**1. Introduction**

Diesel engines are still considered high-efficiency energy conversion systems since their operation under fuel lean combustion allows the production of lower CO2 [1], carbon monoxide (CO) and hydrocarbon (HC) emissions compared to spark ignition engines [2]. On the other hand, other compounds are present in the exhaust emissions of diesel engines, both in the gas phase and in the solid phase as particles [1]. The gas phase is commonly characterized by the presence of N2, CO2, O2, H2O, CO and NOx, as the main components, and the well-known soot is the main component of the solid phase [3]. In the last few years, the use of biodiesel and metallic additives dispersed in diesel have been proved to improve engine efficiency and reduce emissions [4]. However, the former introduces new challenges, such as corrosion in metal alloys, aluminum, and copper [4]. In any case, diesel engines are characterized by high particulate matter (PM) emissions, since some of their peculiarities, in particular, their fuel composition and the operating conditions realized in the combustion chamber, allow nonhomogeneous mixing, resulting in favored PM formation in the hightemperature regions with higher fuel contents [2]. PM is constituted of volatile and aromatic compounds (polycyclic aromatic hydrocarbons (PAHs) and heavier aromatic compounds) and non-volatile compounds (soot) [5]. Soot is characterized by a turbostratic structure, in which the particles are marked by the presence of crystallites of discernable lengths and with some stacking faults in their peripheries, while their centers have random crystallite

**Citation:** Meloni, E.; Rossomando, B.; De Falco, G.; Sirignano, M.; Arsie, I.; Palma, V. Effect of a Cu-Ferrite Catalyzed DPF on the Ultrafine Particle Emissions from a Light-Duty Diesel Engine. *Energies* **2023**, *16*, 4071. https://doi.org/10.3390/en16104071

Academic Editors: Monika Kosowska-Golachowska and Tomasz Czakiert

Received: 5 April 2023 Revised: 26 April 2023 Accepted: 11 May 2023 Published: 13 May 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

orientations and are highly disordered [6]. Moreover, a thin-layer SOF (soluble organic fraction) may be formed on these particles, since small amounts of fuel and engine oil from the crankcase may escape oxidation reactions in the combustion chamber. It is well known that diesel emissions have direct negative effects on human health, causing several kinds of damage, including asthma, lung and heart disease, and cancer, and the main carcinogenic and mutagenic respiratory health risks are related to exposure to PAH, in gas phase and/or adsorbed on soot [7,8]. These harmful effects have led to the limitation of PM emissions from diesel engines all over the world through proper regulations, with the limits becoming more stringent over time [9]. For example, particle mass limits have dropped by more than 95% from the early 1990s (Euro I) to 2009 (Euro V) for light-duty diesel (L-D) vehicles in Europe [7]. A particle number (PN) limit (6 × 1011 particles/km, taking into consideration only particles over 23 nm) was first introduced in 2011 (Euro Vb) for L-D diesel vehicles and was introduced in EURO VI emission standards also for gasoline direct injection (GDI) engines [10,11]. Car manufacturers have widely introduced diesel particulate filters (DPFs) in their vehicles to comply with these limits; thus, DPFs have become a crucial technology for achieving the tradeoff between emission limits and engine performance. In general, the DPF is a ceramic monolith with a honeycomb multi-channel structure composed of silicon carbide (SiC), whose parallel channels are alternately plugged, creating the so-called wallflow configuration. In this way, the exhaust gas is constrained to go through the porous walls of the channels [12]. These devices ensure PM reduction with a quite impressive efficiency—more than 95% in terms of mass and concentrations higher than 99% over a wide range of engine operating conditions [12,13]. Since the particles are trapped by the millions of complex structured pores inside the DPF structure via the physical filtration principle, either the periodic or the continuous elimination of these particles is key to ensuring a long lifetime of the filter [14]. This removal process is called regeneration and consists in the oxidation of the trapped soot [15]. Efficient regeneration is required in order to keep the fuel penalty due to DPF presence as low as possible. In fact, a regeneration process that is not able to fully burn the deposited soot may result in a fast increase in the backpressure, consequently causing different drawbacks, including increased fuel consumption, engine failure or even fire events [16]. During the regeneration step, only the organic components can be burned, while the salts, which are known as DPF ash, survive and require manual dredging. For a diesel vehicle driven 20,000 km per year, regeneration would be expected every 4–14 days if a frequency of regeneration events is assumed to vary from 250 to 800 km [17]. In recent years, the development of models that are able to accurately predict DPF behavior in all its functioning phases (from the soot loading up to the regeneration stages) has become mandatory. Therefore, the attention of researchers has focused on this issue.

