While running, each screw works twice per rotation so the channel space is fully utilized to achieve more exhaling volume compared with the same structural & dimension air compressor. The screw slot depth of screw decreases with the increase of pressure. When exhaling finishes and the star wheel separate from the screw slot, the depth is zero without any clearance volume, then high volume efficiency is achieved. As apply unloading device and servo automatic air volume adjustment, the energy save effects is significant which as much as more than 60% of lower consumption when running at no load.