Analysis of the In-Cylinder Flow, Mixture Formation and Combustion Processes in a Spray-Guided GDI Engine

The purpose of this paper is to investigate the air/fuel mixture formation and combustion characteristics in a spray-guided GDI engine using a commercial code, STAR-CD. This engine adopted the outwardly opening injector located in the center of cylinder head, which forms a hollow cone spray. The spray injection was modeled arranging multiple points using random function along the ring-shaped nozzle exit. To predict the breakup of spray, Reitz-Diwakar's breakup model was used, and the model constants were calibrated against published experimental data in a constant volume chamber. The validated spray models were applied to the analysis of spray behavior and mixture formation process inside the engine combustion chamber under operating condition of ultra-lean mixture (λ ≈ 4). To predict the combustion process, the modified eddy breakup combustion model was applied. The present approach reasonably predicted the spray behavior, the mixture distribution near the spark plug, and flame propagation inside the combustion chamber. The calculated combustion pressure had a good agreement with the engine test results. Furthermore, the variations of the geometries of intake port and combustion chamber, the split injection parameters, and compression ratio were evaluated. The result shows that the enhanced tumble flow can deteriorate the mixture distribution of the hollow cone spray, and therefore decreases the burning rate. Also, the influence of injection timing and compression ratio on the quality of the stratified mixture at the spark was well predicted.