An engine includes a radial arrangement of cylinders each having a reciprocating piston with a piston head and a connecting rod pivotally linked to the piston head at an upper end. A lower end of each connecting rod is pivotally linked to a crank disk that is rotatably mounted on a crank arm of a crankshaft. Steam intake valves at each cylinder are momentarily opened by a bearing cam roller that is moved in a circular path by rotation of the crank disk to sequentially engage spring urged cam followers on inboard ends of radially extending valve stems. Low pressure steam or gas is injected into the top of each cylinder, as the intake valves of the cylinders are opened in sequence, thereby forcing the piston in each cylinder through a power stroke to move the crank disk and turn the crankshaft. Angular displacement of each connecting rod through the return stroke of the piston urges an exhaust reed valve on the piston head to an open position, thereby releasing exhaust steam to a condenser chamber. The engine is self-starting and operates in a low pressure, low temperature range, using waste heat from an external source, such as exhaust from an internal combustion engine, burning of refuse (e.g. garbage or other solid waste material) or solar heat.