Characterization of GDI fuel sprays using Phase Doppler Interferometry

One of the main factors that determine the combustion efficiency of an internal combustion engine and as a result the output power as well as the exhaust gas composition is the degree of mixing between the intake air and the fuel. In order to improve the air/fuel mixing the automotive industry has shifted the last five years from the port fuel injection strategies to the newer direct fuel injection which also yields more advantages. In this strategy one of the most crucial components is the fuel injector, which has to be able to deliver precise quantity of fuel, combined with excellent atomization properties that must behave according to specifications throughout different modes of operation. In this thesis a new experimental fuel injector was provided to the Combustion division of Applied Mechanics and was evaluated in terms of spray characteristics. The most important of these are the fuel droplet diameter which is quantified by the D10 and D32 values and the droplet velocity. The droplets should have a relative small diameter, measured in μm, in order to be able to evaporate properly and fast and they should have a velocity that allows them to reach into the combustion chamber without impinging to the piston and the liner because that causes high unburned hydrocarbon emissions. The method that was used to evaluate the performance of the injector is called Phase Doppler Anemometry. It is an optical, non-intrusive, single point, measuring method that uses a small control volume that is created by a laser source to measure accurately both velocity and diameter at the same time. Three different groups of measurements were performed. In the first the influence of the injection pressure on the velocity and the droplet diameter was evaluated. The pressure was varied from 25bar up to 200bar. In the second the influence of different injection durations while keeping constant pressure were examined again in terms of velocity and droplet diameter and in the final group the ability of the injector to perform double injection and whether that has an effect on velocities and droplet diameters. The results showed a significant reduction in droplet diameter with the increase of the fuel pressure starting from 16,7μm at 25bar and reaching 11,7μm at 200bar while the velocities start from 19,5m/s and reach 55m/s. The different injection duration shows little influence on both the diameters and the velocities and the same can be said for the double injection. The double injection is possible and the droplet diameters as well as the velocity vary very little compared to the single injection