The invention relates to an engine (1) including: —a chamber (3) designed to accommodate a working fluid, —a first piston (4) defining the volume of said chamber (3), —a first passage (5) located in said first piston (4) to supply the chamber (3) with working fluid and/or to discharge from the chamber (3) the burned fluid resulting from the combustion of the working fluid, —a first valve (6) mounted on the first piston (4) to monitor the opening and closing of said first passage (5), —an output shaft (8) that engages with the first piston (4) to convert the motion of the first piston (4) into rotational motion of the output shaft (8), characterized in that the output shaft (8) and the first valve (6) engage to convert the motion of output shaft (8) into motion of the first valve (6).