In addition to the consumers of a closed loop, known air supply systems also can supply an external consumer with fresh pressurized air. These systems are very complex with respect to apparatus and furthermore have the disadvantage that already improved pressurized air can escape from the pressurized air store during the supply of the external consumer. A switchable 3/2-directional valve (31) is arranged in the first consumer pressure line (19) of the drive unit (1) and a switchable 3/2-directional valve (32) is mounted in the store pressure line (11) of the drive unit (1) and a switchable check valve is mounted in the bypass line (16) of the store pressure line (11). This check valve disables the throttle function of the bypass throttle (17).