## 4 Experimental Validation
### Energy Recovery Rate with Different Braking Intensity  
Similar to the simulation process with different braking intensity, the braking intensity ranged from 0.3 to 0.5 at the interval of 0.05 when the initial pressure of the accumulator was kept at 19 MPa and the initial braking speed was kept at 300 rpm. Comparisons of the simulation data and the experimental data were exhibited in Figures 9-13 corresponding to the braking intensity from 0.3 to 0.5. From the comparisons of speeds of the flywheel in Figures 9a, 10a, 11a, 12a and 13a, it could be observed that the actual peak speed was larger than 300 rpm, because response of the accumulator had a time delay. Meanwhile, from the comparisons of pressures of the accumulator in Figures 9b, 10b, 11b, 12b and 13b, it could be found that the actual reduction of the pressures was larger than the simulation data, because there were pressure loss in the hydraulic system and frictional resistance for the inertial flywheel, which indicated that the accumulator must release more energy to drive the flywheel for the given initial braking speed. Similarly, the recycled energy collected by the accumulator was also reduced. In addition, there were obvious fluctuations of the pressure during the braking time, because sudden reversal of the hydraulic fluid led to tremendous impact to the hydraulic system. Moreover, fluctuations of the pressure during the braking time had no influence to working status of the directional valve, because the directional valve was adjusted by controller of the experimental bench according to the driving condition. Meanwhile, fluctuations of the pressure during the braking  
Figure 8: Schematic diagram of the experimental bench.  
time affected the overall system control and the mechanical integrity indeed, but it had no influence on the research results in this study, because calculation of the energy recovery rate used the overall trends of the pressure instead of each individual data. Some new equipment, such as pulsation dampener or pulse attenuator, will be added in the experimental bench to reduce fluctuations of the pressure in further improvement.  
The actual braking time, change of the kinetic energy, recycled energy amount, and the energy recovery rate with different braking intensity were calculated and summarized in Table 6. It could be observed that the optimal energy recovery rate was 83.33% when the braking intensity was 0.4. Comparing with the simulation data in Table 3, it could be found that variation tendencies of the braking time and energy recovery rate in experimental process and those in simulation process were consistent, and the values were decreased accordingly. The major reason for this phenomenon was that pressure loss in the hydraulic system and frictional resistance for the inertial flywheel were not taken into consideration in the simulation process.  
Figure 10: Comparison of the experimental result and simulation result when the braking intensity was 0.35: (**a**) Evolution of speed of the flywheel, (**b**) Evolution of pressure of the accumulator.  
Figure 9: Comparison of the experimental result and the simulation result when the braking intensity was 0.30: (**a**) Evolution of speed of the flywheel, (**b**) Evolution of pressure of the accumulator.  
Figure 11: Comparison of the experimental result and simulation result when the braking intensity was 0.40: (**a**) Evolution of speed of the flywheel, (**b**) Evolution of pressure of the accumulator.  
Figure 12: Comparison of the experimental result and simulation result when the braking intensity was 0.45: (**a**) Evolution of speed of the flywheel, (**b**) Evolution of pressure of the accumulator.  
Figure 13: Comparison of the experimental result and simulation result when the braking intensity was 0.50: (**a**) Evolution of speed of the flywheel, (**b**) Evolution of pressure of the accumulator.