| Comparing to traditional port fuel injection(PFI) engines, gasoline direct injection(GDI) engines have obvious advantages on fuel economy,WOT torque, and cold start. Consequently,GDI technology becomes a hot tendency in the gasoline engine field. However, one drawback for GDI engines is the spray impingement upon the piston and cylinder wall, which would lead to lubricant dilution, fuel economy deterioration and increasing emissions. Matching among spray characteristics, combustion chamber geometry and in cylinder air flow field has to be investigated and optimized to decrease the fuel impingement.CFD simulation has been widely used to conduct combustion system design and optimization because of its benefits on time and costs. However, at most time, the reliability of the CFD simulation cannot be ensured due to lack of model calibration data. In this thesis, the spray characteristics that are penetration,spray angle,spray pattern,velocity and droplet size for a multi-hole GDI injector were precisely measured and investigated with transient white light illumination,LIF,PIV and PDI laser diagnostics technologies. A spray simulation model was built with the commercial CFD software AVL FIRE, and further calibrated and validated using the experimental data. Finally, the spray model was embedded in the 3D dynamic grid of a turbocharged GDI engine to simulate and study the spray and air-fuel mixture formation process.Due to the researches in this thesis, the following conclusions can be achieved:1) The precise and reliable spray simulation model has been obtained after carefully calibrated using the measured data for the spray angle, penetration, velocity field, cross-surface mass and droplet size distribution; 2) The turbocharged GDI engine addressed in this thesis has serious spray impingement on exhaust side cylinder wall and piston crown at low speed full load operating condition;3) Retarding the injection timing can reduce the spray impingement quantity, but lead to decreasing homogeneity of air-fuel mixture;4) Duel-injection injection strategy can obviously reduce fuel impingement, but the homogeneity of the mixture will decrease with delaying second injection timing;5) By optimizing the injection position and nozzle orientation the wall wetting and fuel concentration distribution can be improved to some extent. |
