Degradation of Nonmethane Hydrocarbon Oxidation Efficiency of a Catalyzed Diesel Particulate Filter during Aging

Upcoming 2013 on-board diagnostic (OBD) requirements mandate that catalyzed diesel particulate filter (CDPF) nonmethane hydrocarbon (NMHC) oxidation efficiency degradation be monitored as the filter ages. The malfunction indicator lamp (MIL) is to light when exhaust NMHC levels are greater than two times the legal limit or when the CDPF has no detectable amount of NMHC conversion capability. In this study, a CDPF was thermoelectrically aged at 650°C from the degreened (near-new) to end of useful life (EUL) stage. CDPF total hydrocarbon (THC) oxidation efficiency was measured periodically using a suite of steady-state speed and load conditions ("snapshot test"). The snapshot test was performed after every seven hours of filter aging. Snapshot test data of steady-state THC oxidation efficiency change across the filter at the chosen operating conditions shows no discernible, consistent degradation as the filter is aged. When uncertainty analysis is applied to the results, the story is even clearer: It is impossible to tell, with statistical significance, the age of a filter from THC oxidation efficiency using this methodology. Light-off shift - the gradual upward shift in light-off temperature due to advanced catalyst age - shows promise as a potential method for determining CDPF age with respect to NMHC oxidation efficiency. Light-off temperatures of degreened and EUL filters are separated by more than 50°C. Using this temperature shift as a diagnostic tool has already been successfully applied to diesel oxidation catalysts (DOCs), but will require different strategies and detection algorithms to apply to CDPFs.