A turbulent combustion model is described for SI engines with large variations in mixture strength. The model is for a single gas phase fluid at high Reynolds number and treats combustion in the laminar flamelet regime, which is characterized by high Damkholer and low Karlovitz numbers.An assumed probability density function (pdf) approach is used to extract expressions for mean quantities of interest, which are parameterized on the progress variable and mixture fraction variables. A double delta function pdf is used for the reaction progress variable and a beta function pdf is used for the mixture fraction. The reaction rate term in the progress variable equation is closed using an algebraic expression, which incorporates the effects of mixture strength, pressure and temperature on laminar flame speed.The model is implemented in two versions of a Computational Fluid Dynamics (CFD) code. The first version simulates reciprocating engine flows and the second simulates combustion in closed vessels. Comparison is made against published experimental data for an engine running a homogeneous charge. Agreement with this experimental result is good based on limitations and assumptions used. Qualitatively predictions of combustion trends in stratified charge engines and closed vessels also show agreement with experiment. However, further validation is required with experimental engine data for stratified charge engines.