The invention relates to an engine including: —a cylinder that contributes to define a chamber (3), —a first piston (4), said first piston (4) and cylinder are subjected to a first relative back-and-forth motion, —an output shaft (8), —a second piston (14), said second piston (4) and cylinder are subjected to a second relative back-and-forth motion, said output shaft (8) mounted coaxially to said pistons (4, 14), —a first means for converting (5) said first relative back-and-forth movement into rotational motion of the output shaft (8), including, on one side, a first corrugated guide track (9) and, on the other side, a first guide element (10) designed to move along said guide track (9), —a first adjustment member (5) to position the first guide track (9).