## 1 Introduction  
High pressure injection systems are commonly used in modern internal combustion engines. The systems have essential roles for realizing highly controllable fuel injections, for example, multiple injections in a cycle, short injection duration and accurate injection quantity. On the other hand, the difficulty for recognizing the spray development process, especially near the injector nozzle, is increased because of its drastic transient nature. Since the atomization characteristics in the region have an important influence for the further spray development, the investigation on the atomization processes are one of the focused research targets since long time. However, due to the high speed and high concentrated spray, detailed investigations are still challenging in the region. Several unique techniques have been developed (reviews are given in [1,2]) and applied for the measurement in the region, but the phenomena in the region are not fully understood yet. A schematic diagram of a spray development process is shown in Figure 1[3]. The left side of the figure shows the cut model of an injector tip. The fuel flows through the small channel between the needle and the injector body. Then, the fuel is injected to the chamber through the sac and the nozzle orifice. The liquids are fragmented into huge droplets, ligaments and small droplets near the nozzle exit (primary breakup). These forms of liquids are further fragmented to a large amount of small droplets (secondary breakup). These small droplets are dispersed by the entrained air. The injection process starts when the needle moves vertically and opens a cross section area for the flow inside the injector. Because the flow cross section is drastically changed during the injection period, the temporal velocity and the main flow direction of the liquid fuel are also changed within a short time. In addition, the turbulence intensity of the nozzle flow is high, and it induces the liquid breakup at the nozzle exit rapidly. Cavitation occurs when the local pressure of the liquid decreases to its saturation pressure. The cavitation reduces the effective flow area in the nozzle and enhances the liquid breakup when the low pressure bubbles collapse within the flow. At the outside of the nozzle, the liquid is also fragmented aerodynamically by the velocity difference between the liquid core and the surrounding air. Turbulence, cavitation and aerodynamical shear are seen as essential driving forces of the spray breakup near the nozzle [4,5]. In order to gain more knowledge of the processes inside and near the nozzle, several unique techniques are applied for detailed studies here. The paper is divided into two parts; 1. investigation on the flow inside the nozzle which is related to cavitation and turbulence, and 2. investigation on the spray development near the nozzle. The flow inside the nozzle is affected by the inner structures, such as the needle movement, the sac geometry, the orifice inlet curvature, and the nozzle hole geometry. The investigations of these effects are challenging work due to the conditions of the measurements, such as the high velocity of flow inside the nozzle (up to 800 m/s in some cases), the small size target (the diameter of 100 m and length of 1 mm), and the optical inaccessibility of nozzles. Therefore, only limited experimental methods are applicable. In this paper, X-ray and neutron transmission measurement for inner structures, and cavitation structure measurement on transparent nozzles, are described. Numerical simulations are used to support the measurement data. For the near field behind the nozzle the spray dense core, velocity fields, and droplets velocity and size are investigated. As much as possible, the same injector is used for the different approaches. A combined discussion of the processes and an outlook for further investigation are addressed in the last section. It is highlighted that the combined approach of the  
Figure 1: Overview of spray development [3].