Equations are derived for multicomponent fuel evaporation of airborne fuel droplets and wall films, and are implemented as a multicomponent fuel model into KIVA-3V. Temporal and spatial variations in liquid droplet composition and temperature are not modeled but solved for by discretizing the interior of the droplet in an implicit and computationally efficient way. We find that an interior discretization is necessary to compute the evolution of the droplet composition correctly. This is because an interior discretization gives significantly improved results for the evolution of droplet compositions and temperatures over the alternatives of using infinite diffusivities or of using correlations based on the differences between surface and average values. Numerical simulations of the evaporation of single droplets, axisymmetric sprays, and sprays in 3-D engine geometries demonstrate the need for interior discretization and for using a multicomponent fuel model in gasoline spray simulations.