Particulate filters are a widely used emission control device on heavy-duty diesel engines. The accumulation of particulate matter, mostly consisting of soot, inside the filter results in increased filter pressure-drop (backpressure). This increased backpressure has been used by the on-board control systems as trigger for regeneration procedures, which aim to actively oxidize the accumulated soot. However, it is known that passive soot oxidation during normal operation affects the correlation between backpressure and the deposited soot mass in filter. Therefore, the backpressure alone cannot be a reliable trigger for regeneration. In this work we highlight operating conditions with very poor correlation between backpressure and accumulated soot mass in filter and evaluate the possible root causes.
Experiments with several heavy-duty diesel engines and particulate filters were conducted on engine test bench. These experiments involved prolonged operation under conditions emulating real-world operation. Under certain conditions, the backpressure of the filter did not correlate with the accumulated soot mass: although the soot mass increased monotonically, reaching the regeneration soot mass limit, the measured backpressure corresponded to the backpressure of an almost clean filter. The quality of correlation was found to depend on the operating conditions, PM/NOx engine out emissions as well as on the type of filter used.
To investigate these complex interactions, we used an extensive experimental protocol to calibrate a physico-chemically informed mathematical model of the soot oxidation and the soot properties in the filter. We employed this model to investigate the possible reasons for the apparent lack of correlation between backpressure and accumulated soot mass. The simulation study confirmed the experimental findings: even minor passive soot oxidation may destroy the correlation between soot loading and filter backpressure. Moreover, simulation revealed the relative contribution of the following factors on the filter backpressure: microscopically, changes in the properties of the deposited soot, due to oxidation and compaction; macroscopically, non-uniform soot distribution inside the filter channels, due to transient flow and passive soot oxidation. Finally, simulations showed that the ratio of incoming soot from the engine to oxidizing soot in filter has a major impact on the potential to re-establish a correlation between soot loading and backpressure.