Three main regeneration strategies can be adopted, namely:


Regarding the passive regeneration strategy, the catalyst can be either mixed with the fuel (fuel-borne catalyst, FBC) or deposited on the DPF, thereby obtaining a catalyzed DPF (CDPF) [7]. CeO2 is the most widely used FBC; it can lower soot oxidation temperature and consequently soot emissions. However, recent studies have evidenced that its use reduces soot and THC emissions by up to 30% at part load under low-temperature combustion conditions, but no significant differences are observed at high load [24]. Moreover, the addition of CeO2 and Fe(C5H5)2 nanoparticles to ultralow sulfur diesel (ULSD) may increase the total particle count, due to the formation of self-nucleated metallic nanoparticles, and toxic effects on human health may occur due to modifications in the physicochemical characteristics of PM caused by FBC-doped fuels [25]. Apart from CeO2, different metals have been proposed as fuel additives, including Mn, Fe, Cu, Al, Be and Pt, all

showing positive effects in reducing diesel engine emissions [4]. In recent years, different active species have been proposed for use in soot oxidation, both in powder forms and deposited on DPFs, including Ce0.5Zr0.5O2 catalysts promoted by multivalent transitionmetal (Mn, Fe and Co) oxides [26]; nanostructured structures, such as equimolar ceriapraseodymia [27]; and CoOx-decorated CeO2 (CoCeO2) heterostructured catalysts [28], copper ferrite (CuFe2O4) [22], Ag/Al2O3 [29], nanoscale Mn3O4 [30], Mn2O3 [31], Fe-doped Mn2O3 [32] and CeO2 [33]. All the proposed catalysts have shown good activity in soot burning, with soot temperature oxidation in the range of 300–500 ◦C.

DPF regeneration in L-D vehicles, both in active and passive strategies, is achieved by using an after/post-injection strategy to increase the temperature of the exhausts up to soot burnout temperature. In particular, this strategy involves an after- and a post-injection: the former allows, through the injection of a checked amount of fuel during the expansion stroke, the evaporation of the latter, which occurs during the exhaust valve opening [34]. The unburned and vaporized fuel exiting the combustion chamber is subsequently oxidized in the diesel oxidation catalyst (DOC). This secondary fuel oxidation allows the exhaust gas to reach the ignition soot temperature (500–600 ◦C) across the DPF. A deep understanding of the phenomena related to the post-injection allows extremely important improvements to DPF operation modes, also in terms of extra fuel penalties during active regeneration [35–37].

Although the DPF regeneration condition has not been included yet in vehicle emission regulations, many previous studies have emphasized the emissions that occur during regeneration, evidencing how gaseous pollutants, as well as particulate emissions, in terms of mass and number, largely increase [1,35]. Experimental tests performed with the aim of investigating active regeneration events evidenced remarkable transient particulate emissions. Various mechanisms have been proposed in the literature to explain these observations [9,36,38–42]:


A deep understanding of the particle emissions during the active regeneration of a non-catalyzed wall-flow SiC DPF has been made possible by performing dedicated experimental tests [43]. In particular, a detailed analysis of PSDs at the exhaust of a EURO V L-D diesel engine was performed, with the engine working under different operating conditions, aiming at estimating the PN removal efficiency of the filter in the range of 5–100 nm. Moreover, the experimental tests allowed investigation of the evolution of PN concentration during the regeneration procedure, highlighting the influence of DPF temperature and the fuel post-injection strategy. The results of the PSD analysis have shown that:


In previous works, we have demonstrated good results in terms of soot oxidation using a 30 %wt CuFe2O4 loaded catalytic DPF (CDPF) [23,44]. However, in the literature, the use of a CDPF resulted in high particle emissions, in particular during the first phase of the regeneration step [45]. Therefore, in this work the research was focused on detailed PSD analysis of the particles emitted during the active regeneration of the CDPF at the exhaust of a Euro V L-D diesel engine in order to reach a more complete understanding of the filter behavior, also aiming at its potential use in the real world. Using a scanning mobility particle sizer (SMPS), a PN distribution in the range of 4.5–160 nm was obtained during the soot filtration stage of the CDPF at various engine operating conditions. The filtration efficiency of the CDPF in the above specified range was calculated by alternately sampling the particles at the filter entrance and exit. Moreover, the regeneration process was deeply investigated in terms of the dynamic behavior of the PN size distributions.
