**4. Conclusions**

In this work, starting from the results previously obtained on soot oxidation during DPF regeneration [21–23], the research focused on the detailed PSD analysis of particles emitted during the active regeneration of a CuFe2O4 catalyzed DPF at the exhaust of an L-D common-rail diesel engine. Different tests were performed under different engine operating conditions, aiming at estimating the PN removal efficiency of the CDPF in the range of 5–100 nm. The filtration efficiency in the investigated PN range was evaluated by alternately sampling the particles upstream and downstream of the CDPF. In particular, the dynamics of the PN size distributions during the regeneration process were investigated. The results of the experimental tests evidenced that, during the start-up of the regeneration, differently from the standard DPF, no significant increase in particle emissions at the CDPF outlet was observed. In fact, particle emissions three orders of magnitude lower than those detected when regenerating the standard filter were observed, and they remained two orders of magnitude lower for particle sizes larger than 50 nm. With the regeneration going on, in the range of 200–450 s, the PSDs measured at the CDPF outlet exhibited the bimodality observed during the accumulation phase, with a peak that was three orders of magnitude lower than that detected in the case of the bare DPF. At the end of the regeneration, characterized by a quasi-constant pressure drop across the CDPF, an impressive reduction in PN emissions was still observed vs. the standard DPF, with PN values that were two and three orders of magnitude lower for the particle sizes of 5 nm and above 50 nm, respectively. This very important result highlighted that the use of a well-designed CDPF can allow the oxidation of the accumulated soot at lower temperatures and a decrease in particle emissions during the regeneration phase, not only with respect to a bare DPF, but also with respect to recent studies in the literature, in which increases in emissions of particles above and below 23 nm were observed.

**Supplementary Materials:** The following supporting information can be downloaded at: https://www.mdpi.com/article/en16104071/s1, Table S1: Geometrical data of the bare DPF; Table S2: Engine technical data; Table S3: Sensors accuracy; Table S4: Engine conditions during accumulation phase in 4 test cases; Figure S1: SEM image of the soot emitted by the Diesel engine used in the experimental tests; Figure S2: SEM image (left) and EDX mapping (right) of soot emitted by the Diesel engine as trapped by the CDPF; Figure S3: XRD pattern of the prepared CuFe2O4 compared with cubic, tetragonal and commercial copper ferrite; Figure S4: SEM image and distribution of elements, obtained by EDX element mapping, for the 30%wt CuFe2O4 loaded DPF; Figure S5: Ultrasonic tests performed on 30 %wt CuFe2O4 catalysed monolith.

**Author Contributions:** Conceptualization, E.M., B.R., G.D.F., M.S., I.A. and V.P.; methodology, E.M., B.R., G.D.F., M.S., I.A. and V.P.; software, E.M., B.R., G.D.F., M.S., I.A. and V.P.; validation, E.M., B.R., G.D.F., M.S., I.A. and V.P.; formal analysis, E.M., B.R., G.D.F., M.S., I.A. and V.P.; investigation, E.M., B.R., G.D.F., M.S., I.A. and V.P.; resources, E.M., B.R., G.D.F., M.S., I.A. and V.P.; data curation, E.M., B.R., G.D.F., M.S., I.A. and V.P.; writing—original draft preparation, E.M., B.R., G.D.F., M.S., I.A., V.P.; writing—review and editing, E.M., B.R., G.D.F., M.S., I.A. and V.P.; visualization, E.M., B.R., G.D.F., M.S., I.A. and V.P.; supervision, I.A. and V.P.; project administration, I.A. and V.P. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Data Availability Statement:** Not available.

**Conflicts of Interest:** The authors declare no conflict of interest.
