| An extended coherent flamelet model was implemented in the computational fluid dynamics code KIVA-3V to achieve high fidelity simulation of gasoline direct injection (GDI) combustion. The model allows for the identification of fuel economy improvements and emissions implications for this technology, in addition to the investigation of new operating strategies. An extensive validation of all aspects of the simulation with experimental results was performed.; In the coherent flame model, the flame is represented by a transport equation for flame density, with modeled terms for the production and destruction. Stratification of the engine charge is incorporated by diagnostic equations for the unburned fuel, oxygen, and enthalpy. This allows the characterization of the gas properties on a conditionally-averaged basis, by separately averaging over the burned or unburned fraction in a computational cell. The conditionally-averaged burned gas temperature can then be used to calculate the emissions formation rates, leading to more accurate predictions. The coherent flame model was extended, to capture the effects of exhaust gas recirculation stratification on combustion and pollutant formation, by adding a new diagnostics equation for CO2 originating from exhaust gas recirculation. A near-wall flame treatment was also implemented to represent the realistic behavior of the flame near the walls more accurately.; Since the literature lacks integrated studies thoroughly validating models developed specifically for GDI engines, the combustion model was integrated with updated models for spray breakup and spray wall impingement. This complete model was then used to simulate the engine cycle of an optical 4-valve GDI engine, corresponding to an experimental GDI engine. Laser-induced fluorescence (LIF) experiments, in combination with traditional engine diagnostics, allowed us to validate the simulation by comparing air motion, mixture formation, and combustion data over a range of speed and load conditions, in a realistic engine geometry, including moving valves. The thorough validation of all aspects of the model for the complete engine cycle demonstrates how the model was able to capture the important characteristics of the GDI engine, and pointed to areas that require further improvements. |
