A probability density function method for turbulent reacting flows has been implemented into the CFD code FIRE in order to enable simulation of SI engine combustion under premixed, partially premixed and fully stratified charge conditions. In order to assess the accuracy of the computational method, different aspects of SI engine combustion have been numerically studied under premixed charge conditions for varying mixture composition and engine operation parameters. Calculated flame propagation characteristics, local flame front speeds and global heat release results are presented and compared to the corresponding experimental data for variations in fuel to air equivalence ratio, residual gas mass fraction, engine speed and load. Good overall agreement between the numerical and experimental results is obtained for the operating conditions considered. The application of the computational method to the simulation of combustion in a gasoline engine with internal mixture formation demonstrates its ability to account for the complex premixed, partially premixed and diffusion type flame occuring under stratified charge conditions. Further, the study provides an insight into the interaction of turbulent flow, charge stratification and combustion and enables identification of areas where finite-rate chemistry effects become predominant. Calculated heat release results show good agreement with the data extracted from cylinder pressure measurements.