Cylinder pressure simulation has grown to become an important tool when developing and evaluating new engine concepts and control strategies. A new formulation of zero-dimensional multi-zone models is developed and described. A general model structure is formulated that rely on a set of differential algebraic equations that are easy to solve. The selected formulation also results in models that are easy to scale, i.e. add new zones, and to increase complexity, which is a result of the selected structure. A number of important issues that can cause problems when simulating the model are treated. It is shown: a) How a new zone is initialized. b) How variables of varying magnitude can be scaled to avoid numerical difficulties. c) How numerical errors accumulated during the simulation can be reduced by using a set of consistency equations.