A method for automatic reduction of detailed reaction mechanisms using simultaneous sensitivity, reaction flow and lifetime analysis has been developed and applied to a two-zone model of an SI engine fuelled with Primary Reference Fuel (PRF). Species which are less relevant for the occurrence of autoignition in the end gas are declared redundant. They are identified and eliminated for different pre-set minimum levels of reaction flow and sensitivity. The resulting skeletal mechanism is valid in the ranges of initial and boundary values for which the analyses have been performed. A measure of species lifetime is calculated from the chemical source terms, and the species with the lifetime shorter than and mass-fraction less than specified limits are selected for removal. These are assumed to be in steady state, and their concentrations are modeled by means of algebraic equations that are automatically implemented in FORTRAN subroutines computing the steady-state concentrations by internal iteration. The detailed mechanism is reduced to 19 species, limited by the number of fuels, oxygen, products and stable intermediates. It is found that the error in autoignition time is less than 1 CAD down to 19 species. The error increases monotonously with the increase of the pre-set limits defining the level of reduction. To estimate the overall effect of reduction, sensitivities of selected species on temperature are calculated.