When developing gas exchange and combustion systems at Volvo Car Corporation, CFD (Computational Fluid Dynamics) is today a key tool. Three dimensional CFD is by tradition used to study one single component (e.g. manifolds and ports) at a time. Our experience is that this approach suffers from two main limitations; first that the boundary conditions (both upstream and downstream) are uncertain; and secondly that validation against experimental data is extremely difficult since any measured parameter will depend on the complete engine. Distribution of secondary gases and AFR (Air to Fuel ratio) are typical examples where traditional CFD methods fail.One proposed way to overcome these problems is to use 1D gas exchange models coupled with 3D CFD. The main problem with this approach is however the positioning and treatment of the boundaries between the models. Furthermore, the boundaries themselves will unconditionally cause disturbances in the pressure fields.Instead, a new CFD approach is under development where the complete gas exchange and combustion systems are modeled in CFD. Focus is set on modeling of the gas exchange system where the boundaries are placed as far as possible from the essential components. The effect of combustion on the gas exchange is accounted for by a simplified source term approach. Both a 5-cylinder diesel and a gasoline N/A (Natural Aspirated) engine have been studied. The diesel system includes a rotating turbine and the EGR (Exhaust gas Recirculation) circuit and the N/A system is complete from air intake to tailpipe. The main idea is that the new method will be used as a complement to the traditional methods and improve the understanding of the complex phenomena that are present in the gas exchange systems.This paper includes some results that can be drawn from the simulations and some comparisons with a 1D simulation code.