Abstract: A rotary power unit, comprising a housing having a circular opening and a plurality of bores extending along a radial axis from a center of the opening, a nodular rotor mounted within the opening of the housing and coaxially rotatable within the opening. The nodular rotor comprises a plurality of nodes equally distributed along the bounding circle thereof and the number of nodes is an odd integer less than the number of bores in the housing. A plurality of replaceable cylinder modules are fixedly receivable within a respective bore within the housing. Each cylinder module comprises a piston slidable within a cylinder, a piston actuating member associated with a each piston and a work unit associated with a cylinder head at a distal end the cylinder. Each piston is displaceable along the radial axis between a Top Dead Center (TDC) and a Bottom Dead Center (BDC), the pistons being biased into the BDC.