A power unit (14) which has a spark-ignition engine unit (1) of the Wankel type including a three flanked rotary piston (R) mounted on an eccentric shaft (63) rotating eccentrically within a cavity within a two lobed epitroochoidal inner peripheral surface (14), the cavity including an inlet port (7) through which air at ambient pressure is induced into the working chambers, and an outlet port (45) through which exhaust gasses are exhausted from the working chambers, and the power unit also including a rotary expander unit (4) which has a two flanked rotor (24) mounted on an eccentric shaft (12) rotating eccentrically within a single lobed epitrochoidal chamber (25), both shafts (62, 63) being coupled to rotate together, and the exhaust port (45) of epitrochoidal cavity of the engine unit (1) being connected to an inlet port (26) of the expander unit (4) so that the exhaust gasses from the engine unit (1) are further expanded in the expander unit (4).