Figure 6 shows the variation of piston velocity when the initial pressure in the receiver is 0.05MPa, 0.19MPa and 0.28MPa respectively in the process of exhausted-air reclaiming. From Figure 6, it can be seen that the higher the initial pressure Pc0 is, the lower the piston velocity and the longer the piston motion time interval. As shown in Figure 6, the higher the initial pressure Pc0, the smaller the velocity difference between the maximum and minimum piston velocity