## 3 Results and Discussion
In other words, the properties of hydrogen that enable much faster flame propagation with respect to \"classical\" fuels were considered to fully mitigate possible auto-ignition phenomena. Even if the relatively simple approach of imposing a RON value of 130 in the knock sub-model cannot give detailed insight into such complex and highly stochastic phenomena, the fact that the model predicted a reduced auto-ignition tendency of hydrogen suggests that full-load lambda 1 operation could be possible. Figure 4: Full-load power curve with gasoline, hydrogen, and maximum achievable EGR. Applying EGR ensured a further slight improvement in efficiency, especially in the mid-rpm and peak power region. Evidently, increased boost levels would be needed with an intake pressure of around 30% higher when switching from gasoline to H\({}_{2}\) and of close to 60% when applying EGR. Such elevated manifold pressure (absolute values of close to 2.5 bar) is not uncommon in state-of-the-art automotive SI engines but would be a point to consider when evaluating the conversion of an existing power unit. More to the point, even if the turbine-compressor assembly were capable of operating in the region predicted to ensure the intended boost pressure for EGR, control limitations may prevent actual implementation. Therefore, the control system may require additional hardware modifications (e.g., the pneumatic actuator that controls the wastewater) and not just ECU remapping. The actual relative gain in efficiency of up to 3% compared to non-diluted operation is, however, another important positive aspect, especially as it can be achieved in the high-load region, i.e., most likely to be used during highway driving and therefore with a significant impact on vehicle range. Another essential influence of using H\({}_{2}\) is that its high laminar flame speed results in increased peak pressure, as well as a higher pressure rise rate. Indeed, much higher values were predicted throughout the engine speed range, especially at low rpm (Figure 6; please note that only the results for cylinder #1 are shown). The maximum difference was recorded at 2500 rpm, for which peak pressure was almost double for H\({}_{2}\) compared to gasoline. This can be explained by the fact that, at low rpm, the laminar flame speed exerted a more significant influence on combustion development, and therefore, the difference between the two fuel types was much more evident. This effect would be even more prominent at a lower crankshaft speed, but the lack of boost in that operating region resulted in lower incylinder pressure during the cycle. When looking at the relative difference between H\({}_{2}\) and the H\({}_{2}\) EGR case, it is evident that peak pressure was completely comparable throughout the range; this can be seen as a positive aspect, meaning that even if higher boosting is required when recirculating the exhaust gas, the actual stress on the cranktrain is not augmented significantly. There would still be an increase with respect to the reference level with gasoline, but overall, it could be acceptable. The maximum pressure rise rate (PRR) was instead predicted to be significantly different when using EGR. Recirculation almost lowered this parameter to levels comparable to those recorded for gasoline operation and close to the 6-7 bar/deg range usually recommended as acceptable in terms of combustion noise [37]. Instead, at a low rpm, the PRRmax figure was predicted to be well over this threshold and even over levels usually encountered during HCCI operation. Nonetheless, one of the main conclusions is that EGR has the potential to significantly reduce full-load pressure rise rates while ensuring benefits in terms of efficiency; the only shortcoming is that it requires higher boost levels so as to render possible the augmented flow through the engine. Figure 5: Full-load brake efficiency and absolute intake pressure.