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The design of an air to fuel ratio (AFR) control system is substantially facilitated by a suitable mathematical model of the mixture formation process. Such a model has to be a compromise between short simulation times and good prediction capabilities. Well-known simple “linear” wall-wetting models are easy to use but require substantial calibration time to experimentally determine the operating point dependent model parameters. Full 3D simulations of all physical effects are still computationally not tractable. In this work a control oriented mathematical model of the mixture formation phenomena has been built, which tries to find a middle way between these two extremes. Computation times for one engine cycle are less than half a minute on a standard Pentium-PC. Nevertheless, the model is able to predict nonlinear effects that cannot be described by conventional wall-wetting models.