An increasing emphasis is being placed in the vehicle development process on transient operation of engines and vehicles, and of engine/vehicle integration, because of their importance to fuel economy and emissions. Simulations play a large role in this process, complementing the more usual test-oriented hardware development process. This has fueled the development and continued evolution of advanced engine and powertrain simulation tools which can be utilized for this purpose. This paper describes a new tool developed for applications to transient engine and powertrain design and optimization. It contains a detailed engine simulation, specifically focused on transient engine processes, which includes detailed models of engine breathing (with turbocharging), combustion, emissions and thermal warm-up of components. Further, it contains a powertrain and vehicle dynamic simulation.These two simulations have been coupled into a single tool, which combines the vehicle and powertrain model with the detailed engine model, extending thus the overall modeling capabilities available for system analysis. In addition, this coupled methodology has been further integrated with a general-purpose system dynamics and control simulation tool, to allow control analysis of the whole engine/powertrain system. Several examples of coupled system simulations are presented and discussed.