A rotary positive displacement machine, and more particularly an internal combustion engine in one of its simplest of many forms, has a housing (1) with inner walls closely surrounding four similar rotors (2, 3, 4 and 5) arranged in a symmetrical 'square' formation on axles supported by bearings in the housing. Each rotor has evenly spaced lobes and, during rotation, the lobes of neighbouring rotors are caused to interlocate, with gearing on the axles maintaining the opposite sense of rotation of such rotors. Chambers are defined between rotors, lobes and housing and, during rotation, working fluid enters through inlet ports (9 and 10) to be carried around to compression regions (21 and 22) from which sections of fluid become transferred between rotors and ignited, thereafter to join expansion regions (25 and 26) before becoming exhausted through outlet ports (11 and 12). The compression ration so achieved varies and converges to a predictable value.