**1. Introduction**

Tracing the evolution of the multi-hole nozzles used in direct injection diesel engines, VCO (Valve Covered Orifice) layout was introduced with the advent of the first generation of common rail injectors, replacing the mini-sac layout. This step allowed the substantial reduction of unburned hydrocarbons emission. In fact, the fuel remaining inside the injector tip is released after the end of injection [1]; very recent studies are better focusing the causes of the so called "injector dribble", whose impact on emission is becoming crucial [2]. The implications of in-nozzle phenomena at the early beginning and after the end of injection have been related to pollutant emission and to deposit formation within the holes. In such a scenario, where low lifts and short injection shots are usual, some recent contributions have concerned the phenomenology of the very early phases of injection [3], of the primary atomization of the liquid [4] and of the phases in which fuel is trapped in the holes and in the sac at the beginning and at the end of injection [5].

Significant efforts have been devoted at comparing sac and VCO nozzles, focusing the attention on the internal nozzle flow, on the rate of injection and on the spray behavior [6–8]. Besides some advantages, the major drawback of the VCO layout is represented by the irregularities of the spray that occur at low needle lift; these abnormalities assume relevance because they are related to the

formation of particulate matter [1]. At the time of the introduction of the VCO layout, this aspect had a relatively little importance, since low needle lift was the only operating condition of pilot injections.

The introduction of multiple-injection technique was complemented by the wide adoption of micro-sac geometries. These geometries resulted in being more suited to the characteristics of the second generation injectors (in the compromise between the reduction of unburned hydrocarbons and particulate matter). Indeed, the multiple-injection strategies have required a significant improvement of the injector dynamics, which was also translated into a substantial reduction of the maximum needle lift. Since the needle was located to work almost always at low lift (ballistic displacement), the micro-sac layout represented a better compromise than the VCO layout in producing regular sprays, with low formation of particulate matter [9] and low release of unburned fuel, in agreemen<sup>t</sup> with the emission regulations at that time.

As highlighted by the latest research activities [10], the release level of unburned fuel typical of micro-sac nozzles is no longer tolerable, according to the trend of unburned hydrocarbons emission regulation. From this viewpoint, the VCO design will predictably be reassessed and developed again [11,12], in order to minimize the content of liquid fuel that is trapped when the needle is closed. In the light of this, it is believed that investigations on behavior of VCO nozzles under real operating conditions represent an interesting and significant topic.

The abnormal behavior of the spray produced by the VCO nozzle has been observed by several investigators. The VCO-spray features reported in the literature show differences in the macroscopic characteristics among the fuel jets. The differences relate to the spray penetration and to the spray angle [13,14]. If the presence of significant defects of the nozzle is excluded, this behavior suggests that the fuel flows through each hole in a particular way. After the first experiments carried out on sprays from VCO injectors, investigators had suggested that the spray abnormalities were due to the misalignment of the needle during injection; the misalignment had been considered capable of significantly influencing the fuel flow pattern at the hole inlet section; indeed, in VCO nozzles, the needle remains in the immediate proximity of the inlet section of the holes for the whole injection event [14,15]. The super-accurate design and manufacturing of the nozzles, provided with expensive double guides for the needle in some cases, has been the main technical approach that enabled VCO-injector manufacturers to stabilize the behavior of the spray as much as possible.

Some experimental investigations based on X-ray techniques have confirmed that needle displacement is affected by major radial oscillations [16]. The experiments have later also allowed for detecting the amount of needle movement in the radial direction [17].

In agreemen<sup>t</sup> with these findings, further contributions and simulation campaigns were carried out to quantify the influence of the needle misalignment on the distribution of the fuel flow out of each hole. The results reported in the literature have highlighted that the flow field upstream of the holes is directly affected by the needle position during injection. In more detail, the position of the needle determines the flow characteristics at each hole inlet, influencing the downstream fuel flow [18–20].

The current experimental techniques allow for making a very fine analysis both on the needle displacement during injection and the internal multiphase nozzle flow [21]. Equally advanced are the techniques developed for the diagnosis of the spray [22] features.

In principle, the link between the needle alignment and the characteristics of each spray could be simultaneously studied by combining nozzle and spray diagnostics. Unfortunately, research activities that combine the two types of analysis are not ye<sup>t</sup> reported in the literature.

The 3D-CFD (three-dimensional computational fluid dynamics) is the only practical way to consider, step by step in time, the link between what happens inside the nozzle and what happens outside; this work is oriented in this direction, and the aim is to observe in what measures the off-axis position of the needle is reflected by the spray anomalies, quantifying and delimiting the differences that could be expected due to the off-axis movement of the needle; according to the aforementioned motivations, the study considers a VCO layout.

In the first phase of the work, the reference nozzle geometry and the reference injection event have been identified. Subsequently, the 3D-CFD simulation approach has been defined, relying on the modeling of the internal nozzle flow; once characterized, it has been interfaced to the spray simulation, initializing the primary break-up model on the basis of the transient flow conditions at each outlet section of the multi-hole VCO nozzle. Among the possibilities, the spray modeling has been based on a coupled Euler–Lagrangian approach, whose features have recently been reported in the literature [23]. The simulations have been carried out in *FIRE ™* environment [24] (by AVL List GmbH, Graz, Austria). The following section describes the relevant assumptions and the details of the proposed approach. The results section reports the main points of investigation; and the conclusions are discussed in the last paragraph.
