## 3 Results and Discussion
Once the full-load potential of EGR was evaluated, the analysis was directed towards selected operating points that can be seen as representative of automotive use. As previously mentioned, four vehicle speeds were chosen, at 30, 50, 90, and 130 km/h. Even if they do not cover a wide range of conditions in terms of vehicle dynamics, constant speed operation at these four power levels gives an idea of the relative difference between the three configurations under scrutiny. A maximum EGR level of 30% (monitored in the intman-2 element highlighted in Figure 2; it should be noted that residual gas concentrations below 1% were predicted in this component for the cases without EGR, and therefore, the actual absolute recirculation values were slightly lower) was chosen as the top threshold, as it is well within the dilution tolerance of H\({}_{2}\) and was predicted to be achievable even at full load. Compared to the initial analysis that yielded the results shown in Figures 4-6, the target for the boost controller was set at 1 bar for throttled points and the various EGR rates resulted in variations of intake pressure (e.g., at 30 km/h, the intake pressure increased from 0.41 bar with 0% EGR to over 0.48 bar for 30% recirculation). Figure 7 shows the results obtained for the four vehicle speed values, with gasoline operation as the reference and four different levels of EGR with H\({}_{2}\). A surprising result is that the model predicted lower efficiency for H\({}_{2}\) 0% EGR compared to gasoline for all vehicle conditions except 130 km/h. Stoichiometric operation was considered for all three points (given the reduced load), and therefore, the main advantage of the gaseous fuel in full load conditions was no longer exploitable. High laminar flame speed reduces losses due to finite combustion time [38], but also significantly augments heat transfer [39]. Indeed, the model predicted an increase in this loss category of 20-30% for 30, 50, and 90 km/h, thus explaining the main mechanism behind the observed trend. It should be noted that the flow-based model of in-cylinder heat transfer was chosen for the simulations, as this was found to ensure the fundamental background required for predicting this specific effect when using hydrogen. A positive aspect is that applying EGR was predicted to ensure complete recovery of the lost efficiency at 30 and 50 km/h. This directly translates into an expected range that is directly proportional to the gas tank volume without other fuel effects. Figure 6: Full-load peak pressure and its maximum rise rate in cylinder #1. At 90 km/h, the maximum level of 30% EGR was found to still be below the efficiency mark calculated for gasoline fueling, and complete mitigation would require recirculation rates of around 40%, thus moving closer to the acceptable dilution limit [36]. Nonetheless, the main conclusion that EGR allows for the mitigation of negative effects when using H\({}_{2}\) is a positive aspect that highlights the benefits of this configuration during partial-load operation as well. The 130 km/h condition was quite close to the full-load level, and therefore, significant benefits in terms of efficiency were predicted for H\({}_{2}\) compared to gasoline (which was taken as running the engine at a relative air-fuel ratio of 0.9). Simply switching from the liquid to the gaseous fuel provided an increase of about 3% in fuel economy, whereas applying EGR increased benefits by up to 8%. Again, this translates to a significant augmentation of range, an essential aspect when considering that the 130 km/h condition was representative of highway driving (i.e., during urban use, finding a refueling station within a few kilometers is more likely compared to longer distances between refilling points, specific to highways).