Abstract: A rotary 2-cycle diesel engine has five circumferentially spaced cylindrical combustion chambers orbiting in one direction for engagement with four spherical pistons mounted for parallel orbit in the opposite direction. The pistons and combustion chambers interact through a 72.degree. arc of the combustion chamber while aspiration, exhaust and cooling are effected by centrifugal movement of the air during the remaining 288.degree. arc. The connecting rods of the pistons are provided with a double bend and are geared for rotation to provide a longer piston stroke. A cam wheel and follower arrangement control the necessary piston orbiting speed fluctuation in relation to the combustion chambers' angular velocity.