An internal combustion engine (10) comprises an engine block (12) defining a cylinder (14) having a longitudinal axis A. A piston (16) is arranged slidably within the cylinder 14 and an impeller 18 is arranged at one end of the cylinder (14). The impeller (18) is rotatably mounted on a shaft (30), which extends out of the cylinder (14) and which is driven in rotation by rotation of the impeller (18). The engine further comprises an anti- rotation formation (20) to prevent the piston rotating about a longitudinal axis of the cylinder and a swirl-inducing vane (38) arranged on the face of the piston which faces the end of the cylinder at which the impeller is arranged. Combustion gas generated by combustion of a fuel in the cylinder between the piston and the impeller is caused to swirl by reaction with the swirl-inducing vane and the swirling combustion gases, in turn, cause the impeller to rotate.