| The further development of internal combustion engines is aimed towards higher efficiency and lower emissions. To achieve this goal, a detailed understanding of the mixture formation process is a crucial prerequisite. To enhance mixing and combustion quality, a general trend to higher fuel injection pressures which leads to generation of droplets in a range below 50 μm is evident. In this paper, the experimental setup and the results of a study under realistic conditions for DI engines, with elevated pressure up to 3 bar, a heated wall and droplets in the 50 μm scale will be described. The wall element and the injector are contained in a pressure vessel with adequate optical access. To simplify the complex spray and to allow extensive, detailed measurements, a droplet chain generator is used, producing monodisperse equidistant droplets under elevated pressure with droplet diameters in the range of 40…200 μm. Furthermore, it establishes well defined boundary conditions and allows examination of the impingement process in detail. The droplet impingement process at the wall is illuminated by laser pulse backlight, recorded by a double frame camera and is analyzed by an advanced image processing tool which will be presented in detail. The image processing discriminates between primary droplets, secondary droplets, wall film and atmosphere.For the droplets, it determines the shape, 2D size, 2D position and 2D velocity of each particle within the observation plane. As a result, descriptive maps for the velocity field and diameter field can be determined. Using these data, modeling of the droplet wall interaction under realistic conditions for DI engines is in progress. The most important results are: The impact regime and their limits can be scaled with the dimensionless wall temperature T* for different primary droplet velocities ub with different pressures p. However, T* alone seems not to be sufficient for thermal effects especially if the Leidenfrost temperature is exceeded at elevated pressure. To predict secondary droplet diameters an approach based on the Reynolds number was found to give reasonably good results. |
