Optical absorption under total internal reflection in the characterization of soot in diesel engine oils

Optical absorption of infrared radiation under total internal reflection in a novel sensor has been utilized to investigate the soot contamination of diesel engine oil by response surface methodology in factorial experiments. Sensor response showed highly significant dependence on oil soot concentration and temperature, in which the effect of the soot was greatest. The soot contamination of the optical rod in engine oil was found to be a surface phenomenon which showed little or no dependence on bulk oil shearing displacement below 500 rpm. The quadratic effect of sensor response to soot concentration was very high due to the agglomeration of soot particles, derived from the high surface energy of carbon soot. Test results of this optical absorption technique were in conformance with those other oil analysis techniques such as UV spectrophotometry, total acid number, viscosimetry, optical microscopy and EPMA. The technique proved to be more reliable than RDE emission spectrometry which showed ambiguous results due to colloidal suspension of soot particles in oil. Optical absorption proved to be an effective criterion in characterizing the soot contamination of diesel engine oil.