The main objective of the FVV-project “Cylinder Module” was the development of a profoundly modular designed concept for object-oriented modeling of in-cylinder processes of internal combustion engines. It was designed in such a way, that it can either be used as a stand-alone real working-process calculation tool or in tools for whole vehicle simulations. It is possible to run the “Cylinder Module”-code inside the FVV-“GPA”-software for transient vehicle and driving cycle simulations and it is possible to use the graphical user interface “ATMOS” of the “GPA”-project. The code can also be used as a user-subroutine in 1-D-flow simulation codes. Much effort was spent on the requirements of flexibility and expandability in order to be well prepared to cope with the diversity of both today's and future tasks. The code is freely available for members of the German Research Association for Combustion Engines (FVV).In the working-process calculation an analytical solution for the calculation of the difference quotients was developed, which is suitable for a user-defined number of zones in every thermodynamical system, too. As a consequence, exactly the same source code can be used for one-, two- or multi-zone calculations. In fact, additional zones can be switched on simply in case of need. It is possible to simulate a user-defined number of connected thermodynamical systems contemporaneously.