Abstract: An internal combustion engine has a rotating annular cylinder containing a plurality of mutually oscillating pairs of pistons, one of each pair being rigidly fixed to a first axially-oscillating tube, the other of each pair being fixed to a second such tube concentric with the first. Each of the tubes is connected, by a "Scotch yoke", to a crank throw on a crank shaft. Each Scotch yoke includes an arm fixed at one end to its associated tube for oscillation therewith. The arm includes an elliptical slot that carries an annular ball bearing race for movement therein, one crank throw being journaled within each race. The cylinder is rotated by the crankshaft through a gear train in a fixed ratio to the rotation rate of the shaft. The pistons, having conical end faces, impart a rotational motion to the crankshaft during the power stroke of the cycle, the rotation of the crankshaft, in turn, imparting an oscillation to the piston pairs to actuate them in accordance with the strokes of the engine cycle.