## 3 Results and Discussion  
The starting point for the analysis was to evaluate the two EGR configurations, i.e., low- and high-pressure circuits. Figure 3 shows the full-load operating points at 3000 rpm and peak power (i.e., 5250 rpm) in the compressor map domain; more to the point, it illustrates how the compressor rpm-pressure ratio points shifted from the situation with gasoline fueling (dark blue points) to hydrogen without EGR (light green points) and then at the end of the longer arrows when operating with H\({}_{2}\) at the maximum achievable recirculation level. Evidently, the use of H\({}_{2}\) required higher boost levels, which needed to be further augmented when also applying EGR. In the high-pressure (HP) configuration, the exhaust stream needed to be routed from upstream of the turbine; this had the advantage of a shorter circuit and avoided possible issues with impurities that could have damaged  
Figure 3: Compressor map used for the simulations and illustration of the operating points with gasoline (dark symbols) and H\({}_{2}\) (light symbols) at 3000 rpm full load as well as peak power; the paths for achieving high- (long arrow pointing left) or low-pressure EGR (long arrow pointing right) are also highlighted. the compressor. On the other hand, this also reduced the flow to both components while requiring higher boost pressure to circulate the additional flow through the engine. Figure 3 illustrates this two-step passage (i.e., switching from gasoline to H\({}_{2}\) and then applying EGR), which basically moved the operating point closer to the surge line. It also highlights the fact that low-pressure (LP) EGR tended to move the operating point to a more favorable region of the map, with higher compressor efficiency and away from the surge line (albeit with a longer circuit and possible complications). Initial evaluations of the actual space available in the engine bay for placing the EGR heat exchanger and other routing components revealed that both configurations were feasible, even if low-pressure recirculation required a longer circuit. Another advantage of the LP configuration is that lower pressure in the recirculation circuit favored implementation (i.e., even if the circuit is longer, low pressure can be more easily implemented). For these reasons, the LP version was considered for further analysis. Simulations were performed throughout the engine speed range at full load in three configurations, i.e., gasoline (reference), H\({}_{2}\) fueling, and H\({}_{2}\) with LP EGR. The maximum recirculation potential was sought after; therefore, for the EGR case, the wastage was simply kept closed and the EGR valve was used as a means of controlling the engine output. More to the point, the controller opened the recirculation valve once the imposed brake power was achieved so the compressor would aspirate air mixed with recirculated exhaust gas, resulting in a lower energy input. Figure 4 shows the results for the three configurations. The model predicted that the same level of engine power (i.e., full-load OEM data) could be obtained in all three cases for a crankshaft speed of above 2500 rpm. Below this threshold, the boosting required for compensating the loss in volumetric efficiency could not be achieved when using H\({}_{2}\). This was due to the fact that the model predicted surge margins close to zero and limited wastage closure. Indeed, there was no EGR potential for this range (i.e., between 1000 and 2500 rpm). At the higher end of the engine speed range, significant EGR of close to 30% could be used. This figure is well within the dilution tolerance of hydrogen [36], and therefore, there should not be any issues with combustion stability. As expected, there was a benefit in efficiency when switching from gasoline to hydrogen. Figure 5 shows that relative gains of up to 10% were predicted, mainly due to the fact that stoichiometric H\({}_{2}\) operation was considered possible due to less evident knock limitations compared to gasoline.