## 4 The Simulation Analysis of VAPP
Since the working pressure of Port B was setting at 10 MPa, the flow pulsation decreases along with the increase of the swash plate angle, while the flow pulsation of Port T does not change significantly. It is evident from Figure 10d that although the working pressure of Port B is constant, the pressure of piston chamber increases along with the increase in the swash plate angle. When the swash plate angle is 8\({}^{\circ}\), the maximum pressure of piston chamber is 11 MPa. However, when the swash plate angle is 12\({}^{\circ}\), the maximum pressure of piston chamber is 13 MPa. Figure 4:1 Slots B and T output flow without load at 900 rpm: (**a**) Slots B output flow; and (**b**) Slots T output flow. Figure 4:2 Slots B and T output flow without load: (**a**) Slots B output flow; and (**b**) Slots T output flow. In the VAPP model, when the pump speed is set at 600 r/min, the angle of swash plate is 8\({}^{\circ}\), 10\({}^{\circ}\) and 12\({}^{\circ}\), separately, and the pressure loaded on Port B and Port T are 0 MPa and 10 MPa, respectively. Under these circumstances, the flow of Ports B and T is as shown in Figure 4:3 As evident in Figure 11a, the flow of Port B has flow pulsation of about 2 L/min when swash plate angle is 8\({}^{\circ}\), 10\({}^{\circ}\) and 12\({}^{\circ}\). As Figure 11b details, the flow of Port T changes in the periodic law, although there is an observed discontinuous flow. The reason for this phenomenon is that the pressure is loaded on Port T, thus the internal flow from Port T to Port B is increased. In the actual work of the pump, the phenomenon is not allowed. Therefore, Port T cannot be loaded alone or the pressure loaded on Port T cannot exceed the pressure which is loaded on Port B in the practical applications of VAPP. When the pump speed is set at 600 r/min, the angle of swash plate \(\beta\) is 8\({}^{\circ}\), 10\({}^{\circ}\) and 12\({}^{\circ}\), separately, and the pressure loaded on Ports B and T is 10 MPa. The pressure and flow of Port B, Port T and the piston chamber are as illustrated in Figure 4:4 As presented in Figure 12a,b, when the swash plate angle is big, the response of Port B and Port T is quick. Figures 10a and 12c display that when the pressure loaded on Port T is constant, more pressure is loaded on Port B and the flow becomes less. By comparing Figures 8 and 11, it can be deduced that when the pressure difference between B and T is great, the flow pulsation amplitude is large, and vice versa. The reason for this phenomenon is that the pressure difference between Port B and Port T is large. Therefore, the flow between Port B and Port T flow from the high-pressure area to the low-pressure area. It means that the flow would be stable if the pressure is loaded simultaneously on Port B and Port T of the VAPP. As illustrated in Figure 12e, when the swash plate angle is 12\({}^{\circ}\), the pressure pulsation amplitude in the piston chamber is about 2 MPa. However, when the swash plate angle is 8\({}^{\circ}\) the pressure pulsation amplitude is 1 MPa. In conclusion,  
Figure 4:5 Ten megapascals load in Slots B at 600 rpm: (**a**) Slots B output pressure; (**b**) Slots B output flow; (**c**) Slots T output flow; and (**d**) instantaneous cylinder pressure. In the VAPP model, pump speed is set at 1200 r/min, the angle of swash plate \(\beta\) separately is \(8^{\circ}\), \(10^{\circ}\) and \(12^{\circ}\), and the pressure loaded on Ports B and Port T is 10 MPa. Under this working condition, the pressure change overtime of Port B and Port T is as displayed in Figure 4:6 As shown in Figure 13a, when \(\beta=8^{\circ}\), the pressure gradient stabilization time of Port B and Port T is 140 ms and 170 ms, respectively. When \(\beta=12^{\circ}\), the pressure gradient stabilization time of Port B and Port T is 80 ms and 120 ms, respectively. This indicates that the larger the angle of the swash plate is, the quicker the pressure response speed of the discharging port is. In addition, the system can reach a stable situation more quickly under the condition of even load.