The prediction of combustion within spark ignition engines needs to take into account the interaction of turbulent fluctuations. Previous attempts at this used a model in which the chemical processes were supposed infinitely fast and the combustion was controlled by turbulent mixing only. This paper describes our progress in extending such models in two directions. Firstly, we have included the influence of the nondimensional parameter k½/uL (where k is the turbulent kinetic energy and uL the laminar flame speed), that we expect to be important due to the presence of thin flamelets within the turbulent reacting medium. Secondly, we have tried to take into account non infinitely fast chemistry, especially for the case of engines operating at low equivalence ratios.The model obtained has been included in the KIVA code for internal combustion engines, and we have studied the sensitivity of the full model to its various parameters, for a given geometry.