The current European legislation concerning pollutant emissions from IC engine vehicles is very stringent and demanding. In addition, the CO2 fleet emission must obey to a significant reduction path during the next decade, to cope with the prescribed targets recently agreed. The prediction of pollutant emissions from IC engines has been a challenge since the introduction of the emission regulation legislation. During the last decade, along with the more tightening limits and increased public concern about air quality, the capability of simulating different operating conditions and driving cycles with an acceptable computational effort has become a key feature for modern simulation codes. The role of 1D thermo-fluid dynamic simulation models is extremely important to achieve this task, in order to investigate the performances of the next generation of IC engines working over a wide range of operating conditions, under steady-state and transient conditions. This work is based on the idea of integrating two different 1D simulation tools in a co-simulation environment, realizing a strict numerical coupling between the two codes. The main goal is to allow an accurate 1D simulation of the unsteady flows and the wave action along the intake and exhaust systems, without resorting to over-simplified geometrical discretization, and to rely on advanced thermodynamic combustion models for the calculation of cylinder-out emissions. The simulation of the after-treatment systems is then performed resorting to a steady state model with a detailed chemistry approach. In this scenario, the choice of the coupling strategy is a key issue, since an unsteady model must be coupled to a steady one. In particular, this last element may flatten the unsteady pattern of the flow, imposing unreal boundary conditions at the engine exhaust manifold and increasing the risk of misleading results. For this reason, the chemical and the fluid-dynamic behavior have been decoupled, allowing the 1D unsteady solver to propagate pressure waves along the complete system, preserving all the characteristic lengths (acoustic and fluid dynamic). For every after-treatment device a specific steady-state 1D model is used to predict the chemical conversion phenomena and the heat transfer involved. The boundary conditions for the steady state solver are provided by the 1D unsteady model, suitably averaged over the time. The steady-state solution is then sent back to the 1D unsteady solver and suitably over-imposed: temperature of the substrate, friction coefficient and gas composition. This co-simulation environment is validated against a real engine configuration which was tested at the experimental labs of EMPA research center. A 4- cylinder, SI, turbo-charged, CNG engine has been simulated at different loads and speeds and results are compared with the experimental measurements, to verify the prediction of engine performance and pollutant emissions.