 fluorescence, even at high fluences. The 532-nm laser-induced emission is initially distributed over most of the diesel jet, while the 1064-nm signal is negligible. We speculate that the broadband blue-shifted signal for 532-nm LII is most likely fluorescence from soot precursor species like polycyclic aromatic hydrocarbons (PAHs), whereas the 1064-nm signal is primarily true soot LII. These results suggest that for LTC diesel engines, the emission arising from 532-nm excitation close to the onset of soot formation will likely contain significant broadband fluorescence interference relative to the true LII signal.

Low temperature combustion (LTC) strategies, which can mitigate emissions of particulate matter (PM) and nitrogen oxides (NOx) from diesel engines, typically have longer ignition delays compared to conventional diesel operation. With extended ignition delays, more time is available for premixing, which reduces PM formation. The effect of varying ignition delay on the spatial and temporal evolution of soot in LTC diesel jets is studied by imaging the natural soot luminosity, while the in-cylinder soot mass and temperature are measured using two-color soot thermometry. Ignition delay in the engine is controlled by adjusting the intake air temperature while keeping the same charge density at TDC. This allowed us to study sooting characteristics at various ignition delays while keeping the same diesel jet penetration for all the cases. Results show a 95% decrease in the total in-cylinder soot mass as ignition delay increases from 3 to 15 crank angle degrees (CAD) at an engine speed of 1200 RPM. Furthermore, the structure of the sooty regions in the jet is strongly affected by the ignition delay. For a short ignition delay of 3 CAD, soot formation originates downstream in the jet, 25 mm from the injector. After the end of injection, the sooty region first spreads back to the injector and then it is rapidly oxidized in the near-injector region within a few crank angle degrees. This suggests that rapid mixing occurs in the near injector mixtures just after the end of injection, which promotes soot oxidation. For a longer ignition delay of 15 CAD, soot first appears farther downstream in the jet, and it does not spread back to the injector. Indeed, soot never forms in the jet near the injector when the ignition delay is long, indicating that those regions do not promote soot formation, likely because they become too fuel-lean during the ignition delay.