## 1 Introduction  
Nowadays internal combustion engines play a crucial role in worldwide mobility. A general concern about climate change imposes the adoption of an environmentally sustainable policy, as well relevant efforts towards a reduction of fuel consumption and compliance with strict emission regulations [1, 2]. In the past decades, analytical and numerical models provided fundamental support to the design of IC engines, in addition to the necessary experimental campaign on real engine prototypes. Today reliable 1D CFD tools are commonly used in the community of engineers and researchers to investigate the behavior and performance of new engines [3, 4]. In general, state of the art 1D simulation models can calculate the steady-state engine maps (in terms of air mass flow rate, torque and power, max. cylinder pressure, fuel consumption and emissions), being able to represent every operating condition just starting from a few operating points analyzed in the experimental laboratory (map-based approach) [5; 6]. This is a very powerful and fast simulation approach, but the hypothesis of steady state working condition represents a major limitation. In fact, the IC engine is an unsteady volumetric machine, and its behavior is deeply influenced by transient conditions due to mechanical, thermal and fluid-dynamics inertias, not considered in a map-based approach. A robust simulation method is required to account for the transient conditions of the IC engine coupled to the vehicle and its components, to obtain more realistic results in terms of fuel consumption and emissions [7].  
The present work wants to highlight the application of the new simulation platform based on Gasdyn, for the 1D thermo-fluid dynamic engine simulation, and Velodyn, for the simulation of the vehicle longitudinal dynamics and powertrain composition, to cope with a typical transient condition simulation. The two simulation tools are integrated in a \"co-simulation\" framework, which allows the two software to interact in real-time within the MATLAB(r) Simulink(r) environment. The vehicle simulation tool interacts with the virtual engine to proceed forward in time and follow the prescribed test cycle.  
In particular, the adoption of a specific numerical scheme, based on a staggered leapfrog, finite volume method applied to coarse meshes, allows to preserve the mass conservation and accuracy of the 1D fluid dynamic modeling of the IC engine, resulting in an optimal solution for the simulation of driving cycles in time frames comparable to real-time. In this way, the 1D simulation tool allows the investigation and comparison of different solutions of hybrid architectures in terms of performance, fuel consumption and emissions.  
A six-cylinder, 210 kW diesel engine has been modeled in detail and then validated in steady-state conditions, on the basis of experimental engine map data. Then the 1D engine model was made compatible for the two-way coupling with the vehicle simulation tool, via an S-function block in the Simulink(r) framework.  
Finally, the Co-Simulation environment has been applied and validated against the traditional map-based approach, which is based on the available experimental engine maps of the IC engine. The test case considered is a hybrid powertrain of a commercial bus. An analysis of fuel consumption in real driving emission (RDE) cycles is discussed, considering both the conventional and hybrid architectures of the bus, mainly focusing on the hybrid components design and hybrid control unit (HCU) logic settings.