PURPOSE: To smoothly and efficiently change a reciprocating motion into a rotary motion, by converting the reciprocating motion into a rocking motion through levers etc. and converting the rocking motion into the rotary motion in a prescribed direction through a combination of oppositely operating freewheels, gears, etc.

CONSTITUTION: Cylinders 17W20 are arranged so that when the cylinder 17 is in a position to start a process of explosion, the other cylinders 18W20 are in positions to start processes of intake, exhaust and compression, respectively. The reciprocating motion in the cylinders (17, 19), (18, 20) is changed into the rocking motion of levers 9 through connecting rods (12, 14), (15, 16). When the levers 9 are turned downwards, a gear 7 is rotated clockwise through a freewheel 8 and a shaft coupled with a gear 5 is rotated clockwise through gears 5, 6. When the levers 9 are turned upwards, a gear 2 is rotated counterclockwise through a freewheel 3 and the shaft coupled with the gears 5, 1 is rotated clockwise through the gear 1.