The passive oxidation of particulate matter (PM) in a diesel
catalyzed particulate filter (CPF) was investigated in a series of
experiments performed on two engines. A total of ten tests were
completed on a 2002 Cummins 246 kW (330 hp) ISM and a 2007 Cummins
272 kW (365 hp) ISL. Five tests were performed on each engine to
determine if using engine technologies certified to different
emissions regulations has an impact on the passive oxidation
characteristics of the PM.
A new experimental procedure for passive oxidation testing was
developed and implemented for the experiments. In order to
investigate the parameters of interest, the engines were initially
operated at a steady state loading condition where the PM
concentrations, flow rates, and temperatures were such that the
accumulation of PM within the CPF was obtained in a controlled
manner. This engine operating condition was maintained until a CPF
PM loading of 2.2 ±0.2 g/L was obtained. The engine operating
conditions were then changed in order to vary the parameters of
interest and perform the passive oxidation stage of the test.
The test matrix was designed to concentrate on the variables
that most affect passive oxidation of PM within the CPF. These
variables include NO₂ and NO concentrations into the CPF, the
NO₂/PM and NOx/PM ratios at the inlet of the CPF,
exhaust temperature, and the exhaust flow rate. The test matrix
provided for a wide range of conditions including average CPF
temperatures from 260 to 460°C, NO₂/PM mass ratios from 3 to 59,
NOx/PM mass ratios from 8 to 160, and exhaust mass flow
rates of 5.6 to 18.0 kg/min. By gaining a better understanding of
how these variables affect passive oxidation, engine operating
conditions can be selected that optimize the use of this
regeneration method. This will reduce the dependency on active
regenerations to clean the filter and the fuel penalty associated
with this strategy.
Experimental results during the loading stage of the tests
demonstrated that the PM generated by each engine under similar
operating conditions had similar oxidation rates with different NO₂
and NO concentrations at the CPF inlet. Results from the passive
oxidation portion of the experiments show a strong correlation
between the reaction rate and the CPF average temperature. When
performing a mass balance across the CPF, calculations for some of
the experiments indicate that at temperatures less than 400°C,
assuming that PM oxidation is only accomplished by NO₂ oxidizing
the PM, more NO₂ is consumed in the CPF than what is available at
the CPF inlet. The hypothesis was that back diffusion of NO₂ from
the catalyst on the CPF substrate wall occurs. Calculations of the
Péclet number within the CPF showed that the transport of NO₂ is
dominated by diffusion under the operating conditions in these
experiments. This finding indicates that additional NO₂ could be
back diffusing into the PM cake layer after NO is oxidized into NO₂
at the CPF catalyst surface which would then account for the
additional PM oxidation that occurred in the CPF.