PURPOSE: To obtain rotary motion even at the top dead center and to eliminate frictional force between a piston and a cylinder by changing the reciprocating motion of a piston into the alternate rotation by gears and changing it into rotary motion in a fixed direction by a moving gear engaged with a gear of an alternate rotating shaft and a gear train for reversing the rotating direction of both ends.

CONSTITUTION: Pistons of opposite cylinders are connected to each other by one connecting rod 1, 2, and racks are mounted on the connecting rod 1, 2 to be engaged with both sides of a piston 14, whereby when the pistons are reciprocated, a gear 3 on the same axis as the pinion 14 is alternately rotated. The shaft of a moving gear engaged with the gear 3 is supported by a bearing having a groove long sideways to move the moving gear 4 from side to side. When the gear 3 is rotated clockwise, the moving gear 4 is moved to the right to be engaged with a gear 6. When the gear 3 is rotated counter-clockwise, the moving gear 4 is moved to the left to be engaged with a gear 9. Thus the gears 6, 7, 8, 9 engaged with each other are rotated always in the same direction. Accordingly, even at the top dead center of reciprocating motion, rotary motion is obtained and a connecting rod is kept from tilting not to cause frictional force between the pistons and the cylinders.