Computer simulation models for fuel preparation and combustion ill gasoline direct-injection spark-ignition (GDI or DISI) engines are described. A modified KIVA-3V code that includes improved spray breakup, wall impingement, and combustion models was used, ba particular, a new ignition kernel model, called DPIK (discrete particle ignition kernel), has been developed to describe the early flame kernel growth process. The model uses Lagrangian marker particles to describe the flame kernel location. The spray and engine flow models were validated using available drop size and patternator measurements, and particle tracking velocimetry (PTV) data from a water analog rig. The combustion models were applied and validated for both homogeneous and stratified-charge engines. The stratified-charge engines considered include both wall-guided and spray-guided designs. Applications of the models show that optimized injection timing call lead to reduced wall wetting, higher turbulence intensity near TDC, and better volumetric efficiency and knock resistance. For wall-guided combustion chamber designs, the injector orientation significantly influences the fuel stratification pattern, and hence the combustion characteristics. The gas tumble also affects the fuel distribution and the ignition process. Under certain conditions, the fuel-air mixing is characterized by the existence of many lean regions ill the cylinder and the burning speed is very low; hence the combustion can be poor in these cases. Multidimensional modeling is shown to be a useful tool to help visualize and optimize engine combustion details.