Abstract: A rotary hydraulic engine is disclosed having a stationary crankshaft with an eccentric crank portion disposed thereon. A housing is rotatable about the crankshaft. At least two pairs of cylinders are connected to the housing and extend quadrilaterally therefrom. A piston is slidingly disposed within each cylinder, and is connected to an oppositely disposed piston so as to move in tandem when propelled by a fluid under pressure. An external valve mechanism is provided which is connected to the cylinders so as to sequentially actuate the pistons contained therein when supplied with fluid under pressure. A scotch yoke assembly integrates the motion of the pairs of pistons into a balanced array of weight and force vectors rotating around the axis of the eccentric crank portion of the stationary crankshaft. This sequential reciprocation of the pairs of pistons imparts a rotary motion to the housing, the rotary motion in turn causing the housing to act as a flywheel.