(174b) Durability Analysis of a Field-Aged Diesel Particulate Filter Used in a School Bus Retrofit Program

A field-aged, passive diesel particulate filter (DPF) employed in a school bus retrofit program was evaluated for emission reductions of particulate matter (PM) mass, PM number and total hydrocarbon (HC) concentration. In addition, the effect of the DPF on nitrogen oxides (NOx) emissions, including the fraction of NO2, was measured. As part of the Environmental Protection Agency (EPA) Clean Diesel Program, the DPF accumulated several years of use in the Wissahickon School District in Pennsylvania. It was removed from the exhaust system and mounted on a test engine of similar make and model as that of the school bus for bench dynamometer tests. The performance of the DPF was evaluated as-received and after regeneration over the heavy-duty federal test procedure (FTP) transient and 13-mode steady-state cycles. For the PM mass measurements, filter samples were collected with a BG-3 partial flow sampling system (Sierra Instruments Inc.) and analyzed gravimetrically. For the PM number concentration measurements, the exhaust was sampled using a micro-dilution tunnel. Total number concentration and number-size distributions were measured by a TSI Inc. 3025 condensation particle counter (CPC) and 3936 scanning mobility particle sizer (SMPS), respectively. The results of the evaluation show that, after multi-year use, the DPF was effective in PM removal. The PM mass emissions rate was reduced on average by about 95 ± 2% for the hot and cold FTP cycles. The PM number concentration was reduced approximately 10,000-fold over the 13-mode steady state cycle. PM mass emissions decreased after DPF regeneration. However the number concentration of DPF-emitted particles increased by about a factor of 20; this is comparing the average number concentration during hot FTPs before and after regeneration. The number concentration returned to pre-regeneration values after about 2 hrs of transient cycle operation. For the gaseous species, the effects were less substantial. The HC emissions reduction was about 45%. The concentration of NOx emitted by the DPF remained the same as that of the engine baseline. The fraction of NO2 was elevated from about 0-8% at the DPF inlet to 3-32% at the DPF outlet over the 13-mode cycle. Overall, the study provides new information on field-aged DPF PM emissions before, during and after regeneration and supports evidence of more than adequate durability of retrofit DPFs.