Digital simulation of engine processes employs mathematical models comprised of linear, differential equations. Such models must not unconditionally yield exact, absolute values of processes; often, only changes in values are required and these can be provided fairly accurately. For evaluating an actual system, however, it is necessary to provide as accurate of input values as possible. Furthermore, different simulation models should be developed for different tasks; universal models are not well suited to most applications.The paper applies a model to three different sets of engine processes. Optimization of processes in unsupercharged engines with undivided combustion chambers requires a combustion model with well-simulated ignition delay. Optimization of supercharged systems or gas exchange processes requires a general process model with accurate representation of flow parameters. Engines with precombustion chambers demand more complicated models and engine geometry becomes especially important. Each of these systems is discussed with respect to modeling differences and results.