# On the Link between Diesel Spray Asymmetry and Off-Axis Needle Displacement
## 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; 7; 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 theformation 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 agreement 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].