## 7 Description of Vehicle and Test Cycles  
The vehicle used for the simulations is an interurban hybrid bus, which is coupled to the six-cylinder Diesel engine analyzed for this investigation. The main characteristics of this vehicle (mass, size, gearbox ratios, target mission) with a classical architecture were taken from the literature. Moreover, it is assumed that the vehicle runs on medium payload, with an overall vehicle mass equal to 12,600 kg. In the simulations, the vehicle is equipped with a six-speed manual gearbox and, according to the software gear shifting strategy, the gear shift lines are placed to allow the engine to work in its high efficiency region and to prevent it from running at excessive speeds. These lines are represented by engine torque and speed curves in Figure 11.  
To calculate the vehicle longitudinal dynamics, all the vehicle model blocks are provided with the requested architecture-layout data. Firstly, the simulations are carried out on the Diesel bus version with manual transmission. Then, a full hybrid parallel P2 configuration of the bus has been set up and investigated. The electric motor, placed between the engine flywheel and the gear box is a permanent magnet synchronous one. A \"torque split\" algorithm, together with a first-order thermal model, is used to test and validate the motor, which is simulated through steady-state maps. To match the different engine and motor  
Figure 11: Gear shift lines corresponding to a six-speed manual gearbox mounted on the studied inter-urban bus, reported on the BSFC map of the six-cylinder Diesel engine. speed ranges, a reduction gear connects the latter to the drive shaft between the clutches. The system is powered by a lithium-ion battery pack, characterized by a 1p110s (1 parallel, 110 series) arrangement working at 400 V, which has been designed taking into account the peak-power request, calculated by means of the \"torque split\" algorithm adopted, and of the overall energy storage requirements. Once the electric components are chosen, a proper tuning of the hybrid control unit (HCU) is performed in the vehicle simulation tool, acting on the specific torque-speed curves that define the operating mode of the hybrid system, and the battery SoC (state of charge) threshold levels [28]. As mentioned, a first set of parameters to be defined is represented by the torque curves, which are indexed by the revolution speed, generalized to the drive shaft between the clutches. These curves characterize different regions on a torque-speed plane: depending on the total torque required by the vehicle and its actual speed, the position of such working condition on the torque-speed plane determines the hybrid system operating mode. For the hybrid architecture considered, the torque-speed curves are referred to the engine BSFC map: this allows their correct positioning, aimed at reaching the best overall efficiency. In Figure 12, it is possible to distinguish four different curves, cited with their actual name in the vehicle simulation tool. To correctly place these curves, the underlying rationale of the hybrid powertrain must be pursued: letting the engine work in its high-efficiency regions. The lowest purple line defines a map region below which the engine is generally prevented from working: when the requested engine torque lies below this curve, the HCU determines an increase of torque request, so as to bring the operating point above the purple line. Then, the torque surplus is used to charge the battery through the motor generator mode. This curve has been placed to achieve a compromise: it prevents the engine from working in low-efficiency points, such as the bottom part of the map, so that above this curve an optimal operation of the engine is achieved. The region between the purple line and the yellow one defines the area in which the engine can work. If the required torque is located between the yellow line and the red one the engine is supported by the electric motor to provide the requested torque.