**1. Introduction**

Pollutant regulations applying to compression ignition engines have focused on particulate matter emissions as one of the main pollutant emissions with which to deal. In Europe, particulate matter emissions have been restricted from 140 mg/km in Euro 1 to 4.5 mg/km in current Euro 6b. Besides stringent mass constraints, particulate number is also regulated by Euro 5b applying both mass and number regulations also to direct injection spark ignition engines. Similar trends are found worldwide becoming wall-flow particulate filters in a required exhaust aftertreatment device already present in diesel engines and being progressively adopted in gasoline-powered vehicles.

The reason for the implantation of the wall-flow particulate filters is related to its high filtration efficiency in the whole range of particle size [1]. However, soot collection into and on the porous substrate produces a noticeable pressure drop, increasing as soot loading does. In standardized post-turbine placement, this pressure drop is multiplied by the turbine pressure ratio to define the increase in engine back-pressure directly damaging pumping work. The result is a non-negligible fuel consumption and carbon dioxide (CO2) emission penalty [2]. In addition, the typically low temperature downstream of the turbine makes the use of active regeneration strategies necessary involving additional periodic fuel consumption.

Although most of these drawbacks may be overcome with a pre-turbine Diesel Particulate Filter (DPF) placement [3], which enables conditions for passive regeneration and highly reduces DPF pressure drop impact on the fuel consumption penalty [4], the need for advanced boosting architectures to guarantee fast dynamic response [5] is postponing its further development. In the meantime, efforts are being driven to reduce aftertreatment volume by combining several abatement functions [6]. Thus, new devices, such as the Selective Catalytic Reduction Filter (SCRF) system, are gaining in interest. SCRF consists of the combination into a single wall-flow monolith of soot filtration and nitrogen oxides (NO*x*) abatement capability by coating the porous substrate with a NO*x* Selective Catalytic Reduction (SCR) catalyst [7]. Despite the potential improvement of this solution in terms of DPF passive regeneration, SCR light-off and conversion efficiency at low temperature, the combined chemical behavior of soot and NO*x* is still suggesting doubts concerning the overlapping of temperature ranges, which can lead to a final slight loss of NO*x* abatement and passive regeneration capability [8]. Regardless of the need for further understanding on new particulate filters with extended chemical functions, this kind of solution does not involve improvements concerning the fluid-dynamic behavior, i.e., pressure drop. The current context relies on cell geometry or porous wall optimization concepts, such as asymmetrical cell designs with different inlet channels geometry [9] or a two-layer substrate combining different micro-geometry properties [10]. These solutions contribute to partially mitigate the DPF impact on engine performance by changing the pressure drop to soot loading dependence. However, these parameters are still closely related, and the effects concerning fuel consumption are in the best case only slightly displaced to higher soot loading [11].

Under this context, pre-DPF water injection emerges as a technique able to reduce pressure drop with respect to a baseline DPF, making it independent of the particulate matter loading [12]. It leads to clear advantages in terms of fuel economy and CO2 emission. Secondarily, the particulate matter accumulation capacity is also increased. This feature is beneficial both in terms of ash through the reduction of maintenance requirements and soot, whose active regeneration can become exclusively controlled by soot mass loading instead of pressure drop criteria, thus avoiding an excessive regeneration temperature. Previous works [13] have hypothesized the water drag of the particulate matter towards the inlet channels end as the main cause of the pressure drop reduction without soot mass removal. This kind of restructuring is in agreemen<sup>t</sup> with findings about the influence of the ash deposition pattern along the inlet channels. While ash deposition mixed with soot on the particulate layer produces a grea<sup>t</sup> pressure drop [14], ash deposition in the rear end region leads to lower pressure drop. Rear end deposition of ash is explained by exhaust gas drag during long-term engine operation [15]. This kind of mechanism, i.e., soot restructuring during engine operation, would also contribute to explain why different pressure drops are usually found for the same engine operating conditions and soot loading under real driving operation. In the case of pre-DPF water injection, the drag process is forced by controlled periodic injection events affecting both soot and ash.

In this work, the impact of the particulate layer characteristics along the inlet channel is discussed. The analysis is guided by a computational study carried out with a one-dimensional gas dynamic code for wall-flow DPFs [16]. Parametric studies focus on the effect of the particulate layer thickness profile (soot mass distribution) combined with porosity to identify the solution domain that would provide the pressure drop obtained experimentally and its increased rate after pre-DPF water injection. The conclusions of the theoretical analysis are also supported by visualization of monolith substrates analyzed after pre-DPF water injection and in baseline conditions. As a final step, the influence of the particulate layer restructuring on the regeneration process is also explored by means of the modeling of experimental data obtained during active regeneration.
