## 4 The Simulation Analysis of VAPP
When VAPP speed is set at 1500 r/min and angle as 10\({}^{\circ}\), and the pressure of Ports B and T relief valve are set at 0 MPa in the simulation model, then the pressure and the flow curve of one of the piston chambers is as depicted in Figure 7. The figure shows that the pressure of piston chamber remains constant at around 0 MPa from interval bottom dead center (BDC) to top dead center (TDC) in Port A. The pressure of Port A suddenly rises to about 6 MPa when piston passes TDC. It then reduces to about 4 MPa when piston passes the non-dead point triangular groove and reaches Port T. The pressure of piston chamber reduces to about 0 MPa periodically when piston passes Port B, Port T, BDC and reaches Port A. The reason for the above phenomenon is that the high pressure oil in the oil discharging port pours into piston chamber instantaneously when the piston filled with low pressure oil move from oil suction Port A to oil discharge port. Because the piston rotates constantly, the pressure in the piston chamber increases gradually. The piston begins to discharge oil when the pressure in the chamber is the same as the pressure in the oil discharging port. At this time, because of the inertia of the fluid and the damping effect of the distribution flow port, the resistance of discharged oil is high at the beginning while the pressure in the piston chamber exceeds the discharging pressure, thus becoming positive pressure overshoot. As the piston moves from the oil discharging port to the oil suction port, the high oil pressure port is connected with the low oil pressure port in the piston chamber. Therefore, pressure is released, therefore, pressure is released and therefore overshoots. This periodic change of the pressure and flow causes a periodic pulsation of the oil in the piston chamber. Piston speed is set at 900 r/min in the VAPP and the angle of the swash plate is 8\({}^{\circ}\), 10\({}^{\circ}\) and 12\({}^{\circ}\). Port A of the valve plate is for sucking oil, while Port B and Port T of the valve plate are for discharging oil. The flow of Ports B and T changes over time, as shown in Figure 8. The flow of Ports B and T increases along with the increase in the swash plate angle so that it could realize control of variable mechanism, as detailed in Figure 2. The angle of the indexing circle corresponding with Port B and T of valve plate is 60\({}^{\circ}\) and 40\({}^{\circ}\), respectively. When the angle of swash plate is 12\({}^{\circ}\), the flow of Port B is about 10.5 L/min while the flow of Port T is about 7.3 L/min. Since the indexing circle angle of Port B is bigger than that of Port T, the flow pulse period of Port B is longer than that of Port T. In addition, the flow crest of Port B is higher than that of Port T.  
The angle of swash plate is set at 10\({}^{\circ}\) in the VAPP simulation model, pump speed at 600 rpm, 900 rpm and 1200 rpm, separately, and the pressure loaded on Ports B and T is 0 MPa. The flow of Port B and Port T changes over time, as shown in Figure 9. As detailed in Figure 9, when the swash plate angle is constant, the flow of Ports B and T increases along with the increase of pump speed and the flow pulsation decreasing along with the increase of pump speed. When the pump speed is 1200 r/min, the flow of Port B is about 11 L/min, whereas the flow of Port T is about 7.8 L/min. Figure 7: Instantaneous cylinder pressure and flow without load at 1500 rpm. When the pump speed is set at 600 r/min, the angle of swash plate separately at 8\({}^{\circ}\), 10\({}^{\circ}\) and 12\({}^{\circ}\), and the pressure loaded on Ports B and T are 0 MPa and 10 MPa, separately, the flow of Ports B and T and the pressure of piston chamber are as indicated in Figure 4:0 It is observable from Figure 10a that the larger swash plate angle may make the pump to reach the set speed more quickly. In addition, it is evident from Figure 10a,c that the flow of Ports B and T increase along with the increase of swash plate angle.