A rotary piston internal combustion engine of the Wankel type has a housing (1) with a two lobed epitrochoidal inner peripheral surface (14) a shaft (8) journalled in end casings (3, 4), a rotor (9) eccentrically mounted on the shaft (8) and geared to rotate at one third speed of the shaft (8) whereby working chambers (34, 35, 36) are formed between flanks of the rotor (9) and the end casings (3, 4), which chambers vary in volume as the rotor (9) rotates, the rotor (9) being cooled by a cooling medium in a generally closed cooling circuit, the medium being circulated by a circulating pump (27), via connecting passageways (20, 18) in the end casings (3, 4) and an internal passageway (21) of the rotor (9), and through an external cooling heat exchanger (24), the cooling medium being blow-by gasses from the high pressure working chambers, which gasses have leaked past the rotor side seals (26) into the rotor cooling passages.