1. The screw air compressor has no air inlet and exhaust valve group in the suction process, and the air inlet is adjusted only by the opening and closing of an automatic regulating valve. When the tooth groove space of the main and auxiliary rotors turns to the opening at the inlet end of the casing, the space is the largest. At this time, the tooth groove space under the rotor is connected with the free space of the air inlet. Because all the air in the tooth groove is discharged during exhaust, when the exhaust is completed, the tooth groove is in a vacuum state, and the external air is sucked in and flows into the tooth groove of the main and auxiliary rotors along the axial direction, When the air fills the whole tooth groove, the air inlet side end of the rotor rotates away from the air inlet of the casing, and the air between the tooth grooves is closed. The above is the "suction process". 2. At the end of air suction in the sealing and conveying process, the main and auxiliary rotor teeth are sealed with the casing, and the air in the tooth groove will no longer flow out, that is, the "sealing process". The two rotors continue to rotate, their tooth peaks coincide with the tooth grooves at the suction end, and the coincidence surface gradually moves to the exhaust end, forming a "gas transmission process". 3. During the compression process and fuel injection process, the mating surface gradually moves to the exhaust end, that is, the space between the mating surface and the exhaust port gradually decreases, the air in the groove is gradually compressed and the pressure gradually increases, that is, the "compression process". At the same time of compression, the lubricating oil is also sprayed into the compression chamber due to the effect of pressure difference to mix with air. 4. During the exhaust process, when the end face of the exhaust port of the rotor is connected with the exhaust port of the casing (at this time, the gas pressure is the highest), the compressed gas begins to exhaust until the mating surface of the tooth peak and the tooth groove moves to the end face of the exhaust end of the casing. At this time, the tooth groove space between the mating surface of the two rotors and the exhaust port of the casing is zero. The "exhaust process" is completed. At the same time, the length of the tooth groove between the mating surface of the rotor and the air inlet of the casing reaches the longest, thus starting a new compression cycle.