2 Mixture formation • Phenomenology Injection jets appear at high speed from the injection nozzle, and, as a result of the high rela- tive speed of the surrounding highly compressed air and of high turbulence in the spray, disin- tegrate into small droplets. With progressing penetration into the combustion space, these droplets are then atomized. Fig. 4.20 shows a qualitative sketch of the fuel spray emerging from the injection nozzle. Spray needle Blind hole Droplet collisions, Evaporation Wall impingement Droplet coalescence Spray cone angle Spray hole Primary breakup Secondary breakup Internal nozzle flow Spray penetration S Spray Fig. 4.20: Schematic representation of spray dispersion Spray dispersion and the mixture formation related to it are determined by the injection pa- rameters and the flow field (swirl, turbulence) in the combustion chamber. The turbulent kinetic energy of the spray is, however, at least one order of magnitude higher than that of the combustion air, so that the flow field in the cylinder only becomes significant towards the end of the injection, when the spray has already appreciably slowed down. While the injection pressure in the case of conventional unit pump systems is strongly re- duced towards the end of the injection duration, thereby causing an inferior atomization in this phase, the pressure remains at a constantly high level in the common rail system until the end, thus guaranteeing a continuously fine atomization. At the fringe of the spray, fuel droplets mix with the hot air within the combustion chamber (air entrainment). In this way, the drops are heated up as a result of convective heat transfer and temperature radiation of the hot chamber walls, and the fuel finally begins to evaporate. Besides temperature, the rate of drop evaporation is determined by the diffusion of fuel from the drop surface (high vapor pressure) into the drop surroundings (lower vapor pressure of fuel).