Publications

LII spectra and simultaneous imaging with 532 & 1064 nm excitation at LTC diesel engine conditions

Bobba, Mohan K.; Musculus, Mark P.

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.