FTIR photoacoustic spectroscopy of diesel lubricant oils containing particulate matter((c))

Monitoring of engine lubricants in service is important for evaluating engi ne conditions as well as the performance of new and existing lubricant form ulations. Fourier-transform infrared (FTIR) spectroscopy has previously bee n demonstrated as an effective technique for analyzing lubricant condition, but used oils laden with soot and other particulate matter are difficult t o analyze with common FTIR sampling techniques because of the strong light absorption and scattering by the particulates. FTIR photoacoustic spectrosc opy is examined here as an alternative sampling technique for analyzing par ticulate-laden oils. Diesel-engine oils spiked with carbon black and phoron e are analyzed quantitatively for particulate and carbonyl-species content using FTIR photoacoustic spectroscopy. Photoacoustics is found to be insens itive to the light scattering by particulate matter, but the strength of sp ectrum bands is reduced by the light absorption by particulate matter. A me thod is proposed to compensate for the loss of band strength so that quanti tative measurements can be made.