Mass Concentration of Diesel Particle Emissions from Photoacoustic and Opacity Measurements

Abstract

In this report we compare the capabilities of two optical devices—a conventional opacity meter and a recently developed photoacoustic instrument—for measurement of diesel particulate emissions. Emission measurements were performed using two vehicles (built in 1978 and 1980 and equipped with 5.7 liter diesel engines) operated at a variety of steady-state conditions and also over the federal test procedure driving cycle. The light absorbed by diesel particles heats them thereby increasing the ambient gas pressure. The change in pressure, which can be related to the particle mass concentration, is measured in the photoacoustic instrument. This is about 100 times more sensitive than the opacity meter so that it can be used to measure particulate emissions even in diluted exhaust. Further, its operation in the infrared region ensures that particle size variations do not affect its calibration against mass concentration. However, the observed optical data, both in the visible and in the infrared region, are dependent on engine operating conditions, which indicates that other particle characteristics, such as shape, are significant. Consequently, the measurement of absolute mass concentrations with the photoacoustic instrument has some of the uncertainties experienced with opacity meters.