In pursuing a project to convert a four cylinder gasoline multi-point port fuel injection engine to a CNG direct injection engine, a numerical model has been developed in AVL FIRE software to undertake a detailed numerical investigation on the effects of combustion chamber geometry in such engines. Two main phases have been considered in the present study. In the first phase, aiming to fully investigate flow field and mixing process, multi-dimensional numerical modeling of transient gas injection has been performed. In order to verify the accuracy of the model, two different validation cases have been employed. The results showed that the models are quite capable of grasping all the significant physical phenomena in the process. Adoption of such a modeling was found to be a challenging task because of required computational effort and numerical instabilities. In all validation cases, experimental and numerical results were observed to have excellent agreement with each other. In the second phase, using the moving mesh capability the validated model has been developed to include methane direct injection into the cylinder of a direct injection engine for various combustion chamber geometries. Five different piston head shapes along with two injector types of single hole and multi hole have been taken into consideration. A centrally mounted injector location has been adapted to all cases. The effects of combustion chamber geometry, injection parameters, injector type and cylinder head shape have been studied on mixing of air-fuel inside cylinder. Based on the results, suitable geometrical configuration for the new NG DI engine has been discussed. (c) 2013 Elsevier Ltd. All rights reserved.