## 3 EMS Selection and Description
### Selected Energy Management Strategies
#### 3.2.2 Alternative PFCSs (SA-PFCS and DA-PFCS)  
In order to avoid this inherent drawback of a conventional PFCS, the presented EMSs use a frequency decoupling technique, implemented in the form of a sliding average, to filter the power demanded by the vehicle. The sliding average window is set as one of the control system parameters, allowing for the weighting of the sliding average input data in order to be able to modify the importance of older or newer data during the filtering process. Applying a sliding average to the instantaneous power demand value thereby eliminates the high-frequency component of this measurement, making the strategy suitable for application in FCEVs.  
Additionally, an FSM is used to decide whether the generation unit (FC) shall be turned on or off, varying from EV-MODE and FC-MODE, depending on the battery pack SOC and the calculated power demand for the FC (\(P_{FC}\)). Specifically, the FC is always turned on when the SOC falls below a minimum value (\(SOC_{min}\)), and it is turned off when it exceeds a maximum value (\(SOC_{max}\)). In addition, in order to maintain a predefined target SOC, the FSM is further implemented to allow the FC to be switched on when the SOC is lower than a target value (\(SOC_{obj}>SOC_{min}\)) but the \(P_{FC}\) exceeds a certain threshold, thus aiming to achieve a high overall powertrain efficiency. Furthermore, the FC is also activated when the power demanded from the vehicle exceeds a threshold at which the aid of the hybrid part, in combination with the power available from the battery, is deemed convenient. Figure 2 shows the structure of the FSM for the presented PFCS.  
For the first EMS, known as Standard Alternative PFCS (SA-PFCS), \(P_{FC}\) is the power demanded to the FC when it is switched on. However, for the second EMS, known as the Discretized Alternative PFCS (DA-PFCS), up to six different levels of power demand to the FC are selected depending on the value of \(P_{FC}\) when the FC is turned on. Thus, the DA-PFCS is just a discretization of the SA-PFCS that aims to reduce FC wearing and enhance durability, which could potentially be achieved using several constant operating points instead of directly following the \(P_{FC}\) value. The SA-PFCS is the EMS used in the comparison between the two different models, the AVL Cruise MTM and the MATLAB Simulink model, described in Section 6.  
Both strategies allow for the utilization of the FC between the minimum load value of 15% and the maximum load value of 90%, thus excluding the areas with the lowest efficiency on the FC polarization curve. The DA-PFCS then discretizes the \(P_{FC}\) values into six levels 15% apart, always selecting the one closest to the filtered \(P_{D,Velt}\).  
In this paper, simulations with both of the presented EMSs are carried out for a selected driving cycle. The results, shown in Section 7, are analyzed in order to determine the most convenient PFCS control strategy in terms of fuel consumption, electrical consumption, and durability for both the FC and the HV battery.  
Figure 2: Structure of the FSM for the presented SA-PFCS and DA-PFCS.