The following article details a method for the optimization and improved use of the internal combustion engine as main propulsion. The focus here is not on new propulsion systems or combustion processes, but on the characterization of the typical usage of existing systems in order to enable better utilization. As one major potential for improvement, the transient machinery operation is examined and discussed in this article. Higher fuel consumption and higher emissions occur compared with stationary engine operation in that operation mode. Experimental data from test bed (“Caterpillar MaK 6M20”) measurements are presented which explain the consequences of transient operation. Furthermore, appropriate analyzing methods to evaluate this operation mode are shown. Finally, a modelling approach is presented using the data for calibration and validation of an engine simulation model. The most significant part to predict real transient efficiency and emissions is the in-cylinder process and especially its combustion process. Therefore, the simulation model does not use engine maps but a mostly physically based engine model by using thermodynamic approaches and chemical reaction kinetics. The specific application of that simulation model for four-stroke medium-speed engines covers the behavior of transient operation during ship maneuverings since it is developed for integration into a ship engine simulator.