排气 is a separable verb (离合词) meaning 'to expel gas or air', commonly used for machinery exhaust, vehicle emissions, or ventilation. The verb 排 and object 气 can split when adding complements, measure words, or duration: 排了一次气 ('vented once') or 排了半个小时的气 ('vented for half an hour'). You can also combine 排气 with nouns like 系统 ('system') or 管 ('pipe') to form compound terms.