## 4 Case Study
### Energy Consumption Analysis of the Two Presses  
During the forging process, each motor pump in the drive unit changes the state of its access to the hydraulic system according to the energy demand of different operations, and one drive unit, which is useless in an operation, is set at an unloading state by switching the pressure-relief valve. Table 4 shows the compositions of the drive system of the two hydraulic presses and the number of motor pumps connected for each processing stage. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & **Total Number** & **WT** & **FF** & **PS** & **UL** & **FR** & **SR** \\ \hline Hydraulic press with PFS & 7 & 0 & 3 & 7 & 1 & 7 & 2 \\ Traditional hydraulic press & 9 & 0 & 3 & 9 & 1 & 9 & 3 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Compositions of the drive system and the number of motor pumps used in different operations. WT: waiting within a working cycle, Stages 1 and 8, WT; FF: fast falling, Stage 2; UL: unloading, Stage 5; FR: fast returning, Stage 6; SR: slow returning, Stage 7. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Forging Method** & **Fast Forging** & **Constant Forging** \\ \hline  & 9.7 MPa & 9.1 MPa \\ Traditional pipeline pressure & 20.3 MPa & 18.5 MPa \\ Buffering system pressure & 6.3 MPa & 5.2 MPa \\ Noise intensity & 75 dB & 72 dB \\ Traditional noise intensity & 105 dB & 100 dB \\ Oil temperature & 35.7 \({}^{\circ}\)C & 40.2 \({}^{\circ}\)C \\ Body vibration & Slight & Slight \\ Pressure impact reduction & 10.6 MPa & 9.2 MPa \\ Pressure decline rate & 52.2\% & 49.73\% \\ Noise decline rate & 28.6\% & 28\% \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Results of the experiments at the unloading stage for different forging methods. In this study, the energy consumption of a hydraulic press was scaled by measuring the active electricity consumption of motor pumps. A power meter and an data acquisition card were selected to measure and receive the voltage and current through the motors at any time, respectively. Then, the real-time measured data were transmitted to the PC after processing. During the forming process, the energy efficiency of a hydraulic press can be expressed as follows,  
\[\eta=\frac{E_{Forming}}{\sum_{i}\text{E}_{i-Input}}\cdot 100\% \tag{7}\]  
where \(i\) is all the forming operations, including FF, PS, UL, FR, SR, and WT. E\({}_{i-Input}\) is the energy consumed by the hydraulic machine to complete the \(i\)-th operation, which can be obtained by Equation (8). \[\text{E}_{i-Input}=\int_{t_{i-Start}}^{t_{i-End}}P_{i}(t)dt=\sum_{t_{i-Start}}^{ t_{i-Start}}P_{i}\Delta t \tag{8}\]  
where \(P_{i}\) is the active power of each operation, which can be obtained by the power meter, and \(t_{i-Start}\) and \(t_{i-Start}\) are the start time and the end time of the \(i\)-th operation, respectively. According to Equation (8), the active electrical energy consumption of motors was obtained by experiments. The electricity consumption, useful energy, and energy efficiency of the two presses from experimental testing are shown in Tables 5 and 6. The traditional hydraulic press is equipped with 9 sets of motor pumps, of which the installed power is up to 1050 kW. According to Table 6, the total electric energy consumed to complete a working cycle is 3227.66 KJ, and only 31.01% of the input energy is converted into useful energy. Besides this, as a result of the pipe-valve energy loss, overflow energy loss, and other energy losses, only 17.35% of the input energy is used to form the workpiece. However, by integrating PFS into the press, the energy consumption of each stage significantly improved.