Design & Evaluation of an Exhaust Filtration Analysis (EFA) System

The Diesel Exhaust Filtration Analysis System (DEFA) developed at the University of Wisconsin Madison was modified to perform fundamental filtration experiments using particulate matter (PM) generated by a spark-ignition direct-injection (SIDI) engine fueled with gasoline. The newly modified system, termed the Exhaust Filtration Analysis (EFA) system, enables small-scale fundamental studies of wall-flow filtration processes. A scanning mobility particle sizer (SMPS) was used to characterize running conditions with unique particle size distributions (PSDs). The SMPS and an engine exhaust particle sizer (EEPS) were used to simultaneously measure the PSD downstream of the EFA and the real-time particulate emissions from the SIDI engine, to determine the evolution of filtration efficiency during filter loading. Corrections were developed for each running condition to compare measured PSDs between the EEPS and the SMPS in the raw, as well as, filtered exhaust stream. Background losses in the EFA system (without a filter sample) were quantified for each operating condition. Several steps were taken to minimize these losses using conventional knowledge on Brownian diffusion of particulates. Results from filtration experiments for one of the engine operating conditions using cordierite filter samples showed peak initial penetration for particles with mobility diameter (D_m) of approximately 100 nm. The most penetrating particle size reduced from approximately 90 nm to 60 nm during the filtration tests. A slight increase was observed in the penetration of particles less than 50 nm in mobility diameter, potentially due to increased velocities in the filter as flow area reduces during filter loading, or due to decreasing wall area for capture of particles by diffusion. Transition to soot cake filtration was not seen during filtration of SIDI particulates. Results from previous diesel exhaust filtration experiments using wafers with similar properties showed complete transition to cake filtration over comparable loading durations.