This paper describes the development and validation of a refined combustion model for turbulent flame propagation in SI engines. In this model the original differential equation of flame front entrainment remains, but a new difference equation of burning rate is introduced to replace the original differential equation. The model fully embodies the Tennekes-Chomiak mechanism of premixed turbulent combustion, presenting a flame thinner than the original model.A spark ignition model is also suggested. The model treats the electrical spark ignition as a diffusion wave to calculate the entrainment enhancement by the spark ignition. The simple formula of the initial flame propagation has been validated by measured results.Incorporated with this spark ignition model, the three-stage model of premixed turbulent flame velocity, published earlier by the authors, is used in the combustion model. The model can directly describe the whole process of turbulent flame development without any ignition delay model.The combustion model has been implemented into a comprehensive, general-purpose simulation program of engine system.Comparisons between the predicted and measured results have been carried out for two SI engines. They are in good agreement. A parametric study has been conducted which reveals the influence of variables in the model.