## 3 Results and Discussion  
The results of gasoline- and hydrogen-fueled engine simulations are presented in terms of BTE instead of BSFC because of the different lower heating values of the two fuels. Figure 3 shows the simulated (a) and experimental (b) BTE maps of the gasoline engine as a function of engine speed and BMEP (proportional to engine torque). The maximum efficiency is 0.265, and it is located below the full load condition for the above-mentioned considerations. Figure 4 shows the BTE maps coming from the hydrogen-fueled engine simulation with the TC (a) configuration and the VCSC (b) configuration; some relevant aspects can be highlighted: the maximum BTE of the SC engine (0.277) is higher than the gasoline one but lower than the TC one (0.305); furthermore, the maximum BTE zone of the SC engine is well below the full load condition, such as in the gasoline BTE map while, in the TC configuration, the maximum efficiency zone is located at maximum BMEP. The TC configuration performs better than the VCSC one likely due to high compression work at high MAP levels in the second case. Figure 4: BTE maps of the hydrogen-fueled engine. Figure 3: Simulated (**a**) and experimental (**b**) BTE maps of the gasoline-fueled engine. When the engine equips a conventional vehicle, for a given road load (i.e., a required power), there are infinite possible combinations of engine speed and load (operating points) that produce that power; however, only one is the best efficiency point. This scenario is represented in Figure 5 where the gasoline-fueled engine BTE map is displayed together with two curves at constant required power (10 and 20 kW); the curves cross the BTE surface in many different points but only the two marked with a green cross are the best efficiency points (i.e., the points where the constant power curve is tangent to a constant BTE curve). The main difference between a conventional vehicle and a split-power hybrid one is that, in the first case, the ICE operating point depends on both the vehicle operating point and the current gear ratio between engine shaft and wheels while, in the second case, the engine can always be operated in the best efficiency points due to the decoupling between ICE and wheels and to the application of energy management control strategies (EMCS). To find the mean BTE of the three engine configurations described above, equipping a conventional vehicle, one should evaluate the average BTE over all the points in the maps reported in Figures 3a and 4a,b because all the points are potential operating conditions. Table 2 shows the mean BTE comparison for the three analyzed engine configurations: the same trend of the maximum BTEs is observed with the TC hydrogen-fueled engine that outperforms the gasoline operated one (almost 20% mean BTE increase), and the TC setup is slightly better than the VCSC one. As already stated before, the hybridization obtains the best results from a conventional engine, and the scope of this work is to compare the hydrogen- and gasoline-fueled engines at their best operating conditions. Assuming that, theoretically, a split-power HEV is able to operate the engine in its higher BTE operating point for each required power, the best efficiency path (BEP) is defined as the curve in the BTE map that links all the maximum BTE points for each possible required power from zero to the maximum (43.1 kW). Figure 6 shows the gasoline BTE map with different constant power curves and, in green, the BEP that links all the tangent points between a constant power curve and an \begin{table}
\begin{tabular}{c c c} \hline \hline
**Engine Configuration** & **Mean BTE** & **\% Increase Compared to Gasoline** \\ \hline gasoline & 0.211 & 0 \\ hydrogen VCSC & 0.246 & 16.5\% \\ hydrogen TC & 0.253 & 19.5\% \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The mean BTE of the three engine configurations in a conventional vehicle. Figure 5: The best efficiency points at constant required power (gasoline). iso-efficiency curve.