Experimental and Numerical Analysis of Spray Evolution, Hydraulics and Atomization for a 60 MPa Injection Pressure GDI System

In recent years, the GDI (Gasoline Direct Injection) technology has significantly spread over the automotive market under the continuous push toward the adoption of combustion systems featuring high thermodynamic conversion efficiency and moderate pollutant emissions. Following this path, the injection pressure level has been progressively increased from the initial 5-15 MPa level nowadays approaching 35 MPa. The main reason behind the progressive injection pressure increase in GDI engines is the improved spray atomization, ensuring a better combustion process control and lower soot emissions. On the other hand, increasing injection pressure implies more power absorbed by the pumping system and hence a penalty in terms of overall efficiency. Therefore, the right trade-off has to be found between soot formation tendency reduction thanks to improved atomization and the energetic cost of a high pressure fuel injection system. In this paper, a 5-hole, side-mounted prototype GDI injector was tested in a wide range of injection pressure conditions - from 5 up to 60 MPa - in terms of injection rate and spray development. The injection rate was detected by means of a Zeuch-method-based Injection Analyzer. The spray global shape was investigated by high speed imaging, while the atomization level and droplet velocity were measured by means of a PDA (Phase Doppler Anemometry) system over several measuring stations from 20 to 50 mm downstream the nozzle. A numerical model of the spray was developed and validated against the experimental data in order to simulate the spray penetration, cone angle and atomization over a wide range of injection pressure levels. The results show that the decreasing trend for the drops SMD (Sauter Mean Diameter) from 5 up to 60 MPa approaches its asymptote, suggesting an adequate cost/benefits analysis in terms of soot reduction for further injection pressure level increases.