Attenuation effects on laser-based nitric oxide diagnostics in a heavy-duty diesel engine

A number of laser diagnostic techniques enabling a correction for laser intensity and detection efficiency of the raw NO-LIF data is presented. Laser beam transmission measurements at 226 nm, with the laser beam parallel to the cylinder axis, show that ∼ 20° aTDC the transmission is typically 1%; it increases to 50-80% later in the stroke, depending on engine load. Since absorption by CO₂ shows a strong wavelength dependence (increasing absorption for shorter wavelengths), the attenuation for 237 nm being equal to that for 226 nm suggests that CO₂ does not play a dominant role in the laser beam transmission. Since the overall laser beam transmission measurements show considerable attenuation, the bidirectional results suggest that most of the laser beam extinction occurs outside the field of view, e.g. in the piston bowl. Not only the laser beam, but also NO fluorescence is attenuated. Detection of NO LIF through a spectrograph allows quantitative comparison of the intensities of the observed fluorescence bands at 237, 248, and 259 nm. This is an abstract of a paper presented at the 30th International Symposium on Combustion (Chicago, IL 7/25-30/2004).