Mixture Preparation and Nitric Oxide Formation in a GDI Engine studied by Combined Laser Diagnostics and Numerical Modeling

Abstract

Project objectives:Through the combination of advanced imaging laser diagnostics with multidimensional computer models, a new understanding of the performance of directinjection gasoline engines is pursuit. The work focuses on the fuel injection process, the breakup of the liquid into a fine spray and the mixing of the fuel with the in-cylinder gases. Non-intrusive laser diagnostics will be used to measure the spatial distribution of droplets and vaporized fuel with very high temporal resolution. These data along with temperature measurements will be used to validate a new spray breakup model for gasoline direct-injection. Experimental data on near wall fuel distributions will be used for comparison with a model that predicts the spray-wall interaction and the dynamics of the liquid film on the surface. Quantitative measurements of local nitric oxide concentrations inside the combustion chamber will provide a critical test for a numerical simulation of the nitric oxide formation process. This model is based on a modified flamelet approach and will be used to study the effects of exhaust gas recirculation. Experimental Approaches and Achievements:A direct-injection fuel spray rig was designed and installed to study a wellcharacterized spray and provide data for comparison with a new breakup and impingement model. Measurements of the spatial and temporal structure of sprays were conducted with a combination of planar laser-induced fluorescence and planar Mie scattering. This gives access to fuel distribution as well as to relative droplet diameters. A single KrF excimer laser beam was employed to illuminate the spray and the signals were separated with a newly designed filter-beam splitter setup. This filter allows a complete separation of the signals and produces two parallel images that can then be recorded with a single ICCD camera. Other features of the setup are on-line single-shot energy referencing and flexible wavelength selection.Two sets of experiments were conducted. A first series addressed issues of background signals that occur in engine environments when a reflecting surface lies behind the illuminating laser light sheet. This background signal was determined to be strongly wavelength dependent and an effective suppression is primarily possible when the surface is treated with high pressure water blasting and by optically narrowing the depth-offield for observation by using a large-aperture imaging lens without additional apertures.