## 4 Results and Discussion  
As shown in Figure 6, the polarization curve of the PEM fuel cell in the MFCS model established in this paper is in good accordance with the experimental data from the established 70 kW fuel cell test bench. The simulation results are verified with experimental data to ensure the accuracy of the established MFCS model. Figure 7 shows the parameter variation of the MFCS under the step current. The MFCS exhibits a good dynamic response, as reflected by the response of the voltage of the stack, air mass flow, and pressure. In contrast to the air supply subsystem of the SFCS, there is a common rail buffer in the MFCS. As shown in Figure 8, the volume of the common rail buffer affects the MFCS performance in terms of the maximum pressure drop, maximum power, and total power consumption. The relationship between the maximum pressure drop (Max PD) and the volume of the common rail buffer is shown in Figure 8a, where the maximum pressure drop decreases rapidly with the volume from 50 L to 200 L and then slows down. Figure 8b shows the relationship between the maximum power of the air compressor and the volume of the common rail buffer. It can be seen that as the volume of the common rail buffer increases, the maximum required power of the air compressor decreases gradually. However, when the volume of the common rail buffer reaches 200 L, the maximum power of the air compressor remains constant. As shown in Figure 8c, the power consumption of the air supply subsystem under the CWTVC condition gradually decreases as the volume of the common rail buffer increases. When the volume increases from 50 L to 150 L, the power consumption decreases significantly. Then, when the volume increases from 150 L to 600 L, the power consumption still decreases slowly. After that, the power consumption almost does not change with the increase in the common rail buffer volume. In the air supply subsystem, the common rail buffer plays the role of storing and stabilizing the air  
Figure 5: Radiator MAP diagram. Figure 6: MFCS model validation. pressure; when the pressure and flow rate fluctuations are large, the buffer can absorb and stabilize these fluctuations. Within a certain volume range, the buffer tank is able to better stabilize the air pressure fluctuations in the subsystem, with a significant gain effect. However, as the volume increases further, although the buffer tank is better able to absorb and stabilize air pressure fluctuations, the gain effect is reduced. Therefore, the reduction in maximum pressure drop and power consumption slows down. Obviously, a common rail buffer tank volume of 200 L is a good choice for a 210 kW MFCS. Figure 8: _Cont._  
Figure 7: Dynamic response of MFCS parameters: (**a**) Current; (**b**) Voltage; (**c**) Mass flow; (**d**) Inlet pressure. After determining the volume of the common rail buffer, it is important to observe the internal pressure under the CWTVC condition to ensure that the pressure is always greater than the air inlet pressure of the three stacks. The variation of pressure in the common rail buffer under the CWTVC condition is shown in Figure 9. The pressure of the common rail buffer adopts a constant pressure control strategy with a set pressure of 2.75 bar. An air pressure of 2.64 bar at 1283 s is the minimum instantaneous pressure in the common rail buffer under the CWTVC condition, which satisfies the maximum of cathode inlet pressure. Since the hydrogen subsystem delivers hydrogen that is in the process of depressurization from the high-pressure hydrogen storage tank to the PEM fuel cell, the pressure setting of the hydrogen inlet common rail can be set without considering the energy consumption. The pressure in the hydrogen inlet common rail under CWTVC conditions is shown in Figure 10. The role of the hydrogen inlet common rail is to provide a certain buffer space and time for the hydrogen entering into the stack so that the release of hydrogen from the high-pressure hydrogen storage tank does not have to completely follow the system demand.