Dual-Laser LIDELS: An Optical Diagnostic for Time-Resolved Volatile Fraction Measurements of Diesel Particulate Emissions

Double-pulse laser-induced desorption with elastic laser scattering (LIDELS) is a diagnostic technique capable of making time-resolved, in situ measurements of the volatile fraction of diesel particulate matter (PM). The technique uses two laser pulses of comparable energy, separated in time by an interval sufficiently short to freeze the flow field, to measure the change in PM volume caused by laser-induced desorption of the volatile fraction. The first laser pulse of a pulse-pair produces elastic laser scattering (ELS) that gives the total PM volume, and also deposits the energy to desorb the volatiles. ELS from the second pulse gives the volume of the remaining solid portion of the PM, and the ratio of these two measurements is the quantitative solid volume fraction.

In an earlier study, we used a single laser to make real-time LIDELS measurements during steady-state operation of a diesel engine. In this paper, we discuss the advantages and disadvantages of the two LIDELS techniques and present measurements made in real diesel exhaust and simulated diesel exhaust created by coating diffusion-flame soot with single-component hydrocarbons. Comparison with analysis of PM collected on quartz filters reveals that LIDELS considerably under-predicts the volatile fraction. We discuss reasons for this discrepancy and recommend future directions for LIDELS research.