*3.1. Hydraulic Test Bench*

The test bench for the hydraulic system was designed to measure the flow of energy through the accumulator while charging and discharging. The main components of the system were the battery, the electric motor, the hydraulic pump, and the hydraulic accumulator. The battery was the main source of power for the system, and it was connected to the electric motor with a DC/AC inverter. The electric motor was used to drive the hydraulic pump, which moved the fluid from the reservoir to the hydraulic accumulator. The hydraulic testbench and the DC/AC inverter are shown in Figures 1 and 2, respectively. The schematic representation of the test bench that was designed and constructed for the hydraulic accumulator is presented in Figure 3.

**Figure 1.** Hydraulic test bench.

**Figure 2.** DC/AC inverter used in the hydraulic test bench.

**Figure 3.** Schematic representation for the hydraulic accumulator test bench.

A list of the components used in the test bench is presented in Table 3.


**Table 3.** List of components used in the hydraulic test bench.

The test bench could be operated in either the charging or discharging modes. During charging mode, the electric AC motor was turned on and was used to move the hydraulic pump. Valve *V*<sup>1</sup> was switched on, allowing flow to go through the accumulator while valve *V*<sup>2</sup> was closed. The relief valve RV1 was closed unless a relief pressure of 3000 psi was reached. A schematic representation of the test bench during charging mode is presented in Figure 4. The red lines show the high-pressure lines.

**Figure 4.** Hydraulic test bench during charging mode.

During charging mode, the measured variables were the current and voltage from the battery connected to the electric AC motor powering the pump and the pressure and flow rate going to the accumulator. The battery and the AC motor were connected through a DC/AC inverter. With these variables, it was possible to calculate the power input from the battery, the power going to the accumulator, the consumption of electric energy, and the energy saved in the accumulator. The speed of the electric motor was changed with a motor controller, which was connected between the battery and the electric motor. The schematic representation of the electric system used to power the pump is shown in Figure 5. The technical data of the battery, the inverter, and the electric motor are shown in Table 4.

**Figure 5.** Schematic representation of the electric system.


**Table 4.** Technical data of the electric system.

To measure the efficiency of the hydraulic accumulator, a similar test for the discharge cycle was developed. In the discharge test, the output load was simulated with a variable orifice, *V*3. The load applied to the hydraulic system increased when *V*<sup>3</sup> was progressively closed. A no-load condition was simulated when the valve was completely open. A schematic representation of the system during discharge mode is presented in Figure 6.

**Figure 6.** Hydraulic test bench during discharging mode.

To discharge the accumulator, valves *V*<sup>1</sup> and *V*<sup>2</sup> were open, while the orifice was open just between 0% and 10%. The measured variables during the discharge mode were the pressure and the flow rate just before the orifice; with these variables, it was possible to calculate the instantaneous hydraulic power used from the accumulator. The output power could be calculated by multiplying the flow rate and the pressure, and the efficiency of the hydraulic accumulator could be derived. Equations (1) and (2) were used to calculate the power input and power output in the accumulator, and Equation (3) was used to calculate the efficiency, the nomenclature shown in Table 5 describes the variables used. The results for a single test are shown in Figure 7 as a reference.

$$P\_{In} = Q\_{In} p\_{Acc, In} \tag{1}$$

$$P\_{\text{Out}} = Q\_{\text{Out}} p\_{\text{Acc,Out}} \tag{2}$$

$$p\_{Acc} = \frac{\int Q\_{Out} p\_{Acc,Out} dt}{\int Q\_{Out} \dots \quad \dots} \tag{3}$$

$$\eta\_{A\text{cc}} = \frac{J \cdot \omega \cdot \omega \cdot \omega \cdot \dots \cdot \omega \cdot \dots}{\int Q\_{\text{ln}} p\_{A\text{cc}, \text{ln}} dt} \tag{3}$$


**Table 5.** Variables used for determining the instantaneous efficiency of the accumulator.

**Figure 7.** Pressure in the accumulator during the charging and discharging process.

The results for pressure and flow rate in the accumulator during charging are shown in Figure 8. This plot shows a sudden rise in pressure when *V*<sup>1</sup> was opened and a steady increase in pressure once the pre-charge pressure was reached. It took approximately 17 seconds to fully charge the accumulator to its maximum pressure of 2800 psi. The flow rate plot showed a sudden increase in flow rate and a decrease in flow rate once the pre-charge pressure was reached.

The results for pressure *p*<sup>2</sup> and flow rate *Q*<sup>2</sup> during discharging are shown in Figure 9. The discharge time for the accumulator was approximately 3 seconds with the valve 25% open.

**Figure 8.** Pressure and flow rate while charging.

**Figure 9.** Pressure and flow rate while discharging.

Several experiments like the one described in this section were carried out to calculate the efficiency and performance of the hydraulic accumulator. Load conditions were changed in all the experiments by changing the orifice area. A list of the experiments is shown in Table 6. The 100% value for the orifice area was 11.7 mm2.


**Table 6.** List of experiments performed in the hydraulic test bench.
