Abstract: Two pairs of double-acting pistons are disposed in a hollow toroidal structure. Each pair of the double-acting pistons is operatively connected together and moves in the same rotational direction. Each of the pistons has two heads which cooperate with a head of an adjacently disposed piston to define a cylinder therebetween. Near the closest approach of two opposed heads, the fuel-air mixture is burned and the pistons recede from each other on an expansion stroke. Near their farthest separation, one of the opposed heads of the defined cylinder first opens an exhaust port and then the other one of the opposed heads opens an intake port. On a return compression stroke, during which the opposed heads move toward each other, the exhaust port is closed first and the intake port last.